Tamara S. Nunner

Dopant-Modulated Pair Interaction in Cuprate Superconductors

A comparison of recent experimental STM data with single-impurity and many-impurity Bogoliubov-de Gennes calculations strongly suggests that random out-of-plane dopant atoms in cuprates modulate the pair interaction locally. This type of disorder is crucial to understanding the nanoscale electronic inhomogeneity observed in BSCCO-2212, and can reproduce observed correlations between the positions of impurity atoms and various aspects of the local density of states such as the gap magnitude and the height of the coherence peaks. Our results imply that each dopant atom modulates the pair interaction on a length scale of order one lattice constant.

Observation of Two-Magnon Bound States in the Two-Leg Ladders of (Ca, La)14Cu24O41

Phonon-assisted two-magnon absorption is studied in the spin- 1/2 two-leg ladders of (Ca,La)(14)Cu(24)O(41) for E parallel c (legs) and E parallel a (rungs). We verify the theoretically predicted existence of two-magnon singlet bound states, which give rise to peaks at approximately equal to 2140 and 2800 cm(-1). The two-magnon continuum is observed at approximately equal to 4000 cm(-1). Two different theoretical approaches (Jordan-Wigner fermions and perturbation theory) describe the data very well for J parallel approximately equal to 1020-1100 cm(-1), J parallel/J perpendicular approximately equal to 1-1.2. At high energies, the magnetic contribution to sigma(omega) is strikingly similar in the ladders and in the undoped high-T(c) cuprates, which emphasizes the importance of strong quantum fluctuations in the latter.

Cyclic Spin Exchange in Cuprate Spin Ladders

We investigate the influence of a cyclic spin exchange J_{cyc} on the one- and two-triplet excitations of an undoped two-leg S=1/2 ladder, using the density matrix renormalization group (DMRG). The dispersion of the S=0 two-triplet bound state is dramatically reduced by J_{cyc} due to a repulsion between triplets on neighboring rungs. In (La,Ca)_{14}Cu_{24}O_{41} a consistent description of both the spin gap and the S=0 bound state requires

Influence of Electron-Phonon Interaction on Spin Fluctuation Induced Superconductivity

J_{cyc}/J_\perp \approx 0.20-0.27

Mid-infrared absorption in YBa2Cu3O6: Failure of spin-wave theory in undoped cuprates

and

Local Electronic Structure of Bi~2Sr~2CaCu~2O~8 near Oxygen Dopants: A Window on the High-T~c Pairing Mechanism

J/J_{\perp} \approx 1.25-1.35

Current-induced switching in transport through anisotropic magnetic molecules

. With these coupling ratios the recently developed dynamical DMRG yields an excellent description of the entire S=0 excitation spectrum observed in the optical conductivity, including the continuum contribution.

Local electronic structure near oxygen dopants in BSCCO-2212: a window on the high-Tc pair mechanism?

We investigate the interplay of the electron-phonon and the spin fluctuation interaction for the superconducting state of YBa

Relaxation mechanisms of the persistent spin helix

_2

Thermodynamic transitions in inhomogeneous d -wave superconductors

Cu