Resistance Oscillations Research Articles

Negative viscosity and eddy flow of the imbalanced electron-hole liquid in graphene

We present a hydrodynamic theory for electron-hole magnetotransport in graphene incorporating carrier-population imbalance, energy, and momentum relaxation processes. We focus on the electric response and find that the carrier and energy imbalance relaxation processes strongly modify the shear viscosity, so that an effective viscosity can be negative in the vicinity of charge neutrality. We predict an emergent eddy flow pattern of swirling currents and explore its manifestation in nonlocal resistivity oscillations in a strip of graphene driven by a source current.

A SiC BJT-Based Negative Resistance Oscillator for High-Temperature Applications

This brief presents a 59.5 MHz negative resistance oscillator for high-temperature operation. The oscillator employs an in-house 4H-silicon carbide BJT, integrated with the required circuit passives on a low-temperature co-fired ceramic substrate. Measurements show that the oscillator operates from room temperature up to 400 °C. The oscillator delivers an output power of 11.2 dBm into a 50 $\Omega $ load at 25 °C, which decreases to 8.4 dBm at 400 °C. The oscillation frequency varies by 3.3% in the entire temperature range. The oscillator is biased with a collector current of 35 mA from a 12-V supply and has a maximum dc power consumption of 431 mW.

Sign-alternating photoconductivity and magnetoresistance oscillations induced by terahertz radiation in HgTe quantum wells

We report on the observation of terahertz radiation induced photoconductivity and of terahertz analog of the microwave-induced resistance oscillations (MIRO) in HgTe-based quantum well (QW) structures of different width. The MIRO-like effect has been detected in QWs of 20 nm thickness with inverted band structure and a rather low mobility of about 3 $\times$ 10$^5$ cm$^2$/V s. In a number of other structures with QW widths ranging from 5 to 20 nm and lower mobility we observed an unconventional non-oscillatory photoconductivity signal which changes its sign upon magnetic field increase. This effect was observed in structures characterized by both normal and inverted band ordering, as well as in QWs with critical thickness and linear dispersion. In samples having Hall bar and Corbino geometries an increase of the magnetic field resulted in a single and double change of the sign of the photoresponse, respectively. We show that within the bolometric mechanism of the photoresponse these unusual features imply a non-monotonic behavior of the transport scattering rate, which should decrease (increase) with temperature for magnetic fields below (above) the certain value. This behavior is found to be consistent with the results of dark transport measurements of magnetoresistivity at different sample temperatures. Our experiments demonstrate that photoconductivity is a very sensitive probe of the temperature variations of the transport characteristics, even those that are hardly detectable using standard transport measurements.

Microwave amplification in a magnetic tunnel junction induced by heat-to-spin conversion at the nanoscale.

Heat-driven engines are hard to realize in nanoscale machines because of efficient heat dissipation1. However, in the realm of spintronics, heat has been employed successfully-for example, heat current has been converted into a spin current in a NiFe|Pt bilayer system2, and Joule heating has enabled selective writing in magnetic memory arrays3. Here, we use Joule heating in nanoscale magnetic tunnel junctions to create a giant spin torque due to a magnetic anisotropy change. Efficient conversion from heat dynamics to spin dynamics is obtained because of a large interfacial thermal resistance at an FeB|MgO interface. The heat-driven spin torque is equivalent to a voltage-controlled magnetic anisotropy4,5 of approximately 300 fJ V-1 m-1, which is more than twice the value reported in a (Co)FeB|MgO system6,7. We demonstrate an electric microwave amplification gain of 20% in a d.c. biased magnetic tunnel junction as a result of this spin torque. While electric d.c. power amplification in spintronic devices has been realized previously8, the microwave amplification was limited to relatively small amplification gains (G = radiofrequency output voltage/radiofrequency input voltage) and has never exceeded 1 (refs 9-13). A magnetic tunnel junction driven by radiofrequency spin transfer torque using ferromagnetic resonance enabled a relatively large gain of G ≈ 0.55 (ref. 12). Furthermore, radiofrequency spin waves were tuned by the spin transfer effect14,15. The heat-driven giant spin torque in the FeB|MgO16,17 magnetic tunnel junction, which shows a large magnetization precession and resistance oscillation under a d.c. bias, overcomes the above limitations and provides a gain larger than 1.

Proximity-Induced Superconductivity with Subgap Anomaly in Type II Weyl Semi-Metal WTe2.

Due to the nontrivial topological band structure in type II Weyl semi-metal tungsten ditelluride (WTe2), unconventional properties may emerge in its superconducting phase. While realizing intrinsic superconductivity has been challenging in the type II Weyl semi-metal WTe2, the proximity effect may open an avenue for the realization of superconductivity. Here, we report the observation of proximity-induced superconductivity with a long coherence length along the c axis in WTe2 thin flakes based on a WTe2/NbSe2 van der Waals heterostructure. Interestingly, we also observe anomalous oscillations of the differential resistance during the transition from the superconducting to the normal state. Theoretical calculations show excellent agreement with experimental results, revealing that such a subgap anomaly is the intrinsic property of WTe2 in superconducting state induced by the proximity effect. Our findings enrich the understanding of the superconducting phase of type II Weyl semi-metals and pave the way for their future applications in topological quantum computing.

Effect of illumination on quantum lifetime in GaAs quantum wells

Low-temperature illumination of a two-dimensional electron gas in GaAs quantum wells is known to greatly improve the quality of high-field magnetotransport. The improvement is known to occur even when the carrier density and mobility remain unchanged, but what exactly causes it remains unclear. Here, we investigate the effect of illumination on microwave photoresistance in low magnetic fields. We find that the amplitude of microwave-induced resistance oscillations grows dramatically after illumination. Dingle analysis reveals that this growth reflects a substantial increase in the single-particle (quantum) lifetime, which likely originates from the light-induced redistribution of charge enhancing the screening capability of the doping layers.

Insulator or a metal?

Solid-State Physics When a metal is cooled to low temperatures and placed in an external magnetic field, its resistivity may oscillate as the magnitude of the field is varied. Seeing these so-called quantum oscillations in an insulating material would be very unusual. Xiang et al. report such findings in the insulator ytterbium dodecaboride (YbB12) (see the Perspective by Ong). In addition to oscillations in resistivity, the authors observed oscillations in the magnetic torque. The results present a challenge to theories that aim to explain the insulating state of YbB12. Science , this issue p. [65][1]; see also p. [32][2] [1]: /lookup/volpage/362/65?iss=6410 [2]: /lookup/doi/10.1126/science.aau3840

A real-time humidity sensor based on a microwave oscillator with conducting polymer PEDOT:PSS film

In this paper, we propose a humidity sensor based on a negative resistance oscillator using conducting polymer (CP) PEDOT:PSS film at room temperature. The proposed humidity sensor basically consists of a microstrip line and a negative resistance circuit, which creates a negative resistance for high-frequency oscillation. The microstrip line has a circuit structure that is connected through a via hole between the ground plane and the signal line with a CP thin film. From our experimental results, we find that the oscillation frequency of the sensor gradually decreases as the relative humidity (RH) increases. Owing to the conductivity variation of the CP film with the RH value, the oscillation frequency of the sensor sensitively changes in real time. Therefore, we suggest that our microwave-oscillator-based sensor scheme is a good candidate for the design of a robust and stable humidity sensor.

Effect of density on microwave-induced resistance oscillations in back-gated GaAs quantum wells

We report on microwave-induced resistance oscillations (MIROs) in a tunable-density 30-nm-wide GaAs/AlGaAs quantum well. We find that the MIRO amplitude increases dramatically with carrier density. Our analysis shows that the anticipated increase in the effective microwave power and quantum lifetime with density is not sufficient to explain the observed growth of the amplitude. We further observe that the fundamental oscillation extrema move towards cyclotron resonance with increasing density, which also contradicts theoretical predictions. These findings reveal that the density dependence is not properly captured by existing theories, calling for further studies.

Mechanism research of negative resistance oscillations characteristics of the silicon magnetic sensitive transistor with long base region

A silicon magnetic sensitive transistor (SMST) with the negative resistance oscillation phenomenon is presented in this paper. The SMST of cubic structure is composed of three regions and three electrodes (E, C and B). Two of the regions (collector region and base region) are designed on the top surface of the SMST, and the emitter region is designed at the bottom of the SMST. Using microelectromechanical system (MEMS) technology, the chip is fabricated on [Formula: see text] orientation p-type silicon (near intrinsic) wafer and packaged on printed circuit board (PCB). When collector voltage (V[Formula: see text]) and the base injecting current (I[Formula: see text]) are a certain value, the experimental results show that the collector current (I[Formula: see text]) attains negative resistance oscillation phenomenon and it is influenced by the external magnetic field (B) and temperature (T). Based on the effect of deep-level impurities on the carrier net recombination rate, theoretical analysis demonstrates that the deep-level impurities are the main factors of the appearance for oscillations phenomenon.

A Phase-Noise Reduced Microwave Oscillator Sensor With Enhanced Limit of Detection Using Active Filter

A microwave planar negative resistance oscillator with higher frequency stability and lower phase noise (PN) is introduced for sensing applications. Split-ring resonator is used as sensing element at 2.4 GHz and is used for microfluidic concentration sensing of high lossy solution of water in ethanol. Stabilizing the frequency response with the use of active filter leads to 14-times enhancement in the limit of detection, from 7% (passive filter) to 0.5% (active filter), while the linearity of the sensor is preserved; the sensing range is then increased. Nearly 13-dBc/Hz improvement in the measured PN, in great agreement with theory, also verifies the frequency domain stability of the oscillator.

Quantum oscillations of electrical resistivity in an insulator.

In metals, orbital motions of conduction electrons on the Fermi surface are quantized in magnetic fields, which is manifested by quantum oscillations in electrical resistivity. This Landau quantization is generally absent in insulators. Here, we report a notable exception in an insulator-ytterbium dodecaboride (YbB12). The resistivity of YbB12, which is of a much larger magnitude than the resistivity in metals, exhibits distinct quantum oscillations. These unconventional oscillations arise from the insulating bulk, even though the temperature dependence of the oscillation amplitude follows the conventional Fermi liquid theory of metals with a large effective mass. Quantum oscillations in the magnetic torque are also observed, albeit with a lighter effective mass.

Mathematical modeling of functioning of a tillage machine-tractor unit

The soil-cultivating machine-tractor unit undergoes continuously changing external influences that lead to fluctuations in the speed of movement, slippage of the tractor's driving wheels, loading of the transmission, and increased fuel consumption. The main sources of disturbances that cause the oscillations of the tractor are the unevenness of the traction resistance of the tillers and the unevenness of the field microprofile. The purpose of the research is to improve the process of functioning of the soil-processing machine-tractor unit by modeling of the influence of external action. The mathematical model of the process of operation of a wheeled tractor in the traction mode of operation is a machine-tractor unit in the form of a dynamic system with two input forces, determined by the load on the working elements and the roughness of the relief. The mathematical model includes the equations of the motor and the regulator, the clutch, the power transmission, the driving wheel and the longitudinal-vertical oscillations of the tractor. The solution of the mathematical model of the machine-tractor unit is based on the fourth-order Runge-Kutta numerical integration method at a constant step. Calculations are made for the motion of the unit with periodic sinusoidal disturbances applied to the input of the model. As a result of calculations, it is determined that the effect of the microfield of the field and the traction resistance of the working bodies on the performance of the machine-tractor unit is different. The greatest amplitudes of vertical accelerations are achieved due to the effect of field unevenness. Fluctuations in the speed of movement, hectare fuel consumption, engine torque are caused by the periodic component of traction resistance of low frequency, and slippage and driving torque of the wheels - as well as unevenness of the field microfilm. The average values of the slippage of the tractor's driving wheels and hectare fuel consumption are mainly affected by the oscillations of the traction resistance. The use of diagonal-parallel tires instead of radial makes it possible to reduce the slipping of the tractor's drive wheels from 24 % (when working on serial tires) to 16 % (on test tires) and to reduce the hectare fuel consumption by 6 %.

Microwave-induced zero-resistance states in a high-mobility two-subband electron system

In this study, a selectively-doped GaAs/AlAs heterostructure was used to fabricate a high-mobility two-subband electronic system with a substantially different concentration of electrons in the subbands. We observe microwave photoresistance at high numbers of magneto-intersubband oscillations (MISO). The system under study demonstrates microwave-induced resistance oscillations (MIRO) and MISO interference. The MIRO in the studied two-subband system appears in lower magnetic fields compared to the MISO. This is an indication of some additional mechanism that exists in the two-subband system and is responsible for the MISO amplitude damping in low magnetic fields, while it does not affect the MIRO amplitude. Zero resistance states appear in the system under study under microwave irradiation in a narrow range of magnetic fields near the MISO maxima.

Renormalization of the Effective Electron Mass Governing the Period of Microwave-Induced Resistance Oscillations in ZnO/MgZnO Heterojunctions

The phenomenon of microwave-induced magnetoresistance oscillations is studied in a series of ZnO/MgZnO heterojunctions characterized by different two-dimensional electron densities n. It is found that the effective electron mass m* determined from the period of microwave-induced magnetoresistance oscillations depends essentially on this parameter. For high densities, the value of m* tends to the effective electron mass in bulk ZnO, while for low densities, m* increases pronouncedly and becomes considerably larger than the electron cyclotron mass. The experimental results give clear evidence of a significant impact of the electron–electron interaction on microwave-induced magnetoresistance oscillations.

B-periodic oscillations in the Hall-resistance induced by a dc-current-bias under combined microwave-excitation and dc-current bias in the GaAs/AlGaAs 2D system

We report the observation of dc-current-bias-induced B-periodic Hall resistance oscillations and Hall plateaus in the GaAs/AlGaAs 2D system under combined microwave radiation- and dc bias excitation at liquid helium temperatures. The Hall resistance oscillations and plateaus appear together with concomitant oscillations also in the diagonal magnetoresistance. The periods of Hall and diagonal resistance oscillations are nearly identical, and source power (P) dependent measurements demonstrate sub-linear relationship of the oscillation amplitude with P over the span 0 < P ≤ 20 mW.

Energy band and Hall resistivity, longitudinal resistivity and Shubnikov-de Haas oscillation in graphenes (II)

Energy band and Hall resistivity, longitudinal resistivity and Shubnikov-de Haas oscillation in graphenes (II)

Design of Negative Resistance Oscillator with Rocord Low Phase Noise

The aim of this paper is to use a new design of a negative resistance microwave oscillator in order to fabricate oscillator with very good performance in terms of output power, efficiency, stability and phase noise. In this study the new concept of oscillator using distributed resonator and micro-strip circuit elements improve performances of our structure. A micro-strip microwave oscillator with low phase noise based on an NPN silicon planar epitaxial transistor has been designed, fabricated, and characterized. In this design, each step has been conducted by using Advanced Design System (ADS) and following a theoretical study which enable to optimize the different performances of the whole circuit. The oscillator produce a sinusoidal signal with spectrum power of 12.25 dBm at 2.45 GHz into 50 Ω load when polarized at Vcc=15V with DC to RF efficiency of 16. The obtained phase noise of -120 dBc/Hz at 100 Hz offset is the result of the use of high Q factor resonator and the depth study of the parameters of the oscillator. Simulation and measurement results are in good agreement.

Long-term electrical conductivity stability of graphene under uncontrolled ambient conditions

The stability of graphene under oxidative conditions is crucial to both its applications as an electrical material and the lifetime of graphene-based devices. In this work, large-area high quality graphene film was synthesized through chemical vapor deposition (CVD-G) and transferred onto SiO2/Si substrate. Based on the electrical resistance monitoring, its conductivity stability in air was excellent under uncontrolled ambient conditions within 500 days. After kept in air over years, only tiny resistance oscillation and small increase of Raman D peak intensity of CVD-G have been observed. During a year, CVD-G had the highest resistance in July with high temperature and high relative humidity (the relative resistance changes were no more than 10%). The conductivity of CVD-G returned to the normal value again in the subsequent autumn (October) due to its reversible absorptions of oxidative gas species in air. The morphology characterizations of CVD-G showed it had a well-maintained continuous structure, therefore its electrical resistance was stable.

Floquet theory of microwave absorption by an impurity in the two-dimensional electron gas

We investigate the dynamics of a two-dimensional electron gas (2DEG) under circular polarized microwave radiation in the presence of dilute localized impurities. Inspired by recent developments on Floquet topological insulators we obtain the Floquet wave functions of this system which allow us to predict the microwave absorption and charge density responses of the electron gas; we demonstrate how these properties can be understood from the underlying semiclassical dynamics even for impurities with a size of around a magnetic length. The charge density response takes the form of a rotating charge density vortex around the impurity that can lead to a significant renormalization of the external microwave field which becomes strongly inhomogeneous on the scale of a cyclotron radius around the impurity. We show that this inhomogeneity can suppress the circular polarization dependence which is theoretically expected for microwave induced resistance oscillations but which was not observed in experiments on semiconducting 2DEGs. Our explanation for this so far unexplained polarization independence has close similarities with the Azbel'-Kaner effect in metals where the interaction length between the microwave field and conduction electrons is much smaller than the cyclotron radius due to skin effect generating harmonics of the cyclotron resonance.