Gap Structure of the Spin-Triplet SuperconductorSr2RuO4Determined from the Field-Orientation Dependence of the Specifi

Abstract

We report the field-orientation dependent specific heat of the spin-triplet superconductor Sr2RuO4 under the magnetic field aligned parallel to the RuO2 planes with high accuracy. Below about 0.3 K, striking 4-fold oscillations of the density of states reflecting the superconducting gap structure have been resolved for the first time. We also obtained strong evidence of multi-band superconductivity and concluded that the superconducting gap in the active band, responsible for the superconducting instability, is modulated with a minimum along the [100] direction.PACS numbers: 74.70. Pq,74.25.Bt,74.25.Op,74.25.Dw Since the discovery of its superconductivity [1], the layered ruthenate Sr 2 RuO 4 has attracted a keen interest in the physics community [2]. The superconductivity of Sr 2 RuO 4 has pronounced unconventional features such as: the invariance of the spin susceptibility across its superconducting (SC) transition temperature T c [3,4], appearance of spontaneous internal field [5], evidence for two-component order parameter [6] and absence of a Hebel-Slichter peak [7]. These features are coherently understood in terms of spin-triplet superconductivity with the vector order parameter d(k) =ẑ∆ 0 (k x + ik y ), representing the spin state S z = 0 and the orbital wave function with L z = +1, called a chiral p-wave state.The above vector order parameter leads to the gap1/2 , which is isotropic because of the quasi-two dimensionality of the Fermi surface consisting of three cylindrical sheets [8]. However, a number of experimental results [9,10,11,12,13] revealed the power-law temperature dependence of quasiparticle (QP) excitations, which suggest lines of nodes or nodelike structures in the SC gap. There have been many theoretical attempts (anisotropic p-wave or f-wave states) to resolve this controversy [14,15,16,17,18,19]. Although all these models suggest a substantial gap anisotropy, magnetothermal conductivity measurements with the applied field rotated within the RuO 2 plane down to 0.35 K revealed little anisotropy [20,21]. To explain those experimental facts as well as the mechanism of the spintriplet superconductivity, several theories [22,23], taking the orbital dependent superconductivity (ODS) into account [24], have been proposed. In these models, there are active and passive bands to the superconductivity: the SC instability originates from the active band with a large gap amplitude; pair hopping across active to passive bands leads to a small gap in the passive bands. The gap structure with horizontal lines of nodes [22] or strong in-plane anisotropy [23] in the passive bands was proposed.In order to identify the mechanism of the spin-triplet superconductivity, the determination of the gap structure in the active band is currently of prime importance. The field-orientation dependent specific heat is a direct measure of the QP density of states (DOS) and thus a powerful probe of the SC gap structure [25,26,27,28]. In this Letter, we report high precision experiments of the specific heat as a funct...

Quasiclassical determination of the in-plane magnetic field phase diagram of superconductingSr2RuO4

“…This leads naturally to orbital dependent superconductivity 28,29 ; a theory for the superconducting state has different gaps on the {α, β} and γ sheets of the Fermi surface. This theory has experimental support through specific heat measurements in magnetic fields 30,31 . These measurements indicate that for strong fields applied in the basal plane, superconductivity in the {α, β} bands is suppressed and the γ band has the dominant superconducting gap.…”

Evidence from tunneling spectroscopy for a quasi-one-dimensional origin of superconductivity in Sr2RuO4

“…Specific heat 23,28 and thermal conductivity 25,26,51 measurements have established the existence of nodes or deep minima in the superconducting gap (from the linear dependence of C/T and the nonzero extrapolation of κ/T to zero temperature, respectively). In addition, the fact that C/T extrapolates to nearly zero at zero temperature implies that there is no residual Fermi surface at the lowest temperatures, i.e.…”

“…Exotic phenomena, such as topologically protected Majorana edge modes, which are currently the subject of much speculation, [13][14][15][16] might then be possible in Sr 2 RuO 4 . Issues with this OP identification, however, include the apparent absence of the anticipated edge currents, [17][18][19][20] the absence of a splitting of the transition near T c by an in-plane magnetic field, 21 and the strong evidence [22][23][24][25][26][27][28][29][30] for lines of gap nodes, or near nodes, that are nongeneric in the presence of time-reversal symmetry breaking. We here follow the bulk of the literature and take a chiral p-wave order parameter as a working assumption.…”

Field-orientation dependent heat capacity measurements at low temperatures with a vector magnet system

“…The details of this study are published elsewhere. 41 machining the calorimeter parts. They particularly acknowledge Z. Q. Mao and N. Kikugawa who grew the crystals used in our experiments.…”