Thermodynamic evidence for nematic superconductivity in CuxBi2Se3

Abstract

In condensed matter physics, spontaneous symmetry breaking has been a key concept, and discoveries of new types of broken symmetries have greatly increased our understanding of matter 1,2 . Recently, electronic nematicity, novel spontaneous rotational-symmetry breaking leading to an emergence of a special direction in electron liquids, has been attracting significant attention 3-6 . Here, we show bulk thermodynamic evidence for nematic superconductivity, in which the nematicity emerges in the superconducting gap amplitude, in Cu x Bi 2 Se 3 . Based on high-resolution calorimetry of single-crystalline samples under accurate two-axis control of the magnetic field direction, we discovered clear two-fold symmetry in the specific heat and in the upper critical field despite the trigonal symmetry of the lattice. Nematic superconductivity for this material should possess a unique topological nature associated with odd parity 7-9 . Thus, our findings establish a new class of spontaneously symmetry-broken states of matter-namely, odd-parity nematic superconductivity.

Nematic superconductivity in CuxBi2Se3 : Surface Andreev bound states

“…In the NMR experiment, a prominent in-plane uniaxial anisotropy in the SC order parameter is observed in the Knight shift measurement [11]. The above twofold in-plane rotational symmetry is confirmed further by field-angle dependent specific heat measurement [20]. These new experiments indicate that the SC pairing in Cu x Bi 2 Se 3 might be the first example of a pairing breaking spontaneously the spin rotation symmetry of the parent material, from the threefold rotational symmetry of the normal phase to the uniaxial twofold inplane symmetry [11,12].…”

“…In conclusion, extensive analyses for the bulk and surface spectra have been made for the ∆ 4a (k) nematic pairing, suggested to be the correct SC pairing for the Cu x Bi 2 Se 3 compound based on recent Knight shift and field-angle dependent specific heat measurements [11,20]. The purpose of the present work is to explore the consequences deduced from this type of pairing, taking into account the evolution of the Fermi surface from spheroidal to corrugated cylindrical [28].…”

“…Inspired by the above experimental observations and theoretical arguments, in this work we explore whether the corrugated cylindrical Fermi surface would or would not support the SABSs in the Cu x Bi 2 Se 3 compound with the nematic SC pairing proposed for explaining the recent experiments [11,20]. Using the band parameters which fit qualitatively the experimental Fermi surfaces and the TSSs, we find that the SC is fully gapped, and the bulk gap is rather anisotropic.…”

Global Phase Diagram of a Three-Dimensional Dirty Topological Superconductor

“…While point contact spectroscopy only displays a fully gapped spectrum when x ∼ 0.2 [10], indicative of a trivial s-wave pairing, systems with x ≈ 0.3 display ZBCP [7][8][9] (possible signature of a TpSC). Recent NMR and thermodynamic measurements are also suggestive of a triplet pairing for x ∼ 0.3, with the d-vector locked in the ab-plane [12,14], compatible with a triplet nematic SC, which also represents a class DIII TpSC [31,55,56]. Hence, it is quite reasonable to assume that with increasing Cu concentration (x), the strength of s-wave pairing decreases, while that of random charge impurity increases in the system.…”

“…Jurisdiction of topological classification, however, goes beyond insulators and encompasses semimetals (such as the Weyl semimetal) as well as superconductors (SCs), with our focus being on three dimensional, time-reversal symmetric paired states. Although candidates for such topological paired state are rather sparse, with the triplet B phase of 3 He standing as a prototypical example of charge-neutral topological superfluid [3], a theoretical proposal alluding to the possibility of a charged topological superconductor (TpSC) in strong spin-orbit coupled, doped narrow-gap (or gapless) semiconductors [4], such as Cu x Bi 2 Se 3 , Sn 1−x In x Te, Nd x Bi 2 Se 3 , Sr x Bi 2 Se 3 , led to ample experimental investigations geared toward its possible material realization [5][6][7][8][9][10][11][12][13][14][15][16]. However, the nature of superconducting order in these weakly correlated, but dirty materials still remains elusive, which, thus demands investigations on the role of randomness on topological pairing; constituting the central theme of the current Letter.…”