Topological insulators host protected electronic states at their surface. These states show little sensitivity to disorder. For miniaturization one wants to exploit their robustness at the smallest sizes possible. This is also beneficial for optical applications and catalysis, which favor large surface-to-volume ratios. However, it is not known whether discrete states in particles share the protection of their continuous counterparts in large crystals. Here we study the protection of the states hosted by topological insulator nanoparticles. Using both analytical and tight-binding simulations, we show that the states benefit from the same level of protection as those on a planar surface. The results hold for many shapes and sustain surface roughness which may be useful in photonics, spectroscopy, and chemistry. They complement past studies of large crystals—at the other end of possible length scales. The protection of the nanoparticles suggests that samples of all intermediate sizes also possess protected states.

The severe local lattice distortion, induced mainly by the large atomic size mismatch of the alloy components, is one of the four core effects responsible for the unprecedented mechanical behaviors of high-entropy alloys (HEAs). In this work, we propose a supercell model, in which every lattice site has similar local atomic environment, to describe the random distributions of the atomic species in HEAs. Using these supercells in combination with $\mathit{ab}\mathit{initio}$ calculations, we investigate the local lattice distortion of refractory HEAs with body-centered-cubic structure and $3d$ HEAs with face-centered-cubic structure. Our results demonstrate that the local lattice distortion of the refractory HEAs is much more significant than that of the $3d$ HEAs. We show that the atomic size mismatch evaluated with the empirical atomic radii is not accurate enough to describe the local lattice distortion. Both the lattice distortion energy and the mixing entropy contribute significantly to the thermodynamic stability of HEAs. However the local lattice distortion has negligible effect on the equilibrium lattice parameter and bulk modulus.

We report a rectangular charge density wave (CDW) phase in strained $1\mathrm{T}\text{−}\mathrm{VS}{\mathrm{e}}_2$ thin films synthesized by molecular beam epitaxy on $c$-sapphire substrates. The observed CDW structure exhibits an unconventional rectangular $4a\times \surd 3a$ periodicity, as opposed to the previously reported hexagonal $4a\times 4a$ structure in bulk crystals and exfoliated thin-layered samples. Tunneling spectroscopy shows a strong modulation of the local density of states of the same $4a\times \surd 3a$ CDW periodicity and an energy gap of $2{\mathrm{\Delta }}_{\mathrm{CDW}}=\left(9.1\pm 0.1\right)\mathrm{meV}$. The CDW energy gap evolves into a full gap at temperatures below 500 mK, indicating a transition to an insulating phase at ultra-low temperatures. First-principles calculations confirm the stability of both $4a\times 4a$ and $4a\times \surd 3a$ structures arising from soft modes in the phonon dispersion. The unconventional structure becomes preferred in the presence of strain, in agreement with experimental findings.

We report the synthesis of polycrystalline samples of the recently discovered spin-liquid material ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_7{\mathrm{O}}_{28}$ and present measurements of the ambient and high pressure magnetic susceptibility $\chi$ versus temperature $T$, magnetization $M$ versus magnetic field $H$ at various $T$, and heat capacity $C$ versus $T$ at various $H$. The ambient pressure magnetic measurements indicate the presence of both ferromagnetic and antiferromagnetic exchange interactions with dominant ferromagnetic interactions and with the largest magnetic energy scale $\sim 10$ K. The $\chi \left(T\right)$ measurements under externally applied pressure of up to $P\approx 1$ GPa indicate the robust nature of the spin-liquid state despite relative increase in the ferromagnetic exchanges. $C\left(T\right)$ shows a broad anomaly at $T\approx 2.4$ K which moves to higher temperatures in a magnetic field. The evolution of the low temperature $C(T,H)$ and the magnetic entropy is consistent with frustrated magnetism in ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_7{\mathrm{O}}_{28}$.

The electronic structure near the band gap of the corundumlike $\alpha$ phase of ${\mathrm{Ga}}_2{\mathrm{O}}_3$ has been investigated by means of optical absorption and spectroscopic ellipsometry measurements in the ultraviolet (UV) range (400–190 nm). The absorption coefficient in the UV region and the imaginary part of the dielectric function exhibit two prominent absorption thresholds with wide but well-defined structures at 5.6 and 6.3 eV which have been ascribed to allowed direct transitions from crystal-field split valence bands to the conduction band. Excitonic effects with large Gaussian broadening are taken into account through the Elliott-Toyozawa model, which yields an exciton binding energy of 110 meV and direct band gaps of 5.61 and 6.44 eV. The large broadening of the absorption onset is related to the slightly indirect character of the material.

Reliable and robust methods of predicting the crystal structure of a compound, based only on its chemical composition, is crucial to the study of materials and their applications. Despite considerable ongoing research efforts, crystal structure prediction remains a challenging problem that demands large computational resources. Here we propose an efficient approach for first-principles crystal structure prediction. The new method explores and finds crystal structures by tiling together elementary tetrahedra that are energetically favorable and geometrically matching each other. This approach has three distinguishing features: a favorable building unit, an efficient calculation of local energy, and a stochastic Monte Carlo simulation of crystal growth. By applying the method to the crystal structure prediction of various materials, we demonstrate its validity and potential as a promising alternative to current methods.

Owing to their multivalley, anisotropic, energy band structure, two-dimensional electron systems (2DESs) in modulation-doped AlAs quantum wells (QWs) provide a unique platform to investigate electron interaction physics and ballistic transport. Indeed, a plethora of phenomena unseen in other 2DESs have been observed over the past decade. However, a foundation for sample design is still lacking for AlAs 2DESs, limiting the means to achieve optimal quality samples. Here, we present a systematic study on the fabrication of modulation-doped AlAs and GaAs QWs over a wide range of ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ barrier alloy compositions. Our data indicate clear similarities in modulation doping mechanisms for AlAs and GaAs, and provide guidelines for the fabrication of very high quality AlAs 2DESs. We highlight the unprecedented quality of the fabricated AlAs samples by presenting the magnetotransport data for low-density $\left(\simeq 1\times {10}^{11}{\mathrm{cm}}^{-2}\right)$ AlAs 2DESs that exhibit high-order fractional quantum Hall signatures.

We present a synchrotron-based x-ray scattering technique which allows disentangling magnetic properties of heterogeneous systems with nanopatterned surfaces. This technique combines the nanometer-range spatial resolution of surface morphology features provided by grazing incidence small angle x-ray scattering and the high sensitivity of nuclear resonant scattering to magnetic order. A single experiment thus allows attributing magnetic properties to structural features of the sample; chemical and structural properties may be correlated analogously. We demonstrate how this technique shows the correlation between structural growth and evolution of magnetic properties for the case of a remarkable magnetization reversal in a structurally and magnetically nanopatterned sample system.

We study theoretically and experimentally the mechanisms of nonlinear and nonequilibrium dynamics in geomaterials through dynamic acoustoelasticity testing. In the proposed theoretical formulation, the classical theory of nonlinear elasticity is extended to include the effects of conditioning. This formulation is adapted to the context of dynamic acoustoelasticity testing in which a low-frequency “pump” wave induces a strain field in the sample and modulates the propagation of a high-frequency “probe” wave. Experiments are conducted to validate the formulation in a long thin bar of Berea sandstone. Several configurations of the pump and probe are examined: the pump successively consists of the first longitudinal and first torsional mode of vibration of the sample while the probe is successively based on (pressure) $P$ and (shear) $S$ waves. The theoretical predictions reproduce many features of the elastic response observed experimentally, in particular, the coupling between nonlinear and nonequilibrium dynamics and the three-dimensional effects resulting from the tensorial nature of elasticity.

Two-dimensional (2D) semiconductors possess the potential to ultimately minimize the size of devices and concomitantly drastically reduce the corresponding energy consumption. In addition, materials in their atomic-scale limit often possess properties different from their bulk counterparts paving the way to conceptually novel devices. While graphene and 2D transition-metal dichalcogenides remain the most studied materials, significant interest also exists in the fabrication of atomically thin structures from traditionally 3D semiconductors such as GaN. While in the monolayer limit GaN possesses a graphenelike structure and an indirect band gap, it was recently demonstrated that few-layer GaN acquires a Haeckelite structure in the direction of growth with an effectively direct gap. In this work, we demonstrate the possibility of strain engineering of the atomic and electronic structure of few-monolayer-thick GaN structures, which opens new avenues for their practical application in flexible nanoelectronics and nano-optoelectronics. Our simulations further suggest that due to the weak van der Waals-like interaction between a substrate and an overlayer, the use of a ${\mathrm{MoS}}_2$ substrate may be a promising route to fabricate few-monolayer Haeckelite GaN experimentally.

Two-dimensional transition metal carbides and nitrides (MXenes) are one of the largest and fastest growing families of materials. The presence of molecular hydrogen at ambient conditions in a MXene ($\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{T}_x$, where $T_x$ represents a surface terminating species, including O, OH, and F) material is revealed here by inelastic and elastic neutron scatterings. The inelastic neutron-scattering spectrum measured at 5 K shows a peak at 14.6 meV, presenting a clear indication of the presence of parahydrogen in the MXene synthesized using $48\%$ hydrofluoric acid and annealed at $110{}^{\circ }\mathrm{C}$ in vacuum prior to the measurement. An increase in the measurement temperature gradually reduces the peak intensity and increases the peak width due to the mobility of the molecular hydrogen in confinement. The presence of molecular hydrogen is confirmed further from the observed elastic intensity drop in a fixed energy-window scan of elastic intensity measurements in the temperature range of 10–35 K. Using milder etching conditions, ion intercalation, or an increase in the annealing temperature all result in the absence of the trapped hydrogen molecules in MXene. The results of this paper can guide the development of MXene materials with desired properties and improve our understanding of the behavior of MXenes in applications ranging from supercapacitors to hydrogen evolution reaction catalysis and hydrogen storage.

Cubic $\mathrm{SrCo}{\mathrm{O}}_3$ with an intermediate spin state can only be stabilized by high pressure and high temperature (HPHT) treatment. It is metallic and ferromagnetic with the highest Curie temperature of the transition-metal perovskites. The chemical substitution by Ca on Sr sites would normally lower crystal symmetry from cubic to orthorhombic as seen in the perovskite family of $\mathrm{Ca}M{\mathrm{O}}_3$ ($M=M^{4+}$ of transition metals, $\mathrm{G}{\mathrm{e}}^{4+}, \mathrm{S}{\mathrm{n}}^{4+}$, and $\mathrm{Z}{\mathrm{r}}^{4+}$) at room temperature. This structural change narrows the bandwidth, so as to further enhance the Curie temperature as the crossover to the localized electronic state is approached. We report a successful synthesis of the perovskite $\mathrm{CaCo}{\mathrm{O}}_3$ with a HPHT treatment. Surprisingly, $\mathrm{CaCo}{\mathrm{O}}_3$ crystallizes in a simple cubic structure that remains stable down to 20 K, the lowest temperature in the structural study. The new perovskite has been thoroughly characterized by a suite of measurements including transport, magnetization, specific heat, thermal conductivity, and thermoelectric power. Metallic $\mathrm{CaCo}{\mathrm{O}}_3$ undergoes two successive magnetic transitions at 86 K and 54 K as temperature decreases. The magnetization at 5 K is compatible with the intermediate spin state $t^4{e}^1$ of $\mathrm{C}{\mathrm{o}}^{4+}$ at the octahedral site. The thermal expansion of the Co-O bond length indicates that the population of high spin state $t^3{e}^2$ increases for $T>100\mathrm{K}$. The shortest Co-O bond length in cubic $\mathrm{CaCo}{\mathrm{O}}_3$ is responsible for delocalizing electrons in the ${\pi }^{*}$-band and itinerant-electron ferromagnetism at $T<54\mathrm{K}$. A comprehensive comparison between $\mathrm{SrCo}{\mathrm{O}}_3$ and $\mathrm{CaCo}{\mathrm{O}}_3$ and the justification of their physical properties by first-principles calculation have also been made in this report. Partially filled ${\pi }^{*}$ and ${\sigma }^{*}$ bands would make $\mathrm{CaCo}{\mathrm{O}}_3$ suitable to study the Hund's coupling effect in a metal.

To compete with existing and more mature solar cell technologies such as crystalline Si, thin-film photovoltaics require optimization of every aspect in the device heterostructure to reach maximum efficiencies and cost effectiveness. For absorbers like CdTe, ${\mathrm{Cu}(\mathrm{In},\mathrm{Ga})\mathrm{Se}}_2$ (CIGSe), and ${\mathrm{Cu}}_2{\mathrm{ZnSn}(\mathrm{S},\mathrm{Se})}_4$ (CZTSSe), improving the $n$-type buffer layer partner beyond conventional CdS is one avenue that can reduce photocurrent losses and improve overall performance. Here, we use first-principles calculations based on hybrid functionals to explore alloys spanning the Cd-, Zn-, O-, and S-containing phase space to identify compositions that may be superior to common buffers like pure CdS or Zn(O,S). We address issues highly correlated with device performance such as lattice-matching for improved buffer-absorber epitaxy and interface quality, dopability, the band gap for reduced absorption losses in the buffer, and the conduction-band offsets shown to facilitate improved charge separation from photoexcited carriers. We supplement our analysis with device-level simulations as parameterized from our calculations and real devices to assess our conclusions of low-Zn and O content buffers showing improved performance with respect to CdS buffers.

The interplay of BCS superconductivity and nontrivial band topology is expected to give rise to opportunities for creating topological superconductors, achieved through pairing spin-filtered boundary modes via superconducting proximity effects. The thus-engineered topological superconductivity can, for example, facilitate the search for Majorana fermion quasiparticles in condensed matter systems. Here we report a first-principles study of $\mathrm{M}{\mathrm{g}}_2\mathrm{Pb}$ and predict that it should be a superconducting topological material. The band topology of $\mathrm{M}{\mathrm{g}}_2\mathrm{Pb}$ is identical to that of the archetypal quantum spin Hall insulator HgTe, while isostructural and isoelectronic $\mathrm{M}{\mathrm{g}}_2\mathrm{Sn}$ is topologically trivial; a trivial-to-topological transition is predicted for $\mathrm{M}{\mathrm{g}}_2\mathrm{S}{\mathrm{n}}_{1-x}\mathrm{P}{\mathrm{b}}_x$ for $x\approx 0.77$. We propose that $\mathrm{M}{\mathrm{g}}_2\mathrm{Pb}–\mathrm{M}{\mathrm{g}}_2\mathrm{Sn}$ quantum wells should generate robust spin-filtered edge currents in analogy to HgTe/CdTe quantum wells. In addition, our calculations predict that $\mathrm{M}{\mathrm{g}}_2\mathrm{Pb}$ should become superconducting upon electron doping. Therefore, $\mathrm{M}{\mathrm{g}}_2\mathrm{Pb}$ is expected to provide a practical material platform for studying emergent phenomena arising from the interplay of superconductivity and band topology.

By controlling lattice defects through the annealing condition during crystal growth, a wide-gap and $p$-type semiconductor ${\mathrm{Sn}}_2\left({\mathrm{Nb}}_{2\text{−}x}{\mathrm{Ta}}_x\right){\mathrm{O}}_7$ $(x=$ $0–2)$ was developed. The band gap is variable in the range from 2.4 eV for ${\mathrm{Sn}}_2{\mathrm{Nb}}_2{\mathrm{O}}_7$ $\left(x=0\right)$ to 3.0 eV for ${\mathrm{Sn}}_2{\mathrm{Ta}}_2{\mathrm{O}}_7$ $\left(x=2\right)$, respectively. The carrier density and the hole mobility were estimated to be $0.20–1.4$ $\left(\times {10}^{18}{\mathrm{cm}}^{-3}\right)$ and $0.28–1.9$ $\left(\times {10}^{-1}{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}\right)$, respectively. From the mutual relation between the defect content and the electrical properties, we concluded that the hole carriers in the $p$-type semiconducting ${\mathrm{Sn}}_2\left({\mathrm{Nb}}_{2\text{−}x}{\mathrm{Ta}}_x\right){\mathrm{O}}_7$ are generated by ${\mathrm{Sn}}^{4+}$-on-${\mathrm{Nb}}^{5+}/{\mathrm{Ta}}^{5+}$ substitutional defects in (Nb,${\mathrm{Ta})}_2{\mathrm{O}}_6$ octahedral chains.

We present first-principles calculations of the electronic structure of ${\mathrm{BiVO}}_4$ at various levels of theory. In the calculations, we take into account a series of effects that affect the band gap, i.e., spin-orbit coupling, electron-hole interaction, nuclear quantum motions, and thermal vibrations. All these effects lead to a significant renormalization of the band gap. After including the relevant corrections, the values achieved with the $GW$ level of theory closely match the experiment. Additionally, by treating excitonic effects through the Bethe-Salpeter equation, we obtain an optical band gap and an absorption spectrum in good agreement with experimental data. Through the calculation of Tauc plots, we show that this technique gives the optical band gap within about 0.14 eV, but argue that it is unable to distinguish between direct and indirect bad gaps in the case of ${\mathrm{BiVO}}_4$.

High-level vibrational calculations have been used to investigate anharmonicity in a wide variety of materials using density functional theory methods. We have developed an efficient approach for describing strongly anharmonic systems using a vibrational self-consistent field method. By far the most computationally expensive part of the calculations is the mapping of an accurate Born-Oppenheimer (BO) energy surface within the region of interest. Here we present an improved method which reduces the computational cost of the mapping. In this approach we use data from a set of energy calculations for different vibrational distortions of the materials and the corresponding forces on the atoms. Results using both energies and forces are presented for the test cases of the hydrogen molecule, solid hydrogen under high pressure including mapping of two-dimensional subspaces of the BO surface, and the bcc phases of the metals Li and Zr. The use of force data speeds up the anharmonic calculations by up to 40%.