In this paper we analyze, in detail, the magnetic properties of (Co/Ni) multilayers, a widely used system for spintronics devices. We use spin-polarized photoemission spectroscopy, magneto-optical Kerr effect, x-ray magnetic circular dichroism (XMCD), and anomalous surface diffraction experiments to investigate the electronic, magnetic, and structural properties in [Co($x$)/Ni($y$)] single-crystalline stacks grown by molecular-beam epitaxy. The spin polarization depends sensitively on the surface termination and for Co terminated stacks is found to be much larger than bulk Co, reaching at least 90% for 2 Co atomic planes. We observe a magnetization transition from in plane to out of plane when varying the Ni coverage on a Co layer in the submonolayer range, confirming the interface origin of the perpendicular magnetic anisotropy in this system. Angle-dependent XMCD using strong applied magnetic field allows us to show that the orbital magnetic moment anisotropy in Co is responsible for the anisotropy and that our results are consistent with Bruno's model. Surface x-ray diffraction shows that fcc stacking is preferred for 1-monolayer Co-based superlattices, whereas the hcp stacking dominates for larger Co thicknesses. We finally explored the role of the stacking sequence on the Co and Ni magnetic moments by ab initio calculations.

We introduce a simple and efficient model to describe the potential energy surface of lithium diffusing in a solid-state ionic conductor. First, we assume that the Li atoms are fully ionized and we neglect the weak dependence of the electronic valence charge density on the instantaneous position of the Li ions. Second, we freeze the atoms of the host lattice in their equilibrium positions; consequently, also the valence charge density is frozen. We thus obtain a computational setup (the “pinball model”) for which extremely inexpensive molecular dynamics simulations can be performed. To assess the accuracy of the model, we contrast it with full first-principles molecular dynamics simulations performed either with a free or frozen host lattice; in this latter case, the charge density still readjusts itself self-consistently to the actual positions of the diffusing Li ions. We show that the pinball model is able to reproduce accurately the static and dynamic properties of the diffusing Li ions—including forces, power spectra, and diffusion coefficients—when compared to the self-consistent frozen-host lattice simulations. The frozen-lattice approximation itself is often accurate enough, and certainly a good proxy in most materials. These observations unlock efficient ways to simulate the diffusion of lithium in the solid state, and provide additional physical insight into the respective roles of charge-density rearrangements or lattice vibrations in affecting lithium diffusion.

Here, we experimentally and theoretically clarify III-V/Si crystal growth processes. Atomically resolved microscopy shows that monodomain three-dimensional islands are observed at the early stages of AlSb, AlN, and GaP epitaxy on Si, independently of misfit. It is also shown that complete III-V/Si wetting cannot be achieved in most III-V/Si systems. Surface/interface contributions to the free-energy variations are found to be prominent over strain relief processes. We finally propose a general and unified description of III-V/Si growth processes, including a description of the formation of antiphase boundaries.

Dimensionality and connectivity among octahedra play important roles in determining the properties, electronic structure, and phase transitions of transition-metal oxides. Here we demonstrate the epitaxial growth of (110)-oriented alternating layers of ${\mathrm{IrO}}_2$ and ${\mathrm{TiO}}_2$, both of which have the rutile structure. These ${\left({\mathrm{IrO}}_2\right)}_n/{\left({\mathrm{TiO}}_2\right)}_2$ superlattices consist of ${\mathrm{IrO}}_6$ and ${\mathrm{TiO}}_6$ octahedra tiled in a hyperconnected, edge- and corner-sharing network. Despite the large lattice mismatch between constituent layers $(\mathrm{\Delta }{d}_{\parallel }=-2.1\%$ and $\mathrm{\Delta }c=+6.6\%)$, our reactive molecular-beam epitaxy-grown superlattices show high structural quality as determined by x-ray diffraction and sharp interfaces as observed by transmission electron microscopy. The large strain at the interface is accommodated by an ordered interfacial reconstruction. The superlattices show persistent metallicity down to $n=3$ atomic layers, and angle-resolved photoemission spectroscopy measurements reveal quantized sub-bands with signatures of ${\mathrm{IrO}}_2\text{−}{\mathrm{IrO}}_2$ interlayer coupling.

We realize a two-dimensional electron system (2DES) in ZnO by simply depositing pure aluminum on its surface in ultrahigh vacuum and characterize its electronic structure by using angle-resolved photoemission spectroscopy. The aluminum oxidizes into alumina by creating oxygen vacancies that dope the bulk conduction band of ZnO and confine the electrons near its surface. The electron density of the 2DES is up to two orders of magnitude higher than those obtained in ZnO heterostructures. The 2DES shows two $s$-type subbands, that we compare with the $d$-like 2DESs in titanates, with clear signatures of many-body interactions that we analyze through a self-consistent extraction of the system self-energy and a modeling as a coupling of a two-dimensional Fermi liquid with a Debye distribution of phonons.

Under pressure, the alkali elements sodium, potassium, and rubidium adopt nonperiodic structures based on two incommensurate interpenetrating lattices. While all elements form the same “host” lattice, their “guest” lattices are all distinct. The physical mechanism that stabilizes these phases is not known, and detailed calculations are challenging due to the incommensurability of the lattices. Using a series of commensurate approximant structures, we tackle this issue using density functional theory calculations. In Na and K, the calculations prove accurate enough to reproduce not only the stability of the host-guest phases, but also the complicated pressure dependence of the host-guest ratio and the two guest-lattice transitions. We find Rb-IV to be metastable at all pressures, and suggest it is a high-temperature phase. The electronic structure of these materials is unique: they exhibit two distinct, coexisting types of electride behavior, with both fully localized pseudoanions and electrons localized in 1D wells in the host lattice, leading to low conductivity. While all phases feature pseudogaps in the electronic density of states, the perturbative free-electron picture applies to Na, but not to K and Rb, due to significant $d$-orbital population in the latter.

In this work, we study the nucleation and subsequent evolution behavior of ripplocations – a newly proposed strain accommodating defect in layered materials where one, or more, layers buckle orthogonally to the layers – using atomistic modeling of graphite. To that effect, we model the response to cylindrical indenters with radii $R$ of 50, 100, and 250 nm, loaded edge-on into graphite layers and the strain gradient effects beneath the indenter are quantified. We show that the response is initially elastic followed by ripplocation nucleation, and growth of multiple fully reversible ripplocation boundaries below the indenter. In the elastic region, the stress is found to be a function of indentation volume; beyond the elastic regime, the interlayer strain gradient emerges as paramount in the onset of ripplocation nucleation and subsequent in-plane stress relaxation. Furthermore, ripplocation boundaries that nucleate from the alignment of ripplocations on adjacent layers are exceedingly nonlocal and propagate, wavelike, away from the indented surface. This work not only provides a critical understanding of the mechanistic underpinnings of the deformation of layered solids and formation of kink boundaries, but also provides a more complete description of the nucleation mechanics of ripplocations and their strain field dependence.

Confined two-dimensional assemblies of floating particles, known as granular rafts, are prone to develop a highly nonlinear response under compression. Here we investigate the transition to the friction-dominated jammed state and map the gradual development of the internal stress profile with flexible pressure sensors distributed along the raft surface. Surprisingly, we observe that the surface stress screening builds up much more slowly than previously thought and that the typical screening distance later dramatically decreases. We explain this behavior in terms of progressive friction mobilization, where the full amplitude of the frictional forces is only reached after a macroscopic local displacement. At further stages of compression, rafts of large length-to-width aspect ratio experience much stronger screenings than the full mobilization limit described by the Janssen's model. We solve this paradox using a simple mathematical analysis and show that such enhanced screening can be attributed to a localized compaction front, essentially shielding the far field from compressive stresses.

Superconductivity and its underlying mechanisms are one of the most active research fields in condensed-matter physics. An important question is how to enhance the transition temperature $T_{\mathrm{c}}$ of a superconductor. In this respect, the possibly positive role of valence-skipping elements in the pairing mechanism has been attracting considerable interest. Here we follow this pathway and successfully enhance $T_{\mathrm{c}}$ up to almost 6 K in the simple chalcogenide SnTe known as a topological crystalline insulator by doping the valence-skipping element In substitutionally for the Sn site and codoping Se for the Te site. A high-pressure synthesis method enabled us to form single-phase solid solutions ${\mathrm{Sn}}_{1-x}{\mathrm{In}}_x{\mathrm{Te}}_{1-y}{\mathrm{Se}}_y$ over a wide composition range while keeping the cubic structure necessary for the superconductivity. Our experimental results are supported by density-functional theory calculations which suggest that even higher $T_{\mathrm{c}}$ values would be possible if the required doping range was experimentally accessible.

${\mathrm{BaPb}}_{1-x}{\mathrm{Bi}}_x{\mathrm{O}}_3$ is a superconductor, with transition temperature $T_c=11$ K, whose parent compound ${\mathrm{BaBiO}}_3$ possesses a charge ordering phase and perovskite crystal structure reminiscent of the cuprates. The lack of magnetism simplifies the ${\mathrm{BaPb}}_{1-x}{\mathrm{Bi}}_x{\mathrm{O}}_3$ phase diagram, making this system an ideal platform for contrasting high-$T_c$ systems with isotropic superconductors. Here we use high-quality epitaxial thin films and magnetotransport to demonstrate superconducting fluctuations that extend well beyond $T_c$. For the thickest films (thickness above $\sim 100\mathrm{nm}$) this region extends to $\sim 27\mathrm{K}$, well above the bulk $T_c$ and remarkably close to the higher $T_c$ of ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{BiO}}_3$ ($T_c=31$ K). We drive the system through a superconductor-insulator transition by decreasing thickness and find the observed $T_c$ correlates strongly with disorder. This material manifests strong fluctuations across a wide range of thicknesses, temperatures, and disorder presenting new opportunities for understanding the precursor of superconductivity near the 2D-3D dimensionality crossover.

The combination of strong spin-orbit coupling, large $g$ factors, and the coupling to a superconductor can be used to create a topologically protected state in a semiconductor nanowire. Here we report on growth and characterization of hybrid epitaxial InAsSb/Al nanowires, with varying composition and crystal structure. We find the strongest spin-orbit interaction at intermediate compositions in zinc-blende ${\mathrm{InAs}}_{1-x}{\mathrm{Sb}}_x$ nanowires, exceeding that of both InAs and InSb materials, confirming recent theoretical studies. We show that the epitaxial InAsSb/Al interface allows for a hard induced superconducting gap and $2e$ transport in Coulomb charging experiments, similarly to experiments on InAs/Al and InSb/Al materials, and find measurements consistent with topological phase transitions at low magnetic fields due to large effective $g$ factors. Finally we present a method to grow pure wurtzite InAsSb nanowires which are predicted to exhibit even stronger spin-orbit coupling than the zinc-blende structure.

We report the discovery of a spin one diamond lattice in ${\mathrm{NiRh}}_2{\mathrm{O}}_4$. This spinel undergoes a cubic to tetragonal phase transition at $T=440$ K that leaves all nearest neighbor interactions equivalent. In the tetragonal phase, magnetization measurements show a ${\mathrm{Ni}}^{2+}$ effective moment of $p_{\mathrm{eff}}=3.3\left(1\right)$ and dominant antiferromagnetic interactions with ${\mathrm{\Theta }}_{\mathrm{CW}}=-11.3\left(7\right)$ K. No phase transition to a long-range magnetically ordered state is observed by specific heat measurements down to $T=0.1$ K. Inelastic neutron scattering measurements on substoichiometric ${\mathrm{NiRh}}_2{\mathrm{O}}_4$ reveal possible valence-bond behavior and show no visible signs of magnetic ordering. ${\mathrm{NiRh}}_2{\mathrm{O}}_4$ provides a platform on which to explore the previously unknown and potentially rich physics of spin one interacting on the diamond lattice, including the realization of theoretically predicted quantum spin liquid and topological paramagnet states.

We report on an enhancement of the superconducting transition temperature $\left(T_{\mathrm{c}}\right)$ of the FeSe-based electric-double-layer transistor (FeSe-EDLT) by applying the multivalent oligomeric ionic liquids (ILs). The IL composed of dimeric cation (divalent IL) enables a large amount of charge accumulation on the surface of the FeSe ultrathin film, resulting in inducing electron-rich conduction even in a rather thick 10 nm FeSe channel. The onset $T_{\mathrm{c}}$ in FeSe-EDLT with the divalent IL is enhanced to be approaching about 50 K at the thin limit, which is about 7 K higher than that in EDLT with conventional monovalent ILs. The enhancement of $T_{\mathrm{c}}$ is a pronounced effect of the application of the divalent IL, in addition to the large capacitance, supposing preferable interface formation of ILs driven by geometric and/or Coulombic effect. The present finding strongly indicates that multivalent ILs are powerful tools for controlling and improving physical properties of materials.

In 2012, Björkman et al. posed the question “Are we van der Waals ready?” [T. Björkman et al., J. Phys.: Condens. Matter 24, 424218 (2012)] about the ability of ab initio modeling to reproduce van der Waals (vdW) dispersion forces in layered materials. The answer at that time was no, however. Here we report on a new generation of vdW dispersion models and show that one, i.e., the fractionally ionic atom theory with many-body dispersions, offers close to quantitative predictions for layered structures. Furthermore, it does so from a qualitatively correct picture of dispersion forces. Other methods, such as D3 and optB88vdW, also work well, albeit with some exceptions. We thus argue that we are nearly vdW ready and that some modern dispersion methods are accurate enough to be used for nanomaterial prediction, albeit with some caution required.

We have grown and characterized 30-nm-thick ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (YBCO) films, deposited by pulsed laser deposition on both MgO (110) and ${\mathrm{SrTiO}}_3$ (001) substrates, which induce opposite strain to the superconducting layer. By carefully tuning the in situ post-annealing oxygen pressure, we achieved, in a reproducible way, films at different oxygen doping, spanning from the slightly overdoped down to the strongly underdoped region of the phase diagram. The transport properties of the films, investigated through resistance versus temperature measurements, are in perfect qualitative agreement with single crystals. Starting from these films, we have also successfully fabricated nanowires with widths down to 65 nm, at different oxygen doping. The nanostructures exhibit characteristic temperatures (as the critical temperature $T_{\mathrm{c}}$ and the pseudogap temperature $T^{*})$ similar to those of the as-grown films and carry critical current densities $J_{\mathrm{c}}$ close to the critical depairing value, limited by vortex entry. This implies that the superconducting and the normal state properties of underdoped YBCO are preserved in our films, and they can be studied as a function of the dimensionality of the system, down to the nanoscale.

New chiral de Vries smectic liquid-crystalline compounds are designed, synthesized, and investigated for perspective applications in defect-free bistable surface-stabilized ferroelectric liquid-crystal displays. In these compounds, a 5-phenyl-pyrimidine benzoate core is terminated on one side by a tri- or tetra-carbosilane group linked through an alkoxy group and an alkyl spacer and on the opposite side terminated by a chiral 2-octanol group. The stereogenic center contains either a methyl or perfluoromethyl functional group. These compounds exhibit Iso-$\mathrm{Sm}{A}^{*}-\mathrm{Sm}{C}^{*}-\mathrm{Sm}X$-Cr phases under cooling from the isotropic state. Measurements of the temperature-dependent smectic layer spacing by x-ray diffraction experiments combined with the measured apparent optical tilt angle and the birefringence reveal that $\mathrm{Sm}{A}^{*}$ phase in these compounds is of the de Vries type. In addition, the chiral compound with a tetra-carbosilane backbone, DR277, exhibits good de Vries properties with the $\mathrm{Sm}{C}^{*}$ phase exhibited over a wide temperature range. By varying the carbosilane end group, the de Vries properties are enhanced, that is, the layer shrinkage of $\sim 1.9\%$ for the tri-carbosilane DR276 is reduced to $\sim 0.9\%$ for tetra-carbosilane DR277 at $10{}^{\circ }\mathrm{C}$ below $\mathrm{Sm}{A}^{*}$ to $\mathrm{Sm}{C}^{*}$ transition temperature, $T_{\mathrm{AC}}$. For DR277, the reduction factor $R\approx 0.22$ for $T=\left(T_{\mathrm{AC}}-10\right){}^{\circ }\mathrm{C}$ is reasonably low and the apparent optical tilt angle ${\theta }_{\mathrm{app}}=35.1^{\circ }$, hence this compound is a “good de Vries smectic” LC. Therefore, synthesis of the chiral mesogen with an even higher number of carbosilane groups may lead to a further reduction or even zero-layer shrinkage exhibited at $T_{\mathrm{AC}}$ with $\mathrm{Sm}{C}^{*}$ phase extending over a wide temperature range close to the room temperature for perspective suitability in device applications. Our results for 5-phenyl-pyrimidine benzoate core-based compounds support a recently drawn conclusion by Schubert et al. [J. Mater. Chem. C 4, 8483 (2016)] from a different compound, namely that a carbosilane backbone in chiral mesogens strongly influences the de Vries properties.

Recent studies have highlighted the importance of organic ligands in the field of molecular spintronics, via which delocalized electron-spin density can mediate magnetic coupling to otherwise localized $4f$ moments of lanthanide ions, which show tremendous potential for single-molecule device applications. To this end, high-field/high-frequency electron paramagnetic resonance (EPR) spectroscopy is employed to study a neutral terbium bis-phthalocyaninato metalorganic complex, ${\left[\mathrm{TbP}{\mathrm{c}}_2\right]}^0$, with the aim of understanding the magnetic interaction between the Ising-like moment of the lanthanide ion and the unpaired spin density on the coordinating organic radical ligand. The measurements were performed on a previously unknown ${\left[\mathrm{TbP}{\mathrm{c}}_2\right]}^0$ structural phase crystallizing in the Pnma space group. EPR measurements on powder samples of ${\left[\mathrm{TbP}{\mathrm{c}}_2\right]}^0$ reveal an anisotropic spectrum, which is attributed to the spin-$\frac{1}{2}$ radical coupled weakly to the EPR-silent $\mathrm{T}{\mathrm{b}}^{\mathrm{III}}$ ion. Extensive double-axis rotation studies on a single crystal reveal two independent spin-$\frac{1}{2}$ signals with differently oriented (albeit identical) uniaxial $g$-tensors, in complete agreement with x-ray structural studies that indicate two molecular orientations within the unit cell. The easy-axis nature of the radical EPR spectra thus reflects the coupling to the Ising-like $\mathrm{T}{\mathrm{b}}^{\mathrm{III}}$ moment. This is corroborated by studies of the isostructural ${\left[\mathrm{YP}{\mathrm{c}}_2\right]}^0$ analog (where Y is nonmagnetic yttrium), which gives a completely isotropic radical EPR signal. The experimental results for the terbium complex are well explained on the basis of an effective model that introduces a weak ferromagnetic Heisenberg coupling between an isotropic spin-$\frac{1}{2}$ and an anisotropic spin-orbital moment, $J=6$, that mimics the known, strong easy-axis $\mathrm{Tb}\cdots \mathrm{P}{\mathrm{c}}_2$ crystal-field interaction.

We report on the systematic study of infrared/terahertz spectra of photocurrents in (Bi,Sb)Te based three-dimensional topological insulators. We demonstrate that in a wide range of frequencies, ranging from fractions up to tens of terahertz, the photocurrent is caused by the linear photogalvanic effect (LPGE) excited in the surface states. The photocurrent spectra reveal that at low frequencies the LPGE emerges due to free carrier Drude-like absorption. The spectra allow us to determine the room temperature carrier mobilities in the surface states despite the presence of thermally activated residual impurities in the material bulk. In a number of samples we observed an enhancement of the linear photogalvanic effect at frequencies between 30 and 60 THz, which is attributed to the excitation of electrons from helical surface to bulk conduction band states. Under this condition and applying oblique incidence we also observed the circular photogalvanic effect driven by the radiation helicity.

We investigate the low-temperature properties of two-dimensional Lennard-Jones glass films, prepared in silico both by liquid cooling and by physical vapor deposition. We identify deep in the solid phase a crossover temperature $T^{*}$, at which slow dynamics and enhanced heterogeneity emerge. Around $T^{*}$, localized defects become visible, leading to vibrational anomalies as compared to standard solids. We find that, on average, $T^{*}$ decreases in samples with lower inherent structure energy, suggesting that such anomalies will be suppressed in ultrastable glass films, prepared both by very slow liquid cooling and vapor deposition.

We report on magnetotransport properties associated with the all-in-all-out magnetic order for ${\mathrm{Tb}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ single-crystalline thin films. Two degenerate all-in-all-out domains of an Ir sublattice can be switched to each other by magnetic field along the [111] direction as detected by a characteristic magnetoresistivity with diplike hysteresis. For a given single-domain structure, the linear magnetoresistivity, reflecting the effective magnetic field at ${\mathrm{Ir}}^{4+}$, changes its sign at around 70 K. These observations are consistently explained by the molecular fields from ${\mathrm{Tb}}^{3+}$ on ${\mathrm{Ir}}^{4+}$ moments. The molecular fields from ${\mathrm{Tb}}^{3+}$ moments compete with the external fields and dominate the magnetoresponse of ${\mathrm{Ir}}^{4+}$ moments at lower temperature, whereas the external field does at higher temperature.