CMTC talks are held in room

**Fall 2020**

- November 17, Tuesday, Time: 11am
**Cory Dean**(Columbia University), Zoom seminar-
**Title:**TBA -
**Abstract:**TBA *Host Yang-Zhi Chou*

- October 6, Tuesday, Time: TBA
**Steve Kivelson**(Stanford University), Zoom seminar-
**Title:**TBA -
**Abstract:**TBA *Host Danny Bulmash*

- September 29, Tuesday, 11 am
**Subir Sachdev**(Harvard University), Zoom seminar-
**Title:**TBA -
**Abstract:**TBA *Host Danny Bulmash*

- September 22, Tuesday, 11 am
**David Huse**(Princeton), Zoom seminar-
**Title:**Many-body-localization to thermalization phase transition -
**Abstract:**I will present our current understanding of this novel phase transition as obtained from a strong-randomness real-space renormalization group (RG) approach. This is for the case of isolated one-dimensional quantum many-body systems with quenched randomness and short-range interactions. The RG flow is qualitatively like the famous Kosterlitz-Thouless RG, but is different in some important features, thus making a new universality class of phase transition. Time permitting, I will then try to set this result in the context of the much larger set of transitions and crossovers found in other cases of this transition, such as varying the dimension of space, random vs nonrandom, and changing the range of the interactions. *Host Yang-Zhi Chou*

**Email rcawthor@umd.edu for Zoom details**

- September 8, Tuesday, 11 am
**Andrea Young**(UC Santa Barbara), Zoom seminar-
**Title:**Orbital magnetism and an isospin Pomeranchuk effect in twisted bilayer graphene. -
**Abstract:**Moire flat bands host a wide range of correlated states at low temperatures. My talk will focus on the role of orbital magnetism in these systems, in which the electron system spontaneously polarizes into one or more spin- and valley-isospin flavors. In the first part of the talk, I will focus on the observation of quantized anomalous Hall effects in a variety of moire systems. In this spectacular manifestation of orbital magnetism, the ground state at certain integer filling factors is spontaneously polarized into a single valley-projected moire miniband, leading to robust magnetic hysteresis and a quantized Hall effect at zero magnetic field. We observe a variety of novel phenomena in this regime, including ultra-low power current induced switching and gate-tuned reversal of magnetic order that can be tied to the magnetization of the topological edge states. In the second part of the talk, I will discuss the more general role orbital magnetism plays in the phase diagram of these materials at high temperatures. In particular, we find that even when the ground state is isospin unpolarized, a finite polarization can obtain at high temperatures. We ascribe this effect--observed generically in twisted bilayer graphene--to an isospin analogue of the Pomeranchuk effect of 3He, in which the high entropy associated with isospin excitations of the orbital magnets favors fluctuating magnetic order at high temperatures.

References:

Serlin et al., arXiv:1907.00261

Polshyn et al., arXiv:2004.11353

Tschirhart et al., arXiv:2006.08053

Saito et al., arXiv:2008.10830 *Host Yang-Zhi Chou*

**Summer 2020**- August 18, Tuesday, 11 am
**Igor Mazin**(George Mason University), Zoom seminar-
**Title:**Ising superconductivity in NbSe2 monolayers -
**Abstract:**Recent studies on superconductivity in monolayer NbSe2 have demonstrated a giant anisotropy in the superconducting critical field. This phenomenon was quite well understood in terms of the so-called “Ising superconductivity”, where the spins of Cooper pairs are strictly aligned with one particular crystallographic direction. Besides the (formally infinite) critical field anisotropy Ising superconductors (IS) have demonstrated a number of unusual and seeming exotic phenomena. IS is sometimes misleading perceived as an esoteric subset of the theory of superconductivity, which is hard to explain and even harder to understand for outsiders. In the first part of my talk I will debunk this notion and demonstrate that the physics of IS is exceedingly simple and hardly requires any formulaics to be grasped. In the second part I will make, in terms of DFT calculations, a quantitative connection with the specific material in which most of the IS studies are being performed, monolayer NbSe2; in particular, I will show that, contrary to a common misconception, NbSe2 is close to a magnetic instability and this fact cannot be ignored when discussing IS. In the third part I will discuss to what extent the existing models allow for a sizeable singlet-triplet mixing (NbSe2 had been till recently believed to be, for all intents and purposes, a singlet superconductor, but that is not necessarily the case). Up to now the talk will be based on our paper with Darshana Wickramaratne (NRL) and Daniel Agterberg (UWi), to be published in PRX. If time allows, I will also say a few words about the new experiments from the Fai Mak group in Cornell and present some results from our work in progress with Darshana and Maxim Khodas (Hebrew U) striving to explain his observations microscopically. *Host Victor Yakovenko*

1. ArXiv:2005.05497

- August 11, Tuesday, 11 am
**Dmitry Green**(AppliedTQC), Zoom seminar, Joint CMTC-QuICS seminar-
**A superconducting circuit realization of combinatorial gauge symmetry** - We propose an integrated superconducting circuit design in combination with a general symmetry principle, combinatorial gauge symmetry, to build artificial quantum spin liquids that serve as foundation for the construction of topological qubits. The superconducting wire arrays exhibit rich features. In the classical limit of large capacitances its ground state consists of two superimposed spin liquids; one is a crystal of small loops containing disordered U(1) degrees of freedom, and the other is a soup of loops of all sizes associated to Z_2 topological order. We show that the classical results carry over to the quantum case when fluctuations are gradually tuned via the wire capacitances, yielding Z_2 quantum topological order. In an extreme quantum limit where the capacitances are all small, we arrive at an effective quantum spin Hamiltonian that we conjecture would sustain Z_2 quantum topological order with a gap of the order of the Josephson coupling in the array.
*Host Prof. Victor Galitski***Email rcawthor@umd.edu for Zoom details**

- August 4, Tuesday, 11 am
**Masaki Oshikawa**(University of Tokyo), Zoom seminar-
**Non-Fermi Liquids in 2d Conducting Networks** - We investigate 2-dimensional periodic superstructures consisting of 1-dimensional conducting segments. Such structures naturally appear in twisted transition metal dichalcogenides, some charge-density-wave materials, and a marginally twisted bilayer graphene, in which intriguing non-Fermi liquid transports have been experimentally observed. We model such a system as a network of Tomonaga-Luttinger Liquids, and theoretically derive a variety of non-Fermi liquid behaviors, based on a Renormalization-Group analysis of the junctions of Tomonaga-Luttinger Liquids. In particular, a continuously varying resistivity exponent appears naturally in the 2-dimensional network through the continuously varying Luttinger parameter of the constituent Tomonaga-Luttinger Liquid.
*Host Yang-Zhi Chou*

Email rcawthor@umd.edu for Zoom details

- July 28, Tuesday, 10 am
**Eun-Gook Moon**(KAIST), Zoom seminar-
**Instability of j=3/2 Bogoliubov Fermi surfaces** - Exotic quantum phases including topological states and non-Fermi liquids may be realized by quantum states with total angular momentum j=3/2, as manifested in HgTe and pyrochlore iridates. Recently, an exotic superconducting state with non-zero density of states of zero energy Bogoliubov quasiparticles, Bogoliubov Fermi-surface (BG-FS), was also proposed in a centrosymmetric j=3/2 system, protected by a Z2 topological invariant. Here, we consider interaction effects of a centrosymmetric BG-FS and demonstrate its instability by using mean-field and renormalization group analysis. The Bardeen-Cooper-Schrieffer (BCS) type logarithmical enhancement is shown in fluctuation channels associated with inversion symmetry. Thus, we claim that the inversion symmetry instability is an intrinsic characteristic of a BG-FS under generic attractive interactions between Bogoliubov quasiparticles. In drastic contrast to the standard BCS superconductivity, a Fermi-surface may generically survive even with the instability. We propose the experimental setup, a second harmonic generation experiment with a strain gradient, to detect the instability. Possible applications to iron based superconductors and heavy fermion systems including FeSe are also discussed.
*Host Danny Bulmash***Email rcawthor@umd.edu for Zoom details**

- July 14, Tuesday, 11 am
**Jennifer Cano**(Stony Brook/Flatiron Institute), Zoom seminar-
**Lattice dislocations as a probe of higher order topological insulators** -
Nonzero weak topological indices are thought to be a necessary condition to bind a single helical mode to a lattice dislocation. I will show that higher-order topological insulators (HOTIs) can, in fact, host a single helical mode along screw or edge dislocations in the absence of weak topological indices. When this occurs, the helical mode is necessarily bound to a dislocation characterized by a fractional Burgers vector, macroscopically detected by the existence of a stacking fault. The robustness of a helical mode on a partial defect is demonstrated by an adiabatic transformation that restores translation symmetry in the stacking fault. Since partial defects and stacking faults are commonplace in bulk crystals, the existence of such helical modes can measurably affect the expected conductivity in these materials. I will also discuss our prediction of HOTIs in antiperovskites with spin-orbit coupling.

1. Phys. Rev. Lett. 123, 266802 (2019) (arXiv:1809.03518)

2. Phys. Rev. B 101, 245110 (2020) (arXiv:2002.02969) *Host Danny Bulmash*

**Spring 2020**- January 16, Thursday, 2 pm
**Fengcheng Wu**(CMTC), practice talk for a job interview**Symmetry, Topology, and Many-Body Interactions in Moiré Systems**-
Van der Waals bilayers with small differences in the lattice constants or orientations of the individual layers have long-period Moiré patterns, which provide vast new opportunities to control material properties. In this talk, I will present our work on Moiré pattern physics that arise from an interplay of symmetry, topology and many-body interactions. First, I will describe our theoretical proposal of using twisted bilayer transition metal dichalcogenides as quantum simulators of Hubbard model [1, 2], and discuss recent experimental realizations. Then I will focus on twisted bilayer graphene (TBG) and show how the interplay between many-body interactions and Bloch band symmetry of TBG can lead to unconventional superconductivity [3, 4]. Finally, I will discuss quantum anomalous Hall insulators in TBG and demonstrate their stability against spin/valley magnon excitations [5]. I will describe the effects of quantum geometry on spin stiffness and show that Berry curvature of Moiré bands helps to stiffen spin magnons.

1. F. Wu, T. Lovorn, E. Tutuc, A. H. MacDonald, Phys. Rev. Lett. 121, 026402 (2018).

2. F. Wu, T. Lovorn, E. Tutuc, I. Martin, A. H. MacDonald, Phys. Rev. Lett. 122, 086402 (2019).

3. F. Wu, A. H. MacDonald, and I. Martin, Phys. Rev. Lett. 121, 257001 (2018).

4. F. Wu, E. Hwang, and S. Das Sarma, Phys. Rev. B 99, 165112 (2019).

5. F. Wu, S. Das Sarma, arXiv:1908.05417 (2019). - February 11, Tuesday, 2:30 pm
**Eugene Demler**(Harvard), in conjunction with his JQI seminar-
**Nonlinear optics with collective excitations and photoinduced superconductivity** - This talk will review the recent progress in theoretical modeling of nonequilibrium dynamics of electron-phonon systems. There will be an emphasis on understanding experimental observations of photoinduced superconductivity.
*Host Jay Sau; last update 2020-1-26 by Victor Yakovenko*- March 10, Tuesday, 2 pm
**Ronny Thomale**(Theoretische Physik I, Universitat Wurzburg), see also his JQI and QMC seminars-
**The Quest of the Kagome Hubbard Model** - Since its (re-)discovery in the second half of the 20th century, the lattice of corner-sharing triangles called kagome has become one of the key domains featuring paradigmatic models for exotic quantum electronic states of matter. Depending on the filling, the Hubbard model on the kagome lattice exhibits several fascinating phases subject to contemporary research in condensed matter physics, ranging from topological spin liquids over correlated Dirac metals and unconventional superconductivity to spin-type and charge-type Peierls phases as well as turbulent hydrodynamic flow. I will discuss recent progress in theory to understand such scenarios of correlated electron systems on the kagome lattice.
*Host Jay Sau; last update 2020-3-4 by Victor Yakovenko*- March 11, Wednesday, 11 am
**Gregory Bentsen**(Princeton University), QuICS seminar at CMTC location-
**Tunable geometry and fast scrambling in nonlocal spin networks** - The past decade has seen a dramatic increase in the degree, quality, and sophistication of control over quantum-mechanical interactions available between artificial degrees of freedom in a variety of experimental platforms. The geometrical structure of these interactions, however, remains largely constrained by the underlying rectilinear geometry of three-dimensional Euclidean space. At the same time, there has been growing interest in exploring many-body dynamics in systems, such as the SYK model and tensor network models, for which the interaction structure bears little or no resemblance to Euclidean space. Inspired by these complementary developments, here we study a tunable, nonlocal spin network that can be engineered using cold atoms coupled to an optical cavity. The network exhibits two distinct notions of emergent geometry -- linear and treelike -- that can be accessed using a single tunable parameter. In either of these two extreme limits, we find a succinct description of the resulting dynamics in terms of two distinct metrics on the network, encoding a notion of either linear or treelike distance between spins. Moreover, at the crossover between these two regimes, the spin network becomes highly connected and exhibits signatures of fast scrambling. These observations highlight the essential role played by the geometry of the interaction structure in determining a system's dynamics, and raise prospects for novel studies of nonlocal and highly chaotic quantum dynamics in near-term experiments.
*Host Brain Swingle; last update 2020-3-6 by Victor Yakovenko*

CMTC Seminar Committee as of 2020-1-19:

Jay Sau, Maissam Barkeshli, Brian Swingle, Yi-Ting Hsu, and Danny BulmashFor the earlier CMTC talks, see this page