Performance of the ATLAS Detector on First Single Beam and Cosmic Ray Data

We report on performance studies of the ATLAS detector obtained with first single LHC (Large Hadron Collider) beam data in September 2008, and large samples of cosmic ray events collected in the fall of 2008. In particular, the performance of the calorimeter, crucial for jet and missing transverse energy measurements, is studied. It is shown that the ATLAS experiment is ready to record the first LHC collisions.Comment: 4 pages, 6 figures, proceedings contribution of the SUSY 2009 conference in Bosto

Global Cardinality Constraints Make Approximating Some Max-2-CSPs Harder

Assuming the Unique Games Conjecture, we show that existing approximation algorithms for some Boolean Max-2-CSPs with cardinality constraints are optimal. In particular, we prove that Max-Cut with cardinality constraints is UG-hard to approximate within ~~0.858, and that Max-2-Sat with cardinality constraints is UG-hard to approximate within ~~0.929. In both cases, the previous best hardness results were the same as the hardness of the corresponding unconstrained Max-2-CSP (~~0.878 for Max-Cut, and ~~0.940 for Max-2-Sat). The hardness for Max-2-Sat applies to monotone Max-2-Sat instances, meaning that we also obtain tight inapproximability for the Max-k-Vertex-Cover problem

Peierls-type structural phase transition in a crystal induced by magnetic breakdown

We predict a new type of phase transition in a quasi-two dimensional system of electrons at high magnetic fields, namely the stabilization of a density wave which transforms a two dimensional open Fermi surface into a periodic chain of large pockets with small distances between them. The quantum tunneling of electrons between the neighboring closed orbits enveloping these pockets transforms the electron spectrum into a set of extremely narrow energy bands and gaps which decreases the total electron energy, thus leading to a magnetic breakdown induced density wave (MBIDW) ground state. We show that this DW instability has some qualitatively different properties in comparison to analogous DW instabilities of Peierls type. E. g. the critical temperature of the MBIDW phase transition arises and disappears in a peculiar way with a change of the inverse magnetic field

Competing SDW Phases and Quantum Oscillations in (TMTSF)2ClO4 in Magnetic Field

We propose a new approach for studying spin density waves (SDW) in the Bechgaard salt (TMTSF)2ClO4 where lattice is dimerized in transverse direction due to anion ordering. The SDW response is calculated in the matrix formulation that rigorously treats the hybridization of inter-band and intra-band SDW correlations. Since the dimerization gap is large, of the order of transverse bandwidth, we also develop an exact treatment of magnetic breakdown in the external magnetic field. The obtained results agree with the experimental data on the fast magneto-resistance oscillations. Experimentally found 260T rapid oscillations and the characteristic Tc dependance on magnetic field of relaxed material are fitted with our results for anion potential of the order of interchain hopping

Effects of transverse electron dispersion on photo-emission spectra of quasi-one-dimensional systems

The random phase approximation (RPA) spectral function of the one-dimensional electron band with the three-dimensional long range Coulomb interaction shows a broad feature which is spread on the scale of the plasmon energy and vanishes at the chemical potential. The fact that there are no quasi-particle $\delta$-peaks is the direct consequence of the acoustic nature of the collective plasmon mode. This behaviour of the spectral function is in the qualitative agreement with the angle resolved photo-emission spectra of some Bechgaard salts. In the present work we consider the modifications in the spectral function due to finite transverse electron dispersion. The transverse bandwidth is responsible for the appearance of an optical gap in the long wavelength plasmon mode. The plasmon dispersion of such kind introduces the quasi-particle $\delta$-peak into the spectral function at the chemical potential. The cross-over from the Fermi liquid to the non-Fermi liquid regime by decreasing the transverse bandwidth takes place through the decrease of the quasi-particle weight as the optical gap in the long wavelength plasmon mode is closing.Comment: 2 pages, 2 figures, ISCOM'0

Pauli coupling of the external magnetic field to spin-density waves

The effects of the external magnetic field on the spin-density-wave (SDW) order and on accompanying fluctuations are calculated within the random-phase approximation for the extended Hubbard model with imperfectly nested quasi-one-dimenisonal Fermi surfaces. Both Pauli and orbital mechanisms are treated in parallel. It is shown that the Pauli coupling leads to a finite hybridization of the SDW component in the direction of the external field and the charge-density wave. The mean value of this SDW component remains zero below the critical temperature in the isotropic system, but may be activated in systems with an internal spin anisotropy. The mean-field expression for the corresponding spin-flop field is derived. Furthermore, the Pauli coupling renormalizes two of six fluctuative SDW modes. In order to establish ways of qualitative and quantitative comparison between effects belonging to the Pauli and orbital couplings, we analyze the characteristic parameters for these two modes as well as for the other four modes affected only by the orbital coupling. In particular we evaluate the field dependence of longwavelength gaps, correlation lengths, and attenuation coefficients

Dynamical dislocation lines in the charge density wave systems

The topological aspects of the conversion of collective charge density wave transport into the Ohmic one in front of extended barriers are discussed in the frame of Gor'kov's model. It is shown that simultaneous phase slips form a family of dislocation lines. The shape and dynamics of these lines depend on the morphology of the barrier. They influence the amplitude of the narrow band noise, but do not modify its fundamental frequency

Domain patterns in incommensurate systems with the uniaxial real order parameter

The basic Landau model for the incommensurate-commensurate transition to the uniform or dimerized uniaxial ordering is critically reexamined. The previous analyses identified only sinusoidal and homogeneous solutions as thermodynamically stable and proposed a simple phase diagram with the first-order phase transition between these configurations. By performing the numerical analysis of the free-energy and the Euler-Lagrange equation we show that the phase diagram is more complex. It also contains a set of metastable solutions present in the range of coexistence of homogeneous and sinusoidal solutions. These new configurations are periodic patterns of homogeneous domains connected by sinusoidal segments. They are Lyapunov unstable, very probably due to the nonintegrability of the free-energy functional. We also discuss some other mathematical aspects of the model and compare it with the essentially simpler sine-Gordon model for the transitions to the states with higher commensurabilities. We argue that the present results might be a basis for the explanation of phenomena such as thermal hystereses, cascades of phase transitions, and memory effects

Magnetic-field dependence of phase correlation length in spin- and charge-density waves

Within a simple quasi-two-dimensional model, we study the phason propagator in both spin- and charge-density waves in the presence of a magnetic field perpendicular to the conducting plane. We find, though the magnetic field has little effect in the longitudinal correlation length (i.e., the correlation length in the chain direction), the magnetic field reduces significantly the transverse correlation length. This effect is most easily observable as increases in the fluctuation-induced specific heat and the resistivity in the chain direction in magnetic fields