External Dc Currents Research Articles

Direct-current output of silicon–organic monolayer–platinum Schottky TENGs: Elusive friction-output relationship

Triboelectric nanogenerators (TENGs) are an emerging energy harvesting technology able to convert ubiquitous mechanical energy into electricity. Friction, static charging and flexoelectricity are all involved in the mechanism underpinning TENG operation, but their relative contribution has remained elusive. Here we used dynamic and static conductive atomic force microscopy (C-AFM) measurements on monolayer-modified silicon crystals to detect evidence of a relationship between friction and zero-bias current, and between pressure and the direction of the putative flexovoltage. We demonstrate that a static electricity-related tribovoltage is probably responsible for a friction excess, and that surprisingly this friction excess is found to be dependent on the doping level and type of the silicon substrate. Such friction excess is however no longer measurable once current is allowed to flow across the junction. This observation points to an electrostatic origin of friction in silicon-based Schottky TENGs, and suggests that the zero external bias DC current is at least in part an electronic flow to neutralize static charges. Further, the sign of the zero-bias current, but not its magnitude, is independent of the semiconductor doping type, which is again suggestive of surface statics being a main contributor to the zero-bias output rather than exclusively a space-charge effect. We also reveal the presence of a junction flexovoltage under pressures common in AFM experiments (GPa), even for negligible lateral friction. In a static Pt–monolayer–n-type Si junction the flexovoltage carries the same sign as the tribovoltage, and can reach such magnitude to overwrite external voltages as high as 2 V. The immediate implication is that the flexovoltage is likely to have i) a strong contribution to the zero-bias output of a n-Si Schottky TENG, ii) a negative effect on the output of a p-Si TENG, and iii) its detection can be straightforward, as we discovered that flexoelectricity manifests as an “inverted diode”: a n-type Si–platinum diode with negligible current even when the n-type material is negatively biased as long as the “static” diode remains under a large normal pressure.

Linearization of Patterned Pinning Spin Valve Devices for Low-Field Applications

In spin valve sensors, the bias point caused by the interlayer coupling between the pinned layer and the free layer is a crucial issue encountered in the design process, which degrades the responsivity and the weak-field detection feature. In tackling this issue, we present the magnetic bias technique for efficient control of the bias point of the pinned spin valve (PSV). Specifically, the magnetic field that creates a shift of the bias point is generated by a coil being wrapped around the PSV devices. The PSV multilayer structure, Ta(5 nm)/NiFe(3 nm)/CoFe(4.5 nm)/Cu(1.5 nm)/CoFe(2.4 nm)/IrMn(10 nm)/Ta(5 nm), was fabricated using DC magnetron sputtering deposition. The PSV devices patterned by using a simple lift-off technique have a dimension of 1.5 µm × 200 µm. The bias magnetic field was tuned between ± 5 mT by changing an external DC current. Our experimental results indicate that the bias point can be efficiently controlled via a simple adjustment of the current in the bias coil. Consequently, the device responsivity was remarkably enhanced by a factor of 6.75, from 0.8 V/T to 5.4 V/T, leading to a decrease of the detectivity (field noise) from 366 nT/√Hz@1 Hz to 108 nT/√Hz@1 Hz. Additionally, the field noise was further suppressed by a factor of 5 from 363 nT/√Hz@1 Hz to 72 nT/√Hz@1 Hz by applying the AC-driven mode in the half-bridge PSV. The simplicity of our proposed technique makes it particularly pertinent to PSV sensor designs for low-field applications.

An Electronically Adjustable Waveform Generator

A novel electronically adjustable square/triangular waveform generator has been introduced in this paper. The proposed circuit employs one active element, the multi-output current-controlled current conveyor transconductance amplifier, and one passive component grounded capacitor only. The resistorless realization of the presented generator provides a good advantage in terms of integrated circuit fabrication. In the offered circuit, the frequency and amplitude of the output square wave are electronically tunable by means of relevant bias currents. Additionally, the upper and lower threshold levels are electronically controllable by the respective bias current. On the contrary, electronically adjusting of the duty cycle of output waveform is possible via the external DC current. The generator circuit is simulated with TSMC 0.18[Formula: see text][Formula: see text]m technology parameters and SPICE. Moreover, the introduced circuit is implemented by using commercially available active devices and thus it is also verified experimentally.

Effect of current-induced electron heating on irradiated magnetoresistance in high-mobility 2D electron systems

We present a theoretical work to study the influence of an external DC-current on the irradiated magnetoresistance through a very high-mobility 2D electron system. Recent experiments report that an increasing DC-current through the system under radiation, dramatically reduces magnetoresistance, including background and irradiated contributions. On the one hand, the former ends up progressively negative (negative giant magnetoresistance) and on the other hand, the effect of radiation, i.e., the radiation-induced resistance oscillations, tends to vanish. In our theoretical model, the DC-current–induced hot electrons when scattered by charged impurities end up, to an increasing extent, between Landau levels with a very low density of states to get to. This makes the scattering process much less effective, decreasing both the magnetoresistance itself and the radiation-induced resistance oscillations. We conclude that both effects are not separable, being tightly linked by the scattering process.

Effect of ordered array of magnetic dots on the dynamics of Josephson vortices in stacked SNS Josephson junctions under DC and AC current

We use the anisotropic time-dependent Ginzburg-Landau theory to investigate the effect of a square array of out-of-plane magnetic dots on the dynamics of Josephson vortices (fluxons) in artificial stacks of superconducting-normal-superconducting (SNS) Josephson junctions in the presence of external DC and AC currents. Periodic pinning due to the magnetic dots distorts the triangular lattice of fluxons and results in the appearance of commensurability features in the current-voltage characteristics of the system. For the larger values of the magnetization, additional peaks appear in the voltage-time characteristics of the system due to the creation and annihilation of vortex-antivortex pairs. Peculiar changes in the response of the system to the applied current is found resulting in a “superradiant” vortex-flow state at large current values, where a rectangular lattice of moving vortices is formed. Synchronizing the motion of fluxons by adding a small ac component to the biasing dc current is realized. However, we found that synchronization becomes difficult for large magnetization of the dots due to the formation of vortex-antivortex pairs.

Highly sensitive detection and stochastic analysis of magnetization agitation induced in a single layered magnetic wire

This study shows that broadband magnetic noise in a ferromagnetic wire can be detected over a wide frequency range between 500MHz and 8GHz using a lock-in detection technique. The magnetic noise spectrum from a 20nm-thick single-layered Fe19Ni81 wire biased with a dc current is measured as functions of an external field and dc current. This noise is caused by thermal agitation in magnetization due to ambient temperature and Joule heating. The noise behaviors are well reproduced by a stochastic model. Thus, this paper presents a stochastic analysis of magnetic noise behaviors induced by thermal agitation using a highly sensitive technique for detecting the magnetic noise in a single layered ferromagnetic wire.

Arbitrary frequency tunable radio frequency bandpass filter based on nano-patterned Permalloy coplanar waveguide (invited)

A well designed coplanar waveguide (CPW) based center frequency tunable bandpass filter (BPF) at 4 GHz enabled with patterned Permalloy (Py) thin film has been implemented. The operating frequency of BPF is tunable with only DC current without the use of any external magnetic field. Electromagnetic bandgap resonators structure is adopted in the BPF and thus external DC current can be applied between the input and output of the filter for tuning of Py permeability. Special configurations of resonators with multiple narrow parallel sections have been considered for larger inductance tenability; the tunability of CPW transmission lines of different widths with patterned Py thin film on the top of the signal lines is compared and measured. Py thin film patterned as bars is deposited on the top of the multiple narrow parallel sections of the designed filter. No extra area is required for the designed filter configuration. Filter is measured and results show that its center frequency could be tuned from 4 GHz to 4.02 GHz when the DC current is applied from 0 mA to 400 mA.

Sliding state of the quasi-two dimensional CDW system TbTe3 probed by coherent x-ray diffraction

Coherent x-ray diffraction experiments have been performed to probe the sliding state of a quasi-two dimensional charge density wave (CDW) in TbTe3. The 2kF satellite reflection associated with the CDW has been observed with respect to external dc currents, on both sides of the threshold current. These measurements illustrate how the periodic lattice distortion associated to the CDW supports the collective motion of a sliding CDW in a quasi-two dimensional system. A comparison with the quasi-one dimensional NbSe3 system is done.

Electronically tunable current-mode square-root-domain first-order multifunction filter

In this article, a new current-mode square-root-domain first-order multifunction filter is designed. The filter circuit has a very simple structure; it uses only the Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) and a grounded capacitor. All transistors’ aspect ratios are the same. It provides lowpass, highpass and allpass responses simultaneously for a single input signal. It can be electronically tuned by changing an external DC current. The filter operates with a single supply voltage of 2.5 V. The proposed circuit has been simulated with PSPICE electronics circuit simulation program (Cadence OrCAD Capture CIS Version 9.2) simulation programs using TSMC 0.35 μm (Taiwan Semiconductor Manufacturing Company) Complementary Metal-Oxide-Semiconductor (CMOS) process parameters.

Creep, Flow, and Phase Slippage Regimes: An Extensive View of the Sliding Charge-Density Wave Revealed by Coherent X-ray Diffraction

Coherent x-ray diffraction experiments have been used to probe the dynamics of the charge density wave (CDW) in the quasi-1D system NbSe(3). The 2k(F) satellite reflection associated with the CDW has been measured with respect to external dc currents, below and above the depinning current. These measurements illustrate for the first time the extensive behavior of a moving electronic crystal: the creep regime, with nucleation of CDW dislocations, the flow regime, with motional ordering, along with phase slippage and the role of strong pinning due to an extrinsic defect.

Pinning and depinning process of an incommensurate CDW as revealed by coherent X-ray diffraction

Experiments on the coherent X-ray diffraction, and their modeling, have been performed on the Charge Density Wave (CDW) system NbSe3. The 2kF satellite reflection associated with the CDW has been measured with respect to external dc currents. Below the threshold current, reflection displays several fringes in the transverse direction which disappear when the threshold current is exceeded. In the sliding state, the transverse satellite profile has a form of two nonsymmetric peaks, one of them being centered at the same position as below the threshold and another one being shifted. The shift of the longitudinal peak position below the threshold current and the nonsymmetric peak in the transverse direction above the threshold one is interpreted as the influence of strong linear defect like a crystal step present on the sample surface, combined with induced arrays of dislocations. Coherent X-rays provide a new access to processes in a CDW driven by an external force in a random pinning potential.

Observation of correlations up to the micrometer scale in sliding charge-density waves

High-resolution coherent X-ray diffraction experiment has been performed on the Charge-Density Wave (CDW) system K0.3MoO3K0.3MoO3. The 2kF2kF satellite reflection associated with the CDW has been measured with respect to external dc currents. In the sliding regime, the 2kF2kF satellite reflection displays secondary satellites along the chain axis which corresponds to correlations up to the micrometer scale. This super long range order is 1500 times larger than the CDW period itself.

Observation of Correlations Up To the Micrometer Scale in Sliding Charge-Density Waves

A high resolution coherent x-ray diffraction experiment has been performed on the charge-density wave (CDW) system K0.3MoO3. The 2kF satellite reflection associated with the CDW has been measured with respect to external dc currents. In the sliding regime, the 2kF satellite reflection displays secondary satellites along the chain axis which corresponds to correlations up to the micrometer scale. This super long-range order is 1500 times larger than the CDW period itself. This new type of electronic correlation seems inherent to the collective dynamics of electrons in charge-density wave systems. Several scenarios are discussed.

STUDIES OF THE DC BIASED JOSEPHSON JUNCTION COUPLED TO A RESONANT TANK

We have investigated the dynamics of a Josphson junction circuit, which consists of a dc biased Josephson junction coupled to a resonant tank. Numerical simulations indicate that period-3 and chaotic states coexist when this system is driven by a proper external dc current. The detailed structures of the attractors and the basins of attraction are given to specify the features of these states. This provides one with valuable information to avoid chaos in Josephson junction devices.

Rotobreather dynamics in underdamped Josephson junction ladders

We study rotobreather properties in Josephson ladders with external dc currents and nonzero island-island capacitances. These solutions are localized in space, with tails characterized by exponential localization and a wave length which both vary smoothly upon varying the external bias. We demonstrate the existence of resonances for underdamped systems which are related to the plane wave spectrum of the nonresistive system. We propose experimental observations of these resonances.

Magnetocapacitance in quantum Hall regime with external dc current

We have investigated the magnetocapacitance between gates and two dimensional electron system in a GaAs/AlGaAs heterostructure with an external dc current at quantum Hall (QH) plateau regime. With increasing dc current, the minimum values of capacitance at QH plateaux increase and the range of magnetic fields showing the capacitance minima decreases. The electrons excited by the external dc current enhance the σ XX which causes the breakdown of the QH effect. In a certain magnetic field and current direction, the capacitance dip at QH plateau splits into two valleys with dc current. These results are considered to be due to the change of effective gate voltage caused by the current. The breakdown current of the QH effect evaluated by the resistance measurement is several times larger than that obtained by the dc current dependence of the magnetocapacitance, since the capacitance is very sensitive to the residual σ XX. The vanishing of the capacitance minima may be the precursor of QH breakdown.

Spatiotemporal chaos in Josephson-junction arrays.

We investigate the dynamical behavior of chaotic states in two-dimensional capacitive Josephson-junction arrays (JJA) driven by uniform external ac and dc currents via numerical simulations of the resistively-shunted-junction (RSJ) model. In chaotic states the distribution functions for the fluctuating voltages in individual transverse junctions have exponential tails similar to findings for the temperature in fluid turbulence. We present a mechanism for the origin of this behavior in arrays based on the random generation and motion of vortices. We also provide evidence to argue that collectively chaotic behavior does not occur.

Vortex motion modulated by a self field in a large Josephson junction

Motion of a vortex array (flux flow) in a large Josephson junction is studied both theoretically and experimentally. An expression for dc current-voltage ( I-V ) characteristics of a large junction in a flux-flow state, which is caused by the external dc magnetic field and current, is obtained by taking account of the self field due to the dc current through the junction as well as the large loss due to the surface resistance of junction electrodes. It is shown that the spacially uniform flux flow is crucially modulated by the self field in a high-velocity region of the flux flow. As a result, the magnitude of the current peak in I-V curves decreases significantly and saturates with respect to both the length and the quality factor of the junction, which is confirmed by experiments with large Pb-Pb x O y -Pb junctions.

Microwave-Induced Vortex Motion in Long Josephson Junctions

Experimental and theoretical studies are made of the effect of a microwave on dc current-voltage characteristics of long Josephson junctions. In the case of zero external dc current, dc voltages were observed when the microwave was applied to the junctions. By comparing the observed magnetic-field and microwave-power dependences with theoretical results, it is concluded that these dc voltages resulted from vortex motion which was induced by the applied microwave. When the junctions were in a flux-flow state caused by the external current, a good agreement between theory and experiment concerning the microwave-power and magnetic-field dependences of Shapiro steps demonstrates that an almost ideal traveling-wave type coupling between the applied microwave and a moving vortex array took place in the long Josephson junctions.