Artifacts made of cellulose, such as ancient documents, pose a significant experimental challenge in the terahertz transmission spectra interpretation due to their small optical thickness. In this paper, we describe a method to recover the complex refractive index of cellulose fibers from the terahertz transmission data obtained on single freely standing paper sheets in the (0.2–3.5)-THz range. By using our technique, we eliminate Fabry-Perot effects and recover the absorption coefficient of the cellulose fibers. The obtained terahertz absorption spectra are explained in terms of absorption peaks of the cellulose crystalline phase superimposed to a background contribution due to a disordered hydrogen-bond network. The comparison between the experimental spectra with terahertz vibrational properties simulated by density-functional-theory calculations confirms this interpretation. In addition, evident changes in the terahertz absorption spectra are produced by natural and artificial aging on paper samples, whose final stage is characterized by a spectral profile with only two peaks at about 2.1 and 3.1 THz. These results can be used to provide a quantitative assessment of the state of preservation of cellulose artifacts.

Background forces are linear long-range interactions of the cantilever body with its surroundings that must be compensated for in order to reveal tip-surface force, the quantity of interest for determining material properties in atomic force microscopy. We provide a mathematical derivation of a method to compensate for background forces, apply it to experimental data, and discuss how to include background forces in simulation. Our method, based on linear-response theory in the frequency domain, provides a general way of measuring and compensating for any background force and it can be readily applied to different force reconstruction methods in dynamic AFM.

Accurate quantum gates are basic elements for building quantum computers. Recently, there has been great interest in designing quantum logic gates by using the blockade effect of Rydberg atoms. The fidelity and operation speed of these gates, however, are fundamentally limited by an intrinsic blockade error. Here we propose a type of quantum gate, which is based on the Rydberg blockade effect, yet free from any blockade error. In contrast to the “blocking” method in previous schemes, we use the Rydberg energy shift to realize a rational generalized Rabi frequency so that a $\pi$ phase for one input state of the gate emerges. This leads to an accurate Rydberg quantum logic gate that can operate on a $0.1\text{−}\mu \mathrm{s}$ time scale or faster because it works by a Rabi frequency which is comparable to the blockade shift.

${\mathrm{BiCoO}}_3$ is a perovskite-type cobalt oxide with a polar structure. We investigate the dynamics of its polar state by using photoexcitation with femtosecond visible and terahertz pulses. The intensity of the second-harmonic light, caused by a polar structure without inversion symmetry, is enhanced by more than 50% by irradiating the terahertz pulse showing an ultrafast response following the femtosecond terahertz pulse. By contrast, it is suppressed by more than 60% by exciting visible light with a 100-fs pulse width at room temperature. These results suggest not only an important role of the orbital excitation in the ${\mathrm{Co}}^{3+}$ ion but also a key to improving the nonlinear optical response, e.g., a figure of merit in nonlinear crystals on the time scale of a femtosecond.

In all-oxide ferroelectric ($FE$) superconductors ($S$), due to the low carrier concentration of oxides compared to transition metals, the $FE$ interfacial polarization charges induce an accumulation (or depletion) of charge carriers in the $S$. This effect leads either to an enhancement or a depression of its critical temperature, depending on the $FE$ polarization direction. Here, we exploit this effect at a local scale to define planar weak links in high-temperature-superconducting wires. These experiments are realized in ${\mathrm{BiFeO}}_3(FE)/{\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}(S)$ bilayers in which the remnant $FE$ domain structure is “written” by locally applying voltage pulses with a conductive-tip atomic force microscope. In this fashion, the $FE$ domain pattern defines a spatial modulation of superconductivity. This characteristic allows us to “write” a device whose electrical transport shows different temperature regimes and magnetic-field-matching effects that are characteristic of Josephson coupled weak links. This behavior illustrates the potential of the ferroelectric approach for the realization of high-temperature-superconducting devices.

Optical isolation, nonreciprocal phase transmission, and topological phases for light based on synthetic gauge fields have been raising significant interest in the recent literature. Cavity-optomechanical systems that involve two optical modes coupled to a common mechanical mode form an ideal platform to realize these effects, providing the basis for various recent demonstrations of optomechanically induced nonreciprocal light transmission. Here, we establish a unifying theoretical framework to analyze optical nonreciprocity and the breaking of time-reversal symmetry in multimode optomechanical systems. We highlight two general scenarios to achieve isolation, relying on either optical or mechanical losses. Depending on the loss mechanism, our theory defines the ultimate requirements for optimal isolation and the available operational bandwidth in these systems. We also analyze the effect of sideband resolution on the performance of optomechanical isolators, highlighting the fact that nonreciprocity can be preserved even in the unresolved sideband regime. Our results provide general insights into a broad class of parametrically modulated nonreciprocal devices, paving the way towards optimal nonreciprocal systems for low-noise integrated nanophotonics.

We measure and compare the critical-current oscillation characteristics of Josephson junctions as a function of Ni thickness in different barrier structures. The characteristics dependent on the relative Ni thickness, such as the presence of nodes and the oscillation period, are consistent with a conventional, clean-limit magnetic Josephson-junction model. However, the oscillation phases have different offsets in the Ni thickness between single Ni and $\mathrm{Ni}\text{−}{({\text{Ni}}_{81}{\text{Fe}}_{19})}_x{\mathrm{Nb}}_y$-based barriers, which cannot be explained by the bulk exchange field effect alone. This effect does not originate from the ferromagnetism in ${({\text{Ni}}_{81}{\text{Fe}}_{19})}_x{\mathrm{Nb}}_y$ nor is it cumulative with an additional ${({\text{Ni}}_{81}{\text{Fe}}_{19})}_x{\mathrm{Nb}}_y$ layer. Our results present clear evidence that a nonmagnetic layer can affect the superconducting spin phase across the junction as strongly as the conventional exchange field effect.

The epitaxial growth of the ${\mathrm{Co}}_2{\mathrm{Fe}}_{0.4}{\mathrm{Mn}}_{0.6}\mathrm{Si}$ (CFMS) Heusler alloy on MgO with high crystal orientation requires the use of a buffer layer. The utilization of the Cr buffer layer in a controlled manner without hampering the intrinsic CFMS properties remains a challenge. Here, we epitaxially grow the CFMS films on a Cr-buffered MgO substrate and investigate the thickness ($t$)-dependent variation in structural ordering, and its impact on magnetic Gilbert damping and magnetic anisotropy. We observe a regular improvement in the crystal structure with increasing $t$, which causes a similar increasing trend in saturation magnetization and magnetocrystalline anisotropy. Interestingly, we achieve a very low $t$-independent value of Gilbert damping parameter ($\alpha$) of $\sim 0.0045$ because of an unaltered atomic-site ordering which indicates a different origin of magnetocrystalline anisotropy and $\alpha$ in this system. Notably, $\alpha$ also remains nearly independent of frequency ($f$) for a lower thickness regime in these CFMS films. The observation of a $t$-independent and $f$-independent value of $\alpha$ strongly suggests the suitability of a thinner CFMS film for device applications in the broadband frequency regime.

Electronic properties of III-V semiconductor alloys are examined using first principles, with the focus on the spatial localization of electronic states. We compare localization at the band edges due to various isovalent impurities in a host GaAs, including its impact on the photoluminescence linewidths and carrier mobilities. The extremity of localization at the band edges is correlated with the ability of individual elements to change the band gap and the relative band alignment. Additionally, the formation energies of substitutional defects are calculated and linked to challenges associated with the growth and formability of alloys. A spectrally resolved inverse participation ratio is used to map localization in prospective GaAs-based materials alloyed with B, N, In, Sb, and Bi for $1.55\text{−}\mu \mathrm{m}$-wavelength telecommunication lasers. This analysis is complemented by a band unfolding of the electronic structure and a discussion of the implications of localization on the optical gain and Auger losses. Correspondence with experimental data on the broadening of the photoluminescence spectrum and charge-carrier mobilities show that the localization characteristics can serve as a guideline for the engineering of semiconductor alloys.

Aqueous microdroplets are challenging objects for photoassisted manipulation techniques based on lithium niobate (LN) substrates. In this paper, we demonstrate a successful photovoltaic manipulation of water microdroplets on LN substrates coated with a hydrophobic film. Through the analysis of the dynamic process of the photovoltaic manipulation and the simulation of the electrostatic distribution around the microdroplet-substrate interface, the electrowetting mechanism is suggested to account for the photovoltaic manipulation of water microdroplets. Both the hydrophobicity and the large electrostatic energy capability of the coated film are emphasized for producing a sufficient unbalanced hydrostatic pressure required by the steady water-microdroplet actuation.

We perform electronic-structure calculations based on the first-principles many-body–theory approach in order to study quasiparticle band gaps and optical absorption spectra of hydrogen-passivated zigzag SiC nanoribbons. Self-energy corrections are included using the $GW$ approximation, and excitonic effects are included using the Bethe-Salpeter equation. We systematically study nanoribbons that have widths between 0.6 and 2.2 nm. Quasiparticle corrections widen the Kohn-Sham band gaps because of enhanced interaction effects, caused by reduced dimensionality. Zigzag SiC nanoribbons with widths larger than 1 nm exhibit half-metallicity at the mean-field level. The self-energy corrections increase band gaps substantially, thereby transforming the half-metallic zigzag SiC nanoribbons to narrow gap spin-polarized semiconductors. Optical absorption spectra of these nanoribbons get dramatically modified upon inclusion of electron-hole interactions, and the narrowest ribbon exhibits strongly bound excitons, with binding energy of 2.1 eV. Thus, the narrowest zigzag SiC nanoribbon has the potential to be used in optoelectronic devices operating in the IR region of the spectrum, while the broader ones, exhibiting spin polarization, can be utilized in spintronic applications.

We apply the concept of the “elastic dipole tensor” to study the effect of strain on polaron hopping and electronic conductivity in ionic crystals. As a model system, we choose the commercially used cathode material for Li-ion batteries, namely, ${\mathrm{LiCoO}}_2$, which has a layered structure with alternating planes of oxygen, lithium, oxygen, and cobalt. It is demonstrated that the electronic conductivity of ${\mathrm{LiCoO}}_2$ increases (decreases) exponentially with compressive (tensile) strains. The results of the analytical elastic-dipole-tensor method are shown to be in good agreement with a direct approach in which migration barriers of polaron hopping as a function of strain are evaluated using density-functional-theory-based nudged elastic-band calculations. In a broader sense, the presented analytical approach can be used to study the effect of any induced or applied elastic strain field on the electronic conductivity of polarizable semiconductors and insulators.

We demonstrate a steplike optical modulation based on the activation and deactivation of a bistable localized structure using a photoinduced and reconfigurable miniaturized $30\times 30\mu \mathrm{m}$ electroactivated funnel waveguide. Control of a single $10\text{−}\mu \mathrm{m}$-diameter spot in a $200\text{−}\mu \mathrm{m}$-diameter vertical-cavity surface-emitting laser at 980 nm is achieved modulating the phase of an exciting beam in the specific position of the spot in the cavity. This localized on-off response can be scaled into arrays and offer a possible route to fast integrated optical logical functions and memory at low intensities at near-infrared wavelengths.

Layered $P2$ oxides are promising cathode materials for rechargeable sodium-ion batteries. In this work, we systematically investigate the effects of transition-metal (TM) mixing on Na ordering and kinetics in the ${\mathrm{Na}}_x{\mathrm{Co}}_{1-y}{\mathrm{Mn}}_y{\mathrm{O}}_2$ model system using density-functional-theory (DFT) calculations. The DFT-predicted 0-K stability diagrams indicate that Co-Mn mixing reduces the energetic differences between Na orderings, which may account for the reduction of the number of phase transformations observed during the cycling of mixed-TM $P2$ layered oxides compared to a single TM. Using ab initio molecular-dynamics simulations and nudged elastic-band calculations, we show that the TM composition at the Na(1) (face-sharing) site has a strong influence on the Na site energies, which in turn impacts the kinetics of Na diffusion towards the end of the charge. By employing a site-percolation model, we establish theoretical upper and lower bounds for TM concentrations based on their effect on Na(1) site energies, providing a framework to rationally tune mixed-TM compositions for optimal Na diffusion.

We measure the electroluminescence of light-emitting diodes (LEDs) on substrates with low dislocation densities (LDD) at ${10}^6{\mathrm{cm}}^{-2}$ and low ${10}^8{\mathrm{cm}}^{-2}$, and compare them to LEDs on substrates with high dislocation densities (HDD) closer to ${10}^{10}{\mathrm{cm}}^{-2}$. The external quantum efficiencies (EQEs) are fitted using the $ABC$ model with and without localization. The nonradiative-recombination (NR) coefficient $A$ is constant for HDD LEDs, indicating that the NR is dominated by dislocations at all wavelengths. However, $A$ strongly increases for LDD LEDs by a factor of 20 when increasing the emission wavelength from 440 to 540 nm. We attribute this to an increased density of point defects due to the lower growth temperatures used for longer wavelengths. The radiative recombination coefficient $B$ follows the squared wave-function overlap for all samples. Using the observed coefficients, we calculate the peak efficiency as a function of the wavelength. For HDD LEDs the change of wave-function overlap (i.e., $B$) is sufficient to reduce the EQE as observed, while for LDD LEDs also the NR coefficient $A$ must increase to explain the observed EQEs. Thus, reducing NR is important to improving the EQEs of green LEDs, but this cannot be achieved solely by reducing the dislocation density: point defects must also be addressed.

Entangling remote qubits is an essential technological element in the distributed quantum information processing. Here, we propose a deterministic scheme to generate maximal entanglement between remote superconducting atoms, using a propagating microwave photon as a flying qubit. The building block of this scheme is an atom-photon two-qubit gate, in which the photon qubit is encoded on its carrier frequencies. The gate operation completes deterministically upon reflection of a photon, and the gate type can be continuously varied (including swap, $\sqrt{\mathrm{SWAP}}$, and identity gates) through in situ control of the drive field. Applying such atom-photon gates sequentially, we can perform various gate operations between remote superconducting atoms.

The silicon-on-chip photonic circuit is a very promising platform for scalable quantum-information technology for its low loss, small footprint, and its compatibility with CMOS as well as telecom techniques. Multiplexed multiple entanglement sources, including energy-time, time-bin, and polarization-entangled sources based on 1-cm-length single silicon nanowire, are all compatible with the (100-GHz) dense-wave-division-multiplexing system. Different methods, such as two-photon interference as well as Bell-inequality and quantum-state tomography, are used to characterize the quality of these entangled sources. Multiple entanglements are generated over more than five channel pairs with high raw (net) visibilities of around 97% (100%). The emission spectral brightness of these entangled sources reaches $4.2\times {10}^5/(\mathrm{s}\mathrm{nm}\mathrm{mW})$. The quality of the photon pair generated in the continuous and pulse-pump regimes are compared. The high quality of these multiple entanglement sources makes them very promising for use in minimized quantum-communication and computation systems.

We investigate both theoretically and experimentally a laser-based controlled tuning of the nonlinear behaviors of a single mechanical resonator. Thanks to localized three-dimensional modifications induced by femtosecond-laser irradiation, a Duffing-like oscillator is switched from a hardening resonance to a linear response and then to a softening resonance and exhibits a wide tunability of the resonant frequency and a remarkable increase of its linear dynamic range. The principles that underlie laser-tuned nonlinear oscillators are generic and simple, suggesting its wide applicability not only for micro- or nano-optomechanical systems but also as a generic framework for characterizing and understanding the physics of in-volume laser-affected zones.

We study nonlinear transport effects on the maximum efficiency and power for both inelastic and elastic thermoelectric generators. The former device refers to phonon-assisted hopping in double quantum dots, while the latter device is represented by elastic tunneling through a single quantum dot. We find that nonlinear thermoelectric transport can lead to enhanced efficiency and power for both types of devices. A comprehensive survey of various quantum-dot energy, temperature, and parasitic heat conduction reveals that the nonlinear transport–induced improvements of the maximum efficiency and power are overall much more significant for inelastic devices than for elastic devices, even for temperature biases as small as $T_h=1.2T_c$ ($T_h$ and $T_c$ are the temperatures of the hot and cold reservoirs, respectively). The underlying mechanism is revealed as due to the fact that, unlike the Fermi distribution, the Bose distribution is not bounded when the temperature bias increases. A large flux density of absorbed phonons leads to a great enhancement of the electrical current, output power, and energy efficiency, dominating over the concurrent increase of the parasitic heat current. Our study reveals that nonlinear transport effects can be a useful tool for improving thermoelectric performance.

Spin-transfer torques in a nanocontact to an extended magnetic film can create spin waves that condense to form dissipative droplet solitons. Here we report an experimental study of the temperature dependence of the current and applied field thresholds for droplet soliton formation, as well as the nanocontact’s electrical characteristics associated with droplet dynamics. Nucleation requires lower current densities at lower temperatures, in contrast to typical spin-transfer-torque-induced switching between static magnetic states. Magnetoresistance and electrical noise measurements (10 MHz-1 GHz) show that droplet solitons become more stable at lower temperature. These results are of fundamental interest in understanding the influence of thermal noise on droplet solitons and have implications for the design of devices using the spin-transfer-torque effects to create and control collective spin excitations.