23 research outputs found
Far-from-equilibrium field theory of many-body quantum spin systems: Prethermalization and relaxation of spin spiral states in three dimensions
We study theoretically the far-from-equilibrium relaxation dynamics of spin
spiral states in the three dimensional isotropic Heisenberg model. The
investigated problem serves as an archetype for understanding quantum dynamics
of isolated many-body systems in the vicinity of a spontaneously broken
continuous symmetry. We present a field-theoretical formalism that
systematically improves on mean-field for describing the real-time quantum
dynamics of generic spin-1/2 systems. This is achieved by mapping spins to
Majorana fermions followed by a 1/N expansion of the resulting two-particle
irreducible (2PI) effective action. Our analysis reveals rich
fluctuation-induced relaxation dynamics in the unitary evolution of spin spiral
states. In particular, we find the sudden appearance of long-lived
prethermalized plateaus with diverging lifetimes as the spiral winding is tuned
toward the thermodynamically stable ferro- or antiferromagnetic phases. The
emerging prethermalized states are characterized by different bosonic modes
being thermally populated at different effective temperatures, and by a
hierarchical relaxation process reminiscent of glassy systems. Spin-spin
correlators found by solving the non-equilibrium Bethe-Salpeter equation
provide further insight into the dynamic formation of correlations, the fate of
unstable collective modes, and the emergence of fluctuation-dissipation
relations. Our predictions can be verified experimentally using recent
realizations of spin spiral states with ultracold atoms in a quantum gas
microscope [S. Hild, et al. Phys. Rev. Lett. 113, 147205 (2014)]
Multiple-scale structures: from Faraday waves to soft-matter quasicrystals
For many years, quasicrystals were observed only as solid-state metallic
alloys, yet current research is now actively exploring their formation in a
variety of soft materials, including systems of macromolecules, nanoparticles
and colloids. Much effort is being invested in understanding the thermodynamic
properties of these soft-matter quasicrystals in order to predict and possibly
control the structures that form, and hopefully to shed light on the broader
yet unresolved general questions of quasicrystal formation and stability.
Moreover, the ability to control the self-assembly of soft quasicrystals may
contribute to the development of novel photonic or other applications based on
self-assembled metamaterials. Here a path is followed, leading to quantitative
stability predictions, that starts with a model developed two decades ago to
treat the formation of multiple-scale quasiperiodic Faraday waves (standing
wave patterns in vibrated fluid surfaces) and which was later mapped onto
systems of soft particles, interacting via multiple-scale pair potentials. The
article reviews, and substantially expands, the quantitative predictions of
these models, while correcting a few discrepancies in earlier calculations, and
presents new analytical methods for treating the models. In so doing, a number
of new stable quasicrystalline structures with octagonal, octadecagonal and
higher-order symmetries, some of which may, it is hoped, be observed in future
experiments.Comment: 22 pages, 22 figures, 1 table. Comments welcom
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Non-equilibrium dynamics of artificial quantum matter
The rapid progress of the field of ultracold atoms during the past two decades has set new milestones in our control over matter. By cooling dilute atomic gases and molecules to nano-Kelvin temperatures, novel quantum mechanical states of matter can be realized and studied on a table-top experimental setup while bulk matter can be tailored to faithfully simulate abstract theoretical models. Two of such models which have witnessed significant experimental and theoretical attention are (1) the two-component Fermi gas with resonant -wave interactions, and (2) the single-component Fermi gas with dipole-dipole interactions. This thesis is devoted to studying the non-equilibrium collective dynamics of these systems using the general framework of quantum kinetic theory.Physic
The theory of parametrically amplified electron-phonon superconductivity
The ultrafast optical manipulation of ordered phases in strongly correlated
materials is a topic of significant theoretical, experimental, and
technological interest. Inspired by a recent experiment on light-induced
superconductivity in fullerenes [Mitrano et al., Nature 530, 2016], we develop
a comprehensive theory of light-induced superconductivity in driven
electron-phonon systems with lattice nonlinearities. In analogy with the
operation of parametric amplifiers, we show how the interplay between the
external drive and lattice nonlinearities lead to significantly enhanced
effective electron-phonon couplings. We provide a detailed and unbiased study
of the nonequilibrium dynamics of the driven system using the real-time Green's
function technique. To this end, we develop a Floquet generalization of the
Migdal-Eliashberg theory and derive a numerically tractable set of quantum
Floquet-Boltzmann kinetic equations for the coupled electron-phonon system. We
study the role of parametric phonon generation and electronic heating in
destroying the transient superconducting state. Finally, we predict the
transient formation of electronic Floquet bands in time- and angle-resolved
photo-emission spectroscopy experiments as a consequence of the proposed
mechanism.Comment: 42 pages, 17 figure
Dynamical instabilities and transient short-range order in the fermionic Hubbard model
We study the dynamics of magnetic correlations in the half-filled fermionic
Hubbard model following a fast ramp of the repulsive interaction. We use
Schwinger-Keldysh self-consistent second-order perturbation theory to
investigate the evolution of single-particle Green's functions and solve the
non-equilibrium Bethe-Salpeter equation to study the dynamics of magnetic
correlations. This approach gives us new insights into the interplay between
single-particle relaxation dynamics and the growth of antiferromagnetic
correlations. Depending on the ramping time and the final value of the
interaction, we find different dynamical behavior which we illustrate using a
dynamical phase diagram. Of particular interest is the emergence of a transient
short-range ordered regime characterized by the strong initial growth of
antiferromagnetic correlations followed by a decay of correlations upon
thermalization. The discussed phenomena can be probed in experiments with
ultracold atoms in optical lattices.Comment: 4 pages, 3 figure
Universal behavior of repulsive two-dimensional fermions in the vicinity of the quantum freezing point
We show by a meta-analysis of the available Quantum Monte Carlo (QMC) results that two-dimensional fermions with repulsive interactions exhibit universal behavior in the strongly correlated regime, and that their freezing transition can be described using a quantum generalization of the classical Hansen-Verlet freezing criterion. We calculate the liquid-state energy and the freezing point of the 2D dipolar Fermi gas (2DDFG) using a variational method by taking ground-state wave functions of 2D electron gas (2DEG) as trial states. A comparison with the recent fixed-node diffusion Monte Carlo analysis of the 2DDFG shows that our simple variational technique captures more than 95% of the correlation energy, and predicts the freezing transition within the uncertainty bounds of QMC. Finally, we utilize the ground-state wave functions of 2DDFG as trial states and provide a variational account of the effects of finite 2D confinement width. Our results indicate significant beyond mean-field effects. We calculate the frequency of collective monopole oscillations of the quasi-2D dipolar gas as an experimental demonstration of correlation effects.Physic