High-Field Properties of Single-Crystalline Cavo3

The magnetic properties of perovskite CaVO3 single crystals have been studied by means of magnetoresistance r(T, H) and magnetization M(H) measurements in fields to 18T. At 2 K, the magnetoresistance is positive and a maximum value of Dr(18T)/r(0) = 16.5% is found for H//a. The magnetization exhibits a smooth increase at 2 K, reaching values of M(18T) = 0.03, 0.05, 0.17 mB/f.u. for H//a, H//b, and H//c, respectively. This anisotropy found in M(H) is consistent with that observed for Dr(H//a) > Dr(H//b) > Dr(H//c). These results can be interpreted in terms of the field-dependent scattering mechanism of CaVO3.Comment: 10 pages, including 3 figure

Magnetocrystalline Anisotropy in a Single Crystal of CeNiGe2

We report measurements on single crystals of orthorhombic CeNiGe2, which is found to exhibit highly anisotropic magnetic and transport properties. The magnetization ratio M(H//b)/M(H^b) at 2 K is observed to be about 18 at 4 T and the electrical resistivity ratio r//b/r^b is about 70 at room temperature. It is confirmed that CeNiGe2 undergoes two-step antiferromagnetic transition at 4 and 3 K, as reported for polycrystalline samples. The application of magnetic field along the b axis (the easy magnetization axis) stabilizes a ferromagnetic correlation between the Ce ions and enhances the hopping of carriers. This results in large negative magnetoresistance along the b axis.Comment: 24 pages, including 9 figure

How chemistry controls electron localization in 3d1 perovskites: A Wannier-function study

In the series of 3d1 t2g perovskites, SrVO3--CaVO3--LaTiO3--YTiO3 the transition-metal d electron becomes increasingly localized and undergoes a Mott transition between CaVO3 and LaTiO3. By defining a low-energy Hubbard Hamiltonian in the basis of Wannier functions for the t2g LDA band and solving it in the single-site DMFT approximation, it was recently shown[1] that simultaneously with the Mott transition there occurs a strong suppression of orbital fluctuations due to splitting of the t2g levels. The present paper reviews and expands this work, in particular in the direction of exposing the underlying chemical mechanisms by means of ab initio LDA Wannier functions generated with the NMTO method. The Wannier functions for the t2g band exhibit covalency between the transition-metal t2g, the large cation-d, and the oxygen-p states; this covalency, which increases along the series, turns out to be responsible not only for the splittings of the t2g levels, but also for non-cubic perturbations of the hopping integrals, both of which are decisive for the Mott transition. We find good agreement with the optical and photoemission spectra, with the crystal-field splittings and orbital polarizations recently measured for the titanates, and with the metallization volume for LaTiO3. The metallization volume for YTiO3 is predicted. Using super-exchange theory, we reproduce the observed magnetic orders in LaTiO3 and YTiO3, but the results are sensitive to detail, in particular for YTiO3 which, without the Jahn-Teller distortion, would be AFM C- or A-type, rather than FM. Finally, we show that it possible to unfold the orthorhombic t2g LDA bandstructure to a pseudocubic zone. In this zone, the lowest band is separated from the two others by a direct gap and has a width, W_I, which is significantly smaller than that, W, of the entire t2g band. The progressive GdFeO3-type distortion favours electron localization by decreasing W, by increasing the splitting of the t2g levels and by decreasing W_I. Our conclusions concerning the roles of GdFeO3-type and JT distortions agree with those of Mochizuki and Imada [2].Comment: Published version, final. For high resolution figures see http://www.fkf.mpg.de/andersen/docs/pub/abstract2004+/pavarini_02.pd

Coexistence of ferromagnetism and superconductivity in cerium, tellurium (1.82).

CeTe1 .82 crystallizes in layered tetragonal Cu2Sb-type structure. At ambient pressure, the presence of the charge density wave (CDW) gap accompanying with a lattice distortion in the Te sheet is verified by electron-tunneling spectroscopy measurement. We measured the electrical resistivity on a CeTe1 .82 single crystal as a function of temperature and magnetic field (i // ab-plain, H// c-axis) performed under hydrostatic pressure up to 9 kbar using the 20 T superconduting magnet at the Pulse Field Facility, NHMFL, Los Alamos National Laboratory . Some of the results are shown in Fig.l, which represent the low temperature dependence of the normalized resistance under applied pressure of 5 kbar, the superconducting transition is observed at TC = 2 .7K (well below the magnetic ordering temperatures, the Neel temperature TN = 4 .8K and the shortrange ferromagnetic temperature TSRF = 6K in 5 kbar). This (SC) transition temperature is the highest among f-electron systems . The application of magnetic field suppresses the resistivity drop as expected for a supeconducting transition . The coexisting magnetic structure of a mixed ferromagnetism and antiferromagnetism can provide a clue for this remarkably high Tsc