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2013 Southern California Condensed Matter Theory Meeting at CSU Long Beach, April 12, 2013

Following several LA area Theory meetings we gather once again to exchange ideas on condensed matter theory. All are welcome! You will find below general informations on the meeting, the program and the abstracts.

Post-meeting Survey

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General information

Please RSVP here if you intend to come so we have an idea how many need lunch/coffee/parking (last minute decisions to attend are still accepted).

Venue: The meeting will take place in the new Hall of Science (HSCI) room 102.

Access: Information on the access to campus can be obtained here.

Campus map: A map of the campus can be found here.

Parking: You can obtain a parking permit from the Visitor Information Booth (open until 2pm only!) located at the main entrance of campus, on Beach Drive, entering from Bellflower Blvd. Parking lots 5, 4, 6 and 18 should be used in order of preference. But beware that the first two may be full. It takes 5min walk from lots 5 and 4, and 7-10 min from lots 6 and 18. Also, if you arrive after 2pm you will need to purchase a parking permit at a yellow booth for example on lots 17 or 1,2,3.

Lunch: Will be served at the site of the meeting from 12pm to 1pm. Coffee breaks will be at the site of the meeting as well.

Dinner: We will go to a restaurant nearby after the meeting (Diner pays his/her own meal).

The department of Physics & Astronomy at CSU Long Beach: Information can be found on our website

Organization: Andreas Bill (CSULB), Stephan Haas (USC), Michael Peterson (CSULB)

Organizing Committee: Y. Tserkovnyak (UCLA), A. Chernyshev (UCI), K. Shtengel/V. Ajik (UCR), G. Rafael/J.Alicea (Caltech), D.Arovas (UCSD), N. Kioussis/D. Sheng (CSUN), E. Rezay/JP.Rodriguez (CSULA), I.Tifrea (CSUF), A. Small (CalPoly Pomona)

Program (April 12, 2013)

Time (pm) Speaker / Activity Title of talk
12:00 - 01:00 Lunch Served at the meeting site (HSCI-102)
01:00 - 01:30 S. White
(UC Irvine)
Recent developments in spin liquids: DMRG prospective
01:30 - 02:00 C.Y. Hou
(Caltech / UC Riverside)
Ettingshausen effect due to Majorana modes
02:00 - 02:15 Coffee Break
02:15 - 02:45 J. Rodriguez
(CSU Los Angeles)
Iron-Pnictide High-Tc Superconductivity from the Limit of Local Magnetic Moments
02:45 - 03:15 R. Roy
Topological Adiabatic Cycles
03:15 - 03:45 Coffee Break
03:45 - 04:15 M. Peterson
(CSU Long Beach)
Fractional quantum Hall effect in graphene: effects of Landau level mixing
04:15 - 04:45 D.Clarke
Topological Quantum Engineering: Constructing exotic phases from known states of matter
04:45 - 05:00 Coffee Break
05:00 - 05:30 T. Albash
Fluctuation theorems for quantum processes
05:30 - 06:00 D. Sheng
(CSU Northridge)
Fractional Chern Insulator and its Modular Matrix
06:30 Dinner Place TBA (Diner pays...)


Recent developments in spin liquids: DMRG prospective (S. White)

A quantum spin liquid is a solid whose atoms have magnetic moments but, because of quantum fluctuations, these moments fluctuate like a liquid even at zero temperature. Two dimensional spin liquids have been suggested as a way to produce high temperature superconductivity, and to build quantum computers. Just as helium is the only element which is a liquid at zero temperature, 2D spin liquids have been extremely difficult to find, despite decades of effort, raising the question, do realistic spin liquids even exist? Recently, apparent spin liquids have been found experimentally, stimulating theoretical work to find simple model Hamiltonians of frustrated spin systems that have spin liquid ground states. In this talk, I will give a broad overview of spin liquids and then focus on our simulations of the kagome Heisenberg model, a simple, realistic model of some of the recent experimental spin liquids, where we find a spin liquid ground state.

Ettingshausen effect due to Majorana modes (C.Y. Hou)

The presence of Majorana zero-energy modes at vortex cores in a topological superconductor implies that each vortex carries an extra entropy s0, given by (kB/2) ln 2, that is independent of temperature. By utilizing this special property of Majorana modes, the edges of a topological superconductor can be cooled (or heated) by the motion of the vortices across the edges. As vortices flow in the transverse direction with respect to an external imposed supercurrent, due to the Lorentz force, a thermoelectric effect analogous to the Ettingshausen effect is expected to occur. We propose an experiment to observe this thermoelectric effect, which could directly probe the intrinsic entropy of Majorana zero-energy modes.

Iron-pnictide High-Tc Superconductivity from the limit of Local Magnetic Moments (J. Rodriguez)

We analyze a two-orbital t-J model over a square lattice of iron atoms by Schwinger-boson-slave-fermion mean-field theory and by exact diagonalization on a 4x4x2-site cluster. The low-energy spectrum of one hole exhibits electronic structure at 2D momenta (0,0) , (pi,0) and (0,pi) for Hund's Rule coupling near a critical value of moderate strength. Increasing Hund's Rule coupling off the critical point can lead to the absence of the Fermi-surface pockets centered at momentum (0,0). This effect potentially accounts for the same behavior observed recently in single-layer iron-selenide superconductors, which exhibit a record Tc of 65 K. Mean-field theory further predicts s-wave Cooper pairs that alternate in sign between hole momenta at (0,0) and at (pi,0). Exact diagonalization of two holes confirms this, and it also finds an orbital-singlet/spin-triplet s-wave state close by in energy.

Fractional topological insulators: the role of band geometry (R. Roy)

Recent numerical simulations of flat band models with interactions which show clear evidence of fractionalized topological phases in the absence of a net magnetic field have generated a great deal of interest. We provide an explanation for these observations by showing that the physics of these systems is the same as that of conventional fractional quantum Hall phases in the lowest Landau level under certain ideal conditions which can be specified in terms of the Berry curvature and the Fubini study metric of the topological band. In particular, we show that when these ideal conditions hold, the density operators projected to the topological band obey the celebrated W∞ algebra. Our approach provides a quantitative way of testing the suitability of topological bands for hosting fractionalized phases.

Fractional quantum Hall effect in graphene: effects of Landau level mixing (M. Peterson)

We study the effects of Landau level mixing on the fractional quantum Hall effect in graphene. Landau level mixing in graphene is especially important since the ratio of the Coulomb energy to the cyclotron energy is independent of magnetic field and of order one. We derive an effective Hamiltonian that fully incorporates Landau level mixing by renormalizing the two-body Coulomb potential (renormalizing the Haldane pseudopotentials) and inducing particle-hole symmetry breaking three-body terms, cf. Bishara and Nayak, Phys. Rev. B 80, 121302(R) (2009). As opposed to the FQHE in GaAs semiconductor devices, graphene has no finite-thickness corrections since the two-dimensional graphene sheet is atomically thin and the Dirac nature of the electrons in graphene forces the particle-hole symmetry breaking three-body terms to exactly vanish in the lowest Landau level. We acknowledge support from DARPA, Microsoft Station Q, and Cal State Long Beach Start-up.

Topological Quantum Engineering: Constructing exotic phases from known states of matter (D. Clarke)

Non-Abelian anyons are widely sought for quantum computing applications as well as for the fundamental physics they harbor. As systems containing such excitations are not easily come by in nature, I have been engaged in efforts to produce these extraordinary objects by judiciously interfacing more well-understood phenomena, such as superconductors and fractional quantum Hall states. There are now many blueprints for stabilizing the simplest type of non-Abelian anyon, Majorana zero modes. Here, I will introduce an experimentally feasible device in which defects bind zero energy operators with more general parafermionic commutation relations. These parafermionic defects take us an important step closer to universal quantum computation, and demonstrate the power of the 'engineering' approach in the search for non-Abelian anyons.

Fluctuation theorems for quantum processes (T. Albash)

We present fluctuation theorems and moment generating function equalities for generalized thermodynamic observables and quantum dynamics described by completely positive trace preserving (CPTP) maps. Our results include the quantum Jarzynski equality and Crooks fluctuation theorem, and clarify the special role played by the thermodynamic work and thermal equilibrium states in previous studies. We show that for a specific class of generalized measurements, which include projective measurements, unitality replaces micro-reversibility as the condition for the physicality of the reverse process in our fluctuation theorems. We present an experimental application of our theory to the problem of extracting the system-bath coupling magnitude, which we do for a system of pairs of coupled superconducting flux qubits undergoing quantum annealing.

Fractional Chern Insulator and its Modular Matrix (D. Sheng)

Energy band theory provides successful understanding of the metal and insulator behaviors of solids when electrons are not strongly interacting. Interestingly, a band insulator can be a topological insulator (TI), which is distinguishable from an ordinary insulator by the topological invariant of the system. I will focus on interacting physics of the topological Chern insulator and demonstrate numerical evidences that fractionalized topological phase emerges in flat topological band models with strongly interacting particles, as the examples of fractional quantum Hall effect without a magnetic field. I will present new results of obtaining modular matrix using the minimal entangled states, which characterizes the topological order and quasi-particle statistics of the quantum state.