Avinash V. Mahajan

The interplay between geometric frustration, spin-orbit coupling, and structural disorder can give rise to unconventional magnetic ground states in rare-earth triangular-lattice magnets. We report low-temperature magnetic and $^{7}\mathrm{Li}$ NMR investigations of the disordered triangular-lattice compound ${\mathrm{LiCaYb}}_{5}({\mathrm{BO}}_{3}{)}_{6}$ (LCYBO). Rietveld refinement confirms a hexagonal $P{6}_{5}22$ structure with partial Ca/Yb antisite disorder and fractional Li occupancy. Magnetic susceptibility and magnetization measurements indicate a well-isolated effective ${J}_{\mathrm{eff}}=1/2$ Kramers doublet with weak antiferromagnetic interactions $({\ensuremath{\theta}}_{\mathrm{CW}}\ensuremath{\approx}\ensuremath{-}0.54\phantom{\rule{0.28em}{0ex}}\mathrm{K})$. The specific heat reveals a weak anomaly near 0.43 K, suggesting the development of short-range correlated magnetism rather than conventional long-range order. $^{7}\mathrm{Li}$ NMR spectra broaden strongly upon cooling, consistent with increasingly inhomogeneous internal magnetic fields. The spin-lattice relaxation exhibits two relaxation components consistent with disorder-induced distributions of local magnetic environments arising from antisite disorder and competing exchange pathways. Our results establish LCYBO as a structurally disordered frustrated triangular-lattice magnet with correlated low-energy spin dynamics.

To date, the alternating kagome and triangular lattice cobaltites, RBaCo$_4$O$_7$ (R = Ca, Y, and rare earth elements), have been well studied for their large structural distortions, anisotropic exchange interactions, chiral spin liquid states, and giant multiferroic properties. Here, we report the co-existence of magnetic and dielectric glassy states in LuBaCo$_4$O$_7$ below 50 K. AC magnetization studies show an absence of conventional spin-freezing behavior. The cooling and heating in unequal fields (CHUF), thermal cycling, and time-dependent magnetization measurements at low temperature ($T$) show the presence of magnetic glassy state. The $T$-dependent dielectric constant $ε'$ measurements exhibit a strong frequency-independent response at the first-order structural phase transition $T = 160$ K (trigonal $P31c$ to monoclinic $Cc$) and also significant features at the $T = 110$ K (monoclinic $Cc$ to orthorhombic $Pbn2_1$) phase transition. Further, $ε'$ shows a frequency-independent peak at 43 K ($Pbn2_1$) and also dipolar glassy features below 20 K ($Cc$). The non-equilibrium magnetic glassy dynamics and dipolar glassy state at low-$T$ arises from the kinetic arrest of $Cc$ and $Pbn2_1$ phases. From the dielectric probe, we are able to clearly distinguish the kinetically arrested phases at low-$T$ , whereas the bulk magnetization studies are unable to do so as the arrested phases have low magnetic moments.

We report the magnetic, magnetodielectric, and electric polarization properties of single crystals of the Co-based Kitaev Spin Liquid (KSL) candidate Na$_2$Co$_2$TeO$_6$ (NCTO). The sample shows magnetic transitions at 26 K, 16 K, and 5 K, consistent with the literature. The magnetic measurements along and perpendicular to the Co-honeycomb planes show a strong anisotropy in susceptibility and in Curie-Weiss (C-W) temperatures. The experimental anisotropic C-W temperatures of NCTO qualitatively match with the theoretical C-W temperatures, calculated using the HKTF model [C. Kim \textit{et al.}, J. Phys.: Condens. Matter \textbf{34}, 045802 (2021)]. We find from our temperature- and field-dependent dielectric and pyroelectric ($I_p$) current studies ($H\parallel ab$ and $E\perp ab$) that our single crystal NCTO samples do not have a finite electric polarization below 100 K. These $I_p$ studies confirm the absence of a magnetoelectric coupling and electric polarization properties in the title compound and suggest that the zig-zag AFM structure is more favorable than the triple-$Q$ structure with AFM Kitaev interactions.

We report the discovery of a novel gapless quantum spin liquid in the S=1 honeycomb system Cu$_3$LiRu$_2$O$_6$ with Ru$^{4+}$ ($4d^4$) where moments remain dynamic down to 50 mK. Heat capacity measurements show no sign of magnetic ordering down to 60 mK in spite of a Curie-Weiss temperature = -222 K indicating a strong antiferromagnetic interaction. In zero field, magnetic heat capacity shows a linear T-dependence with Sommerfeld coefficient = 107 mJ/mol K$^2$ is much larger than that found in typical Fermi liquids. Our local probe $^7$Li nuclear magnetic resonance (NMR) measurements find a significant temperature-independent $^7$Li NMR shift (and hence a non-zero spin susceptibility) at low-T and a linear T-variation of the $^7$Li NMR spin-lattice relaxation rate 1/T$_1$ at low-T reminiscent of fermionic excitations. Muon spin relaxation measurements detect neither long-range ordering nor spin freezing down to 50 mK and the temperature variation of the muon depolarization rate shows a gradual increase with decreasing temperature and a leveling off below about 1 K evincing a persistent spin dynamics common to several spin liquid candidates. Our results provide strong signatures of a quantum spin liquid in the titled honeycomb material.

The subtle interaction of low dimensionality and spin correlations in two-dimensional quantum magnets results in novel states of matter. Herein, we investigate the structural, magnetic, heat capacity, ${}^{31}\mathrm{P}$ nuclear magnetic resonance (NMR) measurements, and density functional theory $+$ Hubbard $U$ (DFT $+U$) electronic structure calculations on the ${\mathrm{Cu}}^{2+}$ ($\mathit{S}=\frac{1}{2}$)-based compound ${\mathrm{BaCu}}_2{\left({\mathrm{PO}}_4\right)}_2\left({\mathrm{H}}_2\mathrm{O}\right)$. Based on the hopping parameters obtained from DFT, the titled compound comprises the 2D layer of 1/3-depleted anisotropic kagome lattice with the magnetic couplings $J_1,J_2, J_3$, and $J_4$. At high temperatures, magnetic susceptibility ($\chi$) exhibits paramagnetic behavior, with a Curie-Weiss temperature of ${\theta }_{CW}\approx -67$ K, indicating the presence of dominant antiferromagnetic interactions. The appearance of a broad maximum in the magnetic susceptibility and heat capacity reveals the low-dimensional nature of the compound. While the sharp magnetic anomaly features are not seen clearly in the bulk measurements, the ${}^{31}\mathrm{P}$ NMR local probe measurements unambiguously show the presence of magnetic transition at $T_N=10.5$ K. The temperature ($T$) dependence of nuclear spin-lattice relaxation rate ($1/T_1$) exhibits a sharp peak at $T_{\mathrm{N}}$. While $1/T_1$ falls very steeply below $T_{\mathrm{N}}$, it follows a sublinear power-law behavior above $T_{\mathrm{N}}$, indicating the presence of dynamic short-range correlations, which could be attributed to the complex 2D spin network of the compound.

We report low-temperature (down to $\sim 5$ K) Raman signatures of the recently discovered intercalated honeycomb magnet ${\mathrm{Ag}}_3{\mathrm{LiIr}}_2{\mathrm{O}}_6$, a putative Kitaev quantum spin liquid (QSL) candidate. The Kitaev QSL is predicted to host Majorana fermions as its emergent elementary excitations through a thermal fractionalization of entangled spins $S=1/2$. We observe evidence of this fractionalization in the low-energy magnetic continuum whose temperature evolution harbors signatures of the predicted Fermi statistics obeyed by the itinerant Majorana quasiparticles. The magnetic Raman susceptibility evinces a crossover from the conventional to a Kitaev paramagnetic state below the temperature of $\sim 80$ K. Additionally, the development of the Fano asymmetry in the low-frequency phonon mode and the enhancement of integrated Raman susceptibilities below the crossover temperature signifies prominent coupling between the vibrational and Majorana fermionic excitations.