7,420 research outputs found
Femtosecond Coherent Control of Spin with Light in (Ga,Mn)As ferromagnets
Using density matrix equations of motion, we predict a femtosecond collective
spin tilt triggered by nonlinear, near--ultraviolet (3eV), coherent
photoexcitation of (Ga,Mn)As ferromagnetic semiconductors with linearly
polarized light. This dynamics results from carrier coherences and nonthermal
populations excited in the \{111\} equivalent directions of the Brillouin zone
and triggers a subsequent uniform precession. We predict nonthermal
magnetization control by tuning the laser frequency and polarization direction.
Our mechanism explains recent ultrafast pump--probe experiments.Comment: 4 pages, 3 figures, published in Physical Review Letter
Asteroid flux towards circumprimary habitable zones in binary star systems: I. Statistical Overview
So far, multiple stellar systems harbor more than 130 extra solar planets.
Dynamical simulations show that the outcome of planetary formation process can
lead to various planetary architecture (i.e. location, size, mass and water
content) when the star system is single or double. In the late phase of
planetary formation, when embryo-sized objects dominate the inner region of the
system, asteroids are also present and can provide additional material for
objects inside the habitable zone (hereafter HZ). In this study, we make a
comparison of several binary star systems and their efficiency to move icy
asteroids from beyond the snow-line into orbits crossing the HZ. We modeled a
belt of 10000 asteroids (remnants from the late phase of planetary formation
process) beyond the snow-line. The planetesimals are placed randomly around the
primary star and move under the gravitational influence of the two stars and a
gas giant. As the planetesimals do not interact with each other, we divided the
belt into 100 subrings which were separately integrated. In this statistical
study, several double star configurations with a G-type star as primary are
investigated. Our results show that small bodies also participate in bearing a
non-negligible amount of water to the HZ. The proximity of a companion moving
on an eccentric orbit increases the flux of asteroids to the HZ, which could
result into a more efficient water transport on a short timescale, causing a
heavy bombardment. In contrast to asteroids moving under the gravitational
perturbations of one G-type star and a gas giant, we show that the presence of
a companion star can not only favor a faster depletion of our disk of
planetesimals but can also bring 4 -- 5 times more water into the whole HZ.Comment: Accepted for publication in A&
Observation of inter-Landau-level quantum coherence in semiconductor quantum wells
Using three-pulse four-wave-mixing femtosecond spectroscopy, we excite a
non-radiative coherence between the discrete Landau levels of an undoped
quantum well and study its dynamics. We observe quantum beats that reflect the
time evolution of the coherence between the two lowest Landau level
magnetoexcitons. We interpret our observations using a many-body theory and
find that the inter Landau level coherence decays with a new time constant,
substantially longer than the corresponding interband magnetoexciton dephasing
times. Our results indicate a new intraband excitation dynamics that cannot be
described in terms of uncorrelated interband excitations.Comment: 5 pages, 5 figures, to appear in Phys. Rev. B Rapid Communication
On Color Superconductivity in External Magnetic Field
We study color superconductivity in external magnetic field. We discuss the
reason why the mixing angles in color-flavor locked (CFL) and two-flavor
superconductivity (2SC) phases are different despite the fact that the CFL gap
goes to the 2SC gap for . Although flavor symmetry is
explicitly broken in external magnetic field, we show that all values of gaps
in their coset spaces of possible solutions in the CFL phase are equivalent in
external magnetic field.Comment: 12 pages, LaTe
A Diagrammatic Approach to Crystalline Color Superconductivity
We present a derivation of the gap equation for the crystalline color
superconducting phase of QCD which begins from a one-loop Schwinger-Dyson
equation written using a Nambu-Gorkov propagator modified to describe the
spatially varying condensate. Some aspects of previous variational calculations
become more straightforward when rephrased beginning from a diagrammatic
starting point. This derivation also provides a natural base from which to
generalize the analysis to include quark masses, nontrivial crystal structures,
gluon propagation at asymptotic densities, and nonzero temperature. In this
paper, we analyze the effects of nonzero temperature on the crystalline color
superconducting phase.Comment: 15 pages. 2 eps figure
Instantons and Scalar Multiquark States: From Small to Large N_c
We study scalar quark-anti-quark and two-quark-two-anti-quark correlation
functions in the instanton liquid model. We show that the instanton liquid
supports a light scalar-isoscalar (sigma) meson, and that this state is
strongly coupled to both and . The scalar-isovector
meson, on the other hand, is heavy. We also show that these properties
are specific to QCD with three colors. In the large limit the
scalar-isoscalar meson is not light, and it is mainly coupled to .Comment: 24 page
Pyrochlore S=1/2 Heisenberg antiferromagnet at finite temperature
We use a combination of three computational methods to investigate the notoriously difficult frustrated three-dimensional pyrochlore S = 1/2 aquantum antiferromagnet, at finite temperature T : canonical typicality for a finite cluster of 2 x 2 x 2 unit cells (i.e., 32 sites), a finite-T matrix product state method on a larger cluster with 48 sites, and the numerical linked cluster expansion (NLCE) using clusters up to 25 lattice sites, including nontrivial hexagonal and octagonal loops. We calculate thermodynamic properties (energy, specific heat capacity, entropy, susceptibility, magnetization) and the static structure factor. We find a pronounced maximum in the specific heat at T = 0.57J, which is stable across finite size clusters and converged in the series expansion. At T approximate to 0.25J (the limit of convergence of our method), the residual entropy per spin is 0.47k(B) In 2, which is relatively large compared to other frustrated models at this temperature. We also observe a nonmonotonic dependence on T of the magnetization at low magnetic fields, reflecting the dominantly nonmagnetic character of the low-energy states. A detailed comparison of our results to measurements for the S = 1 material NaCaNi2F7 yields a rough agreement of the functional form of the specific heat maximum, which in turn differs from the sharper maximum of the heat capacity of the spin ice material Dy2Ti2O7
Enforced Electrical Neutrality of the Color-Flavor Locked Phase
We demonstrate that quark matter in the color-flavor locked phase of QCD is
rigorously electrically neutral, despite the unequal quark masses, and even in
the presence of an electron chemical potential. As long as the strange quark
mass and the electron chemical potential do not preclude the color-flavor
locked phase, quark matter is automatically neutral. No electrons are required
and none are admitted.Comment: 4 pages, revtex. v2: very minor changes only. v3: small
clarifications; reference added; version to appear in Phys. Rev. Lett. v4,
posted in 2008: typo in Eq. 14 correcte
Possible Inversion Symmetry Breaking in the S=1/2 Pyrochlore Heisenberg Magnet
We address the ground-state properties of the long-standing and much-studied three-dimensional quantum spin liquid candidate, the S = 1/2 pyrochlore Heisenberg antiferromagnet. By using SU(2) density-matrix renormalization group (DMRG), we are able to access cluster sizes of up to 128 spins. Our most striking finding is a robust spontaneous inversion symmetry breaking, reflected in an energy density difference between the two sublattices of tetrahedra, familiar as a starting point of earlier perturbative treatments. We also determine the ground-state energy, E-0/N-sites = -0.490(6)J, by combining extrapolations of DMRG with those of a numerical linked cluster expansion. These findings suggest a scenario in which a finite-temperature spin liquid regime gives way to a symmetry-broken state at low temperatures
Photoinduced melting of superconductivity in the high-Tc superconductor La2-xSrxCuO4 probed by time-resolved optical and THz techniques
Dynamics of depletion and recovery of superconducting state in La2-xSrxCuO_4
thin films is investigated utilizing optical pump-probe and optical pump - THz
probe techniques as a function of temperature and excitation fluence. The
absorbed energy density required to suppress superconductivity is found to be
about 8 times higher than the thermodynamically determined condensation energy
density and nearly temperature independent between 4 and 25 K. These findings
indicate that during the time when superconducting state suppression takes
place (~0.7 ps), a large part (nearly 90%) of the energy is transferred to the
phonons with energy lower than twice the maximum value of of the SC gap and
only 10% is spent on Cooper pair breaking.Comment: 8 pages, 5 figure
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