There has been a lot of debate on whether non-Fourier thermal conduction can be observed in quasi-one-dimensional materials such as carbon nanotubes (CNTs) and, additionally, whether the phenomenon can be found by equilibrium or nonequilibrium molecular dynamics (EMD or NEMD) simulations. In fact, so far EMD and NEMD simulations have revealed disparities of thermal transport in CNTs, ranging from purely diffusive behavior, to diffusive-ballistic transition, and to non-Fourier thermal conduction. By carefully examining the roles of interfacial thermal resistances and applied temperature differences in NEMD simulations, we show that the two effects often yield spurious results. After removing the unwanted effects that have been overlooked by previous works, we find that most EMD and NEMD simulations on CNTs consistently display diffusive thermal conduction for length (L) > 200 nm. The finding is further supported by the disappearance of nonlocal thermal conduction for L > 200 nm. Our results clarify many discrepancies of previous works and point out that nonideal thermostats commonly used in EMD and NEMD simulations would give an effective contact thermal resistance that misleads data interpretations. Overall, we find EMD and NEMD simulations conducted so far disagree with the current experimental results of nondiffusive thermal conduction in CNTs.

We study the early stages of growth of the IV-VI semiconductor PbSe on (001)-oriented III-V substrates with different surface chemistry and lattice parameter, with the aim of achieving high quality cube-on-cube rocksalt on zincblende epitaxy. We find that PbSe nucleation on bare GaSb, InAs, and GaAs substrates is varied, yet consistently results in mixed orientation growth due to chemistry-dependent interfacial-energy penalties, irrespective of lattice mismatch. To overcome this, we locate a growth window for cube-on-cube single-orientation nucleation of PbSe on III-arsenide surfaces utilizing a high-temperature surface treatment with PbSe flux that we find creates a better template for subsequent low-temperature growth and leads to sharp interfaces. We probe this interface between PbSe and InAs, finding a Chain[Pb,As] atomic arrangement, tantamount to a discontinuous anion sublattice between III-V and IV-VI materials. We also observe a vertical displacement of the first few monolayers of the Se sublattice that we believe has origins in the heterovalency of this interface. Our results point towards surface chemistry as the primary factor governing film orientation, and lattice mismatch governing island coalescence behavior in these heterovalent interfaces with dissimilar crystal structures.

We explore the cross coupling between the ferroelectric and ferromagnetic phases in $\mathrm{Ni}/\mathrm{BaTi}{\mathrm{O}}_3$(001) heterostructures and demonstrate the modulation of the magnetism and incidence of exchange bias in the ultrathin metallic Ni overlayer, depending on the ferroelectric state of the bottom layer. We establish that 5-nm-thick monocrystalline Ni film deposited on $\mathrm{BaTi}{\mathrm{O}}_3$ with ferroelectric polarization pointing towards the surface (P+) favors the organization of Ni into uniform ferromagnetic domains. Ni grown on $\mathrm{BaTi}{\mathrm{O}}_3$ with opposite ferroelectric polarization is featured by emerging exchange-bias coupling between the ferromagnetic Ni top layers and the antiferromagnetic reacted interface, as theoretically explained by first-principles calculations. We explicitly obtain the morphology of the magnetic domains of the crystalline Ni layer in atomic and magnetic force microscopy measurements (AFM/MFM). The resemblance of AFM and MFM images indicate that, although with radically different morphologies, in both cases all spins orient in the Ni plane. Consequently, the distinct signature of the ferroelectric-ferromagnetic coupling extracted from the magneto-optical Kerr effect measurements encodes all the information of sample magnetism. The peculiar magnetic coupling depending on the ferroelectric state indicates new ways of engineering the functionality of metal/ferroelectric interfaces.

Tensile-strained $\mathrm{LaCo}{\mathrm{O}}_{3-\delta }$ thin films are ferromagnetic, in sharp contrast to the zero-spin bulk, although no clear consensus has emerged as to the origin of this phenomenon. While magnetism has been heavily studied, relatively little attention has been paid to electronic transport, due to the insulating nature of the strain-stabilized ferromagnetic state. Here, structure, magnetism, and transport are studied in epitaxial $\mathrm{LaCo}{\mathrm{O}}_{3-\delta }$ films (10–22-nm thick) on various substrates (from 1.4% compressive to 2.5% tensile strain), using synchrotron x-ray diffraction, scanning probe and transmission electron microscopy, magnetometry, polarized neutron reflectometry, resistivity, and Hall effect. High quality, smooth films are obtained, exhibiting superstructures associated with both oxygen vacancy ordering and periodic in-plane ferroelastic domains. Consistent with prior work, ferromagnetism with an approximately 80–85 K Curie temperature is observed under tension; polarized neutron reflectometry confirms a relatively uniform magnetization depth profile, albeit with interfacial dead layer formation. Electrical transport is found to have similar semiconducting nature to bulk, but with reduced resistivity and activation energy. Hall effect measurements, however, reveal a striking inversion of the majority carrier type, from $p$-type in the bulk and under compression to n-type under tension. While thus far overlooked, ferromagnetism in epitaxial $\mathrm{LaCo}{\mathrm{O}}_{3-\delta }$ films is thus directly correlated with $n$-type behavior, providing important insight into the ferromagnetic state in this system. Aided by density functional theory calculations, these results are interpreted in terms of tensile-strain-induced orbital occupation and band structure changes, including a rapid decrease in effective mass at the $e_g$-derived conduction band minimum, and corresponding increase at the valence band maximum.

We determine the work functions of the iron arsenic compounds $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($A=\mathrm{Ca},\mathrm{Ba},\mathrm{Cs}$) using photoemission spectroscopy to be 2.7 eV for ${\mathrm{CaFe}}_2{\mathrm{As}}_2$, 1.8 eV for ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, and 1.3 eV for ${\mathrm{CsFe}}_2{\mathrm{As}}_2$. The work functions of these 122 iron-based superconductors track those of the elementary metal $A$ but are substantially smaller. The most likely explanation of this observation is that the cleaving surface exposes only half an $A$-layer. The low work function and good photoemission cross section of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ and ${\mathrm{CsFe}}_2{\mathrm{As}}_2$ enable photoemission even from a common white LED light.

The polymer relaxation dynamic of a sample, stretched up to the stress hardening regime is measured at room temperature as a function of the strain $\lambda$ for a wide range of the strain rate $\dot{\gamma }$ using an original dielectric spectroscopy set up. The mechanical stress modifies the shape of the dielectric spectra mainly because it affects the dominant polymer relaxation time $\tau$, which depends on $\lambda$ and is a decreasing function of $\dot{\gamma }$. The fastest dynamics is not reached at yield but in the softening regime. The dynamics slows down during the hardening, with a progressive increase of $\tau$. A small influence of $\dot{\gamma }$ and $\lambda$ on the relative dielectric strength cannot be excluded.

The early stages of anodic oxidation in the potentiostatic condition at the interface between a borate buffer solution at pH 8.4 and Fe(100), (110), and (111) surfaces are investigated by sub-second resolution real-time x-ray reflectometry. The growth process is composed of three stages. The electron density of the inner-oxide layer for Fe(111) after a potential step from $-0.8$ to $+0.7$ V versus Ag/AgCl in the steady-state growth, stage III (1.65 s $<t$, where $t$ denotes the time after the potential step), is 1.52 electrons/${\AA }^3$, which is close to the density of magnetite. The growth rate is proportional to $t^{-1}$, which follows the direct logarithmic law or point-defect model. In an earlier stage of the oxide film growth, stage II $(0.4<t\le 1.65$ s), the growth rate is proportional to $t^{-1.5}$, while the electron density maintains the same value as that in stage III. At the first stage that can be discriminated by our time resolution, stage I $(t\le 0.4$ s), the growth rate is the same as that for stage II, i.e., proportional to $t^{-1.5}$, while the electron density significantly decreases. The suggested structure is a highly defective spinel oxide, and defects are filled by the end of stage I; there is no signature of the growth of other phases, such as hydroxide, as the inner layer. This behavior indicates that the early stage of the oxide film growth is faster than the formation of a complete spinel layer at the iron-oxide interface. The deviation of the growth rate from the point-defect model in stages I and II is caused by the strong electric field at the iron-oxide interface.

We reveal the impact of magnetic ordering on stacking fault energy (SFE) and its influence on the deformation mechanisms and mechanical properties in a class of nonequiatomic quinary Mn-containing compositional complex alloys or high entropy alloys (HEAs). By combining ab initio simulation and experimental validation, we demonstrate magnetic ordering as an important factor in the activation and transition of deformation modes from planar dislocation slip to TWIP (twinning-induced plasticity) and/or TRIP (transformation-induced plasticity). A wide compositional space of $\mathrm{C}{\mathrm{r}}_{20}\mathrm{M}{\mathrm{n}}_x\mathrm{F}{\mathrm{e}}_y\mathrm{C}{\mathrm{o}}_{20}\mathrm{N}{\mathrm{i}}_z(x+y+z=60$, at. %) was probed by density-functional theory calculations to search for potential alloys displaying the TWIP/TRIP effects. Three selected promising HEA compositions with varying Mn concentrations were metallurgically synthesized, processed, and probed for microstructure, deformation mechanism, and mechanical property evaluation. The differences in the deformation modes of the probed HEAs are interpreted in terms of the computed SFEs and their dependence on the predicted magnetic state, as revealed by ab initio calculations and validated by explicit magnetic measurements. It is found that the Mn content plays a key role in the stabilization of antiferromagnetic configurations which strongly impact the SFEs and eventually lead to the prevalent deformation behavior.

We carry out x-ray absorption spectroscopy experiment at the oxygen $K$ edge in croconic acid (${\mathrm{C}}_5{\mathrm{H}}_2{\mathrm{O}}_5$) crystal as a prototype of ferroelectric organic molecular solid, whose electric polarization is generated by proton transfer. The experimental spectrum is well reproduced by the electron-hole excitation theory simulations from configuration generated by ab initio molecular dynamics simulation. When inversion symmetry is broken in the ferroelectric state, the hydrogen bonding environment on the two bonded molecules become inequivalent. Such a difference is sensitively probed by the bound excitation in the pre-edge, which is strongly localized on the excited molecules. Our analysis shows that a satellite peak in the pre-edge will emerge at higher excitation energy, which serves as a clear signature of ferroelectricity in the material.

The vertical adsorption distances of the planar conjugated organic molecule 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) on hydroxylated ZnO(0001), determined with the x-ray standing wave technique (XSW), are at variance with adsorption geometries simulated with density functional theory for surface-structure models that consider terminating OH, whereas good agreement is found for PTCDI in direct contact with the topmost Zn layer. The consequential assignment of OH to subsurface sites is supported by additional, independent XSW and energy scanned photoelectron diffraction data and calls for a reconsideration of the prevalent surface models with important implications for the understanding of ZnO(0001) surfaces.

Conducting and magnetic properties of a material often change in some confined geometries. However, a situation where a nonmagnetic semiconductor becomes both metallic and magnetic at the surface is quite rare, or maybe even unique. In this Rapid Communication, we employ first-principles electronic structure theory to predict that such a peculiar magnetic state emerges in a family of quaternary Heusler compounds. We investigate magnetic and electronic properties of CoCrTiP, FeMnTiP, and CoMnVAl. For the latter material, we also analyze the structural stability of various surface terminations. For the ideal CoMn termination we calculate the magnetic exchange interactions and use them for parametrizing an effective spin Hamiltonian. According to our results, magnetism in this material should persist at temperatures at least as high as 160 K.

External magnetic fields influence the microstructure of polycrystalline materials. We explore the influence of strong external magnetic fields on the long time scaling of grain size during coarsening in thin films with an extended phase-field-crystal model. Additionally, the change of various geometrical and topological properties is studied. In a situation which leads to stagnation, an applied external magnetic field can induce further grain growth. The induced driving force due to the magnetic anisotropy defines the magnetic influence of the external magnetic field. Different scaling regimes are identified dependent on the magnetization. At the beginning, the scaling exponent increases with the strength of the magnetization. Later, when the texture becomes dominated by grains preferably aligned with the external magnetic field, the scaling exponent becomes independent of the strength of the magnetization or stagnation occurs. We discuss how the magnetic influence change the effect of retarding or pinning forces, which are known to influence the scaling exponent. We further study the influence of the magnetic field on the grain size distribution (GSD), next-neighbor distribution (NND) as well as grain shape and orientation. If possible, we compare our predictions with experimental findings.

We have synthesized ${\mathrm{Fe}}_{1+y}\mathrm{Te}$ thin films by means of molecular beam epitaxy (MBE) under Te-limited growth conditions. We found that epitaxial layer-by-layer growth is possible for a wide range of excess Fe values, wider than expected from what is known from studies on the bulk material. Using x-ray magnetic circular dichroism spectroscopy at the Fe $L_{2,3}$ and Te $M_{4,5}$ edges, we observed that films with high excess Fe contain ferromagnetic clusters while films with lower excess Fe remain nonmagnetic. Moreover, x-ray absorption spectroscopy showed that it is possible to obtain films with very similar electronic structure as that of a high quality bulk single crystal ${\mathrm{Fe}}_{1.14}\mathrm{Te}$. Our results suggest that MBE with Te-limited growth may provide an opportunity to synthesize FeTe films with smaller amounts of excess Fe as to come closer to a possible superconducting phase.

Recent experimental study on the Tsai-type quasicrystalline approximant Au-Al-Gd, which is a crystal but has the same local structure as quasicrystals, has revealed the presence of magnetic orders and phase transitions with changing the Au/Al concentration. Motivated by the experiment, we theoretically investigate whether a successive change of magnetic orders occurs in a minimal magnetic model including the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction only. We find that the model induces multifarious magnetic orders depending on the Fermi wavenumber and gives a good starting point for understanding the experimental observation. In addition, we predict the presence of an undiscovered magnetic order called cuboc order at the large Fermi wavenumber region.

We report synthesis and crystal structure refinement of $\mathrm{A}{\mathrm{g}}_2\mathrm{Ru}{\mathrm{O}}_4$, followed by combined analysis of its physical properties through bulk experimental tools (magnetic susceptibility, electron transport, and heat capacity measurements), a microscopic experimental tool (temperature dependent neutron diffraction), and ab initio first-principles calculations. We observe a rather unique ${\left(\mathrm{Ru}{\mathrm{O}}_{3/1}{\mathrm{O}}_{2/2}\right)}_{\mathrm{n}}$ polyoxoanion, where Ru is in a distorted trigonal bipyramidal coordination by oxygen. The $\mathrm{Ru}{\mathrm{O}}_5$ polyhedra are linked via the apical oxygen atoms to form chains extending along the crystallographic $a$ axis. Crystal structure, magnetization, and ab initio calculations indicate that Ru is in the $+6$ oxidation state with a nominal valence electron configuration of $4d^2$. Bulk magnetization, specific-heat, and neutron-diffraction measurements provide clear indication of an antiferromagnetic transition around 75 K with moderate spin canting in the order of ${30}^{\circ }$ with respect to the $c$ axis. The neutron-diffraction results as well as the density functional theory based first-principles calculations of exchange interactions revealed that the strong intrachain interaction is predominantly of ferromagnetic (FM) type, and that this spin order along the chains couples with the neighboring chains through comparatively weak FM and antiferromagnetic interactions. Notably, the Landé $g$ factor is found to be 1.8 (with an infinite chain model and even a simple Curie-Weiss approach), away from the ideal value of 2, due to the low dimensionality of the Ru/O substructure.

Spectroscopic ellipsometry and Fourier transform infrared spectroscopy were applied to extract the ultraviolet to far-infrared $(150–33333\mathrm{c}{\mathrm{m}}^{-1})$ complex dielectric functions of high-quality, sputtered indium-doped cadmium oxide (In:CdO) thin crystalline films on MgO substrates possessing carrier densities $\left(N_d\right)$ ranging from $1.1\times {10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$ to $4.1\times {10}^{20}\mathrm{c}{\mathrm{m}}^{-3}$. A multiple oscillator fit model was used to identify and analyze the three major contributors to the dielectric function and their dependence on doping density: interband transitions in the visible, free-carrier excitations (Drude response) in the near- to far-infrared, and IR-active optic phonons in the far-infrared. More specifically, values pertinent to the complex dielectric function such as the optical band gap $\left(E_g\right)$, are shown here to be dependent upon carrier density, increasing from approximately 2.5–3 eV, while the high-frequency permittivity (${\varepsilon }_{\infty }$) decreases from 5.6 to 5.1 with increasing carrier density. The plasma frequency (${\omega }_p$) scales as $\sqrt{N_d}$, resulting in ${\omega }_p$ values occurring within the mid- to near-IR, and the effective mass ($m^{*}$) was also observed to exhibit doping density-dependent changes, reaching a minimum of $0.11m_o$ in unintentionally doped films ($1.1\times {10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$). Good quantitative agreement with prior work on polycrystalline, higher-doped CdO films is also demonstrated, illustrating the generality of the results. The analysis presented here will aid in predictive calculations for CdO-based next-generation nanophotonic and optoelectronic devices, while also providing an underlying physical description of the key properties dictating the dielectric response in this atypical semiconductor system.

The earth-abundant natural mineral chalcopyrite $\mathrm{CuFe}{\mathrm{S}}_2$ is a potential $n$-type thermoelectric material because of its large Seebeck coefficient at high carrier concentrations. For a long time, the large Seebeck coefficient of $\mathrm{CuFe}{\mathrm{S}}_2$ has been attributed to a large electron effective mass, but the reasons for this and the unusual carrier concentration dependent behavior have rarely been discussed. Here, we systematically investigated the special transport behavior of $\mathrm{CuFe}{\mathrm{S}}_2$ and found the classical parabolic band model to be inadequate in explaining it. Our experimental and theoretical studies indicate that there are two flat electronic pockets at the Γ and $Z$ points of the Brillouin zone near the conduction band edge of $\mathrm{CuFe}{\mathrm{S}}_2$ that dominate the charge transport. These electronic pockets result from nonparabolic quasilinearly dispersing bands that give rise to a linear wave vector dependent energy ($E\sim k$) and a carrier density dependent effective mass ($m^{*}\sim m_0{n}^{1/3}$). Such a strong carrier concentration dependent carrier effective mass results in the high Seebeck coefficient of $\mathrm{CuFe}{\mathrm{S}}_2$ compound under a large carrier density. The work demonstrates that quasilinearly dispersing bands can give strongly enhanced Seebeck coefficient, and could be useful in optimizing the properties of thermoelectric materials.

Two-dimensional (2D) metallic states induced by oxygen vacancies ($V_O\mathrm{s}$) at oxide surfaces and interfaces provide opportunities for the development of advanced applications, but the ability to control the behavior of these states is still limited. We used angle resolved photoelectron spectroscopy combined with density-functional theory (DFT) to study the reactivity of $V_O$-induced states at the (001) surface of anatase ${\mathrm{TiO}}_2$, where both 2D metallic and deeper lying in-gap states (IGs) are observed. The 2D and IG states exhibit remarkably different evolutions when the surface is exposed to molecular ${\mathrm{O}}_2$: while IGs are almost completely quenched, the metallic states are only weakly affected. DFT calculations indeed show that the IGs originate from surface $V_O\mathrm{s}$ and remain localized at the surface, where they can promptly react with ${\mathrm{O}}_2$. In contrast, the metallic states originate from subsurface vacancies whose migration to the surface for recombination with ${\mathrm{O}}_2$ is kinetically hindered on anatase ${\mathrm{TiO}}_2$ (001), thus making them much less sensitive to oxygen dosing.

Under certain exposure conditions, a femtosecond laser beam focused in the bulk of fused silica leads to the formation of self-organized structures consisting of a series of “nanolayers,” parallel to one another. Remarkably, this laser-induced nanoscale anisotropy offers the possibility to locally engineer macroscopic properties of a given substrate by selectively exposing it in arbitrarily chosen locations to a laser beam with designed polarization states. Although various physical properties are affected by the laser, this paper specifically discusses in-plane elastic properties of these nanostructures. Using a method based on monitoring resonant properties of vibrating cantilevers combined with a mechanical model of the nanostructures, the Young's moduli of individual nanolayers are calculated and used to define the stiffness matrix of the composite structure. The model shows a good agreement with measured mechanical properties of arbitrarily oriented nanostructures. This work demonstrates the predictability and controllability of laser-induced nanoscale mechanical properties and offers a framework for engineering arbitrary elastic properties through 3D laser writing.

Local lattice distortions in a series of body-centered cubic alloys, including refractory high-entropy alloys, are investigated by means of atomic volumes, atomic charges, and atomic stresses defined by the Bader charge analysis based on first-principles calculations. Analyzing the extensive data sets, we find large distributions of these atomic properties for each element in each alloy, indicating a large impact of the varying local chemical environments. We show that these local-environment effects can be well understood and captured already by the first and the second nearest neighbor shells. Based on this insight, we employ linear regression models up to the second nearest neighbor shell to accurately predict these atomic properties. Finally, we find that the elementwise-averaged values of the atomic properties correlate linearly with the averaged valence-electron concentration of the considered alloys.