Regensburg 2007 – wissenschaftliches Programm

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TT: Fachverband Tiefe Temperaturen

TT 13: Symposium “Superconductivity and Magnetism in Lamellar Cobaltates”

TT 13.4: Vortrag

Dienstag, 27. März 2007, 15:40–16:05, H18

Magnetic phase separation in highly Na doped Na_xCoO₂ with x > 0.75. — •Christian Bernhard¹, Li Yu¹, Alexander Boris², Dapeng Chen², Chengtian Lin², Bernhard Keimer², and Christof Niedermayer³ — ¹Universität Fribourg, Chemin du Musee 3, CH-1700 Fribourg, Switzerland — ²Max-Planck-Insitut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany — ³Paul-Scherrer-Institut, CH-5232 Villigen-PSI, Switzerland

We report muon spin rotation and infrared ellipsometry measurements on Na_xCoO₂ single crystals with 0.75<x<1 that provide evidence for an intrinsic phase separation scenario. At the highest Na content of x=0.97 we observe isolated magnetic clusters which have a small total spin of about S=1 and undergo a spin freezing transition around T_sf ∼ 20 K. With decreasing Na content the cluster begin to overlap developing extended antiferromagnetic patches with T_N=22 K. A bulk long-range ordered state occurs near x=0.75. The optical data establish a highly anomalous charge transport with signatures of a strong spin charge coupling which is explained in terms of a hole doping induced spin state transition of the Co³⁺ ions.