1,575 research outputs found

    Quasiperiodic Hubbard chains

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    Low energy properties of half-filled Fibonacci Hubbard models are studied by weak coupling renormalization group and density matrix renormalization group method. In the case of diagonal modulation, weak Coulomb repulsion is irrelevant and the system behaves as a free Fibonacci chain, while for strong Coulomb repulsion, the charge sector is a Mott insulator and the spin sector behaves as a uniform Heisenberg antiferromagnetic chain. The off-diagonal modulation always drives the charge sector to a Mott insulator and the spin sector to a Fibonacci antiferromagnetic Heisenberg chain.Comment: 4 pages, 4 figures; Final version to appear in Phys. Rev. Let

    Magnetization plateaus in antiferromagnetic-(ferromagnetic)_{n} polymerized S=1/2 XXZ chains

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    The plateau-non-plateau transition in the antiferromagnetic-(ferromagnetic)n_{n} polymerized S=1/2S=1/2 XXZ chains under the magnetic field is investigated. The universality class of this transition belongs to the Brezinskii-Kosterlitz-Thouless (BKT) type. The critical points are determined by level spectroscopy analysis of the numerical diagonalization data for 4p134 \leq p \leq 13 where p(n+1)p(\equiv n+1) is the size of a unit cell. It is found that the critical strength of ferromagnetic coupling decreases with pp for small pp but increases for larger enough pp. It is also found that the plateau for large pp is wide enough for moderate values of exchange coupling so that it should be easily observed experimentally. This is in contrast to the plateaus for p=3p = 3 chains which are narrow for a wide range of exchange coupling even away from the critical point

    Critical Properties of the transition between the Haldane phase and the large-D phase of the spin-1/2 ferromagnetic-antiferromagnetic Heisenberg chain with on-site anisotropy"

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    We analytically study the ground-state quantum phase transition between the Haldane phase and the large-DD (LD) phase of the S=1/2S=1/2 ferromagnetic-antiferromagnetic alternating Heisenberg chain with on-site anisotropy. We transform this model into a generalized version of the alternating antiferromagnetic Heisenberg model with anisotropy. In the transformed model, the competition between the transverse and longitudinal bond alternations yields the Haldane-LD transition. Using the bosonization method, we show that the critical exponents vary continuously on the Haldane-LD boundary. Our scaling relations between critical exponents very well explains the numerical results by Hida.Comment: text 12 pages (Plain TeX), LaTeX sourse files of a table and a figure on reques

    The antiferromagnetic order in an F-AF random alternating quantum spin chain : (CH_3)_2 CHNH_3 Cu(Cl_x Br_{1-x})_3

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    A possibility of the uniform antiferromagnetic order is pointed out in an S=1/2 ferromagnetic (F) - antiferromagnetic (AF) random alternating Heisenberg quantum spin chain compound: (CH_3)_2 CHNH_3 Cu(Cl_x Br_{1-x})_3. The system possesses the bond alternation of strong random bonds that take +/- 2J and weak uniform AF bonds of -J. In the pure concentration limits, the model reduces to the AF-AF alternation chain at x=0 and to the F-AF alternation chain at x=1. The nonequilibrium relaxation of large-scale quantum Monte Carlo simulations exhibits critical behaviors of the uniform AF order in the intermediate concentration region, which explains the experimental observation of the magnetic phase transition. The present results suggest that the uniform AF order may survive even in the presence of the randomly located ferromagnetic bonds.Comment: 4 pages, 3 figure

    Boson Fock representations of stochastic processes

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    A, classification theory of quantum stationary processes similar to the corresponding theory for classical stationary processes is presented, Our main result is the classificatio

    Effects of Single-site Anisotropy on Mixed Diamond Chains with Spins 1 and 1/2

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    Effects of single-site anisotropy on mixed diamond chains with spins 1 and 1/2 are investigated in the ground states and at finite temperatures. There are phases where the ground state is a spin cluster solid, i.e., an array of uncorrelated spin-1 clusters separated by singlet dimers. The ground state is nonmagnetic for the easy-plane anisotropy, while it is paramagnetic for the easy-axis anisotropy. Also, there are the N\'eel, Haldane, and large-DD phases, where the ground state is a single spin cluster of infinite size and the system is equivalent to the spin-1 Heisenberg chain with alternating anisotropy. The longitudinal and transverse susceptibilities and entropy are calculated at finite temperatures in the spin-cluster-solid phases. Their low-temperature behaviors are sensitive to anisotropy.Comment: 8 pages, 4 figure

    Identification of New Near-Infrared Diffuse Interstellar Bands in the Orion Nebula

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    Large organic molecules and carbon clusters are basic building blocks of life, but their existence in the universe has not been confirmed beyond doubt. A number of unidentified absorption features (arising in the diffuse interstellar medium), usually called "Diffuse Interstellar Bands" (DIBs), are hypothesized to be produced by large molecules. Among these, buckminsterfullerene C60 has gained much attention as a candidate for DIB absorbers because of its high stability in space. Two DIBs at lambda similar to 9577 angstrom and 9632 angstrom have been reported as possible features of C-60(+). However, it is still not clear how their existence depends on their environment. We obtained high-resolution spectra of three stars in/around the Orion Nebula, to search for any correlations of the DIB strength with carrier's physical conditions, such as dust abundance and UV radiation field. We find three DIBs at lambda similar to 9017 angstrom, 9210 angstrom, and 9258 angstrom as additional C-60(+) feature candidates, which could support this identification. These DIBs have asymmetric profiles similar to the longer wavelength features. However, we also find that the relative strengths of DIBs are close to unity and differ from laboratory measurements, a similar trend as noticed for the 9577/9632 DIBs.ArticleThe Astrophysical Journal. 700:1988-1993 (2009)journal articl

    Interacting Boson Theory of the Magnetization Process of the Spin-1/2 Ferromagnetic-Antiferromagnetic Alternating Heisenberg Chain

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    The low temperature magnetization process of the ferromagnetic-antiferromagnetic Heisenberg chain is studied using the interacting boson approximation. In the low field regime and near the saturation field, the spin wave excitations are approximated by the δ\delta function boson gas for which the Bethe ansatz solution is available. The finite temperature properties are calculated by solving the integral equation numerically. The comparison is made with Monte Carlo calculation and the limit of the applicability of the present approximation is discussed.Comment: 4 pages, 7 figure

    Dimer Expansion Study of the Bilayer Square Lattice Frustrated Quantum Heisenberg Antiferromagnet

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    The ground state of the square lattice bilayer quantum antiferromagnet with nearest (J1J_1) and next-nearest (J2J_2) neighbour intralayer interaction is studied by means of the dimer expansion method up to the 6-th order in the interlayer exchange coupling J3J_3. The phase boundary between the spin-gap phase and the magnetically ordered phase is determined from the poles of the biased Pad\'e approximants for the susceptibility and the inverse energy gap assuming the universality class of the 3-dimensional classical Heisenberg model. For weak frustration, the critical interlayer coupling decreases linearly with α(=J2/J1)\alpha (= J_2/J_1). The spin-gap phase persists down to J3=0J_3=0 (single layer limit) for 0.45 \simleq \alpha \simleq 0.65. The crossover of the short range order within the disordered phase is also discussed.Comment: 4 pages, 6 figures, One reference adde

    Divergence-free Nonrenormalizable Models

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    A natural procedure is introduced to replace the traditional, perturbatively generated counter terms to yield a formulation of covariant, self-interacting, nonrenormalizable scalar quantum field theories that has the added virtue of exhibiting a divergence-free perturbation analysis. To achieve this desirable goal it is necessary to reexamine the meaning of the free theory about which such a perturbation takes place.Comment: 22 pages. Version accepted for publication; involves modest addition to the end of Sec.
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