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ZnO and BaSnO3 (BSO) thin films grown on Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) substrates have been studied using electrical resistance and photoconductivity (PC) spectra measurements under different applied electric fields on the substrate. The behavior of the resistance and the energy gap (EG) extracted from the PC spectra are modified by the polarization state of the substrate in the case of the ZnO film, while for BSO, these physical parameters depend on the strain imposed by the substrate when a voltage is applied on the PMN-PT. In the latter case, an in-plane tensile (compressive) strain leads to a reduction (increase) in the resistance and the energy gap when an external electric field is applied on the substrate. The behavior of ZnO and BSO can be explained by the different crystalline structure in both films and by the fact that ZnO is also a piezoelectric material. In ZnO, a change in the polarization state of the substrate is associated with an imposed strain and an induced polarization on the film that leads to a modification of the band bending and hence of the energy gap. In the case of BSO, a shift of the impurity and conduction band generates a modification of the energy gap for the different types of strain. [ABSTRACT FROM AUTHOR]

In-plane heterostructures based on transition metal dichalcogenides are promising for applications in tunnel field-effect transistors (TFETs). However, the transport properties of the in-plane heterostructures have not been fully understood due to the presence of generation current derived from the in-gap state of the heterointerface. For further performance improvement, it is important to identify and suppress the origin of the in-gap states at the heterointerface. In this work, we investigated the transport properties of TFETs based on multilayer NbxMo1−xS2/MoS2 in-plane heterostructures on atomically flat hexagonal boron nitride substrate. We observed a transition from staggered gap to broken gap band alignment by electron doping to MoS2 and that band-to-band tunneling current was dominant below 80 K, a higher temperature compared with the heterostructure on an SiO2 surface. These results indicate that the use of atomically flat substrates helps reduce generation current from strain-derived in-gap states in NbxMo1−xS2/MoS2 in-plane heterostructures. [ABSTRACT FROM AUTHOR]

As a fundamental degree of freedom, phonon chirality is expected to promote the development of quantum information technology just like electron spin. Currently, central to this area is the realization of efficient transmission and control of chiral information. In this paper, we propose an approach by integrating topological theory, leveraging topologically invariant Chern numbers, to encode hexagonal lattice systems. Our investigation reveals the presence of topologically protected chiral interface states within the shared band gaps of both trivial and non-trivial system units. By precisely modulating the magnetic field distribution within the encoding system, we can effectively manipulate the topological pathways. Building upon this framework, we design and implement a chiral phonon three-port device. Through dynamic calculations, we demonstrate the transmission process of chiral information, showcasing the chiral phonon switching effect and logical OR operation. Our findings not only establish a fundamental mechanism for the manipulation and control of phonon chiral information but also provide a promising direction for research in harnessing chirality degrees of freedom in practical applications. [ABSTRACT FROM AUTHOR]

The recent emergence of two-dimensional metal-organic framework (MOF) materials with nontrivial magnetic and electronic properties has attracted great interest in spintronics. Here, we theoretically demonstrate the synthesis of a coloring-triangle latticed 2D MOF by assembling 2,3,6,7,10,11-hexahydroxytriphenylene (H6HOTP) species and threefold coordinated Mn atoms, namely, 2D Mn-HOTP. The electronic structure calculations shown that 2D Mn-HOTP exhibits coexistence of bipolar magnetic semiconducting and topological behavior. 2D Mn-HOTP is an intrinsic bipolar magnetic semiconductor with a small spin-flip band gap of 0.21 eV and relatively large spin-conserving band gaps of 0.34 and 0.74 eV. Electrical/hole doping can induce the transformation of 2D Mn-HOTP into half-metal conduction with controllable spin polarization direction. In addition, the organic HOTP ligands containing coloring-triangle lattice enable the formation of p-orbital single polarized Dirac cones and flat bands, which exhibit the topological properties such as nonzero Chern number and nontrivial edge states near the Fermi level. The Dirac points and flat bands can be selectively detected at the Fermi level with experimentally achievable electron and hole concentrations of 5.19 and 0.91 × 1013 cm−2, respectively. These results not only highlight that 2D Mn-HOTP MOF is a promising candidate for developing spintronic devices but also provide an ideal platform to explore kagome-like correlated quantum states. [ABSTRACT FROM AUTHOR]

Accurate quantum efficiency measurement not only provides crucial information for the photovoltaic cell industry but also supports experiments aimed at directly detecting dark matter and elastic neutrino interactions. The dark matter direct searches paradigm has recently expanded to include particles with masses below 1 , MeV / c 2 , where the expected signal in an electron–recoil interaction is approximately in the eV range, just above the energy gap for silicon and germanium. A robust calibration method for ionization signals in this lower energy region is essential. This paper presents a method for measuring quantum efficiency and yield (q/E) in semiconductors using phonon-mediated calorimetry. The Neganov–Trofimov–Luke phonon amplification method in low-temperature semiconductor crystals has been employed to indirectly measure ionization down to single-electron accuracy. Specifically, at zero bias, the phonon readout directly quantifies the total energy deposited within the detector, independent of the ionization yield. This eliminates a significant source of systematic uncertainty in quantum efficiency estimates associated with total energy uncertainty. The paper includes results from an updated ionization efficiency measurement in a germanium detector. [ABSTRACT FROM AUTHOR]

Nonsymmorphic compounds have attracted much interest owing to their potential nontrivial electronic states. Here, we grew V1−δSb2 single crystal with a nonsymmorphic space group I4/mcm and studied their de Haas–van Alphen oscillations. Orientation-dependent magnetization showed quantum oscillations, allowing determination of the three-dimensional Fermi surface and Berry phase. Our theoretical calculation implied that VSb2's band structures manifest flat bands along the Γ–X path, Dirac band crossings near the P and N points, and in the Γ–Z direction, and nontrivial surface states along the Γ ¯ – Z ¯ line. However, the inconsistencies in observed and calculated quantum oscillation frequencies suggest that VSb2's band structures cannot nicely account for these electronic properties of our samples. This study reveals the profound impact of V vacancies on VSb2's electronic states, implying the possible topological quantum phase transition via defect engineering. [ABSTRACT FROM AUTHOR]

Magnon spin currents in the ferrimagnetic garnet Tb3Fe5O12 with 4f electrons were examined through the spin-Seebeck effect and neutron scattering measurements. The compound shows a magnetic compensation, where the spin-Seebeck signal reverses above and below T comp = 249.5 (4) K. Unpolarized neutron scattering unveils two major magnon branches with finite energy gaps, which are well explained in the framework of spin-wave theory. Their temperature dependencies and the direction of the precession motion of magnetic moments, i.e., magnon polarization, defined using polarized neutrons, explain the reversal at T comp and decay of the spin-Seebeck signals at low temperatures. We illustrate an example that momentum- and energy-resolved microscopic information is a prerequisite to understand the magnon spin current. [ABSTRACT FROM AUTHOR]

Magnon spin currents in the ferrimagnetic garnet Tb3Fe5O12 with 4f electrons were examined through the spin-Seebeck effect and neutron scattering measurements. The compound shows a magnetic compensation, where the spin-Seebeck signal reverses above and below T comp = 249.5 (4) K. Unpolarized neutron scattering unveils two major magnon branches with finite energy gaps, which are well explained in the framework of spin-wave theory. Their temperature dependencies and the direction of the precession motion of magnetic moments, i.e., magnon polarization, defined using polarized neutrons, explain the reversal at T comp and decay of the spin-Seebeck signals at low temperatures. We illustrate an example that momentum- and energy-resolved microscopic information is a prerequisite to understand the magnon spin current. [ABSTRACT FROM AUTHOR]

Wurtzite ferroelectrics are an emerging material class that expands the functionality and application space of wide bandgap semiconductors. Promising physical properties of binary wurtzite semiconductors include a large, reorientable spontaneous polarization, direct band gaps that span from the infrared to ultraviolet, large thermal conductivities and acoustic wave velocities, high mobility electron and hole channels, and low optical losses. The ability to reverse the polarization in ternary wurtzite semiconductors at room temperature enables memory and analog type functionality and quasi-phase matching in optical devices and boosts the ecosystem of wurtzite semiconductors, provided the appropriate combination of properties can be achieved for any given application. In this article, advances in the design, synthesis, and characterization of wurtzite ferroelectric materials and devices are discussed. Highlights include: the direct and quantitative observation of polarization reversal of ∼135 μC/cm2 charge in Al1−xBxN via electron microscopy, Al1−xBxN ferroelectric domain patterns poled down to 400 nm in width via scanning probe microscopy, and full polarization retention after over 1000 h of 200 °C baking and a 2× enhancement relative to ZnO in the nonlinear optical response of Zn1−xMgxO. The main tradeoffs, challenges, and opportunities in thin film deposition, heterostructure design and characterization, and device fabrication are overviewed. [ABSTRACT FROM AUTHOR]

Over the past decade, tremendous effort has been put into developing 2D semiconductor materials with a tunable bandgap by alloying different individual components. However, the bandgap bowing effect has hindered the ability to arbitrary control the emission of these alloys. In this study, we report the chemical vapor deposition growth of a graded-composition Mo1−xWxS2 monolayer alloy, in which the photoluminescence emission energy exhibits nearly linear variation in the bandgap, indicating the vanishing of the bandgap bowing effect. Polarized Raman measurements show that the polarization is composition dependent, and a large symmetry breaking occurs at the point where the bandgap bowing effect vanishes. This suggests that the vanishing of the bowing effect may be attributed to the symmetry breaking induced by compressive strain. Our findings demonstrate a significant advancement in the synthesis of alloys for future use. [ABSTRACT FROM AUTHOR]

We theoretically proposed a feasible way to design multi-topological states in one two-dimensional photonic crystal (PC). By sliding and rotating the meta-atoms of its unit cell, the topological phase of the PC can be continuously tuned, but without a closing of bandgap; it is similar to the Kekulé modulation in honeycomb lattices. We first show the existence of helical topological edge states in conventional topological insulator, which intersect together and form a Dirac cone in the two-dimensional synthetic space of the PC. Then, in full synthetic space, the PC behaves high-order topological insulator, and the corner state is then constructed in a heterostructure that consists of three different PCs. Furthermore, we demonstrate the photonic crystal with non-uniform modulation can behaves as a splitter and a rainbow trapper. Our proposal may provide interesting insight for designing of multiple topological states, which have potential in photonic on-chip devices. [ABSTRACT FROM AUTHOR]

Thermal radiation properties of surface micro-Au-line and space structures on direct and indirect electronic transition-type semiconductors of n-type (n-) GaAs and GaP, respectively, are studied. The electron densities range from 0.62 to 1.0 × 1018 cm−3. The emission spectra of samples on n-GaAs wafers show peak energies significantly shifted from the longitudinal optical (LO) phonon energy due to the LO-phonon−plasmon coupling (LOPC) at 630 K, while partial decoupling is observed. The thermal emission from the sample on n-GaP is peaked at 395.9 cm−1, shifted only by 1.5 cm−1 from the LO-phonon energy, with a linewidth of 12.5 cm−1 at 630 K. The effect of LOPC on the emission from the sample on n-GaP is mostly suppressed because of the electron localization in the X-valley, suggesting an advantage of indirect transition-type semiconductors in LO-phonon resonant infrared emitters using the phonons generated via the electron energy relaxation. [ABSTRACT FROM AUTHOR]

Eco-friendly Magnesium antimonide (Mg3Sb2) has been extensively investigated as a promising and low-toxic thermoelectric material for intermediate (500–900 K) thermoelectric applications. Herein, p-type Zn-incorporated Mg3Sb2 was prepared by hot press technique, and its thermoelectric transport properties were investigated. The formation of Mg3−xZnxSb2 solid-solution plays a significant role in enhancing electrical conductivity of 34.59 S cm−1 due to the increased carrier concentration and reduced energy gap. Reduction in lattice thermal conductivity of 0.46 W m−1 K−1 at 753 K was obtained for Mg3−xZnxSb2 (x = 0.15) by combined scattering effect of dislocations, lattice strain, and interfaces, which is clearly seen in HR-TEM and strain analysis. These favorable conditions lead to an enhanced thermoelectric figure-of-merit (zT) of 0.25 at 753 K, which is 400% improved compared to the pure Mg3Sb2 sample. [ABSTRACT FROM AUTHOR]

Eco-friendly Magnesium antimonide (Mg3Sb2) has been extensively investigated as a promising and low-toxic thermoelectric material for intermediate (500–900 K) thermoelectric applications. Herein, p-type Zn-incorporated Mg3Sb2 was prepared by hot press technique, and its thermoelectric transport properties were investigated. The formation of Mg3−xZnxSb2 solid-solution plays a significant role in enhancing electrical conductivity of 34.59 S cm−1 due to the increased carrier concentration and reduced energy gap. Reduction in lattice thermal conductivity of 0.46 W m−1 K−1 at 753 K was obtained for Mg3−xZnxSb2 (x = 0.15) by combined scattering effect of dislocations, lattice strain, and interfaces, which is clearly seen in HR-TEM and strain analysis. These favorable conditions lead to an enhanced thermoelectric figure-of-merit (zT) of 0.25 at 753 K, which is 400% improved compared to the pure Mg3Sb2 sample. [ABSTRACT FROM AUTHOR]

p–n junctions based on nanocrystals now serve as fundamental components in optoelectronics. However, the process of designing these p–n junctions has largely relied on empirical choices, either for ligand exchange or for the selection of charge transport layers. Therefore, a systematic strategy is still lacking. In this study, we explore the utilization of ferroelectric domains as a general method for remotely inducing the formation of a p–n junction. To ensure compatibility with devices of various designs, we employ a commercially available periodically poled LiNbO3 (PPLN) substrate commonly used in nonlinear optics. We engineer a PPLN/graphene/HgTe heterostructure and demonstrate its planar photodiode behavior. Through x-ray photoemission microscopy, we confirm that the rectifying behavior stems from the influence of the ferroelectric domains, by ruling out the possibility of the formation of non-ohmic contacts at the electrode/semiconductor interfaces. This approach proves to be quite general and holds promise for the future design of high-speed nanocrystal-based photodiodes. [ABSTRACT FROM AUTHOR]

As a quantum state of frequency extrema in the momentum space of acoustic systems, sonic valley pseudospin provides a new degree of freedom for controlling acoustic waves. Higher-order topological insulators (HOTIs) have extended the traditional bulk-edge correspondence principle and are a crucial concept for classic wave regulation. However, HOTIs in valley sonic crystals (VSCs) only appear in a single bandgap, which limits the multi-frequency selectivity of the corner state and is not conducive to the design of multi-frequency acoustic communication devices. Here, we demonstrate "Y-shaped" acoustic crystals with C3 symmetry that form a double-band VSC, and the topological phase transitions in both low- and high-frequency band gaps coincide. We realize theoretically and experimentally higher-order states in dual-band valley sonic crystals. Our work enriches the application of HOTIs in acoustic multi-frequency regulatory systems and provides different avenues for designing of multi-band acoustic devices. [ABSTRACT FROM AUTHOR]

Low-dimensional ferroelectrics have a great deal of potential for use in electronic memory devices. However, intrinsic one-dimensional (1D) ferroelectricity is very rare. Using first-principles calculations, we present the discovery of an inborn 1D Ta4OTe9I4 ferroelectric (FE) nanowire. Its distinctive geometry can cause spontaneous electric polarization along the z-axis and allow it to maintain a certain temperature and strain. In addition to its sizable spontaneous polarization and appropriate Curie temperature, the 1D Ta4OTe9I4 nanowire exhibits an energy barrier comparable to those of other ferroelectrics. With polarization reversal, the energy gap can be modulated in the range of 0.38–1.33 eV, corresponding to an apparent peak shift phenomenon in the optical response. We use this nanowire as an exemplary material for building a FE tunnel junction composed of Hf0.5Ta3.5OTe9I4/Ta4OTe9I4/W0.5Ta3.5OTe9I4 with a giant tunneling electroresistance ratio at zero and low bias. Our calculations suggest that this 1D intrinsically FE material obtained from van der Waals crystals can be used in miniaturized and advanced high-density information storage. [ABSTRACT FROM AUTHOR]

Photoelectric properties of the F8BT polymer are investigated by the Constant photocurrent method (CPM) based on the photocurrent spectra Δjph(hν) and absorption spectra αCPM(hν). The negligibly weak dependence of charge carriers lifetime on the photon energy is found for hν = 2.2–2.7 eV. The peculiarities of the absorption spectra and the photoconductivity spectra are revealed to be determined mainly by the Gaussian distribution of the density of states (DOS). The density of trap states in the F8BT polymer thin films in the exponential DOS is estimated to be 103 times higher than in the Gaussian DOS. [ABSTRACT FROM AUTHOR]

Based on the 4I13/2 → 4I15/2 emission (1450–1700 nm) of Er3+ in garnet, which is matching well with third bioimaging window (NIR-III), we propose a ratiometric near-infrared (NIR) thermometer (YAGG:Ce-Er) in NIR-III by utilizing thermal coupling between Stark levels of 4I13/2, where energy levels were split into seven sublevels (I–VII). The energy gaps between Stark sublevel VII and I (ΔEVII-I) and V and I (ΔEV-I) of the 4I13/2 level, calculated from the temperature dependence of intensity ratios of the two selected thermal couples, corresponded well with the theoretical values. This agreement supports the analysis that the system can be utilized as a Boltzmann thermometry. Moreover, the relative sensitivity (Sr) within the temperature range commonly encountered in organisms (25–60 °C) reached a value of 0.63 ± 0.03% K−1 for the VII/I ratio and 0.57 ± 0.07% K−1 for the V/I ratio. These high relative sensitivity values indicate the strong potential for practical application in various biological fields. [ABSTRACT FROM AUTHOR]

Due to the breaking of the time reversal symmetry and spatial inversion symmetry, hexagonal ferrovalley materials have intrinsic large valley polarization. Model analysis shows that tuning the two different band gaps of valleys can realize phase transitions between ferrovalley semiconductors, half valley metals, and valley-polarized quantum anomalous Hall semiconductors. Through first-principle calculations, monolayer ferrovalley materials RuXY (X, Y = F, Cl, Br), which exhibit valley splitting at the top valence band and the bottom conduction band, are predicted to achieve this valley-dependent topological phase transition. Due to the different orbital proportions of d orbitals, the valley splitting at the top valence band is much greater than that at the bottom conduction band. Strain can regulate the interaction between orbitals, thus producing valley-dependent band inversion, leading to the quantum spin or valley Hall effect. The chiral edge states are demonstrated under appropriate biaxial strain. The topological phase transition is related to the inversion of the band structure and Berry curvatures at K and K′ valleys. These results have certain significance for the design of two-dimensional valley-dependent quantum materials and the application of valleytronic devices. [ABSTRACT FROM AUTHOR]

In this Letter, we report on the phase-controlled topological plasmons in 1D graphene nanoribbons (GNRs) based on a Su−Schrieffer−Heeger (SSH) model variant. By considering the dipole–dipole mode interactions, we first study the normal SSH model by an effective Hamiltonian and calculate the Zak phase as a topological invariant, finding that it is nontrivial (trivial) when the coupling distance is bigger (smaller) than half the period. Then, we reveal that the edge modes with fields highly localized at only one side exist in the model with nontrivial topology and shows the robustness of strong field confinement and extreme frequency stability against in-plane and out-of-plane disorders. Finally, we introduce the offset SSH model variant by vertically offsetting one of the GNR in SSH unit, which allows us to greatly engineer both the width of topological gap and the number of topological windows. The underlying physics are uncovered by defining a parameter called phase difference, which reveals that the topological edge modes appear (disappear) generally near the positions where the inter-unit coupling strength is bigger (smaller) than the intra-unit coupling strength, and, more notably, the phase difference is around even (odd) multiple numbers of π, which is much different from the normal SSH model where the topological phase is simply affected by the resonator distance. In addition to opening up a possibility to explore the fundamental physics of topologically protected graphene plasmons, this work also offers potential applications of these concepts to design graphene-based plasmon devices with immunity to structural imperfections. [ABSTRACT FROM AUTHOR]

The exceptional characteristics of two-dimensional materials make them highly efficient and stable for electronic and optoelectronic applications. These materials exhibit a range of beneficial properties, such as ultrafast carrier dynamics, layer-dependent energy bandgap, tunable optical properties, low power dissipation, high mobility, transparency, flexibility, simple fabrication, and ability to confine electromagnetic energy within extremely small volumes. In this work, infrared light (980 nm) photo sensors are fabricated based on a MoS2/p-Si substrate utilizing the plasmonic phenomenon of gold nanoparticles (AuNPs) to enrich the optoelectronic properties and to enhance the photoresponse. The infrared light response for (Au NPs MoS2) comes from the strong interlayer coupling, which narrow the energy gap in the heterojunction area, thus rendering heterostructures to longer wavelength detection ability. [ABSTRACT FROM AUTHOR]

Higher-order topological insulators have drawn great research attention in nanophotonics due to their ability to both support robust edge states and lower dimensional corner states. In this work, we present a theoretical proposal for achieving topologically switchable and valley-selective corner states based on two-dimensional C3-symmetric photonic crystals (PCs), with breaking of inversion symmetry. Through the concatenation of two valley PCs with contrasting topological indices, we demonstrate the emergence of two types of valley-locked chiral topological edge states resulting from the valley–valley interaction. More importantly, we find that the system exhibits two distinct types of corner states, characterized by strong robustness and high localization, when the PCs are spliced at a 60° angle. However, the corner states are absent when the splicing angle is set as 120°. According to the theoretical analysis, the selective activation of topological valley corner states is related to the sign flip of valley Chern number at the corner. Based on this feature, we further propose a topological photonic switching device, in which the corner can be lighted up or off selectively. By combining the benefits of higher-order topology and valley degree of freedom, our work provides an efficient and flexible method for light manipulation. [ABSTRACT FROM AUTHOR]

We present first principles calculations of the interface between GaN and strained AlN, using a slab model in which polarization is compensated via surface fractional-charge pseudo-hydrogen atoms. We show that an interface two-dimensional carrier electron or hole gas emerges naturally in response to different compensating surface charges, but that this need not involve in-gap surface states. [ABSTRACT FROM AUTHOR]

Recent studies have shown the potential for bipolar thermoelectricity in superconducting tunnel junctions with asymmetric energy gaps. The thermoelectric performance of these systems is significantly impacted by the inverse proximity effects present in the normal-superconducting bilayer, which is utilized to adjust the gap asymmetry in the junction. Here, we identify the most effective bilayer configurations, and we find that directly tunnel-coupling the normal metal side of the cold bilayer with the hot superconductor is more advantageous compared to the scheme used in experiments. By utilizing quasiclassical equations, we examined the nonlinear thermoelectric junction performance as a function of the normal metal film thickness and the quality of the normal-superconducting interface within the bilayer, thereby determining the optimal design to observe and maximize this nonequilibrium effect. Our results offer a roadmap to achieve improved thermoelectric performance in superconducting tunnel junctions, with promising implications for a number of applications. [ABSTRACT FROM AUTHOR]

Helical edge states (HESs) in two-dimensional topological spin photonic crystals can be used to realize pseudospin-locked unidirectional propagation of waves. In general, the excitation of HESs is by taking light sources carrying orbital angular momentum of order 1 to stimulate the pseudospin p states in spin photonic crystals. In this paper, we think of the HESs as combinations of the pseudospin p states and the pseudospin d states, which corresponding to the pseudospin modes carrying orbital angular momentum of order 1 and the pseudospin modes carrying orbital angular momentum of order 2, respectively, with their chirality related to the unidirectional propagations of the HESs. By analyzing the field distributions of HESs and via exciting the pseudospin d states with light sources carrying orbital angular momentum of order 2, we demonstrate the unidirectional propagation of the HESs in spin photonic crystals and verify their robustness by checking the unidirectional performance as they propagate along a Z-shape interface containing sharp corners, cavity defect, and disorders. Our study deepens understanding of the nature of HESs and expands methods to excitation and regulation of HESs in topological spin photonics. [ABSTRACT FROM AUTHOR]

A theoretical study is performed on narrow bandgap Janus monolayer MoWCO2 using the density functional theory and the Boltzmann transport equation. The scattering rate is calculated for electron–phonon, phonon–phonon, phonon-boundary, and electron-boundary scattering. It has a power factor (6.5 × 103 μ W/mK2) for p-type and (1.5 × 103 μ W/mK2) for n-type at T = 700 K. A strong effect of surface scattering is observed in phonon transport, and lattice thermal conductivity is reduced to 65 W/m K from 308 W/m K at T = 300K for 1 μ m width (L) of ribbon. In contrast, there is no change observed in electrical conductivity. This reduction in thermal conductivity improves the thermoelectric figure of merit to 0.33 (p-type) and 0.08 (n-type) at T = 700 K for L = 10 nm from 0.04 (p-type) and 0.01 (n-type). The obtained Young's modulus and Poisson's ratio are 244 N/m and 0.55, respectively, indicating that the material can be deformed under small strain. The obtained in-plane piezoelectric coefficients are e11 = 268 pC/m and d11 = 1.6 pm/V. This indicates the material will be suitable for wearable thermoelectric devices and sensor applications. [ABSTRACT FROM AUTHOR]

InAs quantum dot and dash gain media demonstrate performance benefits, such as lower threshold current densities and reduced temperature sensitivity over quantum wells for lasers operating in the C-band telecommunications window. Quantum dashes are of much interest for their higher gain over quantum dots due to an increased density of states. We combine experimental results and simulations to understand how quantum dash morphology and composition can be used to tune the emission wavelengths of these nanoparticles. Atomic force microscopy (AFM) analysis is performed to determine the effect of growth temperature and sublayer type on InAs/InGaAsP/InP nanoparticle morphology and homogeneity. Uncapped InAs nanoparticles grown by CBE on a GaAs sublayer will have dash-like geometries with heights up to 2.36 nm for growth temperatures of 500–540 °C. GaP sublayers will induce taller quantum dots except for a growth temperature of 530 °C, where quantum dashes form. The dimensions extracted from AFM scans are used in conjunction with photoluminescence data to guide parabolic band simulations of an InAs quantum dash with a GaP or GaAs sublayer and InP cap buried within InGaAsP. The calculated emission energy of a buried 30 × 300 nm quantum dash decreases by ∼100 meV for increasing heights from 1.5 to 2.5 nm, or increases by ∼100 meV by addition of 20% phosphorus in the dash and wetting layers. Modifying the quantum dash height and leveraging the As/P intermixing that occurs between the InAs and InP layers are, thus, most effective for wavelength tuning. [ABSTRACT FROM AUTHOR]

We investigate a band structure ω (k) of a photonic time crystal with periodic square (step) modulation in time of its permittivity ε (t) , oscillating between the value ε 1 (sustained for a fraction of time τ of the period) and the value ε 2 [fraction (1 − τ)]. The strength of modulation is m = (ε 1 − ε 2) / (ε 1 + ε 2). We find that ω (k) can be periodic in a wave number k (in addition to the frequency ω), provided that a certain function f (m , τ) of the parameters m and τ is an irreducible rational number. However, even for arbitrary values of m and τ, f (m , τ) can be approximated by a fractional number to any desired degree of periodicity. Hence, for square modulation, a photonic band structure is necessarily periodic or quasi-periodic in the wave number. Moreover, for appropriate sets of the parameters m and τ, the modes associated with k values within the band gaps can have identical values of the imaginary part of ω. For simultaneous excitation of these modes, all the fields would grow in time at the same rate, resulting in powerful amplification. [ABSTRACT FROM AUTHOR]

Topological band theory provides a framework to establish the equivalence/inequivalence of bandgaps in photonic topological insulators. However, experimental discernment of bandgap topological characteristics encounters inherent measurement complexities, particularly beyond the terahertz frequencies. To surmount this difficulty, we resort to the prolific optical technique of spectroscopic ellipsometry and carry out detailed experimental examination of attributes of one-dimensional photonic crystal stopbands and, in consequence, identify an appropriate classifier of the implicit topological characteristics. It is found that governed by the bulk topology, the band edge locations in the dispersion diagram provide a conditional site for the appearance of zeros of a complex reflection ratio. This leads to a selective appearance of topologically robust phase singularities with integer (unity positive) topological charge. We demonstrate that the presence of these phase singularities on either the blue or the red band edges of the stopbands provides us with an experimental marker of their distinctive topological characteristics. [ABSTRACT FROM AUTHOR]

Therefore, a major claim of the published article,[1] i.e., a sizable number of tetrahedral (T SB d sb ) atomic sites are filled by Al atoms even for lower Al compositions along with octahedral (O SB h sb ) atomic sites, and O SB h sb atomic site is preferred by Al atoms in the investigated composition range, is still valid. The Al composition (x) was inadvertently defined as the ratio of the number of Al atoms to the number of Ga atoms, i.e., x = number of Al atoms/number of Ga atoms. However, a sizable number of T SB d sb atomic sites, i.e., 20% for the Al composition of 4.8%, remains occupied by Al atoms, which is found to increase sharply with the Al composition. Thereafter, tiny amounts of 0.4% and 1% of the secondary phase of I i -Al SB 2 sb O SB 3 sb start appearing for the Al composition (x) of 47.4% and 48.7%, respectively, for I i -(Al SB x sb Ga SB 1-x sb ) SB 2 sb O SB 3 sb alloys. [Extracted from the article]

The energy of an Andreev bound state in a clean normal metal in contact with two superconductors disperses with the difference Δ ϕ in the superconducting phase between the superconductors in much the same way as the energies of electrons in a one-dimensional crystal disperse with the crystal momentum k of the electrons. A normal metal with n superconductors maps onto a n − 1 dimensional crystal, each dimension corresponding to the phase difference Δ ϕ i between a specific pair of superconductors. The resulting band structure as a function of the phase differences { Δ ϕ i } in such ballistic devices has been proposed to have a topological nature with gapped regions characterized by different Chern numbers separated by regions where the gap in the quasiparticle spectrum closes. A similar complex evolution of the quasiparticle spectrum with { Δ ϕ i } has also been predicted for diffusive normal metals in contact with multiple superconductors. While the underlying topological description is different in diffusive devices, gapped regions of the band diagram associated with different topological indices are also separated by regions where the gap closes. Here, we show that the variation of the density of states at the Fermi energy of such a system can be directly probed by relatively simple conductance measurements, allowing rapid characterization of the energy spectrum. [ABSTRACT FROM AUTHOR]

Many properties of photonic structures rely on band topology characterized by the integer invariants that can change during the topological transitions and give rise to the disorder-robust topological edge, corner, or interface states. Typically, the periods of such structures are comparable to the wavelength. However, in many cases, the unit cell becomes deeply subwavelength and hence the entire metamaterial can be described in terms of the effective material parameters. Here, focusing on subwavelength topological metamaterials, we identify the behavior of permittivity and permeability accompanying the topological transition on the example of the two structures possessing D6 symmetry. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) magnetic materials with nontrivial topological states have recently drawn considerable attention. Among them, 2D metal-organic frameworks (MOFs) are standing out due to their advantages such as the easy synthesis in practice and less sensitivity to oxidation that are distinctly different from inorganic materials. By means of density-functional theory calculations, we systematically investigate the electronic and topological properties of a class of 2D MOFs X(C21H15N3) (X = transition metal element from 3d to 5d). Excitingly, we find that X(C21H15N3) (X = Ti, Zr, Ag, Au) are Chern insulators with sizable band gaps (∼7.1 meV). By studying a four-band effective model, it is revealed that the Chern insulator phase in X(C21H15N3) (X = Ti, Zr, Ag, Au) is caused cooperatively by the band inversion of the p orbitals of the C21H15N3 molecule and the intrinsic ferromagnetism of X(C21H15N3). Additionally, Mn(C21H15N3) is a Dirac half-metal ferromagnet with a high Curie temperature up to 156 K. Our work demonstrates that 2D MOFs X(C21H15N3) are good platforms for realizing the quantum anomalous Hall effect and designing spintronic devices based on half-metals with high-speed and long-distance spin transport. [ABSTRACT FROM AUTHOR]

A class of active nonreciprocal metamaterial (ANMM) is presented which consists of an acoustic duct with periodically placed active diaphragms that are controlled by a spatiotemporal modulation strategy. The acoustic nonreciprocities can be realized by modulating a system's properties spatiotemporally. Such an approach has been extensively employed by many investigators to break the reciprocity in acoustic and elastic metamaterials. However, our proposed ANMM distinguishes itself from the above-mentioned methods by introducing actively tunable space-time modulated feedback gain of the controllers. The controller is implemented in an analog manner to enable fast response at high modulation frequencies. By discretizing a 1D acoustic duct into multiple acoustic unit cavities, we introduced a time-varying gain with a phase difference between adjacent acoustic cavities. Directional band gaps of the modulated system are numerically analyzed as the asymmetric acoustic wave propagation can be realized by converting the acoustical energy from the fundamental mode to higher order modes. In addition, nonreciprocal behavior of the proposed ANMM was experimentally demonstrated using a waveguide with periodically placed condenser microphones (sensors) and speakers (actuators). [ABSTRACT FROM AUTHOR]

Energetic carriers in semiconductors thermalize by impact-ionization, which generates electron–hole pairs (EHPs), and by energy losses to phonons. The average EHP creation energy is typically about three times the energy gap. In 1960, Shockley derived a simple equation for the average EHP creation energy with a single free parameter that fits experimental values for a wide range of materials, but the underlying assumptions, as stated, have been widely criticized as lacking justification. Modified expressions derived by improved approximations have been proposed but do not fare better. Here, we revisit the foundations of Shockley's equation and provide a robust justification for the kinetic-energy component as a model averaging procedure and then apply a similar procedure to the phonon component of the equation. The phonon result retains Shockley's form, but the interpretation and justification are now on par with those of the kinetic-energy term. The single-parameter fit to the data remains unchanged, i.e., the present analysis accounts for the exceptional applicability of Shockley's equation. [ABSTRACT FROM AUTHOR]

A proof of concept of a new solution for achieving tunable SAW components based on the electrical Bragg band gap concept developed for piezoelectric phononic crystals is presented on a SAW resonator on a LiNbO 3 substrate. In this work, it is shown that for a fixed geometry, it is possible to shift the main resonance frequency by modifying the electrical condition on the electrodes that constitute the cavity's mirrors. The concept was validated by both numerical simulations and experimental characterization of single port resonators fabricated on LiNbO 3 . [ABSTRACT FROM AUTHOR]

Infrared detector barrier heterostructures with strained layer superlattice (SLS) absorbers with different periods were compared. The first was a reference using a conventional barrier heterostructure with a low temperature energy gap corresponding to a wavelength of 10 μm in a 2-μm-thick undoped absorber using a 10.9 nm period with InAs/InAsSb0.36 compositions grown directly on a GaSb substrate. The second structure, in contrast, used a significantly shorter 4.3 nm period absorber with InAsSb0.3/InAsSb0.55 compositions, similar energy gap, and absorber thickness, which were grown on a 6.2 Å lattice constant GaIn0.3Sb virtual substrate on GaSb. It was found that in the short period SLS, the vertical hole mobility and minority carrier lifetime in the temperature range of 80–150 K were a factor on 2–3 greater than in the reference structure. The improvement of the vertical hole mobility was attributed to the effect of hole delocalization. The latter results in an increase in the optical absorption coefficient and the quantum efficiency. [ABSTRACT FROM AUTHOR]

Quantum cascade detectors (QCDs) are mid-infrared and far-infrared, low-noise, photovoltaic detectors utilizing intersubband transitions. This Letter presents an InAs/AlAs0.16Sb0.84 based QCD lattice matched to an InAs substrate. This material system exhibits properties like a low effective electron mass of the well material of 0.023 m0, beneficial for higher optical absorption strength, and a high conduction band offset of 2.1 eV, allowing the design of QCDs in the mid-infrared and near-infrared region. The presented QCD has a peak spectral response at 2.7 μm (0.459 eV), the center of a CO2 absorption band. To enable top side illumination, a grating was implemented. This additionally bypasses absorption by the narrow bandgap 0.345 eV (3.54 μm) InAs substrate material. The QCD has a peak responsivity at a room temperature of 5.63 mA/W and a peak specific detectivity of 1.14 × 108 Jones. [ABSTRACT FROM AUTHOR]

The aim of this Perspective is to provide an overview of approaches that can be employed to tune the energy level alignment at interfaces between inorganic and organic semiconductors for use in electronic and optoelectronic devices. The approaches include tailoring intramolecular dipolar bond distribution, controlling molecular orientation at interfaces, and the insertion of a molecularly thin interlayer that abruptly shifts the electrostatic potential between the two semiconductors and, thus, affords level tuning. With these state of the art methods, the frontier energy levels at an inorganic/organic heterojunction can be varied up to ca. 3 eV, i.e., covering the energy gap of most semiconductors. By combining two or more of these approaches or by employing interfacial molecular switches, it is envisioned that unconventional and dynamically switchable interfacial energy level scenarios can be created, enabling expanded or superior device functionality. [ABSTRACT FROM AUTHOR]

A finite electronic band gap is a standard filter in high-throughput screening of materials using density functional theory (DFT). However, because of the systematic underestimation of band gaps in standard DFT approximations, a number of compounds may be incorrectly predicted metallic. In a more accurate treatment, such materials may instead appear as low band gap materials and could have good thermoelectric properties if suitable doping is feasible. To explore this possibility, we performed hybrid functional calculations on 1093 cubic materials listed in the Materials Project database with four atoms in the primitive unit cell, spin-neutral ground state, and a formation energy within 0.3 eV of the convex hull. Out of these materials, we identified eight compounds for which a finite band gap emerges. Evaluating electronic and thermal transport properties of these compounds, we found the compositions MgSc2Hg and Li2CaSi to exhibit promising thermoelectric properties. These findings underline the potential of reassessing band gaps and band structures of compounds to identify additional potential thermoelectric materials. [ABSTRACT FROM AUTHOR]

A power figure-of-merit (FOM) of ∼62.6–87.3 GW/cm2 is predicted for ScAlN, which represents a value 5–7 times larger than that of GaN. The parameters for the lattice-matched Sc0.18Al0.82N FOM calculation are investigated by first-principles density functional theory (DFT) calculations with the local density approximation. An energy gap of 5.65 eV and an electron effective mass of 0.46m0 are obtained from the DFT band structure calculation of Sc0.1875Al0.8125N. The electron mobility of Sc0.18Al0.82N is simulated based on Boltzmann transport equations, which consider scatterings by ionized impurities, dislocations, alloy scattering, acoustic phonons, and optical phonons. The remarkable power FOM shows that lattice-matched Sc0.18Al0.82N possesses a large breakdown voltage and low specific on-resistance, which suggests the great potential for Sc0.18Al0.82N to be implemented in high-voltage power electronics for improved device performance. [ABSTRACT FROM AUTHOR]

van der Waals materials offer a large variety of electronic properties depending on chemical composition, number of layers, and stacking order. Among them, As2Te3 has attracted attention due to the promise of outstanding electronic properties and high photo-response. Precise experimental determinations of the electronic properties of As2Te3 are yet sorely needed for better understanding of potential properties and device applications. Here, we study the structural and electronic properties of α-As2Te3. Scanning transmission electron microscopy coupled to energy x-ray dispersion and micro-Raman spectroscopy all confirm that our specimens correspond to α-As2Te3. Scanning tunneling spectroscopy (STS) at 4.2 K demonstrates that α-As2Te3 exhibits an electronic bandgap of about 0.4 eV. The valence-band maxima are located at −0.03 eV below the Fermi level, thus confirming the residual p-type character of our samples. The material can be exfoliated, revealing the (100) anisotropic surface. Transport measurements on a thick exfoliated sample (bulk-like) confirm the STS results. These findings allow for a deeper understanding of the As2Te3 electronic properties, underlying the potential of V-VI semiconductors for electronic and photonic technologies. [ABSTRACT FROM AUTHOR]

We present simultaneous suppression of FeSb2 thermal conductivity and electronic correlations in Fe1−xRuxSb2 (0 ≤ x ≤ 0.6) single crystal alloys. Small energy gap Δ 1 in Kondo-insulator-like semiconductor FeSb2 associated with impurity in-gap state increases whereas the intrinsic bandgap Δ 2 decreases upon Ru substitution on Fe atomic site. Thermopower is suppressed along with the intrinsic bandgap and with the thermal conductivity. The more delocalized 4d character of atomic orbital of Ru brings suppression of electronic correlations, but also an increase in impurity density which reduces phonon mean free path and surface scattering length. Our results indicate a range of Ru doping x where nanostructuring could be used to suppress thermal conductivity further, potentially toward the amorphous limit. [ABSTRACT FROM AUTHOR]

Starting from high-quality oxygen-deficient BaSnO3 films, we have monitored the evolution of their electrical conducting and photoconducting properties after subsequent post-thermal annealing in oxygen. In this way, we have been able to modify the electrical conductivity of the film by at least three orders of magnitude (from 18.2 to 0.013 Ω − 1 m−1) by simply reducing the oxygen vacancies concentration after each thermal annealing. Even though the film holds its semiconducting-like behavior, we have observed a modification of the hopping parameters concomitant with a decrease in the Fermi energy level as the electrical conductivity is reduced. Similarly, the effective energy gap extracted from photoconductance spectroscopy measurements decreases as the Fermi energy level decreases suggesting the presence of in-gap states generated by oxygen vacancies. A direct energy bulk gap value of (3.8 ± 0.1) eV was obtained. While the photoconductivity increases from ≃ 4.6 to 73%, its slow time constants become less dominant as the electrical conductivity is decreased in accordance with a reduction of the oxygen vacancies density, which play a key role as electron-traps. [ABSTRACT FROM AUTHOR]

Erbium nitride (ErN) is a rare-earth metal mononitride continuing to receive interest due to its unique electronic, magnetic, and optical properties. ErN has shown promise in the development of new functional materials for optoelectronic and spintronic devices. Here, we report on the optical properties of ErN crystals, grown by sublimation and probed by photoluminescence (PL) spectroscopy at both room temperature and 180 K. Multiple transition lines were observed between 2 and 4.5 eV. Using the PL results together with reported calculations, a coherent picture for the band structure at the Γ-point for ErN crystals was derived. PL results revealed that ErN has a minimum direct energy gap of 2.41 eV and a total of two valence bands and two conduction bands at the Γ-point separated by about 0.15 eV and 0.34 eV, respectively. These transitions reveal optical properties of ErN in the UV region and its band structure at the Γ-point. [ABSTRACT FROM AUTHOR]

Optoelectronic superlattices are proposed based on two-dimensional transition metal dichalcogenides, which can be realized by periodically superimposed, interlaced, or alternate modulations of the gate voltage and the off-resonant right circularly polarized light. Owing to the huge band gap and spin–orbit coupling, the propagation of electrons through the gate tunable WSe2 superlattice under the optical field becomes highly valley-dependent, i.e., the transmission and conductance are suppressed for the K valley but enhanced remarkably for the K′ valley. Moreover, it is shown that the properties of the line-type resonant peaks are extremely sensitive to the valley and spin degrees of freedom, the period number of superlattice, and the mode of modulated external fields and can be further drastically adjusted by the width of the modulated region. This work may shed light on potential applications of the optoelectronic superlattices in the fields of valleytronics and spintronics. [ABSTRACT FROM AUTHOR]

2D honeycomb-Kagome (HK) lattices have attracted extensive attention in recent years due to the peculiar electronic and magnetic properties such as the Dirac band, the half-metallicity, and the high Curie temperature. In this Letter, we theoretically investigate the spin transport properties of a recently proposed 2D Dirac spin gapless semiconductor (also known as a Dirac half-metal with zero energy gap in one spin channel) of the Cr2S3 monolayer with the HK lattice. The excellent spin filtering effect and negative differential resistance effect are found at a bias voltage, and interestingly, a temperature difference can also drive the spin filtering effect. These peculiar transport properties can be understood from the Dirac spin gapless semiconductivity and the spin-dependent transmission spectrum. In addition, we predict that, similar to Cr2S3 and Cr2Se3, 2D Cr2Te3 is also a Dirac spin gapless semiconductor with the above room-temperature Curie temperature and a large magneto-crystalline anisotropic energy (MAE). Under a tensile biaxial strain, the MAE can be greatly increased, and the easy magnetization axis is still along the in-plane. All these results are achieved by the first-principles combined with nonequilibrium Green's function method. The present work will stimulate theoretical and experimental studies on spintronic devices and spin caloritronic devices based on more 2D Dirac HK lattices. [ABSTRACT FROM AUTHOR]

At normal conditions, tin telluride (SnTe) adopts a cubic NaCl-type structure, but under applied pressure above 1.5–2 GPa, it transforms to a distorted crystal structure with an orthorhombic symmetry. Electronic properties of this high-pressure phase, including potential thermoelectricity, remain unexplored to date. Here, we measure the thermoelectric power (the Seebeck coefficient) and electrical resistivity of undoped single crystals of SnTe under applied high pressure up to 9 GPa, i.e., across the above phase transition. We establish that the high-pressure polymorph of SnTe is a p-type semiconductor and estimate its bandgap value at 3 GPa as Eg ∼ 65 meV. In contrast to the NaCl-type phase, the orthorhombic phase is stable in a much wider pressure range up to about 20 GPa, and its energy gap only insignificantly decreases with pressure with a coefficient of dEg/dP ∼ −4 meV/GPa. We find that the thermoelectric power factor of SnTe can be significantly improved in its orthorhombic phase due to the enhancement of the Seebeck coefficient. Furthermore, we show that the high-pressure phase preserves on the pressure releasing down to 0.3 GPa, and its thermopower grows progressively up to about 100 μV/K due to the bandgap expansion to Eg ∼ 105 meV. This results in a colossal rising of the thermoelectric power factor to about 8 mW/(K2m). Probably, this enhancement is contributed by structural distortions in the orthorhombic phase. We discuss how one could fabricate and optimize the orthorhombic polymorph of SnTe for potential use in various technologies, including thermoelectric applications. [ABSTRACT FROM AUTHOR]

Titanium-vanadium dioxide or TixV1−xO2 films for 0 ≤ x ≤ 1 were examined using ellipsometry, and their optical constants (n and k) at visible and near-infrared wavelengths were determined at temperatures (T) below, at, and above the semiconductive-to-metallic phase transition (SMT) temperature (TSM). Ellipsometric analysis was performed for each x at each T using a wavelength dispersion model, i.e., a combination of Lorentz oscillators and a Drude free electron model. The ellipsometric analyses provided information on the electronic band transition caused by the SMT and the influence of cationic replacement (Ti↔V) on the SMT. The results revealed that when x ≤ 0.05, close to the SMT, the energy gap of the interband transition O2p→V3d varied from ≈3.5 eV to ≈3.1 eV, and the quantity of electrons in the interband transition decreased by half. In addition, the energy gap monotonically increased to 4.2 eV when x was increased to 1. Moreover, the energy gap of the split V3d intraband transition varied from ≈1.4 eV to zero, and the quantity of electrons in the intraband transition increased by a factor of four. Furthermore, when x ≥ 0.2, close to the SMT, the energy gap of the intraband transition varied from ≈1.4 eV to a constant positive value, with the generation of a small number of conductive electrons, depending on x. [ABSTRACT FROM AUTHOR]