95 research outputs found
Carrier-density effects in many-polaron systems
Many-polaron systems with finite charge-carrier density are often encountered
experimentally. However, until recently, no satisfactory theoretical
description of these systems was available even in the framework of simple
models such as the one-dimensional spinless Holstein model considered here. In
this work, previous results obtained using numerical as well as analytical
approaches are reviewed from a unified perspective, focussing on spectral
properties which reveal the nature of the quasiparticles in the system. In the
adiabatic regime and for intermediate electron-phonon coupling, a
carrier-density driven crossover from a polaronic to a rather metallic system
takes place. Further insight into the effects due to changes in density is
gained by calculating the phonon spectral function, and the fermion-fermion and
fermion-lattice correlation functions. Finally, we provide strong evidence
against the possibility of phase separation.Comment: 13 pages, 6 figures, accepted for publication in J. Phys.: Condens.
Matter; final versio
Dynamical critical exponent of the Jaynes-Cummings-Hubbard model
An array of high-Q electromagnetic resonators coupled to qubits gives rise to
the Jaynes-Cummings-Hubbard model describing a superfluid to Mott insulator
transition of lattice polaritons. From mean-field and strong coupling
expansions, the critical properties of the model are expected to be identical
to the scalar Bose-Hubbard model. A recent Monte Carlo study of the superfluid
density on the square lattice suggested that this does not hold for the
fixed-density transition through the Mott lobe tip. Instead, mean-field
behavior with a dynamical critical exponent z=2 was found. We perform
large-scale quantum Monte Carlo simulations to investigate the critical
behavior of the superfluid density and the compressibility. We find z=1 at the
tip of the insulating lobe. Hence the transition falls in the 3D XY
universality class, analogous to the Bose-Hubbard model.Comment: 4 pages, 4 figures. To appear as a Rapid Communication in Phys. Rev.
Quantum Monte Carlo results for bipolaron stability in quantum dots
Bipolaron formation in a two-dimensional lattice with harmonic confinement,
representing a simplified model for a quantum dot, is investigated by means of
quantum Monte Carlo simulations. This method treats all interactions exactly
and takes into account quantum lattice fluctuations. Calculations of the
bipolaron binding energy reveal that confinement opposes bipolaron formation
for weak electron-phonon coupling, but abets a bound state at intermediate to
strong coupling. Tuning the system from weak to strong confinement gives rise
to a small reduction of the minimum Frohlich coupling parameter for the
existence of a bound state.Comment: 5 pages, 2 figures, final version to appear in Phys. Rev.
Quantum Phase Transitions in Bosonic Heteronuclear Pairing Hamiltonians
We explore the phase diagram of two-component bosons with Feshbach resonant
pairing interactions in an optical lattice. It has been shown in previous work
to exhibit a rich variety of phases and phase transitions, including a
paradigmatic Ising quantum phase transition within the second Mott lobe. We
discuss the evolution of the phase diagram with system parameters and relate
this to the predictions of Landau theory. We extend our exact diagonalization
studies of the one-dimensional bosonic Hamiltonian and confirm additional Ising
critical exponents for the longitudinal and transverse magnetic
susceptibilities within the second Mott lobe. The numerical results for the
ground state energy and transverse magnetization are in good agreement with
exact solutions of the Ising model in the thermodynamic limit. We also provide
details of the low-energy spectrum, as well as density fluctuations and
superfluid fractions in the grand canonical ensemble.Comment: 11 pages, 14 figures. To appear in Phys. Rev.
Luttinger Liquid Physics and Spin-Flip Scattering on Helical Edges
We investigate electronic correlation effects on edge states of quantum
spin-Hall insulators within the Kane-Mele-Hubbard model by means of quantum
Monte Carlo simulations. Given the U(1) spin symmetry and time-reversal
invariance, the low-energy theory is the helical Tomanaga-Luttinger model, with
forward scattering only. For weak to intermediate interactions, this model
correctly describes equal-time spin and charge correlations, including their
doping dependence. As apparent from the Drude weight, bulk states become
relevant in the presence of electron-electron interactions, rendering the
forward-scattering model incomplete. Strong correlations give rise to slowly
decaying transverse spin fluctuations, and inelastic spin-flip scattering
strongly modifies the single-particle spectrum, leading to graphene-like edge
state signatures. The helical Tomanaga-Luttinger model is completely valid only
asymptotically in the weak-coupling limit.Comment: 5 pages, 5 figures (modified version with additional data
Feshbach Resonance in Optical Lattices and the Quantum Ising Model
Motivated by experiments on heteronuclear Feshbach resonances in Bose
mixtures, we investigate s-wave pairing of two species of bosons in an optical
lattice. The zero temperature phase diagram supports a rich array of superfluid
and Mott phases and a network of quantum critical points. This topology reveals
an underlying structure that is succinctly captured by a two-component Landau
theory. Within the second Mott lobe we establish a quantum phase transition
described by the paradigmatic longitudinal and transverse field Ising model.
This is confirmed by exact diagonalization of the 1D bosonic Hamiltonian. We
also find this transition in the homonuclear case.Comment: 5 pages, 4 figure
Dynamic charge correlations near the Peierls transition
The quantum phase transition between a repulsive Luttinger liquid and an
insulating Peierls state is studied in the framework of the one-dimensional
spinless Holstein model. We focus on the adiabatic regime but include the full
quantum dynamics of the phonons. Using continuous-time quantum Monte Carlo
simulations, we track in particular the dynamic charge structure factor and the
single-particle spectrum across the transition. With increasing electron-phonon
coupling, the dynamic charge structure factor reveals the emergence of a charge
gap, and a clear signature of phonon softening at the zone boundary. The
single-particle spectral function evolves continuously across the transition.
Hybridization of the charge and phonon modes of the Luttinger liquid
description leads to two modes, one of which corresponds to the coherent
polaron band. This band acquires a gap upon entering the Peierls phase, whereas
the other mode constitutes the incoherent, high-energy spectrum with backfolded
shadow bands. Coherent polaronic motion is a direct consequence of quantum
lattice fluctuations. In the strong-coupling regime, the spectrum is described
by the static, mean-field limit. Importantly, whereas finite electron density
in general leads to screening of polaron effects, the latter reappear at half
filling due to charge ordering and lattice dimerization.Comment: 8 pages, 7 figures, final versio
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