Using femtosecond time- and angle-resolved photoemission spectroscopy, we explore the out-of-equilibrium dynamics of surface fermions in the topological system ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$. We show that the presence of localized states from defects at the surface is one of the key material parameters undergirding the long relaxation time of photoexcited Dirac electrons lying in the projected band gap of the bulk-insulating pristine compound. Doping this ternary compound with Sn substituting on Bi sites, while desirably increasing the resistivity at low temperatures decreases decay times of the excited homologous Dirac electrons. On the basis of these observations, we argue that long relaxation times can be ultimately controlled by a charge transfer to the surface.

Control of materials through custom design of ionic distributions represents a powerful new approach to develop future technologies ranging from spintronic logic and memory devices to energy storage. Perovskites have shown particular promise for ionic devices due to their high ion mobility and sensitivity to chemical stoichiometry. In this work, we demonstrate a solid-state approach to control of ionic distributions in $\left(\mathrm{La},\mathrm{Sr}\right)\mathrm{Co}{\mathrm{O}}_3$ thin films. Depositing a Gd capping layer on the perovskite film, oxygen is controllably extracted from the structure, up to 0.5 O/u.c. throughout the entire 36-nm thickness. Commensurate with the oxygen extraction, the Co valence state and saturation magnetization show a smooth continuous variation. In contrast, magnetoresistance measurements show no change in the magnetic anisotropy and a rapid increase in the resistivity over the same range of oxygen stoichiometry. These results suggest significant phase separation, with metallic ferromagnetic regions and oxygen-deficient, insulating, nonferromagnetic regions, forming percolated networks. Indeed, x-ray diffraction identifies oxygen-vacancy ordering, including transformation to a brownmillerite crystal structure. The unexpected transformation to the brownmillerite phase at ambient temperature is further confirmed by high-resolution scanning transmission electron microscopy which shows significant structural—and correspondingly chemical—phase separation. This work demonstrates room-temperature ionic control of magnetism, electrical resistivity, and crystalline structure in a 36-nm-thick film, presenting opportunities for ionic devices that leverage multiple material functionalities.

Quantum spins on a triangular lattice may bring out intriguing and exotic quantum ground states. Here we report a magnetodielectric system of CuOHCl wherein $S=1/2\mathrm{C}{\mathrm{u}}^{2+}$ spins constitute a two-dimensional triangular lattice with the layers weakly coupled via Cl-H-O bonding. Despite strong magnetic interactions, as expected from the relatively high value of ${\theta }_{\mathrm{CW}}=-100\mathrm{K}$, antiferromagnetic transition occurred at $T_N=11\mathrm{K}$, followed by an uprising turn of the magnetic susceptibility below ∼7 K. Neutron-diffraction experiment revealed a coplanar spin structure on the triangular lattice below the $T_N$, with each spin pointing toward the center of a triangle. Of the three spins on a triangle, two are antiparallel and the third one is angled ${120}^{\circ }$ to the antiparallel spins. A concerted effect of geometric frustration in the triangular lattice and superexchange interactions through a zig-zag path via double Cu-O-Cu and double Cu-Cl-Cu bridges counted for this spin arrangement. Further investigation using dielectric constant and heat capacity measurements, as well as a microscopic probe of muon spin rotation, revealed a magnetodielectric effect and the possibility of multiferroic transition at $T^{*}\sim 5\mathrm{K}$, which is suspected to be in close relation to geometric frustration in this triangular lattice. The present paper presents a magnetodielectric system on a two-dimensional triangular lattice with chemical stoichiometry. It can also serve as a rare reference to the hotly debated quantum spin-orbital liquid compound $\mathrm{LiNi}{\mathrm{O}}_2$.

The interplay of magnetism and superconductivity (SC) has been a focus of interest in condensed matter physics for decades. $\mathrm{EuF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ has been identified as a potential platform to investigate interactions between structural, magnetic, and electronic effects as well as the coexistence of magnetism and SC with similar transition temperatures. However, there are obvious inconsistencies in the reported phase diagrams of $\mathrm{Eu}{\left(\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{C}{\mathrm{o}}_x\right)}_2\mathrm{A}{\mathrm{s}}_2$ crystals grown by different methods. For transition metal arsenide (TMA)-flux-grown crystals, even the existence of SC is uncertain. Here we reexamine the phase diagram of single-crystalline $\mathrm{Eu}{\left(\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{C}{\mathrm{o}}_x\right)}_2\mathrm{A}{\mathrm{s}}_2$ grown by TMA flux. We found that the lattice parameter $c$ shrinks linearly with Co doping, almost twice as fast as that of the tin-flux-grown crystals. With Co doping, the spin-density-wave (SDW) order of the Fe sublattice is quickly suppressed, being detected only up to $x=0.08$. The magnetic ordering temperature of the $\mathrm{E}{\mathrm{u}}^{2+}$ sublattice ($T_{\mathrm{Eu}}$) shows a systematic evolution with Co doping, first going down and reaching a minimum at $x=0.08$, then increasing continuously up to $x=0.24$. Over the whole composition range investigated, no signature of SC is observed above 1.8 K.

The isothermal crystallization from the melt and glassy state of poly(trimethylene terephthalate) has been studied with wide-angle x-ray diffraction, small-angle x-ray scattering (SAXS), and ultrasmall-angle x-ray scattering (USAXS). Large scattering intensity in the low-$q$ region has been observed with SAXS and USAXS during the early stage of melt and glass crystallizations. We have quantitatively analyzed the x-ray results using the scattering equations which can simultaneously deal with the hierarchical structures consisting of the crystallites and their aggregates. The results reveal the crystallization mechanism in which the crystalline nodules cover the entire sample with the aggregation regions. The conclusion quantitatively shows that the large SAXS intensity is not due to the density fluctuations of the liquid state but due to the correlations among the heterogeneous aggregation regions of the nodules.

Topological insulators (TIs) with an inverted bulk band and a strong spin-orbit coupling exhibit gapless topological surface states (TSSs) protected by time-reversal symmetry. Helical spin textures driven by spin-momentum locking offer the opportunity to generate spin-polarized currents and therefore TIs are expected to be used for future spintronic applications. For practical applications TIs are urgently required that are operable at room temperature due to a wide bulk band gap as well as a distinct topological surface state that is robust to atmospheric exposure. Here we show two distinguishable TSSs originating from different terminations on ${\mathrm{PbBi}}_4{\mathrm{Te}}_4{\mathrm{S}}_3$ by using spin- and angle-resolved photoemission spectroscopy. We find that one TSS is persistently observed, while the other becomes invisible upon intentional oxygen exposure. The result signifies the presence of a protected TSS buried under the topmost surface. Our finding paves the way for realizing a topological spintronics device under atmospheric conditions.

Optical conductivity measurements on a ${\mathrm{BaCoS}}_2$ single crystal unveil an unusual linear behavior over a broad spectral range. In the paramagnetic phase above 300 K, the spectrum shows no gap, which contradicts the previously proposed scenario of a charge-transfer Mott insulator. Ab initio dynamical mean field theory calculations including a retarded Hubbard interaction explain the data in terms of an incipient opening of a $\mathrm{Co}\left(3d\right)-\mathrm{S}\left(3p\right)$ charge-transfer gap concomitant to incoherent charge transport driven by electronic correlations. These results point to a non-Fermi liquid scenario with Hund's metal properties in the paramagnetic state, which arises from an incipient Mott phase destabilized by low-energy charge fluctuations across the vanishing $3d-3p$ charge-transfer gap.

Applying a stress to a glassy polymer accelerates its dynamics as one goes from low stress up to plastic regime. For decades, the phenomenological Eyring's model has been used to describe plastic flow in polymers. This model, however, raises fundamental issues which makes its use deleterious in glassy polymers. We propose an alternative model in which the elastic energy stored at the length scale of dynamical heterogeneities $\xi \approx 3–5$ nm reduces the free energy barrier for relaxation. Contrary to the Eyring's activation volume, which has no clear interpretation, this length scale is derived from physical arguments, based on a detailed account of relaxation mechanisms at the molecular scale. Recent creep experiments in glassy polymers by Ediger and coworkers allow for discriminating the two pictures. It is shown that the whole evolution of the ${\tau }_{\alpha }$ relaxation time under stress can be reproduced quantitatively, using as the only adjustable parameter the scale $\xi$. The obtained value of $\xi$ is the same as the value previously determined by considering the whole set of properties of glassy polymers. This confirms the coherence and completeness of the theory of relaxation processes in nonpolar glassy polymers that we proposed in previous works.

The structural, elastic, vibrational, and electronic properties of ${\mathrm{RbCaF}}_3$ in the cubic and low-temperature tetragonal phases have been studied at the ab initio level with density functional theory. Using various exchange-correlation functionals of the generalized gradient approximation for structural properties like the ${\mathrm{CaF}}_6$ octahedron rotational angle or ratio $c/a$ of the tetragonal lattice constants, it is found that the best agreement with experiment is obtained with the PBEsol and Wu and Cohen (WC) functionals. The fundamental band gap is calculated to be direct and indirect in the tetragonal and cubic phases, respectively. The relation between the cubic and tetragonal phases is studied by monitoring the cubic zone boundary soft mode phonon $R_{{15}^{\prime }}$ as well as the $c/a$ ratio and tilt angle. The results for the Born effective charge tensors are also reported in order to study the effect of the long-range Coulomb interactions. We also investigated the corresponding pressure driven phase transition at $T=0\mathrm{K}$, which we observe to be of second order in contrast to the first-order character experimentally detected for the structurally similar high pressure transition at ambient temperature. Based on recently developed finite strain Landau theory, we offer a possible explanation for this peculiar change of character.

As a classical structural phase transformation with a long history in physical metallurgy, the face-centered cubic (fcc) to body-centered cubic/tetragonal (bcc/bct) martensitic transformation in steels is one of the most studied crystal structure changes induced by either temperature or stress. However, it has yet to be realized that besides the well-known Bain path that leads to {112} twinning in the bct lattice, a hidden pathway could also be activated during the transformation, which leads to the formation of slip martensite and {110} twinning in the bct lattice. We show by symmetry and crystallographic analysis that activation of the hidden pathway is driven by internal or external stress coupled with short-range migration of interstitial carbon atoms. By considering both the hidden pathway and the Bain path, we construct a transformation pathway network for the fcc to bcc/bct transformation, which allows a systematic analysis of defects generation during transformation cycling. A new mechanism for the formation of slip martensite in steels is demonstrated through the analysis of symmetry breaking along both the hidden pathway and the Bain path, by which two distinctive types of martensites, slip and twinning martensites, are placed on an equal footing from a crystallographic point of view. The pathway network provides new insights into structural transformation mechanisms and the nature of transformation induced defects in steels.

The low thermal conductivity of piezoelectric perovskites is a challenge for high power transducer applications. We report first principles calculations of the thermal conductivity of ferroelectric ${\mathrm{PbTiO}}_3$ and the cubic nearly ferroelectric perovskite ${\mathrm{KTaO}}_3$. The calculated thermal conductivity of ${\mathrm{PbTiO}}_3$ is much lower than that of ${\mathrm{KTaO}}_3$ in accord with experiment. Analysis of the results shows that the reason for the low thermal conductivity of ${\mathrm{PbTiO}}_3$ is the presence of low frequency optical phonons associated with the polar modes. These are less dispersive in ${\mathrm{PbTiO}}_3$, leading to a large three phonon scattering phase space. These differences between the two materials are associated with the $A$-site driven ferroelectricity of ${\mathrm{PbTiO}}_3$ in contrast to the $B$-site driven near ferroelectricity of ${\mathrm{KTaO}}_3$. The results are discussed in the context of modification of the thermal conductivity of electroactive materials.

We critically investigate the purported existence of a multiglass state in the quantum paraelectrics $\mathrm{SrTiO}{}_3$ and $\mathrm{KTaO}{}_3$ doped with magnetic $3d$ transition metals. We observe that the transition metals have limited solubility in these hosts, and that traces of impurity magnetic oxides persist even in the most well-processed specimens. Our dielectric measurements indicate that the polar nanoregions formed as a consequence of doping appear to lack co-operativity, and the associated relaxation process exhibits a thermally activated Arrhenius form. At lower temperatures, the dielectric susceptibility could be fit using the Barrett's formalism, indicating that the quantum-paraelectric nature of the host lattices is unaltered by the doping of magnetic transition metal oxides. All these doped quantum paraelectrics exhibit a crossover from the high-temperature Curie-Weiss regime to one dominated by quantum fluctuations, as evidenced by a $T{}^2$ dependence of the temperature-dependent dielectric susceptibility. The temperature dependence of the magnetic susceptibility indicates that magnetic signatures observed in some of the specimens could be solely ascribed to the presence of impurity oxides corresponding to the magnetic dopants used. Hence, the doped quantum paraelectrics appear to remain intrinsically paramagnetic down to the lowest measured temperatures, ruling out the presence of a multiglass state.

Closed-form analytical expressions for the effective electrical (thermal) conductivity and elastic moduli of a wide class of three-dimensional isotropic particulate composites (dispersions) for all phase contrasts and volume fractions have been previously formulated. These property estimates were extracted from exact strong-contrast expansions that incorporate complete microstructural information. In this paper, we employ these analytical expressions to derive and apply “cross-property maps” that connect combinations of pairs of the aforementioned effective transport and elastic properties to one another for a wide class of dispersions in which the inclusions can have different shapes and sizes and are spatially distributed in a matrix with varying degrees of order/disorder. We illustrate cross-property maps for a variety of cases (e.g., incompressible or compressible composites with conducting/insulating inclusions that may be rigid or compliant or auxetic) for high inclusion volume fractions. All of the cross-property maps that involve the effective conductivity translate immediately into equivalent results for the effective dielectric constant, magnetic permeability, or diffusion coefficient because they are mathematically analogous. We discuss an example that enables us to design a disordered dispersion with desired values of the effective dielectric constant and effective Young's modulus. Cross-property maps and their extensions will facilitate the rational design of particulate media with different desirable multifunctional characteristics. Moreover, our investigation has implications for the application of machine-learning and other data-driven approaches for multifunctional materials discovery.

This article presents a nonequilibrium thermodynamic theory for the mean-field precipitation, aggregation, and pattern formation of colloidal clusters. A variable gradient energy coefficient and the arrest of particle diffusion upon “jamming” of cluster aggregates in the spinodal region predicts observable gel patterns that, at high intercluster attraction, form system-spanning, out-of-equilibrium networks with glasslike, quasistatic structural relaxation. For reactive systems, we incorporate the free energy landscape of stable prenucleation clusters into the Allen-Cahn reaction equation. We show that pattern formation is dominantly controlled by the Damköhler number and the stability of the clusters, which modifies the autocatalytic rate of precipitation. As clusters individually become more stable, bulk phase separation is suppressed.

Understanding the interactions of solutes in a host material is a key factor for successful alloy design. In this work, we focus on the effect of C, B, O, Fe, and Hf interactions in Mo on grain boundary (GB) segregation and cohesion within an ab initio framework. We show that segregation energies reveal only a weak coverage dependence for the interstitial solutes while for substitutionals a pronounced minimum occurs at 1 ML coverage. For Fe, segregation energies remain negative up to very high coverages of up to 8 ML, which correspond to thick segregation films whose geometric structure and magnetic properties are explored. As a next step we evaluate the binding energy for different solute combinations and show that interactions are mainly repulsive at the GB, while strong attractive interactions occur in the bulk. Origins for this phenomena are discussed and cosegregation energies are extracted for different scenarios. The coverage dependence on cohesion is nearly linear for all investigated solutes and increases for all solutes except for O and Hf.

The recent observation of the hexagonal-closed-packed (hcp) phase in CoCrFeNi-based multicomponent alloys has reopened the question of phase stability in these alloys. We investigate the alloying effect of tungsten on the crystal and magnetic structures of (${\mathrm{CoCrFeNi})}_{1-x}{\mathrm{W}}_x$ high entropy alloys using density functional theory by means of the exact muffin-tin orbital method. The body-centered-cubic (bcc), face-centered-cubic (fcc), and hcp phases are investigated in two magnetic states: ferrimagnetic and paramagnetic. Below 8 at. % W the ground state of ${\left(\mathrm{CoCrFeNi}\right)}_{1-x}{\mathrm{W}}_x$ is the ferrimagnetic hcp phase and above that, the ferrimagnetic bcc phase is stabilized. Our calculations show that the fcc and hcp phases are energetically very close in the whole range of studied W compositions and because CoCrFeNi and ${\left(\mathrm{CoCrFeNi}\right)}_{0.93}{\mathrm{W}}_{0.07}$ are observed in the fcc phase at room temperature, the hcp-fcc structural phase transition is expected to occur at lower temperatures. The total magnetic moment in bcc is almost double the value calculated for the fcc and hcp structures, which is due to that Cr moments are nearly quenched in bcc but are coupled antiferromagnetically to Fe, Ni, and Co in both hcp and fcc. We calculated also the Curie temperature of these alloys using the mean-field approximation. The calculated value was found to be 155 K for fcc CoCrFeNi, in excellent agreement with experiments, and the addition of W decreases this value. Our results contribute to the development of these relatively unknown corrosion-resistant materials into industrial applications, such as cemented carbides.

Two distinct trends in the temperature dependence of viscosity, measured directly and inferred from calorimetry by analyzing crystallization kinetics, can be correlated with the temperature evolution of the height of the first peak of the x-ray total structure factor for liquid GeTe and ${\mathrm{Ge}}_{15}{\mathrm{Te}}_{85}$. The phase-change chalcogenide GeTe is a high-fragility liquid with the kinetic fragility value of 76, at the glass-transition temperature, being between those for liquid (Ag,In)-doped ${\mathrm{Sb}}_2\mathrm{Te}$ and ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$. The viscosity of the high-temperature liquid shows Arrhenius kinetics on cooling to the melting point, and the structure factor conforms to the fragile liquid. For liquid ${\mathrm{Ge}}_{15}{\mathrm{Te}}_{85}$, the temperature evolution of the structure factor suggests a transition in the temperature range of about 100 K above the melting. The crystallization shows a wide range of Arrhenius kinetics in the supercooled liquid region. This finding combined with the dynamic viscosity measurements is interpreted by invoking a weak fragile-to-strong crossover on cooling the liquid ${\mathrm{Ge}}_{15}{\mathrm{Te}}_{85}$. The differences in structures and dynamics of liquid GeTe and ${\mathrm{Ge}}_{15}{\mathrm{Te}}_{85}$ appear closely correlated to their distinctly different crystallization mechanisms.