Photoinduced phase-separation in Bi0.4Ca0.6MnO3 thin films

Vera Smolyaninova DMR-0348939Doped rare-earth manganese oxides (manganites) exhibit a wide variety of physical phenomena due to complex interplay of electronic, magnetic, orbital, and structural degrees of freedom. One of the most intriguing properties of manganites is coexistence of two (or several) distinct electronic phases. A photoinduced insulator to conductor transition in charge-

ordered (CO) manganites is especially interesting from the point of view of creating photonic devices. We have created a sub-micron phase coexistence of CO insulating phase (Fig. 1, darker regions) and conducting (lighter regions) by illuminating a Bi0.4Ca0.6MnO3 thin film with visible light (~500 nm) through the tip of a NSOM. Such phase coexistence is possible because of the presence of two local energy minima corresponding to CO insulating and charge-disordered conducting phases in the energy landscape. To better understand the physics of phase coexistence in manganites we studied the process of relaxation from the photoinduced conducting phase (shallower energy minimum) to CO insulating state (deeper energy minimum). Time dependencies of resistivity were measured after sample illumination was switched off. An exponential increase of resistivity was observed at different temperatures (Fig. 2). If approximated with an activation model, the temperature dependence of the relaxation time constant gives a value of E = 235 K (20.3 meV) for the energy barrier separating the CO insulating and conducting states.

Fi

0.005 0.006 0.007 0.008

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55

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ec)

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T = 140 K

49s

τ exp E/kBT

Fig. 1 .1.7x1.7 μ near-field optical microscope (NSOM) image of reflectivity of Bi0.4Ca0.6MnO3 thin film

(a) before illumination with 500 nm wavelength light, (b) after illumination, (c) 7 min after illumination.

Fig.2. Inset shows time dependence of the resistivity at T = 140 K after sample illumination was switched off. Red curve is a fit to an exponential dependence with a time constant of 49 s. (a) Temperature dependence of the time constant for the process shown in the inset. (b) Temperature dependence of the time constant plotted as ln τ vs. 1/T. Red line is a fit to activation model.

(a) (b)

(a) (b) (c)

Education:Towson University is an undergraduate institution with strong emphases on undergraduate research. Two Towson students, Robert Kennedy, and Scott Casey, and one College of Notre Dame (local college, which does not have a research program) student Kim Wall were involved in this research.Robert Kennedy presented his research at the Materials Research Society Fall 2004 Meeting, Boston, MA. Robert Kennedy and Scott Casey presented the results of their research at the University System of Maryland Student Research Day in Annapolis, March 2005, where Maryland students presented their undergraduate research to state legislators.

Outreach:

The PI participated as a mentor scientist for a high school student David Kent in the Academy for Applied Science Intern Program (three week program, Summer 2005).

Photoinduced phase-separation in Bi0.4Ca0.6MnO3 thin films

Vera Smolyaninova DMR-0348939

Undergraduate student Robert Kennedy and high school intern David Kent are conducting NSOM measurements.

Undergraduate students R. Kennedy and S. Casey with House of Representative Delegate Susan Aumann presenting their poster “Photoinduced resistivity changes in Bi0.4Ca0.6MnO3 thin

films” at the USM Student Research Day in

Annapolis, March 2005