Joachim Hemberger, University of Cologne

Jeudi 16 Octobre 2014, 14h
Amphi Holweck, Esc C, 1ème etage

Slowing down or speeding up ? Dynamics of critical fluctuations near complex order

Joachim Hemberger
University of Cologne, Institute of Physics

In systems with competing interactions frustration can create delicate ground states with exotic excitations. Such phases often comprise hybrid order parameters, as e.g. in multiferroic systems, where a non-collinear spin structure is coupled to ferroelectric polarization. In addition to the static manifestation of such complex structures one can raise the question concerning the dynamics of the corresponding fluctuations.
In this talk results on the dynamics of two exemplary magnetoelectric systems shall be reported, which were studied by means of broadband dielectric spectroscopy in the radiofrequency and microwave range.
A) In the complex antiferromagnet MnWO4 a multiferroic, i.e. ferrolectric and cyloidal aniferromagnetic phase sets in at TN2 = 12.6K. We observe a critical slowing down of the corresponding magnetoelectric fluctuations resembling the soft-mode behavior in canonical ferroelectrics. This electric field driven excitation carries much less spectral weight than ordinary phonon modes. Also the critical slowing down of this mode scales with an exponent larger than one which is expected for magnetic second order phase transition scenarios. Therefore the investigated dynamics have to be interpreted as the softening of an electrically active magnetic excitation, an electromagnon.
B) The system Dy2Ti2O7 possesses a degenerate spin-ice ground state ground state in which magnetic degrees of freedom carry zero point entropy in analogy water ice. Recently, it has been found that excitations in spin-ice behave like magnetic monopoles. In a magnetic field along [111] direction the density of these monopoles is a function of temperature and magnetic field and the phase diagram exhibits a phase transition which resembles the gas-liquid, including the corresponding critical end-point. It also has been postulated that the emergent magnetic monopoles in addition carry electric dipole moments.
We demonstrate the existence of such a coupled electric dipole moment and were able to examine the monopole dynamics via this magnetoelectric coupling, tracking the relaxation time of the electrical dipole contribution down to low temperatures in the mK-range as a function of an external magnetic field. Analyzing the dynamics at temperatures above the critical end-point we see the crossover from the conventional slowing down of the fluctuation dynamics to a critical speeding up.

Literature :
"Critical slowing down near the multiferroic phase transition in MnWO4"
D. Niermann, C.P. Grams, P. Becker, L. Bohat_y, H. Schenck, and J. Hemberger
"Critical speeding-up in the magnetoelectric response of spin-ice near its monopole liquid–gas transition"
Christoph P. Grams, Martin Valldor, Markus Garst, and Joachim Hemberger
Nat. Commun. 5:4853, doi : 10.1038/ncomms5853 (2014)

E-mail : hemberger (arobase)

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