We investigate the coupling of spin and thermal currents as a means to raise the thermoelectric efficiency of nanoscale graphene devices. We consider nanostructures composed of overlapping graphene nanoribbons with ferromagnetic contacts in different magnetic configurations. Our results show that the charge Seebeck effect is greatly enhanced when the magnetic leads are in an antiparallel configuration, due to the enlargement of the transport gap. However, for the optimization of the charge figure of merit $ZT$, it is better to choose a parallel alignment of the magnetization in the leads, because the electron-hole symmetry is broken in this magnetic configuration. We also obtain the spin-dependent Seebeck coefficient and spin figure of merit. In fact, the spin $ZT$ can double its value with respect to the charge $ZT$ for a wide temperature range, above 300 K. These findings suggest the potential value of graphene nanosystems as energy-harvesting devices employing spin currents.

We report the development of niobium nitride (NbN)-based ferromagnetic $\pi$ Josephson junctions. For fabricated $\mathrm{NbN}/\mathrm{copper}\text{−}\mathrm{nickel}$ $(\mathrm{CuNi})/\mathrm{NbN}$ junctions, we measure the ferromagnetic CuNi thickness and temperature dependences of the Josephson critical current and find the cusp structure indicating the transition between the 0 and $\pi$ states. We also observe the characteristic temperature dependences for the junctions with CuNi thicknesses near the $0\text{−}\pi$ transition thickness, which originate from the interplay between superconductivity and magnetism. The experimental results are consistently explained by the microscopic theory with reasonable physical parameters. The developed NbN-based $\pi$ junctions are compatible with and provide advantages for superconducting logic circuits and/or quantum computers based on nitride superconductors.

We investigate the influence of the MgO growth process on the bias dependence of the electrical spin injection from a $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{MgO}$ spin injector into a GaAs-based light-emitting diode (spin LED). With this aim, textured MgO tunnel barriers are fabricated either by sputtering or molecular-beam-epitaxy (MBE) methods. For the given growth parameters used for the two techniques, we observe that the circular polarization of the electroluminescence emitted by spin LEDs is rather stable as a function of the injected current or applied bias for the samples with sputtered tunnel barriers, whereas the corresponding circular polarization decreases abruptly for tunnel barriers grown by MBE. We attribute these different behaviors to the different kinetic energies of the injected carriers linked to differing amplitudes of the parasitic hole current flowing from GaAs to Co-Fe-B in both cases.

We present analytical relations for the dark recombination current of a $p\text{−}{n}^{+}$ junction with positively charged columnar grain boundaries in the high-defect-density regime. We consider two defect-state configurations relevant for positively charged grain boundaries: a single donor state and a continuum of both acceptors and donors. Compared to a continuum of $\mathrm{acceptor}+\mathrm{donor}$ states or to the previously studied single $\mathrm{acceptor}+\mathrm{donor}$ state, the grain-boundary recombination of a single donor state is suppressed by orders of magnitude. We show numerically that superposition holds near the open-circuit voltage $V_{\mathrm{OC}}$ so that our dark $J(V)$ relations determine $V_{\mathrm{OC}}$ for a given short-circuit current $J_{\mathrm{SC}}$. Finally, we explicitly show how $V_{\mathrm{OC}}$ depends on the these two grain-boundary defect-state configurations.

We study spatially indirect excitons confined in a $10\text{−}\mu \mathrm{m}$-wide electrostatic trap of a GaAs double quantum well. We introduce a technique to control the amplitude of the electric field interacting with the exciton electric dipole, with nanosecond precision. Our approach relies on electronic waveforms corrected for the distortions occurring at the highest frequencies so that impedance matching is not necessary. Thus, we manipulate the confinement of cold gases without inducing sizable perturbations down to sub-Kelvin bath temperatures.

We report on the microwave characterization of a novel one-dimensional Josephson metamaterial composed of a chain of asymmetric superconducting quantum interference devices with nearest-neighbor coupling through common Josephson junctions. This metamaterial demonstrates a strong Kerr nonlinearity, with a Kerr constant tunable over a wide range, from positive to negative values, by a magnetic flux threading the superconducting quantum interference devices. The experimental results are in good agreement with the theory of nonlinear effects in Josephson chains. The metamaterial is very promising as an active medium for Josephson traveling-wave parametric amplifiers; its use facilitates phase matching in a four-wave-mixing process for efficient parametric gain.

Light scattering by nanostructures has facilitated research on various optical phenomena and applications by interfacing the near fields and free-propagating radiation. However, direct quantitative measurement of far-field scattering by a single nanostructure on the wavelength scale or less is highly challenging. Conventional back-focal-plane imaging covers only a limited solid angle determined by the numerical aperture of the objectives and suffers from optical aberration and distortion. Here, we present a quantitative measurement of the differential far-field scattering cross section of a single nanostructure over the full hemisphere. In goniometer-based far-field scanning with a high signal-to-noise ratio of approximately 27.4 dB, weak scattering signals are efficiently isolated and detected under total-internal-reflection illumination. Systematic measurements reveal that the total and differential scattering cross sections of a Au nanorod are determined by the plasmonic Fabry-Perot resonances and the phase-matching conditions to the free-propagating radiation, respectively. We believe that our angle-resolved far-field measurement scheme provides a way to investigate and evaluate the physical properties and performance of nano-optical materials and phenomena.

Josephson junctions can serve as mixers for electromagnetic radiation, producing difference frequencies $|m{f}_s–n{f}_{\mathrm{LO}}|$ of the signal frequency $f_s$ and the local oscillator frequency $f_{\mathrm{LO}}$, where the latter can be provided by ac Josephson currents, and $m$ and $n$ are natural numbers. In order to obtain a better understanding of the purity of the terahertz radiation generated by stacks of intrinsic Josephson junctions (IJJs), we study self-mixing—i.e., $f_s$ is also produced by Josephson currents inside the stacks—in the difference-frequency range between 0.1 and 3.0 GHz. Simultaneously, we perform off-chip terahertz emission detection and transport measurements. We find that at high-bias currents, when a hot spot has formed in the stack, the power level of self-mixing can be low and sometimes is even absent at the terahertz emission peak, pointing to a good phase locking among all IJJs. By contrast, at low-bias currents where no hot spot exists, the self-mixing products are pronounced even if the terahertz emission peaks are strong. The mixing products at high operation temperature, at which the temperature variation within the stack is moderate, are minor, indicating that the low junction resistance, perhaps in combination with the lowered Josephson critical current density, may play a similar role for synchronization as the hot spot does at low temperature. While these observations are helpful for the task to synchronize thousands of IJJs, the observation of self-mixing in general may offer a simple method in evaluating the coherence of terahertz radiation produced by the IJJ stacks.

Semiconductor nanowires offer the possibility to grow high-quality quantum-dot heterostructures, and, in particular, CdSe quantum dots inserted in ZnSe nanowires have demonstrated the ability to emit single photons up to room temperature. In this paper, we demonstrate a bottom-up approach to fabricate a photonic fiberlike structure around such nanowire quantum dots by depositing an oxide shell using atomic-layer deposition. Simulations suggest that the intensity collected in our $\mathrm{NA}=0.6$ microscope objective can be increased by a factor 7 with respect to the bare nanowire case. Combining microphotoluminescence, decay time measurements, and numerical simulations, we obtain a fourfold increase in the collected photoluminescence from the quantum dot. We show that this improvement is due to an increase of the quantum-dot emission rate and a redirection of the emitted light. Our ex situ fabrication technique allows a precise and reproducible fabrication on a large scale. Its improved extraction efficiency is compared to state-of-the-art top-down devices.

High storage density and high data rate are two of the most desired properties of modern hard disk drives. Heat-assisted magnetic recording (HAMR) is believed to achieve both. Recording media, consisting of exchange-coupled grains with a high and a low $T_C$ part, were shown to have low dc noise—but increased ac noise—compared to hard magnetic single-phase grains like FePt. We extensively investigate the influence of an $\mathrm{Fe}\text{−}\mathrm{Rh}$ interlayer on the magnetic noise in exchange-coupled grains. We find an optimal grain design that reduces the jitter in the down-track direction by up to 30% and in the off-track direction by up to 50%, depending on the head velocity, compared to the same structures without FeRh. Furthermore, the mechanisms causing this jitter reduction are demonstrated. Additionally, we show that, for short heat pulses and low write temperatures, the switching-time distribution of the analyzed grain structure is reduced by a factor of 4 compared to the same structure without an $\mathrm{Fe}\text{−}\mathrm{Rh}$ layer. This feature could be interesting for HAMR use with a pulsed laser spot and could encourage discussion of this HAMR technique.

We introduce the concept of a thermophotovoltaic system whose emitter is separated from the photovoltaic cell by an intermediate thick slab of gallium arsenide. Owing to the engineered structure of the emitter (a multilayer structure of negative- and positive-$\epsilon$ layers) together with a high refractiveindex and transparency of the intermediate slab, we achieve a super-Planckian and frequency-selective spectrum of radiative heat transfer which is desirable for the efficient performance of thermophotovoltaic systems.

Imaging with structured illumination allows for the retrieval of subwavelength features of an object by conversion of evanescent waves into propagating waves. In conditions in which the object plane and the structured-illumination plane do not coincide, this conversion process is subject to progressive filtering of the components with high spatial frequency when the distance between the two planes increases, until the diffraction-limited lateral resolution is restored when the distance exceeds the extension of evanescent waves. We study the progressive filtering of evanescent waves by developing a remote super-resolution terahertz imaging system operating at a wavelength $\lambda =1.00\mathrm{mm}$, based on a freestanding knife edge and a reflective confocal terahertz microscope. In the images recorded with increasing knife-edge-to-object-plane distance, we observe the transition from a super-resolution of $\lambda /17\simeq 60\mu \mathrm{m}$ to the diffraction-limited lateral resolution of $\mathrm{\Delta }x\simeq \lambda$ expected for our confocal microscope. The extreme nonparaxial conditions are analyzed in detail, exploiting the fact that, in the terahertz frequency range, the knife edge can be positioned at a variable subwavelength distance from the object plane. Electromagnetic simulations of radiation scattering by the knife edge reproduce the experimental super-resolution achieved.

We report the ferroelectric glass-glass transitions in $\mathrm{KN}({\mathrm{K}}^{+}/{\mathrm{Nb}}^{5+})$-doped ${\mathrm{BaTiO}}_3$ ferroelectric ceramics, which have been proved by x-ray diffraction profile and Raman spectra data. The formation of glass-glass transitions can be attributed to the existence of cubic ($C$)-tetragonal ($T$)-orthorhombic ($O$)-rhombohedral ($R$) ferroelectric transitions in short-range order. These abnormal glass-glass transitions can perform very small thermal hysteresis (approximately $1.0\mathrm{K}$) with a large dielectric constant (approximately 3000), small remanent polarization $P_r$, and relative high maximum polarization $P_m$ remaining over a wide temperature range (220–350 K) under an electrical stimulus, indicating the potential applications in dielectric recoverable energy-storage devices with high thermal reliability. Further phase field simulations suggest that these glass-glass transitions are induced by the formation of a percolating electric defect-dipole network (PEDN). This proper PEDN breaks the long-range ordered ferroelectric domain pattern and results in the local phase transitions at the nanoscale. Our work may further stimulate the fundamental physical theory and accelerate the development of dielectric energy-storing devices.

Pump-probe electron energy-loss spectroscopy (EELS) with femtosecond temporal resolution will be a transformative research tool for studying nonequilibrium chemistry and electronic dynamics of matter. In this paper, we propose a concept of femtosecond EELS utilizing mega-electron-volt electron beams from a radio-frequency (rf) photocathode source. The high acceleration gradient and high beam energy of the rf gun are critical to the generation of 10-fs electron beams, which enables an improvement of the temporal resolution by more than 1 order of magnitude beyond the state of the art. In our proposal, the “reference-beam technique” relaxes the energy stability requirement of the rf power source by roughly 2 orders of magnitude. The requirements for the electron-beam quality, photocathode, spectrometer, and detector are also discussed. Supported by particle-tracking simulations, we demonstrate the feasibility of achieving sub-electron-volt energy resolution and approximately 10-fs temporal resolution with existing or near-future hardware performance.

An accurate and transferable cluster-expansion model for complex quinary alloys is developed. Lattice Monte Carlo simulation enabled by this cluster-expansion model is used to investigate temperature-dependent atomic structure of alnico alloys, which are considered as promising high-performance non-rare-earth permanent-magnet materials for high-temperature applications. The results of the Monte Carlo simulations are consistent with available experimental data and provide useful insights into phase decomposition, selection, and chemical ordering in alnico. The simulations also reveal a previously unrecognized $D{0}_3$ alloy phase. This phase is very rich in Ni and exhibits very weak magnetization. Manipulating the size and location of this phase provides a possible route to improve the magnetic properties of alnico, especially coercivity.

We demonstrate the full functionality of a circuit that generates single microwave photons on demand, with a wave packet that can be modulated with a near-arbitrary shape. We achieve such a high tunability by coupling a superconducting qubit near the end of a semi-infinite transmission line. A dc superconducting quantum interference device shunts the line to ground and is employed to modify the spatial dependence of the electromagnetic mode structure in the transmission line. This control allows us to couple and decouple the qubit from the line, shaping its emission rate on fast time scales. Our decoupling scheme is applicable to all types of superconducting qubits and other solid-state systems and can be generalized to multiple qubits as well as to resonators.

We study the generation of Greenberger-Horne-Zeilinger (GHZ) states of three path-encoded photons. Inspired by the seminal work of Bouwmeester et al. [Phys. Rev. Lett. 82, 1345 (1999)] on polarization-entangled GHZ states, we find a corresponding path representation for the photon states of an optical circuit, identify the elements required for the state generation, and propose a possible implementation of our strategy. Besides the practical advantage of employing an integrated system that can be fabricated with proven lithographic techniques, our example suggests that it is possible to enhance the generation efficiency by using microring resonators.

Femtosecond-laser-written patterns at the surface of brittle materials may show at random times spontaneous alternations from regular to disordered structures and vice versa. Here, we show that these random transitions carry relevant statistical information, such as the Weibull parameters characterizing the fracture of brittle materials. The regular-erratic cycles of random lengths of the observed patterns suggests a phenomenological analogy with the idle and busy periods arising in queuing systems. This analogy enables us to establish experimentally that the random durations of the successive cycles are statistically independent. Based on these observations, we propose an experimental method bypassing the need for many specimens to build up statistically relevant ensembles of fracture tests. Our method is potentially generic, as it may apply to a broad number of brittle materials.

The static and dynamic electrical characteristics of thin-film transistors (TFTs) are often limited by the parasitic contact resistances, especially for TFTs with a small channel length. For the smallest possible contact resistance, the staggered device architecture has a general advantage over the coplanar architecture of a larger injection area. Since the charge transport occurs over an extended area, it is inherently more difficult to develop an accurate analytical device model for staggered TFTs. Most analytical models for staggered TFTs, therefore, assume that the contact resistance is linear, even though this is commonly accepted not to be the case. Here, we introduce a semiphenomenological approach to accurately fit experimental data based on a highly discretized equivalent network circuit explicitly taking into account the inherent nonlinearity of the contact resistance. The model allows us to investigate the influence of nonlinear contact resistances on the static and dynamic performance of staggered TFTs for different contact layouts with a relatively short computation time. The precise extraction of device parameters enables us to calculate the transistor behavior as well as the potential for optimization in real circuits.