We examine stochastic Markov processes of the type that have been typically been designed to generate nonequilibrium steady states through a deliberate breaking of the detailed balance principle. We show that in order to quantify the heat dissipated by these dynamics, one must supply more details about a system's environment that are normally specified in such approaches. We also show that in an approximate model of a driven system's dynamics, the true entropy production is typically underestimated by the model. These findings are used to critique various procedures that have been used to construct stochastic nonequilibrium dynamics and some notions arising in steady state thermodynamics.

Hosted by Royce Zia.

The discoveries of exotic ground states (p-wave superconductivity, non-Fermi liquid) in layered ruthenates have inspired extensive investigations on 4d and 5d materials. Typified by their extended 5d orbitals, it is commonly expected that the iridates should be more metallic and less magnetic than their 3d, 4d and 4f counterparts, because of the broader 5d-bandwidth and the weaker exchange interaction U, so that U g(E_F) < 1, where g(E_F) is the density of states at the Fermi energy E_F. However, in marked contrast to these expectations, most of the known iridates, such as layered BaIrO₃ and Sr_n+1Ir_nO_3n+1 (n = 1 and 2), are insulators exhibiting weak ferromagnetism. These layered iridates order at relatively high temperatures (T_C = 175, 240, and 285 K for BaIrO₃, Sr₂IrO₄ and Sr₃Ir₂O₇, respectively), but they attain only a small fraction of the expected ordered moment. The iridates exhibit strong phase transition signatures in magnetization but none of their T_C's and resistivities is very sensitive to high magnetic fields. Although a metallic state does not commonly occur in the iridates, the unusual circumstances cited above almost guarantee that it will exhibit extraordinary properties when it does occur. In this talk, anomalous transport and thermodynamic properties of non-Fermi-liquid SrIrO₃ will be presented and discussed along with comparisons drawn with other related iridates and ruthenates.

Hosted by Vicki Soghomonian.

Nuclear magnetic resonance (NMR) provides methods of imaging and spectroscopy that have revolutionized medicine as well as the scope of analysis in physics, chemistry, biology, and materials science. I will present a technique of NMR that relies on detecting mechanical displacements of a small ferromagnet pushed by the spin magnetism of a nearby sample. In BOOMERANG (better observation of magnetization, enhanced resolution, and no gradient), the sample experiences a homogeneous magnetic field as in traditional NMR, allowing application of modern NMR imaging and spectroscopy techniques. Scaling laws for spin detection sensitivity motivate using BOOMERANG at sample diameters below about 0.5 mm, opening up small scales of analysis previously inaccessible by NMR. I will outline theory, experimental progress, and outlooks to future applications.

Hosted by Jean Heremans.

A quantum computer is a device for computation that makes direct use of distinctively quantum mechanical phenomena, such as superposition and entanglement, to perform operations on data. The logical unit of a quantum computer is the qubit, and, as in a classical computer, complex algorithms can be built from simpler single-qubit and two-qubit operations. In many quantum computing architectures individual qubits need to be placed close enough - nanometers apart - to ensure an effective two-qubit operation. This is technologically challenging, especially in the case of semiconductor-based implementations with quantum dots or localized impurities.

One way to overcome these limitations involves the use of Cavity Quantum Electrodynamics (QED), i.e. the physics of confined photons interacting with matter. I will show how cavity QED effects can be used to couple qubits at large separation in a semiconductor-based quantum computer. The key role in the two-qubit operation is played by peculiar states, called cavity polaritons, with a half-matter and half-light character.

Hosted by Giti Khodaparast.

Clostridium perfringensi is a Gram-positive anaerobic bacterium that causes gas gangrene and food poisoning in humans. The species was previously classified as non-motile, since it has no flagella. Genome sequencing of 3 strains of C. perfringens indicated they do not have flagella or identifiable chemotaxis proteins, but could produce type IV pili, which are common in Gram-negative bacteria but had not been seen in Gram-positive bacteria before this. Video microscopy revealed that the bacteria do move with a unique form of gliding motility and that this motility is dependent on production of the type IV pili. At the edge of a colony, the bacteria shift from a randomly oriented conformation into an orderly array of filaments and then proceed to extend the filaments in an outward direction. Our research is currently focused on two biophysical aspects of motility: (1) How the bacteria obtain the propulsive force to move across the surface of an agar plate, despite an extremely high viscosity. (2) How the bacteria arrange themselves into an ordered structure and behave in a group (i.e., social) fashion to sustain motility, rather than functioning as individual bacteria. The talk will discuss our current level of knowledge about these issues and propose underlying hypotheses to explain these functions.

Hosted by Uwe Täuber.

As the field of spintronics progresses, there is growing interest in examining the role spin has on charge transport in both magnetic and non-magnetic materials. Accordingly, this talk will present some electrical transport measurements of spin related effects in both magnetic semiconductors as well as non-magnetic semiconductors. For instance, the nature of the conduction band edge splitting due to the magnetic interactions between the localized 4f spins in the ferromagnetic semiconductor EuS was observed by analyzing the temperature dependence of the tunneling barrier height in the heterojunction formed between EuS and the non-magnetic semiconductor InAs. Separately, measurements of the low field magnetoresistance in thin films of n-type InSb were performed and fit to weak anti-localization theory in order to determine the low temperature spin coherence times (lengths) of the itinerant electrons in such materials. Furthermore, by analyzing the Fermi level dependence of the spin-orbit scattering times for different InSb films the spin-orbit mechanism (Elliot-Yafet vs. D'yakonov-Perel') that is responsible for the spin decoherence in InSb was determined.

Hosted by Jean Heremans.

By simple modeling of dissipative interactions we resolve fundamental questions related to systems far from thermal equilibrium, such as granular materials, foams and colloidal suspensions. We solve the non-Boltzmann energy distribution, demonstrate the violation of time-dependent fluctuation-dissipation relations, show that different measures of effective temperatures generally differ, and address further issues such as ergodicity breaking, detailed balance violation, and entropy decrease upon contact between systems.

Hosted by Beate Schmittmann.

Superfluorescence (SF) is a cooperative quantum optical process in which coherent light is emitted from a macroscopic ensemble of coherent quantum oscillators. In semiconductors, the observation of SF from electron hole recombination is very difficult due to the ultrafast decoherence time. In this presentation, we demonstrate experimental evidences for SF from recombination of high density electron hole plasma in In_xGa_1-xAs/GaAs multiple quantum well at high magnetic field. Above critical magnetic field and excitation laser fluence, we observe that the field or fluence dependent emission strength of photoluminescence changes from linear to superlinear, also the output of photoluminescence changes from omnidirectional emission to a randomly directed emission. We also observe changes in the line-width of photoluminescence with respect to magnetic field and excitation fluence. All of these experimental results are consistent with cooperative recombination.

Hosted by Giti Khodaparast.

Convergent extension is the name given to the initial elongation of embryo tissue which first establishes the body axis. This morphogenetic process is "conserved", meaning that it happens in flies, it happens in frogs, and it happened in you (otherwise you wouldn't be reading this). In Drosophila, convergent extension occurs primarily by cell rearrangement, without cell-shape elongation and prior to the onset of cell division. Recent work (see References) in the laboratory of J. A. Zallen at Sloan-Kettering, using live-imaging confocal microscopy, protein labelling, and genetics, has yielded new insight into this complex and subtle process. After a necessarily simple introduction to morphogenesis, the possible usefulness of topological measures (such as used, for example, in the physics of foams) will be discussed.

References: J. A. Zallen and E. Wieschaus, Dev. Cell. 6, 343 (2004); J. A. Zallen and R. Zallen, J. Phys. Condens. Matter 16, S5073 (2004); J. T. Blankenship et al., Dev. Cell 11, 459 (2006); J. A. Zallen, Cell 159, 1051 (2007).

Since the last decade, molecular magnets or single-molecule magnets have drawn considerable attention due to observed magnetic quantum tunneling and interference and a possibility of using them in various applications. There have been significant experimental efforts to build and characterize thin films or monolayers of single-molecule magnets on surfaces or single-molecule magnets bridged between electrodes. In parallel to the experimental work, theoretical models have been proposed to understand the properties of single-molecule magnets coupled to a metal substrate. However, to the best of our knowledge, atomic-scale simulations do not exist on this complex system. We simulate, within density-functional theory, a nanostructure in which prototype Mn₁₂ molecules are adsorbed via a thiol group onto a gold surface. Based on a supercell calculation, we investigate how strongly a Mn₁₂ molecule is coupled to the metal surface and how much charge and spin moments are transferred between a Mn₁₂ molecule monolayer and the metal surface. In particular, we compare the electronic structure and magnetic properties of the nanostructure with those of an isolated Mn₁₂ molecule in the absence and presence of spin-orbit interaction. Our results may shed light into tailoring the magnetic properties of nanomagnets via electron and spin transfer from a proximal metallic surface.

Hosted by Kyungwha Park.