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Lead magnoniobate-scandoniobate (PMN-PSN) solid solutions are important functional materials for transducer/actuator devices due to their extraordinary dielectric and electromechanical properties. These unique properties are related to the unusual low-frequency relaxation dynamics, which has not been sufficiently understood until now. In our paper the low-frequency relaxation dynamics of 0.45Pb(Mg1/3Nb2/3)O3–0.55Pb(Sc1/2Nb1/2)O3 single crystal is studied in frequency range 10 mHz–1 MHz at temperatures near the dielectric permittivity maximum. Four relaxation processes are identified. Low-frequency mode demonstrating the critical divergence around 306 K is revealed. However, the phase transition into ferroelectric phase at this temperature does not occur. Near 306 K, the appearance of M-type superstructure is found related to the combination of oxygen octahedral rotation and anti-parallel shifts of lead ions. The appearance of the additional order parameter suppresses the slowing down of the ferroelectric mode and the phase transition into the ferroelectric phase occurs only below 295 K. In addition, two relaxation processes, similar to reorientation and "breathing" polar nanoregion (PNR) modes reported for PMN, are found. The sharp softening of "reorientation mode" is observed on cooling from 315 to 295 K with Tf ∼288 K. The fourth relaxation process is the Debye process, and we assume that it is associated with defects relaxation. Below 295 K, all four relaxation processes still exist, but their parameters are practically temperature independent. The low-temperature phase is not exactly a "normal" ferroelectric phase, the PNRs persist in the FE phase. [ABSTRACT FROM AUTHOR]

Polycrystalline materials exhibit physical properties that are driven by both the interatomic crystallographic structure as well as the nature and density of structural defects. Crystallographic evolutions driven by phase transitions and associated twinning process can be observed in situ in three-dimensional (3D) using monochromatic synchrotron radiation at very high temperatures (over 1000 °C). This paper focuses on continuous measurements of the 3D-reciprocal space maps by high-resolution x-ray diffraction as a function of temperature along a phase transition process occurring between 1200 °C and room temperature. These high precision measurements allow observing the reciprocal space node splitting and the evolution of the diffuse scattering signal around that node as a function of temperature. The capability of this experimental method is illustrated by direct in situ high temperature measurements of the 3D splitting of a reciprocal space node due to phase transition recorded on dense pure zirconia polycrystals. [ABSTRACT FROM AUTHOR]

The ilmenite-type CoVO3 undergoes an antiferromagnetic transition below 140 K, and the tetravalent vanadium ions form V–V dimers below 550 K. This paper presents the magnetic spin structure and the phase transition induced by a magnetic field in CoVO3. Neutron powder diffraction at 50 K reveals that the spins of Co2+ arrange in a zigzag order on the honeycomb lattice with no external magnetic field. This magnetic spin structure does not exhibit spontaneous magnetization, corroborating previous magnetic measurements. Magnetization measurements in a pulsed-magnetic field revealed a transition to a weak-ferromagnetic phase at 37 T (100 K) and above 50 T (4.2 K), with spontaneous magnetization approximating 0.3–0.5 μB/f.u. This phase transition could potentially be attributed to a transition to a canted antiferromagnetic phase, which is brought about by the reduction in structural symmetry due to magnetostriction. [ABSTRACT FROM AUTHOR]

KTa1−xNbxO3 crystals exhibit excellent piezoelectric, ferroelectric, and electro-optical properties near the phase transition point, owing to the evolution of polar nanoregions (PNRs). This paper discusses the physical properties of the PNRs in three characteristic temperature ranges and analyzes the dielectric relaxation properties of the crystals. The effects of different heating rates and initial temperatures on the pyroelectric effect are discussed, and the influence of small electric fields on increasing the pyroelectric coefficient is investigated. The polarization characteristics of the crystal were obtained according to the change in the pyroelectric performance, and the influence of different conditions on the dynamic response of the PNRs was analyzed. It is proposed that the evolution of PNRs, induced by external coupling with an internal electric field, significantly affects macroscopic performance. [ABSTRACT FROM AUTHOR]

A significant negative thermal expansion (NTE) in an ilmenite-type MgVO3 ceramic is reported in this paper. MgVO3 undergoes V–V dimerization at 500–550 K accompanied by a structural phase transition from the triclinic (low-temperature) to rhombohedral (high-temperature) phase. This transition induces a large anisotropic crystal structure deformation and results in a large NTE in the ceramic sample, which is caused by the microstructural effect. The NTE was observed over a temperature range of 450–630 K, and the total volume shrinkage reached approximately −0.7%. This study demonstrates that controlling the chemical bonding is a promising method for the creation of NTE materials. [ABSTRACT FROM AUTHOR]

Microwave circuit electrodynamics of disordered superconductors is a very active research topic spawning a wide range of experiments and applications. For compact superconducting circuit elements, the transition to an insulating state poses a limit to the maximum attainable kinetic inductance. It is, therefore, vital to study the fundamental noise properties of thin films close to this transition, particularly in situations where a good coherence and temporal stability is required. In this paper, we present measurements on superconducting granular aluminum microwave resonators with high normal state resistances, where the influence of the superconductor to insulator phase transition is visible. We trace fluctuations of the fundamental resonance frequency and observe, in addition to a 1 / f noise pattern, a distinct excess noise, reminiscent of a random telegraph signal. The excess noise shows a strong dependency on the resistivity of the films as well as the sample temperature but not on the applied microwave power. [ABSTRACT FROM AUTHOR]

This paper presents a thermokinetic computational model of phase transitions in GST225 (germanium–antimony–tellurium) thin films [as well as other phase change materials (PCMs)] induced and initiated by the impact of nano- and femtosecond laser pulses in a wide energy fluence range according to the results of experimental studies using Raman spectra and thin-film samples of TEM cross-sectional image analysis. Applying this phase transition model makes it possible to understand the mechanism of the induced phase transition regarding the usage of PCMs in photonics and optoelectronic devices, which require precise control of the phase state of their PCM-based active elements for their functioning. The proposed model shows the internal structure of the sample, generating both the profile of the crystalline fraction distribution over the sample's depth, providing images of virtual TEM sections, as well as the volume distribution of the crystalline phase. [ABSTRACT FROM AUTHOR]

An insulator–metal transition (IMT) is an emergent characteristic of quantum materials, which have a great amount promise for applications, such as memories, optical switches, and analog brain functions. This is due to their ability to switch between two well-defined states. Thus, the characterization of the state-switching process is essential for the application of these materials. For vanadium dioxide ( VO 2 ), the phase transition can be determined from temperature, magnetic field, and dielectric constant. In this paper, we propose a diamond quantum sensing approach based on nitrogen-vacancy centers for analyzing phase transitions. By using lock-in-based optically detected magnetic resonance and Rabi measurement protocols, temperature and magnetic field can reflect local IMT information of the circuit, and microwave can determine IMT information of an electrical isolation region. Our multifunctional quantum sensor exhibits local, nondestructive, and integrated measurements, which are useful for reliability testing in IMT technology applications. [ABSTRACT FROM AUTHOR]

Two-dimensional van der Waals (2D vdW) materials have opened up an opportunity to explore an innovative spin-based magnetic nanodevice. However, controllable fabrication of 2D vdW ferromagnets with high Curie temperature remains challenging. In this paper, we reported the growth of 2D CrTe2 single-crystal films epitaxially on Al2O3 substrates using pulsed laser deposition. We find that it shows a typical paramagnetic–ferromagnetic (PM–FM) phase transition around 200 K. The precise Curie temperature and Weiss temperature are 189 and 206.7 K, respectively. The saturation magnetization reaches 73.64 emu/g for the film thickness of 30 nm. The critical exponent β = 0.329 indicates that the magnetic interactions obey the 3D-Ising model. Electronic transport measurement confirms that a CrTe2 film always remains a metallic behavior at 5 K ≤ T ≤ 320 K and the resistivity of room temperature is 1.5 mΩ/cm. The first-principles calculation uncovers that the FM ordering state mainly stems from an exchange coupling of the adjacent Cr-spin t2g polarized electrons and the metallic conductivity is due to p–d orbital hybridization between Cr and Te atoms. This work would shed new light on studying large-scale growth of 2D magnets and developing 2D magnet-based nanodevices of room temperature. [ABSTRACT FROM AUTHOR]

Recently, numerous two-dimensional (2D) magnetic materials are predicted with various promising properties, whereas noncollinear antiferromagnetic 2D materials are rarely reported. In this paper, we predicted a stable 2D noncollinear antiferromagnetic triangular lattice T-FeO2. The ground state is 120 ° antiferromagnetic with stronger next nearest neighbor exchange coupling than that of nearest neighbor exchange coupling because of the RKKY interaction. Our Monte Carlo simulations reveal that the magnetic phase transition is from a 120 ° antiferromagnetic state to a stripy state and then to a paramagnetic state with increasing temperature. In addition, by tuning the spin direction from an in plane antiferromagnetic state to a canted weak ferromagnetic state, a nontrivial topological phase transition could be induced. Our investigation about magnetic property and nontrivial topological phase transition is very promising for antiferromagnetic spintronics. [ABSTRACT FROM AUTHOR]

This paper reports a chip-scale radio frequency Lorentz-force gyrator based on an aluminum scandium nitride (Al0.7Sc0.3N) thin film. The two-port gyrator, which is essentially a lateral overtone bulk acoustic resonator, consists of a planar coil for Lorentz-force transduction and two top-bottom electrode pairs for piezoelectric transduction. The non-reciprocity is generated by the phase transition in the Lorentz-force coupling when an external vertical magnetic field is applied. The Lorentz-force gyrators based on both AlN and Al0.7Sc0.3N thin films demonstrate good non-reciprocity, i.e., the 180° phase difference, at approximately 517 and 388 MHz, respectively. Thanks to larger piezoelectric constants, the Al0.7Sc0.3N gyrator demonstrates easier impedance matching and a wider fractional bandwidth of 6.3% at a magnetic field of 1.65 T compared to 1.3% for an AlN device. Finally, an isolator consisting of the Lorentz-force gyrator and a shunt resistor is demonstrated over 35 dB of isolation and flat unidirectional transmission. [ABSTRACT FROM AUTHOR]

This paper presents an integrated measuring system combining a diamond anvil cell (DAC) and a high overtone bulk acoustic resonator (HBAR) operating at the microwave frequency band as 2.8–8.8 GHz. We have studied several metallic (W, Zr) and semiconductor (Si) samples under pressure up to ∼16 GPa. As an HBAR, we have used the "Al/Al0.72Sc0.28N/Mo/(100) diamond" structure utilizing a piezoelectric aluminum–scandium nitride film. We have observed that under pressure, the Q-factor of the HBAR decreases but remains at the value of 2500–3000, which is suitable for our experiments. It is demonstrated that the above system can be used for studying the behavior of various solids under high pressure, the pressure-induced phase transition in Zr, the registration of plastic deformations, and their relaxation in metals. Here, we discussed the phenomenon of an acoustic wave passing through a tungsten layer under a pressure of ∼5.5 GPa. The integrated DAC&HBAR measuring system has demonstrated some practical advantages over known ultrasonic systems combined with the DAC as the possibility of applying a microwave operational frequency, the measurement of a Q-factor change under pressure, and the miniature size of a sensitive HBAR element. The application of the built-in DAC&HBAR system will hopefully allow more accurate studies on materials in the GPa pressure range of a DAC. [ABSTRACT FROM AUTHOR]

Direct studies of the adiabatic temperature change (ΔTad) in the Ni47Mn40Sn12.5Cu0.5 Heusler alloy in steady magnetic fields up to 8 T by the extraction method and in pulsed magnetic fields up to 50 T were carried out in this paper. The alloy Ni47Mn40Sn12.5Cu0.5 demonstrates a magnetostructural phase transition (MSPT) of the first order in the 254–283 K temperature range as well as a second order phase transition near the Curie temperature TC = 313 K. An inverse magnetocaloric effect (MCE) was found in the region of the MSPT, and it reaches the maximum value ΔTad = −12 K in 20 T at the initial temperature T0 = 275 K. The irreversible part of the MCE reached ΔTir = −10 K when the field is completely removed. We consider the dynamics of the MCE in the vicinity of the MSPT and discuss the mechanisms that cause the giant irreversibility of the MCE as well as the possibilities of its application in hybrid cooling systems. [ABSTRACT FROM AUTHOR]

Metal phosphorus trichalcogenides (MPX3: M = Fe, Co, Ni, Cd, Mn; X = S or Se) represent a family of two-dimensional (2D) layered materials with an exceptional response to high pressure and a remarkable structural flexibility originating from the weak interlayer coupling. Despite their interest for applications, the knowledge about pressure-driven phase transitions of cadmium compounds is still limited. In this paper, we fill this gap and provide an accurate description of the structural evolution of CdPS3 by combining high-pressure experiments and first-principle calculations. We have performed high-throughput screening of the low-energy stacking configurations and found a phase evolution starting with C 2 / m space group at 12 GPa using the generalized evolutionary metadynamics method. Then, high-pressure experiments have been used to reveal a structural transition from phase-I (C 2 / m) to phase-II (R 3 ¯ ) to phase-III (R 3 ¯ ), which is marked by the appearance and vanishing of the Raman band at approximately 30–250 cm−1 in good agreement with our theoretical predictions. Our study paves the way to the understanding of pressure-induced phase transitions in weak interlayer coupling 2D CdPS3 materials. [ABSTRACT FROM AUTHOR]

Phase change materials, such as vanadium dioxide (VO2) and Titanium dioxide (Ti2O3) have received extensive attention because of the dramatic changes in their intrinsic properties during phase transitions. However, due to the rapid transition rate and wide dynamics, monitoring of processes is challenging. Previous detection methods are lack of speed and simplicity and require multiple interventions, which largely introduce human factors influencing the results and make it difficult to guarantee the accuracy and visualization. In this paper, the photonic spin Hall effect is used for real-time detection and highly sensitive analysis of the phase transition process of VO2 films. By incorporating with quantum weak measurement, the photonic spin-Hall shift acts as the pointer, and the phase transition process of VO2 is characterized effectively. The high measurement resolution with 63 S/(m μm) is achieved due to weak-value amplification. In our scheme, it does not involve any mechanical adjustment of optical components, thus enabling real-time, visual, non-contact detection of dynamic phase transition processes. [ABSTRACT FROM AUTHOR]

The ilmenite-type CoVO3 undergoes an antiferromagnetic transition below 140 K, and the tetravalent vanadium ions form V–V dimers below 550 K. This paper presents the magnetic spin structure and the phase transition induced by a magnetic field in CoVO3. Neutron powder diffraction at 50 K reveals that the spins of Co2+ arrange in a zigzag order on the honeycomb lattice with no external magnetic field. This magnetic spin structure does not exhibit spontaneous magnetization, corroborating previous magnetic measurements. Magnetization measurements in a pulsed-magnetic field revealed a transition to a weak-ferromagnetic phase at 37 T (100 K) and above 50 T (4.2 K), with spontaneous magnetization approximating 0.3–0.5 μB/f.u. This phase transition could potentially be attributed to a transition to a canted antiferromagnetic phase, which is brought about by the reduction in structural symmetry due to magnetostriction. [ABSTRACT FROM AUTHOR]

This paper proposes an improved heterodyne interferometer to measure the real phase distribution of vortex beams propagating in free space. The fundamental mode Gaussian beam passes through the vortex phase plate and has one or more phase transitions of 2π along the angular direction. Such vortex beams undergo phase distortion during transmission, and their complex phase distribution is difficult to measure. The improved heterodyne interferometer proposed in this study successfully measures the phase distortion, demonstrating high spatial resolution and phase measurement accuracy. Both the theoretical and the experimental results show that in the process of free space propagation, the phase jump intersection boundary transforms from a straight line into a twisted line and that the phase distribution gradually becomes a spiral phase distribution followed by a ring intensity distribution corresponding to further increase in the propagation distance. The proposed method is estimated to be capable of predicting the variation of the vortex beams based on the research conducted on the interaction between the vortex beams and the atmospheric turbulence. [ABSTRACT FROM AUTHOR]

In this paper, we investigated the evolution of microstructural chemistry of metal organic chemical vapor deposition grown (010) (AlxGa1x)2O3 films with varying Al contents, x 0.10-1.0, using atom probe tomography (APT). At a low Al content (x 0.25), the films are homogeneous, where layer inhomogeneity appears at a high Al content (x>0.25). Further increasing the Al content up to x 0.60 results in a homogeneous (AlxGa1x)2O3 layer. This change in microstructural features was linked to the phase transformation of (AlxGa1x)2O3 using a manifold learning approach to capture the governing features hidden in the data dimensionality. Combining APT to unsupervised machine learning enables APT to be an independent material characterization tool to investigate the microstructure, chemical composition, and phase related information. [ABSTRACT FROM AUTHOR]

The acoustic analogy of topological insulators is a hot field of materials research. On one-dimensional acoustic systems, many researchers have lately paid their attention to the applications of the Su-Schrieffer-Heeger (SSH) model, which can support topologically nontrivial phases. In this paper, we design a supercell composed of two identical hollow cylinders with a side split immersed in the air background. The supercell is arranged in a line to form a SSH model, which has three bandgaps including two zone-folding-induced gaps and a local resonant gap in the subwavelength region. By analyzing the eigenstates and calculating the Zak phases, we find that a topological phase transition takes place only in the zone-folding-induced gaps when we rotate the split-cylinders. Thus, a finite-size array, made of two sublattices with distinct topological properties, inevitably produces topological interface states on their interface. In addition, we demonstrate that the topological interface states can be adjusted in a wide frequency range by rotating the cylinders that control the coupling strength between two split-cylinders in one supercell. These tunable topological interface states may have potential application prospects in wave filtering, wave detecting, and so on. [ABSTRACT FROM AUTHOR]

Ge2Sb2Te5 (GST) is a kind of non-volatile chalcogenide phase-change material, which has a significant difference in permittivity between its amorphous and crystalline states in the infrared range. On account of this remarkable property, the combination of GST and metamaterials has great potential in tunable meta-devices. In this paper, a perfect absorber based on a nanocross-resonator array stacked above a GST spacer layer and an Au mirror (i.e., a metal-dielectric-metal configuration) is designed and experimentally demonstrated. A thin indium tin oxide (ITO) protective layer is inserted between the GST spacer and the Au resonator to avoid heat-induced oxidation of the GST layer during phase transition. We found that the ITO layer not only can protect the GST layer from deterioration, but also allows a significant blue shift in the absorption peak from 1.808 μm to 1.559 μm by optimizing the thickness of the two dielectric layers without scaling down the size of the metal structure, which provides a more feasible idea in pushing the absorption peak to higher frequency. The LC circuit model is presented to explain this blue-shift phenomenon, which is mainly attributed to the engineering of the dielectric environment of the parallel plate capacitance. In addition, such good performance in dynamitic modulation makes this perfect absorber a robust candidate for optical switching and modulating in various situations. [ABSTRACT FROM AUTHOR]

Metal phosphorus trichalcogenides (MPX3: M = Fe, Co, Ni, Cd, Mn; X = S or Se) represent a family of two-dimensional (2D) layered materials with an exceptional response to high pressure and a remarkable structural flexibility originating from the weak interlayer coupling. Despite their interest for applications, the knowledge about pressure-driven phase transitions of cadmium compounds is still limited. In this paper, we fill this gap and provide an accurate description of the structural evolution of CdPS3 by combining high-pressure experiments and first-principle calculations. We have performed high-throughput screening of the low-energy stacking configurations and found a phase evolution starting with C 2 / m space group at 12 GPa using the generalized evolutionary metadynamics method. Then, high-pressure experiments have been used to reveal a structural transition from phase-I (C 2 / m) to phase-II (R 3 ¯ ) to phase-III (R 3 ¯ ), which is marked by the appearance and vanishing of the Raman band at approximately 30–250 cm−1 in good agreement with our theoretical predictions. Our study paves the way to the understanding of pressure-induced phase transitions in weak interlayer coupling 2D CdPS3 materials. [ABSTRACT FROM AUTHOR]

In this paper, Re-doped MoS2 nanosheets were synthesized by a facile hydrothermal reaction and an annealing process. Through structural characterization, it was observed that the doping of Re could lead to 2H-1T transformation in both the Re and Mo coordinating structures, and the 1T ratios of Mo and Re atoms increased with the Re-doping concentration. Hysteresis loops demonstrated intrinsic room temperature ferromagnetism in the Re-doped MoS2 nanosheets, and the value of saturated magnetization reached 8 memu/g when the doping concentration of Re was 17.12 at. %. The magnetic properties resulted from the increasing net magnetic moments of Re atoms and 1T-coordinated Mo atoms. Additionally, the 1T-incorporated 2H structure and magnetism were thermally stable in these samples. These results explored a new path for MoS2 nanosheets to be applied in spintronic devices in the future. [ABSTRACT FROM AUTHOR]

Ferroelectric materials are widely used in the paraelectric phase in order to realize tunable capacitors with reduced losses and a low hysteresis effect. Nevertheless, for polycrystalline thin films, the ferro-/para-electric phase transition can be diffuse and some crystallites can exhibit a ferroelectric nature even if the sample seems to be globally in a paraelectric phase. In this case, domain wall motions are responsible for the sensitivity of the dielectric properties to the driving field and are very dissipative phenomena. In this paper, we evaluate, in the high-frequency band, the impact of ferroelectricity on the properties of Ba2/3Sr1/3TiO3 thin films near the phase transition, by measuring the film's dielectric properties as a function of the incident RF power and for different temperatures. [ABSTRACT FROM AUTHOR]

This paper demonstrated the electrocaloric effect (ECE) of (Na0.52K0.48-x)(Nb0.92-xSb0.08)O3-xLiTaO3 lead-free ceramics by direct differential scanning calorimetry measurement and indirect thermodynamic method. Both results show good consistency, where the direct one more accurately depicts ECE value and its evolution according to phase diagram. Due to the diffuse orthorhombic-tetragonal phase transition, the samples show a broad ECE peak which shifts to lower temperature with increasing LiTaO3 amount. Compared to previous direct results in lead-free ceramics at corresponding temperatures, they show a competitive ECE performance with ΔTmax of 0.41 K (@80°C), 0.30 K (@35 °C) and 0.16 K (@15 °C) under 20 kV/cm fields for x = 0.02, 0.0375 and 0.045. [ABSTRACT FROM AUTHOR]

Vanadium dioxide is a promising material for THz modulations due to its remarkable insulator-tometal transition (IMT) properties. Silicon-doped VO2 films, exhibiting excellent IMT properties with giant modulation amplitude and tunable phase transition temperature, greatly adapt in this area. In this paper, we report on a rebound effect of the IMT in Si-doped VO2 films. As the silicon dopants are increasingly introduced into VO2 films, the IMT is first tuned to lower temperature and then is anomalously shifted to higher temperature. This rebound effect is confirmed by crystal structure, valence concentration, and surface morphology. We attribute this rebound behavior to the interstitial and substitutive doping of Si atoms. Due to their distinct impactions on the crystallite, IMT properties of the VO2 films are depressed initially and recovered later. [ABSTRACT FROM AUTHOR]

For NaNbO3, its intrinsic phase transition sequence is still controversial in literature. In this paper, high-densified NaNbO3 ceramics derived from mechanochemical nanostructures presented a stabilized antiferroelectric characteristic. In addition to the Tc of 360 °C, another diffuse dielectric anomaly appeared around 100 °C, which did not vanish after O2 annealing treatment. The fine structure analysis indicated that this dielectric anomaly belongs to first order phase transition from P phase to Q phase. Moreover, even exerting the strong electric field close to breakdown value, the normal ferroelectric loops cannot be induced, proving the large free energy difference between these two phases. [ABSTRACT FROM AUTHOR]

The piezoceramic 0.96(K0.5Na0.5)0.95Li0.05Nb0.93Sb0.07O3-0.04BaZrO3 (KNLNS0.07-BZ), which shows large piezoelectric response (d 33 ≈ 425 pC/N), has been considered as one of the promising Pb-free substitutions for Pb(Zr,Ti)O3. In this paper, we investigate the phase transition sequence for KNLNS0.07-BZ by employing the dielectric measurement, mechanical spectroscopy, as well as Raman spectroscopy. Two ferroelectric-ferroelectric transitions have been detected by inspecting anomalies in the spectra, indicating the existence of three ferroelectric phases. Moreover, in-situ X-ray diffraction study has been further performed on KNLNS0.07-BZ to identify the crystal structure for each phase. The result reveals that the phase sequence for KNLNS0.07-BZ evolves from tetragonal (T) to rhombohedral (R) via an intermediate orthorhombic (O) phase. And the piezoelectric-optimal region for KNLNS0.07-BZ locates on a T-O boundary rather than the previously reported T-R boundary. Strong piezoelectricity may stem from the easier polarization rotation on the T-O boundary with reduced polarization anisotropy. [ABSTRACT FROM AUTHOR]

The gapless energy band spectra make the structures based on graphene and graphene bilayer with the population inversion to be promising media for the interband terahertz (THz) lasing. However, a strong intraband absorption at THz frequencies still poses a challenge for efficient THz lasing. In this paper, we show that in the pumped graphene bilayer, the indirect interband radiative transitions accompanied by scattering of carriers by disorder can provide a substantial negative contribution to the THz conductivity (together with the direct interband transitions). In the graphene bilayer on high-substrates with point charged defects, these transitions substantially compensate the losses due to the intraband (Drude) absorption. We also demonstrate that the indirect interband contribution to the THz conductivity in a graphene bilayer with the extended defects (such as the charged impurity clusters) can surpass by several times the fundamental limit associated with the direct interband transitions, and the Drude conductivity as well. These predictions can affect the strategy of the graphene-based THz laser implementation. [ABSTRACT FROM AUTHOR]

In this paper, the polarization response of dielectric blend based on the polyvinylidene fluoride (PVDF) and aromatic polythiourea (ArPTU) was investigated. It is found that the conduction loss at high field has been reduced 2-4 times along with an interesting irreversible transition under high electric field. The loss at 700 MV/m can be kept below 20%, which is better than most other polar dielectric polymers. It is proved that the transition is a phase change from glass phase to polar ß phase at a critical field. And the highly polar thiourea groups of ArPTU can act as traps for the electrons, reduce conduction loss greatly and prevent dielectric breakdown by stabilizing the electron energy. [ABSTRACT FROM AUTHOR]

The electrostrain behavior through reversible domain switching in aged acceptor-doped ferroelectric ceramics has been widely investigated in the past decade. However, previous works were focused on unpoled ceramics, which could only utilize part of domains to exchange nonequal crystalline axis to generate strain under external electric field. In this paper, we proposed an effective method: (1) Initially, the acceptor-doped ceramics should be poled. (2) Then, the ceramics need to be aged for enough time. (3) Finally, the applied electric field should be perpendicular to the poling direction. Our method can utilize more domains to exchange nonequal crystalline axis to contribute to electrostrain in comparison with unpoled ceramics reported in the literature. According to our method, the unipolar electrostrain of 1.5 mol.% Fe-doped (Pb,Ba,Sr)(Zr,Ti)O3 ceramics in this work could reach 0.33%, which was 3.75 times larger than that of unpoled one at 3.0kV mm-1. Meanwhile, the normalized strain d33* could reach nearly 1100 pm V -1 which was one of the highest values reported in ferroelectric ceramics. Additionally, the ceramics displayed interesting double or slim P-E (polarization-electric field) hysteresis loops at various electric fields. Our work provides a general method via reversible domain switching in aged acceptor-doped ferroelectric ceramics to obtain large electrostrain for high-displacement actuator applications. [ABSTRACT FROM AUTHOR]

In this paper, the temperature dependent lasing characteristics of solution-processed organic-inorganic halide perovskite CH3NH3PbI3 films have been demonstrated. The lasing temperature can be sustained up to a near room temperature at 260 K. Via the temperature dependent photoluminescence (PL) measurements, an emerged phase-transition band can be observed, ascribing to the crystalline structures changed from the orthorhombic to tetragonal phase states in the perovskites as a function of a gradual increase in the ambient temperature. The optical characteristics of the PL emission peaks and the anomalous shifts of the peak intensities are highly correspondent with the phase states in perovskites at different temperatures, showing a low-threshold lasing behavior at the phase transition. The laser cavities may be formed under multiple random scattering provided by the polycrystalline grain boundary and/or phase separation upon the phase transition. Since the threshold gain is potentially high in the random cavities, the large material gain exhibited by the solution-processed perovskite would be very promising in making practical laser devices. [ABSTRACT FROM AUTHOR]

An investigation of the terahertz wave generation in multiple laser-induced air plasmas is presented. First, it is demonstrated that the intensity of the terahertz wave increases as the number of air plasmas increases. Second, the physical mechanism of this enhancement effect of the terahertz generation is studied by quantitatively measuring the intensity of the generated terahertz wave as a function of phase difference between adjacent air plasmas. It is found out that the superposition is the main mechanism to cause this enhancement. Thus, the results obtained in this paper not only provide a technique to generate stronger terahertz wave but also enable a better understanding of the mechanism of the terahertz generation in air plasma. [ABSTRACT FROM AUTHOR]

In this paper, we investigated the evolution of microstructural chemistry of metal organic chemical vapor deposition grown (010) (AlxGa1x)2O3 films with varying Al contents, x 0.10-1.0, using atom probe tomography (APT). At a low Al content (x 0.25), the films are homogeneous, where layer inhomogeneity appears at a high Al content (x>0.25). Further increasing the Al content up to x 0.60 results in a homogeneous (AlxGa1x)2O3 layer. This change in microstructural features was linked to the phase transformation of (AlxGa1x)2O3 using a manifold learning approach to capture the governing features hidden in the data dimensionality. Combining APT to unsupervised machine learning enables APT to be an independent material characterization tool to investigate the microstructure, chemical composition, and phase related information. [ABSTRACT FROM AUTHOR]

In this paper, two displacive phase transitions around the morphotropic phase boundary (MPB) in Ba(Zr0.2Ti08)O3-x(Ba0.7Ca03)TiO3 (BZT-xBCT) ceramics were detected by inspecting two anomalies of the Raman Ti4+-O2- longitudinal optical mode (~725 cm-1). Further, permittivity and X-ray diffraction results demonstrated these two phase transitions originate from tetragonal (T) to rhombohedral (R) through an intermediate orthorhombic (O) phase. Importantly, we found that the maximum piezoelectric response (d33 = 545pC/N) was achieved at the boundary between the T and O phase, indicating that the giant piezoelectricity of BZT-xBCT may mainly stem from the T-O phase boundary due to easier polarization rotation and larger lattice softening. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate high converse magnetoelectric (ME) coupling in a Metglas/Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) laminated ME composite by exploiting stress and field induced reversible ferroelectric-ferroelectric phase transitions in a relaxor ferroelectric single crystal. The approach exploits large transformational strain induced by low applied electric field in a PIN-PMN-PT crystal that was mechanically stressed close to a rhombohedral to orthorhombic phase transformation. The ME coefficient was enhanced by an order of magnitude as compared to the linear piezoelectric regime, with a maximum value of 1.3 x 10-7 sm-1 in non-resonant mode. This phenomenon can thus be exploited to provide improvements in the development of ME devices and magnetic sensors. [ABSTRACT FROM AUTHOR]

Piezoelectric coefficient (d33) of (Na,K)NbO3 (NKN) is enhanced not only at its morphotropic phase boundary (MPB) composition but also enhanced at its polymorphic phase transition (PPT) temperature between orthorhombic and tetragonal phases (TO-T ). Thus, for NKN-based ceramics, even higher d33 could be obtained if both MPB and PPT are simultaneously optimized. This temperature as well as composition dependence of piezoelectric properties of NKN-based ceramics requires a systematic approach that differentiates factors for MPB and PPT. In this paper, the roles of Li and Ta known to affect d33 and TO-T were identified in relation with lattice parameters. [ABSTRACT FROM AUTHOR]

We present transmission visible wavelength filters based on strong interference behaviors in an ultrathin semiconductor material between two metal layers. The proposed devices were fabricated on 2 cm X 2 cm glass substrate, and the transmission characteristics show good agreement with the design. Due to a significantly reduced light propagation phase change associated with the ultrathin semiconductor layer and the compensation in phase shift of light reflecting from the metal surface, the filters show an angle insensitive performance up to ±70°, thus, addressing one of the key challenges facing the previously reported photonic and plasmonic color filters. This principle, described in this paper, can have potential for diverse applications ranging from color display devices to the image sensors. [ABSTRACT FROM AUTHOR]

Phasoscopy is a recently proposed concept correlating electromagnetic (EM) absorption and scattering properties based on energy conservation. Phase information can be extracted from EM absorption induced acoustic wave and scattered EM wave for biological tissue characterization. In this paper, an imaging modality, termed photoacoustic phasoscopy imaging (PAPS), is proposed and verified experimentally based on phasoscopy concept with laser illumination. Both endogenous photoacoustic wave and scattered photons are collected simultaneously to extract the phase information. The PAPS images are then reconstructed on vessel-mimicking phantom and ex vivo porcine tissues to show significantly improved contrast than conventional photoacoustic imaging. [ABSTRACT FROM AUTHOR]

In this paper, we describe an accurate method for determining the critical threshold of the Fréedericksz transition driven by an electric field. It is based on the measurement of the amplitude of the molecule fluctuations as a function of the voltage difference applied to a planar nematic cell. The precise measurement of the director fluctuations of the liquid crystal is made possible by the use of a very precise and sensitive polarization interferometer. The great advantage of the method is that it does not depend on complex fits as it is usually done in literature. [ABSTRACT FROM AUTHOR]

This paper demonstrates the impact of composition on the electrocaloric effect in lead-free (1-x)BaZr0.2Ti0.8O3-xBa0.7Ca0.3TiO3 ceramics, especially on the morphotropic phase boundary. The samples exhibited typical diffused ferroelectric-paraelectric phase transition, which was diffused the most around the tricritical point of rhombohedral, tetragonal, and cubic phases at x = 0.3. The x = 0.3 sample had the highest ΔTmax of 0.30 K under 20 kV/cm field and the widest cooling peak within a wide composition range, which endowed it with the best refrigeration capacity. Although the x = 0.7 sample had even higher ΔTmax, less diffused phase transition narrowed the cooling peak, which reduced the refrigeration capacity. [ABSTRACT FROM AUTHOR]

Exceptional points (EPs) in non-Hermitian systems have attracted enormous attention and spawned intriguing prospects for the manipulation of waves. Despite many efforts focusing on the exotic behaviors about EPs, there are only a few studies of phase transitions involving multiple EPs. Here, by employing staggered couplings as well as two pairs of on-site gain/loss, we propose a one-dimensional parity-time (P T)-symmetric Lieb photonic lattice and demonstrate diverse phase transitions of such a multiband structure. Owing to the non-Hermitian chiral symmetry, symmetry-protected higher-order EPs are constructed, and the system exhibits P T symmetry breaking beyond a certain threshold. More importantly, both the relative couplings and the on-site gain/loss can be flexibly reconfigured on demand, which yields the degeneracy of different bands, i.e., the emergence of multiple EPs. We also unveil that the EPs will no longer exist in the presence of a non-Hermitian diagonal disorder. In contrast, the spectrum remains symmetric and the EPs, along with the flatband, are robust against the off diagonal disorder due to the preserved non-Hermitian particle-hole symmetry. Our work not only provides a controllable platform for studying EPs but also sheds light on the exciting non-Hermitian physics based on exceptional degeneracies. [ABSTRACT FROM AUTHOR]

Due to the breaking of the time reversal symmetry and spatial inversion symmetry, hexagonal ferrovalley materials have intrinsic large valley polarization. Model analysis shows that tuning the two different band gaps of valleys can realize phase transitions between ferrovalley semiconductors, half valley metals, and valley-polarized quantum anomalous Hall semiconductors. Through first-principle calculations, monolayer ferrovalley materials RuXY (X, Y = F, Cl, Br), which exhibit valley splitting at the top valence band and the bottom conduction band, are predicted to achieve this valley-dependent topological phase transition. Due to the different orbital proportions of d orbitals, the valley splitting at the top valence band is much greater than that at the bottom conduction band. Strain can regulate the interaction between orbitals, thus producing valley-dependent band inversion, leading to the quantum spin or valley Hall effect. The chiral edge states are demonstrated under appropriate biaxial strain. The topological phase transition is related to the inversion of the band structure and Berry curvatures at K and K′ valleys. These results have certain significance for the design of two-dimensional valley-dependent quantum materials and the application of valleytronic devices. [ABSTRACT FROM AUTHOR]

Onsite gain-loss-induced topological braiding principle of non-Hermitian energy bands is theoretically formulated in multiband lattice models with Hermitian hopping amplitudes. Braid phase transition occurs when the gain-loss parameter is tuned across exceptional point degeneracy. Laboratory realizable effective-Hamiltonians are proposed to realize braid groups B 2 and B 3 of two and three bands, respectively. While B 2 is trivially Abelian, the group B 3 features non-Abelian braiding and energy permutation originating from the collective behavior of multiple exceptional points. Phase diagrams with respect to lattice parameters to realize braid group generators and their non-commutativity are shown. The proposed theory is conducive to synthesizing exceptional materials for applications in topological computation and information processing. [ABSTRACT FROM AUTHOR]

Single crystalline polar magnets (Mn1−xCox)2Mo3O8 (0 ≤ x ≤ 1) have been synthesized, and their magnetic properties have been studied. Magnetic interactions of the samples can be modulated drastically by Co-doping, and a ferrimagnetic to antiferromagnetic phase transition is identified at x = 0.35. For the samples hosting a ferrimagnetic state (0 ≤ x ≤ 0.3), square-like magnetic hysteresis loops are commonly identified, and the remnant magnetization and coercive field are revealed to strongly depend on the Co-content and temperature. The occupation ratio and magnetic nature of Co2+ are key ingredients to dictate the competition of various magnetic exchange couplings in the samples. [ABSTRACT FROM AUTHOR]

The group IV–VI monochalcogenides have attracted widespread attention because of their diverse physical properties and promising applications in electronics and optoelectronics. As a typical IV–VI semiconductor, SnSe displays ultra-low thermal conductivity and excellent thermoelectric properties, which deeply depends on its layered structure. The layered crystal structure and associated physical properties are sensitive to external pressure. Here, we have systematically investigated the structural behaviors and optical and electrical properties of layered SnSe under high pressure. The SnSe transforms from Pnma phase to Cmcm phase above 10 GPa, and a CsCl-type structure with a space group of Pm 3 ¯ m emerges around 30 GPa and coexists with Cmcm phase up to 42.5 GPa. The optical bandgap of SnSe shows gradual narrowing with increasing pressure, indicating gradual metallization of SnSe under compression. The pressure-induced metallization of SnSe is verified by electric transport experiments. The initial semiconducting SnSe transforms to a metallic state with increasing pressure up to 9.8 GPa. Both phase transitions and optical and electrical properties of SnSe at high pressure are reversible after releasing pressure. Our study provides a modulation strategy of crystal structures and physical properties for the group IV–VI monochalcogenides to broaden their applications in thermoelectric and optoelectronic fields. [ABSTRACT FROM AUTHOR]

Spatially uniform static phases in an antiferromagnet (AFM) at 0 K accessible by varying an external magnetic field along the anisotropy axis are investigated. Using the macrospin model, the energy contributions are comprised of the external field, effective anisotropy, and spatially homogeneous AFM exchange. The critical energy configurations are fully cataloged, and local energy concavity is used to identify stable states. Relative energy levels are taken into account to classify phase transitions. Phase diagrams for energetic stability and phase transitions are provided in terms of the strength of the applied field and the ratio between anisotropy and the AFM exchange. Two nonstandard critical energy states are identified as energy saddle points so are not stable but function as energy barriers between multiple stable states. The results determine thermal switching rates and suggest interesting AFM textures and solitons. [ABSTRACT FROM AUTHOR]

The (PbTiO3)n/(SrTiO3)n (PTO/STO) superlattice system has been shown to exhibit interesting topological phases (e.g., vortices and skyrmions) in addition to normal ferroelectric domain states. Existing studies are mostly focused on the dependence of topological polar distributions and properties of PTO/STO superlattice on its periodicity. Here, we study the strain effect on the topological phase transitions and ferroelectric domain structures employing phase-field simulations. We summarized in an isotropic strain (in-plane misfit strain along the x direction is equal to that along the y direction) periodicity phase diagram displaying the stability regions of different polar topological states, including normal ferroelectric twins, vortices, skyrmions, and mixtures of vortices and twins. We also analyzed the polarization configurations under anisotropic in-plane strains (in-plane misfit strain along the x direction is not equal to that along the y direction) and demonstrated that the strain anisotropy can be used to tune the directions of vortex arrays along either the [100] pc or [010] pc directions or labyrinth vortex arrays. This work offers guidance to manipulating polar structures in the PTO/STO superlattices via strain engineering. [ABSTRACT FROM AUTHOR]

Transition metal-free half-metallicity recently has been the subject of intense research activity due to its potential in spintronics application. By employing density functional theory calculations, we revealed that F-intercalated GaN bilayers exhibit robust p orbital-based half-metallicity against the intercalant concentration and biaxial strain of −10% to 10%. The stable half-metallicity results from a spontaneous phase transition through Stoner instability and can be attributed to unusually high density of states near the Fermi level due to the quasi-flat bands. These findings provide a strategy to design transition-metal free half-metallic materials for futuristic spintronic applications. [ABSTRACT FROM AUTHOR]

Combining alternate-current impedance spectrum measurement and first-principle calculations, we thoroughly analyzed the electrical transport behavior of LaAlO3 under high pressure. A pressure-induced ionic–polaronic–ionic transition has been discovered through impedance spectroscopy measurements. Through first-principle calculations, we have elucidated the physical origin of the emergence of polaronic conduction, which results from the distortion of electron density background around Al and O atoms. Furthermore, the discontinuous changes in the starting frequency of ion migration f W , relaxation frequency f b , and ionic resistance R i have been found at around 13.2 GPa, which can be ascribed to the phase transition of LaAlO3 from rhombohedral to cubic phase. Pressure can enhance the migration of O2− ions, causing an increase in the ionic conductivity of LaAlO3. This research will deepen our comprehension on the ion migration in solid electrolytes. [ABSTRACT FROM AUTHOR]

A special wake-up effect from antiferroelectric-like to ferroelectric (AFE-FE) characteristics in HfxZr1−xO2 thin films has been discussed intensively in terms of endurance performance enhancement. However, its physical origin and general impact on endurance remain unclear. In this work, the influence of various process parameters on the AFE-FE wake-up effect as well as on endurance performance and the material changes during AFE-FE wake-up are systematically studied. It is found that various parameters induce the AFE-FE wake-up effect and enhance endurance performance in the same way with enhancing tetragonal phase formation in HfxZr1−xO2 films, and the cycles of wake-up are universally associated with those of total endurance. In addition, via synchrotron-based grazing incidence x-ray diffraction, a tetragonal-orthorhombic-monoclinic phase transition is observed during AFE-FE wake-up. On the basis of these results, a correlation among crystalline composition, the AFE-FE wake-up effect, and the endurance performance of HfxZr1−xO2 thin films is established. This provides a clear guideline to a viable solution for the high endurance of HfxZr1−xO2 FE memory devices via crystal phase engineering. [ABSTRACT FROM AUTHOR]