arXiv subjects

Miao Gao

Publications and source records attributed to Miao Gao.

46 records · Page 3Linked to original sources

arXiv2014

Electronic transport of a large scale system studied by renormalized transfer matrix method: application to armchair graphene nanoribbons between quantum wires

Study on the electronic transport of a large scale two dimensional system by the transfer matrix method (TMM) based on the Schördinger equation suffers from the numerical instability. To address this problem, we propose a renormalized transfer matrix method (RTMM) by setting up a set of linear equations from U times of multiplication of traditional transfer matrix (U=N/S}with N and S being the atom number of length and the transfer step), and smaller S is required for wider systems. Then we solve the above linear equations by Gauss elimination method and further optimize to reduce the computational complexity from O(U^3M^3) to O(UM^3), in which M is the atom number of the width. Applying RTMM, we study transport properties of large scale pure and long-range correlated disordered armchair graphene nanoribbon (AGR) (carbon atoms up to 10^6 for pure case) between quantum wire contacts. As for pure AGR, the conductance is superlinear with the Fermi energy and the conductance is linear with the width while independent of the length, showing characteristics of ballistic transport. As for disordered AGR with long-range correlation, there is metal-insulator transition induced by the correlation strength of disorder. It is straightforward to extend RTMM to investigate transport in large scale system with irregular structure.

cond-mat.mes-hall↗

arXiv2013

Investigation of atomic and electronic structures of MgOFeSe studied by the first-principles calculations

In order to assist the search of new superconductors in iron selenide materials by intercalation, we calculate the crystal and electronic structures of MgOFeSe using the first-principles density functional theory. MgOFeSe is isotructural to the parent compound of iron pnictide superconductor LaOFeAs. In LaOFeAs, the anion O$^{2-}$ is located at the center of each LaO tetrahedra. But for MgOFeSe, we find that the crystal structure with the cation Mg$^{2+}$ as the tetrahedral center in the MgO layer is energetically more stable. The low energy band structures around the Fermi surfaces of MgOFeSe are contributed mainly by Fe 3$d$ orbitals. The ground state of MgOFeSe is collinearly antiferromagnetically ordered. The height of Se atoms above the Fe-Fe layer is about 1.38 Å, which is close to the height of As from the Fe-Fe layer in the iron pnictide superconductors with optimal superconducting transition temperatures.

cond-mat.supr-con↗

arXiv2013

Random site dilution properties of frustrated magnets on a hierarchical lattice

We present a method to analyze magnetic properties of frustrated Ising spin models on specific hierarchical lattices with random dilution. Disorder is induced by dilution and geometrical frustration rather than randomness in the internal couplings of the original Hamiltonian. The two-dimensional model presented here possesses a macroscopic entropy at zero temperature in the large size limit, very close to the Pauling estimate for spin-ice on pyrochlore lattice, and a crossover towards a paramagnetic phase. The disorder due to dilution is taken into account by considering a replicated version of the recursion equations between partition functions at different lattice sizes. An analysis at first order in replica number allows for a systematic reorganization of the disorder configurations, leading to a recurrence scheme. This method is numerically implemented to evaluate the thermodynamical quantities such as specific heat and susceptibility in an external field.

cond-mat.dis-nn↗

arXiv2013

Prediction of phonon-mediated high temperature superconductivity in stoichiometric Li$_2$B$_3$C

The discovery of superconductivity in Magnesium Diborate (MgB$_2$) has stimulated great interest in the search of new superconductors with similar lattice structures. Unlike cuprate or iron-based superconductors, MgB$_2$ is indisputably a phonon-mediated high temperature superconductor. The emergence of high temperature superconductivity in this material results from the strong coupling between the boron $σ$-bonding electrons around the Fermi level and the bond-stretching optical phonon modes. Here we show, based on the first-principles calculations, that Li$_2$B$_3$C is such a good candidate of superconductor whose superconducting transition temperature (T$_c$) might be even higher than MgB$_2$. Li$_2$B$_3$C consists of alternating graphene-like boron-carbon layers and boron-boron layers with intercalated lithium atoms between them. Similar to MgB$_2$, Li$_2$B$_3$C is inherently metallic and possesses two $σ$- and two $π$-electron bands around the Fermi energy. The superconducting pairs are glued predominately by the strong interaction between boron $σ$-bonding electrons and various optical phonon modes.

cond-mat.supr-con↗

arXiv2012

Effect of the Wyckoff position of K atom on the crystal structure and electronic properties of KFe$_2$Se$_2$ compound

By the first-principle electronic structure calculations, we study the effect of Wyckoff position of K atom on the crystal and electronic structures of KFe$_2$Se$_2$ compound.When the K atoms take up the Wyckoff position $2a$, $2b$ or $4c$ (the related structure of KFe$_2$Se$_2$ is called as Struc-2a, Struc-2b or Struc-4c), the calculated lattice constant $c$ is in the range of 13.5$\sim$14.5 Å, 15.5$\sim$16.7 Å or 18.6$\sim$19.1 Å.Three concentric cylinder-like Fermi surfaces emerge around $Γ$-Z in Brillouin Zone for the Struc-2b in nonmagnetic state, different from the Struc-2a and Struc-4c.The Fe-Se-Fe angle is 107.8$^\circ$, 108.8$^\circ$ or 110.7$^\circ$ in the collinear anti-ferromagnetic state and the superexchange interaction $J_2$ between two next neighbor Fe moments is 13.08 meV/S$^2$, 20.75 meV/S$^2$ or 11.86 meV/S$^2$ in Struct-2a, Struc-2b or Struc-4c structure respectively.The Struc-2b and Struc-4c have a good correspondence to the newly discovered superconducting phases with T$_c$=40 K and T$_c$=30 K in KFe$_2$Se$_2$.Our findings provide a reasonable approach to understand the existence of multiple superconducting phases in alkali metal intercalated FeSe superconductor.

cond-mat.supr-con↗

arXiv2011

Exact results for intrinsic electronic transport in graphene

We present exact results for the electronic transport properties of graphene sheets connected to two metallic electrodes. Our results, obtained by transfer-matrix methods, are valid for all sheet widths and lengths. In the limit of large width-to-length ratio relevant to recent experiments, we find a Dirac-point conductivity of $2e^2/\sqrt{3}h$ and a sub-Poissonian Fano factor of $2 - 3\sqrt{3}/π\simeq 0.346$ for armchair graphene; for the zigzag geometry these are respectively 0 and 1. Our results reflect essential effects from both the topology of graphene and the electronic structure of the leads, giving a complete microscopic understanding of the unique intrinsic transport in graphene.

cond-mat.mes-hall↗

arXiv2011

Ternary iron selenide K$_{0.8}$Fe$_{1.6}$Se$_2$ is an antiferromagnetic semiconductor

We have studied electronic and magnetic structures of K$_{0.8+x}$Fe$_{1.6}$Se$_2$ by performing the first-principles electronic structure calculations. The ground state of the Fe-vacancies ordered K$_{0.8}$Fe$_{1.6}$Se$_2$ is found to be a quasi-two-dimensional blocked checkerboard antiferromagnetic (AFM) semiconductor with an energy gap of 594 meV and a large ordering magnetic moment of 3.37 $μ_B$ for each Fe atom, in excellent agreement with the neutron scattering measurement. The underlying mechanism is the chemical-bonding-driven tetramer lattice distortion. K$_{0.8+x}$Fe$_{1.6}$Se$_2$ with finite $x$ is a doped AFM semiconductor with low conducting carrier concentration which is approximately proportional to the excess potassium content, consistent qualitatively with the infrared observation. Our study reveals the importance of the interplay between antiferromagnetism and superconductivity in these materials. This suggests that K$_{0.8}$Fe$_{1.6}$Se$_2$, instead of KFe$_2$Se$_2$, should be regarded as a parent compound from which the superconductivity emerges upon electron or hole doping.

cond-mat.supr-con↗

arXiv2011

Electronic structures and magnetic orders of Fe-vacancies ordered ternary iron selenides TlFe$_{1.5}$Se$_2$ and AFe$_{1.5}$Se$_2$ (A=K, Rb, or Cs)

By the first-principles electronic structure calculations, we find that the ground state of the Fe-vacancies ordered TlFe$_{1.5}$Se$_2$ is a quasi-two-dimensional collinear antiferromagnetic semiconductor with an energy gap of 94 meV, in agreement with experimental measurements. This antiferromagnetic order is driven by the Se-bridged antiferromagnetic superexchange interactions between Fe moments. Similarly, we find that crystals AFe$_{1.5}$Se$_2$ (A=K, Rb, or Cs) are also antiferromagnetic semiconductors but with a zero-gap semiconducting state or semimetallic state nearly degenerated with the ground states. Thus rich physical properties and phase diagrams are expected.

cond-mat.mtrl-sci↗

arXiv2010

Electronic and magnetic structures of ternary iron selenides AFe$_2$Se$_2$ (A=K, Cs, or Tl)

By the first-principles electronic structure calculations, we find that the ground state of ternary iron selenides AFe$_2$Se$_2$ (A=K, Cs, or Tl) is in a bi-collinear antiferromagnetic order, in which the Fe local moments ($\sim2.8μ_B$) align ferromagnetically along a diagonal direction and antiferromagnetically along the other diagonal direction on the Fe-Fe square lattice. This bi-collinear antiferromagnetic order results from the interplay among the nearest, the next nearest, and the next next nearest neighbor superexchange interactions, mediated by Se $4p$-orbitals.

cond-mat.mtrl-sci↗

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↗