Abstract
The gap structure of Sr$_2$RuO$_4$, which is a longstanding candidate for a chiral p-wave superconductor, has been investigated from the perspective of the dependence of its specific heat on magnetic field angles at temperatures as low as 0.06 K ($\sim 0.04T_{\rm c}$). Except near $H_{\rm c2}$, its fourfold specific-heat oscillation under an in-plane rotating magnetic field is unlikely to change its sign down to the lowest temperature of 0.06 K. This feature is qualitatively different from nodal quasiparticle excitations of a quasi-two-dimensional superconductor possessing vertical lines of gap minima. The overall specific-heat behavior of Sr$_2$RuO$_4$ can be explained by Doppler-shifted quasiparticles around horizontal line nodes on the Fermi surface, whose in-plane Fermi velocity is highly anisotropic, along with the occurrence of the Pauli-paramagnetic effect. These findings, in particular, the presence of horizontal line nodes in the gap, call for a reconsideration of the order parameter of Sr$_2$RuO$_4$.
Highlights
Knight-shift experiments provided favorable evidence that it exhibits spin-triplet pairing.2–5) Numerous experiments have demonstrated that Sr2RuO4 has non s-wave properties,6, 7)
The gap structure of Sr2RuO4, which is a longstanding candidate for a chiral p-wave superconductor, has been investigated from the perspective of the dependence of its specific heat on magnetic field angles at temperatures as low as 0.06 K (∼ 0.04Tc)
Sr2RuO4 comprising of three cylindrical sheets (α, β, and γ)10, 11) together with its well-characterized Fermi-liquid behavior has led to the construction of several theoretical models to describe superconductivity.6) Among these modtransport has raised the possibility of a nodal gap.18) Various gap structures including vertical and horizontal line node gaps have been proposed so far;21–25) the location of gap minima has not yet been established
Summary
Knight-shift experiments provided favorable evidence that it exhibits spin-triplet pairing.2–5) Numerous experiments have demonstrated that Sr2RuO4 has non s-wave properties,6, 7). Searching for gap zeros in Sr2RuO4 via field-angle-dependent specific-heat measurement
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