281 research outputs found
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
Shack-hartmann wavefront sensor with holographic lenslet array for the aberration measurements in a speckle field
The application of a Shack-Hartmann sensor with holographic lenslet array to
the measurements of wavefront aberrations in a speckle field is offered. The main feature
of the method is that the tested wave front can be compared with an arbitrary wavefront
preliminary recorded in the holographic memory of the array. An iterative algorithm of
the sensor work for measuring the variable speckled wavefronts is offered. The
experimental results of measurements of the curvature of a spherical speckled wave are
presented. The possibility to use the method proposed in the analysis of deformations of
a rough surface is shown
Excitation spectrum and ground state properties of the S=1/2 Heisenberg ladder with staggered dimerization
We have studied the excitation spectrum of the quantum spin ladder
with staggered dimerization by dimer series expansions, diagrammatic analysis
of an effective interacting Bose gas of local triplets, and exact
diagonalization of small clusters. We find that the model has two massive
phases, with predominant inter-chain (rung) or intra-chain correlations. The
transition from the rung dimer into the intra-chain dimer phase is
characterized by softening of the triplet spectrum at . The excitation
spectrum as well as the spin correlations away from and close to the critical
line are calculated. The location of the phase boundary is also determined.Comment: 13 pages, 7 figure
Speckle pattern formation in spatially limited optical systems
The dependences of statistical parameters inherent to speckle patterns on the object roughness and aperture size have been investigated. The experimental results that confirm theoretical dependence quality within the limits of errors were obtained. It has been shown that spatial finiteness of the optical system causes significant changes of transferred field
Critical Dynamics of Singlet Excitations in a Frustrated Spin System
We construct and analyze a two-dimensional frustrated quantum spin model with
plaquette order, in which the low-energy dynamics is controlled by spin
singlets. At a critical value of frustration the singlet spectrum becomes
gapless, indicating a quantum transition to a phase with dimer order. This T=0
transition belongs to the 3D Ising universality class, while at finite
temperature a 2D Ising critical line separates the plaquette and dimerized
phases.
The magnetic susceptibility has an activated form throughout the phase
diagram, whereas the specific heat exhibits a rich structure and a power law
dependence on temperature at the quantum critical point.
We argue that the novel quantum critical behavior associated with singlet
criticality discussed in this work can be relevant to a wide class of quantum
spin systems, such as antiferromagnets on Kagome and pyrochlore lattices, where
the low-energy excitations are known to be spin singlets, as well as to the
CAVO lattice and several recently discovered strongly frustrated square-lattice
antiferromagnets.Comment: 5 pages, 5 figures, additional discussion and figure added, to appear
in Phys. Rev.
Radiation of photons in process of charge particle volume reflection in bent single crystal
New type of radiation in crystals is predicted and investigated in computer
simulation. It is shown that process of volume reflection of electrons and
positrons in bent crystals is accomplished with high-power radiation of
photons. Volume reflection radiation has intensity comparable with known
channeling radiation, but it is less sensitive to entrance angle and sign of
charge of a particle. Simulated spectra of radiation power are presented for 10
GeV and 200GeV particles.Comment: Latex, 8 pages, 4 fig
Studies and application of bent crystals for beam steering at 70-GeV IHEP accelerator
This report overviews studies accomplished in the U70 proton synchrotron of
IHEP-Protvino during the recent two decades. Major attention is paid to a
routine application of bent crystals for beam extraction from the machine. It
has been confirmed experimentally that efficiency of beam extraction with a
crystal deflector of around 85% is well feasible for a proton beam with
intensity up to 1012 protons per cycle. Another trend is to use bent crystals
for halo collimation in a high energy collider. New promising options emerge
for, say, LHC and ILC based on the "volume reflection" effect, which has been
discovered recently in machine study runs at U70 of IHEP (50 GeV) and SPS of
CERN (400 GeV).Comment: 12 pages, 14 figure
Excitation spectrum of the S=1/2 quantum spin ladder with frustration: elementary quasiparticles and many-particle bound states
We study the excitation spectrum of the two-chain S=1/2 Heisenberg spin
ladder with additional inter-chain second-neighbor frustrating interactions.
The one and two-particle excitations are analyzed by using a mapping of the
model onto a Bose gas of hard-core triplets. We find that low-lying singlet and
triplet two-particle bound states are present and their binding energy
increases with increasing frustration. In addition, many-particle bound states
are found by a combination of variational and exact diagonalization techniques.
We prove that the larger the number of bound quasiparticles the larger the
binding energy. Thus the excitation spectrum has a complex structure and
consists of elementary triplets and collective many-particle singlet and
triplet excitations which generally mix with the elementary ones.
The model exhibits a quantum phase transition from an antiferromagnetic
ladder phase (small frustration) into Haldane phase (effectively ferromagnetic
ladder for large frustration). We argue that near the transition point the
spectrum in both triplet and singlet channels becomes gapless. The excitation
wave function is dominated by large-size bound states which leads to the
vanishing of the quasiparticle residue.Comment: RevTeX, 23 pages, 12 figure
Boundaries, Cusps and Caustics in the Multimagnon Continua of 1D Quantum Spin Systems
The multimagnon continua of 1D quantum spin systems possess several
interesting singular features that may soon be accessible experimentally
through inelastic neutron scattering. These include cusps and composition
discontinuities in the boundary envelopes of two-magnon continuum states and
discontinuities in the density of states, "caustics", on and within the
continuum, which will appear as discontinuities in scattering intensity. In
this note we discuss the general origins of these continuum features, and
illustrate our results using the alternating Heisenberg antiferromagnetic chain
and two-leg ladder as examples.Comment: 18 pages, 10 figure
Determination of surface defects by using the wavefront scanner
The possibility of changes in the polarization state of the laser beam reflected
from inhomogeneity with the refractive index gradient is theoretically shown, which
allows separating the phase shifts related with relief inhomogeneities and local changes
of the surface refractive index. Modification of the wavefront scanner for analyzing the
wavefront of the laser beam reflected from the samples’ surface is considered. The main
idea of the method is to use the focused laser beams with different polarizations for
illuminating separate areas of the surface. The results of detecting test surfaces with
different structures by the wavefront scanner are presented
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