Crystal plasticity causes structural fluctuations that can be scale-free and therefore admit power-law distributions. Numerous experiments, modeling, and theory have reported a scaling exponent that is in good agreement with mean-field predictions or a jamming-unjamming scenario. Via experiments on pure single crystals, we show here that the scaling exponent of a stress-integrated distribution for dislocation-avalanche sizes is nontrivial and can be in agreement with both models by admitting values between 1.0 and 2.3. This range is dictated by the structure and orientation of the deforming crystal, as long as the applied rate is below a critical value. For the highest symmetry tested, plastic strain can drive a change from truncated power-law scaling to pure exponential scaling. These findings show how the same crystal may yield different scaling exponents depending on intrinsic and extrinsic factors.

Large-area and high-quality single-layer transition metal dichalcogenides can be synthesized by epitaxial growth on single-crystal substrates. An important advantage of this approach is that the interaction between the single layer and the substrate can be strong enough to enforce a single crystalline orientation of the layer. On the other hand, the same interaction can lead to hybridization effects, resulting in the deterioration of the single layer's native properties. This dilemma can potentially be solved by decoupling the single layer from the substrate surface after the growth via intercalation of atoms or molecules. Here we show that such a decoupling can indeed be achieved for single-layer ${\mathrm{WS}}_2$ epitaxially grown on Ag(111) by intercalation of Bi atoms. This process leads to a suppression of the single-layer ${\mathrm{WS}}_2$-Ag substrate interaction, yielding an electronic band structure reminiscent of free-standing single-layer ${\mathrm{WS}}_2$.

Rotational twins are fundamental defects in III-V epitaxy, in particular for the growth on nonpolar (111) surfaces. Based on density functional theory (DFT) calculations, we develop a general model for III-V nucleation on vicinal nonpolar (111)-oriented substrates and focus on the important differences in the atomic step configuration of different miscut directions. We verify this model by a relevant materials system when growing GaP epilayers on As-terminated Si(111): Scanning tunneling microscopy measurements reveal the formation of straight double bilayer steps after As passivation of the Si(111) surface, which persist after III-V growth, as we display when measuring the buried heterointerface with cross-sectional high-resolution transmission electron microscopy. A twin amount in the GaP epilayers is observed in dependence on the misorientation and our nucleation model explains the underlying mechanisms: The number of back bonds at the step edges determines the nucleation site. Accordingly, the substrate misorientation towards $\left[11\overline{2}\right]$ yields twin suppression, which is in full agreement with experiment. Finally, we use DFT input for kinetic Monte Carlo calculations to explain the formation of GaP rotational twins on Si(111):As in order to explain their volume fraction observed by high-resolution x-ray diffraction measurements. We thus derive a complete picture of the formation and suppression of rotational twins relevant for low-defect III-V–on–Si integration.

We present a first-principles approach to compute the transport properties of 2D materials in an accurate and automated framework. We use density-functional perturbation theory in the appropriate bidimensional setup with open-boundary conditions in the third direction. The materials are charged by field effect via planar countercharges. In this approach, we obtain electron-phonon matrix elements in which dimensionality and doping effects are inherently accounted for, without the need for post-processing corrections. This treatment highlights some unexpected consequences, such as an increase of electron-phonon coupling with doping in transition-metal dichalcogenides. We use symmetries extensively and identify pockets of relevant electronic states to minimize the number of electron-phonon interactions to compute; the integrodifferential Boltzmann transport equation is then linearized and solved beyond the relaxation-time approximation. We apply the entire protocol to a set of much studied materials with diverse electronic and vibrational band structures: electron-doped ${\mathrm{MoS}}_2,{\mathrm{WS}}_2,{\mathrm{WSe}}_2$, phosphorene, arsenene, and hole-doped phosphorene. Among these, hole-doped phosphorene is found to have the highest mobility, with a room temperature value around $600{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. Last, we identify the factors that affect most phonon-limited mobilities, such as the number and the anisotropy of electron and hole pockets, to provide a broader understanding of the driving forces behind high mobilities in two-dimensional materials.

We investigate metastable and thermodynamically stable phases that can be expected to occur in electroformed filaments in resistively switching hafnia, and discuss their relevance for the switching process. To this end, we conduct a study, based on density functional theory combined with an evolutionary algorithm determining the composition-dependent (meta)stable phases in ${\mathrm{HfO}}_x$, focusing on the region $0<x<2$. We find that oxygen vacancies in hafnia tend to form regular patterns, which leads to periodic metastable structures featuring one-dimensional open channels, thus favoring ionic conductivity in the host material, i.e., oxygen migration. The band gap of such structures is systematically lowered with increasing oxygen deficiency, resulting in metallic behavior when oxygen migrates out of the channels. Moreover, we find that the solubility of oxygen in metallic Hf is very high, up to one oxygen per six metallic atoms, the concentration corresponding to a thermodynamically stable and ordered metallic compound, ${\mathrm{Hf}}_6\mathrm{O}$. Therefore, thick enough metallic capping of Hf could play the role of an active electrode for hosting oxygen which migrates out of ${\mathrm{HfO}}_2$. In combination with reversible oxygen migration in predicted suboxide phases, this should lead to robust resistive memory cells with high endurance and long retention.

Polar crystals composed of charged ionic planes cannot exist in nature without acquiring surface changes to balance an ever-growing dipole. The necessary changes can manifest structurally or electronically as observed in semiconductors and ferroelectric materials through screening charges and/or domain wall formation. In the case of prototypical polar complex oxides such as the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ system the nature of screening charges for different interface terminations is not symmetric. Electron accumulation is observed near the ${\mathrm{LaAlO}}_3/{\mathrm{TiO}}_2\text{−}{\mathrm{SrTiO}}_3$ interface, while the ${\mathrm{LaAlO}}_3$/SrO-${\mathrm{SrTiO}}_3$ stack is insulating. Here, we observe evidence for an asymmetry in the surface chemical termination for nominally stoichiometric ${\mathrm{LaAlO}}_3$ films in contact with the two different surface layers of ${\mathrm{SrTiO}}_3$ crystals, ${\mathrm{TiO}}_2$ and SrO. Using several element-specific probes, we find that the surface termination of ${\mathrm{LaAlO}}_3$ remains ${\mathrm{AlO}}_2$ irrespective of the starting termination of ${\mathrm{SrTiO}}_3$ substrate surface. We use a combination of cross-plane tunneling measurements and first-principles calculations to understand the effects of this unexpected termination on band alignments and polarity compensation of ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ heterostructures. An asymmetry in ${\mathrm{LaAlO}}_3$ polarity compensation and resulting electronic properties will fundamentally limit atomic level control of oxide heterostructures.

We have studied the physical properties of ${\mathrm{Nd}}_2{\mathrm{O}}_3$ with neutron diffraction, inelastic neutron scattering, heat-capacity, and magnetic susceptibility measurements. ${\mathrm{Nd}}_2{\mathrm{O}}_3$ crystallizes in a trigonal structure with ${\mathrm{Nd}}^{3+}$ ions surrounded by cages of seven oxygen anions. The crystal-field spectrum consists of four excitations spanning the energy range of 3–60 meV. The refined eigenfunctions indicate XY spins in the $ab$ plane. The Curie-Weiss temperature of ${\theta }_{CW}=-23.7\left(1\right)\mathrm{K}$ was determined from magnetic susceptibility measurements. Heat capacity measurements show a sharp peak at 550 mK and a broader feature centered near 1.2 K. Neutron diffraction measurements show that the 550 mK transition corresponds to long-range antiferromagnetic order implying a frustration index of ${\theta }_{CW}/T_N\approx 43$. These results indicate that ${\mathrm{Nd}}_2{\mathrm{O}}_3$ is a structurally and chemically simple model system for frustration caused by competing interactions with moments with predominant XY anisotropy.

We perform experiments and numerical simulations to investigate the low energy states of a triangular colloidal ice, realized by confining repulsive paramagnetic colloids to a lattice of topographic double wells. We find that the collective interactions between the particles lead to a unique ground state characterized by vertices with three colloids pointing inward and three outwards, similar to what was predicted, but never observed, for artificial spin ice. We show that a bias force that magnetizes the system allows us to easily access the elusive GS via a structural martensiticlike transition characterized by the coherent sliding of one particle at each vertex.

Recently published discoveries of acoustic- and optical-mode inversion in the phonon spectrum of certain metals became the first realistic example of noninteracting topological bosonic excitations in existing materials. However, the observable physical and technological use of such topological phonon phases remained unclear. In this paper, we provide strong theoretical and numerical evidence that for a class of metallic compounds (known as triple-point topological metals), the points in the phonon spectrum, at which three (two optical and one acoustic) phonon modes (bands) cross, represent a well-defined topological material phase, in which the hosting metals have very strong thermoelectric response. The triple-point bosonic collective excitations appearing due to these topological phonon band-crossing points significantly suppress the lattice thermal conductivity, making such metals phonon glasslike. At the same time, the topological triple-point and Weyl fermionic quasiparticle excitations present in these metals yield good electrical transport (electron crystal) and cause a local enhancement in the electronic density of states near the Fermi level, which considerably improves the thermopower. This combination of phonon glass and electron crystal is the key for high thermoelectric performance in metals. We call these materials topological thermoelectric metals and propose several compounds for this phase (TaSb and TaBi). We hope that this work will lead researchers in physics and materials science to the detailed study of topological phonon phases in electronic materials, and the possibility of these phases to introduce more efficient use of thermoelectric materials in many everyday technological applications.

Although oxide glasses have many unique properties, their range of applications remains limited by their brittleness. By mimicking the microstructure of composite materials, the presence of controlled nanoscale phase separation in glass could overcome this limitation. However, the nature of the toughening mechanism induced by such nanostructuring remains poorly understood. Here, based on peridynamic simulations, we investigate the effect of nanoscale phase separation on the crack propagation mechanism. We show that phase separation can significantly increase glass's toughness (with up to a 90% increase in the fracture energy for the range of conditions investigated herein). The extent of toughening is found to arise from a balance between the overall cohesion of the phase-separated glass and the propensity for crack deflection. This suggests that controlled nanoscale phase separation is a promising route toward the development of tough, yet optically transparent glasses.

Large unsaturated magnetoresistance has been recently reported in numerous semimetals. Many of them have a topologically nontrivial band dispersion, such as Weyl nodes or lines. Here, we show that elemental antimony displays the largest high-field magnetoresistance among all known semimetals. We present a detailed study of the angle-dependent magnetoresistance and use a semiclassical framework invoking an anisotropic mobility tensor to fit the data. A slight deviation from perfect compensation and a modest variation with magnetic field of the components of the mobility tensor are required to attain perfect fits at arbitrary strength and orientation of magnetic field in the entire temperature window of study. Our results demonstrate that large orbital magnetoresistance is an unavoidable consequence of low carrier concentration and the subquadratic magnetoresistance seen in many semimetals can be attributed to field-dependent mobility, expected whenever the disorder length scale exceeds the Fermi wavelength.

The origin of nematicity, i.e., in-plane rotational symmetry breaking, and in particular the relative role played by spontaneous unidirectional ordering of spin, orbital, or charge degrees of freedom, is a challenging issue of magnetism, unconventional superconductivity, and quantum Hall effect systems, discussed in the context of doped semiconductor systems such as ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}, {\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$, and Ga(Al)As/${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wells, respectively. Here, guided by our experimental and theoretical results for ${\mathrm{In}}_{1-x}{\mathrm{Fe}}_x\mathrm{As}$, we demonstrate that spinodal phase separation at the growth surface (that has a lower symmetry than the bulk) can lead to a quenched nematic order of alloy components, which then governs low-temperature magnetic and magnetotransport properties, in particular the magnetoresistance anisotropy whose theory for the $C_{2v}$ symmetry group is advanced here. These findings, together with earlier data for ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$, show under which conditions anisotropic chemical phase separation accounts for the magnitude of transition temperature to a collective phase or merely breaks its rotational symmetry. We address the question to what extent the directional distribution of impurities or alloy components setting in during the growth may account for the observed nematicity in other classes of correlated systems.

We perform a combined experimental and theoretical study of a magnetic-field $\left(B\right)$-induced evolution of magnetic and ferroelectric properties in an antiferromagnetic material $\mathrm{Pb}\left(\mathrm{TiO}\right){\mathrm{Cu}}_4{\left({\mathrm{PO}}_4\right)}_4$, whose structure is characterized by a staggered array of ${\mathrm{Cu}}_4{\mathrm{O}}_{12}$ magnetic units with convex geometry known as square cupola. Our experiments show a $B$-induced phase transition from a previously reported low-$B$ linear magnetoelectric phase to a high-$B$ magnetoelectric phase, which accompanies a ${90}^{\circ }$ flop of electric polarization and gigantic magnetodielectric effect. Moreover, we observe a $B$-induced sign reversal of ferroelectric polarization in the high-$B$ phase. Our model and first-principles calculations reveal that the observed complex magnetoelectric behavior is well explained in terms of a $B$-dependent electric polarization generated in each ${\mathrm{Cu}}_4{\mathrm{O}}_{12}$ unit by the so-called exchange-striction mechanism. The present study demonstrates that the materials design based on the magnetic structural unit with convex geometry deserves to be explored for developing strong magnetoelectric couplings.

Conventional magnetic imaging techniques have observed magnetic domains in the polished surface of Nd-Fe-B sintered magnets, but the mechanical processing involved introduces numerous defects that facilitate the nucleation of reversed domains, thereby masking the interior domain structure. Here, we utilize high-field synchrotron x-ray magnetic circular dichroism microscopy to map the elemental and magnetic distributions in the polished and fractured surfaces of a Nd-Fe-B sintered magnet throughout its entire demagnetization process. As the applied field is varied, the domains in the fractured surface behave completely differently from those in the polished surface, thereby unmasking the interior domain structure and behavior. The area-averaged fractured surface coercivity is ${\mu }_0{H}_{\mathrm{c}}^{\mathrm{frac}}=0.85\mathrm{T}$ which is much higher than the area-averaged polished surface coercivity, ${\mu }_0{H}_{\mathrm{c}}^{\mathrm{pol}}=0.5\mathrm{T}$. Most of the local magnetic hysteresis loops are positively or negatively biased from the zero of the field axis. The highest coercivity grains, some of which exceed 2 T, are almost always located in the vicinity of strongly oppositely biased adjacent grains. This indicates that these oppositely biased grains are strongly influencing the magnetostatic field at the sample surface.

We use resonant and nonresonant x-ray diffraction measurements in combination with first-principles electronic structure calculations and Monte Carlo simulations to study the relationship between crystal structure and multiferroic orders in the orthorhombic perovskite manganites, $\mathrm{o}\text{−}R{\mathrm{MnO}}_3$ ($R$ is a rare-earth cation or Y). In particular, we focus on how the internal lattice parameters (Mn-O bond lengths and Mn-O-Mn bond angles) evolve under chemical pressure and epitaxial strain, and the effect of these structural variations on the microscopic exchange interactions and long-range magnetic order. We show that chemical pressure and epitaxial strain are accommodated differently by the crystal lattice of $\text{o-}R{\mathrm{MnO}}_3$, which is key for understanding the difference in magnetic properties between bulk samples and strained films. Finally, we discuss the effects of these differences in the magnetism on the electric polarization in $\mathrm{o}\text{−}R{\mathrm{MnO}}_3$.

A lately discovered 1144 phase has generated significant interest for its high superconducting temperatures, disorder-free doping, and various chemical substitutions. However, it has only been found in iron arsenides ($AB{\mathrm{Fe}}_4{\mathrm{As}}_4$), and cations are limited to +1 or +2 valence states (e.g., alkali metals, alkaline earth elements, and Eu). Whether more 1144 phases could be stabilized and whether intriguing properties exist are questions of general interest. In this work, we investigate 1144 iron and cobalt arsenides with tri-valence cations (La, Y, In, Tl, Sm, Gd). We study phase stability among other competing phases: 122 solution phase and phase decomposition. With La as the cation, we predict room-temperature stable 1144 structures: $\mathrm{La}A{\mathrm{Fe}}_4{\mathrm{As}}_4$ ($A=\mathrm{K}$, Rb, and Cs). Other La-contained 1144 structures tend to form solution phase. The solubility of La is estimated and compared with the experiment. By contrast, we do not find stable 1144 structures with Y as the cation. For In and Tl as cations, two 122-phase compounds are remarkably stable: ${\mathrm{InCo}}_2{\mathrm{As}}_2$ and ${\mathrm{TlCo}}_2{\mathrm{As}}_2$, which adds to our knowledge about the In(Tl)-Co-As phase diagram. Stable 1144 phases are found in ${\mathrm{InKCo}}_4{\mathrm{As}}_4$ and ${\mathrm{InRbCo}}_4{\mathrm{As}}_4$. With Sm and Gd as cations, 1144- or 122-phase iron arsenides are generally unstable. Among structures investigated, we recognize two critical factors for 1144-phase stability: size effect and charge balance, which yields a merging picture with the rule found in previous 1144 systems. Moreover, $\mathrm{La}A{\mathrm{Fe}}_4{\mathrm{As}}_4$ ($A=\mathrm{K}$, Rb, and Cs), ${\mathrm{InCo}}_2{\mathrm{As}}_2$, and ${\mathrm{TlCo}}_2{\mathrm{As}}_2$ are exhibiting semimetal features and a two-dimensional Fermi surface, similar to iron superconductors.

The recent discovery of the large anomalous Hall effect (AHE) in noncollinear kagome antiferromagnets $\mathrm{M}{\mathrm{n}}_{3}X$ ($X=\mathrm{Sn}$, Ge) has highlighted compounds and alloys based on a combination of Mn and X. As the spin Hall effect (SHE) and AHE share the same origin, the injection of pure spin current into alloys based on Mn and X could potentially engender a large SHE. Here, we report that our spin Seebeck and the spin pumping measurements both reveal that doping Mn with 18% Sn strikingly increases the spin Hall angle of the amorphous/nanocrystalline Mn-Sn alloy by as much as 20 times while the resistivity is reduced by ten times compared with pure Mn. Our study not only serves as an essential reference for pure spin current phenomena in Mn-based alloys but may also offer a promising method for exploring future energy-saving spin Hall materials.

The reduction of the energy dissipation induced by defects is essential to achieve the ultrahigh-quality-factor mechanical resonators for the applications of quantum platform and high-sensitivity microelectromechanical (MEMS) and nanoelectromechanical system (NEMS) sensors. Single-crystal diamond (SCD) is the ideal material for high-quality-factor mechanical resonators due to its outstanding mechanical properties and intrinsic low-energy dissipation. To achieve mechanical resonators with extreme properties as well as high reliability, it is desirable to develop all-SCD mechanical resonators. By using a smart-cut method and atomic layer etching to remove the defects within the resonators, we achieve the SCD-on-SCD mechanical resonators with ultrahigh quality factors of over one million at room temperature. The quality factors are one or more orders of magnitude higher than those of the state-of-the-art MEMS cantilevers based on polycrystalline diamond, single-crystal silicon, and other crystal materials. The diamond MEMS resonators would be highly promising for sensor application as well as for the scheme for coupling with quantum centers in diamond.

We investigate the metallic surface state in ultrathin films of ${\mathrm{BaTiO}}_3$ by angle-resolved photoemission spectroscopy. We find Fermi-surface contours derived from the Ti $3d{t}_{2g}$ conduction band, similar as in ${\mathrm{SrTiO}}_3$ but with broader spectral features due to enhanced quasiparticle scattering. Oxygen vacancies created in the x-ray beam spot allow for reversible in situ doping control up to surface carrier densities as high as ${10}^{14}{\mathrm{cm}}^{-2}$, but vacancy migration into the subsurface at $T\ge 285\mathrm{K}$ quenches the surface state. Our analysis suggests that the charge state of oxygen vacancies in ultrathin films is predominantly $2+$, which limits charge-carrier trapping and the formation of localized defect states.

This work presents the emergence of superconductivity in Re-substituted topological Weyl semimetal ${\mathrm{MoTe}}_2$. Re substitution for Mo sites leads to a sizable enhancement in the superconducting transition temperature ($T_c$). A record high $T_c$ at ambient pressure in a $1T^{\prime }-{\mathrm{MoTe}}_2$ (room-temperature structure) related sample is observed for the ${\mathrm{Mo}}_{0.7}{\mathrm{Re}}_{0.3}{\mathrm{Te}}_2$ composition ($T_c=4.1\mathrm{K}$; in comparison, ${\mathrm{MoTe}}_2$ shows a $T_c$ of 0.1 K). The experimental and theoretical studies indicate that Re substitution is doping electrons and facilitates the emergence of superconductivity by enhancing the electron-phonon coupling and density of states at the Fermi level. Our findings, therefore, open a new way to further manipulate and enhance the superconducting state together with the topological states in two-dimensional van der Waals materials.