Competing Interactions, the Renormalization Group and the Isotropic-Nematic Phase Transition

We discuss 2D systems with Ising symmetry and competing interactions at different scales. In the framework of the Renormalization Group, we study the effect of relevant quartic interactions. In addition to the usual constant interaction term, we analyze the effect of quadrupole interactions in the self consistent Hartree approximation. We show that in the case of repulsive quadrupole interaction, there is a first order phase transition to a stripe phase in agreement with the well known Brazovskii result. However, in the case of attractive quadrupole interactions there is an isotropic-nematic second order transition with higher critical temperature.Comment: 4 pages, no figures, version to be published in Physical Review Letters. Some scaling dimensions corrected, conclusions are the sam

Soliton Wall Superlattice in Quasi-One-Dimensional Conductor (Per)2Pt(mnt)2

We suggest a model to explain the appearance of a high resistance high magnetic field charge-density-wave (CDW) phase, discovered by D. Graf et al. [Phys. Rev. Lett. v. 93, 076406 (2004)] in (Per)2Pt(mnt)2. In particular, we show that the Pauli spin-splitting effects improve the nesting properties of a realistic quasi-one-dimensional electron spectrum and, therefore, a high resistance Peierls CDW phase is stabilized in high magnetic fields. In low and very high magnetic fields, a periodic soliton wall superlattice (SWS) phase is found to be a ground state. We suggest experimental studies of the predicted phase transitions between the Peierls and SWS CDW phases in (Per)2Pt(mnt)2 to discover a unique SWS phase.Comment: 10 pages, 3 figures. Submitted to Physical Review Letters (February 19, 2007

Soliton phase near antiferromagnetic quantum critical point in Q1D conductors

In the frameworks of a nesting model for Q1D organic conductor at the antiferromagnetic (SDW) quantum critical point the first-order transition separates metallic state from the soliton phase having the periodic domain structure. The low temperature phase diagram also displays the 2nd-order transition line between the soliton and the uniformly gapped SDW phases. The results agree with the phase diagram of (TMTSF)$_2$PF$_6$ near critical pressure [T. Vuletic et al., Eur. Phys. J. B 25, 319 (2002)]. Detection of the 2nd-order transition line is discussed. We comment on superconductivity at lowest temperature.Comment: 4 pages, 1 figur

A Field Theory for Fermionic Ladder with Generic Intrachain Interactions

An effective low energy field theory is developed for a system of two chains. The main novelty of the approach is that it allows to treat generic intrachain repulsive interactions of arbitrary strength. The chains are coupled by a direct tunneling and four-fermion interactions. At low energies the individual chains are described as Luttinger liquids with an arbitrary ratio of spin $v_s$ and charge $v_c$ velocities. A judicious choice of the basis for the decoupled chains greatly simplifies the description and allows one to separate high and low energy degrees of freedom. In a direct analogy to the bulk cuprates the resulting effective field theory distinguishes between three qualitatively different regimes: (i) small doping ($v_c << v_s$), (ii) optimal doping ($v_s \approx v_c$) and (iii) large doping ($v_s << v_c$). I discuss the excitation spectrum and derive expressions for the electron spectral function which turns out to be highly incoherent. The degree of incoherence increases when one considers an array of ladders (stripe phase).Comment: 32 pages, 4 figures. A section explaining adiabatic approximation is modified. Typos correcte

Nature of superconducting state in the new phase in (TMTSF)2_{2}PF6_{6} under pressure

The unusual phase has been recently observed in the organic material (TMTSF)$_{2}$PF$_{6}$, where superconductivity (SC) coexists with spin-density wave (SDW) in the pressure interval $p_{c1}<p<p_{c}$ below the first order transition into SC or normal metal phase. Assuming that the coexistence takes place on the microscopic scale, we consider the properties of the intermediate phase. We show that the new superconducting state inside SDW phase just above $p_{c1}$ must bear a triplet pairing.Comment: 4 pages, 1 figur

Statistics of charged solitons and formation of stripes

The 2-fold degeneracy of the ground state of a quasi-one-dimensional system allows it to support topological excitations such as solitons. We study the combined effects of Coulomb interactions and confinement due to interchain coupling on the statistics of such defects. We concentrate on a 2D case which may correspond to monolayers of polyacetylene or other charge density waves. The theory is developped by a mapping to the 2D Ising model with long-range 4-spin interactions. The phase diagram exhibits deconfined phases for liquids and Wigner crystals of kinks and confined ones for bikinks. Also we find aggregated phases with either infinite domain walls of kinks or finite rods of bikinks. Roughening effects due to both temperature and Coulomb repulsion are observed. Applications may concern the melting of stripes in doped correlated materials.Comment: 16 pages, 7 figure

Nonlinear conduction of sliding electronic crystals: Charge and Spin Density Waves

A model of local metastable states due to the pinning induces plastic deformations allows to describe the nonlinear I-V curves in sliding density waves -DW. With increasing the DW velocity v, the metastable states of decreasing lifetimes ~1/v are accessed. The characteristic second threshold field is reached when configurations of shortest life time are accessed by the fast moving DW. Thus the DW works as a kind of a ``linear accelerator'' testing virtual states.Comment: To be published in Proceedings of ECRYS-99, J. de Physique, Coll., December 1999, http://ipnweb.in2p3.fr/~lptms/membres/brazov

Spin imbalance effect on Larkin-Ovchinnikov-Fulde-Ferrel state

We study spin imbalance effects on the Larkin-Ovchinikov-Fulde-Ferrel (LOFF) state relevant for superconductors under a strong magnetic field and spin polarized ultracold Fermi gas. We obtain the exact solution for the condensates with arbitrary spin imbalance and the fermion spectrum perturbatively in the presence of small spin imbalance. We also obtain fermion zero mode exactly without perturbation.Comment: 5 pages, 2 figures; v2 slightly improved, typos fixed, published versio

Density waves and supersolidity in rapidly rotating atomic Fermi gases

We study theoretically the low-temperature phases of a two-component atomic Fermi gas with attractive s-wave interactions under conditions of rapid rotation. We find that, in the extreme quantum limit, when all particles occupy the lowest Landau level, the normal state is unstable to the formation of "charge" density wave (CDW) order. At lower rotation rates, when many Landau levels are occupied, we show that the low-temperature phases can be supersolids, involving both CDW and superconducting order.Comment: 4 pages, 1 figure, uses feynmp.st

Electron-phonon coupling in semimetals in a high magnetic field

We consider the effect of electron-phonon coupling in semimetals in high magnetic fields, with regard to elastic modes that can lead to a redistribution of carriers between pockets. We show that in a clean three dimensional system, at each Landau level crossing, this leads to a discontinuity in the magnetostriction, and a divergent contribution to the elastic modulus. We estimate the magnitude of this effect in the group V semimetal Bismuth.Comment: 2 figure