Abstract
Strongly enhanced quantum fluctuations often lead to a rich variety of quantum-disordered states. Developing approaches to enhance quantum fluctuations may open paths to realize even more fascinating quantum states. Here, we demonstrate that a coupling of localized spins with the zero-point motion of hydrogen atoms, that is, proton fluctuations in a hydrogen-bonded organic Mott insulator provides a different class of quantum spin liquids (QSLs). We find that divergent dielectric behavior associated with the approach to hydrogen-bond order is suppressed by the quantum proton fluctuations, resulting in a quantum paraelectric (QPE) state. Furthermore, our thermal-transport measurements reveal that a QSL state with gapless spin excitations rapidly emerges upon entering the QPE state. These findings indicate that the quantum proton fluctuations give rise to a QSL—a quantum-disordered state of magnetic and electric dipoles—through the coupling between the electron and proton degrees of freedom.
Highlights
Enhanced quantum fluctuations often lead to a rich variety of quantum-disordered states
The temperature dependence of εr for HCat is a typical dielectric behavior observed in quantum paraelectric (QPE) materials such as SrTiO3, in which longrange electric order is suppressed by strong quantum fluctuations
The coincidence of the QPE and quantum spin liquids (QSLs) states is surprising and strongly suggests that the development of the quantum proton fluctuations triggers the emergence of the QSL
Summary
Enhanced quantum fluctuations often lead to a rich variety of quantum-disordered states. 4), where the 2D spin systems are separated by non-magnetic insulating layers This structural feature of H-Cat is highlighted by deuteration of the hydrogen bonds[21]; in the deuterated analog of H-Cat, κ-D3(Cat-EDT-TTF)[2] (denoted as D-Cat), deuterium localization occurs at Tc = 185 K, accompanied by charge disproportionation within the Cat-EDT-TTF layers, resulting in a non-magnetic ground state (Fig. 1d, f). This demonstrates that the hydrogen bonds in this system strongly couple with the charge and spin degrees of freedom of the π-electrons in the Cat-EDT-TTF dimers
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