Short Coherence Length Research Articles

Earth-abundant Li-ion cathode materials with nanoengineered microstructures

AbstractManganese-based materials have tremendous potential to become the next-generation lithium-ion cathode as they are Earth abundant, low cost and stable. Here we show how the mobility of manganese cations can be used to obtain a unique nanosized microstructure in large-particle-sized cathode materials with enhanced electrochemical properties. By combining atomic-resolution scanning transmission electron microscopy, four-dimensional scanning electron nanodiffraction and in situ X-ray diffraction, we show that when a partially delithiated, high-manganese-content, disordered rocksalt cathode is slightly heated, it forms a nanomosaic of partially ordered spinel domains of 3–7 nm in size, which impinge on each other at antiphase boundaries. The short coherence length of these domains removes the detrimental two-phase lithiation reaction present near 3 V in a regular spinel and turns it into a solid solution. This nanodomain structure enables good rate performance and delivers 200 mAh g−1 discharge capacity in a (partially) disordered material with an average primary particle size of ∼5 µm. The work not only expands the synthesis strategies available for developing high-performance Earth-abundant manganese-based cathodes but also offers structural insights into the ability to nanoengineer spinel-like phases.

Superstrength permanent magnets with iron-based superconductors by data- and researcher-driven process design

Iron-based high-temperature (high-Tc) superconductors have good potential to serve as materials in next-generation superstrength quasipermanent magnets owing to their distinctive topological and superconducting properties. However, their unconventional high-Tc superconductivity paradoxically associates with anisotropic pairing and short coherence lengths, causing challenges by inhibiting supercurrent transport at grain boundaries in polycrystalline materials. In this study, we employ machine learning to manipulate intricate polycrystalline microstructures through a process design that integrates researcher- and data-driven approaches via tailored software. Our approach results in a bulk Ba0.6K0.4Fe2As2 permanent magnet with a magnetic field that is 2.7 times stronger than that previously reported. Additionally, we demonstrate magnetic field stability exceeding 0.1 ppm/h for a practical 1.5 T permanent magnet, which is a vital aspect of medical magnetic resonance imaging. Nanostructural analysis reveals contrasting outcomes from data- and researcher-driven processes, showing that high-density defects and bipolarized grain boundary spacing distributions are primary contributors to the magnet’s exceptional strength and stability.

High space–time bandwidth product imaging in low coherence quantitative phase microscopy

Current low coherence quantitative phase microscopy (LC-QPM) systems suffer from either reduced field of view (FoV) or reduced temporal resolution due to the short temporal coherence (TC) length of the light source. Here, we propose a hybrid, experimental and numerical approach to address this core problem associated with LC-QPM. We demonstrate high spatial resolution and high phase sensitivity in LC-QPM at high temporal resolution. High space–time bandwidth product is achieved by employing incoherent light source for sample illumination in QPM to increase the spatial resolution and single-shot Hilbert spiral transform (HST) based phase recovery algorithm to enhance the temporal resolution without sacrificing spatial resolution during the reconstruction steps. The high spatial phase sensitivity comes by default due to the use of incoherent light source in QPM which has low temporal coherence length and does not generate speckle noise and coherent noise. The spatial resolution achieved by the HST is slightly inferior to the temporal phase-shifting (TPS) method when tested on a specimen but surpasses that of the single-shot Fourier transform (FT) based phase recovery method. Contrary to HST method, FT method requires high density fringes for lossless phase recovery, which is difficult to achieve in LC-QPM over entire FoV. Consequently, integration of HST algorithm with LC-QPM system makes an attractive route. Here, we demonstrate scalable FoV and resolution in single-shot LC-QPM and experimentally corroborate it on a test object and on both live and fixed biological specimen such as MEF, U2OS and human red blood cells (RBCs). LC-QPM system with HST reconstruction offer high-speed single-shot QPM imaging at high phase sensitivity and high spatial resolution enabling us to study sub-cellular dynamic inside U2OS for extended duration (3 h) and observe high-speed (50 fps) dynamics of human RBCs. The experimental results validate the effectiveness of the present approach and will open new avenues in the domain of biomedical imaging in the future.

In situ formed partially disordered phases as earth-abundant Mn-rich cathode materials

AbstractEarth-abundant cathode materials are urgently needed to enable scaling of the Li-ion industry to multiply terawatt hours of annual production, necessitating reconsideration of how good cathode materials can be obtained. Irreversible transition metal migration and phase transformations in Li-ion cathodes are typically believed to be detrimental because they may trigger voltage hysteresis, poor kinetics and capacity degradation. Here we challenge this conventional consensus by reporting an unusual phase transformation from disordered Li- and Mn-rich rock salts to a new phase (named δ), which displays partial spinel-like ordering with short coherence length and exhibits high energy density and rate capability. Unlike other Mn-based cathodes, the δ phase exhibits almost no voltage fade upon cycling. We identify the driving force and kinetics of this in situ cathode formation and establish design guidelines for Li- and Mn-rich compositions that combine high energy density, high rate capability and good cyclability, thereby enabling Mn-based energy storage.

К вопросу оценки надежности суперлюминесцентных диодов

A superluminescent diode (SLED or SLD) are used in low coherence tomography and in fiber-optic gyroscopes, where sources with a short coherence length and relatively high radiation power are required. SLED are, as a rule, the least reliable, and at the same time, extremely important for ensuring operability device. The article discusses the issue of resource reliability of the SLED to analyze the possibility of improving the accuracy of devices based on them during long-term operation. The work is a brief overview of achievements in the field of resource reliability analysis of quantum optical semiconductor devices, obtaining a general representation of the reliability of superluminescent radiation sources, changes in their parameters as a result of degradation, consideration of ways to solve problems associated with their spectral degradation.

Anomalously small magnetic relaxation rate of Ca10(Pt3As8)(Fe2−x Pt x As2)5 superconductor

Large critical current density and low magnetic relaxation are the main challenges for practical applications of high-temperature superconductors (HTSs). Magnetic relaxation refers to the decay of superconducting current density caused by thermal activation and/or quantum fluctuations. This process involves the motion of vortices, particularly vortex creep, which leads to energy dissipation. HTSs typically exhibit strong superconducting fluctuations and magnetic relaxation due to their short coherence length and large anisotropy. These characteristics are usually manifested by the large Ginzburg number Gi and high magnetic relaxation rate S. The correlation between the relaxation rate S and Gi is informative to understand the interplay between vortex dynamics and relevant parameters. Recently, Eley et al (2017 Nat. Mater. 16 409) proposed that almost all the HTSs follow a universal lower limit S∼ Gi 1/2(T/T c), providing new clues to design HTSs with small relaxation rate and evaluate their application potential. Here, we systematically studied the vortex dynamics in the Ca10(Pt3As8)(Fe2−x Pt x As2)5 superconductor with a large Gi∼ 0.16. Strikingly, this material exhibits a small relaxation rate (S ∼0.02 at T c/4 under 1 T), approaching the proposed lower limit of S∼ Gi 1/2(T/T c). We propose that such a small value of S in Ca10(Pt3As8)(Fe2−x Pt x As2)5 may originate from its unique structure with metallic skutterudite blocking layers. Our results suggest a promising new avenue for the search and design of HTSs with low magnetic relaxation.

Superconductivity at Interfaces in Cuprate-Manganite Superlattices.

One of the unsolved problems for using high-Tc superconducting cuprates for spintronic applications are the short coherence lengths of Cooper pairs in oxides (a few Å), which requires atomically sharp and defect-free interfaces. This research demonstrates the presence of high-Tc superconducting La1.84 Sr0.16 CuO4 in direct proximity to SrLaMnO4 and provides evidence for the sharpness of interfaces between the cuprate and the manganite layers at the atomic scale. These findings shed light on the impact of the chemical potential at the interface of distinct materials on highly sensitive physical properties, such as superconductivity. Additionally, this results show the high stability of ultrathin layers from the same K2 NiF4 -type family, specifically one unit cell of Sr2- x Lax MnO4 and three unit cells of La1.84 Sr0.16 CuO4 . This work advances both the fundamental understanding of the proximity region between superconducting cuprates and manganite phases and the potential use of oxide-based materials in quantum computing.

Multiple superconducting transitions in α-Sn/β-Sn mixed films grown by molecular beam epitaxy

We have tried to grow α-Sn films on two different substrates with different sample structures by molecular beam epitaxy. The mixture of an α phase with a β phase in the Sn film has been confirmed. The electrical transport properties have been measured and multiple superconducting transitions have been observed in these α-Sn/β-Sn mixed films. Enhanced critical temperatures and critical fields as well as shorter coherence lengths are observed, as compared to bulk β-Sn. The two-dimensional nature is demonstrated by angle-dependent measurements and a type-II superconductor is indicated by the magnetization measurements. We attribute the multiple superconducting transitions to the β-Sn islands formed within the film.

Coarse-to-fine auto-focus method using a white-and-red light source for white-light scanning interferometers

To realize high-efficiency automation for white-light scanning interferometers, auto-focusing is a critical technique, which is expected to have a large focusing range and high repeatability. In this paper, a coarse-to-fine auto-focusing method using a white-and-red LED light source is proposed. The approximate focus position is firstly searched efficiently in the coarse focusing using a re-blur sharpness algorithm, and then the optimal focusing position is determined with a full range scanning in the fine focusing. By combining the long coherence length of the red LED with the short coherence length of the white LED, the proposed auto-focusing method can realize a large focusing range up to 40 µm and achieve high repeatability within several nanometers level. The comparison experiments of sharpness algorithms and auto-focus experiments are carried out. The results show that the proposed method is robust, low-cost, and insensitive to noise, surface textures, and light intensity, which is highly suitable for the optical surface measurement with White-light scanning interferometer.

Temperature dependence of upper critical fields and coherence lengths for optimally-doped YBa2Cu3O7–δ thin films

We report the comprehensive comparative analysis of the upper critical magnetic fields μHc2(0) obtained within Ginzburg–Landau (GL) and Werthamer–Helfand–Hohenberg (WHH) theories for optimally-doped YBa2Cu3O7–δ thin films. For different orientations of the magnetic field, our calculations give 638 and 153 T for μHc2(0), H || ab and μHc2(0), H || c, respectively, when using Hc2(0). For the first time, the temperature dependences of coherence lengths ξab(T) and ξc(T) within proposed theories were determined using 50 and 90% criteria of the normal state resistivity value ρN. The GL (0.9ρN) approach gives ξab(0) = 11.8 Å and ξc(0) = 3.0 Å which are in a good agreement with literature data. The implications of very short coherence lengths in HTSCs are discussed.

Multi-wavelength off-axis digital holographic microscopy with broadly tunable low-coherent sources: theory, performance and limitations

Multi-wavelength digital holographic microscopy (MDHM) is widely used in biological and industrial applications because of increased unambiguous height measurement range and the ability to measure concentration from the spectral dependence of phase delay. Acousto-optic tunable filters (AOTFs) provide the simultaneous selection of several bands with tunable central wavelengths to create a multiplexed hologram, but may limit the field of view (FOV) in off-axis holography because of the short coherence length of the filtered light. We analyzed the performance of the AOTF-based off-axis MDHM setup with a diffraction grating or a prism in the reference arm necessary to increase the efficiency of angular multiplexing. This allows varying the number of spectral channels selected simultaneously without setup realignment. Mathematical description relates the spectral bandwidth of the AOTF, tilt of the coherence plane induced by the angular dispersion of a prism or a grating, width of the FOV determined by interference pattern visibility, spatial resolution, and optimal intermediate wavelengths. We theoretically and experimentally demonstrated that the FOV may be expanded by changing the angle of light incidence on the AOTF and that the prism changes the wavelength dependence of the FOV. We validated this technique by single-shot acquisition of the height maps of the transparent test chart at four wavelengths with an error similar to that of four sequentially captured single-wavelength holograms. The results may be helpful for multiple applications of MDHM using spectrally tunable light sources.

Large field Digital Image Plane Holography with a double cavity high speed laser

The three velocity components in a fluid plane can be measured by applying Digital Image Plane Holography. This technique is limited by the laser coherence length, which reduces its application with high speed lasers that, generally, have a very short coherence length. In addition, the use of a double cavity can also imply a small wavelength difference between the two laser beams. In this work, we present an improved Optical Path Length Enlarging Device that allows the velocity measurement, in a 2D field whose width is four times larger than the laser coherence length. The optical set-up and the procedure for measuring in a larger field (ten times the laser coherence length) were optimized, and the issues derived from the laser spatial and temporal coherence and wavelength changes were analyzed and solved. Digital Image Plane Holography with the Optical Path Length Enlarging Device and Particle Image Velocimetry were applied for measuring the whole velocity field in the central plane of a cylindrical cavity with a rotating lid, for two Reynolds numbers (800 and 2000), showing both of them a very good agreement with the numerical simulations.Graphical abstract

Correlation of pseudogap, superconducting ordering and superconducting fluctuations with electron–phonon interactions in Bi2Sr2CaCu2O8+δ

The relationship between anomalous pseudogap and superconducting states is a fascinating but controversial subject in high temperature superconductors. Here, we investigate the different quasiparticle dynamics with femtosecond transient optical spectroscopy in underdoped Bi2Sr2CaCu2O8+δ at 3–250 K. Below T c, these results reveal the coexistence of the anomalous pseudogap and superconducting states. On the basis of the Mattis–Bardeen formula for the Cooper-pairing superconducting gap and the Kabanov model for the pseudogap state, the measurements yield a temperature-dependent superconducting gaps and two temperature-independent pseudogaps below and above the superconducting transition T c. Above the superconducting transition temperature, the pseudogap state can be explained as an incipient condensation of Cooper pairs with short coherence length. The quasiparticle dynamics in the superconducting and pseudogap states are correlated well with electron–phonon interaction characteristics.

Substrate-induced broken C4 symmetry and gap variation in superconducting single-layer FeSe/SrTiO3−(13×13)

We performed scanning tunneling microscopy/spectroscopy (STM/STS) measurements to investigate the superconducting properties of single-layer FeSe grown on ${\text{SrTiO}}_{3}$ (STO) $(001)\text{\ensuremath{-}}(\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13})$. We found ``$+$'' and ``z'' patterns arranged in $\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}$ periodicity in the STM images. Surprisingly, they show ${C}_{2}$ symmetry, although free-standing FeSe itself has ${C}_{4}$ symmetry. STS spectra and quasiparticle interference patterns near the Fermi level also indicate the broken ${C}_{4}$ symmetry with the superconducting gap showing a local variation within the STO surface unit cell because of the short coherence length. This clearly shows the non-negligible role of the substrate surface in the superconductivity of the FeSe/STO system.

Efficient and controllable magnetization switching induced by intermixing-enhanced bulk spin–orbit torque in ferromagnetic multilayers

Spin–orbit torque induced ferromagnetic magnetization switching brought by injecting a charge current into strong spin–orbit-coupling materials is an energy-efficient writing method in emerging magnetic memories and spin logic devices. However, because of the short spin coherence length in ferromagnetic layers, the interfacial effective spin–orbit torque typically leads to high critical current density for switching thick ferromagnet, which goes against low-power and high-density requirements. Here, we experimentally demonstrate efficient bulk spin–orbit torque-driven perpendicular magnetization switching under relatively low critical current density in thick Pt/Co multilayers with gradient-induced symmetry breaking. Through tuning the thickness gradient of Pt, the spin–orbit torque efficiency and switching chirality can be highly controlled, which also indicates that net spin current arises from gradient. Meanwhile, x-ray absorption spectroscopy results reveal that the atomic intermixing can significantly enhance the spin–orbit torque efficiency through improving the strength of spin–orbit-coupling of Pt. We also establish a micromagnetic model by taking both gradient-induced and intermixing-enhanced spin–orbit torque into account to well describe the experimental observations. This work would blaze a promising avenue to develop novel spin–orbit torque devices for high-performance spintronic memory and computation systems.

Giant tunable Rashba spin splitting in two-dimensional polar perovskites TlSnX3 (X = Cl, Br, I).

The electronic structures and Rashba effect of two-dimensional polar tetragonal perovskites TlSnX3 (X = Cl, Br, I) are investigated by first-principles density functional theory, and intrinsic Rashba effects are found around the Γ point. In particular, TlSnI3 has the largest Rashba constant of 1.072 eV Å-1. Additionally, TlSnBr3 and TlSnI3 respond strongly to the applied electric field, and the electric field responsivity of TlSnI3 can reach 0.790 e Å2. Therefore, due to the large Rashba constants and strong electric field responses, these 2D polar perovskites are promising semiconductor materials with short channel lengths. The nano-scale short spin coherence length can keep the spin coherence of spin FETs, which is superior to the traditional 3D micron spin FETs, and will show a broad application prospect in the Rashba semiconductor field.

Effects of orientation deviation of a beam splitter and a reference mirror on the stability of a two-interferometer-based handheld FFOCT imaging probe

In this work, we present a handheld full-field optical coherence tomography (FFOCT) system based on a series connection of two interferometers: the Michelson interferometer is used as a compensation part and the Fizeau interferometer is used as a detection part. Owing to the common-path arrangement of the Fizeau interferometer, this handheld FFOCT system has a compact detection arm and is insensitive to the external disturbance. A high-output halogen lamp and high NA microscope objective contribute to achieving the spatial resolution of 0.7µm×0.5µm (transverse × axial). Low imaging stability is caused by an extremely short coherence length. We found that to generate en face images with high quality and high stability using a probe with an extremely short coherence length, the range of deviation of the orientation of the beam splitter must be less than 1°, and the range of orientation deviation of the mirror in the Michelson interferometer corresponds to the displacement between the two field stop images at a distance not to exceed 10 µm.

Performance analysis of dual-frequency lidar in the detection of the complex wind field.

Due to short coherent length of the laser, traditional single frequency lidar (SFL) uses echoes of adjacent samples to retrieve Doppler wind velocity, which inevitably brings about a dilemma of high range resolution and velocity resolution. Dual-frequency lidar (DFL), however, with a frequency difference locating at the radar band, can process the laser echoes with a radar processing framework while maintaining a good wind sensing ability in clear air. In this paper, performances of SFL and DFL are analyzed in terms of different conditions like pulse duration, number of sampling points in processing, wind distribution, signal to noise ratio (SNR), etc. It is shown that, for a complex wind field, DFL has the potential to retrieve the Doppler velocity with high velocity and range resolution, even in a relatively low SNR condition.

Full-field measurement of complex objects illuminated by an ultrashort pulse laser using delay-line sweeping off-axis interferometry.

Measuring the complex field of ultrashort pulse lasers plays a fundamental role in light wavefront manipulation and nonlinear phenomena investigation; yet, it still constitutes a challenge for both full-field and high-resolution characterization due to the short coherent length. We proposed and implemented an off-axis interference system with a delay-line sweeping technique to overcome the fringe contrast degradation caused by the envelope mismatch between interfering pulses, resulting in an increased effective analysis area. The effectiveness of the proposed method was first demonstrated by measuring a complex field generated by a phase-only spatial light modulator, where a four-pixel binning technique was adopted for both amplitude and phase modulation; then it was used for the measurement of the second harmonic generation signal of a urea crystal sample. The experimental results show that the proposed method is capable of measuring complex fields having fine features within the full field. The proposed technique can be applied for strongly scattering medium refocusing and adaptive optics, where measuring the complex field of ultrashort pulse lasers is essential.

Secure key distribution based on bidirectional Michelson interferometers.

A scheme based on bidirectional Michelson interferometers to distribute secret keys through the phase of light wave is proposed and demonstrated. The structure uses two optical cable transmission schemes, lasers of short coherent length, random phase drift, polarization random disturbance, and vibration positioning technology to ensure the safe transmission of the secret key. The feasibility of 80 km secret key transmission is verified by experiments. When the secret key is transmitted at the frequency of 200 kHz, the bit error rate is less than 0.3%, and the disturbance positioning accuracy is within $\pm {100}\;{\rm m}$.