Abstract Body

The kagome lattice, characterized by its unique geometry and rich electronic band structure, has recently garnered significant interest following the discovery of kagome superconductors AV₃Sb₅ (A = K, Rb, Cs) [1-3]. Despite extensive research efforts, the nature of the superconducting ground state remains elusive, and there is still no consensus on the electron pairing symmetry. Angle-resolved photoemission spectroscopy (ARPES) has proven to be a powerful tool to directly measure the superconducting gap in momentum space, offering valuable insights into the nature of superconductivity. In this presentation, I will share our ultrahigh-resolution and low-temperature ARPES studies on two exemplary CsV₃Sb₅-derived kagome superconductors: Cs(V_0.93Nb_0.07)₃Sb₅, which exhibits charge order, and Cs(V_0.86Ta_0.14)₃Sb₅, which does not show charge order in the normal state [4]. Additionally, I will present our latest findings on the superconducting gap of pristine CsV₃Sb₅ [5]. Furthermore, I will discuss our investigations into electronic kinks in the band structure [6], which suggest the presence of moderate electron-phonon couplings. Finally, the possible pairing mechanisms for these kagome superconductors will be discussed based on these results.

References

[1] T. Neupert et al., Nat. Phys. 18, 137 (2021)

[2] J.X. Yin et al., Nature 612, 647 (2022)

[3] S. D. Wilson & B. R. Ortiz, Nat. Rev. Mater. 9, 420 (2024)

[4] Y. Zhong et al., Nature 617, 488 (2023)

[5] A. Mine et al., arXiv: 2404.18472 (2024)

[6] Y. Zhong et al., Nat. Commun. 14, 1945 (2023)

Acknowledgment

The author thanks Jinjin Liu, Xianxin Wu, Akifumi Mine, J.-X. Yin, Hongxiong Liu, Yongkai Li, Sahand Najafzadeh, Takumi Uchiyama, Takeshi Suzuki, Kecheng Liu, Xinloong Han, Takeshi Kondo, Youguo Shi, Jiangping Hu, Zhiwei Wang, Xun Shi, Yugui Yao, and Kozo Okazaki for their invaluable contributions and fruitful discussions during collaborative works.

Figure 1. (a) Kagome lattice layer in Ta or Nb doped CsV₃Sb₅. (b) Schematic plot of the Fermi surface. The van Hove singularity (VHS) locates at the M point. (c) Phase diagram with Nb and Ta substitution of V atoms and the schematic superconducting gap structure for samples Cs(V_0.93Nb_0.07)₃Sb₅, Cs(V_0.86Ta_0.14)₃Sb₅, and CsV₃Sb₅ revealed by ARPES measurements.

Keywords: Kagome superconductors, Superconducting gap, Electron-phonon coupling, Photoemission spectroscopy

Abstract Body

Exploring exotic electronic orders and their underlying driving forces remains a central pursuit in the field of quantum materials. Within this context, the kagome lattice, a corner-sharing triangle network, has emerged as a versatile platform for exploring unconventional correlated and topological quantum states. Due to the unique correlation effects and frustrated lattice geometry inherent to kagome lattices, several families of kagome metals have been found to display a variety of competing electronic instabilities and nontrivial topologies, including quantum spin liquid, unconventional superconductivity, charge density wave orders, and Dirac/Weyl semimetals. Against this backdrop, kagome systems offer an exceptional quantum playground for delving into the intricate interplay among electron correlation effects, geometric frustration, and band topology. In this talk, I will present our recent work, focusing specifically on the unconventional electronic instabilities observed in kagome superconductors AV₃Sb₅ (A = K, Rb, Cs) [1-5] and ATi₃Bi₅ [6,7]. Drawing particularly from the insights derived from angle-resolved photoemission spectroscopy (ARPES), I will highlight the unique characteristics of these systems, shedding light on their intriguing electronic behaviors and elucidating their underlying mechanisms.

References

[1] B. R. Ortiz et al. Phys. Rev. Lett. 125, 247002 (2020).

[2] Y. Hu et al. Nat. Commun. 13, 2220 (2022).

[3] Y. Hu et al. Sci. Bull. 67(5):495–500 (2022).

[4] Y. Hu et al. Phys. Rev. B 106, L241106 (2022). Editors’ suggestion

[5] Y. Hu et al. npj Quant. Mater. 8, 67 (2023).(Invited Review)

[6] Y. Hu et al. Nat. Phys. 19, 1827–1833 (2023).

[7] Y. Hu et al. Supercond. Sci. Technol. (2024). (Invited review)

Figure 1. a Kagome lattice. b Tight-binding band structure of kagome lattice featuring Dirac cone (DC), flat band, and van Hove singularity (VHS), as indicated by red arrow.c Novel electronic states found in kagome superconductors AV₃Sb₅ (i) and ATi₃Bi₅ (ii).

Keywords: Kagome Superconductors, Charge Order, Electronic Nematicity, ARPES

Abstract Body

Despite of the various forms of superconductivity, conventional or unconventional, topologically trivial or nontrivial, the condensation of charge-2e Cooper pairs has remained the origin and character of all superconductivity, as described by the BCS theory. We report our experimental discoveries of the charge-4e and charge-6e superconductivity in ultrathin ring devices fabricated using the kagome superconductor CsV3Sb5 [1]. These new phase coherent states are discovered by the observation of the quantized magnetic flux in units of h/4e and h/6e in systematic magneto-transport measurements. Our observations provide direct experimental evidence for the existence of phase coherent paired quantum matter beyond the charge-2e superconductors, and provide ground work for exploring the physical properties of the charge-4e and charge-6e superconductivity as unprecedented phases of matter beyond the condensation of Cooper pairs described by the BCS theory.

References

[1] Jun Ge#, Pinyuan Wang#, Ying Xing, Qiangwei Yin, Anqi Wang, Jie Shen, Hechang Lei, Ziqiang Wang, and Jian Wang*. "Charge-4e and Charge-6e Flux Quantization and Higher Charge Superconductivity in Kagome Superconductor Ring Devices" Physical Review X 14, 021025 (2024) (arXiv: 2201.10352). (The paper was selected as “Featured in Physics” by the editor of Physical Review X, with an accompanying VIEWPOINT article entitled “Cooper Pairs Pair Up in a Kagome Metal” (link to the accompanying paper: https://physics.aps.org/articles/v17/80). Prof. Chandra M. Varma, a recipient of the John Bardeen Prize, highlighted this work in the Journal Club for Condensed Matter Physics. Link: DOI: 10.36471/JCCM_March_2022_03.)

Figure 1. Higher charge superconductivity in kagome superconductor ring devices.

Abstract Body

The kagome metal CsV₃Sb₅ is an ideal platform to study the interplay between topology and electron correlation. To understand the fermiology of CsV₃Sb₅, intensive quantum oscillation (QO) studies at ambient pressure have been conducted. However, due to the Fermi surface reconstruction by the complicated charge density wave (CDW) order, the QO spectrum is exceedingly complex, hindering a complete understanding of the fermiology. Using a special anvil cell technique we developed [1,2], we directly map the Fermi surface of the pristine CsV₃Sb₅ by measuring Shubnikov-de Haas QOs up to 29 T under pressure, where the CDW order is completely suppressed [3]. The QO spectrum of the pristine CsV₃Sb₅ is significantly simpler than the one in the CDW phase, and the detected oscillation frequencies agree well with our density functional theory calculations. In particular, a frequency as large as 8,200~T is detected. Pressure-dependent QO studies further reveal a weak but noticeable enhancement of the quasiparticle effective masses on approaching the critical pressure where the CDW order disappears, hinting at the presence of quantum fluctuations. Our high-pressure QO results reveal the large, unreconstructed Fermi surface of CsV₃Sb₅, paving the way to understanding the parent state of this intriguing metal in which the electrons can be organized into different ordered states.

[1] W. Zhang et al., Nano Lett. 23, 872 (2023)

[2] J. Xie et al., Nano Lett. 21, 9310 (2021)

[3] W. Zhang et al., Proc. Natl. Acad. Sci. USA 121, e2322270121 (2024)

Abstract Body

Laves phases are a type of intermetallic compounds with the chemical composition AB₂, and classified into cubic MgCu₂-type, hexagonal MgZn₂-type, and MgNi₂-type based on the differences in the B₄ tetrahedral network within the crystal structure. Laves phases are of considerable interest due to their diverse crystal structures and combinations resulting in intriguing superconducting properties. Examples include the high upper critical field in MgCu₂-type (Hf,Zr)V₂ [1], and the time-reversal symmetry breaking which suggests unconventional superconductivity in MgZn₂-type HfRe₂ [2].

In recent years, the introduction of third elements into binary Laves phases has been investigated to induce superconductivity and to improve the superconducting transition temperature (T_c) associated with structural changes. For example, the emergence of superconductivity associated with the formation of derived structures has been observed in Mg₂Cu₃Si-type Mg₂Ir₃Si (T_c = 7 K) [3] and Mg₂Ir_2.3Ge_1.7 (T_c = 5.3 K) [4] which were derived by introducing Si and Ge into MgZn₂-type MgIr₂ [3]. A significant increase in T_c has been observed inMg₂Cu₃Si-type Ta₂V_3.1Si_0.9 (T_c = 7.5 K) [5] achieved by introducing Si into MgCu₂-type TaV₂ (T_c = 3.2 K) [6]. Moreover, the nonmonotonic dependence of Tc on Si substitution in MgCu₂-type Sc₂Ir_4-xSi_x (x = 0 ~ 0.7) [7]have been reported. Thus, the extension to the ternary Laves phase provides a promising route to explore and improve superconductors.

In this study, we attempted to introduce various third elements into BaIr₂ (T_c = 2.7 K) [8], which we recently discovered by high-pressure synthesis, to stabilize the new superconducting phase at ambient pressure by inducing structural modifications. As a result, we found that the new compound Ba₅Ir₇Ge₄ crystallizes at ambient pressure by adding Ge. Structural analysis demonstrated that Ba₅Ir₇Ge₄ belongs to the tetragonal space group I 4₁/a with a = 12.9726(2) Å, c = 8.2703(2) Å, and that 40% of the Ir₄ tetrahedron of BaIr₂ (= Ba₅Ir₁₀) are replaced by Ge₄Ir (Ba₅Ir₆Ge₄Ir = Ba₅Ir₇Ge₄), resulting in a corner-sharing network of Ge₄Ir and Ir₃Ge tetrahedron. Magnetic susceptibility, electrical resistivity (Fig.1), and specific heat measurements revealed that Ba₅Ir₇Ge₄ is a type-II superconductor with T_c = 3.2 K.

References

[1] F. Stein et al., J Mater Sci 56, 5321 (2021)., [2] M. Mandal et al., Phys. Rev. B 105, 094513 (2022)., [3] K. Kudo et al., J. Phys. Soc. Jpn. 89, 013701 (2020)., [4] K. Kudo et al., J. Phys. Soc. Jpn. 89, 123701 (2020)., [5] H. Liu et al., Phys. Rev. B 108, 104504 (2023)., [6] J. Crangle et al., J. Phys. D: Appl. Phys. 29, 2362 (1996)., [7] Z. Zhu et al., Commun Mater 5, 85 (2024)., [8] T. Koshinuma et al., Intermetallics 148, 107643 (2022).

Abstract Body

Recently, various phenomena, including superconducting diode effect (SDE), have been observed in noncentrosymmetric superconductors^1,2. Recent experiments have demonstrated the zero-field SDE, wherein the superconducting current flows only in the forward direction, indicating that the proximity effect at the superconductor/ferromagnet interface can be used to effectively control the SDE^3-5. Zero-field SDEs have the potential to enable novel nonvolatile memory devices and logic circuits with ultralow power consumption.

The proximity effect between ferromagnets and superconductors has been actively studied using ferromagnet/superconductor/ferromagnet structures^6-8. In such structures, the superconducting properties can be controlled by the parallel and antiparallel states of magnetization, and the transition temperature of a superconductor sandwiched by a ferromagnet depends on the magnetization state because of the proximity effect^7-9.

In this study, we successfully fabricated noncentrosymmetric superconducting multilayers containing ferromagnets, superconductors, and heavy metals5 with parallel and antiparallel magnetization states, as shown in Figure 1. Magnetic hysteresis that reflects the magnetization state was observed in the nonreciprocal critical current characteristics. When the magnetization directions of the upper and lower ferromagnets that sandwich the superconducting layer are parallel, the nonreciprocal critical current becomes finite. However, it becomes zero when the magnetization directions are antiparallel. This feature enables to control the polarity-reversible and on–off switching of the zero-field SDE.

References

[1] N. Nagaosa, Y. Yanase, Annu. Rev. Condens. Matter Phys. 15, 63 (2024).

[2] F. Ando et al., Nature 584, 373 (2020).

[3] H. Narita et al., Nat. Nanotechnol. 17, 823 (2022).

[4] K.-R. Jeon et al., Nat. Mater. 21, 1008 (2022).

[5] H. Narita et al., Adv. Mater. 35, 2304083 (2023).

[6] A. I. Buzdin, Rev. Mod. Phys. 77, 935 (2005).

[7] Y. Gu et al., Phys. Rev. Lett. 115, 067201 (2015).

[8] Y. Zhu et al., Nat. Mater. 16, 195 (2017).

[9] A. Di Bernardo et al., Nat. Mater. 18, 1194 (2019).

Acknowledgment

This work was partly supported by JSPS KAKENHI (Grant Nos. 21K13883, 21K18145, 24H00007, and 24K00584) and JST PRESTO (Grant No. JPMJPR2358). This study was also supported by the research grant program of the Futaba Foundation, Iketani Science and Technology Foundation, Tokuyama Science Foundation, Iwatani Naoji Foundation, Murata Science and Education Foundation, and Mazda Foundation. We also acknowledge the MEXT Initiative to Establish Next-generation Novel Integrated Circuits Centers (X-NICS) (Grant No. JPJ011438) and the Collaborative Research Program of the Institute for Chemical Research, Kyoto University.