Polaronic signature in the metallic phase ofLa0.7Ca0.3MnO3films detected by scanning tunneling spectroscopy

Abstract

In this work we map tunnel conductance curves with nanometric spatial resolution, tracking polaronic quasiparticle excitations when cooling across the insulator-to-metal transition in La0.7Ca0.3MnO3 films. In the insulating phase the spectral signature of polarons, a depletion of conductance at low bias flanked by peaks, is detected all over the scanned surface. These features are still observed at the transition and persist on cooling into the metallic phase. Polaron-binding-energy maps reveal that polarons are not confined to regions embedded in a highly-conducting matrix but are present over the whole field of view both above and below the transition temperature. 71.30.+h, 68.37.Ef In manganite compounds such as La 1−x Ca x MnO 3 (0.2 < x < 0.5), the coupling between lattice, magnetism and transport leads to a transition from metallic (dρ/dT > 0) to insulating (dρ/dT > 0) behavior in tune with the suppression of ferromagnetic order at a temperature T MI [1,2]. Understanding the transport properties of the insulating phase required the consideration of the electron-phonon coupling mechanism in addition to double-exchange interactions [3,4]. In the strong coupling limit electrons are bound by a surrounding lattice distortion forming polaronic quasiparticles [3,4]. In the insulating phase, polaron hopping is the dominant transport mechanism and gives rise to the measured thermally-activated resistivity [5,6,7]. Although in these context theoretical predictions [3,4] state that on cooling below T MI spin order leads to electron delocalization, evidence from numerous experimental techniques [8,9,10,11,12,13] suggests the presence of polarons also at T < T MI . Optical reflectivity [14] and structural [15,16] investigations propose a crossover from a large to small-polaron regime on warming across the MIT and a coexistence of both polaronic phases at T ∼ T MI . Therefore, the existence of polarons in the metallic phase and the relevance of nanoscale inhomogeneities [2] to the metal-to-insulator transition (MIT) remain active subjects of debate. Insight into this problem can be gained from scanning tunnelling spectroscopy (STS) studies as the one presented here which probes local electronic properties at the atomic scale.Although the phase separation scenario has gained popularity in recent years, it is mostly supported by results from macroscopic techniques [2]. Local spectroscopy studies in La 1−x Ca x MnO 3 claim electronic phase separation [17,18,19,20], but a strong influence of chemical and structural disorder present in the samples cannot be ruled out. In contrast, a recent study in a fullyrelaxed film reports homogeneous conductance maps [21]. * Electronic address: silvia.seiro@physics.unige.ch However, conductance maps at a fixed energy do not provide enough information on the eventual presence, spectroscopic characteristics and spatial distribution of polaronic quasiparticles. Therefore, in this work we measure tunnel current vs. voltage maps with nanometric spatial resolution in La 0.7 Ca 0.3 MnO 3 (LC...

Griffiths phase, metal-insulator transition, and magnetoresistance of doped manganites

“…Although some experimental data are consistent with a polaron collapse at T MI , other reports ͑see, e.g., Ref. 50 and references therein͒ provide evidence for the presence of polarons in the ferromagnetic metallic phase, as well. Within the framework of the phenomenological model proposed in this work, it is reasonable to attribute a polaronic type of conductivity to superparamagnetic hysterons.…”

Stripe charge ordering in SrO-terminated SrTiO3(001) surfaces

“…Furthermore, a small-polaron ordering was observed along with the charge ordering, which indicates a very strong electronphonon coupling in this system [26]. Polaronic quasiparticle excitations through inelastic tunneling processes were reported in manganite systems, with energy gaps evident in the tunneling spectra even in the metallic phase [27,28].…”

Direct real space observation of magneto-electronic inhomogeneity in ultra-thin film La0.5Sr0.5CoO3−δ on SrTiO3(001)

“…As in prior STM/STS work on electronically inhomogeneous manganites, [24][25][26][27][28] contiguous nanoscale regions of similar differential conductance are evident, conductive clusters forming in an insulating matrix. The extent of this inhomogeneity is reinforced by Fig.…”