147 research outputs found
Dynamic spin susceptibility in the t-J model
A relaxation-function theory for the dynamic spin susceptibility in the
-- model is presented. By a sum-rule-conserving generalized mean-field
approximation (GMFA), the two-spin correlation functions of arbitrary range,
the staggered magnetization, the uniform static susceptibility, and the
antiferromagnetic correlation length are calculated in a wide region of hole
doping and temperaturs. A good agreement with available exact diagonalization
(ED) data is found. The correlation length is in reasonable agreement with
neutron-scattering experiments on La_{2-\delta}Sr_\delta)CuO_4. Going beyond
the GMFA, the self-energy is calculated in the mode-coupling approximation. The
spin dynamics at arbitrary frequencies and wave vectors is studied for various
temperatures and hole doping. At low doping a spin-wave-type behavior is found
as in the Heisenberg model, while at higher doping a strong damping caused by
hole hopping occurs, and a relaxation-type spin dynamics is observed in
agreement with the ED results. The local spin susceptibility and its (\omega/T)
scaling behavior are calculated in a reasonable agreement with experimental and
ED data.Comment: 13 pages, 14 figure
Josephson surface plasmons in spatially confined cuprate superconductors
In this work, we generalize the theory of localized surface plasmons to the
case of high-Tc cuprate superconductors, spatially confined in the form of
small spherical particles. At variance from ordinary metals, cuprate
superconductors are characterized by a low-energy bulk excitation known as the
Josephson plasma wave (JPW), arising from interlayer tunneling of the
condensate along the c-axis. The effect of the JPW is revealed in a
characteristic spectrum of surface excitations, which we call Josephson surface
plasmons. Our results, which apply to any material with a strongly anisotropic
electromagnetic response, are worked out in detail for the case of multilayered
superconductors supporting both low-frequency (acoustic) and transverse-optical
JPW. Spatial confinement of the Josephson plasma waves may represent a new
degree of freedom to engineer their frequencies and to explore the link between
interlayer tunnelling and high-Tc superconductivity
Electronic spectrum in high-temperature cuprate superconductors
A microscopic theory for electronic spectrum of the CuO2 plane within an
effective p-d Hubbard model is proposed. Dyson equation for the single-electron
Green function in terms of the Hubbard operators is derived which is solved
self-consistently for the self-energy evaluated in the noncrossing
approximation. Electron scattering on spin fluctuations induced by kinematic
interaction is described by a dynamical spin susceptibility with a continuous
spectrum. Doping and temperature dependence of electron dispersions, spectral
functions, the Fermi surface and the coupling constant are studied in the hole
doped case. At low doping, an arc-type Fermi surface and a pseudogap in the
spectral function are observed.Comment: 13 pages (revtex), 18 figures, to be published in JET
Spatial Structure of Spin Polarons in the t-J Model
The deformation of the quantum Neel state induced by a spin polaron is
analyzed in a slave fermion approach. Our method is based on the selfconsistent
Born approximation for Green's and the wave function for the quasiparticle. The
results of various spin-correlation functions relative to the position of the
moving hole are discussed and shown to agree with those available from small
cluster calculations. Antiferromagnetic correlations in the direct neighborhood
of the hole are reduced, but they remain antiferromagnetic even for J as small
as 0.1 t. These correlation functions exhibit dipolar distortions in the spin
structure, which sensitively depend on the momentum of the quasiparticle. Their
asymptotic decay with the distance from the hole is governed by power laws, yet
the spectral weight of the quasiparticles does not vanish.Comment: 12 pages, 2 postscipt files with figures; uses REVTeX, to be
published in Phys. Rev. B, Feb. 199
High Temperature Macroscopic Entanglement
In this paper I intend to show that macroscopic entanglement is possible at
high temperatures. I analyze multipartite entanglement produced by the
pairing mechanism which features strongly in the fermionic lattice models of
high superconductivity. This problem is shown to be equivalent to
calculating multipartite entanglement in totally symmetric states of qubits. I
demonstrate that we can conclusively calculate the relative entropy of
entanglement within any subset of qubits in an overall symmetric state. Three
main results then follow. First, I show that the condition for
superconductivity, namely the existence of the off diagonal long range order
(ODLRO), is not dependent on two-site entanglement, but on just classical
correlations as the sites become more and more distant. Secondly, the
entanglement that does survive in the thermodynamical limit is the entanglement
of the total lattice and, at half filling, it scales with the log of the number
of sites. It is this entanglement that will exist at temperatures below the
superconducting critical temperature, which can currently be as high as 160
Kelvin. Thirdly, I prove that a complete mixture of symmetric states does not
contain any entanglement in the macroscopic limit. On the other hand, the same
mixture of symmetric states possesses the same two qubit entanglement features
as the pure states involved, in the sense that the mixing does not destroy
entanglement for finite number of qubits, albeit it does decrease it. Maximal
mixing of symmetric states also does not destroy ODLRO and classical
correlations. I discuss various other inequalities between different
entanglements as well as generalizations to the subsystems of any
dimensionality (i.e. higher than spin half).Comment: 14 pages, no figure
Discontinuity of capacitance at the onset of surface superconductivity
The effect of the magnetic field on a capacitor with a superconducting
electrode is studied within the Ginzburg-Landau approach. It is shown that the
capacitance has a discontinuity at the onset of the surface superconductivity
which is expressed as a discontinuity in the penetration depth of
the electric field into metals. Estimates show that this discontinuity is
observable with recent bridges for both conventional and high-
superconductors of the type-II
Operator projection method applied to the single-particle Green's function in the Hubbard model
A new non-perturbative framework for many-body correlated systems is
formulated by extending the operator projection method (OPM). This method
offers a systematic expansion which enables us to project into the low-energy
structure after extracting the higher-energy hierarchy. This method also opens
a way to systematically take into account the effects of collective
excitations. The Mott-Hubbard metal-insulator transition in the Hubbard model
is studied by means of this projection beyond the second order by taking into
account magnetic and charge fluctuations in the presence of the high-energy
Mott-Hubbard structure. At half filling, the Mott-Hubbard gap is correctly
eproduced between the separated two bands. Near half filling, a strongly
renormalized low-energy single-particle excitations coexisting with the
Mott-Hubbard bands are shown to appear. Signifcance of momentum-dependent
self-energy in the results is stressed.Comment: 6 pages, final version to appear in J. Phys. Soc. Jp
Spectral functions and pseudogap in the t-J model
We calculate spectral functions within the t-J model as relevant to cuprates
in the regime from low to optimum doping. On the basis of equations of motion
for projected operators an effective spin-fermion coupling is derived. The self
energy due to short-wavelength transverse spin fluctuations is shown to lead to
a modified selfconsistent Born approximation, which can explain strong
asymmetry between hole and electron quasiparticles. The coupling to
long-wavelength longitudinal spin fluctuations governs the low-frequency
behavior and results in a pseudogap behavior, which at low doping effectively
truncates the Fermi surface.Comment: Minor corrections; to appear in Phys. Rev. B (RC
Surface deformation caused by the Abrikosov vortex lattice
In superconductors penetrated by Abrikosov vortices the magnetic pressure and
the inhomogeneous condensate density induce a deformation of the ionic lattice.
We calculate how this deformation corrugates the surface of a semi-infinite
sample. The effect of the surface dipole is included
Spatially-resolved relaxation dynamics of photoinduced quasiparticles in underdoped YBaCuO
The spatially-resolved relaxation characteristics of photoinduced
quasiparticles (QPs) in CuO planes of underdoped YBCO are disclosed by
polarized fs time-resolved spectroscopy. The relaxation time (tau) along b axis
diverges at Tc, and appears to be governed by a temperature-dependent gap
Delta(T) at T Tc, a monotonic increase of tau with
decreasing T along the b axis and ab diagonal was observed and can be
attributed to a temperature-independent gap Delta. The results lend
support to recombination dominant scenario of QP dynamics. However, the QP
thermalization may take part along the nodal direction in the highly underdoped
samples.Comment: 16 pages, 4 figures. To be published in Physical Review B, Brief
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