Observation of the Transverse Optical Plasmon inSmLa0.8Sr0.2CuO4−δ

Abstract

We present microwave and infrared measurements on SmLa0.8Sr0.2CuO 4−δ , which are direct evidence for the existence of a transverse optical plasma mode, observed as a peak in the c-axis optical conductivity. This mode appears as a consequence of the existence of two different intrinsic Josephson couplings between the CuO2 layers, one with a Sm2O2 block layer, and the other one with a (La,Sr)2O 2−δ block layer. From the frequencies and the intensities of the collective modes we determine the value of the compressibility of the two dimensional electron fluid in the copper oxygen planes. 74.25.Gz In 1966 A.J. Leggett predicted for superconductors with two bands of charge carriers a collective oscillation corresponding to small fluctuations of the relative phases of the two condensates, briefly indicated as excitons below the superconducting gap [1]. In principle these excitons should be observable with electromagnetic radiation, but in practice the effect on the infrared optical properties of most superconducting materials has been too small to be observable, except for, as we will demonstrate in the present paper, the bi-layer cuprate superconductors. The cuprate high temperature superconductors naturally form weakly coupled stacks of superconducting layers [2]. Some members of this family, e.g. Bi 2 Sr 2 CaCu 2 O 8 , have two superconducting layers per unit cell. These materials are realizations of a two-band superconductor, satisfying the following unique conditions: (i) For polarization of the electric field perpendicular to the conducting planes the metallic screening is very weak due to strong anisotropy of the static and dynamical electrical conductivity. (ii) The dipole selection rules allow optical transitions which resonate at the Josephson exciton energy.In Ref.[3] two of us (DvdM and AAT) calculated the dielectric function for cuprate superconductors with two CuO 2 planes per unit cell, using the Lawrence-Doniach model [2] with alternating coupling constants (the 'multilayer model'). A direct consequence was the presence of a transverse optical plasma mode, polarized perpendicular to the planes for propagation along the planes. Similar to a transverse optical phonon, and in contrast to the transverse Josephson plasma resonance (JPR) in single layer cuprates, this mode is observable as an optically allowed absorption in measurements of the optical conductivity. In Fig. 1 close proximity to the optical phonons it was not possible to separate these electronic collective modes from the optical lattice vibrations [5], which complicates the quantitative analysis of this interesting phenomenon.Both in the two-band exciton model and in the multilayer model the collective modes are oscillations of the relative phase of the two condensates, the inertia of which is due to the finite Josephson coupling between the two condensates. In Ref.[1] the restoring force was provided by the fact, that if δN electrons are added to a plane, the free energy increases with an amount δF = µδN + δN 2 /(2Kn 2 ). Here µ, K, and ...

Reflectivity and Microwave Absorption in Crystals with Alternating Intrinsic Josephson Junctions

“…The Josephson plasma resonance (JPR) observed in the microwave absorption [1], and in optical reflection [2][3][4][5] and transmission measurements [6,7], has proven to be a powerful method to study the properties of highly anisotropic layered superconductors, such as vortex phases [1,8]. In particular, the spatial dispersion of the JPR, ω p (k, k z ), parallel (k) and perpendicular (k z ) of the superconducting layers reflects the inductive coupling of junctions due to intralayer currents and the charge coupling due to the variations of the electrochemical potential on the layers, respectively.…”

“…In Refs. [3][4][5] the reflectivity R and transmission of the electromagnetic wave propagating along the layers and with the electric field along the c-axis (cf. Fig.…”

“…The latter is irrelevant for the experiments in [2][3][4][5], which are performed with incidence parallel to the layers, which will be studied here. In the following we will show, how α can be extracted unambiguously for layered superconductors with a superstructure in c-direction from two independent measurements of the loss function and the magnetic field dependence of the plasma frequencies.…”

“…Recently, these experiments have been performed on SmLa 1−x Sr x CuO 4−δ , where magnetic Sm 2 O 2 and nonmagnetic La 2−x Sr x O 2−δ layers alternate in the barriers between the CuO 2 -layers [3][4][5][6]. In Refs.…”

Electronic compressibility and charge imbalance relaxation in cuprate superconductors

“…in TBCCO [1]). Recently, in the novel material SmLa1−xSrxCuO 4−δ with alternating junctions two plasma peaks were observed with a high ratio of their amplitudes [2,3]. This can only be explained, if the spatial dispersion of the JPR in c direction due to charge fluctuations of the superconducting condensate, is taken into account, see Fig.…”