355 research outputs found
Nuclear spin warm-up in bulk n-GaAs
We show that the spin-lattice relaxation in n-type insulating GaAs is
dramatically accelerated at low magnetic fields. The origin of this effect,
that cannot be explained in terms of well-known diffusion-limited hyperfine
relaxation, is found in the quadrupole relaxation, induced by fluctuating donor
charges. Therefore, quadrupole relaxation, that governs low field nuclear spin
relaxation in semiconductor quantum dots, but was so far supposed to be
harmless to bulk nuclei spins in the absence of optical pumping can be studied
and harnessed in much simpler model environment of n-GaAs bulk crystal.Comment: 5 pages, 4 figure
Ultimate photo-induced Kerr rotation achieved in semiconductor microcavities
Photoinduced Kerr rotation by more than radians is demonstrated in
planar quantum well microcavity in the strong coupling regime. This result is
close to the predicted theoretical maximum of . It is achieved by
engineering microcavity parameters such that the optical impedance matching
condition is reached at the smallest negative detuning between exciton
resonance and the cavity mode. This ensures the optimum combination of the
exciton induced optical non-linearity and the enhancement of the Kerr angle by
the cavity. Comprehensive analysis of the polarization state of the light in
this regime shows that both renormalization of the exciton energy and the
saturation of the excitonic resonance contribute to the observed optical
nonlinearities.Comment: Shortened version prepared to submit in Phys. Rev. Letter
Ballistic spin transport in exciton gases
Traditional spintronics relies on spin transport by charge carriers, such as
electrons in semiconductor crystals. This brings several complications: the
Pauli principle prevents the carriers from moving with the same speed; Coulomb
repulsion leads to rapid dephasing of electron flows. Spin-optronics is a
valuable alternative to traditional spintronics. In spin-optronic devices the
spin currents are carried by electrically neutral bosonic quasi-particles:
excitons or exciton-polaritons. They can form highly coherent quantum liquids
and carry spins over macroscopic distances. The price to pay is a finite
life-time of the bosonic spin carriers. We present the theory of exciton
ballistic spin transport which may be applied to a range of systems where
bosonic spin transport has been reported, in particular, to indirect excitons
in coupled GaAs/AlGaAs quantum wells. We describe the effect of spin-orbit
interaction of electrons and holes on the exciton spin, account for the Zeeman
effect induced by external magnetic fields, long range and short range exchange
splittings of the exciton resonances. We also consider exciton transport in the
non-linear regime and discuss the definitions of exciton spin current,
polarization current and spin conductivity.Comment: 16 pages, 10 figures to be published in Phys. Rev.
Nuclear spin-lattice relaxation in p-type GaAs
Spin-lattice relaxation of the nuclear spin system in p-type GaAs is studied
using a three-stage experimental protocol including optical pumping and
measuring the difference of the nuclear spin polarization before and after a
dark interval of variable length. This method allows us to measure the
spin-lattice relaxation time of optically pumped nuclei "in the dark",
that is, in the absence of illumination. The measured values fall into
the sub-second time range, being three orders of magnitude shorter than in
earlier studied n-type GaAs. The drastic difference is further emphasized by
magnetic-field and temperature dependences of in p-GaAs, showing no
similarity to those in n-GaAs. This unexpected behavior is explained within a
developed theoretical model involving quadrupole relaxation of nuclear spins,
which is induced by electric fields within closely spaced donor-acceptor pairs.Comment: 9 pages, 8 figure
Reactive Rayleigh-Taylor Turbulence
The Rayleigh-Taylor (RT) instability develops and leads to turbulence when a
heavy fluid falls under the action of gravity through a light one. We consider
this phenomenon accompanied by a reactive transformation between the fluids,
and study with Direct Numerical Simulations (DNS) how the reaction (flame)
affects the turbulent mixing in the Boussinesq approximation. We discuss "slow"
reactions where the characteristic reaction time exceeds the temporal scale of
the RT instability. In the early turbulent stage, effects of the flame are
distributed over a maturing mixing zone, whose development is weakly influenced
by the reaction. At later times, the fully mixed zone transforms into a
conglomerate of pure-fluid patches of sizes proportional to the mixing zone
width. In this "stirred flame'' regime, temperature fluctuations are consumed
by reactions in the regions separating the pure-fluid patches. This DNS-based
qualitative description is followed by a phenomenology suggesting that thin
turbulent flame is of a single-fractal character, and thus distribution of the
temperature field is strongly intermittent.Comment: 14 pages, 4 figure
Simultaneous measurements of nuclear spin heat capacity, temperature and relaxation in GaAs microstructures
Heat capacity of the nuclear spin system (NSS) in GaAs-based microstructures
has been shown to be much greater than expected from dipolar coupling between
nuclei, thus limiting the efficiency of NSS cooling by adiabatic
demagnetization. It was suggested that quadrupole interaction induced by some
small residual strain could provide this additional reservoir for the heat
storage. We check and validate this hypothesis by combining nuclear spin
relaxation measurements with adiabatic remagnetization and nuclear magnetic
resonance experiments, using electron spin noise spectroscopy as a unique tool
for detection of nuclear magnetization. Our results confirm and quantify the
role of the quadrupole splitting in the heat storage within NSS and provide
additional insight into fundamental, but still actively debated relation
between a mechanical strain and the resulting electric field gradients in GaAs.Comment: 11 pages, 4 figures, 1 tabl
Synthesis and magnetic properties of BiFe1-xCoxO3 (x = 0-0.07)
This research was carried out under the financial support of the Russian Foundation for Basic Research №17-08-00893
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