The entanglement in a quantum system that possess an internal symmetry, characterized by the S z -magnetization or U (1)-charge, is distributed among different sectors. The aim of this letter is to gain a deeper understanding of the contribution to the entanglement entropy in each of those sectors for the ground state of conformal invariant critical one dimensional systems. We find surprisingly that the entanglement entropy is equally distributed among the different magnetization sectors. Its value is given by the standard area law violating logarithmic term, that depends on the central charge c, minus a double logarithmic correction related to the zero temperature susceptibility. This result provides a new method to estimate simultaneously the central charge c and the critical exponents of U (1)-symmetric quantum chains. The method is numerically simple and gives precise results for the spin-1 2 quantum XXZ chain. We also compute the probability distribution of the magnetization in contiguous sublattices.

The phase diagram of the spin-3/2 Blume-Capel model in two dimensions is explored by conventional finite-size scaling, conformal invariance and Monte Carlo simulations. The model in its τ -continuum Hamiltonian version is also considered and compared with others spin-3/2 quantum chains. Our results indicate that differently from the standard spin-1 Blume-Capel model there is no multicritical point along the order-disorder transition line. This is in qualitative agreement with mean field prediction but in disagreement with previous approximate renormalization group calculations.

We study the entanglement entropies in one-dimensional open critical systems, whose effective description is given by a conformal field theory with boundaries. We show that for pure-state systems formed by the ground state or by the excited states associated to primary fields, the entanglement entropies have a finite-size behavior that depends on the correlation of the underlying field theory. The analytical results are checked numerically, finding excellent agreement for the quantum chains ruled by the theories with central charge c = 1/2 and c = 1.

The extended and standard t-J models are computationally studied on ladders and planes, with emphasis on the small J/t region. At couplings compatible with photoemission results for undoped cuprates, half-doped stripes separating π-shifted antiferromagnetic (AF) domains are found, as in Tranquada's interpretation of neutron experiments. Our main result is that the elementary stripe "building-block" resembles the properties of one hole at small J/t, with robust AF correlations across-the-hole induced by the local tendency of the charge to separate from the spin (G. Martins et al., Phys. Rev. B60, R3716 (1999)). This suggests that the seed of half-doped stripes already exists in the unusual properties of the insulating parent compound. . Whether stripe formation is beneficial or detrimental to superconductivity is unclear, but it appears that stripes are an important ingredient of the normal state that cannot be ignored.The theoretical explanation of stripe formation is much debated. Early work reported stripes in the t-J (at large J/t with 1/r repulsions) and Hubbard (Hartree-Fock) models [3,4]. However, these stripes were insulating with hole density n h ∼1.0, different from the experimental n h ∼0.5 stripes [5]. Recently, considerable progress was made when doped stripes were reported by White and Scalapino within the standard t-J model [6] (see also Ref.[7]). In Ref.[6] the analysis was performed at couplings where two holes form d-wave pairs, and the stripes are sometimes described as a condensation of these pairs into a stripe domain-wall [8]. However, experiments are usually interpreted as holes moving freely along site-centered stripes [1]. In addition, the "extended" t-J model with hopping beyond neighboring sites, or the standard t-J model with very small J/t, are needed [9,10] to reproduce the insulator one-hole photoemission (PES) dispersion [11]. Thus, understanding metallic stripe formation requires further work and searching for stripes in the extended t-J model, particularly in regimes without hole binding and where the absence of phase separation (PS) is not controversial, is important to clarify the driving mechanism for these unusual complex structures.Building upon previous investigations [6,7], in this Letter indications of n h ∼0.5 stripes compatible with experiments [1] are reported in the extended and standard t-J models on ladders and square clusters. These stripes do not seem composed of hole pairs (although pairs forming domain-walls may be present at larger J/t than studied here [8]). They also exist in the t-J z model [12] and using classical spins [7], implying that the details of the AF spin background are unimportant for its stabilization. Moreover, our most important result is that the basic stripe "building-block" exists already in the insulator where one-hole wave functions have a complex spin structure with strong AF correlations across-the-hole, resembling the stripe spin correlations found here numerically. These results provide a rationalization for stripe formation built u...

Using the density matrix renormalization group, we investigate the Rényi entropy of the anisotropic spin-s Heisenberg chains in a z-magnetic field. We considered the half-odd-integer spin-s chains, with s = 1/2, 3/2 and 5/2, and periodic and open boundary conditions. In the case of the spin-1/2 chain we were able to obtain accurate estimates of the new parity exponents p (2010)]. Moreover, the predicted periodicity of the oscillating term was also observed for some non-zero values of the magnetization m. We show that for s > 1/2 the amplitudes of the oscillations are quite small, and get accurate estimates of p α for the spin-3/2 chain are not so accurate, they are consistent with the theoretical predictions.

We report the presence of spin dimerization in the ground state of the one-dimensional Kondo lattice model at quarter filling. The emergence of this new phase of the Kondo lattice can be traced to the form of the RKKY interaction between the localized moments and provides the first example of dimerization induced indirectly by itinerant electrons. We propose this dimer ordering as the driving mechanism of the spin-Peierls phase observed in the quasi-one-dimensional organic compounds (Per)2M(mnt)(2) (M=Pt, Pd). Moreover, this suggests that a richer phase diagram than the Doniach paradigm may be needed to accommodate the physics of heavy fermion materials.

Using the density matrix renormalization group, we calculated the finite-size corrections of the entanglement α-Rényi entropy of a single interval for several critical quantum chains. We considered models with U (1) symmetry such as the spin-1/2 XXZ and spin-1 Fateev-Zamolodchikov models, as well as models with discrete symmetries such as the Ising, the Blume-Capel, and the three-state Potts models. These corrections contain physically relevant information. Their amplitudes, which depend on the value of α, are related to the dimensions of operators in the conformal field theory governing the long-distance correlations of the critical quantum chains. The obtained results together with earlier exact and numerical ones allow us to formulate some general conjectures about the operator responsible for the leading finite-size correction of the α-Rényi entropies. We conjecture that the exponent of the leading finite-size correction of the α-Rényi entropies is p α = 2X /α for α > 1 and p 1 = ν, where X denotes the dimensions of the energy operator of the model and ν = 2 for all the models.

Numerical calculations illustrate the effect of the sign of the next nearest-neighbor hopping term t ′ on the 2-hole properties of the t-t ′ -J model. Working mainly on 2-leg ladders, in the -1.0 ≤t ′ /t ≤ 1.0 regime, it is shown that introducing t ′ in the t-J model is equivalent to effectively renormalizing J, namely t ′ negative (positive) is equivalent to an effective t-J model with smaller (bigger) J. This effect is present even at the level of a 2×2 plaquette toy model, and was observed also in calculations on small square clusters. Analyzing the transition probabilities of a hole-pair in the plaquette toy model, it is argued that the coherent propagation of such hole-pair is enhanced by a constructive interference between both t and t ′ for t ′ >0. This interference is destructive for t ′ <0.PACS numbers: 74.20.Mn, 75.25.Dw One of the most important unsolved problems in theoretical physics is the clarification of the nature of high temperature superconductors. A popular approach in this context is the use of the t-J model, with holes moving in an antiferromagnetic (AF) spin background. In recent years, mainly due to an increase in the sensitivity and resolution of angle resolved photoemission spectroscopy (ARPES), it has been shown that extra hole hoppings beyond nearest-neighbor (NN) are important in the t-J model, giving origin to the "extended" t-J model. For example, ARPES measurements in Sr 2 CuO 2 Cl 2 [1], and their subsequent interpretation [2], have shown the importance of those extra hoppings to reproduce the experimental results. Subsequent efforts have concentrated on the effect of the extra hoppings on various properties of planar and ladder systems, such as stripe stability [3], competition between pairing and stripes [4], spin-charge separation in 2-d [5], stripe formation mechanism [6], spin gap evolution [7], and current-current correlations [8]. Most of these papers have compared and contrasted the dependence of different properties of the extended t-J model with the sign of the next NN (NNN) hopping t ′ . Currently it is well established that a positive t ′ enhances hole pairing and AF correlations, while the opposite occurs for t ′ negative [4,9]. Nevertheless, to the best of our knowledge, these previous publications have not provided an intuitive mechanism that can explain why this happens, namely, for what reason there is an asymmetry between positive and negative t ′ . This is particularly puzzling considering the limit t=0, since in the t ′ -J model the sign of t ′ is irrelevant [10].It is the purpose of this paper to provide a qualitative explanation to this phenomenon, i. e., the sign of t ′ asymmetry. Our main result is that a quantum interference between NN and NNN hoppings identified in the hole-pair propagation was found to be constructive (destructive) for t ′ positive (negative); this accounts for the observed dependence of the hole-pair properties with the sign of t ′ . The t-t ′ -J model used here is defined aswhere t im is t for NN, t ′ for NNN, and zero otherwise. T...