Faculty of Natural SciencesDepartment of Physics

Visiting Researcher

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### Location

B321Bessemer BuildingSouth Kensington Campus

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## Publications

Publication Type
Year
to

34 results found

Azadi S, Drummond ND, Foulkes WMC, 2021, Quasiparticle effective mass of the three-dimensional fermi liquid by quantum Monte Carlo, Physical Review Letters, Vol: 127, Pages: 1-6, ISSN: 0031-9007

According to Landau's Fermi liquid theory, the main properties of thequasiparticle excitations of an electron gas are embodied in the effective mass$m^*$, which determines the energy of a single quasiparticle, and the Landauinteraction function, which indicates how the energy of a quasiparticle ismodified by the presence of other quasiparticles. This simple paradigmunderlies most of our current understanding of the physical and chemicalbehavior of metallic systems. The quasiparticle effective mass of thethree-dimensional homogeneous electron gas has been the subject of theoreticalcontroversy and there is a lack of experimental data. In this work, we deploydiffusion Monte Carlo (DMC) methods to calculate $m^*$ as a function of densityfor paramagnetic and ferromagnetic three-dimensional homogeneous electrongases. The DMC results indicate that $m^*$ decreases when the density isreduced, especially in the ferromagnetic case. The DMC quasiparticle energybands exclude the possibility of a reduction in the occupied bandwidth relativeto that of the free-electron model at density parameter $r_s=4$, whichcorresponds to Na metal.

Journal article

Azadi S, Booth GH, Kuehne TD, 2020, Equation of state of atomic solid hydrogen by stochastic many-body wave function methods, JOURNAL OF CHEMICAL PHYSICS, Vol: 153, ISSN: 0021-9606

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Sahoo SK, Heske J, Azadi S, Zhang Z, Tarakina N, Oschatz M, Khaliullin RZ, Antonietti M, Kuehne TDet al., 2020, On the Possibility of Helium Adsorption in Nitrogen Doped Graphitic Materials, SCIENTIFIC REPORTS, Vol: 10, ISSN: 2045-2322

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Azadi S, Foulkes WMC, 2019, Efficient method for grand-canonical twist averaging in quantum Monte Carlo calculations, Physical Review B: Condensed Matter and Materials Physics, Vol: 100, ISSN: 1098-0121

We introduce a simple but efficient method for grand-canonical twistaveraging in quantum Monte Carlo calculations. By evaluating the thermodynamic grand potential instead of the ground state total energy, we greatly reduce the sampling errors caused by twist-dependent fluctuations in the particle number. We apply this method to the electron gas and to metallic lithium, aluminum, and solid atomic hydrogen. We show that, even when using a small number of twists, grand-canonical twist averaging of the grand potential produces better estimates of ground state energies than the widely used canonical twist-averaging approach.

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Azadi S, Kuehne TD, 2019, Unconventional phase III of high-pressure solid hydrogen, PHYSICAL REVIEW B, Vol: 100, ISSN: 2469-9950

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Azadi S, Kuehne TD, 2018, Quantum Monte Carlo calculations of van der Waals interactions between aromatic benzene rings, PHYSICAL REVIEW B, Vol: 97, ISSN: 2469-9950

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Azadi S, Singh R, Kühne TD, 2018, Nuclear quantum effects induce metallization of dense solid molecular hydrogen, Journal of Computational Chemistry, Vol: 39, Pages: 262-268, ISSN: 0192-8651

We present an accurate computational study of the electronic structure and lattice dynamics of solid molecular hydrogen at high pressure. The band-gap energies of the C2/c, Pc, and P63/m structures at pressures of 250, 300, and 350 GPa are calculated using the diffusion quantum Monte Carlo (DMC) method. The atomic configurations are obtained from ab initio path-integral molecular dynamics (PIMD) simulations at 300 K and 300 GPa to investigate the impact of zero-point energy and temperature-induced motion of the protons including anharmonic effects. We find that finite temperature and nuclear quantum effects reduce the band-gaps substantially, leading to metallization of the C2/c and Pc phases via band overlap; the effect on the band-gap of the P63/m structure is less pronounced. Our combined DMC-PIMD simulations predict that there are no excitonic or quasiparticle energy gaps for the C2/c and Pc phases at 300 GPa and 300 K. Our results also indicate a strong correlation between the band-gap energy and vibron modes. This strong coupling induces a band-gap reduction of more than 2.46 eV in high-pressure solid molecular hydrogen. Comparing our DMC-PIMD with experimental results available, we conclude that none of the structures proposed is a good candidate for phases III and IV of solid hydrogen.

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Azadi S, J Ackland G, 2017, The role of van der Waals and exchange interactions in high-pressure solid hydrogen, Physical Chemistry Chemical Physics, Vol: 19, Pages: 21829-21839, ISSN: 1463-9084

We investigate the van der Waals interactions in solid molecular hydrogen structures. We calculate enthalpy and the Gibbs free energy to obtain zero and finite temperature phase diagrams, respectively. We employ density functional theory (DFT) to calculate the electronic structure and density functional perturbation theory (DFPT) with van der Waals (vdW) functionals to obtain phonon spectra. We focus on the solid molecular C2/c, Cmca-12, P63/m, Cmca, and Pbcn structures within the pressure range of 200 < P < 450 GPa. We propose two structures of the C2/c and Pbcn for phase III which are stabilized within different pressure range above 200 GPa. We find that vdW functionals have a big effect on vibrations and finite-temperature phase stability, however, different vdW functionals have different effects. We conclude that, in addition to the vdW interaction, a correct treatment of the high charge gradient limit is essential. We show that the dependence of molecular bond-lengths on exchange–correlation also has a considerable influence on the calculated metallization pressure, introducing errors of up to 100 GPa.

Journal article

Fitzgerald J, Azadi S, Giannini V, 2017, Quantum plasmonic nanoantennas, Physical review B: Condensed matter and materials physics, Vol: 95, ISSN: 1098-0121

We study plasmonic excitations in the limit of few electrons, in one-atom-thick sodium chains. We compare the excitations to classical localized plasmon modes, and we find for the longitudinal mode a quantum-classical transition around 10 atoms. The transverse mode appears at much higher energies than predicted classically for all chain lengths. The electric field enhancement is also considered, which is made possible by considering the effects of electron-phonon coupling on the broadening of the electronic spectra. Large field enhancements are possible on the molecular level allowing us to consider the validity of using molecules as the ultimate small size limit of plasmonic antennas. Additionally, we consider the case of a dimer system of two sodium chains, where the gap can be considered as a picocavity, and we analyze the charge-transfer states and their dependence on the gap size as well as chain size. Our results and methods are useful for understanding and developing ultrasmall, tunable, and novel plasmonic devices that utilize quantum effects that could have applications in quantum optics, quantum metamaterials, cavity-quantum electrodynamics, and controlling chemical reactions, as well as deepening our understanding of localized plasmons in low-dimensional molecular systems.

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Azadi S, D Kuhne T, 2017, High-pressure hydrogen sulfide by diffusion quantum Monte Carlo, Journal of Chemical Physics, Vol: 146, ISSN: 1089-7690

We revisit the enthalpy-pressure phase diagram of the various products from the different proposed decompositions of H2S at pressures above 150 GPa by means of accurate diffusion Monte Carlo simulations. Our results entail a revision of the ground-state enthalpy-pressure phase diagram. Specifically, we find that the C2/c HS2 structure is persistent up to 440 GPa before undergoing a phase transition into the C2/m phase. Contrary to density functional theory, our calculations suggest that the C2/m phase of HS is more stable than the I41/amd HS structure over the whole pressure range from 150 to 400 GPa. More importantly, we predict that the Im-3m phase is the most likely candidate for H3S, which is consistent with recent experimental x-ray diffraction measurements.

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Azadi S, Drummond ND, Foulkes WMC, 2017, Nature of the metallization transition in solid hydrogen, Physical Review. B, Condensed Matter, Vol: 95, ISSN: 0163-1829

We present an accurate study of the static-nucleus electronic energy band gap of solid molecular hydrogen at high pressure. The excitonic and quasiparticle gaps of the C2/c, Pc, Pbcn, and P63/mstructures at pressures of 250, 300, and 350 GPa are calculated using the fixed-node diffusion quantum Monte Carlo (DMC) method. The difference between the mean-field and many-body band gaps at the same density is found to be almost independent of system size and can therefore be applied as a scissor correction to the mean-field gap of an infinite system to obtain an estimate of the many-body gap in the thermodynamic limit. By comparing our static-nucleus DMC energy gaps with available experimental results, we demonstrate the important role played by nuclear quantum effects in the electronic structure of solid hydrogen.

Journal article

Azadi S, 2016, Low-pressure phase diagram of crystalline benzene from quantum Monte Carlo, Journal of Chemical Physics, Vol: 145, ISSN: 1089-7690

We studied the low-pressure (0–10 GPa) phase diagram of crystalline benzene using quantum Monte Carlo and density functional theory (DFT) methods. We performed diffusion quantum Monte Carlo (DMC) calculations to obtain accurate static phase diagrams as benchmarks for modern van der Waals density functionals. Using density functional perturbation theory, we computed the phonon contributions to the free energies. Our DFT enthalpy-pressure phase diagrams indicate that the Pbca and P21/c structures are the most stable phases within the studied pressure range. The DMC Gibbs free-energy calculations predict that the room temperature Pbca to P21/c phase transition occurs at 2.1(1) GPa. This prediction is consistent with available experimental results at room temperature. Our DMC calculations give 50.6 ± 0.5 kJ/mol for crystalline benzene lattice energy.

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Azadi S, Singh R, Kuehne TD, 2015, Resonating Valence Bond Quantum Monte Carlo: Application to the Ozone Molecule, INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Vol: 115, Pages: 1673-1677, ISSN: 0020-7608

Journal article

Azadi S, Cohen RE, 2015, Chemical accuracy from quantum Monte Carlo for the benzene dimer, Journal of Chemical Physics, Vol: 143, ISSN: 1089-7690

We report an accurate study of interactions between benzene molecules using variational quantum Monte Carlo (VMC) and diffusion quantum Monte Carlo (DMC) methods. We compare these results with density functional theory using different van der Waals functionals. In our quantum Monte Carlo (QMC) calculations, we use accurate correlated trial wave functions including three-body Jastrow factors and backflow transformations. We consider two benzene molecules in the parallel displaced geometry, and find that by highly optimizing the wave function and introducing more dynamical correlation into the wave function, we compute the weak chemical binding energy between aromatic rings accurately. We find optimal VMC and DMC binding energies of −2.3(4) and −2.7(3) kcal/mol, respectively. The best estimate of the coupled-cluster theory through perturbative triplets/complete basis set limit is −2.65(2) kcal/mol [Miliordos et al., J. Phys. Chem. A 118, 7568 (2014)]. Our results indicate that QMC methods give chemical accuracy for weakly bound van der Waals molecular interactions, comparable to results from the best quantum chemistry methods.

Journal article

Azadi S, Foulkes WMC, 2015, Systematic study of finite-size effects in quantum Monte Carlo calculations of real metallic systems, Journal of Chemical Physics, Vol: 143, ISSN: 1089-7690

We present a systematic and comprehensive study of finite-size effects in diffusion quantum Monte Carlo calculations of metals. Several previously introduced schemes for correcting finite-size errors are compared for accuracy and efficiency, and practical improvements are introduced. In particular, we test a simple but efficient method of finite-size correction based on an accurate combination of twist averaging and density functional theory. Our diffusion quantum Monte Carlo results for lithium and aluminum, as examples of metallic systems, demonstrate excellent agreement between all of the approaches considered.

Journal article

Ranber Singh, Sam Azadi, and Thomas D Kühne, 2014, Anharmonicity and finite-temperature effects on the structure, stability, and vibrational spectrum of phase III of solid molecular hydrogen, Physical Review B, Vol: 90

Journal article

Azadi S, Foulkes WMC, Kuehne TD, 2013, Quantum Monte Carlo study of high pressure solid molecular hydrogen, NEW JOURNAL OF PHYSICS, Vol: 15, ISSN: 1367-2630

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Azadi S, Foulkes WMC, 2013, Fate of density functional theory in the study of high-pressure solid hydrogen (vol 88, 014115, 2013), PHYSICAL REVIEW B, Vol: 88, ISSN: 1098-0121

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Azadi S, Kuehne TD, 2012, Absence of metallization in solid molecular hydrogen, JETP LETTERS, Vol: 95, Pages: 449-453, ISSN: 0021-3640

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Marchi M, Azadi S, Sorella S, 2011, Fate of the Resonating Valence Bond in Graphene, PHYSICAL REVIEW LETTERS, Vol: 107, ISSN: 0031-9007

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Azadi S, Cavazzoni C, Sorella S, 2010, Systematically convergent method for accurate total energy calculations with localized atomic orbitals, PHYSICAL REVIEW B, Vol: 82, ISSN: 2469-9950

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Azadi S, Moradian R, Shafaee AM, 2010, The effect of Stone-Wales defect orientations on the electronic properties of single-walled carbon nanotubes, COMPUTATIONAL MATERIALS SCIENCE, Vol: 49, Pages: 699-703, ISSN: 0927-0256

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Azadi S, Moradian R, 2010, First principle study of unzipped boron nitride nanotubes, PHYSICS LETTERS A, Vol: 374, Pages: 605-609, ISSN: 0375-9601

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Marchi M, Azadi S, Casula M, Sorella Set al., 2009, Resonating valence bond wave function with molecular orbitals: Application to first-row molecules, JOURNAL OF CHEMICAL PHYSICS, Vol: 131, ISSN: 0021-9606

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Casula M, Marchi M, Azadi S, Sorella Set al., 2009, A consistent description of the iron dimer spectrum with a correlated single-determinant wave function, CHEMICAL PHYSICS LETTERS, Vol: 477, Pages: 255-258, ISSN: 0009-2614

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Masoudi AA, Farahani SV, Azadi S, 2009, ON THE FLUCTUATIONS OF WATER WAVES GOVERNED BY THE CAMASSA-HOLM AND KdV EQUATIONS IN (1+1)-DIMENSION, INTERNATIONAL JOURNAL OF MODERN PHYSICS B, Vol: 23, Pages: 149-158, ISSN: 0217-9792

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Masoudi AA, Vasheghani Farahani S, Azadi S, 2009, On the fluctuations of water waves governed by the camassaholm and KdV EQUATIONS in (1+1)-dimension, International Journal of Modern Physics B, Vol: 23, Pages: 149-158, ISSN: 0217-9792

In this paper, we are planning to consider the fluctuations of two nonlinear equations which govern the dynamics of water waves named CamassaHolm and KdV. We consider the total number of positive slopes $N-{\rm tot}^{+}$ produced when the fluctuations of the wave velocity u(x) of a surface wave of a fluid, for example water, is crossed by the level $(u(x)-(\bar{u}))=\alpha$ in the CamassaHolm and KdV equations. Here, we just concentrate on the high Reynolds number limit and do the level crossing analysis where υ → 0. In our desired limit, the dissipative term becomes absent or very weak compared to the nonlinear term which is responsible for increasing the amplitude and creating wave steepening, which results in the appearance of shocks. Thus, our analysis works at the times before the appearance of shocks. Our aim in this paper is to show how the quantity, $\nu^{+}-{\alpha}$, counts the fluctuations of the wave velocity in the surface water wave fluctuations which are governed by the KdV and CamassaHolm (CH) equations. © 2009 World Scientific Publishing Company.

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Moradian R, Azadi S, Farahani SV, 2008, Structure and electronic properties of native and defected gallium nitride nanotubes, PHYSICS LETTERS A, Vol: 372, Pages: 6935-6939, ISSN: 0375-9601

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Moradian R, Behzad S, Azadi S, 2008, Ab initio density functional theory investigation of electronic properties of semiconducting single-walled carbon nanotube bundles, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, Vol: 40, Pages: 3055-3059, ISSN: 1386-9477

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