1,174 research outputs found
Unified continuum approach to crystal surface morphological relaxation
A continuum theory is used to predict scaling laws for the morphological
relaxation of crystal surfaces in two independent space dimensions. The goal is
to unify previously disconnected experimental observations of decaying surface
profiles. The continuum description is derived from the motion of interacting
atomic steps. For isotropic diffusion of adatoms across each terrace, induced
adatom fluxes transverse and parallel to step edges obey different laws,
yielding a tensor mobility for the continuum surface flux. The partial
differential equation (PDE) for the height profile expresses an interplay of
step energetics and kinetics, and aspect ratio of surface topography that
plausibly unifies observations of decaying bidirectional surface corrugations.
The PDE reduces to known evolution equations for axisymmetric mounds and
one-dimensional periodic corrugations.Comment: 5 pages, 1 figur
The dependence of strange hadron multiplicities on the speed of hadronization
Hadron multiplicities are calculated in the ALCOR model for the Pb+Pb
collisions at CERN SPS energy. Considering the newest experimental results, we
display our prediction obtained from the ALCOR model for stable hadrons
including strange baryons and anti-baryons.Comment: 8 pages, LaTeX in IOP style, appeared in the Proceedings of
Strangeness'97 Conference, Santorini, April 14-18 1997, J. of Physics G23
(1997) 194
Anisotropic diffusion in continuum relaxation of stepped crystal surfaces
We study the continuum limit in 2+1 dimensions of nanoscale anisotropic
diffusion processes on crystal surfaces relaxing to become flat below
roughening. Our main result is a continuum law for the surface flux in terms of
a new continuum-scale tensor mobility. The starting point is the Burton,
Cabrera and Frank (BCF) theory, which offers a discrete scheme for atomic steps
whose motion drives surface evolution. Our derivation is based on the
separation of local space variables into fast and slow. The model includes: (i)
anisotropic diffusion of adsorbed atoms (adatoms) on terraces separating steps;
(ii) diffusion of atoms along step edges; and (iii) attachment-detachment of
atoms at step edges. We derive a parabolic fourth-order, fully nonlinear
partial differential equation (PDE) for the continuum surface height profile.
An ingredient of this PDE is the surface mobility for the adatom flux, which is
a nontrivial extension of the tensor mobility for isotropic terrace diffusion
derived previously by Margetis and Kohn. Approximate, separable solutions of
the PDE are discussed.Comment: 14 pages, 1 figur
Alignement experience in STAR
The STAR experiment at RHIC uses four layers of silicon strip and silicon drift detectors for secondary vertex reconstruction. An attempt for a direct charm meson measurement put stringent requirements on alignment and calibration. We report on recent alignment and drift velocity calibration work performed on the inner silicon tracking system
Metal-semiconductor-metal photodetectors on a GeSn-on-insulator platform for 2 µm applications
In this work, the metal-semiconductor-metal photodetectors were demonstrated on the Ge0.91Sn0.09-on-insulator (GeSnOI) platform. The responsivity was 0.24 and 0.06 A/W at wavelengths of 1,600 and 2,003 nm, respectively. Through a systematic study, it is revealed that the photodetectors can potentially detect wavelength beyond 2,200 nm. The dark current density was measured to be 4.6 A/cm2 for GeSnOI waveguide-shaped photodetectors. The 3 dB bandwidth was observed to be 1.26 and 0.81 GHz at 1,550 and 2,000 nm wavelengths, respectively. This work opens up an opportunity for low-cost 2 µm wavelength photodetection on the GeSn/Ge interface-free GeSnOI platform
Pair excitations and the mean field approximation of interacting Bosons, I
In our previous work \cite{GMM1},\cite{GMM2} we introduced a correction to
the mean field approximation of interacting Bosons. This correction describes
the evolution of pairs of particles that leave the condensate and subsequently
evolve on a background formed by the condensate. In \cite{GMM2} we carried out
the analysis assuming that the interactions are independent of the number of
particles . Here we consider the case of stronger interactions. We offer a
new transparent derivation for the evolution of pair excitations. Indeed, we
obtain a pair of linear equations describing their evolution. Furthermore, we
obtain apriory estimates independent of the number of particles and use these
to compare the exact with the approximate dynamics
Neutral magic-angle bilayer graphene: Condon instability and chiral resonances
We discuss the full optical response of twisted bilayer graphene at the
neutrality point close to the magic angle within the continuum model. (i)
First, we define the full optical response consistent with the underlying
symmetry, yielding the total, magnetic, and chiral response that transform
according to the irreducible representations , , and ,
respectively. Then, we numerically calculate the dissipative and reactive
response for twist angles around the magic angle and comment on the
possibility of a Condon instability. (ii) Second, we numerically calculate the
full optical response {\it almost at} . The total response is
characterized by three universal plateaus which can be obtained from an
analytical calculation. The magnetic and the chiral response, however, is given
by corresponding non-universal plateaus depending on the twist angle
via the dimensionless parameter . (iii) Following
the discussion on the large magnetic response, we calculate the plasmonic
excitations at the neutrality point inside the optical gap of relaxed twisted
bilayer graphene. We find that acoustic plasmons extend over almost the whole
optical gap and carry the largest oscillator strength. (iv) Finally, we discuss
symmetry relations for the response functions as function of the chemical
potential and highlight the consequences of the approximate particle-hole
symmetry of the continuum model for twisted bilayer graphene. We then discuss a
detailed balance relation where the chiral response at charge neutrality can be
understood in terms of electron (hole) transitions for which the initial
(final) states are energetically closer to charge neutrality than the final
(initial) states.Comment: 17 pages, 7 figure
Drift Chamber Utilizing Microstrip Readout for Testing a New Micro TPC Concept
A drift chamber type radiation detector is being used to examine design criteria for a new type of detector called a micro Time Projection Chamber (micro TPC) which is being proposed for use in high energy nuclear physics experiments. The main advantage of the micro TPC detector is its very low radiation thickness compared to its silicon counterpart. The micro TPC is a charged-particle detector which willbe optimized for good two track resolution which is needed inahigh track density environment. Such performance requires low electron diffusion and high resolution readout. The diffusion willbe reduced bylimiting the drift distance to 15 cm and by using a low diffusion gas such as dimethyl ether. High resolution willbe obtained by using a new readout technology called microstrips. Microstrips are a recent development using photolithography techniques that allow the creation of anodes a few microns in width with submicron precision. The main purpose of this test chamber is to demonstrate the feasibility of a micro TPC design using a low diffusion gas and to insure the sufficient signal remains after electron attenuation. The driftchamber design and the proposed testingprocedures are described
Hadronization of massive quark matter
We present a fast hadronization model for the constituent quark plasma (CQP)
produced in relativistic heavy ion collisions at SPS. The model is based on
rate equations and on an equation of state inspired by the string
phenomenology. This equation of state has a confining character. We display the
time evolution of the relevant physical quantities during the hadronization
process and the final hadron multiplicities. The results indicate that the
hadronization of CQP is fast.Comment: 12 pages, Latex, 2 EPS figures, contribution to the Proceedings of
the 4th International Conference on Strangeness in Quark Matter (SQM'98),
Padova, Italy, 20-24 July 199
Strange hyperon and antihyperon production from quark and string-rope matter
Hyperon and antihyperon production is investigated using two microscopical
models: {\bf (1)} the fast hadronization of quark matter as given by the ALCOR
model; {\bf (2)} string formation and fragmentation as in the HIJING/B model.
We calculate the particle numbers and momentum distributions for Pb+Pb
collisions at CERN SPS energies in order to compare the two models with each
other and with the available experimental data. We show that these two
theoretical approaches give similar yields for the hyperons, but strongly
differ for antihyperons.Comment: 11 pages, Latex, 3 EPS figures, contribution to the Proceedings of
the 4th International Conference on Strangeness in Quark Matter (SQM'98),
Padova, Italy, 20-24 July 199
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