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Search Results: 1 - 10 of 79387 matches for " Yuan-Ming Lu "
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Symmetric $Z_2$ spin liquids and their neighboring phases on triangular lattice
Yuan-Ming Lu
Physics , 2015,
Abstract: Motivated by recent numerical discovery of a gapped spin liquid phase in spin-$1/2$ triangular-lattice $J_1$-$J_2$ Heisenberg model, we classify symmetric $Z_2$ spin liquids on triangular lattice in the Abrikosov-fermion representation. We find 20 phases with distinct spinon symmetry quantum numbers, 8 of which have their counterparts in the Schwinger-boson representation. Among them we identify 2 promising candidates (#1 and #20), which can realize a gapped $Z_2$ spin liquid with up to next nearest neighbor mean-field amplitudes. We analyze their neighboring magnetic orders and valence bond solid patterns, and find one state (#20) that is connected to 120-degree Neel order by a continuous quantum phase transition. We also identify gapped nematic $Z_2$ spin liquids in the neighborhood of the symmetric states and find 3 promising candidates (#1, #6 and #20).
Spin liquids on a honeycomb lattice: Projective Symmetry Group study of Schwinger fermion mean-field theory
Yuan-Ming Lu,Ying Ran
Physics , 2010, DOI: 10.1103/PhysRevB.84.024420
Abstract: Spin liquids are novel states of matter with fractionalized excitations. A recent numerical study of Hubbard model on a honeycomb lattice\cite{Meng2010} indicates that a gapped spin liquid phase exists close to the Mott transition. Using Projective Symmetry Group, we classify all the possible spin liquid states by Schwinger fermion mean-field approach. We find there is only one fully gapped spin liquid candidate state: "Sublattice Pairing State" that can be realized up to the 3rd neighbor mean-field amplitudes, and is in the neighborhood of the Mott transition. We propose this state as the spin liquid phase discovered in the numerical work. To understand whether SPS can be realized in the Hubbard model, we study the mean-field phase diagram in the $J_1-J_2$ spin-1/2 model and find an s-wave pairing state. We argue that s-wave pairing state is not a stable phase and the true ground state may be SPS. A scenario of a continuous phase transition from SPS to the semimetal phase is proposed. This work also provides guideline for future variational studies of Gutzwiller projected wavefunctions.
Symmetry protected fractional Chern insulators and fractional topological insulators
Yuan-Ming Lu,Ying Ran
Physics , 2011, DOI: 10.1103/PhysRevB.85.165134
Abstract: In this paper we construct fully symmetric wavefunctions for the spin-polarized fractional Chern insulators (FCI) and time-reversal-invariant fractional topological insulators (FTI) in two dimensions using the parton approach. We show that the lattice symmetry gives rise to many different FCI and FTI phases even with the same filling fraction $\nu$ (and the same quantized Hall conductance $\sigma_{xy}$ in FCI case). They have different symmetry-protected topological orders, which are characterized by different projective symmetry groups. We mainly focus on FCI phases which are realized in a partially filled band with Chern number one. The low-energy gauge groups of a generic $\sigma_{xy}=1/m\cdot e^2/h$ FCI wavefunctions can be either $SU(m)$ or the discrete group $Z_m$, and in the latter case the associated low-energy physics are described by Chern-Simons-Higgs theories. We use our construction to compute the ground state degeneracy. Examples of FCI/FTI wavefunctions on honeycomb lattice and checkerboard lattice are explicitly given. Possible non-Abelian FCI phases which may be realized in a partially filled band with Chern number two are discussed. Generic FTI wavefunctions in the absence of spin conservation are also presented whose low-energy gauge groups can be either $SU(m)\times SU(m)$ or $Z_m\times Z_m$. The constructed wavefunctions also set up the framework for future variational Monte Carlo simulations.
Theory and classification of interacting 'integer' topological phases in two dimensions: A Chern-Simons approach
Yuan-Ming Lu,Ashvin Vishwanath
Physics , 2012, DOI: 10.1103/PhysRevB.86.125119
Abstract: We study topological phases of interacting systems in two spatial dimensions in the absence of topological order (i.e. with a unique ground state on closed manifolds and no fractional excitations). These are the closest interacting analogs of integer Quantum Hall states, topological insulators and superconductors. We adapt the well-known Chern-Simons {K}-matrix description of Quantum Hall states to classify such `integer' topological phases. Our main result is a general formalism that incorporates symmetries into the {{K}}-matrix description. Remarkably, this simple analysis yields the same list of topological phases as a recent group cohomology classification, and in addition provides field theories and explicit edge theories for all these phases. The bosonic topological phases, which only appear in the presence of interactions and which remain well defined in the presence of disorder include (i) bosonic insulators with a Hall conductance quantized to even integers (ii) a bosonic analog of quantum spin Hall insulators and (iii) a bosonic analog of a chiral topological superconductor, whose K matrix is the Cartan matrix of Lie group E$_8$. We also discuss interacting fermion systems where symmetries are realized in a projective fashion, where we find the present formalism can handle a wider range of symmetries than a recent group super-cohomology classification. Lastly we construct microscopic models of these phases from coupled one-dimensional systems.
Classification and Properties of Symmetry Enriched Topological Phases: A Chern-Simons approach with applications to Z2 spin liquids
Yuan-Ming Lu,Ashvin Vishwanath
Physics , 2013,
Abstract: We study 2+1 dimensional phases with topological order, such as fractional quantum Hall states and gapped spin liquids, in the presence of global symmetries. Phases that share the same topological order can then differ depending on the action of symmetry, leading to symmetry enriched topological (SET) phases. Here we present a K-matrix Chern-Simons approach to identify all distinct phases with Abelian topological order, in the presence of unitary or anti-unitary global symmetries . A key step is the identification of an edge sewing condition that is used to check if two putative phases are indeed distinct. We illustrate this method for the case of $Z_2$ topological order ($Z_2$ spin liquids), in the presence of an internal Z$_2$ global symmetry. We find 6 distinct phases. The well known quantum number fractionalization patterns account for half of these states. Phases also differ due to the addition of a symmetry protected topological (SPT) phase. Also, we allow for the unconventional possibility that anyons are exchanged by the symmetry. This leads to 2 additional phases with symmetry protected Majorana edge modes. Other routes to realizing protected edge states in SET phases are identified. Symmetry enriched Laughlin states and double semion theories are also discussed. Two surprising lessons that emerge are: (i) gauging the global symmetry of distinct SET phases always lead to different topological orders with the same total quantum dimension, (ii) gauge theories with distinct Dijkgraaf-Witten topological terms may have the same topological order.
$Z_2$ spin liquid and chiral antiferromagnetic phase in Hubbard model on the honeycomb lattice: duality between Schwinger-fermion and Schwinger-boson representations
Yuan-Ming Lu,Ying Ran
Physics , 2010, DOI: 10.1103/PhysRevB.84.024420
Abstract: In our previous work, we identify the Sublattice-Pairing State (SPS) in Schwinger-fermion representation as the spin liquid phase discovered in recent numerical study on a honeycomb lattice. In this paper, we show that SPS is identical to the zero-flux $Z_2$ spin liquid in Schwinger-boson representation found by Wang\cite{Wang2010} by an explicit duality transformation. SPS is connected to an \emph{unusual} antiferromagnetic ordered phase, which we term as chiral-antiferromagnetic (CAF) phase, by an O(4) critical point. CAF phase breaks the SU(2) spin rotation symmetry completely and has three Goldstone modes. Our results indicate that there is likely a hidden phase transition between CAF phase and simple AF phase at large $U/t$. We propose numerical measurements to reveal the CAF phase and the hidden phase transition.
Majorana fermions in spin-singlet nodal superconductors with coexisting non-collinear magnetic order
Yuan-Ming Lu,Ziqiang Wang
Physics , 2012, DOI: 10.1103/PhysRevLett.110.096403
Abstract: Realizations of Majorana fermions in solid state materials have attracted great interests recently in connection to topological order and quantum information processing. We propose a novel way to create Majorana fermions in superconductors. We show that an incipient non-collinear magnetic order turns a spin-singlet superconductor with nodes into a topological superconductor with a stable Majorana bound state in the vortex core; at a topologically-stable magnetic point defect; and on the edge. We argue that such an exotic non-Abelian phase can be realized in extended t-J models on the triangular and square lattices. It is promising to search for Majorana fermions in correlated electron materials where nodal superconductivity and magnetism are two common caricatures.
Symmetry-induced anyon "breeding" in fractional quantum Hall states
Yuan-Ming Lu,Lukasz Fidkowski
Physics , 2013, DOI: 10.1103/PhysRevB.89.115321
Abstract: An exotic feature of the fractional quantum Hall effect is the emergence of anyons, which are quasiparticle excitations with fractional statistics. In the presence of a symmetry, such as $U(1)$ charge conservation, it is well known that anyons can carry fractional symmetry quantum numbers. In this work we reveal a different class of symmetry realizations: i.e. anyons can "breed" in multiples under symmetry operation. We focus on the global Ising ($Z_2$) symmetry and show examples of these unconventional symmetry realizations in Laughlin-type fractional quantum Hall states. One remarkable consequence of such an Ising symmetry is the emergence of anyons on the Ising symmetry domain walls. We also provide a mathematical framework which generalizes this phenomenon to any Abelian topological orders.
The Application of TELES in the Enhancement of Disaster Response and Rescue Policy in Tainan Area: A Study Following 0206 Earthquake in Tainan  [PDF]
Yuan-Ming Chiu
Journal of Power and Energy Engineering (JPEE) , 2018, DOI: 10.4236/jpee.2018.61001
Abstract: The 2016 Meinong earthquake in Kaohsiung city caused significant casualties and property damage in Tainan. This study used TELES to simulate the earthquake scenario parameters, such as PGA distribution, casualty case, liquefaction potential index and the total number of occurring fires based on the Meinong fault seismic source. Finally, according to the most damaging areas of the simulation results, we propose to strengthen the important buildings and facilities such as the reservoir area, and the emergency responsive hospitals for a better seismic risk estimation and the earthquake disaster prevention and response. According to the simulation results analysis, we found the areas with the strongest magnitude were in Meinong Dist. in Kaohsiung city. Nevertheless, the areas suffering the serious disasters were in Tainan city. Due to the serious disasters occurring in Tainan city, it is crucial to discuss how to perform anti-earthquake practices in Tainan area and how to coordinate the support from different cities and counties.
Correlation-hole induced paired quantum Hall states in lowest Landau level
Yuan-Ming Lu,Yue Yu,Ziqiang Wang
Physics , 2009, DOI: 10.1103/PhysRevLett.105.216801
Abstract: A theory is developed for the paired even-denominator fractional quantum Hall states in the lowest Landau level. We show that electrons bind to quantized vortices to form composite fermions, interacting through an exact instantaneous interaction that favors chiral p-wave pairing. Two canonically dual pairing gap functions are related by the bosonic Laughlin wavefunction (Jastraw factor) due to the correlation holes. We find that the ground state is the Moore-Read pfaffian in the long wavelength limit for weak Coulomb interactions, a new pfaffian of an oscillatory pairing function for intermediate interactions, and a Read-Rezayi composite Fermi liquid beyond a critical interaction strength. Our findings are consistent with recent experimental observations of the 1/2 and 1/4 fractional quantum Hall effects in asymmetric wide quantum wells.
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