Spin-glass behavior, spin fluctuations, and superconductivity inSr2Y(Ru1−u

Abstract

Muon spin rotation measurements of Sr 2 Y(Ru 1Ϫu Cu u )O 6 ͑for uϭ0.1) reveal two distinct muon sites: one located in a SrO layer ͑which is superconducting at low temperatures͒ and the other in a Y(Ru 1Ϫu Cu u )O 4 layer ͑which is magnetically ordered at low temperatures͒. A precursor spin-glass state due to the Ru moments is detected in high fields ͑Ϸ3.3 kOe͒ in Y(Ru 1Ϫu Cu u )O 4 layers, with a spin-glass temperature of T G ϭ29.25 K. The Y(Ru 1Ϫu Cu u )O 4 layers order ferromagnetically in the a-b planes at the Néel temperature, T N Ϸ23 K. This in-plane ferromagnetism alternates direction between adjacent Y(Ru 1Ϫu Cu u )O 4 planes, resulting in a net antiferromagnetic structure. Although the onset of superconductivity is observed both by electron spin resonance and by dc susceptibility to occur for temperatures up to about T c,onset Ϸ49 K, this superconductivity is adversely affected by the Ru moments that fluctuate for TϾT N producing magnetic fields that break pairs in the SrO layers. The muons, as well as other probes, sense the more-robust static superconductivity for TϽT G . In fact, resistance measurements only show zero resistance below T N , at which temperatures the Ru moments that fluctuated for TϾT N are frozen in-plane. Hence strictly speaking, the superconducting transition temperature is the same as T N , which is far below T c,onset . Below T N there are no pair breaking fluctuating magnetic fields in the SrO layers where the hole condensate resides.

Anomalous magnetic properties ofSr2YRuO6

“…2 Even though there are no detailed experimental observations in Sr 2 YRuO 6 , many experimental results including neutron diffraction exist for Cusubstituted Sr 2 YRu 1−x Cu x O 6 compounds. [7][8][9][10] Assuming that the magnetic properties associated with the ordering of the Ru moments are not drastically altered, we can analyze the results of the SR measurements in the Cu-substituted compounds. 8,10 Both the precession frequency and relaxation rate show anomalies at ϳ30 K for muons trapped in the two possible sites: oxygen in the YRuO 4 layers ͑ O͑1,2͒ ͒ and oxygen in the SrO layers ͑ O͑3͒ ͒.…”

“…Among the Sr 2 LnRuO 6 compounds, Sr 2 YRuO 6 has captured additional interest due to the occurrence of superconductivity when Ru is partially ͑Յ15%͒ replaced by Cu. [5][6][7][8][9][10] Cu is found to get substituted at the Ru site in the YRuO 4 planes and thus the structure of the substituted compounds remains the same as that of the parent compound, without creating any additional Cu-O planes. 7 The parent compound Sr 2 YRuO 6 is known to be an antiferromagnetic insulator with the Ru moments ordering at T N =26 K. 2 The magnetic ordering temperature ͑T N ͒ was inferred as 26 K from the position of the peak in the magnetization measurements.…”

High-temperature superconductivity

“…Complementing the muon data are resistivity data, which show that the resistance vanishes at T c ≈23 K [25], at which temperature the superconductivity becomes complete. Clearly Sr 2 YRu 1−u Cu u O 6 is a superconductor, whose superconductivity originates at ∼49 K and becomes complete at ∼23 K when the Ru librations turn off.…”

“…The muon data for the SrO-plane site (µ O(3) ) show a time-dependence that reveals (i) an onset of superconductivity near ∼49 K [25], (ii) the onset of spin-glass behavior at ≈29.3 K [25], and (iii) the onset of diamagnetism at lower temperatures [25]. Complementing the muon data are resistivity data, which show that the resistance vanishes at T c ≈23 K [25], at which temperature the superconductivity becomes complete.…”

“…Muons in this material stop at one of two nearly identical sites: (i) the µ O(1,2) site which is actually two sites (due to the difference between Y and Ru) near the center of the YRuO 4 layer; and (ii) the µ O(3) site, which is about midway between two oxygen ions on the edge of a SrO plane. Clearly the YRuO 4 layer is highly magnetic (and hence rather hostile to superconductivity), while the SrO layer has an average magnetic field of zero and is the locus of superconductivity.The muon data for the SrO-plane site (µ O(3) ) show a time-dependence that reveals (i) an onset of superconductivity near ∼49 K [25], (ii) the onset of spin-glass behavior at ≈29.3 K [25], and (iii) the onset of diamagnetism at lower temperatures [25]. Complementing the muon data are resistivity data, which show that the resistance vanishes at T c ≈23 K [25], at which temperature the superconductivity becomes complete.…”