Maria Baldini

Preparation and characterization of LaMnO3 thin films grown by pulsed laser deposition

We have grown LaMnO3 thin films on (001) LaAlO3 substrates by pulsed laser deposition. X-ray diffraction confirms that the films are only slightly relaxed and are oriented “square on square” relative to the substrate. The measured Raman spectra closely resemble that observed in bulk LaMnO3, which indicates no relevant distortions of the MnO6 octahedra induced by the epitaxial strain. Therefore, no detectable changes in the lattice dynamics occurred in our LaMnO3 strained films relative to the bulk case. Mn55 nuclear magnetic resonance identifies the presence of localized Mn4+ states. Superconducting quantum interference device magnetization measures TN=131(3)K and a saturation moment μ=1.09μB∕Mn, revealing a small concentration of Mn4+ and placing our films within the antiferromagnetic insulating phase.

Origin of colossal magnetoresistance in LaMnO3 manganite

Significance Magnetoresistance is the change of resistance in the presence of an external magnetic field. In rare-earth manganite compounds, this change is orders of magnitude stronger than usual and it is promising for developing new spintronic and electronic devices. The colossal magnetoresistance (CMR) effect has been observed only in chemically doped manganite compounds. We report the realization of CMR in a compressed single-valent LaMnO3 manganite compound. Pressure generates an inhomogeneous phase constituted by two components: a nonconductive one with a unique structural distortion and a metallic one without distortion. The CMR takes place when the competition between the two phases is at a maximum. We identify phase separation as the driving force for generating CMR in LaMnO3. Phase separation is a crucial ingredient of the physics of manganites; however, the role of mixed phases in the development of the colossal magnetoresistance (CMR) phenomenon still needs to be clarified. We report the realization of CMR in a single-valent LaMnO3 manganite. We found that the insulator-to-metal transition at 32 GPa is well described using the percolation theory. Pressure induces phase separation, and the CMR takes place at the percolation threshold. A large memory effect is observed together with the CMR, suggesting the presence of magnetic clusters. The phase separation scenario is well reproduced, solving a model Hamiltonian. Our results demonstrate in a clean way that phase separation is at the origin of CMR in LaMnO3.

Synthesis and Stability of Lanthanum Superhydrides

Recent theoretical calculations predict that megabar pressure stabilizes very hydrogen-rich simple compounds having new clathrate-like structures and remarkable electronic properties including room-temperature superconductivity. X-ray diffraction and optical studies demonstrate that superhydrides of lanthanum can be synthesized with La atoms in an fcc lattice at 170 GPa upon heating to about 1000 K. The results match the predicted cubic metallic phase of LaH10 having cages of thirty-two hydrogen atoms surrounding each La atom. Upon decompression, the fcc-based structure undergoes a rhombohedral distortion of the La sublattice. The superhydride phases consist of an atomic hydrogen sublattice with H-H distances of about 1.1 Å, which are close to predictions for solid atomic metallic hydrogen at these pressures. With stability below 200 GPa, the superhydride is thus the closest analogue to solid atomic metallic hydrogen yet to be synthesized and characterized.

Probing the electronic and local structural changes across the pressure-induced insulator-to-metal transition in VO2

Local and electronic structures of vanadium in are studied across the high-pressure insulator-to-metal (IMT) transition using V K-edge x-ray absorption spectroscopy. Unlike the temperature-induced IMT, pressure-induced metallization leads to only subtle changes in the V K-edge prepeak structure, indicating a different mechanism involving smaller electronic spectral weight transfer close to the chemical potential. Intriguingly, upon application of the hydrostatic pressure, the electronic structure begins to show substantial changes well before the occurrence of the IMT and the associated structural transition to an anisotropic compression of the monoclinic metallic phase.

High pressure behavior of Ga-doped LaMnO3: a combined X-ray diffraction and optical spectroscopy study

The pressure effects on the Jahn–Teller (JT) distortion of three representative compounds belonging to the LaMn1−xGaxO3 (x = 0.2, 0.3, 0.4) family were widely investigated by means of X-ray diffraction and Raman spectroscopy. A compound with a fully coherent JT distorted structure (x = 0.2), one with regular octahedra (x = 0.6) and one in an intermediate configuration (x = 0.3) were selected. A pressure induced transition from the orthorhombic Pbnm phase towards structures with higher symmetry was observed in all the samples. Both Raman and X-ray data confirm that the most important structural effect of pressure is that of reducing the octahedral distortion. The appearance of a feature in the lattice parameter behavior connected to a structural instability was also detected, pointing out the key role of the JT distortion in stabilizing the manganite structures. On the other hand, the complete suppression of the JT distortion in the high-pressure phases cannot be claimed. The Raman spectra collected on the more distorted compounds (x = 0.2, 0.3) reveal clearly the coexistence of domains of distorted and more regular octahedra in a certain pressure range. The first sketch of the pressure vs. Ga-content phase diagram was drawn.

Evidence for coupling between charge density waves and phonons in two-dimensional rare-earth tritellurides

We report on a Raman scattering investigation of the charge-density-wave (CDW), quasi two-dimensional rare-earth tri-tellurides RTe{sub 3} (R = La, Ce, Pr, Nd, Sm, Gd and Dy) at ambient pressure, and of LaTe{sub 3} and CeTe{sub 3} under externally applied pressure. The observed phonon peaks can be ascribed to the Raman active modes for both the undistorted as well as the distorted lattice in the CDW state by means of a first principles calculation. The latter also predicts the Kohn anomaly in the phonon dispersion, driving the CDW transition. The integrated intensity of the two most prominent modes scales as a characteristic power of the CDW-gap amplitude upon compressing the lattice, which provides clear evidence for the tight coupling between the CDW condensate and the vibrational modes.

Mechanochemical Synthesis of Carbon Nanothread Single Crystals

Synthesis of well-ordered reduced dimensional carbon solids with extended bonding remains a challenge. For example, few single-crystal organic monomers react under topochemical control to produce single-crystal extended solids. We report a mechanochemical synthesis in which slow compression at room temperature under uniaxial stress can convert polycrystalline or single-crystal benzene monomer into single-crystalline packings of carbon nanothreads, a one-dimensional sp3 carbon nanomaterial. The long-range order over hundreds of microns of these crystals allows them to readily exfoliate into fibers. The mechanochemical reaction produces macroscopic single crystals despite large dimensional changes caused by the formation of multiple strong, covalent C-C bonds to each monomer and a lack of reactant single-crystal order. Therefore, it appears not to follow a topochemical pathway, but rather one guided by uniaxial stress, to which the nanothreads consistently align. Slow-compression room-temperature synthesis may allow diverse molecular monomers to form single-crystalline packings of polymers, threads, and higher dimensional carbon networks.

Pressure-induced quenching of the charge-density-wave state in rare-earth tritellurides observed by x-ray diffraction

We report an x-ray diffraction study on the charge-density-wave (CDW) LaTe{sub 3} and CeTe{sub 3} compounds as a function of pressure. We extract the lattice constants and the CDW modulation wave-vector, and provide direct evidence for a pressure-induced quenching of the CDW phase. We observe subtle differences between the chemical and mechanical compression of the lattice. We account for these with a scenario where the effective dimensionality in these CDW systems is dependent on the type of lattice compression and has a direct impact on the degree of Fermi surface nesting and on the strength of fluctuation effects.

Evidence for a monoclinic metallic phase in high-pressure VO2

The pressure dependencies at room temperature of Raman and infrared (IR) data on VO 2 are presented up to 19 and 15 GPa, respectively, together with preliminary X-ray diffraction (XRD) data collected up to 42 GPa. Both Raman and XRD indicate the persistence of a monoclinic lattice up the highest pressures. Observed modifications in the pressure dependence of Raman phonon frequencies, IR spectral weight, and unit cell volume at ∼10 GPa allow us to identify two regimes at low and high pressure and to conjecture a transition to a new unknown monoclinic phase where applying pressure activates a metallization mechanism.

Pressure induced phase separation in optimally doped bilayer manganites

In this paper, we give a direct experimental evidence of the long hypothesized phase separation scenario in strongly correlated manganite-based systems. High resolution, high-pressure synchrotron x-ray diffraction measurements on La1.4Sr1.6Mn2O7 as a function of temperature reveal the instability with P of the ferromagnetic phase leading, above a threshold P, to a phase separation between two phases with different orbital occupancies.