Ultrasmall Josephson Junctions Research Articles

Charge-fluctuation effect on the critical temperature of layered high-Tcsuperconductors

The value of the superconducting critical temperature (${\mathit{T}}_{\mathit{c}}$) of artificially layered superconductors made out of alternating Y-Ba-Cu-O and Pr-Ba-Cu-O layers is calculated as a function of both the number of layers of pure Y-Ba-Cu-O within a unit cell of the superlattice and the thickness of the insulating (Pr-Ba-Cu-O) layer. The calculation is performed within a model of electrostatically coupled two-dimensional (2D) arrays of ultrasmall Josephson junctions. The underlying mechanism is assumed to be the depression of the Beresinskii-Kosterlitz-Thouless transition temperature (${\mathit{T}}_{\mathrm{BKT}}$) by quantum phase fluctuations due to charging effects. The ${\mathit{T}}_{\mathit{c}}$ value of the entire structure can then be tuned by varying the charging energy that depends on the average neighborhood of a typical site in Cu-O planes of Y-Ba-Cu-O layers. Recent experiments on such structures are described very accurately by the model. Furthermore, the ratio (${\mathit{T}}_{\mathit{c}}^{(\mathit{n})}$-${\mathit{T}}_{\mathit{c}}^{(1)}$)/(${\mathit{T}}_{\mathit{c}}^{(2)}$-${\mathit{T}}_{\mathit{c}}^{(1)}$), where ${\mathit{T}}_{\mathit{c}}^{(\mathit{n})}$ is the critical temperature of a thin film made up of n Y-Ba-Cu-O unit cells, is found to be independent of the fitting parameters. This prediction is well confirmed by available experimental data. Furthermore, the model also applies for the series of Bi- and Tl-based cuprates of general formulas ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{\mathit{n}\mathrm{\ensuremath{-}}1}$${\mathrm{Cu}}_{\mathit{n}}$${\mathrm{O}}_{\mathit{y}}$ and ${\mathrm{Tl}}_{2}$ (1)${\mathrm{Ba}}_{2}$${\mathrm{Ca}}_{\mathit{n}\mathrm{\ensuremath{-}}1}$${\mathrm{Cu}}_{\mathit{n}}$${\mathrm{O}}_{\mathit{y}}$ containing n Cu-O planes per unit cell, for which the observed values of the ratio \ensuremath{\rho}=(${\mathit{T}}_{\mathit{c}}^{(3)}$-${\mathit{T}}_{\mathit{c}}^{(1)}$)/(${\mathit{T}}_{\mathit{c}}^{(2)}$-${\mathit{T}}_{\mathit{c}}^{(1)}$) agree within 10% with the predicted ones.

Effect of external circuit on the charge dynamics of ultrasmall Josephson junctions

A real time path integral formalism is developed which allows one to investigate the charge dynamics of ultrasmall Josephson tunnel junctions. For large values of the external inductance hysteretic behaviour of the Josephson junction is found. The effect of charge fluctuations is investigated and quantum corrections to the current-voltage characteristic are calculated. It is shown that the linewidth of Bloch oscillations remains finite even for T = 0.

Phase locking of the Bloch oscillations in ultrasmall Josephson junctions

We have shown that in the case when the quasiparticle conductance GT of an ultrasmall Josephson tunnel junction and ohmic conductance GS of its ohmic enviroment are both present, the Bloch oscillations (in contrast to the SET oscillations which are accompanying them in the junctions with weak Josephson coupling) can be perfectly phase locked only in weakly shunted junctions, GS«GT.

Two ways toward experimental observation of the Bloch oscillations in ultrasmall Josephson junctions

Two possible strategies of experimental observation of the Bloch oscillations with frequency f=I/2e in ultrasmall Josephson tunnel junctions are analyzed. The analysis shows that the effect can be observed in junctions of conventional superconductors, using both the high-ohmic-resistor and normal-junction-array schemes of the dc biasing.

Coulomb blockade of cooper pair tunneling

We examine charging effects for Cooper pairs in ultrasmall Josephson junctions. For small ratio of Josephson coupling energy to charging energy the current-voltage characteristics show a Coulomb gap for Cooper pair tunneling. We observe tunneling of Cooper pairs at voltages where resonance with environmental modes occurs.

Influence on the electrical characteristics of the—NH radicals incorporated into PECVD silicon nitride films : O. Sanchez, C. Gomez-Aleixandre, M. Fernandez and J. M. Albella. Vacuum39(7/8), 727 (1989)

We study the competition between Josephson and charging energies in two-dimensional arrays of ultrasmall Josephson junctions, when the mutual capacitance is dominant over the self-capacitance. Our calculations involve a combination of an analytic WKB renormalization group approach plus nonperturbative Quantum Monte Carlo simulations. We consider the zero frustration case in detail and we are able to make a successful comparison between our results and those obtained experimentally.

Phase-slip mechanism in ultrasmall Josephson junctions with fluctuating coupling

The rate of the phase-slip in an overdamped Josephson junction in the presence of both the thermal quasiparticle noise and the critical current fluctuations is calculated. Dramatic increase of the standard Kramers rate is predicted for large intensity of the critical current noise. For a temperature-independent noise intensity, produced by quantum ionic tunneling processes in the junction barrier, significant departures from the Arrhenius law take place in the T-dependence of the rates below a characteristics temperature which may exceed the temperature for the crossover from the thermal activation to the macroscopic quantum tunneling.

Quasiparticle effects in quantum-induced transitions in superconductors.

The effects of quasi-particle tunneling in the long-range properties of an array of ultrasmall Josephson junctions, with a transverse magnetic field, is studied in two dimensions via extensive quantum Monte Carlo simulations. It is found that quasi-particle tunneling suppresses the zero-point quantum-fluctuation effects in the arrays, as well as triggering a discontinuous transition between the low-temperature metastable phase to the stable high-temperature superconducting phase.

Resonant tunneling phenomena in ultrasmall Josephson junctions - II. radiative transition vs successive tunneling

Resonant tunneling of a macroscopic variable excites a system in a Josephson potential. Two types of quantum processes, a radiative transition to the local ground state and a successive tunneling to the next potential well, can occur and complete in this excited state. Competition between these processes is discussed. It is shown that the radiative transition dominates in a large junction-capacitance region. Crossover from this process to successive tunneling occurs with decreasing capacitance.

New coherent states in periodic arrays of ultrasmall Josephson junctions.

An extensive study of the thermodynamics of a two-dimensional periodic array of ultrasmall Josephson junctions with and without a transverse magnetic field is presented. A quantum Monte Carlo algorithm is introduced to study a model that includes the Josephson energy, ${E}_{J}$, as well as the charging energy, ${E}_{c}$, contributions. The superfluid density, internal energy, and specific heat for different lattice sizes and numbers of Monte Carlo simulation sweeps are studied as a function of the ratio \ensuremath{\alpha}=${E}_{c}$/${E}_{J}$, the temperature and the magnitude of the magnetic field. When \ensuremath{\alpha}\ensuremath{\ne}0, it is found that as the temperature is lowered the model has two phase transitions. First, a second-order Berezinskii-Kosterlitz-Thouless (BKT) transition renormalized by the quantum fluctuations represented by a finite \ensuremath{\alpha}. Below this BKT transition the system has long-range phase coherence; thus it is a state with zero resistance. At lower temperatures, a first-order phase transition appears which is entirely due to the quantum fluctuations that nucleate vortex excitations. Below this ``quantum induced transition'' (QUIT) the model still has a finite but diminished superfluid density, thus indicating that the QUIT is between two different zero-resistance states, one dominated by thermal fluctuations and the other by quantum fluctuations. A QUIT is found to be more pronounced in the case where there is a magnetic field. The case studied here corresponds in the classical limit to the fully frustrated state. Finally, we discuss the physical properties of this new low-temperature phase as well as the necessary conditions to test this prediction experimentally.

Combined effect of Zener and quasiparticle transitions on the dynamics of mesoscopic Josephson junctions.

We study ultrasmall Josephson junctions using the coherent picture to describe the pair dynamics and the semiclassical approach for the quasiparticles. We find that the dynamics depends crucially on the ratio of the superconducting gap energy \ensuremath{\Delta}(T) to the charging energy ${\mathrm{e}}^{2}$/2C. The pair ``Bloch oscillations'' can be practically observed when 2\ensuremath{\Delta}(T)g${\mathrm{e}}^{2}$/2C; for 2\ensuremath{\Delta}(T)${\mathrm{e}}^{2}$/2C the ${\mathrm{I}}_{\mathrm{dc}/\mathrm{e}}$ oscillations due to quasiparticle tunneling can be observed. We discuss under what conditions the I-V characteristic will be S-shaped, exhibiting both oscillations at different values of the current.

Resonant tunneling phenomena in ultrasmall Josephson junctions

A resonant tunneling phenomenon in a current-biased ultrasmall Josephson junction is predicted. It is shown that the tunneling is possible only at discrete values of bias current due to the quantization of the Josephson plasma oscillation.

Resonant Tunneling of Macroscopic Variables in Ultrasmall Josephson Junctions and SQUID Rings

A resonant tunneling phenomenon in a current-biased ultrasmall Josephson junction is predicted. It is shown that the tunneling is possible only at discrete values of bias current due to the quantization of Josephson plasma oscillation. We also predict an emission of a microwave photon accompanying the resonant tunneling, and suggest that observation of this photon may provide a possible proof for the existence of Macroscopic Quantum Coherence (MQC).

Quantum theory of the dc Josephson effect: Static tunneling characteristics of ultrasmall Josephson junctions

We examine the static tunneling characteristics of ultrasmall Josephson junctions that are current biased on the zero-voltage, dc step. Novel scaling, magnetic, and thermal characteristics are exhibited by the super-current as the device is scaled down from the 1.0 to the 0.01 ..mu..m/sup 2/ regime. Furthermore, the noise voltage is found to peak for devices in which the electrostatic energy required to transfer a pair is equal to the interaction energy between the two superconductors. For such junctions, noise voltages are on the order of several mV, and the magnitude of the supercurrent is found to be a very sensitive function of device area and operating temperature. Furthermore, these features of the junction's dynamics have technological implications regarding digital applications of such devices.

Scaling characteristics of ultrasmall Josephson junctions

Recent advances in the technology of ultrasmall Josephson junctions focus interest on the scaling characteristics of these devices. We examine the size dependence of the pair current, the rms noise current as well as the voltage fluctuations of ultrasmall junctions, and find novel scaling behavior. Among other things, the low-temperature zero voltage, dc pair current depends on the cube of the junction area.

Tunneling characteristics of ultrasmall Josephson junctions

Abstract The tunneling characteristics of ultrasmall Josephson junctions in the strong charge fluctuations regime are examined. We find that electrostatic effects strikingly alter the magnitude, area and phase dependence of the dc supercurrent as well as its magnetic and thermal characteristics.

Dynamic Stark Splitting of Plasmon States in Ultrasmall Josephson Junctions

The structure and dynamics of the Anderson plasma mode in a high-Q, high--current-density, low-capacitance Josephson junction is examined. It is found that if the junction is subjected to a microwave field which is resonant with a plasmon transition, the relevant levels will be split in a manner analogous to the Stark effect.