We study the occurrence of symmetry-enforced topological band crossings in tetragonal crystals with strong spin-orbit coupling and time-reversal symmetry. By computing the momentum dependence of the symmetry eigenvalues and the global band topology in the entire Brillouin zone, we determine all symmetry-enforced band crossings in tetragonal space groups. In particular, we classify all Dirac and Weyl degeneracies on points, lines, and planes, and find a rich variety of topological degeneracies. This includes, among others, double Weyl points, fourfold double Weyl points, fourfold quadruple Weyl points, Weyl and Dirac nodal lines, as well as topological nodal planes. For the space groups with symmetry-enforced Weyl points, we determine the minimal number of Weyl points for a given band pair and, remarkably, find that materials in space groups 119 and 120 can have band pairs with only two Weyl points in the entire Brillouin zone. This simplifies the contribution of the Weyl points to the topological responses, which would be useful for device applications. Using the classification of symmetry-enforced band crossings, we perform an extensive database search for candidate materials with tetragonal space groups. Notably, we find that ${\mathrm{Ba}}_5{\mathrm{In}}_4{\mathrm{Bi}}_5$ and ${\mathrm{NaSn}}_5$ exhibit twofold and fourfold Weyl nodal lines, respectively, which cross the Fermi energy. ${\mathrm{Hf}}_3\mathrm{Sb}$ and ${\mathrm{Cs}}_2{\mathrm{Tl}}_3$ have band pairs with few number of Weyl points near the Fermi energy. Furthermore, we show that ${\mathrm{Ba}}_3{\mathrm{Sn}}_2$ has Weyl points with an accordion dispersion and topological nodal planes, while AuBr and ${\mathrm{Tl}}_4{\mathrm{PbSe}}_3$ possess Dirac points with hourglass dispersions. For each of these candidate materials we present the ab initio band structures and discuss possible experimental signatures of the nontrivial band topology.

We have investigated magnetic anisotropy properties of ferromagnetic semiconductor ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x{\mathrm{As}}_{1-y}{\mathrm{P}}_y$ films grown by molecular beam epitaxy on GaAs substrates with constant Mn content $x$ of 0.06 and with P content $y$ graded along the growth direction. Two samples were investigated, one with $y$ increasing in the growth direction from 0.03 to 0.24, the other with $y$ decreasing from 0.25 to 0.04. Such grading of phosphorus concentration leads to a continuous variation of strain, and thus of magnetic anisotropy in the film. Although the phosphorus mole fraction in the films was varied by nearly the same amount in both “forward”- and “reverse”-graded samples, their magnetic anisotropy properties are entirely different. Specifically, while the forward-graded specimen can be described in terms of three distinct magnetic layers in which the magnetic easy axes progress from in plane to out of plane as the phosphorus concentration increases, the reverse-graded sample shows only an out-of-plane anisotropy throughout the entire film. This implies that the initial conditions at the nucleation of sample growth determine the magnetic properties of the entire graded ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x{\mathrm{As}}_{1-y}{\mathrm{P}}_y$ film.

We report the synthesis of a quasi-one-dimensional (1D) iron selenide ${\mathrm{Ba}}_9{\mathrm{Fe}}_3{\mathrm{Se}}_{15}$. It was synthesized at high temperature and high pressure of 5.5 GPa and systematically studied via structural, magnetic, and transport measurements at both ambient and high pressure. ${\mathrm{Ba}}_9{\mathrm{Fe}}_3{\mathrm{Se}}_{15}$ crystallizes in a monoclinic structure and consists of face-sharing $\mathrm{Fe}{\mathrm{Se}}_6$ octahedral chains running in $c$-direction. It shows a sizable unquenched orbital moment and undergoes a ferromagnetic-like phase transition at 14 K. At ambient pressure, ${\mathrm{Ba}}_9{\mathrm{Fe}}_3{\mathrm{Se}}_{15}$ exhibits an insulating behavior with a band gap of ∼460 meV. Under high pressure, a complete metallization occurs at ∼29 GPa, which is accompanied by a spin state crossover from high-spin to low-spin state.

Hybrid materials of MXenes [two-dimensional (2D) carbides and nitrides] and transition-metal oxides have shown great promise in electrical energy storage (EES) and 2D heterostructures have been proposed as the next-generation electrode materials to expand the limits of current technology. Here we use first principles density functional theory to investigate the interfacial structure, energetics, and electronic properties of the heterostructures of MXenes (${\mathrm{Ti}}_{\mathrm{n}+1}{\mathrm{C}}_{\mathrm{n}}{T}_2$; T = terminal groups) and anatase $\mathrm{Ti}{\mathrm{O}}_2$. We find that the greatest work-function differences are between OH-terminated MXene (1.6 eV) and anatase $\mathrm{Ti}{\mathrm{O}}_2\left(101\right)$ (6.4 eV), resulting in the largest interfacial electron transfer ($\sim 0.9e/\mathrm{n}{\mathrm{m}}^2$ across the interface) from MXene to the $\mathrm{Ti}{\mathrm{O}}_2$ layer. This interface also has the strongest adhesion and is further strengthened by hydrogen bond formation. For O–, F–, or mixed O–/F– terminated ${\mathrm{Ti}}_{\mathrm{n}+1}{\mathrm{C}}_{\mathrm{n}}$ MXenes, electron transfer is minimal and interfacial adhesion is weak for their heterostructures with $\mathrm{Ti}{\mathrm{O}}_2$. The strong dependence of the interfacial properties of the $\mathrm{MXene}/\mathrm{Ti}{\mathrm{O}}_2$ heterostructures on the surface chemistry of the MXenes will be useful to tune the heterostructures for EES applications.

We investigate polaronic effects in a monolayer black phosphorus (or phosphorene) due to the charge carrier–intrinsic phonons interaction. We employ a microscopic model to describe vibrational spectra and electron-phonon interactions within the tight-binding approach. Our adiabatic solution for polarons suggests the critical coupling constant of ${\lambda }_c=3.0$ for polaron self-trapping in this anisotropic material. We report nonadiabatic calculations for the band-gap renormalization and the effective mass enhancement as a function of temperature and a range of electron-phonon coupling strengths. Our results demonstrate a surprising large band-gap renormalization, even for the lower bound of the electron-phonon coupling strength reported in the literature. We find that optical phonons give the dominant contributions to the polaronic effect.

The interplay between unconventional superconductivity and quantum critical ferromagnetism in the U-Ge compounds represents an open problem in strongly correlated electron systems. Sample quality can have a strong influence on both of these ordered states in the compound UCoGe, as is true for most unconventional superconductors. We report results of a different approach at UCoGe crystal growth using a floating-zone method with potential for improvements of sample quality and size as compared with traditional means such as Czochralski growth. Single crystals of the ferromagnetic superconductor UCoGe were produced using an ultra-high vacuum electron-beam floating-zone refining technique. Annealed single crystals show well-defined signatures of bulk ferromagnetism and superconductivity at $T_c\sim 2.6\mathrm{K}$ and $T_s\sim 0.61\mathrm{K}$, respectively, in the resistivity and heat capacity. Scanning electron microscopy of samples with different surface treatments shows evidence of an off-stoichiometric uranium-rich phase of UCoGe collected in cracks and voids that might be limiting sample quality.

The quantum anomalous Hall (QAH) effect has recently been realized in thin films of intrinsic magnetic topological insulators (IMTIs) like $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$. Here we point out that the QAH gaps of these IMTIs can be optimized and that both axion insulator/semimetal and Chern insulator/semimetal transitions can be driven by electrical gate fields on the $\sim 10$ meV/nm scale. This effect is described by combining a simplified coupled-Dirac-cone model of multilayer thin films with Schrödinger-Poisson self-consistent-field equations.

High-strength metal alloys achieve their performance via careful control of the nucleation, growth, and kinetics of precipitation. Alloy mechanical properties are then controlled by atomic scale phenomena such as shearing of the precipitates by dislocations. Atomistic modeling to understand the operative mechanisms requires length and timescales far larger than those accessible by first-principles methods. Here, a family of Behler-Parinello neural network potentials (NNPs) for the Al-Mg-Si system is developed to enable quantitative studies of Al-6xxx alloys. The NNP is trained on metallurgically important quantities computed by first-principles density function theory (DFT) leading to high-fidelity predictions of intermetallic compounds, elastic constants, dilute solid-solution energetics, precipitate/matrix interfaces, Al stacking fault energies, antisite defect energies, and other quantities. The generalized stacking fault energy surfaces for the three prevalent ${\beta }^{\prime \prime }$ precipitate compositions in peak-aged Al-6xxx are then computed with the NNP, and are validated by DFT computations at key points. A preliminary examination of early stage clustering kinetics and energetics in Al-6xxx is then made, showing the formation of low-energy Mg-Si structures and the trapping of vacancies in these clusters. The NNP thus shows significant transferability across structures, making it a powerful approach for chemically accurate simulations of metallurgical phenomena in Al-Mg-Si alloys.

Soft, amorphous solids such as tissues, foams, and emulsions are composed of deformable particles. However, the effect of single-particle deformability on the collective behavior of soft solids is still poorly understood. We perform numerical simulations of two-dimensional jammed packings of explicitly deformable particles to study the mechanical response of model soft solids. We find that jammed packings of deformable particles with excess shape degrees of freedom possess low-frequency quartic vibrational modes that stabilize the packings even though they possess fewer interparticle contacts than the nominal isostatic value. Adding intraparticle constraints can rigidify the particles, but these particles undergo a buckling transition and gain an effective shape degree of freedom when their preferred perimeter is above a threshold value. We find that the mechanical response of jammed packings of deformable particles with shape degrees of freedom differs significantly from that of jammed packings of rigid-shape particles, which emphasizes the importance of particle deformability in modeling soft solids.

The phase stability of a bcc AlNbTiV high-entropy alloy at elevated temperatures is studied using a combination of machine-learning interatomic potentials, first-principles calculations, and Monte Carlo simulations. The simulations reveal a B2 ordering below about 1700 K, mainly caused by a strong site preference of Al and Ti. A much weaker site preference for V and Nb is observed, strongly affecting the alloys total configurational entropy. The underlying mechanisms of the B2 phase stability as opposed to the random solid solution are discussed in terms of a high persisting configurational entropy of the B2 phase due to strong sublattice site disorder.

The accurate prediction of solid-solid structural phase transitions at finite temperature is a challenging task, since the dynamics is so slow that direct simulations of the phase transitions by first-principles (FP) methods are typically not possible. Here, we study the $\alpha \text{−}\beta$ phase transition of Zr at ambient pressure by means of on-the-fly machine-learned force fields. These are automatically generated during FP molecular dynamics (MD) simulations without the need of human intervention, while retaining almost FP accuracy. Our MD simulations successfully reproduce the first-order displacive nature of the phase transition, which is manifested by an abrupt jump of the volume and a cooperative displacement of atoms at the phase transition temperature. The phase transition is further identified by the simulated x-ray powder diffraction, and the predicted phase transition temperature is in reasonable agreement with experiment. Furthermore, we show that using a singular value decomposition and pseudo inversion of the design matrix generally improves the machine-learned force field compared to the usual inversion of the squared matrix in the regularized Bayesian regression.

Unlike potassium trichlorocuprate ${\mathrm{KCuCl}}_3$, the isostructural ammonium trichlorocuprate ${\mathrm{NH}}_4{\mathrm{CuCl}}_3$ exhibits the rather unusual effect of orientation-independent steps in the magnetization $\left(M\right)$ as a function of the applied magnetic field $\left(H\right)$. However, replacement of ${\mathrm{NH}}_4^{+}$ by the isomeric ${\mathrm{K}}^{+}$ ion results in a smearing of these steps. With the view of probing this change, we carried out on the mixed system ${\left({\mathrm{NH}}_4\right)}_x{\mathrm{K}}_{1-x}{\mathrm{CuCl}}_3$, with $x=0.0, 0.3, 0.6, 0.9, \mathrm{and} 1.0:$ (1) High-field pulsed measurements of $M$ versus $H$ at 1.5 K, (2) susceptibility measurements from 1.8 to 295 K and (3) zero-field specific heat measurements from 1.8 to 295 K. The magnetization steps were found to be highly sensitive to the ${\mathrm{NH}}_4^{+}$ ion concentration, with the spin gap $\mathrm{\Delta }$ generally decreasing with increasing $x$. Analysis of the susceptibility and specific heat data with several theoretical schemes also supports this general trend. This includes data analysis using the Bleaney-Bowers, Ising chain, and the mean-field model. These results yielded intradimer spin-exchange constants of the order of 20–40 K, somewhat smaller than expected for ${\mathrm{Cu}}^{2+}-{\mathrm{Cu}}^{2+}$. This, in addition to the variation of $x$ with $\mathrm{\Delta }$, points to the need for additional theoretical modeling.

Soft materials often have interesting and unexpected frictional behavior owing to their deformable nature. We use soft polydimethylsiloxane (PDMS) surfaces lubricated by hard glass spheres to study how this deformability influences particle-based lubrication. For particles between 100 and 2000 $\mu \mathrm{m}$ in size, we observe a nontrivial rate dependence and three frictional regimes: (I) a rolling friction regime where the rolling particles keep the surfaces apart sufficiently to give low friction coefficients—this is mainly found for large particles and smooth surfaces; (II) a sliding friction regime with high friction coefficients where the surfaces are partially in contact, which is found for small particles, rough surfaces, and high normal forces; (III) a PDMS-PDMS contact regime where the particles are fully inserted into surfaces and the surfaces are in contact. We interpret the friction dynamics in terms of the Hertzian contact deformation effects in the indentation of the PDMS surfaces.

The potential of a wide range of layered ternary carbide and nitride Mn+1AXn [an early transition metal (M), an element of columns 13 or 14 of the periodic table (A), and either C or N (X)] phases as conductors in interconnect metal lines in advanced complementary metal-oxide-semiconductor (CMOS) technology nodes has been evaluated using automated first-principles simulations based on density-functional theory. The resistivity scaling potential of these compounds, i.e., the expected sensitivity of their resistivity to reduced line dimensions, has been benchmarked against Cu and Ru by evaluating their transport properties within a semiclassical transport formalism. In addition, their cohesive energy has been assessed as a proxy for the resistance against electromigration and the need for diffusion barriers. The results indicate that numerous MAX phases show promise as conductors in interconnects of advanced CMOS technology nodes.

The magnetic properties of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanoparticle assemblies have been investigated in detail through a combination of vibrating sample magnetometry (VSM) and muon spin relaxation ($\mu \mathrm{S}\mathrm{R}$) techniques. Two samples with average particle sizes of 5 and 20 nm, respectively, were studied. For both samples, the VSM and $\mu \mathrm{S}\mathrm{R}$ results exhibit clear signatures of superparamagnetism at high temperature and magnetic blocking at low temperature. The $\mu \mathrm{S}\mathrm{R}$ data demonstrate that the transition from the superparamagnetic to the blocked state occurs gradually throughout the sample volume over an extended temperature range due to the finite particle size distribution of each nanoparticle batch. The transition occurs between approximately 3 and 45 K for the 5-nm nanoparticles and 150 and 300 K for the 20-nm nanoparticles. The VSM and $\mu \mathrm{S}\mathrm{R}$ data are further analyzed to yield estimates of microscopic magnetic parameters including the nanoparticle spin-flip activation energy $E_A$, magnetic anisotropy $K$, and intrinsic nanoparticle spin reversal attempt time ${\tau }_0$. These results highlight the complementary information about magnetic nanoparticles that can be obtained by bulk magnetic probes such as magnetometry and local magnetic probes such as $\mu \mathrm{S}\mathrm{R}$.

Two $\mathrm{CaMn}B\mathrm{Sb}{\mathrm{O}}_6$ ($B=\mathrm{Cr}$ and Fe) high-pressure double double perovskites are reported. Spins in $\mathrm{Ca}\mathrm{Mn}\mathrm{Cr}\mathrm{Sb}{\mathrm{O}}_6$ order below $T_{\mathrm{C}}=49\mathrm{K}$ into a collinear ferrimagnetic arrangement with spins in the $xy$ plane. $\mathrm{Ca}\mathrm{Mn}\mathrm{Fe}\mathrm{Sb}{\mathrm{O}}_6$ has the same ordered magnetic structure below $T_{\mathrm{C}1}=55$ K, but also shows a second magnetic transition at $T_{\mathrm{C}2}=21\mathrm{K}$ where the spins reorient towards the $z$ axis. This may reflect a greater degree of magnetic disorder for $B=\mathrm{Fe}$ and A-B intersite charge transfer may also be significant.

$\mathrm{Cu}\left(\mathrm{In},\mathrm{Ga}\right){\mathrm{S}}_2$ is a promising semiconductor that offers excellent prospects for photovoltaics. However, the performance has been plagued mostly due to large photovoltage deficit. Here, we investigate defects and optoelectronic properties of 1.7-eV band gap near-stoichiometric $\mathrm{Cu}\left(\mathrm{In},\mathrm{Ga}\right){\mathrm{S}}_2$ (CIGS). We have estimated quasi-Fermi-level splitting of 921 meV from steady state photoluminescence (PL) measurements at 1 sun. Detailed analysis of temperature and excitation dependent PL reveals the behavior of a strongly compensated semiconductor. We show that spatially varying energetic disorder, described by electrostatic potential fluctuations, causes band-tail recombination and strongly affects the carrier recombination in compensated CIGS. Apart from band-tail transitions, we also observe two deep defects at about 0.3 and 0.45 eV below the band edge. The defects are also discerned by admittance measurements. Temperature dependent current-voltage measurements show that the open-circuit voltage is further limited by interface recombination. Thus, the performance improvement lies in the mitigation of deep defect states and absorber/buffer interface optimization.

Euler buckling is the elastic instability of a column subjected to longitudinal compression forces at its ends. The buckling instability occurs when the compressing load reaches a critical value, and an infinitesimal fluctuation leads to a large amplitude deflection. Since Euler's original study, this process has been extensively examined in homogeneous, isotropic, linear-elastic solids. Here, we examine the nature of the buckling in inhomogeneous soft composite materials. In particular, we consider a soft host with liquid inclusions both large and small relative to the elastocapillarity length, which lead to softening and stiffening, respectively, of a homogeneous composite. However, by imposing a gradient of the inclusion volume fraction or by varying the inclusion size we can deliberately manipulate the spatial structure of the composite properties of a column and thereby control the nature of Euler buckling.

Here we report the growth of laterally strain-free stanene, that is a two-dimensional (2D) honeycomb structure of tin atoms, on a Pd(111) crystal terminated by a ${\mathrm{Pd}}_2\mathrm{Sn}$ surface alloy. The atomic geometry and the electronic structure have been thoroughly investigated by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), Auger electron spectroscopy, high-resolution synchrotron radiation photoemission spectroscopy, and advanced first principles calculations. The STM images clearly reveal the epitaxial growth of honeycomb stanene on the preformed ${\mathrm{Pd}}_2\mathrm{Sn}$ surface alloy. LEED patterns clearly show commensurate (√3×√3)R30° spots, corresponding to a lattice constant of 0.47 nm, in perfect accord with the cell size of free-standing stanene. The measured very low buckling, within 20 pm, is derived from section profiles of atomic-scale STM images. This contrasts the significantly larger buckling of free-standing stanene, possibly due to a STM tip effect as well as a non-negligible interaction with the underlying surface alloy. The electronic structure exhibits a characteristic 2D band with parabolic dispersion, which is in good accordance with electronic structure calculations in density functional theory.