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Search Results: 1 - 10 of 461963 matches for " A. Sachrajda "
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Anomalous spin-orbit effects in a strained InGaAs/InP quantum well structure
S. A. Studenikin,P. T. Coleridge,P. Poole,A. Sachrajda
Physics , 2003, DOI: 10.1134/1.1577763
Abstract: There currently is a large effort to explore spin-orbit effects in semiconductor structures with the ultimate goal of manipulating electron spins with gates. A search for materials with large spin-orbit coupling is therefore important. We report results of a study of spin-orbit effects in a strained InGaAs/InP quantum well. The spin-orbit relaxation time, determined from the weak antilocalization effect, was found to depend non-monotonically on gate voltage. The spin orbit scattering rate had a maximum value of $5\times 10^{10}s^{-1}$ at an electron density of $n=3\times 10^{15} m^{-2}$. The scattering rate decreased from this for both increasing and decreasing densities. The smallest measured value was approximately $10^9 s^{-1}$ at an electron concentration of $n=6\times 10^{15} m^{-2}$. This behavior could not be explained by either the Rashba nor the bulk Dresselhaus mechanisms but is attributed to asymmetry or strain effects at dissimilar quantum well interfaces.
Spin depolarization in quantum dots
Sachrajda A.S.,Gould C.,Hawrylak P.,Feng Y.
Brazilian Journal of Physics , 1999,
Abstract: We have performed single electron spectroscopy experiments on single lateral quantum dots. We demonstrate that the lateral nature of the tunneling as well as the properties of the 2DEG leads can be used to extract new information in the 'spin-flip' regime. Calculations confirm that one needs to go beyond the Hartree-Fock approximation and include correlation effects to explain the experimental observations. The results are found to be consistent with the presence of spin depolarization events at the 'spin-flips'.
Continuum results for light hadrons from 2+1 flavor DWF ensembles
C. Kelly,P. A. Boyle,C. T. Sachrajda
Physics , 2009,
Abstract: From simultaneous fits to the data from the 2+1 flavor DWF ensembles generated by the RBC and UKQCD collaborations at two different lattice spacings, we present preliminary continuum results for light hadrons. We focus on light pseudoscalar decay constants and quark masses. Several approaches to the calculation of the lattice spacing are discussed and the errors associated with the chiral extrapolation are explained. We make use of reweighting in the dynamical strange quark mass such that our ensembles have a self-consistently determined strange quark mass.
The second moment of the pion's distribution amplitude
L. Del Debbio,M. Di Pierro,A. Dougall,C. Sachrajda
Physics , 1999, DOI: 10.1016/S0920-5632(00)91633-5
Abstract: We present preliminary results for the second moment of the pion's distribution amplitude. The lattice formulation and the phenomenological implications are briefly reviewed, with special emphasis on some subtleties that arise when the Lorentz group is replaced by the hypercubic group. Having analysed more than half of the available configurations, the result obtained is \xi^2_L = 0.06 \pm 0.02.
Weak localisation, interaction effects and the metallic phase in p-SiGe
P. T. Coleridge,A. S. Sachrajda,P. Zawadzki
Physics , 1999,
Abstract: Magnetoresistance results are presented for p-SiGe samples on the metallic side of the B=0 metal-insulator transition. It was possible to separate the weak localisation and Zeeman interaction effects but the results could not be explained quantitatively within the framework of standard theories for quantum corrections of a weakly interacting 2-dimensional system. Analysis using a theory for interaction corrections at intermediate temperatures, recently proposed by Zala, Narozhny and Aleiner, provided values of the Fermi liquid parameter $F_0^{\sigma}$ of order -0.5. Similar values also explain the linear increase of resistance with temperature characteristic of the metallic phase. e
Interaction effects and the metallic phase in p-SiGe
P. T. Coleridge,A. S. Sachrajda,P. Zawadzki
Physics , 2000,
Abstract: Magnetoresistance results are presented for p-SiGe samples on the metallic side of the B=0 metal-insulator transition. The results cannot be understood within the framework of standard theories for quantum corrections of a weakly interacting 2- dimensional system. In particular no logarithmic dependence on temperature is observed, at low fields, in either the longitudinal or Hall resistivities despite evidence in the magnetoresistance of weak localisation effects. Further, the Hall coefficient shows a strong logarithmic dependence on field. The results are better explain by renormalisation group theories and by an anomalous Hall effect associated with strong spin-orbit coupling in the presence of a background spin texture.
A Numerical Study of Partially Twisted Boundary Conditions
J. M. Flynn,A. Juttner,C. T. Sachrajda
Physics , 2005, DOI: 10.1016/j.physletb.2005.10.042
Abstract: We investigate the use of partially twisted boundary conditions in a lattice simulation with two degenerate flavours of improved Wilson sea quarks. The use of twisted boundary conditions on a cubic volume (L^3) gives access to components of hadronic momenta other than integer multiples of 2*pi/L. Partial twisting avoids the need for new gluon configurations for every choice of momentum, while, as recently demonstrated, keeping the finite-volume errors exponentially small for the physical quantities investigated in this letter. In this study we focus on the spectrum of pseudo scalar and vector mesons, on their leptonic decay constants and on Z_P, the matrix element of the pseudo scalar density between the pseudo scalar meson and the vacuum. The results confirm the momentum shift imposed by these boundary conditions and in addition demonstrate that they do not introduce any appreciable noise. We therefore advocate the use of partially twisted boundary conditions in applications where good momentum resolution is necessary.
Prospects for a lattice computation of rare kaon decay amplitudes: I. $K\toπ\ell^+\ell^-$ decays
N. H. Christ,X. Feng,A. Portelli,C. T. Sachrajda
Physics , 2015, DOI: 10.1103/PhysRevD.92.094512
Abstract: The rare kaon decays $K\to\pi\ell^+\ell^-$ and $K\to\pi\nu\bar{\nu}$ are flavor changing neutral current (FCNC) processes and hence promising channels with which to probe the limits of the standard model and to look for signs of new physics. In this paper we demonstrate the feasibility of lattice calculations of $K\to\pi\ell^+\ell^-$ decay amplitudes for which long-distance contributions are very significant. We show that the dominant finite-volume corrections (those decreasing as powers of the volume) are negligibly small and that, in the four-flavor theory, no new ultraviolet divergences appear as the electromagnetic current $J$ and the effective weak Hamiltonian $H_W$ approach each other. In addition, we demonstrate that one can remove the unphysical terms which grow exponentially with the range of the integration over the time separation between $J$ and $H_W$. We will now proceed to exploratory numerical studies with the aim of motivating further experimental measurements of these decays. Our work extends the earlier study by Isidori, Turchetti and Martinelli which focussed largely on the renormalization of ultraviolet divergences. In a companion paper we discuss the evaluation of the long-distance contributions to $K\to\pi\nu\bar{\nu}$ decays; these contributions are expected to be at the level of a few percent for $K^+$ decays.
Long-distance contributions to flavour-changing processes
C. Sachrajda
Physics , 2015,
Abstract: Standard lattice calculations in flavour physics or in studies of hadronic structure are based on the evaluation of matrix elements of local composite operators between hadronic states or the vacuum. In this talk I discuss developments aimed at the computation of long-distance, and hence non-local, contributions to such processes. In particular, I consider the calculation of the $K_L$-$K_S$ mass difference $\Delta m_K=m_{K_L}-m_{K_S}$ and the amplitude for the rare-kaon decay processes $K\to\pi\ell^+\ell^-$, where the lepton $\ell=e$ or $\mu$. Lattice calculations of the long-distance contributions to the indirect $CP$-violating parameter $\epsilon_K$ and to the rare decays $K\to\pi\nu\bar\nu$ are also beginning. Finally I discuss the possibility of including $O(\alpha)$ electromagnetic effects in computations of leptonic and semileptonic decay widths, where the novel feature is the presence of infrared divergences. This implies that contributions to the width from processes with a real photon in the final state must be combined with those with a virtual photon in the amplitude so that the infrared divergences cancel by the Bloch-Nordsieck mechanism. I present a proposed procedure for lattice computations of the $O(\alpha)$ contributions with control of the cancellation of the infrared divergences.
Phenomenology from the Lattice
Christopher Sachrajda
Physics , 2011,
Abstract: In recent years the precision of lattice calculations has improved hugely, and the results are making a very significant impact in particle physics phenomenology. Indeed there is no alternative general method which can be used in the evaluation of nonperturbative strong interaction effects for a wide variety of physical processes. In this talk I discuss a selection of topics in flavour physics, including \textit{mature} quantities for which lattice calculations have been performed for a long time (e.g. the determination of the $V_{us}$ CKM matrix element and $B_K$), quantities which we are now learning to study (e.g. $K\to\pi\pi$ decays amplitudes and the spectrum and mixing of $\eta-\eta^\prime$ mesons) and important phenomenological quantities for which a large amount of experimental data is available but which we do not yet understand how to approach in lattice simulations (e.g. nonleptonic B-decays). The improvement in precision and the extension of the range of processes which can be studied using lattice QCD has to be continued vigorously if precision flavour physics is to play a complementary role to large $p_\perp$ discovery experiments at the LHC in unravelling the next level of fundamental physics.
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