Isotope effect inLixZrNClsuperconductors

Abstract

The N-isotope effect has been investigated for a series of layered superconductors Li x ZrNCl with small carrier density. The N-vibration phonon mode around 615 cm −1 interacting most strongly with electronic system has shown softening by about 20 cm −1 ͑=3.3% ͒ upon 15 N substitution. On the other hand, the decrease in the superconducting critical temperature ͑T c ͒ upon 15 N substitution has been found to be only 0.06± 0.03 K ͑=0.5± 0.3% ͒. This small change in T c corresponds to an isotope shift coefficient ␣ = 0.07± 0.04, which is much smaller than the conventional value of 0.5, implying the relevance of other fluctuations than the phonon to the pairing interaction.

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)

“…As mentioned in the Introduction, relatively high T c is obtained despite the extremely low density of states and weak electron-phonon coupling [6][7][8][9][10], which can be explained within the present theory. The isotope effect is small [11,12], which again seems consistent since the the present pairing mechanism is purely electronic. The cointercalation of organic molecules enhances T c [14], and this also seems to be understandable within this kind of spin fluctuation mediated pairing, where the quasi two dimensionality is favored [22,23].…”

“…In fact, β-MNCl has the highest T c among materials with the specific heat γ as small as ∼1 mJ (mol K) 2 . The electron-phonon coupling is also estimated to be weak [6,[8][9][10], and the isotope effect is found to be small [11,12]. In the superconducting state, the density of states recovers rapidly upon increasing the magnetic field [7], suggesting some kind of anisotropic pairing.…”

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

“…The superconducting transition temperature T c ¼ 12 K decreases by 0.06 AE 0.03 K in Li-doped ZrNCl, when 14 N is substituted by 15 N [164]. This small change in T c corresponds to an isotope shift coefficient a ¼ 0.07 AE 0.04, which is much smaller than the value of 0.5 obtained within the conventional BCS theory.…”

Unconventional superconductivity in electron-doped layered metal nitride halides MNX (M= Ti, Zr, Hf; X= Cl, Br, I)

“…and Li x (THF) y HfNCl, which is much smaller than α = 0.5. N-vibration phonon mode around 615 cm −1 interacting most strongly with electronic system displays an softening by about 20 cm −1 (∆ω/ω = −3.3 %) upon 15 N substitution, which is much stronger than the decrease in T c (∆T c /T c ∼ −0.5 %) [42]. These results may imply the relevance of other interactions than phonons to the pairing interaction.…”

“…In the standard BCS theory, T c ∝ Z −α (Z represents the atomic mass) and α = 0.5 are obtained. The Nisotope effect, replacing 14 N to 15 N, has been investigated for Li x ZrNCl and Li x (THF) y HfNCl [41,42]. and Li x (THF) y HfNCl, which is much smaller than α = 0.5.…”