Low friction is demonstrated with pure polycrystalline tantalum sliding contacts in both molecular dynamics simulations and ultrahigh vacuum experiments. This phenomenon is shown to be correlated with deformation occurring primarily through grain boundary sliding and can be explained using a recently developed predictive model for the shear strength of metals. Specifically, low friction is associated with grain sizes at the interface being smaller than a critical, material-dependent value, where a crossover from dislocation mediated plasticity to grain-boundary sliding occurs. Low friction is therefore associated with inverse Hall-Petch behavior and softening of the interface. Direct quantitative comparisons between experiments and atomistic calculations are used to illustrate the accuracy of the predictions.

The toughness of a polymer material can increase significantly if two networks are combined into one material. This toughening effect is a consequence of a transition from a brittle to a ductile failure response. Although this transition and the accompanying toughening effect have been demonstrated in hydrogels first, the concept has been proven effective in elastomers and in macroscopic composites as well. This suggests that the transition is not caused by a specific molecular architecture, but rather by a general physical principle related to the mechanical interplay between two interpenetrating networks. Here we employ theory and computer simulations, inspired by this general principle, to investigate how disorder controls the brittle-to-ductile transition both at the macroscopic and the microscopic level. A random spring network model featuring two different spring types enables us to study the joined effect of initial disorder and network-induced stress heterogeneity on this transition. We reveal that a mechanical force balance gives a good description of the brittle-to-ductile transition. In addition, the inclusion of disorder in the spring model predicts four different failure regimes along the brittle-to-ductile response in agreement with experimental findings. Finally, we show that the network structure can result in stress concentration, diffuse damage, and loss of percolation depending on the failure regime. This work thus provides a framework for the design and optimization of double-network materials and underlines the importance of network structure in the toughness of polymer materials.

In this work we provide direct evidence of shock-induced melting and associated kinetics in a porous solid (aluminum powder) using time-resolved x-ray diffraction. Unambiguous evidence of melting in 50% porous aluminum (Al) powder samples, shocked to peak pressures between $\sim 13\text{−}19\mathrm{GPa}$, was provided by the broadening of the Debye-Scherrer ring corresponding to the (111) peak. Shocked Al powder did not melt completely in any of our experiments within the durations of measurement. Incomplete (partial) melting of the powder, even after several hundreds of nanoseconds of shock loading, provides insights into thermal transport with Al powder particles under high-pressure dynamic loading. Such insights are quite valuable for developing well-constrained melting models and thermodynamic equations of state for porous Al and other porous solids relevant to planetary and materials science.

In this paper, we present ab initio computer models of Cu-doped amorphous ${\mathrm{Ta}}_2{\mathrm{O}}_5$, a promising candidate for conducting bridge random access memory memory devices, and study the structural, electronic, charge transport, and vibrational properties based on plane-wave density-functional methods. We offer an atomistic picture of the process of phase segregation/separation between Cu and ${\mathrm{Ta}}_2{\mathrm{O}}_5$ subnetworks. Electronic calculations show that the models are conducting with extended Kohn-Sham orbitals around the Fermi level. In addition to that, we also characterize the electronic transport using the Kubo-Greenwood formula modified suitably to calculate the space-projected conductivity (SPC) [K. Prasai, K. N. Subedi, K. Ferris, P. Biswas, and D. A. Drabold, Phys. Stat. Solidi Rapid Res. Lett. 12, 1800238 (2018)]. Our SPC calculations show that Cu clusters and undercoordinated Ta adjoining the Cu are the conduction-active parts of the network. We also report information about the dependence of the electrical conductivity on the connectivity of the Cu submatrix. Vibrational calculations for one of the models has been undertaken with an emphasis on localization and animation of representative modes.

Topological semimetals host unique electronic phases with either Dirac or Weyl characteristic. One way to obtain the Weyl phase is to break the time-reversal symmetry by establishing magnetic ordering. We have investigated the electrical and magnetic properties of a magnetic Weyl semimetal candidate $\mathrm{BaMnS}{\mathrm{b}}_2$ under the application of magnetic field $\left(H\right)$ up to 35 T. We find that $\mathrm{BaMnS}{\mathrm{b}}_2$ undergoes three magnetic phase transitions with a ferromagnetic transition at $T_{\mathrm{C}}\sim 690\mathrm{K}$ and two antiferromagnetic transitions at $T_{\mathrm{N}1}\sim 286\mathrm{K}$ and $T_{\mathrm{N}2}\sim 450\mathrm{K}$. At low temperatures, both the Shubnikov-de Haas and de Haas-van Alphen oscillations are observed by applying H along the $c$ axis of $\mathrm{BaMnS}{\mathrm{b}}_2$. Data analysis indicates that the oscillations result from a single band, and the system can reach the first Landau level at high H. Evidence for Zeeman splitting is also observed at low Landau levels, which yields the Landé factor $g\sim 8.9-10.4$. In addition, thermal property measurements reveal very low phonon thermal conductivity and moderate thermopower. The coupling between charge, spin and lattice is discussed.

Boron phosphide has recently been identified as a potential high-hole-mobility transparent conducting material. This promise arises from its low hole effective masses. However, boron phosphide has a relatively small, 2 eV, indirect bandgap which will affect its transparency. In this work, we computationally study both optical absorption across the indirect gap and phonon-limited electronic transport to quantify the potential of boron phosphide as a $p$-type transparent conductor. We find that phonon-mediated indirect optical absorption is weak in the visible spectrum and that the phonon-limited hole mobility is very high (around 900 ${\mathrm{cm}}^2$/Vs) at room temperature. This exceptional mobility comes from the combination of a low hole effective mass and very weak scattering by polar phonon modes. We rationalize the weak scattering by the less ionic bonding in boron phosphide compared to oxides. We suggest that this could be a general advantage of nonoxides for $p$-type transparent conducting applications. Using our computed properties, we assess the transparent conductor figure of merit of boron phosphide and show that it exceeds by one order of magnitude that of established $p$-type transparent conductors, confirming the potential of this material.

Glassy carbon (GC) is a chemically stable form of fully $s{p}^2$-bonded carbon with locally ordered domains. GC is the intermediate material between graphite and diamond combining various properties such as high temperature resistance, hardness, good electrical conductivity, low density, low gases and liquids permeability, and excellent resistance to a wide range of aggressive chemical environments. These characteristics make it a very promising material for many applications, but unfortunately it is not widely used because of the high temperatures required for its synthesis. In this work, synthesis of glassy carbon thin films by means of laser ablation of carbon targets under vacuum or in gaseous helium, followed by a nanosecond laser irradiation of the deposited films, is presented. In particular, it is demonstrated that the amorphous structure of a thin film can be efficiently modified to the one of glassy carbon film by nanosecond UV laser irradiation. This method is valuable to prepare thin films similar to commercial glassy carbon with a completely different route which does not require the application of temperature beyond $1000{}^{\circ }\mathrm{C}$ which is not compatible with the silicon substrate for example. This opens for glassy carbon the way to microengineering applications (mechanics, electronics,$\cdots$). Particular attention is paid to characterize the vitreous carbon. In the literature, the vitreous nature of carbon layers is often highlighted on the basis of Raman spectroscopy measurements. However, as the Raman spectrum of glassy carbon is similar to that of pyrocarbon, multiwall carbon nanotubes, or functional graphene, this technique is not sufficient to safely characterize a carbonaceous material with a high degree of allotropy. To clear up any doubts, additional characterization methods, such as x-ray spectroscopy, transmission electron microscopy, and Rutherford backscattering spectrometry, are discussed here.

The structure of nitrogen phase $\zeta$ remains unknown decades after it was first observed spectroscopically, despite numerous experimental and theoretical investigations. The present computational study performs crystal structure prediction using ab initio random structure searching and density functional theory to identify candidate structures. These candidates are then analyzed for consistency with experiment in terms of their simulated x-ray diffraction patterns and Raman spectra. While none of the structures generated here is a clear match for the phase-$\zeta$ experimental data, several of the candidates do exhibit features in common with the experiments and could provide an interesting starting point for future studies. The techniques here also rule out several candidate $\zeta$ nitrogen structures that have been identified previously. Finally, one of the structures might be considered a candidate for phase $\kappa$, whose structure is also unknown.

We report a comprehensive high-field/high-frequency electron spin resonance (ESR) study on single crystals of the van der Waals magnet ${\mathrm{CrCl}}_3$. This material, although being known for quite a while, has received recent significant attention in a context of the use of van der Waals magnets in novel spintronic devices. Temperature-dependent measurements of the resonance fields were performed between 4 and 175 K and with the external magnetic field applied parallel and perpendicular to the honeycomb planes of the crystal structure. These investigations reveal that the resonance line shifts from the paramagnetic resonance position already at temperatures well above the transition into a magnetically ordered state. Thereby the existence of ferromagnetic short-range correlations above the transition is established and the intrinsically two-dimensional nature of the magnetism in the title compound is proven. To study details of the magnetic anisotropies in the field-induced effectively ferromagnetic state at low temperatures, frequency-dependent ferromagnetic resonance (FMR) measurements were conducted at 4 K. The observed anisotropy between the two magnetic-field orientations is analyzed by means of numerical simulations based on a phenomenological theory of FMR. These simulations are in excellent agreement with measured data if the shape anisotropy of the studied crystal is taken into account, while the magnetocrystalline anisotropy is found to be negligible in ${\mathrm{CrCl}}_3$. The absence of a significant intrinsic anisotropy thus renders this material as a practically ideal isotropic Heisenberg magnet.

After two decades of extensive research, selected lead-free piezoelectric materials based on the $\left(\mathrm{K},\mathrm{Na}\right)\mathrm{Nb}{\mathrm{O}}_3$ (KNN) system exhibit excellent electromechanical properties that are comparable or even superior to those of $\mathrm{Pb}\left(\mathrm{Zr},\mathrm{Ti}\right){\mathrm{O}}_3$. The origin of the superior properties, however, remains insufficiently understood. Here, we establish a correlation between electromechanical properties and domain-wall dynamics for a prominent piezoceramic $\left(\mathrm{K},\mathrm{Na}\right)\mathrm{Nb}{\mathrm{O}}_3$-based composition, modified with $\left(\mathrm{B}{\mathrm{i}}_{0.5}\mathrm{L}{\mathrm{i}}_{0.5}\right)\mathrm{Ti}{\mathrm{O}}_3$ and different amounts of $\mathrm{BaZr}{\mathrm{O}}_3$. This composition combines excellent piezoelectric properties, high fatigue resistance, and low leakage currents. Domain-wall dynamics were studied over a broad range of electric fields, combining subcoercive field and dynamic pulse measurements. We find that the enhanced electromechanical properties in these materials are caused by easier domain-wall movement. Besides providing insights into fundamental mechanisms of domain-wall dynamics in KNN-based materials, our results can serve as a future guidance for the design of high-performance KNN compositions.

Controlling light propagation using artificial photonic crystals and electromagnetic metamaterials is an important topic in the vibrant field of photonics. Notably, chiral edge states on the surface or at the interface of photonic Chern insulators can be used to make reflection-free waveguides. Here, by both theoretical analysis and electromagnetic simulations, we demonstrate that gyromagnetic hyperbolic metamaterials (GHM) are photonic Chern insulators with superior properties. As a novel mechanism, the simultaneous occurrence of the hyperbolic and gyromagnetic effects in these metamaterials is shown to open the large topological band gaps with a gap Chern number of one. Importantly, the GHM Chern insulators possess nonradiative chiral edge modes on their surfaces, and thus allow us to fabricate unidirectional waveguides without cladding metals which generally incur considerable Ohmic loss. Furthermore, the photonic edge states in the proposed Chern insulators are robust against disorder on a wide range of length scales, in strong contrast to crystalline topological insulators, and the light flow direction on the surface of the Chern insulators can be easily flipped by switching the direction of an applied magnetic field. Fascinatingly, we find that negative refraction of the topological surface wave occurs at the boundary between the GHMs with the opposite signs of gyromagnetic parameters. Finally, we show that compared with other photonic topological materials such as chiral hyperbolic materials, the present GHM Chern insulators can be much easier to fabricate.

Two-dimensional (2D) semiconducting transition metal dichalcogenides such as ${\mathrm{MoS}}_2$ have attracted extensive research interests for potential applications in optoelectronics, spintronics, photovoltaics, and catalysis. To harness the potential of these materials for electronic devices requires a better understanding of how defects control the carrier concentration, character, and mobility. Utilizing a correction scheme developed by Freysoldt and Neugebauer to ensure the appropriate electrostatic boundary conditions for charged defects in 2D materials, we perform density functional theory calculations to compute formation energies and charge transition levels associated with sulfur vacancies in monolayer and layered bulk ${\mathrm{MoS}}_2$. We investigate the convergence of these defect properties with respect to vacuum spacing, in-plane supercell dimensions, and different levels of theory. We also analyze the electronic structures of the defects in different charge states to gain insights into the effect of defects on bonding and magnetism. We predict that both vacancy structures undergo a Jahn-Teller distortion, which helps stabilize the sulfur vacancy in the $-1$ charged state.

We report our study about the growth and characterization of $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ thin films on top of ${\mathrm{Y}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$(111), $\mathrm{T}{\mathrm{m}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$(111), $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$(111), and $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$(100) single-crystal substrates. Using molecular-beam epitaxy, we were able to prepare the topological insulator/ferromagnetic insulator heterostructures with no or minimal chemical reaction at the interface. We observed the anomalous Hall effect on these heterostructures and also a suppression of the weak antilocalization in the magnetoresistance, indicating a topological surface-state gap opening induced by the magnetic proximity effect. However, we did not observe any obvious x-ray magnetic circular dichroism (XMCD) on the Te $M_{45}$ edges. The results suggest that the ferromagnetism induced by the magnetic proximity effect via van der Waals bonding in $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ is too weak to be detected by XMCD, but still can be observed by electrical transport measurements. This is in fact not inconsistent with reported density-functional calculations on the size of the gap opening.

${\mathrm{NiFe}}_2{\mathrm{O}}_4$ thin films with varying thickness were grown on ${\mathrm{SrTiO}}_3\left(001\right)$ by reactive molecular beam epitaxy. Soft and hard x-ray photoelectron spectroscopy measurements reveal a homogeneous cation distribution throughout the whole film with stoichiometric Ni:Fe ratios of 1:2 independent of the film thickness. Low energy electron diffraction and high resolution (grazing incidence) x-ray diffraction in addition to x-ray reflectivity experiments were conducted to obtain information of the film surface and bulk structure, respectively. For ultrathin films up to 7.3 nm, lateral tensile and vertical compressive strain is observed, contradicting an adaption at the interface of ${\mathrm{NiFe}}_2{\mathrm{O}}_4$ film and substrate lattice. The applied strain is accompanied by an increased lateral defect density, which is decaying for relaxed thicker films and attributed to the growth of lateral grains. Determination of cationic site occupancies in the inverse spinel structure by analysis of site sensitive diffraction peaks reveals low tetrahedral occupancies for thin, strained ${\mathrm{NiFe}}_2{\mathrm{O}}_4$ films, resulting in the partial presence of deficient rocksaltlike structures. These structures are assumed to be responsible for the enhanced magnetization of up to $\sim 250$% of the ${\mathrm{NiFe}}_2{\mathrm{O}}_4$ bulk magnetization as observed by superconducting quantum interference device magnetometry for ultrathin films below 7.3 nm thickness.

Spin waves, also known as magnons, are low-lying collective excitations in magnetic materials, for which it is hard to achieve agreement between first-principles electronic structure calculations and experiments. It has been shown in literature [I. Galanakis and E. Şaşıoğlu, J. Mater. Sci. 47, 7668 (2012)] using as a prototype three full Heusler alloys—${\mathrm{Pd}}_2\mathrm{MnSn}, {\mathrm{Ni}}_2\mathrm{MnSn}$, and ${\mathrm{Cu}}_2\mathrm{MnAl}$—that usual density-functional calculations for perfectly ordered compounds fail by a large margin to reproduce neutron scattering measurements of spin waves. We show for these three compounds that the inclusion of correlation effects in the form of the $\mathrm{GGA}+U$ approach and/or substitutional disorder accounted via the coherent potential approximation affects considerably the calculated magnetic properties and their agreement to the experimental data. We expect our results to pave the way for further studies on magnetic materials for which experimental magnonic data exist.

The van der Waals material CeOI is predicted to be a layered antiferromagnetic Mott insulator by density-functional theory $+U$ calculation. We successfully grow the CeOI films down to monolayer on graphene/6H-SiC(0001) substrate by using molecular beam epitaxy. Films are studied by in situ scanning tunneling microscopy and spectroscopy, which shows a band gap of 4.4 eV. A metallic phase with composition unidentified also exists. This rare earth oxyhalide adds a member to the two-dimensional magnetic materials.

We report ultrafast quasiparticle (QP) dynamics of TaAs, from which we obtain its electron-phonon coupling (EPC) strength. Two relaxation components with lifetimes ${\tau }_{\mathrm{fast}}=0.48\mathrm{ps}$ and ${\tau }_{\mathrm{slow}}=250\mathrm{ps}$ are observed, each of which exhibits a smooth temperature dependence. Furthermore, we carry out a fluence-dependence investigation, based on which the quantitative analysis of the fast relaxation component yields a constant EPC strength λ with $\lambda \langle {\mathrm{\Omega }}^3\rangle /{\langle \mathrm{\Omega }\rangle }^2\approx 5.2\mathrm{p}{\mathrm{s}}^{-1}$. In addition, our rotational polarization angle experiment demonstrates that the QP ultrafast dynamics is nearly isotropic under the given excitation geometry. Our investigation of the EPC strength accounts for the so far relatively rarely explored medium fluence regime, which can be generalized to various types of quantum materials.

The bulk photovoltaic effect generates intrinsic photocurrents in materials without inversion symmetry. Shift current is one of the bulk photovoltaic phenomena related to the Berry phase of the constituting electronic bands: photoexcited carriers coherently shift in real space due to the difference in the Berry connection between the valence and conduction bands. Ferroelectric semiconductors and Weyl semimetals are known to exhibit such nonlinear optical phenomena. Here we consider the chalcopyrite semiconductor ${\mathrm{ZnSnP}}_2$, which lacks inversion symmetry, and calculate the shift-current conductivity. We find that the magnitude of the shift current is comparable to the recently measured values on other ferroelectric semiconductors and an order of magnitude larger than bismuth ferrite. The peak response for both optical and shift-current conductivity, which mainly comes from P-$3p$ and Sn-$5p$ orbitals, is several eV above the band gap.

The recently proposed crystal graph convolutional neural network (CGCNN) offers a highly versatile and accurate machine learning (ML) framework by learning material properties directly from graphlike representations of crystal structures (“crystal graphs”). Here, we develop an improved variant of the CGCNN model (iCGCNN) that outperforms the original by incorporating information of the Voronoi tessellated crystal structure, explicit three-body correlations of neighboring constituent atoms, and an optimized chemical representation of interatomic bonds in the crystal graphs. We demonstrate the accuracy of the improved framework in two distinct illustrations: First, when trained/validated on 180 000/20 000 density functional theory (DFT) calculated thermodynamic stability entries taken from the Open Quantum Materials Database (OQMD) and evaluated on a separate test set of 230 000 entries, iCGCNN achieves a predictive accuracy that is significantly improved, i.e., 20% higher than that of the original CGCNN. Second, when used to assist a high-throughput search for materials in the $\mathrm{ThC}{\mathrm{r}}_2\mathrm{S}{\mathrm{i}}_2$ structure-type, iCGCNN exhibited a success rate of $31\%$ which is 155 times higher than an undirected high-throughput search and 2.4 times higher than that of the original CGCNN. Using both CGCNN and iCGCNN, we screened 132 600 compounds with elemental decorations of the $\mathrm{ThC}{\mathrm{r}}_2\mathrm{S}{\mathrm{i}}_2$ prototype crystal structure and identified a total of 97 unique stable compounds by performing 757 DFT calculations, accelerating the computational time of the high-throughput search by a factor of 65. Our results suggest that the iCGCNN can be used to accelerate high-throughput discoveries of new materials by quickly and accurately identifying crystalline compounds with properties of interest.

Two-dimensional (2D) carbon nitrides compounds have attracted wide attention in recent years due to their diverse structures and excellent electronic, thermal, and optical properties. Here, by using first-principles approach, we investigate in details the stability, many-body effect, electronic/thermal transport properties, and thermoelectric performance of monolayer ${\mathrm{C}}_7{\mathrm{N}}_6$, as a new kind of 2D carbon nitride compounds composed of $s{p}^2\text{−}\mathrm{hybridized}$ carbon atoms forming hexagonal lattice. Our results show that ${\mathrm{C}}_7{\mathrm{N}}_6$ monolayer is a direct band-gap semiconductor with a band-gap value of 3.56 eV under the accurate $G_0{W}_0$ method. Ab initio molecular dynamics simulations demonstrate that ${\mathrm{C}}_7{\mathrm{N}}_6$ maintains stable up to $1500\mathrm{K}$. Two exciton absorption peaks can be observed within the band gap with the respective large binding energies of $0.84$ and $0.09\mathrm{eV}$, which means both excitons can exist at room temperature. Monolayer ${\mathrm{C}}_7{\mathrm{N}}_6$ possesses high carrier mobility with the order of ${10}^2–{10}^3{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. Moreover, we find that the lattice thermal conductivity for ${\mathrm{C}}_7{\mathrm{N}}_6$ is as high as $134.55\mathrm{W}/\mathrm{mK}$ at room temperature, thus the thermoelectric figure of merit for ${\mathrm{C}}_7{\mathrm{N}}_6$ is relatively low. Our work suggests that ${\mathrm{C}}_7{\mathrm{N}}_6$ is a promising candidate for nanoscale (opto-)electronic and heat transport devices.