Yequan Chen

Optical harmonic generation in a quasi-phase-matched three-component Fibonacci superlattice LiTaO3

Optical harmonic generation in a three-component Fibonacci optical superlattice (3CFOS) is studied experimentally. Quasi-phase-matched second-harmonic spectrum and third-harmonic generation have been measured in a LiTaO3 3CFOS. Results show that the structure of the spectrum in a 3CFOS is richer than that in a two-component Fibonacci optical superlattice. The intrinsic self-similarity is shown in its Fourier spectrum.

Parametric and cascaded parametric interactions in a quasiperiodic optical superlattice

Quasi-phase-matching optical parametric and cascaded parametric processes in a two-component quasiperiodic superlattice were studied in theory and experiment. This letter demonstrates how to obtain red at 666 nm and blue at 443 nm simultaneously from the superlattice using a 532 nm laser as a pump through these two processes mentioned above. The result confirms that some nonlinear frequency conversion processes occurring in a high-dimension χ(2) nonlinear photonic crystal may be efficiently achieved in such a one-dimension quasiperiodic optical superlattice.

Tuning the transport behavior of centimeter-scale WTe2 ultrathin films fabricated by pulsed laser deposition

We report on an avenue to obtain the centimeter-scale, uniform, and high-quality WTe2 ultrathin films by a pulsed laser deposition technique and the post-annealing under the tellurium (Te) vapor. The WTe2 ultrathin films showed the typical metallic behavior when Te vacancies were mostly eliminated. Magnetoresistance measurements showed that WTe2 ultrathin films underwent the competition between weak localization and weak antilocalization that could be modulated by the amount of Te vacancies. Our study may open an avenue to improve the charge transport of WTe2 for its two-dimensional device applications.

Topological Phase Transition-Induced Triaxial Vector Magnetoresistance in (Bi1–xInx)2Se3 Nanodevices

We report the study of a triaxial vector magnetoresistance (MR) in nonmagnetic (Bi1-xInx)2Se3 nanodevices at the composition of x = 0.08. We show a dumbbell-shaped in-plane negative MR up to room temperature as well as a large out-of-plane positive MR. MR at three directions is about in a -3%:-1%:225% ratio at 2 K. Through both the thickness and composition-dependent magnetotransport measurements, we show that the in-plane negative MR is due to the topological phase transition enhanced intersurface coupling near the topological critical point. Our devices suggest the great potential for room-temperature spintronic applications in, for example, vector magnetic sensors.

Direct Demonstration of the Emergent Magnetism Resulting from the Multivalence Mn in a LaMnO3 Epitaxial Thin Film System

Atomically engineered oxide heterostructures provide a fertile ground for creating novel states. For example, a two-dimensional electron gas at the interface between two oxide insulators, giant thermoelectric Seebeck coefficient, emergent ferromagnetism from otherwise nonmagnetic components, and colossal ionic conductivity. Extensive research efforts reveal that oxygen deficiency or lattice strain play an important role in determining these unexpected properties. Herein, by studying the abrupt presence of robust ferromagnetism (up to 1.5 uB/Mn) in LaMnO3-based heterostructures, we find the multivalence states of Mn that play a decisive role in the emergence of ferromagnetism in the otherwise antiferromagnetic LaMnO3 thin films. Combining spatially resolved electron energy-loss spectroscopy, X-ray absorption spectroscopy and X-ray magnetic circular dichroism techniques, we determine unambiguously that the ferromagnetism results from a conventional Mn3+-O-Mn4+ double-exchange mechanism rather than an interfacial effect. In contrast, the magnetic dead layer of 5 unit cell in proximity to the interface is found to be accompanied with the accumulation of Mn2+ induced by electronic reconstruction. These findings provide a hitherto-unexplored multivalence state of Mn on the emergent magnetism in undoped manganite epitaxial thin films, such as LaMnO3 and BiMnO3, and shed new light on all-oxide spintronic devices.

Direct observation of high spin polarization in Co 2 FeAl thin films

We have studied the Co2FeAl thin films with different thicknesses epitaxially grown on GaAs (001) by molecular beam epitaxy. The magnetic properties and spin polarization of the films were investigated by in-situ magneto-optic Kerr effect (MOKE) measurement and spin-resolved angle-resolved photoemission spectroscopy (spin-ARPES) at 300u2009K, respectively. High spin polarization of 58% (±7%) was observed for the film with thickness of 21 unit cells (uc), for the first time. However, when the thickness decreases to 2.5 uc, the spin polarization falls to 29% (±2%) only. This change is also accompanied by a magnetic transition at 4 uc characterized by the MOKE intensity. Above it, the film’s magnetization reaches the bulk value of 1000u2009emu/cm3. Our findings set a lower limit on the thickness of Co2FeAl films, which possesses both high spin polarization and large magnetization.

Ultrahigh Hall mobility and suppressed backward scattering in layered semiconductor Bi2O2Se

We report on an ultrahigh Hall mobility exceeding 40u2009000u2009cm2/V s and a very long traditional scattering time in a trivial layered semiconductor Bi2O2Se. Shubnikov-de Haas (SdH) oscillations were observed in both the unsaturated longitudinal linear magnetoresistance Rxx and the transverse Hall resistance Rxy. The amplitude ΔRxy of SdH oscillations was phase-shifted approximately 180° with respect to ΔRxx, indicating the strong suppression of electron backward scattering. This was further proved by the evidence of transport lifetime that is 10 times longer than the quantum lifetime. Our results show that the suppressed backward scattering in nontrivial Dirac semimetals can also occur in the trivial semiconductor Bi2O2Se.We report on an ultrahigh Hall mobility exceeding 40u2009000u2009cm2/V s and a very long traditional scattering time in a trivial layered semiconductor Bi2O2Se. Shubnikov-de Haas (SdH) oscillations were observed in both the unsaturated longitudinal linear magnetoresistance Rxx and the transverse Hall resistance Rxy. The amplitude ΔRxy of SdH oscillations was phase-shifted approximately 180° with respect to ΔRxx, indicating the strong suppression of electron backward scattering. This was further proved by the evidence of transport lifetime that is 10 times longer than the quantum lifetime. Our results show that the suppressed backward scattering in nontrivial Dirac semimetals can also occur in the trivial semiconductor Bi2O2Se.

Broadband photocarrier dynamics and nonlinear absorption of PLD-grown WTe2 semimetal films

WTe2 is a unique material in the family of transition metal dichalcogenides and it has been proposed as a candidate for type-II Weyl semimetals. However, thus far, studies on the optical properties of this emerging material have been significantly hindered by the lack of large-area, high-quality WTe2 materials. Here, we grow a centimeter-scale, highly crystalline WTe2 ultrathin film (∼35u2009nm) by a pulsed laser deposition technique. Broadband pump-probe spectroscopy (1.2–2.5u2009μm) reveals a peculiar ultrafast optical response where an initial photo-bleaching signal (lasting ∼3 ps) is followed by a long-lived photoinduced absorption signature. Nonlinear absorption characterization using femtosecond pulses confirms the saturable absorption response of the WTe2 ultrathin films, and we further demonstrated a mode-locked Thulium fiber laser using a WTe2 absorber. Our work provides important insights into linear and nonlinear optical responses of WTe2 thin films.WTe2 is a unique material in the family of transition metal dichalcogenides and it has been proposed as a candidate for type-II Weyl semimetals. However, thus far, studies on the optical properties of this emerging material have been significantly hindered by the lack of large-area, high-quality WTe2 materials. Here, we grow a centimeter-scale, highly crystalline WTe2 ultrathin film (∼35u2009nm) by a pulsed laser deposition technique. Broadband pump-probe spectroscopy (1.2–2.5u2009μm) reveals a peculiar ultrafast optical response where an initial photo-bleaching signal (lasting ∼3 ps) is followed by a long-lived photoinduced absorption signature. Nonlinear absorption characterization using femtosecond pulses confirms the saturable absorption response of the WTe2 ultrathin films, and we further demonstrated a mode-locked Thulium fiber laser using a WTe2 absorber. Our work provides important insights into linear and nonlinear optical responses of WTe2 thin films.