Epitaxial metal/semiconductor superlattices with atomically sharp interfaces and tunable Schottky barrier heights have attracted significant interest in recent years for thermionic emission-based high-temperature thermoelectric devices, optical hyperbolic metamaterials, hot-electron photocatalysis, and optoelectronic heterostructures for visible-to-terahertz frequency range applications. ZrN/ScN is a demonstration of such epitaxial metal/semiconductor superlattices and exhibits atomically sharp lattice-matched interfaces, albeit with the presence of threading dislocations on MgO substrates. Along with its influence on structural integrity and atomic diffusion, the presence of such defects significantly impacts electron and phonon transport in these metamaterials with carrier trapping, scattering, shunt path, etc. Therefore, an in-depth analysis of the atomistic structure and the composition of such defects is extremely necessary to design devices with improved efficiencies. In this paper, high-resolution scanning transmission electron microscopy and atom-probe tomography are employed to determine the structure and three-dimensional (3D) spatial distribution of oxide defect clusters along the voids in ZrN/ScN superlattices. $\mathrm{Sc}{\mathrm{O}}^{+}$ and $\mathrm{Sc}{\mathrm{O}}^{++}$ ions are found to cluster predominantly along such 3D interface defects with zirconium and scandium atoms surrounding them. Defect regions are also found to be depleted of nitrogen atoms and rich with a high concentration of oxygen. The oxygen content was found to be higher inside the ScN layers compared to ZrN. First-principles modeling analysis verified the clustering of oxygen at high oxygen partial pressure and demonstrated a higher affinity of scandium towards oxygen than for zirconium towards oxygen. These results mark significant progress in understanding the atomic structure and composition of defects in nitride superlattices.

This paper reports diffused metal-insulator transitions (MITs) in an oxide with disorder that undergoes a Mott transition. The investigation was carried out in the multilayer film ${\text{NdNiO}}_3/{\text{BaTiO}}_3/{\text{SrTiO}}_3$ (NNO/BTO/STO), where a large mismatch of lattice constants of NNO with those of BTO leads to strain relaxation and creation of quenched disorder in the NNO film. NNO film in the NNO/BTO/STO multilayer structure shows a broad Mott-type MIT at a temperature $T_{\text{MI}}=160\text{K}$ from a high-temperature bad metallic phase ($\frac{1}{{\rho }_{\text{DC}}}\frac{d{\rho }_{\text{DC}}}{dT}<0$) with a high value of resistivity ${\rho }_{\text{DC}}\approx 70 \mathrm{m}\mathrm{\Omega }$ cm at 300 K to a low temperature insulating phase. Using noise spectroscopy and impedance spectroscopy which can probe the dynamics of the coexisting phases near the MIT, it was observed that in addition to the MIT at $T_{\text{MI}}=160\text{K}$, there exists a characteristic temperature $T_G\approx 230\text{K}$ well above the $T_{\text{MI}}$, where large low-frequency correlated fluctuations appear, signifying the appearance of a phase with slow dynamics. $T_G$ signals the onset of a temperature region $T_{\text{MI}}<T<T_G$ with coexisting phases that have been corroborated by the impedance spectroscopy and AC conductivity measurements. It is suggested that the temperature $T_G$ may signify the onset of an electronic Griffiths phase that has been theoretically proposed for Mott transitions with disorder.

The addition of nitrogen as a dopant in monolayer graphene is a flexible approach to tune the electronic properties of graphene as required for applications. Here, we investigate the impact of the doping process that adds N dopants and defects on the key electronic properties, such as the mobility, the effective mass, the Berry phase, and the scattering times of the charge carriers. Measurements at low temperatures and magnetic fields up to 9 T show a decrease of the mobility with increasing defect density due to elastic, short-range scattering. At low magnetic fields weak localization indicates an inelastic contribution depending on both defects and dopants. Analysis of the effective mass shows that the N dopants decrease the slope of the linear bands, which are characteristic for the band structure of graphene around the Dirac point. The Berry phase, however, remains unaffected by the modifications induced through defects and dopants, showing that the overall band structure of the samples is still exhibiting the key properties as expected for Dirac fermions in graphene.

Nodal ring phonons with topologically nontrivial properties have triggered much interest in recent years. Unlike fermions inevitably affected by the spin-orbit interaction, nodal ring phonons are topologically robust. In this paper, we investigate the classification of nodal rings in phonon systems based on symmetry arguments and $\mathit{k}·\mathit{p}$ models. We identify that all the nodal ring phonons can be categorized into three types, i.e., in-plane nodal rings protected by mirror symmetry, twisted nodal rings protected by inversion and time-reversal symmetries, and the hybrid type protected by a combination of these symmetries. On the basis of first-principles calculations, we propose that these three types of nodal ring phonons can emerge in two different phases of silver oxide. The calculation results also show clear drumheadlike surface states along high-symmetry paths and nontrivial arc states in the isofrequency surfaces. This work exposes various appearances of nodal rings and also promotes the development of topological phonons.

Elastic moduli, fracture toughness, crack initiation resistance, and hardness are four important mechanical properties of oxide glasses. However, the correlation between them as well as their relations to glass composition and extrinsic factors are not yet fully understood. In this study, we investigate the variations in these mechanical properties in mixed ${\mathrm{Cs}}_2\mathrm{O}/{\mathrm{Li}}_2\mathrm{O}$-aluminoborate glasses, which are subjected to aging in a humid atmosphere. We observe pronounced nonlinear scaling of the mechanical properties due to the mixed alkali effect, and following the surface hydration, we observe an increase in crack resistance and a decrease in Vickers hardness. These variations are explained by Raman spectroscopy data collected at different depths within the hydrated surface. We found no clear relations between Vickers hardness, crack resistance, and fracture toughness, although Vickers hardness appears to increase and crack resistance appears to decrease with increasing fracture toughness for the present glass series. The glass with equal contents of Cs and Li provides the best compromise between high fracture toughness and Vickers hardness and with a potential to increase the crack resistance through humid aging.

Vibrating sample magnetometery (VSM) combined with Brillouin light scattering (BLS) were used to investigate the Dzyaloshinskii-Moriya interaction (iDMI), the perpendicular magnetic anisotropy (PMA) and the damping in ${\mathrm{Co}}_{0.5}{\mathrm{Fe}}_{0.5}$-based systems grown by sputtering on $\mathrm{Si}/{\mathrm{SiO}}_2$ substrates, using Pt or Cu buffer layers and Hf capping layer of variable thickness $(0\le t_{\mathrm{Hf}}\le 2\mathrm{nm})$. VSM measurements revealed a significant decrease of the areal magnetic moment at saturation as $t_{\mathrm{Hf}}$ increases, most likely due to the increase of the magnetic dead layer thickness at CoFe/Hf interface up to $0.73\pm 0.06\mathrm{nm}$ for $t_{\mathrm{Hf}}=2\mathrm{nm}$. Damping measurements allowed concluding to the weaker spin pumping contribution by the CoFe/Hf interface compared to Pt/CoFe. The analysis of the $t_{\mathrm{Hf}}$ dependence of damping within a model considering the contribution of each interface allowed us to determine the spin diffusion length of Hf, found to be 1.7 nm. The Hf thickness dependence of iDMI and PMA constants revealed significant (weak) contribution of the CoFe/Hf interface to PMA (iDMI). The surface iDMI constant of CoFe/Hf interface was estimated to be $(-0.37\pm 0.12)\times {10}^{–7}\mathrm{erg}/\mathrm{cm}$. This suggests that the iDMI sign of Hf and Pt are opposite, and an additive behavior can be obtained when they are placed in opposite sides of the CoFe layer, allowing thus to strengthen the iDMI, needed for some applications. Quadratic correlations between PMA and iDMI constants and linear dependence of damping versus PMA constant were obtained confirming their relationship with the spin-orbit coupling.

Different from the prototypical elemental semiconductors such as Si and Ge, chalcogenide-based phase-change materials (PCMs) generally show very high resistivity contrast between the amorphous and crystalline phases. In contrast to conventional PCMs, such as Ge-Sb-Te alloys, where the amorphous phase possesses higher resistivity, ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ (CrGT) exhibits the opposite dependence. Namely, the amorphous phase is characterized by a lower resistivity than the crystalline phase. Although density functional theory calculations suggest that Cr clusters are responsible for the low resistivity of amorphous CrGT, the effects of composition on the electrical properties have yet to be investigated. In this work, the dependence of the electrical properties on Cr content and the role of the Cr clusters were investigated experimentally using Hall effect, hard x-ray photoelectron spectroscopy (HAXPES), and optical property measurements. The electrical properties were found to be dependent on the Cr content. From a HAXPES core-level spectra analysis, it was found that the increased carrier density correlated with the extent of Cr clusters, indicating that the hole carriers present likely originated from Cr clusters. The increased concentration of Cr clusters was also found to lead to a shift of the valence band edge toward the Fermi level as well as to a decrease in the optical band gap. It has been suggested that the Cr clusters may induce the formation of new energy states close to the valence band edge. These results indicate that the Cr clusters play an essential role in determining the electrical properties of amorphous CrGT, and that tuning the film composition is an effective way to optimize device properties for nonvolatile memory applications.

The search of new superhard materials has received a strong impulse by industrial demands for low-cost alternatives to diamond and $c$-BN, such as metal borides. In this Letter we introduce a new family of superhard materials, “fused borophenes” (FBs), containing 2D boron layers that are interlinked to form a 3D network. These materials, identified through a high-throughput scan of ${\mathrm{B}}_x{\mathrm{C}}_{1-x}$ structures, exhibit Vicker's hardness comparable to those of the best commercial metal borides. Due to their low formation enthalpies, FBs could be synthesized by high-temperature methods, starting from appropriate precursors, or through quenching of high-pressure phases.

In this work, we have developed a mathematical framework to investigate the deformation response under Berkovich and spherical indenters. Nanoindentation experiments are carried out on two single-crystal magnesium (Mg) samples oriented for basal slip and tensile twinning. The indentation load vs penetration depth curves are then analyzed to explain the pop-in events with the help of Schmid factor analysis and to identify the favorable deformation modes directly beneath and around the indentation. These results explain why there is only a small difference in the values of hardness between the two orientations despite having a significant difference in their crystallographic orientations. In addition to this, the role of indenter tip radius on the formation of extension twins when indented along the crystallographic $c$ axis is analyzed with the help of a simple analytical model.

The mechanical behavior of materials operating under high temperatures is strongly influenced by creep mechanisms such as dislocation climb, which is controlled by the diffusion of vacancies. However, atomistic simulations of these mechanisms have traditionally been impractical due to the long time scales required. To overcome these time scale challenges, we use Parallel Trajectory Splicing (ParSplice), an accelerated molecular dynamics method, to simulate dislocation climb in nickel. We focus on modeling the activity of a vacancy near a jog on an edge dislocation in order to observe vacancy pipe diffusion and vacancy absorption at the jog. From rigorously constructed trajectories encompassing more than 2000 vacancy absorption events over a simulation time of more than $4\mu \mathrm{s}$ at 900 K, a comprehensive sampling of available atomistic mechanisms is collated and analyzed further with molecular statics calculations. We estimate average rates for pipe diffusion and vacancy absorption into the jog using data from the dynamic and static calculations, finding very good agreement. Our results strongly suggest that the dominant mechanism for vacancy absorption by jogs is via biased diffusion to the dislocation core followed by fast pipe diffusion to the jog.

Oxidation and tarnishing are the two most common initial steps in the corrosive process of metals at ambient conditions. These are always initiated with O and S binding to a metallic surface, so that one can use the binding energy as a rough proxy for the metal reactivity. With this in mind, we present a systematic study of the binding energy of O and S across the entire transition metals' composition space, namely, we explore the binding energy of 88 single-phase transition metals and of 646 transition-metal binary alloys. The analysis is performed by defining a suitable descriptor for the binding energy. This is here obtained by fitting several schemes, based on the original Newns-Anderson model, against density functional theory data for the $4d$ transition-metal series. Such descriptor is then applied to a vast database of electronic structures of transition-metal alloys, for which we are able to predict the range of binding energies across both the compositional and the structural spaces. Finally, we extend our analysis to ternary transition-metal alloys and identify the most resilient compounds to O and S binding.

Structural superlubricity is one of the most fascinating tribological concepts, describing the state of almost vanishing friction, which arises from the cancellation of the lateral interactions between incommensurate rigid surfaces. In this paper, the dependence of friction on sliding directions is experimentally studied under ambient conditions using state of the art friction force microscopy of misaligned microscale heterogeneous contacts between the single crystalline $h\mathrm{BN}$ and graphite. The observed minor frictional anisotropy (≤20%) demonstrates that the microscale superlubric translational motion is maintained along all sliding directions. With atomistic simulations, we further show that the frictional anisotropy results from a change in the corrugation of the sliding potential energy surface with changing the sliding direction. Our results validate the robustness of the structural superlubricity against the sliding direction, and provide an effective approach to precise friction control.

A Weyl semimetal with strong electron-phonon interaction can show axionic coupling in its insulator state at low temperatures, owing to the formation of a charge density wave (CDW). Such a CDW emerges in the linear-chain-compound Weyl semimetal ${\mathrm{Ta}}_2{\mathrm{Se}}_8\mathrm{I}$ below 263 K, resulting in the appearance of the dynamical condensed-matter axion quasiparticle. In this paper, we demonstrate that the interchain coupling in ${\mathrm{Ta}}_2{\mathrm{Se}}_8\mathrm{I}$ can be varied to suppress the CDW formation with pressure, while retaining the Weyl semimetal phase at high temperatures. Above 17 GPa, the Weyl semimetal phase does not survive, and we induce superconductivity, due to the amorphization of the iodine sublattice. Structurally, the quasi-one-dimensional Ta-Se chains remain intact and provide a channel for superconductivity. We highlight that our results show a near-complete suppression of the gap induced by the axionic charge density wave at pressures inaccessible to previous studies. Including this CDW phase, our experiments and theoretical predictions and analysis reveal the complete phase diagram of ${\mathrm{Ta}}_2{\mathrm{Se}}_8\mathrm{I}$ and its relationship to the nearby superconducting state. The results demonstrate ${\mathrm{Ta}}_2{\mathrm{Se}}_8\mathrm{I}$ to be a distinctively versatile platform for exploring correlated topological states.

Motivated by the variation in reported lattice parameters of floating-zone-grown ${\mathrm{Nd}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$ crystals, we have performed a detailed study of the relationship between synthesis environment, structural disorder, and magnetic properties. Using a combination of polycrystalline standards, electron-probe microanalysis, and scattering techniques, we show that crystals grown under atmospheric conditions have a reduced lattice parameter relative to pristine polycrystalline powders due to occupation of the Nd site by excess Zr (i.e., negative stuffing). In contrast, crystals grown under high-pressure Ar are nearly stoichiometric with an average lattice parameter approaching the polycrystalline value. While minimal disorder of the oxygen sublattices is observed on the scale of the average structure, neutron pair-distribution function analysis indicates a highly local disorder of the oxygen coordination, which is only weakly dependent on growth environment. Most importantly, our magnetization, heat capacity, and single-crystal neutron scattering data show that the magnetic properties of crystals grown under high-pressure Ar match closely with those of stoichiometric powders. Neutron scattering measurements reveal that the signature of magnetic moment fragmentation—the coexistence of all-in-all-out (AIAO) magnetic Bragg peaks and diffuse pinch-point scattering due to spin-ice correlations–persists in these nearly stoichiometric crystals. However, in addition to an increased AIAO transition temperature, the diffuse signal is seemingly stabilized and remains nearly unchanged upon warming to 800 mK. This behavior indicates that both the AIAO magnetic order and spin-ice correlations are sensitive to deviations of the Nd stoichiometry.

A superconductor, NbReN, in thin film form is synthesized. Here, the deposition parameters and the electrical transport characterization of NbReN films grown by reactive dc sputtering using a NbRe target are reported. The deposition conditions are systematically varied to optimize the superconducting and electrical properties of the resulting samples. Films with polycrystalline structure and well established superconducting ordering are obtained. The results of the electrical transport properties are studied and interpreted in the framework of different theoretical models. From the analysis of the estimated microscopical parameters considerations about research perspectives as well as possible future applications of the material are discussed. In particular, the potential of NbReN thin films for the realization of SNSPDs with improved performances compared to the existing ones is evaluated.

Plasticity in solids is dependent on microstructural history, temperature, and loading rate, and sample-dependent knowledge of yield points in structural materials adds reliability to mechanical behavior. Yielding is commonly measured through controlled mechanical testing, in ways that either distinguish elastic (stress) from total deformation measurements or identify plastic slip contributions. In this paper, we show that yielding can be unraveled through statistical analysis of total-strain fluctuations during the evolution sequence of profiles, measured in situ, through digital image correlation. We demonstrate two distinct ways of quantifying yield locations in widely applicable crystal plasticity models for polycrystalline solids, using either principal component analysis or discrete wavelet transforms. We test and compare these approaches for synthetic data of polycrystals and a variety of yielding responses through changes in applied loading rates and strain-rate sensitivity exponents.

This paper reports a comprehensive study of electrical and thermal transport properties of a series of face-centered cubic structured high-entropy alloys by alloying Mn, Cr, and Pd elements in NiCo, NiFe, and NiCoFe alloys. X-ray diffraction revealed a single-phase Cu-type cubic structure, and scanning electron microscopy displayed elongated grained microstructures in all alloys. Like NiCo, NiFe, and NiCoFe alloys, the alloys containing Cr/Mn/Pd exhibit metallic behavior; however, their electrical transport properties, such as residual resistivity, residual resistivity ratio, and temperature coefficient of resistivity, vary significantly due to the increase of chemical disorder and defects. The analysis of resistivity of these alloys further showed different scattering mechanisms at low temperatures. Interestingly, the electrical resistivity of NiCoCr, NiCoFeCr, and NiCoFeMn alloys is nearly linear at low temperatures, most likely related to the Mott-Ioffe-Regel limit. Additionally, the NiCoMnCr and NiCoFeMnCr alloys exhibit a minimum in resistivity at low temperatures, which can be explained by the weak localization effect. The Seebeck coefficient measurements reveal that the charge carrier for thermoelectric transport in NiCo, NiFe, and NiCoFe is changed from electrons to holes with Mn alloying. In contrast, a sign reversal of the charge carriers observed in the Cr-containing alloys is connected to the compensation of electron and hole carriers. Furthermore, the NiCoCr, NiCoFeCr, NiCoMnCr, and NiCoFeMnCr alloys show a negative phonon drag effect at low temperatures due to electron-phonon interaction. The measured thermal conductivity behaves similarly in all alloys, except for a considerable reduction in magnitude in Cr/Mn/Pd-containing alloys. This is attributed to a significant decrease of electronic thermal conductivity due to an increased electron scattering by disorders and lattice distortions and a substantial modification of band structure. There is almost an equal contribution of electronic and lattice to the total thermal conductivity in Cr/Mn/Pd-containing alloys, suggesting a semimetallic nature. The temperature dependence of lattice thermal conductivity of these alloys is described by different phonon scattering mechanisms.

We have studied the influence of external pressure up to 1 GPa on the magnetic transitions of the orthorhombic Haldane-spin chain compound ${\mathrm{Tb}}_2{\mathrm{BaNiO}}_5$, an exotic multiferroic material. This parent compound is known to undergo Néel ordering at $T_{N1}=63\mathrm{K}$ and another magnetic transition at $T_{N2}=25\mathrm{K}$ at which ferroelectricity sets in, however, without any change in the magnetic symmetry, but with only a sharp change in the canting angle of Tb $4f$ and Ni $3d$ magnetic moments. There is a subtle difference in the antiferromagnetic state above and below $T_{N2}$, which is supported by the fact that there is a metamagnetic transition below $T_{N2}$ only (for 5 K, at ∼60 kOe). We report here that, with the application of external pressure, there is an upward shift of $T_{N1}$, while $T_{N2}$ shifts towards lower temperatures. It is interesting that the two magnetic transitions in the same compound behave differently under pressure and the opposite behavior at $T_{N2}$ is attributed to local distortion leading to ferroelectricity. The results are augmented by temperature dependent x-ray diffraction and positive chemical pressure studies. The chemical pressure caused by the isoelectronic doping at Ba site by Sr reduces both the transition temperatures. Clearly, the external pressure favors antiferromagnetic coupling (that is, leading to $T_{N1}$ enhancement), whereas the chemical pressure reduces $T_{N1}$, suggesting an important role of the changes in local hybridization induced by doping on magnetism in this material.