A sulfur element is a promising anion dopant for synthesizing new multifunctional materials and for exploring unusual physical phenomena. However, owing to its volatility, sulfur substitution to oxide materials is challenging, and thus the sulfurization effects on the associated properties have been limitedly studied. Here, a facile method for sulfurization to a perovskite oxide $\mathrm{Pb}\left(\mathrm{Zr},\mathrm{Ti}\right){\mathrm{O}}_3$ is developed and demonstrated. A thiourea $\left(\mathrm{C}{\mathrm{H}}_4{\mathrm{N}}_2\mathrm{S}\right)$ solution is used as a precursor for the sulfurization and its doping-level control. By manipulating the sulfur concentration ($x$), we systematically examine the physical properties of sulfur-doped $\mathrm{Pb}\left(\mathrm{Zr},\mathrm{Ti}\right){\mathrm{O}}_{3-x}{\mathrm{S}}_x$ films. An enhancement in the tetragonality and ferroelectricity by sulfurization is observed with the band-gap reduction, which is consistent with our theoretical predictions. In the sulfurized films, the ferroelectric phonon modes become softened progressively, probably due to the substitution of apical oxygens with sulfur atoms. Our work is of practical interest for designing ferroelectric photovoltaic devices with high performances.

A computational scheme integrating the atomistic spin model (ASM) and micromagnetic simulations is proposed to predict the coercivity of Nd-Fe-B permanent magnets at high temperatures. ASM simulations are applied to calculate the temperature-dependent intrinsic parameters of ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$, including the saturated magnetization, magnetocrystalline anisotropy, and exchange stiffness, which are shown to agree well with the experimental values. With the ASM results as input, finite-temperature micromagnetic simulations based on the stochastic Landau-Lifshitz-Gilbert equation are performed to calculate the magnetic reversal and coercivity ($H_{\text{c}}$) at high temperatures. It is found that in addition to the decrease of anisotropy field with temperature, thermal fluctuations further reduce $H_{\text{c}}$ by 5–10% and $\beta$ (temperature coefficient of coercivity) by 0.02–0.1% ${\text{K}}^{-1}$ in the presence of a defect layer. The computed thermal-activation volume, which increases with temperature, is shown to be enhanced by several times due to the defect layer with strong magnetization (e.g., 1 T), but can be decreased by introducing a hard shell. Both $H_{\text{c}}$ and $\beta$ can be enhanced by adding the Dy-rich shell, but saturate at a shell thickness ($t^{\text{sh}}$) around 6–8 nm after which further increasing $t^{\text{sh}}$ or adding Dy into the core is not essential.

Nano and microporous connected structures have attracted increasing attention in the past decades due to their high surface area, presenting interesting properties for a number of applications. These structures generally coarsen by surface diffusion, leading to an enlargement of the structure characteristic length scale. We propose to study this coarsening behavior using a phase-field model for surface diffusion. In addition to reproducing the expected scaling law, our simulations enable to investigate precisely the evolution of the topological and morphological characteristics along the coarsening process. In particular, we show that after a transient regime, the coarsening is self-similar as exhibited by the evolution of both morphological and topological features. In addition, the influence of surface anisotropy is discussed and comparisons with experimental tomographic observations are presented.

The predictive modeling of lattice thermal conductivity is of fundamental importance for the understanding and design of materials for a wide range of applications. Two major approaches, namely molecular dynamics (MD) simulations and calculations solving approximately the Boltzmann transport equation (BTE), have been developed to compute the lattice thermal conductivity. We present a detailed direct comparison of these two approaches, using as prototypical cases MgO and PbTe. The comparison, carried out using empirical potentials, takes into account the effects of fourth order phonon scattering, temperature-dependent phonon frequencies (phonon renormalization), and investigates the effects of quantum vs classical statistics. We clarify that equipartition, as opposed to Maxwell-Boltzmann, govern the statistics of phonons in MD simulations. We find that lattice thermal conductivity values from MD and BTE show an apparent, satisfactory agreement; however such an agreement is the result of error cancellations. We also show that the primary effect of statistics on thermal conductivity is via the scattering rate dependence on phonon populations.

Two-dimensional (2D) materials with nodal-loop band crossing have been attracting great research interest. However, it remains a challenge to find 2D nodal loops that are robust against spin-orbit coupling (SOC) and realized in magnetic states. Here, based on first-principles calculations and theoretical analysis, we predict that monolayer MnN is a 2D nodal-loop half-metal with fully spin-polarized nodal loops. We show that monolayer MnN has a ferromagnetic ground state with out-of-plane magnetization. Its band structure shows half metallicity with three low-energy bands belonging to the same spin channel. The crossing between these bands forms two concentric nodal loops centered around the $\mathrm{\Gamma }$ point near the Fermi level. Remarkably, the nodal loops and their spin polarization are robust under SOC due to the protection of a mirror symmetry. We construct an effective model to characterize the fully polarized emergent nodal-loop fermions. We also find that a uniaxial strain can induce a loop transformation from a localized single loop circling around $\mathrm{\Gamma }$ to a pair of extended loops penetrating the Brillouin zone.

Noncollinear, multi-$q$ spin textures can give rise to exotic, topologically protected spin structures such as skyrmions, but the reason for their formation over simple single-$q$ structures is not well understood. While lattice frustration and the Dzyaloshinskii-Moriya interaction are known to produce noncollinear spin textures, the role of electron itinerancy in multi-$q$ formation is much less studied. Here we investigated the noncollinear, helical spin structures in epitaxial films of the perovskite oxides ${\mathrm{SrFeO}}_3$ and ${\mathrm{CaFeO}}_3$ using magnetotransport and resonant soft x-ray magnetic diffraction. Metallic ${\mathrm{SrFeO}}_3$ exhibits features in its magnetoresistance that are consistent with its recently proposed multi-$q$ structure. Additionally, the magnetic Bragg peak of ${\mathrm{SrFeO}}_3$ measured at the Fe $L$-edge resonance energy asymmetrically broadens with decreasing temperature in its multi-$q$ state. In contrast, insulating ${\mathrm{CaFeO}}_3$ has a symmetric scattering peak with an intensity 10 times weaker than ${\mathrm{SrFeO}}_3$. Enhanced magnetic scattering at O $K$-edge prepeak energies demonstrates the role of a negative charge-transfer energy and the resulting oxygen ligand holes in the magnetic ordering of these ferrates. By measuring the magnetic diffraction of ${\mathrm{CaFeO}}_3/{\mathrm{SrFeO}}_3$ superlattices with thick ${\mathrm{CaFeO}}_3$ layers, we find that the ${\mathrm{CaFeO}}_3$ helical ordering is coherent across 1-unit-cell-thick ${\mathrm{SrFeO}}_3$ layers but not 6-unit-cell-thick layers. We conclude that insulating ${\mathrm{CaFeO}}_3$ supports only a simple single-$q$ helical structure, in contrast to metallic ${\mathrm{SrFeO}}_3$ that hosts multi-$q$ structures. Our results provide important insight into the role of electron itinerancy in the formation of multi-$q$ spin structures.

We study the effect of atomic coordination (orbital hybridization) on superconducting properties of boron-doped amorphous carbon. The ratio of threefold coordinated ($s{p}^2$-hybridized) and fourfold coordinated ($s{p}^3$-hybridized) atoms in the system is found to have an impact on their electronic, vibrational, and superconducting properties. Our findings show that a high proportion of fourfold coordination in both carbon and boron atoms is important for realizing a high superconducting transition temperature.

Complex oxide heterostructures provide access to emergent functional and structural phases which are not present in the bulk constituent materials. In this Rapid Communication, we focus on ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{CoO}}_3$ (LSCO)/Gd heterostructures due to the high oxygen ion conductivity, as well as the coupled magnetic and electronic properties of LSCO, which are strongly dependent on the oxygen stoichiometry. This combination of properties enables the ionic control of the functional properties of LSCO thin films through the presence of oxygen getter layers such as Gd. We utilize x-ray nanodiffraction to directly image the nanoscale morphology of LSCO thin films as they are progressively transformed from the equilibrium perovskite phase to the metastable brownmillerite (BM) phase with increasing Gd thickness. Our studies show the coexistence of perovskite and BM phases with a critical oxygen vacancy concentration threshold which leads to the formation of extended BM filaments. In addition to lateral phase separation, we observed phase separation within the film thickness possibly due to pinning of the perovskite and BM phases by the substrate/LSCO and LSCO/Gd interface, respectively. Our studies provide a nanoscale survey of the phase separation in the cobaltites and shed light on the formation of the metastable BM phase.

Most multiferroic materials with coexisting ferroelectric and magnetic order exhibit cycloidal antiferromagnetism with wavelength of several nanometers. The prototypical example is bismuth ferrite (${\mathrm{BiFeO}}_3$ or BFO), a room-temperature multiferroic considered for a number of technological applications. While most applications require small sizes such as nanoparticles, little is known about the state of these materials when their sizes are comparable to the cycloid wavelength. This work describes a microscopic theory of cycloidal magnetism in nanoparticles based on Hamiltonian calculations. It is demonstrated that magnetic anisotropy close to the surface has a huge impact on the multiferroic ground state. For certain nanoparticle sizes the modulus of the ferromagnetic and ferroelectric moments are bistable, an effect that may be used in the design of ideal memory bits that can be switched electrically and read out magnetically.

The equiatomic quaternary Heusler alloy CoFeCrAl is a candidate material for spin-gapless semiconductors (SGSs). However, to date, there have been no experimental attempts at fabricating a junction device. This paper reports a fully epitaxial (001)-oriented MgO barrier magnetic tunnel junction (MTJ) with CoFeCrAl electrodes grown on a Cr buffer. X-ray and electron diffraction measurements show that the (001) CoFeCrAl electrode films with atomically flat surfaces have a $B2$-ordered phase. The saturation magnetization is $380{\mathrm{emu}/\mathrm{cm}}^3$, almost the same as the value given by the Slater–Pauling–like rule, and the maximum tunnel magnetoresistance ratios at 300 K and 10 K are 87% and 165%, respectively. Cross-sectional electron diffraction analysis shows that the MTJs have MgO interfaces with fewer dislocations. The temperature- and bias-voltage-dependence of the transport measurements indicates magnon-induced inelastic electron tunneling overlapping with the coherent electron tunneling. X-ray magnetic circular dichroism (XMCD) measurements show a ferromagnetic arrangement of the Co and Fe magnetic moments of $B2$-ordered CoFeCrAl, in contrast to the ferrimagnetic arrangement predicted for the $Y$-ordered state possessing SGS characteristics. Ab initio calculations taking account of the Cr-Fe swap disorder qualitatively explain the XMCD results. Finally, the effect of the Cr-Fe swap disorder on the ability for electronic states to allow coherent electron tunneling is discussed.

Dopant atoms can be incorporated into nanowires either via the vapor-liquid-solid mechanism through the catalyst droplet or by the vapor-solid growth on the sidewalls. Si is a typical n-type dopant for GaAs, but in nanowires it often suffers from a strongly amphoteric nature in the vapor-liquid-solid process. This issue can be avoided by using Te, which is a promising but less common alternative for $n$-type doping of GaAs nanowires. Here, we present a detailed investigation of Te-doped self-catalyzed GaAs nanowires. We use several complementary experimental techniques, such as atom probe tomography, off-axis electron holography, micro-Raman spectroscopy, and single-nanowire transport characterization, to assess the Te concentration, the free-electron concentration, and the built-in potential in Te-doped GaAs nanowires. By combing the experimental results with a theoretical model, we show that Te atoms are mainly incorporated by the vapor-liquid-solid process through the Ga droplet, which leads to both axial and radial dopant gradients due to Te diffusion inside the nanowires and competition between axial elongation and radial growth of nanowires. Furthermore, by comparing the free-electron concentration from Raman spectroscopy and the Te-atom concentrations from atom probe tomography, we show that the activation of Te donor atoms is $100\%$ at a doping level of $4\times {10}^{18}\mathrm{c}{\mathrm{m}}^{-3}$, which is a significant result in terms of future device applications.

The defects in Si-doped ε-${\mathrm{Ga}}_2{\mathrm{O}}_3$ epitaxial layers have been investigated by electron paramagnetic resonance (EPR) spectroscopy. The results show that Si doping introduces a single, paramagnetic defect, attributed to Si incorporation on the tetrahedral gallium lattice site. It is a spin $S=1/2$ center with an axial $g$ tensor with principal values of $g//c=1.9573$ and $g\perp c=1.9591$. The temperature dependence of the EPR parameter demonstrates that it is a shallow effective mass donor, which is at the origin of the $n$-type conductivity. The EPR spectrum is modified by motional narrowing effects, the analysis of which allows one to reveal different transport regimes, including localization, hopping conductivity, and ionization in the conduction band when the temperature is raised from $T=4\mathrm{K}$ to room temperature. Partial electrical compensation and donor clustering are equally evidenced by the EPR results, which are confirmed by correlated electrical transport measurements. Silicon is thus a promising dopant for the formation of highly conductive $n$-type ε-${\mathrm{Ga}}_2{\mathrm{O}}_3$.

Skyrmions are nanoscale magnetic structures with features promising for future low-power memory or logic devices. In this work, we demonstrate scanning techniques based on nitrogen vacancy center magnetometry that simultaneously probe both the magnetic dynamics and structure of room temperature skyrmion bubbles in a thin-film system Ta/CoFeB/MgO. We confirm the handedness of the Dzyaloshinskii-Moriya interaction in this material and extract the magnitude of the helicity angle of the skyrmion bubbles. Our measurements also show that the skyrmion bubbles in this material change size in discrete steps, dependent on the local pinning environment, with their average size determined dynamically as their domain walls hop between pinning sites. In addition, an increase in magnetic field noise is observed near skyrmion bubble domain walls. These measurements highlight the importance of interactions between internal degrees of freedom of skyrmion bubble domain walls and pinning sites in thin-film systems. Our observations have relevance for future devices based on skyrmion bubbles where pinning interactions will determine important aspects of current-driven motion.

Responding to the rapidly increasing demand for efficient energy usage and increased speed and functionality of electronic and spintronic devices, multiferroic oxides have recently emerged as key materials capable of tackling this multifaceted challenge. In this paper, we describe the development of single-site manganese-based multiferroic perovskite materials with modest amounts of nonmagnetic Ti substituted at the magnetic Mn site in $\mathrm{S}{\mathrm{r}}_{1-x}\mathrm{B}{\mathrm{a}}_x\mathrm{M}{\mathrm{n}}_{1-y}\mathrm{T}{\mathrm{i}}_y{\mathrm{O}}_3$ (SBMTO). Significantly enhanced properties were achieved with ferroelectric-type structural transition temperatures boosted to $\sim 430\mathrm{K}$. Ferroelectric distortions with large spontaneous polarization values of $\sim 30\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2$, derived from a point charge model, are similar in magnitude to those of the prototypical nonmagnetic $\mathrm{BaTi}{\mathrm{O}}_3$. Temperature dependence of the system's properties was investigated by synchrotron x-ray powder diffraction and neutron powder diffraction at ambient and high pressures. Various relationships were determined between the structural and magnetic properties, Ba and Ti contents, and $T_{\mathrm{N}}$ and $T_{\mathrm{C}}$. Most importantly, our results demonstrate the large coupling between the magnetic and ferroelectric order parameters and the wide tunability of this coupling by slight variations of the material's stoichiometry.

We studied how the electronic, superconducting, and magnetic properties of $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_7/\mathrm{N}{\mathrm{d}}_{1-x}{\left(\mathrm{C}{\mathrm{a}}_{1-y}\mathrm{S}{\mathrm{r}}_y\right)}_x\mathrm{Mn}{\mathrm{O}}_3$ multilayers depend on the tolerance factor and the hole doping of the manganite. In particular, we investigated the granular superconducting state and the related magnetic-field-driven insulator-to-superconductor transition that was previously discovered in corresponding multilayers with $\mathrm{P}{\mathrm{r}}_{0.5}\mathrm{L}{\mathrm{a}}_{0.2}\mathrm{C}{\mathrm{a}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ [B. P. P. Mallett et al., Phys. Rev. B 94, 180503(R) (2016)]. We found that this granular superconducting state occurs only when the manganite layer is in a charge/orbital ordered and CE-type antiferromagnetic state (Mn-CO/OO). The coupling mechanism underlying this intriguing proximity effect seems to involve the domain boundaries of the Mn-CO/OO and/or the charge disordered regions of the manganite layer that become more numerous as the hole doping is reduced below $x=0.5$.

The fragile-to-strong crossover seems to be a general feature of metallic glass-forming liquids. Here, we study the behavior of shear viscosity, diffusion coefficient, and vibrational density of states for ${\mathrm{Cu}}_x{\mathrm{Zr}}_x{\mathrm{Al}}_{100-2x}$ alloy through molecular dynamics simulations. The results reveal that the fragile-to-strong temperature ($T_{fs}$) and the glass transition temperature ($T_g$) increase as the aluminum content becomes larger. The inverse of the diffusion coefficient as a function of temperature exhibits a dynamical crossover in the vicinity of $T_g$, at a much lower temperature than that predicted by nearly all previous studies. At the temperature in which the dynamical crossover occurs determined by the inverse of the diffusion coefficient, we found an excess of vibrational states at low frequencies, resembling a pronounced peak in the reduced vibrational density of states characteristic of a strong liquid. Finally, the behavior of the shear viscosity as a function of reduced temperature ($T_g/T$) also shows that, aside from the fragile-to-strong crossover nearby $T_g$, another dynamical crossover is present near the onset of the supercooled regime.

We present a quantum embedding theory called dynamical configuration interaction (DCI) that combines wave function and Green's function theories. DCI captures static correlation in a correlated subspace with configuration interaction and couples to high-energy, dynamic correlation outside the subspace with many-body perturbation theory based on Green's functions. DCI takes the strengths of both theories to balance static and dynamic correlation in a single, fully ab initio embedding concept. The theory adds dynamic correlation around a fixed active space of orbitals with efficient $\mathcal{O}\left(N^5\right)$ scaling, while maintaining a multireference treatment of the active space. We show that treating high-energy correlation up to the $GW$ and Bethe-Salpeter equation level is sufficient even for challenging multireference problems. Our theory treats ground and excited states on equal footing, and we compute the dissociation curve of ${\mathrm{N}}_2$, the vertical excitation energies of small molecules, and the ionization spectrum of benzene in excellent agreement with high-level quantum chemistry methods and experiment.

We report polyamorphic transformations of ${\mathrm{H}}_2\mathrm{O}$ under isothermal compression and decompression at 150 K, observed by in situ x-ray diffraction. Detailed structural information across the transformations among low-, high-, and very-high-density amorphs (LDA, HDA, and VHDA) reveals the mechanisms, the reversibility, and (dis)continuity of the transformations. During isothermal (de)compression, the polyamorphic transformations are characterized by a sharp and reversible LDA-VHDA transformation, with an HDA-like form (referred to as HDA′) appearing as an intermediate state. The LDA-VHDA transformation is found to occur in two steps: a discontinuous transition between LDA and the end member of HDA′ on the low-pressure side, followed by a continuous change within HDA′ involving structural reconfigurations and finally converging to VHDA. Both LDA and VHDA are found to be structurally stable showing mainly elastic behavior under (de)compression, while HDA′ is structurally unstable. Possible relations of these observations to low- and high-density liquids are discussed.

The quasi-two-dimensional Heisenberg spin $S=1/2$ dimer system bis(2-amino-5-fluoro-pyridinium) tetrachlorocuprate(II) is studied by means of inelastic neutron scattering, calorimetry, and nuclear magnetic resonance (NMR) experiments. In the absence of an applied magnetic field we find dispersive triplet excitations with a spin gap of $\mathrm{\Delta }=1.112\left(15\right)$ meV and a bandwidth of 0.715(15) meV within the layers and 0.116(15) meV between the layers, respectively. In an applied magnetic field of ${\mu }_0{H}_c\approx 8.5$ T the spin gap is closed and we find a field-induced antiferromagnetically ordered phase.