Ensemble Of Electrons Research Articles

Ultralong Dephasing Times in Solid-State Spin Ensembles via Quantum Control

Quantum spin dephasing is caused by inhomogeneous coupling to the environment, with resulting limits to the measurement time and precision of spin-based sensors. The effects of spin dephasing can be especially pernicious for dense ensembles of electronic spins in the solid-state, such as for nitrogen-vacancy (NV) color centers in diamond. We report the use of two complementary techniques, spin bath control and double quantum coherence, to enhance the inhomogeneous spin dephasing time ($T_2^*$) for NV ensembles by more than an order of magnitude. In combination, these quantum control techniques (i) eliminate the effects of the dominant NV spin ensemble dephasing mechanisms, including crystal strain gradients and dipolar interactions with paramagnetic bath spins, and (ii) increase the effective NV gyromagnetic ratio by a factor of two. Applied independently, spin bath control and double quantum coherence elucidate the sources of spin dephasing over a wide range of NV and spin bath concentrations. These results demonstrate the longest reported $T_2^*$ in a solid-state electronic spin ensemble at room temperature, and outline a path towards NV-diamond magnetometers with broadband femtotesla sensitivity.

Test‐Particle Simulations of Linear and Nonlinear Interactions Between a 2‐D Whistler‐Mode Wave Packet and Radiation Belt Electrons

AbstractUsing test‐particle simulations, we describe (in 2‐D) the interaction of an ensemble of electrons with a discrete whistler‐mode wave packet, which propagates with a moderate wave‐normal angle with respect to the background magnetic field. We evaluate both the average transport coefficients used in quasi‐linear diffusion transport and also the full nonlinear wave‐particle interactions. The magnitude of the calculated diffusion coefficients is found to increase proportionally to the wave amplitude squared, in agreement with quasi‐linear diffusion theory. Cyclotron resonance of counterstreaming electrons (n = 1 harmonic number) is more important for pitch angle scattering than Landau resonance. Landau resonance of costreaming electrons (n = 0) is comparable to cyclotron resonance for energy diffusion and advection. Strong acceleration of high pitch angle, costreaming electrons arises in nonlinear wave‐particle interactions with high‐amplitude waves. In our simulations with zero parallel electric field, the energy source for electron acceleration is the wave's perpendicular electric field, in both the cyclotron and Landau resonances. The Landau resonance can happen even with zero parallel electric field, if the wave packet propagates with a finite wave‐normal angle. This resonance between the particle's azimuthal velocity and the wave fields leads to trapping and substantial parallel acceleration.

Enhancement of the spontaneous emission in subwavelength quasi-two-dimensional waveguides and resonators

We consider a quantum-electrodynamic problem of the spontaneous emission from a two-dimensional (2D) emitter, such as a quantum well or a 2D semiconductor, placed in a quasi-2D waveguide or cavity with subwavelength confinement in one direction. We apply the Heisenberg-Langevin approach which includes dissipation and fluctuations in the electron ensemble and in the electromagnetic field of a cavity on equal footing. The Langevin noise operators that we introduce do not depend on any particular model of dissipative reservoir and can be applied to any dissipation mechanism. Moreover, our approach is applicable to nonequilibrium electron systems, e.g. in the presence of pumping, beyond the applicability of the standard fluctuation-dissipation theorem. We derive analytic results for simple but practically important geometries: strip lines and rectangular cavities. Our results show that a significant enhancement of the spontaneous emission, by a factor of order 100 or higher, is possible for quantum wells and other 2D emitters in a subwavelength cavity.

Polarized electron beams elastically scattered by atoms as a tool for testing fundamental predictions of quantum mechanics

Quantum information theory deals with quantum noise in order to protect physical quantum bits (qubits) from its effects. A single electron is an emblematic example of a qubit, and today it is possible to experimentally produce polarized ensembles of electrons. In this paper, the theory of the polarization of electron beams elastically scattered by atoms is briefly summarized. Then the POLARe program suite, a set of computer programs aimed at the calculation of the spin-polarization parameters of electron beams elastically interacting with atomic targets, is described. Selected results of the program concerning Ar, Kr, and Xe atoms are presented together with the comparison with experimental data about the Sherman function for low kinetic energy of the incident electrons (1.5eV–350eV). It is demonstrated that the quantum-relativistic theory of the polarization of electron beams elastically scattered by atoms is in good agreement with experimental data down to energies smaller than a few eV.

Tracking of an electron beam through the solar corona with LOFAR

The Sun’s activity leads to bursts of radio emission, among other phenomena. An example is type-III radio bursts. They occur frequently and appear as short-lived structures rapidly drifting from high to low frequencies in dynamic radio spectra. They are usually interpreted as signatures of beams of energetic electrons propagating along coronal magnetic field lines. Here we present novel interferometric LOFAR (LOw Frequency ARray) observations of three solar type-III radio bursts and their reverse bursts with high spectral, spatial, and temporal resolution. They are consistent with a propagation of the radio sources along the coronal magnetic field lines with nonuniform speed. Hence, the type-III radio bursts cannot be generated by a monoenergetic electron beam, but by an ensemble of energetic electrons with a spread distribution in velocity and energy. Additionally, the density profile along the propagation path is derived in the corona. It agrees well with three-fold coronal density model by (1961, ApJ, 133, 983).

Facile Synthesis of Porous Pd3 Pt Half-Shells with Rich "Active Sites" as Efficient Catalysts for Formic Acid Oxidation.

Exploring highly efficient electrocatalysts is greatly important for the widespread uptake of the fuel cells. However, many newly generated nanocrystals with attractive nanostructures often have extremely limited surface area or large particle-size, which leads them to display limited electrocatalytic performance. Herein, a novel anode catalyst of hollow and porous Pd3 Pt half-shells with rich "active sites" is synthesized by using urea as a guiding surfactant. It is identified that the formation of Pd3 Pt half-shells involves the combination of bubble guiding, in situ deposition of particles and bubble burst. The obtained Pd3 Pt half-shells demonstrate a rich edge area with abundant exposed active sites and surface defects, indicating great potential for the electrocatalysis. When used as an electrocatalyst, the Pd3 Pt half-shells exhibit remarkably improved electrocatalytic performance for formic acid oxidation (FAO), where it promotes the dehydrogenation process of FAO by suppressing the formation of poisonous species COads via the electronic effect and ensemble effect.

Dependence of the kinetics of Al2O3 excitation in tracks of swift heavy ions on lattice temperature

This paper investigates an effect of a target temperature on lattice excitation by the relaxing electron ensemble in a swift heavy ion track. A new version of the Monte-Carlo code TREKIS is used for this purpose. Cross sections implemented in TREKIS are constructed within the dynamic structure factor/complex dielectric function (DSF-CDF) formalism. This enables us to take into account unusual pathways of collective responses of the electronic and ionic subsystems to their femtosecond excitations in SHI tracks. The paper demonstrates that the temperature dependent fundamental asymmetry of the lattice DSF manifests itself in different kinetics of energy exchange between the electronic and atomic systems of alumina in SHI tracks at different temperatures (80 K vs. 300 K).

Фотонное эхо на локализованных экситонах в полупроводниковых наноструктурах

AbstractAn overview on photon echo spectroscopy under resonant excitation of the exciton complexes in semiconductor nanostructures is presented. The use of four-wave-mixing technique with the pulsed excitation and heterodyne detection allowed us to measure the coherent response of the system with the picosecond time resolution. It is shown that, for resonant selective pulsed excitation of the localized exciton complexes, the coherent signal is represented by the photon echoes due to the inhomogeneous broadening of the optical transitions. In case of resonant excitation of the trions or donor-bound excitons, the Zeeman splitting of the resident electron ground state levels under the applied transverse magnetic field results in quantum beats of photon echo amplitude at the Larmor precession frequency. Application of magnetic field makes it possible to transfer coherently the optical excitation into the spin ensemble of the resident electrons and to observe a long-lived photon echo signal. The described technique can be used as a high-resolution spectroscopy of the energy splittings in the ground state of the system. Next, we consider the Rabi oscillations and their damping under excitation with intensive optical pulses for the excitons complexes with a different degree of localization. It is shown that damping of the echo signal with increase of the excitation pulse intensity is strongly manifested for excitons, while on trions and donor-bound excitons this effect is substantially weaker.

Why do Electrons with “Anomalous Energies” appear in High-Pressure Gas Discharges?

Experimental studies connected with runaway electron beams generation convincingly shows the existence of electrons with energies above the maximum voltage applied to the discharge gap. Such electrons are also known as electrons with “anomalous energies”. We explain the presence of runaway electrons having so-called “anomalous energies” according to physical kinetics principles, namely, we describe the total ensemble of electrons with the distribution function. Its evolution obeys Boltzmann kinetic equation. The dynamics of self-consistent electromagnetic field is taken into the account by adding complete Maxwell’s equation set to the resulting system of equations. The electrodynamic mechanism of the interaction of electrons with a travelling-wave electric field is analyzed in details. It is responsible for the appearance of electrons with high energies in real discharges.

Statistical fluctuations in cooperative cyclotron radiation

Shot noise is the cause of statistical fluctuations in cooperative cyclotron radiation generated by an ensemble of electrons oscillating in magnetic field. Autophasing time – the time required for the cooperative cyclotron radiation power to peak – is the critical parameter characterizing the dynamics of electron-oscillators interacting via the radiation field.It is shown that premodulation of charged particles leads to a considerable narrowing of the autophasing time distribution function for which the analytic expression is obtained. When the number of particles Ne exceeds a certain value that depends on the degree to which the particles have been premodulated, the relative root-mean-square deviation (RMSD) of the autophasing time δT changes from a logarithmic dependence on Ne (δT∼1∕lnNe) to square-root (δT∼1∕Ne).A slight energy spread (∼4%) results in a twofold drop of the maximum attainable power of cooperative cyclotron radiation.

Ensemble spin fabrication and manipulation of NV centres for magnetic sensing in diamond

PurposeThis study aims to fabricate and manipulate ensemble spin of negative nitrogen-vacancy (NV−) centres optimally for future solid atomic magnetometers/gyroscope. Parameters for sample preparation most related to magnetometers/gyroscope are, in particular, the concentration and homogeneity of the NV− centres, the parameters’ microwave antenna of resonance frequency and the strength of the microwave on NV− centres. Besides, the abundance of other impurities such as neutral NV centres (NV0) and substitutional nitrogen in the lattice also plays a critical role in magnetic sensing.Design/methodology/approachThe authors succeeded in fabricating the assembly of NV centres in diamond and they determined its concentration of (2-3) × 1016 cm−3 with irradiation followed by annealing under a high temperature condition. They explored a novel magnetic resonance approach to detect the weak magnetic fields that takes advantage of the solid-state electron ensemble spin of NV− centres in diamond. In particular, the authors set up a magnetic sensor on the basis of the assembly of NV centres. They succeeded in fabricating the assembly of NV centres in diamond and determined its concentration. They also clarified the magnetic field intensity measured at different positions along the antenna with different lengths, and they found the optimal position where the signal of the magnetic field reaches the maximum.FindingsThe authors mainly reported preparation, initialization, manipulation and measurement of the ensemble spin of the NV centres in diamond using optical excitation and microwave radiation methods with variation of the external magnetic field. They determined the optimal parameters of irradiation and annealing to generate the ensemble NV centres, and a concentration of NV− centres as high as 1016 cm−3 in diamond was obtained. In addition, they found that sensitivity of the magnetometer using this method can reach as low as 5.22 µT/Hz currently.Practical implicationsThis research can shed light on the development of an atomic magnetometer and a gyroscope on the basis of the ensemble spin of NV centres in diamond.Social implicationsHigh concentration spin of NV− in diamond is one of the advantages compared with that of the atomic vapor cells, because it can obtain a higher concentration. When increasing the spin concentration, the spin signal is easy to detect, and macro-atomic spin magnetometer become possible. This research is the first step for solid atomic magnetometers with high spin density and high sensitivity potentially with further optimization. It has a wide range of applications from fundamental physics tests, sensor applications and navigation to detection of NMR signals.Originality/valueAs has been pointed out, in this research, the authors mainly worked on fabricating NV− centres with high concentration (1015-1016 cm−3) in diamond by using optimal irradiation and annealing processes, and they quantitatively defined the NV− concentration, which is important for the design of higher concentration processes in the magnetometer and gyroscope. Until now, few groups can directly define the NV− concentration. Besides, the authors optimized the microwave antenna parameters experimentally and explored the dependence between the splitting of the magnetic resonance and the magnetic fields, which dictated the minimum detectable magnetic field.

Use of discrete Wigner functions in the study of a hybrid dissipative system

The persistence of coherence in a hybrid dissipative system, which is composed of superconducting flux-qubits (SFQs) and an electron ensemble, is analyzed. Both the interactions between the electrons and the SFQs are taken into account. The time evolution of the hybrid system is discussed in terms of the discrete Wigner function of each subsystem and in terms of the entropic uncertainty relations. The inclusion of a linewidth, both for the electrons and the SFQs, influences the coherence of each subsystem and the pattern of spin squeezing of the corresponding steady state.

Dealloyed platinum-copper with isolated Pt atom surface: Facile synthesis and promoted dehydrogenation pathway of formic acid electro-oxidation

In order to obtain the catalyst with Pt isolated-atom surface and high performance towards formic acid electro-oxidation (FAEO), cyclic potential dealloying method was employed for preparing dealloyed PtCu/C (D-PtCu/C) catalysts, and the effect of dealloying parameters on the activity of D-PtCu/C catalysts also were investigated. XRD results show that both of PtCu and D-PtCu catalysts are face-centered cubic (fcc) structure. TEM, XPS and ICP results demonstrate the dissolution of Cu atoms and the formation of core-shell structure. And HRTEM results further exhibit the etched surface of D-PtCu/C catalyst which should be made up of isolated atom and atomic clusters of Pt. By electro-chemical analysis, the performance towards FAEO is extremely enhanced. The onset potential of FAEO on D-PtCu/C is at −0.1V (vs. SCE) and only one peak is confirmed at 0.5V which negatively shifts about 0.19V compared with the peak of commercial Pt/C (at 0.69V). The mass activity of optimized D-PtCu/C reaches up to 1253.89mAmg−1 Pt at 0.5V, which is 6.20 times as high as that of Pt/C (202.40mAmg−1 Pt). And the specific activity (2.36mAcm−2) is 8.43 times of Pt/C (0.28mAcm−2). The increased activity should be ascribed to the electronic effect and ensemble effect, which reduce the chemisorption of CO and promote the dehydrogenation process.

Grand canonical electronic density-functional theory: Algorithms and applications to electrochemistry.

First-principles calculations combining density-functional theory and continuum solvation models enable realistic theoretical modeling and design of electrochemical systems. When a reaction proceeds in such systems, the number of electrons in the portion of the system treated quantum mechanically changes continuously, with a balancing charge appearing in the continuum electrolyte. A grand-canonical ensemble of electrons at a chemical potential set by the electrode potential is therefore the ideal description of such systems that directly mimics the experimental condition. We present two distinct algorithms: a self-consistent field method and a direct variational free energy minimization method using auxiliary Hamiltonians (GC-AuxH), to solve the Kohn-Sham equations of electronic density-functional theory directly in the grand canonical ensemble at fixed potential. Both methods substantially improve performance compared to a sequence of conventional fixed-number calculations targeting the desired potential, with the GC-AuxH method additionally exhibiting reliable and smooth exponential convergence of the grand free energy. Finally, we apply grand-canonical density-functional theory to the under-potential deposition of copper on platinum from chloride-containing electrolytes and show that chloride desorption, not partial copper monolayer formation, is responsible for the second voltammetric peak.

Study of decoherence in a system of superconducting flux-qubits interacting with an ensemble of electrons

The degree of coherence in a hybrid system composed of superconducting flux-qubits and an electron ensemble is analysed. Both, the interactions among the electrons and among the superconducting flux-qubits are taken into account. The time evolution of the hybrid system is solved exactly, and discussed in terms of the reduced density matrix of each subsystem. It is seen that the inclusion of a line width, for the electrons and for the superconducting flux-qubits, influences the pattern of spin-squeezing and the coherence of the superconducting flux qubits.

Beam load structures in a basic relativistic interaction model

Some recent experiments have shown that the beam load in bubble and blow-out experiments is located in a volume as small as a few μm3. Now, we show what kinds of inner structures are possible in such a high dense electron ensemble. Our analysis starts from a first principles model for relativistically corrected mutual electron interaction in a phenomenological bubble model. Discussing 2D and 3D beam load configurations, we show that, depending on the bunch emittance, the beam load might be in a highly ordered and dense configuration, a less ordered but still dense state, or a configuration where each electron performs an individual random motion.

A biocompatible nanocomposite based on allyl chitosan and vinyltriethoxysilane for tissue engineering

The synthesis, structure, and electrophysical properties of a polymer-inorganic biocompatible composite based on unsaturated chitosan ether, namely, allyl chitosan, and vinyltriethoxysilane are studied. During composite synthesis, allyl chitosan forms an individual nanophase with vinyltriethoxysilane and its condensation products in the polymer matrix of allyl chitosan. The size of nanoparticles embedded in a polymer matrix increases from 50 to 1000 nm as the fraction of the added vinyltriethoxysilane grows. Under exposure to UV radiation, both homopolycondensation and heteropolycondensation occur in the composite films via crosslinking according to the radical mechanism and the composite becomes insoluble in water. It has been shown that the resulting composites feature ionic conductivity under application of both direct current and high-frequency electric fields to the sample. Conductivity is provided by a proton–electron ensemble that concentrates at the nanoparticle/polymer matrix interface.

The combination of cold and hot components in the energy spectra of electrons scattered by relativistically intense laser pulses with various transverse distributions of amplitude

AbstractThe energy spectra of a sparse ensemble of electrons scattered by relativistically intense laser pulses are studied numerically by solving the relativistic Newton equations with the Lorentz force generated by an electromagnetic envelope in vacuum. The expressions for the envelope describe focused optical fields, include significant short-pulse corrections, and afford the representation of laser radiation with various types of transverse distributions of amplitude. The dependence of the character of the electron energy spectra on the type of the transverse distribution of laser amplitude is explored. For Gaussian pulses, the electron energy spectra within specific angular ranges tend to either include a relativistic maximum while being localized around it or to have the shapes of evanescent distributions dominated by the cold component. Conversely, the energy spectra of electrons ejected into certain angular ranges by laser pulses having first-order Laguerre profiles combine pronounced cold components and structured strongly relativistic features. The presumed laser pulse transverse structure and the shapes of the calculated electron energy spectra for first-order Laguerre amplitude distributions are shown to match, qualitatively, those reported in a recent experimental study by Kalashnikov et al. in 2015, which revealed the electron energy spectra spanning both the sub-relativistic and the markedly relativistic energy domains.

Color center fluorescence and spin manipulation in single crystal, pyramidal diamond tips

We investigate bright fluorescence of nitrogen (NV)- and silicon-vacancy color centers in pyramidal, single crystal diamond tips, which are commercially available as atomic force microscope probes. We coherently manipulate NV electronic spin ensembles with T2 = 7.7(3) μs. Color center lifetimes in different tip heights indicate effective refractive index effects and quenching. Using numerical simulations, we verify enhanced photon rates from emitters close to the pyramid apex rendering them promising as scanning probe sensors.

Particle dynamics in the electron current layer in collisionless magnetic reconnection

Particle dynamics in the electron current layer in collisionless magnetic reconnection is investigated by using a particle-in-cell simulation. The electron motion and velocity distribution functions are studied by tracking self-consistent trajectories. New classes of electron orbits are discovered: figure-eight-shaped regular orbits inside the electron jet, noncrossing regular orbits on the jet flanks, noncrossing Speiser orbits, and nongyrotropic electrons in the downstream of the jet termination region. The properties of a super-Alfvénic outflow jet are attributed to an ensemble of electrons traveling through the Speiser orbits. The noncrossing orbits are mediated by the polarization electric field near the electron current layer. The noncrossing electrons are found to be non-negligible in number density. The impact of these new orbits to electron mixing, spatial distribution of energetic electrons, and observational signatures is presented.