arXiv subjects

Hyoung Joon Choi

Publications and source records attributed to Hyoung Joon Choi.

46 records · Page 3Linked to original sources

arXiv2009

High resolution angle resolved photoemission studies on quasi-particle dynamics in graphite

We obtained the spectral function of the graphite H point using high resolution angle resolved photoelectron spectroscopy (ARPES). The extracted width of the spectral function (inverse of the photo-hole lifetime) near the H point is approximately proportional to the energy as expected from the linearly increasing density of states (DOS) near the Fermi energy. This is well accounted by our electron-phonon coupling theory considering the peculiar electronic DOS near the Fermi level. And we also investigated the temperature dependence of the peak widths both experimentally and theoretically. The upper bound for the electron-phonon coupling parameter is ~0.23, nearly the same value as previously reported at the K point. Our analysis of temperature dependent ARPES data at K shows that the energy of phonon mode of graphite has much higher energy scale than 125K which is dominant in electron-phonon coupling.

cond-mat.mtrl-sci↗

arXiv2009

Dominant role of local-moment interactions in the magnetism in iron pnictides : comparative study of arsenides and antimonides from first-principles

The magnetic properties of various iron pnictides are investigated using first-principles pseudopotential calculations. We consider three different families, LaFePnO, BaFe$_2$Pn$_2$, and LiFePn with Pn=As and Sb, and find that the Fe local spin moment and the stability of the stripe-type antiferromagnetic phase increases from As to Sb for all of the three families, with a partial gap formed at the Fermi energy. In the meanwhile, the Fermi-surface nesting is found to be enhanced from Pn=As to Sb for LaFePnO, but not for BaFe$_2$Pn$_2$ and LiFePn. These results indicate that it is not the Fermi surface nesting but the local moment interaction that determines the stability of the magnetic phase in these materials, and that the partial gap is an induced feature by a specific magnetic order.

cond-mat.supr-con↗

arXiv2009

Mechanically-Controlled Binary Conductance Switching of a Single-Molecule Junction

Molecular-scale components are expected to be central to nanoscale electronic devices. While molecular-scale switching has been reported in atomic quantum point contacts, single-molecule junctions provide the additional flexibility of tuning the on/off conductance states through molecular design. Thus far, switching in single-molecule junctions has been attributed to changes in the conformation or charge state of the molecule. Here, we demonstrate reversible binary switching in a single-molecule junction by mechanical control of the metal-molecule contact geometry. We show that 4,4'-bipyridine-gold single-molecule junctions can be reversibly switched between two conductance states through repeated junction elongation and compression. Using first-principles calculations, we attribute the different measured conductance states to distinct contact geometries at the flexible but stable N-Au bond: conductance is low when the N-Au bond is perpendicular to the conducting pi-system, and high otherwise. This switching mechanism, inherent to the pyridine-gold link, could form the basis of a new class of mechanically-activated single-molecule switches.

cond-mat.mes-hall↗

arXiv2008

Enhanced spin density wave in LaOFeSb

We predict atomic, electronic, and magnetic structures of a hypothetical compound LaOFeSb by first-principles density-functional calculations. It is shown that LaOFeSb prefers a stripe-type antiferromagnetic phase (i.e., spin density wave (SDW) phase) to the non-magnetic (NM) phase, with a larger Fe spin moment and greater SDW-NM energy difference than those of LaOFeAs. The SDW phase is found to favor the orthorhombic structure while the tetragonal structure is more stable in the NM phase. In the NM-phase LaOFeSb, the electronic bandwidth near the Fermi energy is reduced compared with LaOFeAs, indicating smaller orbital overlap between Fe $d$ states and subsequently enhanced intra-atomic exchange coupling. The calculated Fermi surface in the NM phase consists of three hole and two electron sheets, and shows increased nesting between two hole and two electron sheets compared with LaOFeAs. Monotonous changes found in our calculated material properties of LaOFePn (Pn=P, As, and Sb), along with reported superconducting properties of doped LaOFeP and LaOFeAs, suggest that doped LaOFeSb may have a higher superconducting transition temperature.

cond-mat.supr-con↗

arXiv2008

Origins of anomalous electronic structures of epitaxial graphene on silicon carbide

On the basis of first-principles calculations, we report that a novel interfacial atomic structure occurs between graphene and the surface of silicon carbide, destroying the Dirac point of graphene and opening a substantial energy gap there. In the calculated atomic structures, a quasi-periodic $6\times 6$ domain pattern emerges out of a larger commensurate $6\sqrt{3}\times6\sqrt{3}R30^\circ$ periodic interfacial reconstruction, resolving a long standing experimental controversy on the periodicity of the interfacial superstructures. Our theoretical energy spectrum shows a gap and midgap states at the Dirac point of graphene, which are in excellent agreement with the recently-observed anomalous angle-resolved photoemission spectra. Beyond solving unexplained issues of epitaxial graphene, our atomistic study may provide a way to engineer the energy gaps of graphene on substrates.

cond-mat.mtrl-sci↗

arXiv2007

Amine-Gold Linked Single-Molecule Junctions: Experiment and Theory

The measured conductance distribution for single molecule benzenediamine-gold junctions, based on 59,000 individual conductance traces recorded while breaking a gold point contact in solution, has a clear peak at 0.0064 G$_{0}$ with a width of $\pm$ 40%. Conductance calculations based on density functional theory (DFT) for 15 distinct junction geometries show a similar spread. Differences in local structure have a limited influence on conductance because the amine-Au bonding motif is well-defined and flexible. The average calculated conductance (0.046 G$_{0}$) is seven times larger than experiment, suggesting the importance of many-electron corrections beyond DFT.

cond-mat.mtrl-sci↗

arXiv2005

Electrical Switching in Metallic Carbon Nanotubes

We present first-principles calculations of quantum transport which show that the resistance of metallic carbon nanotubes can be changed dramatically with homogeneous transverse electric fields if the nanotubes have impurities or defects. The change of the resistance is predicted to range over more than two orders of magnitude with experimentally attainable electric fields. This novel property has its origin that backscattering of conduction electrons by impurities or defects in the nanotubes is strongly dependent on the strength and/or direction of the applied electric fields. We expect this property to open a path to new device applications of metallic carbon nanotubes.

cond-mat.mtrl-sci↗

arXiv2003

Reply to "Comment on `First-principles calculation of the superconducting transition in MgB2 within the anisotropic Eliashberg formalism'"

The recent preprint by Mazin et al. [cond-mat/0212417] contains many inappropriate evaluations and/or criticisms on our published work [Phys. Rev. B 66, 020513 (2002) and Nature 418, 758 (2002)]. The preprint [cond-mat/0212417v1] was submitted to Physical Review B as a comment on one of our papers [Phys. Rev. B 66, 020513 (2002)]. In the reviewing process, Mazin et al. have withdrawn many of the statements contained in cond-mat/0212417v1, however two claims remain in their revised manuscript [cond-mat/0212417v3]: (1) the calculated variations of the superconducting energy gap within the sigma- or the pi-bands are not observable in real samples due to scatterings, and (2) the Coulomb repulsion mu(k,k') is negligibly small between sigma- and pi-states and thus should be approximated by a diagonal 2 x 2 matrix in the sigma and pi channels. Here, we point out that the former does not affect the validity of our theoretical work which is for the clean limit, and that the latter is not correct.

cond-mat.supr-con↗

arXiv2001

Superconducting energy gaps, low temperature specific heat, and quasiparticle spectra of MgB2

We report first-principles calculations of the k- and T-dependent superconducting gap Delta(k,T) in MgB2 and its manifestation in various measured quantities. Because the Fermi surface has disconnected sheets with different electron-phonon coupling strengths, our calculations show that near T=0, the values of Delta(k) cluster into two groups: large values (about 6.5 to 7.5 meV) on the strongly coupled sheets and small values (about 1 to 3 meV) on the weakly coupled sheets. The calculated gap, quasiparticle density of states, and specific heat and their temperature dependences are in agreement with the recent measurements which conclude that MgB2 is a multiple gap superconductor. In fact, theory predicts four prominent values for the gap at low T.

cond-mat.supr-con↗

arXiv2001

First-Principles Calculation of the Superconducting Transition in MgB2 within the Anisotropic Eliashberg Formalism

We present a study of the superconducting transition in MgB2 using the ab-initio pseudopotential density functional method and the fully anisotropic Eliashberg equation. Our study shows that the anisotropic Eliashberg equation, constructed with ab-initio calculated momentum-dependent electron-phonon interaction and anharmonic phonon frequencies, yields an average electron-phonon coupling constant lambda = 0.61, a transition temperature Tc = 39 K, and a boron isotope-effect exponent alphaB = 0.31 with a reasonable assumption of mu* = 0.12. The calculated values for Tc, lambda, and alphaB are in excellent agreement with transport, specific heat, and isotope effect measurements respectively. The individual values of the electron-phonon coupling lambda(k,k') on the various pieces of the Fermi surface however vary from 0.1 to 2.5. The observed Tc is a result of both the raising effect of anisotropy in the electron-phonon couplings and the lowering effect of anharmonicity in the relevant phonon modes.

cond-mat.supr-con↗