Berlin 2005 – wissenschaftliches Programm

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TT: Tiefe Temperaturen

TT 32: Correlated Electrons - Low-dimensional Materials II

TT 32.1: Vortrag

Dienstag, 8. März 2005, 16:15–16:30, TU H2053

Magnetic ground state of the quantum spin magnet CaCu₂O₃ probed by high field ESR — •V. Kataev¹, M. Goiran², M. Costes², J. M. Broto², F. C. Chou³, E. Arushanov^1,2, S. Drechsler¹, and B. Büchner¹ — ¹Leibniz Institute for Solid State and Materials Research IFW Dresden, D-01171 Dresden, Germany — ²Laboratoire National des Champs Magnétiques Pulsés, 31432 Toulouse Cedex 04, France — ³Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts, 02139, USA

We report an electron spin resonance (ESR) study of the S=1/2-Heisenberg pseudo-ladder magnet CaCu₂O₃ in pulsed magnetic fields up to 40 T. At sub-Terahertz frequencies we observe an ESR signal originating from a small amount of uncompensated spins residing presumably at the imperfections of the strongly antiferromagnetically correlated host spin lattice. The data give evidence that these few percent of extra spin states are coupled strongly to the bulk spins and are involved in the antiferromagnetic ordering at T_N=25 K. By mapping the frequency/resonance field diagram we have determined the spin gap for magnetic excitations below T_N amounting to ∼0.3 meV. The small value of the gap explains the occurrence of the spin-flop transition in CaCu₂O₃ at the critical magnetic field H_sp∼3 T. Qualitative changes of the ESR response with increasing the field strength give indications that strong magnetic fields reduce antiferromagnetic fluctuations and may even suppress the long-range magnetic order in CaCu₂O₃. ESR data support theoretical predictions of a significant role of the extra spin states for the properties of the low-dimensional quantum magnets.