Electronic correlations on a metallic nanosphere

We consider the correlation functions in a gas of electrons moving within a thin layer on the surface of nanosize sphere. A closed form of expressions for the RKKY indirect exchange, superconducting Cooper loop and `density-density' correlation function is obtained. The systematic comparison with planar results is made, the effects of spherical geometry are outlined. The quantum coherence of electrons leads to the enhancement of all correlations for the points--antipodes on the sphere. This effect is lost when the radius of the sphere exceeds the temperature coherence length.Comment: 5 pages, no figures, to appear in PRB (RC

Anomalous bond stretching phonons as a probe of charge fluctuations in perovskites

Important information on momentum resolved low energy charge response can be extracted from anomalous properties of bond stretching in plane phonons observed in inelastic neutron and X-ray scattering in cuprates and some other perovskites. We discuss a semiphenomenological model based on coupling of phonons to a single charge mode. The phonon dispersion and linewidth allow to locate the energy of the charge excitation in the mid infrared part of the spectrum and to determine some of its characteristics. New experiments on oxygen isotope substitution could allow to achieve a more detailed description. Corresponding relations following from the model can be used for the interpretation of experiments and as test of the model.Comment: presented at the M2S-HTSC-VIII conference in Dresde

Ferrimagnetic mixed-spin ladders in weak and strong coupling limits

We study two similar spin ladder systems with the ferromagnetic leg coupling. First model includes two sorts of spins, s= 1/2 and s= 1, and the second model comprises only s=1/2 legs coupled by a "triangular" rung exchange. The antiferromagnetic (AF) rung coupling destroys the long-range order and eventually makes the systems equivalent to the AF s=1/2 Heisenberg chain. We investigate the situation by different methods in weak and strong rung coupling limits. Particularly we compare the spin-wave theory and the bosonization method in the weak coupling regime of the second model. We analyze the spectra and correlations, and discuss the order parameter of these ladder systems.Comment: 12 pages, 4 figure

Asymmetric spin-1/2 two-leg ladders

We consider asymmetric spin-1/2 two-leg ladders with non-equal antiferromagnetic (AF) couplings J_|| and \kappa J_|| along legs (\kappa <= 1) and ferromagnetic rung coupling, J_\perp. This model is characterized by a gap \Delta in the spectrum of spin excitations. We show that in the large J_\perp limit this gap is equivalent to the Haldane gap for the AF spin-1 chain, irrespective of the asymmetry of the ladder. The behavior of the gap at small rung coupling falls in two different universality classes. The first class, which is best understood from the case of the conventional symmetric ladder at \kappa=1, admits a linear scaling for the spin gap \Delta ~ J_\perp. The second class appears for a strong asymmetry of the coupling along legs, \kappa J_|| << J_\perp << J_|| and is characterized by two energy scales: the exponentially small spin gap \Delta ~ J_\perp \exp(-J_|| / J_\perp), and the bandwidth of the low-lying excitations induced by a Suhl-Nakamura indirect exchange ~ J_\perp^2 /J_|| . We report numerical results obtained by exact diagonalization, density matrix renormalization group and quantum Monte Carlo simulations for the spin gap and various spin correlation functions. Our data indicate that the behavior of the string order parameter, characterizing the hidden AF order in Haldane phase, is different in the limiting cases of weak and strong asymmetry. On the basis of the numerical data, we propose a low-energy theory of effective spin-1 variables, pertaining to large blocks on a decimated lattice.Comment: 18 pages, 11 figure

RKKY interaction in the nearly-nested Fermi liquid

We present the results of analytical evaluation of the indirect RKKY interaction in a layered metal with nearly nested (almost squared) Fermi surface. The final expressions are obtained in closed form as a combination of Bessel functions. We discuss the notion of the ``2k_F'' oscillations and show that they occur as the far asymptote of our expressions. We show the existence of the intermediate asymptote of the interaction which is of the sign-reversal antiferromagnetic type and is the only term surviving in the limit of exact nesting. A good accordance of our analytical formulas with numerical findings is demonstrated until the interatomic distances. The obtained expressions for the Green's functions extend the previous analytical results into the region of intermediate distances as well.Comment: 9 pages, REVTEX, 3 .eps figures, to appear in PRB 1 Oct 199

Spin gap and string order parameter in the ferromagnetic Spiral Staircase Heisenberg Ladder: a quantum Monte Carlo study

We consider a spin-1/2 ladder with a ferromagnetic rung coupling J_\perp and inequivalent chains. This model is obtained by a twist (\theta) deformation of the ladder and interpolates between the isotropic ladder (\theta=0) and the SU(2) ferromagnetic Kondo necklace model (\theta=\pi). We show that the ground state in the (\theta,J_\perp) plane has a finite string order parameter characterising the Haldane phase. Twisting the chain introduces a new energy scale, which we interpret in terms of a Suhl-Nakamura interaction. As a consequence we observe a crossover in the scaling of the spin gap at weak coupling from \Delta/J_\| \propto J_\perp/J_\| for \theta < \theta_c \simeq 8\pi/9 to \Delta/J_\| \propto (J_\perp/J_\|)^2 for \theta > \theta_c. Those results are obtained on the basis of large scale Quantum Monte Carlo calculations.Comment: 4 page

High efficiency and low absorption Fresnel compound zone plates for hard X-ray focusing

Circular and linear zone plates have been fabricated on the surface of silicon crystals for the energy of 8 keV by electron beam lithography and deep ion plasma etching methods. Various variants of compound zone plates with first, second, third diffraction orders have been made. The zone relief height is about 10 mkm, the outermost zone width of the zone plate is 0.4 mkm. The experimental testing of the zone plates has been conducted on SPring-8 and ESRF synchrotron radiation sources. A focused spot size and diffraction efficiency measured by knife-edge scanning are accordingly 0.5 mkm and 39% for the first order circular zone plate.Comment: 5 pages, 7 figure

Indirect RKKY interaction in any dimensionality

We present an analytical method which enables one to find the exact spatial dependence of the indirect RKKY interaction between the localized moments via the conduction electrons for the arbitrary dimensionality $n$. The corresponding momentum dependence of the Lindhard function is exactly found for any $n$ as well. Demonstrating the capability of the method we find the RKKY interaction in a system of metallic layers weakly hybridized to each other. Along with usual $2k_F$ in-plane oscillations the RKKY interaction has the sign-reversal character in a direction perpendicular to layers, thus favoring the antiferromagnetic type of layers' stacking.Comment: 3 pages, REVTEX, accepted to Phys.Rev.