42 research outputs found
Temperature dependent resistivity in the doped two dimensional metallic phase of mTMD bilayers
Two recent experiments from Cornell and Columbia have reported
insulator-to-metal transitions in two-dimensional (2D) moir\'e transition metal
dichalcogenides (mTMD) induced by doping around half-filling, where the system
is a Mott insulator. In the current work, we consider the temperature dependent
resistivity of this metallic phase in the doped situation away from
half-filling, arguing that it arises from the strongly temperature dependent 2D
Friedel oscillations (i.e. finite momentum screening) associated with random
quenched charged impurities, leading to the observed strongly increasing
linear-in- resistivity in the metallic phase. Our theory appears to account
for the temperature-dependent metallic resistivity for doping around
half-filling of the effective moir\'e TMD band, showing that
temperature-dependent screened Coulomb disorder is an essential ingredient of
doped 2D mTMD physics.Comment: 5 pages, 3 figure
Anderson localization crossover in 2D Si systems: The past and the present
Using Ioffe-Regel-Mott (IRM) criterion for strong localization crossover in
disordered doped 2D electron systems, we theoretically study the relationships
among the three key experimentally determined localization quantities: critical
density (), critical resistance (), and sample
quality defined by the effective impurity density (as experimentally diagnosed
by the sample mobility, , at densities much higher than
critical densities). Our results unify experimental results for 2D
metal-insulator transitions (MIT) in Si systems over a 50-year period
(1970-2020), showing that () decrease
(increase) with increasing sample quality, explaining why the early experiments
in the 1970s, using low-quality samples () reported strong localization crossover at with whereas recent experiments
(after 1995), using high-quality samples (), report with
. Our theory establishes the 2D MIT to be primarily a
screened Coulomb disorder-driven strong localization crossover phenomenon,
which happens at different sample-dependent critical density and critical
resistance, thus unifying Si 2D MIT phenomena over a 50-year period.Comment: 5 pages, 1 figur
Density-tuned effective metal-insulator transitions in 2D semiconductor layers: Anderson localization or Wigner crystallization
Electrons (or holes) confined in 2D semiconductor layers have served as model
systems for studying disorder and interaction effects for almost 50 years. In
particular, strong disorder drives the metallic 2D carriers into a strongly
localized Anderson insulator (AI) at low densities whereas pristine 2D
electrons in the presence of no (or little) disorder should solidify into a
Wigner crystal at low carrier densities. Since the disorder in 2D
semiconductors is mostly Coulomb disorder arising from random charged
impurities, the applicable physics is complex as the carriers interact with
each other as well as with the random charged impurities through the same
long-range Coulomb coupling. By critically theoretically analyzing the
experimental transport data in depth using a realistic transport theory to
calculate the low-temperature 2D resistivity as a function of carrier density
in 11 different experimental samples covering 9 different materials, we
establish, utilizing the Ioffe-Regel-Mott criterion for strong localization, a
direct connection between the critical localization density for the 2D
metal-insulator transition (MIT) and the sample mobility deep into the metallic
state, which for clean samples could lead to a localization density low enough
to make the transition appear to be a Wigner crystallization. We believe that
the insulating phase is always an effective Coulomb disorder-induced localized
AI, which may have short-range WC-like correlations at low carrier densities.
Our theoretically calculated disorder-driven critical MIT density agrees with
experimental findings in all 2D samples, even for the ultra-clean samples. In
particular, the extrapolated critical density for the 2D MIT seems to vanish
when the high-density mobility goes to infinity, indicating that transport
probes a disorder-localized insulating ground state independent of how low the
carrier density might be.Comment: 15 pages, 6 figures, 3 table
Planckian properties of 2D semiconductor systems
We describe and discuss the low-temperature resistivity (and the
temperature-dependent inelastic scattering rate) of several different doped 2D
semiconductor systems from the perspective of the Planckian hypothesis
asserting that provides a scattering bound, where
is the appropriate relaxation time. The regime of transport considered
here is well-below the Bloch-Gruneisen regime so that phonon scattering is
negligible. The temperature-dependent part of the resistivity is almost
linear-in- down to arbitrarily low temperatures, with the linearity arising
from an interplay between screening and disorder, connected with carrier
scattering from impurity-induced Friedel oscillations. The temperature
dependence disappears if the Coulomb interaction between electrons is
suppressed. The temperature coefficient of the resistivity is enhanced at lower
densities, enabling a detailed study of the Planckian behavior both as a
function of the materials system and carrier density. Although the precise
Planckian bound never holds, we find somewhat surprisingly that the bound seems
to apply approximately with the scattering rate never exceeding by more than an order of magnitude either in the experiment or in the
theory. In addition, we calculate the temperature-dependent electron-electron
inelastic scattering rate by obtaining the temperature-dependent self-energy
arising from Coulomb interaction, also finding it to obey the Planckian bound
within an order of magnitude at all densities and temperatures. We introduce
the concept of a generalized Planckian bound where is bounded by
with or so in the super-Planckian
regime with the strict Planckian bound of =1 being a nongeneric
finetuned situation.Comment: 21 pages, 9 figures. Revised version with new results, figures, and
reference
Hydrodynamics, viscous electron fluid, and Wiedeman-Franz law in 2D semiconductors
Considering theoretically the transition between hydrodynamic and ballistic
regimes in 2D semiconductors, we show that electrons in high-mobility 2D GaAs
are by far the best system for the direct observation of collective
hydrodynamic effects even in bulk transport properties independent of
complicated transport features in narrow constrictions and small systems where
Gurzhi phenomena are typically studied experimentally. We predict a strong
hydrodynamics-induced generic violation of the Wiedeman-Franz law in bulk 2D
GaAs systems for mobilities as modest as and densities
- in the temperature range of -.Comment: 5 pages, 5 figure