Polarized Electron Source Research Articles

Touschek lifetime and luminosity optimization for Russian Super Charm Tau factory

Electron-positron collider Super Charm Tau factory is planned to be built in the National center for physics and mathematics, Sarov. It is a double ring collider with crab waist collision scheme operating in wide beam energy range from 1.5 GeV to 3.5 GeV with peak luminosity of 1035 cm-2s-1. The polarized electron source and three Siberian Snakes provide 80% longitudinally polarized electron beam at 2 GeV. Superconducting wigglers decrease damping times, effects of intra-beam scattering and increase Touschek beam lifetime, particularly at low energy. The paper presents the status of collider design and optimization of luminosity and beam lifetime.

High figure of merit spin polarized electron sources grown via MOCVD

Spin polarized photocathodes are key to the future operation of electron accelerators such as the ones at Thomas Jefferson National Accelerator Facility and Brookhaven National Laboratory. Currently, these photocathodes come in short supply due to limited production by molecular beam epitaxy. By developing a process to implement similar structures using metal organic chemical vapor deposition, the availability of these devices can be increased. In this paper, we detail the implementation of recent photocathode advancements via metal organic chemical vapor deposition process and show an improvement in both polarization and quantum efficiency of our devices compared to those fabricated via molecular beam epitaxy, with devices reaching 82% polarization and 2.9% quantum efficiency.

Improved Limits on an Exotic Spin- and Velocity-Dependent Interaction at the Micrometer Scale with an Ensemble-NV-Diamond Magnetometer.

Searching for exotic interactions provides a path for exploring new particles beyond the standard model. Here, we used an ensemble-NV-diamond magnetometer to search for an exotic spin- and velocity-dependent interaction between polarized electron spins and unpolarized nucleons at the micrometer scale. A thin layer of nitrogen-vacancy electronic spin ensemble in diamond is utilized as both the solid-state spin quantum sensor and the polarized electron source, and a vibrating lead sphere serves as the moving unpolarized nucleon source. The exotic interaction is searched by detecting the possible effective magnetic field induced by the moving unpolarized nucleon source using the ensemble-NV-diamond magnetometer. Our result establishes new bounds for the coupling parameter f_{⊥} within the force range from 5 to 400 μm. The upper limit of the coupling parameter at 100 μm is |f_{⊥}|≤1.1×10^{-11}, which is 3 orders of magnitude more stringent than the previous constraint. This result shows that NV ensemble can be a promising platform to search for hypothetical particles beyond the standard model.

An improved source of spin-polarized electrons based on spin exchange in optically pumped rubidium vapor

We have improved a polarized electron source in which unpolarized electrons undergo collisions with a mixture of buffer gas molecules and optically spin-polarized Rb atoms. With a nitrogen buffer gas, the source reliably provides spin polarization between 15% and 25% with beam currents >4 μA. Vacuum pump upgrades mitigate problems caused by denatured diffusion pump oil, leading to longer run times. A new differential pumping scheme allows the use of higher buffer gas pressures up to 800 mTorr. With a new optics layout, the Rb polarization is continuously monitored by a probe laser and improved pump laser power provides more constant high polarization. We have implemented an einzel lens to better control the energy of the electrons delivered to the target chamber and to preferentially select electron populations of higher polarization. The source is designed for studies of biologically relevant chiral molecule samples, which can poison photoemission-based GaAs polarized electron sources at very low partial pressures. It operates adjacent to a target chamber that rises to pressures as high as 10−4 Torr and has been implemented in a first experiment with chiral cysteine targets.

Record quantum efficiency from strain compensated superlattice GaAs/GaAsP photocathode for spin polarized electron source

Photocathodes based on GaAs and other III–V semiconductors are capable of producing highly spin-polarized electron beams. GaAs/GaAsP superlattice photocathodes exhibit high spin polarization; however, the quantum efficiency (QE) is limited to 1% or less. To increase the QE, we fabricated a GaAs/GaAsP superlattice photocathode with a Distributed Bragg Reflector (DBR) underneath. This configuration creates a Fabry–Pérot cavity between the DBR and GaAs surface, which enhances the absorption of incident light and, consequently, the QE. These photocathode structures were grown using molecular beam epitaxy and achieved record quantum efficiencies exceeding 15% and electron spin polarization of about 75% when illuminated with near-bandgap photon energies.

Soft magnetism in single phase Fe3Si thin films deposited on SrTiO3(001) by pulsed laser deposition

The development of devices relying on spin phenomena requires of an ideal spin polarized electron source. This can be achieved by taking advantage of half-metallic full Heusler alloy thin films. However, their implementation requires a controlled growth of stoichiometric films with large activation volumes. In this work, we report on the growth of epitaxial Fe3Si ultra-thin films by pulsed laser deposition on SrTiO3(001) substrates, analyzing the effect of deposition temperature in the structural, morphological and magnetic properties of the deposited films. We conclude that optimal compromise between phase purity and interface quality is obtained at 200 ºC, obtaining the best magnetic response under this condition.

Quantum efficiency enhancement in simulated nanostructured negative electron affinity GaAs photocathodes

Nanostructured negative electron affinity GaAs photocathodes for a polarized electron source are studied using finite difference time domain optical simulation. The structures studied are nanosquare columns, truncated nanocones, and truncated nanopyramids. Mie-type resonances in the 700–800 nm waveband, suitable for generation of polarized electrons, are identified. At resonance wavelengths, the nanostructures can absorb up to 99% of the incident light. For nanosquare columns and truncated nanocones, the maximum quantum efficiency (QE) at 780 nm obtained from simulation is 27%, whereas for simulated nanopyramids, the QE is ∼21%. The high photocathode quantum efficiency is due to the shift of Mie resonance toward the longer wavelength, leading to increased light absorption. The field profile distribution shows the excitation of dipole and quadrupole modes within the nanostructures at resonant frequencies. This leads to enhanced photoabsorption and photoelectron generation closer to emission surfaces than for a flat photocathode. The enhanced photoabsorption and reduced electron transport distance for the nanostructured photocathode enhance its QE compared to that for the flat surface wafer.

High voltage dc gun for high intensity polarized electron source

The high intensity polarized electron source is a critical component for future nuclear physics facilities. The Electron Ion Collider (EIC) requires a polarized electron gun with higher voltage and higher bunch charge compared to any existing polarized electron source. At Brookhaven National Laboratory, we have built an inverted high voltage direct current (HVDC) photoemission gun with a large cathode size. We report on the performances of GaAs photocathodes in a high gradient with up to a 16 nC bunch charge. The measurements were performed at a stable operating gap voltage of 300 kV - demonstrating outstanding lifetime, and robustness. We observed obvious lifetime enhancement by biasing the anode. The gun also integrated a cathode cooling system for potential application on high current electron sources. The various novel features implemented and demonstrated in this polarized HVDC gun open the door towards future high intensity-high average current electron accelerator facilities.

Picometer scale electron microscopy exploration for the functionality origin of functional materials

The functionality of functional materials originates from field and local symmetry. The local symmetry in matter science is described by four degrees of freedom: Lattice, charge, orbit and spin. To understand and use the rich physical properties brought by the structural distortion of atomic nearest neighbors and second nearest neighbors under symmetry breaking, we need to grasp the fine structure of matter at the picometer scale. This paper reviews the exploration of the origin of functional materials by picometer scale electron microscopy in the three cases in the lattice degree of freedom, the expansion and contraction of the polyhedron, the tilt and rotation of the polyhedron and the cation displacement. In the expansion and contraction of the polyhedron degree of freedom, we use picometer scale scanning transmission electron microscopy (STEM) studies of lithium ion battery materials, which can be classified as the unit cell breathing model and Jahn-Teller effect, to illustrate the relationship between functionalities and structural origins. In the tilt and rotation of the polyhedron degree of freedom, we mainly focus on the heterointerface among functional oxides. The emergent phenomena arising from this heterointerface form most of the topics of modern condensed matter physics, such as high temperature superconductivity, magnetoresistance, antiferromagnetic, multiferroics, etc. Lattice mismatch caused polyhedron tilt and rotation alter the local symmetry of the oxides, and hence various functionalities are created. In terms of cation displacement degree of freedom, ferroelectricity is the typical example to show the structure-property relationship. The most exciting future of ferroelectric material is the potential to be the next generation memory material because of its topological property. With the help of aberration corrected (AC)-STEM, flux closure type and vortex type ferroelectric domain were directly observed at picometer accuracy. Furthermore, through in-situ mechanical and electric force studies, reverse transition from topological and normal ferroelectric state was observed, which paves the way of the application of the ferroelectric material as future memory material system. At the electronic structure level, the charge structure under the picometer-scale lattice structure is reviewed. Through the simultaneously recorded EELS data, we can know the valence state and content fluctuation of certain element. For example, in lithium ion battery materials, atomic and electronic structure changes all the time during the whole working duration. It is crucial to know the local valence state of the transition metal elements. AC-STEM and electron energy loss spectroscopy (EELS) can help us to solve this problem under one time acquisition. And future perspectives of the electron microscopy research on the orbit and spin structure with high spacial resolution are discussed, such as monochromatic EELS, electron-energy-loss magnetic circular dichroism (EMCD), electron-energy-loss magnetic linear dichroism (EMLD) and convergent beam electron diffraction (CBED). CBED is considered to be the best way to discover the orbit structure with high spacial resolution. In terms of spin structure, EMCD and EMLD are not the direct methods to detect the electron spin, which limits the resolution and signal to noise ratio of acquired results. In the future, spin polarized electron source is considered as the proper way to discover the spin structure with high spacial resolution. Nowadays, spacial resolution is not the limit of the TEM. The ability of accessing more kinds of materials, powerful in-situ method and acquiring orbit and spin structure with picometer resolution are goals for us.

A Compton Backscattering Polarimeter for Electron Beams below 1 GeV

A recently installed polarized electron source will allow internal target experiments to be performed with polarized electrons at the NIKHEF Internal Target Hall, lb measure the longitudinal component of the polarization vector of the stored electron beam, a Polarimeter based on spin-dependent Compton scattering has been developed and successfully commissioned

High intensity polarized electron source

A proposed new high-luminosity electron–ion collider requires a polarized electron source of extremely high intensity. The MIT-Bates Laboratory, in collaboration with Brookhaven National Laboratory (BNL), has developed a new polarized electron gun that can be operated at currents in the mA range. This paper describes the design of the gun and beam line and also presents the results of the beam tests.

Magnetic domain wall contrast under zero domain contrast conditions in spin polarized low energy electron microscopy

Important applications of spin polarized low energy electron microscopy (SPLEEM) employ this technique's vector imaging capability to resolve domain wall (DW) spin textures. Studying several thin film systems including Co/W(110), Co/Cu(001) and (Co/Ni)n/W(110), we show that an additional contrast can appear at magnetic DWs. By imaging the magnetization as a function of electron landing energy, electron energies are selected at which the magnetic domain contrast vanishes. Surprisingly, under such conditions of zero contrast between magnetic domains, we observe the appearance of magnetic contrast outlining the DWs. This DW contrast does not depend on the DW spin texture. Instead, our measurements show that this DW contrast results from a combination of the energy-dependence of the spin reflectivity asymmetry of the magnetic film, the finite energy width of the spin polarized electron source, and the dispersion of the magnetic prism array that separates the illumination and imaging columns of the instrument. Awareness of this DW contrast mechanism is useful to aid correct interpretation of SPLEEM images.

Nonevaporable getter coating chambers for extreme high vacuum

Techniques for nonevaporable getter (NEG) coating a large diameter chamber are presented along with vacuum measurements in the chamber using several pumping configurations, with base pressure as low as 1.56 × 10−12 Torr (N2 equivalent) with only a NEG coating and a small ion pump. The authors then describe modifications to the NEG coating process to coat complex geometry chambers for ultracold atom trap experiments. Surface analysis of NEG coated samples is used to measure the composition and morphology of the thin films. Finally, pressure measurements are compared for two NEG coated polarized electron source chambers: the 130 kV polarized electron source at Jefferson Lab and the upgraded 350 kV polarized electron source, both of which are approaching or within the extreme high vacuum range, defined as P < 7.5 × 10−13 Torr.

Transition metals doped and encapsulated ZnO nanotubes: Good materials for the spintronic applications

Among the ferromagnetic II–VI semiconductors, the (Zn,TM)O diluted magnetic semiconductors (DMSs) is the most promising for application in spintronic, since the high Curie temperature. In this work, the structural, electronic and magnetic properties of pristine armchair (5,5) and zigzag (5,0) ZnO nanotubes (ZnONTs) and doped with a single and a pair 3d transition metals (TMs) were studied based on density functional theory (DFT) method. Moreover a single TM encapsulated inside twice unit cell of (5,5) ZnONT are systematically investigated. The both doped and encapsulated systems are found exceptionally stable and their formation is exothermic. It is revealed that the TM magnetic moment increases first and then decreases, and reaches the maximum value when Mn is doped into ZnONTs. The values of magnetic moment and their variation trend versus the atomic number are similar for 3d TM-doped (5,0) and (5,5) ZnONTs, indicating they are independent of the chirality of tubes. Additionally, the Fe-doped (5,5) and Cu@(5,5) ZnONT with half-metal and thus 100% spin polarization characters seem to be good candidates for spintronic applications. Our research can provide guidance for the experiment upon DMSs and systemic investigation in 3d TMs. We suggest that ZnONTs especially armchair type, doped by TMs would have application potential as a spin polarized electron source for spintronic devices in the future.

Acceleration of polarized electrons UPTO ultrahigh energies

A wide world discussion have been opened few years ago about future e + e − collider after the Higgsboson discovery. Besides utterly high luminosity this machine has to operate with polarized beams. We shall overview in this paper problems and practical possibilities to satisfy second requirements of the future collider. The radiative beam polarization at this 100 km machine will be very long procedure. On other side, at the present time there are developed intensive polarized electron sources based on ArGa photo cathodes with polarization about 90 percents. We show, that fast electron synchrotron equipped pair Siberian Snake is able to provide to accelerate polarized electrons up to the top energy of the collider.

Reply to ``Comment on `Optically pumped spin-exchange polarized-electron source' ''

In the proceeding Comment [1] on our recent report of a Rb spin-exchange polarized-electron source [2], Williams et al. contend: (a) that our source is poorly characterized compared with modern GaAs sources, (b) that we have overstated the difficulties of using GaAs photoemission sources, and (c) that our explanation of various physics issues related to the source's operating principles are not cogent.

Comment on “Optically pumped spin-exchange polarized-electron source”

A of spin-polarized current with a 24% polarization and a 2-eV energy spreaddepending markedly on tenuous roles of various buffer gases which we consider are not explained cogently.Spin-related beam experiments over many decades have significant achievements using GaAs spin sources.DOI: 10.1103/PhysRevA.91.056703 PACS number(s): 34

Nanoscale pressure sensors realized from suspended graphene membrane devices

We study the transport properties of graphene layers placed over ∼200 nm triangular holes via attached electrodes under applied pressure. We find that the injected current division between counter electrodes depends on pressure and can be used to realize a nanoscale pressure sensor. Estimating various potential contributions to the resistivity change of the deflected graphene membrane including piezoresistivity, changing gate capacitance, and the valley Hall effect due to the pressure-induced synthetic magnetic field, we find that the valley Hall effect yields the largest expected contribution to the longitudinal resistivity modulation for accessible device parameters. Such devices in the ballistic transport regime may enable the realization of tunable valley polarized electron sources.

MTV-G experiment: Probing non-standard strong gravitational field at nuclear scale using geodetic precession

A new experiment named MTV-G, probing a large electron spin-precession due to a possible strong gravitational field, which predicted by large extra dimension model, is started at TRIUMF from 2011. In an electron-nuclear scattering experiment, a strong gravitational field is tested as a large spin precession effect caused by geodetic precession predicted by general relativity theory as a result of a warped space-time around nuclei. Experimental design using spin polarized electron source and Mott-spin analyzer, commissioning experiment and the preliminary results are described.

Optically pumped spin-exchange polarized-electron source

We describe the operation of a prototype polarized-electron source. Rubidium vapor, contained in a cell, is optically pumped in the presence of a buffer gas. Unpolarized electrons from a tungsten filament are injected into the cell and extracted after undergoing spin exchange with the Rb atoms. We compare the performance of the source when different buffer gases are used. We measure a decrease in electron polarization as their injection energy increases, but find an unexpected regime at higher injection energies yielding increased electron polarization accompanied by a 40-fold increase in current, suggesting the production of slow secondary electrons in the target cell. With ethylene, we have measured electron currents of 4 \ensuremath{\mu}A simultaneously with electron polarizations of 24%. This work offers the promise of a simple, benchtop, ``turnkey'' source of polarized electrons.