9,124 research outputs found
Classical and quantum depinning of a domain wall with a spin-polarized current
We study in detail the classical and quantum depinning of a domain wall (DW)
induced by a fast-varying spin-polarized current. By confirming the adiabatic
condition for calculating the spin-torque in fast-varying current case, we show
that the time-dependent spin current has two critical values that determine the
classical depinning of DW. This discovery successfully explains the recent
experiments. Furthermore, a feasible way is proposed to lower the threshold of
spin currents and control the direction of DW's motion. Finally, the quantum
properties for the depinning of DW are also investigated in this paper.Comment: 8 pages, 3 figure
Amino acid substitution matrices for protein conformation identification
Methods for alignment of protein sequences typically measure similarity by
using substitution matrix with scores for all possible exchanges of one amino
acid with another. Although widely used, the matrices derived from homologous
sequence segments, such as Dayhoff's PAM matrices and Henikoff's BLOSUM
matrices, are not specific for protein conformation identification. Using a
different approach, we got many amino acid segment blocks. For each of them,
the protein secondary structure is identical. Based on these blocks, we have
derived new amino acid substitution matrices. The application of these matrices
led to marked improvements in conformation segment search and homologues
detection in twilight zone.Comment: 13 pages, 1 figur
Symmetry-protected gapless spin liquids on the strained honeycomb lattice
By including a material-relevant off-diagonal interaction called the
term into the Kitaev model and introducing spatial anisotropy in the
interaction strength on the honeycomb lattice, we obtain a series of nodal
Z quantum spin liquids (QSLs) from parton approach. These QSLs share the
same projective symmetry group and are characterized by certain numbers of
symmetry-protected Majorana cones in their low-energy excitation spectrum. We
illustrate that the physical properties of the QSLs are dependent on the
information of the cones. Using the method, we analyze the
chirality of every cone with respect to mass generating perturbations.
Especially, for an applied external magnetic field, we provide the maximum-mass
field-orientation for every cone. Thus, for arbitrarily oriented weak magnetic
fields, we can immediately read out the Chern number of the system and the
properties of the resultant chiral spin liquids. The new gapless QSLs predicted
in our phase diagrams are promising to be realized experimentally by exerting
uniaxial pressure to tune the anisotropy of the interaction strength. We
further show that all these QSLs can be distinguished by measurable quantities.
Based on the study of these QSL phases, we conclude that a complete
classification of nodal QSLs with certain symmetry should include not only the
projective symmetry groups but also the information of the cones, {\it i.e.},
their total number, their chiralities, and the way in which they are
symmetry-related.Comment: 19 pages, 13 figures, 5 tabl
Dirac and Chiral Quantum Spin Liquids on the Honeycomb Lattice in a Magnetic Field
Motivated by recent experimental observations in -RuCl, we study
the - model on the honeycomb lattice in an external magnetic field.
By a slave-particle representation and Variational Monte Carlo calculations, we
reproduce the phase transition from zigzag magnetic order to a field-induced
disordered phase. The nature of this state depends crucially on the field
orientation. For particular field directions in the honeycomb plane, we find a
gapless Dirac spin liquid, in agreement with recent experiments on
-RuCl. For a range of out-of-plane fields, we predict the existence
of a Kalmeyer-Laughlin-type chiral spin liquid, which would show an
integer-quantized thermal Hall effect.Comment: 5+9 pages, 4+6 figure
A protein structural alphabet and its substitution matrix CLESUM
By using a mixture model for the density distribution of the three pseudobond
angles formed by atoms of four consecutive residues, the local
structural states are discretized as 17 conformational letters of a protein
structural alphabet. This coarse-graining procedure converts a 3D structure to
a 1D code sequence. A substitution matrix between these letters is constructed
based on the structural alignments of the FSSP database.Comment: 10 page
Irreducible Projective Representations and Their Physical Applications
An eigenfunction method is applied to reduce the regular projective
representations (Reps) of finite groups to obtain their irreducible projective
Reps. Anti-unitary groups are treated specially, where the decoupled factor
systems and modified Schur's lemma are introduced. We discuss the applications
of irreducible Reps in many-body physics. It is shown that in symmetry
protected topological phases, geometric defects or symmetry defects may carry
projective Rep of the symmetry group; while in symmetry enriched topological
phases, intrinsic excitations (such as spinons or visons) may carry projective
Rep of the symmetry group. We also discuss the applications of projective Reps
in problems related to spectrum degeneracy, such as in search of models without
sign problem in quantum Monte Carlo Simulations.Comment: 41 pages, 1 figur
Fluctuation of Eigenvalues for Random Toeplitz and Related Matrices
Consider random symmetric Toeplitz matrices
with matrix entries being independent real random
variables such that
\be \mathbb{E}[a_{j}]=0, \ \ \mathbb{E}[|a_{j}|^{2}]=1 \ \ \textrm{for}\,\ \
j=0,1,2,...,\ee (homogeneity of 4-th moments)
\be{\kappa=\mathbb{E}[|a_{j}|^{4}],}\ee
\noindent and further (uniform boundedness)\be\sup\limits_{j\geq 0}
\mathbb{E}[|a_{j}|^{k}]=C_{k}<\iy\ \ \ \textrm{for} \ \ \ k\geq 3.\ee
Under the assumption of , we prove a central limit theorem for
linear statistics of eigenvalues for a fixed polynomial with degree .
Without the assumption, the CLT can be easily modified to a possibly non-normal
limit law. In a special case where 's are Gaussian, the result has been
obtained by Chatterjee for some test functions. Our derivation is based on a
simple trace formula for Toeplitz matrices and fine combinatorial analysis. Our
method can apply to other related random matrix models, including Hankel
matrices and product of several Toeplitz matrices in a flavor of free
probability theory etc. Since Toeplitz matrices are quite different from the
Wigner and Wishart matrices, our results enrich this topic.Comment: 27 pages, corrected small gap in proof of Theorem 1.1, added remark
1.
Manipulating Topological Edge Spins in One-Dimensional Optical Lattice
We propose to observe and manipulate topological edge spins in 1D optical
lattice based on currently available experimental platforms. Coupling the
atomic spin states to a laser-induced periodic Zeeman field, the lattice system
can be driven into a symmetry protected topological (SPT) phase, which belongs
to the chiral unitary (AIII) class protected by particle number conservation
and chiral symmetries. In free-fermion case the SPT phase is classified by a
invariant which reduces to with interactions. The zero edge modes of
the SPT phase are spin-polarized, with left and right edge spins polarized to
opposite directions and forming a topological spin-qubit (TSQ). We demonstrate
a novel scheme to manipulate the zero modes and realize single spin control in
optical lattice. The manipulation of TSQs has potential applications to quantum
computation.Comment: 4+pages+Supplementary material. Details for the model realization has
been added to the supplementary material. Accepted by Phys. Rev. Let
Classification of quantum critical states of integrable antiferromagnetic spin chains and their correspondent two-dimensional topological phases
We examine the effective field theory of the Bethe ansatz integrable
Heisenberg antiferromagnetic spin chains. It shows that the quantum critical
theories for the integer spin-S chains should be characterized by the
SO(3)level-S Wess-Zumino-Witten model, and classified by the third cohomology
group . Depending on the parity of spin S, this integer
classification is further divided into two distinct universality classes, which
are associated with two completely different conformal field theories: the
even-S chains have gapless bosonic excitations and the odd-S chains have both
bosonic and fermionic excitations. We further show that these two classes of
critical states correspond to the boundary states of two distinct topological
phases in two dimension, which can be described by two-dimensional doubled
SO(3) topological Chern-Simons theory and topological spin theory,
respectively.Comment: 5 pages, 1 tabl
Symmetry protected topological phases in spin-1 ladders and their phase transitions
We study two-legged spin-1 ladder systems with symmetry
group, where is discrete spin rotational symmetry and means
interchain reflection symmetry. The system has one trivial phase and seven
nontrivial symmetry protected topological (SPT) phases. We construct
Hamiltonians to realize all of these SPT phases and study the phase transitions
between them. Our numerical results indicate that there is no direct continuous
transition between any two SPT phases we studied. We interpret our results via
topological nonlinear sigma model effective field theory, and further
conjecture that generally there is no direct continuous transition between two
SPT phases in one dimension if the symmetry group is discrete at all length
scales.Comment: 20 pages, 8 figures, published versio
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