Physics Seminar Series
(Refreshments at 3:15 p.m. in 245 Fronczak)
Contact: Prof. Will Kinney, (716) 645-2017 x244 x111;
Prof. Gottfried Strasser, (716) 645-2017 x259
January Go to Feb '08 Mar '08 Apr '08 May '08
01/15/2008, Tuesday 3:30 p.m., 220 NSC
Probing Semiconductors with THz emission
Dr. James Heyman
Department of Physics and Astronomy, Macalester CollegeWe use ultrafast THz emission to investigate photoexcited semiconductors on picosecond time-scales. We observe coherent charge transport when the carrier scattering time is long compared to our temporal resolution, and we have investigated coherent cyclotron motion in semiconductors and intersubband electron oscillations in a quantum wells. THz emission can also be used to study transport when the scattering time is short compared to the temporal resolution. We have developed a sensitive ultrafast technique for measuring photocarrier mobilities in semiconductors placed in a lateral electric field and perpendicular magnetic field.
01/18/2008, Friday 2:00 p.m., 341 Fronczak
ISCAP cosmology seminar: The evolution of cosmological perturbations in f(R) gravity
Dr. Peter Dunsby
University of Cape TownI give a rigorous and mathematically well defined presentation of the Covariant and Gauge invariant theory of perturbations of a FLRW universe for 4th Order Gravity, where matter is described by a perfect fluid witha barotropic equation of state. The general perturbations equations are applied to a simple background solution of R^n gravity. Solutions of the perturbation equations have a number of interesting features. In particular there are values of n for which there is always a growing mode for the density contrast even if the universe undergoes an accelerated expansion. Such a behaviour does not occur in standard GR, where as soon as Dark Energy dominates the density contrast experiences an unrelenting decay. This peculiarity is sufficiently novel to warrant further investigation on 4th order gravity models.
01/22/2008, Tuesday 3:30 p.m., 220 NSC
Inflation in String Theory
Dr. Liam McAllister
Spectacular advances in observational cosmology have given strong support to the idea that the Hot Big Bang was preceded by a period of cosmic inflation. This is a compelling paradigm, but one with serious theoretical shortcomings. The most important issue is that inflation is sensitive to Planck-scale physics, and so a theory valid at this scale -- such as string theory -- is required. Due to much recent progress in string theory, it is now possible to construct realistic string theory models of inflation. These provide a microscopically-sound framework for addressing the theoretical problems of inflation. Moreover, future observations will test our proposed solutions to these problems. I will give an overview of these advances and will argue that the interface between string theory and cosmology is the best hope for a test of string theory.
01/29/2008, Tuesday 3:30 p.m., 220 NSC
Dynamic Susceptibility of Ferromagnets
Dr. Ruslan Prozorov
Ames Laboratory and Department of Physics & Astronomy, Iowa State UniversityWe use an extremely sensitive tunnel-diode resonator technique to study radio-frequency dynamic magnetic susceptibility of various materials. With sensitivity in the pico-emu range (better than 10-7 in dimensionless susceptibility, ) we are able to access various subtle features that are usually masked by static long-range magnetic order. I will show examples of our measurements, such as contactless detection of Shubnikov - de Haas quantum oscillations via normal skin-effect in {Ce,Sm}AgSb2, mapping metamagnetic transitions in CeSb, studying critical scaling in the vicinity of Tc in CeVSb3 and mapping the H-T phase diagram in multiferroic HoMnO3. I will spend more time on a detailed discussion of the experimental discrimination between local-moment and itinerant ferromagnets, which is very important, because the physics of itinerant systems is still poorly understood.
By studying itinerant ferromagnets from weak (ZrZn2), to intermediate (GdFe2Zn20) to strong (Ni) and comparing them to known local-moment metallic and insulating systems (CeAgSb2, CeVSb3, YTiO3) we consistently observe striking differences between local moment and itinerant systems. Whereas local moment systems show a sharp peak at the Curie temperature which evolves to higher temperatures and lower amplitudes with applied dc magnetic field, itinerant systems show a broad maximum at temperatures well below Tc. The itinerant system's maximum is suppressed in amplitude and shifts to lower temperatures with applied dc magnetic field. Although it is clear that Stoner excitations are responsible for the observed behavior, theoretical understanding is limited, because existing Stoner or spin fluctuations theories only derive zero-field susceptibility. We propose a generalization of these models to incorporate the effect of an applied dc field and show that the developed method is a powerful tool to analyze behavior of itinerant ferromagnets.
February Go to Jan '08 Mar '08 Apr '08 May '08
02/05/2008, Tuesday 3:30 p.m., 220 NSC
TBA
02/12/2008, Tuesday 3:30 p.m., 220 NSC
TBA
Dr. Robert McNees
Perimeter
02/19/2008, Tuesday 3:30 p.m., 220 NSC
Dark matter candidates: from the MeV to the TeV
Dr. Francesc Ferrer
Department of Physics, Case Western Reserve UniversityWeakly interacting particles, a natural candidate for composing the dark matter, may lead to observable fluxes of gamma-rays or neutrinos that could be soon identified by Cerenkov telescopes, satellites and neutrino detectors. These fluxes have often been invoked to explain anomalies in different astrophysical observations like the 511KeV line detected by the INTEGRAL satellite. I will consider an extension of the standard model of particle physics, the NMSSM, that predicts a dark matter particle with mass in the MeV-TeV range, and study the observable signals at forthcoming detectors. Special attention will be devoted to the choice of input astrophysical data, like the dark matter spatial distribution, and the progress expected from surveys like the SDSS will be highlighted.
02/26/2008, Tuesday 3:30 p.m., 220 NSC
Why did the universe start in such an extraordinary state?
Dr. Laura Mersini
University of North CarolinaWe believe the universe started with a high energy inflation. Yet, the probability of such an event is 1 part in 10^10^123 as shown by Penrose. Such an extremely special initial condition presents a severe puzzle in our understanding of nature. Meanwhile, progress in string theory produces not one but a whole 'landscape' of vacua solutions with 4D universes.
Then what selected the initial conditions for our universe? In this talk I describe a proposal, based on the dynamics of the wavefunction of the universe propagating on the landscaspe. The landscape is taken as the phase space of the initial conditions since every of its vacua can potentially give rise to a universe. The dynamics of matter and gravity results in a superselection rule for the emergence of our initial state. I will describe possible tests of the astrophysical signatures predicted here, left over from the earliest times. Specifically, one of the predictions made in 2006 for the existence of the giant void was observed at the size and scale predicted in August 2007 by VLA.
Special Seminar
02/27/2008, Wednesday 3:30 p.m., 245 Fronczak
Rate theory what it was, what is it today, does it have a future?
Dr. Eli Pollak
Chemical Physics Department, Weizmann Institute of ScienceA history of the development of rate theory during the past century will be discussed. Starting from Arrhenius, we follow especially the formulation of transition state theory by Wigner and Eyring. Although Wigner gave an abstract definition of the transition state as a surface of minimal unidirectional flux, it took almost a half of a century until the transition state was precisely defined by Pechukas, but even this only in the realm of classical mechanics. Eyring, considered by many to be the father of TST never resolved the question as to the definition of the activation energy for which Arrhenius became famous. In 1978, Chandler finally showed that the activation energy is a free energy, it is the barrier height in the potential of mean force felt by the reacting system.
Parallel to the development of rate theory in the Chemistry community, Kramers published in 1940 a seminal paper on the relation between Einstein's theory of Brownian motion and rate theory. Kramers' paper provided a solution for the effect of friction on reaction rates but left us also with some challenges. He could not derive a uniform expression for the rate, valid for all values of the friction coefficient, known as the Kramers turnover problem. He also did not establish the connection between his approach and the TST developed by the Chemistry community. For many years, Kramers' theory was considered as providing a dynamic correction to the thermodynamic TST. Both of these questions were resolved in the 1980's when I showed that Kramers' expression in the moderate to strong friction regime could be derived from TST, provided that the bath, which is the source of the friction is handled at the same level as the system which is observed. This then led to the solution of the turnover problem posed by Kramers.
Although classical rate theory reached a high level of maturity, its quantum analog leaves the theorist with serious challenges to this very day. The successes and challenges facing quantum rate theory will be discussed.
March Go to Jan '08 Feb '08 Apr '08 May '08
03/04/2008, Tuesday 3:30 p.m., 220 NSC
Gravitational Approaches to Cosmic Acceleration
Dr. Mark Trodden
Syracuse UniversityAmong the possible explanations for the observed acceleration of the universe, perhaps the boldest is the idea that new gravitational physics might be the culprit. In this seminar I will discuss some of the challenges of constructing a sensible phenomenological extension of General Relativity, give examples of some candidate models of modified gravity and survey existing observational constraints on this approach. I will conclude by discussing how we might hope to distinguish between modifications of General Relativity and dark energy as competing hypotheses to explain cosmic acceleration.
03/18/2008, Tuesday 3:30 p.m., 220 NSC
Nanotubes beyond carbon: theory of gallium nitride and boron nanotubes
Dr. Sohrab Ismail-Beigi
Yale
Although atomically-thin nanotubes of other elements are now fabricated, carbon nanotubes are probably the best known examples of nano-materials. They provide ideal cases for studying a variety of nanoscopic effects: e.g., geometric and quantum confinement of electrons, enhanced Coulomb interactions in one dimension, and curvature effects. However, as materials for use in electronics applications, carbon nanotubes have some drawbacks. Here, we describe our recent theoretical work on the physical properties of two other classes of nanotubes based on gallium nitride and boron, highlighting what is novel and potentially useful in each case.
03/25/2008, Tuesday 3:30 p.m., 220 NSC
TBA
April Go to Jan '08, Feb '08 Mar '08 May '08
04/01/2008, Tuesday 3:30 p.m., 220 NSC
Coherent control of nanomagnet dynamics via ultrafast spin torque pulses
Dr. Samir Garzon
Department of Physics & Astronomy and USC Nanocenter, University of South CarolinaThe magnetization orientation of a nanoscale ferromagnet can be manipulated using an electric current via the spin transfer effect. Recent explosive growth in the area of spin transfer torque induced magnetization dynamics has occurred in part because spin torque is a unique way to excite a ferromagnet, simultaneously driving the magnetization out of equilibrium and changing the rate of energy dissipation. However, it has yet to be reported how the magnetization dynamics evolve when the spin torque is applied on a timescale impulsively short compared with the free precessional period, as previous studies convolve the temporal action of the spin torque with the precessional dynamics.
In this talk I will demonstrate an exciting new approach for driving magnetization dynamics: a pair of pulses with precisely adjusted amplitudes and delays delivers spin torque impulses at precise points in the magnetic moment's trajectory, allowing optimal energy transfer into the precessing magnetization and precise control of nanomagnet dynamics. I will discuss our main results: 1) achieved ~100% switching probability with the shortest reported single pulse, using a current two times smaller than expected; 2) tuned the switching probability over a broad range (4% to 93%) by only adjusting the delay between two equal amplitude pulses, showing coherent magnetization dynamics over 1 ns even at room temperature; 3) observed a non-monotonic increase in the switching probability by only increasing pulse pair amplitude, demonstrating, counterintuitively, that increasing the spin torque does not necessarily increase the switching probability. Possible applications of this technique, including resonantly pumped microwave oscillators and efficient reversal of magnetic memory bits in nanoscale magnetic random access memory (MRAM), will also be discussed.
04/08/2008, Tuesday 3:30 p.m., 220 NSC
Multiple scattering approach to the superconductor-normal metal contact conductance
Dr. Vladimir Lukic
University of PennsylvaniaThe discovery of a two-band superconductor MgB2 spurred an interest in possible multiple-band superconductivity in other materials such as CeCoIn5. Point contact spectroscopy is expected to be an effective tool for detection of multiple superconducting gaps, yet the experimental evidence is often inconclusive. In this talk I will show that part of the problem is in the oversimplified conductance formula used to fit gap energies, and derive an expression that takes into account interband scattering by using the coherent multiple scattering (CMS) approach.
I will review the standard wave-function approach to the problem of transport in superconductor-normal metal junctions, and show that the same results are obtained with the CMS formalism. The advantage of CMS is that it can be used in situations where a simple form of wave-function boundary conditions is not known, such as the contact with a two-band superconductor, in which case novel conductance features arise from the coherent superposition of scattering events in different bands. The two-band conductance formula is mapped to that of the graph node with superconducting branches, thus establishing the effective boundary conditions that can be used in the standard approach.
I will compare our results with experiments, and discuss their consequences for the self-consistent gap calculations at the superconductor-normal metal interface.
04/15/2008, Tuesday 3:30 p.m., 220 NSC
Gravity on the Largest Scales and Cosmic Acceleration
Dr. Justin Khoury
PerimeterI present a class of modified theories of gravity in which vacuum energy is screened or degravitated. I argue that any such theory must, at the linear level, reduce to a theory of massive or resonance graviton. The immediate implication is that there are new degrees of freedom, associated with the extra polarization states of the graviton, the most interesting of which is the helicity-0 or longitudinal mode. Phenomenologically, this mode leads to a fifth force which is suppressed in regions of high density but kicks in on large (>Mpc) scales. I discuss the implications for cosmological evolution and structure formation.
04/22/2008, Tuesday 3:30 p.m., 220 NSC
Coherence Time in Open Quantum Dots
Dr. Mike Grobis
StanfordThe advent of quantum mechanics nearly a century ago ushered in a wave of new technologies based on quantum mechanical principles, such as lasers, transistors, and magnetic resonance imaging devices. Researchers today are trying to develop novel technologies that use single quantum states as fundamental building blocks rather than statistical ensembles. An overarching problem with working with single quantum states is the short time scale over which their stored state is randomized due to interactions with the environment, a phenomena known as decoherence. Though coherence times are generally expected to diverge at zero temperature, measurements of several mesoscopic systems have shown unexpected saturations of the coherence time at low temperatures. Our experiments have investigated the nature of the mysterious decoherence time saturation observed in electron transport through large, micron-sized quantum dots. Our measurements show that the saturation of coherence time is linked to the appearance of Coulomb-blockade effects in a regime where these effects had generally been assumed absent: when a dot is coupled by one fully transmitting mode to each of two leads. I will discuss how the subtle effects of these Coulomb interactions affect the extraction of electron coherence time and can incorrectly suggest saturation at low temperatures.
04/29/2008, Tuesday 3:30 p.m., 220 NSC
TBA