1,185 research outputs found
Interaction-tuned compressible-to-incompressible phase transitions in the quantum Hall systems
We analyze transitions between quantum Hall ground states at prominent
filling factors in the spherical geometry by tuning the width parameter
of the Zhang-Das Sarma interaction potential. We find that incompressible
ground states evolve adiabatically under this tuning, whereas the compressible
ones are driven through a first order phase transition. Overlap calculations
show that the resulting phase is increasingly well described by appropriate
analytic model wavefunctions (Laughlin, Moore-Read, Read-Rezayi). This scenario
is shared by both odd () and even
denominator states (). In particular, the Fermi
liquid-like state at gives way, at large enough value of the width
parameter, to an incompressible state identified as the Moore-Read Pfaffian on
the basis of its entanglement spectrum.Comment: 4 pages, 5 figures; modified version as appears in PR
Magnetic Resonant excitations in High-{} superconductors
The observation of an unusual spin resonant excitation in the superconducting
state of various High-Tc ~copper oxides by inelastic neutron scattering
measurements is reviewed. This magnetic mode % (that does not exist in
conventional superconductors) is discussed in light of a few theoretical models
and likely corresponds to a spin-1 collective mode.Comment: 4 figures, Proceedings conference MSM'03 (september 2003) in Monastir
(Tunisia) to be published in Phys. Stat. Solid
Quantum Hall fractions for spinless Bosons
We study the Quantum Hall phases that appear in the fast rotation limit for
Bose-Einstein condensates of spinless bosonic atoms. We use exact
diagonalization in a spherical geometry to obtain low-lying states of a small
number of bosons as a function of the angular momentum. This allows to
understand or guess the physics at a given filling fraction nu, ratio of the
number of bosons to the number of vortices. This is also the filling factor of
the lowest Landau level. In addition to the well-known Bose Laughlin state at
nu =1/2 we give evidence for the Jain principal sequence of incompressible
states at nu =p/(p+- 1) for a few values of p. There is a collective mode in
these states whose phenomenology is in agreement with standard arguments coming
e.g. from the composite fermion picture. At filling factor one, the potential
Fermi sea of composite fermions is replaced by a paired state, the Moore-Read
state. This is most clearly seen from the half-flux nature of elementary
excitations. We find that the hierarchy picture does not extend up to the point
of transition towards a vortex lattice. While we cannot conclude, we
investigate the clustered Read-Rezayi states and show evidence for
incompressible states at the expected ratio of flux vs number of Bose
particles.Comment: RevTeX 4, 11 pages, 13 figure
Crossover to non-Fermi-liquid spin dynamics in cuprates
The antiferromagnetic spin correlation function , the staggered
spin susceptibility and the energy scale are studied numerically within the t-J model and the Hubbard
model, as relevant to cuprates. It is shown that , related to the
onset of the non-Fermi-liquid spin response at , is very low in
the regime below the 'optimum' hole doping , while it
shows a steep increase in the overdoped regime. A quantitative analysis of NMR
spin-spin relaxation-rate for various cuprates reveals a similar
behavior, indicating on a sharp, but continuous, crossover between a
Fermi-liquid and a non-Fermi-liquid behavior as a function of doping.Comment: 4 pages, 4 figures. Submitted to PR
Emergent particle-hole symmetry in spinful bosonic quantum Hall systems
When a fermionic quantum Hall system is projected into the lowest Landau
level, there is an exact particle-hole symmetry between filling fractions
and . We investigate whether a similar symmetry can emerge in bosonic
quantum Hall states, where it would connect states at filling fractions
and . We begin by showing that the particle-hole conjugate to a
composite fermion `Jain state' is another Jain state, obtained by reverse flux
attachment. We show how information such as the shift and the edge theory can
be obtained for states which are particle-hole conjugates. Using the techniques
of exact diagonalization and infinite density matrix renormalization group, we
study a system of two-component (i.e., spinful) bosons, interacting via a
-function potential. We first obtain real-space entanglement spectra
for the bosonic integer quantum Hall effect at , which plays the role of
a filled Landau level for the bosonic system. We then show that at
the system is described by a Jain state which is the particle-hole conjugate of
the Halperin (221) state at . We show a similar relationship between
non-singlet states at and . We also study the case of
, providing unambiguous evidence that the ground state is a composite
Fermi liquid. Taken together our results demonstrate that there is indeed an
emergent particle-hole symmetry in bosonic quantum Hall systems.Comment: 10 pages, 8 figures, 4 appendice
Tunneling in Fractional Quantum Hall line junctions
We study the tunneling current between two counterpropagating edge modes
described by chiral Luttinger liquids when the tunneling takes place along an
extended region. We compute this current perturbatively by using a tunnel
Hamiltonian. Our results apply to the case of a pair of different
two-dimensional electron gases in the fractional quantum Hall regime separated
by a barrier, e. g. electron tunneling. We also discuss the case of strong
interactions between the edges, leading to nonuniversal exponents even in the
case of integer quantum Hall edges. In addition to the expected nonlinearities
due to the Luttinger properties of the edges, there are additional interference
patterns due to the finite length of the barrier.Comment: 7 pages, RevTex, 12 figs, submitted to Phys Rev
Longitudinal and Transverse Zeeman Ladders in the Ising-Like Chain Antiferromagnet BaCo2V2O8
We explore the spin dynamics emerging from the N\'eel phase of the chain
compound antiferromagnet BaCo2V2O8. Our inelastic neutron scattering study
reveals unconventional discrete spin excitations, so called Zeeman ladders,
understood in terms of spinon confinement, due to the interchain attractive
linear potential. These excitations consist in two interlaced series of modes,
respectively with transverse and longitudinal polarization. The latter have no
classical counterpart and are related to the zero-point fluctuations that
weaken the ordered moment in weakly coupled quantum chains. Our analysis
reveals that BaCo2V2O8, with moderate Ising anisotropy and sizable interchain
interactions, remarkably fulfills the conditions necessary for the observation
of these longitudinal excitations.Comment: 5 pages, 4 figures, 2 additional pages of supplemental material with
2 figures; Journal ref. added; 1 page erratum added at the end with 1 figur
Evidence for short-range antiferromagnetic fluctuations in Kondo-insulating YbB12
The spin dynamics of mixed-valence YbB12 has been studied by inelastic
neutron scattering on a high-quality single crystal. In the Kondo-insulating
regime realized at low temperature, the spectra exhibit a spin-gap structure
with two sharp, dispersive, in-gap excitations at E = 14.5 and approximately 20
meV. The lower mode is shown to be associated with short-range correlations
near the antiferromagnetic wave vector q0 = (1/2, 1/2, 1/2). Its properties are
in overall agreement with those expected for a "spin exciton'' branch in an
indirect hybridization gap semiconductor.Comment: 4 pages, 4 figures ; submitted to Physical Review Letter
Dynamics of the two-dimensional S=1/2 dimer system (C5H6N2F)2CuCl4
Inelastic neutron scattering was used to study a quantum S=1/2
antiferromagnetic Heisenberg system-Bis(2-amino-5-fluoropyridinium)
Tetrachlorocuprate(II). The magnetic excitation spectrum was shown to be
dominated by long-lived excitations with an energy gap as 1.07(3) meV. The
measured dispersion relation is consistent with a simple two-dimensional square
lattice of weakly-coupled spin dimers. Comparing the data to a random phase
approximation treatment of this model gives the intra-dimer and inter-dimer
exchange constants J=1.45(2) meV and J'=0.31(3) meV, respectively.Comment: 4 pages, 4 figure
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