2,139 research outputs found
Phonon Universal Transmission Fluctuations and Localization in Semiconductor Superlattices with a Controlled Degree of Order
We study both analytically and numerically phonon transmission fluctuations
and localization in partially ordered superlattices with correlations among
neighboring layers. In order to generate a sequence of layers with a varying
degree of order we employ a model proposed by Hendricks and Teller as well as
partially ordered versions of deterministic aperiodic superlattices. By
changing a parameter measuring the correlation among adjacent layers, the
Hendricks- Teller superlattice exhibits a transition from periodic ordering,
with alterna- ting layers, to the phase separated opposite limit; including
many intermediate arrangements and the completely random case. In the partially
ordered versions of deterministic superlattices, there is short-range order
(among any conse- cutive layers) and long range disorder, as in the N-state
Markov chains. The average and fluctuations in the transmission, the
backscattering rate, and the localization length in these multilayered systems
are calculated based on the superlattice structure factors we derive
analytically. The standard deviation of the transmission versus the average
transmission lies on a {\it universal\/} curve irrespective of the specific
type of disorder of the SL. We illustrate these general results by applying
them to several GaAs-AlAs superlattices for the proposed experimental
observation of phonon universal transmission fluctuations.Comment: 16-pages, Revte
Inherited Twistor-Space Structure of Gravity Loop Amplitudes
At tree-level, gravity amplitudes are obtainable directly from gauge theory
amplitudes via the Kawai, Lewellen and Tye closed-open string relations. We
explain how the unitarity method allows us to use these relations to obtain
coefficients of box integrals appearing in one-loop N=8 supergravity amplitudes
from the recent computation of the coefficients for N=4 super-Yang-Mills
non-maximally-helicity-violating amplitudes. We argue from factorisation that
these box coefficients determine the one-loop N=8 supergravity amplitudes,
although this remains to be proven. We also show that twistor-space properties
of the N=8 supergravity amplitudes are inherited from the corresponding
properties of N=4 super-Yang-Mills theory. We give a number of examples
illustrating these ideas.Comment: 32 pages, minor typos correcte
MHV-Vertices for Gravity Amplitudes
We obtain a CSW-style formalism for calculating graviton scattering
amplitudes and prove its validity through the use of a special type of
BCFW-like parameter shift. The procedure is illustrated with explicit examples.Comment: 21 pages, minor typos corrected, proof added in section
ac Josephson effect in the resonant tunneling through mesoscopic superconducting junctions
We investigate ac Josephson effect in the resonant tunneling through
mesoscopic superconducting junctions. In the presence of microwave irradiation,
we show that the trajectory of multiple Andreev reflections can be closed by
emitting or absorbing photons. Consequently, photon-assisted Andreev states are
formed and play the role of carrying supercurrent. On the Shapiro steps, dc
component appears when the resonant level is near a series of positions with
spacing of half of the microwave frequency. Analytical result is derived in the
limit of infinite superconducting gap, based on which new features of ac
Josephson effect are revealed.Comment: 11 pages, 3 figure
Low-energy properties and magnetization plateaus in a 2-leg mixed spin ladder
Using the density matrix renormalization group technique we investigate the
low-energy properties and the magnetization plateau behavior in a 2-leg mixed
spin ladder consisting of a spin-1/2 chain coupled with a spin-1 chain. The
calculated results show that the system is in the same universality class as
the spin-3/2 chain when the interchain coupling is strongly ferromagnetic, but
the similarity between the two systems is less clear under other coupling
conditions. We have identified two types of magnetization plateau phases. The
calculation of the magnetization distribution on the spin-1/2 and the spin-1
chains on the ladder shows that one plateau phase is related to the partially
magnetized valence-bond-solid state, and the other plateau state contains
strongly coupled S=1 and s=1/2 spins on the rung.Comment: 6 pages with 8 eps figure
A Green's function approach to transmission of massless Dirac fermions in graphene through an array of random scatterers
We consider the transmission of massless Dirac fermions through an array of
short range scatterers which are modeled as randomly positioned -
function like potentials along the x-axis. We particularly discuss the
interplay between disorder-induced localization that is the hallmark of a
non-relativistic system and two important properties of such massless Dirac
fermions, namely, complete transmission at normal incidence and periodic
dependence of transmission coefficient on the strength of the barrier that
leads to a periodic resonant transmission. This leads to two different types of
conductance behavior as a function of the system size at the resonant and the
off-resonance strengths of the delta function potential. We explain this
behavior of the conductance in terms of the transmission through a pair of such
barriers using a Green's function based approach. The method helps to
understand such disordered transport in terms of well known optical phenomena
such as Fabry Perot resonances.Comment: 22 double spaced single column pages. 15 .eps figure
Topological Superfluid in one-dimensional Ultracold Atomic System with Spin-Orbit Coupling
We propose a one-dimensional Hamiltonian which supports Majorana
fermions when -wave superfluid appears in the ultracold atomic
system and obtain the phase-separation diagrams both for the
time-reversal-invariant case and time-reversal-symmetry-breaking case. From the
phase-separation diagrams, we find that the single Majorana fermions exist in
the topological superfluid region, and we can reach this region by tuning the
chemical potential and spin-orbit coupling . Importantly, the
spin-orbit coupling has realized in ultracold atoms by the recent experimental
achievement of synthetic gauge field, therefore, our one-dimensional ultra-cold
atomic system described by is a promising platform to find the
mysterious Majorana fermions.Comment: 5 papers, 2 figure
Spin-Polarized Transport Across an LaSrMnO/YBaCuO Interface: Role of Andreev Bound States
Transport across an
LaSr_{3}/YBa_{3}_{7}_{3}$/YBCO and Ag/YBCO. In all cases, YBCO is used as bottom layer to
eliminate the channel resistance and to minimize thermal effects. The observed
differential conductance re ects the role of Andreev bound states in a-b
planes, and brings out for the first time the suppression of such states by the
spin-polarized transport across the interface. The theoretical analysis of the
measured data reveals decay of the spin polarization near the LSMO surface with
temperature, consistent with the reported photoemission data.Comment: 5 pages LaTeX, 3 eps figures included, accepted by Physical Review
Correlation energy of a two-dimensional electron gas from static and dynamic exchange-correlation kernels
We calculate the correlation energy of a two-dimensional homogeneous electron
gas using several available approximations for the exchange-correlation kernel
entering the linear dielectric response of the system.
As in the previous work of Lein {\it et al.} [Phys. Rev. B {\bf 67}, 13431
(2000)] on the three-dimensional electron gas, we give attention to the
relative roles of the wave number and frequency dependence of the kernel and
analyze the correlation energy in terms of contributions from the plane. We find that consistency of the kernel with the electron-pair
distribution function is important and in this case the nonlocality of the
kernel in time is of minor importance, as far as the correlation energy is
concerned. We also show that, and explain why, the popular Adiabatic Local
Density Approximation performs much better in the two-dimensional case than in
the three-dimensional one.Comment: 9 Pages, 4 Figure
Zircon records of Miocene ultrapotassic rocks from southern Lhasa subterrane, Tiben Plateau
Zircons entrained in mantle-derived magmas offer a prime opportunity to reveal cryptic magmatic episodes in the deep crust. We have investigated zircons from mantle-derived ultrapotassic veins in the Xuena area, southern Lhasa subterrane. Zircons in the Xuena ultrapotassic rocks reveal four major magmatic pulses around <100Ma, 300 ~ 400Ma, 450 ~ 500Ma, and 700 ~ 850Ma. The high U / Yb ratios and low Y contents of these zircons demonstrate their continental origin. Cenozoic-Mesozoic and Late Paleozoic magmatism have been widely identified from the southern Lhasa subterrane, suggesting the contribution from overlying juvenile crust. But similar Proterozoic-Early Paleozoic age distributions (450 ~ 500Ma and 700 ~ 850Ma) between these zircon xenocrysts and those dating records in the Himalayan orogenic belt corroborate the input from underthrusted Indian continental crust. Furthermore, the Increasing (Dy / Yb) N ratio since ~ 60 Ma zircon and Rapid Decreasing epsilon Hf ( t ) values, from + 10 ~ + 5 to -10 ~ -25, are Interpreted to reflect Significant and progressive Crustal Thickening in Response to India-Asia convergence and the contribution from subducted Indian continental crust to postcollisional magmatism in the southern Lhasa sub-terrane
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