Spectral evidence for unidirectional charge density wave in detwinned ${\mathrm{BaNi}}_{2}{\mathrm{As}}_{2}$

Letter

Spectral evidence for unidirectional charge density wave in detwinned ${BaNi}_{2} {As}_{2}$

Yucheng Guo et al.

Phys. Rev. B 108, L081104 – Published 8 August 2023

Abstract

In the iron-based superconductors, unconventional superconductivity emerges in proximity to intertwined electronic orders consisting of an electronic nematic order and a spin density wave (SDW). Recently, ${BaNi}_{2} {As}_{2}$ , like its well-known iron-based analog ${BaFe}_{2} {As}_{2}$ , has been discovered to host a symmetry-breaking structural transition but coupled to a unidirectional charge density wave (CDW) instead of SDW, providing a novel platform to study intertwined orders. Here, through a systematic angle-resolved photoemission spectroscopy study combined with a detwinning $B_{1} g$ uniaxial strain, we identify distinct spectral evidence of band evolution due to the structural transition as well as CDW-induced band folding. In contrast to the nematicity and spin density wave in ${BaFe}_{2} {As}_{2}$ , the structural and CDW order parameters in ${BaNi}_{2} {As}_{2}$ are observed to be strongly coupled and do not separate in the presence of uniaxial strain. Furthermore, no nematic band splitting is resolved above the structural transition. Our measurements point to a likely lattice origin of the CDW order in ${BaNi}_{2} {As}_{2}$ .

Received 22 April 2022
Revised 1 February 2023
Accepted 24 July 2023

DOI:https://doi.org/10.1103/PhysRevB.108.L081104

Physics Subject Headings (PhySH)

Charge density waves Nematic phase transition

Iron-based superconductors

Angle-resolved photoemission spectroscopy

Condensed Matter, Materials & Applied Physics

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Vol. 108, Iss. 8 — 15 August 2023

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Figure 1
(a), (b) Crystal structure and BZ for the tetragonal and triclinic phases. The red (gray) BZ represents the 3D (2D projected) BZ. The green arrows in (b) denote the $q$ vectors for C-CDW and its in-plane component. (c), (d) Temperature-dependent in-plane resistivity $ρ_{a}$ (yellow), $ρ_{b}$ (blue), and $ρ_{b} / ρ_{a}$ (green) with and without uniaxial compressive strain along $b$ . Transition temperatures $T_{S}$ and $T_{IC-CDW}$ are denoted by the gray dashed lines.
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Figure 2
Electronic structure in the tetragonal state (170 K). (a) Fermi surface (FS) measured with 60 eV photons ( $k_{z} = 0$ ). (b) Band dispersions measured along $Γ - X$ as marked by the blue lines in (a) and (e). (c), (d) Same as (a) and (b) but measured with 79 eV photons ( $k_{z} = π$ ), where band dispersions are measured along $Γ - Z$ as marked by the pink lines in (c) and (e). (e) Photon-energy-dependent measurement in the $k_{x} − k_{z}$ plane, with an inner potential of 15 eV. (f) DFT calculation of the 3D FSs. In addition, DFT-calculated band dispersions and FSs are overlaid on all data in green solid lines. Gray arrows and dashed lines mark the surface states. All data were taken with circular-right polarization.
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Figure 3
FSs of detwinned ${BaNi}_{2} {As}_{2}$ . (a) FS taken at 79 eV in the triclinic phase (100 K) with horizontal compressive uniaxial pressure. (b) Same as (a) but taken in the tetragonal phase (170 K). (c) Same as (a) but taken at 50 K with a $45^{\circ}$ analyzer slit direction. (d) FS taken with circular-right 60 eV photons at 50 K without uniaxial pressure. Blue lines mark the Fermi pockets around $\bar{M}$ , while cyan lines indicate bands folded from the $\bar{M}$ points due to the C-CDW. (e), (f) MDCs along $k_{y} = 0.25 Å^{- 1}$ (solid line) and $k_{x} = 0.25 Å^{- 1}$ (dashed line) for the strained sample at 100 K and 170 K respectively, as marked in (a) and (b). The black arrow denotes the folded feature.
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Figure 4
Temperature evolution through the CDW transitions on detwinned ${BaNi}_{2} {As}_{2}$ . (a) Summary of the band evolution across $T_{S/C-CDW}$ . (b)–(d) Temperature dependence of band dispersions along $\bar{Γ} − \bar{X}$ for (b) $T < T_{S/C-CDW}$ , (c) $T_{S/C-CDW} < T < T_{IC-CDW}$ , and (d) $T > T_{IC-CDW}$ . (e)–(g) Same as (b)–(d) but taken along $\bar{Γ} − \bar{Y}$ . Dashed lines are guides to the eyes. (h) Temperature dependence of MDCs as marked in (b)–(g). The locations of MDCs are as marked. (i) Integrated spectral weight as a function of temperature within the gray window for MDC3 and MDC4. Each curve is normalized to the maximum spectral weight. (j) Integrated spectral weight within the gray window in MDC1 and MDC2 and the equivalent MDC taken on the unstained crystal as a function of temperature. Each curve is normalized by calculating $(S - S_{H}) / S_{H}$ , where $S_{H}$ is the average spectral weight between 145 K and 170 K. Solid lines are guides to the eyes. (k), (l) Continuous temperature evolution of the MDCs and EDCs starting slightly below $T_{S/C-CDW}$ . The red line denotes the peak position by fitting (see Supplemental Material [28]). (m) Temperature evolution of $d_{x z}$ and $d_{y z}$ band positions in ${BaFe}_{2} {As}_{2}$ with and without uniaxial strain (reproduced from [11]).
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Physical Review B

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Spectral evidence for unidirectional charge density wave in detwinned ${BaNi}_{2} {As}_{2}$

Yucheng Guo et al.

Phys. Rev. B 108, L081104 – Published 8 August 2023

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Spectral evidence for unidirectional charge density wave in detwinned BaNi2As2

Yucheng Guo et al.

Phys. Rev. B 108, L081104 – Published 8 August 2023

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Spectral evidence for unidirectional charge density wave in detwinned ${BaNi}_{2} {As}_{2}$