Universal Heat Transport inSr2RuO4

Abstract

We present the temperature dependence of the thermal conductivity κ(T ) of the unconventional superconductor Sr2RuO4 down to low temperatures (∼100 mK). In T → 0 K limit we found a finite residual term in κ/T , providing clear evidence for the superconducting state with an unconventional pairing. The residual term remains unchanged for samples with different Tc, demonstrating the universal character of heat transport in this spin-triplet superconductor. The low-temperature behavior of κ suggests the strong impurity scattering with a phase shift close to π/2. A criterion for the observation of universality is experimentally deduced.PACS numbers: 74.20. Rp, 74.25.Fy, 74.70.Pq Through the study of the cuprate superconductor in the last decade, a notable progress has been made in an understanding of the thermodynamic and transport properties of quasi-particles (QP) in unconventional superconductors. One of the important findings is the novel phenomenon called the universal transport, pointed out first by Lee [1]. In an unconventional superconductor with nodes in the superconducting (SC) gap, nonmagnetic impurities suppress the transition temperature (T c ) and induce the finite density of QP states at the Fermi level with the energy width γ imp [2]. The presence of impurities increases both the QP density and the impurity scattering rate, which completely cancel each other [3,4]. In this case the QP transport, such as thermal conductivity (κ), restores the temperature (T ) dependence typical for the normal state (e.g. κ ∝ T ) [5] and becomes independent of impurity concentration. The magnitude of the residual term in κ/T depends only on the QP spectrum near the gap nodes, providing a useful tool for studying the SC order parameter [4].Experimentally, the universal residual term in κ/T at T → 0 K has been reported in optimally-doped high-T c cuprates YBa 2 Cu 3 O 6.9 [6] and Bi 2 Sr 2 CaCu 2 O 8 [7,8].Their residual values are consistent with the d-wave SC gap structure [9]. For non-d-wave gap symmetries, the universal conductivity has never been observed. In contrast, the extrapolated κ/T at T → 0 K of the spin-triplet superconductor UPt 3 [10,11] rapidly increases with the density of defects, showing no universal behavior, and the residual term seems to vanish in zero-disorder limit [11]. The origin of the lack of universality is still unclear, similar to the case of under-doped cuprates [12], and it is strongly desirable to extend studies to other unconventional superconductors.In this respect, of special interest is the study of Sr 2 RuO 4 , a unique quasi-two-dimensional (Q2D) spin triplet superconductor [13,14]. This material has a layered perovskite structure [15] and its electronic state is well described in the Landau-Fermi liquid model [16]. The SC state of Sr 2 RuO 4 (T c = 1.5 K) is chiral with a broken time reversal symmetry [17] and its T c is rapidly suppressed by non-magnetic impurities [18,19]. Views on the SC gap symmetry of Sr 2 RuO 4 changed in time. Early theoretical prediction of the fully ga...

Gap symmetry of superconducting borocarbide YNi2B2C and skutterudite PrOs4Sb12

“…Indeed all four states have the same quasiparticle density of states as d-wave superconductors [16]. As we shall see later, however, the multigap model is incompatible with the angle dependent magnetothermal conductivity observed by Izawa et al [5] and the universal heat conduction in κ zz observed by Suzuki et al [17] in Sr 2 RuO 4 . On the other hand these thermal con-ductivity data are fully consistent with f-wave sc shown in Fig.…”

“…Thus for f'-wave κ zz depends on the scattering rate Γ and never reaches the universality limit contrary to [17].…”

Theory of thermal and charge transport in diffusive normal metal/superconductor junctions

“…47 On the other hand, heat transport in N/S junctions has not received much attention so far, partially due to the lack of experimental data. Recently, sufficient progress was achieved which made it possible to study heat transport in unconventional superconductors [49][50][51][52][53][54][55][56] and this stimulated theoretical study of these phenomena.…”

Measuring thermal conductivity in extreme conditions: Sub-Kelvin temperatures and high (27 T) magnetic fields

“…Thermal conductivity has proved to be a useful tool for investigating the electronic properties of metals and superconductors [1][2][3][4][5][6]. For example, in superconductors, the study of heat transport in the superconducting state can give information on electron scattering mechanisms.…”