5,143 research outputs found
Ferrimagnetism of MnV_2O_4 spinel
The spinel MnV_2O_4 is a two-sublattice ferrimagnet, with site A occupied by
the Mn^{2+} ion and site B by the V^{3+} ion. The magnon of the system, the
transversal fluctuation of the total magnetization, is a complicated mixture of
the sublattice A and B transversal magnetic fluctuations. As a result, the
magnons' fluctuations suppress in a different way the manganese and vanadium
magnetic orders and one obtains two phases. At low temperature (0,T^*) the
magnetic orders of the Mn and V ions contribute to the magnetization of the
system, while at the high temperature (T^*,T_N), the vanadium magnetic order is
suppressed by magnon fluctuations, and only the manganese ions have non-zero
spontaneous magnetization. A modified spin-wave theory is developed to describe
the two phases and to calculate the magnetization as a function of temperature.
The anomalous curve reproduces the experimentally obtained ZFC
magnetization.Comment: 4 pages, one figur
Dislocation density dependent electroabsorption in epitaxial lateral overgrown InGaN/GaN quantum structures
Cataloged from PDF version of article.We study electroabsorption (EA) behavior of InGaN/GaN quantum structures grown using epitaxial lateral overgrowth (ELOG) in correlation with their dislocation density levels and in comparison to steady state and time-resolved photoluminescence measurements. The results reveal that ELOG structures with decreasing mask stripe widths exhibit stronger EA performance, with a maximum EA enhancement factor of 4.8 compared to the reference without ELOG. The analyses show that the EA performance follows similar trends with decreasing dislocation density as the essential parameters of the photoluminescence spectra (peak position, width and intensity) together with the photoluminescence lifetimes. While keeping the growth window widths constant, compared to photoluminescence behavior, however, EA surprisingly exhibits the largest performance variation, making EA the most sensitive to the mask stripe widths. (C) 2013 Optical Society of Americ
Diffusion on a heptagonal lattice
We study the diffusion phenomena on the negatively curved surface made up of
congruent heptagons. Unlike the usual two-dimensional plane, this structure
makes the boundary increase exponentially with the distance from the center,
and hence the displacement of a classical random walker increases linearly in
time. The diffusion of a quantum particle put on the heptagonal lattice is also
studied in the framework of the tight-binding model Hamiltonian, and we again
find the linear diffusion like the classical random walk. A comparison with
diffusion on complex networks is also made.Comment: 5 pages, 6 figure
Inclusive Decays of Bottom Hadrons in New Formulation of Heavy Quark Effective Field Theory
We apply the new formulation of heavy quark effective field theory (HQEFT) to
the inclusive decays of bottom hadrons. The long-term ambiguity of using heavy
quark mass or heavy hadron mass for inclusive decays is clarified within the
framework of the new formulation of HQEFT. The order corrections are
absent and contributions from terms are calculated in detail. This
enables us to reliably extract the important CKM matrix element from
the inclusive semileptonic decay rates. The resulting lifetime ratios
and are found to well agree
with the experimental data. We also calculate in detail the inclusive
semileptonic branching ratios and the ratios of the and decay
rates as well as the charm countings in the , and
systems. For decays, all the observables are found to be consistent with
the experimental data. More precise data for the decays and further
experimental measurements for the and systems will be very
useful for testing the framework of new formulation of HQEFT at the level of
higher order corrections.Comment: 20 pages, RevTex, 8 figures, 3 tables, revised version with `dressed
heavy quark' being addressed, to be published in Int. J. Mod. Phys.
Tailoring a two-dimensional electron gas at the LaAlO3/SrTiO3 (001) interface by epitaxial strain
Recently a metallic state was discovered at the interface between insulating
oxides, most notably LaAlO3 and SrTiO3. Properties of this two-dimensional
electron gas (2DEG) have attracted significant interest due to its potential
applications in nanoelectronics. Control over this carrier density and mobility
of the 2DEG is essential for applications of these novel systems, and may be
achieved by epitaxial strain. However, despite the rich nature of strain
effects on oxide materials properties, such as ferroelectricity, magnetism, and
superconductivity, the relationship between the strain and electrical
properties of the 2DEG at the LaAlO3/SrTiO3 heterointerface remains largely
unexplored. Here, we use different lattice constant single crystal substrates
to produce LaAlO3/SrTiO3 interfaces with controlled levels of biaxial epitaxial
strain. We have found that tensile strained SrTiO3 destroys the conducting
2DEG, while compressively strained SrTiO3 retains the 2DEG, but with a carrier
concentration reduced in comparison to the unstrained LaAlO3/SrTiO3 interface.
We have also found that the critical LaAlO3 overlayer thickness for 2DEG
formation increases with SrTiO3 compressive strain. Our first-principles
calculations suggest that a strain-induced electric polarization in the SrTiO3
layer is responsible for this behavior. It is directed away from the interface
and hence creates a negative polarization charge opposing that of the polar
LaAlO3 layer. This both increases the critical thickness of the LaAlO3 layer,
and reduces carrier concentration above the critical thickness, in agreement
with our experimental results. Our findings suggest that epitaxial strain can
be used to tailor 2DEGs properties of the LaAlO3/SrTiO3 heterointerface
Two-Dimensional Axisymmetric Collapse of Thermally Unstable Primordial Clouds
We have performed two-dimensional hydrodynamic simulations of the collapse of
isolated axisymmetric clouds condensing via radiative cooling in a primordial
background gas. In order to study the development of the so-called
``shape-instability'', we have considered two types of axisymmetric clouds,
oblate and prolate clouds of various sizes and with axial ratios of . We find that the degree of oblateness or
prolateness is enhanced during the initial cooling phase. But it can be
reversed later, if the initial contrast in cooling times between the cloud gas
and the background gas is much greater than one. In such cases an oblate cloud
collapses to a structure composed of an outer thin disk and a central prolate
component. A prolate cloud, on the other hand, becomes a thin cigar-shape
structure with a central dense oblate component. The reversal of shape in the
central part of the cooled clouds is due to supersonic motions either along the
disk plane in the case of oblate clouds or along the symmetry axis in the case
of prolate clouds. For a background gas of K and n_h=0.1
\cm3 in a protogalactic halo environment, the mean density of the cloud gas
that has cooled to K increases to or so, in our simulations
where nonequilibrium cooling is adopted and the background gas cools too. The
spherical Jeans mass of such gas is estimated to be about M_J \sim
5\times10^{7}\Msun. In order for cloud mass to exceed the Jeans mass and at
the same time in order for the thermal instability to operate, the initial
cloud size should be around where is the
cooling length.Comment: 31 pages including 12 figures (reduced resolution), to appear in The
Astrophysical Journal (v584 n2 ApJ February 20, 2003 issue). Pdf with full
resolution figures can be downloaded from
ftp://canopus.chungnam.ac.kr/ryu/ryu.pd
|V_ub| and |V_cb|, Charm Counting and Lifetime Differences in Inclusive Bottom Hadron Decays
Inclusive bottom hadron decays are analyzed based on the heavy quark
effective field theory (HQEFT). Special attentions in this paper are paid to
the b\to u transitions and nonspectator effects. As a consequence, the CKM
quark mixing matrix elements |V_ub| and |V_cb| are reliably extracted from the
inclusive semileptonic decays B\to X_u e \nu and B\to X_c e \nu. Various
observables, such as the semileptonic branch ratio B_SL, the lifetime
differences among B^-, B^0, B_s and \Lambda_b hadrons, the charm counting n_c,
are predicted and found to be consistent with the present experimental data.Comment: 20 pages, Revtex, 4 figures and 2 table
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