Charged magnons and magneto-elastic polarons in the mid-infrared spectrum of YBa2Cu3O6

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Abstract

The optical conductivity of undoped and low-doped YBa2Cu3O6 is studied in detail in the mid-infrared range. Full isotope substitution of 16O with 18O is used to (a) study the optical phonons in the far-infrared and (b) identify the rich structure emerging in the mid-infrared from multiphonon and bimagnon-plus-phonon absorption. Substitution of Zn in the CuO2 planes allows direct observation of bimagnon excitations. Very light chemical doping with oxygen introduces localized charge carriers in the form of magneto-elastic polarons, i.e., holes which are strongly coupled both to the vibrational and magnetic degrees of freedom.

Introduction

The low-energy dynamics and in particular the infrared optical conductivity σ1(ω) of holes doped into a 2D antiferromagnet have been the subject of intensive theoretical investigations (for recent reviews, see Refs. 1, 2, 28, 29). But experimentally, the influence of very low levels of doping on the optical conductivity of the cuprates has not been studied so far. In the undoped parent compounds a contribution of magnetic excitations to the mid-infrared (MIR) optical conductivity has been established. The main absorption peak at about 0.4 eV in the MIR transmission spectra of La2CuO4 and other single layer cuprates [3]has been interpreted by Lorenzana and Sawatzky 4, 5in terms of bimagnon-plus-phonon absorption. A similar feature was reported in the bilayer system YBa2Cu3O6 [6], in the 2D S=1 system La2NiO4 5, 7and in 1D S=1/2 Sr2CuO3 8, 9. In these compounds, direct bimagnon absorption is not dipole active due to symmetry, which can be broken by exciting a phonon simultaneously.

Here, we use full 18O substitution to identify the prominent resonances in the MIR spectrum of YBa2Cu3O6. In particular we use this to distinguish vibrational from electronic degrees of freedom and to give direct experimental evidence for the existence of bimagnon-plus-phonon absorption. Symmetry breaking also occurs if Zn is substituted into the CuO2 planes. We report the observation of direct bimagnon absorption in a sample with 3% of Zn substitution. Recently, similar `charged magnons' have been observed in the two-leg ladder compound [10]α′-NaV2O5. In order to study the influence of low chemical doping we will compare spectra of eight different samples of YBa2Cu3Ox with 6.0≤x≪6.1, corresponding to an effective number of carriers of ≤5×1019 cm−3. In this paper, we will concentrate on the experimental results; a more detailed theoretical analysis will be given elsewhere.

Section snippets

Experimental

Single crystals of YBa2Cu3O7 were grown using the recently developed BaZrO3 (BZO) crucibles 11, 12, which in contrast to other container materials do not pollute the resulting crystals. Crystals grown using this technique exhibit therefore a superior purity (>99.995 at.%) [13]. To exchange the oxygen isotope, the crystals were annealed at 600°C in a sealed quartz ampoule containing a 99.5% isotope pure 18O atmosphere. The obtained weight gain corresponded to a complete exchange of the oxygen

Results

In Fig. 1 we display the optical conductivity σ1(ω) in the mid-infrared for frequencies just above the highest phonon. Please note the very low values of σ1(ω). The three panels show spectra of (from top to bottom): (a) five samples with zero to very little hole doping grown in YSZ crucibles, (b) a sample grown in a BZO crucible, (c) a sample with ≈3% of Zn substitution. At low frequencies the spectra are dominated by two-phonon absorption processes. Taking twice the highest phonon frequency

Acknowledgements

We gratefully acknowledge stimulating discussions with G.A. Sawatzky. This investigation was supported by the Netherlands Foundation for Fundamental Research on Matter (FOM) with financial aid from the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO).

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