Accurate determination of the exciton binding energy and reduced effective mass in bulk halide perovskites is of utmost importance for the selective design of optoelectronic devices. Although these properties are currently determined by several spectroscopic techniques, complementary theoretical models are often required to bridge macroscopic and microscopic properties. Here, we present a method to determine these quantities while considering polarization effects due to carrier interactions with longitudinal optical phonons. Our approach estimates the exciton-polaron binding energy from optical absorption measurements using a recently developed Elliott-based band fluctuations (EBF) model. The reduced effective mass is obtained via the integration of this well-established EBF model with the Pollmann–Büttner exciton-polaron model, which is based on the Fröhlich polaron framework, where the strength of the electron–phonon interaction arises from changes in the dielectric properties. The procedure is applied to the family of perovskites ${\mathrm{ABX}}_3$ $(\mathrm{A}=\mathrm{MA}, \mathrm{FA}, \mathrm{Cs}; \mathrm{B}=\mathrm{Pb}; \mathrm{X}=\mathrm{I}, \mathrm{Br}, \mathrm{Cl})$, showing excellent agreement with high-field magnetoabsorption and other optical-resolved techniques. The results suggest that the Pollmann–Büttner model offers an excellent approach for determining the reduced effective mass in metal trihalide perovskites and other polar materials when combined with the EBF optical dispersion model.

The ternary intermetallic compounds, ${\mathit{R}\mathrm{Mn}}_6{\mathrm{Sn}}_6$ ($R$ = rare-earth or alkali or alkaline-earth metal) hosting Kagome bilayer of Mn, present a class of systems that naturally allows for the interplay of magnetism and topology. In search of ferromagnetic members in these systems, we show that dimensional reduction can be an effective means to achieve ferromagnetism and large anomalous Hall conductivity. Focusing on two specific members with $R$ = Lu and Mg, our first-principles calculations show that dimensional confinement through the construction of experimentally relevant atomistically thin single bilayer kagome geometry of Mn atoms gives rise to a ferromagnetic metal with multiple Weyl points along with spin-orbit-coupling induced gapped bands. The calculated anomalous Hall conductivity in the thin film geometry turns out to be 287 and $178{\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1}$, comparable to that found for bulk ${\mathrm{RMn}}_6{\mathrm{Sn}}_6$ with alkali or alkaline-earth metal at $R$ site. This is of interest as thin-film geometry can be efficiently integrated to devices.

Pulsed laser epitaxial growth is characterized by high instantaneous deposition rates that lead to the nucleation of transient islands, both on surface terraces and on top of stable islands formed during previous deposition pulses. We report results from combined $\mathit{in}$ situ x-ray reflectivity and kinetic Monte Carlo (kMC) simulations. Specular reflectivity monitors interlayer transport, while diffuse scattering reveals the evolution of in-plane length scales, both during the recovery time between individual laser pulses and over multiple deposited layers. The initial stage after each laser pulse is faster than the temporal resolution of the experiment, while subsequent recovery occurs over seconds. The results suggest that transient islands on top of stable two-dimensional islands form immediately after the deposition pulse and then ripen via detachment and diffusion, leading to the slower component. kMC simulations show that the detachment energy barrier plays a dominant role in determining the recovery time constant.

Oxynitrides are used in a variety of applications including photocatalysts, high-k dielectrics, or wear-resistant coatings and often show intriguing multifunctionality. To accelerate the cooptimization of the relevant material properties of these compositionally complex oxynitride systems, high-throughput synthesis and characterization methods are desirable. In the present work, three approaches were investigated to obtain orthogonal anion and cation gradients on the same substrate by magnetron sputtering. The different approaches included varying positions of the local reactive gas inlets and different combinations of target materials. The best performing approach was applied to screen a large two-dimensional area of the quaternary phase space within the Al-Si-O-N system. This material system is a promising candidate for transparent protective coatings with variable refractive indices. With only five depositions of combinatorial libraries, an anion composition range of 0.02–0.46 O/(N+O) and a cation composition range of 0.04–0.44 Si/(Al+Si) is covered. For lower oxygen and silicon contents, a region with hardness of up to 25 GPa is observed, where the material exhibits either wurzite AlN or a composite microstructure. By increasing the deposition temperature to 400°C, an extension of this region can be achieved. At higher oxygen and silicon contents, the structure of the samples is x-ray amorphous. In this structural region, an intimate correlation between hardness and refractive index is confirmed. The results of this study introduce a practical approach to perform high-throughput development of mixed anion materials, which is transferable to many materials systems and applications.

The search for chiral topological superconductivity in magnetic topological insulator (TI)-FeTe heterostructures is a key frontier in condensed matter physics, with potential applications in topological quantum computing. The combination of ferromagnetism, superconductivity, and topologically nontrivial surface states brings together the key elements required for chiral Majorana physics. In this work, we examine the interplay between magnetism and superconductivity at the interfaces between FeTe and a series of Te-based TI overlayers. In both Te/FeTe and superconducting ${\mathrm{MnBi}}_2{\mathrm{Te}}_4/\mathrm{FeTe}$, any interfacial suppression of antiferromagnetism must affect at most a few nanometers. On the other hand, ${(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3/\mathrm{FeTe}$ layers exhibit near-total suppression of antiferromagnetic ordering. Ferromagnetic ${\mathrm{Cr}}_x{(\mathrm{Bi},\mathrm{Sb})}_{2-x}{\mathrm{Te}}_3$ (CBST)/FeTe bilayers exhibit net magnetization in both CBST and FeTe layers, with evidence of interactions between superconductivity and ferromagnetism. These observations identify magnetic TI/FeTe interfaces as an exceptionally robust platform to realize chiral topological superconductivity.

Colossal magnetoresistance (CMR) has been observed across many ${\mathrm{Eu}}^{2+}$-based materials; however, its origin is not completely understood. Here we investigate the antiferromagnetic insulator ${\mathrm{EuZn}}_2{\mathrm{P}}_2$ through single crystal x-ray diffraction, transmission electron microscopy, electrical transport, magnetization, dilatometry, and electron spin resonance measurements complemented by density functional theory calculations. Our electrical resistivity data reveal a large negative magnetoresistance, $\text{MR}=\left[R\right(H)-R(0\left)\right]/R\left(0\right)$, that reaches $\text{MR}=-99.7$% at 9 T near the antiferromagnetic ordering temperature $T_N=23\mathrm{K}$. Dilatometry measurements show an accompanying field-induced lattice strain. Additionally, ${\mathrm{Eu}}^{2+}$ electron spin resonance reveals a strong ferromagnetic exchange interaction between ${\mathrm{Eu}}^{2+}$ and conduction electrons. Our experimental results in ${\mathrm{EuZn}}_2{\mathrm{P}}_2$ are consistent with a magnetic polaron scenario and suggest magnetic polaron formation as a prevailing explanation of CMR in ${\mathrm{Eu}}^{2+}$-based compounds.

We employ first-principles calculations based on hybrid density functional theory to investigate the structural and optical properties of the oxygen vacancy $\left(V_{\mathrm{O}}\right)$ and aluminum vacancy $\left(V_{\mathrm{Al}}\right)$ in $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$, the most stable (corundum) phase of alumina. Our calculations facilitate the identification of experimental excitation and luminescence spectra with specific electronic transitions at the vacancy sites. The absorption line shape for excitation of an electron at $V_{\mathrm{O}}^0$ to the conduction-band minimum (CBM) is in excellent agreement with the 6.1 eV band detected by optical absorption spectroscopy, and we find that the 5.9 eV absorption band is generated by an internal electron transition at $V_{\mathrm{O}}^0$. We confirm that the slowly decaying 3.0 eV emission band of the $F$ center is due to a triplet-singlet transition at $V_{\mathrm{O}}^0$. Our calculations also reveal that the 4.8 eV/5.4 eV absorption and 3.8 eV emission bands assigned to the $F^{+}$ center are generated by internal transitions at $V_{\mathrm{O}}^{+1}$. The line shape for the excitation of an electron from $V_{\mathrm{O}}^{+1}$ to the CBM agrees well with an absorption band at 6.4 eV, while the recombination of an electron with $V_{\mathrm{O}}^{+2}$ also produces luminescence around 3.8 eV, but with a considerably broader line shape. For Al vacancies, we confirm that the most prevalent configuration in $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ is a split-vacancy configuration $V_{\mathrm{Al},\mathrm{s}}$, and we calculate migration barriers for different directions in the corundum crystal. We predict absorption and emission spectra for the excitation and recombination of an electron localized at $V_{\mathrm{Al}}^{-3}$ and $V_{\mathrm{Al},\mathrm{s}}^{-3}$ sites with the CBM. The line shapes of the two $V_{\mathrm{Al}}$ configurations overlap and are considerably broader than the spectra corresponding to $V_{\mathrm{O}}$.

We perform nanoindentation simulations for both the prototypical face-centered cubic metal copper and the body-centered cubic metal tungsten with an adaptive-precision description of interaction potentials including different accuracy and computational costs. We combine both a computationally efficient embedded atom method (EAM) potential and a precise but computationally less efficient machine learning potential based on the atomic cluster expansion (ACE) into an adaptive precision (AP) potential tailored for the nanoindentation. The numerically more expensive ACE potential is employed selectively only in regions of the computational cell where high precision is required. The comparison with pure EAM and pure ACE simulations shows that for Cu, all potentials yield similar dislocation morphologies under the indenter with only small quantitative differences. In contrast, markedly different plasticity mechanisms are observed for W in simulations performed with the central-force EAM potential compared to results obtained using the ACE potential. ACE is able to describe accurately the angular character of bonding, which is in W due to its half-filled $d$ band. All ACE-specific mechanisms are reproduced in the AP nanoindentation simulations, however, with a significant speedup of 20–30 times compared to the pure ACE simulations. Hence, the AP potential overcomes the performance gap between the precise ACE and the fast EAM potential by combining the advantages of both potentials.

Disorder in high entropy alloys, arising from the random distribution of multiple elements, plays a crucial role in their novel properties desirable for various advanced engineering applications. We investigate the role of compositional and structural disorder on the electronic structure of osmium-based superconducting high entropy alloys, ${(\mathrm{Ru}/\mathrm{Re})}_{0.35}{\mathrm{Os}}_{0.35}{\mathrm{Mo}}_{0.10}{\mathrm{W}}_{0.10}{\mathrm{Zr}}_{0.10}$, using photoemission spectroscopy and density functional theory (DFT). Elemental and cumulative core level shifts are found to be commensurate with elemental electronegativities and valence electron counts (VEC), respectively. Valence band spectra together with DFT results indicate that the crystal structure plays an important role in deciding the electronic structure of these high entropy alloys. Through temperature dependent high-resolution spectra, we unveil strongly suppressed spectral density of states (SDOS) in the close vicinity of Fermi level. Energy and temperature dependence of the SDOS in accordance with Altshuler-Aronov theory confirms localization of charge carriers in the presence of strong intrinsic disorder. Computed electron-phonon coupling strength and superconducting transition temperature aligning reasonably well with experiments further shed light on phonon-mediated pairing mechanism and role of disorder in these systems. Our results provide a way forward to the understanding of superconducting high entropy alloys through strategic control of disorder, VEC, and crystal structure.

Magneto-optical materials with low optical absorption are essential for realizing nonreciprocal light propagation in integrated photonic circuits. Among them, cerium-substituted yttrium iron garnet (Ce:YIG) stands out for its exceptionally high Faraday rotation. In this work, we investigate how film thickness and deposition atmosphere influence the structural, magnetic, and magneto-optical properties of Ce:YIG (${\mathrm{Ce}}_1{\mathrm{Y}}_2{\mathrm{Fe}}_5{\mathrm{O}}_{12}$) films. Using pulsed laser deposition in argon and oxygen environments, we grew coherently strained, smooth films on (111)-oriented gadolinium gallium garnet (GGG) substrates, with thicknesses up to 220 nm. Films grown in argon exhibit a significantly expanded out-of-plane lattice parameter, inducing perpendicular magnetic anisotropy, whereas those deposited in oxygen display in-plane magnetization. Remarkably, Ce:YIG films grown in argon show up to a twofold increase in magneto-optical activity compared to their oxygen-grown counterparts, achieving a Faraday rotation of $5.0{}^{\circ }/\mu \mathrm{m}$ at 780 nm. This enhancement is attributed to prolonged electronic relaxation times and longer-lived excitations in the more heavily strained films.

Predicting and explaining charge carrier transport in halide perovskites is a formidable challenge because of the unusual vibrational and electron-phonon coupling properties of these materials. This study explores charge carrier transport in two prototypical halide perovskite materials, methylammonium lead tribromide (${\mathrm{MAPbBr}}_3$) and methylammonium lead triiodide (${\mathrm{MAPbI}}_3$), using a dynamic disorder model. Focusing on the role of real-space transport channels, we analyze temporal orbital occupations to assess the impact of material-specific on-site energy levels and spin-orbit coupling (SOC) strengths. Our findings reveal that both on-site energies and SOC magnitude significantly influence the orbital occupation dynamics, thereby affecting charge dispersal and carrier mobility. In particular, energy gaps across on-site levels and the halide SOC strength govern the filling of transport channels over time. This leads us to identify the $pp\pi$ channel as a critical bottleneck for charge transport and to provide insights into the differences between electron and hole transport across the two materials.

Grain boundaries facet because of anisotropic grain boundary energies: While the faceted boundary has a larger area than the corresponding straight boundary, a significantly lower energy of the facets compared to a straight segment can drive the faceting. This picture is complicated by faceting/defaceting transitions where the free energy difference between the two states depends on the temperature. Such transitions have been observed and modeled before, but their exact nature was not fully understood. Here, we use atomistic computer simulations to show that a well-known faceting transition in $\mathrm{\Sigma }3\left[11\overline{1}\right]$ tilt grain boundaries in Al is in fact a grain boundary phase transition (also called complexion transition). This means that the faceted and defaceted boundaries are associated with different atomic structures, which have different thermodynamic stability ranges. At low temperatures, the grain boundary phase associated with faceting is stable, while at high temperatures the flat grain boundary phase is stable. We also report on our thorough tests of Al interatomic potentials for this purpose, which include comparisons to density-functional theory calculations. Our chosen potential performs well for our grain boundaries. As a consequence, we were able to obtain results that align with previous experimental results.

Here we present the discovery and characterization of a series of antimonides ${Ln}_2{\mathrm{Ti}}_9{\mathrm{Sb}}_{11}$ (Ln: La–Nd) that exhibit well-isolated, $n=2$ rare-earth spin ladders. We discuss the structure of these compounds, with a particular focus on the magnetic Ln spin ladders. ${\mathrm{Nd}}_2{\mathrm{Ti}}_9{\mathrm{Sb}}_{11}$ and ${\mathrm{Ce}}_2{\mathrm{Ti}}_9{\mathrm{Sb}}_{11}$ exhibit antiferromagnetic interactions and a well-defined doublet ground state, whereas ${\mathrm{Pr}}_2{\mathrm{Ti}}_9{\mathrm{Sb}}_{11}$ exhibits a weakly magnetic singlet ground state. ${\mathrm{Nd}}_2{\mathrm{Ti}}_9{\mathrm{Sb}}_{11}$ is a poor metal with an electrical resistivity of $0.1\mathrm{m}\mathrm{\Omega }$ cm at 300 K and weak temperature dependence. The thermal conductivity along the ladder exhibits significant field dependence even at 40 K, considerably higher than the magnetic ordering temperature of 1.1 K. Compared to compounds with transition metal spin ladders, the rare-earth elements impart much lower energy scales, making these compounds highly tunable with external stimuli like magnetic fields. The diverse magnetism of the rare-earth ions and Ruderman-Kittel-Kasuya-Yosida interactions further contribute to the potential for a wide array of rich magnetic ground states, positioning these materials as a rare example of an inorganic square spin-ladder platform.

The $R{M}_6{X}_6$ family of materials ($R$ = rare-earth, $M$ = transition metal, $X=\mathrm{Ga}$, Si, Ge, Sn) produces an array of emergent phenomena, such as charge density waves, intrinsic Hall effects, and complex magnetic order, due to its kagome net of transition metal atoms, its local-moment magnetic anisotropies, and its extensive chemical tunability. Here, we report ${\mathrm{UNb}}_6{\mathrm{Sn}}_6$, a new “166” material containing both an actinide and a $4d$ transition metal, along with its nonmagnetic analog ${\mathrm{ThNb}}_6{\mathrm{Sn}}_6$, to investigate the properties of a $5f\text{−}4d$ electron 166 system. ${\mathrm{UNb}}_6{\mathrm{Sn}}_6$ crystallizes in the hexagonal $P6/mmm$ space group with a small degree of disorder due to shifts in the size of the CoSn-like cages along the $c$ axis. Upon cooling at zero magnetic field, the material undergoes two magnetic phase transitions at $T_2=46\mathrm{K}$ and $T_{\mathrm{N}}=43\mathrm{K}$. The low-temperature, zero-field phase is an antiferromagnet with ordered uranium moments and a k = (0,0,1/2) propagation vector determined by neutron diffraction. Remarkably, with a magnetic field applied along the $c$ axis, five additional magnetic transitions occur, evidenced by magnetization and resistivity data, before the moment saturates at 2.62 ${\mu }_{\mathrm{B}}/\mathrm{U}$ at 2 K and $\ge 13.6\mathrm{T}$. In two magnetic phase regions, the Hall resistivity of ${\mathrm{UNb}}_6{\mathrm{Sn}}_6$ significantly deviates from the magnetization, suggesting that the phases have a large Berry curvature or a change in the Fermi surface. The unknown magnetic ordering of the field-dependent phases of ${\mathrm{UNb}}_6{\mathrm{Sn}}_6$ demonstrates the complexity of the $5f\text{−}4d$ 166 system and encourages further study of its properties.

We use molecular dynamics to show that plastic slip is a crucial component of the transformation mechanism of a square-to-triangular structural transition. The latter is a stylized analog of many other reconstructive phase transitions. To justify our conclusions we use an atomistically informed mesoscopic representation of the field of lattice distortions in molecular dynamics simulations. Our approach reveals a hidden alternating slip distribution behind the seemingly homogeneous product phase which points to the fact that lattice-invariant shears play a central role in this class of phase transformations. While the underlying pattern of antiparallel displacements may be also interpreted as microscopic shuffling, its precise crystallographic nature strongly suggests the plasticity-centered interpretation.

We compute the phase separation of the immiscible liquid alloy Fe-Cu-Ni. Our computational approach uses a virtual semigrand canonical Widom approach to determine differences in excess chemical potentials between different species. Using an embedded atom potential for Fe-Cu-Ni, we simulate liquid states over a range of compositions and temperatures. This raw data is then fit to Redlich-Kister polynomials for the Gibbs free energy with a simple temperature dependence. Using the analytic form, we can determine the phase diagram for the ternary liquid, and compute the miscibility gap and spinodal decomposition as a function of temperature for this EAM potential. In addition, we compute density as a function of composition and temperature, and predict pair correlation functions. We use static structure factors to estimate the second derivative of the Gibbs free energy (the $S^0$ method) and compare it with our fitted Gibbs free energy. Finally, using a nonequilibrium Hamiltonian integration method, we separately compute absolute Gibbs free energies for the pure liquid states; this shows that our end points are accurate to within 1 meV for our ternary Gibbs free energy, as well as the absolute Gibbs free energy for the ternary liquid.

We report on magnetization measurements of single crystalline ${\mathrm{Dy}}_2{\mathrm{Fe}}_{17}$ in pulsed magnetic fields up to $\sim 127\mathrm{T}$. To produce the fields, we used (i) a single- and (ii) a double-coil magnet generating pulses in the 10 ms range, as well as (iii) semidestructive single-turn coils creating pulses of much shorter duration, $\sim 5\mu \mathrm{s}$. We observed steplike transitions at 64.3 and 77.7 T, for fields applied along $\left[100\right]$ and $\left[120\right]$, respectively. We confirmed the latter transition by ultrasound-velocity measurements using the double-coil magnet. Our data allowed us to locate the orthogonality point of ${\mathrm{Dy}}_2{\mathrm{Fe}}_{17}$ at $({\mu }_0{H}_{\perp }=90\mathrm{T},$ $M_{\perp }=31{\mu }_B/\mathrm{f}.\mathrm{u}.)$ and determine the intersublattice exchange field, ${\mu }_0{H}_{ex}=215\mathrm{T}$.

The layered honeycomb material ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$ is of topical interest because its magnetic state is related to that of the Kitaev magnet $\alpha \text{−}{\mathrm{RuCl}}_3$. Using thermal transport to probe how magnetic excitations interact with phonons in the magnetically disordered regime, we have uncovered an unusually large enhancement of the thermal conductivity ${\kappa }_{xx}$ in an in-plane magnetic field $\mathbf{H}$. Just above the Néel temperature $T_{\mathrm{N}}$, a field of 13 T increases ${\kappa }_{xx}$ by a factor of $\sim 211$, which is very large compared to other magnetic insulators. Interestingly, ${\kappa }_{xx}(H,T)$ exhibits a scaling behavior in the entire magnetically disordered region that surrounds the ordered zigzag state. The ratio $\mathrm{\Delta }{\kappa }_{xx}(H,T)/{\kappa }_{xx}(13,T)$, measured throughout the disordered region, collapses to a one-parameter scaling function $\exp (-1/gx)$ (where $x={\mu }_{\mathrm{B}}B/k_{\mathrm{B}}T$ and $g$ is a constant).

Quantum technologies require a new generation of superconducting electronic devices and circuitry. However, the superconducting materials used to construct them are restricted to a class of bulk superconductors. Gold films grown in contact with superconducting materials can exhibit superconducting correlations through the proximity effect, with various possible implementations in quantum technology. Here we study the growth of flat Au films on various surfaces of the elemental superconductors vanadium and niobium through a combination of low-temperature scanning tunneling microscopy and x-ray photoelectron spectroscopy. In particular, we investigate the growth morphology and composition as a function of temperature and coverage. We find that gold films can grow flat and oxygen-free when annealed to sufficiently high temperatures; however, they are also susceptible to partial intermixing with the substrate elements. Low-temperature scanning tunneling spectroscopy measurements elucidate the emergence of a proximitized superconducting gap at the gold surface. Additionally, we demonstrate the survival of the magnetic state of FeTPP-Cl molecules on the surface of these gold films, proving that they behave as a good support for probing molecular magnetism. We found that the exchange interaction between the molecular spin and the superconducting condensate can be inferred by the measurement of subgap Yu-Shiva-Rusinov states. Our paper shows that proximitized Au films constitute a promising platform to explore on-superconducting-surface synthesis and the interaction between superconductivity and magnetism in a large spin-orbit coupling environment.

The recent discovery of altermagnetism has sparked renewed interest in the growth of epitaxial films of the NiAs-phase polymorph of CrSb. This paper describes the magnetron sputtering-based fabrication and characterization of high-quality single crystalline CrSb(0001) thin films supported by an isostructural nonmagnetic PtSb buffer. X-ray diffraction and scanning transmission electron microscopy show that the films are phase pure and possess a very high crystalline quality (mosaicity $\approx 0.{05}^{\circ }$), while also being free of extended crystallographic defects. Both scanning electron microscopy and atomic force microscopy confirm their smooth and homogeneous topography. Additionally, the elemental composition of our films was found to be close to stoichiometric via electron probe microanalysis and x-ray fluorescence. Thus, the developed samples represent an ideal platform for further investigation of the material properties of CrSb.