Articles

September 9, 2016 02:03 PM

Electronic topological transition (ETT), where new features occur in the topology of the Fermi surface, has strong influences on structural, mechanical, and electronic properties of materials. Recent nuclear resonant inelastic x-ray scattering experiments at HPCAT strongly support a thermally driven ETT that drives anomalous changes in phonon dynamics. The thermally induced ETT in FeTi causes an increased electronic screening for the atom displacements in the M5 phonon mode and an adiabatic electron phonon interaction with an unusual temperature dependence. This behavior is consistent with the atomic displacement pattern, in which (110) planes slide in opposite [110] directions. From the phonon polarization vectors, it is found that the magnitude of the Fe displacement is at least twice as that of Ti. [F. C. yang et al, Phys. Rev. Lett., 117, 076402, (2016)]

September 9, 2016 01:53 PM

A hallmark of the iron-based superconductors is the strong coupling between magnetic, structural and electronic degrees of freedom. Using HPCAT capabilities of high-pressure x-ray diffraction and time-domain Mössbauer spectroscopy, the nematicity and magnetism in FeSe under applied pressure are found to be strongly coupled. Distinct structural and magnetic transitions are observed for pressures between 1.0 and 1.7 GPa and merge into a single first-order transition for pressures ≳1.7 GPa, reminiscent of what has been found for the evolution of these transitions in the prototypical system Ba(Fe1−xCox)2As2. Our results are consistent with a spin-driven mechanism for nematic order in FeSe and provide an important step towards a universal description of the normal state properties of the iron-based superconductors. [K. Kothapalli et al, Nature Comm. 7, 12728 (2016)]

 

August 17, 2016 11:13 AM

Perovskite solar cells (PSCs) have attracted much attention owing to their high power conversion efficiency and low manufacturing cost.  Power conversion efficiencies of over 20% have recently been achieved with lead-based organic-inorganic halide PSCs, but these have relatively low structural stability, and the use of lead in the manufacturing process introduces problems of toxicity.  Recently, organotin perovskites have been synthesized as lead-free alternatives, and a number of chemical and processing modifications have been applied to these compounds in an effort to tune their crystal structure and electronic properties, with the hope of yielding more stable compounds which exhibit superior absorption and conductivity.

August 4, 2016 10:53 AM

Molybdenum disulfide (MoS2) attracts extensive interests because of its peculiar bandgap properties and the two-dimensional structure. The layer-dependent band structure has been studied by many theoretical calculations, including its evolution with pressure, e.g., the increase of the direct band gap by applying pressures. It is believed that the band-gap behavior is closely related to lattice strains. However, no general description has been established. A recent study conducted at HPCAT reveals a structural characteristic that is strongly correlated to bandgap behavior, regardless of its form as monolayer or bulk.  There are two unique structural parameters describing the strain of MoS2: in-plane S-Mo-S bond-angle φ and its out-of-plane pair θ. The trends in φ and θ have been determined as a function of external pressure using high-pressure X-ray powder diffraction for bulk MoS2.

August 4, 2016 12:07 PM

The lanthanide elements show many interesting phenomena such as heavy fermion metal properties and unconventional superconductivity due to their ƒ electrons. Among all lanthanide metals, only Ce and Eu are known to become superconducting under pressure. In order to understand the interplay between magnetism and superconductivity in europium at extreme pressures, experiments using X-ray emission spectroscopy (XES) at HPCAT and nuclear forwarding scattering (NFS) at sector 3 of APS have been done recently. With pressure above 80GPa, Eu has a critical superconducting temperature around 2K. Magnetic order in Eu is found to collapse just above 80GPa in association with the superconducting state according NFS spectra; however no change in the Lγ1 line of Eu in XES is observed up to 119GPa. These results indicate that the highly localized character of the 4ƒ7 magnetic state in Eu remains intact and Eu remains nearly divalent to at least 119GPa.

August 1, 2016 05:24 PM

Materials combining the hardness and strength of diamond with the higher thermal stability of cubic boron nitride (cBN) have broad potential value in science and engineering. Reacting nanodiamond with cBN at high pressures and high temperatures provides a pathway to such materials. Postdoctoral associate Dr. Xiaobing Liu, with CDAC partner Steve Jacobsen at Northwestern University, report the fabrication of C2-BN nanocomposite (2:1 ratio of diamond to cBN), measuring up to 10 mm in longest dimension. The nanocomposite consists of randomly-oriented, diamond and cBN domains of 50-250 nm, stitched by sp3 hybridized C-B and C-N bonds. B-C-N solid solution within the sutures leads to bulk p-type semiconductivity with an activation energy of 6.2 meV. The new study, published in the 27 July issue of Scientific Reports, shows that dislocations near the sutures accommodate lattice mismatch between diamond and cBN.

June 8, 2016 03:52 PM

Iron and oxygen are two of the most geochemically important elements on Earth. The core is rich in iron and the atmosphere is rich in oxygen, and between them is the entire range of pressures and temperatures on the planet. Iron, in its multiple oxidation states, controls the oxygen fugacity and oxygen budget. Hydrogen has a key role in the reaction of Fe and O, causing iron to rust in humid air. Recent experiments at HPCAT identified a highly stable, pyrite-structured iron oxide (FeO2) at 76 GPa and 1,800 K that holds an excessive amount of oxygen. The “rust” mineral goethite, FeOOH, decomposes under the deep lower-mantle conditions to form FeO2 and release H2. The reaction could cause accumulation of the heavy FeO2-bearing patches in the deep lower mantle, upward migration of hydrogen, and separation of the oxygen and hydrogen cycles.

May 5, 2016 05:37 PM

Applied pressure is a powerful tool for studying intermediate valent systems because their electronic states are sensitive to small changes in interatomic separation. Recent HPCAT experiments using resonant x-ray emission spectroscopy show that the pressure dependence of the f-electron occupancy in the Kondo insulator SmB6. Applied pressure reduces the f occupancy, but surprisingly, the material maintains a significant divalent character up to a pressure of at least 35 GPa. Thus, the closure of the resistive activation energy gap and onset of magnetic order are not driven by stabilization of an integer valent state. Over the entire pressure range, the material maintains a remarkably stable intermediate valence that can in principle support a nontrivial band structure.

May 4, 2016 01:34 PM

Near the Earth’s core-mantle boundary (CMB) there occur isolated regions characterized by anomalously high density and low seismic wave velocity.  The non-uniform distribution of these so-called ultralow velocity zones (ULVZs) is evidence of chemical and/or thermal heterogeneity near the CMB.  The origin of ULVZs is not well understood, as the complex relationship among elemental composition, mobility, and melt near the CMB makes it difficult to account for the various observed anomalies.  A better understanding of the origin and evolution of ULVZs would play a crucial role in understanding the evolutionary history and planetary dynamics of the Earth as a whole.  Recent experiments at HPCAT’s laser heating endstation (16-ID-B) explore the melting curve of the iron-carbon system.  Results suggest the eutectic melting curve crosses the geotherm near the CMB, and thus it is possible that regions of dense metallic melt, originating from slab material