751 research outputs found
Relativistic quantum model of confinement and the current quark masses
We consider a relativistic quantum model of confined massive spinning quarks
and antiquarks which describes leading Regge trajectories of mesons. The quarks
are described by the Dirac equations and the gluon contribution is approximated
by the Nambu-Goto straight-line string. The string tension and the current
quark masses are the main parameters of the model. Additional parameters are
phenomenological constants which approximate nonstring short-range
contributions. Comparison of the measured meson masses with the model
predictions allows one to determine the current quark masses (in MeV) to be
. The chiral
model[23] makes it possible to estimate from here the - and -quark masses
to be ~ Mev and Mev.Comment: 15 pages, LATEX, 2 tables. (submitted to Phys.Rev.D
Room temperature and low-field resonant enhancement of spin Seebeck effect in partially compensated magnets
Resonant enhancement of spin Seebeck effect (SSE) due to phonons was recently
discovered in Y3Fe5O12 (YIG). This effect is explained by hybridization between
the magnon and phonon dispersions. However, this effect was observed at low
temperatures and high magnetic fields, limiting the scope for applications.
Here we report observation of phonon-resonant enhancement of SSE at room
temperature and low magnetic field. We observed in Lu2BiFe4GaO12 and
enhancement 700 % greater than that in a YIG film and at very low magnetic
fields around 10-1 T, almost one order of magnitude lower than that of YIG. The
result can be explained by the change in the magnon dispersion induced by
magnetic compensation due to the presence of non-magnetic ion substitutions.
Our study provides a way to tune the magnon response in a crystal by chemical
doping with potential applications for spintronic devices.Comment: 17 pages, 4 figure
Quantum Dot as Spin Filter and Spin Memory
We consider a quantum dot in the Coulomb blockade regime weakly coupled to
current leads and show that in the presence of a magnetic field the dot acts as
an efficient spin-filter (at the single-spin level) which produces a
spin-polarized current. Conversely, if the leads are fully spin-polarized the
up or down state of the spin on the dot results in a large sequential or small
cotunneling current, and thus, together with ESR techniques, the setup can be
operated as a single-spin memory.Comment: 4 pages, 3 figures, REVTe
The principle of relative locality
We propose a deepening of the relativity principle according to which the
invariant arena for non-quantum physics is a phase space rather than spacetime.
Descriptions of particles propagating and interacting in spacetimes are
constructed by observers, but different observers, separated from each other by
translations, construct different spacetime projections from the invariant
phase space. Nonetheless, all observers agree that interactions are local in
the spacetime coordinates constructed by observers local to them.
This framework, in which absolute locality is replaced by relative locality,
results from deforming momentum space, just as the passage from absolute to
relative simultaneity results from deforming the linear addition of velocities.
Different aspects of momentum space geometry, such as its curvature, torsion
and non-metricity, are reflected in different kinds of deformations of the
energy-momentum conservation laws. These are in principle all measurable by
appropriate experiments. We also discuss a natural set of physical hypotheses
which singles out the cases of momentum space with a metric compatible
connection and constant curvature.Comment: 12 pages, 3 figures; in version 2 one reference added and some minor
modifications in sects. II and III mad
Drift-Diffusion Approach to Spin-Polarized Transport
We develop a drift-diffusion equation that describes electron spin
polarization density in two-dimensional electron systems. In our approach,
superpositions of spin-up and spin-down states are taken into account, what
distinguishes our model from the traditional two-component drift-diffusion
approximation. The Dresselhaus and Rashba spin-orbit coupling mechanisms are
incorporated into consideration, as well as an applied electric field. The
derived equation is applied to the modelling of relaxation of homogeneous spin
polarization. Our results are consistent with previous studies
Longitudinal spin transport in diluted magnetic semiconductor superlattices: the effect of the giant Zeeman splitting
Longitudinal spin transport in diluted magnetic semiconductor superlattices
is investigated theoretically. The longitudinal magnetoconductivity (MC) in
such systems exhibits an oscillating behavior as function of an external
magnetic field. In the weak magnetic field region the giant Zeeman splitting
plays a dominant role which leads to a large negative magnetoconductivity. In
the strong magnetic field region the MC exhibits deep dips with increasing
magnetic field. The oscillating behavior is attributed to the interplay between
the discrete Landau levels and the Fermi surface. The decrease of the MC at low
magnetic field is caused by the exchange interaction between the electron
in the conduction band and the magnetic ions.Comment: 6 pages, 9 figures, submitted to Phys. Rev.
Mesoscopic spin confinement during acoustically induced transport
Long coherence lifetimes of electron spins transported using moving potential
dots are shown to result from the mesoscopic confinement of the spin vector.
The confinement dimensions required for spin control are governed by the
characteristic spin-orbit length of the electron spins, which must be larger
than the dimensions of the dot potential. We show that the coherence lifetime
of the electron spins is independent of the local carrier densities within each
potential dot and that the precession frequency, which is determined by the
Dresselhaus contribution to the spin-orbit coupling, can be modified by varying
the sample dimensions resulting in predictable changes in the spin-orbit length
and, consequently, in the spin coherence lifetime.Comment: 10 pages, 2 figure
Spin diffusion and injection in semiconductor structures: Electric field effects
In semiconductor spintronic devices, the semiconductor is usually lightly
doped and nondegenerate, and moderate electric fields can dominate the carrier
motion. We recently derived a drift-diffusion equation for spin polarization in
the semiconductors by consistently taking into account electric-field effects
and nondegenerate electron statistics and identified a high-field diffusive
regime which has no analogue in metals. Here spin injection from a ferromagnet
(FM) into a nonmagnetic semiconductor (NS) is extensively studied by applying
this spin drift-diffusion equation to several typical injection structures such
as FM/NS, FM/NS/FM, and FM/NS/NS structures. We find that in the high-field
regime spin injection from a ferromagnet into a semiconductor is enhanced by
several orders of magnitude. For injection structures with interfacial
barriers, the electric field further enhances spin injection considerably. In
FM/NS/FM structures high electric fields destroy the symmetry between the two
magnets at low fields, where both magnets are equally important for spin
injection, and spin injection becomes locally determined by the magnet from
which carriers flow into the semiconductor. The field-induced spin injection
enhancement should also be insensitive to the presence of a highly doped
nonmagnetic semiconductor (NS) at the FM interface, thus FM/NS/NS
structures should also manifest efficient spin injection at high fields.
Furthermore, high fields substantially reduce the magnetoresistance observable
in a recent experiment on spin injection from magnetic semiconductors
High-order Dy multipole motifs observed in DyB2C2 with resonant soft x-ray Bragg diffraction
Resonant soft x-ray Bragg diffraction at the Dy M4,5 edges has been exploited
to study Dy multipole motifs in DyB2C2. Our results are explained introducing
the intra-atomic quadrupolar interaction between the core 3d and valence 4f
shell. This allows us to determine for the first time higher order multipole
moments of dysprosium electrons and to draw their precise charge density.
The Dy hexadecapole and hexacontatetrapole moment have been estimated at -20%
and +30% of the quadrupolar moment, respectively. No evidence for the lock-in
of the orbitals at T_N has been observed, in contrast to earlier suggestions.
The multipolar interaction and the structural transition cooperate along c but
they compete in the basal plane explaining the canted structure along [110].Comment: 4 pages, 3 figure
Hyperfine-mediated transitions between a Zeeman split doublet in GaAs quantum dots: The role of the internal field
We consider the hyperfine-mediated transition rate between Zeeman split spin
states of the lowest orbital level in a GaAs quantum dot. We separate the
hyperfine Hamiltonian into a part which is diagonal in the orbital states and
another one which mixes different orbitals. The diagonal part gives rise to an
effective (internal) magnetic field which, in addition to an external magnetic
field, determines the Zeeman splitting. Spin-flip transitions in the dots are
induced by the orbital mixing part accompanied by an emission of a phonon. We
evaluate the rate for different regimes of applied magnetic field and
temperature. The rates we find are bigger that the spin-orbit related rates
provided the external magnetic field is sufficiently low.Comment: 8 pages, 3 figure
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