arXiv subjects

Fengjie Ma

Publications and source records attributed to Fengjie Ma.

35 records · Page 2Linked to original sources

arXiv2021

First-principles study of the electronic structure of CaKRu$_4$P$_4$

The recent discovery and studies of 1144-phase compounds, e.g., CaKFe$_4$As$_4$, have attracted significant research interest. In this paper, based on the first-principles density functional calculations, we present a systematic study on the electronic structure of the recently synthetized 1144-type quaternary compound CaKRu$_4$P$_4$. We find that the Ru-based 1144-type compound possesses a different electronic structure from that of iron-based superconductors, even though they share very similar crystallographic structures. In CaKRu$_4$P$_4$, there is no hole-type carrier if spin-orbit interaction is not considered. And a long-range magnetic order is absent in its ground state. With the application of pressure, the electronic structure of CaKRu$_4$P$_4$ becomes similar to those of the ternary 122-type compounds LaRu$_2$P$_2$ and LaRu$_2$As$_2$. CaKRu$_4$P$_4$ is very likely to be a phonon-mediated medium coupled BCS superconductor. Furthermore, type-I and type-II Dirac fermions can be created and regulated in this system with pressure. The quaternary compound CaKRu$_4$P$_4$ therefore has a potential to be an attractive platform for the study of topological physics and superconductivity.

cond-mat.mtrl-sci↗

arXiv2021

Coexistence of topological Weyl and nodal-ring states in ferromagnetic and ferrimagnetic double perovskites

Magnetic topological quantum materials have attracted great attention due to their exotic topological quantum physics induced by the interplay among crystalology, magnetism, and topology, which is of profound importance to fundamental research and technology applications. However, limited materials are experimentally available, most of whom are realized by magnetic impurity doping or heterostructural constructions. In this work, based on the first-principles calculations, we predict that double perovskite Ba2CdReO6 is an intrinsic ferromagnetic topological semi-half-metal, while the ferrimagnetic double perovskite with space group symmetry Fm-3m, such as Ba2FeMoO6, belongs to a topological half-metal. One pair of Weyl points and fully spin-polarized nodal-ring states are found in the vicinity of the Fermi level in Ba2CdReO6. Its two-dimensional nearly flat drumhead surface states are fully spin-polarized. In Ba2FeMoO6, however, there exist four pairs of Weyl points and two fully spin-polarized nodal-rings near the Fermi level. These topological properties are stable in the presence of spin-orbit coupling. This makes these materials be an appropriate platform for studying the emerging intriguing properties, especially for the applications in spintronics, information technology, and topological superconductivity.

cond-mat.mtrl-sci↗

arXiv2021

Ab initio electronic density in solids by many-body plane-wave auxiliary-field quantum Monte Carlo calculations

We present accurate many-body results of the electronic densities in several solid materials, including Si, NaCl, and Cu. These results are obtained using the ab initio auxiliary-field quantum Monte Carlo (AFQMC) method working in a plane-wave basis with norm-conserving, multiple-projector pseudopotentials. AFQMC has been shown to be an excellent many-body total energy method. Computation of observables and correlation functions other than the ground-state energy requires back-propagation, whose adaption and implementation in the plane-wave basis AFQMC framework are discussed in the present paper. This development allows us to compute correlation functions, electronic densities and interatomic forces, paving the way for geometry optimizations and calculations of thermodynamic properties in solids. Finite supercell size effects are considerably more subtle in the many-body framework than in independent-electron calculations. We analyze the convergence of the electronic density, and obtain best estimates for the thermodynamic limit. The densities from several typical density functionals are benchmarked against our near-exact results. The electronic densities we have obtained can also be used to help construct improved density functionals.

cond-mat.mtrl-sci↗

arXiv2020

Ground-state properties of the hydrogen chain: insulator-to-metal transition, dimerization, and magnetic phases

Accurate and predictive computations of the quantum-mechanical behavior of many interacting electrons in realistic atomic environments are critical for the theoretical design of materials with desired properties, and require solving the grand-challenge problem of the many-electron Schrodinger equation. An infinite chain of equispaced hydrogen atoms is perhaps the simplest realistic model for a bulk material, embodying several central themes of modern condensed matter physics and chemistry, while retaining a connection to the paradigmatic Hubbard model. Here we report a combined application of cutting-edge computational methods to determine the properties of the hydrogen chain in its quantum-mechanical ground state. Varying the separation between the nuclei leads to a rich phase diagram, including a Mott phase with quasi long-range antiferromagnetic order, electron density dimerization with power-law correlations, an insulator-to-metal transition and an intricate set of intertwined magnetic orders.

cond-mat.str-el↗

arXiv2020

AFeSe2 (A=Tl, K, Rb, or Cs): Iron-based superconducting analog of the cuprates

It has long been a challenging task to find compounds with similar crystal and electronic structures as cuprate superconductors with low dimensionality and strong antiferromagnetic fluctuations. The parent compounds of cuprate superconductors are Mott insulators with strong in-plane antiferromagnetic exchange interactions between Cu moments. Here we show, based on first-principles density functional calculations, that AFeSe2 (A=Tl, K, Rb, or Cs) exhibit many of the physical properties common to the cuprate parent compounds: (1) the FeSe2 layer in AFeSe2 is similar in crystalline and electronic structures to the CuO2 plane in cuprates, although Se atoms are not coplanar to the square Fe-lattice; (2) they are antiferromagnetic insulators, but with relatively small charge excitation gaps; (3) their ground states are Neel antiferromagnetic ordered, similar as in cuprates; and (4) the antiferromagnetic exchange interactions between Fe moments are larger than in other iron-based superconducting materials, but comparable to those in cuprates. Like cuprates, these compounds may become high-Tc superconductors upon doping of charge carriers either by chemical substitution or intercalation or by liquid or solid gating.

cond-mat.supr-con↗

arXiv2017

Towards the solution of the many-electron problem in real materials: equation of state of the hydrogen chain with state-of-the-art many-body methods

We present numerical results for the equation of state of an infinite chain of hydrogen atoms. A variety of modern many-body methods are employed, with exhaustive cross-checks and validation. Approaches for reaching the continuous space limit and the thermodynamic limit are investigated, proposed, and tested. The detailed comparisons provide a benchmark for assessing the current state of the art in many-body computation, and for the development of new methods. The ground-state energy per atom in the linear chain is accurately determined versus bondlength, with a confidence bound given on all uncertainties.

physics.comp-ph↗

arXiv2016

Auxiliary-field quantum Monte Carlo calculations with multiple-projector pseudopotentials

We have implemented recently developed multiple-projector pseudopotentials into the planewave based auxiliary-field quantum Monte Carlo (pw-AFQMC) method. Multiple-projector pseudopotentials can yield smaller planewave cut-offs while maintaining or improving transferability. This reduces the computational cost of pw-AFQMC, increasing its reach to larger and more complicated systems. We discuss the use of non-local pseudopotentials in the separable Kleinman-Bylander form, and the implementation in pw-AFQMC of the multiple-projector optimized norm-conserving pseudopotential ONCVPSP of Hamann. The accuracy of the method is first demonstrated by equation-of-state calculations of the ionic insulator NaCl and more strongly correlated metal Cu. The method is then applied to calibrate the accuracy of density functional theory (DFT) predictions of the phase stability of recently discovered high temperature and pressure superconducting sulfur hydride systems. We find that DFT results are in good agreement with pw-AFQMC, due to near cancellation of electron-electron correlation effects between different structures.

cond-mat.mtrl-sci↗

arXiv2014

Quantum Monte Carlo calculations in solids with downfolded Hamiltonians

We present a systematic downfolding many-body approach for extended systems. Many-body calculations operate on a simpler Hamiltonian which retains material-specific properties. The Hamiltonian is systematically improvable and allows one to dial, in principle, between the simplest model and the original Hamiltonian. As a by-product, pseudopotential errors are essentially eliminated using a frozen-core treatment. The computational cost of the many-body calculation is dramatically reduced without sacrificing accuracy. We use the auxiliary-field quantum Monte Carlo (AFQMC) method to solve the downfolded Hamiltonian. Excellent accuracy is achieved for a range of solids, including semiconductors, ionic insulators, and metals. We further test the method by determining the spin gap in NiO, a challenging prototypical material with strong electron correlation effects. This approach greatly extends the reach of general, ab initio many-body calculations in materials.

cond-mat.mtrl-sci↗

arXiv2012

Excited state calculations in solids by auxiliary-field quantum Monte Carlo

We present an approach for ab initio many-body calculations of excited states in solids. Using auxiliary-field quantum Monte Carlo, we introduce an orthogonalization constraint to prevent collapse of the stochastic Slater determinants in the imaginary-time propagation. Trial wave functions from density-functional calculations are used for the constraints. Detailed band structures calculated for standard semiconductors are in good agreement with GW and experimental results. For the challenging ZnO wurtzite structure, we obtain a fundamental band gap of 3.26(16) eV, consistent with experiments.

cond-mat.mtrl-sci↗

arXiv2011

Finite-size correction in many-body electronic structure calculations of magnetic systems

We extend the post-processing finite-size (FS) correction method, developed by Kwee, Zhang, and Krakauer [Phys. Rev. Lett. 100, 126404 (2008)], to spin polarized systems. The method estimates the FS effects in many-body electronic structure calculations of extended systems by a modified density functional theory (DFT) calculation, without having to repeat expensive many-body simulations. We construct a unified FS DFT exchange-correlation functional for spin unpolarized and fully spin polarized systems, under the local density approximation. The results are then interpolated to arbitrary spin polarizations. Generalization to other functional forms in DFT are discussed. The application of this FS correction method to several typical magnetic systems with varying supercell sizes demonstrates that it consistently removes most of the FS errors, leading to rapid convergence of the many-body results to the infinite size limit.

cond-mat.str-el↗

arXiv2010

Pnicogen-bridged antiferromagnetic superexchange interactions in iron pnictides

The first-principles electronic structure calculations made substantial contribution to study of high $T_c$ iron-pnictide superconductors. By the calculations, LaFeAsO was first predicted to be an antiferromagnetic semimetal, and the novel bi-collinear antiferromagnetic order was predicted for $α$-FeTe. Moreover, based on the calculations the underlying mechanism was proposed to be Arsenic-bridged antiferromagnetic superexchange interaction between the next-nearest neighbor Fe moments. In this article, this physical picture is further presented and discussed in association with the elaborate first-principles calculations on LaFePO. The further discussion of origin of the magnetism in iron-pnictides and in connection with superconductivity is presented as well.

cond-mat.mtrl-sci↗

arXiv2010

Electronic structures of ternary iron arsenides AFe$_2$As$_2$ (A=Ba, Ca, or Sr)

We have studied the electronic and magnetic structures of the ternary iron arsenides AFe$_2$As$_2$ (A = Ba, Ca, or Sr) using the first-principles density functional theory. The ground states of these compounds are in a collinear antiferromagnetic order, resulting from the interplay between the nearest and the next-nearest neighbor superexchange antiferromagnetic interactions bridged by As $4p$ orbitals. The correction from the spin-orbit interaction to the band structure is small. The pressure can reduce dramatically the magnetic moment and diminish the collinear antiferromagnetic order. Based on the calculations, we propose that the low energy dynamics of these materials is described effectively by a $t-J_H-J_1-J_2$-type model.

cond-mat.mtrl-sci↗

arXiv2009

$π$-junction to probe antiphase s-wave pairing in iron pnictide superconductors

Josephson junctions between a $FeAs$-based superconductor with antiphase s-wave pairing and a conventional s-wave superconductor are studied. The translational invariance in a planar junction between a single crystal pnictide and an aluminum metal greatly enhances the relative weight of electron pockets in the pnictide to the critical current. In a wide doping region of the pnictide, a planar and a point contact junctions have opposite phases, which can be used to design a tri-junction ring with $π$ phase to probe the antiphase pairing.

cond-mat.supr-con↗

arXiv2009

Atomic and electronic structures of ternary iron arsenides $A$Fe$_2$As$_2$(001) surfaces ($A$=Ba, Sr, or Ca)

By the first-principles electronic structure calculations, we find that energetically the most favorable cleaved $A$Fe$_2$As$_2$(001) surface ($A$=Ba, Sr, or Ca) is $A$-terminated with a $(\sqrt{2}\times \sqrt{2})R45^{\circ}$ or $(1\times 2)$ order. The $(1\times 2)$ ordered structure yields a $(1\times 2)$ dimerized STM image, in agreement with the experimental observation. The $A$ atoms are found to diffuse on the surface with a small energy barrier so that the cleaving process may destroy the $A$ atoms ordering. At the very low temperatures this may result in an As-terminated surface with the $A$ atoms in randomly assembling. The As-terminated BaFe$_2$As$_2$ surface in orthorhombic phase is $(\sqrt{2}\times\sqrt{2})R45^{\circ}$ buckled, giving rise to a switchable $(\sqrt{2}\times \sqrt{2})R45^{\circ}$ STM pattern upon varying the applied bias. No any reconstruction is found for the other As-terminated surfaces. There are surface states crossing or nearby the Fermi energy in the As-terminated and $(1\times 2)$ $A$-terminated surfaces. A unified physical picture is thus established to help understand the cleaved $A$Fe$_2$As$_2$(001) surfaces.

cond-mat.mtrl-sci↗

arXiv2008

Bi-collinear antiferromagnetic order in the tetragonal $α$-FeTe

By the first-principles electronic structure calculations, we find that the ground state of PbO-type tetragonal $α$-FeTe is in a bi-collinear antiferromagnetic state, in which the Fe local moments ($\sim2.5μ_B$) are ordered ferromagnetically along a diagonal direction and antiferromagnetically along the other diagonal direction on the Fe square lattice. This bi-collinear order results from the interplay among the nearest, next nearest, and next next nearest neighbor superexchange interactions $J_1$, $J_2$, and $J_3$, mediated by Te $5p$-band. In contrast, the ground state of $α$-FeSe is in the collinear antiferromagnetic order, similar as in LaFeAsO and BaFe$_2$As$_2$.

cond-mat.mtrl-sci↗

arXiv2008

Antiferromagnetic superexchange interactions in LaOFeAs

From first-principles calculations, we have studied the electronic and magnetic structures of the ground state of LaOFeAs. The Fe spins are found to be collinear antiferromagnetic ordered, resulting from the interplay between the strong nearest and next-nearest neighbor superexchange antiferromagnetic interactions. The structure transition observed by neutron scattering is shown to be magnetically driven. Our study suggests that the antiferromagnetic fluctuation plays an important role in the Fe-based superconductors. This sheds light on the understanding of the pairing mechanism in these materials.

cond-mat.mtrl-sci↗

arXiv2008

Iron-based layered superconductor LaO$_{1-x}$F$_x$FeAs: an antiferromagnetic semimetal

We have studied the newly found superconductor compound LaO$_{1-x}$F$_x$FeAs through the first-principles density functional theory calculations. We find that the parent compound LaOFeAs is a quasi-2-dimensional antiferromgnetic semimetal with most carriers being electrons and with a magnetic moment of $2.3μ_B$ located around each Fe atom on the Fe-Fe square lattice. Furthermore this is a commensurate antiferromagnetic spin density wave due to the Fermi surface nesting, which is robust against the F-doping. The observed superconduction happens on the Fe-Fe antiferromagnetic layer, suggesting a new superconductivity mechanism, mediated by the spin fluctuations. An abrupt change on the Hall measurement is further predicted for the parent compound LaOFeAs.

cond-mat.mtrl-sci↗