Transport properties of La0:5Ca0:5MnO3, a highly disordered charge–density wave system

Differential resistivity and broadband noise measurements of La0:5Ca0:5MnO3 reveal behaviour typical of a highly disordered charge–density wave system. In addition, the differential resistivity measurements reveal a large hysteresis, with the upper part of the hysteresis curve only appearing when the sample has been annealed by heating to room temperature and then cooling. The variation of the area of the hysteresis loop with temperature is found to be governed by a power law. & 2008 Elsevier B.V. All rights reserved. Charge–density wave (CDW) systems have long been known for their dramatic resistance properties [1]. More recently, the discovery of CDW type behaviour in cuprate [2,3] and manganite [4] systems has indicated that stripe phases which were attributed to strong charge localisation at atomic sites can sometimes be more accurately described by a CDW type model. Stripe phases are currently of great interest since the role of the stripe phase in the high temperature superconductivity mechanism in the cuprates is currently unknown. The signatures of CDW sliding that can be observed in materials such as the cuprates and manganites are somewhat different than those observed in traditional CDW systems, due to the extremely high level of disorder. The short disorder lengthscale (i.e. an impurity spacing rather than a coherence lengthscale) in the manganites of around 25 Å [5] indicates that the disorder may be created by the mismatch between the two varieties of A-site cation. Because of this high level of disorder, it is not possible to observe a narrow band noise signal. The observation of CDW sliding also raises the question: how strong is the electron–phonon coupling in these materials? If the electron–phonon coupling is weak, as expected in a sliding CDW system, it is difficult to explain the fact that these materials are insulating at high temperatures. One possible explanation is that the stripe phase is not fully destroyed at the transition; it still persists over short lengthscales [6,7]. Alternatively, if electron– phonon coupling is strong, sliding would only be observed if the coupling is frustrated, possibly by short range strain fields produced by a high level of disorder [8]. In this case disorder plays a key role in producing the CDW-type behaviour, since in its ll rights reserved. ivision of Cell and Molecular absence the system would manifest strong electron–phonon coupling, and it would not be possible to depin the superstructure. Although the effect of very low levels of disorder on CDW systems has been studied [9,10], the equivalent experiments on highly disordered CDWs were lacking. Here we investigate the properties of the stripe phase of La0:5Ca0:5MnO3, a highly disordered CDW. We find that, as would be expected, the high level of disorder leads to numerous pinning–depinning events, and thus a high level of broadband noise (see Fig. 1). However, we also observe a hysteresis effect which manifests as a difference between the first time the CDW is depinned after cooling, and subsequent electric field sweeps. Although this type of behaviour is expected in a CDW system, the magnitude of the effect is exceptionally large. The area enclosed by the hysteresis loop appears to follow a scaling behaviour with temperature. An 80nm thick La0:5Ca0:5MnO3 thin film was grown on a NdGaO3 substrate as described in Ref. [11] to produce a filmwith a uniaxial stripe phase which becomes detectable at 190K and reaches a stable wavevector value at 90K [11]. The superstructure was identified via superlattice reflections in a transmission electron microscopy selected area diffraction pattern [4]. For the resistance measurements, gold wires were attached to the thin film sample using graphite paint. Resistance measurements were carried out in two configurations: four point measurements for the resistivity and two point measurements for the noise measurements. The differential resistance measurements were carried out using a 17Hz AC current plus a DC bias. The AC component of the voltage (proportional to the differential resistance) was measured using a lock-in amplifier. The variation of the resistivity of La0:5Ca0:5MnO3 with temperature showed no clear feature at the expected ordering temperature, as is seen in other prototypical CDW systems which have been doped with impurities [12–14]. This is interpreted as a large density of impurities, or a high level of disorder (as expected