Electronic Bandwidth Research Articles

Dynamic Kohn anomaly in twisted bilayer graphene

Abstract Twisted bilayer graphene (TBG) has attracted great interest in the last decade due to the novel properties it exhibited. It was revealed that e–phonon interaction plays an important role in a variety of phenomena in this system, such as superconductivity and exotic phases. However, due to its complexity, the e–phonon interaction in TBG is not well studied yet. In this work, we study the electron interaction with the acoustic phonon mode in TBG and one of its consequences, i.e. the Kohn anomaly. The Kohn anomaly in ordinary metals usually happens at phonon momentum q = 2 k F as a dramatic modification of the phonon frequency when the phonon wave vector nests the electron Fermi surface. However, novel Kohn anomaly can happen in topological semimetals, such as graphene and Weyl semimetals. In this work, we show that the novel dynamic Kohn anomaly can also take place in TBG due to the nesting of two different Moire Dirac points by the phonon wave vector. Moreover, by tuning the twist angle, the dynamic Kohn anomaly in TBG shows different features. Particularly, at magic angle when the electron bandwidth is almost flat, the dynamic Kohn anomalies of acoustic phonons disappear. We also studied the effects of finite temperature and doping on the dynamic Kohn anomaly in TBG and discussed the experimental methods to observe the Kohn anomaly in such system.

Manipulating Moirés by Controlling Heterostrain in van der Waals Devices.

Van der Waals (vdW) moirés offer tunable superlattices that can strongly manipulate electronic properties. We demonstrate the in situ manipulation of moiré superlattices via heterostrain control in a vdW device. By straining a graphene layer relative to its hexagonal boron nitride substrate, we modify the shape and size of the moiré. Our sliding-based technique achieves uniaxial heterostrain values exceeding 1%, resulting in distorted moirés values that are larger than those achievable without strain. The stretched moiré is evident in transport measurements, resulting in shifted superlattice resistance peaks and Landau fans, consistent with an enlarged superlattice unit cell. Electronic structure calculations reveal how heterostrain shrinks and distorts the moiré Brillouin zone, resulting in a reduced electronic bandwidth as well as the appearance of highly anisotropic and quasi-one-dimensional Fermi surfaces. Our heterostrain control approach opens a wide parameter space of moiré lattices to explore beyond what is possible by twist angle control alone.

Considerations on time resolution of neutron irradited single pixel 3D structures at fuences up to 1017 neq/cm2 using 120 GeV SPS pion beams

The proven radiation hardness of silicon 3D devices up to fluences of 1 × 1017 neq/cm2 makes them an excellent choice for next generation trackers, providing <10 μm position resolution at a high multiplicity environment. The anticipated pile-up increase at HL-LHC conditions and beyond, requires the addition of <50 ps per hit timing information to successfully resolve displaced and primary vertices. In this study, the timing performance, uniformity and efficiency of neutron and proton irradiated single pixel 3D devices is discussed. Fluences up to 1 × 1017 neq/cm2 in three different geometrical implementations are evaluated using 120 GeV SPS pion beams. A MIMOSA-26 type telescope is used to provide detailed tracking information with a ∼5 μm position resolution. Productions with single- and double-sided processes, yielding active thicknesses of 130 and 230 μm respectively, are examined with varied pixel sizes from 55 × 55 μm2 to 25 × 100 μm2 and a comparative study of field uniformity is presented with respect to electrode geometry. The question of electronics bandwidth is extensively addressed with respect to achievable time resolution, efficiency and collected charge, forming a 3D phase space to which an appropriate operating point can be selected depending on the application requirements.

Brillouin expanded time-domain analysis based on dual optical frequency combs

Brillouin Optical Time-Domain Analysis (BOTDA) is a widely-used distributed optical fiber sensing technology employing pulse-modulated pump waves for local information retrieval of the Brillouin gain or loss spectra. The spatial resolution of BOTDA systems is intrinsically linked to pulse duration, so high-resolution measurements demand high electronic bandwidths inversely proportional to the resolution. This paper introduces Brillouin Expanded Time-Domain Analysis (BETDA) as a modified BOTDA system, simultaneously achieving high spatial resolution and low detection bandwidth. Utilizing two optical frequency combs (OFCs) with different frequency intervals as pump and probe, local Brillouin gain spectra are recorded by their spectral beating traces in an expanded time domain. A 2-cm-long hotspot located in a 230 m single-mode fiber is successfully measured in the time domain with a detection bandwidth of less than 100 kHz using dual OFCs with tailored spectral phase, line spacing, and bandwidth.

Optimizing the curie temperature of La0.67Sr0.33MnO3–based composite materials around room temperature through the addition of Mo, Mn, and Ni metallic elements

Our study sheds light on the role of secondary phases in optimizing the magnetic properties of LSMO, offering valuable insights into the relationship between impurity phases, Curie temperature (TC), and magnetic entropy change. To achieve this goal, the Mn, Ni, and Mo elements were individually incorporated into the LSMO matrix to the formation of secondary phases. The solid-state reaction method was employed to create composite materials of (0.8)La0.67Sr0.33MnO3 + (0.2)A (A=Mn, Ni, and Mo). XRD and SEM results confirmed the successful formation of impurity phases within the main LSMO phase. From temperature-dependent magnetization (M(T)) measurements, it was determined that the TC of the samples approach towards room temperature with the existence of impurity phases. Additionally, the relationship between electron bandwidth (W) and TC was examined. The lowest TC value was observed for the lowest value of W. Besides these, the effect of the formation of impurity phases on the maximum magnetic entropy change (-ΔSMmax\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-\\Delta {S}_{M}^{max}$$\\end{document}) value is examined, and it is seen that impurity phases cause a decrease in the (-ΔSMmax)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(-\\Delta {S}_{M}^{max})$$\\end{document} values.

Optimized TCR and MR of La0.67Ca0.33-xSrxMnO3: Ag0.15 ceramics by Sr doping

Herein, in order to successfully create high-density La0.67Ca0.33-xSrxMnO3: Ag0.15 (LCSMO: Ag) ceramics (0 ≤ x ≤ 0.05), this research combines sol-gel and solid-phase processes. LCSMO: Ag ceramics electrical transport and magnetic properties were thoroughly investigated. XRD verified that the high-purity orthorhombic perovskite structure (space group Pnma) was produced. As Sr doping rises, the grain size grows, the single electron bandwidth increases, the metal-insulator transition temperature (TMI) continues to climb, and temperature coefficient of resistance (TCR) and magnetoresistance (MR) steadily drop. When x = 0.04, the TMI reaches 302.2 K. At this time, the TCR peak (TCRpeak) reaches 24.0 %·K−1 and the MRpeak reaches 49.2 %. These results demonstrate that Sr may effectively raise the TMI of LCMO: Ag ceramics to room temperature while maintaining high TCR and MR Values, which benefits the material's potential applications in devices such as uncooled infrared detectors.

Improved electrical transport properties of La0.7Ca0.3−xKxMnO3 (0.0 ≤ x ≤ 0.3) films grown on La0.3Sr0.7Al0.65Ta0.35O3 (00l) substrates by sol–gel spin coating method

Herein, La0.7Ca0.3−xKxMnO3 (LCKMO) films with highly oriented growth were prepared on La0.3Sr0.7Al0.65Ta0.35O3 (00l) substrates by sol–gel spin coating method. Comprehensive analysis of microstructure, surface morphology, ion valence, and electrical transport properties of LCKMO films indicates that the substitution of K+ ions for Ca2+ ions increased the ratio of Mn4+/(Mn3+ + Mn4+), narrowed the one electron bandwidth, and enhanced double exchange mechanism and electron–lattice coupling. Moreover, the doping of K+ ions induced the dense and uniform conical-structured growth of grains on the surface of the films. Results demonstrated that at x = 0.1, LCKMO film exhibited the maximum TCR of 17.84% K−1 at 253.07 K, which was 31.76% higher than that of previously reported La1−x−yCaxKyMnO3 ceramic with the highest TCR. Further theoretical insights into the effects of Ca/K co-doping on electrical transport properties were supplemented.

Optically Controlled Nano-Transducers Based on Cleaved Superlattices for Monitoring Gigahertz Surface Acoustic Vibrations.

Surface acoustic waves (SAWs) convey energy at subwavelength depths along surfaces. Using interdigital transducers (IDTs) and opto-acousto-optic transducers (OAOTs), researchers have harnessed coherent SAWs with nanosecond periods and micrometer localization depth for various applications. These applications include the sensing of small amount of materials deposited on surfaces, assessing surface roughness and defects, signal processing, light manipulation, charge carrier and exciton transportation, and the study of fundamental interactions with thermal phonons, photons, magnons, and more. However, the utilization of cutting-edge OAOTs produced through surface nanopatterning techniques has set the upper limit for coherent SAW frequencies below 100 GHz, constrained by factors such as the quality and pitch of the surface nanopattern, not to mention the electronic bandwidth limitations of the IDTs. In this context, unconventional optically controlled nanotransducers based on cleaved superlattices (SLs) are here presented as an alternative solution. To demonstrate their viability, we conducted proof-of-concept experiments using ultrafast lasers in a pump-probe configuration on SLs made of alternating AlxGa1-xAs and AlyGa1-yAs layers with approximately 70 nm periodicity and cleaved along their growth direction to produce a periodic nanostructured surface. The acoustic vibrations, generated and detected by laser beams incident on the cleaved surface, span a range from 40 to 70 GHz, corresponding to the generalized surface Rayleigh mode and bulk modes within the dispersion relation. This exploration shows that, in addition to SAWs, cleaved SLs offer the potential to observe surface-skimming longitudinal and transverse acoustic waves at GHz frequencies. This proof-of-concept demonstration below 100 GHz in nanoacoustics using such an unconventional platform might be useful for realizing sub-THz to THz coherent surface acoustic vibrations in the future, as SLs can be epitaxially grown with atomic-scale layer width and quality.

Investigation of the impact of A-site cation disorder on the structure, magnetic properties, and magnetic entropy change of trisubstituted divalent ions in La0.7(Ba,Ca,Sr)0.3MnO3 manganite.

This study investigates the effect of A-site disorder, characterized by the average ionic radius (〈rA〉) and the cation mismatch (σ2), on the structural, magnetic, critical behavior, and magnetic entropy changes in La0.7(Ba,Ca,Sr)0.3MnO3 manganites with trisubstituted Ba, Ca, and Sr. The sol-gel method was used to prepare polycrystalline samples. All series of compounds crystallize in rhombohedral symmetry with the R3̄c space group. A linear relationship between lattice parameters, unit cell volume, and 〈rA〉 was observed. This reveals an unusual behavior in the correlation between 〈rA〉 and σ2 concerning magnetic properties, which is attributed to the complex simultaneous trisubstitution of divalent ions. Energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were utilized to validate the chemical composition of compounds. All the samples crystallized in rhombohedral symmetry, and the lattice parameters increased continuously with increasing 〈rA〉. A-site disorder causes distortions in the Mn-O bond length and Mn-O-Mn bond angle in the MnO6 octahedral structure, which influences the double-exchange interaction and electronic bandwidth (W). The Curie temperature (TC) increases linearly with increasing W. The critical behavior around TC for all the samples was investigated by determining the values of the critical exponents (β, γ, and δ) using the modified Arrott plot (MAP) method. The estimated critical exponents show that the unconventional model establishes a short-range ferromagnetic order. The maximum magnetic entropy change (-ΔSM) was obtained with the lowest 〈rA〉 and σ2 value. The analysis of the critical behavior and universal curve indicates a second-order phase transition (SOPT) nature for all samples.

Effects of Optical Sampling Pulse Power, RF Power, and Electronic Back-End Bandwidth on the Performance of Photonic Analog-to-Digital Converter.

The effects of optical sampling pulse power, RF power, and electronic back-end bandwidth on the performance of time- and wavelength-interleaved photonic analog-to-digital converter (PADC) with eight-channel 41.6 GHz pulses have been experimentally investigated in detail. The effective number of bits (ENOB) and peak-to-peak voltage (Vpp) of converted 10.6 GHz electrical signals were used to characterize the effects. For the 1550.116 nm channel with 5.2 G samples per second, an average pulse power of 0 to -10 dBm input to the photoelectric detector (PD) has been tested. The Vpp increased with increasing pulse power. And the ENOB for pulse power -9~-3 dBm was almost the same and all were greater than four. Meanwhile, the ENOB decreased either when the pulse power was more than -2 dBm due to the saturation of PD or when the pulse power was less than -10 dBm due to the non-ignorable noise relative to the converted weak signal. In addition, RF powers of -10~15 dBm were loaded into the Mach-Zehnder modulator (MZM). The Vpp increased with the increase in RF power, and the ENOB also showed an increasing trend. However, higher RF power can saturate the PD and induce greater nonlinearity in MZM, leading to a decrease in ENOB, while lower RF power will convert weak electrical signals with more noise, also resulting in lower ENOB. In addition, the back-end bandwidths of 0.2~8 GHz were studied in the experiments. The Vpp decreased as the back-end bandwidth decreased from 8 to 3 GHz, and remained nearly constant for the bandwidth between the Nyquist bandwidth and the subsampled RF signal frequency. The ENOB was almost the same and all greater than four for a bandwidth from 3 to 8 GHz, and gradually increased up to 6.5 as the back-end bandwidth decreased from the Nyquist bandwidth to 0.25 GHz. A bandwidth slightly larger than the Nyquist bandwidth was recommended for low costs and without compromising performance. In our experiment, the -3 to -5 dBm average pulse power, about 10 dBm RF power, and 3 GHz back-end bandwidth were recommended to accomplish both a high ENOB more than four and large Vpp. Our research provides a solution for selecting optical sampling pulse power, RF power, and electronic back-end bandwidth to achieve low-cost and high-performance PADC.

Strain control of a bandwidth-driven spin reorientation in Ca3Ru2O7

The layered-ruthenate family of materials possess an intricate interplay of structural, electronic and magnetic degrees of freedom that yields a plethora of delicately balanced ground states. This is exemplified by Ca3Ru2O7, which hosts a coupled transition in which the lattice parameters jump, the Fermi surface partially gaps and the spins undergo a 90∘ in-plane reorientation. Here, we show how the transition is driven by a lattice strain that tunes the electronic bandwidth. We apply uniaxial stress to single crystals of Ca3Ru2O7, using neutron and resonant x-ray scattering to simultaneously probe the structural and magnetic responses. These measurements demonstrate that the transition can be driven by externally induced strain, stimulating the development of a theoretical model in which an internal strain is generated self-consistently to lower the electronic energy. We understand the strain to act by modifying tilts and rotations of the RuO6 octahedra, which directly influences the nearest-neighbour hopping. Our results offer a blueprint for uncovering the driving force behind coupled phase transitions, as well as a route to controlling them.

Superconductivity, Charge Density Wave, and Supersolidity in Flat Bands with a Tunable Quantum Metric.

Predicting the fate of an interacting system in the limit where the electronic bandwidth is quenched is often highly nontrivial. The complex interplay between interactions and quantum fluctuations driven by the band geometry can drive competition between various ground states, such as charge density wave order and superconductivity. In this work, we study an electronic model of topologically trivial flat bands with a continuously tunable Fubini-Study metric in the presence of on-site attraction and nearest-neighbor repulsion, using numerically exact quantum MonteCarlo simulations. By varying the electron filling and the minimal spatial extent of the localized flat-band Wannier wave functions, we obtain a number of intertwined orders. These include a phase with coexisting charge density wave order and superconductivity, i.e., a supersolid. In spite of the nonperturbative nature of the problem, we identify an analytically tractable limit associated with a "small" spatial extent of the Wannier functions and derive a low-energy effective Hamiltonian that can well describe our numerical results. We also provide unambiguous evidence for the violation of any putative lower bound on the zero-temperature superfluid stiffness in geometrically nontrivial flat bands.

Microscopic analysis of the valence transition in tetragonal EuPd2Si2

Under temperature or pressure tuning, tetragonal EuPd$_2$Si$_2$ is known to undergo a valence transition from nearly divalent to nearly trivalent Eu accompanied by a volume reduction. Albeit intensive work, its microscopic origin is still being discussed. Here, we investigate the mechanism of the valence transition under volume compression by $ab~initio$ density functional theory (DFT) calculations. Our analysis of the electronic and magnetic properties of EuPd$_2$Si$_2$ when approaching the valence transition shows an enhanced $c$-$f$ hybridization between localized Eu 4$f$ states and itinerant conduction states (Eu 5$d$, Pd 4$d$, and Si 3$p$) where an electronic charge redistribution takes place. We observe that the change in the electronic structure is intimately related to the volume reduction where Eu-Pd(Si) bond lengths shorten and, for the transition to happen, we trace the delicate balance between electronic bandwidth, crystal field splitting, Coulomb repulsion, Hund's coupling and spin-orbit coupling. In a next step we compare and benchmark our DFT results to surface-sensitive photoemission data in which the mixed-valent properties of EuPd$_2$Si$_2$ are reflected in a simultaneous observation of divalent and trivalent signals from the Eu $4f$ shell. The study serves as well to explore the limits of density functional theory and the choice of exchange correlation functionals to describe such a phenomenon as a valence transition.

Augmented light field tomography through parallel spectral encoding.

Snapshot recording of transient dynamics in three dimensions (3-D) is highly demanded in both fundamental and applied sciences. Yet it remains challenging for conventional high-speed cameras to address this need due to limited electronic bandwidth and reliance on mechanical scanning. The emergence of light field tomography (LIFT) provides a new solution to these long-standing problems and enables 3-D imaging at an unprecedented frame rate. However, based on sparse-view computed tomography, LIFT can accommodate only a limited number of projections, degrading the resolution in the reconstructed image. To alleviate this problem, we herein present a spectral encoding scheme to significantly increase the number of allowable projections in LIFT while maintaining its snapshot advantage. The resultant system can record 3-D dynamics at a kilohertz volumetric frame rate. Moreover, by using a multichannel compressed sensing algorithm, we improve the image quality with an enhanced spatial resolution and suppressed aliasing artifacts.

A Study of Hypersonic Vehicle-Borne SAR Imaging Under Plasma Sheath

Hypersonic vehicle-borne synthetic aperture radar (SAR) imaging has promising application prospects for remote sensing owing to the characteristics of fast response and flexible trajectory. However, a newly observed phenomenon that occurs during hypersonic flight, called the plasma sheath, seriously affects hypersonic vehicle-borne SAR imaging. Therefore, in this study, the effect of plasma sheath on hypersonic vehicle-borne SAR imaging is investigated systematically. First, a hypersonic vehicle-borne SAR signal model under plasma sheath is developed. Then, the phase shift and amplitude attenuation which are critical in the SAR signal model under plasma sheath are deduced. Moreover, the analytical formulas for the linear and quadratic phase errors are derived. Based on the developed signal model, point target response and an SAR image of an area under plasma sheath show that linear phase error will cause image shift, and quadratic phase error will lead to pulse broadening. In addition, the amplitude attenuation will make the point target response submerged in noise, and signal amplitude nonlinear distortion will result in an asymmetric distortion phenomenon. To further explore the effect of plasma sheath on hypersonic vehicle-borne SAR imaging, quantitative evaluation and analysis of SAR imaging degradation are conducted under different electron densities, carrier frequencies, and bandwidths. The evaluation results in this study will benefit the application of hypersonic vehicle-borne SAR imaging under plasma sheath, and the analytical formulas of linear and quadratic phase error provide the possibility to compensate for SAR imaging degradation under plasma sheath in the future.

Superior La0.67K0.33-xSrxMnO3 films with room-temperature TCR prepared by spin coating method

La0.67K0.33-xSrxMnO3 thin films were grown on LaAlO3 (010) substrates via sol–gel and spin coating methods. The replacement of K ions with Sr ions affected the eg electron bandwidth and concentration of Mn3+/Mn4+ pairs as well as effectively modulated the magnetic/orbital order and lattice structure. Consequently, the ferromagnetic metal to paramagnetic insulation transition was affected, which further optimized the resistivity, temperature coefficient of resistivity (TCR), peak TCR temperature (TK), and peak resistivity temperature (TP). At temperatures below 230 K, temperature-independent scattering and electron-electron scattering were practically not observed in the films. Overall, electrical properties of films improved with an increase in the Sr doping content. The superior La0.67K0.27Sr0.06MnO3 film exhibited a maximum TCR of 11.88% K−1 (298.3 K), which is approximately 31% higher than that of bulk ceramic with the same composition. The as-developed films exhibit significant potential for application in uncooled infrared bolometers.

Chemical Bonding Effects and Physical Properties of Noncentrosymmetric Hexagonal Fluorocarbonates ABCO3F (A: K, Rb, Cs; B: Mg, Ca, Sr, Zn, Cd).

The present work applied the methods of density functional theory and the van der Waals interaction PBE + D3(BJ) on the basis of localized orbitals of the CRYSTAL17 package. It featured the effect of interactions between structural elements of fluorocarbonates ABCO3F (A: K, Rb, Cs; B: Mg, Ca, Sr, Zn, Cd) on their elastic and vibrational properties. The hexagonal structures proved to consist of alternating ···B-CO3··· and ···A-F··· layers in planes ab, interconnected along axis c by infinite chains ···F-B-F···, where cations formed polyhedra AOnF3 and BOmF2. The calculations included the band energy structure, the total and partial density of electron states, the energy and band widths of the upper ns- and np-states of alkali and alkaline-earth metals, as well as nd-zinc and nd-cadmium. For hydrostatic compression, we calculated the parameters of the Birch–Murnaghan equation of state and the linear compressibility moduli along the crystal axes and bond lines. We also defined the elastic constants of single crystals to obtain the Voigt–Reuss–Hill approximations for the elastic moduli of polycrystalline materials. The study also revealed the relationship between the elastic properties and the nature of the chemical bond. Hybrid functional B3LYP made it possible to calculate the modes of normal long-wavelength oscillations, which provided the spectra of infrared absorption and Raman scattering. Intramolecular modes ν1 and ν4 with one or two maxima were found to be intense, and their relative positions depended on the lengths of nonequivalent C–O bonds.

Broadband transient absorption spectroscopy using an incoherent white-light source as probe.

Time-resolved spectroscopy and, in particular, transient absorption methods have been widely employed to study the dynamics of materials, usually achieving time resolution down to femtoseconds with measurement windows up to a few nanoseconds. Various techniques have been developed to extend the measurement duration up to milliseconds and beyond to permit probing slower dynamics. However, most of these either demand complicated and expensive equipment or do not provide broadband spectral coverage. This paper proposes a transient absorption technique in which an ultra-short pulse laser and a broadband incoherent continuous-wave light source are employed as pump and probe, respectively. Detection of the transient probe transmission is performed in a time-resolved fashion with a fast photodiode after a monochromator and the data is recorded with an oscilloscope. The time resolution is determined by the electronic bandwidth of the detection and acquisition devices and is ∼1 ns, with a measurement duration window of up to milliseconds and a spectral resolution of <2 nm covering from 0.4 to 2 µm. In addition, the setup can be employed to measure time- and spectrally-resolved photoluminescence.

Structural, Magnetic, and Electrical Properties and Magnetoresistance of Monovalent K-Substituted La0.7Ba0.3−xKxMnO3 (x = 0 and 0.04) Manganite

The effects of K+ substitution at the Ba-site on the structural, magnetic, and electrical properties and magnetoresistance (MR) of La0.7Ba0.3−xKxMnO3 (x = 0 and 0.04) manganites prepared via the solid-state method were investigated. Rietveld refinement of X-ray diffraction data confirmed that both samples were crystallized in the rhombohedral structure with the R3c¯ space group. In addition, the unit cell volume, V, and the average grain size also increased with K+ ions. Magnetization versus applied field (M–H) measurement was carried out, and the saturation magnetization (Ms) was found to increase from 1.81 μB/f.u. (x = 0) to 4.11 μB /f.u. (x = 0.04), implying that K+ ions strengthened the ferromagnetic (FM) interaction. Furthermore, the metal–insulator transition temperature, TMI, increased from 257 K (x = 0) to 271 K (x = 0.04). The observed behaviour may be related to the enhancement of double-exchange (DE) interaction due to the increase in Mn-O-Mn bond angle and electronic bandwidth (W), favouring the increasing rate of the eg electron hopping process. The fitting of the electrical resistivity data in the metallic region describes the significance of residual resistivity, electron–electron and electron–magnon scattering processes to elucidate the electronic transport properties. Within the insulating region, variable range hopping (VRH) and small polaron hopping (SPH) models are proposed to describe the conduction mechanism.

High‐Resolution Time‐Correlated Single‐Photon Counting Using Electro‐Optic Sampling

AbstractA simple, practical method based on electro‐optic gating is experimentally shown to improve the temporal resolution of single‐photon detection by more than 16 times. Delay times between ultrafast single photons and a reference clock are stretched by a desired programmable sampling gate factor, allowing reconstruction of delay histograms with ≈0.001 photons per pulse to within 60 ps. By transferring the bandwidth of RF electronics to single‐photon counting, complex single‐photon signals with large time‐bandwidth products &gt; 2000 are temporally stretched up so that they can be resolved by slow detectors, irrespective of the quality of the detector instrument response function. This method is also applied to biphotons, in order to reconstruct the 2D histogram of joint detection delays with 15 times better resolution. The phenomenon of nonlocal dispersion, which is not resolvable directly with the slow detectors, is then observable to within the 98 ps level. The proof‐of‐concept demonstration uses off‐the‐shelf commercial fiber‐integrated LiNbO3 modulators and RF electronics, and the method is readily integrable on‐chip and to speeds &gt;100 GHz, offering a practical solution to ultrafast time‐correlated single‐photon counting beyond the research laboratory.