Orbital Dependent Superconductivity inSr2RuO4

Abstract

We show that for superconducting Sr2RuO4 any unconventional pairing in the part of the Fermi surface with Ru 4dxy orbital character is weakly coupled to that with Ru {4dxz, 4dyz} orbital character. This naturally gives rise to two disparate energy scales in the superconducting state which leads to novel low temperature properties in a variety of thermodynamic and transport properties and which would also account for the large residual density of states seen in specific heat and NQR measurements. 74.25.Bt,71.27.+d Sr 2 RuO 4 provides the first example of a layered perovskite material that exhibits superconductivity without the presence of copper [1]. Even though there is a close structural similarity with the high T c materials, the electronic properties are very different. While it is clear that electron correlation effects are important in Sr 2 RuO 4 , the normal state near the superconducting transition is well described by a quasi-2D Landau Fermi liquid (e.g. the resistivity in all directions follows a T 2 behavior for T < ∼ 50 K and the resistivity along and perpendicular to the c axis differ by a factor of 850 [1]). Quantum oscillations show three Fermi surface sheets with a 2D topology that agrees well with band structure calculations [2]. It has been pointed out [3] that the mass enhancement is similar to that of 3 He and that there is a metallic ferromagnetic phase in SrRuO 3 [4] (the 3D analogue of Sr 2 RuO 4 ). These observations indicate that an odd-parity (l = 1) superconducting state is likely [3]. This is consistent with the lack of a Hebel-Slichter peak in 1/T 1 in NQR measurements [5]. A weak coupling analysis of the odd-parity state implies the gap should be of constant magnitude [3]. It is therefore surprising that specific heat [6] and NQR measurements [5] reveal that approximately 0.6 of the normal density of states remain in the superconducting phase in clean samples (those in which quantum oscillations were observed). As a consequence it has been proposed that an exotic non-unitary superconducting state similar to the 3 He A 1 phase is stabilized [7,8]. In this scheme, the normal state quasiparticle energy spontaneously splits into two branches (one for spin up and one for spin down) upon entering the superconducting state. One of these branches is gapped while the other is not, leading to a residual density of states that is half the normal density of states.Here we propose an alternative explanation for the large residual density of states. The electronic properties near the Fermi surface of Sr 2 RuO 4 are determined by Wannier functions with Ru d xy , d xz , and d yz orbital character [9,10]. We show that the quasi-2D nature of the electronic dispersion implies that the bands are derived from either the xy or the {xz, yz} Wannier functions and that the pair scattering amplitude between these two classes of bands will be significantly smaller than the intraclass pair scattering amplitude for unconventional superconducting order parameters. It can therefore be expected that the gap on b...

Vortex States of theEuModel forSr2RuO4Kita1 1999Phys. Rev. Lett.232200View full textAdd to dashboardCite

“…25.Dw Active studies have been performed on superconducting Sr 2 RuO 4 [1] where another unconventional pairing may be realized [2]. A possible candidate for its symmetry is the E u model with twofold degeneracy [3], as indicated by various experiments [4][5][6][7][8][9][10]. However, further experiments seem to be required before establishing its validity for Sr 2 RuO 4 .…”

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

“…For a field along the (1, 0, 0) direction, the solution near H c2 is η = (0, 1) (for the three values of ǫ discussed above this was the case) then as field is reduced for fixed temperature, a second transition occurs at H 2 for which the η = (1, 0) component becomes non-zero. Such phase transitions have only been examined within Ginzburg Landau theory 28,40 . The Eilenberger equations discussed above allow for the examination of this phase transition throughout the entire H-T phase diagram.…”

“…Consequently, to first approximation, the γ sheet of the Fermi surface is comprised of xy Wannier functions, while the {α, β} sheets of the Fermi surface are comprised of {xz, yz} Wannier functions. 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 .…”

Edge states and determination of pairing symmetry in superconductingSr2RuO4

“…In response to these experiments, the following alternative order parameters were proposed [15]: anisotropic p-wave [16], p-wave with horizontal lines of nodes [17], and fwaves with vertical [17,18] or horizontal lines of nodes [19]. It was also proposed that the α and β bands of Sr 2 RuO 4 are either not superconducting [20] or have horizontal lines of nodes [21]. Big in-plane anisotropy of ultrasound attenuation [14] may support the vertical lines of nodes.…”