III–V semiconductor nanowires have shown great potential in various quantum transport experiments. However, realizing a scalable high-quality nanowire-based platform that could lead to quantum information applications has been challenging. Here, we study the potential of selective area growth by molecular beam epitaxy of InAs nanowire networks grown on GaAs-based buffer layers, where Sb is used as a surfactant. The buffered geometry allows for substantial elastic strain relaxation and a strong enhancement of field effect mobility. We show that the networks possess strong spin-orbit interaction and long phase-coherence lengths with a temperature dependence indicating ballistic transport. With these findings, and the compatibility of the growth method with hybrid epitaxy, we conclude that the material platform fulfills the requirements for a wide range of quantum experiments and applications.

Controllable local electronic conduction in otherwise insulating materials can be created by arranging two opponent ferroelectric polarizations in a head to head (or tail to tail) configuration. Using an effective trailing field of dc biased tip motion, charged domain walls have been artificially created in the context of tip-based nanolithography. However, the charged domain wall formed by a trailing field is unstable because of elastic interaction at the boundary between poling and nonpoling regions, finally resulting in ferroelastic back-switching. Here, we report that nanoscale plate structures under strain relaxation can provide a promising opportunity for stabilization and manipulation of a charged domain wall using a highly anisotropic mechanical boundary condition that restricts the unique ferroelastic domain configuration. We demonstrate that a ferroelectric ${\mathrm{BiFeO}}_3$ nanoplate subjected to compressive misfit strain at the bottom but less external stress on the side walls exhibits radial-quadrant in-plane ferroelectric domain structures. Electronic conduction is significantly enhanced near the side walls and the magnitude of electrostatic conductivity is adjustable up to about 20 times by 180° ferroelectric switching that is protected by the clamped ferroelastic domain. Our findings provide a pathway to controllable nanoelectronic logic devices by tuning a charged ferroelectric domain wall.

We perform a systematic materials informatics search for two-dimensional (2D) ferromagnetic materials (2DFM) in the Inorganic Crystal Structure Database (ICSD). The key indicator in the search is that the candidates have a 2D network composed of magnetic atoms and heavy atoms. From the 187,093 entries in the ICSD, the search identified 15 candidates whose magnetic order are then determined by first-principles calculations and Curie temperatures obtained by either analytical solution or Monte Carlo simulation of the 2D Ising model. For the 15 candidates, eight materials have ferromagnetic order including three known structural prototypes and the two 2DFM materials discovered in recent experiments. More importantly, the search produced two new structural prototypes and a very promising material, ${\mathrm{Cr}}_3{\mathrm{Te}}_4$, which has a calculated Curie temperature one order of magnitude higher than the others.

The recent realization of topological electronic states such as Dirac and Weyl fermions in real materials and their potential for future energy and electronics applications has motivated interest in the study of new forms of topological behavior embodied through new materials. ${\mathrm{Pd}}_3\mathrm{Pb}$ is one such candidate predicted recently to host unique topological features, including a dispersionless band near the Fermi level and triple nodal points hosting Dirac fermions and open Fermi arcs. Here, we report the crystal growth and electric transport properties of ${\mathrm{Pd}}_3\mathrm{Pb}$. Our low field magnetoresistance measurements indicate an anisotropic Fermi surface. We found that ${\mathrm{Pd}}_3\mathrm{Pb}$ manifests a large transverse magnetoresistance, which reaches 650% at 1.8 K and 14 T, and pronounced Shubnikov-de Haas (SdH) oscillations. Preliminary analysis of the field dependence of the SdH oscillations points to the likelihood of nontrivial Berry phase in ${\mathrm{Pd}}_3\mathrm{Pb}$. Further studies in high field limit are desirable to extend the realization of the topological properties of the predicted novel fermions in this material.

Reconstructing the three-dimensional (3D) size and shape distribution of randomly oriented grains using only images of cross sections remains an important challenge. Even for ellipsoids, a solution is only possible when they are solids of revolution, and may still be numerically unstable. Here we show that crystallographic orientation data, for example from electron back-scatter diffraction (EBSD), provides enough additional information to obtain moments of the 3D grain distribution, provided grain shapes can be assumed to align with crystal axes. We show that this moment method can give an average 3D grain size and shape (with error estimate) which is rigorous for ellipsoids and a good approximation for cuboidal grains, indicating that it may be a useful technique for polycrystalline materials in general. High throughput image analysis and EBSD now make the necessary sample sizes practical. We illustrate by applying the method to a basaltic rock specimen.

The noncentrosymmetric Weyl semimetal candidate ${\mathrm{MoTe}}_2$ was investigated through neutron-diffraction and transport measurements at pressures up to 1.5 GPa and at temperatures down to 40 mK. Centrosymmetric and noncentrosymmetric structural phases were found to coexist in the superconducting state. Density functional theory (DFT) calculations reveal that the strength of the electron-phonon coupling is similar for both crystal structures. Furthermore, it was found that by controlling nonhydrostatic components of stress, it is possible to mechanically control the ground-state crystal structure. This allows for the tuning of crystal symmetry in the superconducting phase from centrosymmetric to noncentrosymmetric. DFT calculations support this strain control of crystal structure. This mechanical control of crystal symmetry gives a route to tuning the band topology of ${\mathrm{MoTe}}_2$ and possibly the topology of the superconducting state.

The line tension model of obstacle hardening is modified to account for the thermally activated, kink-limited glide of $1/2\langle 111\rangle$ screw dislocations, allowing application to the plastic flow of bcc metals. Using atomistically informed dislocation mobility laws, Frenkel-Kontorova simulations, and a simplified dislocation-obstacle model, we identify a size effect for intermediate obstacle densities, where the activation energy for screw dislocation motion halves once the obstacle density falls below a critical value. Our model shows striking agreement with fracture experiments across a wide range of unirradiated and irradiated bcc metals. In particular, we demonstrate that the presence of defects in the crystal lattice can at most double the brittle to ductile transition temperature.

Intralayer deformation in van der Waals (vdW) heterostructures is generally assumed to be negligible due to the weak nature of the interactions between the layers, especially when the interfaces are found incoherent. In the present work, graphene-phosphorene vdW heterostructures are investigated with the density functional theory (DFT). The challenge of treating a nearly incommensurate (very large) supercell in DFT is bypassed by considering different energetic quantities in the grand canonical ensemble, alternative to the formation energy, in order to take into account the mismatch elastic contribution of the different layers. In the investigated heterostructures, it is found that phosphorene contracts by $\sim 4\%$ in the armchair direction when compared to its free-standing form. This large contraction leads to important changes in terms of electronic properties, with the direct electronic optical transition of phosphorene becoming indirect in specific vdW heterostructures. More generally, such a contraction indicates strong substrate effects in supported or encapsulated phosphorene—neglected hitherto—and paves the way to substrate-controlled stresstronic in such 2D crystal. In addition, the stability of these vdW heterostructures is investigated as a function of the rotation angle between the layers and as a function of the stacking composition. The alignment of the specific crystalline directions of graphene and phosphorene is found energetically favored. In parallel, several models based on DFT-estimated quantities are presented; they allow notably a better understanding of the global mutual accommodation of 2D materials in their corresponding interfaces, that is predicted to be non-negligible even in the case of incommensurate interfaces.

We demonstrate experimental evidence for the existence of an optimal thickness that yields the maximum third-harmonic (TH) signal in thick multilayer graphene (MLG). We measure the layer-dependent TH signal from MLG on a quartz substrate for $N$ graphene layers ranging from $\sim 3$ to 50. Theoretical predictions of the layer-dependent TH signal are compared with experiment, including those measured using atomic force and optical microscopy. We find the optimal TH signal for $N\approx 24$, in good agreement with two theoretical models: one in which the graphene layers are assumed isolated but contribute coherently, and one in which the MLG is assumed as one continuous thin film. Our measurements reveal that for MLG, the stacking property of the linear and nonlinear conductivities are found to be remarkably preserved up to the very thick layers $\left(N\sim 50\right)$, i.e., ${\sigma }_{N,g}^{\left(i\right)}=N{\sigma }_g^{\left(i\right)}$, where $i=1$ and 3, respectively, and ${\sigma }_g^{\left(i\right)}$ is the corresponding monolayer conductivity.

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.