Researchers at the Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, have found evidence for an unusual superconducting state in CsV3Sb5, a so-called Kagome metal that exhibits exotic electronic properties. The finding could shed new light on how superconductivity emerges in materials where phenomena such as frustrated magnetism and intertwined orders play a major role.
Kagome metals are named after a traditional Japanese basket-weaving technique that produces a lattice of interlaced symmetrical triangles. Physicists are interested in this configuration (known as a Kagome pattern) because when the atoms of a metal or other conductor are arranged in this fashion, their electrons behave in unusual ways.
An example is frustrated magnetism, which occurs when electrons are “not happy to live together”, observes Ludovic Jaubert, a condensed-matter physicist at the University of Bordeaux in France who was not involved in the present work. In frustrated materials, not all interactions between electron spins can be satisfied at the same time, which prevents the spins from ordering themselves on long length scales. This failure has significant consequences for the material’s properties: if water behaved like this, for example, it would never freeze.
First Kagome metal and a new family
In 2018, researchers at the Massachusetts Institute of Technology (MIT), Harvard University and the Lawrence Berkeley National Laboratory created the first Kagome metal in the laboratory. The material in that work was an electrically conductive crystal consisting of layers of iron and tin atoms arranged in a Kagome lattice pattern, but the discovery of a whole family of Kagome materials with the chemical formula 𝐴V3 Sb5 (where 𝐴 = K, Rb, Cs) came hot on its heels.
These newer Kagome materials behave like conventional superconductors at temperatures below 2.5 K. Under these conditions, their electrons form correlated electron (or Cooper) pairs that carry current without any resistance. However, researchers suspected that the electrons in these compounds might also pair in unconventional ways.
Two-stage-like transition
To investigate further, groups led by Xiaoli Dong, Jinguang Cheng, Jiangping Hu, Hong-Jun Gao and Zhongxian Zhao made a series of magnetic and electrical measurements on single crystals of CsV3Sb5. The researchers began by recording the material’s X-ray diffraction pattern to confirm that it was indeed arranged in a Kagome lattice pattern. They then determined that it became superconducting at around 3 K by measuring its magnetism and electrical resistance as it cooled. This superconducting transition temperature is slightly higher than that observed in previous studies, and Dong and colleagues also noted that the transition to the superconducting phase was not as sharp as expected. Instead, they found that the tell-tale (diamagnetic) signal for superconductivity onset set in gradually below around 3.5 K before dropping abruptly below around 2.8 K.
The team then measured the sample’s angular-dependent magnetoresistance within the plane of the Kagome lattice and identified a twofold rotational symmetry in the mixed state (that is, a state that contains both superconducting and non-superconducting phases). Below 2.8 K, they found that the orientation of this twofold symmetry displays a peculiar twist by an angle of 60o, which is characteristic of the Kagome geometry.
Finally, the researchers measured the sample’s magnetoresistance in two directions – perpendicular to the plane and across it – as they applied magnetic fields of varying strengths (up to 8 Tesla) to it. They did this to obtain the material’s upper critical field, which is defined as the magnetic flux density that would completely suppress superconductivity at 0 K.
Quantum spin liquid state pathway emerges
They found that this field is higher across the plane than perpendicular to it, such that the ratio of the two at 0 K is very large. This result, say the researchers, is best explained if the material is a quasi-two-dimensional multiband superconductor – a type of superconductor that may display one or several phase transitions with increasing temperature from or to chiral superconducting states. These transitions can occur when two or more electronic bands are involved in the superconducting transition.
“Our result, together with other observations, suggests that the superconducting state in CsV3Sb5 comes about partially due to electron-electron correlations,” Dong tells Physics World. “Other effects such as spin-orbit coupling and chiral flux phases may also be present in this multiband Kagome system.”
The researchers, who report their work in Chinese Physics Letters, are now planning to investigate the exotic normal and superconducting states in this multiband system using microscopic spectroscopy techniques.
