Superconductivity

John Clarke, Program Leader

This program brings together two groups working on superconducting materials: one (Clarke) on device aspects—SQUIDs—and the other (R.C. Dynes) on transport properties. Program thrusts include:

  • Ultrasensitive, ultralow-frequency detection of NMR and MRI using a SQUID.

  • Development of quantum-limited SQUID-based amplifiers with dissipative readout in the frequency range 0.5-5 GHz

  • Development of quantum-limited SQUIDs with nanometer linewidths to investigate the magnetic properties of single molecules.

  • Reduction of l/f flux noise in SQUIDs and superconducting circuits.

  • Probing transport in superconductors at the atomic scale using a low-temperature scanning tunneling microscope (STM) with a superconducting tip

  • Measurement of Hall effect, low-temperature conductivity and magnetoresistance at thin-film oxide interfaces and in multiferroics.

CURRENT PROJECTS

Quantum-limited SQUIDs with dissipative readout (Clarke)

  • Design of optimally matched input circuitry to achieve the quantum limit of measurement.

  • Experimental investigation of a theory that a lower noise temperature should be achieved at a frequency slightly lower than the value at which the gain peaks.

Nanoscale SQUIDs with non-dissipative readout (Clarke)

  • SQUIDs are fabricated from Al films in several designs–an aluminum-coated nanotube to be used both as a Josephson junction and an efficient coupling element to a single molecule or a Josephson tunnel junction with oxide barriers and a very narrow, separate constriction to provide coupling to a molecule.

  • Flux changes in the SQUID are determined by incorporating the SQUID in a resonator, applying microwaves and observing the phase shift of the reflected signal.

  • l/f Flux Noise (Clarke)
    · A theoretical study is in progress to elucidate the origin of the large density of electron spins that have recently been shown to be responsible for the noise.

NMR and MRI (Clarke)

  • Perform MRI in zero applied magnetic field by applying appropriate pulses of magnetic field and magnetic field gradient.

  • MRI measurements using a newly constructed 150-mT polarizing coil are expected to result in improved spatial resolution of phantoms containing water or mineral oil. A study of “T1-contrast” of phantoms containing liquids with different values of the longitudinal relaxation time T1 is in progress

  • Investigation of magnetic resonance elastography (MRE), a technique that uses MRI to measure the elasticity of soft materials.

Figure 1 Longitudinal relaxation-time (T1)-weighted contrast images at different
magnetic fields. (a) Phantom consists of plastic tubes 1–6 mm in diameter, containing
water, immersed in a dilute solution of agarose gel. (b) Image at 100 mT. (c) Image at
132 mT, showing greatly enhanced contrast.

Superconducting STM (Dynes)
Measurements on high-transition temperature superconductors using an STM with a superconducting tip exhibit Josephson tunneling between s-wave and d-wave superconductors on an atomic scale. These studies will map the spatial variation of the superconducting order parameter in these oxide materials.

Complex Oxides (Dynes)
Low-temperature measurements of the Hall effect, electrical conductivity, magnetoresistance and tunneling characteristics are performed on the interfaces between a wide range of thin film materials, including high-transition temperature superconductors, conductors, semiconductors, insulators, multiferroics, ferromagnets, antiferromagnets, ferroelectrics, thermoelectrics and piezoelectrics. These measurements lead to the identification of new phases generated by enhanced quantum fluctuations due to reduced dimensionality, frustration in the spin, charge and orbital degrees of freedom, or competition between different forms of electronic order.