Thermal concentrators and cloaks with ellipsoidal shapes are designed by utilizing the transformation thermotics method and finite element simulations. The thermal conductivities for the concentrator and cloak are directly derive in Cartesian coordinates. The simulation results show that the ellipsoidal thermal concentrator can focus heat flux into a central region and that the ellipsoidal thermal cloak can guide heat flux around the cloaked region smoothly without disturbing the external temperature distribution and heat flux. The present method can be extended to design arbitrarily shaped thermal metadevices with novel properties.

Fluidics is one of the most historic subjects that are well-established over centuries on the macroscopic scale. In recent years, fluid detection using a number of micro/nano scale devices has been achieved. However, the interaction of microfluid and solid devices on micro/nano-meter scale still lacks in-depth research. We demonstrate a practical nanomechanical detector for microfluidics via a string resonator with high $Q$-factor, suspended over a hole. This device is placed under a jet nozzle with several microns of diameter, and the interaction between the micro-gas flow and the resonator is observed by monitoring the variation of the fundamental frequency and the quality factor. Moreover, we manage to measure the fluctuations of the micro-gas flow on the nanomechanical resonator by means of stochastic resonance. This work manifests a potential platform for detecting dynamical fluid behaviors at microscopic scale for novel fluid physics.

The Auger decay for the many-electron Xe$^{+}$ (4$p_{3/2}^{-1}$) state is studied in detail, using multistep approaches. It is found that the single Auger decay channels are primarily Coster–Kronig processes, which is in accord with other theoretical and experimental results. The double and triple Auger decays result primarily from cascade processes, i.e., the sequential two-step and three-step Auger decay, and as such, the contributions from direct processes can be neglected. Level-to-level rates for single, double, and triple decays are obtained, based on which comprehensive Auger electron spectra and ion yields are obtained. Our decay paths and Auger electron spectra are in agreement with the experimental analysis [Hikosaka et al., Phys. Rev. A 76 (2007) 032708], and our ion yield ratios (Xe$^{2+}$: Xe$^{3+}$: Xe$^{4+} = 4.6\!:\!87.0\!:\!8.4$) are also in line with their values ($5.0\!:\!86.0\!:\!9.0$). However, with respect to the ion yield ratios, a discrepancy still remains among the experimental and theoretical results. Taking into account the complexity of Xe's electronic structure, further, more detailed experiments are still required.

The plasmonic nanocavity is an excellent platform for the study of light matter interaction within a sub-diffraction volume under ambient conditions. We design a structure of plasmonic tweezers, which can trap molecular J-aggregates and also serve as a plasmonic cavity with which to investigate strong light matter interaction. The optical response of the cavity is calculated via finite-difference time-domain methods, and the optical force is evaluated based on the Maxwell stress tensor method. With the help of the coupled oscillator model and virtual exciton theory, we investigate the strong coupling progress at the lower level of excitons, finding that a Rabi splitting of 230 meV can be obtained in a single exciton system. We further analyze the relationship between optical force and model volume in the coupling system. The proposed method offers a way to locate molecular J-aggregates in plasmonic tweezers for investigating optical force performance and strong light matter interaction.

Near-field thermophotovoltaic systems functioning at 400–900 K based on graphene-hexagonal-boron-nitride heterostructures and thin-film InSb p–n junctions are investigated theoretically. The performances of two near-field systems with different emitters are examined carefully. One near-field system consists of a graphene-hexagonal-boron-nitride-graphene sandwiched structure as the emitter, while the other system has an emitter made of the double graphene-hexagonal-boron-nitride heterostructure. It is shown that both systems exhibit higher output power density and energy efficiency than the near-field system based on mono graphene-hexagonal-boron-nitride heterostructure. The optimal output power density of the former device can reach $1.3\times10^{5}$ W/m$^{2}$, while the optimal energy efficiency can be as large as $42\%$ of the Carnot efficiency. We analyze the underlying physical mechanisms that lead to the excellent performances of the proposed near-field thermophotovoltaic systems. Our results are valuable toward high-performance moderate temperature thermophotovoltaic systems as appealing thermal-to-electric energy conversion (waste heat harvesting) devices.

Symmetry plays fundamental role in physics and the nature of symmetry changes in non-Hermitian physics. Here the symmetry-protected scattering in non-Hermitian linear systems is investigated by employing the discrete symmetries that classify the random matrices. The even-parity symmetries impose strict constraints on the scattering coefficients: the time-reversal ($C$ and $K$) symmetries protect the symmetric transmission or reflection; the pseudo-Hermiticity ($Q$ symmetry) or the inversion ($P$) symmetry protects the symmetric transmission and reflection. For the inversion-combined time-reversal symmetries, the symmetric features on the transmission and reflection interchange. The odd-parity symmetries including the particle-hole symmetry, chiral symmetry, and sublattice symmetry cannot ensure the scattering to be symmetric. These guiding principles are valid for both Hermitian and non-Hermitian linear systems. Our findings provide fundamental insights into symmetry and scattering ranging from condensed matter physics to quantum physics and optics.

Pitch is the most important auditory perception characteristic of sound with respect to speech intelligibility and music appreciation, and corresponds to a frequency of sound stimulus. However, in some cases, we can perceive virtual pitch, where the corresponding frequency component does not exist in the stimulating sound. This virtual pitch contains a deviation from the de Boer pitch shift formula, which is known as second pitch shift. It has been theoretically suggested that nonlinear dynamics in the cochlea or in the neural network produce a nonlinear resonance with a frequency corresponding to the virtual pitch; however, there is no direct experimental observation to support this theory. The second virtual pitch shift, expressed via basilar membrane nonlinear vibration temporal patterns, and consistent with psychoacoustic experiments, is observed in situ in the cochlea via laser interferometry.

Tuning the thermal conductivity of silicon nanowires (Si-NWs) is essential for realization of future thermoelectric devices. The corresponding management of thermal transport is strongly related to the scattering of phonons, which are the primary heat carriers in Si-NWs. Using the molecular dynamics method, we find that the scattering of phonons from internal body defects is stronger than that from surface structures in the low-porosity range. Based on our simulations, we propose the concept of an exponential decay in thermal conductivity with porosity, specifically in the low-porosity range. In contrast, the thermal conductivity of Si-NWs with a higher porosity approaches the amorphous limit, and is insensitive to specific phonon scattering processes. Our findings contribute to a better understanding of the tuning of thermal conductivity in Si-NWs by means of patterned nanostructures, and may provide valuable insights into the optimal design of one-dimensional thermoelectric materials.

We conduct extensive research into the structures of Be$_{x}$Zn$_{1-x}$O ternary alloys in a pressure range of 0–60 GPa, using the ab initio total energy evolutionary algorithm and total energy calculations, finding several metastable structures. Our pressure-composition phase diagram is constructed using the enthalpy results. In addition, we calculate the electronic structures of the Be$_{x}$Zn$_{1-x}$O structures and investigate the bandgap values at varying pressures and Be content. The calculated results show that the bandgap of the Be$_{x}$Zn$_{1-x}$O ternary alloys increases with an increase in Be content at the same pressure. Moreover, the bandgap of the Be$_{x}$Zn$_{1-x}$O ternary alloys increases with the increasing pressure with fixed Be content. At the same Be content, the formation enthalpy of the Be$_{x}$Zn$_{1-x}$O ternary alloys first decreases, then increases with the increasing pressure.

Pressure evolution of local structure and vibrational dynamics of the perovskite-type relaxor ferroelectric single crystal of 0.935(Na$_{0.5}$Bi$_{0.5}$)TiO$_{3}$-0.065BaTiO$_{3}$ (NBT-6.5BT) is systematically investigated via in situ Raman spectroscopy. The pressure dependence of phonon modes up to 30 GPa reveals two characteristic pressures: one is at around 4.6 GPa which corresponds to the rhombohedral-to-tetragonal phase transition, showing that the pressure strongly suppresses the coupling between the off-centered A- and B-site cations; the other structural transition involving the oxygen octahedral tilt and vibration occurs at pressure $\sim $13–15 GPa with certain degree of order-disorder transition, evidenced by the abnormal changes of intensity and FWHM in Raman spectrum.

The properties of six kinds of intrinsic point defects in monolayer GeS are systematically investigated using the “transfer to real state” model, based on density functional theory. We find that Ge vacancy is the dominant intrinsic acceptor defect, due to its shallow acceptor transition energy level and lowest formation energy, which is primarily responsible for the intrinsic p-type conductivity of monolayer GeS, and effectively explains the native p-type conductivity of GeS observed in experiment. The shallow acceptor transition level derives from the local structural distortion induced by Coulomb repulsion between the charged vacancy center and its surrounding anions. Furthermore, with respect to growth conditions, Ge vacancies will be compensated by fewer n-type intrinsic defects under Ge-poor growth conditions. Our results have established the physical origin of the intrinsic p-type conductivity in monolayer GeS, as well as expanding the understanding of defect properties in low-dimensional semiconductor materials.

Symmetry breaking plays a pivotal role in modern physics. Although self-similarity is also a symmetry, and appears ubiquitously in nature, a fundamental question arises as to whether self-similarity breaking makes sense or not. Here, by identifying an important type of critical fluctuation, dubbed ‘phases fluctuations’, and comparing the numerical results for those with self-similarity and those lacking self-similarity with respect to phases fluctuations, we show that self-similarity can indeed be broken, with significant consequences, at least in nonequilibrium situations. We find that the breaking of self-similarity results in new critical exponents, giving rise to a violation of the well-known finite-size scaling, or the less well-known finite-time scaling, and different leading exponents in either the ordered or the disordered phases of the paradigmatic Ising model on two- or three-dimensional finite lattices, when subject to the simplest nonequilibrium driving of linear heating or cooling through its critical point. This is in stark contrast to identical exponents and different amplitudes in usual critical phenomena. Our results demonstrate how surprising driven nonequilibrium critical phenomena can be. The application of this theory to other classical and quantum phase transitions is also anticipated.

We experimentally analyze the heat capacity and thermal expansion of reference solids in a wide temperature range from several Kelvin to melting temperature, and establish a universal double-linear relation between the experimental heat capacity $C_{\rm p}$ and thermal expansion $\beta$, which is different from the previous models. The universal behavior between heat capacity and thermal expansion is important to predict the thermodynamic parameters at constant pressure, and is helpful for understanding the nature of thermal properties in solids.

Topological defects in graphene induce structural and electronic modulations. Knowing exact nature of broken-symmetry states around the individual atomic defects of graphene is very important for understanding the electronic properties of this material. We investigate structural dependence on localized electronic states in the vicinity of topological defects on a highly oriented pyrolytic graphite (HOPG) surface, using scanning tunneling microscopy and spectroscopy. Several inherent topological defects on the HOPG surface and the local density of states surrounding them are explored, visualized as scattering wave-related ($\sqrt{3} \times \sqrt{3}$) R30$^{\circ}$ superstructures and honeycomb superstructures. In addition, the superstructures observed near the grain boundary have a much higher decay length at specific sites than that reported previously, indicating far greater electron scattering on the quasi-periodic grain boundary.

We investigate the electrical conductivity and thermal conductivity of polycrystalline gold nanofilms, with thicknesses ranging from 40.5 nm to 115.8 nm, and identify a thickness-dependent electrical conductivity, which can be explained via the Mayadas and Shatzkes (MS) theory. At the same time, a suppressed thermal conductivity is observed, as compared to that found in the bulk material, together with a weak thickness effect. We compare the thermal conductivity of suspended and supported gold films, finding that the supporting substrate can effectively suppress the in-plane thermal conductivity of the polycrystalline gold nanofilms. Our results indicate that grain boundary scattering and substrate scattering can affect electron and phonon transport in polycrystalline metallic systems.

The directional design of functional materials with multi-objective constraints is a big challenge, in which performance and stability are determined by a complicated interconnection of different physical factors. We apply multi-objective optimization, based on the Pareto Efficiency and Particle-Swarm Optimization methods, to design new functional materials directionally. As a demonstration, we achieve the thermoelectric design of 2D SnSe materials via the above methods. We identify several novel metastable 2D SnSe structures with simultaneously lower free energy and better thermoelectric performance in their experimentally reported monolayer structures. We hope that the results of our work on the multi-objective Pareto Optimization method will represent a step forward in the integrative design of future multi-objective and multi-functional materials.

The recent observation of high critical temperature $T_{\rm c}$ in lanthanum and Yttrium hydrides confirms the key role of hydrogen cage (H-cage) in determining high superconductivity. Here, we present a new class of metastable H$_{12}$ clathrate structures based on the icosahedral $cI24$-Na that can be stabilized by incorporation of metal elements. Analysis shows that the charge transfer from metal atoms to H atoms contributes to forming the H$_{12}$ clathrate. Nine dynamically stable structures are identified to exhibit superconductivity, and a maximum $T_{\rm c}$ of 28 K is found in voids-doped Mo$_{6}$H$_{24}$. Calculations reveal that the low $T_{\rm c}$ is attributed to the weak interaction between H atoms in each cage due to the long H–H distance. The current results provide a possible route to design H-cage containing superconductors.

Two-dimensional (2D) magnetic materials have been experimentally recognized recently, however, the Curie temperatures ($T_{\rm C}$) of known 2D systems are quite low. Generally, magnetic systems can be seen as constituent magnetic elements providing spins and the non-magnetic elements providing frameworks to host the magnetic elements. Short bond lengths between the magnetic and non-magnetic elements would be beneficial for strong magnetic interactions and thus high $T_{\rm C}$. Based on this, we propose to combine the magnetic element Cr and the non-magnetic element boron to design novel 2D magnetic systems. Using our self-developed software package IM$^{2}$ODE, we design a series of chromium-boride based 2D magnetic materials. Nine stable magnetic systems are identified. Among them, we find that CrB$_{4}$-I, CrB$_{4}$-II and CrB$_{5}$-I with common structural units [CrB$_{8}$] are ferromagnetic metals with estimated $T_{\rm C}$ of 270 K, 120 K and 110 K, respectively. On the other hand, five CrB$_{3}$ phases with structural units [Cr$_{2}$B$_{12}$] are antiferromagnetic metals. Additionally, we also find one antiferromagnetic semiconductor CrB$_{2}$-I. Our work may open new directions for identifying 2D magnetic systems with high $T_{\rm C}$.

An ultra-wideband metamaterial absorber is developed, which is polarized-insensitive and angular-stable. Three layers of square resistive films comprise the proposed metamaterial. The optimal values of geometric parameters are obtained, such that the designed absorber can achieve an ultra-broadband absorption response from 4.73 to 39.04 GHz (relative bandwidth of 156.7%) for both transverse electricity and transverse magnetic waves. Moreover, impedance matching theory and an equivalent circuit model are utilized for the absorption mechanism analysis. The compatibility of equivalent circuit calculation results, together with both full-wave simulation and experimental results, demonstrates the excellent performance and applicability of the proposed metamaterial absorber.

Fabrication of atomic dopant wires at large scale is challenging. We explored the feasibility to fabricate atomic dopant wires by nano-patterning self-assembled dopant carrying molecular monolayers via a resist-free lithographic approach. The resist-free lithography is to use electron beam exposure to decompose hydrocarbon contaminants in vacuum chamber into amorphous carbon that serves as an etching mask for nanopatterning the phosphorus-bearing monolayers. Dopant wires were fabricated in silicon by patterning diethyl vinylphosphonate monolayers into lines with a width ranging from 1 µm down to 8 nm. The dopants were subsequently driven into silicon to form dopant wires by rapid thermal annealing. Electrical measurements show a linear correlation between wire width and conductance, indicating the success of the monolayer patterning process at nanoscale. The dopant wires can be potentially scaled down to atomic scale if the dopant thermal diffusion can be mitigated.

The $sp^{2}$–$sp^{3}$-hybridized carbon allotropes with the advantage of two hybrid structures possess rich and fascinating electronic and mechanical properties and they have received long-standing attention. We design a class of versatile $sp^{2}$–$sp^{3}$ carbons composed of graphite and diamond structural units with variable sizes. This class of $sp^{2}$–$sp^{3}$ carbons is energetically more favorable than graphite under high pressure, and their mechanical and dynamical stabilities are further confirmed at ambient pressure. The calculations of band structure and mechanical properties indicate that this class of $sp^{2}$–$sp^{3}$ carbons not only exhibits peculiar electronic characteristics adjusted from semiconducting to metallic nature but also presents excellent mechanical characteristics, such as superhigh hardness and high ductility. These $sp^{2}$–$sp^{3}$ carbons have desirable properties across a broad range of potential applications.