43,433 research outputs found
Micromagnetic Simulations of Ferromagnetic Rings
Thin nanomagnetic rings have generated interest for fundamental studies of
magnetization reversal and also for their potential in various applications,
particularly as magnetic memories. They are a rare example of a geometry in
which an analytical solution for the rate of thermally induced magnetic
reversal has been determined, in an approximation whose errors can be estimated
and bounded. In this work, numerical simulations of soft ferromagnetic rings
are used to explore aspects of the analytical solution. The evolution of the
energy near the transition states confirms that, consistent with analytical
predictions, thermally induced magnetization reversal can have one of two
intermediate states: either constant or soliton-like saddle configurations,
depending on ring size and externally applied magnetic field. The results
confirm analytical predictions of a transition in thermally activated reversal
behavior as magnetic field is varied at constant ring size. Simulations also
show that the analytic one dimensional model continues to hold even for wide
rings
The graduation performance of technology business incubators in China's three tier cities: the role of incubator funding, technical support, and entrepreneurial mentoring
This study examines the effects of technology business incubator (TBI)’s funding, technical support and entrepreneurial mentoring on the graduation performance of new technology-based firms in China’s three tier cities. Using new dataset on all TBIs and incubated new technology-based firms from government surveys conducted over five consecutive years from 2009 to 2013 combined with archival and hand-collected data, we find the effects of incubator services on the early growth of new technology-based firms vary according to the local context. Technical support facilities and entrepreneurial mentoring from TBIs are found to have significantly and positively influenced the early development of the firms in the four most affluent tier 1 cities, whilst these effects become less pronounced for the tier 2 and tier 3 cities. These two services are also found to influence graduation performance in the government and university types of TBI respectively. Results support the notion that the effectiveness of an incubators services is shaped by the level of a city’s socio-economic development and that the city location of a TBI does impact the graduation performance of its incubatees
Symmetry protected topological orders of 1D spin systems with D2+T symmetry
In [Z.-X. Liu, M. Liu, X.-G. Wen, arXiv:1101.5680], we studied 8 gapped
symmetric quantum phases in S=1 spin chains %/ladders which respect a discrete
spin rotation and time reversal symmetries. In this
paper, using a generalized approach, we study all the 16 possible gapped
symmetric quantum phases of 1D integer spin systems with only symmetry.
Those phases are beyond Landau symmetry breaking theory and cannot be
characterized by local order parameters, since they do not break any symmetry.
They correspond to 16 symmetry protected topological (SPT) orders. We show that
all the 16 SPT orders can be fully characterized by the physical properties of
the symmetry protected degenerate boundary states (end `spins') at the ends of
a chain segment. So we can measure and distinguish all the 16 SPT orders
experimentally. We also show that all these SPT orders can be realized in S=1
spin ladder models. The gapped symmetric phases protected by subgroups of
are also studied. Again, all these phases can be distinguished by
physically measuring their end `spins'.Comment: 10+page
Translation-symmetry protected topological orders on lattice
In this paper we systematically study a simple class of translation-symmetry
protected topological orders in quantum spin systems using slave-particle
approach. The spin systems on square lattice are translation invariant, but may
break any other symmetries. We consider topologically ordered ground states
that do not spontaneously break any symmetry. Those states can be described by
Z2A or Z2B projective symmetry group. We find that the Z2A translation
symmetric topological orders can still be divided into 16 sub-classes
corresponding to 16 new translation-symmetry protected topological orders. We
introduced four topological indices at , , , to characterize those 16 new
topological orders. We calculated the topological degeneracies and crystal
momenta for those 16 topological phases on even-by-even, even-by-odd,
odd-by-even, and odd-by-odd lattices, which allows us to physically measure
such topological orders. We predict the appearance of gapless fermionic
excitations at the quantum phase transitions between those symmetry protected
topological orders. Our result can be generalized to any dimensions. We find
256 translation-symmetry protected Z2A topological orders for a system on 3D
lattice
Phase structures of strong coupling lattice QCD with overlap fermions at finite temperature and chemical potential
We perform the first study of lattice QCD with overlap fermions at finite
temperature and chemical potential . We start from the Taylor expanded
overlap fermion action, and derive in the strong coupling limit the effective
free energy by mean field approximation. On the () plane and in the
chiral limit, there is a tricritical point, separating the second order chiral
phase transition line at small and large , and first order chiral
phase transition line at large and small
Complete classification of 1D gapped quantum phases in interacting spin systems
Quantum phases with different orders exist with or without breaking the
symmetry of the system. Recently, a classification of gapped quantum phases
which do not break time reversal, parity or on-site unitary symmetry has been
given for 1D spin systems in [X. Chen, Z.-C. Gu, and X.-G. Wen, Phys. Rev. B
\textbf{83}, 035107 (2011); arXiv:1008.3745]. It was found that, such symmetry
protected topological (SPT) phases are labeled by the projective
representations of the symmetry group which can be viewed as a symmetry
fractionalization. In this paper, we extend the classification of 1D gapped
phases by considering SPT phases with combined time reversal, parity, and/or
on-site unitary symmetries and also the possibility of symmetry breaking. We
clarify how symmetry fractionalizes with combined symmetries and also how
symmetry fractionalization coexists with symmetry breaking.
In this way, we obtain a complete classification of gapped quantum phases in
1D spin systems. We find that in general, symmetry fractionalization, symmetry
breaking and long range entanglement(present in 2 or higher dimensions)
represent three main mechanisms to generate a very rich set of gapped quantum
phases. As an application of our classification, we study the possible SPT
phases in 1D fermionic systems, which can be mapped to spin systems by
Jordan-Wigner transformation.Comment: 15 pages, 3 figure
Quantum orders in an exact soluble model
We find all the exact eigenstates and eigenvalues of a spin-1/2 model on
square lattice: . We show
that the ground states for have different quantum orders
described by Z2A and Z2B projective symmetry groups. The phase transition at
represents a new kind of phase transitions that changes quantum orders
but not symmetry. Both the Z2A and Z2B states are described by lattice
gauge theories at low energies. They have robust topologically degenerate
ground states and gapless edge excitations.Comment: 4 pages, RevTeX4, More materials on topological/quantum orders and
quantum computing can be found in http://dao.mit.edu/~we
Plasmon-exciton polaritons in 2D semiconductor/metal interfaces
The realization and control of polaritons is of paramount importance in the
prospect of novel photonic devices. Here, we investigate the emergence of
plasmon-exciton polaritons in hybrid structures consisting of a two-dimensional
(2D) transition metal dichalcogenide (TMDC) deposited onto a metal substrate or
coating a metallic thin-film. We determine the polaritonic spectrum and show
that, in the former case, the addition of a top dielectric layer, and, in the
latter, the thickness of the metal film,can be used to tune and promote
plasmon-exciton interactions well within the strong coupling regime. Our
results demonstrate that Rabi splittings exceeding 100 meV can be readily
achieved in planar dielectric/TMDC/metal structures under ambient conditions.
We thus believe that this work provides a simple and intuitive picture to
tailor strong coupling in plexcitonics, with potential applications for
engineering compact photonic devices with tunable optical properties.Comment: 6 pages, including 5 figures and reference
- …