Host YangZhi 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 socalled â€śIsing superconductivityâ€ť, 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 singlettriplet 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 CMTCQuICS 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

NonFermi Liquids in 2d Conducting Networks

We investigate 2dimensional periodic superstructures consisting of 1dimensional conducting segments. Such structures naturally appear in twisted transition metal
dichalcogenides, some chargedensitywave materials, and a marginally twisted bilayer graphene, in which intriguing nonFermi liquid transports have been experimentally observed. We model such a system as a network of TomonagaLuttinger Liquids, and theoretically derive a variety of nonFermi liquid behaviors, based on a RenormalizationGroup analysis of the junctions of TomonagaLuttinger Liquids. In particular, a continuously varying resistivity exponent appears naturally in the 2dimensional network through the continuously varying Luttinger parameter of the constituent TomonagaLuttinger Liquid.
 Host YangZhi Chou
Email rcawthor@umd.edu for Zoom details

July 28, Tuesday, 10 am
 EunGook Moon (KAIST), Zoom seminar

Instability of j=3/2 Bogoliubov Fermi surfaces

Exotic quantum phases including topological states and nonFermi 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 nonzero density of states of zero energy Bogoliubov quasiparticles, Bogoliubov Fermisurface (BGFS), was also proposed in a centrosymmetric j=3/2 system, protected by a Z2 topological invariant. Here, we consider interaction effects of a centrosymmetric BGFS and demonstrate its instability by using meanfield and renormalization group analysis. The BardeenCooperSchrieffer (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 BGFS under generic attractive interactions between Bogoliubov quasiparticles. In drastic contrast to the standard BCS superconductivity, a Fermisurface 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 higherorder 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 spinorbit 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 ManyBody Interactions in Moiré Systems

Van der Waals bilayers with small differences in the lattice constants or orientations of the individual layers have longperiod 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 manybody 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 manybody 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 electronphonon systems. There will be an emphasis on understanding experimental observations of photoinduced superconductivity.
 Host Jay Sau; last update 2020126 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 cornersharing 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 spintype and chargetype 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 202034 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 quantummechanical 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 threedimensional Euclidean space. At the same time, there has been growing interest in exploring manybody 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 nearterm experiments.
 Host Brain Swingle; last update 202036 by Victor Yakovenko
CMTC Seminar Committee as of 2020119:
Jay Sau, Maissam Barkeshli, Brian Swingle, YiTing Hsu, and Danny Bulmash
For the earlier CMTC talks, see this page