Berlin 2008 – wissenschaftliches Programm

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TT: Fachverband Tiefe Temperaturen

TT 4: Matter at Low Temperature: Materials

TT 4.1: Hauptvortrag

Montag, 25. Februar 2008, 09:30–10:00, H 3010

Can nuclear spins reveal the nature of tunneling systems in glasses? — •Andreas Fleischmann — Kirchhoff-Institut für Physik, Universität Heidelberg, INF 227, 69120 Heidelberg

Most properties of amorphous materials at temperatures below 1K are caused by tunneling systems, which are assumed to be groups of atoms or molecules, cooperatively moving between two configurations of comparable energy. However, only little is known about the microscopic nature of these tunneling systems.

During the last years unexpected magnetic field effects were found in the dielectric properties of non-magnetic glasses at very low temperatures. These effects can be attributed to the tunnelling of atoms that carry nuclear quadrupole moments. As the electric quadrupole moments are interacting with the local electric field gradients, the nuclear states are coupled to the tunneling motion, resulting in energy levels and transition matrix elements that strongly depend on the atomic composition and the motion of the tunneling systems.

We present dielectric two-pulse polarization echo measurements on glasses, where the tunneling systems are coherently driven with two short microwave pulses. The observed amplitude of the spontaneous echo is strongly affected by the hyperfine structure that is introduced by the nuclear moments. The quantum beating that modulates the echo decay as well as the strong dependence of the echo amplitude on magnetic fields are found to be unique fingerprints for each glass and can be used to reveal information on the microscopic nature of the tunneling systems in these materials.