We report the observation of a large 1/f noise in the ferromagnetic insulating state (FMI) of a hole doped manganite single crystal of La0.80Ca0.20MnO3 which manifests hopping conductivity in presence of a Coulomb gap. The temperature dependent noise magnitude shows a deep within the FMI state, there is a sharp freeze out of the noise magnitude with temperature on cooling. As the material enters the FMI state, the noise becomes non-Gaussian as seen through probability density function and second spectra. It is proposed to arise from charge fluctuations in a correlated glassy phase of the polaronic carriers which develop in these systems as reported in recent simulation studies.PACS numbers: 71.30.+h, 71.27.+a, 72.70.+m Electronic transport through localized states in disordered and correlated electronic systems has been a topic of considerable interest [1,2]. In such systems, long range Coulomb interaction can lead to opening-up of a soft gap in the density of states (referred to as Coulomb gap, ∆ CG ) and hopping conduction in presence of such a gap [3]. Another consequence is emergence of "glassy" slow relaxations of charge carriers arising from a large number of low-lying states separated by barriers [4]. Such a glassy state can lead to enhanced low frequency (f ) non-Gaussian resistance noise (typically with power spectrum varying as 1/f ) arising from charge fluctuations [5,6]. These issues in a Coulomb glass have been reviewed recently [2]. The experimental investigations on these questions are few and were carried out only in the doped semiconductors with electron density close to the critical concentration (n C ) for metal-insulator (MI) transition [7,8] or in 2DEG in MOSFET's[9]. Here, we focus on the issue of non-Gaussian low frequency noise in the Coulomb glass state of a very different material, namely the low hole-doped rare-earth manganites which can have a ferromagnetic insulating (FMI) state below a certain temperature (T ). In these systems, the Coulomb glass phase occurs for the localized polaronic carriers (in sharp contrast to doped semiconductors) which arise due to strong electron-phonon coupling arising from JahnTeller distortion around the Mn 3+ ions.The FMI state of hole doped manganites (La 1−x Ca x MnO 3 ) arises when hole concentration x in LaMnO 3 exceeds the critical concentration for * email: sudeshna@bose.res.in † email: arup@bose.res.in, ferromagnetism x ≈ 0.15 yet it is smaller than the concentration needed for the formation of the metallic ground state x ≥ x C = 0.22. Existence of a Coulomb glass behavior in the FMI state has been inferred from certain transport experiments [10,11] and also predicted theoretically recently [12]. The FMI state in manganite is unique [13]. We observe a large low frequency resistance fluctuations (noise) with non-trivial T dependence in the FMI state and the noise becomes non-Gaussian at temperatures below the onset of the insulating state. At low T well below the transition to the insulating state the noise shows a sharp fall on cooling. We pr...
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