850 research outputs found
Optoelectronic control of spin dynamics at near-THz frequencies in magnetically doped quantum wells
We use time-resolved Kerr rotation to demonstrate the optical and electronic
tuning of both the electronic and local moment (Mn) spin dynamics in
electrically gated parabolic quantum wells derived from II-VI diluted magnetic
semiconductors. By changing either the electrical bias or the laser energy, the
electron spin precession frequency is varied from 0.1 to 0.8 THz at a magnetic
field of 3 T and at a temperature of 5 K. The corresponding range of the
electrically-tuned effective electron g-factor is an order of magnitude larger
compared with similar nonmagnetic III-V parabolic quantum wells. Additionally,
we demonstrate that such structures allow electrical modulation of local moment
dynamics in the solid state, which is manifested as changes in the amplitude
and lifetime of the Mn spin precession signal under electrical bias. The large
variation of electron and Mn-ion spin dynamics is explained by changes in
magnitude of the sp−d exchange overlap.Comment: 4 pages, 3 figure
Spin-Photon Dynamics of Quantum Dots in Two-mode Cavities
A quantum dot interacting with two resonant cavity modes is described by a
two-mode Jaynes-Cummings model. Depending on the quantum dot energy level
scheme, the interaction of a singly doped quantum dot with a cavity photon
generates entanglement of electron spin and cavity states or allows one to
implement a SWAP gate for spin and photon states. An undoped quantum dot in the
same structure generates pairs of polarization entangled photons from an
initial photon product state. For realistic cavity loss rates, the fidelity of
these operations is of order 80%.Comment: 6 pages, 4 figures; extended discussion of experimental
implementatio
Entanglement versus Correlations in Spin Systems
We consider pure quantum states of spins or qubits and study the
average entanglement that can be \emph{localized} between two separated spins
by performing local measurements on the other individual spins. We show that
all classical correlation functions provide lower bounds to this
\emph{localizable entanglement}, which follows from the observation that
classical correlations can always be increased by doing appropriate local
measurements on the other qubits. We analyze the localizable entanglement in
familiar spin systems and illustrate the results on the hand of the Ising spin
model, in which we observe characteristic features for a quantum phase
transition such as a diverging entanglement length.Comment: 4 page
Transport through two-level quantum dots weakly coupled to ferromagnetic leads
Spin-dependent transport through a two-level quantum dot in the sequential
tunneling regime is analyzed theoretically by means of a real-time diagrammatic
technique. It is shown that the current, tunnel magnetoresistance, and shot
noise (Fano factor) strongly depend on the transport regime, providing a
detailed information on the electronic structure of quantum dots and their
coupling to external leads. When the dot is asymmetrically coupled to the
leads, a negative differential conductance may occur in certain bias regions,
which is associated with a super-Poissonian shot noise. In the case of a
quantum dot coupled to one half-metallic and one nonmagnetic lead, one finds
characteristic Pauli spin blockade effects. Transport may be also suppressed
when the dot levels are coupled to the leads with different coupling strengths.
The influence of an external magnetic field on transport properties is also
discussed.Comment: 12 pages, 8 figure
Discrete Fourier Transform in Nanostructures using Scattering
In this paper we show that the discrete Fourier transform can be performed by
scattering a coherent particle or laser beam off a two-dimensional potential
that has the shape of rings or peaks. After encoding the initial vector into
the two-dimensional potential, the Fourier-transformed vector can be read out
by detectors surrounding the potential. The wavelength of the laser beam
determines the necessary accuracy of the 2D potential, which makes our method
very fault-tolerant.Comment: 6 pages, 5 EPS figures, REVTe
Graphene with Structure-Induced Spin-Orbit Coupling: Spin-Polarized States, Spin Zero Modes, and Quantum Hall Effect
Spin splitting of the energy spectrum of single-layer graphene on Au/Ni(111)
substrate has been recently reported. I show that eigenstates of spin-orbit
coupled graphene are polarized in-plane and perpendicular to electron momentum
; the magnitude of spin polarization vanishes when . In
a perpendicular magnetic field , is parallel to , and two
zero modes emerge in the Landau level spectrum. Singular -dependence of
their magnetization suggests existence of a novel magnetic instability. They
also manifest themselves in a new unconventional quantum Hall effect.Comment: 4 pages, 1 figur
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