The microscopic justification for the dimerization in ladder-type rare-earth higher borides $R{\mathrm{B}}_{50}$ assumed by Mori [in Handbook on the Physics and Chemistry of Rare Earths, edited by K. A. Gschneidner, Jr., J.-C. G. Bünzli, and V. K. Pecharsky (North-Holland, Amsterdam, 2008), Vol. 38, p. 105] is presented. We consider a phenomenological model of crystal-field interactions and introduce Heisenberg antiferromagnetic exchange interactions between the rare-earth ions on the ladder rungs and the indirect exchange fields generated by magnetic interactions along the ladder legs. The crystal-field parameters and the exchange integrals for the rare-earth dimers are determined from modeling the temperature dependences of the heat capacity and static magnetic susceptibility. The field dependence of the low-temperature magnetization of polycrystalline samples of borides $R{\mathrm{B}}_{50}$ is simulated by making use of the proposed empirical expression for the renormalized magnetic moments of rare-earth ions. The characteristic anomalies of the thermal and magnetic properties of $\mathrm{Tb}{\mathrm{B}}_{50}, \mathrm{Dy}{\mathrm{B}}_{50}, \mathrm{Ho}{\mathrm{B}}_{50}$ and $\mathrm{Er}{\mathrm{B}}_{50}$ are successfully reproduced within the framework of this approach.

The interplay between groups of water molecules and single-atom contacts, as reflected in the electrical conductances and mechanical forces of copper atomic junctions, is explored by means of first-principles theory and semiempirical calculations. We study the influence of the atomic geometries of copper electrodes with pyramidal and noncrystalline structures in the presence and absence of water on the conductance profiles as the electrodes approach each other. It is shown that the atomic arrangements of nanocontacts have crucial effects on the formation of plateaus and the conductance values. Groups of hydrogen bonded water molecules bridge the junction electrodes before a direct Cu-Cu contact between the electrodes is made. However, the bridging of the two copper electrodes by a single ${\mathrm{H}}_2\mathrm{O}$ molecule only occurs in the junctions with pyramidal electrodes. Our findings reveal that the presence of ${\mathrm{H}}_2\mathrm{O}$ molecules modifies strongly the conductance profile of these junctions. In the absence of water molecules, the pyramidal junctions exhibit continuous transitions between integer conductance plateaus, while in the presence of ${\mathrm{H}}_2\mathrm{O}$ molecules, these junctions show abrupt jump to contact behavior and no well-defined conductance plateaus. By contrast, in the absence of ${\mathrm{H}}_2\mathrm{O}$ molecules, the noncrystalline junctions display jump to contact behavior and no well-defined plateaus, while in the presence of ${\mathrm{H}}_2\mathrm{O}$ molecules they exhibit a jump to contact and abrupt transitions between fractional and integer plateaus.

Although chemical vapor deposition (CVD) is one of the preferred routes to synthetic diamond crystals, a full knowledge of the point defects produced during growth is still incomplete. Here we exploit the charge and spin properties of nitrogen vacancy (NV) centers in type-1b CVD diamond to expose an optically and magnetically dark point defect, so far virtually unnoticed despite an abundance comparable to (if not greater than) that of substitutional nitrogen. Indirectly detected photoluminescence spectroscopy indicates a donor state 1.6 eV above the valence band, although the defect's microscopic structure and composition remain elusive. Our results may prove relevant to the growing set of applications that rely on CVD-grown single-crystal diamond.

Charge-transfer-induced interfacial ferromagnetism and its impact on the exchange bias effect in ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3/{\mathrm{NdNiO}}_3$ correlated oxide heterostructures were investigated by soft x-ray absorption and x-ray magnetic circular dichroism spectra in a temperature range from 10 to 300 K. Besides the antiferromagnetic ${\mathrm{Ni}}^{3+}$ cations which are naturally part of the ${\mathrm{NdNiO}}_3$ layer, ${\mathrm{Ni}}^{2+}$ ions are formed at the interface due to a charge-transfer mechanism involving the Mn element of the adjacent layer. They exhibit a ferromagnetic behavior due to the exchange coupling to the ${\mathrm{Mn}}^{4+}$ ions in the ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ layer. This can be seen as detrimental to the strength of the unidirectional anisotropy since a significant part of the interface does not contribute to the pinning of the ferromagnetic layer. By analyzing the line-shape changes of the x-ray absorption at the Ni $L_{2,3}$ edges, the metal-insulator transition of the ${\mathrm{NdNiO}}_3$ layer is resolved in an element-specific manner. This phase transition is initiated at about 120 K, way above the paramagnetic to antiferromagnetic transition of the ${\mathrm{NdNiO}}_3$ layer which measured to be 50 K. Exchange bias and enhanced coercive fields were observed after field cooling the sample through the Néel temperature of the ${\mathrm{NdNiO}}_3$ layer. Different from ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3/{\mathrm{LaNiO}}_3$, the exchange bias observed in ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3/{\mathrm{NdNiO}}_3$ is due to the antiferromagnetism of ${\mathrm{NdNiO}}_3$ and the frustration at the interface. These results suggest that reducing the interfacial orbital hybridization may be used as a tunable parameter for the strength of the exchange bias effect in all-oxide heterostructures which exhibit a charge-transfer mechanism.

The full dielectric tensor as well as the optical activity of potassium titanyl phosphate $\left[{\mathrm{KTiOPO}}_4\left(\mathrm{KTP}\right)\right]$ was investigated in the spectral range from 0.5 to $8.4\mathrm{eV}$ with generalized spectroscopic ellipsometry. Whereas the optical activity is weakly pronounced in the transparent spectral range, it exhibits a significant increase with the onset of absorption above 3.5 eV. By means of a line-shape analysis, the dielectric function and the optical activity were described with the same set of electronic transitions. Based on the determined dielectric tensor and optical activity, the orientation and dispersion of the singular optic axes were investigated. In the absorption region, each optic axis splits generally into two singular optic axes. At $E\approx 5.85$, 6.60, 7.43, and $8.22\mathrm{eV}$ the singular optic axes fall together pairwise and form two classical optic axes, and consequently KTP exhibits a “classical” biaxial behavior at these energies.

We present a method based on sinusoidal approach to equilibrium molecular dynamics (SAEMD) to compute the thermal conductivity of liquids. Similar to nonequilibrium molecular dynamics, and unlike equilibrium simulations based on the Green-Kubo formalism, the method only requires the calculation of forces and total energies. The evaluation of heat fluxes and energy densities is not necessary, thus offering the promise of efficiently implementing first principles simulations based on density functional theory or deep molecular dynamics. Our approach is a generalization of SAEMD for solids, where the thermal conductivity is computed in the steady state, instead of a transient regime, thus properly taking into account diffusive terms in the heat equation. We present results for liquid water at ambient conditions and under pressure and discuss simulation requirements to obtain converged values of the thermal conductivity as a function of size and simulation time.

Rare-earth pyrochlores are known to exhibit exotic magnetic phenomena. We report a study of crystal growth and characterizations of a new rare-earth compound ${\mathrm{Er}}_2{\mathrm{AlSbO}}_7$, in which ${\mathrm{Al}}^{3+}$ and ${\mathrm{Sb}}^{5+}$ ions share the same positions with a random distribution. The magnetism is studied by magnetic susceptibility, specific heat, and thermal conductivity measurements at low temperatures down to several tens of millikelvin. Different from the other reported Er-based pyrochlores exhibiting distinct magnetically ordered states, a spin-freezing transition is detected in ${\mathrm{Er}}_2{\mathrm{AlSbO}}_7$ below 0.37 K, which is primarily ascribed to the inherent structural disorder. A cluster spin-glass state is proposed in view of the frequency dependence of the peak position in the ac susceptibility. In addition, the temperature and field dependence of thermal conductivity indicates rather strong spin fluctuations which is probably due to the phase competition.

Single-crystalline $\mathrm{M}{\mathrm{g}}_3{\mathrm{N}}_2$ thin films are grown on MgO (100) substrates with plasma-assisted molecular beam epitaxy. To prevent the oxidation of the $\mathrm{M}{\mathrm{g}}_3{\mathrm{N}}_2$ films and allow further physical characterization, a polycrystalline MgO cap is deposited in situ. The growth orientation of the $\mathrm{M}{\mathrm{g}}_3{\mathrm{N}}_2$ films can be tuned from (100) to (111) by changing the growth conditions, and the associated epitaxial relationships have been determined by means of x-ray diffraction. The lattice constant of $\mathrm{M}{\mathrm{g}}_3{\mathrm{N}}_2$ films has been monitored as a function of temperature from 300 to 900 K, determining thereby the linear thermal expansion coefficient. Transmission measurements indicate an optical bandgap of crystalline $\mathrm{M}{\mathrm{g}}_3{\mathrm{N}}_2$ around 2.9 eV at room temperature, consistent with diffuse reflectance measurements on micrometric $\mathrm{M}{\mathrm{g}}_3{\mathrm{N}}_2$ particles. These results demonstrate the possibility of exploiting $\mathrm{M}{\mathrm{g}}_3{\mathrm{N}}_2$ as a new crystalline semiconductor in the blue-violet range.

Vacancy centers in diamond have proven to be a viable solid-state platform for quantum-coherent optoelectronic applications. Among the variety of vacancy centers, silicon-vacancy (SiV) centers have recently attracted much attention as an inversion-symmetric system that is less susceptible to electron-phonon interactions. Nevertheless, phonon-mediated processes still degrade the coherent properties of SiV centers; however, characterizing their electron-phonon coupling is extremely challenging due to their weak spectroscopic signatures and remains an open experimental problem. In this paper we theoretically investigate signatures of electron-phonon coupling in simulated linear and nonlinear spectra of SiV centers. We demonstrate how even extremely weak electron-phonon interactions, such as in SiV centers, may be completely characterized via nonlinear spectroscopic techniques and even resolved between different fine-structure transitions.

$\mathrm{SmF}{\mathrm{e}}_{11}\mathrm{Ti}\text{−}\mathrm{based}$ alloys have potential as permanent magnet materials; however, until now, crystallographically textured bulk permanent magnets have not yet been produced from this alloy system. This is partly due to the lack of information on the morphology and composition of grain boundary phases present in the Fe-rich Sm-Fe-Ti alloys. Here we investigated the microstructure of a $\mathrm{S}{\mathrm{m}}_{1.25}\mathrm{F}{\mathrm{e}}_{11}\mathrm{Ti}$ alloy by using correlative transmission electron microscopy and atom-probe tomography, combined with magneto-optical Kerr effect (MOKE) probing to relate the material's micro- and nanostructure to its properties. The grains of the $\mathrm{Sm}{\left(\mathrm{Fe},\mathrm{Ti}\right)}_{12}$ matrix phase are separated by grain boundaries exhibiting a different composition over 3–4 nm width. They contain $>75\mathrm{at}\%$ of the ferromagnetic element Fe, with an enrichment of Sm of up to 16.6 at% and a depletion in Ti, down to approx. 3.4 at%. We believe that the grain boundary is ferromagnetic at room temperature, which makes the magnetic decoupling of the grains practically impossible, which, in turn, leads to a low coercivity of $\mathrm{SmF}{\mathrm{e}}_{11}\mathrm{Ti}$-based alloys. MOKE measurements reveal the strong ferromagnetic coupling across the grain boundary, causing the nucleation of reversal magnetic domains when exposed to low magnetic fields. In a triple-junction area we identified three other ferromagnetic phases: $\mathrm{S}{\mathrm{m}}_3{\left(\mathrm{Fe},\mathrm{Ti}\right)}_{29},\mathrm{SmF}{\mathrm{e}}_2$, and $\mathrm{F}{\mathrm{e}}_2\mathrm{Ti}$. These details bring out the scope of further adjustment of the coercivity in the Sm-Fe-Ti alloy system by grain boundary segregation engineering through the reduction of the presence of ferromagnetic phases to ensure a magnetic decoupling of the micrometer-sized $\mathrm{Sm}{\left(\mathrm{Fe},\mathrm{Ti}\right)}_{12}$ grains.

Recent advances in electrostatic gating provide a novel way to modify the carrier concentration in materials via electrostatic means instead of chemical doping, thus minimizing the impurity scattering. Here, we use first-principles density functional theory combined with a tight-binding approach to compare and contrast the effects of electrostatic gating and Co chemical doping on the ferromagnetic transition of ${\mathrm{FeS}}_2$, a transition-metal disulfide with the pyrite structure. Using tight-binding parameters obtained from maximally localized Wannier functions, we calculate the magnetic susceptibility across a wide doping range. We find that electrostatic gating requires a higher electron concentration than the equivalent in Co doping to induce ferromagnetism via a Stoner-like mechanism. We attribute this behavior to the formation of a narrow Co band near the bottom of the conduction band under chemical doping, which is absent in the electrostatic gating case. Our results reveal that the effects of electrostatic gating go beyond a simple rigid band shift and highlight the importance of the changes in the crystal structure promoted by gating.

Small-angle x-ray scattering (SAXS) was used to quantitatively study the morphology of aligned, monodisperse conical etched ion tracks in thin films of amorphous $\mathrm{SiO}{}_2$ with aspect ratios of around $6:1$ and in polycarbonate foils with aspect ratios of around $1000:1$. This paper presents the measurement procedure and methods developed for the analysis of the scattering images and shows results obtained for the two material systems. To enable accurate parameter extraction from the data collected from conical scattering objects, a model fitting the two-dimensional (2D) detector images was developed. The analysis involved fitting images from a sequence of measurements with different sample tilts to minimize errors, which may have been introduced due to the experimental setup. The model was validated by the exploitation of the geometric relationship between the sample tilt angle and the cone opening angle, to an angle observed in the features of the SAXS images. We also demonstrate that a fitting procedure for 1D data extracted from the scattering images using a hard cylinder model can also be used to extract the cone size. The application of these techniques enables us to reconstruct the cone morphologies with unprecedented precision.

We report on the pressure evolution of the giant anomalous Hall effect (AHE) in the chiral antiferromagnet ${\mathrm{Mn}}_3\mathrm{Ge}$. The AHE originating from the nonvanishing Berry curvature in ${\mathrm{Mn}}_3\mathrm{Ge}$ can be continuously tuned by application of hydrostatic pressure. At room temperature, the Hall signal changes sign as a function of pressure and vanishes completely at $p=1.53\mathrm{GPa}$. Even though the Hall conductivity changes sign upon increasing pressure, the room-temperature saturation value of $23{\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1}$ at 2.85 GPa is remarkably high and comparable to the saturation value at ambient pressure of about $40{\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1}$. The change in the Hall conductivity can be directly linked to a gradual change of the size of the in-plane components of the Mn moments in the noncollinear triangular magnetic structure. Our findings, therefore, provide a route for tuning of the AHE in the chiral antiferromagnetic ${\mathrm{Mn}}_3\mathrm{Ge}$.

We investigated the electronic structures and optical properties of doped anatase $\mathrm{Ti}{\mathrm{O}}_2$ in the infrared region using first-principles density functional theory with the independent-particle approximation. By examining the dopants from groups V-B, VI-B, and VII-A of the Periodic Table of Elements, we found that the anatase $\mathrm{Ti}{\mathrm{O}}_2$ doped with different dopants Nb, Ta, W, and F showed excellent plasmonic properties in the near-infrared region with nonmetal-to-metal crossover frequencies within a range of 2.0–2.7 µm along the densest-plasma direction. Furthermore, we discussed the prospect of producing low-cost robust mid- to far-infrared plasmonic devices based on doped anatase $\mathrm{Ti}{\mathrm{O}}_2$. Our combined analysis with electromagnetic numerical simulation showed that the doped anatase $\mathrm{Ti}{\mathrm{O}}_2$ exhibited well-defined surface plasmon polariton as well as localized surface plasmon resonances in the midinfrared region. We expect that this $n$-type doped anatase $\mathrm{Ti}{\mathrm{O}}_2$ is a highly promising material for infrared plasmonic applications in harsh environments such as aqueous solutions and/or high temperature.

Rare-metal free ferromagnetic material $L{1}_0$ FeNi with large uniaxial magnetic anisotropy has attracted much attention for broad use of magnetic materials in various applications. While single-crystal films of $L{1}_0$ FeNi can be made by layer-by-layer epitaxial growth, magnitude of the uniaxial anisotropy has been lower than expected because of the imperfection of the crystal atomic structure. Here, a nitrogen (N) -surfactant epitaxial growth below 150 K is employed for overcoming interlayer mixing of the atoms at the interface. Ordered $L{1}_0$ FeNi films are successfully prepared on the Cu(001) substrate precovered by a ${\mathrm{Ni}}_2\mathrm{N}$ monolayer. The role of the N surfactant and the absence of the intermixing among Fe and Ni during the deposition and annealing processes were investigated by scanning tunneling microscopy and x-ray photoemission spectroscopy. Element-specific magnetic properties were in situ studied by soft-x-ray magnetic circular dichroism (XMCD). The observed Fe hysteresis curve and XMCD of the nitrogen-covered Ni/Fe/Ni trilayer indicate an out-of-plane magnetocrystalline anisotropy large enough for dominating the film shape anisotropy. The N-surfactant epitaxy provides a useful method for fabricating well-ordered magnetic materials without rare metals.

We report a model-independent atomic-mapping technique for quantum dots (QDs) by combining Bragg reflection x-ray standing wave (XSW) and grazing incidence diffraction (GID) measurements. In this study, we choose GaAs capped InGaAs QDs/GaAs(001) as a model system to show the locations and arrangements of indium atoms within the QDs along various $\left[hkl\right]$ directions. This technique directly reveals the actual amount of positional anisotropy and ordering fraction of indium atoms within the QDs by probing the ($\overline{1}11$), (111), (311), ($\overline{1}31$), (113), and ($\overline{1}13$) crystallographic planes. We find that indium atoms are outwardly shifted along the [001] direction by small fractions of the lattice constant, $0.04a_{\mathrm{GaAs}}$ and $0.06a_{\mathrm{GaAs}}$ from Ga sites for 50- and 150-Å GaAs capped InGaAs QDs, respectively. We observe that an improved coherency factor of the indium atoms within the QDs by 45–60% along the [001] and [011] directions reduces the photoluminescence linewidth by 22%, thus making the QDs efficient for QD-laser and optoelectronic device applications. We also find that the position and ordering of In atoms along the (113) and ($\overline{1}13$) planes are most sensitive to the thickness of the GaAs cap layer. Our XSW-based results are supported by numerical calculations using a QD-macroscopic structural model based on our GID study. We thus show that this atomic-mapping technique will be useful for studying various quantum structures and tuning their properties.

The crystal structure of the candidate topological superconductor ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ was studied by single-crystal neutron diffraction using samples obtained by inserting the Cu dopant electrochemically. Neither structural refinements nor calculated scattering-density maps find a significant occupation of Cu at the intercalation site between the quintuple layers of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Following Bragg reflection intensities as a function of temperature, there is no signature of a structural phase transition between 295 and 2 K. However, the analysis of large sets of Bragg reflections indicates a small structural distortion breaking the rotational axis due to small displacements of the Bi ions.