This paper is a contribution to the Physical Review Applied collection in memory of Mildred S. Dresselhaus.

In 1993, Hicks and Dresselhaus [Thermoelectric figure of merit of a one-dimensional conductor, Phys. Rev. B 47, 16631 (1993).] suggested that Bi nanowires could result in values of the thermoelectric figure of merit $zT>1$. The Dresselhaus group also calculated a ternary phase diagram for ${\mathrm{Bi}}_{1\text{−}x}{\mathrm{Sb}}_x$ nanowires as a function of $x$ and wire diameter. This manuscript reports a wet-chemical method to synthesize ${\mathrm{Bi}}_{1\text{−}x}{\mathrm{Sb}}_x\text{−}\mathrm{silica}$ nanowire composites. Resistivity, Hall electron concentration, electron mobility, Seebeck and Nernst coefficients, and thermal conductivity of composites are measured and compared to bulk polycrystalline ${\mathrm{Bi}}_{1\text{−}x}{\mathrm{Sb}}_x$ samples prepared either by ingot casting or by the same wet chemistry but without nanostructuring. A clear increase of the thermopower in 20-nm ${\mathrm{Bi}}_{94}{\mathrm{Sb}}_6\text{−}\mathrm{silica}$ is reported when compared to bulk samples, and the values are among the highest found in the literature from 300 to 380 K, even though the electron concentration is higher than in the bulk. This suggests that consistent with theory, size quantization is responsible for the thermopower increase.

This paper is a contribution to the Physical Review Applied collection in memory of Mildred S. Dresselhaus.

Analytical formulas for thermoelectric figures of merit and power factors are derived based on the one-band model. We find that there is a direct relationship between the optimum figures of merit and the optimum power factors of semiconductors despite of the fact that the two quantities are generally given by different values of chemical potentials. By introducing a dimensionless parameter consisting of the optimum power factor and lattice thermal conductivity (without electronic thermal conductivity), it is possible to unify optimum figures of merit of both bulk and low-dimensional semiconductors into a single universal curve that covers many materials with different dimensionalities.

This paper is a contribution to the Physical Review Applied collection in memory of Mildred S. Dresselhaus.

The mechanism of water diffusion across reverse osmosis nanocomposite membranes made of carbon nanotubes (CNTs) and aromatic polyamide is not completely understood despite its high potential for desalination applications. While most of the groups have proposed that superflow inside the CNT might positively impact the water flow across membranes, here we show theoretical evidence that this is not likely the case in composite membranes because CNTs are usually oriented parallel to the membrane surface, not to mention that sometimes the nanotube cores are occluded. Instead, we propose an oriented diffusion mechanism that explains the high water permeation by decreasing the diffusion path of water molecules across the membranes, even in the presence of CNTs that behave as impermeable objects. Finally, we provide a comprehensive description of the molecular dynamics occurring in water desalination membranes by considering the bond polarizability caused by dynamic charge transfer and explore the use of molecular-dynamics-derived stochastic diffusion simulations. The proposed water diffusion mechanism offers an alternative and most likely explanation for the high permeation phenomena observed in CNTs and PA nanocomposite membranes, and its understanding can be helpful to design the next generation of reverse osmosis desalination membranes.

This paper is a contribution to the Physical Review Applied collection in memory of Mildred S. Dresselhaus.

Phonon-related thermal conductivity of graphene is calculated as a function of the temperature and sample size of graphene in which the crossover of ballistic and diffusive thermal conductivity occurs at around 100 K. The diffusive thermal conductivity of graphene is evaluated by calculating the phonon mean free path for each phonon mode in which the anharmonicity of a phonon and the phonon scattering by a ${}^{13}\mathrm{C}$ isotope are taken into account. We show that phonon-phonon scattering of out-of-plane acoustic phonon by the anharmonic potential is essential for the largest thermal conductivity. Using the calculated results, we can design the optimum sample size, which gives the largest thermal conductivity at a given temperature for applying thermal conducting devices.

We provide here a demonstration of $12\text{−}\mu \mathrm{m}$-pitch nanoelectromechanical resonant infrared sensors with fully integrated capacitive transduction. A low-temperature fabrication process is used to manufacture torsional resonator arrays. An $\mathsf{H}$-shaped pixel with $9\text{−}\mu \mathrm{m}$-long nanorods and $(250\times 30)\text{−}{\mathrm{nm}}^2$ cross section is designed to provide high thermal response whose experimental measurements reach up to $1024\mathrm{Hz}/\mathrm{nW}$. A mechanical dynamic range of over 113 dB is obtained, which leads to an Allan deviation of ${\sigma }_A=3\times {10}^{-7}$ at room temperature and 50-Hz noise bandwidth (${\sigma }_A=1.5\times {10}^{-7}$ over 10 Hz). These features allow us to reach a sensitivity of $(8–12)\text{−}\mu \mathrm{m}$ radiation of $27\mathrm{pW}/\sqrt{\mathrm{Hz}}$ leading to a noise-equivalent temperature difference (NETD) of 2 K for a 50-Hz noise bandwidth ($\mathrm{NETD}=1.5\mathrm{K}$ at 10 Hz). We demonstrate that the resolution is no more set by the phonon noise but by the anomalous phase noise already encountered in flexural nanoresonators. By both improving the temperature coefficient of frequency of a factor 10 and using a readout electronics at the pixel level, these resonators will lead to a breakthrough for uncooled infrared detectors. We expect that the NETD will rapidly drop to 180 mK with electronics close to the pixel. As a result of the features of our torsional resonators, an alternative readout scheme of pixels is suggested.

In a specially designed semiconductor device consisting of three capacitively coupled double quantum dots, we achieve strong and tunable coupling between a target qubit and two control qubits. We demonstrate how to completely switch on and off the target qubit’s coherent rotations by presetting two control qubits’ states. A Toffoli gate is, therefore, possible based on these control effects. This research paves a way for realizing full quantum-logic operations in semiconductor multiqubit systems.

Transformation acoustics is a method to design novel acoustic devices, while the complexity of the material parameters hinders its progress. In this paper, we analytically present a three-dimensional perfect undetectable acoustic device from Fabry-Pérot resonances and confirm its functionality from Mie theory. Such a mechanism goes beyond the traditional transformation acoustics. In addition, such a reduced version can be realized by holey-structured metamaterials. Our theory paves a way to the implementation of three-dimensional transformation acoustic devices.

We report on the theory and experimental generation of a class of diffraction-attenuation-resistant beams with state of polarization (SOP) and intensity that can be controlled on demand along the propagation direction. This control is achieved by a suitable superposition of Bessel beams, whose parameters are systematically chosen based on closed-form analytic expressions provided by the frozen waves method. Using an amplitude-only spatial light modulator, we experimentally demonstrate three scenarios. In the first, the SOP of a horizontally polarized beam evolves to radial polarization and is then changed to vertical polarization, with the beam intensity held constant. In the second, we simultaneously control the SOP and the longitudinal intensity profile, which is chosen such that the beam’s central ring can be switched off over predefined space regions, thus generating multiple foci with different SOPs and at different intensity levels along the propagation. Finally, the ability to control the SOP while overcoming attenuation inside lossy fluids is shown experimentally. We envision our proposed method to be of great interest for many applications, such as optical tweezers, atom guiding, material processing, microscopy, and optical communications.

We theoretically propose a thermoelectric spin-current generator consisting of a zigzag germanene nanoribbon with thermal leads that are partially exposed to noncollinear exchange fields. Our results reveal that the exchange field in various regions and directions changes the helical edge states to different characteristics, and hence the pure spin current, the 100% spin-polarized current, and the pure charge current can be generated only under the temperature difference $\mathrm{\Delta }T$. The intertransformation between these three types of currents can be realized by tuning the relative angle of local exchange fields to $\theta =0$, $0.5\pi$, or $\pi$. It is worth noting that the pure spin current cannot be obtained without the temperature difference. We also find that the giant magnetoresistance can reach extremely large values in the bulk gap region by adjusting the exchange field of both leads to the $x$ direction. All of the results indicate that the proposed germanene nanosystem is a promising candidate for spin caloritronic devices.

Recent years have seen a paradigm shift in biomaterials toward stimuli-responsive switchable systems that actively interact with their environment. This work demonstrates how to turn the ubiquitous off-the-shelf material gelatin into such a smart biomaterial. This is achieved by realizing the shape-memory effect, viz., a temperature-induced transition from a secondary into a primary shape that has been programmed in the first place merely by exposure to energetic electrons without addition of potentially hazardous cross-linkers. While this scenario is experimentally quantified for exemplary actuators, a theoretical framework capable of unraveling the molecular foundations and predicting experiments is also presented. It particularly employs molecular dynamics modeling based on force fields that are also derived within this work. Implementing this functionality into a highly accepted material, these findings open an avenue for large-scale application in a broad range of areas.

Graphene modification is one of the most effective routes to enhance the electrochemical properties of the transition-metal sulfide anode for Li-ion batteries and the ${\mathrm{Li}}_2\mathrm{S}$ cathode for Li-S batteries. Boron, nitrogen, oxygen, phosphorus, and sulfur doping greatly affect the electrochemical properties of ${\mathrm{Li}}_2\mathrm{S}/\mathrm{graphene}$. Here, we investigate the interfacial binding energy, lithium adsorption energy, interface diffusion barrier, and electronic structure by first-principles calculations to unveil the diverse effects of different dopants during interfacial lithiation reactions. The interfacial lithium storage follows the pseudocapacitylike mechanism with intercalation character. Two different mechanisms are revealed to enhance the interfacial lithium adsorption and diffusion, which are the electron-deficiency host doping and the vacancylike structure evolutions with bond breaking. The synergistic effect between different dopants with diverse doping effects is also proposed. The results give a theoretical basis for the materials design with doped graphene as advanced materials modification for energy storage.

Achievement of desired $p$-type electrical properties in ${\mathrm{MoS}}_2$ remains a challenge. Here, we demonstrate that $p$ doping in monolayer ${\mathrm{MoS}}_2$ can be enhanced in terms of strain manipulation, through first-principles hybrid functional calculations. Biaxial tensile strain and shear strain with smaller in-plane angles induce the dramatic reduction in formation energy of $p$ dopants such as niobium and tantalum, providing the moderate doping contents required for applications. In addition, the formation of sulfur vacancies which are potential compensators of holes released from the dopants is suppressed by the strains. Our calculations pave an alternative strategy to overcome in the realization of $p$ doping in monolayer ${\mathrm{MoS}}_2$.

Polarization is an intrinsic attribute of optical waves, so manipulating the polarization state of optical surface waves can be of fundamental importance for next-generation information and biophotonics technology. Here, we show theoretically that the polarization of the Bloch surface wave (BSW) on a dielectric multilayer can be transformed between a transverse-electric state and a transverse-magnetic state by using the laterally continuous grooves inscribed on this multilayer. This polarization transformation can be enhanced or inhibited by the interference between the reflected BSW beams, which can be tuned by the periodicity and depth of the grooves. Maximum polarization transformation efficiency can be achieved as high as 43% when the number of grooves is increased to ten. A generalized Fresnel formula is proposed to describe the polarization transformation of the BSW beams. Because of this polarization transformation, an anomalous reflection of BSW beams can be realized, which is the inequality between the incident angle and the reflection angle.

We demonstrate second-harmonic generation (SHG) in an $x$-cut congruent lithium niobate (LN) whispering-gallery mode (WGM) resonator. First, we show theoretically that independent control of the coupling of the pump and signal modes is optimal for high conversion rates. A coupling scheme based on our earlier work [F. Sedlmeir et al., Phys. Rev. Applied 7, 024029 (2017)] is then implemented experimentally to verify this improvement. Thereby, we are able to improve on the efficiency of SHG by more than an order of magnitude by selectively outcoupling using a LN prism, utilizing the birefringence of it and the resonator in kind. This method is also applicable to other nonlinear processes in WGM resonators.

Charge transport in the wide-band-gap $(\mathrm{Al},\mathrm{In})\mathrm{N}/\mathrm{GaN}$ heterostructures with high carrier density approximately $2\times {10}^{13}{\mathrm{cm}}^{-2}$ is investigated over a large range of temperature ($270\mathrm{mK}\le T\le 280\mathrm{K}$) and magnetic field ($0\le B\le 18\mathrm{T}$). We observe the first evidence of weak localization in the two-dimensional electron gas in this system. From the Shubnikov–de Haas (SdH) oscillations a relatively light effective mass of $0.23m_e$ is determined. Furthermore, the linear dependence with temperature ($T<20\mathrm{K}$) of the inelastic scattering rate (${\tau }_i^{-1}\propto T$) is attributed to the phase breaking by electron-electron scattering. Also in the same temperature range the less-than unit ratio of quantum lifetime to Hall transport time $({\tau }_q/{\tau }_t<1)$ is taken to signify the dominance of small-angle scattering. Above 20 K, with increasing temperature scattering changes from acoustic phonon to optical phonon scattering, resulting in a rapid decrease in carrier mobility and increase in sheet resistance. Suppression of such scatterings will lead to higher mobility and a way forward to high-power and high-frequency electronics.

A quartz tuning fork is the key component of high-resolution atomic force microscope. Because of its high quality factor, a quartz tuning fork can also be used for high-sensitivity magnetometry. We develop a highly sensitive torque differential magnetometry using the qPlus mode of a quartz tuning fork. The tuning fork is driven by an ac voltage, and its deflection is measured by the resultant ac current. We observe a sharp resonance of the quartz tuning fork at low temperatures down to 1.6 K. We calibrate our torque differential magnetometry by measuring the angular dependence of the hysteresis loop in single-crystal ${\mathrm{Fe}}_{0.25}{\mathrm{TaS}}_2$. Furthermore, we demonstrate the high sensitivity of the torque differential magnetometry by measuring the quantum oscillations of a bismuth single crystal. The extracted Fermi-surface cross sections are consistent with those of bismuth crystals.

We propose and realize the first classical ghost-imaging (GI) experiment in the frequency or wavelength domain, thus performing ghost spectroscopy using thermal light exhibiting photon bunching. The required wavelength correlations are provided by light emitted by spectrally broadband near-infrared amplified spontaneous emission of a semiconductor-based superluminescent diode. They are characterized by wavelength-resolved intensity cross-correlation measurements utilizing two-photon-absorption interferometry. Finally, a real-world spectroscopic application of this ghost spectroscopy with a classical light scheme is demonstrated in which an absorption band of trichloromethane (chloroform) at 1214 nm is reconstructed with a spectral resolution of 10 nm as a proof-of-principle experiment. This ghost-spectroscopy work fills the gap of a hitherto missing analogy between the spatial and the spectral domain in classical GI modalities, with the expectation of contributing towards a broader dissemination of correlated photon ghost modalities, hence paving the way towards more applications which exploit the favorable advantages.

Coherent radiation produced by relativistic charged bunches is nowadays of great interest for user-oriented applications and high-resolution diagnostics. Here we present experimental results obtained by using a temporal monitor based on the electro-optical sampling that allows us to reveal the features of the radiation emitted in terahertz range by ultrashort electron bunches moving in proximity to a nonlinear crystal. We investigate the radiation properties both in near- and far-field conditions by employing electron beams accelerated by a conventional photoinjector and through laser-plasma interactions. Our results indicate that the emitted radiation moves collinearly with the beam in one case, while its properties resemble those of the classical Cherenkov radiation in a second case.