Non-centrosymmetric Crystal Structure Research Articles

Peripheral chiral spin textures and topological Hall effect in CoSi nanomagnets

The spin structure and transport behavior of B20-ordered CoSi nanomagnets are investigated experimentally and by theoretical calculations. B20 materials are of interest in spin electronics because their noncentrosymmetric crystal structure favors noncoplanar spin structures that yield a contribution to the Hall effect. However, stoichiometric bulk CoSi is nonmagnetic, and combining magnetic order at and above room temperature with small feature sizes has remained a general challenge. Our CoSi nanoclusters have an average size of 11.6 nm and a magnetic ordering temperature of 330 K. First-principle calculations and x-ray circular dichroism experiments show that the magnetic moment is predominantly confined to the shells of the clusters. The CoSi nanocluster ensemble exhibits a topological Hall effect, which is explained by an analytical model and by micromagnetic simulations on the basis of competing Dzyaloshinskii-Moriya and intra- and intercluster exchange interactions. The topological Hall effect is caused by formation of chiral spin textures in the shells of the clusters, which exhibit fractional skyrmion number and are therefore termed as paraskyrmions (closely related to skyrmion spin structures). This research shows how nanostructuring of a chiral atomic structure can create a spin-textured material with a topological Hall effect and a magnetic ordering temperature above room temperature.

Effect of Co and Mg doping at Cu site on structural, magnetic and dielectric properties of α–Cu2V2O7

We have studied the effect of doping of both magnetic (Co) and nonmagnetic (Mg) ions at the Cu site on phase transition in polycrystalline α–Cu2V2O7 through structural, magnetic, and electrical measurements. X-ray diffraction reveals that Mg doping triggers an onset of α- to β-phase structural transition in Cu2−x Mg x V2O7 above a critical Mg concentration x c = 0.15, and both the phases coexist up to x = 0.25. Cu2V2O7 possesses a non-centrosymmetric crystal structure and antiferromagnetic ordering along with a non-collinear spin structure in the α phase, originated from the microscopic Dzyaloshinskii–Moriya interaction between the neighboring Cu spins. Accordingly, a weak ferromagnetic (FM) behavior has been observed up to x = 0.25. However, beyond this concentration, Cu2−x Mg x V2O7 exhibits complex magnetic properties. A clear dielectric anomaly is observed in α–Cu2−x Mg x V2O7 around the magnetic transition temperature, which loses its prominence with the increase in Mg doping. The analysis of experimental data shows that the magnetoelectric coupling is nonlinear, which is in agreement with the Landau theory of continuous phase transitions. Co doping, on the other hand, initiates a sharp α to β phase transition around the same critical concentration x c = 0.15 in Cu2−x Co x V2O7 but the FM behavior is very weak and can be detected only up to x = 0.10. We have drawn the magnetic phase diagram which indicates that the rate of suppression in transition temperature is the same for both types of doping, magnetic (Co) and nonmagnetic (Zn/Mg).

Crystal electric field and possible coupling with phonons in Kondo lattice CeCuGa3

We investigate the magnetic and crystal electric field (CEF) states of the Kondo lattice system CeCuGa3 by muon spin relaxation (muSR), neutron diffraction, and inelastic neutron scattering (INS) measurements. A noncentrosymmetric BaNiSn3-type tetragonal crystal structure (space group I4mm) is inferred from x-ray as well as from neutron powder diffraction. The low-temperature magnetic susceptibility and heat capacity data show an anomaly near 2.3 - 2.5~K associated with long range magnetic ordering, which is further confirmed by muSR and neutron diffraction data. The neutron powder diffraction collected at 1.7 K shows the presence of magnetic Bragg peaks indexed by an incommensurate magnetic propagation vector k = (0.148, 0.148, 0) and the magnetic structure is best described by a longitudinal spin density wave with ordered moments lying in ab-plane. An analysis of the INS data based on a CEF model reveals the presence of two magnetic excitations near 4.5 meV and 6.9 meV. The magnetic heat capacity data suggest an overall CEF splitting of 20.7 meV, however the excitation between 20 and 30 meV is very broad and weak in our INS data, but could provide an evidence of CEF level in this energy range in agreement with the magnetic entropy. Our analysis of INS data based on the CEF-phonon model indicates that the two excitations at 4.5 meV and 6.9 meV have their origin in CEF-phonon coupling (i.e. splitting of one CEF peak into two peaks, called vibron), with an overall splitting of 28.16 meV, similar to the case of CeCuAl3 and CeAuAl3.

Superconductors with noncentrosymmetric crystal structures

Superconductors with noncentrosymmetric crystal structures

Solving the puzzle of the dielectric nature of tantalum oxynitride perovskites

Abstract The dielectric properties of tantalum oxynitride perovskites are of interest because of the unique optical characteristics of these compounds based on their visible light absorption. Unfortunately, such perovskites are thermally metastable, which makes it challenging to prepare high-quality bulk samples for the study of dielectric properties. Recently, studies of small single crystals of BaTaO2N showed the compound to be ferroelectric, and this can be well explained based on the non-centrosymmetric crystal structure proposed previously using first-principles and molecular dynamic investigations.

Synthesis and properties of layered perovskite-like compounds PbBi2Nb2O9 and PbBi3Ti2NbO12

Two lead-containing compounds of the Aurivillus family: PbBi2Nb2O9 and PbBi3Ti2NbO12 were synthesized using a conventional high-temperature solid-state (ss) method and ion-exchange (ie) method. The phase and chemical purity of synthesized compounds were confirmed by XRD and EDX, respectively. The second harmonic generation experiment confirmed that all synthesized compounds have a non-centrosymmetric crystal structure. Rietveld refinement of samples prepared by different methods confirmed that ion-exchange and solid-state products share the same crystal structures; however, indirect evidence for different distributions of Pb/Bi between crystallographic positions was observed. Phase transitions, corresponding to the Curie temperatures were detected using high-temperature XRD. Thermal expansion of ion-exchange reaction products was studied in the temperature range of 298–1273 K. The optical band gap, electronegativity, valence band (EVB), and conduction band (ECB) potentials of materials were calculated from UV–vis spectroscopy data. Both PbBi2Nb2O9 (ss) and PbBi2Nb2O9 (ie) can be used for the decomposition of organic molecules under visible light, while PbBi3Ti2NbO12 (ss) and PbBi3Ti2NbO12 (ie) can work only under UV light irradiation due to their wider band gaps.

A hybrid organic-inorganic perovskite with robust SHG switching

Owing to the diversity of structure and potential applications in the field of electrics, sensors, and light-emitting diodes, lead halide perovskites have attracted great attention in recent years. Especially those lead halide perovskites with non-centrosymmetric crystal structures usually exhibit nonlinear optical (NLO) characteristics, which may endow them photoelectricity switching functionality. In this work, a lead-based hybrid organic-inorganic perovskite (HOIP) material, trimethyliodomethylammonium lead trichloride (TMIM·PbCl3), is obtained on the basis of tetramethylammonium lead chloride through halogen substitution on the cation part. It shows dual-phase-transition behavior around 345 and 358 K, which is significantly improved. TMIM·PbCl3 crystallizes in the chiral space group, P212121, and shows a well-defined second harmonic generation (SHG) response, and good switching endurance, which makes it an excellent candidate for SHG switching material. This work highlights the importance of halogen substitution for crystal engineering and may pave way for the further exploration of the optoelectronic devices.

Structural and thermal stability of B20-type high-pressure phases FeGe and MnGe

Stability and phase transformations in the high-pressure-synthesized phases of FeGe and MnGe with a noncentrosymmetric B20-type crystal structure are studied theoretically using ab initio density-functional calculations and experimentally by means of differential scanning calorimetry. For both germanides in magnetic state, the evolutionary genetic search yields the same set of preferred equiatomic polymorphs. In case of FeGe, this result is consistent with available theoretical and experimental information, whereas for MnGe we find new hypothetical phases, in addition to the known metastable B20 phase. The ground state of nonmagnetic MnGe is unexpectedly represented by a tetragonal structure quite unusual for the pd-bonded AB-type compounds. It follows from our calculations that, for a given input chemical composition, the existence of a structure stable at finite temperatures (and pressures) can be efficiently established by zero-temperature evolutionary search, if this structure appears in the output short list of lowest-energy states. The presented calorimetric study of the metastable FeGe and MnGe phases shows that their behavior upon heating notably depends on preparation method and sample history.

Inorganic Salts of N-phenylbiguanidium(1+)-Novel Family with Promising Representatives for Nonlinear Optics.

Seven inorganic salts containing N-phenylbiguanide as a prospective organic molecular carrier of nonlinear optical properties were prepared and studied within our research of novel hydrogen-bonded materials for nonlinear optics (NLO). All seven salts, namely N-phenylbiguanidium(1+) nitrate (C2/c), N-phenylbiguanidium(1+) perchlorate (P-1), N-phenylbiguanidium(1+) hydrogen carbonate (P21/c), bis(N-phenylbiguanidium(1+)) sulfate (C2), bis(N-phenylbiguanidium(1+)) hydrogen phosphate sesquihydrate (P-1), bis(N-phenylbiguanidium(1+)) phosphite (P21), and bis(N-phenylbiguanidium(1+)) phosphite dihydrate (P21/n), were characterised by X-ray diffraction (powder and single-crystal X-ray diffraction) and by vibrational spectroscopy (FTIR and Raman). Two salts with non-centrosymmetric crystal structures—bis(N-phenylbiguanidium(1+)) sulfate and bis(N-phenylbiguanidium(1+)) phosphite—were further studied to examine their linear and nonlinear optical properties using experimental and computational methods. As a highly SHG-efficient and phase-matchable material transparent down to 320 nm and thermally stable to 483 K, bis(N-phenylbiguanidium(1+)) sulfate is a promising novel candidate for NLO.

Stepwise Li Substitution Induced Structure Evolution and Improved Nonlinear Optical Performance for Diamond-like Sulfides.

One of the key scientific issues for exploration of novel infrared nonlinear optical (NLO) crystals is to obtain ones with good hybrid NLO behaviors. Herein, we report four diamond-like Ag-based sulfides via stepwise Li substitution of Ag in Ag2ZnSnS4, including Ag2ZnSnS4 (1), (Li1.22Ag0.78)ZnSnS4 (2), (Li1.58Ag0.42)ZnSnS4 (3), and Li2ZnSnS4 (4). With the increase of Li content, the sulfide's noncentrosymmetric crystal structure changes from tetragonal I42m for 1, to orthorhombic Pmn21 for 2 and 3, and to monoclinic Pn for 4. Accordingly, their NLO responses are improved along with the increase of Li content, viz. from non-NLO-active for 1, to non-phase-matchable for 2 and 3, and to phase-matchable for 4. Their optical band gaps also increase regularly. The relationship between their chemical compositions, crystal structures, and NLO activities is investigated by means of chemical structural analysis and theoretical calculation. This work offers a new systematic case for designing promising NLO-active compounds via rarely adopted cation's stepwise partial substitution and understanding the chemical composition-structure-NLO property relationship.

Toroidal nonreciprocity of optical second harmonic generation

We demonstrate mechanisms of reciprocity breaking in nonlinear optics driven by the toroidal dipole moment which characterizes nontrivial spatial distributions of spins in solids. Using high-resolution femtosecond spectroscopy at electronic resonances in the magnetoelectric antiferromagnet $\mathrm{Cu}{\mathrm{B}}_{2}{\mathrm{O}}_{4}$, we show that nonreciprocity reaches 100% for opposite magnetic fields due to the interference of nonlinear coherent sources of second harmonic generation originating from the toroidal dipole moment, applied magnetic field, and noncentrosymmetric crystal structure. The experimental results are corroborated by theoretical analysis based on the crystal and magnetic symmetry of $\mathrm{Cu}{\mathrm{B}}_{2}{\mathrm{O}}_{4}$. Our findings open degrees of freedom in nonlinear optics and pave the way for future nonreciprocal spin-optronic devices operating on the femtosecond timescale.

Time-Reversal Symmetry Breaking in Re-Based Superconductors: Recent Developments

In the recent search for unconventional- and topological superconductivity, noncentrosymmetric superconductors (NCSCs) rank among the most promising candidate materials. Surprisingly, some of them—especially those containing rhenium—seem to exhibit also time-reversal symmetry (TRS) breaking in their superconducting state, while TRS is preserved in many other isostructural NCSCs. To date, a satisfactory explanation for such discrepant behavior, albeit crucial for understanding the unconventional superconductivity of these materials, is still missing. Here we review the most recent developments regarding the Re-based class, where the muon-spin relaxation (μSR) technique plays a key role due to its high sensitivity to the weak internal fields associated with the TRS breaking phenomenon. We discuss different cases of Re-containing superconductors, comprising both centrosymmetric- and noncentrosymmetric crystal structures, ranging from pure rhenium, to ReT (T = 3d-5d early transition metals), to the dilute-Re case of ReBe22. μSR results suggest that the rhenium presence and its amount are two key factors for the appearance and the extent of TRS breaking in Re-based superconductors. Besides summarizing the existing findings, we also put forward future research ideas regarding the exciting field of materials showing TRS breaking.

Unusual upper critical fields of the topological nodal-line semimetal candidate Sn x NbSe2−δ

We report superconductivity in Sn x NbSe2−δ , a topological nodal-line semimetal candidate with a noncentrosymmetric crystal structure. The superconducting transition temperature T c of this compound is extremely sensitive to Sn concentration x and Se deficiency δ, 5.0 K for Sn0.13NbSe1.70 and 8.6 K for Sn0.14NbSe1.71 and Sn0.15NbSe1.69. In all samples, the temperature dependence of the upper critical field H c2(T) differs from the prediction of the Werthamer–Helfand–Hohenberg theory. While the zero-temperature value of the in-plane upper critical field of Sn x NbSe2−δ with the higher T c is lower than the BCS Pauli paramagnetic limit H P, that of the lower T c sample exceeds H P by a factor of ∼2. Our observations suggest that a possible odd-parity contribution dominates the superconducting gap function of Sn x NbSe2−δ , and it can be fine-tuned by the Sn concentration and Se deficiency.

Probing the superconducting gap structure in the noncentrosymmetric topological superconductor ZrRuAs

The superconducting gap structure of a topological crystalline insulator (TCI) candidate ZrRuAs ($T^{\rm on}_{\rm c}$ = 7.9(1) K) with a noncentrosymmetric crystal structure has been investigated using muon spin rotation/relaxation ($\mu$SR) measurements in transverse-field (TF) and zero-field (ZF) geometries. We also present the results of magnetization, electrical resistivity and heat capacity measurements on ZrRuAs, which reveal bulk superconductivity below 7.9~K. The temperature dependence of the effective penetration depth obtained from the analysis of the TF-$\mu$SR spectra below $T_{\rm c}$ is well described by an isotropic $s$-wave gap model as also inferred from an analysis of the heat capacity in the superconducting state. ZF $\mu$SR data do not show any significant change in the muon spin relaxation rate above and below the superconducting transition temperature indicating that time-reversal symmetry is preserved in the superconducting state of this material.

Room-temperature antiskyrmions and sawtooth surface textures in a non-centrosymmetric magnet with S4 symmetry.

Topological spin textures have attracted much attention both for fundamental physics and spintronics applications. Among them, antiskyrmions possess a unique spin configuration with Bloch-type and Néel-type domain walls owing to anisotropic Dzyaloshinskii-Moriya interaction in the non-centrosymmetric crystal structure. However, antiskyrmions have thus far only been observed in a few Heusler compounds with D2d symmetry. Here we report a new material, Fe1.9Ni0.9Pd0.2P, in a different symmetry class (S4), in which antiskyrmions exist over a wide temperature range that includes room temperature, and transform into skyrmions on changing magnetic field and lamella thickness. The periodicity of magnetic textures greatly depends on the crystal thickness, and domains with anisotropic sawtooth fractals were observed at the surface of thick crystals and attributed to the interplay between the dipolar interaction and the Dzyaloshinskii-Moriya interaction as governed by crystal symmetry. Our findings provide an arena in which to study antiskyrmions, and should stimulate further research on topological spin textures and their applications.

Synthesis, crystal structure, and electronic structure of Li2PbSiS4: a quaternary thiosilicate with a compressed chalcopyrite-like structure.

The new quaternary thiosilicate, Li2PbSiS4 (dilithium lead silicon tetrasulfide), was prepared in an evacuated fused-silica tube via high-temperature, solid-state synthesis at 800 °C, followed by slow cooling. The crystal structure was solved and refined using single-crystal X-ray diffraction data. By strict definition, the title compound crystallizes in the stannite structure type; however, this type of structure can also be described as a compressed chalcopyrite-like structure. The Li+ cation lies on a crystallographic fourfold rotoinversion axis, while the Pb2+ and Si4+ cations reside at the intersection of the fourfold rotoinversion axis with a twofold axis and a mirror plane. The Li+ and Si4+ cations in this structure are tetrahedrally coordinated, while the larger Pb2+ cation adopts a distorted eight-coordinate dodecahedral coordination. These units join together via corner- and edge-sharing to create a dense, three-dimensional structure. Powder X-ray diffraction indicates that the title compound is the major phase of the reaction product. Electronic structure calculations, performed using the full potential linearized augmented plane wave method within density functional theory (DFT), indicate that Li2PbSiS4 is a semiconductor with an indirect bandgap of 2.22 eV, which compares well with the measured optical bandgap of 2.51 eV. The noncentrosymmetric crystal structure and relatively wide bandgap designate this compound to be of interest for IR nonlinear optics.

Non-Stoichiometric Ferroelectric Bismuth Ferrites (Bi25FeO40) Enhancing Photocatalyst Activity in the Acidic Environment

In this work, non-stoichiometric bismuth ferrites (Bi25FeO40) particles that possess ferroelectric and photocatalysis properties were synthesized by the hydrothermal method. According to our previous work, piezoelectric materials can be employed in numerous applications. When applied to organic dye degradation, the photocatalysis effect is largely enhanced with the assistance of ferroelectricity[1, 2]. The Bi25FeO40 belongs to the I23 space group, which is a non-centrosymmetric piezoelectric crystal structure. The piezoelectricity is investigated by utilizing the piezoresponse force microscope (PFM) to measure its bias to phase relation. The lifetime of electron-hole pairs stimulated by laser could be measured by photoluminescence (PL) and time-resolved photoluminescence (TRPL). Due to its ferroelectricity, when the poling electric field raised, the remnant polarization and lifetime were increased; the intensity of the PL spectrum dropped reversely[2, 3]. This represents the recombination possibility of the electron-hole pairs were decreased by the poling process. Moreover, by adjusting the pH value of the degradation environment, the contact ability of the dye molecules and catalysts was improved. Under the condition of pH=3.5, the Rhodamine B (RhB) dye can be fully degraded in one hour while there is only 16% degraded in one hour of the original RhB solution(pH=4.8). This work demonstrated a photocatalyst assisted by ferroelectric property which could be enhanced through the poling process and prolong the lifetime of the electron-hole pairs. Furthermore, it shows greater degradation ability on RhB dye in acidic environment due to the better contact ability with the dye molecules, which makes it a new choice of acidic industrial wastewater treatment.

NbIr2B2 and TaIr2B2 – New Low Symmetry Noncentrosymmetric Superconductors with Strong Spin–Orbit Coupling

AbstractSuperconductivity was first observed more than a century ago, but the search for new superconducting materials remains a challenge. The Cooper pairs in superconductors are ideal embodiments of quantum entanglement. Thus, novel superconductors can be critical for both learning about electronic systems in condensed matter and for possible application in future quantum technologies. Here two previously unreported materials, NbIr2B2 and TaIr2B2, are presented with superconducting transitions at 7.2 and 5.2 K, respectively. They display a unique noncentrosymmetric crystal structure, and for both compounds the magnetic field that destroys the superconductivity at 0 K exceeds one of the fundamental characteristics of conventional superconductors (the “Pauli limit”), suggesting that the superconductivity may be unconventional. Supporting this experimentally based deduction, first‐principle calculations show a spin‐split Fermi surface due to the presence of strong spin–orbit coupling. These materials may thus provide an excellent platform for the study of unconventional superconductivity in intermetallic compounds.

Quaternary Arsenides REHfCu2+δAs3 (RE = La-Nd; δ ≈ 0.17): Superstructures of the Zr2Ni3P3-Type Structure.

The quaternary rare-earth-metal hafnium copper arsenides REHfCu2+δAs3 (where RE = La-Nd; δ ≈ 0.17) were obtained by direct reactions of the elements at 1073 K. They adopt a noncentrosymmetric orthorhombic crystal structure (space group Pmn21; a = 397.73(3)-392.37(2) pm, b = 1062.68(8)-1055.23(6) pm, c = 1307.25(9)-1291.40(7) pm for RE = La-Nd), which can be considered to be a Cu-deficient superstructure of the ternary Zr2Ni3P3-type structure found among metal-rich pnictides with a metal-to-nonmetal ratio close or equal to 2:1. The RE atoms occupy trigonal prismatic (CN6) and monocapped trigonal prismatic (CN7) sites, the latter resulting from the occurrence of Cu vacancies, the Hf atoms occupy octahedral sites (CN6), and the Cu atoms occupy tetrahedral sites (CN4). Band structure calculations on idealized models LaHfCu2As3 and LaHfCu2.5As3 suggest that the bonding situation improves by fractional occupation of the Cu3 site via depopulation of antibonding Cu-As states.

Evaluation of nonlinear optical properties of quaternary chalcogenide halides Ba4Si3Se9Br2 and Ba4Ge3Se9Br2

The quaternary chalcogenide halides Ba4Si3Se9Br2 and Ba4Ge3Se9Br2 were obtained in the form of crystals from reactions of binary selenides and bromides at 1173 K and in the form of phase-pure polycrystalline samples from reactions at 1023 K. They adopt noncentrosymmetric crystal structures, as determined from single-crystal X-ray diffraction data (hexagonal, space group P63, Z = 2; a = 10.1672 (5) Å, c = 12.4362 (6) Å for Ba4Si3Se9Br2; a = 10.2345 (4) Å, c = 12.5306 (5) Å for Ba4Ge3Se9Br2). The structures contain discrete Tt3Se96− rings (Tt = Si, Ge) which are built up from corner-sharing TtSe4 tetrahedra and which are separated by Ba2+ and Br− ions. Relative to the benchmark infrared nonlinear optical (NLO) material AgGaS2, the second harmonic generation (SHG) response is more intense for Ba4Si3Se9Br2 (3.2 × ) and Ba4Ge3Se9Br2 (3.5 × ) over particle sizes of 20–40 μm. The measured optical band gaps are 2.96 eV for Ba4Si3Se9Br2 and 2.60 eV for Ba4Ge3Se9Br2. Both compounds melt congruently (778 °C for Ba4Si3Se9Br2 and 731 °C for Ba4Ge3Se9Br2). The calculated birefringence is small, which accounts for the non-phase-matching behaviour observed experimentally.