Electron spin relaxation in semiconductors and semiconductor structures

We suggest an approach to the problem of free electron spin evolution in a semiconductor with arbitrary anisotropy or quantum structure in a magnetic field. The developed approach utilizes quantum kinetic equations for average spin components. These equations represent the relaxation in terms of correlation functions for fluctuating effective fields responsible for spin relaxation. In a particular case when autocorrelation functions are dominant, the kinetic equations reduce to the Bloch equations. The developed formalism is applied to the problem of electron spin relaxation due to exchange scattering in a semimagnetic quantum well (QW) as well as to the spin relaxation in a QW due to Dyakonov-Perel mechanism.Comment: 9 pages, 1 postscript figur

On the macroscopic quantization in mesoscopic rings and single-electron devices

In this letter the phenomenon of macroscopic quantization is investigated using the particle on the ring interacting with the dissipative environment as an example. It is shown that the phenomenon of macroscopic quantization has the clear physical origin in that case. It follows from the angular momentum conservation combined with momentum quantization for bare particle on the ring . The existence an observable which can take only integer values in the zero temperature limit is rigorously proved. With the aid of the mapping between particle on the ring and Ambegaokar-Eckern-Schon model, which can be used to describe single-electron devices, it is demonstrated that this observable is analogous to the "effective charge" introduced by Burmistrov and Pruisken for the single-electron box problem. Different consequences of the revealed physics are discussed, as well as a generalization of the obtained results to the case of more complicated systems.Comment: 4.5 page

Persistent currents in nanorings and quantum decoherence by Coulomb interaction

Employing instanton technique we evaluate equilibrium persistent current (PC) produced by a quantum particle moving in a periodic potential on a ring and interacting with a dissipative environment formed by diffusive electron gas. The model allows for detailed non-perturbative analysis of interaction effects and -- depending on the system parameters -- yields a rich structure of different regimes. We demonstrate that at low temperatures PC is exponentially suppressed at sufficiently large ring perimeters $2\pi R> L_{\varphi}$ where the dephasing length $L_{\varphi}$ is set by interactions and does not depend on temperature. This behavior represents a clear example of quantum decoherence by electron-electron interactions at $T\to 0$.Comment: 13 pages, 4 figure

Persistent currents in quantum phase slip rings

We investigate the effect of interacting quantum phase slips on persistent current and its fluctuations in ultrathin superconducting nanowires and nanorings pierced by the external magnetic flux. We derive the effective action for these systems and map the original problem onto an effective sine-Gordon theory on torus. We evaluate both the flux dependent persistent current and the critical radius of the ring beyond which this current gets exponentially suppressed by quantum fluctuations. We also analyze fluctuations of persistent current caused by quantum phase slips. At low temperatures the supercurrent noise spectrum has the form of coherent peaks which can be tuned by the magnetic flux. Experimental observation of these peaks can directly demonstrate the existence of plasma modes in superconducting nanorings.Comment: 11 pages, 5 figure

Fluctuations of persistent current

We theoretically analyze equilibrium fluctuations of persistent current (PC) in nanorings. We demonstrate that these fluctuations persist down to zero temperature provided the current operator does not commute with the total Hamiltonian of the system. For a model of a quantum particle on a ring we explicitly evaluate PC noise power which has the form of sharp peaks at frequencies set by the corresponding interlevel distances. In rings with many conducting channels a much smoother and broader PC noise spectrum is expected. A specific feature of PC noise is that its spectrum can be tuned by an external magnetic flux indicating the presence of quantum coherence in the system.Comment: 12 pages, 3 figur

Bunch-by-bunch detection of coherent transverse modes from digitized single-bpm signals in the Tevatron

A system was developed for bunch-by-bunch detection of transverse proton and antiproton coherent oscillations based on the signal from a single beam-position monitor (BPM) located in a region of the ring with large amplitude functions. The signal is digitized over a large number of turns and Fourier-analyzed offline with a dedicated algorithm. To enhance the signal, the beam is excited with band-limited noise for about one second, and this was shown not to significantly affect the circulating beams even at high luminosity. The system is used to measure betatron tunes of individual bunches and to study beam- beam effects. In particular, it is one of the main diagnostic tools in an ongoing study of nonlinear beam-beam compensation studies with Gaussian electron lenses. We present the design and operation of this tool, together with results obtained with proton and antiproton bunches.Comment: 4 pp. 14th Beam Instrumentation Workshop (BIW10) 2-6 May 2010: Santa Fe, New Mexic

Quantum Phase Slip Noise

Quantum phase slips (QPS) generate voltage fluctuations in superconducting nanowires. Employing Keldysh technique and making use of the phase-charge duality arguments we develop a theory of QPS-induced voltage noise in such nanowires. We demonstrate that quantum tunneling of the magnetic flux quanta across the wire yields quantum shot noise which obeys Poisson statistics and is characterized by a power law dependence of its spectrum $S_\Omega$ on the external bias. In long wires $S_\Omega$ decreases with increasing frequency $\Omega$ and vanishes beyond a threshold value of $\Omega$ at $T \to 0$. Quantum coherent nature of QPS noise yields non-monotonous dependence of $S_\Omega$ on $T$ at small $\Omega$.Comment: 4.5 pages + 2-page supplemental material, 3 figure

Shot noise in ultrathin superconducting wires

Quantum phase slips (QPS) may produce non-equilibrium voltage fluctuations in current-biased superconducting nanowires. Making use of the Keldysh technique and employing the phase-charge duality arguments we investigate such fluctuations within the four-point measurement scheme and demonstrate that shot noise of the voltage detected in such nanowires may essentially depend on the particular measurement setup. In long wires the shot noise power decreases with increasing frequency $\Omega$ and vanishes beyond a threshold value of $\Omega$ at $T \to 0$Comment: 5 pages, 2 figure

Anisotropy of heavy hole spin splitting and interference effects of optical polarization in semiconductor quantum wells subjected to an in-plane magnetic field

Strong effects of optical polarization anisotropy observed previously in the quantum wells subjected to the in-plane magnetic field arrive at complete description within microscopic approach. Theory we develop involves two sources of optical polarization. First source is due to correlations between electron and heavy hole (HH) phases of $\psi$-functions arising due to electron Zeeman spin splitting and joint manifestation of low-symmetry and Zeeman interactions of HH in an in-plane magnetic field. In this case, four possible phase-controlled electron-HH transitions constitute the polarization effect, which can reach its maximal amount ($\pm$1) at low temperatures when only one transition survives. Other polarization source stems from the admixture of excited light-holes (LH) states to HH by low-symmetry interactions. The contribution of this mechanism to total polarization is relatively small but it can be independent of temperature and magnetic field. Analysis of different mechanisms of HH splitting exhibits their strong polarization anisotropy. Joint action of these mechanisms can result in new peculiarities, which should be taken into account for explanation of different experimental situations.Comment: 8 pages, 5 postscript figure

Mechanisms of Carrier-Induced Ferromagnetism in Diluted Magnetic Semiconductors

Two different approaches to the problem of carrier-induced ferromagnetism in the system of the disordered magnetic ions, one bases on self-consistent procedure for the exchange mean fields, other one bases on the RKKY interaction, used in present literature as the alternative approximations is analyzed. Our calculations in the framework of exactly solvable model show that two different contributions to the magnetic characteristics of the system represent these approaches. One stems from the diagonal part of carrier-ion exchange interaction that corresponds to mean field approximation. Other one stems from the off-diagonal part that describes the interaction between ion spins via free carriers. These two contributions can be responsible for the different magnetic properties, so aforementioned approaches are complementary, not alternative. A general approach is proposed and compared with different approximations to the problem under consideration.Comment: 5 pages, RevTe