125 research outputs found
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
Electron transport in waveguides with spatially modulated strengths of the Rashba and Dresselhaus terms of the spin-orbit interaction
We study electron transport through waveguides (WGs) in which the strengths
of the Rashba () and Dresselhaus () terms of the spin-orbit
interaction (SOI) vary in space. Subband mixing, due to lateral confinement, is
taken into account only between the two first subbands. For sufficiently narrow
WGs the transmission exhibits a square-like shape as a function of
or . Particular attention is paid to the case of equal SOI strengths,
, for which spin-flip processes are expected to decrease. The
transmission exhibits resonances as a function of the length of a SOI-free
region separating two regions with SOI present, that are most pronounced for
. The sign of strongly affects the spin-up and spin-down
transmissions. The results show that the main effect of subband mixing is to
shift the transmission resonances and to decrease the transmission from one
spin state to another. The effect of possible band offsets between regions that
have different SOI strengths and effective masses is also discussed
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
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 {( term) and the other due to to the
quantum-well potential that confines electrons in the 2DEG (conventional Rashba
SO interaction or 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
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 , 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
Preparation of decoherence-free, subradiant states in a cavity
The cause of decoherence in a quantum system can be traced back to the
interaction with the environment. As it has been pointed out first by Dicke, in
a system of N two-level atoms where each of the atoms is individually dipole
coupled to the environment, there are collective, subradiant states, that have
no dipole coupling to photon modes, and therefore they are expected to decay
slower. This property also implies that these type of states, which form an N-1
dimensional subspace of the atomic subsytem, also decohere slower. We propose a
scheme which will create such states. First the two-level atoms are placed in a
strongly detuned cavity and one of the atoms, called the control atom is
excited. The time evolution of the coupled atom-cavity system leads to an
appropriately entangled state of the atoms. By applying subsequent laser pulses
at a well defined time instant, it is possible to drive the atomic state into
the subradiant, i. e., decoherence free subspace. Up to a certain average
number of the photons, the result is independent of the state of the cavity.
The analysis of the conditions shows that this scheme is feasible with present
day techniques achieved in atom cavity interaction experiments.Comment: 5 page
Networks of quantum nanorings: programmable spintronic devices
An array of quantum rings with local (ring by ring) modulation of the spin
orbit interaction (SOI) can lead to novel effects in spin state transformation
of electrons. It is shown that already small (3x3, 5x5) networks are remarkably
versatile from this point of view: Working in a given network geometry, the
input current can be directed to any of the output ports, simply by changing
the SOI strengths by external gate voltages. Additionally, the same network
with different SOI strengths can be completely analogous to the Stern-Gerlach
device, exhibiting spatial-spin entanglement.Comment: 4 pages, 3 figures, accepted by Nano Letter
Stability of stationary solutions of the Schrodinger-Langevin equation
The stability properties of a class of dissipative quantum mechanical systems
are investigated. The nonlinear stability and asymptotic stability of
stationary states (with zero and nonzero dissipation respectively) is
investigated by Liapunov's direct method. The results are demonstrated by
numerical calculations on the example of the damped harmonic oscillator.Comment: revised, 12 pages, 7 figure
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