3,781 research outputs found
Deep Traps in AlGaN/GaN Heterostructures Studied by Deep Level Transient Spectroscopy: Effect of Carbon Concentration in GaN Buffer Layers
Electrical properties, including leakage currents, threshold voltages, and deep traps, of AlGaN/GaN heterostructure wafers with different concentrations of carbon in the GaN buffer layer, have been investigated by temperature dependent current-voltage and capacitance-voltage measurements and deep level transient spectroscopy (DLTS), using Schottky barrier diodes (SBDs). It is found that (i) SBDs fabricated on the wafers with GaN buffer layers containing a low concentration of carbon (low-[C] SBD) or a high concentration of carbon (high-[C] SBD) have similar low leakage currents even at 500 K; and (ii) the low-[C] SBD exhibits a larger (negative) threshold voltage than the high-[C] SBD. Detailed DLTS measurements on the two SBDs show that (i) different trap species are seen in the two SBDs: electron traps Ax (0.9 eV), A1 (0.99 eV), and A2 (1.2 eV), and a holelike trap H1 (1.24 eV) in the low-[C] SBD; and electron traps A1, A2, and A3 ( ⌠1.3âeV), and a holelike trap H2 (\u3e1.3âeV) in the high-[C] SBD; (ii) for both SDBs, in the region close to GaN buffer layer, only electron traps can be detected, while in the AlGaN/GaN interface region, significant holelike traps appear; and iii) all of the deep traps show a strong dependence of the DLTS signal on filling pulse width, which indicates they are associated with extended defects, such as threading dislocations. However, the overall density of electron traps is lower in the low-[C] SBD than in the high-[C] SBD. The different traps observed in the two SBDs are thought to be mainly related to differences in microstructure (grain size and threading dislocation density) of GaN buffer layers grown at different pressures
Constraints on Cardassian Scenario from the Expansion Turnaround Redshift and the Sunyaev-Zeldovich/X-ray Data
Cosmic acceleration is one of the most remarkable cosmological findings of
recent years. Although a dark energy component has usually been invoked as the
mechanism for the acceleration, A modification of Friedmann equation from
various higher dimensional models provides a feasible alternative. Cardassian
expansion is one of these scenarios. In this work, we first consider the
constraints on the parameter space from the turnaround redshift, z_{q=0}, at
which the universe switches from deceleration to acceleration. We show that,
for every Omega_m, there exist a unique n_{peak} (Omega_m), which makes z_{q=0}
reach its maximum value, [z_{q=0}]_{max} = exp[1/ (2-3n_{peak})] -1, which is
unlinearly inverse to Omega_m. If the acceleration happans earlier than z_{q=0}
= 0.6, suggested by Type Ia supernovae measurements, we have Omega_m < 0.328 no
matter what the power index is, and moreover, for reasonable matter density,
Omega_m ~ 0.3, it is found n ~ (-0.45,0.25). We next test this scenario using
the Sunyaev-Zeldovich/X-ray data of a sample of 18 galaxy clusters with 0.14 <
z < 0.83 compiled by Reese et al. (2002). The constraints from the current
SZ/X-ray data is weak, though a model with lower matter density is prefered. A
certain range of the model parameters is also consistent with the data.Comment: 18 pages, 4 figures, accepted for publication in Ap
The thermal conductivity of the spin-1/2 XXZ chain at arbitrary temperature
Motivated by recent investigations of transport properties of strongly
correlated 1d models and thermal conductivity measurements of quasi 1d magnetic
systems we present results for the integrable spin-1/2 chain. The thermal
conductivity of this model has , i.e. it is infinite for zero frequency . The weight
of the delta peak is calculated exactly by a lattice path
integral formulation. Numerical results for wide ranges of temperature and
anisotropy are presented. The low and high temperature limits are studied
analytically.Comment: 12 page
Determining the Cosmic Equation of State Using Future Gravitational Wave Detectors
The expected chirp mass distribution of observed events for future
gravitational wave detectors is extensively investigated in the presence of an
exotic fluid component with an arbitrary equation of state, , i.e., the so-called dark energy component. The results
for a flat model dominated by a dark energy are compared to those for the
standard flat model dominated by cold dark matter. It is found that for a flat
universe the chirp mass distribution shows a sensitive dependence on
, which may provide an independent and robust constraint on the
cosmic equation of state.Comment: 5 pages, four figures, aa.sty LaTex fil
Current-Carrying Ground States in Mesoscopic and Macroscopic Systems
We extend a theorem of Bloch, which concerns the net orbital current carried
by an interacting electron system in equilibrium, to include mesoscopic
effects. We obtain a rigorous upper bound to the allowed ground-state current
in a ring or disc, for an interacting electron system in the presence of static
but otherwise arbitrary electric and magnetic fields. We also investigate the
effects of spin-orbit and current-current interactions on the upper bound.
Current-current interactions, caused by the magnetic field produced at a point
r by a moving electron at r, are found to reduce the upper bound by an amount
that is determined by the self-inductance of the system. A solvable model of an
electron system that includes current-current interactions is shown to realize
our upper bound, and the upper bound is compared with measurements of the
persistent current in a single ring.Comment: 7 pager, Revtex, 1 figure available from [email protected]
The Monte Carlo Program KoralW version 1.51 and The Concurrent Monte Carlo KoralW&YFSWW3 with All Background Graphs and First Order Corrections to W-Pair Production
The version 1.51 of the Monte Carlo (MC) program KoralW for all processes is presented. The most important change
since the previous version 1.42 is the facility for writing MC events on the
mass storage device and re-processing them later on. In the re-processing one
may modify parameters of the Standard Model in order to fit them to
experimental data. Another important new feature is a possibility of including
complete corrections to double-resonant W-pair
component-processes in addition to all background (non-WW) graphs. The
inclusion is done with the help of the YFSWW3 MC event generator for fully
exclusive differential distributions (event-per-event). Technically, it is done
in such a way that YFSWW3 runs concurrently with KoralW as a separate slave
process, reading momenta of the MC event generated by KoralW and returning the
correction weight to KoralW. KoralW introduces the
correction using this weight, and finishes processing the event (rejection due
to total MC weight, hadronization, etc.). The communication between KoralW and
YFSWW3 is done with the help of the FIFO facility of the UNIX/Linux operating
system. This does not require any modifications of the FORTRAN source codes.
The resulting Concurrent MC event generator KoralW&YFSWW3 looks from the user's
point of view as a regular single MC event generator with all the standard
features.Comment: 8 figures, 5 tables, submitted to Comput. Phys. Commu
Upper critical field in superconductors near ferromagnetic quantum critical points; UCoGe
We study the strong-coupling superconductivity near ferromagnetic quantum
critical points, mainly focusing on the upper critical fields . Based
on our simple model calculations, we discuss experimentally observed unusual
behaviors of in a recently discovered ferromagnetic superconductor
UCoGe. Especially, the large anisotropy between -axis and
-axis, and the strong-coupling behaviors in
are investigated. We also examine effects of
non-analytic corrections in the spin susceptibility on the superconductivity,
which can arise from effective long range interactions due to particle-hole
excitations.Comment: Proceedings of ICHE2010, Toky
Asymptotically Exact Solution for Superconductivity near Ferromagnetic Criticality
We analyze an asymptotically exact solution for the transition temperature of
p-wave superconductivity near ferromagnetic criticality on the basis of the
three-dimensional electron systems in which scattering processes are dominated
by exchange interactions with small momentum transfers. Taking into account all
Feynman diagrams in the gap equation, we show that vertex corrections neglected
in the conventional Eliashberg's formalism enhance the dynamical retarded
effect of the pairing interaction, and raise the superconducting transition
temperature significantly, though they just give subleading corrections to
properties of the normal state.Comment: 6 pages, 2 figures, published final versio
One-Dimensional Electron Liquid in an Antiferromagnetic Environment: Spin Gap from Magnetic Correlations
We study a one-dimensional electron liquid coupled by a weak spin-exchange
interaction to an antiferromagnetic spin-S ladder with n legs. A perturbative
renormalization group analysis in the semiclassical limit reveals the opening
of a spin gap, driven by the local magnetic correlations on the ladder. The
effect, which we argue is present for any gapful ladder or gapless ladder with
, is enhanced by the repulsive interaction among the conduction
electrons but is insensitive to the sign of the spin exchange interaction with
the ladder. Possible implications for the striped phases of the cuprates are
discussed.Comment: 5 pages, 1 figure, to appear in Phys. Rev. Let
Luminescent properties of Bi-doped polycrystalline KAlCl4
We observed an intensive near-infrared luminescence in Bi-doped KAlCl4
polycrystalline material. Luminescence dependence on the excitation wavelength
and temperature of the sample was studied. Our experimental results allow
asserting that the luminescence peaked near 1 um belongs solely to Bi+ ion
which isomorphically substitutes potassium in the crystal. It was also
demonstrated that Bi+ luminescence features strongly depend on the local ion
surroundings
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