We assess the viability of topological semimetals for application in advanced interconnect technology, where conductor size is on the order of a few nanometers and grain boundaries are expected to be prevalent. We investigate the electron transport properties and grain-boundary scattering in thin films of the topological semimetals $\mathrm{Co}\mathrm{Si}$ and $\mathrm{Co}\mathrm{Ge}$ using first-principles calculations combined with the nonequilibrium Green’s function (NEGF) technique. Unlike conventional interconnect metals such as $\mathrm{Cu}$ and $\mathrm{Al}$, we find that $\mathrm{Co}\mathrm{Si}$ and $\mathrm{Co}\mathrm{Ge}$ conduct primarily through topologically protected surface states in thin-film structures even in the presence of grain boundaries. The area-normalized resistance decreases with decreasing film thickness for $\mathrm{Co}\mathrm{Si}$ and $\mathrm{Co}\mathrm{Ge}$ thin films both with and without grain boundaries; a trend opposite to that of the conventional metals $\mathrm{Cu}$ and $\mathrm{Al}$. The surface-dominated transport mechanisms in thin films of topological semimetals with grain boundaries does not follow the classic resistivity size effect, and suggests that these materials may be promising candidates for applications as nanointerconnects where high electrical resistivity acts as a major bottleneck limiting semiconductor device performance.

Ultrafast acoustic imaging experiments are a powerful tool to investigate, at the nanometer scale, cell mechanical properties such as stiffness, viscosity, and adhesion, properties that play some roles in the life and death of cells. However, due to cell complex structures, the ultrafast acoustic signal analysis is quite intricate and depends on multiple parameters. Complex data analysis with poorly known parameters can be handled by a data clustering method as already shown in particle physics and biology. In this work, ultrafast acoustic data analysis is tackled by a spectral clustering method followed by a hierarchical agglomerating method. Coupled to conventional microscopy performed on the very same cell, the clustered data can be assigned to inner-cell features such as the nucleus, the cytoplasm, and the cytoskeleton. The signal dependency on the cell thickness and stiffness is highlighted. Moreover, thanks to the improvement of the signal-to-noise ratio, the nature of the adhesion is also assessed through observation and characterization of a polymerlike layer as thin as a few nanometers.

Recent studies have revealed that atomically thin two-dimensional (2D) materials exhibit outstanding nonlinear optical (NLO) properties compared with traditional NLO crystals, which provides great potential in numerous photonic devices such as integrated nonlinear photonic chips and modulators. However, the evolution of NLO response with layer number and pump intensity of various 2D materials remains unclear, but offers a basis for hunting powerful NLO materials. Herein, controllable synthesis of a series of $\mathrm{Re}$$X_2$ (X = $\mathrm{Se}$, $\mathrm{S}$) films with different numbers of layers is achieved by chemical vapor deposition. Z-scan techniques are used to investigate the nonlinear absorption coefficient (β) and nonlinear refractive index ($n_2$). Empirically, the absolute values of both β and $n_2$ show a downtrend as the power exponential function with the layer and pump intensity. The NLO parameters of ${\mathrm{Re}\mathrm{Se}}_2$ film are β ∼ 4156 cm/GW and $n_2$ ∼ 1.819 × ${10}^{-10}{\mathrm{cm}}^2/\mathrm{W}$, which are more significant than those of ${\mathrm{Re}\mathrm{S}}_2$ film (β ∼ 806 cm/GW and $n_2$ ∼ 2.22 × ${10}^{-11}{\mathrm{cm}}^2/\mathrm{W}$). This result is ascribed to the smaller band gap, higher carrier density, and larger ground-state absorption of ${\mathrm{Re}\mathrm{Se}}_2$ than that of ${\mathrm{Re}\mathrm{S}}_2$, which is confirmed by the theoretical analysis of the band structure and a three-energy-level system. It is worth pointing out that the β values for $\mathrm{Re}$$X_2$ films are 10–100 times larger than those of ${\mathrm{WS}}_2$, ${\mathrm{Mo}\mathrm{Se}}_2$, and ${\mathrm{Mo}\mathrm{Te}}_2$. The $n_2$ values for $\mathrm{Re}$$X_2$ films are 3–4 orders of magnitude larger than for traditional semiconductors such as $\mathrm{Ga}\mathrm{As}$ and $\mathrm{Si}$. Our results suggest that $\mathrm{Re}$$X_2$ semiconductors are greatly anticipated in designing high-performance on-chip photonic devices.

Phase-change materials with temperature-dependent thermal conductivity have drawn lots of attention recently. They have exhibited promising potentials in devices such as the thermal diode, thermal memristor, etc. Here we investigate the heat exchange between two reservoirs through a bare phase-change material. In the presence of temperature variation in one of the reservoirs, we find a unidirectional thermal flux between those two reservoirs though they share the same average temperature, i.e., no temperature bias. By mapping such a system to the heat exchange between two reservoirs through a regular material with a constant thermal conductivity, we obtain an effective temperature difference $T_{\mathrm{eff}}$, which should present to maintain the unidirectional thermal flux. We then provide a semianalytical formula for $T_{\mathrm{eff}}$, which works well over an extensive parameter range. The parameter dependence of $T_{\mathrm{eff}}$ and the temperature dependence of the thermal capacity are then studied in detail. Our work thus provides a useful tool for calibration of heat conduction within phase-change materials and leads to potential applications related to energy extraction from the thermal variation of the environment.

Isolated attosecond pulses (IAPs) in the soft-x-ray (SXR) region have been successfully produced via high-order harmonic generation in a gas medium at several laboratories, but pulse energies are much lower compared to the IAPs in the extreme ultraviolet. Here, we show that in a gas-filled hollow waveguide, an efficient gating method using a few-cycle three-color synthesizer is able to generate stronger SXR IAPs. We show that an 104-as IAP in the SXR region (central energy of 210 eV) with a higher pulse energy (0.9 nJ) and a lower divergence (within 1.5 mrad) can be generated with a three-color laser beam for input pulse energy at 0.62 mJ, under the condition of optimal gas pressure and waveguide length. The enhancement of the IAP is attributed to favorable phase matching of SXR high harmonics caused by the evolution of the three-color waveform during its propagation in the waveguide, which can be understood by analyzing the time shift of the waveform due to the interplay of the waveguide mode, atomic dispersion, and plasma dispersion. We check that the gating method is quite robust with respect to the phase jitter in the waveform. The advantage of this method is further demonstrated by comparing the generation of SXR high harmonics and attosecond pulses with two other schemes: the three-color waveform in a gas cell and a previously optimized two-color waveform in a gas-filled waveguide.

Along with the scaling of dimensions in quantum systems, transitions between the system’s energy levels would become close in frequency, which are conventionally resolved by weak and lengthy pulses. Here, we extend and experimentally demonstrate analytically based swift quantum control techniques on a four-level trapped ion system, where we perform individual or simultaneous control over two pairs of spectrally nearby transitions with tailored time-varied drive, achieving operational fidelities ranging from 99.2(3)% to 99.6(3)%. We achieve approximately an order of magnitude speed up compared with the case of a weak square pulse for a general control. Therefore, our demonstration may be beneficial to a broad range of quantum systems with crowded spectrum, for spectroscopy, quantum information processing, and quantum simulation.

The interfacial Dzyaloshinskii-Moriya interaction (DMI) has long been observed in normal metal/ferromagnetic multilayers, enabling the formation of chiral domain walls, skyrmions, and other two-dimensional antisymmetric spin textures confined within a single ferromagnetic layer, while more recent works on the interlayer DMI reveal alternative pathways for realizing chiral three-dimensional spin textures by extending the DMI to two spatially distinct magnetic layers. Here, we report on the interlayer DMI between two orthogonally magnetized ferromagnetic layers ($\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{Co}$) mediated by a $\mathrm{Pt}$ layer and confirm the chiral nature of the observed effective field of up to 37 Oe through asymmetric hysteresis loops under an in-plane field. We highlight the importance of growth-induced in-plane symmetry breaking, resulting in a sizable interlayer DMI and a universal characteristic vector through wedge deposition of the samples. We further perform deterministic current-driven magnetization switching in the perpendicularly magnetized $\mathrm{Co}$ layer utilizing solely the effective field from the interlayer DMI. These results demonstrate the interlayer DMI’s potential to facilitate deterministic field-free switching in spin-memory applications.

Several electronic and optoelectronic devices have been proposed in recent years based on vertical heterostructures of two-dimensional (2D) materials. The large number of combinations of available 2D materials and the even larger number of possible heterostructures require effective and predictive device-simulation methods, to inform and accelerate experimental research and to support the interpretation of experiments. Here, we propose a computationally effective and physically sound method to model electron transport in 2D van der Waals heterostructures, based on a multiscale approach and quasiatomistic Hamiltonians. The method uses ab initio simulations to extract the parameters of a simplified tight-binding Hamiltonian based on a uniform three-dimensional lattice geometry that enables device simulations using the nonequilibrium Green’s function approach in a computationally effective way. We describe the application and limitations of the method and discuss the examples of two use cases of practical electronic devices based on 2D materials, such as a field-effect transistor and a floating-gate memory, composed of molybdenum disulphide, hexagonal boron nitride and graphene.

We demonstrate a nonlinear photomodulation spectroscopy method to image the mode profile of a high-$Q$ photonic crystal resonator (PhCR). This far-field imaging method is suitable for ultrahigh-$Q$ cavities which we demonstrate on a $Q=619000$ PhCR. We scan the PhCR surface with a 405-nm pump beam that modulates the refractive index by local thermal tuning, while probing the response of the resonance. We enhance resolution by probing at high power, using the thermo-optical nonlinearity of the PhCR. Spatial resolution of the thermo-optical effect is typically constrained by the broad thermal profile of the optical pump. Here we go beyond the thermal limit and show that we can approach the diffraction limit of the pump light. This is due to free carrier absorption that heats up the PhCR only when there is overlap between the optical pump spot and the optical mode profile. This is supported with a thermo-optical model that reproduces the high-resolution mode mapping. Results reveal that the observed enhanced resolution is reached for surprisingly low carrier density.

Cavity optomechanics enables photon-phonon interaction and correlations by harnessing the radiation-pressure force. Here, we realize a “cavity-in-a-membrane” optomechanical architecture, which allows detection of the motion of lithographically defined, ultrathin membranes via an integrated optical cavity. Using a dissipative filtering method, we are able to eliminate the probe light in situ and observe photon-phonon correlations associated with the low-frequency membrane mechanical mode. The developed method is generally applicable for study of low-frequency light-scattering processes where conventional frequency-selective filtering is unfeasible.

We present a gate-voltage-tunable transmon qubit (gatemon) based on planar $\mathrm{In}\mathrm{As}$ nanowires that are selectively grown on a high-resistivity silicon substrate using III-V buffer layers. We show that low-loss superconducting resonators with an internal quality of $2\times {10}^5$ can readily be realized using these substrates after the removal of buffer layers. We demonstrate coherent control and readout of a gatemon device with a relaxation time, $T_1\approx 700\mathrm{ns}$, and dephasing times, $T_2^{\ast }\approx 20\mathrm{ns}$ and $T_{2,\mathrm{echo}}\approx 1.3\mu \mathrm{s}$. Further, we infer a high junction transparency of $0.4$–$0.9$ from an analysis of the qubit anharmonicity.

A spin ensemble in thermal equilibrium continuously undergoes random fluctuations analogous to those observed in Brownian motion, a process known as spin noise. Here, we investigate the dynamics of a system comprising a spin-1 paramagnetic center and a hyperfine-coupled spin-1/2 nucleus in the vicinity of a level crossing. We theoretically show that nuclear spins polarize efficiently under the combined action of thermal fluctuations and a closed-loop feedback protocol. The latter articulates periodic observations of the electronic magnetization and radio-frequency pulses connecting hybrid states with opposite nuclear spin alignment. Since nuclear polarization emerges from electronic spin fluctuations, not spin order, this microwave-free technique generically benefits from warmer, not colder, operation temperatures. Furthermore, because the spin dynamics at play near a level crossing is rather insensitive to the absolute value of the magnetic field, our work promises opportunities for high-field dynamic nuclear polarization, which is difficult to attain through present methods.

Optical vector beams with a higher-order polarization state, represented by a point on a higher-order Poincaré sphere, have recently found advantages in various applications. However, the considered beams are usually spatially fully coherent (monochromatic), which makes them susceptible to complex environments. In this work, we introduce a coherence-engineering protocol to generate partially coherent vectorial secondary light sources in which a higher-order polarization state is encoded into the structure of electromagnetic spatial coherence. The encoded complex polarization state is well reconstructed in the far field whose global degree of polarization can be controlled with the transverse coherence length of the source. In particular, the produced partially coherent beams are highly robust against obstructions, which is demonstrated theoretically and experimentally by inserting a static obstacle against the source or introducing strong atmospheric turbulence into the transmission link. The results of this work can find useful applications in the transfer of complicated polarization-encoded information in harsh environments.

Owing to their dual-resonance enhanced sensitivity, cavity optomechanical systems provide an ideal platform for ultrasound sensing. In this work, we realize high-sensitivity air-coupled ultrasound sensing from kilohertz to megahertz frequency range based on whispering gallery mode microcavities. Using a $57$-$\mu \mathrm{m}$-diameter microtoroid with high optical Q factor (approximately ${10}^7$) and mechanical Q factor (approximately $700$), we achieve sensitivities of $46\mu \mathrm{Pa}$ ${\mathrm{Hz}}^{-1/2}$–10 mPa ${\mathrm{Hz}}^{-1/2}$ in a frequency range of 0.25–3.2 MHz. Thermal-noise-limited sensitivity is realized around a mechanical resonance at 2.56 MHz, in a frequency range of 0.6 MHz. We also observe the second- and third-order mechanical sidebands, and quantitatively study the intensities of each mechanical sideband as a function of the mechanical displacement. Measuring the combination of signal-to-noise ratios at all sidebands has the potential to extend the dynamic range of ultrasound sensing.