Sep 2014

Sep 1-Sep 5, Bo Liu, Sep 8-Sep 12, Zhenyu Zhou & Jinlong Yu, Sep 15-Sep 19, Jiyao chen & Jianhui zhou, Sep 22-Sep 26, Haiyuan Zou & Ahmet Keles, Sep 29-Oct 3, Zhifang Xu & Xuguang Yue

Oct 3
1.arXiv:1410.0357 [pdf, other]
Interacting bosons in topological optical flux lattices
A. Sterdyniak, B. Andrei Bernevig, Nigel R. Cooper, N. Regnault
Comments: 14 pages, 19 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

An interesting route to the realization of topological Chern bands in ultracold atomic gases is through the use of optical flux lattices. These models differ from the tight-binding real-space lattice models of Chern insulators that are conventionally studied in solid-state contexts. Instead, they involve the coherent coupling of internal atomic (spin) states, and can be viewed as tight-binding models in reciprocal space. By changing the form of the coupling and the number $N$ of internal spin states, they give rise to Chern bands with controllable Chern number and with nearly flat energy dispersion. We investigate in detail how interactions between bosons occupying these bands can lead to the emergence of fractional quantum Hall states, such as the Laughlin and Moore-Read states. In order to test the experimental realization of these phases, we study their stability with respect to band dispersion and band mixing. We also probe novel topological phases that emerge in these systems when the Chern number is greater than 1.

2. arXiv:1410.0377 [pdf, ps, other]
Vanishing edge currents in non-$p$-wave topological chiral superconductors
Wen Huang, Edward Taylor, Catherine Kallin
Comments: 9 pages, 5 figuresSubjects: Superconductivity (cond-mat.supr-con)

The edge currents of two dimensional topological chiral superconductors with nonzero Cooper pair angular momentum---e.g., chiral $p$-, $d$-, and $f$-wave superconductivity---are studied. Bogoliubov-de Gennes and Ginzburg--Landau calculations are used to show that in the continuum limit, \emph{only} chiral $p$-wave states have a nonzero edge current. Outside this limit, when lattice effects become important, edge currents in non-$p$-wave superconductors are comparatively smaller, but can be nonzero. Using Ginzburg--Landau theory, a simple criterion is derived for when edge currents vanish for non-$p$-wave chiral superconductivity on a lattice. The implications of our results for putative chiral superconductors such as Sr2RuO4 and UPt3 are discussed.

Oct 2
1.arXiv:1410.0067 [pdf, other]
Universal Behavior of Perturbation-Induced Defects in Trapped Superfluids
Peter Scherpelz, Karmela Padavić, Andy Murray, Andreas Glatz, Igor S. Aranson, K. Levin
Comments: 6 pages, 2 figures, and 6 pages supplemental material included
Subjects: Quantum Gases (cond-mat.quant-gas)

We address universal features in the defect dynamics formed after sudden perturbations in ultracold gas superfluids which are confined in elongated traps. Our studies, based on simulations of the complex time-dependent Ginzburg-Landau equation focus on both phase imprinting (at angles away from $\pi$), and localized density depletion perturbations. The challenge we address has to do with the equilibration process: how a superfluid system eventually eliminates an extended planar defect which these experiments induce. In contrast to earlier work where soliton--vortex ring oscillations were reported, we observe planar phase gradients coexisting with one or more near-trap-edge vortex rings. The latter repeatedly enter and exit the trap as the phase gradient dissipates.

Oct 1
1.arXiv:1409.8348 [pdf, other]
Compressibility of a fermionic Mott insulator of ultracold atoms
Pedro M. Duarte, Russell A. Hart, Tsung-Lin Yang, Xinxing Liu, Thereza Paiva, Ehsan Khatami, Richard T. Scalettar, Nandini Trivedi, Randall G. Hulet
Subjects: Quantum Gases (cond-mat.quant-gas)

We characterize the Mott insulating regime of a repulsively interacting Fermi gas of ultracold atoms in a three-dimensional optical lattice. We use in-situ imaging to extract the central density, and determine the local compressibility as a function of local density and interactions by making use of the variation of the chemical potential arising from the confining potential of the optical lattice. For strong interactions, we observe the emergence of a density plateau and a reduction of the compressibility at a density of one atom per site, consistent with the formation of a Mott insulator. Comparisons to numerical simulations of the Hubbard model set an upper limit for the temperature. We show that measurements of the local compressibility are useful for thermometry in a temperature regime above that where magnetic ordering develops.

2.arXiv:1409.8399 [pdf, ps, other]
Quantum non-demolition measurement of single atoms beyond the diffraction limit
Yuto Ashida, Masahito Ueda
Comments: 5 pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Optics (physics.optics); Quantum Physics (quant-ph)

Quantum gases in optical lattices offer an ideal playground to investigate strongly correlated systems and quantum information processing. Recently, the single-site resolved detection and manipulation have emerged as a powerful tool for the study of ultracold lattice gases. However, the diffraction limit imposes severe restrictions and all of the present experiments on single-site resolved detection are destructive. Here we show that continuous monitoring of dispersive scattering light induces the wavefunction collapse into a Fock state and enables the nondestructive measurement of the atom-number statistics at a spatial resolution beyond the diffraction limit. We perform numerical simulations and predict that the Pauli exclusion principle accelerates the rate of wavefunction collapse of fermions in comparison with bosons or classical particles. Our method enables a nondestructive measurement at the single-site level with near-unit fidelity and will have important applications ranging from quantum information to quantum simulation.

Sep 30
1.arXiv:1409.7737 [pdf, ps, other]
Stable dilute supersolid of two-dimensional dipolar bosons
Zhen-Kai Lu, D. S. Petrov, G. V. Shlyapnikov
Comments: 5 pages, 2 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Other Condensed Matter (cond-mat.other); Quantum Physics (quant-ph)

We consider two-dimensional bosonic dipoles oriented perpendicularly to the plane. On top of the usual two-body contact and long-range dipolar interactions we add a contact three-body repulsion as expected, in particular, for dipoles in the bilayer geometry with tunneling. We show that this model allows for stable continuous space supersolid states in the dilute regime and calculate the zero-temperature phase diagram. The three-body repulsion is crucial for stabilizing the system and, combined with the two-body attraction, can lead to self-trapped supersolid droplets.

2.arXiv:1409.7768 [pdf, other]
Topological Effects on the Magnetoconductivity in Topological Insulators
Vincent E. Sacksteder IV, Kristin Bjorg Arnardottir, Stefan Kettemann, Ivan A. Shelykh
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Three-dimensional strong topological insulators (TIs) guarantee the existence of a 2-D conducting surface state which completely covers the surface of the TI. The TI surface state necessarily wraps around the TI's top, bottom, and two sidewalls, and is therefore topologically distinct from ordinary 2-D electron gases (2DEGs) which are planar. This has several consequences for the magnetoconductivity $\Delta \sigma$, a frequently studied measure of weak antilocalization which is sensitive to the quantum coherence time $\tau_\phi$ and to temperature. We show that conduction on the TI sidewalls systematically reduces $\Delta \sigma$, multiplying it by a factor which is always less than one and decreases in thicker samples. In addition, we present both an analytical formula and numerical results for the tilted-field magnetoconductivity which has been measured in several experiments. Lastly, we predict that as the temperature is reduced $\Delta \sigma$ will enter a wrapped regime where it is sensitive to diffusion processes which make one or more circuits around the TI. In this wrapped regime the magnetoconductivity's dependence on temperature, typically $1/T^2$ in 2DEGs, disappears. We present numerical and analytical predictions for the wrapped regime at both small and large field strengths. The wrapped regime and topological signatures discussed here should be visible in the same samples and at the same temperatures where the Altshuler-Aronov-Spivak (AAS) effect has already been observed, when the measurements are repeated with the magnetic field pointed perpendicularly to the TI's top face.

3.arXiv:1409.7881 [pdf, ps, other]
Pseudogap phenomena in ultracold atomic Fermi gases
Qijin Chen, Jibiao Wang
Comments: Invited review article, 32 Figures, 29 pages
Journal-ref: Front. Phys. 9(5), 539-570 (2014)
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)

The pairing and superfluid phenomena in a two-component ultracold atomic Fermi gas is an analogue of Cooper pairing and superconductivity in an electron system, in particular, the high $T_c$ superconductors. Owing to the various tunable parameters that have been made accessible experimentally in recent years, atomic Fermi gases can be explored as a prototype or quantum simulator of superconductors. It is hoped that, utilizing such an analogy, the study of atomic Fermi gases may shed light to the mysteries of high $T_c$ superconductivity. One obstacle to the ultimate understanding of high $T_c$ superconductivity, from day one of its discovery, is the anomalous yet widespread pseudogap phenomena, for which a consensus is yet to be reached within the physics community, after over 27 years of intensive research efforts. In this article, we shall review the progress in the study of pseudogap phenomena in atomic Fermi gases in terms of both theoretical understanding and experimental observations. We show that there is strong, unambiguous evidence for the existence of a pseudogap in strongly interacting Fermi gases. In this context, we shall present a pairing fluctuation theory of the pseudogap physics and show that it is indeed a strong candidate theory for high $T_c$ superconductivity.

Sep 29
1. arXiv:1409.7401 [pdf, other]
Exotic Quantum Phase Transitions of $(2+1)d$ Dirac fermions
Kevin Slagle, Yi-Zhuang You, Cenke Xu
Comments: 12 pages, 9 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

Using determinant quantum Monte Carlo (d-QMC) simulations, we demonstrate that an extended Hubbard model on a bilayer honeycomb lattice has two novel quantum phase transitions. The first is a quantum phase transition between the weakly interacting gapless Dirac fermion phase and a strongly interacting fully gapped and symmetric trivial phase, which cannot be described by the standard Gross-Neveu model. The second is a quantum critical point between a quantum spin Hall insulator with spin $S^z$ conservation and the previously mentioned strongly interacting fully gapped phase. At the latter quantum critical point the single particle excitations remain gapped, while spin and charge gap both close. We argue that the first quantum phase transition is related to the $\mathbb{Z}_{16}$ classification of the topological superconductor $^3\text{He-B}$ phase with interactions, while the second quantum phase transition is a topological phase transition described by a bosonic O(4) nonlinear sigma model field theory with a $\Theta$-term.

2.arXiv:1409.7459 [pdf, ps, other]
Orbital Angular Momentum and Spectral Flow in Two Dimensional Chiral Superfluids
Yasuhiro Tada, Wenxing Nie, Masaki Oshikawa
Comments: 5 pages + 4 pages supplemental material
Subjects: Superconductivity (cond-mat.supr-con)

We study the orbital angular momentum (OAM) $L_z$ in two dimensional chiral $(p_x+ip_y)^{\nu}$-wave superfluids (SF) of $N$ fermions on a disc at zero temperature, in terms of spectral asymmetry and spectral flow. It is shown that $L_z=\nu N/2$ for any integer $\nu$, in the BEC regime. In contrast, in the BCS limit, while the OAM is $L_z=N/2$ for the $p+ip$-wave SF, for chiral SF with $\nu\geq2$, the OAM is remarkably suppressed as $L_z=N\times O(\Delta_0/\varepsilon_F)\ll N$, where $\Delta_0$ is the gap amplitude and $\varepsilon_F$ is the Fermi energy. We demonstrate that the difference between the $p+ip$-wave SF and the other chiral SFs in the BCS regimes originates from the nature of edge modes and related depairing effects.

Sep. 26

arXiv:1409.7315 [pdf, ps, other]
Ground-state and spectral properties of an asymmetric Hubbard ladder
Anas Abdelwahab, Eric Jeckelmann, Martin Hohenadler

Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We investigate a ladder system with two inequivalent legs, namely a Hubbard chain and a one-dimensional electron gas. Analytical approximations, the density matrix renormalization group method, and continuous-time quantum Monte Carlo simulations are used to determine ground-state properties, gaps, and spectral functions of this system at half-filling. Evidence for the existence of four different phases as a function of the Hubbard interaction and the rung hopping is presented. First, a Luttinger liquid exists at very weak interchain hopping. Second, a Kondo-Mott insulator with spin and charge gaps induced by an effective rung exchange coupling is found at moderate interchain hopping or strong Hubbard interaction. Third, a spin-gapped paramagnetic Mott insulator with incommensurate excitations and pairing of doped charges is observed at intermediate values of the rung hopping and the interaction. Fourth, the usual correlated band insulator is recovered for large rung hopping. We show that the wavenumbers of the lowest single-particle excitations are different in each insulating phase. In particular, the three gapped phases exhibit markedly different spectral functions. We discuss the relevance of asymmetric two-leg ladder systems as models for atomic wires deposited on a substrate.

arXiv:1409.7063 [pdf, other]
Robust quantum control using smooth pulses and topological winding
Edwin Barnes, Xin Wang, S. Das Sarma

Comments: 18 pages, 4 figuresSubjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Powerful future technologies based on coherent quantum dynamical systems require an unprecedented level of control. Perhaps the greatest challenge in achieving such control is the decoherence induced by the environment, a problem which pervades experimental quantum physics and is particularly severe in the context of solid state quantum computing and nanoscale quantum devices because of the inherently strong coupling to the surrounding material. Recent years have seen rapid improvement in the quality of materials and in the design and fabrication of such systems, and it is crucial to match this progress with similar advances in the external control protocols used to manipulate quantum states so that the high levels of quantum coherence needed for technological applications persist despite the invariable presence of environmental noise. Here, we present an analytical approach that yields explicit constraints on the driving field which ensure that the leading-order noise-induced errors in a qubit's evolution cancel exactly. We derive constraints for two of the most common types of non-Markovian noise that arise in qubits: slow fluctuations of the qubit energy splitting and fluctuations in the driving field itself. By theoretically recasting a phase in the qubit's wavefunction as a topological winding number, we can satisfy the noise-cancelation conditions by adjusting driving field parameters without altering the target state or quantum evolution. We demonstrate our method by constructing robust quantum gates for two types of spin qubit: phosphorous donors in silicon and nitrogen-vacancy centers in diamond. Our results constitute an important step toward achieving robust generic control of quantum systems, bringing their novel applications closer to realization.

Sep. 25

arXiv:1409.6972 [pdf, other]
Spin-orbit physics of j=1/2 Mott insulators on the triangular lattice
Michael Becker, Maria Hermanns, Bela Bauer, Markus Garst, Simon Trebst

Comments: 5+9 pages, 5+8 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el)

The Heisenberg-Kitaev (HK) model on the triangular lattice is conceptually interesting for its interplay of geometric and exchange frustration. HK models are also thought to capture the essential physics of the spin-orbital entanglement in effective $j=1/2$ Mott insulators studied in the context of various 5d transition metal oxides. Here we argue that the recently synthesized Ba$_3$IrTi$_2$O$_9$ is a prime candidate for a microscopic realization of the triangular HK model. We establish that an infinitesimal Kitaev exchange destabilizes the 120$^\circ$ order of the quantum Heisenberg model and results in the formation of an extended $\mathbb{Z}_2$-vortex crystal phase in the parameter regime most likely relevant to the real material. Using a combination of analytical and numerical techniques we map out the entire phase diagram of the model, which further includes various ordered phases as well as an extended nematic phase around the antiferromagnetic Kitaev point.

arXiv:1409.7016 [pdf, ps, other]
Vortex and Meissner phases of strongly-interacting bosons on a two-leg ladder
M. Piraud, F. Heidrich-Meisner, I. P. McCulloch, S. Greschner, T. Vekua, U. Schollwöck

Comments: 4 pages + Supplementary MaterialSubjects: Quantum Gases (cond-mat.quant-gas)

We establish the phase diagram of the strongly-interacting Bose-Hubbard model defined on a two-leg ladder geometry in the presence of a homogeneous flux. Our work is motivated by a recent experiment [Atala et al., Nature Phys. {\bf 10}, 588 (2014)], which studied the same system, in the complementary regime of weak interactions. Based on extensive density matrix renormalization group simulations and a bosonization analysis, we fully explore the parameter space spanned by filling, inter-leg tunneling, and flux. As a main result, we demonstrate the existence of gapless and gapped Meissner and vortex phases, with the gapped states emerging in Mott-insulating regimes. We calculate experimentally accessible observables such as chiral currents and vortex patterns.

Sep. 24

arXiv:1409.6341 [pdf, other]
Dissipative Chern Insulators
Jan Carl Budich, Peter Zoller, Sebastian Diehl

Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

Engineered dissipation can be employed to efficiently prepare interesting quantum many body states in a non-equilibrium fashion. Here, we study the open quantum system dynamics of fermions on a 2D lattice in the framework of a Lindblad master equation. In particular, we propose a novel mechanism to dissipatively prepare a topological state with non-zero Chern number as the unique steady state by means of short-range system bath interaction. This provides a genuine open quantum system approach to the preparation of topological states, which, quite remarkably, gives rise to a stable topological phase in a non-equilibrium phase diagram. We demonstrate how our theoretical construction can be implemented in a microscopic model that is experimentally feasible with cold atoms in optical lattices.

Sep. 23


arXiv:1409.6211 [pdf, other]
Phase separation in a spin-orbit coupled Bose-Einstein condensate
Sandeep Gautam, S. K. Adhikari

Subjects: Quantum Gases (cond-mat.quant-gas)

We study a spin-orbit (SO) coupled hyperfine spin-1 Bose-Einstein condensate (BEC) in a quasi-one-dimensional trap. For a SO-coupled BEC in a one-dimensional box, we show that in the absence of the Rabi term, any non-zero value of SO coupling will result in a phase separation among the components for a ferromagnetic BEC, like $^{87}$Rb. On the other hand, SO coupling favors miscibility in a polar BEC, like $^{23}$Na. In the presence of a harmonic trap, which favors miscibility, a ferromagnetic BEC phase separates, provided the SO-coupling strength and number of atoms are greater than some critical value. The Rabi term favors miscibility irrespective of the nature of the spin interaction: ferromagnetic or polar.

arXiv:1409.6237 [pdf, other]
Realizing Topological Mott Insulators from the RKKY Interaction
Tianhan Liu, Benoît Douçot, Karyn Le Hur

Subjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas)

We engineer topological insulating phases in a fermion-fermion mixture on the honeycomb lattice, without resorting to artificial gauge fields or spin-orbit couplings and considering only local interactions. Essentially, upon integrating out the fast component (characterized by a larger hopping amplitude) in a finite region of dopings, we obtain an effective interaction between the slow fermions at half-filling, which mimics an alternating magnetic flux and triggers a quantum anomalous Hall phase, then referring to a topological Mott insulator [S. Raghu, X.-L. Qi, C. Honerkamp and S.-C. Zhang Phys. Rev. Lett. 100, 156401 (2008)]. If the second species involves spin-1/2 particles, this interaction may induce a quantum spin Hall phase. By increasing the on-site Hubbard interaction, we identify a quantum phase transition towards a Mott state. Such fermion-fermion mixtures can be realized in optical lattices or in graphene heterostructures.

arXiv:1409.6251 [pdf, other]
Fractional angular momentum in cold atom systems
Yuhe Zhang, G. J. Sreejith, N. D. Gemelke, J. K. Jain

Comments: 5 pages, 3 figuresSubjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

The quantum statistics of bosons or fermions are manifest through even or odd relative angular momentum of a pair. We show theoretically that, under certain conditions, a pair of certain test particles immersed in a fractional quantum Hall state possesses, effectively, a fractional relative angular momentum, which can be interpreted in terms of fractional braid statistics. We propose that the fractionalization of the angular momentum can be detected directly through the measurement of the pair correlation function in rotating ultra-cold atomic systems in the fractional quantum Hall regime. Such a measurement will also provide direct evidence for the effective magnetic field, resulting from Berry phases arising from attached vortices, and of excitations with fractional particle number, analogous to fractional charge of electron fractional quantum Hall effect.

Sep. 22
arXiv:1409.5430 [pdf, other]
Density wave instabilities of fractionalized Fermi liquids
Debanjan Chowdhury, Subir Sachdev

Comments: 20 pages, 6 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)

Recent experiments in the underdoped regime of the hole-doped cuprates have found evidence for an incommensurate charge density wave state. We present an analysis of the charge ordering instabilities in a metal with antiferromagnetic correlations, where the electronic excitations are coupled to the fractionalized excitations of a quantum fluctuating antiferromagnet on the square lattice. The resulting charge density wave state emerging out of such a fractionalized Fermi-liquid (FL*) has wavevectors of the form $(\pm Q_0,0),~(0,\pm Q_0)$, with a predominantly $d$-form factor, in agreement with experiments on a number of different families of the cuprates. In contrast, as previously shown, the charge density wave instability of a nearly antiferromagnetic metal with a large Fermi surface, interacting via short-range interactions, has wavevectors of the type $(\pm Q_0,\pm Q_0)$. Our results show that the observed charge density wave appears as a low-energy instability of a fractionalized metallic state linked to the proximity to an antiferromagnetic insulator, and the pseudogap regime can be described by such a metal at least over intermediate length and energy scales.

arXiv:1409.5649 [pdf, ps, other]
Correlations in the low-density Fermi gas: Fermi-Liquid state, Dimerization, and BCS Pairing
H. H. Fan, E. Krotscheck, T. Lichtenegger, D. Mateo, R. E. Zillich

Comments: 23 pages, 18 figuresSubjects: Quantum Gases (cond-mat.quant-gas)

We present ground state calculations for low-density Fermi gases described by two model interactions, an attractive square-well potential and a Lennard-Jones potential, of varying strength. We use the optimized Fermi-Hypernetted Chain integral equation method which has been proved to provide, in the density regimes of interest here, an accuracy better than one percent. We first examine the low-density expansion of the energy and compare with the exact answer by Huang and Yang (H. Huang and C. N. Yang, {\em Phys. Rev.\/} {\bf 105}, 767 (1957)). It is shown that a locally correlated wave function of the Jastrow-Feenberg type does not recover the quadratic term in the expansion of the energy in powers of $a_0 k_F$, where $a_0$ is the vacuum $s$-wave scattering length and $k_F$ the Fermi wave number. The problem is cured by adding second-order perturbation corrections in a correlated basis.
Going to higher densities and/or more strongly coupled systems, we encounter an instability of the system which is characterized by a divergence of the {\em in-medium\/} scattering length. We interpret this divergence as a phonon-exchange driven dimerization of the system, similar to what one has at zero density when the vacuum scattering length diverges.
We then study, in the stable regime, the superfluid gap and its dependence on the density and the interaction strength. We identify two different corrections to low-density expansions: One are medium corrections to the pairing interaction, and the other one finite-range corrections. We show that the most important finite-range corrections are a direct manifestation of the many-body nature of the system.

Sep. 19
1. arXiv:1409.5387 [pdf, ps, other]
Long-range interactions and roton minimum softening in a spin-orbit coupled Bose-Einstein condensate
M. A. Khamehchi, Y. Zhang, C. Hamner, Th. Busch, P. Engels

Comments: 5 pages, 5 figuresSubjects: Quantum Gases (cond-mat.quant-gas)

We experimentally probe the collective excitations of a spin-orbit coupled Bose-Einstein condensate and show the existence of roton structures. These are due to the long-range interactions mediated by the spin-orbit coupling, which is generated by Raman dressing of atomic hyperfine states. When the Raman detuning is decreased, the roton mode softens and in our experiment we directly observe this effect by performing Bragg spectroscopy. We also show that for the parameters of our system, this softening stops at a finite excitation gap, which precludes a transition to a supersolid-like phase. A time-reversal like symmetry inherent in the system's Hamiltonian is confirmed by showing that the roton softening is symmetric under a sign change of the Raman detuning. Finally, using a moving barrier that is swept through the BEC, we also show that the presence of a roton can lead to interesting consequences for the fluid dynamics.

Sep. 18
1. arXiv:1409.5070 [pdf, other]
Phase structure of spin-imbalanced unitary Fermi gases
I. Boettcher, J. Braun, T. K. Herbst, J. M. Pawlowski, D. Roscher, C. Wetterich

Comments: 13 pages, 6 figuresSubjects: Quantum Gases (cond-mat.quant-gas); High Energy Physics - Phenomenology (hep-ph); Nuclear Theory (nucl-th)

We investigate the phase structure of spin-imbalanced unitary Fermi gases beyond mean-field theory by means of the Functional Renormalization Group. In this approach, quantum and thermal fluctuations are resolved in a systematic manner. The discretization of the effective potential on a grid allows us to accurately account for both first- and second-order phase transitions that are present on the mean-field level. We compute the full phase diagram in the plane of temperature and spin-imbalance and discuss the existence of other conjectured phases such as the Sarma phase and a precondensation region. In addition, we explain on a qualitative level how we expect that in-situ density images are affected by our findings and which experimental signatures may potentially be used to probe the phase structure.

2. arXiv:1409.4770 [pdf, other]
Quantum simulation of non-trivial topology
Octavi Boada, Alessio Celi, Maciej Lewenstein, Javier Rodríguez-Laguna, José I. Latorre

Comments: 12 pages, 15 figuresSubjects: Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)

We propose several designs to simulate quantum many-body systems in manifolds with a non-trivial topology. The key idea is to create a synthetic lattice combining real-space and internal degrees of freedom via a suitable use of induced hoppings. The simplest example is the conversion of an open spin-ladder into a closed spin-chain with arbitrary boundary conditions. Further exploitation of the idea leads to the conversion of open chains with internal degrees of freedom into artificial tori and M\"obius strips of different kinds. We show that in synthetic lattices the Hubbard model on sharp and scalable manifolds with non-Euclidean topologies may be realized. We provide a few examples of the effect that a change of topology can have on quantum systems amenable to simulation, both at the single-particle and at the many-body level.

3. arXiv:1409.5001 (cross-list from cond-mat.str-el) [pdf, ps, other]
Escort distribution function of work done and diagonal entropies in quenched Luttinger liquids
Balázs Dóra

Comments: 5 pages, 2 figures, positive comments are welcomeSubjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas)We study the escort probability distribution function of work done during an interaction quantum quench of Luttinger liquids. It crosses over from the thermodynamic to the small system limit with increasing a, the order of the escort distribution, and depends on the universal combination (|Ki−Kf|/(Ki+KF))a with Ki, Kf the initial and final Luttinger liquid parameters, respectively. From its characteristic function, the diagonal R\'enyi entropies and the many body inverse participation ratio (IPR) are determined to evaluate the information content of the time evolved wavefunction in terms of the eigenstates of the final Hamiltonian. The hierarchy of overlaps is dominated by that of the ground states. The IPR exhibits a crossover from Gaussian to power law decay with increasing interaction quench parameter.






Sep. 17
1. arXiv:1409.4743 [pdf, other]
Breakdown of Fermi liquid description for strongly interacting fermions
Yoav Sagi, Tara E. Drake, Rabin Paudel, Roman Chapurin, Deborah S. Jin

Comments: 32 pages including 4 main figures and a supplementary section with 5 extended data figuresSubjects: Quantum Gases (cond-mat.quant-gas)

An ultracold Fermi gas with tunable interactions is a paradigmatic strongly correlated system. These atomic gases provide access to the crossover from Bardeen-Cooper-Schrieffer (BCS) superconductivity to Bose-Einstein condensation (BEC) of tightly bound fermion pairs. The nature of the normal state in this BCS-BEC crossover is an intriguing and controversial topic. While the many-body ground state remains a condensate of paired fermions, the normal state must evolve from a Fermi liquid to a Bose gas of molecules as a function of the interaction strength. How this occurs is still largely unknown. We explore this question with measurements of the distribution of single-particle energies and momenta in a nearly homogeneous gas above Tc. We find that the data fit well to a function that includes a narrow, positively dispersing peak and an "incoherent background" that can accommodate broad, asymmetric line shapes. The peak corresponds to quasiparticles in a Fermi liquid and has a spectral weight Z. We find that Z vanishes abruptly as the strength of interactions is modified, which signals the breakdown of a Fermi liquid description. Such a sharp feature is surprising in a crossover.

2. arXiv:1409.4630 [pdf, ps, other]
(In)stability of itinerant ferromagnetism in ultracold atomic gases
E. Vermeyen, J. Tempere

Subjects: Quantum Gases (cond-mat.quant-gas)It has long been predicted that a two-component non-localized Fermi gas will exhibit spontaneous polarization for sufficiently strong repulsive interactions, a phenomenon which is called itinerant ferromagnetism. Recent experiments with ultracold atomic gases have reached the interaction strength for which theoretical models have predicted the occurrence of the normal-to-itinerant-ferromagnetic phase transition, but so far this transition has not been observed. The instability of the repulsive branch of the Feshbach resonance prevents the formation of the itinerant ferromagnetic state, but it is not clear whether this is the only instability impeding its experimental realization. In this article, we use the path-integral formalism with density fields in the Hubbard-Stratonovich transformation to study the stability of a homogeneous two-component Fermi gas with contact interactions. Within the saddle-point approximation we show that none of the extrema of the action are minima, meaning all extrema are unstable to small density fluctuations. This implies a more general mechanical instability of the polarized (itinerant ferromagnetic) and normal states of the system in the path-integral formalism. We find that it is important to consider the stability of the system when studying itinerant ferromagnetism. Since (mechanical) stability may be influenced by the details of the interaction potential, we suggest the use of a more realistic potential than the contact potential in future theoretical descriptions.




Sep. 16
1. arXiv:1409.4048 (cross-list from nucl-th) [pdf, other]
Low Energy Continuum and Lattice Effective Field Theories
Serdar Elhatisari

Comments: Ph.D. thesis, 201 pages, 19 figures, 17 tablesSubjects: Nuclear Theory (nucl-th); Quantum Gases (cond-mat.quant-gas); High Energy Physics - Lattice (hep-lat); Atomic Physics (physics.atom-ph)In the first part of the thesis we consider the constraints of causality and unitarity for particles interacting via strictly finite-range interactions. We generalize Wigner's causality bound to the case of non-vanishing partial-wave mixing. Specifically we analyze the system of the low-energy interactions between protons and neutrons. We also analyze low-energy scattering for systems with arbitrary short-range interactions plus an attractive 1/rα tail for α≥2. In particular, we focus on the case of α=6 and we derive the constraints of causality and unitarity also for these systems and find that the van der Waals length scale dominates over parameters characterizing the short-distance physics of the interaction. This separation of scales suggests a separate universality class for physics characterizing interactions with an attractive 1/r6 tail. We argue that a similar universality class exists for any attractive potential 1/rα for α≥2.

In the second part of the thesis we present lattice Monte Carlo calculations of fermion-dimer scattering in the limit of zero-range interactions using the adiabatic projection method. The adiabatic projection method uses a set of initial cluster states and Euclidean time projection to give a systematically improvable description of the low-lying scattering cluster states in a finite volume. We use L\"uscher's finite-volume relations to determine the s-wave, p-wave, and d-wave phase shifts. For comparison, we also compute exact lattice results using Lanczos iteration and continuum results using the Skorniakov-Ter-Martirosian equation. For our Monte Carlo calculations we use a new lattice algorithm called impurity lattice Monte Carlo. This algorithm can be viewed as a hybrid technique which incorporates elements of both worldline and auxiliary-field Monte Carlo simulations.




Sep. 15
1. arXiv:1409.3727 [pdf, ps, other]
Phase space manipulations of many-body wavefunctions
G. Condon, A. Fortun, J. Billy, D. Guéry-Odelin

Comments: 7 pages, 6 figures, submitted to Phys. Rev. ASubjects: Quantum Gases (cond-mat.quant-gas)We explore the manipulation in phase space of many-body wavefunctions that exhibit self-similar dynamics, under the application of sudden force and/or in the presence of a constant acceleration field. For this purpose, we work out a common theoretical framework based on the Wigner function. We discuss squeezing in position space, phase space rotation and its implications in cooling for both non-interacting and interacting gases, and time reversal operation. We discuss various optical analogies and calculate the role of spherical-like aberration in cooling protocols. We also present the equivalent of a spin-echo technique to improve the robustness of velocity dispersion reduction protocols.




Sept. 12

1. arXiv:1409.3294 [pdf, ps, other]
Onset of the Limit Cycle and Universal Three-Body Parameter in Efimov Physics
Yusuke Horinouchi, Masahito Ueda
The Efimov effect is the only experimentally realized universal phenomenon that exhibits the renormalization-group limit cycle with the three-body parameter parametrizing a family of universality classes. Recent experiments in ultracold atoms have unexpectedly revealed that the three-body parameter itself is universal when measured in units of an effective range. By performing an exact functional renormalization-group analysis with various finite-range interaction potentials, we demonstrate that the onset of the renormalization-group flow into the limit cycle is universal, regardless of short-range details, which connects the missing link between the two universalities of the Efimov physics. A close connection between the topological property of the limit cycle and few-body physics is also delineated.

Sept. 11

1. arXiv:1409.3100 [pdf, other]
Strongly Correlated Quantum Walks in Optical Lattices
Philipp M. Preiss, Ruichao Ma, M. Eric Tai, Alexander Lukin, Matthew Rispoli, Philip Zupancic, Yoav Lahini, Rajibul Islam, Markus Greiner
Full control over the dynamics of interacting, indistinguishable quantum particles is an important prerequisite for the experimental study of strongly correlated quantum matter and the implementation of high-fidelity quantum information processing. Here we demonstrate such control over the quantum walk - the quantum mechanical analogue of the classical random walk - in the strong interaction regime. We directly observe fundamental effects such as the emergence of correlations due to quantum statistics and interactions in two-particle quantum walks, as well as strongly correlated Bloch oscillations in tilted optical lattices. Our approach can be scaled to larger systems, greatly extending the class of problems accessible via quantum walks, which can now serve as a basis for universal quantum computation and as a quantum simulator for strongly correlated many-body dynamics.

2. arXiv:1409.3149 [pdf, other]
How to make visible and stable stripes in a spin-orbit-coupled Bose-Einstein superfluid
Giovanni I. Martone, Yun Li, Sandro Stringari
The striped phase exhibited by a spin-1/2 Bose-Einstein condensate with spin-orbit coupling is characterized by the spontaneous breaking of two continuous symmetries: gauge and translational symmetry. This feature, which is peculiar of supersolids and is the consequence of interaction effects, shows up in important phenomena, like the occurrence of density fringes and of a double gapless band structure in the excitation spectrum. We propose an approach to increase significantly the contrast of fringes as well as their space separation, making their experimental detection in atomic gases a realistic perspective. The approach is based on the space separation of the two spin components into a 2D bi-layer configuration, causing the reduction of the effective interspecies interaction, and on the application of a π/2 Bragg pulse, causing the increase of the wavelength of the fringes.

Sept. 10

1. arXiv:1409.2519 [pdf, other]
Three-dimensional laser cooling at the Doppler limit
Rockson Chang, Lynn Hoendervanger, Quentin Bouton, Yami Fang, Tobias Klafka, Kevin Audo, Alain Aspect, Christoph I Westbrook, David Clément
Many predictions of the theory of Doppler cooling of 2-level atoms, notably the celebrated minimum achievable temperature TD=ℏΓ/2kB, have never been verified in a three-dimensional geometry. Here, we show that, despite their degenerate level structure, we can use Helium-4 atoms to achieve a situation in which these predictions can be verified. We make measurements of atomic temperatures, magneto-optical trap sizes, and the sensitivity of optical molasses to a power imbalance in the laser beams, finding excellent agreement with the Doppler theory. We show that the special properties of Helium, particularly its small mass and narrow transition linewidth, prevent effective sub-Doppler cooling with red-detuned optical molasses.

Sept. 9

1. arXiv:1409.1945 [pdf, other]
Dynamical phase transition in the open Dicke model
J. Klinder, H. Keßler, M. Wolke, L. Mathey, A. Hemmerich
The Dicke model is realized in the regime of weak dissipation by coupling a Bose-Einstein condensate to an optical cavity with ultra-narrow bandwidth. We explore the dynamical critical properties of the Hepp-Lieb-Dicke phase transition by performing quenches across the phase boundary. We observe hysteresis in the transition between a normal phase and a self-organized collective phase with an enclosed loop area showing power law scaling with respect to the quench time, which suggests an interpretation within a general framework introduced by Kibble and Zurek. Our work provides new insights into the non-equilibrium physics of a many-body-system with infinite range interactions.

2. arXiv:1408.5341 (cross-list from quant-ph) [pdf, other]
Exact Diagonalization of Heisenberg SU(N) models
Pierre Nataf, Frederic Mila
Building on advanced results on permutations, we show that it is possible to construct, for each irreducible representation of SU(N), an orthonormal basis labelled by the set of {\it standard Young tableaux} in which the matrix of the Heisenberg SU(N) model (the quantum permutation of N-color objects) takes an explicit and extremely simple form. Since the relative dimension of the full Hilbert space to that of the singlet space on n sites increases very fast with N, this formulation allows to extend exact diagonalizations of finite clusters to much larger values of N than accessible so far. Using this method, we show that, on the square lattice, there is long-range color order for SU(5), spontaneous dimerization for SU(8), and evidence in favor of a quantum liquid for SU(10).

Sept. 8

1. arXiv:1409.1885 [pdf, other]
Edge binding of sine-Gordon solitons in spin-orbit coupled Bose-Einstein condensates
Sebastiano Peotta, Francisco Mireles, Massimiliano Di Ventra
In recent experiments with ultracold gases a Raman coupling scheme is used to produce both spin-orbit (SO) and Zeeman-type couplings [Y.-J. Lin et al., Nature 471, 83 (2011)]. The competition between them drives a phase transition to a magnetized state with broken Z2 symmetry. Using a hydrodynamic approach we study a confined binary condensate subject to both SO and Zeeman-type couplings. We find that in the limit of strong interactions, and in the phase with unbroken symmetry, the magnetization profile has an analytical solution of the form of a sine-Gordon soliton, which is bound to the edge of the system by the boundary condition induced by SO. In the magnetized phase instead, the boundary structure is well captured by a modified O(3) nonlinear sigma model with the same boundary condition. We further discuss how the non-trivial magnetization structure affects the density profile near the boundary, yet another prediction that can be tested in current experiments of spin-orbit coupled condensates.


Sep 5

1. arXiv:1409.1242 [pdf, ps, other]
Stoner ferromagnetism in a thermal pseudospin-1/2 Bose gas
Juraj Radic, Stefan S. Natu, Victor Galitski
We compute the finite-temperature phase diagram of a pseudospin-1/2 Bose gas with contact interactions, using two complementary methods: the random phase approximation (RPA) and self-consistent Hartree-Fock theory. We show that the inter-spin interactions, which break the (pseudo) spin-rotational symmetry of the Hamiltonian, generally lead to the appearance of a magnetically ordered phase at temperatures above the superfluid transition. In three dimensions, we predict a normal easy-axis/easy-plane ferromagnet for sufficiently strong repulsive/attractive inter-species interactions respectively. The normal easy-axis ferromagnet is the bosonic analog of Stoner ferromagnetism known in electronic systems. For the case of inter-spin attraction, we also discuss the possibility of a \textit{bosonic} analog of the Cooper paired phase. This state is shown to significantly lose in energy to the transverse ferromagnet in three dimensions, but is more energetically competitive in lower dimensions. Extending our calculations to a spin-orbit-coupled Bose gas with equal Rashba and Dresselhaus-type couplings (as recently realized in experiment), we investigate the possibility of stripe ordering in the normal phase. Within our approximations however, we do not find an instability towards stripe formation, suggesting that the stripe order melts below the condensation temperature, which is consistent with the experimental observations of Ji \textit{et al.} [Ji \textit{et al.}, Nature Physics \textbf{10}, 314 (2014)].


2. arXiv:1409.1216 [pdf, other]
Thermal Phase Transitions of Strongly Correlated Bosons with Spin-Orbit Coupling
Ciarán Hickey, Arun Paramekanti
Experiments on ultracold atomic gases have begun to explore lattice effects and thermal fluctuations for two-component bosons with spin-orbit coupling (SOC). Motivated by this, we study a tJ model of strongly correlated lattice bosons, with equal Rashba-Dresselhaus SOC and a uniform magnetic field. At zero temperature, a Gutzwiller ansatz is shown to capture lattice variants of stripe superfluid (SF) ground states. We formulate a finite temperature generalization of the Gutzwiller approach and show that thermal fluctuations in the doped Mott insulator drive a two-step melting of the stripe SF, revealing a wide intermediate regime of a normal fluid with stripe order.

Sep 4

1. arXiv:1409.1192 [pdf, ps, other]
Cold interactions between an Yb$^+$ ion and a Li atom: Prospects for sympathetic cooling, radiative association, and Feshbach resonances
Michał Tomza, Christiane P. Koch, Robert Moszynski
The electronic structure of the (LiYb)+ molecular ion is investigated with two variants of the coupled cluster method restricted to single, double, and noniterative or linear triple excitations. Potential energy curves for the ground and excited states, permanent and transition electric dipole movements, and long-range interaction coefficients C4 and C6 are reported. The data is subsequently employed in scattering calculations and photoassociation studies. Feshbach resonances are shown to be measurable despite the ion's micromotion in the Paul trap. Molecular ions can be formed in their singlet electronic ground state by one-photon photoassociation and in triplet states by two-photon photoassociation; and control of cold atom-ion chemistry based on Feshbach resonances should be feasible. Conditions for sympathetic cooling of an Yb+ ion by an ultracold gas of Li atoms are found to be favorable in the temperature range of 10mK to 10nK; and further improvements using Feshbach resonances should be possible. Overall, these results suggest excellent prospects for building a quantum simulator with ultracold Yb+ ions and Li atoms.

Sep 3

1.arXiv:1409.0649 [pdf, ps, other]
Transport of the repulsive Bose-Einstein condensate in a double-well trap: interaction impact and relation to Josephson effect
V.O. Nesterenko, A.N. Novikov, E. Suraud
Two aspects of the transport of the repulsive Bose-Einstein condensate (BEC) in a double-well trap are inspected: impact of the interatomic interaction and analogy to the Josephson effect. The analysis employs a numerical solution of 3D time-dependent Gross-Pitaevskii equation for a total order parameter covering all the trap. The population transfer is driven by a time-dependent shift of a barrier separating the left and right wells. Sharp and soft profiles of the barrier velocity are tested. Evolution of the relevant characteristics, involving phase differences and currents, is inspected. It is shown that the repulsive interaction substantially supports the transfer making it possible i) in a wide velocity interval and ii) three orders of magnitude faster than in the ideal BEC. The transport can be approximately treated as the d.c. Josephson effect. A dual origin of the critical barrier velocity (break of adiabatic following and d.c.-a.c. transition) is discussed. Following the calculations, robustness of the transport (d.c.) crucially depends on the interaction and barrier velocity profile. Only soft profiles which minimize undesirable dipole oscillations are acceptable.

2.arXiv:1409.0560 [pdf, other]
Damping of Bloch oscillations: variational solutions of the Boltzmann equation beyond linear response
Stephan Mandt
Variational solutions of the Boltzmann equation usually rely on the concept of linear response. We extend the variational approach for tight-binding models at high entropies to a regime far beyond linear response. We analyze the case of interacting Fermions on a lattice, where interactions are too weak to over-damp the driving force; leading to the well-known Bloch oscillations. This regime is computationally demanding and relevant for ultracold atoms in optical lattices. We derive a simple theory in terms of coupled dynamic equations for the particle density, energy density, current and heat current, allowing for analytic solutions. As an application, we identify damping coefficients for Bloch oscillations in the Hubbard model at weak interactions and compute them for a one-dimensional model. We also approximately solve the long-time dynamics of a weakly interacting, strongly Bloch-oscillating cloud of fermionic particles in a tilted lattice, leading to a sub-diffusive scaling exponent.

Sep 2

1.arXiv:1409.0284 [pdf, other]
Feshbach Insulator from Atom-Molecule Coherence of Bosons in Optical Lattices
Laurent de Forges de Parny, Valy G. Rousseau, Tommaso Roscilde
Feshbach resonances - namely resonances between an unbound two-body state (atomic state) and a bound (molecular) state, differing in magnetic moment - are a unique tool to tune the interaction properties of ultracold atoms. Here we show that the spin-changing interactions, coherently coupling the atomic and molecular state, can lead to a novel insulating phase - the Feshbach insulator - for bosons in an optical lattice close to a narrow Feshbach resonance. Making use of quantum Monte Carlo simulations and mean-field theory, we show that the Feshbach insulator appears around the resonance, preventing the system from collapsing when the effective atomic scattering length becomes negative. On the atomic side of the resonance, the transition from condensate to Feshbach insulator has a characteristic first-order nature, due to the simultaneous loss of coherence in the atomic and molecular components. These features appear clearly in the ground-state phase diagram of e.g. 87Rb around the 414 G resonance, and they are therefore directly amenable to experimental observation.


2.arXiv:1409.0335 [pdf, ps, other]

Quantum bright solitons in the Bose-Hubbard model with site-dependent repulsive interactions

L. Barbiero, B. A. Malomed, L. Salasnich


We introduce a one-dimensional (1D) spatially inhomogeneous Bose-Hubbard model (BHM) with the strength of the onsite repulsive interactions growing, with the discrete coordinate zj, as |zj|α with α>0. Recently, the analysis of the mean-field (MF) counterpart of this system has demonstrated self-trapping of robust unstaggered discrete solitons, under condition α>1. Using the numerically implemented method of the density matrix renormalization group (DMRG), we demonstrate that, in a certain range of interaction, the BHM also self-traps, in the ground state, into a soliton-like configuration, at α>1, and remains weakly localized at α<1. An essential quantum feature is a residual density in the background surrounding the soliton-like peak in the BHM ground state, while in the MF limit the finite-density background is absent. Very strong onsite repulsion eventually destroys soliton-like states, and, for integer densities, the system enters the Mott phase with a spatially uniform density.

Sep 1

1.arXiv:1408.6886 [pdf, other]
Elementary excitations of chiral Bose-Einstein condensates
Matthew Edmonds, Manuel Valiente, Patrik Öhberg
We study the collective modes of a Bose-Einstein condensate subject to an optically induced density-dependent gauge potential. The corresponding interacting gauge theory lacks Galilean invariance, yielding an exotic superfluid state. We find that the excitation structure of this condensate displays dynamical instabilities which stem from the current nonlinearity. The nonlinear dynamics in the presence of a current nonlinearity and an external harmonic trap, is found to give rise to dynamics which violates the Kohn theorem where the frequency of the dipole mode strongly depends on the strength of the mass current in the gas.