Lambda-parameter of lattice QCD with Symanzik improved gluon actions

We compute the ratio Lambda_L/Lambda_MS, where the scale parameter Lambda_L is associated with a lattice formulation of QCD. We consider a 3-parameter family of gluon actions, which are most frequently used for O(a) improvement a` la Symanzik. The gluon action is put togeter with standard discretizations for fermions (Wilson/clover, overlap), to provide Lambda_L for several possible combinations of fermion and gluon actions. We employ the background field technique in order to calculate the 1PI 2-point function of the background field; this leads to the coupling constant renormalization function, Z_g, at 1-loop level. Our results are obtained for an extensive range of values for the Symanzik coefficients.Comment: 11 pages, 3 figures, 3 table

Effects of doping on thermally excited quasiparticles in the high-TcT_c superconducting state

The physical properties of low energy superconducting quasiparticles in high- $T_c$ superconductors are examined using magnetic penetration depth and specific heat experimental data. We find that the low energy density of states of quasiparticles of La$_{2-x}$Sr$_x$CuO$_4$ scales with $(x-x_c)/T_c$ to the leading order approximation, where $x_c$ is the critical doping concentration below which $T_c=0$. The linear temperature term of the superfluid density is renormalized by quasiparticle interactions and the renormalization factor times the Fermi velocity is found to be doping independent.Comment: 3 pages, 3 figures, minor change to the content, fig1 is reploted, to appear in Phys Rev

The Critical Hopping Parameter in O(a) improved Lattice QCD

We calculate the critical value of the hopping parameter, $\kappa_c$, in O(a) improved Lattice QCD, to two loops in perturbation theory. We employ the Sheikholeslami-Wohlert (clover) improved action for Wilson fermions. The quantity which we study is a typical case of a vacuum expectation value resulting in an additive renormalization; as such, it is characterized by a power (linear) divergence in the lattice spacing, and its calculation lies at the limits of applicability of perturbation theory. The dependence of our results on the number of colors $N$, the number of fermionic flavors $N_f$, and the clover parameter $c_{SW}$, is shown explicitly. We compare our results to non perturbative evaluations of $\kappa_c$ coming from Monte Carlo simulations.Comment: 11 pages, 2 EPS figures. The only change with respect to the original version is inclusion of the standard formulae for the gauge fixing and ghost parts of the action. Accepted for publication in Physical Review

Penetration Depth Measurements in MgB_2: Evidence for Unconventional Superconductivity

We have measured the magnetic penetration depth of the recently discovered binary superconductor MgB_2 using muon spin rotation and low field $ac$-susceptibility. From the damping of the muon precession signal we find the penetration depth at zero temperature is about 85nm. The low temperature penetration depth shows a quadratic temperature dependence, indicating the presence of nodes in the superconducting energy gap.Comment: 4 pages 3 figure

Hysteresis and Noise in Stripe and Clump Forming Systems

We use simulations to examine hysteresis and noise in a model system that produces heterogeneous orderings including stripe and clump phases. In the presence of a disordered substrate, these heterogeneous phases exhibit 1/f$^\alpha$ noise and hysteresis in transport. The noise fluctuations are maximal in the heterogeneous phases, while in the uniform phases the hysteresis vanishes and both $\alpha$ and the noise power decrease. We compare our results to recent experiments exhibiting noise and hysteresis in high-temperature superconductors where charge heterogeneities may occur.Comment: 4 pages, 5 postscript figure

An Ultra-High-Vacuum Rotating Sample Manipulator with Cryogenic Cooling

We report a homebuilt ultra-high-vacuum (UHV) rotating sample manipulator with cryogenic cooling. The sample holder is thermally anchored to a built-in cryogenic cold head through flexible copper beryllium strips, permitting continuous sample rotation. A similar contact mechanism is implemented for the electrical wiring to the sample holder for thermometry. The apparatus thus enables continuous sample rotation at regulated cryogenic temperatures in a UHV environment. We discuss the potential applications of this apparatus for cryogenic sputtering.Comment: Cryogenics, ultra-high vacuum, sample manipulato

Systematic behaviour of the in-plane penetration depth in d-wave cuprates

We report the temperature T and oxygen concentration dependences of the penetration depth of grain-aligned YBa_2Cu_3O_{7-\delta} with \delta= 0.0, 0.3 and 0.43. The values of the in-plane \lambda_{ab}(0) and out-of-plane \lambda_{c}(0) penetration depths, the low temperature linear term in \lambda_{ab}(T), and the ratio \lambda_{c}(0) /\lambda_{ab}(T) were found to increase with increasing $\delta$. The systematic changes of the linear term in \lambda_{ab}(T) with T_c found here and in recent work on HgBa_2Ca_{n-1} Cu_nO_{2n+2+\delta} (n = 1 and 3) are discussed.Comment: 4 pages, 4 figure

Renormalization of local quark-bilinear operators for Nf=3 flavors of SLiNC fermions

The renormalization factors of local quark-bilinear operators are computed non-perturbatively for $N_f=3$ flavors of SLiNC fermions, with emphasis on the various procedures for the chiral and continuum extrapolations. The simulations are performed at a lattice spacing $a=0.074$ fm, and for five values of the pion mass in the range of 290-465 MeV, allowing a safe and stable chiral extrapolation. Emphasis is given in the subtraction of the well-known pion pole which affects the renormalization factor of the pseudoscalar current. We also compute the inverse propagator and the Green's functions of the local bilinears to one loop in perturbation theory. We investigate lattice artifacts by computing them perturbatively to second order as well as to all orders in the lattice spacing. The renormalization conditions are defined in the RI$'$-MOM scheme, for both the perturbative and non-perturbative results. The renormalization factors, obtained at different values of the renormalization scale, are translated to the ${\bar{\rm MS}}$ scheme and are evolved perturbatively to 2 GeV. Any residual dependence on the initial renormalization scale is eliminated by an extrapolation to the continuum limit. We also study the various sources of systematic errors. Particular care is taken in correcting the non-perturbative estimates by subtracting lattice artifacts computed to one loop perturbation theory using the same action. We test two different methods, by subtracting either the ${\cal O}(g^2\,a^2)$ contributions, or the complete (all orders in $a$) one-loop lattice artifacts.Comment: 33 pages, 27 figures, 6 table

Perturbatively improving renormalization constants

Renormalization factors relate the observables obtained on the lattice to their measured counterparts in the continuum in a suitable renormalization scheme. They have to be computed very precisely which requires a careful treatment of lattice artifacts. In this work we present a method to suppress these artifacts by subtracting one-loop contributions proportional to the square of the lattice spacing calculated in lattice perturbation theory.Comment: 7 pages, 2 figures, LATTICE 201