Single crystal studies of the pairing mechanism in Tl2Ba2CuO6 superconductors

Physlca C 209 (1993) 199-202 North-Holland

S i n g l e C r y s t a l S t u d i e s o f t h e P a i r i n g M e c h a n i s m in T I 2 B a 2 C u O 6 S u p e r c o n d u c t o r s

A.M. Hermann, H.M. Duan, W. I G e h l a n d M . P a r a n t h a m a n D e p a r t m e n t o f Physics, C a m p u s B o x 390, University o f Colorado, Boulder, C O 80309, U S A *

Reasonably large crystals (several mm size) of T12Ba2CuO6 (referred as "1"1-2201)were grown by a self-flux technique. T o's were altered by annealing the crystals in different atmospheres and at different temperatmes. As the oxygen content increased, the T c decreased from 90 K to 0 K. The anisotropic resistivities Pab (along abplane) and Pc (along c-axis) were measured on these crystals by Montgomery method. Pc was found to be 2-3 orders of magnitude greater than Pab. The anisotropy ratio (pc/Pab) increases with T c. For higher T c crystals, the superconducting and the normal state properties of Pab can be explained by the nested Fermi surface behavior with a T-devj~endenceof the in-plane resistivities. For lower T c crystals, the normal Fermi liquid picture emerges from the T -dependence of the in-plane resistivities. Unlike YBa2Cu30 7, Bi-2212, and other copper oxide superconductors, Pc for TI-2201 crystals is metal-like. The paraconductivity data shows that T1-2201 is a twodimensional superconductor. The c-axis Seebeck coefficients ate observed to be p-type for all crystals in the whole temperature range studied whereas some of the corresponding ab-plane Seebeck coefficients change sign from n-type to p-type and some are n-type down to T c. The thermopower behavior is still theoretically unclear.

1. INTRODUCTION In the family of thallium based high-T c superconductors, only two compositions, TI2Ba2CuO 6 (referred as TI-2201) and T1Sr2CaCu207 (T1-1212) show a continuous change from high-T c superconductor to overdoped (with oxygen) nonsuperconducting metal with increase in hole concentration [1-3]. Preliminary experiments on TI-2201 single crystals demonstrate that the Tdependence of in-plane resisti,~ty, Pab has gradually evolved from T-linear to T with doping [4,5]. Similar behavior is observed in granular TI-2201 samples by Kubo et ai. [1,2,6]. The T2-dependence of p can be explained by electron-electron scattering in the usual Fermi liquid theory. The T-linear anomaly arising with decreasing hole concentration is explained in terms of some unusual deviation from the canonical Fermi liquid, such as magnetic fluctuations, localization, or Fermi surface nesting behaviors [6-8]. The negative pressure gradients of T c seen in granular T1-2201 suggest an indirect exchange pairing mechanism for conduction electrons via oxygen ions [9]. Band-structure

calculations [10], XPS measurements [11] and wetchemical analysis [12] have proved the existence of mixed valency (3+ and 1+) for TI and hence the origin of hole concentration in T1-2201 is the overlap of TI-6s band with the conduction band of the CuO 2 sheets. Also, T1-2201 has a high anisotropy with interesting normal state properties [13-16]. In this paper, we report the crystal growth, anisotropic resistivity, paraconductivity and thermopower measurements on TI-2201 phases. 2. EXPERIMENTAL TI2Ba2CuO6 single crystals have been grown by the self-flux technique [14,15]. In this technique, the starting compositions of TI:Ba:Cu = 322 or 413 were packed in an alumina crucible with lid and the crucibles with charge were loaded in a vertical tube furnace. They were typically heated rapidly to 900-950 "C and held for 1-3 hours, then slowly cooled through the melt at the rate of 2-20 "C/hour to 750-780 "C, and finally cooled down to room temperature. The detailed description of the appm'atus is reportedearfier [14,15]. Oxygen or air

* W e gratefully acknowledge the support of the Office o f Naval Research under O N R grant n u m b e r N00014-90-J-1571.

0921-4534/93/$06 00 © 1993 - Elsevier Science Pubhshers B V All nghts reserved

200

A M Hermann et al / Parring mechamsm m TI2Ba2Cu06 superconductors

is passed continuously through the tube during the crystal growth. Thin plates grow along the ab-plane with a typical size of 2x2x0.1 mm 3. Crystals as large as 11 mm in linear dimensions along the abplane were grown. X-ray four cycle diffraction and Laue photographs conf'Lrmed the lattice perfection. The lattice parameters obtained were, a = 3.9 J~, and c = 23.1 ,~, consistent with the literature values [17]. EPMA and SEM probe show single crystals are chemically uniform with clean surfaces. The as-grown crystals were mostly superconducting with Tc's as high as 91 K. These crystals were further annealed in oxygen atmosphere at different temperatures to vary the oxygen content. As the oxygen content increased, the Tcs of the crystals decreased, finally below to 2 K for the heavily oxygen doped crystals. The anisotropic resistivities, Pab (along ab-plane) and Pc (along caxis) were measured by the standard Montgomery method [18]. The samples used were of the size O.6x0.6x0.04 mm3. In-Ag contacts were placed on two adjacent comers of the ab-plane surface, and on the two opposite corners on the other side of the crystal face. The size of the contacts were less than one tenth of the crystal dimensions. The crystals were baked in an oven after the contacts were made. The contact resistance was less than one ohm. The resistivities of the crystals were measured by a dip-tube type apparatus located in a closed-cycle He refrigerator. The thermopower (TEP) measurements

were carried on T1-2201 crystals using a conventional measuring setup reported elsewhere [19]. 3. RESULTS AND DISCUSSION The anisotropic resistivities, Pab and Pc for a T12201 crystal with a T c of 86 K are plotted against temperature in Figure 1. The room temj~ature inplane resistivity is of the order of 6x10 ~ f2 cm and out-of-plane resistivity is of the order of 5x10 "1 f2 cm. Hence, TI-2201 has a high anisotropy. Both the resistivities decrease linearly with the temperature up to the onset of Tc. The same trend is observed for other crystals with lower Tcs. But, both Pal) and Pc decrease with Tc. Pc is always found to have metallike behavior. The anisotropic resistivity Pc is much greater than Pab by a factor of 2-3 orders of magnitude. These results are similar to those obtained by Manako et al. [16]. The anisotropy ratios (Pc/Pab) for TI-2201 crystals with different Tcs are shown in Figure 2. The anisolropy ratio is high for crystals with higher TcS and are found to increase with decreasing temperatures. It is also interesting to note that considerable resistivity anisotropy remains even in the normal metallic samples. This could be due to the large separation of CuO2 sheets in these systems. In the nested Fermi surface theory, when the conduction band is exactly half filled, the Fermi surface would be perfectly nested [8]. The 1400

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600 400

0 50

100

150 200 Temperature (K)

250

300

Figure 1. Temperature dependence of the anisotropic resistivities, Pab and Pc for a 1"1-2201 single crystal with a T c of 86 K.

200

o

5o

~oo

~5o 20o T(k")

~o

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Figure 2. T e m l ~ m r e dependence of the a n i ~ t ~ y ratios (Pc/Pab) for TI-2201 single crystals with different Tc's.

A M Hermann et al /Patrmg mechamsm m Tl2Ba2CuO6 superconductors

201

Table 1 Summary of various parameters obtained from the fit of 1/[ptT)-P 0] vs. 1/T data to the polynomial model (Eq. 1) for various TI-2201 single crystals with different Tc's. The data correspond to least squm-e fits with typical coefficients of correlation of 0.9997. Tc

PO

A

B

(K/fl.cm)

A/B

(K)

fjj~)

80

49.9

0.3904

0.0464

8.4

35

35.3

0.5330

0.1541

3.5

20

25.0

0.6999

0.1479

4.7

<18

6.9

0.4050

0.3835

1.1

conduction electrons on the nested Fermi surface have scattering rate proportional to T while the normal Fermi liquid electron scattering rate is proportional to T . The variation of the in-plane resistivity with temperalme for different Tc's can be fitted only to the following equation [5] 1/[pCr)-po] : A i r + Bfr 2

(1)

Here P0 is the residual resistivity due to impurity scatte~g. The coefficients A and B are proportional to their corresponding areas on the Fermi surface. We have fit many experimental p-T variations to Eq. 1 for a number of samples whose Tc's were varied by oxygen doping. A summary of the parameters obtained from the fit of l/[p(T)-p 0] vs. 1/T to Eq. 1 is reported in Table 1. For a perfectly nested Fermi surface, Bffi0. In the process of changing from a perfectly nested Fermi surface to normal Fermi liquid (as T c decreases), the ratio A/B increases monotonically. Hence the conduction is due to the sum of the contributions of the two parts of Fermi surfaces. We note from the results of Table I that not only are the normal state properties explained by nesting, but that the T c variation is also consistent with the expectation of a van Hove Singularity in two-dimensional nesting [20]. To find the dimensionality of the superconductors, the temperature dependence of the paraconductivity can be fit to [4.5]

(K2/fZ.cm)

(I/K)

A~ 03 = A (T/Tc-1)-a where Aa is the excess conductivity due to fluctuations, T c is the superconducting transition temperatures, A is a constant, and a is equal to 1 for two-dimensional systems and is equal to 1/2 for three-dimensional systems. Figure 3 displays a plot of (Ao)"1 vs. T for a TI-2201 crystal with T c of 14 K. The linearity of the plot indicates two-dimensional fluctuations and similar behavior is observed throughout the TI-2201 system with different Tc's. 5

~

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o

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I I 17

I 18

,

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, 20

Tt~ Figure 3. Plot of 1/Ao vs. T for TI-2201 single crystal with a T c of 14 K.

A M Hermann et al / Patnng mechamsm m T12Ba2CuO 6 superconductors

202

The temperature dependence of the Seebeck Coefficient for two TI-2201 single crystals is shown in Figure4. Crystal#1 h a s a T c onsetof85 Kand crystal #2 has a T c <25 K. The c-axis Seebeck coefficient shows p-type behavior and it decreases with decreasing temperatures for both cases. The magnitude of the c-axis TEP increases with decreasing T c. The c-axis behavior appears to be metallic and is similar to that seen in some other high-Tc systems. The ab-plane Seebeck coefficient is observed to similar to that of granular TI-2201 samples [21]. For higher Tc crystals, the ab-plane behavior is always n-type and it becomes less n-type with decrease in temperature. For lower Tc crystals, ab-plane TEP is n-type at higher temperatures and it crosses over to p-type behavior above the transition. The presence of TI-6s electrons (due to the overlap with the conduction band) could explain the n-type behavior at high temperatures. Kubo et al. [6], however, observed p-type behavior up to room temperature through Hall measurements on single crystal TI-2201 samples. Tight-binding calculations are ctmently underway to resolve these differences.

40

Crystal #2 (c.ax.is! . ~30 • *°"

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leo tso 2o0 Temperature (K)

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t

zso

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Figure 4. Temperature dependence of the Seebeck Coefficient for TI-2201 single crystals. Crystal #1 has a T c onset of 85 K and crystal #2 has a T c <25 K. REFERENCES

1. Y. Kubo, Y. Shimakawa, T. Manako and H. Igarashi, Phys. Rev. B43 (1991) 7875. 2. Y. Kubo, T. Kondo, Y. Shimakawa, T. Manako and H. Igarashi, Phys. Rev. B45 (1992) 5553.

3. M. Paranthaman, W. Kiehl and A.M. Hermann in: Studies of High Temperature Superconductors Vol. 11, ed. A.V. Narlikar (Nova Science Publishers, Inc., New York, 1992)in press, and references therein. 4. R.M. Yandrofski, Ph.D. Thesis, Harvard University, May 1992. 5. H.M. Duan, R.M. Yandrofski, T. Kaplan and A.M. Hermann, preprint. 6. Y. Kubo and T. Manako, Physica C 197 (1992) 378. 7. K. Levin, Ju H. Kim, J.P. Lu and Qimiao Si, Physica C 175 (1991) 449. 8. J. Ruvalds, C.T. Rieck, J. Zhang and A. Virosztek, Science 256 (1992) 1664. 9. L. Jansen, L. Chandran and R. Block, Physica C 201 (1992) 295. 10.D. Jung, M.-H. Whangbo, N. Herron andC.C. Torardi, Physica C 160 (1989) 381. 11. S. Nakajima, M. Kikuchi, T. Olin, N. Kobayashi, T. Suzuki, K. Nagase, K. Hiraga, Y. Muto and Y. Syono, Physica C 160 (1989) 458. 12. M. Pmanthaman, A. Manthiram and J.B. Goodenough, J. Solid State Chem. 87 (1990) 479. 13.H.M. Duan, R.M. Yandrofski, T.S. Kaplan, B. Dlugosch, J.H. Wang and A.M. Hermann, PhysicaC 185-189 (1991) 1283. 14. M. Paranthaman, H.M. Duan and A.M. Hermann in: Thallium-Based High TemperalTare Superconductors, eds. A.M. Hermann and J.V. Yakhmi (Marcel Dekker, Inc., New York, 1992) in press. 15.H.M. Duan, T.S. Kaplan, B. Dlugosch, A.M. Hermann, J. Swope, J. Drexler and P. Boni, Physica C, in press. 16. T. Manako, Y. Shimakawa, Y. Kubo and H. Igamshi, Physica C 185-189 (1991) 1327; ibid., 190 (1991) 62. 17.C.C. Torardi, M.A. Subramanian, J.C. Calabrese, J. Gopalakrishnan,E.M. McCarron, K.J. Morrissey, T.R. Askew, R.B. Flippen, U. Chowdhry and A.W. Sleight, Phys. Rev. B38 (1988) 225. 18.H.C. Montgomery, J. Appl. phys. 42 (1971) 2971. 19. W. Kiehl, Ph.D. Thesis, University of Arkansas, 1993, in preparation. 20. D.M. Newns, C.C. Tsuei, P.C. Pattnaik and C.L. Kane,Comments Cond. Mat. Phys.15 (1992) 273. 21.D.E. Weeks, W. Kiehl, C. Dung and A.M. Hermann, Physica C 176 (1991) 368.