711 research outputs found
Surface effects in nanoparticles: application to maghemite -Fe_{2}O_{3}
We present a microscopic model for nanoparticles, of the maghemite (% -FeO) type, and perform classical Monte Carlo simulations of
their magnetic properties. On account of M\"{o}ssbauer spectroscopy and
high-field magnetisation results, we consider a particle as composed of a core
and a surface shell of constant thickness. The magnetic state in the particle
is described by the anisotropic classical Dirac-Heisenberg model including
exchange and dipolar interactions and bulk and surface anisotropy. We consider
the case of ellipsoidal (or spherical) particles with free boundaries at the
surface. Using a surface shell of constant thickness ( nm) we vary
the particle size and study the effect of surface magnetic disorder on the
thermal and spatial behaviors of the net magnetisation of the particle. We
study the shift in the surface ``critical region'' for different
surface-to-core ratios of the exchange coupling constants. It is also shown
that the profile of the local magnetisation exhibits strong temperature
dependence, and that surface anisotropy is reponsible for the non saturation of
the magnetisation at low temperatures.Comment: 15 pages, 7 figures, to appear in Eur. Phys. J.
Asymmetry of localised states in a single quantum ring: polarization dependence of excitons and biexcitons
We performed spectroscopic studies of a single GaAs quantum ring with an
anisotropy in the rim height. The presence of an asymmetric localised state was
suggested by the adiabatic potential. The asymmetry was investigated in terms
of the polarization dependence of excitons and biexcitons, where a large energy
di erence (0.8 meV) in the exciton emission energy for perpendicular
polarizations was observed and the oscillator strengths were also compared
using the photoluminescence decay rate. For perpendicular polarizations the
biexciton exhibits twice the energy di erence seen for the exciton, a fact that
may be attributed to a possible change in the selection rules for the lowered
symmetry.Comment: accepted in Applied physics Letter
Excited exciton and biexciton localised states in a single quantum ring
We observe excited exciton and biexciton states of localised excitons in an
anisotropic quantum ring, where large polarisation asymmetry supports the
presence of a crescent-like localised structure. We also find that saturation
of the localised ground state exciton with increasing excitation can be
attributed to relatively fast dissociation of biexcitons (? 430 ps) compared to
slow relaxation from the excited state to the ground state (? 1000 ps). As no
significant excitonic Aharonov-Bohm oscillations occur up to 14 T, we conclude
that phase coherence around the rim is inhibited as a consequence of height
anisotropy in the quantum ring.Comment: 4 pages, 4 figure
Magnetic properties of LaFe1-xCrxO3 and Fe2-2xCr2xO3 mixed oxides
Mixed oxides with formula LaFe1-xCrxO3 and Fe2-2xCr2xO3, where 0≤x≤1, are studied. The samples have been prepared using solid state reaction technique in air. The X-ray diffraction spectra indicated that the samples crystallize in a corundum phase with space group ( R3c ) for Fe2-2xCr2xO3 and in the perovskite structure for LaFe1-xCrxO3. Many techniques have been used to explore the magnetic properties of the systems. High field, ZFC and FC magnetization vs. temperature, d.c. susceptibility and Mössbauer spectroscopy were carried out. High temperature magnetic susceptibility measurements and high field magnetic magnetization (H ≤ 20 T) show that the behavior of the susceptibility and the magnetization are complex. Mössbauer spectra of the solid solutions have been measured at 4.2 K and in the temperature range 77 K to 300 K. The shapes of spectra are unusual, showing strong relaxation phenomena in a wide temperature range as recently observed for many frustrated systems. The results are discussed by establishing the existence of various magnetic structures, inducing intermediate magnetic phases between the antiferromagnetic and the paramagnetic states. Preliminary magnetic phase diagrams of the systems have been established.Mixed oxides with formula LaFe1-xCrxO3 and Fe2-2xCr2xO3, where 0≤x≤1, are studied. The samples have been prepared using solid state reaction technique in air. The X-ray diffraction spectra indicated that the samples crystallize in a corundum phase with space group ( R3c ) for Fe2-2xCr2xO3 and in the perovskite structure for LaFe1-xCrxO3. Many techniques have been used to explore the magnetic properties of the systems. High field, ZFC and FC magnetization vs. temperature, d.c. susceptibility and Mössbauer spectroscopy were carried out. High temperature magnetic susceptibility measurements and high field magnetic magnetization (H ≤ 20 T) show that the behavior of the susceptibility and the magnetization are complex. Mössbauer spectra of the solid solutions have been measured at 4.2 K and in the temperature range 77 K to 300 K. The shapes of spectra are unusual, showing strong relaxation phenomena in a wide temperature range as recently observed for many frustrated systems. The results are discussed by establishing the existence of various magnetic structures, inducing intermediate magnetic phases between the antiferromagnetic and the paramagnetic states. Preliminary magnetic phase diagrams of the systems have been established
Trapping of ultra-cold atoms with the magnetic field of vortices in a thin film superconducting micro-structure
We store and control ultra-cold atoms in a new type of trap using magnetic
fields of vortices in a high temperature superconducting micro-structure. This
is the first time ultra-cold atoms have been trapped in the field of magnetic
flux quanta. We generate the attractive trapping potential for the atoms by
combining the magnetic field of a superconductor in the remanent state with
external homogeneous magnetic fields. We show the control of crucial atom trap
characteristics such as an efficient intrinsic loading mechanism, spatial
positioning of the trapped atoms and the vortex density in the superconductor.
The measured trap characteristics are in good agreement with our numerical
simulations.Comment: 4pages, comments are welcom
Thermal excitation of heavy nuclei with 5-15 GeV/c antiproton, proton and pion beams
Excitation-energy distributions have been derived from measurements of
5.0-14.6 GeV/c antiproton, proton and pion reactions with Au target
nuclei, using the ISiS 4 detector array. The maximum probability for
producing high excitation-energy events is found for the antiproton beam
relative to other hadrons, He and beams from LEAR. For protons
and pions, the excitation-energy distributions are nearly independent of hadron
type and beam momentum above about 8 GeV/c. The excitation energy enhancement
for beams and the saturation effect are qualitatively consistent with
intranuclear cascade code predictions. For all systems studied, maximum cluster
sizes are observed for residues with E*/A 6 MeV.Comment: 14 pages including 5 figures and 1 table. Accepted in Physics Letter
B. also available at http://nuchem.iucf.indiana.edu
Conditional large Fock state preparation and field state reconstruction in Cavity QED
We propose a scheme for producing large Fock states in Cavity QED via the
implementation of a highly selective atom-field interaction. It is based on
Raman excitation of a three-level atom by a classical field and a quantized
field mode. Selectivity appears when one tunes to resonance a specific
transition inside a chosen atom-field subspace, while other transitions remain
dispersive, as a consequence of the field dependent electronic energy shifts.
We show that this scheme can be also employed for reconstructing, in a new and
efficient way, the Wigner function of the cavity field state.Comment: 4 Revtex pages with 3 postscript figures. Submitted for publicatio
Effect of nucleon exchange on projectile multifragmentation in the reactions of 28Si + 112Sn and 124Sn at 30 and 50 MeV/nucleon
Multifragmentation of quasiprojectiles was studied in reactions of 28Si beam
with 112Sn and 124Sn targets at projectile energies 30 and 50 MeV/nucleon. The
quasiprojectile observables were reconstructed using isotopically identified
charged particles with Z_f <= 5 detected at forward angles. The nucleon
exchange between projectile and target was investigated using isospin and
excitation energy of reconstructed quasiprojectile. For events with total
reconstructed charge equal to the charge of the beam (Z_tot = 14) the influence
of beam energy and target isospin on neutron transfer was studied in detail.
Simulations employing subsequently model of deep inelastic transfer,
statistical model of multifragmentation and software replica of FAUST detector
array were carried out. A concept of deep inelastic transfer provides good
description of production of highly excited quasiprojectiles. The isospin and
excitation energy of quasiprojectile were described with good overall
agreement. The fragment multiplicity, charge and isospin were reproduced
satisfactorily. The range of contributing impact parameters was determined
using backtracing procedure.Comment: 11 pages, 8 Postscript figures, LaTeX, to appear in Phys. Rev. C (
Dec 2000
Single-photon tunneling
Strong evidence of a single-photon tunneling effect, a direct analog of
single-electron tunneling, has been obtained in the measurements of light
tunneling through individual subwavelength pinholes in a thick gold film
covered with a layer of polydiacetylene. The transmission of some pinholes
reached saturation because of the optical nonlinearity of polydiacetylene at a
very low light intensity of a few thousands photons per second. This result is
explained theoretically in terms of "photon blockade", similar to the Coulomb
blockade phenomenon observed in single-electron tunneling experiments. The
single-photon tunneling effect may find many applications in the emerging
fields of quantum communication and information processing.Comment: 4 pages, 4figure
Dynamics of excitons in individual InAs quantum dots revealed in four-wave mixing spectroscopy
We acknowledge the support by the ERC Starting Grant PICSEN, contract no. 306387. D.E.R. is grateful for financial support from the DAAD within the P.R.I.M.E. program.A detailed understanding of the population and coherence dynamics in optically driven individual emitters in solids and their signatures in ultrafast nonlinear-optical signals is of prime importance for their applications in future quantum and optical technologies. In a combined experimental and theoretical study on exciton complexes in single semiconductor quantum dots we reveal a detailed picture of the dynamics employing three-beam polarization-resolved four-wave mixing (FWM) micro-spectroscopy. The oscillatory dynamics of the FWM signals in the exciton-biexciton system is governed by the fine-structure splitting and the biexciton binding energy in an excellent quantitative agreement between measurement and analytical description. The analysis of the excitation conditions exhibits a dependence of the dynamics on the specific choice of polarization configuration, pulse areas and temporal ordering of driving fields. The interplay between the transitions in the four-level exciton system leads to rich evolution of coherence and population. Using two-dimensional FWM spectroscopy we elucidate the exciton-biexciton coupling and identify neutral and charged exciton complexes in a single quantum dot. Our investigations thus clearly reveal that FWM spectroscopy is a powerful tool to characterize spectral and dynamical properties of single quantum structures.PostprintPostprintPeer reviewe
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