The Short Range RVB State of Even Spin Ladders: A Recurrent Variational Approach

Using a recursive method we construct dimer and nondimer variational ansatzs of the ground state for the two-legged ladder, and compute the number of dimer coverings, the energy density and the spin correlation functions. The number of dimer coverings are given by the Fibonacci numbers for the dimer-RVB state and their generalization for the nondimer ones. Our method relies on the recurrent relations satisfied by the overlaps of the states with different lengths, which can be solved using generating functions. The recurrent relation method is applicable to other short range systems. Based on our results we make a conjecture about the bond amplitudes of the 2-leg ladder.Comment: REVTEX file, 32 pages, 10 EPS figures inserted in text with epsf.st

Low-Energy Properties of Regularly Depleted Spin Ladders

We investigate a model for the regularly depleted two-leg spin ladder systems. By using Lieb-Schultz-Mattis theorem, it is rigorously shown that this model realizes massless excitations or, alternatively, a degenerate ground state, although the original spin ladder system has a spin gap and a unique ground state. The ground state of the depleted model is either a spin singlet or partially ferromagnetic reflecting topological properties of the depleted sites. In order to show that the low-energy excitations are indeed massless, we proceed our analysis in two different ways by resorting to effective field theories. We first investigate an effective weak-coupling model in terms of renormalization group methods. Although the tendency to massless spin excitations is seen in the strong coupling regime, it turns out that the model is still massive for any finite coupling, implying that a conventional weak-coupling approach is not efficient to describe massless modes in our model. To overcome this difficulty, we further study low-energy properties of the depleted spin model by mapping on the non-linear sigma model, and confirm that the massless spin excitation indeed occur.Comment: 13 pages, revtex, 7 figures, to appear in PRB. A new section is added, in which we derived the spin-wave dispersion relation and analyzed the quantum fluctuation around it by means of the O(3) nonlinear sigma mode

Alternating-Spin Ladders

We investigate a two-leg spin ladder system composed of alternating-spin chains with two-different kind of spins. The fixed point properties are discussed by using spin-wave analysis and non-linear sigma model techniques. The model contains various massive phases, reflecting the interplay between the bond-alternation and the spin-alternation.Comment: 6 pages, revtex, to appear in PR

Quasi-excitations and superconductivity in the t-J model on a ladder

We study the t-J model on a ladder by using slave-fermion-CP^1 formalism which is quite useful for study of lightly-doped high-T_c cuprates. By integrating half of spin variables, we obtain a low-energy effective field theory whose spin part is nothing but CP^1 sigma model. We especially focus on dynamics of composite gauge field which determines properties of quasi-excitations. Value of the coefficient of the topological term strongly influences gauge dynamics and explaines why properties of quasi-excitations depend on the number of legs of ladder. We also show that superconductivity appears as a result of short-range antiferromagnetism and order parameter has d-wave type symmetry.Comment: Latex, 28 pages and 1 figur

Low Energy Effective Action of Lightly Doped Two-Leg t-J Ladders

We propose a low energy effective theory of lightly doped two-leg t-J ladders with the help of slave fermion technique. The continuum limit of this model consists of two kinds of Dirac fermions which are coupled to the O(3) non-linear sigma model in terms of the gauge coupling with opposite sign of "charges". In addition to the gauge interaction, there is another kind of attractive force between these Dirac fermions, which arises from the short-ranged antiferromagnetic order. We show that the latter is essential to determine the low energy properties of lightly doped two-leg t-J ladders. The effective Hamiltonian we obtain is a bosonic Gaussian model and the boson field basically describes the particle density fluctuation. We also find two types of gapped spin excitations. Finally, we discuss the possible instabilities: charge density wave (CDW) and singlet superconductivity (SC). We find that the SC instability dominates in our approximation. Our results indicate that lightly doped ladders fall into the universality class of Luther-Emery model.Comment: 16 pages, Revtex, no figure

Intentions on desired length of stay among immigrants in Italy

Abstract The decision to emigrate from the country of origin may not be a permanent one: migrants can decide to return home or to emigrate to a third country. This phenomenon, established for some time in certain other European countries, has become an important one for Italy only recently. This paper contributes to the knowledge of migrants’ intentions in two ways: on the one hand, it analyses the factors associated with indecision about future plans; on the other, it focuses on the desired length of stay and its relationship with attachments (family, economic, socio-cultural and psychological) to host and home country. We used two logistic regression models: one for migrants’ indecision and the other for migrants’ desired length of stay. The data were collected by survey, coordinated by the ISMU Foundation and conducted in 2008 and 2009 with more than 12,000 migrants living in Italy. According to our results, indecision seems to be associated with an intermediate phase of migration at the early stage of family development in the case of negative balance of the migration experience, while attachment to the host country is associated with longer stay, and no attachments or attachment to the country of origin are associated with shorter stay