166 research outputs found
The Droplet State and the Compressibility Anomaly in Dilute 2D Electron Systems
We investigate the space distribution of carrier density and the
compressibility of two-dimensional (2D) electron systems by using the local
density approximation. The strong correlation is simulated by the local
exchange and correlation energies. A slowly varied disorder potential is
applied to simulate the disorder effect. We show that the compressibility
anomaly observed in 2D systems which accompanies the metal-insulator transition
can be attributed to the formation of the droplet state due to disorder effect
at low carrier densities.Comment: 4 pages, 3 figure
Long-Range Order in Electronic Transport through Disordered Metal Films
Ultracold atom magnetic field microscopy enables the probing of current flow
patterns in planar structures with unprecedented sensitivity. In
polycrystalline metal (gold) films we observe long-range correlations forming
organized patterns oriented at +/- 45 deg relative to the mean current flow,
even at room temperature and at length scales orders of magnitude larger than
the diffusion length or the grain size. The preference to form patterns at
these angles is a direct consequence of universal scattering properties at
defects. The observed amplitude of the current direction fluctuations scales
inversely to that expected from the relative thickness variations, the grain
size and the defect concentration, all determined independently by standard
methods. This indicates that ultracold atom magnetometry enables new insight
into the interplay between disorder and transport
SIMBA-C: An updated chemical enrichment model for galactic chemical evolution in the SIMBA simulation
We introduce a new chemical enrichment and stellar feedback model into GIZMO,
using the SIMBA sub-grid models as a base. Based on the state-of-the-art
chemical evolution model of Kobayashi et al., SIMBA-C tracks 34 elements from
HGe and removes SIMBA's instantaneous recycling approximation.
Furthermore, we make some minor improvements to SIMBA's base feedback models.
SIMBA-C provides significant improvements on key diagnostics such as the knee
of the galaxy stellar mass function, the faint end of the main sequence,
and the ability to track black holes in dwarf galaxies. SIMBA-C also matches
better with recent observations of the mass-metallicity relation at . By
not assuming instantaneous recycling, SIMBA-C provides a much better match to
galactic abundance ratio measures such as [O/Fe] and [N/O]. SIMBA-C thus opens
up new avenues to constrain feedback models using detailed chemical abundance
measures across cosmic time.Comment: 16 Pages, 9 Figure, 1 Table, Accepted for MNRAS publication: 8 August
2023, doi:10.1093/mnras/stad239
Physics of the Insulating Phase in the Dilute Two-Dimensional Electron Gas
We propose to use the radio-frequency single-electron transistor as an
extremely sensitive probe to detect the time-periodic ac signal generated by
sliding electron lattice in the insulating state of the dilute two-dimensional
electron gas. We also propose to use the optically-pumped NMR technique to
probe the electron spin structure of the insulating state. We show that the
electron effective mass and spin susceptibility are strongly enhanced by
critical fluctuations of electron lattice in the vicinity of the
metal-insulator transition, as observed in experiment.Comment: 5 pages, 2 figures, uses jetpl.cls (included). v.4: After publication
in JETP Letters, two plots comparing theory and experiment are added, and a
minor error is correcte
Simba-C: the evolution of the thermal and chemical properties in the intragroup medium
© 2024 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/The newly updated GIZMO and Simba based simulation, Simba-C, with its new stellar feedback, chemical enrichment, and recalibrated AGN feedback, allows for a detailed study of the intragroup medium X-ray properties. We discuss the impact of various physical mechanisms, e.g. stellar and AGN feedback, and chemical enrichment, on the composition and the global scaling relations of nearby galaxy groups. We also study the evolution (z = 2 to 0) of the global properties for the temperature groups. Simba-C shows improved consistent matching with the observations of all X-ray scaling relations compared to Simba. It is well known that AGN feedback has a significant influence on LX, 0.5–2.0–Tspec, corr, S500/2500–Tspec, corr, and gas mass fractions, with our Simba-C results consistent with it. Our recalibrated AGN feedback strength also showed an additional improvement in gas entropy, which now aligns with CLoGS observations. The updated stellar feedback and chemical enrichment model is shown to play an important role in our understanding of the chemical abundance ratios and their evolution within galaxy groups. In particular, we find that Simba-C produces an increase in the amount of heavier elements (specifically Si and Fe) relative to O, compared to Simba.Peer reviewe
The nature of localization in graphene under quantum Hall conditions
Particle localization is an essential ingredient in quantum Hall physics
[1,2]. In conventional high mobility two-dimensional electron systems Coulomb
interactions were shown to compete with disorder and to play a central role in
particle localization [3]. Here we address the nature of localization in
graphene where the carrier mobility, quantifying the disorder, is two to four
orders of magnitude smaller [4,5,6,7,8,9,10]. We image the electronic density
of states and the localized state spectrum of a graphene flake in the quantum
Hall regime with a scanning single electron transistor [11]. Our microscopic
approach provides direct insight into the nature of localization. Surprisingly,
despite strong disorder, our findings indicate that localization in graphene is
not dominated by single particle physics, but rather by a competition between
the underlying disorder potential and the repulsive Coulomb interaction
responsible for screening.Comment: 18 pages, including 5 figure
Observation of Electron-Hole Puddles in Graphene Using a Scanning Single Electron Transistor
The electronic density of states of graphene is equivalent to that of
relativistic electrons. In the absence of disorder or external doping the Fermi
energy lies at the Dirac point where the density of states vanishes. Although
transport measurements at high carrier densities indicate rather high
mobilities, many questions pertaining to disorder remain unanswered. In
particular, it has been argued theoretically, that when the average carrier
density is zero, the inescapable presence of disorder will lead to electron and
hole puddles with equal probability. In this work, we use a scanning single
electron transistor to image the carrier density landscape of graphene in the
vicinity of the neutrality point. Our results clearly show the electron-hole
puddles expected theoretically. In addition, our measurement technique enables
to determine locally the density of states in graphene. In contrast to
previously studied massive two dimensional electron systems, the kinetic
contribution to the density of states accounts quantitatively for the measured
signal. Our results suggests that exchange and correlation effects are either
weak or have canceling contributions.Comment: 13 pages, 5 figure
Screening Breakdown on the Route toward the Metal-Insulator Transition in Modulation Doped Si/SiGe Quantum Wells
Exploiting the spin resonance of two-dimensional (2D) electrons in SiGe/Si
quantum wells we determine the carrier-density-dependence of the magnetic
susceptibility. Assuming weak interaction we evaluate the density of states at
the Fermi level D(E_F), and the screening wave vector, q_TF. Both are constant
at higher carrier densities n, as for an ideal 2D carrier gas. For n < 3e11
cm-2, they decrease and extrapolate to zero at n = 7e10 cm-2. Calculating the
mobility from q_TF yields good agreement with experimental values justifying
the approach. The decrease in D(E_F) is explained by potential fluctuations
which lead to tail states that make screening less efficient and - in a
positive feedback - cause an increase of the potential fluctuations. Even in
our high mobility samples the fluctuations exceed the electron-electron
interaction leading to the formation of puddles of mobile carriers with at
least 1 micrometer diameter.Comment: 4 pages, 3 figure
Unexpected Behavior of the Local Compressibility Near the B=0 Metal-Insulator Transition
We have measured the local electronic compressibility of a two-dimensional
hole gas as it crosses the B=0 Metal-Insulator Transition. In the metallic
phase, the compressibility follows the mean-field Hartree-Fock (HF) theory and
is found to be spatially homogeneous. In the insulating phase it deviates by
more than an order of magnitude from the HF predictions and is spatially
inhomogeneous. The crossover density between the two types of behavior, agrees
quantitatively with the transport critical density, suggesting that the system
undergoes a thermodynamic change at the transition.Comment: As presented in EP2DS-13, Aug. 1999. (4 pages, 4 figures
Metallic behavior and related phenomena in two dimensions
For about twenty years, it has been the prevailing view that there can be no
metallic state or metal-insulator transition in two dimensions in zero magnetic
field. In the last several years, however, unusual behavior suggestive of such
a transition has been reported in a variety of dilute two-dimensional electron
and hole systems. The physics behind these observations is presently not
understood. We review and discuss the main experimental findings and suggested
theoretical models.Comment: To be published in Rev. Mod. Phy
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