16,048 research outputs found
Surface segregation and the Al problem in GaAs quantum wells
Low-defect two-dimensional electron systems (2DESs) are essential for studies
of fragile many-body interactions that only emerge in nearly-ideal systems. As
a result, numerous efforts have been made to improve the quality of
modulation-doped AlGaAs/GaAs quantum wells (QWs), with an emphasis
on purifying the source material of the QW itself or achieving better vacuum in
the deposition chamber. However, this approach overlooks another crucial
component that comprises such QWs, the AlGaAs barrier. Here we show
that having a clean Al source and hence a clean barrier is instrumental to
obtain a high-quality GaAs 2DES in a QW. We observe that the mobility of the
2DES in GaAs QWs declines as the thickness or Al content of the
AlGaAs barrier beneath the QW is increased, which we attribute to
the surface segregation of Oxygen atoms that originate from the Al source. This
conjecture is supported by the improved mobility in the GaAs QWs as the Al cell
is cleaned out by baking
Hysteresis and the dynamic phase transition in thin ferromagnetic films
Hysteresis and the non-equilibrium dynamic phase transition in thin magnetic
films subject to an oscillatory external field have been studied by Monte Carlo
simulation. The model under investigation is a classical Heisenberg spin system
with a bilinear exchange anisotropy in a planar thin film geometry with
competing surface fields. The film exhibits a non-equilibrium phase transition
between dynamically ordered and dynamically disordered phases characterized by
a critical temperature Tcd, whose location of is determined by the amplitude H0
and frequency w of the applied oscillatory field. In the presence of competing
surface fields the critical temperature of the ferromagnetic-paramagnetic
transition for the film is suppressed from the bulk system value, Tc, to the
interface localization-delocalization temperature Tci. The simulations show
that in general Tcd < Tci for the model film. The profile of the time-dependent
layer magnetization across the film shows that the dynamically ordered and
dynamically disordered phases coexist within the film for T < Tcd. In the
presence of competing surface fields, the dynamically ordered phase is
localized at one surface of the film.Comment: PDF file, 21 pages including 8 figure pages; added references,typos
added; to be published in PR
Robust Upward Dispersion of the Neutron Spin Resonance in the Heavy Fermion Superconductor CeYbCoIn
The neutron spin resonance is a collective magnetic excitation that appears
in copper oxide, iron pnictide, and heavy fermion unconventional
superconductors. Although the resonance is commonly associated with a
spin-exciton due to the ()-wave symmetry of the superconducting
order parameter, it has also been proposed to be a magnon-like excitation
appearing in the superconducting state. Here we use inelastic neutron
scattering to demonstrate that the resonance in the heavy fermion
superconductor CeYbCoIn with has a ring-like
upward dispersion that is robust against Yb-doping. By comparing our
experimental data with random phase approximation calculation using the
electronic structure and the momentum dependence of the -wave
superconducting gap determined from scanning tunneling microscopy for
CeCoIn, we conclude the robust upward dispersing resonance mode in
CeYbCoIn is inconsistent with the downward dispersion
predicted within the spin-exciton scenario.Comment: Supplementary Information available upon reques
Full momentum- and energy-resolved spectral function of a 2D electronic system
The single-particle spectral function measures the density of electronic states in a material as a function of both momentum and energy, providing central insights into strongly correlated electron phenomena. Here we demonstrate a high-resolution method for measuring the full momentum- and energy-resolved electronic spectral function of a two-dimensional (2D) electronic system embedded in a semiconductor. The technique remains operational in the presence of large externally applied magnetic fields and functions even for electronic systems with zero electrical conductivity or with zero electron density. Using the technique on a prototypical 2D system, a GaAs quantum well, we uncover signatures of many-body effects involving electron-phonon interactions, plasmons, polarons, and a phonon analog of the vacuum Rabi splitting in atomic systems
Gravitational Wavetrains in the Quasi-Equilibrium Approximation: A Model Problem in Scalar Gravitation
A quasi-equilibrium (QE) computational scheme was recently developed in
general relativity to calculate the complete gravitational wavetrain emitted
during the inspiral phase of compact binaries. The QE method exploits the fact
that the the gravitational radiation inspiral timescale is much longer than the
orbital period everywhere outside the ISCO. Here we demonstrate the validity
and advantages of the QE scheme by solving a model problem in relativistic
scalar gravitation theory. By adopting scalar gravitation, we are able to
numerically track without approximation the damping of a simple, quasi-periodic
radiating system (an oscillating spherical matter shell) to final equilibrium,
and then use the exact numerical results to calibrate the QE approximation
method. In particular, we calculate the emitted gravitational wavetrain three
different ways: by integrating the exact coupled dynamical field and matter
equations, by using the scalar-wave monopole approximation formula
(corresponding to the quadrupole formula in general relativity), and by
adopting the QE scheme. We find that the monopole formula works well for weak
field cases, but fails when the fields become even moderately strong. By
contrast, the QE scheme remains quite reliable for moderately strong fields,
and begins to breakdown only for ultra-strong fields. The QE scheme thus
provides a promising technique to construct the complete wavetrain from binary
inspiral outside the ISCO, where the gravitational fields are strong, but where
the computational resources required to follow the system for more than a few
orbits by direct numerical integration of the exact equations are prohibitive.Comment: 15 pages, 14 figure
Working principles of doping-well structures for high-mobility two-dimensional electron systems
Suppressing electron scattering is essential to achieve high-mobility
two-dimensional electron systems (2DESs) that are clean enough to probe exotic
interaction-driven phenomena. In heterostructures it is common practice to
utilize modulation doping, where the ionized dopants are physically separated
from the 2DES channel. The doping-well structure augments modulation doping by
providing additional screening for all types of charged impurities in the
vicinity of the 2DES, which is necessary to achieve record-breaking samples.
Despite its prevalence in the design of ultra-high-mobility 2DESs, the working
principles of the doping-well structure have not been reported. Here we
elaborate on the mechanics of electron transfer from doping wells to the 2DES,
focusing on GaAs/AlGaAs samples grown by molecular beam epitaxy. Based on this
understanding we demonstrate how structural parameters in the doping well can
be varied to tune the properties of the 2DES.Comment: 6 pages, 5 fiture
Electronically highly cubic conditions for Ru in alpha-RuCl3
We studied the local Ru 4d electronic structure of alpha-RuCl3 by means of
polarization dependent x-ray absorption spectroscopy at the Ru-L2,3 edges. We
observed a vanishingly small linear dichroism indicating that electronically
the Ru 4d local symmetry is highly cubic. Using full multiplet cluster
calculations we were able to reproduce the spectra excellently and to extract
that the trigonal splitting of the t2g orbitals is -12 meV, i.e.
negligible as compared to the Ru 4d spin-orbit coupling constant. Consistent
with our magnetic circular dichroism measurements, we found that the ratio of
the orbital and spin moments is 2.0, the value expected for a Jeff = 1/2 ground
state. We have thus shown that as far as the Ru 4d local properties are
concerned, alpha-RuCl3 is an ideal candidate for the realization of Kitaev
physics
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