We show that two different superconducting phases exist at high pressures in the optimized tetradymite topological insulators $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_2\mathrm{Se}$ (BTS) and $\mathrm{B}{\mathrm{i}}_{1.1}\mathrm{S}{\mathrm{b}}_{0.9}\mathrm{T}{\mathrm{e}}_2\mathrm{S}$ (BSTS). The superconducting phases emerge at structural phase transitions; the first at $\sim 8.4\mathrm{GPa}$ for BTS and $\sim 12.4\mathrm{GPa}$ for BSTS, and the second at 13.6 GPa for BTS and 20.4 GPa, for BSTS. Electronic structure calculations show that these phases do not have topological character. Comparison of our results with prior work on $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$, $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$, and ${(\mathrm{Bi},\mathrm{Sb})}_2{\left(\mathrm{Se},\mathrm{Te}\right)}_3$ allows us to uncover a universal phase diagram for pressure-induced superconductivity in tetradymites, providing a basis for understanding the relationships between topological behavior, crystal structure, and superconductivity for these materials.

The emergence of magnetism unique to the interface between the multiferroic $\mathrm{BiFe}{\mathrm{O}}_3$ (BFO) and ferromagnetic $\mathrm{L}{\mathrm{a}}_{1-\mathrm{x}}\mathrm{S}{\mathrm{r}}_{\mathrm{x}}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) offers an opportunity to control magnetism in nanoscale heterostructures with electric fields. In this paper, we investigate the influence of chemical composition and crystallographic orientation on the interfacial magnetism of BFO/LSMO superlattices. Our results reveal that the induced net magnetic moment in the BFO layers increases monotonically with increasing saturation magnetization of the LSMO layers. For the (100)-BFO/LSMO ($x=0.2$) superlattice, the induced moment reaches a record high value of $\sim 2.8{\mu }_{\mathrm{B}}/\mathrm{Fe}$. No interfacial magnetization is observed at the (100)-BFO/LSMO interface when LSMO is an antiferromagnet. In contrast to (100)-oriented superlattices, no induced moment is observed in (111)-BFO layers. Our results suggest the interfacial structural reconstruction may not be a sufficient condition for the enhanced net moment in BFO layer. Instead, spin canting induced by interfacial exchange coupling is proposed in the (100)- but not in the (111)-BFO, leading to the large net magnetization at the (100)-oriented interface. This work further demonstrates the importance of exchange coupling across heterointerfaces for spin canting in nominally antiferromagnets, providing a pathway to control the magnetic properties of artificial oxide heterostructures.

As electrical device components are scaled down to atomic sizes, management of waste heat becomes a major issue in device performance. Minimizing energy consumption of electrical components and waste heat generation are critical issues in the operation of such devices. Interest in alternative cooling mechanisms, such as those provided by electrocaloric, magnetocaloric, elastocaloric, and thermoelectric materials, may also be necessary in mitigating issues associated with waste heat generation. In this work, we provide theoretical predictions for an alternative cooling mechanism, accomplished by utilizing electrostatic gating to induce structural phase transitions in monolayer materials. We refer to this mechanism as the electrostaticaloric effect in reference to the mechanism of electrostatic doping that drives the structural phase transformation and entropy change in the material. Recent predictions and experimental observation that electrostatic gating can induce structural phase transformations in monolayer materials opens the possibility for new application areas. Here, we explore the potential for electrostatically induced structural phase transformations in monolayer ${\mathrm{MoTe}}_2$ to be used in a Carnot refrigeration cycle. We predict that a temperature change of 10–15 K may be possible in devices that utilize monolayer ${\mathrm{MoTe}}_2$ as the active phase change material. This mechanism may prove useful for future electrical devices which require cooling at the component level, and for which small monolayer devices are necessary.

Indirect exchange coupling in magnetic multilayers, also known as the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, is highly effective in controlling the interlayer alignment of the magnetization. This coupling is typically fixed at the stage of the multilayer fabrication and does not allow ex situ control needed for device applications. In addition to the orientational control, it is highly desirable to also control the magnitude of the intralayer magnetization, ideally, being able to switch it on/off by switching the relevant RKKY coupling. Here we demonstrate a magnetic multilayer material incorporating thermally and field-controlled RKKY exchange, focused on a dilute ferromagnetic alloy layer and driving it though its Curie transition. Such on/off magnetization switching of a thin ferromagnet, performed repeatedly and fully reproducibly within a low-field sweep, results in a giant magnetocaloric effect, with an estimated isothermal entropy change of $\mathrm{\Delta }S\approx -10\mathrm{mJ}{\mathrm{cm}}^{-3}{\mathrm{K}}^{-1}$ under an external field of $\sim 10\mathrm{mT}$, which greatly exceeds the performance of the best rare-earth based materials used in the adiabatic-demagnetization refrigeration systems.

We design a compound belonging to a class of materials designated as polar half-metallic ferromagnets, where a 100% spin polarization coexists with polar distortions that globally lift inversion symmetry. Using electronic structure calculations, we predict that the ultrashort period ${\left({\mathrm{LaNiO}}_3\right)}_1/{\left({\mathrm{YCrO}}_3\right)}_1$ superlattice belongs to this materials class, exhibiting an integer magnetic moment of $4{\mu }_B$. The minority channel electric polarization, as computed using Berry phase theory, is as high as $\sim 13.0\mu \mathrm{C}{\mathrm{cm}}^{-2}$, and we discuss experimental strategies to access the polarization. We propose that polar ferromagnetic half-metals exhibit multiferroism and can be exploited to realize nonreciprocal effects and directional anisotropy owing to the absence of both space-inversion and time-reversal symmetries.

Electron magnetic circular dichroism (EMCD), the electron wave analog of x-ray magnetic circular dichroism (XMCD), allows for the element specific measurement of the spin and orbital magnetic moments with up to nanometer resolution. However, due to dynamical diffraction effects, the signal-to-noise ratios of EMCD spectra are often very low. We describe a simple set of rules, how to set up a geometry for a classical EMCD experiment on an arbitrary crystal structure to get a maximum dichroic signal. The procedure is based on an evaluation of the structure factor and extinction distances. Proof-of-concept simulations and experiments on a FeGe crystal present a successful test of these guidelines.

We present an investigation into the structural and magnetic properties of Zn-substituted ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$, a system in which the skyrmion lattice (SkL) phase in the magnetic field–temperature phase diagram was previously seen to split as a function of increasing Zn concentration. We find that splitting of the SkL is only observed in polycrystalline samples and reflects the occurrence of several coexisting phases with different Zn content, each distinguished by different magnetic behavior. No such multiphase behavior is observed in single-crystal samples.

Based on the thermodynamic variation to the free-energy functional, we propose a sharp-interface model for simulating solid-state dewetting of thin films on rigid curved substrates in two dimensions. This model describes the interface evolution which occurs through surface diffusion-controlled mass transport and contact point migration along the curved substrate. Furthermore, the surface energy anisotropy is easily included into the model, and the contact point migration is explicitly described by the relaxed contact angle boundary condition. We implement the mathematical model by a semi-implicit parametric finite-element method to study several interesting phenomena, such as “small” particle migration on curved substrates and templated solid-state dewetting on a prepatterned substrate. Based on ample numerical simulations, we demonstrate that, the migration velocity of a small solid particle is proportional to the substrate curvature gradient ${\widehat{\kappa }}^{\prime }$ and inversely proportional to the square root of the area of the particle $\sqrt{A}$, and it decreases when the isotropic Young's angle ${\theta }_i$ increases. In addition, we also observe four periodic categories of dewetting on a prepatterned sinusoidal substrate. Our approach can provide a convenient and powerful tool for exploring how to produce well-organized nanoparticles by making use of template-assisted solid-state dewetting.

Although the conversion reactions of electron-hole (e-h) pairs into emissive excitons are essential in the operation process of organic light emitting diodes (OLEDs), experimental research on behaviors of e-h pairs in operating OLEDs has not been developed so far. Here, three types of magnetic-resonance techniques that detect changes in current, photoluminescence intensity, and electroluminescence (EL) intensity induced by electron spin-resonance transitions have been applied to a working OLED as a function of operation bias. Combined use of these techniques reveals that e-h pairs exist in the OLED with completely different bias dependence from carriers and excitons. It is shown that the EL process is classified into three regions depending on the bias: pair-accumulation, pair-dissociation/recombination, and non-pair-formation regions.

Diffraction data play an important role in the structural characterizations of solids. While reverse Monte Carlo (RMC) and similar methods provide an elegant approach to (re)construct a three-dimensional model of noncrystalline solids, a satisfactory solution to the RMC problem is still not available. Following our earlier efforts, we present here an accurate structural solution of the inverse problem by developing an information-driven inverse approach (INDIA). The efficacy of the approach is illustrated by choosing amorphous silicon as an example, which is particularly difficult to model using total-energy-based relaxation methods. We demonstrate that, by introducing a subspace optimization technique that sequentially optimizes two objective functions (involving experimental diffraction data, a total-energy functional, and a few geometric constraints), it is possible to produce models of amorphous silicon with very little or no coordination defects and a pristine gap around the Fermi level in the electronic spectrum. The structural, electronic, and vibrational properties of the resulting INDIA models are shown to be fully compliant with experimental data from x-ray diffraction, Raman spectroscopy, differential scanning calorimetry, and inelastic neutron scattering measurements. A direct comparison of the models with those obtained from the Wooten-Winer-Weaire approach and from recent high-quality molecular-dynamics simulations is also presented.

Sensing techniques based on the negatively charged nitrogen-vacancy (NV) center in diamond have emerged as promising candidates to characterize ultrathin and $2\mathrm{D}$ materials. An outstanding challenge to this goal is isolating the contribution of $2\mathrm{D}$ materials from undesired contributions arising from surface contamination and changes to the diamond surface induced by the sample or transfer process. Here we report on such a scenario, in which the abrasive deposition of trace amounts of materials onto a diamond gives rise to a previously unreported source of magnetic noise. By deliberately scratching the diamond surface with macroscopic blocks of various metals (Fe, Cu, Cr, Au), we are able to form ultrathin structures (i.e., with thicknesses down to $<1\mathrm{nm})$, and find that these structures give rise to a broadband source of noise. Explanation for these effects are discussed, including spin and charge noise native to the sample and/or induced by sample-surface interactions, and indirect effects, where the deposited material affects the charge stability and magnetic environment of the sensing layer. This work illustrates the high sensitivity of NV noise spectroscopy to ultrathin materials down to subnanometer regimes—a key step toward the study of $2\mathrm{D}$ electronic systems—and highlights the need to passivate the diamond surface for future sensing applications in ultrathin and 2D materials.

In experiments, strontium cobaltite (${\mathrm{SrCoO}}_3$) has been confirmed to be a ferromagnetic metal (Curie temperature $T_C\approx 305\mathrm{K}$) at ambient conditions and remains in cubic perovskite structure up to $\sim 60\mathrm{GPa}$. Using local density approximation + self-consistent Hubbard $U$ ($\mathrm{LDA}+U_{\mathrm{sc}}$) calculations, we show that ferromagnetic metallic ${\mathrm{SrCoO}}_3$ at low pressure is in an intermediate-spin (IS) state with $d^6\underline{L}$ character: nearly trivalent (${\mathrm{Co}}^{3+}$) instead of tetravalent cobalt (${\mathrm{Co}}^{4+}$) accompanied by spin-down O-$2p$ electron holes (ligand holes $\underline{L}$). Our calculations further predict that upon compression ($\gtrsim 7\mathrm{GPa}$), ${\mathrm{SrCoO}}_3$ undergoes a transition to a low-spin (LS) ferromagnetic half-metal with an energy gap opened in the spin-up channel. Compared to the metallic IS state, the half-metallic LS state exhibits even more prominent $d^6\underline{L}$ character, including nearly nonmagnetic ${\mathrm{Co}}^{3+}$ and exceptionally large oxygen magnetic moments, which contribute most of the magnetization. By analyzing x-ray diffraction data of compressed single-crystal ${\mathrm{SrCoO}}_3$, we point out an anomalous volume reduction of $\sim 1$%. This previously unnoticed volume anomaly is in great agreement with our predictive calculations, providing quantitative evidence for the simultaneous metal–half-metal and spin transition in ${\mathrm{SrCoO}}_3$.

The Cairo tessellation refers to a pattern of type 2 pentagons that can tile an infinite plane without creating a gap or overlap. We reveal the hidden, layered Cairo tessellations in the pyrite structure with a general chemical formula $A{B}_2$ and space group $pa\overline{3}$. We use this hidden tessellation along with density functional theory calculations to examine the possibility of obtaining a two-dimensional (2D) material with the Cairo tessellation from the bulk, using ${\mathrm{PtP}}_2$ as an example. Unlike previously reported single-layer materials such as ${\mathrm{PdSe}}_2$ with a buckled, pentagonal structure—strictly speaking, not belonging to the Cairo tessellation, we find that single-layer ${\mathrm{PtP}}_2$ is completely planar exhibiting dynamically stable phonon modes. We also observe a reduction in the band gaps of ${\mathrm{PtP}}_2$ from bulk to single layer using the Heyd-Scuseria-Ernzerhof (HSE) hybrid density functional, and the band gap type switches from indirect to direct. By contrast, using the standard Perdew-Burke-Ernzerhof functional leads to the conclusion of single-layer ${\mathrm{PtP}}_2$ being metallic. We further study the bonding characteristics of this novel single-layer material by computing the Bader charge transfer, the electron localization function, and the crystal orbit overlap population, which show mixed P-P covalent bonding and Pt-P ionic bonding, with the former being stronger. Finally, we study the surface states of single-layer ${\mathrm{PtP}}_2$ and consider the spin-orbit coupling. We observe no spin-helical Dirac cone states, therefore ruling out single-layer ${\mathrm{PtP}}_2$ as a topological insulator. We expect the example demonstrated in this work will stimulate interest in computationally identifying novel 2D materials from a variety of bulk materials with the pyrite structure.

Single-wall carbon nanotubes (SWCNTs) exhibit a wide range of physical phenomena depending on their chirality. Nanotube networks typically contain a broad mixture of chiralities, which prevents an in-depth understanding of SWCNT ensemble properties. In particular, electronic-type mixing (the simultaneous presence of semiconductor and metallic nanotubes) in SWCNT networks remains the single largest hurdle to developing a comprehensive view of ensemble nanotube electrical transport, a critical step toward their use in optoelectronics. Here, we systematically study temperature-dependent magnetoconductivity (MC) in networks of highly enriched semiconductor and metal SWCNT films. In the semiconductor-enriched network, we observe two-dimensional variable-range hopping conduction from 5 to 290 K. Low-temperature MC measurements reveal a large, negative MC from which we determine the wave-function localization length and Fermi energy density of states. In contrast, the metal-enriched film exhibits positive MC that increases with decreasing temperature, a behavior attributed to two-dimensional weak localization. Using this model, we determine the details of the carrier phase coherence and fit the temperature-dependent conductivity. These extensive measurements on type-enriched SWCNT networks provide insights that pave the way for the use of SWCNTs in solid-state devices.

First-principles density-functional theory is used to study the phase stability/instability and anomalies in the formation of the high-temperature bcc phases of $X\mathrm{Zn}$ ($X$ = Cu, Ag, and Au) alloys. Although from perhaps a naive point of view, their properties are expected to monotonically depend on the noble-metal ($X$) column position in the periodic table, this is not the case. For example, the middle-column AgZn alloy has a lower bcc order-disorder (critical) temperature than the CuZn and AuZn alloys above and below in the column. It is shown that this and other nonmonotonic behaviors can be explained in terms of a competition between atomic-size effects and $X$-atom $d$-orbital spatial extent. For example, charge-density studies and pair-potential modeling of $X\mathrm{Zn}$ alloys show that the effective Ag-Zn bond is significantly weaker than either the Cu-Zn or Au-Zn bond at their respective equilibrium lattice constants. We find that an increased atomic-core size effect initially weakens the $X$-Zn bonding as one goes from CuZn to AgZn, but then the larger $d$-orbital spatial extent for higher principal quantum numbers becomes a more dominant effect and increases the bonding from AgZn to AuZn. This study is focused on the highly symmetric cubic high-temperature phases, where relative bond-strength magnitudes should be far more important than any bond-directionality effects; the lattice parameters, bulk moduli, elastic constants, Debye temperatures, heats of formation, and order-disorder temperatures for the bcc phases of the three $X\mathrm{Zn}$ alloys are calculated and compared with experiment.

The properties of two-dimensional (2D) transition metal dichalcogenide (TMD) monolayers can be dynamically controlled via strain-induced displacive structural transformations between semiconducting (H) and metallic or semimetallic (${\mathrm{T}}^{\prime }$) crystal structures. The shapes, symmetries, and kinetics of crystalline domains generated during these transformations and the mechanical response of transforming monolayers are of fundamental and applied interest in, e.g., phase change memory devices and the study of topologically protected edge states in quantum spin Hall insulating ${\mathrm{T}}^{\prime }$ crystals. We quantitatively characterize ${\mathrm{T}}^{\prime }$ domain morphologies during $\text{H}\to {\mathrm{T}}^{\prime }$ transformations in both flat and bendable TMD monolayers using a combination of first principles and continuum calculations. Wulff constructions for ${\mathrm{MoTe}}_2$ and ${\mathrm{MoS}}_2$ show that ${\mathrm{T}}^{\prime }$ domains within much larger ${\mathrm{T}}^{\prime }$ domains are either rhombi of fixed proportions (if nonmisfitting) or rectangles whose aspect ratio $A_R$ increases with domain size $L_0$ (if misfitting). Isolated ${\mathrm{T}}^{\prime }$ domains within much larger H domains undergo a morphological crossover from compact to elongated shapes at $L_0\approx 100\text{–}200$ nm if the sheet is constrained to be flat or $L_0\gtrsim 2\mu \mathrm{m}$ if the sheet is free to bend. This crossover is driven by a competition between anisotropic interfacial energy and elastic misfit energy, and its position can be tuned via the monolayer-substrate interaction strength. It is shown that the aspect ratio $A_R$ obeys a scaling law $A_R\sim L_0^{2/3}$. Stress-strain response characterized as a function of strain orientation reveals extreme anisotropy in the effective elastic modulus through $\mathrm{H}/{\mathrm{T}}^{\prime }$ coexistence. Ferroelastic multidomain ${\mathrm{T}}^{\prime }\text{−}{\mathrm{WTe}}_2$ monolayers are found to exhibit two to three regimes of reversible mechanical response, and localized buckling in freely suspended ${\mathrm{T}}^{\prime }$ monolayers is shown to qualitatively alter ${\mathrm{T}}^{\prime }$ domain symmetries.

Rare-earth-based (RE) nanolaminates have attracted attention recently because of their complicated magnetism and their potential as precursors for strongly correlated two-dimensional materials. In this work, we synthesized a class of nanolaminates with a $\mathrm{M}{\mathrm{o}}_4\mathrm{R}{\mathrm{E}}_4\mathrm{A}{\mathrm{l}}_7{\mathrm{C}}_3$ chemistry, where RE = Ce or Pr. Powder samples of both phases were characterized with respect to structure and composition. Single crystals of $\mathrm{M}{\mathrm{o}}_4\mathrm{C}{\mathrm{e}}_4\mathrm{A}{\mathrm{l}}_7{\mathrm{C}}_3$ were used for magnetization measurements. The crystal structure was investigated by means of x-ray diffraction and scanning transmission electron microscopy. Magnetization analysis reveals a ferromagnetic ground state with a Curie temperature of $\sim 10.5$ K. X-ray absorption near-edge structure provides experimental evidence that Ce is in a mixed-valence state. X-ray magnetic circular dichroism shows that only the Ce atoms with $4f^1$ configuration occupying one of the two possible sites are ferromagnetically coupled, with a saturation moment of $\sim 1.2{\mu }_B$ per atom. We thus classify $\mathrm{M}{\mathrm{o}}_4\mathrm{C}{\mathrm{e}}_4\mathrm{A}{\mathrm{l}}_7{\mathrm{C}}_3$ as a ferromagnetic, mixed-valence compound.

We report on the formation of regular surface ripples accompanying microscratching of vitreous silica using Berkovich diamond tips. The tips were pulled laterally along the surface with varying normal load of up to 30 mN and scan velocity of up to $500\mu \mathrm{m}/\mathrm{s}$. Post mortem AFM analysis and independent confirmation through laser scanning microscopy reveal the occurrence of regular surface ripples in the wear grooves. The average repetition distance of the ripples is in the sub-$\mu \mathrm{m}$ range and increases linearly with the scan velocity. Their corrugation is on the order of a few nm, increasing with the applied load, and well below the indentation depth (of a few hundred nm). The experimental results are interpreted assuming that the tip moves in a stick-slip fashion, while it is elastically driven along the surface. In this framework the static friction to be overcome in the stick phase corresponds to the lateral hardness of the material, whereas the kinetic friction slowing down the tip results from the plowing motion of the latter.