arXiv subjects

J. Qi

Publications and source records attributed to J. Qi.

28 records · Page 2Linked to original sources

arXiv2014

Time-resolved terahertz dynamics in thin films of the topological insulator Bi$_{2}$Se$_3$

We use optical pump--THz probe spectroscopy at low temperatures to study the hot carrier response in thin Bi$_2$Se$_3$ films of several thicknesses, allowing us to separate the bulk from the surface transient response. We find that for thinner films the photoexcitation changes the transport scattering rate and reduces the THz conductivity, which relaxes within 10 picoseconds (ps). For thicker films, the conductivity increases upon photoexcitation and scales with increasing both the film thickness and the optical fluence, with a decay time of approximately 5 ps as well as a much higher scattering rate. These different dynamics are attributed to the surface and bulk electrons, respectively, and demonstrate that long-lived mobile surface photo-carriers can be accessed independently below certain film thicknesses for possible optoelectronic applications.

cond-mat.mes-hall↗

arXiv2014

Polaronic transport induced by competing interfacial magnetic order in a La$_{0.7}$Ca$_{0.3}$MnO$_{3}$/BiFeO$_{3}$ heterostructure

Using ultrafast optical spectroscopy, we show that polaronic behavior associated with interfacial antiferromagnetic order is likely the origin of tunable magnetotransport upon switching the ferroelectric polarity in a La$_{0.7}$Ca$_{0.3}$MnO$_{3}$/BiFeO$_{3}$ (LCMO/BFO) heterostructure. This is revealed through the difference in dynamic spectral weight transfer between LCMO and LCMO/BFO at low temperatures, which indicates that transport in LCMO/BFO is polaronic in nature. This polaronic feature in LCMO/BFO decreases in relatively high magnetic fields due to the increased spin alignment, while no discernible change is found in the LCMO film at low temperatures. These results thus shed new light on the intrinsic mechanisms governing magnetoelectric coupling in this heterostructure, potentially offering a new route to enhancing multiferroic functionality.

cond-mat.str-el↗

arXiv2012

Mechanical and Electronic Properties of Ferromagnetic GaMnAs Using Ultrafast Coherent Acoustic Phonons

Ultrafast two-color pump-probe measurements, involving coherent acoustic phonon (CAP) waves, have provided information simultaneously on the mechanical properties and on the electronic structure of ferromagnetic GaMnAs. The elastic constant C11 of Ga1-xMnxAs (0.03<x<0.07) are observed to be systematically smaller than those of GaAs. Both C11 and Vs of GaMnAs are found to increase with temperature (78 K<T<295 K), again in contrast to the opposite behavior in GaAs. In addition, the fundamental bandgap (at E0 critical point) of Ga1-xMnxAs is found to shift slightly to higher energies with Mn concentration.

cond-mat.mtrl-sci↗

arXiv2012

Coexistence of coupled magnetic phases in epitaxial TbMnO3 films revealed by ultrafast optical spectroscopy

Ultrafast optical pump-probe spectroscopy is used to reveal the coexistence of coupled antiferromagnetic/ferroelectric and ferromagnetic orders in multiferroic TbMnO3 films through their time domain signatures. Our observations are explained by a theoretical model describing the coupling between reservoirs with different magnetic properties. These results can guide researchers in creating new kinds of multiferroic materials that combine coupled ferromagnetic, antiferromagnetic and ferroelectric properties in one compound.

cond-mat.str-el↗

arXiv2011

Measurement of graphite tight-binding parameters using high field magneto-reflectance

frared reflectance spectroscopy at 4K in fields up to 31T. Both Schrödinger-like (K-point) and Dirac-like (H-point) Landau level transitions have been observed, and their magnetic field dispersion are analyzed by a newly-derived limiting case of the Slonczewski-Weiss-McClure model. The values of the band parameters are evaluated without using sophisticated conductivity peak lineshape analysis. In this work, several less-explored band parameters are determined from the experimental results and they are known to result in electron-hole asymmetry and the opening of an energy gap between the electron and hole bands in multilayer and bilayer graphene systems.

cond-mat.mtrl-sci↗

arXiv2010

Ultrafast carrier and phonon dynamics in Bi2Se3 crystals

Ultrafast time-resolved differential reflectivity of Bi2Se3 crystals is studied using optical pump-probe spectroscopy. Three distinct relaxation processes are found to contribute to the initial transient reflectivity changes. The deduced relaxation timescale and the sign of the reflectivity change suggest that electron-phonon interactions and defect-induced charge trapping are the underlying mechanisms for the three processes. After the crystal is exposed to air, the relative strength of these processes is altered and becomes strongly dependent on the excitation photon energy.

cond-mat.mtrl-sci↗

arXiv2007

Coherent Magnetization Precession in GaMnAs induced by Ultrafast Optical Excitation

We use femtosecond optical pulses to induce, control and monitor magnetization precession in ferromagnetic Ga0.965Mn0.035As. At temperatures below ~40 K we observe coherent oscillations of the local Mn spins, triggered by an ultrafast photoinduced reorientation of the in-plane easy axis. The amplitude saturation of the oscillations above a certain pump intensity indicates that the easy axis remains unchanged above ~TC/2. We find that the observed magnetization precession damping (Gilbert damping) is strongly dependent on pump laser intensity, but largely independent on ambient temperature. We provide a physical interpretation of the observed light-induced collective Mn-spin relaxation and precession.

cond-mat.mtrl-sci↗

arXiv2007

Near-bandgap wavelength-dependent studies of long-lived traveling coherent longitudinal acoustic phonon oscillations in GaSb/GaAs systems

We report first studies of long-lived oscillations in optical pump-probe measurements on GaSb-GaAs heterostructures. The oscillations arise from a photogenerated coherent longitudinal acoustic phonon wave, which travels from the top surface of GaSb across the interface into the GaAs substrate, providing information on the optical properties of the material as a function of time/depth. Wavelength-dependent studies of the oscillations near the bandgap of GaAs indicate strong correlations to the optical properties of GaAs.

cond-mat.mes-hall↗

arXiv2004

Photoassociative Production and Trapping of Ultracold KRb Molecules

We have produced ultracold heteronuclear KRb molecules by the process of photoassociation in a two-species magneto-optical trap. Following decay of the photoassociated KRb*, the molecules are detected using two-photon ionization and time-of-flight mass spectroscopy of KRb$^+$. A portion of the metastable triplet molecules thus formed are magnetically trapped. Photoassociative spectra down to 91 cm$^{-1}$ below the K(4$s$) + Rb (5$p_{1/2}$) asymptote have been obtained. We have made assignments to all eight of the attractive Hund's case (c) KRb* potential curves in this spectral region.

physics.atom-ph↗

arXiv2002

Extracting Classical Correlations from a Bipartite Quantum System

In this paper we discuss the problem of splitting the total correlations for a bipartite quantum state described by the Von Neumann mutual information into classical and quantum parts. We propose a measure of the classical correlations as the difference between the Von Neumann mutual information and the relative entropy of entanglement. We compare this measure with different measures proposed in the literature.

quant-ph↗