Colloquia & Seminars
  • Materials Science and Engineering Seminar
    Speaker Name: Albert Davydov

    Speaker Institution : NIST

    Title : Low-Dimensional Semiconductors for Electronics, Sensors, and Energy

    Abstract : In addition to conventional thin film structures, 1D and 2D nanostructures such as semiconductor nanowires and graphene-like layers have attracted considerable attention due to their unique electronic, magnetic, optical, thermal and mechanical properties, complemented with superior structural quality and high surface-to-volume ratio. Nanowires and 2D layers represent nanoscale building blocks for on-chip integration for optoelectronic, sensor, and energy applications for portable ubiquitous electronics on flexible platforms.

    To realize new applications, the controlled fabrication of nanowires and 2D layers with defined geometries and electronic properties as well as their integration with planar device structures is required. This talk discusses fabrication and characterization of silicon and gallium nitride nanowire materials and devices, including single and arrayed nanowire transistors, chemical and bio- sensors, Li-ion batteries, and LEDs. A special case of developing periodic arrays of vertically aligned GaN core-shell nanostructures for p-i-n photodetectors, realized with a combination of top-down etch and subsequent chemical vapor deposition, is presented.

    The 2D research is illustrated by fabrication and testing of field-effect-transistors (FETs) composed of monolayer to few-layer MoS2 thin films, where device transport characteristics are governed by inter-layer coupling and electrically active surface states.
    When: Fri, October 24, 2014 - 1:00pm
    Where: Room 2108, Chemical and Nuclear Engineering Bldg
  • JQI Seminar
    Speaker Name: Donhee Ham

    Speaker Institution: Harvard

    Title: Manipulating light with mass of massless graphene electrons +

    Abstract: While one of the most celebrated physics of graphene is the behavior of its individual electrons as massless relativistic particles, they do exhibit mass when they move together, which is at the heart of graphene plasmonics. I will discuss our recent measurement of this mass of massless graphene electrons, and its application in ultra-subwavelength plasmonic devices at THz frequencies. And on a different topic, I will introduce our program for biomolecular NMR. NMR spectroscopy is one of the most powerful bio-analytical tools with its ability to elucidate 3D structure & function of bio molecules. Particularly, its use in structural biology and pharmaceutical screening has proven enormously fruitful. But it is inherently too slow while the workload of modern biology & medicine continues to increase. I will discuss our biomolecular NMR work using silicon chips and our vision to enable high-throughput biomolecular NMR.
    Host: James Williams
    When: Mon, October 27, 2014 - 11:00am
    Where: CSS 2400
  • EPT Seminar
    Title: Composite Higgs boson from top dynamics

    Speaker: Bogdan Dobrescu, Fermilab

    Abstract: The Higgs boson may be a composite particle, made of a top quark and a vectorlike quark. The binding should be non-confining, and may be provided by a new
    spontaneously-broken gauge interaction. I will show that the Higgs boon can arise as
    the pseudo Nambu-Goldstone boson of the chiral symmetry associated with the vector-like
    quark and the top quark. Its mass can be computed, and is consistent with the value of 125 GeV measured by the CMS and ATLAS collaborations.
    When: Mon, October 27, 2014 - 3:00pm
    Where: PSC 3150
  • Biophysics Seminar
    Speaker Name: Valeri Barsegov

    Speaker Institution : Barsegov

    Title: Fluctuating nonlinear spring model of mechanical deformation of biological particles

    Abstract : We present a new theory for modeling spectral lineshapes available from single-particle forced indentation experiments. The theory considers weakly non-linear Hertzian deformation due to a physical contact between the indenter and the biological particle, and bending deformations of other portions of the particle structure modeled as ‘vertical beams’. The bending of beams beyond the critical point sets in the particle dynamic transition to the collapsed state, an extreme event leading to the catastrophic force drop as observed in the force (F)-deformation (X) spectra (FX curves).
    The theory interprets fine features of the spectra, i.e. the slope of the FX curves and the force-peak signal, in terms of mechanical characteristics such as the Young’s moduli for Hertzian and bending deformations, and the Weibull probability distribution of the maximum strength with the scale parameter and shape parameter. The theory is applied to model the FX curves for several spherically shaped virus particles – CCMV, TrV, and AdV.
    When: Mon, October 27, 2014 - 4:00pm
    Where: 0112 Chemistry Building
  • CMTC Seminar
    Speaker Name: Kun Yang

    Speaker Institution: NHMFL & Florida State University

    Title: Entanglement Scaling Laws and Eigenstate Thermalization in Many-Particle Systems

    Abstract: Entanglement is perhaps the most counter-intuitive aspect of quantum mechanics, and provides the sharpest distinction between quantum and classical descriptions of nature. The most widely used measure of entanglement is the entanglement entropy. For extended quantum systems, ground states of local Hamiltonians are expected to follow the so called “area law”, which states that the entanglement entropy is proportional to the surface area of the subsystem. However, violations of the area law do exist, and it is important to understand their origin. In 1D they are found to be associated with quantum criticality. Until recently the only established examples of such violation in higher dimensions are free fermion ground states with Fermi surfaces, where it is found that the area law is enhanced by a logarithmic factor. In Ref. [1], we use multi-dimensional bosonization to provide a simple derivation of this result, show that the logarithimic factor has a 1D origin. More importantly the bosonization technique allows us to take into account the Fermi liquid interactions, and obtain the leading scaling behavior of the entanglement entropy of Fermi liquids. The central result of our work is that Fermi liquid interactions do not alter the leading scaling behavior of the entanglement entropy, and the logarithmic enhancement of area law is a robust property of the Fermi liquid phase.

    In sharp contrast to the fermioic systems with Fermi surfaces, quantum critical (or gapless) bosonic systems do not violate the area law above 1D (except for the case discussed below). The fundamental difference lies in the fact that gapless excitations live near a single point (usually origin of momentum space) in such bosonic systems, while they live around an (extended) Fermi surface in Fermi liquids. In Ref. [2], we studied entanglement properties of some specific examples of the so called Bose metal states, in which bosons neither condense (and become a superfluid) nor localize (and insulate) at T=0. The system supports gapless excitations around ``Bose surfaces", instead of isolated points in momentum space. We showed that similar to free Fermi gas and Fermi liquids, these states violate the entanglement area law in a logarithmic fashion. Our results demonstrate that perhaps area-law violation in high dimensions is more common than previously thought; in particular the existence of Fermi surface(s) is not a prerequisite for it.

    Compared to ground states, much less is known concretely about entanglement in (highly) excited states. Going back to free fermion systems, in [3] we show that there exists a duality relation between ground and excited states, and the area law obeyed by ground state turns into a volume law for excited states, something that is widely expected but very hard to prove. Most importantly, we find in appropriate limits the reduced density matrix of a subsystem takes the form of thermal density matrix, providing an explicit example of the eigenstate thermalization hypothesis. Our work [3] explicitly demonstrates how statistical physics emerges from entanglement in a single eigenstate.

    [1] Entanglement Entropy of Fermi Liquids via Multi-dimensional Bosonization, Wenxin Ding, Alexander Seidel, Kun Yang, Phys. Rev. X 2, 011012 (2012).

    [2] Violation of Entanglement-Area Law in Bosonic Systems with Bose Surfaces: Possible Application to Bose Metals, Hsin-Hua Lai, Kun Yang, N. E. Bonesteel, Phys. Rev. Lett. 111, 210402 (2013).

    [3] Entanglement entropy scaling laws and eigenstate thermalization in free fermion systems, Hsin-Hua Lai, Kun Yang, arXiv:1409.1224.

    Host: Bitan Roy

    http://www.physics.umd.edu/cmtc/seminars.html
    When: Tue, October 28, 2014 - 11:00am
    Where: 2205 Toll Physics Building
  • No Physics Colloquium this week

    When: Tue, October 28, 2014 - 4:00pm
    Where:
  • Lab for Physical Sciences Seminar
    Speaker Name: Dr. Michael Dreyer

    Speaker Institution : Lab for Physical Sciences (UMCP)

    Title: Josephson Scanning Tunneling Microscopy at mK Temperatures

    Abstract: Scanning tunneling microscopy allows the observation of the local density of electron states at the atomic scale. Operating at mK temperatures not only enhances the energy resolution, but also allows studying low temperature phenomena such as superconductivity. Using a superconducting tip along with a superconducting sample the STM junction behaves like an ultra-small Josephson junction. Such a setup is referred to as Josephson STM. Due to the small Josephson energy compared to the thermal energy the supercurrent branch of an I(V) curve is suppressed and only a phase diffuse zero bias conduction peak remains.
    We observed the zero bias conduction peak in our STM setup using niobium tips and a niobium (100) single crystal as the sample. Tunneling spectra show additional structures due to multiple Andreev reflection. Near the ZBCP electronic coupling to the environment leads to several dips in the conduction signal. Upon microwave (µW) irradiation the peaks split due to the absorption or emission of µW photons which is well explained by theory and shows that the tunneling charge carriers are indeed cooper pairs. Furthermore, current oscillations were observed at energies of negative differential conductance showing characteristics of relaxation oscillation. In the talk I will describe our setup and discuss the observed effects. Furthermore, I will describe our plans on how to overcome the phase diffusion and locally measure the superconductive
    When: Wed, October 29, 2014 - 3:30pm
    Where: Lab for Physical Sciences downstairs conference room
  • CMTC Seminar
    Speaker Name: Brian Swingle

    Speaker Institution: Stanford

    Title: Renormalization Group Constructions of Topological Quantum Liquids

    Abstract: I will discuss recent work with John McGreevy (1407.8203) on constructing ground state wavefunctions of general gapped Hamiltonians using a renormalization group approach. The formalism provides a number of results including a partial proof of the area law for entanglement entropy, efficient tensor network representations for wavefunctions, a definition of short- and long-range entanglement, and a classification scheme which we conjecture applies to all gapped phases. A special role is played by what we call topological quantum liquids which are gapped phases that are insensitive to the "shape" of space (like quantum Hall fluids).

    Host: Jay D. Sau

    http://www.physics.umd.edu/cmtc/seminars.html
    When: Thu, October 30, 2014 - 11:00am
    Where: 2205 Toll Physics Building
  • Applied Dynamics Seminar
    Speaker Name: Andrey Vilesov

    Speaker Institution : USC, Dept of Chemistry

    Title: X-ray diffraction imaging of quantum vortices in superfluid He droplets
    When: Thu, October 30, 2014 - 12:30pm
    Where: IREAP Large Conference Room, ERF 1207
  • Refreshments for CNAM Cond. Matter Colloquium

    When: Thu, October 30, 2014 - 1:30pm
    Where: physics room 1305F, the "new" Toll Room
  • CNAM Condensed Matter Colloquium
    Speaker Name: Michael Lilly

    Speaker Institution: Sandia National Lab

    Title: Electron spin qubits using donors in silicon

    Abstract: Semiconductor nanoelectronic devices can be combined, or integrated, in a variety of ways to control a single electron and form a qubit. We have focused on silicon devices where the electron is confined on a single donor atom and its spin forms the qubit. Pulsed gate techniques allow spin readout for a single electron. At the end of the readout pulse, the electron is initialized to spin down. We demonstrate spin control by applying pulsed microwaves to coherently rotate the electron spin. Using these techniques, we have measured long spin lifetimes, measured single shot spin readout and recently observed coherent oscillations of a single electron spin. Devices for moving to two-qubits will be discussed at the end of the talk.

    Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
    When: Thu, October 30, 2014 - 2:00pm
    Where: physics room 1201
  • Super-Lie algebra cohomology and de Rham cohomology on supermanifolds
    Speaker: Paul Green
    Abstract: We will try to discuss how one could extend the work of Chavelley-Eilenberg to the case of supermanifolds.
    When: Thu, October 30, 2014 - 3:30pm
    Where: PHY 1117
  • JQI Special Seminar
    Speaker Name: Kartik Srinivasan

    Speaker Institution: NIST

    Title: Quantum frequency conversion and nanocavity optomechanics”.

    Abstract to follow

    Host: Luis Orozco
    When: Mon, November 3, 2014 - 11:00am
    Where: CSS 2400
  • EPT Seminar
    Title: LHC phenomenology of exotic fermions

    Speaker: Tanumoy Mandal, HRI

    Abstract: Many BSM extensions predict the existence of exotic fermions near the
    TeV scale. I will discuss the LHC phenomenology of such heavy exotic
    fermions, namely the vectorlike quarks that arise in various warped
    extra dimensional theories and the color octet electrons which appear
    in some quark-lepton compositeness models. In this regard, I will
    present a generic coupling extraction method with a toy example of
    exotic b' and a generic combined search strategy in the context of
    color octet electron.
    When: Mon, November 3, 2014 - 3:00pm
    Where: PSC 3150
  • Space and Cosmic Ray Physics Seminar
    Speaker Name: Daniel Berdichevsky

    Speaker Institution: Goddard Space Flight Center

    Title: On a few properties of very dilute matter frozen in space magnetic fields

    Abstract: For a case study, the flux-rope (FR) that passed Earth on June 2, 2014 (1) (see also listing of magnetic clouds and their properties in the Wind SC MFI science team site at http://wind.gsfc.nasa.gov/mfi/mag_cloud_S1.html), we proceed to interpret plasma and magnetic field observations in the context of MHD. The observations used are 3s average interplanetary magnetic field (Wind/MFI instrument) and 3s plasma (Wind/SWE instruments) data (2). After identifying the observed correlation between electron density, temperature and pressure in the plasma frame of reference we proceed to establish the existence of a relationship between these plasma observables with the magnetic field pressure. By assuming ideal MHD conditions to be valid we proceed to confirm that the medium is diamagnetic, as is to be expected for the MHD state of matter and magnetic field which is assumed to be a superconducting medium. Additionally we infer the presence of magnetization work, as well as a few other constitutive properties of this state of matter.

    (1) Berdichevsky D. B., R. P. Lepping, and C. J. Farrugia, Geometric considerations of the evolution of magnetic flux ropes, Phys. Rev. E67, doi:10.1103/PhysRevE.036405. Lepping R. P. et al, A summary of Wind magnetic clouds for years 1995 – 2003: model-fitted parameters, associated errors and classifications, Ann. Geophysicae, 24, 215-245, 2006.

    (2) Ogilvie, K. W., et al, SWE, A comprehensive plasma instrument for the Wind spacecraft, Space Sci. Rev., 71, 55 – 77, 1995; Lepping R. P., et al , The Wind Magnetic Field Investigation, Space Sci. Rev., 71, 207 – 229, 1995.

    Notes: Coffee, Tea & Cookies 4:15-4:30 PM
    When: Mon, November 3, 2014 - 4:30pm
    Where: CSS Room 2400
  • CMTC Seminar
    Speaker Name: Titus Neupert

    Speaker Institution: Princeton

    Title: Interacting surface states of three-dimensional topological insulators

    Abstract: The surface states of three-dimensional topological insulators are celebrated for their robustness against perturbations, provided that time-reversal symmetry and particle number conservation are not violated. In my talk, I want to survey their possible phases in the limit where interactions between the surface electrons are strong. To that end, I choose a spherical topological insulator geometry to make the surface amenable to numerical studies of finite size systems. In this case, the single-particle problem maps to that of Landau orbitals on the sphere with a magnetic monopole at the center that has unit strength and opposite sign for electrons with opposite spin.

    Restricting the single particle Hilbert space to the small region in the surface Brillouin zone that is covered by the surface Dirac cone enforces a nontrivial quantum geometry on the problem, resulting in distinct real-space localization properties of the electron orbitals. Assuming density-density contact interactions, we find superconducting and anomalous (quantum) Hall phases for attractive and repulsive interactions, respectively. Our setup is ideally adapted to the search for recently proposed topologically ordered surface terminations that could be microscopically stabilized by tailored surface interaction profiles.

    Host: Philip Brydon

    http://www.physics.umd.edu/cmtc/seminars.html
    When: Tue, November 4, 2014 - 11:00am
    Where: 2205 Toll Physics Building
  • Physics Colloquium
    Speaker Name: Steven Kivelson

    Speaker Institution: Stanford University

    Title: Quenched Disorder and Vestigial Nematicity

    Abstract: Intermediate phases with “vestigial order” occur when the spontaneously broken symmetries of a “fully ordered” groundstate are restored sequentially as a function of increasingly strong thermal or quantum fluctuations, or of increasing magnitude of quenched randomness. As an important example, incommensurate charge-density-wave short-range order (i.e. with a finite correlation length) and a sharp phase transition to a phase with long-range nematic order is shown to be natural in the presence of weak quenched disorder in systems which, in the absence of disorder, would have unidirectional (stripe) ordered ground states. Recent experiments probing charge order in the pseudo-gap regime of the hole-doped cuprate high-temperature superconductors and nematic order in the Fe based superconductors are interpreted in light of these results.
    When: Tue, November 4, 2014 - 4:00pm
    Where: PSC Lobby
  • CMTC Seminar
    Speaker Name: Steven Kivelson (Stanford)

    Title: Theory of Intertwined Orders in High Temperature Superconductors

    Abstract: The electronic phase diagrams of many highly correlated systems, and in particular the cuprate high temperature superconductors, are complex, with many different phases appearing with similar - sometimes identical - ordering temperatures even as material properties, such as a dopant concentration, are varied over wide ranges. This complexity is sometimes referred to as "competing orders." However, since the relation is intimate, and can even lead to the existence of new phases of matter such as the putative "pair-density-wave," the general relation is better thought of in terms of "intertwined orders." We selectively analyze some of the experiments in the cuprates which suggest that essential aspects of the physics are reflected in the intertwining of multiple orders-not just in the nature of each order by itself. We also summarize and critique several theoretical ideas concerning the origin and implications of this complexity.

    Host: Jay D. Sau

    http://www.physics.umd.edu/cmtc/seminars.html
    When: Wed, November 5, 2014 - 10:00am
    Where: 2205 Toll Physics Building
  • Nuclear Physics Seminar
    Title: Massive and massless modes of the triplet phase of neutron matter

    Speaker: Srimoyee Sen, UMD

    Abstract: tk
    When: Wed, November 5, 2014 - 1:00pm
    Where: PSC 3150
  • Fall CMTC Symposium -- Day 1
    Talk schedule:

    11:00 AM Pallab Goswami, Axial anomaly and negative longitudinal magnetoresistance: theory vs. experiment

    11:45 AM Alejandro Lobos, Magnetic edge-states in strongly interacting one-dimensional topological Kondo insulators

    1:30 PM Jed Pixley, Unconventional Superconductivity near a Kondo Destroyed Quantum Critical Point

    2:15 PM Jimmy Williams, Quantum Nanoelectronics in Oxides

    3:00 PM Vlad Manucharyan, Single Cooper pair proximity effect in mesoscopic quantum dots

    The entire talk schedule can be found at http://www.physics.umd.edu/cmtc/seminars.html
    When: Wed, November 5, 2014 - 11:00am
    Where: PHY 2205
 

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