arXiv subjects

Junhua Zhang

Publications and source records attributed to Junhua Zhang.

44 records · Page 3Linked to original sources

arXiv2013

Post-transient relaxation in graphene after an intense laser pulse

High intensity laser pulses were recently shown to induce a population inverted transient state in graphene [T. Li et al. Phys. Rev. Lett. 108, 167401 (2012)]. Using a combination of hydrodynamic arguments and a kinetic theory we determine the post-transient state relaxation of hot, dense, population inverted electrons towards equilibrium. The cooling rate and charge-imbalance relaxation rate are determined from the Boltzmann-equation including electron-phonon scattering. We show that the relaxation of the population inversion, driven by inter-band scattering processes, is much slower than the relaxation of the electron temperature, which is determined by intra-band scattering processes. This insight may be of relevance for the application of graphene as an optical gain medium.

cond-mat.mes-hall↗

arXiv2012

State-independent experimental tests of quantum contextuality in a three dimensional system

We experimentally observed state-independent violations of Kochen-Specker inequalities for the simplest indivisible quantum system manifesting quantum contextuality, a three-level (qutrit) system. We performed the experiment with a single trapped ^{171}Yb^{+} ion, by mapping three ground states of the ^{171}Yb^{+} ion to a qutrit system and carrying out quantum operatations by applying microwaves resonant to the qutrit transition frequencies. Our results are free from the detection loophole and cannot be explained by the non-contextual hidden variable models.

quant-ph↗

arXiv2010

Spectral analysis for the iron-based superconductors: Anisotropic spin fluctuations and fully gapped s^{\pm}-wave superconductivity

Spin fluctuations are considered to be one of the candidates that drive a sign-reversed s^{\pm} superconducting state in the iron pnictides. In the magnetic scenario, whether the spin fluctuation spectrum exhibits certain unique fine structures is an interesting aspect for theoretical study in order to understand experimental observations. We investigate the detailed momentum dependence of the short-range spin fluctuations using a 2-orbital model in the self-consistent fluctuation exchange approximation and find that a common feature of those fluctuations that are capable of inducing a fully gapped s^{\pm} state is the momentum anisotropy with lengthened span along the direction transverse to the antiferromagnetic momentum transfer. Performing a qualitative analysis based on the orbital character and the deviation from perfect nesting of the electronic structure for the 2-orbital and a more complete 5-orbital model, we gain the insight that this type of anisotropic spin fluctuations favor superconductivity due to their enhancement of intra-orbital, but inter-band, pair scattering processes. The momentum anisotropy leads to elliptically shaped magnetic responses which have been observed in inelastic neutron scattering measurements. Meanwhile, our detailed study on the magnetic and the electronic spectrum shows that the dispersion of the magnetic resonance mode in the nearly isotropic s^{\pm} superconducting state exhibits anisotropic propagating behavior in an upward pattern and the coupling of the resonance mode to fermions leads to a dip feature in the spectral function.

cond-mat.supr-con↗

arXiv2009

Orbital coupling and superconductivity in the iron pnictides

We demonstrate that strong inter-orbital interaction is very efficient to achieve superconductivity due to magnetic fluctuations in the iron pnictides. Fermi surface states that are coupled by the antiferromagnetic wave vector are often of different orbital nature, causing pair-hopping interactions between distinct Fe-3d orbitals to become important. Performing a self-consistent FLEX calculation below $T_{c}$ we determine the superconducting order parameter as function of intra- and inter-orbital couplings. We find an $s^{\pm}$-pairing state with $T_{c}\simeq80\mathrm{K}$ for realistic parameters.

cond-mat.supr-con↗

arXiv2006

Spin Structure Factor of the Frustrated Quantum Magnet Cs_2CuCl_4

The ground state properties and neutron structure factor for the two-dimensional antiferromagnet on the triangular lattice, with uni-directional anisotropy in the nearest-neighbor exchange couplings and a weak Dzyaloshinskii-Moriya (DM) interaction, are studied. This Hamiltonian has been used to interpret neutron scattering measurements on the spin 1/2 spiral spin-density-wave system, Cs_2CuCl_4, [R. Coldea, et al., Phys. Rev. B 68, 134424 (2003)]. Calculations are performed using a 1/S expansion, taking into account interactions between spin-waves. The ground state energy, the shift of the ordering wave-vector, Q, and the local magnetization are all calculated to order 1/S^2. The neutron structure factor, obtained using anharmonic spin-wave Green's functions to order 1/S, is shown to be in reasonable agreement with published neutron data, provided that slightly different parameters are used for the exchange and DM interactions than those inferred from measurements in high magnetic field.

cond-mat.str-el↗

arXiv2001

Calculating the I=2 Pion Scattering Length Using Tadpole Improved Clover Wilson Action on Coarse Anisotropic Lattices

In an exploratory study, using the tadpole improved clover Wilson quark action on small, coarse and anisotropic lattices, the $ππ$ scattering length in the I=2 channel is calculated within quenched approximation. A new method is proposed which enables us to make chiral extrapolation of our lattice results without calculating the decay constant on the lattice. Finite volume and finite lattice spacing errors are analyzed and the results are extrapolated towards the infinite volume and continuum limit. Comparisons of our lattice results with the new experiment and the results from Chiral Perturbation Theory are made. Good agreements are found.

hep-lat↗

arXiv2001

I=2 Pion Scattering Length from a Coarse Anisotropic Lattice Calculation

Using the tadpole improved clover Wilson quark action on coarse anisotropic lattices, the $ππ$ scattering length in the I=2 channel is calculated within quenched approximation. We show that such a calculation is feasible using small lattices on small computers provided that the finite volume and finite lattice spacing errors are under control. Our results are extrapolated towards the chiral, infinite volume and continuum limit. Comparisons of our results with previous lattice results from JLQCD collaboration, the new results from E865 experiment, and the results from Chiral Perturbation Theory are made. Good agreements are found.

hep-lat↗

arXiv2001

Tuning the Tadpole Improved Clover Wilson Action on Coarse Anisotropic Lattices

Wilson quark action, with tadpole improved clover term added, is studied on coarse anisotropic lattices. The bare velocity of light parameter in this action is determined non-perturbatively using the pseudo-scalar and vector meson dispersion relations for various values of the gauge coupling $β$ and bare quark mass parameter $κ$.

hep-lat↗