We have successfully synthesized a hyperkagome antiferromagnet ${\mathrm{Zn}}_2{\mathrm{Mn}}_3{\mathrm{O}}_8$ by a topochemical method, where ${\mathrm{Mn}}^{4+}$ ions with $S=3/2$ spins form a three-dimensional (3D) corner-sharing triangle network. Magnetic susceptibility and heat capacity measurements revealed an antiferromagnetic transition at $T_{\mathrm{N}}=5.8\mathrm{K}$. Although no evidence was found for any structural distortion accompanying the transition, the low-temperature magnetic heat capacity showed a nearly $T^2$ dependence, suggesting the realization of a 2D magnonlike excitation in the 3D hyperkagome lattice.

Oxide superlattices have drawn great attention owing to the intriguing coupling among elastic, electrical, and magnetic orderings at the interfaces and the emergence of improper ferroelectricity. Here, superlattices composed of hexagonal $\mathrm{LuFe}{\mathrm{O}}_3$ $\left(\mathrm{h}\text{−}\mathrm{LuFe}{\mathrm{O}}_3\right)$ and $\mathrm{Lu}{\mathrm{Fe}}_2{\mathrm{O}}_4$ are investigated via density functional theory calculations. h-$\mathrm{LuFe}{\mathrm{O}}_3$ is a well-known multiferroic material that is stable only in thin film or doped bulk state, while $\mathrm{Lu}{\mathrm{Fe}}_2{\mathrm{O}}_4$ is a charge ordered (CO) material where the existence of ferroelectricity is still a controversy. We have found that the CO-induced polarizations in $\mathrm{Lu}{\mathrm{Fe}}_2{\mathrm{O}}_4$ layers coexist with the geometric polarizations in $\mathrm{h}\text{−}\mathrm{LuFe}{\mathrm{O}}_3$ layers in the ${\left(\mathrm{Lu}{\mathrm{Fe}}_2{\mathrm{O}}_4\right)}_m/{\left(\mathrm{LuFe}{\mathrm{O}}_3\right)}_n$ superlattices with different periodicities, and the ferroelectric states are generally preferred over the antiferroelectric states for $\mathrm{Lu}{\mathrm{Fe}}_2{\mathrm{O}}_4$ in superlattices. The out-of-plane polarizations in h-$\mathrm{LuFe}{\mathrm{O}}_3$ and $\mathrm{Lu}{\mathrm{Fe}}_2{\mathrm{O}}_4$ layers tend to be aligned in parallel, and the overall polarization increases with the ratio of $\mathrm{h}\text{−}\mathrm{LuFe}{\mathrm{O}}_3$. The influence of layered polarizations on the local electrostatic potential is not significant except the detected small trend caused by the CO-induced polarization within a FeO bilayer. Additionally, the local electronic structures show that the Fermi level position in a certain layer can be tuned by the valences of Fe in this layer and the polarization distributions in neighboring layers. $\mathrm{Lu}{\mathrm{Fe}}_2{\mathrm{O}}_4$ layers sandwiched between thick $\mathrm{h}\text{−}\mathrm{LuFe}{\mathrm{O}}_3$ layers are more susceptible. The calculated configurations of the superlattices are supported by atomic-resolution transmission electron microscopy experiments. Our results pave the way for tunable ferroelectricity in superlattice systems and create a playground for manipulating the coupling between various degrees of freedom.

Nickelates have been studied extensively due to their intriguing physical properties, but less attention has been paid to the properties of divalent A-site cation perovskite nickelates with a formal valence state of 4+ for Ni. Here, we study the electronic and magnetic properties of perovskite ${\mathrm{SrNiO}}_{3-\delta }$ with an oxygen deficiency δ up to 0.375 using density functional theory. Because of the strong covalency and negative charge transfer energetics, the structure is predicted to exhibit ligand holes , with Ni present as $d^8{\underline{L}}^2$ or $d^7\underline{L}$ and significant magnetic moment at the oxygen sites. The ground state for $\delta =0–0.375$ consists of ferromagnetically ordered Ni with the Ni and O moments coupled antiferromagnetically, and the removal of oxygen increases the net magnetization. These behaviors are also predicted for other A-site cations such as Ca and Ba. This work demonstrates the importance of ligand holes in oxides with formally high valence Ni, including their influence on the magnetic properties, and motivates further experimental study of the electronic and magnetic properties of nickelates with divalent A-site cations.

Hydrogen has the largest gravimetric energy density among all chemical fuels. At the same time, the density of gaseous ${\mathrm{H}}_2$ is extremely low, which makes its compression to high pressures, liquefaction, or solid-state storage necessary for transport purposes. Liquid hydrogen ($\mathrm{L}{\mathrm{H}}_2$) can be transported in a dewar under atmospheric pressure, but this requires energy-intensive cooling down to $20\mathrm{K}$. Magnetocaloric materials have great potential to revolutionize gas liquefaction to make $\mathrm{L}{\mathrm{H}}_2$ more competitive as fuel. In this paper, we investigate a series of Laves-phase materials regarding their structural, magnetic, and magnetocaloric properties in high magnetic fields. The three compounds $\mathrm{Ho}{\mathrm{Al}}_2$, ${\mathrm{Ho}}_{0.5}{\mathrm{Dy}}_{0.5}{\mathrm{Al}}_2$, and $\mathrm{Dy}{\mathrm{Al}}_2$ are suited for building a stack for cooling from liquid-nitrogen temperature ($77\mathrm{K}$) down to the boiling point of hydrogen at $20\mathrm{K}$. This is evident from our direct measurements of the adiabatic temperature change in pulsed magnetic fields, which we compare with calorimetric data measured in a static field. With this methodology, we are now able to study the suitability of magnetocaloric materials down to low temperatures up to the highest magnetic fields of 50 T.

The increasing interest in nanostructured ceramics and their applications highlights the need to understand the hot-press sintering of nanoscale AlN powders. We use molecular dynamics simulations to investigate the hot-press sintering of AlN nanoceramics and to clarify the underlying sintering mechanisms. We consider samples with 32 nanoparticles with diameters 8, 12, and 16 nm, arranged in a face centered cubic supercell: samples AlN-8, AlN-12, and AlN-16. Sintering simulations are performed at $T=1900\mathrm{K}$ under 1 GPa for 6 ns. An additional simulation is performed for a sample with 8 nm sized nanoparticles at a lower pressure of 0.1 GPa, namely, sample AlN-8-0.1. After 6 ns, densifications of 99%, 96.2%, 95.6%, and 93.2% are achieved for samples AlN-8, AlN-8-0.1, AlN-12, and AlN-16, respectively. Analysis shows that the fast densification process is rooted at the high diffusivity of nanoparticles surface atoms. The AlN-8 sample undergoes intense microstructural evolution during the sintering process from 3 to 6 ns, resulting in a wide distribution of grain sizes from 4 to 15 nm and a larger, 11 nm average grain size. A slower grain growth process is observed in the AlN-8-0.1 sample from $\sim 4.0\mathrm{ns}$. These results indicate a change in the densification mechanism from surface diffusion to grain boundary migration and relaxation of grain boundaries and triple junctions, resulting in a two-stage sintering process, i.e., initially the sample experiences a fast densification, which is followed by intense microstructural evolution. The densification mechanism crossover occurs at 98.7% and 95% densification for the AlN-8 and AlN-8-0.1 samples, respectively. The results indicate that the onset of the second stage depends on a densification threshold, which can be delayed by applying higher external pressure. Sintering of the AlN-16 sample indicates the presence of structural phase transformation at the nanoparticles contact points, which reach over 12 GPa of local pressure during the 1 GPa compression. These results provide atomistic insights into the hot-press sintering of nanoscale ceramics, highlighting the intrinsic swift densification and microstructural evolution processes.

The many surface reconstructions of (110)-oriented lanthanum strontium manganite (${\mathrm{La}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{MnO}}_3$, LSMO) were followed as a function of the oxygen chemical potential (${\mu }_{\text{O}}$) and the surface cation composition. Decreasing ${\mu }_{\text{O}}$ causes Mn to migrate across the surface, enforcing phase separation into $A$-site-rich areas and a variety of composition-related, structurally diverse $B$-site-rich reconstructions. The composition of these phase-separated structures was quantified with scanning tunneling microscopy, and these results were used to build a two-dimensional phase diagram of the LSMO(110) equilibrium surface structures.

We apply van der Waals density-functional theory (vdW-DFT) calculations to predict crystal structure parameters and spontaneous polarization values for seven hydrogen-bonded single-component organic ferroelectrics. The results show good agreement with experimental results, implying that an important step for the computational materials design of organic ferroelectrics has been achieved. This approach also enables the simulation of electromechanical responses. Calculations using the vdW-DFT method are performed for croconic acid (CRCA), 2-phenylmalondialdehyde (PhMDA), and 5,6-dichloro-2-methylbenzimidazole (DC-MBI) under uniaxial stresses or electric fields. Direct piezoelectric $d_{33}$ constants are evaluated from the polarization change as a function of stress, whereas converse piezoelectric $d_{33}$ constants are evaluated from the change in lattice parameter as a function of electric field. The obtained values show acceptable agreement with the experimental values if possible objective factors are considered. The stress-induced or electric-field-induced variation of polarization is analyzed considering two types of contributions. One is from proton transfer as a classical point charge motion and the other residual part corresponds to the redistribution of $\pi$ electrons.

We report the energy landscape of an upcoming solar cell material series, namely, silver-bismuth-iodide rudorffites $\left(A_m{B}_n{X_m}_{+3n}\right)$. We formed the compounds through compositional engineering and characterized them. We employed Kelvin probe force microscopy (KPFM) to derive the surface work function and thereby the Fermi energy; scanning tunneling spectroscopy (STS), on the other hand, provided conduction and valence band edges with respect to the Fermi energy of the rudorffites. By combining KPFM and STS studies, we obtained a composition-dependent tuning of the Fermi energy in rudorffite thin films, namely, ${\mathrm{Ag}}_3\mathrm{Bi}{\mathrm{I}}_6$, ${\mathrm{Ag}}_2\mathrm{Bi}{\mathrm{I}}_5$, $\mathrm{AgBi}{\mathrm{I}}_4$, $\mathrm{Ag}{\mathrm{Bi}}_2{\mathrm{I}}_7$, and $\mathrm{Ag}{\mathrm{Bi}}_3{\mathrm{I}}_{10}$ with a gradual change in electronic conductivity from $p$ to $n$ type. While such behavior has been correlated to point defects, which are possible to form in the different members of the rudorffite series, we more importantly show that a combination of KPFM and STS studies can provide the complete energy landscape of a semiconductor.

Two-dimensional multiferroics inherit prominent physical properties from both low-dimensional materials and magnetoelectric materials, and they surpass their three-dimensional counterparts for their unique structures. Here, based on density functional theory calculations, a MXene derivative, i.e., i-MXene $({\mathrm{Ta}}_{2/3}{\mathrm{Fe}}_{1/3}{)}_2{\mathrm{CO}}_2$, is predicted to be a type-I multiferroic material. Originating from the reliable $5d^0$ rule, its ferroelectricity is robust, with a moderate polarization up to $\sim 12.33\mu \mathrm{C}/{\mathrm{cm}}^2$ along the $a$-axis, which can be easily switched and may persist above room temperature. Its magnetic ground state is layered antiferromagnetism. Although it is a type-I multiferroic material, its Néel temperature can be significantly tuned by the paraelectric-ferroelectric transition, manifesting a kind of intrinsic magnetoelectric coupling. Such a magnetoelectric effect originates from the conventional magnetostriction, but it is unexpectedly magnified by the exchange frustration. Our work not only reveals a nontrivial magnetoelectric mechanism, but it also provides a strategy to search for more multiferroics in the two-dimensional limit.

In this paper, we report a formation mechanism of $1/2\langle 111\rangle$ interstitial dislocation loops via the collapse of C15 clusters in bulk tungsten. The interstitial atoms formed C15 clusters in only a few picoseconds within a displacement cascade, and the transformation from C15 clusters to $\langle 111\rangle$ interstitial loops was observed in both classical molecular dynamics and accelerated molecular dynamics simulations. This transformation was further confirmed under compressive stress, and it was determined to be accelerated once the $\langle 111\rangle$ loops reacted with the C15 clusters. Three possible transformation processes were proposed, namely the C15 cluster directly transforming to $\langle 111\rangle$ loops/clusters, a partial transformation to a $\langle 111\rangle$ cluster with high mobility, thus leaving a shrunken C15 cluster behind, and a partially transformed $\langle 111\rangle$ cluster but pinned by the adjacent shrunken C15 cluster. The formation mechanism provides an essential reference for predicting the evolution of microstructures in tungsten-based materials under irradiation.

The rate and pathways of relaxation of a magnetic medium to its equilibrium following excitation with intense and short laser pulses are the key ingredients of ultrafast optical control of spins. Here we study experimentally the evolution of the magnetization and magnetic anisotropy of thin films of a ferromagnetic metal galfenol (${\mathrm{Fe}}_{0.81}{\mathrm{Ga}}_{0.19}$) resulting from excitation with a femtosecond laser pulse. From the temporal evolution of the hysteresis loops we deduce that the magnetization $M_S$ and magnetic anisotropy parameters $K$ recover within a nanosecond, and the ratio between $K$ and $M_S$ satisfies the thermal equilibrium's power law in the whole time range spanning from a few picoseconds to 3 nanoseconds. We further use the experimentally obtained relaxation times of $M_S$ and $K$ to analyze the laser-induced precession and demonstrate how they contribute to its frequency evolution at the nanosecond timescale.

One of the most important functionalities of the atomically thin insulator hexagonal boron nitride (hBN) is its ability to chemically and electronically decouple functional materials from highly reactive surfaces. It is therefore of utmost importance to uncover its structural properties on surfaces on an atomic and mesoscopic length scale. In this paper, we quantify the relative coverages of structurally different domains of a hBN layer on the Ni(111) surface using low-energy electron microscopy and the normal incidence x-ray standing wave technique. We find that hBN nucleates on defect sites of the Ni(111) surface and predominantly grows in two epitaxial domains that are rotated by ${60}^{\circ }$ with respect to each other. The two domains reveal identical adsorption heights, indicating a similar chemical interaction strength with the Ni(111) surface. The different azimuthal orientations of these domains originate from different adsorption sites of N and B. We demonstrate that the majority ($\approx 70\%$) of hBN domains exhibit a $(\mathrm{N},\mathrm{B})=(\mathrm{top},\mathrm{fcc})$ adsorption site configuration while the minority ($\approx 30\%$) show a $(\mathrm{N},\mathrm{B})=(\mathrm{top},\mathrm{hcp})$ configuration. Our study hence underlines the crucial role of the atomic adsorption configuration in the mesoscopic domain structures of in situ fabricated two-dimensional materials on highly reactive surfaces.

Epitaxial thin films of ${\left({\mathrm{Sn}}_x{\mathrm{Pb}}_{1-x}\right)}_{1-y}{\mathrm{In}}_y\mathrm{Te}$ were successfully grown by molecular-beam epitaxy in a broad range of compositions ($0\le x\le 1$, $0\le y\le 0.23$). We investigated electronic phases of the films by the measurements of electrical transport and optical second-harmonic generation. In this system, one can control the inversion of the band gap, the electric polarization that breaks the inversion symmetry, and the Fermi-level position by tuning the Pb/Sn ratio and In composition. A plethora of topological electronic phases is expected to emerge, such as the topological crystalline insulator, the topological semimetal, and superconductivity. For the samples with large Sn compositions $(x>0.5)$, hole density increases with In composition $\left(y\right)$, which results in the appearance of superconductivity. On the other hand, for those with small Sn compositions $(x<0.5)$, an increase in In composition reduces the hole density and changes the carrier type from $p$ type to $n$ type. In a narrow region centered at $\left(x,y\right)=\left(0.16,0.07\right)$ where the $n$-type carriers are slightly doped, charge transport with high mobility exceeding $5000\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{\text{–}1}{\mathrm{s}}^{\text{–}1}$ shows up, representing the possible semimetal states. In those samples, the optical second-harmonic generation measurement showing the breaking of inversion symmetry along the out-of-plane [111] direction, which is a necessary condition for the emergence of the polar semimetal state. The thin films of ${\left({\mathrm{Sn}}_x{\mathrm{Pb}}_{1-x}\right)}_{1-y}{\mathrm{In}}_y\mathrm{Te}$ material systems with a variety of electronic states would become a promising materials platform for the exploration of novel quantum phenomena.

Transition metal doped ${\mathrm{In}}_2{\mathrm{O}}_3$ with high mobility can be used as a transparent conductor with enhanced transparency spectral window. In this work, we carried out a comprehensive study on the electrical and optical properties of ${\mathrm{In}}_2{\mathrm{O}}_3$ doped with several transition metal (TM) species (${\mathrm{In}}_2{\mathrm{O}}_3:\mathrm{TM}$) including W, Zr, Mo, and Ti. Detailed optical properties obtained by spectroscopic ellipsometry (SE) are correlated with electrical properties obtained by Hall effect measurements. We find that the mobility of ${\mathrm{In}}_2{\mathrm{O}}_3:\mathrm{TM}$ thin films lies in the range of $50\text{–}75\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{\text{−}1}{\mathrm{s}}^{\text{−}1}$, much higher than the typical mobility of $30\text{–}40\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{\text{−}1}{\mathrm{s}}^{\text{−}1}$ for conventional ITO. The complex dielectric functions of the thin films reveal remarkable carrier density dependent changes in the optical properties. SE analyses show that the electron effective mass of ${\mathrm{In}}_2{\mathrm{O}}_3:\mathrm{TM}$ at the bottom of the conduction band $m_o^{*}$ ($0.11\text{–}0.14m_o$) is much smaller than the reported $m_o^{*}\sim 0.18-0.30m_o$ for ITO, which directly results in their higher mobility. This low $m_o^{*}$ is consistent with recent theoretical studies which proposed that $4d$ donor states of the TMs are resonance in the CB. For films with comparably low resistivity of $1\text{–}2\times {10}^{\text{−}4}\mathrm{\Omega }\mathrm{cm}$, we find that ${\mathrm{In}}_2{\mathrm{O}}_3:\mathrm{TM}$ films have ∼4–10 times lower absorption coefficient at $\lambda =1300\mathrm{nm}$ due to free carrier absorption and have their plasma reflection edge extended to ∼1.7 μm compared to ∼1.2–1.4 μm for ITO. Hence, using TM doping we have achieved transparent conductors with conductivity comparable to ITO but with transmission extended to >1600 nm. These materials will be potentially important as transparent conductors for optoelectronic devices utilizing NIR photons.

Hydrogen isotopes are retained in plasma-facing fusion materials, triggering hydrogen embrittlement and changing tritium inventory as a function of exposure to neutron irradiation. But modeling highly damaged materials—exposed to over 0.1 displacements per atom (dpa)—where saturation of damage is often observed, is difficult because a microstructure containing high density of defects evolves nonlinearly as a function of dose. In this study we show how to determine the defect and hydrogen isotope content in tungsten exposed to high irradiation dose, using no adjustable or fitting parameters. First, we generate converged high dose ($>1$ dpa) microstructures, using a combination of the creation-relaxation algorithm and collision cascade simulations. Then we make robust estimates of vacancy and void regions using a modified Wigner-Seitz decomposition. The resulting estimates of the void surface area enable predicting the deuterium retention capacity of tungsten as a function of radiation exposure. The predictions are compared to ${}^3\mathrm{He}$ nuclear reaction analysis measurements of tungsten samples, self-irradiated at 290 K to different damage doses and exposed to low-energy deuterium plasma at 370 K. The theory gives an excellent match to the experimental data, with both model and experiment showing that 1.5–2.0 at.% deuterium is retained in irradiated tungsten in the limit of high dose.

While first-principles calculations with different levels of sophistication predict a topologically trivial ${\mathbb{Z}}_2$ state for bulk bismuth, some photoemission experiments show surface states consistent with the interpretation of bismuth being in a topologically nontrivial ${\mathbb{Z}}_2$ state. We resolve this contradiction between theory and experiment by showing, based on quasiparticle self-consistent $GW$ calculations, that the experimental surface states interpreted as supporting a nontrivial phase are actually consistent with a trivial ${\mathbb{Z}}_2$ invariant. We identify this contradiction as the result of a crosstalk effect arising from the extreme penetration depth of the surface states into the bulk of Bi. A film of Bi can be considered bulklike only for thicknesses of about 1000 bilayers ($\approx 400$ nm) and more.

Germanium-rich GeSbTe alloys have recently emerged as promising materials for embedded phase change memories of interest for applications in the automotive sector thanks to the high crystallization temperature of their amorphous phase. Crystallization of Ge-rich GeSbTe alloys can lead to phase separation with segregation of crystalline Ge. The inhomogeneity of the resulting system brings, however, some drawbacks such as the drift of the electrical resistance with time of the crystalline phase. To mitigate these effects, a better understanding of the decomposition process is needed. In this work, we performed density functional calculations on possible decomposition pathways of the ${\mathrm{Ge}}_5{\mathrm{Sb}}_2{\mathrm{Te}}_3$ alloy as an example of Ge-rich GeSbTe alloy on the Ge-${\mathrm{Sb}}_2{\mathrm{Te}}_3$ pseudobinary line. The energetics of the transformation into crystalline Ge and a less Ge-rich alloy is investigated as well as the structural, electronic, and vibrational properties of some possible decomposition products such as ${\mathrm{Ge}}_3{\mathrm{Sb}}_2{\mathrm{Te}}_3$ or ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_1$, among several others, in both the crystalline and amorphous phases.

Temperature dependent (4–295 K) photoluminescence and transmission spectra are analyzed to study the effect of changing the different components of a perovskite compound, be it $A, B$, or $X$. Four different films are compared: $\mathrm{F}\mathrm{A}\mathrm{M}\mathrm{A}\mathrm{Pb}\mathrm{Sn}{\mathrm{I}}_3, \mathrm{F}\mathrm{A}\mathrm{Pb}{\mathrm{I}}_3, \mathrm{F}\mathrm{A}\mathrm{M}\mathrm{A}\mathrm{Pb}{\mathrm{I}}_3$, and $\mathrm{F}\mathrm{A}\mathrm{Pb}{\mathrm{Br}}_3$. The low temperature results highlight the changes that occur, especially underlying ones that are easily masked at room temperature. The overall Stokes shift is of similar magnitude at room temperature for the three Pb only based samples. This is governed by the interaction strength ${\mathrm{\Gamma }}_{\mathrm{LO}}$, phonon energy $E_{\mathrm{LO}}$, and exciton binding energy $E_{\mathrm{ex}}$. One exception to this behavior is the Sn based $\mathrm{F}\mathrm{A}\mathrm{M}\mathrm{A}\mathrm{Pb}\mathrm{Sn}{\mathrm{I}}_3$ film, which shows a lack of Stokes shift between the absorption and photoluminescence. However, the strong absorption (more than 100 meV) below the band gap is indicative of an excitonic feature that has a large density of states. Transient absorption measurements confirm the trends observed in continuous wave (CW) measurements; the three Pb only films all show the convolution of an excitonic feature within 20 meV of the band gap as a contributing factor to the photobleach along with a region of high energy photoinduced absorption (PIA). However, the behavior for the Sn based film is notably different (just as it is in the CW measurements) with an unusual low energy PIA and a lack of high energy PIA. The large unusual low energy PIA is attributed to the large sub-band-gap absorption observed in the CW transmission/absorption measurements. Notably, regardless of interchanging components, the slow cooling of carriers in metal-halide perovskites shows little effect of ${\mathrm{\Gamma }}_{\mathrm{LO}}, E_{\mathrm{LO}}$, and $E_{\mathrm{ex}}$. As such, here it is proposed that while the initial cooling of carriers is attributed to LO phonons, the overall cooling of carriers is dominated by the intrinsic low thermal conductivity of all metal-halide perovskites, which limits the dissipation of acoustic phonons in these systems.