Optical and magneto-optical properties of UPtGe J. Schoenes, M. Marutzky, U. Barkow, S. Weber, and R. Troc1 UPtGe has an orthorhombic structure and a substantial magnetic and electric anisotropy. We have determined the optical properties of UPtGe single crystals in dependence of the polarization and propagation vector of the incident light relative to the crystal axes from 6 meV to 32 eV. In addition, the polar magneto-optical Kerr effect has been measured between 1 and 4.5 eV at 12 T. At the lower energies both measurements confirm the strong anisotropy of the thermodynamic data. At higher energies additional band structure effects are found. UPtGe orders antiferromagnetically at TN = 50 K  (see Fig. 1). It has an incommensurable [2,3] cycloic spin structure with a propagation vector along the a axis which is observed rarely in 5f systems. At high fields above 20 T metamagnetic transitions have been reported. Fig. 1: Temperature dependence of the magnetization measured on three single crystals along the different crystal directions. The crystal structure has been determined to be of the noncentrosymmetric EuAuGe type with two uranium sites with different magnetic moments . The electrical and magnetic properties show a small anisotropy in the ac plane, while the electric and magnetic behavior along the b axis is distinct from the ac plane. Fig. 1 shows the temperature dependence of the normalized magnetization along the three crystallographic directions for three single crystals cut perpendicular to the main axes. One clearly recognizes the particular behavior when the magnetic field is aligned parallel to the b axis. This singularity is also manifest in the dielectric function which was determined by optical ellipsometry. Fig.2 displays the real (ε1) and imaginary (ε2) part of the dielectric function between 1 and 10 eV for different orientations of the electric field of the electromagnetic radiation. We recognize most easily in the dispersive part (ε1) of the complex dielectric function that UPtGe is more metallic for E parallel to b. The same conclusion can be drawn by inspecting the optical conductivity derived from a Kramers-Kronig transformation of reflectivity data. Fig. 3 shows the so-obtained real part of the optical conductivity for E parallel to either a or b. The horizontal lines on the left indicate the electrical dc conductivity values. One sees that, while the latter are still somewhat higher than the conductivities at the lowest optical frequencies, the anisotropy is the same. Besides this conduction electron effect Fig.2 evidences a strong anisotropy in the interband part of the spectra. Thus, for E parallel to b a large structure appears in the infrared near 300 cm-1 while for E parallel to a a much smaller structure is found near 400 cm-1. Also for E parallel to b a broad peak occurs around 5000 cm-1 in contrast to a narrower peak at 7000 cm-1 for E parallel to a. Measurements  between 15 and 32 eV performed at the VUVellipsometer in Berlin show in addition maxima at 19 and 20 eV for E parallel to a and E parallel to b, respectively. As an example for the polar Kerr effect measurements, Fig. 4 displays the Kerr rotation spectra θba and Kerr ellipticity spectra ηba for E parallel to b and the magnetic field B of 12 T parallel to c at 51 K. We find a maximum signal of 0.15 deg in agreement with the expectation for a hard uranium based antiferro-magnet . The narrow shape and Fig. 2: Dielectric function with polarization vectors essentially parallel to the a, b, and c-axis. The measurements from 1 to 4 eV have been performed with the home ellipsometer and between 4 and 10 eV at the VUV-ellipsometer at BESSY II. Additionally, the dielectric function calculated by Kramers-Kronig transformation of the reflectivity data is shown. Fig. 3: Real part of the optical conductivity, calculated from the reflectivity spectra by KKR. the size of the magneto-optical spectra at energies where the diagonal conductivity is rather small indicates that the structures near 4 eV involve spin-polarized 5f states. The small U-Pt and U-Ge separation is likely to favor a strong hybridization of 5d(Pt) and 4p(Ge) states with 5f(U) states near the Fermi energy EF. The anisotropic hybridization is made responsible for the anisotropic transport and optical properties in the infrared. Empty 5f states above EF can serve as final state for interband transitions from occupied states (like Pt 5d states) 4 eV below EF. Fig. 4: Kerr rotation θba and Kerr ellipticity ηba for E parallel to b and the magnetic field B of 12 T parallel to c at 51 K. 1 W. Trzebiatowski Institute of Low Temperature and Structure research, Polish Academy of Sciences, Wroclaw, Poland  R. Troc and V. H. Tran, J. Magn. Magn. Mater. 73, 389 (1988).  R. A. Robinson, A. C. Lawson, J. W. Lynn, and K. H. J. Buschow, Phys. Rev. B 47, 6138 (1993).  D. Mannix, S. Coad, G. H. Lander, J. Rebizant, P. J. Brown, J. A. Paixao, S. Langridge, S. Kawamata, and Y. Yamaguchi, Phys. Rev. B 62, 3801 (2000).  R. Troc, J. Stepien-Damm, C. Sulkowski, and A. M. Strydom, Phys. Rev. B 69, 094422 (2004).  M. Marutzky, U. Barkow, S. Weber, J. Schoenes, and R. Troc, Phys. Rev. B 74, 205115 (2006).  J. Schoenes, "Magneto-Optical Properties of Metals, Alloys and Compounds" in "Materials Science and Technology", Eds.: R.W. Cahn, P. Haasen and E.J. Kramer, Vol. 3A, VCH Weinheim , Germany 1992.