arXiv subjects

W. Liu

Publications and source records attributed to W. Liu.

At least 109 records · Page 6Linked to original sources

arXiv2010

Optical properties of the iron-arsenic superconductor BaFe1.85Co0.15As2

The transport and complex optical properties of the electron-doped iron-arsenic superconductor BaFe1.85Co0.15As2 with Tc = 25 K have been examined in the Fe-As planes above and below Tc. A Bloch-Gruneisen analysis of the resistivity yields a weak electron-phonon coupling constant lambda_ph ~ 0.2. The low-frequency optical response in the normal state appears to be dominated by the electron pocket and may be described by a weakly-interacting Fermi liquid with a Drude plasma frequency of omega_p,D ~ 7840 cm-1 (~ 0.972 eV) and scattering rate 1/tau_D ~ 125 cm-1 (~ 15 meV) just above Tc. The frequency-dependent scattering rate 1/tau(omega) has kinks at ~ 12 and 55 meV that appear to be related to bosonic excitations. Below Tc the majority of the superconducting plasma frequency originates from the electron pocket and is estimated to be omega_p,S ~ 5200 cm-1 (lambda0 ~ 3000 Angstroms) for T << Tc, indicating that less than half the free carriers in the normal state have collapsed into the condensate, suggesting that this material is not in the clean limit. Supporting this finding is the observation that this material falls close to the universal scaling line for a BCS dirty-limit superconductor in the weak-coupling limit. There are two energy scales for the superconductivity in the optical conductivity and photo-induced reflectivity at Delta1 ~ 3.1 +/- 0.2 meV and Delta2 ~ 7.4 +/- 0.3 meV. This corresponds to either the gaping of the electron and hole pockets, respectively, or an anisotropic s-wave gap on the electron pocket; both views are consistent with the s+/- model.

cond-mat.supr-con↗

arXiv2010

A single-sided linear synchronous motor with a high temperature superconducting coil as the excitation system

Thrust measurements were performed on a coil made of YBa2Cu3O7-d coated conductor acting as the excitation system of a single-sided linear synchronous motor. The superconducting coil was a single pancake in the shape of a racetrack with 100 turns, the width and effective lengths were 42 mm and 84 mm, respectively. The stator was made of conventional copper wire. At 77 K and a gap of 10 mm, with an operating direct current of I_DC=30 A for the superconducting coil and alternating current of I_AC=9 A for the stator coils, thrust of 24 N was achieved. With addition of an iron core, thrust was increased by 49%. With addition of an iron back plate, thrust was increased by 70%.

cond-mat.supr-con↗

arXiv2009

High-P_T Physics with Identified Particles

The suppression of high-P_T particles in heavy ion collisions was one of the key discoveries at the Relativistic Heavy Ion Collider. This is usually parameterized by the average rate of momentum-transfer squared to this particle, qhat. Here we argue that measurements of identified particles at high P_T can lead to complementary information about the medium. The leading particle of a jet can change its identity through interactions with the medium. Tracing such flavor conversions could allow us to constrain the mean free path. Here we review the basic concepts of flavor conversions and discuss applications to particle ratios and elliptic flow. We make a prediction that strangeness is enhanced at high P_T at RHIC energies while its elliptic flow is suppressed.

nucl-th↗

arXiv2009

The effective spin concept to analyze coherent charge transport in mesoscopic systems

An effective spin concept is introduced to examine the mathematical and physical analogy between phase coherent charge transport in mesoscopic systems and quantum operations on spin based qubits. When coupled with the Bloch sphere concept, this isomorphism allows formulation of transport problems in a language more familiar to researchers in the field of spintronics and quantum computing. We exemplify the synergy between charge tunneling and spin qubit unitary operations by recasting well-known problems of tunneling through a delta scatterer, a resonant tunneling structure, a superlattice structure, and arrays of elastic scatterers, in terms of specific unitary operations (rotations) of a spinor on the Bloch sphere.

cond-mat.mes-hall↗

arXiv2009

Parameter Estimation in Diagonalizable Stochastic Hyperbolic Equations

A parameter estimation problem is considered for a linear stochastic hyperbolic equation driven by additive space-time Gaussian white noise. The damping/amplification operator is allowed to be unbounded. The estimator is of spectral type and utilizes a finite number of the spatial Fourier coefficients of the solution. The asymptotic properties of the estimator are studied as the number of the Fourier coefficients increases, while the observation time and the noise intensity are fixed.

math.PR↗

arXiv2008

Estimating Speed and Damping in the Stochastic Wave Equation

A parameter estimation problem is considered for a one-dimensional stochastic wave equation driven by additive space-time Gaussian white noise. The estimator is of spectral type and utilizes a finite number of the spatial Fourier coefficients of the solution. The asymptotic properties of the estimator are studied as the number of the Fourier coefficients increases, while the observation time and the noise intensity are fixed.

math.PR↗

arXiv2008

Elliptic Flow of Rare High-Momentum Probes in Nuclear Collisions

In high energy nuclear collisions the leading parton of a jet can change flavor through interactions with the surrounding medium. This can considerably boost the relative yield of rare high momentum particles, like strange quarks and photons. We revisit these jet conversions and discuss implications for the azimuthal asymmetry $v_2$ of rare probes. We predict that the $v_2$ of kaons at RHIC falls significantly below that of pions and protons at high transverse momentum. Experimental information on the relative $v_2$ of strange hadrons could provide constraints on the mean free path of fast quarks and gluons in a quark gluon plasma.

nucl-th↗

arXiv2008

Heavy Quark Production from Jet Conversions in a Quark-Gluon Plasma

Recently, it has been demonstrated that the chemical composition of jets in heavy ion collisions is significantly altered compared to jets in the vacuum. This signal can be used to probe the medium formed in nuclear collisions. In this study we investigate the possibility that fast light quarks and gluons can convert to heavy quarks when passing through a quark gluon plasma. We study the rate of light to heavy jet conversions in a consistent Fokker-Planck framework and investigate their impact on the production of high-$p_T$ charm and bottom quarks at RHIC and LHC.

nucl-th↗

arXiv2008

Probing Nuclear Matter with Jet Conversions

We discuss the flavor of leading jet partons as a valuable probe of nuclear matter. We point out that the coupling of jets to nuclear matter naturally leads to an alteration of jet chemistry even at high transverse momentum $p_T$. In particular, QCD jets coupling to a chemically equilibrated quark gluon plasma in nuclear collisions, will lead to hadron ratios at high transverse momentum $p_T$ that can differ significantly from their counterparts in $p+p$ collisions. Flavor measurements could complement energy loss as a way to study interactions of hard QCD jets with nuclear matter. Roughly speaking they probe the inverse mean free path $1/λ$, while energy loss probes the average squared momentum transfer $μ^2/λ$. We present some estimates for the rate of jet conversions in a consistent Fokker-Planck framework and their impact on future high-$p_T$ identified hadron measurements at RHIC and LHC. We also suggest some novel observables to test flavor effects.

nucl-th↗

arXiv2007

Low-Energy Thermal Photons from Meson-Meson Bremsstrahlung

Within an effective hadronic model including electromagnetic interactions via a U$_{\rm em}$(1) gauge, we reinvestigate photon Bremsstrahlung from a hot hadronic gas as expected to be formed in relativistic heavy-ion collisions at SPS energies. We calculate photon emission from the reactions $ππ\toππγ$ and $πK \toπKγ$ by an explicit (numerical) evaluation of the multi-dimensional phase space integral. This, in particular, allows to avoid the commonly employed soft photon approximation (SPA), as well as to incorporate final-state thermal enhancement factors. % during the hadronic stage of the fireball. Both improvements are shown to result in an appreciable increase of the photon production rate over previous hadronic calculations. Upon convolution over a thermal fireball we find an improvement in the description of recent low transverse-momentum WA98 data at SPS. The influence of both Landau-Pomeranchuk-Migdal and in-medium effects on "$σ$" and $ρ$-meson exchanges are briefly discussed.

nucl-th↗

arXiv2007

Thermal Dileptons at LHC

We predict dilepton invariant-mass spectra for central 5.5 ATeV Pb-Pb collisions at LHC. Hadronic emission in the low-mass region is calculated using in-medium spectral functions of light vector mesons within hadronic many-body theory. In the intermediate-mass region thermal radiation from the Quark-Gluon Plasma, evaluated perturbatively with hard-thermal loop corrections, takes over. An important source over the entire mass range are decays of correlated open-charm hadrons, rendering the nuclear modification of charm and bottom spectra a critical ingredient.

hep-ph↗

arXiv2007

$D_{sJ}$(2317) meson production at RHIC

Production of $D_{sJ}$(2317) mesons in relativistic heavy ion collisions at RHIC is studied. Using the quark coalescence model, we first determine the initial number of $D_{sJ}$(2317) mesons produced during hadronization of created quark-gluon plasma. The predicted $D_{sJ}$(2317) abundance depends sensitively on the quark structure of the $D_{sJ}$(2317) meson. An order-of-magnitude larger yield is obtained for a conventional two-quark than for an exotic four-quark $D_{sJ}$(2317) meson. To include the hadronic effect on the $D_{sJ}$(2317) meson yield, we have evaluated the absorption cross sections of the $D_{sJ}$(2317) meson by pion, rho, anti-kaon, and vector anti-kaon in a phenomenological hadronic model. Taking into consideration the absorption and production of $D_{sJ}$(2317) mesons during the hadronic stage of heavy ion collisions via a kinetic model, we find that the final yield of $D_{sJ}$(2317) mesons remains sensitive to its initial number produced from the quark-gluon plasma, providing thus the possibility of studying the quark structure of the $D_{sJ}$(2317) meson and its production mechanism in relativistic heavy ion collisions.

nucl-th↗

arXiv2007

Jet conversions in a quark-gluon plasma

Quark and gluon jets traversing through a quark-gluon plasma not only lose their energies but also can undergo flavor conversions. The conversion rates via the elastic $q(\bar q)g\to gq(\bar q)$ and the inelastic $q\bar q\leftrightarrow gg$ scatterings are evaluated in the lowest order in QCD. Including both jet energy loss and conversions in the expanding quark-gluon plasma produced in relativistic heavy ion collisions, we have found a net conversion of quark to gluon jets. This reduces the difference between the nuclear modification factors for quark and gluon jets in central heavy ion collisions and thus enhances the $p/π^+$ and ${\bar p}/π^-$ ratios at high transverse momentum. However, a much larger net quark to gluon jet conversion rate than the one given by the lowest-order QCD is needed to account for the observed similar ratios in central Au+Au and p+p collisions at same energy. Implications of our results are discussed.

nucl-th↗

arXiv2006

Effect of three-body elastic scattering on heavy quark momentum degradation in the quark-gluon plasma

Heavy quark drag coefficients in the quark-gluon plasma are evaluated in the perturbative QCD. It is found that for charm quarks the contribution from three-body elastic scattering is comparable to those from two-body elastic and radiative scatterings while for bottom quarks it becomes dominant. Using a schematic expanding fireball model, effects of both two-body and three-body scatterings on the transverse momentum spectra of heavy quarks produced in Au+Au collisions at center of mass energy $\sqrt{s_{NN}}=200$ GeV are studied. Results on electrons from resulting heavy meson decays are compared with available experimental data.

nucl-th↗

arXiv2005

Eta absorption by mesons

Using the $[SU(3)_{\mathrm{L}} \times SU(3)_{\mathrm{R}}]_{\mathrm{global}% }\times [SU(3)_V]_{\mathrm{local}}$ chiral Lagrangian with hidden local symmetry, we evaluate the cross sections for the absorption of eta meson ($% η$) by pion ($π$), rho ($ρ$), omega ($ω$), kaon ($K$), and kaon star ($K^*$) in the tree-level approximation. With empirical masses and coupling constants as well as reasonable values for the cutoff parameter in the form factors at interaction vertices, we find that most cross sections are less than 1 mb, except the reactions $ρη\to K\bar K^*(\bar KK^*)$, $ωη\to K\bar K^*(\bar KK^*)$, $K^*η\toρK$, and $K^*η\toωK$, which are a few mb, and the reactions $πη\to K\bar K$ and $Kη\toπK$, which are more than 10 mb. Including these reactions in a kinetic model based on a schematic hydrodynamic description of relativistic heavy ion collisions, we find that the abundance of eta mesons likely reaches chemical equilibrium with other hadrons in nuclear collisions at the Relativistic Heavy Ion Collider.

nucl-th↗

arXiv2005

Magnetic topologies and two-class coronal mass ejections: a numerical magnetohydrodynamic study

White-light observations of the solar corona show that there are two characteristic types of Coronal Mass Ejections (CMEs) in terms of speed-height profiles: so-called fast CMEs that attain high speeds low in the corona and slow CMEs that gradually accelerate from low initial speeds. Low and Zhang (2002) have recently proposed that fast and slow CMEs result from initial states with magnetic configurations characterized by normal prominences (NPs) and inverse prominences (IPs), respectively. To test their theory, we employed a two-dimensional, time-dependent, resistive magnetohydrodynamic code to simulate the expulsion of CMEs in these two different prominence environments. Our numerical simulations demonstrate that (i) a CME-like expulsion is more readily produced in an NP than in an IP environment, and, (ii) a CME originating from an NP environment tends to have a higher speed early in the event than one originating from an IP environment. Magnetic reconnection plays distinct roles in the two different field topologies of these two environments to produce their characteristic CME speed-height profiles. Our numerical simulations support the proposal of Low and Zhang (2002) although the reconnection development for the NP associated CME is different from the one sketched in their theory. Observational implications of our simulations are discussed.

astro-ph↗

arXiv2004

Pentaquark baryon production in nuclear reactions

Using a hadronic model with empirical coupling constants and form factors, we have evaluated the cross sections for the production of exotic pentaquark $Θ^+$ and/or $Ξ_5^+$ and $Ξ_5^{--}$ in reactions induced by photons, nucleons, pions, and kaons on nucleon targets. We have also predicted the $Θ^+$ yield in relativistic heavy ion collisions using a kinetic model that takes into account both $Θ^+$ production from the coalescence of quarks and antiquarks in the quark-gluon plasma and the effects due to subsequent hadronic absorption and regeneration.

nucl-th↗

arXiv2004

Transverse Beam Envelope Measurements and the Limitations of the 3-Screen Emittance Method for Space-Charge Dominated Beams

In its normal mode of operation the Argonne Wakefield Accelerator Facility uses a high charge (10-100 nC), short pulse (3-5 psec) drive bunch to excite high-gradient accelerating fields in various slow-wave structures. To generate this bunch, we designed a 1.5 cell, L-band, rf photocathode gun with an emittance compensating solenoid to give optimal performance at high-charge; it has recently completed commissioning. More recently, we have begun to investigate the possibility of using this gun in a high-brightness, low-charge operating mode, with charge equal to approximately 1 nC, for high-precision measurements of wakefields. Two related measurements are reported on in this paper: (1) measurements of the transverse beam envelope are compared to predictions from the beam dynamics code PARMELA; and (2) investigations into the use of a modified 3-screen emittance measurement method that uses a beam envelope model that includes both space-charge and emittance effects. Both measurements were made for the 1 nC, 8 MeV beam in the drift region directly following the rf photocathode gun.

physics.acc-ph↗