Temperature dependence of ratio, Hc2∥Hc2⊥ , for NbSe2

Volume 45A, number 2

PHYSICS LETTERS

TEMPERATURE

10 September 1973

DEPENDENCE OF RATIO, k&&&u,

FOR NbSe2

Y. MUTO, N. TOYOTA, K. NOT0 and A. HOSHI The. Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai, Japan

Received 4 July 1973 It is found that the ratio of Hc2,, to Hca is dependent on temperature for the superconducting NbSe2.

The temperature and angular dependence of upper critical fields, Hc2, have been measured for temperatures close to T, and for those below 4.2 K on single crystals, 2H-NbSe,, by use of electrical conduction measurements. It has become clear that our experimental results are not satisfactorily described on the basis of the effective mass model used by Morris et al.

NC a5s? ?2

111.

g-

Single crystals of 2H-NbSe2( 10 X 10 X 0.1 0.2 mm3) were prepared by the method of chemical gas-transport reactions using iodine as the carrier. The residual resistance ratio of the sample reported here is 30. In absence of magnetic field, Tc is found to be 7.340 rt 0.005 K which is pretty higher than values reported by many authors, while the highest T, value of 7.38 K was recently reported by de Trey et al. [2] . Hc2 values close to T, and those below 4.2 K were measured in magnetic fields up to 8 kOe produced by an ironcore Bitter type magnet and in those up to 95 kOe produced by a superconducting solenoid, respectively. Hc2 values were also studied as a function of crystal orientation in several temperatures. The effective mass tensor model gives the angular dependence of Hc2 as follows: Q(W~,~

= (sin28 te2cos28)-1’2

(1)

where E = (m/M)l/z. m andM are the effective mass of parallel and perpendicular direction to the layer, respectively. 8 is measured from the layer plane and Hc2(90) = I&. From eq. (l), e-l becomes = H&O). Since mass is in42lPcU where 42s dependent on temperature, the ratio of Hc2,, to If,__ should be independent on temperature according to the model. In fig. 1 are shown Hc2 values at 7.07 K as a func-

dichalcogenide,

‘I 7 07K

0 Experiment

6-

- Theory G2=0.155

T 0

“3t I

\

I

90”

I

60”

30’

O” -30” -60° 8 (degree)

-90”

Fig. 1. H,2 values (0) at 7.07 K as a function of 8. The solid curve is calculated from eq. (1).

tion of 0. The solid curve is obtained from eq. ( 1) by use of our experimental values of Hc2,, and Hc22. Our result shows that the above law does not hold but some deviations occur. This trend is common for every temperature studied. This fact was also recognized by the Trey et al. [2], though they could not observe Hc2 u values but used the E value as a parameter. In fig. 2 are shown both Hc2,, and Hcu values as a function of temperature. Hc2 values below 4.2 K for both directions decrease almost linearly with increasing temperature, while they decrease with positive curvature as the temperature approaches to T,. Furthermore, as can be seen in fig. 2, HC2,,/Hc. increases with decreasing temperature. Such behavior on anisotropic upper critical fields of layer structure superconductors has never been reported. 99

10 September 1973

PHYSICS LETTERS

Volume 45A, number 2

‘i&,,

!

-3 '. '\

‘\

“5

theory

does not explain our experimental

results for

NbSe,. We are grateful to Professor Y. Onodera and Mr. K. Kikuchi for supplying specimes and to Dr. P. de Trey for sending a preprint. The superconducting solenoid of Tohoku University Cryogenic Center was used.

References '0

I2

3

4

5

6

7

TW)

r, I_\ Fig. 2. Hc211(o) and Hc21(A) values and ratio, Hc2ll/flc2~(0’ as a function of temperature.

We conclude that Hc2,,/Hc_(= e-1) depends on temperature and that the effective mass tensor

100

[l] R.C. Morris, R.V. Coleman and R. Bhandari, Phys. Rev. B5 (1972) 895. [2] P. de Trey, S. Gygax and J.-P. Jan, J. Low Temp. Phys., to be published.