- Monday, February 8, 2016
ABSTRACT: The quantization of electron motion in magnetic fields generates a plethora of fascinating phenomena observed in condensed materials. One of the well-known examples is the Shubnikov-de Haas (SdH) resistance oscillations. In two dimensional electron systems, SdH oscillations can be very pronounced leading to the Quantum Hall Effect (QHE) at low temperatures.
Landau quantization produces a remarkable effect on Joule heating of two dimensional (2D) electrons. The heating forces 2D electrons into exotic electronic states in which voltage (current) does not depend on current (voltage). In contrast to the linear response at low temperatures (SdH, QHE), the quantization affects Joule heating in a significantly broader temperature range. At temperatures significantly exceeding the cyclotron energy the dc heating produces a multi-tiered electron distribution containing as many tiers as the number of Landau levels inside the energy interval kT. This quantal heating preserves the overall broadening of the electron distribution. Surprisingly the distribution resulting from quantal heating is, in some respect, similar to the one created by the quantum microwave pumping between Landau levels. Indicated phenomena produce a broad variety of nonlinear effects in quantizing magnetic fields and present an exciting area of the contemporary research. In this talk a recent experimental investigations of the dynamics of quantal heating are presented indicating an important role of the electron-electron interaction in the relaxation of the electron distribution.
- Monday, February 29, 2016
ABSTRACT: Noble metal nanoparticles can support localized surface plasmons, which lead to enhanced electromagnetic fields at the nanoparticle surface and allow for a host of surface-enhanced spectroscopies, such as surface-enhanced Raman scattering (SERS). While extensive theoretical calculations have been performed that predict how these enhanced electromagnetic fields are distributed on the nanoparticle surface, confirming these results using optical techniques is extremely challenging due to the diffraction limit of light. Because the metal nanoparticles are smaller than the wavelength of light, they appear as diffraction limited spots in optical images, obscuring the local electromagnetic field enhancements. This talk will describe recent efforts to use high resolution single molecule imaging techniques to measure how electromagnetic fields are locally enhanced on the surface of noble metal nanoparticles for applications in SERS. Single molecule spectroscopy allows us to beat the diffraction limit by over an order of magnitude, providing the necessary resolution to optically image electromagnetic field enhancements on noble metal nanoparticle surfaces.
- Monday, March 28, 2016
ABSTRACT: Because of the possibility of electromagnetic interference between neighboring electronic systems, there is an urgent need to quantify the entry and distribution of electromagnetic (EM) energy within complicated metallic enclosures and to understand the manner in which this energy couples to sensitive electronic devices within such enclosures. When the wavelength of the impinging radiation is much smaller than the typical length scale of the enclosure, the distribution of energy within such cavities is highly sensitive to small changes in the frequency, the structure of the cavity, as well as the nature of the channels which couple EM energy into the cavity. Thus, a statistical approach to understanding this problem is called for.
There is great interest in the wave and quantum properties of systems that show chaos in the classical (short wavelength) limit. These ‘wave chaotic’ systems appear in many contexts: nuclear physics, acoustics, two-dimensional quantum dots, and electromagnetic enclosures, for example. Random Matrix Theory (RMT) predicts the universal fluctuating properties of quantum/wave systems that show chaos in the classical/ray limit.
In this context we developed a stochastic model, the “Random Coupling Model” (RCM) [1,2], which can accurately predict the probability density functions (PDFs) of voltages and electromagnetic field quantities on objects within such cavities, given a minimum of information about the cavity and the nature of its internal details. The RCM is formulated in terms of electrical impedance, essentially equivalent to Wigner’s reaction matrix in quantum mechanics, rather than the more commonly studied scattering matrix. The RCM predictions have been tested in a series of experiments using normal metal and superconducting quasi-two-dimensional and three-dimensional electromagnetic billiards . We have extended the RCM in a number of directions, for example by examining the effects of ‘short orbit’ ray trajectories that enter the cavity, bounce a small number of times, and then leave the cavity. We are able to account for the effects of these orbits using a semi-classical theory, and find excellent agreement between theory and experiment . Our current efforts are focused on testing predictions for the statistical properties of multiple inter-connected enclosures , enclosures irradiated through apertures , and enclosures characterized by a mixed chaotic and regular phase space , using scaled model structures.
For more information see: http://anlage.umd.edu/AnlageQChaos.htm.
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I. Cevdet Noyan
- Monday, April 11, 2016
ABSTRACT: 100-plus years of theoretical and experimental advances have reduced kinematical scattering formalisms for powder diffraction to routine, vendor-supplied, black-box analysis programs accessible to users at all training levels. Understanding what really goes on in the analysis, however, is a non-trivial task. We used computer modeling to analyze the powder diffraction process from nanoparticle ensembles.
Our results showed, surprisingly, that the classical formulations described in diffraction textbooks were inadequate; venerable concepts like reflection multiplicity, the "Lorentz factor", sampling statistics, etc. actually depended on the size of the crystalline particles contributing to the diffraction profile. We expect modeling of scattering experiments to yield more surprises as the phase space hidden behind canonical assumptions becomes accessible for exploration.