Spin Correlations in the Two-Dimensional Spin-5/2 Heisenberg Antiferromagnet Rb2MnF4

We report a neutron scattering study of the instantaneous spin correlations in the two-dimensional spin S=5/2 square-lattice Heisenberg antiferromagnet Rb_2MnF_4. The measured correlation lengths are quantitatively described, with no adjustable parameters, by high-temperature series expansion results and by a theory based on the quantum self-consistent harmonic approximation. Conversely, we find that the data, which cover the range from about 1 to 50 lattice constants, are outside of the regime corresponding to renormalized classical behavior of the quantum non-linear sigma model. In addition, we observe a crossover from Heisenberg to Ising critical behavior near the Neel temperature; this crossover is well described by a mean-field model with no adjustable parameters.Comment: 8 pages, LaTeX, with 6 included EPS figures, submitted to EPJ

Direct Evidence for the Source of Reported Magnetic Behavior in "CoTe"

In order to unambiguously identify the source of magnetism reported in recent studies of the Co-Te system, two sets of high-quality, epitaxial CoTe$_x$ films (thickness $\simeq$ 300 nm) were prepared by pulse laser deposition (PLD). X-ray diffraction (XRD) shows that all of the films are epitaxial along the [001] direction and have the hexagonal NiAs structure. There is no indication of any second phase metallic Co peaks (either $fcc$ or $hcp$) in the XRD patterns. The two sets of CoTe$_x$ films were grown on various substrates with PLD targets having Co:Te in the atomic ratio of 50:50 and 35:65. From the measured lattice parameters $c = 5.396 \AA$ for the former and $c = 5.402\AA$ for the latter, the compositions CoTe$_{1.71}$ (63.1% Te) and CoTe$_{1.76}$ (63.8% Te), respectively, are assigned to the principal phase. Although XRD shows no trace of metallic Co second phase, the magnetic measurements do show a ferromagnetic contribution for both sets of films with the saturation magnetization values for the CoTe$_{1.71}$ films being approximately four times the values for the CoTe$_{1.76}$ films. $^{59}$Co spin-echo nuclear magnetic resonance (NMR) clearly shows the existence of metallic Co inclusions in the films. The source of weak ferromagnetism reported in several recent studies is due to the presence of metallic Co, since the stoichiometric composition "CoTe" does not exist.Comment: 19 pages, 7 figure

Anomalous Phase Transition in Strained SrTiO3_3 Thin Films

We have studied the cubic to tetragonal phase transition in epitaxial SrTiO$_3$ films under various biaxial strain conditions using synchrotron X-ray diffraction. Measuring the superlattice peak associated with TiO$_6$ octahedra rotation in the low temperature tetragonal phase indicates the presence of a phase transition whose critical temperature is a strong function of strain, with T$_C$ as much as 50K above the corresponding bulk temperature. Surprisingly, the lattice constants evolve smoothly through the transition with no indication of a phase change. This signals an important change in the nature of the phase transition due to the epitaxy strain and substrate clamping effect. The internal degrees of freedom (TiO$_6$ rotations) have become uncoupled from the overall lattice shape.Comment: 4 pages, 3 figures, REVTeX

Phase Coexistence Near a Morphotropic Phase Boundary in Sm-doped BiFeO3 Films

We have investigated heteroepitaxial films of Sm-doped BiFeO3 with a Sm-concentration near a morphotropic phase boundary. Our high-resolution synchrotron X-ray diffraction, carried out in a temperature range of 25C to 700C, reveals substantial phase coexistence as one changes temperature to crossover from a low-temperature PbZrO3-like phase to a high-temperature orthorhombic phase. We also examine changes due to strain for films greater or less than the critical thickness for misfit dislocation formation. Particularly, we note that thicker films exhibit a substantial volume collapse associated with the structural transition that is suppressed in strained thin films

Nitrogen Fertilizer Use by a High Yielding No-Till Corn Crop

At the present time there is public concern about the effect which nitrogen (N) fertilizer use by farmers has on groundwater quality. This has resulted in research efforts to describe what happens to nitrogen fertilizer after it is applied to a crop. We have conducted a study for the past 3 years which enables us to make a reasonable estimate of how much fertilizer N is taken up by a high yielding corn crop. And, depending on whether the corn is harvested for silage or grain, we have estimated how much of the fertilizer N is removed from the field. Such an evaluation provides a good picture of the extent to which fertilizer N is utilized by the crop and its potential as a groundwater pollutant

Local Structure and It's Effect on The Ferromagnetic Properties of La0.5_{0.5}Sr0.5_{0.5}CoO3_3 thin films}

We have used high-resolution Extended X-ray Absorption Fine-Structure and diffraction techniques to measure the local structure of strained La$_{0.5}$Sr$_{0.5}$CoO$_3$ films under compression and tension. The lattice mismatch strain in these compounds affects both the bond lengths and the bond angles, though the larger effect on the bandwidth is due to the bond length changes. The popular double exchange model for ferromagnetism in these compounds provides a correct qualitative description of the changes in Curie temperature $T_C$, but quantitatively underestimates the changes. A microscopic model for ferromagnetism that provides a much stronger dependence on the structural distortions is needed.Comment: 4 pages, 4 figure

Staging superstructures in high-TcT_c Sr/O co-doped La2−x_{2-x}Srx_xCuO4+y_{4+y}

We present high energy X-ray diffraction studies on the structural phases of an optimal high-$T_c$ superconductor La$_{2-x}$Sr$_x$CuO$_{4+y}$ tailored by co-hole-doping. This is specifically done by varying the content of two very different chemical species, Sr and O, respectively, in order to study the influence of each. A superstructure known as staging is observed in all samples, with the staging number $n$ increasing for higher Sr dopings $x$. We find that the staging phases emerge abruptly with temperature, and can be described as a second order phase transition with transition temperatures slightly depending on the Sr doping. The Sr appears to correlate the interstitial oxygen in a way that stabilises the reproducibility of the staging phase both in terms of staging period and volume fraction in a specific sample. The structural details as investigated in this letter appear to have no direct bearing on the electronic phase separation previously observed in the same samples. This provides new evidence that the electronic phase separation is determined by the overall hole concentration rather than specific Sr/O content and concommittant structural details.Comment: 8 pages, incl. 4 figure