K. Behnia

Line nodes in the superconducting gap function of noncentrosymmetric CePt3Si

The superconducting gap structure of recently discovered heavy fermion CePt_3Si without spatial inversion symmetry was investigated by thermal transport measurements down to 40 mK. In zero field a residual T-linear term was clearly resolved as T->0, with a magnitude in good agreement with the value expected for a residual normal fluid with a nodal gap structure, together with a T^2-dependence at high temperatures. With an applied magnetic fields, the thermal conductivity grows rapidly, in dramatic contrast to fully gapped superconductors, and exhibits one-parameter scaling with T/sqrt{H}. These results place an important constraint on the order parameter symmetry, that is CePt_3Si is most likely to have line nodes.

THERMAL CONDUCTIVITY OF SUPERCONDUCTING (TMTSF)2CLO4 : EVIDENCE FOR A NODELESS GAP

We report on the first measurements of thermal conductivity in the superconducting state of (TMTSF){sub 2}ClO{sub 4}. The electronic contribution to heat transport is found to decrease rapidly below T{sub c}, indicating the absence of low-energy electronic excitations. We argue that this result provides strong evidence for a nodeless superconducting gap function, but does not exclude a possible unconventional order parameter. {copyright} {ital 1997} {ital The American Physical Society}

Angular Position of Nodes in the Superconducting Gap of YBCO

The thermal conductivity of a YBa{sub 2}Cu{sub 3}O{sub 6.9} detwinned single crystal has been studied as a function of the relative orientation of the crystal axes and a magnetic field rotating in the Cu-O planes. Measurements were carried out at several different temperatures below T{sub c} for a field of 30kOe. A fourfold symmetry characteristic of a superconducting gap with nodes at odd multiples of 45{degree} in k space was resolved. Experiments were performed to exclude a possible macroscopic origin for such a fourfold symmetry and our results impose an upper limit of 10% on the weight of the s-wave component of the essentially d-wave superconducting order parameter of YBCO. {copyright} {ital 1997} {ital The American Physical Society}

Thermal transport in the hidden-order state of URu2Si2.

We present a study of thermal conductivity in the normal state of the heavy-fermion superconductor URu2Si2. Ordering at 18 K leads to a steep increase in thermal conductivity and (in contrast with all other cases of magnetic ordering in heavy-fermion compounds) to an enhancement of the Lorenz number. By linking this observation to several other previously reported features, we conclude that most of the carriers disappear in the ordered state and this leads to a drastic increase in both the phononic and electronic mean free path.

Flux Line Lattice Melting and the Formation of a Coherent Quasiparticle Bloch State in the Ultraclean URU2Si2 Superconductor

We find that in the ultraclean heavy-fermion superconductor URu(2)Si(2) (T_{c0}=1.45 K) a distinct flux line lattice melting transition with outstanding characters occurs well below the mean-field upper critical fields. We show that a very small number of carriers with heavy mass in this system results in exceptionally large thermal fluctuations even at sub-Kelvin temperatures, which are witnessed by a sizable region of the flux line liquid phase. The uniqueness is further highlighted by an enhancement of the quasiparticle mean free path below the melting transition, implying a possible formation of a quasiparticle Bloch state in the periodic flux line lattice.

(TM)2X organic superconductors: interplay between 1-D charge localization and higher dimensionality cross-over

Abstract A survey of transport and magnetic properties of the (TM) 2 X series together with a recent investigation in high magnetic fields shows that the charge localization occurring at low temperature in a 1-D half-filled band is suppressed by the 1-D to 2-D (3-D) cross-over in selenium compounds. However, a localization around 30 K in the non-ordered phase can be reactivated by the application of a high transverse magnetic field. The intermediate (strong) coupling limit is valid for the spin degree of freedom whereas the charge is governed by the strength of the Umklapp scattering varying by a large factor from sulfur to selenium compounds and under pressure.

Thermal conductivity of heavy-fermion superconductor URu2Si2

Abstract The thermal conductivity of a URu 2 Si 2 single crystal was studied as a function of temperature (between 40 mK and 2 K) and magnetic field (up to 7 T). At the onset of superconductivity, the electronic contribution counts roughly for half the overall thermal conductivity revealing an important lattice contribution. An electron-phonon coupling parameter is found which is much smaller than that for conventional superconductors. The field dependence of the thermal conductivity at various temperatures is highly anisotropic and confirms the presence of multiple types of heat carriers.

Probing the Upper Critical Field of κ-(BEDT-TTF)2Cu(NCS)2

Studies of heat conduction and microwave absorption in the vortex state of κ-(BEDT-TTF)2Cu(NCS)2 reveal some of the specific aspects of superconductivity in this system. Moreover they allow us to probe the destruction of bulk superconductivity by a magnetic field. The two set of results yield identical magnitudes for the upper critical field with a temperature dependence distinctly different from the one suggested by the onset of resistive transition.

Evidence for a new magnetic field scale in CeCoIn5.

The Nernst coefficient of displays two distinct anomalies in magnetic field. The feature detected at Hk approximately 23 T is similar to what is observed in CeRu2Si2 at Hm = 7.8 T where a metamagnetic transition occurs. In CeCoIn5, new frequencies are observed in de Haas-van Alphen oscillations when the field exceeds 23 T where the Dingle temperature decreases by about 30%. Based on the Nernst coefficient anomalies, the magnetic phase diagram of CeCoIn5 is revised.

Heat conduction in κ-(BEDT-TTF)2X superconductors

Abstract We present the first study of thermal conductivity of a quasi two dimensional organic superconductor. κ-(BEDT-TTF) 2 Cu(NCS) 2 presents features which have already been detected in the high T c cuprates. Notably we observed an increase of the thermal conductivity at the superconducting transition which is suppressed by a moderate magnetic field. At low temperatures we clearly resolved a residual electronic contribution close to the universal limit value which is consistent with an anisotropic pairings with lines of nodes in the superconducting gap function.