203 research outputs found
Electron-phonon and electron-electron interaction effects in twisted bilayer graphene
By comparing with recently available experimental data from several groups,
we critically discuss the manifestation of continuum many body interaction
effects in twisted bilayer graphene (tBLG) with small twist angles and low
carrier densities, which arise naturally within the Dirac cone approximation
for the non-interacting band structure. We provide two specific examples of
such continuum many body theories: one involving electron-phonon interaction
and one involving electron-electron interaction. In both cases, the
experimental findings are only partially quantitatively consistent with rather
clear-cut leading-order theoretical predictions based on well-established
continuum many body theories. We provide a critical discussion, based mainly on
the currently available tBLG experimental data, on possible future directions
for understanding many body renormalization involving electron-phonon and
electron-electron interactions in the system. One definitive conclusion based
on the comparison between theory and experiment is that the leading order
1-loop perturbative renormalization group theory completely fails to account
for the electron-electron interaction effects in the strong-coupling limit of
flatband moir\'e tBLG system near the magic twist angle even at low doping
where the Dirac cone approximation should apply. By contrast, approximate
nonperturbative theoretical results based on Borel-Pad\'e resummation or
expansion seems to work well compared with experiments, indicating rather small
interaction corrections to Fermi velocity or carrier effective mass. For
electron-phonon interactions, however, the leading-order continuum theory works
well except when van Hove singularities in the density of states come into
play.Comment: 18 pages, 7 figure
Identification of superconducting pairing symmetry in twisted bilayer graphene using in-plane magnetic field and strain
We show how the pairing symmetry of superconducting states in twisted bilayer
graphene can be experimentally identified by theoretically studying effects of
externally applied in-plane magnetic field and strain. In the low field regime,
superconducting critical temperature is suppressed by in-plane magnetic
field in singlet channels, but is enhanced by weak
in triplet channels, providing an important
distinction. The in-plane angular dependence of the critical
has a six-fold rotational symmetry, which is
broken when strain is present. We show that anisotropy in
generated by strain can be similar for - and
-wave channels in moir\'e superlattices. The -wave state is pinned to be
nematic by strain and consequently gapless, which is distinguishable from the
fully gapped -wave state by scanning tunneling measurements.Comment: 5+2 pages, 4 figure
Ferromagnetism and superconductivity in twisted double bilayer graphene
We present a theory of competing ferromagnetic and superconducting orders in
twisted double bilayer graphene (TDBG). In our theory, ferromagnetism is
induced by Coulomb repulsion, while superconductivity with intervalley
equal-spin pairing can be mediated by electron-acoustic phonon interactions. We
calculate the transition temperatures for ferromagnetism and superconductivity
as a function of moir\'e band filling factor, and find that superconducting
domes can appear on both the electron and hole sides of the ferromagnetic
insulator at half filling. We show that the ferromagnetic insulating gap has a
dome shape dependence on the layer potential difference, which provides an
explanation to the experimental observation that the ferromagnetic insulator
only develops over a finite range of external displacement field. We also
verify the stability of the half-filled ferromagnetic insulator against two
types of collective excitations, i.e., spin magnons and valley magnons.Comment: 9 pages, 6 figure
Surface plasmon polaritons in topological Weyl semimetals
We consider theoretically surface plasmon polaritons in Weyl semimetals.
These materials contain pairs of band touching points - Weyl nodes - with a
chiral topological charge, which induces an optical anisotropy and anomalous
transport through the chiral anomaly. We show that these effects, which are not
present in ordinary metals, have a direct fundamental manifestation in the
surface plasmon dispersion. The retarded Weyl surface plasmon dispersion
depends on the separation of the Weyl nodes in energy and momentum space. For
Weyl semimetals with broken time-reversal symmetry, the distance between the
nodes acts as an effective applied magnetic field in momentum space, and the
Weyl surface plasmon polariton dispersion is strikingly similar to
magnetoplasmons in ordinary metals. In particular, this implies the existence
of nonreciprocal surface modes. In addition, we obtain the nonretarded Weyl
magnetoplasmon modes, which acquire an additional longitudinal magnetic-field
dependence. These predicted surface plasmon results are observable
manifestations of the chiral anomaly in Weyl semimetals and might have
technological applications.Comment: 8 pages, 2 figure
Quantum phases of interacting electrons in three-dimensional dirty Dirac semimetals
We theoretically study the stability of three dimensional Dirac semimetals
against short-range electron-electron interaction and quenched time-reversal
symmetric disorder (but excluding mass disorder). First we focus on the clean
interacting and the noninteracting dirty Dirac semimetal separately, and show
that they support two distinct quantum critical points. Using renormalization
group techniques, we find that while interaction driven quantum critical points
are \emph{Gaussian} (mean-field) in nature, describing quantum phase
transitions into various broken symmetry phases, the ones controlled by
disorder are \emph{non-Gaussian}, capturing the transition to a metallic phase.
We classify such diffusive quantum critical points based on the transformation
of disorder vertices under a \emph{continuous} chiral rotation. Our wek
coupling renormalization group analysis suggests that two distinct quantum
critical points are stable in an interacting dirty Dirac semimetal (with chiral
symmetric randomness), and a multicritical point (at finite interaction and
disorder) results from their interplay. By contrast, the chiral symmetry
breaking disorder driven critical point is unstable against weak interactions.
Effects of weak disorder on the ordering tendencies in Dirac semimetal are
analyzed. The clean interacting critical points, however, satisfy the
\emph{Harris criterion}, and are therefore expected to be unstable against bond
disorder. Although our weak coupling analysis is inadequate to establish the
ultimate stability of these fixed points in the strong coupling regime (when
both interaction and disorder are strong), they can still govern crossover
behaviors in Dirac semimetals over a large length scale, when either
interaction or randomness is sufficiently weak.Comment: Published Version: 28 Pages, 10 Figures, 5 Tables, added discussion,
new reference
Island Nucleation in Silicon on Si(111) Growth under Chemical Vapor Deposition
Recent experiments show that the islanding behavior during chemical vapor
deposition (CVD) of Si on Si(111) using disilane (SiH) is quite
different from that due to molecular beam epitaxy (MBE). While the latter can
be understood using rate equation theories (RET), the islanding exponent
(connecting the power law growth of island density with growth rate) obtained
for the CVD growth is a puzzle, with the CVD exponent being almost twice the
MBE exponent. We carry out (2+1) dimensional kinetic Monte Carlo(MC)
simulations to study this CVD growth. Hydrogen plays a critical role during
growth. Disilane breaks up into hydrides on the Si surface. We use MC
simulations to explore a number of cases involving one or two migrating species
and show that the large islanding exponent is probably due to the presence of
two hydrides, one of which has a much shorter lifetime than the other. We
modify RET taking this possibility into account in order to shed light on the
experimental observation. We calculate the scaling properties of the island
distributions using MC simulations and the modified RET, and conclude that the
large effective CVD exponents arise from the failure of the simple island
number scaling scenario which no longer applies to the two-component situation
prevailing under CVD growth conditions.Comment: 20 pages, 3 figure
Building topological quantum circuits: Majorana nanowire junctions
Topological quantum computation using non-Abelian Majorana zero modes
localized in proximitized semiconductor nanowires requires careful
electrostatic control of wire-junctions so as to manipulate and braid the zero
modes enabling anyonic fault-tolerant gate operations. We theoretically
investigate the topological superconducting properties of such elementary
wire-junctions, the so-called T junctions, finding that the existence of the
junction may nonperturbatively affect the Majorana behavior by introducing
spurious non-topological subgap states mimicking zero-modes. We propose a
possible solution to this potentially serious problem by showing that junctions
made lithographically from two-dimensional (2D) electron gas systems may
manifest robust subgap topological properties without any spurious zero modes.
We propose a 2D structure that enables multiprobe tunneling experiments
providing position-dependent spectroscopy, which can decisively settle
outstanding open questions related to the origin of the zero-bias conductance
peaks observed experimentally. We also find that junctions with trivial
superconductors may result in local perturbations that induce extrinsic
low-energy states similar to those associated with wire junctions.Comment: Published version, 8 pages, 9 figure
Wiedemann-Franz law and Fermi liquids
We consider in depth the applicability of the Wiedemann-Franz (WF) law,
namely that the electronic thermal conductivity () is proportional to
the product of the absolute temperature () and the electrical conductivity
() in a metal with the constant of proportionality, the so-called
Lorenz number , being a materials-independent universal constant in all
systems obeying the Fermi liquid (FL) paradigm. It has been often stated that
the validity (invalidity) of the WF law is the hallmark of an FL
(non-Fermi-liquid (NFL)). We consider, both in two (2D) and three (3D)
dimensions, a system of conduction electrons at a finite temperature
coupled to a bath of acoustic phonons and quenched impurities, ignoring effects
of electron-electron interactions. We find that the WF law is violated
arbitrarily strongly with the effective Lorenz number vanishing at low
temperatures as long as phonon scattering is stronger than impurity scattering.
This happens both in 2D and in 3D for , where is the
Bloch-Gr\"uneisen temperature of the system. In the absence of phonon
scattering (or equivalently, when impurity scattering is much stronger than the
phonon scattering), however, the WF law is restored at low temperatures even if
the impurity scattering is mostly small angle forward scattering. Thus,
strictly at the WF law is always valid in a FL in the presence of
infinitesimal impurity scattering. For strong phonon scattering, the WF law is
restored for (or the Debye temperature , whichever is lower)
as in usual metals. At very high temperatures, thermal smearing of the Fermi
surface causes the effective Lorenz number to go below manifesting a
quantitative deviation from the WF law. Our work establishes definitively that
the uncritical association of an NFL behavior with the failure of the WF law is
incorrect.Comment: 11 pages, 4 figures, 12 pages of appendice
Configuration-Controlled Many-Body Localization and the Mobility Emulsion
We uncover a new non-ergodic phase, distinct from the many-body localized
(MBL) phase, in a disordered two-leg ladder of interacting hardcore bosons. The
dynamics of this emergent phase, which has no single-particle analog and exists
only for strong disorder and finite interaction, is determined by the many-body
configuration of the initial state. Remarkably, this phase features the
of localized and extended many-body states at fixed
energy density and thus does not exhibit a many-body mobility edge, nor does it
reduce to a model with a single-particle mobility edge in the noninteracting
limit. We show that eigenstates in this phase can be described in terms of
interacting emergent Ising spin degrees of freedom ("singlons") suspended in a
mixture with inert charge degrees of freedom ("doublons" and "holons"), and
thus dub it a (ME). We argue that grouping
eigenstates by their doublon/holon density reveals a transition between
localized and extended states that is invisible as a function of energy
density. We further demonstrate that the dynamics of the system following a
quench may exhibit either thermalizing or localized behavior depending on the
doublon/holon density of the initial product state. Intriguingly, the
ergodicity of the ME is thus tuned by the initial state of the many-body
system. These results establish a new paradigm for using many-body
configurations as a tool to study and control the MBL transition. The ME phase
may be observable in suitably prepared cold atom optical lattices.Comment: 20 pages, 12 figure
Non-adiabatic Effects in the Braiding of Non-Abelian Anyons in Topological Superconductors
Qubits in topological quantum computation are built from non-Abelian anyons.
Adiabatic braiding of anyons is exploited as topologically protected logical
gate operations. Thus, the adiabaticity upon which the notion of quantum
statistics is defined, plays a fundamental role in defining the non-Abelian
anyons. We study the non-adiabatic effects in braidings of Ising-type anyons,
namely Majorana fermions in topological superconductors, using the formalism of
time-dependent Bogoliubov-de Gennes equations. Using this formalism, we
consider non-adiabatic corrections to non-Abelian statistics from: (1)
tunneling splitting of anyons imposing an additional dynamical phase to the
transformation of ground states; (2) transitions to excited states that are
potentially destructive to non-Abelian statistics since the non-local fermion
occupation can be spoiled by such processes. However, if the bound states are
localized and being braided together with the anyons, non-Abelian statistics
can be recovered once the definition of Majorana operators is appropriately
generalized taking into account the fermion parity in these states. On the
other hand, if the excited states are extended over the whole system and form a
continuum, the notion of local fermion parity no longer holds. We then
quantitatively characterize the errors introduced in this situation
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