5,461 research outputs found
Spin Effects in the Local Density of States of GaAs
We present spin-resolved measurements of the local density of states in Si
doped GaAs. Both spin components exhibit strong mesoscopic fluctuations. In the
magnetic quantum limit, the main features of the spin-up and spin-down
components of the local density of states are found to be identical apart from
Zeeman splitting. Based on this observation, we introduce a mesoscopic method
to measure the -factor in a material where macroscopic methods are severely
restricted by disorder. Differences between the spin-up and spin-down
components are discussed in terms of spin relaxation due to spin-orbit
coupling.Comment: 4 pages and 5 figure
Polarons in semiconductor quantum-dots and their role in the quantum kinetics of carrier relaxation
While time-dependent perturbation theory shows inefficient carrier-phonon
scattering in semiconductor quantum dots, we demonstrate that a quantum kinetic
description of carrier-phonon interaction predicts fast carrier capture and
relaxation. The considered processes do not fulfill energy conservation in
terms of free-carrier energies because polar coupling of localized quantum-dot
states strongly modifies this picture.Comment: 6 pages, 6 figures, accepted for publication in Phys.Rev.
General properties of response functions of nonequilibrium steady states
We derive general properties, which hold for both quantum and classical
systems, of response functions of nonequilibrium steady states. We clarify
differences from those of equilibrium states. In particular, sum rules and
asymptotic behaviors are derived, and their implications are discussed. Since
almost no assumptions are made, our results are applicable to diverse physical
systems. We also demonstrate our results by a molecular dynamics simulation of
a many-body interacting system.Comment: After publication of this paper, several typos were found, which have
been fixed in the erratum (J. Phys. Soc. Jpn., 80 (2011) 128001). All the
corrections have been made in this updated arXive version. 13 pages with 3
figure
Rabi oscillations under ultrafast excitation of graphene
We study coherent nonlinear dynamics of carriers under ultrafast interband
excitation of an intrinsic graphene. The Rabi oscillations of response appear
with increasing of pumping intensity. The photoexcited distribution is
calculated versus time and energy taking into account the effects of energy
relaxation and dephasing. Spectral and temporal dependencies of the response on
a probe radiation (transmission and reflection coefficients) are considered for
different pumping intensities and the Rabi oscillations versus time and
intensity are analyzed.Comment: 6 pages, 6 figure
Double non-equivalent chain structure on vicinal Si(557)-Au surface
We study electronic and topographic properties of the vicinal Si(557)-Au
surface using scanning tunneling microscopy and reflection of high energy
electron diffraction technique. STM data reveal double wire structures along
terraces. Moreover behavior of the voltage dependent STM tip - surface distance
is different in different chains. While the one chain shows oscillations of the
distance which are sensitive to the sign of the voltage bias, the oscillations
in the other chain remain unchanged with respect to the positive/negative
biases. This suggests that one wire has metallic character while the other one
- semiconducting. The experimental results are supplemented by theoretical
calculations within tight binding model suggesting that the observed chains are
made of different materials, one is gold and the other one is silicon chain.Comment: 9 pages, 12 figures, accepted for publication in Phys. Rev.
Thermal expansion in carbon nanotubes and graphene: nonequilibrium Green's function approach
The nonequilibrium Green's function method is applied to investigate the
coefficient of thermal expansion (CTE) in single-walled carbon nanotubes
(SWCNT) and graphene. It is found that atoms deviate about 1% from equilibrium
positions at T=0 K, resulting from the interplay between quantum zero-point
motion and nonlinear interaction. The CTE in SWCNT of different sizes is
studied and analyzed in terms of the competition between various vibration
modes. As a result of this competition, the axial CTE is positive in the whole
temperature range, while the radial CTE is negative at low temperatures. In
graphene, the CTE is very sensitive to the substrate. Without substrate, CTE
has large negative region at low temperature and very small value at high
temperature limit, and the value of CTE at T=300 K is
K which is very close to recent experimental result,
K (Nat. Nanotechnol. \textbf{10}, 1038 (2009)). A very weak substrate
interaction (about 0.06% of the in-plane interaction) can largely reduce the
negative CTE region and greatly enhance the value of CTE. If the substrate
interaction is strong enough, the CTE will be positive in whole temperature
range and the saturate value at high temperature reaches
K.Comment: final version, to appear in PR
Pseudogap in the chain states of YBCO
As established by scanning tunneling microscopy (STM) cleaved surfaces of the
high temperature superconductor YBaCuO develop charge
density wave (CDW) modulations in the one-dimensional (1D) CuO chains. At the
same time, no signatures of the CDW have been reported in the spectral function
of the chain band previously studied by photoemission. We use soft X-ray angle
resolved photoemission (SX-ARPES) to detect a chain-derived surface band that
had not been detected in previous work. The for the new surface
band is found to be 0.55\,\AA, which matches the wave vector of the CDW
observed in direct space by STM. This reveals the relevance of the Fermi
surface nesting for the formation of CDWs in the CuO chains in
YBaCuO. In agreement with the short range nature of the
CDW order the newly detected surface band exhibits a pseudogap, whose energy
scale also corresponds to that observed by STM
Carrier dynamics and coherent acoustic phonons in nitride heterostructures
We model generation and propagation of coherent acoustic phonons in
piezoelectric InGaN/GaN multi-quantum wells embedded in a \textit{pin} diode
structure and compute the time resolved reflectivity signal in simulated
pump-probe experiments. Carriers are created in the InGaN wells by ultrafast
pumping below the GaN band gap and the dynamics of the photoexcited carriers is
treated in a Boltzmann equation framework. Coherent acoustic phonons are
generated in the quantum well via both deformation potential electron-phonon
and piezoelectric electron-phonon interaction with photogenerated carriers,
with the latter mechanism being the dominant one. Coherent longitudinal
acoustic phonons propagate into the structure at the sound speed modifying the
optical properties and giving rise to a giant oscillatory differential
reflectivity signal. We demonstrate that coherent optical control of the
differential reflectivity can be achieved using a delayed control pulse.Comment: 14 pages, 11 figure
Nematicity as a route to a magnetic field-induced spin density wave order; application to the high temperature cuprates
The electronic nematic order characterized by broken rotational symmetry has
been suggested to play an important role in the phase diagram of the high
temperature cuprates. We study the interplay between the electronic nematic
order and a spin density wave order in the presence of a magnetic field. We
show that a cooperation of the nematicity and the magnetic field induces a
finite coupling between the spin density wave and spin-triplet staggered flux
orders. As a consequence of such a coupling, the magnon gap decreases as the
magnetic field increases, and it eventually condenses beyond a critical
magnetic field leading to a field-induced spin density wave order. Both
commensurate and incommensurate orders are studied, and the experimental
implications of our findings are discussed.Comment: 5 pages, 3 figure
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