The Verwey transition in magnetite, Fe 3 O 4 , has been studied using temperature-dependent high-resolution photoemission spectroscopy. On heating through the transition temperature T V the band gap is not collapsed, but is merely reduced by ϳ50 meV, showing that a metal-insulator transition does not occur. The change in the gap is perfectly consistent with the two orders of magnitude conductivity jump at T V . Thus even above T V short-range charge ordering rather than site equivalency dominates the single-particle excitations and the electrical properties. We also point out important implications for efforts to model the electrical transport above T V . ͓S0163-1829͑97͒03419-X͔ Magnetite, Fe 3 O 4 , is the archetype mixed valent 3d transition metal compound. Fe 3 O 4 crystallizes in an inverted cubic spinel structure in which tetrahedral A sites contain onethird of the Fe ions as Fe 3ϩ , while octahedral B sites contain the remaining Fe ions, with equal numbers of Fe 3ϩ and Fe 2ϩ in B3 and B2 sites, respectively. Below 860 K, magnetite is ferrimagnetic with the A-site magnetic moments aligned antiparallel to the B-site moments. Immediately apparent is the fundamental tension between the mixed valence of the B sites and their crystallographic equivalence in this crystal structure. This tension is manifested in a first order phase transition, the so-called Verwey transition 1 at T V Ϸ120 K, in which the dc conductivity abruptly increases by two orders of magnitude on heating through T V . 2 Although a large number of papers 3,4 have been published since its discovery in 1941, the Verwey transition was the subject of an entire international workshop 3 as recently as 1979 and continues as a paradigm of the classic condensed matter problem-how to describe electron motion if the kinetic energy, electron-electron interactions, and electron-lattice interaction are all comparably important.Verwey and Haayman 1 interpreted the transition as an order-disorder transformation of Fe ions on the B sites. Indeed, studies by electron and neutron diffraction and nuclear magnetic resonance [5][6][7] show that below T V the B2 and B3 sites are structurally distinguishable in a distorted crystal PHYSICAL REVIEW B CONDENSED MATTER THIRD SERIES, VOLUME 55, NUMBER 19 15 MAY 1997-I BRIEF REPORTSBrief Reports are accounts of completed research which, while meeting the usual Physical Review B standards of scientific quality, do not warrant regular articles. A Brief Report may be no longer than four printed pages and must be accompanied by an abstract. The same publication schedule as for regular articles is followed, and page proofs are sent to authors.
The valence v and the 4f 14-' --)4f173/4 transition energy TK of YbAI~ at low temperature were determined by the high resolution valence band photoemission spectroscopy. The obtained values v = 2,65-+0.03 and T K = 30 -+ 15meV are consistent with the zero-temperature magnetic susceptibility X,,(0), supporting the Anderson Hamiltonian description for its electronic structure.
We present a microsystem capable of electrochemically patterning the dissolved oxygen gradients present in cell culture. Multiple microelectrodes in an array are separated from the cells by an oxygen permeable membrane. Each electrode generates distinct amounts of dissolved oxygen via electrolysis; these different sources superimpose to generate one-and two-dimensional microgradient profiles not possible with other methods. We believe this is the first technology that enables researchers to pattern localized oxygen doses and program arbitrary oxygen gradients with microscale resolution during cell culture. We present fluorescent and colorimetric data on the construction of two-dimensional oxygen microgradients in aqueous solution and preliminary cell culture results with C2C12 rat myoblasts.
We present a microdevice capable of electrochemically generating user-defined oxygen gradients for use in cell and tissue culture. Electrolytic dissolved oxygen generation at multiple electrodes evolves 1D and 2D oxygen gradients across several millimeters with microscale precision and has the potential to test the effect of localized oxygen doses on a wide range of tissue and cell samples. The developed microgradients are stable for days, enabling experiments of physiologically relevant duration. We present the basic theory of operation and initial results.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.