Structural and dielectric properties of La- and Ti-modified K0.5Na0.5NbO3 ceramics

We report the results of studies of rectifying behavior, positive magneoresistance (MR), the charge-transport mechanism and the effect of an electric field on a ZnO (n)/La_0.5Pr_0.2Sr_0.3MnO₃ (LPSMO) (p)/SrNb_0.002Ti_0.998O₃ (SNTO) (n) heterostructure comprising two p–n junctions fabricated using the pulsed laser deposition technique. The heterostructure exhibits rectifying behavior over a wide temperature and field range having hysteresis in I–V behavior (forward bias) due to the tunneling of charge carriers. It is also observed that, depending on the nature of the electric field bias to n-type ZnO and SNTO, the junction resistance becomes modified, which has been explained on the basis of spin injection in the heterostructure. The observation of unconventional positive MR at room temperature has been justified on the basis of interface effects and the reduction in barrier height obtained using fitting of the I–V data by a thermionic emission model.     

Spontaneously gapped ground state in suspended bilayer graphene

Bilayer graphene bears an eightfold degeneracy due to spin, valley, and layer symmetry, allowing for a wealth of broken symmetry states induced by magnetic or electric fields, by strain, or even spontaneously by interaction. We study the electrical transport in clean current annealed suspended bilayer graphene. We find two kinds of devices. In bilayers of type B1 the eightfold zero-energy Landau level is partially lifted above a threshold field revealing an insulating ν=0 quantum-Hall state at the charge neutrality point. In bilayers of type B2 the Landau level lifting is full and a gap appears in the differential conductance even at zero magnetic field, suggesting an insulating spontaneously broken symmetry state. Unlike B1, the minimum conductance in B2 is not exponentially suppressed, but remains finite with a value G is < or approximately equall to e(2)/h even in a large magnetic field. We suggest that this phase of B2 is insulating in the bulk and bound by compressible edge states.     

Stability diagram for elastic domains in epitaxial ferroelectric thin films

The statics of isolated elastic domains (twins) in epitaxial thin tetragonal films grown on a cubic substrate is investigated theoretically. Different possible variants of the geometric shape of a domain are studied: plate, trapezoidal, and triangular. The nonuniform internal stresses, which also exist in polydomain epitaxial systems, are calculated by the effective-dislocation method. Hence the elastic energies stored in heterostructures with different domains are determined. The equilibrium width of a domain is calculated by minimizing the total internal energy of the heterostructure. Next, the stability diagram for isolated domains in epitaxial films is constructed from energy considerations. It is shown that in a large part of this diagram trapezoidal domains are energetically more advantageous than plate-shaped domains. The effect of an external electric field on the stability of 90° domains in epitaxial ferroelectric films is investigated. Fiz. Tverd. Tela (St. Petersburg) 39, 127–134 (January 1997)     

Quasi-static electric field–temperature diagrams in epitaxial relaxor ferroelectric films

Quasi-static polarization induced by dc electric field was studied in a broad range of temperatures in epitaxial films of relaxor ferroelectric PbMg_1/3Nb_2/3O₃ and PbSc_1/2Nb_1/2O₃. The electric field was applied and the response was measured along the out-of-plane <100> crystal direction of the epitaxial perovskite-structure (001) oriented films. The films remained in the relaxor state in zero-field cooling. A new polar state can be induced by electric field at a critical temperature below 100 K. The critical field and the induced polarization increased on cooling and reached the bulk-like values at 20 K. The orientational anisotropy of thin-film dipolar systems is discussed as a possible reason for the observed stable relaxor state.     

Energy band alignment of MgO (111)/ZnO (0002) heterojunction determined by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) was used to measure the energy discontinuity in the MgO (111)/ZnO (0002) heterostructure. The valence band offset (VBO) was determined to be 1.22±0.23 eV and a type-I heterojunction with a conduction band offset (CBO) of 3.24±0.23 eV was obtained. The discrepancy of VBO values between MgO/ZnO and ZnO/MgO heterojunctions was mainly attributed to the internal electric field induced by spontaneous polarization effect in ZnO layer.     

Thickness-dependent of conduction mechanism and bias electric field modulation of transport properties for SrRuO3/PMN-PT heterostructures

Ultra-thin SrRuO₃ (SRO) films have been grown on ferroelectric and piezoelectric PMN-PT substrates. The structural properties of these films have been characterized by atomic force microscopy, x-ray diffraction and cross-sectional transmission electron microscopy. The nature of electric transport was analyzed in detail and the conduction mechanism of SRO films evolves through three regimes: from a three-dimensional (3D) metallic through a weakly localized to a strongly localized behavior as film thickness is reduced. The bias electric field modulations of transport properties and magnetic properties were explored for these films. We also demonstrate that ferroelectric (FE) domain switching induces a reversible tuning of the magnetic and electric properties in SRO/PMN-PT heterostructure. The FE domain switching in the substrate contributes to an in-plane strain that changes the spin exchange coupling in the SRO layer, and therefore results in a reversible resistance difference of up to 16%. This modulation effect on the electric properties by an electric field demonstrates great potential for the applications of all-oxides spintronics devices.     

Magnetoresistance of Electrons Channelled by Microscopic Magnetic Field Modulation

We report the magnetoresistance of two-dimensional electron gas, which is made of GaAs based epitaxial mul-tilayers and laterally subjected to a periodic magnetic field. The modulation field is produced by an array of submicrometre ferromagnets fabricated at the surface of the heterostructure. The magnetoresistance of about 20% is found at low temperature 80K. The measurement is in quantitative agreement with semiclassical simulations, which reveal that the magnetoresistance is due to electrons trapped in snake orbits along lines of zero magnetic field.     

Tilted electric field effects on the electronic states in a GaAs quantum disk

In the present work we report the effects of a geometrical confinement and tilted applied electric field on the electronic energy levels in a semiconducting quantum disk. Calculations are performed in the effective mass approximation and using a variational method. The results can be summarized as follows: (1) due to the infinite confinement along the all directions of the heterostructure, the variational calculation with two parameters for tilted applied electric field can be treated with two independent each other variational parameters; (2) the magnitude of the energy shift is an increasing function of the applied electric field; (3) the effects of the applied electric field are magnified as the dimensions of the heterostructure (height and radius) grow; and finally (4) for large enough applied electric field the energy shift is a linear function of the applied electric field.     

Electrical domain morphologies in compositionally graded ferroelectric films

We present a nonlinear thermodynamic formalism coupled with an electrostatic analysis of uniaxial n-layered compositionally graded heteroepitaxial ferroelectric films and extend this formalism to continuously graded ferroelectric films. We show that the domain morphology and its subsequent evolution in the presence of an electric field are determined by the spontaneous polarisation of the film induced through the compositional grading. The results for compositionally graded epitaxial (001) (Ba,Sr)TiO(3) and (001) Pb(Zr,Ti)O(3) films on (001)SrTiO(3) demonstrate that, while the domain morphologies in these two films are different in appearance, the dielectric displacement and the dielectric permittivity of such graded ferroelectric films exhibit a strong nonlinear behaviour which results in a high dielectric tunability. These findings indicate that it is possible to design specific domain structures that will yield desirable dielectric properties by controlling the strength of the compositional grading in the films.     

Electric field control of nonvolatile four-state magnetization at room temperature

We find the realization of large converse magnetoelectric (ME) effects at room temperature in a magnetoelectric hexaferrite Ba0.52Sr2.48Co2Fe24O41 single crystal, in which rapid change of electric polarization in low magnetic fields (about 5 mT) is coined to a large ME susceptibility of 3200 ps/m. The modulation of magnetization then reaches up to 0.62μ(B)/f.u. in an electric field of 1.14 MV/m. We find further that four ME states induced by different ME poling exhibit unique, nonvolatile magnetization versus electric field curves, which can be approximately described by an effective free energy with a distinct set of ME coefficients.     

Antiferroelectric behavior in symmetric KNbO(3)/KTaO(3) superlattices

Antiferroelectric behavior is observed in artificially layered KTaO (3)/KNbO (3) perovskite superlattices. While KTaO (3) and KNbO (3) are ferroelectric and paraelectric, respectively, the superlattice appears antiferroelectric based on an increase in the dielectric constant with applied dc bias. This dielectric behavior is inconsistent with the nonlinear response for either paraelectric or ferroelectric materials. However, an increase in the dielectric constant with applied electric field is consistent with antiferroelectric behavior. The antiferroelectric ordering appears to be induced by cation modulation imposed by the superlattice.     

Interface contact and modulated electronic properties by external vertical strains and electric fields in graphene/MoS2 heterostructure

Based to the first-principles calculations, we study the electronic properties of graphene/MoS₂ heterostructure by modulating the vertical strains and applying external electric field. Graphene/MoS₂ heterostructure is a van der Waals heterostructure (vdWH) with the interlayer spacing is 3.2 Å for the equilibrium state, and the contact property of the interface is n-type Schottky contact. The Schottky barrier height (SBH) changes with vertical strains which induces a change of charge transfer between graphene and MoS₂ layer. In addition, with strain or without strain, the applied positive electric field can effectively promote the charge transfer from graphene to MoS₂, while the negative electric field has the opposite effect. These findings support for the design of field effect transistors based on graphene vdWHs.     

顺磁性La2/3Sr1/3MnO3层对Bi0.8Ba0.2FeO3薄膜多铁性能的影响

界面效应在提升异质结构材料的多铁性能方面有着重要的作用. 本文采用脉冲激光沉积技术在SrTiO₃(STO)基片上制备了Bi_0.8Ba_0.2FeO₃(BBFO)/La_2/3Sr_1/3MnO₃(LSMO)异质结. X-射线衍射图谱表明异质结呈现单相外延生长, 利用高分辨透射电镜进一步证实了BBFO为四方相结构. X-射线光电子能谱证实异质结中只存在Fe³⁺ 离子, 没有产生价态的变化, 揭示了异质结铁电和铁磁性的增强与BBFO/LSMO的界面有关. 同时, 测试了磁电阻(MR)和磁介电(MD), 当磁场强度为0.8 T, 温度为70 K时, MR约为-42.2%, MD约为21.2%. 并且发现在180 K时出现磁相的转变. 实验结果揭示出异质界面效应在提升材料的多铁性和磁电耦合效应方面具有超常的优点, 是加快多铁材料实际应用的有效途径.     

DC electric-field-induced heterophase structure on the surface of a peierls metal

A theoretical study has been made of the formation of a heterostructure of alternating metallic and semiconducting phases in atomic chains of a Peierls-metal surface monatomic layer placed in an electrostatic field normal to the surface. It is shown that an increase in the electric field results in an increase in the critical temperature of the metal-semiconductor phase transition on the sample surface, an increase in the temperature interval within which the heterostructure exists, a decrease in its spatial period, and an increase in the depth of spatial modulation of the gap in the electronic spectrum.     

Experimental observation of the spin-Hall effect in a spin–orbit coupled two-dimensional hole gas

Electrically induced ordering and manipulation of electron spins in semiconductors has a number of practical advantages over the established techniques using circularly polarized light sources, external magnetic fields and spin injection from a ferromagnet. The spin-Hall effect utilizes spin–orbit coupling to induce edge spin accumulation in response to a longitudinal electric field which can be applied locally and lead to low energy consumption devices. We study spin accumulation near the edge of a weakly disordered two-dimensional hole gas (2DHG) in a GaAs/AlGaAs heterostructure where the magnitude of the transverse spin current approaches the intrinsic, disorder independent value, in contrast to the impurity dominated regime observed in 3D electron doped systems. In our experiment, the induced spin polarization is detected by the electroluminescence resulting from two p–n junctions bordering the 2DHG channel. When an electric field is applied across the 2DHG channel, a non-zero out-of-plane component of the spin is optically detected. The sign of the spin depends on the direction of the field and is opposite for the two edges, consistent with theory predictions. We also report and analyze an in-plane spin-polarization effect induced in the device by asymmetric electron–hole recombination.     

Enhanced magnetoresistance in La0.82Sr0.18MnO3-π-conjugated semiconducting polymer heterostructure

We report a large enhancement (∼90%) in magnetoresistance in La_0.82Sr_0.18MnO₃ (LSMO) layers by incorporating a π-conjugated semiconducting polymer layer in between them. The epitaxial LSMO layers were deposited by DC magnetron sputtering on SrTiO₃ single crystal substrates and have FM transition temperature (T_C)∼310 K. A semiconducting polymer poly(3-octylthiophene) (P3OT) layer was deposited over the epitaxial LSMO layer by solution dip coating technique and with subsequent deposition of another epitaxial LSMO layer, forming a LSMO-P3OT-LSMO heterostructure. The effect of P3OT incorporation on magnetotransport properties of this heterostructure has been examined in the temperature range 77-350 K. Large MR enhancement observed near room temperature in the FM regime is explained in terms of efficient magnetic field dependent carrier injection at LSMO/P3OT interface.     

Beam damage by the induced electric field in transmission electron microscopy

Electric fields can be induced by electron irradiation of insulating thin film materials. In this work, the electric fields under a broad beam illumination in transmission electron microscopy (TEM) are analyzed for insulating samples. Some damage phenomena observed can be interpreted by the mechanism of damage by the induced electric field (DIEF). For broad-beam illumination in an ultra-thin specimen, the electric field near the center of the illumination may not be strong, but at the periphery of the illumination the electric field can be significant. Therefore, damage may be easily observed in these regions rather than at the center of the illumination. For a beam which is broad compared to the specimen thickness, e.g. 100 ∼ 1000 nm, a strong electric field pointing inward into the specimen near the surface region may result in cation diffusion into the specimen and/or anion diffusion out to the surface region. Meanwhile, a strong electric field perpendicular to the beam direction near the edge of the illumination may attract anions into the illuminated region, but eject cations to the periphery. For a wedge-shaped specimen, the electric field points inward into thicker region, driving cations toward the thicker region, while attracting anions to the edge region. On the sharp edge, a strong electric field pointing outward may be responsible for the edge-smoothing effect observed in insulating materials.     

Magnetic resonance imaging of electroconvection in a polar organic solvent

Molecular motion in the polar organic solvent nitrobenzene induced by an electric field is studied by magnetic resonance imaging. Rf pulse sequences that correlate images obtained at two different times under conditions of both continuous and pulsed electric fields are introduced. The resultant image correlation spectra indicate that the time scale of motion in a 9.6 kV/cm electric field is tens of milliseconds. Comparison of the results to an analytic solution for the Fokker-Planck probability function for one-dimensional bounded diffusion yields an electric field dependent effective diffusion coefficient for perdeuterated nitrobenzene of D = 1.08 x 10(-5) cm(2)/s + (3.33 x 10(-3) cm(4)/kV(2)s) E(2) at room temperature. Characteristics of this electroconvection and its consequences for combining multidimensional nuclear magnetic resonance with electrical orientation are also discussed.     

Experimental evidence for a metal–insulator transition and geometric effect in a half-filled Landau level

We have measured the low-temperature transport properties of a high-mobility front-gated GaAs/Al Ga As heterostructure. By changing the applied gate voltage, we can vary the amount of disorder within the system. At a Landau level filling factor , where the system can be described by the composite fermion picture, we observe a crossover from metallic to insulating behaviour as the disorder is increased, in agreement with theory. We also report experimental evidence of geometric effect for composite fermions induced by an effective magnetic field.     

Nonvolatile control of transport and magnetic properties in magnetoelectric heterostructures by electric field

Nonvolatile manipulation of transport and magnetic properties by external electric field is significant for information storage. In this study, we investigate the electric field control of resistance and magnetization in a magnetoelectric heterostructure comprising an electronic phase-separated La_(0.325)Pr_(0.3)Ca_(0.375)MnO_3(LPCMO) thin film and a ferroelectric(011)-oriented 0.7Pb(Mg_(1/3)Nb_(2/3))O_3-0.3PbTiO_3(PMN-PT) substrate. In a room-temperature poled sample, the metal-toinsulator transition temperature of an LPCMO film increases and the resistance decreases with variation in the effect of the remnant strain. Meanwhile, the increase in the magnetization of the sample is observed as well. This effect would be beneficial for the development of novel storage devices with low power consumption.