Fundamental energies are determined optically for an ensemble of correlated electrons in an antiferromagnetic insulator, V203. The observed variation of the energy gap and the kinetic energy are compared quantitatively to some approximate solutions of the Hubbard Hamiltonian.PACS numbers: 71.27.+a, 71.30.+h, 78.30.Hv The approach to the metal-insulator transition by a collection of correlated electrons has been a subject of considerable interest since Mott's classic paper [1] We have made measurements with energy resolution less than 1 meV on crystals which we found to be very difficult to anneal, polish, and etch, with the additional difficulty that they crack on passing through the metalinsulator transition that occurs on cooling from room temperature.Based on optical and dc transport characterization of over 10 crystals, we have developed annealing and surface preparation procedures which yield reproducible spectra [17]. We have made reflectivity, R, measurements from the far-infrared to E = 3.5 eV (a frequency, ta, about 27000 crn ') and used the measurements of Shin et al. [18] from 3.5 to 25 eV in our Kramers-Kronig transformations to obtain the optical conductivity, tr.The main part of Fig. 1 [4,5]) which can introduce conducting states into a filled band, and it also differs from disordered doping which primarily introduces impurity states into the gap [1]. We see negligible absorption (except that due to phonons) in the energy gap, and our 0. extrapolated to E = 0 agrees with the (essentially zero) measured dc conductivity, trd"of similarly prepared samples.Qualitatively, the spectra show an energy gap, 2b"with a "soft" edge and a broad peak at higher energy. This

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We present heat-capacity measurements in a magnetic field up to 7.7 T on the recently discovered borocarbide intermetallic superconductors LuNi2B2C and LaPtl7Au03B2C. For LuNi2B2C with