By Josh Perry, Editor
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Researchers at the University of Utah (Salt Lake City, Utah) have observed the microscopic process in which nanomaterial loses its superconductivity by demonstrating how superconducting nanowires of MoGe alloy have a quantum phase transition (QPT) to normal metal state in magnetic fields at low temperatures.

An illustration that describes Del Maestro’s pair-breaking critical theory in nanowires. (Adrian Del Maestro)

According to a report from the university, the magnetic field break apart Cooper pairs of electrons, which then interact with other Cooper pairs and the unpaired electrons cause a damping effect on conductivity.

This study confirms in lab experiments a theory that was originally proposed by a University of Vermont professor; the first time that a prediction about QPT has been proven in a lab. “The theory correctly described how the evolution of superconductivity depends on critical temperature, magnetic field magnitude and orientation, nanowire cross sectional area, and the microscopic characteristics of the nanowire material,” the article explained.

The original theory stated that the nanoscale of the wire would make it difficult for electrons to travel together and that a powerful magnetic field would disentangle the electrons by curving their paths, ultimately damping the superconductivity of the nanowire.

Researchers tested this theory by creating MoGe nanowires with diameters smaller than 10 nanometers through an innovative negative e-beam lithography process. The nanowires were cooled and a magnetic field was applied at the Institut Néel in Grenoble, France.

The next step for researchers is to test nanowires composed of cuprates, which have a QPT between a magnetic state that has been theorixed could promote superconductivity. “The cuprates are often called high-temperature superconductors because they go to the superconducting state at the record-high temperature of 90-155 K, a contrast to the rather small critical temperature of MoGe alloys at 3 – 7 K,” the article added.

The research was recently published in Nature Physics. The abstract read:

“Quantum phase transitions (QPT) between distinct ground states of matter are widespread phenomena, yet there are only a few experimentally accessible systems where the microscopic mechanism of the transition can be tested and understood.

“These cases are unique and form the experimentally established foundation for our understanding of quantum critical phenomena. Here we report that a magnetic-field-driven QPT in superconducting nanowires—a prototypical one-dimensional system (d=1)—can be fully explained by the critical theory of pair-breaking transitions characterized by a correlation length exponent v≈1 and dynamic critical exponent z≈2.

“We find that in the quantum critical regime, the electrical conductivity is in agreement with a theoretically predicted scaling function and, moreover, that the theory quantitatively describes the dependence of conductivity on the critical temperature, field magnitude and orientation, nanowire cross-sectional area, and microscopic parameters of the nanowire material. At the critical field, the conductivity follows a T^(d–2)/z dependence predicted by phenomenological scaling theories and more recently obtained within a holographic framework.

“Our work uncovers the microscopic processes governing the transition: the pair-breaking effect of the magnetic field on interacting Cooper pairs overdamped by their coupling to electronic degrees of freedom. It also reveals the universal character of continuous quantum phase transitions.”