# Presentation: 590199

## Doped 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.

- Doped 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

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

**Notes:**
Doped rare-earth manganites R1-xAxMnO3 (R being a trivalent rare earth and A being a divalent alkaline-earth ion) exhibit a large diversity in electronic, magnetic, and orbital states .  Application of modest external fields may drastically modify the state of such materials (especially of charge-ordered (CO) manganites), e. g., induce an insulator to metal transition.  One can anticipate that a high refractive index contrast between the photoinduced conducting and insulating phases of charge-ordered manganites would be sufficient for development of a photonic crystal material, which allows control of dispersion and propagation of light. Since visible light penetrates into the bulk of this material only to a depth of about 300 nm, we created thin films of Bi1-xCaxMnO3 [1].  These thin films have the charge ordering transition around room temperature, a useful feature for device applications.  Another important point for development of manganite-based photonic devices is the possibility of coexistence of two phases with high refractive index contrast on a sub-micron scale, in this case charge-ordered insulating and charge-disordered metallic phases.   A unique and attractive feature of manganites is that different competing phases, such as phases with different magnetic ordering, charge-ordered and disordered phases, or metallic and insulating electronic phases can coexist for certain compositional and temperature ranges, or when material is influenced by external stimuli, such as magnetic, electric field, or electromagnetic wave irradiation. From this point of view it is important to understand the physics of photoinduced conductive and insulating charge-ordered phase coexistence in manganites, its dynamics and stability.
We have created a sub-micron phase coexistence of CO insulating and conducting phases after illuminating Bi0.4Ca0.6MnO3 thin film with visible light around 500 nm wavelength through an NSOM (Fig.1) tip.  All reflectivity images were taken using 633 nm light of low intensity.  Before illumination, the optical reflectivity of the sample is uniform (Fig. 1a).  Immediately after illumination bright spots corresponding to a conducting charge-disordered phase appear (Fig. 1b).  However, after approximately 7 min. the optical contrast diminishes (Fig. 1c), because the sample relaxes back to its CO ground state. 
In order to better understand the dynamics of this phase coexistence, we studied the time dependence of resistivity of Bi0.4Ca0.6MnO3 thin films after sample illumination was switched off at different temperatures.  After sample illumination is switched off, resistance of the sample increases exponentially to a value corresponding to a resistance value of CO phase (Fig. 2a, inset).  The time constant of such process was determined for different temperatures (Fig. 2a). In the temperature range from 120 K to 200 K the temperature dependence of this time constant can be approximated with an activation model: τ  exp E/kBT, where kB is the Boltzmann constant and  E is an energy barrier separating two local energy minima corresponding to CO insulating and charge-disordered conducting phases in the energy landscape.  This is the first measurement of the energy barrier in manganites. Recently it was predicted, that the energy landscape determines the scale of phase-separation in manganites, with larger energy barriers corresponding to phase coexistence on a sub-micron scale [2].  In our case, the value of the energy barrier is large, almost as large as the charge ordering temperature (TCO= 256 K in this case), and we have observed phase coexistence in this materials on the sub-micron scale, which is in agreement with predictions, made in [2]. 

[1]  V. N. Smolyaninova at al., Appl. Phys. Lett. 86, 071922 (2005).
[2]  K. H. Ahn at el., Nature 428, 401 (2004).  

## Outreach:

- 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).

- 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