Electronic structure and parity effects in correlated nanosystems

We discuss the spectral, transport and magnetic properties of quantum nanowires composed of N\leq 13 atoms and containing either even or odd numbers of valence electrons. In our approach we combine Exact Diagonalization and Ab Initio calculations (EDABI method). The analysis is performed as a function of the interatomic distance. The momentum distribution differs drastically for those obtained for even N with those for odd N, whereas the Drude weight evolves smoothly. A role of boundary conditions is stressed.Comment: 4 pages, 3 figure

Competition Between Antiferromagnetic Order and Spin-Liquid Behavior in the Two-Dimensional Periodic Anderson Model at Half-Filling

We study the two-dimensional periodic Anderson model at half-filling using quantum Monte Carlo (QMC) techniques. The ground state undergoes a magnetic order-disorder transition as a function of the effective exchange coupling between the conduction and localized bands. Low-lying spin and charge excitations are determined using the maximum entropy method to analytically continue the QMC data. At finite temperature we find a competition between the Kondo effect and antiferromagnetic order which develops in the localized band through Ruderman-Kittel-Kasuya-Yosida interactions.Comment: Revtex 3.0, 10 pages + 5 figures, UCSBTH-94-2

Spin and charge dynamics of the ferromagnetic and antiferromagnetic two-dimensional half-filled Kondo lattice model

We present a detailed numerical study of spin and charge dynamics of the two-dimensional Kondo lattice model with hopping t and exchange J. At T=0 and J > 0, the competition between the RKKY interaction and Kondo effect triggers a quantum phase transition between magnetically ordered and disordered insulators: J_c/t = 1.45(5). The quasiparticle gap scales as |J|. S(q,\omega), evolves smoothly from its strong coupling form with spin gap at q = (\pi,\pi) to a spin wave form. At J>0, A(\vec{k},\omega) shows a dispersion relation following that of hybridized bands. For J < J_c this feature is supplemented by shadows thus pointing to a coexistence of Kondo screening and magnetism. For J < 0 A(\vec{k},\omega) is similar to that of non-interacting electrons in a staggered magnetic field. Spin, T_S, and charge, T_C, scales are defined. For weak to intermediate couplings, T_S marks the onset of antiferromagnetic fluctuations and follows a J^2 law. At strong couplings T_S scales as J. T_C scales as J both at weak and strong couplings. At and slightly below T_C we observe i) a rise in the resistivity as a function of decreasing temperature, ii) a dip in the integrated density of states at the Fermi energy and iii) the occurrence of hybridized bands in A(k,\omega). It is shown that in the weak coupling limit, the charge gap of order J is of magnetic origin. The specific heat shows a two peak structure, the low temperature peak being of magnetic origin. Our results are compared to various mean-field theories.Comment: 30 pages, 24 figure

Phase separation and pairing in coupled chains and planes

A generalization of the $t-J$ model in a system of two coupled chains or planes is studied by numerical diagonalization of small clusters. In particular, the effect of density fluctuations between these one- or two-dimensional coupled layerson intralayer phase separation and pairing is analyzed. The most robust signals of superconductivity are found at quarter filling for couplings just before the fully interlayer phase separated regime. The possibility of an enhancement of the intralayer superconducting pairing correlations by the interlayer couplings is investigated.Comment: 13 pages + 3 figures, available upon request, LATEX, preprint ORNL/CCIP/93/1

Estimation of properties of low-lying excited states of Hubbard models : a multi-configurational symmetrized projector quantum Monte Carlo approach

We present in detail the recently developed multi-configurational symmetrized projector quantum Monte Carlo (MSPQMC) method for excited states of the Hubbard model. We describe the implementation of the Monte Carlo method for a multi-configurational trial wavefunction. We give a detailed discussion of issues related to the symmetry of the projection procedure which validates our Monte Carlo procedure for excited states and leads naturally to the idea of symmetrized sampling for correlation functions, developed earlier in the context of ground state simulations. It also leads to three possible averaging schemes. We have analyzed the errors incurred in these various averaging procedures and discuss and detail the preferred averaging procedure for correlations that do not have the full symmetry of the Hamiltonian. We study the energies and correlation functions of the low-lying excited states of the half-filled Hubbard model in 1-D. We have used this technique to study the pair-binding energies of two holes in $4n$ and $4n+2$ systems, which compare well the Bethe ansatz data of Fye, Martins and Scalettar. We have also studied small clusters amenable to exact diagonalization studies in 2-D and have reproduced their energies and correlation functions by the MSPQMC method. We identify two ways in which a multiconfigurational trial wavefunction can lead to a negative sign problem. We observe that this effect is not severe in 1-D and tends to vanish with increasing system size. We also note that this does not enhance the severity of the sign problem in two dimensions.Comment: 29 pages, 2 figures available on request, submitted to Phys. Rev.

Variational local moment approach: from Kondo effect to Mott transition in correlated electron systems

The variational local moment approach (VLMA) solution of the single impurity Anderson model is presented. It generalizes the local moment approach of Logan et al. by invoking the variational principle to determine the lengths of local moments and orbital occupancies. We show that VLMA is a comprehensive, conserving and thermodynamically consistent approximation and treats both Fermi and non-Fermi liquid regimes as well as local moment phases on equal footing. We tested VLMA on selected problems. We solved the single- and multi-orbital impurity Anderson model in various regions of parameters, where different types of Kondo effects occur. The application of VLMA as an impurity solver of the dynamical mean-field theory, used to solve the multi-orbital Hubbard model, is also addressed.Comment: 11 pages, 12 figure

Fine Structure and Fractional Aharonov-Bohm Effect

We find a fine structure in the Aharonov-Bohm effect, characterized by the appearence of a new type of periodic oscillations having smaller fractional period and an amplitude, which may compare with the amplitude of the conventional Aharonov-Bohm effect. Specifically, at low density or strong coupling on a Hubbard ring can coexist along with the conventional Aaronov-Bohm oscillations with the period equal to an integer, measured in units of the elementary flux quantum, two additional oscillations with periods $1/N$ and $M/N$. The integers $N$ and $M$ are the particles number and the number of down-spin particles, respectively. {}From a solution of the Bethe ansatz equations for $N$ electrons located on a ring in a magnetic field we show that the fine structure is due to electron-electron and Zeeman interactions. Our results are valid in the dilute density limit and for an arbitrary value of the Hubbard repulsion $U$Comment: 40 pages (Latex,Revtex) 12 figures by request, in Technical Reports of ISSP , Ser. A, No.2836 (1994

Scaling theory of the Mott-Hubbard metal-insulator transition in one dimension

We use the Bethe ansatz equations to calculate the charge stiffness $D_{\rm c} = (L/2) d^2 E_0/d\Phi_{\rm c}^2|_{\Phi_{\rm c}=0}$ of the one-dimensional repulsive-interaction Hubbard model for electron densities close to the Mott insulating value of one electron per site ($n=1$), where $E_0$ is the ground state energy, $L$ is the circumference of the system (assumed to have periodic boundary conditions), and $(\hbar c/e)\Phi_{\rm c}$ is the magnetic flux enclosed. We obtain an exact result for the asymptotic form of $D_{\rm c}(L)$ as $L\to \infty$ at $n=1$, which defines and yields an analytic expression for the correlation length $\xi$ in the Mott insulating phase of the model as a function of the on-site repulsion $U$. In the vicinity of the zero temperature critical point U=0, $n=1$, we show that the charge stiffness has the hyperscaling form $D_{\rm c}(n,L,U)=Y_+(\xi \delta, \xi/L)$, where $\delta =|1-n|$ and $Y_+$ is a universal scaling function which we calculate. The physical significance of $\xi$ in the metallic phase of the model is that it defines the characteristic size of the charge-carrying solitons, or {\em holons}. We construct an explicit mapping for arbitrary $U$ and $\xi \delta \ll 1$ of the holons onto weakly interacting spinless fermions, and use this mapping to obtain an asymptotically exact expression for the low temperature thermopower near the metal-insulator transition, which is a generalization to arbitrary $U$ of a result previously obtained using a weak- coupling approximation, and implies hole-like transport for $0<1-n\ll\xi^{-1}$.Comment: 34 pages, REVTEX (5 figures by request

Is the hijab protective? An investigation of body image and related constructs among British Muslim women

Previous studies have reported equivocal findings concerning the impact of wearing a hijab, or Islamic head- and body-cover, on Muslim women's body image. Here, we sought to examine that impact using a larger sample of Muslim women than has been relied upon and a wider range of body image measures. A total of 587 British Muslim women completed a battery of scales assessing their frequency and conservativeness of hijab use, body image variables, attitudes towards the media and beauty ideals, importance of appearance, and religiosity. Preliminary results indicated that 218 women never used the hijab and 369 women used some form of the hijab at least rarely. Controlling for religiosity, women who wore the hijab had more positive body image, lower internalization of media messages about beauty standards, and placed less importance on appearance than women who did not wear the hijab. Among women who wore the hijab, hijab use significantly predicted weight discrepancy and body appreciation over and above religiosity. These results are discussed in terms of the possible protective impact among British Muslim women of wearing the hijab