Optical spectroscopy of (La,Ca)14Cu24O41 spin ladders: comparison of experiment and theory

Abstract

Transmission and reflectivity of La x Ca 14−x Cu 24 O 41 two-leg spin-1/2 ladders were measured in the mid-infrared regime between 500 and 12 000 cm −1 . This allows us to determine the optical conductivity σ 1 directly and with high sensitivity. Here we show data for x=4 and 5 with the electrical field polarized parallel to the rungs (E || a) and to the legs (E || c). Three characteristic peaks are identified as magnetic excitations by comparison with two different theoretical calculations.Key words: spin ladder, bound state, phonon-assisted 2-magnon absorptionThe quantum nature of magnetic excitations in spin-1/2 systems and in particular the role of quantum fluctuations in low dimensions are a fascinating subject. Antiferromagnetic (AF) S=1/2 Heisenberg ladders represent an intermediate step between one-dimensional (1D) chains and the 2D CuO 2 layers of undoped high-T c superconductors. The elementary excitations of the ladders can be described as triplets or as interacting spinons. Topics of current interest are theoretical predictions of 2-triplet bound states [1], the size of the exchange coupling along the rungs (J ⊥ ) and the legs (J ) as well as the role of the ring exchange J cyc [2]. We address these issues in La x Ca 14−x Cu 24 O 41 which contains layers with Cu 2 O 3 two-leg AF S=1/2 ladders [3].

Dynamics ofS=1/2 Two-Leg Spin Ladder Systems

“…1) -7) Twomagnon singlet bound-state was observed recently by phonon-assisted optical absorption. 8), 9) From the theoretical analysis of the experimental findings, it was confirmed that the above picture of the elementary excitation holds even in J ⊥ /J ∼ 1. 7), 9) Furthermore, momentum dependence of the one-triplet and two-magnon spectral densities was calculated for J ⊥ /J ≥ 1.…”

“…8), 9) From the theoretical analysis of the experimental findings, it was confirmed that the above picture of the elementary excitation holds even in J ⊥ /J ∼ 1. 7), 9) Furthermore, momentum dependence of the one-triplet and two-magnon spectral densities was calculated for J ⊥ /J ≥ 1. 7), 9) In this paper, we investigate dynamical properties of S = 1/2 two-leg spin ladder systems from strong to weak interchain-coupling regions.…”

“…7), 9) Furthermore, momentum dependence of the one-triplet and two-magnon spectral densities was calculated for J ⊥ /J ≥ 1. 7), 9) In this paper, we investigate dynamical properties of S = 1/2 two-leg spin ladder systems from strong to weak interchain-coupling regions. Using a continued fraction method based on Lanczos algorithm, 10) the dynamical structure factor (DSF) is calculated.…”

Dynamical structure factors ofS=12two-leg spin-ladder systems

“…Such an S = 0 two-triplet bound-state was identified in the optical conductivity measured for an S = 1/2 two-leg spin ladder material (Ca, La) 14 Cu 24 O 41 [9]. In this experiment, the strength of the interchain coupling (J ⊥ ) and the intrachain coupling (J ) was estimated as 9,10]. In addition to the interchain and intrachain couplings, a cyclic four-spin interaction, which acts among four S = 1/2 spins forming a plaquette, has been introduced to explain experimental findings for cuprate two-leg spin ladder systems.…”

Boundaries, cusps, and discontinuities in the multimagnon continua of one-dimensional quantum spin systems

“…Theorists have long predicted that some of these systems will possess bound states, 1 and there are now experimental indications of such bound states in the alternating chain material copper nitrate 2,3 and in the spin ladder (Ca,La) 14 Cu 24 O 41 . 4,5 In addition to the bound modes there is a continuum of two-magnon states, which has been reported in inelastic neutron scattering from polycrystalline 6 Sr 0.73 CuO 2 ͑assuming an alternating chain model͒ and copper nitrate single crystals. 2,3 Although this continuum has been considered a rather uninteresting feature in comparison with the bound states, it actually possesses an interesting and rather complicated internal structure.…”