July 1 - July 5, Li-Jun Lang
July 1
1. arXiv:1306.6804 [pdf, ps, other]
Low-dimensional physics of ultracold gases with bound states and the sine-Gordon model
Thierry Jolicoeur, Evgeni Burovski, Giuliano Orso
One-dimensional systems of interacting atoms are an ideal laboratory to study the Kosterlitz-Thouless phase transition. In the renormalization group picture there is essentially a two-parameter phase diagram to explore. We first present how detailed experiments have shown direct evidence for the theoretical treatment of this transition. Then generalization to the case of two-component systems with bound state formation is discussed. Trimer formation in the asymmetric attractive Hubbard model involve in a crucial way this kind of physics.
2. arXiv:1306.6824 [pdf, other]
From dia- to paramagnetic orbital susceptibility of Dirac cones
A. Raoux, M. Morigi, J.N. Fuchs, F. Piéchon, G. MontambauxWe study the orbital susceptibility of coupled energy bands with a pair of Dirac points, as in graphene. We show that different systems having the same zero-field energy spectrum exhibit strong differences in their orbital magnetic response at zero energy, ranging from diamagnetism (graphene) to paramagnetism (dice lattice). A lattice model is introduced which interpolates continuously between these two limits. This striking behavior is related to a Berry phase varying continuously from pi to 0. These predictions could be tested with cold atoms in an optical lattice.
July 2
1. arXiv:1307.0821 [pdf, ps, other]
Self trapping in the two-dimensional Bose-Hubbard model
We study the expansion of harmonically trapped bosons in a two-dimensional lattice after suddenly turning off the confining potential. We show that, in the presence of multiple occupancies per lattice site and strong interactions, the system exhibits a clear dynamical separation into trapped and expanding clouds. We discuss how this effect can be understood within a simple picture by invoking doublons and Bose enhancement. This picture is corroborated by an analysis of the momentum distribution function of trapped and expanding bosons.
2. arXiv:1307.0372 [pdf, other]
Anderson localization of a Majorana fermion
D. A. Ivanov, P. M. Ostrovsky, M. A. Skvortsov
Isolated Majorana fermion states can be produced at the boundary of a topological superconductor in a quasi-one-dimensional geometry. If such a superconductor is connected to a disordered quantum wire, the Majorana fermion is spread into the wire, subject to Anderson localization. We study this effect in the limit of a thick wire with broken time-reversal and spin-rotational symmetries. With the use of a supersymmetric nonlinear sigma model, we calculate the average local density of states in the wire as a function of energy and of the distance from the interface with the superconductor. Our results may be qualitatively explained by the repulsion of states from the Majorana level and by Mott hybridization of localized states.
3. arXiv:1307.0448 [pdf]
Topological surface state in the Kondo Insulator Samarium Hexaboride
D. J. Kim, J. Xia, Z. Fisk
Strongly correlated electron systems show many exotic properties such as unconventional superconductity, quantum criticality, and Kondo insulating behavior. In addition, the Kondo insulator SmB6 has been predicted theoretically to be a 3D topological insulator with a metallic surface state. We report here transport measurements on doped SmB6, which show that ~3% magnetic and non-magnetic dopants in SmB6 exhibit clearly contrasting behavior, evidence that the metallic surface state is only destroyed when time reversal symmetry is broken. We find as well a quantum percolation limit of impurity concentration which transform the topological insulator into a conventional band insulator by forming impurity band. Our careful thickness dependence results show that SmB6 is the first demonstatrated perfect 3D topological insulator with virtually zero residual bulk conductivity.
July 3
1. arXiv:1307.0899 [pdf, other]
Quasi-One-Dimensional Dipolar Quantum Gases
Liming Guan, Xiaoling Cui, Ran Qi, Hui Zhai
In this letter we consider dipolar quantum gases in a quasi-one-dimensional tube with dipole moment perpendicular to the tube direction. We deduce the effective one-dimensional interaction potential and show that this potential is not purely repulsive, but rather has an attractive part due to high-order scattering processes through transverse excited states. The attractive part can induce bound state and cause scattering resonances. This represents the dipole induced resonance in low-dimension. We work out an unconventional behavior of low-energy phase shift for this effective potential and show how it evolves across a resonance. Based on the phase shift, the interaction energy of spinless bosons is obtained using asymptotic Bethe ansatz. Despite of long-range nature of dipolar interaction, we find that a behavior similar as short-range Lieb-Linger gas emerges at the resonance regime.
2. arXiv:1307.0744 [pdf, other]
Spin fragmentation of Bose-Einstein condensates with antiferromagnetic interactions
Luigi De Sarlo, Lingxuan Shao, Vincent Corre, Tilman Zibold, David Jacob, Jean Dalibard, Fabrice Gerbier
We study spin fragmentation of an antiferromagnetic spin 1 condensate in the presence of a quadratic Zeeman (QZ) effect breaking spin rotational symmetry. We describe how the QZ effect turns a fragmented spin state, with large fluctuations of the Zeemans populations, into a regular polar condensate, where atoms all condense in the $m=0$ state along the field direction. We calculate the average value and variance of the Zeeman state $m=0$ to illustrate clearly the crossover from a fragmented to an unfragmented state. The typical width of this crossover is $q \sim k_B T/N$, where $q$ is the QZ energy, $T$ the spin temperature and $N$ the atom number. This shows that spin fluctuations are a mesoscopic effect that will not survive in the thermodynamic limit $N\rightarrow \infty$, but are observable for sufficiently small atom number.
July 4
1. arXiv:1307.1207 [pdf, ps, other]
Topological Superfluids with Finite Momentum Pairing and Majorana Fermions
Chunlei Qu, Zhen Zheng, Ming Gong, Yong Xu, Li Mao, Xubo Zou, Guangcan Guo, Chuanwei Zhang
Majorana fermions, quantum particles that are their own anti-particles, are not only of fundamental importance in elementary particle physics and dark matter, but also building blocks for fault-tolerant quantum computation. Recently Majorana fermions have been intensively studied in solid state and cold atomic systems. These studies are generally based on superconducting pairing between two Fermions with opposite momenta (\textit{% i.e.}, zero total momentum). On the other hand, finite total momentum Cooper pairings, known as Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states, were predicted 50 years ago and then widely studied in many branches of physics. However, whether FFLO superconductors can also support Majorana fermions has not been explored. Here we show that Majorana fermions can exist in certain types of gapped FFLO states, yielding a new topological quantum matter: topological FFLO superfluids/superconductors. We demonstrate the existence of such topological FFLO superfluids and the associated Majorana fermions using spin-orbit coupled degenerate Fermi gases and derive their physical parameter regions. The potential implementation of topological FFLO superconductors in semiconductor/superconductor heterostructures are also discussed.
2. arXiv:1307.1180 [pdf, ps, other]
A geometric wave function for few interacting bosons in a harmonic trap
B. Wilson, A. Foerster, C. C. N. Kuhn, I. Roditi, D. Rubeni
We establish a new geometric wave function that combined with a variational principle efficiently describes a system of bosons interacting in a one-dimensional trap. By means of a a combination of the exact wave function solution for contact interactions and the asymptotic behaviour of the harmonic potential solution we obtain the ground state energy, probability density and profiles of a few boson system in a harmonic trap. We are able to access all regimes, ranging from the strongly attractive to the strongly repulsive one with an original and simple formulation.
July 5
1. arXiv:1307.1301 [pdf, ps, other]
Spin Susceptibility and Strong Coupling Effects in an Ultracold Fermi Gas
Hiroyuki Tajima, Ryo Hanai, Ryota Watanabe, Yoji Ohashi
We investigate magnetic properties and strong coupling corrections in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an ultracold Fermi gas. Within the framework of an extended $T$-matrix theory, we calculate the spin susceptibility $\chi$ above the superfluid phase transition temperature $T_{\rm c}$. In the crossover region, the formation of preformed Cooper pairs is shown to cause a non-monotonic temperature dependence of $\chi$, which is similar to the so-called spin-gap phenomenon observed in the under-doped regime of high-$T_{c}$ cuprates. From this behavior of $\chi$, we determine the spin-gap temperature as the temperature at which $\chi$ takes a maximum value, in the BCS-BEC crossover region. Since the spin susceptibility is sensitive to the formation of singlet Cooper pairs, our results would be useful in considering the temperature region where pairing fluctuations are important in the BCS-BEC crossover regime of an ultracold Fermi gas.
2. arXiv:1307.1308 [pdf, other]
Probing Majorana fermions in the tunneling spectra of a resonant level
Richard Korytár, Peter Schmitteckert
Unambiguous identification of Majorana physics presents an outstanding problem whose solution could render topological quantum computing feasible. We develop a numerical approach to treat finite-size superconducting chains supporting Majorana fermions, which is based on iterative application of a two-site Bogoliubov transformation. We demonstrate the applicability of the method by studying a resonant level attached to the superconductor subject to external perturbations. In the topological phase, we show that the spectrum of a single resonant level allows to distinguish peak coming from Majorana physics from the Kondo resonance.
3. arXiv:1307.1442 [pdf, ps, other]
Topological Superconductivity and Majorana Fermions in RKKY Systems
Jelena Klinovaja, Peter Stano, Ali Yazdani, Daniel Loss
We consider quasi one-dimensional RKKY systems in proximity to an s-wave superconductor. We show that a $2k_F$-peak in the spin susceptibility of the superconductor in the one-dimensional limit supports helical order of localized magnetic moments via RKKY interaction, where $k_F$ is the Fermi wavevector. The magnetic helix is equivalent to a uniform magnetic field and very strong spin-orbit interaction (SOI) with an effective SOI length $1/2k_F$. We find the conditions to establish such a magnetic state in atomic chains and semiconducting nanowires with magnetic atoms or nuclear spins. Generically, these systems are in a topological phase with Majorana fermions. The inherent self-tuning of the helix to $2k_F$ eliminates the need to tune the chemical potential.
July 8 - July 12, Saubhik Sarkar
July 8
1.arXiv:1307.1491 [pdf, ps, other]
Long time non-equilibrium dynamics of binary Bose condensates
Stefan S. Natu, S. Das Sarma
We explore the out-of-equilibrium temporal dynamics of demixing and phase separation in a two dimensional binary Bose fluid at zero temperature, following a sudden quench across the miscible-immiscible phase boundary. On short timescales, the system rapidly settles into a steady state characterized by short-range correlations in the relative density. The subsequent dynamics is extremely slow: domains of the relative density appear to grow with time, however, the rate of growth is much slower than that predicted by conventional theories of phase ordering kinetics. Moreover, we find that the growth dynamics slows down with increasing time, and is consistent with logarithmic growth. Our study sheds light on ongoing investigations of how isolated quantum systems approach equilibrium, and indicates that studying the quantum phase diagram of the binary Bose fluids following a quench, may be difficult due to equilibration problems.
2.arXiv:1307.1607 [pdf, ps, other]
Spin-orbit coupled fermions in ladder-like optical lattices at half-filling
G. Sun, J. Jaramillo, L. Santos, T. Vekua
We study the ground-state phase diagram of two-component fermions loaded in a ladder-like lattice at half filling in the presence of spin-orbit coupling. For repulsive fermions with unidirectional spin-orbit coupling along the legs we identify a N\'{e}el state which is separated from rung-singlet and ferromagnetic states by Ising phase transition lines. These lines cross for maximal spin-orbit coupling and a direct Gaussian phase transition between rung-singlet and ferro phases is realized. For the case of Rashba-like spin-orbit coupling, besides the rung singlet phases two distinct striped ferromagnetic phases are formed. In case of attractive fermions with spin-orbit coupling at half-filling for decoupled chains we identify a dimerized state that separates a singlet superconductor and a ferromagnetic states.
July 9
1.arXiv:1307.1704 [pdf, other]
An Exact Classification of Landau-Majorana-Stückelberg-Zener Resonances By Floquet Determinants
Sriram Ganeshan, Edwin Barnes, S. Das Sarma
Recent experiments have shown that Landau-Majorana-St?uckelberg-Zener (LMSZ) interferometry is a powerful tool for demonstrating and exploiting quantum coherence not only in atomic systems but also in a variety of solid state quantum systems such as spins in quantum dots, superconducting qubits, and nitrogen vacancy centers in diamond. In this work, we propose and develop a general (and, in principle, exact) theoretical formalism to identify and characterize the interference resonances that are the hallmark of LMSZ interferometry. Unlike earlier approaches, our scheme does not require any approximations, allowing us to uncover important and previously unknown features of the resonance structure. We also discuss the experimental observability of our results.
2.arXiv:1211.3464 (replaced) [pdf, other]
Matrix Product State Representation without explicit local Hilbert Space Truncation with Applications to the Sub-Ohmic Spin-Boson Model
Max F. Frenzel, Martin B. Plenio
We present an alternative to the conventional matrix product state representation, which allows us to avoid the explicit local Hilbert space truncation many numerical methods employ. Utilising chain mappings corresponding to linear and logarithmic discretizations of the spin-boson model onto a semi-infinite chain, we apply the new method to the sub-ohmic SBM. We are able to reproduce many well-established features of the quantum phase transition, such as the critical exponent 1/2 predicted by mean-field theory. Via extrapolation of finite-chain results, we are able to determine the infinite-chain critical couplings at which the transition occurs and, in general, study the behaviour of the system well into the localised phase.
July 10
1. arXiv:1307.2439 [pdf, other]
Topological Fulde-Ferrel-Larkin-Ovchinnikov states in Spin-orbit Coupled Fermi Gases
Wei Zhang, Wei Yi
Pairing in an attractively interacting two-component Fermi gas in the absence of the inversion symmetry and/or the time-reversal symmetry may give rise to exotic superfluid states. Notable examples range from the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state with a finite center-of-mass momentum in a polarized Fermi gas, to the topological superfluid state in a two-dimensional Fermi gas under Rashba spin-orbit coupling and an out-of-plane Zeeman field. Here, we show that a topological FFLO state can be stabilized in a two-dimensional Fermi gas with Rashba spin-orbit coupling and both in-plane and out-of-plane Zeeman fields. We characterize the topological FFLO state by a non-trivial Berry phase, and demonstrate the stability region of the state on the zero-temperature phase diagram. Given its unique properties in both the quasi-particle dispersion spectra and the momentum distribution, signatures of the topological FFLO state can be detected using existing experimental techniques.
2. arXiv:1307.2256 [pdf, ps, other]
Far from equilibrium topological p-wave superfluids
Matthew S. Foster, Victor Gurarie, Maxim Dzero, Emil A. Yuzbashyan
Recent years have witnessed an explosion in the study of topological superconductors, which carry energy currents along their boundaries in a topologically protected way, while these currents are suppressed in the bulk. Topological superconductors can host Majorana fermions, actively sought-after fractional particles. The simplest superconductor of this kind is a two-dimensional p-wave fermionic superfluid, with order parameter symmetry of the px+ipy type. It was expected that systems of ultracold atoms close to a p-wave Feshbach resonance would form such a superfluid, but these were found to be unstable in experiments. Here we show that if weakly interacting atoms are brought suddenly close ("quenched") to such a Feshbach resonance, in the available time before the instability kicks in, they can form a new type of out-of-equilibrium p-wave superfluid which may retain the topological properties of its equilibrium counterpart. We describe the phase diagram of this superfluid and the conditions under which it can be termed topological. The superfluid produced by this quench turns out to be either time-independent and non-topological or of a topological Floquet type.
July 11
1. arXiv:1307.2699 [pdf, other]
From Classical to Quantum Glasses with Ultracold Polar Molecules
Wolfgang Lechner, Peter Zoller
The dynamics of a bilayer system of ultracold dipolar molecules exhibits classical and quantum glassy behavior, characterized by long tails in the relaxation time and dynamical heterogeneity. In the proposed setup, quantum fluctuations are of the order of thermal fluctuations and the degree of frustration can tuned by the interlayer distance. We present experimental accessible order parameters based on marker molecules, distinguished by properly chosen internal levels, and find quantum features of dynamical heterogeneity.
2. arXiv:1307.2654 (cross-list from physics.atom-ph) [pdf, ps, other]
Long range interactions of ytterbium in mixed quantum gases
S.G. Porsev, M.S. Safronova, A. Derevianko, Charles W. Clark
We present methods for accurate evaluation of van der Waals coefficients of dimers with excited atoms that have a strong decay channel. We calculate C_6 coefficients for the Yb-Yb 1S_0+ 3P_{0,1}, 3P_0+3P_0 and Yb-Rb 3P_1+5s, 1S_0+ 5p_{1/2} dimers and C_8 coefficients for the Yb-Yb 1S_0+1S_0, 1S_0+3P_1 and Yb-Rb 1S_0+5s dimers. We evaluate uncertainties of all properties.
July 12
1. arXiv:1307.3193 [pdf, other]
Stability of a unitary Bose gas
Richard J. Fletcher, Alexander L. Gaunt, Nir Navon, Robert P. Smith, Zoran Hadzibabic
We study the stability of a thermal $^{39}$K Bose gas across a broad Feshbach resonance, focusing on the unitary regime, where the scattering length $a$ exceeds the thermal wavelength $\lambda$. We measure the general scaling laws relating the particle-loss and heating rates to the temperature, scattering length, and atom number. Both at unitarity and for positive $a \ll \lambda$ we find agreement with three-body theory. However, for $a<0$ and away from unitarity, we observe significant four-body decay. At unitarity, the three-body loss coefficient, $L_3 \propto \lambda^4$, is three times lower than the universal theoretical upper bound. This reduction is a consequence of species-specific Efimov physics and makes $^{39}$K particularly promising for studies of many-body physics in a unitary Bose gas.
2. arXiv:1307.2910 [pdf, ps, other]
Absence of damping of low energy excitations in a quasi-2D dipolar Bose gas
Stefan S. Natu, S. Das Sarma
We develop a theory of damping of low energy, collective excitations in a quasi-2D, homogenous, dipolar Bose gas at zero temperature, via processes whereby an excitation decays into two excitations with lower energy. We find that owing to the nature of the low energy spectrum of a quasi-2D dipolar gas, such processes cannot occur unless the momentum of the incoming quasi-particle exceeds a critical value k_{crit}. We find that as the dipolar interaction strength is increased, this critical value shifts to larger momenta. Our predictions can be directly verified in current experiments on dipolar Bose condensates using Bragg spectroscopy, and provide valuable insight into the quantum many-body physics of dipolar gases.
July 15 - July 19, Xiaopeng Li
Jul 19
1. arXiv:1307.5047 [pdf, ps, other]
Fractionalized excitations in the spin liquid state of a kagomé lattice antiferromagnet
Tian-Heng Han, Joel S. Helton, Shaoyan Chu, Daniel G. Nocera, Jose A. Rodriguez-Rivera, Collin Broholm, Young S. Lee
New physics can emerge in magnetic materials where quantum fluctuations are enhanced due to reduced dimensionality and strong frustration. One long sought example is the resonating-valence-bond (RVB) state, where atomic magnetic moments are strongly correlated but do not order or freeze even in the limit of T -> 0. The RVB ground state does not break conventional symmetries, such as lattice translation or spin-rotation. The realization of such a quantum spin liquid in two-dimensions would represent a new state of matter. It is believed that spin liquid physics plays a role in the phenomenon of high-Tc superconductivity, and the topological properties of the spin liquid state may have applications in the field of quantum information. We present neutron scattering measurements of the spin excitations on single crystal samples of the spin-1/2 kagom\'{e} lattice antiferromagnet ZnCu3(OD)6Cl2 (also called herbertsmithite). Our observation of a spinon continuum in a two-dimensional magnet is remarkable first. The results serve as a key fingerprint of the quantum spin liquid state in herbertsmithite.
2. arXiv:1307.4982 [pdf, ps, other]
Efficient variational approach to the impurity problem and its application to the dynamical mean-field theory
Chungwei Lin, Alexander A. Demkov
Within the framework of exact diagonalization (ED), we compute the ground state of Anderson impurity problem using the variational approach based on the configuration interaction (CI) expansion. We demonstrate that an accurate ground state can be obtained by iteratively diagonalizing a matrix with the dimension that is less than 10$%$ of the full Hamiltonian. The efficiency of the CI expansion for different problems is analyzed. By way of example, we apply this method to the single-site dynamical mean field theory using ED as the impurity solver. Specifically, to demonstrate the usefulness of this approach, we solve the attractive Hubbard model in the grand-canonical ensemble, where the s-wave superconducting solution is explicitly obtained.
Jul 18
1. arXiv:1307.4597 [pdf, other]
Anisotropic Quantum Spin Hall Effect, Spin-Orbital Textures and Mott Transition
Tianhan Liu, Benoît Douçot, Karyn Le Hur
We investigate the interplay between topological effects and Mott physics in two dimensions on a graphene-like lattice, via a tight-binding model containing an anisotropic spin-orbit coupling on the next-nearest-neighbour links and the Hubbard interaction. We thoroughly analyze the resulting phases, namely a topological band insulator phase or anisotropic quantum Spin Hall phase until moderate interactions, a N\'eel and Spiral phase at large interactions in the Mott regime, as well as the formation of a spin-orbital texture in the bulk at the Mott transition. The emergent magnetic orders at large interactions are analyzed through a spin wave analysis and mathematical arguments. At weak interactions, by analogy with the Kane-Mele model, the system is described through a Z_2 topological invariant. In addition, we describe how the anisotropic spin-orbit coupling already produces an exotic spin texture at the edges. The physics at the Mott transition is described in terms of a U(1) slave rotor theory. Taking into account gauge fluctuations around the mean-field saddle point solution, we show how the spin texture now proliferates into the bulk above the Mott critical point. The latter emerges from the response of the spinons under the insertion of monopoles and this becomes more pronounced as the spin-orbit coupling becomes prevalent. We discuss implications of our predictions for thin films of the iridate compound Na2IrO3 and also graphene-like systems.
2. arXiv:1307.4403 [pdf, other]
Universal topological quantum computation from a superconductor/Abelian quantum Hall heterostructure
Roger S. K. Mong, David J. Clarke, Jason Alicea, Netanel H. Lindner, Paul Fendley, Chetan Nayak, Yuval Oreg, Ady Stern, Erez Berg, Kirill Shtengel, Matthew P. A. Fisher
Non-Abelian anyons promise to reveal spectacular features of quantum mechanics that could ultimately provide the foundation for a decoherence-free quantum computer. A key breakthrough in the pursuit of these exotic particles originated from Read and Green's observation that the Moore-Read quantum Hall state and a (relatively simple) two-dimensional p+ip superconductor both support so-called Ising non-Abelian anyons. Here we establish a similar correspondence between the Z_3 Read-Rezayi quantum Hall state and a novel two-dimensional superconductor in which charge-2e Cooper pairs are built from fractionalized quasiparticles. In particular, both phases harbor Fibonacci anyons that---unlike Ising anyons---allow for universal topological quantum computation solely through braiding. Using a variant of Teo and Kane's construction of non-Abelian phases from weakly coupled chains, we provide a blueprint for such a superconductor using Abelian quantum Hall states interlaced with an array of superconducting islands. Fibonacci anyons appear as neutral deconfined particles that lead to a two-fold ground-state degeneracy on a torus. In contrast to a p+ip superconductor, vortices do not yield additional particle types yet depending on non-universal energetics can serve as a trap for Fibonacci anyons. These results imply that one can, in principle, combine well-understood and widely available phases of matter to realize non-Abelian anyons with universal braid statistics. Numerous future directions are discussed, including speculations on alternative realizations with fewer experimental requirements.
Jul 17
1. arXiv:1307.4263 [pdf, ps, other]
Universal Bose Gases Near Resonance: A Rigorous Solution
Shao-Jian Jiang, Wu-Ming Liu, G. W. Semenoff, Fei Zhou
Bose gases at large scattering lengths or near resonances, where the standard dilute gas theory breaks down, have posed a long-standing challenge for quantum many-body physics. In this Letter, we offer an answer to this enigmatic topic of quantum gases at large scattering lengths. We should outline a rigorous solution to Bose gases near resonance which we obtain by using an epsilon expansion near four spatial dimension. In dimension d = 4 - epsilon, the chemical potential of Bose gases near resonances is shown to approach the universal value epsilon^(2/(4-epsilon)) epsilon_F sqrt(2/3) (1 + 0.474 epsilon - i 1.217 epsilon + ...), where epsilon_F is the Fermi energy defined for a Fermi gas of density n, and the condensation fraction is equal to 2/3 (1 + 0.0877 epsilon + ...). This is in contrast to three spatial dimension where the chemical potential further depends on a non-universal ultraviolet length scale due to Efimov physics.
2. arXiv:1307.4185 [pdf, ps, other]Squeezing light with Majorana fermions
Audrey Cottet, Takis Kontos, Benoit Douçot
Coupling a semiconducting nanowire to a microwave cavity provides a powerfull means to assess about the presence or absence of isolated Majorana fermions in the nanowire. These exotic bound states can cause a significant cavity frequency shift but also a strong cavity nonlinearity leading for instance to light squeezing. The dependence of these effects on the nanowire gate voltages gives direct signatures of the unique properties of Majorana fermions, such as their self-adjoint character and their exponential confinement.
3. arXiv:1307.4092 [pdf, other]
Localization and topology protected quantum coherence at the edge of 'hot' matter
Yasaman Bahri, Ronen Vosk, Ehud Altman, Ashvin Vishwanath
Topological phases are often characterized by special edge states confined near the boundaries by an energy gap in the bulk. On raising temperature, these edge states are lost in a clean system due to mobile thermal excitations. Recently however, it has been established that disorder can localize an isolated many body system, potentially allowing for a sharply defined topological phase even in a highly excited state. Here we show this to be the case for the topological phase of a one dimensional magnet with quenched disorder, which features spin one-half excitations at the edges. The time evolution of a simple, highly excited, initial state is used to reveal quantum coherent edge spins. In particular, we demonstrate, using theoretical arguments and numerical simulation, the coherent revival of an edge spin over a time scale that grows exponentially bigger with system size. This is in sharp contrast to the general expectation that quantum bits strongly coupled to a 'hot' many body system will rapidly lose coherence.
Jul 16
1. arXiv:1307.3744 [pdf, ps, other]
Topological Fulde-Ferrell superfluid in spin-orbit coupled atomic Fermi gases
Xia-Ji Liu, Hui Hu
We theoretically predict a new topological matter - topological inhomogeneous Fulde-Ferrell superfluid - in one-dimensional atomic Fermi gases with equal Rashba and Dresselhaus spin-orbit coupling near s-wave Feshbach resonances. The realization of such a spin-orbit coupled Fermi system has already been demonstrated recently by using a two-photon Raman process and the extra one-dimensional confinement is easy to achieve using a tight two-dimensional optical lattice. The topological Fulde-Ferrell superfluid phase is characterized by a nonzero center-of-mass momentum and a non-trivial Berry phase. By tuning the Rabi frequency and the detuning of Raman laser beams, we show that such an exotic topological phase occupies a significant part of parameter space and therefore it could be easily observed experimentally, by using, for example, momentum-resolved and spatially resolved radio-frequency spectroscopy.
2. arXiv:1307.3738 [pdf, other]
Pre-thermalization in a non-integrable quantum spin chain after a quench
Matteo Marcuzzi, Jamir Marino, Andrea Gambassi, Alessandro Silva
We study the dynamics of a quantum Ising chain after the sudden introduction of a non-integrable long-range interaction. Via an exact mapping onto a fully-connected lattice of hard-core bosons, we show that a pre-thermal state emerges and we investigate its properties by focusing on a class of physically relevant observables. We show that some features of this quasi-stationary state are captured by perturbation theory and that the corrections to the statistical distribution function are unexpectedly independent of the interaction strength, whereas actual observables are perturbatively modified.
3. arXiv:1307.3697 [pdf, other]Weyl semimetals and superconductors designed in an orbital selective superlattice
Tanmoy Das
We propose two complementary design principles for engineering three-dimensional (3D) Weyl semimetals and superconductors in a layer-by-layer setup which includes even and odd parity orbitals in alternating layers - dubbed orbital selective superlattice. Such structure breaks mirror symmetry along the superlattice growth axis which, with the help of either a basal plane spin-orbit coupling or a spinless p+ip superconductivity, stabilizes a 3D Dirac node. To explore this idea, we develop a 3D generalization of Haldane model and a Bogoliubov-de-Gennes (BdG) Hamiltonian for the two cases, respectively, and show that a tunable single or multiple Weyl nodes with linear dispersion in all spatial directions can be engineered desirably in a widespread parameter space. We also demonstrate that a single helical Weyl band can be created at the $\Gamma$-point at the Fermi level in the superconducting case via gapping out either of the orbital state by violating its particle-hole symmetry but not any other symmetries. Finally, implications of our results for the realization of anomalous Hall effect and Majorana bound state are discussed.
Jul 15
1.arXiv:1307.3245 [pdf, ps, other]
Algebra of Majorana Doubling
Jaehoon Lee, Frank Wilczek
Motivated by the problem of identifying Majorana mode operators at junctions, we analyze a basic algebraic structure leading to a doubled spectrum. In general the emergent mode creation operator is highly non-linear in the original effective mode operators, and therefore also in the underlying electron creation and destruction operators. We briefly compare and contrast related issues in the Pfaffian quantum Hall state.
July 22 - July 26, Bo Liu
Jul 26
1. arXiv:1307.6813 [pdf, ps, other]
Contact parameters in two dimensions for general three-body systems
F. F. Bellotti, T. Frederico, M. T. Yamashita, D. V. Fedorov, A. S. Jensen, N. T. Zinner
We study the two dimensional three-body problem in the general case of three distinguishable particles interacting through zero-range potentials. The Faddeev decomposition is used to write the momentum-space wave function. We show that the large-momentum asymptotic spectator function has the same functional form as derived previously for three identical particles. We derive analytic relations between the three different Faddeev components for three distinguishable particles. We investigate the one-body momentum distributions both analytically and numerically and analyze the tail of the distributions to obtain two- and three-body contact parameters. We specialize from the general cases to examples of two identical, interacting or non-interacting, particles. We find that the two-body contact parameter is not a universal constant in the general case and show that the universality is recovered when a subsystem is composed of two identical non-interacting particles. We also show that the three-body contact parameter is negligible in the case of one non-interacting subsystem compared to the situation where all subsystem are bound. As example, we present results for mixtures of Lithium with two Cesium or two Potassium atoms, which are systems of current experimental interest.
Jul 25
1. arXiv:1307.6395 [pdf, ps, other]
Spin Drag of a Fermi Gas in a Harmonic Trap
Olga Goulko, Frédéric Chevy , Carlos Lobo
Using a Boltzmann equation approach, we analyze how the spin drag of a trapped interacting fermionic mixture is influenced by the non-homogeneity of the system in a classical regime where the temperature is much larger than the Fermi temperature. We show that for very elongated geometries, the spin damping rate can be related to the spin conductance of an infinitely long cylinder. We characterize analytically the spin conductance both in the hydrodynamic and collisionless limits and discuss the influence of the velocity profile. Our results are in good agreement with recent experiments and provide a quantitative benchmark for further studies of spin drag in ultracold gases.
Jul 24
1. arXiv:1307.6128 [pdf, ps, other]
Dynamically generated flat-band phases in optical kagome lattices
Gia-Wei Chern, Chih-Chun Chien, Massimiliano Di Ventra
We suggest that a dynamically generated flat-band insulator can be realized using ultracold fermions loaded in an optical kagome lattice by depleting the mobile atoms at one edge of the lattice with a focused laser beam. Since the flat band of the kagome lattice is a high-energy one compared to the dispersive bands, this dynamically generated flat-band insulator is a population-inversion phase with no pumping required to maintain it after its formation. We also show that in a similar setup a dynamical stripe phase emerges in the flat band when two-component fermions with weakly repulsive interactions evolve in a static kagome lattice or even in the absence of interactions when the optical lattice is modulated. Given the broad variety of lattice geometries supporting frustration-induced localized states, our work opens the door to atomtronic devices utilizing geometrical effects and offers new insight on the dynamics of geometrically frustrated systems.
2. arXiv:1307.6004 [pdf, other]
Time-of-Flight Roton Spectroscopy in Dipolar Bose-Einstein Condensates
M. Jona-Lasinio, K. Łakomy, L. Santos
Dipolar Bose-Einstein condensates may present a rotonlike dispersion minimum, which is yet to be observed in experiments. We discuss a simple method to reveal roton excitations, based on the response of quasi-2D dipolar condensates against a weak lattice potential. By employing numerical simulations for realistic scenarios, we analyze the response of the system as a function of both the lattice spacing and the s-wave scattering length, showing that the roton minimum may be readily revealed in current experiments by the resonant population of Bragg peaks in time-of-flight measurements.
Jul 23
1. arXiv:1307.5794 [pdf, ps, other]
Quantum Monte Carlo study of the three-dimensional spin-polarized homogeneous electron gas
G G Spink, R J Needs, N D Drummond
We have studied the spin-polarized three-dimensional homogeneous electron gas using the diffusion quantum Monte Carlo method, with trial wave functions including backflow and three-body correlations in the Jastrow factor, and we have used twist averaging to reduce finite-size effects. Calculations of the pair correlation function, including the on-top pair density, as well as the structure factor and the total energy, are reported for systems of 118 electrons in the density range $r_\text{s}=0.5$--20 a.u., and for spin polarizations of 0, 0.34, 0.66, and 1. We consider the spin resolution of the pair correlation function and structure factor, and the energy of spin polarization. We show that a control variate method can reduce the variance when twist-averaging, and we have achieved higher accuracy and lower noise than earlier quantum Monte Carlo studies.
Jul 22
1. arXiv:1307.5301 [pdf, other]Gauge turbulence, topological defect dynamics, and condensation in Higgs models
Thomas Gasenzer, Larry McLerran, Jan M. Pawlowski, Dénes Sexty
The real-time dynamics of topological defects and turbulent configurations of gauge fields for electric and magnetic confinement are studied numerically within a 2+1D Abelian Higgs model. It is shown that confinement is appearing in such systems equilibrating after a strong initial quench such as the overpopulation of the infrared modes. While the final equilibrium state does not support confinement, metastable vortex defect configurations appear in the gauge field which are found to be closely related to the appearance of physically observable confined electric and magnetic charges. These phenomena are seen to be intimately related to the approach of a non-thermal fixed point of the far-from-equilibrium dynamical evolution, signalled by universal scaling in the gauge-invariant correlation function of the Higgs field. Even when the parameters of the Higgs action do not support condensate formation in the vacuum, during this approach, transient Higgs condensation is observed. We discuss implications of these results for the far-from-equilibrium dynamics of Yang-Mills fields and potential mechanisms how confinement and condensation in non-abelian gauge fields can be understood in terms of the dynamics of Higgs models. These suggest that there is an interesting new class of dynamics of strong coherent turbulent gauge fields with condensates.
