Cu nuclear quadrupole resonance study of site disorder and chemical pressure effects onY(Ba1−xSrx

Abstract

We report a zero-field Cu nuclear quadrupole resonance (NQR) study on the effects of nonmagnetic Sr substitution for high-Tc superconductors, Y(Ba1−xSrx)2Cu4O8 (Tc=82-80 K for x=0-0.4), using a spin-echo technique. The site-disordering and chemical pressure effects associated with doping Sr were observed in the broadened, shifted Cu NQR spectra. Nevertheless, the site disorder did not significantly affect the homogeneity of Cu electron spin dynamics, in contrast to the in-plane impurity. The peak shift of Cu NQR spectrum due to Sr was different between the chain-and the plane-Cu sites, more remarkably than those under a hydrostatic physical pressure, suggesting anisotropic or nonuniform local structural strains. The small decrease of Tc due to Sr can be traced back to either a cancellation effect on Tc between the disorder and the pressure, or an anisotropic or nonuniform chemical pressure effect on Tc. The role of imperfections in quantum many-body systems has attracted strong attention [1,2]. In high-T c cuprate superconductors, an additional impurity potential is introduced through the in-plane or out-of-plane element substitution for the CuO 2 plane. There is a belief that site disorder does not cause serious damage to the electronic states of the CuO 2 plane. However, the reason why chemical pressure effect on T c due to the element substitution is not the same as the physical (external) pressure one has been frequently ascribed to the randomness effect. How the site disorder changes the electronic states has been a problem.Here, we focus on the divalent Sr 2+ -doped high-T c superconductors, Y(Ba 1−x Sr x ) 2 Cu 4 O 8 . Since the stoichiometric and naturally underdoped YBa 2 Cu 4 O 8 (Y124) with T c =82 K has the exceptionally tight oxygen content, the Sr-doped Y124 is a suitable system to study the site-disordering effect on the electronic states. To be exact, one can expect two effects of Sr substitution for Ba in Y124 without change of oxygen content; (1) the chemical pressure and (2) the crystalline potential disorder. The substitution of Sr 2+ ions causes no additional local (formal) charge in Ba 2+ O 2− layers. However, the size of Sr 2+ ion is smaller than that of Ba 2+ by about 10 % [3], so that the substituted Sr ions make local crystalline strains on the BaO layers. Thus, the Sr doping introduces crystalline potential disorder and chemical pressure to the lattice. Actually, the lattice constants shrink with doping Sr [4], being in parallel to the physical pressure effect [5].The effect of hydrostatic physical pressure on the spin dynamics is similar to the carrier doping effect [6][7][8]:The physical pressure effects on T c and on the pseudo spin-gap temperature T s agree with the carrier doping effects, i.e. with applying the physical pressure to Y124 (T c =82 K and T s ∼160 K at an ambient pressure P =0

Zn-neighbor Cu NQR in Zn-substitutedYBa2Cu3O7

“…The Cu NQR frequencies of stoichiometic cuprate oxides have been analyzed with several models [38]. For nonstoichiometic or impure compounds, although there is an attempt with numerical simulation [32], it is more complicated, because local crystal strain [39] or electric charge oscillation [40] can affect the line profile of Cu NQR spectrum. Thus, we do not employ a specific model to analyze how the satellite Cu NQR frequency is shifted to the lower side than the main one.…”

Emergence of Antiferromagnetic Correlation in LiTi2-xVxO4 via 7Li NMR

“…Even for the random crystalline potentials introduced through the substitution, NMR linewidths may be broader but the NMR relaxation can have a single spin-lattice relaxation time. 19) In this Letter, we report the 7 Li NMR studies of LiTi 2Àx V x O 4 (x ¼ 0 { 0:4). The V substitution was found to reduce the relative volume fraction of superconductivity faster than T c and to enhance fairly homogeneously and largely the 7 Li nuclear spin-lattice relaxation but not so much the Knight shift.…”

Driving force for the proper ferroelastic phase transitions induced by cation exchange

“…Based on papers [2][3][4][5][6][7][8][9], which dealt with the internal pressure in crystals, it follows that the internal pressure is a real value which is capable of going into action differently from the ( * ) E-mail: ovsyuk@uiggm.nsc.ru c EDP Sciences external pressure. For example, in ref.…”