Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session T34: Correlated Electrons: Theory |
Hide Abstracts |
Sponsoring Units: GMAG Chair: Juana Moreno, Louisiana State University Room: E144 |
Wednesday, March 17, 2010 2:30PM - 2:42PM |
T34.00001: Theoretical study of phase transition from normal to topological insulators in Na$_2$IrO$_3$ Choong Hyun Kim, Heungsik Kim, Hogyun Jeong, Hosub Jin, Jaejun Yu Recently Na$_2$IrO$_3$ has been suggested to have possible quantum spin Hall effect arising from the novel $j_{\rm eff}=1/2$ state of 5$d$ Ir atoms. The electronic structure of the layered iridium oxides with honeycomb lattice is investigated based on a tight-binding model with spin-orbit coupling included. Our tight-binding model, fitted to the first-principles calculation results, reveals that the electronic states near the Fermi level are not the $j_{\rm eff}=1/2$ states but the $e'_g$ states. The delocalized 5$d$ orbitals lying in the edge sharing octahedron structure leads to (i) a significant direct hopping between neighboring Ir 5$d$ states, (ii) a strong trigonal crystal field, and (iii) non-negligible next-nearest-neighbor and next-next-nearest-neighbor hoppings. A peculiar band structure is found to play a crucial role in determination of the topological nature of the spin-orbit coupled ground state in Na$_2$IrO$_3$. [Preview Abstract] |
Wednesday, March 17, 2010 2:42PM - 2:54PM |
T34.00002: Thermoelectric coefficient for the 3d FCC lattice Hubbard model Louis-Fran\c{c}ois Arsenault, B. Sriram Shastry, Patrick S\'{e}mon, A.M.S Tremblay Thermal transport in strongly correlated materials impacts on both fundamental science and applications. Since calculations based on the standard Kubo formula have proven extremely difficult, Shastry and co-workers [1] have developed two novel approximate ways to obtain the thermopower in interacting systems. One method is based on the high-frequency limit and includes vertex corrections. The other, based on ideas of Kelvin, is purely thermodynamical. We compare the results of both methods for the Hubbard model, the prototype of correlated systems, on a lattice that is of great interest for applications, the 3d FCC. This lattice is frustrated even with only nearest- neighbor hopping. In addition to being 3d, each site has 12 nearest-neighbors. We thus used dynamical mean field theory with CTQMC in the hybridization expansion as the impurity solver [2] to study the Seebeck coefficient as a function of the band structure, doping and temperature for different interaction strengths. Connexions with experiments are made and propositions for good candidate for applications are discussed. [1] B.S. Shastry, Rep. Prog. Phys. 72, 016501 (2009) [2] P. Werner et al., Phys. Rev. Lett. 97, 076405 (2006) [Preview Abstract] |
Wednesday, March 17, 2010 2:54PM - 3:06PM |
T34.00003: Effect of magnetic field on thermopower in strongly correlated electron systems Mari Matsuo, Wataru Koshibae, Michiyasu Mori, Sadamichi Maekawa We theoretically study the magnetic field and temperature dependences of the thermopower in the Hubbard model by using the dynamical mean field theory. The asymptotic behavior of the Seebeck coefficient in high temperature region is well described by the extended Heikes formula [PRB 62, 6869 ('00).]. With decreasing temperature, the Seebeck coefficient changes its sign non-monotonically. The strong Coulomb interaction suppresses the bandwidth of quasi-particles, and enhances the response of the electron system to the external field. This response can be associated with the large magnetic-field dependence of the Seebeck coefficient. The relation between the Seebeck coefficient and the density of states of the electron system is discussed. The large response of the thermopower to the magnetic field observed in the cobalt oxides [Wang et al., Nature 423, 425 ('03).], is examined in the light of our theory. [Preview Abstract] |
Wednesday, March 17, 2010 3:06PM - 3:18PM |
T34.00004: Inhomogeneous phase formation on the border of itinerant ferromagnetism Andrew G. Green, Gareth J. Conduit, Ben D. Simons A variety of analytical techniques suggest that quantum fluctuations lead to a fundamental instability of the Fermi liquid that drives ferromagnetic transitions first order at low temperatures. We present both analytical and numerical evidence that, driven by the same quantum fluctuations, this first order transition is pre-empted by the formation of an inhomogeneous magnetic phase. This occurs in a manner that is closely analogous to the formation of the inhomogeneous superconducting Fulde-Ferrel-Larkin-Ovchinnikov state. We derive these results from a field theoretical approach supplemented with numerical Quantum Monte Carlo simulations. [Preview Abstract] |
Wednesday, March 17, 2010 3:18PM - 3:30PM |
T34.00005: Dynamic Correlations and the Ground State of MnSi Robert D. Collyer, Dana A. Browne MnSi is a metallic helimagnet below 29 K. Above 1.46 GPa, the moment is completely suppressed resulting in an unusual magnetic state. Density functional theory (DFT) predicts that MnSi has a moment of 1.0 $\mu_B$/Mn, which is much larger than the measured value of $\sim$0.4 4$\mu_B$/Mn. Additionally, DFT predicts a smaller magnetovolume coupling than found in experiment. We found that the addition of a Hubbard-U reproduces the experimental moment, but worsens the pressure dependence. We explored the effect of dynamic correlations on the pressure dependence of the moment using a fluctuating exchange approximation (FLEX), and contrast our results for MnSi with the B20 form of FeGe, which exhibits similar behaviors. [Preview Abstract] |
Wednesday, March 17, 2010 3:30PM - 3:42PM |
T34.00006: Kondo resonance narrowing in d- and f-electron systems Andriy Nevidomskyy, Piers Coleman By developing a simple scaling theory for the effect of Hund's interactions on the Kondo effect, we show how an exponential narrowing of the Kondo resonance develops in magnetic ions with large Hund's interaction. Our theory predicts an exponential reduction of the Kondo temperature with spin $S$ of the Hund's coupled moment, a little-known effect first observed in d-electron alloys in the 1960's, and more recently encountered in numerical calculations on multi-band Hubbard models with Hund's interactions. We discuss the consequences of Kondo resonance narrowing for the Mott transition in $d$-band materials, particularly iron pnictides, and the narrow ESR linewidth recently observed in ferromagnetically correlated $f$-electron materials.\\ For more information see: Phys. Rev. Lett. {\bf 103}, 147205 (2009). [Preview Abstract] |
Wednesday, March 17, 2010 3:42PM - 3:54PM |
T34.00007: Low energy properties of the two-impurity Anderson model Lijun Zhu, Jian-Xin Zhu We investigate the low energy properties of the two-impurity Anderson model with the complete-Fock-space numerical renormalization group method. From the calculated spectral function, correlation functions and self-energy, two energy scales are identified, as onsets of the Kondo resonance and the Fermi liquid behaviors. With the tuning of the RKKY interaction, the latter is uniformly suppressed to zero in the particle-hole symmetric case, resembling the Jones-Varma quantum critical point. In cases with the particle-hole asymmetry, an inter-impurity hybridization term is generated, which turns the quantum critical point into a crossover. Similar behaviors are found with either a direct hopping term or a local magnetic field, to lift the parity or spin degeneracies, respectively. Application to the Anderson lattice model is also presented. [Preview Abstract] |
Wednesday, March 17, 2010 3:54PM - 4:06PM |
T34.00008: Pseudogaps from thermally disordered spin density waves: optical and Hall conductivity Jie Lin, Andrew Millis We use the spin-fermion model and an approximation originally introduced in the charge denstiy wave context by Lee, Rice, and Anderson to study the conductivity and the Hall conductivity of two-dimensional systems close to antiferromagnetically ordered states. The electron spectral function reveals a pseudogap. Calculation of the longitudinal and Hall conductivities requires a vertex correction which has a nontrivial structure which we discuss. Results are compared to data and to other calculations. [Preview Abstract] |
Wednesday, March 17, 2010 4:06PM - 4:18PM |
T34.00009: Relaxation dynamics of excited states in spin-electron coupled systems W. Koshibae, N. Furukawa, N. Nagaosa We have examined the quantum dynamics of the excited electronic states in the double-exchange model at half-filling by solving coupled equations for the quantum evolution of electrons and Landau-Lifshits-Gilbert equation for classical spins. The Gilbert damping term gives an energy dissipation of the excited electronic state and relaxation process. We numerically investigate the relaxation dynamics of excited states by calculating time evolution of the electronic states and local spin structure. We find a new relaxation process, i.e., the non-adiabatic quantum transitions through a resonant mutual precession analogous to the electron spin resonance (ESR) process. With the relaxation dynamics similar to the ESR process, spatial inhomogeneity of the electronic structure is developed by the time evolution. Consequently, the nano-scale spatial structure of the spins evolves spontaneously accompanied by the localization of the electronic wavefunctions. The robustness of the quantum dynamics to the parameters in the model is also investigated. [Preview Abstract] |
Wednesday, March 17, 2010 4:18PM - 4:30PM |
T34.00010: Non-conservation of Fermionic Degrees of Freedom at Low-energy in Doped Mott Insulators Seungmin Hong, Shiladitya Chakraborty, Philip Phillips We show that the low-energy fermionic degrees of freedom in a doped Mott insulator described by the Hubbard model possesses a chemical potential that is less than that of the bare electrons. Consequently, the Landau one-to-one correspondence between bare electrons and fermionic quasiparticles breaks down explicitly. This state of affairs obtains because the hole number is not conserved as it contains a dynamical contribution. Any experimental probe that couples to the low-energy dynamics of a doped Mott insulator, quantum oscillation experiments included, should be interpreted in terms of the total dynamically generated hole number than the bare value. [Preview Abstract] |
Wednesday, March 17, 2010 4:30PM - 4:42PM |
T34.00011: Local density approximation combined with Gutzwiller method for correlated electron systems Xiaoyu Deng, Lei Wang, Xi Dai, Zhong Fang We introduce in detail our newly developed \textit{ab initio} LDA+Gutzwiller (local density approximation plus Gutzwiller) method, in which the Gutzwiller variational approach is naturally incorporated with the density functional theory (DFT) through the ``Gutzwiller density functional theory (GDFT)'' (which is a generalization of original Kohn-Sham formalism). This method can be used for ground state determination of electron systems ranging from weakly correlated metal to strongly correlated insulators with long-range ordering. The method is fully variational, the charge-density self-consistency can be naturally achieved, and the quantities, such as total energy, linear response, can be accurately obtained similar to LDA-type calculations. We will present some applications on typical correlated systems including d-electron systems (iron, nickel, $Na_xCoO_2$ and so on) and f-electron systems (delta plutonium). The obtained results using LDA+Gutzwiller are in good agreement with existing experiments. [Preview Abstract] |
Wednesday, March 17, 2010 4:42PM - 4:54PM |
T34.00012: Hybrid density functional theory applied and the role of electron correlations in LCMO Timothy Pennycook, Weidong Luo, Sokrates Pantelides Transition metal oxides (TMOs) are generally considered too complex for density functional theory (DFT) in the local density approximation (LDA or GGA) because of the 3d electrons. It is widely believed that a Hubbard U is needed to account for strong correlations. A prominent example is that GGA underestimates the Jahn-Teller distortions in LaMnO3 (LMO) causing it to predict the wrong magnetic ordering; GGA+U predicts both the structure and magnetic ordering correctly for LMO, but for the rest of the La$_x$Ca$_{1-x}$MnO$_3$ series it was recently demonstrated that the opposite is true (PRL 99, 036402). Recently, new exchange-correlation functionals have become available that do very well in correcting the well-known band gap problem of semiconductors and insulators. We have performed calculations using the parameter-free HSE06 hybrid density functional for LMO and find that it accurately reproduces both the magnetic ordering and the Jahn-Teller distortions seen experimentally. We will present results using the HSE06 functional from across the LCMO series. This work was supported in part by DOE grant DE-FG02-09ER46554. [Preview Abstract] |
Wednesday, March 17, 2010 4:54PM - 5:06PM |
T34.00013: Frustrated classical Heisenberg model with biquadratic interactions in a rhombic lattice: exact ground-state phase diagram L.X. Hayden, T.A. Kaplan, S.D. Mahanti The model cited has nearest-neighbor (nn) ferromagnetic and nnn Heisenberg interactions plus nn biquadratic interactions. The rhombic symmetry comes from assuming the nnn interaction only between sites on a square lattice connected by (1,1) (not (1,-1)) diagonals, as done for various multiferroic manganites{\footnote{T. Kimura et al., Phys. Rev. B~\textbf{68}, 060403(R) (2003)}$^,$\footnote{M. Mochizuki and N. Furukawa, J. Phys. Soc. Japan~\textbf{78}, 053704 (2009)}}. The biquadratic interactions replace the much smaller anisotropic terms usually used$^2$. The ground state problem in the thermodynamic limit is reduced, exactly, to a 3-spin problem, enabled by the LK cluster method\footnote{D. H. Lyons and T. A. Kaplan, J. Phys. Chem. Solids~\textbf{25}, 645 (1964)}, leading to the phase diagram. We find 4 phases: (1) ferromagnetic, (2) general-wave-vector ($\mathbf{Q}$) spiral, (3) up-up-down- down or ``E-type", degenerate with $\mathbf{Q}=(\pi,0)$, and (4) disordered. The uudd- $(\pi,0)$ degeneracy is removed in favor of uudd by a small ferromagnetic nnn interaction connecting sites along the (1,-1) diagonal (such an interaction was in fact found in ref. 1, where the observed uudd state was discussed). It is argued that the present model is probably realistic for these materials. [Preview Abstract] |
Wednesday, March 17, 2010 5:06PM - 5:18PM |
T34.00014: Angular momentum conservation in light-induced spin manipulation in NiO(001) Georg Lefkidis, Guo Ping Zhang, Wolfgang H\"{u}bner We compute from first principles the strongly-correlated intragap levels of a NiO cluster and we propagate the population in time under the influence of the laser pulse [1]. In this ultrafast magnetization-dynamics scenario we demonstrate an exact microscopic spin-switch mechanism. Combining \emph{ab initio} electronic many-body theory and quantum optics analysis we show in detail how the coherently induced material polarization leads to angular-momentum exchange between the light and the irradiated antiferromagnetic NiO (001) surface. Thus we answer the long standing question where the angular momentum goes. The calculation also predicts a dynamic Kerr-effect, which provides a signature for monitoring spin-dynamics, by simply measuring the transient rotation and ellipticity of the reflected pump beam [2].\\[4pt] [1] G. Lefkidis and W. H\"{u}bner, PRB {\bf 76}, 014418 (2007)\\[0pt] [2] G. Lefkidis, G. P. Zhang, and W. H\"{u}bner, PRL (2009, in press) [Preview Abstract] |
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