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.

Metal halide perovskites (MHPs) have emerged as highly promising materials for optoelectronic applications, with all-inorganic MHPs presenting enhanced stability compared to their hybrid counterparts. Here, we investigate the atomic dynamics and structural fluctuations in single crystals of $\mathrm{CsSnBr}{}_3$ and $\mathrm{CsPbBr}{}_3$ through systematic inelastic neutron scattering (INS) measurements as a function of temperature. Our experiments are compared with first-principle simulations, augmented with large-scale molecular dynamics modeling, based on machine-learned neural network potentials. Through both INS and simulations, we find quasi-elastic diffuse rods in reciprocal space in both compounds, originating from fluctuating planar domains featuring correlated tilts of Br octahedron. The diffuse rods exhibit a slow, overdamped dynamic process, modulated across $\mathit{Q}$ space, reflecting the strong lattice anharmonicity of the inorganic framework. We do not find evidence for dynamic off-centering of the ${\mathrm{Sn}}^{2+}$ ions besides phonon vibrations at the center of the Br octahedron. These results offer valuable insights into the unusual anharmonic atomic dynamics and intricate correlated structural distortions in MHPs, which will be critical for rationalizing and further tailoring their thermal and optoelectronic properties.

Hexagonal boron nitride (hBN) hosts luminescent defects possessing spin qualities compatible with quantum sensing protocols at room temperature. Vacancies, in particular, are readily obtained via exposure to high-energy ion beams. While the defect creation mechanism via such irradiation is well understood, the occurrence rate of optically active negatively charged vacancies (${\mathrm{V}}_{\mathrm{B}}^{-}$) is still an open question. In this work, we exploit focused helium ions to systematically create optically active vacancy defects in hBN flakes at varying densities. By comparing the density-dependent spin splitting measured by magnetic resonance to calculations based on a microscopic charge model, in which we introduce a correction term due to a constant background charge, we are able to quantify the number of ${\mathrm{V}}_{\mathrm{B}}^{-}$ defects created by the ion irradiation. We find a lower bound for the fraction (0.2%) of all vacancies in the optically active, negatively charged state. Our results provide a protocol for measuring the creation efficiency of ${\mathrm{V}}_{\mathrm{B}}^{-}$, which is necessary for understanding and optimizing luminescent centers in hBN.

The application of density functional theory to materials in the warm dense matter regime has motivated the development of exchange-correlation functionals which incorporate proper, explicit temperature dependence. Previous work has yielded fully thermal exchange-correlation free energy functionals at the local density approximation and generalized gradient approximation (GGA) levels of refinement. Recently, an additive thermal correction scheme was utilized to construct a meta-GGA exchange-correlation (XC) functional in which thermal effects are treated at the GGA level. The $f\mathrm{TSCAN}$ free-energy XC functional presented here includes thermal effects through the meta-GGA level in the context of the SCAN (strongly constrained and appropriately normed) ground-state functional. The $f\mathrm{TSCAN}$ functional provides generality while achieving similar performance to a thermal GGA functional at high temperatures, e.g., pressures within 1% of path integral Monte Carlo simulations of warm dense hydrogen and a significant improvement over ground-state functionals. At low temperatures, $f\mathrm{TSCAN}$ demonstrates improvements in accuracy relative to lower-level and deorbitalized functionals, indicating that calculations using $f\mathrm{TSCAN}$ may be expected to perform well across experimentally relevant densities and pressures.

The hallmark feature of polymorphic systems is their ability to assemble into many possible structures at the same thermodynamic state. Designer polymorphic materials can in principle be engineered via programmable self-assembly, but the robustness of the assembly process depends on dynamical factors that are poorly understood. Here we predict a failure mode for the growth of multicomponent polymorphic materials in which dynamical coexistence occurs between ordered and disordered assembly trajectories. We show that this transition is preceded by the formation of a steady-state disordered wetting layer, suggesting a nonequilibrium analogy to premelting phenomena at equilibrium. This dynamical phase transition is likely to occur in a variety of systems and may fundamentally limit the complexity of polymorphic materials that can be designed through programmable self-assembly.

The quasiharmonic approximation is the most common method for modeling the specific heat of solids; however, it fails to capture the effects of intrinsic anharmonicity. In this study, we introduce the “elastic softening approximation,” an alternative approach to modeling intrinsic anharmonic effects on thermodynamic quantities, which is grounded in Wallace's thermodynamic framework that tracks entropy changes resulting from the continuous change (e.g., softening) of phonons as a function of temperature. A key finding of our study is a correlation between Poisson's ratio and the differential rate of phonon softening at finite frequencies, compared to lower frequencies relevant to elastic moduli measurements. We observe that elemental solids such as $\alpha$-Be, diamond, Al, Cu, In, W, Au, and Pb, which span a wide range of Poisson's ratios and exhibit varying degrees of intrinsic anharmonicity, consistently follow this trend. When applied to $\alpha$-U, $\alpha$-Pu, and $\delta$-Pu, our method reveals unusually large anharmonic phonon contributions at elevated temperatures across all three light actinide metals. These findings are attributed to the unique combination of enhanced covalency and softer elastic moduli inherent in the actinides, potentially influenced by their $5f$-electron bonding.

Nanoporous materials are expected to be game changers in the energy and environmental crisis, with central applications in fluid adsorption, separation, and catalysis. Yet, besides the abundant literature on fluids nanoconfined in their extreme porosity, the acoustic behavior of nanoporous solids subjected to fluid adsorption remains to be explored. In fact, while sound testing is a routine method used to probe fluid-filled porous materials—typically for geoscience applications, the molecular origin of the acoustic signature of fluid confinement at the nanoscale has yet to be unveiled. In particular, a molecular formalism describing how nanoconfinement and surface interactions affect longitudinal and transverse wave propagation is still missing. Here, using different fluids $({\mathrm{CH}}_4,$ ${\mathrm{CO}}_2)$ in a prototypical nanoporous material (zeolite), we conduct a molecular simulation study to identify the acoustic modes (velocity and attenuation) from the system's dynamic structure factor. First, by varying the fluid nature and temperature as well as the fluid-solid interaction strength, we decipher the parameters that drive acoustic propagation and derive a simple kinematic model that predicts accurately the linear decay observed in the sound velocity upon increasing the fluid mass density (regardless of the fluid description level, e.g., flexible versus rigid, coarse-grained versus all-atom, etc.). Second, from the broadening of the acoustic modes in the dynamic structure factor, we show that attenuation increases with the fluid adsorbed amount and with the solid-fluid interaction strength due to phonon scattering at the fluid-solid interface. We establish that all data can be quantitatively rationalized by considering the change in the phonon lifetime through an additional relaxation time arising from the interaction between fluid molecules and the zeolite lattice. This simple stochastic model, which provides a robust framework to describe the acoustics of fluid-solid composites based on physical quantities amenable to simple experiments, offers new perspectives for the design of novel applications involving the acoustics of fluids in interaction with nanoporous solids.

In recent years, as the attention in spintronics expands from collinear to noncollinear magnetism, the interlayer Dzyaloshinskii-Moriya interaction (IL-DMI) has been proven capable of mediate long-range chiral spin textures between magnetic layers. Here, we experimentally confirm that analogous to the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, where indirect coupling oscillates between parallel and antiparallel states with spacer thickness, a damped oscillation in IL-DMI strength and direction is observed. Specifically, an oscillation period of $0.96\pm 0.01\mathrm{nm}$ is obtained when mediated by a Ru spacer, close to the 1.15 nm reported for conventional RKKY while well fitted by theoretical predictions. In contrast, no oscillation is observed when IL-DMI is mediated by a Pt spacer, consistent with the negligible RKKY coupling across Pt. This work experimentally confirms the physical origin of IL-DMI as stemming from the RKKY mechanism, while also providing a tuning knob in controlling IL-DMI's polarity and strength by adjusting spacer thickness, beneficial to the development of future three-dimensional spintronics memory devices.

Increased electrical resistance from grain boundary phases (complexions) limit the performance of many thermoelectric materials. A promising strategy for mitigating the excess boundary resistance in polycrystalline materials is altering the composition at a grain boundary by causing a complexion transition. In NbFeSb, increasing the Ti content has been shown to result in Ti-rich boundaries that effectively eliminate boundary resistance and make Ti-doped NbFeSb one of the best $p$-type thermoelectric materials in the medium temperature range. In this study, we have developed a model based on the average band offset between the grain boundaries and grain to identify complexion transitions from resistive, insulating boundaries to metallic boundaries. This model has been applied to NbFeSb, where a transition occurs at a Ti chemical potential with an approximate composition of ${\mathrm{Nb}}_{0.9}{\mathrm{Ti}}_{0.10}\mathrm{FeSb}$. This approach is an interesting method to evaluate the behavior of grain boundaries in thermoelectric materials and can be applied to any material with thermoelectric transport data and grain size measurements.

Vanadium in silicon carbide is a promising spin photon interface candidate with optical transitions in the telecom range and a long lived electron spin, hosted in an advanced semiconductor platform. In this detailed investigation of the defect's 16-dimensional ground state spin manifold at millikelvin temperatures, a wide range of previously unreported transitions are observed which are accurately described using a theoretical model that includes strain. Using a superconducting microcoil we achieve Rabi frequencies exceeding $20\mathrm{MHz}$ and perform the first coherent manipulation of a direct hyperfine transition. These insights further underscore the defect's potential for strain engineering and sensing, as well as for fault-tolerant qudit encoding.

Chiral multifold fermions in solids exhibit unique band structures and topological properties, making them ideal for exploring fundamental physical phenomena related to nontrivial topology, chirality, and symmetry breaking. However, the challenge of obtaining clean, flat surfaces through cleavage has hindered the investigation of their unique electronic states. In this study, we utilize high-resolution angle-resolved photoemission spectroscopy and density functional theory calculations to investigate the low-energy electronic structure of the cleavable single-crystal PdSbSe. Our combined experimental and theoretical analysis reveals the presence of multifold-degenerate fermions within this chiral crystal. We also observe multiple chiral Fermi arc surface states and spin-splitting behavior in the associated bulk bands. These findings provide unique insights into chiral, multifold fermionic states in easily cleavable crystals and offer a robust platform for further research into their unique electronic properties and potential applications in novel electronic devices.

We report the measurement, using angle-resolved photoemission spectroscopy, of the metallic electronic structure of the hole-doped thermoelectric oxide ${\mathrm{CuRh}}_{0.9}{\mathrm{Mg}}_{0.1}{\mathrm{O}}_2$. The material is found to have a “pudding mold” type band structure, with a nearly flat band edge located near the Fermi level, which is thought to be the origin of the thermoelectric behavior of this material. The experimental data match the density functional theory of the undoped parent compound, simply corrected by a rigid shift of the bands. Transport calculations based on the observed band structure yield a Seebeck coefficient of $\sim 200\mu \mathrm{V}/\mathrm{K}$ for the undoped parent material, consistent with experimental measurements. Our results show that ${\mathrm{CuRhO}}_2$ is a textbook example of how pure band-structural effects can result in a large thermoelectric figure of merit, demonstrating that flat band edges in oxides are a realistic route for the efficient conversion of thermal energy.