We use density functional theory (DFT) calculations to investigate structural models consisting of anatase $\mathrm{Ti}{\mathrm{O}}_2\left(101\right)$ slabs covered by reduced overlayers formed by (101) crystallographic shear planes (CSPs). Ab initio thermodynamics supports the stability of these structures under a wide range of experimental conditions. The overlayers are found to have $\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_3$ stoichiometry with a crystal structure different from the known corundumlike $\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_3$ (here denoted a $\alpha \text{−}\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_3$) phase. DFT calculations predict this new “$\mathrm{csp}\text{−}\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_3$” phase to be energetically close to $\alpha \text{−}\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_3$ and to have also a similar band gap. These results suggest a possible role of the $\mathrm{csp}\text{−}\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_3$ phase in the properties of black $\mathrm{Ti}{\mathrm{O}}_2$, a promising photocatalytic material made of nanoparticles with a crystalline $\mathrm{Ti}{\mathrm{O}}_2$ core and a highly reduced $\mathrm{Ti}{\mathrm{O}}_x$ shell that is capable of absorbing the whole spectrum of visible light.

We propose a crystal structure prediction method based on Bayesian optimization. Our method is classified as a selection-type algorithm which is different from evolution-type algorithms such as an evolutionary algorithm and particle swarm optimization. Crystal structure prediction with Bayesian optimization can efficiently select the most stable structure from a large number of candidate structures with a lower number of searching trials using a machine learning technique. Crystal structure prediction using Bayesian optimization combined with random search is applied to known systems such as NaCl and ${\mathrm{Y}}_2{\mathrm{Co}}_{17}$ to discuss the efficiency of Bayesian optimization. These results demonstrate that Bayesian optimization can significantly reduce the number of searching trials required to find the global minimum structure by 30–40% in comparison with pure random search, which leads to much less computational cost.

The role of $d$ electrons in determining distributions of formation and migration energies for point defects in equiatomic NiCoCr and NiCoFeCr concentrated solid solution alloys (CSAs) are studied regarding electron density deformation flexibility based on first-principles calculations. The disordered state is taken into account by constructing special quasirandom structures. The migration barriers are determined by directly optimizing the saddle point. It is found that the formation energies of interstitials in CSAs are lower than those in pure Ni, whereas the formation energies of vacancies are higher. In both NiCoCr and NiCoFeCr, Co-related dumbbell interstitials exhibit lower formation energies. Notably, the distributions of migration energies for Cr interstitials and vacancies exhibit a remarkable overlap region. A detailed analysis of electronic properties reveals that the electronic charge deformation flexibility regarding $e_g$ to $t_2$_g transition has a dominant effect on defect energetics for different elements in CSAs. Thus the electron deformation ability is suggested as a key factor in understanding the peculiar defect behavior in CSAs.

Evolving cracks in consolidated granular media cause a modification of the microstructure in the area surrounding the damaged zone. As a consequence, the mechanical properties (flexural strength) and the elastic characteristics (linear modulus and nonlinear parameters) change as well. The same happens during repair, which could be considered as the symmetric counterpart of mechanical damaging. Starting from ultrasonic measurements of the resonance and nonlinear properties of concrete prisms during progression of damage and repair, we propose a description of the microstructure evolution, which allows to predict the observed phenomenology. Also, we demonstrate the existence of a correlation between ultrasonic elastic parameters and mechanical properties of the samples at each damage/repair state, pointing out symmetries and differences observed in the two processes.

Bi/Ni bilayers with varying Bi and Ni layer thicknesses have been prepared by (a) pulsed-laser deposition (PLD) at 300 K and (b) thermal evaporation at 4.2 K. A two-step superconducting transition appears on the electrical transport measurements in the samples prepared by PLD. High-resolution transmission and scanning transmission electron microscopy, supported by energy-dispersive x-ray spectroscopy (EDXS) analysis, reveal that two superconducting intermetallic alloys, namely NiBi and ${\mathrm{NiBi}}_3$, are formed by interdiffusion, if the bilayers are prepared at 300 K. The $T_c$ of the two phases behaves very differently in an external magnetic field and the upper critical magnetic fields at zero temperature $\left[{\mathrm{B}}_{c2}\left(0\right)\right]$ were estimated as 1.1 and 7.4 T, respectively. The lower value corresponds to the ${\mathrm{B}}_{c2}$(0) of ${\mathrm{NiBi}}_3$ phase and the higher one is supposed to be of NiBi. These alloys are responsible for the superconductivity and the two-step transition appearing in the Bi/Ni bilayer system. Surprisingly, the Bi-rich phase (${\mathrm{NiBi}}_3$) is formed near the Ni layer, while the Ni-rich phase (NiBi) is formed far from the Ni layer. The EDXS analysis at nanometer scale clearly shows an unusual increase of Ni concentration near the interface of Bi/substrate. The limited thickness of Bi layer in the interdiffusion process results in an unexpected distribution of Ni concentration. Samples prepared at 4.2 K after annealing at 300 K do not show any superconductivity, which indicates that a nonepitaxial Bi/Ni interface does not induce superconductivity in the case interdiffusion does not occur. These results offer a deeper understanding of the superconductivity in the Bi/Ni bilayer system.

We investigate the microscopic mechanism responsible for the change of macroscopic electrical properties of the $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{r}}_2\mathrm{CaC}{\mathrm{u}}_2{\mathrm{O}}_{8+\delta }$ high-temperature superconductor induced by intense synchrotron hard x-ray beams. The possible effects of secondary electrons on the oxygen content via the knock-on interaction are studied by Monte Carlo simulations. The change in the oxygen content expected from the knock-on model is computed convoluting the fluence of photogenerated electrons in the material with the Seitz-Koehler cross section. This approach has been adopted to analyze several experimental irradiation sessions with increasing x-ray fluences. A close comparison between the expected variations in oxygen content and the experimental results allows determining the irradiation regime in which the knock-on mechanism can satisfactorily explain the observed changes. Finally, we estimate the threshold displacement energy of loosely bound oxygen atoms in this material $T_d=0.{15}_{-0.01}^{+0.025}\mathrm{eV}$.

We have performed realistic atomistic simulations at finite temperatures using Monte Carlo and atomistic spin dynamics simulations incorporating quantum (Bose-Einstein) statistics. The description is much improved at low temperatures compared to classical (Boltzmann) statistics normally used in these kind of simulations, while at higher temperatures the classical statistics are recovered. This corrected low-temperature description is reflected in both magnetization and the magnetic specific heat, the latter allowing for improved modeling of the magnetic contribution to free energies. A central property in the method is the magnon density of states at finite temperatures, and we have compared several different implementations for obtaining it. The method has no restrictions regarding chemical and magnetic order of the considered materials. This is demonstrated by applying the method to elemental ferromagnetic systems, including Fe and Ni, as well as Fe-Co random alloys and the ferrimagnetic system ${\mathrm{GdFe}}_3$.

An essential property of magnetic devices is the relaxation rate in magnetic switching, which depends strongly on the damping in the magnetization dynamics. It was recently measured that damping depends on the magnetic texture and, consequently, is a nonlocal quantity. The damping enters the Landau-Lifshitz-Gilbert equation as the phenomenological Gilbert damping parameter $\alpha$, which does not, in a straightforward formulation, account for nonlocality. Efforts were spent recently to obtain Gilbert damping from first principles for magnons of wave vector $\mathit{q}$. However, to the best of our knowledge, there is no report about real-space nonlocal Gilbert damping ${\alpha }_{ij}$. Here, a torque-torque correlation model based on a tight-binding approach is applied to the bulk elemental itinerant magnets and it predicts significant off-site Gilbert damping contributions, which could be also negative. Supported by atomistic magnetization dynamics simulations, we reveal the importance of the nonlocal Gilbert damping in atomistic magnetization dynamics. This study gives a deeper understanding of the dynamics of the magnetic moments and dissipation processes in real magnetic materials. Ways of manipulating nonlocal damping are explored, either by temperature, materials doping, or strain.

Doping bismuth selenide ($\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$) with elements such as copper and strontium (Sr) can induce superconductivity, making the doped materials interesting candidates to explore potential topological superconducting behaviors. It was thought that the superconductivity of doped $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ was induced by dopant atoms intercalated in van der Waals gaps. However, several experiments have shown that the intercalation of dopant atoms may not necessarily make doped $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ superconducting. Thus, the structural origin of superconductivity in doped $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ remains an open question. Herein, we combined material synthesis and characterization, high-resolution transmission electron microscopy, and first-principles calculations to study the doping structure of Sr-doped $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$. We found that the emergence of superconductivity is strongly related with $n$-type dopant atoms. Atomic-level energy-dispersive x-ray mapping revealed various $n$-type Sr dopants that occupy intercalated and interstitial positions. First-principles calculations showed that the formation energy of a specific interstitial Sr doping position depends strongly on the Sr doping level. This site changes from a metastable position at low Sr doping level to a stable position at high Sr doping level. The calculation results explain why quenching is necessary to obtain superconducting samples when the Sr doping level is low and also why slow furnace cooling can yield superconducting samples when the Sr doping level is high. Our findings suggest that Sr atoms doped at interstitial locations, instead of those intercalated in van der Waals gaps, are most likely to be responsible for the emergence of superconductivity in Sr-doped $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$.

Alignment of anisometric particles into liquid crystals (LCs) often results from an entropic competition between their rotational and translational degrees of freedom at dense packings. Here we show that by selectively functionalizing the heads of colloidal rods with magnetic nanoparticles this tendency can be broken to direct the particles into novel, low-density LC phases. Under an external magnetic field, the magnetic heads line up in columns whereas the nonmagnetic tails point out randomly in a plane perpendicular to the columns, forming bottle-brush-like objects; laterally, the bottle brushes are entropically stabilized against coalescence. Experiments and simulations show that upon increasing the particle density the system goes from a dilute gas to a dense two-dimensional liquid of bottle brushes with a density well below the zero-field nematic phase. Our findings offer a strategy for self-assembly into three-dimensional open phases that may find applications in switchable photonics, filtration, and light-weight materials.

First-principles-based thermodynamic modeling of cubic $\alpha$ and $\beta$ phases of Mn represent a challenge due to their structural complexity and the necessity of simultaneous treatment of several types of disorder (electronic, magnetic, and vibrational) that have very different characteristic time scales. Here we employ mean-field theoretical models to describe the different types of disorder and then we connect each layer of theory to the others using the adiabatic principle of separating faster and slower degrees of freedom. The slowest (vibrational) degrees of freedom are treated using the Moruzzi, Janak, and Schwarz formalism [Phys. Rev. B 37, 790 (1988)] of the Debye-Grüneisen model parametrized based on the first-principles calculated equation of state which includes the free-energy contributions due to the fast (electronic and magnetic) degrees of freedom via the Fermi-Dirac distribution function and a mean-field theory of transverse spin fluctuations. The magnetic contribution due to transverse spin fluctuations has been computed self-consistently within the disordered local moment picture of the paramagnetic state. The obtained results for thermodynamic properties such as lattice parameter, linear thermal expansion coefficient, and heat capacity of both phases show a good agreement with available experimental data. We also tested the assumption about the nature (localized versus delocalized) of magnetic moment on site IV in $\alpha \text{−}\mathrm{Mn}$ and site I in $\beta \text{−}\mathrm{Mn}$ on the thermodynamic properties of these two phases. Similar to the findings of experimental studies, we conclude that magnetic moment on site IV in $\alpha \text{−}\mathrm{Mn}$ is not of a localized character. However, a similar analysis suggests that the magnetic moment of site I in $\beta \text{−}\mathrm{Mn}$ should be treated as localized.

Particle-tracking microrheology (PT-$\mu \mathrm{r}$) exploits the thermal motion of embedded particles to probe the local mechanical properties of soft materials. Despite its appealing conceptual simplicity, PT-$\mu \mathrm{r}$ requires calibration procedures and operating assumptions that constitute a practical barrier to its wider application. Here we demonstrate differential dynamic microscopy microrheology (DDM-$\mu \mathrm{r}$), a tracking-free approach based on the multiscale, temporal correlation study of the image intensity fluctuations that are observed in microscopy experiments as a consequence of the translational and rotational motion of the tracers. We show that the mechanical moduli of an arbitrary sample are determined correctly over a wide frequency range provided that the standard DDM analysis is reinforced with an iterative, self-consistent procedure that fully exploits the multiscale information made available by DDM. Our approach to DDM-$\mu \mathrm{r}$ does not require any prior calibration, is in agreement with both traditional rheology and diffusing wave spectroscopy microrheology, and works in conditions where PT-$\mu \mathrm{r}$ fails, providing thus an operationally simple, calibration-free probe of soft materials.

The surface structure of black phosphorus materials is determined using surface-sensitive dynamical microspot low energy electron diffraction ($\mu \mathrm{LEED}$) analysis using a high spatial resolution low energy electron microscopy (LEEM) system. Samples of (i) crystalline cleaved black phosphorus (BP) at 300 K and (ii) exfoliated few-layer phosphorene (FLP) of about 10 nm thickness which were annealed at 573 K in vacuum were studied. In both samples, a significant surface buckling of 0.22 Å and 0.30 Å, respectively, is measured, which is one order of magnitude larger than previously reported. As direct evidence for large buckling, we observe a set of (for the flat surface forbidden) diffraction spots. Using first-principles calculations, we find that the presence of surface vacancies is responsible for the surface buckling in both BP and FLP, and is related to the intrinsic hole doping of phosphoresce materials previously reported.

The magnetization of thin films of cobalt ferrite frequently falls far below the bulk value of $455\mathrm{kA}{\mathrm{m}}^{-1}$, which corresponds to an inverse cation distribution in the spinel structure with a significant orbital moment of about $0.6{\mu }_{\mathrm{B}}$ that is associated with the octahedrally coordinated $\mathrm{C}{\mathrm{o}}^{2+}$ ions. The orbital moment is responsible for the magnetostriction and magnetocrystalline anisotropy and its sensitivity to imposed strain. We have systematically investigated the structure and magnetism of films produced by pulsed-laser deposition on different substrates ($\mathrm{Ti}{\mathrm{O}}_2$, MgO, $\mathrm{MgA}{\mathrm{l}}_2{\mathrm{O}}_4$, $\mathrm{SrTi}{\mathrm{O}}_3$, LSAT, $\mathrm{LaAl}{\mathrm{O}}_3$) and as a function of temperature $(500-700{}^{\circ }\mathrm{C})$ and oxygen pressure $({10}^{-4}-10\mathrm{Pa})$. Magnetization at room-temperature ranges from 60 to $440\mathrm{kA}{\mathrm{m}}^{-1}$, and uniaxial substrate-induced anisotropy ranges from $+220\mathrm{kJ}{\mathrm{m}}^{-3}$ for films on deposited on MgO (100) to $-2100\mathrm{kJ}{\mathrm{m}}^{-3}$ for films deposited on $\mathrm{MgA}{\mathrm{l}}_2{\mathrm{O}}_4$ (100), where the room-temperature anisotropy field reaches 14 T. No rearrangement of high-spin $\mathrm{F}{\mathrm{e}}^{3+}$ and $\mathrm{C}{\mathrm{o}}^{2+}$ cations on tetrahedral and octahedral sites can reduce the magnetization below the bulk value, but a switch from $\mathrm{F}{\mathrm{e}}^{3+}$ and $\mathrm{C}{\mathrm{o}}^{2+}$ to $\mathrm{F}{\mathrm{e}}^{2+}$ and low-spin $\mathrm{C}{\mathrm{o}}^{3+}$ on octahedral sites will reduce the low-temperature magnetization to $120\mathrm{kA}{\mathrm{m}}^{-1}$, and a consequent reduction of Curie temperature can bring the room-temperature value to near zero. Possible reasons for the appearance of low-spin cobalt in the thin films are discussed.

NiO is a canonical Mott (or charge-transfer) insulator and, as such, is notoriously difficult to describe using density functional theory (DFT)–based electronic structure methods. Doped Mott insulators such as NiO are of interest for various applications but rigorous theoretical descriptions are lacking. Here, we use quantum Monte Carlo methods, which very accurately include electron-electron interactions, to examine energetics, charge structures, and spin structures of NiO with various point defects, such as vacancies and substitutional doping with potassium. The formation energy of a potassium dopant is significantly lower than that of a Ni vacancy, making potassium an attractive monovalent dopant for NiO. We compare our results with DFT results that include an on-site Hubbard $U$ ($\mathrm{DFT}+U$) to account for correlations and find relatively large discrepancies for defect formation energies as well as for charge and spin redistributions in the presence of point defects. Beyond fitting to a single property, it is unlikely that single-parameter tuning of the $\mathrm{DFT}+U$ will be able to obtain accurate accounts of complex properties in these materials. Responses that depend in subtle and complex ways on ground-state properties, such as charge and spin densities, are likely to contain quantitative and qualitative errors.