Competing phases in the high field phase diagram of (TMTSF)2_2ClO4_4

A model is presented for the high field phase diagram of (TMTSF)$_2$ClO$_4$, taking into account the anion ordering, which splits the Fermi surface in two bands. For strong enough field, the largest metal-SDW critical temperature corresponds to the N=0 phase, which originates from two intraband nesting processes. At lower temperature, the competition between these processes puts at disadvantage the N=0 phase vs. the N=1 phase, which is due to interband nesting. A first order transition takes then place from the N=0 to N=1 phase. We ascribe to this effect the experimentally observed phase diagrams.Comment: 5 pages, 3 figures (to appear in Phys. Rev. Lett.

Renormalization Group calculations with k|| dependent couplings in a ladder

We calculate the phase diagram of a ladder system, with a Hubbard interaction and an interchain coupling $t_\perp$. We use a Renormalization Group method, in a one loop expansion, introducing an original method to include $k_{||}$ dependence of couplings. We also classify the order parameters corresponding to ladder instabilities. We obtain different results, depending on whether we include $k_{||}$ dependence or not. When we do so, we observe a region with large antiferromagnetic fluctuations, in the vicinity of small $t_\perp$, followed by a superconducting region with a simultaneous divergence of the Spin Density Waves channel. We also investigate the effect of a non local backward interchain scattering : we observe, on one hand, the suppression of singlet superconductivity and of Spin Density Waves, and, on the other hand, the increase of Charge Density Waves and, for some values of $t_\perp$, of triplet superconductivity. Our results eventually show that $k_{||}$ is an influential variable in the Renormalization Group flow, for this kind of systems.Comment: 20 pages, 19 figures, accepted in Phys. Rev. B 71 v. 2

Field-induced confinement in (TMTSF)2ClO4 under accurately aligned magnetic fields

We present transport measurements along the least conducting c direction of the organic superconductor (TMTSF)2ClO4, performed under an accurately aligned magnetic field in the low temperature regime. The experimental results reveal a two-dimensional confinement of the carriers in the (a,b) planes which is governed by the magnetic field component along the b' direction. This 2-D confinement is accompanied by a metal-insulator transition for the c axis resistivity. These data are supported by a quantum mechanical calculation of the transverse transport taking into account in self consistent treatment the effect of the field on the interplane Green function and on the intraplane scattering time

Towards a consistent picture for quasi-1D organic superconductors

The electrical resistivity of the quasi-1D organic superconductor (TMTSF)2PF6 was recently measured at low temperature from the critical pressure needed to suppress the spin-density-wave state up to a pressure where superconductivity has almost disappeared. This data revealed a direct correlation between the onset of superconductivity at Tc and the strength of a non-Fermi-liquid linear term in the normal-state resistivity, going as r(T) = r0 + AT + BT2 at low temperature, so that A goes to 0 as Tc goes to 0. Here we show that the contribution of low-frequency antiferromagnetic fluctuations to the spin-lattice relaxation rate is also correlated with this non-Fermi-liquid term AT in the resistivity. These correlations suggest that anomalous scattering and pairing have a common origin, both rooted in the low-frequency antiferromagnetic fluctuations measured by NMR. A similar situation may also prevail in the recently-discovered iron-pnictide superconductors.Comment: ISCOM'09 proceedings to be published in Physica

Renormalization of the hopping parameters in quasi-one-dimensional conductors in the presence of a magnetic field

Abstract. We consider the competition between the one dimensionalization effect due to a magnetic field and the hopping parameters in quasi-one-dimensional conductors. Our study is based on a perturbative renormalization group method with three cut-off parameters, the bandwidth E0, the 1D-2D crossover temperature T * 1 , which is related to the hopping process t1, and the magnetic energy ωc. We have found that the renormalized crossover temperatures T * 1 and T * 2 , at which the respectively hopping processes t1 and t2 become coherent, are reduced compared to the bare values as the field is increased. We discuss the consequences of these renormalization effects on the temperature-field phase diagram of the organic conductors. PAC

Field-induced spin density wave in (TMTSF)2_2NO3_3

Interlayer magnetoresistance of the Bechgaard salt (TMTSF)$_2$NO$_3$ is investigated up to 50 teslas under pressures of a few kilobars. This compound, the Fermi surface of which is quasi two-dimensional at low temperature, is a semi metal under pressure. Nevertheless, a field-induced spin density wave is evidenced at 8.5 kbar above $\sim$ 20 T. This state is characterized by a drastically different spectrum of the quantum oscillations compared to the low pressure spin density wave state.Comment: to be published in Phys. Rev. B 71 (2005

Phase Diagram for Charge Density Waves in a Magnetic Field

The influence of an external magnetic field on a quasi one-dimensional system with a charge density wave (CDW) instability is treated within the random phase approximation which includes both CDW and spin density wave correlations. We show that the CDW is sensitive to both orbital and Pauli effects of the field. In the case of perfect nesting, the critical temperature decreases monotonously with the field, and the wave vector of the instability starts to shift above some critical value of magnetic field. Depending on the ratio between the spin and charge coupling constants and on the direction of the applied magnetic field, the wave vector shift is either parallel ($CDW_x$ order) or perpendicular ($CDW_y$ order) to the most conducting direction. The $CDW_x$ order is a field dependent linear combination of the charge and spin density waves and is sensible only to the Pauli effect. The wave vector shift in $CDW_y$ depends on the interchain coupling, but the critical temperature does not. This order is affected by the confinement of the electronic orbits. By increasing the relative strength of the orbital effect with respect to the Pauli effect, one can destroy the $CDW_y$, establishing either a $CDW_x$, or a $CDW_0$ (corresponding to perfect nesting wave vector). We also show that by increasing the imperfect nesting parameter, one passes from the regime where the critical temperature decreases with the field to the regime where it is initially enhanced by the orbital effect and eventually suppressed by the Pauli effect. For a bad nesting, the quantized phases of the field-induced CDW appear.Comment: 30 pages (LaTeX) + 15 figure

Sign reversals of the Quantum Hall Effect in quasi-1D conductors

The sign reversals of the Quantum Hall Effect observed in quasi-one-dimensional conductors of the Bechgaard salts family are explained within the framework of the quantized nesting model. The sequence of reversals is driven by slight modifications of the geometry of the Fermi surface. It is explained why only even phases can have signign reversals and why negative phases are less stable than positive ones.Comment: 4 LaTex pages, 3 Postscript figure

Magneto-Roton Modes of the Ultra Quantum Crystal: Numerical Study

The Field Induced Spin Density Wave phases observed in quasi-one-dimensional conductors of the Bechgaard salts family under magnetic field exhibit both Spin Density Wave order and a Quantized Hall Effect, which may exhibit sign reversals. The original nature of the condensed phases is evidenced by the collective mode spectrum. Besides the Goldstone modes, a quasi periodic structure of Magneto-Roton modes, predicted to exist for a monotonic sequence of Hall Quantum numbers, is confirmed, and a second mode is shown to exist within the single particle gap. We present numerical estimates of the Magneto-Roton mode energies in a generic case of the monotonic sequence. The mass anisotropy of the collective mode is calculated. We show how differently the MR spectrum evolves with magnetic field at low and high fields. The collective mode spectrum should have specific features, in the sign reversed "Ribault Phase", as compared to modes of the majority sign phases. We investigate numerically the collective mode in the Ribault Phase.Comment: this paper incorporates material contained in a previous cond-mat preprint cond-mat/9709210, but cannot be described as a replaced version, because it contains a significant amount of new material dealing with the instability line and with the topic of Ribault Phases. It contains 13 figures (.ps files

Sign reversals of the quantum Hall effect and helicoidal magnetic-field-induced spin-density waves in quasi-one-dimensional organic conductors

We study the effect of umklapp scattering on the magnetic-field-induced spin-density-wave phases, which are experimentally observed in the quasi-one-dimensional organic conductors of the Bechgaard salts family. Within the framework of the quantized nesting model, we show that umklapp processes may naturally explain sign reversals of the quantum Hall effect (QHE) observed in these conductors. Moreover, umklapp scattering can change the polarization of the spin-density wave (SDW) from linear (sinusoidal SDW) to circular (helicoidal SDW). The QHE vanishes in the helicoidal phases, but a magnetoelectric effect appears. These two characteristic properties may be utilized to detect the magnetic-field-induced helicoidal SDW phases experimentally.Comment: 4 pages, latex, 3 figure