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Pyroelectricity is a first rank tensor (vector) property that connects electric displacement to temperature. Like all vector properties, it is allowed only in certain point group symmetries. In related literature, a number of formulations exist that apparently treat various contributions to the pyroelectric coefficient but without strictly considering the crystal symmetry. We revisit the formulation of the pyroelectric coefficient in the presence of external fields and find that a consistent treatment of the pyroelectric coefficient, allowing one to arrive at a single convergent formula starting from either thermodynamic or electrostatic arguments, may not be straightforward. Motivated by this outcome, we develop an approach allowing both electrostatic and thermodynamic arguments for pyroelectricity to converge to one single expression with mathematical consistency of the partial derivatives. Albeit not very significant at first sight, the approach and the manner in which fields are introduced prove vital in evaluating the field contributions to the pyroelectric effect and their deconvolution in experiments. Importance of the crystal point groups is also highlighted in the context of vector properties, specifically pyroelectric coefficient. Finally, the so-called induced pyroelectricity and a correct mathematical expansion of the polarization is discussed to clarify the contributions of the pyroelectric effect. [ABSTRACT FROM AUTHOR]

Altermagnets (AM) are a recently discovered third class of collinear magnets and have been attracting significant interest in the field of condensed matter physics. Here, based on first-principles calculations and theoretical analysis, we propose four two-dimensional (2D) magnetic materials—monolayers V2Te2O, V2STeO, V2SSeO, and V2S2O—as candidates for altermagnetic materials. We show that these materials are semiconductors with spin-splitting in their nonrelativistic band structures. Furthermore, in the band structure, there is a pair of Dirac-type valleys located at the time-reversal invariant momenta (TRIM) X and Y points. These two valleys are connected by crystal symmetry instead of time-reversal symmetry. We investigate the strain effect on the band structure and find that uniaxial strain can induce valley polarization, topological states in these monolayer materials. Moreover, piezomagnetism can be realized upon finite doping. Our result reveals interesting valley physics in monolayers V2Te2O, V2STeO, V2SSeO, and V2S2O, suggesting their great potential for valleytronics, spintronics, and multifunctional nanoelectronics applications. [ABSTRACT FROM AUTHOR]

Understanding the structure formation and its underlying physical mechanism is a fundamental topic in condensed matter systems, with both academic and practical implications. Soft matter is playing a remarkable role in current era of information explosion, demonstrating enormous potential in integrated functional photonics. As unique soft photonic crystals with cubic symmetries, not only liquid crystalline blue phases (BPs) have circularly polarized selective reflection and ultra-fast electro-optical response, but also their three-dimensional photonic structures increase degrees-of-freedom for multiplexed optical modulation. In the thriving field of soft-matter-based photonics, precise and programmable engineering of BP crystal orientation is of vital importance for planar optical elements, which remains a challenging task due to the complexity of the nucleation process as well as the interaction between the BP building blocks and the boundary conditions. Aiming to gain a comprehensive understanding of how to tailor the orientation of BP crystals for the photonic applications of next generation, here we discuss the solutions for uniformity improvement and orientation control of BP crystals, about which a few of examples in combination with the underlying mechanisms are explained. In addition, the remaining challenges and the efforts that are expected are also reviewed. We expect this work provides a deeper understanding of phase transitions and resulting structures in soft crystals, which may open encouraging perspectives for their applications in photonics, biosensing, interfacial, and chemical engineering. [ABSTRACT FROM AUTHOR]

The prototype high-entropy oxide (HEO) Y0.2La0.2Ce0.2Pr0.2Sm0.2O2−δ represents a particularly complex class of HEOs with significant anion sublattice entropy. The system takes either a fluorite or bixbyite-type crystal structure, depending on synthesis kinetics and thermal history. Here, we synthesize bulk ceramics and epitaxial thin films of Y0.2La0.2Ce0.2Pr0.2Sm0.2O2−δ and use diffraction to explore crystal symmetry and phase. Thin films exhibit the high symmetry fluorite phase, while bulk ceramics adopt the lower symmetry bixbyite phase. The difference in chemical ordering and observed symmetry between vapor-deposited and reactively sintered specimens suggests that synthesis kinetics can influence accessible local atomic configurations, i.e., the high kinetic energy adatoms quench in a higher-effective temperature, and thus higher symmetry structure with more configurational entropy. More generally, this demonstration shows that recovered HEO specimens can exhibit appreciably different local configurations depending on synthesis kinetics, with potential ramifications on macroscopic physical properties. [ABSTRACT FROM AUTHOR]

Two-dimensional layered transition metal dichalcogenides (TMDCs) offer the ideal platform to optically initialize the qubits utilizing the superposition of valley polarized photons. However, in the commonly available 2H phase TMDCs, thickness sensitive crystal inversion symmetry demands the precise control over the number of layers, which makes the study of valley physics challenging. On the other hand, 3R-polymorphic phase TMDCs are non-centrosymmetric irrespective of the number of layers and therefore retain valley degree of freedom for all the thicknesses. Here, using polarization resolved photoluminescence measurements under linearly polarized excitations, we demonstrate the coherent superposition of the valley photons (known as valley coherence) in synthetic 3R-phase MoS2 with different numbers of layers ranging from 3 layers to 11 layers. With near-resonant excitation (633 nm for AX0), the degree of linear polarization (a measure of the valley coherence) is observed up to 70% at 80 K measurement temperatures and is found to be layer thickness independent. Robust valley coherence is attributed to the high optical quality and the 3R crystal symmetry of the MoS2. Using angle resolved polarization dependent measurements, it is established that the net dipolar emission aligns with the polarization orientations of the incident laser, which signifies the valley coherence to be independent of the crystal orientation. This study paves the way toward using 3R-phase MoS2 as a key material for the development of future quantum technologies utilizing valley polarized photons. [ABSTRACT FROM AUTHOR]

As a quantum state of frequency extrema in the momentum space of acoustic systems, sonic valley pseudospin provides a new degree of freedom for controlling acoustic waves. Higher-order topological insulators (HOTIs) have extended the traditional bulk-edge correspondence principle and are a crucial concept for classic wave regulation. However, HOTIs in valley sonic crystals (VSCs) only appear in a single bandgap, which limits the multi-frequency selectivity of the corner state and is not conducive to the design of multi-frequency acoustic communication devices. Here, we demonstrate "Y-shaped" acoustic crystals with C3 symmetry that form a double-band VSC, and the topological phase transitions in both low- and high-frequency band gaps coincide. We realize theoretically and experimentally higher-order states in dual-band valley sonic crystals. Our work enriches the application of HOTIs in acoustic multi-frequency regulatory systems and provides different avenues for designing of multi-band acoustic devices. [ABSTRACT FROM AUTHOR]

As a promising van der Waals (vdW) magnet, Fe3GeTe2 (FGT) has an inversion symmetry in structure, thus nominally prohibiting the generation of the Dzyaloshinskii–Moriya interaction and the resultant topological Hall effect (THE), which is manifested as hump-like features in transport. By virtue of doping, like the method of fabricating heterostructures, the crystal inversion symmetry of FGT could be broken artificially; therefore, humps of Hall resistance due to the THE are expected. However, alternative interpretations indicate inhomogeneity-induced multi-channel anomalous Hall effect (AHE) could also give rise to these humps. Here, besides the magnetic properties modified by Co doping, we report emergent humps in (Fe0.74Co0.26)3GeTe2 (FCGT) nanodevices. By scanning minor loops, partial anomalous Hall loops exceeding the full loop cannot be attributed to the THE, while it is in line with the multi-channel AHE scenario. These channels likely develop as a result of the Co atoms in FGT being randomly distributed, leading to multiple spatially separated magnetic regions with distinct coercive fields and magnetizations. By combing these AHE channels, the hump features in FCGT are effectively reproduced. Our work implies that the underlying multi-channel transport could also facilitate the development of versatile nanodevices utilizing the doped vdW ferromagnets. [ABSTRACT FROM AUTHOR]

As a promising van der Waals (vdW) magnet, Fe3GeTe2 (FGT) has an inversion symmetry in structure, thus nominally prohibiting the generation of the Dzyaloshinskii–Moriya interaction and the resultant topological Hall effect (THE), which is manifested as hump-like features in transport. By virtue of doping, like the method of fabricating heterostructures, the crystal inversion symmetry of FGT could be broken artificially; therefore, humps of Hall resistance due to the THE are expected. However, alternative interpretations indicate inhomogeneity-induced multi-channel anomalous Hall effect (AHE) could also give rise to these humps. Here, besides the magnetic properties modified by Co doping, we report emergent humps in (Fe0.74Co0.26)3GeTe2 (FCGT) nanodevices. By scanning minor loops, partial anomalous Hall loops exceeding the full loop cannot be attributed to the THE, while it is in line with the multi-channel AHE scenario. These channels likely develop as a result of the Co atoms in FGT being randomly distributed, leading to multiple spatially separated magnetic regions with distinct coercive fields and magnetizations. By combing these AHE channels, the hump features in FCGT are effectively reproduced. Our work implies that the underlying multi-channel transport could also facilitate the development of versatile nanodevices utilizing the doped vdW ferromagnets. [ABSTRACT FROM AUTHOR]

The observation of second-order nonlinear Hall effect generally requires broken inversion symmetry and other spatial symmetries (such as out-of-plane C2 symmetry) rather than time-reversal symmetry for the conventional Hall effect, which brings fascinating opportunities for the fundamental investigation and innovative concepts for device applications. However, the imposed symmetry constraints severely limit the accessibility of the second-order nonlinear Hall effect in many materials. Here, we report the observation of the second-order nonlinear Hall effect in bulk Td-WTe2 with symmetry constraints under the application of direct current (DC). The observed second-order nonlinear Hall effect is strongly dependent on the external DC with its magnitude being vanished at zero driving DC and linearly increased with increasing driving DC. Furthermore, the driving DC induced second-order nonlinear Hall effect in bulk Td-WTe2 exhibits the same twofold rotational symmetry as that of the intrinsic third-order nonlinear Hall effect, which is distinct from the second-order nonlinear Hall effect in few-layer Td-WTe2. In addition, similar driving DC induced second-order nonlinear features are also observed in the longitudinal direction. These imply that the driving DC induced second-order nonlinear transport properties may share the same physical origins as that of the intrinsic third-order nonlinear transport. Scaling relationship analyses further demonstrate that extrinsic scattering-related mechanism plays a dominant role in the observed high-order nonlinear transport properties. Our work outlines a potential roadmap to control the second-order nonlinear Hall effect in materials where crystal symmetry forbids this effect with both fundamental research interest and technological device applications. [ABSTRACT FROM AUTHOR]

The observation of second-order nonlinear Hall effect generally requires broken inversion symmetry and other spatial symmetries (such as out-of-plane C2 symmetry) rather than time-reversal symmetry for the conventional Hall effect, which brings fascinating opportunities for the fundamental investigation and innovative concepts for device applications. However, the imposed symmetry constraints severely limit the accessibility of the second-order nonlinear Hall effect in many materials. Here, we report the observation of the second-order nonlinear Hall effect in bulk Td-WTe2 with symmetry constraints under the application of direct current (DC). The observed second-order nonlinear Hall effect is strongly dependent on the external DC with its magnitude being vanished at zero driving DC and linearly increased with increasing driving DC. Furthermore, the driving DC induced second-order nonlinear Hall effect in bulk Td-WTe2 exhibits the same twofold rotational symmetry as that of the intrinsic third-order nonlinear Hall effect, which is distinct from the second-order nonlinear Hall effect in few-layer Td-WTe2. In addition, similar driving DC induced second-order nonlinear features are also observed in the longitudinal direction. These imply that the driving DC induced second-order nonlinear transport properties may share the same physical origins as that of the intrinsic third-order nonlinear transport. Scaling relationship analyses further demonstrate that extrinsic scattering-related mechanism plays a dominant role in the observed high-order nonlinear transport properties. Our work outlines a potential roadmap to control the second-order nonlinear Hall effect in materials where crystal symmetry forbids this effect with both fundamental research interest and technological device applications. [ABSTRACT FROM AUTHOR]

Multimetal oxyhalide intergrowths show promise for photocatalytic water splitting. However, the relationships between intergrowth stoichiometry and their electronic and nanoscale structures are yet to be identified. This study investigates Bi4TaO8Cl–Bi2GdO4Cl intergrowths and demonstrates that stoichiometry controls the tilting of [TaO6] octahedra, influencing the bandgap of the photocatalyst and its valence and conduction band positions. To determine how the [TaO6] octahedral tilting in the intergrowths manifests as a function of intergrowth stoichiometry, we investigated changes in crystal symmetry by analyzing features arising at the higher order Laue zone (HOLZ) of convergent-beam electron diffraction patterns. Higher Ta content intergrowths displayed a more intense outer HOLZ ring compared to lower Ta content intergrowths, indicating transformation from P21cn (orthorhombic) to P4/mmm (tetragonal). This finding suggests that more distortion occurs along the ⟨001⟩ directions of the crystal than the ⟨100⟩ and ⟨010⟩ directions. This variation directly impacts the electronic structure, affecting both conduction and valence band energy levels. By combining ultraviolet photoelectron spectroscopy, UV-visible diffuse reflectance spectroscopy, and electron energy loss spectroscopy, the absolute band positions of the intergrowths were determined. Agreement between the bandgaps obtained via ensemble and nanoscale measurements indicates nanoscale homogeneity of the electronic structure. Overall, the integrated approach establishes that the bandgap energy increases with increasing Ta content, which is correlated with the crystal symmetry and [TaO6] octahedral tilting. Broadly, the modular nature of intergrowths provides building block layers to tune octahedral tilting within perovskite layers for manipulation of optoelectronic properties. [ABSTRACT FROM AUTHOR]

Altermagnetism can achieve spin-split bands in collinear symmetry-compensated antiferromagnets. Here, we predict altermagnetic order in Janus monolayer Cr2SO with eliminated inversion symmetry, which can realize the combination of piezoelectricity and altermagnetism in a two-dimensional (2D) material, namely, 2D piezoelectric altermagnetism. It is found that Cr2SO is an altermagnetic semiconductor, and the spin-split bands of both valence and conduction bands are near the Fermi level. The Cr2SO has large out-of-plane piezoelectricity ( | d 31 | = 0.97 pm/V), which is highly desirable for ultrathin piezoelectric device application. Due to spin-valley locking, both spin and valley can be polarized by simply breaking the corresponding crystal symmetry with uniaxial strain. Our findings provide a platform to integrate spin, piezoelectricity, and valley in a single material, which is useful for multi-functional device applications. [ABSTRACT FROM AUTHOR]

The presence of topological band crossings near the Fermi energy is essential for the realization of large anomalous transport properties in the materials. The topological semimetals host such properties owing to their unique topological band structure, such as Weyl points or nodal lines (NLs), those are protected by certain symmetries of the crystal. When the NLs break out in the system, a large Berry curvature arises in the surrounding area of the gapped NL. In the present work, we studied anomalous transport properties of Cu2CoSn compound, which has a cubic Heusler crystal structure (space group: Fm 3 ¯ m). Cu2CoSn full Heusler compound possesses NLs at the Fermi level that is protected by mirror reflection symmetries of the lattice. Upon introducing the spin–orbit coupling in the Hamiltonian and by setting the magnetization axis along the [001] direction, we found that the NLs are gapped out and large Berry curvature arises in the system. The integral of Berry curvature gives the intrinsic anomalous Hall conductivity (AHC) about 1003 S/cm and the anomalous Nernst conductivity (ANC) about 3.98 A/m K at the Fermi level. These values of AHC and ANC are comparable to the largest reported values for the Co2MnGa Heusler compound. Therefore, Cu2CoSn becomes a newborn member of the family of full Heusler compounds, which possesses giant AHC and ANC that can be useful for the spintronics application. [ABSTRACT FROM AUTHOR]

We report the detailed mechanism behind the β to γ phase transformation in Sn-doped and Si-implanted Ga2O3 that we determined based on the direct observation of the atomic scale structure using scanning transmission electron microscopy (STEM). Quantitative analysis of the STEM images revealed that the high concentration of impurity atoms favored the formation of interstitial–divacancy complexes, which then leads to the secondary relaxation that creates additional interstitial atoms and cation vacancies, resulting in a local structure that closely resembles γ-Ga2O3. We explain the mechanism of how the impurity atoms facilitate the transformation, as well as the detailed sequence of the local γ phase transformation. The findings here offer an insight on how the lattice respond to the external stimuli, such as doping and strain, and transform into different structures, which is important for advancing Ga2O3 but also a variety of low symmetry crystals and oxides with multiple polymorphs. [ABSTRACT FROM AUTHOR]

It is known that a crystal structure and symmetry determine the physical properties of materials. Lattice distortion can strongly affect the symmetry of the crystal structure. Polar materials show changes in polarization with temporal fluctuations of temperature due to the asymmetry. As a polar crystal, hemimorphite shows excellent pyroelectric properties. However, to date, there are a few studies on its intrinsic physical properties, and the mechanism of its pyroelectricity remains unclear. In this paper, single-crystal x-ray diffraction measurement was carried out to obtain the atomic positions at 100–400 K. Furthermore, the electric dipole moments of [ZnO4] and [SiO4] polyhedrons along a, b, and c axes have been calculated. The calculated pyroelectric coefficient derived from the intrinsic electric dipole moment was compared with the experimental measurement. The results indicate that the pyroelectric coefficients of hemimorphite at different temperatures mainly come from the variation of the electric dipole moment of [ZnO4] and [SiO4] polyhedrons along the c axis. The electric dipole moment changes as a function of temperature from 100 to 400 K, which is induced by the random lattice distortion. It is found that pyroelectricity is strongly correlated with the random lattice distortion. The establishment of the relationship between lattice distortion and pyroelectricity helps us to regulate the specific electrical parameters of the material, which may lead to future work in energy harvesting and further properties. [ABSTRACT FROM AUTHOR]

The choice and quality of single-crystal substrates for epitaxial thin film growth drastically influence the properties of the synthesized films. Among them, high-quality substrates with a polar surface are particularly difficult to prepare due to their inherent structural instability against electrostatic forces of the polar discontinuity. As a commonly used insulating substrate, the (111) cut of MgO shows alternating planes of Mg2+ and O2− that form a hexagonal unit cell at the surface, which is useful for thin films with similar crystal symmetries. We present a method to prepare such a surface by first introducing (3 × 3) R 30 ° reconstructed MgO (111) surface via face-to-face annealing in an O2 environment. We then dissolve the top layers in acid to achieve an unreconstructed and also passivated surface. The structure, stoichiometry, and stability of the resulting surfaces are characterized. This work provides an easy and reliable way to prepare a MgO (111) polar surface ready for subsequent epitaxial growth. [ABSTRACT FROM AUTHOR]

Fe3Sn2, a ferromagnetic alloy with a kagome lattice, has attracted much attention from research communities owing to its special crystal structure and symmetry, which gives rise to numerous intriguing magneto-electronic properties, including the topological Hall effect, skyrmionic bubbles, Dirac cones, and Weyl points. To study such physical properties, Fe3Sn2 epitaxial films without buffer layers are in urgent need. In this work, we fabricated the Fe3Sn2 epitaxial films without buffer layers using the facing-target magnetron sputtering method, and their magneto-electronic transport properties were investigated systematically. It is found that the sign of the Hall resistivity slope in a high-field region is reversed when the temperature decreases below 100 K, suggesting a temperature-induced transition of charge carriers from electrons to holes. Moreover, a non-negligible topological Hall resistivity subtracted from the anomalous Hall resistivity is depicted, and its value is comparable to that of the Fe3Sn2 single crystals. These results not only offer a clear understanding of ferromagnetic Fe3Sn2 films with the kagome lattice but also provide guidance for fabrication and application of epitaxial Fe3Sn2 films. [ABSTRACT FROM AUTHOR]

The spin Hall effect appears in nature in two forms. Its intrinsic form is highly dependent on the crystal symmetry while its extrinsic form stems from impurity scattering. Its efficiency is defined by the spin Hall angle, θ S H , and has profound impact on spintronic technologies. However, an accurate measurement of θ S H is not straightforward nor the identification of its origin. In this work, we apply a spin-torque driven ferromagnetic resonance method that is probed in two different ways, optically and electrically, to study the dependence of θ S H in the crystallographic direction in epitaxial Al2O3/Pt (111), MgO(110)/Pt (110), and MgO(001)/Pt (001) films. We show that the electrical technique is limited in its ability to accurately quantify θ S H at high current densities, and in some cases, it may even result in erroneous θ S H values. Such cases include films that exhibit a large inhomogeneous broadening. We find that θ S H is strongly affected by the crystallographic direction. Our study extends the understanding of one of the most commonly used methods for the exploration of the spin Hall effect. [ABSTRACT FROM AUTHOR]

The development of spintronics applications using the spin–orbit torque from the spin Hall effect is expected to advance the low-power consumption of various information devices. However, the dependence of the spin Hall effect on the crystal symmetry of the material has not been studied in detail. In this work, we investigate the dependence of the spin Hall angle on crystal orientation for epitaxial Pt in Pt/FeNi systems by using multiple ferromagnetic resonance techniques. Our estimation of the spin Hall angle indicates that the efficiency of spin current generation via the spin Hall effect can vary depending on the direction in which the spin current flows out, while the direction of the charge current has less impact on this efficiency. [ABSTRACT FROM AUTHOR]

Based on first-principles calculations, we predict a giant crystal-induced transverse current in antiferromagnetic γ-FeMn. This abnormal transverse current cannot be understood by the conventional anomalous Hall effect (e.g., Berry curvature, skew scattering, and side jump), which widely exists in ferromagnetic and antiferromagnetic materials. Moreover, the efficiency of the transverse current generation therein can be as large as 18.4% at low temperatures; this is an order of magnitude larger than the anomalous Hall angle in conventional ferromagnetic materials, such as Fe or Fe-based alloys. Furthermore, using the Boltzmann transport equation and a tight-binding model, we conclude that the asymmetric group velocities on the Fermi surface are the origin of this crystal-induced transverse current in γ-FeMn. Additionally, with a systematic discussion, we show that this unusual effect is not dependent on specific materials but is universal in any crystal with anisotropic symmetry. [ABSTRACT FROM AUTHOR]

Bulk single-crystal relaxor-ferroelectrics, like Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT), are widely known for their large piezoelectricity. This is attributed to polarization rotation, which is facilitated by the presence of various crystal symmetries for compositions near a morphotropic phase boundary. Relaxor-ferroelectric thin films, which are necessary for low-voltage applications, suffer a reduction in their piezoelectric response due to clamping by the passive substrate. To understand the microscopic behavior of this adverse phenomenon, we employ the AC electric field driven in-operando synchrotron x-ray diffraction on patterned device structures to investigate the piezoelectric domain behavior under an electric field for both a clamped (001) PMN-PT thin film on Si and a (001) PMN-PT membrane released from its substrate. In the clamped film, the substrate inhibits the field-induced rhombohedral (R) to tetragonal (T) phase transition resulting in a reversible R to Monoclinic (M) transition with a reduced longitudinal piezoelectric coefficient d33 < 100 pm/V. Releasing the film from the substrate results in recovery of the R to T transition and results in a d33 > 1000 pm/V. Using diffraction with spatial mapping, we find that lateral constraints imposed by the boundary between the active and inactive materials also inhibit the R to T transition. Phase-field calculations on both clamped and released PMN-PT thin films simulate our experimental findings. Resolving the suppression of thin film piezoelectric response is critical to their application in piezo-driven technologies. [ABSTRACT FROM AUTHOR]

Hyperbolic crystals, such as α-MoO3, can support large wavevectors and photon density as compared to the commonly used dielectric crystals, which makes them a highly desirable platform for compact photonic devices. The extreme anisotropy of the dielectric constant in these crystals is intricately linked with the anisotropic character of the phonons, which along with photon confinement leads to the rich physics of phonon polaritons. However, the chiral nature of phonons in these hyperbolic crystals have not been studied in detail. In this study, we report our observations of helicity selective Raman scattering from flakes of α-MoO3. Both helicity-preserving and helicity-reversing Raman scattering are observed. Our studies reveal that helical selectivity is largely governed by the underlying crystal symmetry. This study shed light on the chiral character of the high symmetry phonons in these hyperbolic crystals. It paves the way for exploiting proposed schemes of coupling chiral phonon modes into propagating surface plasmon polaritons and realizing compact photonic circuits based on helical polarized light. [ABSTRACT FROM AUTHOR]

Hyperbolic crystals, such as α-MoO3, can support large wavevectors and photon density as compared to the commonly used dielectric crystals, which makes them a highly desirable platform for compact photonic devices. The extreme anisotropy of the dielectric constant in these crystals is intricately linked with the anisotropic character of the phonons, which along with photon confinement leads to the rich physics of phonon polaritons. However, the chiral nature of phonons in these hyperbolic crystals have not been studied in detail. In this study, we report our observations of helicity selective Raman scattering from flakes of α-MoO3. Both helicity-preserving and helicity-reversing Raman scattering are observed. Our studies reveal that helical selectivity is largely governed by the underlying crystal symmetry. This study shed light on the chiral character of the high symmetry phonons in these hyperbolic crystals. It paves the way for exploiting proposed schemes of coupling chiral phonon modes into propagating surface plasmon polaritons and realizing compact photonic circuits based on helical polarized light. [ABSTRACT FROM AUTHOR]

Taking inspiration from the brain, neuromorphic systems are thought to be a potential computational platform to solve the problems of the "von-Neumann bottleneck." Artificial synaptic devices hold the potential to emulate the multi-synapse system to perform complex functions by applying anisotropic materials. Here, we demonstrate a synaptic device based on a two-dimensional ReS2 material, exhibiting synaptic functions such as short-term plasticity, long-term plasticity, paired-pulse facilitation, and spike-rate dependent plasticity. More importantly, benefiting from the broken symmetry of the ReS2 crystal, the devices showed obvious anisotropy of response behavior to the same input signal. In addition, the synaptic performance can also be modulated by laser irradiation. Our results provide possibilities for two-dimensional materials to mimic the biological axon-multisynapse systems with additional optical modulation. [ABSTRACT FROM AUTHOR]

We investigated the crystalline direction dependent Dzyaloshinskii–Moriya interaction (DMI) in single crystal CoFeB(211)/Pt(110) films utilizing the Brillouin light scattering technique. The in-plane magnetic anisotropy and the DMI coefficient show a clear 1/tCFB relation. The quantified DMI strength is −2.5 pJ/m along Pt [ 001 ] and −1.2 pJ/m along Pt [ 1 1 ¯ 0 ] with a large anisotropy ratio of 2.1. Such a strong anisotropy of DMI is attributed to the C2v crystal symmetry at the CoFeB/Pt(110) interface. Our results provide a route toward designing topological magnetic textures by tailoring the DMI strength along the specific crystal orientation in ferromagnet/Pt(110) systems. [ABSTRACT FROM AUTHOR]

Twistronic assembly of 2D materials employs the twist angle between adjacent layers as a tuning parameter for designing the electronic and optical properties of van der Waals heterostructures. Here, we study how interlayer hybridization, weak ferroelectric charge transfer between layers, and a piezoelectric response to deformations set the valence and conduction band edges across the moiré supercell in twistronic homobilayers of MoS2, MoSe2, WS2, and WSe2. We show that, due to the lack of inversion symmetry in the monolayer crystals, bilayers with parallel (P) and antiparallel (AP) unit cell orientations display contrasting behaviors. For P-bilayers at small twist angles, we find band edges in the middle of triangular domains of preferential stacking. In AP-bilayers at marginal twist angles ( θ A P < 1 °), the band edges are located in small regions around the intersections of domain walls, giving highly localized quantum dot states. [ABSTRACT FROM AUTHOR]

We propose a method to control a bilayer superconducting spin valve (SSV) which does not perturb its superconducting state and is suitable for energy saving cryogenic electronics. This SSV consists of a superconducting layer and a helimagnetic layer of B20 family compounds, namely, Nb and spiral antiferromagnet MnSi. Thanks to unique properties of MnSi—broken inversion symmetry and cubic crystal lattice—there are a few ground state magnetic configurations with different directions of the magnetic spiral, divided by a potential barrier. Superconductivity in such a bilayer is controlled by the reorientation of the spiral vector in the MnSi layer, which leads to a change in the critical temperature of the Nb layer due to the proximity effect. The switching is proposed to be carried out by a several hundred ps in duration magnetic field pulse of several kOe in magnitude. Such a pulse does not destroy the superconducting state of the Nb layer by itself but leads to the excitation of magnons in the MnSi layer, which triggers the reorientation process of the magnetic spiral. After the completion of this process, the Nb layer switches into a normal state. Inverse switching returns the spiral to the initial state, opening the valve and turning on the superconducting state. The system can be switched there and back by a magnetic field of opposite signs along one direction in the layers plane, which allows an easy control. The switching time is estimated as several nanoseconds, which coincides with the scales of the STT-MRAM recording time. [ABSTRACT FROM AUTHOR]

High-order harmonic generation (HHG) from crystals is emerging as a new ultrashort source of coherent extreme ultraviolet (XUV) light. Doping the crystal structure can offer a new way to control the source properties. Here, we present a study of HHG enhancement in the XUV spectral region from an ionic crystal, using dopant-induced vacancy defects, driven by a laser centered at a wavelength of 1.55 μm. Our numerical simulations based on solutions of the semiconductor Bloch equations and density-functional theory are supported by our experimental observations and demonstrate an increase in the XUV high harmonic yield from doped bulk magnesium oxide (MgO) compared to undoped MgO, even at a low defect concentration. The anisotropy of the harmonic emission as a function of the laser polarization shows that the pristine crystal's symmetry is preserved. Our study paves the way toward the control of HHG in solids with complex defects caused by transition-metal doping. [ABSTRACT FROM AUTHOR]

We present studies of second harmonic generation in core/shell ZnTe/Zn1−xMgxTe nanowires (NWs) with a waveguiding geometry. The NWs are excited by a pulsed femtosecond laser over a broad range of excitation wavelengths from 800 to 1140 nm. This allows us to demonstrate a giant increase in the second harmonic light intensity. We attribute the observed effect to the combined action of absorption by the NW, the confinement of light due to the waveguiding geometry, and the bending of the NWs. We show that a multi-photon luminescence (MPL) is generated alongside the second harmonic light. Furthermore, we demonstrate a strong polarization anisotropy of the second harmonic and MPL signals. Unlike in bulk semiconductors and quantum wells, this anisotropy is not due to the underlying symmetries of the crystal structure, but can be attributed solely to the shape anisotropy of the NW. [ABSTRACT FROM AUTHOR]

Crystalline phase identification for hafnium-based ferroelectrics by diffraction techniques has been elusive. We use density-functional-theory (DFT)-assisted extended X-ray absorption fine-structure spectroscopy (EXAFS) to determine the crystal symmetry of thin hafnium zirconium oxide (Hf0.46Zr0.54O2) films grown by atomic layer deposition. Ferroelectric switching in TiN/Hf0.46Zr0.54O2/TiN metal–insulator–metal capacitors is verified. Grazing-incidence fluorescence-yield mode Hf L3 and Zr K absorption edge EXAFS data are compared with reference data calculated from DFT-based atomic coordinates for various structural phases of Hf0.5Zr0.5O2. Via EXAFS multiphase fitting, we confirm that the frequently invoked polar orthorhombic Pca21 phase is present in ferroelectric hafnium zirconium oxide, along with an equal amount of the nonpolar monoclinic P21/c phase. For comparison, we verify that paraelectric HfO2 films exhibit the P21/c phase. [ABSTRACT FROM AUTHOR]

The local crystal symmetry and crystal structure at CaTiO3 twin boundaries were examined using convergent-beam electron diffraction (CBED). CBED patterns obtained right at the twin boundaries exhibited one mirror symmetry not observed in the bulk form. Two types of crystal structures were identified at the boundaries on different boundary widths and symmetry breaking. Quantitative analysis using CBED patterns revealed atomic shifts of Ti and Ca. A distortion of oxygen octahedra was also produced, which was smaller than the corresponding distortion obtained in the bulk form. The refined crystal structure conforms to the polar structure expected only at CaTiO3 twin boundaries. [ABSTRACT FROM AUTHOR]

Phononic crystals have attracted wide attention in diverse scientific communities due to their ability to efficiently reflect, focus, and guide acoustic and elastic waves. Their use in ultrasonic applications such as medical imaging, however, remains elusive since three-dimensional phononic crystals with lattice constants in the range of hundreds of micrometers are required to operate with MHz ultrasound, smaller than what most additive manufacturing solutions can provide. This work presents a three-dimensional phononic crystal with a cubic symmetry having an experimental ultra-wide complete bandgap extending from 0.6 MHz to 7.5 MHz. Crystal specimens are manufactured using two-photon lithography at the microscale, with a lattice constant of 300 μm, and are characterized using heterodyne interferometry. [ABSTRACT FROM AUTHOR]

We introduce a carbon allotrope with rhombic symmetry and a high crystal density reached 3.56 g/cm3, named Pn-C10, which was found using an unbiased particle-swarm structural-searching technique. This fully sp3-bonded carbon phase is dynamically and mechanically stable in its ground state. Results from our calculations reveal the excellent mechanical nature of Pn-C10 with a claimed shear modulus and Vickers hardness of 484 GPa and 88.45 GPa, respectively, which also suggested its slight anisotropy in elasticity. The predicted electronic band structure indicates that Pn-C10 is an indirect-bandgap material with a bandgap of 5.05 eV. Natural tiling analysis and simulations of x-ray diffraction and Raman spectra were performed to provide insights for further studies into Pn-C10 and other carbon phases. [ABSTRACT FROM AUTHOR]

One-inch free-standing (001) diamond layers on a (11 2 ¯ 0) (a-plane) sapphire substrate with an Ir buffer layer (Kenzan Diamond®) were grown. The full-width at half maximum values of (004) and (311) x-ray rocking curves were 113.4 and 234.0 arc sec, respectively. The dislocation density of the substrates was 1.4 × 107 cm−2, determined by plan-view transmission electron microscopy observation. These values are much lower than the reported values among heteroepitaxial diamonds. Furthermore, x-ray pole figure measurements showed four symmetry of the crystal, showing single crystallinity without any twinning. The curvature radius of diamond was measured to be 90.6 cm, which is much larger than previous values, ca. 20 cm. Surprisingly, a cubic-lattice (001) diamond crystal was epitaxially grown on a trigonal-lattice (11 2 ¯ 0) sapphire substrate. However, we found that the epitaxial relation is diamond (001) [110]//Ir (001) [110]//sapphire (11 2 ¯ 0) [0001]. Now, high-quality one-inch diamond wafers will be available as a substrate used for diamond electronic devices. [ABSTRACT FROM AUTHOR]

Current-induced spin-torques in two-dimensional (2D) heterostructures have attracted extensive attention due to their importance in understanding the underlying fundamental physics and developing low-power dissipation nanoelectronics. Here, the Permalloy/black phosphorus (BP) bilayer devices are fabricated, and spin-torque ferromagnetic resonance (ST-FMR) measurements are utilized to investigate the spin-torque effect in the heterostructure. An obvious out-of-plane antidamping torque is observed, which could be associated with the broken mirror symmetry of BP. These results show the possibility of manipulating magnetization by semiconductor field-effect devices based on 2D materials and provide a clear avenue for engineering spintronic devices based on 2D materials. [ABSTRACT FROM AUTHOR]

The structural properties of single crystals of BiFeO3–BaTiO3 (BF–BT) ferroelectric oxide solid solution with a slightly BF-rich composition (72BF–28BT; toward the rhombohedral-phase side) from the rhombohedral–pseudocubic phase boundary were investigated by transmission electron microscopy (TEM) and reciprocal space mapping (RSM), including three-dimensional (3D) RSM. According to the TEM results, the 72BF–28BT specimen had a domain structure similar to that of a rhombohedral crystal and the polarization direction appeared to be approximately ⟨111⟩, corresponding to a ferroelectric crystal with rhombohedral symmetry, although some distorted behavior suggested that the crystal symmetry may not have been strictly rhombohedral. Fine and complex domain structures were also observed inside the rectangular domains. The RSM results also indicated that the crystal had rhombohedral-like symmetry, although several discrepancies were observed, such as unexpectedly split diffraction peaks and distortion-related diffraction distribution in the reciprocal space. The structural behavior observed in the TEM images and 2D and 3D RSM images suggested that the symmetry of the 72BF–28BT specimen was lower than rhombohedral and may have been monoclinic. Structural disorder was detected along the distorted direction from rhombohedral symmetry, which could interfere with obtaining structural information via powder diffraction experiments and lead to inaccurate identification of the crystal structure as a higher-symmetry one. [ABSTRACT FROM AUTHOR]

Conventionally, there is no lateral beam shift (LBS) at normal incidence for a wave beam pass through a slab. However, by simultaneously breaking spatial inversion, time-reversal, and mirror symmetries of the photonic crystal slab, we realized nonreciprocal LBS for the transmitted wave beam with high transmission. We showed that the nonreciprocal LBS could be positive or negative, which could be tuned by the arrangement of a magnetic basis in the unit cell. We verified the nonreciprocal LBS at normal incidence by experiments. Our study provides a useful way to manipulate the wave propagation and wave-matter interaction by artificial materials and leads to a breakthrough in LBS, which has promising potential in optical devices, such as transducers, switches, and unidirectional couplers. [ABSTRACT FROM AUTHOR]

Monolayer transition metal dichalcogenides (TMDs) are intrinsically piezoelectric within the plane of their atoms, but out-of-plane piezoelectric response should not occur due to the symmetry of the crystal structure. Recently, however, MoS2 was shown to exhibit out-of-plane electromechanical coupling consistent with the flexoelectric effect. In this study, MoSe2, WS2, and WSe2 are investigated to determine the existence and strength of out-of-plane electromechanical coupling in other monolayer TMD semiconductor materials. Piezoresponse force microscopy measurements show that monolayer MoS2, MoSe2, WS2, and WSe2 all exhibit out-of-plane electromechanical response. The relative magnitudes of their out-of-plane electromechanical couplings are calculated and compared with one another and to predictions made from a simple model of flexoelectricity. This simple model correctly predicts the magnitude of out-of-plane electromechanical response in these materials, and the measured values provide useful guidance for both more detailed understanding of flexoelectric response in monolayer TMDs, and assessment of their consequences in devices incorporating these materials. [ABSTRACT FROM AUTHOR]

Although the connection between symmetry and entropy is not clear, researchers calculate configurational entropy with an ideal gas mixing model all along regardless of which structure they are considering. However, it is obvious that crystalline structures have symmetry, while an ideal gas does not. Therefore, the same ideal gas mixing value should not be assigned to other structures, such as face-centered-cubic (fcc) and hexagonal-close-packed (hcp) structures. Here, we offer a precise definition for determining the configurational entropy of crystals. We calculate the difference in configurational entropy between fcc and hcp structures based on Burnside's lemma in combinatorial mathematics and crystallographic rotation-point groups. [ABSTRACT FROM AUTHOR]

A set of monoclinic β-(AlxGa1–x)2O3 films coherently grown by plasma-assisted molecular beam epitaxy onto (010)-oriented β-Ga2O3 substrates for compositions x ≤ 0.21 is investigated by generalized spectroscopic ellipsometry at room temperature in the spectral range of 1.5 eV–9.0 eV. We present the composition dependence of the excitonic and band to band transition energy parameters using a previously described eigendielectric summation approach for β-Ga2O3 from the study by Mock et al. All energies shift to a shorter wavelength with the increasing Al content in accordance with the much larger fundamental band to band transition energies of Al2O3 regardless of crystal symmetry. The observed increase in the lowest band to band transition energy is in excellent agreement with recent theoretical predictions. The most important observation is that charge confinement in heterostructures will strongly depend on the growth condition due to the strongly anisotropic properties of the band to band transitions. [ABSTRACT FROM AUTHOR]

The existence of a variety of phases and their transitions in the electric field-temperature (E-T) space of relaxor ferroelectrics make them an interesting research subject. To directly observe the evolution of domain morphology and crystal symmetry in ferroelectrics under either temperature change or an applied electric field, in-situ transmission electron microscopy (TEM) is often the preferred characterization technique. Accessing all the phases in the E-T space of a relaxor crystal requires in-situ TEM with capabilities of simultaneous heating and biasing. In this letter, such a dual-stimuli in-situ TEM technique is demonstrated on 0.75(Bi1/2Na1/2)TiO3–0.25SrTiO3, a relaxor ceramic with a thermal depolarization temperature Td of about 40 °C. Not only the domain structure change during thermal depolarization is directly seen, but also different microstructural responses under an applied electric field in the nonergodic and ergodic states are observed. The results prove that the dual-stimuli in-situ TEM technique is indispensable for exploring the rich physics in functional materials. [ABSTRACT FROM AUTHOR]

Determining the unreacted equation of state of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) is challenging because it exhibits low crystal symmetry and low X-ray scattering strength. Here, we present the first high-pressure single-crystal X-ray diffraction (SXD) study of this material. Our SXD results reveal a previously unknown transition to a monoclinic phase above 4 GPa. No abrupt change of the volume occurs but the compressibility changes. Concomitant first principles evolutionary crystal structure prediction USPEX calculations confirm this transition and show that it involves a pressure-induced in-plane shift of the layers of TATB molecules with respect to the ambient-pressure phase. [ABSTRACT FROM AUTHOR]

Magnetocaloric response in anisotropic single-crystals with cubic crystal symmetry is studied. As compared to isotropic magnets where only paramagnetic-to-ferromagnetic phase transition occurs at Curie temperature ( T C = 185 K) and magnetization increases monotonously with increasing field and/or decreasing temperature, the spin reorientation phase transitions may occur at lower temperatures (∼ 115 K) as the magnetic field gets oriented along one of the cubic anisotropy easy axes and induces nonmonotonic increases in magnetization with increasing field and temperature, leading to a spike-like entropy change (−ΔSM) and positive ΔSM at TC, or move the −ΔSM peak towards higher temperatures. Finally, the power laws of maximum −ΔSM and relative cooling power with the field and the double-shifted loop results evidence that only second-order phase transition occurs and the abnormal magnetization behavior completely comes from the spin order evolution due to energy competition. Therefore, the Maxwell relation is applicable. The results are also suitable for multiple anisotropy or polycrystalline materials with well-defined domains. [ABSTRACT FROM AUTHOR]

Disentangling the primary order parameter from secondary order parameters in phase transitions is critical to the interpretation of transition mechanisms in strongly correlated systems and quantum materials. Here, we present a study of structural phase transition pathways in superionic Cu2S nanocrystals that exhibit intriguing properties. Utilizing ultrafast electron diffraction techniques sensitive to both the momentum-space and the time-domain, we distinguish the dynamics of crystal symmetry breaking and lattice expansion in this system. We are able to follow the transient states along the transition pathway, and so observe the dynamics of both the primary and secondary order parameters. Based on these observations, we argue that the mechanism of structural phase transition in Cu2S is dominated by the electron-phonon coupling. This mechanism advances the understanding from previous results, where the focus was solely on dynamic observations of the lattice expansion. [ABSTRACT FROM AUTHOR]

We have conducted an in-situ upconversion (UC) mechanism study through fluorescence enhancement induced by electric field in the Er3+ doped 0.75Pb(Mg1/3Nb2/3)O3-0.25PbTiO3 transparent ceramic. Under the excitation of a 980 nm diode laser, the sample shows a strong green UC fluorescence, which increases with electric field. A maximum enhancement of 150% was achieved in fluorescence intensity by the applied electric field due the lowering of crystal symmetry by the electric field induced strains. Combined with an in-situ analysis of time-resolved spectra, we proved that the dominant UC mechanism in this system should be the energy transfer (ET) process. [ABSTRACT FROM AUTHOR]

We investigated polarization-sensitive electro-optic (EO) detection of terahertz (THz) waves by using two uniaxial crystals: a c-cut gallium selenide and a c-cut lithium niobate crystals. We formulated a general frequency-domain description of EO detection by in-plane isotropic EO crystals, which holds regardless of the frequency. Based on this description, the polarization of THz waves can be derived by analyzing EO sampling signals measured with two orthogonal configurations of the in-plane isotropic EO crystals as well as typical (111) zinc-blende EO crystals. In addition, we experimentally demonstrated that the frequency-dependent polarization of THz waves can be reproducibly retrieved using three EO crystals with different crystal symmetries and with different phase matching conditions. Our description provides essential information for practical polarization sensing in the THz frequency range as well as in the mid-infrared range. [ABSTRACT FROM AUTHOR]

Cube-on-cube epitaxy of perovskite sub-cell of Pr-doped and undoped NaNbO3 is obtained in 130-nm-thick films on top of (La0.18Sr0.82)(Al0.59Ta0.41)O3 (001) substrates. Experimental studies show that the edge of optical absorption red-shifts and some interband transitions change in the films compared to crystals. Bright red luminescence is achieved at room-temperature under ultraviolet excitation in the Pr-doped film. An interband mechanism of luminescence excitation is detected in the film, which is in contrast to the intervalence charge transfer mechanism in the crystal. The results are discussed in terms of epitaxially induced changes of crystal symmetry and ferroelectric polarization in the films. It is suggested that the band structure and interband transitions in NaNbO3 and the transition probabilities in the Pr ions can be significantly modified by these changes. [ABSTRACT FROM AUTHOR]

Here, we report a determination of monoclinic nanodomains in PMN-xPT with x = 31%PT by using scanning convergent beam electron diffraction (SCBED). We show the presence of 60 ± a degree nanodomains with Cm-like symmetry as well as significant variations (a) in local polarization directions across lengths of 10 nm. The principle of our technique is general and can be applied for the determination of polarization domains in other ferroelectric materials of different symmetry. [ABSTRACT FROM AUTHOR]

We demonstrated the generation of the intense radially polarized mid-infrared optical vortex at a wavelength of 10.6 µm by use of a passive axially symmetric zinc selenide (ZnSe) waveplate with high energy pulse throughput. The phase of the radially polarized optical vortex with the degree of polarization of 0.95 was spirally distributed in regard to the angle. The converted laser beam energy of about 2.6 mJ per pulse was obtained at the input pulse energy of 4.9 mJ, corresponding to the energy conversion efficiency of 56%. [ABSTRACT FROM AUTHOR]

We report on how the relaxation of patterns prepared on a thin film can be controlled by manipulating the symmetry of the initial shape. The validity of a lubrication theory for the capillary-driven relaxation of surface profiles is verified by atomic force microscopy measurements, performed on films that were patterned using focused laser spike annealing. In particular, we observe that the shape of the surface profile at late times is entirely determined by the initial symmetry of the perturbation, in agreement with the theory. The results have relevance in the dynamical control of topographic perturbations for nanolithography and high density memory storage. [ABSTRACT FROM AUTHOR]