Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G54: Theoretical Methods in Correlated Electron Magnetism |
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Sponsoring Units: GMAG Chair: Yi Nina Li, Princeton University Room: Mile High Ballroom 1B |
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G54.00001: Exact Results of Itinerant Ferromagnetism in Multi-orbital Hubbard Systems Yi Nina Li, Elliott H. Lieb, Congjun Wu We study itinerant ferromagnetism in multi-orbital Hubbard models in certain two-dimensional square and three dimensional cubic lattices. In the strong coupling limit where doubly occupied orbitals are not allowed we prove that the fully spin-polarized states are the unique ground states, apart from the trivial spin degeneracies, for any generic filling factor {\$}0 \textless n \textless 2 (0 \textless n \textless 3){\$} in two (three) dimensions. This theorem applies to both certain d-orbital transition-metal oxide layer systems and the p-orbital bands with ultra-cold fermions in optical lattices [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G54.00002: Sign-problem free quantum Monte-Carlo simulations on itinerant ferromagnetism in multi-orbital band systems Shenglong Xu, Yi Nina Li, Congjun Wu In a recent paper by Li, Lieb and Wu, it has been proved recently that certain multiorbital Hubbard models exhibit ferromagnetic ground states in strong coupling limit at zero temperature and any generic fillings. In a suitably defined basis, it can be proved that the sign problem of quantum Monte-Carlo simulations is absent. Quantum Monte Carlo simulations are performed to investigate the nature of itinerant ferromagnetism in such systems. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G54.00003: Observation of Block Magnetic States within the Orbital-Selective Mott Regime of Multiorbital Hubbard Models Julian Rincon, Gonzalo Alvarez, Adriana Moreo, Elbio Dagotto The orbital-selective Mott phase (OSMP) of multiorbital Hubbard models has been extensively analyzed using static and dynamical mean-field approximations. In parallel, the existence of Block states (antiferromagnetically coupled ferromagnetic spin clusters) in Fe-based superconductors has also been much discussed. This effort uses numerically exact techniques in one-dimensional systems to show that the OSMP remains stable even in the presence of full quantum fluctuations. Our main result is the observation of Block states within the OSMP regime, connecting two seemingly independent areas of research, and providing analogies with the physics of Double-Exchange models. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G54.00004: Ultrasoft pseudopotentials and Hubbard U values for rare-earth elements (Re=La-Lu) guided by HSE06 calculations Mehmet Topsakal, Koichiro Umemoto, Renata Wentzcovitch The lanthanide series of the periodic table comprises fifteen members ranging from La to Lu - the rare-earth (Re) elements. They exhibit unique (and mostly unexplored) chemical properties depending on the fillings of 4f-orbitals. Due to strong electronic correlation, 4f valence electrons are incorrectly described by standard DFT functionals. In order to cope with these inefficiencies, the DFT+U method is often employed where Hubbard-type U is introduced into the standard DFT. Another approach is to use hybrid functionals. Both improve the treatment of strongly correlated electrons. However, DFT+U suffers from ambiguity of U while hybrid functionals suffer from extremely demanding computational costs. Here we provide Vanderbilt type ultrasoft pseudopotentials for Re elements with suggested U values allowing efficient plane-wave calculations. Hubbard U values are determined according to HSE06 calculations on Re-nitrides (ReN). Generated pseudopotentials were further tested on some Re-cobaltite (Re-CoO3) perovskites. Alternative pseudopotentials with f-electrons kept frozen in the core of pseudopotential are also provided and possible outcomes are addressed. We believe that these new pseudopotentials with suggested U values will allow further studies on rare-earth materials. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G54.00005: Atomic-scale magnetism of Fe and Co on a complex surface Barbara Jones, Shruba Gangopadhyay, Oliver Albertini Miniaturization is one of present challenges for development of future spintronic devices. Our goal is to exploit the unusual properties of magnetism of transition metal atoms on complex surfaces. In collaboration with Almaden's Scanning Tunneling Microscopy team, we use DFT+U to calculate the properties of transition atoms on nanolayers of insulator on top of a metal such as silver. In this talk we report the results of detailed calculations of Fe and Co on MgO/Ag. MgO is a common spintronic insulator, but in a nanolayer on metallic Ag, its behavior is not that of the bulk. We find that Fe and Co have very different local spin and charge interactions with this surface. Using an onsite Hubbard U parameter which we determine from first principles, we are able to study the variability of the magnetic moment and nature of bonding. The magnetic adatoms affect the surrounding interface layer in unexpected ways. We are able to obtain interesting insights which help us understand how magnetism propagates along surfaces as well as between interfaces. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G54.00006: Quantum Monte Carlo calculations of magnetic couplings in cuprates Kateryna Foyevtsova, Jaron Krogel, Jeongnim Kim, Fernando Reboredo Spin excitations are generally believed to play a fundamental role in the mechanism of high temperature superconductivity in cuprates. However, accurate description of the cuprates' magnetic properties and, in particular, calculation of spin exchange couplings have been a long-standing challenge to the electronic structure theory. While the quantum-mechanically more rigorous cluster methods suffer from finite-size effects, the density functional theory approach, on the other hand, is ambiguous due to a rich variety of approximations to the exchange-correlation functional available which often give very different numbers for the spin exchange constants. For example, in some cuprates the theoretically predicted values of the nearest-neighbor superexchange range from 1 eV (local density approximation) to 0.05 eV (periodic unrestricted Hartree Fock) [C. de Graaf \textit{et al}, PRB \textbf{63} 014404 (2000)]. We compute spin exchange constants with the fixed-node diffusion Monte Carlo method (FN-DMC). In one-dimensional cuprates, we find that the FN-DMC computed nearest-neighbor spin superexchange is in an excellent agreement with experiment. This both demonstrates that FN-DMC is capable of describing properly the magnetism of strongly correlated oxides as well as positions this technique as the method of choice for theoretical parameterization of spin models. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G54.00007: Valence-Bond Monte Carlo Study of the 1D t-J Model Julia Wildeboer, Nicholas Bonesteel, Anders Sandvik We show that the valence-bond Monte Carlo (VBMC) method [1] can be applied to the one-dimensional $t$-$J$ model. In this projector Monte Carlo approach, the ground state of the model is sampled directly from a generalized valence-bond basis consisting of states with fixed hole configurations and electron spins singlet correlated in pairs to form valence bonds. For $n < 0.6$, where $n$ is the number of electrons per site, as $J/t$ is increased from 0, the 1D $t$-$J$ exhibits a quantum phase transition at which a spin-gap opens, followed by a transition to a phase separated state for large $J/t$ [2]. Using VBMC, we calculate the valence-bond entanglement entropy [3] (roughly, the average number of valence bonds leaving a block of size $L$) as the system is tuned through the transition to the spin gap phase. [1] A. Sandvik, PRL 95, 207203 (2005). See, e.g., A. Moreno, A. Muramatsu, and S.R. Manmana, PRB 83, 205113 (2011). [3] F. Alet, S. Capponi, N. Laflorencie, and M. Mambrini, PRL 99, 117204 (2007). [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G54.00008: Perturbation energy as an alternative to the total energy calculations Andrey Kutepov, Vladimir Antropov, Mark van Schilfgaarde, Victor Antonov We analyze different approaches to determine the energy from a perturbation using modern electronic structure methods. We compare the energy of perturbation from standard perturbation theory with what is obtained directly in self consistent band structure methods. The method is applied for studies such perturbations as internal magnetic field and spin orbital coupling in solids. This method is further compared with integration over the coupling constant. Numerical tests have been performed for magnetic Fe and Gd systems using the local density approximation. The main advantage of present scheme is its usefulness in methods for strongly correlated electronic systems studies where total energy calculations are not always possible. Specific calculations are performed using self consistent quasiparticle GW and LDA+U calculations for MnBi where the right value of magnetic moment and sign/value of magnetic anisotropy as a function of temperature have been obtained. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G54.00009: Double expansion with respect to $U$ and $1/(N-1)$ for an SU($N$) impurity Anderson model Akira Oguri, Miyuki Awane We apply a new large-$N$ scheme for an SU($N$) impurity Anderson model [1,2] to the Green's function for finite frequency $\omega$ and finite Coulomb interaction $U$. This approach is essentially different from the conventional large-$N$ theories, such as the non-crossing approximation and its extensions which are based on a perturbation expansion in the hybridization strength $V$. Our expansion scheme, which uses $1/(N-1)$ and the scaled interaction $u \equiv (N-1)U$ as a set of two independent variables, gives the Hartree-Fock (HF) results at zeroth order. Then, to leading order in $1/(N-1)$ it describes the Hartree-Fock random phase approximation (HF-RPA). The higher-order corrections systematically describe the fluctuations beyond the HF-RPA. It was shown that the renormalized local-Fermi-liquid parameters, calculated up to order $1/(N-1)^2$, agree closely with the exact NRG results at $N=4$ where the degeneracy is still not so large [1,2]. We discuss the $\omega$ dependence of the Green's function to clarify both the low- and high-energy features. \\[4pt] [1] A.O., R.\ Sakano, and T.\ Fujii, PRB {\bf 84}, 113301 (2011).\\[0pt] [2] A.O., PRB {\bf 85}, 155404 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G54.00010: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G54.00011: Critical Exponents of Strongly Correlated Fermion Systems from Diagrammatic Multi-Scale Methods Andrey Antipov, Stefan Kirchner, Emanuel Gull The dynamical mean field theory (DMFT) has become the standard tool in describing strongly correlated electron materials. While it captures the quantum dynamics of local fields, it neglects spatial correlations. To describe e.g. anti-ferromagnetism, unconventional superconductivity or frustration a proper treatment of non-local correlations is necessary. Diagrammatic multi-scale approaches offer an elegant option to accomplish this: the difficult correlated part of the system is solved using a non-perturbative many-body method, whereas 'easier', 'weakly correlated' parts of the problem are tackled using a secondary perturbative scheme. Here we employ such a method, the dual fermion approach, to problems of charge ordering in Falicov-Kimball model [1] by constructing a systematic diagrammatic extension on top of DMFT. Near the critical point of the Falicov-Kimball model we study the interplay between charge excitations and long-range fluctuations. We show that such multi-scale approach is indeed capable of capturing the non mean-field nature of the critical point of the lattice model and correctly describes the transition to mean-field like behavior as the number of spatial dimensions increases. [1] A. Antipov, S. Kirchner, E. Gull, arXiv:1309.5976 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G54.00012: Representing highly excited eigenstates of many-body localized systems using matrix product states Bryan Clark, David Pekker Many-body localization remains a mysterious topic largely due to the lack of tools for describing highly excited eigenstates of interacting quantum systems. Matrix Product States (MPS) are a family of low-entanglement variational ansatz. Typically, excited states of many-body systems exhibit volume law scaling of the entanglement entropy and therefore cannot be efficiently described by an MPS of low bond-dimension. In many-body localized systems, though, the eigenstates generically have area law scaling suggesting the existence of an efficient MPS representation. Here we investigate how to to find these states. Our achievement opens a new numerical window on many-body localization. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G54.00013: The spin Drude weight in the spin-1/2 XXZ chain: a combined exact diagonalization and time-dependent DMRG study Fabian Heidrich-Meisner, Christoph Karrasch, Johannes Hauschild, Stephan Langer Various theoretical approaches predict a finite Drude weight for spin transport in the gapless phase of the spin-1/2 XXZ chain, suggesting ballistic transport properties. We compute the Drude weight at finite temperatures with two approaches: Time-dependent density matrix renormalization group simulations and exact diagonalization. For the latter, we present a detailed comparison of different schemes of evaluating finite-size data, namely either in a grand-canonical ensemble or in a canonical one. We argue that the grand-canonical data, obtained from averaging over all subspaces with different magnetizations, have a more systematic finite-size dependence than the canonical one. The results for D(T) from exact diagonalization and tDMRG are in good quantitative agreement in the massless phase [1]. Financial support from the DFG through FOR 912 is gratefully acknowledged. \\[4pt] [1] Karrasch, Hauschild, Langer, HM, Phys. Rev. B 87, 245128 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G54.00014: Magnetic properties of FeO: a DFT+DMFT study Peng Zhang, R.E. Cohen, Kristjan Haule The magnetic properties of the transition metal oxide FeO greatly effect its equation of state and elasticity, and thus have been of great interest [1-3]. But FeO is not treated well by density functional theory which makes it a metal, instead of an insulator at low pressures. Employing the newly developed method of the density functional theory plus dynamical mean field theory, the magnetic properties of FeO within a wide range of pressure and temperature are investigated. Relative to density functional theory, the local correlations of the Fe d-electrons is exactly included in the new method in a fully self-consistent way. Adopting the hybridization expansion continuous time quantum Monte Carlo method as the impurity solver, the ab initio calculated impurity magnetic susceptibility is inserted in the Bethe-Salpeter equation, to derive the bulk magnetic susceptibility. By exploring the antiferromagnetic ordering and the Neel temperature as a function of pressure and temperature, the magnetic phase diagram of FeO is plotted. Our preliminary results indicate $T_{N}=203.091K$ at V=540 $b.a.u.^{3}$ and $T_{N}=223.345K$ at V=520 $b.a.u.^{3}$. \\[4pt] [1] J. Badro et al, PRL, 83, 4101(1999). [2] M.P. Pasternak et al, PRL, 79, 5046 (1997). [3] A. Mattinla et al, PRL, 98, 196404(2007). [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G54.00015: Dynamical Mean-Field Approach to Core-Level Spectroscopy of NiO and its Insulating Character Atsushi Hariki, Takayuki Uozumi Core-level X-ray photoemission spectroscopy (XPS) is a powerful tool to investigate electronic structure of strongly correlated electron systems, such as 3$d$ transition metal oxides. In the Ni2$p $XPS of NiO, a characteristic double-peak structure has been observed in the 2$p_{3/2}$ main line, [1] which is considered to be related with the insulating property of NiO. However, previous studies contradicts each other for the spectral assignment of the double peaks. [1,2] Thus, a further investigation about the microscopic origin of the double peaks from a different viewpoint is required. In this talk, we investigate the double-peak structure using a framework, which was recently proposed in our research group, based on the dynamical mean-field theory (DMFT) under realistic crystal structure. We show that, besides the so-called nonlocal screening indicated by Van Veenendaal et al., [2] the antiferromagnetic ordering of NiO plays a crucial role of the formation of the double peaks. We conclude from the spectral analysis that the lowest first ionization state of NiO is given by an electron removal from the Zhang-Rice doublet band. \\[4pt] [1] M. Taguchi et al.: Phys. Rev. Lett. {\bf 100} (2008) 206401.\\[0pt] [2] M. A. van Veenendaal and G. A. Sawatzky: Phys. Rev. Lett. {\bf 70} (1993) 2459. [Preview Abstract] |
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