In a class of carbon-based materials called polymerized triptycene, which consist of triptycene molecules and phenyls, exotic electronic structures such as Dirac cones and flat bands arise from the kagome-type network. In this paper, we theoretically investigate the tight-binding models for polymerized triptycene, focusing on the origin of flat bands and the topological properties. The mechanism of the existence of the flat bands is elucidated by using the “molecular-orbital” representation, which we have developed in the prior works. Further, we propose that the present material is a promising candidate to realize the two-dimensional second-order topological insulator, which is characterized by the boundary states localized at the corners of the sample. To be concrete, we propose two methods to realize the second-order topological insulator and elucidate the topological properties of the corresponding models by calculating the corner states as well as the bulk topological invariant, namely, the ${\mathbb{Z}}_3$ Berry phase.

Understanding of the metal-insulator transition (MIT) in correlated transition-metal oxides is a fascinating topic in condensed matter physics and a precise control of such transitions plays a key role in developing novel electronic devices. Here we report an effective tuning of the MIT in epitaxial $\mathrm{SrV}{\mathrm{O}}_3$ (SVO) films by expanding the out-of-plane lattice constant without changing in-plane lattice parameters, through helium ion irradiation. Upon increase of the ion fluence, we observe a MIT with a crossover from metallic to insulating state in SVO films. A combination of transport and magnetoresistance measurements in SVO at low temperatures reveals that the observed MIT is mainly ascribed to electron-electron interactions rather than disorder-induced localization. Moreover, these results are well supported by the combination of density functional theory and dynamical mean-field theory calculations, further confirming the decrease of the bandwidth and the enhanced electron-electron interactions resulting from the expansion of the out-of-plane lattice constant. These findings provide insights into the understanding of MIT in correlated oxides and perspectives for the design of unexpected functional devices based on strongly correlated electrons.

${\mathrm{Cu}}_2\mathrm{O}$ is a widely known $p$-type semiconductor with a band-gap value suitable for photovoltaic applications. However, due to its parity-forbidden nature of the first interband transition and high carrier recombination currents, ${\mathrm{Cu}}_2\mathrm{O}$ has failed to reach commercial application. Hybrid density functional theory has been used to study the effect of transition metal dopants on the electronic and optical properties of ${\mathrm{Cu}}_2\mathrm{O}$. Substitutional transition metal dopants, incorporated on the copper site, allow band-gap tunability by creating a range of defect states in the electronic structure, from shallow levels to deep intermediate bands. The preferred position of the dopants is in the vicinity of copper vacancies, which are naturally found in ${\mathrm{Cu}}_2\mathrm{O}$ and are responsible for its $p$-type conductivity. Impurity levels created via extrinsic transition metal dopants increase substantially the capacity of ${\mathrm{Cu}}_2\mathrm{O}$ to absorb light, reaching values close to 10%. First row transition metal dopants thus show potential for considerable improvement of the overall photovoltaic performance of ${\mathrm{Cu}}_2\mathrm{O}$.

Interstitial alloying in CrMnFeCoNi-based high-entropy alloys is known to modify their mechanical properties. Specifically, strength can be increased due to interstitial solid-solution hardening, while simultaneously affecting ductility. In this paper, first-principles calculations are carried out to analyze the impact of interstitial C atoms on CrMnFeCoNi in the fcc and the hcp phases. Our results show that C solution energies are widely spread and sensitively depend on the specific local environments. Using the computed solution-energy distributions together with statistical mechanics concepts, we determine the impact of C on the phase stability. C atoms are found to stabilize the fcc phase as compared to the hcp phase, indicating that the stacking-fault energy of CrMnFeCoNi increases due to C alloying. Using our extensive set of first-principles computed solution energies, correlations between them and local environments around the C atoms are investigated. This analysis reveals, e.g., that the local valence-electron concentration around a C atom is well correlated with its solution energy.

We report the discovery of ${\mathrm{Ce}}_3{\mathrm{Ir}}_4{\mathrm{Ge}}_{13}$, a Remeika phase with a complex array of structural, electronic, and magnetic properties. Our single crystal x-ray diffraction measurements show that ${\mathrm{Ce}}_3{\mathrm{Ir}}_4{\mathrm{Ge}}_{13}$ forms in the tetragonally distorted $I{4}_1/amd$ space group, which has three distinct rare earth sites. The electrical resistivity is almost temperature independent over three decades in temperature, from 0.4 K to 400 K, while the Hall coefficient measurements are consistent with a low-carrier semimetal. Magnetic susceptibility measurements reveal an effective moment, ${\mu }_{\mathrm{eff}}^{\exp }=1.87{\mu }_B/\mathrm{Ce}$, that falls significantly short of the expected value for ${\mathrm{Ce}}^{3+}$, suggesting that two of the rare earth sites are occupied by magnetic ${\mathrm{Ce}}^{3+}$ while the third is occupied by nonmagnetic ${\mathrm{Ce}}^{4+}$. Upon cooling, ${\mathrm{Ce}}_3{\mathrm{Ir}}_4{\mathrm{Ge}}_{13}$ first enters a short range magnetically ordered state below $T_{\text{SRO}}=10\mathrm{K}$, marked by a deviation from Curie-Weiss behavior in susceptibility and a broad field-independent heat capacity anomaly. At lower temperatures, we observe a second, sharper peak in the heat capacity at $T^{*}=1.7\mathrm{K}$, concurrent with a splitting of the field-cooled and zero-field-cooled susceptibilities. A small resistivity drop at $T^{*}$ suggests a loss of spin disorder scattering consistent with a magnetic ordering or spin freezing transition. ${\mathrm{Ce}}_3{\mathrm{Ir}}_4{\mathrm{Ge}}_{13}$ is therefore a rare example of an inhomogeneous mixed valence compound with a complex array of thermodynamic and transport properties.

The symmetry of local moments plays a defining role in the nature of exotic ground states stabilized in frustrated magnetic materials. We present inelastic neutron scattering (INS) measurements of the crystal electric field (CEF) excitations in the family of compounds $\mathrm{Mg}{R}_2{\mathrm{Se}}_4$ $(R\in \{\mathrm{Ho},\mathrm{Tm},\mathrm{Yb}\left\}\right)$. These compounds form in the spinel structure, with the rare-earth ions comprising a highly frustrated pyrochlore sublattice. Within the symmetry constraints of this lattice, we fit both the energies and intensities of observed transitions in the INS spectra to determine the most likely CEF Hamiltonian for each material and comment on the ground-state wavefunctions in the local electron picture. In this way, we experimentally confirm ${\mathrm{MgTm}}_2{\mathrm{Se}}_4$ has a nonmagnetic ground state, and ${\mathrm{MgYb}}_2{\mathrm{Se}}_4$ has effective $S=\frac{1}{2}$ spins with $g_{\parallel }=5.188\left(79\right)$ and $g_{\perp }=0.923\left(85\right)$. The spectrum of ${\mathrm{MgHo}}_2{\mathrm{Se}}_4$ indicates a ground-state doublet containing Ising spins with $g_{\parallel }=2.72\left(46\right)$, though low-lying CEF levels are also seen at thermally accessible energies $E=0.591\left(36\right)$, 0.945(30), and 2.88(7) meV, which can complicate interpretation. These results are used to comment on measured magnetization data of all compounds and are compared to published results on the material ${\mathrm{MgEr}}_2{\mathrm{Se}}_4$.

There has been a puzzling discrepancy on the physical property of quasi-one-dimensional Cr-based compound ${\mathrm{KCr}}_3{\mathrm{As}}_3$. While the polycrystalline ${\mathrm{KCr}}_3{\mathrm{As}}_3$ was originally reported to be nonsuperconducting with a spin-glass ground state, the single-crystalline ${\mathrm{KCr}}_3{\mathrm{As}}_3$ was later found to exhibit superconductivity at $\sim 5$ K. Here we demonstrate that both the polycrystals and single crystals can be made either nonsuperconducting or superconducting. The pristine samples show a spin-glass behavior only, and superconductivity below 5 K is actually induced by the post-aging and/or post-annealing. The result can be understood in terms of hydrogen intercalation into the one-dimensional ${\mathrm{Cr}}_3{\mathrm{As}}_3$ tubes, which was discovered very recently by K. M. Taddei et al. (arXiv:1905.03360). In addition, significant K deficiency and microscopic inhomogeneity have been revealed, suggesting a phase separation scenario with columnar superconductivity in ${\mathrm{K}}_{1-\delta }{\mathrm{Cr}}_3{\mathrm{As}}_3{\mathrm{H}}_x$. The possible processes of the hydrogen intercalation are given.

Materials with two interpenetrating phases often exhibit excellent mechanical and physical properties, and can be used as precursors for the production of porous materials that have many practical applications. Such bicontinuous structures can be generated by liquid metal dealloying, followed by quenching. However, this procedure is neither efficient nor cost-effective, because it requires a sizeable liquid-metal corrosion reservoir and long-range mass transport that restricts sample size and synthesis rate. In this paper, we propose a mechanism that generates bicontinuous structures by simple thermal treatments. By heating a TiAg intermetallic compound to above its peritectic temperature, we demonstrate that it decomposes into solid Ti and liquid Ag that interpenetrate at the micrometer scale. The resulting structure can be quenched to room temperature. In this case, peritectic melting proceeds by cooperative growth of solid Ti and liquid Ag into a solid TiAg compound. The behavior of structural evolution resembles the process of dealloying, particularly liquid metal dealloying, except that corrosion media and long-range diffusion are no longer required. Our findings suggest that the directional transition from a solid to another solid and a fluid phase, instead of “selective etching,” might be responsible for the evolution of a bicontinuous structure in all dealloying processes. This study also provides a versatile metallurgical route to generate bicontinuous solid/solid or solid/liquid structures and hierarchical nanoporous structures in many materials for mechanical and functional applications.

Fracture toughness testing at small scales is widely adopted for materials of limited size. Recent experimental results demonstrate that the fracture toughness of single-crystal tungsten decreases by half as the specimen size drops equally from the order of several dozen micrometers to 1 $\mu \mathrm{m}$. To explore this size-dependent fracture feature, we construct a fracture model by combining the Griffith strength theory and elastic-damage law. It is shown that as the specimen scales down to a critical size, the fracture behavior changes from the fracture toughness domination to the strength domination, leading to a seeming decrease of the fracture toughness. Reviewing mechanical fracture testing of different brittle materials at small scales, we found that their fracture is dominated by fracture strength instead of fracture toughness, which explains why the reported values of fracture toughness obtained from the microscale testing are significantly less than the corresponding ones measured at the macroscale. Our study has profound guiding significance for the characterization of the fracture toughness at small scales.

Metal-organic frameworks (MOFs) are crystalline materials consisting of metal centers and organic linkers forming open and porous structures. They have been extensively studied because of various possible applications exploiting their large amount of internal surface area. Phonon properties of MOFs are, however, still largely unexplored, despite their relevance for thermal and electrical conductivities, thermal expansion, and mechanical properties. Here, we use quantum-mechanical simulations to provide an in-depth analysis of the phonon properties of isoreticular MOFs. We consider phonon band structures, spatial confinements of modes, projected densities of states, and group velocity distributions. Additionally, the character of selected modes is discussed based on real-space displacements, and we address how phonon properties of MOFs change when their constituents are altered, in terms of mass and spatial extent, bonding structure, etc. We find that more complex linkers shift the spectral weight of the phonon density of states toward higher frequencies, while increasing the mass of the metal atoms in the nodes has the opposite effect. As a consequence of the high porosity of MOFs, we observe a particularly pronounced polarization dependence of the dispersion of acoustic phonons with rather high group velocities for longitudinal acoustic modes (around $6000\mathrm{m}{\mathrm{s}}^{-1}$ in the long-wavelength limit). Interestingly, also for several optical phonon modes group velocities amounting to several thousand meters per second are obtained. For heterogeneous systems such as MOFs, correlating group velocities and the displacement of modes is particularly relevant. Here we find that high group velocities are generally associated with delocalized vibrations, while the inverse correlation does not necessarily hold. To quantify anharmonicities, we calculate mode Grüneisen parameters, which we find to be significant only for phonons with frequencies below $\sim 3$ THz. The presented results provide the foundations for an in-depth understanding of the vibrational properties of MOFs and, therefore, pave the way for a future rational design of systems with well-defined phonon properties.

Two-dimensional $\mathrm{Mo}{\mathrm{S}}_2$ is an excellent channel material for ultrathin field-effect transistors, but high contact resistance across the deposited metal-$\mathrm{Mo}{\mathrm{S}}_2$ interface continues to limit its full realization. Using atomic-resolution scanning transmission electron microscopy and first-principles calculations, we showed that deposited metals with a high affinity for sulfur could have a fundamental limitation. Ti-$\mathrm{Mo}{\mathrm{S}}_2$ contact shows a destruction of the $\mathrm{Mo}{\mathrm{S}}_2$ layers, a formation of clusters and void pockets, and penetration of Ti into $\mathrm{Mo}{\mathrm{S}}_2$, resulting in many localized pinning states in the band gap. InAu-$\mathrm{Mo}{\mathrm{S}}_2$ contact shows that it is possible to achieve a van der Waals-type interface and dramatically reduced pinning states.

The local atomic structure around individual dopant atoms can directly influence the electronic properties of a doped material. Here, we use quantitative scanning transmission electron microscopy to study the local lattice relaxations around Sm dopant atoms in $\mathrm{SrTi}{\mathrm{O}}_3$ thin films. These films have recently been shown to undergo successive ferroelectric and superconducting transitions when strained. We show that neighboring Ti-O columns move away from the columns that contain Sm dopants. The observed displacements are, however, more complex than a simple outward expansion of all four surrounding Ti-O columns. We discuss potential implications, especially for the ferroelectric transition observed in strained films.

Ferroelastic domains generate polarity near domain walls via the flexoelectric effect. Applied electric fields change the wall dipoles and generate additional dipoles in the bulk. Molecular dynamics simulations show that the thickness of domain walls changes when an electric field is applied to the sample. Fields parallel to the walls lead to expansion of the wall thickness while fields perpendicular to the wall lead to shrinking of the wall thickness. The interactions between polar domain walls expand over more than 45 unit cells, the resulting forces change the wall-wall distances if pinning effects are small. The interaction increases nonlinearly with decreasing wall-wall distances favoring equal wall distances as the consequence of energy minimization under the constraints of a constant number of domain walls. Even for small groups of three walls the sequence of walls is locally periodic: assemblies of three parallel domain walls arrange themselves so that the intermediate domain wall is located exactly in the middle between the two outer walls. The driving force is appreciable if the distance between the outer domain walls is below approximately 30 lattice units. Pairs of domain walls often form needle domains where the shaft of the needle is ca. 3 lattice units wide. The movement of needle domains under applied electric field was simulated. The advancement and retraction of needles is larger in finite samples with charge-free surfaces than under periodic boundary conditions in the bulk. The needle tip moves even more freely when the sample surface is charged.

A ${\mathrm{Co}}_{2-x}{\mathrm{Ti}}_x\mathrm{FeGe}(0\le x\le 1)$ Heusler alloy series has been synthesized and investigated in steps of $x=0.125$. While the parent ${\mathrm{Co}}_2\mathrm{FeGe}$ composition was previously reported to exhibit multi-phase structures, the novel substitution of Ti for Co is successful in stabilizing single-phase microstructures for compositions from $x=0.125$ to 0.625. The single-phase compositions crystallized in a face-centered cubic crystal structure. Soft-ferromagnetic behavior and a very high Curie temperature, as high as 881 K, is obtained for alloys with the lowest Ti concentration. Magnetic and electrical properties of these alloys suggest the possibility of half-metallic behavior in them. Both experimental findings and theoretical calculations support that the substitution of Ti atoms displaces Fe atoms towards vacated Co sites. The theoretical calculations also predict half-metallic character in the alloys after Ti substitution. The alloys are found to be unusually hard, with Vicker's hardness approaching 12 GPa for the most Ti-rich material.

Spin ensembles of nitrogen vacancy (NV) centers in diamond are emerging as powerful spin-based sensors for magnetic, electric and thermal field imaging with high spatial and temporal resolution. Here we characterize the formation of depth-confined NV center ensembles, activated by electron irradiation in diamond layers grown by plasma enhanced chemical vapor deposition with nitrogen co-doping. To do so, we exploit the high magnetic sensitivity of ensembles of NV centers to probe their spin environment as a function of growth and irradiation parameters. We engineer an NV ensemble whose magnetic sensitivity is within a factor of two of the static NV-NV dipolar interaction limit, thus demonstrating a powerful platform for quantum sensing.

Metal halide perovskites have emerged as versatile photonic device materials because of their outstanding band structure and excellent optical properties. Here, we determined the excitation wavelength dependences of the two-photon absorption coefficient and the Kerr-effect-induced nonlinear refractive index of $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{PbC}{\mathrm{l}}_3$ perovskite single crystals by means of the Z-scan method. From theoretical analysis, we found that the electron-hole interaction, so-called exciton effect, significantly enhances the nonlinear optical responses even for the interband transitions. This interaction explains the universal relation between the exciton reduced mass and the bandgap for lead halide perovskites.

The iron-based superconductor ${\mathrm{FeTe}}_{1-x}{\mathrm{Se}}_x$ has attracted considerable attention as a candidate topological superconductor owing to a unique combination of topological surface states and bulk high-temperature superconductivity. The superconducting properties of as-grown single crystals, however, are highly variable and synthesis dependent due to excess interstitial iron impurities incorporated during growth. Here we report a physicochemical process for pumping this interstitial iron out of the ${\mathrm{FeTe}}_{1-x}{\mathrm{Se}}_x$ matrix and achieving bulk superconductivity. Our method should have significant value for the synthesis of high-quality single crystals of ${\mathrm{FeTe}}_{1-x}{\mathrm{Se}}_x$ with large superconducting volume fractions.

Nanoscale curvature is an important and powerful tool in understanding and tailoring chemical/surface functionalities of nanostructures that dictate a host of important applications from biochemical recognitions, catalysis to spectroscopy. However, it is a critical challenge in materials chemistry to rationally shape the local nanoscale curvatures of colloidal nanoparticles during the growth owing to the constraints of their flat facets. Here we demonstrate a synthetic mechanism that could cooperatively mediate local nanoparticle surface curvature patchiness and shape symmetries during one-step colloidal growth. The idea is to tailor host-guest supramolecular recognition using fluorocarbon and hydrocarbon molecules that regulate interfacial energy during the nanoparticle growth. Such delicate regulation enables a degree of freedom in control over the local nanoparticle curvatures during the growth, resulting in intriguing plasmonic properties. More interestingly, a morphological shape transformation was induced by such curvature changes from anisotropic nanorods to isotropic nanospheres. This unique approach of the spontaneous curvature/structural transformation of plasmonic nanoparticles exploits the mutual interplay between competing supramolecules and colloidal growth. It may ultimately allow for accurate controlling nanoscale objects with varied degree of complexity that could open the door to a myriad of surface chemical, optical, and biomedical applications.

In the present work, single-crystalline quasi-van der Waals ferromagnet $\mathrm{F}{\mathrm{e}}_{0.26}\mathrm{Ta}{\mathrm{S}}_2$ was successfully synthesized with Fe atoms intercalated at ordered positions between $\mathrm{Ta}{\mathrm{S}}_2$ layers. Its critical behavior was systematically studied by measuring the magnetization around ferromagnetic to paramagnetic phase transition temperature, $T_C\sim 100.7\mathrm{K}$, under different magnetic fields. The critical exponent β for the spontaneous magnetization below $T_C$, γ for the inverse initial susceptibility above $T_C$, and δ for the magnetic isotherm at $T_C$ were determined with modified Arrott plots, the Kouvel-Fisher method, the Widom scaling law, and critical isotherm analysis, and found to be the following values: $\beta =0.459\left(6\right),\gamma =1.205\left(11\right)$, and $\delta =3.69\left(1\right)$. The obtained critical exponents are self-consistent and follow the scaling equation, indicating the reliability and intrinsicality of these parameters. A close analysis within the framework of renormalization group theory reveals that the spin coupling inside $\mathrm{F}{\mathrm{e}}_{0.26}\mathrm{Ta}{\mathrm{S}}_2$ crystal is of the three-dimensional Heisenberg $\left(\left\{d:n\right\}=\left\{3:3\right\}\right)$ type with long-range magnetic interaction and that the exchange interaction decays with distance as $J\left(r\right)\sim r^{-4.71}$.