1,296 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
Multi-photon Rabi oscillations in high spin paramagnetic impurity
We report on multiple photon monochromatic quantum oscillations (Rabi
oscillations) observed by pulsed EPR (Electron Paramagnetic Resonance) of
Mn (S=5/2) impurities in MgO. We find that when the microwave magnetic
field is similar or large than the anisotropy splitting, the Rabi oscillations
have a spectrum made of many frequencies not predicted by the S=1/2 Rabi model.
We show that these new frequencies come from multiple photon coherent
manipulation of the multi-level spin impurity. We develop a model based on the
crystal field theory and the rotating frame approximation, describing the
observed phenomenon with a very good agreement.Comment: International Conference: Resonance in Condensed Matter Altshuler 10
Magnetic strong coupling in a spin-photon system and transition to classical regime
We study the energy level structure of the Tavis-Cumming model applied to an
ensemble of independent magnetic spins coupled to a variable number of
photons. Rabi splittings are calculated and their distribution is analyzed as a
functin of photon number and spin system size . A sharp
transition in the distribution of the Rabi frequency is found at . The width of the Rabi frequency spectrum diverges as
at this point. For increased number of photons , the Rabi
frequencies converge to a value proportional to . This
behavior is interpreted as analogous to the classical spin resonance mechanism
where the photon is treated as a classical field and one resonance peak is
expected. We also present experimental data demonstrating cooperative, magnetic
strong coupling between a spin system and photons, measured at room
temperature. This points towards quantum computing implementation with magnetic
spins, using cavity quantum-electrodynamics techniques.Comment: Received 8 April 2010; revised manuscript received 17 June 2010;
published 14 July 201
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
is smaller than the fermion density , 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 .Comment: 5 pages, 3 figures, published versio
Entrapment of magnetic micro-crystals for on-chip electron spin resonance studies
On-chip Electron Spin Resonance (ESR) of magnetic molecules requires the
ability to precisely position nanosized samples in antinodes of the
electro-magnetic field for maximal magnetic interaction. A method is developed
to entrap micro-crystals containing spins in a well defined location on a
substrate's surface. Traditional cavity ESR measurements are then performed on
a mesoscopic crystal at 34 GHz. Polycrystalline diluted Cr spins were
entrapped as well and measured while approaching the lower limit of the ESR
sensitivity. This method suggests the feasibility of on-chip ESR measurements
at dilution refrigerator temperatures by enabling the positioning of samples
atop an on-chip superconducting cavity.Comment: to appear in Journal of Applied Physic
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[VAsO(DO)]
8DO, 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
High Curie temperature Mn 5 Ge 3 thin films produced by non-diffusive reaction
Polycrystalline Mn 5 Ge 3 thin films were produced on SiO 2 using magnetron
sputtering and reactive diffusion (RD) or non-diffusive reaction (NDR). In situ
X-ray diffraction and atomic force microscopy were used to determine the layer
structures, and magnetic force microscopy, superconducting quantum interference
device and ferromagnetic resonance were used to determine their magnetic
properties. RD-mediated layers exhibit similar magnetic properties as MBE-grown
monocrystalline Mn 5 Ge 3 thin films, while NDR-mediated layers show magnetic
properties similar to monocrystalline C-doped Mn 5 Ge 3 C x thin films with
NDR appears as a CMOS-compatible efficient method to
produce good magnetic quality high-curie temperature Mn 5 Ge 3 thin films
Photon and spin dependence of the resonance lines shape in the strong coupling regime
We study the quantum dynamics of a spin ensemble coupled to cavity photons.
Recently, related experimental results have been reported, showing the
existence of the strong coupling regime in such systems. We study the
eigenenergy distribution of the multi-spin system (following the Tavis-Cummings
model) which shows a peculiar structure as a function of the number of cavity
photons and of spins. We study how this structure causes changes in the
spectrum of the admittance in the linear response theory, and also the
frequency dependence of the excited quantities in the stationary state under a
probing field. In particular, we investigate how the structure of the higher
excited energy levels changes the spectrum from a double-peak structure (the
so-called vacuum field Rabi splitting) to a single peak structure. We also
point out that the spin dynamics in the region of the double-peak structure
corresponds to recent experiments using cavity ringing while in region of the
single peak structure, it corresponds to the coherent Rabi oscillation in a
driving electromagnetic filed. Using a standard Lindblad type mechanism, we
study the effect of dissipations on the line width and separation in the
computed spectra. In particular, we study the relaxation of the total spin in
the general case of a spin ensemble in which the total spin of the system is
not specified. The theoretical results are correlated with experimental
evidence of the strong coupling regime, achieved with a spin 1/2 ensemble
- …