819 research outputs found
Theory of Raman scattering from Leggett's collective mode in a multiband superconductor: Application to MgB
In 1966 Leggett used a two-band superconductor to show that a new collective
mode could exist at low temperatures, corresponding to a counter-flow of the
superconducting condensates in each band. Here, the theory of electronic Raman
scattering in a superconductor by Klein and Dierker (1984) is extended to a
multiband superconductor. Raman scattering creates particle/hole pairs. In the
relevant \ symmetry, the attraction that produces pairing necessarily
couples excitations of superconducting pairs to these p/h excitations. In the
Appendix it is shown that for zero wave vector transfer this coupling
modifies the Raman response and makes the long-range Coulomb correction null.
The 2-band result is applied to MgB where this coupling activates
Leggett's collective mode. His simple limiting case is obtained when the
interband attractive potential is decreased to a value well below that given by
LDA theory. The peak from Leggett's mode is studied as the potential is
increased through the theoretical value: With realistic MgB\ parameters,
the peak broadens through decay into the continuum above the smaller (
band) superconducting gap. Finite effects are also taken into account,
yielding a Raman peak that agrees well in energy with the experimental result
by Blumberg \textit{et el.} (2007). This approach is also applied to the ,
2-band model of the Fe-pnictides considered by Chubukov \textit{et al.}(2009).Comment: 10 pages, 3 figures. To appear in Physical Review
Phase transition in the Higgs model of scalar dyons
In the present paper we investigate the phase transition
"Coulomb--confinement" in the Higgs model of abelian scalar dyons -- particles
having both, electric and magnetic , charges. It is shown that by dual
symmetry this theory is equivalent to scalar fields with the effective squared
electric charge e^{*2}=e^2+g^2. But the Dirac relation distinguishes the
electric and magnetic charges of dyons. The following phase transition
couplings are obtained in the one--loop approximation:
\alpha_{crit}=e^2_{crit}/4\pi\approx 0.19,
\tilde\alpha_{crit}=g^2_{crit}/4\pi\approx 1.29 and \alpha^*_{crit}\approx
1.48.Comment: 16 pages, 2 figure
Quantum Breathing Mode of Interacting Particles in a One-dimensional Harmonic Trap
Extending our previous work, we explore the breathing mode---the [uniform]
radial expansion and contraction of a spatially confined system. We study the
breathing mode across the transition from the ideal quantum to the classical
regime and confirm that it is not independent of the pair interaction strength
(coupling parameter). We present the results of time-dependent Hartree-Fock
simulations for 2 to 20 fermions with Coulomb interaction and show how the
quantum breathing mode depends on the particle number. We validate the accuracy
of our results, comparing them to exact Configuration Interaction results for
up to 8 particles
Heavy-to-light form factors: sum rules on the light cone and beyond
We report the first systematic analysis of the off-light-cone effects in sum
rules for heavy-to-light form factors. These effects are investigated in a
model based on scalar constituents, which allows a technically rather simple
analysis but has the essential features of the analogous QCD calculation. The
correlator relevant for the extraction of the heavy-to-light form factor is
calculated in two different ways: first, by adopting the full Bethe-Salpeter
amplitude of the light meson and, second, by performing the expansion of this
amplitude near the light cone . We demonstrate that the contributions to
the correlator from the light-cone term and the off-light-cone terms
have the same order in the expansion. The light-cone
correlator, corresponding to , is shown to systematically overestimate
the full correlator, the difference being , with
the continuum subtraction parameter of order 1 GeV. Numerically, this
difference is found to be 10-20%.Comment: revtex 14 pages, version to be published in Phys. Rev. D (discussion
in Sect. 3 extended, example in Sect. 4 added
Localization of Bogoliubov quasiparticles in interacting Bose gases with correlated disorder
We study the Anderson localization of Bogoliubov quasiparticles (elementary
many-body excitations) in a weakly interacting Bose gas of chemical potential
subjected to a disordered potential . We introduce a general mapping
(valid for weak inhomogeneous potentials in any dimension) of the Bogoliubov-de
Gennes equations onto a single-particle Schr\"odinger-like equation with an
effective potential. For disordered potentials, the Schr\"odinger-like equation
accounts for the scattering and localization properties of the Bogoliubov
quasiparticles. We derive analytically the localization lengths for correlated
disordered potentials in the one-dimensional geometry. Our approach relies on a
perturbative expansion in , which we develop up to third order, and we
discuss the impact of the various perturbation orders. Our predictions are
shown to be in very good agreement with direct numerical calculations. We
identify different localization regimes: For low energy, the effective
disordered potential exhibits a strong screening by the quasicondensate density
background, and localization is suppressed. For high-energy excitations, the
effective disordered potential reduces to the bare disordered potential, and
the localization properties of quasiparticles are the same as for free
particles. The maximum of localization is found at intermediate energy when the
quasicondensate healing length is of the order of the disorder correlation
length. Possible extensions of our work to higher dimensions are also
discussed.Comment: Published versio
Nonlinear Bogolyubov-Valatin transformations and quaternions
In introducing second quantization for fermions, Jordan and Wigner
(1927/1928) observed that the algebra of a single pair of fermion creation and
annihilation operators in quantum mechanics is closely related to the algebra
of quaternions H. For the first time, here we exploit this fact to study
nonlinear Bogolyubov-Valatin transformations (canonical transformations for
fermions) for a single fermionic mode. By means of these transformations, a
class of fermionic Hamiltonians in an external field is related to the standard
Fermi oscillator.Comment: 6 pages REVTEX (v3: two paragraphs appended, minor stylistic changes,
eq. (39) corrected, references [10]-[14], [36], [37], [41], [67]-[69] added;
v4: few extensions, references [62], [63] added, final version to be
published in J. Phys. A: Math. Gen.
Renormalization and additional degrees of freedom within the chiral effective theory for spin-1 resonances
We study in detail various aspects of the renormalization of the spin-1
resonance propagator in the effective field theory framework. First, we briefly
review the formalisms for the description of spin-1 resonances in the path
integral formulation with the stress on the issue of propagating degrees of
freedom. Then we calculate the one-loop 1-- meson self-energy within the
Resonance chiral theory in the chiral limit using different methods for the
description of spin-one particles, namely the Proca field, antisymmetric tensor
field and the first order formalisms. We discuss in detail technical aspects of
the renormalization procedure which are inherent to the power-counting
non-renormalizable theory and give a formal prescription for the organization
of both the counterterms and one-particle irreducible graphs. We also construct
the corresponding propagators and investigate their properties. We show that
the additional poles corresponding to the additional one-particle states are
generated by loop corrections, some of which are negative norm ghosts or
tachyons. We count the number of such additional poles and briefly discuss
their physical meaning.Comment: 65 pages, 12 figure
Bogolyubov approximation for diagonal model of an interacting Bose gas
We study, using the Bogolyubov approximation, the thermodynamic behaviour of
a superstable Bose system whose energy operator in the second-quantized form
contains a nonlinear expression in the occupation numbers operators. We prove
that for all values of the chemical potential satisfying ,
where is the lowest energy value, the system undergoes
Bose--Einstein condensation
Electronic Orbital Currents and Polarization in Mott Insulators
The standard view is that at low energies Mott insulators exhibit only
magnetic properties while charge degrees of freedom are frozen out as the
electrons become localized by a strong Coulomb repulsion. We demonstrate that
this is in general not true: for certain spin textures {\it spontaneous
circular electric currents} or {\it nonuniform charge distribution} exist in
the ground state of Mott insulators. In addition, low-energy ``magnetic''
states contribute comparably to the dielectric and magnetic functions
and leading to interesting phenomena
such as rotation the electric field polarization and resonances which may be
common for both functions producing a negative refraction index in a window of
frequencies
Anderson Localization of Bogolyubov Quasiparticles in Interacting Bose-Einstein Condensates
We study the Anderson localization of Bogolyubov quasiparticles in an
interacting Bose-Einstein condensate (with healing length \xi) subjected to a
random potential (with finite correlation length \sigma_R). We derive
analytically the Lyapunov exponent as a function of the quasiparticle momentum
k and we study the localization maximum k_{max}. For 1D speckle potentials, we
find that k_{max} is proportional to 1/\xi when \xi is much larger than
\sigma_R while k_{max} is proportional to 1/\sigma_R when \xi is much smaller
than \sigma_R, and that the localization is strongest when \xi is of the order
of \sigma_R. Numerical calculations support our analysis and our estimates
indicate that the localization of the Bogolyubov quasiparticles is accessible
in current experiments with ultracold atoms.Comment: published version (no significant changes compared to last version
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