X-Ray and Neutron Scattering Studies of Underdoped La2-xBaxCuO4 Single Crystals

Abstract

<p> The interplay between superconductivity, magnetism and crystal structure is a central issue in the study of the high Tc cuprates. The transition metal compound La2-xBaxCuO4 (LBCO) was the very first high Tc superconductor, discovered by J. G. Bednorz and K. A. Muller in 1986. However, it has been much less extensively studied than other high Tc materials, due to the difficulty of growing large single crystals. With our Image Furnace facility, we have successfully grown high quality, large LBCO crystals (with x~0.095 , 0.08, 0.05 and 0.025) on the underdoped side of the well known 1/8 (x=0.125) anomaly in this material's phase diagram.</p> <p> Using our rotating anode X-ray source at McMaster University we have performed high-resolution X-ray diffraction studies on our x=0.095 and 0.08 samples and on a 1/8 doped LBCO (x=0.125) crystal grown by our collaborators. The X-ray study mapped out a sequence of tetragonal and orthorhombic crystal structures with temperature, which were known from earlier measurements.</p> <p>We have also performed neutron scattering studies at several Laboratories on x=0.095 , 0.08, 0.05 and 0.025 samples. We observed elastic spin incommensurate Bragg peaks in all samples, and inelastic measurements on the x=0.095 sample allowed us to explore the material's low energy spin fluctuations. The elastic neutron scattering results of higher doped samples (x=0.095 and 0.08) indicate that "collinear" static incommensurate magnetic ordering develops below the low temperature structural phase transition, and this order persists into the superconducting state. Static incommensurate magnetic order is also observed in the La2-xBaxCuO4 (x=0.05 and 0.025) compounds with ordering wavevectors which are rotated by 45° about the commensurate (0.5 ,0.5 ,0) position, with respect to that in the superconducting x=0.095 and 0.08 samples. These spin modulations are one dimensional in the x=0.05 and 0.025 samples, with ordering wavevectors lying along the orthorhombic b* direction. Such a rotation in the orientation of the static spin ordering as a function of increasing Ba doping, from diagonal to collinear is roughly coincident with the transition from an insulating to a superconducting ground state and is similar to that observed in the related La2-xSrxCuO4 system. The low energy, inelastic neutron scattering studies show that the dynamic spin susceptibility for x=0.095 is constant within the superconducting state and decreases as the temperature rises above Tc. </p>