Pressure-Driven Metal-Insulator Transition in Hematite from Dynamical Mean-Field Theory

The Local Density Approximation combined with Dynamical Mean-Field Theory (LDA+DMFT method) is applied to the study of the paramagnetic and magnetically ordered phases of hematite Fe$_2$O$_3$ as a function of volume. As the volume is decreased, a simultaneous 1st order insulator-metal and high-spin to low-spin transition occurs close to the experimental value of the critical volume. The high-spin insulating phase is destroyed by a progressive reduction of the charge gap with increasing pressure, upon closing of which the high spin phase becomes unstable. We conclude that the transition in Fe$_2$O$_3$ at $\approx$50 GPa can be described as an electronically driven volume collapse.Comment: 5 pages, 4 figure

Role of c-axis pairs in V2O3 from the band-structure point of view

The common interpretation of the LDA band structure of V$_{2}$O$_{3}$ is that the apparent splitting of the $a_{1g}$ band into a low intensity structure deep below the Fermi energy and a high intensity feature above it, is due to the bonding-antibonding coupling of the vertical V-V pair. Using tight-binding fitting to --as well as first-principles NMTO downfolding of-- the spin-up LDA+U $a_{1g}$ band, we show that there are other hopping integrals which are equally important for the band shape as the integral for hopping between the partners of the pair

First principle computation of stripes in cuprates

We present a first principle computation of vertical stripes in $La_{15/8}Sr_{1/8}CuO_4$ within the LDA+U method. We find that Cu centered stripes are unstable toward O centered stripes. The metallic core of the stripe is quite wide and shows reduced magnetic moments with suppressed antiferromagnetic (AF) interactions. The system can be pictured as alternating metallic and AF two-leg ladders the latter with strong AF interaction and a large spin gap. The Fermi surface shows warping due to interstripe hybridization. The periodicity and amplitude of the warping is in good agreement with angle resolved photoemission experiment. We discuss the connection with low-energy theories of the cuprates.Comment: 5 pages,4 figure

Neutron Star Mergers Are the Dominant Source of the r-process in the Early Evolution of Dwarf Galaxies

There are many candidate sites of the r-process: core-collapse supernovae (including rare magnetorotational core-collapse supernovae), neutron star mergers, and neutron star/black hole mergers. The chemical enrichment of galaxies---specifically dwarf galaxies---helps distinguish between these sources based on the continual build-up of r-process elements. This technique can distinguish between the r-process candidate sites by the clearest observational difference---how quickly these events occur after the stars are created. The existence of several nearby dwarf galaxies allows us to measure robust chemical abundances for galaxies with different star formation histories. Dwarf galaxies are especially useful because simple chemical evolution models can be used to determine the sources of r-process material. We have measured the r-process element barium with Keck/DEIMOS medium-resolution spectroscopy. We present the largest sample of barium abundances (almost 250 stars) in dwarf galaxies ever assembled. We measure [Ba/Fe] as a function of [Fe/H] in this sample and compare with existing [alpha/Fe] measurements. We have found that a large contribution of barium needs to occur at more delayed timescales than core-collapse supernovae in order to explain our observed abundances, namely the significantly more positive trend of the r-process component of [Ba/Fe] vs. [Fe/H] seen for [Fe/H] <~ -1.6 when compared to the [Mg/Fe] vs. [Fe/H] trend. We conclude that neutron star mergers are the most likely source of r-process enrichment in dwarf galaxies at early times.Comment: Accepted to ApJ on 2018 October 2