Evidence for low-temperature internal dynamics in Cu12As4S13 according to copper NQR and nuclear relaxation

Contribution of Copper Nqr Spectroscopy to the Geological Studies of Complex Sulfides and Oxides

“…In Figure 2 we have plotted ν Q (T) dependence for triangularcoordinated Cu sites in covellite Cu 1.00 S. 30 In general, the quadrupole frequency ν Q decreases with increasing temperature without any significant anomalies. This unusual fact is accompanied by the drastic broadening of the NQR spectrum below 65 K. 44,45 Consistently with the crystal structure, the experimental data can be explained by an occurrence of a magnetic phase transition, which takes place near 65 K. 44,45 The analysis shows that the low-temperature phase is characterized by Cu 2+ electron magnetic moments freezing in the form of a spin-glass-like constitution. However, we focus here on two weak effects: the change of the slope in the ν Q versus T dependence at 65 K and at 210 K. The analysis of such ν Q (T) dependence permits us to conclude that change of slope in the ν Q versus T dependence at 65 K is related to the strong deformations of CuS 4 tetrahedrons in covellite Cu 1.00 S, whereas the change of slope at 210 K is attributed to Cu valence state fluctuations near average value 1.3+ owing to an electron transfer between two Cu non-equivalent ions.…”

“…43 Although quite informative, studies of ν Q (T) and ν Q (p) in sulfides are relatively rare. 44,45 It was found that the dependence is almost linear in the range 60-210 K, but it begins to slow down at lower temperatures ( Figure 3). We thus exclude a major structural transition.…”

“…21,50 It was shown that the conduction electrons in metallic crystals are the main source of this "simple" temperature dependence of nuclear relaxation rate. 44,45 The temperature dependence of spin-lattice nuclear relaxation rate 1/T 1 is shown in Figure 5. 40 However, below T = 55 K, the relaxation rate in covellite Cu 1.00 S demonstrates a surprising crossover from constant T 1 ·T relation to unusual T-dependent T 1 ·T behavior (Figure 4), in spite of the fact that Cu 1.00 S appears to be a metal down to superconducting transition T c ≈ 1.6 K. This anomalous dependence was treated as a "pseudogap" opening.…”

“…The T-dependence of the relaxation rate T 1 −1 in this case is determined by fluctuations of the hyperfine magnetic field or the EFG with the frequencies ~10 15 s −1 created by the conducting spin-carrying charges at the nucleus site. 44,45 40 The surprising result of the covellite Cu 1.00 S NQR relaxation study is that the ratio of the relaxation times of two non-equivalent coppers is constant.…”

Phase transition and anomalous electronic behavior in the layered superconductor CuS probed by NQR

“…This is consistent with the interpretation of some crystal-chemical features of CuS, 21 according to which the valence of Cu(1) and Cu(2) should satisfy the value of Cu 1.3+ . It is interesting to note that some recent studies reveal the "d count" in the Cu sulfides to be intermediate between 3d 9 and 3d 10 , although these sulfides were initially classified as nominally monovalent or divalent Cu compounds (for example, chalcopyrite CuFeS 2 74 and tennantite Cu 12 As 4 S 13 76,77 ). Probably, this tendency is the consequence of some "antipathy of Cu for a 3d 9 configuration and the stability of Cu 3d 10 in Cu sulfides".…”