Impurity state in Haldane gap for S=1 Heisenberg antiferromagnetic chain with bond doping

Using a new impurity density matrix renormalization group scheme, we establish a reliable picture of how the low lying energy levels of a $S=1$ Heisenberg antiferromagnetic chain change {\it quantitatively} upon bond doping. A new impurity state gradually occurs in the Haldane gap as $J' < J$, while it appears only if $J'/J>\gamma_c$ with $1/\gamma_c=0.708$ as $J'>J$. The system is non-perturbative as $1\leq J'/J\leq\gamma_c$. This explains the appearance of a new state in the Haldane gap in a recent experiment on Y$_{2-x}$Ca$_x$BaNiO$_5$ [J.F. DiTusa, et al., Phys. Rev. Lett. 73 1857(1994)].Comment: 4 pages of uuencoded gzip'd postscrip

Edge states in Open Antiferromagnetic Heisenberg Chains

In this letter we report our results in investigating edge effects of open antiferromagnetic Heisenberg spin chains with spin magnitudes $S=1/2, 1,3/2,2$ using the density-matrix renormalization group (DMRG) method initiated by White. For integer spin chains, we find that edge states with spin magnitude $S_{edge}=S/2$ exist, in agreement with Valence-Bond-Solid model picture. For half-integer spin chains, we find that no edge states exist for $S=1/2$ spin chain, but edge state exists in $S=3/2$ spin chain with $S_{edge}=1/2$, in agreement with previous conjecture by Ng. Strong finite size effects associated with spin dimmerization in half-integer spin chains will also be discussed.Comment: 4 pages, RevTeX 3.0, 5 figures in a separate uuencoded postscript file. Replaced once to enlarge the acknowlegement

The Haldane gap for the S=2 antiferromagnetic Heisenberg chain revisited

Using the density matrix renormalization group (DMRG) technique, we carry out a large scale numerical calculation for the S=2 antiferromagnetic Heisenberg chain. Performing systematic scaling analysis for both the chain length $L$ and the number of optimal states kept in the iterations $m$, the Haldane gap $\Delta (2)$ is estimated accurately as $(0.0876\pm0.0013)J$. Our systematic analysis for the S=2 chains not only ends the controversies arising from various DMRG calculations and Monte Carlo simulations, but also sheds light on how to obtain reliable results from the DMRG calculations for other complicated systems.Comment: 4 pages and 1 figur

Distribution of exchange energy in a bond-alternating S=1 quantum spin chain

The quasi-one-dimensional bond-alternating S=1 quantum antiferromagnet NTENP is studied by single crystal inelastic neutron scattering. Parameters of the measured dispersion relation for magnetic excitations are compared to existing numerical results and used to determine the magnitude of bond-strength alternation. The measured neutron scattering intensities are also analyzed using the 1st-moment sum rules for the magnetic dynamic structure factor, to directly determine the modulation of ground state exchange energies. These independently determined modulation parameters characterize the level of spin dimerization in NTENP. First-principle DMRG calculations are used to study the relation between these two quantities.Comment: 10 pages, 10 figure

Magnetization and dimerization profiles of the cut two-leg spin ladder and spin-1 chain

The physical properties of the edge states of the cut two-leg spin ladder are investigated by means of the bosonization approach. By carefully treating boundary conditions, we derive the existence of spin-1/2 edge states in the spin ladder with a ferromagnetic rung exchange and for the open spin-1 Heisenberg chain. In contrast, such states are absent in the antiferromagnetic rung coupling case. The approach, based on a mapping onto decoupled semi-infinite off-critical Ising models, allows us to compute several physical quantities of interest. In particular, we determine the magnetization and dimerization profiles of the cut two-leg spin ladder and of the open biquadratic spin-1 chain in the vicinity of the SU(2)$_2$ WZNW critical point.Comment: RevTeX 4, no figure, 26 page

Superconductivity at 18 K in potassium-doped C60

The synthesis of macroscopic amounts of C60 and C70 (fullerenes) has stimulated a variety of studies on their chemical and physical properties. We recently demonstrated that C60 and C70 become conductive when doped with alkali metals. Here we describe low-temperature studies of potassium-doped C60 both as films and bulk samples, and demonstrate that this material becomes superconducting. Superconductivity is demonstrated by microwave, resistivity and Meissner-effect measurements. Both polycrystalline powders and thin-film samples were studied. A thin film showed a resistance transition with an onset temperature of 16 K and essentially zero resistance near 5 K. Bulk samples showed a well-defined Meissner effect and magnetic-field-dependent microwave absorption beginning at 18 K. The onset of superconductivity at 18 K is the highest yet observed for a molecular superconductor.

Superconductivity at 28 K in RbxC60

Meissner-effect and microwave-absorption measurements on bulk samples show that RbxC60 is superconducting with a maximum transition temperature of 28 K. This is a 10-K (60%) increase over the K-doped material. Only Ba0.6K0.4BiO3 and the cuprate superconductors have higher transition temperatures.

Charge Transfer Salts of Benzene-Bridged 1,2,3,5-Dithiadiazolyl Diradicals. Preparation, Structures, and Transport Properties of 1,3- and 1,4-[(S2N2C)C6H4(CN2S2)][X] (X = I, Br)

Cosublimation of 1,3- and 1,4-benzene-bis(1,2,3,5-dithiadiazolyl) and iodine/bromine affords crystals of the mixed valence salts 1,3- and 1,4-[(S2N2C)C6H4(CN2S2)][X] (X = I, Br). The crystal structures of the two iodide salts consist of perfectly superimposed stacks of molecular units with interannular spacing along the stacks of 3.487(3) and 3.415(2) Å, for the 1,3- and 1,4-derivatives. In both compounds the iodines are disordered along the stacking direction. The 1,3-derivative has a highly one-dimensional structure; there are no short intercolumnar S-S interactions. In the 1,4-derivative, however, lateral S-S contacts of 3.911 Å, afford some measure of three-dimensionality. The bromide salt of the 1,4-derivative consists of ribbons of alternating 1,4-[(S2N2C)C6H4(CN2S2)]+ units and bromide ions. Within each molecule one heterocyclic ring is closed shell, i.e., a [CN2S2]+ cation, while the other is a discrete radical. The ribbons are layered in zigzag fashion that maximizes ion pairing and isolates the radical centers. The bromide salt of the 1,3-derivative also forms ribbon-like arrays, but the unit cell repeat consists of four layers of ribbons. Within these layers the [CN2S2] rings are approximately stacked. The four rings within the repeat unit along each stack consists of three rings clustered into a trimeric [CN2S2]3+ cation, while the remaining ring is a discrete [CN2S2]+ cation. Magnetic susceptibility and conductivity measurements on the two iodide salts indicate weakly metallic behavior at room temperature, with a charge density wave (CDW) driven metal-insulator phase transition occurring near 270 and 190 K for the 1,3- and 1,4-derivatives, respectively. For the 1,4-derivative, analysis of the CDW wavevector associated with the transition affords a degree of charge transfer of 1/4 of electron per radical, i.e., an overall formulation of [(S2N2C)C6H4(CN2S2)]0.5+[I]0.5-. The bromide salt of the 1,3-derivative is a closed shell insulator, while in the 1,4-bromide the isolated radical centers are antiferromagnetically coupled.