Infrared investigation of the charge ordering pattern in the organic spin ladder candidate (DTTTF)2Cu(mnt)2

Abstract

We measured the variable temperature infrared response of the spin ladder candidate (DTTTF)2Cu(mnt)2 in order to distinguish between two competing ladder models, rectangular versus zigzag, proposed for this family of materials. The distortion along the stack direction below 235 K is consistent with a doubling along b through the metal-insulator transition. While this would agree with either of the ladder models, the concomitant transverse distortion rules out the rectangular ladder model and supports the zigzag scenario. Intramolecular distortions within the DTTTF building block molecule also give rise to on-site charge asymmetry.Quantum spin ladders have attracted considerable interest as intermediaries between one-dimensional chains and two-dimensional square lattices [1,2,3,4]. Additional interest has arisen from theoretical studies which find that hole-doped spin ladders can support superconductivity [2,5]. Whereas most systems are structural ladders, organic ladder-like compounds such as dithiophentelrathiafulvalene copper maleonitrile dithiolate, (DTTTF) 2 Cu(mnt) 2 are formed by the coupling of molecular building blocks ( Fig. 1(a)). This system is particularly attractive because it belongs to a family of quasi-isostructural compounds, with tunable properties depending on the counterion (M = Pt, Cu, Au, Ni) [3,4]. That the cation stacks of (DTTTF) 2 Cu(mnt) 2 have a 1 4 -filled band with delocalized charge and spin at high temperatures, and are therefore described by an electronic extended Hubbard Hamiltonian, as opposed to the localized Heisenberg spin Hamiltonian, brings additional complexity to the field.Two different theoretical models, the rectangular [4,6,7,8] and the zigzag ladder [9], have been proposed for this family of coupled stack materials (Figs. 1(d) and (e)) Although determination of the wavevector dependence of the magnon bands [10] is the strongest test of spin ladder character and can in principle distinguish between the models, inelastic neutron scattering experiments on (DTTTF) 2 Cu(mnt) 2 are limited by sample quantity and isotopic decoration requirements [11]. The cations have homogeneous charge within the rectangular model and are charge ordered within the zigzag model [9]. An alternate approach to distinguishing between the models involves detailed comparison of vibrational property measurements with theoretical predictions.(DTTTF) 2 Cu(mnt) 2 is a donor-acceptor salt that displays the characteristic herringbone chain structure of many organic molecular solids ( Fig. 1(a)). The

Spin-ladder behaviour in molecular materials

“…7c). This ordering scheme is a realization of the Bond-Charge-Density Wave broken symmetry state previously suggested for 1 4 filled zig-zag ladders 40 and in agreement with previous infra-red spectroscopy studies suggesting charge ordering in this compound 41 as well as in the Au analogue. 34 It entirely rules out the possible alternative dimerisation arrangement based on a bond ordering scheme (Fig.…”

Ferromagnetic Spin Ladder System: Stack of Chlorido‐Bridged Dinuclear Copper(II) Complexes with 2‐Methylisothiazol‐3(2H)‐one

“…The former method is suitable for the formation of systems in which strong π-π or S···S interactions are expected to form two chains. [9][10][11] On the other hand, the latter is appli-cable to systems such as the stack of dinuclear planar complexes [12][13][14] in which the selection of ligands can tune the strength and sign of the intramolecular magnetic coupling interactions.…”

Ground State and Finite Temperature Behavior of 1/4-Filled Band Zigzag Ladders

“…35 In contrast, optical response indicates symmetry breaking in both rung and stack directions at T MI for M=Cu. 37 We believe that the 1 4 -filled band zigzag ladder model is nevertheless a valid description for both M = Au and Cu at low temperatures. The only other competing model for these systems is the rectangular ladder model, 35 wherein each DTTTF molecule is coupled to a single other such molecule on the neighboring stack.…”

“…33 In this series of compounds, the M=Au and M=Cu (for both the metal ion is diamagnetic) have been studied in most detail. [33][34][35][36][37] Structurally, these materials consist of pairs of DTTTF stacks, each with 1 4 -filled band of holes, separated by stacks of M(mnt) 2 which are Mott-Hubbard semiconductors with 1 2 -filled electron bands. It is then likely that the each pair of DTTTF stacks behaves as ladders.…”

“…36 Changes in the optical properties at T MI and T SG for the two salts are also different. 35,37 For M=Au, at T MI symmetry breaking occurs along the rung direction (perpen-dicular to the DTTTF stacks), while at T SG , symmetry breaking is predominantly along the stacks. 35 In contrast, optical response indicates symmetry breaking in both rung and stack directions at T MI for M=Cu.…”