Seminars, held on Wednesdays, begin at 4:00 p.m. in the Robeson Room, 304
(unless otherwise indicated)

Refreshments are served at 3:30 p.m.

< Fall 2005 | January | February | March | April | May | Fall 2006 >

Surprises in the Kondo physics of single-molecule transistors

(3:30pm, McBryde 216)

Inelastic effects in molecular junctions: weak and strong interaction

(3:30pm, Derring 3092)

Bridging the gap between ab initio computables and experimental observables

(2:30pm, Robeson 210)

Path Integral Tools for Nano-Electronics

(2:30pm, Robeson 210)

Aging and dynamical scaling in nonequilibrium systems

(3:30pm, Derring 3092)

Precise Adaptation in bacterial chemotaxis through "assistance neighborhoods"

(2:30pm, Robeson 210)

Life in a Crowd: Macromolecular Crowding and Confinement Effects on Protein Interactions in Living Systems

(3:30pm, Robeson 304)

Do spin glasses order in a field?

Quantum antiferromagnetism and high T_C superconductivity: a close connection between the t-J model and the projected BCS Hamiltonian

A connection between quantum antiferromagnetism and high T_C superconductivity is theoretically investigated by analyzing the t-J model and its relationships to the Gutzwiller-projected BCS Hamiltonian. After numerical corroboration via exact diagonalization, it is analytically shown that the ground state of the t-J model at half filling (i.e., the 2D antiferromagnetic Heisenberg model) is entirely equivalent to the ground state of the Gutzwiller-projected BCS Hamiltonian with strong pairing. Combined with the high wavefunction overlap between the ground states of the t-J model and the projected BCS Hamiltonian at moderate doping, this equivalence provides strong support for the existence of superconductivity in the t-J model. The relationship between the ground state of the projected BCS Hamiltonian and Anderson's resonating valence bond state (i.e., the projected BCS ground state) is discussed.

Special Seminar
(2:30pm, Robeson 304)

Investigation of Metallic Magnetic Microstructures by Optical Methods

Other than traditional applications of acting as hard disk memory media and magnetic field sensor, metallic magnetic microstructures in recent years open up a number of utilization frontiers such as random access memorization, logic computation and mesoscopic microwave generation. In this seminar, three independent studies on metallic magnetic microstructures by means of optical method will be presented: (a) Kerr imaging of layer-by-layer magnetic reversal in Co/Pt multilayers, (b) Brillouin light scattering studies of spin precession under tunable magnetic field imbalance and (c) Observation of spin-polarized current induced domain wall motion in magnetic micro-wires. Their physical implications and applications will be discussed.

Special Seminar
(4:00pm, Robeson 304)

DETERMINING SPIN POLARIZATION OF FERROMAGNETS USING SUPERCONDUCTING SPECTROSCOPY

The tremendous interest in using the spin degree of freedom in electronic devices has led to an extensive endeavor to investigate the intrinsic spin polarization of various magnetic materials. The work done here expands upon the existing methods of Andreev reflection spectroscopy and spin polarized tunneling to develop a more general technique of precise electrical determination of spin polarization using superconducting spectroscopy with or without the presence of a magnetic field. As part of this effort, the use of Andreev reflection in planar junction configuration was explored on several ferromagnetic materials including the dilute magnetic semiconductor (DMS) Ga1xMnxAs and the concentrated magnetic semiconductor EuS, revealing high spin polarization in both materials. This work also led to the exploration of the effects of barrier strength on the measured spin polarization in Al/AlOx/NiFe where the barrier thickness was varied by oxidation time.

Electrons, Molecules, Surfaces, and Light

Individual molecules, nanoparticles, and their assemblies are being explored as the building blocks for new proposed electronic and photonic devices. Yet electrical characteristics of molecules are poorly understood. Scanning tunneling microscopy (STM) is an ideal tool to measure these systems because it interacts with the sample via electron tunneling and its spatial resolution is inherently atomic scale. STM has been applied with some success to measure the conductance of molecules and monolayer films. However because STM measurements are a convolution of the spatial and electronic properties of the tunneling junction, many details remain unresolved. It is my group's interest to develop optical spectroscopy with nanometer-scale resolution in combination with STM, to open up a new channel of information. To this end we have developed atomically-flat optically-resonant gold nanoparticles to use as benchtop-friendly substrates which are also excellent substrates for growing self-assembled monolayers. We have studied the optical modes (plasmons) of these particles using scanning near-field optical microscopy (NSOM) and dark-field light-scattering microscopy. The optical resonances should facilitate optical coupling to/from molecules on the surface of these particles, thus they could be used as photonic antenna.

Exchange interaction between localized magnetic moments and band carriers in GaMnAs: or what makes GaMnAs ferromagnetic?

Even though GaMnAs is the most studied ferromagnetic semiconductor, there remain many fundamental issues that are not yet fully understood. One of these issues is the nature of ferromagnetic coupling between the Mn moments. Another issue needed for understanding of the microscopic mechanisms of ferromagnetic long range order in this material is to have a clearer picture of the electronic properties of Mn (3d) impurities and of the p-d exchange interaction between the localized Mn moments and band carriers. In this talk I will present magneto-optical data obtained on a series of GaMnAs samples with a wide range of Mn and carrier concentrations that correspond to both para- and ferromagnetic phases in GaMnAs. The data show that in a certain range of carrier concentrations the p-d exchange is suppressed, resulting in the absence of Zeeman splitting of the valence band states. We will argue that the hole bound to a Mn acceptor (the so-called A0 center) is not only crucial for mediating the long range ferromagnetic order, but also provides a ferromagnetic channel for the interaction of Mn with electrons in the valence band. In highly compensated samples an electron also becomes bound to the A0 center, thus blocking the hole from participating in the p-d exchange and from mediating the Mn-Mn ferromagnetic coupling.

Electronic Properties of InSb Quantum Wells and Mesoscopic Structures

In narrow-gap semiconductors, electrons have properties that are much different than in free space. For example, the effective mass in InSb is nearly two orders of magnitude smaller than the mass in free space. This property can be exploited in applications, such as magnetic read heads or ballistic transport devices, where a high mobility or a long mean free path is required. The strength of the interaction between an electron's spin and a magnetic field is also enhanced in InSb. The consequences of a small effective mass and large spin-orbit coupling are seen in far-infrared spectroscopy and charge transport measurements performed on structures with nanometer-scale dimensions in one or more directions.