5,030 research outputs found
Spin-Charge Separation in Two-dimensional Frustrated Quantum Magnets
The dynamics of a mobile hole in two-dimensional frustrated quantum magnets
is investigated by exact diagonalization techniques. Our results provide
evidence for spin-charge separation upon doping the kagome lattice, a prototype
of a spin liquid. In contrast, in the checkerboard lattice, a symmetry broken
Valence Bond Crystal, a small quasi-particle peak is seen for some crystal
momenta, a finding interpreted as a restoration of weak holon-spinon
confinement.Comment: 4 pages, 6 figure
The Gap Function Phi(k,w) for a Two-leg t-J Ladder and the Pairing Interaction
The gap function phi(k,omega), determined from a Lanczos calculation for a
doped 2-leg t-J ladder, is used to provide insight into the spatial and
temporal structure of the pairing interaction. It implies that this interaction
is a local near-neighbor coupling which is retarded. The onset frequency of the
interaction is set by the energy of an S=1 magnon-hole-pair and it is spread
out over a frequency region of order the bandwith
Magnetic ordering in a doped frustrated spin-Peierls system
Based on a model of a quasi-one dimensional spin-Peierls system doped with
non-magnetic impurities, an effective two-dimensional Hamiltonian of randomly
distributed S=1/2 spins interacting via long-range pair-wise interaction is
studied using a stochastic series expansion quantum Monte Carlo method. The
susceptibility shows Curie-like behavior at the lowest temperatures reached
although the staggered magnetisation is found to be finite for . The
doping dependance of the corresponding three-dimensional Neel temperature is
also computed.Comment: Published version, 4 pages, 5 figure
The Dominant Role of Critical Valence Fluctuations on High Superconductivity in Heavy Fermions
Despite almost 40 years of research, the origin of heavy-fermion
superconductivity is still strongly debated. Especially, the pressure-induced
enhancement of superconductivity in CeCuSi away from the magnetic
breakdown is not sufficiently taken into consideration. As recently reported in
CeCuSi and several related compounds, optimal superconductivity occurs
at the pressure of a valence crossover, which arises from a virtual critical
end point at negative temperature . In this context, we did a
meticulous analysis of a vast set of top-quality high-pressure electrical
resistivity data of several Ce-based heavy fermion compounds. The key novelty
is the salient correlation between the superconducting transition temperature
and the valence instability parameter , which is in
line with theory of enhanced valence fluctuations. Moreover, it is found that,
in the pressure region of superconductivity, electrical resistivity is governed
by the valence crossover, which most often manifests in scaling behavior. We
develop the new idea that the optimum superconducting of a given
sample is mainly controlled by the compound's and limited by
non-magnetic disorder. In this regard, the present study provides compelling
evidence for the crucial role of critical valence fluctuations in the formation
of Cooper pairs in Ce-based heavy fermion superconductors besides the
contribution of spin fluctuations near magnetic quantum critical points, and
corroborates a plausible superconducting mechanism in strongly correlated
electron systems in general.Comment: Supplementary Material follows after the bibliograph
Stability of Inhomogeneous Superstructures from Renormalized Mean-field Theory of the t--J Model
Using the t--J model (which can also include Coulomb repulsion) and the
``plain vanilla'' renormalized mean-field theory of Zhang et al. (1988),
stability of inhomogeneous 4a x 4a superstructures as those observed in
cuprates superconductors around hole doping 1/8 is investigated. We find a
non-uniform 4a x 4a bond order wave involving simultaneously small (~ 10^-2 t)
inhomogeneous staggered plaquette currents as well as a small charge density
modulation similar to pair density wave order. On the other hand, no supersolid
phase involving a decoupling in the superconducting particle-particle channel
is found.Comment: 4 page
Quantum transport of slow charge carriers in quasicrystals and correlated systems
We show that the semi-classical model of conduction breaks down if the mean
free path of charge carriers is smaller than a typical extension of their
wavefunction. This situation is realized for sufficiently slow charge carriers
and leads to a transition from a metallic like to an insulating like regime
when scattering by defects increases. This explains the unconventional
conduction properties of quasicrystals and related alloys. The conduction
properties of some heavy fermions or polaronic systems, where charge carriers
are also slow, present a deep analogy.Comment: 4 page
Two-dimensional quantum liquids from interacting non-Abelian anyons
A set of localized, non-Abelian anyons - such as vortices in a p_x + i p_y
superconductor or quasiholes in certain quantum Hall states - gives rise to a
macroscopic degeneracy. Such a degeneracy is split in the presence of
interactions between the anyons. Here we show that in two spatial dimensions
this splitting selects a unique collective state as ground state of the
interacting many-body system. This collective state can be a novel gapped
quantum liquid nucleated inside the original parent liquid (of which the anyons
are excitations). This physics is of relevance for any quantum Hall plateau
realizing a non-Abelian quantum Hall state when moving off the center of the
plateau.Comment: 5 pages, 6 figure
Quantum spin ladders of non-Abelian anyons
Quantum ladder models, consisting of coupled chains, form intriguing systems
bridging one and two dimensions and have been well studied in the context of
quantum magnets and fermionic systems. Here we consider ladder systems made of
more exotic quantum mechanical degrees of freedom, so-called non-Abelian
anyons, which can be thought of as certain quantum deformations of ordinary
SU(2) spins. Such non-Abelian anyons occur as quasiparticle excitations in
topological quantum fluids, including p_x + i p_y superconductors, certain
fractional quantum Hall states, and rotating Bose-Einstein condensates. Here we
use a combination of exact diagonalization and conformal field theory to
determine the phase diagrams of ladders with up to four chains. We discuss how
phenomena familiar from ordinary SU(2) spin ladders are generalized in their
anyonic counterparts, such as gapless and gapped phases, odd/even effects with
the ladder width, and elementary `magnon' excitations. Other features are
entirely due to the topological nature of the anyonic degrees of freedom.Comment: 12 pages, 17 figures, 3 tables, 2 references adde
Non-Parametric Analyses of Log-Periodic Precursors to Financial Crashes
We apply two non-parametric methods to test further the hypothesis that
log-periodicity characterizes the detrended price trajectory of large financial
indices prior to financial crashes or strong corrections. The analysis using
the so-called (H,q)-derivative is applied to seven time series ending with the
October 1987 crash, the October 1997 correction and the April 2000 crash of the
Dow Jones Industrial Average (DJIA), the Standard & Poor 500 and Nasdaq
indices. The Hilbert transform is applied to two detrended price time series in
terms of the ln(t_c-t) variable, where t_c is the time of the crash. Taking all
results together, we find strong evidence for a universal fundamental
log-frequency corresponding to the scaling ratio . These values are in very good agreement with those obtained in
past works with different parametric techniques.Comment: Latex document 13 pages + 58 eps figure
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