Oct 31 - Nov 4, Johannes Schachenmayer
Nov 4
1. arXiv:1111.0798 [pdf, ps, other]
Title: Fractional quantum Hall states of few bosonic atoms in geometric gauge fields
B. Juliá-Díaz, T. Graß, N. Barberán, M. Lewenstein
We employ the exact diagonalization method to analyze the possibility of generating strongly correlated states in two-dimensional clouds of ultracold bosonic atoms which are subjected to a geometric gauge field created by coupling two internal atomic states to a laser beam. Tuning the gauge field strength, the system undergoes stepwise transitions between different ground states, which we describe by analytical trial wave functions, amongst them the Pfaffian, the Laughlin, and a Laughlin quasiparticle many-body state. The adiabatic following of the center of mass movement by the lowest energy dressed internal state, is lost by the mixing of the second internal state. This mixture can be controlled by the intensity of the laser field. The non-adiabaticity is inherent to the considered setup, and is shown to play the role of circular asymmetry. We study its influence on the properties of the ground state of the system. Its main effect is to reduce the overlap of the numerical solutions with the analytical trial expressions by occupying states with higher angular momentum. Thus, we propose generalized wave functions arising from the Laughlin and Pfaffian wave function by including components, where extra Jastrow factors appear, while preserving important features of these states. We analyze quasihole excitations over the Laughlin and generalized Laughlin states, and show that they possess effective fractional charge and obey anyonic statistics. Finally, we study the energy gap over the Laughlin state as the number of particles is increased keeping the chemical potential fixed. The gap is found to decrease as the number of particles is increased, indicating that the observability of the Laughlin state is restricted to a small number of particles.
2. arXiv:1111.0778 [pdf, other]
Title: Non-local Order in Elongated Dipolar Gases
Jonathan Ruhman, Emanuele G. Dalla Torre, Sebastian D. Huber, Ehud Altman
Dipolar particles in an elongated trap are expected to undergo a quantum phase transition from a linear to a zigzag structure with decreasing transverse confinement. We derive the low energy effective theory of the transition showing that in presence of quantum fluctuations the Zigzag phase can be characterized by a long ranged string order, while the local Ising correlations decay as a power law. This is also confirmed using DMRG calculations on a microscopic model. The non local order in the bulk gives rise to zero energy states localized at the interface between the ordered and disordered phases. Such an interface naturally arises when the particles are subject to a weak harmonic confinement along the tube axis. We compute the signature of the edge states in the single particle tunneling spectra pointing to differences between a system with bosonic versus fermionic particles. Finally we asses the magnitude of the relevant quantum fluctuations in realistic systems of dipolar particles, including ultracold polar molecules as well as alkali atoms weakly dressed by a Rydberg excitation.
3. arXiv:1111.0776 [pdf, other]
Title: Light-cone-like spreading of correlations in a quantum many-body system
Marc Cheneau, Peter Barmettler, Dario Poletti, Manuel Endres, Peter Schauß, Takeshi Fukuhara, Christian Gross, Immanuel Bloch, Corinna Kollath, Stefan Kuhr
How fast can correlations spread in a quantum many-body system? Based on the seminal work by Lieb and Robinson, it has recently been shown that several interacting many-body systems exhibit an effective light cone that bounds the propagation speed of correlations. The existence of such a "speed of light" has profound implications for condensed matter physics and quantum information, but has never been observed experimentally. Here we report on the time-resolved detection of propagating correlations in an interacting quantum many-body system. By quenching a one-dimensional quantum gas in an optical lattice, we reveal how quasiparticle pairs transport correlations with a finite velocity across the system, resulting in an effective light cone for the quantum dynamics. Our results open important perspectives for understanding relaxation of closed quantum systems far from equilibrium as well as for engineering efficient quantum channels necessary for fast quantum computations.
4. arXiv:1111.0941 [pdf, ps, other]
Title: Few-body physics with ultracold atomic and molecular systems in traps
D. Blume
Few-body physics has played a prominent role in atomic, molecular and nuclear physics since the early days of quantum mechanics. It is now possible---thanks to tremendous progress in cooling, trapping, and manipulating ultracold samples---to experimentally study few-body phenomena in trapped atomic and molecular systems with unprecedented control. This review summarizes recent studies of few-body phenomena in trapped atomic and molecular gases, with an emphasis on small trapped systems. We start by introducing the free-space scattering properties and then investigate what happens when two particles, bosons or fermions, are placed in an external confinement. Next, various three-body systems are treated analytically in limiting cases. Our current understanding of larger two-component Fermi systems and Bose systems is reviewed, and connections with the corresponding bulk systems are established. Lastly, future prospects and challenges are discussed. Throughout this review, commonalities with other systems such as nuclei or quantum dots are highlighted.
Nov 3
1. arXiv:1111.0578 [pdf, other]
Title: Dispersion and wavefunction symmetry in cold atoms experiencing artificial gauge fields
Yariv Yanay, Erich Mueller
We analyze the single particle quantum mechanics of an atom whose dispersion is modified by spin orbit coupling to Raman lasers. We calculate how the novel dispersion leads to unusual single particle physics. We focus on the symmetry of the ground state wavefunction in different potentials.
2. arXiv:1111.0550 [pdf, ps, other]
Title: Phase transitions in dipolar gases in optical lattices
Y. Sherkunov, Vadim V. Cheianov, Vladimir Fal'ko
We investigate the phase diagrams of two-dimensional lattice dipole systems with variable geometry. For bipartite square and triangular lattices with tunable vertical sublattice separation, we find rich phase diagrams featuring a sequence of easy-plane magnetically ordered phases separated by incommensurate spin-wave states.
3. arXiv:1111.0529 (cross-list from quant-ph) [pdf, ps, other]
Title: Coherent control of atomic spin currents in a double well
H. T. Ng, Shih-I Chu
We propose an experimental feasible method for controlling the atomic currents of a two-component Bose-Einstein condensate in a double well by applying an external field to the atoms in one of the potential wells. We study the ground-state properties of the system and show that the directions of spin currents and net-particle tunneling can be manipulated by adiabatically varying the coupling strength between the atoms and the field. This system can be used for studying spin and tunneling phenomena across a wide range of interaction parameters. In addition, spin-squeezed states can be generated. It is useful for quantum information processing and quantum metrology.
Nov 2
1. arXiv:1111.0227 [pdf, ps, other]
Title: Resonant control of polar molecules in an optical lattice
Thomas M. Hanna, Eite Tiesinga, William F. Mitchell, Paul S. Julienne
We study the resonant control of two nonreactive polar molecules in an optical lattice site, focussing on the example of RbCs. Collisional control can be achieved by tuning bound states of the intermolecular dipolar potential, by varying the applied electric field or trap frequency. We consider a wide range of electric fields and trapping geometries, showing that a three-dimensional optical lattice allows for significantly wider avoided crossings than free space or quasi-two dimensional geometries. Furthermore, we find that dipolar confinement induced resonances can be created with reasonable trapping frequencies and electric fields, and have widths that will enable useful control in forthcoming experiments.
2. arXiv:1111.0239 (cross-list from cond-mat.mes-hall) [pdf, other]
Title: Projective dynamics on the dice lattice
Gunnar Moller, Nigel R Cooper
We study correlated phases occurring in the flat lowest band of the dice lattice model at flux density one half. We discuss how to realize this model in cold atomic gases. We construct the projection of the model to the lowest dice band, which yields a Hubbard-Hamiltonian with interaction-assisted hopping processes. We solve this model for bosons in two limits. In the limit of large density, we use Gross-Pitaevskii mean-field theory to reveal time-reversal symmetry breaking vortex lattices phases. At low density, we use exact diagonalization to identify three stable phases at fractional filling factors $\nu$ of the lowest band, including a classical crystal at \nu=1/3, a supersolid state at \nu=1/2 and a Mott insulator at \nu=1.
Nov 1
1. arXiv:1110.6466 [pdf, ps, other]
Title: Novel Polaron State for Single Impurity in a Bosonic Mott Insulator
Yasuyuki Kato, K. A. Al-Hassanieh, A. E. Feiguin, Eddy Timmermans, C. D. Batista
We show that a single impurity embedded in a cold atom bosonic Mott insulator leads to a novel polaron that exhibits correlated motion with an effective mass and a linear size that nearly diverge at critical value of the on-site impurity-boson interaction strength. Cold atom technology can tune the polaron's properties and break up the composite particle into a deconfined impurity-hole and boson particle state at finite, controllable polaron momentum.
2. arXiv:1110.6829 [pdf, other]
Title: Efimov Effect for P-wave Interactions
Eric Braaten, P. Hagen, H.-W. Hammer, L. Platter
Nonrelativistic particles with short-range interactions that produce a P-wave threshold resonance can exhibit the Efimov effect: if the inverse scattering volume 1/a_P and the P-wave effective range r_P are simultaneously tuned to zero, there is an infinite sequence of three-body bound states called Efimov states that have an accumulation point at the threshold. The discrete scaling factor that characterizes the Efimov effect depends on the mass ratios and the symmetries of the three particles. There is no Efimov effect if all three particles are identical, but it can occur if two identical particles have a resonant P-wave interaction with a third particle. The spectrum of Efimov trimers is compatible with discrete scale invariance. The Efimov trimers disappear through the three-particle threshold at values of a_P and r_P that differ by appropriate powers of the discrete scaling factor.
3. arXiv:1110.6524 [pdf, other]
Title: Fractional quantum Hall states of photons in an array of dissipative coupled cavities
R. O. Umucalilar, I. Carusotto
The study of the physics of quantum fluids of light has been pioneered by the recent observation of superfluid hydrodynamics effects in a Bose condensed gas of dressed photons confined in a planar semiconductor microcavity. All these experiments were performed in a dilute regime where a mean-field description based on a generalized Gross-Pitaevskii equation is accurate. Strong optical nonlinearities, on the other hand, have been anticipated to drive the system into a strongly correlated state, e.g. a Mott insulator in an array of coupled cavities or a Tonks-Girardeau gas of impenetrable photons in a one-dimensional geometry. Here we investigate the even richer variety of strongly correlated states that arise when an artificial magnetic field is imposed on strongly interacting photons in a coupled cavity array and we point out unambiguous signatures of photon states that are the optical analogs of fractional quantum Hall states of electrons in strong magnetic fields.
Oct 31
1. arXiv:1110.6415 [pdf, ps, other]
Title: Repulsive polarons in two-dimensional Fermi gases
V. Ngampruetikorn, J. Levinsen, Meera M. Parish
We consider a single spin-down impurity atom interacting via an attractive, short-range potential with a spin-up Fermi sea in two dimensions (2D). Similarly to 3D, we show that the impurity can form a metastable state (the "repulsive polaron") with energy greater than that of the non-interacting impurity. Moreover, we find that the repulsive polaron can acquire a finite momentum for sufficiently weak attractive interactions. Even though the energy of the repulsive polaron can become sizeable, we argue that saturated ferromagnetism is unfavorable in 2D because of the polaron's finite lifetime and small quasiparticle weight.
2. arXiv:1110.6364 [pdf, ps, other]
Title: Artificial spin-orbit coupling in ultra-cold Fermi superfluids
Kangjun Seo, Li Han, C. A. R. Sá de Melo
We develop a theory for interacting fermions in the presence of spin-orbit coupling and Zeeman fields, and show that many new superfluids phases, which are topological in nature, emerge. Depending on values of spin-orbit coupling, Zeeman fields, and interactions, initially gapped s-wave superfluids acquire p-wave, d-wave, f-wave and higher angular momentum components, which produce zeros in the excitation spectrum, rendering the superfluid gapless. Several multi-critical points, which separate topological superfluid phases from normal or non-uniform, are accessible depending on spin-orbit coupling, Zeeman fields or interactions, setting the stage for the study of tunable topological superfluids.
Oct 24 - Oct 28, Saubhik Sarkar
Oct 28
1. arXiv:1110.5926 [pdf, ps, other]
Title: "Hard probes" of strongly-interacting atomic gases
Yusuke Nishida
We investigate properties of an energetic atom propagating through strongly-interacting atomic gases. We use operator product expansions to systematically compute quasiparticle energies and its scattering rates both in two-component Fermi gas and identical Bose gas. Reasonable agreement with recent quantum Monte Carlo simulations even at relatively small momentum k/kF>1.5 indicates that our large momentum expansions are valid in a wide range of momentum. We also study differential scattering rates when a third species of atom is shot into atomic gases. Because the number density of the target atomic gas contributes to the forward scattering only, its contact density gives the leading contribution to the backward scattering. Therefore, such an experiment can be used to measure the contact density and thus provides a new local probe of strongly-interacting atomic gases.
Oct 27
1. arXiv:1110.5854 [pdf, ps, other]
Title: The Fate of the Efimov Effect in a Many-Body World
Nicolai Gayle Nygaard, Nikolaj Thomas Zinner
The stabilization of Cooper pairs of bound electrons in the background of a Fermi sea is the origin of superconductivity and the paradigmatic example of the striking influence of many-body physics on few-body properties. In the quantum-mechanical three-body problem the famous Efimov effect defines highly non-trivial scaling relations among a tower of universal states. Within the realm of ultracold atomic gases, both of these problems have been studied in great detail within the last decade. In light of the tremendous effect of a background Fermi sea on two-body properties, a natural question is how such a background modifies three-body physics. Here we demonstrate how the generic problem of three interacting particles changes when one of the particles is embedded in a background Fermi sea. Our results show that there is a new universal scaling behavior connected with the background density of fermionic particles. The novel method presented here is completely general and applies to arbitrary masses of the three particles. Furthermore, it can accommodate corrections to universality, more than one Fermi sea, and many-body corrections in a systematic manner.
2. arXiv:1110.5444 [pdf, other]
Title: Distinguishing mesoscopic quantum superpositions from statistical mixtures in periodically shaken double wells
Christoph Weiss
For Bose-Einstein condensates in double wells, N-particle Rabi-like oscillations often seem to be damped. Far from being a decoherence effect, the apparent damping can indicate the emergence of quantum superpositions in the many-particle quantum dynamics. However, in an experiment it would be difficult to distinguish the apparent damping from decoherence effects. The present paper suggests using controlled periodic shaking to quasi-instantaneously switch the sign of an effective Hamiltonian, thus implementing an `echo' technique which distinguishes quantum superpositions from statistical mixtures. The scheme for the effective time-reversal is tested by numerically solving the time-dependent N-particle Schrodinger equation.
3. arXiv:1110.5705 [pdf, ps, other]
Title: Dynamically generating arbitrary spin-orbit couplings for neutral atoms
Z. F. Xu, L. You
Spin-orbit coupling (SOC) is responsible for interesting physics, from spin Hall to topological insulators, in condensed matter systems. This topical areas of research is transformed into atomic quantum gases with artificial/synthetic gauge potentials. The prospects of tunable interactions and quantum state controls promote neutral atoms as nature's quantum emulators for SOC. Y.-J. Lin {\it et al.} recently demonstrated a special form of the SOC $k_x\sigma_y$: an equal superposition of Rashba and Dresselhaus couplings, in bose condensed atoms [Nature (London) \textbf{471}, 83 (2011)]. Based on their breakthrough, we report an idea of implementing arbitrary forms of SOC relying on coherent control methods adapted to atomic pseudo-spins with simple laser pulses. Our scheme involves no added complication, can be demonstrated within current experimental setups, and is equally applicable to bosonic or fermionic atoms.
Oct 26
1. arXiv:1110.5330 [pdf, other]
Title: d-Wave Superfluidity in Optical Lattices of Ultracold Polar Molecules
Kevin A. Kuns, Ana Maria Rey, Alexey V. Gorshkov
Recent work on ultracold polar molecules, governed by a generalization of the t-J Hamiltonian, suggests that molecules may be better suited than atoms for studying d-wave superfluidity due to stronger interactions and larger tunability of the system. We compute the phase diagram for polar molecules in a checkerboard lattice consisting of weakly coupled square plaquettes. In the simplest experimentally realizable case where there is only tunneling and an XX-type spin-spin interaction, we identify the parameter regime where d-wave superfluidity occurs. We also find that the inclusion of a density-density interaction destroys the superfluid phase and that the inclusion of a spin-density or an Ising-type spin-spin interaction can enhance the superfluid phase. We also propose schemes for experimentally realizing the perturbative calculations exhibiting enhanced d-wave superfluidity.
Oct 25
1. arXiv:1110.5314 [pdf, other]
Title: Experimental realization of strong effective magnetic fields in an optical lattice
Monika Aidelsburger, Marcos Atala, Sylvain Nascimbène, Stefan Trotzky, Yu-Ao Chen, Immanuel Bloch
We use Raman-assisted tunneling in an optical superlattice to generate large tunable effective magnetic fields for ultracold atoms. When hopping in the lattice, the accumulated phase shift by an atom is equivalent to the Aharonov-Bohm phase of a charged particle exposed to a staggered magnetic field of large magnitude, on the order of one flux quantum per plaquette. We study the ground state of this system and observe that the frustration induced by the magnetic field can lead to a degenerate ground state for non-interacting particles. We provide a measurement of the local phase acquired from Raman-induced tunneling, demonstrating time-reversal symmetry breaking of the underlying Hamiltonian. Furthermore, the quantum cyclotron orbit of single atoms in the lattice exposed to the magnetic field is directly revealed.
2. arXiv:1110.5261 [pdf, other]
Title: Commensurate Supersolid of Three-Dimensional Lattice Bosons
Takahiro Ohgoe, Takafumi Suzuki, Naoki Kawashima
Using quantum Monte Carlo simulations, we show that a perfect {\it commensurate} checkerboard supersolid is stable in the soft-core Bose-Hubbard model with nearest-neighbor repulsions on a cubic lattice. In conventional cases, supersolids are realized by doped-defect-condensation mechanism where doped bosons or holes into a perfect crystal act as interstitials or vacancies, delocalizing in the crystal order. However, in the above model, a supersolid state is stabilized even at the commensurate filling 1/2 {\it in the absence of doping}. By our grand canonical simulations, we obtain a ground-state phase diagram that suggests the existence of supersolid at commensurate density (commensurate supersolid). In order to obtain direct evidence of the commensurate supersolid, we perform simulation at a fixed density $\rho=1/2$ and deny a possibility of phase separations. From snapshots, we confirm that the commensurate supersolid is realized by unconventional mechanism where interstitial-vacancy pairs are created in a perfect crystal and separate from each other, delocalizing in the crystal order.
3. arXiv:1110.5192 [pdf, other]
Title: Optimized Bose-Einstein-condensate production in a dipole trap based on a 1070-nm multifrequency laser: Influence of enhanced two-body loss on the evaporation process
Thomas Lauber, Johannes Kueber, Oliver Wille, Gerhard Birkl
We present an optimized strategy for the production of tightly confined Bose-Einstein condensates (BEC) of 87Rb in a crossed dipole trap with direct loading from a magneto-optical trap. The dipole trap is created with light of a multifrequency fiber laser with a center wavelength of 1070nm. Evaporative cooling is performed by ramping down the laser power only. A comparison of the resulting atom number in an almost pure BEC to the initial atom number and the value for the gain in phase space density per atom lost confirm that this straightforward strategy is very efficient. We observe that the temporal characteristics of evaporation sequence are strongly influenced by power-dependent two-body losses resulting from enhanced optical pumping to the higher-energy hyperfine state. We characterize these losses and compare them to results obtained with a single-frequency laser at 1030nm.
4. arXiv:1110.5245 [pdf, other]
Title: Particle injection into a chain: decoherence versus relaxation for Hermitian and non-Hermitian dynamics
F. Gebhard, K. zu Muenster, J. Ren, N. Sedlmayr, J. Sirker, B. Ziebarth
We investigate a model system for the injection of fermionic particles from filled source sites into an empty chain. We study the ensuing dynamics for Hermitian as well as for non-Hermitian time evolution where the particles cannot return to the bath sites (quantum ratchet). A non-homogeneous hybridization between bath and chain sites permits transient currents in the chain. Non-interacting particles show decoherence in the thermodynamic limit: the average particle number and the average current density in the chain become stationary for long times, whereas the single-particle density matrix displays large fluctuations around its mean value. Using the numerical time-dependent density-matrix renormalization group ($t$-DMRG) method we demonstrate, on the other hand, that sizable density-density interactions between the particles introduce relaxation which is by orders of magnitudes faster than the decoherence processes.
Oct 24
1. arXiv:1110.4420 [pdf, other]
Title: Long-lived dipolar molecules and Feshbach molecules in a 3D optical lattice
Amodsen Chotia, Brian Neyenhuis, Steven A. Moses, Bo Yan, Jacob P. Covey, Michael Foss-Feig, Ana Maria Rey, Deborah S. Jin, Jun Ye
We have realized long-lived ground-state polar molecules in a 3D optical lattice, with a lifetime of up to 25 s, which is limited only by off-resonant scattering of the trapping light. Starting from a 2D optical lattice, we observe that the lifetime increases dramatically as a small lattice potential is added along the tube-shaped lattice traps. The 3D optical lattice also dramatically increases the lifetime for weakly bound Feshbach molecules. For a pure gas of Feshbach molecules, we observe a lifetime of >20 s in a 3D optical lattice; this represents a 100-fold improvement over previous results. This lifetime is also limited by off-resonant scattering, the rate of which is related to the size of the Feshbach molecule. Individually trapped Feshbach molecules in the 3D lattice can be converted to pairs of K and Rb atoms and back with nearly 100% efficiency.
Oct 17 - 21, Xiaopeng Li
Oct 21
1. arXiv:1110.4369 [pdf, other]
Title: Spin Waves in 2D ferromagnetic square lattice stripe
Author: Maher Z. Ahmed
In this work, the area and edges spin wave calculations were carried out using the Heisenberg Hamiltonian and the tridiagonal method for the 2D ferromagnetic square lattice stripe, where the SW modes are characterized by a 1D in-plane wave vector $q_x$. The results show a general and an unexpected feature that the area and edge spin waves only exist as optic modes. This behavior is also seen in 2D Heisenberg antiferromagnetic square lattice. This absence of the acoustic modes in the 2D square lattice is explained by the fact that the geometry constrains for NN exchange inside the square lattice allow only optical modes. We suggest that this unexpected behavior of spin waves in the 2D square lattice may be useful in realizing an explanation for HTS.
2. arXiv:1110.4464 [pdf, ps, other]
Title: Quantum fluctuations around black hole horizons in Bose-Einstein condensates
Author: P.-É. Larré, A. Recati, I. Carusotto, N. Pavloff
We study several realistic configurations allowing to realize an acoustic horizon in the flow of a one dimensional Bose-Einstein condensate. In each case we give an analytical description of the flow pattern, of the spectrum of Hawking radiation and of the associated quantum fluctuations. Our calculations confirm that the non local correlations of the density fluctuations previously studied in a simplified model provide a clear signature of Hawking radiation also in realistic configurations. In addition we explain by direct computation how this non local signal relates to short range modifications of the density correlations.
3. arXiv:1110.4469 [pdf, ps, other]
Title: Flat band in topological matter: possible route to room-temperature superconductivity
Author: G. E. Volovik
Topological media are systems whose properties are protected by topology and thus are robust to deformations of the system. In topological insulators and superconductors the bulk-surface and bulk-vortex correspondence gives rise to the gapless Weyl, Dirac or Majorana fermions on the surface of the system and inside vortex cores. In gapless topological media, the bulk-surface and bulk-vortex correspondence produce topologically protected gapless fermions without dispersion - the flat band. Fermion zero modes forming the flat band are localized on the surface of topological media with protected nodal lines and in the vortex core in systems with topologically protected Fermi points (Weyl points). Flat band has an extremely singular density of states, and this property may give rise in particular to surface superconductivity which in principle could exist even at room temperature.
4. arXiv:1110.4552 [pdf, ps, other]
Title: Quantum degenerate Bose-Fermi mixture of chemically different atomic species with widely tunable interactions
Author: Jee Woo Park, Cheng-Hsun Wu, Ibon Santiago, Tobias G. Tiecke, Peyman Ahmadi, Martin W. Zwierlein
Subjects: Quantum Gases (cond-mat.quant-gas)
We have created a quantum degenerate Bose-Fermi mixture of 23Na and 40K with widely tunable interactions via broad interspecies Feshbach resonances. Twenty Feshbach resonances between 23Na and 40K were identified. The large and negative triplet background scattering length between 23Na and 40K causes a sharp enhancement of the fermion density in the presence of a Bose condensate. As explained via the asymptotic bound-state model (ABM), this strong background scattering leads to a series of wide Feshbach resonances observed at low magnetic fields. Our work opens up the prospect to create chemically stable, fermionic ground state molecules of 23Na-40K where strong, long-range dipolar interactions will set the dominant energy scale.
5. arXiv:1110.4607 [pdf, other]
Title: Bose Metals and Insulators on Multi-Leg Ladders with Ring Exchange
Author: Ryan V. Mishmash, Matthew S. Block, Ribhu K. Kaul, D. N. Sheng, Olexei I. Motrunich, Matthew P. A. Fisher
We establish compelling evidence for the existence of new quasi-one-dimensional descendants of the d-wave Bose liquid (DBL), an exotic two-dimensional quantum phase of uncondensed itinerant bosons characterized by surfaces of gapless excitations in momentum space [O. I. Motrunich and M. P. A. Fisher, Phys. Rev. B {\bf 75}, 235116 (2007)]. In particular, motivated by a strong-coupling analysis of the gauge theory for the DBL, we study a model of hard-core bosons moving on the $N$-leg square ladder with frustrating four-site ring exchange. Here, we focus on four- and three-leg systems where we have identified two novel phases: a compressible gapless Bose metal on the four-leg ladder and an incompressible gapless Mott insulator on the three-leg ladder. The former is conducting along the ladder and has five gapless modes, one more than the number of legs. This represents a significant step forward in establishing the potential stability of the DBL in two dimensions. The latter, on the other hand, is a fundamentally quasi-one-dimensional phase that is insulating along the ladder but has two gapless modes and incommensurate power law transverse density-density correlations. In both cases, we can understand the nature of the phase using slave-particle-inspired variational wave functions consisting of a product of two distinct Slater determinants, the properties of which compare impressively well to a density matrix renormalization group solution of the model Hamiltonian. Stability arguments are made in favor of both quantum phases by accessing the universal low-energy physics with a bosonization analysis of the appropriate quasi-1D gauge theory. We will briefly discuss the potential relevance of these findings to high-temperature superconductors, cold atomic gases, and frustrated quantum magnets.
Oct 20
1. arXiv:1110.4110 [pdf, ps, other]
Title: Topologically protected surface Majorana arcs and bulk Weyl fermions in ferromagnetic superconductors
Author: Jay D. Sau, Sumanta Tewari
A number of ferromagnetic superconductors have been recently discovered which are believed to be in the so-called "equal spin pairing" (ESP) state. In the ESP state the Cooper pairs condense forming order parameters $\Delta_{\uparrow\uparrow}, \Delta_{\downarrow\downarrow}$ which are decoupled in the spin-sector. We show that these three-dimensional systems should generically support topologically protected surface Majorana arcs and bulk Weyl fermions as gapless excitations. Similar protected low-energy exotic quasiparticles should also appear in the recently discovered non-centrosymmteric superconductors in the presence of a Zeeman field. The protected surface arcs can be probed by angle-resolved photoemission (ARPES) as well as scanning tunneling microscope (STM) experiments.
2. arXiv:1110.4120 [pdf, other]
Title: Multiferroic behavior in trimerized Mott insulators
Author: Y. Kamiya, C. D. Batista
We demonstrate multiferroic behavior in trimerized Mott insulators through interplay between spins and electric dipole moments resulting from electronic charge fluctuations in frustrated units [L. N. Bulaevskii, C. D. Batista, M. V. Mostovoy, and D. I. Khomskii, Phys. Rev. B 78, 024402 (2008)]. The model consists of stacked triangular layers of trimers with small inter-trimer exchange interactions $J'$ and $J"$. Ferroelectric states coexist with ferro- or antiferromagnetic orderings depending on the value of the magnetic field $H$ and the sign of the inter-layer exchange $J"$. The electric polarization undergoes abrupt changes as a function of $H$.
3. arXiv:1110.4229 [pdf, ps, other]
Title: Spectroscopy of a fractional Josephson vortex molecule
Author: U. Kienzle, J. M. Meckbach, K. Buckenmaier, T. Gaber, H. Sickinger, Ch. Kaiser, K. Ilin, M. Siegel, D. Koelle, R. Kleiner, E. Goldobin
In long Josephson junctions with multiple discontinuities of the Josephson phase, fractional vortex molecules are spontaneously formed. At each discontinuity point a fractional Josephson vortex carrying a magnetic flux $|\Phi|<\Phi_0$, $\Phi_0\approx 2.07\times 10^{-15}$ Wb being the magnetic flux quantum, is pinned. Each vortex has an oscillatory eigenmode with a frequency that depends on $\Phi/\Phi_0$ and lies inside the plasma gap.
We experimentally investigate the dependence of the eigenfrequencies of a two-vortex molecule on the distance between the vortices, on their topological charge $\wp=2\pi\Phi/\Phi_0$ and on the bias current $\gamma$ applied to the Josephson junction. We find that with decreasing distance between vortices, a splitting of the eigenfrequencies occurs, that corresponds to the emergence of collective oscillatory modes of both vortices. We use a resonant microwave spectroscopy technique and find good agreement between experimental results and theoretical predictions.
4. arXiv:1110.4340 [pdf, ps, other]
Title: Tomonaga-Luttinger-liquid criticality: numerical entanglement entropy approach
Author: Satoshi Nishimoto
Comments: 5 pages, 5 figures, 3 tablesSubjects: Strongly Correlated Electrons (cond-mat.str-el)
The von Neumann entanglement entropy is studied with the density-matrix renormalization group technique. We propose a simple approach to calculate the central charge using the entanglement entropy for one-dimensional (1D) quantum system. This approach is applied to a couple of quantum systems: (i) 1D frustrated spin model and (ii) 1D half-filled spinless fermions with nearest-neighbor repulsion; and, it is confirmed that the central charge is estimated very accurately for the both systems. Also, a new method to determine the critical point between TL-liquid and gapped (or ordered) phases from the proposed approach is suggested. Furthermore, we mention that the Tomonaga-Luttinger parameter can be obtained in a like manner as the central charge, using the charge-density fluctuation of a part of the 1D system.
5. arXiv:1110.4362 [pdf, ps, other]
Title: Cluster update for tensor network states
Author: Ling Wang, Frank Verstraete
Comments: 6 pages, 10 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el)
We propose a novel recursive way of updating the tensors in projected entangled pair states by evolving the tensor in imaginary time evolution on clusters of different sizes. This generalizes the so- called simple update method of Jiang et al. [Phys. Rev. Lett. 101, 090603 (2008)] and the updating schemes in the single layer picture of Pi\v{z}orn et al. [Phys. Rev. A 83, 052321 (2011)]. A finite-size scaling of the observables as a function of the cluster size provides a remarkable improvement in the accuracy as compared to the simple update scheme. We benchmark our results on the hand of the spin 1/2 staggered dimerized antiferromagnetic model on the square lattice, and accurate results for the magnetization and the critical exponents are determined.
Oct 19
1. arXiv:1110.4092 [pdf, ps, other]
Title: Dynamics as a probe for population-imbalanced fermionic systems
Author: Raka Dasgupta, J. K. Bhattacharjee
We investigate a population-imbalanced two-species fermionic system where the resonantly-paired fermions combine to form bosonic molecules via Feshbach interaction. The natural dynamics of the system is studied and it is shown that the oscillation of the condensate fraction is periodic or quasi periodic, depending on the value of Feshbach coupling. We describe how a time dependent magnetic field can be used to study the natural frequencies and thus explore the momentum space structure of the population imbalanced system.
2. arXiv:1110.4091 [pdf, ps, other]
Title: Two-orbital Schwinger Boson Representation of Spin-One: Application to a Non-abelian Spin Liquid with Quaternion Gauge Field
Author: Fa Wang, Cenke Xu
A non-abelian spin liquid in triangular lattice spin-1 systems was recently formulated in the form of continuum field theory [T. Grover, and T. Senthil, Phys. Rev. Lett. 107, 077203 (2011); Cenke Xu, A.W.W. Ludwig, arXiv:1012.5671]. It has spin-1/2 bosonic spinons coupled to emergent quaternion gauge fields, and can be obtained by quantum disordering a non-collinear spin nematic order hypothesized to describe NiGa_2S_4 [H. Tsunetsugu, and M. Arikawa, J. Phys. Soc. Jpn. 75, 083701 (2006)], However a microscopic lattice description, e.g. the lattice spinon (mean-field) Hamiltonian and the spin wavefunction, has been missing, and it has been noted that the standard Schwinger boson or bosonic triplon representations of spin-1 cannot describe this spin liquid. In this paper a two-orbital Schwinger boson representation for spin-1 systems is developed and used to construct a mean-field description of this quaternion spin liquid. Projecting the mean-field state produces a prototype wavefunction, which is a superposition of close-packed AKLT loop configurations with nontrivial amplitudes. This new formalism and related wavefunctions may be generalized to higher spin systems and can possibly produce spin liquid states with even richer emergent gauge structures.
3. arXiv:1110.4045 [pdf, other]
Title: The importance of on-site inter-orbital single hopping in multi-orbital systems
Author: Qingguo Feng, Peter M. Oppeneer
A general multi-orbital Hubbard model is introduced with the inclusion of the on-site inter-orbital single hoppings in a realistic physical picture. It is shown that the on-site inter-orbital single hopping is one of the most basic and important interactions. The spin-flip term and pair-hopping term are only an explicit expression of the double hoppings at an equal time limit, as higher order terms of the on-site inter-orbital single hoppings. The two-orbital Hubbard model as an example is studied numerically to show the influence of the single hopping effect in changing the shape of the bands and shrinking the difference between the bands. The inclusion of the on-site inter-orbital hopping suppresses the so-called orbital-selective Mott transition.
4. arXiv:1110.3858 [pdf, other]
Title: Two-Dimensional Skyrmion in an Antiferromagnetic Spinor Bose-Einstein Condensate
Author:Jae-yoon Choi, Woo Jin Kwon, Yong-il Shin
We present the creation and time evolution of two-dimensional Skyrmion excitations in an antiferromagnetic spinor Bose-Einstein condensate. Quasi-two-dimensional F=1 sodium condensates confined in an optical dipole trap were prepared in a polar phase and the Skyrmion spin textures were imprinted by passing the zero-field center of a three-dimensional quadrupole magnetic field through the condensates. The Skyrmion was found to be unstable in a harmonic potential to dynamically deform its structure and decay to an uniform spin texture. We discuss the possible formation of half-quantum vortices in the decay process.
5. arXiv:1110.3814 (cross-list from hep-th) [pdf, other]
Title: Lectures on holographic non-Fermi liquids and quantum phase transitions
Author: Nabil Iqbal, Hong Liu, Márk Mezei
In these lecture notes we review some recent attempts at searching for non-Fermi liquids and novel quantum phase transitions in holographic systems using gauge/gravity duality. We do this by studying the simplest finite density system arising from the duality, obtained by turning on a nonzero chemical potential for a U(1) global symmetry of a CFT, and described on the gravity side by a charged black hole. We address the following questions of such a finite density system:
1. Does the system have a Fermi surface? What are the properties of low energy excitations near the Fermi surface?
2. Does the system have an instability to condensation of scalar operators? What is the critical behavior near the corresponding quantum critical point?
We find interesting parallels with those of high T_c cuprates and heavy electron systems. Playing a crucial role in our discussion is a universal intermediate-energy phase, called a "semi-local quantum liquid", which underlies the non-Fermi liquid and novel quantum critical behavior of a system. It also provides a novel mechanism for the emergence of lower energy states such as a Fermi liquid or a superconductor.
Oct 18
1. arXiv:1110.3364 [pdf, ps, other]
Title: Time reversal symmetry breaking of $p$-orbital bosons in a one-dimensional optical lattice
Author: Xiaopeng Li, Zixu Zhang, W. Vincent Liu
We study bosons loaded in a one-dimensional optical lattice of two-fold $p$-orbital degeneracy at each site. Our numerical simulations find an anti-ferro-orbital p$_x$+ip$_y$, a homogeneous p$_x$ Mott insulator phase and two kinds of superfluid phases distinguished by the orbital order (anti-ferro-orbital and para-orbital). The anti-ferro-orbital order breaks time reversal symmetry. Experimentally observable evidence is predicted for the phase transition between the two different superfluid phases. We also discover that the quantum noise measurement is able to provide a concrete evidence of time reversal symmetry breaking in the first Mott phase.
2. arXiv:1110.3339 [pdf, other]
Title: One-dimensional physics in transition-metal nanowires: Renormalization group and bosonization analysis
Author: Jun-ichi Okamoto, A. J. Millis
We study the one-dimensional two-orbital Hubbard model with general local interactions including a pair-hopping term. The model might be realized in one-dimensional transition-metal nanowires. Phase diagrams at T=0 are obtained by numerical integration of renormalization group equations and bosonization. Particular attention is paid to the effects of orbital degeneracy (or near-degeneracy), interactions favoring locally high-spin configurations, and velocity differences. Dynamical symmetry enlargement and duality approaches are employed to determine ground states and to understand quantum phase transitions between them. An important result is that the pair-hopping term and associated orbital symmetry can lead to new insulating states. The ground state for spin-polarized case is also discussed.
3. arXiv:1110.3328 [pdf, ps, other]
Title: Spin Liquid Phases for Spin-1 systems on the Triangular lattice
Author: Cenke Xu, Fa Wang, Yang Qi, Leon Balents, Matthew P. A. Fisher
Motivated by recent experiments on material Ba3NiSb2O9, we propose two novel spin liquid phases (A and B) for spin-1 systems on a triangular lattice. At the mean field level, both spin liquid phases have gapless fermionic spinon excitations with quadratic band touching, thus in both phases the spin susceptibility and C_v/T saturate to a constant at zero temperature, which are consistent with the experimental results on Ba3NiSb2O9. On the lattice scale, these spin liquid phases have Sp(4) ~ SO(5) gauge fluctuation; while in the long wavelength limit this Sp(4) gauge symmetry is broken down to U(1)xZ_2 in type A spin liquid phase, and broken down to Z_4 in type B phase. We also demonstrate that the $A$ phase is the parent state of the ferro-quadrupole state, nematic state, and the noncollinear spin density wave state.
Oct 17
1. arXiv:1110.3309 [pdf, other]
Title: Revealing the Superfluid Lambda Transition in the Universal Thermodynamics of a Unitary Fermi Gas
Author: Mark J. H. Ku, Ariel T. Sommer, Lawrence W. Cheuk, Martin W. Zwierlein
We have observed the superfluid phase transition in a strongly interacting Fermi gas via high-precision measurements of the local compressibility, density and pressure down to near-zero entropy. Our data completely determine the universal thermodynamics of strongly interacting fermions without any fit or external thermometer. The onset of superfluidity is observed in the compressibility, the chemical potential, the entropy, and the heat capacity. In particular, the heat capacity displays a characteristic lambda-like feature at the critical temperature of $T_c/T_F = 0.167(13)$. This is the first clear thermodynamic signature of the superfluid transition in a spin-balanced atomic Fermi gas. Our measurements provide a benchmark for many-body theories on strongly interacting fermions, relevant for problems ranging from high-temperature superconductivity to the equation of state of neutron stars.
2. arXiv:1110.3058 [pdf, other]
Title: Evolution of Fermion Pairing from Three to Two Dimensions
Author: Ariel T. Sommer, Lawrence W. Cheuk, Mark Jen-Hao Ku, Waseem S. Bakr, Martin W. Zwierlein
We follow the evolution of fermion pairing in the dimensional crossover from 3D to 2D as a strongly interacting Fermi gas of $^6$Li atoms becomes confined to a stack of two-dimensional layers formed by a one-dimensional optical lattice. Decreasing the dimensionality leads to the opening of a gap in radiofrequency spectra, even on the BCS-side of a Feshbach resonance. With increasing lattice depth, the measured binding energy \eb{} of fermion pairs increases in surprising agreement with mean-field theory for the BEC-BCS crossover in two dimensions.
3. arXiv:1110.2901 (cross-list from quant-ph) [pdf, other]
Title: Why momentum width matters for atom interferometry with Bragg pulses
Author: Stuart S. Szigeti, John E. Debs, Joseph J. Hope, Nicholas P. Robins, John D. Close
Improving the precision of atom-based sensors by using large momentum transfer Bragg mirrors and beamsplitters is an exciting prospect. However, the effect of source momentum width on the efficiency of Bragg mirrors and beamsplitters and, more generally, on the phase sensitivity of an atom interferometer has been largely ignored. We have developed a theoretical method for optimizing Bragg beamsplitter and mirror pulses that includes the momentum width of the atomic cloud. Using these tools, we theoretically demonstrate that momentum width considerations are important, and in fact are crucial for optimizing Bragg pulses and comparing different atomic sources.
Oct 10 - Oct 14, Zixu Zhang
Oct 14
1. arXiv:1110.3021 [pdf, ps, other]
Title: Phase-Sensitive Detection for Unconventional Bose-Einstein Condensations
Authors:Zi Cai, Lu-Ming Duan, Congjun Wu
We propose a phase-sensitive detection scheme to identify the unconventional $p_{x}\pm ip_{y}$ symmetry of the condensate wavefunctions of bosons, which have already been proposed and realized in high bands in optical lattices. Using the impulsive Raman operation combining with time-of-flight imaging, the off-diagonal correlation functions in momentum space give rise to the relative phase information between different components of condensate wavefunctions. This scheme is robust against the interaction and interband effects, and provides smoking gun evidence for unconventional Bose-Einstein condensations with nontrivial condensation symmetries.
Oct 13
1. arXiv:1110.2495 [pdf, ps, other]
Competition between two- and three-sublattice ordering for S=1 spins on the square lattice
Tamas A. Toth, Andreas M. Laeuchli, Frederic Mila, Karlo Penc
In spite of the bipartite nature of the square lattice and the possibility to minimize the `classical' (Hartree) energy for all parameters with only two sublattices, since interactions are limited to nearest neighbors, the S=1 bilinear-biquadratic Heisenberg model on the square lattice is shown to have an extended three-sublattice phase induced by quantum fluctuations for sufficiently large biquadratic interactions. The proof relies on exact diagonalizations of finite clusters and on a semiclassical treatment of quantum fluctuations within linear flavor-wave theory. In zero field, this three-sublattice phase is purely quadrupolar, and upon increasing the field it replaces most of the plateau at 1/2 that is predicted by the classical theory.
Oct 12
1. arXiv:1110.2183 [pdf, ps, other]
Nature of 3D Bose Gases near Resonance
Dmitry Borzov, Mohammad S. Mashayekhi, Shizhong Zhang, Jun-Liang Song, Fei Zhou
In this Letter, we illustrate an approach to Bose gases at large scattering lengths which is based on resummation of a subset of the most dominating loop diagrams involving a minimum number of virtually excited atoms. We apply this approach to explore the nature of 3D near-resonance Bose gases, investigate the issue of onset stability and the role of Lee-Huang-Yang correction. At the point of instability, we also estimate the effect of the three-body forces to be within a few percent.
2. arXiv:1110.2241 [pdf, ps, other]
Unconventional superfluidity induced by spin-orbital coupling in a polarized two-dimensional Fermi gas
J.-N. Zhang, Y.-H. Chan, L.-M. Duan
We show the spin-orbital coupling induced by an artificial light-induced gauge field can fully restore superfluidity suppressed by population imbalance in a two-dimensional (2D) Fermi gas, leading to unconventional superfluid states either with topological Majorana fermion excitations or showing a novel mixture of triplet pairing with spin-up (down) components respectively in the $p_{x}\pm ip_{y}$ pairing channels. We self-consistently calculate the zero temperature phase diagram at the BCS\ side of Feshbach resonance and show that the phase transitions between different superfluid states can be revealed through measurement of the in-situ density profile of the 2D atomic cloud in a weak global trap.
3. arXiv:1110.2418 [pdf, ps, other]
Observation of a pairing pseudogap in a two-dimensional Fermi gas
Michael Feld, Bernd Fröhlich, Enrico Vogt, Marco Koschorreck, Michael Köhl
Pairing of fermions is ubiquitous in nature and it is responsible for a large variety of fascinating phenomena like superconductivity, superfluidity of $^3$He, the anomalous rotation of neutron stars, and the BEC-BCS crossover in strongly interacting Fermi gases. When confined to two dimensions, interacting many-body systems bear even more subtle effects, many of which lack understanding at a fundamental level. Most striking is the, yet unexplained, effect of high-temperature superconductivity in cuprates, which is intimately related to the two-dimensional geometry of the crystal structure. In particular, the questions how many-body pairing is established at high temperature and whether it precedes superconductivity are crucial to be answered. Here, we report on the observation of pairing in a harmonically trapped two-dimensional atomic Fermi gas in the regime of strong coupling. We perform momentum-resolved photoemission spectroscopy, analogous to ARPES in the solid state, to measure the spectral function of the gas and we detect a many-body pairing gap above the superfluid transition temperature. Our observations mark a significant step in the emulation of layered two-dimensional strongly correlated superconductors using ultracold atomic gases.
Oct 11
1. arXiv:1110.1646 [pdf, other]
Bosonic and fermionic transport phenomena of ultra-cold atoms in 1D optical lattices
Chih-Chun Chien, Michael Zwolak, Massimiliano Di Ventra
Experiments demonstrating transport phenomena of ultra-cold atoms are gaining considerable momentum. Using the micro-canonical picture of transport -- a framework ideally suited to describe the dynamics of closed quantum systems such as those realized in ultra-cold atom experiments -- we show that non-interacting fermions and bosons exhibit very different transport properties when the system is initially in the ground state and set out of equilibrium by removing the particles from half of the lattice. Fermions rapidly develop a finite quasi steady-state current reminiscent of electronic transport in nanoscale systems, whereas the bosonic current exhibits strong oscillatory behavior that decays into a steady-state of zero current only in the thermodynamic limit. We find that these differences appear most strikingly in the different particle number fluctuations on half of the lattice as a consequence of the spin statistics in the two cases. These predictions can be readily verified experimentally.
2. arXiv:1110.1649 [pdf, other]
Fermi polarons in two dimensions
Richard Schmidt, Tilman Enss, Ville Pietilä, Eugene Demler
We theoretically analyze inverse radiofrequency (rf) spectroscopy experiments in two-component Fermi gases. We consider a small number of impurity atoms interacting strongly with a bath of majority atoms. In two-dimensional geometries we find that the main features of the rf spectrum correspond to an attractive polaron and a metastable repulsive polaron. Our results suggest that the attractive polaron has been observed in a recent experiment [Phys. Rev. Lett. 106, 105301 (2011)].
3. arXiv:1110.1653 [pdf, ps, other]
Many-body Landau-Zener Transition in Cold Atom Double Well Optical Lattices
Yinyin Qian, Ming Gong, Chuanwei Zhang
Ultra-cold atoms in optical lattices provide an ideal platform for exploring many-body physics of a large system arising from the coupling among a series of small identical systems whose few-body dynamics are exactly solvable. Using Landau-Zener (LZ) transition of bosonic atoms in double well optical lattices as an experimentally realizable model, we investigate such few to many body route by exploring the relation and difference between the few-body (in one double well) and many-body (in double well lattice) non-equilibrium dynamics of cold atoms in optical lattices. We find the many-body coupling between double wells greatly enhances the LZ transition probability, while keeping the main features of the few-body dynamics. Various experimental signatures of the many-body LZ transition, including atom density, momentum distribution, and density-density correlation, are obtained.
4. arXiv:1110.2109 [pdf, ps, other]
Quantum Phases of Bosons with Anisotropic Dipolar Interactions on 2D Lattices
Takahiro Ohgoe, Takafumi Suzuki, Naoki Kawashima
We investigate the hard-core Bose-Hubbard model with {\it anisotropic} long-range dipole-dipole interactions on square lattices. In our model, we assume that the dipole moments are oriented along the one particular axis on the 2D plane. To treat this model exactly, we perform unbiased quantum Monte Carlo simulations with an O(N) method. We obtain the ground-state phase diagram that includes a superfluid phase and a striped solid phase at the particle density $\rho=1/2$ in broad regions. The obtained phase diagram indicates that a supersolid state is unstable. We give the qualitative discussion of the reason from a perturbative treatment. Finite-temperature transitions to the phases are also investigated. For large dipole-dipole interactions, we observe a small $\rho = 1/3$ striped solid phase and incompressible regions adjacent to it. In spite of its incompressibility, the particle density increases as the chemical potential increases in the regions. This indicates the devil's staircase caused by the presence of numerous metastable states.
Oct 10
1. arXiv:1110.1414 [pdf, other]
Modification of the 3He Phase Diagram by Anisotropic Disorder
R.G. Bennett, N. Zhelev, E.N. Smith, J. Pollanen, W.P. Halperin, J.M. Parpia
Motivated by the recent prediction that uniaxially compressed aerogel can stabilize the anisotropic A phase over the isotropic B phase, we measure the pressure dependent superfluid fraction of 3He entrained in 10% axially compressed, 98% porous aerogel. We observe that a broad region of the temperature-pressure phase diagram is occupied by the metastable A phase. The reappearance of the A phase on warming from the B phase, before superfluidity is extinguished at Tc, is in contrast to its absence in uncompressed aerogel. The phase diagram is modified from that of pure 3He, with the disappearance of the polycritical point (PCP) and the appearance of a region of A phase extending below the PCP of bulk 3He, even in zero applied magnetic field. The expected alignment of the A phase texture by compression is not observed.
Oct 3 - Oct 7, Bin Wang
Oct 7
1. arXiv:1110.1348 [pdf, ps, other]
Title: Dynamics of Non-Equilibrium Dicke Models
Author: M. J. Bhaseen, J. Mayoh, B. D. Simons, J. Keeling
Motivated by experiments observing self-organization of cold atoms in optical cavities we investigate the collective dynamics of the associated non-equilibrium Dicke model. The model displays a rich semiclassical phase diagram of long time attractors including distinct superradiant fixed points, bistable and multistable coexistence phases and regimes of persistent oscillations. We explore the intrinsic timescales for reaching these asymptotic states and discuss the implications for finite duration experiments. On the basis of a semiclassical analysis of the effective Dicke model we find that sweep measurements over 200ms may be required in order to access the asymptotic regime. We briefly comment on the corrections that may arise due to quantum fluctuations and states outside of the effective two-level Dicke model description.
2. arXiv:1110.1323 [pdf, other]
Title: Theory of real supersolids
Author: Gustavo During, Christophe Josserand, Yves Pomeau, Sergio Rica
We review the main properties of a supersolid. We describe first the macroscopic equation that satisfies a supersolid based on general arguments and symmetries and show that such solids might exhibit simultaneously or independently both elastic behavior and superfluidity. We then explain why a supersolid state should exist for solids at very low temperature but with a very small superfluid fraction. Finally, we propose a mean-field model, based on the Gross-Pitaevski\u{\i} equation, which presents the general properties expected for a supersolid and should therefore provide a consistent framework to study its dynamical properties.
Oct 6
1.arXiv:1110.0840 [pdf, ps, other]
Title: Slow interaction ramps in trapped many-particle systems: universal deviations from adiabaticity
Author: Masudul Haque, F.E. Zimmer
For harmonic-trapped atomic systems, we report universal non-adiabatic features in the response to interaction ramps. We provide results for several different systems in one, two, and three dimensions: bosonic and fermionic Hubbard models realized through optical lattices, a Bose-Einstein condensate, a fermionic superfluid and a fermi liquid. The deviation from adiabaticity is characterized through the heating or excitation energy produced during the ramp. We find that the dependence of the heat on the ramp time is sensitive to the ramp protocol but has aspects unexpectedly common to all systems considered. We explain these common features in terms of universal dynamics of the system size or cloud radius.
Oct 5
1. arXiv:1110.0805 [pdf, ps, other]
Title: Rashba spin-orbit coupled atomic Fermi gases
Author: Lei Jiang, Xia-Ji Liu, Hui Hu, Han Pu
We investigate theoretically BEC-BCS crossover physics in the presence of a Rashba spin-orbit coupling in a system of two-component Fermi gas with and without a Zeeman field that breaks the population balance between the two components. A new bound state (Rashba pair) emerges because of the spin-orbit interaction. We study the properties of Rashba pairs using a standard pair fluctuation theory. At zero temperature, the Rashba pairs condense into a macroscopic mixed spin state. We discuss in detail the experimental signatures for observing the condensation of Rashba pairs by calculating various physical observables which characterize the properties of the system and can be measured in experiment.
2. arXiv:1110.0558 [pdf, ps, other]
Title: Spin-orbit-coupled dipolar Bose-Einstein condensates
Author: Y. Deng, J. Cheng, H. Jing, C.-P. Sun, S. Yi
We propose an experimental scheme to create spin-orbit coupling in spin-3 Cr atoms using Raman processes. Employing linear Zeeman effect and optical Stark shift, two spin states within the ground electronic manifold are selected, which results in a pseudo-spin-1/2 model. We further study the ground state structures of a spin-orbit-coupled Cr condensate. We show that, in addition to the stripe structures induced by the spin-orbit coupling, the magnetic dipole-dipole interaction gives rise to the vortex phase, in which spontaneous spin vortex is formed.
3. arXiv:1110.0494 [pdf, ps, other]
Title: Pairing instabilities in quasi-two-dimensional Fermi gases
Author: Ville Pietilä, David Pekker, Yusuke Nishida, Eugene Demler
We study non-equilibrium dynamics of ultracold two-component Fermi gases in low-dimensional geometries after the interactions are quenched from weakly interacting to strongly interacting regime. We develop a T-matrix formalism that takes into account the interplay between Pauli blocking and tight confinement in low-dimensional geometries. We employ our formalism to study the formation of molecules in quasi-two-dimensional Fermi gases near Feshbach resonance and show that the rate at which molecules form depends strongly on the transverse confinement. Furthermore, Pauli blocking gives rise to a sizable correction to the binding energy of molecules.
Oct 4
none
Oct 3
1. arXiv:1109.6792 [pdf, ps, other]
Title: Universal relations for the two-dimensional spin-1/2 Fermi gas with contact interactions
Author: Manuel Valiente, Nikolaj Zinner, Klaus Molmer
We present universal relations for a two-dimensional Fermi gas with pairwise contact interactions. The derivation of these relations is made possible by obtaining the explicit form of a generalized function -- selector -- in the momentum representation. The selector implements the short-distance boundary condition between two fermions in a straightforward manner, and leads to simple derivations of the universal relations, in the spirit of Tan's original method for the three-dimensional gas.
