High-resolution x-ray scattering studies of structural phase transitions in underdoped La_ {2− x} Ba_ {x} CuO_ {4}

Physical review. B, Condensed matter (Impact Factor: 3.77). 11/2007; 76(18). DOI: 10.1103/PhysRevB.76.184121
Source: arXiv

ABSTRACT We have studied structural phase transitions in high quality underdoped La2−xBaxCuO4 single crystals using high-resolution x-ray scattering techniques. Critical properties associated with the continuous high temperature tetragonal (HTT) (I4∕mmm) to middle temperature orthorhombic (MTO) (Bmab) phase transition were investigated in single crystal samples with x=0.125, 0.095, and 0.08, and we find that all behavior is consistent with three-dimensional XY criticality, as expected from theory. Power law behavior in the orthorhombic strain, 2(a−b)∕(a+b), is observed over a remarkably wide temperature range, spanning most of the MTO regime in the phase diagram. Low temperature measurements investigating the low temperature tetragonal (LTT) (P42∕ncm) phase, below the strongly discontinuous MTO→LTT phase transition, in x=0.125 and x=0.095 samples show that the LTT phase is characterized by relatively broad Bragg scattering, compared with that observed at related wave vectors in the HTT phase. This shows that the LTT phase is either an admixture of tetragonal and orthorhombic phases or that it is orthorhombic with very small orthorhombic strain, consistent with the “less orthorhombic” low temperature structure previously reported in mixed La2−xSrx−yBayCuO4 single crystals. We compare the complex temperature-composition phase diagram for the location of structural and superconducting phase transitions in underdoped La2−xBaxCuO4 and find good agreement with results obtained on polycrystalline samples.

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    ABSTRACT: We present neutron-scattering measurements on single crystals of lightly doped La2-xBaxCuO4, with 0≤x≤0.035. These reveal the evolution of the magnetism in this prototypical doped Mott insulator from a three-dimensional (3D) commensurate (C) antiferromagnetic ground state, which orders at a relatively high TN, to a two-dimensional (2D) incommensurate (IC) ground state with finite-ranged static correlations, which appear below a relatively low effective TN. At low temperatures, the 2D IC magnetism coexists with the 3D C magnetism for doping concentrations as low as ∼0.0125. We find no signal of a 3D C magnetic ground state by x∼0.025, consistent with the upper limit of x∼0.02 observed in the sister family of doped Mott insulators, La2-xSrxCuO4. The 2D IC ground states observed for 0.0125≤x≤0.035 are diagonal, and are rotated by 45 degrees within the orthorhombic basal plane compared with those previously reported for samples with superconducting ground states: La2-xBaxCuO4, with 0.05≤x≤0.095. We construct a phase diagram based solely on magnetic order-parameter measurements, which displays much of the complexity of standard high-temperature superconductivity phase diagrams discussed in the literature. Analysis of high-energy resolution inelastic neutron scattering at moderately low temperatures shows a progressive depletion of the very low-energy dynamic magnetic susceptibility as x increases from 0.0125 to 0.035. This low-energy, dynamic susceptibility falls off with increasing temperature on a scale much higher than the effective 2D IC TN appropriate to these materials. Appreciable dynamic 2D IC magnetic fluctuations inhabit much of the “pseudogap” regime of the phase diagram.
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    ABSTRACT: Using neutron scattering techniques, we have studied incommensurate spin ordering as well as low energy spin dynamics in single crystal underdoped \LBCO with x$\sim$0.095 and 0.08; high temperature superconductors with T$_C \sim$ 27 K and 29 K respectively. Static two dimensional incommensurate magnetic order appears below T$_N$=39.5 $\pm$ 0.3 K in \LBCO (x=0.095) and a similar temperature for x=0.08 within the low temperature tetragonal phase. The spin order is unaffected by either the onset of superconductivity or the application of magnetic fields of up to 7 Tesla applied along the c-axis in the x=0.095 sample. Such magnetic field {\it independent} behaviour is in marked contrast with the field induced enhancement of the staggered magnetisation observed in the related \LSCO system, indicating this phenomenon is not a universal property of cuprate superconductors. Surprisingly, we find that incommensurability $\delta $ is only weakly dependent on doping relative to \LSCO. Dispersive excitations in \LBCO (x=0.095) at the same incommensurate wavevector persist up to at least 60 K. The dynamical spin susceptibility of the low energy spin excitations saturates below \tc, in a similar manner to that seen in the superconducting state of La$_2$CuO$_{4+y}$.
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