Xiaoqian Zhang

Large electrocaloric effect in Ba(Ti1−xSnx)O3 ceramics over a broad temperature region

A large electrocaloric effect (ECE) near room temperature is reported in Sn doped BaTiO3 ceramics. By tuning Ba(Ti1−xSnx)O3 compositions which also exhibit relaxor ferroelectric response to near the invariant critical point, the Ba(Ti1−xSnx)O3 bulk ceramics at x ∼ 0.12 exhibit a large EC coefficient (ΔT/ΔE =0.27*10−6K⋅mV−1) over a 50 K temperature range. In addition to that, the diffuse phase transition is successfully applied to broaden the EC peaks under low electric field. These properties added together indicate a general solution of the electrocaloric materials with high performance for practical cooling applications.

Unique Current-Direction-Dependent ON–OFF Switching in BiSbTeSe 2 Topological Insulator-Based Spin Valve Transistors

In this letter, spin valve transistors are fabricated based on BiSbTeSe2 topological insulator (TI) with enhanced surface mobility (~4039 cm2V-1s-1). The output in our spin valve transistors exhibits a dominant steplike behavior when sweeping the magnetic field to change the magnetization orientation of the Ni21Fe79 electrode. Most importantly, the ON (low resistance)-OFF (high resistance) state can be even switched when reversing the direction of the dc current. The TI-based spin valve transistors enable the current-direction-dependent switching of ON-OFF state, allowing for the applicability in magnetic sensors and spin-logic circuits, and show the potential use of TIs as innovative current-driven spin generators.

Completely (001)-textured growth and electrical properties of Bi 4 Ti 3 O 12 /LaNiO 3 heterostructures prepared by pulsed laser deposition on LaAlO 3 single crystal substrates

Heterostructures of Bi4Ti3O12(BTO)/LaNiO3(LNO) have been prepared on (001) LaAlO3 single crystal substrates by pulsed laser deposition (PLD). Analyses of X-ray diffraction (XRD), (including θ-2θ scan, rocking curve), atomic force microscopy (AFM) and scanning electron microscopy (SEM) reveal that the high quality LNO and BTO films are completely c-axis oriented. With Pt top electrodes, the BTO films exhibit a highly insulative characteristic of low leakage current (∼10-7 A/cm2 at an applied voltage of 6 V) and excellent dielectric properties. At 100 kHz, the dielectric constant and dielectric loss are 238 and 0.033, respectively. Also a well symmetric ferroelectric hysteresis loop is observed with remnant polarization Pr of 4.5 µC/cm2 and coercive field Ec of 72.5 kV/cm. Our experiments confirm that the use of the LNO as an electrode material can reduce the amount of the interfacial charges, and thus remove the imprint failure due to thermal processes.

Fabrication and electrical properties of LaNiO 3 /Pb(Zr 0.61 Ti 0.39 )O 3 /LaNiO 3 /LaAlO 3 all-Perovskite heterostructures

All-perovskite heterostructures of LaNiO3(LNO)/Pb(Zr0.61Ti0.39)O3(PZT)/LNO were fabricated on (001)- LaAlO3 (LAO) single crystal substrates by pulsed laser deposition (PLD). The structures of both LNO/LAO and LNO/PZT/LNO/LAO were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). The Zr:Ti ratio and the surface oxygen ions of the PZT films were analyzed by X-ray photoelectron spectroscopy (XPS) in detail. The XPS peak corresponding to lattice oxygen ions showed obvious asymmetry, which indicated the different binding energy of Ti-O and Pb/Zr-O bonds. The PZT films showed a square hysteresis loop with remnant polarization (Pr) and saturated polarization (Ps) of 13.8 µC/cm2 and 31.5 µC/cm2, respectively. Good saturation property with the increase of applied electrical field was exhibited.

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.

Evidence for Layered Quantized Transport in Dirac Semimetal ZrTe 5

ZrTe5 is an important semiconductor thermoelectric material and a candidate topological insulator. Here we report the observation of Shubnikov-de Hass (SdH) oscillations accompanied by quantized Hall resistance in bulk ZrTe5 crystal, with a mobility of 41,000 cm2V−1s−1. We have found that the quantum oscillations does not originate from the surface states, but from the bulk states. Each single layer ZrTe5 acted like an independent 2D electron system in the quantum Hall regime having the same carrier density and mobilities, while the bulk of the sample exhibits a multilayered quantum Hall effect.

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 300 K, 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 1000 emu/cm3. Our findings set a lower limit on the thickness of Co2FeAl films, which possesses both high spin polarization and large magnetization.

Temperature-induced band shift in bulk γ-InSe by angle-resolved photoemission spectroscopy

Indium selenide (InSe) has recently become popular research topics because of its unique layered crystal structure, direct band gap and high electron mobilities. In this work, we have acquired the electronic structure of bulk γ-InSe at various temperatures using angle-resolved photoemission spectroscopy (ARPES). We have also found that as the temperature decreases, the valence bands of γ-InSe exhibit a monotonic shift to lower binding energies. This band shift is attributed to the change of lattice parameters and has been validated by variable temperature X-ray diffraction measurements and theoretical calculations.

Interface Magnetic and Electrical Properties of CoFeB /InAs Heterostructures

Amorphous magnetic CoFeB ultrathin films have been synthesized on the narrow band gap semiconductor InAs(100) surface, and the nature of the interface magnetic anisotropy and electrical contact has been studied. Angle-dependent hysteresis loops reveal that the films have an in-plane uniaxial magnetic anisotropy (UMA) with the easy axis along the InAs [0-11] crystal direction. The UMA was found to be dependent on the annealing temperatures of the substrates, which indicates the significant role of the Fe, Co-As bonding at the interface related to the surface condition of the InAs(100). I-V measurements show an ohmic contact interface between the CoFeB films and the InAs substrates, which is not affected by the surface condition of the InAs (100).

Weak Anti-Localization and Quantum Oscillations in Topological Crystalline Insulator PbTe

Topological crystalline insulators (TCIs) have attracted worldwide interest since their theoretical predication and have created exciting opportunities for studying topological quantum physics and for exploring spintronic applications. In this work, we successfully synthesize PbTe nanowires via the chemical vapor deposition method and demonstrate the existence of topological surface states by their 2D weak anti-localization effect and Shubnikov–de Haas oscillations. More importantly, the surface state contributes ~61% of the total conduction, suggesting dominant surface transport in PbTe nanowires at low temperatures. Our work provides an experimental groundwork for researching TCIs and is a step forward for the applications of PbTe nanowires in spintronic devices.