115 research outputs found
Factorization Approach for Inclusive Production of Doubly Heavy Baryon
We study inclusive production of doubly heavy baryon at a collider
and at hadron colliders through fragmentation. We study the production by
factorizing nonpertubative- and perturbative effects. In our approach the
production can be thought as a two-step process: A pair of heavy quarks can be
produced perturbatively and then the pair is transformed into the baryon. The
transformation is nonperturbative. Since a heavy quark moves with a small
velocity in the baryon in its rest frame, we can use NRQCD to describe the
transformation and perform a systematic expansion in the small velocity. At the
leading order we find that the baryon can be formed from two states of the
heavy-quark pair, one state is with the pair in state and in color
, another is with the pair in state and in color . Two matrix elements are defined for the transformation from the two
states, their perturbative coefficients in the contribution to the
cross-section at a collider and to the function of heavy quark
fragmentation are calculated. Our approach is different than previous
approaches where only the pair in state and in color is
taken into account. Numerical results for colliders at the two
-factories and for hadronic colliders LHC and Tevatron are given.Comment: Add results for large p_t, minor change
Magnetic vortex oscillator driven by dc spin-polarized current
Transfer of angular momentum from a spin-polarized current to a ferromagnet
provides an efficient means to control the dynamics of nanomagnets. A peculiar
consequence of this spin-torque, the ability to induce persistent oscillations
of a nanomagnet by applying a dc current, has previously been reported only for
spatially uniform nanomagnets. Here we demonstrate that a quintessentially
nonuniform magnetic structure, a magnetic vortex, isolated within a nanoscale
spin valve structure, can be excited into persistent microwave-frequency
oscillations by a spin-polarized dc current. Comparison to micromagnetic
simulations leads to identification of the oscillations with a precession of
the vortex core. The oscillations, which can be obtained in essentially zero
magnetic field, exhibit linewidths that can be narrower than 300 kHz, making
these highly compact spin-torque vortex oscillator devices potential candidates
for microwave signal-processing applications, and a powerful new tool for
fundamental studies of vortex dynamics in magnetic nanostructures.Comment: 14 pages, 4 figure
Field-driven femtosecond magnetization dynamics induced by ultrastrong coupling to THz transients
Controlling ultrafast magnetization dynamics by a femtosecond laser is
attracting interest both in fundamental science and industry because of the
potential to achieve magnetic domain switching at ever advanced speed. Here we
report experiments illustrating the ultrastrong and fully coherent light-matter
coupling of a high-field single-cycle THz transient to the magnetization vector
in a ferromagnetic thin film. We could visualize magnetization dynamics which
occur on a timescale of the THz laser cycle and two orders of magnitude faster
than the natural precession response of electrons to an external magnetic
field, given by the Larmor frequency. We show that for one particular
scattering geometry the strong coherent optical coupling can be described
within the framework of a renormalized Landau Lifshitz equation. In addition to
fundamentally new insights to ultrafast magnetization dynamics the coherent
interaction allows for retrieving the complex time-frequency magnetic
properties and points out new opportunities in data storage technology towards
significantly higher storage speed.Comment: 25 page
Doubly Heavy Baryon Production at \gamma \gamma Collider
The inclusive production of doubly heavy baryons and at
collider is investigated. It is found that the contribution from
the heavy quark pair in color triplet and color sextet are important.Comment: errors/typos are correcte
Large microwave generation from d.c. driven magnetic vortex oscillators in magnetic tunnel junctions
Spin polarized current can excite the magnetization of a ferromagnet through
the transfer of spin angular momentum to the local spin system. This pure
spin-related transport phenomena leads to alluring possibilities for the
achievement of a nanometer scale, CMOS compatible and tunable microwave
generator operating at low bias for future wireless communications. Microwave
emission generated by the persitent motion of magnetic vortices induced by spin
transfer effect seems to be a unique manner to reach appropriate spectral
linewidth. However, in metallic systems, where such vortex oscillations have
been observed, the resulting microwave power is much too small. Here we present
experimental evidences of spin-transfer induced core vortex precessions in
MgO-based magnetic tunnel junctions with similar good spectral quality but an
emitted power at least one order of magnitude stronger. More importantly,
unlike to others spin transfer excitations, the thorough comparison between
experimental results and models provide a clear textbook illustration of the
mechanisms of vortex precessions induced by spin transfer
Thermally driven spin injection from a ferromagnet into a non-magnetic metal
Creating, manipulating and detecting spin polarized carriers are the key
elements of spin based electronics. Most practical devices use a perpendicular
geometry in which the spin currents, describing the transport of spin angular
momentum, are accompanied by charge currents. In recent years, new sources of
pure spin currents, i.e., without charge currents, have been demonstrated and
applied. In this paper, we demonstrate a conceptually new source of pure spin
current driven by the flow of heat across a ferromagnetic/non-magnetic metal
(FM/NM) interface. This spin current is generated because the Seebeck
coefficient, which describes the generation of a voltage as a result of a
temperature gradient, is spin dependent in a ferromagnet. For a detailed study
of this new source of spins, it is measured in a non-local lateral geometry. We
developed a 3D model that describes the heat, charge and spin transport in this
geometry which allows us to quantify this process. We obtain a spin Seebeck
coefficient for Permalloy of -3.8 microvolt/Kelvin demonstrating that thermally
driven spin injection is a feasible alternative for electrical spin injection
in, for example, spin transfer torque experiments
Spin torque resonant vortex core expulsion for an efficient radio-frequency detection scheme
Spin-polarised radio-frequency currents, whose frequency is equal to that of
the gyrotropic mode, will cause an excitation of the core of a magnetic vortex
confined in a magnetic tunnel junction. When the excitation radius of the
vortex core is greater than that of the junction radius, vortex core expulsion
is observed, leading to a large change in resistance, as the layer enters a
predominantly uniform magnetisation state. Unlike the conventional spin-torque
diode effect, this highly tunable resonant effect will generate a voltage which
does not decrease as a function of rf power, and has the potential to form the
basis of a new generation of tunable nanoscale radio-frequency detectors
Excitations of incoherent spin-waves due to spin-transfer torque
As predicted by Slonczewski and Berger, the possibility of exciting microwave
oscillations in a nanomagnet by a spin-polarized current has been recently
demonstrated. This observation opens very important perspectives of
applications in RF components. However, some unresolved inconsistencies are
found when interpreting the magnetization dynamics results within the coherent
spin-torque model (CSM). In some cases, the telegraph noise caused by
spin-currents could not be described quantitatively by the CSM. This led to
controversies about the need of an effective magnetic temperature model (ETM).
Here we interpret the experimental results of Kiselev et al. [Nature 425, 380
(2003)] using micromagnetic simulations. We point out the key role played by
incoherent spin-waves excitation due to spin-transfer effects. The incoherence
is caused by the spatial inhomogeneities of the local fields, generating a
distribution of local precession frequencies. It results in telegraph noise at
zero temperature associated with transitions between attraction wells in phase
space.Comment: Nature Materials advance online publication, 7 November 200
Electrical switching of vortex core in a magnetic disk
A magnetic vortex is a curling magnetic structure realized in a ferromagnetic
disk, which is a promising candidate of a memory cell for future nonvolatile
data storage devices. Thus, understanding of the stability and dynamical
behaviour of the magnetic vortex is a major requirement for developing magnetic
data storage technology. Since the experimental proof of the existence of a
nanometre-scale core with out-of-plane magnetisation in the magnetic vortex,
the dynamics of a vortex has been investigated intensively. However, the way to
electrically control the core magnetisation, which is a key for constructing a
vortex core memory, has been lacking. Here, we demonstrate the electrical
switching of the core magnetisation by utilizing the current-driven resonant
dynamics of the vortex; the core switching is triggered by a strong dynamic
field which is produced locally by a rotational core motion at a high speed of
several hundred m/s. Efficient switching of the vortex core without magnetic
field application is achieved thanks to resonance. This opens up the
potentiality of a simple magnetic disk as a building block for spintronic
devices like a memory cell where the bit data is stored as the direction of the
nanometre-scale core magnetisation.Comment: 20 pages, 4 figures. Supplementary discussion included. Accepted for
publication in Nature Material
Spintronics: Fundamentals and applications
Spintronics, or spin electronics, involves the study of active control and
manipulation of spin degrees of freedom in solid-state systems. This article
reviews the current status of this subject, including both recent advances and
well-established results. The primary focus is on the basic physical principles
underlying the generation of carrier spin polarization, spin dynamics, and
spin-polarized transport in semiconductors and metals. Spin transport differs
from charge transport in that spin is a nonconserved quantity in solids due to
spin-orbit and hyperfine coupling. The authors discuss in detail spin
decoherence mechanisms in metals and semiconductors. Various theories of spin
injection and spin-polarized transport are applied to hybrid structures
relevant to spin-based devices and fundamental studies of materials properties.
Experimental work is reviewed with the emphasis on projected applications, in
which external electric and magnetic fields and illumination by light will be
used to control spin and charge dynamics to create new functionalities not
feasible or ineffective with conventional electronics.Comment: invited review, 36 figures, 900+ references; minor stylistic changes
from the published versio
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