Dr Dharmalingam Prabhakaran

Abstract:

We report the first direct resonant soft x-ray scattering observations of orbital ordering. We have studied the low temperature phase of La0.5Sr1.5MnO4, a compound that displays charge and orbital ordering. Previous claims of orbital ordering in such materials have relied on observations at the manganese K edge. These claims have been questioned in several theoretical studies. Instead we have employed resonant soft x-ray scattering at the manganese L(III) and L(II) edges which probes the orbital ordering directly. Energy scans at constant wave vector are compared to theoretical predictions and suggest that at all temperatures there are two separate contributions to the scattering: direct orbital ordering and strong cooperative Jahn-Teller distortions of the Mn3+ ions.

Abstract:

To study the static and dynamic properties of the charge stripe phases in the nickelates by neutron scattering, single crystals are needed with a large volume ( >1 cm(3)). We will report the optimization and growth of large R2NiO4+delta (R = Pr, Nd) crystals by the floating-zone technique in an image furnace. Crack-free single crystals of diameter 8-10 mm and length 8090 mm have been obtained. The phase-purity of the crystals has been checked by powder X-ray diffraction, and information on the crystal quality has been obtained by X-ray and neutron diffraction. We will give an account of our investigations into the conditions needed to obtain the highest quality crystals, and we will discuss the magnetic studies by SQUID magnetometry. (C) 2003 Kluwer Academic Publishers.

Abstract:

Large single crystals of La2-2xSr1+2xMn2O7 (x= 0.3 to 0.525) have been prepared under controlled atmospheric conditions. The crystals were grown by the floating-zone technique in an image furnace under a mixed oxygen/argon atmosphere pressurized to 6-8 x 10(5) Pa. Rectangular single crystals with sizes up to 50 x 9 x 4 mm(3) have been obtained. The phase-purity, composition, and quality of the crystals were analyzed by X-ray diffraction and electron probe microanalysis. The magnetic behavior is found to be sensitive to the composition of the atmosphere during growth. (C) 2003 Kluwer Academic Publishers.

Abstract:

In this paper we present x-ray scattering results of charge and orbital ordering in the bilayer manganite La2−2xSr1+2xMn2O7 with doping levels x = 0.5 and x = 0.475. Using high-energy x-ray scattering, the structural modulation due to the Jahn-Teller ordering and the charge ordering due to the Mn3+/Mn4+ pattern have been measured. Both the x = 0.5 and x = 0.475 samples are found to display charge and Jahn-Teller order. We have confirmed that the wave vectors of the Jahn-Teller order, charge order, and orbital order are (Formula presented), (Formula presented) and (Formula presented). The origin of these has been confirmed by resonant x-ray scattering in the vicinity of the Mn K edge using polarization analysis. Contrary to previous studies, the Jahn-Teller order is found to be not reentrant, but is found to reduce in intensity at temperatures below 140 K for both samples. Charge ordering was also detected in the x = 0.5 sample below this temperature. © 2003 The American Physical Society.