Observation of cusps in irreversible vortex states of κ-(BEDT-TTF)2Cu(NCS)2 single crystals

Solid State Communications, Vol. 89, No. 11, pp. 955-958, 1994 Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/94 $6.00 + .00

Pergamon

0038-1098(93)E0136-L OBSERVATION OF CUSPS IN IRREVERSIBLE VORTEX STATES OF ~;-(BEDT-TTF)2Cu(NCS)2 SINGLE CRYSTALS S. Kawamata and K. Okuda Department of Physics and Electronics, University of Osaka Prefecture, Gakuen-cho, Sakai, Osaka 593, Japan and T. Sasaki and N. Toyota Institute for Materials Research, Tohoku University, Katahira, Sendai 980, Japan

(Received 11 November 1993; accepted for publication 3 December 1993 by T. Tsuzuki) Magnetic torque of a quasi two-dimensional superconductor t~(BEDT-TTF)2Cu(NCS)2 was measured as a function of field direction 0 with respect to the a*-axis under several magnetic fields H up to 8 kOe in the temperature range from 1.3 to 8 K. A sharp cusp Cl in the irreversible vortex region was found at 0cl near H II bc-plane between 1.3 and 7K. In addition, extra cusps C2 and Ca were also observed at 0c2 and 0c3, respectively, between 2.5 and 6K. At each temperature, the perpendicular component of H to give each cusp is kept constant as: HcosOcn=Const.-Hcpn(n= 1,2,3), i.e., cusps C1, C2 and C3 are ruled by the characteristic field perpendicular to the bc-plane HceI, Hc, 2 and Hoe3, respectively. These behaviors are almost the same as those we reported previously on the oxide superconductor Bi2Sr2CaCu208 (Bi22t2). The results reveal that the cusp effect is intrinsic for these layered superconductors.

1. INTRODUCTION Bisethylenedithio-tetrathiafulvalene (BEDT-TTF) based organic conductors are characterized by two dimensional nature, because they have a layered structure in which conducting layers of BEDT-TTF molecules are separated by the insulating anion layers. In particular, n type configuration of BEDT-TTF molecules brings the salts highly two dimensional character. ~-(BEDT-TTF)2Cu(NCS)2 takes a monoclinic structure: the conducting layer is the bc-plane formed by stacked BEDT-TTFs and the interlayer direction is along a*. It is the typical superconductor with Tc of about 10 K, in which a large anisotropy is reported by the transport and magnetic measurements [1-4]. Magnetic torque is a sensitive probe to investigate the anisotropy of the magnetization. Farrell et al. reported a huge anisotropy parameter 3' of more than 200 on ~-(BEDT-TTF)ECu(NCS)2 by the magnetic torque measurements [3]. We have measured the

magnetic torque for oxide superconductor Bi2Sr2CaCu208+6 (Bi2212) ( T c = 8 9 K ) [5] which has the layered structure and found the sharp cusp in the case that the magnetic field lies close to the layer plane in the temperature range between 15 and 82 K and also found the extra cusps in the narrow temperature range from 21 to 40 K. It was proposed that the cusp around layer plane is due to the intrinsic flux pinning between the superconducting CuO2 layers. There are a lot of similarities between n-type BEDT-TTF salts and the copper oxide superconductors. It is interesting to check whether these cusps are common properties in the layered superconductors. In this paper, we report the observation of cusps in the irreversible region for n-(BEDT-TTF) 2 Cu(NCS)2 single crystals by the magnetic torque measurements and compare these results with those for Bi2212. 2. EXPERIMENTAL

955

The single crystals were grown by an electro-

VORTEX STATES OF ~-(BEDT-TTF)2Cu(NCS)2 S I N G L E CRYSTALS Vol. 89, No. 11

956

21}1}

(a) 3

i

J

I

'

'

I

I

~f / B E D T - T T F ) 2 C u ( N C S ) ~

'

'

[

I

'

I

i

I

'

- (#2 B E D I - I'I F b C u ( N C S b

l Cl

I

'

'

I

'"

I

~

2 I00 11=8 kOc

o / a ~..~-~'~°°"~,~

11=4 kOc

I

° o

o ~-'v--O_.

4

~

~ v e. Ou.: O--x~" ,

% -

- ......

cttccfivc

-2

I

-3

I

~ i

60

. . . . <

I

(~t!l)l

Ill

¢"

. . . .

T=S.0 K

i

I

J

i

,('uN('S,

i

120

I

i

I

-201}

i 18()

150

I1(',,(oi0'' ! 0

o

8

o

°

o

a

T:20

K

O tooO

o

T=30 T=42

K

, (I

I 30

,

,

I 60 0

I 90 (degrees)

~ 120

,

K

i 150

,

, 180

Fig. 2. Angular dependence of the torque for the sample no. 2 under 4kOe at several temperatures between 1.3 and 4.2 K.

. . . .

dP ](', tl,_

#1

oo

T=9.[I K

91) (degrees)

0

(b) mo

-100

I

i o

T=7.[I K

o (.~.

t

~

30

% O

II]il~N" ~]

7 =350 qH~ =32 kOc

o~ o~ ~'~'~::~---~..~°" o o oo.oN, f

I

5O

iI=S kOc

("('

"

l~°~a

/>x\

~=

,

H [[ c. As the torque is caused by the force making the bc-plane parallel to the field preferably, the angle 0 = 90 ° is the most stable direction.

.....

% -

~

C

1=50

K

3. RESULTS A N D DISCUSSIONS

I={~ (I K (11

i

-101) 70

i

[ i Nil

i

i

{}

i

[ i t){} {degrees)

i

i

i

[ i I(}(I

i

i

i I()

Fig. 1. Angular dependence of the torque for the sample no. 1 under 8kOe at several temperatures between 7 and 10K (a) and between 4.2 and 6 K (b). Line indicates the fitted torque by the effective mass model. chemical oxidation method and the detail on the sample preparation was described in [6]. Two crystals were used for the measurements and each sample is in the form of a flat thin plate with well-developed habits. The sample no. 1 has the size of 2.5 × 1.4 (bc plane) x 0.2 (along a*)mm 3 weighing 0.6 mg and no. 2 has that of 1.7 × 1.0(bc plane) x 0.1 (along a*)mm 3 weighing 0.2mg. The measurements were performed using a conventional torque magnetometer operated by a null detection system in a field range from 0 to 8 kOe. The experimental procedure is the same as the previous measurements [5] on Bi2212: the sample was cooled down in zero field through T c to the temperature at which measurement was done, then the magnetic field H was applied along the a*-axis up to a given magnitude. Then, the magnetic torque was measured by rotating the field direction in the a'cplane from H][ a* to the opposite direction passing through the c-axis. The data were also collected by turning back the field. The angle is defined such that 0 = 0 ° is for H [[ a* (H ± bc-plane) and 0 = 90 ° is for

The torque of the samples no. 1 and no. 2 was measured between 4.2 and 10 K and between 1.3 and 4.2K, respectively. Figures l(a) and l(b) show the angular dependence of the magnetic torque r in 8 kOe between 7 and 10K (0°< 0 < 180°), and between 4.2 and 6 K (70°< 0 < 110°), respectively. The torque curves in 4 kOe between 1.3 and 4.2 K are shown in Fig. 2. Data at 4.2 K for the sample no. 1 coincided with those for the sample no. 2. As the temperature decreases, the amplitude of the torque becomes large and the angular region in which the torque shows hysteresis becomes large. The shape of the torque curves is sinusoidal one above 9 K, while the torque curve exhibits the sharp change both of the amplitude and sign around HI[ bc-plane below 8 K. A broken line at 7 K is the fitting curve which will be described later in terms of the anisotropy parameter % The magnetic torque in the irreversible region was investigated in detail. Below 8 K, the sharp cusp Ci is found in torque curves in an angular range where the magnetic field lies close to the layer plane as indicated by arrows in Fig. l(a) and (b). In addition to the cusp Cl, the shoulder in the torque appears below 6 K and it grows up to become extra cusp 6"3 at 4.2K as indicated by the arrows in Fig. l(b). Furthermore, another shoulder C2 appears below 4.2K. These extra cusps C2 and C3 disappear as the temperature decreases further down to 2.5 K. The cusp CI exists between 1.3 and 8K. Although the torque below 1.3 K has not been measured, it is expected that the cusp CI exists even below 1.3 K. On the other hand,

Vol. 89, No. 11 V O R T E X STATES OF ~-(BEDT-TTF)2Cu(NCS)2 S I N G L E CRYSTALS • '

I

I

L '

' |

7

301)0 -~ -(BEDT-TTF),CulNCS): T=4.2 K

1000

a

C:

•

c,

'

llcos(l,~=l 3(1 ()c (=llcr,)

'

I

#1

?

T=4.2 K

/

/

///

,/.

=l: 40011

I

'

'

I

'

' J

\ 6

/

Hcos(lirr=]~(H) Ott (-tti,~)

'L/

/

;

'\

///

"~%.....

S(l

I

r

6000

20110 0

70

'

.c,

~"

611

I

"tO -(BEDT-lq'FbCu(NCS),

, ~

CI

I(}(RX)

I

80O0 ;

O

'

i

2000

0

I

957

90

100

1111

1211

J

i

I

0

i

J

I

31)

i

,

[

611

L

i

90

0~ (degrees)

I

I

I t 1511

12(1

I 180

(degrees)

Oir r

Fig. 3. Magnetic field dependence of cusp angles at 4.2 K. Lines indicate fitting curves by the equation: Hcos 0~ = Hc~, (n = 1,2, 3).

Fig. 4. Magnetic field dependence of the angle where the hysteresis appears at 4.2 K. Lines indicate fitting curves by the equation: H c o s Oi,.r = Hi,.,..

the extra cusps C2 and C 3 appear in the limited temperature range between 2.5 and 4.2K and between 2.5 and 6 K, respectively. We define the cusp angles 0d, 0c2 and 0c3 as the angle to give the cusps C1, C2 and C3, respectively. The magnetic field dependence of the cusp angles is shown in Fig. 3 for the case at 4.2K as a typical example. Open circles, open triangles and solid triangles in the figure indicate 0cl, 0c2 and 0c3 , respectively. All three cusp angles 0o, (n = 1,2,3) get near 0 = 90 ° with increasing the field• It turned out that the perpendicular component of the applied field to give all three cusps HcosOcn (n = 1,2,3) is constant at a fixed temperature• Therefore, the characteristic fields perpendicular to the bc-plane to give each cusp Hc,,. (n = 1,2,3) can be uniquely defined by the equation: HcosOcn = Hc~ (n = 1,2, 3) as reported previously for Bi2212 [5]. The lines in Fig. 3 indicate the fitted curves using the equation. The origin for the cusp C1 is considered as follows. When the magnetic field is applied parallel to the layer planes, the magnetic fluxes lie between the superconducting layers consisting of B E D T - T T F molecules by the intrinsic flux trapping [7]. As the direction of the field rises from the bc-plane and the perpendicular component H c o s 0 reaches a certain critical value, the flux begins to intersect the layer planes making the vortex pancake. The field H~e ' is considered to correspond to the field where the number of the vortex pancakes starts to increase suddenly and represents the magnitude of the intrinsic pinning effect. Although the origin for the extra cusps C2 and C 3 is not yet clear, H~e 2 and Hoe3 seem to reveal some kind of change in the vortex state related to the collective pinning [8] or to the dimensional crossover of the vortex pancakes. Recently, it is proposed that the extra cusps are

caused by the matching of the distance between pinning centers with that between vortices [9]• In addition, the magnetic field dependence of the angle Oirr where the hysteresis appears in the torque curve is also investigated at each temperature• Figure 4 shows the data at 4.2 K. The perpendicular component of the field at Oirr is also kept constant at a fixed temperature. Therefore, it is found that Oir,. is also determined by the irreversibility field perpendicular to the layer plane, Hirr as the equation: HcosOi,.,. = Hirr. The line in the figure indicates fit to data by the equation• The values of H~e, (n = 1,2, 3) and Hir r for ~(BEDT-TTF)2Cu(NCS)2 determined at each temperature are shown by circles in Fig. 5 as a function of the reduced temperature T/Tc. Those for Bi2212 [5] is also indicated by triangles in the figure as a reference. For the case of Bi2212, the average values of Hcp 2 and Hc~ 3 at each temperature are plotted. 21100

i

i

I

'

i 1501)

I ---o---.

- -~o---

~ -(BI!I) I

,

-

•

(I-I0

=_e1011(I

Bi2212

.....

I-I'l:):('u(N('Sh K)

( [,=S~; K)

2 ~7

5011

(1

\

\ Ik~, --7-

'X. a,~^o,~

* -a- ,.e. q., t.i,

'%a, 9 - - . o L ~ . . . . .

• tpA

-'--

(1.5

• ~^_,,±~.=* • .

t

l T/T~.

Fig. 5. He~" (n = 1,2, 3) and Hirr for ~-(BEDT-TTF)2 Cu(NCS)2 and Bi2212 as a function of the reduced temperature T/Tc. For the case of Bi2212, the average values of Hop2 and H ~ 3 at each temperature are plotted•

958

VORTEX STATES OF ~-(BEDT-TTF)2Cu(NCS)2 SINGLE CRYSTALS Vol. 89, No. 11

Hoe' takes the value near the lower critical field 4. SUMMARY perpendicular to the bc-plane, Hc~ determined by the We have measured the magnetic torque 'for d.c. magnetization measurements [2]. This supports ~-(BEDT-TTF)2Cu(NCS)2 single crystals in the the interpretation that Hoe, is the field where the temperature range from 1.3 to 10K in several fields number of the vortex pancakes starts to increase up to 8 kOe. A sharp cusp near HI[ bc-plane and suddenly. Temperature dependence of Hoe, for I¢extra two cusps were observed in the torque curves as (BEDT-TTF)2Cu(NCS)2 is different from that for reported previously for Bi2212 [5]. The magnetic field Bi2212 below T/Tc = 0.4. Hc~2(T/Tc) and dependence of the angle to give all cusps was well H¢~3(T/Tc) for ~;-(BEDT-TTF)2Cu(NCS)2 is about ruled by the characteristic fields perpendicular to the one third of those for Bi2212. This may be attributed bc-plane Hc~n as HcosOcn = Hce, (n = 1,2, 3). The to the different pinning character. Hitr almost reduced temperature dependence of Hce~ for ~coincides with that determined by the d.c. magnetiza(BEDT-TTF)2Cu(NCS)2 shows nearly the same tion measurements [4]. Hirr for ~-(BEDT-TTF)2Cu behavior as that for Bi2212. The present results (NCS)2 is almost the same as that for Bi2212. Above reveal that the cusp effect is intrinsic for the layered results reveal that the cusp effect is intrinsic for these structure of the superconductors and the magnetic layered superconductors. properties of ~-(BEDT-TTF)2Cu(NCS)2 in the At present time, we have no theoretical formulae irreversible region are quite similar to those of to describe the magnetic torque appropriate for Bi2212. highly anisotropic layered superconductors. However, the anisotropic London model brings us the rough estimation of the anisotropy parameter 7. The Acknowledgements - We are very grateful to Prof. T. angular dependence of the torque at 7 K was fitted by Ishida for valuable discussions. We thank Dr S. Noguchi, H. Niimi, T. Kanayama and K. Tanaka for the effective mass model formulae by Kogan [1]: the support in the measurements. (boHV 7 2 - 1 sin20 In [7r/Hc~] 7"(0) 647r2 A----~

e(8) = (sin 2 8 + 7 2 cos2 O)ll2,

(1)

where ~0 is the flux quantum, V is the volume of the sample, )~ is average London penetration depth, r/is the constant of order of unity, Hc~ is the upper critical field perpendicular to the bc-plane and "y = X/-~a'/mb~ (ma* and mbc express the effective mass along the a*-axis and within the bc-plane, respectively). In Fig. l(a), circles and broken lines indicate the measured torque at 7K in 8kOe and fitted curve, respectively. The fitting parameters are indicated in the figure. The hysteresis in the angular dependence of the torque was observed only in narrow angular region from 89.7 to 90.3° and fitting was carried out in the reversible region. The 7 becomes very huge value of 350 that is consistent with the almost perfect cylindrical Fermi surface observed in Shubnikov-de Haas oscillations [6]. It suggests that the ~;-(BEDT-TTF)2Cu(NCS)2 is actually highly anisotropic superconductor.

REFERENCES 1. 2. 3. 4.

5. 6. 7. 8. 9. 10.

K. Oshima, H. Urayama, H. Yamochi & G. Saito, J. Phys. Soc. Jpn 57, 730 (1988). M.V. Kartsovnik, V.M. Krasnov & N.D. Kushch, Synthetic Metals 36, 27 (1990). D.E. Farrell, C.J. Allen, R.C. Hadden and S.V. Chichester, Phys. Rev. 1142, 8694 (1990). M. Lang, N. Toyota, T. Sasaki & H. Sato, Phys. Rev. Lett. 69, 1443 (1992), M. Lang, N. Toyota & T. Sasaki, Synth. Metals 55-57, 2401 (1993). S. Kawamata, N. Itoh, K. Okuda, T. Mochiku & K. Kadowaki, Physica C195, 103 (1992). T. Sasaki, H. Sato & N. Toyota, Solid State Commun. 76, 507 (1990). M. Tachiki & S. Takahashi, Solid State Commun. 70, 291 0989) and 72, 1083 (1989). A.I. Larkin & Y.N. Ovchinikov, J. Low. Temp. Phys. 34, 409 (1979). P. Shang, G. Yang, I.P. Jones, C.E. Gough & J.S. Abell, Appl. Phys. Lett. 63, 827 (1993). V.G. Kogan, Phys. Rev. B38, 7049 (1988).