Chalcopyrites are ternary crystalline compounds which have found use in a diverse array of applications, from solar cells to thermoelectric devices. These are known to be ternary-compound analogs to diamond or sphalerite. “Defect chalcopyrites,” which are chalcopyrite structures stabilized with ordered vacancies and substitutions, are well attested and can serve as a method of tuning material properties through control of stoichiometry. In particular, defect chalcopyrites related to the thermoelectric compound ${\mathrm{CuInTe}}_2$ comprise a large range of compositions in the pseudobinary system ${\left({\mathrm{Cu}}_2\mathrm{Te}\right)}_x{\left({\mathrm{In}}_2{\mathrm{Te}}_3\right)}_{1-x}$, where $x<0.5$. In contrast, the converse case of “augmented chalcopyrites,” namely, $x>0.5$, is much less known or studied. We report the discovery of a range of ${\mathrm{Cu}}_2\mathrm{Te}$-rich compositions in this binary system where stable phases can potentially be found. Here, the stoichometry of augmented chalcopyrites is likely to be modulated by the concentration of defect clusters ${[{\mathrm{Cu}}_{\text{In}}^{2-}·2{\mathrm{Cu}}_i^{+}]}^0$ in chalcopyrite ${\mathrm{CuInTe}}_2$.

The hexagonal $\mathrm{Eu}MX$ $(M=\mathrm{Cu}, \mathrm{Ag}, \mathrm{Au}; X=\mathrm{P}, \mathrm{As}, \mathrm{Sb}, \mathrm{Bi})$ compounds host interesting electronic and magnetic properties, with seemingly intertwined topology and transport properties. One key feature of such behavior is the nature of the ordered magnetic structure. In EuCuAs, a topological Hall effect is caused by a conical spin structure that emerges when a field is applied within the easy-plane $(H$ $\perp$ $c)$ of the helical ground state that exists below the Neel temperature of $T_N=14\mathrm{K}$. On the other hand, EuCuP is an easy-axis ferromagnet with a Curie temperature $T_C$ near 31 K. Here, we investigate the evolution of the magnetic properties in ${\mathrm{EuCuAs}}_{1-x}{\mathrm{P}}_x$ single crystals with $0.16 \le x\le 0.75$. Crystals grown by cooling slowly in a Sn flux possessed macroscale inhomogeneity of As/P, particularly for arsenic-rich crystals. However, growth in a Sn flux via an isothermal dwell at $600{}^{\circ }\mathrm{C}$ produced crystals that were homogeneous within the resolution of the probes utilized to investigate these crystals. The unit cell volumes, Curie-Weiss temperatures, and magnetic transitions trend linearly with composition and the magnetic anisotropy is reduced in the alloys. The magnetization data of crystals with $x=0.16$ and 0.24 indicate an easy-plane antiferromagnetic ground state while behavior similar to ferromagnetism is observed for crystals with $x\ge 0.41$. The temperature-dependent magnetization data possess multiple transitions for compositions near ${\mathrm{EuCuAs}}_{0.75}{\mathrm{P}}_{0.25}$, revealing a competition of ground states in this arsenic-rich region of the phase diagram. Neutron diffraction data for EuCuP are also presented as a follow up to previous results that revealed a two-step transition at $T_C$; the observed data were consistent with ferromagnetic order at $T=5\mathrm{K}$.

Lead halide perovskites have recently been reported to demonstrate an exceptionally high nonlinear (Kerr) refractive index ${\mathrm{n}}_2$ of up to ${10}^{-8}{\mathrm{cm}}^2/\mathrm{W}$ in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbBr}}_3$. Other researchers, however, observe different, substantially more conservative numbers. In order to resolve this disagreement, the nonlinear Kerr index of a bulk sample of lead halide perovskite was measured directly by means of an interferometer. This approach has many advantages as compared to the more standard z-scan technique. In particular, this method allows studying the induced changes to the refractive index in a time-resolved manner, thus enabling to separate the different contributions to $n_2$. The extracted ${\mathit{n}}_2$ values for ${\mathrm{CsPbBr}}_3$ and ${\mathrm{MAPbBr}}_3$ at $\lambda \approx 1\mu \mathrm{m}$ are $n_2=+2.1\times {10}^{-14}{\mathrm{cm}}^2/\mathrm{W}$ and $n_2=+6\times {10}^{-15}{\mathrm{cm}}^2/\mathrm{W}$, respectively. Hence, these values are substantially lower than what has been indicated in most of the previous reports, implying the latter one should be regarded with great care.

We have investigated impedance, electric modulus, and dielectric spectroscopies, along with AC conductivity, on single crystals of methylammonium lead iodide (MAPI) and formamidinium lead iodide (FAPI) in the frequency range $50\mathrm{Hz}\le f\le 1\mathrm{MHz}$ in the dark and under low-intensity illumination ($\le 80\mu \mathrm{W}/{\mathrm{cm}}^2$). It is demonstrated that the relaxation observed in this frequency range in these single crystals can be attributed to space charge effects in the bulk of the crystals, which are caused by the finite time scale associated with charge relaxation, which can occur in this frequency range due to the large static dielectric constant and low conductivity of these solids. The relaxation was found to be faster in FAPI (with higher conductivity) compared to that in MAPI (with lower conductivity). The electron-hole pair generated by illumination enhances electronic conductivity and accelerates ionic migration by lowering the barrier; this, in turn, decreases the charge relaxation time and enhances the relaxation process. The barrier lowering inferred from the reduction in relaxation times by illumination is proposed to be associated with changes in the chemical potential attributed to carrier generation.

Below $T_{\mathrm{s}}\sim 90\mathrm{K}$, single-crystal FeSe undergoes a structural phase transition coinciding with the emergence of lattice domain boundaries, termed twin boundaries. Polarized-light microscopy validated the presence of twin boundaries in pristine FeSe and indicated the existence of extensive microsized twining in proton-irradiated FeSe. In twinned FeSe, vortex penetration displays a fractal, mazelike pattern, likely guided by these twin domains. We speculate substantial microtwinning in the proton-irradiation crystals, yet macroscopic twin boundaries persist as primary conduits for vortex motion.

There are many reports of a surprisingly high charge-carrier density with sizable mobility in photoexcited two-dimensional (2D) halide perovskites despite their unusually high exciton binding energy. In this work we study the thermodynamic quasiequilibrium of the relative population of photoexcited free quasielectron/quasihole pairs, neutral excitons, and excitonic trions, in 2D materials that support such excitonic complexes with large binding energy. We derive and solve the general Saha equations which describe the detailed balance of such a system of photoexcited electronic degrees of freedom forming a multicomponent fluid of excitations in thermodynamic quasiequilibrium. The solution to these equations, for the special case of 2D perovskites where the reported exciton and excitonic trion binding energies are of the order of 0.3–0.4 eV for the former and 30–40 meV for the latter, reveals that while the charge-neutral excitonic population dominates all other excitations, at room temperature and below, the excitonic trion component can be the dominant population among charge carriers. We also argue that trionic hopping can take place via a tunneling mechanism which is speculated to play a role in a novel charge-transport mechanism.

The two-dimensional van der Waals (vdW) materials $M{\mathrm{PS}}_3(M=\mathrm{Mn},\mathrm{Fe},\mathrm{Co},\mathrm{Ni})$ display antiferromagnetic ordering of the magnetic moments at the transition metal ions. The possibility to exfoliate thin layers that preserve the magnetic order makes these materials interesting for numerous applications in devices that require integration of flexible patches of magnetic materials, e.g., in antiferromagnetic spintronics. Hence, an improved understanding of their magnetic properties is desirable. Here, we parametrize spin Hamiltonians for a monolayer of all four materials of this class using density functional theory plus Hubbard $U$ calculations. We provide a step-by-step guide for calculating the magnetic exchange interactions and magnetic anisotropy energy using the (non)collinear $\mathrm{DFT}+U(+\mathrm{SOC})$ approach with a suitably chosen $U$ for each material. It is found that the biquadratic interactions gain in importance while moving through the $3d$ series. Retaining the leading terms of a Holstein-Primakoff-transformed spin Hamiltonian, the magnon spectra are calculated. While ${\mathrm{MnPS}}_3$ is found to be an almost isotropic antiferromagnet with a tiny gap, the biquadratic interaction opens an increasingly wider gap for ${\mathrm{FePS}}_3, {\mathrm{CoPS}}_3$, and ${\mathrm{NiPS}}_3$. In line with this observation, Monte Carlo simulations demonstrate that the biquadratic interactions contribute to a systematic rise in the Néel temperature from ${\mathrm{FePS}}_3$ to ${\mathrm{NiPS}}_3$.

One of the most important nonlinear optical (NLO) effects is the second-harmonic generation (SHG), and crystals with strong SHG effects are called NLO crystals. The traditional anion group theory has guided the early discovery of NLO crystals, but the variety of NLO-active motifs discovered so far is limited. In this study, material-type unbiased high-throughput first-principles calculations are performed to screen thousands of materials in a materials database for NLO crystals at target frequencies. The electronic, linear, and nonlinear optical properties of these materials are calculated. Among them, 40 NLO crystals suitable for mid-infrared (MIR) frequencies and 5 for deep-ultraviolet (DUV) frequencies are identified, from 229 materials to which scissors correction is applied. As an extension, several NLO-active motifs that dominate the SHG response are identified, and they all show good transferability among similar materials. Furthermore, for materials where scissors correction cannot be applied due to the lack of accurate bandgap value in the database, a recommendation list of NLO crystals based on the scaling law of bandgap and SHG susceptibility is presented. The discovery of new NLO crystals and NLO-active motifs beyond traditional methods will greatly accelerate the applications of NLO crystals at DUV and MIR frequencies and enrich our understanding in the search and design of new NLO crystals.

Motivated by contradicting reports in the literature, we have investigated the structural stability of tungsten nanoparticles using density functional theory calculations. The comparison of BCC, FCC, A15, disordered, and icosahedral configurations unequivocally shows that BCC is, energetically, the most stable structure when the number of atoms is greater than 40. A disordered structure is more stable for smaller sizes. This result conflicts with an earlier theoretical study on transition metal nanoparticles, based on a semi-empirical modeling of nanoparticles energetics [D. Tománek et al., Phys. Rev. B 28, 665 (1983)]. Examining this latter work in the light of our results suggests that an inappropriate description of cluster geometry is the source of the discrepancy. Finally, we improve the accuracy of the semi-empirical model proposed in this work, which will be useful to calculate nanoparticle energies for larger sizes.

Electron-correlation-driven phonon soft modes have been recently reported in the antiferromagnetic kagome FeGe compound and associated with the observed charge density wave (CDW). In this paper, we present a systematic investigation of the CDW origin in the context of the ab initio lattice dynamics study. Performing the group theory analysis of the aforementioned soft modes, we found that the stable structure has the Immm symmetry and can be achieved by shifts of Ge atoms. The occurrence of two soft modes induces the first-order phase transition, which leads to the CDW order. Additionally, we show that the final structure realizes a distorted honeycomb Ge lattice, as well as a nonflat kagome-like Fe net. For completeness, we present the electronic properties calculations. From the theoretical STM topography simulation, we indicate that the observed CDW occurs in the deformed honeycomb Ge sublattice.

Recently, a new type of unconventional superconductivity with a field-induced transition between two different superconducting (SC) states was discovered in the heavy fermion system ${\mathrm{CeRh}}_2{\mathrm{As}}_2$. This unusual SC state was proposed to be based on specific symmetries of the underlying structure, i.e., a globally centrosymmetric layered structure, but where the Ce layers themselves lack inversion symmetry. This new type of SC state has attracted strong interest, prompting the search for further heavy fermion systems crystallizing in structures with appropriate symmetries. We report the discovery and the study of a new Ce-based heavy fermion system with a globally centrosymmetric structure but without inversion symmetry on the Ce-site, ${\mathrm{Ce}}_2{\mathrm{Ir}}_3{\mathrm{Ga}}_5$. A single crystal x-ray diffraction study revealed an orthorhombic ${\mathrm{U}}_2{\mathrm{Co}}_3{\mathrm{Si}}_5$ type structure. Resistivity, specific heat, and magnetization measurements indicate a moderate-heavy fermion behavior with a Kondo energy scale of the order of 40 K. Most experimental results suggest the absence of magnetic order, but a tiny anomaly in the specific heat opens the possibility for a very weak, itinerant type of ordering.

Co-based materials have recently been explored because of their potential to realize complex bond-dependent anisotropic magnetism. Prominent examples include ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6, {\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2, {\mathrm{Na}}_2\mathrm{BaCo}{\left({\mathrm{PO}}_4\right)}_2$, and ${\mathrm{Co}\mathit{X}}_2$ $(X=\mathrm{Cl},\mathrm{Br},\mathrm{I})$. In order to provide insight into the magnetic interactions in these compounds, we make a comparative analysis of their local crystal electric field excitations spectra via Raman scattering measurements. Combining these measurements with theoretical analysis confirms the validity of $j_{\mathrm{eff}}=1/2$ single-ion ground states for all compounds, and provides accurate experimental estimates of the local crystal distortions, which play a prominent role in the magnetic couplings between spin-orbital coupled Co moments.

We detail the optimized synthesis of the ${\mathrm{Mo}}_3{\mathrm{O}}_8$-type cluster Mott insulator (CMI) ${\mathrm{Na}}_3{\mathrm{Sc}}_2{\left({\mathrm{MoO}}_4\right)}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$, which has been considered a candidate for realizing the spin liquid ground state. The optimized ${\mathrm{Na}}_3{\mathrm{Sc}}_2{\left({\mathrm{MoO}}_4\right)}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$, characterized by x-ray diffraction, energy-dispersive x-ray spectroscopy, and magnetic and heat capacity measurements exhibited an effective magnetic moment close to the ideal 1.73 ${\mu }_{\mathrm{B}}$ for S = 1/2 spin and magnetic ordering at ∼5 K. These observations categorize ${\mathrm{Na}}_3{\mathrm{Sc}}_2{\left({\mathrm{MoO}}_4\right)}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ as the second ${\mathrm{Mo}}_3{\mathrm{O}}_8$-type CMI to achieve a magnetic ground state, following ${\mathrm{Li}}_2{\mathrm{InMo}}_3{\mathrm{O}}_8$. They highlight the stabilization of the magnetic ground state over the theoretically anticipated quantum spin liquid state through precise valence and chemical disorder tuning. Our findings challenge the existing theory that the magnetic ground state of ${\mathrm{Mo}}_3{\mathrm{O}}_8$-type CMIs is determined by the breathing parameter, instead showing that magnetic order is suppressed by spin defects. This study underscores the crucial role of chemical precision in investigating quantum magnetism. It suggests that precise tuning of valence states could induce magnetic ordering in previously nonmagnetic ${\mathrm{Mo}}_3{\mathrm{O}}_8$-type CMIs. Additionally, the negative findings regarding the existence of quantum spin liquids highlight the need for applied research and a reevaluation of our fundamental understanding of electronic states from both theoretical and experimental aspects.

Gratings composed of cholesteric liquid crystals as an important optical element for emerging applications such as augmented and virtual reality and are renowned for their characteristic single reflective diffraction band. Heliconical liquid crystal is a newly discovered state where the constituent molecules self-organize into helical structures with a non-90° polar angle between the director and the helical axis. Here, we present a numerical study on the reflective diffraction of gratings made of heliconical liquid crystals. Remarkably, numerical results demonstrate that there exist two diffraction bands at the same diffraction angle, with one peak wavelength being twice the other. We show that the short-wavelength diffraction originates from the Pancharatnam-Berry phase acquired by the reflected light while the long-wavelength diffraction stems from the reflection of the slanted volume grating, and that the wavelengths of these two diffraction bands can be attributed to the first and second band gaps of the slanted volume grating as a one-dimensional photonic crystal. We further show that the polarization of the reflected diffraction light is circular, exhibiting the same handedness as the liquid crystal for the short-wavelength band, whereas it is perfectly linearly polarized along the grating direction for the long-wavelength band.

Topological materials are often associated with exceptional thermoelectric properties. Orthorhombic $\mathrm{Ba}\mathrm{Mn}{\mathrm{Sb}}_2$ is a topological semimetal consisting of alternating layers of Ba, Sb, and MnSb. A recent experiment demonstrates that $\mathrm{Ba}\mathrm{Mn}{\mathrm{Sb}}_2$ has a low thermal conductivity and modest thermopower, promising as a thermoelectric material. Through first-principles calculations with Coulomb repulsion and spin-orbit coupling included, we studied the electronic structure, phononic structure, and thermoelectric transport properties of $\mathrm{Ba}\mathrm{Mn}{\mathrm{Sb}}_2$ in depth. We find that $\mathrm{Ba}\mathrm{Mn}{\mathrm{Sb}}_2$ exhibits a low lattice thermal conductivity, owing to the scattering of the acoustic phonons with low-frequency optical modes. Using the linearized Boltzmann transport theory with a constant relaxation time approximation, the thermopower is further calculated and an intriguing goniopolar transport behavior, which is associated with both $n$-type and $p$-type conduction along separate transport directions simultaneously, is observed. We propose that the figure of merit can be enhanced via doping in which electrical conductivity is increased while the thermopower remains undiminished. $\mathrm{Ba}\mathrm{Mn}{\mathrm{Sb}}_2$ is a potential platform for elucidating complex band structure effects and topological phenomena, paving the way to explore rich physics in low-dimensional systems.

The Nabarro-Herring (NH) diffusional creep theory postulates the vacancy-mediated transport of atoms under a stress gradient as the creep mechanism under low-stress and high-temperature conditions. In multicomponent alloys, we premise that this stress-assisted flow of vacancies to and from grain boundaries will produce elemental segregation. An observation of such segregation, validated with theoretical predictions, can provide the necessary experimental evidence for the occurrence of NH creep. Theoretical calculations of the segregation tendencies via analyzing the dominant solute diffusion mechanisms and the difference in diffusivities of the elements are therefore essential. To this end, this study applies density functional theory calculations of migration barriers and solute-vacancy binding energies as input to the self-consistent mean-field theory to assess the vacancy-mediated diffusion mechanisms, transport coefficients, and segregation tendencies of Co, Cr, Mo, Re, Ta, and W solutes in face-centered-cubic Ni. We find Co, Re, and W to be slow diffusers at high temperatures and Cr, Mo, and Ta to be fast diffusers. Further analysis shows that the slow diffusers tend to always enrich at vacancy sinks over a wide range of temperatures. In contrast, the fast diffusers show a transition from depletion to enrichment as the temperature lowers. Furthermore, our analysis of the segregation tendencies under tensile hydrostatic strains shows that slow diffusers are largely unaffected by the strain and favor enrichment. On the other hand, the fast diffusers exhibit high sensitivity to strain and their segregation tendency can transition from depletion to enrichment at a given temperature. The transport coefficients calculated in this work are expected to serve as input to mesoscale microstructure models to provide a more rigorous assessment of solute segregation under NH creep conditions.

The intrinsic anomalous Hall conductivity (AHC) in a ferromagnetic metal is completely determined by its band structure. Since the spin orientation direction is an important band–structure tuning parameter, it is highly desirable to study the anomalous Hall effect in a system with multiple spin reorientation transitions. We study a layered tetragonal room temperature ferromagnet $\mathrm{Sm}{\mathrm{Mn}}_2{\mathrm{Ge}}_2$, which gives us the opportunity to measure magnetotransport properties where the long $c$-axis and the short $a$-axis can both be magnetically easy axes depending on the temperature range we choose. We show a moderately large fully intrinsic AHC up to room temperature when the crystal is magnetized along the $c$-axis. Interestingly, the AHC can be tuned to completely extrinsic with extremely large values when the crystal is magnetized along the $a$-axis, regardless of whether the $a$-axis is magnetically easy or hard axis. First-principles calculations show that nodal line states originate from Mn-$d$ orbitals just below the Fermi energy ($E_{\mathrm{F}}$) in the electronic band structure when the spins are oriented along the $c$-axis. Intrinsic AHC originates from the Berry curvature effect of the gapped nodal lines in the presence of spin-orbit coupling. AHC almost disappears when the spins are aligned along the $a$-axis because the nodal line states shift above $E_{\mathrm{F}}$ and become unoccupied. Since the AHC can be tuned from fully extrinsic to intrinsic even at 300 K, $\mathrm{Sm}{\mathrm{Mn}}_2{\mathrm{Ge}}_2$ becomes a potential candidate for room-temperature spintronics applications.

Magnetoelastic effects in antiferromagnetic CuMnAs are investigated by applying dynamic strain in the 0.01% range through surface acoustic waves in the GaAs substrate. The magnetic state of the CuMnAs/GaAs is characterized by a multitude of submicron-sized domains, which we image by x-ray magnetic linear dichroism combined with photoemission electron microscopy. Within the explored strain range, CuMnAs shows magnetoelastic effects in the form of Néel vector waves with micrometer wavelength, which corresponds to an averaged overall spin-axis rotation up to $2.4^{\circ }$ driven by the time-dependent strain from the surface acoustic wave. Measurements at different temperatures indicate a reduction of the wave amplitude when lowering the temperature. However, no domain wall motion has been detected on the nanosecond timescale.