arXiv subjects

K. -W. Lee

Publications and source records attributed to K. -W. Lee.

49 records · Page 3Linked to original sources

arXiv2006

Charge and Spin Ordering in Insulating Na$_{0.5}$CoO$_2$: Effects of Correlation and Symmetry

Ab initio band theory including correlations due to intra-atomic repulsion is applied to study charge disproportionation and charge- and spin-ordering in insulating Na$_{0.5}$CoO$_2$. Various ordering patterns (zigzag and two striped) for four-Co supercells are analyzed before focusing on the observed "out-of-phase stripe" pattern of antiferromagnetic Co$^{4+}$ spins along charge-ordered stripes. This pattern relieves frustration and shows distinct analogies with the cuprate layers: a bipartite lattice of antialigned spins, with axes at 90 degree angles. Substantial distinctions with cuprates are also discussed, including the tiny gap of a new variant of "charge transfer" type within the Co 3d system.

cond-mat.str-el↗

arXiv2006

Study of Correlation Effects in the High Formal Oxidation State Compound Sr$_2$CoO$_4$

Two recent reports confirm that the newly synthesized Sr$_2$CoO$_4$ (formal oxidation state Co$^{4+}$) shows a high Curie temperature (~ 250 K), but they report different moments of 1.8 $μ_B$ and 1 $μ_B$ per Co. Using both commonly used functionals in the correlated band approach (LDA+U) as well as the local density approximation (LDA),the combined effects of correlation and hybridization with O 2p states are calculated and analyzed. Sr$_2$CoO$_2$ is already ferromagnetic within LDA (M=1.95 $μ_B$). Increasing U from zero, the two LDA+U schemes affect the moment oppositely out to a critical value $U_c$=2.5 eV, at which point they transform discontinuously from different states to the same large U state. Fixing U at $U_c$, fixed spin moment calculations show similar behavior out to a minimum at 1$μ_B$ (a half metallic state), beyond which the fully-localized-limit scheme jumps to a state with energy minimum very near 2$μ_B$ very close to the LDA moment). Although the energy minima occur very near integer values of the moment/Co (1$μ_B, 2$μ_B$), the strong 3d-2p mixing and resulting 3d orbital occupations seem to preclude any meaningful S=1/2 or S=1 assignment to the Co ion.

cond-mat.str-el↗

arXiv2006

Boron Spectral Density and Disorder Broadening in B-doped Diamond

Comparison of periodic B dopants with a random alloy of substitional boron in diamond is carried out using several supercells and the coherent potential approximation (CPA) for the random alloy case. The main peak in the B local density of states is shifted to lower binding energy compared to the corresponding C peak in intrinsic diamond. In supercells, this shows up as strongly B-character bands split from bulk C bands away from the zone center,in an energy region around -1 eV. Even for a 4*4*4 supercell (BC$_{127}$), effects of the dopant order are evident in the form of primarily B-character bands just below the Fermi level at the supercell zone boundary. The bands resulting from the CPA are of continuous mixed C-B character. They resemble virtual crystal bands, but broadened somewhat reflecting the disorder-induced lifetime, and are consistent with angle-resolved photoemission band maps. The B character is 1.7 times larger than for C (per atom) near the top of the valence bands for CPA, and roughly the same for supercells. CPA results are particularly useful since they characterize the wavevector and energy dependence of disorder broadening.

cond-mat.supr-con↗

arXiv2005

Comparison of the Electronic Structures of Two Non-cuprate Layered Transition Metal Oxide Superconductors

Comparison is made of the electronic structure of the little-studied layered transition metal oxide LiNbO$_2$ with that of Na$_x$CoO$_2$, which has attracted tremendous interest since superconductivity was discovered in its hydrate. Although the active transition metal $d$ states are quite different due to different crystal fields and band filling, both systems show a strong change of electronic structure with changes in the distance between the transition metal ion layer and the oxygen layers. The niobate is unusual in having a large second-neighbor hopping amplitude, and a nearest neighbor hopping amplitude that is sensitive to the Nb-O separation. Li$_x$NbO$_2$ also presents the attractive simplicity of a single band triangular lattice system with variable carrier concentration that is superconducting.

cond-mat.supr-con↗

arXiv2005

Crystal Symmetry, Electron-Phonon Coupling, and Superconducting Tendencies in Li$_2$Pd$_3$B and Li$_2$Pt$_3$B

After theoretical determination of the internal structural coordinates in Li$_2$Pd$_3$B, we calculate and analyze its electronic structure and obtain the frequencies of the two $A_g$ phonons (40.6 meV for nearly pure Li mode, 13.0 meV for the strongly mixed Pd-Li mode). Pd can be ascribed a $4d^{10}$ configuration, but strong 4d character remains up to the Fermi level. Small regions of flat bands occur at -70 meV at both the $Γ$ and X points. Comparison of the Fermi level density of states to the linear specific heat coefficient gives a dynamic mass enhancement of $λ$ = 0.75. Rough Fermi surface averages of the deformation potentials $D$ of individual Pd and Li displacements are obtained. While <$D_{Li}$> is small, <$D_{Pd}> ~ 1.15 eV/Åis sizable, and a plausible case exists for its superconductivity at 8 K being driven primarily by coupling to Pd vibrations. The larger d bandwidth in Li$_2$Pt$_3$B leads to important differences in the bands near the Fermi surface. The effect on the band structure of spin-orbit coupling plus lack of inversion is striking, and is much larger in the Pt compound.

cond-mat.supr-con↗

arXiv2005

Disproportionation and Critical Interaction Strength in Na$_x$CoO$_2$: Concentration Dependence

We present results of studies of charge disproportionation (CD) and spin differentiation in Na$_x$CoO$_2$ using the correlated band theory approach (local-density approximation+Hubbard U:LDA+U). The simultaneous CD and gap opening can be followed through a first order charge disproportionation transition. By comparison with experiments, we propose a value of the Coulomb repulsion strength U($x$) that has significant dependence on the carrier concentration $x$, for which we obtain a phase diagram. The connection between the oxygen height and effects of on-site correlation is also reported.

cond-mat.str-el↗

arXiv2005

Na$_x$CoO$_2$ in the x -> 0 Regime: Coupling of Structure and Correlation effects

The study of the strength of correlations in Na$_x$CoO$_2$ is extended to the x=0 end of the phase diagram where Mott insulating behavior has been widely anticipated. Inclusion of correlation as modeled by the LDA+U approach leads to a Mott transition in the $a_g$ subband if U is no less than U$_c$=2.5 eV. Thus U smaller than U$_c$ is required to model the metallic, nonmagnetic CoO$_2$ compound reported by Tarascon and coworkers. The orbital-selective Mott transition of the $a_g$ state, which is essentially degenerate with the $e'_{g}$ states, occurs because of the slightly wider bandwidth of the $a_g$ bands. The metal-insulator transition is found to be strongly coupled to the Co-O bond length, due to associated changes in the $t_{2g}$ bandwidth, but the largest effects occur only at a reduced oxygen height that lies below the equilibrium position.

cond-mat.str-el↗

arXiv2004

Origin of Superconductivity in Boron-doped Diamond

Superconductivity of boron-doped diamond, reported recently at T_c=4 K, is investigated exploiting its electronic and vibrational analogies to MgB2. The deformation potential of the hole states arising from the C-C bond stretch mode is 60% larger than the corresponding quantity in MgB2 that drives its high Tc, leading to very large electron-phonon matrix elements. The calculated coupling strength λ~ 0.5 leads to T_c in the 5-10 K range and makes phonon coupling the likely mechanism. Higher doping should increase T_c somewhat, but effects of three dimensionality primarily on the density of states keep doped diamond from having a T_c closer to that of MgB2.

cond-mat.supr-con↗

arXiv2004

On Heavy Carbon Doping of MgB$_2$

Heavy carbon doping of MgB$_2$ is studied by first principles electronic structure studies of two types, an ordered supercell (Mg(B$_{1-x}$C$_{x}$)$_{2}$,x=0.0833) and also the coherent potential approximation method that incorporates effects of B-C disorder. For the ordered model, the twofold degenerate $σ$-bands that are the basis of the high temperature superconductivity are split by 60 meV (i.e.7 meV/%C) and the $σ$ Fermi cylinders contain 0.070 holes/cell, compared to 0.11 for MgB$_2$. A virtual crystal treatment tends to overestimate the rate at which $σ$ holes are filled by substitutional carbon. The coherent potential approximation (CPA) calculations give the same rate of band filling as the supercell method. The occupied local density of states of C is almost identical to that of B in the upper 2 eV of the valence bands, but in the range -8 eV to -2 eV, C has a considerably larger density of states. The calculations indicate that the $σ$ Fermi surface cylinders pinch off at the zone center only above the maximum C concentration x ~ 0.10. These results indicate that Mg(B$_{1-x}$C$_{x}$)$_{2}$ as well as Mg$_{1-x}$Al$_x$B$_2$ is a good system in which to study the evolution of the unusual electron-phonon coupling character and strength as the crucial $σ$ hole states are filled.

cond-mat.supr-con↗

arXiv2004

Disproportionation Transition at Critical Interaction Strength: Na$_{1/2}$CoO$_2$

Charge disproportionation (CD) and spin differentiation in Na$_{1/2}$CoO$_2$ are studied using the correlated band theory approach. The simultaneous CD and gap opening seen previously is followed through a first order charge disproportionation transition 2Co$^{3.5+} \to$ Co$^{3+}$+Co$^{4+}$, whose ionic identities are connected more closely to spin (S=0, S=1/2 respectively) than to real charge. Disproportionation in the Co $a_g$ orbital is compensated by opposing charge rearrangement in other 3d orbitals. At the transition large and opposing discontinuities in the (all-electron) kinetic and potential energies are slightly more than balanced by a gain in correlation energy. The CD state is compared to characteristics of the observed charge-ordered insulating phase in Na$_{1/2}$CoO$_2$, suggesting the Coulomb repulsion value $U$ is concentration-dependent, with $U(x=1/2)\simeq$3.5 eV.

cond-mat.str-el↗

arXiv2004

Charge Disproportionation and Spin Ordering Tendencies in Na(x)CoO2

The strength and effect of Coulomb correlations in the (superconducting when hydrated) x~1/3 and ``enhanced'' x~2/3 regimes of Na(x)CoO2 are evaluated using the correlated band theory LDA+U method. Our results, neglecting quantum fluctuations, are: (1) allowing only ferromagnetic order, there is a critical U_c = 3 eV, above which charge disproportionation occurs for both x=1/3 and x=2/3, (2) allowing antiferromagnetic order at x=1/3, U_c drops to 1 eV for disproportionation, (3) disproportionation and gap opening occur simultaneously, (4) in a Co(3+)-Co(4+) ordered state, antiferromagnetic coupling is favored over ferromagnetic, while below U_c ferromagnetism is favored. Comparison of the calculated Fermi level density of states compared to reported linear specific heat coefficients indicates enhancement of the order of five for x~0.7, but negligible enhancement for x~0.3. This trend is consistent with strong magnetic behavior and local moments (Curie-Weiss susceptibility) for x>0.5 while there no magnetic behavior or local moments reported for x<0.5. We suggest that the phase diagram is characterized by a crossover from effective single-band character with U >> W for x>0.5 into a three-band regime for x<0.5, where U --> U_eff <= U/\sqrt(3) ~ W and correlation effects are substantially reduced.

cond-mat.str-el↗

arXiv2004

Infinite Layer LaNiO(2): Ni(1+)is not Cu(2+)

The Ni ion in LaNiO$_2$ has the same formal ionic configuration $3d^9$ as does Cu in isostructural CaCuO$_2$, but it is reported to be nonmagnetic and probably metallic whereas CaCuO$_2$ is a magnetic insulator. From ab initio calculations we trace its individualistic behavior to (1) reduced $3d-2p$ mixing due to an increase of the separation of site energies ($ε_d - ε_p$) of at least 2 eV, and (2) important Ni $3d(3z^2-r^2)$ mixing with La $5d(3z^2-r^2)$ states that leads to Fermi surface pockets of La $5d$ character that hole-dope the Ni 3d band.Correlation effects do not appear to be large in LaNiO$_2$. However, ad hoc increase of the intraatomic repulsion on the Ni site (using the LDA+U method) is found to lead to a novel correlated state: (i) the transition metal $d(x^2-y^2)$ and $d(3z^2-r^2)$ states undergo consecutive Mott transitions, (ii) their moments are antialigned leading (ideally) to a "singlet" ion in which there are two polarized orbitals, and (iii) mixing of the upper Hubbard $3d(3z^2-r^2)$ band with the La $5d(xy)$ states leaves considerable transition metal 3d character in a band pinned to the Fermi level. The magnetic configuration is more indicative of a Ni$^{2+}$ ion in this limit, although the actual charge changes little with U.

cond-mat.str-el↗

arXiv2004

Charge Disproportionation and Spin-Ordering Tendencies in Na(x)CoO2 at x=1/3

The strength and effect of Coulomb correlations in the (superconducting when hydrated) x~1/3 regime of Na(x)CoO(2) have been evaluated using the correlated band theory LDA+U method. Our results, neglecting quantum fluctuations, are: (1) there is a critical $U_{c}$ = 3 eV, above which charge ordering occurs at x=1/3, (2) in this charge-ordered state, antiferromagnetic coupling is favored over ferromagnetic, while below $U_{c}$, ferromagnetism is favored; and (3) carrier conduction behavior should be very asymmetric for dopings away from x=1/3. For x < 1/3, correlated hopping of parallel spin pairs is favored, suggesting a triplet superconducting phase.

cond-mat.str-el↗