Infrared (IR) detectors and focal-plane arrays based on superlattices have progressed significantly during the past decade. However, there are still fundamental challenges associated with this system, especially in understanding the transport of minority carriers due to the anisotropy in the band structure and the quantum-confined miniband transport. In this paper, we investigate the key parameters influencing the vertical minority-electron transport and lifetime in $nBp$ midwave infrared (MWIR) detectors with a $p$-type $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ type-II-superlattice (T2SL) absorber. We measure the minority carrier diffusion length using the temperature-dependent electron-beam-induced-current (EBIC) method at three electron-beam ($e$-beam) energies. Our results show that the diffusion length is independent of the temperature from 80 to 140 K and increases linearly beyond that. By varying the $e$-beam energies and the beam current in EBIC, we examine the effect of carrier distributions from the surface in our measurement. We further study the minority-carrier lifetime using the time-resolved-microwave-reflectance (TMR) measurement as a function of the temperature and the excitation density. The TMR results indicate that the lifetime is Shockley-Read-Hall (SRH) limited for the temperature range of interest in this work (80–150 K) and the Auger recombination is dominant above 150 K, while the radiative recombination is negligible. The combined results of the diffusion length and the lifetime are used to determine the temperature-dependent mobility along the growth direction. The transport is found to be limited by deep-level states for temperatures below 140 K and the activation energy of 115 meV is calculated from the minority-carrier mobility results for temperatures above 140 K. This effort is a theoretical and experimental study to address some of the essential questions regarding the transport in $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ T2SLs, the result of which would help optimize the design and growth of T2SL structures with improved performance.

We consider the scattering problem for an asymmetric-composite photonic structure with a component experiencing a thermally driven phase transition. Using a numerical example, we show that if the heating is caused by the incident light, the transmittance can become highly asymmetric within a broad range of light intensities. This effect can be utilized for nonreciprocal light transport, optical limiting, or power switching.

We use localized surface-plasmon resonances in metallic nanoantennas to suppress the defect environment of colloidal quantum dots and to enhance and polarize their spontaneous emission. For this we study the interaction of such quantum dots with functional metal-oxide plasmonic metastructures consisting of an $\mathrm{Au}$/$\mathrm{Si}$ Schottky junction in close vicinity of a $\mathrm{Si}$/$\mathrm{Al}$ oxide charge barrier. We show that optically excited quantum dots can couple with such metastructures via their electric dipole fields, offering super-plasmonic-processes that include the impact of hot electron generation and fixed negative charges at the $\mathrm{Si}$/$\mathrm{Al}$ oxide junction. For metallic nanoantennas with small aspect ratios our results show that these metastructures can reduce the defect-induced nonradiative decay rates of quantum dots such that their lifetime can become significantly longer than those in the absence of such nanoantennas. For metallic nanoantennas with larger aspect ratios these structures lead to ultrahigh enhancement of exciton-plasmon coupling, making the spontaneous emission of quantum dots strongly polarized while increasing their emission intensities by about 50 times. These results show by keeping excitons in the cores of quantum dots, the metastructures suppress migration of photoexcited electrons to surface defects and, therefore, reduce Auger recombination. Coherent dynamics associated with the quantum-dot-induced exciton-plasmon coupling is theoretically investigated.

Cellulose-nanofibril aerogels have been found to be highly effective thermal insulators, where some reports state that they have lower thermal conductivity than air. To further enhance the performance of the material, it is important to understand the contribution of the three heat-transfer components: solid-heat conduction by the fibrils, gas-heat conduction by the Knudsen gas in pores, and thermal radiation between the fibrils. The overall effective thermal conductivity is measured by an in-house steady-state setup under atmospheric and vacuum conditions. Contributions from each heat transfer component are quantified by constructing a simple open-cell model and fitting it to the experimental measurements, which vary based on the solid-volume fraction. The thermal conductivity values of a single cellulose-nanofibril filament that constitutes the struts of the open-cell structure are well within the range of previous studies, which confirms the validity of the analysis results. The analysis model can also be used to reveal target dimensions when fabricating aerogels with minimum thermal conductivity. All in all, the simple analysis method can be further applied to improve other porous thermal-insulation materials.

We elucidate a significant thermal rectification effect in molecular bridges (MBs) that covalently bond gold and carbon nanotubes (CNTs) by nonequilibrium molecular dynamics (MD) simulations. We find asymmetric heat flux across these MBs, resulting in significant thermal rectification. Thermal rectification ratios are found to decrease with the number of the MBs and increase with the lengths of CNTs, reaching a value as high as 3.75. In addition, the calculations show that the thermal conductances vary with the orientation of the crystal planes of $\mathrm{Au}$, the number of molecular bridges, and the length of the CNTs. They are in a range of 40–250 MW m K, agreeing well with the reported experimental results. Detailed analysis suggests that the thermal rectification can be attributed to the mismatch of vibrational modes between neighboring sulfur and carbon atoms in the MBs. This discovery could provide theoretical guidance in designing molecular thermal diodes, paving alternative routes to potential applications in nanoscale heat manipulation, waste energy harvesting, chip cooling, and molecular phononics.

Nanoscale thermal transport is becoming ever more technologically important with the development of next-generation nanoelectronics, nanomediated thermal therapies, and high efficiency thermoelectric devices. However, direct experimental measurements of nondiffusive heat flow in nanoscale systems are challenging, and first-principle models of real geometries are not yet computationally feasible. In recent work, we used ultrafast pulses of short-wavelength light to uncover a previously-unobserved regime of nanoscale thermal transport that occurs when the width and separation of heat sources are comparable to the mean free paths of the dominant heat-carrying phonons in the substrate. We now systematically compare thermal transport from gratings of metallic nanolines with different periodicities, on both silicon and fused-silica substrates, to map the entire nanoscale thermal transport landscape – from closely spaced through increasingly isolated to fully isolated heat-transfer regimes. By monitoring the surface profile dynamics with subangstrom sensitivity, we directly measure thermal transport from the nanolines into the substrate. This allows us to quantify for the first time how the nanoline separation significantly impacts thermal transport into the substrate, making it possible to reach efficiencies that are within a factor of 2 of the diffusive (i.e., thin-film) limit. We also show that partially isolated nanolines perform significantly worse, because cooling occurs in a regime that is intermediate between close-packed and fully isolated heat sources. This work thus confirms the surprising prediction that closely spaced nanoscale heat sources can cool more quickly than when far apart. These results show that our predictive model is validated by experiment over a broad parameter space, which is important for benchmarking theories that go beyond the Fourier model of heat diffusion, and for informed design of nanoengineered systems.

We present exact numerical calculations of the supercurrent density, inductance, and impurity-induced flux noise of cylindrical superconducting wires in the nonlocal Pippard regime, which occurs when the Pippard coherence length is greater than the London penetration depth. In this regime, the supercurrent density displays a peak away from the surface and changes sign inside the superconductor, signaling a breakdown of the usual approximation of local London electrodynamics with a renormalized penetration depth. Our calculations show that the internal inductance and the bulk flux noise power are enhanced in nonlocal superconductors. In contrast, the kinetic inductance is reduced and the surface flux noise remains the same. As a result, impurity spins in the bulk may dominate the flux noise in superconducting qubits in the Pippard regime, such as the ones using aluminum superconductors with a large electron mean free path.

We present a procedure for addressing extrinsic defects in amorphous oxides, in which the most stable defect configurations are identified through ab initio molecular dynamics in various charge states and studied through hybrid functional calculations. The protocol is further complemented with an electron counting scheme based on maximally localized Wannier functions, which allows one to identify the nominal charge state and the composition of the defect core unit. Here, we apply this approach to the study of hydrogen, carbon and nitrogen impurities in amorphous alumina $(a{\text{−}\mathrm{Al}}_2{\mathrm{O}}_3)$, a highly relevant material for technological applications. Hydrogen is found to be amphoteric with a thermodynamic $+1$/$-1$ charge transition level lying at $\sim 4.6$ eV above the valence band maximum, in qualitative agreement with results obtained with crystalline models, but at a substantially different energy level. Hydroxyl groups are further shown to lead to the same defect states as observed for hydrogen. Application of our procedure to carbon and nitrogen impurities leads to structural configurations that are found to depend on the total charge set in the simulation cell. Through the adopted electron counting rule, we assess that carbon and nitrogen impurities are only found in neutral and in singly positive charge states, respectively. This indicates that neither carbon nor nitrogen give charge transition levels in the band gap, in strong contrast with results achieved for crystalline models. In addition, the defect core units are shown to incorporate a varying number of oxygen atoms, by which their formation energy depends on the oxygen chemical potential ${\mu }_O$. In oxygen-poor conditions, both the carbon and the nitrogen impurities favor bonding to $\mathrm{Al}$ atoms, while they tend to form single or double bonds with oxygen atoms as ${\mu }_O$ increases. Based on the band alignment of $a{\text{−}\mathrm{Al}}_2{\mathrm{O}}_3$ with three technologically relevant semiconductors ($\mathrm{Ga}\mathrm{As}$, $\mathrm{Ga}\mathrm{N}$ and $\alpha$-${\mathrm{Fe}}_2{\mathrm{O}}_3$), we discuss the possible role of point defects in degrading the performance of electronic devices and in favoring hole transport across the oxide in water-splitting setups.

The magnetization dynamics in $\mathrm{W}/{\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$, ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}/\mathrm{Pt}$ and $\mathrm{W}/{\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}/\mathrm{Pt}$ multilayers are investigated using spin-orbit-torque ferromagnetic resonance (SOT FMR) technique. An analytical model based on magnetization dynamics due to SOT is used to fit the heavy-metal (HM) thickness dependence of the symmetric and antisymmetric components of the SOT FMR signal. The analysis results in a determination of the properties of the HM layers, such as the spin Hall angle and the spin diffusion length. Spin Hall angles of $-0.36$ and 0.09 are found in the $\mathrm{W}/{\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ and ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}/\mathrm{Pt}$ bilayers, respectively, which add up in the case of a $\mathrm{W}/{\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}/\mathrm{Pt}$ trilayer. More importantly, we determine the effective interfacial spin-orbit fields at both the $\mathrm{W}/{\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ and ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}/\mathrm{Pt}$ interfaces, which are shown to cancel the Oersted field for particular thicknesses of the heavy-metal layers, leading to pure spin-current-induced dynamics and indicating the possibility of more efficient magnetization switching.

We report on the experimental observation of selective delamination of semitransparent materials on the example of yttria-stabilized zirconia ceramics upon femtosecond laser processing of its surface with a low numerical aperture lens. The delamination of a ceramic layer of dozens of micrometers takes place as a side effect of surface processing and is observed above the surface ablation threshold. The onset of delamination (delamination threshold) depends on the degree of overlap of the irradiation spots from consecutive laser pulses upon beam scanning over material surface. Analysis of the delaminated layer indicates that the material undergoes melting on both of its surfaces. The mechanism of delamination is identified as a complex interplay between the optical response of laser-generated free-electron plasma and nonlinear effects upon laser beam propagation in semitransparent ceramics. The discovered effect enables controllable laser microslicing of brittle ceramic materials.

Correlated disorder, a type of crystallographic state, widely exists in complex material systems and is beneficial for the improvement of material properties. The strain gradient ∇S, caused by structural heterogeneity, controls the features of correlated disorder. However, the exact mechanism whereby ∇S controls the disorder dynamics, especially in bulk materials, is still largely unknown, hindering property optimization. We design and achieve a macroscopic ∇S in the ${\mathrm{KTa}}_{1-X}{\mathrm{Nb}}_X{\mathrm{O}}_3$ (KTN) single crystal by the coupling effect of the $\mathrm{Nb}$-composition gradient ∇X and 180° domains near the Curie temperature $T_C$. The electric-field dependences of the electro-optic (EO) effect are investigated to reveal the ∇S-controlled disorder dynamics. The EO characteristics indicate that the flexoelectric field $E_{\mathrm{flexo}}$ caused by ∇S is sufficiently strong to control the local correlated dipoles. Further, it is demonstrated by dielectric, Raman, and absorption spectroscopies that the ∇S in the KTN sample plays a dominant role in the disorder dynamics. At the nanoscale, the large local ∇S, approximately ${10}^{-3}–{10}^{-2}{\mathrm{nm}}^{-1}$ in magnitude, caused by random substitutions of B cations, can determine the local structure and dynamics, deeply influencing the properties of materials. This work reveals how ∇S acts on the disorder dynamics and local dipole behaviors, which provides the basis for the understanding and harnessing of disordered ferroelectrics.

The investigation of the characteristic luminescent response of $\mathrm{Ce}$-doped silica fibers exposed to electrons in the 20–200-GeV energy range is reported in this work to explore the feasibility of using silica-based fibers for a simultaneous dual-readout approach. The sol-gel method allows the preparation of either doped or undoped fibers with high aspect ratio and high purity, providing good flexibility and spatial resolution for the realization of a dual-readout detector. The dual Cherenkov and scintillation light emitted by silica-based fibers potentially offers applications in high-energy-physics calorimetry as well as in other fields, such as radiation monitoring in medicine, security, and industrial control. The response of the fibers, embedded in a tungsten-copper absorber block to obtain a spaghetti-like geometry in a high-energy-physics environment, is investigated through a test-beam campaign at the CERN Super Proton Synchrotron facility. The discrimination of Cherenkov and scintillation light is demonstrated and discussed, along with a detailed investigation of the scintillation properties of the material: time-resolved spectroscopy, relative light output, and attenuation length are evaluated. The results presented in this study can pave the way for further material engineering and future applications.

The presence of domains in ferroic materials can negatively affect their macroscopic properties and hence their usefulness in device applications. From an experimental perspective, the measurement of materials comprising multiple domains can complicate the interpretation of the material properties and their underlying mechanisms. In general, ${\mathrm{Bi}\mathrm{Fe}\mathrm{O}}_3$ films tend to grow with multiple magnetic domains and often contain multiple ferroelectric- and ferroelastic-domain variants. By growing (111)-oriented ${\mathrm{Bi}\mathrm{Fe}\mathrm{O}}_3$ films on an orthorhombic ${\mathrm{Tb}\mathrm{Sc}\mathrm{O}}_3$ substrate, we are able to overcome this and, by exploiting the magnetoelastic coupling between the magnetic and crystal structures, bias the growth of a given magnetic-, ferroelectric-, and structural-domain film. We further demonstrate the coupling of the magnetic structure to the ferroelectric polarization by showing that the magnetic polarity in this domain is inverted upon ${180}^{\circ }$ ferroelectric switching.

Compression of electron pulses with terahertz radiation offers short pulse durations and intrinsic subcycle stability in time. We report the generation of 12-fs (rms), 28-fs (FWHM) electron pulses at a kinetic energy of 75 keV by using single-cycle terahertz radiation and a simple planar mirror. The mirror interface provides transverse velocity matching and spatially uniform compression in time with purely longitudinal field-electron interaction. The measured short-term and long-term temporal drifts are substantially below the pulse duration without any active synchronization. A simple phase-space model explains the measured temporal focusing dynamics for different compressor strengths and shows a path toward generating isolated attosecond electron pulses from beams of almost arbitrary transverse emittance.

The introduction of defects into photonic and phononic crystals leads to defect states arising within the former forbidden bands and has been found in applications for making functional devices. These defect states, due to Bragg resonances, commonly have an antinode of energy localization in the center of the defect. We report our experimental observation and theoretical and numerical validation of a dipole-like antisymmetric localization of acoustic waves in a defect-embedded periodic duct. Our measurements and analysis reveal that the antisymmetric localization that we observe arises from an extraordinary transmission by non-Bragg resonances. Our study can inspire the exploration of similar defect states in other wave systems and broaden the applications for making functional devices, such as acoustical and optical tweezers, spatial sound and/or light modulators, and signal and/or structure detectors.

It is known that the permittivity of a magnetic nanofluid is controllable by external magnetic fields. The resulting effect of magneto-dielectric anisotropy originates in the magnetic nanoparticle assembly formation induced by the applied magnetic field. The particle assembly in magnetic nanofluids can be induced even by a dc electric field. Then, analogous to the magneto-dielectric effect, a question arises concerning the effect of the dc bias electric field on the nanofluid permittivity in a wide frequency range. We present dielectric spectra (1 mHz–1 MHz) measured on a transformer oil-based magnetic nanofluid at various temperatures. Electrode polarization and an interfacial relaxation process are analyzed. We show that the applied dc bias voltage results in permittivity sign switching at low frequencies. The critical frequency at which the sign is reversed depends on the nanofluid temperature and the dc bias voltage level. The nanoparticle assembly and conductive percolative paths are considered as key mechanisms leading to the transition from capacitive to inductive reactance. The measurement of apparent negative permittivity in a dc electric field can be used as a simple method for the detection of electric field-induced particle assembly and percolation. The permittivity sign control by means of a dc bias voltage may open an alternative avenue for research and applications of magnetic nanofluids.

$1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ is a layered material that exhibits charge density wave (CDW) -induced distinct electrical resistivity phases and has attracted a lot of attention for interesting device applications. However, such resistivity switching effects are often weak, and cannot be modulated by an external gate voltage — limiting their widespread usage. Using a back-gated $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2/2H\text{−}\mathrm{Mo}{\mathrm{S}}_2$ heterojunction, we show that the usual resistivity switching in $\mathrm{Ta}{\mathrm{S}}_2$ due to different phase transitions is accompanied with a surprisingly strong modulation in the Schottky barrier height (SBH) at the $\mathrm{Ta}{\mathrm{S}}_2/\mathrm{Mo}{\mathrm{S}}_2$ interface — providing an additional knob to control the degree of the phase-transition-driven resistivity switching by an external gate voltage. In particular, the commensurate (C) to triclinic (T) phase transition results in an increase in the SBH owing to a collapse of the Mott gap in $\mathrm{Ta}{\mathrm{S}}_2$. The change in SBH allows us to estimate an electrical Mott-gap opening of approximately 71 ± 7 meV in the C phase of $\mathrm{Ta}{\mathrm{S}}_2$. On the other hand, the nearly commensurate (NC) to incommensurate (IC) phase transition results in a suppression in the SBH, and the heterojunction shows a gate-controlled resistivity switching ratio up to 17.3, which is approximately $14.5$ times higher than that of stand-alone $\mathrm{Ta}{\mathrm{S}}_2$. The findings mark an important step forward showing a promising pathway to externally control as well as amplify the CDW-induced resistivity switching. This will boost device applications that exploit these phase transitions, such as ultra-broadband photodetection, negative differential conductance, fast oscillator and threshold switching in neuromorphic circuits.

We present a phononic crystal based on $\mathrm{Zn}\mathrm{O}$ pillars on a $\mathrm{Si}$ substrate that creates local-resonance band gaps, prohibiting surface-acoustic-wave propagation. The created local-resonance band gaps originate from Fano-resonance between the pillars and the substrate, and are shown to be highly influenced by the structural parameters of the pillars. Regarding surface acoustic waveguiding, we introduce a line defect to the crystal where elastic energy can be confined, leading to defect modes with slow group velocity. It is shown that the emerged defect guiding modes can be conducted into the local-resonance gap by adjusting the structural parameters of the defects. Moreover, it is proven that hollow cylinders in the linear defect introduce higher degrees of freedom in the structural parameters of the defect for adjusting the guiding mode, and can lead to narrow-band guiding modes in comparison with the gap width at the vicinity of the mid gap. The achieved narrow-band guiding modes are attributed to the reduced number of allowed resonance frequencies in hollow cylinders due to their reduced effective radius and inherent spatial confinement. Additionally, benefiting from acoustoelectric interaction in piezoelectric materials, we can modulate the elasticity of the $\mathrm{Zn}\mathrm{O}$ structures by varying their conductivity, which can lead to efficient tuning of the designed guiding modes. The required conductivity modulation in the $\mathrm{Zn}\mathrm{O}$ structures is proposed to achieved by UV illumination so that the photogenerated excess carriers screen the electric field associated with the elastic wave, perturbing the wave propagation and affecting the defect guiding modes. The proposed system opens up perspectives for designing reconfigurable, high-quality-factor surface-acoustic-wave devices such as filters and duplexers. The achieved maximum shift of the guiding frequency is about 100 MHz for a defect frequency of 4.18 GHz and a bandwidth of about 100 MHz, suitable for wireless communication applications.

Two reciprocity theorems are important for fundamental understanding of the solar cell operation and applications to device evaluation: (1) the carrier-transport reciprocity connecting the dark-carrier injection with the short-circuit photocarrier collection and (2) the optoelectronic reciprocity connecting the electroluminescence with the photovoltaic quantum efficiency at short circuit. These theorems, however, fail in devices with thick depletion regions such as p-i-n junction solar cells. By properly linearizing the carrier-transport equation in such devices, we report that the dark-carrier injection is related to the photocarrier collection efficiency at the operating voltage, not at short circuit as suggested in the original theorem. This leads to the general form of the optoelectronic reciprocity relation connecting the electroluminescence with the voltage-dependent quantum efficiency, providing a correct interpretation of the optoelectronic properties of p-i-n junction devices. We also discuss the validity of the well-known relation between the open-circuit voltage and the external luminescence efficiency. The impact of illumination intensity and device parameters on the validity of the reciprocity theorems is quantitatively investigated.

$\mathrm{Fe}\mathrm{Pt}$ is the primary material being considered for the development of information storage technologies based on heat-assisted magnetic recording (HAMR). A practical realization of HAMR requires understanding the high-temperature phase transition behavior of $\mathrm{Fe}\mathrm{Pt}$, including critical exponents and Curie temperature distributions as the fundamental HAMR media design characteristics. The studies so far found a significant degree of variability in the values of critical exponents of $\mathrm{Fe}\mathrm{Pt}$ and remain controversial. Here, we show that at the heart of this variability is the phase transition crossover phenomenon induced by two-ion anisotropy of $\mathrm{Fe}\mathrm{Pt}$. Through Monte Carlo simulations based on a realistic $\mathrm{Fe}\mathrm{Pt}$ effective Hamiltonian, we demonstrate that in order to identify the critical exponents accurately, it is necessary to base the analysis on field-dependent magnetization data. We have developed a two-variable finite-size scaling method that accounts for the field effect. Through the use of this method, we show unambiguously that true critical exponents of $\mathrm{Fe}\mathrm{Pt}$ are fully consistent with the three-dimensional Heisenberg universality class.