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New Research into Kagome Lattice Superconductor Reveals Correlated Electron States

Sourse: N.Mori, Public Domain

Electrical resistance at normal temperatures is a characteristic most materials exhibit when confronted with an electrical current. Because of this resistance, some energy is lost as heat. Moreover, for many materials, this resistance remains even if the material is cooled to very low temperatures. In some materials, however, direct current electricity is conducted without energy loss even when the materials are cooled below a critical temperature. This phenomenon is called superconductivity. In superconductors an electric current can exist indefinitely in a loop of superconducting wire with no power source, while offering no resistance to the flow of electricity.

Now (2021), scientists have discovered that on a kagome lattice superconductor a complex landscape of electronic states can exist at the same time. The centre of the study was a bulk single crystal of a topological kagome metal CsV3Sb5 which becomes superconducting below minus 235 degrees Celsius. The material consisted of atomic planes composed of vanadium atoms on a so-called kagome lattice which was made up of a pattern of interlaced triangles and hexagons on top of one another, with caesium and antimony spacer layers lodged between the kagome planes. The material analysed was not magnetic, which is why the scientists were able to have a look at how electrons in such systems behaved in the absence of magnetism. The team employed scanning tunnelling spectroscopy to analyse the quantum interference effects of the electron liquid. In the course of the experiments the scientists discovered a great variety of symmetry-broken phases of the electron liquid driven by the correlation between the electrons in the material. As the temperature of the material was lowered, ripples, or standing waves, so-called charge density waves, were witnessed to be the first to develop in the electron liquid, however with a different periodicity from the underlying atomic lattice. At a lower temperature, a new standing wave component nucleated only along one direction of the crystal axes, so that electrical conduction there was different from that in any other direction. These phases developed in the normal state, the non-superconducting metallic state and persisted below the superconducting transition. All in all, the experiments showed that superconductivity in CsV3Sb5 was initiated by, and coexisted with, a correlated quantum electronic state that broke the spatial symmetries of the crystal.

The study of kagome superconductors is a fairly new discipline. In 2019, scientists discovered a new kagome prototype structure, KV3Sb5, as well as the isostructural compounds RbV3Sb5 and CsV3Sb5. The materials showed a two-dimensional kagome net of vanadium. Density-functional theory calculations indicated that the materials were metallic, with the Fermi level, which is the thermodynamic work required to add one electron to the body, being close to several Dirac points, the crossing-point for electrons. Magnetization measurements indicated that KV3Sb5 exhibited a behaviour consistent with the Curie-Weiss model at high temperatures, which describes the magnetic susceptibility of a ferromagnet in the paramagnetic region above the Curie point, although the effective moment was low. An anomaly occurred in both magnetisation and heat capacity measurements at 80 K, below which the moment was largely quenched. Through single-crystal resistivity measurements, the effect of deintercalation on the electron transport was shown.

In 2020, scientists used scanning tunnelling microscopy/spectroscopy to discover unusual electronic coupling to flat-band phonons in a layered kagome paramagnet, CoSn. The kagome structure was captured with unprecedented atomic resolution which allowed to analyse how the bosonic mode interacted with dispersive kagome electrons near the Fermi surface. At this mode energy, the fermionic quasi-particle dispersion displayed a marked renormalisation, which gave evidence of a coupling to bosons. Through the self-energy analysis, first-principles calculation, and a lattice vibration model, the scientists proved that this mode was initiated by the geometrically frustrated phonon flat-band, which was the lattice bosonic analogous of the Kagome electron flat-band.

There are many potential benefits to the findings: first and foremost, they could have strong bearing on how the electrons form “Cooper” pairs, i.e. superconducting electrons, and help them turn into a charged superfluid at an even lower temperature, or lead to the creation of a superconductor capable of electrical conduction without resistance. Also, other possibilities of unconventional electron pairing in this family of kagome superconductors might present themselves, as has been suggested by related research. Possible applications of the high-temperature superconductors include magnetic energy-storage systems, levitated passenger trains for high-speed travel, motors, generators, transformers, and transmission lines. The principal advantages of these superconducting devices would be their low power dissipation, high operating speed, and extreme sensitivity.

The next step in this research will focus on a phenomenon called time-reversal symmetry breaking. Time-reversal symmetry means that the laws of physics are the same regardless of if the system is going forward or backward in time. During time reversal symmetry breaking, the direction of electron spins changes and the symmetry is broken. One of the ways to break time-reversal symmetry is by developing magnetism, especially ferromagnetism, where all electron spins align in a parallel fashion. However, kagome metals have no substantial magnetic moments, a contradiction the scientists now want to research more closely as well as the different electronic states and their relation to time-reversal symmetry breaking.