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Register a new userHigh Field ESR in the Two-Dimensional Triangular Lattice Antiferromagnet NiGa2S4
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In this Letter, we report the results of ESR measurements in high magnetic fields up to about 53 T on single crystals of NiGa2S4 to clarify the spin dynamics in more detail. We have found that the dynamics of Z2 vortices affects the temperature dependence of the ESR absorption linewidth and the frequency dependence of the ESR resonance fields at 1.3 K is well explained by a conventional spin wave theory. These results suggest an occurrence of Z2 vortex-induced topological transition.
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We successfully synthesized and characterized the triangular lattice anitferromagnet Ba$_8$MnNb$_6$O$_{24}$, which comprises equilateral spin-5/2 Mn$^{2+}$ triangular layers separated by six non-magnetic Nb$^{5+}$ layers. The detailed susceptibility, specific heat, elastic and inelastic neutron scattering measurements, and spin wave theory simulation on this system reveal that it has a 120 degree ordering ground state below T$_N$ = 1.45 K with in-plane nearest-neighbor exchange interaction ~0.11 meV. While the large separation 18.9 A between magnetic layers makes the inter-layer exchange interaction virtually zero, our results suggest that a weak easy-plane anisotropy is the driving force for the k$_m$ = (1/3 1/3 0) magnetic ordering. The magnetic properties of Ba$_8$MnNb$_6$O$_{24}$, along with its classical excitation spectra, contrast with the related triple perovskite Ba$_3$MnNb$_2$O$_9$, which shows easy-axis anisotropy, and the iso-structural compound Ba$_8$CoNb$_6$O$_{24}$, in which the effective spin-1/2 Co$^{2+}$ spins do not order down to 60 mK and in which the spin dynamics shows sign of strong quantum effects.
Using electron-spin-resonance (ESR) technique we investigate the magnetic structure of CuCrO2, quasi-two-dimensional antiferromagnet with weakly distorted triangular lattice. Resonance frequencies and the excitation conditions in CuCrO2 at low temperatures are well described in the frame of cycloidal spin structure, defined by two susceptibilities parallel and perpendicular to the spin plane and by a biaxial crystal-field anisotropy. In agreement with the calculations, the character of the eigenmodes changes drastically at the spin-flop transition. The splitting of the observed modes can be well attributed to the resonances from different domains. The domain structure in CuCrO2 can be controlled by annealing of the sample in magnetic field.
Xenes, graphene-like two-dimensional (2D) monoelemental crystals with a honeycomb symmetry, have been the focus of numerous experimental and theoretical studies. In comparison, single-element 2D materials with a triangular lattice symmetry have not received due attention. Here, taking Pb as an example, we investigate the triangular-lattice monolayer made of group-IV atoms employing first-principles density functional theory calculations. The flat Pb monolayer supports a mirror-symmetry-protected spinless nodal line in the absence spin-orbit coupling (SOC). The introduction of an out-of-plane buckling creates a glide mirror, protecting an anisotropic Dirac nodal loop. Both flat and buckled Pb monolayers become topologically trivial after including SOC. A large buckling will make the Pb sheet a 2D semiconductor with symmetry-protected Dirac points below the Fermi level. The electronic structures of other group-IV triangular lattices such as Ge and Sn demonstrate strong similarity to Pb. We further design a quasi-3D crystal PbHfO$_2$ by alternately stacking Pb and 1T-HfO$_2$ monolayers. The new compound PbHfO$_2$ is dynamically stable and retains the properties of Pb monolayer. By applying epitaxial strains to PbHfO$_2$, it is possible to drive an insulator-to-metal transition coupled with an anti-ferroelectric-to-paraelectric phase transition. Our results suggest the potential of the 2D triangular lattice as a complimentary platform to design new type of broadly-defined Xenes.
We have grown single crystals of Na$_2$BaNi(PO$_4$)$_2$, a new spin-1 equilateral triangular lattice antiferromagnet (ETLAF), and performed magnetic susceptibility, specific heat and thermal conductivity measurements at ultralow temperatures. The main results are (i) at zero magnetic field, Na$_2$BaNi(PO$_4$)$_2$ exhibits a magnetic ordering at 430 mK with a weak ferromagnetic moment along the $c$ axis. This suggests a canted 120$^circ$ spin structure, which is in a plane including the crystallographic $c$ axis due to the existence of an easy-axis anisotropy and ferromagnetically stacked along the $c$ axis; (ii) with increasing field along the $c$ axis, a 1/3 magnetization plateau is observed which means the canted 120$^circ$ spin structure is transformed to a up up down (UUD) spin structure. With even higher fields, the UUD phase further evolves to possible V and V phases; (iii) with increasing field along the $a$ axis, the canted 120$^circ$ spin structure is possibly transformed to a umbrella phase and a V phase. Therefore, Na$_2$BaNi(PO$_4$)$_2$ is a rare example of spin-1 ETLAF with single crystalline form to exhibit easy-axis spin anisotropy and series of quantum spin state transitions.
The anomalous thermodynamic properties of the paradigmatic frustrated spin-1/2 triangular lattice Heisenberg antiferromagnet (TLH) has remained an open topic of research over decades, both experimentally and theoretically. Here we further the theoretical understanding based on the recently developed, powerful exponential tensor renormalization group (XTRG) method on cylinders and stripes in a quasi one-dimensional (1D) setup, as well as a tensor product operator approach directly in 2D. The observed thermal properties of the TLH are in excellent agreement with two recent experimental measurements on the virtually ideal TLH material Ba$_8$CoNb$_6$O$_{24}$. Remarkably, our numerical simulations reveal two crossover temperature scales, at $T_l/J sim 0.20$ and $T_h/Jsim 0.55$, with $J$ the Heisenberg exchange coupling, which are also confirmed by a more careful inspection of the experimental data. We propose that in the intermediate regime between the low-temperature scale $T_l$ and the higher one $T_h$, the gapped roton-like excitations are activated with a strong chiral component and a large contribution to thermal entropies, which suppress the incipient 120$^circ$ order that emerges for temperatures below $T_l$.
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