Organic molecular devices show strong potential for future nanoelectronics applications, many of which require multifunction capability. Organometallic ${[({\mathrm{C}}_5{\mathrm{H}}_5)\mathrm{Ti}({\mathrm{C}}_8{\mathrm{H}}_8)]}_n$ clusters are one of the few molecular material clusters that have been synthesized experimentally. Here we present a theoretical study of the spin-polarized transport on the clusters of ${[({\mathrm{C}}_5{\mathrm{H}}_5)\mathrm{Ti}({\mathrm{C}}_8{\mathrm{H}}_8)]}_n$ by using nonequilibrium Green’s functions combined with the density-functional theory. Results indicate that one-dimensional, organic $({\eta }^5\text{−}{\mathrm{C}}_5{\mathrm{H}}_5)\mathrm{Ti}({\eta }^8\text{−}{\mathrm{C}}_8{\mathrm{H}}_8)\mathrm{Ti}$ wire has unique half-metallic properties with 100% spin polarization near the Fermi level in the ground state. We predict that finite-layer ${[({\mathrm{C}}_5{\mathrm{H}}_5)\mathrm{Ti}({\mathrm{C}}_8{\mathrm{H}}_8)]}_n$ clusters coupled to gold electrodes exhibit a near-perfect spin-filter effect. Furthermore, due to the large spin coherence originated from its organic nature, these devices possess a spin-valve effect with large magnetoresistance. Our findings suggest that such a multifunctional molecular device is an excellent candidate for future molecule-based spintronic applications.

We demonstrate fast readout of a double quantum dot (DQD) that is coupled to a superconducting resonator. Utilizing a parametric amplifier beyond its range of linear amplification, we improve the signal-to-noise ratio (SNR) by a factor of 2000 compared to the situation with the parametric amplifier turned off. With an integration time of 400 ns comparable to the inverse effective bandwidth, we achieve a SNR of 76. By measuring the SNR as a function of the integration time, we extract an equivalent charge sensitivity of $8\times {10}^{-5}e/\sqrt{\mathrm{Hz}}$. The high SNR allows us to acquire a DQD charge-stability diagram in just 20 ms. At such a high data rate, it is possible to acquire charge-stability diagrams in a live “video mode,” enabling real-time tuning of the DQD confinement potential.

We examine the effect that bias current has on the spectral response of spin-torque oscillators designed to have large precessional orbits. These devices exhibit a nonmonotonic dependence of auto-oscillator frequencies and linewidths as a function of bias current, indicating that oscillator nonlinearity first decreases and then increases with increasing oscillator power. Macrospin simulations indicate that this behavior is characteristic of orbits with large precession angles, as oscillation frequency is a function of the deviation away from a 90° alignment of the free- and reference-layer moments. Using a recent nonlinear oscillator theory, we analyze these results by using two different techniques, both of which confirm that these theories predict the behavior of large-angle oscillations and that the linewidths of these oscillators have a different dependence on oscillator power than shown in previous studies of small-angle precession. From these results, we find that the details of the excited orbital motion can have a significant impact on a spin-torque oscillator’s properties, providing additional methods for optimizing device performance.

Graphene is used as flexible electrodes in various optoelectronic devices. In these applications, ultrafast charge transfer from semiconducting light absorbers to graphene can impact the overall device performance. Here, we propose a mechanism in which the charge-transfer rate can be controlled by varying the number of graphene layers and their stacking. Using an organic semiconducting molecule as a light absorber, the charge-transfer rate to graphene is measured by using time-resolved photoemission spectroscopy. Compared to graphite, the charge transfer to monolayer graphene is about 2 times slower. Surprisingly, the charge transfer to $A\text{−}B$–stacked bilayer graphene is slower than that to both monolayer graphene and graphite. This anomalous behavior disappears when the two graphene layers are randomly stacked. The observation is explained by a charge-transfer model that accounts for the band-structure difference in mono- and bilayer graphene, which predicts that the charge-transfer rate depends nonintuitively on both the layer number and stacking of graphene.

Step-recovery diodes operating in the snappy recovery regime are used as opening switches for generating narrow pulses with high-voltage amplitude. Physical modeling of the switching process is complex due to the large number of parameters involved, including diode structure, the extreme physical conditions, and the effect of external driving conditions. In this work, we address the problem by using a physical device simulator for solving the coupled device and electrical driving circuit equations. This method allows deciphering of the physical processes to take place in the diode during the fast current interruption phase. Herein we analyze the complete hard (snappy) reverse recovery process in short-base devices and determine the fast-transition-phase mechanism. It was found that the fast current interruption phase is constructed of two processes; the main parameters governing the switching time duration and the prepulse magnitude are the diode’s reverse current density and its base-doping concentration. We describe the dependence of the switching performance in these parameters.

Rectification of microwave oscillations of magnetization in a permalloy film is realized using the planar Hall effect. Two different rectified signals are obtained: a signal from the linearly excited uniform magnetization precession at the frequency of the external pumping and a signal from the pairs of contrapropagating short-wavelength spin waves parametrically generated at one-half of the pumping frequency. The second, most unusual, rectified signal is caused by the uniform component of the dynamic magnetization created due to the interference of the phase-correlated pairs of parametric spin waves.

We demonstrate a promising fiber architecture for generating strong photon-photon interactions. Exposed-core silica optical fibers possess low-loss guidance between 400 and 1700 nm crucial for quantum-logic applications. The potential of this fiber is demonstrated by exciting a two-photon transition within a rubidium vapor using an exposed-core silica optical fiber. Transit-time broadened spectral features enable measurement of the evanescent-field scale length of $(120\pm 20)\mathrm{nm}$ which shows excellent agreement with the characteristics of the modeled fiber mode $(118\pm 2)\mathrm{nm}$. We observe a two-photon absorption coefficient of $8.3{\mathrm{cm}}^{-1}$ for one optical mode in response to a transmitted power of 1.3 mW in the second mode. A clear pathway to an exposed-core fiber exhibiting substantial absorption mediated by a single photon is identified.

Several rare-earth-doped, heavy-metal halides have recently been identified as potential next-generation luminescent materials with high efficiency at low cost. ${\mathrm{AB}}_2{\mathrm{I}}_5:{\mathrm{Eu}}^{2+}$ ($A=\mathrm{Li}–\mathrm{Cs}$; $B=\mathrm{Sr}$, Ba) is one such family of halides. Its members, such as ${\mathrm{CsBa}}_2{\mathrm{I}}_5:{\mathrm{Eu}}^{2+}$ and ${\mathrm{KSr}}_2{\mathrm{I}}_5:{\mathrm{Eu}}^{2+}$, are currently being investigated as high-performance scintillators with improved sensitivity, light yield, and energy resolution less than 3% at 662 keV. Within the ${\mathrm{AB}}_2{\mathrm{I}}_5$ family, our first-principles-based calculations reveal two remarkably different trends in Eu site occupation. The substitutional Eu ions occupy both eightfold-coordinated ${B1}^{(\mathrm{VIII})}$ and the sevenfold-coordinated ${B2}^{(\mathrm{VII})}$ sites in the Sr-containing compounds. However, in the Ba-containing crystals, Eu ions strongly prefer the ${B2}^{(\mathrm{VII})}$ sites. This random versus preferential distribution of Eu affects their electronic properties. The calculations also suggest that in the Ba-containing compounds one can expect the formation of Eu-rich domains. These results provide atomistic insight into recent experimental observations about the concentration and temperature effects in Eu-doped ${\mathrm{CsBa}}_2{\mathrm{I}}_5$. We discuss the implications of our results with respect to luminescent properties and applications. We also hypothesize Sr, Ba-mixed quaternary iodides ${\mathrm{ABa}}^{\mathrm{VIII}}{\mathrm{Sr}}^{\mathrm{VII}}{\mathrm{I}}_5:\mathrm{Eu}$ as scintillators having enhanced homogeneity and electronic properties.

Noise is mostly considered to be an adverse factor in electronics. Yet, effects like logical stochastic resonance (LSR) can render electronic fluctuations useful. Here, we study LSR in a system consisting of two Coulomb-coupled quantum dots (QDs). We observe that voltage fluctuations applied to one of the QDs lead to a rectified and controllable current in the other QD. The interplay between applied noise and gate voltages enables our system to offer logic and, or, nand, and nor gate functionalities, which can be switched by either a variation of the noise or of a single gate voltage. For an optimal amount of noise, all four functionalities can be toggled by changing solely one single gate voltage. The presented results may prove beneficial for future autonomous, noise-tolerant, and energy-efficient electronics.

We demonstrate a vibrational microscopy technique with subdiffraction spatial resolution by the use of saturation of coherent anti-Stokes Raman scattering (CARS). The saturated CARS signals effectively produce a reduced point-spread function at harmonic frequencies, which is extracted by temporal modulation of the pump beam and demodulation of the CARS signal. An increase in spectral resolution and suppression of the nonresonant background signal accompany the spatial- resolution enhancement. Our simple, enhanced CARS technique promises to be useful in studying molecules in gas and liquid phases as well as soft condensed-matter systems.

We show that uniaxial color centers in silicon carbide with hexagonal lattice structure can be used to measure not only the strength but also the polar angle of the external magnetic field with respect to the defect axis with high precision. The method is based on the optical detection of multiple spin resonances in the silicon vacancy defect with quadruplet ground state and suggests significant improvement of the angle sensitivity compared to spin-1 color centers. We demonstrate a good agreement between the experimental and calculated spin resonance spectra, providing angle resolution better than 1° per ${\mathrm{Hz}}^{1/2}$ in submillitesla magnetic fields. Our approach is suitable for ensemble as well as single spin-$3/2$ color centers, allowing for angle-resolved magnetometry on the nanoscale at ambient conditions.

The magnetic and magneto-optic properties of epitaxial ${\mathrm{CeY}}_2{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ ($\mathrm{Ce}:\mathrm{YIG}$) and ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (yttrium iron garnet or YIG) thin films grown by pulsed laser deposition on gadolinium gallium garnet substrates are determined. An enhanced Faraday effect is known to result from Ce substitution into the yttrium iron garnet lattice, and here we characterize the magneto-optic Kerr effect, as well as the magnetic hysteresis and ferromagnetic resonance response that result from the Ce substitution. X-ray diffraction analysis reveals a high crystallographic quality for the $\mathrm{Ce}:\mathrm{YIG}$ films. Measurements of the magneto-optic Kerr effect for two different wavelengths demonstrate that the $\mathrm{Ce}:\mathrm{YIG}$ exhibits an up-to-tenfold increase in Kerr rotation compared to YIG. The $\mathrm{Ce}:\mathrm{YIG}$ has a slightly larger magnetic moment, as well as increased magnetic damping and higher magnetic anisotropy compared to YIG with a dependence on the crystalline orientation. By specific cerium substitution in YIG, our results show that the engineering of a large Kerr effect and tailored magnetic anisotropy becomes possible as required for magneto-optically active spintronic devices.

Anatase ${\mathrm{TiO}}_2$ attracts considerable interest for a range of technological applications such as photocatalysis and electronic memory devices. Recent studies have shown that the (001) surface plays a crucial role in its photocatalytic activity and this has been attributed to its higher surface energy and its ability to adsorb water dissociatively. However, a fundamental understanding of why this surface is so unique is still lacking. In this study, the anatase ${\mathrm{TiO}}_2$ (001) and (101) surfaces, both present in its equilibrium crystal shape, are studied using state-of-the-art ab initio hybrid density-functional-theory calculations. It is found that the electronic states at the (001) surface strongly deviate from the bulk, while the electronic states at the (101) surface do not. We illustrate the role of anisotropy in the crystal on the electronic structures of anatase ${\mathrm{TiO}}_2$ and demonstrate how the formation of the (001) surface disrupts local orbital interactions and gives rise to distinctive surface electronic states, which give the (001) surface its unique properties.

The thermal conductivity ($\kappa$) of graphene dramatically decreases once supported on a substrate, hindering its use for thermal management. To clarify the underlying mechanisms, we investigate the $\kappa$ of graphene on amorphous ${\mathrm{SiO}}_2$ by using molecular dynamics with particular attention to the graphene-substrate topography. Our analysis reveals that the suppression in $\kappa$ increases with the nonuniformity of the forces acting on graphene, which tends to increase as the substrate-surface roughness and graphene conformity increase. Our findings highlight the importance of the interfacial morphology on $\kappa$ and can provide guidance on the design of substrates to improve thermal transport through graphene.

We prove that balanced loss-gain distributions provide an elegant path towards realizing loss-free unidirectional cloaks for objects much larger than the wavelength. By coating a scatterer with an ultrathin metasurface with balanced loss and gain, such as a parity-time ($PT$)-symmetric metasurface in the special case of a symmetric scatterer, we show that it is possible to ideally cloak the scatterer from a particular direction. The passive portion of the cloak ensures that the incident power on the illuminated side of the object is fully absorbed, while the active portion radiates the required pattern on the opposite side, automatically synchronized with the impinging signal, eliminating shadows and making the object invisible. The proposed approach substitutes complex metamaterial cloaks and achieves cloaking with relaxed limitations on size, and in an ultrathin, loss-free, and stable design. Beyond introducing a new venue to cloaking of large objects, our paper shows the unique potential for scattering manipulation introduced by structures with balanced loss and gain, pointing to a new direction in the field of $PT$-symmetric systems.

The new generation of x-ray free-electron lasers opens up unique avenues for exploring matter under exotic and extreme conditions. Extensive spatial characterization of focused, typically (sub)micron-sized, laser beams is indispensable but, nevertheless, difficult to be accomplished due to excessive radiation intensities. Methods exist allowing indirect or semidirect focus characterization from a safe distance far from the focal point. Here we present a direct method of in-focus numerical phase recovery exploiting multishot desorption imprints in poly(methyl methacrylate). Shapes of the imprints serve as input data for the newly developed code PhaRe (phase recovery), inspired by the iterative Gerchberg-Saxton algorithm. A procedure of dynamic input-output mixing guarantees that the algorithm always converges to a self-consistent paraxial Helmholtz equation solution, which is thereafter optimized for transverse spatial coherence. Very good agreement with single-shot ablation imprints in lead tungstate (${\mathrm{PbWO}}_4$) is found. The experiment is carried out at the Linac Coherent Light Source with a focused beam monochromatized at 800 eV. The results of the coherence optimization indicate that the act of monochromatization may have an effect on otherwise very good transverse coherence of free-electron laser beams.

We present an approach to infer the surface density of magnetic moments $I_s$ in ultrathin ferromagnetic films with perpendicular anisotropy. It relies on quantitative stray-field measurements with an atomic-size magnetometer based on the nitrogen-vacancy center in diamond. The method is applied to microstructures patterned in a 1-nm-thick film of CoFeB. We report measurements of $I_s$ with a few percent uncertainty and a spatial resolution in the range of $(100\mathrm{nm}{)}^2$, an improvement by several orders of magnitude over existing methods. As an example of application, we measure the modifications of $I_s$ induced by local irradiation with ${\mathrm{He}}^{+}$ ions in an ultrathin ferromagnetic wire. This method offers a route to study variations of magnetic properties at the nanoscale.

We find that the motion of the valley electrons—electronic states close to the $K$ and $K^{\prime }$ points of the Brillouin zone—is confined into two dimensions when the layers of ${\mathrm{MoS}}_2$ form the $3R$ stacking, while in the $2H$ polytype, the bands have dispersion in all three dimensions. According to our first-principles band-structure calculations, the valley states have no interlayer hopping in $3R\text{−}{\mathrm{MoS}}_2$, which is proven to be the consequence of the rotational symmetry of the Bloch functions. By measuring the reflectivity spectra and analyzing an anisotropic hydrogen-atom model, we confirm that the valley excitons in $3R\text{−}{\mathrm{MoS}}_2$ have two-dimensional hydrogenlike spectral series, and the spreads of the wave function are smaller than the interlayer distance. In contrast, the valley excitons in $2H\text{−}{\mathrm{MoS}}_2$ are well described by the three-dimensional model and, thus, not confined in a single layer. Our results indicate that the dimensionality of the valley degree of freedom can be controlled simply by the stacking geometry, which can be utilized in future valleytronics.

Core-shell nanoparticle systems of Zn-ZnO and $\mathrm{ZnO}\text{−}{\mathrm{TiO}}_2$ are studied computationally using finite-element methods. The inclusion of a surface free energy and the elastic mismatch of the core and shell create an imprinting effect within the shell structure that produces a wide variation of strains. Due to this diversity of strains, the sharp, direct, band-gap edges of the bulk semiconductor are observed to be broadened. We show that a variety of factors, such as particle size, core-to-shell volume ratio, applied hydrostatic pressure, shell microstructure, as well as the effect of elastic anisotropy, can influence the distribution of optical band-gap values throughout the particle.

Near-field heat engines are devices that convert the evanescent thermal field supported by a primary source into usable mechanical energy. By analyzing the thermodynamic performance of three-body near-field heat engines, we demonstrate that the power they supply can be substantially larger than that of two-body systems, showing their strong potential for energy harvesting. Theoretical limits for energy and entropy fluxes in three-body systems are discussed and compared with their corresponding two-body counterparts. Such considerations confirm that the thermodynamic availability in energy-conversion processes driven by three-body photon tunneling can exceed the thermodynamic availability in two-body systems.