Diamond has attracted attention as a next-generation semiconductor because of its various exceptional properties such as a wide bandgap and high breakdown electric field. Diamond field-effect transistors, for example, have been extensively investigated for high-power and high-frequency electronic applications. The quality of their charge transport (i.e., mobility), however, has been limited due to charged impurities near the diamond surface. Here, we fabricate diamond field-effect transistors by using a monocrystalline hexagonal boron nitride as a gate dielectric. The resulting high mobility of charge carriers allows us to observe quantum oscillations in both the longitudinal and Hall resistivities. The oscillations provide important information on the fundamental properties of the charge carriers, such as effective mass, lifetime, and dimensionality. Our results indicate the presence of a high-quality two-dimensional hole gas at the diamond surface and thus pave the way for studies of quantum transport in diamond and the development of low-loss and high-speed devices.

In this work, we comprehensively investigate and discuss the structural, magnetostatic, dynamic, and magnetoresistive properties of epitaxial Co/Mo superlattices. The magnetization of the Co sublayers is coupled antiferromagnetically with a strength that depends on the thickness of the nonmagnetic Mo spacer. The magnetization and magnetoresistance hysteresis loops clearly reflect interlayer exchange coupling and the occurrence of uniaxial magnetic anisotropy induced by the strained Co sublayers. Upon accounting for a deviation of the sublayer thicknesses from the nominal value, theoretical modeling, including both micromagnetic and macrospin approaches, precisely reproduces experimental magnetic hysteresis loops, magnetoresistance curves, and ferromagnetic resonance dispersion relations. The interlayer magnetic coupling as a function of the Mo spacer thickness is determined as a fitting parameter by modeling the experimental results.

We revisited ab initio evaluations of the barrier energies along the possible diffusion paths of the defects in rutile ${\mathrm{TiO}}_2$ by using a diffusion Monte Carlo method. We found that Ti interstitials hopping along the $c$ axis are identified as the major diffusion directing to the (001) surface, contradicting any of the previous density functional theory studies. Our finding reasonably explains recent experiments reporting that the photocatalytic activity in the (001) surface is superior to that in the (110) surface: The faster Ti diffusion directing to the (001) surface leads to the better self-compensation ability and maintains its photocatalytic activity.

${\mathrm{Bi}}_{1-\mathit{x}}{\mathrm{Sr}}_{\mathit{x}}{\mathrm{FeO}}_{3-\delta }$ (BSFs) are well known not only as multiferroic materials but also as mixed oxide-ion and electronic conductors. BSFs show remarkable ionic conductivity despite their limited electrochemical activity due to their low electronic conductivity. In this study, the discontinuous change in the electrical conductivity of ${\mathrm{Bi}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{FeO}}_{3-\delta }$ (BSF30) at around 770 ${}^{\circ }\mathrm{C}$ was investigated in the context of their unique defect order-disorder transition. Based on the electrical conductivity and the Seebeck coefficient of BSF30, both functions of temperature and oxygen partial pressure, the discontinuous change in both hole and oxide-ion conductivities was the result of a reduction in their mobility but unrelated to their concentration term. High-temperature x-ray diffraction and scanning transmission electron microscopy revealed that periodic oxygen-deficient planes with fivefold $d$-spacing of $\left\{001\right\}$ emerge in an ordered low-temperature phase while the ordered defect structure disappears in a high-temperature phase. The discontinuous change of electrical conductivity of BSF30 was then found to be the result of the order-disorder transition of its oxygen sublattice, i.e., oxygen vacancies. Because there was no such transition in the cation sublattice, the cation did not affect the electrical conductivity of BSF30. This study suggests that BSFs with the desired electrochemical and multiferroic functionalities can be designed by controlling their order-disorder transition.

We fabricated spin-polarized surface electronic states with tunable Fermi levels from a semiconductor to low-dimensional metal in the Bi/GaSb(110)-$(2\times 1)$ surface using angle-resolved photoelectron spectroscopy (ARPES) and spin-resolved ARPES. The spin-polarized surface band of Bi/GaSb(110) exhibits quasi-one-dimensional character with the Rashba parameter ${\alpha }_{\mathrm{R}}$ of 4.1 and 2.6 eVÅ at the $\overline{\mathrm{\Gamma }}$ and $\overline{\mathrm{Y}}$ points of the surface Brillouin zone, respectively. The Fermi level of the surface electronic state is tuned in situ by element-selective Ar-ion sputtering on the GaSb substrate. The giant Rashba-type spin splitting with switchable metallic/semiconducting character on a semiconductor substrate makes this system a promising candidate for future research in low-dimensional spintronic phenomena.

We report the single-crystal growth and characterization of a hexagonal phase ${\mathrm{Cu}}_{0.82}{\mathrm{Mn}}_{1.18}\mathrm{As}$, in the Cu-Mn-As system. This compound contains the same square-pyramidal ${\mathrm{MnAs}}_5$ units as the tetragonal and orthorhombic polymorphs of CuMnAs. Calorimetry, magnetometry, and neutron diffraction measurements reveal antiferromagnetic ordering at 270 K. The magnetic structure consists of a triangular arrangement of spins in the $ab$ plane. Hexagonal ${\mathrm{Cu}}_{0.82}{\mathrm{Mn}}_{1.18}\mathrm{As}$ shows resistivity that varies only weakly from 5 to 300 K, and is many times higher than tetragonal CuMnAs, indicative of a strongly scattering metal. First-principles calculations confirm the metallic band structure with a small density of states at the Fermi energy. The neutron-refined magnetic ground state is close to the computationally determined minimum energy configuration. This compound should serve as a clear control when disentangling the effects of current-driven Néel switching of metallic antiferromagnets since it exhibits in-plane spins but the magnetic ordering does not break degeneracy along the $a$ and $b$ directions, unlike tetragonal CuMnAs.

The quantum nature of hydrogen bonds in water manifests itself in peculiar physicochemical isotope effects: While deuteration often elongates and weakens the hydrogen bonds of typical hydrogen-bonded systems composed of bulky constituent molecules, it elongates but strengthens the hydrogen bonds of water molecular aggregates. The origin of this unique isotope effect of water molecules remains to be elucidated at the molecular level. By means of isotope-selective measurements on the sublimation of water ices with various H/D compositions, we disentangle two opposite deuteration effects on the hydrogen-bond breaking process of water molecules: (1) Deuterating a desorbing water molecule increases the energy needed for desorption $E_d$, while (2) deuterating water molecules neighboring a desorbing molecule reduces its $E_d$. The increase in $E_d$ originates from zero-point energy in the hindered rotation of the desorbing molecule, whereas the decrease in $E_d$ is caused by quantum anharmonic couplings between the inter- and intramolecular vibrational modes involved in the hydrogen-bonding interactions of desorbing water molecules. On the basis of these findings, we discuss the peculiar nature of hydrogen bonds of water molecules in comparison with bulky hydrogen-bonded molecules.

We investigate the dielectric response in the glass-electret state of two dipolar glass-forming materials. This unusual polar glassy state of matter is produced when a dipolar liquid is supercooled under the influence of a high electric dc field, which leads to partial orientational order of the molecules carrying a dipole moment. Investigation of the prepared glass electrets by using low-field dielectric spectroscopy reveals a clear modification of their dielectric response in the regime of the Johari-Goldstein β relaxation, pointing to a small but significant increase of its relaxation strength compared to the normal glass. We discuss the implications of this finding for the still controversial microscopic interpretation of the Johari-Goldstein relaxation, an inherent property of glassy matter.

Research on low-dimensional materials has increased drastically in the last decade, with the discovery of two-dimensional transition metal carbides and nitrides (MXenes) produced by atom-selective chemical etching of laminated parent $M_{n+1}A{X}_n$ (MAX) phases. Here, we apply density functional theory and subsequent materials synthesis and analysis to explore the phase stability and Mo/Sc intermixing on the M site in the chemically ordered quaternary $i-\mathrm{MAX}$ phase (${\mathrm{Mo}}_x{\mathrm{Sc}}_{1-x}{)}_2\mathrm{AlC}$. Transmission electron microscopy confirms the theoretical predictions of preferential in-plane ordering of Mo and Sc, with the highest crystal quality obtained for the ideal Mo:Sc ratio of 2:1 (predicted as the most stable), as well as a retained $i-\mathrm{MAX}$ structure even for an increased relative Sc content, with Sc partially occupying Mo sites. The results are supported by refined neutron diffraction data, which show space group $C2/c$ (no. 15), and a C occupancy of 1. Subsequent chemical etching produces MXene for $x=0.66$, while for $x=0.33$ and 0.5 no MXene is observed. These results demonstrate that a precise control of the $i-\mathrm{MAX}$ composition is crucial for derivation of MXene, with a MXene quality optimized for a Mo:Sc ratio of 2:1 with minimal intermixing between Mo and Sc.

A method suitable for computing the ideal strength of random substitutional alloys is introduced. The method relies on nonlinear continuum elasticity theory and allows for the high-throughput computation of ideal strength. The method also allows for the high-throughput computation of an intrinsic ductility parameter defined for a given applied stress state as the ratio of the strain associated with the cleavage instability to the strain associated with the first shear instability. The intrinsic ductility parameter is shown to correlate well with the measured elongations to failure for elemental body-centered-cubic and hexagonal close-packed metals. Application to four high-entropy alloys indicates that the intrinsic ductility parameter describes their experimental compressions to failure well. The method is used to argue that the brittle refractory high-entropy alloy Ta-Nb-V-W-Mo could be made much more ductile through replacement of Mo with Nb. The potential for the high-throughput optimization of high entropy and chemically complex alloys is discussed.

This paper describes the development of a third-generation charge optimized many-body (COMB3) potential for Al-C and its application to the investigation of aluminum/graphene nanostructures. In particular, the new COMB3 potential was used to investigate the interactions of aluminum surfaces with pristine and defective graphene sheets. Classical molecular dynamics simulations were performed at temperatures of 300–900 K to investigate the structural evolution of these interfaces. The results indicate that, although the interfaces between Al and graphene are mostly weakly bonded, aluminum carbide can form under the right conditions, including the presence of vacancy defects in graphene, undercoordinated Al in surface regions with sharp boundaries, and at high temperatures. COMB3 potentials were further used to examine a new method to transfer graphene between Al surfaces as well as between Al and Cu surfaces by controlling the angle of graphene between the two surfaces. The findings indicate that, by controlling the peeling angles, it is possible to transfer graphene without any damage from the surface having greater graphene/surface adhesion to another surface with less adhesion.

Small polarons and point defects in ${\mathrm{BaTiO}}_3$ are investigated using hybrid functional calculations. Based on the experimentally-confirmed order-disorder-type phase transitions, Ti displacements along $\langle 111\rangle$ directions are included in the cubic model. We reveal that the self-trapped electrons at Ti sites are stable in both rhombohedral and cubic ${\mathrm{BaTiO}}_3$ and the Ti off-centering, which introduces antibonding hybridization between lowest-lying $\mathrm{Ti}\text{−}3d$ and $\mathrm{O}\text{−}2p$ orbitals at the conduction band minimum, is essential for stabilizing the self-trapped electrons. Our calculations are in contrast to previous theoretical studies, even qualitatively, but reasonably consistent with the long-standing experimentally-observed small polarons in ${\mathrm{BaTiO}}_3$. This finding may explain why self-trapped electrons are not stable in ${\mathrm{SrTiO}}_3$ but are in ${\mathrm{BaTiO}}_3$ from the symmetry viewpoint.

Ion transport in solid materials is frequently facilitated by defects. It is thus attractive to increase ionic conductivity by introducing proper types of defects. In antiperovskite ${\mathrm{Li}}_3\mathrm{OCl}$, a promising solid-state electrolyte material, a Li interstitial is predicted to be superior to Li vacancy for Li ion transport. However, the Li interstitial is difficult to dope into ${\mathrm{Li}}_3\mathrm{OCl}$ and it typically comes with a charge-compensating ${\mathrm{O}}_{\mathrm{Cl}}^{\prime }$ substitutional defect, which is a Li trap and thus hampers Li ion transport. In this study, a protocol to enhance Li interstitial formation via S doping is proposed. With a ${\mathrm{S}}_{\mathrm{Cl}}^{\prime }$ charge-compensating defect, the Li interstitial becomes more stable than Li vacancy. At the same time, thanks to a cancellation of two different contributions, ${\mathrm{S}}_{\mathrm{Cl}}^{\prime }$ has little effect on the Li local electrostatic environment and thus ion transport. As a result, Li interstitial conductivity can be boosted from $3.822\times {10}^{-4}$ to 1.286 mS/cm upon S doping, which leads to a 1-order-of-magnitude increase of the overall ionic conductivity. Such a significant performance enhancement via defect engineering opens an alternative avenue for the design of battery materials.

This paper presents a proof-of-concept demonstration of triaxial strain tomography from Bragg-edge neutron imaging within a three-dimensional sample. Bragg-edge neutron transmission can provide high-resolution images of the average through thickness strain within a polycrystalline material. This poses an associated rich tomography problem which seeks to reconstruct the full triaxial strain field from these images. The presented demonstration is an important step toward solving this problem, and toward a technique capable of studying the residual strain and stress within engineering components. A Gaussian process based approach is used that ensures the reconstruction satisfies equilibrium and known boundary conditions. This approach is demonstrated experimentally on a nontrivial steel sample with use of the RADEN instrument at the Japan Proton Accelerator Research Complex. Validation of the reconstruction is provided by comparison with conventional strain scans from the KOWARI constant-wavelength strain diffractometer at the Australian Nuclear Science and Technology Organisation and simulations via finite-element analysis.

In this paper we report the synthesis, magnetization, and heat capacity of the frustrated magnets $A{\mathrm{ErSe}}_2$($A=\mathrm{Na},\mathrm{K}$) which contain perfect triangular lattices of ${\mathrm{Er}}^{3+}$. The magnetization data suggests no long-range magnetic order exists in $A{\mathrm{ErSe}}_2$($A=\mathrm{Na},\mathrm{K}$), which is consistent with the heat capacity measurements. Large anisotropy is observed between the magnetization within the $ab$ plane and along the $c$ axis of both compounds. When the magnetic field is applied along the $ab$ plane, anomalies are observed at 1.8 ${\mu }_B$ in ${\mathrm{NaErSe}}_2$ at 0.2 T and 2.1 ${\mu }_B$ in ${\mathrm{KErSe}}_2$ at 0.18 T. Unlike ${\mathrm{NaErSe}}_2$, a plateaulike field-induced metamagnetic transition is observed for $\mathrm{H}\parallel c$ below 1 K in ${\mathrm{KErSe}}_2$. Two broad peaks are observed in the heat capacity below 10 K indicating possible crystal electric field (CEF) effects and magnetic entropy released under different magnetic fields. All results indicate that $A{\mathrm{ErSe}}_2$ are strongly anisotropic, frustrated magnets with field-induced transition at low temperature. The lack of signatures for long-range magnetic order implies that these materials are candidates for hosting a quantum spin liquid ground state.

Epitaxial ${\mathrm{Fe}}_4\mathrm{N}$ thin films grown on ${\mathrm{LaAlO}}_3$ (LAO) substrate using sputtering and molecular beam epitaxy techniques have been studied in this work. Within the sputtering process, films were grown with conventional direct current magnetron sputtering (dcMS) and using a high power impulse magnetron sputtering (HiPIMS) process. Surface morphology and depth profile studies on these samples reveal that HiPIMS deposited film has the lowest roughness, the highest packing density, and the sharpest interface. We found that the substrate-film interface and the microstructure play a vital role in affecting the electronic hybridization and magnetic properties of ${\mathrm{Fe}}_4\mathrm{N}$ films. La from the LAO substrate and Fe from the film interdiffuse and form an undesired interface. The magnetic moment ($M_s$) was compared using bulk, element-specific and magnetic depth profiling techniques. We found that $M_s$ was the highest when the thickness of the interdiffused layer was lowest and such conditions can only be achieved in the HiPIMS grown samples. The presence of a small moment at the N site was also evidenced by element-specific x-ray circular dichroism measurement in the HiPIMS grown sample. A large variation in the $M_s$ values of ${\mathrm{Fe}}_4\mathrm{N}$ films found in the experimental works carried out so far could be due to such an interdiffused layer which is generally not expected to form in otherwise stable oxide substrate at a low substrate temperature $\approx 675\mathrm{K}$. In addition, a consequence of substrate-film interdiffusion and microstructure resulted in the different kinds of magnetic anisotropies in ${\mathrm{Fe}}_4\mathrm{N}$ films grown using different techniques. A detailed investigation of the substrate-film interface and microstructure on the magnetization of ${\mathrm{Fe}}_4\mathrm{N}$ film is presented and discussed in this work.

An antiferromagnetic NiO spiral wall in $\mathrm{Fe}/\mathrm{NiO}/\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{N}{\mathrm{i}}_{0.5}\mathrm{O}/\mathrm{vicinal}$ Ag(001) was created by rotating Fe magnetization and investigated using x-ray magnetic linear dichroism (XMLD). Different from the Mauri's 180° spiral wall, we find that the NiO spiral wall always switches its chirality at $\sim {90}^{\circ }$ rotation of the Fe magnetization, and unwinds the spiral wall back to a single domain with a further rotation of the Fe magnetization from 90° to 180°. The effect of this chirality switching on the magnetic anisotropy was studied using rotational magneto-optic Kerr effect (ROTMOKE) on $\mathrm{Py}/\mathrm{NiO}/\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{N}{\mathrm{i}}_{0.5}\mathrm{O}/\mathrm{vicinal}$ Ag(001). We find that the original Mauri's model has to be corrected by an energy folding due to the chirality switching, which consequently converts the exchange bias from the Mauri's 180° spiral wall into a uniaxial anisotropy and a negative fourfold anisotropy.

We analyze the effect of interphase boundary anisotropy on the dynamics of lamellar eutectic solidification fronts, in the limit that the lamellar spacing varies slowly along the envelope of the front. In the isotropic case, it is known that the spacing obeys a diffusion equation, which can be obtained theoretically by making two assumptions: (i) The lamellae always grow normal to the large-scale envelope of the front, and (ii) the Jackson-Hunt law that links lamellar spacing and front temperature remains locally valid. For anisotropic boundaries, we replace hypothesis (i) by the symmetric pattern approximation, which has recently been found to yield good predictions for lamellar growth direction in presence of interphase anisotropy. We obtain a generalized Jackson-Hunt law for tilted lamellae, and an evolution equation for the envelope of the front. The latter contains a propagative term if the initial lamellar array is tilted with respect to the direction of the temperature gradient. However, the propagation velocity of the propagative wave modes are found to be small, so that the dynamics of the front can be reasonably described by a diffusion equation with a diffusion coefficient that is modified with respect to the isotropic case.

Single crystals of the single kagome layer compound FeSn are investigated using x-ray and neutron scattering, magnetic susceptibility and magnetization, heat capacity, resistivity, Hall, Seebeck, thermal expansion, thermal conductivity measurements, and density functional theory (DFT). FeSn is a planar antiferromagnet below $T_{\mathrm{N}}=365\mathrm{K}$ and exhibits ferromagnetic magnetic order within each kagome layer. The in-plane magnetic susceptibility is sensitive to synthesis conditions. Resistivity, Hall and Seebeck results indicate multiple bands near the Fermi energy. The resistivity of FeSn is ≈3 times lower for current along the stacking direction than in the plane, suggesting that transport and the bulk electronic structure of FeSn is not quasi-two-dimensional (2D). FeSn is an excellent metal with ρ(300 K)/ρ(2 K) values ≈100 in both directions. While the ordered state is antiferromagnetic, high temperature susceptibility measurements indicate a ferromagnetic Curie-Weiss temperature of 173 K, reflecting the strong in-plane ferromagnetic interactions. DFT calculations show a 3D electronic structure with the Dirac nodal lines along the K-H directions in the magnetic Brillouin zone about 0.3 eV below the Fermi energy, with the Dirac dispersions at the $K$ points gapped by spin-orbit coupling except at the $H$ point. The magnetism, however, is highly 2D with $J_{\mathrm{in}\text{−}\mathrm{plane}}/J_{\mathrm{out}\text{−}\mathrm{of}\text{−}\mathrm{plane}}\approx 10$. The predicted spin-wave spectrum is presented.