Using one-and two-dimensional NMR spectroscopy applied to 13 C spin-labeled (TMTTF)2AsF6 and (TMTTF)2PF6, we demonstrate the existence of an intermediate charge-ordered phase in the TMTTF family of charge-transfer salts. At ambient temperature, the spectra are characteristic of nuclei in equivalent environments, or molecules. Below a continuous charge-ordering transition temperature Tco, the spectra are explained by assuming there are two inequivalent molecules with unequal electron densities. The absence of an associated magnetic anomaly indicates only the charge degrees of freedom are involved and the lack of evidence for a structural anomaly suggests that charge/lattice coupling is too weak to drive the transition.

(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...

TMTTF)2SbF6 is known to undergo a charge ordering (CO) phase transition at TCO ≈ 156K and another transition to an antiferromagnetic (AF) state at TN ≈ 8K. Applied pressure P causes a decrease in both TCO and TN . When P > 0.5GP a, the CO is largely supressed, and there is no remaining signature of AF order. Instead, the ground state is a singlet. In addition to establishing an expanded, general phase diagram for the physics of TMTTF salts, we establish the role of electron-lattice coupling in determining how the system evolves with pressure.

Static electrical and magnetic properties of single crystal BaVS3 were measured over the structural (TS = 240K), metal-insulator (TMI = 69K), and suspected orbital ordering (TX = 30K) transitions. The resistivity is almost isotropic both in the metallic and insulating states. An anomaly in the magnetic anisotropy at TX signals a phase transition to an ordered low-T state. The results are interpreted in terms of orbital ordering and spin pairing within the lowest crystal field quasi-doublet. The disordered insulator at TX < T < TMI is described as a classical liquid of non-magnetic pairs.Spatial ordering of the occupancy of degenerate electronic orbitals plays important role in the diverse magnetic phenomena of transition metal compounds [1]. To cite a well-known example: the interplay of magnetic and orbital long range ordering, and strong coupling to the lattice, account for the metal-insulator transitions of the V 2 O 3 system [2,3]. In contrast, the metal-insulator transition of the S = 1/2, 3d 1 electron system BaVS 3 is not associated either with magnetic long range order, or with any detectable static spin pairing. As an alternative, the possibility of an orbitally ordered ground state was discussed [4], while other proposals emphasized the quasione-dimensional character of the material [5][6][7]. The crystal structure is certainly suggestive of a linear chain compound since along the c axis, the intrachain V-V distance is only 2.81Å, while in the a-b plane the interchain separation is 6.73Å [8,9]. It is thus somewhat surprising that our present studies show that electrically BaVS 3 is nearly isotropic. This means that BaVS 3 provides one of the few realizations of a Mott transition within the non-magnetic phase of a three-dimensional system. Since this case (or rather its D → ∞ counterpart) is much studied theoretically, but scarcely investigated experimentally, a good understanding of BaVS 3 should be valuable for strong correlation physics in general.BaVS 3 has a metal-insulator transition at T MI = 69K, accompanied by a sharp spike in the magnetic susceptibility [5,10]. The high temperature phase is a strongly correlated metal with mean free path in the order of the lattice constant. There is no sign of a sharp Fermiedge in the UPS/XPS spectra [6] and instead of a Pauli-susceptibility it exhibits Curie-Weiss like behavior. Though the magnetic susceptibility is similar to that of an antiferromagnet [10,11], no long-range magnetic order develops at the transition [9,12]. The transition is clearly seen in the thermal expansion anomaly [5], and in the specific heat [7]. The d-electron entropy right above T MI is estimated as ∼ 0.8R ln 2, and it seems that a considerable fraction of the electronic degrees of freedom is frozen even at room temperature [7]. It appears that the 69K transition is not symmetry breaking [13]: it is a pure Mott transition which does not involve either magnetic order or any static displacement of the atoms.Hints of long range order were found well below T MI , at T X = 30K, in rece...

Both the Hall effect and the ab(')-plane conduction anisotropy are directly addressing the unconventional normal phase properties of the Bechgaard salt (TMTSF)2PF6. We found that the dramatic reduction of the carrier density deduced from recent optical data is not reflected in an enhanced Hall resistance. The pressure and temperature dependence of the b(')-direction resistivity reveal isotropic relaxation time and do not require explanations beyond the Fermi liquid theory. Our results allow a coherent-diffusive transition in the interchain carrier propagation, however the possible crossover to Luttinger liquid behavior is placed at an energy scale above room temperature.

The pressure induced quantum phase transition of the weakly ferromagnetic metal MnSi is studied using zero-field 29Si NMR spectroscopy and relaxation. Below P(*) approximately 1.2 GPa, the intensity of the signal and the nuclear spin-lattice relaxation are independent of pressure, even though the amplitude of the magnetization drops by 20% from the ambient-pressure amplitude. For P>P(*), the decreasing intensity within the experimentally detectable bandwidth signals the onset of an inhomogeneous phase that persists to the highest pressure measured, P>/=1.75 GPa, which is well beyond the known critical pressure P(c)=1.46 GPa. Implications for the non-Fermi liquid behavior observed for P>P(c) are discussed.

Abstract. (T M T T F ) 2 AsF 6 and (T M T T F ) 2 SbF 6 are both known to undergo a charge ordering phase transition, though their ground states are different. The ground state of the first is Spin-Peierls, and the second is an antiferromagnet. We study the effect of pressure on the ground states and the charge-ordering using 13 C NMR spectroscopy. The experiments demonstrate that the the CO and SP order parameters are repulsive, and consequently the AF state is stabilized when the CO order parameter is large, as it is for (T M T T F ) 2 SbF 6 . An extension of the well-known temperature/pressure phase diagram is proposed.

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