arXiv subjects

T. Blum

Publications and source records attributed to T. Blum.

At least 91 records · Page 5Linked to original sources

arXiv1997

Perturbative studies of the conductivity in the vortex-liquid regime

We calculate the Aslamazov-Larkin term of the conductivity in the presence of a magnetic field applied along the c-axis from the time-dependent Ginzburg-Landau equation perturbatively using two approaches. In the first a uniform electric field is explicitly applied; in the second the Kubo formula is used to extract the linear response. The former yields a version of the flux-flow formula for the uniform ab-plane conductivity, sigma_{xx}(k=0), that holds to all orders of perturbation theory. Obtaining the same result from the Kubo formula requires considerable cancellation of terms. We also use the Kubo calculation to examine the nonlocal ab-plane conductivity, sigma_{xx}( k \neq 0) (where the cancellations no longer occur), as well as the nonlocal c-axis conductivity sigma_{zz}( k \neq 0), and look for the perturbative precursors of the growing viscous length scales. In addition, we consider the effects of weak disorder -- both uncorrelated (point defects) and correlated (columnar and planar defects).

cond-mat.supr-con↗

arXiv1996

The equation of state for two flavor QCD at N_t=6

We calculate the two flavor equation of state for QCD on lattices with lattice spacing a=(6T)^{-1} and find that cutoff effects are substantially reduced compared to an earlier study using a=(4T)^{-1}. However, it is likely that significant cutoff effects remain. We fit the lattice data to expected forms of the free energy density for a second order phase transition at zero-quark-mass, which allows us to extrapolate the equation of state to m_q=0 and to extract the speed of sound. We find that the equation of state depends weakly on the quark mass for small quark mass.

hep-lat↗

arXiv1996

SU(3) Flavor Breaking in Hadronic Matrix Elements for $B - \bar B$ Oscillations

We present an analysis, using quenched configurations at $6/g^2=$5.7, 5.85, 6.0, and 6.3 of the matrix element $\MP\equiv\langle \bar P_{hl}|\bar h γ_μ(1-γ_5)l \bar h γ_μ(1-γ_5)l|P_{hl}\rangle$ for heavy-light pseudoscalar mesons. The results are extrapolated to the physical $B$ meson states, $\Bd$ and $\Bs$. We directly compute the ratio $\MS/\MB$, and obtain the preliminary result $\MS/\MB=1.54(13)(32)$. A precise value of this SU(3) breaking ratio is important for determining $V_{td}$ once the mixing parameter $x_s$ for $\Bs-\bar\Bs$ is measured experimentally. We also determine values for the corresponding B parameters, $B_{bs}(2 \rm{GeV})=B_{bd}(2 \rm{GeV})=1.02(13)$, which we cannot distinguish in the present analysis.

hep-lat↗

arXiv1996

Simulating QCD at finite density with static quarks

We study lattice QCD in the limit that the quark mass and chemical potential are simultaneously made large, resulting in a controllable density of quarks which do not move; this is similar in spirit to the quenched approximation for zero density QCD. In this approximation we find that the deconfinement transition seen at zero density becomes a smooth crossover for any nonzero density at which we simulated, and that at low enough temperature chiral symmetry remains broken at all densities.

hep-lat↗

arXiv1996

Recent MILC spectrum results

We report on results from three spectrum calculations with staggered quarks: 1) a quenched calculation with the standard action for the gluons and quarks; 2) a quenched calculation with improved actions for both the gluons and quarks; and 3) a calculation with two flavors of dynamical quarks using the standard actions for the gluons and quarks.

hep-lat↗

arXiv1996

Update on $f_B$

We describe the current status of the MILC collaboration computation of $f_B$, $f_{B_s}$, $f_D$, $f_{D_s}$ and their ratios. Progress over the past year includes: better statistics and plateaus at $β=6.52$ (quenched), $β=5.6$ ($N_F=2$) and $β=5.445$ ($N_F=2$), new runs with a wide range of dynamical quark masses at $β=5.5$ ($N_F=2$), an estimate of the systematic errors due to the chiral extrapolation, and an improved analysis which consistently takes into account both the correlations in the data at every stage and the systematic effects due to changing fitting ranges.

hep-lat↗

arXiv1996

Assorted weak matrix elements involving the bottom quark

As part of a larger project to estimate the fB decay constant, we are recalculating fB_static using a variational smearing method in an effort to improve accuracy. Preliminary results for the static B_B parameter and HQET two point functions are also presented.

hep-lat↗

arXiv1996

Thermodynamics for two flavor QCD

We conclude our analysis of the N_t=6 equation of state for two flavor QCD, first described at last year's conference. We have obtained new runs at am_q=0.025 and improved runs at am_q=0.0125. The results are extrapolated to m_q=0, and we extract the speed of sound as well. We also present evidence for a restoration of the SU(2) X SU(2) chiral symmetry just above the crossover, but not of the axial U(1) chiral symmetry.

hep-lat↗

arXiv1995

Nonlocal conductivity in the vortex-liquid regime

We investigate the nonlocal conductivity calculated from the time-dependent Ginzburg-Landau equation. When the fluctuations of the vector potential are negligible, the high-temperature (Gaussian) and low-temperature (flux-flow) forms of the uniform conductivity withan an ab plane (sigma_yy) are essentially identical. We find to what extent the nonlocal conductivity shares this feature. The results suggest that for pure samples in theses regimes the length scales of the nonlocal resistivity, rho_yy(y-y',z-z'), remain short-ranged in the y and z directions in contrast to the assumptions made by the hydrodynamic modelling of the multiterminal transport measurements. On the other hand, the resistivity is seen to have a long length scale in the x direction rho_yy(x-x'). The implications for the interpretation of recent experiments are discussed.

cond-mat↗

arXiv1995

Semiquantum Chaos in the Double-Well

The new phenomenon of semiquantum chaos is analyzed in a classically regular double-well oscillator model. Here it arises from a doubling of the number of effectively classical degrees of freedom, which are nonlinearly coupled in a Gaussian variational approximation (TDHF) to full quantum mechanics. The resulting first-order nondissipative autonomous flow system shows energy dependent transitions between regular behavior and semiquantum chaos, which we monitor by Poincaré sections and a suitable frequency correlation function related to the density matrix. We discuss the general importance of this new form of deterministic chaos and point out the necessity to study open (dissipative) quantum systems, in order to observe it experimentally.

chao-dyn↗

arXiv1995

The $N_t=6$ equation of state for two flavor QCD

We improve the calculation of the equation of state for two flavor QCD by simulating on $N_t=6$ lattices at appropriate values of the couplings for the deconfinement/chiral symmetry restoration crossover. For $am_q=0.0125$ the energy density rises rapidly to approximately 1 ${\rm GeV/fm^3}$ just after the crossover($m_π/m_ρ\approx 0.4$ at this point). Comparing with our previous result for $N_t=4$~\cite{eos}, we find large finite $N_t$ corrections as expected from free field theory on finite lattices. We also provide formulae for extracting the speed of sound from the measured quantities.

hep-lat↗

arXiv1995

Two-flavor staggered-fermion thermodynamics at $N_t = 12$

We present new results in an ongoing study of the nature of the high temperature crossover in QCD with two light fermion flavors. These results are obtained with the conventional staggered fermion action at the smallest lattice spacing to date---approximately 0.1 fm. Of particular interest are a study of the temperature of the crossover, an important indicator of continuum scaling, a determination of the induced baryon charge and baryon susceptibility, used to study the dissolution of hadrons at the crossover, the scalar susceptibility, a signal for the appearance of soft modes, and the chiral order parameter, used to test models of critical behavior.

hep-lat↗

arXiv1995

$f_B$ quenched and unquenched

Results for $f_B$, $f_{B_s}$, $f_D$, $f_{D_s}$, and their ratios are presented. High statistics quenched runs at $β=5.7$, $5.85$, $6.0$, and $6.3$, plus a run still in progress at $β=6.52$ make possible a preliminary extrapolation to the continuum. The data allows good control of all systematic errors except for quenching, although not all of the error estimates have been finalized. Results from configurations which include effects of dynamical quarks show a significant deviation from the quenched results and make possible a crude estimate of the quenching error.

hep-lat↗

arXiv1995

Variational Schemes in the Fokker-Planck Equation

We investigate variational methods for finding approximate solutions to the Fokker-Planck equation, especially in cases lacking detailed balance. These schemes fall into two classes: those in which a Hermitian operator is constructed from the (non-Hermitian) Fokker-Planck operator, and those which are based on soluble approximations to this operator. The various approaches are first tested on a simple quantum-mechanical problem and then applied to a toy Fokker-Planck equation. The problem of a particle moving in a potential and subject to external non-white noise is then investigated using the formalism developed earlier on in the paper.

cond-mat↗

arXiv1995

Improved Perturbation Theory for the Kardar-Parisi-Zhang Equation

We apply a number of schemes which variationally improve perturbation theory for the Kardar-Parisi-Zhang equation in order to extract estimates for the dynamic exponent z. The results for the various schemes show the same broad features, giving closer agreement with numerical simulations in low dimensions than self-consistent methods. They do, however, continue to predict that z=2 in some critical dimension d_c in disagreement with the findings of simulations.

cond-mat↗

arXiv1995

Status and prospect for determining $f_B$, $f_{B_s}$, $f_{B_s} / f_B$ on the lattice

Preliminary results from the MILC collaboration for $f_B$, $f_{B_s}$, $f_D$, $f_{D_s}$ and their ratios are presented. We compute in the quenched approximation at $β=6.3$, 6.0 and 5.7 with Wilson light quarks and static and Wilson heavy quarks. We attempt to quantify all systematic errors other than quenching, and have a first indication of the size of quenching errors.

hep-ph↗

arXiv1994

Failure of hydrodynamics within the vortex liquid phase

The recent discovery that some of the coefficients of the viscosity tensor are negative is shown to invalidate the hydrodynamic approach to the vortex liquid phase of a type-II superconductor. A satisfactory theory requires retention of all the spatial gradients of the velocities or electric fields and not just the first derivatives, as assumed in a hydrodynamic theory. We illustrate such a procedure by using time-dependent Ginzburg-Landau theory to determine the electric field distribution near a single ``twin-plane boundary" due to a current passing through the boundary.

cond-mat↗

arXiv1994

Thermal Phase Transition in Mixed Action SU(3) Lattice Gauge Theory and Wilson Fermion Thermodynamics

We study the thermal phase diagram of pure SU(3) lattice gauge theory with fundamental and adjoint couplings. We improve previous estimates of the position of the bulk transition line and determine the thermal deconfinement transition lines for $N_t=2,4,6,$ and 8. The endpoint of the bulk transition line $(β_f, β_a)=(4.00(7), 2.06(8))$ improves upon earlier estimates obtained using smaller lattice sizes. For $N_t > 4$ the deconfinement transition line splits cleanly away from the bulk transition line. With increasing $N_t$ the thermal deconfinement transition lines shift to increasingly weaker coupling, joining onto the bulk transition line at increasingly larger $β_a$ in a pattern consistent with the usual universality picture of lattice gauge theories. We also discuss the possible consequences of an induced adjoint term from the fermionic determinant and determine this induced term numerically with a microcanonical demon method for two flavors of dynamical Wilson fermions.

hep-lat↗