Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J24: Quantum Monte Carlo Simulations of Fermion and Boson Systems I 
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Chair: Juana Moreno, Louisiana State University Room: 203AB 
Tuesday, March 3, 2015 2:30PM  3:06PM 
J24.00001: Quantum phases of dipolar bosons in multilayered optical lattice geometries Invited Speaker: Barbara CapogrossoSansone Dipolar interactions are responsible for stabilizing novel quantum manybody states in ultracold systems. In this talk we consider dipolar bosons trapped in $N\geq 2$ optical lattice layers. These configurations provide an ideal setup to explore novel physics resulting from the long range and anisotropic character of the dipolar interaction. We present results based on Path Integral Quantum Monte Carlo by the twoworm and a novel Nworm algorithm for dipolar hardcore bosons whose dipole moments are aligned perpendicularly to the optical lattice layers. Several nontrivial phases are stabilized. For twodimensional bilayer geometries these phases include pairsuperfluidity, pairsupersolidity, and solid phases. For stacks of onedimensional layers (tubes) superfluidity of multimers, solids, countersuperfluids are stabilized and they have a thresholdless nature with respect to the dipolar interaction. [Preview Abstract] 
Tuesday, March 3, 2015 3:06PM  3:18PM 
J24.00002: Detecting Goldstone Modes Using Entanglement Entropy in Quantum Monte Carlo Bohdan Kulchytskyy, Chris Herdman, Stephen Inglis, Roger Melko Bipartite entanglement entropy has emerged as a multifunctional tool in the study of condensed matter systems. In the context of systems with a spontaneously broken continuous symmetry, the scaling of this quantity has been predicted by Metlitski and Grover to have logarithmic subleading universal contribution to the boundary law [1]. To test this, we conduct largescale Quantum Monte Carlo simulations for a twodimensional spin1/2 XYmodel at temperatures below the finitesystem energy gap. Based on the predicted Renyi entropy scaling form, we are able to extract the number of Goldstone modes through the coefficient of the subleading logarithm. Further, we confirm that an additional subleading geometrical constant is present, which can be expressed in terms of a quantity in a free scalar field theory. This work illustrates the striking quantitative agreement that can be achieved between analytical continuum theory and lattice numerics, through calculations of Renyi entanglement entropies. [1] M. Metlitski and T. Grover, arXiv:1112.5166 (2011) [Preview Abstract] 

J24.00003: ABSTRACT WITHDRAWN 
Tuesday, March 3, 2015 3:30PM  3:42PM 
J24.00004: Solving fermion sign problem in quantum Monte Carlo by Majorana representation Hong Yao, ZiXiang Li, YiFan Jiang We discover a new quantum Monte Carlo (QMC) method to solve the fermion sign problem in interacting fermion models by employing Majorana representation of complex fermions. We call it Majorana QMC (MQMC). Especially, MQMC is fermion sign free in simulating a class of spinless fermion models on bipartite lattices at half filling and with arbitrary range of (unfrustrated) interactions. To the best of our knowledge, MQMC is the first auxiliary field QMC method to solve fermion sign problem in spinless (more generally, odd number of species) fermion models. MQMC simulations can be performed efficiently both at finite and zero temperatures. We believe that MQMC could pave a new avenue to solve fermion sign problem in more generic fermionic models. (ZiXiang Li, YiFan Jiang, and Hong Yao, arXiv:1408.2269). [Preview Abstract] 
Tuesday, March 3, 2015 3:42PM  4:18PM 
J24.00005: Quantum Monte Carlo simulations of bosons with complex interactions Invited Speaker: Valery Rousseau Many of the most exciting materials and phenomena being studied today, from oxide heterostructures to topological insulators or ironbased superconductors, are the ones in which an understanding of how quantum particles interact with each other is essential. In the last decade, the development and the improvement of quantum Monte Carlo algorithms combined with the increased power of computers has opened the way to the exact simulation of Hamiltonians that include various types of interactions, such as interspecies conversion terms or ringexchange terms. Simultaneously, developments made in the field of optical lattices, laser cooling and magneto/optical trapping techniques have led to ideal realizations of such Hamiltonians. A wide variety of phases can be present, including Mott insulators and superfluids, as well as more exotic phases such as Haldane insulators, supersolids, countersuperfluids, or the recently proposed Feshbach insulator. These experimental realizations of the various forms of the Hubbard model can have interesting applications, in particular they provide a possible way of performing quantum computing, and have also given rise to a new field known as Atomtronics, the equivalent of Electronics where the carriers are replaced by atoms. I will illustrate these ideas with examples of Hamiltonians that have been studied and some results. In order to study these systems, it is crucial to identify the various phases that are present, which can be characterized by a set of order parameters. Of particular importance in this task is the superfluid density. It is well known that the superfluid density can be related to the response of the free energy to a boundary phase twist, or to the fluctuations of the winding number. However, these relationships break down when complex interactions are involved. To address this problem, I will propose a general expression of the superfluid density, derived from real and thought experiments. I will discuss two interesting applications of my method to the SF transition of softcore bosons with 2nd neighbor hopping and to atommolecule mixtures.\\[4pt] Reference: ``Superfluid density in continuous and discrete spaces: Avoiding misconceptions," Phys. Rev. B 90, 134503 (2014) [Preview Abstract] 
Tuesday, March 3, 2015 4:18PM  4:30PM 
J24.00006: Thermochemistry and Charge Delocalization in Cyclization Reactions Using Accurate Quantum Monte Carlo Calculations Kayahan Saritas, Jeffrey C. Grossman Molecules that undergo pericyclic isomerization reactions find interesting optical and energy storage applications, because of their usually high quantum yields, large spectral shifts and small structural changes upon light absorption. These reactions induce a drastic change in the conjugated structure such that substituents that become a part of the conjugated system upon isomerization can play an important role in determining properties such as enthalpy of isomerization and HOMOLUMO gap. Therefore, theoretical investigations dealing with such systems should be capable of accurately capturing the interplay between electron correlation and exchange effects. In this work, we examine the dihydroazulene isomerization as an example conjugated system. We employ the highly accurate quantum Monte Carlo (QMC) method to predict thermochemical properties and to benchmark results from density functional theory (DFT) methods. Although DFT provides sufficient accuracy for similar systems, in this particular system, DFT predictions of ground state and reaction paths are inconsistent and nonsystematic errors arise. We present a comparison between QMC and DFT results for enthalpy of isomerization, HOMOLUMO gap and charge densities with a range of DFT functionals. [Preview Abstract] 
Tuesday, March 3, 2015 4:30PM  4:42PM 
J24.00007: Quantum Monte Carlo studies for the CO adsorption on late transition metal (111) surfaces ChengRong Hsing, ChunMing Chang, Ching Cheng, ChingMing Wei With the rapid development of computers, Density Functional Theory(DFT) has been extensively used to study the physical and chemical properties of materials. However, the major challenge for DFT approaches is whether the exchangecorrelation(XC) functionals can describe the system correctly. It is therefore vital to use the highly accurate Quantum Monte Carlo(QMC) method to examine the accuracy of the presently available XC functionals. In this talk, we will present the DMC(diffusion QMC) results in studying the adsorption of CO on the late transition metal (111) surface. It is well known that the LDA and GGA results predict the preference of FCC site. However, the diffuseLEED experiment concluded that CO adsorbed on atop(88{\%}) and bridge(12{\%}) site. The current DMC results agree with the experimental findings. In order to investigate the failure of LDA and GGA predictions, the DMC calculations are also performed to study the CO adsorption on other (111) surfaces(Rh, Ir, Cu). It is interesting to observe that GGA predicts similar adsorption energies on the atop site as DMC, but overestimates the adsorption energy for the other adsorption sites(bridge, HCP and FCC). The results explain why GGA tends to favor the higher coordination adsorption sites. [Preview Abstract] 
Tuesday, March 3, 2015 4:42PM  4:54PM 
J24.00008: Quantum Monte Carlo calculations in solids: downfolded Hamiltonians and multipleprojector pseudopotentials Fengjie Ma, Wirawan Purwanto, Shiwei Zhang, Henry Krakauer Accurate and efficient treatment of core electrons presents a significant challenge in manybody calculations. We discuss two approaches for addressing this problem. In the first, a systematic downfolding method is developed for extended systems, which allows manybody calculations to operate on a simpler and systematically improvable Hamiltonian, while retaining materialspecific properties. As a byproduct, pseudopotential errors are essentially eliminated using a frozencore treatment\footnote{W. Purwanto et al., JCTC 9, 4825 (2013)}. Dramatic savings of computational cost and excellent accuracy are achieved for a range of solids with auxiliaryfield quantum Monte Carlo (AFQMC). With this method, we determine the spin gap in NiO, a challenging material with strong electron correlation effects. In the second approach, we have implemented the recently developed multipleprojector pseudopotentials\footnote{D. R. Hamann, PRB 88, 085117 (2013)} into planewave based AFQMC (pwAFQMC), which improves transferability and leads to much smaller planewave cutoff, hence less computational cost, in the pwAFQMC calculations. We will present comparative studies of the NaCl equation of state between the two approaches. Results will also be presented for more strongly correlated metal systems. [Preview Abstract] 

J24.00009: ABSTRACT WITHDRAWN 
Tuesday, March 3, 2015 5:06PM  5:18PM 
J24.00010: Reptation Quantum Monte Carlo Calculation of Charge Transfer in The NaCl Dimer Yi Yao, Yosuke Kanai Reptation Quantum Monte Carlo (QMC) calculations are performed to describe the charge transfer behavior in a NaCl dimer. Influence of fixed node approximation on the charge transfer was examined by obtaining electron density via reputation QMC. We employ SlaterJastrow wavefunction as the trial wavefunction, and the fermion nodes are obtained from single particle orbitals of HartreeFock and Density Functional Theory (DFT) with several exchangecorrelation approximations. We will discuss our QMC results together with DFT calculations to give insights into observed dependence of the charge transfer behavior on the fixednode approximation. [Preview Abstract] 
Tuesday, March 3, 2015 5:18PM  5:30PM 
J24.00011: Monte Carlo simulations of twodimensional Hubbard models with string bond tensornetwork states JeongPil Song, Daehyun Wee, R. T. Clay We study charge and spinordered states in the twodimensional extended Hubbard model on a triangular lattice at 1/3 filling. While the nearestneighbor Coulomb repulsion V induces chargeordered states, the competition between onsite U and nearestneighbor V interactions lead to quantum phase transitions to an antiferromagnetic spinordered phase with honeycomb charge order. In order to avoid the fermion sign problem and handle frustrations here we use quantum Monte Carlo methods with the stringbond tensor network ansatz for fermionic systems in two dimensions. We determine the phase boundaries of the several spin and chargeordered states and show a phase diagram in the onsite U and the nearestneighbor V plane. The numerical accuracy of the method is compared with exact diagonalization results in terms of the size of matrices D. We also test the use of lattice symmetries to improve the stringbond ansatz. [Preview Abstract] 
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