Showing papers on "Electron published in 2018"

Improved limit on the electric dipole moment of the electron

Abstract: The standard model of particle physics accurately describes all particle physics measurements made so far in the laboratory. However, it is unable to answer many questions that arise from cosmological observations, such as the nature of dark matter and why matter dominates over antimatter throughout the Universe. Theories that contain particles and interactions beyond the standard model, such as models that incorporate supersymmetry, may explain these phenomena. Such particles appear in the vacuum and interact with common particles to modify their properties. For example, the existence of very massive particles whose interactions violate time-reversal symmetry, which could explain the cosmological matter–antimatter asymmetry, can give rise to an electric dipole moment along the spin axis of the electron. No electric dipole moments of fundamental particles have been observed. However, dipole moments only slightly smaller than the current experimental bounds have been predicted to arise from particles more massive than any known to exist. Here we present an improved experimental limit on the electric dipole moment of the electron, obtained by measuring the electron spin precession in a superposition of quantum states of electrons subjected to a huge intramolecular electric field. The sensitivity of our measurement is more than one order of magnitude better than any previous measurement. This result implies that a broad class of conjectured particles, if they exist and time-reversal symmetry is maximally violated, have masses that greatly exceed what can be measured directly at the Large Hadron Collider.

Measurement of the fine-structure constant as a test of the Standard Model

Abstract: Measurements of the fine-structure constant α require methods from across subfields and are thus powerful tests of the consistency of theory and experiment in physics. Using the recoil frequency of cesium-133 atoms in a matter-wave interferometer, we recorded the most accurate measurement of the fine-structure constant to date: α = 1/137.035999046(27) at 2.0 × 10 −10 accuracy. Using multiphoton interactions (Bragg diffraction and Bloch oscillations), we demonstrate the largest phase (12 million radians) of any Ramsey-Borde interferometer and control systematic effects at a level of 0.12 part per billion. Comparison with Penning trap measurements of the electron gyromagnetic anomaly g e − 2 via the Standard Model of particle physics is now limited by the uncertainty in g e − 2; a 2.5σ tension rejects dark photons as the reason for the unexplained part of the muon’s magnetic moment at a 99% confidence level. Implications for dark-sector candidates and electron substructure may be a sign of physics beyond the Standard Model that warrants further investigation.

Strong spin-photon coupling in silicon.

Abstract: Long coherence times of single spins in silicon quantum dots make these systems highly attractive for quantum computation, but how to scale up spin qubit systems remains an open question. As a first step to address this issue, we demonstrate the strong coupling of a single electron spin and a single microwave photon. The electron spin is trapped in a silicon double quantum dot, and the microwave photon is stored in an on-chip high-impedance superconducting resonator. The electric field component of the cavity photon couples directly to the charge dipole of the electron in the double dot, and indirectly to the electron spin, through a strong local magnetic field gradient from a nearby micromagnet. Our results provide a route to realizing large networks of quantum dot–based spin qubit registers.

Experimental Evidence of Radiation Reaction in the Collision of a High-Intensity Laser Pulse with a Laser-Wakefield Accelerated Electron Beam

Abstract: The dynamics of energetic particles in strong electromagnetic fields can be heavily influenced by the energy loss arising from the emission of radiation during acceleration, known as radiation reaction. When interacting with a high-energy electron beam, today's lasers are sufficiently intense to explore the transition between the classical and quantum radiation reaction regimes. We present evidence of radiation reaction in the collision of an ultrarelativistic electron beam generated by laser-wakefield acceleration (epsilon > 500 MeV) with an intense laser pulse (a(0) > 10). We measure an energy loss in the postcollision electron spectrum that is correlated with the detected signal of hard photons (gamma rays), consistent with a quantum description of radiation reaction. The generated gamma rays have the highest energies yet reported from an all-optical inverse Compton scattering scheme, with critical energy epsilon(crit) > 30 MeV.

Demonstration of high mobility and quantum transport in modulation-doped β-(AlxGa1-x)2O3/Ga2O3 heterostructures

Abstract: In this work, we demonstrate a high mobility two-dimensional electron gas (2DEG) formed at the β-(AlxGa1-x)2O3/Ga2O3 interface through modulation doping. Shubnikov-de Haas (SdH) oscillations were observed in the modulation-doped β-(AlxGa1-x)2O3/Ga2O3 structure, indicating a high-quality electron channel formed at the heterojunction interface. The formation of the 2DEG channel was further confirmed by the weak temperature dependence of the carrier density, and the peak low temperature mobility was found to be 2790 cm2/Vs, which is significantly higher than that achieved in bulk-doped Beta-phase Gallium Oxide (β-Ga2O3). The observed SdH oscillations allowed for the extraction of the electron effective mass in the (010) plane to be 0.313 ± 0.015 m0 and the quantum scattering time to be 0.33 ps at 3.5 K. The demonstrated modulation-doped β-(AlxGa1-x)2O3/Ga2O3 structure lays the foundation for future exploration of quantum physical phenomena and semiconductor device technologies based on the β-Ga2O3 material system.

Constraints on sub-GeV dark matter-electron scattering from the DarkSide-50 experiment

Abstract: We present new constraints on sub-GeV dark-matter particles scattering off electrons based on 6780.0 kg d of data collected with the DarkSide-50 dual-phase argon time projection chamber. This analysis uses electroluminescence signals due to ionized electrons extracted from the liquid argon target. The detector has a very high trigger probability for these signals, allowing for an analysis threshold of three extracted electrons, or approximately 0.05 keVee. We calculate the expected recoil spectra for dark matter-electron scattering in argon and, under the assumption of momentum-independent scattering, improve upon existing limits from XENON10 for dark-matter particles with masses between 30 and 100 MeV/c^{2}.

Experimental Signatures of the Quantum Nature of Radiation Reaction in the Field of an Ultraintense Laser

Abstract: Substantial energy loss in an electron beam passing through a high-intensity laser provides clear evidence of the radiation reaction, shedding light on how electrons interact with extreme electromagnetic fields.

Electrically controlled long-distance spin transport through an antiferromagnetic insulator

Abstract: Spintronics uses spins, the intrinsic angular momentum of electrons, as an alternative for the electron charge. Its long-term goal is in the development of beyond-Moore low dissipation technology devices. Recent progress demonstrated the long-distance transport of spin signals across ferromagnetic insulators. Antiferromagnetically ordered materials are however the most common class of magnetic materials with several crucial advantages over ferromagnetic systems. In contrast to the latter, antiferromagnets exhibit no net magnetic moment, which renders them stable and impervious to external fields. In addition, they can be operated at THz frequencies. While fundamentally their properties bode well for spin transport, previous indirect observations indicate that spin transmission through antiferromagnets is limited to short distances of a few nanometers. Here we demonstrate the long-distance, over tens of micrometers, propagation of spin currents through hematite (\alpha-Fe2O3), the most common antiferromagnetic iron oxide, exploiting the spin Hall effect for spin injection. We control the spin current flow by the interfacial spin-bias and by tuning the antiferromagnetic resonance frequency with an external magnetic field. This simple antiferromagnetic insulator is shown to convey spin information parallel to the compensated moment (N\'eel order) over distances exceeding tens of micrometers. This newly-discovered mechanism transports spin as efficiently as the net magnetic moments in the best-suited complex ferromagnets. Our results pave the way to ultra-fast, low-power antiferromagnet-insulator-based spin-logic devices that operate at room temperature and in the absence of magnetic fields.

Solar UV and X-ray spectral diagnostics

Abstract: X-ray and ultraviolet (UV) observations of the outer solar atmosphere have been used for many decades to measure the fundamental parameters of the solar plasma. This review focuses on the optically thin emission from the solar atmosphere, mostly found at UV and X-ray (XUV) wavelengths, and discusses some of the diagnostic methods that have been used to measure electron densities, electron temperatures, differential emission measure (DEM), and relative chemical abundances. We mainly focus on methods and results obtained from high-resolution spectroscopy, rather than broad-band imaging. However, we note that the best results are often obtained by combining imaging and spectroscopic observations. We also mainly focus the review on measurements of electron densities and temperatures obtained from single ion diagnostics, to avoid issues related to the ionisation state of the plasma. We start the review with a short historical introduction on the main XUV high-resolution spectrometers, then review the basics of optically thin emission and the main processes that affect the formation of a spectral line. We mainly discuss plasma in equilibrium, but briefly mention non-equilibrium ionisation and non-thermal electron distributions. We also summarise the status of atomic data, which are an essential part of the diagnostic process. We then review the methods used to measure electron densities, electron temperatures, the DEM, and relative chemical abundances, and the results obtained for the lower solar atmosphere (within a fraction of the solar radii), for coronal holes, the quiet Sun, active regions and flares.

Coherent control of single electrons: a review of current progress.

Abstract: In this report we review the present state of the art of the control of propagating quantum states at the single-electron level and its potential application to quantum information processing. We give an overview of the different approaches that have been developed over the last few years in order to gain full control over a propagating single-electron in a solid-state system. After a brief introduction of the basic concepts, we present experiments on flying qubit circuits for ensemble of electrons measured in the low frequency (DC) limit. We then present the basic ingredients necessary to realise such experiments at the single-electron level. This includes a review of the various single-electron sources that have been developed over the last years and which are compatible with integrated single-electron circuits. This is followed by a review of recent key experiments on electron quantum optics with single electrons. Finally we will present recent developments in the new physics that has emerged using ultrashort voltage pulses. We conclude our review with an outlook and future challenges in the field.

The Spatially Oriented Charge Flow and Photocatalysis Mechanism on Internal van der Waals Heterostructures Enhanced g-C3N4

Abstract: Undirected charge transfer and inhibited interlayer electron migration largely limit the photocatalytic efficiency of two-dimensional (2D) layered graphitic carbon nitride (g-C3N4). Herein, we present a facile chemical approach to forming internal van der Waals heterostructures (IVDWHs) within g-C3N4, which enhance the interlayer coulomb interaction and facilitate the spatially oriented charge separation. Such a structure, generated through simultaneous g-C3N4 intralayer modification by O and interlayer intercalation by K, enables the oriented charge flow between the layers, enhancing the accumulation of the localized electrons and promoting the production of active radicals for the activation of reactants as suggested by density functional theory calculations. The resultant O, K-functionalized g-C3N4 with IVDWHs shows an enhanced photocatalytic activity, with nearly 100% enhancement of NO purification efficiency compared with pristine g-C3N4. The reaction mechanism for NO purification is also provided, i...

Electrical control of interlayer exciton dynamics in atomically thin heterostructures.

Abstract: Excitons in semiconductors, bound pairs of excited electrons and holes, can form the basis for new classes of quantum optoelectronic devices. A van der Waals heterostructure built from atomically thin semiconducting transition metal dichalcogenides (TMDs) enables the formation of excitons from electrons and holes in distinct layers, producing interlayer excitons with large binding energy and a long lifetime. Employing heterostructures of monolayer TMDs, we realize optical and electrical generation of long-lived neutral and charged interlayer excitons. We demonstrate the transport of neutral interlayer excitons across the whole sample that can be controlled by excitation power and gate electrodes. We also realize the drift motion of charged interlayer excitons using Ohmic-contacted devices. The electrical generation and control of excitons provides a new route for realizing quantum manipulation of bosonic composite particles with complete electrical tunability.

Intrinsic and anisotropic Rashba spin splitting in Janus transition-metal dichalcogenide monolayers

Abstract: Transition-metal dichalcogenides (TMDs) monolayers have been considered as important two-dimensional semiconductor materials for the study of fundamental physics in the field of spintronics. However, the out-of-plane mirror symmetry in TMDs may constrain electrons' degrees of freedom and it may limit spin-related applications. Recently, a newly synthesized Janus TMDs MoSSe was found to intrinsically possess both the in-plane inversion and the out-of-plane mirror-symmetry breaking. Here we performed first-principles calculations in order to systematically investigate the electronic band structures of a series of Janus monolayer TMDs with chemical formula $MXY\phantom{\rule{0.16em}{0ex}}(M=\mathrm{Mo},\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}\mathrm{and}\phantom{\rule{0.16em}{0ex}}X,Y=\mathrm{S},\phantom{\rule{0.16em}{0ex}}\mathrm{Se},\phantom{\rule{0.16em}{0ex}}\mathrm{Te})$. It is found that they possess robust electronic properties like their parent phases. We explored also the effect of perpendicular external electric field and in-plane biaxial strain on the Rashba spin splittings. The Zeeman-type spin splitting and valley polarization at $K({K}^{\ensuremath{'}})$ point are well preserved and we observed a Rashba-type spin splitting around the $\mathrm{\ensuremath{\Gamma}}$ point for all the $MXY$ systems. We have also found that these spin splittings can be enhanced by an external electric field collinear with the local electric field derived by the polar bonds and by the compressive strain. The Rashba parameters change linearly with the external electric field, but nonlinearly with the biaxial strain. The compressive strain is found to enhance significantly the anisotropic Rashba spin splitting.

Thermal and electrical signatures of a hydrodynamic electron fluid in tungsten diphosphide.

Abstract: In stark contrast to ordinary metals, in materials in which electrons strongly interact with each other or with phonons, electron transport is thought to resemble the flow of viscous fluids. Despite their differences, it is predicted that transport in both conventional and correlated materials is fundamentally limited by the uncertainty principle applied to energy dissipation. Here we report the observation of experimental signatures of hydrodynamic electron flow in the Weyl semimetal tungsten diphosphide. Using thermal and magneto-electric transport experiments, we find indications of the transition from a conventional metallic state at higher temperatures to a hydrodynamic electron fluid below 20 K. The hydrodynamic regime is characterized by a viscosity-induced dependence of the electrical resistivity on the sample width and by a strong violation of the Wiedemann–Franz law. Following the uncertainty principle, both electrical and thermal transport are bound by the quantum indeterminacy, independent of the underlying transport regime. Advances in the fabrication of low-disorder metallic materials have made it possible to reach the hydrodynamic regime of electronic transport. Here the authors investigate a hydrodynamic electron fluid in tungsten diphosphide and find that both electrical and thermal transport are limited by the quantum indeterminacy.

Detection of sub-MeV Dark Matter with Three-Dimensional Dirac Materials

Abstract: We propose the use of three-dimensional Dirac materials as targets for direct detection of sub-MeV dark matter. Dirac materials are characterized by a linear dispersion for low-energy electronic excitations, with a small band gap of O(meV) if lattice symmetries are broken. Dark matter at the keV scale carrying kinetic energy as small as a few meV can scatter and excite an electron across the gap. Alternatively, bosonic dark matter as light as a few meV can be absorbed by the electrons in the target. We develop the formalism for dark matter scattering and absorption in Dirac materials and calculate the experimental reach of these target materials. We find that Dirac materials can play a crucial role in detecting dark matter in the keV to MeV mass range that scatters with electrons via a kinetically mixed dark photon, as the dark photon does not develop an in-medium effective mass. The same target materials provide excellent sensitivity to absorption of light bosonic dark matter in the meV to hundreds of meV mass range, superior to all other existing proposals when the dark matter is a kinetically mixed dark photon.

Laser Cooled YbF Molecules for Measuring the Electron's Electric Dipole Moment.

Abstract: We demonstrate one-dimensional sub-Doppler laser cooling of a beam of YbF molecules to 100 μK. This is a key step towards a measurement of the electron's electric dipole moment using ultracold molecules. We compare the effectiveness of magnetically assisted and polarization-gradient sub-Doppler cooling mechanisms. We model the experiment and find good agreement with our data.

Uncovering electron scattering mechanisms in NiFeCoCrMn derived concentrated solid solution and high entropy alloys

Abstract: Whilst it has long been known that disorder profoundly affects transport properties, recent measurements on a series of solid solution 3d-transition metal alloys reveal two orders of magnitude variations in the residual resistivity. Using ab-initio methods, we demonstrate that, while the carrier density of all alloys is as high as in normal metals, the electron mean-free-path can vary from ~10 A (strong scattering limit) to ~10$^3$ A (weak scattering limit). Here, we delineate the underlying electron scattering mechanisms responsible for this disparate behavior. While spin dependent site-diagonal disorder is always dominant, for alloys containing only Fe, Co, and Ni the majority spin channel experiences negligible disorder scattering, thereby providing a short circuit, while for Cr/Mn containing alloys both spin channels experience strong disorder scattering due to an electron filling effect. Unexpectedly, other scattering mechanisms (e.g. displacement scattering) are found to be relatively weak in most cases.

Skyrmion lattice with a giant topological Hall effect in a frustrated triangular-lattice magnet

Abstract: Geometrically frustrated magnets provide abundant opportunities for discovering complex spin textures, which sometimes yield unconventional electromagnetic responses in correlated electron systems. It is theoretically predicted that magnetic frustration may also promote a topologically nontrivial spin state, i.e., magnetic skyrmions, which are nanometric spin vortices. Empirically, however, skyrmions are essentially concomitant with noncentrosymmetric lattice structures or interfacial-symmetry-breaking heterostructures. Here, we report the emergence of a Bloch-type skyrmion state in the frustrated centrosymmetric triangular-lattice magnet Gd2PdSi3. We identified the field-induced skyrmion phase via a giant topological Hall response, which is further corroborated by the observation of in-plane spin modulation probed by resonant x-ray scattering. Our results exemplify a new gold mine of magnetic frustration for producing topological spin textures endowed with emergent electrodynamics in centrosymmetric magnets.

Fluidity onset in graphene

Abstract: Viscous electron fluids have emerged recently as a new paradigm of strongly-correlated electron transport in solids. Here we report on a direct observation of the transition to this long-sought-for state of matter in a high-mobility electron system in graphene. Unexpectedly, the electron flow is found to be interaction-dominated but non-hydrodynamic (quasiballistic) in a wide temperature range, showing signatures of viscous flows only at relatively high temperatures. The transition between the two regimes is characterized by a sharp maximum of negative resistance, probed in proximity to the current injector. The resistance decreases as the system goes deeper into the hydrodynamic regime. In a perfect darkness-before-daybreak manner, the interaction-dominated negative response is strongest at the transition to the quasiballistic regime. Our work provides the first demonstration of how the viscous fluid behavior emerges in an interacting electron system.

Subcycle observation of lightwave-driven Dirac currents in a topological surface band

Abstract: Harnessing the carrier wave of light as an alternating-current bias may enable electronics at optical clock rates1. Lightwave-driven currents have been assumed to be essential for high-harmonic generation in solids2-6, charge transport in nanostructures7,8, attosecond-streaking experiments9-16 and atomic-resolution ultrafast microscopy17,18. However, in conventional semiconductors and dielectrics, the finite effective mass and ultrafast scattering of electrons limit their ballistic excursion and velocity. The Dirac-like, quasi-relativistic band structure of topological insulators19-29 may allow these constraints to be lifted and may thus open a new era of lightwave electronics. To understand the associated, complex motion of electrons, comprehensive experimental access to carrier-wave-driven currents is crucial. Here we report angle-resolved photoemission spectroscopy with subcycle time resolution that enables us to observe directly how the carrier wave of a terahertz light pulse accelerates Dirac fermions in the band structure of the topological surface state of Bi2Te3. While terahertz streaking of photoemitted electrons traces the electromagnetic field at the surface, the acceleration of Dirac states leads to a strong redistribution of electrons in momentum space. The inertia-free surface currents are protected by spin-momentum locking and reach peak densities as large as two amps per centimetre, with ballistic mean free paths of several hundreds of nanometres, opening up a realistic parameter space for all-coherent lightwave-driven electronic devices. Furthermore, our subcycle-resolution analysis of the band structure may greatly improve our understanding of electron dynamics and strong-field interaction in solids.

Non-thermal particle acceleration in collisionless relativistic electron-proton reconnection

Abstract: Magnetic reconnection in relativistic collisionless plasmas can accelerate particles and power high-energy emission in various astrophysical systems. Whereas most previous studies focused on relativistic reconnection in pair plasmas, less attention has been paid to electron-ion plasma reconnection, expected in black hole accretion flows and relativistic jets. We report a comprehensive particle-in-cell numerical investigation of reconnection in an electron-ion plasma, spanning a wide range of ambient ion magnetizations $\sigma_i$, from the semirelativistic regime (ultrarelativistic electrons but nonrelativistic ions, 0.001 >1). We investigate how the reconnection rate, electron and ion plasma flows, electric and magnetic field structures, electron/ion energy partitioning, and nonthermal particle acceleration depend on $\sigma_i$. Our key findings are: (1) the reconnection rate is about 0.1 of the Alfvenic rate across all regimes; (2) electrons can form concentrated moderately relativistic outflows even in the semirelativistic, small-$\sigma_i$ regime; (3) while the released magnetic energy is partitioned equally between electrons and ions in the ultrarelativistic limit, the electron energy fraction declines gradually with decreased $\sigma_i$ and asymptotes to about 0.25 in the semirelativistic regime; (4) reconnection leads to efficient nonthermal electron acceleration with a $\sigma_i$-dependent power-law index, $p(\sigma_i) \simeq $const$+0.7 {\sigma_i}^{-1/2}$. These findings are important for understanding black hole systems and lend support to semirelativistic reconnection models for powering nonthermal emission in blazar jets, offering a natural explanation for the spectral indices observed in these systems.

Emergence of Kondo lattice behavior in a van der Waals itinerant ferromagnet, Fe3GeTe2

Abstract: Searching for heavy fermion (HF) states in non–f-electron systems becomes an interesting issue, especially in the presence of magnetism, and can help explain the physics of complex compounds. Using angle-resolved photoemission spectroscopy, scanning tunneling microscopy, physical properties measurements, and the first-principles calculations, we observe the HF state in a 3d-electron van der Waals ferromagnet, Fe 3 GeTe 2 . Upon entering the ferromagnetic state, a massive spectral weight transfer occurs, which results from the exchange splitting. Meanwhile, the Fermi surface volume and effective electron mass are both enhanced. When the temperature drops below a characteristic temperature T *, heavy electrons gradually emerge with further enhanced effective electron mass. The coexistence of ferromagnetism and HF state can be well interpreted by the dual properties (itinerant and localized) of 3d electrons. This work expands the limit of ferromagnetic HF materials from f- to d-electron systems and illustrates the positive correlation between ferromagnetism and HF state in the 3d-electron material, which is quite different from the f-electron systems.

Measurement of the fine-structure constant as a test of the Standard Model

Abstract: Measurements of the fine-structure constant alpha require methods from across subfields and are thus powerful tests of the consistency of theory and experiment in physics. Using the recoil frequency of cesium-133 atoms in a matter-wave interferometer, we recorded the most accurate measurement of the fine-structure constant to date: alpha = 1/137.035999046(27) at 2.0 x 10^-10 accuracy. Using multiphoton interactions (Bragg diffraction and Bloch oscillations), we demonstrate the largest phase (12 million radians) of any Ramsey-Borde interferometer and control systematic effects at a level of 0.12 parts per billion. Comparison with Penning trap measurements of the electron gyromagnetic anomaly ge-2 via the Standard Model of particle physics is now limited by the uncertainty in ge-2; a 2.5 sigma tension rejects dark photons as the reason for the unexplained part of the muon's magnetic moment at a 99 percent confidence level. Implications for dark-sector candidates and electron substructure may be a sign of physics beyond the Standard Model that warrants further investigation.

Chirality-Dependent Electron Spin Filtering by Molecular Monolayers of Helicenes

Abstract: The interaction of low-energy photoelectrons with well-ordered monolayers of enantiopure helical heptahelicene molecules adsorbed on metal surfaces leads to a preferential transmission of one longitudinally polarized spin component, which is strongly coupled to the helical sense of the molecules. Heptahelicene, composed of only carbon and hydrogen atoms, exhibits only a single helical turn but shows excess in longitudinal spin polarization of about PZ = 6 to 8% after transmission of initially balanced left- and right-handed spin polarized electrons. Insight into the electronic structure, that is, the projected density of states, and the spin-dependent electron scattering in the helicene molecule is gained by using spin-resolved density functional theory calculations and a model Hamiltonian approach, respectively. Our results support the semiclassical picture of electronic transport along a helical pathway under the influence of spin–orbit coupling induced by the electrostatic molecular potential.

Diagnostics for plasma-based electron accelerators

Abstract: Plasma-based accelerators that impart energy gain as high as several GeV to electrons or positrons within a few centimeters have engendered a new class of diagnostic techniques very different from those used in connection with conventional radio-frequency (rf) accelerators. The need for new diagnostics stems from the micrometer scale and transient, dynamic structure of plasma accelerators, which contrasts with the meter scale and static structure of conventional accelerators. Because of this micrometer source size, plasma-accelerated electron bunches can emerge with smaller normalized transverse emittance (en<0.1 mm mrad) and shorter duration (τb∼1 fs) than bunches from rf linacs. Single-shot diagnostics are reviewed that determine such small en and τb noninvasively and with high resolution from wide-bandwidth spectral measurement of electromagnetic radiation the electrons emit: en from x rays emitted as electrons interact with transverse internal fields of the plasma accelerator or with external optical fields or undulators; τb from THz to optical coherent transition radiation emitted upon traversing interfaces. The duration of ∼1 fs bunches can also be measured by sampling individual cycles of a copropagating optical pulse or by measuring the associated magnetic field using a transverse probe pulse. Because of their luminal velocity and micrometer size, the evolving structure of plasma accelerators, the key determinant of accelerator performance, is exceptionally challenging to visualize in the laboratory. Here a new generation of laboratory diagnostics is reviewed that yield snapshots, or even movies, of laser- and particle-beam-generated plasma accelerator structures based on their phase modulation or deflection of femtosecond electromagnetic or electron probe pulses. Spatiotemporal resolution limits of these imaging techniques are discussed, along with insight into plasma-based acceleration physics that has emerged from analyzing the images and comparing them to simulated plasma structures.

In Situ Engineering of Double-Phase Interface in Mo/Mo2C Heteronanosheets for Boosted Hydrogen Evolution Reaction

Abstract: Two-dimensional Mo/Mo2C heteronanosheets (Mo/Mo2C-HNS) were successfully prepared via a NaCl template-assisted synthesis route followed by a controllable simultaneous reduction and carbonization of MoO3 nanosheets for efficient hydrogen evolution reaction (HER) catalysis under both acidic and alkaline conditions. The Mo species in the atomically thin Mo/Mo2C-HNS not only guarantees the rapid transport of electrons but also optimizes the electronic configuration of β-Mo2C. Besides, the abundant Mo/β-Mo2C heterointerfaces in nanodimension afford large numbers of additional heterogeneous catalytic sites. HER electrocatalytic performance with overpotential of merely 89 mV to drive a current density of 10 mA/cm2 in 0.5 M H2SO4 is therefore achieved. Strikingly, stable chronoamperometric electrolysis for 20 h and also an impressive cycling stability with negligible overpotential decay over 4000 sweeps demonstrate its considerable durability in an acidic environment. These findings highlight the promising potent...

Directly detecting sub-GeV dark matter with electrons from nuclear scattering

Abstract: Dark matter (DM) particles with mass in the sub-GeV range are an attractive alternative to heavier weakly interacting massive particles, but direct detection of such light particles is challenging. If, however, DM-nucleus scattering leads to ionization of the recoiling atom, the resulting electron may be detected even if the nuclear recoil is unobservable. We demonstrate that including this effect significantly enhances direct detection sensitivity to sub-GeV DM. Existing experiments set world-leading limits, and future experiments may probe the cross sections relevant for thermal freeze-out.

Extremely Vivid, Highly Transparent, and Ultrathin Quantum Dot Light-Emitting Diodes.

Abstract: Displaying information on transparent screens offers new opportunities in next-generation electronics, such as augmented reality devices, smart surgical glasses, and smart windows. Outstanding luminance and transparency are essential for such “see-through” displays to show vivid images over clear background view. Here transparent quantum dot light-emitting diodes (Tr-QLEDs) are reported with high brightness (bottom: ≈43 000 cd m−2, top: ≈30 000 cd m−2, total: ≈73 000 cd m−2 at 9 V), excellent transmittance (90% at 550 nm, 84% over visible range), and an ultrathin form factor (≈2.7 µm thickness). These superb characteristics are accomplished by novel electron transport layers (ETLs) and engineered quantum dots (QDs). The ETLs, ZnO nanoparticle assemblies with ultrathin alumina overlayers, dramatically enhance durability of active layers, and balance electron/hole injection into QDs, which prevents nonradiative recombination processes. In addition, the QD structure is further optimized to fully exploit the device architecture. The ultrathin nature of Tr-QLEDs allows their conformal integration on various shaped objects. Finally, the high resolution patterning of red, green, and blue Tr-QLEDs (513 pixels in.−1) shows the potential of the full-color transparent display.

Surface Facet of CuFeO2 Nanocatalyst: A Key Parameter for H2O2 Activation in Fenton-Like Reaction and Organic Pollutant Degradation

Abstract: The development of efficient heterogeneous Fenton catalysts is mainly by “trial-and-error” concept and the factor determining H2O2 activation remains elusive. In this work, we demonstrate that suitable facet exposure to elongate O–O bond in H2O2 is the key parameter determining the Fenton catalyst’s activity. CuFeO2 nanocubes and nanoplates with different surface facets of {110} and {012} are used to compare the effect of exposed facets on Fenton activity. The results indicate that ofloxacin (OFX) degradation rate by CuFeO2 {012} is four times faster than that of CuFeO2 {110} (0.0408 vs 0.0101 min–1). In CuFeO2 {012}-H2O2 system, OFX is completely removed at a pH range 3.2–10.1. The experimental results and theoretical simulations show that •OH is preferentially formed from the reduction of absorbed H2O2 by electron from CuFeO2 {012} due to suitable elongation of O–O (1.472 A) bond length in H2O2. By contrast, the O–O bond length is elongated from 1.468 to 3.290 A by CuFeO2 {110} facet, H2O2 tends to be d...