1:1 Alkali-TCNQ salts and the bond order wave (BOW) phase of half-filled linear Hubbard-type models (original) (raw)
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Realization of the bond order wave (BOW) phase of extended Hubbard models in Rb–TCNQ(II)
Chemical Physics Letters, 2009
Rb-TCNQ(II) is shown to be a realization of the BOW phase of half-filled extended Hubbard models. The BOW phase has a regular array of sites with inversion (C i) symmetry, a finite magnetic gap E m and broken electronic C i symmetry. The phase is conditionally stable against dimerization for linear electron-phonon (e-ph) coupling. At 100 K, Rb-TCNQ(II) crystals have regular TCNQ À stacks at C i centers, negligible spin susceptibility that indicates finite E m , and infrared (ir) spectra that indicate broken C i symmetry. The 100 and 295 K crystal structures rule out a dimerization transition around 220 K that had previously been inferred from magnetic and ir data.
Physical Review B, 2010
The bond order wave (BOW) phase of the extended Hubbard model (EHM) in one dimension (1D) is characterized at intermediate correlation U = 4t by exact treatment of N -site systems. Linear coupling to lattice (Peierls) phonons and molecular (Holstein) vibrations are treated in the adiabatic approximation. The molar magnetic susceptibility χM (T ) is obtained directly up to N = 10. The goal is to find the consequences of a doubly degenerate ground state (gs) and finite magnetic gap Em in a regular array. Degenerate gs with broken inversion symmetry are constructed for finite N for a range of V near the charge density wave (CDW) boundary at V ≈ 2.18t where Em ≈ 0.5t is large. The electronic amplitude B(V ) of the BOW in the regular array is shown to mimic a tight-binding band with small effective dimerization δ ef f . Electronic spin and charge solitons are elementary excitations of the BOW phase and also resemble topological solitons with small δ ef f . Strong infrared intensity of coupled molecular vibrations in dimerized 1D systems is shown to extend to the regular BOW phase, while its temperature dependence is related to spin solitons. The Peierls instability to dimerization has novel aspects for degenerate gs and substantial Em that suppresses thermal excitations. Finite Em implies exponentially small χM (T ) at low temperature followed by an almost linear increase with T . The EHM with U = 4t is representative of intermediate correlations in quasi-1D systems such as conjugated polymers or organic ion-radical and charge-transfer salts. The vibronic and thermal properties of correlated models with BOW phases are needed to identify possible physical realizations.
2011
Abstract: The molar spin susceptibilities chi(T)\ chi (T) chi(T) of Na-TCNQ, K-TCNQ and Rb-TCNQ (II) are fit quantitatively to 450 K in terms of half-filled bands of three one-dimensional Hubbard models with extended interactions using exact results for finite systems. All three models have bond order wave (BOW) and charge density wave (CDW) phases with boundary $ V= V_c (U) $ for nearest-neighbor interaction $ V $ and on-site repulsion $ U .Athigh. At high .Athigh T ,allthreesaltshaveregularstacksof, all three salts have regular stacks of ,allthreesaltshaveregularstacksof\ rm TCNQ^-$ anion radicals.
The Journal of Chemical Physics, 2011
The molar spin susceptibilities χ(T) of Na-TCNQ, K-TCNQ and Rb-TCNQ(II) are fit quantitatively to 450 K in terms of half-filled bands of three one-dimensional Hubbard models with extended interactions using exact results for finite systems. All three models have bond order wave (BOW) and charge density wave (CDW) phases with boundary V = Vc(U) for nearest-neighbor interaction V and on-site repulsion U. At high T , all three salts have regular stacks of TCNQ − anion radicals. The χ(T) fits place Na and K in the CDW phase and Rb(II) in the BOW phase with V ≈ Vc. The Na and K salts have dimerized stacks at T < T d while Rb(II) has regular stacks at 100K. The χ(T) analysis extends to dimerized stacks and to dimerization fluctuations in Rb(II). The three models yield consistent values of U , V and transfer integrals t for closely related TCNQ − stacks. Model parameters based on χ(T) are smaller than those from optical data that in turn are considerably reduced by electronic polarization from quantum chemical calculation of U , V and t on adjacent TCNQ − ions. The χ(T) analysis shows that fully relaxed states have reduced model parameters compared to optical or vibration spectra of dimerized or regular TCNQ − stacks.
Quantum phase diagram of the generalized ionic Hubbard model for ABn chains
Physical Review B, 2006
We investigate the ground-state phase diagram of the Hubbard model for the ABN−1 chain with filling 1/N , where N is the number of atoms per unit cell. In the strong-coupling limit, a charge transition takes place from a band insulator (BI) to a correlated insulator (CI) for increasing on-site repulsion U and positive on-site energy difference ∆ (energy at A sites lower than at B sites). In the weak-coupling limit, a bosonization analysis suggests that for N > 2 the physics is qualitatively similar to the case N = 2 which has already been studied: an intermediate phase emerges, which corresponds to a bond-ordered ferroelectric insulator (FI) with spontaneously broken inversion symmetry. We have determined the quantum phase diagram for the cases N = 3 and N = 4 from the crossings of energy levels of appropriate excited states, which correspond to jumps in the charge and spin Berry phases, and from the change of sign of the localization parameter z c L . From these techniques we find that, quantitatively, the BI and FI phases are broader for N > 2 than when N = 2, in agreement with the bosonization analysis. Calculations of the Drude weight and z c L indicate that the system is insulating for all parameters, with the possible exception of the boundary between the BI and FI phases.
Bond-order-wave phase and quantum phase transitions in the one-dimensional extended Hubbard model
Physical Review B, 2002
We use a stochastic series expansion quantum Monte Carlo method to study the phase diagram of the one-dimensional extended Hubbard model at half filling for small to intermediate values of the on-site (U) and nearest-neighbor (V) repulsions. We confirm the existence of a novel, long-rangeordered bond-order-wave (BOW) phase recently predicted by Nakamura (J. Phys. Soc. Jpn. 68, 3123 (1999)) in a small region of the parameter space between the familiar charge-density-wave (CDW) state for V U/2 and the state with dominant spin-density-wave (SDW) fluctuations for V U/2. We discuss the nature of the transitions among these states and evaluate some of the critical exponents. Further, we determine accurately the position of the multi-critical point, (Um, Vm) = (4.7±0.1, 2.51±0.04) (in energy units where the hopping integral is normalized to unity), above which the two continuous SDW-BOW-CDW transitions are replaced by one discontinuous (first-order) direct SDW-CDW transition. We also discuss the evolution of the CDW and BOW states upon hole doping. We find that in both cases the ground state is a Luther-Emery liquid, i.e., the spin gap remains but the charge gap existing at half-filling is immediately closed upon doping. The charge and bond-order correlations decay with distance r as r −Kρ , where Kρ is approximately 0.5 for the parameters we have considered. We also discuss advantages of using parallel tempering (or exchange Monte Carlo)-an extended ensemble method that we here combine with quantum Monte Carlo-in studies of quantum phase transitions.
Croatica Chemica Acta, 2013
Similar quantum phase diagrams and transitions are found for three classes of one-dimensional models with equally spaced sites, singlet ground states (GS), inversion symmetry at sites and a bond order wave (BOW) phase in some sectors. The models are frustrated spin−1/2 chains with variable range exchange, half-filled Hubbard models with spin-independent interactions and modified Hubbard models with site energies for describing organic charge transfer salts. In some range of parameters, the models have a first order quantum transition at which the GS expectation value of the sublattice spin S A 2 of odd or even-numbered sites is discontinuous. There is an intermediate BOW phase for other model parameters that lead to two continuous quantum transitions with continuous S A 2 . Exact diagonalization of finite systems and symmetry arguments provide a unified picture of familiar 1D models that have appeared separately in widely different contexts.
From the Square Lattice to the Checkerboard Lattice : A Spin Wave Analysis
2001
Within a linear spin wave analysis, it is shown that the antiferromagnetic Heisenberg model on the checkerboard lattice is a good candidate for a quantum spin liquid ground state. Through an additional exchange interaction that corresponds to an intra-tetrahedron coupling, all cases from the square lattice to the checkerboard lattice have been explored and it is shown that there exists a critical coupling for which the local magnetization vanishes. This behavior is similar to that already observed in the kagomé lattice and suggests that the quantum spin liquid behavior of the checkerboard limit is robust against small variations away from this limit.
The SPIN-1/2 and SPIN-1 Quantum J1−J′1−J2 Heisenberg Models on the Square Lattice
Recent Progress in Many-Body Theories - Proceedings of the 14th International Conference, 2008
We study the J:-J~-h quantum 'spin model on the two-dimensional square lattice using the coupled cluster method. We compare and contrast the influence of the interchain coupling Jf on the zero-temperature phase diagrams for the two spin values s = 1/2 and s = 1. Our most important result for the s = 1/2 case is the predicted existence of a quantum triple point (QTP) at (Jf ;:::j 0.60 ± 0.03, J: ;:::j 0.33 ± 0.02) when h = 1. Below the QTP (JU J1 ;S 0.60) we predict a second-order phase transition between the quasi-classical Neel and stripe-ordered phases, whereas the corresponding classical model, which contains only these two phases for all spin values s, yields a first-order transition. Above the QTP (JU J1 2: 0.60) an intermediate disordered phase emerges, which has no classical counterpart. By contrast, the situation for s = 1 is qualitatively different. Instead of a QTP where three phases co-exist, we now predict a quantum tricritical point at (Jf ;:::j 0.66 ± 0.03, Jz~0.35 ± 0.02) when Jv = 1, where a line of second-order phase transitions between the quasi-classical Neel and stripe-ordered phases (for JU J: ;S 0.66) meets a line of first-order phase transitions between the same two states (for JU h 2: 0.66). Surprisingly, we find no evidence at all for any intermediate disordered phase in the s = 1 case. .