Generation Of Charge Currents Research Articles

Terahertz photon to dc current conversion via magnetic excitations of multiferroics

Direct conversion from terahertz photon to charge current is a key phenomenon for terahertz photonics. Quantum geometrical description of optical processes in crystalline solids predicts existence of field-unbiased dc photocurrent arising from terahertz-light generation of magnetic excitations in multiferroics, potentially leading to fast and energy-efficient terahertz devices. Here, we demonstrate the dc charge current generation from terahertz magnetic excitations in multiferroic perovskite manganites with spin-driven ferroelectricity, while keeping an insulating state with no free carrier. It is also revealed that electromagnon, which ranges sub-terahertz to 2 THz, as well as antiferromagnetic resonance shows the giant conversion efficiency. Polar asymmetry induced by the cycloidal spin order gives rise to this terahertz-photon-induced dc photocurrent, and no external magnetic and electric bias field are required for this conversion process. The observed phenomena are beyond the conventional photovoltaics in semi-classical regime and demonstrate the essential role of quantum geometrical aspect in low-energy optical processes. Our finding establishes a paradigm of terahertz photovoltaic phenomena, paving a way for terahertz photonic devices and energy harvesting.

Antiferromagnetic magnonic charge current generation via ultrafast optical excitation

Néel spin-orbit torque allows a charge current pulse to efficiently manipulate the Néel vector in antiferromagnets, which offers a unique opportunity for ultrahigh density information storage with high speed. However, the reciprocal process of Néel spin-orbit torque, the generation of ultrafast charge current in antiferromagnets has not been demonstrated. Here, we show the experimental observation of charge current generation in antiferromagnetic metallic Mn2Au thin films using ultrafast optical excitation. The ultrafast laser pulse excites antiferromagnetic magnons, resulting in instantaneous non-equilibrium spin polarization at the antiferromagnetic spin sublattices with broken spatial symmetry. Then the charge current is generated directly via spin-orbit fields at the two sublattices, which is termed as the reciprocal phenomenon of Néel spin-orbit torque, and the associated THz emission can be detected at room temperature. Besides the fundamental significance on the Onsager reciprocity, the observed magnonic charge current generation in antiferromagnet would advance the development of antiferromagnetic THz emitter.

Generation of charge currents and magnetic pulses

The generation of charge currents and hence induced magnetic field in a spherically bound hydrogen atom by an ultrafast right circularly polarized laser pulse has been studied. The atom is assumed to be under the effect of a Hulthén plus a ring-shaped potential. The simultaneous effect of pulse parameters such as El0, tp and ω0, the cavity radius, r0, and parameters related to the non-central potential has been investigated. These parameters have been found to significantly affect the magnitude of generated current as well as the magnetic field and thus play an important role in determining the response of the system to external laser. The laser pulse is found to produce femtosecond magnetic pulses. Such magnetic pulses have many biomedical applications. Hulthén screening or ring deformation potential reduces the magnitude of current and induced magnetic field for an unbounded atom. On the other hand, for the bounded case, the potential parameters are found to have a positive impact on these quantities.

Effect of picosecond magnetic pulse on dynamics of electron's subbands in semiconductor bilayer nanowire

We report on possibility of charge current generation in nanowire made of two tunnel coupled one-dimensional electron waveguides by means of single magnetic pulse lasting up to 20ps. Existence of interlayer tunnel coupling plays a crucial role in the effect described here as it allows for hybridization of the wave functions localized in different layers which can be dynamically modified by applying a time changeable in-plane magnetic field. Results of time-dependent DFT calculations performed for a bilayer nanowire confining many electrons show that the effect of such magnetic hybridization relies on tilting of electrons’ energy subbands, to the left or to the right, depending on a sign of time derivative of oscillating magnetic field due to the Faraday law. Consequently, the tilted subbands become a source of charge flow along the wire. Strength of such magneto-induced current oscillations may achieve even 0.6μA but it depends on duration of magnetic pulse as well as on charge density confined in nanowire which has to be unequally distributed between both transport layers to observe this effect.

Compression and shear mode ionic polymer-metal composite (IPMC) pressure sensors

In this work, two types of dynamic pressure sensors based on ionic polymer-metal composites (IPMCs) in compression and shear deformation modes are designed, prototyped, and tested in a shock pressure tube. Although IPMCs offer appropriate properties such as flexibility, light weight, easy processing, resilience and high sensitivity to be used as sensors, there are only limited studies on applicable IPMC sensors and almost all of these devices were developed based on bending deformation. For a better understanding of the electromechanical sensing performance of the IPMCs, the charge current generation mechanisms due to external pressure in both compression and shear modes are developed based on the streaming potential hypothesis. The direct assembly process (DAP) is applied to fabricate some pressure sensor samples of each type for experimental evaluation. An equivalent circuit with parallel resistance and capacitance of the IMPCs samples, estimated by an impedance analyzer, is used to design an appropriate signal conditioner. A shock pressure tube setup is then utilized for performance evaluation of the fabricated devices. The results demonstrate that IPMCs as dynamic pressure sensors represent an appropriate linearity, sensitivity and consistency in both compression and shear modes. It is found that IPMC sensors in shear mode have a higher sensitivity compared with its compression mode counterpart.

Ultrafast optically induced resonant and non-resonant current generation in atoms and nanostructures: role of the photons orbital angular momentum

Aside from technological applications, steering non-equilibrium currents in electronic matter delivers information on the system electric and magnetic properties and their relation to electronic correlation and the underlying symmetry of the electrons’ confinement. This theoretical study demonstrates that optical vortices are a versatile tool for spatio-temporal generation of charge currents via resonant and stimulated Raman-type processes for semi-conductor-based nanostructures as well as for atoms, thereby the orbital angular momentum associated with the optical vortex plays a key role.

Thermal control and generation of charge currents in coupled quantum dots

AbstractThis article reviews recent thermoelectric experiments on quantum dot (QD) systems. The experiments focus on two types of inter‐dot coupling: tunnel coupling and Coulomb coupling. Tunnel‐coupled QDs allow particles to be exchanged between the attached reservoirs via the QD system. Hence, an applied temperature bias results in a thermovoltage. When being investigated as a function of QD energies, this leads to the thermopower stability diagram. Here, largest thermovoltage is observed in the regions of the triple points. In a QD system which exhibits only capacitive inter‐dot coupling, electron transfer is suppressed. Such a device is studied in a three‐terminal geometry: while one QD connects to the heat reservoir, the other one can exchange electrons with two reservoirs at a lower temperature. When the symmetry of the tunneling coefficients in the cold system is broken, the device becomes an energy harvester: thermal energy is extracted from the heat reservoir and is converted into a directed charge current between the two cold reservoirs. This review illustrates the large potential of multi‐QD devices for thermoelectrics and thermal management at the nanometer‐scale.

Nonlocal optical generation of spin and charge currents on the surface of magnetic insulators using total absorption and surface plasmons

We study the nonlocal spin and charge current generation in a finite metallic element on the surface of magnetic insulators such as yttrium iron garnet due to the absorption of the magnetic surface plasmon (MSP). Whereas a surface plasmon is completely reflected by a metal, an MSP can be absorbed due to the absence of backward states. The injection of MSP generates a voltage in the longitudinal direction parallel to the wave vector, with the voltage proportional to input power. If the metal is a ferromagnet, a spin current can also be induced in the longitudinal direction. Our results provide a way to improve upon integrated circuits of spintronics and spin wave logic devices.

Charge-current generation in atomic systems induced by optical vortices

We envisage the possibility of producing and tuning electronic orbital currents in atoms by weak, spatially inhomogeneous laser pulses with optical vortices. As already known, when interacting with such pulses an electronic system attains a definite amount of angular momentum. Here we explore this effect in the case when the system size is atomically small, i.e., much smaller than the scale of the inhomogeneity of the light pulses. We show that, nonetheless, angular momentum is transferred to the atomic system. The amount of the attained angular momentum depends on the position of the atom inside the field. Our numerical calculations and analytical analysis show that, indeed, light-induced orbital magnetism emerges in a dilute atomic gas when it is exposed to resonant or broadband optical vortices. This effect can be assessed, for instance, in a Faraday rotation experiment.

Local Charge and Spin Currents in Magnetothermal Landscapes

A scannable laser beam is used to generate local thermal gradients in metallic (Co2FeAl) or insulating (Y3Fe5O12) ferromagnetic thin films. We study the resulting local charge and spin currents that arise due to the anomalous Nernst effect (ANE) and the spin Seebeck effect (SSE), respectively. In the local ANE experiments, we detect the voltage in the Co2FeAl thin film plane as a function of the laser-spot position and external magnetic field magnitude and orientation. The local SSE effect is detected in a similar fashion by exploiting the inverse spin Hall effect in a Pt layer deposited on top of the Y3Fe5O12. Our findings establish local thermal spin and charge current generation as well as spin caloritronic domain imaging.