An X-Ray Induced Structural Transition in La_0.875Sr_0.125MnO_3

We report a synchrotron x-ray scattering study of the magnetoresistive manganite La_0.875Sr_0.125MnO_3. At low temperatures, this material undergoes an x-ray induced structural transition at which charge ordering of Mn^3+ and Mn^4+ ions characteristic to the low-temperature state of this compound is destroyed. The transition is persistent but the charge-ordered state can be restored by heating above the charge-ordering transition temperature and subsequently cooling. The charge-ordering diffraction peaks, which are broadened at all temperatures, broaden more upon x-ray irradiation, indicating the finite correlation length of the charge-ordered state. Together with the recent reports on x-ray induced transitions in Pr_(1-x)Ca_xMnO_3, our results demonstrate that the photoinduced structural change is a common property of the charge-ordered perovskite manganites.Comment: 5 pages, 4 embedded EPS figures; significant changes in the data analysis mad

Persistent X-Ray Photoconductivity and Percolation of Metallic Clusters in Charge-Ordered Manganites

Charge-ordered manganites of composition $\rm Pr_{1-x}(Ca_{1-y}Sr_{y})_{x}MnO_3$ exhibit persistent photoconductivity upon exposure to x-rays. This is not always accompanied by a significant increase in the {\it number} of conduction electrons as predicted by conventional models of persistent photoconductivity. An analysis of the x-ray diffraction patterns and current-voltage characteristics shows that x-ray illumination results in a microscopically phase separated state in which charge-ordered insulating regions provide barriers against charge transport between metallic clusters. The dominant effect of x-ray illumination is to enhance the electron {\it mobility} by lowering or removing these barriers. A mechanism based on magnetic degrees of freedom is proposed.Comment: 8 pages, 4 figure

Martensitic accommodation strain and the metal-insulator transition in manganites

In this paper, we report polarized optical microscopy and electrical transport studies of manganese oxides that reveal that the charge ordering transition in these compounds exhibits typical signatures of a martensitic transformation. We demonstrate that specific electronic properties of charge-ordered manganites stem from a combination of martensitic accommodation strain and effects of strong electron correlations. This intrinsic strain is strongly affected by the grain boundaries in ceramic samples. Consistently, our studies show a remarkable enhancement of low field magnetoresistance and the grain size effect on the resistivity in polycrystalline samples and suggest that the transport properties of this class of manganites are governed by the charge-disordered insulating phase stabilized at low temperature by virtue of martensitic accommodation strain. High sensitivity of this phase to strains and magnetic field leads to a variety of striking phenomena, such as unusually high magnetoresistance (10^10 %) in low magnetic fields.Comment: Short paper, 4 figures, to appear in Rapid Communicatio

Multiphase segregation and metal-insulator transition in single crystal La(5/8-y)Pr(y)Ca(3/8)MnO3

The insulator-metal transition in single crystal La(5/8-y)Pr(y)Ca(3/8)MnO3 with y=0.35 was studied using synchrotron x-ray diffraction, electric resistivity, magnetic susceptibility, and specific heat measurements. Despite the dramatic drop in the resistivity at the insulator-metal transition temperature Tmi, the charge-ordering (CO) peaks exhibit no anomaly at this temperature and continue to grow below Tmi. Our data suggest then, that in addition to the CO phase, another insulating phase is present below Tco. In this picture, the insulator-metal transition is due to the changes within this latter phase. The CO phase does not appear to play a major role in this transition. We propose that a percolation-like insulator-metal transition occurs via the growth of ferromagnetic metallic domains within the parts of the sample that do not exhibit charge ordering. Finally, we find that the low-temperature phase-separated state is unstable against x-ray irradiation, which destroys the CO phase at low temperatures.Comment: 9 pages, 9 encapsulated eps figure

Field-induced structural evolution in the spin-Peierls compound CuGeO3_3: high-field ESR study

The dimerized-incommensurate phase transition in the spin-Peierls compound CuGeO$_3$ is probed using multifrequency high-resolution electron spin resonance (ESR) technique, in magnetic fields up to 17 T. A field-induced development of the soliton-like incommensurate superstructure is clearly indicated as a pronounced increase of the ESR linewidth $\Delta B$ (magnon excitations), with a $\Delta B_{max}$ at $B_{c}\sim$ 13.8 T. The anomaly is explained in terms of the magnon-soliton scattering, and suggests that the soliton-like phase exists close to the boundary of the dimerized-incommensurate phase transition. In addition, magnetic excitation spectra in 0.8% Si-doped CuGeO$_3$ are studied. Suppression of the $\Delta B$ anomaly observed in the doped samples suggests a collapse of the long-range-ordered soliton states upon doping, that is consistent with high-field neutron scattering experiments.Comment: Accepted to Phys. Rev.

Immediate Results of Colonic Flat Epithelial Neoplasms Removal Using Diathermic Snare Endoscopic Mucosal Resection and Mucosectomy Combined with Dissection in the Submucosal Layer: Comparative Assessment

Aim: to compare the immediate outcomes of removing colonic flat epithelial neoplasms by using diathermic snare endoscopic mucosal resection (EMR) and mucosectomy with endoscopic submucosal dissection (ESD).Materials and methods. Ninety-six endoscopic procedures were conducted on the colon of 93 patients with flat epithelial neoplasms. The mean age of patients was 64.9 ± 10.7 years, with an age range of 39 to 88 years. The size range of epithelial neoplasms was 20 to 70 mm, with a median of 37.4 ± 14.8 mm. Only patients with benign epithelial neoplasms were included in the study since patients with suspected malignancy in laterally spreading tumors are indicated for ESD intervention to avoid fragmentary excision and risk of colorectal cancer progression due to possible positive resection margins. The patients were separated into two equally sized groups and treated with EMR and ESD methods.Results. Epithelial neoplasms in the ESD group had an average size of 41.6 ± 15 mm while those in the EMR group had an average size of 33.1 ± 13.5 mm. The ESD group had an average intervention time of 143.6 ± 102.9 min, whereas the EMR group had an average intervention time of 52.6 ± 34.4 min. Both groups (96 adenomas) had a total of 10 (10.4 %) patients who experienced colonic perforations during the intervention, with 4 (4.1 %) cases observed in the EMR group and 6 (6.2 %) in the ESD group. No statistically significant differences were identified in the occurrence of perforations during the operation (p = 0.7401). ESD resulted in an en bloc removal rate of 44/48 (91.6 %), whereas EMR only achieved a rate of 14/48 (29.1 %).Conclusion. Endoscopic mucosectomy with submucosal dissection (ESD), in contrast to endoscopic mucosal resection (EMR) with diathermic snare, provides a higher frequency of tumor removal en bloc, regardless of tumor size, but is characterized by a longer duration of intervention (p < 0.0001). In the ESD group, there were twice as many intraoperative bleedings (p = 0.0061) due to the longer duration and technical complexity of the procedure. There were no statistically significant differences in the incidence of late complications between the groups (p = 0.9999). Local recurrence of adenoma developed in two patients (4.1 %) after pEMR, statistically significant differences were noted (p < 0.0006)

Analysis of magnetization and a spin state crossover in the multiferroic Ca3_3Co2−x_{2-x}Mnx_xO6_6

Ca_3Co_{2-x}Mn_xO_6 (x ~ 0.96) is a multiferroic with spin-chains of alternating Co(2+) and Mn(4+) ions. The spin state of Co(2+) remains unresolved, due to a discrepancy between high temperature X-ray absorption (S=3/2) and low temperature neutron (S=1/2) measurements. Using a combination of magnetic modeling and crystal-field analysis, we show that the existing low temperature data cannot be reconciled within a high spin scenario by invoking spin-orbit or Jahn-Teller distortions. To unify the experimental results, we propose a spin-state crossover with specific experimental predictions.Comment: 4 pages, 3 figure

Thermodynamic Properties of the Incommensurate Phase of CuGeO_3

We present high resolution measurements of the specific heat and the thermal expansion of the inorganic spin--Peierls cuprate CuGeO_3 in a magnetic field of 16 Tesla. At the transition from the incommensurate to the uniform phase both quantities show pronounced anomalies, which allow to derive the uniaxial pressure dependencies of the transition temperature. In high magnetic fields the specific heat is dominated by magnetic excitations and follows a T^3 law at low temperatures. The thermal expansion measurements show the occurrence of spontaneous strains along all three lattice constants and yield high resolution measurements of the temperature dependence of the incommensurate structural distortion. The sizes of the spontaneous strains in the incommensurate phase are significantly reduced, but both their anisotropy as well as their temperature dependencies are very similar to those in zero field.Comment: 12 pages (Latex), 4 Figs. (PS), to appear in Phys. Rev. B54 (Vol.21