Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session K20: Quantum ManyBody Systems and Methods IFocus

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Sponsoring Units: DCOMP Chair: Nicola Lanata, Rutgers University Room: 319 
Wednesday, March 16, 2016 8:00AM  8:12AM 
K20.00001: Continuoustime auxiliary field~quantum Monte Carlo study of charge ordering in twodimensional extended Hubbard model. Hanna Terletska, Tianran Chen, Emanuel Gull The competition between local and nonlocal longrange Coulomb repulsions in strongly interacting electron systems leads to emergence of complex charge ordering phases. To perform the quantum manybody simulations of such effects, we extend the~existing continuoustime auxiliary field quantum Monte Carlo method to incorporate the nonlocal densitydensity interactions.~We apply the developed method to the twodimensional extended Hubbard model (with all range of nonlocal interactions) at~and away from the halffilling. We explore the properties of the model in different parameter regimes of short and long range interactions, temperature and fillings. [Preview Abstract] 
Wednesday, March 16, 2016 8:12AM  8:24AM 
K20.00002: Diagrammatic Monte Carlo for Dual Fermions Sergei Iskakov, Emanuel Gull The dual fermion series is a diagrammatic extension of the dynamical mean field theory that includes nonlocal dynamic correlations. Evaluating this series analytically has proven to be challenging. In this talk we show results for a diagrammatic Monte Carlo method that stochastically samples twoparticle vertex diagrams of the dual fermion perturbation series. We present an introduction to the method and show applications to correlated systems. [Preview Abstract] 
Wednesday, March 16, 2016 8:24AM  8:36AM 
K20.00003: The numerical renormalization group and multiorbital impurity models Andreas Weichselbaum, K. M. Stadler, J. von Delft, Z. P. Yin, G. Kotliar, Andrew Mitchell The numerical renormalization group (NRG) is a highly versatile and accurate method for the simulation of (effective) fermionic impurity models. Despite that the cost of NRG is exponential in the number of orbitals, by now, symmetric threeband calculations have become available on a routine level. [1] Here we present a recent detailed study on the spinorbital separation in a threeband Hund metal with relevance for ironpnictides via the dynamical mean field theory (DMFT). [2] In cases, finally, where the orbital symmetry is broken, we demonstrate that interleaved NRG [3] still offers an accurate alternative approach within the NRG with dramatically improved numerical efficiency at comparable accuracy relative to conventional NRG. \\[2ex] [1] Weichselbaum, Annals of Physics {\bf 327}, 2972 (2012) \\ [2] Stadler et al, PRL {\bf 115}, 136401 (2015) \\ [3] Mitchell et al, PRB {\bf 89}, 121105(R) (2014) [Preview Abstract] 
Wednesday, March 16, 2016 8:36AM  8:48AM 
K20.00004: DFT$+$DMFT calculation of band gaps for the transition metal monoxides NiO, CoO, FeO and MnO. Long Zhang, Peter Staar, Anton Kozhevnikov, Thomas Schulthess, HaiPing Cheng We report calculated spectral functions of the four late transition metal monoxides MnO, FeO, CoO and NiO in the paramagnetic phase. We used density functional theory (DFT) in combination with dynamic mean field theory (DMFT), which gives much better description of band gaps. Both projected Wannier orbitals and the onsite screened Coulomb interactions are obtained from DFT ground states to ensure consistency. Because of the pd hybridization in these materials, we calculated Coulomb interactions for the dp model as well as the ddp model using the cRPA method. With the standard fully localized limit double counting correction, we found that the ddp model gives results in better agreement with experiments. This work was supported by the US Department of Energy (DOE), Office of Basic Energy Sciences (BES), under Contract No. DEFG0202ER45995. [Preview Abstract] 
Wednesday, March 16, 2016 8:48AM  9:00AM 
K20.00005: Probing many body effects using Fourier Transform Scanning Tunneling Spectroscopy: Can spinorbit splitting in dispersion be observed in qspace? Gelareh Farahi Well studied surface systems such as Ag and Cu provide a safe platform to test novel spectroscopy methods that can have extended applications in near future. Our current focus is given to Fourier Transform Scanning Tunneling Spectroscopy (FTSTS) that allows us to study scattering effects (quasiparticle interactions  namely QPI) of CO and Co on Cu(111) surface. Magnetic Co adatoms are expected to generate a spinorbit split in dispersion in QPI(q) space, the existence of which is confirmed by the kspace angleresolved photoemission spectroscopy (ARPES) of Cu(111) surface in the recent years. Hence the previously observed electronphonon kink and spinorbit splitting of the dispersion, as well as the scattering properties of CO molecules and Co adatoms, should also be observable in QPI space via FTSTS of Cu(111), and compatible with previous studies on similar systems. We are using a low temperature (4.2 K) commercial Scanning Tunneling Microscope (CREATEC STM) that operates using Nanonis electronic controllers and software which allows us to perform FTSTS as well as topological imaging. [Preview Abstract] 
Wednesday, March 16, 2016 9:00AM  9:12AM 
K20.00006: Deconfined criticality in "easyplane" SU($N$) antiferromagnets Jonathan D'Emidio, Ganpathy Murthy, Ribhu Kaul Motivated by evidence for deconfined criticality in SU($N$) antiferromagnets, we investigate the phase diagram of these models in the case where the SU($N$) symmetry is reduced to rotations about the diagonal generators ("easyplane" symmetry). We carry out extensive numerical simulations using quantum Monte Carlo, revealing a firstorder magnetic to valence bond solid phase transition that becomes a continuous deconfined transition at large $N$. We support our numerical data by performing epsilon expansions of the easyplane deformed $CP^{N1}$ field theory near both the upper and lower critical dimensions. This renormalization group analysis shows that the symmetric deconfined fixed point is unstable in the presence of easyplane anisotropy, resulting in a runaway flow for intermediate values of $N$ and a flow towards a stable easyplane deconfined fixed point at large $N$, which is consistent with the critical behavior of our lattice models. [Preview Abstract] 
Wednesday, March 16, 2016 9:12AM  9:24AM 
K20.00007: Magnetic transitions and quantum criticality in the threedimensional Hubbard model Thomas Sch\"afer, Andrey Katanin, Karsten Held, Alessandro Toschi We analyze the (quantum) critical properties of the simplest model for electronic correlations, the Hubbard model, in three spatial dimensions by means of the dynamical mean field theory (DMFT, including all local correlations) and the dynamical vertex approximation (D$\Gamma$A, including nonlocal correlations on all length scales). Both in the halffilled/unfrustrated [1] and in the holedoped system [2] the transition temperature is significantly lowered by including nonlocal fluctuations.\\\\ In the latter case, however, the magnetic order becomes incommensurate, eventually leading to a complete suppression of the order and giving rise to a magnetic quantum critical point (QCP) at zero temperature [2]. We analyze the (quantum) critical properties of this QCP (e.g. critical exponents) and relate our findings to the standard theory of quantum criticality in metals, the HertzMillisMoriya theory. \\\\ $[1]$ G. Rohringer, A. Toschi, A. A. Katanin, and K. Held, Phys. Rev. Lett. {\bf 107}, 256402 (2011).\\ $[2]$ T. Sch\"afer, A. A. Katanin, K. Held, and A. Toschi, in preparation. [Preview Abstract] 
Wednesday, March 16, 2016 9:24AM  9:36AM 
K20.00008: A Multiorbital DMFT Analysis of ElectronHole Asymmetry in the Dynamic Hubbard Model Christopher Polachic, Frank Marsiglio The dynamic Hubbard model (DHM) improves on the description of strongly correlated electron systems provided by the conventional singleband Hubbard model through additional electronic degrees of freedom, namely a second, higher energy orbital and associated hybridization parameters for interorbital transitions. The additional orbital in the DHM provides a more realistic modeling of electronic orbital "relaxation" in real lattices. One result of orbital relaxation is a clear electronhole asymmetry, absent in the singleband case. We have employed the computational technique of dynamical mean field theory, generalized to the twoorbital case, to study this asymmetry with respect to varying system parameters, including both intersite and intrasite orbital hybridization as well as the role played by Mott physics. Our results stand in good agreement with previous exact diagonalization studies of the DHM. [Preview Abstract] 
Wednesday, March 16, 2016 9:36AM  9:48AM 
K20.00009: Interlaced coarsegraining for the dynamical cluster approximation Urs Haehner, Peter Staar, Mi Jiang, Thomas Maier, Thomas Schulthess The negative sign problem remains a challenging limiting factor in quantum Monte Carlo simulations of strongly correlated fermionic manybody systems. The dynamical cluster approximation (DCA) makes this problem less severe by coarsegraining the momentum space to map the bulk lattice to a cluster embedded in a dynamical meanfield host. Here, we introduce a new form of an interlaced coarsegraining and compare it with the traditional coarsegraining. We show that it leads to more controlled results with weaker cluster shape and smoother cluster size dependence, which with increasing cluster size converge to the results obtained using the standard coarsegraining. In addition, the new coarsegraining reduces the severity of the fermionic sign problem. Therefore, it enables calculations on much larger clusters and can allow the evaluation of the exact infinite cluster size result via finite size scaling. To demonstrate this, we study the holedoped twodimensional Hubbard model and show that the interlaced coarsegraining in combination with the DCA$^+$ algorithm permits the determination of the superconducting $T_c$ on cluster sizes, for which the results can be fitted with the KosterlitzThouless scaling law. [Preview Abstract] 
Wednesday, March 16, 2016 9:48AM  10:00AM 
K20.00010: Diagrammatic Monte Carlo sampling of the dualfermion expansion for the Hubbard model Jan Gukelberger, Evgeny Kozik, Hartmut Hafermann The dualfermion approach provides a formally exact prescription for calculating the properties of a correlated electron system in terms of a diagrammatic expansion around dynamical meanfield theory (DMFT). The approach can address the full range of interactions, is exact in both the weak and strongcoupling limits, and naturally incorporates longrange correlations beyond the reach of current cluster extensions to DMFT. Practical implementations have so far been limited to leadingorder or laddertype approximations to the expansion. In this work we compute the dualfermion expansion for the Hubbard model to higher orders by means of a diagrammatic Monte Carlo algorithm which stochastically samples all diagram topologies. This approach allows a systematic check for the convergence of the series and hence provides a route towards a fully controlled treatment of correlated electrons. [Preview Abstract] 
Wednesday, March 16, 2016 10:00AM  10:12AM 
K20.00011: Hubbard operator density functional theory for Fermionic lattice models Zhengqian Cheng, Chris Marianetti We formulate an effective action as a functional of Hubbard operator densities whose stationary point delivers all local static information of the interacting lattice model. Using the variational principle, we get a selfconsistent equation for Hubbard operator densities. The computational cost of our approach is set by diagonalizing the local Fock space. We apply our method to the one and two band Hubbard model (including crystal field and onsite exchange) in infinite dimensions where the exact solution is known. Excellent agreement is obtained for the oneband model. In the twoband model, good agreement is obtained in the metallic region of the phase diagram in addition to the metalinsulator transition. While our approach does not address frequency dependent observables, it has a negligible computational cost as compared to dynamical mean field theory and could be highly applicable in the context total energies of strongly correlated materials and molecules. [Preview Abstract] 
Wednesday, March 16, 2016 10:12AM  10:24AM 
K20.00012: Towards an abinitio treatment of nonlocal electronic correlations with dynamical vertex approximation Anna Galler, Patrik Gunacker, Jan Tomczak, Patrik Thunstr{\"o}m, Karsten Held Recently, approaches such as the dynamical vertex approximation (D$\Gamma$A) [1] or the dualfermion method [2] have been developed. These diagrammatic approaches are going beyond dynamical mean field theory (DMFT) by including nonlocal electronic correlations on all length scales as well as the local DMFT correlations. Here we present our efforts to extend the D$\Gamma$A methodology to abinitio materials calculations (abinitio D$\Gamma$A) [3]. Our approach is a unifying framework which includes both GW and DMFTtype of diagrams, but also important nonlocal correlations beyond, e.g. nonlocal spin fluctuations. In our multiband implementation we are using a worm sampling technique [4] within continuoustime quantum Monte Carlo in the hybridization expansion to obtain the DMFT vertex, from which we construct the reducible vertex function using the two particlehole ladders. As a first application we show results for transition metal oxides. [1] A. Toschi, A. A. Katanin, and K. Held, Physical Review B 75, 045118 (2007). [2] A. N. Rubtsov, M. I. Katsnelson, A. I. Lichtenstein, Physical Review B 77, 033101 (2008). [3] A. Toschi et al., Annalen der Physik 523, 698 (2011) [4] P. Gunacker et al., Physical Review B 92, 155102 (2015) [Preview Abstract] 
Wednesday, March 16, 2016 10:24AM  10:36AM 
K20.00013: Extremely Correlated Fermi Liquid theory: Imposing the hole density sumrule B. Sriram Shastry The analytical theory of extremely strongly correlated Fermi liquids (ECFL) for the large U models, when applied to Cuprate superconductors in the nodal direction, provides ARPES spectral line shapes that are very close to experiments. Approximate lowest order calculations within this formalism also closely reproduce the spectral line shapes for the single impurity Anderson model found using the Numerical Renormalization Group. Similarly excellent comparison is possible with the Dynamical Mean Field Theory self energy for the Hubbard model in high dimensions. However these calculations yields too large an energy scale for frequency dependence, in the proximity of integer (or Mott) filling. We show that the theory permits the imposition of sum rules for hole density, rather than the electron density used earlier, on the Greens functions of the theory. The numerical results of these variants are presented, and compared to the earlier calculations. The new results go a long way towards resolving the energy scale problem, while retaining the excellence of line shapes. [Preview Abstract] 
Wednesday, March 16, 2016 10:36AM  10:48AM 
K20.00014: Spacetime separation of electronic correlations Jan M. Tomczak, Thomas Sch{\"a}fer, Benjamin Klebel, Alessandro Toschi While secondorder phase transitions always cause strong nonlocal fluctuations, their effect on spectral properties crucially depends on the dimensionality. First, we show that for the important case of three dimensions the electron selfenergy is well separable into a local dynamical part and static nonlocal contributions$[1]$. In particular, using the dynamical vertex approximation for the doped 3D Hubbard model, we demonstrate that the quasiparticle weight remains essentially momentum independent, despite overall large nonlocal corrections to the selfenergy when approaching the spinordered state. This generalizes earlier empirical findings of this property in the iron pnictides$[2]$ and transition metal oxides$[3]$ based on Hedin's {\it GW} approximation. With this insight, we here propose a "spacetimeseparated" scheme for manybody perturbation theory that is up to ten times more efficient than current implementations. Finally, we discuss limits of the spacetime separation of correlation effects by studying the crossover from three to two dimensions.\\[0.25cm] $[1]$ T. Sch{\"a}fer, A. Toschi, JMT. PRB 91, 121107(R) (2015)\\ $[2]$ JMT, M. van Schilfgaarde, G. Kotliar. PRL 109, 237010 (2012)\\ $[3]$ JMT, M. Casula, T. Miyake, S. Biermann PRB 90, 165138 (2014) [Preview Abstract] 
Wednesday, March 16, 2016 10:48AM  11:00AM 
K20.00015: Computation of ab initio energy savings due to magnetic interactions Alexander Munoz, Lucas Wagner The doubleexchange mechanism [1] is the traditional explanation for antiferromagnetic coupling between magnetic ions. In this theory, the energy savings within the context of a hopping model is derived from kinetic energy terms. However, the connection to ab initio energy savings to our knowledge has not been studied using an explicitly correlated theory that can obtain quantitative accuracy. Our study focuses on determining, from ab initio calculations, whether the origin of interactions in magnetic systems is explainable through conventional arguments. To this end, we investigate the contributions (kinetic, ionic, electronelectron) to the total energy of the (MnOMn)$^{\mathrm{+2}}$system using quantum Monte Carlo techniques. We will report on progress in elucidating the connection between the ab initio energy savings and the effective model energy savings that result in an antiferromagnetic interaction in this system. [1] Zener. Phys. Rev. , 403 (1951) [Preview Abstract] 
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