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Register a new userTheoretical study of the electronic states of hollandite vanadate K$_2$V$_8$O$_{16}$
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We consider electronic properties of hollandite vanadate K$_2$V$_8$O$_{16}$, a one-dimensional zigzag-chain system of $t_{2g}$ orbitals in a mixed valent state. We first calculate the Madelung energy and obtain the relative stability of several charge-ordering patterns to determine the most stable one that is consistent with the observed superlattice structure. We then develop the strong-coupling perturbation theory to derive the effective spin-orbit Hamiltonian, starting from the triply-degenerate $t_{2g}$ orbitals in the VO$_6$ octahedral structure. We apply an exact-diagonalization technique on small clusters of this Hamiltonian and obtain the orbital-ordering pattern and spin structures in the ground state. We thereby discuss the electronic and magnetic properties of K$_2$V$_8$O$_{16}$ including predictions on the outcome of future experimental studies.
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An textit{ab initio} electronic structure calculation based on the generalized gradient approximation in the density functional theory is carried out to study the basic electronic states of hollandite vanadate K$_2$V$_8$O$_{16}$. We find that the states near the Fermi energy consist predominantly of the three $t_{2g}$-orbital components and the hybridization with oxygen $2p$ orbitals is small. The $d_{yz}$ and $d_{zx}$ orbitals are exactly degenerate and are lifted from the $d_{xy}$ orbital. The calculated band dispersion and Fermi surface indicate that the system is not purely one-dimensional but the coupling between the VO double chains is important. Comparison with available experimental data suggests the importance of electron correlations in this system.
Based on the first-principles electronic structure calculations, we predict that a chromium oxide K$_2$Cr$_8$O$_{16}$ of hollandite type should be a half-metallic ferromagnet where the Fermi level crosses only the majority-spin band, whereas the minority-spin band has a semiconducting gap. We show that the double-exchange mechanism is responsible for the observed saturated ferromagnetism. We discuss possible scenarios of the metal-insulator transition observed at low temperature and we argue that the formation of the incommensurate, long-wavelength density wave of spinless fermions caused by the Fermi-surface nesting may be the origin of the opening of the charge gap.
The magnetic properties of the two-dimensional, S=1 honeycomb antiferromagnet BaNi$_2$V$_2$O$_8$ have been comprehensively studied using DC susceptibility measurements and inelastic neutron scattering techniques. The magnetic excitation spectrum is found to be dispersionless within experimental resolution between the honeycomb layers, while it disperses strongly within the honeycomb plane where it consists of two gapped spin-wave modes. The magnetic excitations are compared to linear spin-wave theory allowing the Hamiltonian to be determined. The first- and second-neighbour magnetic exchange interactions are antiferromagnetic and lie within the ranges 10.90meV$le$J$_n$$le$13.35 meV and 0.85meV$le$J$_{nn}$$le$1.65 meV respectively. The interplane coupling J$_{out}$ is four orders of magnitude weaker than the intraplane interactions, confirming the highly two-dimensional magnetic behaviour of this compound. The sizes of the energy gaps are used to extract the magnetic anisotropies and reveal substantial easy-plane anisotropy and a very weak in-plane easy-axis anisotropy. Together these results reveal that BaNi$_2$V$_2$O$_8$ is a candidate compound for the investigation of vortex excitations and Berezinsky-Kosterliz-Thouless phenomenona.
Large single crystals of the new compound SrMn$_2$V$_2$O$_8$ have been grown by the floating-zone method. This transition-metal based oxide is isostructural to SrNi$_2$V$_2$O$_8$, described by the tetragonal space group $I4_1cd$. Magnetic properties were investigated by means of susceptibility, magnetization, and specific heat measurements. The title compound behaves like a one-dimensional magnetic system above the ordering temperature ($T_N$ = 43 K). The magnetic ground state can be described as a classical long-range ordered antiferromagnet with weak anisotropy.
We report $^{51}$V nuclear magnetic resonance (NMR) and inelastic neutron scattering (INS) measurements on a quasi-1D antiferromagnet BaCo$_2$V$_2$O$_8$ under transverse field along the [010] direction. The scaling behavior of the spin-lattice relaxation rate above the N{e}el temperatures unveils a 1D quantum critical point (QCP) at $H_c^{1D}approx 4.7$ T, which is masked by the 3D magnetic order. With the aid of accurate analytical analysis and numerical calculations, we show that the zone center INS spectrum at $H_c^{1D}$ is precisely described by the pattern of the 1D quantum Ising model in a magnetic field, a class of universality described in terms of the exceptional $E_8$ Lie algebra. These excitations keep to be non-diffusive over a certain field range when the system is away from the 1D QCP. Our results provide an unambiguous experimental realization of the massive $E_8$ phase in the compound, and open new experimental route for exploring the dynamics of quantum integrable systems as well as physics beyond integrability.
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