Harold U. Baranger

Communication through a diffusive medium : Coherence and capacity

Coherent wave propagation in disordered media gives rise to many fascinating phenomena as diverse as universal conductance fluctuations in mesoscopic metals and speckle patterns in light scattering. Here, the theory of electromagnetic wave propagation in diffusive media is combined with information theory to show how interference affects the information transmission rate between antenna arrays. Nontrivial dependencies of the information capacity on the nature of the antenna arrays are found, such as the dimensionality of the arrays and their direction with respect to the local scattering medium. This approach provides a physical picture for understanding the importance of scattering in the transfer of information through wireless communications.

Quantum-Interference-Controlled Molecular Electronics

Quantum interference in coherent transport through single molecular rings may provide a mechanism to control the current in molecular electronics. We investigate its applicability, using a single-particle Green function method combined with ab initio electronic structure calculations. We find that the quantum interference effect (QIE) is strongly dependent on the interaction between molecular pi-states and contact sigma-states. It is masked by sigma tunneling in small molecular rings with Au leads, such as benzene, due to strong pi-sigma hybridization, while it is preserved in large rings, such as [18]annulene, which then could be used to realize quantum interference effect (QIE) transistors.

Electron transport through molecules: Self-consistent and non-self-consistent approaches

A self-consistent method for calculating electron transport through a molecular device is developed. It is based on density functional theory electronic structure calculations under periodic boundary conditions and implemented in the framework of the nonequilibrium Green function approach. To avoid the substantial computational cost in finding theI-V characteristic of large systems, we also develop an approximate but much more efficient non-self-consistent method. Here the change in effective potential in the device region caused by a bias is approximated by the main features of the voltage drop. As applications, the I-V curves of a carbon chain and an aluminum chain sandwiched between two aluminum electrodes are calculated—two systems in which the voltage drops very differently. By comparing to the self-consistent results, we show that this non-self-consistent approach works well and can give quantitatively good results.

Mesoscopic transport through chaotic cavities: A random S-matrix theory approach.

We deduce the effects of quantum interference on the conductance of chaotic cavities by using a statistical ansatz for the S matrix. Assuming that the circular ensembles describe the S matrix, we find that the conductance fluctuation and weak-localization magnitudes are universal: they are independent of the size and shape of the cavity if the number of incoming modes, N, is large. The limit of small N is more relevant experimentally; here we calculate the full distribution of the conductance and find striking differences as N changes or a magnetic field is applied.

Weak localization in chaotic versus nonchaotic cavities: A striking difference in the line shape.

We report experimental evidence that chaotic and non-chaotic scattering through ballistic cavities display distinct signatures in quantum transport. In the case of non-chaotic cavities, we observe a linear decrease in the average resistance with magnetic field which contrasts markedly with a Lorentzian behavior for a chaotic cavity. This difference in line-shape of the weak-localization peak is related to the differing distribution of areas enclosed by electron trajectories. In addition, periodic oscillations are observed which are probably associated with the Aharonov-Bohm effect through a periodic orbit within the cavities.

Experimental studies of the acoustic signature of proton beams traversing fluid media

Abstract Recent experiments at Brookhaven National Laboratory and Harvard University demonstrate that a detectable sonic signal is produced by energetic proton beams while traversing a fluid medium. The observed acoustic wave agrees with the predictions of a thermal expansion model. Results are inconsistent with any significant contribution from either microbubble implosion or molecular dissociation, two other suggested means of sonic production. Frequency and amplitude distributions, radiation patterns, temperature, pressure, and medium dependencies are explored. This phenomenon may have immediate applications in beam monitoring and in detecting energetic heavy ions. Signal thresholds may be enough to permit detection of particle showers induced by single particles at the next generation of high energy accelerators or from high energy cosmic rays. The inexpensive transducers and long sonic transmission lengths obtainable in liquids suggest that high energy particle detectors may be feasible with masses many orders of magnitude greater than those currently in use.

Wireless propagation in buildings: a statistical scattering approach

A new approach to the modeling of wireless propagation in buildings is introduced. We treat the scattering by walls and local clutter probabilistically through either a relaxation-time approximation in a Boltzmann equation or by using a diffusion equation. The result is a range of models in which one can vary the tradeoff between the complexity of the building description and the accuracy of the prediction. The two limits of this range are ray tracing at the most accurate end and a simple decay law at the most simple. By comparing results for two of these new models with measurements, we conclude that a reasonably accurate description of propagation can be obtained with a relatively simple model. The most effective way to use the models is by combining them with a few measurements through a sampling technique.

Weak localization and integrability in ballistic cavities.

We demonstrate the existence of an interference contribution to the average magnetoconductance, G(B), of ballistic cavities and use it to test the semiclassical theory of quantum billiards. G(B) is qualitatively different for chaotic and regular cavities (saturation versus linear increase) which is explained semiclassically by the differing classical distribution of areas. The magnitude of G(B) is poorly explained by the semiclassical theory of coherent backscattering (elastic enhancement factor); interference between trajectories which are not exactly time reversed must be included

Near-perfect conduction through a ferrocene-based molecular wire

Here we describe the design, single-molecule transport measurements, and theoretical modeling of a ferrocene-based organometallic molecular wire, whose bias-dependent conductance shows a clear Lorentzian form with magnitude exceeding 70% of the conductance quantum

Contact Atomic Structure and Electron Transport Through Molecules

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