Science Gazette

Unusual superconductivity gets a new look

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In a kagome superconductor, signatures for a unique electronic phase that allows charge to flow spontaneously in loops have been found. Researchers identified time-reversal symmetry-breaking magnetic fields within the material using ultra-sensitive muon spin spectroscopy, revealing the presence of long-sought ‘orbital currents.’ The finding, which was published in Nature today, will help researchers better comprehend high-temperature superconductivity and quantum processes that drive next-generation gadget development.

Traditional Japanese basket weavers — and condensed matter physicists — are familiar with the kagome pattern, which is a network of corner-sharing triangles. The kagome lattice’s peculiar structure of metal atoms, as well as the consequent electron behavior, gives it a great place to investigate strange and amazing quantum phenomena that are at the heart of next-generation device development.

Unconventional superconductivity, such as high-temperature superconductivity, is a prime example, since it defies standard superconducting equations. At temperatures of a few degrees Kelvin, most superconducting materials demonstrate their almost miraculous feature of zero resistance: temperatures that are simply unsuitable for most uses. At temperatures feasible with liquid nitrogen cooling (or even at room temperature), materials that exhibit so-called “high-temperature” superconductivity are a tantalizing potential. Finding and synthesising novel materials with unusual superconductivity has become the Holy Grail for condensed matter physicists, but getting there requires a greater understanding of exotic, topological electrical behavior in materials.

Long thought to be a prelude to high-temperature superconductivity and a cause behind another unexplained phenomena, the quantum anomalous Hall effect, an unusual sort of electron transport behavior that results in a spontaneous flow of charge in loops has been contested. This topological phenomenon, which was the focus of F. Duncan M. Haldane’s Nobel Prize-winning study in 2016, happens in certain two-dimensional electronic materials and refers to the creation of a current even when no magnetic field is present. Understanding the quantum anomalous Hall effect is crucial not just for basic physics, but also for innovative electronics and gadgets. Now, a PSI-led worldwide effort has found compelling evidence for this enigmatic electron transport behavior.

Charge ordering in the kagome superconductor KV3Sb5 with time-reversal symmetry breakdown

The team, lead by experts from PSI’s Laboratory for Muon Spin Spectroscopy, detected weak internal magnetic fields in a linked kagome superconductor, indicating an unconventional charge ordering. These magnetic fields violate so-called time-reversal symmetry, which states that the rules of physics are the same whether the system is viewed forward or backward in time.

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A unique sort of charge order provides a natural explanation for the emergence of time-reversal symmetry-breaking fields. A periodic modulation of the electron density via the lattice and rearranging of the atoms into a higher-order (superlattice) structure may be interpreted as charge ordering. The researchers concentrated their research on the kagome lattice, KV3Sb5, which superconducts at temperatures below 2.5 Kelvin. A huge quantum anomalous Hall effect is detected in the material below a higher critical temperature of around 80 Kelvin, which was previously unexplained. Below this crucial temperature of around 80 Kelvin, known as the ‘charge ordering temperature,’ unusual charge ordering emerges.

The discovery of time-reversal symmetry-breaking fields suggests an unusual sort of charge order known as orbital currents, in which currents flow across the unit cells of the kagome lattice. The prolonged orbital motion of electrons in a lattice of atoms dominates the magnetism produced by them.

“Experimental realization of this phenomena is exceedingly difficult, since materials showing orbital currents are uncommon, and the distinctive signals [of orbital currents] are typically too faint to be detected,” says corresponding author Zurab Guguchia of PSI’s Muon Spin Spectroscopy Lab.

Although prior research has proven that time-reversal symmetry can be broken below superconducting temperatures, this is the first time that charge order has been used to break time-reversal symmetry. This implies that the alleged unusual charge arrangement is a new quantum phase of matter.

This is a really strong piece of evidence

The researchers employed very sensitive muon spin rotation/relaxation spectroscopy (SR) to identify the modest, tell-tale magnetic signals that the long debated orbital currents would create. Muons injected into the sample act as a localized, very sensitive magnetic probe for the material’s intrinsic field, allowing magnetic fields as tiny as 0.001 Bohr to be measured. The muon spin depolarizes in the presence of an internal magnetic field. The muons decay into intense positrons, which are released in the direction of the muon spin and convey information about the muon spin polarization in the surrounding environment.

When the temperature was reduced below 80K, the charge ordering temperature, the researchers saw a consistent change in the magnetic signal. The scientists could employ an external high magnetic field to increase the shift in the small internal magnetic fields and offer even stronger proof that the magnetic field was due to internal orbital currents by using the world’s most powerful SR facility at PSI, which can apply fields up to 9.5 Tesla.

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“”We started with no external field and were really inspired to keep going when we noticed the systematic shift develop below the charge ordering temperature,” Dr. Guguchia reveals. We were ecstatic, though, when we used the high field and were able to generate this electrical reaction. It’s a pretty strong piece of proof for something that has eluded scientists for a long time.”

Unconventional superconductivity and the quantum anomalous Hall effect: a better understanding

The findings are likely the most conclusive proof yet that long-debated orbital currents really occur in the kagome substance KV3Sb5. The quantum anomalous Hall effect is thought to be caused by orbital currents, according to theory. As a result, orbital currents have been hypothesized in a variety of unconventional superconductors that display a very strong quantum anomalous Hall effect, such as graphene, cuprates, and kagome lattices, but there had been no real proof that they existed until recently.

The discovery of time-reversal symmetry-breaking fields, which imply orbital currents, as well as the unique charge ordering that causes them, opens up new physics and next-generation device research options. Orbital currents are thought to be important in the mechanism of a variety of unusual transport phenomena, including as high-temperature superconductivity, with applications ranging from power transmission to MAGLEV trains. Orbital currents are also the foundation of orbitronics, which uses the orbital degree of freedom as an information carrier in solid-state electronics.

This research was done in collaboration with Zahid Hasan’s group at Princeton University, where Guguchia is a visiting scientist, as well as colleagues from the University of Zürich 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 Würzburg, and the Max-Planck-Institut für Festkörperforschung.

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