Modeling Intercalated Group-4-Metal Nitride Halide Superconductivity with Interlayer Coulomb Coupling

J. Phys.: Condens. Matter

Fiory

2017

Unique among alkali-doped A C fullerene compounds, the A15 and fcc forms of CsC exhibit superconducting states varying under hydrostatic pressure with highest transition temperatures at [Formula: see text] = 38.3 and 35.2 K, respectively. Herein it is argued that these two compounds under pressure represent the optimal materials of the A C family, and that the C-associated superconductivity is mediated through Coulombic interactions with charges on the alkalis. A derivation of the interlayer Coulombic pairing model of high-T superconductivity employing non-planar geometry is introduced, generalizing the picture of two interacting layers to an interaction between charge reservoirs located on the C and alkali ions. The optimal transition temperature follows the algebraic expression, T = (12.474 nm K)/ℓζ, where ℓ relates to the mean spacing between interacting surface charges on the C and ζ is the average radial distance between the C surface and the neighboring Cs ions. Values of T for the measured cation stoichiometries of Cs C with x ≈ 0 are found to be 38.19 and 36.88 K for the A15 and fcc forms, respectively, with the dichotomy in transition temperature reflecting the larger ζ and structural disorder in the fcc form. In the A15 form, modeled interacting charges and Coulomb potential e/ζ are shown to agree quantitatively with findings from nuclear-spin relaxation and mid-infrared optical conductivity. In the fcc form, suppression of [Formula: see text] below T is ascribed to native structural disorder. Phononic effects in conjunction with Coulombic pairing are discussed.

Section: Interlayer Coulombic Pairing Modelmentioning

confidence: 88%

Section: Application To Cubic a 3 C 60mentioning

confidence: 99%

High-T_Csuperconductivity in Cs₃C₆₀compounds governed by local Cs–C₆₀Coulomb interactions

J. Phys.: Condens. Matter

Fiory

2017

“…The essentially instantaneous nature of quasiparticle creation, compared to the significantly slower relaxation process, allows for 6 See e.g. equation (25) in [28] unscreened Coulomb interactions between electronic charges in adjacent reservoirs. Equation (1) thus derives fundamentally from quantum fluctuations within the virtual photon field between the two charge reservoirs inducing Compton scattering of quasiparticles confined essentially within their respective reservoirs.…”

Section: Derivation Of T C0mentioning

confidence: 99%

Compressed H₃S: inter-sublattice Coulomb coupling in a high-T_Csuperconductor

J. Phys.: Condens. Matter

Fiory²

2017

Upon thermal annealing at or above room temperature (RT) and at high hydrostatic pressure P ~ 155 GPa, sulfur trihydride HS exhibits a measured maximum superconducting transition temperature T ~ 200 K. Various theoretical frameworks incorporating strong electron-phonon coupling and Coulomb repulsion have reproduced this record-level T. Of particular relevance is that experimentally observed H-D isotopic correlations among T , P, and annealed order indicate an H-D isotope effect exponent α limited to values ⩽ 0.183, leaving open for consideration unconventional high-T superconductivity with electronic-based enhancements. The work presented herein examines Coulombic pairing arising from interactions between neighboring S and H species on separate interlaced sublattices constituting HS in the Im[Formula: see text]m structure. The optimal value of the transition temperature is calculated from T = [Formula: see text]Λe/[Formula: see text] ζ, with Λ = 0.007465 Å, inter-sublattice S-H separation spacing ζ = a /[Formula: see text], interaction charge linear spacing [Formula: see text] = a (3/σ), average participating charge fraction σ = 3.43 ± 0.10 estimated from calculated H-projected electron states, and lattice parameter a = 3.0823 Å at P = 155 GPa. The resulting value of T = 198.5 ± 3.0 K is in excellent agreement with transition temperatures determined from resistivity (196-200 K onsets, 190-197 K midpoints), susceptibility (200 K onset), and critical magnetic fields (203.5 K by extrapolation). Analysis of mid-infrared reflectivity data confirms the expected correlation between boson energy and ζ . Suppression of T below T , correlating with increasing residual resistance for< RT annealing, is treated in terms of scattering-induced pair breaking. Correspondences between HS and layered high-T superconductor structures are also discussed, and a model considering Compton scattering of virtual photons of energies ⩽ e/ζ by inter-sublattice electrons is introduced, illustrating that Λ ∝ ƛ , where ƛ is the reduced electron Compton wavelength.

“…This theory predicts T C0 for optimal superconductors, which are identified experimentally by maxima in transition temperatures governed by controlling doping, structure, and externally applied pressure. Calculations of T C0 have been previously validated against experiment with statistical accuracy of ±1.30 K or ±4% in T C0 for 51 different superconductors from nine superconducting families, consisting of the aforecited 3D compounds [50,51], layered cuprates, ruthenates, rutheno-cuprates, iron pnictides, BEDT-based [bis(ethylenedithio)tetrathiafulvalene] organics [49,52,53], iron chalcogenides [54], and intercalated group-4-metal nitride-chlorides [55,56], with measured superconducting transitions ranging from ~7 to 200 K.…”

Section: Interlayer Coulomb Pairing Modelmentioning

confidence: 99%

“…Pairing in this model is ascribed to Coulomb interactions between charges in adjacent reservoirs separated by a distance ζ via the exchange of virtual photons, with T C0 scaling with the root of the participating charge density 1/ℓ multiplied by the potential energy e 2 /ζ (where e is the electron charge). In prior work, this approach has been successfully applied to 51 compounds from nine unique superconducting families, ascertained by accurately deriving T C0 [49][50][51][52][53][54][55][56]. For TBG devices, the two gate-charged graphene layers, separated by average distance ζ, comprise the interacting reservoir structure.…”

Section: Introductionmentioning

confidence: 99%

High-TC Superconductivity Originating from Interlayer Coulomb Coupling in Gate-Charged Twisted Bilayer Graphene Moiré Superlattices

Fiory

2019

J Supercond Nov Magn

Self Cite

Unconventional superconductivity in bilayer graphene has been reported for twist angles θ near the first magic angle and charged electrostatically with holes near half filling of the lower flat bands. A maximum superconducting transition temperature T C ≈ 1.7 K was reported for a device with θ = 1.05° at ambient pressure and a maximum T C ≈ 3.1 K for a device with θ = 1.27° under 1.33 GPa hydrostatic pressure. A high-T C model for the superconductivity is proposed herein, where pairing is mediated by Coulomb coupling between charges in the two graphene sheets. The expression derived for the optimal transition temperature, T C0 = k B −1 Λ(|n opt − n 0 |/2) 1/2 e 2 /ζ, is a function of mean bilayer separation distance ζ, measured gated charge areal densities n opt and n 0 corresponding to maximum T C and superconductivity onset, respectively, and the length constant Λ = 0.00747(2) Å. Based on existing experimental carrier densities and theoretical estimates for ζ, T C0 = 1.94(4) K is calculated for the θ = 1.05° ambient-pressure device and T C0 = 3.02(3) K for the θ = 1.27° pressurized device. Experimental mean-field transition temperatures T C mf = 1.83(5) K and T C mf = 2.86(5) K are determined by fitting superconducting fluctuation theory to resistance transition data for the ambient-pressure and pressurized devices, respectively; the theoretical results for T C0 are in remarkable agreement with these experimental values. Corresponding Berezinskii-Kosterlitz-Thouless temperatures T BKT of 0.96(3) K and 2.2(2) K are also determined and interpreted.