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Stoichiometric Cr2Se3 single crystals are particular layer-structured antiferromagnets, which possess a noncollinear spin configuration, weak ferromagnetic moments, moderate magnetoresistance (MR ∼14.3%), and poor metallic conductivity below the antiferromagnetic phase transition. Here, we report an interesting >16 000% colossal magnetoresistance (CMR) effect in Ti (1.5 atomic percent) lightly doped Cr2Se3 single crystals. Such a CMR is approximately 1143 times larger than that of the stoichiometric Cr2Se3 crystals and is rarely observed in layered antiferromagnets and is attributed to the frustrated spin configuration. Moreover, the Ti doping not only dramatically changes the electronic conductivity of the Cr2Se3 crystal from a bad metal to a semiconductor with a gap of ∼15 meV but also induces a change in the magnetic anisotropy of the Cr2Se3 crystal from strong out-of-plane to weak in-plane. Further, magnetotransport measurements reveal that the low-field MR scales with the square of the reduced magnetization, which is a signature of CMR materials. The layered Ti:Cr2Se3 with the CMR effect could be used as two-dimensional (2D) heterostructure building blocks to provide colossal negative MR in spintronic devices.

We report the synthesis of transition-metal-doped ferromagnetic elemental single-crystal semiconductors with quantum oscillations using the physical vapor transport method. The 7.7 atom% Cr-doped Te crystals (Cr_Te) show ferromagnetism, butterfly-like negative magnetoresistance in the low temperature (< 3.8 K) and low field (< 0.15 T) region, and high Hall mobility, e.g., 1320 cm2 V-1 s-1 at 30 K and 350 cm2 V-1 s-1 at 300 K, implying that Cr_Te crystals are ferromagnetic elemental semiconductors. When B // c // I, the maximum negative MR is -27% at T = 20 K and B = 8 T. In the low temperature semiconducting region, Cr_Te crystals show strong discrete scale invariance dominated logarithmic quantum oscillations when the direction of the magnetic field B is parallel to the [100] crystallographic direction and show Landau quantization dominated Shubnikov-de Haas (SdH) oscillations for B // [210] direction, which suggests the broken rotation symmetry of the Fermi pockets in the Cr_Te crystals. The findings of coexistence of multiple quantum oscillations and ferromagnetism in such an elemental quantum material may inspire more study of narrow bandgap semiconductors with ferromagnetism and quantum phenomena., 25 pages, 25 figures

Ferroelectrics are appealing candidates for photovoltaic applications. In this study, Bi5FeTi3O15 (BFTO) single layer and ZnO/BFTO bilayer films were deposited onto 0.7 wt% Nb-doped SrTiO3 (NSTO) electrically conducting single-crystal substrates to form Ag/BFTO/NSTO and Ag/ZnO/BFTO/NSTO photovoltaic devices. It was found that the insertion of an n-type semiconducting ZnO layer eliminates the built-in electric field between the Ag electrode and the BFTO film and results in the formation of a BFTO/ZnO p–n junction with a built-in electric field whose direction points to the BFTO film. These effects, together with the depolarization field within the BFTO film, provide a favorable energy level alignment for efficient electron extraction. As a result, the short circuit photocurrent density (Jsc) was improved by nearly 50 fold to 2.2 mA cm−2 and the output power density was increased by 15 fold to 0.09 mW cm−2 under ultraviolet light (λ = 405 nm) illumination. Upon the reversal of the ferroelectric polarization direction from downward to upward, the Jsc and the open circuit voltage (Voc) were increased by 2.5 and 4 fold for the Ag/BFTO/NSTO device and by 2.83 and 8.5 fold for the Ag/ZnO/BFTO/NSTO device, respectively. This work provides an effective way for developing efficient ferroelectric and semiconductor based photovoltaic devices.

The responses of materials properties to multi-field stimulation are often exploited to construct new type multi-functional devices. Here, we demonstrate electric-optical-thermal modulation of the electronic properties of optothermal ferroelectric field-effect transistors (FeFETs) which are fabricated by growing Bi2Te3 films on (111)-oriented 0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (PMN-PT) ferroelectric single-crystal substrates. Utilizing the electric field to switch the polarization direction of PMN-PT, the carrier density and resistance of Bi2Te3 films are in situ, reversibly, and nonvolatilely modulated via the ferroelectric field effect. Moreover, through the infrared light illumination on the bottom of PMN-PT substrates, the resistance of Bi2Te3 films under two polarization states could be further modulated, which is ascribed to the decreased polarization intensity under higher temperature due to the pyroelectric effect. Taking advantage of these two effects, the Bi2Te3/PMN-PT optothermal FeFETs exhibit multiresponsive to optical and electric field stimulation at room temperature. Our work provides a new strategy to design optoelectronic devices with both photodetector and memory functionalities.

We report the structural, dielectric, ferroelectric and magnetic properties of (1-x)BiFe0.95Ga0.03Mn0.02O3−xBaTiO3 (BFGMO−xBTO) lead-free ceramics. Structural characterizations demonstrate that the BFGMO forms single-phase solid solutions with the BTO within the doping level of x = 0.3–0.65. With increasing the content of BTO the dielectric constant and ferroelectric Curie temperature shift towards lower temperatures, exhibiting relaxor-like ferroelectric behaviors. Moreover, the remnant polarization reaches the maximum value of 2Pr ~27.67 μC/cm2 at x = 0.35 near the morphotropic phase boundary. At room temperature, remnant magnetization increases - to the maximal value of 2Mr ~0.584 emu/g at x = 0.45, which is attributed to local lattice distortion and destruction of the spiral antiferromagnetic spin structure upon the doping of BTO. The coexistence of ferroelectric and ferromagnetic performance at room temperature makes the environment-friendly BFGMO−xBTO ceramics potential candidate for multiferroic device applications.

Bi thin films were grown on (111)-oriented 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 (PMN–PT) single-crystal substrates by pulsed laser deposition. Resistance of Bi/PMN–PT could be modulated by asymmetrical bipolar electric field in a reversible and nonvolatile manner at 300 K. By tuning asymmetrical bipolar electric field, different nonvolatile strain states could be generated in PMN–PT and transferred to Bi thin film, which leads to different nonvolatile resistance states. Furthermore, different resistance states of the Bi films could be achieved under different magnetic field at 300 K. The ferroelastic strain- and magnetic-field-modulated resistive properties in Bi/PMN–PT suggest a promising approach for multistate resistive memories.

SrIrO3 (SIO) thin films were epitaxially grown on (001)-oriented 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) single-crystal substrates. Upon applying electric fields to the piezoelectric PMN-PT along the thickness direction, the electronic transport properties of SIO films can be in situ tuned and modulated by non-180° ferroelectric domain rotation-induced strain, piezoelectric strain, and rhombohedral-to-tetragonal structural phase transition-induced strain in the PMN-PT layer, respectively. Moreover, the weak negative magnetoresistance (MR) of the 60-nm SIO films could be modified by applying an electric field to the PMN-PT layer. At T = 2 K, upon the application of E = 4 kV/cm to the PMN-PT, MR at H = 9 T is reduced by 14.2% as compared to that under zero electric field, indicating in-plane compressive strain-induced suppression of the influence of quantum corrections to the conductivity in the SIO film. These results demonstrate that the electric-field controllable lattice strain is a simple approach to get insight into the strain-property relationships of 5 d iridate thin films.

Stoichiometric Cr

A series of Cr-doped In2-xCrxO3 (ICO) semiconductor thin films were epitaxially grown on (111)-oriented 0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (PMN-0.29PT) single-crystal substrates by the pulsed laser deposition. Upon the application of an electric field to the PMN-0.29PT substrate along the thickness direction, we realized in situ, reversible, and nonvolatile control of the electronic properties and Fermi level of the films, which are manifested by abundant physical phenomena such as the n-type to p-type transformation, metal-semiconductor transition, metal-insulator transition, crossover of the magnetoresistance (MR) from negative to positive, and a large nonvolatile on-and-off ratio of 5.5 × 104% at room temperature. We also strictly disclose that both the sign and the magnitude of MR are determined by the electron carrier density of ICO films, which could modify the s-d exchange interaction and weak localization effect. Our results demonstrate that the ferroelectric gating approach using PMN-PT can be utilized to gain deeper insight into the carrier-density-related electronic properties of In2O3-based semiconductors and provide a simple and energy efficient way to construct multifunctional devices which can utilize the unique properties of composite materials.

Enhanced green and red upconversion emissions under 980 nm excitation are observed in Ba0.85−xCa0.15TiO3:0.75%Er3+/xIn3+ lead-free ferroelectric ceramics which were synthesized by solid state reaction method. With increasing In3+ ions doping level from x = 0% mol the green and red emission intensities increase and reach the maximum at x = 1.75% mol. The maximum intensity of green emission at 550 nm is 20 times larger than In3+ ions undoped samples, which is unprecedented for Er3+-doped perovskite ferroelectric materials. Based on the ultraviolet-visible-near infrared absorption spectra, the enhancement of the luminescence intensity of the A-site In3+ doped Ba0.85−xCa0.15TiO3:0.75%Er/xIn ceramics is attributed to In3+-doping-induced relative increase in isolate Er3+ and decrease in the clustered-Er3+. The effects of In3+ doping on the crystal structure, dielectric, ferroelectric, piezoelectric properties of the ceramics were also studied and discussed.

Single-phase (00 l)-oriented Bi2Te3 topological insulator thin films have been deposited on (111)-oriented ferroelectric 0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (PMN-PT) single-crystal substrates. Taking advantage of the nonvolatile polarization charges induced by the polarization direction switching of PMN-PT substrates at room temperature, the carrier density, Fermi level, magnetoconductance, conductance channel, phase coherence length, and quantum corrections to the conductance can be in situ modulated in a reversible and nonvolatile manner. Specifically, upon the polarization switching from the positively poled Pr+ state (i.e., polarization direction points to the film) to the negatively poled Pr- (i.e., polarization direction points to the bottom electrode) state, both the electron carrier density and the Fermi wave vector decrease significantly, reflecting a shift of the Fermi level toward the Dirac point. The polarization switching from Pr+ to Pr- also results in significant increase of the conductance channel α from -0.15 to -0.3 and a decrease of the phase coherence length from 200 to 80 nm at T = 2 K as well as a reduction of the electron-electron interaction. All these results demonstrate that electric-voltage control of physical properties using PMN-PT as both substrates and gating materials provides a simple and a straightforward approach to realize reversible and nonvolatile tuning of electronic properties of topological thin films and may be further extended to study carrier density-related quantum transport properties of other quantum matter.

Perovskite oxides with luminescent ions hold great promise in optoelectronic devices because of their outstanding thermal stabilities and electro-optic performance. As one typical perovskite upconversion (UC) host material, lead-free potassium sodium niobate ((K, Na)NbO3/(KxNa1-x)NbO3 or KNN) has attracted much attention in recent years. In the present work, a novel routine was developed to tune the upconversion photoluminescence (UC PL) performance by controlling the oxygen vacancy concentration in the KNN matrix, based on the 0.1% Er3+-doped KNN (Er-KNN) single crystals grown for the first time. UC PL properties, conductivity and defect chemistry of the single crystals were systematically investigated. The UC PL intensity of the as-grown Er-KNN material could be enhanced by 20 times after oxygen atmosphere annealing at 800 °C and fully quenched after vacuum annealing. What's more, by annealing under an oxygen atmosphere and vacuum, the conductivity of the Er-KNN sample was successfully tuned for more than 8 orders of magnitude. The super-wide range tunability of UC PL performance and conductivity could be explained by oxygen vacancies which gave rise to Nb5+-Nb4+ valence alternation. Because of the modulated photoluminescence properties and conductivity, our grown Er-KNN single crystals have great potential for use in multifunctional devices.

We prepared 300-nm GeTe thin films on (111)-oriented and piezoelectrically active 0.71 Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (PMN-0.29 PT) single-crystal substrates by the pulsed laser deposition and investigated the effects of in situ electric-field-controllable non-180° ferroelastic domain switching of the PMN-0.29 PT on the electronic properties of the GeTe films. The in-plane strain of the PMN-0.29 PT could be modulated continuously and reversibly by electric fields in a nonvolatile manner and could be effectively transferred to the GeTe films. Based on this, we realized reversible and nonvolatile resistance switching and obtained multilevel stable nonvolatile resistance states with good stability and endurance at T = 300 K by applying appropriate asymmetrical bipolar electric fields to the PMN-0.29 PT(111) substrates along the thickness direction. Such heterostructures may be used for multilevel data storage that allows each unit to store multiple bits of information and thus improve the memory density. Our investigation would be beneficial for the fabrication of nonvolatile memory devices using PMN-xPT-based heterostructures. Keywords: Ferroelastic strain, Electronic transport, PMN-PT, Heterostructure, Strain effect

Layered magnetic materials with nontrivial spin configurations usually emerge novel magnetic and transport properties. Herein, we investigate the anisotropy of magnetic properties and the anomalous Hall effect (AHE) in the layered itinerant ferromagnetic (FM) monoclinic ${\mathrm{Cr}}_{5}{\mathrm{Te}}_{8}\phantom{\rule{0.28em}{0ex}}(m\text{\ensuremath{-}}{\mathrm{Cr}}_{5}{\mathrm{Te}}_{8})$ single crystal. There are some observations: (1) Based on the measurement of angular-dependent magnetization $M(\ensuremath{\varphi})$, the system shows strong twofold out-of-plane symmetry. Meanwhile, the maximum of the rotational magnetic entropy change $\mathrm{\ensuremath{\Delta}}{S}_{M}^{\mathrm{R}}(T,H)$ between the $c$ axis and the $ab$ plane is about $1.1\phantom{\rule{0.28em}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}{\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ with a magnetic-field change of 4.5 T. Both indicate that there exists a strong uniaxial anisotropy in $m\text{\ensuremath{-}}{\mathrm{Cr}}_{5}{\mathrm{Te}}_{8}$. (2) The paramagnetic-FM phase-transition temperature (${T}_{\mathrm{C}}$) of $m\text{\ensuremath{-}}{\mathrm{Cr}}_{5}{\mathrm{Te}}_{8}$ is about 186 K and very sensitive to the applied pressure ($P$) with the derivative of ${T}_{\mathrm{C}}$ with respect to $P\phantom{\rule{0.16em}{0ex}}d{T}_{\mathrm{C}}/dP\ensuremath{\sim}\ensuremath{-}50\phantom{\rule{0.28em}{0ex}}\mathrm{K}\phantom{\rule{0.16em}{0ex}}{\mathrm{GPa}}^{\ensuremath{-}1}$. (3) As to the AHE, by considering the scaling behavior between the anomalous Hall resistivity ${\ensuremath{\rho}}_{xy}^{\mathrm{A}}$ and the longitudinal resistivity ${\ensuremath{\rho}}_{xx}$, the origin of the AHE in $m\text{\ensuremath{-}}{\mathrm{Cr}}_{5}{\mathrm{Te}}_{8}$ can be described by the skew-scattering mechanism. Moreover, the measurements of the AHE and isothermal magnetization confirm that the spin-flop behavior exists in $m\text{\ensuremath{-}}{\mathrm{Cr}}_{5}{\mathrm{Te}}_{8}$. The critical field ${H}_{\mathrm{C}}$ is about 0.26 T, and a possible explanation is given with the magnetic phase diagram of $m\text{\ensuremath{-}}{\mathrm{Cr}}_{5}{\mathrm{Te}}_{8}$ drawn.

We report on the growth of high-quality stoichiometric layered Cr

The intriguing electronic phase diagram involving charge density wave (CDW) transitions in the $\mathrm{Ta}{\mathrm{S}}_{2}$ system has been widely investigated over the past decade, especially for the $1T$ and $2H$ phases. $4{H}_{b}\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$, regarded as the natural heterostructure that combines the characteristics of $1T$ and $2H\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$, has also been the focus recently, due to the prospects for fundamental research and device applications. Here, we have systematically investigated the electrical transport properties of $4{H}_{b}\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$ single crystals combined with the band structure calculations and found that the low-temperature phase of candidate chiral superconductor $4{H}_{b}\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$ (${T}_{c}\ensuremath{\sim}3.5$ K) at normal state is not the simple CDW but exhibits a strong magnetic field dependence. The most significant result is the emergence of the large magnetoresistance (MR), which may originate from the high mobility of holes and partial compensation. In addition, the symmetry of MR under the low magnetic field has also changed significantly in $4{H}_{b}\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$, which is closely related to the CDW structures formed in the $H$ layers at 22 K. The results are conducive to the understanding of the mechanism of MR appearing in layered CDW compounds, and the presence of MR in $4{H}_{b}\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$ suggests the potential applications for functional devices in the future.

We report the epitaxial growth of the $\mathrm{Cr}$-doped ${\mathrm{In}}_{2\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{O}}_{3}$ (x = 0.05) ($\mathrm{Cr}$:${\mathrm{In}}_{2}{\mathrm{O}}_{3}$) semiconducting thin films on perovskite-type (111)-oriented $0.7\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}$\ensuremath{-}$0.3{\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_{3}$ (PMN-PT) ferroelectric single-crystal substrates in the form of ferroelectric field-effect devices that allow us to obtain an in situ tuning of the electron carrier density and magnetoresistance (MR) as well as the resistance in a reversible and nonvolatile manner, thereby stringently disclosing the relationship between the MR and the electron carrier density. Specifically, for the thinnest 25-nm $\mathrm{Cr}$:${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ film the polarization switching of the PMN-PT from the positively polarized ${P}_{r}^{+}$ state to the negatively polarized ${P}_{r}^{\ensuremath{-}}$ state results in a large increase in the resistance and MR. Particularly, at T = 10 K, the polarization switching induces reversible and nonvolatile changes in the magnitude and sign of MR, demonstrating strong coupling between the MR and the electron carrier density. Moreover, regardless of the polarization states of PMN-PT, MR for films with different thicknesses can be quite well described by a combination of the two-band model and the semiempirical model proposed by Khosla and Fischer based on which the positive MR (PMR) and negative MR (NMR) could be disentangled into positive component [MR(+)] and negative component [MR(\ensuremath{-})], respectively. We find that the polarization-switching-induced large decrease in the PMR and the change in the sign of MR from positive to negative is mainly due to the rapid decrease in the MR(+), demonstrating that the coupling between MR(+) and electron carrier density plays a dominant role in controlling the magnitude and sign of MR.

We report the superconductivity of PbTaS2 single crystals with the centrosymmetric structure. The systematic measurements of magnetization, electric transport and specific heat indicate that PbTaS2 is a weakly coupled type-II superconductor with transition temperature Tc = 2.6 K. Furthermore, the band structure calculations predicted four nodal lines near the Fermi energy with drumhead-like surface states, suggesting centrosymmetric PbTaS2 is a candidate of topological nodal line semimetals. These results demonstrate that PbTaS2 may open up another avenue for further exploring the properties of superconductivity and topological nodal-line states., 21 pages, 3 figures, 1 table

Reversible and nonvolatile electric-field control of the physical properties of topological insulators is essential for fundamental research and development of practical electronic devices. Here, we report the integration of topological insulator films with ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) single crystals in the form of ferroelectric field-effect devices that allow us to tune the electronic properties of topological insulator films in a reversible and nonvolatile manner. Specifically, gating of Cr-doped Bi2Se3 films with the PMN-PT layer is shown to provide a means to reversibly tune and modulate the carrier density and carrier type, as well as its other properties, such as the conductance, magnetoconductance, Fermi level, phase coherence length, and screening factor of electron–electron interaction by polarization switching at room temperature. These findings provide a simple and direct approach for probing the quantum transport properties of topological insulator films through ferroelectric gating by using PMN-PT. The combination of topological insulators with both ferroelectrically and piezoelectrically active PMN-PT thus offers a promising step toward exploring topological insulator/ferroelectric(piezoelectric) hybrid devices that could utilize not only the ferroelectric field-effect of topological insulator/PMN-PT structures but also the unique properties of respective materials. Electric gating can tune the properties of topological insulators, modifying the carriers’ density and type and Fermi level. Several gating materials can be used, but finding materials that allow the manipulation of these properties in a reversible and nonvolatile way at room temperature while being compatible with solid-state electronics is difficult. Now, an international team of researchers led by Ren-Kui Zheng, Yu Wang and Yu-Kuai Liu demonstrated the ferroelectric gating of the topological insulator Bi2Se3 doped with Cr, using single crystals of the perovskite material Pb(Mg1/3Nb2/3)O3-PbTiO3. They fabricated field-effect transistors in which they achieved electric-field control of the carrier density and type and of the Fermi level of the topological insulator through polarization switching of the ferroelectric layer. These results represent a step forward in the development of practical electronic devices based on topological insulators.

Superconducting thin films of FeSe0.5Te0.5 are epitaxially grown on piezoelectric 0.71Pb(Mg1/3Nb1/2)O3-0.29PbTiO3 (PMN-PT) single-crystal substrates. Upon the application of an electric field to the PMN-PT along the thickness direction the superconducting properties of the FeSe0.5Te0.5 films, such as the normal-state resistance, superconducting phase transition temperature, upper critical field, critical current, and coherence length are in situ modified via the ferroelectric polarization rotation, converse piezoelectric effect, and structural phase transition of the PMN-PT. Our findings demonstrate that the combination of superconducting thin films with piezoelectrically active PMN-PT offers a promising approach for exploring the intrinsic lattice strain effects of iron-based superconducting systems.

A series of In3+-doped Ba0.85Ca0.15TiO3:0.75%Er3+/xIn3+ (BCT:Er/xIn) lead-free piezoelectric ceramics with excellent upconversion luminescence were synthesized by the solid state reaction method. The effects of In3+ content on the crystal structure, ferroelectric, dielectric, piezoelectric, and upconversion luminescence properties were systematically studied. Under 980 nm excitation, a giant enhancement of the green emission (550 nm) by 10 times is achieved upon 2.5% mol In3+ doping, which is rarely observed in rare-earth ions-doped perovskite ferroelectric materials. The ultraviolet-visible-near infrared absorption measurements show that the In3+ doping may improve the dissolution of Er3+ ions and modify the isolate-/clustered-Er3+ ratio for x ≤ 2.5%, resulting in the enhancement of the absorption cross-section, thereby contributing to the enhancement of green luminescence. Unfortunately, the In3+ doping suppresses the ferroelectric and piezoelectric properties of the BCT:Er/xIn ceramics. This problem can be resolved by adding a small amount (1 mol%) of Yb3+ to the BCT:Er/xIn ceramics to restore their good ferroelectric and piezoelectric properties. Such In3+ and rare-earth ions co-doped ceramics with greatly enhanced upconversion luminescence and good ferroelectricity and piezoelectricity may have potential applications in electro-optical devices.

The electric-field control of resistance change (ΔR/R) and magnetic properties has been investigated in the LaMnO3(LMO)/0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (PMN-PT)(0 1 1) heterostructure. When the unipolar electric field (E) increases beyond the coercive field of PMN-PT, the ΔR/R vs E curve transforms from a loop-like behavior to a butterfly-like one. The electric-field-induced non-linear strain effect in the PMN-PT substrate realizes the modulation between low and high resistance states at room temperature. Moreover, different freezing temperatures of the LMO film are observed as the PMN-PT substrate is in the fully negative ( P r - ) and partially positive ( P r / / ) polarization states. The observation of less magnetization for P r / / state than that for P r - state indicates that the in-plane tensile strain is beneficial to the Jahn-Teller distortion.

2D ferroelectric field‐effect transistors devices are fabricated by epitaxial growth of Bi2O2Se on Pb(Mg1/3Nb2/3)O3‐PbTiO3. The devices exhibit ferroelectric polarization‐dependent photoresponse upon visible light (λ = 405 nm) and infrared light (IR, λ = 980 nm) illumination. Combining optical stimuli with ferroelectric gating, the devices show not only nonvolatile memory and optoelectronic response, but also coincidence detection of visible and infrared light.

(1 − x)K0.44Na0.52Li0.04Nb0.96Sb0.04O3–xBi0.25Na0.25Ba0.5ZrO3 [(1 − x)KNLNS-xBNBZ] (0 ≤ x ≤ 0.05) lead-free piezoelectric ceramics were prepared by using the conventional solid-state reaction method. The relationships among the composition, phase structure, phase volume fraction, microstructure, dielectric, ferroelectric, and piezoelectric properties as well as the phase diagram are established. Rietveld refinements of X-ray diffraction patterns show that with increasing doping level the volume fractions of the orthorhombic and tetragonal phases evolve from 58.7 and 41.3% for x = 0 to 1.7 and 98.3% for x = 0.05, which has strong impacts on the piezoelectric coefficient d 33 and Curie temperature T C. Upon the doping of BNBZ an orthorhombic-tetragonal (O–T) phase boundary forms near room temperature, resulting in an optimum d 33 ~ 288 pC/N, T C ~ 305 °C, a converse piezoelectric coefficient d 33 * ~ 391 pm/V, and a field-induced strain S ~ 0.156% for the x = 0.035 ceramics. These results demonstrate that a balanced development of d 33 and T C can be realized by choosing suitable doping materials and content.

Here we report the structural, dielectric, ferroelectric, piezoelectric, and magnetic properties of zirconium-doped 0.67BiFe0.97Ga0.03O3–0.33BaZr x Ti1−x O3 (BFG–BZT) lead-free ceramics. Structural investigations reveal that the doped Zr4+ ions (up to x = 0.15) can be fully incorporated into the host lattice. Dielectric measurements demonstrate that with increasing Zr4+ content, both the ferroelectric Curie temperature T C and the dielectric constant decrease monotonously, and that the dielectric constant exhibits relaxor behaviors upon Zr4+ doping. At the same time, the remnant polarization P r increases and then decreases, with the maximum P r ∼ 26 µC/cm2 for x = 0.025. One of the noteworthy results is that the remanent magnetic moment at room temperature increases up to 0.17 emu/g with increasing x from 0 to 0.15, which is attributed to enhanced lattice distortions due to Zr4+ doping. The coexistence of strong ferroelectricity and ferromagnetism at room temperature implies that the BFG–BZT could be potential candidate materials for multiferroic device applications.

We synthesized In3+-doped Ba0.85Ca0.15TiO3:Er3+/Yb3+/xIn3+ lead-free ferroelectric ceramics that possess strong ferroelectricity and excellent upconversion luminescence under the excitation of 980 nm. With increasing x from 0, the green and red emission intensity increases and reaches the maximum at x=1.5% mol, then decreases slightly with further increase in x. For the x=1.5% sample the green emission intensity at 550 nm is ∼10 times larger than that of In3+ undoped samples. Such a large In3+ doping-induced enhancement of the luminescence is unprecedented for perovskite ferroelectrics. Based on the ultraviolet-visible-near infrared absorption spectra, the variation of the luminescence intensity with the In3+ content was discussed.

Layered $\mathrm{V}{\mathrm{I}}_{3}$ has emerged as a new two-dimensional (2D) van der Waals bonded ferromagnetic semiconductor, which is an important class of materials for spintronics applications. However, the origin of ferromagnetism in $\mathrm{V}{\mathrm{I}}_{3}$ is still missing. In this work, we systematically investigate the anisotropy of magnetic properties and entropy change in a $\mathrm{V}{\mathrm{I}}_{3}$ single crystal. Based on the measurement of the angular-dependent magnetization, there is a weak sixfold in-plane symmetry and fourfold out-of-plane symmetry, indicating $\mathrm{V}{\mathrm{I}}_{3}$ possesses weak in-plane anisotropy and large out-of-plane anisotropy. Meanwhile, anisotropy of magnetic entropy change $\mathrm{\ensuremath{\Delta}}{S}_{M}(T,H)$ is further studied by the isothermal magnetization, showing that the difference of $\ensuremath{-}\mathrm{\ensuremath{\Delta}}{S}_{M}^{\mathrm{max}}$ between the $c$ axis and $ab$ plane reaches a maximum value $\ensuremath{\sim}0.95\phantom{\rule{0.16em}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}\mathrm{k}{\mathrm{g}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ with a magnetic field change of 4.5 T. With the scaling analysis of $\mathrm{\ensuremath{\Delta}}{S}_{M}$, the critical exponents can be obtained and indicate a crossover between 2D-Ising and tricritical mean-field magnetic interactions, and the rescaled $\mathrm{\ensuremath{\Delta}}{S}_{M}(T,H)$ curves fall onto a universal curve, demonstrating a second-order type of magnetic transition.

A series of Cr-doped In

Strongly correlated perovskite transition-metal oxides have received considerable attention due to their intriguing physical properties, including a metal-insulator transition that can be used in devices. Key questions remain, though, including whether in-plane tensile strain helps or hinders the transition. This study uses both static and dynamic lattice strain to manipulate the transport properties of NdNiO${}_{3}$ films systematically. The electric-field-tunable ferroelastic/piezoelectric strain approach can be used not only to obtain deeper insight into the intrinsic properties of nickelate films, but also as a simple, energy-efficient means to construct multistate resistive memory.

Single-phase (00 l)-oriented Bi

The origin of the structural phase transition of $\mathrm{TaT}{\mathrm{e}}_{2}$ has been investigated through the measurements of the temperature-dependent Raman spectroscopy and first-principles calculations. Based on the experimental and the calculation results, some interpretations are present. Firstly, abnormal redshifts of few Raman peaks are observed around the structural distortion temperature ${T}_{S}\ensuremath{\sim}170\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which is thought to originate from the clustering of Ta atoms below ${T}_{S}$. Secondly, new modes of Raman spectrum below ${T}_{S}$ are present, which is suggested to be related to the possible transition of charge density wave (CDW) in $\mathrm{TaT}{\mathrm{e}}_{2}$ from the room temperature to the low temperature. The ground state of the $\mathrm{TaT}{\mathrm{e}}_{2}$ is the special charge modulation, which is related to the formation of Ta-Ta dimer structure at low temperature. All explanations have been supported by first-principles calculations. Our results suggest that $\mathrm{TaT}{\mathrm{e}}_{2}$ is an alternative platform to explore the charge-lattice coupling with rich structural and charge configurations in $M{X}_{2}$ systems, where $M$ is the transition metal ion (Nb, Ta, Mo, etc.) and $X$ is the chalcogenide one (S, Se, or Te).

We investigate the Ti-doping effect on the charge density wave (CDW) of 1T-TaS2 by combining scanning tunneling microscopy (STM) measurements and first-principle calculations. Although the Ti-doping induced phase evolution seems regular with increasing of the doping concentration (x), an unexpected chiral CDW phase is observed in the sample with x = 0.08, in which Ti atoms almost fully occupy the central Ta atoms in the CDW clusters. The emergence of the chiral CDW is proposed to be from the doping-enhanced orbital order. Only when x = 0.08, the possible long-range orbital order can trigger the chiral CDW phase. Compared with other 3d-elements doped 1T-TaS2, the Ti-doping retains the electronic flat band and the corresponding CDW phase, which is a prerequisite for the emergence of chirality. We expect that introducing elements with a strong orbital character may induce a chiral charge order in a broad class of CDW systems. The present results open up another avenue for further exploring the chiral CDW materials., Comment: 12 pages, 4 figures

NiO–Ni composite films were heteroepitaxially grown on (111)-oriented ferroelectric 0.31Pb(In1/2Nb1/2)O3-0.35Pb(Mg1/3Nb2/3)O3-0.34PbTiO3 (PIN-PMN-PT) single-crystal substrates by pulsed laser deposition. The NiO films prepared in high vacuum are n-type conducting and possess room-temperature ferromagnetism, which originates from oxygen vacancies and the presence of the second Ni phase, respectively. Taking advantage of the electric-field-induced ferroelectric polarization charges, we realized in situ reversible and nonvolatile modulation of both the electrical resistance and magnetism of the film. A relative resistance change of ∼470% is obtained at room temperature, while an appreciable magnetization change of ∼15% was achieved at 50 K by switching the polarization states of PIN-PMN-PT. The coexistence of charge-density-tunable electronic and magnetic properties of NiO–Ni/PIN-PMN-PT heterostructures may provide a strategy to design charge-mediated multiferroic devices for nonvolatile memory and spintronic applications.

We report on the growth of high-quality stoichiometric layered Cr2Se3 single crystals with metallic and noncollinear antiferromagnetic ground state using the chemical vapor transport (CVT) method. The crystals show weak ferromagnetism in the in-plane and out-of-plane directions below the Neel temperature (T N), however, the field-cooled out-of-plane magnetization at 500 Oe and 10 K (∼0.24 μ B/f.u.) is approximately 15 times larger than that of the in-plane one, indicating strong c-axis easy uniaxial magnetic anisotropy, which is further supported by the in-plane and out-of-plane isothermal anisotropic magnetic hysteresis loops and the angular dependent magnetoresistance (MR). The latter also reveals a decrease of the coercive field of the crystal upon the tilting of the weak ferromagnetic easy axis away from the direction of the magnetic field. Further, the out-of-plane isothermal MR are negative below T N and show butterfly shapes for T < 10 K and couple with the magnetic hysteresis M(H) loop. These results may help researchers better understand the interplay between the weak ferromagnetism and the magnetotransport properties of 2D itinerant noncollinear antiferromagnetic systems.

0.55BiFe0.95Ga0.03Mn0.02O3–0.45BaTiO3 (BFGMO–BTO) thin films have been epitaxially grown on (0 0 1)-oriented Nb-doped SrTiO3 (NSTO) single-crystal substrates by the pulsed laser deposition. X-ray diffraction, selected area electron diffraction, magnetic hysteresis loop, and piezoresponse force microscopy results show that the structural and magnetic properties of the BFGMO–BTO epitaxial films can be progressively tuned by the oxygen pressure ( P O 2 ) during film deposition. The films prepared at P O 2 = 5 Pa phase separated into BFGMO and BTO phases, however, with increasing P O 2 to 20 Pa the films evolve from two phases to single phase with the best crystalline quality, largest saturation magnetization, lowest coercive field and relatively easy-switchable ferroelectric domains.

We report the fabrication of 0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (PMN-0.29PT)-based ferroelectric field effect transistors (FeFETs) by the epitaxial growth of cobalt-doped tin dioxide (SnO2) semiconductor thin films on PMN-0.29PT single crystals. Using such FeFETs we realized in situ, reversible, and nonvolatile manipulation of the electron carrier density and achieved a large nonvolatile modulation of the resistance (∼330%) of the SnO2:Co films through the polarization switching of PMN-0.29PT at 300 K. Particularly, combining the ferroelectric gating with piezoresponse force microscopy, X-ray diffraction, Hall effect, and magnetoresistance (MR), we rigorously disclose that both sign and magnitude of the MR are intrinsically determined by the electron carrier density, which could modify the s–d exchange interaction of the SnO2:Co films. Furthermore, we realized multilevel resistance states of the SnO2:Co films by combining the ferroelectric gating with ultraviolet light illumination, demonstrating that the Fe...

Topological Weyl semimetals (WSMs), a new quantum state of matter, provide the realization of relativistic Weyl fermions in solid-state physics. The appearance of the Weyl nodes will demonstrate novel transport phenomena related to the chiral anomaly. Theoretical prediction hints that the giant planar Hall effect (PHE) can be induced by the chiral anomaly in the WSMs. In this work, we performed PHE measurements to probe chiral charge in type-II WSM ${T}_{d}\text{\ensuremath{-}}\mathrm{MoT}{\mathrm{e}}_{2}$. Robust transverse voltage can be acquired when the magnetic field and electric current is coplanar. Subsequently, the amplitude of intrinsic PHE increased sharply at $T=50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ with the decreasing temperature. The increasing trend in the temperature dependence is attributed to coupling effect between the chiral charge and the electric one, which drives the system into intermediate-coupling states at higher field. Our work confirms that the PHE measurement is a practical transport tool to probe chiral charge pumping effect in the WSMs systems.

Copper-based ZrCuSiAs-type compounds of LnCuChO ($\mathrm{Ln}=\mathrm{Bi}$ and lanthanides, $\mathrm{Ch}=\mathrm{S}$, Se, Te) with a layered crystal structure continuously attract worldwide attention in recent years. Although their high-temperature (T \ensuremath{\ge} 300 K) electrical properties have been intensively studied, their low-temperature electronic transport properties are little known. In this paper, we report the integration of ZrCuSiAs-type copper oxyselenide thin films of $\mathrm{B}{\mathrm{i}}_{0.94}\mathrm{P}{\mathrm{b}}_{0.06}\mathrm{CuSeO}$ (BPCSO) with perovskite-type ferroelectric $\mathrm{Pb}(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}){\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{PbTi}{\mathrm{O}}_{3}$ (PMN-PT) single crystals in the form of ferroelectric field effect devices that allow us to control the electronic properties (e.g., carrier density, magnetoconductance, dephasing length, etc.) of BPCSO films in a reversible and nonvolatile manner by polarization switching at room temperature. Combining ferroelectric gating and magnetotransport measurements with the Hikami-Larkin-Nagaoka theory, we demonstrate two-dimensional (2D) electronic transport characteristics and weak antilocalization effect as well as strong carrier-density-mediated competition between weak antilocalization and weak localization in BPCSO films. Our results show that ferroelectric gating using PMN-PT provides an effective and convenient approach to probe the carrier-density-related 2D electronic transport properties of ZrCuSiAs-type copper oxyselenide thin films.

$\mathrm{MnC}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$ that exhibits spin frustration and complex spiral spin order is of great interest from both fundamental as well as application-oriented perspectives. Unlike $\mathrm{CoC}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$, whose ground state presents the coexistence of commensurate spiral spin order (CSSO) and ferroelectric order, $\mathrm{MnC}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$ shows no multiferroicity. One reason is that the spiral spin order is highly sensitive to the oxygen concentration in $\mathrm{MnC}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$. Here, we have successfully grown high-quality single-crystalline $\mathrm{MnC}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$ by the chemical vapor transport method. We observe a first-order magnetic transition from the incommensurate spiral spin order (ICSSO) at 19.4 K to the CSSO at 17.4 K. This magnetic transition is verified by magnetization, specific heat, and magnetoelectric measurements, which also confirm that the ground state exhibits the coexistence of the CSSO and magnetoelectricity below 17.4 K. Interestingly, the temperature evolution of Raman spectra between 5.4 and 300 K suggests that the structure remains the same. We also find that the phase-transition temperature of the CSSO decreases as applied magnetic field increases up to 45 kOe.

CrI3, which belongs to a rare category of two-dimensional (2D) ferromagnetic semiconductors, is of great interest for spintronic device applications. Unlike CrCl3 whose magnetism presents a 2D-Heisenberg behavior, CrI3 exhibits a larger van der Waals gap, smaller cleavage energy, and stronger magnetic anisotropy which could lead to a 3D magnetic characteristic. Hence, we investigate the critical behavior of CrI3 in the vicinity of magnetic transition. We use the modified Arrott plot and Kouvel-Fisher method, and conduct critical isotherm analysis to estimate the critical exponents near the ferromagnetic phase transition. This shows that the magnetism of CrI3 follows the crossover behavior of a 3D-Ising model with mean field type interactions where the critical exponents \b{eta}, {\gamma}, and {\delta} are 0.323, 0.835, and 3.585, respectively, at the Curie temperature of 64 K. We propose the crossover behavior can be attributed to the strong uniaxial anisotropy and inevitable interlayer coupling. Our experiment demonstrates the applicability of crossover behavior to a 2D ferromagnetic semiconductor., Comment: Accepted by Appl. Phys. Lett

Cr0.68Se single crystals with two-dimensional (2D) character have been grown, and the detailed magnetization M(T), electrical transport properties (including longitudinal resistivity and Hall resistivity and thermal transport ones (including heat capacity Cp(T) and thermoelectric power (TEP) S(T)) have been measured. There are some interesting phenomena: (i) Cr0.68Se presents a non-collinear antiferromagnetic (AFM) semiconducting behavior with the Neel temperature TN = 42 K and the activated energy Eg=3.9 meV; (ii) It exhibits the anomalous Hall effect (AHE) below TN and large negative magnetoresistance (MR) about 83.7% (2 K, 8.5 T). The AHE coefficient RS is 0.385 cm-3/C at T=2 K and the AHE conductivity {\sigma}H is about 1 ohm-1cm-1 at T=40 K, respectively; (iii) The scaling behavior between the anomalous Hall resistivity and the longitudinal resistivity is linear and further analysis implies that the origin of the AHE in Cr0.68Se is dominated by the skew-scattering mechanism. Our results may be helpful for exploring the potential application of these kind of 2D AFM semiconductors., Comment: Accepted by Appl.Phys.Lett

${\mathrm{CrGeTe}}_{3}$ recently emerges as a new two-dimensional (2D) ferromagnetic semiconductor that is promising for spintronic device applications. Unlike ${\mathrm{CrSiTe}}_{3}$ whose magnetism can be understood using the 2D-Ising model, ${\mathrm{CrGeTe}}_{3}$ exhibits a smaller van der Waals gap and larger cleavage energy, which could lead to a transition of magnetic mechanism from 2D to 3D. To confirm this speculation, we investigate the critical behavior of ${\mathrm{CrGeTe}}_{3}$ around the second-order paramagnetic-ferromagnetic phase transition. We obtain the critical exponents estimated by several common experimental techniques including the modified Arrott plot, Kouvel-Fisher method, and critical isotherm analysis, which show that the magnetism of ${\mathrm{CrGeTe}}_{3}$ follows the tricritical mean-field model with the critical exponents $\ensuremath{\beta}$, $\ensuremath{\gamma}$, and $\ensuremath{\delta}$ of $0.240\ifmmode\pm\else\textpm\fi{}0.006$, $1.000\ifmmode\pm\else\textpm\fi{}0.005$, and $5.070\ifmmode\pm\else\textpm\fi{}0.006$, respectively, at the Curie temperature of 67.9 K. We therefore suggest that the magnetic phase transition from 2D to 3D for ${\mathrm{CrGeTe}}_{3}$ should locate near a tricritical point. Our experiment provides a direct demonstration of the applicability of the tricritical mean-field model to a 2D ferromagnetic semiconductor.