Gapless magnetic excitations in the kagome antiferromagnet Ca-kapellasite probed by Cl35 NMR spectroscopy

Abstract

The low-energy magnetic excitations of the spin-1/2 kagome antiferromagnet CaCu 3 (OH) 6 Cl 2 · 0.6H 2 O (Ca-kapellasite) have been investigated by a 35 Cl NMR experiment in fields up to 18.9 T. Recently, Ca-kapellasite was found to be one of the most promising candidates to explore the intrinsic magnetic properties in kagome antiferromagnets, because magnetic defects, which deteriorate the intrinsic magnetic properties, are minimized by choosing Ca ions with a large ionic radius as the counterions. From our nuclear spin-lattice relaxation rate 1/T 1 measurement, we found a power-law temperature dependence below the magnetic ordering temperature T M = 7.2 K, which leads us to suggest that gapless magnetic excitations survive even in the ordered state. This power-law behavior is suppressed in high magnetic fields. We discuss a possible magnetic state that can generate gapless excitations at low fields. DOI: 10.1103/PhysRevB.96.180409 In quantum magnets with 1/2 spins, when the canonical magnetic ordering is disturbed by geometrical frustration and/or low-dimensionality effects, strong quantum fluctuations at low temperatures stabilize exotic spin states. The most apparent quantum fluctuation effect is expected for antiferromagnetically interacting spins on a kagome network, because the corner-sharing triangle network reduces the number of neighboring spins (Z = 4). In these kagome antiferromagnets (KAFMs), macroscopic numbers of spin states are degenerate or pseudodegenerate at very low energy, and therefore the exotic spin states, which never appear in canonical antiferromagnets, can be stabilized by a small energy perturbation, such as magnetic fields. Although interesting, an unperturbed ground state is hard to be picked out from the huge numbers of possible candidates [1]. In fact, theoretical studies have proposed various quantum spin-liquid states with gaps or without gaps in the spin excitation spectrum. A gapped topological Z 2 state was suggested from the density matrix renormalization group theory [2], while a gapless U (1)-Dirac state was suggested from the Gutzwiller projection technique [3] and also from the recently developed tensor-network method [4]. To reveal the intrinsic ground state, which appears in real materials, an experimental search for a perfect KAFM is compulsory. At this time, the most promising candidate material for the perfect KAFM is the mineral herbertsmithite [ZnCu 3 (OH) 6 Cl 2 ] [5], for which many experimental studies have been performed [6][7][8][9][10][11], and a gapped spin-liquid ground state has been suggested from the latest 17 O NMR spectroscopy study for the single crystal [12]. In herbertsmithite, however, the site exchange between magnetic Cu and nonmagnetic Zn creates isolated spins [13], which can deteriorate the proposed intrinsic ground state for a clean system. Searches for the perfect KAFM have been carried out for long time [14][15][16][17].The recently reported candidate for a KAFM is Cakapellasite [CaCu 3 (OH) 6 Cl 2 · 0.6H 2 O], for which an x-ray diff...

Spin Thermal Hall Conductivity of a Kagome Antiferromagnet

“…The temperature dependence of the heat capacity also features a peak at T * , indicating a magnetic transition at T * . This magnetic transition was confirmed in NMR, reporting a small broadening of the spectrum of ∼ 0.05µ B below T * [30]. Although the magnetic ground state of Ca kapellasite is not a disordered state, the ordering temperature is much smaller than J 1 , suggesting that a spin liquid state is realized over a wide temperature range, T * < T < J 1 /k B .…”

“…1(e)) compared to the values found in other quantum spin-liquid candidates [44,45], indicating the absence of itinerant gapless excitations with linear-T dependence. This suggests that the gapless excitations obtained from heat capacity [23] and NMR [30] measurements are localized and do not contribute to κ xx .…”

“…A zero-temperature extrapolation of the heat capacity data above T * reveals a large residual in the linear T term, γ, implying the presence of gapless spin excitations in the spin liquid phase. Also a finite γ is suggested to remain even in the ordered phase below T * [23,30], implying the presence of unusual gapless spin excitations.…”

Thermal-transport studies of kagomé antiferromagnets

“…Therefore, the spin Hamiltonian of Ca kapellasite is well approximated to an ideal KHA. The temperature dependence of χ and C feature a peak at T N ∼ 7.4 K, which is shown later as an onset of a longranged ordered state by NMR measurement [58].…”

$\mathbf{q}=\mathbf{0}$ long-range magnetic order in centennialite CaCu$_3$(OD)$_6$Cl$_2$$\cdot$0.6D$_2$O: A spin-1/2 perfect kagome antiferromagnet with $J_1$-$J_2$-$J_d$

“…Since kapellasite and haydeeite have ferromagnetic J 1 [18,23], centennialite is considered to be an ideal compound for investigating the J 1 -J 2 -J d kagome lattice with antiferromagnetic J 1 [28]. The heat capacity [28] and nuclear magnetic resonance [29] measurements show that centennialite undergoes long-range magnetic ordering below T N = 7.2 K. However, detailed crystal and magnetic structures of centennialite have not been reported yet by diffraction techniques. In this paper, we investigated in detail the crystal and magnetic structures by synchrotron x-ray and neutron diffraction measurements using single crystals.…”