The rapid development of new photonic materials calls for simple yet effective approaches to characterize ultrafast dynamics in materials and devices. Here we demonstrate time-frequency transient reflectometry based on a compact, few-cycle pump-probe reflectometer. The system simultaneously features a broad spectral range (300 nm) and fine temporal resolution (10 fs), and it requires only a femtosecond oscillator. With this capability, we report a time-frequency spectroscopic measurement of the transient dispersion of $\mathrm{Ga}\mathrm{As}$ near its band gap. Our results map important carrier dynamics such as carrier cooling, carrier decay, band filling, and band-gap renormalization on a single two-dimensional graph, offering a comprehensive picture of these processes and revealing details unattainable with conventional methods.

Modern spectroscopic techniques for the investigation of magnetization dynamics in micro and nanostructures or thin films typically use microwave antennas. They are directly fabricated on top of the sample by means of electron-beam lithography (EBL). Following this approach, every magnetic structure on the sample needs its own antenna, resulting in additional EBL steps and layer-deposition processes. Here, we demonstrate an approach for magnetization excitation that is suitable for optical and nonoptical spectroscopy techniques. By patterning the antenna on a separate flexible glass cantilever and insulating it electrically, we solve the mentioned issues. Since we use flexible transparent glass as the antenna substrate, optical spectroscopy techniques like microfocused Brillouin-light-scattering microscopy ($\mu \mathrm{BLS}$), time-resolved magneto-optical Kerr-effect measurements, or optically detected magnetic resonance measurements can be carried out at visible laser wavelengths. As the antenna is detached from the sample it can be freely positioned in all three dimensions to address only the desired magnetic structures and to achieve an effective excitation. We demonstrate the functionality of these antennas using $\mu \mathrm{BLS}$ and compare coherently and thermally excited magnon spectra to reveal an enhancement of the signal by a factor of about 400 due to the strong excitation by the antenna. Moreover, we succeed in characterizing yttrium-iron-garnet thin films with spatial resolution using optical ferromagnetic resonance experiments. We analyze the spatial excitation profile of the antenna by measuring the magnetization dynamics in two dimensions. The technique is furthermore applied to investigate injection locking of spin Hall nano-oscillators in the most favourable geometry with the highest spin-torque efficiency.

We investigate temperature-dependent thermal conductivity $\kappa (T)$ in a single $\mathrm{Ge}$ nanowire (NW) using optothermal Raman spectroscopy, which utilizes the temperature dependence of Raman lines as a local probe for temperature. The experiment is carried out from 300 K to above 700 K, a temperature range in which thermal conductivity of single NWs has been rarely explored. The thermal conductivity of $\mathrm{Ge}$ NWs (grown by vapor-liquid-solid mechanism) at around room temperature is observed to lie in the range 1.8–4.2 W/m K for diameters between 50 and 110 nm. The thermal conductivity at a given temperature is found to follow a linear dependence on NW diameter, suggesting that the low magnitude of $\kappa (T)$ is determined by diffused scattering of phonons from the surface of NWs that reduces it severely from its bulk value. $\kappa (T)$ shows approximately $1/T$ behavior which arises from the Umklapp processes. The quantitative estimation of errors arising from the optothermal measurement and methods to mitigate them is discussed. We also suggest a quick way to estimate approximately the thermal conductivity of $\mathrm{Ge}$ and $\mathrm{Si}$ NWs using the above observations.

Versatile optical devices are designed from anisotropic and gradient materials based on transformation optics, which are usually hard to realize. Here, by utilizing conformal equivalence, we demonstrate that a flat gradient index optical thin-film waveguide is equivalent to a curved homogenous optical thin-film waveguide for light rays. Such a relationship provides us an alternative method to design optical devices with either an inhomogeneous medium or equivalently curved surfaces made from only a homogeneous material, leading to applications on on-chip optical devices. Moreover, we provide a prototype of the curved optical thin-film waveguide without rotational symmetry, which can serve as a concave lens. Our work gives a conformal landscape of a two-dimensional gradient refractive-index profile from the geometric optics perspective.

Electronic cooling in hybrid normal-metal–insulator–superconductor junctions is a promising technology for the manipulation of thermal loads in solid-state nanosystems. One of the main bottlenecks for efficient electronic cooling is the electron-phonon coupling, as it represents a thermal leakage channel to the phonon bath. Graphene is a two-dimensional material that exhibits a weaker electron-phonon coupling compared to standard metals. For this reason, we study the electron cooling in graphene-based systems consisting of a graphene sheet contacted by two insulator-superconductor junctions. We show that, by properly biasing the graphene, its electronic temperature can reach base values lower than those achieved in similar systems based on metallic ultrathin films. Moreover, the lower electron-phonon coupling is mirrored in a lower heat power pumped into the superconducting leads, thus avoiding their overheating and preserving the cooling mechanisms. Finally, we analyze the possible application of cooled graphene as a bolometric radiation sensor. We study its main figures of merit, i.e., responsivity, noise equivalent power, and response time. In particular, we show that the built-in electron refrigeration allows reaching a responsivity of the order of $50\mathrm{nA}/\mathrm{pW}$ and a noise equivalent power of order of ${10}^{-18}\mathrm{W}{\mathrm{Hz}}^{-1/2}$ while the response speed is about 10 ns corresponding to a thermal bandwidth in the order of 20 MHz.

Copper-indium-gallium-diselenide (CIGS) thin-film solar cells suffer from high recombination losses at the back contact and parasitic absorption in the front-contact layers. Dielectric passivation layers overcome these limitations and enable an efficient control over interface recombination, which becomes increasingly relevant as thin-film solar cells increase in efficiency and become thinner to reduce the consumption of precious resources. We present the optoelectronic and chemical interface properties of oxide-based passivation layers deposited by atomic layer deposition on CIGS. A suitable postdeposition annealing removes detrimental interface defects and leads to restructuring and oxidation of the CIGS surface. The optoelectronic interface properties are very similar for different passivation approaches, demonstrating that an efficient suppression of interface states is possible independent of the metal used in the passivating oxide. If aluminum oxide (${\mathrm{Al}}_2{\mathrm{O}}_3$) is used as the passivation layer we confirm an additional field-effect passivation due to interface charges, resulting in an efficient interface passivation superior to that of a state-of-the-art cadmium-sulfide (CdS) buffer layer. Based on this chemical interface model we present a full-area rear-interface passivation layer without any contact patterning, resulting in a 1% absolute efficiency gain compared to a standard molybdenum back contact.

The giant magnetocaloric effect is widely achieved in hexagonal $\mathrm{Mn}MX$-based (M = $\mathrm{Co}$ or $\mathrm{Ni}$, X = $\mathrm{Si}$ or $\mathrm{Ge}$) ferromagnets at their first-order magnetostructural transition. However, the thermal hysteresis and low sensitivity of the magnetostructural transition to the magnetic field inevitably lead to a sizeable irreversibility of the low-field magnetocaloric effect. Here, we show an alternative way to realize a reversible low-field magnetocaloric effect in $\mathrm{Mn}MX$-based alloys by taking advantage of the second-order phase transition. With introducing $\mathrm{Cu}$ into $\mathrm{Co}$ in stoichiometric $\mathrm{Mn}\mathrm{Co}\mathrm{Ge}$ alloy, the martensitic transition is stabilized at high temperature, while the Curie temperature of the orthorhombic phase is reduced to room temperature. As a result, a second-order magnetic transition with a negligible thermal hysteresis and a large magnetization change can be observed, enabling a reversible magnetocaloric effect. By both calorimetric and direct measurements, a reversible adiabatic temperature change of about 1 K is obtained under a field change of 0–1 T at 304 K, which is larger than that obtained in a first-order magnetostructural transition. To gain a better insight into the origin of these experimental results, first-principles calculations are carried out to characterize the chemical bonds and the magnetic exchange interaction. Our work provides an understanding of the $\mathrm{Mn}\mathrm{Co}\mathrm{Ge}$ alloy and indicates a feasible route to improve the reversibility of the low-field magnetocaloric effect in the $\mathrm{Mn}MX$ system.

We construct optimal protocols for verifying qubit and qudit Greenberger-Horne-Zeilinger (GHZ) states using local projective measurements. When the local dimension is a prime, an optimal protocol is constructed from Pauli measurements only. Our protocols provide a highly efficient way to estimate the fidelity and to certify genuine multipartite entanglement. In particular, they enable the certification of genuine multipartite entanglement using only one test when the local dimension is sufficiently large. By virtue of adaptive local projective measurements, we then construct protocols for verifying GHZ-like states that are optimal over all protocols based on one-way communication. The efficiency can be improved further if additional communications are allowed. Finally, we construct optimal protocols for verifying GHZ states and nearly optimal protocols for GHZ-like states in the adversarial scenario.

We present a set of experiments to optimize the performance of a noninvasive thermometer based on proximity superconductivity. Current through a standard tunnel junction between an aluminum superconductor and a copper electrode is controlled by the strength of the proximity induced to this normal metal, which in turn is determined by the position of a direct superconducting contact from the tunnel junction. Several devices with different distances are tested. We develop a theoretical model based on Usadel equations and dynamic Coulomb blockade that reproduces the measured results and yields a tool to calibrate the thermometer and to optimize it further in future experiments. We also propose an analytic formula that reproduces the experimental data for a wide range of temperatures.

It is shown theoretically that in a layered heterostructure comprising piezoelectric, dielectric antiferromagnetic crystal, and heavy metal (PZ/AFM/HM), it is possible to control the anisotropy of the AFM layer by applying a dc voltage across the PZ layer. In particular, we show that by varying the dc voltage across the heterostructure and/or the dc current in the HM, it is possible to vary the frequency of the antiferromagnetic resonance of the AFM in a passive (subcritical) regime and, also, to reduce the threshold of the current-induced terahertz-frequency generation. Our analysis also shows that, unfortunately, the voltage-induced reduction of the generation threshold leads to the proportional reduction of the amplitude of the terahertz-frequency signal generated in the active (supercritical) regime. The general results are illustrated by a calculation of the characteristics of experimentally realizable PZT-5H/$\mathrm{Ni}\mathrm{O}$/$\mathrm{Pt}$.

We theoretically investigate the threshold current density for spin-orbit-torque- (SOT) induced perpendicular magnetization switching. According to the relative sign of fieldlike torque (FLT) and dampinglike torque (DLT), we derive the corresponding analytical formulas of the threshold current density $J_{\mathrm{th}}$ required to switch magnetization from equilibrium states to nearly in-plane states. These formulas, which agree well with numerical results among a wide range of material parameters, indicate that SOT current density can be significantly reduced in the presence of a large FLT. The conditions are then explored to achieve complete magnetization reversal after removing the SOT currents. When the ratio of FLT to DLT η is negative in our sign convention, we find that SOT pulses with long enough fall time, for example, 0.2 ns in our simulations, can guarantee stable switching. In contrast, a small in-plane magnetic field or a large damping is necessary when η > 0. Based on the conditions, the measured threshold current densities in several reported experiments, which deviate from previous models, can be well reproduced by our derived $J_{\mathrm{th}}$. We further study the possible incubation delay induced by SOT, clarifying the recent contradiction on incubation time during SOT switching. Our work sheds insights on the SOT material optimization and the comprehension of magnetization switching dynamics induced by SOT.

Slow extraction of beams from synchrotrons or storage rings as required by many fixed target experiments is performed by controlled excitation and feeding of a structural lattice resonance. Due to the sensitive nature of this resonant extraction process, the temporal structure of the extracted beam is modulated by the minuscule current fluctuations present on the quadrupole magnet power supplies. Such a modulation leads to pile-up in detectors and significant reduction in accumulated event statistics. This contribution proposes and experimentally demonstrates that by introduction of a further modulation on quadrupole currents with a specific amplitude and frequency, the influence of inherent power supply fluctuations on spill is mitigated. The slow-extraction beam dynamics associated with this method are explained along with the operational results.

Recently, nanolaminated ternary carbides have attracted immense interest due to the concomitant presence of both ceramic and metallic properties. Here, we grow nanolaminate ${\mathrm{Ti}}_3{\mathrm{Al}\mathrm{C}}_2$ thin films by pulsed laser deposition on c-axis-oriented sapphire substrates and, surprisingly, the films are found to be highly oriented along the (103) axis normal to the film plane, rather than the (000l) orientation. Multiple characterization techniques are employed to explore the structural and chemical quality of these films, the electrical and optical properties, and the device functionalities. The 80-nm thick ${\mathrm{Ti}}_3{\mathrm{Al}\mathrm{C}}_2$ film is highly conducting at room temperature, with a resistivity of about 50 µΩ cm and a very-low-temperature coefficient of resistivity. The ultrathin (2 nm) ${\mathrm{Ti}}_3{\mathrm{Al}\mathrm{C}}_2$ film has fairly good optical transparency (∼70%) and high conductivity (sheet resistance ∼735 Ω/sq) at room temperature. Scanning tunneling microscopy reveals the metallic characteristics (finite density of states at the Fermi level) at room temperature. The metal-semiconductor junction of the p-type ${\mathrm{Ti}}_3{\mathrm{Al}\mathrm{C}}_2$ film and n-$\mathrm{Si}$ show the expected rectification (diode) characteristics, in contrast to the ohmic contact behavior in the case of ${\mathrm{Ti}}_3{\mathrm{Al}\mathrm{C}}_2/$p-$\mathrm{Si}$. A triboelectric-nanogenerator-based touch-sensing device, comprising of the ${\mathrm{Ti}}_3{\mathrm{Al}\mathrm{C}}_2$ film, shows a very impressive peak-to-peak open-circuit output voltage (∼80 V). These observations reveal that pulsed laser deposited ${\mathrm{Ti}}_3{\mathrm{Al}\mathrm{C}}_2$ thin films have excellent potential for applications in multiple domains, such as bottom electrodes, resistors for high-precision measurements, Schottky diodes, ohmic contacts, fairly transparent ultrathin conductors, and next-generation biomechanical touch sensors for energy harvesting.

The spin-orbit-torque (SOT) device is a promising candidate for the next-generation magnetic random-access memory; however, the static in-plane field needed to induce deterministic switching is a main obstacle for its application in circuits. In this work, we employ the exchange coupling between the $\mathrm{Co}$/$\mathrm{Ni}$/$\mathrm{Co}$ trilayer and $\mathrm{Tb}$/$\mathrm{Co}$ multilayers in a device to form the domain wall (DW), by whose current-driven propagation the field-free magnetization switching is achieved. The SOT efficiency of the device is highly dependent on the chirality of the Néel-type DW. Meanwhile, the coexistence of two switching modes results in an asymmetric switching phase diagram. We find the competition between the uniform external field and the Dzyaloshinskii–Moriya interaction induces negative feedback to the pinning effect, resulting in a sharp switching process and straight DW profile. Finally, a synthetic antiferromagnetic device is investigated by experiment and micromagnetic simulation to verify the feasibility of using this proposal as the free layer of a magnetic tunneling junction.

Carbon-based nanomaterials have attracted a lot of interest lately due to their highly promising applications. Here we report on the modifications of single-walled carbon nanotubes (SWCNTs) induced by swift (highly energetic) heavy ions. Using scanning force microscopy and Raman spectroscopy, we observe a dramatic change in the structure of the irradiated SWCNTs, accompanied by an increase of the adhesion force as a function of ion fluence and electronic energy loss. With increasing ion fluence, the SWCNTs exhibit a partial transformation from metallic to more semiconducting. Moreover, at high fluence they break into segments 10–20 nm long.