1,276 research outputs found

### BCS-BEC crossover in a two-dimensional Fermi gas

We investigate the crossover from Bardeen-Cooper-Schrieffer (BCS)
superfluidity to Bose-Einstein condensation (BEC) in a two-dimensional Fermi
gas at T=0 using the fixed-node diffusion Monte Carlo method. We calculate the
equation of state and the gap parameter as a function of the interaction
strength, observing large deviations compared to mean-field predictions. In the
BEC regime our results show the important role of dimer-dimer and atom-dimer
interaction effects that are completely neglected in the mean-field picture.
Results on Tan's contact parameter associated with short-range physics are also
reported along the BCS-BEC crossover.Comment: 4 pages, 4 figure

### Density profiles of polarized Fermi gases confined in harmonic traps

On the basis of the phase diagram of the uniform system we calculate the
density profiles of a trapped polarized Fermi gas at zero temperature using the
local density approximation. By varying the overall polarization and the
interaction strength we analyze the appearance of a discontinuity in the
profile, signalling a first order phase transition from a superfluid inner core
to a normal outer shell. The local population imbalance between the two
components and the size of the various regions of the cloud corresponding to
different phases are also discussed. The calculated profiles are quantitatively
compared with the ones recently measured by Shin {\it et al.}, Phys. Rev. Lett.
{\bf 101}, 070404 (2008).Comment: 6 pages, 4 figures. We added references and modified the figure

### First and second sound in cylindrically trapped gases

We investigate the propagation of density and temperature waves in a
cylindrically trapped gas with radial harmonic confinement. Starting from
two-fluid hydrodynamic theory we derive effective 1D equations for the chemical
potential and the temperature which explicitly account for the effects of
viscosity and thermal conductivity. Differently from quantum fluids confined by
rigid walls, the harmonic confinement allows for the propagation of both first
and second sound in the long wave length limit. We provide quantitative
predictions for the two sound velocities of a superfluid Fermi gas at
unitarity. For shorter wave-lengths we discover a new surprising class of
excitations continuously spread over a finite interval of frequencies. This
results in a non-dissipative damping in the response function which is
analytically calculated in the limiting case of a classical ideal gas.Comment: 4 pages, 2 figures. Published version in Phys. Rev. Let

### Bose-Fermi mixtures in the molecular limit

We consider a Bose-Fermi mixture in the molecular limit of the attractive
interaction between fermions and bosons. For a boson density smaller or equal
to the fermion density, we show analytically how a T-matrix approach for the
constituent bosons and fermions recovers the expected physical limit of a
Fermi-Fermi mixture of molecules and atoms. In this limit, we derive simple
expressions for the self-energies, the momentum distribution function, and the
chemical potentials. By extending these equations to a trapped system, we
determine how to tailor the experimental parameters of a Bose-Fermi mixture in
order to enhance the 'indirect Pauli exclusion effect' on the boson momentum
distribution function. For the homogeneous system, we present finally a
Diffusion Monte Carlo simulation which confirms the occurrence of such a
peculiar effect.Comment: 13 pages, 7 figures; final versio

### Quantum Monte Carlo Study of a Resonant Bose-Fermi Mixture

We study a resonant Bose-Fermi mixture at zero temperature by using the
fixed-node diffusion Monte Carlo method. We explore the system from weak to
strong boson-fermion interaction, for different concentrations of the bosons
relative to the fermion component. We focus on the case where the boson density
$n_B$ is smaller than the fermion density $n_F$, for which a first-order
quantum phase transition is found from a state with condensed bosons immersed
in a Fermi sea, to a Fermi-Fermi mixture of composite fermions and unpaired
fermions. We obtain the equation of state and the phase diagram, and we find
that the region of phase separation shrinks to zero for vanishing $n_B$.Comment: 5 pages, 3 figures, published versio

### Implementation of the Linear Method for the optimization of Jastrow-Feenberg and Backflow Correlations

We present a fully detailed and highly performing implementation of the
Linear Method [J. Toulouse and C. J. Umrigar (2007)] to optimize
Jastrow-Feenberg and Backflow Correlations in many-body wave-functions, which
are widely used in condensed matter physics. We show that it is possible to
implement such optimization scheme performing analytical derivatives of the
wave-function with respect to the variational parameters achieving the best
possible complexity O(N^3) in the number of particles N.Comment: submitted to the Comp. Phys. Com

### Spin-Orbit Coupling Fluctuations as a Mechanism of Spin Decoherence

We discuss a general framework to address spin decoherence resulting from
fluctuations in a spin Hamiltonian. We performed a systematic study on spin
decoherence in the compound K$_6$[V$_{15}$As$_6$O$_{42}$(D$_2$O)] $\cdot$
8D$_2$O, using high-field Electron Spin Resonance (ESR). By analyzing the
anisotropy of resonance linewidths as a function of orientation, temperature
and field, we find that the spin-orbit term is a major decoherence source. The
demonstrated mechanism can alter the lifetime of any spin qubit and we discuss
how to mitigate it by sample design and field orientation.Comment: submitte

### Quantum Monte Carlo simulations of two-dimensional repulsive Fermi gases with population imbalance

The ground-state properties of two-component repulsive Fermi gases in two
dimensions are investigated by means of fixed-node diffusion Monte Carlo
simulations. The energy per particle is determined as a function of the
intercomponent interaction strength and of the population imbalance. The regime
of universality in terms of the s-wave scattering length is identified by
comparing results for hard-disk and for soft-disk potentials. In the large
imbalance regime, the equation of state turns out to be well described by a
Landau-Pomeranchuk functional for two-dimensional polarons. To fully
characterize this expansion, we determine the polarons' effective mass and
their coupling parameter, complementing previous studies on their chemical
potential. Furthermore, we extract the magnetic susceptibility from
low-imbalance data, finding only small deviations from the mean-field
prediction. While the mean-field theory predicts a direct transition from a
paramagnetic to a fully ferromagnetic phase, our diffusion Monte Carlo results
suggest that the partially ferromagnetic phase is stable in a narrow interval
of the interaction parameter. This finding calls for further analyses on the
effects due to the fixed-node constraint.Comment: 10 pages, 5 figure

### Reduced rovibrational coupling Cartesian dynamics for semiclassical calculations: Application to the spectrum of the Zundel cation

We study the vibrational spectrum of the protonated water dimer, by means of a divide-and-conquer semiclassical initial value representation of the quantum propagator, as a first step in the study of larger protonated water clusters. We use the potential energy surface from the work of Huang et al. [J. Chem. Phys. 122, 044308 (2005)]. To tackle such an anharmonic and floppy molecule, we employ fully Cartesian dynamics and carefully reduce the coupling to global rotations in the definition of normal modes. We apply the time-averaging filter and obtain clean power spectra relative to suitable reference states that highlight the spectral peaks corresponding to the fundamental excitations of the system. Our trajectory-based approach allows for the physical interpretation of the very challenging proton transfer modes. We find that it is important, for such a floppy molecule, to selectively avoid initially exciting lower energy modes, in order to obtain cleaner spectra. The estimated vibrational energies display a mean absolute error (MAE) of 3c29 cm-1 with respect to available multiconfiguration time-dependent Hartree calculations and MAE 3c14 cm-1 when compared to the optically active experimental excitations of the Ne-tagged Zundel cation. The reasonable scaling in the number of trajectories for Monte Carlo convergence is promising for applications to higher dimensional protonated cluster systems

### Quantum Monte Carlo study of the indirect Pauli exclusion effect in Bose-Fermi mixtures

We study the momentum distributions of a three-dimensional resonant
Bose-Fermi mixture in the molecular limit at zero temperature. For
concentration of the bosons with respect to the fermions less or equal to one,
each boson is bound to a fermion and the system is composed of fermionic
molecules plus excess fermions. Not only the bosonic condensate fraction goes
to zero, signaling a quantum phase transition towards a normal phase, but a
finite region of low momenta is depleted, depending on the concentration. This
phenomenon is named indirect Pauli exclusion effect and is demonstrated via
Fixed-Node Diffusion Monte Carlo simulations and T-matrix calculations.Comment: 5 pages, 3 figures, published in EPJ ST volume entitled "Novel
Quantum Phases and Mesoscopic Physics in Quantum Gases

- â€¦