We investigate the influence of a cyclic spin exchange Jcyc 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 Jcyc due to a repulsion between triplets on neighboring rungs. In (La,Ca)14Cu24O41 a consistent description of both the spin gap and the S=0 bound state requires Jcyc/J ⊥ ≈ 0.20-0.27 and J/J ⊥ ≈ 1.25-1.35. 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.PACS numbers: 75.10. Jm, 75.40.Gb, 75.40.Mg, 74.72.Jt, 75.30.Et The antiferromagnetic parent compounds of the high-T c cuprates are thought to be the best representatives of the two-dimensional S=1/2 square-lattice Heisenberg model. Understanding their magnetic properties is of utmost importance due to the intimate relationship of magnetic correlations and high-T c superconductivity. Recently, the question how to set up a minimal model which accounts for these magnetic properties has been readdressed. High-resolution inelastic neutron scattering experiments performed on the two-dimensional S=1/2 antiferromagnet La 2 CuO 4 [1] exhibit a magnon dispersion at the zone boundary which cannot be obtained within a nearest-neighbor Heisenberg model. It has been argued that the inclusion of a cyclic spin exchange term of about 20% would reproduce this dispersion [2]. This cyclic spin exchange emerges as a correction to the nearestneighbor Heisenberg Hamiltonian in order t 4 /U 3 from a t/U -expansion of the one-band Hubbard model [3]. It is expected to be the dominant correction within a more realistic three-band description of the CuO 2 -planes because there the cyclic permutation of 4 spins on a plaquette can take place without double occupancy [4,5]. Similar cyclic spin exchange processes have proven to be significant in other systems, e.g. in cuprate spin chains a ferromagnetic 2-spin cyclic exchange is responsible for the unusually strong exchange anisotropy [6].Cuprate spin ladders offer an alternative approach to decide about the existence and potential implications of a cyclic spin exchange term. They are composed of the same corner-sharing Cu-O plaquettes as the 2D cuprates, thus similar exchange couplings are expected for all spin products. In fact the inclusion of a cyclic spin exchange has been suggested in order to explain the smallness of the ladder spin gap observed in (La,Ca) 14 Cu 24 O 41 [7][8][9]. However, it is impossible to extract a unique set of coupling parameters or even to settle the existence of a cyclic spin exchange term from an analysis of the spin gap only. Here, the optical conductivity σ(ω) [10,11] can provide the missing information. Magnetic excitations can be observed in σ(ω) via the simultaneous excitation of a phonon [12,13]. In (La,Ca) 14 Cu 24 O 41 , two peaks in σ(ω) were identified as S=0 bound states of tw...

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.

One challenge in condensed-matter physics is the experimental confirmation of a new kind of elementary excitation orbital waves, or orbitons, which are predicted to exist in an orbitally ordered state. Saitoh et al. have observed three peaks at 160, 144 and 126 meV in the Raman scattering of orbitally ordered lanthanum manganate (LaMnO(3)), and interpret these as evidence of orbitons. However, we find similar peaks in the optical conductivity, sigma(omega), of LaMnO(3) and point out that the direct observation of orbitons in sigma(omega) is prohibited by a selection rule. This suggests that the Raman peaks observed by Saitoh et al. arise from multiphonons, and that the existence of orbitons has yet to be experimentally confirmed.

The ab-plane optical spectra of one underdoped and one nearly optimally doped single crystal of HgBa2Ca2Cu3O 8+δ were investigated in the frequency range from 40 to 40,000 cm −1 . The frequency dependent scattering rate was obtained by Kramers Kronig analysis of the reflectance. Both crystals have a scattering rate gap of about 1000 cm −1 which is much larger than the 700 cm −1 gap seen in optical studies of several cuprates with maximum Tc around 93 K. There appears to be a universal scaling between scattering rate gap and maximum Tc for the cuprate superconductors.PACS numbers: 74.25. Gz, 74.72.Gr, 78.20.Ci Although high temperature superconductivity in the copper oxides was discovered over a decade ago, an understanding of the mechanism that gives rise to the high transition temperature, T c , is still elusive. Most of the research has been directed towards the one-and twolayer systems which have a maximum T c in the 93 K range. Recently, however, high quality single crystals of the three-layer Hg based materials with a maximum T c ≈ 135 K have become available [1] making it possible to examine, spectroscopically, oxide superconductors that have significantly higher T c .Of particular interest is the magnitude of the normal state pseudogap. This is a partial suppression of the density of low energy excitations seen well above T c in all underdoped high temperature superconductors. A pseudogap has been seen with a variety of techniques such as angle-resolved photoemission (ARPES), tunneling spectroscopy, specific heat, dc resistivity, nuclear magnetic resonance and optical spectroscopy [2]. Measurements of dc resistivity [3] and NMR 63 Cu T 1 [4] on the threelayer underdoped HgBa 2 Ca 2 Cu 3 O 8+δ (Hg-1223) show evidence of a pseudogap with onset temperatures, T * , of 320 K and 230 K respectively.The size of the pseudogap in the one-and two-layer materials is of the order of 9.5k B T c at optimal doping and essentially independent of temperature. As T c is reduced below optimal doping the pseudogap increases in size [5][6][7].In ab-plane infrared response, the pseudogap is most clearly seen in the frequency-dependent scattering rate which can be calculated from the reflectance after Kramers-Kronig analysis. Here the pseudogap is a suppression of scattering below a characteristic energy which is taken to be a measure of the "size" of the pseudogap.

Magnetic excitations in two-leg S=1/2 ladders are studied both experimentally and theoretically. Experimentally, we report on the reflectivity, the transmission and the optical conductivity sigma(omega) of undoped La_x Ca_14-x Cu_24 O_41 for x=4, 5, and 5.2. Using two different theoretical approaches (Jordan-Wigner fermions and perturbation theory), we calculate the dispersion of the elementary triplets, the optical conductivity and the momentum-resolved spectral density of two-triplet excitations for 0.2 <= J_parallel/J_perpendicular <= 1.2. We discuss phonon-assisted two-triplet absorption, the existence of two-triplet bound states, the two-triplet continuum, and the size of the exchange parameters.Comment: 6 pages, 7 eps figures, submitted to SNS 200