Low Symmetry Research Articles

Large out-of-plane spin–orbit torque in topological Weyl semimetal TaIrTe4

The unique electronic properties of topological quantum materials, such as protected surface states and exotic quasiparticles, can provide an out-of-plane spin-polarized current needed for external field-free magnetization switching of magnets with perpendicular magnetic anisotropy. Conventional spin–orbit torque (SOT) materials provide only an in-plane spin-polarized current, and recently explored materials with lower crystal symmetries provide very low out-of-plane spin-polarized current components, which are not suitable for energy-efficient SOT applications. Here, we demonstrate a large out-of-plane damping-like SOT at room temperature using the topological Weyl semimetal candidate TaIrTe4 with a lower crystal symmetry. We performed spin–torque ferromagnetic resonance (STFMR) and second harmonic Hall measurements on devices based on TaIrTe4/Ni80Fe20 heterostructures and observed a large out-of-plane damping-like SOT efficiency. The out-of-plane spin Hall conductivity is estimated to be (4.05 ± 0.23)×104 (ℏ ⁄ 2e) (Ωm)−1, which is an order of magnitude higher than the reported values in other materials.

Further on the Choice of Space Group for Scapolite Group Members and Genetic Considerations about the Si-Al Ordering in Their Framework Construction

This paper poses a major question regarding the choice of space group for scapolite mineral group members. An artificial boundary is typically drawn between space groups I4/m and P42/n when solving the structures of scapolites within the marialite–meionite series. The authors debate if solving the crystal structure in lower symmetries is justified. The choice of space group here is attributed to Si-Al ordering of the framework, and it is shown that the interstitial framework cations and anions have an accompanying role in that decision. Some answers on the ranges and limits of distribution of space groups of scapolite members in the marialite–meionite series, and the manifestations of violation of the Lowenstein rule or the so-called aluminum avoidance rule are presented. Modern physical methods (SEM-EDS and SXDA) are employed in the study to properly analyze the solid solution series in detail. New crystal–chemical data are reported for scapolite samples from different localities. An analysis was made for the types of possible Al-O-Al bonds that can occur in the structures at different Al:Si ratios and their influence on Al-Si ordering. Finally, genetic considerations about Al-Si ordering in the framework construction during the mineral formation processes are proposed.

Exploring fracture anisotropy in tantalum carbide compounds: A density functional theory approach

AbstractIn this paper, we examine the cleavage fracture anisotropy in tantalum carbides, namely, TaC, α‐Ta2C, and ζ‐Ta4C3 − x, using density functional theory (DFT) calculations. Our investigation identifies the presence of multiple low‐energy cleavage planes indicating multiple potential pathways for crack propagation in these ceramics, even the low symmetry compounds. The anisotropy characteristics of cleavage fractures exhibited by α‐Ta2C and ζ‐Ta4C3 − x closely align with the intrinsic fracture anisotropy observed in TaC. Notably, there exist at least three pyramidal planes in ζ‐Ta4C3 − x whose cleavage energies are less than those of the carbon‐depleted basal planes, previously reported to have the lowest cleavage energy. The observed preference in experiments for cleavage along carbon‐depleted basal planes, exclusive of other identified low‐energy planes, points to factors beyond cleavage energy influencing cleavage plane preference.

Synthesis and Properties of the Ba2PrWO6 Double Perovskite.

We report details on the synthesis and properties of barium praseodymium tungstate, Ba2PrWO6, a double perovskite that has not been synthesized before. Room-temperature (RT) powder X-ray diffraction identified the most probable space group (SG) as monoclinic I2/m, but it was only slightly distorted from the cubic structure. X-ray photoelectron spectroscopy confirmed that the initial (postsynthesis) material contained praseodymium in both 3+ and 4+ charge states. The former (Pr3+) disappeared after exposure to UV light at RT. Photoluminescence studies of Pr3+ revealed that Ba2PrWO6 is an insulator with a band gap exceeding 4.93 eV. Pressure-dependent Raman spectroscopy excluded the possibility of a phase transition up to 20 GPa; however, measurements between 8 and 873 K signified that there might be a change toward the lower symmetry SG below 200 K. Electron paramagnetic resonance spectra revealed the presence of interstitial oxygen which acts as a deep electron trap.

Low-temperature properties of magnetically frustrated rare-earth zirconates A 2Zr2O7

Rare-earth A 2Zr2O7 zirconates have attracted considerable attention of the scientific community for their complex magnetic, electronic and material properties applicable in modern technologies. The light rare-earth members of the series, crystallising in the pyrochlore variant of cubic crystal structure, have been studied in detail. The heavier A 2Zr2O7 compounds have been investigated mainly from the material properties viewpoint, focussing on their thermal properties and stability at high temperature and pressure. Low-temperature studies were mostly missing until recently. We present the low-temperature magnetic and thermodynamic properties of A 2Zr2O7 with A = Y, La, Nd, Eu, Gd, Tb, Dy, Ho, Tm, Yb, and Lu, well covering the whole series, newly synthesised by high-temperature sintering and melting methods. X-ray diffraction reveals and confirms the ordered pyrochlore structure in the light members, the disordered cubic structure of the defect-fluorite type in A 2Zr2O7 with A = Y, Gd–Yb, and finally the lower symmetry rhombohedral structure in the end-member Lu2Zr2O7. The specific heat of the investigated compounds is dominated by a low-temperature anomaly associated with magnetic ordering: long-range in light rare-earth zirconates; and short-range in heavier members. The effective magnetic moment in the studied compounds, determined by fitting the magnetisation data to the Curie–Weiss formula, is in good agreement with the expected value of the A 3+ free ion. The magnetic properties have been revealed to be strongly influenced by the geometric frustration of the magnetic moments of both the pyrochlore structure, as well as the face centred cubic lattice created by the cations of the defect-fluorite structure, but connected also to intrinsic atomic disorder. The experimental results are discussed in the framework of previous studies on A 2Zr2O7 zirconates, as well as other A 2 B 2O7 compounds.

Improved detection of magnetic interactions in proteins based on long-lived coherences

Living systems rely on molecular building blocks with low structural symmetry. Therefore, constituent amino acids and nucleotides yield short-lived nuclear magnetic responses to electromagnetic radiation. Magnetic signals are at the basis of molecular imaging, structure determination and interaction studies. In solution state, as the molecular weight of analytes increases, coherences with long lifetimes are needed to yield advantageous through-space magnetisation transfers. Interactions between magnetic nuclei can only be detected provided the lifetimes of spin order are sufficient. In J-coupled pairs of nuclei, long-lived coherences (LLC’s) connect states with different spin-permutation symmetry. Here in, we show sustained LLC’s in protein Lysozyme, weighing 14.3 kDa, with lifetimes twice as long as those of classical magnetisation for the aliphatic protons of glycine residues. We found for the first time that, in a protein of significant molecular weight, LLC’s yield substantial through-space magnetisation transfers: spin-order transfer stemming from LLC’s overcame transfers from classical coherences by factors > 2. Furthermore, in agreement with theory, the permutation symmetry of LLC-based transfers allows mapping interacting atoms in the protein structure with respect to the molecular plane of glycine residues in a stereospecific manner. These findings can extend the scope of liquid-state high-resolution biomolecular spectroscopy.

Low Temperature Raman Spectroscopy of Tetrahydrofuran: Phonon Spectra Compared to Matrix Isolation Spectra in Air

The conformation of tetrahydrofuran (THF) molecules in vapor has been the subject of considerable computational and experimental studies, the most recent by Park and Kwon stated that the difference between the most stable, twisted C2 conformer and the bent Cs conformer is 17 ± 15 cm−1. Because of low symmetry, all modes from both conformers are allowed in the Raman and infrared spectra. In 1982, Aleksanyan and Antipov observed the emergence of two Raman bands at 249 and 303 cm−1 at 20 K, while only one band at 293 cm−1 was present in solid THF at 142. They assigned the 249 cm−1 band to the restricted pseudorotational motion of THF in the solid state, because on heating, the band diminishes and is too weak to be observed near melting point (at 142 K). Cadioli et al. reported a study of the vibrational spectrum of tetrahydrofuran, giving a complete assignment of all bands including those present in the low-temperature Raman spectrum at 85 K and infrared bands observed at 90 K. They assigned the band at 242 cm−1 in the Raman spectrum at 85 K as an overtone of the lowest normal mode (pseudorotational mode), while the 299 cm−1 band in the same spectrum was assigned as a radial mode. In the following, low-temperature Raman spectra of solid THF together with the Raman matrix isolated spectrum of THF in air will be presented and compared to published data. Our results indicate that the band observed at 245 cm−1 at 10 K is too strong to be assigned as an overtone, since its intensity is of the same magnitude as the 299 cm−1 band.

Uranium Oxide Hydrate Frameworks with Dy(III) or Lu(III) Ions: Insights Into the Framework Structures With Lanthanide Ions.

Two uranium oxide hydrate frameworks (UOHFs) with either Dy3+ or Lu3+ ions, Dy1.36(H2O)6[(UO2)10UO13(OH)4] (UOHF-Dy) or Lu2(H2O)8[(UO2)10UO14(OH)3] (UOHF-Lu), were synthesized hydrothermally and characterized with a range of structural and spectroscopic techniques. Although SEM-EDS analysis confirmed the same atomic ratio of ~5.5 for U : Dy and U : Lu, they displayed different crystal morphologies, needles for UOHF-Dy in the orthorhombic C2221 space group and plates for UOHF-Lu in the triclinic P-1 space group. Both frameworks are composed of β-U3O8 type layers linked by pentagonal bipyramidal uranium polyhedra, with the Dy3+/Lu3+ ions inside the channels. However, the arrangements of Dy3+/Lu3+ ions are different, with disordered Dy3+ ions well aligned at the centers of the channels and single Lu3+ ions well-separated in a zigzag pattern in the channels. While the characteristic vibrational modes were revealed by Raman spectroscopy, the presence of a pentavalent uranium center in UOHF-Lu was confirmed with diffuse reflectance spectroscopy. The formation of two types of UOHFs with lanthanide ions, high or low symmetry, and the structure trend were discussed regards to synthesis conditions and lanthanide ionic radius. This work highlights the complex chemistry driving the formation of UOHFs with lanthanide ions and has implications to the spent nuclear fuel under geological disposal.

Ferroelectricity with Long ion Displacements in Crystals of Non‐Polar Point Groups

AbstractIn the classical model, ferroelectricity is associated with small ion displacements from paraelectric phases with high symmetry, and ferroelectric crystals must adopt one of the ten polar point groups with low symmetry according to Neumann's principle. In this work, it is proposed that this conclusion is based on perfect bulk crystals without taking the boundaries into account. First‐principles evidence shows that ferroelectric polarizations may also be formed in some non‐polar point groups as the edges generally break the crystal symmetry. Meanwhile, such polarizations can be maintained at macroscale and are switchable when the transition between multiple equivalent states with high symmetry can be realized via long ion displacements， essentially akin to ion conductors. For example, the switching barriers can be much reduced in sliding ferroelectric bilayer systems or ionic compounds with covalent‐like directionality. Such unconventional ferroelectricity can be attributed to the boundaries and long ion displacements, and its existence in several systems like CuCrS2 is supported by experimental observations. It may explain a series of unclarified phenomena reported previously as well as significantly expand the scope of ferroelectrics, especially those with high polarizations induced by long ion displacements.

Enhanced self-collimation effect by low rotational symmetry in hexagonal lattice photonic crystals

In this study, we present the design of a photonic crystal (PC) structure with a hexagonal lattice, where adjustments to the PC unit cell symmetry reveal an all-angle self-collimation (SC) effect. By optimizing opto-geometric parameters, such as the rotational angle of auxiliary rods and adjacent distances, we analyze the SC property in detail, leveraging group velocity dispersion (GVD) and third-order dispersion (TOD) characteristics. We also investigate the relationship between symmetry properties and their influence on dispersion characteristics. Through symmetry manipulation, we gain a comprehensive understanding of the underlying mechanisms governing light collimation and confinement in the proposed configurations. The PC structure with a C1 symmetry group exhibits all-angle SC effect within the range of a/λ = 0.652 and a/λ = 0.668 normalized frequencies, with a bandwidth of Δω/ω c =2.4% Further breaking the symmetry, transforming from C1 to C2 group symmetry enhances the SC bandwidth to Δω/ω c =6.5% and reveals the perfect linear equi-frequency contours (EFC) at two different frequency bands: all angle SC between a/λ = 0.616 and a/λ = 0.344 normalized frequencies in the 4th transverse magnetic (TM) band and between a/λ = 0.712 and a/λ = 0.760 in the 5th TM band. Here, GVD and TOD values of the TM 4th band vary between 7.3 (a/2πc2)–254.3 (a/2πc2) and 449.2 (a2/4π 2c3)–1.3×105 (a2/4π 2c3), respectively. Also, GVD and TOD values of the TM 5th band vary between 182.5 (a/2πc2)–71.3 (a/2πc2) and −24380(a2/4π 2c3)–−9619 (a2/4π 2c3) values, respectively. Additionally, we propose a composite/hybrid PC structure resembling C2 group symmetry, where two auxiliary rods are replaced by rectangular photonic wires with the same refractive index and width equal to the diameter of auxiliary rods. This hybrid structure exhibits an all-angle SC effect with an operating bandwidth of Δω/ω c =11.7%, which displays near-zero GVD and TOD performance and offers enhanced robustness against potential fabrication precision issues.

A first-principles study of HfB2 anisotropic surface stability and its oxygen adsorption behavior

In this work, the HfB2 anisotropic surface stability, and the adsorption behavior of oxygen molecule on the most likely exposed HfB2 surface were investigated based on density functional theory. The study found that the HfB2(0001) surface terminated by Hf (labeled as: Hf-(0001)) is more stable, and more likely interact with oxygen. By rotating the oxygen molecule orientation and changing the adsorption site, all possible high and low-symmetry adsorption configurations are considered. The results show that the low symmetry structure is unstable, and eventually turn into the high symmetry ones. For the highly symmetric structures, oxygen molecule tends to be adsorbed in parallel, and the dissociated oxygen molecule tends to be located at the bridge and hollow site of the Hf-(0001) surface, and there is no energy barrier to this process. Charge density difference and partial density of states proved that oxygen absorbed structures present similar electronic interaction characteristics, and oxygen adsorption process mainly affects the Hf atom at the outmost layer of the Hf-(0001) plane. Oxygen atoms bind to the Hf-(0001) surface mainly in the form of ionic bonds and covalent bonds, originating from the orbital hybridization of O-p and Hf-d. The oxidation of HfB2 starts from the interaction between oxygen and Hf.

Topology Prediction of Gas-Separating Metal-Organic Frameworks with Low Symmetry Vertices.

Topology serves as a blueprint for the construction of reticular structures such as metal-organic frameworks, especially for those based on building blocks with highly symmetrical shapes. However, it remains a challenge to predict the topology of the frameworks from less symmetrical units, because their corresponding vertex figures are largely deformed from the perfect geometries with no "default" net embedding. Furthermore, vertices involving flexible units may have multiple shape choices, and the competition among their designated topologies makes the structure prediction in large uncertainty. Herein, the deformation index is proposed to characterize the symmetry loss of the vertex figure by comparing it with its ideal geometry. The mathematical index is employed to predict the shapes of two in situ formed Co-based metalloligands (pseudo-tetrahedron and pseudo-square), which further dictate the framework topology (flu and scu) when they are joined with the [Zr6O8]-based cuboid units. The two frameworks with very similar constituents provide an ideal platform to investigate how the pore shapes and interconnectivity influence the gas separation. The net with cylindrical channels outperforms the other with discreate cages in C3H8/C2H6/CH4 separation, benefiting from the facile accessibility of its interaction sites to the guests imposed by the specific framework topology.

Role of Zr3+ in excitation of Eu3+ ions in stabilized ZrO2:Eu nanoparticles

Microwave hydrothermal technique has been applied to crystallize ZrO2 nanoparticles with the sizes below 10 nm and activated with Eu. Trivalent europium ions acted also as the stabilization agent of the high temperature ZrO2 polymorphs at the room temperature. To achieve this, high concentrations - up to 20 % of Eu3+ - were introduced to the zirconia host lattice. Reference samples of ZrO2 containing yttrium were synthesized to obtain pure cubic, tetragonal and monoclinic phases at constant europium concentration. Eu3+ ions have not been reduced to divalent form, however, we have observed reduction of Zr4+ ions by means of luminescence spectroscopy and electron paramagnetic resonance (EPR). The variation of Zr3+ concentration in the samples with increasing Eu3+ content seems to support the concept of trivalent ion pairs as charge compensation centers in ZrO2. Excitation of Eu3+ ions was conducted via low and high symmetry sites. Trivalent zirconium ions have been found to play a role in the excitation of Eu3+ ions at low symmetry sites.

Cascading symmetry constraint during machine learning-enabled structural search for sulfur-induced Cu(111)-(43×43) surface reconstruction.

In this work, we investigate how exploiting symmetry when creating and modifying structural models may speed up global atomistic structure optimization. We propose a search strategy in which models start from high symmetry configurations and then gradually evolve into lower symmetry models. The algorithm is named cascading symmetry search and is shown to be highly efficient for a number of known surface reconstructions. We use our method for the sulfur-induced Cu (111) (43×43) surface reconstruction for which we identify a new highly stable structure that conforms with the experimental evidence.

Distorted dislocation cores and asymmetric glide resistances in titanium

The determination of Critical Resolved Shear Stress (CRSS) in titanium for basal, prismatic, and pyramidal slip-planes without empirical constants is presented by combining Density Functional Theory (DFT) and anisotropic elasticity. A new mechanism leading to tension-compression (T-C) asymmetry in the CRSS levels has been revealed for the first time. The conditions for this asymmetry are established, involving a complex interplay between the dislocation core-structure and core-advance behavior. The three conditions for T-C asymmetry that need to be simultaneously satisfied can be summarized as: (1) a medium stacking fault width, d, (3<d/bF<10, where bF is the magnitude of the Burgers vector of the full dislocation), (2) an asymmetric core-structure of the extended dislocation (ξ1≠ξ2, where ξ1 and ξ2 are the core-widths of the first and second partials, respectively), and (3) intermittent motion of the partials (Δd/bF≠0, where Δd is the magnitude of fluctuation in stacking fault width during intermittent motion). Pyramidal-slip in titanium satisfies all three conditions, resulting in significant T-C asymmetry. The CRSS theory considers a Wigner-Seitz (W-S) cell based domain area assigned to each lattice site for the calculations of core-energies accurately capturing the slip-plane lattice. The W-S based approach is essential due to the lower symmetry of the HCP crystal circumventing potential errors associated with the one-dimensional atomic-row approximation. Dislocation core structures are obtained for all the slip-systems in titanium showing significant distortion of the disregistry profile governing the core-advance behavior. The CRSS values predicted from the theory show agreement with the experimental CRSS levels reported in the literature.

Preparation, thermal properties, and structures of ionic plastic crystals with sandwich and half-sandwich Ru complexes

We have recently been investigating the phase behavior of salts of cationic sandwich complexes in pursuit of novel ionic plastic crystals (IPCs). In this study, we synthesized salts containing sandwich complexes [Ru(Cp)(C6H6)]X ([1]X; X = CPFSA− (1,1,2,2,3,3-hexafluoropropane-1,3-disulfonimide), SbCl6−) and half-sandwich complexes [Ru(Cp)(DMSO)3]X ([2]X; X = CPFSA−, FSA− ((FSO2)2N−), PF6−). In addition, we examined their thermal properties and crystal structures. Among them, [1]CPFSA exhibited an IPC phase between 365 and 630 K, whereas the others did not undergo phase transitions at 123–403 K. The cations and anions were alternately arranged in the crystals of [1]X, whereas [2]X did not exhibit the alternating arrangement except for [2]PF6. The results showed that alternating molecular arrangements are crucial for the IPC formation, although low symmetry of the ion environment and intermolecular steric hindrance may inhibit it.

Probing ligand conformation and net dimensionality in a series of tetraphenylethene-based metal-organic frameworks.

Tetraphenylethene-based ligands with lowered symmetry are promising building blocks for the construction of novel luminescent metal-organic frameworks (MOFs). However, few examples have been reported, and predicting the ligand conformation and the dimensionality of the resulting MOF remains challenging. In order to uncover how synthetic conditions and accessible ligand conformations may affect the resulting MOF structure, four new MOF structures were synthesized under solvothermal conditions using the meta-coordinated tetraphenylethene-based ligand m-ETTC and paddlewheel SBUs composed of Co(II), Cu(II), and Zn(II). WSU-10 (WSU = Washington State University) is formed with either Zn or Cu comprising stacked psuedo-2D layers. The dimensionality of WSU-10 can be intentionally increased through the addition of pyrazine as a pillar ligand into the synthesis, forming the 3D structure WSU-11. The third structure, WSU-20, is formed by the combination of Zn or Co with m-ETTC and is intrinsically 3D without the use of a pillar ligand; interestingly, this is the result of a distortion in the paddlewheel SBU. Finally, Cu was also found to form a new structure (WSU-12), which displays an m-ETTC conformation unique from that found in the other isolated MOFs. Structural features are compared across the series and a mechanistic relationship between WSU-10 and -20 is proposed, providing insight into the factors that can encourage the generation of frameworks with increased dimensionality.

Tamping the movement of the laser absorption cutoff position using gold foam hohlraum

In indirect-driven laser fusion experiments, the movement of the laser absorption layer will distort the radiation uniformity on the capsule. The gold foam has advantages in symmetry control and lowering wall plasma blowoff when used in an inertial confinement fusion (ICF) hohlraum. This work investigates the motion of the laser absorption cutoff position using low-density foam gold walls. It is found that the motion of the laser absorption cutoff position can be significantly mitigated through optimal initial low density, tailored to a specific laser shape. For a short square laser pulse, the laser absorption cutoff position remains almost stationary at an initial density of approximately 0.6 g cm−3. For a long-shaped laser pulse, the minimal motion of the laser absorption cutoff position is observed at an initial density of about 0.1 g cm−3. This approach allows for the adjustment of the symmetry of the hohlraum radiation source. The insights gained from this study serve as a crucial reference for optimizing the hohlraum wall density.

Asymmetric and anisotropic quench sensitivity of ZA81M magnesium alloy

The quench sensitivity of the ZA81M magnesium alloy is studied by combining experiment and crystal plasticity finite element simulation. The quench sensitivity is high in tension and low in compression making the single-valued quench sensitivity calculated from hardness insufficient. So, there is different loss rate of strength, along different test directions of the alloy, due to slow quenching, i.e., the quench sensitivity is asymmetric and anisotropic. The coarsening of the precipitate and widening of the precipitate-free zone are the origin of the quench sensitivity. Crystal plasticity finite element simulation is proposed to calculate the quench sensitivity. The phenomenological constitutive model is used to explicitly consider the strengthening effect of precipitates on different slip and twinning systems. So, the precipitate characteristics of the water-quenched and air-cooled alloy are easily captured. The anisotropy and asymmetry of the quench sensitivity in two perpendicular sample planes are determined by simulation. Such a method can be useful for determining the quench sensitivity of precipitation-hardening alloys with low crystal symmetry and with precipitates having different strengthening efficiencies on different slip or twinning systems.

Automatic Determination of the Non-Covalent Stable Conformations of the NO2-Pyrene Cluster in Full Dimensionality (81D) Using the vdW-TSSCDS Approach.

We present the detailed topographical characterisation (stationary points and minimum energy paths connecting them) of the full dimensional (81D) intermolecular potential energy surface associated with the non-covalent interactions between the NO2 radical and the pyrene (C16H10) molecule. The whole procedure is (quasi) fully automated. We have used our recent algorithm vdW-TSSCDS as implemented on the freely-available AutoMekin software package. To this end, a series of inexpensive classical trajectories using forces from a low-level (semi-empirical) theory are used to sample the configuration space of the system in the search for candidates to first order saddle points. These guess structures are determined by means of a graph-theory based algorithm using the concept of adjacency matrix. Low-level optimizations are followed by re-optimizations at a final high-level of theory (DFT and CCSD(T)-F12 in our case.). The resulting set of stationary points and paths connecting them constitutes the so-called reaction network. In the case of NO2-pyrene, this network exhibits four major basins which can be characterized by their point-group symmetry. A central one, of global C2 symmetry, comprises the global minimum (as well as all other permutationally related conformers) together with the corresponding C2v saddle points connecting them. This central basin is connected to three others of lower C1 symmetry. The latter can be distinguished by the projection of the position of the NO2 nitrogen atom on the pyrene plane in combination with the relative orientation of the oxygen pair pointing either inwards, outwards, upwards or downwards.