Aging in a Two-Dimensional Ising Model with Dipolar Interactions

Aging in a two-dimensional Ising spin model with both ferromagnetic exchange and antiferromagnetic dipolar interactions is established and investigated via Monte Carlo simulations. The behaviour of the autocorrelation function $C(t,t_w)$ is analyzed for different values of the temperature, the waiting time $t_w$ and the quotient $\delta=J_0/J_d$, $J_0$ and $J_d$ being the strength of exchange and dipolar interactions respectively. Different behaviours are encountered for $C(t,t_w)$ at low temperatures as $\delta$ is varied. Our results show that, depending on the value of $\delta$, the dynamics of this non-disordered model is consistent either with a slow domain dynamics characteristic of ferromagnets or with an activated scenario, like that proposed for spin glasses.Comment: 4 pages, RevTex, 5 postscript figures; acknowledgment added and some grammatical corrections in caption

Topological Excitations of One-Dimensional Correlated Electron Systems

Properties of low-energy excitations in one-dimensional superconductors and density-wave systems are examined by the bosonization technique. In addition to the usual spin and charge quantum numbers, a new, independently measurable attribute is introduced to describe elementary, low-energy excitations. It can be defined as a number w which determines, in multiple of $\pi$, how many times the phase of the order parameter winds as an excitation is transposed from far left to far right. The winding number is zero for electrons and holes with conventional quantum numbers, but it acquires a nontrivial value w=1 for neutral spin-1/2 excitations and for spinless excitations with a unit electron charge. It may even be irrational, if the charge is irrational. Thus, these excitations are topological, and they can be viewed as composite particles made of spin or charge degrees of freedom and dressed by kinks in the order parameter.Comment: 5 pages. And we are not only splitting point

One-dimensional Kondo lattice at partial band filling

An effective Hamiltonian for the localized spins in the one-dimensional Kondo lattice model is derived via a unitary transformation involving a bosonization of delocalized conduction electrons. The effective Hamiltonian is shown to reproduce all the features of the model as identified in various numerical simulations, and provides much new information on the ferro- to paramagnetic phase transition and the paramagnetic phase.Comment: 11 pages Revtex, 1 Postscript figure. To appear in Phys. Rev. Let

Translational Symmetry Breaking in the Superconducting State of the Cuprates: Analysis of the Quasiparticle Density of States

Motivated by the recent STM experiments of J.E. Hoffman et.al. and C. Howald et.al., we study the effects of weak translational symmetry breaking on the quasiparticle spectrum of a d-wave superconductor. We develop a general formalism to discuss periodic charge order, as well as quasiparticle scattering off localized defects. We argue that the STM experiments in $Bi_2Sr_2CaCu_2O_{8+\delta}$ cannot be explained using a simple charge density wave order parameter, but are consistent with the presence of a periodic modulation in the electron hopping or pairing amplitude. We review the effects of randomness and pinning of the charge order and compare it to the impurity scattering of quasiparticles. We also discuss implications of weak translational symmetry breaking for ARPES experiments.Comment: 12 pages, 9 figs; (v2) minor corrections to formalism, discussions of dispersion, structure factors and sum rules added; (v3) discussion of space-dependent normalization added. To be published in PR

Critical Ising modes in low-dimensional Kondo insulators

We present an Ising-like intermediate phase for one-dimensional Kondo insulator systems. Resulting from a spinon splitting, its low-energy excitations are critical Ising modes, whereas the triplet sector has a spectral gap. It should occur as long as the RKKY oscillation amplitude dominates over any direct exchange between localized spins. The chiral fixed point, however, becomes unstable in the far Infra-Red limit due to prevalent fluctuations among localized spins which induce gapless triplet excitations in the spectrum. Based on previous numerical results, we obtain a paramagnetic disordered state ruled by the correlation length of the single impurity Kondo model.Comment: 7 pages, RevTeX; last version: to be published in Physical Review

Classification and Stability of Phases of the Multicomponent One-Dimensional Electron Gas

The classification of the ground-state phases of complex one-dimensional electronic systems is considered in the context of a fixed-point strategy. Examples are multichain Hubbard models, the Kondo-Heisenberg model, and the one-dimensional electron gas in an active environment. It is shown that, in order to characterize the low-energy physics, it is necessary to analyze the perturbative stability of the possible fixed points, to identify all discrete broken symmetries, and to specify the quantum numbers and elementary wave vectors of the gapless excitations. Many previously-proposed exotic phases of multichain Hubbard models are shown to be unstable because of the ``spin-gap proximity effect.'' A useful tool in this analysis is a new generalization of Luttinger's theorem, which shows that there is a gapless even-charge mode in any incommensurate N-component system.Comment: 15 pages revtex. Final version as publishe

Co-operative Kondo Effect in the two-channel Kondo Lattice

We discuss the possibility of a co-operative Kondo effect driven by channel interference in a Kondo lattice where local moments are coupled to a single Fermi sea via two orthogonal scattering channels. In this situation, the channel quantum number is not conserved. We argue that the absence of channel conservation causes the Kondo effect in the two channels to constructively interfere, giving rise to a superconducting condensate of composite pairs, formed between the local moments and the conduction electrons. Our arguments are based on the observation that a heavy Fermi surface gives rise to zero modes for Kondo singlets to fluctuate between screening channels of different symmetry, producing a divergent composite pair susceptibility. Secondary screening channels couple to these divergent fluctuations, promoting an instability into a state with long-range composite order. We present detailed a detailed mean-field theory for this superconducting phase, and discuss the possible implications for heavy fermion physics.Comment: 23 double column pages. 9 fig

Kagom{\'e} Lattice Antiferromagnet Stripped to Its Basics

We study a model of a spin S = 1/2 Heisenberg antiferromagnet on a one dimensional lattice with the local symmetry of the two dimensional kagom{\'e} lattice. Using three complementary approaches, it is shown that the low energy spectrum can be described by two critical Ising models with different velocities. One of these velocities is small, leading to a strongly localized Majorana fermion. These excitations are singlet ones whereas the triplet sector has a spectral gap.Comment: 4 page

Non-perturbative approach to Luttinger's theorem in one dimension

The Lieb-Schultz-Mattis theorem for spin chains is generalized to a wide range of models of interacting electrons and localized spins in one-dimensional lattice. The existence of a low-energy state is generally proved except for special commensurate fillings where a gap may occur. Moreover, the crystal momentum of the constructed low-energy state is $2k_F$, where $k_F$ is the Fermi momentum of the non-interacting model, corresponding to Luttinger's theorem. For the Kondo lattice model, our result implies that $k_F$ must be calculated by regarding the localized spins as additional electrons.Comment: Note added on the rigorous proof given by H. Tasaki; also added some references; 5 pages, REVTEX (no figure