The boundary between carbon nanotubes is an important factor affecting the transport properties of carbon-nanotube-based materials. Recently, carbon nanotube bundles have been shown to have very high thermal anisotropy due to boundaries at $1000{}^{\circ }\mathrm{C}$. However, it is unclear whether the anisotropy is peculiar to this high-temperature region. Here, we conduct an in situ study of the anisotropic thermal properties of bundles of carbon nanotubes by utilizing the phase transition of Sn nanoparticles. The results indicate that the anisotropic thermal properties are not unique to $1000{}^{\circ }\mathrm{C}$. Moreover, observation of two different phase transitions of Sn particles enables the direct comparison of two temperature regions and indicates that carbon nanotube bundles have higher anisotropy at lower temperatures. A simple finite element calculation suggests that the size-dependent thermal conductivity of carbon nanotube bundles can be attributed to the extremely large anisotropy estimated in this study.

Charged molecular species, such as ${{\left[{\mathrm{N}}_2\right]}^x}^{\text{–}}$, ${{\left[{\mathrm{O}}_2\right]}^x}^{\text{–}}$, ${{\left[{\mathrm{C}}_2\right]}^x}^{\text{–}}$, and ${{\left[{\mathrm{S}}_2\right]}^x}^{\text{–}}$, follow the paradigm of carrying integer values of electrons. Here, the ${\mathrm{Na}}_3{\left({\mathrm{N}}_2\right)}_4$, ${\mathrm{Ca}}_3{\left({\mathrm{N}}_2\right)}_4$, ${\mathrm{Sr}}_3{\left({\mathrm{N}}_2\right)}_4$, and $\mathrm{Ba}{\left({\mathrm{N}}_2\right)}_3$ compounds were produced and characterized <70 GPa and evidenced to be composed of paradigm-breaking ${{\left[{\mathrm{N}}_2\right]}^x}^{\text{–}}$ dimers with noninteger charges of −0.75, −1.5, −1.5, and −0.67, respectively. The anion-driven metallicity of the compounds is proposed as the physical mechanism enabling the noninteger electron count of the ${{\left[{\mathrm{N}}_2\right]}^x}^{\text{–}}$ dimers. The properties of these dimers and the compounds bearing them are demonstrated to depend on their noninteger charge, paving the way to materials with electron-tunable features.

We study the structural and transport properties of a heavily oxygen-reduced region that is formed by electrical injection of a large amount of oxygen vacancies into a ${\mathrm{Bi}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{FeO}}_{3-\delta }$ thin film. In electroforming, the epitaxial as-grown state is transformed into a disordered, polycrystalline phase. Nonohmic current-voltage relations, which can be interpreted as a space-charge-limited conduction, appear at low temperatures. As temperature increases, the curvature of the current-voltage relationship gradually changes to be nearly ohmic at and above 100 K. The maximum value of magnetoresistance, as large as −2.6% at 90 kOe, is attained at the same temperature of ∼100 K, which is significantly larger than −0.1% at 25 K and −1.3% at 300 K.

The transition-metal-intercalated dichalcogenide ${\mathrm{CoNb}}_3{\mathrm{S}}_6$ is a triangular antiferromagnet (AFM) that has recently been shown to exhibit a large anomalous Hall effect (AHE) below the Néel temperature, even though the response to an external field is very small. This suggests that there is an interesting magnetic structure that interacts with the electronic structure to yield the AHE, as collinear AFMs cannot exhibit a nonzero AHE. We propose a model for magnetic transition-metal-intercalated dichalcogenides and examine its ground state as a function of interaction parameters. The model exhibits transitions between planar spin spirals, nonplanar spin spirals, and a particular noncoplanar so-called $3q$ state. This latter state must exhibit a nonzero AHE, while the spin spirals do not.

Understanding and controlling the morphology of crystals growing from prepatterned shapes, as in selective-area epitaxy, is fundamental for the development of novel device architectures. Here, an in-depth analysis of the faceted growth of ring-shaped crystal structures is presented. This is a prototypical case for assessing the role of thermodynamic and kinetic driving forces, as they induce different faceting between the outer convex region and the inner concave one. The well-established concepts of equilibrium crystal shape, set by surface energy minimization, and its kinetic counterpart, determined by orientation-dependent growth rates, provide a qualitative indication of the outcome of growth experiments. However, they are insufficient to capture the whole evolution pathway. A phase-field growth model including deposition, incorporation, and surface diffusion dynamics is considered, and extensive two- and three-dimensional simulations are performed. A continuum transition in the crystal morphology is recognized by varying the magnitude of the anisotropic surface energy vs the incorporation times. The in-depth analysis of a realistic case, mimicking experiments in the literature, highlights the prominent role of kinetics and of material redistribution from the outer to the inner perimeter of the ring. Finally, the templating effect of the initial pattern is investigated by considering polygonal profiles with different orientations, showing their effect on the facet formation and evolution.

A theory for the lateral strain-induced grain boundary instability is proposed by considering two crucial factors, namely, the dependence of the boundary energy on coincidence sites and the differences in elastic responses of the grain and the boundary regions. In contrast to the Asaro-Tiller-Grinfeld instability, where strains, however small, lead to breakup of the interfaces and the amplitudes of the perturbations are only a function of wavelength and time, we find that there exists a critical strain for the grain boundary instability to occur which is due to the periodicity of coincidence site lattices, and the growth rate of perturbations is dependent on the amplitude. These theoretical predictions are validated by the phase field crystal simulations in two dimensions. In addition, the amplitude-dependent growth rate gives rise to two distinct outcomes for the late stage evolution predicted by the proposed theory, namely, the grain boundary structural transformation and the dislocation emission, as seen in the phase field crystal simulations. Not only is the predicted oscillatory behavior of the growth rate observed in simulations, but also the phase diagram predicted by the theory is in quantitative agreement with the phase field crystal simulations.

Localized surface plasmon resonances (LSPRs) are fascinating optical phenomena occurring in metal nanostructures, like gold nanoparticles (NPs). Plasmonic excitation can be tailored efficiently in the visible range by acting on size, shape, and NP surroundings, whereas carrier density is fixed, thus restricting the LSPR modulation. Transparent conductive oxides (TCOs), on the other hand, are gaining increasing interest for their transparency, charge carrier tunability, and plasmonic features in the infrared. The combination of these two materials into a metal-TCO nanocomposite can give access to unique electrical and optical characteristics, to be tailored in view of the desired optoelectronic application. In this paper, Au NPs and Ta-doped ${\mathrm{TiO}}_2$ TCO films have been merged with the aim to master the Au plasmon resonance by acting on the dielectric properties of the surrounding TCO. Morphology, structure, and electrical properties have been investigated as well to understand the optical response of the nanosystems. The role of the embedding geometry has been explored, revealing that the largest LSPR shift (550–760 nm) occurs when the NPs are sandwiched in the middle of the film and not at the bottom of the film (substrate-film interface). Ta doping in the TCO has been varied (5–10% at. and bare ${\mathrm{TiO}}_2$) to induce a permittivity change of the matrix. As a result, Au LSPR is clearly blueshifted when decreasing the dielectric permittivity at higher Ta content in the sandwich configuration. Despite the nonoptimal electrical performance caused by defectivity of the films, Au-$\mathrm{Ta}:\mathrm{Ti}{\mathrm{O}}_2$ multifunctional nanocomposites are promising candidates for their optical behavior as highly tunable plasmonic conductive metamaterials for advanced light management.

In recent years, superlattices and layered materials have been highlighted as potential candidates for thermoelectric applications, this thanks to their low thermal conductivity. Moreover, external applied pressure and biaxial strain can be used to enhance their properties by achieving a band engineering and electronic tuning. With this in mind, we performed an $ab$ initio based study on InSe/GaSe superlattices under biaxial strain along the layer planes. Layers of InSe and GaSe with $D_{3h}$ symmetry were stacked along the $c$ axis to create the superlattice. The atomic stacking along the $c$ axis is Se-Ga-Ga-Se-Se-In-In-Se, which corresponds to the space group #187. Our ab initio calculations predict the superlattice to be a semiconductor with an electronic band gap of $E_g$ = 0.54 eV. With the aim to increase thermoelectric performance, we apply positive and negative biaxial strain on the ab plane. Under compressive strain, the electronic structure evolves to an insulating behavior by increasing the band gap. When tensile strain is applied, we observe a transition towards a metallic character with a systematic reduction of the band gap. Interestingly, the semiconductor-metal transition only occurs when spin-orbit coupling (SOC) is switched off. With the inclusion of SOC, the system experiences an electronic topological transition around 3% tensile strain, with a double band gap along the $K\text{−}\mathrm{\Gamma }\text{−}M$ high-symmetry paths. We have found that for both, $n$-type and $p$-type doping, compressive strain improves the electronic figure of merit ($Z{T}_e$ under constant relaxation time approximation). Not only the electronic part increases thermoelectric performance, but also the lattice contribution to the thermal conductivity decreases with compressive strain.

Inspired by the observation of the extremely large anomalous Hall effect in the absence of applied magnetic fields or uniform magnetization in ${\mathrm{CoNb}}_3{\mathrm{S}}_6$ [Nat. Commun. 9, 3280 (2018); Phys. Rev. Research 2, 023051 (2020)], we perform a first-principles study of this and related compounds of the $M{\mathrm{Nb}}_3{\mathrm{S}}_6$ type with different transition metal $M$ ions to determine their magnetic orders and the anomalous Hall conductivity (AHC). We find that noncoplanar antiferromagnetic ordering is favored relative to collinear or coplanar order in the case of $M$ = Co, Fe, and Ni, while ferromagnetic ordering is favored in ${\mathrm{MnNb}}_3{\mathrm{S}}_6$ at low temperatures. The AHC in these materials with noncoplanar spin ordering can reach about $e^2/h$ per crystalline layer, while being negligible for coplanar and collinear cases. We also find that the AHC depends sensitively on doping and reaches a maximum for intermediate values of the local spin exchange potential between 0.3 and 0.8 eV. Our AHC results are consistent with the reported Hall measurements in ${\mathrm{CoNb}}_3{\mathrm{S}}_6$ and suggest a possibility of similarly large anomalous Hall effects in related compounds.

The ongoing quest for unambiguous signatures of topological superconductivity and Majorana modes in magnet-superconductor hybrid systems creates a high demand for suitable superconducting substrates. Materials that incorporate $s$-wave superconductivity with a wide energy gap, large spin-orbit coupling, and high surface quality, which enable the atom-by-atom construction of magnetic nanostructures using the tip of a scanning tunneling microscope, are particularly desired. Since single materials rarely fulfill all these requirements, we propose and demonstrate the growth of thin films of a high-Z metal, Ir, on a surface of the elemental superconductor with the largest energy gap, Nb. We find a strained Ir(110)/Nb(110)-oriented superlattice for thin films of one to two atomic layers, which transitions to a compressed Ir(111) surface for thick films of ten atomic layers. Using tunneling spectroscopy, we observe proximity-induced superconductivity in the latter Ir(111) film with a hard gap $\mathrm{\Delta }$ that is $85.3\%$ of that of bare Nb(110).

We report on the depth-sensitive, temperature-dependent exchange coupling in an FePt/FeRh thin-film exchange-spring structure. The depth-dependent in-plane magnetization is measured as a function of applied magnetic field and sample temperature using polarized neutron reflectometry (PNR). The magnetization profiles are interpreted in terms of the competition between anisotropy, exchange coupling, and dipolar coupling as the FeRh undergoes the magnetic phase transition from antiferromagnetic to ferromagnetic ordering. The PNR data are combined with bulk magnetometry and x-ray characterization, allowing us to determine characteristic length scales over which the exchange-spring mechanism is effective at ambient and elevated temperatures.

On the basis of density functional calculations, we report on a comprehensive study of the influences of atomic arrangement and Ni substitution for Al on the ground-state structural and magnetic properties for ${\mathrm{Fe}}_2{\mathrm{Ni}}_{1+x}{\mathrm{Al}}_{1-x}$ Heusler alloys. We discuss systematically the competition between five Heusler-type structures formed by shuffles of Fe and Ni atoms and their thermodynamic stability. All Ni-rich ${\mathrm{Fe}}_2{\mathrm{Ni}}_{1+x}{\mathrm{Al}}_{1-x}$ tend to decompose into a dual-phase mixture consisting of ${\mathrm{Fe}}_2\mathrm{Ni}\mathrm{Al}$ and FeNi. The successive replacement of Ni by Al leads to a change of ground-state structure and eventually an increase in magnetocrystalline anisotropy energy (MAE). We predict for stoichiometric ${\mathrm{Fe}}_2\mathrm{Ni}\mathrm{Al}$ a ground-state structure with nearly cubic lattice parameters but alternating layers of Fe and Ni possessing a uniaxial MAE that is even larger than tetragonal $L{1}_0$-FeNi. This opens an alternative route for improving the phase stability and magnetic properties in FeNi-based permanent magnets.

Material data are used to accelerate the development of materials. For the crystalline solid domain, however, informatics schemes are challenged by the encoding of crystal structures because embedding the invariances with respect to translation, rotation, and unit-cell choice is intractable by data augmentation schemes. We propose an efficient search space in which identical structures are reduced given such crystal operators. This crystal-invariant search space is created by morphing between the topologies of known crystal structures in accordance with the gradient of a smooth overlap of atomic positions descriptor formulated in reciprocal space as a metric. This crystal morphing is examined using an identification task of the structures from an x-ray diffraction spectrum. By applying Bayesian optimization on the search space, good initial structures for the Rietveld analysis are achieved via automatic determination without expert operations. Our method is applicable in the autonomous search of crystals, various structural analyses, and design of metamaterials.

We report on the observation of the nonlinear intensity dependence of the bulk photogalvanic current and photoconductivity in the twisted graphene with small twist angles close to the second magic angle. We show that terahertz radiation results in photoresponses, which are caused by indirect optical transitions (free carrier absorption), direct interband transitions, and optical transitions between moiré subbands. The relative contribution of these absorption channels depends on the Fermi-level position with respect to the multiple moiré subbands of the twisted graphene. The interplay of these absorption channels results in oscillations of the photoresponses with variation of the gate voltage. We show that the photoresponse saturates at high intensities. For different absorption channels it has different intensity dependencies and saturation intensities. The latter depends nonmonotonically on the Fermi-level position, which is controlled by the gate voltage.

We study the differences in electrical charge transport dynamics of the conductivity enhancement of poly(3,4-ethylenedioxythiophene) (PEDOT) derivatives under geometrical confinement. The results of polymer blend poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and a polymer-monomer blend, poly(3,4-ethylenedioxythiophene):tosylate, highlight the role of dopants and processing conditions of these systems under confinement. The prevailing transport length scales in confined geometry of characteristic dimensions originate from varying disorder in these polymer systems. These observable differences in two different PEDOTs introduced by molecular level reorganization can be utilized to tune conducting polymer systems for efficient electrical and thermoelectric properties. The electrical conductivity σ of the polymer system, which is a function of the electronic structure at molecular level and a connectivity parameter, has been probed in cylindrical-alumina nanoscaffolds of various channel diameters, at different frequencies ω and temperatures $T$. The observations also emphasize the role of disorder in these conducting polymer systems.

Lithium boracite crystals have been identified as promising ion conductors for possible use in all-solid-state batteries. With the help of first-principles modeling techniques, we are able to show that these materials have structures with natural interstitial sites which play important roles in Li ion migration processes. The arrangements of these natural interstitial sites follow from group theory analyses of the computed and experimentally analyzed structures. Specifically, the low-temperature $\alpha$ phase of ${\mathrm{Li}}_4{\mathrm{B}}_7{\mathrm{O}}_{12}\mathrm{Cl}$ is computed to have the face-centered rhombohedral $R3c$ structure which is closely related to an ideal face-centered cubic $F\overline{4}3c$ structure with 24 natural interstitial sites per conventional unit cell. Li ion diffusion in this material is found to proceed largely by a concerted motion involving these interstitial sites and two neighboring host lattice sites, consistent with the large measured ionic conductivity of this material. Adding one addition ${\mathrm{Li}}_2\mathrm{O}$ cluster per formula unit to ${\mathrm{Li}}_4{\mathrm{B}}_7{\mathrm{O}}_{12}\mathrm{Cl}$ forms ${\mathrm{Li}}_5{\mathrm{B}}_7{\mathrm{O}}_{12.5}\mathrm{Cl}$, which crystallizes in the face-centered cubic $F23$ structure, a subgroup of the $F\overline{4}3c$ structure. While these $F23$ crystals have 16 natural interstitial sites per conventional unit cell, their distribution is such that they do not participate in Li ion diffusion mechanisms, as is consistent with the negligible ionic conductivity measured for this material. Phonon spectral analyses of ${\mathrm{Li}}_4{\mathrm{B}}_7{\mathrm{O}}_{12}\mathrm{Cl}$ in its $R3c$ structure and ${\mathrm{Li}}_5{\mathrm{B}}_7{\mathrm{O}}_{12.5}\mathrm{Cl}$ in its $F23$ structure show that both crystals are dynamically stable. Chemical stability of these materials is indicated by convex hull analysis of ${\mathrm{Li}}_4{\mathrm{B}}_7{\mathrm{O}}_{12}\mathrm{Cl}$ and ${\mathrm{Li}}_5{\mathrm{B}}_7{\mathrm{O}}_{12.5}\mathrm{Cl}$ with respect to their building blocks of LiCl, ${\mathrm{Li}}_2\mathrm{O}$, and ${\mathrm{B}}_2{\mathrm{O}}_3$.

A polycrystalline sample of the hyperhoneycomb iridate $\beta \text{−}{\mathrm{ZnIrO}}_3$ was synthesized via a topochemical reaction, and its structural, magnetic, and thermodynamic properties were investigated. The magnetization and heat capacity data show the absence of long-range magnetic order at least down to 2 K. A positive Curie-Weiss temperature ${\theta }_{\mathrm{W}}\sim 45\mathrm{K}$ probed by the temperature dependence of magnetic susceptibility indicates that a Kitaev interaction is dominant. These observations suggest that a quantum spin liquid may have been realized. Furthermore, the observation of linear temperature-dependent contribution to the heat capacity with no magnetic field effect evidences gapless excitation. These facts are surprisingly contrary to the chemical disorder evidenced by the crystallographic analysis. By discussing the differences in the size effect of ${\mathrm{Z}}_2$ fluxes in two- and three-dimensional Kitaev magnets, we propose that there is a hidden mechanism to protect the quantum spin liquid state from chemical disorder.