1,449 research outputs found
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
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
Tunable multi-photon Rabi oscillations in an electronic spin system
We report on multi-photon Rabi oscillations and controlled tuning of a
multi-level system at room temperature (S=5/2 for Mn2+:MgO) in and out of a
quasi-harmonic level configuration. The anisotropy is much smaller than the
Zeeman splittings, such as the six level scheme shows only a small deviation
from an equidistant diagram. This allows us to tune the spin dynamics by either
compensating the cubic anisotropy with a precise static field orientation, or
by microwave field intensity. Using the rotating frame approximation, the
experiments are very well explained by both an analytical model and a
generalized numerical model. The calculated multi-photon Rabi frequencies are
in excellent agreement with the experimental data
Stack-CNN algorithm: A new approach for the detection of space objects
We present a new trigger algorithm combining a stacking procedure and a Convolutional Neural Network that could be applied to any space object moving linearly or with a known trajectory in the field of view of a telescope. This includes the detection of high velocity fragmentation debris in orbit. A possible implementation is as trigger system for an orbiting Space Debris remediation system. The algorithm was initially developed as offline system for the Multiwavelength Imaging New Instrument for the Extreme Universe Space Observatory (Mini-EUSO), on the International Space Station. We evaluated the performance of the algorithm on simulated data and compared it with those obtained by means of a more conventional trigger algorithm. Results indicate that this method would allow to recognise signals with 1% Signal over Background Ratio (SBR) on poissonian random fluctuations with a negligible fake trigger rate. Such promising results lead us to not only consider this technique as an online trigger system, but also as an offline method for searching moving signals and their characteristics (speed and direction). More generally, any kind of telescope (on the ground or in space) such as those used for space debris, meteors monitoring or cosmic ray science, could benefit from this automatized technique. The content of this paper is part of the recent Italian patent proposal submitted by the authors (patent application number: 102021000009845)
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
Bose-Fermi mixtures with pairing
I will review recent work by us on the properties of Bose-Fermi mixtures with a tunable pairing interaction
between bosons and fermions. A many-body diagrammatic approach, able to describe the condensed phase of
a Bose-Fermi mixture from weak to strong boson-fermion couplings, will be presented [1]. This approach will
be validated by comparing it with previous [2] and new dedicated fixed-node diffusion Monte Carlo
calculations. By using both methods, a universal behavior of the condensate fraction and bosonic momentum
distribution with respect to the boson concentration is found in an extended range of boson-fermion couplings
and concentrations. For vanishing boson density, the bosonic condensate fraction reduces to the quasiparticle
weight Z of the Fermi polaron studied in the context of polarized Fermi gases, unifying in this way two
apparently unrelated quantities. Finally, I will discuss an interesting effect occurring in the molecular limit of
the boson- fermion coupling, where the condensation is completely suppressed [3]. This phenomenon is an
indirect effect on bosons of the Pauli exclusion principle acting on fermions, and is the counterpart in BoseFermi
mixtures of the so called “Sarma phase” discussed for polarized Fermi gases.
[1] A. Guidini, G. Bertaina, D. Galli, and P. Pieri, arXiv:1412.2542.
[2] G. Bertaina, E. Fratini, S. Giorgini, and P. Pieri, Phys. Rev. Lett. 110, 115303 (2013).
[3] A. Guidini, G. Bertaina, E. Fratini, and P. Pieri, Phys. Rev. A 89, 023634 (2014)
Decoherence window and electron-nuclear cross-relaxation in the molecular magnet V 15
Rabi oscillations in the V_15 Single Molecule Magnet (SMM) embedded in the
surfactant DODA have been studied at different microwave powers. An intense
damping peak is observed when the Rabi frequency Omega_R falls in the vicinity
of the Larmor frequency of protons w_N, while the damping time t_R of
oscillations reaches values 10 times shorter than the phase coherence time t_2
measured at the same temperature. The experiments are interpreted by the N-spin
model showing that t_R is directly associated with the decoherence via
electronic/nuclear spin cross-relaxation in the rotating reference frame. It is
shown that this decoherence is accompanied with energy dissipation in the range
of the Rabi frequencies w_N - sigma_e < Omega_R < w_N, where sigma_e is the
mean super-hyperfine field (in frequency units) induced by protons at SMMs.
Weaker damping without dissipation takes place outside this dissipation window.
Simple local field estimations suggest that this rapid cross-relaxation in
resonant microwave field observed for the first time in SMMV_15 should take
place in other SMMs like Fe_8 and Mn_12 containing protons, too
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