409 research outputs found
Bosonization Theory of Excitons in One-dimensional Narrow Gap Semiconductors
Excitons in one-dimensional narrow gap semiconductors of anti-crossing
quantum Hall edge states are investigated using a bosonization method. The
excitonic states are studied by mapping the problem into a non-integrable
sine-Gordon type model. We also find that many-body interactions lead to a
strong enhancement of the band gap. We have estimated when an exciton
instability may occur.Comment: 4pages, 1 figure, to appear in Phys. Rev. B Brief Report
Quantum Hall Fluids
We review the effective field theory treatment of topological quantum fluids,
focussing on the Hall fluids.Comment: 82 pages, TeX, Preprint ITP (This version comes with ALL the MACROS
appended at the end of the file)
Isothermal Fatigue and Creep-Fatigue Interaction Behavior of Nickel-Base Directionally Solidified Superalloy
The creep-fatigue interaction in directionally solidified nickel-base superalloy was analyzed with the modified Chaboche-based unified viscoplasticity constitutive model. The model features the anisotropic material behavior, hardening/ softening, and stress relaxation. Simple low-cyclic fatigue and specified hold time experiments were conducted on a directionally solidified superalloy (DZ125) at temperatures over 760°C. The material parameters were optimized considering its tensile, cyclic and creep behavior with the Levenberg–Marquardt optimization procedure. The model was constructed in FORTRAN and integrated in FEA software UMAT/ABAQUS. The results show that experimental and simulated hysteresis loop size/shape, peak stresses, stress relaxation, and related areas are closely matched. The modified constitutive model was found to be instrumental for revealing the fatigue and creep-fatigue interaction behavior of such materials and can be used for practical applications.Взаимодействие ползучести и усталости в суперсплаве на основе никеля, полученного направленной кристаллизацией, проанализировано с помощью модифицированной унифицированной определяющей модели вязкопластичности Шабоша. Модель характеризует анизотропное поведение материала, упрочнение/разупрочнение и релаксацию напряжений. Эксперименты по определению малоцикловой усталости и времени пребывания выполнены на суперсплаве (DZ125) при температурах выше 760°С. Параметры материала оптимизированы с учетом его поведения при растяжении, циклической нагрузке и ползучести при помощи метода оптимизации Левенберга–Марквардта. Модель построена на языке ФОРТРАН и интегрирована в программу FEА UMAT/ABAQUS. Показано, что экспериментальные и модельные размеры/ форма петли гистерезиса, пиковые напряжения, релаксация напряжений и их область находятся в соответствии. Установлено, что модифицированную определяющую модель целесообразно применять при анализе усталости и взаимодействия ползучести и усталости подобных материалов, она может также найти практическое применение
Effective field theory
I give a brief review of effective field theory, disscussing the contribution
of Feza G\"ursey in particular and focusing on the literature I am most
familiar with.Comment: 17 pages, no figs, macros appended, plain te
Haldane's Fractional Exclusion Statistics for Multicomponent Systems
The idea of fractional exclusion statistics proposed by Haldane is applied to
systems with internal degrees of freedom, and its thermodynamics is examined.
In case of one dimension, various bulk quantities calculated show that the
critical behavior of such systems can be described by conformal field
theories and conformal weights are completely characterized by statistical
interactions. It is also found that statistical interactions have intimate
relationship with a topological order matrix in Chern-Simons theory for the
fractional quantum Hall effect.Comment: 12 pages, Revtex, preprint YITP/K-107
Structural, Magnetic and Transport Properties of B-Site Substituted Perovskite La0.7Sr0.3MnO3
In this chapter, in order to understand the structural related magnetic and transport properties of B site substituted perovskites La0.7Sr0.3MnO3 (LSMO), we have systematically investigated the effects of replacing some of the Mn with nonmagnetic elements Ti, Zr, Cu, Al, Zn and magnetic elements Co, Ni, Cr, Fe. The structural, magnetic and electrical phase transitions and transport properties of these compounds were investigated by neutron diffraction, magnetization and electric resistivity measurements
Skyrmion Excitations in Quantum Hall Systems
Using finite size calculations on the surface of a sphere we study the
topological (skyrmion) excitation in quantum Hall system with spin degree of
freedom at filling factors around . In the absence of Zeeman energy, we
find, in systems with one quasi-particle or one quasi-hole, the lowest energy
band consists of states with , where and are the total orbital and
spin angular momentum. These different spin states are almost degenerate in the
thermodynamic limit and their symmetry-breaking ground state is the state with
one skyrmion of infinite size. In the presence of Zeeman energy, the skyrmion
size is determined by the interplay of the Zeeman energy and electron-electron
interaction and the skyrmion shrinks to a spin texture of finite size. We have
calculated the energy gap of the system at infinite wave vector limit as a
function of the Zeeman energy and find there are kinks in the energy gap
associated with the shrinking of the size of the skyrmion. breaking ground
state is the state with one skyrmion of infinite size. In the presence of
Zeeman energy, the skyrmion size is determined by the interplay of the Zeeman
energy and electron-electronComment: 4 pages, 5 postscript figures available upon reques
Semi-inclusive B Decays and Direct CP Violation in QCD Factorization
We have systematically investigated the semi-inclusive B decays B->MX, which
are manifestations of the quark decay b->Mq, within the framework of
QCD-improved factorization. These decays are theoretically clean and have
distinctive experimental signatures. We focus on a class of these that do not
require any form factor information and therefore may be especially suitable
for extracting information on the angles and of the unitarity
triangle. The nonfactorizable effects, such as vertex-type and penguin-type
corrections to the two-body b decay and hard spectator corrections to the
3-body decay are calculable in the heavy quark limit. QCD factorization is
applicable when the emitted meson is a light meson or a charmonium. We discuss
the issue of the CPT constraint on partial rate asymmetries. The strong phase
coming from final-state rescattering due to hard gluon exchange between the
final states can induce large rate asymmetries for tree-dominated
color-suppressed modes . The nonfactorizable
hard spectator interactions in the 3-body decay, though phase-space suppressed,
are extremely important for the tree-dominated modes
, and the
penguin-dominated mode . In fact, they are dominated by the
hard spectator corrections. Our result for is in
agreement with experiment. The semi-inclusive decay modes: , ,
and are the most promising ones in searching for
direct CP violation. In fact, they have branching ratios of order
and CP rate asymmetries of order .Comment: 28 page
Final State Rescattering and Color-suppressed \bar B^0-> D^{(*)0} h^0 Decays
The color-suppressed \bar B^0-> D^{(*)0}\pi^0, D^{(*)0}\eta, D^0\omega decay
modes have just been observed for the first time. The rates are all larger than
expected, hinting at the presence of final state interactions. Considering \bar
B^0-> D^{(*)0}\pi^0 mode alone, an elastic D^{(*)}\pi -> D^{(*)}\pi
rescattering phase difference \delta \equiv \delta_{1/2} - \delta_{3/2} \sim
30^\circ would suffice, but the \bar B^0-> D^{(*)0}\eta, D^0\omega modes compel
one to extend the elastic formalism to SU(3) symmetry. We find that a universal
a_2/a_1=0.25 and two strong phase differences 20^\circ \sim \theta < \delta <
\delta^\prime \sim 50^\circ can describe both DP and D^*P modes rather well;
the large phase of order 50^\circ is needed to account for the strength of {\it
both} the D^{(*)0}\pi^0 and D^{(*)0}\eta modes. For DV modes, the nonet
symmetry reduces the number of physical phases to just one, giving better
predictive power. Two solutions are found. We predict the rates of the \bar
B^0-> D^{+}_s K^-, D^{*+}_s K^-, D^0\rho^0, D^+_s K^{*-} and D^0\phi modes, as
well as \bar B^0-> D^{0}\bar K^0, D^{*0}\bar K^0, D^{0}\bar K^{*0} modes. The
formalism may have implications for rates and CP asymmetries of charmless
modes.Comment: REVTeX4, 18 pages, 5 figures, to appear in Phys. Rev.
Anisotropic Transport of Quantum Hall Meron-Pair Excitations
Double-layer quantum Hall systems at total filling factor can
exhibit a commensurate-incommensurate phase transition driven by a magnetic
field oriented parallel to the layers. Within the commensurate
phase, the lowest charge excitations are believed to be linearly-confined Meron
pairs, which are energetically favored to align with . In order
to investigate this interesting object, we propose a gated double-layer Hall
bar experiment in which can be rotated with respect to the
direction of a constriction. We demonstrate the strong angle-dependent
transport due to the anisotropic nature of linearly-confined Meron pairs and
discuss how it would be manifested in experiment.Comment: 4 pages, RevTex, 3 postscript figure
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