Zoom link:

https://umich.zoom.us/j/91655313495 with the passcode: CM-AMO

ABC-stacked trilayer graphene/hexagonal boron nitride moiré superlattice (TLG/hBN) has emerged as a playground for correlated electron physics. A spectroscopy study of this system is, however, challenging due to the device configuration. In this talk, I will introduce our recent efforts on FTIR photocurrent spectroscopy measurements of dual-gated TLG/hBN. We observed strong gate-tunable optical transitions that originated from the moire flat band. At half-filling of the valence flat band, a broad absorption peak emerges at ~18 meV, indicating direct optical excitation across an emerging Mott gap. Furthermore, I will talk about a unique moire-enabled interlayer electron-phonon coupling phenomenon in this system. The ZO phonon in hBN hybridizes with electron-hole excitations in trilayer graphene and appear as an asymmetric peak in the photocurrent spectrum. This Fano line-shaped peak evolves continuously as we tune the displacement field. I'll talk about the implications of this phenomenon to engineering the physics of moire quantum matters.

We investigate the standard and dual BMS supertranslation generators on the black hole horizon and draw some conclusions about black hole physics. Recently, it has been shown that in addition to the conventional BMS supertranslation symmetries, there exists another infinite set of magnetic asymptotic symmetries that are referred to as dual BMS supertranslations. We show that the Dirac bracket between these generators exhibits a central term when the parameter functions have singularities in the complex stereographical coordinates on the sphere. Such singularities are related to Dirac-string-like configurations in the bulk, and should therefore be included in the set of acceptable transformations. We then demonstrate that this anomalous central term can be removed by including an appropriate gravitational Chern-Simons theory on the horizon. This implies that consistency of the asymptotic symmetry algebra requires a new structure on the horizon.

]]>This colloquium is a hybrid event. It will take place in 340 West Hall and via a YouTube live stream link: https://youtu.be/heYnnM-xC5A.

In 1972, Willie Hobbs Moore became the first African-American woman to receive a PhD in physics, right here at the University of Michigan. On the 50th anniversary of this historic event, I will:

- discuss some of Dr. Hobbs Moore's biographical highlights,

- provide an overview of her dissertation research,

- share some memories of my friendship with Willie and her family before and after her untimely passing in 1994.

Holographic interpretations of Randall-Sundrum (RS) branes provide a laboratory to explore the way quantum information evolves in field theories coupled to gravity. Despite this importance, the holographic interpretation of RS branes in terms of a theory of gravity coupled to a CFT is rather ad-hoc. In this talk, we use top-down constructions of RS branes in order to work out a precise dictionary for this "intermediate" holographic prescription, resolving serious causality problems of the naive picture often used in the literature while preserving many of the successes of the RS construction

]]>Observations have found black holes spanning ten orders of magnitude in mass across most of cosmic history. The leading black hole model, the Kerr solution, is however provisional because its behavior at infinity is incompatible with an expanding universe. Recent work on black hole models with realistic behavior at infinity predicts that the gravitating mass of a black hole can increase with the expansion of the universe independently of accretion or mergers, in a manner that depends on the black hole’s interior solution. Here we test this prediction by considering the growth of supermassive black holes in passively evolving elliptical galaxies over 0 < z < 2.5. We find evidence for cosmologically coupled mass growth among these black holes, with zero cosmological coupling excluded at 99.98% confidence. The redshift dependence of the mass growth implies that, at z < 7, black holes contribute an effectively constant cosmological energy density to Friedmann’s equations. The continuity equation then requires that black holes contribute cosmologically as vacuum energy. We further show that, within current observational

limits, black hole production from the cosmic star formation history gives the cosmological constant value measured by Planck. We thus propose that stellar remnant black holes are the astrophysical origin of dark energy, explaining the late-time onset of accelerating expansion.

Magnetic domain walls and their crossings as magnetic vortices are typical magnetic topological defects existing in many magnetic materials. While visualizing them and their evolution under field at the atomic level are rarely reported except for magnetic skyrmions. Here I will present the proliferation of topological magnetic defects in a 2D square lattice under field, seen by neutrons.

By introducing Ising spins in a 2-dimensional (2D) bi-layer square lattice, we realized a frustrated magnet where no long-range magnetic order was found upon cooling to 100 mK. Using the local magnetic susceptibility method with polarized neutrons, we revealed canted Ising spins. With this information, we were able to simulate the neutron diffuse scattering patterns observed under selected magnetic fields through machine learning assisted spin Hamiltonian optimization. Our studies revealed a short-range ordered 2D stripe magnetic phase wrapped by domain-wall phases. By applying magnetic field perpendicular to the square-lattice plane, the stripe magnetic phase melts and the condensed domain wall phases form a short-range ordered vortex lattice, so-called magnetic vortex liquid state, at a critical field of 2 T. Further application of the magnetic field to 4 T, makes all of the spins canted to the field direction, i.e., a polarized paramagnetic phase. Here the evolution of stripe phase and domain wall phase can be precisely controlled by a magnetic field and tracked by neutron scattering. A Z4 vortex was found to be originated from two crossed domain walls. While the density of the domain wall and vortices increase with the field and reach their maximum before entering the fully polarized paramagnetic phase.

*The research was supported by the U.S. Department of Energy (DOE), Early Career Research Program Award KC0402020 and used resources at the HFIR and SNS, DOE Office of Science User Facilities operated by ORNL.

Abstract:

In quantum science we strive to construct systems that utilize the quantum behavior of nature to encode and measure information. This enables the exploration of new scientific and technological regimes by controlling and coherently manipulating the internal quantum states of the system's constituent elements (i.e. atoms, photons, solid-state spins, etc.). The highly controllable and coherent nature of neutral atoms make them an attractive quantum element for pursuits at the frontier of quantum information science. Experimental tools like the optical tweezer have led to unprecedented control and manipulation of single atoms giving the ability to create defect-free arrays of atoms in one and two dimensions. Exciting the atoms to a high principal quantum number state known as a Rydberg state creates strong inter-atomic interactions used to achieve entangling operations for quantum computing and simulating quantum systems. I will discuss the construction of our neutral ytterbium (Yb) tweezer apparatus and why Yb is a particularly exciting atom for pursuing quantum information science. In addition to utilizing Rydberg states, I will describe how we can leverage the precise control afforded by optical tweezers to integrate these cold atom systems with a nanophotonic platform and create conditions in the array where the atoms experience cooperative optical resonances which can be used to store and distribute quantum information.

Fluids are constantly mixing in our everyday lives. Some examples are oil and vinegar or coffee and cream. While we often don't think too much about how these fluids mix, they can have profound consequences in material ejecta in the Universe, fusion energy, and at your local pub. This talk will give a fundamental description of fluid mixing, discuss examples found in nature and engineering, and describe the effects mixing can have.

This talk will be live in ROOMS 170 & 182 Weiser Hall. You can also watch the talk/Q&A, live, on YouTube: https://www.youtube.com/watch?v=zCLXmkQUwlg

Many animal species move together in groups (schools, herds, flocks) without leaders but displaying complex collective behavior, sometimes visually very striking. This behavior is the emergent of individual interactions, so that the phenomenon can be fruitfully approached from statistical physics. In this talk we shall summarize experimental results on flocks of starlings (Sturnus vulgaris) in the field, and show that it is possible to develop a sensible statistical theory of their collective movement, despite the difficulties related to out-of-equilibrium effects and relatively small system size. We shall discuss in particular the presence of scale-free correlations in speed and velocity fluctuations and their relation to symmetry breaking and speed control mechanisms.

CM-AMO Seminar