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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...

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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.

Recent advances in III(1-x)Mn(x)V ferromagnetic semiconductors (for example in Ga(1-x)Mn(x)As) have demonstrated that electrical control of their spin properties can be used for manipulation and detection of magnetic signals. The Mn(2+) ions in these alloys provide magnetic moments, and at the same time act as a source of valence-band holes that mediate the Mn(2+)-Mn(2+) interactions. This coupling results in the ferromagnetic phase. In earlier workit was shown that the ferromagnetic state can be enhanced or suppressed by varying the carrier density. Here we demonstrate that, by using hydrostatic pressure to continuously tune the wavefunction overlap, one can control the strength of ferromagnetic coupling without any change in the carrier concentration. Tuning the exchange coupling by this process increases the magnetization spectacularly, and can even induce the ferromagnetic phase in an initially paramagnetic alloy. These results may open new directions for strain-engineering of nanodevices.

The phase diagram of BaVS3 is studied under pressure using resistivity measurements. The temperature of the metal to nonmagnetic Mott insulator transition decreases under pressure, and vanishes at the quantum critical point pcr = 20kbar. We find two kinds of anomalous conducting states. The high-pressure metallic phase is a non-Fermi liquid described by ∆ρ ∝ T n where n =1.2-1.3 at 1K< T <60K. At p < pcr, the transition is preceded by a wide precursor region with critically increasing resistivity which we ascribe to the opening of a soft Coulomb gap.Understanding the Mott transition, and clarifying the nature of the phases on either side of the transition, is a matter of great importance. Though metal-insulator transitions are often accompanied by an ordering transition and/or influenced by disorder, one may speak about a "pure" Mott transition which is a local correlation effect in an ideal lattice fermion system, and takes place without breaking any global symmetry. Many aspects of this problem can be studied on the multifaceted behavior of BaVS 3 [1-3].The metal-insulator transition of the nearly isotropic 3D compound BaVS 3 offers a realization of the pure Mott transition in nature [3]. Under atmospheric pressure BaVS 3 has three transitions: the hexagonal-toorthorhombic transition at T S = 240K which has only a slight effect on the electrical properties; the metalinsulator transition at T MI = 69K, which does not seem to break any of the symmetries of the metallic phase; and the ordering transition at T X = 30K [4]. In spite of decades of effort, the character of the phases and the driving force of the transitions at T MI and T X , remain mysterious.Here we report the results of single crystal resistivity measurements under hydrostatic pressure in the range of 1bar ≤ p < 25kbar. These pressures encompass the entire insulating phase and part of a high-pressure low-T conducting phase. We report the first observation of the quantum critical point in BaVS 3 , and we characterize the strange metallic phase lying beyond the critical pressure p cr . On the metallic side of the phase boundary, we identify two regimes with anomalous properties: (i) a broad region at p < p cr in which the resistivity increases strongly with decreasing temperature, and (ii) a high-pressure non-Fermi-liquid state.Single crystals of BaVS 3 were grown by Tellurium flux method. The crystals, obtained from the flux by sublimation, have typical dimensions of 3 × 0.5 × 0.5 mm 3 . The resistivity was measured in four probe arrangement. The current was kept low enough to avoid the self-heating of the sample. For the high-pressure measurements the crystal was inserted into a self-clamping cell with kerosene as a pressure medium. The pressure was monitored in-situ by an InSb sensor. During cooling down the cell there was a slight pressure loss, but its influence on the temperature dependence of the resistivity was negligible. Above about 15 kbar the pressure was stable within 0.1 kbar in the whole temperature range.

We demonstrate that the onset of complex spin orders in ACr2O4 spinels with magnetic and Jahn-Teller active A=Fe and Cu ions lowers the lattice symmetry. This is clearly indicated by the emergence of anisotropic lattice dynamics-i.e., by the pronounced phonon splittings-even when experiments probing static distortions fail. The crystal symmetry in the magnetic phase is reduced from tetragonal to orthorhombic for both compounds. The conical spin ordering in FeCr2O4 is also manifested in the hardening of the phonon frequencies. In contrast, the multiferroic CoCr2O4 with no orbital degrees of freedom shows tiny deviations from cubic structure even in its ground state.