4,001 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
Giant Flexoelectric Effect in Ferroelectric Epitaxial Thin Films
We report on nanoscale strain gradients in ferroelectric HoMnO3 epitaxial
thin films, resulting in a giant flexoelectric effect. Using grazing-incidence
in-plane X-ray diffraction, we measured strain gradients in the films, which
were 6 or 7 orders of magnitude larger than typical values reported for bulk
oxides. The combination of transmission electron microscopy, electrical
measurements, and electrostatic calculations showed that flexoelectricity
provides a means of tuning the physical properties of ferroelectric epitaxial
thin films, such as domain configurations and hysteresis curves.Comment: Accepted by Phys. Rev. Let
Putative spin liquid in the triangle-based iridate BaIrTiO
We report on thermodynamic, magnetization, and muon spin relaxation
measurements of the strong spin-orbit coupled iridate BaIrTiO,
which constitutes a new frustration motif made up a mixture of edge- and
corner-sharing triangles. In spite of strong antiferromagnetic exchange
interaction of the order of 100~K, we find no hint for long-range magnetic
order down to 23 mK. The magnetic specific heat data unveil the -linear and
-squared dependences at low temperatures below 1~K. At the respective
temperatures, the zero-field muon spin relaxation features a persistent spin
dynamics, indicative of unconventional low-energy excitations. A comparison to
the isostructural compound BaRuTiO suggests that a concerted
interplay of compass-like magnetic interactions and frustrated geometry
promotes a dynamically fluctuating state in a triangle-based iridate.Comment: Physical Review B accepte
Double polarization hysteresis loop induced by the domain pinning by defect dipoles in HoMnO3 epitaxial thin films
We report on antiferroelectriclike double polarization hysteresis loops in
multiferroic HoMnO3 thin films below the ferroelectric Curie temperature. This
intriguing phenomenon is attributed to the domain pinning by defect dipoles
which were introduced unintentionally during film growth process. Electron
paramagnetic resonance suggests the existence of Fe1+ defects in thin films and
first principles calculations reveal that the defect dipoles would be composed
of oxygen vacancy and Fe1+ defect. We discuss migration of charged point
defects during film growth process and formation of defect dipoles along
ferroelectric polarization direction, based on the site preference of point
defects. Due to a high-temperature low-symmetry structure of HoMnO3, aging is
not required to form the defect dipoles in contrast to other ferroelectrics
(e.g., BaTiO3).Comment: 4 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
Dynamic Response of Wigner Crystals
The Wigner crystal, an ordered array of electrons, is one of the very first
proposed many-body phases stabilized by the electron-electron interaction. This
electron solid phase has been reported in ultra-clean two-dimensional electron
systems at extremely low temperatures, where the Coulomb interaction dominants
over the kinetic energy, disorder potential and thermal fluctuation. We closely
examine this quantum phase with capacitance measurements where the device
length-scale is comparable with the crystal's correlation length. The
extraordinarily high performance of our technique makes it possible to
quantitatively study the dynamic response of the Wigner crystal within the
single crystal regime. Our result will greatly boost the study of this
inscrutable electron solid
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