Graphic shows the first kind of supercurrent forming vortices. Image: Egor Babaev

In this 100th anniversary year of the discovery of
superconductivity, physicists at the University
of Massachusetts Amherst and Sweden’s Royal
Institute of Technology have published a fully self-consistent theory of the
new kind of superconducting behavior, Type 1.5, in Physical Review B.

In three recent papers, the authors report on their detailed
investigations to show that a Type 1.5 superconducting state is indeed possible
in a class of materials called multiband superconductors.

For years, most physicists believed that superconductors
must be either Type I or Type II. Type 1.5 superconductivity is the subject of
intense debate because until now there was no theory to connect the physics
with micro-scale properties of real materials, says Egor Babaev of UMass
Amherst, currently a fellow at the technology institute in Stockholm, with
Mikhail Silaev, a postdoctoral researcher there.

Their new papers now provides a theoretical framework to allow
scientists to calculate conditions necessary for the appearance of Type 1.5
superconductivity, which exhibits characteristics of Types I and II previously
thought to be antagonistic.

Superconductivity is a state where electric charge flows
without resistance. In Type I and Type II, charge flow patterns are
dramatically different. Type I, discovered in 1911, has two state-defining
properties: Lack of electric resistance and the fact that it does not allow an
external magnetic field to pass through it. When a magnetic field is applied to
these materials, superconducting electrons produce a strong current on the
surface which in turn produces a magnetic field in the opposite direction.
Inside this type of superconductor, the external magnetic field and the field
created by the surface flow of electrons add up to zero. That is, they cancel
each other out.

Type II superconductivity was predicted to exist by a
Russian theoretical physicist who said there should be superconducting
materials where a complicated flow of superconducting electrons can happen deep
in the interior. In Type II material, a magnetic field can gradually penetrate,
carried by vortices like tiny electronic tornadoes, Babaev explains. The
combined works that theoretically described Type I and II superconductivity won
the Nobel Prize in 2003.

Classifying superconductors in this way turned out to be
very robust: All superconducting materials discovered in the last half-century
can be classified as either, Babaev says. But he believed a state must exist
that does not fall into either camp: Type 1.5. By working out the theoretical
bases for superconducting materials, he had predicted that in some materials,
superconducting electrons could be classed as two competing types or
subpopulations, one behaving like electrons in Type I material, the other
behaving like electrons in a Type II material.

Babaev also says that Type 1.5 superconductors should form
something like a super-regular Swiss cheese, with clusters of tightly packed
vortex droplets of two kinds of electron: one type bunched together and a
second type flowing on the surface of vortex clusters in a way similar to how
electrons flow on the exterior of Type I superconductors. These vortex clusters
are separated by “voids,” with no vortices, no currents and no
magnetic field.

The major objection raised by skeptics, he recalls, is that
fundamentally there is only one kind of electron, so it’s difficult to accept
that two types of superconducting electron populations could exist with such
dramatically different behaviors.

To answer this, Silaev and Babaev developed their theory to
explain how real materials can give raise to Type-1.5 superconductivity, taking
into account interactions at microscales. In a parallel effort, their colleagues
at UMass Amherst and in Sweden
including Johan Carlstrom and Julien Garaud, with Babaev, used supercomputers
to perform large-scale numerical calculations modeling the behavior of
superconducting electrons to better understand the structure of vortex clusters
and what they look like in a Type-1.5 superconductor.

They found that under certain conditions they could describe
new, additional forces at work between the Type-1.5 vortices, which can give
vortex clusters very complicated structure. As more work is done on
superconductivity, the team of physicists in Stockholm and at UMass Amherst say the family
of multi-band superconducting materials will grow. They expect that some of the
newly discovered materials will belong in Type 1.5.

“With the development of theory that works on the
microscopic level, as well as our better understanding of inter-vortex
interaction, we can now connect the properties of vortex clusters with the
properties of electronic structure of concrete materials. This can be useful in
establishing whether materials belong in the Type 1.5 superconductivity
domain,” says Babaev.

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