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Q. Li

Publications and source records attributed to Q. Li.

At least 163 records · Page 9Linked to original sources
arXiv2005

Probing the density dependence of the symmetry potential in intermediate energy heavy ion collisions

Based on the ultrarelativistic quantum molecular dynamics (UrQMD) model, the effects of the density-dependent symmetry potential for baryons and of the Coulomb potential for produced mesons are investigated for neutron-rich heavy ion collisions at intermediate energies. The calculated results of the $Δ^-/Δ^{++}$ and $π^{-}/π^{+}$ production ratios show a clear beam-energy dependence on the density-dependent symmetry potential, which is stronger for the $π^{-}/π^{+}$ ratio close to the pion production threshold. The Coulomb potential of the mesons changes the transverse momentum distribution of the $π^{-}/π^{+}$ ratio significantly, though it alters only slightly the $π^-$ and $π^+$ total yields. The $π^-$ yields, especially at midrapidity or at low transverse momenta and the $π^-/π^+$ ratios at low transverse momenta, are shown to be sensitive probes of the density-dependent symmetry potential in dense nuclear matter. The effect of the density-dependent symmetry potential on the production of both, K$^0$ and K$^+$ mesons, is also investigated.

nucl-th↗
arXiv2005

Interpretation of the "S-shaped" temperature dependence of luminescent peaks from semiconductors

The ``S-shape'' (decrease-increase-decrease) temperature dependence of luminescence peak shift from semiconductors is considered. A luminescence model for localized state ensemble was employed to interpret this anomalous temperature dependence of emission peak. Excellent agreement between the theoretical calculation and the experiments was achieved over the whole studied temperature region. The physical origin of the ``S-shaped'' shift is revealed.

cond-mat.mtrl-sci↗
arXiv2004

A model for luminescence of localized state ensemble

A distribution function for localized carriers, $f(E,T)=\frac{1}{e^{(E-E_a)/k_BT}+τ_{tr}/τ_r}$, is proposed by solving a rate equation, in which, electrical carriers' generation, thermal escape, recapture and radiative recombination are taken into account. Based on this distribution function, a model is developed for luminescence from localized state ensemble with a Gaussian-type density of states. The model reproduces quantitatively all the anomalous temperature behaviors of localized state luminescence. It reduces to the well-known band-tail and luminescence quenching models under certain approximations.

cond-mat.other↗
arXiv2004

Investigation of Galilean Invariance of multi-phase lattice Boltzmann methods

We examine the Galilean invariance of standard lattice Boltzmann methods for two-phase fluids. We show that the known Galilean invariant term that is cubic in the velocities, and is usually neglected, is the main source of Galilean invariance violations. We show that incorporating a correction term can improve the Galilean invariance of the method by up to an order of magnitude. Surprisingly incorporating this correction term can also noticeably increase the range of stability for multi-phase algorithms. We found that this is true for methods in which the non-ideality is incorporated by a forcing term as well as methods in which non-ideality is directly incorporated in a non-ideal pressure tensor.

cond-mat.soft↗
arXiv2004

Properties of MgB2 thin films with carbon doping

We have studied structural and superconducting properties of MgB2 thin films doped with carbon during the hybrid physical-chemical vapor deposition process. A carbon-containing metalorganic precursor bis(cyclopentadienyl)magnesium was added to the carrier gas to achieve carbon doping. As the amount of carbon in the films increases, the resistivity increases, Tc decreases, and the upper critical field increases dramatically as compared to the clean films. The self-field Jc in the carbon-doped films is lower than that in the clean films, but Jc remains relatively high to much higher magnetic fields, indicating stronger pinning. Structurally, the doped films are textured with nano-grains and highly resistive amorphous areas at the grain boundaries. The carbon doping approach can be used to produce MgB2 materials for high magnetic field applications.

cond-mat.supr-con↗
arXiv2004

The Role of Thermal Phase Fluctuations in Underdoped YBCO Films

The effect of thermal phase fluctuations (TPF's) on the ab-plane penetration depth, lambda(T), of thin YBa2Cu3O(7-delta) (YBCO) films is found to be much smaller than expected from the paradigm of cuprates as weakly-coupled 2D superconducting layers. A 2D vortex-pair-unbinding transition is observed, but the effective thickness for fluctuations is the film thickness, not a CuO bilayer thickness. In a strongly underdoped YBCO film, Tc= 34 K, TPF's suppress Tc by only about 3 K. They cannot be a significant factor in the suppression of Tc and emergence of the pseudogap with underdoping.

cond-mat.supr-con↗
arXiv2004

The Effect of Rotational Gravity Darkening on Magnetically Torqued Be Star Disks

In the magnetically torqued disk (MTD) model for hot star disks, as proposed and formulated by \citet{cassi02}, stellar wind mass loss was taken to be uniform over the stellar surface. Here account is taken of the fact that as stellar spin rate S_o (=\sqrt {Ω_o^2 R^3/GM}) is increased, and the stellar equator is gravity darkened, the equatorial mass flux and terminal speed are reduced, compared to the poles, for a given total \mdot. As a result, the distribution of equatorial disk density, determined by the impact of north and southbound flows, is shifted further out from the star. This results, for high S_o (\gtrsim 0.5), in a fall in the disk mass and emission measure, and hence in the observed emission line EW, scattering polarization, and IR emission. Consequently, contrary to expectations, critical rotation S_o \to 1 is not the optimum for creation of hot star disks which, in terms of EM for example, is found to occur in a broad peak around S_o\approx 0.5-0.6 depending slightly on the wind velocity law. The relationship of this analytic quasi-steady parametric MTD model to other work on magnetically guided winds is discussed. In particular the failures of the MTD model for Be-star disks alleged by \citet{owo03} are shown to revolve largely around open observational tests, rather in the basic MTD physics, and around their use of insufficiently strong fields.

astro-ph↗
arXiv2004

Formation of MgB2 at low temperatures by reaction of Mg with B6Si

Formation of MgB2 by reactions of Mg with B6Si and Mg with B were compared, the former also producing Mg2Si as a major product. Compared to the binary system, the ternary reactions for identical time and temperature were more complete at 750 C and below, as indicated by higher diamagnetic shielding and larger x-ray diffraction peak intensities relative to those of Mg. MgB2 could be produced at temperatures as low as 450 C by the ternary reaction. Analyses by electron microscopy, x-ray diffraction, and of the upper critical field show that Si does not enter the MgB2 phase.

cond-mat.supr-con↗
arXiv2003

Mid-Infrared Conductivity from Mid-Gap States Associated with Charge Stripes

The optical conductivity of La(2-x)Sr(x)NiO(4) has been interpreted in various ways, but so far the proposed interpretations have neglected the fact that the holes doped into the NiO(2) planes order in diagonal stripes, as established by neutron and X-ray scattering. Here we present a study of optical conductivity in La(2)NiO(4+d) with d=2/15, a material in which the charge stripes order three-dimensionally. We show that the conductivity can be decomposed into two components, a mid-infrared peak that we attribute to transitions from the filled valence band into empty mid-gap states associated with the stripes, and a Drude peak that appears at higher temperatures as carriers are thermally excited into the mid-gap states. The shift of the mid-IR peak to lower energy with increasing temperature is explained in terms of the Franck-Condon effect. The relevance of these results to understanding the optical conductivity in the cuprates is discussed.

cond-mat.str-el↗
arXiv2003

Thickness dependence of the properties of epitaxial MgB2 thin films grown by hybrid physical-chemical vapor deposition

We have studied the effect of deposition rate and layer thickness on the properties of epitaxial MgB2 thin films grown by hybrid physical-chemical vapor deposition on 4H-SiC substrates. The MgB2 film deposition rate depends linearly on the concentration of B2H6 in the inlet gas mixture. We found that the superconducting and normal-state properties of the MgB2 films are determined by the film thickness, not by the deposition rate. When the film thickness was increased, the transition temperature, Tc, increased and the residual resistivity, rho0, decreased. Above about 300 nm, a Tc of 41.8 K, a rho0 of 0.28 mikroOhm.cm, and a residual resistance ratio RRR of over 30 were obtained. These values represent the best MgB2 properties reported thus far.

cond-mat.supr-con↗
arXiv2003

Critical Current Density and Resistivity of MgB2 Films

The high resistivity of many bulk and film samples of MgB2 is most readily explained by the suggestion that only a fraction of the cross-sectional area of the samples is effectively carrying current. Hence the supercurrent (Jc) in such samples will be limited by the same area factor, arising for example from porosity or from insulating oxides present at the grain boundaries. We suggest that a correlation should exist, Jc ~ 1/{Rho(300K) - Rho(50K)}, where Rho(300K) - Rho(50K) is the change in the apparent resistivity from 300 K to 50 K. We report measurements of Rho(T) and Jc for a number of films made by hybrid physical-chemical vapor deposition which demonstrate this correlation, although the "reduced effective area" argument alone is not sufficient. We suggest that this argument can also apply to many polycrystalline bulk and wire samples of MgB2.

cond-mat.supr-con↗
arXiv2002

Infrared optical properties of Pr2CuO4

The ab-plane reflectance of a Pr2CuO4 single crystal has been measured over a wide frequency range at a variety of temperatures, and the optical properties determined from a Kramers-Kronig analysis. Above ~ 250 K, the low frequency conductivity increases quickly with temperature; the resistivity follows the form e^(E_a/k_BT), where E_a ~ 0.17 eV is much less than the inferred optical gap of ~ 1.2 eV. Transport measurements show that at low temperature the resistivity deviates from activated behavior and follows the form e^[(T_0/T)^1/4], indicating that the dc transport in this material is due to variable-range hopping between localized states in the gap. The four infrared-active Eu modes dominate the infrared optical properties. Below ~ 200 K, a striking new feature appears near the low-frequency Eu mode, and there is additional new fine structure at high frequency. A normal coordinate analysis has been performed and the detailed nature of the zone-center vibrations determined. Only the low-frequency Eu mode has a significant Pr-Cu interaction. Several possible mechanisms related to the antiferromagnetism in this material are proposed to explain the sudden appearance of this and other new spectral features at low temperature.

cond-mat.supr-con↗
arXiv2002

Coherence-Incoherence and Dimensional Crossover in Layered Strongly Correlated Metals

Correlations between electrons and the effective dimensionality are crucial factors that shape the properties of an interacting electron system. For example, the onsite Coulomb repulsion, U, may inhibit, or completely block the intersite electron hopping, t, and depending on the ratio U/t, a material may be a metal or an insulator. The correlation effects increase as the number of allowed dimensions decreases. In 3D systems, the low energy electronic states behave as quasiparticles (QP), while in 1D systems, even weak interactions break the quasiparticles into collective excitations. Dimensionality is particularly important for a class of new exotic low-dimensional materials where 1D or 2D building blocks are loosely connected into a 3D whole. Small interactions between the blocks may induce a whole variety of unusual transitions. Here, we examine layered systems that in the direction perpendicular to the layers display a crossover from insulating-like, at high temperatures, to metallic-like character at low temperatures, while being metallic over the whole temperature range within the layers. We show that this change in effective dimensionality correlates with the existence or non-existence of coherent quasiparticles within the layers.

cond-mat.str-el↗
arXiv2002

Irreversibility line of the MgB$_{2}$ superconductor

Magneto-resistivity of a c-axis oriented MgB$_{2}$ thin film was studied in perpendicular and parallel magnetic fields up to $\sim $23 T with temperatures down to 0.38 K. Resistive critical magnetic fields were determined. Large separations between irreversibility lines and upper critical magnetic fields are observed. An effective quantum parameter as a function of temperature is proposed to explain the separations under the theoretical framework of quantum fluctuations, in good agreement with our experimental and previously published experimental data.

cond-mat.supr-con↗
arXiv2002

Freezing of a Stripe Liquid

The existence of a stripe-liquid phase in a layered nickelate, La(1.725)Sr(0.275)NiO(4), is demonstrated through neutron scattering measurements. We show that incommensurate magnetic fluctuations evolve continuously through the charge-ordering temperature, although an abrupt decrease in the effective damping energy is observed on cooling through the transition. The energy and momentum dependence of the magnetic scattering are parametrized with a damped-harmonic-oscillator model describing overdamped spin-waves in the antiferromagnetic domains defined instantaneously by charge stripes.

cond-mat.str-el↗
arXiv2001

Fluctuations Indicate Strong Interlayer Coupling in Cuprate Superconductors

From study of the Kosterlitz-Thouless-Berezinskii (KTB) transition in the superfluid density, n_s(T), of ultrathin c-axis oriented YBa_{2}Cu_{3}O_{7-δ} (YBCO) films, we find that interlayer coupling is unexpectedly strong. The KTB transition occurs at a high temperature, as if the films were isotropic rather than quasi-two-dimensional. This result agrees with a comparison of the superfluid density of YBCO with Bi_{2}Sr_{2}CaCu_{2}O_{8} and with numerical simulations of Josephson junction arrays, and challenges the thermal phase fluctuation interpretation of critical behavior near T_c in YBCO.

cond-mat.supr-con↗
arXiv2001

On the Doping and Temperature Dependence of the Mass Enhancement Observed in the Cuprate Bi2212

High-resolution photoemission is used to study the electronic structure of the cuprate superconductor, Bi2212, as a function of hole doping and temperature. A kink observed in the band dispersion in the nodal direction in the superconducting state is associated with coupling to a resonant mode observed in neutron scattering. From the measured real part of the self energy it is possible to extract a coupling constant which is largest in the underdoped regime, then decreasing continuously into the overdoped regime.

cond-mat.supr-con↗