Unconventional Superconductivity in Electron-Doped LayeredLi0.48(THF)yHfNCl

Abstract

We report magnetic susceptibility measurements on a layered superconductor Li0.48(THF)0.3HfNCl having Tc approximately 26 K. The present study revealed that (a) the Fermi level density of states is small, N*(EF) approximately 0.25 states/(eV spin f.u.), (b) mass enhancement is negligible, gamma; approximately 1, (c) electron-phonon coupling is weak, lambda(ep)<<1, (d) exchange enhancement is negligible, 1/(1+F(a)0) approximately 1, and (e) electronic density parameter is large, r(2D)s approximately 10.3 (i.e., low-carrier density). It is difficult to explain the origin of the high Tc in terms of the conventional phonon (BCS) mechanism of superconductivity.

Electronic collective modes and superconductivity in layered conductors

“…The value of γ was estimated in Ref. 18 to be γ ≃ 1.1 mJ/molK 2 , whereas band structure calculations 35 give N (0) ≃ 0.74 eV −1 . Thus, λ ≃ 0.25.…”

“…26) the intercalation of alkali atoms and organic molecules into the parent compound (Zr,Hf)NCl leads to a superconductor with rather high critical temperature (T c ∼ 25K). Based on experimental 18,26,27,28,29,30,31,32,33 studies and band structure calculations 35 it was concluded that electron-phonon mediated pairing is insufficient to explain the observed T c , since the electron-phonon coupling constant appears to be too small. Note also that a small nitrogen isotope effect of T c has been observed.…”

“…We note from Eqs. (9,(17)(18)(19)(20), that the evaluation of T c for a specific compound needs the knowledge of following parameters: the interlayer distance L, the band mass m b and Fermi velocity v F , and the dielectric constant of the spacers ǫ M . In addition, the evaluation of the phonon contribution to the pairing requires the knowledge of the characteristic phonon frequency Ω, the electron-phonon coupling constant λ and the Coulomb pseudopotential µ ⋆ .…”

Superconducting nitride halides MNX (M = Ti, Zr, Hf; X = Cl, Br, I)

“…N(E F ) % 0.25 states/(eV spin f.u.) for Li 0.48 (THF) y HfNCl [160], which is an unconventional feature of superconductivity within the BCS theory. The comparison of the electronic structures of A x ZrNCl and A x HfNCl suggests higher densities of states for the Hf-bearing compounds [82,161].…”

Unconventional pairing in doped band insulators on a honeycomb lattice: the role of the disconnected Fermi surface and a possible application to superconducting β-MNCl (M=Hf, Zr)

“…The bilayer honeycomb lattice consisting of M and N (figure 1) is considered the main cause of superconductivity, and the two-dimensional nature of the superconductivity has been revealed by nuclear magnetic resonance [3] and muon spin relaxation studies [4,5]. Despite the relatively high T c , both experimental and theoretical studies indicate extremely low density of states (DOS) at the Fermi level [6][7][8]. In fact, β-MNCl has the highest T c among materials with the specific heat γ as small as ∼1 mJ (mol K) 2 .…”