arXiv subjects

H. -P. Cheng

Publications and source records attributed to H. -P. Cheng.

7 recordsLinked to original sources

arXiv2013

Coherent electron transport through an azobenzene molecule: A light-driven molecular switch

We apply a first-principles computational approach to study a light-sensitive molecular switch. The molecule that comprises the switch can convert between a trans and a cis configuration upon photo-excitation. We find that the conductance of the two isomers vary dramatically, which suggests that this system has potential application as a molecular device. A detailed analysis of the band structure of the metal leads and the local density of states of the system reveals the mechanism of the switch.

cond-mat.mes-hall↗

arXiv2010

Modulation of pairing interaction in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ by an O dopant: a density functional theory study

Scanning tunneling spectroscopy measurements on the high temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ have reported an enhanced spectral gap in the neighborhood of O dopant atoms. We calculate, within density functional theory (DFT), the change in electronic structure due to such a dopant. We then construct and discuss the validity of several tight binding (TB) fits to the DFT bands with and without an O dopant. With the doping-modulated TB parameters, we finally evaluate the spin susceptibility and pairing interaction within spin fluctuation theory. The d-wave pairing eigenvalues are enhanced above the pure system without O dopant, supporting the picture of enhanced local pairing around such a defect.

cond-mat.supr-con↗

arXiv2010

Sensitivity of the superconducting state and magnetic susceptibility to key aspects of electronic structure in ferropnictides

Experiments on the iron-pnictide superconductors appear to show some materials where the ground state is fully gapped, and others where low-energy excitations dominate, possibly indicative of gap nodes. Within the framework of a 5-orbital spin fluctuation theory for these systems, we discuss how changes in the doping, the electronic structure or interaction parameters can tune the system from a fully gapped to nodal sign-changing gap with s-wave ($A_{1g}$) symmetry ($s^\pm$). In particular we focus on the role of the hole pocket at the $(π,π)$ point of the unfolded Brillouin zone identified as crucial to the pairing by Kuroki {\it et al.}, and show that its presence leads to additional nesting of hole and electron pockets which stabilizes the isotropic $s^\pm$ state. The pocket's contribution to the pairing can be tuned by doping, surface effects, and by changes in interaction parameters, which we examine. Analytic expressions for orbital pairing vertices calculated within the RPA fluctuation exchange approximation allow us to draw connections between aspects of electronic structure, interaction parameters, and the form of the superconducting gap.

cond-mat.supr-con↗

arXiv2010

Spin fluctuations and superconductivity in a 3D tight-binding model for BaFe2As2

Despite the wealth of experimental data on the Fe-pnictide compounds of the KFe2As2-type, K = Ba, Ca, or Sr, the main theoretical work based on multiorbital tight-binding models has been restricted so far to the study of the related 1111 compounds. This can be ascribed to the more three dimensional electronic structure found by ab initio calculations for the 122 materials, making this system less amenable to model development. In addition, the more complicated Brillouin zone (BZ) of the body-centered tetragonal symmetry does not allow a straightforward unfolding of the electronic band structure into an effective 1Fe/unit cell BZ. Here we present an effective 5-orbital tight-binding fit of the full DFT band structure for BaFeAs including the kz dispersions. We compare the 5-orbital spin fluctuation model to one previously studied for LaOFeAs and calculate the RPA enhanced susceptibility. Using the fluctuation exchange approximation to determine the leading pairing instability, we then examine the differences between a strictly two dimensional model calculation over a single kz cut of the BZ and a completely three dimensional approach. We find pairing states quite similar to the 1111 materials, with generic quasi-isotropic pairing on the hole sheets and nodal states on the electron sheets at kz = 0 which however are gapped as the system is hole doped. On the other hand, a substantial kz dependence of the order parameter remains, with most of the pairing strength deriving from processes near kz = pi. These states exhibit a tendency for an enhanced anisotropy on the hole sheets and a reduced anisotropy on the electron sheets near the top of the BZ.

cond-mat.supr-con↗

arXiv2007

Structure of BSCCO supermodulation from ab initio calculations

We present results of density functional theory (DFT) calculation of the structural supermodulation in BSCCO-2212 structure, and show that the supermodulation is indeed a spontaneous symmetry breaking of the nominal crystal symmetry, rather than a phenomenon driven by interstitial O dopants. The structure obtained is in excellent quantitative agreement with recent x-ray studies, and reproduces several qualitative aspects of scanning tunnelling microscopy (STM) experiments as well. The primary structural modulation affecting the CuO_2 plane is found to be a buckling wave of tilted CuO_5 half-octahedra, with maximum tilt angle near the phase of the supermodulation where recent STM experiments have discovered an enhancement of the superconducting gap. We argue that the tilting of the half-octahedra and concommitant planar buckling are directly modulating the superconducting pair interaction.

cond-mat.supr-con↗

arXiv2006

Local electronic structure near oxygen dopants in BSCCO-2212: a window on the high-Tc pair mechanism?

The cuprate material BSCCO-2212 is believed to be doped by a combination of cation switching and excess oxygen. The interstitial oxygen dopants are of particular interest because scanning tunnelling microscopy (STM) experiments have shown that they are positively correlated with the local value of the superconducting gap, and calculations suggest that the fundamental attraction between electrons is modulated locally. In this work, we use density functional theory to try to ascertain which locations in the crystal are energetically most favorable for the O dopant atoms, and how the surrounding cage of atoms deforms. Our results provide support for the identification of STM resonances at -1eV with dopant interstitial O atoms, and show how the local electronic structure is modified nearby.

cond-mat.supr-con↗

arXiv2005

Ab Initio Calculation of Impurity Effects in Copper Oxide Materials

We describe a method for calculating, within density functional theory, the electronic structure associated with typical defects which substitute for Cu in the CuO2 planes of high-Tc superconducting materials. The focus is primarily on Bi2Sr2CaCu2O8, the material on which most STM measurements of impurity resonances in the superconducting state have been performed. The magnitudes of the effective potentials found for Zn, Ni and vacancies on the in-plane Cu sites in this host material are remarkably consistent with phenomenological fits of potential scattering models to STM resonance energies. The effective potential ranges are quite short, of order 1 A with weak long range tails, in contrast to some current models of extended potentials which attempt to fit STM data. For the case of Zn and Cu vacancies, the effective potentials are strongly repulsive, and states on the impurity site near the Fermi level are simply removed. The local density of states (LDOS) just above the impurity is nevertheless found to be a maximum in the case of Zn and a local minimum in case of the vacancy, in agreement with experiment. The Zn and Cu vacancy patterns are explained as due to the long-range tails of the effective impurity potential at the sample surface. The case of Ni is richer due to the Ni atom's strong hybridization with states near the Fermi level; in particular, the short range part of the potential is attractive, and the LDOS is found to vary rapidly with distance from the surface and from the impurity site. We propose that the current controversy surrounding the observed STM patterns can be resolved by properly accounting for the effective impurity potentials and wave-functions near the cuprate surface. Other aspects of the impurity states for all three species are discussed.

cond-mat.supr-con↗