The moiré superstructure of a single layer of carbon on ruthenium, where $25\times 25$ graphene honeycombs sit on $23\times 23$ substrate unit cells, is determined from first principles. The density functional theory (DFT) calculations predict two kinds of structural units, $\Omega$ and $Y$, in the supercell, which are identified as moiré beatings or moirons. The related topographic bucklings, or “hills,” have distinct carbon conformations and a height of 1.16 Å. The different moirons are observed with scanning tunneling microscopy (STM), and surface x-ray diffraction (SXRD) also discriminates the two. This connects ab initio DFT calculations with STM and SXRD experiments in unit cells containing more than 4000 atoms.

We investigate the many-body instabilities of electrons interacting near Van Hove singularities arising in monolayer and twisted bilayer graphene. We show that a pairing instability must be dominant over the tendency to magnetic order as the Fermi level is tuned to the Van Hove singularity in the conduction band of graphene. As a result of the extended character of the saddle points in the dispersion, we find that the pairing of the electrons takes place preferentially in a channel of $f$-wave symmetry, with an order parameter vanishing at the position of the saddle points along the Fermi line. In the case of the twisted bilayers, the dispersion has instead its symmetry reduced down to the $C_{3v}$ group and, most importantly, it leads to susceptibilities that diverge at the saddle points but are integrable along the Fermi line. This implies that a ferromagnetic instability becomes dominant in the twisted graphene bilayers near the Van Hove singularity, with a strength which is amplified as the lowest subband of the electron system becomes flatter for decreasing twist angle.

Octahedral distortion plays a key role in engineering the physical properties of heterostructures composed of perovskite oxides. We observe a strong in-plane uniaxial magnetic anisotropy in a strain-enabled multiferroic EuTiO${}_3$ thin film epitaxially grown on a (110)${}_{\mathrm{o}}$ DyScO${}_3$ substrate. First-principles calculations show that the magnetic anisotropy is closely correlated with the uniaxial TiO${}_6$ octahedral tilting and the ferroelectric polarization of the film, indicating potential strong magnetoelectric coupling in the strain-engineered multiferroic system.

We study the spin $S=1$ antiferromagnetic Heisenberg model on the square lattice with, in addition to the nearest-neighbor interaction, a three-site interaction of the form $({\mathbf{S}}_i·{\mathbf{S}}_j)({\mathbf{S}}_j·{\mathbf{S}}_k)+\text{H.c.}$ This interaction appears naturally in a strong coupling expansion of the two-orbital, half-filled Hubbard model. For spin 1/2, this model reduces to a Heisenberg model with bilinear interactions up to third neighbors, with a second-neighbor interaction twice as large as the third-neighbor one, a very frustrated model with an infinite family of helical classical ground states in a large parameter range. Using a variety of analytical and numerical methods, we show that the spin-1 case is also very frustrated, and that its phase diagram is even richer, with possibly the succession of seven different phases as a function of the ratio of the three-site interaction to the bilinear one. The phases are either purely magnetic phases with collinear order or of mixed magnetic and quadrupolar character with helical order.

We report ${}^{29}$Si nuclear magnetic resonance measurements in single crystals and aligned powders of URu${}_2$Si${}_2$ in the hidden order and paramagnetic phases. The spin-lattice relaxation data reveal evidence of pseudospin fluctuations of U moments in the paramagnetic phase. We find evidence for partial suppression of the density of states below 30 K and analyze the data in terms of a two-component spin-fermion model. We propose that this behavior is a realization of a pseudogap between the hidden-order transition $T_{\mathrm{HO}}$ and 30 K. This behavior is then compared to other materials that demonstrate precursor fluctuations in a pseudogap regime above a ground state with long-range order.

Chiral properties of the two phases—collinear motif (below Morin transition temperature, $T_M\approx 250$ K) and canted motif (above $T_M$)—of magnetically ordered hematite ($\alpha$-Fe${}_2$O${}_3$) have been identified in single-crystal resonant x-ray Bragg diffraction using circular polarized incident x rays tuned near the iron $K$ edge. Magnetoelectric multipoles, including an anapole, fully characterize the high-temperature canted phase, whereas the low-temperature collinear phase supports both parity-odd and parity-even multipoles that are time odd. Orbital angular momentum accompanies the collinear motif, whereas it is conspicuously absent with the canted motif. Intensities have been successfully confronted with analytic expressions derived from an atomic model fully compliant with chemical and magnetic structures. Values of Fe atomic multipoles previously derived from independent experimental data are shown to be completely trustworthy.

We report on muon-spin rotation and relaxation ($\mu$SR) and ${}^{119}$Sn nuclear magnetic resonance (NMR) measurements to study the microscopic superconducting and magnetic properties of the Heusler compound with the highest superconducting transition temperature, YPd${}_2$Sn ($T_c=5.4$ K). Measurements in the vortex state provide the temperature dependence of the effective magnetic penetration depth $\lambda \left(T\right)$ and the field dependence of the superconducting gap $\Delta \left(0\right)$. The results are consistent with a very dirty $s$-wave BCS superconductor with a gap $\Delta \left(0\right)=0.85\left(3\right)$ meV, $\lambda \left(0\right)=212\left(1\right)$ nm, and a Ginzburg-Landau coherence length ${\xi }_{\mathrm{GL}}\left(0\right)\cong 23$ nm. In spite of its very dirty character, the effective density of condensed charge carriers is high compared to that in the normal state. The $\mu$SR data in a broad range of applied fields are well reproduced by taking into account a field-related reduction of the effective superconducting gap. Zero-field $\mu$SR measurements, sensitive to the possible presence of very small magnetic moments, do not show any indications of magnetism in this compound.

We address two important issues that arise in recent studies of iron-based superconductivity. (1) Why are the $T_c$ of $A_x$Fe${}_{2-y}$Se${}_2$ ($A=\mathrm{K}$, Rb, Cs) and the single unit cell FeSe on SrTiO${}_3$ so high despite both only having electron pockets? (2) What (if any) are the effects of orbital order and orbital fluctuation on the Cooper pairing. Our main conclusions are the following: (1) removing hole pockets releases frustration of Cooper pairing from their band vorticity, therefore can enhance $T_c$, and (2) orbital fluctuation has negligible effect on Cooper pairing.

We report multiscale quantum mechanical modeling of hydrogen assisted cracking in aluminum which is central to H embrittlement phenomena. We find that dislocation emission and brittle cleavage can occur simultaneously. H embrittlement takes place when H occupies the top sites on the crack front surface and even a very low H coverage at 0.2 monolayers can lead to brittle cleavage. H atoms adsorbed on the crack surfaces tend to suppress dislocation emission, whereas the solute H atoms on the slip plane can promote dislocation emission. Top-site H atoms at the front surface are found to facilitate the migration of other H atoms towards the front surface, providing a mechanism for H accumulation at the crack tip. The study resolves a long-standing puzzle of why H embrittlement could occur in Al where the equilibrium H solubility is extremely low under normal conditions.

We have measured the temperature dependence of the direct band gap $E_g$ in SrTiO${}_3$ and BaTiO${}_3$ and related materials with quantum-paraelectric and ferroelectric properties using optical spectroscopy. We show that $E_g$ exhibits an anomalous temperature dependence with pronounced changes in the vicinity of the ferroelectric transition that can be accounted for in terms of the Fröhlich electron-phonon interaction with an optical phonon mode, the so-called soft mode. In addition, we demonstrate that these characteristic changes of $E_g$ can be readily detected even in very thin films of SrTiO${}_3$ with a strain-induced ferroelectric order. Optical spectroscopy thus can be used as a rather sensitive probe of ferroelectric order in very thin films of these titanates and probably also in subsequent multilayers and devices.

It is convenient to define the spin-torque switching efficiency in nanostructured magnetic tunnel junctions as the ratio between the free-layers thermal activation barrier height $E_b$ and the threshold switching current $I_{c0}$. Recent device exploration has led to occasional observations of spin-torque induced magnetic switching efficiency in magnetic tunnel junctions that exceeds the macrospin limit by a factor of 2–10. In this paper we examine the possible origins for such enhancement, and materials properties that may allow the full realization of such enhancements.

Interactions between electrons in one dimension are fully described at low energies by only a few parameters of the Tomonaga-Luttinger model, which is based on linearization of the spectrum. We consider a model of spinless fermions with short-range interaction via the Bethe-Ansatz technique and show that a Luttinger parameter emerges in an observable beyond the low-energy limit. A distinct feature of the spectral function, the edge that marks the lowest possible excitation energy for a given momentum, is parabolic for arbitrary momenta and the prefactor is a function of the Luttinger parameter, $K$.

We present a comprehensive study of different $3d$ transition metal monosilicides in their ground state crystal structure (B20), ranging from equilibrium bulk over biaxially strained bulk to epitaxial thin films on Si(111), by means of density functional theory. The magnetic properties of MnSi and FeSi films are found to be considerably modified due to the epitaxial strain induced by the substrate. In MnSi bulk material, which can be seen as a limit of thick films, we find a strain-induced volume expansion, an increase of the magnetic moments, and a significant rise of the energy difference between different spin configurations. The latter can be associated with an increase of the Curie temperature, which is in accordance with recent experimental results. While a ferromagnetic spin alignment is found to be the ground state also for ultrathin films, we show that for films of intermediate thickness a partially compensating magnetic ordering is more favorable; however, the films retain a net magnetic moment. Furthermore, we analyze the orbital structure in FeSi around the band gap, which can be located somewhere in the density of states for all studied B20 transition metal monosilicides, and find that FeSi becomes metallic and ferromagnetic under epitaxial strain. Finally, the influence of on-site electronic correlation and the reliability of ab initio calculations for $3d$ transition metal monosilicides are discussed.

Two-dimensional graphenelike BC${}_3$ honeycomb structures, which have been grown on the NbB${}_2$(0001) surface, possess intriguing properties that offer new opportunities for applications in nanoelectronics, mechanical materials, and superconductivity. The bonding configuration of BC${}_3$ on the substrate has not yet been determined, however. We report ab initio thermodynamics calculations and analysis that lead to a prediction about the most stable bonding configuration of a monolayer BC${}_3$ on NbB${}_2$, namely the C-top configuration in which carbon is bonded directly on the top of the niobium atoms. We also find that the BC${}_3$ monolayer on NbB${}_2$ is thermodynamically stable, accounting for the experimental observation that the BC${}_3$ sheet can be grown on NbB${}_2$ by carbon substitution and surface segregation of carbon impurities. We further provide detailed information about the atomic structures and electronic properties of the BC${}_3$-bonded NbB${}_2$ surface.

We report here in situ measurements of the evolution of the Ag(110) surface during Si growth, using scanning tunneling microscopy and grazing incidence x-ray diffraction. We provide compelling evidence of an Ag(110) surface reconstruction associated with the release of Ag atoms induced by the growth of Si nanoribbons. Our results are in agreement with a missing row reconstruction of the Ag layer underneath the nanoribbons. This challenges the current understanding of the Si growth on nonreconstructed Ag(110), interpreted within the framework of silicene models.

We present numerical studies of conduction in graphene nanoribbons with reconstructed edges based on the standard tight-binding model of the graphene and the extended Hückel model of the reconstructed defects. We performed atomic geometry relaxation of individual defects using density functional theory and then explicitly calculated the tight-binding parameters used to model electron transport in graphene with reconstructed edges. The calculated conductances reveal strong backscattering and electron-hole asymmetry depending on the edge and defect type. This is related to an additional defect-induced band whose wave function is poorly matched to the propagating states of the pristine ribbon. We find a transport gap to open near the Dirac point and to scale inversely with the ribbon width, similarly to what has been observed in experiments. We predict the largest transport gap to occur for armchair edges with Stone-Wales defects, while heptagon and pentagon defects cause about equal backscattering for electrons and holes, respectively. Choosing the heptagon defect as an example, we show that although electron interactions in the Hartree approximation cause accumulation of charge carriers on the defects, surprisingly, their effect on transport is to reduce carrier backscattering by the defects and thus to enhance the conductance.

The application of a perpendicular electric field can drive silicene into a gapless state, characterized by two nearly fully spin-polarized Dirac cones owing to both relatively large spin-orbital interactions and inversion symmetry breaking. Here we argue that since intervalley scattering from nonmagnetic impurities is highly suppressed by time-reversal symmetry, the physics should be effectively single-Dirac-cone like. Through numerical calculations, we demonstrate that there is no significant backscattering from a single impurity that is nonmagnetic and unit-cell uniform, indicating a stable delocalized state. This conjecture is then further confirmed from a scaling of conductance for disordered systems using the same type of impurities.

The impact of surface reflection on the statistics of transmission eigenvalues is a largely unexplored subject of fundamental and practical importance in statistical optics. Here, we develop a first-principles theory and confirm numerically that the distribution of transmission eigenvalues of diffusive waves exhibits a nonanalytic transition as the strength of surface reflection at one surface passes through a critical value while that at the other is fixed. Above the critical value, the highest transmission eigenvalue is strictly smaller than unity and decreases with increasing internal reflection. When the input and output surfaces are equally reflective, the highest transmission eigenvalue is unity and the transition disappears irrespective of the strength of surface reflection.

We analyze the structural, electronic, and magnetic properties of the europium chalcogenide series Eu$X$ ($X=\text{O,}\text{S,}\text{Se,}\text{and}\text{Te}$) using density-functional theory (DFT). To describe the localized $4f$ states of Eu, we utilize the parameter-free PBE0 and HSE hybrid functionals and found a systematic qualitative and quantitative improvement over the conventional local and semilocal functionals. Both hybrid functionals predict the lattice constant and the bulk modulus of all four compounds accurately. The semiconducting behavior, the opening of the band gap, as well as the nature of the band gap across the series is reproduced. By mapping the magnetic interactions to a Heisenberg model up to the next-nearest neighbors, we find the correct trend of the magnetic order of the Eu compounds (in magnetic coupling of the Eu $4f$ moments), from ferromagnetic in EuO to antiferromagnetic in EuTe, with critical temperatures that are in fair agreement with experiment.

A series of high-quality NaFe${}_{1-x}$Cu${}_x$As single crystals have been grown with the self-flux technique, which were systematically characterized via structural, transport, thermodynamic, and high-pressure measurements. Both the structural and magnetic transitions are suppressed by Cu doping, and bulk superconductivity is induced by Cu doping. The superconducting transition temperature ($T_c$) is initially enhanced from 9.6 to 11.5 K by Cu doping, and then suppressed with further doping. A phase diagram similar to NaFe${}_{1-x}$Co${}_x$As is obtained except that semiconducting instead of metallic behavior is observed in the extremely overdoped samples. $T_c$s of underdoped, optimally doped, and overdoped samples are all notably enhanced by applying pressure. Although a universal maximum transition temperature ($T_c^{\max }$) of about 31 K under external pressure is observed in underdoped and optimally doped NaFe${}_{1-x}$Co${}_x$As, $T_c^{\max }$ of NaFe${}_{1-x}$Cu${}_x$As is monotonically suppressed by Cu doping, suggesting that impurity potential of Cu is stronger than that of Co in NaFeAs. The comparison between Cu and Co doping effect in NaFeAs indicates that Cu serves as an effective electron dopant with strong impurity potential, but part of the doped electrons are localized and do not fill the energy bands as predicted by the rigid-band model.