Correlations between pressure and bandwidth effects in metal-insulator transitions in manganites

The effect of pressure on the metal-insulator transition in manganites with a broad range of bandwidths is investigated. A critical pressure is found at which the metal-insulator transition temperature, T$_{MI}$, reaches a maximum value in every sample studied. The origin of this universal pressure and the relation between the pressure effect and the bandwidth on the metal-insulator transition are discussed

Pressure Induced Reentrant Electronic and Magnetic State in Pr0.7Ca0.3MnO3 Manganite

In Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$, pressure induces reentrant magnetic and electronic state changes in the range 1 atm to $\sim$ 6 GPa. The metal-insulator and magnetic transition temperatures coincide from $\sim$1 to 5 GPa, decouple outside of this range and do not change monotonically with pressure. The effects may be explained by pressure tuned competition between double exchange and super exchange. The insulating state induced by pressure above $\sim$5 GPa is possibly ferromagnetic, different from the ferromagnetic and antiferromagnetic phase-separated insulating state below $\sim$0.8 GPa

High pressure effects on electron transport and structure of colossal magnetoresistive materials

Pressure effects on the electronic, magnetic properties and structure of several typical colossal magnetoresistive manganites, La0.60Y0.07Ca0.33MnO3, Pr1-xCaxMn03 (X = 0.25, 0.30, 0.35), Nd1-xSrxMnO3 (x = 0.45, 0.50), were explored through high pressure resistivity and structure measurements. It was shown that pressure up to ~7 GPa induces more complicated charge, spin and lattice state changes than in the low pressure range explored previously. In La0.60Y0.07Ca0.33MnO3, pressure induces a local atomic structure transformation at a critical point P*, and hence, a non-monotonic change in metal insulator (MI) transition temperature (Tmi) and spin state. In Pr0.75Ca0.25MnO3, with pressure increase, Tmi, increases and Tc decreases below P* and the trend is reversed above P*. In Pr0.7Ca0.3MnO3, pressure induces reentrant electronic and magnetic states: between ~0.8-5 GPa, TmI, and Tc are coupled and have a behavior similar to La0.60Y0.07Ca0.33MnO3, outside of this range, TmI and Tc are decoupled and at low and high pressure the material is insulating. In all three Pr1-xCaxMnO3 compounds, charge ordering is suppressed below P*. Above P*, an insulating state with unknown conducting mechanism is induced. In Nd1-xSrxMnO3, at x = 0.45, in addition to the effect on TmI, pressure possibly induces an A-type antiferromagnetic phase. For x = 0.5, the charge ordering transition temperature is increased, which is different from Pr1-xCaxMnO3 system. The effects of chemical doping (bandwidth) and pressure are not equivalent in the high pressure range. This is unlike the results in the low pressure range acquired by other groups previously. A universal P* exists for samples with metal-insulator transitions

Pressure effects on charge, spin, and metal-insulator transitions in narrow bandwidth manganite Pr1−x_{1-x}Cax_{x}MnO3_{3}

Pressure effects on the charge and spin states and the relation between the ferromagnetic and metallic states were explored on the small bandwidth manganite Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ (x = 0.25, 0.3, 0.35). Under pressure, the charge ordering state is suppressed and a ferromagnetic metallic state is induced in all three samples. The metal-insulator transition temperature (T$_{MI}$) increases with pressure below a critical point P*, above which T$_{MI}$ decreases and the material becomes insulating as at the ambient pressure. The e$_{g}$ electron bandwidth and/or band-filling mediate the pressure effects on the metal-insulator transition and the magnetic transition. In the small bandwidth and low doping concentration compound (x = 0.25), the T$_{MI}$ and Curie temperature (T$_{C}$) change with pressure in a reverse way and do not couple under pressure. In the x = 0.3 compound, the relation of T$_{MI}$ and T$_{C}$ shows a critical behavior: They are coupled in the range of $\sim$0.8-5 GPa and decoupled outside of this range. In the x = 0.35 compound, T$_{MI}$ and T$_{C}$ are coupled in the measured pressure range where a ferromagnetic state is present

Transport and structural study of pressure-induced magnetic states in Nd0.55Sr0.45MnO3 and Nd0.5Sr0.5MnO3

Pressure effects on the electron transport and structure of Nd1-xSrxMnO3 (x = 0.45, 0.5) were investigated in the range from ambient to ~6 GPa. In Nd0.55Sr0.45MnO3, the low-temperature ferromagnetic metallic state is suppressed and a low temperature insulating state is induced by pressure. In Nd0.5Sr0.5MnO3, the CE-type antiferromagnetic charge-ordering state is suppressed by pressure. Under pressure, both samples have a similar electron transport behavior although their ambient ground states are much different. It is surmised that pressure induces an A-type antiferromagnetic state at low temperature in both compounds

High-resolution ab initio three-dimensional X-ray diffraction microscopy

Coherent X-ray diffraction microscopy is a method of imaging non-periodic isolated objects at resolutions only limited, in principle, by the largest scattering angles recorded. We demonstrate X-ray diffraction imaging with high resolution in all three dimensions, as determined by a quantitative analysis of the reconstructed volume images. These images are retrieved from the 3D diffraction data using no a priori knowledge about the shape or composition of the object, which has never before been demonstrated on a non-periodic object. We also construct 2D images of thick objects with infinite depth of focus (without loss of transverse spatial resolution). These methods can be used to image biological and materials science samples at high resolution using X-ray undulator radiation, and establishes the techniques to be used in atomic-resolution ultrafast imaging at X-ray free-electron laser sources.Comment: 22 pages, 11 figures, submitte

Massively parallel X-ray holography

Advances in the development of free-electron lasers offer the realistic prospect of high-resolution imaging to study the nanoworld on the time-scale of atomic motions. We identify X-ray Fourier Transform holography, (FTH) as a promising but, so far, inefficient scheme to do this. We show that a uniformly redundant array (URA) placed next to the sample, multiplies the efficiency of X-ray FTH by more than one thousand (approaching that of a perfect lens) and provides holographic images with both amplitude- and phase-contrast information. The experiments reported here demonstrate this concept by imaging a nano-fabricated object at a synchrotron source, and a bacterial cell at a soft X-ray free-electron-laser, where illumination by a single 15 fs pulse was successfully used in producing the holographic image. We expect with upcoming hard X-ray lasers to achieve considerably higher spatial resolution and to obtain ultrafast movies of excited states of matter.Comment: 5 pages, 3 figures, revte