Apr 2014

Mar 31-Apr 4, Jiyao chen, Apr 7-Apr 11, Ahmet Keles, Apr 14-Apr 18, Zhifang Xu, Apr 21-Apr 25, Bo Liu, Apr 28-May 2, Jinlong Yu

Mar 31

1. arXiv:1403.7246 [pdf, other]
Observation of Efimov Resonances in a Mixture with Extreme Mass Imbalance
R. Pires, J. Ulmanis, S. Häfner, M. Repp, A. Arias, E.D. Kuhnle, M. Weidemüller
We observe two consecutive heteronuclear Efimov resonances in an ultracold Li-Cs mixture by measuring three-body loss coefficients as a function of magnetic field near a Feshbach resonance. The first resonance is detected at a scattering length of a(1)−=−320(10) a0 corresponding to ∼7 (∼3) times the Li-Cs (Cs-Cs) van der Waals range. The second resonance appears at 5.8(1.0)a(1)− close to the unitarity-limited regime at the sample temperature of 450 nK. Indication of a third resonance is found in the atom loss spectra. The scaling of the resonance positions is close to the universal scaling value of 4.9 predicted for zero temperature. Deviations from universality might be caused by finite-range and temperature effects, as well as magnetic field dependent Cs-Cs interactions.

2. arXiv:1403.7199 [pdf, other]
Emergence of coherence and the dynamics of quantum phase transitions
S. Braun, M. Friesdorf, S.S. Hodgman, M. Schreiber, J.P. Ronzheimer, A. Riera, M. del Rey, I. Bloch, J. Eisert, U. Schneider
The dynamics of quantum phase transitions poses one of the most challenging problems in modern many-body physics. Here, we study a prototypical example in a clean and well-controlled ultracold atom setup by observing the emergence of coherence when crossing the Mott insulator to superfluid quantum phase transition. In the one-dimensional Bose-Hubbard model, we find perfect agreement between experimental observations and numerical simulations for the resulting coherence length. We thereby perform a largely certified analogue quantum simulation of this strongly correlated system reaching beyond the regime of free quasiparticles. Experimentally, we additionally explore the emergence of coherence in higher dimensions where no classical simulations are available, as well as for negative temperatures. For intermediate quench velocities, we observe a power-law behaviour of the coherence length, reminiscent of the Kibble-Zurek mechanism. However, we find exponents that strongly depend on the final interaction strength and thus lie outside the scope of this mechanism.

3. arXiv:1403.7262 (cross-list from cond-mat.mes-hall) [pdf, other]
Effects of Symmetry on Bulk-Edge Correspondence in Periodically Driven Systems
Derek Y.H. Ho, Jiangbin Gong
Topological states of matter in periodically driven systems have been attracting great theoretical and experimental interests, but more insights need to be gained to better understand the issue of bulk-edge correspondence in driven systems. To that end, this work investigates in detail how specific properties of chiral symmetry operators may lead to a big difference in the behavior of the edge states. In particular, we exploit two dynamical models, namely, a variant of the so-called kicked-rotor model in the quantum chaos literature and the kicked Harper model, both possessing fractal-like quasi-energy spectra closely resembling the Hofstadter butterfly. Despite the topological equivalence between the two dynamical systems [H. Wang, D. Y. H. Ho, W. Lawton, J. Wang and J. B. Gong, Phys. Rev. E 88, 052920 (2013)] and the fact that these two systems can be mapped onto each other, we show that different chiral symmetry properties have remarkably different consequences. In (only) one model there can exist an arbitrary number of edge modes with 0 or pi quasi-energy values accompanied by a proliferation of Dirac points, but (only) in the other model there can be counter-propagating chiral edge modes at the same boundary. Our results should be useful towards the ongoing efforts to topologically classify periodically driven systems, and should further motivate experimental studies of topological states using quantum systems under external control fields.


 Apr 1
 1. arXiv:1403.8041 [pdf, other]
Phase fluctuations and first-order correlation functions of dissipative Bose-Einstein condensates
A.-W. de Leeuw, H.T.C. Stoof, R.A. Duine
We investigate the finite lifetime effects on first-order correlation functions of dissipative Bose-Einstein condensates. By taking into account the phase fluctuations up to all orders, we show that the finite lifetime effects are neglible for the spatial first-order correlation functions, but have an important effect on the temporal correlations. As an application, we calculate the one-particle density matrix of a quasi-condensate of photons. Finally, we also consider the photons in the normal state and we demonstrate that the finite lifetime effects decrease both the spatial and temporal first-order correlation functions.


 2. arXiv:1403.7979 (cross-list from quant-ph) [pdf, other]
U(3) artificial gauge fields for cold atoms
Yu-Xin Hu, Christian Miniatura, David Wilkowski, Benoît Grémaud
We propose to generate an artificial non-Abelian U(3) gauge field by using a 2-tripod scheme, namely two tripod configurations sharing a common ground state level and driven by resonant 1-photon transitions. Using an appropriate combination of four Laguerre-Gauss and two Hermite-Gauss laser beams, we are able to produce a U(3)-monopole and a U(3) spin-orbit coupling for both alkali and alkaline-earth atoms. This 2-tripod scheme could open the way to the study of interacting spinor condensates subjected to U(3)-monopoles.



 Apr 3
1.arXiv:1404.0647 [pdf, ps, other]

 Nonlinear Zeno dynamics due to atomic interactions in Bose-Einstein condensate
V. G. Navarro, V. S. Shchesnovich
We show that nonlinear interactions induce both the Zeno and anti-Zeno effects in the generalized Bose-Josephson model (with the on-sight interactions and the pair tunneling) describing Bose-Einstein condensate in double-well trap subject to particle removal from one of the wells. We find that the on-sight interactions induce \textit{only} the Zeno effect, which appears at long evolution times, whereas the pair tunneling leads to a strong decay of the atomic population at short evolution times, reminiscent of the anti-Zeno effect, and destroys the nonlinear Zeno effect due to the on-sight interactions at long times.

2. arXiv:1404.0557 [pdf, ps, other]

 Two dispersion curves for a one-dimensional interacting Bose gas under zero boundary conditions
Maksim Tomchenko
The influence of boundaries and non-point character of interatomic interaction on the dispersion law has been studied for a uniform Bose gas in a one-dimensional vessel. The non-point character of interaction was taken into account using the Gross equation, which is more general than the Gross-Pitaevskii one. In the framework of this approach, the well-known Bogolyubov dispersion mode \hbar\omega(k)=[(\hbar^{2}k^{2}/2m) ^{2}+qn\nu(k)\hbar^{2}k^{2}/m]^{1/2} (q=1) was obtained, as well as a new one, which is described by the same formula, but with q= 1/2. The new mode emerges owing to the account of boundaries and the non-point character of interaction: this mode is absent when either the Gross equation for a cyclic system or the Gross-Pitaevskii equation for a cyclic system or a system with boundaries is solved. Capabilities for the new mode to be observed are discussed.


Apr 4
1. arXiv:1404.1038 [pdf, other]

Life Cycle of Superfluid Vortices in the Unitary Fermi Gas
Gabriel Wlazłowski, Aurel Bulgac, Michael McNeil Forbes, Kenneth J. Roche
We demonstrate the critical role played by the geometry of the trap in the formation and dynamics of a vortex in the UFG. The evolution of a domain wall phase imprinted perpendicularly to the long axis depends on the axial symmetry. In an axially symmetric trap a vortex ring is formed. In a slightly asymmetric trap the vortex ring evolves into vortex lines that demonstrate crossing and reconnection. Anharmonicity results in a single vortex that moves with a comparable speed of that of a vortex ring along the long axis of the trap. New imagining techniques demonstrated by the MIT group should be able to directly visualize this crossing and reconnection of vortex lines, conjectured by Feynman in 1955 to be at the origin of quantum turbulence in superfluids at zero temperature.

2. arXiv:1404.0970 [pdf, ps, other]

Bose-Bose Mixtures with Synthetic Spin-Orbit Coupling in Optical Lattices
Liang He, Anchun Ji, Walter Hofstetter
We investigate the ground state properties of Bose-Bose mixtures with Rashba-type spin-orbit (SO) coupling in a square lattice. The system displays rich physics from the deep Mott-insulator (MI) all the way to the superfluid (SF) regime. In the deep MI regime, novel spin-ordered phases arise due to the effective Dzyaloshinskii-Moriya type super-exchange interactions. By employing the non-perturbative Bosonic Dynamical Mean-Field-Theory (BDMFT), we numerically study and establish the stability of these magnetic phases against increasing hopping amplitude. We show that as hopping is increased across the MI to SF transition, exotic superfluid phases with magnetic textures emerge. In particular, we identify a new spin-spiral magnetic texture with spatial period 3 in the superfluid close to the MI-SF transition.


3. arXiv:1404.0678 [pdf, ps, other]
Superfluid-Bose glass transition in one dimension
Zoran Ristivojevic, Aleksandra Petkovic, Pierre Le Doussal, Thierry Giamarchi


We consider a one-dimensional system of interacting bosons in a random potential. At zero temperature, it can be either in the superfluid or in the insulating phase. We study the transition at weak disorder and moderate interaction. Using a systematic approach, we derive the renormalization group equations at two loop order and discuss the phase diagram. We find the universal form of the correlation functions at the transitions and compute the logarithmic corrections to the main universal power-law behavior. In order to mimic large density fluctuations on a single site we study a simplified model of disordered two leg bosonic ladders with correlated disorder across the rung. Contrarily to the single chain case, the latter exhibits a superfluid-Bose glass transition where the exponents of the correlation functions at the transition do not take universal values.

April 7
1. arXiv:1404.1131 [pdf, ps, other]
Exotic electronic states in the world of flat bands: from theory to material
Zheng Liu, Feng Liu, Yong-Shi Wu
It has long been noticed that special lattices contain single-electron flat bands (FB) without any dispersion. Since the kinetic energy of electrons is quenched in the FB, this highly degenerate energy level becomes an ideal platform to achieve strongly correlated electronic states, such as magnetism, superconductivity and Wigner crystal. Recently, the FB has attracted increasing interests, because of the possibility to go beyond the conventional symmetry-breaking phases, towards topologically ordered phases, such as lattice versions of fractional quantum Hall states. This article reviews different aspects of FBs in a nutshell. Starting from the standard band theory, we aim to bridge the frontier of FBs with the textbook solid-state physics. Then, based on concrete examples, we show the common origin of FBs in terms of destructive interference, and discuss various many-body phases associated with such a singular band structure. In the end, we demonstrate real FBs in quantum frustrated materials and organometallic frameworks.

2. arXiv:1404.1076 [pdf, ps, other]
A New Class of Particle in 2+1 Dimensions
Philip Schuster, Natalia Toro
In two spatial dimensions, spin characterizes how particle states re-phase under changes of frame that leave their momentum and energy invariant. Massless particles can in principle have non-trivial spin in this sense, but all existing field theories only describe the trivial case. This letter presents a field theory for a massless particle with non-trivial physical spin. These particles are the 2+1-dimensional analogues of "continuous-spin" particles in 3+1 dimensions, but here they have only two real degrees of freedom, related by parity. They can be understood as massless generalizations of anyons, but are simpler in key respects.

3. arXiv:1404.1210 [pdf, ps, other]
Anisotropic two-dimensional RF-dressed potentials for ultracold atoms
Arijit Chakraborty, Satya Ram Mishra
We have theoretically studied the generation of various types of two-dimensional RF-dressed adiabatic potentials for ultracold atoms by considering the radio frequency (RF) fields of different polarizations and phases in the presence of a static magnetic field of a quadrupole trap. It is found that, besides the previously investigated double well and ring shape trapping potentials, some interesting anisotropic two-dimensional potentials can be generated by appropriately choosing the polarization and phase of the RF field. These anisotropic potentials, which include static (e.g, split arcs) as well as time dependent (e.g, asymmetric ring) potentials, can facilitate the splitting, rotation and oscillation of the trapped cold atom cloud along the circular path. The ultracold atoms trapped in these potentials may be used to investigate the interesting physics phenomena such as tunneling and super-fluidity.


4. arXiv:1404.1284 [pdf, other]

Introduction to superfluidity -- Field-theoretical approach and applications
Andreas Schmitt
In this pedagogical introduction, I discuss theoretical aspects of superfluidity and superconductivity, mostly using a field-theoretical formalism. While the emphasis is on general concepts and mechanisms behind superfluidity, I also discuss various applications in low-energy and high-energy physics. Besides some introductory and standard topics such as superfluid helium and superfluidity in a simple scalar field theory, the lecture notes also include more advanced chapters, for instance discussions of the covariant two-fluid formalism and Cooper pairing with mismatched Fermi surfaces.

April 8
1. arXiv:1404.1388 [pdf, ps, other]
Stripe order in superfluid 3He confined in narrow cylinders
Kazushi Aoyama
We theoretically investigate pairing states of the spin-triplet p-wave superfluid 3He confined in narrow cylinders. The surface-induced distortion and the multiple internal degrees of freedom of the order parameter lead to the occurrence of a stripe structure along the cylinder axis in the superfluid 3He-B phase. We show that in sufficiently small cylinders with an anisotropic surface scattering, the stripe order with broken translational symmetry may be stabilized as the lowest energy state. Periodic spin-current textures caused in this inhomogeneous superfluid phase are also discussed.

2. arXiv:1404.1662 [pdf, ps, other]
From p- to s-wave behaviors in a three-dimensional topological superconductor with non-magnetic impurities
Yuki Nagai, Yukihiro Ota, Masahiko Machida
Responses to impurity scattering reveal unconventional features in superconductivity. We study a robustness against non-magnetic impurities in a three-dimensional topological superconductor, focusing on an effective model (massive Dirac Hamiltonian with s-wave on-site pairing) of Copper-doped bismuth-selenium compounds. Using a self-consistent T-matrix approach for impurity scattering, we examine the presence of in-gap states in density of states. We find that the results are summarized well by a single material parameter, which quantifies relativistic effects in the Dirac Hamiltonian. In non-relativistic regime, an odd-parity superconducting state is fragile against non-magnetic impurities. We show that this behavior is caused by a $p$-wave character involved in the topological superconducting state. In contrast, we show that in relativistic regime the superconductivity is robust against non-magnetic impurities, owing to an s-wave character. Therefore, the system has two aspects, p- and s-wave characters, depending on the amount of relativistic effects.


3. arXiv:1404.1394 [pdf, ps, other]
Creation and optical detection of spin cat states in Bose-Einstein condensates
Hon Wai Lau, Zachary Dutton, Tian Wang, Christoph Simon
We propose a method to create "spin cat states", i.e. macroscopic superpositions of coherent spin states, in two-component Bose-Einstein condensates using the Kerr nonlinearity due to atomic collisions. Based on a detailed study of atom loss, we conclude that cat sizes of hundreds of atoms should be realistic. The existence of the spin cat states can be demonstrated by optical readout. Our analysis also includes the effects of higher-order nonlinearities, atom number fluctuations and limited readout efficiency.

April 9

1. arXiv:1404.2010 [pdf, ps, other]
Floquet Majorana Edge Mode and Non-Abelian Anyons in a Driven Kitaev Model
Masahiro Sato, Yuki Sasaki, Takashi Oka
We theoretically study laser driven nonequilibrium states in the Kitaev honeycomb model with a magnetoelectric cross coupling. We show that a topological spin liquid with a gapless chiral edge mode emerges when we apply an elliptically or circularly polarized laser. This is a strongly correlated quantum spin version of the Floquet topological insulator. In the topological phase, the edge mode is made from Majorana fermions and the bulk has gapped non-Abelian anyon excitations.


2. arXiv:1404.2230 [pdf, ps, other]
Dispersion-driven ferromagnetism in a flat-band Hubbard system
Oleg Derzhko, Johannes Richter
We investigate a mechanism to establish ground-state ferromagnetism in flat-band Hubbard systems based on a kind of {\it order-from-disorder} effect driven by dispersion. As a paradigm we consider a frustrated diamond chain, where for ideal diamond geometry the lowest one-electron band is flat, but the ground state remains paramagnetic for arbitrary on-site repulsion $U$. We focus on half filling of the flat band. By using numerical and analytical arguments we present the ground-state phase diagram for a distorted diamond chain, i.e., the former flat band becomes dispersive. Driven by the interplay of dispersion and interaction the ground state is ferromagnetic if the interaction exceeds a critical value $U_c$.


April 10

1. arXiv:1404.2442 [pdf, ps, other]
Gapless topological Fulde-Ferrell superfluidity induced by in-plane Zeeman field
Hui Hu, Lin Dong, Ye Cao, Han Pu, Xia-Ji Liu
Topological superfluids are recently discovered quantum matters that host topologically protected gapless edge states known as Majorana fermions - exotic quantum particles that act as their own anti-particles and obey non-Abelian statistics. Their realizations are believed to lie at the heart of future technologies such as fault-tolerant quantum computation. To date, the most efficient scheme to create topological superfluids and Majorana fermions is based on the Sau-Lutchyn-Tewari-Das Sarma model with a Rashba-type spin-orbit coupling on the }\textbf{\textit{x-y}}\textbf{ plane and a large out-of-plane (perpendicular) Zeeman field along the }\textbf{\textit{z}}\textbf{-direction. Here we propose an alternative setup, where the topological superfluid phase is driven by applying an in-plane Zeeman field. This scheme offers a number of new features, notably Cooper pairings at finite centre-of-mass momentum (i.e., Fulde-Ferrell pairing) and gapless excitations in the bulk. As a result, a novel gapless topological quantum matter with inhomogeneous pairing order parameter appears. It features unidirected Majorana surface states at boundaries, which propagate in the same direction and connect two Weyl nodes in the bulk. We demonstrate the emergence of such an exotic topological matter and the associated Majorana fermions in spin-orbit coupled atomic Fermi gases and determine its parameter space. The implementation of our scheme in semiconductor/superconductor heterostructures is briefly discussed.


2. arXiv:1404.2546 [pdf, ps, other]
Mode-locked Bloch oscillations in a ring cavity
M. Samoylova, N. Piovella, D. Hunter, G.R.M. Robb, R. Bachelard, Ph.W. Courteille
We study Bloch oscillations of ultracold atoms stored in a one-dimensional vertical optical lattice under the action of a constant gravitational force. The atoms also interact with a unidirectionally pumped optical ring cavity whose vertical arm is collinear with the optical lattice. We find that, in certain parameter regimes, the feedback provided by the cavity field on the optical lattice stabilizes the Bloch oscillations via a mode-locking mechanism and generates periodic bursts of light emitted into the counterpropagating cavity mode. While the light bursts provide a non- destructive monitor of the dynamics, the mode-locking eliminates noise and is even capable of steering the atoms to the lowest Bloch band. Both features may be crucial for future improvements of the design of atomic gravimeters based on recording Bloch oscillations.


3. arXiv:1404.2548 [pdf, other]
A Mixture of Bose and Fermi Superfluids
Igor Ferrier-Barbut, Marion Delehaye, Sebastien Laurent, Andrew T. Grier, Matthieu Pierce, Benno S. Rem, Frédéric Chevy, Christophe Salomon
In recent years, ultracold atoms have emerged as a unique tool to engineer and study novel experimental systems, furthering our understanding of quantum many-body physics. For bosons, their advent has permitted the study of weakly interacting Bose-Einstein condensates, two-dimensional gases or the superfluid-Mott insulator transition. For fermions with attractive interaction the observation of BEC-BCS crossover demonstrated the connexion between Bose-Einstein condensation (BEC) and fermionic superfluidity described by Bardeen, Cooper and Schrieffer's theory (BCS). Here, we report on the first production of a mixture of bosons and fermions where both species are superfluid. Such a mixture has been long sought in liquid helium where superfluidity has been achieved separately in both bosonic 4He and fermionic 3He. However, due to strong interactions between the two isotopes, 3He-4He mixtures contain a small fraction of 3He (typically 6%) which, so far, has prevented reaching simultaneous superfluidity for the two species. With cold atoms, a Fermi sea mixed with a BEC has been observed. In this work we tune a cloud of lithium isotopes to a regime where the mixture of bosonic and fermionic superfluids is stable. We probe the collective dynamics of this system by exciting center-of-mass oscillations that exhibit extremely low damping. Using high precision spectroscopy of these low-lying modes we observe coherent energy exchange and measure the coupling between the two superfluids. We interpret our observations by a simple coupled oscillator model

4. arXiv:1404.2586 [pdf, ps, other]
A simple model for interactions and corrections to the Gross-Pitaevskii Equation
Hagar Veksler, Shmuel Fishman, Wolfgang Ketterle
One of the assumptions leading to the Gross-Pitaevskii Equation (GPE) is that the interaction between atom pairs can be written effectively as a delta function so that the effective size of the particles is assumed to vanish. A simple model that takes into account the extension of the inter-particle potential is introduced. The correction to GPE predictions for the energy of a condensate confined by harmonic trap in the Thomas-Fermi (TF) regime is estimated. Although it is found to be small, we believe that in some situations it can be measured using its dependance on the frequency of the confining trap. The model is applied to atoms and polaritons. We believe that due to its simplicity it may have a wide range of applications.

April 11

1. arXiv:1404.2711 [pdf, ps, other]
Theory of Anderson pseudospin resonance with Higgs mode in a superconductor
Naoto Tsuji, Hideo Aoki
A superconductor irradiated by an ac electric field with frequency $\Omega$ is theoretically found to generate a collective precession of Anderson's pseudospins, and hence coherent amplitude oscillation of the order parameter, with a doubled frequency $2\Omega$ through a nonlinear light-matter coupling. We provide a fundamental theory to show that the induced pseudospin precession resonates with the Higgs amplitude mode of the superconductor at $2\Omega=2\Delta$ with $2\Delta$ being the superconducting gap. The resonant precession is accompanied by a divergent enhancement of the third harmonic generation. The resonance width is determined by Higgs mode's lifetime.


2. arXiv:1404.2818 [pdf, other]
Realization of 2-Dimensional Bosonic Topological Insulators
Zheng-Xin Liu, Zheng-Cheng Gu, Xiao-Gang Wen
It is well known that a Bosonic Mott insulator can be realized by condensing vortices of a boson condensate. Usually, a vortex becomes an anti-vortex (and vice-versa) under time reversal symmetry, and the condensation of vortices results in a trivial Mott insulator. However, if each vortex/anti-vortex interacts with a spin trapped at its core, the time reversal transformation of the composite vortex operator will contain an extra minus sign. It turns out that such a composite vortex condensed state is a bosonic topological insulator (BTI) with gapless boundary excitations protected by U(1) and time reversal symmetry. We point out that in BTI, an external pi flux monodromy defect carries a Kramers doublet. We propose two realistic Hamiltonians that might realize BTI in cold atom systems. We also discuss the realization of BTI phase in spin-1 solid state systems.


3. arXiv:1404.2777 [pdf, other]
Effects of interactions on the dynamics of driven cold atoms
Alexandra Bakman, Shmuel Fishman
The quantum fidelity was introduced by Peres to study some fingerprints of classically chaotic behavior in the quantum dynamics of the corresponding systems. In the present paper the signatures of classical dynamics near elliptic points and of interactions between particles are characterized for kicked systems. In particular, the period of the fidelity resulting of the interactions is found using analytical and numerical calculations. The relation between the dynamics of a kicked system and the dynamics of a harmonic oscillator with some corrections is developed and used to understand the mechanism for generation of the different frequencies of the fidelity.
4. arXiv:1404.2868 [pdf, other]
Fermion-fermion scattering with superconducting circuits
L. García-Álvarez, J. Casanova, A. Mezzacapo, I. L. Egusquiza, L. Lamata, G. Romero, E. Solano
Quantum field theories (QFTs) are among the deepest descriptions of nature. In this sense, different computing approaches have been developed, as Feynman diagrams or lattice gauge theories. In general, the numerical simulations of QFTs are computationally hard, with the processing time growing exponentially with the system size. Nevertheless, a quantum simulator could provide an efficient way to emulate these theories in polynomial time. Here, we propose the quantum simulation of fermionic field modes interacting via a continuum of bosonic modes with superconducting circuits, which are among the most advanced quantum technologies in terms of quantum control and scalability. An important feature of superconducting devices is that, unlike other quantum platforms, they offer naturally a strong coupling of qubits to a continuum of bosonic modes. Therefore, this system is a specially suited platform to realize quantum simulations of scattering processes involving interacting fermionic and bosonic quantum field theories, where access to the continuum of modes is required.


April 14
1.arXiv:1404.3009 [pdf, ps, other]
Spin-orbit coupling induced unconventional pairings in a one-dimensional lattice
Junjun Liang, Xiaofan Zhou, Pak Hong Chui, Kuang Zhang, Shi-jian Gu, Ming Gong, Gang Chen, Suotang Jia
Comments: 6 Pages, 5 Figures
Subjects: Quantum Gases (cond-mat.quant-gas)

Pairings are the heart for exploring novel superfluid physics of degenerate fermi gas. In the presence of spin-orbit coupling (SOC), some exotic pairings, including the triplet p-wave pairing, have been predicted, which opens a new way to search topological superfluids and associated Majorana fermions. In this work, we reveal unconventional pairings induced by SOC in a one-dimensional optical lattice, using the exact density-matrix renormalization group. In the weak SOC, we find the interband Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) and intraband Bardeen-Cooper-Schrieffer (BCS) pairings may coexist when both bands are partially occupied. This interesting coexistence can lead to a new quantum state, called the FFLO-BCS phase, which exhibits a unique three-peak structure in pairing momentum distribution. However, in the intermediate and strong SOCs, the intraband BCS pairing dominates in the whole parameter regime. These results shed great light on the realization of topological superfluids in the spin-orbit coupled lattice.

2.arXiv:1404.3178 [pdf, ps, other]
Pair condensation of polarized fermions in the BCS-BEC crossover
G. Bighin, L. Salasnich, G. Mazzarella, L. Dell'Anna
Comments: 7 pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We investigate a two-component Fermi gas with unequal spin populations along the BCS-BEC crossover. By using the extended BCS equations and the concept of off-diagonal-long-range-order we derive a formula for the condensate number of Cooper pairs as a function of energy gap, average chemical potential, imbalance chemical potential and temperature. Then we study the zero-temperature condensate fraction of Cooper pairs by varying interaction strength and polarization, finding a depletion of the condensate fraction by increasing the population imbalance. We also compare our theoretical results with the available experimental data on the condensate fraction of polarized ^6Li atoms [Science {\bf 311}, 492 (2006)]. These experimental data are in agreement with our predictions in a suitable range of polarizations, but only in the BCS side of the crossover up to unitarity. Finally, we consider explicitly the presence of an external harmonic confinement and we study, within the local-density approximation, the phase separation between superfluid and normal phase regions of the polarized fermionic cloud. In particular, we calculate both condensate density profiles and total density profiles from the inner superfluid core to the normal region passing for the interface, where a finite jump in the density is a clear manifestation of this phase-separated regime.

April 15
1.arXiv:1404.3217 [pdf, ps, other]
Floquet Edge States with Ultracold Atoms
Matthew Reichl, Erich Mueller
Comments: 6 pages, 5 figures, revtex4
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

We describe an experimental setup for imaging topologically protected Floquet edge states using ultracold bosons in an optical lattice. Our setup involves a deep two dimensional optical lattice with a time dependent superlattice that modulates the hopping between neighboring sites. The finite waist of the superlattice beam yields regions with different topological numbers. One can observe chiral edge states by imaging the real-space density of a bosonic packet launched from the boundary between two topologically distinct regions.

2.arXiv:1404.3528 (cross-list from cond-mat.dis-nn) [pdf, other]
Measurement of the mobility edge for 3D Anderson localization
Giulia Semeghini, Manuele Landini, Patricia Castilho, Sanjukta Roy, Giacomo Spagnolli, Andreas Trenkwalder, Marco Fattori, Massimo Inguscio, Giovanni Modugno
Comments: 13 pages
Subjects: Disordered Systems and Neural Networks (cond-mat.dis-nn); Quantum Gases (cond-mat.quant-gas)

Anderson localization is a universal phenomenon affecting non-interacting quantum particles in disorder. In three spatial dimensions it becomes particularly interesting to study because of the presence of a quantum phase transition from localized to extended states, predicted by P.W. Anderson in his seminal work, taking place at a critical energy, the so-called mobility edge. The possible relation of the Anderson transition to the metal-insulator transitions observed in materials has originated a flurry of theoretical studies during the past 50 years, and it is now possible to predict very accurately the mobility edge starting from models of the microscopic disorder. However, the experiments performed so far with photons, ultrasound and ultracold atoms, while giving evidence of the transition, could not provide a precise measurement of the mobility edge. In this work we are able to obtain such a measurement using an ultracold atomic system in a disordered speckle potential, thanks to a precise control of the system energy. We find that the mobility edge is close to the mean disorder energy at small disorder strengths, while a clear effect of the spatial correlation of the disorder appears at larger strengths. The precise knowledge of the disorder properties in our system offers now the opportunity for an unprecedented experiment-theory comparison for 3D Anderson localization, which is also a necessary step to start the exploration of novel regimes for many-body disordered systems.

April 16
1.arXiv:1404.3740 [pdf, other]
Non-Equilibrium Universality in the Heating Dynamics of Interacting Luttinger Liquids
Michael Buchhold, Sebastian Diehl
Comments: 4.5 pages, 2 figures (main text); 8 pages, 3 figures (supplementary material)
Subjects: Quantum Gases (cond-mat.quant-gas)

We establish a new non-equilibrium scaling regime in the short time evolution of one-dimensional interacting open quantum systems subject to a generic heating mechanism. This dynamical regime is characterized by uncompensated phonon production and a subdiffusive, universal scaling of quasiparticle lifetimes with momentum \sim q^{-5/3}, distinct from finite and zero temperature cases. It is separated from a high momentum regime by a time dependent scale fading out as q_0(t) \sim t^{-4/5}. In the latter region we observe thermalization to an effective time-dependent equilibrium with linearly increasing temperature. By mapping out the dynamical phase diagram and computing the dynamical structure factor within an open system Keldysh functional integral approach, we show how these predictions can be explored in cold atom experiments by means of Bragg spectroscopy.

2.arXiv:1404.3728 [pdf, ps, other]
Criterion for non-Fermi liquid phases via interactions with Nambu-Goldstone bosons
Haruki Watanabe, Ashvin Vishwanath
Comments: 5 pages + 8 pages (Supplemental Material), 2 + 1 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); High Energy Physics - Theory (hep-th)

Interactions between Fermi liquid quasiparticle and gapless bosons, such as photons or quantum critical fluctuations, are expected to destabilize the Fermi liquid and lead to overdamping of the bosonic modes. However, coupling electrons to Nambu-Goldstone bosons (NGBs), typically does not have such a dramatic effect. This arises because symmetry usually dictates the existence of derivatives in the coupling, which makes them vanish in the limit of small energy-momentum transfer. Here we formulate a general criterion which specifies when this coupling can be free of derivatives, which makes it similar to the coupling to gauge fields or quantum critical modes. This criterion is satisfied by the example of the nematic Fermi fluid that spontaneously breaks rotation symmetry while preserving translations, where non-Fermi liquid physics and overdamping of NGBs was discussed by Oganesyan-Kivelson-Fradkin. In addition, the criterion also allows us to identify a new kind of symmetry breaking - of magnetic translations - where non vanishing couplings are expected, which is confirmed by an explicit calculation.



3.arXiv:1404.3770 [pdf, other]
The Kondo Temperature of SU(4) Symmetric Quantum Dots
Michele Filippone, Catalin Pascu Moca, Gergely Zarand, Christophe Mora
Comments: 4 pages, 5 figures and Suppl. Material
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

A path integral approach is used to derive a closed analytical expression for the Kondo temperature of the SU(4) symmetrical Anderson model. In contrast to the SU(2) case, the prefactor of the Kondo temperature is found to display a peculiar orbital energy (gate voltage) dependence, reflecting the presence of various SU(4) mixed valence fixed points. Our analytical expressions are tested against and confirmed by numerical renormalization group computations.

4.arXiv:1404.4023 [pdf, other]
Spin pumping and spin-transfer torques in antiferromagnets
Ran Cheng, Jiang Xiao, Qian Niu, Arne Brataas
Comments: 5 pages, 4 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Materials Science (cond-mat.mtrl-sci)

Spin pumping and spin-transfer torques are two widely studied reciprocal phenomena in ferromagnets. However, pumping phenomena in homogeneous antiferromagnets and their relations to current-induced torques have not been explored. By calculating how electrons scatter off a normal metal-antiferromagnetic interface, we derive pumped spin and staggered spin currents in terms of the staggered field, the magnetization, and their rates of change. For both compensated and uncompensated interfaces, spin pumping is large and of a similar magnitude with a direction controlled by the microwave polarization. The pumped currents are connected to current-induced torques via Onsager reciprocity relations.

5.arXiv:1404.4039 [pdf, other]
Topological superconductivity and unconventional pairing in oxide interfaces
Mathias S. Scheurer, Jörg Schmalian
Comments: 17 pages, 6 figures
Subjects: Superconductivity (cond-mat.supr-con); Strongly Correlated Electrons (cond-mat.str-el)

To pinpoint the microscopic mechanism for superconductivity has proven to be one of the most outstanding challenges in the physics of correlated quantum matter. Thus far, the most direct evidence for an electronic pairing mechanism is the observation of a new symmetry of the order-parameter, as done in the cuprate high-temperature superconductors. Like distinctions based on the symmetry of a locally defined order-parameter, global, topological invariants allow for a sharp discrimination between states of matter that cannot be transformed into each other adiabatically. Here we propose an unconventional pairing state for the electron fluid in two-dimensional oxide interfaces and establish a direct link to the emergence of nontrivial topological invariants. Topological superconductivity and Majorana edge states can then be used to detect the microscopic origin for superconductivity. In addition, we show that also the density wave states that compete with superconductivity sensitively depend on the nature of the pairing interaction. Our conclusion is based on the special role played by the spin-orbit coupling and the shape of the Fermi surface in SrTiO$_{3}$/LaAlO$_{3}$-interfaces and closely related systems.

April 17
1.arXiv:1404.4050 [pdf, other]
Formation of a topological non-Fermi liquid in MnSi
R. Ritz, M. Halder, M. Wagner, C. Franz, A. Bauer, C. Pfleiderer
Journal-ref: Nature 497, 231 (2013)
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Materials Science (cond-mat.mtrl-sci)

Fermi liquid theory provides a remarkably powerful framework for the description of the conduction electrons in metals and their ordering phenomena, such as superconductivity, ferromagnetism, and spin- and charge-density-wave order. A different class of ordering phenomena of great interest concerns spin configurations that are topologically protected, that is, their topology can be destroyed only by forcing the average magnetization locally to zero. Examples of such configurations are hedgehogs (points at which all spins are either pointing inwards or outwards) or vortices. A central question concerns the nature of the metallic state in the presence of such topologically distinct spin textures. Here we report a high-pressure study of the metallic state at the border of the skyrmion lattice in MnSi, which represents a new form of magnetic order composed of topologically non-trivial vortices. When long-range magnetic order is suppressed under pressure, the key characteristic of the skyrmion lattice - that is, the topological Hall signal due to the emergent magnetic flux associated with their topological winding - is unaffected in sign or magnitude and becomes an important characteristic of the metallic state. The regime of the topological Hall signal in temperature, pressure and magnetic field coincides thereby with the exceptionally extended regime of a pronounced non-Fermi-liquid resistivity. The observation of this topological Hall signal in the regime of the NFL resistivity suggests empirically that spin correlations with non-trivial topological character may drive a breakdown of Fermi liquid theory in pure metals.

2. arXiv:1404.4054 [pdf, other]
Variational study of polarons in Bose-Einstein condensates
Weiran Li, S. Das Sarma
Subjects: Quantum Gases (cond-mat.quant-gas)

We use a class of variational wave functions to study the properties of an impurity in a Bose-Einstein condensate, i.e. the "Bose polaron". The impurity interacts with the condensate through a contact interaction, which can be tuned by a Feshbach resonance. We find a stable attractive polaron branch that evolves continuously across the resonance to a tight-binding diatomic molecule deep in the positive scattering length side. A repulsive polaron branch with finite lifetime is also observed and it becomes unstable as the interaction strength increases. The effective mass of the attractive polaron also changes smoothly across the resonance connecting the two well-understood limits deep on both sides.

April 18
1.arXiv:1404.4363 [pdf, other]
Real-Space and Reciprocal-Space Berry Phases in the Hall Effect of Mn$_{\rm 1-x}$Fe$_{\rm x}$Si
C. Franz, F. Freimuth, A. Bauer, R. Ritz, C. Schnarr, C. Duvinage, T. Adams, S. Bügel, A. Rosch, Y. Mokrousov, C. Pfleiderer
Comments: Phys. Rev. Lett., accepted for publication
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Materials Science (cond-mat.mtrl-sci)

We report an experimental and computational study of the Hall effect in Mn$_{\rm 1-x}$Fe$_{\rm x}$Si, as complemented by measurements in Mn$_{\rm 1-x}$Co$_{\rm x}$Si, when helimagnetic order is suppressed under substitutional doping. For small $x$ the anomalous Hall effect (AHE) and the topological Hall effect (THE) change sign. Under larger doping the AHE remains small and consistent with the magnetization, while the THE grows by over a factor of ten. Both the sign and the magnitude of the AHE and the THE are in excellent agreement with calculations based on density functional theory. Our study provides the long-sought material-specific microscopic justification, that while the AHE is due to the reciprocal-space Berry curvature, the THE originates in real-space Berry phases.

2.arXiv:1404.4367 [pdf, ps, other]
Perfect Metal Phases of One-Dimensional and Anisotropic Higher-Dimensional Systems
Eugeniu Plamadeala, Michael Mulligan, Chetan Nayak
Comments: 6 pages. Supplementary information: large matrices in a Mathematica notebook and in 12 text files
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We show that a 1D quantum wire with $23$ channels of interacting fermions has a perfect metal phase in which all weak perturbations that could destabilize this phase are irrelevant. Consequently, weak disorder does not localize it, a weak periodic potential does not open a gap, and contact with a superconductor also fails to open a gap. Similar phases occur for $N \geq 24$ channels of fermions, except for $N=25$, and for $8k$ channels of interacting bosons, with $k\geq 3$. Arrays of perfect metallic wires form higher-dimensional fermionic or bosonic perfect metals, albeit highly-anisotropic ones.

3.arXiv:1404.4373 [pdf, other]
Periodically-driven quantum systems: Effective Hamiltonians and engineered gauge fields
N. Goldman, J. Dalibard
Comments: 24 pages, 6 figures, includes Appendices (A-K)
Subjects: Quantum Gases (cond-mat.quant-gas); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)

Driving a quantum system periodically in time can profoundly alter its long-time dynamics and trigger topological order. Such schemes are particularly promising for generating non-trivial energy bands and gauge structures in quantum-matter systems. Here, we develop a general formalism that captures the essential features ruling the dynamics: the effective Hamiltonian, but also the effects related to the initial phase of the modulation and the micro-motion. This framework allows for the identification of driving schemes, based on general N-phase modulations, which lead to configurations relevant for quantum simulation. In particular, we explore methods to generate synthetic spin-orbit couplings and magnetic fields in cold-atom setups.



April 21


1. arXiv:1404.5326 [pdf, other]
Quantum Spin Ice and dimer models with Rydberg atoms
Alexander W. Glaetzle, Marcello Dalmonte, Rejish Nath, Ioannis Rousochatzakis, Roderich Moessner, Peter Zoller
Quantum spin ice represents a paradigmatic example on how the physics of frustrated magnets is related to gauge theories. In the present work we address the problem of approximately realizing quantum spin ice in two dimensions with cold atoms in optical lattices. The relevant interactions are obtained by weakly admixing van der Waals interactions between laser admixed Rydberg states to the atomic ground state atoms, exploiting the strong angular dependence of interactions between Rydberg p-states together with the possibility of designing step-like potentials. This allows us to implement Abelian gauge theories in a series of geometries, which could be demonstrated within state of the art atomic Rydberg experiments. We numerically analyze the family of resulting microscopic Hamiltonians and find that they exhibit both classical and quantum order by disorder, the latter yielding a quantum plaquette valence bond solid. We also present strategies to implement Abelian gauge theories using both s- and p-Rydberg states in exotic geometries, e.g. on a 4-8 lattice.



April 22

1.arXiv:1404.5233 [pdf, other]
Renormalization theory of a two dimensional Bose gas: quantum critical point and quasi-condensed state
Serena Cenatiempo, Alessandro Giuliani

We present a renormalization group construction of a weakly interacting Bose gas at zero temperature in the two-dimensional continuum, both in the quantum critical regime and in the presence of a condensate fraction. The construction is performed within a rigorous renormalization group scheme, borrowed from the methods of constructive field theory, which allows us to derive explicit bounds on all the orders of renormalized perturbation theory. Our scheme allows us to construct the theory of the quantum critical point completely, both in the ultraviolet and in the infrared regimes, thus extending previous heuristic approaches to this phase. For the condensate phase, we solve completely the ultraviolet problem and we investigate in detail the infrared region, up to length scales of the order $(\lambda \sqrt{\rho_0})^{-1}$ (here $\lambda$ is the interaction strength and $\rho_0$ the condensate density), which is the largest length scale at which the problem is perturbative in nature. We exhibit violations to the formal Ward Identities, due to the momentum cutoff used to regularize the theory, which suggest that previous proposals about the existence of a non-perturbative non-trivial fixed point for the infrared flow should be seriously reconsidered.


April 23

1. arXiv:1404.5583 [pdf, other]
Nondestructive imaging of an ultracold lattice gas
Y. S. Patil, L. M. Aycock, S. Chakram, M. Vengalattore

We demonstrate the nondestructive imaging of a lattice gas of ultracold bosons. Atomic fluorescence is induced in the simultaneous presence of degenerate Raman sideband cooling. The combined influence of these processes controllably cycles an atom between a dark state and a fluorescing state while eliminating heating and loss. Through spatially resolved sideband spectroscopy following the imaging sequence, we demonstrate the efficacy of this imaging technique in various regimes of lattice depth and fluorescence acquisition rate. Our work provides an important extension of quantum gas imaging to the nondestructive detection, control and manipulation of atoms in optical lattices. In addition, our technique can also be extended to atomic species that are less amenable to molasses-based lattice imaging.




April 24

1.arXiv:1404.5622 [pdf, other]
Properties of the density-wave phase of a two-dimensional dipolar fermi gas
J. K. Block, G. M. Bruun
The rapid progress in the production and cooling of molecular gases indicates that experimental studies of quantum gases with a strong dipolar interaction is soon within reach. Dipolar gases are predicted to exhibit very rich physics including quantum liquid crystal phases such as density-waves as well as superfluid phases, both of which play an important role for our understanding of strongly correlated systems. Here, we investigate the zero temperature properties of the density-wave phase of a two-dimensional (2D) system of fermonic dipoles using a conserving Hartree-Fock theory. We calculate the amplitude of the density waves as a function of the dipole moment and orientation with respect to the 2D plane. The stripes give rise to a 1D Brillouin zone structure, and the corresponding quasiparticle spectrum is shown to have gapped as well as gapless regions around the Fermi surface. As a result, the system remains compressible in the density-wave phase, and it collapses for strong attraction. We show that the density-waves has clear signatures in the momentum distribution and in the momentum correlations. Both can be measured in time-of-flight experiments. Finally, we discuss how the striped phase can be realised with experimentally available systems.


2. arXiv:1404.5631 [pdf, other]
Coherent magnon optics in a ferromagnetic spinor Bose-Einstein condensate
G. Edward Marti, Andrew MacRae, Ryan Olf, Sean Lourette, Fang Fang, Dan M. Stamper-Kurn
We measure the mass, gap, and magnetic moment of a magnon in the ferromagnetic $F=1$ spinor Bose-Einstein condensate of $^{87}$Rb. We find an unusually heavy magnon mass of $1.038(2)_\mathrm{stat}(8)_\mathrm{sys}$ times the atomic mass, as determined by interfering standing and running coherent magnon waves within the dense and trapped condensed gas. This measurement is shifted significantly from theoretical estimates. The magnon energy gap of $h\times 2.5(1)_\mathrm{stat}(2)_\mathrm{sys}\;\mathrm{Hz}$ and the effective magnetic moment of $-1.04(2)_\mathrm{stat}(8)\,\mu_\textrm{bare}$ times the atomic magnetic moment are consistent with mean-field predictions. The nonzero energy gap arises from magnetic dipole-dipole interactions.




April 25


1. arXiv:1404.6211 [pdf, ps, other]
Spin-orbit-coupled topological Fulde-Ferrell states of fermions in a harmonic trap
Lei Jiang, Eite Tiesinga, Xia-Ji Liu, Hui Hu, Han Pu
Motivated by recent experimental breakthroughs in generating spin-orbit coupling in ultracold Fermi gases using Raman laser beams, we present a systematic study of spin-orbit-coupled Fermi gases confined in a quasi-one-dimensional trap in the presence of an in-plane Zeeman field (which can be realized using a finite two-photon Raman detuning). We find that a topological Fulde-Ferrell state will emerge, featuring finite-momentum Cooper pairing and zero-energy Majorana excitations localized near the edge of the trap based on the self-consistent Bogoliubov-de Genes (BdG) equations. We find analytically the wavefunctions of the Majorana modes. Finally using the time-dependent BdG we show how the finite-momentum pairing field manifests itself in the expansion dynamics of the atomic cloud.



Apr 28-May 2, Jinlong Yu
Apr 28


1. arXiv:1404.6400 [pdf, other]
Observation of Quantized Conductance in Neutral Matter
Sebastian Krinner, David Stadler, Dominik Husmann, Jean-Philippe Brantut, Tilman Esslinger
In transport experiments the quantum nature of matter becomes directly evident when changes in conductance occur only in discrete steps, with a size determined solely by Planck's constant h. The observations of quantized steps in the electric conductance have provided important insights into the physics of mesoscopic systems and allowed for the development of quantum electronic devices. Even though quantized conductance should not rely on the presence of electric charges, it has never been observed for neutral, massive particles. In its most fundamental form, the phenomenon requires a quantum degenerate Fermi gas, a ballistic and adiabatic transport channel, and a constriction with dimensions comparable to the Fermi wavelength. Here we report on the observation of quantized conductance in the transport of neutral atoms. We employ high resolution lithography to shape light potentials that realize either a quantum point contact or a quantum wire for atoms. These constrictions are imprinted on a quasi two-dimensional ballistic channel connecting two adjustable reservoirs of quantum degenerate fermionic lithium atoms. By tuning either a gate potential or the transverse confinement of the constrictions, we observe distinct plateaus in the conductance for atoms. The conductance in the first plateau is found to be equal to 1/h, the universal conductance quantum. For low gate potentials we find good agreement between the experimental data and the Landauer formula, with all parameters determined a priori. Our experiment constitutes the cold atom version of a mesoscopic device and can be readily extended to more complex geometries and interacting quantum gases.


2. arXiv:1404.6346 [pdf, ps, other]
Extended Bose-Hubbard model with pair tunneling: spontaneous symmetry breaking, effective ground state and fragmentation
Qizhong Zhu, Qi Zhang, Biao Wu
The extended Bose-Hubbard model for a double-well potential with pair tunneling is studied through both exact diagonalization and mean field theory (MFT). When pair tunneling is strong enough, the ground state wavefunction predicted by the MFT is complex and doubly degenerate while the quantum ground state wavefunction is always real and unique. The time reversal symmetry is spontaneously broken when the system transfers from the quantum ground state into one of the mean field ground states upon a small perturbation. As the gap between the lowest two levels decreases exponentially with particle number, the required perturbation inducing the spontaneous symmetry breaking (SSB) is infinitesimal for particle number of typical cold atom systems. The quantum ground state is further analyzed with the Penrose-Onsager criterion, and is found to be a fragmented condensate. The state also develops the pair correlation and has non-vanishing pair order parameter instead of the conventional single particle order parameter. When this model is generalized to optical lattice, a pair superfluid can be generated. The mean field ground state can be regarded as effective ground state in this simple model. The detailed computation for this model enables us to offer an in-depth discussion of the relation between SSB and effective ground state, giving a glimpse on how nonlinearity arises in the SSB of a quantum system.

3. arXiv:1404.6315 [pdf, other]
Topological Properties of Ultracold Bosons in One-Dimensional Quasiperiodic Optical Lattice
Fuyuki Matsuda, Masaki Tezuka, Norio Kawakami
We analyze topological properties of the one-dimensional Bose-Hubbard model with a quasiperiodic superlattice potential. This system can be realized in interacting ultracold bosons in optical lattice in the presence of an incommensurate superlattice potential. We first analyze the quasiperiodic superlattice made by the cosine function, which we call Harper-like Bose-Hubbard model. We compute the Chern number and observe a gap-closing behavior as the interaction strength U is changed. Also, we discuss the bulk-edge correspondence in our system. Furthermore, we explore the phase diagram as a function of U and a continuous deformation parameter β between the Harper-like model and another important quasiperiodic lattice, the Fibonacci model. We numerically confirm that the incommensurate charge density wave (ICDW) phase is topologically non-trivial and it is topologically equivalent in the whole ICDW region.
Apr 29

1. arXiv:1404.6676 [pdf, ps, other]
Multichannel quantum-defect theory for magnetic Feshbach resonances in heteronuclear group I systems
Constantinos Makrides, Bo Gao
We present a multichannel quantum-defect theory for magnetic Feshbach resonances in the interaction of two heteronuclear group I atoms. The theory provides a unified and a uniform description of resonances in all partial waves, and enables the characterization of large number of resonances in terms of very few parameters. For the sample system of 6Li40K, we present descriptions of all resonances in aa, ab, and ba channels, in partial waves s (l=0) through h (l=5), and in a magnetic field of 0 through 1000 Gauss. All resonances, including those in nonzero partial waves, are fully characterized using the newly developed parametrization of Gao [Phy. Rev. A \textbf{84}, 022706 (2011)].

2. arXiv:1404.6622 (cross-list from physics.atom-ph) [pdf, ps, other]
Tuning a magnetic Feshbach resonance with spatially modulated laser light
Yi-Cai Zhang, Wu-Ming Liu, Hui Hu
We theoretically investigate the control of a magnetic Feshbach resonance using a bound-to-bound molecular transition driven by spatially modulated laser light. Due to the spatially periodic coupling between the ground and excited molecular states, there exists a series of band structure of bound states, which can uniquely be characterized by some extra bumps in radio-frequency spectroscopy. With the increasing of coupling strength, the series of bound states will cross zero energy and directly result in a number of scattering resonances, whose position and width can be conveniently tuned by the coupling strength of the laser light and the applied magnetic field (i.e., the detuning of the ground molecular state). The spatially modulated coupling also implies a local spatially varying interaction between atoms. Our work proposes a practical way of optically controlling interatomic interactions with high spatial resolution and negligible atomic loss.

3. arXiv:1404.7102 [pdf, ps, other]
Mapping the phase diagram of spinor condensates via adiabatic quantum phase transitions
J. Jiang, L. Zhao, M. Webb, Y. Liu
We experimentally study two quantum phase transitions in a sodium spinor condensate immersed in a microwave dressing field. We also demonstrate that many previously unexplored regions in the phase diagram of spinor condensates can be investigated by adiabatically tuning the microwave field across one of the two quantum phase transitions. This method overcomes two major experimental challenges associated with some widely used methods, and is applicable to other atomic species. Agreements between our data and the mean-field theory for spinor Bose gases are also discussed.


Apr 30
1. arXiv:1404.7385 [pdf, ps, other]
Spin dynamics in a two dimensional quantum gas
Poul L. Pedersen, Miroslav Gajdacz, Frank Deuretzbacher, Luis Santos, Carsten Klempt, Jacob F. Sherson, Andrew J. Hilliard, Jan J. Arlt
We have investigated spin dynamics in a 2D quantum gas. Through spin-changing collisions, two clouds with opposite spin orientations are spontaneously created in a Bose-Einstein condensate. After ballistic expansion, both clouds acquire ring-shaped density distributions with superimposed angular density modulations. The density distributions depend on the applied magnetic field and are well explained by a simple Bogoliubov model. We show that the two clouds are anti-correlated in momentum space. The observed momentum correlations pave the way towards the creation of an atom source with non-local Einstein-Podolsky-Rosen entanglement.

2. arXiv:1404.7248 [pdf, other]
Aspects of Floquet Bands and Topological Phase Transitions in a Continuously Driven Superlattice
Longwen Zhou, Hailong Wang, Derek Y.H. Ho, Jiangbin Gong
Recently the creation of novel topological states of matter by a periodic driving field has attracted great attention. To motivate further experimental and theoretical studies, we investigate interesting aspects of Floquet bands and topological phase transitions in a continuously driven Harper model. In such a continuously driven system with an odd number of Floquet bands, the bands are found to have nonzero Chern numbers in general and topological phase transitions take place as we tune various system parameters, such as the amplitude or the period of the driving field. The nontrivial Floquet band topology results in a quantized transport of Wannier states in the lattice space. For certain parameter choices, very flat yet topologically nontrivial Floquet bands may also emerge, a feature that is potentially useful for the simulation of physics of strongly correlated systems. Some cases with an even number of Floquet bands may also have intriguing Dirac cones in the spectrum. Under open boundary conditions, anomalous counter-propagating chiral edge modes and degenerate zero modes are also found as the system parameters are tuned. These results should be of experimental interest because a continuously driven system is easier to realize than a periodically kicked system.