Improving Lattice Quark Actions

We explore the first stage of the Symanzik improvement program for lattice Dirac fermions, namely the construction of doubler-free, highly improved classical actions on isotropic as well as anisotropic lattices (where the temporal lattice spacing, a_t, is smaller than the spatial one). Using field transformations to eliminate doublers, we derive the previously presented isotropic D234 action with O(a^3) errors, as well as anisotropic D234 actions with O(a^4) or O(a_t^3, a^4) errors. Besides allowing the simulation of heavy quarks within a relativistic framework, anisotropic lattices alleviate potential problems due to unphysical branches of the quark dispersion relation (which are generic to improved actions), facilitate studies of lattice thermodynamics, and allow accurate mass determinations for particles with bad signal/noise properties, like glueballs and P-state mesons. We also show how field transformations can be used to completely eliminate unphysical branches of the dispersion relation. Finally, we briefly discuss future steps in the improvement program.Comment: Tiny changes to agree with version to appear in Nucl. Phys. B (33 pages, LaTeX, 13 eps files

Quantum computation in a Ising spin chain taking into account second neighbor couplings

We consider the realization of a quantum computer in a chain of nuclear spins coupled by an Ising interaction. Quantum algorithms can be performed with the help of appropriate radio-frequency pulses. In addition to the standard nearest-neighbor Ising coupling, we also allow for a second neighbor coupling. It is shown, how to apply the 2\pi k method in this more general setting, where the additional coupling eventually allows to save a few pulses. We illustrate our results with two numerical simulations: the Shor prime factorization of the number 4 and the teleportation of a qubit along a chain of 3 qubits. In both cases, the optimal Rabi frequency (to suppress non-resonant effects) depends primarily on the strength of the second neighbor interaction.Comment: 19 pages, 6 figure

First Principles Calculation of Elastic Properties of Solid Argon at High Pressures

The density and the elastic stiffness coefficients of fcc solid argon at high pressures from 1 GPa up to 80 GPa are computed by first-principles pseudopotential method with plane-wave basis set and the generalized gradient approximation (GGA). The result is in good agreement with the experimental result recently obtained with the Brillouin spectroscopy by Shimizu et al. [Phys. Rev. Lett. 86, 4568 (2001)]. The Cauchy condition was found to be strongly violated as in the experimental result, indicating large contribution from non-central many-body force. The present result has made it clear that the standard density functional method with periodic boundary conditions can be successfully applied for calculating elastic properties of rare gas solids at high pressures in contrast to those at low pressures where dispersion forces are important.Comment: 4 pages, 5 figures, submitted to PR

Spontaneous magnetization of aluminum nanowires deposited on the NaCl(100) surface

We investigate electronic structures of Al quantum wires, both unsupported and supported on the (100) NaCl surface, using the density-functional theory. We confirm that unsupported nanowires, constrained to be linear, show magnetization when elongated beyond the equilibrium length. Allowing ions to relax, the wires deform to zig-zag structures with lower magnetization but no dimerization occurs. When an Al wire is deposited on the NaCl surface, a zig-zag geometry emerges again. The magnetization changes moderately from that for the corresponding unsupported wire. We analyse the findings using electron band structures and simple model wires.Comment: submitted to PHys. Rev.

Structure and dynamics of Rh surfaces

Lattice relaxations, surface phonon spectra, surface energies, and work functions are calculated for Rh(100) and Rh(110) surfaces using density-functional theory and the full-potential linearized augmented plane wave method. Both, the local-density approximation and the generalized gradient approximation to the exchange-correlation functional are considered. The force constants are obtained from the directly calculated atomic forces, and the temperature dependence of the surface relaxation is evaluated by minimizing the free energy of the system. The anharmonicity of the atomic vibrations is taken into account within the quasiharmonic approximation. The importance of contributions from different phonons to the surface relaxation is analyzed.Comment: 9 pages, 7 figures, scheduled to appear in Phys. Rev. B, Feb. 15 (1998). Other related publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm

Dynamics of Collective Decoherence and Thermalization

We analyze the dynamics of N interacting spins (quantum register) collectively coupled to a thermal environment. Each spin experiences the same environment interaction, consisting of an energy conserving and an energy exchange part. We find the decay rates of the reduced density matrix elements in the energy basis. We show that if the spins do not interact among each other, then the fastest decay rates of off-diagonal matrix elements induced by the energy conserving interaction is of order N^2, while that one induced by the energy exchange interaction is of the order N only. Moreover, the diagonal matrix elements approach their limiting values at a rate independent of N. For a general spin system the decay rates depend in a rather complicated (but explicit) way on the size N and the interaction between the spins. Our method is based on a dynamical quantum resonance theory valid for small, fixed values of the couplings. We do not make Markov-, Born- or weak coupling (van Hove) approximations

The Uses of Conformal Symmetry in QCD

The Lagrangian of Quantum Chromodynamics is invariant under conformal transformations. Although this symmetry is broken by quantum corrections, it has important consequences for strong interactions at short distances and provides one with powerful tools in practical calculations. In this review we give a short exposition of the relevant ideas, techniques and applications of conformal symmetry to various problems of interest.Comment: 110 pages, invited review for "Prog. Part. Nucl. Phys

Highly connected 3D chromatin networks established by an oncogenic fusion protein shape tumor cell identity.

Cell fate transitions observed in embryonic development involve changes in three-dimensional genomic organization that provide proper lineage specification. Whether similar events occur within tumor cells and contribute to cancer evolution remains largely unexplored. We modeled this process in the pediatric cancer Ewing sarcoma and investigated high-resolution looping and large-scale nuclear conformation changes associated with the oncogenic fusion protein EWS-FLI1. We show that chromatin interactions in tumor cells are dominated by highly connected looping hubs centered on EWS-FLI1 binding sites, which directly control the activity of linked enhancers and promoters to establish oncogenic expression programs. Conversely, EWS-FLI1 depletion led to the disassembly of these looping networks and a widespread nuclear reorganization through the establishment of new looping patterns and large-scale compartment configuration matching those observed in mesenchymal stem cells, a candidate Ewing sarcoma progenitor. Our data demonstrate that major architectural features of nuclear organization in cancer cells can depend on single oncogenes and are readily reversed to reestablish latent differentiation programs

Search for the glueball candidates f0(1500) and fJ(1710) in gamma gamma collisions

Data taken with the ALEPH detector at LEP1 have been used to search for gamma gamma production of the glueball candidates f0(1500) and fJ(1710) via their decay to pi+pi-. No signal is observed and upper limits to the product of gamma gamma width and pi+pi- branching ratio of the f0(1500) and the fJ(1710) have been measured to be Gamma_(gamma gamma -> f0(1500)). BR(f0(1500)->pi+pi-) < 0.31 keV and Gamma_(gamma gamma -> fJ(1710)). BR(fJ(1710)->pi+pi-) < 0.55 keV at 95% confidence level.Comment: 10 pages, 3 figure