Quasi-two-dimensional semiconductor nanoplatelets (NPLs) are intriguing systems for studying the influence of Auger recombination processes on the multiexciton emission efficiencies in the weak in-plane confinement regime. We investigate CdSe/CdS core/shell NPLs using cryogenic temperature single particle spectroscopy and observe bright biexciton emission at high excitation powers. The average binding energy of the biexcitons is determined to be 16.5 meV. The observed switching between the biexciton and trion states indicates charging-decharging dynamics of the NPLs mediated by the Auger ionization process. These findings are highly relevant for harvesting efficient biexciton emission for energy, lighting, and quantum applications.

Materials exhibiting negative thermal expansion (NTE) are important for the fabrication and operation of microelectronic devices and optical systems. As an important group of Ruddlesden-Popper (RP) perovskites, calcium titanates ${\mathrm{Ca}}_{n+1}{\mathrm{Ti}}_n{\mathrm{O}}_{3n+1}$ [(CTO), $n=1,2,...,\infty$] have layered structures and may exhibit quasi-two-dimensional (quasi-2D) NTE within their three-dimensional structural architectures. In this paper, combining density-functional-theory calculations and the self-consistent quasiharmonic approximation method, we investigate the variation of the quasi-2D character of the phonon spectra and thermal expansion in the ${\mathrm{Ca}}_{n+1}{\mathrm{Ti}}_n{\mathrm{O}}_{3n+1}$ family ($n=1–3$, and $\infty$) with respect to $n$. We find that a quasi-2D NTE mechanism is active in the RP-CTOs at $n$ of 1–3, whereas a quasi-rigid-unit mode mechanism is active at $n=\infty$ (i.e., the perovskite phase). We find a NTE trend with layer number for the orthorhombic materials comprising the RP series, but the monoclinic polymorph is an outlier. For the orthorhombic members, we find the critical pressure for NTE increases with increasing $n$, but the NTE critical temperature decreases (when materials are compared at the same pressure). Additionally, the elastic moduli can be used as effective descriptors for this layer-dependent behavior of the NTE, i.e., the stiffer the RP-CTO then the lower its NTE. We also propose the integrated NTE capacity to capture the correlation between the quasi-2D NTE and $n$, and it monotonically decreases with increasing $n$.

Doped transition-metal dichalcogenide monolayers exhibit exciting magnetic properties for the benefit of two-dimensional spintronic devices. Using density functional theory (DFT) incorporating Hubbard-type correction ($\mathrm{DFT}+U$) to account for the electronic correlation, we study the magnetocrystalline anisotropy energy (MAE) characterizing Mn-doped $M{\mathrm{S}}_2$ ($M$ = Mo, W) monolayers. A single isolated Mn dopant exhibits a large perpendicular magnetic anisotropy of 35 meV (8 meV) in the case of Mn-doped ${\mathrm{WS}}_2$ (${\mathrm{MoS}}_2$) monolayer. This value originates from the Mn in-plane orbitals degeneracy lifting due to the spin-orbit coupling. In pairwise doping, the magnetization easy axis changes to the in-plane direction with a weak MAE compared to single Mn doping. Our results suggest that diluted Mn-doped $M{\mathrm{S}}_2$ monolayers, where the Mn dopants are well separated, could potentially be a candidate for the realization of ultimate nanomagnet units for future magnetic storage applications.

The generation of significant spin imbalance in nonmagnetic semiconductors is crucial for the functioning of many spintronic devices, such as magnetic diodes and transistors, spin-based logic gates, and spin-polarized lasers. An attractive design of spin injectors into semiconductors is based on spin pumping from a precessing ferromagnet, but the classical excitation of magnetization precession by a microwave magnetic field leads to the high-power consumption of the device. Here, we describe theoretically a spin injector with greatly reduced energy losses, in which the magnetic dynamics is excited by an elastic wave generated in a ferromagnet-semiconductor heterostructure by an attached piezoelectric transducer. To demonstrate the efficient functioning of such an injector, we first perform micromagnetoelastic simulations of the coupled elastic and magnetic dynamics in $\mathrm{Ni}$ films and $\mathrm{Ni}/\mathrm{GaAs}$ bilayers traversed by plane longitudinal and shear waves. For thick $\mathrm{Ni}$ films, it is shown that a monochromatic acoustic wave generates a spin wave with the same frequency and wavelength, which propagates together with the driving wave over distances of several micrometers at excitation frequencies $\nu \approx 10$ GHz close to the frequency of ferromagnetic resonance. The simulations of $\mathrm{Ni}/\mathrm{GaAs}$ bilayers with $\mathrm{Ni}$ thicknesses comparable to the wavelength of the injected acoustic wave demonstrate the development of a steady-state magnetization precession at the $\mathrm{Ni}|\mathrm{GaAs}$ interface. The amplitude of such a precession has a maximum at $\mathrm{Ni}$ thickness amounting to three quarters of the wavelength of the elastic wave, which is explained by an analytical model. Using simulation data obtained for the magnetization precession at the $\mathrm{Ni}|\mathrm{GaAs}$ interface, we evaluate the spin current pumped into $\mathrm{GaAs}$ and calculate the spin accumulation in the semiconducting layer by solving the spin diffusion equation. Then the electrical signals resulting from the spin flow and the inverse spin Hall effect are determined via the numerical solution of the Laplace's equation. It is shown that amplitudes of these ac signals near the interface are large enough for experimental measurement, which indicates an efficient acoustically driven spin pumping into $\mathrm{GaAs}$ and a rather high spin accumulation in this semiconductor.

Line defects (LDs) are a kind of lattice imperfection widely existing in two-dimensional materials, which may play a significant role in the properties of materials. Different from the negative effect of LDs for stability in most 2D materials, we have proposed a simple universal rule to construct stable 2D boron isomers based on the LDs engineering, where the self-assembled monolayers are unconventionally enhanced. Based on the self-assembly of LDs from ${\beta }_{12}$ and ${\chi }_3$ monolayers, we have found several boron monolayers with higher stability on the Ag (111) substrate than the experimental phases, and two monolayers in our prediction have been confirmed by the recent experiments. Notably, the mechanical response of the boron monolayers also can be enhanced by the LDs, indicating the potential application for advanced flexible electronic devices.

PdO, widely used in catalysis in powder form for decades, has been predicted recently to be a Dirac semimetal. Synthesis of high-quality single crystals of this material is thus of great interest. Here, by using ozone-assisted molecular beam epitaxy, PdO thin films were grown on a MgO (001) substrate. X-ray diffraction and transmission electron microscopy indicate the film is $a/b$ axis oriented, with the $c$ axis lying in plane. Fully oxygenated PdO films have a low density of holelike carriers, and are insulating at the lowest temperatures. Our density functional theory calculations using the Heyd-Scuseria-Ernzerhof exchange-correlation functional suggest a ∼1.0 eV band gap at the $M$ point, where the gap can be reduced by tensile strain along the $c$ axis and the bands begin to invert when the tensile strain is as high as 18%. Although tensile strain of this magnitude is experimentally not viable using epitaxy, electrons can be doped by oxygen reduction. Our results emphasize the need for careful consideration of electron correlation effects and stoichiometry in ab initio modeling of topological semimetals involving transition metal oxides.

We report a decreased surface wettability when polymer films on a glass substrate are treated by ultrafast laser pulses in a back-illumination geometry. We propose that back illumination through the substrate confines chemical changes beneath the surface of polymer films, leaving the surface blistered but chemically intact. To confirm this hypothesis, we measure the phase contrast of the polymer when imaged with a focused ion beam. We observe a void at the polymer-quartz interface that results from the expansion of an ultrafast laser-induced plasma. A modified polymer layer surrounds the void, but otherwise the film seems unmodified. We also use x-ray photoelectron spectroscopy to confirm that there is no chemical change to the surface. When patterned with partially overlapping blisters, our polymer surface shows increased hydrophobicity. The increased hydrophobicity of back-illuminated surfaces can only result from the morphological change. This contrasts with the combined chemical and morphological changes of the polymer surface caused by a front-illumination geometry.

This manuscript demonstrates ferroelectricity in B-substituted AlN thin films and a complementary set of first-principles calculations to understand their structure-property relationships. ${\mathrm{Al}}_{1–x}{\mathrm{B}}_x\mathrm{N}$ films are grown by dual-cathode reactive magnetron sputtering on $\left(110\right)\mathrm{W}/\left(001\right){\mathrm{Al}}_2{\mathrm{O}}_3$ substrates at 300°C at compositions spanning $x=0$ to $x=0.20$. X-ray diffraction studies indicate a decrease in both the $c$ and $a$ lattice parameters with increasing B concentration, resulting in a decrease in unit cell volume and a constant $c$/$a$ axial ratio of 1.60 over this composition range. Films with $0.02\le x\le 0.15$ display ferroelectric switching with remanent polarizations exceeding $125\mu \mathrm{C}\mathrm{c}{\mathrm{m}}^{–2}$ while maintaining band gap energies of $>5.2\mathrm{eV}$. The large band gap allows low frequency hysteresis measurement (200 Hz) with modest leakage contributions. At B concentrations of $x>0.15$, $c$-axis orientation deteriorates and ferroelectric behavior is degraded. Density-functional theory calculations corroborate the structural observations and provide predictions for the wurtzite $u$ parameter, polarization reversal magnitudes, and composition-dependent coercive fields.

We start by analyzing experimental data of Spinelli et al. [Phys. Rev. B 81, 155110 (2010)] for the conductivity of $n$-type bulk crystals of ${\mathrm{SrTiO}}_3$ (STO) with broad electron concentration $n$ range of $4\times {10}^{15}$–$4\times {10}^{20}{\mathrm{cm}}^{-3}$, at low temperatures. We obtain a good fit of the conductivity data, $\sigma \left(n\right)$, by the Drude formula for $n\ge n_c\simeq 3\times {10}^{16}{\mathrm{cm}}^{-3}$ assuming that used for doping insulating STO bulk crystals are strongly compensated and the total concentration of background charged impurities is $N={10}^{19}{\mathrm{cm}}^{-3}$. At $n<n_c$, the conductivity collapses with decreasing $n$ and the Drude theory fit fails. We argue that this is the metal-insulator transition (MIT) in spite of the very large Bohr radius of hydrogenlike donor state $a_B\simeq 700$ nm with which the Mott criterion of MIT for a weakly compensated semiconductor, $n{a}_B^3\simeq 0.02$, predicts ${10}^5$ times smaller $n_c$. We try to explain this discrepancy in the framework of the theory of the percolation MIT in a strongly compensated semiconductor with the same $N={10}^{19}{\mathrm{cm}}^{-3}$. In the second part of this paper, we develop the percolation MIT theory for films of strongly compensated semiconductors. We apply this theory to doped STO films with thickness $d\le 130$ nm and calculate the critical MIT concentration $n_c\left(d\right)$. We find that, for doped STO films on insulating STO bulk crystals, $n_c\left(d\right)$ grows with decreasing $d$. Remarkably, STO films in a low dielectric constant environment have the same $n_c\left(d\right)$. This happens due to the Rytova-Keldysh modification of a charge impurity potential which allows a larger number of the film charged impurities to contribute to the random potential.

The novel nickelate superconductors of infinite-layer type feature challenging electronic pecularities in the normal-state phase diagram with doping. Distinct many-body behavior and different dispersion regimes of the entangled $\{\mathrm{Ni}\text{−}{d}_{z^2}$, Ni-$d_{x^2-y^2}\}$ orbital sector give rise to highly rich physics, which is here studied for the case of the $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_2$ system. An analysis based on advanced realistic dynamical mean-field theory unveils that the superconducting hole-doped region is the meeting place of a (self-)doped Mott insulator from the underdoped side, and a bad Hund metal from the overdoped side. Fermi-level crossing of the Ni-$d_{z^2}$ flat-band ties both regimes together to form a singular arena for unconventional superconductivity. We furthermore shed light on the intriguing problem of elusive magnetism in infinite-layer nickelates. Antiferromagnetic (AFM) order with small Ni moments is shown to be a vital competitor at low temperature. At stoichiometry, C-AFM order with ferromagnetic spin-alignment along the $c$-axis benefits from a conceivable coexistence with Kondo(-lattice) screening.

We investigate flux-grown Sm-deficient ${\mathrm{Sm}}_x{\mathrm{B}}_6$ ($x<1$) by global and local tools, including x-ray diffraction (XRD), electronic transport, and scanning tunneling microscopy (STM) and spectroscopy (STS). All these tools indicate a remarkable persistence of the ${\mathrm{SmB}}_6$ local structure in the flux-grown samples even for nominal Sm concentrations as low as $x=0.75$. As a consequence, the overall electronic properties of ${\mathrm{Sm}}_x{\mathrm{B}}_6$, and particularly the surface conductance at low temperature, are only affected locally by the Sm deficiency.

${\mathrm{Fe}}_2\mathrm{P}$ alloys have been identified as promising candidates for magnetic refrigeration at room-temperature and for custom magnetostatic applications. The intent of this study is to accurately characterize the magnetic ground state of the parent compound, ${\mathrm{Fe}}_2\mathrm{P}$, with two spectroscopic techniques, $\mu \mathrm{SR}$ and NMR, in order to provide solid bases for further experimental analysis of ${\mathrm{Fe}}_2\mathrm{P}$-type transition metal based alloys. We perform zero applied field measurements using both techniques below the ferromagnetic transition $T_C=220$ K. The experimental results are reproduced and interpreted using first principles simulations, validating this approach for quantitative estimates in alloys of interest for technological applications.

High-energy x-ray diffraction coupled with atomic pair distribution analysis and large-scale computer simulations are used to study the relationship between the local structure and piezoelectric response of an exemplary ferroelectric from the $\mathrm{BaTi}{\mathrm{O}}_3$ family where Ti is partially replaced by nonferroelectric Ce. Our results indicate that, likely, the increase in the piezoelectric response observed for Ce concentration <10% is due to an increased local rhombohedral distortion of the perovskite lattice. Despite a further increase in the distortion, the piezoelectric response for Ce concentration >10% decays quickly, likely because of rapidly increasing nonuniform strain fields due to the size mismatch between Ti- and Ce-centered octahedra and loss of electric dipoles due to the nonferroactivity of the latter. Thus, the transition between the observed two regimes of piezoelectric response does not appear to involve a crossing of a morphotropic phase boundary where the crystallographic symmetry changes abruptly but is likely to be percolative in nature. A similar behavior, referred to as tricritical phenomenon, is observed with other B-site substituted ferroelectrics from the $\mathrm{BaTi}{\mathrm{O}}_3$ family, indicating the presence of a common structural origin. Our results highlight the importance of chemical substitution-driven rhombohedral distortions in achieving control over the piezoelectric response of perovskite ferroelectrics, thereby providing a different perspective on the ongoing effort to improve their performance in practical applications.

This paper describes manipulation of the antiferromagnetic (AFM) moment by external magnetic field in thin epitaxial nanofilms of $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$—the iron oxide polymorph known for its canted antiferromagnetism above the Morin temperature. The field-driven rotation of the AFM moment has been detected in thin $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$/GaN layers by monitoring the azimuthal field dependence of the x-ray magnetic linear dichroism line shape at $L_{23}$ absorption edge of iron. The presence of the tiny canted ferrimagnetic (FM) moment in $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ makes it possible to manipulate the related AFM moment by applying external magnetic field, monitoring the field-driven rotation of both AFM and FM moments that otherwise can be hardly detected in a nanoscale film by conventional magnetometry. A controllable manipulation of the AFM moment in a thin film by external magnetic field is supposed to be an important keystone for development of the emerging field of AFM spintronics.

We combine magnetometry and magnetic small-angle neutron scattering to study the influence of the microstructure on the macroscopic magnetic properties of a nanocrystalline Ni bulk sample, which was prepared by straining via high-pressure torsion (HPT). As seen by magnetometry, the mechanical deformation leads to a significant increase of the coercivity compared with nondeformed polycrystalline Ni. The neutron data reveal a significant spin-misalignment scattering caused by the high density of crystal defects inside the sample, which were created by the severe plastic deformation during the sample preparation. The corresponding magnetic correlation length, which characterizes the spatial magnetization fluctuations in real space, indicates an average defect size of 11 nm, which is smaller than the average crystallite size of 60 nm. In the remanent state, the strain fields around the defects cause spin disorder in the surrounding ferromagnetic bulk, with a penetration depth of ∼22 nm. The range and amplitude of the disorder is systematically suppressed by an increasing external magnetic field. Our findings are supported and illustrated by micromagnetic simulations, which for the case of nonmagnetic defects (holes) embedded in a ferromagnetic Ni phase, further highlight the role of localized spin perturbations for the magnetic microstructure of defect-rich magnets such as HPT materials.

The ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ and GeTe compounds are of interest for applications in phase change memories. In the reset process of the memory the crystal is rapidly brought above the melting temperature $T_m$ by Joule heating and then the liquid phase rapidly cools down leading to the formation of the amorphous phase. Since the liquid above $T_m$ is metallic and the amorphous phase is semiconducting a semiconductor-to-metal transition occurs in the supercooled liquid. Based on density functional simulations, we estimated the metal-semiconductor transition temperature $T_{\text{M-SC}}$ by monitoring the opening of a band gap in the supercooled liquid phase. Due to previous evidence on the importance of the van der Waals (vdW) interaction in describing the liquid phase of these materials, we used both the revised Vydrov-van Voorhis functional which includes vdW nonlocal interactions and the Perdew-Burke-Ernzerhof functional without vdW corrections. The estimated $T_{\text{M-SC}}$ is about 100–150 K higher with the former than with the latter framework for both compounds. By including vdW interactions the estimated $T_{\text{M-SC}}$ is closer to $T_m$ than to the glass transition temperature for both systems. The analysis of the structural properties as a function of temperature suggests a correlation between the metal-semiconductor transition and a Peierls distortion. However, the data support more a continuous structural transformation than the presence of a first order liquid-liquid phase change associated the metal-semiconductor transition.

The complex interplay between energetic and kinetic factors that governs the phase and morphology selections can originate at the earliest stage of crystallization in the amorphous parent phases. Because of the extreme difficulties in capturing the microscopic nucleation process, a detailed picture of how initial disordered structures affect the transformation pathway remains unclear. Here, we report the experimental observation of widely varying phase selection and grain size evolution during the devitrification of a homogeneous melt-spun glassy ribbon. Two different crystalline phases $\theta \text{−}{\mathrm{Al}}_5\mathrm{Sm}$ and $\varepsilon \text{−}{\mathrm{Al}}_{60}{\mathrm{Sm}}_{11}$ are found to form in the different regions of the same metallic glass (MG) ribbon during the devitrification. The grain size of the $\varepsilon \text{−}{\mathrm{Al}}_{60}{\mathrm{Sm}}_{11}$ phase shows a strong spatial heterogeneity. The coarse-grained $\varepsilon \text{−}{\mathrm{Al}}_{60}{\mathrm{Sm}}_{11}$ phase coupled with the small volume fraction of the $\theta \text{−}{\mathrm{Al}}_5\mathrm{Sm}$ phase is preferably formed close to the wheel side of the melt-spun ribbon. Combining experimental characterization and computational simulations, we show that phase selection and microstructure evolution can be traced back to different types and populations of atomic clusters that serve as precursors for the nucleation of different crystalline phases. Inhomogeneous cooling rates cause different structure orders across the glass sample during the quenching process. Our findings provide direct insight into the effect of structural order on the crystallization pathways during the devitrification of MG. It also opens an avenue to study the detailed nucleation process at the atomic level using the MG as a platform and suggests the opportunity of microstructure and property design via controlling the cooling process.