5,405 research outputs found
Collective excitations in a fermion-fermion mixture with different Fermi surfaces
In this paper, collective excitations in a homogeneous fermion-fermion
mixture with different Fermi surfaces are studied. In the Fermi liquid phase,
the zero-sound velocity is found to be larger than the largest Fermi velocity.
With attractive interactions, the superfluid phase appears below a critical
temperature, and the phase mode is the low-energy collective excitation. The
velocity of the phase mode is proportional to the geometric mean of the two
Fermi velocities. The difference between the two velocities may serve as a tool
to detect the superfluid phase.Comment: 4 pages. To be published in Phys. Rev.
Simulated acoustic emissions from coupled strings
We consider traveling transverse waves on two identical uniform taut strings that are elastically coupled through springs that gradually decrease their stiffness over a region of finite length. The wave system can be decomposed into two modes: an in-phase mode ( + ) that is transparent to the coupling springs, and an out-of-phase mode ( − ) that engages the coupling springs and can resonate at a particular location depending on the excitation frequency. The system exhibits linear mode conversion whereby an incoming ( + ) wave is reflected back from the resonance location both as a propagating ( + ) wave and an evanescent ( − ) wave, while both types emerge as propagating forward through the resonance location. We match a local transition layer expansion to the WKB expansion to obtain estimates of the reflection and transmission coefficients. The reflected waves may be an analog for stimulated emissions from the ear
Evolution of Higgs mode in a Fermion Superfluid with Tunable Interactions
In this letter we present a coherent picture for the evolution of Higgs mode
in both neutral and charged -wave fermion superfluids, as the strength of
attractive interaction between fermions increases from the BCS to the BEC
regime. In the case of neutral fermionic superfluid, such as ultracold
fermions, the Higgs mode is pushed to higher energy while at the same time,
gradually loses its spectral weight as interaction strength increases toward
the BEC regime, because the system is further tuned away from Lorentz
invariance. On the other hand, when damping is taken into account, Higgs mode
is significantly broadened due to coupling to phase mode in the whole BEC-BCS
crossover. In the charged case of electron superconductor, the Anderson-Higgs
mechanism gaps out the phase mode and suppresses the coupling between the Higgs
and the phase modes, and consequently, stabilizes the Higgs mode.Comment: 5 figures, 9 pages, including supplementary materia
Collective charge density fluctuations in superconducting layered systems with bilayer unit cells
Collective modes of bilayered superconducting superlattices (e.g., YBCO) are
investigated within the conserving gauge-invariant ladder diagram approximation
including both the nearest interlayer single electron tunneling and the
Josephson-type Cooper pair tunneling. By calculating the density-density
response function including Coulomb and pairing interactions, we examine the
two collective mode branches corresponding to the in-phase and out-of-phase
charge fluctuations between the two layers in the unit cell. The out-of-phase
collective mode develops a long wavelength plasmon gap whose magnitude depends
on the tunneling strength with the mode dispersions being insensitive to the
specific tunneling mechanism (i.e., single electron or Josephson). We also show
that in the presence of tunneling the oscillator strength of the out-of-phase
mode overwhelms that of the in-phase-mode at and finite ,
where and are respectively the mode wave vectors perpendicular
and along the layer. We discuss the possible experimental observability of the
phase fluctuation modes in the context of our theoretical results for the mode
dispersion and spectral weight.Comment: 9 pages, 3 figure
Stability of Discrete Solitons in the Presence of Parametric Driving
In this brief report, we consider parametrically driven bright solitons in
the vicinity of the anti-continuum limit. We illustrate the mechanism through
which these solitons become unstable due to the collision of the phase mode
with the continuous spectrum, or eigenvelues bifurcating thereof. We show how
this mechanism typically leads to complete destruction of the bright solitary
wave.Comment: 4 pages, 4 figure
Isochronism and tangent bifurcation of band edge modes in Hamiltonian lattices
In {\em Physica D} {\bf 91}, 223 (1996), results were obtained regarding the
tangent bifurcation of the band edge modes () of nonlinear Hamiltonian
lattices made of coupled oscillators. Introducing the concept of {\em
partial isochronism} which characterises the way the frequency of a mode,
, depends on its energy, , we generalize these results and
show how the bifurcation energies of these modes are intimately connected to
their degree of isochronism. In particular we prove that in a lattice of
coupled purely isochronous oscillators ( strictly constant),
the in-phase mode () never undergoes a tangent bifurcation whereas the
out-of-phase mode () does, provided the strength of the nonlinearity in
the coupling is sufficient. We derive a discrete nonlinear Schr\"odinger
equation governing the slow modulations of small-amplitude band edge modes and
show that its nonlinear exponent is proportional to the degree of isochronism
of the corresponding orbits. This equation may be seen as a link between the
tangent bifurcation of band edge modes and the possible emergence of localized
modes such as discrete breathers.Comment: 23 pages, 1 figur
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