Search results for "superfluid"
showing 10 items of 171 documents
Quantum field theory of dilute homogeneous Bose-Fermi mixtures at zero temperature: General formalism and beyond mean-field corrections
2002
We consider a dilute homogeneous mixture of bosons and spin-polarized fermions at zero temperature. We first construct the formal scheme for carrying out systematic perturbation theory in terms of single particle Green's functions. We introduce a new relevant object, the renormalized boson-fermion T-matrix which we determine to second order in the boson-fermion s-wave scattering length. We also discuss how to incorporate the usual boson-boson T-matrix in mean-field approximation to obtain the total ground state properties of the system. The next order term beyond mean-field stems from the boson-fermion interaction and is proportional to $a_{\scriptsize BF}k_{\scriptsize F}$. The total groun…
K-ϵ-L model in turbulent superfluid helium
2020
We generalize the K−ϵ model of classical turbulence to superfluid helium. In a classical viscous fluid the phenomenological eddy viscosity characterizing the effects of turbulence depends on the turbulent kinetic energy K and the dissipation function ϵ, which are mainly related to the fluctuations of the velocity field and of its gradient. In superfluid helium, instead, we consider the necessary coefficients for describing the effects of classical and quantum turbulence, involving fluctuations of the velocity, the heat flux, and the vortex line density of the quantized vortex lines. By splitting the several fields into a time-average part and a fluctuating part, some expressions involving t…
Effective thermal conductivity of superfluid helium in short channels
2014
The aim of this paper is to explore how the effective thermal conductivity of small channels filled with superfluid helium II in the laminar regime separates from the classical Landau expression as the channel becomes shorter. The Landau expression is valid for fully developed Poiseuille flow for the normal component, and therefore is suitable for long channels. By taking into account entrance effects, we show a transition from a heat flux proportional to ∆T /l (Landau regime) for long channels, to a heat flux proportional to l^(1/3) (∆T /l)^(2/3) for short channels.
Strongly interacting Fermi gases with density imbalance
2005
We consider density-imbalanced Fermi gases of atoms in the strongly interacting, i.e. unitarity, regime. The Bogoliubov-deGennes equations for a trapped superfluid are solved. They take into account the finite size of the system, as well as give rise to both phase separation and FFLO type oscillations in the order parameter. We show how radio-frequency spectroscopy reflects the phase separation, and can provide direct evidence of the FFLO-type oscillations via observing the nodes of the order parameter.
Pairing gap and in-gap excitations in trapped fermionic superfluids
2004
We consider trapped atomic Fermi gases with Feshbach-resonance enhanced interactions in pseudogap and superfluid temperatures. We calculate the spectrum of RF(or laser)-excitations for transitions that transfer atoms out of the superfluid state. The spectrum displays the pairing gap and also the contribution of unpaired atoms, i.e. in-gap excitations. The results support the conclusion that a superfluid, where pairing is a many-body effect, was observed in recent experiments on RF spectroscopy of the pairing gap.
Supercurrent Induced Charge-Spin Conversion in Spin-Split Superconductors
2017
We study spin-polarized quasiparticle transport in a mesoscopic superconductor with a spin- splitting field in the presence of co-flowing supercurrent. In such a system, the nonequilibrium state is characterized by charge, spin, energy and spin energy modes. Here we show that in the presence of both spin splitting and supercurrent, all these modes are mutually coupled. As a result, the supercurrent can convert charge imbalance, that in the presence of spin splitting decays on a relatively short scale, to a long-range spin accumulation decaying only via inelastic scattering. This effect enables coherent charge-spin conversion controllable by a magnetic flux, and it can be detected by studyin…
Thermodynamics of computation and linear stability limits of superfluid refrigeration of a model computing array
2019
We analyze the stability of the temperature profile of an array of computing nanodevices refrigerated by flowing superfluid helium, under variations in temperature, computing rate, and barycentric velocity of helium. It turns out that if the variation in dissipated energy per bit with respect to temperature variations is higher than some critical values, proportional to the effective thermal conductivity of the array, then the steady-state temperature profiles become unstable and refrigeration efficiency is lost. Furthermore, a restriction on the maximum rate of variation in the local computation rate is found.
Magneto‐elastic oscillations modulating the emission of magnetars
2017
Magneto-elastic oscillations of neutron stars are believed to explain observed quasi-periodic oscillations (QPOs) in the decaying tail of the giant flares of highly magnetized neutron stars (magnetars). Strong efforts of the theoretical modelling from different groups have increased our understanding of this phenomenon significantly. Here, we discuss some constraints on the matter in neutron stars that arise if the interpretation of the observations in terms of superfluid, magneto-elastic oscillations is correct. To explain the observed modulation of the light curve of the giant flare, we describe a model that allows the QPOs to couple to the stellar exterior through the magnetic field. In …
Dark matter, dark photon and superfluid He-4 from effective field theory
2020
We consider a model of sub-GeV dark matter whose interaction with the Standard Model is mediated by a new vector boson (the dark photon) which couples kinetically to the photon. We describe the possibility of constraining such a model using a superfluid He-4 detector, by means of an effective theory for the description of the superfluid phonon. We find that such a detector could provide bounds that are competitive with other direct detection experiments only for ultralight vector mediator, in agreement with previous studies. As a byproduct we also present, for the first time, the low-energy effective field theory for the interaction between photons and phonons.
Positive ion extraction across the superfluid-vapor helium interface
2009
The extraction efficiency of positive (219)Rn ions across the superfluid-vapor helium interface above similar to 1.3 K indicates that extraction results from thermal activation across a barrier of about 20 K. Below similar to 1.3 K, the extraction efficiency is constant at about 0.7%. The evaporation of the superfluid surface by second sound pulses has a negative impact on the ion extraction, but not on the ions themselves. It takes 3.2( 6) s at 1.60 K and 15( 6) s at 1.15 K for the extraction process to recover from a disturbed state of yet unknown nature.