Competition and coexistence of bond and charge orders in(TMTTF)2AsF6

Abstract

(TMTTF)2AsF6 undergoes two phase transitions upon cooling from 300 K. At TCO=103 K a charge-ordering (CO) occurs, and at TSP (B=9 T)=11 K the material undergoes a spin-Peierls (SP) transition. Within the intermediate, CO phase, the charge disproportionation ratio is found to be at least 3:1 from 13 C NMR T −1 1 measurements on spin-labeled samples. Above TSP up to about 3TSP T −1 1 is independent of temperature, indicative of low-dimensional magnetic correlations. With the application of about 0.15 GPa pressure, TSP increases substantially, while TCO is rapidly suppressed, demonstrating that the two orders are competing. The experiments are compared to results obtained from calculations on the 1D extended Peierls-Hubbard model. PACS numbers: 71.20.Rv, 71.30.+h, 71.45.Lr, Inhomogenous charge and spin structures are a consequence of competing interactions and therefore of general interest in correlated electron systems. Examples include the high-T c cuprates 1 and manganites 2 as well as the quasi-2D organic conductors 3 . The quasi-1D salts made from TMTTF or TMTSF molecules are also susceptible to charge-ordered states. Independent of that, they are well-known for the sequence of ground states accessible by applying pressure or selecting different counterions. For example, the material (TMTTF) 2 PF 6 undergoes transitions from spin-Peierls, antiferromagnetic (AF), spin-density wave (SDW), and finally to superconducting (SC) ground states as the pressure is increased to 4-5 GPa 4,5 . For a long time, it was known that another phase transition occurs in a number of TMTTF salts with both centrosymmetric (e.g., AsF 6 , SbF 6 ) and non-centrosymmetric (e.g., ReO 4 ) counterions. Only recently 6,7 was the broken symmetry associated with this transition identified as a charge disproportionation.In TMTTF salts the characteristic temperature of the onset of the charge-ordered (CO) phase is high, on the order of 100 K. It indicates that the interactions driving the CO are relatively strong, and therefore potentially impact the electronic and magnetic properties of the disordered phase. Issues associated with CO correlations in these systems take particular relevance when considering that the nature of the metallic phase of TMTSF salts remains controversial 8,9,10 . Below we report the results of a number of NMR measurements on 13 C spin-labeled samples of (TMTTF) 2 AsF 6 in the CO phase. Our principle result is a mapping of the temperature/pressure phase diagram of the SP and CO phases that includes a tetracritical point with a region of coexistence of the two forms of order. There is good agreement between the experiments and the results of calculations on the 1D extended Hubbard 11 and Peierls-Hubbard models 12,13 .A review of the characteristics of the CO phase and the phase transition is in order. With counterions PF 6 , AsF 6 , and SbF 6 , the ordering temperature is 62 K, 103 K, and 154 K, respectively. Upon cooling, the salts made with the first two are already well into a region of thermally activated resistivitie...

13C NMR study of commensurate antiferromagnetism in (TMTTF)2Br

“…The salts of (TMTTF) 2 AsF 6 and (TMTTF) 2 SbF 6 show CO states below 100 and 157 K, respectively, as well as being in a nonmagnetic insulating state and an antiferromagnetic state, respectively, at low temperature. [9][10][11] The charge imbalance in the disproportionation state of (TMTTF) 2 AsF 6 , ρ = ρ rich − ρ poor , was estimated to be 0.16e by infrared spectroscopy. 12 We suggested a commensurate antiferromagnetic fluctuation in the CO paramagnetic state of (TMTTF) 2 AsF 6 from NMR study.…”

Temperature-driven transition from the Wigner crystal to the bond-charge-density wave in the quasi-one-dimensional quarter-filled band

“…Spatial broken symmetries in the quasi-one-dimensional ͑quasi-1D͒ 1 4 -filled organic charge-transfer solids ͑CTSs͒ have been of strong experimental [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] and theoretical [18][19][20][21][22][23][24][25][26] interests. The broken symmetry states include charge order ͑hereafter CO, this is usually accompanied by intramolecular distortions͒, intermolecular lattice distortions ͑hereafter bond order wave or BOW͒, antiferromagnetism ͑AFM͒, and spinPeierls ͑SP͒ order.…”

Interaction of magnetic field-dependent Peierls and spin-Peierls ground states in(Per)2[Pt(mnt)Green1, Brooks2, Kuhns3 et al. 2011Phys. Rev. BSelf Cite103100View full textAdd to dashboardCite

“…This is in fact a general characteristic of related organic conductors in this class, where 4k F instabilities can lead to CO and SP ground states [3], driven by Coulomb interactions. Moreover, based on the linear scaling of 1/T 1 vs. T χ, Bourbonnais et al [10] show the spin degrees of freedom remain diffusive and weak until T SP −CDW , and hence it is hard to see how the SP dimerization can be a result of, for instance quantum spin dynamics alone.…”

“…2d,e shows that after the SP state starts to break above 18 T, there is a further broadening and evolution of the spectra (feature 3) that indicates a Zeeman-like increase of local moment strength. This is characteristic of the generation of triplet excitations in the incommensurate (I) phase [3]. Although a non-SP model has been proposed [20], it is interesting to speculate about how a changing periodic singlet-triplet soliton configuration might affect the anomalous step-like behavior [13] Comparison of magnetic field dependent phase boundaries of (P er) 2 [P t(mnt) 2 ] determined from electrical transport and torque measurements [13,17] , 1 H NMR measurements (this work), and mean-field predictions [11] for T CDW and T SP .…”

“…i) In DA compounds [1], charge transfer can leave the donor with an unpaired spin (leading to a SP transition), while the acceptor is diamagnetic. ii) In D 2 A compounds generally the acceptor chain is a half-filled insulator, and the donor chain is a quarter-filled metal that can undergo sequential CO and SP transitions [2,3]. iii) In the inorganic insulator CuGeO 3 the Cu ++ spin 1/2 chains undergo a SP transition [4].…”