Tentative Structural Features of a Gapped RVB State in the Anisotropic Triangular Lattice

The self-consistency equations for the independent order parameters as well as the free energy expression for the mean-field RVB model of the spin-1/2 Heisenberg Hamiltonian on the anisotropic triangular lattice is considered in the quasi-one-dimensional approximation. The solutions of the self-consistency equations in the zero-temperature limit are in fair agreement with the previous numerical analysis of the same model by other authors. In particular, the transition from the ungapped 1D-RVB state to the gapped 2D-RVB state occurs at an arbitrarily weak transversal exchange ($J_{2}\rightarrow0)$ although the amount of the gap is exponentially small: $\frac{12J_{1}}{\pi}\exp(-\frac{2J_{1}}{J_{2}})$, where $J_{1}$ is the longitudinal exchange parameter. The structural consequences of the formation of the 2D-RVB state are formulated by extending the famous bond order \emph{vs}. bond length relation known for polyenes (one-dimensional Hubbard chains). Analytical estimates of this effect are given.Comment: 14 pages, 2 figure

Unconventional Magnetism in a Nitrogen-Based Analogue of Cupric Oxide

We have investigated the magnetic properties of CuNCN, the first nitrogen-based analogue of cupric oxide, CuO. Our muon spin relaxation, nuclear magnetic resonance and electron spin resonance studies reveal that classical magnetic ordering is absent down to lowest temperatures. However, large enhancement of spin correlations and unexpected inhomogeneous magnetism have been observed below 80 K. We attribute this to a peculiar fragility of the electronic state against weak perturbations due to geometrical frustration, which selects between competing spin-liquid and more conventional frozen states.Comment: 4 pages + 1 page of supplementary information, accepted for publication in PR

Efficient magnetic superstructure optimization with ΘΦ

Simulating the incommensurate spin density waves (ISDW) states is not a simple task within the standard \emph{ab-initio} methods. Moreover, in the context of new material discovery, there is a need for fast and reliable tool capable to scan and optimize the total energy as a function of the pitch vector, thus allowing to automatize the search for new materials. In this paper we show how the ISDW can be efficiently obtained within the recently released $\Theta\Phi$ program. We illustrate this on an example of the single orbital Hubbard model and of $\gamma$-Fe, where the ISDW emerge within the mean-field approximation and by using the twisted boundary conditions. We show the excellent agreement of the $\Theta\Phi$ with the previously published ones and discuss possible extensions. Finally, we generalize the previously given framework for spin quantization axis rotation to the most general case of spin-dependent hopping matrix elements.Comment: 8 pages, 4 figures, to appear in Computational Materials Scienc

Magnetism and lattice dynamics of FeNCN compared to FeO

Three-dimensional non-oxidic extended frameworks offer the possibility to design novel materials with unique properties, which can be different from their oxide analogues. Here, we present first experimental results concerning unusual magnetic properties of FeNCN, investigated using Mössbauer spectroscopy and magnetometry between 5 and 380 K. This study reveals an unconventional behaviour of the magnetic parameters below the Néel temperature of 350 K, i.e., the hyperfine field on iron decreases with decreasing temperature. At room temperature, quadrupole and hyperfine magnetic field interaction energies are comparable in magnitude, which leads to a rare five-line absorption spectrum. We suggest that these features in the hyperfine field are caused by the combination of a small Fermi contact term and a temperature-dependent contribution from the orbital momentum and the dipole term. One additional spectral component is observed, which exhibits a magnetic relaxation behaviour and slows down at low temperatures to yield a sextet. The magnetometry data suggest that the antiferromagnetic FeNCN is rich in structural distortions, which results in a splitting of the field-cooled and zero-field-cooled curves. The lattice dynamics of FeNCN were investigated using nuclear inelastic scattering. The comparison of the obtained data with literature data of iron monoxide reveals very similar iron phonon modes with a small softening and a slightly reduced sound velocity

Magnetic inhomogeneity in the copper pseudochalcogenide CuNCN

Copper carbodiimide, CuNCN, is a geometrically frustrated nitrogen-based analogue of cupric oxide, whose magnetism remains ambiguous. Here, we employ a combination of local-probe techniques, including $^{63,\, 65}$Cu nuclear quadrupole resonance, $^{13}$C nuclear magnetic resonance and muon spin rotation to show that the magnetic ground state of the Cu$^{2+}$ ($S=1/2$) spins is frozen and disordered. Moreover, these complementary experiments unequivocally establish an onset of intrinsically inhomogeneous magnetic state at $T_h=80$ K. Below $T_h$, the low-temperature frozen component coexist with the remnant high-temperature dynamical component down to $T_l = 20$ K, where the latter finally ceases to exist. Based on a scaling of internal magnetic fields of both components we conclude that the two components coexist on a microscopic level