Accessible Energy Range Research Articles

Table-Top Ultrafast Soft X-Ray Spectroscopy Toward Synchrotron-like in-Situ Photoelectrochemical Measurements

Advancements in in-situ X-ray capabilities have enabled the measurement of electrochemical systems in more realistic operating conditions. While these measurements have progressed our understanding of electrochemical reactions from carrier dynamics to reaction mechanisms to degradation processes, they have mostly been limited to synchrotron facilities. However, performing in-situ table-top measurements has remained a challenge in the field. Table-top XUV spectrometers based on high-harmonic generation (HHG) are powerful tools for measuring ultrafast electronic and structural properties of photoelectrocatalysts ex-situ. However, extending the accessible energy range of HHG-based spectrometers beyond 300 eV is difficult due to inherent physical scaling laws. Laser-produced plasma sources can extend into the soft X-ray range to cover the water window. This capability could enable in-situ studies of photocatalysts, but their inherent source fluctuations have prevented ultrafast studies in the past. In this work, we employ methods pioneered by HHG-based setups to create a novel laser-produced plasma technique for table-top time-resolved measurements in the soft X-ray regime (200 – 1000 eV). We combine EMCCD detection, background-free self-referencing, edge-referencing analysis, amongst other advances, to reduce shot-to-shot noise so that sub-mOD signals are detectable. This presentation will also describe the technical advances pioneered at synchrotron sources that have enabled in-situ X-ray measurements of electrochemical systems. These advancements have inspired the effort to achieve the first table-top soft X-ray measurements in device-relevant liquid environments, enabling the correlation of photocatalyst properties, such as electron and hole energies and polarons, with photoelectrochemical product formation.

Ultraviolet interlayer excitons in bilayer WSe2.

Interlayer excitons in van der Waals heterostructures are fascinating for applications like exciton condensation, excitonic devices and moiré-induced quantum emitters. The study of these charge-transfer states has almost exclusively focused on band edges, limiting the spectral region to the near-infrared regime. Here we explore the above-gap analogues of interlayer excitons in bilayer WSe2 and identify both neutral and charged species emitting in the ultraviolet. Even though the transitions occur far above the band edge, the states remain metastable, exhibiting linewidths as narrow as 1.8 meV. These interlayer high-lying excitations have switchable dipole orientations and hence show prominent Stark splitting. The positive and negative interlayer high-lying trions exhibit significant binding energies of 20-30 meV, allowing for a broad tunability of transitions via electric fields and electrostatic doping. The Stark splitting of these trions serves as a highly accurate, built-in sensor for measuring interlayer electric field strengths, which are exceedingly difficult to quantify otherwise. Such excitonic complexes are further sensitive to the interlayer twist angle and offer opportunities to explore emergent moiré physics under electrical control. Our findings more than double the accessible energy range for applications based on interlayer excitons.

Nagaoka ferromagnetism in an array of phosphorene quantum dots.

We consider an array of four quantum dots defined in phosphorene containing three excess electrons, i.e., in the conditions of near half filling when itinerant Nagaoka ferromagnetism is expected to appear in a square array with isotropic interdot hopping. The interdot hopping in the array arranged in a square inherits the anisotropy from the form of the phosphorene conduction band. We apply the configuration interaction method for discussion of the appearance and stability of the spin-polarized ground state and discuss the compensation of the effective mass anisotropy by the geometry of the quantum dot array. Our study shows strong stability of Nagaoka ferromagnetism for optimized geometry of the array, with the Nagaoka gap as large as ∼230µeV. A phase diagram for the ground-state spin ordering versus the geometric parameters of the array is presented. We study the suppression of the ferromagnetism in a transition of the [Formula: see text] array to a quasi-1D chain and indicate that the shift of one of the quantum dots away from the array center is enough to transform the system to a quantum dot chain. A shift in the zigzag crystal direction induces the low-spin ground state more effectively than a shift along the armchair direction. We also discuss the robustness of the spin ordering against detuning one of the dots. The ferromagnetic ground-state survives as long as the detuning is not large enough to trap one of the electrons within a single quantum dot (for positive detuning) or remove one of the quantum dots of the accessible energy range (for negative detuning).

Tailoring the Weight of Surface and Intralayer Edge States to Control LUMO Energies.

The energies of the frontier molecular orbitals determine the optoelectronic properties in organic films, which are crucial for their application, and strongly depend on the morphology and supramolecular structure. The impact of the latter two properties on the electronic energy levels relies primarily on nearest-neighbor interactions, which are difficult to study due to their nanoscale nature and heterogeneity. Here,an automated method is presented for fabricating thin films with a tailored ratio of surface to bulk sites and a controlled extension of domain edges, both of which are used to control nearest-neighbor interactions. This method uses a Langmuir-Schaefer-type rolling transfer of Langmuir layers (rtLL) to minimize flow during the deposition of rigid Langmuir layers composed of π-conjugated molecules. Using UV-vis absorption spectroscopy, atomic force microscopy, and transmission electron microscopy, it is shown that the rtLL method advances the deposition of multi-Langmuir layers and enables the production of films with defined morphology. The variation in nearest-neighbor interactions is thus achieved and the resulting systematically tuned lowest unoccupied molecular orbital (LUMO) energies (determined via square-wave voltammetry) enable the establishment of a model that functionally relates the LUMO energies to a morphological descriptor, allowing for the prediction of the range of accessible LUMO energies.

Spin- and angle-resolved photoemission spectroscopy study of heptahelicene layers on Cu(111) surfaces.

It has been demonstrated previously that electrons interact differently with chiral molecules depending on their polarization. For enantiomeric pure monolayers of heptahelicene, opposite asymmetries in spin polarization were reported and attributed to the so-called chirality-induced spin selectivity effect. However, these promising proof-of-concept photoemission experiments lack the angular and energy resolution that could provide the necessary insights into the mechanism of this phenomenon. In order to fill in the missing gaps, we provide a detailed spin- and angle-resolved photoemission spectroscopy study of heptahelicene layers on a Cu(111) substrate. Throughout the large accessible energy and angle range, no chirality induced spin asymmetry in photoemission could be observed. Possible reasons for the absence of signatures of the spin-dependent electron transmission through the chiral molecular layer are briefly discussed.

Decay of the pseudoscalar glueball into vector, axial-vector, scalar and pseudoscalar mesons

We resume the investigation of the ground-state pseudoscalar glueball, J^{PC}=0^{-+}, by computing its two- and three-body decays into vector and axial-vector quark-antiquark meson fields additional to scalar and pseudoscalar mesons through the construction of an interaction chiral Lagrangian that produces these decays. We evaluate the branching ratio, via a parameter-free calculation, by setting the mass of the pseudoscalar glueball to 2.6 GeV as predicted by lattice QCD simulations. We duplicate the computation for the branching ratios for a pseudoscalar glueball mass 2.37 GeV which matches to the measured mass of the resonance X(2370) in the BESIII experiment. The present channels and states are potentially reached and are interesting for the running BESIII and Belle-II experiments and the planned PANDA experiment at FAIR/GSI which will be able to detect the pseudoscalar glueball within the accessible energy range.

Doubly-polarized WZ hadronic cross sections at NLO QCD + EW accuracy

We present new results for next-to-leading order (NLO) electroweak (EW) corrections to double polarization signals in the WZ production channel at the LHC using the e^+nu _e mu ^+ mu ^- final state. It is found that the EW corrections are most sizable in the transverse momentum distributions of the doubly longitudinal polarization, being around -,10% compared to the NLO QCD prediction at p_{T,e}approx 200 GeV, which is in the accessible energy range of the current LHC data.

Speed limits for two-qubit gates with weakly anharmonic qubits

We consider the implementation of two-qubit gates when the physical systems used to realize the qubits possess additional quantum states in the accessible energy range. We use optimal control theory to determine the maximum achievable gate speed for two-qubit gates in the qubit subspace of the many-level Hilbert space, and we analyze the effect of the additional quantum states on the gate speed. We identify two competing mechanisms. On one hand, higher energy levels are generally more strongly coupled to each other. Under suitable conditions, this stronger coupling can be utilized to make two-qubit gates significantly faster than the reference value based on simple qubits. On the other hand, a weak anharmonicity constrains the speed at which the system can be adequately controlled: according to the intuitive picture, faster operations require stronger control fields, which are more likely to excite higher levels in a weakly anharmonic system, which in turn leads to faster decoherence and uncontrolled leakage outside the qubit space. In order to account for this constraint, we modify the pulse optimization algorithm to avoid pulses that lead to appreciable population of the higher levels. In this case we find that the presence of the higher levels can lead to a significant reduction in the maximum achievable gate speed. We also compare the optimal-control gate speeds with those obtained using the cross-resonance/selective-darkening gate protocol. We find that the latter, with some parameter optimization, can be used to achieve a relatively fast implementation of the CNOT gate. These results can help the search for optimized gate implementations in realistic quantum computing architectures, such as those based on superconducting circuits. They also provide guidelines for desirable conditions on anharmonicity that allow optimal utilization of the higher levels to achieve fast quantum gates.

Enhanced Versatility of Table-Top X-Rays from Van der Waals Structures.

Van der Waals (vdW) materials have attracted much interest for their myriad unique electronic, mechanical, and thermal properties. In particular, they are promising candidates for monochromatic, table‐top X‐ray sources. This work reveals that the versatility of the table‐top vdW X‐ray source goes beyond what has been demonstrated so far. By introducing a tilt angle between the vdW structure and the incident electron beam, it is theoretically and experimentally shown that the accessible photon energy range is more than doubled. This allows for greater versatility in real‐time tuning of the vdW X‐ray source. Furthermore, this work shows that the accessible photon energy range is maximized by simultaneously controlling both the electron energy and the vdW structure tilt. These results will pave the way for highly tunable, compact X‐ray sources, with potential applications including hyperspectral X‐ray fluoroscopy and X‐ray quantum optics.

LST-1, the Large-Sized Telescope prototype of CTA. Status and first observations

CTA (Cherenkov Telescope Array) is the next generation ground-based observatory for gamma-ray astronomy at very-high energies. Once completed, CTA will outperform present-day facilities by an order of magnitude in sensitivity, and significantly enlarge the accessible energy range and survey capabilities. Deployed in the CTA north site, on the island of La Palma (Spain), LST-1 is the prototype for the CTA Large-Sized Telescopes, which will cover the lower end of the energy range of the array, down to 20 GeV. LST-1 started astronomical observations in late 2019, and is currently completing its commissioning phase. We present the status of the instrument and an overview of the first physics results.

Multiscale Conformational Sampling Reveals Excited-State Locality in DNA Self-Repair Mechanism.

Ultraviolet (UV) irradiation is known to be responsible for DNA damage. However, experimental studies in DNA oligonucleotides have shown that UV light can also induce sequence-specific self-repair. Following charge transfer from a guanine adenine sequence adjacent to a cyclobutane pyrimidine dimer (CPD), the covalent bond between the two thymines could be cleaved, recovering the intact base sequence. Mechanistic details promoting the self-repair remained unclear, however. In our theoretical study, we investigated whether optical excitation could directly lead to a charge-transfer state, thereby initiating the repair, or whether the initial excited state remains localized on a single nucleobase. We performed conformational sampling of 200 geometries of the damaged DNA double strand solvated in water and used a hybrid quantum and molecular mechanics approach to compute excited states at the complete active space perturbation level of theory. Analysis of the conformational data set clearly revealed that the excited-state properties are uniformly distributed across the fluctuations of the nucleotide in its natural environment. From the electronic wavefunction, we learned that the electronic transitions remained predominantly local on either adenine or guanine, and no direct charge transfer occurred in the experimentally accessed energy range. The investigated base sequence is not only specific to the CPD repair mechanism but ubiquitously occurs in nucleic acids. Our results therefore give a very general insight into the charge locality of UV-excited DNA, a property that is regarded to have determining relevance in the structural consequences following absorption of UV photons.

Connection between νn→ν¯n¯ reactions and n−n¯ oscillations via additional Higgs triplet bosons

In this work, we investigate the connection and compatibility between $\ensuremath{\nu}n\ensuremath{\rightarrow}\overline{\ensuremath{\nu}}\overline{n}$ reactions and $n\text{\ensuremath{-}}\overline{n}$ oscillations based on the $SU(3{)}_{c}\ifmmode\times\else\texttimes\fi{}SU(2{)}_{L}\ifmmode\times\else\texttimes\fi{}U(1)$ symmetry model with additional Higgs triplets. We explore the possibility that the scattering process $\ensuremath{\nu}n\ensuremath{\rightarrow}\overline{\ensuremath{\nu}}\overline{n}$ produced by low-energy solar neutrinos gives rise to an unavoidable background in the measurements of $n\text{\ensuremath{-}}\overline{n}$ oscillations. We focus on two different scenarios, depending on whether the ($B\ensuremath{-}L$) symmetry could be broken. We analyze the interplay of the various constraints on the two processes and their observable consequences. In the scenario where both ($B+L$) and ($B\ensuremath{-}L$) could be broken, we point out that if all the requirements, mainly arising from the type-II seesaw mechanism, are satisfied, the parameter space would be severely constrained. In this case, although the masses of the Higgs triplet bosons could be within the reach of a direct detection at the LHC or future high-energy experiments, the predicted $n\text{\ensuremath{-}}\overline{n}$ oscillation times would be completely beyond the detectable regions of the present experiments. In both scenarios, the present experiments are unable to distinguish a $\ensuremath{\nu}n\ensuremath{\rightarrow}\overline{\ensuremath{\nu}}\overline{n}$ reaction event from an $n\text{\ensuremath{-}}\overline{n}$ oscillation event within the accessible energy range. Nevertheless, if any of the two processes is detected, there could be signal associated with new physics beyond the Standard Model.

New Insights about CuO Nanoparticles from Inelastic Neutron Scattering.

Inelastic Neutron Scattering (INS) spectroscopy has provided a unique insight into the magnetodymanics of nanoscale copper (II) oxide (CuO). We present evidence for the propagation of magnons in the directions of the ordering vectors of both the commensurate and helically modulated incommensurate antiferromagnetic phases of CuO. The temperature dependency of the magnon spin-wave intensity (in the accessible energy-range of the experiment) conforms to the Bose population of states at low temperatures (T ≤ 100 K), as expected for bosons, then intensity significantly increases, with maximum at about 225 K (close to TN), and decreases at higher temperatures. The obtained results can be related to gradual softening of the dispersion curves of magnon spin-waves and decreasing the spin gap with temperature approaching TN on heating, and slow dissipation of the short-range dynamic spin correlations at higher temperatures. However, the intensity of the magnon signal was found to be particle size dependent, and increases with decreasing particle size. This “reverse size effect” is believed to be related to either creation of single-domain particles at the nanoscale, or “superferromagnetism effect” and the formation of collective particle states.

Anomalous quartic gauge couplings and unitarization for the vector boson scattering process pprightarrow W^+W^+jjXrightarrow ell ^+nu _ell ell ^+nu _ell jjX

Weak vector boson scattering (VBS) at the LHC provides an excellent source of information on the structure of quartic gauge couplings and possible effects of physics beyond the SM in electroweak symmetry breaking. Parameterizing deviations from the SM within an effective field theory at tree level, the dimension-8 operators, which are needed for sufficiently general modeling, lead to unphysical enhancements of cross sections within the accessible energy range of the LHC. Preservation of unitarity limits is needed for phenomenological studies of the VVjj events which signify VBS. Here we develop a numerical unitarization scheme for the full off-shell VBS processes and apply it to same-sign W scattering, i.e. processes like qqrightarrow qqW^+W^+. The scheme is implemented within the Monte Carlo program VBFNLO, including leptonic decay of the weak bosons and NLO QCD corrections. Distributions differentiating between higher dimensional operators are discussed.

Recent enhancements and performance gains from upgrades to ANSTO’s thermal neutron instrument TAIPAN and the triple-axis and Be-filter spectrometers

TAIPAN is the thermal-neutron instrument located at the reactor face of Australia’s OPAL reactor (ANSTO). TAIPAN hosts two interchangeable secondary spectrometers; a classic triple-axis spectrometer (TAS) and a beryllium-filter spectrometer, the latter being a high-throughput, low-energy, band-pass filter spectrometer based on the NIST design (Udovic et al., 2008). The TAS option has been operating in the user programme since 2010 whilst the Be-filter is a newer addition, which began operating at the end of 2015. TAIPAN is renowned for its versatility and high neutron flux which has allowed the TAS to measure successfully a broad range of samples including single crystals, powders, thin films, and co-aligned multi-crystal arrays (Danilkin and Yethiraj, 2009; Danilkin et al., 2012, 2007). While the TAS option is used mostly to study structural and magnetic excitations in materials, the Be-filter option is used to measure vibrational density of states from mainly powder samples (Stampfl et al., 2016).TAIPAN has recently undergone some significant upgrades to improve the accessible momentum and energy range of both the TAS and the Be-filter spectrometers. Four key features have been modified to improve performance: the accessible momentum transfer has been increased by re-designing the enclosure and expanding the available instrument range of motion (section A); a new sapphire-filter translation-stage mechanism (Section B) has been installed to allow epithermal neutrons to pass to the two monochromators; a new Cu-200 double-focussing monochromator has been installed to allow monochromatic selection of neutrons up to 180 meV (Section C); and finally a new tertiary shutter and snout (Section D) have been designed to improve the signal-to-noise ratio and reduced background outside the instrument enclosure. Extensive testing and alignment of all new motion stages were undertaken with reproducibility within ±0.05 degrees or ±0.25 mm obtained for both the monochromator rotation angle & sapphire-filter translation.

LINUS, the Integrated LNL Neutron Source facility

LINUS is a project at the INFN Legnaro National Laboratories (LNL, Italy) to create a suite of different neutron sources (LSNS, NEPIR, SLOWNE) driven by existing accelerators. LSNS, driven by a 40 mA, 5 MeV proton RFQ, will use Li and Be targets to deliver cold, thermal, epithermal and fast neutrons. The SPES high current (0.75 mA), 70 MeV proton cyclotron will drive the NEPIR and SLOWNE facilities. NEPIR will alternatively deliver quasi mono-energetic neutrons with energy peak down to 20 MeV, and a neutron beam with a continuous energy distribution similar to that of neutrons present in the Earth atmosphere in the accessible energy range. SLOWNE is an intense neutron source for applications outside the LSNS range.

Commissioning of a powerful electron gun for electron–ion crossed-beams experiments

The sensitivity of an electron–ion crossed-beams experiment is mainly determined by the densities of both beams in the interaction region. Aiming at the extension of the available range of accessible electron energies and densities, a new high-power electron gun has been developed and fabricated. It delivers a ribbon-shaped beam with currents of up to 1A at variable energies reaching 3500eV at the maximum. The design goals of the gun are being met by using a configuration of electrodes, which allows for a variety of operation modes. Here, we report on the results of the ongoing tests of the new electron gun. In particular, electron-impact ionization cross sections of He+ and Xe5+ ions were measured employing the animated crossed-beams technique. The results are in excellent agreement with literature data.

Two crystal x-ray spectrometers for OMEGA experiments.

Two x-ray spectrometers have been built for x-ray spectroscopy of laser-produced plasmas on OMEGA at the Laboratory for Laser Energetics (LLE) by Commissariat à l'Energie Atomique et aux énergies alternatives (CEA). The accessible photon energy range is from 1.5 to 20 keV. The first spectrometer, called X-ray CEA Crystal Spectrometer with a Charge-Injection Device (XCCS-CID), records three spectra with three crystals coupled to a time integrated CID camera. The second one, called X-ray CEA Crystal Spectrometer (XCCS) with a framing camera, is time resolved and records four spectra with two crystals on the four frames of a framing camera. Cylindrical crystals are used in Johan geometry. Each spectrometer is positioned with a ten-inch manipulator inside the OMEGA target chamber. In each experiment, after choosing a spectral window, a specific configuration is designed and concave crystals are precisely positioned on a board with angled wedges and spacers. Slits on snouts enable 1D spatial resolution to distinguish spectra emitted from different parts of the target.

Above-Threshold Ionization of Quasiperiodic Structures by Low-Frequency Laser Fields.

We investigate the theoretical problem of the photoelectron cutoff change in periodical structures induced by an infrared laser field. We use a one-dimensional Kronig-Penney potential including a finite number of wells, and the analysis is fulfilled by resolving the time-dependent Schrödinger equation. The electron spectra, calculated for an increasing number of wells, clearly show that a plateau quickly appears as the periodic nature of the potential builds up, even at a moderate intensity (10 TW/cm(2)). Varying the intensity from 10 to 30 TW/cm(2) we observe a net increase of both the yield and accessible energy range of the ionization spectrum. In order to gain insight into the dynamics of the system at these intensities, we use an analytical approach, based on exact solutions of the full Hamiltonian in a periodic potential. We show that the population transfers efficiently from lower to upper bands when the Bloch and laser frequencies become comparable. The model leads to a quantitative prediction of the intensity range where ionization enters the nonperturbative regime. Moreover, it reveals the physics underlying the increase of the photoelectron energy cutoff at moderate intensities, as observed experimentally.

New mechanism for Type-II seesaw dominance in SO(10) with low-mass $$Z^{\prime }$$ Z ′ , RH neutrinos, and verifiable LFV, LNV and proton decay

Dominance of type-II seesaw mechanism for neutrino masses has attracted considerable attention because of a number of advantages. We show a novel approach to achieve Type-II seesaw dominance in non-supersymmetric $SO(10)$ grand unification where a low mass $Z^{\prime}$ boson and specific patterns of right-handed neutrino masses are predicted within the accessible energy range of the Large Hadron Collider. In spite of the high value of the seesaw scale, $M_{\Delta_L} \simeq 10^8-10^9$ GeV, the model predicts new dominant contributions to neutrino-less double beta decay in the $W_L-W_L$ channel close to the current experimental limits via exchanges of heavier singlet fermions used as essential ingredients of this model even when the light active neutrino masses are normally hierarchical or invertedly hierarchical. We obtain upper bounds on the lightest sterile neutrino mass $m_s\lesssim 3.0$ GeV, $2.0$ GeV, and $0.7$ GeV for normally hierarchical, invertedly hierarchical, and quasi-degenerate patterns of light neutrino masses, respectively. The underlying non-unitarity effects lead to lepton flavor violating decay branching ratios within the reach of ongoing or planned experiments and the leptonic CP-violation parameter nearly two orders larger than the quark sector. Some of the predicted values on proton lifetime for $p\to e^+\pi^0$ are found to be within the currently accessible search limits. Other aspects of model applications including leptogenesis etc. are briefly indicated.