Exploring magnetic anisotropy (MA) in single-atom-doped two-dimensional materials provides a viable ground for realizing information storage and processing at ultimate length scales. Herein, the MA of $5d$ transition-metal doped monolayer ${\mathrm{WSe}}_2$ is investigated by first-principles calculations. Large MA energy (MAE) is achieved in several doping systems. The direction of MA is determined by the dopant in-plane $d$ states in the vicinity of the Fermi level in line with previous studies. An occupation rule that the parity of the occupation number of the in-plane $d$ orbital of the dopant determines the preference between in-plane and out-of-plane anisotropy is found in this $5d$-doped system. Furthermore, this rule is understood by second-order perturbation theory and proved by charge-doping analysis. Considering relatively little research on two-dimensional MA and not sufficiently large MAE, suitable contact medium dopant pairs with large MAE and tunable MA pave the way to novel data storage paradigms.

The ultrafast relaxation dynamics of a nonequilibrium phonon gas towards thermal equilibrium involves many-body collisions that cannot be properly described by perturbative approaches. Here, we develop a nonperturbative method to elucidate the microscopic mechanisms underlying the decay of laser-excited coherent phonons in the presence of electron-hole pairs, which so far are not fully understood. Our theory relies on ab initio molecular dynamics simulations on laser-excited potential-energy surfaces. Those simulations are compared with runs in which the laser-excited coherent phonon is artificially deoccupied. We apply this method to antimony and show that the decay of the $A_{1g}$ phonon mode at low laser fluences can be accounted mainly to three-body down-conversion processes of an $A_{1g}$ phonon into acoustic phonons. For higher excitation strengths, however, we see a crossover to a four-phonon process, in which two $A_{1g}$ phonons decay into two optical phonons.

X-ray diffraction studies of nanoporous gold face the poorly understood diffraction scenario where large coherent crystals are riddled with nanoscale holes. Theoretical considerations derived in this study show that the ligament size of the porous network influences the scattering, despite being quasi single crystalline. Virtual diffraction of numerically generated samples confirms the results but also shows a loss of long-range coherency and the appearance of microstrain due to thermal relaxation. Subsequently, a large set of laboratory x-ray investigations of nanoporous gold fabricated by different approaches and synthesis parameters reveal a clear correlation between ligament size and size of the coherent scattering domains, as well as extremely high microstrains in samples with ligament sizes below 10 nm.

The combination of ferromagnets with topological superconductors or insulators allows for new phases of matter that support excitations such as chiral edge modes and Majorana fermions. EuS, a wide-bandgap ferromagnetic insulator with a Curie temperature around 16 K, and ${\mathrm{SrTiO}}_3$ (STO), an important substrate for engineering heterostructures, may support these phases. We present scanning superconducting quantum interference device measurements of EuS grown epitaxially on STO that reveal micron-scale variations in ferromagnetism and paramagnetism. These variations are oriented along the STO crystal axes and only change their configuration upon thermal cycling above the STO cubic-to-tetragonal structural transition temperature at 105 K, indicating that the observed magnetic features are due to coupling between EuS and the STO tetragonal structure. We speculate that the STO tetragonal distortions may strain the EuS, altering the magnetic anisotropy on a micron scale. This result demonstrates that local variation in the induced magnetic order from EuS grown on STO needs to be considered when engineering new phases of matter that require spatially homogeneous exchange.

Materials with a large magnetocaloric response are associated with a temperature change upon the application of a magnetic field and are of interest for applications in magnetic refrigeration and thermomagnetic power generation. The usual metric of this response is the gravimetric isothermal entropy change $\mathrm{\Delta }{S}_M$. The use of a simple proxy for the $\mathrm{\Delta }{S}_M$ that is based on density functional theory (DFT) calculations of the magnetic electronic structure suggests that half-Heusler MnNiSb should be a better magnetocaloric than the corresponding Heusler compound ${\mathrm{MnNi}}_2\mathrm{Sb}$. Guided by this observation, we present a study of ${\mathrm{MnNi}}_{1+x}\mathrm{Sb}$ ($x=0$, 0.25, 0.5, 0.75, and 1.0) to evaluate relevant structural and magnetic properties. DFT stability calculations suggest that the addition of Ni takes place at a symmetrically distinct Ni site in the half-Heusler structure and support the observation using synchrotron x-ray diffraction of a homogeneous solid solution between the half-Heusler and Heusler end members. There is a maximum in the saturation magnetization at $x=0.5$ and the Curie temperature systematically decreases with increasing $x$. $\mathrm{\Delta }{S}_M$ for a maximum magnetic field change of $\mathrm{\Delta }H=5\mathrm{T}$ monotonically decreases in magnitude from $-2.93\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ in the half-Heusler to $-1.35\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ in the Heusler compound. The concurrent broadening of the magnetic transition results in a maximum in the refrigerant capacity at $x=0.75$. The Curie temperature of this system is highly tunable between 350 K and 750 K, making it ideal for low grade waste heat recovery via thermomagnetic power generation. The increase in $\mathrm{\Delta }{S}_M$ with decreasing $x$ may be extendable to other ${\mathrm{MnNi}}_{2}Z$ Heusler systems that are currently under investigation for use in magnetocaloric refrigeration applications.

In situ surface-sensitive x-ray diffraction and grazing incidence x-ray fluorescence spectroscopy (GIXFS) methods are combined to determine the ionic distributions across the liquid/vapor interfaces of thiolated-polyethylene-glycol–capped gold nanoparticle (PEG-AuNP) solutions. Induced by the addition of salts (i.e., ${\text{Cs}}_2{\mathrm{SO}}_4$) to PEG-AuNPs solutions, two-dimensional hexagonal lattices of PEG-AuNPs form spontaneously at the aqueous surfaces, as is demonstrated by x-ray reflectivity and grazing incidence small-angle x-ray scattering. By taking advantage of element specificity with the GIXFS method, we find that the cation ${\mathrm{Cs}}^{+}$ concentration at the crystalline film is significantly reduced in parts of the PEG-AuNP film compared with that in the bulk.

Segregated impurities at grain boundaries can dramatically change the mechanical behavior of metals, while the mechanism is still obscure in some cases. Here, we suggest a unified approach to investigate segregation and its effects on the mechanical properties of polycrystalline alloys using the example of $3sp$ impurities (Mg, Al, Si, P, or S) at a special type $\mathrm{\Sigma }5\left(310\right)\left[001\right]$ tilt grain boundary in Cu. We show that for these impurities segregating to the grain boundary, the strain contribution to the work of grain boundary decohesion is small and that the chemical contribution correlates with the electronegativity difference between Cu and the impurity. The strain contribution to the work of dislocation emission is calculated to be negative, while the chemical contribution is calculated to be always positive. Both the strain and chemical contributions to the work of dislocation emission generally become weaker with the increasing electronegativity from Mg to S. By combining these contributions together, we find, in agreement with experimental observations, that a strong segregation of S can reduce the work of grain boundary separation below the work of dislocation emission, thus embrittling Cu, while such an embrittlement cannot be produced by a P segregation because it lowers the energy barrier for dislocation emission relatively more than for work separation.

We theoretically investigate the low-temperature phase of the recently synthesized ${\mathrm{Lu}}_2{\mathrm{Mo}}_2{\mathrm{O}}_5{\mathrm{N}}_2$ material, an extraordinarily rare realization of a $S=1/2$ three-dimensional pyrochlore Heisenberg antiferromagnet in which ${\mathrm{Mo}}^{5+}$ are the $S=1/2$ magnetic species. Despite a Curie-Weiss temperature (${\mathrm{\Theta }}_{\mathrm{CW}}$) of $-121\left(1\right)$ K, experiments have found no signature of magnetic ordering $\mathit{or}$ spin freezing down to $T^{*}\approx 0.5$ K. Using density functional theory, we find that the compound is well described by a Heisenberg model with exchange parameters up to third nearest neighbors. The analysis of this model via the pseudofermion functional renormalization group method reveals paramagnetic behavior down to a temperature of at least $T=|{\mathrm{\Theta }}_{\mathrm{CW}}|/100$, in agreement with the experimental findings hinting at a possible three-dimensional quantum spin liquid. The spin susceptibility profile in reciprocal space shows momentum-dependent features forming a “gearwheel” pattern, characterizing what may be viewed as a molten version of a chiral noncoplanar incommensurate spiral order under the action of quantum fluctuations. Our calculated reciprocal space susceptibility maps provide benchmarks for future neutron scattering experiments on single crystals of ${\mathrm{Lu}}_2{\mathrm{Mo}}_2{\mathrm{O}}_5{\mathrm{N}}_2$.

Realistic oxide materials are often semiconductors, in particular at elevated temperatures, and their surfaces contain undercoordinated atoms at structural defects such as steps and corners. Using hybrid density-functional theory and ab initio atomistic thermodynamics, we investigate the interplay of bond-making, bond-breaking, and charge-carrier trapping at the corner defects at the (100) surface of a $p$-doped MgO in thermodynamic equilibrium with an ${\mathrm{O}}_2$ atmosphere. We show that by manipulating the coordination of surface atoms, one can drastically change and even reverse the order of stability of reduced versus oxidized surface sites.

Owing to unique electronic, excitonic, and valleytronic properties, atomically thin transition metal dichalcogenides are becoming a promising two-dimensional (2D) semiconductor system for diverse electronic and optoelectronic applications. In an ideal 2D semiconductor, efficient carrier transport is very difficult because of lacking free charge carriers. Doping is necessary for electrically driven device applications based on such 2D semiconductors, which requires investigation of electronic structure changes induced by dopants. Therefore probing correlations between localized electronic states and doping is important. Here, we address the electronic nature of broad bound exciton bands and their origins in exfoliated monolayer (1L) $\mathrm{W}{\mathrm{S}}_2$ and $\mathrm{Mo}{\mathrm{S}}_2$ through monitoring low-temperature photoluminescence and manipulating electrostatic doping. The dominant bound excitons in 1L $\mathrm{W}{\mathrm{S}}_2$ vary from donor to acceptor bound excitons with the switching from $n$- to $p$-type doping. In 1L $\mathrm{Mo}{\mathrm{S}}_2$, two localized emission bands appear which are assigned to neutral and ionized donor bound excitons, respectively. The deep donor and acceptor states play critical roles in the observed bound exciton bands, indicating the presence of strongly localized excitons in such 2D semiconductors.

We report a crystal growth method to obtain homochiral single crystals of ${\mathrm{CsCuCl}}_3$ and polarized neutron diffraction studies to examine the chiral helimagnetism of this compound. The homochiral crystals were grown by two-step crystallization. First, millimeter-sized seed crystals were synthesized by spontaneous crystallization with stirring. The handedness of the seed crystals was determined by x-ray diffraction. Then, centimeter-sized homochiral crystals were obtained from the selected homochiral seed crystals. The large homochiral crystals made it possible to perform polarized neutron diffraction. We clarified a strong correlation between the crystal and magnetic chiralities, which governs the nature of antisymmetric Dzyaloshinskii-Moriya interactions.

The development of novel quantum bits is key to extending the scope of solid-state quantum-information science and technology. Using first-principles calculations, we propose that large metal ion–vacancy pairs are promising qubit candidates in two binary crystals: $4H$-SiC and $w$-AlN. In particular, we found that the formation of neutral Hf- and Zr-vacancy pairs is energetically favorable in both solids; these defects have spin-triplet ground states, with electronic structures similar to those of the diamond nitrogen-vacancy center and the SiC divacancy. Interestingly, they exhibit different spin-strain coupling characteristics, and the nature of heavy metal ions may allow for easy defect implantation in desired lattice locations and ensure stability against defect diffusion. To support future experimental identification of the proposed defects, we report predictions of their optical zero-phonon line, zero-field splitting, and hyperfine parameters. The defect design concept identified here may be generalized to other binary semiconductors to facilitate the exploration of new solid-state qubits.

In search of new prospects for thermoelectric materials, using ab initio calculations and semiclassical Boltzmann theory, we have systematically investigated the electronic structure and transport properties of 18-valence electron count cobalt-based half-Heusler alloys with prime focus on CoVSn, CoNbSn, CoTaSn, CoMoIn, and CoWIn. The effect of doping on transport properties has been studied under the rigid band approximation. The maximum power factor, $S^2\sigma$, for all systems is obtained on hole doping and is comparable to the existing thermoelectric material CoTiSb. The stability of all the systems is verified by phonon calculations. Based on our calculations, we suggest that CoVSn, CoNbSn, CoTaSn, CoMoIn, and CoWIn could be potential candidates for high-temperature thermoelectric materials.

Ultralong linear carbon chains of more than 6000 carbon atoms have recently been synthesized within double-walled carbon nanotubes (DWCNTs), and they show a promising route to one-atom-wide semiconductors with a direct band gap. Theoretical studies predicted that this band gap can be tuned by the length of the chains, the end groups, and their interactions with the environment. However, different density functionals lead to very different values of the band gap of infinitely long carbyne. In this work, we applied resonant Raman excitation spectroscopy with more than 50 laser wavelengths to determine the band gap of long carbon chains encapsulated inside DWCNTs. The experimentally determined band gaps ranging from 2.253 to 1.848 eV follow a linear relation with Raman frequency. This lower bound is the smallest band gap of linear carbon chains observed so far. The comparison with experimental data obtained for short chains in gas phase or in solution demonstrates the effect of the DWCNT encapsulation, leading to an essential downshift of the band gap. This is explained by the interaction between the carbon chain and the host tube, which greatly modifies the chain's bond-length alternation.

Large angular variations in film composition have been found for ablation of a metallic AuCu alloy (Au/Cu ratio $\sim 1)$ in vacuum and background gases of Ne and Xe. The AuCu films grown in vacuum at a laser fluence of $5\mathrm{Jc}{\mathrm{m}}^{-2}$ exhibit a large loss in the Cu content, with the Au/Cu ratio $\sim 2.4$ at angles close to normal incidence. At this fluence, a distortion of the plume front is observed followed by the appearance of a secondary emission at the substrate, suggesting that resputtering of the film by energetic ions and reflection of ions/atoms at the substrate can lead to a nonstoichiometric transfer in pulsed laser deposition. Further, we have found that depending on the mass of the background gas employed during growth (Ne or Xe), the ratio of elements in the film can vary significantly over a wide range of angles of deposition. In the presence of the light gas Ne, the degree of nonstoichiometric transfer is gradually reduced with increasing background pressure, resulting in a nearly stoichiometric AuCu films at a Ne pressure of 2 mbar. The behavior in the heavy gas Xe is more complex, and both theoretical and experimental data indicate that the loss of Cu in the deposits is caused by the preferential scattering, as well as by backscattering of the light Cu atoms in the plume upon collisions with the background gas.