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Akos Rapp

Publications and source records attributed to Akos Rapp.

13 recordsLinked to original sources
arXiv2014

Absence of quasiclassical coherence in mean-field dynamics of bosons in a kinetically frustrated regime

We study numerically the dynamics of bosons on a triangular lattice after quenching both the on-site interactions and the external trapping potential to negative values. In a similar situation on the square lattice, the dynamics can be understood in terms of an effectively reversed Hamiltonian. On the triangular lattice, however, the kinetic part of the reversed Hamiltonian is frustrated and whether coherence can develop is an open question. The strength of the frustration can be changed by tuning the ratio of the hopping rates along different directions. We calculate time-of-flight images at different times after the quench for different values of the hopping anisotropy. We observe peaks at the maxima of the original non-interacting dispersion relation both in the isotropic case and also in the rhombic limit of high hopping anisotropy. For an intermediate value, however, no coherence develops up to the longest simulation times. These results imply that experiments along similar lines could study unconventional superfluidity of bosons and aspects of the conjectured spin-liquid behavior in the hard-core limit.

cond-mat.quant-gas↗
arXiv2013

Mean-field dynamics to negative absolute temperatures in the Bose-Hubbard model

We apply time-dependent Gutzwiller mean-field theory to provide a qualitative understanding for bosons in optical lattices that approach states corresponding to negative absolute temperatures. We perform the dynamical simulations to relate to the recent experiments by Braun et al. [ S. Braun, J. P. Ronzheimer, M. Schreiber, S. S. Hodgman, T. Rom, I. Bloch and U. Schneider, Science 339 52 (2013)]. Time-of-flight images calculated from the two-dimensional numerical simulations reproduce characteristics of the experimental observations, in particular, the emergence of the four peaks at the corners of the Brillouin zone.

cond-mat.quant-gas↗
arXiv2013

Asymptotic scattering and duality in the one-dimensional three-state quantum Potts model on a lattice

We determine numerically the single-particle and the two-particle spectrum of the three-state quantum Potts model on a lattice by using the density matrix renormalization group method, and extract information on the asymptotic (small momentum) S-matrix of the quasiparticles. The low energy part of the finite size spectrum can be understood in terms of a simple effective model introduced in a previous work, and is consistent with an asymptotic S-matrix of an exchange form below a momentum scale $p^*$. This scale appears to vanish faster than the Compton scale, $mc$, as one approaches the critical point, suggesting that a dangerously irrelevant operator may be responsible for the behavior observed on the lattice.

cond-mat.stat-mech↗
arXiv2012

Ultracold Lattice Gases with Periodically Modulated Interactions

We show that a time-dependent magnetic field inducing a periodically modulated scattering length may lead to interesting novel scenarios for cold gases in optical lattices, characterized by a nonlinear hopping depending on the number difference at neighboring sites. We discuss the rich physics introduced by this hopping, including pair superfluidity, exactly defect-free Mott-insulator states for finite hopping, and pure holon and doublon superfluids. We also address experimental detection, showing that the introduced nonlinear hopping may lead in harmonically trapped gases to abrupt drops in the density profile marking the interface between different superfluid regions.

cond-mat.quant-gas↗
arXiv2012

Quantum Simulators at Negative Absolute Temperatures

We propose that negative absolute temperatures in ultracold atomic clouds in optical lattices can be used to simulate quantum systems in new regions of phase diagrams. First we discuss how the attractive SU(3) Hubbard model in three dimensions can be realized using repulsively interacting 173-Yb atoms, then we consider how an antiferromagnetic S=1 spin chain could be simulated using spinor 87-Rb or 23-Na atoms. The general idea to achieve negative absolute temperatures is to reverse the sign of the external harmonic potential. Energy conservation in a deep optical lattice imposes a constraint on the dynamics of the cloud, which can relax toward a T<0 state. As the process is strongly non-adiabatic, we estimate the change of the entropy.

cond-mat.quant-gas↗
arXiv2011

Interacting Fermionic Atoms in Optical Lattices Diffuse Symmetrically Upwards and Downwards in a Gravitational Potential

We consider a cloud of fermionic atoms in an optical lattice described by a Hubbard model with an additional linear potential. While homogeneous interacting systems mainly show damped Bloch oscillations and heating, a finite cloud behaves differently: It expands symmetrically such that gains of potential energy at the top are compensated by losses at the bottom. Interactions stabilize the necessary heat currents by inducing gradients of the inverse temperature 1/T, with T<0 at the bottom of the cloud. An analytic solution of hydrodynamic equations shows that the width of the cloud increases with t^(1/3) for long times consistent with results from our Boltzmann simulations.

cond-mat.quant-gas↗
arXiv2011

Ground state phase diagram of the repulsive SU(3) Hubbard model in Gutzwiller approximation

We perform a variational Gutzwiller calculation to study the ground state of the repulsive SU(3) Hubbard model on the Bethe lattice with infinite coordination number. We construct a ground-state phase diagram focusing on phases with a two-sublattice structure and find five relevant phases: (1) a paramagnet, (2) a completely polarized ferromagnet, (3) a two-component antiferromagnet where the third component is depleted, (4) a two-component antiferromagnet with a metallic third component (an "orbital selective" Mott insulator), and (5) a density-wave state where two components occupy dominantly one sublattice and the last component the other one. First-order transitions between these phases lead to phase separation. A comparison of the SU(3) Hubbard model to the better-known SU(2) model shows that the effects of doping are completely different in the two cases.

cond-mat.quant-gas↗
arXiv2010

Equilibration rates and negative absolute temperatures for ultracold atoms in optical lattices

As highly tunable interacting systems, cold atoms in optical lattices are ideal to realize and observe negative absolute temperatures, T < 0. We show theoretically that by reversing the confining potential, stable superfluid condensates at finite momentum and T < 0 can be created with low entropy production for attractive bosons. They may serve as `smoking gun' signatures of equilibrated T < 0. For fermions, we analyze the time scales needed to equilibrate to T < 0. For moderate interactions, the equilibration time is proportional to the square of the radius of the cloud and grows with increasing interaction strengths as atoms and energy are transported by diffusive processes.

cond-mat.quant-gas↗
arXiv2009

Universal diffusive decay of correlations in gapped one-dimensional systems

We apply a semiclassical approach to express finite temperature dynamical correlation functions of gapped spin models analytically. We show that the approach of [A. Rapp, G. Zarand, Phys. Rev. B 74, 014433 (2006)] can also be used for the S=1 antiferromagnetic Heisenberg chain, whose lineshape can be measured experimentally. We generalize our calculations to O(N) quantum spin models and the sine-Gordon model in one dimension, and show that in all these models, the finite temperature decay of certain correlation functions is characterized by the same universal semiclassical relaxation function.

cond-mat.stat-mech↗
arXiv2008

Trionic phase of ultracold fermions in an optical lattice: A variational study

To investigate ultracold fermionic atoms of three internal states (colors) in an optical lattice, subject to strong attractive interaction, we study the attractive three-color Hubbard model in infinite dimensions by using a variational approach. We find a quantum phase transition between a weak-coupling superconducting phase and a strong-coupling trionic phase where groups of three atoms are bound to a composite fermion. We show how the Gutzwiller variational theory can be reformulated in terms of an effective field theory with three-body interactions and how this effective field theory can be solved exactly in infinite dimensions by using the methods of dynamical mean field theory.

cond-mat.supr-con↗
arXiv2007

Color Superfluidity and "Baryon" Formation in Ultracold Fermions

We study fermionic atoms of three different internal quantum states (colors) in an optical lattice, which are interacting through attractive on site interactions, U<0. Using a variational calculation for equal color densities and small couplings, |U| < |U_C|, a color superfluid state emerges with a tendency to domain formation. For |U| > |U_C|, triplets of atoms with different colors form singlet fermions (trions). These phases are the analogies of the color superconducting and baryonic phases in QCD. In ultracold fermions, this transition is found to be of second order. Our results demonstrate that quantum simulations with ultracold gases may shed light on outstanding problems in quantum field theory.

cond-mat.str-el↗
arXiv2006

Dynamical correlations and quantum phase transition in the quantum Potts model

We present a detailed study of the finite temperature dynamical properties of the quantum Potts model in one dimension.Quasiparticle excitations in this model have internal quantum numbers, and their scattering matrix {\gf deep} in the gapped phases is shown to take a simple {\gf exchange} form in the perturbative regimes. The finite temperature correlation functions in the quantum critical regime are determined using conformal invariance, while {\gf far from the quantum critical point} we compute the decay functions analytically within a semiclassical approach of Sachdev and Damle [K. Damle and S. Sachdev, Phys. Rev. B \textbf{57}, 8307 (1998)]. As a consequence, decay functions exhibit a {\em diffusive character}. {\gf We also provide robust arguments that our semiclassical analysis carries over to very low temperatures even in the vicinity of the quantum phase transition.} Our results are also relevant for quantum rotor models, antiferromagnetic chains, and some spin ladder systems.

cond-mat.str-el↗