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Heat conduction in κ-(BEDT-TTF)2X superconductors
ELSEVIER
Synthetic Metals 103 (1999) 2046-2047
Heat Conduction in K-(BEDT-TTQX
superconductors
Sttphane Belin”, Kamran Behnia”, Michel Ribault”. Andre Deluzetb and Patrick Batailb “Laboratoire
de Physique des Solides(CNRS),
UniversitC Paris&d,
F-91405 Orsay, France
bInstitut des Materiaux de Nantes, Universitt de Names, F-44322 Nantes. France
We present the first study of thermal prcscnts ltiatures which have already been at the superconducting transition which is
conductivity of a quasi two dimensional detected in the high T, cuprates. Notably suppressed by a moderate magnetic field. cicclronic contribution close to the universal limit value which is consistent with \uperconJucting gap function. K(JI.II’IIII/.S: Heat conduction, Superconducting
transition, Organic superconductors.
K-i BEDT-TTF)2Cu(NCS)2 is a quasi 2D organic superconductor (Tc=lOK). K-(BEDT-TTF) 2X family is known to have similar features to cuprates superconductor[l]: they exhibit low carrier densities. possibly strong electronic correlations and proximiry of &ferromagnetic ground state. Superconducting properties arc also similar: the superconductivity is confined in cnndu~ting planes sandwiched between insulating layers. In the K-( BEDT-TTF) ?X~ family the role of the pressure and the choice ot’ X can be compared to the role of the doping in high T, mnlcriais: X=Cu[N(CN)$ZI can be compared to the strongly underdoped, X=Cu[N(CN)zJBr to the underdoped and X=CulNCS)? to the optimaly or overdoped compounds. In spite 01’ the increasing evidence of a unconventional pairing of electrons in the superconducting state[2], in this family there is no 1rc:11consensus on the symmetry of the order parameter in compkson
organic superconductor. r+{BEDT-TI’F)2Cu(NCS)2 we observed an increase of the thermal conductivity At low temperatures we clearly resolved a residual an anisotropic pairings with lines of nodes in the
studied presented the same behavior, but the magnitude of the peak was found to be strongly sample-dependent. The increase in thermal conductivity below T, indicates fhat the condensatcon 01 quasi-particles in the superconducting state increases thermal conductivity by reducing the scattering_of heat carriers. In the case of YBCO the debate was centered on the identity of these heat carriers. An orthodox scenario invoked an increase in the lattice conductivity due to condensation of electrons. But -a very unusual increase in the electronic r@axation time iii- the superconducting state[5] seemed to indicate that the main part of the upturn in ic could be due to an increase of electronic mean free path. Strong support for the latter point of view was provided by thermal Hall effect measurements]7].
with the high T, cuprates.
In thia short paper we report the first study of thermal conductivity of u member of this family. According to this study, K-( BEDT-T’TF)Q(NC& presents features which have been .~hc;!tiy detected in the high-T, cuprates. Notably, we observed a rclc~
0.0
0.2
0.4
0.6
0.8
1.0
mu1.2
Fig. 1 Normalized temperature dependence of the normalized thermal conductivity of K-(BEDT-TTF)2Cu(NCS)2 , compared to YBC0[6] and NiobiumI61. The deviation~from the straight line . .~_. marks T,. Note the similar upturn in thermal conductrvrty tor K-(BEDT-TTF)~CU(NCS)~ and YBCO. pate the conventional decrease of heat transport of Niobium at the superconducting transition.
0379-6779/99J% - see front matter 0 1999 Elsevi&r Science S.A. All rights reserved. PII: SO379-6779(98)00399-3
S. Belin
et al. i Synthetic
Metals
In the cxx ol’ K-(BEDT-TTF)$XNCS)~ the origin of the LI~I~~II raises the h;~mc questions. Surface resistance studies have rcportctl ;I mcxlest but unusual increase in the microwave conductivity ol’ thia ayatcm below T,[8]. This analogy with the c’upr;~tcs suggests L~;II a part of the upturn in K(T) reported here should he due to quasi-particles,
35
30
25
2
20
Lrs 5
15
Y 10
i 15
Fig. 2 The tcmperaturc dependence of the thermal c,onductivlty for dil’t’erent magnetic field. Inset shows the T2 dependence of K/T in the both superconducting and normal phase. The shaded area represents the universal linear term in the clean limit. The straight and doted lines show the expected aT+bT’ behavior respectively in the superconducting and normal phase.
103
(1999)
2046-2047
2047
Nevertheless this very visible effect oi‘ the quasi-particle condensation on lattice conductivity indicates the prescncc al :l strong electron-phonon coupling in this system already observed by Raman scattering[9] study. This is to be contrasted to the LXSC of (TMTSF)2CIOq, where lattice conductivity was found to bc unchanged by the superconducting transition[lO]. Evidence for unconventional superconductivity comes from our low-temperature results. The inset of fig. 2 presents the low temperature behavior of thermal conductivity in normal imcl superconducting states. The remarkable feature is the prescncc 01 a residual linear term in the thermal conductivity of the superconducting state. K(T) presents a aT+ by tempernturc a= 0.20F0.09mWiK’cm Wd dependence with b= 1 liSmW/K’cm. The cubic term is related to the maximum phonon mean-free-path using the kinetic gas equation Kp~,=l/3c~i,v,l,~,. where cpI,=PT’ is the lattice specific heat (p= 73.6 IJJ/K’cm [l 11) and v, the velocity of sound (v, = 5.lO’m/s[ 121). We found lpi,= 28pm which is comparable to the sample tickness (20wm). It is then coherent to attribute the linear term to ;I residual electronic contribution in the superconducting states: &t/T= 0.2~0.09mW/K2cm. Now in the normal state, we can reasonably expect that the ballistic regime should be attained at ;I similar temperature range and the magnitude should he identical in the normal and superconducting states. In this way, the zerotemperature Kec,l"/T can be estimated to be 0.95 mW/K’cm. According to the theory of heat transport in Unconvention‘LI superconductors, the impurity scattering of residual qunsiparticles creates a finite zero-temperature value for ic’,liTl~l3]. Moreover. for certain anisotropic gap the magnitude of this lineat term is found to be universal at small concentrations of impurity: Kf&= frka2c+‘l(6e2S), where o+ is the plasma frequency and S=d&dd$ is the slope of the gap at the node. The expcrimcntal validity of this theory has been recently reported in the case ot YBa2Cu30&‘14]. In our case, assuming a standard d-wave gap with S= 24, and using fiq= O.biO.leV[l5] and 2&1= 4.8&1.lmeV[16], we found K&= 0.16+0.05 mW/K’cm in very, good agreement with the experimental result. In conclusion, our study of heat conductivity in K-(BEDTprovides strong support for nodes in the TTF)#Zu(NCS)2 superconducting gap function, strong electron-phonon coupling and possibly an increase of quasi-particle mean free path below T,. We thank H.Aubin, L. Taillefer, C. Pasquier. D. JCrome and L. Fruchter for stimulating discussions and L. Bouvot l’or technical assistance.
References 1 1 R. H. Mckenzie. Science 278, 820 (1997). 7-J H. Mayal’t’re et al.. Phys. Rev. Lett. 75, 4122 (1995): Y. ~akazawa and K. Kancda, Phys. Rev. B 55, 8670 (1997). 3 j For more inl’ormntion on the experimental appartus: <. Bclin K.Bchnia and A.Deluzet to be published. II G. Yu. Logenov et al., J. Phys. I (Paris) 6, 2051 (1996). 51 D. A. Bonn et al.. Phys. Rev. Lett. 68, 2390 (1993). (,j H.Aubin et al., 2. Phys. B 104, 175 (1997). 71 K, Krlahima et al.. Phys. Rev. Lett. 75, 483 (1995). X] II. Achkir et al.. Phys. Rev. B 47, 1159.5 (1993); M. Dressel :t al., Phys. Rev. B 48, 9906 (1993). 91 D. Pedron et al., Physica C 276, I (1997).
[lo] S. Belin and K. Behnia, Phys. Rev. Lett. 79. 2125 (1997). [I l] R. G. Goodrich et al., Mat. Res. Sot. Symp. Proc. 173. 125 (1990). [12] M. Yoshizawa et al., Solid State Comm. 89, 701 (1994). [13] M. J. Grafet al., Phys. Rev. B 53, 15147 (1996); M. R. Norman and P. J. Hirschfeld, ibid 53, 5706 (1996). [I41 L. Taillefer et al., Phys. Rev. Lett. 79,483 (1997). [I51 A. Ugawaetal.Phys. Rev.B38,5122(1988). [ 161 H. Bando et al. in “The Physics and Chemistry of Organic Superconductors”, eds: G. Saito and S. Kagoshima. (Springer. Heidelberg, 1990), 167.
Synthetic Metals 103 (1999) 2046-2047
Heat Conduction in K-(BEDT-TTQX
superconductors
Sttphane Belin”, Kamran Behnia”, Michel Ribault”. Andre Deluzetb and Patrick Batailb “Laboratoire
de Physique des Solides(CNRS),
UniversitC Paris&d,
F-91405 Orsay, France
bInstitut des Materiaux de Nantes, Universitt de Names, F-44322 Nantes. France
We present the first study of thermal prcscnts ltiatures which have already been at the superconducting transition which is
conductivity of a quasi two dimensional detected in the high T, cuprates. Notably suppressed by a moderate magnetic field. cicclronic contribution close to the universal limit value which is consistent with \uperconJucting gap function. K(JI.II’IIII/.S: Heat conduction, Superconducting
transition, Organic superconductors.
K-i BEDT-TTF)2Cu(NCS)2 is a quasi 2D organic superconductor (Tc=lOK). K-(BEDT-TTF) 2X family is known to have similar features to cuprates superconductor[l]: they exhibit low carrier densities. possibly strong electronic correlations and proximiry of &ferromagnetic ground state. Superconducting properties arc also similar: the superconductivity is confined in cnndu~ting planes sandwiched between insulating layers. In the K-( BEDT-TTF) ?X~ family the role of the pressure and the choice ot’ X can be compared to the role of the doping in high T, mnlcriais: X=Cu[N(CN)$ZI can be compared to the strongly underdoped, X=Cu[N(CN)zJBr to the underdoped and X=CulNCS)? to the optimaly or overdoped compounds. In spite 01’ the increasing evidence of a unconventional pairing of electrons in the superconducting state[2], in this family there is no 1rc:11consensus on the symmetry of the order parameter in compkson
organic superconductor. r+{BEDT-TI’F)2Cu(NCS)2 we observed an increase of the thermal conductivity At low temperatures we clearly resolved a residual an anisotropic pairings with lines of nodes in the
studied presented the same behavior, but the magnitude of the peak was found to be strongly sample-dependent. The increase in thermal conductivity below T, indicates fhat the condensatcon 01 quasi-particles in the superconducting state increases thermal conductivity by reducing the scattering_of heat carriers. In the case of YBCO the debate was centered on the identity of these heat carriers. An orthodox scenario invoked an increase in the lattice conductivity due to condensation of electrons. But -a very unusual increase in the electronic r@axation time iii- the superconducting state[5] seemed to indicate that the main part of the upturn in ic could be due to an increase of electronic mean free path. Strong support for the latter point of view was provided by thermal Hall effect measurements]7].
with the high T, cuprates.
In thia short paper we report the first study of thermal conductivity of u member of this family. According to this study, K-( BEDT-T’TF)Q(NC& presents features which have been .~hc;!tiy detected in the high-T, cuprates. Notably, we observed a rclc~
0.0
0.2
0.4
0.6
0.8
1.0
mu1.2
Fig. 1 Normalized temperature dependence of the normalized thermal conductivity of K-(BEDT-TTF)2Cu(NCS)2 , compared to YBC0[6] and NiobiumI61. The deviation~from the straight line . .~_. marks T,. Note the similar upturn in thermal conductrvrty tor K-(BEDT-TTF)~CU(NCS)~ and YBCO. pate the conventional decrease of heat transport of Niobium at the superconducting transition.
0379-6779/99J% - see front matter 0 1999 Elsevi&r Science S.A. All rights reserved. PII: SO379-6779(98)00399-3
S. Belin
et al. i Synthetic
Metals
In the cxx ol’ K-(BEDT-TTF)$XNCS)~ the origin of the LI~I~~II raises the h;~mc questions. Surface resistance studies have rcportctl ;I mcxlest but unusual increase in the microwave conductivity ol’ thia ayatcm below T,[8]. This analogy with the c’upr;~tcs suggests L~;II a part of the upturn in K(T) reported here should he due to quasi-particles,
35
30
25
2
20
Lrs 5
15
Y 10
i 15
Fig. 2 The tcmperaturc dependence of the thermal c,onductivlty for dil’t’erent magnetic field. Inset shows the T2 dependence of K/T in the both superconducting and normal phase. The shaded area represents the universal linear term in the clean limit. The straight and doted lines show the expected aT+bT’ behavior respectively in the superconducting and normal phase.
103
(1999)
2046-2047
2047
Nevertheless this very visible effect oi‘ the quasi-particle condensation on lattice conductivity indicates the prescncc al :l strong electron-phonon coupling in this system already observed by Raman scattering[9] study. This is to be contrasted to the LXSC of (TMTSF)2CIOq, where lattice conductivity was found to bc unchanged by the superconducting transition[lO]. Evidence for unconventional superconductivity comes from our low-temperature results. The inset of fig. 2 presents the low temperature behavior of thermal conductivity in normal imcl superconducting states. The remarkable feature is the prescncc 01 a residual linear term in the thermal conductivity of the superconducting state. K(T) presents a aT+ by tempernturc a= 0.20F0.09mWiK’cm Wd dependence with b= 1 liSmW/K’cm. The cubic term is related to the maximum phonon mean-free-path using the kinetic gas equation Kp~,=l/3c~i,v,l,~,. where cpI,=PT’ is the lattice specific heat (p= 73.6 IJJ/K’cm [l 11) and v, the velocity of sound (v, = 5.lO’m/s[ 121). We found lpi,= 28pm which is comparable to the sample tickness (20wm). It is then coherent to attribute the linear term to ;I residual electronic contribution in the superconducting states: &t/T= 0.2~0.09mW/K2cm. Now in the normal state, we can reasonably expect that the ballistic regime should be attained at ;I similar temperature range and the magnitude should he identical in the normal and superconducting states. In this way, the zerotemperature Kec,l"/T can be estimated to be 0.95 mW/K’cm. According to the theory of heat transport in Unconvention‘LI superconductors, the impurity scattering of residual qunsiparticles creates a finite zero-temperature value for ic’,liTl~l3]. Moreover. for certain anisotropic gap the magnitude of this lineat term is found to be universal at small concentrations of impurity: Kf&= frka2c+‘l(6e2S), where o+ is the plasma frequency and S=d&dd$ is the slope of the gap at the node. The expcrimcntal validity of this theory has been recently reported in the case ot YBa2Cu30&‘14]. In our case, assuming a standard d-wave gap with S= 24, and using fiq= O.biO.leV[l5] and 2&1= 4.8&1.lmeV[16], we found K&= 0.16+0.05 mW/K’cm in very, good agreement with the experimental result. In conclusion, our study of heat conductivity in K-(BEDTprovides strong support for nodes in the TTF)#Zu(NCS)2 superconducting gap function, strong electron-phonon coupling and possibly an increase of quasi-particle mean free path below T,. We thank H.Aubin, L. Taillefer, C. Pasquier. D. JCrome and L. Fruchter for stimulating discussions and L. Bouvot l’or technical assistance.
References 1 1 R. H. Mckenzie. Science 278, 820 (1997). 7-J H. Mayal’t’re et al.. Phys. Rev. Lett. 75, 4122 (1995): Y. ~akazawa and K. Kancda, Phys. Rev. B 55, 8670 (1997). 3 j For more inl’ormntion on the experimental appartus: <. Bclin K.Bchnia and A.Deluzet to be published. II G. Yu. Logenov et al., J. Phys. I (Paris) 6, 2051 (1996). 51 D. A. Bonn et al.. Phys. Rev. Lett. 68, 2390 (1993). (,j H.Aubin et al., 2. Phys. B 104, 175 (1997). 71 K, Krlahima et al.. Phys. Rev. Lett. 75, 483 (1995). X] II. Achkir et al.. Phys. Rev. B 47, 1159.5 (1993); M. Dressel :t al., Phys. Rev. B 48, 9906 (1993). 91 D. Pedron et al., Physica C 276, I (1997).
[lo] S. Belin and K. Behnia, Phys. Rev. Lett. 79. 2125 (1997). [I l] R. G. Goodrich et al., Mat. Res. Sot. Symp. Proc. 173. 125 (1990). [12] M. Yoshizawa et al., Solid State Comm. 89, 701 (1994). [13] M. J. Grafet al., Phys. Rev. B 53, 15147 (1996); M. R. Norman and P. J. Hirschfeld, ibid 53, 5706 (1996). [I41 L. Taillefer et al., Phys. Rev. Lett. 79,483 (1997). [I51 A. Ugawaetal.Phys. Rev.B38,5122(1988). [ 161 H. Bando et al. in “The Physics and Chemistry of Organic Superconductors”, eds: G. Saito and S. Kagoshima. (Springer. Heidelberg, 1990), 167.