The transition temperature ($T_{\mathrm{C}}$) between normal and superconducting states usually exhibits a dramatic increase or decrease with increasing applied pressure. Here we present, in contrast, a type of superconductor that exhibits the exotic feature that $T_{\mathrm{C}}$ is robust against large volume shrinkages (RSAVS, so naming them “RSAVS superconductors”) induced by applied pressure. Extraordinarily, our previous studies found that the $T_{\mathrm{C}}$ in the two materials stays almost constant over a large pressure range, e.g., over 136 GPa in the ${\left(\mathrm{TaNb}\right)}_{0.67}{\left(\mathrm{HfZrTi}\right)}_{0.33}$ high-entropy alloy and 141 GPa in the NbTi commercial alloy. Here, we show that the RSAVS behavior also exists in another high-entropy alloy, ${\left(\mathrm{ScZrNbTa}\right)}_{0.6}{\left(\mathrm{RhPd}\right)}_{0.4}$, and in superconducting elemental Ta and Nb, indicating that this behavior occurs universally in a certain kind of superconductor, composed of only transition metal elements, with a body-centered cubic lattice. Our electronic structure calculations indicate that in the RSAVS state the contribution of the degenerate $d_{x^2-y^2}$ and $d_{z^2}$ orbital electrons remains almost unchanged at the Fermi level, suggesting that these are the electrons that may play a crucial role in stabilizing the $T_{\mathrm{C}}$ in the RSAVS state. We preliminarily analyzed the reasonability and validity of this suggestion by the Homes law.

Transition metal dichalcogenide (TMD) materials have received enormous attention due to their extraordinary optical and electrical properties, with ${\mathrm{MoS}}_2$ being one of the most representative examples. As the thickness increases from monolayer to multilayer, the photoluminescence (PL) of ${\mathrm{MoS}}_2$ is gradually quenched due to the direct-to-indirect band gap transition. How to enhance PL response and decrease the layer dependence in multilayer ${\mathrm{MoS}}_2$ remains a challenge. In this work, we report simultaneous generation of three PL peaks at around 1.3, 1.4, and 1.8 eV on multilayer ${\mathrm{MoS}}_2$ bubbles. The temperature dependent PL measurements indicate that the two peaks at 1.3 and 1.4 eV come from phonon-assisted indirect-gap transitions while the peak at 1.8 eV comes from the direct-gap transition. The weakening of interlayer coupling on multilayer ${\mathrm{MoS}}_2$ bubbles, which may account for the emergence of PL peaks, is confirmed by the low-frequency Raman spectroscopy. Using first-principles calculations, the band structure evolution of multilayer ${\mathrm{MoS}}_2$ under strain is studied, from which the origin of the three PL peaks of ${\mathrm{MoS}}_2$ bubbles is further confirmed. Moreover, PL standing waves are observed in ${\mathrm{MoS}}_2$ bubbles that create Newton-Ring-like patterns. This work demonstrates that the bubble structure may provide new opportunities for engineering the electronic structure and optical properties of layered materials.

A suspended layer made up of ferromagnetically ordered spins could be created between two- monolayer or multilayer graphene through intercalation. Stability and electronic structure studies show that, when fluorine molecules are intercalated between two mono/multilayer graphene, their bonds get stretched enough $(\sim 1.9\text{–}2.0\AA )$ to weaken their molecular singlet eigenstate. Geometrically, these stretched molecules form a pseudoatomized fluorine layer by maintaining a van der Waals separation of $\sim 2.6\AA$ from the adjacent carbon layers. As there is a significant charge transfer from the adjacent carbon layers to the fluorine layers, a mixture of triplet and doublet states stabilizes to induce local spin moments at each fluorine site and in turn form a suspended two-dimensional spin lattice. The spins of this lattice align ferromagnetically with nearest-neighbor coupling strength as large as $\sim 100\mathrm{meV}$. Our finite-temperature ab initio molecular dynamics study reveals that the intercalated system can be stabilized up to a temperature of 100 K with an average magnetic moment of $\sim 0.6{\mu }_B/\mathrm{F}$. However, if the graphene layers can be held fixed, the room-temperature stability of such a system is feasible.

Lead halide perovskites have emerged as new optoelectronic materials owing to their outstanding optical properties. There has been increased interest in their temperature-sensitive optical properties and new optical applications have been proposed thereby. Here, we report the origin of the unusual negative thermo-optic coefficient of the halide perovskite $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{PbC}{\mathrm{l}}_3$, i.e., a decrease in the refractive index by an increase in temperature. From the temperature dependences of the absorption spectrum and the lattice constant and using the Lorentz oscillator model, we conclude that the negative thermo-optic coefficient below the absorption edge is predominantly determined by the large thermal expansion coefficient inherent to this soft material system. This work demonstrates that the negative thermo-optic coefficient is a distinctive phenomenon reflecting the unique electronic and lattice properties of halide perovskites.

The formation of an ultrathin aluminum oxide film at the ${\mathrm{Fe}}_{0.85}{\mathrm{Al}}_{0.15}$(110) surface (${\mathrm{A}}_2$ random alloy) has been studied by a variety of surface-sensitive techniques (x-ray photoemission, low-energy electron diffraction, surface x-ray diffraction, and scanning tunneling microscopy) supplemented by ab initio atomistic simulations. Since iron is not oxidized in the conditions used, the study focused on the coupling between aluminum oxidation and segregation processes. Compared to the bare surface, whose average composition (${\mathrm{Fe}}_{0.6}{\mathrm{Al}}_{0.4}$) is closer to the ${\mathrm{B}}_2$-CsCl structure over a $\sim 3$ nm depth, the oxidation hardly affects the subsurface segregation of aluminum. All the structural and chemical fingerprints point to an oxide film similar to that found on NiAl(110). It is a bilayer ($\sim 7.5\AA$ thick) with a composition close to ${\mathrm{Al}}_{10}{\mathrm{O}}_{13}$ and a large $(18.8\times 10.7){\AA }^2$ nearly rectangular unit cell; an almost perfect match between substrate periodicity and the $(1\times 2)$ oxide supercell is found. Nevertheless, microscopy reveals the presence of antiphase domain boundaries. Measured Al $2p$ and O $1s$ core-level shifts match calculated ones; their origin and the relative contributions of initial/final state effects are discussed. The ubiquity of the present oxide on different supports asks for the origin of its stability.

Enhancing the electromechanical response by engineering domain boundaries in multiferroics has become a highly active research field in recent years. The starting point is the discovery that ferroelastic twin walls are polar inside a nonpolar matrix. The density of such twin walls is then greatly enhanced by forming complex twin patterns. Our computer simulations show that the interaction of nanocavities with differently charged configurations with twin boundaries generates strong piezoelectricity in ferroelastic (nonferroelectric) crystals. Cavity-induced domain patterns statistically break the inversion symmetry of a sample even when the cavities themselves obey inversion symmetry with relatively weak emerging piezoelectricity $(d\sim {10}^{-3}\mathrm{pm}/\mathrm{V})$ . Stronger piezoelectricity occurs in noncentrosymmetric charged cavity arrangements with a coefficient of $d\sim {10}^{-1}\mathrm{pm}/\mathrm{V}$. Structurally, the electric field polarizes and shifts the nanocavities by the displacement of trapped surface charges. The related strain fields interact with the ferroelastic domains, which act as soft bridges between the nanocavities. This leads to a significant deformation of the entire sample and hence to enhanced piezoelectricity. Our simulation results point to new directions for designing and enhancing electromechanical nanodevices based on ferroelastic templates even when the bulk material is structurally centrosymmetric.

While $f$-electron intermetallics are well-known as hosts for complex valence behavior, magnetism, and unconventional superconductivity, it remains a persistent challenge to design specific phenomena. Some guidance is provided by studies of large families of materials, such as those based on the ${\mathrm{AlB}}_2, {\mathrm{AuCu}}_3$, and ${\mathrm{BaAl}}_4$ structural subunits, where systematic trends in their electronic behavior have been observed. This suggests that an effective approach in developing designer materials is to incorporate multimodal stacking arrangements of well-understood building blocks. Here, we report results for ${\mathrm{Ce}}_7{\mathrm{Co}}_6{\mathrm{Ge}}_{19}$, the $n=6$ member of a new homologous series, ${A_n}_{+1}{B}_n{X}_{3n+1}$, that consists of subunits previously identified as having potential for unconventional magnetic behavior. We show that ${\mathrm{Ce}}_7{\mathrm{Co}}_6{\mathrm{Ge}}_{19}$ exhibits complex magnetism with evidence for frustration and Kondo lattice behavior that is directly related to what is seen in other $n$ value analogs. Here, the added structural units introduce additional magnetic degrees of freedom, demonstrating that it is possible to rationally design magnetic complexity, and possibly other phenomena in this family of intermetallics.

Tilt $\left\{112\right\}$ grain boundaries (GBs) in bcc metals perform shear-coupled grain-boundary migration by the creation and glide of disconnections. Disconnection dipoles may be created at the pristine GB at high stresses or may be generated at the core of a GB dislocation that acts as a source of disconnections. We characterize this source in terms of its Burgers vector, denoted ${\vec{b}}_{1/-1}$, and describe the mechanism that allows the source to move conservatively with the GB. The ${\vec{b}}_{1/-1}$ grain-boundary dislocation is created, for instance, during the absorption of a crystal dislocation by the $\left\{112\right\}$ grain boundary. In addition, ${\vec{b}}_{1/-1}$ accommodates $\left\{112\right\}$ vicinal grain boundaries that are formed by segments of $\left\{112\right\}$ planes separated by ${\vec{b}}_{1/-1}$ grain-boundary dislocations. The presence of these ${\vec{b}}_{1/-1}$ dislocations facilitates the conservative displacement of both the pristine and the vicinal GBs. We show that the creation of disconnections is the key for the absorption of edge and screw dislocations by the GB and the drag of mixed dislocations by the GB during its migration. These conservative processes are efficient ways to accommodate plastic deformation by the growth and shrink of $\left\{112\right\}$ twins, and shear-coupled motion of the $\left\{112\right\}$ GB and its vicinal GBs.

In spite of intensive studies on the chalcogenides as conventional thermoelectrics, it remains a challenge to find a proper material with high electrical but low thermal conductivities. In this work, we introduced a new class of thermoelectrics, Ge-based inorganic halide perovskites $\mathrm{CsGe}{X}_3$ $(X=\mathrm{I}, \mathrm{Br}, \mathrm{and} \mathrm{Cl})$, which were already known as a promising candidate for photovoltaic applications. By performing the lattice-dynamics calculations and solving the Boltzmann transport equation, we revealed that these perovskites have ultralow thermal conductivities below 0.18 W ${\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ while very high carrier mobilities above 860 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, being much superior to the conventional thermoelectrics of chalcogenides. These results highlight the way of searching high-performance and low-cost thermoelectrics based on inorganic halide perovskites.

Hybrid improper ferroelectricity (HIF) is recognized as a promising way to design room-temperature multiferroic materials with strong magnetoelectric coupling. However, predicting new HIF materials with high ferroelectric polarization remains to be a great challenge. Here, the new compound $\mathrm{B}{\mathrm{a}}_3\mathrm{C}{\mathrm{e}}_2{\mathrm{O}}_7$ with Ruddlesden-Popper (R-P) structure, which crystallizes in polar $A{2}_{1}am$ phase has been proposed and its HIF has been demonstrated using the first-principles calculation. The ferroelectric polarization stems from the condensation of in-phase rotation and out-of-phase tilt modes. The correlations between orbital interactions, structural distortions and ferroelectric properties have also been clarified. Moreover, the presence of negative hydrostatic pressure will generate large octahedral distortions and noncancelled anti-ferrodistortive displacements at $A$-site, resulting in the significant enhancement of polarization. The pressure engineering renders a guideline for tuning HIF and designing ferroelectrics in R-P compounds.

The incorporation of point defects into semiconductors could substantially tailor their optical and electrical properties as well as the spin-based quantum properties. In terms of structural properties, however, efforts have seldom been devoted to the relevant aspects. Herein, we propose point defects engineering by intentionally introduced vacancies to improve the structural properties. GaN-on-Si are selected as a paradigm to demonstrate the applicability of this approach. By tuning the growth stoichiometry, nonequilibrium Ga vacancies are intentionally introduced and absorbed by dislocation cores, which leads to dislocation inclination and annihilation in GaN. In addition, this dislocation inclination can proceed without relaxing the compressive lattice stress. These together enable high quality GaN thick layers on Si substrates with dislocation density of $1.6\times {10}^8\mathrm{c}{\mathrm{m}}^{-2}$ and a record electron mobility of $1090\mathrm{c}{\mathrm{m}}^2/\mathrm{V}\mathrm{s}$ at a carrier density of $1.3\times {10}^{16}\mathrm{c}{\mathrm{m}}^{-3}$. With these advances, a quasivertical GaN Schottky barrier diode with the lowest specific on-resistance of $0.95\mathrm{m}\mathrm{\Omega }/\mathrm{c}{\mathrm{m}}^2$ and highest on/off ratio of ${10}^{10}$ on Si substrates is demonstrated. These results demonstrate the promise of point defect engineering as a strategy to improve the structural properties and pave the way for high-performance III-nitride based electronic and optoelectronic devices on Si platforms.

Ultrathin films and multilayers, with controlled thickness down to single atomic layers, are critical for advanced technologies ranging from nanoelectronics to spintronics to quantum devices. However, for thicknesses less than 10 nm, surfaces and dopants contribute significantly to the film properties, which can differ dramatically from that of bulk materials. For amorphous films being developed as low dielectric constant interfaces for nanoelectronics, the presence of surfaces or dopants can soften films and degrade their mechanical performance. Here we use coherent short-wavelength light to fully and nondestructively characterize the mechanical properties of individual films as thin as 5 nm within a bilayer. In general, we find that the mechanical properties depend both on the amount of doping and the presence of surfaces. In very thin (5-nm) silicon carbide bilayers with low hydrogen doping, surface effects induce a substantial softening—by almost an order of magnitude—compared with the same doping in thicker (46-nm) bilayers. These findings are important for informed design of ultrathin films for a host of nano- and quantum technologies, and for improving the switching speed and efficiency of next-generation electronics.

Graphene functionalization by hydrogen and fluorine has been proposed as a route to modulate its reactivity and electronic properties. However, until now, proposed systems have presented degradation and limited hydrogen adsorption capacity. In this paper, combining first-principles calculations based on density-functional theory and reactive molecular dynamics, we analyze the tuning of hydrogen adsorption and electronic properties in fluorinated and hydrogenated monolayer graphene. Our results indicate that fluorine adsorption promotes stronger carbon–hydrogen bonds. By changing the concentration of fluorine and hydrogen, charge-density transfer and electronic properties such as the band gap and spin-splitting can be tailored, increasing their potential applicability for electronic and spintronic devices. Despite fluorine not affecting the total H incorporation, the ab initio molecular dynamics results suggest that 3% fluorinated graphene increases hydrogen anchoring, indicating the hydrogenated and fluorinated graphene potential for hydrogen storage and related applications.

The electronic and magnetic properties of the magnetoelectric solid ${\mathrm{GaFeO}}_3$ are investigated within the generalized gradient approximation including the Hubbard interaction ($U$) on the localized $d$ orbitals of iron. It was found that using an onsite $U=8$ eV describes consistently the experimental results. The origin of ferrimagnetism was attributed to the cationic site disorders. The density of states at the iron sites, in a octahedral geometry, show that the occupied $e_g$ states are below the $t_{2g}$ states in contradiction with the crystal-field splitting obtained by a point-charge model. However, for the unoccupied states the ab initio data agree qualitatively with the model, showing the complexity of the electron-electron interaction in ${\mathrm{GaFeO}}_3$. The computed electric polarization of the system as a function of the temperature in the linear regime shows a monotonic decreasing trend. To determine the nature of the magnetoelectric coupling, the computed electric polarization as a function of the rotation of the magnetization axis indicates that the magnetoelectric effect observed experimentally could not have been due to a direct coupling between the electric and magnetic order parameters. Finally, the calculated x-ray absorption and x-ray magnetic circular dichroism spectra for the disordered system are shown to be in good agreement with experiment.

We theoretically investigate the application of topological plasmon polaritons (TPPs) to achieving robust temperature sensing. Based on an analogy of the topological edge states in the Su-Schrieffer-Heeger model, TPPs are realized in a one-dimensional intrinsic indium antimonide (InSb) microsphere chain. The existence of TPPs is demonstrated by analyzing the topology of the photonic band structures and the eigenmode distribution. Due to the temperature dependence of the permittivity of InSb in the terahertz range, the resonance frequency of TPPs can be largely tuned by the temperature. The temperature susceptibility of the TPP resonance frequency can be as high as $0.0264\mathrm{THz}$/K at room temperature, leading to a figure of merit over 150. Based on calculations on the optical local density of states (LDOS) near the chain, it is shown that the temperature susceptibility of TPPs is experimentally detectable via near-field probing techniques. Numerical results also indicate the sensing performance is immune to disorder. We further propose a robust and practical calibration method to correctly obtain the peak frequency of the LDOS spectrum due to the TPPs and then the temperature susceptibility, which is based on the ratio between the LDOS of a topological chain and that of a nontopological one. These TPPs can be potential candidates for robust temperature sensing, for which several experimental considerations are further discussed.

Extended planar defects found in epitaxially grown $\mathrm{SrFe}{\mathrm{O}}_{3\text{−}\delta }$ thin films are expected to exhibit distinct conductivity properties. Here, we use a combination of scanning transmission electron microscopy techniques and electron energy-loss spectroscopy (EELS) to uncover the peculiar structure of these planar defects and to explore their electronic properties. We find that the defects are formed by $\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_{2+\alpha }$ layers consisting of $\mathrm{Fe}{\mathrm{O}}_5$ polyhedra alternating with SrO and $\mathrm{Fe}{\mathrm{O}}_2$ perovskite-type layers, analogous to the $\mathrm{S}{\mathrm{r}}_4\mathrm{F}{\mathrm{e}}_6{\mathrm{O}}_{12+\delta }$ crystal structure. Our experimental and theoretical EELS data, combined with projected density of states calculations, reveal peak width changes and energies shifts, which suggest an increased electron doping of the Fe $3d{e}_g$ band in the $\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_{2+\alpha }$ layers as compared to the $\mathrm{SrFe}{\mathrm{O}}_{3-\delta }$ film. Thus we show that the presence of $\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_{2+\alpha }$ planar defects indeed can effectively modify the electron-hole conductivity in $\mathrm{SrFe}{\mathrm{O}}_{3\text{−}\delta }$ films.

The past decade has witnessed the rapid introduction of organic-inorganic hybrid compounds in photovoltaic applications. Motivated by the strong demand for stable and nontoxic materials in this class, we report a theoretical study on the structural, elastic, electronic, thermodynamic and dielectric properties of alkali-metal-based bismuth ternary chalcogenides. In particular, we employ state-of-the-art density functional theory to explore the potential of $A\mathrm{Bi}{X}_2$ and $A\mathrm{Bi}{X}_3$ ($A=\mathrm{Na}$, K and $X=$ O, S) as light-absorbing media. All the compounds under investigation are found to be thermodynamically and mechanically stable, with a semiconductor band structure. The Kohn-Sham band gaps range between 0.80 eV and 1.80 eV, when calculated with semilocal functionals, values that increase to 1.24–2.47 eV with hybrid ones. Although all but ${\mathrm{NaBiO}}_2$ and ${\mathrm{KBiO}}_2$ are indirect band-gap semiconductors, the onset of the imaginary part of their dielectric functions, the optical gap, is only marginally larger than the quasiparticle gap. This is due to the generally flat nature of both the conduction and the valence bands. We then expect these compounds to absorb light in the upper part of the visible spectrum. In all cases the valence band is dominated by $\mathrm{O}\text{−}p$ and S-$p$ orbitals and the conduction one by Bi-$p$, suggesting the possibility of excitons with low binding energy. The only exceptions are ${\mathrm{NaBiO}}_2$ and ${\mathrm{KBiO}}_2$ for which the $\mathrm{O}\text{−}p$ states dominate the density of states at both sides of the band gap.