*Qi Zhou, Purdue University*

**Speaker:***Synthetic quantum matters in ultracold atoms: from topological defects to curved spaces.*

**Title:***The well controllable atomic samples have allowed physicists to access a wide range of quantum phenomena unattainable in other systems. I will discuss a few such examples. The first example includes topological defects in high dimensions, ranging from Yang monopoles to continuous topological defects in five dimensions. A Yang monopole as a fundamental ingredient in non-abelian gauge theories was proposed back to seventy years ago. It was recently delivered for the first time in an experiment by tailoring laser-atom couplings in a five-dimensional parameter space. Moreover, highly tunable interactions between atoms could turn a Yang monopole into a much broader range of continuous topological defects. In another example, I will show how to create synthetic curved spaces, including a Hall cylinder and a Hall torus. Coupling internal degrees of freedom and orbital motions of ultracold atoms allows us to bypass constraints from physical laws and thread a finite effective magnetic flux through the surface of a torus or a cylinder. This development opens the door to exploring intriguing quantum phenomena inherent to the topology of spaces.*

**Abstract:***Ar. Abanov*

**Host:**17 Jan 2020, 4:00PM | MIST M102

Hosted By: Ar. Abanov

*Michael Kolodrubetz, UT Dallas*

**Speaker:***Floquet systems with quantized drive.*

**Title:***Time-periodic (Floquet) drive has become a powerful tool to engineer phases of matter, both in equilibrium and far from equilibrium. In this talk, I show that treating drive photons as quantized degrees of freedom, as in cavity QED, leads to interesting phases of coupled light and matter. This is non-trivial even for such “basic” phenomena as many body localization [Ng and MHK, PRL 122, 240402 (2019)], where the localizing interactions compete against global coupling to the quantized cavity mode. I will show how this competition gives rise to unusual phenomena such as an inverted mobility edge, with delocalization at low energies. I then discuss topological versions of the problem, such as the Floquet-Thouless energy pump [MHK et al, PRL 120, 150601 (2018)], where topological pumping naturally occurs by a quantized backaction on the drive. These phases may be realizable experimentally in many body cavity QED and superconducting circuit QED.*

**Abstract:***Ar. Abanov*

**Host:**24 Jan 2020, 4:00PM | MIST M102

Hosted By: Ar. Abanov

*Kate Ross, Colorado State*

**Speaker:***Quantum Dimers and Field Induced Bose Einstein Condensation in a Spin-Orbit Coupled Oxide*

**Title:***The quantum dimer magnet (QDM) is the canonical example of 'quantum magnetism'. This state consists of entangled nearest-neighbor spin dimers and often exhibits a field-induced 'triplon' Bose-Einstein condensate (BEC) phase. I will discuss a new QDM in the strongly spin-orbit coupled, distorted honeycomb-lattice material Yb2Si2O7. Single crystal neutron scattering, specific heat, and ultrasound velocity measurements reveal a gapped singlet zero field ground state with sharp, dispersive excitations. We find a field-induced magnetically ordered phase reminiscent of a BEC phase, with exceptionally low critical fields of Hc1 ~0.4 T and Hc2 ~1.4 T. Using inelastic neutron scattering we observe a Goldstone mode that persists throughout the entire field-induced magnetically ordered phase, suggestive of the spontaneous breaking of U(1) symmetry expected for a triplon BEC. However, in contrast to other well- known cases of this phase, the high-field (H > 1.2T) part of the phase diagram in Yb2Si2O7 is interrupted by an unusual regime signaled by a change inthe field dependence of the ultrasound velocity and net magnetization, as well as the disappearance of a sharp anomaly in the specific heat. These measurements raise the question of how anisotropy in strongly spin-orbit coupled materials modifies the field induced phases of QDMs.*

**Abstract:***Ar. Abanov*

**Host:**31 Jan 2020, 4:00PM | MIST M102

Hosted By: Ar. Abanov

*Evan Philip, Stony Brook*

**Speaker:***Chiral Photocurrents and Terahertz Emission in Dirac/Weyl Semimetals*

**Title:***I will start with an introduction to Dirac/Weyl semimetals and chiral anomaly. We will then look at a new mechanism for photocurrents in Dirac/Weyl semimetals that relies on the chiral anomaly. I will also discuss terahertz emission from the Weyl semimetal TaAs induced by a near-infrared laser, where Weyl fermions play a central role. We will conclude with remarks on some potential applications of these works.*

**Abstract:***A. Belyanin*

**Host:**7 Feb 2020, 4:00PM | MIST M102

Hosted By: A. Belyanin

*Vladimir Zyuzin, Nordita, Stockholm*

**Speaker:***Odd-frequency spin-triplet instability in disordered electron liquid*

**Title:***I will discuss the possibility of the odd frequency pairing, proposed initially by Berezinskii, in disordered conductors. In the first example, we considered the case of a very disordered system near an insulating state. There, an interplay of the Coulomb interaction with the strong disorder could be the source of the Cooper pairing in the triplet channel, which in turn may lead to the odd-frequency pairing. In the other considered case, such instability may develop in the vicinity of the transition into a superconducting state. I will discuss the existing experimental data in view of the considered exotic pairing. A hypothesis is put forward that the odd frequency pairing, unlike the even frequency pairing, leads to insulating behavior. Such an insulating state can be considered as an independent phase occurring before a regular superconducting transition.*

**Abstract:***A. Finkelstein*

**Host:**14 Feb 2020, 4:00PM | MIST M102

Hosted By: A. Finkelstein

*Brad Ramshaw , Cornell (notice the room change!)*

**Speaker:***Determining the symmetry of a superconducting order parameter with resonant ultrasound spectroscopy.*

**Title:***Sr2RuO4 has stood as the leading candidate for a spin-triplet superconductor for 26 years. Recent NMR experiments have cast doubt on this candidacy, however, and it is difficult to find a theory of superconductivity that is consistent with all experiments. What is needed are symmetry-based experiments that can rule out broad classes of possible superconducting order parameters. We use resonant ultrasound spectroscopy ––- a unique method for measuring all elastic moduli in a single experiment ––- to measure the entire symmetry-resolved elastic tensor of Sr2RuO4 through the superconducting transition. We have discovered a thermodynamic discontinuity in the shear elastic modulus c66, which requires that the superconducting order parameter has two components. The standard interpretation of such a result would be the two-component p-wave order parameter px + ipy. However, this order parameter appears to be precluded by recent NMR experiments, and we therefore suggest two other rather exotic two-component order parameters - d_xz + i d_yz, or d_x^2-y^2 + i g _xz(x^2-y^2). I will discuss the implications of this result for the long-standing problem of superconductivity in Sr2RuO4, as well as the recently discovered superconductor UTe2.*

**Abstract:***Ar. Abanov*

**Host:**21 Feb 2020, 4:00PM | MIST M108

Hosted By: Ar. Abanov

*Alexei Kitaev, Caltech*

**Speaker:***Complex SYK model*

**Title:***We study the Sachdev-Ye-Kitaev model for complex fermions with N flavors and a conserved global U(1) charge. One of its interesting properties is an analogue of Luttinger's theorem originally obtained by Georges, Parcollet, and Sachdev, namely, a relation between the total charge Q and the spectral asymmetry parameter E. Three new derivations are given: using a time-domain regularization, RG flow, and bulk picture (namely, Dirac fermions on the hyperbolic plane). The last method directly gives the "zero-temperature entropy". This talk is based on arXiv:1910.14099 with Gu, Sachdev, and Tarnopolsky.*

**Abstract:***V. Pokrovsky*

**Host:**28 Feb 2020, 4:00PM | MIST M102

Hosted By: V. Pokrovsky

*APS March Meeting*

**Speaker:**

**Title:**

**Abstract:**

**Host:**6 Mar 2020, 4:00PM | Denver, CO

*Spring Break*

**Speaker:**

**Title:**

**Abstract:**

**Host:**13 Mar 2020, 4:00PM | Everywhere

*Shambhu Ghimire, Stanford (Canceled)*

**Speaker:***Ultrafast Spectroscopy of Quantum Materials*

**Title:***Quantum materials such as topological insulators, Weyl-semimetals, and atomically thin two-dimensional crystals have intriguing electronic properties, which makes them promising candidates for their potential applications in next-generation technology. Therefore, it is desirable to investigate the electronic properties of these materials, perhaps through various methods such that their full potential, as well as the limitations, can be identified. In this talk, I will introduce a novel spectroscopic approach based on the use of strong ultrafast laser pulses and the generation of high-order harmonics. Analysis of temporal and spectral properties of high-order harmonics from these materials reveals their valence charge density distributions, electronic band-structure, topology, as well as driven electron dynamics in the natural time scales of electrons. The advantages of this approach over conventional methods include the use of the all-optical setting, no physical contacts to samples, and much of the measurements that can be performed in ambient conditions. More importantly, the non-perturbative interaction to strong laser pulses forms novel quantum phases, which could be probed by analyzing high-order harmonics.*

**Abstract:***H. Schuessler*

**Host:**20 Mar 2020, 4:00PM | MIST M102

Hosted By: H. Schuessler

*OPEN*

**Speaker:**

**Title:**

**Abstract:**

**Host:**27 Mar 2020, 4:00PM | MIST M102

*Physics Festival (Canceled)*

**Speaker:**

**Title:**

**Abstract:**

**Host:**3 Apr 2020, 4:00PM | MIST M102

*Good Friday*

**Speaker:**

**Title:**

**Abstract:**

**Host:**10 Apr 2020, 4:00PM | MIST M102

*Peter Armitage, Johns Hopkins University (rescheduled for 11/13/2020),*

**Speaker:***TBA*

**Title:**

**Abstract:***Ar. Abanov*

**Host:**17 Apr 2020, 4:00PM | MIST M102

Hosted By: Ar. Abanov

*OPEN*

**Speaker:**

**Title:**

**Abstract:**

**Host:**24 Apr 2020, 4:00PM | MIST M102

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