Superconducting nanowires pulse to a new beat

“Initially, this project started as a search for the well known Little-Parks oscillations in superconducting nanodevices,” said Alexey Bezryadin. “Unexpectedly, our measurements on these two-nanowire devices revealed a strange class of periodic oscillations in resistance with applied magnetic field that were qualitatively different from the expected Little-Parks effect.”

Bezryadin and colleagues made the devices by arranging two DNA molecules across a roughly 100 nm-wide trench in SiN/SiO₂ on a silicon chip. Then they sputter-coated the molecules and substrate with superconducting Mo₂₁Ge₇₉.

The resulting nanowires became superconducting at low temperatures, with their resistance decreasing exponentially with temperature. As is typical for nanowires, they did not exhibit zero resistance.

“In the absence of a magnetic field, these ultra-narrow wires exhibited a nonzero resistance over a broad temperature range,” said Bezryadin. “At temperatures where thicker wires would already be superconducting, these DNA-templated wires remained resistive.”

When a magnetic field was present, the device showed regular oscillations of resistance with the magnetic field. To investigate the effect, the researchers tested devices with different geometries, varying the lead width and interwire spacing. This enabled them to formulate an explanation for the behaviour.

“The applied magnetic field causes a small current to flow along the trench banks, and this current then causes a large change in resistance,” explained Paul Goldbart. “The strength of the current is controlled only by the magnetic field and the width of the banks supporting the wires.”

The researchers say their device is very sensitive to magnetic fields and, if coupled to a scanning probe microscope, can be used to detect local variations in magnetic field. “The device is also sensitive to phase gradients of the superconducting order parameter,” said Bezryadin. “Thus it may be used as a superconducting phase gradiometer.”

Now the scientists plan to produce phase variations in the device by injecting electrical currents into the electrodes, without using magnetic fields. “If successful, this approach will prove that our NQUID - nanowire quantum interference device - can be used as a phase gradiometer,” said Bezryadin. “We also plan to develop DNA self-assembly strategies and use them for fabrication of nanowire networks with complex structures.”

The researchers reported their work in Science.