We have investigated the anomalous Nernst effect (ANE) in $\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{G}{\mathrm{a}}_x$ alloy films with different Ga atomic compositions ($x=0-0.44$) deposited on MgO(001) single-crystalline substrates. We found that the magnitude of the ANE increases with increasing $x$ up to $x=0.32$ even though the saturation magnetization decreases with increasing $x$, suggesting a dominant contribution of the intrinsic mechanism on the observed ANE. The magnitude of the ANE reaches $2.4\mu \mathrm{V}/\mathrm{K}$ in bcc ${\mathrm{Fe}}_{0.68}{\mathrm{Ga}}_{0.32}$ film, which is two orders of magnitude greater than that of pure Fe film. The magnetotransport measurements and the first-principles calculations revealed that the large ANE in bcc $\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{G}{\mathrm{a}}_x$ is caused by the large transverse Peltier coefficient. The drastic enhancement of the transverse Peltier coefficient with increasing $x$ can be attributed to small Fermi level tuning through the electron doping effect. Thus, we anticipate that our finding will provide a crucial piece of information to enhance the thermopower through the ANE.

The $\mathrm{N}{\mathrm{i}}_{1-x}\mathrm{C}{\mathrm{u}}_x{\mathrm{S}}_2$ pyrite ($x\le 0.07$) is a model material where the impact of electron filling on a half-filled $e_{\mathrm{g}}$ band system can be separated from changes in the bandwidth, since the unit cell volume does not change with $x$. This contrasts with the $\mathrm{Ni}{\mathrm{S}}_{2-x}\mathrm{S}{\mathrm{e}}_x$ system where the size difference between ${\mathrm{S}}^{2-}$ and $\mathrm{S}{\mathrm{e}}^{2-}$ makes the bandwidth vary at constant band filling. Our magnetic measurements show that there exists a clear relation between the presence of the antiferromagnetic phase developing below $T_{\mathrm{N}2}=29.75\mathrm{K}$ and charge localization. With the addition of Cu in $\mathrm{N}{\mathrm{i}}_{1-x}\mathrm{C}{\mathrm{u}}_x{\mathrm{S}}_2$, the ground state evolves towards a metallic paramagnetic state. These findings are discussed with the scenario based on charge ordering at low temperature in the charge transfer insulator $\mathrm{Ni}{\mathrm{S}}_2$ pyrite, and also considering possible conducting surface states at low temperatures. At 300 K, the thermal conductivity decreases by 2.5 as $x$ increases from 0.00 to 0.07, even though the latter is much more electrically conductive than the former. This considerably extends the range of values for the thermal conductivity of the $\mathrm{M}{\mathrm{S}}_2$ pyrites, from the largest in $\mathrm{Fe}{\mathrm{S}}_2$ to the lowest in the present $\mathrm{N}{\mathrm{i}}_{1-x}\mathrm{C}{\mathrm{u}}_x{\mathrm{S}}_2$ samples.

The properties of the high-temperature superconductor $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{7-x}$ (YBCO) depend on the concentration of oxygen vacancies ($V_{\mathrm{O}}$). It is generally agreed upon that $V_{\mathrm{O}}$ form in the CuO chains, even at low concentrations where the critical temperature for superconductivity peaks ($x\approx 0.07$), with only a handful of reports suggesting the presence of $V_{\mathrm{O}}$ at the apical sites. In this paper, we show direct evidence of apical $V_{\mathrm{O}}$ in optimally doped YBCO samples. Using density-functional-theory calculations, we predict that isolated $V_{\mathrm{O}}$ are equally favorable to form in either the CuO chains or the apical sites, which we confirm using atomic-resolution scanning transmission electron microscope imaging and spectroscopy. We further show that apical $V_{\mathrm{O}}$ lead to significant lattice distortions and changes in the electronic structure of YBCO, indicating they should be considered on an equal footing with chain $V_{\mathrm{O}}$ to understand the superconducting properties of YBCO in the optimal doping region.

A series of oxytetrahalides $\mathrm{WO}{X}_4$ ($X$: a halogen element) that form quasi-one-dimensional chains is investigated using first-principles calculations. The crystal structures, electronic structures, as well as ferroelectric and piezoelectric properties are discussed in detail. Group theory analysis shows that the ferroelectricity in this family originates from an unstable polar phonon mode ${\mathrm{\Gamma }}_1^{-}$ induced by the W's $d^0$ orbital configuration. Their polarization magnitudes are found to be comparable to widely used ferroelectric perovskites. Because of its quasi-one-dimensional characteristics, the interchain domain wall energy density is low, leading to loosely coupled ferroelectric chains. This is potentially beneficial for high-density ferroelectric memories: We estimate that the upper limit of memory density in these compounds could reach hundreds of terabytes per square inch.

The bismuthate superconductor $\mathrm{BaP}{\mathrm{b}}_{1-x}\mathrm{B}{\mathrm{i}}_x{\mathrm{O}}_3$ and its bismuthate heterostructure have gained much attention due to their potential applications, such as topologically protected quantum devices. To fabricate $\mathrm{BaP}{\mathrm{b}}_{1-x}\mathrm{B}{\mathrm{i}}_x{\mathrm{O}}_3$-based junctions or devices with atomically sharp interfaces, researchers have long searched for innovative methods to achieve coherent-strained superconducting $\mathrm{BaP}{\mathrm{b}}_{1-x}\mathrm{B}{\mathrm{i}}_x{\mathrm{O}}_3$ films. However, the large lattice mismatches between $\mathrm{BaP}{\mathrm{b}}_{1-x}\mathrm{B}{\mathrm{i}}_x{\mathrm{O}}_3$ and widely used perovskite substrates have been huge obstacles hindering the achievement of coherent-strained superconducting films. Here, we successfully fabricated coherent-strained superconducting $\mathrm{BaP}{\mathrm{b}}_{1-x}\mathrm{B}{\mathrm{i}}_x{\mathrm{O}}_3$ films on $\mathrm{SrTi}{\mathrm{O}}_3$ substrates by inserting $\mathrm{BaCe}{\mathrm{O}}_3/\mathrm{BaZr}{\mathrm{O}}_3$ buffer layers. By performing both grazing-incidence in-plane x-ray diffraction and transmission electron microscopy, we demonstrated that without buffer layers the $\mathrm{BaP}{\mathrm{b}}_{1-x}\mathrm{B}{\mathrm{i}}_x{\mathrm{O}}_3$ films exhibited fully relaxed structures with a reconstructed interface layer. With buffer layers, the superconducting transition temperatures of coherent-strained $\mathrm{BaP}{\mathrm{b}}_{1-x}\mathrm{B}{\mathrm{i}}_x{\mathrm{O}}_3$ films were higher than that of relaxed films. Based on these interface-engineering results, this paper provides opportunities for exploring the emergent properties of bismuthate-based superconducting devices, such as quantum computing circuits.

Layered materials, such as transition metal dichalcogenides, can be combined at will in van der Waals heterostructures and lead to a variety of new phenomena. To better understand the coupling between layers and the variation of electronic and optical properties, noninvasive techniques with the best possible spatial resolution are needed. Here we show that due to an enhanced interaction cross section with electrons in a type-I van der Waals heterostructure, made of single-layer molybdenum disulfide and thin boron nitride, cathodoluminescence is strongly enhanced. It can be mapped with a spatial resolution far exceeding what can be achieved in more commonly used photoluminescence experiments, thereby providing invaluable insights into the optoelectronic properties at the nanoscale. We demonstrate that the technique is noninvasive, i.e., it does not induce any defect, only if the interface between boron nitride and the molybdenum disulfide layer is pristine. In the presence of trapped species, structural defects are locally induced by the electron beam in the layer. Such defects quench the luminescence and present clear Raman signatures. We show that optimizing the heterostructure preparation techniques can lead to extended areas with clean interfaces that lead to a more homogeneous cathodoluminescence signal.

Single crystals of a honeycomb lattice antiferromagnet, ${\mathrm{Tb}}_2{\mathrm{Ir}}_3{\mathrm{Ga}}_9$, were synthesized, and the physical properties have been studied. From magnetometry, a long-range antiferromagnetic ordering at $\approx 12.5$ K with highly anisotropic magnetic behavior was found. Neutron powder diffraction confirms that the Tb spins lie along the $\mathbf{a}$-axis, parallel to the shortest Tb-Tb contact. Two field-induced spin-flip transitions are observed when the field is applied parallel to this axis, separated by a plateau corresponding roughly to $M\approx M_{\mathrm{s}}/2$. Transport measurements show the resistivity to be metallic with a discontinuity at the onset of Néel order. Heat capacity shows a $\lambda$-like transition confirming the bulk nature of the magnetism. We propose a phenomenological spin Hamiltonian that describes the magnetization plateau as a result of strong Ising character arising from a quasidoublet ground state of the ${\mathrm{Tb}}^{3+}$ ion in a site of $C_s$ symmetry and expressing a significant bond-dependent anisotropy.

We present a many-body perturbation theory study of the excitonic properties of wurtzite GaN containing a single charged nitrogen vacancy. We determine that the lowest-energy exciton consists of a bulk to defect transition, resulting in a slight redshift ($<0.1$ eV) of the optical absorption onset and a 50 meV increase in the exciton binding energy when compared with pristine bulk. Furthermore, by analysis of the electron-hole correlation function, we quantify the defect-induced localization of the Wannier-Mott exciton in two ways. First, we show that the electron-hole separation is reduced, and that the exciton envelope wave function can be related to a simple model of a defect-bound exciton. Second, we show that the exciton center-of-mass does not display the periodicity of the lattice due to defect-induced localization. We anticipate that our approach, which quantitatively describes the influence of a point defect on the exciton wave function, will be generally applicable.

The structural and nanochemical properties of thin $\mathrm{Al}{\mathrm{O}}_x$ layers are decisive for the performance of advanced electronic devices. For example, they are frequently used as tunnel barriers in Josephson junction-based superconducting devices. However, systematic studies of the influence of oxidation parameters on structural and nanochemical properties are rare up to now, as most studies focus on the electrical properties of $\mathrm{Al}{\mathrm{O}}_x$ layers. This study aims to close this gap by applying transmission electron microscopy in combination with electron energy loss spectroscopy to analyze the structural and nanochemical properties of differently fabricated $\mathrm{Al}{\mathrm{O}}_x$ layers and correlate them with fabrication parameters. With respect to the application of $\mathrm{Al}{\mathrm{O}}_x$ as tunnel barrier in superconducting Josephson junctions, $\mathrm{Al}/\mathrm{Al}{\mathrm{O}}_x/\mathrm{Al}$-layer systems were deposited on Si substrates. We will show that the oxygen content and structure of amorphous $\mathrm{Al}{\mathrm{O}}_x$ layers is strongly dependent on the fabrication process and oxidation parameters. Dynamic and static oxidation of Al yields oxygen-deficient amorphous $\mathrm{Al}{\mathrm{O}}_x$ layers, where the oxygen content ranges from $x=0.5$ to $x=1.3$ depending on oxygen pressure and substrate temperature. Thicker layers of stoichiometric crystalline $\gamma \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ layers were grown by electron-beam evaporation of $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ and reactive sputter deposition.

We have studied the thermalization of hot carriers in both pristine and defective titanium nitride (TiN) using a two-temperature model. All parameters of this model, including the electron-phonon coupling parameter, were obtained from first-principles density-functional theory calculations. The virtual-crystal approximation was used to describe defective systems. We find that thermalization of hot carriers occurs on much faster timescales than in gold as a consequence of the significantly stronger electron-phonon coupling in TiN. Specifically, the largest thermalization times, on the order of 200 femtoseconds, are found in TiN with nitrogen vacancies for electron temperatures around 4000 K.

We have grown untwinned ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (YBCO) films on (110) MgO substrates that were preannealed at high temperature in oxygen atmosphere. The annealing results in surface reconstruction with shallow facets, which induce the suppression of the YBCO twinning domains, and the preferential alignment of the CuO chains along one of the in-plane directions of the substrate. Because of the large mismatch between the in-plane lattice parameters of film and substrate, the strain induced by the MgO into the YBCO layer is strong and very peculiar. The YBCO film is compressed, with respect to the bulk, and presents a unidirectional buckling of the atomic planes, along the chains' direction, due to a deformation of the copper-oxygen octahedra. The YBCO films, which can be grown with thicknesses down to few unit cells and oxygen doping levels spanning most of the superconducting dome, are patterned into nanowires with dimensions down to 50 nm. The anisotropies due to the untwinning state are preserved in these structures; moreover, additional anisotropies appear in ultrathin structures where strain effects become more pronounced. Such untwinned and compressively strained films can therefore be used as a platform to study the interplay between strain and the various local orders in the normal state of YBCO.

We have synthesized a single-crystalline sample of ${\mathrm{Mn}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ using the floating zone technique, and we investigated the ground state using magnetic susceptibility, heat capacity, and neutron diffraction. Our magnetic susceptibility and heat capacity reveal two successive magnetic transitions at $T_{N1}=17\mathrm{K}$ and $T_{N2}=11.8\mathrm{K}$ indicating two distinct magnetically ordered phases. The single-crystal neutron diffraction study shows that in the temperature ($T$) range $11.8\le T\le 17\mathrm{K}$ the magnetic structure is commensurate with propagation vector $k_1=(0,0.5,0)$, while upon lowering temperature below $T_{N2}=11.8\mathrm{K}$ an incommensurate magnetic order emerges with $k_2=(0.38,0.48,0.5)$ and the magnetic structure can be represented by cycloidal modulation of the Mn spin in the $ac$ plane. We discuss the role of magnetic exchange interactions and spin-orbital coupling on the stability of the observed magnetic phase transitions.

An isobar $x\left(T\right)$ of deuterium solubility in iron is constructed at $P=6.3\mathrm{GPa}$ and $100\le T\le 800{}^{\circ }\mathrm{C}$ based on the results of thermal desorption analysis of $\mathrm{Fe}{\mathrm{D}}_x$ samples produced by quenching under high ${\mathrm{D}}_2$ pressure to the temperature of liquid nitrogen. The experiment confirms the value of $x=0.64$ at $T=715{}^{\circ }\mathrm{C}$ proposed previously in a neutron diffraction work [Machida et al., Nature Commun. 5, 5063 (2014)] for γ iron deuteride under the assumption that deuterium atoms occupy both octa- and tetrahedral interstices in its fcc metal lattice. An estimate of $\mathrm{\Delta }V/x=2.2{\AA }^3/\mathrm{atom}$ D made in that work for the deuterium-induced volume expansion $\mathrm{\Delta }V\left(x\right)$ of fcc iron is also confirmed. To prove that the absorption of protium leads to a similar volume expansion, we constructed an isotherm $x\left(P\right)$ of hydrogen solubility in fcc iron at $T=600{}^{\circ }\mathrm{C}$ and ${\mathrm{H}}_2$ pressures from 4.3 to 7.4 GPa. The available $\mathrm{\Delta }V\left(P,T\right)$ data of in situ x-ray diffraction studies of iron hydrides [T. Hiroi et al., J. Alloys Compd. 404–406, 252 (2005); H. Saitoh et al., J. Alloys Compd. 706, 520 (2017)] agree with this isotherm under the assumption that $\mathrm{\Delta }V/x=2.2{\AA }^3/\mathrm{atom}$ H. The transformation between the high-temperature fcc (γ) and low-temperature dhcp (ε′) deuterides of iron is shown to occur at 260 °C, which is approximately 100 °C lower than the temperature of the γ ↔ ε′ transformation in the Fe-H system at the same pressure of 6.3 GPa.

Octahedral tilting transitions are observed in most inorganic halide perovskites and play an important role in determining their functional and thermodynamic properties. Despite existing near room temperature, the cubic and tetragonal forms of halide perovskites become dynamically unstable at low temperature, making it impossible to study their thermodynamic properties with commonly used quasiharmonic models. An anharmonic vibrational Hamiltonian is constructed that accurately reproduces the low-energy portion of the potential-energy surface of the halide perovskite ${\mathrm{CsPbBr}}_3$. The Hamiltonian is validated using a large first-principles dataset of energies calculated within density functional theory for large-amplitude deformations of the ${\mathrm{CsPbBr}}_3$ crystal. Monte Carlo simulations performed on the Hamiltonian reproduce the orthorhombic-tetragonal-cubic phase transitions observed in ${\mathrm{CsPbBr}}_3$ and many other halide perovskites, demonstrating the importance of anharmonic vibrational excitations in stabilizing the tetragonal and cubic phases in these materials. Measures of local structure and octahedral tilting in the cubic and tetragonal phases, obtained from Monte Carlo simulations, confirm the connection between large anisotropic displacement factors and octahedral tilting, as observed experimentally.

We present optical measurements of the transition metal dichalcogenide ${\mathrm{PdTe}}_2$. The reflectivity displays an unusual temperature and energy dependence in the far infrared, which we show can only be explained by a collapse of the scattering rate at low temperature, resulting from the vicinity of a van Hove singularity near the Fermi energy. An analysis of the optical conductivity suggests that below 150 K a reduction in the available phase space for scattering takes place, resulting in long-lived quasiparticle excitations. We suggest that this reduction in phase space provides experimental evidence for a van Hove singularity close to the Fermi level. Our data furthermore indicates a very weak electron-phonon coupling. Combined this suggests that the superconducting transition temperature is set by the density of states associated with the van Hove singularity.

The binary alloys with heavy elements have been considered promising candidates for spin-orbit torque application due to the tunable spin Hall effect. In light of previous studies, the effect of crystalline structure on spin Hall effect in nonmagnetic alloys has not been thoroughly studied. Here, we present a systematic investigation of the spin-orbit torques in chemically disordered $\mathrm{C}{\mathrm{u}}_{100\text{−}x}\mathrm{P}{\mathrm{t}}_x$ and $\mathrm{L}{1}_1\text{−}\mathrm{C}{\mathrm{u}}_{50}\mathrm{P}{\mathrm{t}}_{50}$ by the spin torque ferromagnetic resonance technique. The results indicate that both the atomic concentration and the degree of the chemical ordering substantially influence the spin-orbit torque efficiency of the CuPt alloys. In chemically disordered $\mathrm{C}{\mathrm{u}}_{100\text{−}x}\mathrm{P}{\mathrm{t}}_x$, the primary mechanism of spin Hall effect changes from extrinsic to intrinsic when the Pt concentration is increased to larger than $80\%$. In $\mathrm{L}{1}_1\text{−}\mathrm{C}{\mathrm{u}}_{50}\mathrm{P}{\mathrm{t}}_{50}$ with weak chemical ordering, the side-jump and intrinsic mechanisms dominate, whereas the skew scattering mechanism dominates for strong chemical ordering. This work provides a perspective to control the spin-orbit torques in alloys.

The electronic structure of Mott and charge-transfer insulators can be tuned through charge doping to achieve a variety of fascinating physical properties, e.g., superconductivity, colossal magnetoresistance, and metal-to-insulator transitions. Strong correlations between $d$ electrons give rise to these properties but they are also the reason why they are inherently difficult to model. This holds true especially for the evolution of properties upon charge doping. Here, we hole-dope nickel oxide with potassium and elucidate the resulting structure by using a range of experimental and theoretical tools; potassium is twice as big as nickel and is expected to lead to distortions in its vicinity. Our measurements of the x-ray absorption fine structure (XAFS) show a significant distortion around the dopant and that the dopant is fully incorporated in the nickel oxide matrix. In parallel, the theoretical investigations include developing a Gaussian process for quantum Monte Carlo calculations to determine the lowest energy local structure around the potassium dopant. While the optimal structures determined from density functional theory and quantum Monte Carlo calculations agree very well, we find a large discrepancy between the experimentally determined structures and the theoretical doped structures. Further modeling indicates that the discrepancy is likely due to vacancy defects. Our work shows that potassium doping is a possible avenue to doping NiO, in spite of the size of the potassium dopant. In addition, the Gaussian process opens up a new route towards obtaining structure predictions outside of density functional theory.

Spin state transition in perovskite cobaltite is delicately controlled by the competition between exchange interaction energy and crystal-field energy. The latter is mainly governed by the change of bond length and bonding angle. Previous work has revealed that the electronic configuration associated with spin state transition in $\mathrm{LaCo}{\mathrm{O}}_3$ (LCO) thin films can be effectively modulated by epitaxial strain. However, a systematic study on the spin state transition of $\mathrm{C}{\mathrm{o}}^{3+}$ ions in LCO films with different crystallographic symmetry is still missing. Here, keeping the in-plane strain unchanged, we report that the magnetization of LCO films can be manipulated with crystallographic symmetry. The ultrathin LCO layers, constrained by the cubic substrate, have pseudotetragonal structure and small magnetization. Upon increasing the layer thickness, the monoclinic structure dominates the LCO film and maximizes its ferromagnetism. For the LCO films with a thickness beyond 35 unit cells, the symmetry relaxes gradually towards its rhombohedral bulk form, and meanwhile the magnetization reduces. These results highlight the importance of spin-lattice entanglement in a ferroelastic material and provide a concise way to maximize its functionality using symmetry engineering.

Recent experimental work has realized a new insulating state of samarium nickelate $\left({\mathrm{SmNiO}}_3\right)$, accessible in a reversible manner via high-density electron doping. To elucidate this behavior, we use the first-principles density functional theory $\left(\mathrm{DFT}\right)+U$ method to study the effect of added electrons on the crystal and electronic structure of ${\mathrm{SmNiO}}_3$. First, we track the changes in the crystal and electronic structure with added electrons compensated by a uniform positive background charge at concentrations of $\frac{1}{4}$, $\frac{1}{2}$, $\frac{3}{4}$, and 1 electrons per Ni. The change in electron concentration does not rigidly shift the Fermi energy; rather, the added electrons localize on ${\mathrm{NiO}}_6$ octahedra causing an on-site Mott transition and a change in the density of states resulting in a large gap between the occupied and unoccupied Ni $e_g$ orbitals at full doping. This evolution of the density of states is essentially unchanged when the added electrons are introduced by doping with interstitial H or Li ions.

Tin selenide is a layered material that captured the interest of the scientific community for its stunning thermoelectric properties and fascinating phase transitions under pressure. Recently, an experimental study revealed the existence of a topological and superconducting phase in its pressure-stabilized CsCl-type phase. By means of ab initio techniques, we investigate the structural properties of this compound and its pressure phase diagram, comparing our findings with the experimental results. We then focused on the electronic features of the topological CsCl-type phase and analyze its dynamical and superconducting properties. To understand the origin of the superconducting transition, we predict the critical temperature as a function of the pressure, $T_c$(P), by the superconducting density-functional theory and analyze the behavior of the resistance with pressure and temperature by means of a percolative model. The careful comparison of calculations with available experiments reveals that inhomogeneities and nonhydrostatic pressure effects are relevant in this system.