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Hai-cang Ren

Publications and source records attributed to Hai-cang Ren.

61 records · Page 4Linked to original sources
arXiv1999

The Transition Temperature to the Superconducting Phase of QCD at High Baryon Density

Recent interest in the study of color superconductivity has focused on the regime of high baryon density where perturbative QCD may be employed. Based on the dominant one-gluon-exchange interaction, both the transition temperature and zero temperature gap have been determined to leading order in the coupling, g. While the leading non-BCS behavior, $T_C\simμg^{-5}e^{-κ/g}$, is easily obtained, the pre-exponential factor has proved more difficult to evaluate. Focusing on the transition temperature, we present a perturbative derivation of this factor, exact to leading order in g. This approach is first motivated by the study of a toy model and involves working to second order in the perturbative expansion. We compare this result to the zero temperature gap. Additionally, we extend the analysis to the case of higher angular momentum for longitudinal and transverse quark pairing.

hep-ph↗
arXiv1999

On the Perturbative Nature of Color Superconductivity

Color superconductivity is a possible phase of high density QCD. We present a systematic derivation of the transition temperature, T_C, from the QCD Lagrangian through study of the di-quark proper vertex. With this approach, we confirm the dependence of T_C on the coupling g, namely $T_C \sim μg^{-5} e^{-κ/g}$, previously obtained from the one-gluon exchange approximation in the superconducting phase. The diagrammatic approach we employ allows us to examine the perturbative expansion of the vertex and the propagators. We find an additional O(1) contribution to the prefactor of the exponential from the one-loop quark self energy and that the other one-loop radiative contributions and the two gluon exchange vertex contribution are subleading.

hep-ph↗
arXiv1998

Path Integrals, BRST Identities and Regularization Schemes in Nonstandard Gauges

The path integral of a gauge theory is studied in Coulomb-like gauges. The Christ-Lee terms of operator ordering are reproduced {\it{within}} the path integration framework. In the presence of fermions, a new operator term, in addition to that of Christ-Lee, is discovered. Such kind of terms is found to be instrumental in restoring the invariance of the effective Lagrangian under a field dependent gauge transformation, which underlies the BRST symmetry. A unitary regularization scheme which maintains manifest BRST symmetry and is free from energy divergences is proposed for a nonabelian gauge field.

hep-th↗
arXiv1998

On T-violation signals originating from magnetic orders through double exchange mechanism

The photon self energy tensor in a crystal with double exchange and ferromagnetic or canting magnetic orders is analyzed at zero temperature with emphasis on the signal of the time-reversal invariance breaking. Three comparable contributions, spin susceptibility, local magnetic field and spin-orbit coupling are estimated and the prospect of their experimental detection is discussed.

cond-mat.str-el↗
arXiv1998

Physical properties of the double exchange and its relevance to the specific heat of double perovskite materials

In response to the experimental discovery of double perovskite Ru- base material, The magnetic and electronic properties of the double exchange mechanism are studied theoretically. The electron band structure and the spin wave spectrum response at zero temperature with the canting background are computed from the first principle Hamiltonian. The long range Coulomb interaction between charge carriers is found to be crucial in stabilizing the canting order. A Monte Carlo simulation is performed on de-Gennes's effective magnetic Hamiltonian with classical spin approximation. The peaks of the specific heat as a function of temperature, which correspond to FM(AF) and canting transitions, and its response to the external magnetic field are consistent with the observation. For certain region of parameters, a more complex canting order than layer type is found at sufficiently low temperature. The effective interaction between carriers via exchanging a magnon wave is found to be rather similar to that via exchanging an acoustic phonon. The pros and cons to a genuine superconducting order in this system is analyzed.

cond-mat.str-el↗
arXiv1996

A Possible Mechanism for Pseudo-Gap in Cuprates

The electron spectrum in the normal phase of cuprates is analyzed in terms of the boson-fermion model. It is argued that the existence of the uncondensed pairs and the quasi-two-dimensional nature of the crystal structure are responsible for the pseudo-gap above T_C observed recently.

cond-mat↗