April2011

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=April 25-29, Zixu Zhang=

Title: Underdoped cuprates, manifestations of boson-fermion crossover, `quantum oscillations' and the robust fractional quantum Hall state nu = 2/5
Authors: [|John A. Wilson] Comments: 51 pages and 18 figures Subjects: Superconductivity (cond-mat.supr-con)

The quantum oscillation data from a whole variety of underdoped HTSC systems and physical measurements has almost universally been presented in very classical terms following the Lifshitz-Kosevich formulation for Fermi-Landau quasiparticles, quantized into vortices and Landau levels under the high applied magnetic fields. The Fermi surface always emerges as a very small fraction of the large parent Fermi surface detected at higher doping. This then calls for some Fermi surface reconstruction to have occurred, as from a charged density wave, etc.. The paper refutes this reciprocal space approach, and shows that many of the more recent detailed works taking this line are not consistent with the data. The present author developed an alternative real-space interpretation of these matters a couple of years ago. This was in terms of interaction of the vortex array with the stripe array, using a 2D modelling for the latter. This approach has here been revised slightly and copes well with addressing the wide variety of new data. The paper pursues and justifies the BEC/BCS, negative-U based, two-subsystem modelling advocated earlier, along with the strength and geometry of the striping acquired in a strong magnetic field. A wide variety of recent experiments are expounded to produce an overall appreciation of a situation far from being appropriately treated within a Fermi-Landau quasiparticle framework. Indeed it is demonstrated that the universality of the 'QO' observations ensues from the fractional quantum Hall effect. The data all point to FQHE state with filling factor nu = 2/5. This state in its genesis and expression finds a much greater alignment with the BEC/BCS crossover condition and the proximity of the HTSC materials to Mott-Anderson localization than does any model for this highly correlated material appealing to well-metallized Fermi-Landau quasiparticle physics.

Title: Observation of interference between two molecular Bose-Einstein condensates
Authors: [|Christoph Kohstall], [|Stefan Riedl], [|Edmundo Sánchez Guajardo], [|Leonid Sidorenkov], [|Johannes Hecker Denschlag], [|Rudolf Grimm] Subjects: Quantum Gases (cond-mat.quant-gas)

We have observed interference between two Bose-Einstein condensates of weakly bound Feshbach molecules of fermionic $^6$Li atoms. Two condensates are prepared in a double-well trap and, after release from this trap, overlap in expansion. We detect a clear interference pattern that unambiguously demonstrates the de Broglie wavelength of molecules. We verify that only the condensate fraction shows interference. For increasing interaction strength, the pattern vanishes because elastic collisions during overlap remove particles from the condensate wave function. For strong interaction the condensates do not penetrate each other as they collide hydrodynamically.

Title: Stripes in the two-dimensional t-J model with infinite projected entangled-pair states
Authors: [|P. Corboz], [|S. R. White], [|G. Vidal], [|M. Troyer] Comments: 5 pages, 5 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We simulate the $t$ $J$ model in two dimensions by means of infinite projected entangled-pair states (iPEPS) generalized to arbitrary unit cells, finding results similar to those previously obtained by the density-matrix renormalization group (DMRG) for wide ladders. In particular, we show that states exhibiting stripes, {\it i.e}. a unidirectional modulation of hole-density and antiferromagnetic order with a $\pi$-phase shift between adjacent stripes, have a lower variational energy than uniform phases predicted by variational and fixed-node Monte Carlo simulations. For a fixed unit-cell size the energy per hole is minimized for a hole density $\rho_l\sim 0.5$ per unit length of a stripe. The superconducting order parameter is maximal around $\rho_l\sim 0.75-0.8$.

Title: Specific heat jump at superconducting transition in the presence of Spin-Density-Wave in iron-pnictides
Authors: [|M. G. Vavilov], [|A. V. Chubukov], [|A. B. Vorontsov] Comments: 4+ pages, 3 figures Subjects: Superconductivity (cond-mat.supr-con)

We analyze the magnitude of the specific heat jump \Delta C at the superconducting transition temperature T_c in the situation when superconductivity develops in the pre-existing antiferromagnetic phase. We show that \Delta C/T_c differs from the BCS value and is peaked at the tri-critical point where this coexistence phase first emerges. Deeper in the magnetic phase, the onset of coexistence, T_c, drops and \Delta C/T_c decreases, roughly as \Delta C/T_c \propto T^2_c at intermediate T_c and exponentially at the lowest T_c, in agreement with the observed behavior of \Delta C/T_c in iron-based superconductors.

Title: Models for gapped boundaries and domain walls
Authors: [|Alexei Kitaev], [|Liang Kong] Comments: 20 pages Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Category Theory (math.CT); Quantum Algebra (math.QA)

We define a class of lattice models for two-dimensional topological phases with boundary such that both the bulk and the boundary excitations are gapped. The bulk part is constructed using a unitary tensor category $\calC$ as in the Levin-Wen model, whereas the boundary is associated with a module category over $\calC$. We also consider domain walls (or defect lines) between different bulk phases. A domain wall is transparent to bulk excitations if the corresponding unitary tensor categories are Morita equivalent. Defects of higher codimension will also be studied. In summary, we give a dictionary between physical ingredients of lattice models and tensor-categorical notions.

Title: Topological density wave states of non-zero angular momentum
Authors: [|Chen-Hsuan Hsu], [|S. Raghu], [|Sudip Chakravarty] Comments: 4 pages, 4 figures Subjects: Superconductivity (cond-mat.supr-con) ; Strongly Correlated Electrons (cond-mat.str-el)

The pseudogap state of high temperature superconductors is a profound mystery. It has tantalizing evidence of a number of broken symmetry states, not necessarily conventional charge and spin density waves. Here we explore a class of more exotic density wave states characterized by topological properties observed in recently discovered topological insulators. We suggest that these rich topological density wave states deserve closer attention in not only high temperature superconductors but in other correlated electron states, as in heavy fermions.

Title: Fermi surfaces and gauge-gravity duality
Authors: [|Liza Huijse], [|Subir Sachdev] Comments: 37 pages, 12 figures Subjects: High Energy Physics - Theory (hep-th) ; Strongly Correlated Electrons (cond-mat.str-el)

We give a unified overview of the zero temperature phases of compressible quantum matter: i.e. phases in which the expectation value of a globally conserved U(1) density, Q, varies smoothly as a function of parameters. Provided the global U(1) and translational symmetries are unbroken, such phases are expected to have Fermi surfaces, and the Luttinger theorem relates the volumes enclosed by these Fermi surfaces to . We survey models of interacting bosons and/or fermions and/or gauge fields which realize such phases. Some phases have Fermi surfaces with the singularities of Landau's Fermi liquid theory, while other Fermi surfaces have non-Fermi liquid singularities. Compressible phases found in models applicable to condensed matter systems are argued to also be present in models obtained by applying chemical potentials (and other deformations allowed by the residual symmetry at non-zero chemical potential) to the paradigmic supersymmetric gauge theories underlying gauge-gravity duality: the ABJM model in spatial dimension d=2, and the N=4 SYM theory in d=3.

Title: Thermally Fluctuating Inhomogeneous Superfluid State of Strongly Interacting Fermions in an Optical Lattice
Authors: [|Viveka Nand Singh], [|Sanjoy Datta], [|Pinaki Majumdar] Comments: 4 pages. pdflatex, 4 figs Subjects: Quantum Gases (cond-mat.quant-gas)

The presence of attractive interaction between fermions can lead to pairing and superfluidity in an optical lattice. The temperature needed to observe superfluidity is about a tenth of the tunneling energy in the optical lattice, and currently beyond experimental reach. However, at strong coupling the precursors to global superfluidity should be visible at achievable temperatures, in terms of fluctuating domains with strong pairing correlations. We explore this regime of the attractive two dimensional fermion Hubbard model, in the presence of a confining potential, using a new Monte Carlo technique. We capture the low temperature inhomogeneous superfluid state with its unusual spectral signatures but mainly focus on the experimentally accessible intermediate temperature state. In this regime, and for the trap center density we consider, there is a large pairing amplitude at the center, spatially correlated into domains extending over several lattice spacings. We map out the thermal evolution of the local density, the double occupancy, the pairing correlations, and the momentum distribution function across this phase fluctuation window.

Title: Global phase diagram of three-dimensional extended Boson Hubbard model - a continuous time Quantum Monte Carlo study
Authors: [|Bin Xi], [|Fei Ye], [|Weiqiang Chen], [|Fuchun Zhang], [|Gang Su] Subjects: Quantum Gases (cond-mat.quant-gas) ; Strongly Correlated Electrons (cond-mat.str-el)

We present the global phase diagram of the extended boson Hubbard model on a simple cubic lattice by quantum Monte Carlo simulation with worm update algorithm. Four kinds of phases are supported by this model, including superfluid, supersolid, Mott, and charge density wave (CDW) states, which are identified in the phase diagram of chemical potential $\mu$ versus nearest neighbor interaction V. By changing the chemical potential, a continuous transition is found from the Mott phase to a superfluid phase without breaking the translational symmetry. For an insulating CDW state, adding particles to it gives rise to a continuous transition to a supersolid phase, while removing particles usually leads to a first-order one to either supersolid or superfluid phase. By tuning the nearest neighbor interaction, one can realize the transition between two insulating phases, Mott and CDW with the same particle density, which turns out to be of the first-order. We also demonstrate that a supersolid phase with average particle density less than 1/2 can exist in a small region of $\mu$ - V phase diagram.

Title: Pairing symmetry of superconductivity coexisting with antiferromagnetism
Authors: [|Keisuke Shigeta], [|Seiichiro Onari], [|Yukio Tanaka] Comments: 4 pages, 5 figures Subjects: Superconductivity (cond-mat.supr-con)

Pairing symmetry in the superconducting state coexisting with antiferromagnetic order is studied based on a microscopic theory. We calculate the linearized Eliashberg's equation within the random phase approximation in the Hubbard model with a staggered field. We find that odd-frequency spin-triplet (equal-spin) s-wave pairing state can be realized. This result contradicts a naive expectation that antiferromagnetic order induces antiferromagnetic spin fluctuation and favors spin-singlet d-wave pairing as in the standard strongly correlated systems.

Title: Excess energy of an ultracold Fermi gas in a trapped geometry
Authors: [|Shyamal Biswas], [|Debnarayan Jana], [|Raj Kumar Manna] Comments: 6 pages, 3 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Statistical Mechanics (cond-mat.stat-mech)

We have explored finite size and inter-particle interaction corrections to the average energy of a harmonically trapped Fermi gas below and above the Fermi temperature, and have obtained a better fitting for the excess energyreported by DeMarco and Jin [Science $\textbf{285}$, 1703 (1999)]. We have presented a perturbative calculation within a mean field approximation. Our calculation also reproduces the Thomas-Fermi and classical results at the two extreme limits of temperature.

Title: Finite-momentum Bose-Einstein condensates in shaken 2D square optical lattices
Authors: [|M. Di Liberto], [|O. Tieleman], [|V. Branchina], [|C. Morais Smith] Comments: 7 pages, 7 figures Subjects: Quantum Gases (cond-mat.quant-gas)

We consider ultracold bosons in a 2D square optical lattice described by the Bose-Hubbard model. In addition, an external time-dependent sinusoidal force is applied to the system, which shakes the lattice along one of the diagonals. The effect of the shaking is to renormalize the nearest-neighbor hopping coefficients, which can be arbitrarily reduced, can vanish, or can even change sign, depending on the shaking parameter. It is therefore necessary to account for higher-order hopping terms, which are renormalized differently by the shaking, and introduce anisotropy into the problem. We show that the competition between these different hopping terms leads to finite-momentum condensates, with a momentum that may be tuned via the strength of the shaking. We calculate the boundaries between the Mott-insulator and the different superfluid phases, and present the time-of-flight images expected to be observed experimentally. Our results open up new possibilities for the realization of bosonic analogs of the FFLO phase describing inhomogeneous superconductivity.

Title: Phase Separation of Multi-Component Bose-Einstein Condensates of Trapped Atoms and Molecules with a Homonuclear Feshbach Resonance
Authors: [|Ryosuke Shibato], [|Takushi Nishimura], [|Takayuki Watanabe], [|Toru Suzuki] Comments: 11pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)

We investigate phase separation of Bose-Einstein condensates (BECs) of two-component atoms and one-component molecules with a homonuclear Feshbach resonance. We develop a full model for dilute atomic and molecular gases including correlation of the Feshbach resonance and all kinds of interparticle interactions, and numerically calculate order parameters of the BECs in spherical harmonic oscillator traps at zero temperature with the Bogoliubov's classical field approximation. As a result, we find out that the Feshbach resonance can induce two types of phase separation. The actual phase structures and density profiles of the trapped gases are predicted in the whole parameter region, from the atom dominant regime to the molecule dominant regime. We focus on the role of the molecules in the phase separation. Especially in the atom dominant regime, the role of the molecules is described through effective interactions derived from our model. Furthermore we show that a perturbative and semi-classical limit of our model reproduces the conventional atomic BEC (single-channel) model.

Title: Fractional and Majorana Fermions: The Physics of Zero Energy Modes
Authors: [|R. Jackiw] Comments: 11 pages, 12 figures, talks given at the Nobel Symposium, Stockholm, Sweden, May 2010 and Semat Lecture, CCNY, New York, NY, April 2011; uses float.sty Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; High Energy Physics - Theory (hep-th)

We describe the occurrence and physical role of zero-energy modes in the Dirac equation with a topologically non-trivial background.

===4. [|arXiv:1104.4430] (cross-list from physics.atom-ph) [[|pdf], [|ps], [|other]]===

Title: Quantum degenerate mixtures of alkali and alkaline-earth-like atoms
Authors: [|Hideaki Hara], [|Yosuke Takasu], [|Yoshifumi Yamaoka], [|John Doyle], [|Yoshiro Takahashi] Comments: 4 pages, 3 figures Subjects: Atomic Physics (physics.atom-ph) ; Quantum Gases (cond-mat.quant-gas)

We realize simultaneous quantum degeneracy in mixtures consisting of the alkali and alkalineearth-like atoms Li and Yb. This is accomplished within an optical trap by sympathetic cooling of the fermionic isotope 6Li with evaporatively cooled bosonic 174Yb and, separately, fermionic 173Yb.Using cross-thermalization studies, we also measure the elastic s-wave scattering lengths of both Li-Yb combinations, |a6Li-174Yb| = 1.0\pm0.2 nm and |a6Li-173Yb| = 0.9\pm0.2 nm. The equality of these lengths is found to be consistent with mass-scaling analysis. The quantum degenerate mixtures of Li and Yb, as realized here, can be the basis for creation of ultracold molecules with electron spin degrees of freedom, studies of novel Efimov trimers, and impurity probes of superfluid systems.

=April 18-22, Chungwei Lin=

April 22
1. [|arXiv:1104.4179] [[|pdf], [|ps], [|other]] Authors: [|Takuto Kawakami], [|Takeshi Mizushima], [|Kazushige Machida] Comments: 5 pages, 5 figures Subjects: Quantum Gases (cond-mat.quant-gas) We study the textures of F=2 spinor Bose Einstein condensates (BECs) with spin-orbit coupling (SOC) induced by a synthetic non-Abelian gauge field. On the basis of the analysis of the SOC energy and the numerical calculation of the Gross-Pitaevskii equation, we demonstrate that the textures originate from the helical modulation of the order parameter (OP) due to the SOC.In particular, the cyclic OP consists of novel two dimensional lattice textures, such as the hexagonal lattice and the 1/3 vortex lattice, commonly understandable as the two dimensional network of the helical modulations.
 * Title: Textures of Spin-Orbit Coupled F=2 Spinor Bose Einstein Condensates**

2. [|arXiv:1104.4128] (cross-list from cond-mat.supr-con) [[|pdf], [|ps], [|other]] Authors: [|Rodrigo A. Muniz], [|Ivar Martin] Comments: 4 pages, 4 figures Subjects: Superconductivity (cond-mat.supr-con) ; Strongly Correlated Electrons (cond-mat.str-el) We theoretically study the effect of stripe-like superconducting inclusions on the non-linear resistivity in single crystals. Even when the stripe orientation varies throughout the sample between two orthogonal directions due to twinning, we predict that there should be a universal scaling relationship between the nonlinear resistivity curves measured at different angles relative to the crystal axes. This prediction can be used to verify or rule out the existence of superconducting stripes at and above the superconducting transition temperature in cuprate superconductors.
 * Title: A non-linear transport method for detecting superconducting stripes**

April 21
1. [|arXiv:1104.3881] (cross-list from cond-mat.supr-con) [[|pdf], [|ps], [|other]] Authors: [|L. Andrew Wray], [|Suyang Xu], [|Yuqi Xia], [|Dong Qian], [|Alexei V. Fedorov], [|Hsin Lin], [|Arun Bansil], [|Yew San Hor], [|Robert J. Cava], [|M. Zahid Hasan] Comments: 13 pages, 4 Figures, High resolution figures at [|this http URL] Journal-ref: Nature Physics 6, 855 (2010) Subjects: Superconductivity (cond-mat.supr-con) ; Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Strongly Correlated Electrons (cond-mat.str-el) Topological insulators embody a new state of matter characterized entirely by the topological invariants of the bulk electronic structure rather than any form of spontaneously broken symmetry. Unlike the 2D quantum Hall or quantum spin-Hall-like systems, the three dimensional (3D) topological insulators can host magnetism and superconductivity which has generated widespread research activity in condensed-matter and materials-physics communities. Thus there is an explosion of interest in understanding the rich interplay between topological and the broken-symmetry states (such as superconductivity), greatly spurred by proposals that superconductivity introduced into certain band structures will host exotic quasiparticles which are of interest in quantum information science. The observations of superconductivity in doped Bi_2Se_3 (Cu$_x$Bi$_2$Se$_3$) and doped Bi_2Te_3 (Pd$_x$-Bi$_2$Te$_3$ T$_c$ $\sim$ 5K) have raised many intriguing questions about the spin-orbit physics of these ternary complexes while any rigorous theory of superconductivity remains elusive. Here we present key measurements of electron dynamics in systematically tunable normal state of Cu$_x$Bi$_2$Se$_3$ (x=0 to 12%) gaining insights into its spin-orbit behavior and the topological nature of the surface where superconductivity takes place at low temperatures. Our data reveal that superconductivity occurs (in sample compositions) with electrons in a bulk relativistic kinematic regime and we identify that an unconventional doping mechanism causes the topological surface character of the undoped compound to be preserved at the Fermi level of the superconducting compound, where Cooper pairing occurs at low temperatures. These experimental observations provide important clues for developing a theory of topological-superconductivity in 3D topological insulators.
 * Title: Observation of topological order in a Superconducting doped topological insulator (based on the Bi2Se3 class)**

2. [|arXiv:1104.4071] [[|pdf], [|other]] Authors: [|R. O. Umucalilar], [|I. Carusotto] Subjects: Quantum Gases (cond-mat.quant-gas) We propose and characterize solid-state photonic structures where light experiences an artificial gauge field. A suitable coupling of the propagation and polarization degrees of freedom introduces a geometrical phase for photons tunneling between adjacent sites of a coupled cavity array. We then discuss the feasibility of observing strong gauge field effects in the optical spectra of realistic systems, including the Hofstadter butterfly spectrum.
 * Title: Artificial Gauge Field for Photons in Coupled Cavity Arrays**

April 20
1. [|arXiv:1104.3693] (cross-list from cond-mat.supr-con) [[|pdf], [|other]] Authors: [|Youichi Yanase], [|Manfred Sigrist] Comments: 8 pages Subjects: Superconductivity (cond-mat.supr-con) ; Strongly Correlated Electrons (cond-mat.str-el) Incommensurate antiferromangetic (AFM) order in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductor is investigated on the basis of the Ginzburg-Landau theory. We formulate the two component Ginzburg-Landau model to discuss two degenerate incommensurate AFM states in the tetragonal crystal structure. Owing to the broken translation symmetry in the FFLO state, a multiple phase diagram of single-q phase and double-q phase is obtained under the magnetic field along [100] or [010] direction. Magnetic properties in each phase are investigated and compared with the neutron scattering and NMR measurements for a heavy fermion superconductor CeCoIn_5. An ultrasonic measurement is proposed for a future experimental study to identify the AFM-FFLO state. The field orientation dependence of the AFM order in CeCoIn_5 is discussed.
 * Title: Ginzburg-Landau Analysis for the Antiferromagnetic Order in the Fulde-Ferrell-Larkin-Ovchinnikov Superconductor**

2. [|arXiv:1104.3625] [[|pdf], [|ps], [|other]] Authors: [|Zhu Chen], [|Biao Wu] Subjects: Quantum Gases (cond-mat.quant-gas) Mean-field Bloch bands of a Bose-Einstein condensate in a honeycomb optical lattice are computed. We find that the topological structure of the Bloch bands at the Dirac point is changed completely by the atomic interaction of arbitrary small strength: the Dirac point is extended into a closed curve and an intersecting tube structure arises around the original Dirac point. These tubed Bloch bands are caused by the superfluidity of the system. Furthermore, they imply the inadequacy of the tight-binding model to describe an interacting Boson system around the Dirac point and the breakdown of adiabaticity by interaction of arbitrary small strength.
 * Title: Bose-Einstein Condensate in a Honeycomb Optical Lattice: Fingerprint of Superfluidity at the Dirac Point**

[|3. arXiv:1104.3838] (cross-list from cond-mat.str-el) [[|pdf], [|ps], [|other]] Authors: [|Michael Knap], [|Enrico Arrigoni], [|Wolfgang von der Linden] (TU Graz) Comments: 9 pages, 5 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Other Condensed Matter (cond-mat.other); Quantum Gases (cond-mat.quant-gas) A numerical approach is presented that allows to compute non-equilibrium steady state properties of strongly correlated quantum many-body systems. The method is imbedded in the Keldysh Green's function formalism and is based upon the idea of the variational cluster approach as far as the treatment of strong correlations is concerned. It appears that the variational aspect is crucial as it allows for a self-consistent adjustment of the equilibrium reference system to the nonequilibrium target state. The approach is neither perturbative in the many-body interaction nor in the field, that drives the system out of equilibrium, and it allows to study strong perturbations and nonlinear responses of systems in which also the correlated region is spatially extended. We apply the presented approach to non-linear transport across a strongly correlated quantum wire described by the fermionic Hubbard model.
 * Title: Nonequilibrium steady state for strongly-correlated many-body systems: variational cluster approach**

April 19
1. [|arXiv:1101.1757] (cross-list from cond-mat.quant-gas) [[|pdf], [|other]] Authors: [|J. Boening], [|A. Filinov], [|M. Bonitz] Subjects: Quantum Gases (cond-mat.quant-gas) ; Strongly Correlated Electrons (cond-mat.str-el) Indirect excitons -- pairs of electrons and holes spatially separated in semiconductor bilayers or quantum wells -- are known to undergo Bose-Einstein condensation and to form a quantum fluid. Here we show that this superfluid may crystallize upon compression. However, further compression results in quantum melting back to a superfluid. This unusual behavior is explained by the effective interaction potential between indirect excitons which strongly deviates from a dipole potential at small distances due to many-particle and quantum effects. Based on first principle path integral Monte Carlo simulations, we compute the complete phase diagram of this system and predict the relevant parameters necessary to experimentally observe exciton crystallization in semiconductor quantum wells.
 * Title: Crystallization of an exciton superfluid**

2. [|arXiv:1104.3389] [[|pdf], [|ps], [|other]] Authors: [|R. N. Bisset], [|D. Baillie], [|P. B. Blakie] Comments: 5 pages, 2 figures Subjects: Quantum Gases (cond-mat.quant-gas) We calculate the stability diagram for a trapped normal Bose gas with dipole-dipole interactions. Our study characterizes the roles of trap geometry, temperature, and short-ranged interactions on the stability. We predict a robust double instability feature in oblate trapping geometries arising from the interplay of thermal gas saturation and the anisotropy of the interaction. Our results are relevant to current experiments with polar molecules and will be useful in developing strategies to obtain a polar molecule Bose-Einstein condensate.
 * Title: Finite temperature stability of a trapped dipolar Bose gas**

April 18
1. [|arXiv:1104.2988] [[|pdf], [|ps], [|other]] Authors: [|Benjamin Obert], [|So Takei], [|Walter Metzner] Comments: 8 pages, 2 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) We analyze the scaling behavior at and near a quantum critical point separating a semimetallic from a superfluid phase. To this end we compute the renormalization group flow for a model of attractively interacting electrons with a linear dispersion around a single Dirac point. We study both ground state and finite temperature properties. In two dimensions, the electrons and the order parameter fluctuations exhibit power-law scaling with anomalous scaling dimensions. The quasi-particle weight and the Fermi velocity vanish at the quantum critical point. The order parameter correlation length turns out to be infinite everywhere in the semimetallic ground state.
 * Title: Anomalous criticality near semimetal-to-superfluid quantum phase transition in a two-dimensional Dirac cone model**

2. [|arXiv:1104.3080] (cross-list from physics.atom-ph) [[|pdf], [|other]] Authors: [|S. Ji], [|C. Ates], [|I. Lesanovsky] Subjects: Atomic Physics (physics.atom-ph) ; Quantum Gases (cond-mat.quant-gas) We study a two-dimensional lattice gas of atoms that are photo-excited to high-lying Rydberg states in which they interact via the van-der-Waals interaction. We explore the regime of dominant nearest neighbor interaction where this system is intimately connected to a quantum version of Baxter's hard squares model. We show that the strongly correlated ground state of the Rydberg gas can be analytically described by a projected entangled pair state that constitutes the ground state of the quantum hard squares model. This correspondence allows us to identify a first order phase boundary where the Rydberg gas undergoes a transition from a disordered (liquid) phase to an ordered (solid) phase.
 * Title: Two-dimensional Rydberg gases and the quantum hard squares model**

2. [|arXiv:1104.3032] [[|pdf], [|ps], [|other]] Authors: [|Xinxing Liu], [|Xiaoji Zhou], [|Wei Zhang], [|Thibault Vogt], [|Bo Lu], [|Xuguang Yue], [|Xuzong Chen] Comments: 6 pages, 5 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Atomic Physics (physics.atom-ph) We use a coherent Bragg diffraction method to impart an external momentum to ultracold bosonic atoms trapped in a one-dimensional optical lattice. This method is based on the application of a single light pulse, with conditions where scattering of photons can be resonantly amplified by the atomic density grating. An oscillatory behavior of the momentum distribution resulting from the time evolution in the lattice potential is then observed. By measuring the oscillating frequencies, we extract multi-band energy structures of single-particle excitations with zero pseudo-momentum transfer for a wide range of lattice depths. The excitation energy structures reveal the interaction effect through the whole range of lattice depth.
 * Title: Exploring multi-band excitations of interacting Bose gases in a 1D optical lattice by coherent scattering**

=April 11-15, Xiaopeng Li=

April 15
Authors:  [|Eran Sela], [|Matthias Punk] , [|Markus Garst]
 * 1.__[|arXiv:1104.2603]__ [ [|pdf], [|ps] , [|other] ] **
 * Title: Dislocation-mediated melting of one-dimensional Rydberg crystals **

Subjects: **Quantum Gases (cond-mat.quant-gas)** ; Strongly Correlated Electrons (cond-mat.str-el) We consider cold Rydberg atoms in a one-dimensional optical lattice in the Mott regime with a single atom per site at zero temperature. An external laser drive with Rabi frequency \Omega and laser detuning \Delta, creates Rydberg excitations whose dynamics is governed by an effective spin-chain model with (quasi) long-range interactions. This system possesses intrinsically a large degree of frustration resulting in a ground-state phase diagram in the (\Delta,\Omega) plane with a rich topology. As a function of \Delta, the Rydberg blockade effect gives rise to a series of crystalline phases commensurate with the optical lattice that form a so-called devil's staircase. The Rabi frequency, \Omega, on the other hand, creates quantum fluctuations that eventually lead to a quantum melting of the crystalline states. Upon increasing \Omega, we find that generically a commensurate-incommensurate transition to a floating Rydberg crystal occurs first, that supports gapless phonon excitations. For even larger \Omega, dislocations within the floating Rydberg crystal start to proliferate and a second, Kosterlitz-Thouless-Nelson-Halperin-Young dislocation-mediated melting transition finally destroys the crystalline arrangement of Rydberg excitations. This latter melting transition is generic for one-dimensional Rydberg crystals and persists even in the absence of an optical lattice. The floating phase and the concomitant transitions can, in principle, be detected by Bragg scattering of light.

**2.** ** [|arXiv:1104.2617] [ [|pdf], [|ps] , [|other] ]** **Titile**: Two-Fermi-surface superconducting state and a nodal d-wave gap in the electron-doped Sm(1.85)Ce(0.15)CuO(4-d) cuprate superconductor Authors: __[|Andres F. Santander-Syro]__, [|Masaki Ikeda], [|Teppei Yoshida] , [|Atsushi Fujimori] , [|Kyoko Ishizaka] , [|Mario Okawa] , [|Shik Shin] , [|Richard L. Greene] , [|Nicole Bontemps] We report on laser-excited angle-resolved photoemission spectroscopy (ARPES) in the electron-doped cuprate Sm(1.85)Ce(0.15)CuO(4-d). The data show the existence of a nodal hole-pocket Fermi-surface both in the normal and superconducting states. We prove that its origin is long-range antiferromagnetism by an analysis of the coherence factors in the main and folded bands. This coexistence of long-range antiferromagnetism and superconductivity implies that electron-doped cuprates are two-Fermi-surface superconductors. The measured superconducting gap in the nodal hole-pocket is compatible with a d-wave symmetry.

**3.**  **__ [|arXiv:1104.2632] __ [ [|pdf], [|ps] , [|other] ]** **Titile**: (3+1)-TQFTs and Topological Insulators Authors: Kevin Walker, Zhenghan Wang Levin-Wen models are microscopic spin models for topological phases of matter in (2+1)-dimension. We introduce a generalization of such models to (3+1)-dimension based on unitary braided fusion categories, also known as unitary premodular categories. We discuss the ground state degeneracy on 3-manifolds and statistics of excitations which include both points and defect loops. Potential connections with recently proposed fractional topological insulators and projective ribbon permutation statistics are described.

**4. __[|arXiv:1104.2672]__ [ [|pdf], [|other] ]**

**Titile**: Magnetic structure of antiferromagnetic Fulde-Ferrell-Larkin-Ovchinnikov state Authors:  [|Youichi Yanase], [|Manfred Sigrist] The properties of incommensurate antiferromagnetic (AFM) order in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is studied by solving the Bogoliubov-de-Gennes (BdG) equations. The relationship between the electronic structure and the magnetic structure is clarified. We find that the magnetic structure in the AFM-FFLO state includes three cases. (I) In the strongly localized case, the AFM staggered moment is confined into the FFLO nodal planes where the superconducting order parameter vanishes. (II) In the weakly localized case, the AFM staggered moment appears in the whole spatial region, and its magnitude is enhanced around the FFLO nodal planes. (III) In the extended case, the AFM staggered moment is nearly homogeneous and slightly suppressed in the vicinity of FFLO nodal planes. The structure of Bragg peaks in the momentum resolved structure factor is studied in each case. We discuss the possibility of AFM-FFLO state in the heavy fermion superconductor CeCoIn_5 by comparing these results with the neutron scattering data. Experimentally the magnetic structure and its dependence on the magnetic field orientation in the high field superconducting phase of CeCoIn_5 are consistent with the case (II).

April 14
Authors:  [|Bryce Gadway], [|Daniel Pertot] , [|Jeremy Reeves] , [|Dominik Schneble] Atomic quantum gases in optical lattices serve as a versatile testbed for important concepts of modern condensed-matter physics. The availability of methods to characterise strongly correlated phases is crucial for the study of these systems. Diffraction techniques to reveal long-range spatial structure, which may complement \emph{in situ} detection methods, have been largely unexplored. Here we experimentally demonstrate that Bragg diffraction of neutral atoms can be used for this purpose. Using a one-dimensional Bose gas as a source of matter waves incident on an atomic Mott insulator, we are able to infer the atoms' spatial ordering and on-site localisation. In tuning the depth of the optical lattice localising the atoms, we furthermore observe a suppression of elastic two-body collisions in the band structure. Our method should be well suited for the non-destructive detection of spin-ordered phases in strongly correlated atomic gases.
 * 1.** **__[|arXiv:1104.2564]__ [ [|pdf], [|other] ]**
 * Titile**: Probing an Ultracold-Atom Crystal with Matter Waves

**April 13**
**1.** <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: normal;"> ** [|arXiv:1104.2279] [ [|pdf], [|ps] , [|other] ]** **Titile**: Properties of Ferromagnetic Superconductors Authors: <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 16px; line-height: 18px;"> [|Dai Aoki], [|Frederic Hardy] , [|Atsushi Miyake] , [|Valentin Taufour] , [|Tatsuma D. Matsuda] , [|Jacques Flouquet] <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: 19px; margin: 0px; padding: 0px;">Thanks to the discovery in the last decade of three uranium ferromagnetic superconductors, UGe2, URhGe and UCoGe, the fascinating aspects of the interplay between the triplet state of Cooper pairing and ferromagnetism have emerged. Furthermore, as the ferromagnetic properties in the normal state are quite different with respect to the proximity of the ferromagnetic-paramagnetic instabilities, the feedback with the coexistence of superconductivity gives rise to quite different boundaries in pressure and magnetic field. Special attention is given on the location of the materials with respect to the tricriticality and on the reinforcement of superconductivity in a transverse field response with respect to the direction of the ferromagnetic sublattice magnetization. The other facts of the interplay between ferromagnetism and superconductivity is briefly mentioned.

2. <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: normal;">** [|arXiv:1104.2257] [ [|pdf], [|other] ]**

**Titile**: Topologically protected flat zero-energy surface bands in non-centrosymmetric superconductors Authors: <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 16px; line-height: 18px;"> [|P. M. R. Brydon], [|Andreas P. Schnyder] , [|Carsten Timm] <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: 19px; margin: 0px; padding: 0px;">Nodal non-centrosymmetric superconductors (NCS) have recently been shown to be topologically non-trivial. An important consequence is the existence of topologically protected flat zero-energy surface bands, which are related to the topological characteristics of the line nodes of the bulk gap via a bulk-boundary correspondence [Schnyder and Ryu, [|arXiv:1011.1438] ]. In this paper we examine these zero-energy surface bands using a quasiclassical theory. We determine their spectrum and derive a general condition for their existence in terms of the sign change of the gap functions. A key experimental signature of the zero-energy surface bands is a zero-bias peak in the tunneling conductance, which depends strongly on the surface orientation. This can be used as a fingerprint of a topologically non-trivial NCS.

**April 12**
**1.** <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: normal;">** [|arXiv:1104.2031] [ [|pdf], [|ps] , [|other] ]** **Titile**: How "quantum" is the exciton dynamics in the Fenna-Matthews-Olson complex? Authors: <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 16px; line-height: 18px;"> [|Peter Nalbach], [|Daniel Braun] , [|Michael Thorwart]

<span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We present numerically exact results for the quantum coherent energy transfer in the Fenna-Matthews-Olson molecular aggregate under realistic physiological conditions, including also vibrational fluctuations of the protein and the pigments. We find that although the coherence times are to some extent reduced as compared to the often assumed Ohmic fluctuation spectrum, the exciton dynamics is far from being a classical hoping process. We quantify the "quantumness" of the energy transfer as distance of the density matrix to the classical pointer states for the energy current operator.

**April 11**
**1.** <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: normal;"> ** [|arXiv:1104.1614] [ [|pdf], [|ps] , [|other] ]** **Titile**: The orbital analog of ice: p-band Mott-insulators on the diamond lattice Authors: <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 16px; line-height: 18px;"> [|Gia-Wei Chern], [|Congjun Wu]

<span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; margin: 0px; padding: 0px;">The Mott-insulating states of p-orbital spinless fermions in a three-dimensional optical lattice exhibit an unusual degeneracy due to the anisotropic orbital interaction and geometrical frustration. Here we investigate the orbital exchange physics in cubic and diamond optical lattices. In particular, the effective orbital interaction in a diamond lattice is governed by a quantum 'tetrahedral' Hamiltonian which is a three-dimensional generalization of the planar 120-degree model. A macroscopic degeneracy remains in its quantum ground states due to frustrated orbital interactions. We show that this degenerate manifold can be mapped to Ising spins obeying the ice rule on a pyrochlore lattice. An experimental signature is the pinch-point singularities in diffusion-scattering measurement which originate from a dipolar-like correlation function in the orbital ice state.

=April 4 - 8, Zixu Zhang=

Title: Effective action approach to the p-band Mott insulator and superfluid transition
Authors: [|Xiaopeng Li], [|Erhai Zhao], [|W. Vincent Liu] Comments: 9 pages, 7 figures Subjects: Quantum Gases (cond-mat.quant-gas)

Motivated by the recent experiment on p-orbital band bosons in optical lattices, we study theoretically the quantum phases of Mott insulator and superfluidity in two-dimensions. The system features a novel superfluid phase with transversely staggered orbital current at weak interaction, and a Mott insulator phase with antiferro-orbital order at strong coupling and commensurate filling. We go beyond mean field theory and derive from a microscopic model an effective action that is capable of describing both the p-band Mott insulating and superfluid phases in strong coupling. We further calculate the excitation spectra near the quantum critical point and find two gapless modes away from the tip of the Mott lobe but four gapless modes at the tip. Our effective theory reveals how the phase coherence peak builds up in the Mott regime when approaching the critical point. We also discuss the finite temperature phase transition of p-band superfluidity.

Title: Comment on "Fermi-Bose Mixtures near Broad Interspecies Feshbach Resonances"
Authors: [|J. Dukelsky], [|C. Esebbag], [|P. Schuck], [|T. Suzuki] Journal-ref: Phys. Rev. Lett. 106, 129601 (2011) Subjects: Quantum Gases (cond-mat.quant-gas)

In a recent Letter (Phys. Rev. lett. 105, 195301 (2010)), Song et al. introduced a new variational approach to treat strong attractive boson-fermion (BF) correlations in BF atomic mixtures. The proposed theory predicts a first order phase transition to a condensate of composite BF pairs with center of mass momentum Q=0 as opposed to a composite fermionic molecular Fermi gas. We show in this comment that their approach is incorrect and moreover, by resorting to an exactly solvable model we demonstrate that there cannot be more than one correlated Q=0 BF pair in complete contradiction with their conclusions.

Title: Collapse and revival dynamics of superfluids of ultracold atoms in optical lattices
Authors: [|E. Tiesinga], [|P. R. Johnson] Comments: 5 pages, 3 figures Subjects: Quantum Gases (cond-mat.quant-gas)

Recent experiments have shown a remarkable number of collapse-and-revival oscillations of the matter-wave coherence of ultracold atoms in optical lattices [Will et al., Nature 465, 197 (2010)]. Using a mean-field approximation to the Bose-Hubbard model, we show that the visibility of collapse-and-revival interference patterns reveal number squeezing of the initial superfluid state. To describe the dynamics, we use an effective Hamiltonian that incorporates the intrinsic two-body and induced three-body interactions, and we analyze in detail the resulting complex pattern of collapse-and-revival frequencies generated by virtual transitions to higher bands. Our work shows that a combined analysis of both the multiband, non-stationary dynamics in the final deep lattice, and the number-squeezing of the initial superfluid state, explains important characteristics of optical lattice collapse-and-revival physics.

4.

Title: Topological Order and Degenerate Entanglement Spectrum in Two-Dimensional Dimerized Quantum Heisenberg Model
Authors: [|Ching-Yu Huang], [|Feng-Li Lin] Comments: 25 pages, 15 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We study the connection between topological order and degeneracy of the entanglement spectrum by explicitly solving the two-dimensional dimerized quantum Heisenberg model in the form of tensor product state ansatz. By evaluating the topological entanglement entropy, we identify a new phase with topological order in this model, in which the entanglement spectrum is doubly degenerate. Degeneracy of the entanglement spectrum is robust against various types of perturbations, in accordance with our expectation for topological order. Our results support the connection among topological order, long range entanglement and the dominant degenerate singular values of entanglement spectrum. In the context of tensor product state ansatz, the numerical evaluation of entanglement spectrum costs far less computation power than the one for topological entanglement entropy. Our results provide a more viable way to numerically identify the topological order for the generic frustrated systems.

Title: Theory of 2D transport in graphene for correlated disorder
Authors: [|Qiuzi Li], [|E. H. Hwang], [|S. Das Sarma] Comments: 5 pages, 2 figures Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall) We theoretically revisit graphene transport properties as a function of carrier density, taking into account possible correlations in the spatial distribution of the Coulomb impurity disorder in the environment. We find, quite unexpectedly, that charged impurity correlations give rise to a density dependent graphene conductivity which agrees very well qualitatively with the existing experimental data without the additional {\it ad hoc} inclusion of any short-range disorder in the model. In particular, the linearity (sublinearity) of graphene conductivity at lower (higher) gate voltage away from the charge neutrality point is naturally explained as arising solely from impurity correlation effects in the Coulomb disorder.

Title: Quantum paramagnetic ground states on the honeycomb lattice and field-induced transition to Néel order
Authors: [|R. Ganesh], [|D. N. Sheng], [|Young-June Kim], [|A. Paramekanti] Comments: 7 pages, longer version of [|arXiv:1012.0316], to appear in Phys. Rev. B Subjects: Strongly Correlated Electrons (cond-mat.str-el)

Motivated by recent experiments on Bi$_3$Mn$_4$O$_{12}$(NO$_3$), and a broader interest arising from numerical work on the honeycomb lattice Hubbard model, we have studied the effect of a magnetic field on honeycomb lattice spin models with quantum paramagnetic ground states. For a model with frustrating second-neighbor exchange, $J_2$, we use a Lindemann-like criterion within spin wave theory to show that N\'eel order melts beyond a critical $J_2$. The critical $J_2$ increases with a magnetic field, implying the existence of a field-induced paramagnet-N\'eel transition over a range of $J_2$. We also study bilayer model using a spin-$S$ generalization of bond operator mean field theory. We show that there is a N\'eel-dimer transition for various spin values with increasing bilayer coupling, and that the resulting interlayer dimer state undergoes a field induced transition into a state with transverse N\'eel order. Finally, we study a spin-3/2 model which interpolates between the Heisenberg model and the Affleck-Kennedy-Lieb-Tasaki (AKLT) parent Hamiltonian. Using exact diagonalization, we compute the fidelity susceptibility to locate the Neel-AKLT quantum critical point, obtain the spin gap of the AKLT parent Hamiltonian, and argue that AKLT state also undergoes field-induced Neel ordering.

Title: Effects of Interactions on the Critical Temperature of a Trapped Bose Gas
Authors: [|Robert P. Smith], [|Robert L. D. Campbell], [|Naaman Tammuz], [|Zoran Hadzibabic] Comments: 4 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Statistical Mechanics (cond-mat.stat-mech); Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)

We perform high-precision measurements of the condensation temperature of a harmonically-trapped atomic Bose gas with widely-tuneable interactions. For weak interactions we observe a negative shift of the critical temperature in excellent agreement with mean-field theory. However for sufficiently strong interactions we clearly observe an additional positive shift, characteristic of beyond-mean-field critical correlations. We also discuss non-equilibrium effects on the apparent critical temperature for both very weak and very strong interactions.

Title: Antiferromagnetic phase transition in a nonequilibrium lattice of Rydberg atoms
Authors: [|Tony E. Lee], [|H. Häffner], [|M. C. Cross] Comments: 4 pages + appendix Subjects: Quantum Gases (cond-mat.quant-gas) ; Pattern Formation and Solitons (nlin.PS); Quantum Physics (quant-ph)

We study a nonequilibrium system of atoms in the presence of laser excitation to a Rydberg state and spontaneous emission. The atoms interact via the blockade effect, whereby an atom in the Rydberg state shifts the Rydberg level of neighboring atoms. We use mean-field theory to study how the Rydberg population varies in space. As the laser frequency changes, there is a continuous transition between the uniform and antiferromagnetic phases. The nonequilibrium nature also leads to a novel oscillatory phase and bistability between the uniform and antiferromagnetic phases.

Title: Effective Field Theory of Fractional Quantized Hall Nematics
Authors: [|Michael Mulligan], [|Chetan Nayak], [|Shamit Kachru] Comments: 4 pages + refs Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; High Energy Physics - Theory (hep-th)

We present a Landau-Ginzburg theory for a fractional quantized Hall nematic state and the transition to it from an isotropic fractional quantum Hall state. This justifies Lifshitz-Chern-Simons theory -- which is shown to be its dual -- on a more microscopic basis and enables us to compute a ground state wave function in the symmetry-broken phase. In such a state of matter, the Hall resistance remains quantized while the longitudinal DC resistivity due to thermally-excited quasiparticles is anisotropic. We interpret recent experiments at Landau level filling factor \nu =7/3 in terms of our theory.

Title: An exactly solvable BCS-type model of Dirac fermions in three dimension and a new type of FFLO states
Authors: [|Qiang Han], [|Jia Liu], [|Z. D. Wang] Comments: 5 pages, 2 figures Subjects: Superconductivity (cond-mat.supr-con) ; Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)

We solve exactly a BCS-type model of Dirac fermions in three dimension (3D) and show that 3D s-wave supercoductors with spin-orbit interactions are equivalent to chiral $p+ip$-wave ones. In the presence of a Zeeman field, it is found that the reduced BCS Hamiltonian is also exactly solvable. Most intriguingly, a new type of FFLO ground state is rigorously revealed, in which the center-of-mass momentum of the fermion pair is proportional to the Zeeman field.

Title: Spin-imbalanced topological phases and chiral Rashba states in a fermionic lattice
Authors: [|N. Goldman], [|W. Beugeling], [|C. Morais Smith] Comments: 8 pages, 3 figures Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ; Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

The quantum Hall effect - the exact quantization of the transverse Hall conductivity- finds its roots in time-reversal breaking perturbations. Indeed, a magnetic field can produce chiral edge states, which transport current without dissipation. Recently, the realm of quantum Hall physics has revealed novel quantum phases, with the discovery of the quantum spin Hall effect. The latter was observed in materials with important spin-orbit interactions and leads to finite spin-conductivity. This intrinsic spin-orbit interaction acts as equal but opposite magnetic fields on each spin component and produces counter-propagating edge states with opposite spins. Here we reveal new topological phases that arise when the effects produced by spin-orbit couplings and magnetic fields are combined. We demonstrate that this system provides an elegant setup to generate rich and robust spin-structures propagating along the edge of a sample. The exquisite control over these polarized current-carrying states will pave an interesting route for the development of quantum spintronics.

Title: Quantum Spin Hall Effect in Silicene
Authors: [|Cheng-Cheng Liu], [|Wanxiang Feng], [|Yugui Yao] Comments: 4 pages, 4 figures Subjects: Materials Science (cond-mat.mtrl-sci) ; Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)

Recent years have witnessed great interest in the quantum spin Hall effect (QSHE) which is a new quantum state of matter with nontrivial topological property due to the scientific importance as a novel quantum state and the technological applications in spintronics. Taking account of Si, Ge significant importance as semiconductor material and intense interest in the realization of QSHE for spintronics, here we investigate the spin-orbit opened energy gap and the band topology in recently synthesized silicene using first-principles calculations. We demonstrate that silicene with topologically nontrivial electronic structures can realize QSHE by exploiting adiabatic continuity and direct calculation of the Z2 topological invariant. We predict that QSHE in silicene can be observed in an experimentally accessible low temperature regime with the spin-orbit band gap of 1.55 meV, much higher than that of graphene due to large spin-orbit coupling and the low-buckled structure. Furthermore, we find that the gap will increase to 2.90 meV under certain pressure strain. Finally, we also study germanium with similar low buckled stable structure, and predict that SOC opens a band gap of 23.9 meV, much higher than the liquid nitrogen temperature.

Title: Quantum Hall effects in fast rotating Fermi gases with anisotropic dipolar interaction
Authors: [|R.-Z. Qiu], [|Su-Peng Kou], [|Z.-X. Hu], [|Xin Wan], [|S. Yi] Comments: 10 pages, 9 figures Subjects: Quantum Gases (cond-mat.quant-gas) We investigate fast rotating quasi-two-dimensional dipolar Fermi gases in the quantum Hall regime. By tuning the direction of the dipole moments with respect to the z-axis, the dipole-dipole interaction becomes anisotropic in the $x$-$y$ plane. For a soft confining potential we find that, as we tilt the angle of the dipole moments, the system evolves from a $\nu = 1/3$ Laughlin state with dipoles being polarized along the z axis to a series of ground states characterized by distinct mean total angular momentum, and finally to an anisotropic integer quantum Hall state. During the transition from the fractional regime to the integer regime, we find that the density profile of the system exhibits crystal-like structures. We map out the ground states as a function of the tilt angle and the confining potential, revealing the competition of the isotropic confining potential and both the isotropic and anisotropic components of the dipole-dipole interaction.