Equidistant spectrum of localized states due to fluctuations of the parameters in quantum-well structures

Narrow equidistant peaks are recorded in the microphotoluminescence spectra of GaAs/Al_xGa_1−x As superlattices with thin barrier layers. An approximate method, consisting of the reduction of a three-dimensional problem to a one-dimensional problem, is developed for calculating the electronic spectrum of quantum dots of arbitrary shape. It is shown on the basis of this method that the observed peaks may be due to the formation of electronic states of a new type, which are produced as a result of the overlapping of fluctuations of the thicknesses of adjacent wells in structures with thin barriers. The equidistant spectrum of the new states is accounted for by the nearly quadratic dependence of the diameter of the overlap regions on the coordinate in the plane of the layers. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 4, 260–265 (25 February 1996)     

From quantum fluctuations to large-scale structures

An acceleration phase in the early universe allows microscopic quantum fluctuations inside a causal domain to expand into macroscopic ripples in the spacetime metric. These, in turn, can evolve into large-scale structures in the universe. After its generation from quantum fluctuations, a ripple in the metric spends a long period outside the causal domain where its evolution is characterized by a conserved amplitude, a fact closely related to the large-scale Friedmann-like evolution of the perturbed Friedmann universe. We show that, under the assumption of linear processes, the generation and evolution of large-scale structures can be described quite simply.     

No cryptoferromagnetic state due to fluctuations

It is shown that the cryptoferromagnetic state which arises in a mean-fieldtreatment of magnetic superconductors cannot form due to strong fluctuations. The Bragg-like neutron reflection is due these fluctuations rather than a helical texture.     

Quantum conformal fluctuations revisited

The conformal quantization method of Narlikar and Padmanabhan is reformulated with a view to take into account theexact propagator and to provide explicitnumerical estimates of various predictions for dust cosmologies. It is found that in spite of the divergence of quantum fluctuations at the big-bang epoch it is possible to construct wave packets which remain sharp fromt=10^–70s, say, up to the present epoch provided the present state is finely tuned to the classical one. Also, if the transition probability from the Minkowski to the FRW metric is calculated using Gaussian wave functions (with zero mean) then thet ^2/3 models withk = 0, ± 1 cannot be distinguished, i.e., a fine tuning to the flat (k=0) model does not seem to result if the conformal factor depends on time only.     

Biased surface fluctuations due to current stress

Direct correlation between temporal structural fluctuations and electron wind force is demonstrated, for the first time, by STM imaging and analysis of atomically resolved motion on a thin film surface under large applied current (10(5) A/cm2). The magnitude of the momentum transfer between current carriers and the geometrically constrained atoms in the fluctuating structure is at least 5x to 15x (+/-1sigma range) larger than for freely diffusing adatoms. Corresponding changes in surface resistivity will contribute significant fluctuation signature to nanoscale electronic properties.     

Quantum fluctuations in nonlinear optics

We consider nonlinear optical devices which are excited by coherent light from a laser and in which several other waves are generated by nonlinear processes. We develop a general theory to treat the noise properties of the generated waves. In particular a probability distribution function is found which in principle can be measured experimentally. This formulation allows a thermodynamic study of phase transitions.     

Quantum fluctuations near the classical space-time singularity

The method of path integration is used to study the effects of quantum fluctuations in the space-time geometry near the classical singularity of general relativity. It is shown that in certain special cases explicit Feynman propagators can be constructed which enable us to evaluate these fluctuationsquantitatively. The cases discussed are (i) the gravitational collapse of a uniform dust ball, (ii) the Friedmann cosmologies, (iii) the axisymmetric Bianchi type I cosmological model, and (iv) the general anisotropic Bianchi type I cosmological model. In all cases discussed here the quantum uncertainty grows to infinity as the classical space-time singularity is approached. In this wider regime of quantum gravitation nonsingular solutions can occur with finite probabilities.     

Quantum fluctuations in the new inflationary universe

The evolution of quantum fluctuations of a scalar field in de Sitter space is analyzed in the context of the new inflationary scenario. The duration of the inflationary phase is estimated and the problem of density perturbations resulting from quantum fluctuations of the Higgs field is discussed.     

Quantum electrodynamic fluctuations of the macroscopic Josephson phase

We study the equilibrium dynamics of the relative phase in a superconducting Josephson link taking into account the quantum fluctuations of the electromagnetic vacuum. The photons act as a superohmic heat bath on the relative Cooper pair number and thus, indirectly, on the macroscopic phase difference φ. This leads to an enhancement of the mean square 〈φ²〉 that adds to the spread due to the Coulomb interaction carried by the longitudinal electromagnetic field. We also include the interaction with the electronic degrees of freedom due to quasiparticle tunneling, which couple to the phase and only indirectly to the particle number. The simultaneous inclusion of both the radiation field fluctuations and quasiparticle tunneling leads to a novel type of particle-bath Hamiltonian in which the quantum particle couples through its position and momentum to two independent bosonic heat baths. We study the interplay between the two mechanisms in the present context and find interference contributions to the quantum fluctuations of the phase. We explore the observability of the QED effects discussed here.     

Sound amplitude fluctuations due to a temperature microstructure

The experiments reported in this paper were carried out in a water tank in which a random medium was generated by convective mixing from an array of heaters. An approximate thermodynamic model of the medium was derived. Temperature measurements were made which showed that the temperature microstructure created in this way could be considered as a passive additive of turbulence. Furthermore, it was possible to characterize the random refractive index in terms of a spectral distribution by using an adapted version of a spectrum proposed by Medwin for the upper ocean. By using the adapted Medwin model and the single-scatter theoretical results of Tatarski, theoretical estimates were obtained of the fluctuations of an acoustic signal propagating in this particular medium. Experiments were carried out to measure acoustic signal amplitude fluctuations at frequencies of 9 MHz and 1 MHz. The empirical results were in agreement with the theoretical estimates. Measurements are also reported for the spatial correlation functions of the acoustic signal amplitude fluctuations. The results are discussed in the light of currently available theoretical results.     

Luminescence of CdS crystals due to near-surface potential fluctuations

The nature of the wide emission band observed in the low-temperature (T ≤ 60 K) near-edge photoluminescence spectra of CdS crystals placed in water and irradiated by a He-Cd laser has been investigated. The spectral and temporal characteristics of the band and its dependence on temperature, excitation intensity, and transverse electric field are considered. The relationship between the band and the radiative recombination of carriers localized on near-surface potential fluctuations, which are due to the defect-forming character of the joint effect of laser irradiation and water on near-surface layer of semiconductor is grounded.     

Quantum fluctuations in atomistic semiconductor devices

Atomistic Green function simulations of model 25 nm×25 nm Si MOSFETs predict strong fluctuation effects derived from mode fluctuations in the quantum transport through the inhomogeneous 2DEG channel caused by the spatial distribution of non-self-averaged discrete dopants.