Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Freitag
We propose that diffuse neutron scattering can be used to resolve the
inherent ambiguity in determining the ratio and sign of J1 and J2 from
thermodynamic properties alone, and use a finite temperature Lanczos
algorithm to make predictions for the relevant high temperature spin-spin
correlation functions.
TT 33 Amorphe- und Tunnelsysteme, Gläser
We investigate the crossover between the various magnetic and spinliquid
phases for both ferromagnetic and antiferromagnetic couplings J1
and J2. We also analyse the nonlinear magnetic susceptibility to resolve
the ambiguity of the frustration ratio J2/J1.
Zeit: Freitag 10:15–11:30 Raum: H19
TT 33.1 Fr 10:15 H19
Isotopeneffekt der Magnetfeldabhängigkeit von dielektrischen
Polarisationsechos in amorphem Glycerin — •Maximilian
Brandt 1 , Peter Nagel 2 , Andreas Fleischmann 1 , Christian
Enss 3 und Siegfried Hunklinger 1 — 1 Kirchhoff Institut für Physik,
Universität Heidelberg,D- 69120 Heidelberg — 2 Forschungszentrum
Karlsruhe, D- 76021 Karlsruhe — 3 Brown University, Providence, RI
02912, USA
Die physikalischen Eigenschaften von amorphen Festkörpern werden
unterhalb einiger Kelvin durch atomare Tunnelsysteme bestimmt. In
diesem Temperaturbereich zeigen dielektrische Multikomponentengläser
mit Ausnahme von Quarzglas magnetfeldabhängige Eigenschaften. Zur
Erklärung dieser unerwarteten Magnetfeldeffekte in nominell unmagnetischen
amorphen Systemen wurden in den letzten Jahren verschiedene
Modelle entwickelt. Eines dieser Modelle setzt voraus, dass die Atomkerne
der tunnelnden Teilchen ein elektrisches Kernquadrupolmoment besitzen.
Mit dielektrischen Polarisationsechoexperimenten haben wir an amorphem
Glycerin den Zerfall der Echoamplitude in Abhängigkeit des Magnetfeldes
untersucht. Durch partielles Deuterieren wurden gezielt Atome
mit Kernquadrupolmoment und bekannter Quadrupolaufspaltung in
das System eingebracht. Der beobachtete Isotopeneffekt stützt die Erklärung
der Magnetfeldeffekte basierend auf einem Kernquadrupolmement.
Auch das in Glycerin beobachtete Quantum-Beating im Zerfall von
Zweipulsechos kann im Rahmen dieses Modells semiquantitativ erkärt
werden.
TT 33.2 Fr 10:30 H19
Dielektrische Eigenschaften von Glyzerin bei tiefen Temperaturen
im Magnetfeld — •Marek Bartkowiak 1 , Andreas Fleischmann
1 , Christian Enss 2 und Siegfried Hunklinger 1 — 1 Kirchhoff
Institut für Physik, Universität Heidelberg — 2 Brown University, Providence,
USA
Bei tiefen Temperaturen werden die thermischen, akustischen und dielektrischen
Eigenschaften von amorphen Festkörpern durch atomare
Tunnelsysteme bestimmt. Völlig unerwartet wurde vor wenigen Jahren
gefunden, dass die dielektrischen Eigenschaften von bestimmten Multikomponentengläsern
bei sehr tiefen Temperaturen vom Magnetfeld
abhängen. Eine derartige Abhängigkeit wurde nicht nur in der dielektrischen
Suszeptibilität sondern auch bei Polarizationsechos beobachtet.
Letztere Messungen deuten darauf hin, dass tunnelnde Atome mit Kernquadrupolmoment
für die Magnetfeldabhängigkeit verantwortlich sind.
Um den Einfluss des Kernquadrupolmoments zu untersuchen haben wir
Messungen der dielektrischen Suszeptibilität bei verschiedenen Frequenzen
an natürlichem und vollständig deuteriertem Glyzerin in Magnetfeldern
durchgeführt. Wir stellen die Messungen vor und diskutieren die
Isotopenabhängigkeit.
TT 33.3 Fr 10:45 H19
Low-temperature investigation on thermal conductivity of
glasses — •H.-Y. Hao 1 , M. Neumann 1 , A. Rost 1 , A. Fleischamnn
1 , C. Enss 2 , and S. Hunklinger 1 — 1 Kirchhoff-Institut fuer
Physik, Universitaet Heidelberg — 2 Department of Physics, Brown
University
The thermal conductivity of glasses at temperatures below 1 K is generally
described through the resonant scattering between the heat-carrying
phonons and the tunnuling systems in the glasses. At further low temperature,
it is evident that the interactions between the tunneling systems
themselves could be responsible for describing the anomalies observed in
certain properties of glasses. It is therefore conceivable that such interactions
could also contribute to the heat transport and become visible
when the phonons are frozen out at very low temperatures or suppressed
by some additional scattering mechanism introduced to the system. For
measuring thermal conductivity of such diminutive magnitude, our recently
developed contact-free technique is proved to be ideal owing to its
surpassingly small parasitic heating to the system. Measurements on bulk
BK7 glass and synthetic quartz glass have been successfully preformed
down to 5 mK. For reducing the phonon mean-free-path, a glass capillary
array is used to introduce extra scattering of the thermal phonons. For
such a sample, an advanced design of the setup is developed to improve
the precision of the measurements. Preliminary findings will be presented
and discussed.
TT 33.4 Fr 11:00 H19
Systematic Location and Description of Two Level Tunneling
States in Simulated Lennard-Jones and Silicate Glasses. —
•Jens Reinisch and Andreas Heuer — Institut für Physikalische
Chemie and International Graduate School of Chemistry, Westfälische
Wilhelms-Universität, Corrensstraße 30, 48149 Münster
Though the existence of Two Level Tunneling States (TLS) is widely
accepted to explain low temperature anomalies in the sound absorption,
heat capacity, thermal conductivity and other quantities, an exact description
of their microscopic nature is still lacking. We performed computer
simulations on a binary Lennard-Jones and a silicate system, using
a newly developed algorithm to find TLS on a systematic basis. A detailed
microscopic description of the Tunneling States in the simulated
systems is thus available. Beyond structure we are also interested in general
movement schemes for the participating particles. We found local
density anomalies at the TLS, which are connected to the movement of
the participating particles and we suggest that these anomalies are important
for the TLS formation. For the LJ-system we will show strong
evidence that only one particle is dominant in TLS, although several
particles are moving. Furthermore, semi-quantitative estimations for the
density of states of TLS are presented.
TT 33.5 Fr 11:15 H19
Four level tunneling systems and electronic structure in
clathrates — •Ivica Zerec 1 , Alexander Yaresko 2 , and Peter
Thalmeier 1 — 1 Max-Planck-Institut fuer Chemische Physik fester
Stoffe, Noethnitzer Str. 40, 01187 Dresden — 2 Max-Planck-Institut fuer
Physik komplexer Systeme, Noethnitzer Str. 38, 01187 Dresden
The clathrates are cage compounds, interesting because of their structural
and thermoelectrical properties. In the oversized cages formed by
Si, Ge, Ga, etc. atoms, the ”guest” atoms rattle and strongly scatter
phonons, reducing the phonon dominated thermal conductivity.
The elastic response of the Eu clathrate provides clear evidence for the
existence of a new type of four-well tunneling states, described by two
nearly degenerate four level systems (FLS). The FLS’s are closely linked
with the fourfold split positions of Eu known from neutron diffraction
density profiles. Using a realistic potential we estimate the tunneling frequencies
and show that the energy gap between the two FLS’s is of the
same order as the Einstein oscillator frequency. This explains why the observed
harmonic oscillator type specific heat is not modified by tunneling
states. The quadrupolar interaction of FLS’s with elastic strains explains
the pronounced dip observed in the elastic constant measurements.
We present also the electronic band structure of Ba6Ge25 clathratelike
compound and investigate the modifications due to the structural
phase transitions, reflected in the shifting of some Ba and Ge atoms to
the split sites. The corresponding modifications in optical conductivity
is calculated. It agrees well with experiment.