Large Perpendicular Electric Fields Research Articles

Enhanced superconductivity in spin-orbit proximitized bilayer graphene.

In the presence of a large perpendicular electric field, Bernal-stacked bilayer graphene (BLG) features several broken-symmetry metallic phases1-3 as well as magnetic-field-induced superconductivity1. The superconducting state is quite fragile, however, appearing only in a narrow window of density and with a maximum critical temperature Tc ≈ 30 mK. Here we show that placing monolayer tungsten diselenide (WSe2) on BLG promotes Cooper pairing to an extraordinary degree: superconductivity appears at zero magnetic field, exhibits an order of magnitude enhancement in Tc and occurs over a density range that is wider by a factor of eight. By mapping quantum oscillations in BLG-WSe2 as a function of electric field and doping, we establish that superconductivity emerges throughout a region for which the normal state is polarized, with two out of four spin-valley flavours predominantly populated. In-plane magnetic field measurements further reveal that superconductivity in BLG-WSe2 can exhibit striking dependence of the critical field on doping, with the Chandrasekhar-Clogston (Pauli) limit roughly obeyed on one end of the superconducting dome, yet sharply violated on the other. Moreover, the superconductivity arises only for perpendicular electric fields that push BLG hole wavefunctions towards WSe2, indicating that proximity-induced (Ising) spin-orbit coupling plays a key role in stabilizing the pairing. Our results pave the way for engineering robust, highly tunable and ultra-clean graphene-based superconductors.

Transport Studies of Dual-Gated ABC and ABA Trilayer Graphene: Band Gap Opening and Band Structure Tuning in Very Large Perpendicular Electric Fields

We report on the transport properties of ABC and ABA stacked trilayer graphene using dual, locally gated field effect devices. The high efficiency and large breakdown voltage of the HfO(2) top and bottom gates enable independent tuning of the perpendicular electric field and the Fermi level over an unprecedentedly large range. We observe a resistance change of 6 orders of magnitude in the ABC trilayer, which demonstrates the opening of a band gap. Our data suggest that the gap saturates at a large displacement field of D ~ 3 V/nm, in agreement with self-consistent Hartree calculations. In contrast, the ABA trilayer remains metallic even under a large perpendicular electric field. Despite the absence of a band gap, the band structure of the ABA trilayer continues to evolve with increasing D. We observe signatures of two-band conduction at large D fields. Our self-consistent Hartree calculation reproduces many aspects of the experimental data but also points to the need for more sophisticated theory.

Auroral hot-ion dynamo model with finite gyroradii

Discrete auroras have (1) narrow size s≲30km in at least one dimension (e.g., north-south) and (2) often rapid variation of forms, especially where the size is extremely small, s⩽1km. These points mesh with spatial and temporal features observed at several Earth radii in earthward flows (bursts) of hot plasma along high-latitude geomagnetic field lines. The flows (include PSBL) usually have some filamentary structure with transverse widths of a few local gyroradii of the hot protons (kT∼1–30keV), i.e., widths that encompass auroral-arc size when scaled by magnetic field-line separation. At these widths, modest density gradients (δn∼0.01–0.1cm−3) lead to charge separation by differential mirroring of hot protons and electrons and large perpendicular electric fields. Thermal escape of ionspheric electrons into positive charge layer builds up magnetic field-aligned potential difference that accelerates hot electrons from negative charge layer into the ionosphere within auroral arc thickness. As a corollary, the model delineates a mechanism for charge-driven plasma instabilities.

Comparison of electric fields and density structures seen in simulations and satellite observations

[1] There are now clear observations of large parallel electric fields (E∥) from Polar and FAST in the auroral upward-current region (UCR). Such fields in some events are associated with large perpendicular electric fields (E⊥) and density cavities while in others they could occur without them. The observed features of the fields and density are found to be compatible with the spatial structures seen in a simulated U-shaped potential structure (PS) using a 2.5D electrostatic code. The spatial and temporal evolution of the U-shaped PS (USPS) including its upward and downward motions and its striations into thin long potential structures suggest a variety of signatures in the satellite detection of E∥ and E⊥.

Small‐scale electric fields in downward auroral current channels

The origin and spatiotemporal properties of small‐scale, intense electric fields and currents commonly observed by low‐altitude, polar‐orbiting satellites in the vicinity of discrete auroral arcs are investigated numerically. It is shown that these electromagnetic structures can be generated by the development of ionospheric feedback instability inside the resonant cavity formed by the ionospheric E‐layer and an auroral acceleration region. The major factor regulating feedback unstable dynamics is the downward current channel of a large‐scale, slowly evolving auroral current system interacting with the ionosphere. The downward current lowers the instability threshold by depleting the E‐region plasma density and conductivity, thereby allowing the development of a large perpendicular electric field in the E‐layer. The downward field‐aligned current also produces a collisionless resistive layer in the lower magnetosphere where the parallel drift speed of current‐carrying electrons exceeds a critical threshold for onset of microinstability. This resistive layer provides a highly reflective upper boundary for the resonant cavity confining small‐scale Alfvén waves generated at the ionosphere by the feedback mechanism. The quality of the ionospheric resonator without the turbulent upper boundary is shown to be very poor for resonator modes with transverse wavelengths of the order of 10 km. Application of the simulation results to the interpretation of satellite measurements of small‐scale electromagnetic structures in the auroral zone is demonstrated.

Properties of large electric fields in the plasma sheet at 4–7 RE measured with Polar

Measurements from the Polar satellite provide evidence for large electric field structures in the plasma sheet at geocentric distances of 4–7 RE‐ These structures had amplitudes perpendicular to the ambient magnetic field that can exceed 100 m V m−1 (6 s averaged). Two years (from May 1, 1996, to April 30, 1998) of electric field data (EZ component, approximately along GSE z) were surveyed. The distribution in invariant latitude (ILAT) and magnetic local time (MLT) of large perpendicular electric field events (defined as ≥20 mV m−1 for a 6‐s average) delineates the statistical auroral oval with the majority of events occurring in the nightside centered around midnight and a smaller concentration around 1500 MLT. The magnitude‐versus‐altitude distribution of the electric fields between 4 and 7 RE in the nightside could be explained by models which assume either shear Alfvén waves propagating into regions of larger background magnetic fields or electrostatic structures being mapped quasi‐statically along equipotential magnetic field lines. In addition, this survey yielded 24 very large amplitude events with |E⊥| ≥100 mV m−1 (6 s averaged), all of which occurred in the nightside. In the spacecraft frame, the electric field structures occurred on timescales ranging from 10 to 60 s. About 85% of these events occurred in the vicinity of the outer boundary of the plasma sheet; the rest occurred in the central plasma sheet. The polarity of the electric fields was dominantly perpendicular to the nominal plasma sheet boundary. For a large fraction of events (≤50%) the ratios of electric and magnetic fields in the period range from 10 to 60 s were consistent with Alfvén waves. Large Poynting flux (up to 2.5 ergs cm−2s−1) dominantly directed downward along the background magnetic field was associated with 21 events. All 24 events occurred during geomagnetic disturbances such as magnetic substorms. A conjugate study with ground stations for 14 events (out of the 24 events) showed that these structures occurred during times of rapid changes in the H component (or X component) of magnetometer data. For most events this time corresponded to the expansion phase; two events occurred during a quick recovery of the negative H bay signature. Thus there is evidence that large electromagnetic energy transfer processes in the plasma sheet occur during the most dynamic phase of geomagnetic disturbances. From the statistical analysis it was found that Polar observed events larger than 100 mV m−1 (50 mV m−1) in the plasma sheet between 2100 and 0300 MLT with a 2–4% (15%) probability per crossing. These probabilities will be compared to the probability of substorm occurrence during Polar plasma sheet crossings.

How does the U-shaped potential close above the acceleration region? A study using Polar data

Abstract. We present a statistical study of Polar electric field observations using auroral oval passes over Scandinavia above the acceleration region. We are especially interested in seeing whether we can find large perpendicular electric fields associated with an upward extended classical U-shaped potential drop for these passes, during which Polar is in the northern hemisphere usually at about 4 RE altitude. We also use Polar magnetic field data to infer the existence of a field-aligned current (FAC) and conjugate ground-based magnetometers (the IMAGE magnetometer network) to check whether the event is substorm-related or not. We find several events with a FAC but only weak perpendicular electric fields at Polar. In those rare cases where the Polar electric field was large, its direction was mostly found to be incompatible with the U-shaped potential model, or it was associated with disturbed conditions (substorms), where one cannot easily distinguish between inductive and static perpendicular electric fields. We found only two cases which are compatible with the upward extended U-shaped potential picture, and even in those cases the potential value is quite small (1-2 kV). To check the validity of the analysis method we also estimate the perpendicular electric field on the southern hemisphere, where Polar flies within or below the acceleration region, and we found a large number of inverted-V-type signatures as expected from previous studies. To explain the lack of perpendicular electric fields at high altitudes we suggest an O-shaped potential model instead of the U-shaped one.Key words. Ionosphere (particle acceleration) · Magnetospheric physics (auroral phenomena; magnetosphere · ionosphere interactions)

How does the U-shaped potential close above the acceleration region? A study using Polar data

We present a statistical study of Polar electric field observations using auroral oval passes over Scandinavia above the acceleration region. We are especially interested in seeing whether we can find large perpendicular electric fields associated with an upward extended classical U-shaped potential drop for these passes, during which Polar is in the northern hemisphere usually at about 4 RE altitude. We also use Polar magnetic field data to infer the existence of a field-aligned current (FAC) and conjugate ground-based magnetometers (the IMAGE magnetometer network) to check whether the event is substorm-related or not. We find several events with a FAC but only weak perpendicular electric fields at Polar. In those rare cases where the Polar electric field was large, its direction was mostly found to be incompatible with the U-shaped potential model, or it was associated with disturbed conditions (substorms), where one cannot easily distinguish between inductive and static perpendicular electric fields. We found only two cases which are compatible with the upward extended U-shaped potential picture, and even in those cases the potential value is quite small (1-2 kV). To check the validity of the analysis method we also estimate the perpendicular electric field on the southern hemisphere, where Polar flies within or below the acceleration region, and we found a large number of inverted-V-type signatures as expected from previous studies. To explain the lack of perpendicular electric fields at high altitudes we suggest an O-shaped potential model instead of the U-shaped one.Key words. Ionosphere (particle acceleration) · Magnetospheric physics (auroral phenomena; magnetosphere · ionosphere interactions)

Numerical simulation of the formation of thin ionization layers at high latitudes

A one‐dimensional simulation of high latitude sporadic‐E layers has been developed to investigate the role of electric field in layer formation. The simulation model computes the ion densities (O+, O2+, N+, N2+, NO+, Fe+) and temperatures as a function of altitude. The stationary state momentum equation and continuity equation is solved for each ion species. Then the energy equation is solved for electrons, neutrals, and a generic ion having the mean ion mass and velocity. The model does not include any photoionization or particle precipitation. Chemical production and loss are included for the five standard ion species, but not for the Fe+ ions. An initial altitude profile is assumed for the ions, and the model computes the altitude distribution as a function of time. The horizontal direction of the electric field is important for determining the layer structure and its altitude. An electric field directed between magnetic west and north results in convergent plasma flow and thus layer formation at about 120–130 km altitude. For fields directed to the west and south layers form at lower altitudes(100–115km). With an assumed dip angle of 75° the layers formed by the N‐W quadrant fields form quickly (<10min), are quite thin, and remain stationary in altitude. Layers formed by S‐W quadrant fields are initially thick but compress in time (15–20 min). Additionally they move down in altitude as they compress. The large perpendicular electric field (50 mV/m) used in the simulation leads to significant heating of the ions and to a smaller extent the electrons and neutrals. The layer formed by the field directed in the N‐W quadrant occurs in the region of significant heating, and the temperature profile shows some enhancement at the layer altitude.

Simulations of auroral plasma processes: Electric fields, waves and particles

Satellite and rocket observations have revealed a host of auroral plasma processes, including large dc perpendicular electric fields ( E ⊥) associated with electrostatic shocks, relatively weak parallel electric fields ( E ∥) associated with double layers, upflowing ions in the form of beams and conics, downflowing and upflowing accelerated electron beams, several wave modes such as the elctrostatic ion-cyclotron (EIC), lower hybrid (LH), VLH, ELF and high frequency waves and associated non-linear phenomena. We have attempted to simulate these various processes using a two-dimensional particle-in-cell code. In these simulations the plasma is driven by current sheets (carrying upward currents) of a finite thicknesses. Striking similarities between the observed auroral plasma processes and those seen in the simulations are found. Large perpendicular electric fields ( E ⊥ e ) develop near the edges of the current sheets, which have features similar to electrostatic shocks. Double layer formation occurs inside the current sheets and also outside the sheet where downward current flows. The polarities of the parallel electric fields of the double layers inside and outside the current sheet are found to be mainly upward and downward, respectively. The double layer electric field strength ( E ∥) is found to be highly variable, depending on the plasma parameters. However, it is generally found that E ⊥ m ⪢ and E ∥ m where, E ⊥ m and E ∥ m are respectively, the maximum strengths of E ⊥ and E ∥ seen in the simulations irrespective of their relative locations. Normally, the electric field strength of double layers in larger than the parallel electric fields occurring at the locations where E ⊥ e maximizes. The non-linear phenomenon associated with the waves near the electron plasma frequency is found to cause a low frequency relaxation-type of oscillition in the potential structure. The low frequency waves seen in the simulations range from below the ion-cyclotron frequency to above the lower hybrid frequency. In some simulations, spiky ion-cyclotron waves are seen. Ion beam formation with a positive slope in the parallel velocity distribution function is seen only in a narrow currentsheet with a thickness l ∼ p i, where p i is an appropriate ion Larmor radius. In wide sheets l ⪢ p i; the upflowing ions are found to be considerably heated. The most energetic ions seen in the simulations have pitch angles near 90°, implying perpendicular ion acceleration, which is the main mechanism for the generation of ion conics. The inverted-V feature in the horizontal distribution of the downflowing accelerated electrons is seen. However, the accelerated electrons are found to be neither monoenergetic nor perfectly field-aligned. Double layers in the current sheets are found to be the triggering mechanism for the plasma processes which shape the distribution function of the accelerated electrons. Counterstreaming accelerated electrons are also seen. Upward electron beams in the downward current regions are found to be nearly a permanent feature of the simulations with wide sheets. On the other hand, in simulations with narrow sheets such beams are transient.

Plasma Processes Driven by Current Sheets and Their Relevance to the Auroral Plasma

Satellite and rocket observations have revealed a host of auroral plasma processes, including large dc perpendicular electric fields (E⊥) associated with electrostatic shocks, relatively weak parallel electric fields (E║) associated with double layers, upflowing ions in the form of beams and conics, downflowing and upflowing accelerated electron beams, several wave modes such as the electrostatic ion-cyclotron (EIC), lower hybrid (LH), very low frequency (VLF), extremely low frequency (ELF), and high-frequency waves and associated nonlinear phenomena. Recently, we have attempted to simulate the various processes using a two-dimensional particle-in-cell code in which the plasma is driven by current sheets of a finite thickness. Striking similarities between the observed auroral plasma processes and those seen in the simulations are found. In this paper we give a review of the plasma processes dealing with dc and ac electric fields, formation of ion beams and conics, and electron acceleration. Electrostatic shock-type electric fields (E┴e) occur near the current sheet edges. Such fields arise because of the contact between the high-and low-density plasmas inside and outside the sheet, respectively. Double layers having upward electric fields form inside the sheet and they are distinguishable from the large perpendicular electric fields (E⊥e) only in wide sheets with thicknesses l >> ρi, the ion Larmor radius. Double layers with a reverse polarity form outside the sheet where downward currents flow. The most energetic ions are found to have pitch angles near 90°, implying a large perpendicular acceleration of the ions.

Classification of auroral electrostatic shocks by their ion and electron associations

We have examined ion data associated with 1073 electrostatic shocks (regions of large perpendicular electric field) below 8000 km altitude and have found that at least 85% of these shocks are associated with upflowing ions in the form of either ion beams or ion conics. Analysis of these data indicates that significant differences exist between the distributions of shocks associated with ion beams and shocks associated with ion conics. Shocks associated with ion beams are concentrated in the evening, while shocks associated with ion conics are observed with comparable frequency at all MLTs. Although both types of shocks are seen more frequently at higher altitudes than at lower altitudes, the probability of seeing a shock increases more rapidly with altitude for shocks associated with ion beams than for shocks associated with ion conics. Shocks associated with ion beams have not been seen below 3000 km. Shocks associated with ion beams have, on the average, larger electric fields than shocks associated with ion conics. Furthermore, the average electric field tends to increase with altitude for shocks associated with ion beams. Shocks are far more likely to occur with ion conics rather than with ion beams during periods of high Kp. Ion beams associated with electrostatic shocks typically have higher energies than ion conics associated with electrostatic shocks. Also, examination of the energy spectra of electrons associated with these events revealed that 86% of the electrostatic shocks associated with ion beams have peak electron energies above 360 eV, while 66% of the electrostatic shocks associated with ion conics have peak electron energies below 360 eV. The average peak electron energy is somewhat higher than the peak ion energy. Most shocks associated with ion beams are either S shocks or V shocks. Although the peak ion energy of the ion beam is typically larger than the potential through the shock, there is a fair correlation between the two. The direction of the electric field of the shock is strongly correlated with the location of the ion beam. Of the shocks associated with ion beams, V shocks have the largest and most coherent structures, followed by S shocks and embedded shocks. Shocks associated with ion conics have smaller and less coherent structures than shocks associated with ion beams. The invariant latitude magnetic local time distribution of electrostatic shocks approximates an offset circle with a radius that has a linear dependence on Kp and an offset that is inversely related to Kp.

Ion‐neutral coupling in the high‐latitude F region: Evaluation of ion heating terms from Dynamics Explorer 2

We study a set of south (summer) polar passes from the Dynamics Explorer 2 data base that provide measurements of ion and neutral velocities along the track of the spacecraft for various geomagnetic conditions. The ion velocities are obtained from the ion drift meter and the retarding potential analyzer instruments; the neutral velocities are obtained from the Fabry‐Perot interferometer and the wind and temperature spectrometer instruments. These data enable a direct determination of the in situ ion‐neutral difference velocity. Simultaneous measurements of atmospheric composition using the neutral atmosphere composition spectrometer and of electron temperatures and densities using the Langmuir probe enable the ion heating and cooling rates to be determined. The rates are discussed in relationship to values for the electron, ion, and neutral kinetic temperatures. The comprehensive nature of the data set allows for a quantitative study of ion‐neutral coupling and, in particular, affords a more rigorous test of the common assumptions used in the ion energy equation than has been previously possible. From our study we find the following: (1) The ion‐neutral difference velocities, which drive the Joule heating, have a complex morphology that depends on the stability of the high‐latitude ion convection and the inertia of the neutral gas. (2) Ionospheric “hot spots” are observed to be related to large perpendicular electric fields that give rise to frictional (Joule) heating. (3) These events are often seen to occur in conjunction with troughs in the electron density and with decreases in the ratio of atomic oxygen to molecular nitrogen. In such cases, momentum transfer to the neutral gas is reduced, and the ion‐neutral velocity difference and associated Joule heating can be maintained over longer time periods. (4) An approximate balance between local frictional heating of the ions and local cooling of the ions to the neutrals is seen in regions of strong ion convection. The commonly used approximate form of the ion energy equation involving only local processes appears at times, however, to be inadequate for large regions of the summer polar cap. The ion temperatures observed in these regions are higher than expected from simple local heating/local cooling arguments. Additional source(s) of ion heating would be required to provide energy balance.