Hall conductivity in Tl2Ba2CaCu2O8 thin films

PHYSICA

Physica B 194-196 (1994) 2319-2320 North-Holland

Hall conductivity in T12Ba2CaCu208 thin films A. V. Samoilov a, Z. G. Ivanovb and L-G. Johansson c ap..L. Kapitza Institute for Physical Problems, ul. Kosygina 2, 117334, Moscow, Russia bDepartment of Physics, Chalmers University of Technology and University of Gothenburg S-412 96 Gothenburg, Sweden CDepartment of Inorganic Chemistry, Chalmers University of Technology and University of Gothenburg S-412 96 Gothenburg, Sweden We have studied the Hall and longitudinal conductivities in c-axis oriented T12Ba2CaCu208 thin films. While the longitudinal conductivity displays thermally activated behavior increasing exponentially in the mixed state, the Hall conductivity tends to saturate with decreasing temperature. The data provide evidence that the Hall conductivity in the mixed state is independent of pinning strength and allow the determination of the bare Hall drag coefficient. I. I N T R O D U C T I O N M e a s u r e m e n t s of the resistivity tensor components of a type-II superconductor in the mixed state provide important information about the vortex motion under the influence of a transport current. It would be of great interest to compare experimental data on the Hall and longitudinal resistivities, Pxy and P xx, respectively, with theoretical models of viscous flux flow [l-3]. However, such a comparison is difficult to carry out because under decreasing temperature and/or magnetic field pinning becomes increasingly important. Vinokur, Geshkeinbein, Feigelman and Blatter [4] have shown that Pxy = c~ I(~oH)p2xx, where ~ is the bare Hall drag coefficient, ~ 0 = h/2e, H is the magnetic field strength. It means that the Hall conductivity Crxy=Pxy/p2xx (I Pxyl <
conductivity Crxy for the 240 nm thick film. The data for the other film were similar. In the normal state, CYxy is proportional to the magnetic field strength. At T = 102-103 K the Crxy(T)-dependence exhibits a minimum which, at smaller fields, evolves into the s i g n - c h a n g e behavior. At l o w e r temperatures, while Crxx increases exponentially being strongly affected by pinning, the Hall conductivity tends to saturate. The T-dependency of ~xy at low fields is shown in Fig. 2. One can see that at fixed T Crxx- -1/H, in agreement with the low-field result of the theory [2, 3]. It means that c~ is field-independent. We plot the field dependency of the Hall coefficient in Fig. 3. The I/H term is present both at 10

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II. E X P E R I M E N T A L

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b

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III. R E S U L T S AND DISCUSSION Fig. 1 shows the Arrhenius plots of the longitudinal conductivity O'xx = 1/Pxx, and Halll

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i

I 20

n 30

IO00/T (K-') Figure 1. The Arrhenius plots of the conductivities in H = 3.5 T and 5T.

0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved

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We measured Pxy and Pxx in two c-axis oriented T12Ba2CaCu208 films grown on LaA10 3 substrates. The films were 240 and 360 nm thick. The 10-3 I.tf~cm resistivity level in H = 0 was reached at T --101 K. The measurements were carried out at dc current densities j = 750 - 1500 A/cm 2 at which both resistivities were ohmic.

# T 6

2320 low and high temperatures. As temperature decreases, the field-independent component of c~ changes sign from negative to positive at T = 80-85 K. At a fixed temperature, under increasing magnetic field, the deviation from the l/H-law most probably takes place because of the normal Hall effect in the vortex cores [1, 2]. If so, one can understand the fact that Crxyseems to tend to a constant value at T---~0 (Fig. 1) in the following way. Crxy = 1/(cotOHPxx), where OH is the Hall angle. In the normal state, cotOH = C~+ bT 2 is a well-documented behavior for the high-To superconductors which is also valid for our samples. Then the normal state Hall conductivity tends to the constant value (o~p0) -1, where P0 is the residual resistivity. There is a clear analogy with the recent experiments on a 2D electron system [5] if one changes (localized electrons) --+ (localized vortices) and (resistances) ---)(conductivities). It has bee shown [5] that as temperature decreases the longitudinal resistance increases exponentially, while the Hall resistance is close to the classical value -H/ne, where n is the electron density.

ACKNOWLEDGMENTS

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Figure 2.

The T-dependencies of Go , in low magnetic fields indicated near the curves in T.

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The project utilized the Swedish Nanometer Laboratory and was supported by the Swedish Board for Industrial and Technical Development.

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REFERENCES

1. J. Bardeen and M. J. Stephen, Phys. Rev., 140 (1965) A1197. 2. A. T. Dorsey, Phys. Rev. B, 46 (1992) 8376. 3. N.B.Kopnin, B.I. Ivlev, and V.A. Katalsky, Pis'ma v ZhETF, 55 (1992) 717. 4. V. M. Vinokur, V. B. Geshkenbein, M. V. Feigel'man, and G. Blatter (to be published). 5. S. I. Dorozhkin, A. A. Shashkin, G. V. Kravchenko, V. T. Dolgopolov, R. J. Haug, K. von Klitzing, and K. Ploog, Pis'ma v Zhetf, 57 (1993) 55.

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The magnetic field dependencies of Crxy. at different temperatures indicated near the curves. Lines are guide to the eye.