We study the effect of weak disorder on tunneling conductance of a single-channel quantum ring threaded by magnetic flux. We assume that temperature is higher than the level spacing in the ring and smaller than the Fermi energy. In the absence of disorder, the conductance shows sharp dips (antiresonances) as a function of magnetic flux. We discuss different types of disorder and find that the short-range disorder broadens antiresonances, while the long-range one leads to arising of additional resonant dips. We demonstrate that the resonant dips have essentially non-Lorentzian shape. The results are generalized to account for the spin-orbit interaction which leads to splitting of the disorder-broadened resonant dips, and consequently to coexisting of two types of oscillations (both having the form of sharp dips): Aharonov-Bohm oscillations with magnetic flux and AharonovCasher oscillations with the strength of the spin-orbit coupling. We also discuss the effect of the Zeeman coupling.

We study theoretically the combined effect of the spin-orbit and Zeeman interactions on the tunneling electron transport through a single-channel quantum ring threaded by magnetic flux. We focus on the high temperature case (temperature is much higher than the level spacing in the ring) and demonstrate that spin-interference effects are not suppressed by thermal averaging. In the absence of the Zeeman coupling the high-temperature tunneling conductance of the ring exhibits two types of oscillations: Aharonov-Bohm oscillations with magnetic flux and AharonovCasher oscillations with the strength of the spin-orbit interaction. For weak tunneling coupling both oscillations have the form of sharp periodic antiresonances. In the vicinity of the antiresonances the tunneling electrons acquire spin polarization, so that the ring serves as a spin polarizer. We also demonstrate that the Zeeman coupling leads to appearance of two additional peaks both in the tunneling conductance and in the spin polarization.

We overview transport properties of an Aharonov-Bohm interferometer made of a single channel quantum ring. Remarkably, in this setup, essentially quantum effects survive thermal averaging: the high temperature tunneling conductance G of a ring shows sharp dips (antiresonances) as a function of magnetic flux. We dis cuss effects of the electron-electron interaction, disorder, and spin-orbit coupling on the Aharonov-Bohm transport through the ring. The interaction splits the dip into series of dips broadened by dephasing. The physics behind this behavior is the persistent current blockade: the current through the ring is blocked by the circular current inside the ring. Dephasing is then dominated by tunneling induced fluctuations of the circu lar current. The short range disorder broadens antiresonances, while the long range one induces additional dips. In the presence of a spin-orbit coupling, G exhibits two types of sharp antiresonances: Aharonov-Bohm and Aharonov-Casher ones. In the vicinity of the antiresonances, the tunneling electrons acquire spin polar ization, so that the ring serves as a spin polarizer. Fig. 1. Single channel quantum ring and its energy levels.

We study collective spin excitations in two-dimensional diluted magnetic semiconductors, placed into external magnetic field. Two coupled modes of the spin waves (the electron and ion modes) are found to exist in the system along with a number of the ion spin excitations decoupled from the electron system. We calculate analytically the spectrum of the waves taking into account the exchange interaction of itinerant electrons both with each other and with electrons localized on the magnetic ions. The interplay of these interactions leads to a number of intriguing phenomena including tunable anticrossing of the modes and a field-induced change in a sign of the group velocity of the ion mode.PACS numbers: 75.30. Ds, 76.50.+g Diluted magnetic semiconductors (DMS) have recently been the subject of great interest [1,2] due to their potential in combining magnetic and semiconductor properties in a single material. The DMS are formed by replacing of cations in ordinary semiconductors with magnetic ions, typically Mn ions. Strong exchange interaction between the itinerant electrons and the electrons localized on d-shells of the magnetic ions leads to a number of remarkable features of the DMS. In particular, it results in the effective indirect interaction between the ion spins thus promising for creating room-temperature ferromagnetic systems that may offer advantages of semiconductors. It also dramatically enhances the effective coupling of the itinerant electrons with the external magnetic field. In contrast to conventional GaAs/GaAlAs systems, where small values of g−factor prevents manipulation of the spin degree of freedom, the giant electron Zeeman splitting arising in the DMS as a manifestation of the exchange interaction can be on the order of the Fermi energy [3,4], offering a wide range of spintronics applications.Here we discuss spin excitations in the twodimensional DMS. Our studies are motivated by recent experiments [5][6][7] and a theoretical discussion [7][8][9] focused on the spin dynamics in diluted magnetic Cd 1−x Mn x Te quantum wells placed into the magnetic field [10]. In Ref. 5, the spectrum of the spin waves, ω(k), was measured. Only one excitation mode was observed. It was demonstrated that the excitations exist in a finite range of wavelengths, k < k m , and their group velocity is negative: dω(k)/dk < 0. The experimental data were interpreted [5,8] in terms of conventional spin waves in the Fermi liquids [11], while k m was attributed to the edge of the Stoner continuum of the single-particle spin excitations. Such interpretation implies that the only effect of the magnetic ions on the electron spin waves is the strong renormalization of the electron Zeeman splitting. However, more recent experimental observations [6,7] supported by theoretical studies [7,9] appear to be in disagreement with this conclusion. Indeed, in Refs. [6,7] two modes of the collective homogeneous (k = 0) spin excitations were observed in Cd 1−x Mn x Te wells. The modes were identified [6,7,9] as the spin excitations o...

We study electron spin dynamics in diluted magnetic quantum wells. The electrons are coupled by exchange interaction with randomly distributed magnetic ions polarized by magnetic field. This coupling leads to both spin relaxation and spin decoherence. We demonstrate that even very small spatial fluctuations of quantum well width dramatically increase rate of decoherence. Depending on the strength of exchange interaction and the amplitude of the fluctuations the decoherence can be homogeneous or inhomogeneous. In the homogeneous regime, the transverse (with respect to magnetic field) component of the electron spin decays on the short time scale exponentialy, while the long-time spin dynamics is non-exponential demonstrating long-lived power law tail. In the inhomogeneous case, the transverse spin component decays exponentially with the exponent quadratic in time