11,658 research outputs found
Topological kink states at a tilt boundary in gated multi-layer graphene
The search for new realization of topologically protected edge states is an
active area of research. We show that a tilt boundary in gated multi-layer
graphene supports topologically protected gapless kink states, associated with
quantum valley Hall insulator (QVH). We investigate such kink states from two
perspectives: the microscopic perspective of a tight-binding model and an
ab-initio calculation on bilayer, and the perspective of symmetry protected
topological (SPT) states for general multi-layer. We show that a AB-BA bilayer
tilt boundary supports gapless kink states that are undeterred by strain
concentrated at the boundary. Further, we establish the kink states as concrete
examples of edge states of {\it time-reversal symmetric} -type
SPT, protected by no valley mixing, electron number conservation, and time
reversal symmetries. This allows us to discuss possible phase transitions
upon symmetry changes from the SPT perspective. Recent experimental
observations of a network of such tilt boundaries suggest that transport
through these novel topological kink states might explain the long standing
puzzle of sub-gap conductance. Further, recent observation of gap closing and
re-opening in gated bi-layer might be the first example of a transition between
two distinct SPT's: QVH and LAF.Comment: Improved a discussion of the structural aspects of the tilt boundary.
Included a discussion of boundary condition dependence. Added new section on
connection to experiment
Entanglement entropy of the composite fermion non-Fermi liquid state
The so-called ``non-Fermi liquid'' behavior is very common in strongly
correlated systems. However, its operational definition in terms of ``what it
is not'' is a major obstacle against theoretical understanding of this
fascinating correlated state. Recently there has been much interest in
entanglement entropy as a theoretical tool to study non-Fermi liquids. So far
explicit calculations have been limited to models without direct experimental
realizations. Here we focus on a two dimensional electron fluid under magnetic
field and filling fraction , which is believed to be a non-Fermi
liquid state. Using the composite fermion (CF) wave-function which captures the
state very accurately, we compute the second R\'enyi entropy using
variational Monte-Carlo technique and an efficient parallel algorithm. We find
the entanglement entropy scales as with the length of the boundary
as it does for free fermions, albeit with a pre-factor twice that of the
free fermion. We contrast the results against theoretical conjectures and
discuss the implications of the results.Comment: 4+ page
Signatures of unconventional pairing in near-vortex electronic structure of LiFeAs
A major question in Fe-based superconductors remains the structure of the
pairing, in particular whether it is of unconventional nature. The electronic
structure near vortices can serve as a platform for phase-sensitive
measurements to answer this question. By solving Bogoliubov-de Gennes equations
for LiFeAs, we calculate the energy-dependent local electronic structure near a
vortex for different nodeless gap-structure possibilities. At low energies, the
local density of states (LDOS) around a vortex is determined by the
normal-state electronic structure. However, at energies closer to the gap
value, the LDOS can distinguish an anisotropic from a conventional isotropic
s-wave gap. We show within our self-consistent calculation that in addition,
the local gap profile differs between a conventional and an unconventional
pairing. We explain this through admixing of a secondary order parameter within
Ginzburg-Landau theory. In-field scanning tunneling spectroscopy near vortices
can therefore be used as a real-space probe of the gap structure
Spin-Orbit Coupling in LaAlO/SrTiO interfaces: Magnetism and Orbital Ordering
The combination of Rashba spin-orbit coupling and electron correlations can
induce unusual phenomena in the metallic interface between SrTiO and
LaAlO. We consider effects of Rashba spin-orbit coupling at this interface
in the context of the recent observation of anisotropic magnetism. Firstly, we
show how Rashba spin-orbit coupling in a system near a band-edge can account
for the observed magnetic anisotropy. Secondly, we investigate the coupling
between in-plane magnetic-moment anisotropy and nematicity in the form of an
orbital imbalance between d / d orbitals. We estimate this
coupling to be substantial in the low electron density regime. Such an orbital
ordering can affect magneto transport
Commensurate period Charge Density Modulations throughout the Pseudogap Regime
Theories based upon strong real space (r-space) electron electron
interactions have long predicted that unidirectional charge density modulations
(CDM) with four unit cell (4) periodicity should occur in the hole doped
cuprate Mott insulator (MI). Experimentally, however, increasing the hole
density p is reported to cause the conventionally defined wavevector of
the CDM to evolve continuously as if driven primarily by momentum space
(k-space) effects. Here we introduce phase resolved electronic structure
visualization for determination of the cuprate CDM wavevector. Remarkably, this
new technique reveals a virtually doping independent locking of the local CDM
wavevector at throughout the underdoped phase diagram of the
canonical cuprate . These observations have significant
fundamental consequences because they are orthogonal to a k-space (Fermi
surface) based picture of the cuprate CDM but are consistent with strong
coupling r-space based theories. Our findings imply that it is the latter that
provide the intrinsic organizational principle for the cuprate CDM state
Topological Defects Coupling Smectic Modulations to Intra-unit-cell Nematicity in Cuprate
We study the coexisting smectic modulations and intra-unit-cell nematicity in
the pseudogap states of underdoped Bi2Sr2CaCu2O8+{\delta}. By visualizing their
spatial components separately, we identified 2\pi topological defects
throughout the phase-fluctuating smectic states. Imaging the locations of large
numbers of these topological defects simultaneously with the fluctuations in
the intra-unit-cell nematicity revealed strong empirical evidence for a
coupling between them. From these observations, we propose a Ginzburg-Landau
functional describing this coupling and demonstrate how it can explain the
coexistence of the smectic and intra-unit-cell broken symmetries and also
correctly predict their interplay at the atomic scale. This theoretical
perspective can lead to unraveling the complexities of the phase diagram of
cuprate high-critical-temperature superconductors
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