Northeastern Physics Department Events

Thu 1 April 2010 4:00 PMPhysics Colloquium TBA TBADA 114Host: Professor Albert-Laszlo Barabasi
Tue 1 December 2009 4:00 PMCIRCS Scale-Invariant Aspects of Cardiac Dynamics: Observing Physiologic States Under Health, Disease, and Aging Plamen IvanovPhysics Department, Boston UniversityDA 114CIRCS
Mon 30 November 2009 4:00 PMPhysics Colloquium Mott Transition in VO2 Observed By Infrared Spectroscopy and Nano-Imaging Mumtaz QazilbashUniversity of CA - San DiegoDA 114A grand challenge of contemporary condensed matter physics is the understanding of the emergence of metallic transport in correlated insulators or Mott insulators in which, for example, a temperature change or chemical doping induces anomalous conducting phases. Where charge, spin, orbital and phononic degrees of freedom result in competing interactions, exotic phases emerge including the pseudogap state in cuprates and manganites, high-temperature superconductivity, even phase separation in some manganites and cuprates. I will report on the electronic properties of a prototypical correlated insulator vanadium dioxide (VO2) in which the metallic state is induced by increasing temperature. The pioneering technique of scanning near-field infrared microscopy allows us to directly image nano-scale metallic puddles that appear at the onset of the insulator-to-metal transition in VO2. In combination with far-field infrared spectroscopy, the data reveal divergent optical mass in the metallic puddles which is evidence of a Mott transition. Our methods and results illuminate a new path towards spectroscopic exploration of electronic inhomogeneities in correlated electron systems.
Mon 23 November 2009 4:00 PMPhysics Colloquium Electronic Spin Control in Nanostructures: From Spin Currents to Single Spins Sergey FrolovDelft University of TechnologyDA 114Spins are most often manipulated by oscillating magnetic fields; the successes of nuclear magnetic resonance and electronic spin resonance spread well beyond the boundaries of fundamental physics. But for spin-based electronics it is highly desired to rotate spins using a.c. electric fields, which are much easier to generate on the nanoscale and at high frequencies. Spin can be coupled to electric field by a mediating interaction such as spin-orbit or hyperfine. We have observed how both of these interactions influence spin states of single electrons confined by electrostatic gates in InAs nanowires. Our next goal is coherent spin rotation using electric-dipole spin resonance (EDSR)--pushing the electron wavefunction with a gate voltage at gigahertz frequencies.
While EDSR is the way to go for confined spins, it is possible to manipulate spin currents without any high frequency fields, electric or magnetic. Spin resonance can be induced by spin-orbit fields which are generated intrinsically when electrons bounce in narrow ballistic channels. Ballistic spin resonance observed in a GaAs two-dimensional electron gas is used to probe electron-electron interactions at low density, the anisotropy of spin-orbit coupling and to build a spin-field effect transistor. Coherent spin control is a future goal that requires precisely defined electron trajectories.
Fri 20 November 2009 2:00 PMPhysics Colloquium Stability in a Turbulent (Fermi) Sea: The Ever More Remarkable High Temperature Superconductors Professor Eric HudsonMIT, Department of PhysicsDA 114Host: Professor Robert MarkiewiczFor over two decades high temperature superconductivity has captured the attention of scientists the world round. However, rather than finding a simple explanation for the properties of these materials, as was done for their low temperature cousins half a century ago, intensive research has instead led to an increasingly complex picture of materials characterized by an intricate phase diagram, full of competing or coexisting states, yet still dominated by a superconducting state which persists, at least in some materials, almost half way to room temperature.
In this talk I will describe nanoscale investigations of the electronic structure of high temperature superconductors using scanning tunneling microscopy (STM). We have recently found that a still not understood high temperature phase in these materials, the pseudogap, is characterized by strong charge inhomogeneity. Surprisingly, although this disorder persists into the superconducting state, it does not seem to perturb coexisting homogeneous superconductivity. The resolution of this apparent contradiction gives new insight into the onset of superconductivity and its relationship with the pseudogap phase.
Thu 19 November 2009 1:00 PMBiophysical Group Meeting Statistical Theory of Learning and Memory Professor Armen StepanyantsNortheastern UniversityDA 114
Wed 18 November 2009 4:00 PMPhysics Colloquium Synchronization of Nonlinear Magnetic Nano-Oscillators Sergei UrazhdinWest Virginia University, Morgantown, WVDA 114We all have observed synchronization of oscillators, such birds in a flock flapping their wings, fireflies simultaneously emitting light pulses, violins in an orchestra, or the daily rhythms of the biological organisms. In all of these cases, interaction among similar oscillating systems or with an external periodic force produces a certain relationship between the frequencies and phases of the oscillations. The phenomenon of synchronization plays a central role in communication technology. Understanding synchronization in biological organisms can help with treatment of heart disease, Parkinson’s disease, and other disorders.
I will discuss the syncronization phenomena in perhaps the world's smallest oscillator, a nanomagnet consisting of only about a million atoms. The nanomagnetic oscillator is incorporated into an electric structure that includes electrical leads and at least one other "fixed" magnet. The oscillation of the magnetic moment is driven by the spin torque effect which is produced when the applied electrical current is spin-polarized by the "fixed" magnet.
After a brief introduction to the relevant physics of nanomagnets and spintronics, I will demonstrate that, despite their nanoscale dimensions, nanomagnetic oscillators are perhaps some of the most complicated oscillating systems in existence. In particular, I will discuss two phenomena, fractional synchronization (Devil’s staircase) and synchronization hysteresis, which we have recently observed in magnetic oscillators synchronized by an external oscillating field. I will show that the first effect is caused by the unharmonicity of the oscillation and nonlinearity of the coupling to the field, while the second is caused by the nonlinearity of the oscillation, i.e. dependence of frequency on oscillation amplitude. I will show that these effects provide some unique information about the properties of the nanoscale magnets. Finally, I will discuss how the nanomagnets can be used in practical applications.
Wed 18 November 2009 12:00 PMGraduate Journal Club Vibrational Coherence Spectroscopy of Green Fluorescent Protein Dr. Arthur McClellandNortheastern UniversityDA 114Graduate Journal Club
Tue 17 November 2009 4:00 PMCIRCS Mechanics of Twisted Biopolymers Moneesh UpmanyuMechanical and Industrial Engineering, Northeastern UniversityDA 114
Mon 16 November 2009 4:00 PMPhysics Colloquium Intrinsic and Extrinsic Scattering Mechanisms in Graphene Xia HongPennsylvania State UniversityDA 114Graphene, an ideal two-dimensional electron system, is known for its unconventional linear dispersion relation and high mobility. Recent studies show that the intrinsic mobility of graphene, set by longitudinal acoustic (LA) phonon scattering, can reach~10⁵ cm²/Vs at room temperature. However, extrinsic scattering sources, including charged impurity and remote interfacial phonon scattering from the widely used SiO₂ substrate, limit mobility to the current range of 2x10³ – 2x10⁴ cm2/Vs. Understanding the various scattering mechanisms has important implications in both fundamental studies and technological development of graphene.
Thu 12 November 2009 1:00 PMBiophysical Group Meeting Analysis of Vibrational Spectra of Iron Porphyrins with Density Functional Theory Dr. Alex DemidovNortheastern UniversityDA 114
Wed 11 November 2009 3:15 PMCCNR The Unreasonable Effectiveness of Self-Excited Branching Models of Triggered Seismicity vs. Claims of New Universal Laws by Physicists Didier SornetteETH Zurich, Professor of Finance (Chair of Entrepreneurial Risks)DA 5th floorCCNR
Fri 6 November 2009 12:00 PMIGERT Nanomedicine Distinguished Lecture Imaging with Magnetic Nanoparticles Mukesh G. Harisinghani, MD, Associate Professor, Harvard Medical SchoolDirector of Abdominal MRI at Massachusetts General HospitalBK 320
Fri 6 November 2009 10:30 AMPhysics Colloquium Structural and Surface Transformations in Complex Nanostructures: A Basic Science Approach Towards Controlled Nanoscale Engineering Nicola FerralisDepartment of Chemical Engineering, Center of Integrated Nanomechanical Systems, University of California, BerkeleyDA 114The characterization of the growth mechanisms of nanostructures often relies on the understanding of the basic and fundamental surface and interface phenomena taking place during the process. Here, I will present two cases in which such understanding is successfully achieved by direct investigations of the surface and interface structural transformations and dynamics at the surface of nanostructures: controlled hierarchical branching of silicon nanowires, and strain control in epitaxial graphene. In the first case, I will describe a novel in-situ and real-time investigation of the spreading dynamics of a single layer of gold atoms from large self-pinned gold reservoirs flat Si surfaces. The gained understanding of the reactive, non-diffusive nature of the spreading process is used for nanoscale control of a self-seeding growth of branches in nanowires via vapor-liquid-solid mechanism. In the second case, I will describe how the substrate-induced strain in epitaxial graphene grown on SiC surfaces are strongly affected by optimized synthesis conditions. A comprehensive description of the temperature dependent evolution of the epigraphene morphology and the competing graphene-substrate interactions will be provided. The implications of this fundamental investigation to nanoscale strain engineering will be discussed.
Thu 5 November 2009 4:00 PMPhysics Colloquium Statistical Physics of Large Scale Inverse Problems Professor Riccardo ZecchinaTheoretical Physics, Politechnico di TorinoDA 114Host: Professor Ginestra BianconiWe will review some recent results in the statistical mechanics approaches to inverse problems and discuss applications in computational biology.
Specifically we will show how statistical physics algorithms for inverse Ising, Inverse Potts and for the identification of optimal connected sub-graphs of a given network can be used to unveil interactions in biological systems from large scale data (in neural circuits, in protein contacts and in cell signaling respectively.)