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Harshman D.R.,Physikon Research Corporation
Journal of physics. Condensed matter : an Institute of Physics journal | Year: 2012

In a recent contribution to this journal, it was shown that the transition temperatures of optimal high-T(C) compounds obey the algebraic relation T(C0) = k(-1)(B)/ℓζ, where ℓ is related to the mean spacing between interacting charges in the layers, ζ is the distance between interacting electronic layers, β is a universal constant and k(B) is Boltzmann's constant. The equation was derived assuming pairing based on interlayer Coulomb interactions between physically separated charges. This theory was initially validated for 31 compounds from five different high-T(C) families (within an accuracy of ±1.37 K). Herein we report the addition of Fe(1+x)Se(1-y) and Fe(1+x)Se(1-y)Te(y) (both optimized under pressure) and A(z)Fe(2-x)Se(2) (for A = K, Rb or Cs) to the growing list of Coulomb-mediated superconducting compounds in which T(C0) is determined by the above equation. Doping in these materials is accomplished through the introduction of excess Fe and/or Se deficiency, or a combination of alkali metal and Fe vacancies. Consequently, a very small number of vacancies or interstitials can induce a superconducting state with a substantial transition temperature. The confirmation of the above equation for these Se-based Fe chalcogenides increases to six the number of superconducting families for which the transition temperature can be accurately predicted. © 2012 IOP Publishing Ltd Source


Harshman D.R.,Physikon Research Corporation | Harshman D.R.,University of Notre Dame | Harshman D.R.,Arizona State University | Fiory A.T.,New Jersey Institute of Technology | And 2 more authors.
Journal of Physics Condensed Matter | Year: 2011

It is demonstrated that the transition temperature (TC) of high-TC superconductors is determined by their layered crystal structure, bond lengths, valency properties of the ions, and Coulomb coupling between electronic bands in adjacent, spatially separated layers. Analysis of 31 high-TC materials (cuprates, ruthenates, ruthenocuprates, iron pnictides, organics) yields the universal relationship for optimal compounds, kBTC0=β/ℓζ, where ℓ is related to the mean spacing between interacting charges in the layers, ζ is the distance between interacting electronic layers, β is a universal constant and T C0 is the optimal transition temperature (determined to within an uncertainty of ±1.4K by this relationship). Non-optimum compounds, in which sample degradation is evident, e.g.by broadened superconducting transitions and diminished Meissner fractions, typically exhibit reduced T C < TC0. It is shown that TC0 may be obtained from an average of the Coulomb interaction forces between the two layers. © 2011 IOP Publishing Ltd. Source


Harshman D.R.,Physikon Research Corporation | Harshman D.R.,University of Notre Dame | Harshman D.R.,Arizona State University | Fiory A.T.,New Jersey Institute of Technology
Journal of Physics Condensed Matter | Year: 2011

Muon spin rotation (μ+SR) measurements of square-root second moments of local magnetic fields σ in superconducting mixed states, as published for oriented crystals and powder samples of YBa2Cu 3O7 - δ (δ≈0.05), YBa2Cu 4O8 and La2 - xSrxCuO4 (x ∼ 0.15-0.17), are subjected to comparative analysis for superconducting gap symmetry. For oriented crystals it is shown that anomalous dependences of σ on temperature T and applied field H, as-measured and extracted a-and b-axial components, are attributable to fluxon depinning and disorder that obscure the intrinsic character of the superconducting penetration depth. Random averages derived from oriented crystal data differ markedly from corresponding non-oriented powders, owing to the weaker influence of pinning in high-quality crystals. Related indicators for pinning perturbations, such as non-monotonic H dependence of σ, irreproducible data and strong H dependence of apparent transition temperatures, are also evident. Strong intrinsic pinning suppresses thermal anomalies in c-axis components of σ, which reflect nodeless gap symmetries in YBa2Cu3O7 - δ and YBa 2Cu4O8. For YBa2Cu3O 7 - δ, the crystal (a-b components, corrected for depinning) and powder data all reflect a nodeless gap (however, a-b symmetries remain unresolved for crystalline YBa2Cu4O8 and La1.83Sr0.17CuO4). Inconsistencies contained in multiple and noded gap interpretations of crystal data, and observed differences between bulk μ+SR and surface-sensitive measurements are discussed. © 2011 IOP Publishing Ltd. Source


Harshman D.R.,Physikon Research Corporation | Harshman D.R.,College of William and Mary | Fiory A.T.,New Jersey Institute of Technology
Journal of Superconductivity and Novel Magnetism | Year: 2015

Behavior consistent with Coulomb-mediated high- TC superconductivity is shown to be present in the intercalated group-4-metal nitride halides Ax(S)yMNX, where the MNX host (M = Ti, Zr, Hf; X = Cl, Br) is partially intercalated with cations Ax and optionally molecular species (S)y in the van der Waals gap between the halide X layers, expanding the basal-plane spacing d. The optimal transition temperature is modeled by TC0 ∝ ζ−1(σ/ A)1/2, where the participating fractional charge per area per formula unit σ/A and the distance ζ, given by the transverse Ax-X separation (ζ < d), govern the interlayer Coulomb coupling. From experiment results for β-form compounds based on Zr and Hf, in which concentrations x of Ax are varied, it is shown that σ = γ[ v(xopt − x0)], where xopt is the optimal doping, x0 is the onset of superconducting behavior, v is the Ax charge state, and γ = 1/8 is a factor determined by the model. Observations of TC < TC0 in the comparatively more disordered α-Ax(S)yTiNX compounds are modeled as pair breaking by remote Coulomb scattering from the Ax cations, which attenuates exponentially with increasing ζ. The TC0 values calculated for nine Ax(S)yMNCl compounds, shown to be optimal, agree with the measured TC to within experimental error. The model for TC0 is also found to be consistent with the absence of high- TC characteristics for AxMNX compounds in which a spatially separated intercalation layer is not formed. © 2015, Springer Science+Business Media New York. Source


Harshman D.R.,Physikon Research Corporation | Harshman D.R.,College of William and Mary | Fiory A.T.,New Jersey Institute of Technology
Journal of Physics and Chemistry of Solids | Year: 2015

Superconductivity in the Tl-based cuprates encompasses a notably broad range of measured optimal transition temperatures TC0, ranging from lowest in the charge-depleted Tl-1201 compounds (Tl1-x(Ba/Sr)1+yLa1-yCuO5-δ), such as Tl0.7LaSrCuO5 (37 K) and TlBa1.2La0.8CuO5 (45.4 K), to highest in the Tl-1223 compound TlBa2Ca2Cu3O9±δ (133.5 K). Seven Tl-based cuprates are considered and compared using the model of superconductive pairing via electronic interactions between two physically separated charge reservoirs, where TC0 (ση/A)1/2ζ-1 is determined by the superconducting interaction charge fraction σ, the number η of CuO2 layers, and the basal-plane area A, each per formula unit, and the transverse distance ζ between interacting layers. Herein it is demonstrated that σ follows from the elemental electronegativity and the oxidation state of Tl, and other structurally analogous cations. The comparatively lower elemental electronegativity of Tl, in conjunction with its oxidation state, explains the higher σ and TC0 values in the Tl-based compounds relative to their Bi-based cuprate homologs. A derivation of σ is introduced for the optimal Tl2Ba2Caη-1CuηO2η+4 (for η=1, 2, 3) compounds, which exhibit a Tl oxidation state at or near +3, obtaining the fundamental value σ0=0.228 previously established for YBa2Cu3O6.92. Also reported is the marked enhancement in σ associated with Tl+1 and analogous inner-layer cations relative to higher-valence cations. For a model proposition of σ=σ0, the fractional Tl+1 content of the mixed-valence compound, TlBa2Ca2Cu3O9±δ, is predicted to be 1/3 at optimization, in agreement with existing data. Charge depletion is illustrated for the two Tl-1201 compounds, where σ<σ0 values are determined according to substitution of Ba+2 or Sr+2 by La+3, and/or Tl depletion. Additionally, statistical analysis of calculated and experimental transition temperatures of 48 optimal superconductors shows an absence of bias in determining σ, A, and ζ. © 2015 Elsevier Ltd.All rights reserved. Source

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