Emergent Spin-Gapped Magnetization Plateaus in a Spin-$1/2$ Perfect Kagome Antiferromagnet

Emergent Spin-Gapped Magnetization Plateaus in a Spin- $1 / 2$ Perfect Kagome Antiferromagnet

S. Suetsugu, T. Asaba, Y. Kasahara, Y. Kohsaka, K. Totsuka, B. Li, Y. Zhao, Y. Li, M. Tokunaga, and Y. Matsuda

Phys. Rev. Lett. 132, 226701 – Published 28 May 2024

Abstract

The two-dimensional spin- $1 / 2$ kagome Heisenberg antiferromagnet is believed to host quantum spin liquid (QSL) states with no magnetic order, but its ground state remains largely elusive. An important outstanding question concerns the presence or absence of the $1 / 9$ magnetization plateau, where exotic quantum states, including topological ones, are expected to emerge. Here we report the magnetization of a recently discovered kagome QSL candidate ${YCu}_{3} {(OH)}_{6.5} {Br}_{2.5}$ up to 57 T. Above 50 T, a clear magnetization plateau at $1 / 3$ of the saturation moment of ${Cu}^{2 +}$ ions is observed, supporting that this material provides an ideal platform for the kagome Heisenberg antiferromagnet. Remarkably, we found another magnetization plateau around 20 T, which is attributed to the $1 / 9$ plateau. The temperature dependence of this plateau reveals the presence of the spin gap. The observation of $1 / 9$ and $1 / 3$ plateaus highlights the emergence of novel states in quantum spin systems.

Received 20 October 2023
Accepted 30 April 2024

DOI:https://doi.org/10.1103/PhysRevLett.132.226701

Physics Subject Headings (PhySH)

Frustrated magnetism Quantum spin liquid

Kagome lattice

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

S. Suetsugu ^1,*, T. Asaba ^1,*, Y. Kasahara¹, Y. Kohsaka ¹, K. Totsuka², B. Li³, Y. Zhao³, Y. Li ³, M. Tokunaga⁴, and Y. Matsuda ¹

¹Department of Physics, Kyoto University, Kyoto 606-8502, Japan
²Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto 606-8502, Japan
³Wuhan National High Magnetic Field Center and School of Physics, Huazhong University of Science and Technology, 430074 Wuhan, China
⁴Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan

^*These authors contributed equally to this work.

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Vol. 132, Iss. 22 — 31 May 2024

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Images

Figure 1
Crystal structure and magnetic properties of YCOB. (a) Crystal structure of YCOB. The intersite mixing of OH2 and Br2 displaces 70% of $Y^{3 +}$ ions from the optimal Y1 position on the kagome plane. (b) The kagome plane of ${Cu}^{2 +}$ in YCOB. YCOB consists of a 2D perfect kagome lattice of ${Cu}^{2 +}$ ions without antisite mixing by other nonmagnetic ions. (c) Temperature dependence of the inverse of the magnetic susceptibility $χ^{- 1}$ at 1 T for both $H | | c$ and $H | | a b$ . Linear fits of the high temperature data are denoted by dashed lines. The large negative Curie-Weiss temperatures $θ_{c} = - 77$ and $θ_{a b} = - 65 K$ indicate a predominant antiferromagnetic interaction. (d) Field dependence of magnetization $M_{a b}$ and $M_{c}$ at 2 K up to 7 T. The small anisotropy $M_{c} / M_{a b} = 1.1$ is observed.
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Figure 2
Magnetization process of YCOB up to 57 T. (a) Magnetization process at 0.6, 1.6, and 3.8 K for $H | | c$ . At 0.6 K, distinct magnetization plateaus at $1 / 9$ and $1 / 3$ of the ${Cu}^{2 +}$ saturation moment are observed, as highlighted by light and dark gray regions, respectively. The magnetization curve at 0.6 K nearly perfectly overlaps with that at 1.6 K in both plateau regions, which provides evidence for the formation of the spin gap. For comparison, we also plot the magnetization process obtained by numerical calculations from Fig. 1 in Ref. [10]. The best fit $J = 37 K$ is obtained by adjusting the center of the $1 / 9$ plateau for the experimental results and the numerical calculations. (b) The field dependence of the derivative $d M_{c} / d H$ at 0.6 and 1.6 K. $d M_{c} / d H$ is significantly reduced within the $1 / 9$ and $1 / 3$ plateaus regions. (c) Magnetization process at 0.6, 1.0, 1.6 K, and 3.8 K for $H | | a b$ . Both $1 / 9$ and $1 / 3$ magnetization plateaus are less pronounced for $M_{a b}$ compared to $M_{c}$ . (d) The field dependence of the derivative $d M_{a b} / d H$ at 0.6 K. The absolute values of $M_{c}$ and $M_{a b}$ were calibrated by the MPMS data at 2 K (purple line) in (a) and (c).
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Figure 3
Temperature dependence of magnetization in the $1 / 9$ plateau regime. (a) and (b) Temperature dependence of magnetization at several fields in the $1 / 9$ plateau region for $H | | c$ and $H | | a b$ . The magnetization values are independent of temperature for $T \leq 1.6 K$ , indicating the formation of the spin gap.
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Figure 4
Crystal state of localized magnons. (a) Magnon crystal state at $1 / 3$ , $5 / 9$ , and $7 / 9$ magnetization plateaus. Six entangled spins (red hexagons) form the extended magnons (blue) within the hexagon of the kagome lattice, that crystallizes into a $\sqrt{3} \times \sqrt{3}$ superstructure. The other spins (green arrows) point in the field direction. (b)–(d) The hexagonal magnons at $1 / 3$ , $5 / 9$ , and $7 / 9$ plateaus. Three, two, or one magnons (red hexagons and circles) are localized on each hexagon at $1 / 3$ , $5 / 9$ , and $7 / 9$ plateaus, respectively.
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