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BIT’s progress in Kagome superconductivity research
News Resource: School of Physics
Editor: News Agency of BIT
Translator: Gong Dantong, News Agency of BIT
Recently, the particular researcher Wang Zhiwei and Professor Yao Yugui’s team from the School of Physics of BIT has made a series of progress in the superconductor CsV3Sb5 of Nb doping Kagome structure. The quasi-two-dimensional material AV3Sb5 (A="K, Rb, Cs) is found to be a representative example of Kagome superconductor and a new quantum platform for studying the interaction between electron correlation effects, topology and geometric frustration. Although various research results displays the characteristics of coexistence between the superconductivity of AV3Sb5 presents and the charge density wave (CDW), the generation mechanism of superconductivity and CDW is still controversial.
In order to explore the CsV3Sb5 superconductivity of Kagome and the generation mechanism of CDW as well as their inner relationship, Ph.Li Yongkai (now is BIT postdoctorate from School of Physics ) , led by researcher Wang Zhiwei, succeeded in synthesizing Cs(V1-xNbx)3Sb5 single crystal sample of Nb doping, which was found that the limitation of Nb doping in this system was 7%. Apart from it, the XRD structure refinement confirmed that Nb replaced the V site, so as to effectively control the VSb two-dimensional Kagome lattice layer of this system. Furthermore, they systematically studied the change of CDW and of superconductivity of the system, with the concentration of Nb doping. With electrical transport measurements, it finds that Nb doping can gradually suppress the CDW and enhance the superconductivity. The highest superconducting temperature is 4.45K, indicating a competitive relationship between the CDW and superconductivity, as shown in Figure 1. Simultaneously, with the increase of Nb doping amount, the anomalous Hall effect and the CDW are significantly weakened, which provides experimental evidence for the possible close relationship between them, as shown in Figure 2.
Figure 1. The change of CDW and of superconductivity in Cs(V1-xNbx)3Sb5 with different Nb doping concentrations
Figure 2. The anomalous Hall effect in Cs(V1-xNbx)3Sb5 weakens with the increase of Nb doping concentration
To explain this competitive relationship between CDW and superconductivity, we performed first-principles calculations, as shown in Figure 3: The results of the first-principles calculations showed that the Nb doping changed van Hooftsch at M point－Nb doping makes the Van Hove singular point move from below the Fermi energy to above it, so that the filled energy density of states is depleted, thus weakening the CDW; at the same time, the Nb doping expands the electron pocket at , which provides more electronic density of states for the superconducting pairing, to improve the superconductivity. Our results reveal the unique relationship between CDW and superconductivity in the system of Cs(V1-xNbx)3Sb5. Having cooperated with the research group of Professor Wen Haihu of Nanjing University, the results were published in Physical Review B [PRB 105, L180507 (2022)] as Letter.
Figure 3. Theoretical calculation of the electronic band structure of Cs(V1-xNbx)3Sb5 (x="0 and x=0.07)
In order to verify our theoretical calculations further and study the effect of Nb doping on the electronic band structure of the CsV3Sb5 system, the research team cooperated with T.Sato from TOhoko University and studied systematically electronic band structure of Cs(V1--xNbx)3Sb5 by Angle resolved photoemission spectroscopy (ARPES). As ARPES showed, with the doping of Nb element, as shown in Fig. 4, the electron pockets (α bands) derived from Sb atoms at Γ point move downward, while the van Hove singularity points derived from V atoms at M point (δ band) moves upward. In addition, with the doping of Nb and the δ band moves upward, it causes the Van Hove singularity to move away from the Fermi level and furthermore, leads transition temperature of CDW－TCDW decrease and CDW intensity weaken, as shown in Figures 4 and 5 . This conclusion indicates that the generation of charge density waves is closely related to the van Hove singularity. Besides, we found that the intrinsic physical nature of the increase of the superconducting transition temperature Tc caused by Nb doping is: caused by doping Nb, the increase of superconductivity was led by the expansion of the electron pocket derived from Sb of α-band and the recovery of the electronic density of states derived from V at the Fermi surface. Relevant work was published in Physical Review Letters [PRL, 129, 206402 (2022)].
Figure 4. Electronic energy band structure of Cs(V1-xNbx)3Sb5 (x="0 vs. x=0.07) at 120 K
Figure 5. Evolution of electronic band structure of Cs(V1-xNbx)3Sb5 with temperature and Nb doping concentration
All in all,with Nb doping, we modulated the energy band structure of Kagome lattice superconductor CsV3Sb5, and then changed its superconductivity and phase transition conditions of CDW, providing the important experimental evidence to reveal the origin of superconductivity and CDW in CsV3Sb5.
The above work has been supported by the National Key R&D Program of China, the National Natural Science Foundation of China, the Beijing Municipal Natural Science Foundation, and the Academic Startup Program for Young Teachers of Beijing Institute of Technology.
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