Single-molecule sensitivity of nuclear magnetic resonance (NMR) and angstrom resolution of magnetic resonance imaging (MRI) are the highest challenges in magnetic microscopy. Recent development in dynamical-decoupling- (DD) enhanced diamond quantum sensing has enabled single-nucleus NMR and nanoscale NMR. Similar to conventional NMR and MRI, current DD-based quantum sensing utilizes the “frequency fingerprints” of target nuclear spins. The frequency fingerprints by their nature cannot resolve different nuclear spins that have the same noise frequency or differentiate different types of correlations in nuclear-spin clusters, which limit the resolution of single-molecule MRI. Here we show that this limitation can be overcome by using “wave-function fingerprints” of target nuclear spins, which is much more sensitive than the frequency fingerprints to the weak hyperfine interaction between the targets and a sensor under resonant DD control. We demonstrate a scheme of angstrom-resolution MRI that is capable of counting and individually localizing single nuclear spins of the same frequency and characterizing the correlations in nuclear-spin clusters. A nitrogen-vacancy-center spin sensor near a diamond surface, provided that the coherence time is improved by surface engineering in the near future, may be employed to determine with angstrom resolution the positions and conformation of single molecules that are isotope labeled. The scheme in this work offers an approach to breaking the resolution limit set by the “frequency gradients” in conventional MRI and to reaching the angstrom-scale resolution.

We report a theoretical and experimental study on the use of nonequilibrium tunneling injection to control the local Cooper pair amplitude ${\mathrm{\Phi }}_S$ in an $SF$ sandwich involving superconducting ($S$) and ferromagnetic ($F$) layers coupled by the proximity effect. In an SISFIFS structure (where $I$ is an insulator), this same $SF$ sandwich serves as the acceptor electrode of an $SISF$ Josephson junction whose critical current $I_{\mathrm{ca}}$ depends on ${\mathrm{\Phi }}_S$. We derive the self-consistency equation describing the critical temperature $T_c$ of the $SF$ sandwich under nonequilibrium conditions. In addition, we compute $I_{\mathrm{ca}}$ by solving the Boltzmann equation for the electron distribution function $f_{ϵ}$, which then allows a determination of the relative change of $\delta I_{\mathrm{ca}}/\delta I_i(V_i)$ versus the bias voltage $V_i$ and the injection current $I_i(V_i)$. The computed gain $\delta I_{\mathrm{ca}}/\delta I_i(V_i)$ strongly depends on $\delta I_{\mathrm{ca}}/\delta I_i(V_i)$, and agrees well with the experimental data.

The blocking of fine-capillary tubes used as flow impedances in ${}^4\mathrm{He}$ evaporation cryostats to achieve temperatures below 4.2 K is generally attributed to nitrogen or air impurities entering these tubes from the main bath. The failure of even the most rigorous low-temperature laboratory best practices aimed at eliminating the problem by maintaining the cleanliness of the helium bath and preventing impurities from entering the capillary tubes suggests that a different cause is responsible for the inexplicable reduction of impedance flow. Many low-temperature research laboratories around the world have suffered this nuisance at a considerable financial cost due to the fact that the affected systems have to be warmed to room temperature in order to recover their normal low-temperature operation performance. Here, we propose an underlying physical mechanism responsible for the blockages based upon the freezing of molecular ${\mathrm{H}}_2$ traces present in the liquid-helium bath. Solid ${\mathrm{H}}_2$ accumulates at the impedance low-pressure side, and, after some time, it produces a total impedance blockage. The presence of ${\mathrm{H}}_2$ traces is unavoidable due its occurrence in the natural gas wells where helium is harvested, forcing gas suppliers to specify a lower bound for impurity levels at about 100 ppb even in high-grade helium. In this paper, we present a simple apparatus to detect hydrogen traces present in liquid helium and easily check the quality of the liquid. Finally, we propose a solution to eliminate the hydrogen impurities in small- and large-scale helium recovery plants. The solution has been implemented in several laboratories that previously experienced a chronic occurrence of blocking, eliminating similar occurrences for more than one year.

We investigate the ferromagnetic resonance (FMR) of a single chiral structure of a ferromagnetic metal—the magnetochiral (MCh) metamolecule. Using a strain-driven self-coiling technique, micrometer-sized MCh metamolecules of metallic permalloy (Py) are fabricated without any residual Py films. The magnetization curves of ten Py MCh metamolecules obtained by an alternating gradient magnetometer show soft magnetic behavior. In cavity FMR with a magnetic-field sweep and coplanar-waveguide (CPW) FMR with a frequency sweep, the Kittel-mode FMR of the single Py metamolecule is observed. The CPW-FMR results, which are consistent with the cavity-FMR results, bring about the effective $g$ factor, effective magnetization, and Gilbert damping of the single metamolecule. Together with calculations using these parameters, the angle-resolved cavity FMR reveals that the magnetization in the Py MCh metamolecule is most likely to be the hollow-bar type of configuration when the external magnetic field is applied parallel to the chiral axis, although the expected magnetization state at remanence is the corkscrew type of configuration.

A concept of multilevel thermally assisted magnetoresistive random-access memory is proposed and investigated by micromagnetic simulations. The storage cells are magnetic tunnel junctions in which the storage layer is exchange biased and in a vortex configuration. The reference layer is an unpinned soft magnetic layer. The stored information is encoded via the position of the vortex core in the storage layer. This position can be varied along two degrees of freedom: the radius and the in-plane angle. The information is read out from the amplitude and phase of the tunnel magnetoresistance signal obtained by applying a rotating field on the cell without heating the cell. Various configurations are compared in which the soft reference layer consists of either a simple ferromagnetic layer or a synthetic antiferromagnetic sandwich (SAF). Among those, the most practical one comprises a SAF reference layer in which the magnetostatic interaction between the SAF and storage layer is minimized. This type of cell should allow one to store at least 40 different states per cell representing more than five bits per cell.

We consider a near-field enhanced negative luminescent refrigeration system made of a polar material supporting surface-phonon polariton resonances and a narrow-band-gap semiconductor under a reverse bias. We show that in the near-field regime, such a device yields significant cooling power density and a high efficiency close to the Carnot limit. In addition, the performance of our system still persists even in the presence of strong nonidealities such as Auger recombination and sub-band-gap thermal radiation from free carriers.

${\alpha }^{\prime \prime }\text{−}{\mathrm{Fe}}_{16}{\mathrm{N}}_2$ is considered as one of the most promising candidates for future rare-earth-free magnets, showing the highest saturation magnetization reported so far. We propose and demonstrate a “strained-wire method” to synthesize ${\alpha }^{\prime \prime }\text{−}{\mathrm{Fe}}_{16}{\mathrm{N}}_2$ compound anisotropic magnets with an enhanced hard magnetic property, with a direct experimental observation of the intercoupling between tensile strain and the martensitic phase transition. The principle is helpful for the generation of another martensitic phase. In this paper, the method is demonstrated on an ${\alpha }^{\prime \prime }\text{−}{\mathrm{Fe}}_{16}{\mathrm{N}}_2$ compound permanent magnet preparation by starting from pure bulk iron, with urea as the nitrogen provider. A uniaxial tensile stress is applied on the wire-shaped sample during the postannealing stage, producing a promising permanent magnet with a hard magnet property which lacks any rare-earth elements. The sample synthesized in the lab exhibits a coercivity of 1220 Oe and an energy product of up to 9 MGOe. The mechanism of the strained-wire method is analyzed based on scanning-transmission-electron-microscopy characterizations of samples with different strains. We observe a strain-induced recrystallization of ${\alpha }^{\prime \prime }\text{−}{\mathrm{Fe}}_{16}{\mathrm{N}}_2$ samples at a low annealing temperature ($150°\mathrm{C}$). We demonstrate that this strained-wire method can be used on ${\alpha }^{\prime \prime }\text{−}{\mathrm{Fe}}_{16}{\mathrm{N}}_2$ samples to increase the ${\alpha }^{\prime \prime }\text{−}{\mathrm{Fe}}_{16}{\mathrm{N}}_2$ phase-volume ratio and to fine-tune its microstructure at a low temperature. Some further characterization results are also included in this paper. The physics of the influence of tensile stress on the martensitic phase transition is discussed.

The active control of optical signals in the time domain is what science and technology demand in fast all-optical information processing. Nanostructured materials can modify the group velocity and slow the light down, as the artificial light dispersion emerges. We observe the ultrafast temporal behavior of the Faraday rotation within a single femtosecond laser pulse under conditions of slow light in a one-dimensional magnetophotonic crystal. The Faraday effect changes by 20% over the time of 150 fs. This might be applicable to the fast control of light in high-capacity photonic devices.

The interface between two wide band-gap insulators, ${\mathrm{LaAlO}}_3$ and ${\mathrm{SrTiO}}_3$ (LAO/STO) offers a unique playground to study the interplay and competitions between different ordering phenomena in a strongly correlated two-dimensional electron gas. Recent studies of the LAO/STO interface reveal the inhomogeneous nature of the 2DEG that strongly influences electrical-transport properties. Nanowires needed in future applications may be adversely affected, and our aim is, thus, to produce a more homogeneous electron gas. In this work, we demonstrate that nanostructures fabricated in the quasi-2DEG at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface, capped with a ${\mathrm{SrCuO}}_2$ layer, retain their electrical resistivity and mobility independent of the structure size, ranging from 100 nm to $30\mu \mathrm{m}$. This is in contrast to noncapped LAO/STO structures, where the room-temperature electrical resistivity significantly increases when the structure size becomes smaller than $1\mu \mathrm{m}$. High-resolution intermodulation electrostatic force microscopy reveals an inhomogeneous surface potential with “puddles” of a characteristic size of 130 nm in the noncapped samples and a more uniform surface potential with a larger characteristic size of the puddles in the capped samples. In addition, capped structures show superconductivity below 200 mK and nonlinear current-voltage characteristics with a clear critical current observed up to 700 mK. Our findings shed light on the complicated nature of the 2DEG at the LAO/STO interface and may also be used for the design of electronic devices.

The magnetoelectric effects in multiferroics have a great potential in creating next-generation memory devices. We use an alternative concept of nonvolatile memory based, on a type of nonlinear magnetoelectric effects showing a butterfly-shaped hysteresis loop. The principle is to utilize the states of the magnetoelectric coefficient, instead of magnetization, electric polarization, or resistance, to store binary information. Our experiments in a device made of the PMN-PT/Terfenol-$D$ multiferroic heterostructure clearly demonstrate that the sign of the magnetoelectric coefficient can be repeatedly switched between positive and negative by applying electric fields, confirming the feasibility of this principle. This kind of nonvolatile memory has outstanding practical virtues such as simple structure, easy operation in writing and reading, low power, fast speed, and diverse materials available.

Near transform-limited single-photon sources are required for perfect photon indistinguishability in quantum networks. Having such sources in nanodiamonds is particularly important since it can enable engineering hybrid quantum-photonic systems. In this paper, we report the generation of optically stable, nearly transform-limited single silicon-vacancy emitters in nanodiamonds. Lines as narrow as 325 MHz are reported, which is close to the lifetime-limited linewidth (141 MHz). Moreover, the emitters exhibit minimal spectrum diffusion and high photostability, even if pumped well above saturation. Our results suggest that nanodiamonds can host color centers with superior properties suitable for hybrid photonic devices and quantum information.

A switch capable of routing microwave signals at cryogenic temperatures is a desirable component for state-of-the-art experiments in many fields of applied physics, including but not limited to quantum-information processing, communication, and basic research in engineered quantum systems. Conventional mechanical switches provide low insertion loss but disturb operation of dilution cryostats and the associated experiments by heat dissipation. Switches based on semiconductors or microelectromechanical systems have a lower thermal budget but are not readily integrated with current superconducting circuits. Here we design and test an on-chip switch built by combining tunable transmission-line resonators with microwave beam splitters. The device is superconducting and as such dissipates a negligible amount of heat. It is compatible with current superconducting circuit fabrication techniques, operates with a bandwidth exceeding 100 MHz, is capable of handling photon fluxes on the order of $10^5\mu {\mathrm{s}}^{-1}$, equivalent to powers exceeding $-90\mathrm{dBm}$, and can be switched within approximately 6–8 ns. We successfully demonstrate operation of the device in the quantum regime by integrating it on a chip with a single-photon source and using it to route nonclassical itinerant microwave fields at the single-photon level.

Understanding the fracture toughness (resistance) of glasses is a fundamental problem of prime theoretical and practical importance. Here we theoretically study its dependence on the loading rate, the age (history) of the glass, and the notch radius $\rho$. Reduced-dimensionality analysis suggests that the notch fracture toughness results from a competition between the initial, age- and history-dependent, plastic relaxation time scale ${\tau }_0^{\text{pl}}$ and an effective loading time scale ${\tau }^{\text{ext}}({\dot{K}}_I,\rho )$, where ${\dot{K}}_I$ is the tensile stress-intensity-factor rate. The toughness is predicted to scale with $\sqrt{\rho }$ independently of $\xi \equiv {\tau }^{\text{ext}}/{\tau }_0^{\text{pl}}$ for $\xi \ll 1$, to scale as $T\sqrt{\rho }\log (\xi )$ for $\xi \gg 1$ (related to thermal activation, where $T$ is the temperature), and to feature a nonmonotonic behavior in the crossover region $\xi \sim \mathcal{O}(1)$ (related to plastic yielding dynamics). These predictions are verified using 2D computations, providing a unified picture of the notch fracture toughness of glasses. The theory highlights the importance of time-scale competition and far-from-steady-state elasto-viscoplastic dynamics for understanding the toughness and shows that the latter varies quite significantly with the glass age (history) and applied loading rate. Experimental support for bulk metallic glasses is presented, and possible implications for applications are discussed.

Perovskite solar cells (PSCs) often suffer from large performance variations which impede to define a clear charge-transfer mechanism. Ferroelectric polarization is measured numerically using ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ ($MA{\mathrm{PbI}}_3$) pellets to overcome the measurement issues such as pinholes and low uniformity of thickness, etc., with $MA{\mathrm{PbI}}_3$ thin films. $MA{\mathrm{PbI}}_3$ perovskite is an antiferroelectric semiconductor which is different from typical semiconducting materials and ferroelectric materials. The effect of polarization carrier separation on the charge-transfer mechanism in the PSCs is elucidated by using the results of ferroelectric and structural studies on the perovskite. The ferroelectric polarization contributes to an inherent carrier-separation effect and the $I–V$ hysteresis. The ferroelectric and semiconducting synergistic charge-separation effect gives an alternative category of solar cells, ferroelectric semiconductor solar cells. Our findings identify the ferroelectric semiconducting behavior of the perovskite absorber as being significant to the improvement of the ferroelectric PSCs performances in future developments.

We embed a small Josephson junction in a microwave resonator that allows simultaneous dc biasing and dispersive readout. Thermal fluctuations drive the junction into phase diffusion and induce a temperature-dependent shift in the resonance frequency. By sensing the thermal noise of a remote resistor in this manner, we demonstrate primary thermometry in the range of 300 mK to below 100 mK, and high-bandwidth (7.5 MHz) operation with a noise-equivalent temperature of better than $10\mu \mathrm{K}/\sqrt{\mathrm{Hz}}$. At a finite bias voltage close to a Fiske resonance, amplification of the microwave probe signal is observed. We develop an accurate theoretical model of our device based on the theory of dynamical Coulomb blockade.

Collective behavior, where a set of elements interact and generate effects that are beyond the reach of the individual noninteracting elements, is always of great interest in physics. Quantum collective effects that have no classical analog are even more intriguing. In this work, we show how to construct collective quantum heat machines and explore their performance boosts with respect to regular machines. Without interactions between the machines, the individual units operate in a stochastic, nonquantum manner. The construction of the collective machine becomes possible by introducing two simple quantum operations: coherence extraction and coherence injection. Together, these operations can harvest coherence from one engine and use it to boost the performance of a slightly different engine. For weakly driven engines, we show that the collective work output scales quadratically with the number of engines rather than linearly. Eventually, the boost saturates and then becomes linear. Nevertheless, even in saturation, work is still significantly boosted compared to individual operation. To study the reversibility of the collective machine, we introduce the “entropy-pollution” measure. It is shown that there is a regime where the collective machine is $N$ times more reversible while producing $N$ times more work, compared to the individual operation of $N$ units. Moreover, the collective machine can even be more reversible than the most reversible unit in the collective. This high level of reversibility becomes possible due to a special symbiotic mechanism between engine pairs.

The on-demand generation of viscous microdroplets to print functional or biological materials remains challenging using conventional inkjet-printing methods, mainly due to aggregation and clogging issues. In an effort to overcome these limitations, we implement a jetting method to print viscous microdroplets by laser-induced shockwaves. We experimentally investigate the dependence of the jetting regimes and the droplet size on the laser-pulse energy and on the inks’ physical properties. The range of printable liquids with our device is significantly extended compared to conventional inkjet printers’s performances. In addition, the laser-induced flow-focusing phenomenon allows us to controllably generate viscous microdroplets up to 210 mPa s with a diameter smaller than the nozzle from which they originated ($200\mu \mathrm{m}$). Inks containing proteins are printed without altering their functional properties, thus demonstrating that this jetting technique is potentially suitable for bioprinting.

Nondestructive analysis (NDA) based on x-ray emission is widely used, for example, in the semiconductor and concrete industries. Here, we demonstrate significant quantitative and qualitative improvements in broadband x-ray NDA by combining particle-induced emission with detection based on superconducting microcalorimeter arrays. We show that the technique offers great promise in the elemental analysis of thin-film and bulk samples, especially in the difficult cases where tens of different elements with nearly overlapping emission lines have to be identified down to trace concentrations. We demonstrate the efficiency and resolving capabilities by spectroscopy of several complex multielement samples in the energy range 1–10 keV, some of which have a trace amount of impurities not detectable with standard silicon drift detectors. The ability to distinguish the chemical environment of an element is also demonstrated by measuring the intensity differences and chemical shifts of the characteristics x-ray peaks of titanium compounds. In particular, we report measurements of the $K\alpha /K\beta$ intensity ratio of thin films of TiN and measurements of Ti $K\alpha$ satellite peak intensities in various Ti thin-film compounds. We also assess the detection limits of the technique, comment on detection limits possible in the future, and discuss possible applications.

Understanding the fill factor in organic solar cells remains challenging due to its complex dependence on a multitude of parameters. By means of drift-diffusion simulations, we thoroughly analyze the fill factor of such low-mobility systems and demonstrate its dependence on a collection coefficient defined in this work. We systematically discuss the effect of different recombination mechanisms, space-charge regions, and contact properties. Based on these findings, we are able to interpret the thickness dependence of the fill factor for different experimental studies from the literature. The presented model provides a facile method to extract the photoactive layer’s electronic quality which is of particular importance for the fill factor. We illustrate that over the past 15 years, the electronic quality has not been continuously improved, although organic solar-cell efficiencies increased steadily over the same period of time. Only recent reports show the synthesis of polymers for semiconducting films of high electronic quality that are able to produce new efficiency records.

The identification of new solid-state defect-qubit candidates in widely used semiconductors has the potential to enable the use of nanofabricated devices for enhanced qubit measurement and control operations. In particular, the recent discovery of optically active spin states in silicon carbide thin films offers a scalable route for incorporating defect qubits into on-chip photonic devices. Here, we demonstrate the use of 3C silicon carbide photonic crystal cavities for enhanced excitation of color-center defect spin ensembles in order to increase measured photoluminescence signal count rates, optically detected magnetic-resonance signal intensities, and optical spin initialization rates. We observe an up to a factor of 30 increase in the photoluminescence and optically detected magnetic-resonance signals from Ky5 color centers excited by cavity-resonant excitation and increase the rate of ground-state spin initialization by approximately a factor of 2. Furthermore, we show that the 705-fold reduction in excitation mode volume and enhanced excitation and collection efficiencies provided by the structures can be used to overcome inhomogenous broadening in order to facilitate the study of defect-qubit subensemble properties. These results highlight some of the benefits that nanofabricated devices offer for engineering the local photonic environment of color-center defect qubits to enable applications in quantum information and sensing.