Tunable Van Hove Singularity without Structural Instability in Kagome Metal ${\mathrm{CsTi}}_{3}{\mathrm{Bi}}_{5}$

Tunable Van Hove Singularity without Structural Instability in Kagome Metal ${CsTi}_{3} {Bi}_{5}$

Bo Liu et al.

Phys. Rev. Lett. 131, 026701 – Published 12 July 2023

Abstract

In kagome metal ${CsV}_{3} {Sb}_{5}$ , multiple intertwined orders are accompanied by both electronic and structural instabilities. These exotic orders have attracted much recent attention, but their origins remain elusive. The newly discovered ${CsTi}_{3} {Bi}_{5}$ is a Ti-based kagome metal to parallel ${CsV}_{3} {Sb}_{5}$ . Here, we report angle-resolved photoemission experiments and first-principles calculations on pristine and Cs-doped ${CsTi}_{3} {Bi}_{5}$ samples. Our results reveal that the van Hove singularity (vHS) in ${CsTi}_{3} {Bi}_{5}$ can be tuned in a large energy range without structural instability, different from that in ${CsV}_{3} {Sb}_{5}$ . As such, ${CsTi}_{3} {Bi}_{5}$ provides a complementary platform to disentangle and investigate the electronic instability with a tunable vHS in kagome metals.

Received 12 December 2022
Revised 24 March 2023
Accepted 5 June 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.026701

Physics Subject Headings (PhySH)

van Hove singularity

Kagome lattice

Angle-resolved photoemission spectroscopy First-principles calculations

Condensed Matter, Materials & Applied Physics

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Vol. 131, Iss. 2 — 14 July 2023

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Figure 1
Electronic structure of ${CsTi}_{3} {Bi}_{5}$ . (a) Crystal structure of ${CsTi}_{3} {Bi}_{5}$ . (b)–(c) Photoelectron intensity plot of the band structure along the $\bar{Γ} − \bar{M} − \bar{K} − \bar{Γ}$ direction (b) and the Fermi surface (c) of ${CsTi}_{3} {Bi}_{5}$ measured with 78 eV photons at 7 K. The dashed lines in (c) are a guide to the eye for the different Fermi surface sheets. (d) The bulk band structure of ${CsTi}_{3} {Bi}_{5}$ obtained from first-principles calculations with spin-orbit coupling included. The electronlike band around $\bar{Γ}$ and the holelike bands near $\bar{M}$ are labeled as $α$ , $β$ , $γ$ , and $δ$ band, respectively. The orange shade is a guide to the eye for the vHS.
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Figure 2
Evolution of the electronic structure with Cs surface doping at 7 K. (a) Photoelectron intensity plot of the band structure around $\bar{Γ}$ (along the $\bar{M} − \bar{Γ} − \bar{M}$ direction) as a function of continuous Cs doping on the same sample, measured with 21.2 eV photons. The results at doping sequences 0–4 are shown in (i)–(v), respectively. Doping sequence 0 indicates the pristine ${CsTi}_{3} {Bi}_{5}$ sample. The dashed lines are a guide to the eye. (b) Momentum distribution curves (MDCs) at $E_{F}$ extracted from (a). The numbers 0–4 denote the doping sequences. (c) The projected in-plane BZ and the momentum locations of the cuts. (d) Same as (a), but for the band structure around $\bar{M}$ (along the $\bar{Γ} − \bar{M} − \bar{Γ}$ direction). (e) Integrated EDC around the $\bar{M}$ point in (d). The integration window is marked by the gray dashed box in the first panel of (d). The numbers 0-4 denote the doping sequences. (f) Enhanced EDC peak intensity around $\bar{M}$ as a function of the doping sequence. The absolute EDC peak intensity at the doping sequence $x$ ( $x = 0 - 4$ ) is calculated by integrating the area between $- 20 meV$ and $E_{F}$ [the pink shaded region in (e)] of the corresponding EDC, and labeled as $I_{x}$ . The enhanced EDC peak intensity is defined as $(I_{x} - I_{0}) / (I_{4} - I_{0})$ .
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Figure 3
Doping evolution of the vHS. (a)–(b) Photoelectron intensity plot of the band structure around $\bar{M}$ (along the $\bar{Γ} − \bar{M} − \bar{Γ}$ direction) before (a) and after (b) the Cs surface doping, measured with 21.2 eV photons at 200 K. (c)–(d) Same as (a)–(b), but measured along the $\bar{Γ} − \bar{K} − \bar{M}$ direction. (e) Orbital-resolved band structure obtained by first-principles calculations. Different orbitals are presented in different colors. The size of the markers represents the spectral weight of the orbitals. (f) Quantified dispersion of the $γ$ and δ bands near $\bar{M}$ after the sufficient Cs doping, extracted from (b) and (d). The orange shade is a guide to the eye for the vHS. (g) The projected in-plane BZ and the momentum locations of the cuts.
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Figure 4
Calculated total energy profiles, phonon spectra, and x-ray diffraction measurements. (a) The $2 \times 2 \times 1$ supercells for kagome structure, Star of David structure and Inverse Star of David structure. The black arrows indicate the lattice distortion due to the breathing mode. (b)–(d) Total energy as a function of the interpolated structure in pristine ${CsV}_{3} {Sb}_{5}$ (b), pristine ${CsTi}_{3} {Bi}_{5}$ (c), and electron doped ${CsTi}_{3} {Bi}_{5}$ (d). The $Δ E$ stands for the relative total energy with respect to the kagome structure per supercell (36 atoms). (e) Calculated phonon spectra along the high-symmetry directions in pristine ${CsTi}_{3} {Bi}_{5}$ (magenta line) and electron doped ${CsTi}_{3} {Bi}_{5}$ (orange line). (f) Comparison between the observed x-ray diffraction intensities from 322 indexed Bragg peaks measured at 18 K and the calculation from the crystal structure of ${CsTi}_{3} {Bi}_{5}$ with the space group of $P 6 / m m m$ .
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Physical Review Letters

Tunable Van Hove Singularity without Structural Instability in Kagome Metal CsTi3Bi5

Bo Liu et al.

Phys. Rev. Lett. 131, 026701 – Published 12 July 2023

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Tunable Van Hove Singularity without Structural Instability in Kagome Metal ${CsTi}_{3} {Bi}_{5}$