Single hole transistor in a p-Si/SiGe quantum well

A single hole transistor is patterned in a p-Si/SiGe quantum well by applying voltages to nanostructured top gate electrodes. Gating is achieved by oxidizing the etched semiconductor surface and the mesa walls before evaporation of the top gates. Pronounced Coulomb blockade effects are observed at small coupling of the transistor island to source and drain.Comment: 3 pages, 3 figure

n-Si/SiGe quantum cascade structures for THz emission

In this work we report on modelling the electron transport in n-Si/SiGe structures. The electronic structure is calculated within the effective-mass complex-energy framework, separately for perpendicular (Xz) and in-plane (Xxy) valleys, the degeneracy of which is lifted by strain, and additionally by size quantization. The transport is described via scattering between quantized states, using the rate equations approach and tight-binding expansion, taking the coupling with two nearest-neighbour periods. The acoustic phonon, optical phonon, alloy and interface roughness scattering are taken in the model. The calculated U/I dependence and gain profiles are presented for a couple of QC structures

Analysis of the resistance in p-SiGe over a wide temperature range

The temperature dependence of a system exhibiting a `metal-insulator transition in two dimensions at zero magnetic field' (MIT) is studied up to 90K. Using a classical scattering model we are able to simulate the non-monotonic temperature dependence of the resistivity in the metallic high density regime. We show that the temperature dependence arises from a complex interplay of metallic and insulating contributions contained in the calculation of the scattering rate 1/\td(E,T), each dominating in a limited temperature range.Comment: 4 pages with 5 figure

Second harmonic generation on incommensurate structures: The case of multiferroic MnWO4

A comprehensive analysis of optical second harmonic generation (SHG) on an incommensurate (IC) magnetically ordered state is presented using multiferroic MnWO4 as model compound. Two fundamentally different SHG contributions coupling to the primary IC magnetic order or to secondary commensurate projections of the IC state, respectively, are distinguished. Whereas the latter can be described within the formalism of the 122 commensurate magnetic point groups the former involves a breakdown of the conventional macroscopic symmetry analysis because of its sensitivity to the lower symmetry of the local environment in a crystal lattice. Our analysis thus foreshadows the fusion of the hitherto disjunct fields of nonlinear optics and IC order in condensed-matter systems

Periodic vacuum and particles in two dimensions

Different dynamical symmetry breaking patterns are explored for the two dimensional phi4 model with higher order derivative terms. The one-loop saddle point expansion predicts a rather involved phase structure and a new Gaussian critical line. This vacuum structure is corroborated by the Monte Carlo method, as well. Analogies with the structure of solids, the density wave phases and the effects of the quenched impurities are mentioned. The unitarity of the time evolution operator in real time is established by means of the reflection positivity.Comment: Final version, additional references and the proof of reflection positivity added, 41 pages, 16 figure

Coexistence of Weak Localization and a Metallic Phase in Si/SiGe Quantum Wells

Magnetoresistivity measurements on p-type Si/SiGe quantum wells reveal the coexistence of a metallic behavior and weak localization. Deep in the metallic regime, pronounced weak localization reduces the metallic behavior around zero magnetic field without destroying it. In the insulating phase, a positive magnetoresistivity emerges close to B=0, possibly related to spin-orbit interactions.Comment: 4 pages, 3 figure

Analysis of the Metallic Phase of Two-Dimensional Holes in SiGe in Terms of Temperature Dependent Screening

We find that temperature dependent screening can quantitatively explain the metallic behaviour of the resistivity on the metallic side of the so-called metal-insulator transition in p-SiGe. Interference and interaction effects exhibit the usual insulating behaviour which is expected to overpower the metallic background at sufficiently low temperatures. We find empirically that the concept of a Fermi-liquid describes our data in spite of the large r_s = 8.Comment: 4 pages, 3 figure

On the driven Frenkel-Kontorova model: I. Uniform sliding states and dynamical domains of different particle densities

The dynamical behavior of a harmonic chain in a spatially periodic potential (Frenkel-Kontorova model, discrete sine-Gordon equation) under the influence of an external force and a velocity proportional damping is investigated. We do this at zero temperature for long chains in a regime where inertia and damping as well as the nearest-neighbor interaction and the potential are of the same order. There are two types of regular sliding states: Uniform sliding states, which are periodic solutions where all particles perform the same motion shifted in time, and nonuniform sliding states, which are quasi-periodic solutions where the system forms patterns of domains of different uniform sliding states. We discuss the properties of this kind of pattern formation and derive equations of motion for the slowly varying average particle density and velocity. To observe these dynamical domains we suggest experiments with a discrete ring of at least fifty Josephson junctions.Comment: Written in RevTeX, 9 figures in PostScrip

Does Being Bored Make Us More Creative?

Boredom has traditionally been associated with a range of negative outcomes, both within the workplace and outside it. More recently, however, it has been suggested that boredom can have positive outcomes, one of which might be increased creativity. This study addressed this proposition by examining the relationship between boredom and creative potential on a range of tasks. Two studies were carried out; the first involved 80 participants taking part in either a boring writing activity or not (control group) followed by a creative task. The second study involved a further 90 participants who varied in the type of boring activity they undertook (either a boring written activity, a boring reading activity or control) and the type of creative task that followed. Results suggested that boring activities resulted in increased creativity and that boring reading activities lead to more creativity in some circumstances (such as convergent tasks) than boring written activities. The role of daydreaming as a mediator between boredom and creativity is discussed and implications are outline