Kai Ji

Quantum Monte Carlo study on speckle variation due to photorelaxation of ferroelectric clusters in paraelectric barium titanate

Time-dependent speckle pattern of paraelectric barium titanate observed in a soft x-ray laser pump-probe measurement is theoretically investigated as a correlated optical response to the pump and probe pulses. The scattering probability is calculated based on a model with coupled soft x-ray photon and ferroelectric phonon mode. It is found that the speckle variation is related to the relaxation dynamics of ferroelectric clusters created by the pump pulse. Additionally, critical slowing down of cluster relaxation arises on decreasing temperature toward the paraelectric-ferroelectric transition temperature. The relation between critical slowing down, local dipole fluctuation, and crystal structure is revealed by quantum Monte Carlo simulation.

Isotopic shift in angle-resolved photoemission spectra of Bi2Sr2CaCu2O8 due to quadratic electron–phonon coupling

Abstract In connection with the experiment on oxygen isotope effect of Bi 2 Sr 2 CaCu 2 O 8 with the angle-resolved photoemission spectroscopy (ARPES), we theoretically study the isotope-induced band shift in ARPES by the Hartree–Fork and quantum Monte Carlo methods. We find that this band shift can be clarified based on a quadratically coupled electron–phonon ( e –ph) model. The large ratio of band shift versus phonon energy change is connected with the softening effect of phonon, and the positive–negative sign change is due to the momentum dependence of the e –ph coupling.

DYNAMICS OF FERROELECTRIC NANO CLUSTER IN BaTiO3 OBSERVED AS A REAL TIME CORRELATION BETWEEN TWO SOFT X-RAY LASER PULSES

We carry out a theoretical investigation to clarify the dynamic property of photo-created nano-sized ferroelectric cluster observed in the paraelectric BaTiO3 as a real time correlation of speckle pattern between two soft X-ray laser pulses, at just above the paraelectric-ferroelectric phase transition temperature. Based on a model with coupled soft X-ray photon and ferroelectric phonon mode, we study the time-dependence of scattering probability by using a perturbative expansion approach. The cluster-associated phonon softening as well as central peak effects are well-reproduced in the phonon spectral function via quantum Monte Carlo simulation. Besides, it is found that the time-dependence of speckle correlation is determined by the relaxation dynamics of ferroelectric clusters. Near the transition point, cluster excitation is stable, leading to a long relaxation time, while at high temperature, cluster structure is subject to the thermal fluctuation, ending up with a short relaxation time.

Probing Ultrafast Dynamics of Polarization Clusters in BaTiO3 by Pulsed Soft X-Ray Laser Speckle Technique

The technical development of ultrashort laser pulses covering infrared to extreme ultraviolet has opened a door to study the photo-induced dynamic physical and chemical processes. By using an optical excitation at a particular wavelength, exotic states can be trigged coherently in atoms, molecules, clusters as well as complex condensed systems. The evolution of excited states then can be imaged in the real-time domain by subsequent single or trains of pulse. This laser pump-probe technique features an unprecedented spatial and temporal resolution, thus not only allows us a fundamental insight into the microscopic ultrafast dynamics, but also brings about a potential of selective controlling on the microstructures at atomic scale (Krausz & Ivanov, 2009). In this chapter, a recent advance of soft x-ray laser speckle pump-probe measurement on barium titanate (BaTiO3) is reviewed, with primary concerns on the theoretical description of the photo-matter interaction and photo-induced relaxation dynamics in the crystal. Here, the observed time-dependent speckle pattern is theoretically investigated as a correlated optical response to the pump and probe pulses. The scattering probability is calculated based on a model with coupled soft x-ray photon and ferroelectric phonon mode of BaTiO3. It is found that the speckle variation is related with the relaxation dynamics of ferroelectric clusters created by the pump pulse. Additionally, a critical slowing down of cluster relaxation arises on decreasing temperature towards the paraelectric-ferroelectric transition temperature. Relation between the critical slowing down, local dipole fluctuation and crystal structure are revealed by a quantum Monte Carlo simulation. This chapter is organized as follows. In Section 1, the properties ferroelectric material and experimental techniques are introduced. The theoretical model and methods are elaborated in Section 2. In Section 3, the numerical results on speckle correlation, relaxation dynamics

Theoretical study on isotopic shift in angle-resolved photoemission spectra of Bi2Sr2CaCu2O8

We develop a path-integral theory to study the angle-resolved photoemission spectra (ARPES) of high-Tc superconductors based on a two-dimensional model for the CuO2 conduction plane, including both electron-electron (e-e) and electron-phonon (e-ph) interactions. Comparing our result with the experimental one of Bi2Sr2CaCu2O8, we find that the experimentally observed isotopic band shift in ARPES is due to the off-diagonal quadratic e-ph coupling, whereas the presence of e-e repulsion partially suppresses this effect.

Novel theoretical approach in photoemission spectroscopy: application to isotope effect and boron-doped diamond

A new path-integral theory is developed to calculate the photoemission spectra (PES) of correlated many-electron systems. The application to the study on Bi2Sr2CaCu2O8 (Bi2212) and boron-doped diamond (BDD) is discussed in details. It is found that the isotopic shift in the angle-resolved photoemission spectra of Bi2212 is due to the off-diagonal quadratic electron-phonon (e-ph) coupling, whereas the presence of electron-electron repulsion partially suppresses this effect. For the BDD, a semiconductor-metal phase transition, which is induced by increasing the e-ph coupling and dopant concentration, is reproduced by our theory. Additionally, the presence of Fermi edge and phonon step-like structure in PES is found to be due to a co-existence of itinerant and localized electronic states in BDD.