Novel epitaxial quasicrystalline films can be grown using the surfaces of intermetallic quasicrystals as templates. Here, we present a study of Pb adsorption on the threefold i-Ag-In-Yb surface, where Pb grows in a manner contrasting with conventional thin-film growth modes. Pb atoms are found to adsorb at sites over a range of heights, which are explained by bulk atomic positions left vacant by surface truncation, producing three-dimensional, isolated quasicrystalline Pb structures. This finding is contrasted with the growth of Pb on the more commonly used fivefold surface of the same quasicrystal, where smooth epitaxial layers result. We suggest that this unique structure originates due to the lower atomic density of the threefold surface, compared to the fivefold surface. Similar atomic density can be found in lower symmetry planes of periodic systems, but these planes are often unstable and become facetted. This stable low-density quasicrystalline substrate provides a facile route to achieve this type of templated growth.

Molybdenum ditelluride, ${\mathrm{MoTe}}_2$, is a versatile material where the topological phase can be readily tuned by manipulating the associated structural phase transition. The fine details of the band structure of ${\mathrm{MoTe}}_2$, key to understanding its topological properties, have proven difficult to disentangle experientially due to the multiband character of the material. Through experimental optical conductivity spectra, we detect two strong low-energy interband transitions. Both are linked to excitations between spin-orbit split bands. The lowest interband transition shows a strong thermal shift, pointing to a chemical potential that dramatically decreases with temperature. With the help of ab initio calculations and a simple two-band model, we give qualitative and quantitative explanations of the main features in the temperature-dependent optical spectra up to 400 meV.

In ferromagnetic solids, a transverse voltage can be generated by a longitudinal temperature gradient even in the absence of a magnetic field. This thermoelectric counterpart of the anomalous Hall effect is dubbed the anomalous Nernst effect (ANE), and both of these effects are expected to scale with spontaneous magnetization. Here, we report the observation of a giant ANE in a newly discovered magnetic Weyl semimetal ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ crystal. The Hall effect and the Nernst effect show sharp jumps at a threshold field and exhibit a clear hysteresis loop below the Curie temperature $T_C$. The anomalous Nernst signal, $S_{yx}^A$, peaks at a maximum value of $\sim 5$ $\mu \mathrm{V}/\mathrm{K}$, which is comparable to the largest seen in previous magnetic materials and seriously violates the conventional scale relation in conventional ferromagnets. Moreover, the anomalous transverse thermoelectric conductivity ${\alpha }_{yx}^A$ reaches about $\sim 10$ A/K m at 70 K, the largest in known semimetals. These results together with first-principles calculation provide strong evidence that the giant ANE originated from the nontrivial Berry curvature near the chiral Weyl points, which are very close to the Fermi level in this compound.

Due to the high density of native defects, the prototypical topological insulator (TI), ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, is naturally $n$-type. Although ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ can be converted into $p$-type by substituting $2+$ ions for Bi, only light elements such as Ca have so far been effective as the compensation dopant. Considering that strong spin-orbit coupling (SOC) is essential for the topological surface states, a light element is undesirable as a dopant because it weakens the strength of SOC. In this sense, Pb, which is the heaviest $2+$ ion, located right next to Bi in the Periodic Table, is the most ideal $p$-type dopant for ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. However, Pb-doping has so far failed to achieve $p$-type ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ not only in thin films but also in bulk crystals. Here, by utilizing an interface engineering scheme, we have achieved the first Pb-doped $p$-type ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ thin films. Furthermore, at heavy Pb-doping, the mobility turns out to be substantially higher than that of Ca-doped samples, suggesting that Pb is a less disruptive dopant than Ca. The availability of Pb-doped $p$-type ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ films will provide opportunities to study a Fermi-level tunable TI system while preserving the SOC strength.

We present a first-principles study of the surface magnetism induced by Cr, Mn, Fe, and Co adatoms on the (111) surfaces of Rh, Pd, Ag, Ir, Pt, and Au. We first describe how the different contributions to the surface magnetism enter the magnetic stray field, with special attention paid to the induced orbital moments. Then we present results for the spin and orbital magnetic moments of the adatoms, and for the induced surface spin and orbital magnetic moments, the latter being further divided into atomic and interatomic contributions. We investigate how the surface magnetism is determined by the chemical nature of the elements involved, such as the filling of the magnetic $d$-orbitals of the adatoms and the properties of the itinerant electrons at the surface (whether they are $sp$- or $d$-like, and whether the spin-orbit interaction is relevant), and how it is modified if the magnetic adatoms are brought together to form a cluster, with Cr, Mn, Fe, and Co trimers on Pt(111) as an example. We also explore the impact of computational approximations, such as the distance between the adatom and the Pt(111) surface, or confinement effects due to the finite thickness of the slab used to model it. Our discussion of the magnetic stray field generated by a single adatom and its environment suggests a possible way of disentangling the induced surface magnetism from the adatom one, which could be feasible with scanning nitrogen-vacancy-center microscopy.

Chromium selenide thin films were grown epitaxially on $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\left(0001\right)$ and Si(111)-(7 × 7) substrates using molecular beam epitaxy. Sharp streaks in reflection high-energy electron diffraction and triangular structures in scanning tunneling microscopy indicate flat smooth film growth along the $c$ axis, which is very similar to that from a hexagonal surface. The x-ray diffraction pattern confirms the growth along the $c$ axis with a $c$-axis lattice constant of 17.39 Å. The grown film is semiconducting, having a small band gap of about 0.034 eV, as calculated from the temperature-dependent resistivity. The antiferromagnetic nature of the film with a Néel temperature of about 40 K is estimated from the magnetic exchange bias measurements. A larger out-of-plane exchange bias, along with a smaller in-plane exchange bias is observed below 40 K. Exchange bias training effects are analyzed based on different models and are observed to be following a modified power-law decay behavior.

Molybdenum contamination of silicon can have serious detrimental consequences for the efficiency of solar cells, raising durability concerns for novel solar cell designs that utilize $\mathrm{Mo}{\mathrm{O}}_3$ in contact with Si. Density functional theory simulations of Mo defects in Si revealed that Mo is preferentially accommodated in tetrahedrally coordinated interstitial sites and that the contamination may reach a sufficiently high concentration to cause a 20% relative solar cell efficiency degradation if processing steps are performed between 950 and 1300 K. The formation energy of the most energetically favored Mo defect in Si has a minimum value of 1.58 eV at the valence band maximum and a maximum of 2.10 eV at higher Fermi levels, indicating that higher Mo defect concentrations may occur in $p$-type Si than intrinsic or $n$-type Si. The diffusion processes for Mo in Si were investigated, and it was identified that interstitial diffusion dominates over a vacancy-mediated mechanism under all equilibrium conditions. Migration barriers were calculated to be 2.29 eV for charge neutral and 2.03 eV for charge +1 defects, occurring under $n$-type and $p$-type doping, respectively, indicating that Mo diffusion is faster in $p$-type Si, and hence potentially more effectively gettered than it would be in $n$-type Si.

We report charge transfer up to a single electron per interfacial unit cell across nonpolar heterointerfaces from the Mott insulator $\mathrm{LaTi}{\mathrm{O}}_3$ to the charge transfer insulator $\mathrm{LaCo}{\mathrm{O}}_3$. In high-quality bi- and trilayer systems grown using pulsed laser deposition, soft x-ray absorption, dichroism, and scanning transmission electron microscopy-electron energy loss spectroscopy are used to probe the cobalt-$3d$ electron count and provide an element-specific investigation of the magnetic properties. The experiments show the cobalt valence conversion is active within 3 unit cells of the heterointerface, and able to generate full conversion to $3d^7$ divalent Co, which displays a paramagnetic ground state. The number of $\mathrm{LaTi}{\mathrm{O}}_3/\mathrm{LaCo}{\mathrm{O}}_3$ interfaces, the thickness of an additional, electronically insulating “break” layer between the $\mathrm{LaTi}{\mathrm{O}}_3$ and $\mathrm{LaCo}{\mathrm{O}}_3$, and the $\mathrm{LaCo}{\mathrm{O}}_3$ film thickness itself in trilayers provide a trio of control knobs for average charge of the cobalt ions in $\mathrm{LaCo}{\mathrm{O}}_3$, illustrating the efficacy of $O-2p$ band alignment as a guiding principle for property design in complex oxide heterointerfaces.

Spin-wave resonance measurements were performed in the mixed magnetic phase regime of a Pd-doped FeRh epilayer that appears as the first-order ferromagnetic-antiferromagnetic phase transition takes place. It is seen that the measured value of the exchange stiffness is suppressed throughout the measurement range when compared to the expected value of the fully ferromagnetic regime, extracted via the independent means of a measurement of the Curie point, for only slight changes in the ferromagnetic volume fraction. This behavior is attributed to the influence of the antiferromagnetic phase: inspired by previous experiments that show ferromagnetism to be most persistent at the surfaces and interfaces of FeRh thin films, we modelled the antiferromagnetic phase as forming a thin layer in the middle of the epilayer through which the two ferromagnetic layers are coupled up to a certain critical thickness. The development of this exchange stiffness is then consistent with that expected from the development of an exchange coupling across the magnetic phase boundary, as a consequence of a thickness dependent phase transition taking place in the antiferromagnetic regions and is supported by complimentary computer simulations of atomistic spin-dynamics. The development of the Gilbert damping parameter extracted from the ferromagnetic resonance investigations is consistent with this picture.

Gaining fundamental insights into phonon interactions is of high importance for the improvement of thermoelectric materials. The particular challenge is to enable a phonon glass state with low thermal conductivity in crystalline materials with high electron conductivity. We present here the relation between atomic structure and $c$-axis lattice thermal conductivity ${\kappa }_{c,l}\left(T\right)$ of epitaxially grown nearly single crystalline ${\left(\mathrm{S}{\mathrm{b}}_{1-x}\mathrm{B}{\mathrm{i}}_x\right)}_2\mathrm{T}{\mathrm{e}}_3$ thin films. Aberration corrected high-resolution transmission electron microscopy shows a highly ordered crystalline lattice, with doping dependent statistical occupation of the Sb sublattice by Bi atoms and a very low density of planar defects. The observed strong decrease of ${\kappa }_{c,l}\left(T\right)$ with doping is due to an increase both of the Rayleigh scattering rate at point defects as well as of the phonon-phonon scattering rate. For $x=0.24$ a transition to a low, almost temperature independent ${\kappa }_{c,l}\left(T\right)$ is observed, indicating a transition to a phonon glass state. The theoretical calculations reveal that the phonon mean free path is reduced below the phonon wavelength for the majority of phonon frequencies, suggesting a breakdown of the phonon approximation to heat transport due to the strongly anharmonic lattice.

In this article, we report a comparative study of the electronic structure of Cr-doped and pristine ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Circular dichroism and photon-energy-dependent angle-resolved photoemission experiments were performed. Even though the surface states seen on the Cr-doped samples are gapped, they exhibit strong circular dichroism, for which we provide its origin in accordance with the nontrivial band structure of the bulk. The surface electronic structure measurements with linear and circular polarized light show signatures that the orbital composition of the surface states is changed with Cr doping. Our observations not only provide further spectroscopic information about topological materials, but also promotes an alternative experimental tool to control their spin-orbital texture.

Nucleant substrates may induce formation of several quasilayers in the liquid metals in contact with the solid substrates. These liquid layers, called prenucleation layers (PNLs), normally form above the liquidus temperatures of the metal alloys and thus are believed to significantly affect the heterogeneous nucleation mechanism. In this work, a comprehensive study of the (0001) sapphire-liquid Al interfaces was conducted using the ab initio molecular dynamics method to unveil the key factors determining the substrate-liquid metal interfacial structural features, focusing primarily on revealing the underlying mechanisms of the PNL formation. Two types of (0001) sapphire surfaces, i.e., nonhydroxylated and hydroxylated surfaces, were investigated. Essentially the same equilibrated interfacial structure was observed for all the interfaces with the nonhydroxylated sapphire surfaces, featuring an Al-rich termination of the substrates and two distinguishable adjacent PNLs. These typical structural features are similar to the $\left(\sqrt{\text{31}}\times \sqrt{\text{31}}\right)R\pm \text{9}{}^{\circ }$ reconstructed surface, which supports the previous experimental observation that the presence of liquid Al promotes the reconstruction of the (0001) sapphire surface. A completely different structure was found at the interface with fully hydroxylated sapphire surface: the PNLs are absent, replaced by an Al-depleted region. To unveil the factors influencing the interfacial structure, we first analyzed the nature of interfacial bonding. Strong covalent bonds were observed between the nonhydroxylated sapphire surfaces and liquid Al, which constitute a relatively large proportion of the interfacial bonds at such systems. On the contrary, weak van der Waals force is the primary interaction between the fully hydroxylated sapphire surface and liquid Al. Then we characterized the dynamical behaviors of the substrate surfaces in the substrate-liquid Al interfaces. Better mobility and larger vibration amplitude were found for the fully hydroxylated sapphire surface compared to the equilibrated nonhydroxylated sapphire surface. We demonstrated that both the stronger bonding strength and the lower mobility of interfacial atoms contribute to the formation of the PNLs at the nonhydroxylated sapphire-liquid Al interfaces. Finally, the formation mechanism of the PNLs at the substrate-liquid metal interface is summarized.

We present a generalization of a model that takes into account the magnetic disorder of paramagnetic host with interstitial point defects towards the case of alloy hosts. In the framework of disordered local moment approach combined with magnetic sampling method, we calculate solution enthalpy of carbon impurity in the paramagnetic fcc Fe-Mn steels. First, we use the magnetic special quasirandom structure (M-SQS) method for simulation of the paramagnetic state in Fe-Mn alloys without impurity. Here, Fe and Mn atoms are randomly distributed at the sites of a supercell following the chemical SQS method. Next, to calculate the energies for various magnetic realizations around the interstitial carbon impurity, we vary the position of the impurity within the SQS. Our calculations show that in alloys containing $\sim 20$ at. % Mn, the solution enthalpy of carbon reduces compared to the pure fcc-Fe. By analyzing the local and global effects of impurity on the properties of the matrix, we discuss various factors that could increase the carbon solubility in high-manganese austenitic steels.

An exciting development over the past few decades has been the use of high-throughput computational screening as a means of identifying promising candidate materials for a variety of structural or functional properties. Experimentally, it is often found that the highest-performing materials contain substantial atomic site disorder. These are frequently overlooked in high-throughput computational searches, however, due to difficulties in dealing with materials that do not possess simple, well-defined crystallographic unit cells. Here we demonstrate that the screening of magnetocaloric materials with the help of the density-functional-theory-based magnetic deformation proxy can be extended to systems with atomic site disorder. This is accomplished by thermodynamic averaging of the magnetic deformation for ordered supercells across a solid solution. We show that the highly nonmonotonic magnetocaloric properties of the disordered solid solutions $\mathrm{Mn}({\mathrm{Co}}_{1-x}{\mathrm{Fe}}_x)\text{Ge}$ and $\left({\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x\right)\text{CoGe}$ are successfully captured using this method.

Materials showing reversible magnetocaloric effect near room temperature are desirable for green refrigeration technology. The compounds in the magnetic Heusler family displaying significant magnetocaloric effect driven by first-order magnetostructural transitions are in vogue. Comparatively, Heusler compounds undergoing second-order magnetic transition near room temperature are less explored in the context of discovering significant magnetocaloric effects, though they can offer certain advantages like less energy cost and hysteresis loss. Using density-functional theory in conjunction with model Hamiltonians, we explore potential room-temperature magnetocaloric materials near second-order magnetic transition by cosubstitution in the Ni-Mn-Sb family. Our investigation on two cosubstituted families, ${\mathrm{Ni}}_{2-x}{\mathrm{Fe}}_x{\mathrm{Mn}}_{1+z-y}{\mathrm{Cu}}_y{\mathrm{Sb}}_{1-z}$ and ${\mathrm{Ni}}_{2-x}{\mathrm{Co}}_x{\mathrm{Mn}}_{1+z-y}{\mathrm{Cu}}_y{\mathrm{Sb}}_{1-z}$, discovers significant numbers of compounds with second-order phase transition near room temperature exhibiting significant changes in magnetic entropy, comparable to that observed in the compounds showing inverse magnetocaloric effect near a first-order magnetostructural transition. We provide a systematic way to select potential reversible magnetocaloric compounds and analyze our results from their electronic structures and magnetic exchange interactions.

We present a detailed study of the growth of the tetragonal polymorph of antiferromagnetic CuMnAs by the molecular beam epitaxy technique. We explore the parameter space of growth conditions and their effect on the microstructural and transport properties of the material. We identify its typical structural defects and compare the properties of epitaxial CuMnAs layers grown on GaP, GaAs, and Si substrates. Finally, we investigate the correlation between the crystalline quality of CuMnAs and its performance in terms of electrically induced resistance switching.

Artificial neural-network (ANN) interatomic potentials for simulating atomic structures and energetics of grain boundaries (GBs) in silicon were constructed and integrated into structural optimization and molecular dynamics (MD) algorithms. A training dataset including various atomic environments of symmetric tilt GBs was generated by performing density-functional-theory (DFT) calculations. The ANN potential after training was found to be capable of approximating the potential-energy surface at GBs even with dangling bonds and large atomic displacements at high temperatures, which cannot be well reproduced with empirical interatomic potentials. Additionally, reliability of the ANN potential for molecular simulations was evaluated. GB structures optimized or equilibrated by the ANN molecular simulations were also energetically lower for DFT calculations, without significant errors. The ANN potential is therefore expected to greatly reduce structural-optimization iterations and required time steps to acquire stable or equilibrium GB structures in MD simulations, enabling us to address even large-scale systems of general GBs in silicon, with high accuracy and low computational cost.

We predict the existence of large barocaloric effects above room temperature in the thermoelectric fast-ion conductor ${\mathrm{Cu}}_2\mathrm{Se}$ by using classical molecular dynamics simulations and first-principles computational methods. A hydrostatic pressure of 1 GPa induces large isothermal entropy changes of $\left|\mathrm{\Delta }S\right|\sim 15–45\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ and adiabatic temperature shifts of $\left|\mathrm{\Delta }T\right|\sim 10\mathrm{K}$ in the temperature interval $400\le T\le 700\mathrm{K}$. Structural phase transitions are absent in the analyzed thermodynamic range. The causes of such large barocaloric effects are significant $P$-induced variations on the ionic conductivity of ${\mathrm{Cu}}_2\mathrm{Se}$ and the inherently high anharmonicity of the material. Uniaxial stresses of the same magnitude, either compressive or tensile, produce comparatively much smaller caloric effects, namely, $\left|\mathrm{\Delta }S\right|\sim 1\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ and $\left|\mathrm{\Delta }T\right|\sim 0.1\mathrm{K}$, due to practically null influence on the ionic diffusivity of the material. Our simulation work shows that thermoelectric compounds presenting high ionic disorder, like copper and silver-based chalcogenides, may render large mechanocaloric effects and thus are promising materials for engineering solid-state cooling applications that do not require the application of electric fields.