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The ground-state properties of the spin-1 antiferromagnetic Heisenberg model on the corner-sharing tetrahedra, pyrochlore lattice, is investigated. By breaking up each spin into a pair of 1/2-spins, the problem is reduced to the equivalent one of the spin-1/2 tetrahedral network in analogy with the valence bond solid state in one dimension. The twofold degeneracy of the spin-singlets of a tetrahedron is lifted by a Jahn-Teller mechanism, leading to a cubic to tetragonal structural transition. It is proposed that the present mechanism is responsible for the phase transition observed in the spin-1 spinel compounds ZnV$_2$O$_4$ and MgV$_2$O$_4$., Comment: 4 pages, 3 eps figures, REVTeX, to appear in Phys. Rev. Lett

The two dimensional Heisenberg model for ${\mathrm{SrCu}}_{2}({\mathrm{BO}}_{3}{)}_{2}$ has an exact dimer ground state which was proven by Shastry and Sutherland almost twenty years ago. The critical value of the quantum phase transition from the dimer state to the N\'eel ordered state is determined. Analysis of the experimental data shows that $\mathrm{SrCu}{}_{2}(\mathrm{BO}{}_{3}{)}_{2}$ has the dimer ground state but its coupling constants are close to the transition point, which leads to an unusual temperature dependence of the susceptibility. The almost localized nature of the triplet excitations explains the plateaus observed in the magnetization curve.

Combination of the finite-temperature density-matrix renormalization-group and the maximum entropy presents a new method to calculate dynamic quantities of one-dimensional many-body systems. In the present paper, density of states, local dynamic spin and charge correlation functions of the one-dimensional Kondo insulator are discussed. Excitation gaps open with decreasing temperature and the gaps take different values depending on channels. The excitation spectra change qualitatively around the characteristic temperature corresponding to the spin gap which is the lowest energy scale of the Kondo insulator., Comment: 6 pages, 3 Postscript figures, REVTeX, submitted to Phys. Rev. Lett

Spin gap behavior of CaV4O9 is studied by including the effects of frustrating magnetic interactions and lattice distortions. The spectrum of triplet excitations is calculated for a Heisenberg model on the 1/5-depleted square lattice. In the spin gap phase, the location of the minima of the spectrum in the Brillouin zone is found to depend nontrivially on the exchange parameters. Experimental consequences of the temperature-dependent lattice distortion including its effect on the uniform susceptibility and the spin gap are explored.

Using the quantum Monte Carlo loop algorithm we have determined the phase diagram of the 1/5-depleted Heisenberg square lattice model representing CaV{sub 4}O{sub 9} as a function of the ratio of the two different couplings: bonds within a plaquette and dimer bonds between plaquettes. At isotropic coupling long range order {ital survives} the depletion, but the system is close to the quantum critical point. Already a small frustration can thus drive the system into the quantum disordered phase and explain the spin gap behavior of CaV{sub 4}O{sub 9}. {copyright} {ital 1996 The American Physical Society.}

We have spectroscopically investigated the thermally and doping-induced metal-insulator transitions for pyrochlore-type Nd2Ir2O7 as well as its Rh-doped analogs Nd2(Ir(1-x)Rh(x))(2)O(7), where the spin-orbit interaction as well as the electron correlation is effectively tuned by the doping level (x). The charge dynamics dramatically changes on an energy scale of 1 eV in the course of thermally and doping-induced metal-insulator transitions, while the insulating ground state shows a small but well-defined charge gap of 45 meV. Anomalous doping variation of the low-energy (

It is proved that the ground state of the symmetric periodic Anderson model forms a total spin singlet at half filling. This result can also be applied to the Kondo lattice model, at least for the weak and the very strong antiferromagnetic-coupling limit. For the strong coupling limit of the Kondo lattice it is additionally proved that the singlet ground state exists even when two holes are doped to the half-filled band.

We calculate the temperature dependence of NMR relaxation rate and electrical resistivity for coupling to a local, strongly anharmonic phonon mode. We argue that the two-phonon Raman process is dominating NMR relaxation. Due to the strong anharmonicity of the phonon an unusual temperature dependence is found having a low temperature peak and becoming constant towards higher temperatures. The electrical resistivity is found to vary like T^2 at low temperatures and following a sqrt{T} behavior at high temperatures. Both results are in qualitative agreement with recent observations on beta-pyrochlore oxide superconductors., 4 pages, 4 figures; new version with some minor additional clarifications; accepted for publication in Phys. Rev. Lett

We discuss microscopic aspects of odd-parity triplet pair in orbital degenerate systems. From the concept of off-diagonal long-range order, a pair state is unambiguously defined as the eigenstate with the maximum eigenvalue of pair correlation function. Performing this scheme by a numerical technique, we clarify that the odd-parity triplet pair occurs as an out-of-phase combination of local triplets induced by Hund's rule coupling for the lattice including two sites in the unit cell., 4 pages, 3 figures, to appear in Phys. Rev. Lett

Using a relativistic linear augmented-plane-wave method, we clarify energy band structures and Fermi surfaces of recently discovered plutonium-based superconductor PuCoGa5 and the isostructural material NpCoGa5. For PuCoGa5, we find several cylindrical sheets of Fermi surfaces with large volume, similar to CeMIn5, while for NpCoGa5, the Fermi surfaces are found to be similar to those of UMGa5. These similarities are discussed based on the j-j coupling scheme, suggesting some hints for the superconducting mechanism in HoCoGa5-type f-electron compounds.

Using a relativistic linear augmented-plane-wave method, we clarify energy band structures and Fermi surfaces of recently discovered plutonium-based superconductor PuCoGa5 and the isostructural material NpCoGa5. For PuCoGa5, we find several cylindrical sheets of Fermi surfaces with large volume, similar to CeMIn5, while for NpCoGa5, the Fermi surfaces are found to be similar to those of UMGa5. These similarities are discussed based on the j-j coupling scheme, suggesting some hints for the superconducting mechanism in HoCoGa5-type f-electron compounds.

The ground state of the Kondo-lattice model with one conduction electron is analyzed. A rigorous proof is given that this system forms an incomplete ferromagnetic order with ${\mathit{S}}_{\mathrm{tot}}$=(N-1)/2 for antiferromagnetic exchange coupling. The wave function of the ground state is derived and some of its properties are discussed.

A theoretical model is presented to explain the spin gap observed for CaV$_4$O$_9$. The underlying lattice of the 1/5-depleted square lattice favors a formation of plaquette resonating valence bond state. Inclusion of the frustrating second neighbor interaction enhances this tendency, leading to a quantum disordered state of a two dimensional spin-1/2 Heisenberg model with a sufficiently big spin gap compatible with experiments.

Modele d'un reseau de Fibonacci 2D; mise en evidence de l'existence de deux regimes en fonction de la force du potentiel et de fluctuation de la conductance avec la taille du systeme, de l'ordre de e 2 /h pour les potentiels forts

Solutions of the Ginzburg-Landau equations for a complex superconducting phase are presented for regions close to a domain wall and to the surface. The finite local magnetization found at these inhomogeneities yields a small magnetic response via a movement of domain walls in an external magnetic field. We examine line defects in the walls which are vortices enclosing a fraction of the universal flux quantum.