1,959 research outputs found
Finite-temperature Wigner solid and other phases of ripplonic polarons on a helium film
Electrons on liquid helium can form different phases depending on density,
and temperature. Also the electron-ripplon coupling strength influences the
phase diagram, through the formation of so-called "ripplonic polarons", that
change how electrons are localized, and that shifts the transition between the
Wigner solid and the liquid phase. We use an all-coupling, finite-temperature
variational method to study the formation of a ripplopolaron Wigner solid on a
liquid helium film for different regimes of the electron-ripplon coupling
strength. In addition to the three known phases of the ripplopolaron system
(electron Wigner solid, polaron Wigner solid, and electron fluid), we define
and identify a fourth distinct phase, the ripplopolaron liquid. We analyse the
transitions between these four phases and calculate the corresponding phase
diagrams. This reveals a reentrant melting of the electron solid as a function
of temperature. The calculated regions of existence of the Wigner solid are in
agreement with recent experimental data.Comment: 12 pages, 6 figures. arXiv admin note: text overlap with
arXiv:1012.4576, arXiv:0709.4140 by other author
Energy-weighted density matrix embedding of open correlated chemical fragments
We present a multi-scale approach to efficiently embed an ab initio
correlated chemical fragment described by its energy-weighted density matrices,
and entangled with a wider mean-field many-electron system. This approach,
first presented in Phys. Rev. B, 98, 235132 (2018), is here extended to account
for realistic long-range interactions and broken symmetry states. The scheme
allows for a systematically improvable description in the range of correlated
fluctuations out of the fragment into the system, via a self-consistent
optimization of a coupled auxiliary mean-field system. It is discussed that the
method has rigorous limits equivalent to existing quantum embedding approaches
of both dynamical mean-field theory, as well as density matrix embedding
theory, to which this method is compared, and the importance of these
correlated fluctuations is demonstrated. We derive a self-consistent local
energy functional within the scheme, and demonstrate the approach for Hydrogen
rings, where quantitative accuracy is achieved despite only a single atom being
explicitly treated.Comment: 14 pages, 8 figure
Mean-field phase diagram of the 1-D Bose gas in a disorder potential
We study the quantum phase transition of the 1D weakly interacting Bose gas
in the presence of disorder. We characterize the phase transition as a function
of disorder and interaction strengths, by inspecting the long-range behavior of
the one-body density matrix as well as the drop in the superfluid fraction. We
focus on the properties of the low-energy Bogoliubov excitations that drive the
phase transition, and find that the transition to the insulator state is marked
by a diverging density of states and a localization length that diverges as a
power-law with power 1. We draw the phase diagram and we observe that the
boundary between the superfluid and the Bose glass phase is characterized by
two different algebraic relations. These can be explained analytically by
considering the limiting cases of zero and infinite disorder correlation
length.Comment: 10 pages, 10 figure
Vortices in nonequilibrium photon condensates
We present a theoretical study of vortices in arrays of photon condensates.
Even when interactions are negligible, as is the case in current experiments,
pumping and losses can lead to a finite vortex core size. While some properties
of photon condensate vortices, such as their self-acceleration and the
generation of vortex pairs by a moving vortex, resemble those in interacting
polariton condensates far from equilibrium, in several aspects they differ from
previously studied systems: the vortex core size is determined by the balance
between pumping and tunneling, the core appears oblate in the direction of its
motion and new vortex pairs can spontaneously nucleate in the core region.Comment: 5 pages plus supplementary figure
From single particle to superfuid excitations in a dissipative polariton gas
Using angle-resolved heterodyne four-wave-mixing technique, we probe the low
momentum excitation spectrum of a coherent polariton gas. The experimental
results are well captured by the Bogoliubov transformation which describes the
transition from single particle excitations of a normal fluid to
sound-wave-like excitations of a superfluid. In a dense coherent polariton gas,
we find all the characteristics of a Bogoliubov transformation, i.e. the
positive and negative energy branch with respect to the polariton gas energy at
rest, sound-wave-like shapes for the excitations dispersion, intensity and
linewidth ratio between the two branches in agreement with the theory. The
influence of the non-equilibrium character of the polariton gas is shown by a
careful analysis of its dispersion.Comment: 4 pages, 3 figure
Energy Relaxation in a 1-D Polariton Condensate
We study the kinetics of polariton condensation accounting for the
condensation process as well as the energy relaxation of condensed polaritons
due to their scattering with phonons and excitons. By assuming a Boltzmann
kinetic description of the scattering process, we show that intra-condensate
relaxation can be accounted for by an additional time-dependent term in the
Gross-Pitaevskii equation. As an example, we apply the formalism to the
experimental results recently obtained in polariton microwires [E. Wertz, et
al., Nature Phys. 6, 860 (2010)]. In the presence of a local non-resonant
optical pump, a dynamic balance between spatially dependent relaxation and
particle loss develops and excites a series of modes, roughly equally spaced in
energy. Upon comparison, excellent agreement is found with the experimental
data.Comment: 6 pages, 4 figure
Avalanches, breathers, and flow reversal in a continuous Lorenz-96 model
For the discrete model suggested by Lorenz in 1996, a one-dimensional long-wave approximation with nonlinear excitation and diffusion is derived. The model is energy conserving but non-Hamiltonian. In a low-order truncation, weak external forcing of the zonal mean flow induces avalanchelike breather solutions which cause reversal of the mean flow by a wave-mean flow interaction. The mechanism is an outburst-recharge process similar to avalanches in a sandpile model
Unstable and stable regimes of polariton condensation
Modulational instabilities play a key role in a wide range of nonlinear
optical phenomena, leading e.g. to the formation of spatial and temporal
solitons, rogue waves and chaotic dynamics. Here we experimentally demonstrate
the existence of a modulational instability in condensates of cavity
polaritons, arising from the strong coupling of cavity photons with quantum
well excitons. For this purpose we investigate the spatiotemporal coherence
properties of polariton condensates in GaAs-based microcavities under
continuous-wave pumping. The chaotic behavior of the instability results in a
strongly reduced spatial and temporal coherence and a significantly
inhomogeneous density. Additionally we show how the instability can be tamed by
introducing a periodic potential so that condensation occurs into negative mass
states, leading to largely improved coherence and homogeneity. These results
pave the way to the exploration of long-range order in dissipative quantum
fluids of light within a controlled platform.Comment: 7 pages, 5 figure
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