238 research outputs found
Spin and Spin-Wave Dynamics in Josephson Junctions
We extend the Keldysh formulation to quantum spin systems and derive exact
equations of motion. This allows us to explore the dynamics of single spins and
of ferromagnets when these are inserted between superconducting leads. Several
new effects are reported. Chief amongst these are nutations of single S=1/2
spins in Josephson junctions. These nutations are triggered by the
superconducting pairing correlations in the leads. Similarly, we find that on
rather universal grounds, magnets display unconventional spin wave dynamics
when placed in Josephson junctions. These lead to modifications in the
tunneling current.Comment: (14 pages, 5 figures
Photoluminescence of the incompressible Laughlin liquid: Excitons, charged excitons, and fractionally charged excitons
The photoluminescence (PL) of a two-dimensional electron gas (2DEG) in a high
magnetic field is studied as a function of the filling factor and the
separation d between the electron layer and the valence hole. Depending on the
magnitude of d relative to the magnetic length lambda, two distinct regimes in
the response of the 2DEG to the valence hole occur, with different elementary
emission processes contributing to the PL spectrum. At d<lambda ("strong
coupling" regime), the hole binds one or two electrons to form an exciton (X)
or one of three possible charged exciton (X-) states, a spin-singlet or one of
two spin-triplets. At d>lambda ("weak coupling" regime), the hole decouples or
binds one or two Laughlin quasielectrons to form fractionally charged excitons
(FCX's). The binding energies as well as the emission energies and intensities
of all X- and FCX states are calculated.Comment: 9 pages, 6 figures, sumbitted to physica status solidi (b
Josephson Current in the Presence of a Precessing Spin
The Josephson current in the presence of a precessing spin between various
types of superconductors is studied. It is shown that the Josephson current
flowing between two spin-singlet pairing superconductors is not modulated by
the precession of the spin. When both superconductors have equal-spin-triplet
pairing state, the flowing Josephson current is modulated with twice of the
Larmor frequency by the precessing spin. It was also found that up to the
second tunneling matrix elements, no Josephson current can occur with only a
direct exchange interaction between the localized spin and the conduction
electrons, if the two superconductors have different spin-parity pairing
states.Comment: 5 pages, 1 figur
Exchange Based Noise Spectroscopy of a Single Precessing Spin with STM
ESR-STM is an emerging technique which is capable of detecting the precession
of a single spin. We discuss the mechanism of ESR-STM based on a direct
exchange coupling between the tunneling electrons and the local precessing spin
S. We claim that since the number of tunneling electrons in a single precessing
period is small (~20) one may expect a net temporary polarization within this
period that will couple via exchange interaction to the localized spin. This
coupling will randomly modulate the tunneling barrier and create a dispersion
in the tunneling current which is a product of a Larmor frequency component due
to the precession of the single spin and the dispersion of the spin of the
tunneling electrons. This noise component is spread over the whole frequency
range for random white noise spin polarization of electrons. In opposite case
the power spectrum of the spins of the tunneling electrons has a peak at zero
frequency an elevated noise in the current at omega_L will appear. We discuss
the possible source of this spin polarization. We find that for relevant values
of parameters signal to noise ratio in the spectral characteristic is 2-4 and
is comparable to the reported signal to noise ratio. The magnitude of the
current fluctuation is a relatively weak increaing function of the DC current
and the magnetic field. The linewidth produced by the back action effect of
tunneling electrons on the precessing spin is also discussed.Comment: Added discussion on spin polarization of tunneling electrons and the
role of paramagnetic centers on the tip. To be published; 5 pages latex file,
1 ps figure fil
Solid-State Quantum Computer Based on Scanning Tunneling Microscopy
We propose a solid-state nuclear spin quantum computer based on application
of scanning tunneling microscopy (STM) and well-developed silicon technology.
It requires the measurement of tunneling current modulation caused by the
Larmor precession of a single electron spin.
Our envisioned STM quantum computer would operate at the high magnetic field
(T) and at low temperature K.Comment: 3pages RevTex including 2 figure
Spectrum of qubit oscillations from Bloch equations
We have developed a formalism suitable for calculation of the output spectrum
of a detector continuously measuring quantum coherent oscillations in a
solid-state qubit, starting from microscopic Bloch equations. The results
coincide with that obtained using Bayesian and master equation approaches. The
previous results are generalized to the cases of arbitrary detector response
and finite detector temperature.Comment: 8 page
Absorption spectrum of a weakly n-doped semiconductor quantum well
We calculate, as a function of temperature and conduction band electron
density, the optical absorption of a weakly n-doped, idealized semiconductor
quantum well. In particular, we focus on the absorption band due to the
formation of a charged exciton. We conceptualize the charged exciton as an
itinerant excitation intimately linked to the dynamical response of itinerant
conduction band electrons to the appearance of the photo-generated valence band
hole. Numerical results for the absorption in the vicinity of the exciton line
are presented and the spectral weights associated with, respectively, the
charged exciton band and the exciton line are analyzed in detail. We find, in
qualitative agreement with experimental data, that the spectral weight of the
charged exciton grows with increasing conduction band electron density and/or
decreasing temperature at the expense of the exciton.Comment: 5 pages, 4 figure
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