Wenliang Zhou

Structural phase transition, narrow band gap, and room-temperature ferromagnetism in [KNbO3]1−x[BaNi1/2Nb1/2O3−δ]x ferroelectrics

Structural phase transition, narrow band gap (Eg), and room-temperature ferromagnetism (RTFM) have been observed in the [KNbO3]1−x[BaNi1/2Nb1/2O3−δ]x (KBNNO) ceramics. All the samples have single phase perovskite structure, but exhibit a gradual transition behaviour from the orthorhombic to a cubic structure with the increase of x. Raman spectroscopy analysis not only corroborates this doping-induced change in normal structure but also shows the local crystal symmetry for x ≥ 0.1 compositions to deviate from the idealized cubic perovskite structure. A possible mechanism for the observed specific changes in lattice structure is discussed. Moreover, it is noted that KBNNO with compositions x = 0.1–0.3 have quite narrow Eg of below 1.5 eV, much smaller than the 3.2 eV band gap of parent KNbO3 (KNO), which is due to the increasing Ni 3d electronic states within the gap of KNO. Furthermore, the KBNNO materials present RTFM near a tetragonal to cubic phase boundary. With increasing x from 0 to 0.3, the magnetis...

Magnetism switching and band-gap narrowing in Ni-doped PbTiO3 thin films

Ions doping-driven structural phase transition accompanied by magnetism switching and band-gap narrowing effects has been observed in PbTi1−xNixO3−δ (xPTNO, x = 0.00, 0.06, and 0.33) thin films. With the increase of x, the xPTNO thin films exhibit not only a phase transition from the pseudotetragonal structure to a centrosymmetric cubic structure but also a drastic decrease of grain size. Moreover, the as-grown Ni-doped PbTiO3 (PTO) thin films show obvious room-temperature ferromagnetism and an increased saturation magnetization with increasing the Ni content, in contrast to undoped PTO, which shows diamagnetism. A bound magnetic polaron model was proposed to understand the observed ferromagnetic behavior of PTO-derived perovskite thin films. Furthermore, the 0.33PTNO thin film presents a narrowed band-gap, much smaller than that of PTO, which is attributed to new states of both the highest occupied molecular orbital and the lowest unoccupied molecular orbital in an electronic structure with the presence ...

Transport properties of AlGaAs/GaAs parabolic quantum wells

We report on the magnetotransport properties of 100 nm wide parabolic quantum wells and observe an enhancement of the Hall resistance in one sample but not the other. This phenomenon is likely related to the effective thickness of the electronic slab. We also observe a parabolic negative magnetoresistance originating from electron-electron interactions when only one subband is occupied in one of the samples. The interaction correction to the Drude conductivity is extracted using two methods. We find that the extracted interaction correction increases with increasing tilted angle, for which two possible explanations are given.

Strain-induced structural phase transition, ferromagnetic and optical properties of Bi1−x Tb x FeO3 thin films

In this paper, we report on the structural phase transition, surface morphologies, ferromagnetic and optical properties that have been observed in Bi1−x Tb x FeO3 (BTFO) thin films with (x = 0, 0.03, 0.06 and 0.09). X-ray diffraction (XRD) indicated that all the films have a single phase with rhombohedral structure and exhibit gradual transition behavior to pseudo-tetragonal structure with the increase of x, the strain values were estimated to be 0.1–0.8, depending on composition. A possible mechanism, strain-induced structural phase transition, is discussed. The micro-Raman spectroscopy analysis was in good agreement with structural phase transition verified by the XRD patterns. Moreover, with an increase in Tb3+ ions from x = 0–0.06, the saturation magnetization of the BTFO thin films shows a linear increase, which is mainly due to the Tb3+ ions. However, a sharp increase emerges with x from 0.06 to 0.09, the phase transition mainly dominates the higher ferromagnetism in BTFO films. Moreover, BTFO films with x > 0 exhibit a lower band-gap than that with x = 0, originating from local rhombohedral to pseudo-tetragonal structure transition. These results are helpful for a deeper understanding of structural transition, magnetic, and optical properties in perovskite oxides and show the potential roles, which such materials can play in multiferroic applications and solar energy devices.

Weak antilocalization and beating pattern in high electron mobility AlxGa1−xN/GaN two-dimensional electron gas with strong Rashba spin-orbit coupling

The weak antilocalization (WAL) effects of the two-dimensional electron gas (2DEG) in high mobility AlxGa1−xN/GaN heterostructure as well as beating patterns in the Shubnikov–de Haas (SdH) oscillatory magnetoresistance have been investigated by means of magnetotransport measurements before and after illumination. The zero-field spin splitting mainly arising from the Rashba spin-orbit coupling effect is studied using the weak antilocalization and beating patterns analysis, respectively. The Rashba spin-orbit coupling constant α deduced using the weak antilocalization analysis showed a good agreement with that estimated from the analysis of the beating patterns for the sample before and after illumination. For our sample, the electron motion in the high mobility system is in the ballistic regime, the experimental WAL curves were fitted by a simulated quantum conductance correction according to a model proposed by [Golub [Phys. Rev. B 71, 235310 (2005)].

Band-gap narrowing and magnetic behavior of Ni-doped Ba(Ti0.875Ce0.125)O3 thin films

Band-gap narrowing and magnetic effects have been observed in a Ni-doped Ba(Ti0.875Ce0.125)O3 (BTC) thin film. Structural characterizations and microstructural analysis show that the as-prepared Ba(Ti0.75Ce0.125Ni0.125)O3−δ (BTCN) thin film exhibits a cubic perovskite structure with an average grain size of 25 nm. The Ce doping at the Ti-site results in an increasing perovskite volume to favour an O-vacancy-stabilized Ni2+ substitution. Raman spectroscopy, however, shows the cubic symmetry of crystalline structures is locally lowered by the presence of dopants, significantly deviating from the ideal Pm3m space group. Moreover, BTCN presents a narrowed band-gap, much smaller than that of BaTiO3 and BTC, due to new states of both the highest occupied molecular orbital and the lowest unoccupied molecular orbital in an electronic structure with the presence of Ni. Also, magnetic enhancement driven by co-doping has been confirmed in the films, which mainly stems from the exchange interaction of Ni2+ ions via an electron trapped in a bridging oxygen vacancy. These findings may open an avenue to discover and design optimal perovskite compounds for solar-energy devices and information storage.

Structural, optical and electrical properties of Sb2Te3 films prepared by pulsed laser deposition

Revealing travel patterns and city structure with taxi trip data Xi Liu, Li Gong , Yongxi Gong , Yu Liu a,b* a Institute of Remote Sensing and Geographical Information Systems, Peking University, Beijing 100871, PR China b Beijing Key Lab of Spatial Information Integration and Its Applications, Peking University, Beijing 100871, PR China c Shenzhen Key Laboratory of Urban Planning and Decision Making, Harbin Institute of Technology Shenzhen Graduate School, Shenzhen 518055, PR China Abstract: Delineating travel patterns and city structure has long been a core research topic in transport geography. Different from the physical structure, the city structure beneath the complex travel-flow system shows the inherent connection patterns within the city. On the basis of massive taxi trip data of Shanghai, we built spatially-embedded networks to model the intra-city spatial interactions and introduced network science methods into the issue. The community detection method is applied to reveal sub-regional structures, and several network measures are used to examine the properties of sub-regions. Considering the differences between longand short-distance trips, we reveal a two-level hierarchical polycentric city structure of Shanghai. Further explorations on sub-network structures demonstrate that urban sub-regions have broader internal spatial interactions, while suburban centers are more influential in local traffic. By incorporating the land use of centers from the travel pattern perspective, we investigate sub-region formation and center–local places interaction patterns. This study provides insights into using emerging data sources to reveal travel patterns and city structures, which could potentially aid in applying urban and transportation policies. The sub-regional structures revealed in this study are more easily interpreted for transportation-related issues than other structures, such as administrative divisions.

Influence of the illumination on weak antilocalization in an AlxGa1−xN∕GaN heterostructure with strong spin-orbit coupling

The weak antilocalization effects of the two-dimensional electron gas in a high mobility AlxGa1−xN∕GaN heterostructure have been investigated by means of magnetotransport measurements before and after illumination. The zero-field spin splitting mainly arising from the Rashba spin-orbit coupling effect as a function of electron concentration as well as a function of temperature is studied using the weak antilocalization analysis. The Rashba spin-orbit coupling constant α deduced using the weak antilocalization analysis shows a rapid decrease with the increase of the measured electron concentration.

Transport properties of a spin-split two-dimensional electron gas in an In0.53Ga0.47As∕InP quantum well structure

We study the magnetotransport properties of a gated In0.53Ga0.47As∕InP quantum well structure in the presence of spin splitting when only one electronic subband is occupied. We develop an analytical method to extract the quantum mobilities for the two spin subbands. Ionized impurity scattering and alloy disorder scattering are determined to be important in this system. Larger quantum mobility is found for the higher-energy spin subband. We also demonstrate that the difference between the quantum mobilities for the two spin subbands can be altered with the gate.

Experimental approaches to zero-field spin splitting in a gated high-mobility In0.53Ga0.47As/InP quantum well structure: Weak antilocalization and beating pattern

We report on the strong spin-orbit (SO) interaction in a gated high-mobility In0.53Ga0.47As/InP quantum well two-dimensional electron gas. We establish that the SO interaction is dominated by the Rashba mechanism. The Rashba coupling parameters determined from analysis of both weak antilocalization and the beating pattern in the Shubnikov–de Haas oscillations are in reasonable agreement, and the small difference between them was explained by a magnetic-field-dependent effective g factor. The zero-field spin splitting shows nonmonotonic behavior with a maximum as the electron density is varied with the applied gate voltage. This is related to strong Rashba SO coupling in our sample.