39 research outputs found
Strong-Coupling Features Due to Quasiparticle Interaction in Two Dimensional Superconductors
I calculate the effect of interactions among superconducting quasiparticles
in two-dimensional(2D) a superconductor at T=0. The strength of the effective
interaction among the quasiparticles is essentially given by the screened
Coulomb interaction which has strength at low frequency because of the gapless
nature of the plasmon. This is in contrast to three dimensions where the
effective interaction has negligible weight at frequencies , the
superconducting gap. The quasiparticle interactions give rise to
strong-coupling effects in experimental quantities which are beyond the
conventional Eliashberg treatment of superconductivity. The present calculation
offers an explanation of why these effects are much larger in 2D than in 3D
superconductors and, in particular, why the analogous strong-coupling effects
due to quasiparticle interactions are seen in data on the quasi-2D cuprate
superconductors. the strong-coupling features seen in data on the cuprates are
discussed in light of the present calculation.Comment: 18 pages including 11 figures Revte
Collective Excitations in High-Temperature Superconductors
Collective, low-energy excitations in quasi-two-dimensional d-wave
superconductors are analyzed. While the long-range Coulomb interaction shifts
the charge-density-wave and phase modes up to the plasma energy, the
spin-density-wave excitation that arises due to a strong local
electron-electron repulsion can propagate as a damped collective mode within
the superconducting energy gap. It is suggested that these excitations are
relevant to high-Tc superconductors, close to the antiferromagnetic phase
boundary, and may explain some of the exotic features of the experimentally
observed spectral-density and neutron-scattering data.Comment: 5 jolly page
Theory of Luminescence Spectra of High-Density Electron-Hole Systems: Crossover from Excitonic Bose-Einstein Condenstation to Electron-Hole BCS State
We present a unified theory of luminescence spectra for highly excited
semiconductors, which is applicable both to the electron-hole BCS state and to
the exciton Bose-Einstein condensate. The crossover behavior between
electron-hole BCS state and exciton Bose-Einstein condensate clearly manifests
itself in the calculated luminescence spectra. The analysis is based on the
Bethe-Salpeter equation combined with the generalized
random-phase-approximation, which enables us to consider the multiple Coulomb
scattering and the quantum fluctuation associated with the center-of-mass
motion of electron-hole pairs. In the crossover regime, the calculated spectra
are essentially different from results obtained by the BCS-like mean-field
theory and the interacting Boson model. In particular, it is found that the
broad spectrum, arising from the recombination of electron-hole BCS state,
splits into the P- and P_2-luminescence bands with decreasing the particle
density. The dependence of these bands on the carrier density is in good
agreement with experiments for highly excited semiconductors.Comment: 9 pages, 4 figures, To appear in Solid State Communication
From Cooper Pairs to Composite Bosons: A Generalized RPA Analysis of Collective Excitations
The evolution of the ground state and the excitation spectrum of the two and
three dimensional attractive Hubbard model is studied as the system evolves
from a Cooper pair regime for weak attraction to a composite boson regime for a
strong attraction.Comment: 20 pages RevTex, 7 figures on reques
Many-body theory of pump-probe spectra for highly excited semiconductors
We present a unified theory for pump-probe spectra in highly excited
semiconductors, which is applicable throughout the whole density regime
including the high-density electron-hole BCS state and the low-density
excitonic Bose-Einstein condensate (BEC). The analysis is based on the BCS-like
pairing theory combined with the Bethe-Salpeter (BS) equation, which first
enables us to incorporate the state-filling effect, the band-gap
renormalization and the strong/weak electron-hole pair correlations in a
unified manner. We show that the electron-hole BCS state is distinctly
stabilized by the intense pump-light, and this result strongly suggests that
the macroscopic quantum state can be observed under the strong photoexcitation.
The calculated spectra considerably deviate from results given by the BCS-like
mean field theory and the simple BS equation without electron-hole pair
correlation especially in the intermediate density states between the
electron-hole BCS state and the excitonic BEC state. In particular, we find the
sharp stimulated emission and absorption lines which originate from the optical
transition accompanied by the collective phase fluctuation mode in the
electron-hole BCS state. From the pump-probe spectral viewpoint, we show that
this fluctuation mode changes to the exciton mode with decreasing carrier
densityComment: RevTeX 11 pages, 10 figures. To appear in Phys.Rev.B1
Where is the pi particle?
We discuss the interplay of particle-particle and particle-hole spin-triplet
channels in high-T_c superconductors using a quasiparticle dispersion motivated
by angle-resolved photoemission. Within a generalized RPA, we find a well
defined antibound state of two holes, the pi resonance of Demler and Zhang, as
well as a bound state of a particle and a hole, the spin exciton. We show that
the energy of the pi resonance always exceeds 2 Delta, twice the maximum d-wave
gap, therefore the neutron resonance observed in the cuprates around energy
Delta is most likely a spin exciton. At the same time, we speculate that the pi
particle can exist at higher energies and might be observed in neutron
scattering around 100 meV.Comment: RevTeX, 5 pages, 4 eps figure
Pi excitation of the t-J model
In this paper, we present analytical and numerical calculations of the pi
resonance in the t-J model. We show in detail how the pi resonance in the
particle-particle channel couples to and appears in the dynamical spin
correlation function in a superconducting state. The contribution of the pi
resonance to the spin excitation spectrum can be estimated from general
model-independent sum rules, and it agrees with our detailed calculations. The
results are in overall agreement with the exact diagonalization studies of the
t-J model. Earlier calculations predicted the correct doping dependence of the
neutron resonance peak in the YBCO superconductor, and in this paper detailed
energy and momentum dependence of the spin correlation function is presented.
The microscopic equations of motion obtained within current formalism agree
with that of the SO(5) nonlinear sigma model, where the pi resonance is
interpreted as a pseudo Goldstone mode of the spontaneous SO(5) symmetry
breaking.Comment: 33 pages, LATEX, 14 eps fig
Plasmon excitations in homogeneous neutron star matter
We study the possible collective plasma modes which can affect neutron-star
thermodynamics and different elementary processes in the baryonic density range
between nuclear saturation () and . In this region, the
expected constituents of neutron-star matter are mainly neutrons, protons,
electrons and muons ( matter), under the constraint of beta
equilibrium. The elementary plasma excitations of the three-fluid
medium are studied in the RPA framework. We emphasize the relevance of the
Coulomb interaction among the three species, in particular the interplay of the
electron and muon screening in suppressing the possible proton plasma mode,
which is converted into a sound-like mode. The Coulomb interaction alone is
able to produce a variety of excitation branches and the full spectral function
shows a rich structure at different energy. The genuine plasmon mode is pushed
at high energy and it contains mainly an electron component with a substantial
muon component, which increases with density. The plasmon is undamped for not
too large momentum and is expected to be hardly affected by the nuclear
interaction. All the other branches, which fall below the plasmon, are damped
or over-damped.Comment: misprint corrected in Eq. (1
Helical spin-density wave in doped V2O3
Recent neutron scattering and nuclear magnetic resonance experiments have
revealed that the low temperature phase of doped V_{2-y}O_3 is an itinerant
antiferromagnet with a helical spin structure. We use a band structure
calculation as the point of departure to show that these experiments are in
agreement with mean field results for an Overhauser spin-density wave state.
The influences of a finite life-time and of dilute magnetic impurities are
discussed.Comment: 6 pages RevTex incl. 7 postscript figures, to be published by PR
On the relative positions of the peaks in Raman and tunneling spectra of d-wave superconductors
We study Raman intensity and the density of states
in isotropic 2D d-wave superconductors. For an ideal gas,
and have sharp peaks at and , respectively, where is the maximum value of the gap.
We study how the peak positions are affected by the fermionic damping due to
impurity scattering.
We show that while the damping generally shifts the peak positions to larger
frequencies, the peak in still occurs at almost twice the peak
position in and therefore cannot account for the experimentally
observed downturn shift of the peak frequency in in underdoped
cuprates compared to twice that in . We also discuss how the
fermionic damping affects the dynamical spin susceptibility.Comment: 5 pages, 2 figure