Low-energy Window Research Articles

Spin-valley locked topological phase transitions in reversible strain-tailoring honeycomb motifs

Using an effective low-energy k·p model on the frontier px,y orbitals, we establish a general phase diagram of spin-valley locked band inversion by introducing a mechanical strain field into nonmagnetic honeycomb motifs with robust spin–orbit coupling and intrinsically broken inversion symmetry. Using first-principles calculations, we realize such multiple topological phase transitions in a strained InTe monolayer within experimental reach with the Weyl semimetal as the nontrivial boundary state at two critical strains. The massless Weyl fermions endow the spin and valley Hall effects with ultrafast and dissipationless transport over a broad low-energy window. The valley selective circular dichroism can be regulated by strain-induced band inversion. A crossover between the topologically trivial and nontrivial regimes with sizable bandgaps makes InTe suitable for room-temperature (RT) topological strain-effect transistors. Our work not only demonstrates a fundamental mechanism for exploring tunable topological states and valley physics but also provides a potential platform for realizing many exotic phenomena and RT quantum devices.

Near-Infrared Materials: The Turning Point of Organic Photovoltaics.

Near-infrared (NIR)-absorbing organic semiconductors have opened up many exciting opportunities for organic photovoltaic (OPV) research. For example, new chemistries and synthetical methodologies have been developed; especially, the breakthrough Y-series acceptors, originally invented by ourgroup, specifically Y1, Y3, and Y6, have contributed immensely to boosting single-junction solar cell efficiency to around 19%; novel device architectures such as tandem and transparent organic photovoltaics have been realized. The concept of NIR donors/acceptors thus becomes a turning point in the OPV field. Here, the development of NIR-absorbing materials for OPVs is reviewed. According to the low-energy absorption window, here, NIR photovoltaic materials (p-type (polymers) and n-type (fullerene and nonfullerene)) are classified into four categories: 700-800 nm, 800-900 nm, 900-1000 nm, and greater than 1000nm. Each subsection covers the design, synthesis, and utilization of various types of donor (D) and acceptor (A) units. The structure-property relationship between various kinds of D, A units and absorption window are constructed to satisfy requirements for different applications. Subsequently, a variety of applications realized by NIR materials, including transparent OPVs, tandem OPVs, photodetectors, are presented. Finally, challenges and future development of novel NIR materials for the next-generation organic photovoltaics and beyond are discussed.

Detection Efficiency Modeling and Joint Activity and Attenuation Reconstruction in Non-TOF 3-D PET From Multiple-Energy Window Data

Emission-based attenuation correction (AC) meth-ods offer the possibility of overcoming quantification errors induced by conventional MR-based approaches in PET/MR imaging. However, the joint problem of determining AC and the activity of interest is strongly ill-posed in non-TOF PET. This can be improved by exploiting the extra information arising from low energy window photons, but the feasibility of this approach has only been studied with relatively simplistic analytic simulations so far. This manuscript aims to address some of the remaining challenges needed to handle realistic measurements; in particular, the detection efficiency (“normalisation”) estimation for each energy window is investigated. An energy-dependent detection efficiency model is proposed, accounting for the presence of unscattered events in the lower energy window due to detector scatter. Geometric calibration factors are estimated prior to the reconstruction for both scattered and unscattered events. Different reconstruction methods are also compared. Results show that geometric factors differ markedly between the energy windows and that our analytical model correspond in good approximation to Monte Carlo simulation; the multiple energy window reconstruction appears sensitive to input/model mismatch. Our method applies to Monte Carlo generated data but can be extended to measured data. This study is restricted to single scatter events.

The influence of micro/nano-structure on transmission ability of low-energy Electron Transmission Window

It is a extremely difficult to make the Electron Transmission Window (ETW) have the transmission ability for low-energy electrons, excellent mechanical strength, and thicker thickness at a large barometric pressure gradient, i.e., from vacuum to atmosphere. We present a new physical model of the ETW, which has a periodic array microstructure to restrain the temperature increase and nanostructures to provide transmission channels for electron sources. Two different periodic array micro-pattern ETWs with nanostructures have been prepared to testify the physical mechanisms, which are a thicker periodic array triangle micro-pattern unit with a nested triangle pillar type (NETW) and a thinner one without nested part type (OETW). A nanosecond-pulse power source and an array field emission electrode are adopted to focus the energy and reduce the energy dissipation of the electron source. The transmission efficiencies of the NETW and the OETW are 5.86% and 2.82% at −3.5 kV, 8.90% and 5.77% at −5.5 kV, and 12.69% and 11.97% at −8.5 kV pulsed voltages, respectively. In addition, the transmission abilities of the two ETWs have been compared by visualization diagnoses in transient and steady states, which match well with the results of transmission efficiencies. It is found that the results are consistent with those of this novel model mechanism.

Evolution of the electronic structure of twisted bilayer MoSe2

In this work we examine the evolution of the electronic structure in twisted bilayers of $\mathrm{Mo}{\mathrm{Se}}_{2}$, assuming the moir\'e potential to be a small perturbation to the untwisted limit. Its role in modifying the electronic structure is probed by mapping the calculated band structure for the moir\'e cell onto the primitive cell direction which represents the untwisted limit. At large twist angles such as $19.{03}^{\ensuremath{\circ}}$, we find that the moir\'e cell band structure is identical to the primitive cell one in the low-energy window. There are, however, significant deviations for small twist angles such as $3.{48}^{\ensuremath{\circ}}$ which have large patches of high-symmetry regions of AA and ${\mathrm{AB}}^{\ensuremath{'}}$ stackings. These lead to enhanced interlayer hopping interaction strengths in some regions, and hence stronger perturbations leading to subband formation of the highest occupied band, which has a bandwidth of 19 meV and is found to be localized both in real space as well as in momentum space.

The impact of anion elements on the engineering of the electronic and optical characteristics of the two dimensional monolayer janus MoSSe for nanoelectronic device applications

Two-dimensional (2D) materials have gained prominent attention in the nano-electronics arena, owing to their tunable electronic and optical features. Here, the physical properties of a janus MoSSe monolayer are examined upon the chemically co-doping of S/Se sites by non-metallic and halogen elements (C, Si, N, P, As, and F) employing first-principles calculations. Accordingly, an alteration of both the upper valence and the lower conduction states is revealed for janus MoSSe monolayer upon the replacement of both S and Se anion host atoms by sp-elements (C, Si, N, P, As, and F). A shift in the lowest conduction band underneath the Fermi energy level (EF) occurs in janus MoSSe monolayer when both S and Se elements are replaced by (F, F) atoms. This effectively conducted to a system with an n-type character. In contrast, the highest valence bands moved upward EF owing to the co-doping effect of C, Si, N, P, and As atoms on the janus MoSSe monolayer with p-type nature. The key features of the optical spectra, such as the optical absorption, reflectivity, and electron loss functions of the co-doped janus MoSSe monolayer are inspected. Our results imply a modification in the low-energy photon regime of the co-doped janus MoSSe monolayer at S and Se host atoms by non-metallic sp-elements comparatively to the free-standing monolayer. A reduction in the optical absorption and an increase in the reflectivity at low-energy photon window are detected when the janus MoSSe monolayer is co-doped by (C, Si), (N, P), (P,As), and (F,F) elements, respectively at S and Se chalcogen atoms. The current study infers that the co-doping S and Se sites of janus MoSSe monolayers, with sp- elements, can be beneficial in the future applications of 2D materials for the field-effect transistors and nano-electronic devices.

Crosstalk Reduction Using a Dual Energy Window Scatter Correction in Compton Imaging.

Compton cameras can simultaneously detect multi-isotopes; however, when simultaneous imaging is performed, crosstalk artifacts appear on the images obtained using a low-energy window. In conventional single-photon emission computed tomography, a dual energy window (DEW) subtraction method is used to reduce crosstalk. This study aimed to evaluate the effectiveness of employing the DEW technique to reduce crosstalk artifacts in Compton images obtained using low-energy windows. To this end, in this study, we compared reconstructed images obtained using either a photo-peak window or a scatter window by performing image subtraction based on the differences between the two images. Simulation calculations were performed to obtain the list data for the Compton camera using a 171 and a 511 keV point source. In the images reconstructed using these data, crosstalk artifacts were clearly observed in the images obtained using a 171 keV photo-peak energy window. In the images obtained using a scatter window (176–186 keV), only crosstalk artifacts were visible. The DEW method could eliminate the influence of high-energy sources on the images obtained with a photo-peak window, thereby improving quantitative capability. This was also observed when the DEW method was used on experimentally obtained images.

Using the full power of the cosmic microwave background to probe axion dark matter

The cosmic microwave background (CMB) places strong constraints on models of dark matter (DM) that deviate from standard cold DM (CDM), and on initial conditions beyond the scalar adiabatic mode. Here, the full \textit{Planck} data set (including temperature, $E$-mode polarisation, and lensing deflection) is used to test the possibility that some fraction of the DM is composed of ultralight axions (ULAs). This represents the first use of CMB lensing to test the ULA model. We find no evidence for a ULA component in the mass range $10^{-33}\leq m_a\leq 10^{-24}\text{ eV}$. We put percent-level constraints on the ULA contribution to the DM, improving by up to a factor of two compared to the case with temperature anisotropies alone. Axion DM also provides a low-energy window onto the high-energy physics of inflation through the interplay between the vacuum misalignment production of axions and isocurvature perturbations. We perform the first systematic investigation into the parameter space of ULA isocurvature, using an accurate isocurvature transfer function at all $m_{a}$ values. We precisely identify a "window of co-existence" for $10^{-25}\text{ eV}\leq m_a\leq10^{-24}\text{ eV}$ where the data allow, simultaneously, a $\sim10\%$ contribution of ULAs to the DM, and $\sim 1\%$ contributions of isocurvature and tensors to the CMB power. ULAs in this window (and \textit{all} lighter ULAs) are shown to be consistent with a large inflationary Hubble parameter, $H_I\sim 10^{14}\text{ GeV}$. The window of co-existence will be fully probed by proposed CMB-S4 observations with increased accuracy in the high-$\ell$ lensing power and low-$\ell$ $E$ and $B$-mode polarisation. If ULAs in the window exist, this could allow for two independent measurements of $H_I$ in the CMB using the axion DM content and isocurvature, and the tensor contribution to $B$-modes.

Two-dimensional electronic spectroscopy as a tool for tracking molecular conformations in DNA/RNA aggregates.

A computational strategy to simulate two-dimensional electronic spectra (2DES) is introduced, which allows us to analyse ground state dynamics and to sample and measure different conformations attained by flexible molecular systems in solution. An explicit mixed quantum mechanics/molecular mechanics (QM/MM) approach is employed for the evaluation of the necessary electronic excited state energies and transition dipole moments. The method is applied towards a study of the highly flexible water-solvated adenine-adenine monophosphate (ApA), a system featuring two interacting adenine moieties that display various intermolecular arrangements, known to deeply affect their photochemical outcome. Molecular dynamics simulations and cluster analysis have been used to select the molecular conformations, reducing the complexity of the flexible ApA conformational space. By using our sum-over-states (SOS) approach to obtain the 2DES spectra for each of these selected conformations, we can discern spectral changes and relate them to specific nuclear arrangements: close lying π-stacked bases exhibit a splitting of their respective 1La signal traces; T-stacked bases exhibit the appearance of charge transfer states in the low-energy Vis probing window while displaying no 1La splitting, being particularly favoured when promoting amino to 5-ring interactions; unstacked and distant adenine moieties exhibit signals similar to those of the adenine monomer, as is expected for non-interacting nucleobases. 2DES maps reveal the spectral fingerprints associated with specific molecular conformations, and are thus a promising option to enable their quantitative spectroscopic detection beyond standard 1D pump-probe techniques. This is expected to aid the understanding of how nucleobase aggregation controls and modulates the photostability and photo-damage of extended DNA/RNA systems.

Neutrino physics with DARWIN

DARWIN (DARk matter WImp search with liquid xenoN) will be a multi-ton dark matter detector with the primary goal of exploring the entire experimentally accessible parameter space for weakly interacting massive particles (WIMPs) over a wide mass-range. With its 40 tonne active liquid xenon target, low-energy threshold and ultra-low background level, DARWIN can also search for other rare interactions. Here we present its sensitivity to low-energy solar neutrinos and to neutrinoless double beta decay. In a low-energy window of 2-30 keV a rate of 105/year, from pp and 7Be neutrinos can be reached. Such a measurement, with 1% precision will allow testing neutrinos models. DARWIN could also reach a competitive half-life sensitivity of 8.5 · 1027 y to the neutrinoless double beta decay (0νββ) of 136Xe after an exposure of 140 t×y of natural xenon. Nuclear recoils from coherent scattering of solar neutrinos will limit the sensitivity to WIMP masses below 5 GeV/c2, and the event rate from 8B neutrinos would range from a few to a few tens of events per tonne and year, depending on the energy threshold of the detector. Deviations from the predicted but yet unmeasured neutrino flux would be an indication for physics beyond the Standard Model

Inelastic electron tunneling spectroscopy in molecular junctions showing quantum interference

Destructive quantum interference effect is implemented in large area molecular junctions to improve signatures of electron-phonon interaction. Vertical molecular junctions based on a cross-conjugated anthraquinone layer were fabricated and low-noise transport measurements were performed by acquiring simultaneously the current-voltage characteristics, its second derivative, and the differential conductance. Signatures of vibrational modes excited by inelastic events are present in the whole measured voltage range and superpose to the conductance suppression induced by destructive quantum interference. As a consequence vibrational modes have improved visibility in the low energy window $(<80\phantom{\rule{0.28em}{0ex}}\mathrm{meV})$. Inelastic electron transport spectroscopy data are compared to infrared attenuated total reflection spectroscopy on Au/anthraquinone thin films. Common vibrational modes can be clearly identified, but inelastic electron tunneling spectroscopy reveals the existence of vibrational modes in a wider energy range $(0--400\phantom{\rule{0.28em}{0ex}}\mathrm{meV})$ where infrared spectroscopy is lacking.

Aharonov-Bohm effect in monolayer phosphorene nanorings

This work presents theoretical demonstration of Aharonov-Bohm (AB) effect in monolayer phosphorene nanorings (PNR). Atomistic quantum transport simulations of PNR are employed to investigate the impact of multiple modulation sources on the sample conductance. In presence of a perpendicular magnetic field, we find that the conductance of both armchair and zigzag PNR oscillate periodically in a low-energy window as a manifestation of the AB effect. Our numerical results have revealed a giant magnetoresistance (MR) in zigzag PNR (with a maximum magnitude approaching two thousand percent). It is attributed to the AB effect induced destructive interference phase in a wide energy range below the bottom of the second subband. We also demonstrate that PNR conductance is highly anisotropic, offering an additional way to modulate MR. The giant MR in PNR is maintained at room temperature in the presence of thermal broadening effect.

Electronic Excited States in Amorphous MEH-PPV Polymers from Large-Scale First Principles Calculations

The electronic excited states of amorphous polymeric semiconductor MEH-PPV are investigated by first principles quantum chemical calculations based on trajectories from classical molecular dynamics simulations. We inferred an average conjugation length of ∼5-7 monomers for lowest vertical excitations of amorphous MEH-PPV at room temperature and verified that the normal definition of a chromophore in a polymer based on purely geometric "conjugation breaks" is not always valid in amorphous polymers and a rigorous definition can be only on the basis of the evaluation of the polymer excited state wave function. The charge transfer character is observed to be nearly invariant for all excited states in low energy window while the exciton delocalization extent is found to increase with energy. The interchain excitonic couplings for amorphous MEH-PPV are shown to be usually smaller than 10 meV suggesting that the transport mechanism across chain can be described by incoherent hopping. All these observations about the energetic and spatial distribution of the excitons in polymer as well as their couplings provide important qualitative insights and useful quantitative information for constructing a realistic model for exciton migration dynamics in amorphous polymer materials.

Colella-Overhauser-Werner test of the weak equivalence principle: A low-energy window to look into the noncommutative structure of space-time?

We construct the quantum mechanical model of the COW experiment assuming that the underlying space time has a granular structure, described by a canonical noncommutative algebra of coordinates $x^{\mu}$. The time-space sector of the algebra is shown to add a mass-dependent contribution to the gravitational acceleration felt by neutron deBrogli waves measured in a COW experiment. This makes time-space noncommutativity a potential candidate for an apparent violation of WEP even if the ratio of the inertial mass $m_{i}$ and gravitational mass $m_{g}$ is a universal constant. The latest experimental result based on COW principle is shown to place an upper-bound several orders of magnitude stronger than the existing one on the time-space noncommutative parameter. We argue that the evidence of NC structure of space-time may be found if the COW-type experiment can be repeated with several particle species.

Neutrino physics with multi-ton scale liquid xenon detectors

We study the sensitivity of large-scale xenon detectors to low-energy solar neutrinos, to coherent neutrino-nucleus scattering and to neutrinoless double beta decay.As a concrete example, we consider the xenon part of the proposed DARWIN (Dark Matter WIMP Search with Noble Liquids) experiment.We perform detailed Monte Carlo simulations of the expected backgrounds, considering realistic energy resolutions and thresholds in the detector.In a low-energy window of 2–30 keV, where the sensitivity to solar pp and 7Be-neutrinos is highest, an integrated pp-neutrino rate of 5900 events can be reached in a fiducial mass of 14 tons of natural xenon, after 5 years of data. The pp-neutrino flux could thus be measured with a statistical uncertainty around 1%, reaching the precision of solar model predictions. These low-energy solar neutrinos will be the limiting background to the dark matter search channel for WIMP-nucleon cross sections below ∼ 2 × 10−48 cm2 and WIMP masses around 50 GeV⋅c−2, for an assumed 99.5% rejection of electronic recoils due to elastic neutrino-electron scatters. Nuclear recoils from coherent scattering of solar neutrinos will limit the sensitivity to WIMP masses below ∼ 6 GeV⋅c−2 to cross sections above ∼ 4 × 10−45cm2.DARWIN could reach a competitive half-life sensitivity of 5.6 × 1026 y to the neutrinoless double beta decay of 136Xe after 5 years of data, using 6 tons of natural xenon in the central detector region.

Probing Molecular Solids with Low-Energy Ions

Ion/surface collisions in the ultralow- to low-energy (1-100-eV) window represent an excellent technique for investigation of the properties of condensed molecular solids at low temperatures. For example, this technique has revealed the unique physical and chemical processes that occur on the surface of ice, versus the liquid and vapor phases of water. Such instrument-dependent research, which is usually performed with spectroscopy and mass spectrometry, has led to new directions in studies of molecular materials. In this review, we discuss some interesting results and highlight recent developments in the area. We hope that access to the study of molecular solids with extreme surface specificity, as described here, will encourage investigators to explore new areas of research, some of which are outlined in this review.

Performance evaluation and optimization of the MiniPET-II scanner

This paper presents results of the performance of a small animal PET system (MiniPET-II) installed at our Institute. MiniPET-II is a full ring camera that includes 12 detector modules in a single ring comprised of 1.27×1.27×12mm3 LYSO scintillator crystals. The axial field of view and the inner ring diameter are 48mm and 211mm, respectively. The goal of this study was to determine the NEMA-NU4 performance parameters of the scanner. In addition, we also investigated how the calculated parameters depend on the coincidence time window (τ=2, 3 and 4ns) and the low threshold settings of the energy window (Elt=250, 350 and 450keV). Independent measurements supported optimization of the effective system radius and the coincidence time window of the system. We found that the optimal coincidence time window and low threshold energy window are 3ns and 350keV, respectively. The spatial resolution was close to 1.2mm in the center of the FOV with an increase of 17% at the radial edge. The maximum value of the absolute sensitivity was 1.37% for a point source. Count rate tests resulted in peak values for the noise equivalent count rate (NEC) curve and scatter fraction of 14.2kcps (at 36MBq) and 27.7%, respectively, using the rat phantom. Numerical values of the same parameters obtained for the mouse phantom were 55.1kcps (at 38.8MBq) and 12.3%, respectively. The recovery coefficients of the image quality phantom ranged from 0.1 to 0.87. Altering the τ and Elt resulted in substantial changes in the NEC peak and the sensitivity while the effect on the image quality was negligible. The spatial resolution proved to be, as expected, independent of the τ and Elt. The calculated optimal effective system radius (resulting in the best image quality) was 109mm. Although the NEC peak parameters do not compare favorably with those of other small animal scanners, it can be concluded that under normal counting situations the MiniPET-II imaging capability assures remarkably good image quality, sensitivity and spatial resolution.

Molecular Optical Imaging with Radioactive Probes

BackgroundOptical imaging (OI) techniques such as bioluminescence and fluorescence imaging have been widely used to track diseases in a non-invasive manner within living subjects. These techniques generally require bioluminescent and fluorescent probes. Here we demonstrate the feasibility of using radioactive probes for in vivo molecular OI.Methodology/Principal FindingsBy taking the advantages of low energy window of light (1.2–3.1 eV, 400–1000 nm) resulting from radiation, radionuclides that emit charged particles such as β+ and β− can be successfully imaged with an OI instrument. In vivo optical images can be obtained for several radioactive probes including 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG), Na18F, Na131I, 90YCl3 and a 90Y labeled peptide that specifically target tumors.Conclusions/SignificanceThese studies demonstrate generalizability of radioactive OI technique. It provides a new molecular imaging strategy and will likely have significant impact on both small animal and clinical imaging.

Phenomenology of1032dark sectors

We postulate an exact permutation symmetry acting on 10(32) standard model copies as the largest possible symmetry extension of the standard model. This setup automatically lowers the fundamental gravity cutoff down to TeV, and thus, accounts for the quantum stability of the weak scale. We study the phenomenology of this framework and show that below TeV energies the copies are well hidden, obeying all the existing observational bounds. Nevertheless, we identify a potential low energy window into the hidden world, the oscillation of the neutron into its dark copies. At the same time, proton decay can be suppressed by gauging the diagonal baryon number of the different copies. This framework offers an alternative approach to several particle physics questions. For example, we suggest a novel mechanism for generating naturally small neutrino masses that are suppressed by the number of neutrino species. The mirror copies of the standard model naturally house dark matter candidates. The general experimentally observable prediction of this scenario is an emergence of strong gravitational effects at the LHC. The low energy permutation symmetry powerfully constrains the form of this new gravitational physics and allows to make observational predictions, such as, production of micro black holes with very peculiar properties.

Monte-Carlo simulation for scatter correction compensation studies in SPECT imaging using GATE software package

Simulations using Monte-Carlo packages are increasingly used in Nuclear Imaging, both for PET and SPECT applications. Either for modeling imaging systems or developing algorithms for analysis and improvement of image quantification. The aim of this work is the study of several scatter phenomena and the evaluation and improvement of two scatter correction compensation methods in pixelized scintillators. Firstly, the Dual Energy Window Subtraction Technique (DEWST) and then the Multi Energy Window and Filtering Technique (MEWFT) are investigated. Minimization Method has been used in order to achieve the best weighting co-efficient and the appropriate position and width of each low energy window. The position of them was found to be around 80–120 keV. Following scatter rejection, both quality and quantitative accuracy of the images improved.