QM Seminar speaker Laurens Vanderstraeten (Ghent University), Wednesday, June 12

Time/Venue: Wednesday, June 12 at 2:00 pm in LeConte 402
Title: Simulating excitation spectra with projected entangled-pair states
Abstract: In this talk we show how to simulate the excitation spectrum for two-dimensional quantum spin systems using the formalism of tensor networks. We first explain the one-dimensional case of spin chains, for which we can compute the quasiparticle dispersion, spectral weight and scattering properties. Then we go on to two dimensions, showing how to capture strongly-correlated ground states using projected entangled-pair states (PEPS) and how to simulate the quasiparticle excitations on top of such a PEPS.

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QM Seminar speaker Frank Schindler (University of Zurich), Wednesday, June 5

Time/Venue:  Wednesday, June 5 at 2:00 pm in LeConte 402

Title: Topological zero-dimensional defect states
Abstract: Crystal defects in topological insulators (TIs) are known to bind anomalous electronic states with two fewer dimensions than the bulk; the most commonly cited examples are the helical modes bound to screw dislocations in weak TIs. In this talk, we extend the classification of topological electronic defect states. By mapping the Hamiltonians of planes in momentum space to the real-space surfaces between screw or edge dislocations with integer Burgers vectors, we show that these crystalline defects can bind higher-order end states with fractional charge. We support our findings with extensive numerical calculations. Using density functional theory, we demonstrate the presence of first-order 0D defect states in 2D PbTe monolayers, and higher-order 0D states in 3D SnTe crystals.

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Special QM Seminar Speaker Joaquin Rodriguez Nieva (Harvard) Thursday, May 23

Time: 11:00 am
Venue: 325 LeConte
Title: Probing hydrodynamics and thermalization in quantum spin systems
Abstract: The presence of symmetries and conservation laws can have striking manifestations in the dynamic and thermalization behavior of interacting quantum systems. I will discuss two such manifestations arising in the context of ferromagnets driven out of equilibrium when SU(2) symmetry is present, and I will propose experiments to observe these behaviors. First, I will show that ferromagnets can enter the hydrodynamic regime when the magnon density is large. A key fingerprint of this regime is the existence of a sound mode which governs dynamics at small frequencies and is manifested as a gapless excitation of the longitudinal spin component even in the presence of a Zeeman gap. Such mode can be accessed with recently-introduced spin qubit magnetometers, which are ideally suited to probe dynamics in quantum spin systems. Second, I will describe thermalization dynamics when ferromagnets are pumped into a non-thermal incoherent state. Despite being far from equilibrium, thermalization can nonetheless exhibit universal features, namely, universal scaling in time and momentum of quasiparticle occupation numbers. These two predictions, if experimentally confirmed, can inspire new platforms to explore hydrodynamics and thermalization in readily-available ferromagnetic materials, for instance Yttrium-Iron-Garnet (YIG)

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QM Seminar Speaker Constantin Schrade (MIT) Wednesday, May 15

Time: 2:00 pm, Wednesday, May 15
Venue: LeConte 402

Title: New quantum materials and devices: From “Majorana Superconducting Qubits” to “Spin-valley density waves” in moiré materials.
Abstract: The first part of this talk will introduce new devices for establishing a feasible platform towards Majorana-based quantum computing; one of the most urgent challenges in the field of topological superconductivity. Specifically, I will propose a new “Majorana Superconducting Qubit” which uses the well-developed superconducting qubit technology to address a robust qubit formed by spatially well-separated Majorana bound states.
The second part of this talk will address moiré materials as a new platform for strongly-correlated insulators and superconductors. Specifically, I will introduce a minimal Hubbard model to describe the essential features of trilayer ABC-stacked graphene on hexagonal boron nitride. By solving the proposed model in the semiclassical approximation, I will argue that a “Spin-valley density wave ground state” arises that can be stabilized against zero-temperature quantum fluctuations. I will also examine the properties of this state in the presence of spin- and valley-Zeeman fields.

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Special QM Seminar Speaker Bjorn Sbierski (UCB) Monday, May 6

Time: 11:00 am
Venue:  LeConte 375
Title: Non-perturbative approach to disorder (in nodal semimetals)
Abstract: The self-consistent Born approximation quantitatively fails to
capture disorder effects in semimetals. We present an alternative, simple-to-use non-perturbative approach to calculate the disorder induced self-energy. It requires a sufficient broadening of the quasiparticle pole and the solution of a differential equation on the imaginary frequency axis. We demonstrate the performance of our method for various paradigmatic semimetal Hamiltonians and compare our results to exact numerical
reference data. For intermediate and strong disorder, our approach yields quantitatively correct momentum resolved results. It is thus complementary to existing RG treatments of weak disorder in semimetals.

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Special QM Seminar Speaker Congjun Wu (UCSD) Tuesday, April 30

Time: 3:00 pm, Tuesday, April 30
Venue: CQCS (Ground floor/ 101 Campbell Hall)
Title: Orbital-active honeycomb materials
Abstract: We provide a unified view for a class of orbital-active honeycomb materials, including bismuthene, stanene, exciton-polarition lattice, and the recent focus of the twisted bilayer graphene. These materials are orbital-active possessing a pair of degenerate px and py orbitals on each site, which are unified under the E-representation of the C6v group symmetry. We started the research on orbital-active honeycomb systems in ultra-cold atom optical lattices, and found that similar physics also applies to the above solid state materials unified under the same symmetry structure. For solid state applications, we propose a new mechanism to boost the topological gap to the full scale of the atomic spin-orbit coupling, i.e., the order of 1eV. This mechanism has been recently realized in experiments of bismuthene on SiC, which shows the evidence of the gap up to 0.67eV. Mechanism for unconventional superconductivity assisted by the orbital structure in this class of materials is also studied. The flat band structure arises in 2D orbital-active honeycomb materials, such as organic metal frameworks and transition metal oxide films, in which strong correlation phenomena including Wigner crystallization and ferromagnetism appear.

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Special QM Seminar Speaker Janos Asboth (Wigner Research Centre for Physics, Budapest, Hungary) Tuesday, April 30

Time: 11 am
Venue: 375 LeConte
Title: Topological phases of quantum walks and how they can be detected using losses
Abstract: Topological insulators have Hamiltonians with bulk topological invariants, which control the interesting processes at the surface of the system, but are hard to measure directly. We have found a way to measure the bulk topological invariant (winding number) of one-dimensional topological insulator Hamiltonians and quantum walks with chiral symmetry: it is given by the displacement of a single particle, observed via losses [1]. Losses represent the effect of repeated weak measurements on one sublattice only, which interrupt the dynamics periodically. When these do not detect the particle, they realize negative measurements. In our repeated measurement scheme these losses occur at the end of every timestep. In the limit of rapidly repeated, vanishingly weak measurements, this corresponds to non-Hermitian Hamiltonians, such as the lossy Su-Schrieffer-Heeger model [2]. Contrary to intuition, the time needed to detect the winding number can be made shorter by decreasing the efficiency of the measurement. Our scheme has since been used to measure the bulk topological invariants of a discrete-time quantum walk on photons [3].

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QM Seminar Speaker Paul Fendley (Oxford) Wednesday, April 24

Time: 2:00 pm, Wednesday, April 24
Venue: Old LeConte 402
Title: Supersymmetry and free fermions from interacting Majorana fermions 
Abstract: Perturbing a quantum spin/Majorana chain with a self-dual four-fermi terms leads to tricritical Ising point separating a critical phase from a gapped phase with order-disorder coexistence. I explain how supersymmetry is not only an emergent property of the scaling limit, but give an example where explicit lattice expressions for the supersymmetry generators and currents can be found. Writing the Hamiltonian in terms of these generators allows us to find the ground states exactly at a frustration-free coupling. At the strongly interacting pure four-fermi point, the model can be solved using free-fermion raising and lowering operators, giving a gapless system with dynamical critical exponent z=3/2. I will also explain how a parafermionic generalization yields interesting multicritical points along with an AKLT phase. At the AKLT point, a map to the ordered dual model makes it straightforward to find some exact excited states as well as the ground state.

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Chern-Simons Lectures: Jean-Bernard Zuber (Laboratoire de Physique Théorique et Hautes Energies, Sorbonne Université, Paris) Tuesday, April 2-Thursday, April 4

Lecture 1.   Counting curves and knots and links
Tuesday April 2, 11:00-12.30 am, Le Conte 402

 This talk will be devoted to the problem of counting curves, knots and links, a classical mathematical problem in which physics may bring new ideas and methods. After reviewing how matrix integrals enable one to count planar “maps” and alternating links, I turn to the more difficult problem of counting and listing curves and knots, i.e. objects with a single component.

Lecture 2. Revisiting Horn’s problem
Wednesday April 3, 2:00-3.30 pm, Le Conte 402

Horn’s problem deals with the following question: what can be said about the spectrum of eigenvalues of the sum C=A+B of two Hermitian matrices of given spectrum?  The support of the spectrum of C is well understood, after a long series of works from Weyl (1912) to Horn (1952) to Klyachko (1998) and Knutson and Tao (1999). In this talk, after a short review of the problem, I show how to compute the probability distribution function of the eigenvalues of C, when A and B are independently and uniformly distributed on their orbit under the action of the unitary group. Comparison with the similar problem for real symmetric matrices and the action of the orthogonal group reveals unexpected differences…

Lecture 3. Horn’s problem and representation theory
Thursday April 4, 11:00-12.30 am, Le Conte 402

Horn’s problem has also amazing connections with group  theory and the decomposition of tensor product of representations. Recent progress in that direction will be discussed in that third lecture.



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QM Seminar Speaker Antti Niemi (Nordita) Thursday, March 28

Time: Thursday, March 28, 11:00 am
Venue: LeConte 325
Title: Hamiltonian time crystals
Abstract: Time crystal is presumed to be a state of matter that cannot be isolated from its environment. A time crystal is supposed to be an open, non-equilibrium system. In this talk we show that there are also energy conserving, autonomous time crystals. For this we construct examples of Hamiltonian systems that are time crystals, and  we propose their actual material realizations.

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