Quantum rings with time dependent spin-orbit coupling: Rabi oscillations, spintronic Schrodinger-cat states, and conductance properties

The strength of the (Rashba-type) spin-orbit coupling in mesoscopic semiconductor rings can be tuned with external gate voltages. Here we consider the case of a periodically changing spin-orbit interaction strength as induced by sinusoidal voltages. In a closed one dimensional quantum ring with weak spin-orbit coupling, Rabi oscillations are shown to appear. We find that the time evolution of initially localized wave packets exhibits a series of collapse and revival phenomena. Partial revivals -- that are typical in nonlinear systems -- are shown to correspond to superpositions of states localized at different spatial positions along the ring. These "spintronic Schrodinger-cat sates" appear periodically, and similarly to their counterparts in other physical systems, they are found to be sensitive to environment induced disturbances. The time dependent spin transport problem, when leads are attached to the ring, is also solved. We show that the "sideband currents" induced by the oscillating spin-orbit interaction strength can become the dominant output channel, even in the presence of moderate thermal fluctuations and random scattering events.Comment: 11 pages, 9 figures, submitted to PR

Mesoscopic superposition and sub-Planck-scale structure in molecular wave packets

We demonstrate the possibility of realizing sub-Planck-scale structures in the mesoscopic superposition of molecular wave packets involving vibrational levels. The time evolution of the wave packet, taken here as the SU(2) coherent state of the Morse potential describing hydrogen iodide molecules, produces macroscopicquantum- superposition-like states, responsible for the above phenomenon. We investigate the phase-space dynamics of the coherent state through the Wigner function approach and identify the interference phenomena behind the sub-Planck-scale structures. The optimal parameter ranges are specified for observing these features.Comment: 4 pages, 3 figure

Decoherence of molecular wave packets in an anharmonic potential

The time evolution of anharmonic molecular wave packets is investigated under the influence of the environment consisting of harmonic oscillators. These oscillators represent photon or phonon modes and assumed to be in thermal equilibrium. Our model explicitly incorporates the fact that in the case of a nonequidistant spectrum the rates of the environment induced transitions are different for each transition. The nonunitary time evolution is visualized by the aid of the Wigner function related to the vibrational state of the molecule. The time scale of decoherence is much shorter than that of dissipation, and gives rise to states which are mixtures of localized states along the phase space orbit of the corresponding classical particle. This behavior is to a large extent independent of the coupling strength, the temperature of the environment and also of the initial state.Comment: 7 pages, 4 figure

Quantum rings as electron spin beam splitters

Quantum interference and spin-orbit interaction in a one-dimensional mesoscopic semiconductor ring with one input and two output leads can act as a spin beam splitter. Different polarization can be achieved in the two output channels from an originally totally unpolarized incoming spin state, very much like in a Stern-Gerlach apparatus. We determine the relevant parameters such that the device has unit efficiency.Comment: 4 pages, 3 figures; minor change

Filtering of spin currents based on ballistic ring

Quantum interference effects in rings provide suitable means for controlling spin at mesoscopic scales. Here we apply such a control mechanism to the spin-dependent transport in a ballistic quasi one dimensional ring patterned in two dimensional electron gases (2DEGs). The study is essentially based on the {\it natural} spin-orbit (SO) interactions, one arising from the laterally confining electric field {($\beta$ term) and the other due to to the quantum-well potential that confines electrons in the 2DEG (conventional Rashba SO interaction or $\alpha$ term).} We focus on single-channel transport and solve analytically the spin polarization of the current. As an important consequence of the presence of spin splitting, we find the occurrence of spin dependent current oscillations. We analyze %the effects of disorder by discussing the transport in the presence of one non-magnetic obstacle in the ring. We demonstrate that a spin polarized current can be induced when an unpolarized charge current is injected in the ring, by focusing on the central role that the presence of the obstacle plays.Comment: 9 pages, 7 figures, PACS numbers: 72.25.-b, 72.20.My, 73.50.Jt, accepted for publication in J. Phys. - Cond. Ma

Spintronic single qubit gate based on a quantum ring with spin-orbit interaction

In a quantum ring connected with two external leads the spin properties of an incoming electron are modified by the spin-orbit interaction resulting in a transformation of the qubit state carried by the spin. The ring acts as a one qubit spintronic quantum gate whose properties can be varied by tuning the Rashba parameter of the spin-orbit interaction, by changing the relative position of the junctions, as well as by the size of the ring. We show that a large class of unitary transformations can be attained with already one ring -- or a few rings in series -- including the important cases of the Z, X, and Hadamard gates. By choosing appropriate parameters the spin transformations can be made unitary, which corresponds to lossless gates.Comment: 4 pages, 4 figure

Time evolution in the Morse potential using supersymmetry: dissociation of the NO molecule

We present an algebraic method for treating molecular vibrations in the Morse potential perturbed by an external laser field. By the help of a complete and normalizable basis we transform the Schr\"{o}dinger equation into a system of coupled ordinary differential equations. We apply our method to calculate the dissociation probability of the NO molecule excited by chirped laser pulses. The dependence of the molecular dipole-moment on the interatomic separation is determined by a quantum-chemical method, and the corresponding transition dipole moments are given by approximate analytic expressions. These turn out to be very small between neighboring stationary states around the vibrational quantum number $m=42$, therefore we propose to use additional pulses in order to skip this trapping state, and to obtain a reasonable dissociation probability.Comment: 4 pages, 3 figure

Microwave emission from a crystal of molecular magnets -- The role of a resonant cavity

We discuss the effects caused by a resonant cavity around a sample of a magnetic molecular crystal (such as Mn${}_{12}$-Ac), when a time dependent external magnetic field is applied parallel to the easy axis of the crystal. We show that the back action of the cavity field on the sample significantly increases the possibility of microwave emission. This radiation process can be supperradiance or a maser-like effect, depending on the strength of the dephasing. Our model provides further insight to the theoretical understanding of the bursts of electromagnetic radiation observed in recent experiments accompanying the resonant quantum tunneling of magnetization. The experimental findings up to now can all be explained as being a maser effect rather than superradiance. The results of our theory scale similarly to the experimental findings, i.e., with increasing sweep rate of the external magnetic field, the emission peaks are shifted towards higher field values.Comment: 12 pages, 6 figures. To appear in Phys. Rev.

Re-Introduction of Ancient Wheat Cultivars into Organic Agriculture — Emmer and Einkorn Cultivation Experiences under Marginal Conditions

Modern agriculture depends on the production of very few crop species, which provide lower nutritive value for consumers. The present work summarizes the results of a three-year experiment on hulled wheat varieties as potential candidates for food system diversification. The organic field cultivation tests with 10 emmer and five einkorn landraces and varieties were conducted on ~10m2 plots on sandy soil, and from 2017, under on-farm conditions in eastern Hungary. Most accessions adapted well to the marginal conditions, with some landraces even yielding higher than registered varieties—over 3 t per ha on average over three years. Compared to emmer, einkorn had higher maximum grain yields, but its yield performance varied more than that of emmer. Grain protein and the total phenolic content were high in both species. Compared to emmer, einkorn seeds exhibited a 3.8 times higher content of bound flavonoids and had 3.4 times higher antioxidant activity. Four einkorn accessions were resistant to leaf spot, as well as yellow and leaf rusts. Fusarium infected both species similarly. Our findings indicate that not only registered varieties of ancient wheat species but also their landraces can provide sustainable alternatives both for organic farmers and also for the diversification of agriculture