Zero-resistance State Research Articles

Magneto-transport characteristics of a 2D electron system driven to negative magneto-conductivity by microwave photoexcitation

Negative diagonal magneto-conductivity/resistivity is a spectacular- and thought provoking-property of driven, far-from-equilibrium, low dimensional electronic systems. The physical response of this exotic electronic state is not yet fully understood since it is rarely encountered in experiment. The microwave-radiation-induced zero-resistance state in the high mobility GaAs/AlGaAs 2D electron system is believed to be an example where negative magneto-conductivity/resistivity is responsible for the observed phenomena. Here, we examine the magneto-transport characteristics of this negative conductivity/resistivity state in the microwave photo-excited two-dimensional electron system (2DES) through a numerical solution of the associated boundary value problem. The results suggest, surprisingly, that a bare negative diagonal conductivity/resistivity state in the 2DES under photo-excitation should yield a positive diagonal resistance, with a concomitant sign reversal in the Hall voltage.

Erratum: Theoretical Approach to Microwave-Radiation-Induced Zero-Resistance States in 2D Electron Systems [Phys. Rev. Lett.94, 016806 (2005)

Erratum: Theoretical Approach to Microwave-Radiation-Induced Zero-Resistance States in 2D Electron Systems [Phys. Rev. Lett.94, 016806 (2005)

Emergence of Domains and Nonlinear Transport in the Zero-Resistance State

We study transport in the domain state, the so-called zero-resistance state, that emerges in a two-dimensional electron system in which the combined action of microwave radiation and magnetic field produces a negative absolute conductivity. We show that the voltage-biased system has a rich phase diagram in the system size and voltage plane, with second- and first-order transitions between the domain and homogeneous states for small and large voltages, respectively. We find the residual negative dissipative resistance in the stable domain state.

Towards a synchronization theory of microwave-induced zero-resistance states

We develop the synchronization theory of microwave induced zero-resistance states (ZRS) for two-dimensional electron gas in a magnetic field. In this theory the dissipative effects lead to synchronization of cyclotron phase with driving microwave phase at certain resonant ratios between microwave and cyclotron frequencies. This synchronization produces stabilization of electron transport along edge channels and at the same time it gives suppression of dissipative scattering on local impurities and dissipative conductivity in the bulk, thus creating the ZRS phases at that frequency ratios. The electron dynamics along edge and around circular disk impurity is well described by the Chirikov standard map. The theoretical analysis is based on extensive numerical simulations of classical electron transport in a strongly nonlinear regime. We also discuss the value of activation energy obtained in our model and the experimental signatures that could establish the synchronization origin of ZRS.

Possible room temperature superconductivity in conductors obtained by bringing alkanes into contact with a graphite surface

Electrical resistances of conductors obtained by bringing alkanes into contact with a graphite surface have been investigated at room temperatures. Ring current in a ring-shaped container into which n-octane-soaked thin graphite flakes were compressed did not decay for 50 days at room temperature. After two HOPG plates were immersed into n-heptane and n-octane at room temperature, changes in resistances of the two samples were measured by four terminal technique. The measurement showed that the resistances of these samples decrease to less than the smallest resistance that can be measured with a high resolution digital voltmeter (0.1μV). The observation of persistent currents in the ring-shaped container suggests that the HOPG plates immersed in n-heptane and n-octane really entered zero-resistance state at room temperature. These results suggest that room temperature superconductor may be obtained by bringing alkanes into contact with a graphite surface.

Supra-oscillatory critical temperature dependence of Nb-Ho bilayers

We investigate the critical temperature Tc of a thin s-wave superconductor (Nb) proximity coupled to a helical rare-earth ferromagnet (Ho). As a function of the Ho layer thickness, we observe multiple oscillations of Tc superimposed on a slow decay, that we attribute to the influence of the Ho on the Nb proximity effect. Because of the inhomogeneous magnetization in Ho, singlet and triplet pair correlations are present in the bilayers. We take both into consideration when solving the self-consistent Bogoliubov-de Gennes equations, and we find a reasonable agreement with the experiments. We also observe non-trivial transitions into the superconducting state, the zero resistance state being attained after two successive transitions which appear to be associated with the magnetic structure of Ho.

Linear Polarization Rotation Study of the Microwave-Induced Magnetoresistance Oscillations in the GaAs/AlGaAs System

ABSTRACTMicrowave-induced zero-resistance states appear when the associated B-1-periodic magnetoresistance oscillations grow in amplitude and become comparable to the dark resistance of the two-dimensional electron system (2DES). Existing theories have made differing predictions regarding the influence of the microwave polarization in this phenomenon. We have investigated the effect of rotating, in-situ, the polarization of linearly polarized microwaves relative to long-axis of Hall bars. The results indicate that the amplitude of the magnetoresistance oscillations is remarkably responsive to the relative orientation between the linearly polarized microwave electric field and the current-axis in the specimen. At low microwave power, P, experiments indicate a strong sinusoidal variation in the diagonal resistance Rxx vs. θ at the oscillatory extrema of the microwave-induced magnetoresistance oscillations. Interestingly, the phase shift θ0 for maximal oscillatory Rxx response under photoexcitation is a strong function of the magnetic field, the extremum in question, and the magnetic field orientation.

Enhancement of Flux Pinning in YBa2Cu3O7−δ + Ag Nanocomposites

YBa2Cu3O7−δ/Ag thin film is prepared through pulsed laser deposition. The presence of Ag brings about significant inter- and intragranular modification in the mi- crostructure of the composites. The resistive broadening in the tail part is suggested to be associated with the link be- tween the grains and to be extremely sensitive to magnetic fields. It is assumed that the zero resistance state at the Tc0 region of the above high temperature superconductor is gov- erned by the excitations in the weak link network. Invoking Beans critical state model, enhancement in critical current density Jc is observed.

Non-linear conduction in the critical region of the superconductor-insulator transition in TiN thin films

We investigate experimentally electric transport at both the superconducting and insulating sides of the superconductor to insulator transition in TiN thin films. Relatively deeply on the superconducting side we find sharp transitions of R(T) to a zero-resistance state. Near the transition temperature the current-voltage I(V)-characteristics develops power-law type of non-linearity even for low currents. Close to the transition the superconducting state is destroyed by small magnetic fields and develops into an insulator with a thermally activated conductance. At low temperatures and low bias voltage we observe I ∝ Vα dependences in the insulating state with an exponent α rapidly growing with decreasing temperature. This behavior is strikingly similar to the V(I)'s observed in the superconducting state, if I and V are interchanged.

Nonequilibrium phenomena in high Landau levels

Developments in the physics of 2D electron systems during the last decade have revealed a new class of nonequilibrium phenomena in the presence of a moderately strong magnetic field. The hallmark of these phenomena is magnetoresistance oscillations generated by the external forces that drive the electron system out of equilibrium. The rich set of dramatic phenomena of this kind, discovered in high mobility semiconductor nanostructures, includes, in particular, microwave radiation-induced resistance oscillations and zero-resistance states, as well as Hall field-induced resistance oscillations and associated zero-differential resistance states. We review the experimental manifestations of these phenomena and the unified theoretical framework for describing them in terms of a quantum kinetic equation. The survey contains also a thorough discussion of the magnetotransport properties of 2D electrons in the linear response regime, as well as an outlook on future directions, including related nonequilibrium phenomena in other 2D electron systems.

Enhancement of edge channel transport by a low-frequency irradiation

The magnetotransport properties of high mobility two dimensional electron gas have recently attracted a significant interest due to the discovery of microwave induced zero resistance states. Here we show experimentally that microwave irradiation with a photon energy much smaller than the spacing between Landau levels can induce a strong decrease in the four terminal resistance. This effect is not predicted by the bulk transport models introduced to explain zero resistance states, but can be naturally explained by an edge transport model. This highlights the importance of edge channels for zero resistance state physics that was proposed recently.

Coulombic effects on magnetoconductivity oscillations induced by microwave excitation in multisubband two-dimensional electron systems

We develop a theory of magneto-oscillations in photoconductivity of multisubband two-dimensional electron systems which takes into account strong Coulomb interaction between electrons. In the presence of a magnetic field oriented perpendicular, internal electric fields of fluctuational origin cause fast drift velocities of electron orbit centers which affect probabilities of inter-subband scattering and the photoconductivity. For the electron system formed on the liquid helium surface, internal forces are shown to suppress the amplitude of magnetooscillations, and change positions of magnetoconductivity minima which evolve in zero-resistance states for high radiation power.

STRUCTURE AND CONDUCTIVITY FLUCTUATIONS OF THE Y3Ba5Cu8O18 SUPERCONDUCTOR

Synthesis of the Y 3 Ba 5 Cu 8 O 18 superconducting material by the standard solid state reaction is reported. DC resistivity measurements reveal a bulk Tc = 97.86 K, which was determined by the criterion of the maximum in the numerical temperature derivative of electrical resistivity. Structure characterization was performed by means of the X-ray diffraction (XRD) technique. A Rietveld refinement of XRD patterns shows that the material crystallizes in an orthorhombic structure, space group Pmm2 with cell parameters a = 3.9211(3) Å, b = 3.8514(1) Å and c = 31.0170(0) Å. In order to study the pairing mechanism close to Tc, conductivity fluctuation analysis was performed by the method of logarithmic temperature derivative of the conductivity excess. Close and above Tc, the conductivity fluctuation analysis revealed the occurrence of two fluctuation regimes characterized by the critical exponents λ 3D = 0.48 and λ 2D = 0.97. These regions were interpreted as corresponding to 3D and 2D Gaussian regimes, respectively. By the utilization of the Ginzburg–Landau theory, we experimentally determined the Ginzburg number. Our results are in agreement with that reported on YBa 2 Cu 3 O 7-δ, but an enhancement of the Gaussian fluctuation regimes was experimentally detected. The correlations of the critical exponents with the dimensionality of the fluctuation system for each Gaussian regime were performed by using the Aslamazov–Larkin theory. Closer to Tc, a genuinely critical regime was identified. Scaling of our results permits to establish that the dynamical of fluctuation system has the universality class described by the 3D-XY model. Below Tc, it is observed that for several values of transport current density, this material evidences a paracoherent regime followed by a coherent transition, which occurs in the regime of the zero resistance state.

Magnetoplasmon resonance in a two-dimensional electron system driven into a zero-resistance state

We report on a very strong, and a rather sharp, photoresistance peak originating from a dimensional magnetoplasmon resonance (MPR) in a high-mobility GaAs/AlGaAs quantum well driven by microwave radiation into a zero-resistance state (ZRS). The analysis of the MPR signal reveals a negative background, providing experimental evidence for the concept of absolute negative resistance associated with the ZRS. When the system is further subject to a dc field, the maxima of microwave-induced resistance oscillations decay away and the system reveals a state with close-to-zero differential resistance. The MPR peak, on the other hand, remains essentially unchanged, indicating robust Ohmic behavior under the MPR conditions.

Phase diagram of the strongly disordereds-wave superconductor NbN close to the metal-insulator transition

We present a phase diagram as a function of disorder in three-dimensional NbN thin films, as the system enters the critical disorder for the destruction of the superconducting state. The superconducting state is investigated using a combination of magnetotransport and tunneling spectroscopy measurements. Our studies reveal 3 different disorder regimes. At low disorder the (k_{F}l~10-4), the system follows the mean field Bardeen-Cooper-Schrieffer behavior where the superconducting energy gap vanishes at the temperature where electrical resistance appears. For stronger disorder (k_{F}l<4) a "pseudogap" state emerges where a gap in the electronic spectrum persists up to temperatures much higher than Tc, suggesting that Cooper pairs continue to exist in the system even after the zero resistance state is destroyed. Finally, very strongly disordered samples (k_{F}l<1) exhibit a pronounced magnetoresistance peak at low temperatures, suggesting that localized Cooper pairs continue to survive in the system even after the global superconducting ground state is completely destroyed.

Theory of Zero-Resistance States Generated by Radiation in GaAs/AlGaAs

Mani observed zero-registance states similar to those quantum-Hall-effect states in GaAs/AlGaAs but without the Hall resistance plateaus upon the application of radiations [R. G. Mani, Physica E 22, 1 (2004)]. An interpretation is presented. The applied radiation excites “holes”. The condensed composite (c)-bosons formed in the excited channel create a superconducting state with an energy gap. The supercondensate suppresses the non-condensed c-bosons at the higher energy, but it cannot suppress the c-fermions in the base channel, and the small normal current accompanied by the Hall field yeilds a B-linear Hall resistivity.

Zero-resistance states in bilayer electron systems induced by microwave irradiation

We experimentally demonstrate the existence of microwave-induced zero-resistance states (ZRS) in bilayer electron systems created in wide quantum wells. In contrast to single-layer two-dimensional electron systems, ZRS are developed from the strongest magneto-intersubband oscillation peaks inverted by microwave radiation. Our experimental work is discussed within the theories of microscopic mechanisms of photoresistance, and the calculations reveal that the condition for absolute negative resistivity correlates with the appearance of ZRS.

Macroscopic Superconducting Current through a Silicon Surface Reconstruction with Indium Adatoms:Si(111)−(7×3)−In

Macroscopic and robust supercurrents are observed by direct electron transport measurements on a silicon surface reconstruction with In adatoms [Si(111)-(√7 × √3)-In]. The superconducting transition manifests itself as an emergence of the zero resistance state below 2.8 K. I-V characteristics exhibit sharp and hysteretic switching between superconducting and normal states with well-defined critical and retrapping currents. The two-dimensional (2D) critical current density J(2D,c) is estimated to be as high as 1.8 A/m at 1.8 K. The temperature dependence of J(2D,c) indicates that the surface atomic steps play the role of strongly coupled Josephson junctions.

Negative conductivity and anomalous screening in two-dimensional electron systems subjected to microwave radiation

A 2D electron system in a quantized magnetic field can be driven by microwave radiation into a non-equilibrium state with strong magnetooscillations of the dissipative conductivity. We demonstrate that in such system a negative conductivity can coexist with a positive diffusion coefficient. In a finite system, solution of coupled electrostatic and linear transport problems shows that the diffusion can stabilize a state with negative conductivity. Specifically, this happens when the system size is smaller than the absolute value of the non-equilibrium screening length that diverges at the point where the conductivity changes sign. We predict that a negative resistance can be measured in such a state. Further, for a non-zero difference between the work functions of two contacts, we explore the distribution of the electrostatic potential and of the electron density in the sample. We show that in the diffusion-stabilized regime of negative conductivity the system splits into two regions with opposite directions of electric field. This effect is a precursor of the domain structure that has been predicted to emerge spontaneously in the microwave-induced zero-resistance states.

FLUCTUATION-INDUCED CONDUCTIVITY OF POLYCRYSTALLINE Er1-xCexBa2Cu3O7-δ SUPERCONDUCTOR

A detailed study of the effect caused by the partial substitution of Er by Ce on fluctuation conductivity of the Er 1-x Ce x Ba 2 Cu 3 O 7-δ; 0 ≤ x ≤ 0.10; is presented. The combined experimental results of structural and electrical measurements indicate that Ce substitutes Er for x ≤ 0.10 with no significant structural distortions. However, there is suppression of the superconducting state with zero-resistance state at temperatures ≥ 86.3 K. The results were analyzed in terms of the temperature derivative of the resistivity and the logarithmic temperature derivative of the conductivity to identify power-law divergences of the conductivity. The data revealed the occurrence of a two-stage intragranular-intergranular transition. Above the critical temperature, the Gaussian and critical regimes were observed. Also, from the critical exponents analysis, we observed a splitting of the pairing transition for Ce -doped samples, suggesting the occurrence of a phase separation. On approaching the zero resistance state, our results showed a power-law behavior that corresponds to a phase transition from a paracoherent to a coherent state of the granular array and which does not depend on Ce doping.