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Register a new userNaYbS$_2$ - a new planar spin-$frac{1}{2}$ triangular-lattice magnet and putative spin liquid
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Added by
Kizhake Malayil Ranjith Kumar
Publication date
2018
fields
Physics
and research's language is
English
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Platelike high-quality NaYbS$_{2}$ rhombohedral single crystals with lateral dimensions of a few mm have been grown and investigated in great detail by bulk methods like magnetization and specific heat, but also by local probes like nuclear magnetic resonance (NMR), electron-spin resonance (ESR), muon-spin relaxation ($mu$SR), and inelastic neutron scattering (INS) over a wide field and temperature range. Our single-crystal studies clearly evidence a strongly anisotropic quasi-2D magnetism and an emerging spin-orbit entangled $S=1/2$ state of Yb towards low temperatures together with an absence of long-range magnetic order down to 260~mK. In particular, the clear and narrow Yb ESR lines together with narrow $^{23}$Na NMR lines evidence an absence of inherent structural distortions in the system, which is in strong contrast to the related spin-liquid candidate YbMgGaO$_{4}$ falling within the same space group $Roverline{3}m$. This identifies NaYbS$_{2}$ as a rather pure spin-1/2 triangular lattice magnet and a new putative quantum spin liquid.
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Here, we report the synthesis and magnetic properties of a Yb-based triangular-lattice compound LiYbS$_2$. At low temperatures, it features an effective spin-$frac{1}{2}$ state due to the combined effect of crystal electric field and spin orbit coupling. Magnetic susceptibility measurements and $^7$Li nuclear magnetic resonance experiments reveal the absence of magnetic long range ordering down to 2~K, which suggests a possible quantum spin liquid ground state. A dominant antiferromagnetic nearest neighbour exchange interaction $J/k_{rm B}simeq$ 5.3~K could be extracted form the magnetic susceptibility. The NMR linewidth analysis yields the coupling constant between the Li nuclei and Yb$^{3+}$ ions which was found to be purely dipolar in nature.
Rare-earth delafossites were recently proposed as promising candidates for the realization of an effective $S$=1/2 quantum spin liquid (QSL) on the triangular lattice. In contrast to the most actively studied triangular-lattice antiferromagnet YbMgGaO$_4$, which is known for considerable structural disorder due to site intermixing, NaYbS$_2$ delafossite realizes structurally ideal triangular layers. We present detailed $mu$SR studies on this regular (undistorted) triangular Yb sublattice based system with effective spin $J_{mathrm{eff}}=1/2$ in the temperature range 0.05 - 40 K. Zero-field (ZF) and longitudinal field (LF) $mu$SR studies confirm the absence of any long range magnetic order state down to 0.05K ($sim J$/80). Current $mu$SR results together with the so far available bulk characterization data suggest that NaYbS$_2$ is an ideal candidate to identify QSL ground state.
Polycrystalline samples of NaYbO$_2$ are investigated by bulk magnetization and specific-heat measurements, as well as by nuclear magnetic resonance (NMR) and electron spin resonance (ESR) as local probes. No signatures of long-range magnetic order are found down to 0.3~K, evidencing a highly frustrated spin-liquid-like ground state in zero field. Above 2,T, signatures of magnetic order are observed in thermodynamic measurements, suggesting the possibility of a field-induced quantum phase transition. The $^{23}$Na NMR relaxation rates reveal the absence of magnetic order and persistent fluctuations down to 0.3~K at very low fields and confirm the bulk magnetic order above 2~T. The $H$-$T$ phase diagram is obtained and discussed along with the existing theoretical concepts for layered spin-$frac{1}{2}$ triangular-lattice antiferromagnets
A layered triangular lattice with spin-1/2 ions is an ideal platform to explore highly entangled exotic states like quantum spin liquid (QSL). Here, we report a systematic in-field neutron scattering study on a perfect two-dimensional triangular-lattice antiferromagnet, CsYbSe$_2$, a member of the large QSL candidate family rare-earth chalcogenides. The elastic neutron scattering measured down to 70 mK shows that there is a short-range 120$^{circ}$ magnetic order at zero field. In the field-induced ordered states, the spin-spin correlation lengths along the $c$ axis are relatively short, although the heat capacity results indicate long-range magnetic orders at 3 T $-$ 5 T. The inelastic neutron scattering spectra evolve from highly damped continuum-like excitations at zero field to relatively sharp spin wave modes at the plateau phase. Our extensive large-cluster density-matrix renormalization group calculations with a Heisenberg triangular-lattice nearest-neighbor antiferromagnetic model reproduce the essential features of the experimental spectra, including continuum-like excitations at zero field, series of sharp magnons at the plateau phase as well as two-magnon excitations at high energy. This work presents comprehensive experimental and theoretical overview of the unconventional field-induced spin dynamics in triangular-lattice Heisenberg antiferromagnet and thus provides valuable insight into quantum many-body phenomena.
We report a comprehensive investigation of the magnetism of the $S$ = 3/2 triangular-lattice antiferromagnet, $alpha$-CrOOH(D) (delafossites green-grey powder). The nearly Heisenberg antiferromagnetic Hamiltonian ($J_1$ $sim$ 23.5 K) with a weak single-ion anisotropy of $|D|$/$J_1$ $sim$ 4.6% is quantitatively determined by fitting to the electron spin resonance (ESR) linewidth and susceptibility measured at high temperatures. The weak single-ion anisotropy interactions, possibly along with other perturbations, e.g. next-nearest-neighbor interactions, suppress the long-range magnetic order and render the system disordered, as evidenced by both the absence of any clear magnetic reflections in neutron diffraction and the presence of the dominant paramagnetic ESR signal down to 2 K ($sim$ 0.04$J_1$$S^2$), where the magnetic entropy is almost zero. The power-law behavior of specific heat ($C_m$ $sim$ $T^{2.2}$) observed below the freezing temperature of $T_f$ = 25 K in $alpha$-CrOOH or below $T_f$ = 22 K in $alpha$-CrOOD is insensitive to the external magnetic field, and thus is consistent with the theoretical prediction of a gapless U(1) Dirac quantum spin liquid (QSL) ground state. At low temperatures, the spectral weight of the low-energy continuous spin excitations accumulates at the K points of the Brillouin zone, e.g. $|mathbf{Q}|$ = 4$pi$/(3$a$), and the putative Dirac cones are clearly visible. Our work is a first step towards the understanding of the possible Dirac QSL ground state in this triangular-lattice magnet with $S$ = 3/2.
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