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An enhancement mechanism of two-particle exchange interactions in single- and multi-layer cuprate superconductors
Authors:
Shingo Teranishi,
Satoaki Miyao,
Kazutaka Nishiguchi,
Koichi Kusakabe
Abstract:
To explore material dependence of layered cuprate superconductors, we examine effective two-particle interactions for Hg1201 and Tl1201, where Tl1201 having a nearly half value of Tc of Hg1201 even at the optimal oxygen concentration. Although the 3dx_2-y_2 band, the Fermi surface, and its Wannier-orbitals are similar for these superconductors, there is an apparent difference in the unoccupied lev…
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To explore material dependence of layered cuprate superconductors, we examine effective two-particle interactions for Hg1201 and Tl1201, where Tl1201 having a nearly half value of Tc of Hg1201 even at the optimal oxygen concentration. Although the 3dx_2-y_2 band, the Fermi surface, and its Wannier-orbitals are similar for these superconductors, there is an apparent difference in the unoccupied levels above EF. Based on a multi-reference density-functional-theory formulation, effective two-particle exchange interactions are estimated to derive enhancement in intra-layer exchange interactions for HgBa2CuO4, while it is weakened in TlBaLaCuO5 and furthermore it is weak in TlBa2CuO5. The characteristic difference in the band structure is correlated with oxygen contents in the buffer layer. We also comment on the similar feature in triple-layered compounds. Our spin-fluctuation enhancement mechanism in an electron-correlation regime is consistent with the experimental fact.
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Submitted 21 April, 2017;
originally announced April 2017.
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A study on the growth mechanism and the process parameters controlling aluminum oxide thin films deposition by pulsed pressure MOCVD
Authors:
Hari Murthy,
S. S Miya,
Susan Krumdieck
Abstract:
Aluminum oxide thin films were deposited on silicon substrates under different deposition conditions using pulse pressure metal organic chemical vapour deposition (PP-MOCVD). The current study investigates into the growth mechanism of the deposited film and the control of the film morphology by varying the processing parameters of PP-MOCVD - choice of solvent, concentration, and presence of a shie…
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Aluminum oxide thin films were deposited on silicon substrates under different deposition conditions using pulse pressure metal organic chemical vapour deposition (PP-MOCVD). The current study investigates into the growth mechanism of the deposited film and the control of the film morphology by varying the processing parameters of PP-MOCVD - choice of solvent, concentration, and presence of a shield. Aluminum sec-butoxide (ASB) was used as the aluminum source while hexane and toluene were used as the solvents. The films were deposited at 475oC at different precursor concentrations. It was observed that the choice of solvent has no effect on the surface morphology, but it influenced the deposition rate. The improved deposition rate, relatively close enthalpy of vaporisation (ΔH) values and uniformity of the film, irrespective of the growth conditions, showed that hexane was a better solvent for ASB than toluene. A hybrid mode of vapour deposition and vapour condensation model for thin film growth is proposed where five different mechanisms lead to a solid film formation. These include vapour phase deposition under low arrival rate, vapour phase deposition under high arrival rate, Leidenfrost aerosol formation, heterogeneous particle formation and liquid droplet impingement. The important parameter that needs to be controlled is the precursor flux arrival rate which can be controlled by varying the precursor concentration, use of a solvent with a low ΔH and the presence of a shield over the substrate, which influences the surface morphology and the growth rate of the films.
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Submitted 21 August, 2016;
originally announced August 2016.
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Theoretical Analysis on Pseudo-Degenerate Zero-Energy Modes in Vacancy-Centered Hexagonal Armchair Nanographene
Authors:
Naoki Morishita,
Gagus Ketut Sunnardianto,
Satoaki Miyao,
Koichi Kusakabe
Abstract:
Deriving mathematical expressions of two zero modes for a $π$-band tight-binding model, we identify a class of bipartite graphs having the same number of subgraph sites, where each graph represents one of the quasi-hexagonal nanographene molecule with a center vacancy (VANG). Indeed, in a VANG molecule, C$_{60}$H$_{24}$, showing stability in a density-functional simulation at the highest occupied…
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Deriving mathematical expressions of two zero modes for a $π$-band tight-binding model, we identify a class of bipartite graphs having the same number of subgraph sites, where each graph represents one of the quasi-hexagonal nanographene molecule with a center vacancy (VANG). Indeed, in a VANG molecule, C$_{60}$H$_{24}$, showing stability in a density-functional simulation at the highest occupied level, there appear two pseudo-degenerate zero modes, a vacancy-centered quasi-localized zero mode, and extending zero mode with a $\sqrt{3} \times \sqrt{3}$ structure. Since there is a finite energy gap between these two zero-energy modes and the other modes, low-lying states composed of quasi-degenerate zero modes appear as magnetic multiplets. Thus, the unique magnetic characteristics derived in our theory are expected to hold for synthesized VANG molecules in reality.
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Submitted 22 April, 2016;
originally announced April 2016.
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Orbital mixture effect on the Fermi surface-$T_c$ correlation in the cuprate superconductors --- bilayer vs single layer
Authors:
Hirofumi Sakakibara,
Katsuhiro Suzuki,
Hidetomo Usui,
Satoaki Miyao,
Isao Maruyama,
Koichi Kusakabe,
Ryotaro Arita,
Hideo Aoki,
Kazuhiko Kuroki
Abstract:
By constructing $d_{x^2-y^2}-d_{z^2}$ two-orbital models from first principles, we have obtained a systematic correlation between the Fermi surface warping and the evaluated $T_c$ for various bilayer as well as single-layer cuprates. This reveals that smaller mixture of the $d_{z^2}$ orbital component on the Fermi surface leads to both of larger Fermi surface warping and higher $T_c$. The theoreti…
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By constructing $d_{x^2-y^2}-d_{z^2}$ two-orbital models from first principles, we have obtained a systematic correlation between the Fermi surface warping and the evaluated $T_c$ for various bilayer as well as single-layer cuprates. This reveals that smaller mixture of the $d_{z^2}$ orbital component on the Fermi surface leads to both of larger Fermi surface warping and higher $T_c$. The theoretical correlation strikingly resembles a systematic plot for the experimentally observed $T_c$ against the Fermi surface warping due to Pavarini {\it et al.} [Phys. Rev. Lett. {\bf 87}, 047003 (2001)], and the present result unambiguously indicates that the $d_{z^2}$ mixture is a key factor that determines $T_c$ in the cuprates.
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Submitted 3 June, 2014; v1 submitted 11 March, 2014;
originally announced March 2014.
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Material Optimization for Fermi Surface Shape Control of Tl-based Cuprate Superconductors
Authors:
Satoaki Miyao,
Hirofumi Sakakibara,
Isao Maruyama,
Kazuhiko Kuroki,
Koichi Kusakabe
Abstract:
To show an optimization method of element substitution for cuprate superconductors, we investigate Fermi surface shape of TlR2A2Cu3O9 with R=La, Y, and A=Li, Na, K, Rb, Cs. We adopt the generalized gradient approximation in the density-functional theory (DFT-GGA) for the study of over-doped phases of these unknown cuprates. The electronic structures of crystals optimized by DFT-GGA show systematic…
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To show an optimization method of element substitution for cuprate superconductors, we investigate Fermi surface shape of TlR2A2Cu3O9 with R=La, Y, and A=Li, Na, K, Rb, Cs. We adopt the generalized gradient approximation in the density-functional theory (DFT-GGA) for the study of over-doped phases of these unknown cuprates. The electronic structures of crystals optimized by DFT-GGA show systematic element dependence in a Fermi surface shape controlling parameter, r, of Cu dx2-y2 bands, where nearly absent dz2 component at the Fermi level and smaller r keeping t1 suggest enhancement of the superconducting transition temperature within the spin-fluctuation mechanism. For TlYRb2Cu3O9, smaller r by a reduction factor larger than 10% compared to a reference system of TlBa2Ca2Cu3O9 (TBCCO) appears in the outer CuO2 plane, but with 12 % reduction in t1. For TlR2Li2Cu3O9, (R=Y, La), smaller r by a factor larger than 1% appears keeping t1 as large as TBCCO in the inner plane. Our method may be used to predict an optimized material structure referencing known cuprate superconductors.
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Submitted 24 April, 2013; v1 submitted 18 April, 2013;
originally announced April 2013.