Photo-modulated dynamic competition between metallic and insulating phases in a layered manganite
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Photo-modulated dynamic competition between metallic and insulating phases in a layered manganite Yuelin Li1*, Donald Walko1, Qing’an Li2, Yaohua Liu2, Stephan Rosenkranz2, Hong Zheng2, J. F Mitchell2, Haidan Wen1, Eric Dufresne1, and Bernhard Adams1 1
X-ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA
2
Material Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA
*[email protected]
ABSTRACT We show evidence that the competition between the antiferromagnetic metallic phase and the charge- and orbital-ordered insulating phase at the reentrant phase boundary of a layered manganite, LaSr2Mn2O7, can be manipulated using ultrafast optical excitation. The timedependent evolution of the Jahn-Teller superlattice reflection, the indicator of the formation of charge and orbital order, was measured at different laser fluences. The laser-induced change in the Jahn-Teller reflection intensity shows a reversal of sign between earlier (~10 ns) and later (~150 ns) times during the relaxation of the sample. This is consistent with a physics picture whereby the laser excitation modulates the local competition between the metallic and the insulating phases.
INTRODUCTION The charm of strongly correlated materials originates from competition among multiple symmetry-breaking quantum states, such as high-temperature superconductivity, and charge-, spin- and orbital-ordered phases [1]. Among them, colossal magnetoresistive (CMR) manganites are a prototypical system with complex coupling among charge, orbital, spin, and lattice orders. They have been widely studied due to the intriguing physics and technological potential for magnetic memory, logic, etc., arising from their half-metallicity controllable via magnetic field and light. By directly or indirectly modifying different degrees of freedom, many experiments have demonstrated the feasibility of using ultrafast laser pulses to create and probe novel transient states that are inaccessible via adiabatic or static thermal methods [2–4]. Manganites come in several families of structures. Bilayer manganites (La22xSr1+2xMn2O7) with reduced dimensionality [5,6] amplify the effect of entropy change in the temperature regime near the phase transitions and offers a unique opportunity to study the ordering dynamics. For x close to 0.5 and 0.6, a COO insulating phase is sandwiched between a
low-temperature antiferromagnetic metallic (AFMM) phase and a high-temperature (> 220 K) paramagnetic metallic (PMM) phase (Fig. 1(a)). The AFMM phase transforms into the COO phase when the transition temperature is approached from below. At x~0.5, the COO is of checkerboard form characterized by a 4 × 4 superlattice that causes Jahn-Teller (JT) distortion and a superlattice reflection [7–9]. EXPERIMENTAL DETAILS thisis
250
AFMM
1
3
100
2
2
COO
150
(c)
(b)
PMM
I (10 cps)
(a)
I (103 cps)
T (K)
200
warming cooling
50 0
0
0
0.48
0.50
x
0.52
0.54
1
0
50
100
150
200
Temperature (K)
250
0
2 4 6 2 Fluence (mJ/cm )
8
Fig
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