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Effect of Fe doping on gaps in GdBa2Cu3O7−δ
ELSEVIER
Physica C 223 (1994) 83-89
Effect of Fe doping on gaps in
GdBaECU307_a
A.I. A k i m e n k o *, V.A. G u d i m e n k o B. VerkinInstitute for Low TemperaturePhysics and Engineering, UkrainianAcademy of Sciences, 47, Lenin Ave., Kharkov, 310164, Ukraine Received 6 November 1993; revised manuscript received 12 February 1994
Abstract The effect of Fe doping on the gap suppression has been investigated by the point-contact spectroscopy method for two kinds of GdBa2 (Cut _xFex)307-a specimens (x = 0.03, 0.06, 0.09, 0.12 ). Only in the case of Fe ion duster formation in chains, largegap (.4~u) suppression has been observed, and the rate of the z f ~ decrease is almost the same as was found in the Zn doped YBa2Cu3OT_6samples. The more homogeneous distribution of Fe in the chains leads to gapless superconductivityat x> 0.06.
1. Introduction
The investigation of the change of the superconductor parameters with composition gives the possibility to clarify the nature of superconductivity and to get new superconductors. To analyze the role of the magnetic moments in the creation or breaking of superconducting pairs, the paramagnetic ion doping is often used. Though in the former low-To superconductors with electron pairing due to the electronphonon interaction, the paramagnetic impurities (unlike the nonmagnetic ones) effectively destroy the Cooper pairs (breaking the time-reversal symmetry of the electron system) [ 1 ], the role of magnetic interactions in the high- T¢ superconductivityis still unclear. Moreover, there are some works showing that the nonmagnetic ions (for instance, when zinc replaces copper in YBa2CU3OT_6) suppress the critical temperature T~even more than the paramagnetic ones (e.g., Fe) [2]. In the case of Fe doped 123 compounds the situation is complicated by the Fe3+ ion tendency to form linear clusters in the Cu( l )-O ( 1)* Correspondingauthor.
O (5) planes. The specific distribution of Fe ions on the Cu ( 1) and Cu (2) sites to a great extent depends on the procedure of specimen preparation and this may be the reason of the existing irreproducibility of experimental results for the same Fe concentration [31. If there is considerable inhomogeneity in the impurity distribution, it seems to be reasonable to use the point-contact (PC) method (with a several-nanometer contact diameter) to get information about the local superconducting properties (such as the transition temperature or energy gap). It was shown by this method that the substitution of Cu by Zn in YBa2Cu307_a yields a gap-distribution shift to lower energies, thus resulting in gapless superconductivity
[4]. In the present work, we have investigated the gap structure behavior in PC spectra of the samples in which Cu ions are partially substituted by paramagnetic Fe ions and compared them with the appropriate PC spectra of the Zn doped samples. We believe that our spectra contain information about the gaps in a single superconducting domain situated between the Fe rich regions in the samples with Fe clusters.
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A.L Akimenko, V.A. Gudimenko / Physica C 223 (1994) 83-89
We have also investigated the samples with a more homogeneous distribution o f Fe impurities. A significant gap suppression has been found in ref. [ 5 ] where the gap dependence on the Fe concentration has been investigated in the 123 c o m p o u n d by the far-infrared reflection ( F I R ) method, but the gap was too small ( ~ 12 m e V ) in the undoped sample. On the other hand, the more sensitive method o f F I R spectrum measurement gives evidence o f the gap distribution in the 123 c o m p o u n d s (in the range ~ 1030 m e V ) [6 ] that agrees well with our earlier data [ 7 ] obtained from PC spectra. In the work ofref. [ 8 ] only little change o f the tunnel spectra has been found for the Fe doped (4%) YBa2Cu307.
2. Results and discussion
Polycrystal GdBa2 (Cu 1- xFex) 3 0 7 -- ~ samples have been prepared by two different methods, and the distribution o f Fe over the Cu sites was found by M6ssbauer spectroscopy in ref. [ 9 ]. The standard ceramic procedure (SCP) was shown to give rise to linear Fe clusters in the Cu ( 1 ) - O ( 1 ) - O ( 5 ) planes while a socalled "prereaction" method ( P R M ) results in a more homogenous Fe ion distribution on the Cu ( 1 ) sites. There was no essential difference in the rather rand o m Fe distribution on the C u ( 2 ) sites in the CuO2 planes. The SCP samples had higher values o f Tcbulk than the P R M ones (see for details ref. [ 3 ] ). We have studied the SCP samples with x = 0 . 0 3 , 0.06, 0.09, 0.12 and the P R M ones with x - - 0 . 0 6 and 0.09. The Tcbulk values corresponding to the midpoint in the AC susceptibility drop at the temperature o f the superconducting transition were 93.5 ( 1 ) K (x = 0), 83 (9) (x=0.03), 57(2) (x=0.06), 45(5) (x=0.09), 2 3 ( 6 ) ( x = 0 . 1 2 ) for the SCP samples and 3 5 ( 6 ) ( x = 0 . 0 6 ) , 3 8 ( 5 ) ( x = 0 . 0 9 ) for the P R M ones, respectively (the figures in the first brackets are the temperature interval for the 10% to 90% change o f the AC susceptibility around T¢b~k ) [ 3 ]. The J values were less than 0.06 for all the samples. The measurement o f the PC spectra (i.e., the differential resistance d V / d I versus voltage V applied to the contact) was conventional (by current modulation ), and Ag or Au was used as a counterelectrode to make an N - c - S contact (N is a normal metal, S a superconductor, c a constriction).
A PC spectrum of the N - c - S contact is well known to have a clear singularity (a narrow m i n i m u m or kink if there is no potential barrier at the N - S boundary) at the biases F = + z l / e (zl is the energy gap) due to the so-called Andreev reflection o f electrons at the N - S boundary [10]. In the case o f the 123 compound, there is no single gap, but rather a set (distribution) of gaps in the range of ~ 10-30 meV that can be a possible reason o f the wide m i n i m u m gap structure in the same range o f biases [ 7 ]. Two such spectra are shown in Fig. 1 (curves 1 and 2) (the minim u m around V = 0 is likely to be connected with gapless superconductivity in a small part of this contact as the m i n i m u m disappears at a rather low temperature, and its temperature dependence is similar to that o f a gapless superconductor [ 11 ], see also Fig. 7). The rest o f the curves are for the PC spectra of the Fe doped SCP samples.
0.' i
-!
0.09
0.06 /-
0.03 :
~.~
4
X=O -150 400 -50
0
50 1 0 0 V( mV)
Fig. 1. Point-contact spectra (dV/dl vs. V) of GdBa2(CUl_xFex)3OT_a made by SCP (x=0, 0.03, 0.06, 0.09, 0.12 ). Point-contact resistances at V~ 0, Ro, are 1.6 ( l ), 3.0 (2), 1.8 (3), 4.0 (4), 9.8 (5), 6.7 (6), 24.0 (7), 6.5 (8), 155.0 (9). The local temperatures of the superconducting transition T~pc are 94 K (contacts 1, 2), 83 (3, 4), 57 (5, 6), 42 (7), 22 f~ (9). The bath temperature T is 4.2 K. Inset: Point-contact spectra calculated from BTK theory [ 9 ] using an even gap distribution in the energy ranges 7-35 meV (curve 1), 7-25 (2), 7-15 (3). The barrier parameter z is 0.5.
A.L Akimenko, KA. Gudimenko / Physica C 223 (1994) 83-89
It should be noted that the higher the Fe concentration x is in a sample, the less PC spectra with a clear gap minimum occur during experiments. The resistances at V ~ 0 Ro= (dV/dlv=o of PC's with such spectra systematically increase with x (or the contact diameter d decreases since R ~ 1/d 2 in the ballistic regime). For example, the diameters of the contacts shown in Fig. 1 range from ~ 20 n m ( 1 , 2 ) to ~ 3 nm (9). This is correlated with the expected decrease of spacing between adjacent Fe rich clusters (or the size of domains between the clusters ) with x increasing. Thus, it is reasonable to suppose that the spectra in Fig. 1 correspond to the contacts in the central parts of such domains. The presence of the domains in our samples is additionally confirmed by sharp peaks often observed in the PC spectra (e.g., see curves 4 6 the appearance of which may be evoked by the transition of a superconducting domain into the normal state when the critical current in a domain is reached. The domain structure was also observed using electron microscopy in ref. [ 12 ]. Before analyzing the behavior of the PC spectrum singularities it is necessary to make sure that they reflect the bulk material properties and are not connected with specific surface properties, current or multicontact effects, which are possible in a considerably inhomogeneous sample. For that we have measured the PC spectrum versus temperature and Ro dependences, and the Ro versus Tcurves. According to BCS theory, with increasing temperature the gap structure must go to lower biases and disappear at T = T~ c where T Pc is the temperature of the transition into the normal state of the S part of an N - c - S contact. According to the BTK results [ 10], the Ro(T) values have a sharp enough increase with T just before T~ c (for rather low barriers) and a kink at T - T ~ c. So from these dependences one can determine the local critical temperature T~ c and the corresponding gap value(s) A. In Fig. 1 T~ c is nearly equal to T~ ~ . The temperature dependences of the PC spectrum and Ro are shown in ref. [7] for x = 0 and in Fig. 2 for x=0.12. However, sometimes we had contacts with T~ c differing from T~ ~n~.One can see that the Ro (T) dependence in inset (a) of Fig. 2 has a kink at T ~ 20 K that is close to T~~ = 23 K, and the gap singularity disappears at this temperature too (inset ( b ) ) . But the Ro value has also a rather large increase at T ~ 35 K that confirms the existence
450 400-50
85
0
$0 100VI~V }
Fig. 2. Point-contact spectrum no. 9 from Fig. 1 at various temperatures. The curves are shifted down for clarity. Insets: (a) Temperature dependence of resistance at V~0, Ro, for this point-
contact, (b) temperature dependence of the gap-related minimum Amax.
of another domain inside the S region near the N - S boundary (not directly at the N - S boundary because of the absence of a proper gap structure in the PC spectrum). In some cases singularities looking like gaps can occur due to current effects in the point-contact region. Then the voltage position of such a singularity depends on the PC resistance because the singularity is determined by the current (e.g., by the critical current in a small superconducting domain or in the Josephson S-c-S ( S - I - S ) contacts). One can see from Fig. 3 that the almost double decrease of the PC resistance (without change of the electrode touch point) does not shift the gap minimum position at ~ 16 mV (while the peak structure at ~ 100-140 mV depends on Ro). In Fig. 1 are also shown couples of PC spectra (for other concentrations) having different Ro (the PC's have been obtained at different points on the S surface except for the x = 0 . 0 6 sample). The irreproducible peak structure in the curves is eventually a result of current effects; however, the common shapes of the gap minima for each x (shown by broken lines) are very similar. So we are sure that such minima are the gap-related structure. One can see that the characteristic energy interval of a gap minimum (at halfwidth) decreases with x increasing, from ~ 10-35 mV ( x = 0 ) to ~ 8 - 1 6 mV ( x = 0 . 1 2 ) , and
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A.L AMmenko, V.A. Gudimenko / Physica C 223 (1994) 83-89
~ y--o
nt5
I
T A v
1"4
0.025~ -150400 -50 0 50 lOOv(wiV) Fig. 3. Variation of the point-contact spectrum (d V/dI vs. V) of GdBa2(Cul _xFex)307- a (SCP sample with x = 0.12 ) with resistance Ro. As a result spectrum no. 9 of Fig. 1 has been obtained (curve 3). Ro=262 (curve 1), 176 (2), 155 ~1 (3).
is almost the same ( ~ 8 - 2 2 mV) for x = 0 . 0 6 and 0.09. In Fig. 4 are shown the PC spectra with the wide minimum gap structure for the Zn doped YBa2CuaO7_a though the narrow minima have been also observed in the previous works [4,7]. The relatively narrow gap minima may occur in PC spectra of the "channel-like" contacts with ballistic electron (hole) transport in the N part of a contact and diffusive scattering from the channel walls. In this case the electron trajectories close to the channel axis contribute somewhat more in the PC spectrum compared with the "orifice-like" PC's. After a lot of PC spectra of the "channel-like" PC's (having different direction axes) were analyzed, the energy-gap distribution for the YBa2 (Cu ~_yZny) 307- a compounds has been constructed [4 ]. In the case of the "orifice-like" PC's (especially with the diffusive regime of electron transport in the N part of a contact), almost all trajectories are rather effective (except for close to an interface plane in the ballistic regime) and then the wide minimum gap structure is observed that gives evidence for the gap distribution too. In ref. [ 4 ] the gap histogram was shown to move to lower energies with increasing Y resulting in gapless superconductivity at Y> 0.05 (probably, first in the c-direction). In Fig. 4 one can compare such an histogram behav-
5
0.05.~.~ 8
150 100 50
0
50 lOOv(mv)
Fig. 4. Point-contact spectra (dV/dI vs. V) of YBa2(Cul _yZny)307_a with a wide-minimum gap-related structure. The shaded pictures are the histograms for the occurrence of the narrow gap-related structure vs. applied voltage V taken from ref. [4]. The Vscale is the same for all these dependences. R0=5.0 (curve 1), 1.5 (2), 2.0 (3), 6.0 (4), 4.3 (5), 4.5 (6), 2.1 (7), 4.1 f~ (8), T=4.2 K.
ior with the change of the PC spectra of possible "oririce-like" PC's. One can notice that the decrease of all gaps with Y increasing gives rise to flattening of the PC spectra a r o u n d V = 0 m V (curves 5, 6). The gaps are expected to decrease with temperature rising as well, a n d just the same flattening can be found in the temperature change of the PC spectrum for x = 0 shown in Fig. 3 of ref. [7] ~1. So the curves of Fig. 4 give additional evidence of the reliability of the histogram results. O n the other hand, one can see that the halfwidth of the gap m i n i m u m should be carefully used to evaluate a gap distribution. We could not find any flattening of the PC spectra in Fig. 1. So we may conclude that suppression of only the large gaps with x increasing occurs in the SCP samples while the small gaps are almost not altered. Such a supposition is confirmed by the BTK-like calculations of the PC spectra if one assumes an even #~ It is necessaryalso to have in mind the temperature smearing of curves.
A.L Akimenko, V.A. GudimenkoI Physica C 223 (1994)83-89 gap distribution with decreasing large gaps #2 (see inset in Fig. 1 ). Suppression of only the large gaps (which are most likely connected with hole-pairing in the CuO2 planes) may lead to gapless superconductivity in the CuO2 planes with a finite gap in the c-direction. Possible evidence of such a supposition can be found in Fig. 5 where the PC spectra of the x = 0 . 0 9 SCP samples are shown (curves 1-3). One can notice that, apart from the gap minima at eV= 1018 meV ( 1, 2), a very wide minimum-like singularity at leVI < 100-150 meV exists. In particular, the singularity is clearly seen in spectrum no. 3 (with a maximum barrier at an N - S boundary) in which a lowenergy gap minimum is very weak (perhaps, due to quasitunneling along the a-b plane [ 12 ] ). We think that the appearance of such a singularity in the gapless regime could be expected. According to the theory [ 14], after transition into the gapless regime
87
(with magnetic impurity doping) the PC spectrum of an undoped superconductor transforms into that with a wide minimum around V= 0 and the same behavior at high enough biases V ~ . 4 / e (almost constant value of the PC spectrum intensity). The almost constant value of the PC spectrum intensity (or the d V / d I versus V dependence of a normal N - c - N contact at T> T~r~ ) for undoped 123 compounds occurs at biases higher than 100-150 mV (see, e.g., the spectra 1, 2 in Fig. 1 or spectra 1, 2 in Fig. 4) for Am~,= 30-35 meV. So it is reasonable to connect the very wide minimum at V< 100-150 mV in the curves of Fig. 5 with the large-gap suppression. The conclusion about gapless superconductivity in the x > 0.05 SCP samples has also been made in ref. [15 ] from the specific-heat measurements. These results are in agreement with our direct measurements. The PC spectra of the PRM samples (with the more homogeneous distribution of Fe ions in the chains) are shown in Fig. 6 (curves 1-4). Here are also two PC spectra (5, 6) of the Zn doped sample ( y = 0.05 ) in the gapless regime which have a minimum around
S
450 400-50 Fig.
5.
Point-contact
0 spectra
50 100 V(mV) (dV/d/
vs.
V)
of
GdBa2(Cu~ ~Fe~) 307 samples illustrating the occurrenceof a singularity (indicated by arrows connected with the axis of the ordinates) caused by probable gapless superconductivityin the CuO2 planes. The dashed fines exhibit a trend of the PC spectrum changewithout such a singularity. For explanation see the text. Curves 1-3 are the spectra of the SCP samplewith x-0.09. Curves 4, 5 are the spectra of the PRM sampleswith x=0.09 and 0.06, respectively.Ro=24.3 (1), 13.7 (2), 176 (3), 50 (4), 110 (5) T=4.2 IC _
_
a
#2 The calculationshave been made in the way as in ref. [7].
-~-20
-10
0
m
2O Vl~V)
Fig. 6. Poim-~ntact spectra of GdBa2(Cu~_xFe~) 307_a samples made by PRM (curves I-4) in comparison to those of the YBa2(Cu1_~Zny)3OT_a sample in the gapless regime (4, 5). Ro=94 (1), 126 (2), 43 (3), 57 (4), 2 (5), 14£1 (6). 7=4.2 IC Insets:temperature dependenceof the Imint-contactresistance at V=0, Ro.
88
A.L Akimenko, V.A. Gudimenko / Physica C 223 (1994) 83-89
V=0 most probably connected with the small-gaps (8-10 meV) suppression. Such a conclusion seems to be true because of the PC spectrum saturation at the rather low biases V= 30-40 mV. As a whole, the PC spectra form of all the compounds are similar at low biases. The PC spectrum dependences on temperature of the Fe doped samples are also about the same as was found earlier for the Zn doped samples in the gapless regime [ I l ]. Only "filling" of the well around V = 0 with increasing temperature is observed (Fig. 7). This tells us that our PRM samples are also in the gapless regime as a result of the smallgap suppression. Gapless superconductivity in the CuO2 planes should give a much wider minimum as was found in the case of SCP samples and really one can find such a singularity at V< 100 mV for the PRM sample spectra too (Fig. 5, curves 4, 5). It is interesting to note that the critical temperatures for these Zn and Fe doped samples with similar PC spectra (curves 3, 4 and 5, 6 in Fig. 6 ) and nearly the same concentration, x = 0.05 and y = 0.06, are the same too (37 and 35 K). However, it is necessary to take into account that Zn doping does not change the carrier concentration and Fe doping may decrease it significantly [ 16 ]. The decrease may occur because of the six ( five )-fold preferable coordination for the Fe3+-ions so that the 0 ( 5 ) positions may be occupied by oxygen replacing from the 0 ( 4 ) sites too (as the 8 value is almost constant for our samples). In the case of the SCP samples the Fe ion clustering may decrease this effect and thus we could compare the character of the large-gap suppression in our Zn
..~ ~
T,K 4.2 12 ,o
28 35
[
"0 -30-20-10 0 10 20 30
vl~v)
Fig. 7. Point-contact spectrum of no. 4 in Fig. 6 at various temperatures. The curves are shifted down for clarity.
Aml~ A~
?
0.5
o
o
i
X(2)(%) .--~-
Fig. 8. Normalized maximum-gapvalues ,¢~,/Am~ ,Y x,y=ovs. impurity concentration in the CuO2 plane only X (2)/2 (on the Cu (2) sites) for Zn ( • ) and Fe ( O ) doped samples. and Fe doped samples. Since large gaps are most likely determined by the hole interaction in the CuO2 planes, it is natural to construct appropriate dependences on the impurity concentration in the CuO2 planes X (2) only (Fig. 8 ). We should note that, following the results of ref. [ 9 ], our SCP samples with x--0.06 and 0.09 have almost the same impurity concentration in a CuO2 plane (0.78/2% and 0.9/ 2%) which is obviously a reason of th gap structure similarity. From Fig. 8, one can see that both dependences do not differ too much. So we can conclude that Fe impurities with magnetic moments of 4--5//a/ atom [ 2 ] and Zn impurities without constant magnetic moments (perhaps they have fluctuating ones [ 17,18 ] ) play similar roles in the pair-breaking effect in 123 compounds. In summary, in the present work we have found no significant difference between Zn and Fe impurities concerning their gap suppression effect in the 123 compounds. However, it has been found that in the case of Fe ion clustering in the Cu ( 1 ) - O ( 1 )-O (5) planes Fe impurities mainly suppress large gaps having only little effect on the small gaps. On the other hand, if Fe ions are placed more randomly in the chains, small gaps are suppressed giving rise to gapless superconductivity like in the Zn doped samples [4,11].
Acknowledgements The authors wish to thank G. Czjzek and H.J. Bornemann for permission to use their samples, H. v.
A.L Akimenko, V.A. Gudimenko / Physica C223 (1994) 83-89
l_25hneysen and H. Wtihl for support and helpful discussions, and G. Goll for help in experiments. This work was started when one of the authors (AIA) stayed at the Universifiit Karlsruhe and was supported by the Deutsche Forschungsgemeinschaft and Bundesrninister Ftir Forschung and Technologie, Project No. 13N54864.
References [ 1 ] A.A. Abrikosov and L.P. Gor'kov, Zh. Eksp. Tcor. Fiz. 39 (1960) 1781 (Soviet Phys. JETP 12 ( 1961 ) 1243). [2] A.V. Nadikar, C.V. Narasiraha Rao and S.K. Agarwal, in: Studies of high temperature superconductor, Vol. 1, ed. A. Narlikar (1989) p. 341. [3] H.J. Bornemann, R. Kmiec, G. Czjzek, P. Adelman and A. Seidei, Physica C 159 (1989) 173. [4] A.L Akimenko, G. Goll, I.K. Yanson, H.v. L/Shneysen, R. Ahrens, T. Wolf and H. Wiihl, Z. Phys. B Condens. Matt. 82 (1991) 5. [ 5 ] E. Seider, M. Bauer, L. Genzel, P. Wyder, A. Jansen and C. Richter, SSC 72 (1989) 85. [6] L. Genzel, M. Bauer, H.-U. Habermeier and E.H. Brandt, Z. Phys. B Condens. Matt., to be published.
89
[7]A.I. Aldmenko, G. Goll, H. v. LiShneysen and V.A. Gudimenko, Phys. Rev. B 46 (1992) 6409. [8] A.M. Cucolo, R. Di Leo, A. Nigro and F. Licci, Phys. Rev. B 46 (1992) 9250. [9] H.J. Bornemann, R. Kmiec and G. Czjzek, Physica B 165&166 (1990) 1323. [ 10] G.E. Blonder, M. Tinkham and T.M. Klapwijk, Phys. Rev. B25 (1982) 4515. [ l l ] A . I . Aldmenko, G. Goll, H. v. L~hneysen and V.A. Gudimenko. Abstract in Fiz. Nizk. Temp, 18 (1992) 588 (Soy. J. Low Temp. Phys. ); Physica C 213 (1993) 399. [ 12 ] T. Krekels, G. Van Tendeloo, D. Broddin, S. Amelinckx, L. Tanner, M. Mehbod, E. Vanlathem and R. Deltour, Physica C 173 (1991) 361. [13IT. Claeson, in: Tunneling phenomena in solids, eds. E. Burstein and S. Lundovist (Plenum, New York, 1969). [ 14] S.L Beloborod'ko and A.N. Omelyanchuk, Fiz. Nizk. Temp. 17 (1991) 994 (Soy. J. Low Temp. Phys. 17 (1992) 518). [ 15 ] C. Meingast, R. Ahrens, B. Blank, H. Burlde, B. Rudolf and H. Wiihl, Physica C 173 ( 1991 ) 309. [ 16 ] N. Kawaji, K. Muranaka, Y. Oda and K. Asayama, Physiea B 165&166 (1990) 1543. [ 17 ] G. Balakrishnan, L.W.J. Caves, R. Dupree, D. Mc.K. Paul and M.E. Smith, Pliysiea C 161 (1989) 9. [18]H. Alloul, P. Mendels, H. Casalta, J.F. Marusev and J. Arabstei, Phys. Rev. Lett. 67 (1991) 3140.
Physica C 223 (1994) 83-89
Effect of Fe doping on gaps in
GdBaECU307_a
A.I. A k i m e n k o *, V.A. G u d i m e n k o B. VerkinInstitute for Low TemperaturePhysics and Engineering, UkrainianAcademy of Sciences, 47, Lenin Ave., Kharkov, 310164, Ukraine Received 6 November 1993; revised manuscript received 12 February 1994
Abstract The effect of Fe doping on the gap suppression has been investigated by the point-contact spectroscopy method for two kinds of GdBa2 (Cut _xFex)307-a specimens (x = 0.03, 0.06, 0.09, 0.12 ). Only in the case of Fe ion duster formation in chains, largegap (.4~u) suppression has been observed, and the rate of the z f ~ decrease is almost the same as was found in the Zn doped YBa2Cu3OT_6samples. The more homogeneous distribution of Fe in the chains leads to gapless superconductivityat x> 0.06.
1. Introduction
The investigation of the change of the superconductor parameters with composition gives the possibility to clarify the nature of superconductivity and to get new superconductors. To analyze the role of the magnetic moments in the creation or breaking of superconducting pairs, the paramagnetic ion doping is often used. Though in the former low-To superconductors with electron pairing due to the electronphonon interaction, the paramagnetic impurities (unlike the nonmagnetic ones) effectively destroy the Cooper pairs (breaking the time-reversal symmetry of the electron system) [ 1 ], the role of magnetic interactions in the high- T¢ superconductivityis still unclear. Moreover, there are some works showing that the nonmagnetic ions (for instance, when zinc replaces copper in YBa2CU3OT_6) suppress the critical temperature T~even more than the paramagnetic ones (e.g., Fe) [2]. In the case of Fe doped 123 compounds the situation is complicated by the Fe3+ ion tendency to form linear clusters in the Cu( l )-O ( 1)* Correspondingauthor.
O (5) planes. The specific distribution of Fe ions on the Cu ( 1) and Cu (2) sites to a great extent depends on the procedure of specimen preparation and this may be the reason of the existing irreproducibility of experimental results for the same Fe concentration [31. If there is considerable inhomogeneity in the impurity distribution, it seems to be reasonable to use the point-contact (PC) method (with a several-nanometer contact diameter) to get information about the local superconducting properties (such as the transition temperature or energy gap). It was shown by this method that the substitution of Cu by Zn in YBa2Cu307_a yields a gap-distribution shift to lower energies, thus resulting in gapless superconductivity
[4]. In the present work, we have investigated the gap structure behavior in PC spectra of the samples in which Cu ions are partially substituted by paramagnetic Fe ions and compared them with the appropriate PC spectra of the Zn doped samples. We believe that our spectra contain information about the gaps in a single superconducting domain situated between the Fe rich regions in the samples with Fe clusters.
0921-4534/94/$07.00 © 1994 Elsevier ScienceB.V. All fights reserved SSD10921-4534 ( 93 )E0787-2
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A.L Akimenko, V.A. Gudimenko / Physica C 223 (1994) 83-89
We have also investigated the samples with a more homogeneous distribution o f Fe impurities. A significant gap suppression has been found in ref. [ 5 ] where the gap dependence on the Fe concentration has been investigated in the 123 c o m p o u n d by the far-infrared reflection ( F I R ) method, but the gap was too small ( ~ 12 m e V ) in the undoped sample. On the other hand, the more sensitive method o f F I R spectrum measurement gives evidence o f the gap distribution in the 123 c o m p o u n d s (in the range ~ 1030 m e V ) [6 ] that agrees well with our earlier data [ 7 ] obtained from PC spectra. In the work ofref. [ 8 ] only little change o f the tunnel spectra has been found for the Fe doped (4%) YBa2Cu307.
2. Results and discussion
Polycrystal GdBa2 (Cu 1- xFex) 3 0 7 -- ~ samples have been prepared by two different methods, and the distribution o f Fe over the Cu sites was found by M6ssbauer spectroscopy in ref. [ 9 ]. The standard ceramic procedure (SCP) was shown to give rise to linear Fe clusters in the Cu ( 1 ) - O ( 1 ) - O ( 5 ) planes while a socalled "prereaction" method ( P R M ) results in a more homogenous Fe ion distribution on the Cu ( 1 ) sites. There was no essential difference in the rather rand o m Fe distribution on the C u ( 2 ) sites in the CuO2 planes. The SCP samples had higher values o f Tcbulk than the P R M ones (see for details ref. [ 3 ] ). We have studied the SCP samples with x = 0 . 0 3 , 0.06, 0.09, 0.12 and the P R M ones with x - - 0 . 0 6 and 0.09. The Tcbulk values corresponding to the midpoint in the AC susceptibility drop at the temperature o f the superconducting transition were 93.5 ( 1 ) K (x = 0), 83 (9) (x=0.03), 57(2) (x=0.06), 45(5) (x=0.09), 2 3 ( 6 ) ( x = 0 . 1 2 ) for the SCP samples and 3 5 ( 6 ) ( x = 0 . 0 6 ) , 3 8 ( 5 ) ( x = 0 . 0 9 ) for the P R M ones, respectively (the figures in the first brackets are the temperature interval for the 10% to 90% change o f the AC susceptibility around T¢b~k ) [ 3 ]. The J values were less than 0.06 for all the samples. The measurement o f the PC spectra (i.e., the differential resistance d V / d I versus voltage V applied to the contact) was conventional (by current modulation ), and Ag or Au was used as a counterelectrode to make an N - c - S contact (N is a normal metal, S a superconductor, c a constriction).
A PC spectrum of the N - c - S contact is well known to have a clear singularity (a narrow m i n i m u m or kink if there is no potential barrier at the N - S boundary) at the biases F = + z l / e (zl is the energy gap) due to the so-called Andreev reflection o f electrons at the N - S boundary [10]. In the case o f the 123 compound, there is no single gap, but rather a set (distribution) of gaps in the range of ~ 10-30 meV that can be a possible reason o f the wide m i n i m u m gap structure in the same range o f biases [ 7 ]. Two such spectra are shown in Fig. 1 (curves 1 and 2) (the minim u m around V = 0 is likely to be connected with gapless superconductivity in a small part of this contact as the m i n i m u m disappears at a rather low temperature, and its temperature dependence is similar to that o f a gapless superconductor [ 11 ], see also Fig. 7). The rest o f the curves are for the PC spectra of the Fe doped SCP samples.
0.' i
-!
0.09
0.06 /-
0.03 :
~.~
4
X=O -150 400 -50
0
50 1 0 0 V( mV)
Fig. 1. Point-contact spectra (dV/dl vs. V) of GdBa2(CUl_xFex)3OT_a made by SCP (x=0, 0.03, 0.06, 0.09, 0.12 ). Point-contact resistances at V~ 0, Ro, are 1.6 ( l ), 3.0 (2), 1.8 (3), 4.0 (4), 9.8 (5), 6.7 (6), 24.0 (7), 6.5 (8), 155.0 (9). The local temperatures of the superconducting transition T~pc are 94 K (contacts 1, 2), 83 (3, 4), 57 (5, 6), 42 (7), 22 f~ (9). The bath temperature T is 4.2 K. Inset: Point-contact spectra calculated from BTK theory [ 9 ] using an even gap distribution in the energy ranges 7-35 meV (curve 1), 7-25 (2), 7-15 (3). The barrier parameter z is 0.5.
A.L Akimenko, KA. Gudimenko / Physica C 223 (1994) 83-89
It should be noted that the higher the Fe concentration x is in a sample, the less PC spectra with a clear gap minimum occur during experiments. The resistances at V ~ 0 Ro= (dV/dlv=o of PC's with such spectra systematically increase with x (or the contact diameter d decreases since R ~ 1/d 2 in the ballistic regime). For example, the diameters of the contacts shown in Fig. 1 range from ~ 20 n m ( 1 , 2 ) to ~ 3 nm (9). This is correlated with the expected decrease of spacing between adjacent Fe rich clusters (or the size of domains between the clusters ) with x increasing. Thus, it is reasonable to suppose that the spectra in Fig. 1 correspond to the contacts in the central parts of such domains. The presence of the domains in our samples is additionally confirmed by sharp peaks often observed in the PC spectra (e.g., see curves 4 6 the appearance of which may be evoked by the transition of a superconducting domain into the normal state when the critical current in a domain is reached. The domain structure was also observed using electron microscopy in ref. [ 12 ]. Before analyzing the behavior of the PC spectrum singularities it is necessary to make sure that they reflect the bulk material properties and are not connected with specific surface properties, current or multicontact effects, which are possible in a considerably inhomogeneous sample. For that we have measured the PC spectrum versus temperature and Ro dependences, and the Ro versus Tcurves. According to BCS theory, with increasing temperature the gap structure must go to lower biases and disappear at T = T~ c where T Pc is the temperature of the transition into the normal state of the S part of an N - c - S contact. According to the BTK results [ 10], the Ro(T) values have a sharp enough increase with T just before T~ c (for rather low barriers) and a kink at T - T ~ c. So from these dependences one can determine the local critical temperature T~ c and the corresponding gap value(s) A. In Fig. 1 T~ c is nearly equal to T~ ~ . The temperature dependences of the PC spectrum and Ro are shown in ref. [7] for x = 0 and in Fig. 2 for x=0.12. However, sometimes we had contacts with T~ c differing from T~ ~n~.One can see that the Ro (T) dependence in inset (a) of Fig. 2 has a kink at T ~ 20 K that is close to T~~ = 23 K, and the gap singularity disappears at this temperature too (inset ( b ) ) . But the Ro value has also a rather large increase at T ~ 35 K that confirms the existence
450 400-50
85
0
$0 100VI~V }
Fig. 2. Point-contact spectrum no. 9 from Fig. 1 at various temperatures. The curves are shifted down for clarity. Insets: (a) Temperature dependence of resistance at V~0, Ro, for this point-
contact, (b) temperature dependence of the gap-related minimum Amax.
of another domain inside the S region near the N - S boundary (not directly at the N - S boundary because of the absence of a proper gap structure in the PC spectrum). In some cases singularities looking like gaps can occur due to current effects in the point-contact region. Then the voltage position of such a singularity depends on the PC resistance because the singularity is determined by the current (e.g., by the critical current in a small superconducting domain or in the Josephson S-c-S ( S - I - S ) contacts). One can see from Fig. 3 that the almost double decrease of the PC resistance (without change of the electrode touch point) does not shift the gap minimum position at ~ 16 mV (while the peak structure at ~ 100-140 mV depends on Ro). In Fig. 1 are also shown couples of PC spectra (for other concentrations) having different Ro (the PC's have been obtained at different points on the S surface except for the x = 0 . 0 6 sample). The irreproducible peak structure in the curves is eventually a result of current effects; however, the common shapes of the gap minima for each x (shown by broken lines) are very similar. So we are sure that such minima are the gap-related structure. One can see that the characteristic energy interval of a gap minimum (at halfwidth) decreases with x increasing, from ~ 10-35 mV ( x = 0 ) to ~ 8 - 1 6 mV ( x = 0 . 1 2 ) , and
86
A.L AMmenko, V.A. Gudimenko / Physica C 223 (1994) 83-89
~ y--o
nt5
I
T A v
1"4
0.025~ -150400 -50 0 50 lOOv(wiV) Fig. 3. Variation of the point-contact spectrum (d V/dI vs. V) of GdBa2(Cul _xFex)307- a (SCP sample with x = 0.12 ) with resistance Ro. As a result spectrum no. 9 of Fig. 1 has been obtained (curve 3). Ro=262 (curve 1), 176 (2), 155 ~1 (3).
is almost the same ( ~ 8 - 2 2 mV) for x = 0 . 0 6 and 0.09. In Fig. 4 are shown the PC spectra with the wide minimum gap structure for the Zn doped YBa2CuaO7_a though the narrow minima have been also observed in the previous works [4,7]. The relatively narrow gap minima may occur in PC spectra of the "channel-like" contacts with ballistic electron (hole) transport in the N part of a contact and diffusive scattering from the channel walls. In this case the electron trajectories close to the channel axis contribute somewhat more in the PC spectrum compared with the "orifice-like" PC's. After a lot of PC spectra of the "channel-like" PC's (having different direction axes) were analyzed, the energy-gap distribution for the YBa2 (Cu ~_yZny) 307- a compounds has been constructed [4 ]. In the case of the "orifice-like" PC's (especially with the diffusive regime of electron transport in the N part of a contact), almost all trajectories are rather effective (except for close to an interface plane in the ballistic regime) and then the wide minimum gap structure is observed that gives evidence for the gap distribution too. In ref. [ 4 ] the gap histogram was shown to move to lower energies with increasing Y resulting in gapless superconductivity at Y> 0.05 (probably, first in the c-direction). In Fig. 4 one can compare such an histogram behav-
5
0.05.~.~ 8
150 100 50
0
50 lOOv(mv)
Fig. 4. Point-contact spectra (dV/dI vs. V) of YBa2(Cul _yZny)307_a with a wide-minimum gap-related structure. The shaded pictures are the histograms for the occurrence of the narrow gap-related structure vs. applied voltage V taken from ref. [4]. The Vscale is the same for all these dependences. R0=5.0 (curve 1), 1.5 (2), 2.0 (3), 6.0 (4), 4.3 (5), 4.5 (6), 2.1 (7), 4.1 f~ (8), T=4.2 K.
ior with the change of the PC spectra of possible "oririce-like" PC's. One can notice that the decrease of all gaps with Y increasing gives rise to flattening of the PC spectra a r o u n d V = 0 m V (curves 5, 6). The gaps are expected to decrease with temperature rising as well, a n d just the same flattening can be found in the temperature change of the PC spectrum for x = 0 shown in Fig. 3 of ref. [7] ~1. So the curves of Fig. 4 give additional evidence of the reliability of the histogram results. O n the other hand, one can see that the halfwidth of the gap m i n i m u m should be carefully used to evaluate a gap distribution. We could not find any flattening of the PC spectra in Fig. 1. So we may conclude that suppression of only the large gaps with x increasing occurs in the SCP samples while the small gaps are almost not altered. Such a supposition is confirmed by the BTK-like calculations of the PC spectra if one assumes an even #~ It is necessaryalso to have in mind the temperature smearing of curves.
A.L Akimenko, V.A. GudimenkoI Physica C 223 (1994)83-89 gap distribution with decreasing large gaps #2 (see inset in Fig. 1 ). Suppression of only the large gaps (which are most likely connected with hole-pairing in the CuO2 planes) may lead to gapless superconductivity in the CuO2 planes with a finite gap in the c-direction. Possible evidence of such a supposition can be found in Fig. 5 where the PC spectra of the x = 0 . 0 9 SCP samples are shown (curves 1-3). One can notice that, apart from the gap minima at eV= 1018 meV ( 1, 2), a very wide minimum-like singularity at leVI < 100-150 meV exists. In particular, the singularity is clearly seen in spectrum no. 3 (with a maximum barrier at an N - S boundary) in which a lowenergy gap minimum is very weak (perhaps, due to quasitunneling along the a-b plane [ 12 ] ). We think that the appearance of such a singularity in the gapless regime could be expected. According to the theory [ 14], after transition into the gapless regime
87
(with magnetic impurity doping) the PC spectrum of an undoped superconductor transforms into that with a wide minimum around V= 0 and the same behavior at high enough biases V ~ . 4 / e (almost constant value of the PC spectrum intensity). The almost constant value of the PC spectrum intensity (or the d V / d I versus V dependence of a normal N - c - N contact at T> T~r~ ) for undoped 123 compounds occurs at biases higher than 100-150 mV (see, e.g., the spectra 1, 2 in Fig. 1 or spectra 1, 2 in Fig. 4) for Am~,= 30-35 meV. So it is reasonable to connect the very wide minimum at V< 100-150 mV in the curves of Fig. 5 with the large-gap suppression. The conclusion about gapless superconductivity in the x > 0.05 SCP samples has also been made in ref. [15 ] from the specific-heat measurements. These results are in agreement with our direct measurements. The PC spectra of the PRM samples (with the more homogeneous distribution of Fe ions in the chains) are shown in Fig. 6 (curves 1-4). Here are also two PC spectra (5, 6) of the Zn doped sample ( y = 0.05 ) in the gapless regime which have a minimum around
S
450 400-50 Fig.
5.
Point-contact
0 spectra
50 100 V(mV) (dV/d/
vs.
V)
of
GdBa2(Cu~ ~Fe~) 307 samples illustrating the occurrenceof a singularity (indicated by arrows connected with the axis of the ordinates) caused by probable gapless superconductivityin the CuO2 planes. The dashed fines exhibit a trend of the PC spectrum changewithout such a singularity. For explanation see the text. Curves 1-3 are the spectra of the SCP samplewith x-0.09. Curves 4, 5 are the spectra of the PRM sampleswith x=0.09 and 0.06, respectively.Ro=24.3 (1), 13.7 (2), 176 (3), 50 (4), 110 (5) T=4.2 IC _
_
a
#2 The calculationshave been made in the way as in ref. [7].
-~-20
-10
0
m
2O Vl~V)
Fig. 6. Poim-~ntact spectra of GdBa2(Cu~_xFe~) 307_a samples made by PRM (curves I-4) in comparison to those of the YBa2(Cu1_~Zny)3OT_a sample in the gapless regime (4, 5). Ro=94 (1), 126 (2), 43 (3), 57 (4), 2 (5), 14£1 (6). 7=4.2 IC Insets:temperature dependenceof the Imint-contactresistance at V=0, Ro.
88
A.L Akimenko, V.A. Gudimenko / Physica C 223 (1994) 83-89
V=0 most probably connected with the small-gaps (8-10 meV) suppression. Such a conclusion seems to be true because of the PC spectrum saturation at the rather low biases V= 30-40 mV. As a whole, the PC spectra form of all the compounds are similar at low biases. The PC spectrum dependences on temperature of the Fe doped samples are also about the same as was found earlier for the Zn doped samples in the gapless regime [ I l ]. Only "filling" of the well around V = 0 with increasing temperature is observed (Fig. 7). This tells us that our PRM samples are also in the gapless regime as a result of the smallgap suppression. Gapless superconductivity in the CuO2 planes should give a much wider minimum as was found in the case of SCP samples and really one can find such a singularity at V< 100 mV for the PRM sample spectra too (Fig. 5, curves 4, 5). It is interesting to note that the critical temperatures for these Zn and Fe doped samples with similar PC spectra (curves 3, 4 and 5, 6 in Fig. 6 ) and nearly the same concentration, x = 0.05 and y = 0.06, are the same too (37 and 35 K). However, it is necessary to take into account that Zn doping does not change the carrier concentration and Fe doping may decrease it significantly [ 16 ]. The decrease may occur because of the six ( five )-fold preferable coordination for the Fe3+-ions so that the 0 ( 5 ) positions may be occupied by oxygen replacing from the 0 ( 4 ) sites too (as the 8 value is almost constant for our samples). In the case of the SCP samples the Fe ion clustering may decrease this effect and thus we could compare the character of the large-gap suppression in our Zn
..~ ~
T,K 4.2 12 ,o
28 35
[
"0 -30-20-10 0 10 20 30
vl~v)
Fig. 7. Point-contact spectrum of no. 4 in Fig. 6 at various temperatures. The curves are shifted down for clarity.
Aml~ A~
?
0.5
o
o
i
X(2)(%) .--~-
Fig. 8. Normalized maximum-gapvalues ,¢~,/Am~ ,Y x,y=ovs. impurity concentration in the CuO2 plane only X (2)/2 (on the Cu (2) sites) for Zn ( • ) and Fe ( O ) doped samples. and Fe doped samples. Since large gaps are most likely determined by the hole interaction in the CuO2 planes, it is natural to construct appropriate dependences on the impurity concentration in the CuO2 planes X (2) only (Fig. 8 ). We should note that, following the results of ref. [ 9 ], our SCP samples with x--0.06 and 0.09 have almost the same impurity concentration in a CuO2 plane (0.78/2% and 0.9/ 2%) which is obviously a reason of th gap structure similarity. From Fig. 8, one can see that both dependences do not differ too much. So we can conclude that Fe impurities with magnetic moments of 4--5//a/ atom [ 2 ] and Zn impurities without constant magnetic moments (perhaps they have fluctuating ones [ 17,18 ] ) play similar roles in the pair-breaking effect in 123 compounds. In summary, in the present work we have found no significant difference between Zn and Fe impurities concerning their gap suppression effect in the 123 compounds. However, it has been found that in the case of Fe ion clustering in the Cu ( 1 ) - O ( 1 )-O (5) planes Fe impurities mainly suppress large gaps having only little effect on the small gaps. On the other hand, if Fe ions are placed more randomly in the chains, small gaps are suppressed giving rise to gapless superconductivity like in the Zn doped samples [4,11].
Acknowledgements The authors wish to thank G. Czjzek and H.J. Bornemann for permission to use their samples, H. v.
A.L Akimenko, V.A. Gudimenko / Physica C223 (1994) 83-89
l_25hneysen and H. Wtihl for support and helpful discussions, and G. Goll for help in experiments. This work was started when one of the authors (AIA) stayed at the Universifiit Karlsruhe and was supported by the Deutsche Forschungsgemeinschaft and Bundesrninister Ftir Forschung and Technologie, Project No. 13N54864.
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[7]A.I. Aldmenko, G. Goll, H. v. LiShneysen and V.A. Gudimenko, Phys. Rev. B 46 (1992) 6409. [8] A.M. Cucolo, R. Di Leo, A. Nigro and F. Licci, Phys. Rev. B 46 (1992) 9250. [9] H.J. Bornemann, R. Kmiec and G. Czjzek, Physica B 165&166 (1990) 1323. [ 10] G.E. Blonder, M. Tinkham and T.M. Klapwijk, Phys. Rev. B25 (1982) 4515. [ l l ] A . I . Aldmenko, G. Goll, H. v. L~hneysen and V.A. Gudimenko. Abstract in Fiz. Nizk. Temp, 18 (1992) 588 (Soy. J. Low Temp. Phys. ); Physica C 213 (1993) 399. [ 12 ] T. Krekels, G. Van Tendeloo, D. Broddin, S. Amelinckx, L. Tanner, M. Mehbod, E. Vanlathem and R. Deltour, Physica C 173 (1991) 361. [13IT. Claeson, in: Tunneling phenomena in solids, eds. E. Burstein and S. Lundovist (Plenum, New York, 1969). [ 14] S.L Beloborod'ko and A.N. Omelyanchuk, Fiz. Nizk. Temp. 17 (1991) 994 (Soy. J. Low Temp. Phys. 17 (1992) 518). [ 15 ] C. Meingast, R. Ahrens, B. Blank, H. Burlde, B. Rudolf and H. Wiihl, Physica C 173 ( 1991 ) 309. [ 16 ] N. Kawaji, K. Muranaka, Y. Oda and K. Asayama, Physiea B 165&166 (1990) 1543. [ 17 ] G. Balakrishnan, L.W.J. Caves, R. Dupree, D. Mc.K. Paul and M.E. Smith, Pliysiea C 161 (1989) 9. [18]H. Alloul, P. Mendels, H. Casalta, J.F. Marusev and J. Arabstei, Phys. Rev. Lett. 67 (1991) 3140.