Exploring Super Atomic Crystals

About the Speaker

The Chiral Universe

Ultracool Dwarfs as Tracers for Galactic Structure and Star Formation History: Prospects with Large-Scale Surveys

About the Speaker

Abstract 

In recent years, the usefulness of astrophysical objects as dark matter (DM) probes has become more and more evident, especially in view of null results from direct-detection and particle-production experiments. The potentially observable signatures of DM gravitationally trapped inside a star, or another compact astrophysical object, have been used to forecast stringent constraints on the nucleon–dark matter interaction cross section. Currently, the probes of interest are at high redshifts, Population III (Pop III) stars that form in isolation or in small numbers, in very dense DM minihalos at z ∼ 15–40, and, in our own Milky Way, neutron stars, white dwarfs, brown dwarfs, exoplanets, etc. None of these objects are truly single component and, as such, capture rates calculated with the common assumption made in the literature of single-component capture, i.e., capture of DM by multiple scatterings with one single type of nucleus inside the object, are not accurate. In this paper, we present an extension of this formalism to multicomponent objects and apply it to Pop III stars, thereby investigating the role of He in the capture rates of Pop III stars. As expected, we find that the inclusion of the heavier He nuclei leads to an enhancement of the overall capture rates, further improving the potential of Pop III stars as dark matter probes.

The Innovate Seminar Series speaker for October is Dr. Richard Anantua. He is a postdoctoral fellow at the Center for Astrophysics | Harvard & Smithsonian and Black Hole Initiative at Harvard. As a member of the Event Horizon Telescope Collaboration, Richard coordinates the Outreach Group and links observational features of near-horizon shadows of supermassive black holes to the plasma physics of nearby accretion disks and jets which light them up. Prior to Harvard, Richard held a postdoctoral/instructor position at U.C. Berkeley after completing his Ph.D. in Physics at Stanford under Prof. Roger Blandford and a B.S. from Yale in (Physics & Philosophy) and (Economics & Mathematics). The title of this talk is "Modeling Black Hole Plasma Jets for EHT Observations". Registration is required to participate in this talk. Click the image above to register.

Abstract:

Recent radio observations of inflowing and outflowing plasma in the vicinity of supermassive black holes are linked to plasma physics models via simulations through a methodology called “Observing” Jet/Accretion flow/Black hole (JAB) Systems. For M87, HARM jet simulations are viewed from Very Large Array (43 GHz) to Event Horizon Telescope (230 GHz) scales to replicate the observed collimation and magnetic field configuration, while serving as the basis for a semi-analytic model used to generate polarization maps and spectra. This model varies plasma content from ionic (e-p) to pair (e-e+). Emission at the observed frequency is assumed to be synchrotron radiation from electrons and positrons, whose pressure is set to relate to the local magnetic pressure through parametric prescriptions. Polarization maps and spectra are found to be observationally distinguishable through positron effects such as decreasing intrinsic circular polarization and Faraday conversion. For Sagittarius A* in our Galactic Center, we include a turbulent heating electron temperature model. Intensity map movies simulating hourly timescales show that these models can be classified into at least four types: 1.) thin, asymmetric photon ring with best fit spectrum; 2.) coronal boundary layer with thin photon ring and steep spectrum; 3.) thick photon torus with flat spectrum; and 4.) extended outflow with flat spectrum. These models may be distinguishable by the Event Horizon Telescope.

The Innovate Seminar Series speaker for September 2021 is Caprice Phillips. Caprice Phillips is a third year PhD student at The Ohio State University working with Dr. Ji Wang in the Department of Astronomy. Her research involves the detectability of potential biosignatures in the atmospheres of gas dwarf exoplanets with upcoming telescopes like JWST and Twinkle. The title of her talk is "Detecting Biosignatures in the Atmospheres of Gas Dwarf Planets with the James Webb Space Telescope".

Abstract

Exoplanets with radii between those of Earth and Neptune have stronger surface gravity than Earth, can retain a sizable hydrogen-dominated atmosphere. In contrast to gas giant planets, we call these planets gas dwarf planets. The James Webb Space Telescope (JWST) will offer unprecedented insight into these planets. Here, we investigate the detectability of ammonia (NH3, a potential biosignature) in the atmospheres of seven temperate gas dwarf planets using various JWST instruments. We use petitRADTRANS and PandExo to model planet atmospheres and simulate JWST observations under different scenarios by varying cloud conditions, mean molecular weights (MMWs), and NH3 mixing ratios. A metric is defined to quantify detection significance and provide a ranked list for JWST observations in search of biosignatures in gas dwarf planets. In this talk, I will show that it is very challenging to search for the 10.3-10.8 micron NH3 feature using eclipse spectroscopy with MIRI in the presence of photon and a systemic noise floor of 12.6 ppm for 10 eclipses. NIRISS, NIRSpec, and MIRI are feasible for transmission spectroscopy to detect NH3 features from 1.5 to 6.1 microns under optimal conditions such as a clear atmosphere and low MMWs for a number of gas dwarf planets. In this talk, I will show that searching for potential biosignatures such as NH3 is feasible with a reasonable investment of JWST time for gas dwarf planets given optimal atmospheric conditions.

Abstract

The mathematical topic of Supersymmetry (SUSY) is about a half century old. Yet, it still holds unsolved mysteries. Gell-Mann's 'Eight-fold Way' diagrams opened the door of discovery for the Standard Model. The analogous diagrammatic structures for SUSY are poorly understood and only now starting to be revealed. This search is presented in this talk.

Abstract

Outside of theoretical physics, it's hard to really understand the buzz around string theory and supersymmetry. In the past couple of decades, we've gotten quite a bit of mileage through the framework of holography, namely the Anti de Sitter/Conformal Field Theory (AdS/CFT) correspondence, where a theory of quantum gravity can be described by a purely quantum theory on its boundary. I don't know very much at all about the details of string theory, but I do know how to compute quantities in certain conformal field theories, so the audience is in luck: we will set out to compute quantities in string theory only with the conformal field theory on the boundary, so we won't need to know a thing about string theory. Specifically, we will compute integrated correlators in N=4 SO(2N) Super-Yang-Mills theory, which correspond to scattering amplitudes in the bulk supergravity theory. This is a pretty overkill method as far as ordinary quantum field theories are concerned, so we'll also see how supersymmetry can be a powerful tool for theories like QCD, which describes the strong nuclear force.

 

Abstract:

In this talk I will revisit the hybrid inflation theory first proposed by A.Linde. In his original paper, A.Linde considers an inflationary model involving two coupled massive scalar fields. This theory fails to predict a spectral index coherent with the Planck data. I will focus on a model developed by Stewart that allows for a viable spectral tilt that fits the current data. We will see that we can study this hybrid inflation as an effective field theory showing that its predictions agree with the Planck data. Furthermore, I will explore the quantum stability of this model and outline a possible mechanism realizing the scalars as compact axions dual to massive 4-forms.

Talk Abstract:

Monolayer FeSe on a SrTiO₃ (STO) substrate is a high-temperature superconductor with reported T_c as high as 100 K, but the mechanism for such enhanced T_c remains poorly understood. Samantha's research characterizes the atomic structural and chemical composition of the FeSe/STO interface using transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). These measurements reveal the presence of selenium in the top layers of STO, located on interstitial sites and in the TiO₂ layers. We support our measurement with density functional theory (DFT) calculations. We discuss implications of our findings on substrate-induced electron doping in the FeSe/STO heterostructure.

Speaker Bio:

Next-Generation Simulations of The Remarkable Deaths of Massive Stars

Core-collapse supernova explosions (CCSN) are one possible fate of a massive star. Simulations of CCSNe rely on the properties of the massive star at core-collapse. As such, a critical component is the realization of realistic initial conditions. Multidimensional progenitor models can enable us to capture the chaotic nuclear shell burning occurring deep within the stellar interior. I will discuss ongoing efforts to progress our understanding of the nature of massive stars through next-generation hydrodynamic stellar models. In particular, I will present recent results of three-dimensional hydrodynamic models of massive stars evolved for the final moments before collapse. These recent results suggest that realistic 3D progenitor models can be favorable for obtaining robust models of CCSN explosions and are an important aspect of massive star explosions that must be taken into consideration. I will conclude with a brief discussion of the implications our models have for predication of multi-messenger signals from CCSNe.

Speaker Bio

Carl E. Fields is a Ph.D. Candidate in the Department of Physics & Astronomy at Michigan State University working with Prof. Sean Couch. Carl's research focusses on astrophysical sources of gravitational waves, stellar nucleosynthesis, and multi-dimensional simulations of core-collapse supernova explosions and their massive star progenitors. Carl is jointly supported by the National Science Foundation and Los Alamos National Laboratory. Carl received his undergraduate degrees in Physics and Earth & Space Exploration (Astrophysics) where he worked with Prof. Frank Timmes. In 2020, Fields was award the Price Prize by Ohio State University and named to the Forbes 2021 Class of 30 under 30 for Science.

Seminar Abstract

It is well known that a light, pseudo-scalar particle called the Axion can solve several fundamental physics problems. Proposed to explain the lack of a neutron EDM, such a weakly interacting particle has the right characteristics to explain formation of galaxies, by providing the needed mass in the form of Cold Dark Matter. Additionally, there has been data collected on the decay of several radioactive nuclei suggesting the need for weakly interacting particles streaming from the sun and throughout the galaxy. This talk will review the theory behind axion particles, examples of early experimental searches and some new search techniques. The nuclei data reviewed here can provide complimentary results to any existing axion searches as well as a novel type of search that can be conducted.

The National Society of Black Physicists (NSBP) is pleased to present Dr. Charles Brown as the next speaker for the NSBP ‘Innovate Seminar Series’! The Innovate Seminar Series is a new forum for NSBP members to share their research ideas and projects in a non-specialist way. Dr. Brown is an experimental quantum physicist, science communicator, and champion for increased Black American representation in physics. He is now a postdoctoral scholar and Ford Foundation fellow at the University of California, Berkeley. Special thanks to the Kavli Institute of Theoretical Physics for their continued support of the NSBP Innovate Seminar Series.

Speaker Bio

Dr. Charles Brown is an experimental quantum physicist, science communicator, and champion for increased Black American representation in physics. Charles earned his B.S. with honors in physics at the University of Minnesota, Twin Cities. He also earned a Ph.D. in physics at Yale University, where he conducted experiments with superfluid helium-filled optical cavities, and magnetically levitated superfluid helium drops in vacuum. He is now a postdoctoral scholar and Ford Foundation fellow at the University of California, Berkeley. At Berkeley, he is a member of the Ultracold Atomic Physics Group, where he investigates ultracold atoms trapped in optical lattices, which offers an avenue to study a rich variety of many-body quantum physics phenomena. Charles has a long history of both empowering students - spanning the elementary through graduate levels - to pursue their STEM interests, and advocating for the interests of Black students. Charles recently wrote a widely read op-ed about Black underrepresentation in physics that appeared in Physics Today, which has been sparking important conversations regarding necessary changes in the physics community.

Seminar Abstract

Geometric frustration of particle motion in a kagome lattice causes the single-particle band structure to exhibit a dispersion-less, flat band. Generally, frustration can cause a vast degeneracy of low-energy states, and instabilities in the presence of atomic interactions may lead to the manifestation of exotic states of matter. The kagome lattice, a pattern of vertex-sharing triangular plaquettes, offers the highest degree of frustration among two-dimensional lattice geometries. We create an optical kagome lattice by superimposing two optical triangular lattices made from laser light with commensurate wavelengths. We probe the band structure of the kagome lattice by preparing a Bose-Einstein condensate in excited Bloch states of the lattice, and then measuring the atoms’ group velocity via the atomic momentum distribution. We find that atomic interactions renormalize the kagome lattice band structure, significantly increasing the dispersion of the third band, which, according to non-interacting band theory, should be nearly flat (dispersion-less). Measurements at various lattice depths and gas densities agree quantitatively with predictions from the lattice Gross-Pitaevskii equation, which indicates that the observed band structure distortion, onset by atomic interactions, is caused by the distortion of the overall lattice\

Seminar Abstract

High-temperature superconductivity in the cuprates remains an unsolved problem because the cuprates start off their lives as Mott insulators in which no organizing principle such a Fermi surface can be invoked to treat the electron interactions. Consequently, it would be advantageous to solve even a toy model that exhibits both Mottness and superconductivity. In 1992 Hatsugai and Khomoto wrote down a momentum-space model for a Mott insulator which is safe to say was largely overlooked, their paper garnering just 21 citations (6 due to our group). I will show exactly[1] that this model when appended with a weak pairing interaction exhibits not only the analogue of Cooper's instability but also a superconducting ground state, thereby demonstrating that a model for a doped Mott insulator can exhibit superconductivity. The properties of the superconducting state differ drastically from that of the standard BCS theory. The elementary excitations of this superconductor are not linear combinations of particle and hole states but rather superpositions of doublons and holons, composite excitations signaling that the superconducting ground state of the doped Mott insulator inherits the non-Fermi liquid character of the normal state.
Additional unexpected features of this model are that it exhibits a superconductivity-induced transfer of spectral weight from high to low energies and a suppression of the superfluid density as seen in the cuprates.

View the talk below and download a PDF version of the talk here.

Seminar Abstract

With the success of the Event Horizon Telescope, identifying new observational signatures of black holes are of increasing interest. In this talk we investigate blueshift of photons from an equatorial accretion disk of emitters orbiting on stable circular orbits terminating at the ISCO. We analytically calculate the blueshift of photons that escape to the celestial sphere (instead of falling into the black hole) and numerically calculate the maximum blueshift received by an observer at fixed angle on the celestial sphere.

Special thanks to the Kavli Institute for Theoretical Physics at the University of California Santa Barbara for their support of this seminar series.