Transverse Currents Research Articles

The investigation of ferromagnetic resonance linewidth in Ni80Fe20 films with submicron rectangular elements

Patterned magnetic films with nano-scaled dots exhibit some special magnetic properties. In this paper, we investigate the in-plane shape anisotropy and the magnetization dynamic damping in permalloy (Ni80Fe20) arrays of submicron rectangular elements using ferromagnetic resonance (FMR). The FMR linewidth exhibits a dependence on the element size, and mainly comes from the contribution of the intrinsic damping. Also the contribution of two-magnon scattering plays an important role and is reduced with increasing aspect ratio. The damping coefficient decreases from 0.0129 to 0.0118 with the element length increasing from 300 nm to 1200 nm, and the theoretical calculation suggests that the change of damping results from the longitudinal and transverse interlayer spin current due to the spatially inhomogeneous magnetization dynamics.

Perfect inverse spin Hall effect and inverse Edelstein effect due to helical spin-momentum locking in topological surface states

We present a theory for the inverse spin Hall effect in a thin film of topological insulator (TI) ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, connected to a reservoir with applied spin bias, in the ballistic regime. In the case that either the spin polarization of the spin bias is along the longitudinal direction, or the hybridization gap $\mathrm{\ensuremath{\Delta}}$ of the surface states vanishes, the spin Hall angle ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{sh}}$ tends to infinity, indicating that the spin bias is perfectly converted into a measurable transverse charge current, essentially without generating a longitudinal spin current in the TI. In other cases, with increasing the Fermi energy ${E}_{\mathrm{F}}$ from the bottom of the conduction band of surface states, ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{sh}}$ grows continuously from zero and exhibits an interesting linear dependence on ${E}_{\mathrm{F}}/\mathrm{\ensuremath{\Delta}}$ for ${E}_{\mathrm{F}}\ensuremath{\gg}\mathrm{\ensuremath{\Delta}}$. We also find that the inverse Edelstein effect occurs, when the in-plane transverse component of the spin polarization vector is nonzero. The spin-to-charge conversion becomes complete, when the spin polarization vector is along the transverse direction, or the hybridization gap $\mathrm{\ensuremath{\Delta}}$ vanishes.

Spin generation via bulk spin current in three-dimensional topological insulators.

To date, spin generation in three-dimensional topological insulators is primarily modelled as a single-surface phenomenon, attributed to the momentum-spin locking on each individual surface. In this article, we propose a mechanism of spin generation where the role of the insulating yet topologically non-trivial bulk becomes explicit: an external electric field creates a transverse pure spin current through the bulk of a three-dimensional topological insulator, which transports spins between the top and bottom surfaces. Under sufficiently high surface disorder, the spin relaxation time can be extended via the Dyakonov–Perel mechanism. Consequently, both the spin generation efficiency and surface conductivity are largely enhanced. Numerical simulation confirms that this spin generation mechanism originates from the unique topological connection of the top and bottom surfaces and is absent in other two-dimensional systems such as graphene, even though they possess a similar Dirac cone-type dispersion.

Microscopic collective dynamics of water

Data obtained on microscopic collective excitations in water by molecular dynamics simulation within the framework of the coarse-grained mW-model of the intermolecular interaction potential for water are reported. The calculated spectra of the dynamic structure factor and spectral densities of time correlation functions of longitudinal and transverse currents reveal the existence of propagating collective excitations of longitudinal and transverse polarization in water for a wide range of wavenumbers. The dynamics of fluctuations in the particle number density is analyzed within the framework of a microscopic theory that takes into account only the structural features of a system. The theoretically calculated data on the spectra of dynamic structure factor in a wavenumber range of 0.13–0.48 A–1 are in good agreement with the results of molecular dynamics simulation.

Single-Site Resolution Detection of Methylation in DNA with Graphene Nanopores

Epigenetic modification of DNA where methyl groups are added at the 5-carbon position of cytosine is known as DNA methylation. It is associated with carcinogenesis, thus capable of serving as markers for detection of cancer. Nanopore analytics provides an easier and quicker route to detect methylated sites by avoiding complicated bisulfite treatment and polymerase chain reaction amplification. Graphene has a thickness of a single atom, thereby holding the potential to detect the methylation at single-site resolution. In this work, we use a self-consistent Poisson-Boltzmann formalism to simulate the translocation of methyl-CpG (MBD) proteins bound to a DNA molecule in a graphene nanopore, and detect the methylated sites along the DNA strand by computing both ionic current using molecular dynamics simulations and transverse sheet current via tight binding Hamiltonian based none-equilibrium Green's function approach.1 Evident dips are recorded in the ionic current trace through the nanopore for each methylation site, as expected, suggesting a single-site resolution. The graphene membrane with added quantum point contacts, by means of transverse sheet currents, can also detect the methylated site through local jumps in the variance of the measured transverse current. The proposed measurement strategy allows for real time, fast and high resolution DNA methylation detection.21. Girdhar, A., Sathe, C., Schulten, K., & Leburton, J. P. (2013). Graphene quantum point contact transistor for DNA sensing. Proceedings of the National Academy of Sciences, 110(42), 16748-16753.2. Sarathy, A., Qiu, H., Schulten, K., & Leburton, J. P. Single-site detection of methylation in DNA with graphene nanopores. To be published.

Multilane driven diffusive systems

We consider networks made of parallel lanes along which particles hop according to driven diffusive dynamics. The particles also hop transversely from lane to lane, hence indirectly coupling their longitudinal dynamics. We present a general method for constructing the phase diagram of these systems which reveals that in many cases their physics reduce to that of single-lane systems. The reduction to an effective single-lane description legitimizes, for instance, the use of a single TASEP to model the hopping of molecular motors along the many tracks of a single microtubule. Then, we show how, in quasi-2D settings, new phenomena emerge due to the presence of non-zero transverse currents, leading, for instance, to strong ‘shear localization’ along the network.

Topological currents in black phosphorus with broken inversion symmetry

We examine the nature of topological currents in black phosphorus when its inversion symmetry is deliberately broken. Here, the conduction- and valence-band edges are located at the $\mathrm{\ensuremath{\Gamma}}$ point of the rectangular Brillouin zone, and they exhibit strong anisotropy along its two crystal axes. We will show below that these salient features lead to linear transverse neutral topological currents, accompanied also by nonlinear transverse charge currents at the Fermi surface. These topological currents are maximal when the in-plane electric field is applied along the zigzag crystal axes but zero along the armchair direction.

Characteristics of the three-half-turn-antenna-driven RF discharge in the Uragan-3M torsatron

In the l = 3 Uragan-3M torsatron hydrogen plasma is produced by RF fields in the Alfven range of frequencies (ω ≤ ωci). The initial (target) plasma with the line-averaged density of units 1012 cm−3 is produced by a frame antenna with a broad spectrum of generated parallel wavenumbers. After this, to heat the plasma and bring its density to ~1013 cm–3, another, shorter wavelength three-half-turn antenna with large transverse currents is used. The behavior of the density, electron temperature, and loss of the plasma supported by the three-half-turn antenna is studied depending on the RF power fed to the antenna and initial values of the density and electron temperature supplied by the frame antenna.

Quantum Nonlinear Hall Effect Induced by Berry Curvature Dipole in Time-Reversal Invariant Materials

It is well known that a nonvanishing Hall conductivity requires broken time-reversal symmetry. However, in this work, we demonstrate that Hall-like currents can occur in second-order response to external electric fields in a wide class of time-reversal invariant and inversion breaking materials, at both zero and twice the driving frequency. This nonlinear Hall effect has a quantum origin arising from the dipole moment of the Berry curvature in momentum space, which generates a net anomalous velocity when the system is in a current-carrying state. The nonlinear Hall coefficient is a rank-two pseudotensor, whose form is determined by point group symmetry. We discus optimal conditions to observe this effect and propose candidate two- and three-dimensional materials, including topological crystalline insulators, transition metal dichalcogenides, and Weyl semimetals.

Spin Hall effects

In solid-state materials with strong relativistic spin-orbit coupling, charge currents generate transverse spin currents. The associated spin Hall and inverse spin Hall effects distinguish between charge and spin current where electron charge is a conserved quantity but its spin direction is not. This review provides a theoretical and experimental treatment of this subfield of spintronics, beginning with distinct microscopic mechanisms seen in ferromagnets and concluding with a discussion of optical-, transport-, and magnetization-dynamics-based experiments closely linked to the microscopic and phenomenological theories presented.

Propagation properties of right-hand circularly polarized Airy-Gaussian beams through slabs of right-handed materials and left-handed materials.

The propagation of right-hand circularly polarized Airy-Gaussian beams (RHCPAiGBs) through slabs of right-handed materials (RHMs) and left-handed materials (LHMs) is investigated analytically and numerically with the transfer matrix method. An approximate analytical expression for the RHCPAiGBs passing through a paraxial ABCD optical system is derived on the basis of the Huygens diffraction integral formula. The intensity and the phase distributions of the RHCPAiGBs through RHMs and LHMs are demonstrated. The influence of the parameter χ0 on the propagation of RHCPAiGBs through RHM and LHM slabs is investigated. The RHCPAiGBs possess transverse-momentum currents, which shows that the physics underlying this intriguing accelerating effect is that of the combined contributions of the transverse spin and transverse orbital currents. Additionally, we go a step further to explore the radiation force including the gradient force and scattering force of the RHCPAiGBs.

High field terahertz emission from relativistic laser-driven plasma wakefields

We propose a method to generate high field terahertz (THz) radiation with peak strength of GV/cm level in the THz frequency gap range of 1–10 THz using a relativistic laser interaction with a gaseous plasma target. Due to the effect of local pump depletion, an initially Gaussian laser pulse undergoes leading edge erosion and eventually evolves to a state with leading edge being step function. Interacting with such a pulse, electrons gain transverse residual momentum and excite net transverse currents modulated by the relativistic plasma frequency. These currents give rise to the low frequency THz emission. We demonstrate this process with one and two dimensional particle-in-cell simulations.

Topological spin Hall effect resulting from magnetic skyrmions

The intrinsic spin Hall effect (SHE) originates from the topology of the Bloch bands in momentum space. The duality between real space and momentum space calls for a spin Hall effect induced from a real space topology in analogy to the topological Hall effect (THE) of skyrmions. We theoretically demonstrate the topological spin Hall effect (TSHE) in which a pure transverse spin current is generated from a skyrmion spin texture.

Influence of a strong longitudinal magnetic field on laser wakefield acceleration

Optimization of the beam quality and electronic trapped charge in the cavity are key issues of laser wake field acceleration. The effect of an initially applied uniform magnetic field, parallel to the direction of propagation of the pump pulse, on the laser wakefield is explored. First, an analytic model for the laser wakefield is built up in the case when such an external magnetic field is applied. Then, simulations are performed with a 3D quasi-cylindrical particle in cell code in the blowout (or bubble) regime. Transverse currents are generated at the rear of the bubble which amplify the longitudinal magnetic field. For several plasma and laser parameters, the wake shape is altered and trapping can be reduced or cancelled by the magnetic field. When considering optical injection, and when two counterpropagating waves interact with a rather high plasma density, trapping is not affected by the magnetic field. In this range of plasma and laser parameters, it is shown that the longitudinal magnetic field can reduce or even prevent self-injection and enhance beam quality.

High-intensity terahertz generation by nonlinear frequency-mixing of lasers in plasma with DC magnetic field

AbstractWe propose a mechanism of highly focused, tunable and high-intensity terahertz (THz) radiation generation by frequency-mixing of two super-Gaussian lasers with frequencies ω1, ω2 and wave numbers k1, k2 (laser profile index p > 2) in a corrugated plasma in the presence of external static magnetic field ${B_0}\hat z$. In this process, a strong nonlinear ponderomotive force is offered to the plasma electrons at frequency ω′ = ω1 − ω2 and wave number k′ = k1 − k2 by laser beams. The ponderomotive force results in a strong, controllable nonlinear transverse oscillatory current, which can be optimized by optimizing the external magnetic field, ripple parameters, and laser indexes. This controllable current produces focused and intense THz radiation of tunable frequency and power along with a remarkable efficiency ~25%.

DNA/RNA transverse current sequencing: intrinsic structural noise from neighboring bases.

Nanopore DNA sequencing via transverse current has emerged as a promising candidate for third-generation sequencing technology. It produces long read lengths which could alleviate problems with assembly errors inherent in current technologies. However, the high error rates of nanopore sequencing have to be addressed. A very important source of the error is the intrinsic noise in the current arising from carrier dispersion along the chain of the molecule, i.e., from the influence of neighboring bases. In this work we perform calculations of the transverse current within an effective multi-orbital tight-binding model derived from first-principles calculations of the DNA/RNA molecules, to study the effect of this structural noise on the error rates in DNA/RNA sequencing via transverse current in nanopores. We demonstrate that a statistical technique, utilizing not only the currents through the nucleotides but also the correlations in the currents, can in principle reduce the error rate below any desired precision.

THz radiation by amplitude-modulated self-focused Gaussian laser beam in ripple density plasma

AbstractThe effect of self-focusing and defocusing on terahertz (THz) generation by amplitude-modulated Gaussian laser beam in rippled density plasma is investigated. A stronger transient transverse current is generated by transverse component of ponderomotive force exerted by laser on electrons that drives radiation at the modulation frequency (which is chosen to be in the THz domain) because of the variation in intensity in the direction transverse to the laser propagation. Numerical simulations indicate the enhancement of THz yield by many folds due to self-focusing of laser beam in comparison with that without self-focusing. The transient focusing of laser beam and its effect on the generated THz amplitude has also been studied.

Strong terahertz emission from electromagnetic diffusion near cutoff in plasma

A new mechanism for electromagnetic emission in the terahertz (THz) frequency regime from laser-plasma interactions is described. A localized and long-lasting transverse current is produced by two counter-propagating short laser pulses in weakly magnetized plasma. We show that the electromagnetic wave radiating from this current source, even though its frequency is close to cut-off of the ambient plasma, grows and diffuses towards the plasma-vacuum boundary, emitting a strong monochromatic THz wave. With driving laser pulses of moderate power, the THz wave has a field strength of tens of MV m−1, a frequency of a few THz and a quasi-continuous power that exceeds all previous monochromatic THz sources. The novelty of the mechanism lies in a diffusing electromagnetic wave close to cut-off, which is modelled by a continuously driven complex diffusion equation.

Recent progress in atomistic simulation of electrical current DNA sequencing.

We review recent advances in the DNA sequencing method based on measurements of transverse electrical currents. Device configurations proposed in the literature are classified according to whether the molecular fingerprints appear as the major (Mode I) or perturbing (Mode II) current signals. Scanning tunneling microscope and tunneling electrode gap configurations belong to the former category, while the nanochannels with or without an embedded nanopore belong to the latter. The molecular sensing mechanisms of Modes I and II roughly correspond to the electron tunneling and electrochemical gating, respectively. Special emphasis will be given on the computer simulation studies, which have been playing a critical role in the initiation and development of the field. We also highlight low-dimensional nanomaterials such as carbon nanotubes, graphene, and graphene nanoribbons that allow the novel Mode II approach. Finally, several issues in previous computational studies are discussed, which points to future research directions toward more reliable simulation of electrical current DNA sequencing devices.

Entangled polymer complexes as Higgs phenomena

We derive an effective Maxwell-London equation for entangled polymer complexes under topological constraints, borrowing the theoretical framework from topological field theory. We find that the transverse current flux of a test polymer chain, surrounded by entangled chains, decays exponentially from its centerline position with a finite penetration depth, which is analogous to the magnetic-field decay in a superconductor (SC), referred to as the Meissner effect. Just as the mass acquirement of photons in a SC is the origin of the magnetic-field decay, the polymer obtains uncrossable intersections along the chain due to the preservation of the linking number, which restricts the deviation of the transverse polymer current in the normal direction. The underlying physics is as follows: less flexible polymers have stronger current-current correlations, giving rise to a heavier effective mass of the gauge fields and resulting in a shorter decay length. Interestingly, this picture is well incorporated within the most successful phenomenological theory of the, so called, tube model, the microscopic origins of which researchers have long pursued. The correspondence of our equation of motion to the tube model claims that the confining tube potential is a consequence of the topological constraint (linking number). The tube radius is attributed to the decay length. On increasing the effective mass (by strengthening the interaction at an uncrossable intersection or a number of intersections), the tube becomes narrower. Using this argument, the exponential decay of the chain leakage out of the tube is well understood.