arXiv subjects

J. E. Han

Publications and source records attributed to J. E. Han.

29 records · Page 2Linked to original sources

arXiv2003

Saturation of electrical resistivity

Resistivity saturation is observed in many metallic systems with a large resistivity, i.e., when the resistivity has reached a critical value, its further increase with temperature is substantially reduced. This typically happens when the apparent mean free path is comparable to the interatomic separations - the Ioffe-Regel condition. Recently, several exceptions to this rule have been found. Here, we review experimental results and early theories of resistivity saturation. We then describe more recent theoretical work, addressing cases both where the Ioffe-Regel condition is satisfied and where it is violated. In particular we show how the (semiclassical) Ioffe-Regel condition can be derived quantum-mechanically under certain assumptions about the system and why these assumptions are violated for high-Tc cuprates and alkali-doped fullerides.

cond-mat.str-el↗

arXiv2002

Superconductivity in molecular solids with Jahn-Teller phonons

We analyze fulleride superconductivity at experimental doping levels, treating the electron-electron and electron-phonon interactions on an equal footing, and establish the existence of novel physics which helps explain the unusually high superconducting transition temperatures in these systems. The Jahn-Teller phonons create a local (intramolecular) pairing that is surprisingly resistant to the Coulomb repulsion, despite the weakness of retardation in these low-bandwidth systems. The requirement for coherence throughout the solid to establish superconductivity then yields a very strong doping dependence to Tc, one consistent with experiment and much stronger than expected from standard Eliashberg theory.

cond-mat.str-el↗

arXiv2000

Tuning Fermi-surface properties through quantum confinement in metallic meta-lattices: New metals from old atoms

We describe a new class of nanoscale structured metals wherein the effects of quantum confinement are combined with dispersive metallic electronic states to induce modifications to the fundamental low-energy microscopic properties of a three-dimensional metal: the density of states, the distribution of Fermi velocities, and the collective electronic response.

cond-mat.mtrl-sci↗

arXiv2000

The mean free path for electron conduction in metallic fullerenes

We calculate the electrical resistivity due to electron-phonon scattering for a model of A3C60 (A= K, Rb), using an essentially exact quantum Monte-Carlo calculation. In agreement with experiment, we obtain exceptionally large metallic resistivities at large temperatures T. This illustrates that the apparent mean free path can be much shorter than the separation of the molecules. An interpretation of this result is given. The calculation also explains the linear behavior in T at small T.

cond-mat.str-el↗

arXiv2000

Metal-insulator transitions in systems with electron-phonon and Coulomb interactions

We consider a model which includes the electron-phonon coupling to Ag or Jahn-Teller Hg phonons, the Coulomb interaction U and an exchange integral K. We study the metal-insulator transition for the integer fillings n=3 and 4. We find that the coupling to Ag phonons increases the critical value Uc where the transition takes place, while the coupling to Hg phonons decreases Uc. Without electron-phonon coupling the Hund's rule coupling also decreases Uc. There is, however, an interesting competition between the Hund's rule coupling and the Jahn-Teller effect. Thus the reduction of Uc due to the Hg phonons becomes smaller when the Hund's rule coupling is turned on. The implications of this is discussed in the context of AnC60.

cond-mat.str-el↗

arXiv1999

Metal-insulator transitions: Influence of lattice structure, Jahn-Teller effect, and Hund's rule coupling

We study the influence of the lattice structure, the Jahn-Teller effect and the Hund's rule coupling on a metal-insulator transition in AnC60 (A= K, Rb). The difference in lattice structure favors A3C60 (fcc) being a metal and A4C60 (bct) being an insulator, and the coupling to Hg Jahn-Teller phonons favors A4C60 being nonmagnetic. The coupling to Hg (Ag) phonons decreases (increases) the value Uc of the Coulomb integral at which the metal-insulator transition occurs. There is an important partial cancellation between the Jahn-Teller effect and the Hund's rule coupling.

cond-mat.str-el↗

arXiv1999

Phonon spectral function for an interacting electron-phonon system

Using exact diagonalzation techniques, we study a model of interacting electrons and phonons. The spectral width of the phonons is found to be reduced as the Coulomb interaction U is increased. For a system with two modes per site, we find a transfer of coupling strength from the upper to the lower mode. This transfer is reduced as U is increased. These results give a qualitative explanation of differences between Raman and photoemission estimates of the electron-phonon coupling constants for A3C60 (A= K, Rb).

cond-mat.str-el↗

arXiv1999

Phonon line shape in disordered A3C60 (A= K, Rb)

We present a calculation of the Hg phonon self-energy for a model of A3C60 (A= K, Rb). The orientational disorder of the C60 molecules is included, and the lowest order self-energy diagram is considered. The calculations illustrate that due to the violation of momentum conservation by the orientational disorder, Raman scattering can measure the decay of a phonon in an electron-hole pair, allowing the estimate of the electron-phonon coupling from such experiments. Comparison with experimental line shapes further provides support for a local correlation of the molecular orientations, with neighboring molecules preferentially having orientations differing by a 90 degree rotation.

cond-mat.mtrl-sci↗

arXiv1998

Spectral evolution in (Ca,Sr)RuO_3 near the Mott-Hubbard transition

We investigated the optical properties of (Ca,Sr)RuO_3 films on the borderline of a metal-insulator (M-I) transition. Our results show all of the predicted characteristics for a metallic Mott-Hubbard system, including (i) a mass enhancement in dc-limit, (ii) an U/2 excitation, and (iii) an U excitation. Also, a self-consistency is found within the Gutzwiller-Brinkman-Rice picture for the Mott transition. Our finding displays that electron correlation should be important even in 4d materials.

cond-mat.str-el↗

arXiv1998

Multi-Orbital Hubbard Model in Infinite Dimensions: Quantum Monte Carlo Calculation

Using Quantum Monte Carlo we compute thermodynamics and spectra for the orbitally degenerate Hubbard model in infinite spatial dimensions. With increasing orbital degeneracy we find in the one-particle spectra: broader Hubbard bands (consistent with increased kinetic energy), a narrowing Mott gap, and increasing quasi-particle spectral weight. In opposition, Hund's rule exchange coupling decreases the critical on-site Coulomb energy for the Mott transition. The metallic regime resistivity for two-fold degeneracy is quadratic-in-temperature at low temperatures.

cond-mat.str-el↗

arXiv1996

Ab Initio Calculation of Crystalline Electric Fields and Kondo Temperatures in Ce-Compounds

We have calculated the band-$f$ hybridizations for Ce$_x$La$_{1-x}$M$_3$ compounds ($x=1$ and $x\rightarrow 0$; M=Pb, In, Sn, Pd) within the local density approximation and fed this into a non-crossing approximation for the Anderson impurity model applied to both dilute and concentrated limits. Our calculations produce crystalline electric field splittings and Kondo temperatures with trends in good agreement with experiment and demonstrate the need for detailed electronic structure information on hybridization to describe the diverse behaviors of these Ce compounds.

cond-mat↗