Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session D47: Theory of Unconventional Superconductivity: Mainly Topical Phases |
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Sponsoring Units: DCMP Chair: Michael Senthef, Stanford University Room: Mile High Ballroom 4F |
Monday, March 3, 2014 2:30PM - 2:42PM |
D47.00001: Intrinsic triplet p$+$ip topological superconductivity in low filled graphene Tianxing Ma, Fan Yang, Hong Yao, Hai-Qing Lin Inspired by the continuously distributed Van-Hove saddle points at the band bottom, we studied the low filled Hubbard-model on the Honeycomb-lattice with negative next-nearest-neighbor hopping integrals, which represents the graphene system. Within different parameter regimes, our combined weak coupling random phase approximation and strong coupling determinant quantum Monte-Carlo approaches consistently reveal the triplet p$+$ip topological superconductivity in the ground state of the system. Further more, when a weak Kane-Mele spin-orbit coupling is included, the time-reversal invariant Z2 weak topological superconductivity will be realized in the system. Our unbiased numerical results provide basis to realize the intrinsic exotic topological superconductivity in the graphene and similar systems. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D47.00002: Possible exotic superconductivity in the monolayer and bilayer silicene Fan Yang, Yugui Yao, Li-Da Zhang, Cheng-Cheng Liu, Feng Liu Silicene, the silicon-based counterpart of graphene, has attracted a lot of research interest since synthesized recently. Similar honeycomb lattice structures of the two systems let them share most of their marvelous physical properties. The most important structural difference between the two systems lie in the noncoplanar lowbuckled geometry in silicene, which brings up a lot of interesting physical consequence to the system. Here we focus on possible exotic superconductivity (SC) in the family, via random phase approximation (RPA) study on the relevant Hubbard-models. Two systems of this family are studied, including the monolayer and bilayer silicene. For the former system, we found an electric-field driven quantum phase transition (QPT) from chiral d+id to f-wave SC when the field is perpendicular to the silicene plane. For the latter system, we found that even the undoped system is intrinsically metallic and superconducting with chiral d+id symmetry and tunable Tc which can be high . Our study not only provides a new playground for the study of the exotic SC, but also brings a new epoch to the familiar Si industry. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D47.00003: Possible Topological Superconducting Phases of Heavy Gated MoS$_2$ Noah Yuan, K.F. Mak, K.T. Law Molybdenum disulfide (MoS2) has attracted a lot of attention recently because of its grapheme-like crystal structure and massive Dirac spectrum at low energy. Recently, it was found that thin films of MoS2 become superconducting when they are heavy gated with a critical temperature of about 10 K at optimal gating [1]. In this presentation, we discuss the possible pairing symmetries of MoS2 according to group-theoretical calculations. Depending on the sign and strength of the on-site and next nearest neighbor interaction, we found that MoS2 can support two topological phases. In the chiral d-wave phase, the system breaks time-reversal symmetry spontaneously and supports chiral Majorana edge states. In a spin singlet and triplet mixing phases, the system respects time-reversal symmetry and support helical Majorana edge states. Experimental signatures of the topological superconducting phases are discussed. \\[4pt] [1] J. T. Ye et al. Science 338, 1193 (2012). [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D47.00004: Superconductivity on the brink of spin-charge order in doped honeycomb bilayer Oskar Vafek, James Murray, Vladimir Cvetkovic Using a controlled weak-coupling renormalization group approach, we establish the mechanism of unconventional superconductivity in the vicinity of spin or charge ordered excitonic states for the case of electrons on the Bernal stacked bilayer honeycomb lattice. With one electron per site this system exhibits nearly parabolically touching conduction and valence bands. Such a state is unstable towards a spontaneous symmetry breaking, and repulsive interactions favor excitonic order, such as a charge nematic and/or a layer antiferromagnet. We find that upon adding charge carriers to the system, the excitonic order is suppressed, and unconventional superconductivity appears in its place, before it is replaced by a Fermi liquid. We focus on firmly establishing this phenomenon using the RG formalism within an idealized model with parabolic touching. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D47.00005: Renormalization group study of excitonic and superconducting order in doped honeycomb bilayer James Murray, Oskar Vafek We explore the competition between spin-charge order and unconventional superconductivity in the context of the AB stacked bilayer honeycomb lattice, realized experimentally as bilayer graphene, which features approximately parabolically touching electron bands. Using a weak-coupling renormalization group theory, we show that unconventional superconductivity arises generically for repulsively interacting fermions as excitonic order is suppressed by adding charge carriers to the system. We investigate the effects of finite temperature and further-neighbor hopping, the latter of which leads to so-called ``trigonal warping'' and destroys the perfect circular symmetry of the Fermi surfaces. We show that superconductivity survives for a finite range of trigonal warping, and that the nature of the superconducting phase may change as a function of further neighbor hopping. Depending on the range of interactions and the degree of trigonal warping, we find that the most likely superconducting instabilities are to f-wave, chiral d-wave, and pair density wave phases. It is shown that unconventional superconductivity is significantly enhanced by fluctuations in particle-hole channels, with the critical temperature reaching a maximum near the excitonic phase. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D47.00006: Topologically stable gapped state in a layered superconductor Mauro Doria, Marco Cariglia, Alfredo A. Vargas-Paredes We show that a layered charged superconductor, described by a spinorial (two-component) order parameter, has a gapped state above the ground state, topologically protected from decay. This state is made of skyrmions, breaks the time reversal symmetry and produces a weak local magnetic field. This excited but stable state contains spontaneous circulating supercurrents, with flow and counter flow in the layers, even without the presence of an external magnetic field. We derive the order parameter and the local magnetic field of this skyrmion state from the Abrikosov-Bogomolny (first order) equations, instead of the second order variational equations. We find a gap density of the order of $0.1\,h_{max}\,\mbox{meV.nm}^{-3}$, where $h_{max}$ is the maximum local magnetic field between layers expressed in Gauss. The present threshold of detection, set by $\mu$SR and NMR/NQR, $h_{max}\sim 0.01\,\mbox{G}$, gives a gap density of the order $10^{-3}\,\mbox{meV.nm}^{-3}$ for the single-layer cuprates (inter-layer distance $d \approx 1.0 nm$). We suggest that the pseudogap is a skyrmion state, and so, estimate that the density of carriers that condense in the pseudogap is of the order of $10^{-4} \,\mbox{nm}^{-3}$, or $0.01\%$ of the Cooper pair density in the cuprates. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D47.00007: Novel phases in topological superconducting quantum dots Karen Michaeli, Liang Fu Recent progress in realizing topological superconductors has paved the road to study new physical phenomena resulting from the non-abelian statistics of the Majorana modes they host. A particularly interesting situation arises when Majorana bound states in a closed topological superconducting dot are coupled to external normal leads. In this talk, we will show that interactions with the quantum dot drive the lead electrons into a non-Fermi liquid phase, which can be understood by mapping the problem to a variant of a Kondo system. Interestingly, the non-Fermi liquid states in these systems are more robust than in the conventional two channel Kondo problem. This is because realizations with different numbers of metallic leads are connected to each other by a line of fixed points. We will conclude with a discussion of the experimental consequences of our theory. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D47.00008: Magnetic edge states and mixed-parity pairing in spin-triplet superconductors Mario Cuoco, Paola Gentile, Canio Noce, Ilya Vekhter, Alfonso Romano We show that a spontaneous magnetic moment may appear at the edge of a spin-triplet superconductor if the system allows for pairing in a subdominant channel and non-uniform spatial profile. To unveil the microscopic mechanism behind such effect we combine numerical solution of the Bogoliubov-De Gennes equations for a tight-binding model with nearest-neighbor attraction, and the symmetry based Ginzburg-Landau approach. We find that a modulation of the electronic density near the edge of the system leads to a non-unitary superconducting state where spin-singlet pairing coexists with the dominant triplet superconducting order. We demonstrate that the spin polarization at the edge appears due to the inhomogeneity of the non-unitary state and originates in the lifting of the spin-degeneracy of the Andreev bound-states. For chiral spin-triplet superconductors spin current flows along the interface and surface charge currents exhibit anomalous dependence on the magnetization. - A. Romano, P. Gentile, C. Noce, I. Vekhter, M. Cuoco, Phys. Rev. Lett. 110, 267002 (2013). [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D47.00009: Field-induced phase transitions in spin-orbit coupled superconductors Florian Loder, Arno P. Kampf, Thilo Kopp Spin-orbit coupling (SOC) or magnetic fields both split the otherwise degenerate spin eigenstates in metals. A pairing interaction may then lead to Cooper pairs which are either of intra- or of inter-band pairing type. These pairing states are separated by a first-order phase transition depending on the relative strength of SOC and the magnetic field [1]. We analyze this phase transition for a two-dimensional electron system in an in-plane magnetic field and show that the spin-triplet component of the superconducting order parameter reaches its maximum exactly at the phase transition. The superconducting energy gap closes at this transition and thereby allows for a change in the topological character of the superconductor. We suggest that this in-plane magnetic field driven transition is well suited for experimental detection because of the absence of orbital depairing effects.\\[4pt] [1] F. Loder {\it et al}., J. Phys. Condens. Matter {\bf 25}, 362201 (2013) [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D47.00010: Topological superconducting states with magnetic moments on a conventional $s$-wave superconductor Sho Nakosai, Yukio Tanaka, Naoto Nagaosa The search for topological properties in superconductors has been one of the most highlighted topics in nearly a decade. Especially Majorana fermions, appearing as topologically protected boundary modes associated with nontrivial features of superconductors, provide a promising platform for quantum computations. Therefore there is a real need for designing adapted superconductivity with ordinary materials. In this talk, we will present theoretical calculations on unconventional superconductivity induced by the magnetic moments in a conventional spin-singlet $s$-wave superconductor [1]. By choosing the spin directions of these moments, one can design spinless pairing states appearing within the $s$-wave superconducting energy gap. It is found that the helix spins produce a $p_x+p_y$-wave state while the skyrmion crystal configuration a $p_x+ip_y$-wave-like state. Nodes in the energy gap and the zero-energy flat band of Majorana edge states exist in the former case, while the chiral Majorana channels along edges of the sample and the zero-energy Majorana bound state at the core of the vortex appear in the latter case. \\[4pt] [1] Sho Nakosai, Yukio Tanaka, and Naoto Nagaosa, PRB 88, 180503(R) (2013) [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D47.00011: Non-uniform superconducting states in Nematic Electronic Liquid Crystal Phases Rodrigo Soto Garrido, Eduardo Fradkin We study the possible superconducting states that arise in a nematic Fermi fluid state in the spin-triplet channel. First, we study the nematic $\alpha$ phase in the $l=2$ state, which is invariant under a $\pi/2$ rotation followed by a spin flip. In this phase the only infinitesimal superconducting instability is in the spin-triplet p-wave channel. However, close enough to the nematic transition both a uniform d-wave superconducting state and a non-uniform state (pair density wave or checkerboard), also with d-wave symmetry, can arise. In addition, we study the nematic $\beta$ phase, in which the spin polarization winds around the Fermi surface, and we also find that it is possible to have a non-uniform superconducting state above a critical value of the coupling constant, which is controlled by the order parameter of nematic phase. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D47.00012: Quantum quenching through a topological phase transition in a system with open boundaries Vasudha Shivamoggi, Smitha Vishveshwara, Diptiman Sen We study the effect of open boundaries on the non-adiabatic dynamics of a system driven across a topological phase transition. The closing of the bulk gap at the critical point implies that a quantum quench across the critical point necessarily results in a states with defects. The presence of mid-gap surface states in a topological phase modifies the usual Kibble-Zurek scaling used to describe the defect density. We investigate the phase transition in a 1D spinless p-wave superconductor between a non-topological phase and a topological phase with Majorana fermions localized at the ends. We calculate the non-adiabatic evolution of the ground state across the transition and the overlap of this state with the instantaneous ground state. We also discuss the consequences of the topologically protected edge states on defect production. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D47.00013: Odd-frequency superconductivity in two-channel Kondo lattice Shintaro Hoshino, Yoshio Kuramoto Unconventional superconductivity has been sought as an intriguing ground state or thermodynamic state in condensed matter physics. Among those states, we address the odd-frequency (OF) pairing state, which breaks the gauge symmetry, but has zero pairing amplitude at equal time. Possible realizations of the OF superconductivity have been theoretically proposed in a variety of strongly correlated electron systems. In particular, Emery and Kivelson have shown for the two-channel Kondo impurity that the OF pairing susceptibility is enhanced at the impurity site. However, no microscopic theory has established the OF pairing in the two-channel Kondo lattice. Recently, we have demonstrated the emergence of odd-frequency s-wave superconductivity in the two-channel Kondo lattice using the dynamical mean-field theory explicitly by divergence of the OF susceptibility. The corresponding order parameter is given by staggered composite-pair amplitude with even frequencies, which involves both localized spins and conduction electrons. The Kondo effect in the presence of two channels is essential for the present unconventional superconductivity. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D47.00014: The interplay between topological p-wave superconductivity and odd-frequency pairing in superconducting proximity systems Valentin Stanev, Victor Galitski We study the proximity-induced superconductivity in semiconductor nanowires. The interplay between superconductivity and spin-orbit coupling plays a crucial role in proposals for creating Majorana fermions in semiconducting heterostructures. To further elucidate the physics of such devices we employ the quasiclassical Green's functions methods. We show that the spatial variations of the superconducting order parameter leads to non-trivial effects in the nanowire. We demonstrate the appearance of odd-frequency pairing correlations close to the boundaries, and discuss their effect on the density of states. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D47.00015: Quantum Theory of Heat Magnetization Atsuo Shitade We give the thermodynamic definition of the heat magnetization, and calculate with use of the Keldysh formalism in a curved spacetime. As the charge current is coupled to a vector potential, the heat current is coupled to a part of a vielbein. Consequently, as we define the orbital magnetization by a magnetic field, we can define the heat magnetization by a torsional magnetic field induced by a vielbein. Such heat magnetization, together with the Kubo formula for the thermal conductivity calculated by a torsional electric field, leads to the proper thermal Hall conductivity satisfying the Wiedemann-Franz law. Our results indicate that the quantum thermal Hall effect in (2$+$1)-D time-reversal-broken topological insulators or superconductors is described by the Chern-Simons action of a vielbein. \\[4pt] [1] A. Shitade, arXiv:1310.8043.\\[0pt] [2] A. Shitade, arXiv:1310.8046. [Preview Abstract] |
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