2PO9-90 Angular dependent magnetothermal conductivity in unconventional superconductors

I. Vekhter, Department of Physics, University of Guelph, Guelph, Ontario, Canada N1G 2W1, and P.J. Hirschfeld, Department of Physics, University of Florida, Gainesville, FL 32611-8440, USA

Presenting Author: I. Vekhter

We analyse the behavior of the thermal conductivity in the vortex state of a two-dimensional d-wave superconductor in the geometry when both the heat current and the applied magnetic field are in the basal plane. At low temperature the effect of the magnetic field is accounted for in a semiclassical approximation, via a Doppler shift in the spectrum of the nodal quasiparticles. We find that in that regime the thermal conductivity exhibits twofold oscillations as a function of the angle between the direction of the field in the plane and the direction of the heat current, in agreement with existent experimental data. We also discuss the possible origin of a four-fold angular dependence observed in high-T_c materials at higher temperatures.

2PO9-91 Is the Heavy Fermion Specific Heat of Bosonic Origin?

Alpo Kallio, Johannes Hissa, Vinski Braysy, and Tapio Hayrynen, Department of Physical Sciences, Theoretical Physics, University of Oulu, P.O. Box 3000, FIN-90401 Oulu, Finland

Presenting Author: A.J. Kallio

We propose that the giant specific heat observed for heavy fermions (HF) comes from bosons rather than supermassive fermions. Several compounds such as UBe₁₃, CeCu₂Si₂, CeAl₃ and CeCu₆ exhibit temperature dependence of the form g (T)=g _F+A/T for their linear electronic specific heat coefficient g =C_e/T. In the case of UBe₁₃ and CeCu₂Si₂ one obtains good description with g ~5mJ/molK² and A~ 1300mJ/molK in the temperature range 2K<T<10K (exp. See G. R. Stewart, Rev. Mod. Phys. 56 (1984) 753). This gives a hint that the chemical equilibrium model [1] with boson decay reaction B^++® 2h⁺ might work also for HF-compounds with the g _F-term originating from fermions and the second term from bosons with A=x n_Bk_B, where n_B is the boson density and x depends upon the di-mensionality and spin of the bosons. The case UBe₁₃ is particularly interesting since it becomes su-perfluid at T_c~9K and the boson delocalization temperature is T_BL~2K, estimated from the Hall co-efficient maximum. The model [1] states that above T_c bosons are 2D, but below T_c they become 3D. Due to the asymmetry of the chemical lattice the delocalization takes place in two stages at T_BL and at T_c. Since in UBe₁₃, T_BL is close to T_c a considerable fraction of bosons can remain localized even below T_c and give a major contribution to the electronic specific heat. Since the bose system becomes 3D at T_c the multiplicity factor x _S(3D) in the superfluid state is larger than x _n(2D) in the normal state. In this way the size of the peak is of large bosonic order 2.2 J/molK.

[1] A. Kallio et al. Superlattices and Microstructures, Vol 21, Supl. A (1997) 111.

2PO9-92 Electrical Properties of Phase-Slip Center and Artificial SNS Structure in the Presence of Different Relaxation Mechanisms of Quasiparticles

Oleksiy B. Agafonov 1, Georgii E. Churilov 1, Dmytro A. Dikin 1,2, Vitaly M. Dmitriev 1,3. 1 B. Verkin Institute for Low Temperature Physics and Engineering of National Academy of Sciences of Ukraine, 47 Lenin Ave., 310164, Kharkov, Ukraine. 2 Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA. 3 International Laboratory of High Magnetic Fields and Low Temperatures, Gajowicka str., 95, 53-421, Wroclaw, Poland.

Presenting Author: O.B. Agafonov

The electrical characteristics of homogeneous superconducting tin (Sn) films both with phase-slip centers (PSC) and with structures of the Superconductor-Normal metal-Superconductor (SNS) type, created in the same film, have been studied experimentally in the temperature range close to the superconducting transition temperature (T/T_c=0,95-0,999). Here the normal region of the SNS arose as a result of the local critical temperature depression due to the proximity effect. We observed that the resistive transition to the superconducting state R(T) of the samples with SNS structures reflects the temperature dependence of the weak longitudinal electric field penetration depth into the superconductor lÅ(T), which is related to the inelastic scattering mechanism. For small currents the residual ohmic resistance caused by the first PSC was less than that of the SNS structures. This means that the PSC has a shorter nonequilibrium region than the SNS structure due to the contribution of the elastic scattering mechanism at the presence of the supercurrent of Cooper pairs to the relaxation of the nonequilibrium quasiparticles. This leads to the diminution of the nonequilibrium region resistance which we have observed experimentally.

*2PO9-93 Thermal Conductivity of type II Superconductors in the Vortex State

Etienne Boaknin 1, R.W. Hill 1, Christian Lupien 1, Louis Taillefer 1, P.C. Canfield 2, P. Meeson 2, and S.M. Hayden 2. 1 Department of Physics, University of Toronto, Canada. 2 Ames Laboratory, Iowa State University, USA. 2 Department of Physics, University of Bristol, United Kingdom.

Presenting Author: E. Boaknin

The thermal conductivity of the borocarbide material LuNi₂B₂C and the A-15 compound V₃Si was measured as a function of magnetic field, in order to investigate the quasiparticle behavior of s-wave superconductors in the vortex state. This study aims to shed light on the evolution from localized quasiparticle states at low fields to extended quasiparticle states near the upper critical field. This is directly related to the occurrence of de Haas-van Alphen oscillations in these compounds deep inside the superconducting state, an issue of current debate.

2PO9-94 Phase Transitions and Superconductivity of LuFe₄Al₈

Vitaly M. Dmitriev 1,2, Victor M. Eropkin 1, Alla M. Gurevich 1, Mikola M. Prentslau 1, and Larisa V. Shlyk 1. 1 B. Verkin Institute for Low Temperature Physics and Engineering of National Academy of Sciences of Ukraine, 47 Lenin Ave., 310164, Kharkov, Ukraine. 2 International Laboratory of High Magnetic Fields and Low Temperatures, Gajowicka str., 95, 53-421, Wroclaw, Poland.

Presenting Author: V.M. Dmitriev

The temperature dependences of heat capacity and low - frequency impedance (f = 0 - 107 Hz) of polycrystal samples of the LuFe₄Al₈ compound are studied at temperatures ranged from 300 to 4.2K. The antiferromagnetic ordering is shown to occur at T_N = 100.5K.

It is found, that a magnetostructural phase transition takes place at temperature T₁, several Kelvins below T_N. In this case the antiferromagnetically coupled Fe layers are formed, which are separated by nonmagnetic Lu interlayers. In this phase the LuFe₄Al₈ structure is a specific Fe-Lu-Fe superlattice. Below T₁ the weak static magnetic field is found to influence ohmic losses demonstrating the negative magnetic resistance.

It is found, that at T < 25K the compound becomes superconducting. This was verified by the existence of the Meissner effect, by the specific features of heat capacity, by levitation and by the resistive and magnetic measurements.

2PO9-95 Reduction of critical temperatures in pure and thoriated UBe₁₃ due to heavy ion irradiation

H.A. Radovan 1, E. Behne 1, R.J. Zieve 1, J.S. Kim 2, G.R. Stewart 2, and W.K. Kwok 3. 1 Physics Dept., University of California, Davis, CA 95616. 2 Physics Dept., University of Florida, Gainesville, FL 32611. 3 Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439.

Presenting Author: H.A. Radovan

The nature of superconductivity in the heavy fermion material UBe₁₃ is not yet understood. In particular, a second thermodynamic transition appears below the superconducting transition in UBe₁₃ doped with thorium, suggesting an unconventional order parameter. We used heat capacity measurements down to 100 mK to investigate the influence of columnar defects, created by heavy ion irradiation, on the transitions. A highly anisotropic order parameter could be strongly suppressed by correlated scattering from columnar defects. For defects with a matching field of 5 Tesla, the zero-field reduction of T_c is 15 mK in UBe13 (T_c 770 mK), and 10 mK and 15 mK for the upper (T_c 625 mK) and lower (T_c=415 mK) transitions, respectively, in U_0.97Th_0.03Be₁₃. This small decrease of T_c is similar for all these transitions and comparable in relative magnitude to the effects of columnar defects in other superconducting systems. Thus the columnar defects do not appear to have a strong interaction with the order parameter in these heavy fermion superconductors.

2PO9-96 Upper Critical Field and Thermal Conductivity in the vortex state of f-wave superconductor: UPt₃

Guolin Yang and Kazumi Maki, Departure of Physics and Astronomy, University of Southern California, Los Angeles, CA 90089-0484, USA

Presenting Author: G. Yang

The superconductivity in UPt₃ is now established as E_2u or of the spin triplet f-wave. We shall report some of recent works on f-wave superconductivity. First we have shown that f-wave superconductivity with the {d} vector parallel to the c axis describes the observed anisotropy in upper critical field H_c2.

Perhaps more crucial insight to the f-wave symmetry is provided by the thermal conductivity tensor in the vortex state parallel to say the b axis in a magnetic field within the a-b plane, then the diagonal term exhibits clear cos 2q dependence while off diagonal term (Righi-Leduc effect) does sin 2q dependence where q is the angle between the magnetic field and the heat current. Indeed this cos 2q dependence is observed in a recent thermal conductivity measurement in UPt₃¹. The field and the temperature dependence of this cos 2q term will provide a unique test of f-wave supercondutivity. A similar cos 2q has been predicted for p-wave superconductor in Sr₂RuO₄, which has not been tested experimentally.

2PO9-97 Resistivity and electron-phonon coupling in YNi₂B₂C single crystals

Renato S. Gonnelli 1, Valeri A. Stepanov 2, Andrea Morello 1, Giovanni A. Ummarino 1, Gunter Behr 3, Gerald Graw 3, Seguei V. Shulga 3, and Stefan L. Drechsler 3. 1 INFM - Dipartimento di Fisica, Politecnico di Torino, c. so Duca degli Abruzzi, 24 - 10129 Torino, Italy. 2 P.N. Lebedev Physical Institute, Russian Academy of Sciences, Leninski Pr. 53, Moscow, Russia. 3 Institut für Festkörper- und Werkstofforschung Dresden, Postfach 270016, D-01171 Dresden, Germany.

Presenting Author: R.S. Gonnelli

In this work we present the results of precise measurements of the resistivity of YNi₂B₂C single crystals with T_c = 15.5 K as a function of temperature. As observed in previous experiments, the resistivity shows a perfect Bloch-Grüneisen (BG) behaviour with a linear high-temperature part (dr /dT » 0.12 m W cm/K) and a small residual value r (0) » 3 m W cm that indicates the high quality and low impurity contents of the samples.

From the linear part of the resistivity and the experimental value of the plasma frequency h w _p = 4.25 eV we extracted the transport electron-phonon coupling constant l _tr = 0.53. The resistivity in the whole temperature range is perfectly fitted in the framework of the BG model by using as a ²F(w )_tr the neutron density of states weighted by a multistep-like function determined by the fit.

The resulting spectral function is used in the direct solution of the Eliashberg equations both in s- and d-wave symmetry demonstrating that this approach can explain both T_c and the superconducting gap D as determined in tunneling experiments.

2PO9-98 Transport Properties of Ho_xDy_(1-x)Ni₂B₂C Single Crystals

D.G. Naugle 1, K.D.D. Rathnayaka 1, A. Parasiris 1, and P.C. Canfield 2. 1 Department of Physics, Texas A&M University, College Station, TX 77843. 2 Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011.

Presenting Author: D.G. Naugle

The Néel temperature (T_n) of the magnetic superconductor HoNi₂B₂C is below its superconducting transition temperature [1] (T_c) while DyNi₂B₂C becomes superconducting below the magnetic ordering temperature [2] (T_c<Tn). Ho_xDy_(1-x)Ni₂B₂C demonstrates a rich interplay between superconductivity and magnetism as x is increased [3]. Transport properties of single crystal Ho_xDy_(1-x)Ni₂B₂C were measured for different values of x, in order to investigate the magnetic phase of this system and results will be reported.

[1] P.C. Canfield, B.K. Cho, D.C. Johnston, D.K. Finnemore and M.F. Hundley, Physica C230, 397 (1994).

[2] B.K. Cho, P.C. Canfield and D.C. Johnston, Phys. Rev. B52, R3844 (1995).

[3] B.K. Cho, P.C. Canfield and D.C. Johnston, PHys. Rev. Lett 77, 163 (1996).

*This work is supported by the Robert A. Welch Foundation, Texas Advanced Technology Program and the Texas Center for Superconductivity at the University of Houston (TCSUH).

2PO9-99 Superconductivity of alpha-TiAl Alloys

Zhang Dianlin 1,2,3, Liu Shumei 1, Jing Xiunian 1, Lu Li 1,2, Li Shanlin 1, and J.J. Lin 4. 1 Institute of Physics & Center for Condensed Matter Physics, CAS, Beijing 100080, China. 2 Laboratory of Ultra-Low Temperature Physics, CAS, Beijing 100080, China. 3 National Laboratory for Superconductivity, CAS, Beijing 100080, China. 4 Institute of Physics, National Chiao Tung University, Hsinchu 300, Taiwan.

Presenting Author: Z. Dianlin

Although the theory of superconductivity is mature in explaining the basic properties as well as their material dependence of conventional superconductors, large experimental efforts have been devoted to materials with cubic structures because of the importance for application. We have made a systematic investigation of the superconductivity of hcp TiAl alloys as a function of Al content. The system is chosen for the following reasons:

1. The hcp a -Ti phase is stable up to ~ 11at.% Al. In the whole doping range the lattice constants change very little. The widths of the X-ray diffraction peaks do not show appreciable degradation by introducing 10at.% Al. This means that the phonon spectra should not be much disturbed by the addition of Al.

2. Band structure calculation shows that the Fermi level of pure Ti is located at a position where a small shift could cause appreciable change in the DOS at Fermi level.

3. The addition of Al into Ti does not change the d-band structure, resulting in a simple shift of the Fermi level at the d-band.

4. The transition temperatures of Ti and TiAl alloys in their alpha phase are very low, leaving enough room for independent measurements of other properties related to the superconductivity of the materials.

The transition temperature, critical field, Debye temperature, Hall effect, electronic specific heat, and sound velocity of the alloys have been measured. The results are discussed in connection with the theory of superconductivity