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Register a new userQuantum Paraelectricity in the Kitaev Quantum-Spin-Liquid Candidates H$_{3}$LiIr$_{2}$O$_{6}$ and D$_{3}$LiIr$_{2}$O$_{6}$
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H3LiIr2O6 is the first honeycomb-lattice system without any signs of long-range magnetic order down to the lowest temperatures, raising the hope for the realization of an ideal Kitaev quantum spin liquid. Its honeycomb layers are coupled by interlayer hydrogen bonds. Static or dynamic disorder of these hydrogen bonds was proposed to strongly affect the magnetic exchange and to make Kitaev-type interactions dominant. Using dielectric spectroscopy, here we provide experimental evidence for dipolar relaxations in H3LiIr2O6 and deuterated D3LiIr2O6, which mirror the dynamics of protons and deuterons within the double-well potentials of the hydrogen bonds. The detected hydrogen dynamics reveals glassy freezing, characterized by a strong slowing down under cooling, with a crossover from thermally-activated hopping to quantum-mechanical tunneling towards low temperatures. Thus, besides being Kitaev quantum-spin-liquid candidates, these materials also are quantum paraelectrics. However, the small relaxation rates in the mHz range, found at low temperatures, practically realize quasi-static hydrogen disorder, as assumed in recent theoretical works to explain the quantum-spin-liquid ground state of both compounds.
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Kitaev magnets are materials with bond-dependent Ising interactions between localized spins on a honeycomb lattice. Such interactions could lead to a quantum spin-liquid (QSL) ground state at zero temperature. Recent theoretical studies suggest two potential signatures of a QSL at finite temperatures, namely a scaling behavior of thermodynamic quantities in the presence of quenched disorder, and a two-step release of the magnetic entropy. Here, we present both signatures in Ag$_{3}$LiIr$_{2}$O$_{6}$ which is synthesized from $alpha$-Li$_{2}$IrO$_{3}$ by replacing the inter-layer Li atoms with Ag atoms. In addition, the DC susceptibility data confirm absence of a long-range order, and the AC susceptibility data rule out a spin-glass transition. These observations suggest a closer proximity to the QSL in Ag$_{3}$LiIr$_{2}$O$_{6}$ compared to its parent compound $alpha$-Li$_{2}$IrO$_{3}$ that orders at 15 K. We discuss an enhanced spin-orbit coupling due to a mixing between silver d and oxygen p orbitals as a potential underlying mechanism.
Searching for an ideal Kitaev spin liquid candidate with anyonic excitations and long-range entanglement has motivated the synthesis of a new family of intercalated Kitaev magnets such as H$_{3}$LiIr$_{2}$O$_{6}$, Cu$_{2}$IrO$_{3}$, and Ag$_{3}$LiIr$_{2}$O$_{6}$. The absence of a susceptibility peak and a two-step release of the magnetic entropy in these materials has been proposed as evidence of proximity to the Kitaev spin liquid. Here we present a comparative study of the magnetic susceptibility, heat capacity, and muon spin relaxation ($mu$SR) between two samples of Ag$_{3}$LiIr$_{2}$O$_{6}$ in the clean and disordered limits. In the disordered limit, the absence of a peak in either susceptibility or heat capacity and a weakly depolarizing $mu$SR signal may suggest a proximate spin liquid ground state. In the clean limit, however, we resolve a peak in both susceptibility and heat capacity data, and observe clear oscillations in $mu$SR that confirm long-range antiferromagnetic ordering. The $mu$SR oscillations fit to a Bessel function, characteristic of an incommensurate order, as reported in the parent compound $alpha$-Li$_{2}$IrO$_{3}$. Our results clarify the role of structural disorder in the intercalated Kitaev magnets.
Recently, there have been contrary claims of Kitaev spin-liquid behaviour and ordered behavior in the honeycomb compound Ag$_3$LiIr$_2$O$_6$ based on various experimental signatures. Our investigations on this system reveal a low-temperature ordered state with persistent dynamics down to the lowest temperatures. Magnetic order is confirmed by clear oscillations in the muon spin relaxation ($mu$SR) time spectrum below 9 K till 52 mK. Coincidentally in $^7$Li nuclear magnetic resonance, a wipe-out of the signal is observed below $sim$ 10 K which again strongly indicates magnetic order in the low temperature regime. This is supported by our density functional theory calculations which show an appreciable Heisenberg exchange term in the spin Hamiltonian that favors magnetic ordering. The $^7$Li shift and spin-lattice relaxation rate also show anomalies at $sim$ 50 K. They are likely related to the onset of dynamic magnetic correlations, but their origin is not completely clear. Detailed analysis of our $mu$SR data is consistent with a co-existence of incommensurate Neel and striped environments. A significant and undiminished dynamical relaxation rate ($sim 5$ MHz) as seen in $mu$SR deep into the ordered phase indicates enhanced quantum fluctuations in the ordered state.
We report gapless quantum spin liquid behavior in the layered triangular Sr$_{3}$CuSb$_{2}$O$_{9}$ (SCSO) system. X-ray diffraction shows superlattice reflections associated with atomic site ordering into triangular Cu planes well-separated by Sb planes. Muon spin relaxation ($mu$SR) measurements show that the $S = frac{1}{2}$ moments at the magnetically active Cu sites remain dynamic down to 65 mK in spite of a large antiferromagnetic exchange scale evidenced by a large Curie-Weiss temperature $theta_{mathrm{cw}} simeq $ -143 K as extracted from the bulk susceptibility. Specific heat measurements also show no sign of long-range order down to 0.35 K. The magnetic specific heat ($mathit{C}$$_{mathrm{m}}$) below 5 K reveals a $mathit{C}$$_{mathrm{m}}$ $=$ $gamma T$ + $alpha T$$^{2}$ behavior. The significant $T$$^{2}$ contribution to the magnetic specific heat invites a phenomenology in terms of the so-called Dirac spinon excitations with a linear dispersion. From the low-$T$ specific heat data, we estimate the dominant exchange scale to be $sim $ 36 K using a Dirac spin liquid ansatz which is not far from the values inferred from microscopic density functional theory calculations ($sim $ 45 K) as well as high-temperature susceptibility analysis ($sim$ 70 K). The linear specific heat coefficient is about 18 mJ/mol-K$^2$ which is somewhat larger than for typical Fermi liquids.
We have used high-resolution neutron Larmor diffraction and capacitative dilatometry to investigate spontaneous and forced magnetostriction in undoped, antiferromagnetic YBa$_2$Cu$_3$O$_{6.0}$, the parent compound of a prominent family of high-temperature superconductors. Upon cooling below the Neel temperature, $T_N = 420$~K, Larmor diffraction reveals the formation of magneto-structural domains of characteristic size $sim 240$~nm. In the antiferromagnetic state, dilatometry reveals a minute ($4 times 10^{-6}$) orthorhombic distortion of the crystal lattice in external magnetic fields. We attribute these observations to exchange striction and spin-orbit coupling induced magnetostriction, respectively, and show that they have an important influence on the thermal and charge transport properties of undoped and lightly doped cuprates.
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