Physics Colloquium Series
(Refreshments at 3:15 p.m. in 245 Fronczak)
Contact: Prof. Igor Zutic , (716) 645-2017 x183;
Prof. Will Kinney, (716) 645-2017 x111;
Prof. Hao Zeng, (716) 645-2017 x243
September Go to Oct. '09 Nov. '09 Dec. '09
09/03/2009, Thursday, 3:30 p.m., 201 NSC
Nano-Spintronics with Quantum Dots
Dr. Pawel Hawrylak
Institute for Microstructural Sciences (IMS), National Research Council (NRC) of Canada, CanadaThe emerging area of nano-spintronics focuses on developing the means of exploiting spin properties at the nanoscale, with a single electron spin, single magnetic ion spin, and polarization of a single photon as ultimate limits. I will review our recent work on nano-spintronics with gated , self-assembled and grapheme quantum dots carried out at IMS NRC with focus on two ways of controlling spin: through Pauli exclusion principle and via spin-orbit interaction. Some effects covered include "single spin transistor", magnetic frustration and topological Hunds rules in quantum dot networks, spin textures in lateral and graphene quantum dots, optical detection of spin polarization, RKKY spin-spin interactions and SO coupling of spin, angular momentum, and parity in quantum dot molecules.
09/10/2009, Thursday, 3:30 p.m., 201 NSC
Searching for the origin of electroweak symmetry breaking
Dr. Csaba Csaki
Deparment of Physics, Cornell UniversityThe standard model of particle physics has been one of the most important scientific discoveries of the 20th century. While most aspects of this theory have been verified to a great precision in successive collider experiments over the past 30 years, one of the main ingredients of this theory remain untested to date. The question is how the electroweak symmetry underlying the standard model is broken in nature, which is also responsible for the mass generation for most observed particles. In this colloquium I will first review the history of the standard model, and then simplest idea for electroweak symmetry breaking, which is called the Higgs mechanism. I will explain the problems that the Higgs mechanism faces, and summarize the most successful theories solving these problems, including supersymmetry, extra dimensional models, higgsless models and little higgs theories, and their prospects for discovery at the upcoming LHC experiments.
09/17/2009, Thursday, 3:30 p.m., 201 NSC
Oxide Nanoelectronics
Dr. Jeremy Levy
Deparment of Physics, University of PittsburghElectronic confinement at nanoscale dimensions remains a central means of science and technology. In this talk, I will describe a new method for producing extreme nanoscale electronic confinement at the interface between two separately insulating oxides, LaAlO3 and SrTiO3. Using an approach reminiscent of the popular toy "Etch-a-Sketch", we scan an electrically biased probe on the surface of this heterostructure to create nanoscale conducting islands, nanowires, tunnel junctions and field-effect transistors at the interface. The smallest feature size approaches one nanometer. These structures are created in ambient conditions at room temperature, and can be erased and rewritten repeatedly. At low temperatures, a variety of quantum phases have been observed, including integer and fractional quantum Hall states and superconductivity. This new, on-demand nanoelectronics platform has the potential for widespread scientific and technological exploitation.
09/24/2009, Thursday, 3:30 p.m., 201 NSC
Magnetic Dopants and Charge Carriers in Carriers in Colloidal II-VI Quantum Dots
Dr. Daniel R. Gamelin
Deparment of Chemistry, University of WashingtonThe generation, manipulation, and detection of electron spins in semiconductor nanostructures is a central theme in the emerging field of spintronics. This talk will describe the synthesis and physical properties of colloidal diluted magnetic semiconductor nanocrystals such as transition-metal-doped CdSe, ZnO, and ZnSe quantum dots. The talk will describe the use of photochemical carrier generation, magneto-optical spectroscopies, and magnetic resonance spectroscopies to probe carrier spin relaxation dynamics and carrier-dopant magnetic exchange interactions in these colloidal quantum dots. Such effects underlie many important magneto-electronic phenomena in magnetic semiconductor nanostructures, including carrier-mediated ferromagnetism, magnetic polaron formation, and proposed spin-based quantum information processing schemes. Basic aspects of doped quantum dot electronic structures will be discussed in this context.
Related references:
"Colloidal Mn2+-Doped CdSe Quantum Dots: New Inorganic Materials for Spin-Electronics and Spin-Photonics." Beaulac, R.; Archer, P. I.; Ochsenbein, S. T.; Gamelin, D. R., Adv. Funct. Mater., 2008, 18, 3873-3891 (Review).
"Light-Induced Spontaneous Magnetization in Colloidal Doped Quantum Dots." Beaulac, R.; Schneider, L.; Archer, P. I.; Bacher, G.; Gamelin, D. R., Science, 2009, 325, 973.
"Charge-Controlled Magnetism in Colloidal Doped Semiconductor Nanocrystals." Ochsenbein, S. T.; Feng, Y.; Whitaker, K. M.; Badaeva, E.; Liu, W. K.; Li, X.; Gamelin, D. R., Nature Nanotechnology, 2009, published on line (DOI: 10.1038/NNANO.2009.221).
October Go to Sept. '09 Nov. '09 Dec. '09
10/01/2009, Thursday, 3:30 p.m., 201 NSC
Nanoscale Interference Effects in Electron-Phonon Kinetics and Transport
Dr. Andrei Sergeev
Deparment of Electrical Engineering, University at BuffaloElectron-phonon (e-ph) interaction plays a key role in many phenomena, such as electron heating and dephasing, electric and thermal transport, superconductivity etc. Although it is well characterized in clean bulk conductors, its current understanding in disordered and nanoscale systems is limited. Quantum-mechanical interference of pure electron-phonon scattering and electron scattering from impurities, defects, and boundaries drastically changes kinetic and transport properties of nanostructures and nanostructured materials. For example, the interference violates the Wiedemann-Franz law and the Mathiessen rule, according to which the contributions to conductivity due to the random potential and e-ph interaction are additive. I will summaries the results of our theoretical works related to the interference effects in metallic films, semiconductors structures, quantum wires etc. I review a number of recent experimental papers, which confirm the theoretical conclusions. Finally, I describe some advanced devices (hot-electron nanobolometers, quantum nanocalorimeters, single photon counters), which are based on electron heating in nanostructures.
10/08/2009, Thursday, 3:30 p.m., 201 NSC
The quest to understand physics in the microworld: Physics at Hadron Colliders
Dr. Yuri Gershtein
Rutgers UniversityHadron colliders have been essential in our quest to understand the physics of the micro-world. I will talk about the recent progress of the experimental program to address one of the fundamental questions of Physics: "what is the Nature of the Electroweak Symmetry breaking?". I will present results from the Tevatron collider at Fermilab and prospects for the Large Hadron Collider at CERN.
10/15/2009, Thursday, 3:30 p.m., 201 NSC
What if the Milky Way isn't integrable?
Dr. David Hogg
New York UniversityOur best bet for getting detailed information about the gravitational potential and dark matter of the Milky Way is to measure the positions and velocities of as many stars as possible (with the Gaia mission we will get a billion). Existing methods for inferring the gravitational potential all depend on the potential being integrable and the orbiting stars being phase-mixed. Neither of these things can conceivably be true; integrability and phase-mixing are both strongly ruled out, observationally and theoretically. I describe some ideas we are exploring to perform this dynamical inference in our "worst of all possible worlds". The key concept is that locally, phase-space structure evolves predictably. I will talk much more about data than about theory, with two fully worked examples; one is a (toy) dynamical inference in a simpler system (the Solar System) and the other is a constraint on the Milky Way potential from a disrupted stellar cluster.
10/22/2009, Thursday, 3:30 p.m., 201 NSC
ELECTROPHORETIC DEPOSITION OF NANOPARTICLE THIN FILMS: RECENT DEVELOPMENTS AND NEW DIRECTIONS
Dr. James H. Dickerson
Department of Physics and Astronomy, Vanderbilt UniversitySemiconducting, insulating, and metallic nanoparticles have attracted considerable interest recently due to their size-dependent, quantum confinement characteristics, which make them attractive for a broad platform of optical, magnetic, and electronic devices. Proposed commercial applications include solid state lighting devices, magnetic recording media, ultra-light video displays, and bio-imaging reagents. For nanoparticles to be employed in an array of commercial and industrial applications, a technique for the facile, rapid, and site-selective assembly of homogeneous, densely packed, defect-free thin films must be realized. The most widely used methods for casting nanoparticle (NP) constituents into densely packed, thermally stable films, such as evaporation-driven self assembly and Langmuir-Blodgett casting, have some recognized limitations, including the inability to achieve both large-scale ordering of the nanoparticles as well as robust chemical and structural properties. NP deposition schemes also require an understanding of both the NP dynamics in solution and the interactions that govern nanoparticle-substrate and nanoparticle-nanoparticle binding. Further, these procedures require knowledge of the intrinsic and collective properties of NPs that arise from of electrostatic, magnetic, and fluctuating electric dipole effects. The organization and stability of colloidal NP assemblies are markedly affected by the surface charge state of the constituents. Although much research has been done on the assembly of nanoparticles with a distribution of surface charge states, little has been done on the assembly of like-charged nanoparticles. In this case, repulsive Coulomb interactions, as well as van der Waals, dipole-dipole, and steric interactions govern the types of assemblies that can form. The only nanoparticle deposition scheme that considers the primary physical characteristics of the NPs in the film formation and incorporates the most favorable attributes of NP deposition is electrophoretic deposition.
Recent progress in the electrophoretic deposition of nanoparticles and other nanoscale materials will be the emphasis of this presentation. Highlighted are recent developments, including the fabrication of free-standing nanoparticle thin films, comprised solely of electrophoretically deposited nanoparticles.
10/29/2009, Thursday, 3:30 p.m., 201 NSC
The two-dimensional Metal-Insulator-Transition in Si MOSFETs and GaAs Quantum Wells: Repulsive Interactions and the Density-of-States Pseudogap
Dr. Ted Castner
Department of Physics, University of RochesterThe dominant feature of these 2d MIT (metal-insulator transition) systems arises from the repulsive interactions between charged traps and itinerant carriers. The itinerant carriers form conducting filaments in the lowest energy valleys of the random potential from the charged traps analogous to the Swiss cheese model. The dramatic differences between the transport behavior of these 2d MIT systems and the 3d MIT systems (Si:P, etc.) will be discussed. It will be shown the Kubo-Greenwood expression for the DC conductivity is proportional to N(EF, x)2, N the DOS at EF and x = n/nc-1 the reduced density and the Kubo-Greenwood expression is required to explain the data. N(EF) scales to zero as x goes to zero. Other physical quantities like TF, the Thomas-Fermi screening wave vector, the effective mass enhancement m*(x)/mb*, and the Kondo temperature TK all depend on N(EF). The mobility μ(n,T) has been calculated and compared with experiment. μ(n, T=0) is proportional to N(EF, x)2. The separatrix conductivity (x=0, T) yields a power law behavior in T. The data will be compared with different theoretical models including Fermi Liquid theory, scaling, weak localization, percolation, interaction corrections, and Wigner crystal melting. The data appears to support a Quantum Phase Transition.
November Go to Sept. '09 Oct. '09 Dec. '09
11/5/2009, Thursday, 3:30 p.m., 201 NSC
Magnetism in two unusual types of magnetic semiconductors: bulk rare-earth nitrides and Mn doped ScN.
Dr. Walter R. L. Lambrecht
Department of Physics, Case Western Reserve UniversityI will discuss our recent work on rare-earth nitrides using the LSDA+U approach and progress in understanding these materials using various specroscopies from my experimental collaborators. First I will discuss the role of Hund's rules and orbital magnetic moments. Then I will discuss the exchange couplings in GdN, EuO and related Gd pnictides. I will explain the advantages of linear response theory for these calculations and explain what is surprising about these materials. I will also discuss various synchrotron based spectroscopies, such as XES, XAS and RXES. Next, I will discuss their Raman spectra and what we learn from studying the phonons. An outlook on what is needed next, namely tackling the multiplet problem will finish this part of the talk. If time permits I will tell you in the second part about the Mn-doped ScN dilute magnetic semiconductor. I will discuss our rationale for choosing this system, and how we arrive at the prediction that it could be an interesting above room temperature ferromagnetic semiconductor.
11/12/2009, Thursday, 3:30 p.m., 201 NSC
Terahertz Spectroscopy of Complex Materials
Richard D. Averitt
Department of Physics and Photonics Center, Boston UniversityTerahertz time-domain spectroscopy is a powerful tool to investigate complex materials broadly defined. This includes artificial electromagnetic composites such as metamaterials, and correlated electron materials where the interplay between microscopic degrees of freedom leads to phenomena such as superconductivity or metal-insulator transitions. I will discuss our recent results in these areas.
Metamaterials are a relatively new type of artificial composite with electromagnetic properties that derive from their sub-wavelength structure. The judicious combination of metamaterials with MEMS technology has enabled the creation of non-planar flexible composites and micromechanically active metamaterials where the orientation of the individual (and strongly anisotropic) "atoms" can be precisely controlled. Such adaptive metamaterials potentially serve as the starting point for the development of a host of new functional electromagnetic devices.
V2O3 undergoes a transition from antiferromagnetic insulator at low temperatures to strongly correlated metal above ~160K. Optical-pump THz-probe studies on V2O3 thin films reveal coherent oscillations in the far-infrared conductivity following excitation with 35-fs optical pulses. The ~100 ps conductivity oscillations result from optically induced strain revealing a strong dynamical coupling of carriers to the lattice in the correlated metallic state.
These results reveal the utility of far-infrared spectroscopy to investigate complex materials and point the way towards future studies of hybrid composites incorporating artificial and quantum-based complex matter. Such multi-scale structures may offer complementary benefits where quantum materials confer additional functionality to artificial electromagnetic composites or, conversely, metamaterials serve as a novel tool to facilitate fundamental studies of the electrodynamic response of complex quantum materials.
11/19/2009, Thursday, 3:30 p.m., 201 NSC
Coulomb drag and spin Hall Drag: new coupling mechanisms for nanoelectronics
Dr. Giovanni Vignale
Department of Physics, University of Missouri-ColumbiaDouble-layer structures consisting of two parallel quantum wells separated by a narrow potential barrier are an important class of nanoscale electronic devices. Each layer hosts a quasi-two dimensional electron gas and electrons interact across the barrier via the Coulomb interaction. When an electric current is driven in one of the layers the Coulomb interaction causes a charge accumulation in the other layer. This phenomenon, known as Coulomb drag, is of fundamental interest as a probe of electron correlations and provides a new coupling mechanism for nano-electronics, alternative to the conventional inductive and capacitive couplings. Another effect of great interest is the Spin Hall Effect, i.e. the generation of spin accumulation by an electric current. This is due to spin-orbit interactions and has recently received great attention not only because of its theoretical subtlety but also for its usefulness as a source of spin-polarized currents. In this talk I describe a new effect, which arises from the combination of spin Hall effect and Coulomb drag. I call it Spin Hall Drag. The effect consists in the generation of transversal spin accumulation in one layer by an electric current in the other layer. Microscopic calculations indicate that the induced spin accumulation, although considerably smaller than the one observed in the ordinary spin Hall effect, is large enough to be detected in optical rotation experiments.
December Go to Sept. '09 Oct. '09 Nov. '09