Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session S10: Heavy Fermion Effects in Pu, Ce, and U |
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Sponsoring Units: DMP Chair: Cedomir Petrovic, Brookhaven National Laboratory Room: Colorado Convention Center Korbel 1E |
Wednesday, March 7, 2007 2:30PM - 2:42PM |
S10.00001: Theory of magnetism in Pu at high magnetic fields Per Soderlind Density-functional theory (DFT), in conjunction with the fixed-spin-moment (FSM) method, spin-orbit coupling (SO), and orbital polarization (OP), is shown to provide an apparent accurate picture of $\delta $-Pu, while opening the possibility of a zero net magnetic moment due to complete spin- and orbital-moment cancellation. Calculated total energies, photoemission spectra, and magnetic form factors appear consistent with available experimental data. Calculations including SO, OP and the Zeeman term in the Hamiltonian, address Pu in high magnetic fields $\sim $ 100 T. The Pu phase stability, as a function of applied field, is also investigated. This work was performed under the auspices of the US DOE by the UC LLNL under contract no. W-7405-Eng-48. [Preview Abstract] |
Wednesday, March 7, 2007 2:42PM - 2:54PM |
S10.00002: Strongly correlated state in $\delta$-Pu and Am A. Shick, J. Kolorenc, L. Havela, V. Drchal We investigate the electron-electron correlation effects in the vicinity of the localization threshold of the 5f series, which is crossed between Pu and Am. The ground state electronic and magnetic properties are calculated making use of static {\em around-mean-field} LDA+U approximation. We obtain non-magnetic $\delta$-Pu with 5f occupation $n_f$ = 5.4, and non-magnetic $fcc$-Am with $n_f$=6.0. The equilibrium volumes and bulk moduli are obtained in a good agreement with experiment. For Pu-Am alloys, neither tendency to a 5f localization nor formation of Pu local magnetic moments was found despite a lattice expansion caused by the Am atoms. The excitation spectra of $\delta$-Pu and $fcc$-Am are calculated on the basis of the Dynamical Mean-Field theory (DMFT). Starting from LDA+U ground state we included multiplet transitions using the Hubbard-I approximation [1], which gives a good agreement with experimental photoelectron spectra of $\delta$-Pu, Am, and their selected compounds. The spectral density at Fermi level explains the high $\gamma$-coefficient of the electronic specific heat found experimentally in $\delta$-Pu. The calculations show that atomic-like excitations can be observed in a solid-state environment even if the 5f states are not fully localized as in $\delta$-Pu. [1] A. Shick, J. Kolorenc, L. Havela {\em et al.}, arXiv: cond- mat/0610794 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 2:54PM - 3:06PM |
S10.00003: Dynamical Mean Field Treatment of the Valence Transition in Yb Erik Ylvisaker, Andrew McMahan, Warren Pickett At ambient pressure, Yb metal is a divalent rare-earth with configuration $f^{14}v^2$ where $v$ represents the valence ($s$, $p$, $d$) occupation. When pressure is applied it undergoes a gradual transition to a trivalent configuration ($f^ {13}v^3$), completing by $34$ GPa. We investigate this valence transition using the all-electron DMFT(HI) method (which includes 6s, 6p, 5d and 4f in the basis) with the Hubbard I atomic solver. Experimental evidence suggests that this transition involves a linear combination of these two many-body states ($\alpha f^{13} + \beta f^{14}$) with $\alpha$ increasing as pressure is applied. We present evidence that DMFT can appropriately model this type of wavefunction, and that this is necessary to capture the gradual nature of the valence transition in Yb. The DMFT(HI) results are sensitive to the parameter chosen for the $4f$ energy level and its volume dependence; however, a good description of the valence transition can be achieved with reasonable adjustments in this function. We also compare and contrast DMFT results with LDA+U, which seems to have fundamental difficulties in modeling this transition. The equation of state provided by the DMFT(HI) method is significantly more accurate than the LDA or LDA+U methods give. More rigorous DMFT (QMC) calculations are currently underway to establish what the accuracy of the HI approximation is. [Preview Abstract] |
Wednesday, March 7, 2007 3:06PM - 3:18PM |
S10.00004: From $\delta$-Pu to PuCoGa$_5$: Kondo effect investigations of strong electronic correlations in Pu Eric D. Bauer, J. N. Mitchell, D. S. Schwartz, J. D. Thompson, J. L. Sarrao Within the actinide series, plutonium is located at the itinerant/localized boundary between the strongly hybridized 5 $f$ states of U and localized 5$f$ states of Am. The hybridization is intimately connected with the range of interesting behavior found in elemental Pu such as six allotropic phases, the largest effective mass enhancement of any element, and a liquid phase whose density is less than its solid phase, much like ordinary water. Despite decades of research, the strongly correlated electron state of Pu is still not understood. The exciting discovery of superconductivity in PuCoGa$_5$ with T$_c$ = 18.5 K, offers a possibility of understanding strong electronic correlations in Pu. The 5$f$ electrons in PuCoGa$_5$ are neither fully localized nor fully itinerant, similar to the cubic delta phase of Pu. Investigations of the Kondo effect, considered to be the hallmark of strong electronic correlations, in dilute intermetallic systems such as {\bf{Th}}Pu and Lu$_{1-x}$Pu$_x$CoGa$_5$ will yield information about the orbital degeneracy of the Pu 5$f$ electrons, their degree of localization, and a characteristic energy scale of the electronic correlations. Measurements of magnetic susceptibility, electrical resistivity, and specific heat will be presented. [Preview Abstract] |
Wednesday, March 7, 2007 3:18PM - 3:30PM |
S10.00005: Ultra-High Magnetic Field Study of Actinide Elements Charles Mielke, Ross McDonald The magnetic susceptibility and electrical conductivity of elemental plutonium and uranium are predicted to reveal the highly correlated nature of the elements upon application of very intense magnetic fields. A specialized Ultra-High Field generation system has been built and commissioned to study the effects of applied magnetic fields to actinide specimens. Magnetic fields to 150 tesla are routine with maximum field intensity extending to well above 200 tesla. The single turn system is designed to generate fields above 100 tesla while not damaging or dispersing the sample under study. First experimental results will be discussed as well as brief review of the system and techniques. [Preview Abstract] |
Wednesday, March 7, 2007 3:30PM - 3:42PM |
S10.00006: Volume collapse in Ce alloys under pressure by neutron diffraction James L. Smith, Anna Llobet, Sergei M. Stishov, Darrick Williams, Jason C. Lashley Neutron-diffraction measurements under hydrostatic pressure up to 10 kbar were performed on the Ce$_{0.9-x}$La$_{x}$Th$_{0.10}$ system to investigate the tricritical point at $x_{c}$ = 0.14. For $x < \quad x_{c}$, we observe first-order transitions with a pressure derivative of the transition temperature, d$T$/d$P$ = 20 K/kbar. For $x \quad > \quad x_{c}$ we observe a continuous transition that is second order, which again demonstrates a tri-critical point in the pressure-temperature phase diagram. The results will be presented and discussed. [Preview Abstract] |
Wednesday, March 7, 2007 3:42PM - 3:54PM |
S10.00007: Phonon Anomalies in $\alpha$-Uranium Xiaodong Yang, Peter Riseborough The temperature dependence of the phonon spectra of $\alpha$- uranium has recently been measured by Manley {\it et al.}[1] using inelastic neutron scattering and inelastic x-ray scattering techniques. Although there is little evidence of any anharmonicity, the phonon shows some softening in the optic modes at the zone boundary. In a later publication [2], an extra mode was reported to form at high temperatures, which is incompatible with a structure composed of a monoclinic Bravais lattice with a two-atom basis. We investigate the effect that the f electron-phonon interaction has on the phonon spectrum and its role on the possible formation of a breathing mode of mixed electron and phonon character. \newline \newline [1] M. E. Manley, B. Fultz, R. J. McQueeney, C. M. Brown, W. L. Hults, J. L. Smith, D. J. Thoma, R. Osborn, and J. L. Robertson, {\it Phys. Rev. Lett.} 86 (2001), p3076. \newline [2] M. E. Manley, M. Yethiraj, H. Sinn, H. M. Volz, A. Alatas, J. C. Lashley, W. L. Hults, G. H. Lander, and J. L. Smith, {\it Phys. Rev. Lett.} 96 (2006), p125501. [Preview Abstract] |
Wednesday, March 7, 2007 3:54PM - 4:06PM |
S10.00008: Spectral Properties of Elemental Pu Studied by LDA+DMFT Jian-Xin Zhu, A.K. McMahan, M.D. Jones, J.M. Wills, R.C. Albers The merger of density functional theory in the local density approximation and the many-body dynamical mean field theory is a powerful theoretical technique for the study of strongly correlated electron materials. We present calculations of spectral properties of the $\delta$-phase plutonium by combining for the first time the sophisticated tight-binding method with a recent implementation of quantum Monte Carlo techinque. The tight-binding parameters are determined from the fit to the full-potential linearized augmented plane-wave calculation for the face-centered-cubic crystal structure of the slightly compressed $\delta$-phase plutonium. The computationally more expensive but rigorous quantum Monte Carlo simulation is supplemented by the more efficient but approximate Hubbard-I method. By comparing the calculations without and with spin-orbit interaction included, we discuss our results in the context of several key features observed in the photoemission spectroscopy. [Preview Abstract] |
Wednesday, March 7, 2007 4:06PM - 4:18PM |
S10.00009: Role of Phonons in Heavy Fermion Volume Collapse within the Periodic Anderson Model M. A. Majidi, J. Moreno, B. Moritz, A. Macridin, M. Jarrell, A. K. McMahan X-ray and neutron diffraction studies by Jeong et al. indicate the involvement of phonons in the volume collapse of Cerium. Lattice vibrations may also be important in other heavy fermion materials with large volume changes such as Praseodymium. Whether phonons drive the volume change, or the effect has an electronic origin and phonons play a secondary role, is unknown. We address this problem within the Periodic Anderson Model by introducing Holstein phonons coupled with the localized f electrons. We solve this model in three dimensions using the dynamical cluster Quantum Monte Carlo technique to incorporate non-local correlations. In the Kondo regime we calculate the renormalized d-f hybridization with respect to the on-site Coulomb repulsion and the electron-phonon coupling at various temperatures. We also investigate the temperature dependence of the isotropic ionic displacements and compare them with Jeong's experimental results. [Preview Abstract] |
Wednesday, March 7, 2007 4:18PM - 4:30PM |
S10.00010: Fermi Surface of the Kondo Lattice Antiferromagnets and Ferromagnets: A Continuum Field Theory Approach Seiji Yamamoto, Qimiao Si Studies in the quantum phase transitions of heavy fermion metals have raised the question about the nature of the Fermi surface in a heavy fermion antiferromagnet (AF). Related questions are relevant to their ferromagnetic (FM) counterparts. Here, we study the Kondo lattice model in the limit that the Kondo coupling is small compared to the direct (RKKY) exchange coupling. We map the spin-$1/2$ Heisenberg Hamiltonian for the local moment component to a quantum nonlinear sigma model. This leads to an effective coupling between a vector boson field and the conduction electrons, which is dominated by the forward-scattering channel. There is also a Berry phase term which can be ignored in the AF case, but must be included in the FM case. We establish that the Fermi surface in the AF case is small [1], and also present the results for the FM case. The implications for the global zero-temperature phase diagram of the FM/AF heavy fermions are discussed. [1] S. J. Yamamoto and Q. Si, cond-mat/0610001 [Preview Abstract] |
Wednesday, March 7, 2007 4:30PM - 4:42PM |
S10.00011: Non-linear Sigma Model of Kondo Lattice in Antiferromagnetic Regime Tzen Ong, Barbara Jones We analyze the antiferromagnetic transition in heavy fermion compounds in two dimensions, which we study using the Kondo-Heisenberg model. The system is assumed to be in the antiferromagnetic regime, with a Heisenberg coupling (J$_{H})$ that is larger than the Kondo coupling (J$_{K})$. The Heisenberg terms are mapped onto a non-linear sigma model, and the fermions are then formally integrated out to obtain an effective theory for the Neel field. We then study the evolution of the Heisenberg and Kondo couplings under renormalization, and calculate the critical exponents at the phase transition. We also find that a mean field calculation of this model gives a pairing state (superconductivity) in part of the J$_{H}$ /J$_{K}$ phase space. By symmetry arguments, unless the transition from AFM to SC is first order, this suggests the existence of a new state in between the antiferromagnetic and SC states, and we consider the possibility of a novel new ground state in this regime. [Preview Abstract] |
Wednesday, March 7, 2007 4:42PM - 4:54PM |
S10.00012: Nonperturbative Scaling of the Disorder Quenched Kondo Effect Stefan Kettemann, Andrey Zhuravlev, Isa Zharekeshev, Eduardo Mucciolo The quenching of the Kondo effect in weakly disordered metals with dilute magnetic impurities is studied by means of the numerical renormalisation group method. Using the one particle basis of a disordered tight binding model on a square latttice of size L, we calculate the temperature dependence of the local magnetic susceptibility. We find a finie probability that the magnetic moment remains unscreened at the lowest temperatures. This probability is calculated as function of the exchange coupling J,lattice size L and disorder amplitude W. These results are compared with analytical and numerical methods, based on the solution of the self consistent 1-loop equation (Nagaoka-Suhl), as well as with the numerical soultion of the 2-loop renormalisation group equation. Experimental consequences for disordered metals are studied. In particular, it is shown that the presence of magnetic impurities with small Kondo temperatures enhances the electron's dephasing rate at low temperatures in comparison to the clean metal case. [Preview Abstract] |
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