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.
Wed 18 November 2009 4:00 PMPhysics Colloquium Complex Synchronization Patterns in Spin Torque 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.
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.
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.)
Thu 5 November 2009 1:00 PMBiophysical Group Meeting Universality of the Distribution of Potential Synapse Numbers Tarec FaresNortheastern UniversityEG 406
Wed 4 November 2009 4:00 PMPhysics Colloquium Nature of 1D Charge Transport Near Metal-Insulator Transitions in Carbon Nanotubes Dr. Swastik KarDepartment of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic InstituteDA 114
The interplay of disorder and interactions, and its role in determining the nature of charge transport¹ especially in quantum-confined systems near a metal-insulator transition (MIT) remains one of the most fascinating areas of research in condensed matter physics. In this context, carbon nanotubes – a quasi 1D electronic system – demonstrates a variety of exotic physical phenomena and provides an ideal platform for investigations of various aspects of 1D physics. Exciting new results of 1D mesoscopic physics can be obtained by controlling locally inhomogeneous electrostatic and magnetic fields, e-e interaction strength and disorder density, and the electronic band structures of carbon nanotubes. In this talk, a number of innovative ways will be described that drives carbon nanotubes through different kinds of MITs. The behavior of various transport properties, as individual single- and multi-wall carbon nanotubes are driven through the MIT, will be presented. Topics will include metal-insulator transitions in single-wall carbon nanotubes², Lüttinger-liquid to Al'tshuler-Aronov transition and suppression of phase coherence by e-e interactions in disordered, many-channel carbon nanotubes³, effect of local nanoscale magnetism and superparamagnetism on transport in carbon nanotubes⁴, and the effect of metal nanocluster decoration on the electronic structure and transport properties of single-wall carbon nanotubes⁵. These results will be discussed within the framework of relevant models and phenomenological descriptions of electronic transport in 1D systems.

References:
1. Observation of non-Gaussian conductance fluctuations at low temperatures in Si:P(B) at the metal-insulator transition, S. Kar, A. K. Raychaudhuri, A. Ghosh, H. v. Lohneysen and G. Weiss, Physical Review Letters 91, 216603 (2003)
2. Metal-Semiconductor Transition in Single-Walled Carbon Nanotubes Induced by Low-Energy electron Irradiation, A. Vijayaraghavan, K. Kanzaki, S. Suzuki,Y. Kobayashi, H. Inokawa, Y. Ono, S. Kar, P.M Ajayan, Nano Letters 5, 1575 (2005)
3. Lüttinger Liquid to Al’tshuler-Aronov Transition in Disordered, Many-Channel Carbon Nanotubes, S. Kar*, C. Soldano*, L. Chen, S. Talapatra, R. Vajtai, S.K. Nayak and P.M. Ajayan, ACS Nano 3, 207 (2009)
4. Detection of Nanoscale Ferromagnetic Activity using a Single Carbon Nanotube, C. Soldano*, S. Kar*, S. Talapatra, S.K. Nayak and P.M. Ajayan, Nano Letters 8, 4498 (2008)
5. Highly Aligned Scalable Platinum-decorated Single-wall Carbon Nanotube Arrays for Nanoscale Electrical Interconnects, Young Lae Kim, Bo Li, Xiaohong An, Myung Gwan Hahm, Li Chen, Morris Washington, P.M. Ajayan, Saroj K. Nayak, Ahmed Busnaina, Swastik Kar*, and Yung Joon Jung*, ACS Nano 3 2818 (2009)
Wed 4 November 2009 12:00 PMGraduate Journal Club Polymer Brushes, Hydrogels and Cartilage: How They Lead to Superlubrication in the Human Body Prof. Jeffrey SokoloffNortheastern UniversityDA 114Graduate Journal Club
Tue 3 November 2009 4:00 PMCIRCS Structure-Function Studies on Nucleic Acid Binding Proteins: Model Systems and Potential Applications Richard L. KarpelDepartment of Chemistry and Biochemistry, University of MarylandDA 114CIRCS
Thu 29 October 2009 1:00 PMBiophysical Group Meeting Force-Induced Unfolding of TAR RNA in the Presence of NC Dr. Micah McCauleyNortheastern UniversityEG 406