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)     

Quantum vacuum fluctuations

The existence of irreducible field fluctuations in vacuum is an important prediction of quantum theory. These fluctuations have many observable consequences, like the Casimir effect, which is now measured with good accuracy and agreement with theory, provided that the latter accounts for differences between real experiments and the ideal situation considered by Casimir. But the vacuum energy density calculated by adding field mode energies is much larger than the density observed around us through gravitational phenomena. This ‘vacuum catastrophe’ is one of the unsolved problems at the interface between quantum theory on one hand, inertial and gravitational phenomena on the other hand. It is however possible to put properly formulated questions in the vicinity of this paradox. These questions are directly connected to observable effects bearing upon the principle of relativity of motion in quantum vacuum.     

Quantum fluctuations of the antiferro-antiferromagnetic double-layer

This paper stuides the magnetization and quantum fluctuations of an antiferro-antiferromagnetic (AF-AF) double-layer at zero temperature. It is found that the exchanges and anisotropy constants affect the quantum fluctuations of spins. If the anisotropy exists, there will be no acoustic energy branch in the system. The anisotropy constant, antiferromagnetic intralayer and interlayer coupling have important roles in a balance of the quantum competition.     

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.     

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.     

Quantum fluctuations and inertia

One of the three key assumptions involved in the stochastic formulation of quantum mechanics, the inverse proportionality of the quantum diffusion constant to the inertial mass, is shown to be amenable to experimental test. By relaxing this assumption, a non-linear generalization of the Schrödinger equation is found. The present experimental uncertainty in the measurement of the Lamb shift is then used to bound the deviation from the aforementioned inverse proportionality to be less than 4 × 10⁻¹³.     

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 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 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 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.