arXiv subjects

Jun Chang

Publications and source records attributed to Jun Chang.

25 records · Page 2Linked to original sources

arXiv2010

Ultrafast Cascading Theory in Intersystem Crossings in Transition-Metal Complexes

We investigate the cascade decay mechanism for ultrafast intersystem crossing mediated by the spin-orbit coupling in transition-metal complexes. A quantum-mechanical description of the cascading process that occurs after photoexcitation is presented. The conditions for ultrafast cascading are given, which relate the energy difference between the levels in the cascading process to the electron-phonon self energy. These limitations aid in the determination of the cascade path. For $Fe^{2+}$ spin-crossover complexes, this leads to the conclusion that the ultrafast decay primarily occurs in the manifold of antibonding metal-to-ligand charge-transfer states. We also give an interpretation why some intermediate states are bypassed.

cond-mat.str-el↗

arXiv2010

A probability-conserving dissipative Schrödinger equation

Dissipative effects on a microscopic level are included in the Schrödinger equation. When the decay between different local levels as a result of the coupling to a bath, the Schrödinger equation no longer conserves energy, but the probability of the states is conserved. The procedure is illustrated with several examples that include direct electronic decay and damping of local phonons (vibrational levels). This method significantly reduces the calculational effort compared to conventional density matrix techniques.

cond-mat.other↗

arXiv2010

Ultrafast Electron Dynamics Theory of Photo-excited Ruthenium Complexes

An explanation is provided for the ultrafast photo-excited electron dynamics in low-spin Ruthenium (II) organic complexes. The experimentally-observed singlet to triplet decay in the metal-to-ligand charge-transfer (MLCT) states contradicts the expectation that the system should oscillate between the singlet and triplet states in the presence of a large spin-orbit coupling and the absence of a significance change in metal-ligand bond length. This dilemma is solved with a novel quantum decay mechanism that causes a singlet to triplet decay in about 300 femtoseconds. The decay is mediated by the triplet metal-centered state ($^3$MC) state even though there is no direct coupling between the $^1$MLCT and $^3$MC states. The coupling between the $^3$MLCT and $^3$MC via excited phonon states leads to vibrational cooling that allows the local system to dissipate the excess energy. In the relaxed state, the population of the $^3$MC state is low and the metal-ligand bond length is almost unchanged with respect to the initial photoexcited state, in agreement with experiment.

cond-mat.str-el↗

arXiv2010

Fast Intersystem Crossing in Transition-Metal Complexes

The mechanism behind fast intersystem crossing in transition-metal complexes is shown to be a result of the dephasing of the photoexcited state to the phonon continuum of a different state with a significantly different transition metal-ligand distance. The coupling is a result of the spin-orbit interaction causing a change in the local moment. Recurrence to the initial state is prevented by the damping of the phonon oscillation. The decay time is faster than the oscillation frequency of the transition metal-ligand stretch mode, in agreement with experiment. For energies above the region where the strongest coupling occurs, a slower "leakage-type" decay is observed. If the photoexcited state is lower in energy than the state it couples to, then there is no decay.

cond-mat.str-el↗

arXiv2008

Superconductivity in $β$-pyrochlore superconductor KOs$_{2}$O$_{6}$: treatment within strong-coupling Eliashberg theory

We study the influence of the rattling phonons on superconductivity in $β$-pyrochlore KOs$_{2}$O$_{6}$ compound based on the strong-coupling Eliashberg approach. In particular, analyzing the specific heat data we find that the rattling phonon frequency changes discontinuously at the critical temperature of the first order phase transition. Solving the strong-coupling Eliashberg equations with temperature dependent $α^{2}F(ω)$, we investigate the consequence of this first order phase transition for the anomalous temperature dependence of the superconducting gap. We discuss our results in context of the recent experimental data.

cond-mat.supr-con↗

arXiv2008

Eliashberg theory of superconductivity and inelastic rare-earth impurity scattering in filled skutterudite La$_{1-x}$Pr$_{x}$Os$_{4}$Sb$_{12}$

We study the influence of inelastic rare-earth impurity scattering on electron-phonon mediated superconductivity and mass renormalization in (La$_{1-x}$Pr$_{x}$)Os$_{4}$Sb$_{12}$ compounds. Solving the strong coupling Eliashberg equations we find that the dominant quadrupolar component of the inelastic scattering on Pr impurities yields an enhancement of the superconducting transition temperature T$_c$ in LaOs$_{4}$Sb$_{12}$ and increases monotonically as a function of Pr concentration. The calculated results are in good agreement with the experimentally observed T$_c (x)$ dependence. Our analysis suggests that phonons and quadrupolar excitations cause the attractive electron interaction which results in the formation of Cooper pairs and singlet superconductivity in PrOs$_{4}$Sb$_{12}$.

cond-mat.supr-con↗

arXiv2007

Theory of magnetic excitons in the heavy-fermion superconductor $UPd_{2}Al_{3}$

We analyze the influence of unconventional superconductivity on the magnetic excitations in the heavy fermion compound UPd$_2$Al$_3$. We show that it leads to the formation of a bound state at energies well below 2$Δ_0$ at the antiferromagnetic wave vector {\textbf Q}=$(0,0,π/c)$. Its signature is a resonance peak in the spectrum of magnetic excitations in good agreement with results from inelastic neutron scattering. Furthermore we investigate the influence of antiferromagnetic order on the formation of the resonance peak. We find that its intensity is enhanced due to intraband transitions induced by the reconstruction of Fermi surface sheets. We determine the dispersion of the resonance peak near {\textbf Q} and show that it is dominated by the magnetic exciton dispersion associated with local moments. We demonstrate by a microscopic calculation that UPd$_2$Al$_3$ is another example in which the unconventional nature of the superconducting order parameter can be probed by means of inelastic neutron scattering and determined unambiguously.

cond-mat.supr-con↗