Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session U27: Correlated Electrons: Theory, Actinides, and Ising Systems |
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Sponsoring Units: DCMP Chair: Makariy Tanatar, University of Sherbrooke Room: Morial Convention Center 219 |
Thursday, March 13, 2008 8:00AM - 8:12AM |
U27.00001: Quantitative Calculation of the Spatial Extension of the Kondo Cloud Bergmann Gerd A recently developed compact solution for the singlet state of the Friedel-Anderson and the Kondo impurity is applied to investigate the old question of a Kondo cloud in the Kondo ground state. Wilson's states with an exponentially decreasing frame of energy cells towards the Fermi level are used. The Wilson states are expressed as free electron waves with a linear dispersion and integrated over the width of their energy cells. For the magnetic state of the Friedel-Anderson impurity one finds essentially no spin polarization in the vicinity of the d-impurity. However, for the magnetic \emph{component} of the singlet state a spin polarization cloud is observed which screens the spin (magnetic moment) of the d-electron. The range $\xi_{K}$ of this polarization cloud is investigated in detail for the Kondo impurity. The range is inversely proportional to the Kondo energy $\Delta _{K}$. The extent of the electron density in real space is a detector for a resonance in energy. The spatial extension $\xi $ and the resonance width $\Delta $ are reciprocal and given by the relation $\xi \Delta \approx \hbar v_{F}$. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U27.00002: Perturbative Cumulant Monte Carlo Study of LiHoF$_4$ in a Weak Transverse Magnetic Field S.M. Ali Tabei, Michel Gingras, Ying-Jer Kao, Taras Yavorskii Results from a recent quantum Monte Carlo (QMC) (P.B. Chakraborty {\it et al.}, Phys. Rev. B {\bf 70}, 144411 (2004)) study of the LiHoF$_4$ Ising magnetic material in an applied transverse magnetic field $B_x$ show a discrepancy with the experimental results, even for small $B_x$ where quantum fluctuations are small. This discrepancy persists asymptotically close to the classical ferromagnet to paramagnet phase transition. We numerically reinvestigate the phase diagram of LiHoF$_4$ in the regime of weak $B_x$. In this regime, we derive an effective temperature-dependent classical Hamiltonian that incorporates perturbatively the small quantum fluctuations in the vicinity of the classical phase transition at $B_x=0$. Via this effective classical Hamiltonian, we study the $B_x-T$ phase diagram via classical Monte Carlo simulations. In particular, we investigate the influence of various effects that may be at the source of the discrepancy. We also show how our method can be generalized to numerically study the diluted LiHo$_x$Y$_{1-x}$F$_4$ in the small $B_x$ regime. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U27.00003: A New Heavy Fermion Compound Yb$_{3}$Pt$_{4}$ Marcus Bennett, Peter Khalifah, Dmitriy Sokolov, Yiu Yuen, Moosung Kim, Carl Henderson, William Gannon, Meigan Aronson We report the synthesis of single crystals of a new binary heavy fermion system, Yb$_{3}$Pt$_{4}$. Magnetic susceptibility measurements find Yb$^{3+}$ local moment behavior above 150 K. Heat capacity measurements find a large weakly first order anomaly at 2.4 K, and the associated entropy indicates that magnetic order emerges from a doublet ground state. Magnetic field suppresses both the magnitude of the anomaly and the temperature at which the anomaly occurs, mapping out a first order phase line that ends at a tri-critical point, 1.75 T, 1.3 K. A weak cusp in the AC magnetic susceptibility indicates antiferromagnetic ordering. Above 0.2 T, the cusp becomes a step, which increases in height with increasing field indicating ferromagnetic order. The electrical resistivity of Yb$_{3}$Pt$_{4}$ is that of a good metal, and the quadratic temperature dependence of a Fermi liquid is found throughout the antiferromagnetically ordered state and continues into the high field paramagnetic state. Both the magnitude of the quadratic temperature dependence of the resistivity and of $\gamma$ are comparable to that found in heavy fermion compounds, indicating substantial quasiparticle mass enhancement. The Sommerfeld-Wilson ratio approaches 30 in the ordered state, suggesting strong ferromagnetic correlations among the quasiparticles. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U27.00004: Pressure and magnetic field effects in heavy-fermion UCu$_{3.5}$Al$_{1.5 }$ A. Alsmadi, H. Nakotte, V. Zapf, F. Fabris, T.D. Didn, A. Lacerda, J. Kamarad UCu$_{3.5}$Al$_{1.5}$ crystallizes in the hexagonal CaCu$_{5}$ structure and is described as a heavy fermion, which shows non-Fermi liquid behavior[1]. Here, we report on electrical resistivity, magnetic susceptibility, and magnetization results on polycrystalline UCu$_{3.5}$Al$_{1.5}$. The resistivity was measured under hydrostatic pressure up to 10kbar and in fields up to 18T. At ambient pressure and in zero field, the resistivity shows an anomaly at T$_{1}$=19K and then it goes through a maximum at T$_{max}$=2K. These two anomalies were also observed in the susceptibility data. The anomaly in the resistivity at T$_{1}$ goes to lower values with increasing fields and disappear at fields about 12T. T$_{max}$ on the other hand goes to higher values with increasing fields. We find relatively weak pressure dependence, where both T$_{1}$ and T$_{max}$ go down with increasing pressure. In the field scan at 2K and at ambient pressure, we find a change in the slope of the magnetoresistance at about 6.9T. Application of pressure causes a reduction of the magnetoresistance effect. \newline [1] H. Nakotte, et al., Phys. Rev. B 54, 12176 (1996) [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U27.00005: Optical spectra of the heavy fermion uniaxial ferromagnet UGe2 Violeta Guritanu, Peter Armitage, Riccardo Tediosi, Siddharth Saxena, Andrew Huxley, Dirk van der Marel We report on a detailed optical study of UGe$_{2}$ single crystalline material using infrared reflectivity and spectroscopic ellipsometry. The optical conductivity suggests the presence of a low frequency interband transition (~ 300 $cm^{-1}$) and a narrow free-carrier response with strong frequency dependence of the scattering rate and effective mass. We observe sharp changes in the low frequency mass and scattering rate below the upper ferromagnetic transition T$_{C_{1}}$. They recover their unrenormalized value above T$_{C_{1}}$ and for $\omega >$ 250 cm$^{-1}$. In contrast no sign of an anomaly is seen at T$_{C_ {2}} \sim$ 30 K, which is the lower transition of unknown nature. These observations are consistent with the weak anomaly observed at T$_{C_{2}}$ in transport and thermodynamic experiments. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U27.00006: Ferromagnetism of Silicon Doped with Uranium Investigated to Extremes of Magnetic Field (Beyond 100 tesla) Charles Mielke, Jason Cooley, William Hults The ferromagnetic (FM) phase of Si:U x At. \% (where x = 0.25, 0.5, 0.75, 1.0, and 50.0 (i.e. USi)) were studied in high magnetic fields as a function of temperature and U concentration. The effect of doping U into Si is investigated vis a vis the FM transition temperature (127K for x = 50.0) and high magnetic field saturation is discussed. The effect of the FM transition temperature is approached from the point of view of correlation effects in f-electron systems. Attention to the high magnetic field saturation is investigated as it is unusually high in the x = 50.0 intermetallic compound. Ultra-high field data extending to 185 tesla is reported for the system. Issues with homogenization of the dilute samples are presented as well. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U27.00007: Electronic Structure of Actinide Materials J.J. Joyce, T. Durakiewicz, K.S. Graham, D.P. Moore, L.A. Morales, J.M. Wills, R.L. Martin, J.-X. Zhu, L.E. Roy, C.G. Olson, G.E. Scuseria, I.D. Prodan Photoelectron spectroscopy results for both metallic and insulating actinide materials are reviewed and compared against model calculations. The dual nature of 5f electron characteristics is discussed for photoemission results and three different electronic structure calculations. Magnetic configurations as a means of f-electron localization are discussed for metallic materials. The photoemission results for U and Pu intermetallics are compared against mixed-level-model and dynamical-mean-field-theory calculations. The experimental results for the actinide oxide Mott insulators are compared against screened hybrid functional calculations. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U27.00008: Effects of full Coulomb interactions on electronic structure of \textit{$\delta $}-Pu Eugeny Gorelov, Tim Wehling, Hartmut Hafferman, Alexander Lichtenstein, Alexey Rubtsov, Alexander Landa, Chris Marianetti, Michael Fluss, Alexey Shorikov, Alexey Lukoyanov, Michael Korotin, Vladimir Anisimov We used the CTQMC method for the realistic simulation of electronic properties of correlated actinides. In particular, we focus on the spectral function of \textit{$\delta $}-Pu, which is described in terms of a 7-orbital $f$-impurity model interacting with a metallic bath. Our CTQMC implementation solves this model by calculating a weak coupling expansion of the partition function in the fermionic multiorbital path-integral representation and provides numerically exact results for relatively high temperature. We discussed how different terms in the full on-site Coulomb vertex affect the local density of states. The comparison of CTQMC results with only diagonal density-density like Coulomb interactions and with additional non-diagonal terms in the interaction part of the Hamiltonian related with so-called spin flips terms shows the importance of the full rotationally invariant Coulomb vertex on the electronic structure of \textit{$\delta $}-Pu. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U27.00009: First-principles study of electronic structure and local moment interactions in PuAm alloy Myung Joon Han, Xiangang Wan, Sergej Y. Savrasov Expected to provide a clue about the origin of zero moment in the bulk phase of Plutonium, Pu$_{1-x}$Am$_{x}$ alloy has attracted a great attention, in which as Am ratio, $x$, increases, Pu approaches from bulk to atomic limit. To understand the electronic structure and the magnetic properties of Pu in different crystal environments, we perform fully self-consistent first-principles calculations of the PuAm system based on the density functional theory. Electronic structure strongly depends on the level of approximation for correlation effects. The exchange interactions between Pu 5f electrons and the Kondo screening strength were estimated and compared, which provide a new insight to Pu magnetism. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U27.00010: Knight shifts around nonmagnetic impurities in a trinagular lattice spin 1/2 antiferromagnet: Case of $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ Karol Gregor, Olexei Motrunich We study effects of nonmagnetic impurities in a spin-1/2 frustrated triangular antiferromagnet with the aim of understanding the observed broadening of $^{13}$C NMR lines in the organic spin liquid material $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$. For high temperatures down to $J/2$, we calculate local susceptibility near a nonmagnetic impurity and near a grain boundary for the nearest neighbor Heisenberg model in the high temperature series expansion. We find that the local susceptibility decays to the uniform one very quickly (few lattice spacings), and with the suggested density of impurities would not explain the observed line broadening present already at elevated temperatures; more extended defects and/or longer-ranged interactions are probably needed. At low temperatures, we assume a gapless spin liquid with a Fermi surface of spinons. We calculate the local susceptibility in the mean field and also go beyond the mean field by Gutzwiller projection. Here the Knight shift decays with a power law and oscillates at $2 k_F$. However, with single site impurities the results fall short of the observed inhomogeneous broadening, calling for a better understanding of the appropriate models for the spins and impurities and of the possible ground states that are probed by such experiments. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U27.00011: Excitons in the 1D Hubbard Model: a Real-Time Study Khaled Al-Hassanieh, Adrian Feiguin, Fernando Reboredo, Ivan Gonzalez, Elbio Dagotto We study the real-time dynamics of a pair hole/doubly-occupied-site, namely a holon and a doublon, in a 1D Hubbard insulator with on-site and nearest-neighbor Coulomb repulsion. Our analysis shows that the pair is long-lived and the expected decay mechanism to underlying spin excitations is actually inefficient. For a nonzero inter-site Coulomb repulsion, we observe that part of the wave-function remains in a bound state. Our study also provides insight on the holon-doublon propagation in real space. Due to the one-dimensional nature of the problem, these particles move in opposite directions even in the absence of an applied electric field. The potential relevance of our results to solar cell applications is discussed. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U27.00012: Investigation of Dipole-Forbidden $d-d$ Excitations in Strongly Correlated Transition-Metal Oxides Using Higher-Order Multipole, Non-resonant Inelastic X-Ray Scattering B.C. Larson, J.Z. Tischler, C.-L. Yeh, C.-C. Lee, Wei Ku We have shown that quadrupole and higher order multipole non-resonant inelastic x-ray scattering (NIXS) at large wavevectors, $q$, provides direct access to dipole-forbidden $d-d$ excitations (Larson et al. \textit{Phys. Rev. Lett.} \textbf{99}, 026401 (2007)). NIXS measurements using the XOR/UNI beamline at the APS have shown that the large-$q $intensity of on-site excitons in NiO and CoO is highly anisotropic in $q$ and dominates the energy loss spectrum. Energy-resolved Wannier function analyses have shown that the anisotropies, including a nodal direction for NiO, provide direct information on the point-group symmetry of the particle-hole wave functions for transition-metal oxides. The interpretation of these large-$q$ NIXS measurements will be discussed in connection with energy-resolved Wannier function analyses and LDA+$U$ dynamical response calculations. Implications for the extension of such investigations to manganite systems will be considered. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U27.00013: Exact many-electron ground states on the diamond Hubbard chain Zsolt Gulacsi, Arno Kampf, Dieter Vollhardt Exact ground states of interacting electrons on the diamond Hubbard chain in a magnetic field are constructed which exhibit a wide range of properties such as flat-band ferromagnetism, correlation induced metallic, half-metallic, or insulating behavior [1]. The properties of these ground states can be tuned by changing the magnetic flux, local potentials, or electron density.The results show that the studied simple one-dimensional structure displays remarkably complex physical properties. The virtue of tuning different ground states through external parameters points to new possibilities for the design of electronic devices which can switch between insulating or conducting and nonmagnetic or (fully or partially spin polarized) ferromagnetic states, open new routes for the design of spin-valve devices and gate induced ferromagnetism. \break [1] Z. Gulacsi, A. Kampf, D. Vollhardt, Phys. Rev. Lett. {\bf 99}, 026404(2007). [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U27.00014: Phonon effect on elementary excitations in one-dimensional Mott insulators Hiroaki Matsueda, Akihiro Ando, Takami Tohyama, Sadamichi Maekawa We examine the interplay among phonons and elementary excitations in one-dimensional Mott insulators. For this purpose, we perform dynamical density matrix renormalization group calculations for the single-particle excitation and optical absorption spectra in the extended Hubbard-Holstein model. We find that the elementary excitations in the Mott insulators are quite easily modified by the phonons. In particular, the spinon branch in the single-particle excitation spectrum can be broadened, even when the phonons only couple with charge degrees of freedom. In addition, the exciton in the photoexcited state becomes polaronic, and this polaronic feature is enhanced by increasing the on-site Coulomb repulsion. We show the origins of these novel properties, and discuss implications of the present results in light of spectroscopic measurements in 1D cuprates. [Preview Abstract] |
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