New insight into unconventional superconductivity

Date:
February 9, 2022
Source:
Paul Scherrer Institute
Summary:
Signatures for a novel electronic phase that enables charge
to flow spontaneously in loops have been observed in a kagome
superconductor.

Using ultra-sensitive muon spin spectroscopy, researchers discovered
time-reversal symmetry-breaking magnetic fields inside the material,
indicating the existence of long-searched-for 'orbital currents'.



FULL STORY ========================================================================== Signatures for a novel electronic phase that enables charge
to flow spontaneously in loops have been observed in a kagome
superconductor. Using ultra-sensitive muon spin spectroscopy, researchers discovered time-reversal symmetry-breaking magnetic fields inside
the material, indicating the existence of long-searched-for 'orbital
currents'. The discovery, published today in Nature, aids understanding
of high-temperature superconductivity and quantum phenomena underpinning next-generation device research.


==========================================================================
The kagome pattern, a network of corner-sharing triangles, is well
known amongst traditional Japanese basket weavers -- and condensed
matter physicists.

The unusual geometry of metal atoms in the kagome lattice and resulting electron behaviour makes it a playground for probing weird and wonderful quantum phenomena that form the basis of next-generation device research.

A key example is unconventional -- such as high-temperature - - superconductivity, which does not follow the conventional laws of superconductivity. Most superconducting materials exhibit their seemingly magical property of zero resistance at a few degrees Kelvin: temperatures
that are simply impractical for most applications. Materials that
exhibit so-called 'high-temperature' superconductivity, at temperatures achievable with liquid nitrogen cooling (or even at room temperature),
are a tantalising prospect.

Finding and synthesising new materials that exhibit unconventional superconductivity has become the condensed matter physicist's Holy
Grail -- but getting there involves a deeper understanding ofexotic, topological electronic behaviour in materials.

An exotic type of electron transport behaviour that results in a
spontaneous flow of charge in loops has long been debated as a precursor
to high- temperature superconductivity and as a mechanism behind another mysterious phenomenon: the quantum anomalous Hall effect. This topological effect, the subject of F. Duncan M. Haldane's 2016 Nobel Prize winning
work, occurs in certain two-dimensional electronic materials and relates
to the generation of a current even in the absence of an applied magnetic field. Understanding the quantum anomalous Hall effect is important not
only for fundamental physics, but also for the potential applications in
novel electronics and devices. Now, a PSI-led international collaboration
has discovered strong evidence supporting this elusive electron transport behaviour.

Time-reversal symmetry-breaking charge ordering in the kagome
superconductor KV3Sb5 The team, led by researchers from PSI's Laboratory
for Muon Spin Spectroscopy, discovered weak internal magnetic fields
indicative of an exotic charge ordering in a correlated kagome
superconductor. These magnetic fields break so- called time-reversal
symmetry, a type of symmetry that means that the laws of physics are
the same whether you look at a system going forward or backward in time.



==========================================================================
A natural explanation of the occurrence of time-reversal symmetry-breaking fields is a novel type of charge order. The charge ordering can be
understood as a periodic modulation of the electron density through the
lattice and rearrangement of the atoms into a higher-order (superlattice) structure. The team focused their study on the kagome lattice, KV3Sb5,
which superconducts below 2.5 Kelvin. Below a higher critical temperature
of approximately 80 Kelvin, a giant quantum anomalous Hall effect is
observed in the material, which was previously unexplained. The exotic
charge ordering appears below this critical temperature of approximately
80 Kelvin, termed the 'charge ordering temperature'.

The discovered time-reversal symmetry-breaking fields implies an exotic
type of charge order where currents move around the unit cells of the
kagome lattice, known as orbital currents. These produce magnetism
dominated by the extended orbital motion of the electrons in a lattice
of atoms.

"Experimental realization of this phenomenon is exceptionally challenging,
as materials exhibiting orbital currents are rare and the characteristic signals [of orbital currents] are often too weak to be detected,"
explains corresponding author, Zurab Guguchia, from the Lab of Muon Spin Spectroscopy at PSI, who led the team.

Although previous studies have shown the breaking of time-reversal
symmetry below the superconducting temperature, this is the first example
in which time- reversal symmetry is broken by charge order. This means
that this putative exotic charge order classes as a new quantum phase
of matter.

An extremely convincing piece of evidence To search for the long
disputed orbital currents, the physicists used highly sensitive muon spin rotation/relaxation spectroscopy (mySR) to detect the weak, tell-tale
magnetic signals that they would generate. Muons implanted into the sample serve as a local and highly sensitive magnetic probe to the internal
field of the material, enabling magnetic fields as small as 0.001
myBohr to be detected. In the presence of an internal magnetic field,
the muon spin depolarises. The muons decay into energetic positrons,
which are emitted along the direction of the muon spin, carrying with
them information on the muon spin polarisation in the local environment.



==========================================================================
The researchers observed how, as the temperature is decreased to below
80K, the charge ordering temperature, a systematic shift in the magnetic
signal appeared. Using the world's most advanced mySR facility at PSI,
which enables application of fields up to 9.5 Tesla, the team could
use an external high magnetic field to enhance the shift in the tiny
internal magnetic fields and provide even stronger evidence that the
magnetic field was due to internal orbital currents.

"We first performed the experiment with no external field," explains Dr.

Guguchia, "and when we saw the systematic shift appear below the charge ordering temperature, we felt very motivated to continue. But when we then applied the high field and could promote this electronic response, we were delighted. It's a very, very convincing piece of evidence for something
that has remained elusive for a long time." A deeper understanding of unconventional superconductivity and the quantum anomalous Hall effect The research provides arguably the strongest evidence yet that long debated
orbital currents actually exist in the kagome material KV3Sb5. Theory
suggests that the quantum anomalous Hall effect originates from orbital currents.

Therefore, orbital currents have been proposed in a number of
unconventional superconductors that exhibit a strangely large quantum
anomalous Hall effect; namely graphene, cuprates and kagome lattices,
but actual evidence that they existed had been missing until now.

The discovery of time-reversal symmetry-breaking fields, which imply
orbital currents -- and the peculiar charge ordering that gives rise
to them, opens doors to exotic avenues of physics and next-generation
device research. Orbital currents are considered to play a fundamental
role in the mechanism of various unconventional transport phenomena
including high-temperature superconductivity, with applications from
power transmission to MAGLEV trains.

The concept of orbital currents also forms the basis of orbitronics --
an area that exploits the orbital degree of freedom as an information
carrier in solid- state devices.

This work was carried out in collaboration with the group of Zahid Hasan
at Princeton University, in which Guguchia is a visiting scientist, and
with other colleagues from the University of Zu"rich Physics Institute, Institute of Physics Chinese Academy of Sciences, Songshan Lake Materials Laboratory in China, Renmin University of China, Rice University, Oak
Ridge National Laboratory, University of Wu"rzburg and Max-Planck-Institut
fu"r Festko"rperforschung.

========================================================================== Story Source: Materials provided by Paul_Scherrer_Institute. Original
written by Miriam Arrell. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. C. Mielke, D. Das, J.-X. Yin, H. Liu, R. Gupta, Y.-X. Jiang,
M. Medarde,
X. Wu, H. C. Lei, J. Chang, Pengcheng Dai, Q. Si, H. Miao,
R. Thomale, T.

Neupert, Y. Shi, R. Khasanov, M. Z. Hasan, H. Luetkens, Z. Guguchia.

Time-reversal symmetry-breaking charge order in a kagome
superconductor.

Nature, 2022; 602 (7896): 245 DOI: 10.1038/s41586-021-04327-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220209112105.htm

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