5,643 research outputs found
Theory of incommensurate magnetic correlations across the insulator-superconductor transition in underdoped La_{2-x}Sr_xCuO_4
The main feature in the elastic neutron scattering of La_{2-x}Sr_xCuO_4 is
the existence of incommensurate peaks with positions that jump from 45 to 0
degrees at 5% doping. We show that the spiral state of the t-t'-t''-J model
with realistic parameters describes this data perfectly. We explain why in the
insulator the peak is at 45 degrees while it switches to 0 degrees precisely at
the insulator-metal transition. The calculated positions of the peaks are in
agreement with the data in both phases.Comment: 5 pages, 1 figure; minor change
Theory of Anisotropic Hopping Transport due to Spiral Correlations in the Spin-Glass Phase of Underdoped Cuprates
We study the in-plane resistivity anisotropy in the spin-glass phase of the
high- cuprates, on the basis of holes moving in a spiral spin
background. This picture follows from analysis of the extended model with
Coulomb impurities. In the variable-range hopping regime the resistivity
anisotropy is found to have a maximum value of around 90%, and it decreases
with temperature, in excellent agreement with experiments in
LaSrCuO. In our approach the transport anisotropy is due to the
non-collinearity of the spiral spin state, rather than an intrinsic tendency of
the charges to self-organize.Comment: 5 pages, 4 figures; expanded versio
Lattice Quantum Monte Carlo Study of Chiral Magnetic Effect in Dirac Semimetals
In this paper Chiral Magnetic Effect (CME) in Dirac semimetals is studied by
means of lattice Monte Carlo simulation. We measure conductivity of Dirac
semimetals as a function of external magnetic field in parallel
and perpendicular to the external field
directions. The simulations are carried out in three regimes: semimetal phase,
onset of the insulator phase and deep in the insulator phase. In the semimetal
phase grows whereas drops with magnetic
field. Similar behaviour was observed in the onset of the insulator phase but
conductivity is smaller and its dependence on magnetic field is weaker. Finally
in the insulator phase conductivities are close to
zero and do not depend on magnetic field. In other words, we observe
manifestation of the CME current in the semimetal phase, weaker manifestation
of the CME in the onset of the insulator phase. We do not observe signatures of
CME in the insulator phase. We believe that the suppression of the CME current
in the insulator phase is connected to chiral symmetry breaking and generation
of dynamical fermion mass which take place in this phase.Comment: 6 pages, 4 figure
AC Hopping Magnetotransport Across the Spin Flop Transition in Lightly Doped La_2CuO_4
The weak ferromagnetism present in insulating La_{2}CuO_4 at low doping leads
to a spin flop transition, and to transverse (interplane) hopping of holes in a
strong external magnetic field. This results in a dimensional crossover 2D
3D for the in-plane transport, which in turn leads to an increase of the
hole's localization length and increased conduction. We demonstrate
theoretically that as a consequence of this mechanism, a frequency-dependent
jump of the in-plane ac hopping conductivity occurs at the spin flop
transition. We predict the value and the frequency dependence of the jump.
Experimental studies of this effect would provide important confirmation of the
emerging understanding of lightly doped insulating La_{2-x}Sr_xCuO_4.Comment: 4 pages, 1 figur
Spin polaron in the J1-J2 Heisenberg model
We have studied the validity of the spin polaron picture in the frustrated
J1-J2 Heisenberg model. For this purpose, we have computed the hole spectral
functions for the Neel, collinear, and disordered phases of this model, by
means of the self-consistent Born approximation and Lanczos exact
diagonalization on finite-size clusters. We have found that the spin polaron
quasiparticle excitation is always well defined for the magnetically ordered
Neel and collinear phases, even in the vicinity of the magnetic quantum
critical points, where the local magnetization vanishes. As a general feature,
the effect of frustration is to increase the amplitude of the multimagnon
states that build up the spin polaron wave function, leading to the reduction
of the quasiparticle coherence. Based on Lanczos results, we discuss the
validity of the spin polaron picture in the disordered phase.Comment: 9 pages, 12 figure
New quantum phase transitions in the two-dimensional J1-J2 model
We analyze the phase diagram of the frustrated Heisenberg antiferromagnet,
the J1-J2 model, in two dimensions. Two quantum phase transitions in the model
are already known: the second order transition from the Neel state to the spin
liquid state at (J_2/J_1)_{c2}=0.38, and the first order transition from the
spin liquid state to the collinear state at (J_2/J_1)_{c4}=0.60. We have found
evidence for two new second order phase transitions: the transition from the
spin columnar dimerized state to the state with plaquette type modulation at
(J_2/J_1)_{c3}=0.50(2), and the transition from the simple Neel state to the
Neel state with spin columnar dimerization at (J_2/J_1)_{c1}=0.34(4). We also
present an independent calculation of (J_2/J_1)_{c2}=0.38 using a new approach.Comment: 3 pages, 5 figures; added referenc
Negative Hopping Magnetoresistance and Dimensional Crossover in Lightly Doped Cuprate Superconductors
We show that, due to the weak ferromagnetism of LaSrCuO, an
external magnetic field leads to a dimensional crossover 2D 3D for the
in-plane transport. The crossover results in an increase of the hole's
localization length and hence in a dramatic negative magnetoresistance in the
variable range hopping regime. This mechanism quantitatively explains puzzling
experimental data on the negative magnetoresistance in the N\'eel phase of
LaSrCuO.Comment: 6 pages, 3 figures; published versio
Strange magnification pattern in the large separation lens SDSS J1004+4112 from optical to X-rays
We present simultaneous XMM-Newton UV and X-ray observations of the quadruply
lensed quasar SDSS J1004+4112 (RBS 825). Simultaneously with the XMM-Newton
observations we also performed integral field spectroscopy on the two closest
lens images A and B using the Calar Alto PMAS spectrograph. In X-rays the
widely spaced components C and D are clearly resolved, while the closer pair of
images A and B is marginally resolved in the XMM-EPIC images. The integrated
X-ray flux of the system has decreased by a factor of 6 since it was observed
in the ROSAT All Sky Survey in 1990, while the X-ray spectrum became much
harder with the power law index evolving from Gamma=-2.3 to -1.86. By
deblending the X-ray images of the lensed QSO we find that the X-ray flux
ratios between the lens images A and B are significantly different from the
simultaneously obtained UV ratios and previously measured optical flux ratios.
Our optical spectrum of lens image A shows an enhancement in the blue emission
line wings, which has been observed in previous epochs as a transient feature.
We propose a scenario where intrinsic UV and X-ray variability gives rise to
line variations which are selectively magnified in image A by microlensing. The
extended emission of the lensing cluster of galaxies is clearly detected in the
EPIC images, we measure a 0.5-2.0 keV luminosity of 1.4 E44 erg/s. Based on the
cluster X-ray properties, we estimate a mass of 2-6 E14 solar masses.Comment: 11 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
New limits for neutrinoless tau decays
Neutrinoless 3-prong tau lepton decays into a charged lepton and either two charged particles or one neutral meson have been searched for using 4.79fb^(-1) of data collected with the CLEO II detector at Cornell Electron Storage Ring. This analysis represents an update of a previous study and the addition of six decay channels. In all channels the numbers of events found are compatible with background estimates and branching fraction upper limits are set for 28 different decay modes. These limits are either more stringent than those set previously or represent the first attempt to find these decays
Spectrum of elementary and collective excitations in the dimerized S=1/2 Heisenberg chain with frustration
We have studied the low-energy excitation spectrum of a dimerized and
frustrated antiferromagnetic Heisenberg chain. We use an analytic approach,
based on a description of the excitations as triplets above a strong-coupling
singlet ground state. The quasiparticle spectrum is calculated by treating the
excitations as a dilute Bose gas with infinite on-site repulsion. Additional
singlet (S=0) and triplet (S=1) modes are found as two-particle bound states of
the elementary triplets. We have also calculated the contributions of the
elementary and collective excitations into the spin structure factor. Our
results are in excellent agreement with exact diagonalizations and dimer series
expansions data as long as the dimerization parameter is not too small
(), i.e. while the elementary triplets can be treated as localized
objects.Comment: 18 pages, 13 figure
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