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Universal inverse deuterium isotope effect on the Tc of BEDT-TTF-based molecular superconductors
Physica C 351 (2001) 261±273
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Universal inverse deuterium isotope eect on the Tc of BEDT-TTF-based molecular superconductors J.A. Schlueter a,*, A.M. Kini a, B.H. Ward a, U. Geiser a, H.H. Wang a, J. Mohtasham b, R.W. Winter b, G.L. Gard b a
Materials Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Building 200, Argonne, IL 60439-4831, USA b Department of Chemistry, Portland State University, Portland, OR 97207-0751, USA Received 22 May 2000; received in revised form 1 September 2000; accepted 11 September 2000
Abstract Deuterium substitution of the ethylene end groups of the ET [ET: BEDT-TTF or bis(ethylenedithio)tetrathiafulvalene] electron-donor molecule results in a 0.25 K increase in the superconducting transition temperature of three molecular superconductors derived from this molecule. Simplistically speaking, this change in Tc is in contradiction to that predicted by the electron±phonon coupling mechanism of the BCS theory. We suggest that the slight shortening of the C±D bond relative to the C±H bond, coupled with the recent ®ndings of a large, negative uniaxial pressure derivative of Tc in j-(ET)2 Cu(SCN)2 and b00 -(ET)2 SF5 CH2 CF2 SO3 , can explain this unique eect. Herein we report the ®rst study of the eect of deuterium substitution on the superconducting transition temperature in a molecular-based superconductor in which the electron-donor molecules are packed in a b00 motif, viz., b00 -(ET)2 SF5 CH2 CF2 SO3 . This compound is ideally suited for this study because it contains discrete (non-polymeric) anions, has a completely ordered structure, is inde®nitely stable in air at room temperature, and is free from possible magnetic impurities. Substitution of the eight hydrogen atoms of the ET molecule by deuterium causes the Tc of b00 -(ET)2 SF5 CH2 CF2 SO3 to increase from 4:34 0:05 to 4:61 0:03 K. These values were determined by measuring several representative crystals from various parallel electrocrystallization experiments containing h8 - or d8 -ET that was prepared in parallel syntheses. This is the ®rst example which demonstrates that the inverse (positive) isotope eect previously observed in j-phase salts is also present in a b00 -phase superconductor. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Organic superconductors; Superconducting transitions Tc ; Bis(ethylenedithio)tetrathiafulvalene; Penta¯uoroalkylsulfonate anion; Isotope eect
1. Introduction
* Corresponding author. Tel.: +1-630-252-3588; fax: +1-630252-9151. E-mail address: [email protected] (J.A. Schlueter).
The number of known molecular-based superconductors is rapidly approaching 100. Over half of these superconductors contain the ET [BEDTTTF, or bis(ethylenedithio)tetrathiafulvalene] electron-donor molecule (see Fig. 1 for an illustration
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 0 ) 0 1 6 2 0 - 8
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Fig. 1. Diagram of d8 -ET.
of the fully deuterated ET molecule, d8 -ET). The majority of these salts contain either simple inorganic anions (such as I±3 or ReO±4 ), or complex polymeric anions (such as Cu(SCN)±2 or Cu[N(CN)2 ]Br± ) [1]. The record superconducting transition temperature (Tc ) for cation-radical molecular superconductors at ambient pressure stands at 11.6 K for j-(ET)2 Cu[N(CN)2 ]Br [2]. A slightly higher Tc (12.8 K) has been observed in the isostructural j-(ET)2 Cu[N(CN)2 ]Cl salt under modest applied pressure of 300 bar [3]. We have recently focussed our search for new superconducting salts on molecular-based materials containing discrete organometallic or organic anions which can be rationally designed and synthesized prior to the electrocrystallization procedure. Through use of the M(CF3 )±4 (M: Cu, Ag or Au) anions, we were able to double the number of known cation-radical-based molecular superconductors [4]. Ambient-pressure superconductivity is observed in the j-(ET)2 M(CF3 )4 (1,1,2-trihaloethane) family superconductors with superconducting transition temperatures ranging from 2.1 K in jL -(ET)2 Au(CF3 )4 (1,1,2-trichloroethane) [5] to 11.1 K in jH -(ET)2 Ag(CF3 )4 (1,1,2-trichloroethane) [6]. More recently, we have turned our attention to the synthesis of completely organic salts in which both the electron-donor molecule and the charge compensating anion are organic in nature. Such salts are essentially free of the possible magnetic impurities which can complicate the physical property measurements of charge-transfer salts. Initially, we have chosen to use sulfonate anions which can be modi®ed through chemical methods. Through this rationale, we have synthesized the ®rst completely organic superconductor, b00 -(ET)2 SF5 CH2 CF2 SO3 , which shows an onset of superconductivity near 4.5 K [7]. The eect of isotopic substitution on the Tc of molecular superconductors has been an active
area of research over the past decade. The earliest isotope studies on ET-based salts were performed on the b-(ET)2 I3 superconductor. These studies showed that a large (0.28 K), positive shift in Tc occurs upon deuteration of the ET molecules in the Ôlow-Tc phaseÕ of this salt [8]. This shift is inverse to that predicted by the BCS model assuming a mass isotope eect. Interestingly, the Ôhigh-Tc phaseÕ of the same salt seems to show a normal (negative) shift [9]. These results are dicult to interpret because a large range to Tc s (1±8 K) has been reported for this salt due in part to a kinetically slow crystallographic ordering of the terminal ethylene groups of the ET electron-donor molecule [10±12]. Furthermore, it has been shown that domains of the a-, b-, b -, and at -phases of (ET)2 I3 can all occur in one apparently ÔsingleÕ crystal, resulting again in a range of Tc s from 1.5±8 K [13]. Similar experimental diculties have plagued efforts to determine the eect of 13 C substitution when either the central C@C bond [14±18] or when all double-bonded carbon atoms [19] of ET in the b-(ET)2 I3 superconductor are labeled. A large inverse (positive) shift of Tc has been reported upon deuteration of the ET molecules in j-(ET)2 Ag(CN)2 H2 O [20]. The superconducting transition temperature increases from 5 K for j-(h8 -ET)2 Ag(CN)2 H2 O to 6 K for j-(d8 -ET)2 Ag(CN)2 H2 O. It has been demonstrated that the superconducting transition in the j-(ET)2 Cu[N(CN)2 ]Cl salt at 0.3 kbar increases 0.5±1.5 K upon deuteration of the ET molecules [21]. Although sample dependent Tc s and the diculties associated with performing these experiments under pressure complicated this experiment, it is clear that a definite inverse isotope eect is present in this salt with a shift in Tc of at least 0.5 K. The eect of deuterium substitution on Tc has also been studied for the j-(ET)2 Cu[N(CN)2 ]Br superconductor. Although an abnormally large negative isotope eect (DTc 0:3 to 0.9 K) has been reported, there is some ambiguity in these results due to the use of randomly oriented crystals with relatively broad superconducting transitions (DT 10±90% 3 K) [22±25]. These results are complicated by the fact that at ambient pressure, j-(d8 ET)2 Cu[N(CN)2 ]Br lies near an antiferromagnetic/
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superconducting phase transition, and some ÔsingleÕ crystals contain a signi®cant fraction of the insulating antiferromagnetic phase [26,27]. The isotope eect due to 13 C or 34 S labeling is less sensitive to the proximity of this phase boundary. No discernable shift of Tc has been observed upon 13 C substitution of only the central C@C bond [28,29] or all double-bonded carbon atoms of ET in j-(ET)2 Cu[N(CN)2 ]Br [30]. The substitution of the four ethylene carbon atoms of ET with 13 C resulted in no detectable isotope eect on the superconductivity in j-(ET)2 Cu[N(CN)2 ]Br [25]. We previously reported a small negative shift in Tc , 0:08 0:07 K, upon substitution of the sulfur atoms with 34 S. This indicates that the C±S stretching modes do not provide a dominant coupling mechanism in this salt, but a mass eect consistent with the BCS theory is possible [31]. Among molecular superconductors, the eect of isotopic substitution on Tc has been most thoroughly studied in the j-(ET)2 Cu(SCN)2 salt. It has been convincingly shown that an inverse (positive) shift in Tc occurs upon deuteration [23,32±37]. Although the magnitude of this shift has been reported to range from 0.3 to 0.8 K, our carefully performed, systematic studies have indicated that the deuterium isotope eect for this salt is on the lower end of this range (DTc 0:30 0:07 K) [38]. Extensive 13 C and 34 S labeling studies have also been performed on the j-(ET)2 Cu(SCN)2 superconductor. Initial studies of 13 C labeling of the terminal ethylene carbon atoms of ET showed either slightly positive [35,39] or slightly negative [23] shifts in Tc . More recently, our extensive studies found that this labeling had no measurable eect [38]. Labeling all the doubly bonded carbon atoms with 13 C also resulted in no observable shift of Tc [30]. Also, fully labeling ET with eight atoms of 34 S showed no discernable eect [31]. However, we were able to show the ®rst genuine mass isotope eect on Tc for an electron-donor-based superconductor by simultaneously labeling both the ethylene groups with 13 C and all the sulfur atoms with 34 S [38,40]. It is interesting that j-(ET)2 Cu(SCN)2 exhibits two dierent isotope eects on Tc depending on where the labels are placed. It was therefore of
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interest to investigate the eect of 13 C and 15 N labels on the Cu(SCN)±2 anion. Substitution of 15 N for 14 N did not result in any observable shift in Tc [39]. We have subsequently shown that simultaneous substitution by 13 C and 15 N in this anion also does not aect the superconducting transition temperature within experimental uncertainty [41]. To date, this is the only study of the eect of isotopic substitution within the anion layer on Tc . More recently, we have measured the eect on Tc of deuterium substitution on the ET molecules of the jL -(ET)2 Ag(CF3 )4 (1-bromo-1,2-dichloroethane) superconductor [42]. The electron-donor layer of this salt is isostructural to j-(ET)2 Cu[N(CN)2 ]Br, but the anion layer contains discrete Ag(CF3 )±4 anions and neutral 1-bromo-1,2-dichloroethane cocrystallized solvent molecules. This study showed similar results to that observed for j-(ET)2 Cu(SCN)2 : an inverse (positive) shift of Tc of about 0.2 K from 2:90 0:04 to 3:11 0:04 K. A summary of the previous studies of the eect of isotopic substitution on Tc of ET-based superconductors is provided in Table 1. We felt that it was important to extend these isotope studies to the b00 -(ET)2 SF5 CH2 CF2 SO3 system for several reasons. First, the SF5 CH2 CF2 SO±3 anions are ordered at room temperature and there does not seem to be any cooling rate dependence of the superconducting transition. Secondly, the crystals are quite stable under ambient conditions: there is no cocrystallized solvent in the structure and the anion is stable toward oxidation, disproportionation or other means of decomposition. Thirdly, these crystals are stoichiometric and no other phases are known. Thus, there are no complications from crystals which contain varying stoichiometries, phases or Tc s. Fourthly, hydrostatic pressure studies have revealed that the Tc of b00 (ET)2 SF5 CH2 CF2 SO3 decreases monotonically with pressure and thus there are no complications arising from a close proximity of the superconducting state to an antiferromagnetic regime [43]. Finally, studies of the eect of isotopic substitution on Tc have largely focussed on j-phase materials, and to a limited extent on b-phase salts. Herein is reported the ®rst study of the eect of isotopic substitution on the superconducting
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Table 1 Summary of experiments on ET-based molecular superconductors which have probed the eect of various isotopic substitutions on the superconducting transition temperature Compound
Isotopea
Tc (K)b
DTc (K)
Methodc
Ref.
bL -(ET)2 I3 bH -(ET)2 I3
d8 d8 13 C2 13 C2 13 C2 13 C6
1.15 7 8:0 0:1 8:0 0:1 6:95 0:05 7:2 0:05
0.28 (±) 0:6 0:1 0:2 0:1 0 0:1 0:1 0:1
AC mag DC mag Res AC res DC mag DC mag
[8] [9] [14] [15,16] [17,18] [19]
j-(ET)2 Ag(CN)2 H2 O
d8 d8
5.0 5.0
1.0 1.0
DC res DC mag
[20] [20]
j-(ET)2 Cu[N(CN)2 ]Br
d8 d8 d8 d8 13 C2 13 C4 13 C6 34 S8
11.2 10:9 0:3 11.7 11.3 11:40 0:05 11.3 11:39 0:01 11:28 0:05
0:9 0:3 0:5 0:4 0:05 0:08 0.0 0:03 0:02 0:08 0:07
DC mag DC mag DC res DC mag AC mag DC mag AC mag AC mag
[22] [23] [25] [25] [28,29] [25] [30] [31]
j-(ET)2 Cu(SCN)2
d8 d8 d8 d8 13 C2 13 C4 13 C4 13 C6 13 C4 34 S8 d8 13 C4 34 S8 15 13 N C
8:7 0:2 10:3 0:1 8.8 9:20 0:05 9:30 0:05 8:7 0:2 9:16 0:07 9:32 0:05 9:16 0:04 9:24 0:06 9:08 0:10
0:3 0:3 0.7 0.8 0:30 0:07 0:02 0:08 0:1 0:3 0:00 0:09 0.03 0:12 0:08 0:15 0:08 0:05 0:12
DC mag DC res, RF AC mag AC mag AC mag DC mag AC mag AC mag AC mag AC mag AC mag
[23] [33] [36] [38] [28,29] [23] [38] [30] [38] [38] [41]
jL -(ET)2 Ag(CF3 )4 (solvent)d
d8
2:90 0:04
0:21 0:06
AC mag
[42]
b00 -(ET)2 SF5 CH2 CF2 SO3
d8
4:34 0:05
0:27 0:06
AC mag
This work
a
13
d8 : substitution of all eight hydrogen atoms of ET with deuterium. C2 : substitution of the central double bonded carbon atoms with 13 C. 13 C4 : substitution of the four ethylene carbon atoms with 13 C. 13 C6 : substitution of all doubly bonded carbon atoms with 13 C. 13 C4 34 S8 : substitution of all eight sulfur atoms with 34 S8 and the four ethylene carbon atoms with 13 C4 , d8 13 C4 34 S8 : substitution of all eight hydrogen atoms with deuterium, all eight sulfur atoms with 34 S8 and the four ethylene carbon atoms with 13 C4 . 15 N13 C: substitution of 15 N13 C in the anion layer. b Superconducting transition temperature of the salt with natural isotopic abundance. When multiple de®nitions of the superconducting transition temperature are given, the linearly extrapolated value is displayed. c AC mag: AC magnetization; DC mag: DC magnetization; AC res: AC resistivity; DC res: DC resistivity; RF: RF penetration. d Solvent is 1-bromo-1,2-dichloroethane.
transition temperature in a b00 -phase salt. As an initial test, we chose to study the eect of deuterium substitution on the ET electron-donor molecule because of the aforementioned discoveries of signi®cant shifts in Tc , inverse to that predicted by the BCS theory for both the j-(ET)2 Cu(SCN)2 (Tc 10 K) [32±36] and j-(ET)2 Ag(CF3 )4 (1-bromo-1,2-dichloroethane) (Tc 3 K) [42] salts. This
study was designed to probe whether the inverse (positive) isotope shift observed in several ETbased superconductors is speci®c to the j-phase packing motif. We were interested to know how the sign and magnitude of any shift in Tc would compare to those previously found for the j-phase salts. Such information will be necessary to thoroughly understand the cause of this eect and to
J.A. Schlueter et al. / Physica C 351 (2001) 261±273
design materials with increased superconducting transition temperatures. 2. Experimental ET containing both the natural isotopic composition (h8 -ET) and d8 -ET were synthesized in strictly parallel experiments with the use of previously reported methods from 1,2-dibromoethane and d4 -1,2-dibromoethane (D, 99%), respectively [44±47]. The precursor anion salt, LiSF5 CH2 CF2 SO3 was prepared from NaSF5 CH2 CF2 SO3 as previously reported [7,48]. Single crystals of b00 (ET)2 SF5 CH2 CF2 SO3 were grown as previously described [7] (by the electrocrystallization method with LiSF5 CH2 CF2 SO3 and 12-crown-4 as the electrolytes in 1,1,2-trichloroethane) from normal ET and d8 -ET in identically parallel experiments. A modi®ed H-cell design was used in which four disposable Whatman GF/F ®lters replaced the traditional glass frit. The two halves of the H-cell were sealed together with a horseshoe style spherical joint clamp and a size 116 O-ring. Superconducting transitions were determined as described previously [31] with the use of a commercial AC susceptometer (Lake Shore Cryo-
265
tronics, Inc.) equipped with a low temperature helium subpot option (allowing measurements to be achieved at temperatures as low as 1.2 K), and operated with a frequency of 125 Hz. Rod-like single crystals were supported on the end of a Delrinâ isolation rod with a minimal amount of Apiezonâ N grease. The magnetic ®eld was aligned perpendicular to the plate face (H ? ab). These crystals were cooled from room temperature to 1.5 K over a period of 15 min. The real (in-phase, v0 ) and imaginary (out-of-phase, v00 ) components of the volume AC susceptibility for each crystal were recorded at temperature intervals of 0.05 K on slow warming from 1.5 to 5.5 K. The lower critical ®elds of b00 -(h8 -ET)2 SF5 CH2 CF2 SO3 have previously been determined [49]. For a ®eld applied perpendicular to the ET planes, Bc1 T 0 20 5 Oe. As illustrated in Fig. 2, the AC susceptibility was measured for a 0.308 mg crystal of b00 -(h8 -ET)2 SF5 CH2 CF2 SO3 with AC ®elds of 1, 2, 5, 10 and 20 G, applied perpendicular to the ET planes. The data obtained at 1 G were rather noisy, making it dicult to obtain a de®nitive value for Tc . The transition curve obtained with a 2 G ®eld is much smoother, with only a slightly suppressed Tc . Thus, we chose to perform this study with a 2 G AC ®eld.
Fig. 2. Volume AC susceptibility data, obtained from a single crystal of the b00 -(ET)2 SF5 CH2 CF2 SO3 at ®elds of 1, 2, 5, 10, and 20 G.
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A plot of typical data obtained for a high quality single crystal of b00 -(h8 -ET)2 SF5 CH2 CF2 SO3 is shown in Fig. 3. Sample volumes were based on the experimental mass of each crystal, which ranged from 0.479 to 0.668 mg for b00 -(d8 ET)2 SF5 CH2 CF2 SO3 and from 0.151 to 0.956 mg for b00 -(h8 -ET)2 SF5 CH2 CF2 SO3 , combined with the density (1.941 g/cm3 for the h8 salt and 1.971 g/ cm3 for the d8 salt at 123 K) calculated from X-ray unit-cell data. Temperatures were determined with the use of a diode sensor, calibrated to an accuracy of better than 15 mK at temperatures below 20 K. In total, the superconducting transitions were determined for ®ve single crystal specimens of b00 (d8 -ET)2 SF5 CH2 CF2 SO3 and seven single crystals of b00 -(h8 -ET)2 SF5 CH2 CF2 SO3 . For ready visual comparison, the ``internal'' susceptibility (vint ) data (as de®ned below) for the seven h8 and ®ve d8 crystals studied herein are plotted in Fig. 4 over the temperature region 2.5 to 5.5 K. This was done by linearly correcting the v0 data to a normal-state value of 0 (at 4.5 K) and calculating an experimental demagnetization factor, assuming a saturation susceptibility of ±1 (at 1.5 K), for each crystal according to the following equation:
vint
v0 1 Dv0
1
where vint is the internal susceptibility, v0 is the measured volume susceptibility, and D is the demagnetization factor. Finally, vint was calculated for each data point through the use of this experimentally determined demagnetization factor. These D values (listed in Table 2) ranged from 0.36 to 0.65. The Tc s of each of these samples were determined according to four dierent de®nitions: Tco is the diamagnetic onset, Tcl is the linearly extrapolated mean-®eld value, Tcm is the midpoint of the superconducting transition, and v00max is the maximum in the imaginary component of the AC susceptibility. These values, for each of the 12 crystals measured, are listed in Table 2. The tabulated transition widths, DTc 10±90% , de®ned as temperature dierence between 10% and 90% of the superconducting transition, are indicators of the high crystal quality of the specimens selected. In addition, this table lists, for each de®nition, the average Tc for the groups of crystals, the standard deviation in the mean, the isotopic shift, DTc Tc (d8 -ET) Tc (h8 -ET), and the propagated standard deviation in this temperature dierence.
Fig. 3. Volume AC susceptibility data, obtained from a single crystal of b00 -(h8 -ET)2 SF5 CH2 CF2 SO3 with an ac ®eld of 2 G. A sharp superconducting transition is observed in both the real (in-phase, v0 ) and imaginary (out-of-phase, v00 ) components.
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Fig. 4. Volume susceptibility v0 (corrected for demagnetization) for ®ve crystals of b00 -(d8 -ET)2 SF5 CH2 CF2 SO3 and seven crystals the b00 -(h8 -ET)2 SF5 CH2 CF2 SO3 showing the high reproducibility in susceptibility of these single crystal samples and a clear increase in the superconducting transition temperature for the deuterated samples. Table 2 Superconducting transition temperatures (Tc ) at ambient pressure for the b00 -(ET)2 SF5 CH2 CF2 SO3 organic superconductor with compositions of natural isotopic abundance (h8 ) and with deuterium (d8 ) substitution of the hydrogen atoms in the ET electron-donor moleculea Crystal
Tco (K)
Tcl (K)
Tcm (K)
Tcv00 (K)
DTc 10±90% (K)
D
d8 -b -(ET)2 SO3 CF2 CH2 SF5 1 2 3 4 5
4.72 4.66 4.75 4.69 4.69
4.65 4.60 4.60 4.64 4.57
4.20 4.26 4.24 4.19 4.19
4.18 4.22 4.17 4.13 4.16
0.93 0.76 0.76 0.86 0.89
0.36 0.50 0.37 0.46 0.51
Mean
4:70 0:03
4:61 0:03
4:22 0:03
4:17 0:03
0:84 0:08
h8 -b -(ET)2 SO3 CF2 CH2 SF5 1 2 3 4 5 6 7
4.47 4.51 4.54 4.48 4.46 4.50 4.52
4.35 4.38 4.39 4.26 4.28 4.36 4.36
3.88 3.85 3.80 3.84 3.84 3.85 3.93
3.91 3.77 3.73 3.78 3.84 3.84 3.93
1.06 1.02 1.17 0.96 0.92 0.90 0.80
Mean DTc
4:50 0:03 0:20 0:04
4:34 0:05 0:27 0:06
3:86 0:04 0:36 0:05
3:83 0:07 0:34 0:08
0:98 0:12
00
00
0.43 0.49 0.61 0.63 0.65 0.49 0.43
a
Superconducting transition temperatures were determined on AC susceptibility data, uncorrected for demagnetization eects, according to the following de®nitions: Tco is the diamagnetic onset temperature, Tcl is the linearly extrapolated mean-®eld Tc , Tcm is the temperature of the transition midpoint, Tc v00 is the temperature at the maximum in the imaginary component of the AC susceptibility. The 10±90% transition width, DT 10±90% , and the experimentally determined demagnetization factor, D, are also tabulated.
High quality crystals with sharp superconducting transitions, DTc 10±90% 6 1:0 K, were used in
these experiments. For the analysis of DTc following isotopic substitution, we have used the
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linearly extrapolated values, DTcl , because of our four de®nitions of Tc the estimated standard deviations are the lowest for Tcl . It should be noted that the superconducting onset temperature, Tco , is the value reported in our initial publication of b00 (ET)2 SF5 CH2 CF2 SO3 [7]. The uncertainty in Tco is quite large due to the diculty in determining the temperature at which the susceptibility ®rst deviates from the normal-state background value. Upon inspection of Table 2, it is clear that a sizable shift (>3r), DTcl 0:27 0:06 K, occurs upon deuteration of the ET donor molecule, and a comparable shift occurs in all cases for the various de®nitions of Tc . Fig. 4 clearly illustrates that sharp superconducting transitions are observed with a discernible shift in Tc to higher temperatures upon deuteration.
3. Discussion and conclusions A packing diagram of the b00 -(ET)2 SF5 CH2 CF2 SO3 superconductor is provided in Fig. 5 [7].
Short contacts between the sulfur atoms in adjacent ET stacks create a two dimensionally connected layer. Between these layers are located insulating sheets of charge compensating SF5 CH2 CF2 SO±3 anions. The hydrogen atoms of the ET molecule point directly at the anion layer and make short (hydrogen-bond like) contacts with the ¯uorine and oxygen atoms of the anion. Interlayer coupling, which is likely a prerequisite of superconductivity, will be directly aected by the exchange of deuterium for hydrogen. As summarized in Table 3, we have now measured the eect of deuterium substitution on the superconducting transition temperature by an essentially identical procedure for three molecular superconductors: jL -(ET)2 Ag(CF3 )4 (1-bromo-1,2dichloroethane) (Tcl 2:90 0:04 K) [42], b00 (ET)2 SF5 CH2 CF2 SO3 (Tcl 4:34 0:04 K) and j-(ET)2 Cu(SCN)2 (Tcl 9:20 0:05 K) [38]. The only signi®cant dierence in these measurements was that the AC ®eld used in the b00 -(ET)2 SF5 CH2 CF2 SO3 experiment was 2 G, while a 1 G AC ®eld was used in the other two experiments. Within experimental error, the Tc observed in all
Fig. 5. Packing diagram of b00 -(ET)2 SF5 CH2 CF2 SO3 as projected on the bc-plane. Depicted with ®ne lines are S S contacts shorter These contacts are responsible for the two-dimensional nature of this salt. The then the sum of the van der WaalÕs radii (d 3:60 A). hydrogen atoms of ET, which have been exchanged with deuterium, have been ®lled in for emphasis. These hydrogen atoms point directly at the anion layer, and are likely important for interlayer coupling.
J.A. Schlueter et al. / Physica C 351 (2001) 261±273 Table 3 Tabulation of the linearly extrapolated Tc (Tcl ), the shift in Tc upon deuteration of the ET electron-donor molecule (DTc ), and the relative shift in Tc (DTc /Tc ), for three molecular superconductors: jL -(ET)2 Ag(CF3 )4 (1-bromo-1,2-dichloroethane), b00 (ET)2 SF5 CH2 CF2 SO3 and j-(ET)2 Cu(SCN)2 jL -(ET)2 Ag(CF3 )4 (solvent) b00 -(ET)2 SF5 CH2 CF2 SO3 j-(ET)2 Cu(SCN)2
a
Tc (K)b
DTc (K)
2:90 0:04 4:34 0:05 9:20 0:05
0:21 0:06 0:27 0:06 0:30 0:07
a
Solvent 1-bromo-1,2-dichloroethane. Tc is the linearly extrapolated value for crystals of natural isotopic abundance. b
cases is about 0.26 K higher for the deuterated salt than that in crystals of natural hydrogen abundance. This shift in Tc is independent of packing motif (j or b00 ), transition temperature (2.9 to 9.2 K), and anion type (polymeric, organometallic, or organic). The direction of these shifts is opposite to that expected by the BCS model which predicts that the lower frequency phonons obtained in the deuterated salt would cause a lower transition temperature to be observed [50,51]. The use of BCS theory with molecular superconductors is complicated by the correct choice of the relevant mass entity. For the b00 -(ET)2 SF5 CH2 CF2 SO3 superconductor, several logical choices for the mass entity exist. Among these are the mass of the conductive TTF moiety (200 amu) of the ET molecule, the mass of one ET molecule (384 amu) or the mass of one formula unit of (ET)2 SF5 CH2 CF2 SO3 (1039 amu). If the TTF moiety is chosen as the relevant entity, deuteration of the ET molecule would not be expected to aect the superconducting transition temperature. If the mass of the ET molecule is chosen as the relevant mass (M) for intermolecular phonon±electron pairing, the magnitude of the ideal (BCS) theoretical conventional isotope eect (with Tc / M a , a 0:5) 0:5 would be DTc Tc 384=392 1 0:04 K. A similar shift is expected if one formula unit is chosen as the relevant mass. Thus, any mass-eect shift in Tc occurring as a result of the increased mass associated with deuterium substitution is masked by a much larger inverse (positive) shift in Tc . We have previously observed a BCS-type mass isotope eect on Tc upon the simultaneous sub-
269
stitution of the eight 32 S atoms and four 12 C atoms in j-(ET)2 Cu(SCN)2 with 34 S and 13 C, respectively [38]. This demonstrates that the eects of isotopic substitution in the core of the conducting layer appears to be described by a BCS-type mechanism. In contrast, our studies involving deuteration of the ethylene end groups of ET in jL -(ET)2 Ag(CF3 )4 (1-bromo-1,2-dichloroethane), b00 -(ET)2 SF5 CH2 CF2 SO3 and j-(ET)2 Cu(SCN)2 show an inverse (non-BCS) isotope eect. It can be envisioned that these hydrogen/deuterium atoms are more closely associated with the insulating anion layers than the conducting layer. Thus, it appears that a conventional (BCS) isotope eect is observed in the conducting layer while a non-conventional mechanism may be active across the non-conducting layers [40]. Our interest in probing this concept further was illustrated by an anion labeling experiment, in which 13 C and 15 N isotopes were substituted into the thiocyanate groups of j(ET)2 Cu(SCN)2 [41]. No observable shift in Tc was observed. The b00 -(ET)2 SF5 CH2 CF2 SO3 salt provides an ideal opportunity to further probe this concept. Deuteration of the SF5 CH2 CF2 SO±3 anion provides an alternative method to place deuterium atoms in the non-conducting layer. Thus, the synthesis and Tc determination of the b00 -(ET)2 SF5 CD2 CF2 SO3 salt will be of interest. The mechanism by which the conduction electrons are coupled in molecular superconductors is still open to debate. These systems are quite complex, and the coupling could occur by a combination of acoustic phonons, librational modes, and intramolecular vibrations. It is still not clear why deuteration of these molecular superconductors cause a positive (inverse) shift in the superconducting transition temperature, but several explanations have been proposed. This inverse eect has been attributed to an increase in the electron±phonon coupling in the deuterated salts [33,52]. According to the BCS theory of superconductivity, the superconducting transition temperature, Tc , is related to the Debye temperature, h, and the electron±phonon coupling constant, k, by the McMillan equation [50,51,53]: 1k T c h exp : k l
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The Coulomb pseudopotential, l , is a small correction term. In the weak coupling limit, this equation is reduced to: 1 T c / h exp : k For molecular superconductors, low-frequency intermolecular vibrations are expected to be the phonons responsible for superconductivity. For the ET-based materials discussed in this paper, the interactions between the hydrogen atoms of ET and the charge compensating anions will directly in¯uence the frequency of these lattice modes. The observed shift in Tc upon deuteration suggests that phonons that involve the motion of the hydrogen atoms of ET may play a signi®cant role in the coupling of the superconducting electrons. Softer intermolecular phonons are expected when the C±H anion contacts lengthen, thus weakening these interactions [54]. The vibrational frequency of a C±D bond is lower than that of a C±H bond. As a consequence of the lower zero point energy and the anharmonic contributions to the potential well, the C±D bond is slightly shorter than a C±H bond resulting in a longer C±D anion contact in an isostructural deuterated salt with identical lattice constants. However, in a carefully performed single crystal four-circle X-ray diraction study (300±12 K), Watanabe et al. have shown that deuterium substitution in both j-(ET)2 Cu(SCN)2 and j-(ET)2 Cu[N(CN)2 ]Br results in no signi®cant changes in lattice parameters perpendicular to the conducting plane [55]. Small, but measurable changes were observed at the lowest temperatures just above Tc in the lattice parameters within the conducting plane: 0.11% and 0.03% for c- and a-axes, respectively, in j-(ET)2 Cu[N(CN)2 ]Br and ±0.1% for c-axis in j-(ET)2 Cu(SCN)2 . If one assumes that deuterium substitution leads to a ÔsofterÕ lattice and thus an increased electron±phonon coupling, a higher superconducting transition temperature would be expected upon deuteration. Raman spectroscopy has been used to provide the ®rst direct experimental evidence for this suggested lattice softening induced by deuteration [56]. An inelastic neutron scattering study of j(ET)2 Cu(SCN)2 has shown that there is a strong
coupling between the superconducting electrons and intermolecular phonons [57]. Thus, an inverse isotope eect does not, by itself, rule out a phonon mediated pairing mechanism. Another possible explanation for the observed increase in Tc upon deuteration involves coupling through intramolecular vibrations of the organic electron-donor molecules [58,59]. Extensive 13 C, 34 S, and 2 H labeling studies of the j-(ET)2 Cu(SCN)2 superconductor have shown that the intramolecular C@C and C±S bond stretching vibrational modes of the ET molecule do not provide a dominant exchange mechanism for Cooper pairing [41]. These negative results do not, however, rule out the possibility that intramolecular vibrations play some role in the coupling mechanism [60]. Inelastic neutron scattering experiments have suggested that vibrations associated with the hydrogen atoms of the terminal ethylene groups may be associated with the superconducting charge carriers [61]. We propose that another contributing factor to the deuterium inverse isotope eect observed in ET-based superconductors may result from the highly anisotropic uniaxial pressure derivatives of Tc which have recently been derived from thermal expansion data [62]. For measurements perpendicular to the conducting planes of b00 -(ET)2 SF5 CH2 CF2 SO3 and j-(ET)2 Cu(SCN)2 , the uniaxial pressure dependencies of Tc were determined to be 5:9 0:25 and 6:2 0:25 K/kbar, respectively, which are essentially identical within experimental uncertainty. Half of the C±H(D) bonds of the ET molecules point toward the anion layer forming several C±H(D) anion contacts. The in-plane compressibility is governed by SáááS contacts on adjacent ET molecules, thus the C±H(D) bonds that are directed in the in-plane direction are expected to have little eect on the Ôinternal lattice pressureÕ. In contrast, the C±H(D) bonds directed toward the anion layer will directly aect the Ôlattice pressureÕ in the interplane direction. Since C±D bonds have a smaller zero-point displacement compared to C±H bonds, H to D substitution would lead to longer (or softer) intermolecular contacts with the atoms of the anion and with other donor molecules. Thus, the Ôinternal lattice pressureÕ perpendicular to the conducting planes
J.A. Schlueter et al. / Physica C 351 (2001) 261±273
would be reduced upon H to D substitution, and this change is expected to be of the same magnitude irrespective of the anion or the packing motif. The 0.25 K increase in Tc that we have reported for these salts upon deuteration would result from a very slight ÔnegativeÕ pressure of only about 40 bar. It should be noted that an inverse isotope has also been reported upon deuteration of the inorganic palladium hydride superconductor [63]. The increase in Tc upon deuteration of PdHx is much larger (2 K) than that observed in the organics. The superconducting transition temperature for this compound can be tuned by varying the H/Pd ratio. Similar to the molecular superconductors discussed in this paper, the shift in Tc upon deuteration seems to be independent of Tc [64]. It has been suggested that the inverse isotope eect in this material is caused by anharmonicity eects in the phonon spectrum and concomitant electronic structure in¯uenced by the vibrating Pd and H(D) atoms [65±67]. It is unlikely that deuterium substitution in the molecular superconductors addressed in this paper would change the electronic structure of the ET molecule enough to account for the observed shift in Tc , but is rather a geometric eect [55]. The results presented in this paper clearly demonstrate that regardless of the crystal structure adopted by the various ET salts (b00 - and j-motifs) the interlayer coupling through hydrogen-anion interactions play a signi®cant role in the superconducting state as exhibited by the increase in Tc s obtained upon deuteration. This inverse eect has now been documented in several molecular superconductors, even though they contain radically dierent anion layers (polymeric or discrete), superconducting transition temperatures, and packing motifs. Reducing the Ôlattice pressureÕ along the interlayer direction continues to be a prominent theme in increasing the superconducting transition temperatures. It appears that small changes in the interaction between the electron-donor molecules and the anions (C±H X) may cause further increases in Tc , and possibly lead to a route for better investigating and understanding the superconducting pairing mechanism in these novel molecular superconductors. We hope to use these
271
experiments to understand the mechanism by which deuteration of the ET molecules leads to an increase in Tc and apply this knowledge to designing new materials with higher superconducting transition temperatures.
Acknowledgements The novel H-cell used in these experiments was designed by J. Gregar. This work was supported by the US Department of Energy, Oce of Basic Energy Sciences, Division of Materials Sciences, under contract no. W-31-109-ENG-38. Research at the Portland State University was supported by NSF grant no. CHE-9904316 and the Petroleum Research Fund ACS-PRF 34624-AC7.
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www.elsevier.nl/locate/physc
Universal inverse deuterium isotope eect on the Tc of BEDT-TTF-based molecular superconductors J.A. Schlueter a,*, A.M. Kini a, B.H. Ward a, U. Geiser a, H.H. Wang a, J. Mohtasham b, R.W. Winter b, G.L. Gard b a
Materials Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Building 200, Argonne, IL 60439-4831, USA b Department of Chemistry, Portland State University, Portland, OR 97207-0751, USA Received 22 May 2000; received in revised form 1 September 2000; accepted 11 September 2000
Abstract Deuterium substitution of the ethylene end groups of the ET [ET: BEDT-TTF or bis(ethylenedithio)tetrathiafulvalene] electron-donor molecule results in a 0.25 K increase in the superconducting transition temperature of three molecular superconductors derived from this molecule. Simplistically speaking, this change in Tc is in contradiction to that predicted by the electron±phonon coupling mechanism of the BCS theory. We suggest that the slight shortening of the C±D bond relative to the C±H bond, coupled with the recent ®ndings of a large, negative uniaxial pressure derivative of Tc in j-(ET)2 Cu(SCN)2 and b00 -(ET)2 SF5 CH2 CF2 SO3 , can explain this unique eect. Herein we report the ®rst study of the eect of deuterium substitution on the superconducting transition temperature in a molecular-based superconductor in which the electron-donor molecules are packed in a b00 motif, viz., b00 -(ET)2 SF5 CH2 CF2 SO3 . This compound is ideally suited for this study because it contains discrete (non-polymeric) anions, has a completely ordered structure, is inde®nitely stable in air at room temperature, and is free from possible magnetic impurities. Substitution of the eight hydrogen atoms of the ET molecule by deuterium causes the Tc of b00 -(ET)2 SF5 CH2 CF2 SO3 to increase from 4:34 0:05 to 4:61 0:03 K. These values were determined by measuring several representative crystals from various parallel electrocrystallization experiments containing h8 - or d8 -ET that was prepared in parallel syntheses. This is the ®rst example which demonstrates that the inverse (positive) isotope eect previously observed in j-phase salts is also present in a b00 -phase superconductor. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Organic superconductors; Superconducting transitions Tc ; Bis(ethylenedithio)tetrathiafulvalene; Penta¯uoroalkylsulfonate anion; Isotope eect
1. Introduction
* Corresponding author. Tel.: +1-630-252-3588; fax: +1-630252-9151. E-mail address: [email protected] (J.A. Schlueter).
The number of known molecular-based superconductors is rapidly approaching 100. Over half of these superconductors contain the ET [BEDTTTF, or bis(ethylenedithio)tetrathiafulvalene] electron-donor molecule (see Fig. 1 for an illustration
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 0 ) 0 1 6 2 0 - 8
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Fig. 1. Diagram of d8 -ET.
of the fully deuterated ET molecule, d8 -ET). The majority of these salts contain either simple inorganic anions (such as I±3 or ReO±4 ), or complex polymeric anions (such as Cu(SCN)±2 or Cu[N(CN)2 ]Br± ) [1]. The record superconducting transition temperature (Tc ) for cation-radical molecular superconductors at ambient pressure stands at 11.6 K for j-(ET)2 Cu[N(CN)2 ]Br [2]. A slightly higher Tc (12.8 K) has been observed in the isostructural j-(ET)2 Cu[N(CN)2 ]Cl salt under modest applied pressure of 300 bar [3]. We have recently focussed our search for new superconducting salts on molecular-based materials containing discrete organometallic or organic anions which can be rationally designed and synthesized prior to the electrocrystallization procedure. Through use of the M(CF3 )±4 (M: Cu, Ag or Au) anions, we were able to double the number of known cation-radical-based molecular superconductors [4]. Ambient-pressure superconductivity is observed in the j-(ET)2 M(CF3 )4 (1,1,2-trihaloethane) family superconductors with superconducting transition temperatures ranging from 2.1 K in jL -(ET)2 Au(CF3 )4 (1,1,2-trichloroethane) [5] to 11.1 K in jH -(ET)2 Ag(CF3 )4 (1,1,2-trichloroethane) [6]. More recently, we have turned our attention to the synthesis of completely organic salts in which both the electron-donor molecule and the charge compensating anion are organic in nature. Such salts are essentially free of the possible magnetic impurities which can complicate the physical property measurements of charge-transfer salts. Initially, we have chosen to use sulfonate anions which can be modi®ed through chemical methods. Through this rationale, we have synthesized the ®rst completely organic superconductor, b00 -(ET)2 SF5 CH2 CF2 SO3 , which shows an onset of superconductivity near 4.5 K [7]. The eect of isotopic substitution on the Tc of molecular superconductors has been an active
area of research over the past decade. The earliest isotope studies on ET-based salts were performed on the b-(ET)2 I3 superconductor. These studies showed that a large (0.28 K), positive shift in Tc occurs upon deuteration of the ET molecules in the Ôlow-Tc phaseÕ of this salt [8]. This shift is inverse to that predicted by the BCS model assuming a mass isotope eect. Interestingly, the Ôhigh-Tc phaseÕ of the same salt seems to show a normal (negative) shift [9]. These results are dicult to interpret because a large range to Tc s (1±8 K) has been reported for this salt due in part to a kinetically slow crystallographic ordering of the terminal ethylene groups of the ET electron-donor molecule [10±12]. Furthermore, it has been shown that domains of the a-, b-, b -, and at -phases of (ET)2 I3 can all occur in one apparently ÔsingleÕ crystal, resulting again in a range of Tc s from 1.5±8 K [13]. Similar experimental diculties have plagued efforts to determine the eect of 13 C substitution when either the central C@C bond [14±18] or when all double-bonded carbon atoms [19] of ET in the b-(ET)2 I3 superconductor are labeled. A large inverse (positive) shift of Tc has been reported upon deuteration of the ET molecules in j-(ET)2 Ag(CN)2 H2 O [20]. The superconducting transition temperature increases from 5 K for j-(h8 -ET)2 Ag(CN)2 H2 O to 6 K for j-(d8 -ET)2 Ag(CN)2 H2 O. It has been demonstrated that the superconducting transition in the j-(ET)2 Cu[N(CN)2 ]Cl salt at 0.3 kbar increases 0.5±1.5 K upon deuteration of the ET molecules [21]. Although sample dependent Tc s and the diculties associated with performing these experiments under pressure complicated this experiment, it is clear that a definite inverse isotope eect is present in this salt with a shift in Tc of at least 0.5 K. The eect of deuterium substitution on Tc has also been studied for the j-(ET)2 Cu[N(CN)2 ]Br superconductor. Although an abnormally large negative isotope eect (DTc 0:3 to 0.9 K) has been reported, there is some ambiguity in these results due to the use of randomly oriented crystals with relatively broad superconducting transitions (DT 10±90% 3 K) [22±25]. These results are complicated by the fact that at ambient pressure, j-(d8 ET)2 Cu[N(CN)2 ]Br lies near an antiferromagnetic/
J.A. Schlueter et al. / Physica C 351 (2001) 261±273
superconducting phase transition, and some ÔsingleÕ crystals contain a signi®cant fraction of the insulating antiferromagnetic phase [26,27]. The isotope eect due to 13 C or 34 S labeling is less sensitive to the proximity of this phase boundary. No discernable shift of Tc has been observed upon 13 C substitution of only the central C@C bond [28,29] or all double-bonded carbon atoms of ET in j-(ET)2 Cu[N(CN)2 ]Br [30]. The substitution of the four ethylene carbon atoms of ET with 13 C resulted in no detectable isotope eect on the superconductivity in j-(ET)2 Cu[N(CN)2 ]Br [25]. We previously reported a small negative shift in Tc , 0:08 0:07 K, upon substitution of the sulfur atoms with 34 S. This indicates that the C±S stretching modes do not provide a dominant coupling mechanism in this salt, but a mass eect consistent with the BCS theory is possible [31]. Among molecular superconductors, the eect of isotopic substitution on Tc has been most thoroughly studied in the j-(ET)2 Cu(SCN)2 salt. It has been convincingly shown that an inverse (positive) shift in Tc occurs upon deuteration [23,32±37]. Although the magnitude of this shift has been reported to range from 0.3 to 0.8 K, our carefully performed, systematic studies have indicated that the deuterium isotope eect for this salt is on the lower end of this range (DTc 0:30 0:07 K) [38]. Extensive 13 C and 34 S labeling studies have also been performed on the j-(ET)2 Cu(SCN)2 superconductor. Initial studies of 13 C labeling of the terminal ethylene carbon atoms of ET showed either slightly positive [35,39] or slightly negative [23] shifts in Tc . More recently, our extensive studies found that this labeling had no measurable eect [38]. Labeling all the doubly bonded carbon atoms with 13 C also resulted in no observable shift of Tc [30]. Also, fully labeling ET with eight atoms of 34 S showed no discernable eect [31]. However, we were able to show the ®rst genuine mass isotope eect on Tc for an electron-donor-based superconductor by simultaneously labeling both the ethylene groups with 13 C and all the sulfur atoms with 34 S [38,40]. It is interesting that j-(ET)2 Cu(SCN)2 exhibits two dierent isotope eects on Tc depending on where the labels are placed. It was therefore of
263
interest to investigate the eect of 13 C and 15 N labels on the Cu(SCN)±2 anion. Substitution of 15 N for 14 N did not result in any observable shift in Tc [39]. We have subsequently shown that simultaneous substitution by 13 C and 15 N in this anion also does not aect the superconducting transition temperature within experimental uncertainty [41]. To date, this is the only study of the eect of isotopic substitution within the anion layer on Tc . More recently, we have measured the eect on Tc of deuterium substitution on the ET molecules of the jL -(ET)2 Ag(CF3 )4 (1-bromo-1,2-dichloroethane) superconductor [42]. The electron-donor layer of this salt is isostructural to j-(ET)2 Cu[N(CN)2 ]Br, but the anion layer contains discrete Ag(CF3 )±4 anions and neutral 1-bromo-1,2-dichloroethane cocrystallized solvent molecules. This study showed similar results to that observed for j-(ET)2 Cu(SCN)2 : an inverse (positive) shift of Tc of about 0.2 K from 2:90 0:04 to 3:11 0:04 K. A summary of the previous studies of the eect of isotopic substitution on Tc of ET-based superconductors is provided in Table 1. We felt that it was important to extend these isotope studies to the b00 -(ET)2 SF5 CH2 CF2 SO3 system for several reasons. First, the SF5 CH2 CF2 SO±3 anions are ordered at room temperature and there does not seem to be any cooling rate dependence of the superconducting transition. Secondly, the crystals are quite stable under ambient conditions: there is no cocrystallized solvent in the structure and the anion is stable toward oxidation, disproportionation or other means of decomposition. Thirdly, these crystals are stoichiometric and no other phases are known. Thus, there are no complications from crystals which contain varying stoichiometries, phases or Tc s. Fourthly, hydrostatic pressure studies have revealed that the Tc of b00 (ET)2 SF5 CH2 CF2 SO3 decreases monotonically with pressure and thus there are no complications arising from a close proximity of the superconducting state to an antiferromagnetic regime [43]. Finally, studies of the eect of isotopic substitution on Tc have largely focussed on j-phase materials, and to a limited extent on b-phase salts. Herein is reported the ®rst study of the eect of isotopic substitution on the superconducting
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Table 1 Summary of experiments on ET-based molecular superconductors which have probed the eect of various isotopic substitutions on the superconducting transition temperature Compound
Isotopea
Tc (K)b
DTc (K)
Methodc
Ref.
bL -(ET)2 I3 bH -(ET)2 I3
d8 d8 13 C2 13 C2 13 C2 13 C6
1.15 7 8:0 0:1 8:0 0:1 6:95 0:05 7:2 0:05
0.28 (±) 0:6 0:1 0:2 0:1 0 0:1 0:1 0:1
AC mag DC mag Res AC res DC mag DC mag
[8] [9] [14] [15,16] [17,18] [19]
j-(ET)2 Ag(CN)2 H2 O
d8 d8
5.0 5.0
1.0 1.0
DC res DC mag
[20] [20]
j-(ET)2 Cu[N(CN)2 ]Br
d8 d8 d8 d8 13 C2 13 C4 13 C6 34 S8
11.2 10:9 0:3 11.7 11.3 11:40 0:05 11.3 11:39 0:01 11:28 0:05
0:9 0:3 0:5 0:4 0:05 0:08 0.0 0:03 0:02 0:08 0:07
DC mag DC mag DC res DC mag AC mag DC mag AC mag AC mag
[22] [23] [25] [25] [28,29] [25] [30] [31]
j-(ET)2 Cu(SCN)2
d8 d8 d8 d8 13 C2 13 C4 13 C4 13 C6 13 C4 34 S8 d8 13 C4 34 S8 15 13 N C
8:7 0:2 10:3 0:1 8.8 9:20 0:05 9:30 0:05 8:7 0:2 9:16 0:07 9:32 0:05 9:16 0:04 9:24 0:06 9:08 0:10
0:3 0:3 0.7 0.8 0:30 0:07 0:02 0:08 0:1 0:3 0:00 0:09 0.03 0:12 0:08 0:15 0:08 0:05 0:12
DC mag DC res, RF AC mag AC mag AC mag DC mag AC mag AC mag AC mag AC mag AC mag
[23] [33] [36] [38] [28,29] [23] [38] [30] [38] [38] [41]
jL -(ET)2 Ag(CF3 )4 (solvent)d
d8
2:90 0:04
0:21 0:06
AC mag
[42]
b00 -(ET)2 SF5 CH2 CF2 SO3
d8
4:34 0:05
0:27 0:06
AC mag
This work
a
13
d8 : substitution of all eight hydrogen atoms of ET with deuterium. C2 : substitution of the central double bonded carbon atoms with 13 C. 13 C4 : substitution of the four ethylene carbon atoms with 13 C. 13 C6 : substitution of all doubly bonded carbon atoms with 13 C. 13 C4 34 S8 : substitution of all eight sulfur atoms with 34 S8 and the four ethylene carbon atoms with 13 C4 , d8 13 C4 34 S8 : substitution of all eight hydrogen atoms with deuterium, all eight sulfur atoms with 34 S8 and the four ethylene carbon atoms with 13 C4 . 15 N13 C: substitution of 15 N13 C in the anion layer. b Superconducting transition temperature of the salt with natural isotopic abundance. When multiple de®nitions of the superconducting transition temperature are given, the linearly extrapolated value is displayed. c AC mag: AC magnetization; DC mag: DC magnetization; AC res: AC resistivity; DC res: DC resistivity; RF: RF penetration. d Solvent is 1-bromo-1,2-dichloroethane.
transition temperature in a b00 -phase salt. As an initial test, we chose to study the eect of deuterium substitution on the ET electron-donor molecule because of the aforementioned discoveries of signi®cant shifts in Tc , inverse to that predicted by the BCS theory for both the j-(ET)2 Cu(SCN)2 (Tc 10 K) [32±36] and j-(ET)2 Ag(CF3 )4 (1-bromo-1,2-dichloroethane) (Tc 3 K) [42] salts. This
study was designed to probe whether the inverse (positive) isotope shift observed in several ETbased superconductors is speci®c to the j-phase packing motif. We were interested to know how the sign and magnitude of any shift in Tc would compare to those previously found for the j-phase salts. Such information will be necessary to thoroughly understand the cause of this eect and to
J.A. Schlueter et al. / Physica C 351 (2001) 261±273
design materials with increased superconducting transition temperatures. 2. Experimental ET containing both the natural isotopic composition (h8 -ET) and d8 -ET were synthesized in strictly parallel experiments with the use of previously reported methods from 1,2-dibromoethane and d4 -1,2-dibromoethane (D, 99%), respectively [44±47]. The precursor anion salt, LiSF5 CH2 CF2 SO3 was prepared from NaSF5 CH2 CF2 SO3 as previously reported [7,48]. Single crystals of b00 (ET)2 SF5 CH2 CF2 SO3 were grown as previously described [7] (by the electrocrystallization method with LiSF5 CH2 CF2 SO3 and 12-crown-4 as the electrolytes in 1,1,2-trichloroethane) from normal ET and d8 -ET in identically parallel experiments. A modi®ed H-cell design was used in which four disposable Whatman GF/F ®lters replaced the traditional glass frit. The two halves of the H-cell were sealed together with a horseshoe style spherical joint clamp and a size 116 O-ring. Superconducting transitions were determined as described previously [31] with the use of a commercial AC susceptometer (Lake Shore Cryo-
265
tronics, Inc.) equipped with a low temperature helium subpot option (allowing measurements to be achieved at temperatures as low as 1.2 K), and operated with a frequency of 125 Hz. Rod-like single crystals were supported on the end of a Delrinâ isolation rod with a minimal amount of Apiezonâ N grease. The magnetic ®eld was aligned perpendicular to the plate face (H ? ab). These crystals were cooled from room temperature to 1.5 K over a period of 15 min. The real (in-phase, v0 ) and imaginary (out-of-phase, v00 ) components of the volume AC susceptibility for each crystal were recorded at temperature intervals of 0.05 K on slow warming from 1.5 to 5.5 K. The lower critical ®elds of b00 -(h8 -ET)2 SF5 CH2 CF2 SO3 have previously been determined [49]. For a ®eld applied perpendicular to the ET planes, Bc1 T 0 20 5 Oe. As illustrated in Fig. 2, the AC susceptibility was measured for a 0.308 mg crystal of b00 -(h8 -ET)2 SF5 CH2 CF2 SO3 with AC ®elds of 1, 2, 5, 10 and 20 G, applied perpendicular to the ET planes. The data obtained at 1 G were rather noisy, making it dicult to obtain a de®nitive value for Tc . The transition curve obtained with a 2 G ®eld is much smoother, with only a slightly suppressed Tc . Thus, we chose to perform this study with a 2 G AC ®eld.
Fig. 2. Volume AC susceptibility data, obtained from a single crystal of the b00 -(ET)2 SF5 CH2 CF2 SO3 at ®elds of 1, 2, 5, 10, and 20 G.
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J.A. Schlueter et al. / Physica C 351 (2001) 261±273
A plot of typical data obtained for a high quality single crystal of b00 -(h8 -ET)2 SF5 CH2 CF2 SO3 is shown in Fig. 3. Sample volumes were based on the experimental mass of each crystal, which ranged from 0.479 to 0.668 mg for b00 -(d8 ET)2 SF5 CH2 CF2 SO3 and from 0.151 to 0.956 mg for b00 -(h8 -ET)2 SF5 CH2 CF2 SO3 , combined with the density (1.941 g/cm3 for the h8 salt and 1.971 g/ cm3 for the d8 salt at 123 K) calculated from X-ray unit-cell data. Temperatures were determined with the use of a diode sensor, calibrated to an accuracy of better than 15 mK at temperatures below 20 K. In total, the superconducting transitions were determined for ®ve single crystal specimens of b00 (d8 -ET)2 SF5 CH2 CF2 SO3 and seven single crystals of b00 -(h8 -ET)2 SF5 CH2 CF2 SO3 . For ready visual comparison, the ``internal'' susceptibility (vint ) data (as de®ned below) for the seven h8 and ®ve d8 crystals studied herein are plotted in Fig. 4 over the temperature region 2.5 to 5.5 K. This was done by linearly correcting the v0 data to a normal-state value of 0 (at 4.5 K) and calculating an experimental demagnetization factor, assuming a saturation susceptibility of ±1 (at 1.5 K), for each crystal according to the following equation:
vint
v0 1 Dv0
1
where vint is the internal susceptibility, v0 is the measured volume susceptibility, and D is the demagnetization factor. Finally, vint was calculated for each data point through the use of this experimentally determined demagnetization factor. These D values (listed in Table 2) ranged from 0.36 to 0.65. The Tc s of each of these samples were determined according to four dierent de®nitions: Tco is the diamagnetic onset, Tcl is the linearly extrapolated mean-®eld value, Tcm is the midpoint of the superconducting transition, and v00max is the maximum in the imaginary component of the AC susceptibility. These values, for each of the 12 crystals measured, are listed in Table 2. The tabulated transition widths, DTc 10±90% , de®ned as temperature dierence between 10% and 90% of the superconducting transition, are indicators of the high crystal quality of the specimens selected. In addition, this table lists, for each de®nition, the average Tc for the groups of crystals, the standard deviation in the mean, the isotopic shift, DTc Tc (d8 -ET) Tc (h8 -ET), and the propagated standard deviation in this temperature dierence.
Fig. 3. Volume AC susceptibility data, obtained from a single crystal of b00 -(h8 -ET)2 SF5 CH2 CF2 SO3 with an ac ®eld of 2 G. A sharp superconducting transition is observed in both the real (in-phase, v0 ) and imaginary (out-of-phase, v00 ) components.
J.A. Schlueter et al. / Physica C 351 (2001) 261±273
267
Fig. 4. Volume susceptibility v0 (corrected for demagnetization) for ®ve crystals of b00 -(d8 -ET)2 SF5 CH2 CF2 SO3 and seven crystals the b00 -(h8 -ET)2 SF5 CH2 CF2 SO3 showing the high reproducibility in susceptibility of these single crystal samples and a clear increase in the superconducting transition temperature for the deuterated samples. Table 2 Superconducting transition temperatures (Tc ) at ambient pressure for the b00 -(ET)2 SF5 CH2 CF2 SO3 organic superconductor with compositions of natural isotopic abundance (h8 ) and with deuterium (d8 ) substitution of the hydrogen atoms in the ET electron-donor moleculea Crystal
Tco (K)
Tcl (K)
Tcm (K)
Tcv00 (K)
DTc 10±90% (K)
D
d8 -b -(ET)2 SO3 CF2 CH2 SF5 1 2 3 4 5
4.72 4.66 4.75 4.69 4.69
4.65 4.60 4.60 4.64 4.57
4.20 4.26 4.24 4.19 4.19
4.18 4.22 4.17 4.13 4.16
0.93 0.76 0.76 0.86 0.89
0.36 0.50 0.37 0.46 0.51
Mean
4:70 0:03
4:61 0:03
4:22 0:03
4:17 0:03
0:84 0:08
h8 -b -(ET)2 SO3 CF2 CH2 SF5 1 2 3 4 5 6 7
4.47 4.51 4.54 4.48 4.46 4.50 4.52
4.35 4.38 4.39 4.26 4.28 4.36 4.36
3.88 3.85 3.80 3.84 3.84 3.85 3.93
3.91 3.77 3.73 3.78 3.84 3.84 3.93
1.06 1.02 1.17 0.96 0.92 0.90 0.80
Mean DTc
4:50 0:03 0:20 0:04
4:34 0:05 0:27 0:06
3:86 0:04 0:36 0:05
3:83 0:07 0:34 0:08
0:98 0:12
00
00
0.43 0.49 0.61 0.63 0.65 0.49 0.43
a
Superconducting transition temperatures were determined on AC susceptibility data, uncorrected for demagnetization eects, according to the following de®nitions: Tco is the diamagnetic onset temperature, Tcl is the linearly extrapolated mean-®eld Tc , Tcm is the temperature of the transition midpoint, Tc v00 is the temperature at the maximum in the imaginary component of the AC susceptibility. The 10±90% transition width, DT 10±90% , and the experimentally determined demagnetization factor, D, are also tabulated.
High quality crystals with sharp superconducting transitions, DTc 10±90% 6 1:0 K, were used in
these experiments. For the analysis of DTc following isotopic substitution, we have used the
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J.A. Schlueter et al. / Physica C 351 (2001) 261±273
linearly extrapolated values, DTcl , because of our four de®nitions of Tc the estimated standard deviations are the lowest for Tcl . It should be noted that the superconducting onset temperature, Tco , is the value reported in our initial publication of b00 (ET)2 SF5 CH2 CF2 SO3 [7]. The uncertainty in Tco is quite large due to the diculty in determining the temperature at which the susceptibility ®rst deviates from the normal-state background value. Upon inspection of Table 2, it is clear that a sizable shift (>3r), DTcl 0:27 0:06 K, occurs upon deuteration of the ET donor molecule, and a comparable shift occurs in all cases for the various de®nitions of Tc . Fig. 4 clearly illustrates that sharp superconducting transitions are observed with a discernible shift in Tc to higher temperatures upon deuteration.
3. Discussion and conclusions A packing diagram of the b00 -(ET)2 SF5 CH2 CF2 SO3 superconductor is provided in Fig. 5 [7].
Short contacts between the sulfur atoms in adjacent ET stacks create a two dimensionally connected layer. Between these layers are located insulating sheets of charge compensating SF5 CH2 CF2 SO±3 anions. The hydrogen atoms of the ET molecule point directly at the anion layer and make short (hydrogen-bond like) contacts with the ¯uorine and oxygen atoms of the anion. Interlayer coupling, which is likely a prerequisite of superconductivity, will be directly aected by the exchange of deuterium for hydrogen. As summarized in Table 3, we have now measured the eect of deuterium substitution on the superconducting transition temperature by an essentially identical procedure for three molecular superconductors: jL -(ET)2 Ag(CF3 )4 (1-bromo-1,2dichloroethane) (Tcl 2:90 0:04 K) [42], b00 (ET)2 SF5 CH2 CF2 SO3 (Tcl 4:34 0:04 K) and j-(ET)2 Cu(SCN)2 (Tcl 9:20 0:05 K) [38]. The only signi®cant dierence in these measurements was that the AC ®eld used in the b00 -(ET)2 SF5 CH2 CF2 SO3 experiment was 2 G, while a 1 G AC ®eld was used in the other two experiments. Within experimental error, the Tc observed in all
Fig. 5. Packing diagram of b00 -(ET)2 SF5 CH2 CF2 SO3 as projected on the bc-plane. Depicted with ®ne lines are S S contacts shorter These contacts are responsible for the two-dimensional nature of this salt. The then the sum of the van der WaalÕs radii (d 3:60 A). hydrogen atoms of ET, which have been exchanged with deuterium, have been ®lled in for emphasis. These hydrogen atoms point directly at the anion layer, and are likely important for interlayer coupling.
J.A. Schlueter et al. / Physica C 351 (2001) 261±273 Table 3 Tabulation of the linearly extrapolated Tc (Tcl ), the shift in Tc upon deuteration of the ET electron-donor molecule (DTc ), and the relative shift in Tc (DTc /Tc ), for three molecular superconductors: jL -(ET)2 Ag(CF3 )4 (1-bromo-1,2-dichloroethane), b00 (ET)2 SF5 CH2 CF2 SO3 and j-(ET)2 Cu(SCN)2 jL -(ET)2 Ag(CF3 )4 (solvent) b00 -(ET)2 SF5 CH2 CF2 SO3 j-(ET)2 Cu(SCN)2
a
Tc (K)b
DTc (K)
2:90 0:04 4:34 0:05 9:20 0:05
0:21 0:06 0:27 0:06 0:30 0:07
a
Solvent 1-bromo-1,2-dichloroethane. Tc is the linearly extrapolated value for crystals of natural isotopic abundance. b
cases is about 0.26 K higher for the deuterated salt than that in crystals of natural hydrogen abundance. This shift in Tc is independent of packing motif (j or b00 ), transition temperature (2.9 to 9.2 K), and anion type (polymeric, organometallic, or organic). The direction of these shifts is opposite to that expected by the BCS model which predicts that the lower frequency phonons obtained in the deuterated salt would cause a lower transition temperature to be observed [50,51]. The use of BCS theory with molecular superconductors is complicated by the correct choice of the relevant mass entity. For the b00 -(ET)2 SF5 CH2 CF2 SO3 superconductor, several logical choices for the mass entity exist. Among these are the mass of the conductive TTF moiety (200 amu) of the ET molecule, the mass of one ET molecule (384 amu) or the mass of one formula unit of (ET)2 SF5 CH2 CF2 SO3 (1039 amu). If the TTF moiety is chosen as the relevant entity, deuteration of the ET molecule would not be expected to aect the superconducting transition temperature. If the mass of the ET molecule is chosen as the relevant mass (M) for intermolecular phonon±electron pairing, the magnitude of the ideal (BCS) theoretical conventional isotope eect (with Tc / M a , a 0:5) 0:5 would be DTc Tc 384=392 1 0:04 K. A similar shift is expected if one formula unit is chosen as the relevant mass. Thus, any mass-eect shift in Tc occurring as a result of the increased mass associated with deuterium substitution is masked by a much larger inverse (positive) shift in Tc . We have previously observed a BCS-type mass isotope eect on Tc upon the simultaneous sub-
269
stitution of the eight 32 S atoms and four 12 C atoms in j-(ET)2 Cu(SCN)2 with 34 S and 13 C, respectively [38]. This demonstrates that the eects of isotopic substitution in the core of the conducting layer appears to be described by a BCS-type mechanism. In contrast, our studies involving deuteration of the ethylene end groups of ET in jL -(ET)2 Ag(CF3 )4 (1-bromo-1,2-dichloroethane), b00 -(ET)2 SF5 CH2 CF2 SO3 and j-(ET)2 Cu(SCN)2 show an inverse (non-BCS) isotope eect. It can be envisioned that these hydrogen/deuterium atoms are more closely associated with the insulating anion layers than the conducting layer. Thus, it appears that a conventional (BCS) isotope eect is observed in the conducting layer while a non-conventional mechanism may be active across the non-conducting layers [40]. Our interest in probing this concept further was illustrated by an anion labeling experiment, in which 13 C and 15 N isotopes were substituted into the thiocyanate groups of j(ET)2 Cu(SCN)2 [41]. No observable shift in Tc was observed. The b00 -(ET)2 SF5 CH2 CF2 SO3 salt provides an ideal opportunity to further probe this concept. Deuteration of the SF5 CH2 CF2 SO±3 anion provides an alternative method to place deuterium atoms in the non-conducting layer. Thus, the synthesis and Tc determination of the b00 -(ET)2 SF5 CD2 CF2 SO3 salt will be of interest. The mechanism by which the conduction electrons are coupled in molecular superconductors is still open to debate. These systems are quite complex, and the coupling could occur by a combination of acoustic phonons, librational modes, and intramolecular vibrations. It is still not clear why deuteration of these molecular superconductors cause a positive (inverse) shift in the superconducting transition temperature, but several explanations have been proposed. This inverse eect has been attributed to an increase in the electron±phonon coupling in the deuterated salts [33,52]. According to the BCS theory of superconductivity, the superconducting transition temperature, Tc , is related to the Debye temperature, h, and the electron±phonon coupling constant, k, by the McMillan equation [50,51,53]: 1k T c h exp : k l
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The Coulomb pseudopotential, l , is a small correction term. In the weak coupling limit, this equation is reduced to: 1 T c / h exp : k For molecular superconductors, low-frequency intermolecular vibrations are expected to be the phonons responsible for superconductivity. For the ET-based materials discussed in this paper, the interactions between the hydrogen atoms of ET and the charge compensating anions will directly in¯uence the frequency of these lattice modes. The observed shift in Tc upon deuteration suggests that phonons that involve the motion of the hydrogen atoms of ET may play a signi®cant role in the coupling of the superconducting electrons. Softer intermolecular phonons are expected when the C±H anion contacts lengthen, thus weakening these interactions [54]. The vibrational frequency of a C±D bond is lower than that of a C±H bond. As a consequence of the lower zero point energy and the anharmonic contributions to the potential well, the C±D bond is slightly shorter than a C±H bond resulting in a longer C±D anion contact in an isostructural deuterated salt with identical lattice constants. However, in a carefully performed single crystal four-circle X-ray diraction study (300±12 K), Watanabe et al. have shown that deuterium substitution in both j-(ET)2 Cu(SCN)2 and j-(ET)2 Cu[N(CN)2 ]Br results in no signi®cant changes in lattice parameters perpendicular to the conducting plane [55]. Small, but measurable changes were observed at the lowest temperatures just above Tc in the lattice parameters within the conducting plane: 0.11% and 0.03% for c- and a-axes, respectively, in j-(ET)2 Cu[N(CN)2 ]Br and ±0.1% for c-axis in j-(ET)2 Cu(SCN)2 . If one assumes that deuterium substitution leads to a ÔsofterÕ lattice and thus an increased electron±phonon coupling, a higher superconducting transition temperature would be expected upon deuteration. Raman spectroscopy has been used to provide the ®rst direct experimental evidence for this suggested lattice softening induced by deuteration [56]. An inelastic neutron scattering study of j(ET)2 Cu(SCN)2 has shown that there is a strong
coupling between the superconducting electrons and intermolecular phonons [57]. Thus, an inverse isotope eect does not, by itself, rule out a phonon mediated pairing mechanism. Another possible explanation for the observed increase in Tc upon deuteration involves coupling through intramolecular vibrations of the organic electron-donor molecules [58,59]. Extensive 13 C, 34 S, and 2 H labeling studies of the j-(ET)2 Cu(SCN)2 superconductor have shown that the intramolecular C@C and C±S bond stretching vibrational modes of the ET molecule do not provide a dominant exchange mechanism for Cooper pairing [41]. These negative results do not, however, rule out the possibility that intramolecular vibrations play some role in the coupling mechanism [60]. Inelastic neutron scattering experiments have suggested that vibrations associated with the hydrogen atoms of the terminal ethylene groups may be associated with the superconducting charge carriers [61]. We propose that another contributing factor to the deuterium inverse isotope eect observed in ET-based superconductors may result from the highly anisotropic uniaxial pressure derivatives of Tc which have recently been derived from thermal expansion data [62]. For measurements perpendicular to the conducting planes of b00 -(ET)2 SF5 CH2 CF2 SO3 and j-(ET)2 Cu(SCN)2 , the uniaxial pressure dependencies of Tc were determined to be 5:9 0:25 and 6:2 0:25 K/kbar, respectively, which are essentially identical within experimental uncertainty. Half of the C±H(D) bonds of the ET molecules point toward the anion layer forming several C±H(D) anion contacts. The in-plane compressibility is governed by SáááS contacts on adjacent ET molecules, thus the C±H(D) bonds that are directed in the in-plane direction are expected to have little eect on the Ôinternal lattice pressureÕ. In contrast, the C±H(D) bonds directed toward the anion layer will directly aect the Ôlattice pressureÕ in the interplane direction. Since C±D bonds have a smaller zero-point displacement compared to C±H bonds, H to D substitution would lead to longer (or softer) intermolecular contacts with the atoms of the anion and with other donor molecules. Thus, the Ôinternal lattice pressureÕ perpendicular to the conducting planes
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would be reduced upon H to D substitution, and this change is expected to be of the same magnitude irrespective of the anion or the packing motif. The 0.25 K increase in Tc that we have reported for these salts upon deuteration would result from a very slight ÔnegativeÕ pressure of only about 40 bar. It should be noted that an inverse isotope has also been reported upon deuteration of the inorganic palladium hydride superconductor [63]. The increase in Tc upon deuteration of PdHx is much larger (2 K) than that observed in the organics. The superconducting transition temperature for this compound can be tuned by varying the H/Pd ratio. Similar to the molecular superconductors discussed in this paper, the shift in Tc upon deuteration seems to be independent of Tc [64]. It has been suggested that the inverse isotope eect in this material is caused by anharmonicity eects in the phonon spectrum and concomitant electronic structure in¯uenced by the vibrating Pd and H(D) atoms [65±67]. It is unlikely that deuterium substitution in the molecular superconductors addressed in this paper would change the electronic structure of the ET molecule enough to account for the observed shift in Tc , but is rather a geometric eect [55]. The results presented in this paper clearly demonstrate that regardless of the crystal structure adopted by the various ET salts (b00 - and j-motifs) the interlayer coupling through hydrogen-anion interactions play a signi®cant role in the superconducting state as exhibited by the increase in Tc s obtained upon deuteration. This inverse eect has now been documented in several molecular superconductors, even though they contain radically dierent anion layers (polymeric or discrete), superconducting transition temperatures, and packing motifs. Reducing the Ôlattice pressureÕ along the interlayer direction continues to be a prominent theme in increasing the superconducting transition temperatures. It appears that small changes in the interaction between the electron-donor molecules and the anions (C±H X) may cause further increases in Tc , and possibly lead to a route for better investigating and understanding the superconducting pairing mechanism in these novel molecular superconductors. We hope to use these
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experiments to understand the mechanism by which deuteration of the ET molecules leads to an increase in Tc and apply this knowledge to designing new materials with higher superconducting transition temperatures.
Acknowledgements The novel H-cell used in these experiments was designed by J. Gregar. This work was supported by the US Department of Energy, Oce of Basic Energy Sciences, Division of Materials Sciences, under contract no. W-31-109-ENG-38. Research at the Portland State University was supported by NSF grant no. CHE-9904316 and the Petroleum Research Fund ACS-PRF 34624-AC7.
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