Hole Asymmetry Research Articles

The Association of Filaments, Polarity Inversion Lines, and Coronal Hole Properties with the Sunspot Cycle: An Analysis of the McIntosh Database

Abstract Filaments and coronal holes, two principal features observed in the solar corona, are sources of space weather variations. Filament formation is closely associated with polarity inversion lines (PILs) on the solar photosphere which separate positive and negative polarities of the surface magnetic field. The origin of coronal holes is governed by large-scale unipolar magnetic patches on the photosphere from where open magnetic field lines extend to the heliosphere. We study the properties of filaments, PILs, and coronal holes in solar cycles 20, 21, 22, and 23 utilizing the McIntosh archive. We detect a prominent cyclic behavior of filament length, PIL length, and coronal hole area with significant correspondence with the solar magnetic cycle. The spatio-temporal evolution of the geometric centers of filaments shows a butterfly-like structure and distinguishable poleward migration of long filaments during cycle maxima. We identify this rush to the poles of filaments to be co-temporal with the initiation of polar field reversal as gleaned from Mount Wilson and Wilcox Solar Observatory polar field observations, and quantitatively establish their temporal correspondence. We analyze the filament tilt angle distribution to constrain their possible origins. The majority of the filaments exhibit negative and positive tilt angles in the northern and the southern hemispheres, respectively, strongly suggesting that their formation is governed by the overall large-scale magnetic field distribution on the solar photosphere and not by the small-scale intra-active region magnetic field configurations. We also investigate the hemispheric asymmetry in filaments, PILs, and coronal holes. We find that the hemispheric asymmetry in filaments and PILs is positively correlated with sunspot area asymmetry, whereas coronal hole asymmetry is uncorrelated.

Impact of single-mask hole asymmetry on the properties of GEM detectors

A single-mask Gas Electron Multiplier (GEM) technique overcomes the cumbersome practice of alignment of two masks and allows the production of foils with very large area as needed for the CMS muon forward region upgrade. However, the holes obtained with refinements in the single-mask technique are asymmetrically bi-conical in shape compared to symmetrically bi-conical holes of double-mask technology. The hole geometry and their uniformity define the performance of the detectors which are constructed with such GEM foils. To evaluate the effect of this asymmetry, the foils have been characterized experimentally using a special prototype with three single-mask GEM foils. The structure allowed to change the orientation of foils, testing from above with foils having a large hole opening, testing from the bottom with all the foils having small hole opening. The effective gain, energy resolution and the charging up behavior are compared for the two different hole orientations.

Natural orbital-based Lanczos method for Anderson impurity models

We implement the Lanczos algorithm on natural orbital basis to solve the zero-temperature Green’s function of Anderson impurity models, following the work of Lu et al. (2014). We present the technical details, generalize the algorithm to the cases of particle–hole asymmetry, with local magnetic field, and of two impurities. The results are benchmarked with conventional Lanczos, quantum Monte Carlo, and numerical renormalization group methods, demonstrating its potential as a powerful impurity solver for the dynamical mean-field theory.

Topology‐Controlled Thermopower Oscillations in Multiterminal Andreev Interferometers

We theoretically investigate coherent oscillations of the thermopower S as a function of the magnetic flux Ф in six‐terminal Andreev interferometers. We demonstrate that the thermopower behavior is determined by a number of contributions originating from the Josephson‐ and Aharonov–Bohm‐like effects as well as from electron–hole asymmetry. The relative weight of these contributions depends on the relation between temperature, voltage bias, and an effective Thouless energy of our setup. We particularly emphasize the role of the system topology that may have a dramatic impact on the behavior of S(Ф).

Spin-flip enhanced thermoelectricity in superconductor-ferromagnet bilayers

We study the effects of spin-splitting and spin-flip scattering in a superconductor (S) on the thermoelectric (TE) properties of a tunneling contact to a metallic ferromagnet (F) using the Green’s function method. A giant thermopower has been theoretically predicted and experimentally observed in such structures. This is attributed to the spin-dependent particle–hole asymmetry in the tunneling density of states (DOS) in the S/F heterostructure. Here, we evaluate the S DOS and thermopower for a range of temperatures, Zeeman-splitting, and spin-flip scattering. In contrast to the naive expectation based on the negative effect of spin-flip scattering on Cooper pairing, we find that the spin-flip scattering strongly enhances the TE performance of the system in the low-field and low-temperature regime. This is attributed to a complex interplay between the charge and spin conductances caused by the softening of the spin-dependent superconducting gaps. The maximal value of the thermopower exceeds kB/e by a factor of ≈5 and has a non-monotonic dependence on spin-splitting and spin-flip rate.

Ambipolar ferromagnetism by electrostatic doping of a manganite

Complex-oxide materials exhibit physical properties that involve the interplay of charge and spin degrees of freedom. However, an ambipolar oxide that is able to exhibit both electron-doped and hole-doped ferromagnetism in the same material has proved elusive. Here we report ambipolar ferromagnetism in LaMnO3, with electron–hole asymmetry of the ferromagnetic order. Starting from an undoped atomically thin LaMnO3 film, we electrostatically dope the material with electrons or holes according to the polarity of a voltage applied across an ionic liquid gate. Magnetotransport characterization reveals that an increase of either electron-doping or hole-doping induced ferromagnetic order in this antiferromagnetic compound, and leads to an insulator-to-metal transition with colossal magnetoresistance showing electron–hole asymmetry. These findings are supported by density functional theory calculations, showing that strengthening of the inter-plane ferromagnetic exchange interaction is the origin of the ambipolar ferromagnetism. The result raises the prospect of exploiting ambipolar magnetic functionality in strongly correlated electron systems.

The axial anomaly in chiral tilted Weyl semimetals

The axial anomaly is investigated based on Fujikawa’s method for chiral tilted Weyl semimetals, in which the Weyl cones with different chiralities tilt to opposite directions. When the electric field (EF) is parallel to the magnetic field (MF), the axial anomaly contributed by electromagnetic fields along tilt direction is reduced less than that along other directions. This results in that the associated effects, such as anomalous Hall effect, chiral magnetic effect and negative magnetoresistivity, are suppressed anisotropically. More interestingly, when the EF is perpendicular to MF, the axial anomaly has a new contribution from the mixed product of tilt vector, EF and MF, which is asymmetry under reversing tilt direction (electron–hole asymmetry) and leads to a current between Dirac cones with different tilt directions and novel magnetoresistivity (MR). Specifically, this contribution can change the properties of MR in some experimental establishments and makes trivial components of MR in un-tilted Weyl semimetal develop non-trivial properties. These changes are consistent with the MR phenomena observed in experiments, such as gate-tunable negative longitudinal MR, Hall MR, extremely large and non-saturating MR.

Josephson current through a quantum dot molecule with a Majorana zero mode and Andreev bound states

Based on the Green's function method, we investigate the interplay between Majorana zero mode (MZM) and Andreev bound states (ABSs) in a quantum dot molecule side coupled to a topological superconducting nanowire with a pair of MZMs forming a Josephson junction. Since the strong electron–hole asymmetry induced by the nanowire with a topologically non-trivial phase, the MZM suppress the ABSs. The suppression induced by the MZM is robust against the Coulomb repulsion. The interplay between the MZM and the ABSs in Josephson junction presents a feasible experimental means for distinguish between the presence of MZM and ABSs.

DMFT Study on the Electron–hole Asymmetry of the Electron Correlation Strength in the High Tc Cuprates

Recent experiments revealed a striking asymmetry in the phase diagram of the high temperature cuprate superconductors. The correlation effect seems strong in the hole-doped systems and weak in the electron-doped systems. On the other hand, a recent theoretical study shows that the interaction strengths (the Hubbard U) are comparable in these systems. Therefore, it is difficult to explain this asymmetry by their interaction strengths. Given this background, we analyze the one-particle spectrum of a single band model of a cuprate superconductor near the Fermi level using the dynamical mean field theory. We find the difference in the "visibility" of the strong correlation effect between the hole- and electron-doped systems. This can explain the electron-hole asymmetry of the correlation strength without introducing the difference in the interaction strength.

Gigantic 2D laser-induced photovoltaic effect in magnetically doped topological insulators for surface zero-bias spin-polarized current generation

A new kind of 2D photovoltaic effect (PVE) with the generation of anomalously large surface photovoltage up to 210 meV in magnetically doped topological insulators (TIs) has been studied by the laser time-resolved pump-probe angle-resolved photoelectron spectroscopy. The PVE has maximal efficiency for TIs with high occupation of the upper Dirac cone (DC) states and the Dirac point located inside the fundamental energy gap. For TIs with low occupation of the upper DC states and the Dirac point located inside the valence band the generated surface photovoltage is significantly reduced. We have shown that the observed giant PVE is related to the laser-generated electron–hole asymmetry followed by accumulation of the photoexcited electrons at the surface. It is accompanied by the 2D relaxation process with the generation of zero-bias spin-polarized currents flowing along the topological surface states (TSSs) outside the laser beam spot. As a result, the spin-polarized current generates an effective in-plane magnetic field that is experimentally confirmed by the kII-shift of the DC relative to the bottom non-spin-polarized conduction band states. The realized 2D PVE can be considered as a source for the generation of zero-bias surface spin-polarized currents and the laser-induced local surface magnetization developed in such kind 2D TSS materials.

Transport spectroscopy in bilayer graphene using double layer heterostructures

We provide a comprehensive study of the chemical potential of bilayer graphene in a wide range of carrier density, at zero and high magnetic (B)-fields, and at different transverse electric (E)-fields, using high quality double bilayer graphene heterostructures. Using a direct thermodynamic transport spectroscopic technique, we probe the chemical potential as a function of carrier density in six samples. The data clearly reveal the non-parabolicity and electron–hole asymmetry of energy-momentum dispersion in bilayer graphene. The tight-binding hopping amplitudes, t0, t1, and t4, renormalized by electron–electron interaction are extracted from the chemical potential versus density dependence. A diverse set of electron–electron interaction driven phenomena were also clearly discerned at zero and high B-fields. We measure the gaps at integer fillings with orbital index N = 0, 1, and discuss about the dependence of the N = 0, 1 quantum Hall phases on the carrier density (or filling factor), E-field, and B-field.

Manipulating topological-insulator properties using quantum confinement

Recent discoveries have spurred the theoretical prediction and experimental realization of novel materials that have topological properties arising from band inversion. Such topological insulators are insulating in the bulk but have conductive surface or edge states. Topological materials show various unusual physical properties and are surmised to enable the creation of exotic Majorana-fermion quasiparticles. How the signatures of topological behavior evolve when the system size is reduced is interesting from both a fundamental and an application-oriented point of view, as such understanding may form the basis for tailoring systems to be in specific topological phases. This work considers the specific case of quantum-well confinement defining two-dimensional layers. Based on the effective-Hamiltonian description of bulk topological insulators, and using a harmonic-oscillator potential as an example for a softer-than-hard-wall confinement, we have studied the interplay of band inversion and size quantization. Our model system provides a useful platform for systematic study of the transition between the normal and topological phases, including the development of band inversion and the formation of massless-Dirac-fermion surface states. The effects of bare size quantization, two-dimensional-subband mixing, and electron–hole asymmetry are disentangled and their respective physical consequences elucidated.

Electron–hole asymmetry of the topological surface states in strained HgTe

Topological insulators are a new class of materials with an insulating bulk and topologically protected metallic surface states. Although it is widely assumed that these surface states display a Dirac-type dispersion that is symmetric above and below the Dirac point, this exact equivalence across the Fermi level has yet to be established experimentally. Here, we present a detailed transport study of the 3D topological insulator-strained HgTe that strongly challenges this prevailing viewpoint. First, we establish the existence of exclusively surface-dominated transport via the observation of an ambipolar surface quantum Hall effect and quantum oscillations in the Seebeck and Nernst effect. Second, we show that, whereas the thermopower is diffusion driven for surface electrons, both diffusion and phonon drag contributions are essential for the hole surface carriers. This distinct behavior in the thermoelectric response is explained by a strong deviation from the linear dispersion relation for the surface states, with a much flatter dispersion for holes compared with electrons. These findings show that the metallic surface states in topological insulators can exhibit both strong electron-hole asymmetry and a strong deviation from a linear dispersion but remain topologically protected.

Effects of Electron–Hole Asymmetry on Magnetic Response in Graphene

Electron–hole asymmetry present in the band structure of graphene in a higher-order k · p scheme is shown to cause a discrete jump in the orbital susceptibility at zero energy. Trigonal band-warpin...

Robust fractional quantum Hall effect in the N=2 Landau level in bilayer graphene.

The fractional quantum Hall effect is a canonical example of electron–electron interactions producing new ground states in many-body systems. Most fractional quantum Hall studies have focussed on the lowest Landau level, whose fractional states are successfully explained by the composite fermion model. In the widely studied GaAs-based system, the composite fermion picture is thought to become unstable for the N≥2 Landau level, where competing many-body phases have been observed. Here we report magneto-resistance measurements of fractional quantum Hall states in the N=2 Landau level (filling factors 4<|ν|<8) in bilayer graphene. In contrast with recent observations of particle–hole asymmetry in the N=0/N=1 Landau levels of bilayer graphene, the fractional quantum Hall states we observe in the N=2 Landau level obey particle–hole symmetry within the fully symmetry-broken Landau level. Possible alternative ground states other than the composite fermions are discussed.

Theory of the power characteristics of quantum-well lasers with asymmetric barrier layers: Inclusion of asymmetry in electron- and hole-state filling

The power characteristics of quantum-well lasers with asymmetric barrier layers, which represent a novel type of injection laser, are calculated on the basis of an extended model taking into account asymmetry in the filling of electron and hole states. The electron–hole asymmetry is shown to have no significant effect on the characteristics of these lasers. Even in the presence of intermediate layers (located between the quantum well and each of the two asymmetric barrier layers), where parasitic electron–hole recombination does occur, the internal differential quantum efficiency of such a laser exhibits only a weak dependence on the pump current and remains close to unity; therefore, the light–current characteristic remains linear up to high pumping levels.

Electron-hole asymmetry in the electron-phonon coupling in top-gated phosphorene transistor

Using in situ Raman scattering from phosphorene channel in an electrochemically top-gated field effect transistor, we show that phonons with Ag symmetry depend much more strongly on concentration of electrons than that of holes, wheras phonons with Bg symmetry are insensitive to doping. With first-principles theoretical analysis, we show that the observed electon–hole asymmetry arises from the radically different constitution of its conduction and valence bands involving π and σ bonding states respectively, whose symmetry permits coupling with only the phonons that preserve the lattice symmetry. Thus, Raman spectroscopy is a non-invasive tool for measuring electron concentration in phosphorene-based nanoelectronic devices.

Superfluidity of electron–hole pairs between two critical temperatures

We study a system of spatially separated electrons and holes, assuming the carriers are confined to two parallel planes. The existence of the superfluid state of electron–hole pairs between two critical temperatures is predicted for such system in a case of electron–hole asymmetry caused by the difference in the carrier masses and their chemical potentials. The stability of the superfluid state is studied with respect to the changes of the asymmetry between electrons and holes. It is found that one type of the asymmetry can compensate another one, so the superfluid state is possible in a wide range of the asymmetry parameters when they satisfy a simple linear equation.

Electron–Hole Asymmetry Driven Surface Charge Expulsion

We study an Ising-like dynamic Hubbard (IDHB) model using dynamical mean field theory with an embedding potential. The prominent characteristic of the IDHB model is the broken electron–hole symmetry. Our calculations indicate that the electron–hole asymmetry enhances electron expulsion toward the surface of a semi-infinite bulk. Since correlated hopping amplitude produces the electron–hole asymmetry, it strongly affects the electron expulsion toward the surface.

Cyclotron resonance in topological insulators: non-relativistic effects

The low-energy Hamiltonian used to describe the dynamics of the helical Dirac fermions on the surface of a topological insulator contains a subdominant non-relativistic (Schrödinger) contribution. This term can have an important effect on some properties while having no effect on others. The Hall plateaus retain the same relativistic quantization as the pure Dirac case. The height of the universal interband background conductivity is unaltered, but its onset is changed. However, the non-relativistic term leads directly to particle–hole asymmetry. It also splits the interband magneto-optical lines into doublets. Here, we find that, while the shape of the semiclassical cyclotron resonance line is unaltered, the cyclotron frequency and its optical spectral weight are changed. There are significant differences in both of these quantities for a fixed value of chemical potential or fixed doping away from charge neutrality depending on whether the Fermi energy lies in the valence or conduction band.