2,715 research outputs found
Hysteresis and re-entrant melting of a self-organized system of classical particles confined in a parabolic trap
A self-organized system composed of classical particles confined in a
two-dimensional parabolic trap and interacting through a potential with a
short-range attractive part and long-range repulsive part is studied as
function of temperature. The influence of the competition between the
short-range attractive part of the inter-particle potential and its long-range
repulsive part on the melting temperature is studied. Different behaviors of
the melting temperature are found depending on the screening length ()
and the strength () of the attractive part of the inter-particle potential.
A re-entrant behavior and a thermal induced phase transition is observed in a
small region of ()-space. A structural hysteresis effect is observed
as a function of temperature and physically understood as due to the presence
of a potential barrier between different configurations of the system.Comment: 8 pages, 6 figure
Structural and dynamical properties of a quasi-one-dimensional classical binary system
The ground state configurations and the \lq{}\lq{}normal\rq{}\rq{} mode
spectra of a -one-dimensional (Q1D) binary system of charged particles
interacting through a screened Coulomb potential are presented. The minimum
energy configurations were obtained analytically and independently through
molecular dynamic simulations. A rich variety of ordered structures were found
as a function of the screening parameter, the particle density, and the ratio
between the charges of the distinct types of particles. Continuous and
discontinuous structural transitions, as well as an unexpected symmetry
breaking in the charge distribution are observed when the density of the system
is changed. For near equal charges we found a disordered phase where a mixing
of the two types of particles occurs. The phonon dispersion curves were
calculated within the harmonic approximation for the one- and two-chain
structures.Comment: 11 pages, 11 fig
Hysteresis in mesoscopic superconducting disks: the Bean-Livingston barrier
The magnetization behavior of mesoscopic superconducting disks can show
hysteretic behavior which we explain by using the Ginzburg-Landau (GL) theory
and properly taking into account the de-magnetization effects due to
geometrical form factors. In large disks the Bean-Livingston surface barrier is
responsible for the hysteresis. While in small disks a volume barrier is
responsible for this hysteresis. It is shown that although the sample
magnetization is diamagnetic (negative), the measured magnetization can be
positive at certain fields as observed experimentally, which is a consequence
of the de-magnetization effects and the experimental set up.Comment: Latex file, 4 ps file
Metallic nanograins: spatially nonuniform pairing induced by quantum confinement
It is well-known that the formation of discrete electron levels strongly
influences the pairing in metallic nanograins. Here we focus on another effect
of quantum confinement in superconducting grains that was not studied
previously, i.e., spatially nonuniform pairing. This effect is very significant
when single-electron levels form bunches and/or a kind of shell structure: in
highly symmetric grains the order parameter can exhibit variations with
position by an order of magnitude. Nonuniform pairing is closely related to a
quantum-confinement induced modification of the pairing-interaction matrix
elements and size-dependent pinning of the chemical potential to groups of
degenerate or nearly degenerate levels. For illustration we consider spherical
metallic nanograins. We show that the relevant matrix elements are as a rule
enhanced in the presence of quantum confinement, which favors spatial
variations of the order parameter, compensating the corresponding energy cost.
The size-dependent pinning of the chemical potential further increases the
spatial variation of the pair condensate. The role of nonuniform pairing is
smaller in less symmetric confining geometries and/or in the presence of
disorder. However, it always remains of importance when the energy spacing
between discrete electron levels is approaching the scale of the bulk
gap , i.e., -
Yukawa particles confined in a channel and subject to a periodic potential: ground state and normal modes
We consider a classical system of two-dimensional (2D) charged particles,
which interact through a repulsive Yukawa potential ,
confined in a parabolic channel which limits the motion of the particles in the
-direction. Along the -direction, the particles are also subject to a
periodic potential substrate. The ground state configurations and the normal
mode spectra of the system are obtained as function of the periodicity and
strength of the periodic potential (), and density. An interesting set of
tunable ground state configurations are found, with first and second order
structural transitions between them. A magic configuration with particles
aligned in each minimum of the periodic potential is obtained for V_0 larger
than some critical value which has a power law dependence on the density. The
phonon spectrum of different configurations were also calculated. A
localization of the modes into a small frequency interval is observed for a
sufficient strength of the periodic potential. A tunable band-gap is found as a
function of . This model system can be viewed as a generalization of the
Frenkel and Kontorova model.Comment: Submmited to PR
Reply to the comment by D. Kreimer and E. Mielke
We respond to the comment by Kreimer et. al. about the torsional contribution
to the chiral anomaly in curved spacetimes. We discuss their claims and refute
its main conclusion.Comment: 9 pages, revte
Off center centers in a quantum well in the presence of a perpendicular magnetic field: angular momentum transition and magnetic evaporation
We investigate the effect of the position of the donor in the quantum well on
the energy spectrum and the oscillator strength of the D- system in the
presence of a perpendicular magnetic field. As a function of the magnetic field
we find that when the D- centers are placed sufficiently off-center they
undergo singlet-triplet transitions which are similar to those found in
many-electron parabolic quantum dots. The main difference is that the number of
such transitions depends on the position of the donor and only a finite number
of such singlet-triplet transitions are found as function of the strength of
the magnetic field. For sufficiently large magnetic fields the two electron
system becomes unbound. For the near center D- system no singlet-triplet and no
unbinding of the D- is found with increasing magnetic field. A magnetic field
vs. donor position phase diagram is presented that depends on the width of the
quantum well.Comment: 16 pages, 17 figures. Accepted for publication in Phys. Rev.
CO2 Conversion in Nonuniform Discharges: Disentangling Dissociation and Recombination Mechanisms
Motivated by environmental applications such as synthetic fuel synthesis, plasma-driven conversion shows promise for efficient and scalable gas conversion of CO2 to CO. Both discharge contraction and turbulent transport have a significant impact on the plasma processing conditions, but are, nevertheless, poorly understood. This work combines experiments and modeling to investigate how these aspects influence the CO production and destruction mechanisms in the vortex-stabilized CO2 microwave plasma reactor. For this, a two-dimensional axisymmetric tubular chemical kinetics model of the reactor is developed, with careful consideration of the nonuniform nature of the plasma and the vortex-induced radial turbulent transport. Energy efficiency and conversion of the dissociation process show a good agreement with the numerical results over a broad pressure range from 80 to 600 mbar. The occurrence of an energy efficiency peak between 100 and 200 mbar is associated with a discharge mode transition. The net CO production rate is inhibited at low pressure by the plasma temperature, whereas recombination of CO to CO2 dominates at high pressure. Turbulence-induced cooling and dilution of plasma products limit the extent of the latter. The maxima in energy efficiency observed experimentally around 40% are related to limits imposed by production and recombination processes. Based on these insights, feasible approaches for optimization of the plasma dissociation process are discussed.</p
Characterization of the CO2 microwave plasma based on the phenomenon of skin-depth-limited contraction
The subatmospheric CO2 microwave plasma is known to contract to a narrow filament with rising pressure as result of a mode transition. This changing state of contraction is investigated in relation to its dielectric properties, in order to directly relate the discharge parameters to the discharge radius. The electron density and gas temperature are measured, respectively, by 168 GHz microwave interferometry and Doppler broadening of the 777 nm oxygen emission lines. The plasma is operated in steady state with 1400 W at 2.45 GHz, between 100 mbar and 400 mbar. Electron density values in the central region range from 1018 to 1020 mâ3 between the discharge modes, while the gas temperature increases from 3000 K to 6500 K, in good agreement with previously reported values. Based on the dielectric properties of the discharge in relation to the plasma radius, it is found that the discharge column constitutes a radius of a single skin depth. Implications of these insights on the conditions of previously reported CO2 dissociation experiments are discussed.</p
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