Quarter-filled Ladder Compound Research Articles

R-matrices for integrable SU(2) × U(1)-symmetric spin-orbital chains

The Yang–Baxter equation for an SU(2) × U(1)-symmetric spin-orbital chain was solved using the special computer algorithm developed by the author. The seven new R-matrices separated into four groups are presented. Among the obtained integrable models there are special cases related to 1D ferromagnet TDAE-C60, 1D superconductors AC60 (A = K, Cs, Rb), the quarter-filled ladder compound NaV2O5 and the model of correlated electrons on a chain of Berry phase molecules.

Ordering in two-dimensional Ising models with competing interactions

We study the 2D Ising model on a square lattice with additional non-equal diagonal next-nearest neighbor interactions. The cases of classical and quantum (transverse) models are considered. Possible phases and their locations in the space of three Ising couplings are analyzed. In particular, incommensurate phases occurring only at non-equal diagonal couplings, are predicted. We also analyze a spin-pseudospin model comprised of the quantum Ising model coupled to XY spin chains in a particular region of interactions, corresponding to the Ising sector’s super-antiferromagnetic (SAF) ground state. The spin-SAF transition in the coupled Ising-XY model into a phase with co-existent SAF Ising (pseudospin) long-range order and a spin gap is considered. Along with destruction of the quantum critical point of the Ising sector, the phase diagram of the Ising-XY model can also demonstrate a re-entrance of the spin-SAF phase. A detailed study of the latter is presented. The mechanism of the re-entrance, due to interplay of interactions in the coupled model, and the conditions of its appearance are established. Applications of the spin-SAF theory for the transition in the quarter-filled ladder compound NaV2O5 are discussed.

Simultaneous charge ordering and spin dimerization in quasi-two-dimensional quarter-filled ladders

We study the spin-pseudospin Hamiltonian of the Ising model in transverse field (IMTF) for pseudospins, coupled to the $\mathrm{XY}$ spins on a triangular lattice. This model appears from analyses of the quarter-filled ladder compound ${\mathrm{NaV}}_{2}{\mathrm{O}}_{5},$ and pseudospins represent its charge degrees of freedom. In the molecular-field approximation we find that the model possesses two phases: charge-disordered phase without spin gap and a low-temperature phase containing both the antiferroelectric (zigzag) charge order and spin dimerization (spin gap). The phase transition is of the second kind, and the calculated physical quantities are just as those one expects from the Landau theory. One of the particular features of the phase diagram is that the interladder spin-pseudospin coupling, responsible for the spin-gap generation, also destroys the IMTF quantum critical point, resulting in the exponential behavior of ${T}_{c}$ in the region of Ising's coupling where the IMTF is always disordered. We conclude that our mean-field results give a qualitatively correct description of the phase transition in ${\mathrm{NaV}}_{2}{\mathrm{O}}_{5},$ while a more sophisticated analysis is warranted to take into account the thermal fluctuations and, probably, the proximity of the IMTF quantum critical point.

Magnetic excitations in the high-temperature phase of α′-NaV 2O 5

We measured the 51 V nuclear spin relaxation rate in the high-temperature phase of the quarter-filled ladder compound α′-NaV 2O 5. We compare the results with theories for the spin- 1 2 Heisenberg chain and find no serious discrepancies between them.

Comment on “Low-Temperature Structure of the Quarter-Filled Ladder Compound α'-NaV2O5”

Comment on “Low-Temperature Structure of the Quarter-Filled Ladder Compound α'-NaV₂O₅”

Nature of insulating state inNaV2O5above charge-ordering transition: A cluster dynamical mean-field study

The nature of the insulating state driven by electronic correlations in the quarter-filled ladder compound ${\ensuremath{\alpha}}^{\ensuremath{'}}{\mathrm{NaV}}_{2}{\mathrm{O}}_{5}$ is investigated within a cluster dynamical mean-field approach. An extended Hubbard model with first-principles tight-binding parameters has been used. It is shown that the insulating state in the charge-disordered phase of this compound is formed due to the transfer of spectral density and dynamical charge fluctuations where for the latter, the role of intersite Coulomb interaction is found to be of crucial importance.

Zigzag charge ordering structure of the quarter-filled ladder compound NaV2O5

Zigzag charge ordering structure of the quarter-filled ladder compound NaV₂O₅

Coupled spin-charge dynamics inNaV2O5and its effect on the optical absorption

We argue that in the quarter filled ladder compound ${\mathrm{NaV}}_{2}{\mathrm{O}}_{5}$ the quasi-one-dimensional spin system is strongly coupled to an antiferroelectric mode with a broad band. This mode is the single-electron excitation on a rung of the two-leg ladder, which can hop from rung to rung due to the Coulomb interactions between electrons. We show that the coupling of the antiferroelectric mode to spins gives rise to the infrared absorption continuum, observed in ${\mathrm{NaV}}_{2}{\mathrm{O}}_{5}.$

Low-Temperature Structure of the Quarter-Filled Ladder Compound α'-NaV2O5

The low-temperature (LT) superstructure of $\alpha'$-NaV$_2$O$_5$ was determined by synchrotron radiation x-ray diffraction. Below the phase transition temperature associated with atomic displacement and charge ordering at 34K, we observed the Bragg peak splittings, which evidence that the LT structure is monoclinic. It was determined that the LT structure is $(a-b)\times 2b \times 4c$ with the space group $A112$ where $a, b$ and $c$ represent the high temperature orthorhombic unit cell. The valence estimation of V ions according to the bond valence sum method shows that the V sites are clearly separated into two groups of V$^{4+}$ and V$^{5+}$ with a $zigzag$ charge ordering pattern. This LT structure is consistent with resonant x-ray and NMR measurements, and strikingly contrasts to the LT structure previously reported, which includes V$^{4.5+}$ sites.

Effective charge and spin Hamiltonian for the quarter-filled ladder compound α'-NaV2O5

An effective intra- and inter-ladder charge-spin hamiltonian for the quarter-filled ladder compound $\alpha'$-NaV$_2$O$_5$ has been derived by using the standard canonical transformation method. In the derivation, it is clear that a finite inter-site Coulomb repulsion is needed to get a meaningful result otherwise the perturbation becomes ill-defined. Various limiting cases depending on the values of the model parameters have been analyzed in detail and the effective exchange couplings are estimated. We find that the effective intra-ladder exchange may become ferromagnetic for the case of zig-zag charge ordering in a purely electronic model. We estimate the magnitude of the effective inter-rung Coulomb repulsion in a ladder and find it to be about one-order of magnitude too small in order to stabilize charge-ordering.

NaV2O5as a Quarter-Filled Ladder Compound

A new X-ray diffraction study of the one-dimensional spin-Peierls compound \alpha-NaV_2O_5 reveals a centrosymmetric (Pmmn) crystal structure with one type of V site, contrary to the previously postulated non-centrosymmetric P2_1mn structure with two types of V sites (V^{+4} and V^{+5}). Density functional calculations indicate that NaV_2O_5 is a quarter-filled ladder compound with the spins carried by V-O-V molecular orbitals on the rungs of the ladder. Estimates of the charge-transfer gap and the exchange coupling agree well with experiment and explain the insulating behavior of NaV_2O_5 and its magnetic properties.