Epitaxially strained ${\mathrm{SrRuO}}_3$ films have been a model system for understanding the magnetic anisotropy in metallic oxides. In this paper, we investigate the anisotropy of the Ru $4d$ and O $2p$ electronic structure and magnetic properties using high-quality epitaxially strained (compressive and tensile) ${\mathrm{SrRuO}}_3$ films grown by machine-learning-assisted molecular beam epitaxy. The element-specific magnetic properties and the hybridization between the Ru $4d$ and O $2p$ orbitals were characterized by Ru $M_{2,3}$-edge and O $K$-edge soft x-ray absorption spectroscopy and magnetic circular dichroism measurements. The magnetization curves for the Ru $4d$ and O $2p$ magnetic moments are identical, irrespective of the strain type, indicating the strong magnetic coupling between the Ru and O ions. The electronic structure and the orbital magnetic moment relative to the spin magnetic moment are isotropic despite the perpendicular and in-plane magnetic anisotropy in the compressive-strained and tensile-strained ${\mathrm{SrRuO}}_3$ films; i.e., the orbital magnetic moments make a negligibly small contribution to the magnetic anisotropy. This result contradicts Bruno's model, where magnetic anisotropy arises from the difference in the orbital magnetic moment between the perpendicular and in-plane directions. Contributions of strain-induced electric quadrupole moments to the magnetic anisotropy are discussed, too.

Organic semiconductors consisting mainly of relatively light chemical elements were expected to exhibit low spin-orbit coupling (SOC) and large spin relaxation time, making them attractive as spin conveyors in spintronic devices. However, a series of theoretical and experimental spin diffusion length and spin Hall angle results contradicting this expectation have triggered an ongoing discussion to understand the coupling of spins to their environment in these systems. In this context we use the prototype material poly(3,4-ethylenedioxythiophene):polystyrene sulfonate as a model conducting polymer system to monitor the impact of chemical doping and the resulting modification of the polymer's backbone conformation on the spin diffusion length. We observed an increase by a factor of 2 of the spin diffusion length during dedoping of the polymer that is accompanied by a respective order of magnitude decrease in the spin Hall angle and the spin admixture parameter indicating a reduction of the SOC strength. This trend is associated with an increased planarity at the dedoped level monitored by Raman spectroscopy revealing that spin transport is inextricably linked to the structural conformation of the organic semiconductor. Spin phenomena of a conjugated polymer can be modified through control of the polymer's structural conformation, paving the way for a new functionality for spintronic based devices.

We study the superconducting pairing instabilities and gap functions for prototypical two-dimensional (2D) transition-metal dichalcogenides (TMDCs) ${\mathrm{WS}}_2$, ${\mathrm{MoTe}}_2$, and ${\mathrm{MoS}}_2$ in the 2H phase under both hole and electron doping at 10 K. Our first-principles quantum many-body Green's function approach allows us to treat the full $d$ and $p$ manifold of orbitals with strong spin-orbit coupling, yielding pairing predictions with material-specific detail. The resulting gap functions exhibit a variety of mixed-parity superconducting states, including $s$, $p$, $d$, $f$, $d\pm id$, and $p\pm ip$ pairing modes. In particular, we predict 3% and 4% hole-doped ${\mathrm{WS}}_2$ to be a chiral $p\pm ip$ topological superconductor. For 1% hole-doped ${\mathrm{MoS}}_2$, we find a competition between three doubly degenerate chiral and nonchiral instabilities. Overall, the relative pairing strengths are found to follow the Fermi-surface topology, due to nesting between the Fermi-surface sheets. Finally, we discuss our predictions in relation to available experimental data and classify the topology of the predicted superconducting pairing symmetries.

The perovskite-type iron oxide ${\mathrm{Sr}}_{2/3}{\mathrm{La}}_{1/3}{\mathrm{FeO}}_3$ is known to show characteristic spin-charge ordering (SCO), where sixfold collinear spin ordering and threefold charge ordering are coupled with each other. Here, we report the discovery of a spin-charge decoupling and an antiferromagnetic (AFM) state competing with the SCO phase in perovskites ${\left({\mathrm{Sr}}_{1-x}{\mathrm{Ba}}_x\right)}_{2/3}{\mathrm{La}}_{1/3}{\mathrm{FeO}}_3$. By comprehensive measurements including neutron diffraction, Mössbauer spectroscopy, and x-ray absorption spectroscopy, we found that the isovalent ${\mathrm{Ba}}^{2+}$ substitution systematically reduces the critical temperature of the SCO phase and additionally yields the spin-charge decoupling in $x>0.75$. Whereas the ground state remains in the SCO phase in the whole $x$ region, an unexpected G-type AFM phase with incoherent charge ordering or charge fluctuation appears as the high-temperature phase in the range of $x>0.75$. Reflecting the competing nature between them, the G-type AFM phase partially exists as a metastable state in the SCO phase at low temperatures. We discuss the origin of the spin-charge decoupling and the emergence of the G-type AFM phase with charge fluctuation in terms of the bandwidth reduction by the Ba substitution.

The materials preserving a uniform spin configuration in the momentum space, known as persistent spin texture (PST), provide long carrier spin lifetimes through persistent spin-helix mechanism. However, most of the PSTs studied until now are attributed to the linear in $\mathit{k}$ splitting and cease to exist locally around certain high-symmetry points of the first Brillouin zone (BZ). The PSTs with purely cubic spin splittings (PCS) have drawn attention owing to unique benefits in spin transport. Here, using relativistic first-principles calculations supplemented with $\mathit{k}·\mathit{p}$ analysis, we report the emergence of PCS belonging to $D_{3h}$ point group, which is enforced by in-plane mirror and threefold rotation operations. In addition, the in-plane mirror symmetry operation sustains the PST in a larger region, i.e., full planes of BZ alongside giant spin splitting. The observed PSTs provide a route to nondephasing spin transport with larger spin Hall conductivity, thereby offering a promising platform for future spintronics devices.

The generation of spin and orbital currents is of crucial importance in the field of spin-orbitronics. In this work, using relativistic density functional theory and the Kubo linear-response formalism, we systematically investigate the spin Hall and orbital Hall effects for 40 monoatomic metals. The spin Hall conductivity and orbital Hall conductivity (OHC) are computed as a function of the electrochemical potential and the influence of the spin-orbit interaction strength is also investigated. Our calculations predict a rather small OHC in $sp$ metals, but a much larger OHC in $d$-band metals, with maximum values $[\sim 8000(\hslash /e){\left(\mathrm{\Omega }\mathrm{cm}\right)}^{-1}]$ near the middle of the $d$ series. Using the Mott formula, we evaluate the thermal counterparts of the spin and orbital Hall effects, the spin Nernst effect (SNE), and the orbital Nernst effect (ONE). We find that the as-yet unobserved ONE is significantly larger $(\sim 10\times )$ than the SNE and has maximum values for group 10 elements (Ni, Pd, and Pt). Our work provides a broad overview of electrically and thermally induced spin and orbital transport in monoatomic metals.

Motivated by the recent discovery of superconductivity in the pentalayer nickelate ${\mathrm{Nd}}_6{\mathrm{Ni}}_5{\mathrm{O}}_{12}$ [G. A. Pan et al., Nat. Mater. 21, 160 (2022)], we calculate its electronic structure and superconducting critical temperature. We find that electronic correlations are essential for pushing ${\mathrm{Nd}}_6{\mathrm{Ni}}_5{\mathrm{O}}_{12}$ into the superconducting doping range as they shift the electron pockets above the Fermi energy. As a consequence, ${\mathrm{Nd}}_6{\mathrm{Ni}}_5{\mathrm{O}}_{12}$ can be described with a single $d_{x^2-y^2}$ orbital per Ni. We predict the pentalayer to be overdoped; further improvements in $T_c$ can be expected for a few additional layers. Instead, for the bilayer nickelate ${\mathrm{Nd}}_3{\mathrm{Ni}}_2{\mathrm{O}}_6$ we find correlations to drive the system into a three-orbital regime also involving the Ni $d_{xz,yz}$ states. We suggest, however, that single-orbital physics with optimal doping can be restored by substituting 60% of the trivalent Nd or La by tetravalent Zr.

Single-crystal delafossite ${\mathrm{PdCoO}}_2$ is known to have an extremely low intrinsic impurity concentration of ∼0.001%, demonstrating extraordinarily high conductivity with a mean free path of ∼20 μm at low temperatures. However, when grown as thin films, the resistivity at room temperature increases by a factor of 3–80 times, depending on the film thickness. Using scanning transmission electron microscopy, we identify different classes of defects for the single crystal vs epitaxial thin film. The dominant defect for single-crystal ${\mathrm{PdCoO}}_2$ is found to be ribbonlike defects. For the thin films, we identify different types of defects arising in epitaxial thin films mainly due to substrate termination that disrupt the lateral connectivity of the conducting planes. Our results are consistent with the high conductivity of single crystals and increased electrical resistivity of the thin films compared to that of single crystals, suggesting that selecting a proper substrate, improving surface quality, and reducing the step density are the keys to enhance the film quality for utilizing ${\mathrm{PdCoO}}_2$ as a platform for future applications.

The bypassing of an oxygen interstitial by $\langle a\rangle$-type screw dislocations in Ti is studied as a function of the dislocation core structure using an empirical potential and the nudged elastic band method. It is shown that an interstitial oxygen will be pushed from the octahedral site into a new interstitial site in the dislocation core. From there, with the passage of a dislocation spread primarily on the prismatic plane there is a high energy barrier for oxygen to return to the octahedral site. Instead, it is likely to shuffle into the neighboring hexahedral site, a transition expected to lead to slip plane softening. For dislocations spread primarily within the pyramidal planes, the octahedral to hexahedral transition is much less likely as there is little to no barrier for the oxygen to return to the original octahedral site. This difference in barriers is then explained by a reduction in the attraction between the hexahedral site and the dislocation core, calculated within micromechanics, as the core transitions from a prismatic to a pyramidal state.

Silicon carbide coatings and thin films are used for a wide array of applications ranging from thermal barrier coatings to microelectronics. In this paper, we report on the role of mass density and atomic coordination on the fundamental vibrational characteristics and thermal conductivity of amorphous silicon carbide systems through a combination of experiments and systematic atomistic simulations. We use time domain thermoreflectance to show that the thermal conductivity of hydrogenated amorphous silicon carbide can be increased twofold with $\sim 40\%$ increase in the mass density. A simple description of thermal transport applicable to a range of amorphous solids where diffusion of thermal energy is predominantly driven by nonpropagating modes cannot fully describe our experimental measurements. Our molecular dynamics simulations in conjunction with our lattice dynamics calculations shed light on the intrinsic role of atomic coordination in dictating the contributions from both propagating and nonpropagating modes in amorphous silicon carbide structures. More specifically, we find that as the concentration of $s{p}^3$ hybridized carbon atoms is increased by up to $10\%$ with increasing mass densities, the contribution from propagons can be increased from $\sim 25\%$ to $\sim 40\%$, after which further increments in the mass density and the $s{p}^3$ fraction does not lead to higher contributions from propagons. In contrast, contributions from the nonpropagating modes increases monotonically with increasing mass density and $s{p}^3$ hybridization. Our results pave a path forward to manipulate the thermal conductivity of amoprhous silicon carbide systems based on varying the atomic coordination.

Recent advances in epitaxial oxide deposition have enabled the fabrication of thick films of ferroelectric perovskite ${\mathrm{BaTiO}}_3$ capable of providing a robust electro-optic response via the Pockels effect in silicon photonic devices. We report a microstructure analysis of such films integrated on Si(001) by molecular beam epitaxy, showing how the crystallographic and polarization orientation change as a function of thickness. The measured electro-optic properties of the film correlate well with the microstructural analysis and demonstrate the potential of Si-integrated ${\mathrm{BaTiO}}_3$ for silicon photonics. An effective Pockels coefficient of up to 268 pm/V has been demonstrated in 110-nm-thick films in transmission measurements and 352 pm/V in waveguide measurements.

Many crystalline solids possess strongly anharmonic “soft” phonon modes characterized by diminishing frequency as temperature approaches a critical point associated with a symmetry breaking phase transition. While electron–soft phonon coupling can introduce unique scattering channels for charge carriers in ferroelectrics, recent studies on the nonferroelectric lead halide perovskites have also suggested the central role of anharmonic phonons bearing resemblance to soft modes in charge carrier screening. Here we apply coherent phonon spectroscopy to directly study electron coupling to the soft transverse optical phonon mode in a ferroelectric semiconductor SbSI. Photogenerated charge carriers in SbSI are found to be exceptionally long lived and are associated with a transient electro-optical effect that can be explained by interactions between charge carriers and thermally stimulated soft phonon excitations. These results provide strong evidence for the role of electron–soft phonon coupling in the efficient screening of charge carriers and in reducing charge recombination rates, both desirable properties for optoelectronics.

Spin glasses (SGs) arise from the frustration of competing magnetic interactions without long-range order; hence they tend to be destabilized by thermal fluctuation and exhibit a rather low glass transition temperature, presenting a major challenge for SG research and applications. Here, we report an unusual SG state in quaternary Fe-Co-Ni-Mn chemically complex alloys (CCAs). The SG exhibits an ultrahigh freezing temperature above room temperature, well exceeding that of conventional bulk SGs, as well as a unique and fast relaxation dynamics. The thermally stable SG state can be attributed to the strong frustration of exchange interactions owing to the high concentration of magnetic atoms and their chemical randomness in the solid-solution lattice. In addition, owing to the high phase stability of CCAs, the SG is robust over a wide compositional range, enabling a variety of magnetic phase transitions and largely tunable glass properties. These properties make CCAs important for understanding the nature of the SG state and intriguing for practical applications of SGs in spintronics.

In this paper, the influence of thickness in epitaxial ${\mathrm{K}}_{0.65}{\mathrm{Na}}_{0.35}{\mathrm{NbO}}_3$ ferroelectric thin films grown on a (110) ${\mathrm{TbScO}}_3$ substrate was systematically studied. Detailed atomic force microscopy revealed a complex change in growth mode with increasing film thickness, with the surface roughness remaining <0.30 nm throughout. By combining piezoresponse force microscopy and high-resolution x-ray diffraction, the occurrence of 90° stripe domains was demonstrated for the films with a thickness of ≥11 nm, while the domain periodicity is in good agreement with Kittel's law. Furthermore, up to the thickness of 93 nm, elastic strain relaxation induced by the formation of ferroelectric domains was observed, whereas plastic strain relaxation plays only a minor role. As the film thickness increases, three successive phases of ferroelectric domains were observed: (i) irregularly arranged orthorhombic $c$ domains in the thinnest film, (ii) periodically arranged 90° monoclinic $M_C$ domains up to a thickness of 25 nm, and (iii) a flux closure vortexlike structure in thicker films to achieve the lowest equilibrium energy. These results demonstrate the importance of understanding the lattice relaxation mechanism for intentional tuning of ferroelectric thin film properties.

Recently, magnetic topological quantum materials have attracted substantial research attention due to their great application potential. Here, using high-resolution angle-resolved photoemission spectroscopy and ab initio calculation, we systematically investigate the electronic structure of antiferromagnet Dirac semimetal candidate $\mathrm{Gd}{\mathrm{In}}_3$. According to our ab initio calculation, there exist two and one pair(s) of Dirac fermions in the paramagnetic and antiferromagnetic state, respectively. In the antiferromagnetic state, the magnetic Dirac semimetal phase is protected by $C_{4z}$ rotation symmetry in the type-IV magnetic space group. Our experiment above the Néel temperature is well reproduced by the calculated band structure in the paramagnetic state, from which we identify a pair of Dirac fermions at 1.5 eV below the Fermi level. However, we do not observe the signature of electronic reconstruction in the antiferromagnetic state, suggesting a weak interaction between the localized Gd $4f$ states of spin configuration $S=7/2{\mu }_{\mathrm{B}}$ and the itinerant conduction electrons. Our results confirm the Dirac semimetal nature of the paramagnetic $\mathrm{Gd}{\mathrm{In}}_3$ and provide important insights into its antiferromagnetic Dirac semimetal phase.

$\mathrm{Hf}{\mathrm{O}}_2/\mathrm{Si}$ interface is among the most studied heterostructure materials due to the use of ${\mathrm{HfO}}_2$ in the mainstream Si microelectronic technology. Following the discovery of new functionalities in ${\mathrm{HfO}}_2$ such as ferroelectric and reversible resistance-switching properties, we study ultrathin ${\mathrm{HfO}}_2$ films grown on highly doped ($p^{+}$ and $n^{+}$) Si by means of synchrotron-based soft-x-ray spectroscopy techniques, such as x-ray photoelectron spectrosopy (XPS) and angle resolved photoelectron spectroscopy (ARPES). With angular resolution, we directly obtain the electronic dispersions $E$(k) of the single-crystalline Si substrate in contact with the ${\mathrm{HfO}}_2$ overlayer, depending on the Si doping and heat treatment, and determine the k-resolved band offset at the interface. Analysis of the Hf and Si core-level energies and line shapes as a function of photon energy yields band bending in ${\mathrm{HfO}}_2$ and Si. The evolution of the Hf $4f$ linewidth upon annealing points to development of a potential distribution across ${\mathrm{HfO}}_2$ due to charged defects at the surface and interface with Si. The effect of intense x-ray beam on the $\mathrm{Hf}{\mathrm{O}}_2/\mathrm{Si}$ interfaces, distorting their pristine electronic structure, is evaluated from the time evolution of line shape and position under irradiation. We propose a model explaining the effects of both heat treatment and x-ray irradiation on the $\mathrm{Hf}{\mathrm{O}}_2/\mathrm{Si}$ electronic structure in terms of oxygen vacancies generated at the surface of ${\mathrm{HfO}}_2$ and its interface to Si, where the released O atoms react with Si to form ${\mathrm{SiO}}_x$ at the interface. The knowledge of the irradiation-dependent band bending is essential for precise determination of the k-dependent band offset locally at the $\mathrm{Hf}{\mathrm{O}}_2/\mathrm{Si}$ interface.

A first-principles approach combining density-functional and dynamical mean-field theories in conjunction with a quasiatomic approximation for the strongly localized $4f$ shell is applied to ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$-based hard magnets to evaluate crystal-field and exchange-field parameters at rare-earth sites and their corresponding single-ion contribution to the magnetic anisotropy. In pure ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$, our calculations reproduce the easy-cone to easy-axis transition; theoretical magnetization curves agree quantitatively with experiment. Our study reveals that the rare-earth single-ion anisotropy in the 2-14-1 structure is strongly site dependent, with the $g$ rare-earth site exhibiting a larger value. In particular, we predict that increased $f$- and $g$-site occupancy of $R=$ Ce and Dy, respectively, leads to an increase of the magnetic anisotropy of the corresponding (${\mathrm{Nd},R)}_2{\mathrm{Fe}}_{14}\mathrm{B}$-substituted compounds.

The spin Hall effects of various materials, including Weyl semimetals, have been studied by the spin absorption method with the nonlocal spin valve structures. Here, we study ${\mathrm{Co}}_2\mathrm{MnGa}$ (CMG) by using the standard spin absorption method. A considerable amount of thermoelectric signal superimposes on the inverse spin Hall signal in the measurement configuration: the applied electric current between the spin injector (permalloy) and bridge (copper) wires produces heating or cooling at the interface via the Peltier effect, which causes an out-of-plane temperature gradient in CMG. As a result, a voltage signal induced by the anomalous Nernst effect (ANE) of CMG comes into the inverse spin Hall signal. We quantitatively separate these signals by combining the experiment and a numerical simulation. About 75% of the detected signal is found attributable to the thermoelectric effects. The superposing thermoelectric contribution is a general and unavoidable problem in this method when the spin Hall materials exhibit ANE. After eliminating the thermoelectric signal, the spin Hall angle of CMG turns out to be $\theta =-0.09$ at room temperature.

Tailoring the physical properties of complex materials for targeted applications requires optimizing the microstructure and crystalline defects that influence electrical and thermal transport and mechanical properties. Laser surface remelting can be used to modify the subsurface microstructure of bulk materials and hence manipulate their properties locally. Here, we introduce an approach to perform remelting in a reactive nitrogen atmosphere to form nitrides and induce segregation of nitrogen to structural defects. These defects arise from the fast solidification of the full-Heusler ${\mathrm{Fe}}_2\mathrm{VAl}$ compound that is a promising thermoelectric material. Advanced scanning electron microscopy, including electron channeling contrast imaging and three-dimensional electron backscatter diffraction, is complemented by atom probe tomography to study the distribution of crystalline defects and their local chemical composition. We reveal a high density of dislocations, which are stable due to their character as geometrically necessary dislocations. At these dislocations and low-angle grain boundaries, we observe segregation of nitrogen and vanadium, which can be enhanced by repeated remelting in nitrogen atmosphere. We propose that this approach can be generalized to other additive manufacturing processes to promote local segregation and precipitation states, thereby manipulating physical properties.