Field-Angle-Dependent Specific Heat Measurements and Gap Determination of a Heavy Fermion SuperconductorURu2Si2

Abstract

To identify the superconducting gap structure in URu2Si2, we perform field-angle-dependent specific heat measurements for the two principal orientations in addition to field rotations, and a theoretical analysis based on microscopic calculations. The Sommerfeld coefficient gamma(H)'s in the mixed state exhibit a distinctly different field dependence. This comes from point nodes and the substantial Pauli paramagnetic effect of URu2Si2. These two features combined give rise to a consistent picture of superconducting properties, including a possible first order transition of Hc2 at low temperatures.

Superconductivity in Ce- and U-Based “122” Heavy-Fermion Compounds

“…We have provided examples that involve first-order antiferromagnetic transitions, first-order valence transitions as well as the enigmatic case of URu 2 Si 2 . Here, d-wave superconductivity with broken time-reversal symmetry occurs near an antiferromagnetic quantum phase transition at nonzero pressure, 122,123 raising the possibility for the relevance of antiferromagnetic order to superconductivity. However, T c appears to diminish on approach to the antiferromagnetic order, and is nonzero only in the presence of the hidden order (cf.…”

“…120 The "weak" antiferromagnetic order, which seems to occur within the hidden order phase, is most likely due to static antiferromagnetically ordered regions that exist in a small part of the sample, phase separated from the surrounding hidden order region. 121 Thus, d-wave superconductivity 122,123 seems to microscopically coexist with the hidden order but not with antiferromagnetism. 121 …”

Colloquium: Hidden order, superconductivity, and magnetism: The unsolved case ofURu2Si2

“…Only out of the HO state evolves a highly unconventional (d-wave, even parity, spin-singlet) multi-gap superconducting ground state with T c ≈ 1.5 K Matsuda et al, 1996), being the focus of recent investigations (Kasahara et al, 2007(Kasahara et al, , 2009Morales and Escudero, 2009;Okazaki et al, 2008;Yano et al, 2008). Now the question arises: Is the HO phase generic, i.e., can it, as defined above, be found in other materials with different types of interactions?…”

“…Such investigations were pioneered by Amitsuka and Sakakibara (1994) and spanned a decade of experimentation (Yokoyama et al, 2002). The important questions posed here are: Can dilute U in a non-magnetic matrix support a single-impurity Kondo effect or even a multichannel Kondo behavior (Cox, 1987)?…”

FFLO and vortex states in superconductors with strong paramagnetic effect

“…From these behaviors, we can estimate the strength of the paramagnetic effect, µ. The H-dependence of γ(H) for H � c and H � ab was used to identify the pairing symmetry and paramagnetic effect in URu 2 Si 2 (Yano et al, 2008).…”