We develop a probabilistic machine learning model and use it to screen for new hybrid organic-inorganic perovskites (HOIPs) with targeted electronic band gap. The data set used for this work is highly diverse, containing multiple atomic structures for each of 192 chemically distinct HOIP formulas. Therefore, any property prediction on a given formula must be associated with an irreducible “uncertainty” that comes from its unknown atomic details. As a result, dozens of new HOIP formulas with band gap falling between 1.25 and 1.50 eV were identified and validated against suitable first-principles computations. Through this demonstration we show that the probabilistic deep learning approach is robust, versatile, and can be used to properly quantify this uncertainty. In conclusion, the probabilistic standpoint and approach described herein could be widely useful for the very common and inevitable data uncertainty which is rooted at the incompleteness of information during experiments and/or computations.

The realization of magnetic frustration in a metallic van der Waals (vdW) coupled material has been sought as a promising platform to explore novel phenomena, both in bulk matter and in exfoliated devices. However, a suitable material realization has been lacking so far. Here, we demonstrate that the vdW compound CeSiI hosts itinerant electrons coexisting with exotic magnetism. In CeSiI, the magnetic cerium atoms form a triangular bilayer structure sandwiched by vdW stacked iodine layers. From resistivity and magnetometry measurements, we confirm the coexistence of itinerant electrons with magnetism with dominant antiferromagnetic exchange between the strongly Ising-like Ce moments below 7 K. Neutron diffraction confirms magnetic order with an incommensurate propagation vector $k\sim \left(0.28,0,0.19\right)$ at 1.6 K, which points to the importance of further-neighbor magnetic interactions in this system. The presence of a two-step magnetic-field-induced phase transition along the $c$ axis further suggests magnetic frustration in the ground state. Our findings provide a material platform which hosts the coexistence of an itinerant electron and frustrated magnetism in a vdW system, where exotic phenomena arising from the rich interplay between the spin, charge, and lattice in low dimension can be explored.

X-ray powder diffraction experiments at high pressures combining conventional sources and synchrotron radiation, together with theoretical simulations have allowed us to study the anomalous compression of the entire α-${RE}_2$(${\mathrm{WO}}_4{)}_3$ ($\mathit{RE}$ = La-Ho) family with modulated scheelite structure (α phase). The investigated class of materials is of great interest due to their peculiar structural behavior with temperature and pressure, which is highly sought after for specialized high-tech applications. Experimental data were analyzed using full-profile refinements and were complemented with computational methods based on density functional theory (DFT) total energy calculations for a subset of the samples investigated. An unusual change in the compression curves of the lattice parameters $a, c$, and β was observed in both the experiments and theoretical simulations. In particular, in all the studied compounds the lattice parameter $a$ decreased with pressure to a minimum value and then increased upon further compression. Pressure evolution of the experimental x-ray diffraction (XRD) patterns and cell parameters is correlated with the ionic radius of the rare earth element: (1) the lighter La-Nd tungstates underwent two phase transitions, and both transition pressures decreased as the rare earth's ionic radius increased. The XRD patterns of the first high pressure phase could be indexed with propagation vectors parallel to the $a$ axis (tripling the unit cell). At higher pressures, the lattice parameters for the second phase (referred to as the preamorphous phase) showed little variation with pressure. (2) The heavier tungstates, from Sm to Dy, undergo a transition to the preamorphous phase without any intermediate phase. The reversibility of both phase transitions was investigated. DFT calculations support this unusual response of the crystal structures under pressure and shed light on the structural mechanism of negative linear compressibility (NLC) and the resulting softening. The pressure dependence of the structural modifications is related to tilting, along with small elongation and alignment, of the ${\mathrm{WO}}_4^{2-}$ tetrahedrons. These changes correlate with those in the alternating RE…RE…RE chains and blocks of cationic vacancies arranged along the $a$ axis. Possible stacking defects, which emerge between them, helped to explain this anomalous compression and the pressure induced amorphization. Such mechanisms were compared with other ferroelastic families of molybdates, niobates, vanadates, and other compounds with similar structural motifs classified as having “hinge frames.”

The lack of time-reversal symmetry and Weyl fermions give exotic transport properties to Co-based Heusler alloys. In the present study, we have investigated the role of chemical disorder on the variation of Weyl points in ${\mathrm{Co}}_2{\mathrm{Ti}}_{1-\mathrm{x}}{\mathrm{V}}_{\mathrm{x}}\mathrm{Sn}$ magnetic Weyl semimetal candidate. We employ the first principle approach to track the evolution of the nodal lines responsible for the appearance of Weyl node in ${\mathrm{Co}}_2\mathrm{TiSn}$ as a function of V substitution in place of Ti. By increasing the V concentration in place of Ti, the nodal line moves toward Fermi level and remains at Fermi level around the middle composition. Further increase of the V content, leads shifting of nodal line away from Fermi level. Density of state calculation shows half-metallic behavior for the entire range of composition. The magnetic moment on each Co atom as a function of V concentration increases linearly up to $x=0.4$, and after that, it starts decreasing. We also investigated the evolution of the Weyl nodes and Fermi arcs with chemical doping. The first-principles calculations reveal that via replacing almost half of the Ti with V, the intrinsic anomalous Hall conductivity increased twice as compared to the undoped composition. Our results indicate that the composition close to the 50% V doped ${\mathrm{Co}}_2\mathrm{TiSn}$ will be an ideal composition for the experimental investigation of Weyl physics.

Optimally doped manganite ${[\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ (LSMO)] is a ferromagnetic material with perspective use in applications due to colossal magnetoresistance and $T_c$ around room temperature. However, utilizing LSMO in thin-film form and complex heterostructures is limited due to the appearance of a magnetic dead layer, but interfacing LSMO with other transition metal oxides (TMOs) can reduce the magnetic dead layer. For instance, the LSMO layer in proximity to ${\mathrm{SrRuO}}_3$ (SRO) remains magnetic down to 1–2 u.c. To illuminate this effect, we study the magnetic properties and orbital anisotropy of ultrathin LSMO deposited on ${\mathrm{SrTiO}}_3$ (STO) compared with and without an SRO intermediate layer by resonant x-ray spectroscopy. We found that two events occur at the LSMO-SRO interface: Orbital rearrangement and charge transfer. Both effects cooperate, enhancing in-plane double exchange in ultrathin LSMO. Based on quantitative analysis and theoretical simulation of the x-ray spectra, magnetic stability mechanisms in LSMO/SRO are discussed in detail.

This paper provides an accurate theoretical defect energy database for pure and Bi-containing III-V (III-V:Bi) materials and investigates efficient methods for high-throughput defect calculations based on corrections of results obtained with local and semilocal functionals. Point defects as well as nearest-neighbor and second-nearest-neighbor pair defects were investigated in charge states ranging from $-5$ to 5. Ga-V:Bi systems (GaP:Bi, GaAs:Bi, and GaSb:Bi) were thoroughly investigated with significantly slower, higher fidelity hybrid Heyd-Scuseria-Ernzerhof (HSE) and significantly faster, lower fidelity local density approximation (LDA) calculations. In both approaches, spurious electrostatic interactions were corrected with the Freysoldt correction. The results were verified against available experimental results and used to assess the accuracy of a previous band alignment correction. Here, a modified band alignment method is proposed to better predict the HSE values from the LDA ones. The proposed method allows prediction of defect energies with values that approximate those from the HSE functional at the computational cost of LDA (about $20\times$ faster for the systems studied here). Tests of selected point defects in In-V:Bi materials resulted in corrected LDA values having a mean absolute error (MAE) $=0.175$ eV for defect levels versus HSE. The method was further verified on an external database of defects and impurities in $\mathrm{Cd}X$ ($X$=S, Se, Te) systems, yielding a MAE $=0.194$ eV. These tests demonstrate the correction to be sufficient for qualitative and semiquantitative predictions, and may suggest transferability to many semiconductor systems without significant loss in accuracy. Properties of the remaining In-V:Bi defects and all Al-V:Bi defects were predicted with the use of the modified band alignment method.

We present a detailed study of the emergence of bulk ferromagnetism in low-carrier-density samples of undoped indium tin oxide. We use annealing to increase the density of oxygen vacancies and change the sample morphology without introducing impurities through the metal insulator transition (MIT). We utilize a novel and highly sensitive “Corbino-disk torque magnetometry” technique to simultaneously measure the thermodynamic and transport effects of magnetism on the same sample after successive annealing. With increased sample granularity, the carrier density increases, the sample becomes more metallic, and ferromagnetism appears as the resistance approaches the MIT. Ferromagnetism is observed through the detection of magnetization hysteresis, the anomalous Hall effect (AHE), and hysteretic magnetoresistance. A sign change of the AHE as the MIT is approached may elucidate the interplay between the impurity band and the conduction band on the weakly insulating side of the MIT.

We have attempted to improve the thermoelectric performance of an Al–Si–Ru semiconducting approximant through optimizing the carrier concentration. The effects of Cu doping on the thermoelectric properties of an Al–Si–Ru semiconducting quasicrystalline approximant with the nominal composition of ${\mathrm{Al}}_{69-0.75x}{\mathrm{Si}}_{7.5}{\mathrm{Cu}}_x{\mathrm{Ru}}_{23.5-0.25x}(x=0,2,4,6,8)$ are investigated. At approximately room temperature, an increase of $x$ leads to a decrease of the Seebeck coefficient $S$ and an increase of the electrical conductivity. Density functional theory and the Boltzmann transport theory are used to calculate $S$ as the hole concentration and the band gap are varied. The temperature dependence of $S$ is quantitatively described by an increase of the hole concentration and narrowing of the band gap with increasing $x$. Cu doping is shown to increase the hole concentration and narrow the band gap. The maximum dimensionless figure of merit increases from 0.03 at 400 K for $x=0$ to 0.2 at 500 K for $x=4$. These results indicate that the semiconducting quasicrystalline approximant could be a suitable candidate as a thermoelectric material for low grade waste heat recovery.

We have investigated the origin of magnetization and exchange bias (EB) reversals in the $\mathrm{YbCr}{\mathrm{O}}_3$ compound at the magnetization compensation temperature, $T_{\mathrm{COMP}}$ of 16.5 K by dc magnetization, ac susceptibility, neutron diffraction (ND), neutron depolarization, specific heat, and dielectric measurements. A weak magnetization $\left(M_{\mathrm{Cr}}\right)$ is observed below the Néel temperature $T_{\mathrm{N}}$ (∼120 K) in dc magnetization, which is consistent with a canted antiferromagnetic (AFM) state due to ordering of ${\mathrm{Cr}}^{3+}$ moments. Specific heat data exhibit a λ-shaped peak at $T_{\mathrm{N}}$. Further, a monotonic increase of ac susceptibility at low temperatures and the Schottky anomaly in the specific heat data account for the polarized nature of the ${\mathrm{Yb}}^{3+}$ moment $\left(M_{\mathrm{Yb}}\right)$ under the molecular field of the AFM ${\mathrm{Cr}}^{3+}$ sublattice. Our ND study has confirmed a finite polarized ${\mathrm{Yb}}^{3+}$ ordered moment, and the $G_{\mathrm{z}}$-type AFM ordering of ${\mathrm{Cr}}^{3+}$ moments below $T_{\mathrm{N}}$. The temperature variations of the lattice constants $a$ and $b$ show a crossover from positive to negative thermal expansion (NTE) across the $T_{\mathrm{COMP}}$ while cooling. Below $T_{\mathrm{COMP}}$, the separation between Cr-Cr atoms lying in the $ab$ plane increases with decrease in temperature, which corroborates with the observed NTE of the lattice constants. In the present compound, the sign change of the net magnetization arising out of the AFM coupled polarized ${\mathrm{Yb}}^{3+}$ $\left(M_{\mathrm{Yb}}\right)$ and the ferromagnetic ${\mathrm{Cr}}^{3+}$ $\left(M_{\mathrm{Cr}}\right)$ sublattice moments results in the sign reversal of magnetization. Interestingly, anomalous behavior of the coercive field with its minimum value at $T\sim T_{\mathrm{COMP}}$ is observed. EB also changes sign at $T\sim T_{\mathrm{COMP}}$. Moreover, the temperature variation of the real part of the dielectric constant reveals an anomaly at $T_{\mathrm{COMP}}$, indicating a weak magnetodielectric coupling in the $\mathrm{YbCr}{\mathrm{O}}_3$ compound. A training effect analysis ensures the conventional nature of the observed EB. Neutron depolarization study sheds light on the temperature-dependent domain magnetization with its zero value at the $T_{\mathrm{COMP}}$. The presence of the observed important phenomena viz. magnetization and EB reversals in a single-phase compound suggests their possible use in making magnetization switching, spin-value, thermomagnetic, and other spintronic devices.

We investigated the Ru $4d$ and O $2p$ electronic structure and magnetic properties of an ultrahigh-quality $\mathrm{SrRu}{\mathrm{O}}_3$ film on $\mathrm{SrTi}{\mathrm{O}}_3$ grown by machine-learning assisted molecular-beam epitaxy. The high itinerancy and long quantum lifetimes of the quasiparticles in the Ru $4d$ $t_{2g}$-O $2p$ hybridized valence band are confirmed by observing the prominent well-screened peak in the Ru $3d$ core-level photoemission spectrum, the coherent peak near the Fermi energy in the valence-band spectrum, and quantum oscillations in the resistivity. The element-specific magnetic properties and the hybridization between the Ru $4d$ and O $2p$ orbitals were characterized by Ru $M_{2,3}$-edge and O $K$-edge soft x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements. The ultrahigh-quality $\mathrm{SrRu}{\mathrm{O}}_3$ film with the residual resistivity ratio of 86 shows the large orbital magnetic moment of oxygen ions induced by the strong orbital hybridization of the O $2p$ states with the spin-polarized Ru $4d$ $t_{2g}$ states. The film also shows single-domain perpendicular magnetization with an almost ideal remanent magnetization ratio of 0.97. These results provide detailed insights into the relevance between orbital hybridization and the perpendicular magnetic anisotropy in $\mathrm{SrRu}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ systems.

Vanadium dioxide undergoes a metal-insulator transition from an insulating (monoclinic) to a metallic (tetragonal) phase close to room temperature, which makes it a promising functional material for many applications, e.g., as chemical sensors. Not much is known about its surface and interface properties, although these are critical in many applications. In this paper, we present an atomic-scale investigation of the tetragonal rutile $\mathrm{V}{\mathrm{O}}_2{\left(110\right)}_{\mathrm{T}}$ single-crystal surface and report results obtained with scanning tunneling microscopy, low-energy electron diffraction, and x-ray photoelectron spectroscopy, supported by density-functional-theory-based calculations. The surface reconstructs into an oxygen-rich (2 × 2) superstructure that coexists with small patches of the underlying unreconstructed $\left(110\right)\text{−}\left(1\times 1\right)$ surface when the crystal is annealed $>600{}^{\circ }\mathrm{C}$. The best structural model for the (2 × 2) surface termination, conceptually derived from a vanadium pentoxide (001) monolayer, consists of rings of corner-shared tetrahedra. Over a wide range of oxygen chemical potentials, this reconstruction is more stable than the unreconstructed (110) surface and models proposed in the literature.

Increasing the number of known ferroelectrics requires expanding the chemical design space and mechanisms producing ferroelectricity. To that end, we examine the displacive, noncentrosymmetric-to-centrosymmetric phase transition in the oxyfluoride ${\mathrm{KNaNbOF}}_5$ using ab initio calculations and Landau theory. We predict an intermediate $Pnma$ phase occurs in the transition from the known low- and high-temperature phases, $Pna{2}_1$ and $Cmcm$, respectively, which we subsequently confirm using high-resolution and in situ ${}^{19}\mathrm{F}$ magic-angle spinning nuclear magnetic resonance. Using the intermediate structure, we then calculate the monodomain polarization switching barrier in ${\mathrm{KNaNbOF}}_5$ to be $\sim 93$ meV per formula unit, comparable to existing ferroelectrics. The reversal of the polarization is due to uncompensated antipolar displacements of oxygen and fluorine and does not require oxygen and fluorine site exchange as in other nonswitchable heteroanionic materials, which makes ${\mathrm{KNaNbOF}}_5$ a viable oxyfluoride for a switchable electric polarization.

The thermodynamic stability of the premartensite (PM) phase has been a topic of extensive investigation in shape memory alloys as it affects the main martensite phase transition and the related physical properties. In general, the PM phase is stable over a rather narrow temperature-composition range. We present here evidence for chemical pressure induced suppression of the main martensite transition and stabilization of the PM phase over a very wide temperature range from 300 to $\sim 5\mathrm{K}$ in a magnetic shape memory alloy (MSMA), ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{34}{\mathrm{In}}_{16}$, using magnetic susceptibility, synchrotron x-ray powder diffraction (SXRPD) studies, and first-principles calculations. The ac-susceptibility studies show a highly skewed and smeared peak around 300 K without any further transition up to the lowest temperature of our measurement (5 K) for $\sim 5\%$ Al substitution. The temperature evolution of the SXRPD patterns confirms the appearance of the PM phase related satellite peaks at $T\le 300\mathrm{K}$ without any splitting of the main austenite (220) peak showing preserved cubic symmetry. This is in marked contrast to the temperature evolution of the SXRPD patterns of the martensite phase of the Al free as well as $\sim 3\%$ Al substituted compositions where the austenite (220) peak shows a clear splitting due to Bain distortion signalling symmetry breaking transition. Our theoretical calculations support the experimental findings and reveal that the substitution at the In site by a smaller size atom, like Al, can stabilize the PM phase with preserved cubic symmetry. Our results demonstrate that Al-substituted Ni-Mn-In MSMAs provide an ideal platform for investigating the physics of various phenomena related to the PM state.

Harsh service conditions in aerospace, defense and military, and other fields are calling for materials with excellent mechanical properties to undergo extreme deformation (such as elevated and cryogenic temperatures and high strain rates) without sustaining damage while retaining high strength. Outstanding mechanical properties of high or medium entropy alloys (MEAs) render them potential candidates. In this paper, as the temperature decreases, a breakthrough of the strength-ductility tradeoff is achieved in a CrFeNi MEA with partially recrystallized face-centered cubic phases, together with body-centered cubic (BCC) precipitates. At a strain rate of $3000{\mathrm{s}}^{-1}$, the yield strength (YS) is increased from 920 MPa at 298 K to 1320 MPa at 77 K, while the uniform elongation (UE) increases by 28.5%. This phenomenon also occurs at quasistatic tension. Under low-temperature loading, nanotwins are popularly activated due to the decreased stacking fault energy with decreasing temperatures. A positive strain-rate dependent YS arises owing to the contributions from the short-range dislocation obstacles, BCC phases, and refined grains. High-density dislocations and BCC phases result in the degeneration of UE, especially with the increase of strain rates. Strain-rate- and temperature-dependent constitutive models were successfully established to predict the deformation behaviors of CrFeNi MEAs under such a wide range of strain rates at room and cryogenic temperatures.

Thermal expansion of materials is of fundamental practical relevance and arises from an interplay of several material properties. For nanocrystalline materials, accurate measurements of thermal expansion based on high-precision reference dilatometry allow inferring phenomena taking place at internal interfaces such as vacancy annihilation at grain boundaries. Here we report on measurements obtained for a severely deformed 316L austenitic steel, showing an anomaly in difference dilatometry curves which we attribute to the exceptionally high density of stacking faults. On the basis of ab intio simulations we report evidence that the peculiar magnetic state of the 316L austenitic steel causes stacking faults to expand more than the matrix. So far, the effect has only been observed for this particular austenitic steel but we expect that other magnetic materials could exhibit an even more pronounced anomaly.

The interplay between topology and energy hierarchy plays a vital role in the collective magnetic order in artificial ferroic systems. Here we investigate, experimentally, the effect of having one or two activation energies of interacting Ising-like magnetic islands—mesospins—in thermalized, vertex-frustrated lattices. The thermally arrested magnetic states of the elements were determined using synchrotron-based magnetic microscopy after cooling the samples from temperatures above the Curie temperature of the material. Statistical analysis of the correlations between mesospins across several length scales reveals changes in the magnetic order, reflecting the amount of ground state plaquettes realized for a vertex-frustrated lattice. We show that the latter depends on the presence, or not, of different activation energies.

Polycrystalline ${\mathrm{La}}_7{\mathrm{Ir}}_3$ is reported to show superconductivity breaking time-reversal symmetry while also having an isotropic $s$-wave gap. Single crystals of this noncentrosymmetric superconductor are highly desirable to understand the nature of the electron pairing mechanism in this system. Here we report the growth of high-quality single crystals of ${\mathrm{La}}_7{\mathrm{Ir}}_3$ by the Czochralski method. The structural and superconducting properties of these large crystals have been investigated using x-rays, magnetization, resistivity, and heat capacity measurements. We observe a clear anisotropy in the lower and upper critical fields for magnetic fields applied parallel and perpendicular to the hexagonal $c$ axis. We also report the presence of a robust electronic critical field, that diverges from the upper critical field derived from heat capacity, which is the hallmark of surface superconductivity.

We demonstrate growth of epitaxial (100)SmN thin films directly on (100)Si surfaces. By using physical vapor deposition of Sm metal in an ${\mathrm{N}}_2$ atmosphere we show that careful control of substrate temperature, ${\mathrm{N}}_2$ pressure, and postannealing steps leads to epitaxial SmN without the formation of samarium silicide impurity phases. While rare-earth silicide formation competes with and is favored over nitride formation at high growth temperatures, we find that low-temperature grown SmN seed layers are stable against high-temperature annealing, and thus allow for subsequent high-temperature growth of SmN with a clear epitaxial relationship to the Si substrate. The relatively low lattice mismatch of SmN with (100)Si, compared to other commonly available substrates, coupled with the low cost and maturity of Si processing technology provide a promising route for further studies of the fundamental properties of SmN and other isostructural members of the rare-earth nitride series. Because SmN is a ferromagnetic semiconductor, which also becomes superconducting close to 4 K under sufficient doping, integration with Si technology presents new opportunities for spin-transport devices.

The advanced data structure of the zero-suppressed binary decision diagram (ZDD) enables us to efficiently enumerate nonequivalent substitutional structures. Not only can the ZDD store a vast number of structures in a compressed manner, but also a set of structures satisfying given constraints can be extracted from the ZDD efficiently. Here, we present a ZDD-based efficient algorithm for exhaustively searching for special quasirandom structures (SQSs) that mimic the perfectly random substitutional structure. We demonstrate that the current approach can extract only a tiny number of SQSs from a ZDD composed of many substitutional structures ($>{10}^{12}$). As a result, we find SQSs that are optimized better than those proposed in the literature. A series of SQSs should be helpful for estimating the properties of substitutional solid solutions. Furthermore, the present ZDD-based algorithm should be useful for applying the ZDD to the other structure enumeration problems.

Lattice structure can dictate electronic and magnetic properties of a material. Especially, reconstruction at a surface or heterointerface can create properties that are fundamentally different from those of the corresponding bulk material. We have investigated the lattice structure on the surface and in the thin films of epitaxial $\mathrm{SrRu}{\mathrm{O}}_3$ with the film thickness up to 22 pseudocubic unit cells (u.c.), using the combination of surface sensitive low energy electron diffraction and bulk sensitive scanning transmission electron microscopy. Our analysis indicates that, in contrast to many perovskite oxides, the $\mathrm{Ru}{\mathrm{O}}_6$ tilt and rotational distortions appear even in single unit cell $\mathrm{SrRu}{\mathrm{O}}_3$ thin films on cubic $\mathrm{SrTi}{\mathrm{O}}_3$, while the full relaxation to the bulklike orthorhombic structures takes 3–4 u.c. from the interface for thicker films. Yet the $\mathrm{Ti}{\mathrm{O}}_6$ octahedra of the substrate near the interface with $\mathrm{SrRu}{\mathrm{O}}_3$ films show no sign of distortion, unlike those near the interface with $\mathrm{CaRu}{\mathrm{O}}_3$ films. Two orthogonal in-plane rotated structural domains are identified. These octahedral distortions are essential for the understanding of the thickness dependent transport and magnetism in ultrathin films.