Local Symmetry Breaking Research Articles

Origin of off-centering effect and the influence on heat transport in thermoelectrics

Recently, off-centering behavior has been discovered in a series of thermoelectric materials. This behavior indicates that the constituent atoms of the lattice displace from their coordination centers, leading to the locally distorted state and local symmetry breaking, while the material still retains its original crystallographic symmetry. This effect has been proved to be the root cause of ultralow thermal conductivity in off-centering materials, and is considered as an effective tool to regulate the thermal conductivity and improve the thermoelectric performance. Herein, we present a collection of recently discovered off-centering compounds, discuss their electronic origins and local coordination structures, and illuminate the underlying mechanism of the off-centering effect on phonon transport and thermal conductivity. This paper presents a comprehensive view of our current understanding to the off-centering effect, and provides a new idea for designing high performance thermoelectrics.

Edge of chaos as critical local symmetry breaking in dissipative nonautonomous systems

The fully nonlinear notion of resonance−geometrical resonance−in the general context of dissipative systems subjected to spatially periodic phase-modulated potentials is discussed. It is demonstrated that there is an exact local invariant associated with each geometrical resonance solution which reduces to the system’s energy when the potential is stationary. The geometrical resonance solutions represent a local symmetry whose critical breaking leads to a new analytical criterion for the onset of chaotic instabilities. This physical criterion is deduced in the co-moving frame from the local energy conservation over the shortest significant timescale. Remarkably, the new physical criterion for the onset of chaotic instabilities is shown to be valid over large regions of parameter space, thus being useful beyond the scope of current mathematical techniques. More importantly, the present theory helps to understand the unreasonable effectiveness of the Melnikov’s method beyond the perturbative regime.

Ultrahigh-Efficiency Superior Energy Storage in Lead-Free Films with a Simple Composition.

Dielectric capacitors are highly desired in modern electronic devices and power systems to store and recycle electric energy. However, achieving simultaneous high energy density and efficiency remains a challenge. Here, guided by theoretical and phase-field simulations, we are able to achieve a superior comprehensive property of ultrahigh efficiency of 90-94% and high energy density of 85-90 J cm-3 remarkably in strontium titanate (SrTiO3), a linear dielectric of a simple chemical composition, by manipulating local symmetry breaking through introducing Ti/O defects. Atomic-scale characterizations confirm that these Ti/O defects lead to local symmetry breaking and local lattice strains, thus leading to the formation of the isolated ultrafine polar nanoclusters with varying sizes from 2 to 8 nm. These nanoclusters account for both considerable dielectric polarization and negligible polarization hysteresis. The present study opens a new realm of designing high-performance dielectric capacitors utilizing a large family of readily available linear dielectrics with very simple chemistry.

Structural and physico-chemical characterization of hybrid materials based on globular quinuclidinium cation derivatives and tetrachloridocobaltate(ii) anions

The present study explores the effect of local symmetry breaking of globular cations on the formation and properties of hybrid organic–inorganic materials based on tetrahalometallate anions.

Crystallization of polarons through charge and spin ordering transitions in 1T-TaS2

The interaction of electrons with the lattice in metals can lead to reduction of their kinetic energy to the point where they may form heavy, dressed quasiparticles—polarons. Unfortunately, polaronic lattice distortions are difficult to distinguish from more conventional charge- and spin-ordering phenomena at low temperatures. Here we present a study of local symmetry breaking of the lattice structure on the picosecond timescale in the prototype layered dichalcogenide Mott insulator 1T-TaS2 using X-ray pair-distribution function measurements. We clearly identify symmetry-breaking polaronic lattice distortions at temperatures well above the ordered phases, and record the evolution of broken symmetry states from 915 K to 15 K. The data imply that charge ordering is driven by polaron crystallization into a Wigner crystal-like state, rather than Fermi surface nesting or conventional electron-phonon coupling. At intermediate temperatures the local lattice distortions are found to be consistent with a quantum spin liquid state.

Synthesis, Crystal Structure, Local Structure, and Magnetic Properties of Polycrystalline and Single-Crystalline Ce2Pt6Al15

Asymmetry, such as non-centrosymmetry in the crystal or chiral structure and local symmetry breaking, plays an important role in the discovery of new phenomena. The honeycomb structure is an example of an asymmetric structure. Ce2Pt6Al15 is a candidate for a frustrated system with honeycomb Ce-layers, which have been reported to show near the quantum critical point. However, the ground state of Ce2Pt6Al15 depends on the sample, and analysis of the crystal structure is difficult due to the presence of stacking disorder. We synthesized polycrystalline Ce2Pt6Al15 using arc melting method (AM-Ce2Pt6Al15) and single-crystalline Ce2Pt6Al15 using flux method (F-Ce2Pt6Al15). The prepared samples were characterized by electron probe micro-analysis (EPMA), single and powder X-ray diffraction methods, measured magnetic properties and X-ray absorption spectroscopy (XAS). The composition ratio of AM-Ce2Pt6Al15 was stoichiometric, although it contained a small amount (i.e., a few percent) of the impurity Ce2Pt9Al16. Meanwhile, the composition ratio of F-Ce2Pt6Al15 deviated from stoichiometry. The X-ray absorption fine structure (XAFS) spectrum of AM-Ce2Pt6Al15 at the Ce L3-edge was similar to that of CeF3, which possesses the Ce3+ configuration, indicating that the valence of Ce in Ce2Pt6Al15 is trivalent; this result is consistent with that for the magnetic susceptibility. To determine the precise structure, we analyzed the extended X-ray absorption fine structure (EXAFS) spectra of Ce L3- and Pt L3-edges for Ce2Pt6Al15, and found that the EXAFS spectra of Ce2Pt6Al15 can be explained not as a hexagonal Sc0.6Fe2Si4.9-type structure but, instead, as an orthorhombic structure with honeycomb structure.

Reprogrammable multistable ribbon kirigami with a wide cut

Upon stretching, a ribbon kirigami with parallel major cuts exhibits multistable behaviors that can maintain at multiple stable configurations. This work investigates the phenomena of a ribbon kirigami with one of the major cuts replaced by a wide cut, of which the multistable behaviors of the wide-cut located cell, the stable configurations, and the energy barriers between the stable configurations are explored. It is observed that the introduction of the wide cut results in local symmetry breaking, enabling bidirectional transition of the stable configurations in such a kirigami. The results also reveal that the geometries of major cuts and the stretch level enable reprogramable dynamic behaviors, such as the number of transitions once triggered. A kirigami-Morse code system is hereby presented, utilizing dynamic reconfiguration and showing a refreshable mechanical readout utilizing reprogrammability. The kirigami has potential for developing metamaterials with unique dynamic features.

Net Dzyaloshinskii–Moriya interaction in defect-enriched ferromagnet

The Dzyaloshinskii–Moriya interaction (DMI), which typically occurs in lattices without space inversion symmetry, can also be induced in a highly symmetric lattice by local symmetry breaking due to any lattice defect. We recently presented an experimental study of polarized small angle neutron scattering (SANS) on the nanocrystalline soft magnet Vitroperm (Fe73Si16B7Nb3Cu1), where the interface between the FeSi nanoparticles and the amorphous magnetic matrix serves as such a defect. The SANS cross sections exhibited the polarization-dependent asymmetric term originating from the DMI. One would naturally expect the defects characterized by a positive and a negative DMI constant D to be randomly distributed and this DMI-induced asymmetry to disappear. Thus, the observation of such an asymmetry indicates that there exists an extra symmetry breaking. In the present work we experimentally explore the possible causes by measuring the DMI-induced asymmetry in the SANS cross sections of the Vitroperm sample tilted in different directions with respect to the external magnetic field. Furthermore, we analyzed the scattered neutron beam using a spin filter based on polarized protons and confirm that the asymmetric DMI signal originates from the difference between the two spin-flip scattering cross-sections.

Local symmetry breaking drives picosecond spin domain formation in polycrystalline halide perovskite films.

Photoinduced spin-charge interconversion in semiconductors with spin-orbit coupling could provide a route to optically addressable spintronics without the use of external magnetic fields. However, in structurally disordered polycrystalline semiconductors, which are being widely explored for device applications, the presence and role of spin-associated charge currents remains unclear. Here, using femtosecond circular-polarization-resolved pump-probe microscopy on polycrystalline halide perovskite thin films, we observe the photoinduced ultrafast formation of spin domains on the micrometre scale formed through lateral spin currents. Micrometre-scale variations in the intensity of optical second-harmonic generation and vertical piezoresponse suggest that the spin-domain formation is driven by the presence of strong local inversion symmetry breaking via structural disorder. We propose that this leads to spatially varying Rashba-like spin textures that drive spin-momentum-locked currents, leading to local spin accumulation. Ultrafast spin-domain formation in polycrystalline halide perovskite films provides an optically addressable platform for nanoscale spin-device physics.

Emergent Magnetism as a Cooperative Effect of Interactions and Reservoir.

Closed shell molecular structures are under normal conditions time-reversal invariant. Experimental evidence points, however, toward that this invariance may be locally violated when the structure is in contact with a particle reservoir. The mechanisms behind such local symmetry breaking are not clear by any means. By considering a minimal model for a closed shell structure, here we propose that the symmetry breaking may result from a combination of internal and/or external interactions. It is shown that a magnetic moment of a localized electron level can be generated and maintained under the influence of such a combination. The theoretical results should cast new light on the mechanisms that may form magnetic properties in molecular compounds.

Multi-interfacial TiO2/carbon fibers encapsulated with needle-like FeCo2O4 for excellent microwave absorption

The introduced heterojunction structures can effectively improve the interface polarization and microwave absorption ability. In this work, the titanium dioxide modified carbon fibers (TiO2/CNFs) covered with needle-like FeCo2O4 were synthesized by electrospinning process and sequential hydrothermal method. When the filling ratio in paraffin wax is only 1 wt%, the FeCo2O4/TiO2/CNFs composite demonstrates remarkable absorbing properties. The composite material displays low density (∼0.79 g/cm3) with optimal reflection loss (RL) of − 47.0 dB and a large effective absorption bandwidth (RL＜−10 dB) of 4.1 GHz (8.3–12.4 GHz). Density functional theory (DFT) calculations were performed to investigate the dielectric loss mechanism caused by interface polarization at the atomic level. The calculations revealed that the density of states (DOS) of the p orbitals near the Fermi level decreases significantly, caused by breaking of local microstructural symmetry and inducing additional electric dipoles at the interface of CNFs and needle-like FeCo2O4. Under an external electromagnetic field, the dipole oscillation strengthens the polarization loss. The dielectric loss mechanism caused by interface polarization is explored from the atomic level and it paves a new way for designing microwave absorbers. These findings pave the way for designing effective microwave absorbers.

SrTiO3:Pr,Al phosphor mesocrystals: The role of Al-doping in short-range and electronic structure and its influence on photoluminescence properties

Phosphor materials with narrow emission bands and high quantum efficiency have received a good amount of attention, suggesting the possibility of several applications in the field of optoelectronic devices. SrTiO3:Pr is an example of a material with these characteristics. In this study, mesocrystal samples of Sr0.998Pr0.002Ti1−yAlyO3 (SrTiO3:Pr,Al) were prepared via a hydrothermal route, and the morphology presented a microcube shape and pristine structure with Pm3m space group without any spurious phases for all Al concentrations. Measurements of X-ray absorption spectroscopy (XAS) at Ti K-, LII,III-, and O K-edges, calculated projected density of states, and Raman spectroscopy revealed that the hydrothermal method and Al incorporation cause a local symmetry breaking, deviating from the cubic structure, Ti off-center displacement, tilting of TiO6 octahedra, and O vacancies. As the Al content increases, the intensity of some emissions in the photoluminescence spectra also increases up to 3 at%. The disorder produced by Al concentration leads to a lower symmetry around Pr3+ sites, resulting in an increase in the probabilities of transitions for Pr3+ ions due to the mixing of the opposite parity in 4 f configurational levels. On the other hand, higher Al concentrations cause a decrease in the intensity of Pr emissions, which is related to their quenching because of O vacancies. Additionally, intrinsic defects due to the hydrothermal route and Al incorporation generate a broad emission in the photoluminescence spectra. This broad emission is associated with intermediary electronic levels in the band gap caused by these intrinsic defects.

Metavalent Bonding-Mediated Dual 6s2 Lone Pair Expression Leads to Intrinsic Lattice Shearing in n-Type TlBiSe2.

Metavalent bonding has attracted immense interest owing to its capacity to impart a distinct property portfolio to materials for advanced functionality. Coupling metavalent bonding to lone pair expression can be an innovative way to propagate lattice anharmonicity from lone pair-induced local symmetry-breaking via the soft p-bonding electrons to achieve long-range phonon dampening in crystalline solids. Motivated by the shared chemical design pool for topological quantum materials and thermoelectrics, we based our studies on a three-dimensional (3D) topological insulator TlBiSe2 that held prospects for 6s2 dual-cation lone pair expression and metavalent bonding to investigate if the proposed hypothesis can deliver a novel thermoelectric material. Herein, we trace the inherent phononic origin of low thermal conductivity in n-type TlBiSe2 to dual 6s2 lone pair-induced intrinsic lattice shearing that strongly suppresses the lattice thermal conductivity to a low value of 1.1-0.4 Wm-1 K-1 between 300 and 715 K. Through synchrotron X-ray pair distribution function and first-principles studies, we have established that TlBiSe2 exists not in a monomorphous R-3m structure but as a distribution of distorted configurations. Via a cooperative movement of the constituent atoms akin to a transverse shearing mode facilitated by metavalent bonding in TlBiSe2, the structure shuttles between various energetically accessible low-symmetry structures. The orbital interactions and ensuing multicentric bonding visualized through Wannier functions augment the long-range transmission of atomic displacement effects in TlBiSe2. With additional point-defect scattering, a κlatt of 0.3 Wm-1 K-1 was achieved in TlBiSeS with a maximum n-type thermoelectric figure of merit (zT) of ∼0.8 at 715 K.

Spin-torque efficiency enhanced in sputtered topological insulator by interface engineering

The way inter-mixing chemical state influences the damping-torque efficiency of a spin–orbit torque (SOT) heterostructure is a matter of debate. This is because it acts as a local symmetry-breaking that differs from the global symmetry-breaking at the interface. A combination of angle-resolved X-ray photoelectron spectroscopy (AR-XPS), X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) was utilized to investigate the composition-gradient effects on the damping-torque efficiency of the Bi2Se3/insertion (Ta, Pt, Cu)/CoFeB SOT devices, with the insertion to be 0.5 and 1.0 nm. These embedded metals have the advantages of (i) Ta: suppressing CoFeB magnetic dead-layer (ii) Pt: large spin-hall angle (iii) Cu: larger spin-diffusion length. From AR-XPS we observed a sharper composition gradient for Pt-0.5 nm than Pt-1.0 nm in terms of the Se-Pt state. By XAS branching ratio and XMCD sum-rule analyses, we obtained larger spin–orbit coupling (SOC) and spin-polarization (SP) effects on the Pt-0.5, Pt-1.0 nm samples than Cu & Ta insertion. The sharper composition gradient and promoted SOC and SP validate the larger damping-torque efficiency in the Pt-0.5 nm than the Pt-1.0 nm sample. This work manifests interfacial engineering as a powerful avenue for modulating SOT efficiency in spintronic-based applications.

Interlayer Exciton-Phonon Bound State in Bi2Se3/Monolayer WS2 van der Waals Heterostructures.

The ability to assemble layers of two-dimensional (2D) materials to form permutations of van der Waals heterostructures provides significant opportunities in materials design and synthesis. Interlayer interactions can enable desired properties and functionality, and understanding such interactions is essential to that end. Here we report formation of interlayer exciton-phonon bound states in Bi2Se3/WS2 heterostructures, where the Bi2Se3 A1(3) surface phonon, a mode particularly susceptible to electron-phonon coupling, is imprinted onto the excitonic emission of the WS2. The exciton-phonon bound state (or exciton-phonon quasiparticle) presents itself as evenly separated peaks superposed on the WS2 excitonic photoluminescence spectrum, whose periodic spacing corresponds to the A1(3) surface phonon energy. Low-temperature polarized Raman spectroscopy of Bi2Se3 reveals intense surface phonons and local symmetry breaking that allows the A1(3) surface phonon to manifest in otherwise forbidden scattering geometries. Our work advances knowledge of the complex interlayer van der Waals interactions and facilitates technologies that combine the distinctive transport and optical properties from separate materials into one device for possible spintronics, valleytronics, and quantum computing applications.

Structural‐functional unit ordering for high‐performance electron‐correlated materials

AbstractElectron‐correlated materials have been drawing ever‐increasing attention due to their fascinating physical behaviors and extensive application scenarios. In this review, a new method for material research and design (R&D), named structural‐functional unit ordering (SFU ordering), which is presented, overcomes the shortcomings—for example, the limitation of finite chemical elements and long R&D circle‐of conventional strategy and thus provides guidance for the design of these high‐performance functional materials on demand. Meanwhile, with the development of material characterization technologies, SFUs of different scales and types can be directly observed, which, moreover, regulate the corresponding orderings. The review, starts with an introduction of the profile for SFU ordering and the synergistic effect between SFUs. Then, studies on several new high‐performance electronic‐correlated materials, for example, a ferromagnetic semiconductor with local spin, ferromagnetic metals with spin topologies, ferroelectric thin films with polar topologies, piezoelectric thin films with nanopillars enclosed by charged boundaries, thermoelectric materials with local ferromagnetic nanoparticles and topotactic phase transformation with conducting nanofilaments are stated in detail one by one. The vital aspect is the breaking of local symmetry, the construction, the structure, of SFUs and their orderings existing or theoretically existing, together with the enhanced/new performance. All in all, the main comments of the review tend to the remaining challenges, promising design approaches for the SFUs, and their orderings for high‐performance functional materials.

Is the doped MoS2 basal plane an efficient hydrogen evolution catalyst? Calculations of voltage-dependent activation energy.

Transition metal dichalcogenides are cheap and earth-abundant candidates for the replacement of precious metals as catalyst materials. Experimental measurements of the hydrogen evolution reaction (HER), for example, have demonstrated significant electrocatalytic activity of MoS2 but there is large variation depending on the preparation method. In order to gain information about the mechanism and active sites for the HER, we have carried out calculations of the reaction and activation energy for HER at the transition metal doped basal plane of MoS2 under electrochemical conditions, i.e. including applied electrode potential and solvent effects. The calculations are based on identifying the relevant saddle points on the energy surface obtained from density functional theory within the generalized gradient approximation, and the information on energetics is used to construct voltage-dependent volcano plots. Doping with 3d-metal atoms as well as Pt is found to enhance hydrogen adsorption onto the basal plane by introducing electronic states within the band gap, and in some cases (Co, Ni, Cu, Pt) significant local symmetry breaking. The Volmer-Heyrovsky mechanism is found to be most likely and the associated energetics show considerable dopant and voltage-dependence. While the binding free energy of hydrogen can be tuned to be seemingly favorable for HER, the calculated activation energy turns out to be significant, at least 0.7 eV at a voltage of -0.5 V vs. SHE, indicating low catalytic activity of the doped basal plane. This suggests that other sites are responsible for the experimental activity, possibly edges or basal plane defects.

Pyroelectric effect in doped nonpolar glycine crystals

Doping-induced local symmetry breaking of centrosymmetric molecular crystals endows them with piezoelectric and pyroelectric properties. In this work, we measured temperature dependences of pyroelectric and piezoelectric coefficients in α--glycine crystals single-doped by threonine and double-doped by threonine and alanine. We analyzed primary and secondary pyroelectric effects in these crystals and suggested a model of the dopant complexes related to the observed effects. Keywords: pyroelectricity, α-glycine, doped molecular crystals.

Исследование пироэлектрического эффекта в легированных неполярных кристаллах глицина

Doping-induced local symmetry breaking of centrosymmetric molecular crystals endows them with piezoelectric and pyroelectric properties. In this work, we measured temperature dependences of pyroelectric and piezoelectric coefficients in α-glycine crystals single-doped by threonine and double-doped by threonine and alanine. We analyzed primary and secondary pyroelectric effects in these crystals and suggested a model of the dopant complexes related to the observed effects.

Role of Hidden Spin Polarization in Nonreciprocal Transport of Antiferromagnets.

The discovery of hidden spin polarization (HSP) in centrosymmetric nonmagnetic crystals, i.e., spatially distributed spin polarization originated from local symmetry breaking, has promised an expanded material pool for future spintronics. However, the measurements of such exotic effects have been limited to subtle space- and momentum-resolved techniques, unfortunately, hindering their applications. Here, we theoretically predict macroscopic non-reciprocal transports induced by HSP when coupling another spatially distributed quantity, such as staggered local moments in a space-time PT-symmetric antiferromagnet. By using a four-band model Hamiltonian, we demonstrate that HSP plays a crucial role in determining the asymmetric bands with respect to opposite momenta. Such band asymmetry leads to non-reciprocal nonlinear conductivity, exemplified by tetragonal CuMnAs via first-principles calculations. We further provide the material design principles for large nonlinear conductivity, including two-dimensional nature, multiple band crossings near the Fermi level, and symmetry protected HSP. Our Letter not only reveals direct spintronic applications of HSP (such as Néel order detection), but also sheds light on finding observables of other hidden effects, such as hidden optical polarization and hidden Berry curvature.