Two-dimensional (2D) Rashba systems have been intensively studied in the last decade due to their unconventional physics, tunability capabilities, and potential for spin-charge interconversion when compared to conventional heavy metals. With the advent of a new generation of spin-based logic and memory devices, the search for Rashba systems with more robust and larger conversion efficiencies is expanding. Conventionally, demanding techniques such as angle- and spin-resolved photoemission spectroscopy are required to determine the Rashba parameter ${\alpha }_R$ that characterizes these systems. Here, we introduce a simple method that allows a quantitative extraction of ${\alpha }_R$, through the analysis of the bilinear response of angle-dependent magnetotransport experiments. This method is based on the modulation of the Rashba-split bands under a rotating in-plane magnetic field. We show that our method is able to correctly yield the value of ${\alpha }_R$ for a wide range of Fermi energies in the 2D electron gas at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface. By applying a gate voltage, we observe a maximum ${\alpha }_R$ in the region of the band structure where interband effects maximize the Rashba effect, consistent with theoretical predictions.

An analytical model invoking variations in the charge-carrier density is used to generate magnetoresistance curves that are almost indistinguishable from those produced by sophisticated numerical models. This demonstrates that, though disorder is pivotal in causing linear magnetoresistance, the form of the magnetoresistance thus generated is insensitive to details of the disorder. Taken in conjunction with the temperature ($T$) dependence of the zero-field resistivity, realistic levels of disorder are shown to be sufficient to explain the linear magnetoresistance and field-$T$ resistance scaling observed in high-temperature pnictide and cuprate superconductors. Hence, though the $T$-linear zero-field resistance is a definite signature of the “strange metal” state of high-temperature superconductors, their linear magnetoresistance and its scaling is unlikely to be so.

Single-phase concentrated solid-solution alloys (CSA), i.e., alloys without a principle alloying element but one randomly populated crystal structure, exhibit attractive material properties such as very high ductility at cryogenic temperatures, a gentle decrease of strength with temperature, or an unexpectedly high resistance against irradiation. For clarification of those observations assessment of atomic transport mechanisms including formation and migration of equilibrium point defects is indispensable. Positron annihilation lifetime spectroscopy measurements are performed to quantify the concentration of quenched-in thermal vacancies in fcc CSAs after quenching from temperatures close to their onset of melting. For various alloy compositions the concentration of quenched-in vacancies decreases with increasing entropy of mixing $\mathrm{\Delta }{S}_{\mathrm{mix}}$. Whereas alloys with three constituents in nonequimolar fractions (CrFeNi) exhibit vacancy concentrations in the ${10}^{-5}$ range, the studied alloys with four (CoCrFeNi) and five constituents (CoCrFeMnNi, AlCoCrFeNi) do not show a vacancy-specific positron lifetime. Therefore, the concentration of quenched-in vacancies must be in the range of ${10}^{-6}$ or less. It can be concluded that there is either only a vanishingly small fraction of vacancies present at temperatures near the onset of melting or the generated vacancies are inherently unstable.

In the $A{B}_4{Q}_8$ lacunar spinels, the electronic structure is described on the basis of inter- and intracluster interactions of tetrahedral $B_4$ clusters, and tuning these can lead to myriad fascinating electronic and magnetic ground states. In this work, we employ magnetic measurements, synchrotron x-ray and neutron scattering, and first-principles electronic structure calculations to examine the coupling between structural and magnetic phase evolution in ${\mathrm{GaMo}}_4{\mathrm{Se}}_8$, including the emergence of a skyrmionic regime in the magnetic phase diagram. We show that the competition between two distinct Jahn-Teller distortions of the room temperature cubic $F\overline{4}3m$ structure leads to the coexistence of the ground-state $R3m$ phase and a metastable $Imm2$ phase. The magnetic properties of these two phases are computationally shown to be very different, with the $Imm2$ phase exhibiting uniaxial ferromagnetism and the $R3m$ phase hosting a complex magnetic phase diagram including equilibrium Néel-type skyrmions stable from nearly $T$ = 28 K down to $T$ = 2 K, the lowest measured temperature. The large change in magnetic behavior induced by a small structural distortion reveals that ${\mathrm{GaMo}}_4{\mathrm{Se}}_8$ is an exciting candidate material for tuning unconventional magnetic properties via mechanical means.

By means of temperature and wavelength-dependent small-angle neutron scattering (SANS) experiments on sintered isotropic and textured Nd-Fe-B magnets we provide evidence for the existence of an anisometric structure in the microstructure of the textured magnets. This conclusion is reached by observing a characteristic cross-shaped angular anisotropy in the total unpolarized SANS cross section at temperatures well above the Curie temperature. Comparison of the experimental SANS data to a microstructural model based on the superquadrics form factor allows us to estimate the shape and lower bounds for the size of the structure. Subtraction of the scattering cross section in the paramagnetic regime from data taken at room temperature provides the magnetic SANS cross section. Surprisingly, the anisotropy of the magnetic scattering is very similar to the nuclear SANS signal, suggesting that the nuclear structure is decorated by the magnetic moments via spin-orbit coupling. Based on the computation of the two-dimensional correlation function we estimate lower bounds for the longitudinal and transversal magnetic correlation lengths.

The influence of spatial confinement on the thermally excited stochastic cation dynamics of the room-temperature ionic liquid 1-N-butylpyridinium bis-[(trifluoromethyl)sulfonyl]imide ([BuPy][${\mathrm{Tf}}_2\mathrm{N}$]) inside porous carbide-derived carbons with various pore sizes in the sub- to a few nanometer range is investigated by quasielastic neutron spectroscopy. Using the potential of fixed window scans, i.e., scanning a sample parameter, while observing solely one specific energy transfer value, an overview of the dynamic landscape within a wide temperature range is obtained. It is shown that already these data provide a quite comprehensive understanding of the confinement-induced alteration of the molecular mobility in comparison to the bulk. A complementary, more detailed analysis of full energy transfer spectra at selected temperatures reveals two translational diffusive processes on different time scales. Both are considerably slower than in the bulk liquid and show a decrease of the respective self-diffusion coefficients with decreasing nanopore size. Different thermal activation energies for molecular self-diffusion in nanoporous carbons with similar pore size indicate the importance of pore morphology on the molecular mobility, beyond the pure degree of confinement. In spite of the dynamic slowing down we can show that the temperature range of the liquid state upon nanoconfinement is remarkably extended to much lower temperatures, which is beneficial for potential technical applications of such systems.

In ferroelectrics, domain walls have significant effects on ferroic properties. The mechanical behavior of domain walls has been investigated because of its scientific and technological importance. Numerous experiments and simulations have examined domain walls and their motion under mechanical strain. However, since nano- and micromechanical testing is challenging, previous studies have mainly involved indirect or intermittent observations, and there have been few real-time in situ observations. As a result, the mechanism by which mechanical loads induce wall motion remains elusive. Here, we directly observe stress-induced domain wall motion in real time by using rationally designed specimens and transmission electron microscopy (TEM). We imaged the domain walls directly as they moved and disappeared depending on the mechanical conditions during the experiments. Furthermore, we experimentally determined the mechanical criteria for wall motion. Our results not only provide fundamental knowledge about ferroelectrics but also provide information on how to control their dynamics from an engineering standpoint.

We report on the evolution of the average and depth-dependent magnetic order in thin-film samples of biaxially stressed and electron-doped ${\mathrm{EuTiO}}_3$ for samples across a doping range $<0.1$ to $7.8\times {10}^{20}{\mathrm{cm}}^{-3}$. Under an applied in-plane magnetic field, the $G$-type antiferromagnetic ground state undergoes a continuous spin-flop phase transition into in-plane, field-polarized ferromagnetism. The critical field for ferromagnetism slightly decreases with an increasing number of free carriers, yet the field evolution of the spin-flop transition is qualitatively similar across the doping range. Unexpectedly, we observe interfacial ferromagnetism with saturated ${\mathrm{Eu}}^{2+}$ moments at the substrate interface at low fields preceding ferromagnetic saturation throughout the bulk of the degenerate semiconductor film. We discuss the implications of these findings for the unusual magnetotransport properties of this compound.

We have synthesized a monoclinic phase of IrSi by means of high-pressure and high-temperature treatment above 25 GPa, whereas the theoretically predicted high-pressure phase (i.e., FeSi-type structure) was not identified up to 48 GPa. Magnetotransport measurements clarify the semimetallic nature (carrier density of $\sim 6-9\times {10}^{19}/\mathrm{c}{\mathrm{m}}^3$) of monoclinic IrSi, in agreement with our band structure calculations. Furthermore, the surface electronic bands are predicted to show the large Rashba spin splitting, offering potential spintronic applications of this material.

Designing and understanding functional electronic and magnetic properties in perovskite oxides requires controlling and tuning the underlying crystal lattice. Here we report the structure, including oxygen and cation positions, of a single-crystal, entropy stabilized perovskite oxide film of $\mathrm{La}\left(\mathrm{C}{\mathrm{r}}_{0.2}\mathrm{M}{\mathrm{n}}_{0.2}\mathrm{F}{\mathrm{e}}_{0.2}\mathrm{C}{\mathrm{o}}_{0.2}\mathrm{N}{\mathrm{i}}_{0.2}\right){\mathrm{O}}_3$ grown on $\mathrm{SrTi}{\mathrm{O}}_3$ (001). The parent materials range from orthorhombic ($\mathrm{LaCr}{\mathrm{O}}_3, \mathrm{LaMn}{\mathrm{O}}_3$, and $\mathrm{LaFe}{\mathrm{O}}_3$) to rhombohedral ($\mathrm{LaCo}{\mathrm{O}}_3$ and $\mathrm{LaNi}{\mathrm{O}}_3$), and first principles calculations indicate that these structural motifs are nearly degenerate in energy and should be highly distorted site to site. Despite this extraordinary local configurational disorder on the $B$-site sublattice, we find a structure with unexpected macroscopic crystalline homogeneity with a clear orthorhombic unit cell, whose orientation is demonstrated to be controlled by the strain and crystal structure of the substrate for films grown on $\left(\mathrm{L}{\mathrm{a}}_{0.3}\mathrm{S}{\mathrm{r}}_{0.7}\right)\left(\mathrm{A}{\mathrm{l}}_{0.65}\mathrm{T}{\mathrm{a}}_{0.35}\right){\mathrm{O}}_3$ and $\mathrm{NdGa}{\mathrm{O}}_3$ (110). Furthermore, quantification of the atom positions within the unit cell reveals that the orthorhombic distortions are small, close to $\mathrm{LaCr}{\mathrm{O}}_3$, which may be driven by a combination of disorder averaging and the average ionic radii. This is a step toward understanding the rules for designing crystal motifs and tuning functional properties through controlled configurational complexity.

Understanding the physics of associative polymers is often limited by our inability to directly measure bond dissociation under deformation. In this work, we developed a rheo-fluorescence technique and applied it to characterize the nonlinear shear response of linear side-functionalized polymer chains cross-linked via nickel-terpyridine complexation. As the network was sheared, the fraction of dissociated bonds was quantitatively measured based upon a change in fluorescence with metal dissociation. Shear thinning of the gel was accompanied by only a small increase in the fraction of dissociated bonds. Comparison with several transient network models shows that the shear thinning within the constraint of the measured fraction of dissociated bonds cannot be explained by classical theories that include retraction of dangling chains alone; the rheological response likely involves alternative modes of stress relaxation.

This paper is a contribution to the joint Physical Review Applied and Physical Review Materials collection titled Two-Dimensional Materials and Devices.

Metal and magnetic overlayers alter the surface of the topological insulator (TI) bismuth selenide $\left(\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3\right)$ through proximity effects but also by changing the composition and chemical structure of the $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ subsurface. The interface between $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ and Mn metal or manganese selenide was explored using x-ray photoelectron spectroscopy revealing chemical and electronic changes at the interface. Depositing Mn metal on $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ without an external source of Se shows unexpected bonding within the Mn layer due to Mn-Se bonding as Se diffuses out of the $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ layer into the growing Mn film. The Se outdiffusion is further evidenced by changes in Bi core levels within the $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ layers indicating primarily Bi-Bi bonding over Bi-Se bonding. No outdiffusion of Se occurred when excess Se is supplied with Mn, indicating the importance of supplying enough chalcogen atoms with deposited metals. However, $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ core-level photoelectrons exhibited a rigid chemical shift toward higher binding energy after depositing a monolayer of $\mathrm{MnS}{\mathrm{e}}_{2-x}$, indicating a dipole within the overlayer. Stoichiometry calculations indicated that the monolayer forms MnSe preferentially over the transition metal dichalcogenide (TMD) phase $\mathrm{MnS}{\mathrm{e}}_2$, providing a consistent picture of the dipole formation in which a plane of Se anions sits above Mn cations. This study shows that chemical diffusion and dipole formation are important for $\mathrm{Mn}\text{−}\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ and $\mathrm{MnS}{\mathrm{e}}_{2-x}\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ and should be considered carefully for TMD/TI interfaces more generally.

The kinetics and mechanisms of phase transitions that occur during lithiation of amorphous silicon thin films were extensively studied using electrochemical techniques and transmission electron microscopy. Measurements of current made while changing applied potentials at a constant rate (cyclic voltammograms) showed peaks that correspond to the formation of two amorphous Li-Si alloys with different stoichiometries. Peaks associated with these phase transitions were also observed in current-time measurements made in potentiostatic experiments. The times at which currents reached maxima were found to be independent of film thickness, suggesting that the phase transitions occur throughout the volume of the films. Analysis of current-time measurements using the Johnson-Mehl-Avrami-Kolmogorov method indicate that the transitions occur through a first-order nucleation and growth process. This conclusion is supported by cross-section transmission electron microscopy in which high-energy electron-beam-induced sputtering of lithium led to contrast between the parent and product phases. In contrast with other studies, the polyamorphic amorphous-to-amorphous first-order phase transitions observed in this study are driven by changes in composition rather than changes in pressure or temperature. The potentiostatic techniques employed in this study can be used to characterize the nature and kinetics of phase transitions that occur in other electrode materials for lithium-ion batteries.

Reversible topotactic transitions between oxygen-vacancy-ordered structures in transition-metal oxides provide a promising strategy for active manipulation of material properties. While transformations between various oxygen-deficient phases have been attained in bulk $\mathrm{AB}{\mathrm{O}}_{3-\delta }$ perovskites, substrate clamping restricts the formation of distinct ordering patterns in epitaxial films. Using in situ scanning transmission electron microscopy (STEM), we image a thermally driven reversible transition in $\mathrm{L}{\mathrm{a}}_{0.5}\mathrm{S}{\mathrm{r}}_{0.5}\mathrm{Co}{\mathrm{O}}_{3-\delta }$ films on $\mathrm{SrTi}{\mathrm{O}}_3$ from a multidomain brownmillerite (BM) structure to a uniform new phase wherein oxygen vacancies order in every third $\mathrm{Co}{\mathrm{O}}_x$ plane. Because temperature cycling is performed over a limited temperature range $(25-385{}^{\circ }\mathrm{C})$, the oxygen deficiency parameter $\delta$ does not vary measurably. Under constant $\delta$, the topotactic transition proceeds via local reordering of oxygen vacancies driven by thermal strain. Atomic-resolution imaging reveals a two-step process whereby alternating vertically and horizontally oriented BM domains first scale in size to accommodate the strain induced by different thermal expansions of $\mathrm{L}{\mathrm{a}}_{0.5}\mathrm{S}{\mathrm{r}}_{0.5}\mathrm{Co}{\mathrm{O}}_{3-\delta }$ and $\mathrm{SrTi}{\mathrm{O}}_3$, before the new phase nucleates and quickly grows above $360{}^{\circ }\mathrm{C}$. Upon cooling, the film transforms back to the mixed BM phase. As the structural transition is fully reversible and $\delta$ does not change upon temperature cycling, we rule out electron-beam irradiation during STEM as the driving mechanism. Instead, our findings demonstrate that thermal strain can solely drive topotactic phase transitions in perovskite oxide films, presenting opportunities for switchable ionic devices.

Three-dimensional topological insulators (3D TIs) possess a specific topological order of electronic bands, resulting in gapless surface states via bulk-edge correspondence. Exotic phenomena have been realized in ferromagnetic TIs, such as the quantum anomalous Hall (QAH) effect with a chiral-edge conduction and a quantized value of the Hall resistance $R_{yx}$. Here, we report on the emergence of distinct topological phases in paramagnetic Fe-doped ${\left(\mathrm{Bi},\mathrm{Sb}\right)}_2\mathrm{S}{\mathrm{e}}_3$ heterostructures with varying structure architecture, doping, and magnetic and electric fields. Starting from a 3D TI, a two-dimensional insulator appears at layer thicknesses below a critical value, which turns into an Anderson insulator for Fe concentrations sufficiently large to produce localization by magnetic disorder. With applying a magnetic field, a topological transition from the Anderson insulator to the QAH state occurs, which is driven by the formation of an exchange gap owing to a giant Zeeman splitting and reduced magnetic disorder. A topological phase diagram of ${\left(\mathrm{Bi},\mathrm{Sb}\right)}_2\mathrm{S}{\mathrm{e}}_3$ allows exploration of intricate interplay of topological protection, magnetic disorder, and exchange splitting.

Two-dimensional electron gases (2DEGs) can form at the surface of oxides and semiconductors or in carefully designed quantum wells and interfaces. Depending on the shape of the confining potential, 2DEGs may experience a finite electric field, which gives rise to relativistic effects such as the Rashba spin-orbit coupling. Although the amplitude of this electric field can be modulated by an external gate voltage, which in turn tunes the 2DEG carrier density, sheet resistance and other related properties, this modulation is volatile. Here, we report the design of a “ferroelectric” 2DEG whose transport properties can be electrostatically switched in a nonvolatile way. We generate a 2DEG by depositing a thin Al layer onto a $\mathrm{SrTi}{\mathrm{O}}_3$ single crystal in which 1% of Sr is substituted by Ca to make it ferroelectric. Signatures of the ferroelectric phase transition at 25 K are visible in the Raman response and in the temperature dependences of the carrier density and sheet resistance that shows a hysteretic dependence on electric field as a consequence of ferroelectricity. We suggest that this behavior may be extended to other oxide 2DEGs, leading to novel types of ferromagnet-free spintronic architectures.

Materials with simple crystal structure usually exhibit substantial lattice thermal conductivity $\left({\kappa }_{\mathrm{latt}}\right)$ due to a scarcity of strong phonon-phonon scattering channels. Yet here we find simple CsCl-structure TlBr to have an extremely low room temperature ${\kappa }_{\mathrm{latt}}\sim 0.5\mathrm{W}/\text{m-K}$, while at low temperature we find a high ${\kappa }_{\mathrm{latt}}$ of $155\mathrm{W}/\text{m-K}$. This extreme range of conductivity behavior results from lattice-instability-related anharmonicity, effective only at temperatures above $\sim 30\mathrm{K}$. Manipulating these instabilities (e.g., pressure, strain) may yield a pathway to ultralow thermal conductivity via intrinsic phonon resistance in similar materials.

At its boundaries and vortices, a topological superconductor features Majorana fermions, which are potentially applicable for topological quantum computations. The scarcity of the known experimentally verified physical systems with topological superconductivity, time-reversal invariant ones in particular, is giving rise to a strong demand for identifying new candidate materials. In this research, we study a heterostructure consisting of a transition-metal-oxide two-dimensional electron gas (2DEG) sandwiched by insulators near the paraelectric/ferroelectric (PE/FE) phase transition. Its relevant characteristics are the combination of the transition-metal spin-orbit coupling and the soft odd-parity phonons arising from the ferroelectric fluctuation; it gives rise to the fluctuating Rashba effect, which can mediate the pairing interaction for time-reversal invariant topological superconductivity. As the PE/FE phase transition can be driven by applying strain on the heterostructure, this system provides a tunable electron-phonon coupling. Through the first-principle calculations on the (001) $\left[{\mathrm{BaOsO}}_3\right]{\left[{\mathrm{BaTiO}}_3\right]}_4$, we find such electron-phonon coupling to be strong over a wide range of applied tensile biaxial strain in the monolayer ${\mathrm{BaOsO}}_3$ sandwiched between the (001) ${\mathrm{BaTiO}}_3$, hence qualifying it as a good candidate material. Furthermore, the stability of topological superconductivity in this material is enhanced by its orbital physics that gives rise to the anisotropic dispersion.

This paper is a contribution to the joint Physical Review Applied and Physical Review Materials collection titled Two-Dimensional Materials and Devices.

We perform coherent nonlinear spectroscopy of excitons in single-layers of ${\mathrm{MoSe}}_2$ and ${\mathrm{WSe}}_2$ encapsulated between thin films of hexagonal boron nitride. Employing four-wave-mixing microscopy we identify virtually disorder-free areas, generating an exciton optical response at the homogeneous limit. Focussing on such areas, we measure exciton homogeneous broadening as a function of environmental factors, namely temperature and exciton density. Exploiting four-wave-mixing imaging, we find that at such locations, nonlinear absorption of the exciton excited states and their coherent couplings can be observed. Using the ${\mathrm{WSe}}_2$ heterostructure, we infer coherence and density dynamics of the exciton $2S$ state. Owing to its increased radiative lifetime, at low temperatures, the dephasing of the $2S$ state is longer than of the $1S$ transition. While scanning various heterostructures across tens of micrometers, we conclude that the disorder, principally induced by strain variations, remain to be present creating spatially varying inhomogeneous broadening.

Phosphorene is a promising two-dimensional material for electronics, energy, and catalysis applications, but its instability in ambient conditions is a major obstacle in its commercialization. The degradation begins with dissociation of ${\mathrm{O}}_2$ onto the surface of phosphorene which is spontaneous in its singlet state. Focusing on singlet ${\mathrm{O}}_2$ splitting, compressive strain is proposed as a method of inducing an activation barrier to this process. Applying compressive strain introduced a barrier as large as 0.45 eV causing a decrease in the rate constant by seven orders of magnitude. Introducing entropic effects via metadynamics biased ab initio molecular dynamics allowed for the generation of free-energy landscapes. Through this method, the largest barrier was observed to be 0.33 ± 0.04 eV causing a decrease in the kinetic rate constant by five orders of magnitude. These results highlight compression as a method of inhibiting singlet oxidation of phosphorene and pave the way towards further experimental investigation.