There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

N G van Kampen 1981 Amsterdam: North-Holland xiv + 419 pp price Dfl 180 This is a book which, at a lower price, could be expected to become an essential part of the library of every physical scientist concerned with problems involving fluctuations and stochastic processes, as well as those who just enjoy a beautifully written book. It provides an extensive graduate-level introduction which is clear, cautious, interesting and readable.

https://iopscience.iop.org/article/10.1088/0031-9112/34/4/040/pdf

Stochastic Processes in Physics and Chemistry

Brownian motors: noisy transport far from equilibrium

The field of magnetoelectronics has been growing in practical importance in recent years1 For example, devices that harness electronic spin—such as giant-magnetoresistive sensors and magnetoresistive memory cells—are now appearing on the market2 In contrast, magnetoelectronic devices based on spin-polarized transport in semiconductors are at a much earlier stage of development, largely because of the lack of an efficient means of injecting spin-polarized charge Much work has focused on the use of ferromagnetic metallic contacts3,4, but it has proved exceedingly difficult to demonstrate polarized spin injection More recently, two groups5,6 have reported successful spin injection from an NiFe contact, but the observed effects of the spin-polarized transport were quite small (resistance changes of less than 1%) Here we describe a different approach, in which the magnetic semiconductor BexMnyZn1-x-ySe is used as a spin aligner We achieve injection efficiencies of 90% spin-polarized current into a non-magnetic semiconductor device The device used in this case is a GaAs/AlGaAs light-emitting diode, and spin polarization is confirmed by the circular polarization state of the emitted light

Injection and detection of a spin-polarized current in a light-emitting diode

https://www.physik.uni-augsburg.de/theo1/hanggi/Papers/213.pdf

Driven quantum tunneling

Activated escape from a metastable state underlies many physical, chemical and biological processes: examples include diffusion in solids, switching in superconducting junctions1,2, chemical reactions3,4 and protein folding5,6. Kramers presented the first quantitative calculation7 of thermally driven transition rates in 1940. Despite widespread acceptance of Kramers’ theory8, there have been few opportunities to test it quantitatively as a comprehensive knowledge of the system dynamics is required. A trapped brownian particle (relevant to our understanding of the kinetics, transport and mechanics of biological matter9,10) represents an ideal test system. Here we report a detailed experimental analysis of the brownian dynamics of a sub-micrometre sized dielectric particle confined in a double-well optical trap. We show how these dynamics can be used to directly measure the full three-dimensional confining potential—a technique that can also be applied to other optically trapped objects11,12. Excellent agreement is obtained between the predictions of Kramers’ theory and the measured transition rates, with no adjustable or free parameters over a substantial range of barrier heights.

Thermally activated transitions in a bistable three-dimensional optical trap

The dielectric constant ∈ and conductivity σ of undoped C60 single crystals have been measured as a function of temperature, 10 K < T < 330 K, and frequency, 0.2 kilohertz < f < 100 kilohertz. On cooling below the first-order structural phase transition at 260 K, a Debye-like relaxational contribution to the dielectric response is observed, which requires the presence of permanent electric dipoles. The relaxation rate is thermally activated with a broad distribution of energies centered at 270 millielectron volts. The existence of a dipole moment in C60 is unexpected, because it is precluded by symmetry for the pure ordered cubic phase. These data suggest that the high degree of frozen-in orientational disorder of the C60 molecules is responsible for the existence of electric dipolar activity.

https://science.sciencemag.org/content/sci/257/5069/511.full.pdf

Existence of an orientational electric dipolar response in c60 single crystals.

Raman microscopy was used to identify corrosion products and their locations on and within samples from ancient Chinese bronze money trees to help determine the corrosive environments the trees experienced in the past. The compounds found included Cu2 O, CuCO3 · Cu(OH)2, 2CuCO3 · Cu(OH)2, PbO, PbCO 3 and PbSO4. In addition, a number of pigments on the exterior were identified (ultramarine, Prussian blue, phthalocyanine blue, phthalocyanine green, Hansa yellow G and chrome yellow) which were applied to the artifacts in more recent times. Two-dimensional Raman mapping was applied to cross-sections of the artifacts to determine the spatial extent of corrosion products in the interior and to investigate the selective corrosion of different phases of the bronze. Raman microscopy is shown to be a useful tool to complement more traditional characterizations such as scanning electron microscopy and x-ray energy-dispersive spectrometry in the analysis of metal artifacts. Copyright © 1999 John Wiley & Sons, Ltd.

Corrosion of ancient Chinese bronze money trees studied by Raman microscopy

We provide an explicit solution of the problem of activation escape from a metastable state of a periodically driven Brownian particle, including both the exponent and the prefactor. We find the instantaneous and time-average escape rates, and a crossover in their field dependence, from weak to exponentially strong, with amplitude and period of the driving field. The results apply for an arbitrary ratio between the field amplitude and the noise intensity, and between the field period and the relaxation time of the system.

Time Oscillations of Escape Rates in Periodically Driven Systems

The transition rates of individual two-state systems in a mesoscopic disordered metal (Bi) are studied via time-dependent conductance fluctuations. A striking increase in the transition rates is observed as the temperature T is decreased below 1 K for defects with energy splitting \ensuremath{\varepsilon}${\mathit{k}}_{\mathit{B}}$T. In contrast, the transition rates decrease monotonically as T is lowered if \ensuremath{\varepsilon}g${\mathit{k}}_{\mathit{B}}$T. We show that these phenomena are a manifestation of dissipative quantum tunneling and estimate, for one particular defect, the defect--conduction-electron coupling parameter, \ensuremath{\alpha}\ensuremath{\approxeq}0.24.

B. Golding

Papers

Thermally activated transitions in a bistable three-dimensional optical trap

Existence of an orientational electric dipolar response in c60 single crystals.

Corrosion of ancient Chinese bronze money trees studied by Raman microscopy

Time Oscillations of Escape Rates in Periodically Driven Systems

Dissipative quantum tunneling of a single microscopic defect in a mesoscopic metal.