Microscopic structure and formation mechanism of the current-stabilized conducting path in the light-induced insulator–metal transition of Pr$_{0.7}$Ca$_{0.3}$MnO$_3$

In charge-ordered Pr$_{0.7}$Ca$_{0.3}$MnO$_3$, Fiebig, Miyano, Tomioka and Tokura (Science 280, 1925, 1998) showed that a nanosecond laser pulse under an applied bias creates a well-localized conducting path between electrodes while the crystal bulk stays insulating, and that the path can be visualized through the reflectivity change of the insulator–metal transition. The path was only resolved optically (about 30 µm at 10 mA), and several fundamental questions were left open: is it a single continuous metallic filament or a dense percolating network of metallic patches; what makes it spontaneously shift position with increasing current; by what mechanism an existing path induces a new one without illumination; whether its edges are a sharp phase boundary or a partially converted percolation zone; and how the antiferromagnetic charge-ordered background controls path formation, stability and competition. No work is known that resolves these questions for the bulk-crystal current-stabilized path, so the problem remains open.

Background

Colossal magnetoresistance (CMR) denotes the enormous drop of electrical resistance that mixed-valence perovskite manganites such as La$_{1-x}$Ca$_x$MnO$_3$ display under a magnetic field near their ordering temperatures; the effect arises because the material sits close to a first-order boundary between a ferromagnetic metallic phase and an insulating phase with charge, orbital and spin order. Charge ordering is the periodic arrangement of Mn$^{3+}$ and Mn$^{4+}$ valences, which in Pr$_{1-x}$Ca$_x$MnO$_3$ (PCMO) at $x\approx 0.3$ stabilizes an antiferromagnetic insulating ground state at low temperature. Phase separation means that submicrometer regions of the competing metallic and insulating phases coexist and that transport through the mixed state is percolative, that is, governed by the appearance of a connected network of the conducting minority phase. The insulating state of PCMO can be melted by many stimuli (magnetic field, pressure, X-rays, visible light, electric current). Miyano et al. discovered the photoinduced version (Phys. Rev. Lett. 78, 4257 (1997)): a short light pulse converts a thin surface layer to a metallic state. Fiebig et al. (Science 280, 1925 (1998)) then showed that when a voltage of 10–100 V is applied across gold electrodes with a 150 µm gap on a PCMO crystal at $T=30$ K, a 7 ns, 1064 nm laser pulse nucleates a localized conducting path that bridges the electrodes and persists under bias after the light is off, while the rest of the crystal remains insulating. Because the metallic phase has a different reflectivity spectrum from the insulating phase, differential reflection imaging (imaging the ratio of reflectivities before and after the transition) makes the path visible: its width grows from about 30 µm at $I=10$ mA to about 250 µm at 100 mA, while the reflectivity contrast at its center stays constant, indicating a fully converted core with partially converted edges. The path shows negative differential resistance ($\mathrm{d}U/\mathrm{d}I<0$): it carries more current at lower voltage drop, so two nearby paths on separate electrode pairs compete, and the weaker one disappears unless the resistance connecting their anodes exceeds a critical value. Most strikingly, a new path can be created between a second electrode pair without any laser illumination if another path is already carrying current elsewhere on the same crystal, over distances much larger than the metallic region detected optically. Two companion papers by the same group (J. Appl. Phys. 85, 5561 (1999), and Appl. Phys. Lett. 74, 2310 (1999)) proposed a two-stage mechanism for the local transition and showed that the spectrum of the photoinduced local metallic phase differs from the magnetic-field-induced bulk metallic phase, which they attributed to the competition of the ferromagnetic metallic region with its antiferromagnetic insulating surroundings. A separate, later line of research on thin-film PCMO junctions (electric-pulse-induced resistive switching, the basis of 'ReRAM' nonvolatile memories) found interface-limited, oxygen-vacancy-related switching mechanisms; electroforming is the initial high-voltage step that creates the conducting state in such devices. That literature concerns films and junctions, mostly at room temperature, and is distinct from the current-stabilized path in the bulk crystal, which is the subject of this problem.

Problem Statement

Determine the microscopic structure and the formation, stability and induction mechanisms of the localized conducting path that a light pulse nucleates and a transport current stabilizes in a charge-ordered Pr$_{0.7}$Ca$_{0.3}$MnO$_3$ single crystal, as observed by Fiebig et al. (Science 280, 1925 (1998)). The common objective of all requested results is a microscopic, mechanism-level account of the current-stabilized metallic state and of its interface with the antiferromagnetic charge-ordered insulating matrix. Specifically: (i) resolve the structure of the path below the optical resolution of roughly 30 µm — is it a single continuous metallic filament or a dense, percolating cluster of nanoscale metallic patches, and how does this structure evolve with current (the path widens from about 30 µm at $I=10$ mA to about 250 µm at 100 mA while its core reflectivity stays fully metallic)? (ii) identify the mechanism of the spontaneous positional movement of the path that is occasionally observed when the current is increased; (iii) identify the physical mechanism by which an existing current-carrying path induces a new conducting path at a different electrode pair without any illumination (distinguishing strain propagation, electric-field, magnetic-coupling and electrochemical or thermal contributions, and explaining the observed millimeter-range induction distance and its dependence on current and separation); (iv) establish the nature of the gradual reflectivity transition at the path edges — a partially converted region with a sharp phase boundary, or a percolation zone of nanoscale metallic clusters whose density falls with distance; and (v) determine the role of magnetic correlations, in particular of the antiferromagnetic charge-ordered background, in the formation, stability and competition of paths (including the origin of their negative differential resistance and of the rule that the path carrying the smaller current vanishes). Items (i)–(v) are jointly required results. An optional application-level extension is (vi) assessing whether such optically switchable paths can form the basis of a practical switch (scaling, response time and endurance); it is downstream of the microscopic questions and not required.

References

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