A number of recent experiments report spin polarization in quantum wires in the absence of magnetic fields. These observations are in apparent contradiction with the Lieb-Mattis theorem, which forbids spontaneous spin polarization in one dimension. We show that sufficiently strong interactions between electrons induce deviations from the strictly one-dimensional geometry and indeed give rise to a ferromagnetic ground state in a certain range of electron densities.

We consider interacting electrons in a quantum wire in the case of a shallow confining potential and low electron density. In a certain range of densities, the electrons form a two-row ͑zigzag͒ Wigner crystal whose spin properties are determined by nearest and next-nearest neighbor exchange as well as by three-and fourparticle ring exchange processes. The phase diagram of the resulting zigzag spin chain has regions of complete spin polarization and partial spin polarization in addition to a number of unpolarized phases, including antiferromagnetism and dimer order as well as a novel phase generated by the four-particle ring exchange.

We consider a long quantum wire at low electron densities. In this strong interaction regime a Wigner crystal may form, in which electrons comprise an antiferromagnetic Heisenberg spin chain. The coupling constant J is exponentially small, as it originates from tunneling of two neighboring electrons through the segregating potential barrier. We study this exponential dependence, properly accounting for the many-body effects and the finite width of the wire.PACS numbers: 73.21. Hb,75.10.Pq,75.30.Et,71.70.Gm Quantum wires exhibit a plethora of interesting phenomena. In particular, quantization of conductance, which is a fundamental manifestation of the quantum nature of the electron, has been actively studied ever since its first observation in quantum point contacts [1]. The phenomenon presents itself as very flat plateaus of linear conductance G at integer multiples of G 0 = 2e 2 /h, as a function of gate voltage which tunes the electron density in the wire. Since the first observation of the phenomenon, its various facets have been studied by measurements of thermal transport [2,3], noise [4], and experiments on systems with superconducting elements [5].A new generation of experiments in quantum wires has revealed an unexpected structure at low electron densities: a plateau at about 0.7 G 0 for short [6,7,8,9, 10] and at 0.5 G 0 for long quantum wires [11,12,13,14]. These new features have generated much interest as they are likely caused by electron correlation effects. The origin of the new plateau has not yet been established; however, the experiments [6,7,8] point to the important role played by the electron spins.In recent experiments of a different kind, involving tunneling between two parallel ballistic quantum wires, mapping of the spectrum of spin and charge excitations was achieved [15]. In addition, unexpected behavior indicating electron localization was observed at low densities. Of particular interest is the concurrence of the localization with the drops in the conductance steps, which indicates [15] a possible connection with the 0.7 structure [6,7,8,9,10].Recent theoretical work suggests that qualitatively new transport properties of quantum wires at low electron densities may be due to the formation of a Wigner crystal state of electrons. The latter is expected to occur when the density is low enough for the potential energy of the Coulomb repulsion to overwhelm the kinetic energy of the electrons in the wire. In the crystalline configuration, the electron spins form an antiferromagnetic Heisenberg spin chain, with the exchange coupling constant J emerging as a new energy scale. The Heisenberg exchange can be viewed as arising from tunneling of two neighboring electrons through the potential barrier created by their mutual repulsion, and by the repulsion from all other electrons of the wire. As a result, J is expected to be small compared to the Fermi energy E F , and qualitatively new transport properties of the quantum wires, such as the plateau at 0.5 G 0 , are expected [16,17] at tempera...

We study the effect of disorder on the London penetration depth in iron-based superconductors. The theory is based on a two-band model with quasi-two-dimensional Fermi surfaces, which allows for the coexistence region in the phase diagram between magnetic and superconducting states in the presence of intraband and interband scattering. Within the quasiclassical approximation we derive and solve Eilenberger's equations, which include a weak external magnetic field, and provide analytical expressions for the penetration depth in the various limiting cases. A complete numerical analysis of the doping and temperature dependence of the London penetration depth reveals the crucial effect of disorder scattering, which is especially pronounced in the coexistence phase. The experimental implications of our results are discussed.