Session H15: Metals: Electronic Structure, Fermiology, Transport

Angle Resolved Photoemission Spectroscopy of Single Crystal Uranium (001)

Using Angle Resolved Photoemission Spectroscopy (ARPES) we measured the energy dispersion for binding energy (BE) $<$ 8 eV at 173 and 373 K on a high quality uranium single crystal (001) along the principle crystallographic directions $\Gamma $ to Y, $\Sigma $, and S. These measurements are compared with WIEN2K band structure calculations. We report significant correspondence between our band dispersion and theoretical calculations. Temperature and electron correlation effects will be discussed.   

Orbital-Resolved Soft X-ray Spectroscopy Measurnents of Sr$_{2}$RuO$_{4}$ Electronic Structure.

Strontium ruthenate (Sr$_{2}$RuO$_{4})$ is a spin-triplet superonductor$^{1}$ of the Ruddlesden-Popper (RP) series with n=1. We have studied the electronic structure of single crystals of this system by density functional theory (DFT) calculations and soft x-ray emission and absorption measurements at the oxygen K edge (530 eV) and find that DFT calculations provide an excellent model to characterize the molecular orbitals corresponding to the inequivalent O sites in this perovskite compound. The linearly polarized radiation excites primarily O[1] (planar) sites at almost normal incidence and O[2] (apical) sites at grazing incidence. Our results suggest a larger level of hybridization between the Ru d and the O p$_{ }$states in the RuO$_{2}$ planes than between the Ru and apical O atoms, which also implies more localized bonding along the apical direction than the planar direction. This work was supported by DOE grant No. DE-FG02-05ER-46183. 1)Y. Maeno, et al, Nature 372, 532 (1994).   

Electronic Structure and Bulk Spin Valve Behavior in Ca$_3$Ru$_2$O$_7$

The perovskite based ruthenates show a remarkable range of electronic and magnetic properties, even though they are all based on Ru$^{4+}$ ions in octahedral environments with corner sharing bonding topologies. These behaviors include robust itinerant ferromagnetism, paramagnetic “bad metal” properties, unconventional superconductivity, Mott insulating properties and metamagnetic quantum critical behavior. The bilayer, so- called 327 compounds are of particular interest because of their borderline properties. We report density functional calculations of the magnetic properties and Fermiology of Ca$_3$Ru$_2$O$_7$. The ground state consists of ferromagnetic bilayers, stacked antiferromagnetically. The bilayers are almost but not exactly half-metallic. In the ferromagnetic state opposite spin polarizations are found for in-plane and out-of-plane transport. Relatively high out of plane conductivity is found for the majority spin, which is relatively weakly conductive in- plane. In the ground state in-plane quantities are essentially the same, but the out of plane transport is strongly reduced. This work was supported by the Department of Energy.   

First Observation of MCD at O and Ru sites in Sr$_{4}$Ru$_{3}$O$_{10}$

Perovskite strontium ruthenates of the Ruddlesden-Popper (RP) series Sr $_{n+1}$Ru$_{n }$O$_{3n+1 }$show a variety of complex behavior including superconductivity and metamagnetism. Sr$_{4}$Ru$_{3}$O$_{10}$ is a trilayer member of the RP series with n=3 that is metamagnetic and shows an electronic phase separation$^{1}$. We wanted to test if this material showed magnetic behavior at low fields and performed magnetic circular dichroism measurements (MCD) at the O K and Ru L edges. The measurements were carried out at the Sector 4 polarization beam line of the Advanced Photon Source at grazing (10\r{ }) and normal incidence angles at the characteristic temperature 50K ~below which ~metamagnetic behavior occurs$^{2}$ and above the ordering temperature (150K). We observed dichroic behavior at both temperatures although the dichroism was reduced by a factor of 10 at 150K relative to the measurements at 50K. It is significant to note that only the lowest unoccupied orbital (LUMO) associated with the planar O site showed dichroic behavior. This research was supported in part by DOE grant No. DE-FG02-05ER-46183, and the Advanced Photon Source is supported by DOE grant W-31-109-ENG-38. \begin{enumerate} \item Z.Q. Mao et al, submitted to Physical Review Letter. \item G. Cao et al, Phys. Rev. B \textbf{68}, 174409 (2003) \end{enumerate}   

Zero bias anomaly in the low-temperature scanning tunneling spectra of Sr3Ru2O7

We have studied a zero bias anomaly in scanning tunneling spectra of Sr3Ru2O7 as function of temperature. The dip-like feature in the dI/dV spectra is observed in an energy range of --20meV to +20meV around Fermi energy. The depth of the dip decreases with increasing temperature in a temperature range of 5 to 30 Kelvin. The temperature dependence of the dip is not due to the thermal broadening connected to the tunneling process. The results on Sr3Ru2O7 are compared with temperature dependent scanning tunneling spectra of Sr2RuO4. Finally, we discuss different effects as possible explanations for the zero bias anomaly.   

Fermi-liquid-like quasiparticle excitations in Sr$_{2}$RhO$_{4}$

The layered 4d transition metal oxide Sr$_{2}$RhO$_{4}$ supports a highly anisotropic, well-defined Fermi-liquid-like electron fluid. Angle resolved photoemission spectroscopy shows a Fermi surface and quasiparticle effective masses in quantitative agreement with bulk de Haas-van Alphen data. Sr$_{2}$RhO$_{4}$ thus presents a prime opportunity to study the electronic self energy in a well-defined model system. To this end, we present a comprehensive spectral function analysis of high resolution angle resolved photoemission data from Sr$_{2}$RhO$_{4}$. Preliminary data at very low photon energies are presented and compared to state-of-the-art data taken at conventional UV photon energies.   

Quantum oscillations in RAgSb$_2$ (R = La, Sm, Y) as seen via magnetostriction measurements

We present low temperature longitudinal magnetostriction measurements taken using a capacitance dilatometer in a PPMS-14 instrument for several RAgSb$_2$ (R = rare earth) compounds. The quantum oscillations in magnetostriction were observed up to temperatures as high as 25 K. These data are compared with the results of de Haas - van Alphen and Shubnikov - de Haas measurements. Using these data the uniaxial stress dependencies of the extremal Fermi surface orbits can be evaluated.   

Local Flattening of the Fermi Surface and Quantum Oscillations in the Magnetoacoustic Response of a Metal

In the present work we theoretically analyze the effect of the Fermi surface local geometry on quantum oscillations in the velocity of an acoustic wave travelling in metal across a strong magnetic field. We show that local flattenings of the Fermi surface could cause significant amplification of quantum oscillations [1]. This occurs due to enhancement of commensurability oscillations modulating the quantum oscillations in the electron density of states on the Fermi surface. The amplification in the quantum oscillations could be revealed at fitting directions of the magnetic field. \newline \newline [1] N. A. Zimbovskaya, cond-mat/0505301.   

Theory of the Cyclotron Resonance in Al,Pb,Zn and Cd

A quantum theory of the cyclotron resonance is developed. For a face-centered-cubic (fcc) metal the obvious candidates for the Cyclotron Planes (CP) in which the conduction electron (``electron'',``hole'') circulates are the three families of planes $\{100\}$, $\{110\}$ and $\{111\}$. Following Dresselhaus-Kip-Kittel (DKK,1955) we assume a quadratic energy-momentum ($\hbar k$) relation with the effective mass $(m_{1},m_{2},m_{3})$ and analyze the angle- dependent resonance peaks in terms of Shockley's formula (a generalization of the DKK formula) .For aluminum Al (fcc) an ``electron'' ellipsoid with the major axes in [110] with $(m_1,m_2,m_3)=(0.108,0.156,1.96)m$ is obtained. For lead (Pb) (fcc) a hyperboloid in [110] with $(m_1,m_2,m_3)=(1.18,0.244,- 8.71)m$ and an ``electron'' sphere with $m^*=1.30m$ associated with the CP $\{100\}$ are obtained. For a hexagonal- closed-pack (hcp) metal,the CP is the hexagonal base plane. The effective mass $m_{b}$ for the basal-plane motion and the mass $m_{c}$ along the c-axis for zinc (Zn)[cadmium (Cd)] (both hcp)are $(m_{b},m_{c})$=(1.04,0.212)m [(1.14,0.217)m], which characterize the spheroids with the major axis along the c-axis.   

Magnetic and Transport Properties of RCoIn$_5$(R=Pr,Nd) and RCoGa$_5$ (R=Tb-Tm)

We report on magnetic and transport properties of single crystals of the light rare earth containing series of compounds RCoIn$_5$ (R=Pr,Nd) and heavy rare earth con- taining series RCoGa$_5$ (R=Tb-Tm). All the compounds crystallize in the tetragonal HoCoGa$_5$ crystal structure and are very good metals with small defect scattering at low temperatures. NdCoIn$_5$ and members of the RCoGa$_5$ series with large de Gennes factors order antiferromagnetically.   

Anomalous Nernst Effect in Ferromagnets

We study the intrinsic contribution to the anomalous Nernst effect in ferromagnets at low temperatures. It is shown that the anomalous Nernst conductivity is of topological nature and can be related to the Berry phase of occupied Bloch states. In ferromagnets, there may be nonzero magnetization current even in thermodynamic equilibrium. It can significantly change the thermoelectric transport coefficients. The Onsager relation is established for the charge and heat transport coefficients. Our theory well explains recent experimental result.   

Giant Nernst effect in the Pr-filled skutterudite PrFe$_4$P$_{12}$

The phase transition of PrFe$_{4}$P$_{12}$ at T$_{O}=6.5$ K, probably associated with antiferroquadrupolar ordering, is explored by measuring thermal and thermoelectric transport coefficients. The transition leads to a drastic increase in lattice thermal conductivity pointing to a large drop in carrier concentration induced by the transition and a strong electron-phonon coupling. The low level of carrier density in the ordered state is confirmed by the anomalously large values of the Seebeck and Nernst coefficients. The latter attains a magnitude larger than what is reported for any metal. The results are reminiscent of the hidden-order state of URu$_{2}$Si$_{2}$. According to our analysis of the data, most carriers disappear at the transition, paving the way for an enhanced mean-free-path of both phonons and the residual quasi-particles. Thus, the ordered state of PrFe$_{4}$P$_{12}$ emerges as a heavy-fermion semi-metal.   

Magnetoresistance and Hall Effect in cubic FeGe

An anomalous contribution to Hall Effect is observed in a variety of ferromagnetic systems ranging from simple metals and oxides to manganites, ruthenates and spinel compounds. Its nature (intrinsic or extrinsic) and microscopic mechanism is still an open issue and is being heavily investigated. It was originally attributed to skew or side jump scattering, which corresponds to an asymmetric scattering of conduction electrons by magnetic moments. More recent developments in the field seem to favor a scenario where an electron orbiting around the Fermi Surface at a finite field acquires a Berry phase when the direction of its moment is locked in presence of a strong spin-orbit interaction. FeGe is an itinerant helimagnet with a high Curie temperature of 280K. The lack of inversion symmetry of the crystal structure leads to helimagnetic order via Dzyaloshinskii-Moriya interaction, signature of a strong spin-orbit coupling in an otherwise cubic lattice. We will present new results of magnetoresistance, Hall Effect and magnetization in a single crystal of cubic FeGe and discuss the origin of the Anomalous Hall effect in this system.   

Electrical conductivity of warm expanded aluminum

The electronic and ionic structures of warm expanded aluminum are determined self-consistently using an average-atom formalism based on density-functional theory and Gibbs-Bogolyubov inequality. Ion configurations are generated using a least-square fit of the pair distribution function deduced from the average-atom model calculations. The electrical conductivity is computed from the Kubo-Greenwood formula for the optical conductivity implemented in a molecular dynamics scheme based on density-functional theory. This method goes beyond the Ziman approach used in the average-atom formalism. Moreover, it is faster than performing quantum molecular dynamics simulations to obtain ion configurations for the conductivity calculation. Numerical results and comparisons with experiments are presented and discussed.   

The Kondo effect in the presence of magnetic impurities

We measure transport through gold grain quantum dots fabricated using electromigration, with magnetic impurities in the leads. A Kondo interaction is observed between dot and leads, but the presence of magnetic impurities results in a gate-dependent zero-bias conductance peak that is split due to an RKKY interaction (I) between the spin of the dot and the static spins of the impurities. Both ferromagnetic and anti- ferromagnetic interactions have been observed in different samples. A magnetic field restores the single Kondo peak in the case of an anti-ferromagnetic RKKY interaction, whereas the splitting is enhanced in the case of ferromagnetic interaction. A gate electrode can change the relative interaction strength $T_K/I$. This system provides a new platform to study Kondo and RKKY interactions in metals at the level of a single spin.