William & Mary

Spring 2019

January 18, 2019 (Friday) 4:00-5:00p.m. Small Hall 111
Speaker: Meg Urry, Yale University
Host: I. Novikova
Title: New Insights into the Cosmic Growth of Supermassive Black Holes
Abstract: Using a “wedding cake” combination of multi-wavelength X-ray+infrared+optical surveys, we measure the growth of supermassive black holes at the centers of galaxies over the last ~12 billion years. Most actively growing black holes (“Active Galactic Nuclei” or AGN) are heavily obscured and thus look like inactive galaxies in optical surveys, so our census has effectively quadrupled the amount of accretion, and thus the amount of energy deposited in their host galaxies. Theorists have suggested that this energy could quench star formation and strongly affect galaxy evolution (“feedback”), and that AGN episodes are triggered by major galaxy mergers. Such a picture may hold for the most luminous quasars but our morphological analyses show that, both in the local universe and 7-9 billion years ago (at the peak of star formation and black hole growth), most galaxies are disk-dominated (i.e., not merger products) and they evolve too slowly for AGN to play a significant role. That is, there are two distinct modes of galaxy evolution, with mergers and AGN feedback affecting only a minority. Bio: https://cuwip2019.wm.edu/speakers/meg-urry/ Prof. Meg Urry's visit is supported by the 100th anniversary of women committee

January 22, 2019(Tuesday) 4:00-5:00p.m. Small Hall 111
Speaker: Seung Sae Hong, Stanford University
Host: E. Rossi
Complex oxide membranes: more freedom for artificial materials
Abstract: The ability to create materials in two-dimensional (2D) form has repeatedly had a transformative impact on science and technology. In parallel with the exfoliation of layered crystals, atomic-scale thin film growth of complex materials has enabled the creation of artificial 2D heterostructures with emergent phenomena, as seen in perovskite oxide heterostructures. By releasing the oxide thin films from substrates, we want to grant more freedom in design, manipulation, and characterization of the artificial material.

Here I will present early harvests in the research of oxide membranes. The first topic is the ultrathin limit of oxide membranes, where the freestanding layer faces an inherent 2D limit of the crystalline lattices described by a 2D topological phase transition. In the second part, I will discuss how to utilize the freestanding geometry of the oxide membranes, in particular controlling the lattice of 2D quantum materials. In the example of magnetic oxides, an extreme degree of strains was induced to control electromagnetic ground states, revealing a new phase diagram inaccessible in bulk crystals and thin films.

January 25, 2019 (Friday) 4:00-5:00p.m. Small Hall 111
Speaker: Derek Meyers, University of California
Host: E. Rossi
Forging next generation materials through atomic layer engineering
Abstract: Society’s progress throughout history has been driven by our ability to craft the world around us into functional technologies. In this talk, we will explore one of the most recent emergent methods to create artificial crystalline structures of complex oxides with unprecedented properties and functionalities. Pulsed laser deposition allows stacking of single atomic layers of disparate materials with sharp interfaces and high crystalline quality. To directly probe these nanoscale interfaces, advanced synchrotron X-ray characterization will be introduced as a powerful method for investigating the charge and magnetic behavior of these artificial structures. Fascinating physical phenomena derived from the strongly correlated electrons, such as superconductivity and 2D magnetism, will be showcased as recent paragons of this growth and characterization methodology. In particular, the role of electron-phonon coupling in the recent SrTiO3-based superconductors and the magnetic behavior of isolated strongly spin-orbit coupled SrIrO3 layers will be discussed. We will conclude this talk with a discussion on the promising future applications for these methods, with an emphasis on topological phenomena and quantum information science.

January 29, 2018 (Tuesday) 4:00-5:00p.m. Small Hall 111
Speaker: Guangxin Ni, Columbia University 
Host: E. Rossi
Title:  Nano-light in van der Waals heterostructures
Abstract: Near-field nano-optics as a new and vibrant area of research has enabled the control and manipulation of electromagnetic radiations at the nano-meter length scales. In this talk, Dr. Ni will discuss recent nano-optical experiments on two-dimensional graphene/hexagonal boron nitride (G/hBN) van der Waals heterostructures. By harnessing infrared nano-optics one can directly image surface plasmon polaritonic standing waves and uncover its fundamental limits of graphene plasmonics [Nature (2018)]. The presence of periodic moiré patterns enables further fine tuning of the host materials and yielding rich insights into the electronic phenomenons. This has been manifested in both G/hBN moiré patterns [Nature Materials (2015)] and twisted bilayer graphene moiré structures [Science (2018)]. Furthermore, by examining the sub picosecond dynamics of plasmons in a unique set of pump-probe spectroscopy apparatus we were able to switch on plasmon on demand [Nature Photonics (2016)].

February 1, 2019 (Friday) 4:00-5:00p.m. Small Hall 111
Speaker: Daniel Rhodes, Columbia University
Host: E. Rossi
Title: Disorder and Superconductivity in 2D TMD Heterostructures
Two dimensional transition metal dichalcogenides (TMD) interest due to their novel optical and electronic properties, and their potential for application. However, observations of the emergent phenomena in these materials is limited by scattering and nonradiative recombination processes due to a large density of defects and disorder at the interface. In this work, using a combination of scanning tunneling microscopy (STM) and scanning transmission electron microscopy (STEM), we characterize the atomic and electronic nature of intrinsic point defects in single crystal TMDs. We demonstrate that these defects can be reduced by almost three orders of magnitude (1013/cm2 to 5 x 1010/cm2) through a self-flux growth method. This method of growth can be applied across a variety of TMDs and we further utilize this method to grow the superconducting TMD—MoTe2. In the bulk, MoTe2 is a type II Weyl semimetal with a superconducting transition temperature (Tc) of 120 mK.  I will show that in the clean limit, the superconducting transition temperature is enhanced by a factor of 60x in monolayer Td-MoTe2, while still retaining a low carrier density (~1013/cm2).  Reflecting the low carrier density, the critical temperature, magnetic field, and current density are all tunable by an applied gate voltage. Furthermore, the temperature dependence of the in-plane upper critical field is distinct from that of 2H-TMDs, consistent with a complex spin texture predicted by ab initio theory.

February 4, 2019 (Monday) 4:00-5:00p.m. Small Hall 111
Speaker: Hyunsoo Kim, University of Maryland-College Park
Host: E. Rossi
Anomalous electrical transport and unconventional superconductivity in Half-Heusler YPtBi
Abstract: The Half-Heusler RTBi compound (R=rare earth, T=transition metal) is a promising platform to study interactions between topological ordered phase and various symmetry-breaking ordered ground states. YPtBi is a prototypical topological semimetal with j=3/2 conduction fermions driven by strong spin-orbit coupling. It undergoes a superconducting phase transition at T = 0.8 K surprisingly only with a carrier density as low as 1018 cm-3. This challenges conventional Eliashberg formalism of superconductivity based on the electron-phonon interaction. Naturally, the normal state and superconducting state properties in the topological semimetal YPtBi are of great interest. In this talk, I will present recently discovered anomalous transport and superconducting properties in YPtBi. In normal state, the angle-dependent magnetoresistance breaks the rotational symmetry of the underlying cubic crystal structure, which can be understood by two-channel conduction model with at least one channel having a lower rotational symmetry than the crystal symmetry. At high magnetic fields, Shubnikov-de Haas quantum oscillations are readily visible. However, the amplitude of the quantum oscillation abruptly vanishes along certain crystallographic orientations. I will explain the unusual quantum oscillations in YPtBi within a semiclassical picture with the effective spin g-factor of conduction fermions affected by strong spin-orbit coupling. Finally, I will talk about unconventional nodal superconductivity with j=3/2 fermions in cubic YPtBi, which can be explained by unprecedented higher spin Cooper pairing beyond spin-triplet.

February 6, 2019 (Wednesday) 4:00-5:00p.m. Small Hall 111
Speaker: Alessandro Pilloni, 
European Centre for Theoretical Studies in Nuclear Theory and related areas (ECT*), Italy
Host: D. Armstrong
Title: Challenges in Hadron Spectroscopy
Abstract: Quantum Chromodynamics is universally acknowledged as the theory of strong interactions. However, the way how the fundamental constituents (quark and gluons) arrange themselves into the hadrons that are actually observed in experiments, is still a mystery. Even at a phenomenological level, the presence of multiple overlapping states leads to intricate interference patterns that make the extraction of meaningful information complicated.
In this colloquium, I will explain what challenges we face every day to understand the spectrum of strong interacting particles. I will discuss the role of amplitude analysis in converting the raw experimental data into robust physics information. I will finally show how these tools allow us to solve a longstanding puzzle about the elusive hybrid mesons. 

February 11, 2019 (Monday) 4:00-5:00p.m. Small Hall 111
Speaker: Xiaoqian Chen, Material Sciences Division, Lawrence Berkeley National Laboratory
Host: E. Rossi
Charge Density Wave Memory in a Cuprate Superconductor La2-xBaxCuO4
Abstract: Charge density wave (CDW) order is known to coexist with superconductivity in essentially all underdoped cuprates. Yet, its precise nature and the relationship with superconductivity is still unclear. Specifically, whether the CDW is static or fluctuating is a long-standing question whose answer will provide deep insight into whether the CDW order competes or cooperates with superconductivity.

In the first part of my talk, I will show how performing diffraction with coherent x-rays can be a test for dynamics. Coherent x-ray scattering from electronic and magnetic orders result in complex interference (speckle) patterns. These speckle patterns are dependent on the detailed order parameter configuration and therefore provide insight into their structure, motion, and dynamics. By correlating speckle positions over time, we showed that the CDW domains in underdoped cuprate La2-xBaxCuO4 (LBCO) are surprisingly static, with no evidence of significant fluctuation well into the superconducting state.

Is the static nature of CDW order in LBCO universal for all underdoped cuprates? Motivated by this question, I will discuss in the second part of my talk, the CDW pinning mechanism in LBCO. By tracking the history of CDW speckle patterns upon thermal cycling, we have found an unexpected pinning mechanism where memory is only lost on cycling across the structural transition at 240(3) K that restores the 4-fold symmetry of the copper-oxide planes instead of the CDW ordering temperature at 54K.

Finally, I would like to end my talk by diving into the exciting world of coherent x-ray physics and discuss its capability for current and future research.

February 15, 2019 (Friday) 4:00-5:00p.m. Small Hall 111
Speaker: Chia Chang, Lawrence Berkeley National Laboratory
Host: J. Dudek
Predicting the neutron lifetime from the Standard Model
Abstract: From quarks and gluons, the emergence of nucleons and their properties can be predicted from the modern theory of the strong interaction, quantum chromodynamics. Precise predictions from the Standard Model in tandem with observations from experiments provides a window for revealing new physics beyond. One such example is the neutron lifetime, where tension at 4 standard deviations is measured between independent experiments. With the maturation of lattice quantum chromodynamics over the past decades, I will present our published result of $g_A$, which governs the neutron lifetime, to one percent precision commensurate with measurements from experiment. With the arrival of near-exascale computing made available by the largest supercomputers in the world, I will show improvements to $g_A$ which are approaching enough precision to start revealing possible hints of physics beyond the Standard Model. I will conclude by discussing future calculations with promising potential towards predicting the radius of the proton, and other observables that may help us understand the origin of matter.

February 20, 2019 (Wednesday) 4:00-5:00p.m. Small Hall 111
Speaker: Jeremy Green, Deutsches Eletronen-Synchrotron, Germany
Host: J. Dudek
Title: Calculating the quark substructure of the proton and neutron
Abstract: Protons and neutrons (together called nucleons) are the basic building blocks of ordinary matter, along with electrons. Unlike electrons, they are not fundamental particles in the Standard Model of particle physics — they are bound states of quarks and gluons interacting as described by the theory of quantum chromodynamics (QCD). Because QCD is strongly coupled at low energies, it is difficult to determine the structure of nucleons from first principles. This can be done using lattice QCD: a formulation of QCD suitable for performing calculations on supercomputers, which provides a way to calculate nucleon structure with controllable uncertainties.

In this talk I will discuss some basic properties of nucleons such as the proton radius (which is the subject of a large disagreement between different experiments), and how they can be computed using lattice QCD. I will give an overview of the current status of these calculations, and present some progress in understanding the role that strange quarks play in nucleons.

February 22, 2019 (Friday) 4:00-5:00p.m. Small Hall 111
Speaker: Jacobo Ruiz de Elvira, University of Bern, Switzerland
Host: K. Orginos
Title: Strong interactions and the precision frontier of Hadron Physics
Although QCD is well established as the theory of strong interactions, the hadron spectrum is still not understood from first principles. Nevertheless, the understanding of strong interactions with high precision has become in recent years a prerequisite for progress in nuclear and particle physics in many different areas. Dispersion relations are a consequence of causality, which mathematically translates into analyticity conditions on scattering amplitudes. In this talk, I will show how the fruitful combination of dispersion-theoretical methods with modern high-precision experimental data  allows one to determine low-energy hadron observables with unprecedented accuracy.  I will highlight this area of current research with two examples: pion-pion(kaon) scattering and the role of the lightest scalars mesons;  2. the mass of the nucleon and its connection to pion-nucleon scattering.

March 1, 2019 (Friday) 4:00-5:00p.m. Small Hall 111
Speaker: Christopher Monahan, University of Washington
Host: D. Armstrong
Title: Lattice quantum chromodynamics and the search for new physics

Abstract: The Standard Model of Particle Physics, the mathematical framework that describes the basic building blocks of the visible Universe, has been enormously successful . But we know that it is incomplete: it doesn’t explain the origin of neutrino masses, for example, nor does it  incorporate gravity. In fact, the Standard Model explains only 5% of the current energy density of the Universe! The Large Hadron Collider (LHC) has also been hugely successful - discovering the long-expected Higgs particle and greatly refining our knowledge of the Standard Model. But the LHC has been marked by the lack of direct experimental signatures of new fundamental particles. So where is all the new physics hiding? I will discuss the role that lattice quantum chromodynamics (QCD) plays in attempting to answer this question. In particular, I will highlight two arenas in which lattice QCD can help us search for new physics, through precision tests of the Standard Model, and suggest new lattice calculations that could help us understand what lies beyond the Standard Model.

 April 12, 2019 (Friday) 4:00-5:00p.m. Small Hall 111
Speaker: Peter Armitage, Department of Physics, Johns Hopkins University
Host: M. Qazilbash
Title: On Ising's model of ferromagnetism

Abstract: The 1D Ising model is a classical model of great historical significance for both classical and quantum statistical mechanics. Developments in the understanding of the Ising model have fundamentally impacted our knowledge of thermodynamics, critical phenomena, magnetism, conformal quantum field theories, particle physics, and emergence in many-body systems. Despite the theoretical impact of the Ising model there have been very few good 1D realizations of it in actual real material systems. However, it has been pointed out recently, that the material CoNb2O6, has a number of features that may make it the most ideal realization we have of the Ising model in one dimension.   In this talk I will discuss the surprisingly complex physics resulting in this simple model and review the history of "Ising’s model” from both a scientific and human perspective.  In the modern context I will review recent experiments by my group and others on CoNb2O6.  In particular I will show how low frequency light in the THz range gives unique insight into the tremendous zoo of phenomena arising in this simple model system.

April 19, 2019 (Friday) 4:00-5:00p.m. Small Hall 111
Speaker: Victor Galitski, Joint Quantum Institute & Condensed Matter Theory Center, University of Maryland
Host: E. Rossi
Title: Strong Correlations Meet Topology in Kondo Insulators

Abstract: Topological states of quantum matter represent a rapidly developing area of research, where a fascinating variety of exotic phenomena occur, ranging from unusual transport properties to fractionalized excitations that may emerge at system’s defects. Of particular recent interest has been the topic of strongly-interacting topological phases, where electronic correlations and topology both play an important role. In this talk, I will review recent theoretical and experimental work on a relatively new class of such interacting topological material system – topological Kondoinsulators, which appear as a result of interplay between strong correlations and spin-orbit interactions. I will start by explaining in simple terms the basics of topological quantum matter, including the by now standard theory of topological band insulators. Then, I will use these concepts to show that the conduction electrons and localized magnetic moments in certain heavy fermion compounds hybridize to give rise to a topological insulating behavior. I will explain key experimental results, which have confirmed our predictions in the Samarium hexaboride heavy fermion compound, where the long-standing puzzle of the residual low-temperature conductivity has been shown to originate from topological surface states. I will also mention several recent theory-experiment collaborative projects that led to the development of a “topological device” and new methods to extend topological behavior in Kondo insulators from a few Kelvin to room temperature. In conclusion, I will discuss a series of recent puzzling experiments, which unexpectedly observed quantum oscillations, typical to a metal, coming from an inert, insulating bulk of Kondo insulators, which may represent a smoking gun of a new fractionalized state of matter.