J. Y. Juang

Electrically controllable spontaneous magnetism in nanoscale mixed phase multiferroics

Magnetoelectrics and multiferroics present exciting opportunities for electric-field control of magnetism. However, there are few room-temperature ferromagnetic-ferroelectrics. Among the various types of multiferroics the bismuth ferrite system has received much attention primarily because both the ferroelectric and the antiferromagnetic orders are quite robust at room temperature. Here we demonstrate the emergence of an enhanced spontaneous magnetization in a strain-driven rhombohedral and super-tetragonal mixed phase of BiFeO₃. Using X-ray magnetic circular dichroism-based photoemission electron microscopy coupled with macroscopic magnetic measurements, we find that the spontaneous magnetization of the rhombohedral phase is significantly enhanced above the canted antiferromagnetic moment in the bulk phase, as a consequence of a piezomagnetic coupling to the adjacent tetragonal-like phase and the epitaxial constraint. Reversible electric-field control and manipulation of this magnetic moment at room temperature is also shown.

Snapshots of Dirac Fermions near the Dirac Point in Topological Insulators

The recent focus on topological insulators is due to the scientific interest in the new state of quantum matter as well as the technology potential for a new generation of THz optoelectronics, spintronics and quantum computations. It is important to elucidate the dynamics of the Dirac fermions in the topologically protected surface state. Hence we utilized a novel ultrafast optical pump mid-infrared probe to explore the dynamics of Dirac fermions near the Dirac point. The femtosecond snapshots of the relaxation process were revealed by the ultrafast optics. Specifically, the Dirac fermion-phonon coupling strength in the Dirac cone was found to increase from 0.08 to 0.19 while Dirac fermions were away from the Dirac point into higher energy states. Further, the energy-resolved transient reflectivity spectra disclosed the energy loss rate of Dirac fermions at room temperature was about 1 meV/ps. These results are crucial to the design of Dirac fermion devices.

Quasiparticle Dynamics and Phonon Softening in FeSe Superconductors

Quasiparticle dynamics of FeSe single crystals revealed by dual-color transient reflectivity measurements (ΔR/R) provides unprecedented information on Fe-based superconductors. The amplitude of the fast component in ΔR/R clearly gives a competing scenario between spin fluctuations and superconductivity. Together with the transport measurements, the relaxation time analysis further exhibits anomalous changes at 90 and 230 K. The former manifests a structure phase transition as well as the associated phonon softening. The latter suggests a previously overlooked phase transition or crossover in FeSe. The electron-phonon coupling constant λ is found to be 0.16, identical to the value of theoretical calculations. Such a small λ demonstrates an unconventional origin of superconductivity in FeSe.

Monophasic TiO2 films deposited on SrTiO3(100) by pulsed laser ablation

Single phase TiO2 thin films, of either rutile or anatase structure, have been prepared on SrTiO3(STO)(100) substrates by in situ pulsed laser deposition (PLD). Thermodynamically unfavorable, for films deposited on STO(100) substrate directly, pure anatase TiO2(00l) films were formed even when a rutile TiO2(110) substrate was used as a target. On the other hand, pure rutile TiO2(110) films were obtained by oxidizing PLD TiN films in-situ at temperatures higher than 700 °C. The optimized deposition conditions for preparing TiN and TiO2 films were reported. The crystalline structure, surface morphology, and electronic structure of these films were examined. A mechanism of the process of film formation is also proposed.

Enhancement of critical current density in direct‐current‐sputtered TlBa2Ca2Cu3O9±δ superconducting thin films

A multistep postannealing scheme has been developed for preparing nearly single phased, c‐axis oriented TlBa2Ca2Cu3Ox (Tl‐1223) superconducting thin films fabricated by the direct‐current‐sputtering process. Films obtained by the present process have shown, for the first time in this system, a critical current density (Jc) above 106 A/cm2 at 77 K with a zero‐resistance transition temperature Tc0≊110 K. The order of magnitude enhancement in Jc is attributed to the improvement of film morphology which, in turn, removed most of the weak links encountered previously.

Room-temperature ferromagnetism in Zn and Mn codoped SnO2 films

Room-temperature ferromagnetism was observed in Mn and Zn codoped SnO2 films grown on c-cut sapphires by using pulsed laser deposition technique. The valence of Mn ions in the codoped films is determined to be 2+ from x-ray absorption near edge spectroscopy. Moreover, the ferromagnetism is affected by the carrier concentration of the films. This result is consistent with the carrier-mediated model.

Optimization of depositing Y1Ba2Cu3O7-δ superconducting thin films by excimer laser ablation with Co2 laser-heated substrates

Abstract The optimum deposition conditions for growing Y 1 Ba 2 Cu 3 O 7 -δ superconducting thin films in situ by using xenon, Nd:YAG, and KrF excimer lasers with CO 2 laser-heated substrates were studied.The dependences of the T co and microstructure of the films on the deposition parameters such as substrate temperature, oxygen partial pressure, laser energy density and repetition rates have been investigated systematically. It was found that both the surface mobility on the substrate and the reaction temperatures for the particulates arriving at the substrates are the key factors for obtaining high-quality films. High-quality Y 1 Ba 2 Cu 3 O 7 -δ thin films with mirror-like surface morphology, T co ≈90K, and nearly perfect c -axis oriented normal to the film surface were routinely grown in situ on SrTiO 3 substrates with the following deposition conditions: substrate temperature at 670°C, oxyegen partial pressure of 0.1 torr, laser energy density of 4J/cm 2 per pulse, and repition rate of 10hz. More significantly, it too only about 3 min to grow a 0.3μm thick film and less than 50 s to down the film to room temperature after deposition by the present process. Subsequent processes, such as slow cooling in oxygen atmosphere or any further heat treatments, were unnecessary for the present process. The unique advantage of the present process further offered us good oppurtunities to delineate the effects of each deposition parameter on the detailed film growth mechanisms without having to deal with the complications introduced by prolonged post-deposition treatments. The details of all the interesting features observed will be presented and their physical implications will be discussed.

Ultrafast photoinduced mechanical strain in epitaxial BiFeO3 thin films

We studied ultrafast dynamics and photoinduced mechanical strain of BiFeO3 thin films by dual-color transient reflectivity measurements (ΔR/R). Anisotropic photostriction in BiFeO3 is found to be mainly driven by the optical rectification effect. Results of the photostriction at various thicknesses show that the estimated sound velocity along [110] direction of BiFeO3 is 4.76 km/s.

Magnetic ordering anisotropy in epitaxial orthorhombic multiferroic YMnO3 films

Orthorhombic YMnO3 thin films with (200), (020), and (001) orientations were, respectively, obtained by pulsed laser deposition on SrTiO3(110), LaAlO3(110), and SrTiO3(001) substrates. The results demonstrate that the strain between film and substrate can serve as an alternative in transforming the thermodynamically stable hexagonal YMnO3 into the orthorhombic phase, which previously could be obtained with high-pressure high temperature syntheses and epitaxy-stabilized thin film processes. More importantly, these films allow us, for the first time, to unambiguously disclose the intrinsic magnetic property along different crystallographic orientations. Our results show that, although the antiferromagnetic (AFM) ordering remains the same, there is an additional spin reordering transition which is very much dependent on the crystallographic orientation along which the measuring field was applied and on the in-plane crystallographic alignment of the films. Detailed analyses indicate that the origin of the obs...

Ultrafast thermoelastic dynamics of HoMnO(3) single crystals derived from femtosecond optical pump-probe spectroscopy

In this paper, the photo-induced ultrafast thermoelastic dynamics of hexagonal HoMnO3 single crystals has been studied by using optical pump–probe spectroscopy. The thermoelastic effect in the ab-plane of hexagonal HoMnO3, associated with giant magnetoelastic coupling around the Neel temperature (TN), is intimately correlated to the emergence of a negative component in the transient reflectivity changes (ΔR/R) obtained in temperature-dependent pump–probe experiments. Moreover, the variation in the oscillation period in ΔR/R caused by a strain pulse propagating along the c-axis exhibits an abrupt drop around TN, presumably due to the antiferromagnetic-ordering-induced changes in ferroelectricity.