Quantum Spin Liquid Emerging from Antiferromagnetic Order by Introducing Disorder

T. Furukawa, K. Miyagawa, T. Itou, M. Ito, H. Taniguchi, M. Saito, S. Iguchi, T. Sasaki, and K. Kanoda

Phys. Rev. Lett. 115, 077001 – Published 11 August 2015

Abstract

Quantum spin liquids, which are spin versions of quantum matter, have been sought after in systems with geometrical frustration. We show that disorder drives a classical magnet into a quantum spin liquid through conducting NMR experiments on an organic Mott insulator, $κ − ({ET)}_{2} Cu [N ({CN)}_{2}] Cl$ . Antiferromagnetic ordering in the pristine crystal, when irradiated by x rays, disappears. Spin freezing, spin gap, and critical slowing down are not observed, but gapless spin excitations emerge, suggesting a novel role of disorder that brings forth a quantum spin liquid from a classical ordered state.

Received 19 February 2015

DOI:https://doi.org/10.1103/PhysRevLett.115.077001

Authors & Affiliations

T. Furukawa^1,*, K. Miyagawa¹, T. Itou², M. Ito³, H. Taniguchi³, M. Saito⁴, S. Iguchi⁴, T. Sasaki⁴, and K. Kanoda^1,†

¹Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan
²Department of Applied Physics, Tokyo University of Science, Tokyo 125-8585, Japan
³Department of Physics, Saitama University, Saitama 338-8570, Japan
⁴Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

^*Present address: Department of Applied Physics, Tokyo University of Science, Tokyo 125-8585, Japan. Corresponding author. tetsuya.furukawa@rs.tus.ac.jp
^†Corresponding author. kanoda@ap.t.u-tokyo.ac.jp

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Vol. 115, Iss. 7 — 14 August 2015

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Figure 1
Structure and transport properties of $κ − Cl$ . (a) Layered structure of $κ − Cl$ . (b) Structure of a conducting layer of $κ − Cl$ . Transfer integrals between nearest-neighbor dimers (next-nearest-neighbor dimers) are shown as $t$ ( $t^{'}$ ). (c) Pressure-temperature phase diagram of nonirradiated $κ − Cl$ [16, 17]. SC and AF denote superconductor and antiferromagnetic ordered state, respectively. (d) Temperature dependence of resistivity of the 500-h x-ray-irradiated (blue line) and nonirradiated (black line) $κ − Cl$ crystals. The former crystal was used for the ${}^{1}H$ NMR study. The data for the latter were obtained in the previous study [19].
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Figure 2
Disappearance of antiferromagnetic order in the $κ − Cl$ crystal after 500-h x-ray irradiation. (a) The temperature dependence of ${}^{1}H$ NMR spectra of the 500-h x-ray-irradiated and nonirradiated $κ − Cl$ crystals. Some of the data for the latter are from our previous study [14]. (b) ${}^{1}H$ NMR spectra of the 500-h x-ray-irradiated $κ − Cl$ crystal normalized to the maximum values at temperatures between 340 mK and 100 K. $f_{peak}$ is the frequency at which the spectra have the maximum intensity. (c) The temperature dependence of the square root of the second moment of NMR spectra, $⟨ f_{2 nd} ⟩^{1 / 2}$ , for the 500-h x-ray-irradiated crystal (blue circles) and the nonirradiated crystal (black triangles); the latter is calculated from the spectra obtained in the previous study [14]. The dashed line guides the eye. The slight difference in the line shape and $⟨ f_{2 nd} ⟩^{1 / 2}$ above 30 K between the two measurements is most likely due to a slight difference in the field geometry against the crystal axes, to which the line shape is sensitive.
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Figure 3
Temperature dependence of the ${}^{1}H$ nuclear spin-lattice relaxation rate of $κ − Cl$ . The main panel shows the ${}^{1}H$ nuclear spin-lattice relaxation rate $T_{1}^{- 1}$ of 500-h x-ray-irradiated and nonirradiated $κ − Cl$ crystals. The inset shows $(T_{1} {T)}^{- 1}$ , where $T$ is temperature. The light blue circles and dark blue diamonds correspond to the data acquired with the use of ${}^{3}{He}$ and ${}^{4}{He}$ cryostats [22], respectively. The black triangles are from the previous study of a nonirradiated $κ − Cl$ crystal [14].
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Figure 4
${}^{1}H$ NMR relaxation curves. The recovery of nuclear magnetization $M (t)$ is plotted in the form of $\log [1 - M (t) / M_{0}]$ vs $t$ for several temperatures. All of the data, including those not shown here, are well fitted by straight lines: $1 - M (t) / M_{0} \propto \exp (- t / T_{1})$ with $T_{1}$ nuclear spin-lattice relaxation time.
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