Effects of the coexistence of dry and wet oxidants on the reaction process at ${\mathrm{SiO}}_2$/4H-SiC(0001) and ($000\overline{1}$) interfaces during SiC oxidation are systematically investigated by performing ab initio calculations. We find characteristic features of the interfacial reaction mechanisms, which are dependent on the plane orientation and wet oxidation condition. The incorporation of wet oxidants leads to lower barrier heights for the reaction of ${\mathrm{O}}_2$ molecules, resulting in the promotion of interfacial reaction at the ${\mathrm{SiO}}_2$/4H-SiC(0001) interface. In contrast, the coexisting ${\mathrm{O}}_2$ molecule assists the reactions by ${\mathrm{H}}_2\mathrm{O}$ molecule and OH groups at the ${\mathrm{SiO}}_2$/4H-SiC($000\overline{1}$) interface. Furthermore, we estimate the linear rate constants in the Deal-Grove model using the calculated barrier heights and reveal that the preferable wet ambient condition at the ${\mathrm{SiO}}_2$/4H-SiC(0001) interface is different from that at the ${\mathrm{SiO}}_2$/4H-SiC($000\overline{1}$) interface. The difference in the rate constants is caused by the difference in the rate-limiting reaction pathway between ${\mathrm{SiO}}_2$/4H-SiC(0001) and ($000\overline{1}$) interfaces. These calculated results offer better understanding of the atom-scale mechanisms for the significant enhancement of SiC oxidation by the interplay of dry and wet oxidants.

Phonon dispersions and linewidths of layered thermoelectric Sb/Bi codoped ${\mathrm{Ge}}_{0.86}{\mathrm{Sb}}_{0.08}{\mathrm{Bi}}_{0.06}\mathrm{Te}$ covering 300 to 630 K in the $R3m$ phase are mapped by inelastic neutron scattering measurement. The acoustic phonons depart greatly from the harmonic frequencies with downturns in dispersion revealing soft phonon energies at larger wave vectors for phonons propagating in the crystallographic [110] direction. Two components that scatter phonons are identified. One is smaller in magnitude having a linear wave vector $q$ dependency that links to the three-phonon scattering. The dominant component for phonon softening has a wave vector square $q^2$ dependency with a Fermi-Dirac thermal reduction rate linked to the electron-phonon scattering. The scattering of phonons is so strong that they only propagate over a few unit cells in length, with lifetimes as short as ∼1 ps. Our results reveal the origin of the extremely low thermal conductivity of ${\mathrm{Ge}}_{0.86}{\mathrm{Sb}}_{0.08}{\mathrm{Bi}}_{0.06}\mathrm{Te}$ in the $R3m$ phase.

Single crystals of the three-dimensional frustrated magnet and spin liquid candidate ${\mathrm{PbCuTe}}_2{\mathrm{O}}_6$ were grown using both the traveling solvent floating zone and the top-seeded solution growth (TSSG) techniques. The growth conditions were optimized by investigating the thermal properties. The quality of the crystals was checked by polarized optical microscopy, x-ray Laue, and x-ray powder diffraction and compared to the polycrystalline samples. Excellent quality crystals were obtained by the TSSG method. Magnetic measurements of these crystals revealed a small anisotropy for different crystallographic directions in comparison with the previously reported data. The heat capacity of both single-crystal and powder samples reveal a transition anomaly around 1 K. Curiously the position and magnitude of the transition are strongly dependent on the crystallite size, and it is almost absent for the smallest crystallites. A structural transition is suggested which accompanies the reported ferroelectric transition, and a scenario whereby it becomes energetically unfavorable in small crystallites is proposed.

The coupling of a superconductor (SC) to a different material often results in a system with unconventional superconducting properties. A conventional SC is a perfect diamagnet expelling magnetic fields out of its volume, a phenomenon known as the Meissner effect. Here, we show that the simple adsorption of a monolayer of chiral molecules (ChMs), which are nonmagnetic in solution, onto the surface of a conventional SC can markedly change its diamagnetic Meissner response. By measuring the internal magnetic field profile in superconducting Nb thin films under an applied transverse field by low-energy muon spin rotation spectroscopy, we demonstrate that the local field profile inside Nb is considerably modified upon molecular adsorption in a way that also depends on the applied field direction. The modification is not limited to the ChMs/Nb interface, but it is long ranged and occurs over a length scale comparable with the superconducting coherence length. Zero-field muon spin spectroscopy measurements in combination with our theoretical analysis show that odd-frequency spin-triplet states induced by the ChMs are responsible for the modification of the Meissner response observed inside Nb. These results indicate that a ChMs/SC system supports odd-frequency spin-triplet pairs due to the molecules acting as a spin-active layer, and therefore, they imply that such a system can be used as a simpler alternative to SC/ferromagnet or SC/topological insulator hybrids for the generation and manipulation of unconventional spin-triplet superconducting states.

Ir-based materials have drawn much attention due to the observation of insulating phase believed to be driven by spin-orbit coupling, while Ir $5d$ states are expected to be weakly correlated due to their large orbital extensions. ${\mathrm{IrO}}_2$, a simple binary material, shows a metallic ground state which seems to deviate from the behavior of most other Ir-based materials and varied predictions in this material class. We studied the electronic structure of ${\mathrm{IrO}}_2$ at different temperatures, employing high-resolution photoemission spectroscopy with photon energies spanning from ultraviolet to hard $x$-ray range. Experimental spectra exhibit a signature of enhancement of Ir-O covalency in the bulk compared to the surface electronic structure. The branching ratio of the spin-orbit split Ir core-level peaks is found to be larger than its atomic values and it enhances further in the bulk electronic structure. Such deviation from the atomic description of the core-level spectroscopy manifests the enhancement of the orbital moment due to the uncompensated electric field around Ir sites. The valence-band spectra could be captured well within the density functional theory. The photon energy dependence of the features in the valence-band spectra and their comparison with the calculated results show dominant Ir $5d$ character of the features near the Fermi level; O $2p$ peaks appear at higher binding energies. Interestingly, the O $2p$ contributions of the feature at the Fermi level are significant, and it enhances at low temperatures. This reveals an orbital selective enhancement of the covalency with cooling, which is an evidence against the purely spin-orbit coupling based scenario proposed for these systems.

Grain boundaries have been shown to dramatically influence the behavior of relatively simple materials such as monatomic metals and binary alloys. The increased chemical complexity associated with multiprincipal element alloys is hypothesized to lead to unique grain boundary phenomena. To explore the relationship between grain boundary structure and chemistry in these materials, hybrid molecular dynamics/Monte Carlo simulations of a faceted $\mathrm{\Sigma }11$ $\langle 110\rangle$ tilt boundary, chosen to sample both high- and low-energy boundary configurations, are performed in face-centered cubic (fcc) CrFeCoNiCu and CrFeCoNi equiatomic alloys. Unexpected enrichment of Fe is discovered in the fcc regions adjacent to the interface and found to be correlated with a structurally distinct region of reduced atomic volume. Comparison with the boundary of the same type in monatomic Cu demonstrates that altered near-boundary regions exist in simpler systems as well, with the chemical complexity of the multiprincipal element alloys highlighting its existence and importance.

Honeycomb structure has frequently been observed in two-dimensional (2D) materials. CuAu in the buckled honeycomb (BHC) structure was synthesized recently, which is a case of a 2D intermetallic compound. Here, the dynamical stability of 2D $AX$ in the BHC structure, where $A=\mathrm{Cu}$, Ag, and Au, and $X$ is a metallic element in the Periodic Table, is systematically studied by calculating phonon dispersions from first-principles. Among 135 $AX$, 48 $AX$ are identified to be dynamically stable. In addition, we discuss (i) the relationship between the dynamical stability and the formation energy, (ii) a correlation of the dynamical stability between different constituents $A$, and (iii) trends of the structural, elastic, electronic, and magnetic properties. Furthermore, a stable phase of the B11-type AuZr is predicted based on both the result (ii) and the stability relationship between 2D and 3D structures. The stability of B11 AuZr is consistent with other predictions using the machine learning approach. The present findings should stimulate future studies exploring the physics and chemistry of 2D intermetallic compounds.

We measured the heat capacity, magnetic susceptibility, magnetization, magnetoelectric effect, and optical spectra of samarium chromium borate crystals. It was found that, in $\mathrm{Sm}{\mathrm{Cr}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ single crystals, structures with the space groups $R32$ and $C2/c$ may coexist. Magnetic phase transitions were discovered at temperatures $T_{\mathrm{N}1}=7.8\pm 0.5\mathrm{K}$, $T_{\mathrm{N}2}=6.7\pm 0.5\mathrm{K}$, and $T_3=4.3\pm 0.2\mathrm{K}$, and assumptions about their nature were made. Based on the high-resolution optical spectra and magnetometry data, the crystal-field parameters in the ${\mathrm{Sm}}^{3+}$ and ${\mathrm{Cr}}^{3+}$ positions and the parameters of ${\mathrm{Cr}}^{3+}\text{−}{\mathrm{Cr}}^{3+}$ and ${\mathrm{Cr}}^{3+}\text{−}{\mathrm{Sm}}^{3+}$ exchange interactions were determined. The magnetic properties of the quasi-one-dimensional chromium subsystem were analyzed in the frame of the previously developed self-consistent four-particle cluster model. Despite a strong suppression of ${\mathrm{Cr}}^{3+}$ magnetic moments by the interchain antiferromagnetic exchange interactions, the contributions of the chromium subsystem to the bulk magnetization are dominant.

Ultrafast vibrational dynamics of inorganic-organic hybrid lead halide perovskite $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ in the room-temperature tetragonal phase is investigated under weak photoexcitation at the band edge. Time-resolved optical Kerr effect signals exhibit periodic modulations at 4 and 8 THz due to the libration and twisting of the methylammonium molecule, in addition to the inorganic sublattice deformation at 1.2 THz. These oscillations are heavily damped and resemble the short-lived ground-state oscillation reported in a recent THz-pump study [Z. Liu et al., Phys. Rev. B 101, 115125 (2020)]. The vibrational frequencies exhibit blueshifts with increasing pump fluence, contrary to the expectation from conventional laser heating. Our ab initio lattice dynamics simulations reveal strongly anharmonic vibrational potentials for the zone-center optical phonon modes, and thereby confirm the anharmonicity to be the origin of the experimentally observed frequency blueshifts.

The spin anomalous Hall effect (SAHE) in ferromagnetic metals, which can generate a spin-orbit torque to rotate the magnetization of another ferromagnetic layer through a nonmagnetic spacer in magnetic junctions, has attracted much attention. We theoretically investigated the spin anomalous Hall conductivity (SAHC) of the $L{1}_0$-type alloys $X\mathrm{Pt}$ ($X=\mathrm{Fe},\mathrm{Co},\mathrm{Ni}$) on the basis of first-principles density functional theory and linear response theory. We found that the SAHC of FePt is much smaller than the anomalous Hall conductivity (AHC), leading to very small polarization for the anomalous Hall effect $\zeta =\mathrm{SAHC}/\mathrm{AHC}$ of around 0.1. On the other hand, the SAHC increases with an increasing number of valence electrons ($N_{\mathrm{v}}$), and CoPt and NiPt show relatively large values of $\left|\zeta \right|$, greater than 1. The negative contribution of the spin-down-down component of AHC is the origin of the large SAHC and $\zeta$ in CoPt and NiPt, which is due to the antibonding states of Pt around the Fermi level in the minority-spin states.

Bulk metallic glasses (BMGs) surpass the strength of steels and possess the elasticity and formability of polymers. The key to obtain these properties is to conserve the amorphous structure of a metallic melt and avoid crystallization during processing. In this work, a change in the crystallization sequence in the widely used BMG ${\mathrm{Zr}}_{52.5}{\mathrm{Cu}}_{17.9}{\mathrm{Ni}}_{14.6}{\mathrm{Al}}_{10}{\mathrm{Ti}}_5$ (Vit105) upon the addition of sulfur (${\mathrm{Zr}}_{51.45}{\mathrm{Cu}}_{17.54}{\mathrm{Ni}}_{14.31}{\mathrm{Al}}_{9.8}{\mathrm{Ti}}_{4.9}{\mathrm{S}}_2$; Vit105 ${\mathrm{S}}_2$) is revealed by in situ high-energy x-ray diffraction, both upon heating from the glassy state and upon cooling from the liquid state during electrostatic levitation. This methodology proves to be a powerful tool to elucidate the complete crystallization behavior of complex BMG-forming liquids. The experiments show that the addition of sulfur changes the crystallization sequence and phases in a different manner upon cooling from the liquid state than upon heating from the glassy state. The thermal stability at low temperatures upon heating is increased, as the supercooled liquid region is extended from 60 to 77 K and the glass transition temperature increases from 671 to 692 K. However, the thermal stability is decreased upon cooling, causing a reduced glass-forming ability.

Two-dimensional van der Waals (2D vdW) single layered materials with ferromagnetism and piezoelectricity have been a subject of recent attention. Despite numerous reports of 2D ferromagnetic materials, developing an air stable and transferable vdW material has been challenging. To address this problem, we studied layered transition metal silicates that are derivatives of kaolinites and lizardites with transition metals substituting on ${\mathrm{Al}}^{3+}$ and ${\mathrm{Mg}}^{2+}$ sites using ab initio density functional theory calculations. This class of materials is appealing because they meet the symmetry requirements for piezoelectricity and can host a range of transition metal cations. As oxides, these materials are inherently stable in air. Following our previous experimental work, we predict that these compounds are stable under varying ${\mathrm{O}}_2$ partial pressure and can be synthesized using a surface-assisted method. We also show that the oxidation states of the substituted transition metal ions can be tuned through the level of hydrogenation.

The two-dimensional honeycomb lattice is an important system for investigating exotic quantum physics. Boron, the neighboring element of carbon, may form a flat monolayer honeycomb lattice, namely the honeycomb-borophene ($h$-B), on the surface of specific materials. In this work, we evaluate the potential of eight fcc metal materials, including fcc Ge, Nb, Ta, Ag, Ti, W, Al, and Mo, as the possible substrate for epitaxial $h$-B growth through the first-principles calculations. We find that Al(111) is one of the good candidates, while other fcc(111) surfaces of Nb, Ta, Ti, W, and Mo are also very likely to serve as viable substrates to support the $h$-B growth, since their film-substrate binding energies are even larger than that on Al(111). It has been identified that there is a strong charge accumulation effect between $h$-B and the metal substrate to stabilize the plane $h$-B. Combining the analysis from the thermodynamical and the electronic aspects, it is likely that the fcc phase of the Ta and W, other than Al, can serve as the best substrates for $h$-B growth, owing to their large adhesive energy ($E_{ad}$) and strong charge-transferring effect between $h$-B and substrate. However, it is nearly impossible to obtain a freestanding $h$-B for the same reason.

${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ hosts a novel $J_{\mathrm{eff}}=1/2$ Mott state and quasi-two-dimensional antiferromagnetic order, providing a unique avenue of exploring emergent states of matter and functions that are extraordinarily sensitive to any structural variations. Although the correlation between the physical property and the lattice structure in ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ has been a focused issue in the past decade, a common perception assumes that the magnetic ordering is essentially determined by the Ir-O-Ir bond angle. Therefore, a delicate modulation of this angle and, consequently, a major modulation of the magnetic ordering by chemical doping, such as Ga at the Ir site, has been extensively investigated and well believed. In this paper, however, we present a whole package of structure and magnetism data on a series of single-crystal and polycrystalline ${\mathrm{Sr}}_2{\mathrm{Ir}}_{1-x}{\mathrm{Ga}}_x{\mathrm{O}}_4$ samples, revealing the substantial difference in the Néel temperature $T_N$ between the two types of samples, and the $T_N$ value for the polycrystalline sample $x=0.09$ is even 64 K higher than that of the single-crystal sample $x=0.09$ ($\mathrm{\Delta }{T}_{\mathrm{N}}\sim 64\mathrm{K}$ at $x=0.09$). Our systematic investigations demonstrate the crucial role of the c/a ratio in tuning the interlayer coupling and thereby the Néel point $T_N$, i.e., a higher $T_N$ can be achieved as c/a is reduced. The notable differences in structural parameters between the two groups of samples are probably caused by additional strain due to the massive grain boundaries in polycrystalline samples. The present paper suggests an additional ingredient of physics that is essential in modulating the emergent properties in ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ and probably other iridates.

Ferromagnet/heavy metal (FM/HM) multilayer thin films with $C_{2v}$ symmetry have the potential to host antiskyrmions and other chiral spin textures via an anisotropic Dzyaloshinkii-Moriya interaction (DMI). Here, we present a candidate material system that also has a strong uniaxial magnetocrystalline anisotropy aligned in the plane of the film. This system is based on a new Co/Pt epitaxial relationship, which is the central focus of this work: hexagonal closed-packed $\mathrm{Co}\left(10\overline{1}0\right)\left[0001\right]$ $\parallel$ face-centered cubic $\mathrm{Pt}\left(110\right)\left[001\right]$. We characterized the crystal structure and magnetic properties of our films using x-ray diffraction techniques and magnetometry, respectively, including $q$-scans to determine stacking fault densities and their correlation with the measured magnetocrystalline anisotropy constant and thickness of Co. In future ultrathin multilayer films, we expect this epitaxial relationship to further enable an anisotropic DMI while supporting interfacial perpendicular magnetic anisotropy. The anticipated confluence of these properties, along with the tunability of multilayer films, make this material system a promising testbed for unveiling new spin configurations in FM/HM films.

We present a density functional theory study of the low-temperature structural, magnetic, and proposed charge-quadrupolar ordering in the double perovskite, ${\mathrm{Ba}}_2\mathrm{Mg}\mathrm{Re}{\mathrm{O}}_6$. ${\mathrm{Ba}}_2\mathrm{Mg}\mathrm{Re}{\mathrm{O}}_6$ is a spin-orbit-driven Mott insulator with a symmetry-lowering structural phase transition at 33 K and a canted antiferromagnetic ordering of $5d^1$ Re magnetic moments at 18 K. Our calculations confirm the existence of the proposed charge quadrupolar order and reveal an additional, previously hidden, ordered charge quadrupolar component. By separately isolating the structural distortions and the orientations of the magnetic dipoles, we determine the relationship between the charge quadrupolar, structural, and magnetic orders, finding that either a local structural distortion or a specific magnetic dipole orientation is required to lower the symmetry and enable the existence of charge quadrupoles. Our paper establishes the crystal structure–magnetic dipole–charge multipole relationship in ${\mathrm{Ba}}_2\mathrm{Mg}\mathrm{Re}{\mathrm{O}}_6$ and related $5d^1$ double perovskites, and illustrates a method for separating and analyzing the contributions and interactions of structural, magnetic, and charge orders beyond the usual dipole level.

By means of in situ synchrotron x-ray diffraction and Raman spectroscopy under hydrostatic pressure, we investigate the structural stability of the quadruple perovskite $\mathrm{La}{\mathrm{Mn}}_7{\mathrm{O}}_{12}$. At 34 GPa, the data unveil a first-order structural phase transition from monoclinic $I2/m$ to cubic $Im\overline{3}$ symmetry characterized by a pronounced contraction of the unit cell and by a significant modification in the Raman phonon modes. The phase transition is also marked by the suppression of a Jahn-Teller distortion which is present in the ambient monoclinic phase. In addition, above 20 GPa pressure, a sudden and simultaneous broadening is observed in several Raman modes which suggests the onset of a sizable electron-phonon interaction and incipient charge mobility. Considering that $\mathrm{La}{\mathrm{Mn}}_7{\mathrm{O}}_{12}$ is a paramagnetic insulator at ambient conditions, and that the Jahn-Teller distortion is frozen in the high-pressure $Im\overline{3}$ phase, we argue that this phase could be a potential candidate to host a purely electronic insulator-metal transition with no participation of the lattice.

To better understand the many effects of temperature on the optical properties of metals, we experimentally and theoretically quantify the electron vs phonon contributions to the thermoreflectance spectra of gold. We perform a series of pump/probe measurements on nanoscale Pt/Au bilayers at wavelengths between 400 and 1000 nm. At all wavelengths, we find that changes in phonon temperature, not electron temperature, are the primary contributor to the thermoreflectance of Au. The thermoreflectance is most sensitive to the electron temperature at a wavelength of ∼480 nm due to interband transitions between $d$ states and the Fermi level. At 480 nm, the electron temperature is responsible for ∼20% of the total thermoreflectance. In the near infrared, the electron temperature is responsible for $<2\%$ of the total thermoreflectance. We also compute the thermoreflectance spectra of Au from first principles. Our calculations further confirm that phonon temperature dominates thermoreflectance of Au. Most of the thermoreflectance of Au is due to the effect of the phonon population on electron lifetime.