3,951 research outputs found
Boson-fermion model beyond mean-field approximation
A model of hybridized bosons and fermions is studied beyond the mean field
approximation. The divergent boson self-energy at zero temperature makes the
Cooper pairing of fermions impossible.The frequency and momentum dependence of
the self- energy and the condensation temperature of initially
localized bosons are calculated analytically. The value of the boson
condensation temperature is below which rules out the
boson-fermion model with the initially localized bosons as a phenomenological
explanation of high-temperature superconductivity. The intra-cell
density-density fermion-boson interaction dominates in the fermion self-energy.
The model represents a normal metal with strongly damped bosonic excitations.
The latter play the role of normal impurities.Comment: 16 pages, Latex, 5 figures available upon reques
Superconducting Volume Fraction in Overdoped Regime of La_2-x_Sr_x_CuO_4_: Implication for Phase Separation from Magnetic-Susceptibility Measurement
We have grown a single crystal of La_2-x_Sr_x_CuO_4_ in which the Sr
concentration, x, continuously changes from 0.24 to 0.29 in the overdoped
regime and obtained many pieces of single crystals with different x values by
slicing the single crystal. From detailed measurements of the magnetic
susceptibility, chi, of each piece, it has been found that the absolute value
of chi at the measured lowest temperature 2 K, |chi_2K_|, on field cooling
rapidly decreases with increasing x as well as the superconducting (SC)
transition temperature. As the value of |chi_2K_| is regarded as corresponding
to the SC volume fraction in a sample, it has been concluded that a phase
separation into SC and normal-state regions occurs in a sample of
La_2-x_Sr_x_CuO_4_ in the overdoped regime.Comment: 4 pages, 3 figures, ver. 2 has been accepted in J. Phys. Soc. Jp
Magnetic Phase Diagram of the Hole-doped CaNaCuOCl Cuprate Superconductor
We report on the magnetic phase diagram of a hole-doped cuprate
CaNaCuOCl, which is free from buckling of CuO
planes, determined by muon spin rotation and relaxation. It is characterized by
a quasi-static spin glass-like phase over a range of sodium concentration
(), which is held between long range antiferromagnetic
(AF) phase () and superconducting phase where the system is
non-magnetic for . The obtained phase diagram qualitatively agrees
well with that commonly found for hole-doped high-\tc cuprates, strongly
suggesting that the incomplete suppression of the AF order for is an
essential feature of the hole-doped cuprates.Comment: 5 pages, submitted to Phys. Rev. Let
Energy Injection Episodes in Gamma Ray Bursts: The Light Curves and Polarization Properties of GRB 021004
Several GRB afterglow light curves deviate strongly from the power law decay
observed in most bursts. We show that these variations can be accounted for by
including refreshed shocks in the standard fireball model previously used to
interpret the overall afterglow behavior. As an example we consider GRB 021004
that exhibited strong light curve variations and has a reasonably well
time-resolved polarimetry. We show that the light curves in the R-band, X-rays
and in the radio can be accounted for by four energy injection episodes in
addition to the initial event. The polarization variations are shown to be a
consequence of the injections.Comment: 4 pages, 2 figures. To appear in ApJ
Muon spin relaxation studies of incommensurate magnetism and superconductivity in stage-4 LaCuO and LaSrCuO
This paper reports muon spin relaxation (MuSR) measurements of two single
crystals of the title high-Tc cuprate systems where static incommensurate
magnetism and superconductivity coexist. By zero-field MuSR measurements and
subsequent analyses with simulations, we show that (1) the maximum ordered Cu
moment size (0.36 Bohr magneton) and local spin structure are identical to
those in prototypical stripe spin systems with the 1/8 hole concentration; (2)
the static magnetism is confined to less than a half of the volume of the
sample, and (3) regions with static magnetism form nano-scale islands with the
size comparable to the in-plane superconducting coherence length. By
transverse-field MuSR measurements, we show that Tc of these systems is related
to the superfluid density, in the same way as observed in cuprate systems
without static magnetism. We discuss a heuristic model involving percolation of
these nanoscale islands with static magnetism as a possible picture to
reconcile heterogeneity found by the present MuSR study and long-range spin
correlations found by neutron scattering.Comment: 19 pages, 15 figures, submitted to Phys. Rev. B. E-mail:
[email protected]
Effect of Zn substitution on the suppression of Tc of Y1-xCaxBa2(Cu1-yZny)3O7-delta superconductors: pseudogap and the systematic shift of the optimum hole content
The effect of Zn substitution on the superconducting transition temperature,
Tc, was investigated for the sintered Y1-xCaxBa2(Cu1-yZny)3O7-delta compounds
over a wide range of hole concentration per CuO2 plane, p. Ca substitution
enabled us to study the deeply overdoped region. p was changed by changing the
oxygen deficiency (delta). A strongly p-dependent rate of suppression of Tc
with Zn (dTc/dy) was found. From the analysis of the dTc(p)/dy and Tc(p,y)
data, we found that the optimum hole content, popt, shifts to higher values
with increasing Zn and superconductivity is at its strongest when p = 0.185 +/-
0.005. Various complementary experiments have identified this as the hole
content where the pseudogap vanishes quite abruptly. We have discussed the
possible relevance to these ideas with our findings.Comment: Replacement with some minor correction
Competing ferromagnetism in high temperature copper oxide superconductors
The extreme variability of observables across the phase diagram of the
cuprate high temperature superconductors has remained a profound mystery, with
no convincing explanation of the superconducting dome. While much attention has
been paid to the underdoped regime of the hole-doped cuprates because of its
proximity to a complex Mott insulating phase, little attention has been paid to
the overdoped regime. Experiments are beginning to reveal that the
phenomenology of the overdoped regime is just as puzzling. For example, the
electrons appear to form a Landau Fermi liquid, but this interpretation is
problematic; any trace of Mott phenomena, as signified by incommensurate
antiferromagnetic fluctuations, is absent, and the uniform spin susceptibility
shows a ferromagnetic upturn. Here we show and justify that many of these
puzzles can be resolved if we assume that competing ferromagnetic fluctuations
are simultaneously present with superconductivity, and the termination of the
superconducting dome in the overdoped regime marks a quantum critical point
beyond which there should be a genuine ferromagnetic phase at zero temperature.
We propose new experiments, and make new predictions, to test our theory and
suggest that effort must be mounted to elucidate the nature of the overdoped
regime, if the problem of high temperature superconductivity is to be solved.
Our approach places competing order as the root of the complexity of the
cuprate phase diagram.Comment: The expanded published version with very minor difference
Parameter-free expression for superconducting Tc in cuprates
A parameter-free expression for the superconducting critical temperature of
layered cuprates is derived which allows us to express Tc in terms of
experimentally measured parameters. It yields Tc values observed in about 30
lanthanum, yttrium and mercury-based samples for different levels of doping.
This remarkable agreement with the experiment as well as the unusual critical
behaviour and the normal-state gap indicate that many cuprates are close to the
Bose-Einstein condensation regime.Comment: 5 pages, 2 figures. Will be published in Physical Review
Quantum interference between charge excitation paths in a solid state Mott insulator
The competition between electron localization and de-localization in Mott
insulators underpins the physics of strongly-correlated electron systems.
Photo-excitation, which re-distributes charge between sites, can control this
many-body process on the ultrafast timescale. To date, time-resolved studies
have been performed in solids in which other degrees of freedom, such as
lattice, spin, or orbital excitations come into play. However, the underlying
quantum dynamics of bare electronic excitations has remained out of reach.
Quantum many-body dynamics have only been detected in the controlled
environment of optical lattices where the dynamics are slower and lattice
excitations are absent. By using nearly-single-cycle near-IR pulses, we have
measured coherent electronic excitations in the organic salt ET-F2TCNQ, a
prototypical one-dimensional Mott Insulator. After photo-excitation, a new
resonance appears on the low-energy side of the Mott gap, which oscillates at
25 THz. Time-dependent simulations of the Mott-Hubbard Hamiltonian reproduce
the oscillations, showing that electronic delocalization occurs through quantum
interference between bound and ionized holon-doublon pairs.Comment: 4 figures and supplementary informatio
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