Low-energy Features Research Articles

Experimental observation of quantum many-body excitations of E8 symmetry in the Ising chain ferromagnet CoNb2O6

Close to the quantum critical point of the transverse-field Ising spin-chain model, an exotic dynamic spectrum was predicted to emerge upon a perturbative longitudinal field. The dynamic spectrum consists of eight particles and is governed by the symmetry of the $E_8$ Lie algebra. Here we report on high-resolution terahertz spectroscopy of quantum spin dynamics in the ferromagnetic Ising-chain material CoNb$_2$O$_6$. At 0.25 K in the magnetically ordered phase we identify characteristics of the first six $E_8$ particles, $\mathbf{m}_1$ to $\mathbf{m}_6$, and the two-particle ($\mathbf{m}_1+\mathbf{m}_2$) continuum in an applied transverse magnetic field of $B_c^{1D}=4.75$ T, before the three-dimensional magnetic order is suppressed above $B_c^{3D}\approx 5.3$ T. The observation of the higher-energy particles ($\mathbf{m}_3$ to $\mathbf{m}_6$) above the low-energy two-particle continua features quantum many-body effects in the exotic dynamic spectrum.

Direct inner-shell photoionization of Xe atoms embedded in helium nanodroplets

We present the first measurements of photoelectron spectra of atomic clusters embedded in superfluid helium (He) nanodroplets. Owing to the large absorption cross section of xenon (Xe) around 100 eV photon energy (4d inner-shell ionization), direct dopant photoionization exceeds charge transfer ionization via the ionized He droplets. Despite the predominant creation of Xe2+ and Xe3+ by subsequent Auger decay of free Xe atoms, for Xe embedded in He droplets only singly charged , k = 1, 2, 3 fragments are observed. Broad Xe+ ion kinetic-energy distributions indicate Coulomb explosion of the ions due to electron transfer to the primary Auger ions from surrounding neutral atoms. The electron spectra correlated with Xe ions emitted from the He nanodroplets contain a low-energy feature and nearly unshifted Xe photolines. These results pave the way to extreme ultraviolet and x-ray photoelectron spectroscopy of clusters and molecular complexes embedded in He nanodroplets.

Hardware-Efficient 2D-DCT/IDCT Architecture for Portable HEVC-Compliant Devices

Low power hardware acceleration cores for integration into real-time High Efficiency Video Coding (HEVC) codec for smartphones, tablets, camcorders, and televisions are in great demand. This need motivates one for an efficient realization of Discrete Cosine Transform (DCT) and Inverse-DCT (IDCT) for HEVC. This paper presents an algorithm to calculate the required minimum number of low-frequency DCT-output/IDCT-input coefficients for 4, 8, 16, and 32-point DCT/IDCT in HEVC, such that there is a slight decrease in peak-signal-to-noise-ratio (<; 0.15 decibel) and a minor increment in bitrate (<; 1.5%) as compared to the reference HEVC-Test-Model (HM) Software. However, the encoding time gets reduced at most by 17.95% for Class-A type sequences, while reporting mean-squared-error and structural-similarity of 1.42 and 0.9913, respectively for 4K ultra-high-definition videos. Moreover, HEVC-compliant computationally efficient architectures are introduced for n-point DCT/IDCT. The presented flexible Transpose Memory architecture uses only sixteen random-access-memories to support all transform-unit sizes in HEVC. The proposed two-dimensional DCT/IDCT architecture can process up to 288@$4K$ frames-per-second, and it consumes the minimum power, energy, and area of 11.23 milliwatts, 2.34 picojoules, and 120 kilo-gate-equivalents, respectively. Such design with low power, area, and energy features can be included in a real-time HEVC codec for HEVC-compliant consumer electronic devices.

Landau theory for non-equilibrium steady states

We examine how non-equilibrium steady states close to a continuous phase transition can still be described by a Landau potential if one forgoes the assumption of analyticity. In a system simultaneously coupled to several baths at different temperatures, the non-analytic potential arises from the different density of states of the baths. In periodically driven-dissipative systems, the role of multiple baths is played by a single bath transferring energy at different harmonics of the driving frequency. The mean-field critical exponents become dependent on the low-energy features of the two most singular baths. We propose an extension beyond mean field.

Strongly anharmonic collective modes in a coupled electron-phonon-spin problem

We solve for the finite temperature collective mode dynamics in the Holstein-double exchange problem, using coupled Langevin equations for the phonon and spin variables. We present results in a strongly anharmonic regime, close to a polaronic instability. For our parameter choice the system transits from an `undistorted' ferromagnetic metal at low temperature to a structurally distorted paramagnetic insulator at high temperature, through a short range charge ordered (CO) phase near the ferromagnetic crossover at $T_{FM}$. The small amplitude harmonic phonons at low temperature cross over to large amplitude dynamics around $0.5 T_{FM}$ due to thermally generated short range correlated polarons. The rare thermal ``tunneling'' of CO domains generates a hitherto unknown momentum selective spectral weight at very low energy. We compare our results to inelastic neutron data in the manganites and suggest how the singular low energy features can be probed.

Molecular vibrations reduce the maximum achievable photovoltage in organic solar cells

The low-energy edge of optical absorption spectra is critical for the performance of solar cells, but is not well understood in the case of organic solar cells (OSCs). We study the microscopic origin of exciton bands in molecular blends and investigate their role in OSCs. We simulate the temperature dependence of the excitonic density of states and low-energy absorption features, including low-frequency molecular vibrations and multi-exciton hybridisation. For model donor-acceptor blends featuring charge-transfer excitons, our simulations agree very well with temperature-dependent experimental absorption spectra. We unveil that the quantum effect of zero-point vibrations, mediated by electron-phonon interaction, causes a substantial exciton bandwidth and reduces the open-circuit voltage, which is predicted from electronic and vibronic molecular parameters. This effect is surprisingly strong at room temperature and can substantially limit the OSC’s efficiency. Strategies to reduce these vibration-induced voltage losses are discussed for a larger set of systems and different heterojunction geometries.

Tunable Perovskite-Derived Bismuth Halides: Cs3Bi2(Cl1-xIx)9.

Bismuth-based perovskites are of interest as safer alternatives to lead-based optoelectronic materials. Prior studies have reported on the compounds Cs3Bi2Cl9, Cs3Bi2I9, and Cs3Bi2Cl3I6. Here we examine a range of compounds of the formula Cs3Bi2(Cl1-xIx)9, where x takes values from 0.09 to 0.52. Powder and single-crystal X-ray diffraction were used to determine that all of these compounds adopt the layered vacancy-ordered perovskite structure observed for Cs3Bi2Cl3I6, which is also the high-temperature phase of Cs3Bi2Cl9. We find that, even with very small iodine incorporation, the structure is switched to that of Cs3Bi2Cl3I6, with I atoms displaying a distinct preference for the capping sites on the BiX6 octahedra. Optical absorption spectroscopy was employed to study the evolution of optical properties of these materials, and this is complemented by density functional theory electronic structure calculations. Three main absorption features were observed for these compounds, and with increasing x, the lowest-energy features are red-shifted.

Short-term outcomes of laparoscopic intersphincteric resection with intraoperative radiotherapy using low-energy X-rays for primary locally advanced low rectal cancer: a single center experience

BackgroundEven with the augmentative application of anal-preservation surgery in low rectal cancer, the role and indications of laparoscopic intersphincteric resection (Lap ISR) are still under debate, especially for T3 or node-positive (T3N0M0, T1–3N+M0) cancer, mainly due to the oncological safety and functional outcomes. INTRABEAM (Carl Zeiss, Germany) intraoperative radiotherapy (IORT) using low-energy X-rays features in accurate irradiation, less exposure, and reduced complications. Taking advantages of Lap ISR and INTRABEAM IORT, this innovative approach aims to increase the probability of the anal preservation with acceptable postoperative outcomes.Materials and methodsFrom December 2015 to August 2019, we retrospectively analyzed the short-term outcomes of 12 patients evaluated preoperatively with T3 or node-positive (T3N0M0, T1–3N+M0) primary locally advanced low rectal cancer. They all had received Lap ISR and INTRABEAM IORT with a dose of 16–18 Gy applied by an applicator through the anus (natural orifice). Then, with no pre- or postoperative radiotherapy given, the patients were suggested to receive 6–8 cycles of the XELOX chemotherapy regimen (oxaliplatin, 130 mg/m2 and capecitabine, 1000 mg/m2).ResultsAll patients achieved R0 resection. The median radiation time was 27 min and 15 s, and the mean radiative dose was 17.3 Gy (range 16–18 Gy). The median follow-up time was 18.5 months (range 3–45 months). Two patients experienced local recurrence. Two male patients experienced anastomotic stenosis. Furthermore, one of them experienced perianal abscess and the other one experienced pulmonary metastasis after refusing to receive chemotherapy. One female patient with internal anal sphincter invasion experienced distant metastases to the liver and gluteus maximus muscle 35 months after IORT. No acute radiation injuries or symptoms were observed. Although they experienced a reduction in anal function, every patient was satisfied with the postoperative outcomes.ConclusionsFor patients evaluated preoperatively with T3 or node-positive (T3N0M0, T1–3N+M0) primary locally advanced low rectal cancer, Lap ISR with INTRABEAM IORT may be a safe and feasible approach for anal preservation without compromising oncological outcomes.

Increasing the Efficiency of Photocatalytic Reactions via Surface Microenvironment Engineering.

The use of photocatalysis for water purification and environmental protection is of key interest. However, the reaction kinetics can be limited by the restricted accessibility of electron acceptor oxygen and the low adsorption of organic compounds-crucial factors underlying photocatalytic performance. Here we simultaneously alleviate these constraints via reaction interface microenvironment design using superhydrophobic (SHB) TiO2 nanoarrays as a model photocatalyst. The low surface energy and rough surface microstructure features of the SHB nanoarrays give the photocatalytic system long-range hydrophobic force and an air-water-solid triphase reaction interface. This simultaneously changes the adsorption model of organic compounds and the access pathway of oxygen, leading to a markedly enhanced adsorption capacity and higher interfacial oxygen levels. These synergistic qualities result in over 30-fold higher reaction kinetics versus a normal diphase system. In addition, this photocatalytic system is stable via repeated cycling. Our findings highlight the importance of reaction interface microenvironment design and reveal an effective path for the development of efficient photocatalysis systems.

Conductance-Matrix Symmetries of a Three-Terminal Hybrid Device.

We present conductance-matrix measurements of a three-terminal superconductor-semiconductor hybrid device consisting of two normal leads and one superconducting lead. Using a symmetry decomposition of the conductance, we find that antisymmetric components of pairs of local and nonlocal conductances qualitatively match at energies below the superconducting gap, and we compare this finding with symmetry relations based on a noninteracting scattering matrix approach. Further, the local charge character of Andreev bound states is extracted from the symmetry-decomposed conductance data and is found to be similar at both ends of the device and tunable with gate voltage. Finally, we measure the conductance matrix as a function of magnetic field and identify correlated splittings in low-energy features, demonstrating how conductance-matrix measurements can complement traditional single-probe measurements in the search for Majorana zero modes.

Feature-Supervised Compressed Sensing for Microwave Imaging Systems

Traditional compressed sensing (CS) applications use sparse information for downsampling but ignore overall system objectives such as feature extraction. This article jointly designs the sensing and feature-extraction process to improve the efficiency of the microwave imaging systems in time, storage, and feature extraction. A feature-supervised CS (FsCS) is proposed for the cases where not all data contribute to feature extraction. Compared with the traditional spatial–spectral sweep and CS solutions, a feature constraint is added in designing the CS measurement matrix. More efficient sensing and feature extraction are achieved, because only the data contributing to feature extraction are sampled and reconstructed. To improve the time efficiency of CS reconstruction, an aligned spatial–spectral sensing (ASSS) is involved in FsCS to enable joint reconstruction. The proposed scheme is validated in an open-ended waveguide imaging system for low-energy impact damage feature detection. The experimental results demonstrate one order of magnitude improvement in time and two orders of improvement in the data compression ratio compared with the state-of-the-art method, while preserving the interested feature. This article can inspire the joint sensing-processing designs for more intelligent industrial processes.

Dynamical and thermal magnetic properties of the Kitaev spin liquid candidate \u03b1-RuCl3

What is the correct low-energy spin Hamiltonian description of alpha-RuCl_{3}? The material is a promising Kitaev spin liquid candidate, but is also known to order magnetically, the description of which necessitates additional interaction terms. The nature of these interactions, their magnitudes and even signs, remain an open question. In this work we systematically investigate dynamical and thermodynamic magnetic properties of proposed effective Hamiltonians. We calculate zero-temperature inelastic neutron scattering (INS) intensities using exact diagonalization, and magnetic specific heat using a thermal pure quantum states method. We find that no single current model satisfactorily explains all observed phenomena of alpha-RuCl_{3}. In particular, we find that Hamiltonians derived from first principles can capture the experimentally observed high-temperature peak in the magnetic specific heat, while overestimating the magnon energy at the zone center. In contrast, other models reproduce important features of the INS data, but do not adequately describe the magnetic specific heat. To address this discrepancy we propose a modified ab initio model that is consistent with both magnetic specific heat and low-energy features of INS data.

Continuous-wave terahertz quantitative dual-plane ptychography

Terahertz (THz) radiation lies between the millimeter and infrared region of the electromagnetic spectrum, which is typically defined as the frequency range of 0.1–10 THz and the corresponding wavelength ranges from 30 μm to 3 mm. Terahertz radiation due to wide spectrum, high penetration, low energy, and other important features, has been a valuable tool for imaging and non-destructive testing on a submillimeter scale. Continuous-wave (CW) terahertz ptychography is a type of phase-contrast technique with advantages of simple set-up and large field-of-view. It retrieves the complex-valued transmission function of the specimen and the probe function at the same time. The extended ptychographic iterative engine (ePIE) algorithm is used as the reconstruction algorithm in the field of ptychography, because it is relatively simple, and can use computer memory efficiently. However, the problem of algorithm convergence delay makes us unable to acquire the reconstruction result very quickly. Since the ptychography is a problem of retrieving phase information, physical constraints affect the convergence speed of the algorithm strongly. In this paper, we propose a dual-plane ePIE (dp-ePIE) algorithm for CW THz ptychography. By moving detector along the axis and capturing diffraction patterns of one zone of an object at two recording planes, then, two sets of patterns used as the constraints simultaneously can increase the diversity of experimental parameter. Hence, the convergence rate can be improved. The simulation results suggest better reconstruction fidelity with a faster convergence rate by the dp-ePIE algorithm. The dual-plane terahertz ptychography experimental setup is built based on 2.52 THz optically pumped laser and Pyrocam-III pyroelectric array detector. Compared with other methods to increase the diversity of measurement, the setup of dual-plane ptychography can be compact and simple, thus reducing the terahertz wave transmission loss. A polypropylene sample is adopted and it is approximated as a pure phase object. No-reference structural sharpness (NRSS) is utilized as a quantitative evaluation index. It takes 45.086 s to achieve NRSS value of 0.9831 by using the dp-ePIE algorithm in 10 iterations, while the NRSS value and calculation time for e-PIE algorithm are 0.9531 and 57.117 s (20 loops), respectively. The experimental results show that the dp-ePIE algorithm can obtain high-quality amplitude and phase distribution with less iterations than the traditional ePIE algorithm.

Quantification of non-bridging oxygens in silicates using X-ray Raman scattering

Detecting and quantifying non-bridging oxygen (NBO) atoms is of particular interest for understanding the physical properties of melts or compressed materials, and requires an unequivocal spectral signature usable during in situ measurements. In this paper, we evidence a low-energy feature of NBO in lithium silicate crystals using X-ray Raman scattering (XRS) spectroscopy around the energy losses of the oxygen K-edge. A specific peak at 534–535 eV in the edge onset is unequivocally attributed to the presence of NBO. Its intensity is used to quantify NBO in lithium silicate glasses. A similar feature at low energy has also been evidenced in Na2SiO3 and MgSiO3, generalizing the method to other alkali and alkali-earth silicates. This non-destructive method of NBO quantification, which is based on an X-ray inelastic scattering technique, can be extended to other spectroscopies such as electron-energy loss spectroscopy, and soft X-ray absorption spectroscopy at the oxygen K edge.

An early fault diagnosis method of gear based on improved symplectic geometry mode decomposition

Symplectic geometry mode decomposition (SGMD) is an effective signal processing method, and it has been applied in compound fault diagnosis successfully. However, for early gear fault vibration signals, SGMD has two shortcomings. On the one hand, SGMD directly reconstructs the trajectory matrix through the original time series, which may cause the weak fault features submerged in global time series. Therefore, add a slip window to preprocess the original time series. On the other hand, the symplectic geometry components (SGCs) with low energy and fault feature information are eliminated for denoise. Therefore, variable entropy (VE) weighting is proposed to obtain the weighted symplectic geometry components (WSGCs) containing the vast majority of fault feature information. In conclusion, an improved symplectic geometry mode decomposition (ISGMD) is proposed to overcome the above two shortcomings. Simulated and experimental results indicate that ISGMD is effective for raw vibration signals.

Thickness-dependent orbital hybridization in ultrathin SrRuO3 epitaxial films

We investigated the electronic structures of ultrathin SrRuO3 (SRO) films with n = 1, 2, 3, 4, and 8 monolayers (MLs) on SrTiO3 substrates using O K-edge X-ray absorption spectroscopy (XAS). The intensities of the low-energy features reflect the strengths of the Ru 4d-O 2p orbital hybridization. The Ru 4d orbital state evolves with the increasing SRO thickness, exhibiting a crossover at approximately n = 2. For thick SRO films (n ≥ 3), this constitutes a metallic band, while for the 1 or 2 ML film, the band features shift to a higher energy to form a bandgap (&amp;gt; 0.2 eV), reflecting the emergent insulating nature. The polarization dependence of the peak intensities further shows that in the metallic films (n ≥ 3), Ru t2g - O 2p hybridizations are strong and anisotropic with stronger (weaker) equatorial (apical) hybridizations, possibly owing to compressive strain effects from the SrTiO3 substrate, while in thinner films (n ≤ 2), the hybridization effects become weak and rather isotropic because of the localization of Ru 4d orbitals. Thus, the evolution of anisotropic hybridizations in SRO films in the vicinity of the thickness-driven metal-insulator transition was substantiated.

When "Donor-Acceptor" Dyes Delocalize: A Spectroscopic and Computational Study of D-A Dyes Using "Michler's Base".

In this study, we show that the "Michler's base" motif can be combined in a donor-acceptor arrangement with a range of acceptor units (indandione, indandione with cyano substituents, barbituric acid, or rhodanine) to give photophysical properties that are dominated by delocalized excited states. By changing the acceptor unit and by altering the planarity of this system, it is possible to tune the low-energy absorption feature in terms of intensity from 23 000 to 67 000 M-1 cm-1 and energy between 500 and 700 nm. Resonance Raman spectroscopy and time-dependent density functional theory indicate that this absorption feature has two underlying transitions: a weaker charge-transfer transition around 500 nm and a strong mixed or delocalized transition between 550 and 700 nm. Generally, these compounds are not strongly emissive; however, dual emission is observed, and the relative intensity of the two states can be modulated by solvent polarity. The energy of these emissive states does not correlate with the Lippert-Mataga analysis in which the Stokes shift is related to the solvent polarity (Δf).

Photophysical Processes in Rhenium(I) Diiminetricarbonyl Arylisocyanides Featuring Three Interacting Triplet Excited States.

We present a series of four transition-metal complexes based on the rhenium(I) tricarbonyl 1,10-phenanthroline (phen) template, with a lone ancillary arylisocyanide (CNAr) ligand to yield metal-organic chromophores of the generic molecular formula [Re(phen)(CO)3(CNAr)]+ [CNAr = 2,6-diisopropylphenyl isocyanide (1), 4-phenyl-2,6-diisopropylphenyl isocyanide (2), 4-phenylethynyl-2,6-diisopropylphenyl isocyanide (3), and 4-biphenyl-2,6-diisopropylphenyl isocyanide (4)]. This particular series features varied degrees of π-conjugation length in the CNAr moiety, resulting in significant modulation in the resultant photophysical properties. All molecules possess long-lived [8-700 μs at room temperature (RT)], strongly blue-green photoluminescent and highly energetic excited states (λmax,em = 500-518 nm; Φ = 14-64%). Each of these chromophores has been photophysically investigated using static and dynamic spectroscopic techniques, the latter probed from ultrafast to supra-nanosecond time scales using transient absorption and photoluminescence (PL). Time-resolved PL intensity decays recorded as a function of the temperature were consistent with the presence of at least two emissive states lying closely spaced in energy with a third nonemissive state lying much higher in energy and likely ligand-field in character. The combined experimental evidence, along with the aid of electronic structure calculations (density functional theory and time-dependent density functional theory performed at the M06/Def2-SVP/SDD level), illustrates that the CNAr ligand is actively engaged in manipulating the excited-state decay in three of these molecules (2-4), wherein the triplet metal-to-ligand charge-transfer (3MLCT) state along with two distinct triplet ligand-centered (3LC) excited-state configurations (phen and CNAr) conspire to produce the resultant photophysical properties. Because the π conjugation within the CNAr ligand was extended, an interesting shift in the dominant photophysical processes was observed. When the CNAr conjugation length is short, as in 1, the phenanthroline 3LC state dominates, resulting in a configurationally mixed triplet excited state of both LC and MLCT character. With more extended π conjugation in the CNAr subunit (2-4), the initially generated 3LC(phen)/3MLCT excited state ultimately migrates to the CNAr 3LC state on the order of tens of picoseconds. Molecules 3 and 4 in this series also feature unique examples of inorganic excimer formation, as evidenced by dynamic self-quenching in the corresponding PL intensity decays accompanied by the observation of a short-lived low-energy emission feature.

Emergence of Broadband Plasmon Absorption in Gold nanoparticle-decorated Molybdenum Diselenide Nanosheets

Layered transition metal dichalcogenides (TMDs) represent a diverse, emerging source of two-dimensional (2D) nanostructures with intriguing optoelectronic and catalytic properties. Gold nanoparticles (AuNPs), supporting localized surface plasmon resonances, can complement TMDs, allowing their optoelectronic properties to be tuned for use in a broad range of applications. Plasmonic AuNPs offer an extraordinary ability to concentrate electromagnetic fields and confine light to sub-wavelength dimensions. While non-plasmonic enhancement of electrocatalytic HER performance in AuNP-decorated TMDs has been reported, light-induced plasmonic excitation and dissipation mechanisms and their effect on light absorption, charge transfer, and band structure in adjacent TMDs are of keen interest. Mechanisms of plasmon-active enhancement includes light scattering, local field confinement, resonant energy transfer, and interfacial charge transfer pathways. This work utilizes transient absorption spectroscopy, with pump tunability and broadband visible probing, to monitor the carrier excitation and dynamics in various AuNP-TMD heterostructures. In particular, a combination of both time-resolved and steady-state optical spectroscopies unveils the presence of a sub-gap absorption feature in AuNP-decorated molybdenum diselenide (MoSe2) nanosheets. Optical excitation of this low-energy feature generates long-lived, energetic carriers in MoSe2. This development could represent an important advance in the design and implementation of hybrid metal NP-TMD heterostructures for use in photoelectrochemistry.

Level-resolved Photoionization Cross Sections for Fe i

Abstract Significant contributions to the UV opacity in the solar atmosphere have been found to stem from bound-free transitions in neutral iron. As such, accurate cross sections for the photoionisation process are required for a detailed and meaningful analysis. However, existing photoionisation cross sections display large discrepancies across the low-energy region, highlighting the need for further calculations. In this work, we present level-resolved photoionisation cross sections for neutral iron across a wide energy range from a 262 level Dirac R-matrix calculation. Comparisons with existing experimental measurements reveal good agreement in the positions of the various low-energy resonance features. However, additional comparisons with theoretical data sets highlight wide variations. Significant resonance structures at high photon energies are explored by employing an additional series of 262 level and 896 level Dirac R-matrix calculations with a smaller six configuration target. The resulting photoionisation cross sections reproduce the main features from existing experimental observations. The results presented throughout will be useful to those requiring an extensive set of level-resolved photoionisation cross sections for astrophysical applications.