Nov 2015
Nov 2-Nov 6 Bo Liu, Nov 9-Nov 13 Max, Nov 16-Nov 20 Haiyuan Zou, Nov 23-Nov 27 Ahmet Keles
Nov 23-27
arXiv:1511.08170 [pdf, other]
Realization of Two-Dimensional Spin-orbit Coupling for Bose-Einstein Condensates
Zhan Wu, Long Zhang, Wei Sun, Xiao-Tian Xu, Bao-Zong Wang, Si-Cong Ji, Youjin Deng, Shuai Chen, Xiong-Jun Liu, Jian-Wei Pan
Comments: 27 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)
Cold atoms with laser-induced spin-orbit (SO) interactions provide intriguing new platforms to explore novel quantum physics beyond natural conditions of solids. Recent experiments demonstrated the one-dimensional (1D) SO coupling for boson and fermion gases. However, realization of 2D SO interaction, a much more important task, remains very challenging. Here we propose and experimentally realize, for the first time, 2D SO coupling and topological band with $^{87}$Rb degenerate gas through a minimal optical Raman lattice scheme, without relying on phase locking or fine tuning of optical potentials. A controllable crossover between 2D and 1D SO couplings is studied, and the SO effects and nontrivial band topology are observed by measuring the atomic cloud distribution and spin texture in the momentum space. Our realization of 2D SO coupling with advantages of small heating and topological stability opens a broad avenue in cold atoms to study exotic quantum phases, including the highly-sought-after topological superfluid phases.
arXiv:1511.08010 [pdf, ps, other]
Topological phases of lattice bosons with a dynamical gauge field
David Raventós, Tobias Graß, Bruno Juliá-Díaz, Luis Santos, Maciej Lewenstein
Comments: 9 pages, 8 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
Optical lattices with a complex-valued tunnelling term have become a standard way of studying gauge-field physics with cold atoms. If the complex phase of the tunnelling is made density-dependent, such system features even a self-interacting or dynamical magnetic field. In this paper we study the scenario of a few bosons in either a static or a dynamical gauge field by means of exact diagonalization. The topological structures are identified computing their Chern number. Upon decreasing the atom-atom contact interaction, the effect of the dynamical gauge field is enhanced, giving rise to a phase transition between two topologically non-trivial phases.
arXiv:1511.08001 [pdf, ps, other]
Antiferromagnetism, $f$-wave and chiral $p$-wave superconductivity in a Kagome lattice with possible application to $sd^2$-graphenes
Wan-Sheng Wang, Yuan-Chun Liu, Yuan-Yuan Xiang, Qiang-Hua Wang
Comments: 6 pages, 5 color figures
Subjects: Superconductivity (cond-mat.supr-con); Strongly Correlated Electrons (cond-mat.str-el)
We investigate the electronic instabilities in a Kagome lattice with Rashba spin-orbital coupling by the unbiased singular-mode functional renormalization group. At the parent $1/3$-filling, the normal state is a quantum spin Hall system. Since the bottom of the conduction band is near the van Hove singularity, the electron-doped system is highly susceptible to competing orders upon electron interactions. The topological nature of the parent system enriches the complexity and novelty of such orders. We find $120^o$-type intra-unitcell antiferromagnetic order, $f$-wave superconductivity and chiral $p$-wave superconductivity with increasing electron doping above the van Hove point. In both types of superconducting phases, there is a mixture of comparable spin singlet and triplet components because of the Rashba coupling. The chiral $p$-wave superconducting state is characterized by a Chern number $Z=1$, supporting a branch of Weyl fermion states on each edge. The model bares close relevance to the so-called $sd^2$-graphenes proposed recently.
arXiv:1511.07890 [pdf, other]
$η$-pairing Superfluid in Periodically-Driven Fermionic Hubbard Model with Strong Attraction
Sota Kitamura, Hideo Aoki
Comments: 7 pages, 8 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Superconductivity (cond-mat.supr-con)
We propose a novel possibility of dynamically changing the pairing of fermionic superfluids/superconductors from $s$-wave to $\eta$-pairing by driving the system with ac fields. We consider a periodically-driven attractive Hubbard model in the strong-coupling limit, and show that the pair-hopping and pair-repulsion terms in the effective Hamiltonian in the Floquet formalism are drastically renormalized, which can change the ground states from an $s$-wave superfluid to an $\eta$-pairing superfluid or a charge-ordered phase. While a simple quench scheme would fail to realize the dynamical phase transition to $\eta$-pairing superfluid due to the inverted population in isolated systems as in cold atoms, we can circumvent this with dynamical instabilities in a two-stage protocol via a charge-ordered phase to realize the $\eta$-pairing superfluid.
arXiv:1511.07874 [pdf, other]
Dynamical Cooper pairing in non-equilibrium electron-phonon systems
Michael Knap, Mehrtash Babadi, Gil Refael, Ivar Martin, Eugene Demler
Subjects: Superconductivity (cond-mat.supr-con); Strongly Correlated Electrons (cond-mat.str-el); Quantum Physics (quant-ph)
Ultrafast laser pulses have been used to manipulate complex quantum materials and to induce dynamical phase transitions. One of the most striking examples is the transient enhancement of superconductivity in several classes of materials upon irradiating them with high intensity pulses of terahertz light. Motivated by these experiments we analyze the Cooper pairing instabilities in non-equilibrium electron-phonon systems. We demonstrate that the light induced non-equilibrium state of phonons results in the following phenomena: an increase in the time averaged superconducting coupling constant, its periodic modulation, and an increase in the electron scattering. While the first two enhance Cooper pairing, the third one suppresses it. We analyze the competition between these effects and show that in a broad range of parameters the dynamic enhancement of pair formation dominates over the increase in the scattering rate. This opens the possibility of transient light induced superconductivity at temperatures that are considerably higher than the equilibrium transition temperatures. Our results pave new pathways for engineering high-temperature light-induced superconducting states.
arXiv:1511.07287 [pdf, other]
Direct probing of the Mott crossover in the SU($N$) Fermi-Hubbard model
Christian Hofrichter, Luis Riegger, Francesco Scazza, Moritz Höfer, Diogo Rio Fernandes, Immanuel Bloch, Simon Fölling
Subjects: Quantum Gases (cond-mat.quant-gas)
The Fermi-Hubbard model (FHM) is a cornerstone of modern condensed matter theory. Developed for interacting electrons in solids, which typically exhibit SU($2$) symmetry, it describes a wide range of phenomena, such as metal to insulator transitions and magnetic order. Its generalized SU($N$)-symmetric form, originally applied to multi-orbital materials such as transition-metal oxides, has recently attracted much interest owing to the availability of ultracold SU($N$)-symmetric atomic gases. Here we report on a detailed experimental investigation of the SU($N$)-symmetric FHM using local probing of an atomic gas of ytterbium in an optical lattice to determine the equation of state through different interaction regimes. We prepare a low-temperature SU($N$)-symmetric Mott insulator and characterize the Mott crossover, representing important steps towards probing predicted novel SU($N$)-magnetic phases.
arXiv:1511.07197 [pdf, ps, other]
The Sagnac Effect in Optical Lattices with Laser-Assisted Tunneling
Bo-Nan Jiang, Xiao-Gang Wei, Jia-Hua Li, Guo-Wan Zhang, Yong-Jie Cheng, Cheng Xu
Subjects: Quantum Gases (cond-mat.quant-gas)
We propose a scheme to realize the enhanced full-loop Sagnac phase shift using optical lattices with laser-assisted tunneling. We demonstrate that the Sagnac effect manifests itself in both the spin-dependent crossovers in the single-particle energy spectrum and the spin-dependent response of the cyclotron orbits of the atom flux. The full-loop Sagnac phase shift between the spinup and spin-down components of the spin-orbit-coupled bosons exceeds that of the atom-based Mach-Zehnder system by a factor of 2, being further enhanced by the cyclotron motion. We also theoretically show that the sensitivity limit of the spin-orbit-coupled system to the rotational motion can reach at least 7*10^-8 rad s^-1 Hz^1/2.
arXiv:1511.06515 [pdf, other]
Quantum solitons with emergent interactions in a model of cold atoms on the triangular lattice
Hiroaki T. Ueda, Yutaka Akagi, Nic Shannon
Comments: 8 pages, 5 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Statistical Mechanics (cond-mat.stat-mech)
Cold atoms bring new opportunities to study quantum magnetism, and in particular, to simulate quantum magnets with symmetry greater than $SU(2)$. Here we explore the topological excitations which arise in a model of cold atoms on the triangular lattice with $SU(3)$ symmetry. Using a combination of homotopy analysis and analytic field-theory we identify a new family of solitonic wave functions characterised by integer charge ${\bf Q} = (Q_A, Q_B, Q_C)$, with $Q_A + Q_B + Q_C = 0$. We use a numerical approach, based on a variational wave function, to explore the stability of these solitons on a finite lattice. We find that, while solitons with charge ${\bf Q} = (Q, -Q, 0)$ are stable, wave functions with more general charge spontaneously decay into pairs of solitons with emergent interactions. This result suggests that it could be possible to realise a new class of interacting soliton, with no classical analogue, using cold atoms. It also suggests the possibility of a new form of quantum spin liquid, with gauge--group U(1)$\times$U(1).
arXiv:1511.06366 [pdf, other]
Site-resolved imaging of a fermionic Mott insulator
Daniel Greif, Maxwell F. Parsons, Anton Mazurenko, Christie S. Chiu, Sebastian Blatt, Florian Huber, Geoffrey Ji, Markus Greiner
Comments: 6+7 pages
Subjects: Quantum Gases (cond-mat.quant-gas)
The complexity of quantum many-body systems originates from the interplay of strong interactions, quantum statistics, and the large number of quantum-mechanical degrees of freedom. Probing these systems on a microscopic level with single-site resolution offers unique insight with unprecedented control. Here we report site-resolved imaging of two-component fermionic Mott insulators, metals, and band insulators using ultracold atoms in a square lattice. We observe large, defect-free 2D Mott insulators for strong repulsive interactions. For intermediate interactions we observe a coexistence of phases. From comparison to theory we find trap-averaged entropies per particle of $1.0\,k_{\mathrm{B}}$ and local entropies as low as $0.5\,k_{\mathrm{B}}$. This experiment is a vital step towards probing quantum-mechanical models in regimes inaccessible by modern theoretical methods.
Nov 20
arXiv:1511.05961 [pdf, other]
Anomalous conductances in a tunable Fermi gas
Sebastian Krinner, Martin Lebrat, Dominik Husmann, Charles Grenier, Jean-Philippe Brantut, Tilman Esslinger
Comments: 12 pages, 8 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
The conductance of a quantum point contact is quantized in units of 1/h, with h being Planck's constant, which is the universal upper bound to transport set by Heisenberg's and Pauli's principles. Can interactions cause a breakdown of this quantization? Here we answer this question using a cold atom quantum simulation. Our simulation yields the spin and particle conductance of a Fermi gas flowing through a single mode quantum point contact as a function of the strength of attractive interactions. Spin conductance exhibits a broad maximum when varying the chemical potential at moderate interactions. In contrast, the particle conductance is unexpectedly enhanced. It shows non-universal quantization with values of the conductance plateaux ranging from 1/h to 4/h, as the interaction strength is increased from weak to intermediate. For strong interactions, the particle conductance plateaux disappear and the spin conductance gets gradually suppressed. In that regime we interpret our data as demonstrating the emergence of a superfluid with a spin-insulating character. Our observations document the breakdown of universal conductance quantization as many-body correlations appear. The anomalous quantization is incompatible with a Fermi liquid description, shedding new light on the nature of the strongly attractive Fermi gas in the normal phase.
Nov 19
arXiv:1511.05569 [pdf, other]
Classification of gapless Z2 spin liquids in three-dimensional Kitaev models
Kevin O'Brien, Maria Hermanns, Simon Trebst
Comments: 35 pages, 62 figures, VESTA-files for all lattice structures included in the ancillary material
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
Frustrated quantum magnets can harbor unconventional spin liquid ground states in which the elementary magnetic moments fractionalize into new emergent degrees of freedom. While the fractionalization of quantum numbers is one of the recurring themes in modern condensed matter physics, it often remains a challenge to devise a controlled analytical framework tracking this phenomenon. A notable exception is the exactly solvable Kitaev model, in which spin degrees of freedom fractionalize into Majorana fermions and a Z2 gauge field. Here we discuss the physics of fractionalization in three-dimensional Kitaev models and demonstrate that the itinerant Majorana fermions generically form a (semi)metal which, depending on the underlying lattice structure, exhibits Majorana Fermi surfaces, nodal lines or topologically protected Weyl nodes. We show that the nature of these Majorana metals can be deduced from an elementary symmetry analysis of the projective time-reversal and inversion symmetries for a given lattice. This allows us to comprehensively classify the gapless spin liquids of Kitaev models for the most elementary tricoordinated lattices in three dimensions. We further expand this classification by addressing the effects of time-reversal symmetry breaking and additional interactions.
Nov 18
arXiv:1511.05320 [pdf, other]
Multiple Transitions of Coupled Atom-Molecule Bosonic Mixtures in Two Dimensions
Laurent de Forges de Parny, Adam Rançon, Tommaso Roscilde
Comments: 18 pages, 16 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech)
Motivated by the physics of coherently coupled, ultracold atom-molecule mixtures, we investigate a classical model possessing the same symmetry -- namely a $U(1)\times \mathbb{Z}_2$ symmetry, associated with the mass conservation in the mixture ($U(1)$ symmetry), times the $\mathbb{Z}_2$ symmetry in the phase relationship between atoms and molecules. In two spatial dimensions the latter symmetry can lead to a finite-temperature Ising transition, associated with (quasi) phase locking between the atoms and the molecules. On the other hand, the $U(1)$ symmetry has an associated Berezinskii-Kosterlitz-Thouless (BKT) transition towards quasi-condensation of atoms or molecules. The existence of the two transitions is found to depend crucially on the population imbalance (or detuning) between atoms and molecules: when the molecules are majority in the system, their BKT quasi-condensation transition occurs at a higher temperature than that of the atoms; the latter has the unconventional nature of an Ising (quasi) phase-locking transition, lacking a finite local order parameter below the critical temperature. When the balance is gradually biased towards the atoms, the two transitions merge together to leave out a unique BKT transition, at which both atoms and molecules acquire quasi-long-range correlations, but only atoms exhibit conventional BKT criticality, with binding of vortex-antivortex pairs into short-range dipoles. The molecular vortex-antivortex excitations bind as well, but undergo a marked crossover from a high-temperature regime in which they are weakly bound, to a low-temperature regime of strong binding, reminiscent of their transition in the absence of atom-molecule coupling.
Nov 17
arXiv:1511.04969 [pdf, ps, other]
Topology of chiral superfluid: skyrmions, Weyl fermions and chiral anomaly
G.E. Volovik
Comments: 7 pages, 3 figures, JETP Letters style
Subjects: Other Condensed Matter (cond-mat.other)
Chiral anomaly observed in the chiral superfluid $^3$He-A is the result of the combined effect of the real space and momentum space topologies. This effect incorporates several topological charges in the extended $({\bf k},{\bf r})$-space, which is beyond the conventional chiral anomaly in the relativistic systems.
Nov 16
arXiv:1511.04276 [pdf, other]
Excitations and impurity dynamics in a fermionic Mott insulator with nearest-neighbor interactions
A.-M. Visuri, T. Giamarchi, P. Törmä
Comments: 5 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)
We study analytically and with the numerical time-evolving block decimation method the dynamics of an impurity in a bath of spinless fermions with nearest-neighbor interactions in a one-dimensional lattice. The bath is in a Mott insulator state with alternating sites occupied and the impurity interacts with the bath by repulsive on-site interactions. We find that when the magnitudes of the on-site and nearest-neighbor interactions are close to each other, the system shows excitations of two qualitatively distinct types. For the first type, a domain wall and an anti-domain wall of density propagate in opposite directions, while the impurity stays at the initial position. For the second one, the impurity is bound to the anti-domain wall while the domain wall propagates, an excitation where the impurity and bath are closely coupled.
Nov 6
1. arXiv:1511.01588 (cross-list from physics.atom-ph) [pdf, other]
Rashba realization: Raman with RF
Daniel L. Campbell, Ian B. Spielman
can be continuously opened or closed by modifying laser polarizations. Our technique uses far-detuned "Raman" laser coupling to create the Rashba potential, which has the benefit of low spontaneous emission rates. At these detunings, the Raman matrix elements that link mF magnetic sublevel quantum numbers separated by two are also suppressed. These matrix elements are necessary to produce the Rashba Hamiltonian within a single total angular momentum f manifold. However, the far-detuned Raman couplings can link the three XYZ states familiar to quantum chemistry, which possess the necessary connectivity to realize the Rashba potential. We show that these XYZ states are essentially the hyperfine spin eigenstates of 87Rb dressed by a strong radio-frequency magnetic field.
Nov 5
1. arXiv:1511.01109 (cross-list from quant-ph) [pdf, ps, other]
Cavity-Optomechanics with Spin-Orbit Coupled Spinor Bose-Einstein Condensate
Kashif Ammar Yasir, Wu-Ming Liu
Cavity-optomechanics, an exploitation of mechanical-effects of light to couple optical-field with mechanical-objects, has made remarkable progress. Besides, spin-orbit (SO)-coupling, interaction between spin of a quantum-particle and its momentum, has provided foundation to analyze various phenomena like spin-Hall effect and topological-insulators. However, SO-coupling and corresponding topological-features have not been examined in optical-cavity with one vibrational-mirror. Here we report cavity-optomechanics with SO-coupled Bose-Einstein condensate, inducing non-Abelian gauge-field in cavity. We ascertain the influences of SO-coupling and long-range atomic-interactions on low-temperature dynamics which can be experimentally measured by maneuvering area underneath density-noise spectrum. It is detected that not only optomechanical-coupling is modifying topological properties of atomic dressed-states but SO-coupling induced topological-effects are also enabling us to control effective-temperature of mechanical-mirror and dynamic structure factor, which is measurable by detecting neutron-scattering. Our findings are testable in a realistic setup and provide foundations to manipulate SO-coupling in the field of quantum optics and quantum computation.
Nov 4
1. arXiv:1511.00980 (cross-list from quant-ph) [pdf, other]
Engineering Many-Body Dynamics with Quantum Light Potentials and Measurements
Thomas J. Elliott, Igor B. Mekhov
Interactions between many-body atomic systems and light in cavities induce new atomic dynamics, which we show can be tailored by projective light measurement backaction, leading to collective effects such as density-density interactions, perfectly-correlated atomic tunneling, superexchange, and effective pair creation and annihilation. These can be long- and short-range, with tunable strengths, based on the optical setup. We show this provides a framework to enhance quantum simulations of novel physical phenomena, including reservoir models and dynamical gauge fields, beyond current methods.
Nov 3
1. arXiv:1511.00595 [pdf, other]
Engineering Photonic Floquet Hamiltonians through Fabry Pérot Resonators
Ariel Sommer, Jonathan Simon
In this letter we analyze an optical Fabry-P\'erot resonator as a time-periodic driving of the (2D) optical field repeatedly traversing the resonator, uncovering that resonator twist produces a synthetic magnetic field applied to the light within the resonator, while mirror aberrations produce relativistic dynamics, anharmonic trapping, and spacetime curvature. We develop a Floquet formalism to compute the effective Hamiltonian for the 2D field, generalizing the idea that the intra-cavity optical field corresponds to an ensemble of non-interacting, massive, harmonically trapped particles. This work illuminates the extraordinary potential of optical resonators for exploring the physics of quantum fluids in gauge fields and exotic space-times.
Nov 2
1. arXiv:1510.09183 [pdf, other]
A mesoscopic Rydberg impurity in an atomic quantum gas
Richard Schmidt, H. R. Sadeghpour, E. Demler
Giant impurity excitations with large binding energies are powerful probes for exploring new regimes of far out of equilibrium dynamics in few- and many-body quantum systems, as well as for in-situ observations of correlations. Motivated by recent experimental progress in spectroscopic studies of Rydberg excitations in ensembles of ultracold atoms, we develop a new theoretical approach for describing multiscale dynamics of Rydberg excitations in quantum Bose gases. We find that the crossover from few- to many-body dynamics manifests in a dramatic change in spectral profile from resolved molecular lines to broad Gaussian distributions representing a superpolaronic state in which many atoms bind to the Rydberg impurity. We discuss signatures of this crossover in the temperature and density dependence of the spectra.
