Superconductivity and fluctuations in misfit layer compounds (MS)nTS2

Synthetic Metals', 41-43 (1991) 3775-3780

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S U P E R C O N D U C T I V I T Y AND F L U C T U A T I O N S IN MISFIT L A Y E R C O M P O U N D S (MS)nTS2 D. Reefman, P. Koorevaar, H. B. Brom Kamerlingh Onnes Laboratory, Leiden University, P. O. Box 9506, 2300 RA Leiden, The Netherlands. G. A. Wiegers Laboratory of Inorganic Chemistry, Materials Science Centre of the University, Nyenborgh 16, 9747 AG Groningen, The Netherlands.

Abstract Critical field data of the highly two-dimensional misfit layer materials (MS)nTS2, M = SII, Pb, Bi, La, Sm; T = Ta, Nb, are discussed together with earlier results from specific heat and AC susceptibility measurements. All materials become superconducting above 70 inK, except for M = La,Sm. Compounds with M = Sn, Pb; T = Ta, Nb behave as anisotropic 3-D BCS superconductors with small coherence lengths, whereas (BiS)I.(~TaS2 shows large two-dimensional fluctuations below 1 K. Below 0.78 K the system becomes 3-D. CDW formation and concomittant charge fluctuations in the BiS layer might explain this effect. Introduction The layered transition-metal dichalcogenides TX2 (T = Nb, Ta; X = S, Se, Te) have received much attention in the last two decades [1]. These materials are highly anisotropic metals with a very narrow conduction band of mostly Ta dz: origin. The compounds exhibit a variety of properties which are closely related to their two-dimensionality, e.g. lattice instabilities (CDW's). Also, many of these chalcogenides become superconducting at low temperatures. As the degree of anisotropy can be varied easily by intercalation these materials are well suited for a study of dimensionality aspects of superconductivity. Only recently it has been found that the well-known ternary sulfides 'MTS3' (M = Sn, Pb, Bi, La, Sm; T = Nb, Ta) also belong to the forementioned class of intercalation compounds [2]. These materials can be looked upon as 'classical' intercalates, but the organic (or metallic) intercalate is now replaced by a two-dimensional MS lattice. In only one direction the MS lattice fits to the host lattice TS2; for the other direction the lattices are incommensurate (see fig.l). For this reason they are named 'misfit compounds'. The misfit compounds are all highly two-dimensional metals: a typical value for the anisotropy in resistance is about l0 4 (compared to 10 for NbS2). In addition, due to the incommensurability, the materials are even more prone to charge fluctuations than the classical intercalates which are known to display large fluctuation effects [3]. The extent of these fluctuations is largely controlled by the metal M in the MS layer. These properties make the misfit compounds of considerable physical interest for a study of dimensionality aspects and fluctuation phenomena. If a CDW is present then its wave length or correlation length introduces a new length scale. The finite size effects due to this scale can become dominant around the transition temperature[4]. Furthermore, the misfit compounds are good model 0379-6779/91/$3.50

© Elsevier Sequoia/Printed in The Netherlands

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systems for some (non-magnetic) aspects found in high Tc superconductivity, where high anisotropy and incommensurability also seem to be important. In this contribution we present recently obtained data on the critical fields of various misfit compounds, and we will summarize in the light of these data some earlier obtained results on the specific heat and susceptibility [5]. Experimental Single crystals of (MS)~TS2 were obtained by vapour transport in a gradient of 930 °C to 800 °C. To about 200 mg of starting material about 5-10 mg of (NH4)2PbCls was added. (NH4)2PbC16 decomposes at high temperature into C12, PbC12 and NH4C1. Chlorine is supposed to be the transport gas. High quality crystals grow at the low temperature side of the quartz tube as thin platelets with a diameter up to 4 mm. X-ray analysis could not reveal any other phase. For details on the specific heat and susceptibility measurements, we refer to ref [5]. Critical field measurements were performed in home-built apparatus. At constant temperature and current, the field was swept and the voltage over the sample was recorded using a four-point technique. The current was always in the plane of the crystals and much smaller than the critical current at that temperature. He2 was taken to be the field where the voltage over the sample was 10 percent of the maximum voltage in high field. By taking an other criterium for He2 only the absolute values were influenced, not the general shape of the temperature dependence of the critical field. All data proofed to be reproducible for samples of different batches. b

Structure

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g ....o/ .... 0 ......9 .....9 ..... 0 ..... 0 .....0 .... o .

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Fig.1 General structure of the (MS)nTS2 compounds along [001] and [100]. Large open, small open and filled circles are S, T and M respectively. The general structure [2] of the misfit layer compounds is depicted in fig.1. The TS~ part of the structure is identical to the layers in unintercalated TS2; the T atoms are trigonal prismatic coordinated by sulfur. The MS part inbetween the layers TS2 can be described as slices of a (hypothetical) deformed NaCl type SnS with a thickness of half a cell edge. Going in the c-direction of the crystal, we thus have a layer of TS2, a double layer MS, and so forth with a TS2 layer separation of about 12/~. In the b-direction both two-dimensional crystal structures have the same lattice parameters. In the a-direction however, the ratio of the unit cell dimensions varies between 1.83 and 1.7, depending on T and M. Results In Table I we tabulated the superconducting transition temperatures of various misfit compounds, as measured by AC susceptibility (second column) and specific heat (third column). All transitions are sharp, except for the T = Ta, M = Bi compound which displays already at 1.2 K some diamagnetism. This diamagnetism can be fitted to the following expression: X = - 1 . 5 . 10-6(T - To) -L°. The

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non-superconducting (LaS)la4NbS2 and (SmS)I.lsNbS2 behave quite differently. Below 1 K their susceptibility diverges according to the powerlaw: X 0¢ T -°'7°. Table I

Transition temperatures from A C susceptibility (midpoint transition) and specific heat (maximum of electronic contribution), parameters c~ and/3 for the normal state specific heat, the ratio A C/7 Tc, and parallel and perpendicular coherence lengths. Compound

'7 mJ/mol.K 2 7.4 6.9

AC/TT

2.78 2.73

/3 mJ/mol.K 4 1.40 1.36

0.75

1.78

10.1

0.6

0.77

23.0

Tc(susc.)

Tc(spec.heat)

2.75 2.72 2.85 3.08 0.76

(SnS)l.17NbS2 (PbS)l.14NbS2 (SnS)l.15TaS2 (PbS)I.13TaS2 (BiS)I.0sTaS2 (SmS)HsNbS2 (LaSh a4NbS2

~11(0) ~±(0) =~ 290 t 27 [

1.3 1.2

340 500

26 23

In the sixth column of Table I we have tabulated the ratio A C / T T (7 is defined as the coefficient of the linear term in the normal state electronic specific heat) for the superconducting compounds. For (SnS)HTNbS2 and (PbS)H4NbS2 this ratio amounts to 1.2, close to the BCS prediction 1.43. For (BiS)I.0sTaS2 however this ratio is only 0.6. As in the AC susceptibility measurements, this is the only superconducting compound not showing a sharp transition. In order to get an estimate of the magnitude of the zero temperature gap A(0), we fitted the electronic contribution to the lowtemperature specific heat of (SnS)HTNbS2 and (PbS)m4NbS2 to Ces = eonst • ekBr . The value obtained for ksT~ zx(0) was about 1.2, compared to the BCS-value of 1.76. For the Bi compound such an estimate was impossible to make due to the lack of low temperature data. Further characterization of the compounds is given by fitting the measured specific heat (for the superconducting compounds in a field of 4 T) to the expression: C = 7T ~ +/3T 3. Numerical values for the parameters a and/~ are also tabulated. For all compounds but (LaS)m4NbS2, where the the exponent a was found to be 0.27, a was unity, as expected for an electronic contribution. Also, this compound shows a maximum in the specific heat as a function of field. In figs.2-4 we show the temperature dependence of the critical fields of some compounds. The anisotropy in tic2 is about a factor of 10 and only for (PbS)H4NbS2 temperature independent. Near Tc, the temperature dependence of the critical fields has a positive curvature for both orientations. 1.2 1.0

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Fig.2 Temperature dependence of the critical field of (SnS)I.lvNbS2.

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Fig.3 Temperature dependence of the critical field of (PbS)la3TaS2.

3778 2.0

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Fig.4 Temperature dependence of the critical field of (BiSh.0sTaS2. The M = Bi; T = Ta compound behaves differently. Firstly, the apparent Tc seems to be 1 K, instead of around 0.75 K found with susceptibility and specific heat. In the region 1 K - 0.8 K, He2± is extremely small; at 0.8 K Hc2± starts to rise very steeply and continues increasing with a diminished slope from about 0.7 K. Strikingly, when we extrapolate the low-temperature linear regime, it crosses the temperature axis at T = 1 K. This anomalous behavior is much less pronounced in the He211curves, although there seems to be a slight discontinuity at 0.8 K. However, in view of our experimental accuracy, this might be an artefact. Discussion Before we analyse the results it will be useful to estimate the region where a mean field (MF) approach should be valid. In general, we can distinguish two different regions. The most well-known region is the critical region, where the ability of the mean-field GL theory to calculate universal quantities (e.g. critical exponents) breaks down. The criterion to be used is the Ginzburg criterion, which for a Josephson coupled array of 2-D superconductors looks like [6]:

q~

( a(0)~ ~2(a(0)~ ~- \ ~ /

kkBTc]

(1)

where ( = IT - Tel~To is the region within which the MF theory breaks down, s is the interlayer distance and •F is the Fermi energy. The ability of MF theory to calculate non-universal parameters, such as prefactors, breaks down in a much earlier state. This last region is characterized by the Brout criterium: ~-B _~ ~ ' ~ [ 6 ] : As is clear from the definitions, ~-B is in general much larger than ~'~. Now let us return to the data. From the results presented in the previous section, the misfit layer compounds fall into three classes. Non-superconductors. The only compounds in the series we have investigated that are not superconducting are (SmS)I.lsNbS2 and (LaS)l.14NbS2. The specific heat and susceptibility can be understood in terms of a random exchange between localized spins. If these interactions are dominant then C(T) c~ T ~-~ and x(T) c( T - % In these relations, a is an adjustable parameter. The results are well described by choosing a = 0.70. A large electron donation from the MS layer to the NbS2 layer is probably the origin of the lack of superconductivity [5]. Anisotropic 3-D BCS superconductors. To this class belong the compounds with T = Nb,Ta; M = Sn,Pb. These superconductors show a sharp, well-defined transition without large fluctuations, and a A C / T T ratio of about 1.2, close to the BCS value 1.43.

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From the He2± data we can deduce the anisotropy of the superconductors. If we regard the curvature of near Tc as due to flux flow, which is very likely in an incommensurate structure where we do not have regular strong pinning potentials, we can fit the low-temperature behavior to the usual anisotropic 3-D BCS predictions:

~o

T)

He2± - 2~-~(0)(1 - Tc

~o

(2)

____T)

"1 H:211- 27r~1i(~±(0) ( - Tc

(3)

The obtained zero temperature coherence lengths are tabulated in Table I. We expect any deviations from MF to be confined to a region centered within 10 -3 K around To, as follows from the Brout criterium when we put in the appropriate parameters and take eF of the order of 1000 K. As expected, for both compounds the in plane coherence length is much larger than ~±(0). However, as the calculated ~±(0) is only twice as large as the TaS2 layer separation, relations (2) and (3) might not be valid anymore. Thus, some of the positive curvature in the He211 dependences may be related to the high anisotropy. 2-D superconductors. The only compound we have investigated so far that falls in this class is (BiS)l.0sTaS2. In the susceptibility diamagnetism shows up at temperatures as high as 1.2 K; also in the specific heat fluctuation effects are present. Furthermore, the ratio AC/',/T (0.6) is much lower than expected for pure BCS behavior. The fluctuations in the AC susceptibility can be described very well by the powerlaw dependence expected for 2-dimensionai fluctuations [7]: kT

~±(T)~_ ~

~ (,~ 0 ) . (T - Tc) -1

(4)

where d is the thickness of the superconducting layer. In order to obtain the large prefactor found in the susceptibility which sets the ratio ~ as large as 10 -6 m, one has to assume either that the thickness of the superconducting layers is of the order of the distance between the TaS2 layers, or that the coherence length is extremely large. As ~i might be very small (giving a large cB), we cannot make a realistic estimate of the Brout criterion. This implies that the interpretation of the numerical . ~,(o)

value oi ~ may be doubtful. The temperature dependence of the critical field is completely different from the curves found for compounds of the above mentioned class (see fig.4). When we fit the temperature dependence of the critical fields below 0.7 K to eqn. (2) and (3), we obtain the coherence lengths ~± ~ 23 ,~ and ~11 -~ 500 It. These values are not inconsistent with anisotropic 3-D behavior, as the layer separation is about

12 A. From table I we see that the lattice term of (BiS)l.0sTaS2 is of equal magnitude as found for the other superconducting samples. As we assume for Sn and Pb in these samples a valency of 2 +, the ionicity of Bi also is 2 +. If we assume a different valency for Bi, this would mean a charge transfer between the layers resulting in electrostatic interaction, diminishing the lattice term. Also, transport measurements support a Bi valency of 2 +. However, this is a very unusual state for Bi. Hypothetical Bi 2+ would carry a spin which is, up to room temperature, not observed in susceptibility. As an alternative we can consider a combination of Bi 1+ and Bi3+ , both usual valence states. A single crystal X-ray study showed a very complex diffraction pattern [8], which might be due to a CDW and concomittant disproportionation of Bi 2+ into Bi 1+ and Bi 3+. This CDW may cause charge fluctuations which deminish the coupling between the TaS2 layers. We now want to give two possible explanations for the anomalous behavior observed in the measurements. A first model would be that the TaS2 layers are weakly coupled to the BiS layers, and are not interacting with each other (only in second order). This situation might arise due to charge

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fluctuations in the BiS layer. At about 1 K the TaS2 layers get superconducting, possibly stabilized by the interaction with the BiS layer. We thus have 2-D superconductivity. The observed Hc2fj curve is not inconsistent with the square-root like behavior expected for purely 2-D systems [9]. Around 0.8 K the system gets 3-D, and Hc~± rises. The reason for this event may be a simple temperature dependence of the interaction between the different layers, or a deminishment of the fluctuations in the BiS layer. Only this 3-D transition would give rise to a discontinuity in the specific heat. A second explanation might be that in the crystals used there are regions where some layers of the same type (we do not indicate if these layers would be TaS2 or BiS layers) are stacked upon each other. If these 'packets' of layers are well separated, they do not have interaction and can be regarded as two-dimensional. These packets should have their Tc at 1 K. Because of their thinness, they are apt to fluctuations and correspondingly have only a very low Hc2z. Hc2[I however can be very large. At about 0.8 K the bulk material becomes superconducting, so He2± rises sharply due to the onset of three-dimensionality of the system. Also in this model, it is the last transition which causes the jump in the specific heat. In both of the explanations for the observed behavior, fluctuations may be even more enhanced due a finite scale set by the period or correlation length of the CDW in the BiS layer. Conclusions The misfit layer compound with T = Ta; M = Bi displays anomalous behavior, probably due to strong two-dimensional fluctuations. CDW formation in the BiS layer may be at the basis of these phenomena, and may enhance the fluctuations due to finite size effects. Compounds with T -Ta,Nb; M = Sn,Pb behave as anisotropic BCS superconductors. Both the in-plane and perpendicular coherence lengths are about a factor of 10 larger than found for the high Tc superconductors. Also the size of the critical region is much smaller then found for the tITc's. Compounds with M = La,Sm; T = Nb are not superconducting. Their magnetic properties can be explained on basis of a model of random exchange between localized spins. Acknowledgements The authors want to thank Dr. S. I. Mukhin, Dr. J. Aarts and Dr. P. Kes for clarifying discussions about the critical field curves. This work is part of the research project of the Leiden Material Center and is supported by the Foundation of Fundamental Research on Matter (FOM), which is sponsored by the Netherlands Organization for the Advancement of Pure Research (ZWO). References [1] R. L. Withers, J. A. Wilson, J. Phys. C 19, 4809 (1986). [2] G. A. Wiegers, R. J. Haange, J. L. de Boer, Mat. Res. Bull. 23, 1551 (1988). [3] T. H. Geballe, A. Menth, F. J. Di Salvo and F. R. Gamble, Phys. Rev. Lett. 27, 314 (1971). [4] M. B. Salamon, S. E. Inderhees, J. P. Rice and D. M. Ginsberg, Physica A168, 283 (1990). [5] D. Reefman, J. Baak, It. B. Brom, G. A. Wiegers, Solid State Communications 75, 47 (1990). [6] A. Kapitulnik, M. R. Beasley, C. Castellani, C. Di Castro, Phys. Rev. B37, 537 (1988). [7] M. Tinkham, 'Introduction to Superconductivity', Florida, Robert E. Krieger Publishing Company (1975), p.249. [8] J. Wulff, A. Meetsma, R. J. Haange, J. L. de Boer, G. A. Wiegers, Synth. Met. 39, l (1990). [9] R. A. Klemm, A. Luther, M. R, Beasley, Phys. Rev. B12, 877 (1975).