Structural Distortion-Modulated Magnetic and Dielectric Properties in Nonstoichiometric Yb2-xTi2O7-δ Pyrochlore.

Rare-earth titanate pyrochlores have attracted considerable attention for their unique magnetic frustration. Among those compounds, Yb2Ti2O7, a candidate for quantum spin ice, has been extensively studied in its magnetic ground state. However, works on its dielectric property and structure-property relationship lag far more behind. Here, by preparing and investigating nonstoichiometric Yb2-xTi2O7-δ (x = 0-0.15) ceramics, we demonstrate that the samples with x ≤ 0.05 maintain a single-pyrochlore phase, but the nonstoichiometry arouses significant structural distortion and increased oxygen vacancy. As a result, the ferromagnetism, indicated by a positive Curie-Weiss temperature, decreases almost linearly with increasing x value. Remarkably composition-dependent low-temperature dielectric relaxations have been observed. In addition, through introducing nonstoichiometry, the relaxor degree of dielectric behavior is enhanced, and the dielectric curve shows an altered shape. The origin of this dielectric relaxation is attributed to the increased structural distortion reflected by the changed bond length/angle, since there is no phase transition in 90-300 K. Our work gives a comprehensive view on the structural, magnetic, and dielectric properties of Yb2Ti2O7, which is instructive for further work on pyrochlores.

Enhanced photocatalytic efficiency of C3N4/BiFeO3 heterojunctions: the synergistic effects of band alignment and ferroelectricity.

As one of the most promising photocatalysts, graphitic carbon nitride (g-C3N4) shows a visible light response and great chemical stability. However, its relatively low photocatalytic efficiency is a major obstacle to actual applications. Here an effective and feasible method to dramatically increase the visible light photocatalytic efficiency by forming C3N4/BiFeO3 ferroelectric heterojunctions is reported, wherein the band alignment and piezo-/ferroelectricity have synergistic positive effects in accelerating the separation of the photogenerated carriers. At the optimum composition of 10 wt% BiFeO3, the heterojunction shows 1.4 times improved photocatalytic efficiency than that of the pure C3N4. Most importantly, mechanical pressing and electrical poling can also improve the photocatalytic efficiencies by 1.3 times and 1.8 times, respectively. The optimized photocatalytic efficiency is even comparable with that of some noble metal based compounds. These results not only prove the improved photocatalytic activity of the C3N4-ferroelectric heterojunctions, but also provide a new approach for designing high-performance photocatalysts by taking advantage of ferroelectricity.

Experimental Observation of Anisotropic Adler-Bell-Jackiw Anomaly in Type-II Weyl Semimetal WTe_{1.98} Crystals at the Quasiclassical Regime.

The asymmetric electron dispersion in type-II Weyl semimetal theoretically hosts anisotropic transport properties. Here, we observe the significant anisotropic Adler-Bell-Jackiw (ABJ) anomaly in the Fermi-level delicately adjusted WTe_{1.98} crystals. Quantitatively, C_{W}, a coefficient representing the intensity of the ABJ anomaly along the a and b axis of WTe_{1.98} are 0.030 and 0.051 T^{-2} at 2 K, respectively. We found that the temperature-sensitive ABJ anomaly is attributed to a topological phase transition from a type-II Weyl semimetal to a trivial semimetal, which is verified by a first-principles calculation using experimentally determined lattice parameters at different temperatures. Theoretical electrical transport study reveals that the observation of an anisotropic ABJ along both the a and b axes in WTe_{1.98} is attributed to electrical transport in the quasiclassical regime. Our work may suggest that electron-doped WTe_{2} is an ideal playground to explore the novel properties in type-II Weyl semimetals.

Chemical composition-induced evolution of the structure, domain and electrical properties of Sm-doped (1 - x)Pb(Ni1/3Nb2/3)O3-xPbTiO3.

As the two typical basic binary solid solutions of the relaxor-PbTiO3 family, Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) has been widely investigated, whereas Pb(Ni1/3Nb2/3)O3-PbTiO3 (PNN-PT) has not. Here, 1.5 mol% Sm-doped (1 - x)Pb(Ni1/3Nb2/3)O3-xPbTiO3, (1 - x)PNN-xPT:0.015Sm with x = 0.33-0.39, ceramics have been prepared and the chemical composition-induced evolution of crystal structure, domain, and electrical properties investigated systematically. With increasing PT content, evolution of the rhombohedral-tetragonal structure was observed. A rhombohedral-tetragonal morphotropic phase boundary occurred around x = 0.36-0.37, which showed a peak piezoelectric property with piezoelectric constant d33 = 531 pC N-1 and planar electromechanical coupling factor kp = 0.37 at room temperature. At the same time, the x = 0.36 composition showed improved ferroelectric behavior with remanent polarization Pr = 13.4 µC cm-2 and coercive field Ec = 3.2 kV cm-1. Interestingly, different from its PMN-PT counterpart, there is no temperature-driven phase transition between room temperature and the Curie temperature for (1 - x)PNN-xPT:0.015Sm. These parameters indicated that the PNN-PT system is worthy of more attention and is a promising platform for further development of high-performance piezo/ferroelectric materials.

Ultrahigh Energy Storage Density and High Efficiency in Lead-Free (Bi0.9Na0.1)(Fe0.8Ti0.2)O3-Modified NaNbO3 Ceramics via Stabilizing the Antiferroelectric Phase and Enhancing Relaxor Behavior.

Dielectric capacitors have attracted growing attention because of their important applications in advanced high power and/or pulsed power electronic devices. Nevertheless, the synergistic enhancement of recoverable energy storage density (Wrec > 10 J/cm3) and efficiency (η > 80%) is still a great challenge for lead-free dielectric bulk ceramics. Herein, by introducing complex perovskite compound (Bi0.9Na0.1)(Fe0.8Ti0.2)O3 with a smaller tolerance factor into an NaNbO3 matrix (NN-BNFT), we have achieved and explored stable relaxor antiferroelectric ceramics with enhanced relaxor behavior. Of particular importance is the composition of 0.88NN-0.12BNFT, which exhibits a large electric breakdown strength Eb of 87.3 kV/mm, an ultrahigh Wrec of 12.7 J/cm3, and a high efficiency η of 82.5%, as well as excellent thermal reliability and an ultrafast discharge speed, resulting from the dense microstructure, the moderate dielectric constant, the reduced grain size, the dielectric loss, and the sample thickness. The outstanding energy storage properties of NN-BNFT display great promise in advanced dielectric capacitors for energy storage applications.

Non-hydrostatic pressure-dependent structural and transport properties of BiCuSeO and BiCuSO single crystals.

High-pressure experiments usually expect a hydrostatic condition, in which the physical properties of materials can be easily understood by theoretical simulations. Unfortunately, non-hydrostatic effect is inevitable in experiments due to the solidification of the pressure transmitting media under high pressure. Resultantly, non-hydrostaticity affects the accuracy of the experimental data and sometimes even leads to false phenomena. Since the non-hydrostatic effect is extrinsic, it is quite hard to analyze quantitatively. Here, we have conducted high pressure experiments on the layered BiCuXO (X = S and Se) single crystals and quantitatively analyzed their pronounced non-hydrostatic effect by high throughput first-principles calculations and experimental Raman spectra. Our experiments find that the BiCuXO single crystals sustain the tetragonal structure up to 55 GPa (maximum pressure in our experiment). However, their pressure-dependent Raman shift and electric resistance show anomalous behaviors. Through optimization of thousands of crystal structures in the high throughput first-principles calculations, we have obtained the evolution of the lattice constants under external pressures, which clearly substantiates the non-hydrostatical pressure exerted in BiCuXO crystals. Our work indicates that the high throughput first-principles calculations could be a handy method to investigate the non-hydrostatic effect on the structural and electronic properties of materials in high pressure experiments.

Realizing a ferroelectric state and high pyroelectric performance in antiferroelectric-oxide composites.

We report a robust room temperature ferroelectric (FE) state in (1 - x)Pb0.99Nb0.02[(Zr0.57Sn0.43)0.933Ti0.067]0.98O3-xZnO ((1 - x)PNZST-xZnO) composites, where PNZST shows a predominant antiferroelectric (AFE) nature due to ZnO-induced internal strain. Upon heating, a FE-AFE transition occurs and generates high pyroelectric performance. The composite with x = 0.1 shows a peak pyroelectric coefficient of p = 2450.7 × 10-4 C m-2 K-1 and figures of merit of current responsivity Fi = 926.9 × 10-10 m V-1, voltage responsivity Fv = 1334.3 × 10-2 m2 C-1, and detectivity Fd = 1194.8 × 10-5 Pa-1/2, which are about two orders of magnitude higher than those of most perovskite pyroelectric oxides. More interestingly, the FE-AFE transition temperature, i.e., the temperature corresponding to peak pyroelectric performance, is tunable in a wide temperature range from 30 °C to 65 °C. This work not only provides a promising material candidate for high performance pyroelectric devices, but also an alternative idea to develop ferroelectric and pyroelectric properties based on antiferroelectric materials.

Crossover from negative to positive magnetoresistance in Sr2CrWO6/Sr2Fe10/9Mo8/9O6 superlattices.

Sr2CrWO6/Sr2Fe10/9Mo8/9O6 (SCWO/SFMO) superlattices with 4, 6, 7, 10 periods (abbreviated as S-1, S-2, S-3, and S-4) were prepared on (0 0 1) SrTiO3 (STO) substrates by pulsed laser deposition. All superlattices show macroscopic ferromagnetic behavior, and the magnetization increases with increasing period. The S-1 superlattice demonstrates semiconductor-like temperature-dependent resistivity in the whole measuring temperature range and negative magnetoresistance of -5.3% at 2 K with 2 T magnetic field, while the other superlattices illustrate metallic behaviors and increasing positive magnetoresistance of 223.1%, 275.4%, and 766.1% under the same conditions. This work not only provides a feasible way to tune the MR effect in magnetic perovskite oxides, but also may stimulate further work on artificially micro-structured thin films with designable magnetic properties.

The Microstructural Characterization of Multiferroic LaFeO3-YMnO3 Multilayers Grown on (001)- and (111)-SrTiO3 Substrates by Transmission Electron Microscopy.

The microstructure of multiferroic LaFeO3-YMnO3 (LFO-YMO) multilayers grown on (001)- and (111)-SrTiO3 substrates is characterized by the transmission electron microscopy (TEM). Detailed TEM characterization reveals that LFO-YMO multilayers grown on both substrates have clear layer-by-layer morphology and distinct chemical-composition layered structure. The most notable feature is that LFO-YMO multilayers grown on (001)-SrTiO3 substrate have three types of domains, while those on (111)-SrTiO3 have only one. The multi-/twin- domain structure can be qualitatively explained by the lattice mismatch in this system. The details of the domain structure of LFO-YMO multilayers are crucial to understanding their magnetic properties.

Giant positive magnetoresistance in half-metallic double-perovskite Sr2CrWO6 thin films.

Magnetoresistance (MR) is the magnetic field-induced change of electrical resistance. The MR effect not only has wide applications in hard drivers and sensors but also is a long-standing scientific issue for complex interactions. Ferromagnetic/ferrimagnetic oxides generally show negative MR due to the magnetic field-induced spin order. We report the unusually giant positive MR up to 17,200% (at 2 K and 7 T) in 12-nm Sr2CrWO6 thin films, which show metallic behavior with high carrier density of up to 2.26 × 1028 m-3 and high mobility of 5.66 × 104 cm2 V-1 s-1. The possible mechanism is that the external magnetic field suppresses the long-range antiferromagnetic order to form short-range antiferromagnetic fluctuations, which enhance electronic scattering and lead to the giant positive MR. The high mobility may also have contributions to the positive MR. These results not only experimentally confirm that the giant positive MR can be realized in oxides but also open up new opportunities for developing and understanding the giant positive MR in oxides.

Broadband gradient impedance matching using an acoustic metamaterial for ultrasonic transducers.

High-quality broadband ultrasound transducers yield superior imaging performance in biomedical ultrasonography. However, proper design to perfectly bridge the energy between the active piezoelectric material and the target medium over the operating spectrum is still lacking. Here, we demonstrate a new anisotropic cone-structured acoustic metamaterial matching layer that acts as an inhomogeneous material with gradient acoustic impedance along the ultrasound propagation direction. When sandwiched between the piezoelectric material unit and the target medium, the acoustic metamaterial matching layer provides a broadband window to support extraordinary transmission of ultrasound over a wide frequency range. We fabricated the matching layer by etching the peeled silica optical fibre bundles with hydrofluoric acid solution. The experimental measurement of an ultrasound transducer equipped with this acoustic metamaterial matching layer shows that the corresponding -6 dB bandwidth is able to reach over 100%. This new material fully enables new high-end piezoelectric materials in the construction of high-performance ultrasound transducers and probes, leading to considerably improved resolutions in biomedical ultrasonography and compact harmonic imaging systems.

Room-Temperature Multiferroics and Thermal Conductivity of 0.85BiFe1-2xTixMgxO3-0.15CaTiO3 Epitaxial Thin Films (x = 0.1 and 0.2).

Thin films of 0.85BiFe1-2xTixMgxO3-0.15CaTiO3 (x = 0.1 and 0.2, abbreviated to C-1 and C-2, respectively) have been fabricated on (001) SrTiO3 substrate with and without a conductive La0.7Sr0.3MnO3 buffer layer. The X-ray θ-2θ and ϕ scans, atomic force microscopy, and cross-sectional transmission electron microscopy confirm the (001) epitaxial nature of the thin films with very high growth quality. Both the C-1 and C-2 thin films show well-shaped magnetization-magnetic field hysteresis at room temperature, with enhanced switchable magnetization values of 145.3 and 42.5 emu/cm3, respectively. The polarization-electric loops and piezoresponse force microscopy measurements confirm the room-temperature ferroelectric nature of both films. However, the C-1 films illustrate a relatively weak ferroelectric behavior and the poled states are easy to relax, whereas the C-2 films show a relatively better ferroelectric behavior with stable poled states. More interestingly, the room-temperature thermal conductivity of C-1 and C-2 films are measured to be 1.10 and 0.77 W/(m·K), respectively. These self-consistent multiferroic properties and thermal conductivities are discussed by considering the composition-dependent content and migration of Fe-induced electrons and/or charged point defects. This study not only provides multifunctional materials with excellent room-temperature magnetic, ferroelectric, and thermal conductivity properties but may also stimulate further work to develop BiFeO3-based materials with unusual multifunctional properties.

Temperature dependent structures and properties of Bi0.5Na0.5TiO3-based lead free piezoelectric composite.

The thermal depolarization around 100 °C of the Bi0.5Na0.5TiO3-based piezoelectric solid solutions leads to the disappearance of macroscopic ferroelectric/piezoelectric properties and remains a long-standing obstacle for their actual applications. In this communication, we report lead-free piezoelectric composites of 0.94Bi0.5Na0.5TiO3-0.06BaTiO3:0.5ZnO (BNT-6BT:0.5ZnO, where 0.5 is the mole ratio of ZnO to BNT-6BT) with deferred thermal depolarization, which is experimentally confirmed by systematic temperature dependent dielectric, ferroelectric, piezoelectric measurements. Especially, based on temperature dependent X-ray diffraction measurements on unpoled and poled samples, thermal depolarization is confirmed to have no relationship with the structural phase transition, the possible mechanism for the deferred thermal depolarization is correlated with the ZnO-induced local electric field which can suppress the depolarization field. We believe our results may be helpful for understanding the origin of thermal depolarization in BNT-based piezoelectric materials, and thus provide an effective way to overcoming this obstacle.

Dramatically decreased magnetoresistance in non-stoichiometric WTe2 crystals.

Recently, the layered semimetal WTe2 has attracted renewed interest owing to the observation of a non-saturating and giant positive magnetoresistance (~10(5)%), which can be useful for magnetic memory and spintronic devices. However, the underlying mechanisms of the giant magnetoresistance are still under hot debate. Herein, we grew the stoichiometric and non-stoichiometric WTe2 crystals to test the robustness of giant magnetoresistance. The stoichiometric WTe2 crystals have magnetoresistance as large as 3100% at 2 K and 9-Tesla magnetic field. However, only 71% and 13% magnetoresistance in the most non-stoichiometry (WTe1.80) and the highest Mo isovalent substitution samples (W0.7Mo0.3Te2) are observed, respectively. Analysis of the magnetic-field dependent magnetoresistance of non-stoichiometric WTe2 crystals substantiates that both the large electron-hole concentration asymmetry and decreased carrier mobility, induced by non-stoichiometry, synergistically lead to the decreased magnetoresistance. This work sheds more light on the origin of giant magnetoresistance observed in WTe2.

Semiconductor/relaxor 0-3 type composites without thermal depolarization in Bi0.5Na0.5TiO3-based lead-free piezoceramics.

Commercial lead-based piezoelectric materials raised worldwide environmental concerns in the past decade. Bi0.5Na0.5TiO3-based solid solution is among the most promising lead-free piezoelectric candidates; however, depolarization of these solid solutions is a longstanding obstacle for their practical applications. Here we use a strategy to defer the thermal depolarization, even render depolarization-free Bi0.5Na0.5TiO3-based 0-3-type composites. This is achieved by introducing semiconducting ZnO particles into the relaxor ferroelectric 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 matrix. The depolarization temperature increases with increasing ZnO concentration until depolarization disappears at 30 mol% ZnO. The semiconducting nature of ZnO provides charges to partially compensate the ferroelectric depolarization field. These results not only pave the way for applications of Bi0.5Na0.5TiO3-based piezoceramics, but also have great impact on the understanding of the mechanism of depolarization so as to provide a new design to optimize the performance of lead-free piezoelectrics.

Metal-insulator transition in SrIrO3 with strong spin-orbit interaction.

The thickness-dependent metal-insulator transition is observed in meta-stable orthorhombic SrIrO3 thin films synthesized by pulsed laser deposition. SrIrO3 films with thicknesses less than 3 nm demonstrate insulating behaviour, whereas those thicker than 4 nm exhibit metallic conductivity at high temperature, and insulating-like behaviour at low temperature. Weak/Anderson localization is mainly responsible for the observed thickness-dependent metal-insulator transition in SrIrO3 films. Temperature-dependent resistance fitting shows that electrical-conductivity carriers are mainly scattered by the electron-boson interaction rather than the electron-electron interaction. Analysis of the magneto-conductance proves that the spin-orbit interaction plays a crucial role in the magneto-conductance property of SrIrO3.