Colloquium Fall 2007

All Department Seminars and Colloquium

 

September 3
Labor Day, University Holiday

September 10
Electron Dephasing Time Near Zero Temperature: Recent Experimental and Theoretical Studies
Prof Juhn-Jong Lin
Institute of Physics and Department of Electrophysics, National Chiao Tung University , Taiwan

Abstract

The electron dephasing time, t, in metal and semiconductor mesoscopic structures is a quantity of fundamental interest and importance. In addition to the inelastic electron scattering times at finite temperatures, the behavior of the dephasing time near zero temperature, t (T→0), has recently attracted vigorous experimental and theoretical attention. The focus of this renewed interest is concerned with the issue of whether  should reach a finite or an infinite value as T→0. While it is accepted that t (T→0) should diverge if there exist only the quasielastic electron-electron scattering and the electron-phonon scattering, several recent measurements performed on different metals and semiconductors have found that t (T→0) depends only very weakly on temperature, if any, when the temperature is sufficiently low. This colloquium talk will survey the recent experimental studies and briefly discuss the current theoretical situation of t (T→0) in mesoscopic structures. In particular, the roles of inelastic spin-spin scattering due to dilute magnetic impurities in the samples will be examined. We will also present our recent experimental observation of strong electron dephasing in a series of highly disordered Cu93Ge4Au3 thin films. This dephasing is much stronger than any known inelastic electron scattering process, and is nonmagnetic in origin. It may be caused by the coupling of the conduction electrons to dynamical structure defects in the sample.

September 17
Status of the Search for Gravitational Waves with LIGO
Prof Alan Weinstein
LIGO / Caltech Analysis Group, Caltech

Abstract

The LIGO detectors are at design sensitivity and, along with the VIRGO and GEO detectors in Europe, are currently observing the sky for gravitational waves from astrophysical sources. The Advanced LIGO project will begin next year. We will review the physics of gravitational waves, the current and future detectors, the various types of astrophysical sources that we search for, and the status of those searches.

September 24
First Principles Studies of Magnetic Properties of Nanostructures
Prof Ruqian Wu
Physics and Astronomy, University of California, Irvine

Abstract

Nanomagnetism is a cutting-edge research field, and may potentially revolutionize storage, sensing, spintronics, and optoelectronic technologies. The application of innovative experimental tools and the desire to grow nanostructures and novel materials in a controlled manner have created an urgent need for highly reliable, robust, and predictive theoretical models and tools to achieve a quantitative understanding of their growth dynamics and the size, shape, and process dependences of their physical properties. Much of the basis of theoretical tools is founded on density-functional theory and related new methods being developed specifically for magnetic materials. It has been increasingly recognized in many fields of materials science that state-of-the-art ab initio electronic structure calculations based on the density-functional theory have been enormously successful, in both explaining existing phenomena and, more importantly, in predicting the properties of new systems. For example, the prediction of enhanced magnetic moments with lowered coordination number at clean metal surfaces and interfaces has stimulated both theoretical and experimental investigations for new magnetic systems and phenomena in man-made nanostructures. Synergistic applications of theory and experiment, as have been demonstrated repeatedly in many areas of materials science, become a ''must'' to further advance our microscopic understanding in nanomagnetism. I will review the background and recent progresses in this field, with examples mostly from our own previous work.

October 1
Two-Time Physics: The Unified View from Higher Dimensional Space and Time
Prof Itzhak Bars
Physics, University of Southern California

Abstract

Evidence has been accumulating that the ordinary formulation of physics, in a space-time with 3 space and 1 time dimensions, is insufficient to describe our world, just like shadows on walls alone are insufficient to capture the true essence of an object in a 3-dimensional room. Two-Time Physics reveals that our physical world in 3+1 dimensions is like a shadow of a highly symmetric universe in 4 space and 2 time dimensions. In this higher dimensional approach, not only space and time, but all of phase space (space, time, momentum, energy) is unified by gauge symmetries that make all of them indistinguishable from each other, at every instant, for all motions in 4+2 dimensions. The distinction among these degrees of freedom occurs only when a highly symmetric 4+2 system is gauge fixed to multiple non-symmetric "shadows" in 3+1 dimensions. Amazingly, the best understood fundamental theory in Physics, the Standard Model of Particles and Forces in 3+1 dimensions, is reproduced when viewed as one of the "shadows" of a more symmetric field theory in 4+2 dimensions. This emergent Standard Model solves the nagging "strong CP violation problem" of QCD, thus explaining the negative results in searches of the hypothetical "axion" in the past 30 years. The Two-Time Physics point of view provides new mathematical tools and new insights for understanding our universe at all scales of physics. Evidence of the 4+2 dimensional world can be found both at the macroscopic and microscopic scales in the form of hidden symmetries and "dualities", and such predictions can be tested through theory and experiment. A live demonstration of how to see evidence of 2T-physics in the structure of the Hydrogen atom will be given during this lecture. Tests that could distinguish Two-Time Physics from other approaches at the level of particle physics at the energy scales of the Large Hadron Collider and beyond will also be outlined.

October 8
Challenges in predicting rainfall changes under global warming---and how physicists might help
Prof David Neelin
Atmospheric and Oceanic Sciences, Institute of Geophysics and Planetary Physics, UCLA

Abstract

Climate models cannot explicitly resolve the small-scale processes that produce rainfall. The ensemble average effect of these scales must be represented as a function of large-scale variables, such as temperature and moisture, in approximations known as convective parameterizations. Despite the enormous amount known about rainfall producing processes, and decades of work on convective parameterizations, climate models exhibit an embarrassing scatter in their results when faced with problems for which they have not been previously tested, such as changes under global warming. Examples of what the climate models currently do---and don't---capture will be provided. In contrast to the apparent messiness of this problem, I'll present results from a collaboration with O. Peters, a statistical physicist. The properties of the transition to strong convection are shown to conform rather neatly to those of a continuous phase transition, including power law pickup of an order parameter (ensemble average precipitation) above the critical value of a tuning parameter (water vapor), and nontrivial scaling of the variance near critical. Signatures of self organization of the system towards the critical point are also seen. In addtion to helping reinterpret some of the basic postulates of convective parameterization, these properties provide means of empirically probing the transition, and thus, potentially, of better constraining climate models. (Prof. Neelin would be interested in chatting with physics undergraduates, especially seniors who might be considering climate science as a direction for graduate school.)

October 15
Engineering Design in the Presence of Errors
Dr Omid Nohadani

Abstract

In principle, complex systems can be optimized to improve their performance with respect to their desired functionalities. There is numerous evidence showing that if possible errors are not taken into account during the design process, we might lose the actual phenomenon. With this in mind, the field of robust optimization started within the mathematical programming community. Most results, however, were confined to problems with convex objectives. But modern engineering problems have objectives and constraints that are not explicitly given, e.g. numerical simulations describing the problem. In this talk, I will describe the recent advancements in this field which allowed to bridge the gap and provided methods for generic robust design. I will discuss examples in ultrafast optics using chirped mirrors and in cancer treatments with intensity modulated radiation therapy. Surprisingly, the robust solutions in question come at a fairly low price indicating that the advantages are worth the negligible sub-optimality. Because of their generality, the presented methods can be applied to a wide range of engineering design problems.

October 22
Bose Condensation, Superfluidity, and the Quantum Hall Effect
Prof Jim Eisenstein
Physics, Caltech

Abstract

Composite particles consisting of an even number of fermions (e.g. 4He atoms) can pretend to be bosons. Bosons, of course, can Bose condense and do remarkable things. Superconductivity, which is certainly remarkable when you stop to think about it, results (sort of) from the Bose condensation of electron pairs. With this in mind, theorists have speculated since the early 1960s that excitons (electron-hole pairs in a semiconductor) might be able to do the same thing. In this talk I will describe experiments done at Caltech on a special collection of excitons that exists in equilibrium and does indeed show many (but not all) of the expected signs of excitonic superfluidity. Surprisingly, the system in question is a double layer two dimensional electron gas. With no valence band holes in sight, where do the excitons come from?

October 29
Buckyballs, Nanotubes, and the Nanotechnology Revolution
Prof David Tomanek
Physics and Astronomy, Michigan State University

Abstract

The continuous reduction of device sizes, which is rapidly approaching the atomic level, raises particular challenges in terms of component interconnection and fault tolerance. Due to fundamental limitations imposed on observations by the quantum behavior of these systems, predictive computer simulations emerge as a powerful approach to design complex nanostructures and to understand their behavior [1]. Combined electronic structure and quantum transport calculations for metal-nanotube junctions reveal that the optimum electrical contact should be neither too strong, nor too weak [2]. In contrast to bulk systems, carbon nanotubes and related nanostructures demonstrate an unexpected defect tolerance, assisted by a self-healing capability when thermally and electronically excited [3]. In nanotubes, electronic excitations decay very slowly [4]. Since bonding in the excited state differs significantly from the ground state, photo-chemical reactions [5] and ion beam irradiation [6] emerge as powerful tools to selectively modify nanostructures. Time-dependence of Kohn-Sham state following an electron-hole excitation in a (3,3) carbon nanotube [4]. Photo-chemistry in nanotechnology: Selective deoxidation by photoexcitation [5]. References [1] David Tománek, Carbon-based nanotechnology on a supercomputer, Topical Review, J. Phys.: Condens. Matter 17, R413-R459 (2005). [2] Norbert Nemec, David Tománek, and Gianaurelio Cuniberti, Contact Dependence of Carrier Injection in Carbon Nanotubes: An Ab Initio Study, Phys. Rev. Lett. 96, 076802 (2006). [3] Yoshiyuki Miyamoto, Savas Berber, Mina Yoon, Angel Rubio, David Tománek, Can Photo Excitations Heal Defects in Carbon Nanotubes? Chem. Phys. Lett. 392, 209?13 (2004). [4] Yoshiyuki Miyamoto, Angel Rubio, and David Tománek, Real-time ab initio simulations of excited carrier dynamics in carbon nanotubes, Phys. Rev. Lett. 97, 126104 (2006). [5] Yoshiyuki Miyamoto, Noboru Jinbo, Hisashi Nakamura, Angel Rubio, and David Tománek, Photodesorption of oxygen from carbon nanotubes, Phys. Rev. B 70, 233408 (2004). [6] Arkady V. Krasheninnikov, Yoshiyuki Miyamoto, and David Tománek, Role of electronic excitations in ion collisions with carbon nanostructures, Phys. Rev. Lett. 99, 016104 (2007).

November 5
Novel correlated electron phenomena in filled skutterudite compounds
Prof Brian Maple
Department of Physics, Institute for Pure and Applied Physical Sciences, University of California, San Diego

Abstract

The filled skutterudite compounds, which have the formula MT4X12, where M = alkali metal, alkaline earth, lanthanide, actinide, T = Fe, Ru, Os, and X = P, As, Sb, have attracted a great deal of interest in recent years because they exhibit a wide range of correlated electron phenomena and are promising candidates for thermoelectric applications. Correlated electron phenomena that have been observed include superconductivity, magnetic order, quadrupolar order, valence fluctuations, heavy fermion behavior, non-Fermi liquid behavior, and metal-insulator transitions. In this talk, we describe recent experiments on the Pr-based filled skutterudites, particularly PrOs4Sb12, Pr(Os1-xRux)4Sb12, and PrOs4As12, whose ground states are determined by a delicate interplay between hybridization of Pr3+ localized 4f- and itinerant-electron states, crystalline electric field splitting of the Pr3+ energy levels, magnetic and quadrupolar interactions, and electronic band stricture. The compound PrOs4Sb12 is of considerable interest for several reasons: (1) it is the first example of a heavy fermion superconductor based on Pr (all of the other known heavy fermion superconductors are compounds of Ce or U); (2) it exhibits a type of unconventional strong coupling superconductivity that breaks time reversal symmetry, apparently consists of several distinct superconducting phases, some of which appear to have point nodes in the energy gap, and may involve triplet spin pairing of electrons; (3) there is a high field ordered phase between 4.5 T and 16 T and below ~1 K that has been identified with antiferroquadrupolar order, indicating that the superconductivity occurs in the proximity of a quadrupolar quantum critical point; and (4) the pairing of superconducting electrons may be mediated by electric quadrupole fluctuations, rather than magnetic dipole fluctuations. In the Pr(Os1-xRux)4Sb12 pseudoternary system, increasing the Ru concentration x results in a monotonic increase in the splitting between the Pr3+ singlet ground state and triplet first excited state from ~7 K at x = 0 to ~60 K at x = 1, a minimum in the Tc vs x curve at x ≈ 0.6, and an apparent change in the nature of the superconductivity from unconventional to conventional BCS at x ≈ 0.3. In contrast, the compound PrOs4As12 undergoes transitions at 2.3 K and 2.2 K in zero-field into two ordered phases that can be suppressed to 0 K with magnetic fields of 2 T and 3.2 T. The low field ordered phase is antiferromagnetic, while the nature of the higher field ordered phase has not yet been determined. The temperature and field dependences of the specific heat and electrical resistivity indicate that PrOs4As12 is a Kondo lattice system with a small Kondo temperature TK ~ 1 K and an enormous electronic specific heat coefficient of ~1 J/mol K2.

November 12
Manipulation and Detection of Individual Electron Spins in Nanostructured Semiconductors
Prof HongWen Jiang
Physics, UCLA

Abstract

Isolated electron spins in low temperature semiconductors are now recognized to have considerable potential in storing and manipulating quantum information. One of the attractions of a spin in a semiconductor is its extremely long decoherence time, since it has zero hyperfine interaction to nuclear spins in isotopically-purified structures. The tunable spin-orbital coupling and the ability to control the electron wavefunctions in semiconductors allow gate operations on the spins. Furthermore, the extensive collection of chipmaking techniques, cumulated over decades, is expected to be extremely invaluable for building a scalable processor. In order to physically implement spin-based quantum information processing, it is essential to control and measure the state of individual spins, which is a significant scientific challenge. In the last several years, key experimental demonstrations made by several groups around the world have considerably improved the prospects of quantum information processing based electron spins. In this talk, I will review these recent experimental breakthroughs.

November 19
Exotic Phases of Frustrated Mott Insulators
Dr Cristian D Batista
Host: Stephan Haas

Abstract

Mott insulators are the paradigm of strongly correlated materials. Strong Coulomb interactions localize the valence electrons in their ions and the low energy physics can be described in terms of the remaining spin degree of freedom. For this reason, Mott insulators have been traditionally considered as materials that have only magnetic properties at low energies due to their spin moments. Despite this common conviction, we will show here that certain ground states of Mott insulators exhibit real electric currents in loops (orbital currents) that produce orbital magnetic moments, while others show modulation of electron charge (polarization). Consequently, spins in Mott insulators are not only coupled to dc magnetic fields, but also to dc electric fields, and it is possible to have magnetically driven electronic ferroelectricity. Moreover, non-vanishing matrix elements of the polarization between the ground state and excited magnetic states result in a rotation of the electric field polarization, which is a characteristic signature of spin textures with orbital currents. In addition, we will discuss some alternative realizations of exotic magnetic orderings induced by geometric frustration.

November 26
The Secondary Role of CO2 and CH4 Forcing in Climate Change: Past, Present and Future
Dr Willie Soon
Smithsonian Astrophysics , Host: Werner Däppen

Abstract

A review of the recent refereed literature fails to confirm quantitatively that carbon dioxide (CO2) radiative forcing was the prime mover in the changes in temperature, ice-sheet volume, and related climatic variables in the glacial and interglacial periods of the past 650,000 years, even under the "fast response" framework where the convenient if artificial distinction between forcing and feedback is assumed. Atmospheric CO2 variations generally follow changes in temperature and other climatic variables rather than preceding them. Likewise, there is no confirmation of the often-posited significant supporting role of methane (CH4) forcing, which despite its faster atmospheric response time is simply too small, amounting to less than 0.2 W/m2 from a change of 400 ppb. We cannot quantitatively validate the numerous qualitative suggestions that the CO2 and CH4 forcings that occurred in response to the Milankovich orbital cycles accounted for more than half of the amplitude of the changes in the glacial/interglacial cycles of global temperature, sea level, and ice volume. Consequently, we infer that natural climatic variability notably the persistence of insolation forcing at key seasons and geographical locations, taken with closely-related thermal, hydrological, and cryospheric changes (such as the water vapor, cloud, and ice-albedo feedbacks) suffices in se to explain the proxy-derived, global and regional, climatic and environmental phase-transitions in the paleoclimate. If so, it may be appropriate to place anthropogenic greenhouse-gas emissions in context by separating their medium-term climatic impacts from those of a host of natural forcings and feedbacks that may, as in paleoclimatological times, prove just as significant.

December 3
How Hard is Quantum Many-Body Theory?
Matthew Hastings
LANL , Host: Stephan Haas

Abstract

The basic problem of much of condensed matter physics, high-energy physics, and quantum chemistry is to find the ground state of a quantum Hamiltonian. How difficult is this problem? I will discuss recent results on entanglement in quantum systems, and relate the entanglement of the system to the difficulty of solving it. I will discuss recent results on area laws for entanglement entropy, and use this to discuss the effectiveness of certain algorithms, while also arguing that certain other quantum systems are likely to have no effective algorithm to find their ground state.

  • Department of Physics and Astronomy
  • University of Southern California
  • 825 Bloom Walk
  • ACB 439
  • Los Angeles, CA 90089-0484