13 research outputs found
Continuum Theory of Edge States of Topological Insulators: Variational Principle and Boundary Conditions
We develop a continuum theory to model low energy excitations of a generic
four-band time reversal invariant electronic system with boundaries. We propose
a variational energy functional for the wavefunctions which allows us derive
natural boundary conditions valid for such systems. Our formulation is
particularly suited to develop a continuum theory of the protected edge/surface
excitations of topological insulators both in two and three dimensions. By a
detailed comparison of our analytical formulation with tight binding
calculations of ribbons of topological insulators modeled by the
Bernevig-Hughes-Zhang (BHZ) hamiltonian, we show that the continuum theory with
the natural boundary condition provides an appropriate description of the low
energy physics. As a spin-off, we find that in a certain parameter regime, the
gap that arises in topological insulator ribbons of finite width due to the
hybridization of edges states from opposite edges, depends non-monotonically on
the ribbon width and can nearly vanish at certain "magic widths".Comment: 8 pages, 5 figure
Fermionic Superfluid from a Bilayer Band Insulator in an Optical Lattice
We propose a model to realize a fermionic superfluid state in an optical
lattice circumventing the cooling problem. Our proposal exploits the idea of
tuning the interaction in a characteristically low entropy state, a
band-insulator in an optical bilayer system, to obtain a superfluid. By
performing a detailed analysis of the model including fluctuations and
augmented by a variational quantum Monte Carlo calculations of the ground
state, we show that the superfluid state obtained has high transition
temperature of the order of the hopping energy. Our system is designed to
suppress other competing orders such as a charge density wave. We suggest a
laboratory realization of this model via an orthogonally shaken optical lattice
bilayer.Comment: 5 pages, 7 figures, typos fixed, figures modifie
Synchronous and Asynchronous Mott Transitions in Topological Insulator Ribbons
We address how the nature of linearly dispersing edge states of two
dimensional (2D) topological insulators evolves with increasing
electron-electron correlation engendered by a Hubbard like on-site repulsion
in finite ribbons of two models of topological band insulators. Using an
inhomogeneous cluster slave rotor mean-field method developed here, we show
that electronic correlations drive the topologically nontrivial phase into a
Mott insulating phase via two different routes. In a synchronous transition,
the entire ribbon attains a Mott insulating state at one critical that
depends weakly on the width of the ribbon. In the second, asynchronous route,
Mott localization first occurs on the edge layers at a smaller critical value
of electronic interaction which then propagates into the bulk as is further
increased until all layers of the ribbon become Mott localized. We show that
the kind of Mott transition that takes place is determined by certain
properties of the linearly dispersing edge states which characterize the
topological resilience to Mott localization.Comment: 4+ pages, 5 figure
Sensory organ like response determines the magnetism of zigzag-edged honeycomb nanoribbons
We present an analytical theory for the magnetic phase diagram for zigzag
edge terminated honeycomb nanoribbons described by a Hubbard model with an
interaction parameter U . We show that the edge magnetic moment varies as ln U
and uncover its dependence on the width W of the ribbon. The physics of this
owes its origin to the sensory organ like response of the nanoribbons,
demonstrating that considerations beyond the usual Stoner-Landau theory are
necessary to understand the magnetism of these systems. A first order magnetic
transition from an anti-parallel orientation of the moments on opposite edges
to a parallel orientation occurs upon doping with holes or electrons. The
critical doping for this transition is shown to depend inversely on the width
of the ribbon. Using variational Monte-Carlo calculations, we show that
magnetism is robust to fluctuations. Additionally, we show that the magnetic
phase diagram is generic to zigzag edge terminated nanostructures such as
nanodots. Furthermore, we perform first principles modeling to show how such
magnetic transitions can be realized in substituted graphene nanoribbons.Comment: 5 pages, 5 figure
Coexistence of magnetism and superconductivity in a t-J bilayer
We investigate coexistence of antiferromagnetic and superconducting
correlations in bilayered materials using a two-dimensional t-J model with
couplings across the layers using variational Monte Carlo calculations. It is
found that the underdoped regime supports a coexisting phase, beyond which the
(d-wave) superconducting state becomes stable. Further, the effects of
interplanar coupling parameters on the magnetic and superconducting
correlations as a function of hole doping are studied in details. The magnetic
correlations are found to diminish with increasing interplanar hopping away
from half filling, while the exchange across the layers strengthens interplanar
antiferromagnetic correlations both at and away from half filling. The
superconducting correlations show more interesting features where larger
interplanar hopping considerably reduces planar correlations at optimal doping,
while an opposite behaviour, i.e. stabilisation of the superconducting state is
realised in the overdoped regime, with the interplanar exchange all the while
playing a dormant role.Comment: 8 pages, 9 figures, RevTex4, Submitted to Phys. Rev.
EvoCut : A new Generalization of Albert-Barab\'asi Model for Evolution of Complex Networks
With the evolution of social networks, the network structure shows dynamic
nature in which nodes and edges appear as well as disappear for various
reasons. The role of a node in the network is presented as the number of
interactions it has with the other nodes. For this purpose a network is modeled
as a graph where nodes represent network members and edges represent a
relationship among them. Several models for evolution of social networks has
been proposed till date, most widely accepted being the Barab\'asi-Albert
\cite{Network science} model that is based on \emph{preferential attachment} of
nodes according to the degree distribution. This model leads to generation of
graphs that are called \emph{Scale Free} and the degree distribution of such
graphs follow the \emph{power law}. Several generalizations of this model has
also been proposed. In this paper we present a new generalization of the model
and attempt to bring out its implications in real life
Utility of telemedicine in COVID-19 pandemic: our experience at a tertiary cancer center in North East India
Background: Telemedicine is a very useful tool of communication between the doctor and the patient. The aim of this study was to find out the utility of telemedicine during the lockdown period of COVID-19 pandemic in North East India.Methods: It is a cross sectional study among the cancer patients at our center on follow up or ongoing treatment and analysis of all the data acquired from telephonic conversation with our patients from 30th March, 2020 to 3rd May, 2020. Have contacted 4181 patients during this period over phone. All phone calls were done by respective department doctors.Results: From the demographic data, we get that 35.4% of patients were at good physical condition, 3.5% with poor general condition, 11.6% patients having ongoing treatment in our institute, 21.1% patients expired, 0.9% patients have nonmalignant diagnosis, 1.4% patients left the institute due to various reasons. Analyzed this data with brain storming sessions amongst the COVID-19 task force doctors and tried to find out solutions of each problem.Conclusions: Telemedicine cannot replace conventional method of in person treatment, but it proved to be a useful tool during the COVID-19 pandemic for patient follow up and treatment of cancer patients