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 behavior 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 tantalizing prospect. Finding and
synthesizing new materials that exhibit unconventional superconductivity has
become the condensed matter physicist's Holy Grail—but getting there
involves a deeper understanding of exotic, topological electronic behavior
in materials.
An exotic type of electron transport behavior 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
behavior.
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 (µSR) 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
µBohr 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 polarization 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 µSR 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.
Reference:
Zurab Guguchia, Time-reversal symmetry-breaking charge order in a kagome
superconductor, Nature (2022).
DOI: 10.1038/s41586-021-04327-z.
Tags:
Physics