Spin Susceptibilities and Phase Transitions in Some Simple Alkali-TCNQ Salts

Vegter, Johan G.; Kommandeur, Jan

doi:10.1080/15421407508082839

Cited by 65 publications

(27 citation statements)

References 20 publications

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“…We also consider χ(T ) of dimerized stacks for T < T d without, however, treating the transition. There are two Rb salts: Rb-TCNQ(I) is strongly dimerized [15] at 300 K while Rb-TCNQ(II) has regular stacks [16,17] with A − at inversion centers at both 100 and 295 K. The recent 100 K structure [17] rules out a dimerization transition around 220 K that was inferred from magnetic susceptibility [18] and infrared [19] data. We reinterpret these observations.…”

Section: Introductionmentioning

confidence: 95%

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Magnetic susceptibility of alkali-tetracyanoquinodimethane salts and extended Hubbard models with bond order and charge density wave phases

Kumar

Topham

et al. 2011

The Journal of Chemical Physics

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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.

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Section: Introductionmentioning

confidence: 95%

“…Crystal data [13,14] at T > T d indicate eclipsed (ring over ring, Fig. 1) stacks with TCNQ − at inversion centers and interplanar separation R(Na) = 3.385Å at 353 K, R(K) = 3.479Å at 413 K. The solid line for Rb(II) is esr intensity [18]. The dotted line is static susceptibility [17] corrected for diamagnetism.…”

Section: Introductionmentioning

confidence: 99%

Magnetic susceptibility of alkali-tetracyanoquinodimethane salts and extended Hubbard models with bond order and charge density wave phases

Kumar

Topham

et al. 2011

The Journal of Chemical Physics

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“…Finally, we made an effort to compare our computational results with experiment, in particular as regards the spin susceptibilities. For some simple TCNQ salts these are available, they show an increase as a function of temperature (9). This means we are in a region of fairly low temperatures.…”

Section: Half-filled Bandsmentioning

confidence: 93%

CALCULATIONS OF THE ELECTRONIC (4k_F) AND MAGNETIC (2k_F) PHASE TRANSITIONS FROM A HUBBARD HAMILTONIAN WITH AN EXPONENTIALLY DEPENDENT INTERSITE-TRANSFER INTEGRAL

1983

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Résumé -La dépendance exponentielle de l'intégrale de transfert et la corrélation électron-électron U / 3t dans l'Hamiltonien de Hubbard conduisent à une séparation des composantes 2kF et 4kF de la transition de phase Peierls. Un diagramme de phase des températures de transition vers la valeur de U/t peut être construit. Les résultats calculés pour quelques propriétés physiques peuvent être comparés favorablement avec les résultats des expériences.Abstract -An exponential intersite-dependence of the transfer integral in the Hubbard Hamiltonian together with U / 3t in a quarter filled band lead to a separation of the 2kF and 4kF components of the Peierls transition. A phase diagram of the transition temperatures versus U/t can be constructed. The results for some physical properties such as the spin susceptibility can be favorably compared with experiment. IntroductionRecently we published (1) a technique to calculate electronic Peierls and spin Peierls phase transitions from a simple Hubbard model, in which the transfer integral was taken to depend exponentially on the intersite distance. Basically the method solves for the eigenvalues of the Hubbard Hamiltonian and with a partition function we then calculate the electronic free energy. A lattice energy term is added and minima of the free energy as a function of temperature are found by allowing the system to distort with 4kF and 2kF distortions for a quarter filled band and 2kF and kF distortions for a half filled band. Cooperativity is ensured by requiring that all members of the ensemble, which the free energy calculation implicitly requires, show the same distortions at the same time. For details of the calculation and its group theoretical justification we refer to our earlier work (1).

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“…TCNQ forms segregated stacks with many closed-shell cations and CT salts with strong π-donors. Vegter and Kommandeur [8] reported the magnetism and dimerization transition at T d of 1:1 salts M-TCNQ, where M = Na, K, Rb or Cs. These salts have half-filled regular A -stacks for T > T d .…”

Section: Introductionmentioning

confidence: 99%

“…These salts have half-filled regular A -stacks for T > T d . They are prime candidates for modeling, especially the polymorph Rb-TCNQ(II) that has regular stacks [9] at 100 K instead of [8] T d ~ 220 K. Models have to be understood before they can be applied, however, and correlated 1D models are very challenging. Several theoretical advances make it timely to study 1:1 alkali-TCNQ salts again.…”

Section: Introductionmentioning

confidence: 99%

1:1 Alkali-TCNQ salts and the bond order wave (BOW) phase of half-filled linear Hubbard-type models

Soos¹,

Kumar²,

Ramasesha³

et al. 2010

Physica B: Condensed Matter

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The bond order wave (BOW) phase of half-filled linear Hubbard-type models is narrow and difficult to characterize aside from a few ground state properties. The BOW phase of a frustrated Heisenberg spin chain is wide and tractable. It has broken inversion symmetry C i in a regular array and finite gap E m to the lowest triplet state. The spin-BOW is exact in finite systems at a special point. Its elementary excitations are spin-1/2 solitons that connect BOWs with opposite phase. The same patterns of spin densities and bond orders appear in the BOW phase of Hubbard-type models. Infrared (IR) active lattice phonons or molecular vibrations are derivatives of P, the polarization along the stack. Molecular vibrations that are forbidden in regular arrays become IR active when C i symmetry is broken. 1:1 alkali-TCNQ salts contain half-filled regular TCNQ -stacks at high temperature, down to 100 K in the Rb-TCNQ(II) polymorph whose magnetic susceptibility and polarized IR spectra indicate a BOW phase. More complete modeling will require explicit electronic coupling to phonons and molecular vibrations. IntroductionFace-to-face stacks of organic π-radicals suggest a one-dimensional (1D) electronic structure [1,2]. It was realized [1] early on that 1D Hubbard models rationalize the striking magnetic, optical and electronic properties of π-radical solids in terms of the LUMO of π-acceptors A or the HOMO of π-donors D. Segregated stacks of A or D can have different degree of filling by electrons or holes and different unit cells along the stack. Half-filled systems with one electron per site are Mott insulators. Good conductors deviate from half filling in a regular array. Triplet spin excitons signal dimerized stacks. Mixed DA stacks have alternating site energy ±ε and undergo a neutral-ionic transition (NIT) in charge-transfer (CT) salts of special interest. The bandwidth of π-stacks is 4t ~ 1 eV, an order of magnitude smaller than in conjugated polymers with π-bonds along the backbone. 1D quantum cell or Hubbard-type models for π-radical organic solids or conjugated polymers have been extensively developed and applied for decades, sometimes almost quantitatively [3][4][5][6][7]. Both electron-phonon coupling and electron correlation are important.

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Spin Susceptibilities and Phase Transitions in Some Simple Alkali-TCNQ Salts

Cited by 65 publications

References 20 publications

Magnetic susceptibility of alkali-tetracyanoquinodimethane salts and extended Hubbard models with bond order and charge density wave phases

Magnetic susceptibility of alkali-tetracyanoquinodimethane salts and extended Hubbard models with bond order and charge density wave phases

CALCULATIONS OF THE ELECTRONIC (4k_F) AND MAGNETIC (2k_F) PHASE TRANSITIONS FROM A HUBBARD HAMILTONIAN WITH AN EXPONENTIALLY DEPENDENT INTERSITE-TRANSFER INTEGRAL

1:1 Alkali-TCNQ salts and the bond order wave (BOW) phase of half-filled linear Hubbard-type models

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Spin Susceptibilities and Phase Transitions in Some Simple Alkali-TCNQ Salts

Cited by 65 publications

References 20 publications

Magnetic susceptibility of alkali-tetracyanoquinodimethane salts and extended Hubbard models with bond order and charge density wave phases

Magnetic susceptibility of alkali-tetracyanoquinodimethane salts and extended Hubbard models with bond order and charge density wave phases

CALCULATIONS OF THE ELECTRONIC (4kF) AND MAGNETIC (2kF) PHASE TRANSITIONS FROM A HUBBARD HAMILTONIAN WITH AN EXPONENTIALLY DEPENDENT INTERSITE-TRANSFER INTEGRAL

1:1 Alkali-TCNQ salts and the bond order wave (BOW) phase of half-filled linear Hubbard-type models

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CALCULATIONS OF THE ELECTRONIC (4k_F) AND MAGNETIC (2k_F) PHASE TRANSITIONS FROM A HUBBARD HAMILTONIAN WITH AN EXPONENTIALLY DEPENDENT INTERSITE-TRANSFER INTEGRAL