Congratulations to Andrey Rogachev, associate professor, and Kevin Davenport, graduate research assistant! Their research, in association with an international team of physicists, was published in the October 8, 2018, issue of Nature Physics

Abrikosov vortices help scientists to explain inconsistencies in “dirty” superconductors theory

An international team of physicists, including scientists from the University of Grenoble, the Landau Institute for Theoretical Physics, the Weizmann Institute of Sciences, and the University of Utah (Andrey Rogachev and Kevin Davenport) have explained anomalous low-temperature behavior of “dirty” superconductors. These materials possess various non-trivial properties which make them a necessary part of quantum computers with superconductive qubits. In a paper published in Nature Physics, scientists report how “dirty” superconductors can violate the conventional theory of superconductivity. With these results, it becomes possible to engineer superconductive qubits that are perfectly isolated from the outer disturbances and thus can be fully used for quantum computing.

Andrey Rogachev, associate professor (left) and Kevin Davenport, graduate research assistant (right)

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Space jets accelerate particles and send a high energy signal to Earth.

The night sky seems serene, but telescopes tell us that the universe is filled with collisions and explosions. Distant, violent events signal their presence by spewing light and particles in all directions. When these messengers reach Earth, scientists can use them to map out the action-packed sky, helping to better understand the volatile processes happening deep within space.
 
For the first time, an international collaboration of scientists, including physicists from the University of Utah, has detected highly energetic light coming from the outermost regions of an unusual star system within our own galaxy. The source is a microquasar—a black hole that gobbles up matter from a nearby companion star and blasts out powerful jets of material. The team’s observations, described in the Oct. 4 issue of the journal Nature, strongly suggest that electron acceleration and collisions at the ends of the microquasar’s jets produced the powerful gamma rays. Scientists think that studying messengers from this microquasar may offer a glimpse into more extreme events happening at the centers of distant galaxies.

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University of Utah physicist led the design, construction, upgrade of the VERITAS instrument

Dr. Dave Kieda

Dr. Anushka Udara Abeysekara

The VERITAS array has confirmed the detection of high-energy gamma rays from the vicinity of a supermassive black hole located in a distant galaxy, TXS 0506+056. While these detections are relatively common for VERITAS, this blackhole is potentially the first known astrophysical source of high-energy cosmic neutrinos, a type of ghostly subatomic particle that can be made at astrophysical sources of ultra-high energy cosmic rays.

The University of Utah is one of the founding collaborating institutions of the VERITAS observatory. Co-author Dave Kieda, professor of physics and astronomy and the dean of the U’s graduate school, led the design, construction and upgrade of VERITAS that gave the instrument enhanced sensitivity to the lower-energy gamma rays critical to the discovery. Anushka Udara Abeysekara, research assistant professor of physics and astronomy at the U, is also a coauthor on the paper.

“This is the first time we’ve seen high-energy gamma-rays and neutrinos being generated by a common astrophysical source. This is evidence that nearby and faraway galaxies with supermassive blackholes at their centers are actively creating high-energy cosmic rays,” said Kieda. “It’s one of the pieces of the puzzle needed to solve the mystery of where these cosmic rays come from.”

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ALL DETAILS PREDICTED BY CRITICAL THEORY WERE CONFIRMED ON REAL OBJECTS IN A LAB

PHOTO CREDIT: Andrey Rogachev, adapted from a figure in Nature Physics 10.1038/s41567-018-0179-8.
This schematic diagram shows the quantum phase transition of a superconducting metal to a normal metal at zero temperature. As the magnetic field increases in strength, the superconductivity breaks down until the critical point at which the material becomes a normal metal.

The struggle to keep drinks cold during the summer is a lesson in classical phase transitions. To study phase transitions, apply heat to a substance and watch how its properties change. Add heat to water and at the so-called “critical point,” watch as it transforms into a gas (steam). Remove heat from water and watch it turn into a solid (ice).

Now, imagine that you’ve cooled everything down to very low temperatures — so low that all thermal effects vanish. Welcome to the quantum realm, where pressure and magnetic fields cause new phases to emerge in a phenomenon called quantum phase transitions (QPT). More than a simple transition from one phase to another, QPT form completely new properties, such as superconductivity, in certain materials.

Apply voltage to a superconductive metal, and the electrons travel through the material with no resistance; electrical current will flow forever without slowing down or producing heat. Some metals become superconducting at high temperatures, which has important applications in electric power transmission and superconductor-based data processing. Scientists discovered the phenomenon 30 years ago, but the mechanism for superconductivity remains an enigma because the majority of materials are too complex to understand QPT physics in details. A good strategy would be first to look at less complicated model systems.

Now, University of Utah physicists and collaborators have discovered that superconducting nanowires made of MoGe alloy undergo quantum phase transitions from a superconducting to a normal metal state when placed in an increasing magnetic field at low temperatures. The study is the first to uncover the microscopic process by which the material loses its superconductivity; the magnetic field breaks apart pairs of electrons, called Cooper pairs, which interact with other Cooper pairs and experience a damping force from unpaired electrons present in the system.

The findings are fully explained by the critical theory proposed by coauthor Adrian Del Maestro, associate professor at the University of Vermont. 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. This is the first time in the field of superconductivity that all details of QPT predicted by a theory were confirmed on real objects in the lab.

“Quantum phase transitions may sound really exotic, but they are observed in many systems, from the center of stars to the nucleus of atoms, and from magnets to insulators,” said Andrey Rogachev, associate professor at the U and senior author of the study.“By understanding quantum fluctuations in this simpler system, we can talk about every detail of the microscopic process and apply it to more complicated objects.”

The study published online July 9, 2018 in Nature Physics.

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