BIT’s progress in the Kagome structure CsV3Sb5 superconducting material

News Resource: School of Physics

Editor: News Agency of BIT

Translator: Xiang Tong, News Agency of BIT

A few days ago, the team of Professor Wang Zhiwei and Professor Yao Yugui from the School of Physics, BIT made a series of progress in the Kagome structure superconductor CsV3Sb5 material. Cage mesh structure materials have always been a research hotspot in the field of condensed matter physics. Due to special structural characteristics, electrons are localized in honeycomb hexagons. Such materials usually form flat bands and saddle points, and a Dirac point with a linear dispersion relationship, and thus exhibit exotic quantum states such as quantum spin liquids, charge density waves, spin density waves, and unconventional superconductivity. The recently discovered family of A V3Sb5 ( A ="K," Rb, Cs) Kagome-structured superconductors has both topological nontrivial Dirac bands, charge density waves (CDW), superconductivity (SC), and anomalous Hall effect (AHE)and other quantum phenomena. Thus, it quickly attracted extensive attention and research in the field of condensed matter physics.

Figure1 Electrical and magnetic properties of CsV3Sb5 samples

Figure 2 Two modulation structures and chiral charge order observed in CsV3Sb5

Figure 3 Domain wall formed by 1×4 modulation structure

The acquisition of high-quality single crystal samples is the premise of carrying out relevant experimental research. Under the guidance of Researcher Wang Zhiwei, Doctoral Student Li Yongkai obtained a high-quality CsV3Sb5 single crystal sample after a series of crystal growth parameters adjustment and optimization, whose full width at half maximum of the single crystal diffraction peak is only 0.07°. Electrical transport and magnetic tests show that the material has a CDW phase transition around 91K and a superconducting transition around 3K, and an obvious anomalous Hall signal is also observed, as shown in Figure 1. In further cooperation with the team of M. Z. Hasan of Princeton University and others, the phenomenon of chiral charge order was discovered by low-temperature STM measurement, and two modulation structures of 2 × 2 and 1 × 4 were observed, as shown in Figure 2. The 2 × 2 charge modulation structure is closely related to the CDW in this system, and the amplitude of the energy gap opened by the charge order exhibits real-space modulation characteristics with 2 × 2 chirality, which reflects the electronic nature of the chiral charge order. However, the 1×4 modulation structure forms different domain walls, and there is a fixed angle of 120° between the domain walls, as shown in Figure 3. This work was published in PRB [Phys. Rev. B 104, 075148 (2021)] and was selected as an Editor's Recommendation.

The driving mechanism of CDW in CsV3Sb5 material and its relationship with superconductivity have been the focus of attention. In order to study them, the research team cooperated with the research group of Professor Wen Haihu of Nanjing University, and firstly conducted an in-depth study of the CDW in CsV3Sb5 using infrared spectroscopy. It is found that above T CDW = 91 K, the low-frequency photoconductivity of the material exhibits a distinct Drude response, which is consistent with the metallicity of the material. Typical optical signatures of charge density waves are observed below T CDW. Further by performing Drude-Lorentz fitting on the photoconductivity spectra at all temperatures, as shown in Figure 4,it is found that the CDW energy gap opens at the saddle point of the M point, while neither the electron band near the Γ point nor the Dirac band near the K point is affected. This result suggests that the nesting of the saddle point at point M through the wave vector Q may be the driving mechanism of the charge density wave in CsV3Sb5. This work was published in the form of Letter in PRB [Phys. Rev. B 104, L041101 (2021)] and was selected as an Editor's Recommendation. They then measured the resistivity of CsV3Sb5 in the c-direction (as shown in Figure 5), where the current is along the c-direction and is always perpendicular to the magnetic field, which is parallel to the ab plane and rotates within the plane. It can be seen that the curve of resistivity versus angle under different magnetic field conditions has obvious double symmetry, which destroys the in-plane six-fold (or triple) symmetry of the crystal structure. When the applied magnetic field is less than 2.4 T (corresponding to the superconducting state) and greater than 2.4 T (corresponding to the normal state), the magnetic field directions corresponding to the minimum resistivity values are perpendicular to each other, indicating that the superconducting state and the normal state have two-fold symmetry perpendicular to each other. The relationship of these two double symmetries with temperature was further investigated, and the results are shown in Figure 6. The resistance double symmetry corresponding to the superconducting state at low magnetic field disappears rapidly near the superconducting transition temperature, while the double symmetry measured in the normal state at high magnetic field weakens with increasing temperature and disappears near the CDW transition temperature. These results greatly enrich people's understanding of this cage-structured superconducting material and its physical properties. Related results were published in NC [Nat. Commun. 12, 6727 (2021)].

Figure 4 Fitting of photoconductivity spectrum, calculated energy band structure diagram, comparison of experimental and theoretical photoconductivity spectrum, and temperature dependence of various parameters obtained by fitting

Figure 5 c-axis resistivity as a function of magnetic field azimuth

Figure 6 Two-fold symmetry of c-axis resistivity as a function of temperature

Flat bands, saddle points and Dirac points with linear dispersion relationship are usually formed in Kagome structural materials. In order to conduct a comprehensive and in-depth study of the energy band structure of this system material, the research team, together with T. Sato's group from Tohoku University, Japan, used angle-resolved photoelectron spectroscopy (ARPES) to study the electronic energy band structure of CsV3Sb5 material and its regulation.

Figure 7 Band structure and multiple Dirac spots of CsV3Sb5

Figure 8 Band structure along the MK direction near the Fermi surface

The saddle point and Dirac point were first observed in the CsV3Sb5 parent phase material, as shown in Figure 7. Further, when the system temperature drops below the CDW transition temperature, the CDW energy gap is observed, and the energy gap has strong Fermi-surface and momentum-dependent properties, as shown in Figure 8. The related results were published in the form of Letter in PRB [Phys. Rev. B 104, L161112 (2021)]. Furthermore, by in-situ Cs atomic deposition on the surface of CsV3Sb5 material, the electronic doping of the system is realized for the first time, and thus the CDW and energy band of the system are manipulated. Cs deposition exhibits orbital selectivity for the electron doping of this material, characterized by a significant increase in electron filling in the Sb 5 p z and V 3 d xz/yz bands, while the V 3 d xy/x2−y2 band is relatively stable. By studying the variation of the CDW energy gap around the M point with temperature, it is found that the Cs modification can completely suppress the CDW while keeping the saddle point at the Fermi level, as shown in Figure 9. This result indicates that the multiorbital effect plays a crucial role in the generation of CDWs, and at the same time, due to the competition between CDWs and superconductivity, it provides a possibility to manipulate CDWs and superconductivity in the A V3Sb5 system in the future. The results were published in PRX [Phys. Rev. X 12, 011001 (2022)].

Figure 9 Electronic band structures of untreated and Cs-treated CsV3Sb5 samples

The above work has been supported by the National Key Research and Development Program, the National Natural Science Foundation of China, the Beijing Natural Science Foundation, and the BIT Young Teachers Academic Start-up Program.

Related article links: