38 research outputs found
Spectral functions, Fermi surface and pseudogap in the t-J model
Spectral functions within the generalized t-J model as relevant to cuprates
are analyzed using the method of equations of motion for projected fermion
operators. In the evaluation of the self energy the decoupling of spin and
single-particle fluctuations is performed. It is shown that in an undoped
antiferromagnet (AFM) the method reproduces the selfconsistent Born
approximation. For finite doping with short range AFM order the approximation
evolves into a paramagnon contribution which retains large incoherent
contribution in the hole part of the spectral function as well as the
hole-pocket-like Fermi surface at low doping. On the other hand, the
contribution of (longitudinal) spin fluctuations, with the coupling mostly
determined predominantly by J and next-neighbor hopping t', is essential for
the emergence of the pseudogap. The latter shows at low doping in the effective
truncation of the large Fermi surface, reduced electron density of states and
at the same time quasiparticle density of states at the Fermi level.Comment: RevTex, 13 pages, 11 figures (5 color
Spin correlations in the algebraic spin liquid - implications for high Tc superconductors
We propose that underdoped high superconductors are described by an
algebraic spin liquid (ASL) at high energies, which undergoes a spin-charge
recombination transition at low energies. The spin correlation in the ASL is
calculated via its effective theory - a system of massless Dirac fermions
coupled to a U(1) gauge field. We find that without fine tuning any parameters
the gauge interaction strongly enhances the staggered spin correlation even in
the presence of a large single particle pseudo-gap. This allows us to show that
the ASL plus spin-charge recombination picture can explain many highly unusual
properties of underdoped high superconductors.Comment: 22 pages, 18 figures, submitted to PR
Spin-Charge Separation in the Model: Magnetic and Transport Anomalies
A real spin-charge separation scheme is found based on a saddle-point state
of the model. In the one-dimensional (1D) case, such a saddle-point
reproduces the correct asymptotic correlations at the strong-coupling
fixed-point of the model. In the two-dimensional (2D) case, the transverse
gauge field confining spinon and holon is shown to be gapped at {\em finite
doping} so that a spin-charge deconfinement is obtained for its first time in
2D. The gap in the gauge fluctuation disappears at half-filling limit, where a
long-range antiferromagnetic order is recovered at zero temperature and spinons
become confined. The most interesting features of spin dynamics and transport
are exhibited at finite doping where exotic {\em residual} couplings between
spin and charge degrees of freedom lead to systematic anomalies with regard to
a Fermi-liquid system. In spin dynamics, a commensurate antiferromagnetic
fluctuation with a small, doping-dependent energy scale is found, which is
characterized in momentum space by a Gaussian peak at (, ) with
a doping-dependent width (, is the doping
concentration). This commensurate magnetic fluctuation contributes a
non-Korringa behavior for the NMR spin-lattice relaxation rate. There also
exits a characteristic temperature scale below which a pseudogap behavior
appears in the spin dynamics. Furthermore, an incommensurate magnetic
fluctuation is also obtained at a {\em finite} energy regime. In transport, a
strong short-range phase interference leads to an effective holon Lagrangian
which can give rise to a series of interesting phenomena including linear-
resistivity and Hall-angle. We discuss the striking similarities of these
theoretical features with those found in the high- cuprates and give aComment: 70 pages, RevTex, hard copies of 7 figures available upon request;
minor revisions in the text and references have been made; To be published in
July 1 issue of Phys. Rev. B52, (1995
Competing orders in a magnetic field: spin and charge order in the cuprate superconductors
We describe two-dimensional quantum spin fluctuations in a superconducting
Abrikosov flux lattice induced by a magnetic field applied to a doped Mott
insulator. Complete numerical solutions of a self-consistent large N theory
provide detailed information on the phase diagram and on the spatial structure
of the dynamic spin spectrum. Our results apply to phases with and without
long-range spin density wave order and to the magnetic quantum critical point
separating these phases. We discuss the relationship of our results to a number
of recent neutron scattering measurements on the cuprate superconductors in the
presence of an applied field. We compute the pinning of static charge order by
the vortex cores in the `spin gap' phase where the spin order remains
dynamically fluctuating, and argue that these results apply to recent scanning
tunnelling microscopy (STM) measurements. We show that with a single typical
set of values for the coupling constants, our model describes the field
dependence of the elastic neutron scattering intensities, the absence of
satellite Bragg peaks associated with the vortex lattice in existing neutron
scattering observations, and the spatial extent of charge order in STM
observations. We mention implications of our theory for NMR experiments. We
also present a theoretical discussion of more exotic states that can be built
out of the spin and charge order parameters, including spin nematics and phases
with `exciton fractionalization'.Comment: 36 pages, 33 figures; for a popular introduction, see
http://onsager.physics.yale.edu/superflow.html; (v2) Added reference to new
work of Chen and Ting; (v3) reorganized presentation for improved clarity,
and added new appendix on microscopic origin; (v4) final published version
with minor change
Detection of HIV, hepatitis B and hepatitis C markers in discarded syringes and bloodstains
Puncture injuries from discarded syringes continue to increase and testing these syringes for viral markers of HIV, HBV and HCV infection is now frequently requested. Workers in forensic medicine also regularly deal with potentially infected material. A simulation study was carried out. Syringes (with attached needles) and cotton gauze were contaminated with HIV, HBV and HCV infected blood or serum and tested at intervals over five weeks. Sensitive ELISA techniques, giving a result within three hours, were used to detect viral antibodies (HIV, HCV) or antigens (HBV). Blood residues inside the syringes and on the gauze gave positive results for viral markers throughout the period of study; the needle-tips gave intermittently positive reactions. It was concluded that standard ELISAs for detecting markers of HIV, HBV and HCV could provide valuable guidance on potential infection hazards of material being examined in a forensic situatio
The normal liquid 3 He one-body momentum distribution at zero and finite temperature
61.20.Gy Theory and models of liquid structure, 61.20.Ne Structure of simple liquids,