A novel method for patterning nanoscale silicon in ultrathin metal films using focused ion beams controlled by ion beam dose is proposed. Here, a focused beam of Si ions is used to implant Si into a silver layer on a sapphire support. Subsequent thermal treatment causes the underlying, silicon-implanted silver to evaporate, leaving behind the previously implanted silicon. The resulting structures exhibit bonding between the implanted silicon and the underlying sapphire surface, consistent with the formation of an ultrathin, patterned silicon-containing film. This controlled method for fabricating ultrathin films ensures minimal lateral diffusion of Si atoms and preserves the integrity of the film during processing. The successful formation of these films, as confirmed by a range of analytical techniques, demonstrates the potential of this method for producing high-quality ultrathin films in advanced material applications.

Oxygen deficient strontium titanate (SrTiO3-delta; STO) shows a range of electronic properties, including metallic conductivity, semiconducting behavior, and superconductivity, in contrast to stoichiometric SrTiO3, which is a band insulator. Several phenomena, including a resistivity transition (referred to in the literature as a "metal-insulator transition"), multiferroic, magnetic behavior, and enhanced superconducting transition temperatures have been observed in oxygen deficient STO thin films. Non-equilibrium conditions during film growth, lattice-mismatch strain, defects, and interface effects are expected to influence the electronic properties of thin films. Understanding the role of these factors is necessary to achieve reproducible modulation of film properties. Here, we report a controlled investigation of STO films on [100] LaAlO3 (STO-LAO) and [100] SrTiO3 (STO-STO), revealing striking differences between the properties of the films on the two substrates. Our results include the first observation of thickness dependent evolution of the resistive transition in partially strain-relaxed STO-LAO films. We show that the extrapolated zero temperature conductivity of these films is finite, indicating that the low temperature phase is not a true insulator. Instead, the temperature dependence indicates a low temperature quantum diffusive metallic state, which may be attributed to disorder-induced weak-localization and enhanced electron-electron interactions. Thus, the resistive transition may be more appropriately termed a "quantum diffusive transition" rather than a "metal-insulator transition." STO-STO films, in stark contrast, exhibit metallic conductivity for the whole thickness and temperature range. Films on both substrates show magnetoresistance at low temperatures with characteristic differences. We present structural and microstructural characterization supporting our results. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International (CC BY-NC-ND) license (https://creativecommons.org/licenses/by-nc-nd/4.0/).

The study of the evolution and metamorphosis of nanoparticles under high pressure and in nanoscale confinement is a rapidly developing field that promises a diverse range of fundamental research and application opportunities. Here, we demonstrate how a linked and strained graphene oxide (GO)-based confinement system, functioning as a nanoscale reactor at high pressures, allows the evolution of the magnetic properties of an in situ generated composite cobalt (Co) nanoparticle system and further enables the retention of such properties when the pressure of the system is returned to ambient. We posit that this phenomenon is due to an 'induced pressure' created by strain, on the flexible planes of the 2D GO system, created by the intercalated Co-based nanoparticles. For graphene/GO this strain is characterized by shifts in the Raman "G-band". Specifically, the studied system comprises in situ generated Co-containing nanoparticles confined between linked GO layers upon pressurization between 0 and 25 GPa. After quenching to ambient pressure, each of these samples exhibited innate ferromagnetic behaviors, demonstrating that our confinement system can be used to 'lock in' phase changes created by the application of transient high pressure. Importantly, while the unpressurized sample exhibited antiferromagnetic Co3O4 nanoparticles of similar to 4 nm size, the samples pressurized to 10 GPa and 25 GPa, presented ferromagnetic order on the shell of an antiferromagnetic core of Co3O4 and CoO.

The mid-T-C superconductor Ba1-XKXBiO3 (BKBO) exhibits different superconducting mechanisms depending on x, in the range similar to 0.35-0.65. The optimal doping for the highest T-C is reported to be around x = 0.4. To understand more about the dependence of the superconducting mechanism on x, high quality and reproducible epitaxial films with controlled x are needed. This has been challenging owing to the volatility of K and (to a lesser extent) Bi. In this work, we use pulsed laser deposition (PLD) with several novel process steps to achieve high-quality films in a reproducible way. These include a modified method for target preparation, a low NO2 growth pressure, and precise positioning of substrates in the PLD plume. Optimum T-C films (32 K onset) were grown from an x = 0.4 target, i.e. with no excess K, as is normally used. Stable, higher K content films (made from an x = 0.45 target), were also grown. These x = 0.45 films had a lower T-C (22.5 K onset), as expected for (K) overdoped films, with very high upper critical field, H-C2 (0 K), and irreversibility field, H-irr (0 K), values, from linear extrapolation, of similar to 31.7 T and similar to 28.8 T, respectively. The growth methodology demonstrated in this work is highly beneficial for fundamental mechanistic studies of this complex superconductor on which there is renewed interest, and where controlled compositions and crystalline quality are currently limited.

Fusion reactors are limited by the magnetic field available to confine their plasma. The commercial fusion industry uses the larger magnetic field and higher operating temperature of the cuprate superconductor $\mathbf{YBa_{2}Cu_{3}O_{7-\delta}}$ (YBCO) in order to confine their plasma into a dense volume. A superconductor is a macroscopic quantum state that is protected from the metallic (resistive) state by an energy gap. Unfortunately, YBCO has an anisotropic gap, known as D-wave because it has the shape of a $\mathbf{d_{x^2-y^2}}$ chemical orbital. This D-wave gap means that poly-crystalline wire cannot be made because a few degree misalignment between grains in the wire leads to a drastic loss in its supercurrent carrying ability, and thereby its magnetic field limit. The superconductor industry has responded by growing nearly-single-crystal superconducting YBCO films on carefully prepared substrate tapes kilometers in length. Heroic development programs have made such tapes commercially available, but they are very expensive and delicate. MRI magnet superconductors, such as $\mathbf{NbTi}$ and $\mathbf{Nb_{3}Sn}$, are formed into poly-crystalline wires because they have an isotropic gap in the shape of an s chemical orbital (called S-wave) that makes them insensitive to grain misalignment. However, these materials are limited to lower magnetic fields and liquid-He temperatures. Here, we modified YBCO by doping the Y site with Ca and Ce atoms to form $\mathbf{(Y_{1-x-y}Ca_{x}Ce_{y})Ba_{2}Cu_{3}O_{7-\delta}}$, and show evidence that it changes to an S-wave gap. Its superconducting transition temperature, $\mathbf{T_c}$, of $\mathbf{\sim 70K}$, while lower than that of D-wave YBCO at $\mathbf{\sim 90K}$, is easily maintained using common, economic cryogenic equipment.

The transmission coefficient for a 6-function barrier is a convenient model for many technologically important applications relying on photoemission, simulations of wave packets, or modeling the narrow barrier of a normal- superconducting point contact. We examine an extension of the model to treat instead a function sequence (a rectangular barrier that approaches the behavior of a function in the limit of vanishing width). It is shown how the eigenstates of the sequence converge on the function barrier eigenstates, but more importantly, how the even and odd parity states depart from the 6-function limiting case. The exact eigenstates enable the time evolution of exponentially attenuated tunneling to be exactly evaluated, in contrast to numerical methods. The application is the inclusion of tunneling time effects in simulations of time-varying electron emission.

The analytic nature of the transmission coefficient for a δ-function barrier makes it a useful tool to examine a variety of technologically important applications, such as photoemission from semiconductors with an alkali coating, the examination of tunneling times for wave packets incident on a barrier, and for parameterizing tunneling through the narrow barrier of a normal-superconducting point contact. The analytic model of a δ-function barrier inside a confining well is extended to the finite height and width rectangular barrier (a delta-function sequence). Methods to exactly evaluate the eigenstates are given and their dependencies are examined. The time evolution of a superposition of the lowest eigenstates is considered for barriers having comparable Gamow tunneling factors so as to quantify the impact of barrier height and shape on time evolution in a simple and exact system and, therefore, serve as a proxy for tunneling time. Last, density profiles and associated quantum potentials are examined for coupled wells to show changes induced by weaker and wider barriers.

Chemiresistive graphene sensors are promising for chemical sensing applications due to their simple device structure, high sensitivity, potential for miniaturization, low-cost, and fast response. In this work, we investigate the effect of (1) ZnO nanoparticle functionalization and (2) engineered defects onto graphene sensing channel on device resistance and low frequency electrical noise. The engineered defects of interest include 2D patterns of squares, stars, and circles and 1D patterns of slots parallel and transverse to the applied electric potential. The goal of this work is to determine which devices are best suited for chemical sensing applications. We find that, relative to pristine graphene devices, nanoparticle functionalization leads to reduced contact resistance but increased sheet resistance. In addition, functionalization lowers 1/f current noise on all but the uniform mesa device and the two devices with graphene strips parallel to carrier transport. The strongest correlations between noise and engineering defects, where normalized noise amplitude as a function of frequency f is described by a model of AN/fγ, are that γ increases with graphene area and contact area but decreases with device total perimeter, including internal features. We did not find evidence of a correlation between the scalar amplitude, AN, and the device channel geometries. In general, for a given device area, the least noise was observed on the least-etched device. These results will lead to an understanding of what features are needed to obtain the optimal device resistance and how to reduce the 1/f noise which will lead to improved sensor performance.

Graphene, the first isolated two-dimensional material, has captivated researchers for the last decade due to its unique structure that leads to novel electronic, chemical, mechanical, and thermal properties. The most intriguing properties are the large electronic mobilities that are achievable for low carrier concentrations and the large tunability of graphene's electrical properties via electrostatic gating, in which the Fermi energy is shifted relative to the charge neutrality, or Dirac, point and the high electronic mobilities obtained when the Fermi energy is close to that point. In this report, we show that both covalent and non-covalent functionalization of graphene leads to adsorbate-induced doping. This results in a three-fold increase in the graphene systems' mobilities and the observation of quantum transport phenomena (Hall effect plateaus, Shubnikov-de Haas oscillations, and Berry's phase) which were not observed in the unfunctionalized graphene. This ability to control the electronic properties without electrostatic gating is critical for chemical and biological sensing, optical, and electronic applications, which require both low carrier concentrations and the attachment of nanocrystals, biomolecules, increased adhesion and wettability of graphene layers, and enable strong cohesion between graphene layers in stacked graphene structures.

The metamaterial approach to dielectric response engineering for enhancing the transition temperature, ${T}_{c}$, of a superconductor has been demonstrated in several recent reports. One example of this effect is ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ coated aluminum nanoparticles that form an epsilon near zero (ENZ) core-shell metamaterial superconductor with a ${T}_{c}$ that is nearly three times that of pure aluminum. Since the ${T}_{c}$ of a conventional low temperature superconductor is determined by the Debye temperature and the coupling strength, which is expressed as the product of the single particle density of state (DOS) at the Fermi energy and an attractive electron-electron potential, it is natural to explore these properties to determine whether the attractive potential is responsible for the enhancement. In this report, we present specific heat results obtained from ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ coated aluminum nanoparticle composite pellets with an enhanced ${T}_{c}$ that demonstrate that the Debye temperature and DOS are similar to that of pure aluminum indicating that the source of the ${T}_{c}$ enhancement is indeed a modification of the attractive electron-electron interaction.

The electronic transport and optical properties of high quality multilayers of NbTiN/AlN with ultrathin NbTiN layers were characterized. The anisotropy of the dielectric function of the multilayers confirmed their hyperbolic metamaterial properties. The superconductive transition temperature, Tc, of these engineered superconductors was enhanced up to 32% compared to the Tc of a single ultrathin NbTiN layer while the resistivity per NbTiN layer remained unchanged. We have demonstrated that this Tc increase can be attributed to enhanced electron–electron interaction in superconducting hyperbolic metamaterials. The measured critical fields are high and have an anomalous temperature dependence on the direction perpendicular to the magnetic field. These results demonstrate that the metamaterial engineering approach can be used to enhance Hc2.

The leading order nonlinear (NL) susceptibility, χ 3 , in a paramagnet is negative and diverges as T → 0. This divergence is destroyed when spins correlate and the NL response provides unique insights into magnetic order. Dimensionality, exchange interaction, and preponderance of quantum effects all imprint their signatures in the NL magnetic response. Here, we study the NL susceptibilities in the proximate Kitaev magnet α -RuCl 3 , which differs from the expected antiferromagnetic behavior. For T < T c = 7.5 K and field B in the ab -plane, we obtain contrasting NL responses in low (<2 T) and high field regions. For low fields, the NL behavior is dominated by a quadratic response (positive χ 2 ), which shows a rapid rise below T c . This large χ 2 > 0 implies a broken sublattice symmetry of magnetic order at low temperatures. Classical Monte Carlo (CMC) simulations in the standard K − H − Γ model secure such a quadratic B dependence of M , only for T ≈ T c with χ 2 being zero as T → 0. It is also zero for all temperatures in exact diagonalization calculations. On the other hand, we find an exclusive cubic term ( χ 3 ) that describes the high field NL behavior well. χ 3 is large and positive both below and above T c crossing zero only for T > 50 K. In contrast, for B ∥ c -axis, no separate low/high field behaviors are measured and only a much smaller χ 3 is apparent.

We report on the anomalous magnetization dynamics of the cycloidally modulated spin textures under the influence of uniaxial anisotropy in multiferroic GaV4S8. The temperature and field dependence of the linear ac susceptibility [chi(1 omega)' (T, H)], AC magnetic loss [chi(1 omega)'' (T, H)], and nonlinear AC magnetic response [M-3 omega (T, H)] are examined across the magnetic phase diagram in the frequency range f = 10 - 10 000 Hz. According to recent theory, skyrmion vortices under axial crystal symmetry are confined along specific orientations, resulting in enhanced robustness against oblique magnetic fields and altered spin dynamics. We characterize the magnetic response of each spin texture and find that the dynamic rigidity of the Neel skyrmion lattice appears enhanced compared to Bloch-type skyrmions in cubic systems, even in the multidomain state. Anomalous M-3 omega and strong dissipation emerge over the same phase regime where strong variations in the cycloid pitch were observed on lowering temperature in recent small-angle neutron-scattering experiments [White et al., Phys. Rev. B 97, 020401(R) (2018)]. Here, we show that strong anisotropy also drives an extended crossover of the zero-field cycloid texture in GaV4S8. The frequency dependence of these dynamic signatures is consistent with that of a robust anharmonic spin texture exhibiting a correlated domain arrangement. The results underpin the essential role of magnetic anisotropy in enhancing the rigidity of topological spin textures for diverse applications.

Recent experiments have demonstrated that the superconducting critical temperature may be improved in various metamaterial superconductor geometries. Here, we present the results of a study of tin-based metamaterial superconductors in the epsilon-near-zero (ENZ) and hyperbolic metamaterial configurations. It was observed that Tc enhancement is significantly reduced when the metamaterial structural dimensions exceed 240 nm, the superconducting coherence length in pure tin.

A metamaterial approach is capable of drastically increasing the critical temperature, T c , of composite metal-dielectric superconductors as demonstrated by the tripling of T c that was observed in bulk Al-Al2O3 coreshell metamaterials. A theoretical model based on the Maxwell-Garnett approximation provides a microscopic explanation of this effect in terms of electron-electron pairing mediated by a hybrid plasmon-phonon excitation. We report an observation of this excitation in Al-Al2O3 core-shell metamaterials using inelastic neutron scattering. This result provides support for this mechanism of superconductivity in metamaterials.