We demonstrate a new fabrication process for hybrid semiconductor-superconductor heterostructures based on anodic oxidation (AO), allowing controlled thinning of epitaxial Al films. Structural and transport studies of oxidized epitaxial Al films grown on insulating GaAs substrates reveal spatial nonuniformity and enhanced critical temperature and magnetic fields. Oxidation of epitaxial Al on hybrid InAs heterostructures with a conducting quantum well show similarly enhanced superconducting properties transferred to the two-dimensional electron gas (2DEG) by proximity effect, with critical perpendicular magnetic fields up to 3.5 T. An insulating AlOx film that passivates the heterostructure from exposure to air is obtained by complete oxidation of the Al. It simultaneously removes the need to strip Al which damages the underlying semiconductor. AO passivation yielded 2DEG mobilities two times higher than similar devices with Al removed by wet etching. An AO-passivated Hall bar showed quantum Hall features emerging at a transverse field of 2.5 T, below the critical transverse field of thinned films, eventually allowing transparent coupling of quantum Hall effect and superconductivity. AO thinning and passivation are compatible with standard lithographic techniques, giving lateral resolution below $<50$ nm. We demonstrate local patterning of AO by realizing a semiconductor-based Josephson junction operating up to 0.3 T perpendicular.

Topological surface Dirac states (TSDSs) generated in three-dimensional topological insulators (3D-TIs) are currently of significant interest for new science and advanced technologies. In contrast to many other thermoelectric materials, 3D-TIs exhibit a significant potential to achieve a large enhancement in thermoelectric power factor ($PF=\sigma S^2$) due to their special topological symmetry. However, only limited experiments and discussions have been made so far for elucidating the thermoelectric properties of TSDS. Herein, we report a large $S$ and $\mathit{PF}$ observed for high-quality single-crystal flakes of $\mathrm{Sn}\text{−}\mathrm{B}{\mathrm{i}}_{1.1}{\mathrm{Sb}}_{0.9}\mathrm{Te}{\mathrm{S}}_2$ (Sn-BSTS). Accurate interpretations that the energy-dependent relaxation times $\tau \left(E\right)$ play an important role in thermoelectrical transport of 3D-TIs are provided. Among 3D-TIs, Sn-BSTS has the highest bulk insulation and shows intrinsic TSDS transport without bulk contributions, along with its hallmark of quantum integer Hall effect at high temperatures. Based on the longitudinal/transverse electrical transport and the thermoelectric coefficient, $\tau \left(E\right)\propto E^{0.21}$ is accurately deduced. As a consequence of the energy-dependent $\tau \left(E\right)$, a large enhancement in both $S$ and $\mathit{PF}$ is obtained ($S=58\mu \mathrm{V}{\mathrm{K}}^{-1}$ and $PF=5.0\mathrm{mW}{\mathrm{m}}^{-1}{\mathrm{K}}^{-2}$ at 77 K), leading to a large increase of 160% for $S$ and 280% for $\mathit{PF}$ when compared to those of graphene at 77 K. The potential thermoelectric performance of the pure TSDS is discussed based on the Boltzmann transport equations.

We report current-induced spin torques in epitaxial NiMnSb films on a commercially available epiready GaAs substrate. The NiMnSb was grown by cosputtering from three targets using optimized parameters. The films were processed into microscale bars to perform current-induced spin-torque measurements. Magnetic dynamics were excited by microwave currents, and electric voltages along the bars were measured to analyze the symmetry of the current-induced torques. We found that the extracted symmetry of the spin torques matches those expected from spin-orbit interaction in a tetragonally distorted half-Heusler crystal. Both fieldlike and dampinglike torques are observed in all the samples characterized, and the efficiency of the current-induced torques is comparable to that of ferromagnetic metal/heavy-metal bilayers.

A series of poly(methyl methacrylate) (PMMA) surfaces decorated by Cu nanoparticles (NP) with gradually varied morphology were prepared by high-pressure ${\mathrm{CO}}_2$ treatment at various time spans. Combining the characterizations of transmission electron microscopy (TEM) and atomic force microscopy (AFM), an accurate three-dimensional view of the morphology of the surfaces was presented. Subsequently, the wettability of the surfaces decreases near linearly with the increase of the apparent height of the decorating NPs in both static (static contact angle) and dynamic (contact angle hysteresis) aspects. The observed tendency contradicts to the Wenzel or Cassie-Baxter model and is explained by the contribution of nanomeniscus formed between the decorating NP and the flat substrate. The capillary pressure from this meniscus is negative and results in the increase of the contact angle with the apparent height $\left(H_N\right)$ of the Cu NPs decorating the PMMA surface. In addition, the effect of the coverage $\left(C_N\right)$ by NPs on the wettability can be explained on the same basis. Our experiment demonstrates the important influence of the nanomeniscus on the wettability, which is usually not taken into account. The results in this work provide a comprehensive understanding of how nanostructure affects the wettability of the decorated surfaces and shed light on how to obtain certain wettability through nanostructuring of the surface morphology.

We study magnetic and magnetotransport properties of an epitaxial interfacial multiferroic system consisting of a ferromagnetic Heusler-alloy ${\mathrm{Co}}_2\mathrm{Fe}\mathrm{Si}$ and a ferroelectric-oxide $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$. $L{2}_1$-ordered ${\mathrm{Co}}_2\mathrm{Fe}\mathrm{Si}$ epilayers on $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3\left(001\right)$ show an in-plane uniaxial magnetic anisotropy with strong temperature dependence, induced by the presence of the magnetoelastic effect via the spin-orbit interaction at the ${\mathrm{Co}}_2\mathrm{Fe}\mathrm{Si}/\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3\left(001\right)$ interface. In the ${\mathrm{Co}}_2\mathrm{Fe}\mathrm{Si}$ Hall-bar devices, the anisotropic magnetoresistance (AMR) hysteretic curves depending on in-plane magnetization reversal processes on the $a$ and $c$ domains of $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3\left(001\right)$ are clearly observed at room temperature. Notably, the magnitude of the AMR ratio (%) for ${\mathrm{Co}}_2\mathrm{Fe}\mathrm{Si}$ Hall-bar devices can be tuned through the $a-c$ domain wall motion of $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3\left(001\right)$ by applying electric fields. We propose that the tunable AMR effect is associated with the modulation of the spin-orbit interaction, exchange interaction, and/or the electronic band structure near the Fermi level by applying electric fields in the epitaxial ${\mathrm{Co}}_2\mathrm{Fe}\mathrm{Si}/\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3\left(001\right)$ interfacial multiferroic system.

The $B$-site ordered double perovskites ${\mathrm{Sr}}_2\mathrm{Cu}\left({\mathrm{Te}}_{1-x}{\mathrm{W}}_x\right){\mathrm{O}}_6$ provide an excellent arena for investigating exotic phases expected for the $J_1\text{−}{J}_2$ square-lattice Heisenberg antiferromagnet. Here, combining magnetic susceptibility and specific-heat measurements with electron spin resonance (ESR) and muon spin rotation/relaxation $\left(\mu \mathrm{SR}\right)$ techniques, we explore a spin-liquid-like state in the vicinity of the Néel critical end point $(x=0.05–0.1)$. The specific heat and the ESR and muon relaxation rates give evidence for an energy hierarchy of low-energy excitations, reminiscent of randomness-induced singlet states. In addition, the weak transverse $\mu \mathrm{SR}$ data show a fraction of frozen magnetic moments in the random-singlet background. The origin of a random-singlet-like state near the phase boundary is discussed in terms of concomitant exchange randomness and local strain generated by the ${\mathrm{W}}^{6+}\text{−}\mathrm{for}\text{−}{\mathrm{Te}}^{6+}$ substitution.

Magnetothermoelectric transport provides an understanding of coupled electron-hole-phonon current in topological materials and has applications in energy conversion and cooling. In this work, we study the Nernst coefficient, the magneto-Seebeck coefficient, and the magnetoresistance of single-crystalline ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ under external magnetic field in the range of $-3\mathrm{T}$ to 3 T and in the temperature range of 55 K to 380 K. Moreau's relation is employed to justify both the overall trend of the Nernst coefficient and the temperature at which this coefficient changes sign. We observe a nonlinear relationship between the Nernst coefficient and the applied magnetic field in the temperature range of 55 K to 255 K. An increase in both the Nernst coefficient and the magneto-Seebeck coefficient is observed as the temperature is reduced which can be attributed to the increased mobility of the carriers at lower temperatures. First-principles density functional theory calculations were carried out to physically model the experimental data including electronic and transport properties. Simulation findings agreed with the experiments and provide a theoretical insight to justify the measurements.

In this Letter, we demonstrate that it is possible to form a two-dimensional (2D) silicenelike ${\mathrm{Si}}_5\mathrm{Ge}$ compound by replacing the Si atoms occupying on-top sites in the planarlike structure of epitaxial silicene on ${\mathrm{ZrB}}_2$(0001) by deposited Ge atoms. For coverages below 1/6 monolayer, the Ge deposition gives rise to a ${\mathrm{Si}}_{6-x}{\mathrm{Ge}}_x$ alloy (with $x$ between 0 and 1) in which the on-top sites are randomly occupied by Si or Ge atoms. The progressive increase of the valence band maximum with $x$ observed experimentally originates from a selective charge transfer from Ge atoms to Si atoms. These achievements provide evidence for the possibility of engineering the band structure in 2D SiGe alloys in a way that is similar for their bulk counterpart.

Radiation-induced segregation (RIS) of solutes in materials exposed to irradiation is a well-known problem. It affects the lifetime of nuclear reactor core components by favoring radiation-induced degradation phenomena such as hardening and embrittlement. In this work, RIS tendencies in face centered cubic (fcc) Ni-$X$ ($X=\mathrm{Cr}$, Fe, Ti, Mn, Si, P) dilute binary alloys are examined. The goal is to investigate the driving forces and kinetic mechanisms behind the experimentally observed segregation. By means of ab initio calculations, point-defect stabilities and interactions with solutes are determined, together with migration energies and attempt frequencies. Transport and diffusion coefficients are then calculated in a mean-field framework, to get a full picture of solute-defect kinetic coupling in the alloys. Results show that all solutes considered, with the exception of Cr, prefer vacancy-mediated over interstitial-mediated diffusion during both thermal and radiation-induced migration. Cr, on the other hand, preferentially migrates in a mixed-dumbbell configuration. P and Si are here shown to be enriched, and Fe and Mn to be depleted at sinks during irradiation of the material. Ti and Cr, on the other hand, display a crossover between enrichment at lower temperatures, and depletion in the higher temperature range. Results in this work are compared with previous studies in body centered cubic (bcc) Fe, and discussed in the context of RIS in austenitic alloys.

Boron arsenide $\left(c\text{−}\mathrm{BAs}\right)$ is at the forefront of research on ultrahigh thermal conductivity materials. We present a Raman scattering study of isotopically tailored cubic boron arsenide single crystals for 11 isotopic compositions spanning the range from nearly pure $c\text{−}{}^{10}\mathrm{BAs}$ to nearly pure $c\text{−}{}^{11}\mathrm{BAs}$. Our results provide insights on the effects of strong mass disorder on optical phonons and the appearance of two-mode behavior in the Raman spectra of mixed crystals. Strong isotope disorder also relaxes the one-phonon Raman selection rules, resulting in disorder-activated Raman scattering by acoustic phonons.

Recent experiments indicate that the reactivity of metal surfaces changes profoundly when they are covered with two-dimensional (2D) materials. Nickel, the widespread catalyst choice for graphene (G) growth, exhibits complex surface restructuring even after the G sheet is fully grown. In particular, due to excess carbon segregation from bulk nickel to surface upon cooling, a nickel carbide (${\mathrm{Ni}}_2\mathrm{C}$) phase is detected under rotated graphene (RG) but not under epitaxial graphene (EG). Motivated by this experimental evidence, we construct different G/Ni(111) interface models accounting for the two types of G domains. Then, by applying density functional theory, we illuminate the microscopic mechanisms governing the structural changes of nickel surface induced by carbon segregation. A high concentration of subsurface carbon reduces the structural stability of Ni(111) surface and gives rise to the formation of thermodynamically advantageous ${\mathrm{Ni}}_2\mathrm{C}$ monolayer. We show the restructuring of the nickel surface under RG cover and reveal the essential role of G rotation in enabling high density of favorable C binding sites in the Ni(111) subsurface. As opposed to RG, the EG cover locks the majority of favorable C binding sites preventing the build-up of subsurface carbon density to a phase transition threshold. Therefore we confirm that the conversion of C-rich Ni surface to ${\mathrm{Ni}}_2\mathrm{C}$ takes place exclusively under RG cover, in line with the strong experimental evidence.

We have studied the magnetism of a cobalt ultrathin film deposited on different two-dimensional (2D) materials, namely graphene, h-BN, and ${\mathrm{WSe}}_2$ by the Brillouin light scattering technique. The studied samples are prepared by a pick-up method of large flakes deposited on ${\mathrm{SiO}}_2$ and the subsequent physical vapor deposition of metal layers, in a similar way to what is done to make spintronic devices out of such materials. Compared to the reference layer $(\mathrm{Co}/{\mathrm{SiO}}_2)$, the perpendicular magnetic anisotropy is enhanced in the Co/2D systems, although less than what could be expected on single crystal samples. This result is quantitatively discussed by comparison with ab initio calculations in the case of the Co/graphene interface. We also measure an increase of the magnetic damping and a small Dzyaloshinskii-Moriya interaction in such samples which are discussed with respect to the recent literature.

Discovery and design of two-dimensional (2D) materials with suitable band gaps and high carrier mobility are of vital importance for the photonics, optoelectronics, and high-speed electronics. In this work, based on first principles calculations using density functional theory with Perdew-Burke-Ernzerhof and Heyd-Scuseria-Ernzerhoffunctionals, we introduce a family of monolayer isostructural semiconducting tellurides MN${\mathrm{Te}}_4$, with $M=\left\{\mathrm{Ti},\mathrm{Zr},\mathrm{Hf}\right\}$ and $N=\left\{\mathrm{Si},\mathrm{Ge}\right\}$. These compounds have been identified to possess direct band gaps from 1.0 to 1.31 eV, which are well suited for photonics and optoelectronics applications. Additionally, anisotropic in-plane transport behavior is observed, and small electron and hole $(0.11–0.15m_e)$ effective masses are identified along the dominant transport direction. Ultrahigh carrier mobility is predicted for this family of 2D compounds, which host great promise for potential applications in high-speed electronic devices. Detailed analysis of electronic structures reveals the origins of the promising properties of this unique class of 2D telluride materials.

The robustness of topological materials against disorder and defects is presumed but has not been demonstrated explicitly in realistic systems. In this work, we use state-of-the-art density functional theory and recursive nonequilibrium Green's functions methods to study the effect of disorder on the electronic transport of long nanoribbons, up to $157\mathrm{nm}$, as a function of vacancy concentration. In narrow nanoribbons, a finite-size effect gives rise to hybridization between the edge states erasing topological protection. Hence, even small vacancy concentrations enable backscattering events. We show that the topological protection is more robust for wide nanoribbons, but surprisingly it breaks down at moderate structural disorder. Our study helps to establish some bounds on defective bismuthene nanoribbons as promising candidates for spintronic applications.

The rhombohedral distortion-driven occurrence of the spontaneous electric polarization at a reasonably high temperature $\left(T\right)$ and an exchange bias (EB) effect below the antiferromagnetic (AFM) Néel temperature $\left(T_N\right)$ are revealed in ${\mathrm{ZnFe}}_2{\mathrm{O}}_4$. We observe the magnetic memory effect, suggesting a cooperative glassy magnetic state below $T_N$. The phase separation between the long-range AFM and the glassy magnetic component leads to the EB effect below $T_N$. The synchrotron diffraction studies confirm a structural transition to a polar $R3m$ structure from the cubic spinel structure, involving a strong rhombohedral distortion. This distortion correlates with the occurrence of the spontaneous electric polarization $\left(P\right)$ below $\sim 110$ K $\left(T_{\text{FE}1}\right)$, which is accompanied by a short-range order (SRO). The $P$-value enhances further due to an additional rhombohedral distortion below $T_N$. A considerable magnetoelectric (ME) coupling is detected below $T_{\text{FE}1}$. The increase of $P$ is $\sim 7.2$% for 5 T at the liquid nitrogen temperature. There has been a fundamental interest among the community in the unique result of the occurrence of ferroelectric (FE) order coexisting with SRO and having a significant ME coupling, which is accompanied by a strong rhombohedral structural distortion analogous to that observed in ${\mathrm{BiFeO}}_3$.

PbTe is a leading thermoelectric material at intermediate temperatures, largely thanks to its low lattice thermal conductivity. However, its efficiency is too low to compete with other forms of power generation. This efficiency can be effectively enhanced by designing nanostructures capable of scattering phonons over a wide range of length scales to reduce the lattice thermal conductivity. The presence of grain boundaries can reduce the thermal conductivity to $\sim 0.5{\mathrm{W}\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ for small vacancy concentrations and grain sizes. However, grains anneal at finite temperature, and equilibrium and metastable grain size distributions determine the extent of the reduction in thermal conductivity. In the present work, we propose a phase-field model informed by molecular dynamics simulations to study the annealing process in PbTe and how it is affected by the presence of grain boundaries and voids. We find that the thermal conductivity of PbTe is reduced by up to 35% in the porous material at low temperatures. We observe that a phase transition at a finite density of voids governs the kinetics of impeding grain growth by Zener pinning.

The Heulser intermetallic ${\mathrm{MnAu}}_2\mathrm{Al}$ is shown to undergo a dramatic change in net magnetization in response to plastic deformation. A mechanism is proposed involving antiferromagnetic interactions in the otherwise ferromagnetic compound (when ordered) that arise due to chemical changes at the antiphase boundaries created by the deformation. The coupling between chemical and magnetic order across antiphase boundaries is likely to occur in other ordered magnetic systems and may provide an explanation for otherwise anomalous magnetic behavior across several systems, including other Heusler intermetallics.

The ability to control carrier concentration based on the extent of Cu solubility in the ${\mathrm{Cu}}_{2x}{\mathrm{Hg}}_{2-x}{\mathrm{GeTe}}_4$ alloy compound (where $0\le x\le 1$) makes ${\mathrm{Cu}}_{2x}{\mathrm{Hg}}_{2-x}{\mathrm{GeTe}}_4$ an interesting case study in the field of thermoelectrics. While Cu clearly plays a role in this process, it is unknown exactly how Cu incorporates into the ${\mathrm{Cu}}_{2x}{\mathrm{Hg}}_{2-x}{\mathrm{GeTe}}_4$ crystal structure and how this affects the carrier concentration. In this work, we use a combination of resonant energy x-ray diffraction (REXD) experiments and density functional theory (DFT) calculations to elucidate the nature of Cu incorporation into the ${\mathrm{Cu}}_{2x}{\mathrm{Hg}}_{2-x}{\mathrm{GeTe}}_4$ structure. REXD across the ${\mathrm{Cu}}_k$ edge facilitates the characterization of Cu incorporation in the ${\mathrm{Cu}}_{2x}{\mathrm{Hg}}_{2-x}{\mathrm{GeTe}}_4$ alloy and enables direct quantification of antisite defects. We find that Cu substitutes for Hg at a 2:1 ratio, wherein Cu annihilates a vacancy and swaps with a Hg atom. DFT calculations confirm this result and further indicate that the incorporation of Cu occurs preferentially on one of the $z=1/4$ or $z=3/4$ planes before filling the other plane. Furthermore, the amount of ${\mathrm{Cu}}_{\mathrm{Hg}}$ antisite defects quantified by REXD was found to be directly proportional to the experimentally measured hole concentration, indicating that the ${\mathrm{Cu}}_{\mathrm{Hg}}$ defects are the driving force for tuning carrier concentration in the ${\mathrm{Cu}}_{2x}{\mathrm{Hg}}_{2-x}{\mathrm{GeTe}}_4$ alloy. The link uncovered here between crystal structure, or more specifically antisite defects, and carrier concentration can be extended to similar cation-disordered material systems and will aid the development of improved thermoelectric and other functional materials through defect engineering.

Topological insulators are good candidates for charge to spin conversion with high efficiency due to their spin-polarized topological surface states (TSSs). In this work, we provide experimental evidence for two-dimensional (2D) TSSs in ${\left(\mathrm{Sn}{\mathrm{Bi}}_2{\mathrm{Te}}_4\right)}_n {\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)}_m$ natural van der Waals superlattices grown by molecular beam epitaxy using angle resolved photoelectron spectroscopy and magnetotransport. While the TSSs overlap with bulk conduction band (BCB) states at the Fermi energy, it is shown that by increasing the Sn composition, the influence of BCB states is reduced and becomes minimum for $\mathrm{Sn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$. The latter compound, found to be in the form of septuplet layers, shows weak antilocalization effect with a prefactor $\alpha \sim –0.41$, indicating that the TSSs and the bulk behave as one 2D channel in which magnetotransport properties are influenced by large spin-orbit coupling.