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Superconductors

Superconductor Computer Elements Based on the Josephson Effect

Objective

•      R&D in the pre-eminent Josephson Superconductor Computer Project of IBM over 5 years until its termination in 1983.

Technique

•      Prime emphasis directed at the design, modelling (ASTAP, Monte Carlo) and measurement of devices and integrated circuits such as

-      Josephson interferometers, shaped tunnel junctions to suppress side lobes in the magnetic interference patterns, peripheral circuits including memory array line drivers and sense bus, single-magnetic flux quantum main memory and cache memory (world’s first results).

Results

•      Example: fast pulse measurements on simulated 4k sub-memory arrays including peripheral circuits. Access times in ns-regime.

Selected References

→   Henkels, Geppert, Kadlec, Epperlein, Beha, Chang, Jaeckel: “Josephson 4 K-bit cache memory design for a prototype signal processor. I. General overview“. J. Appl. Phys. 58, 2371-2378, (1985).

→   Henkels, Geppert, Kadlec, Epperlein, Beha, Chang, Jaeckel: “Josephson 4 K-bit cache memory design for a prototype signal processor. II. Cell array and drivers“. J. Appl. Phys. 58, 2379-2388, (1985).

→   Henkels, Geppert, Kadlec, Epperlein, Beha, Chang, Jaeckel: “Josephson 4 K-bit cache memory design for a prototype signal processor. III. Decoding, sensing and timing“. J. Appl. Phys. 58, 2389-2399, (1985).

Superconducting Tunnel Junction Barrier Uniformity

Problem

•      Abnormal side lobe height variations in supercurrent versus magnetic flux characteristics of Josephson superconducting tunnel junction devices. Current-controlled magnetic flux dependence used for driver operations to address superconducting memories. Abnormal side lobes led to loss in magnetic flux and hence to unreliable memory operation.

Technique

•      Resolution:  Possible root-cause – patchy, non-uniform tunnel barrier layer (tunnelling probability).

•      First indication of existence of a non-uniform tunnel barrier by observation of so-called sub-harmonic gap structure in the current-voltage characteristics of superconducting tunnel junctions. Can be interpreted within a multi-particle tunnelling model.

•      Pioneered in collaboration with the University of Tübingen, Germany, an innovative technique to image directly the tunnel barrier uniformity by measuring the local tunnel current density. The novel, world’s first experiments gave evidence of the existence of a non-uniform tunnelling probability across the tunnel junction area by recording the voltage signal generated by the electron-beam scanned over the current-biased sample held at liquid helium temperatures in a scanning electron microscope.

•      Technique was used

-       to establish optimal conditions to grow uniform tunnel barriers.

-       as quality gate in an established manufacturing process to assure the formation of uniform high-quality tunnel barriers.

Outcome

•      Measured current density distributions were in qualitative agreement with the experimental magnetic field dependence of the DC Josephson current of the respective tunnel junction.

•      Controlled growth conditions enabled uniform tunnel barriers and hence reliable, repeatable and optimum driver and memory functionality.

Selected References

→   Epperlein, Seifert, Huebener: “Two-dimensional imaging of the current-density distribution in superconducting tunnel junctions“. Physics Letters 92A, 146-150, (1982).

→   Seifert, Huebener, Epperlein: “Imaging of spatial structures in superconducting tunnel junctions by electron-beam scanning“. Physics Letters 95A, 326-330, (1983).

→   Seifert, Huebener, Epperlein: “High-resolution imaging of inhomogeneities in superconducting tunnel junctions by scanning with a modulated electron beam“. Physics Letters 97A, 421-423, (1983).

Sub-Harmonic Energy Gap Structure

Problem

•      Root-cause of well-pronounced current steps at distinct voltages in the single-particle tunnelling regime of current-voltage characteristics of superconducting tunnel junctions.

Technique

•      Fabrication of symmetric and asymmetric tunnel junctions by thin film deposition and tunnel barrier formation by oxidation in an oxygen glow discharge. I/V measurements in thermal tunnelling regime with high current and voltage resolution.

Results

•      Current step structure can clearly be attributed to the subharmonic gap structure SGS.

•      According to detailed analysis the SGS is due to multi-particle tunnelling processes indicating a non-uniform, ‘patchy’ tunnel barrier (tunnelling probability) across the junction area.

•      Heights of SGS current steps directly impact side lobes in the magnetic interference patterns (DC Josephson current versus magnetic flux) of shaped tunnel junctions.

Selected References

→   Epperlein: “Supercurrent interference patterns and quasiparticle excess currents in Josephson tunnel junctions” in “Squid’80“, eds. Hahlbohm, Luebbig, Walter de Gruyter & Co., 131-146, (1980).

→   Epperlein: “Multiparticle tunneling observed in superconducting junctions“. Physica 108B, 999-1000, (1981).

→   Epperlein: “Model for the temperature dependence of the subharmonic gap structure in superconducting tunnel junctions”. Bull. American Phys. Soc. 28, 464 (1983).

Superconducting Magnetic Penetration Depths

Problem

•      Measurement of magnetic penetration depth in evaporated thin superconducting films.

Technique

•      Pioneered an innovative 2-Josephson junction interferometer technique to measure absolute magnetic penetration depths l. (World’s-first results).

-      Special configurations of ground plane, junction layers and control line layers for the magnetic field enabled an absolute measurement of l on both sides of a film with an accuracy of ± 7%.

Results

•      l values measured in Nb films as function of residual resistivity r_Tc and temperature T are within the local London theory and the experimental l(T) dependence follows the empirical approximation l(T) = l(0) [1-(T/T_c)⁴]^-1/2 in the range 4 K £ T £ 8 K for a film with the critical temperature T_c = 9 K and r_Tc = 5.8 mWcm.

Selected References

→   Epperlein: “Magnetic penetration depths in superconducting Nb films“. Physica 108B, 931-932, (1981).

Non-Equilibrium Quasiparticle Distribution in Thin Superconducting Films  

Problem

•      Measure the energy gap reduction generated by non-equilibrium quasiparticle distributions.

Technique

•      Use of superconducting double tunnel junction stack structures. Excess, non-thermal quasiparticles injected via single-particle tunnelling in lower junction. Energy gap reduction and non-equilibrium quasiparticle population in middle film probed by top tunnel junction by measuring I-V characteristics at low temperatures well below the critical temperature.

Results

•      Energy gap reduction stronger than in thermal case.

•      Gap instability at the critical gap reduction ratio.

•      Both, in support of the chemical potential µ^*-model (Owen, Scalapino) for a first-order phase transition.

Selected References

→   Fuchs, Epperlein, Welte, Eisenmenger: “Energy gap reduction in superconducting tin films by quasiparticle injection”.  Phys. Rev. Lett. 38, 919-922, (1977).

Electron Recombination Times in Superconducting Tunnel Junctions

Problem

•      Measure effective electron (quasiparticle) recombination times t_eff in thin superconducting Al-, Sn- and Pb films as function of temperature, phonon re-absorption, phonon loss processes, and film disorder.

Technique

•      t_eff derived directly from time decay measurements of excess quasiparticle concentrations injected by current or laser pulses in single tunnel junctions below the critical temperature T_c.

•      Disordered Al films (strength of granularity) produced by a controlled oxygen background evaporation.

Results (selected, world’s-first)

•      Exponential increase of effective times with decreasing temperature with typical values in the range 10 to 1000 ns at 1.5 K and dependent on material Al, Sn, Pb, respectively.

•      Experimental times are within a ray acoustic model considering film thickness dependence of phonon re-absorption, phonon volume losses and phonon escape processes.

•      Reduced times in perturbed Al films are due to enhanced phonon escape by diffuse scattering at the microcrystalline, irregular junction boundaries, and an increase of the electron-phonon coupling.

Selected References

→   Epperlein, Lassmann, Eisenmenger: “Quasiparticle recombination time in superconducting tin and normal electronic density of states at the Fermi surface from tunnel junction experiments“. Z. Physik B 31, 377-384, (1978).

→   Epperlein, Eisenmenger: “Measurement of effective quasiparticle recombination times and the densities of electronic states at the Fermi level in superconducting Al- and Pb-films“. Z. Physik B 32, 167-174, (1979).

Density of States at the Fermi Level in Thin Superconducting Films

Problem

•      Measure density of states at Fermi Level N₀ in different thin film superconductors. Compare with respective bulk values. Evaluate relevance of N₀ of perturbed films to critical temperature T_c enhancement.