35,297 research outputs found
Dynamical Mean Field Theory of the Gutzwiller-projected BCS Hamiltonian: Phase Fluctuations and the Pseudogap
One of the most prominent problems in high temperature superconductivity is
the nature of the pseudogap phase in underdoped regimes; particularly important
is the role of phase fluctuations. The Gutzwiller-projected BCS Hamiltonian is
a useful model for high temperature superconductivity due to an exact mapping
to the Heisenberg model at half filling and generally a very close connection
to the t-J model at moderate doping. We develop the dynamical mean field theory
for the d-wave BCS Hamiltonian with on-site repulsive interaction, ,
physically imposing the partial Gutzwiller projection. For results, two
pseudogap energy scales are identified: one associated with the bare pairing
gap for the singlet formation and the other with the local phase coherence. The
real superconducting gap determined from sharp coherence peaks in the density
of states shows strong renormalization from the bare value due to .Comment: 8 pages, 5 figure
A Passage to Topological Matter: Colloquium
Topological matter has become one of the most important subjects in
contemporary condensed matter physics. Here, I would like to provide a
pedagogical review explaining some of the main ideas, which were pivotal in
establishing topological matter as such an important subject. Specifically, I
explain how the integer quantum Hall state played the role as a prototype for
topological insulator, eventually leading to the concept of topological matter
in general. The topological nature of the integer quantum Hall state is best
represented by the Thouless-Kohmoto-Nightingale-den Nijs, or so-called TKNN
formula, which connects between the Berry phase and the Hall conductivity. The
topological non-triviality of topological insulator stems from the existence of
a Dirac monopole in an appropriate, but often hidden Hamiltonian parameter
space. Interestingly, having the identical Dirac monopole structure, the
Hamiltonian describing the Rabi oscillation bears the essence of topological
insulator. The concept of topological matter has expanded to include
topological semimetals such as Weyl and Dirac semimetals. A final frontier in
the research of topological matter is the interaction-induced topological
phases of matter, namely, the fractional Chern and topological insulators. The
existence of the fractional Chern and topological insulators has been proposed
theoretically by drawing an analogy from the fractional quantum Hall states.
The gist of this proposal is explained along with some of its issues. I
conclude this review by discussing some of the future directions in the
research of topological matter.Comment: 16 pages, 8 figure
Theory of Tunneling in the Exciton Condensate of Bilayer Quantum Hall Systems
We develop a theory of interlayer tunneling in the exciton condensate of
bilayer quantum Hall systems, which predicts strongly enhanced, but finite,
tunneling conductance peaks near zero bias even at zero temperature. It is
emphasized that, though this strongly enhanced tunneling originates from
spontaneous interlayer phase coherence, it is fundamentally not the Josephson
effect. Because of strong interlayer correlation, the bilayer system behaves as
a single system so that conventional tunneling theories treating two layers as
independent systems are not applicable. Based on our theory, we compute the
height of conductance peak as a function of interlayer distance, which is in
good agreement with experiment.Comment: 5 pages, 5 figure
Ground states of quantum antiferromagnets in two dimensions
We explore the ground states and quantum phase transitions of
two-dimensional, spin S=1/2, antiferromagnets by generalizing lattice models
and duality transforms introduced by Sachdev and Jalabert (Mod. Phys. Lett. B
4, 1043 (1990), http://onsager.physics.yale.edu/p32.pdf). The `minimal' model
for square lattice antiferromagnets is a lattice discretization of the quantum
non-linear sigma model, along with Berry phases which impose quantization of
spin. With full SU(2) spin rotation invariance, we find a magnetically ordered
ground state with Neel order at weak coupling, and a confining paramagnetic
ground state with bond charge (e.g. spin Peierls) order at strong coupling. We
study the mechanisms by which these two states are connected in intermediate
coupling. We extend the minimal model to study different routes to
fractionalization and deconfinement in the ground state, and also generalize it
to cases with a uniaxial anisotropy (the spin symmetry group is then U(1)). For
the latter systems, fractionalization can appear by the pairing of vortices in
the staggered spin order in the easy-plane; however, we argue that this route
does not survive the restoration of SU(2) spin symmetry. For SU(2) invariant
systems we study a separate route to fractionalization associated with the
Higgs phase of a complex boson measuring non-collinear, spiral spin
correlations: we present phase diagrams displaying competition between magnetic
order, bond charge order, and fractionalization, and discuss the nature of the
quantum transitions between the various states. A strong check on our methods
is provided by their application to S=1/2 frustrated antiferromagnets in one
dimension: here, our results are in complete accord with those obtained by
bosonization and by the solution of integrable models.Comment: 44 pages, 19 figures; SS web page: http://pantheon.yale.edu/~subir
(v2) updated discussion of literatur
Distributed Topology Design for Network Coding Deployed Large-scale Sensor Networks
In this paper, we propose a solution to the distributed topology formation
problem for large-scale sensor networks with multi-source multicast flows. The
proposed solution is based on game-theoretic approaches in conjunction with
network coding. The proposed algorithm requires significantly low computational
complexity, while it is known as NP-hard to find an optimal topology for
network coding deployed multi-source multicast flows. In particular, we
formulate the problem of distributed network topology formation as a network
formation game by considering the nodes in the network as players that can take
actions for making outgoing links. The proposed solution decomposes the
original game that consists of multiple players and multicast flows into
independent link formation games played by only two players with a unicast
flow. We also show that the proposed algorithm is guaranteed to determine at
least one stable topology. Our simulation results confirm that the
computational complexity of the proposed solution is low enough for practical
deployment in large-scale networks
Two-species branching annihilating random walks with one offspring
We study the effects of hard core (HC) interactions between different species
of particles on two-species branching annihilating random walks with one
offspring(BAW(1)). The single-species model belongs to the directed
percolation (DP) universality class. In the BAW(1) model, a particle
creates one particle of the same species in its neighborhood with the
probability and of the different species with ,
where is the hopping probability. Without HC interactions, this model
always exhibits the DP-type absorbing transition for all . Even with HC
interactions, the nature of the phase transitions does not change except near
, where the HC interaction destabilizes and completely wipes away the
absorbing phase. The model is always active except at the annihilation fixed
point of zero branching rate (). Critical behavior near the annihilation
fixed point is characterized by exponents and
.Comment: 4 pages, APCTP international workshop on similarity in diversity
(Seoul, Korea; Aug.24, 2000): To appear in Journal of the Korean Physical
Societ
Floquet Topological Semimetal with Nodal Helix
Topological semimetals with nodal line are a novel class of topological
matter extending the concept of topological matter beyond topological
insulators and Weyl/Dirac semimetals. Here, we show that a Floquet topological
semimetal with nodal helix can be generated by irradiating graphene or the
surface of a topological insulator with circularly polarized light. Nodal helix
is a form of nodal line running across the Brillouin zone with helical winding.
Specifically, it is shown that the dynamics of irradiated graphene is described
by the time Stark Hamiltonian, which can host a Floquet topological insulator
and a weakly driven Floquet topological semimetal with nodal helix in the high
and low frequency limits, respectively. It is predicted that, at low frequency,
the shift of the Zak phase generates a topological discontinuity along
the projected nodal helix in the momentum spectrum of the Floquet states. At
intermediate frequency, this topological discontinuity can create an
interesting change of patterns in the quasienergy dispersion of the Floquet
states.Comment: 11 pages, 5 figure
Spin separation due to an inherent spontaneous symmetry breaking of the fractional topological insulator
Motivated by the close analogy with the fractional quantum Hall states
(FQHSs), fractional Chern insulators (FCIs) are envisioned as strongly
correlated, incompressible states emerging in a fractionally filled, (nearly)
flat band with non-trivial Chern number. Built upon this vision, fractional
topological insulators (FTIs) have been proposed as being composed of two
independent copies of the FCI with opposite Chern numbers for different spins,
preserving the time-reversal symmetry as a whole. An important question is if
the correlation between electrons with different spins can be really ignored.
To address this question, we investigate the effects of correlation in the
presence of spin-dependent holomorphicity, i.e., electrons of one spin species
reside in the holomorphic lowest Landau level, while those of the other in the
antiholomorphic counterpart. By constructing and performing exact
diagonalization of an appropriate model Hamiltonian, here, we show that
generic, strongly correlated, fractionally filled states with spin-dependent
holomorphicity cannot be described as two independent copies of the FQHS,
suggesting that FTIs in the lattice cannot be described as those of the FCI
either. Fractionally filled states in this system are generally compressible
except at half filling, where an insulating state called the half-filled
spin-holomorphic FTI occurs. It is predicted that the half-filled
spin-holomorphic FTI is susceptible to an inherent spontaneous symmetry
breaking, leading to the spatial separation of spins.Comment: 9 pages and 5 figure
Network Coding Based Evolutionary Network Formation for Dynamic Wireless Networks
In this paper, we aim to find a robust network formation strategy that can
adaptively evolve the network topology against network dynamics in a
distributed manner. We consider a network coding deployed wireless ad hoc
network where source nodes are connected to terminal nodes with the help of
intermediate nodes. We show that mixing operations in network coding can induce
packet anonymity that allows the inter-connections in a network to be
decoupled. This enables each intermediate node to consider complex network
inter-connections as a node-environment interaction such that the Markov
decision process (MDP) can be employed at each intermediate node. The optimal
policy that can be obtained by solving the MDP provides each node with optimal
amount of changes in transmission range given network dynamics (e.g., the
number of nodes in the range and channel condition). Hence, the network can be
adaptively and optimally evolved by responding to the network dynamics. The
proposed strategy is used to maximize long-term utility, which is achieved by
considering both current network conditions and future network dynamics. We
define the utility of an action to include network throughput gain and the cost
of transmission power. We show that the resulting network of the proposed
strategy eventually converges to stationary networks, which maintain the states
of the nodes. Moreover, we propose to determine initial transmission ranges and
initial network topology that can expedite the convergence of the proposed
algorithm. Our simulation results confirm that the proposed strategy builds a
network which adaptively changes its topology in the presence of network
dynamics. Moreover, the proposed strategy outperforms existing strategies in
terms of system goodput and successful connectivity ratio.Comment: IEEE Transactions on Mobile Computing (Early Access
Towards Interactive Object-Oriented Programming
To represent interactive objects, we propose a choice-disjunctive declaration
statement of the form S R where S;R are the (procedure or field) declaration
statements within a class. This statement has the following semantics: request
the user to choose one between S and R when an object of this class is created.
This statement is useful for representing interactive objects that require
interactions with the user
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