8,053 research outputs found
Common features of deconfining and chiral critical points in QCD and the three state Potts model in an external field
In the presented study we investigated the second order endpoints of the
lines of first order phase transitions which emerge for the QCD in the heavy
and light quark mass regime and for the three-dimensional three state Potts
model with an external field. We located the endpoints with Binder cumulants
and constructed the energy-like and ordering field like observables. The joint
probability distributions of these scaling fields and the values of the Binder
cumulant confirm that all three endpoints belong to the universality class of
the 3-dimensional Ising model.Comment: Based on a poster presented by S.Stickan at the CCP2001 Aachen,4
pages,6 figures, to be published in computer physics communicatio
QCD Thermodynamics with 2 and 3 Quark Flavors
We discuss the flavor dependence of the pressure and critical temperature
calculated in QCD with 2, 2+1 and 3 flavors using improved gauge and staggered
fermion actions on lattices with temporal extent Nt=4. For T > 2 Tc we find
that bulk thermodynamics of QCD with 2 light and a heavier strange quark is
well described by 3-flavor QCD while the transition temperature is closer to
that of 2-flavor QCD. Furthermore, we present evidence that the chiral critical
point of 3-flavor QCD, i.e. the second order endpoint of the line of first
order chiral phase transitions, belongs to the universality class of the 3d
Ising model.Comment: 6 pages, LaTeX2e File, 7 EPS-figures, presented at SEWM 2000,
Marseille, June 13-17th, 200
Quantum-Classical Liouville Approach to Molecular Dynamics: Surface Hopping Gaussian Phase-Space Packets
In mixed quantum-classical molecular dynamics few but important degrees of freedom of a molecular system are modeled quantum-mechanically while the remaining degrees of freedom are treated within the classical approximation. Such models can be systematically derived as a first order approximation to the partial Wigner transform of the quantum Liouville-von Neumann equation. The resulting adiabatic quantum-classical Liouville equation (QCLE) can be decomposed into three individual propagators by means of a Trotter splitting:
Phase oscillations of the coherences resulting from the time evolution of the quantum-mechanical subsystem.
Exchange of densities and coherences reflecting non-adiabatic effects in quantum-classical dynamics.
Classical Liouvillian transport of densities and coherences along adiabatic potential energy surfaces or arithmetic means thereof.
A novel stochastic implementation of the QCLE is proposed in the present work. In order to substantially improve the traditional algorithm based on surface hopping trajectories [J. C. Tully, J. Chem. Phys. 93 (2), 1061 (1990)], we model the evolution of densities and coherences by a set of surface hopping Gaussian phase-space packets (GPPs) with variable width and with adjustable real or complex amplitudes, respectively. The dense sampling of phase-space offers two main advantages over other numerical schemes to solve the QCLE. First, it allows to perform a quantum-classical simulation employing a constant number of particles, i. e. the generation of new trajectories at each surface hop is avoided. Second, the effect of non-local operators in the exchange of densities and coherences can be treated without having to invoke the momentum jump approximation.
For the example of a single avoided crossing we demonstrate that convergence towards fully quantum-mechanical dynamics is much faster for surface hopping GPPs than for trajectory-based methods. For dual avoided crossings the Gaussian-based dynamics correctly reproduces the quantum-mechanical result even when trajectory-based methods not accounting for the transport of coherences fail qualitatively
InGaAs/GaAs/alkanethiolate radial superlattices: Experimental
A radial InGaAs/GaAs/1-hexadecanethiol superlattice is fabricated by the
roll-up of a strained InGaAs/GaAs bilayer passivated with a molecular
self-assembled monolayer. Our technique allows the formation of multi-period
inorganic/organic hybrid heterostructures. This paper contains the detailed
experimental description of how to fabricate these structures.Comment: 2 pages, no figures, Version 2; minor changes (fixed typos and update
references
Multidimensional Classical Liouville Dynamics with Quantum Initial Conditions
A simple and numerically efficient approach to Wigner transforms and classical Liouville dynamics in phase-space is presented.
The Wigner transform can be obtained with a given accuracy by optimal decomposition of an initial quantum-mechanical wavefunction in terms of a minimal set of Gaussian wavepackets.
The solution of the classical Liouville equation within the locally quadratic approximation of the potential energy function requires a representation of the density in terms of an ensemble of narrow Gaussian phase-space packets. The corresponding equations of motion can be efficiently solved by a modified Leap-Frog integrator.
For both problems the use of Monte-Carlo based techniques allows numerical calculation in multidimensional cases where grid-based methods such as fast Fourier transforms are prohibitive. In total, the proposed strategy provides a practical and efficient tool for classical Liouville dynamics with quantum-mechanical initial states
Nonadiabatic Effects on Peptide Vibrational Dynamics Induced by Conformational Changes
Quantum dynamical simulations of vibrational spectroscopy have been carried out for glycine dipeptide (CH3-CO-NH-CH2-CO-NH-CH3). Conformational structure and dynamics are modeled in terms of the two Ramachandran dihedral angles of the molecular backbone. Potential energy surfaces and harmonic frequencies are obtained from electronic structure calculations at the density functional theory (B3LYP/6-31+G(d)) level. The ordering of the energetically most stable isomers (C7 and C5) is reversed upon inclusion of the quantum mechanical zero point vibrational energy. Vibrational spectra of various isomers show distinct differences, mainly in the region of the amide modes, thereby relating conformational structures and vibrational spectra. Conformational dynamics is modeled by propagation of quantum mechanical wave packets. Assuming a directed energy transfer to the torsional degrees of freedom, transitions between the C7 and C5 minimum energy structures occur on a sub-picosecond timescale (700 ... 800 fs). Vibrationally non-adiabatic effects are investigated for the case of the coupled, fundamentally excited amide I states. Using a two state-two mode model, the resulting wave packet dynamics is found to be strongly non-adiabatic due to the presence of a seam of the two potential energy surfaces. Initially prepared adiabatic vibrational states decay upon conformational change on a timescale of 200 ... 500 fs with population transfer of more than 50 % between the coupled amide I states. Also the vibrational energy transport between localized (excitonic) amide I vibrational states is strongly influenced by torsional dynamics of the molecular backbone where both enhanced and reduced decay rates are found. All these observations should allow the detection of conformational changes by means of time-dependent vibrational spectroscopy
Archeops, mapping the CMB sky from large to small angular scales
Archeops is a balloon-borne experiment designed to measure the temperature
fluctuations of the CMB on a large region of the sky () with a high
angular resolution (10 arcminutes) and a high sensitivity ( per
pixel). Archeops will perform a measurement of the CMB anisotropies power
spectrum from large angular scales () to small angular scales
(). Archeops flew for the first time for a test flight in July
1999 from Sicily to Spain and the first scientific flight took place from
Sweden to Russia in January 2001. The data analysis is on its way and I present
here preliminary results, realistic simulations showing the expected accuracy
on the measurement of the power spectrum and perspectives for the incoming
flights (Winter 2001/2003).Comment: 6 pages, 6 figures, proceedings to TAUP2001 conference, LNGS, Italy,
Sept. 200
Bending and wrinkling as competing relaxation pathways for strained free-hanging films
An equilibrium phase diagram for the shape of compressively strained
free-hanging films is developed by total strain energy minimization. For small
strain gradients {\Delta}{\epsilon}, the film wrinkles, while for sufficiently
large {\Delta}{\epsilon}, a phase transition from wrinkling to bending occurs.
We consider competing relaxation mechanisms for free-hanging films, which have
rolled up into tube structures, and we provide an upper limit for the maximum
achievable number of tube rotations.Comment: 4 pages, 4 figure
Transition temperature in QCD with physical light and strange quark masses
We present results from a calculation of the transition temperature in QCD
with two light and one heavier (strange) quark mass on lattices with temporal
extent N_t =4 and 6. Calculations with improved staggered fermions have been
performed with a strange quark mass fixed close to its physical value and for
various light to strange quark mass ratios that correspond to light
pseudo-scalar masses in the range (150-500) MeV. From a combined extrapolation
to the chiral (m_l -> 0) and continuum (aT -> 0) limits we obtain for the
transition temperature at the physical point T_c = 192(7)(4) MeV.
We also present first results from an ongoing calculation of the QCD equation
of state with almost realistic light and strange quark masses.Comment: 4 pages, 4 figures, to appear in the proceedings of the 19th
International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions,
Shanghai, Nov. 200
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