We present electronic structure theory calculations and scanning tunneling microscopy experiments for the adsorption of $\alpha$-sexithiophene on the 100 surface of gold. Our density-functional theory calculations show that $\alpha$-sexithiophene prefers to adjust an energetically unfavorable adsorption site by modifying the gold surface over seeking out more favorable adsorption sites. Molecular adsorption results in a complex charge transfer pattern, with more charge transfer in more stable sites. Our results challenge the current paradigm that weakly interacting (e.g., physisorbed) molecules perceive metal surfaces as rigid templates with preordained adsorption sites.

The structure and microstructure of $\mathrm{Fe}-x\mathrm{Al}-y\mathrm{Cr}$ alloys with $x=25$ and 27 and 0 ≤ $y\le 15$ have been studied by neutron diffraction at several fixed temperatures with high $\mathrm{\Delta }d/d$ resolution and with high-intensity continuous scanning in a wide temperature range. The joint analysis of the obtained data established that the microstructure of these compounds is organized in the form of nanoscopic-sized clusters with an ordered atomic structure coherently embedded in a disordered (or less ordered) matrix. In the quenched samples, the matrix is a disordered A2 phase with clusters of a partially ordered B2 phase, whereas, in the annealed samples, the clusters of the ordered $\mathrm{D}{0}_3$ phase are dispersedly distributed in the B2 matrix. The characteristic size of the clusters depends on the state of the alloy, the chromium content, and the temperature and ranges in size from 100 to 1000 Å. For the matrix, the coherent scattering domain size exceeds 3500 Å. Therefore, the state of the alloys is actually a two-phase state, $\mathrm{A}2+\mathrm{B}2$ or $\mathrm{B}2+\mathrm{D}{0}_3$, although these phases are indistinguishable in elemental composition. One of the factors stabilizing the phase-separated state in a compositionally homogeneous medium can be local composition fluctuations at the level of elementary cells, which in turn lead to fluctuations in the interactions of neighboring pairs of atoms.

The cubic antifluorite structure comprises a fcc sublattice of anions with cations on the tetrahedral sites. The voids in the antifluorite structure that are crucial for superionicity in ${\mathrm{Li}}_2\mathrm{O}$ might also act as atomic traps. Trapping of guest atoms and small molecules within voids of a host structure leads to the formation of what are known as clathrate compounds. Here we investigate the possibility of trapping helium or larger neon guest atoms under pressure within alkali-metal oxide and sulfide structures. We find stable helium and neon-bearing compounds at very low pressures. These structures are stabilized by a reduction in volume from incorporation of helium or neon atoms within the antifluorite structure. We predict that ${\mathrm{NeCs}}_2\mathrm{S}$ could be stable at ambient pressure. Our study suggests a novel class of alkali oxide and sulfide materials incorporating noble-gas atoms that might potentially be useful for gas storage.

We are motivated by the need to design magnesium alloys that are free of rare-earth additions, but, nevertheless, forgeable and free of strong basal texture. It has become recently clear that calcium is a promising candidate to replace the economically strategic rare-earth elements. To this end, we focus on the planar faults that typically bound partial glide dislocations of the hcp lattice. We have made first-principles calculations to examine the generalized stacking fault energy (SFE) of the basal, first- and second-order pyramidal planes. We examine the changes in fault energy and anisotropy for increasing alloy concentrations and the effect of low concentrations of calcium. For the calculation of the SFEs from first principles, we use the non-self-consistent Harris-Foulkes approximation to the local density functional theory. We demonstrate that while this approximation leads to high computational efficiency, there is no significant loss of precision compared to the self-consistent Hohenberg-Kohn functional. We go beyond all previous work in which the alloying element is assumed to reside within the fault and instead address the more realistic situation in which the SFE is modified by the remote presence of the impurity. This allows us to determine whether segregation is expected and we find that while elements do segregate to the basal fault they do not to pyramidal faults. Nevertheless, in either case, the fault energy is strongly modified by alloying. This argues that either a long-ranged electronic structure effect is in play, or the fault energy modification is affected by the atomic size difference—particularly large in the case of Ca. We find that Mg-Li-Ca and Mg-Zn-Ca alloys show a remarkable decrease in anisotropy, which is consistent with their known high strength and formability. In favorable cases, this comes about by strengthening basal slip rather than weakening nonbasal slip. The Ca contribution increases inversely with the atomic size of the alloying element, allowing us to speculate that alloying effects are generally atomic size effects.

Endohedral fullerenes, such as ${\mathrm{Dy}}_2\mathrm{ScN}@{\mathrm{C}}_{80}$, are single-molecule magnets with long relaxation times of their magnetization. An open and anisotropic $4f$ electron shell in the lanthanides (here Dy) imposes a magnetic moment that maintains its orientation at liquid-helium temperatures for macroscopic times. If these molecules shall be used as single-bit information storage elements or for quantum operations, the orientation of the endohedral units and the orientation of the magnetic moments has to be controlled. X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD)—with variation of the angle of x-ray incidence—allows for the detection of these two structural elements. We present XMCD data of ${\mathrm{Dy}}_2\mathrm{ScN}@{\mathrm{C}}_{80}$ on an $h\text{−}\mathrm{BN}/\mathrm{Rh}\left(111\right)$ nanomesh that display at 2 K a large hysteresis with a coercive field of 0.4 T. The angular dependence of the XAS data at the Dy $M_5$ edge indicates partial ordering of the endohedral units. In order to quantify anisotropic orientation we introduce the “deviation” $\mathcal{D}$ as an operational quantity that measures differences between two spectra.

Strain engineering of epitaxial heterostructures offers opportunities to control the orbital degree of freedom by lifting the degeneracy of $e_g$ states. Here, we show that the orbital occupation in $\mathrm{LaCo}{\mathrm{O}}_3$ (LCO) films can be switched between two degenerate $e_g$ bands with epitaxial strain. The orbital polarization of nearly −100% (or 100%) is controlled by depleting occupation of the $d_{x^2-y^2}\left(\mathrm{or}{d}_{3z^2-r^2}\right)$ orbital entirely in LCO for large compressive (or moderate tensile) strain. The change of electronic configuration associated with the spin-state transition modulates the magnetization of strained LCO films. Under compressive strain, LCO films exhibit a small magnetization without long-range ferromagnetic ordering. With tensile-strain increases, the magnetization of LCO films increases and reaches the maximum value when the bonding angle (Co–O–Co) is close to ${180}^{\circ }$ and the in-plane bond length (Co–O) is unstretched. Our results highlight the role of octahedral distortion and spin-state crossover in tailoring the magnetic properties of cobaltite thin films, suggesting an attractive route to deliberately control the orbital polarization that can be tuned to maximize the functionality of oxide heterostructures.

Magnetic skyrmions have been the focus of intense research due to their potential applications in ultrahigh-density data and logic technologies, as well as for the unique physics arising from their antisymmetric exchange term and topological protections. In this work we prepare a chiral jammed state in chemically disordered (Fe, Co)Si consisting of a combination of randomly oriented magnetic helices, labyrinth domains, rotationally disordered skyrmion lattices, and/or isolated skyrmions. Using small angle neutron scattering, we demonstrate a symmetry-breaking magnetic field sequence which disentangles the jammed state, resulting in an ordered, oriented skyrmion lattice. The same field sequence was performed on a sample of powdered $\mathrm{C}{\mathrm{u}}_2\mathrm{OSe}{\mathrm{O}}_3$ and again yields an ordered, oriented skyrmion lattice, despite the relatively noninteracting nature of the grains. Micromagnetic simulations confirm the promotion of a preferred skyrmion lattice orientation after field treatment, independent of the initial configuration, suggesting this effect may be universally applicable. Energetics extracted from the simulations suggests that approaching a magnetic hard axis causes the moments to diverge away from the magnetic field, increasing the Dzyaloshinskii-Moriya energy, followed subsequently by a lattice reorientation. The ability to facilitate an emergent ordered magnetic lattice with long-range orientation in a variety of materials despite overwhelming internal disorder enables the study of skyrmions even in imperfect powdered or polycrystalline systems and greatly improves the ability to rapidly screen candidate skyrmion materials.

In this work, we report on the thermally induced phase behavior and crystalline structures of a series of solution-processable naphthalene diimide (NDI)-based molecules with alkyl side chains of varying length and branching. Taking advantage of synchrotron-based in situ grazing-incidence wide-angle x-ray scattering, the most kinetically favorable thin film crystalline structures of each molecule at room temperature and at elevated temperatures were resolved. The extracted unit cell parameters of the kinetically favorable crystal structures as a function of temperature were used to quantify the anisotropic thermal expansion coefficients along the three principal crystallographic directions. In particular, it is found that long alkyl side chains can effectively suppress crystalline polymorphism and enhance thermal reversibility. The acquired knowledge regarding the thermally induced phase transitions of thin films based on NDI-based small molecules with varying solubilizing side chains serves as guidance for further molecular design and synthesis of high performance $n$-type small molecules.

We present measurements of ferromagnetic resonance driven spin pumping and inverse spin Hall effect in $\mathrm{NbN}/{\mathrm{Y}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$ (YIG) bilayers. A clear enhancement of the (effective) Gilbert damping constant of the thin-film YIG was observed due to the presence of the NbN spin sink. By varying the NbN thickness and employing spin-diffusion theory, we have estimated the room-temperature values of the spin-diffusion length and the spin Hall angle in NbN to be 14 nm and $-1.1\times {10}^{-2}$, respectively. Furthermore, we have determined the spin mixing conductance of the NbN/YIG interface to be $10\mathrm{n}{\mathrm{m}}^{-2}$. The experimental quantification of these spin transport parameters is an important step towards the development of superconducting spintronic devices involving NbN thin films.

Near-surface InAs two-dimensional electron gas (2DEG) systems have great potential for realizing networks of multiple Majorana zero modes towards a scalable topological quantum computer. Improving mobility in the near-surface 2DEGs is beneficial for stable topological superconducting states as well as for correlation of multiple Majorana zero modes in a complex network. Here, we investigate near-surface InAs 2DEGs (13 nm away from the surface) grown on GaSb(001) substrates, whose lattice constant is closely matched to InAs, by molecular beam epitaxy. The effect of a 10-nm-thick top barrier to the mobility is studied by comparing $\mathrm{A}{\mathrm{l}}_{0.9}\mathrm{G}{\mathrm{a}}_{0.1}\mathrm{Sb}$ and $\mathrm{I}{\mathrm{n}}_{0.75}\mathrm{G}{\mathrm{a}}_{0.25}\mathrm{As}$ as a top barrier on otherwise identical InAs quantum wells grown with identical bottom barrier and buffer layers. A 3-nm-thick capping layer on an $\mathrm{A}{\mathrm{l}}_{0.9}\mathrm{G}{\mathrm{a}}_{0.1}\mathrm{Sb}$ top barrier also affects the 2DEG electronic transport properties by modifying scattering from 2D remote ionized impurities at the surface. The highest transport mobility of $650000\mathrm{c}{\mathrm{m}}^2/\mathrm{V}\mathrm{s}$ with an electron density of $3.81\times {10}^{11}\mathrm{c}{\mathrm{m}}^{-2}$ was observed in an InAs 2DEG with an $\mathrm{A}{\mathrm{l}}_{0.9}\mathrm{G}{\mathrm{a}}_{0.1}\mathrm{Sb}$ top barrier and an $\mathrm{I}{\mathrm{n}}_{0.75}\mathrm{G}{\mathrm{a}}_{0.25}\mathrm{As}$ capping layer. Analysis of Shubnikov–de Haas oscillations in the high-mobility sample suggests that long-range scattering, such as remote ionized impurity scattering, is the dominant scattering mechanism in the InAs 2DEGs grown on GaSb(001) substrates. In comparison to InAs quantum wells grown on lattice-mismatched InP, the ones grown on GaSb show smoother surface morphology and higher quantum mobility. However, the $\mathrm{I}{\mathrm{n}}_{0.75}\mathrm{G}{\mathrm{a}}_{0.25}\mathrm{As}$ top barrier in the InAs quantum well grown on GaSb limits the transport mobility by charged dislocations formed in it, in addition to the major contribution to scattering from the alloy scattering.

Single crystals of intermetallic $\mathrm{Re}{}_3\mathrm{Ge}{}_7$ were grown and characterized by measuring magnetization, electrical resistivity, Hall coefficient and specific heat. Magnetization measurements show the material is weakly diamagnetic. A phase transition is indicated by a kink in magnetic susceptibility at $T_c=58.5$ K and is confirmed by a $\lambda$-like anomaly in specific heat. In zero field, the temperature dependence of electrical resistivity $\rho \left(T\right)$ follows a typical metallic behavior above $T_c$ and sharply increases below $T_c$, showing a metal-to-insulator-like transition. When a magnetic field is applied, strong effects on the transport properties are observed. The temperature dependence of magnetoresistivity $\mathrm{\Delta }\rho =\rho (T,H)-\rho (T,H=0)$ develops a maximum around 30 K, deviating from ordinary metallic behavior. Analysis of the Hall coefficient measurements indicates that the carrier density is 0.04 per formula unit at 300 K and drops by two orders of magnitude below $T_c$. The effective mass of charge carriers is inferred from the analysis of the Shubnikov–de Haas quantum oscillations to be close to the bare electron mass.

Tin-chalcogenides $\mathrm{Sn}X$ ($X=\text{Te}$, Se, and S) have been attracting research interest due to their thermoelectric physical properties. Their two-dimensional (2D) counterparts, which are expected to enhance those properties, nevertheless have not been fully explored because of many possible structures. A variable-composition exploration of 2D ${\mathrm{Sn}}_{1-x}{X}_x$ systems ($X=\text{Te}$, Se, and S) has been performed using a global searching method based on an evolutionary algorithm combined with density-functional calculations. A new hexagonal phase denoted by ${\beta }^{\prime }\text{−}\mathrm{Sn}X$ is found using Universal Structure Predictor: Evolutionary Xtallography (USPEX), and the structural stability has been further checked by calculations of phonons and elasticity. ${\beta }^{\prime }$-SnTe is the most stable among all possible 2D phases of SnTe, including experimentally available phases. Further, ${\beta }^{\prime }$ phases of SnSe and SnS are also found to be energetically close to the most stable phases. A high thermoelectronic (TE) performance has been predicted in the ${\beta }^{\prime }\text{−}\mathrm{Sn}X$ phases, which have a dimensionless figure of merit as high as ∼0.96 to 3.81 for SnTe, ∼0.93 to 2.51 for SnSe, and ∼1.19 to 3.18 for SnS at temperatures ranging from 300 to 900 K with a practically attainable carrier concentration of $5\times {10}^{12}{\mathrm{cm}}^{-2}$. The high TE performance results from a high power factor that is attributed to the quantum confinement of 2D materials and the band convergence near the Fermi level, as well as low thermal conductivity mainly from both low elastic constants due to weak inter-Sn bonding strength and strong lattice anharmonicity.

The recent discoveries of proximate quantum spin-liquid compounds and their potential application in quantum computing informs the search for new candidate materials for quantum spin-ice and spin-liquid physics. While the majority of such work has centered on members of the pyrochlore family due to their inherently frustrated linked tetrahedral structure, the rare-earth pyrogermanates also show promise for possible frustrated magnetic behavior. With the familiar stoichiometry $R_2{\mathrm{Ge}}_2{\mathrm{O}}_7$, these compounds generally have tetragonal symmetry with a rare-earth sublattice built of a spiral of alternating edge and corner-sharing rare-earth site triangles. Studies on ${\mathrm{Dy}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$ and ${\mathrm{Ho}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$ have shown tunable low temperature antiferromagnetic order, a high frustration index, and spin-ice-like dynamics. Here we use neutron diffraction to study magnetic order in ${\mathrm{Er}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$ (space group $P{4}_1{2}_{1}2$) and find the lowest yet Neél temperature in the pyrogermanates of 1.15 K. Using neutron powder diffraction, we find the magnetic structure to order with $k=(0,0,0)$ ordering vector, magnetic space group symmetry $P{4}_1^{{}^{\prime }}{2}_1{2}^{{}^{\prime }}$, and a refined Er moment of $m=8.1{\mu }_B$ near the expected value for the ${\mathrm{Er}}^{3+}$ free ion. Provocatively, the magnetic structure exhibits similar “local Ising” behavior to that seen in the pyrocholres where the Er moment points up or down along the short Er-Er bond. Upon applying a magnetic field, we find a first-order metamagnetic transition at $\sim 0.35\mathrm{T}$ to a lower symmetry $P{2}_1^{{}^{\prime }}{2}_1^{{}^{\prime }}2$ structure. This magnetic transition involves an inversion of Er moments aligned antiparallel to the applied field describing a class I spin-flip-type transition, indicating a strong local anisotropy at the Er site—reminiscent of that seen in the spin-ice pyrochlores.

Elastic strain and defects are important key words for controlling structure and properties in films. While epitaxial strain and misfit dislocations have been discussed in conventional films, the evolution of strain and defect can be significantly varied by nanocomposite strain and complicated defects in oxides. In the present study, long stacking faults with a spacing of 5–30 nm and a length of $>500\mathrm{nm}$ were self-organized by ex situ annealing highly strained nanocomposite films of $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{7–\delta }\left(\mathrm{YBCO}\right)+\mathrm{BaM}{\mathrm{O}}_3\left(M=\mathrm{Hf},\mathrm{Sn}\right)$. It is surprising that the nonlocal nature of stacking faults, namely, the structural correlation over $>500\mathrm{nm}$, was observed in spite of the local configuration of the nanocomposite interface. This kind of structural variation was not observed in the pure $\mathrm{YBCO}$ film without nanorods, even when the same annealing was performed. A strain energy analysis showed that the stacking fault formation led to the strain energy minimum by reducing the nanocomposite strain. The layered structure of YBCO stacking faults and the large nanocomposite strain realized the present nonlocal self-organization, which is not observed in the conventional systems with epitaxial strain and misfit dislocations.

We investigate the elastic properties of selected zinc-blende III-V semiconductors. Using hybrid functional density functional theory, we calculate the second- and third-order elastic constants and first- and second-order internal strain tensor components for Ga, In, and Al containing III-V compounds. For many of these parameters, there are no available experimental measurements, and this work is the first to predict their values. The stricter convergence criteria for the calculation of higher-order elastic constants are demonstrated, and arguments are made based on this for extracting these constants via the calculated stresses, rather than the energies, in the context of plane-wave-based calculations. The calculated elastic properties are used to determine the strain regime at which higher-order elasticity becomes important by comparing the stresses predicted by a lower- and a higher-order elasticity theory. Finally, the results are compared with available experimental literature data and previous theory.

We report a symmetry-resolved electronic Raman scattering (ERS) study of a back-gated bilayer graphene device. We show that the ERS continuum is dominated by interband chiral excitations of $A_2$ symmetry and displays a characteristic Pauli-blocking behavior similar to the monolayer case. Crucially, we show that nonchiral excitations make a vanishing contribution to the Raman cross-section due to destructive interference effects in the Raman amplitude matrix elements. This is in marked contrast to the optical absorption measurements and opens interesting prospects for the use of Raman scattering as a selective probe for the detection of the chiral degrees of freedom in graphene, topological materials, and other two-dimensional crystals.

In relaxor ferroelectrics, the interplay between the short-range-order dipolar state and long-range-order ferroelectric state leads to outstanding response functions, which enable valuable capacitor, electromechanical, pyroelectric, and electrocaloric applications. Advanced applications are envisaged with epitaxial relaxor ferroelectric films, whose fundamentals are still poorly explored. In particular, frustration of ferroelectricity, contrasting the bulk behavior, has been found and ascribed to nonpolar crystal orientation in (001) epitaxial films of archetypical lead-based relaxors. Here, a peculiar hybrid polar state, where coexisting long-range and short-range orders persist to very low temperatures, is demonstrated in epitaxial polar-oriented (111) ${\mathrm{PbSc}}_{0.5}{\mathrm{Nb}}_{0.5}{\mathrm{O}}_3$ films. The hybrid state is evidenced by relaxorlike frustrations of ferroelectric transition and the Curie-Weiss behavior, validity of the Vogel-Fulcher relationship, a non-Rayleigh dynamic nonlinearity, and a ferroelectriclike low-temperature polarization. Local fluctuations of lattice strain arising from the relaxation of large epitaxial misfit are suggested to be responsible for the hybrid polar state. Introducing strain fluctuations by doping or nanostructuring is anticipated to boost the dynamic dielectric and piezoelectric performance of many perovskite oxide ferroelectrics.

We use density functional theory to calculate the structure, band-gap, and magnetic properties of oxygen-deficient ${\mathrm{SrTi}}_{1-x-y}{\mathrm{Fe}}_x{\mathrm{Co}}_y{\mathrm{O}}_{3-\delta }$ with $x=y=0.125$ and $\delta =\{0,0.125,0.25\}$. The valence and the high or low spin states of the Co and Fe ions, as well as the lattice distortion and the band gap, depend on the oxygen deficiency, on the locations of the vacancies, and on the direction of the Fe-Co axis. A charge redistribution that resembles a self-regulatory response lies behind the valence spin-state changes. Ferromagnetism dominates, and both the magnetization and the band gap are greatest at $\delta =0.125$. This qualitatively mimics the previously reported magnetization measured for ${\mathrm{SrTiFeO}}_{3-\delta }$, which was maximum at an intermediate deposition pressure of oxygen.