Despite exhibiting magnetic long-range order below $T_{\mathrm{N}}=26.7\mathrm{K}$, the honeycomb cobaltate ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ is predicted to enter a Kitaev spin liquid state when subjected to small external perturbations. While most of the reported literature investigates the effects of magnetic fields applied parallel to the honeycomb layers, we present high-resolution capacitance dilatometry studies for fields perpendicular to the Co-planes up to 15 T. Grüneisen analysis reveals the effect of uniaxial out-of-plane strain and shows that antiferromagnetic order in ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ is stabilized at a rate of $\partial T_{\mathrm{N}}/\partial p_{\mathrm{c}}=0.28\left(5\right)$ K/GPa. Further, failure of the Grüneisen scaling at low temperatures around $T_{\mathrm{cr}}\simeq 7.5\mathrm{K}$ demonstrates the presence of competing energy scales. In contrast to an only weak field dependence of the anomaly at $T_{\mathrm{N}}$, a broad hump at $T_{\mathrm{cr}}$($B=0\mathrm{T}$) evolves into a sharp peak at high fields applied $B\parallel c$. Our magnetostriction data show that a kink in the magnetization at $B_{\mathrm{C}}\simeq 4.6\mathrm{T}$ is accompanied by an inflection point in the field-induced length changes, which is likely related to weak unequal spin canting. All observed phenomena leave their signatures in the magnetoelastic phase diagram as constructed by our experimental results.

We study the ultrafast demagnetization dynamics of $Ln{\mathrm{Rh}}_2{\mathrm{Si}}_2$ ($Ln=\mathrm{Pr}$, Nd, Sm, Gd, Tb, Dy, Ho) antiferromagnets after excitation by a laser pulse, using a combination of density functional theory and atomistic spin and spin-lattice dynamics simulations. In the first step, we calculate the Heisenberg interactions using the magnetic force theorem and compare two approaches, where the $4f$ states of the rare earths are treated as frozen core states or as valence states with added correlation corrections. We find marked quantitative differences in terms of predicted Curie temperature for most of the systems, especially for those with a large orbital moment of the rare-earth cations. This can be attributed to the importance of indirect interactions of the $4f$ states through the Si states, which depends on the binding energy of the $4f$ states and coexists with Ruderman-Kittel-Kasuya-Yosida–type interactions mediated by the conduction states. However, qualitatively both approaches agree in terms of the predicted antiferromagnetic ordering at low temperature, which is in line with previous experiments. In the second step, the atomistic dynamics simulations are used in combination with a heat-conserving two-temperature model, which allows for the calculation of spin and electronic temperatures during the magnetization dynamics simulations. Our simulations demonstrate that despite quite different demagnetization times, magnetization dynamics of all studied $Ln{\mathrm{Rh}}_2{\mathrm{Si}}_2$ antiferromagnets exhibit similar two-step behavior, in particular the first fast drop followed by slower demagnetization. In addition, we observe that the demagnetization amplitude depends linearly on laser fluence, for low fluences, something that is also in agreement with experimental observations. We also investigate the impact of lattice dynamics on ultrafast demagnetization using coupled atomistic spin-lattice dynamics simulations and the heat-conserving three-temperature model, which confirm the linear dependence of magnetization on laser fluence. The microscopic mechanisms behind these behaviors are investigated in detail.

Magnetic thin films hosting topological spin textures, such as magnetic skyrmions, hold high potential for breakthroughs in the field of spintronics, due to good scalability and energy efficiency. Novel computational architectures such as memory-in-logic devices rely on material platforms able to host those topological spin textures. Furthermore, recently proposed designs of novel quantum information technologies are based on heterostructures where topological spin textures are in direct proximity to a superconducting layer. Here, we demonstrate the stabilization of out-of-plane magnetic bubbles in highly ordered ${[\mathrm{Ni}/\mathrm{Co}]}_n$ multilayers on a Nb(110) single crystal. This is achieved without the need for the removal of the well-known Nb(110)-oxide surface reconstruction, due to the introduction of an ultra-thin Cu interlayer in between the Nb substrate and the magnetic multilayer. The Cu interlayer generates a well-ordered hexagonal surface, which is key for the epitaxial growth of the ${[\mathrm{Ni}/\mathrm{Co}]}_n$ multilayers hosting the desired out-of-plane anisotropy. The magnetic ground state of the prepared material stacks is directly imaged via spin-polarized low-energy electron microscopy, revealing the presence of magnetic bubble domains with lateral sizes as small as 450 nm.

We synthesize the high-quality $\mathrm{G}{\mathrm{d}}_3\mathrm{BW}{\mathrm{O}}_9$ single crystal and investigate its low-temperature magnetic and thermodynamic properties. Below $T_{\mathrm{N}}=1.08\mathrm{K}$, the anisotropic behavior of magnetic susceptibilities reveals that the $\mathrm{G}{\mathrm{d}}^{3+}$ moments exhibit the dominant antiferromagnetic coupling along the $c$ axis, while displaying a ferromagnetic arrangement in kagome plane. With pronounced magnetic frustration, in adiabatic demagnetization refrigeration experiments starting from initial conditions of 9 T and 2 K, $\mathrm{G}{\mathrm{d}}_3\mathrm{BW}{\mathrm{O}}_9$ polycrystal reaches a minimum temperature of 0.151 K, significantly lower than its $T_{\mathrm{N}}$. Due to the high density of $\mathrm{G}{\mathrm{d}}^{3+}$ ions ($S=7/2$), the maximum magnetic entropy change reaches over $50\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}{\mathrm{K}}^{-1}$ under fields up to 7 T in $\mathrm{G}{\mathrm{d}}_3\mathrm{BW}{\mathrm{O}}_9$, nearly 1.5 times as large as commercial sub-kelvin magnetic coolant $\mathrm{G}{\mathrm{d}}_3\mathrm{G}{\mathrm{a}}_5{\mathrm{O}}_{12}$. The H-T phase diagram of $\mathrm{G}{\mathrm{d}}_3\mathrm{BW}{\mathrm{O}}_9$ under $\mathit{H}\parallel c$ exhibits field-induced critical behavior near the phase boundaries. This observation aligns with the theoretical scenario in which a quantum critical point acts as the endpoint of a line of classical second-order phase transitions. Such behavior suggests the importance of further investigations into the divergence of the magnetic Grüneisen parameter in the vicinity of critical field at ultralow temperatures.

The interplay between superconductivity and charge correlations in the kagome metal ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ can be tuned by external perturbations such as doping or pressure. Here we present a study of charge correlations and superconductivity upon hole doping via Ti substitution on the V kagome sites in ${\mathrm{CsV}}_{3-x}{\mathrm{Ti}}_x{\mathrm{Sb}}_5$ via synchrotron x-ray diffraction and scanning superconducting quantum interference device measurements. While the superconducting phase, as viewed via the vortex structure, remains conventional and unchanged across the phase diagram, the nature of charge correlations evolves as a function of hole doping from the first superconducting dome into the second superconducting dome. For Ti doping in the first superconducting dome, competing $2\times 2\times 2$ and $2\times 2\times 4$ supercells form within the charge density wave state and are suppressed rapidly with carrier substitution. In the second superconducting dome, no charge correlations are detected. Comparing these results to those observed for ${\mathrm{CsV}}_3{\mathrm{Sb}}_{5-x}{\mathrm{Sn}}_x$ suggests important differences between hole doping via chemical substitution on the V and Sb sites, particularly in the disorder potential associated with each dopant.

Hematite is predicted to have a high photoelectrochemical water splitting efficiency but currently is limited by a low polaron carrier conductivity. Consequently, doping hematite is necessary to increase the carrier concentration and thereby the conductivity. However, at modest dopant concentrations of 1.0% and above, a significant improvement is not observed. Recent theoretical calculations have predicted that there is a critical concentration, ${\mathrm{C}}_{\mathrm{crit}}$, of dopants that can be incorporated in the hematite structure for a given annealing temperature and oxygen partial pressure, above which precipitates may begin to form, thereby limiting the substitution of dopant into the hematite lattice. This critical concentration varies with the type of dopant, as well as the annealing temperature. In this paper we test the validity of these predictions by examining four dopants; Sn, Ge, Zr, and Nb, for samples synthesized in air at 800 ${}^{\circ }\mathrm{C}$. We find in good agreement with theory, that dopant-Fe-oxide or dopant-oxide precipitates do form at moderate dopant concentrations, reasonably close to the calculated critical concentrations (0.22%, 0.44%, 0.03%, and 0.07%, respectively) and explore the evolution of these oxide formations as the concentration increases above the critical point. Using EXAFS data analysis, we first show that at low total dopant concentrations the dopant substitutes uniformly throughout the hematite structure on the Fe site. As the concentration increases and oxide precipitates form we modeled the EXAFS data as a sum of a fraction, f1, of doped hematite and a second fraction, f2, of oxide. Surprisingly, the dopant-Fe-oxides/dopant-oxides that form are quite different for different dopants—and for Zr and Nb two different nanocrystal precipitates develop; two polymorphs of ${\mathrm{ZrO}}_2$ (orthorhombic and tetragonal) for Zr, and ${\mathrm{NbFeO}}_4$ and ${\mathrm{Nb}}_{12}{\mathrm{O}}_{29}$ for Nb. At moderate concentrations (0.3%) one oxide is present (orthorhombic ${\mathrm{ZrO}}_2$ and ${\mathrm{NbFeO}}_4$). As the dopant concentration increases further, these fractions saturate while the second oxide begins to grow. Finally, the fraction of dopant going into the oxide phase first increases logarithmically with increasing total dopant concentration and then begins to saturate. This means that as the total concentration increases above the critical concentration, a significant fraction of the dopant still substitutes into the hematite lattice until the total concentration is 3–5 times the critical concentration. This is important for applications that need to maximize parameters such as the catalytic efficiency.

We report a size-dependent solute clustering mechanism at grain boundaries in a sputtered ultrafine-grained Ag-Cu alloy, where large Cu clusters form despite weak individual solute-solute interactions. X-ray diffraction confirms limited Cu solubility in the Ag matrix, while scanning transmission electron microscopy reveals Cu clustering at both ordinary GBs and GB junctions. Density functional theory calculations show that 12-atom Cu clusters are energetically preferred, while smaller three-atom clusters are significantly less stable. Additional calculations demonstrate a marked increase in solute-cluster attraction energy with cluster size. These findings point to a previously unrecognized pathway for grain-boundary solute clustering in immiscible systems, driven by collective solute-cluster interactions, with implications for segregation behavior and stability in nanocrystalline and ultrafine-grained alloys.

The lightweight high-entropy alloy represents an innovative class of multicomponent systems that combine low density with the exceptional mechanical properties of high-entropy alloys. We present a detailed synthesis and investigation of a $3d$ rich equiatomic high entropy alloy superconductor Sc-Ti-V-Nb-Cu, which crystallizes in a body-centered cubic structure. Magnetization, electrical resistivity, and heat capacity measurements confirm weakly coupled bulk type II superconductivity with a transition temperature of 7.21(3) K and an upper critical field of 12.9(1) T close to the Pauli paramagnetic limit. Its remarkably high critical current density of $1.812\left(1\right)\times {10}^5$ A-${\mathrm{cm}}^{-2}$ exceeds practical requirements. The combination of low-density, moderate transition temperature, high upper critical field, and large critical current density positions Sc-Ti-V-Nb-Cu as a promising candidate for next-generation superconducting device applications.

The energy landscape of screw dislocation cores plays a central role in dislocation-mediated deformation mechanisms in body-centered cubic (bcc) metals. In bcc high-entropy alloys (HEAs), this energy landscape is modulated by local compositional fluctuations, which has important implications for deformation processes in these materials. Through first-principles calculations, this study investigates high-symmetry screw dislocation core structures in NbTaMoW and NbTaTiHf bcc HEAs. The results show that alloying group IV transition metals lead to large local lattice distortions at dislocation cores, which is demonstrated to be an important factor governing fluctuations in core configurations along a dislocation line. Importantly, group IV elements near the core induce features in the energy landscape that are exclusive for HEAs, specifically lowering the energy of core configurations that are unstable in elemental bcc metals. A combined influence of these chemical effects with crystallographic details enables the activation of $\left\{112\right\}$ glide planes, a feature that has been linked to ductility improvements in bcc HEAs. These findings provide new insights into the atomic-scale mechanisms underlying dislocation mobility in bcc HEAs, offering a pathway for designing materials with tailored mechanical properties.

The possibility of growing high-quality $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ thin films is crucial to exploring the properties of topological surface states. Since sputtering is a well-established growth technique and it can be used on an industrial scale, the study of sputtered $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ thin film represents a path for direct large-scale device production. This work presents a systematic study of RF-sputtered growth of $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ thin film at room temperature on (100)-oriented MgO substrates. The structural and chemical properties of $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ thin films are characterized by high-resolution transmission electron microscopy (HRTEM); in-depth energy dispersion x-ray spectroscopy; x-ray photoemission spectroscopy (XPS); x-ray diffraction (XRD); x-ray reflectivity (XRR); and Raman spectroscopy techniques, which confirm the growth of high-quality and stoichiometric compounds. Atomic force microscopy (AFM) and scanning tunneling microscopy (STM) reveal films characterized by granular surface morphology. Scanning tunneling spectroscopy (STS), a technique that measures the local density of states (LDOS) of the surface, confirms the presence of surface states. The STS maps show that the Dirac point shifts as we approach a grain boundary.

Realizing an epitaxial interface between a superconductor (SC) and a topological insulator (TI) is crucial for studying emergent quantum phenomena such as topological superconductivity. So far, this epitaxial heterostructure has been mainly investigated by growing TI films on SC substrates. Here, we reverse the order by epitaxially growing ultrathin films of $\alpha$-Sn, a SC layer, on top of TIs. $In situ$ scanning tunneling microscopy reveals that similar thin films of $\alpha$-Sn (111), from monolayer to quintuple layer, can be realized on a wide range of substrates from n-type ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ to p-type ${\mathrm{Sb}}_2{\mathrm{Te}}_3$. We obtain superconductivity with a transition temperature around 1 K from these heterostructures across the n-type to p-type crossover. The large spin-orbit coupling of our systems can give rise to enhanced in-plane upper critical magnetic field. Our work reveals the potential of employing the $\alpha \text{−}\text{Sn/}(\text{Bi},{\mathrm{Sb})}_2{\mathrm{Te}}_3$ heterostructures for transport studies on emergent phenomena due to nontrivial topology.

This study proposes a materials search method combining a data assimilation technique based on a multivariate Gaussian distribution with Bayesian optimization. The efficiency of the search using this method was demonstrated using a pair of example functions. By combining Bayesian optimization with the data assimilation technique, the maximum value of the example function was found more efficiently compared to ordinary Bayesian optimization without the data assimilation. A practical demonstration was also conducted by constructing a data assimilation model for the band gap of $\left({\mathrm{Sr}}_{1-x_1-x_2}{\mathrm{La}}_{x_1}{\mathrm{Na}}_{x_2}\right)\left({\mathrm{Ti}}_{1-x_1-x_2}{\mathrm{Ga}}_{x_1}{\mathrm{Ta}}_{x_2}\right){\mathrm{O}}_3$. The concentration dependence of the band gap was analyzed, and synthesis was performed with chemical compositions in the sparse region of the training data points to validate the predictions.

Ceria (${\mathrm{CeO}}_2$) is a material of significant technological importance. A detailed understanding of the material's defect physics and chemistry is key to understanding and optimizing its properties. Here, we report a hybrid density-functional study of native point defects, hydrogen impurities, and metal dopants in ${\mathrm{CeO}}_2$. We find that electron polarons (${\eta }_{\mathrm{Ce}}^{-}$) and oxygen vacancies ($V_{\mathrm{O}}^{2+}$) are the dominant native defects under conditions ranging from extreme oxidizing to highly reducing. Hydrogen is stable either in the hydroxyl (${\mathrm{H}}_i^{+}$) or hydride (${\mathrm{H}}_{\mathrm{O}}^{+}$) structure, but the substitutional ${\mathrm{H}}_{\mathrm{O}}^{+}$ is energetically more favorable than ${\mathrm{H}}_i^{+}$ only under highly reducing conditions. The interstitial ${\mathrm{H}}_i^{+}$ is highly mobile in the bulk. Yttrium (Y) is energetically most favorable at the substitutional Ce site. Copper (Cu) and nickel (Ni) can be incorporated at the substitutional site and/or an interstitial site, depending on actual conditions during preparation, and the dopants can exist in different charge and spin states. In light of the results, we discuss electronic and ionic conduction and the effects of metal doping on the formation of electron polarons and oxygen vacancies.

Disorder can be utilized as an effective parameter to probe the interplay between two long-range orders such as superconductivity and charge density wave. In the present work, we report the experimental evidence for filamentary superconductivity in polycrystalline $\mathrm{TiS}{\mathrm{e}}_{1.2}{\mathrm{S}}_{0.8}$ with superconducting transition $T_{\mathrm{c}}\sim 7\mathrm{K}$. This is validated from magnetization and magnetotransport measurements. Strain-induced dislocations, substitution defects, and randomly distributed Ti ions (with local moments) are considered as possible sources of disorder. A detailed analysis of the temperature-dependent resistivity evaluates the degree of disorder and the consequent localization effects. These findings are in contrast to the fact that superconductivity has not been established at ambient pressure in ${\mathrm{TiSe}}_{2-x}{\mathrm{S}}_x$ system. It is suggested that disorder serves as a stabilizing factor for the superconducting phase due to incommensuration of the charge density wave.

We formulate a novel methodology called the regularized Green's function method for dislocations (RGFMD) for determining the continuum elastic fields of a dislocation at the lattice scales. RGFMD utilizes the quantum mechanical force density (QMFD) to represent atoms within the elastic media without creating any singularities in the elastic fields. QMFD is related to the electron wave functions, which is a fundamental result of density functional theory (DFT) calculations. RGFMD solves the force equilibrium equation where QMFD is used to represent atomic displacements via computationally efficient Fourier transforms. The quasiperiodic nature of dislocations is accounted for by introducing the periodic images of individual atoms along the dislocation line. The advantage of RGFMD over existing classical elasticity solutions and atomistic simulations for dislocations is threefold. First, RGFMD guarantees the production of well-behaving bounded elastic fields, whereas classical elasticity frameworks diverge at the dislocation core. Second, classical elasticity frameworks are built upon the assumption of plane strain conditions, which limits their ability to realistically capture variations in the elastic fields along the dislocation line. RGFMD accurately accounts for the local atomic configuration and does not imply simplifying conditions such as plane strain. Third, atomistic calculations can report elastic quantities such as displacements only at the lattice sites, which precludes their full integration with well-established continuum field theories. RGFMD bridges this gap between atomistic and continuum theories by mapping out continuum elastic fields using the discrete displacement data provided by DFT calculations.