Our group has made significant progress in the research of quasi one-dimensional superconductor Ta2PdS5 nanowires. On November 7, 2018, the relevant research findings were published online in the internationally authoritative journal Nature Communications (DOI: 10.1038/s41467-018-07123-y) titled Signature of Quantum Griffiths singular state in a layered homogeneous superconductor. The first unit of the paper is the Department of Physics at Fudan University, with Professor Faxian Xiu as the corresponding author and Professor Faxian Xiu 's doctoral student Enze Zhang as the first author.
Griffiths singularity was first proposed by American physicist Griffiths in 1969, specifically referring to the behavior where the degree invariance is broken during phase transitions, and the critical exponent tends to diverge rather than remain constant. Quantum Griffith singularity refers to the Griffith singularity in the quantum phase transition that occurs in a system at zero temperature. In theory, Griffith singularity does not specify that it only occurs in a specific dimension. In the decades since the theory was proposed, experimental scientists have only observed critical exponential divergence in phase transitions in a few three-dimensional ferromagnetic and two-dimensional superconducting systems, such as gallium thin films. However, the existence of Griffith strange states in lower dimensional systems has not been verified.
Figure. (a) Crystal Structure Diagram of Ta2PdS5. (b) Schematic diagram of Ta2PdS5 nanowire device structure. (c) Superconductivity Metal Transition in Ta2PdS5 Nanowires
In order to search for quantum Griffith singularity in lower dimensional systems, Professor Faxian Xiu 's research group studied a new type of quasi one-dimensional superconductor Ta2PdS5. Due to its unique crystal structure, Ta2PdS5 crystals from bulk materials can be prepared into elongated nanowires with a thickness of 70-300 nanometers and a width of 0.1-2 micrometers using tape mechanical peeling method. Interestingly, through transport measurements, it was found that, unlike in bulk materials, the superconductivity in nanoscale Ta2PdS5 exhibits quasi one-dimensional superconductivity. For example, the I-V relationship curve of the device has multiple jumps and hysteresis, and the critical current of the device varies with temperature according to the quasi-one-dimensional Bardeen formula and other characteristics. The research team confirmed that the superconductivity in Ta2PdS5 nanowires indeed possesses quasi one-dimensional characteristics through further research on various aspects such as the superconducting penetration depth of Ta2PdS5.
Based on the quasi one-dimensional superconductivity in Ta2PdS5 nanowires, the research team investigated the superconducting metal quantum phase transition of Ta2PdS5 nanowires at extremely low temperatures. Research has found that the intersection of the adjacent temperature isothermal magnetoresistance curves near the critical point of the superconducting metal quantum phase transition is a continuous line on the phase diagram, rather than intersecting at the same critical point as in the general superconducting metal phase transition. The magnetic field at the intersection changes with temperature rather than the same value; Through further finite size scaling analysis of the sample magnetoresistance, it was found that the dynamic critical index in the system is not a constant but exhibits divergent behavior when approaching absolute zero or critical magnetic field near the quantum phase transition critical point. These characteristics are direct and powerful evidence of quantum Griffith singularity.
This research achievement extends the quantum Griffith singularity to quasi one-dimensional superconducting systems for the first time, which is of great significance for deepening the understanding of disordered fluctuations near quantum phase transition critical points and quasi one-dimensional superconductivity; At the same time, the unique physical properties exhibited by the new quasi one-dimensional superconducting Ta2PdS5 nanowires in research also indicate that they have considerable application prospects in quantum calculator devices.
Paper link: https://www.nature.com/articles/s41467-018-07123-y
