We report the synthesis and physical characterization of single-crystalline ${\mathrm{Ce}}_3{\mathrm{MgBi}}_5$, a previously unexplored member of the ${\mathrm{Ce}}_3{MPn}_5$ family. This compound crystallizes in the hexagonal $P{6}_3/mcm$ structure, featuring an anisotropic Ce sublattice composed of zigzag chains along the $c$ axis and a distorted kagome-like network in the basal plane. Magnetization measurements reveal antiferromagnetic order below $T_N\approx 4.2\text{K}$ with strong magnetic anisotropy and multiple field-induced metamagnetic transitions for fields applied perpendicular to [001], leading to a dome-shaped $H–T$ phase diagram. Electrical transport exhibits characteristic signatures of a Ce-based Kondo lattice, including broad resistivity maxima and pronounced field-dependent anomalies in the magnetoresistance and Hall response that track the magnetic phase boundaries. Specific-heat measurements confirm the magnetic transition and show that the full $\text{R}ln2$ entropy expected for a ${\mathrm{Ce}}^{3+}$ Kramers doublet is recovered by 20 K, indicating an extended temperature range of magnetic fluctuations consistent with Kondo correlations. Our results establish ${\mathrm{Ce}}_3{\mathrm{MgBi}}_5$ as a platform within the ${\mathrm{Ce}}_3{MPn}_5$ family for exploring the interplay of geometric frustration, magnetic anisotropy, and Kondo-lattice physics under applied magnetic fields.

Single crystals of $\mathrm{Ag}\mathrm{In}{\mathrm{Cr}}_4{\mathrm{S}}_8$ are grown by chemical vapor transport and crystallographic ordering of Ag/In that results in a breathing pyrochlore motif of ${\mathrm{Cr}}^{3+}$ is verified by x-ray and neutron diffraction. Long-range antiferromagnetic order is observed below a Néel temperature of $T_{\mathrm{N}}$ ≈ 9.6 K. The magnetic properties are characterized using ac and dc magnetization, specific heat capacity, and single crystal neutron diffraction measurements. The specific heat data are characterized by a small lambda anomaly near 9.5 K and the estimated magnetic entropy reaches $\approx$ $\frac{1}{3}$ of the expected value by $3T_{\mathrm{N}}$, suggesting significant short-range order in the paramagnetic phase. Single crystal neutron diffraction evidences an incommensurate spin structure with propagation vector $\mathit{k}=(0,0,\delta )$ and $\delta =0.343$ at 5 K. The minimal model that accounts for the data consists of ferromagnetic layers of Cr atoms, with magnetic moments lying in the plane of the layers and modulating in the perpendicular direction to form a helical structure propagating along $\mathit{k}$. This study represents a rare investigation of single crystals within the family of breathing pyrochlore materials.

Polar orders play a central role in modern technologies, spanning applications from information storage and energy harvesting to neuromorphic computing. While ferroelectric (FE) implementation into devices is more mature, the development of antiferroelectric (AFE)-based devices is still in its infancy. However, AFEs are emerging as promising candidates for multiple applications such as energy storage (supercapacitors), all-solid refrigeration (electrocaloric cooling) or thermal switches. In this work, we probe the transition between the antipolar and polar states in epitaxial layers of the prototypical antiferroelectric oxide, $\mathrm{PbZr}{\mathrm{O}}_3$. We discuss operando second-harmonic generation (SHG) imaging as a powerful tool, providing spatially resolved insights into the electric-field-induced AFE–FE phase transitions. This technique enables the mapping of local symmetry-breaking and phase coexistence during switching, offering a route to unravel the microscopic mechanisms governing AFE–FE transitions in such functional oxides.

The search for intrinsic excitonic insulators (EI) has long been confounded by coexisting electron–phonon coupling in bulk materials. Although the ground state of an EI may be difficult to differentiate from density-wave orders or other structural instabilities, excited states offer distinctive signatures. One way to provide clarity is to directly inspect the phonon spectral function for long wavelength broadening caused by phonon interaction with the high velocity EI phason. Here, we report that the quasi-one-dimensional (quasi-1D) EI candidate ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ shows extremely anisotropic phonon broadening and softening in the semimetallic normal state. In contrast, such behavior is completely absent in the broken symmetry state of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ and in the isostructural ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$ , where the latter has a fully gapped normal state. By contrasting the expected phonon lifetimes in the BCS and BEC limits of a putative EI, our results suggest that the phase transition in ${\mathrm{Ta}}_2\mathrm{Ni}{(\mathrm{Se},\mathrm{S})}_5$ family is closely related to strong interband electron–phonon coupling. We experimentally determine the dimensionless coupling $\frac{g}{{\omega }_0}\sim 10$, revealing ${\mathrm{Ta}}_2\mathrm{Ni}{(\mathrm{Se},\mathrm{S})}_5$ as a rare “ultrastrong coupling” material.

Strong coupling between optical and magnetic excitations could enable contactless, spatially resolved, or ultrafast interrogation and control of magnetism in two-dimensional (2D) materials and devices. The layered 2D A-type antiferromagnet ${\mathrm{CrPS}}_4$ stands out among van der Waals (vdW) magnets for its rich optical fine structure, but its spectroscopy is not yet understood and has so far been interpreted without consideration of magnetic exchange. Here, we show that this fine structure comes primarily from exchange-mediated coupling between on-site optical “spin-flip” transitions of $\mathrm{C}{\mathrm{r}}^{3+}$ and low-energy spin transitions involving the surrounding lattice. Well-resolved magnon sidebands to optical ${}^4\mathrm{A}_2\leftrightarrow {}^2\mathrm{E}$ transitions are observed in photoluminescence (PL) and PL excitation spectra, as well as a pronounced PL sideband associated with short-range exchange splitting. Energy migration is probed using $\mathrm{Y}{\mathrm{b}}^{3+}$ dopants as traps, revealing subpicosecond intersite excitation hopping. Formation of dispersive Frenkel excitons of coupled on-site d-d transitions because of intersite exchange is discussed. In addition to impacting how optical fine structure is interpreted in this and potentially other vdW magnets, these findings may have ramifications for future applications of layered 2D magnets by suggesting potential routes to drive mode-specific spin-wave excitations using light.

We present a comprehensive investigation of reconstructions on $\beta {\text{-Ga}}_2{\mathrm{O}}_3\left(001\right)$ combining first-principles calculations with experimental observations. Using ab initio atomistic thermodynamics and replica-exchange grand-canonical molecular dynamics simulations, we explore the configurational space of possible reconstructions under varying chemical potentials of oxygen and gallium. Our calculations reveal several stable surface reconstructions, most notably a previously unreported $1\times 2$ reconstruction consisting of paired ${\mathrm{GaO}}_4$ tetrahedra that exhibits remarkable stability across a wide range of experimental growth conditions. In this reconstruction, two Ga atoms share one oxygen bond and are separated by a distance of $2.64\AA$ along the [010] direction. High-angle annular dark-field scanning transmission electron microscopy imaging of homoepitaxially grown (001) layers is consistent with the predicted structure. Additional investigations of possible indium substitution at the surface sites, which can occur during indium-mediated metal-exchange catalysis via molecular beam epitaxial growth, reveal a cooperative effect in In incorporation, with distinct stability regions for In-substituted structures under O-rich conditions. Our findings provide an understanding for controlling surface properties during epitaxial growth of $\beta {\text{-Ga}}_2{\mathrm{O}}_3\left(001\right)$.

The emergence of hexagonal Ge (2H-Ge) as a candidate direct-gap group-IV semiconductor for Si photonics mandates a rigorous understanding of its optoelectronic properties. Theoretical predictions of a “pseudodirect” band gap, characterized by weak oscillator strength, contrast with a claimed high radiative recombination coefficient $B$ comparable to conventional (cubic) InAs. We compute $B$ in 2H-Ge from first principles and quantify its dependence on temperature, carrier density, and strain. For unstrained 2H-Ge, our calculated spontaneous emission spectra corroborate that measured photoluminescence corresponds to direct-gap emission, but with $B$ being approximately three orders of magnitude lower than in InAs. We confirm a pseudodirect-to-direct-gap transition under $\sim 2\%$ [0001] uniaxial tension, which can enhance $B$ by up to 3 orders of magnitude, making it comparable to that of InAs. Beyond quantifying the strong enhancement of $B$ via strain engineering, our analysis suggests the dominance of additional, as-yet unquantified recombination mechanisms in this nascent material.

The antiferromagnets with the time-reversal symmetry broken magnetic structures possess a finite spin splitting in the momentum space, and may contribute to a realization of a finite tunnel magnetoresistance (TMR) effect even with magnets with zero net spin polarization. In this paper, we study the TMR effect with the noncollinear antiferromagnet ${\mathrm{Mn}}_3\mathrm{Sn}$ whose inverse ${120}^{\circ }$ antiferromagnetic order breaks the time-reversal symmetry. In particular, we employ the representative barrier material $\mathrm{MgO}$ as the tunnel insulator, and calculate the TMR effect in the ${\mathrm{Mn}}_3\mathrm{Sn}\left(01\overline{1}0\right)/\mathrm{MgO}\left(110\right)/{\mathrm{Mn}}_3\mathrm{Sn}$ magnetic tunnel junctions (MTJs), which has an optimal geometry for the spin-orbit torque switching of the magnetic configurations. We show that a finite TMR ratio reaching $\gtrsim 1000\%$ appears in the ${\mathrm{Mn}}_3\mathrm{Sn}/\mathrm{MgO}/{\mathrm{Mn}}_3\mathrm{Sn}$ MTJs, which is due to the spin splitting properties of ${\mathrm{Mn}}_3\mathrm{Sn}$ in the momentum space combined with the screening effect of $\mathrm{MgO}$.

We report bulk magnetic properties of the monoclinic lanthanide tantalates, $M^{\prime }\text{−}Ln{\mathrm{TaO}}_4$ ($Ln$= Tb, Dy, Ho, Er), where the magnetic $L{n}^{3+}$ ions are arranged on a distorted 2D square lattice. The heavier analog $M^{\prime }\text{−}{\mathrm{YbTaO}}_4$ has been investigated as a spin-orbit-coupled, quasi-two-dimensional frustrated magnet, and the properties of the other $M^{\prime }\text{−}Ln{\mathrm{TaO}}_4$ are expected to vary depending on the electronic configuration of the $Ln$ ion, namely, Kramers vs non-Kramers behavior and different crystal electric field parameters. In this work, powder neutron diffraction is used to confirm the crystal structure for $Ln=$ Tb, Ho, Er, and to determine the magnetic structure of $M^{\prime }\text{−}{\mathrm{TbTaO}}_4$, which displays long-range antiferromagnetic (AFM) order below $T_{\mathrm{N}}=2.1\mathrm{K}$. The ${\mathrm{Tb}}^{3+}$ moments are aligned primarily along the $c$ axis with AFM nearest-neighbor interactions. Susceptibility data suggest that $M^{\prime }\text{−}{\mathrm{DyTaO}}_4$ may display short-range ordering around 2.7 K, while $M^{\prime }\text{−}{\mathrm{HoTaO}}_4$ and $M^{\prime }\text{−}{\mathrm{ErTaO}}_4$ show AFM correlations but do not order above 1.8 K. Measurements of the magnetic specific heat provide evidence for a Kramers doublet ground state in $M^{\prime }\text{−}{\mathrm{ErTaO}}_4$, similar to its heavier analog $M^{\prime }\text{−}{\mathrm{YbTaO}}_4$.

Dielectric particles of the same material exchange electrical charge during collisions or sliding contacts, yet the underlying charge-exchange mechanism is still not understood. The fact that particles can become highly charged as a result of this effect has significant consequences for many settings, both in nature and industry, such as thunderstorms, volcanic eruptions, particle aggregation during meteorite and planet formation, and the clogging of industrial granular systems. Toward understanding these systems, great efforts have been made to develop precise in situ measurements for particle charge, e.g., to determine ensemble charge distributions or measure exchange during individual contacts. Here, we present experimental results concerning the particle size scaling of the stationary-state charge distributions of oxide particles in the sub-millimeter range. We measure the charge distributions for large ensembles of monodisperse ${\mathrm{ZrO}}_2$:${\mathrm{SiO}}_2$ composite spheres, ranging from 172 to $545\mathrm{\mu }\mathrm{m}$ in diameter. These distributions are non-Gaussian and collapse to a single master curve when plotted as functions of the surface charge density $\mathrm{\Sigma }=q/4\pi R^2$. X-ray fluorescence and atomic force microscopy measurements show that the differences in the measured charge distributions are not due to variations in chemical composition or surface roughness, but rather to size alone. Our findings provide constraints on microscopic models for charge exchange, namely that they should lead to steady-state distributions that are non-Gaussian and scale in a specific way with particle size.

Colloidal gels form through the sol-gel transition of attractive particle suspensions, where local aggregation leads to a space-spanning network with solidlike properties. Their microstructure and mechanical properties are highly sensitive to external perturbations, which can substantially alter the pathway of network formation. Here, we investigate how nonlinear oscillatory shear affects the sol-gel transition of colloidal silica suspensions. Using large-amplitude oscillatory shear (LAOS), we vary both the strain amplitude and the duration of oscillatory forcing, varying between one and two times the gelation time. We find that sufficiently large strain amplitudes or prolonged exposure to oscillations in the nonlinear regime irreversibly alter the gel properties: The storage modulus $G^{\prime }$ decreases while its frequency dependence remains unchanged. In contrast, the loss modulus $G^{″}$, which decreases monotonically with frequency under quiescent gelation, exhibits an upturn at high frequencies when the gel is formed under strong oscillatory shear. The viscoelastic spectra of gels formed under quiescent conditions are well captured by a fractional Maxwell model, while gels formed under LAOS require an additional fractional element to account for damage-induced dissipation. Rheo-imaging experiments corroborate this interpretation by revealing the growth of cracks in gels formed under LAOS. We further show that these gels display a progressively more ductile nonlinear response for prolonged exposure to LAOS during gelation. These results demonstrate that the interplay between nonlinear shear and gelation can permanently imprint a macroscopic fracture pattern into colloidal gels, offering a route to tune their viscoelastic properties.

Superconducting nickelate thin films with the layered square-planar structure are synthesized via the topotactic reduction of a more oxygen rich perovskite or Ruddlesden-Popper (RP) phase due to the instability of the target ${\mathrm{Ni}}^{1+\delta }$ valence. The topotactic reduction induces a dramatic structural transformation, which yields high defect densities in the resulting film and has introduced uncertainty regarding film uniformity, particularly as it relates to the calculation of physical parameters based on estimates of superconducting volume fraction. Here we use neutron reflectometry (NR) and secondary ion mass spectrometry (SIMS) to obtain a sub-nm resolved understanding of the structural depth profile across a wide range of layered square-planar nickelates. While the as-grown parent compounds are bulklike and of very high quality, the depth profiles of reduced films are generally nonuniform and indicative of partial amorphization. We identify careful selection of substrate and reduction conditions as promising directions toward mitigating amorphization and improving uniformity.

The large van der Waals gap in transition metal dichalcogenides (TMDs) offers an avenue to tune the ground state of 2D materials through the intercalation of magnetic atoms. Here, we investigate the charge correlations in ${\mathrm{Fe}}_{1/3}{\mathrm{TaS}}_2, {\mathrm{Co}}_{1/3}{\mathrm{TaS}}_2$, and ${\mathrm{Fe}}_{0.35}{\mathrm{NbS}}_2$ by combining angle-resolved photoemission spectroscopy (ARPES), x-ray scattering, magnetometry, and density functional theory (DFT). We find that, while short-range charge fluctuations develop in Ta-based compounds, ${\mathrm{Fe}}_{0.35}{\mathrm{NbS}}_2$ exhibits long-range charge order which is strongly coupled with magnetic order and tunable by external magnetic field. Our electronic structure analysis reveals that intercalation reconstructs the Fermi surface via charge transfer and band renormalization, yet does not generate the nesting conditions compatible with the observed ordering vectors. Complementary phonon calculations further exclude a conventional electron-phonon origin of charge order. Together, these results establish magnetoelastic coupling as the dominant mechanism behind charge ordering in ${\mathrm{Fe}}_{0.35}{\mathrm{NbS}}_2$ and highlight the contrasting role of Nb and Ta hosts in stabilizing correlated ground states in intercalated TMDs.

Directional memory in amorphous solids is commonly quantified through the Bauschinger effect, yet the observation of the inverse Bauschinger effect suggests that the sign of memory can invert, pointing to distinct underlying plastic organization. Here, we connect directional memory to the nature of yielding in steadily sheared amorphous solids. Using simulations of two-dimensional polydisperse glasses, we show that the type of directional memory (Bauschinger versus inverse Bauschinger) is jointly controlled by deformation history, strain rate, and parent temperature. We identify a critical history amplitude ${\gamma }_{N,\mathrm{crit}}(T_p,\dot{\gamma })$ and construct a phase diagram that delineates regimes with memory inversion from those showing only conventional Bauschinger response. Microscopically, memory inversion correlates with networklike shear-band morphology and plastic healing, whereas conventional memory is associated with persistent localization and cumulative damage. These results establish directional memory as an order parameter for a shear-rate and annealing-controlled brittle-ductile crossover and suggest that plastic healing provides a generic route to memory inversion in disordered solids.

The realization of air-stable two-dimensional (2D) metals confined at the interface between graphene and SiC has attracted significant attention owing to their distinct properties compared to their bulk counterparts. In this work, we identify two distinct structural phases in a monolayer silver intercalated between graphene and SiC: (i) a moiré pattern arising from the lattice mismatch between graphene and monolayer silver and (ii) a striped reconstruction formed through the interaction with the underlying SiC. Using cryogenic scanning tunneling microscopy combined with density functional theory, we elucidate the origin of the silver striped reconstruction. Driven by the competition between the silver-SiC and the silver-silver interactions within a monolayer, the silver atoms form a one-dimensional Frenkel-Kontorova domain, in which an $(n+1)\times \surd 3$ silver supercell accommodates an $n\times \surd 3$ supercell $(n=20)$ of the underlying SiC, which partially relieves tensile strain, otherwise imposed by ideal epitaxy. The strain-induced reconstruction creates a long-range modulation of 2D silver electronic density, giving rise to an electronic state at $\sim 0.75\mathrm{eV}$ above the Fermi level in the transition regions between different silver-SiC registries. These findings uncover the important interplay between the structural and electronic properties of intercalated 2D metals and the underlying SiC substrate, providing new insights into substrate-mediated reconstruction and electronic modulation at confined interfaces.

Using first-principles calculations and companion experimental validations, we demonstrate that the valence electron concentration (VEC) can be used to self-consistently parameterize the bonding characteristics of high-entropy diborides (HEBs) and in turn predict their mechanical properties. When VEC is 10.0 per formula unit (f.u.), HEBs enter the superhard category where ${\mathrm{H}}_{\mathrm{v}}>40\mathrm{GPa}$ because bonding states are optimally filled to resist shear deformation. Conversely, above or below a VEC of 10.0, the hardness decreases due to the filling of antibonding or the emptying of bonding states of the HEBs, respectively. This result is supported by analyses of the electronic structure, bonding, antibonding, and electronic orbitals’ (metal $d$ and B $2p$) occupation as a function of VEC. To complement the first-principles calculations, four HEBs were synthesized using high power impulse magnetron sputtering to interrogate their crystalline structure and mechanical properties. All HEBs retain a single-phase $\mathrm{Al}{\mathrm{B}}_2$-type hexagonal structure with highly dense nodular grain morphology. Hardness measurements reveal general agreement between theory and experiment. An HEB with VEC of 10.0/f.u. shows superhard characteristics, with micro- and nanoindentation hardness greater than 40 GPa. This combined entropy and VEC-based materials design formulation establishes a foundation for the discovery of high-entropy ceramics with enhanced thermomechanical properties, making them suitable for extreme engineering applications.

In his groundbreaking work on the theory of ferri- and antiferromagnetism, Néel proposed the existence of ferrimagnets with zero net magnetization. These $L$-type ferrimagnets were later termed fully compensated. This study focuses on $L$-type ferrimagnets, particularly Heusler alloys based on vanadium (V), manganese (Mn), iron (Fe), and cobalt (Co). First-principles calculations reveal that these ferrimagnets exhibit anomalous temperature dependence, differing from antiferromagnets. Spin dynamics calculations, based on the calculated exchange coupling coefficients, show that this behavior stems from the ferrimagnets' inequivalent magnetic sublattices, which have distinct exchange interactions. The temperature dependence of the magnetic properties is analyzed through a comparison of the first-principles calculations and Néel's molecular field theory, extended to include Heusler compounds. In addition to exhibiting compensated ferrimagnetic behavior, examining the electronic structure using the Bloch spectral function shows that the investigated Heusler alloys are half-metals which have a full spin polarization at the Fermi energy. The combination of complete magnetic compensation and half-metallicity makes these alloys interesting for fundamental research and spintronics applications because their zero net magnetization minimizes stray fields while retaining half-metallicity.

We introduce and systematically investigate a new class of topological magnetic textures—skyrmionium metamatter—composed of skyrmioniums ($Q=0$) and skyrmions ($Q=-1$) arranged in periodic compound lattices that emulate the structural diversity of atomic materials. We show that pure skyrmionium lattices are intrinsically unstable against elongation distortions and relax into spiral states, whereas the inclusion of even a small fraction of skyrmions suppresses this instability and stabilizes a broad family of mixed skyrmion–skyrmionium crystals. These states are classified by their topological stoichiometry and exhibit multiple metastable polymorphs with distinct plane-group symmetries. Smooth transformations between different polymorphs can be achieved by tuning the lattice periodicity, highlighting the reconfigurability of the metamatter. We further analyze the collective spin dynamics of these composite lattices and uncover a rich hierarchy of excitation modes that extends far beyond those of conventional skyrmion lattices. The coexistence of skyrmions and skyrmioniums gives rise to hybrid collective modes with nontrivial phase relations and deformation-assisted dynamics. Our results establish skyrmionium-based metamatter as a versatile platform for designing tunable, topologically heterogeneous magnetic lattices with emergent structural and dynamical properties, opening new opportunities for reconfigurable magnonic and spintronic functionalities.

${\mathrm{Co}}_2\mathrm{MnGa}$ (CMG), a magnetic Weyl semimetal, exhibits high Curie temperature, ultralow damping, and strong spin-dependent transport properties, favorable for spintronics applications. While its room-temperature magnetization dynamics have been explored, the temperature ($T$) dependence of key dynamic parameters such as damping and exchange stiffness remains underexplored. In this work, we investigate the temperature-dependent magnetization dynamics of sputtered CMG thin films (20, 60, and 80 nm) on MgO substrates. We use broadband ferromagnetic resonance spectroscopy with a magnetic field applied perpendicular to the sample plane to characterize these properties. X-ray diffraction reveals a strain-induced tetragonal distortion originating from the substrate-film lattice mismatch, most pronounced in the 20 nm film. As temperature decreases, the saturation magnetization ($M_s$) deviates from Bloch's law below 170 K, due to a temperature-dependent tetragonal distortion. Furthermore, the effective magnetization $M_{\mathrm{eff}}>M_s$ indicates a net in-plane anisotropy that strengthens with decreasing temperature, while the perpendicular uniaxial anisotropy ($K_{u_{\left[001\right]}}$) also increases, with the 20 nm film exhibiting the highest values across all temperatures. Perpendicular standing spin waves observed in the 60 and 80 nm films enable extraction of the exchange stiffness ($A_{ex}$) and exchange length ($l_{ex}$), both of which increase with decreasing temperature. Furthermore, $A_{ex}$ follows a $T^2$ dependence indicative of a dominant contribution from electron-magnon interactions. Compared to the 80 nm film, the 60 nm film has a larger magnetic domain size as revealed by magnetic force microscopy. All films exhibit ultralow damping at room temperature, with the 80 nm film showing a temperature-independent behavior.

Physical properties intermediate between constituents of alloys can be achieved as downward convex positive bowing, upward concave negative bowing, or zero bowing. Such bowing effects are essential for band gap engineering in semiconductor alloys. Upward band gap bowing effects are rather rare, hindering the exploration on half of the available physical property space of alloys. Part of this being a rare event is related to the need to stabilize an alloy with low mixing enthalpy, so it does not phase separate. In this paper we find via density functional theory that one can satisfy the simultaneous conditions of negative mixing enthalpy and upward band gap bowing in four-component $\mathrm{AB}{\mathrm{X}}_3$ halide perovskite alloys in the cubic perovskite structure. Such perovskite alloys have the B-site occupied by a mixture of group IVB and IIB elements that have the IVB-$s$ and IIB-s states in the valence bands and conduction bands, respectively, leaving the delocalized $s$ states to strongly repel each other. This $s-s$ repulsion leads to the upward band gap bowing and negative mixing enthalpies simultaneously. Remarkably, we identify a perovskite alloy that has a band gap much larger than all its components. Analogous trends of upward band gap bowing and negative mixing enthalpy also appear in the corresponding three-component and two-component $\mathrm{AB}{\mathrm{X}}_3$ halide perovskite alloys. These observations of upward band gap bowing and negative mixing enthalpy will significantly accelerate the design of stable upward band gap bowing alloys in a broad range of material families.