Research overview

Pairing symmetries of unconventional superconductors via quasiparticle interference:

One of our research problem in my group belongs to the theory of STM and particularly the quasiparticle interference (QPI). In recent years, QPI spectroscopy has emerged as an important tool to understand the pairing symmetry in unconventional and high Tc superconductors. This method is based on scanning tunneling microscopy (STM) and essentially involves investigating how the local density of states (LDOS) is modulated due to the presence of impurities. Recently we have focused on the theory of quasiparticle interference as one of the main future techniques to estimate ordered phases such as magnetic and superconducting phases. Up to now, we have developed several theoretical analyses of QPI for different unconventional superconductors, heavy fermion systems and so on.

Non-equilibrium phenomena in closed quantum systems:

We are also interested in the theory of time-resolved spectroscopy and short time dynamics. In recent years, numerous studies of non-equilibrium dynamics of multi-band superconductors have been performed using femtosecond time-resolved spectroscopy. The relaxation kinetics measured in these experiments gives important information on the electronic band structures, electron-phonon coupling strengths, as well as on the symmetry of the superconducting order parameters. Using density-matrix theory, we are interested in understanding the relaxation dynamics and the non-equilibrium evolution of multiband superconductors at times shorter than the relaxation time.

Collective spin resonance excitation:

Another active area of our research is the theory of spin resonance, which has been observed in numerous materials such as unconventional superconductors, heavy fermions, and Kondo lattice compounds. We have succeeded in explaining many features of these phenomena in various systems. Specifically, we have shown that the hybridization gap and hidden order parameters play the same role in Kondo systems and heavy Fermion metals, as does the superconducting gap in unconventional superconductors.

Many body effects and broken symmetry states in condensed matter:

Theoretical work is focused on the effect of electronic correlations on quasiparticle renormalization in unconventional superconductors as well as heavy fermions compounds and appearance of broken symmetries. In particular unconventional superconductivity and hidden multipolar order is investigated. Furthermore, the collective modes like spin excitons appearing in the gap of the ordered phase are studied. Furthermore, the physical effects of helical Dirac surface states in topological insulators like response functions, transport properties and resonance states from impurity scattering are investigated.

Quantum criticality and quantum correlations:

Another research interest line in our group is quantum phase transition and quantum criticality. Where the gapped critical environment could remarkably prevent an enhanced decay of decoherence factor and quantum correlations at the critical point, which is nontrivially different from the ones in a gapless critical environment. The quantum correlations display very fast decaying to their local minimum at the critical point while maximum decaying occurs away from this point. In particular, our results imply that the collapse of decoherence factor is not an indicator of a quantum phase transition of the environment as opposed to what happens in a gapless criticality.

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