Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session V22: Nematic Superconductivity in Doped Topological MaterialsInvited

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Sponsoring Units: DCMP Chair: Andriy Nevidomskyy, Rice University Room: New Orleans Theater A 
Thursday, March 16, 2017 2:30PM  3:06PM 
V22.00001: Oddparity nematic and chiral superconductivity in spinorbit coupled materials Invited Speaker: Jorn Venderbos Recent experimental advances indicate that superconductivity in doped topological insulator materials such as Cu$_x$Bi$_2$Se$_3$ is characterized by a full oddparity pairing gap with spontaneous rotational symmetry breaking. This suggests that the pairing symmetry is unconventional and has two symmetryrelated components, akin to unconventional pwave superconductors with degenerate ($p_x$,$p_y$) components. Motivated by these recent experiments, this talk will present a theory of spinorbit coupled oddparity superconductors with twocomponent pairing symmetry. These spinorbit coupled superconductors can come in two varieties: nematic and chiral. Nematic superconductors spontaneously break the rotational symmetry of the crystal, and are either fully gapped topological superconductors or have Dirac point nodes protected by crystal symmetry. Chiral superconductors spontaneously break timereversal symmetry, are generally characterized by nonunitary pairing states, and have point nodes. The nodal quasiparticles of chiral superconductors satisfy the Majorana condition and realize Majorana fermions in three dimensions. A comprehensive classification of such Majorana fermions in terms of dispersion and topology will be presented, and experimental signatures as well as candidate materials will be discussed. [Preview Abstract] 
Thursday, March 16, 2017 3:06PM  3:42PM 
V22.00002: Spinrotation symmetry breaking and triplet superconducting state in doped topological insulator CuxBi2Se3 Invited Speaker: Guoqing Zheng Spontaneous symmetry breaking is an important concept for understanding physics ranging from the elementary particles to states of matter. For example, the superconducting state breaks global gauge symmetry, and unconventional superconductors can break additional symmetries. In particular, spin rotational symmetry is expected to be broken in spintriplet superconductors. However, experimental evidence for such symmetry breaking has not been obtained so far in any candidate compounds. We report 77Se nuclear magnetic resonance measurements which showed that spin rotation symmetry is spontaneously broken in the hexagonal plane of the electrondoped topological insulator Cu0.3Bi2Se3 below the superconducting transition temperature Tc$=$3.4 K. Our results not only establish spintriplet (odd parity) superconductivity in this compound, but also serve to lay a foundation for the research of topological superconductivity (Ref.*). We will also report the doping mechanism and superconductivity in Sn1xInxTe. * K. Matano, M. Kriener, K. Segawa, Y. Ando and Guoqing Zheng, Nature Physics 12, 852 (2016). [Preview Abstract] 
Thursday, March 16, 2017 3:42PM  4:18PM 
V22.00003: Nematic superconductivity in Cu$_x$Bi$_2$Se$_3$ revealed by fieldangleresolved calorimetry Invited Speaker: Shingo Yonezawa Unconventional superconductivity is characterized by the spontaneous symmetry breaking of the macroscopic superconducting wavefunction in addition to the ordinary gaugesymmetry breaking. In particular, superconductivity in which the wavefunction phase breaks rotational symmetry has been widely investigated and is now believed to be realized in various classes of materials such as high$T_{\mathrm{c}}$ copper oxides. However, superconductivity with rotationalsymmetry breaking in the gap amplitude, which can be termed ``nematic'' superconductivity in analogy to the nematic liquidcrystal phases, has not been reported previously. Here, by measuring the specific heat of the doped topological insulator Cu$_{x}$Bi$_2$Se$_3$ ($T_{\mathrm{c}} \sim 3$ K) under accurate magneticfielddirection control, we observed clear twofoldsymmetric behavior in the inplane field angle dependence of the specific heat and the upper critical field [1]. Considering the trigonal symmetry of the lattice, the observed twofold behavior of bulk quantities indicate rotational symmetry breaking in the superconducting gap amplitude. Thus, this result provide the first thermodynamic evidence for nematic superconductivity, which actually belongs to a class of topological superconductivity in the case of this compound. This work has been performed under collaboration with K. Tajiri, S. Nakata, Y. Nagai, Z. Wang, K. Segawa, Y. Ando, and Y. Maeno. \newline [1] S.~Yonezawa \textit{et al.}, Nature Phys. doi:10.1038/nphys3907 (2016). [Preview Abstract] 
Thursday, March 16, 2017 4:18PM  4:54PM 
V22.00004: Rotational Symmetry Breaking in a Trigonal superconductor Nbdoped Bi$_{2}$Se$_{3}$ Invited Speaker: Lu Li The search for unconventional superconductivity has been focused on materials with strong spinorbit coupling and unique crystal lattices. Doped bismuth selenide (Bi$_2$Se$_3$) is a strong candidate given the topological insulator nature of the parent compound and its triangular lattice. The coupling between the physical properties in the superconducting state and its underlying crystal symmetry is a crucial test for unconventional superconductivity. In this paper, we report direct evidence that the superconducting magnetic response couples strongly to the underlying trigonal crystal symmetry in the recently discovered superconductor with trigonal crystal structure, niobium (Nb)doped bismuth selenide (Bi$_2$Se$_3$). As a result, the inplane magnetic torque signal vanishes at every 60$^\circ$. More importantly, we observed that the superconducting hysteresis loop amplitude is enhanced along one preferred direction spontaneously breaking the rotational symmetry. This observation confirms the breaking of the rotational symmetry and indicates the presence of nematic order in the superconducting ground state of Nbdoped Bi$_2$Se$_3$. [Preview Abstract] 
Thursday, March 16, 2017 4:54PM  5:30PM 
V22.00005: Beyond Triplet: Unconventional Superconductivity in a Spin3/2 Topological Semimetal Invited Speaker: Hyunsoo Kim In all known fermionic superfluids, Cooper pairs are composed of spin1/2 quasiparticles that pair to form either spinsinglet or spintriplet bound states. The "spin" of a Bloch electron, however, is fixed by the symmetries of the crystal and the atomic orbitals from which it is derived, and in some cases can behave as if it were a spin3/2 particle. The superconducting state of such a system allows pairing beyond spintriplet, with higher spin quasiparticles combining to form quintet or even septet pairs. Here, we report evidence of unconventional superconductivity emerging from a spin3/2 quasiparticle electronic structure in the halfHeusler semimetal YPtBi, a lowcarrier density noncentrosymmetric cubic material with a high symmetry that preserves the $p$like $j=3/2$ manifold in the Bibased $\Gamma_8$ band in the presence of strong spinorbit coupling. With a striking linear temperature dependence of the London penetration depth, the existence of line nodes in the superconducting order parameter $\Delta$ is directly explained by a mixedparity Cooper pairing model with high total angular momentum, consistent with a highspin fermionic superfluid state. We propose a $\mathbf{k\cdot p}$ model of the $j=3/2$ fermions to explain how a dominant $J$=3 septet pairing state is the simplest solution that naturally produces nodes in the mixed evenodd parity gap. Together with the underlying topologically nontrivial band structure, the unconventional pairing in this system represents a truly novel form of superfluidity that has strong potential for leading the development of a new generation of topological superconductors. [Preview Abstract] 
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