46 research outputs found
Effect of quantum fluctuations on structural phase transitions in SrTiO_3 and BaTiO_3
Using path-integral Monte Carol simulations and an ab initio effective
Hamiltonian, we study the effects of quantum fluctuations on structural phase
transitions in the cubic perovskite compounds SrTiO3 and BaTiO3. We find
quantum fluctuations affect ferroelectric (FE) transitions more strongly than
antiferrodistortive (AFD) ones, even though the effective mass of a single FE
local mode is larger. For SrTiO3 we find that the quantum fluctuations suppress
the FE transition completely, and reduce the AFD transition temperature from
130K to 110K. For BaTiO3, quantum fluctuations do not affect the order of the
transition, but do reduce the transition temperature by 35-50 K. The
implications of the calculations are discussed.Comment: Revtex (preprint style, 14 pages) + 2 postscript figures. A version
in two-column article style with embedded figures is available at
http://electron.rutgers.edu/~dhv/preprints/index.html#wz_qs
Comparison of two non-primitive methods for path integral simulations: Higher-order corrections vs. an effective propagator approach
Two methods are compared that are used in path integral simulations. Both
methods aim to achieve faster convergence to the quantum limit than the
so-called primitive algorithm (PA). One method, originally proposed by
Takahashi and Imada, is based on a higher-order approximation (HOA) of the
quantum mechanical density operator. The other method is based upon an
effective propagator (EPr). This propagator is constructed such that it
produces correctly one and two-particle imaginary time correlation functions in
the limit of small densities even for finite Trotter numbers P. We discuss the
conceptual differences between both methods and compare the convergence rate of
both approaches. While the HOA method converges faster than the EPr approach,
EPr gives surprisingly good estimates of thermal quantities already for P = 1.
Despite a significant improvement with respect to PA, neither HOA nor EPr
overcomes the need to increase P linearly with inverse temperature. We also
derive the proper estimator for radial distribution functions for HOA based
path integral simulations.Comment: 17 pages, latex, 6 postscript figure
Quantum heuristic algorithm for traveling salesman problem
We propose a quantum heuristic algorithm to solve a traveling salesman
problem by generalizing Grover search. Sufficient conditions are derived to
greatly enhance the probability of finding the tours with extremal costs,
reaching almost to unity and they are shown characterized by statistical
properties of tour costs. In particular for a Gaussian distribution of the
tours along the cost we show that the quantum algorithm exhibits the quadratic
speedup of its classical counterpart, similarly to Grover search.Comment: Published versio
Localized Basis for Effective Lattice Hamiltonians: Lattice Wannier Functions
A systematic method is presented for constructing effective Hamiltonians for
general phonon-related structural transitions. The key feature is the
application of group theoretical methods to identify the subspace in which the
effective Hamiltonian acts and construct for it localized basis vectors, which
are the analogue of electronic Wannier functions. The results of the symmetry
analysis for the perovskite, rocksalt, fluorite and A15 structures and the
forms of effective Hamiltonians for the ferroelectric transition in
and , the oxygen-octahedron rotation transition in , the
Jahn-Teller instability in and the
antiferroelectric transition in are discussed. For the oxygen-
octahedron rotation transition in , this method provides an
alternative to the rotational variable approach which is well behaved
throughout the Brillouin zone. The parameters appearing in the Wannier basis
vectors and in the effective Hamiltonian, given by the corresponding invariant
energy expansion, can be obtained for individual materials using first-
principles density-functional-theory total energy and linear response
techniques, or any technique that can reliably calculate force constants and
distortion energies. A practical approach to the determination of these
parameters is presented and the application to ferroelectric
discussed.Comment: extensive revisions in presentation, 32 pages, Revtex, 7 Postscript
figure
Developing an interatomic potential for martensitic phase transformations in zirconium by machine learning
Interatomic potentials: predicting phase transformations in zirconium Machine learning leads to a new interatomic potential for zirconium that can predict phase transformations. A team led by Hongxian Zong at Xi’an Jiaotong University, China, and Turab Lookman at Los Alamos National Laboratory, U.S.A, used a Gaussian-type machine learning approach to produce an interatomic potential that predicted phase transformations in zirconium. They expressed each atomic energy contribution via changes in the local atomic environment, such as bond length, shape, and volume. The resulting machine-learning potential successfully described pure zirconium’s physical properties. When used in molecular dynamics simulations, it predicted a zirconium phase diagram as a function of both temperature and pressure that agreed well with previous experiments and simulations. Developing learnt interatomic potentials in phase-transforming systems could help us better simulate complex systems
Two Engineered OBPs with opposite temperature-dependent affinities towards 1-aminoanthracene
Engineered odorant-binding proteins (OBPs) display tunable binding affinities triggered by temperature alterations. We designed and produced two engineered proteins based on OBP-I sequence: truncated OBP (tOBP) and OBP::GQ20::SP-DS3. The binding affinity of 1-aminoanthracene (1-AMA) to these proteins revealed that tOBP presents higher affinity at 25°C (kd=0.45M) than at 37°C (kd=1.72M). OBP::GQ20::SP-DS3 showed an opposite behavior, revealing higher affinity at 37°C (kd=0.58M) than at 25°C (kd=1.17M). We set-up a system containing both proteins to evaluate their temperature-dependent binding. Our data proved the 1-AMA differential and reversible affinity towards OBPs, triggered by temperature changes. The variations of the binding pocket size with temperature, confirmed by molecular modelling studies, were determinant for the differential binding of the engineered OBPs. Herein we described for the first time a competitive temperature-dependent mechanism for this class of proteins.This study was supported by the Portuguese Foundation for Science and Technology (FCT) under the scope of the strategic funding of UID/BIO/04469/2013 unit and COMPETE 2020 (POCI-01-0145-FEDER-006684). A.R. and F.G. thank FCT for funding their scholarships with the references SFRH/BPD/98388/2013 and SFRH/BD/114684/2016, respectively. T.G.C. thanks senior position funded by the European Union through the European Regional Development Fund (ERDF) under the Competitiveness Operational Program (COP-A1-A1.1.4-E nr.30/01.09.2016). C.S. thanks to the BioTecNorte operation (NORTE-01-0145-FEDER-000004) funded by European Regional Development Fund under the scope of Norte2020 - Programa Operacional Regional do Norte. Access to computing resources funded by the Project "Search-ON2: Revitalization of HPC infrastructure of UMinho" (NORTE-07-0162-FEDER-000086), co-funded by the North Portugal Regional Operational Programme (ON.2 - O Novo Norte), under the National Strategic Reference Framework (NSRF), through the European Regional Development Fund (ERDF), is also gratefully acknowledged.info:eu-repo/semantics/publishedVersio