ELFT HÍRADÓ

[Szeminarium koordinator <szfi-seminar AT wigner.mta.hu>]

DAN HÜVONEN (National Institute of Chemical Physics and Biophysics, Tallinn, 
vendéglátó: Penc Karlo):
"Bond Disorder in Quantum Magnets: Dynamics and Criticality"

Idõpont: 2014. december 09. (kedd) 10:00
Hely: MTA Wigner FK SZFI, I. épület 1. emeleti Tanácsterem
       
Részletes információ (tartalmi kivonat):
    <p>Spatial disorder of exchange interactions can be achieved by chemical 
substitution of superexchange mediating ligands in some prototypical quantum 
magnets with gapped spectrum. This enables to study the effect of disorder on 
quantum phase transitions, probe for theoretically predicted novel phases, 
study the stability of spin gap, lifetime of excitations and seek for 
appearance of in-gap localized states. I will show experimental data from 
bulk thermodynamic and neutron scattering experiments on two fascinating 
model compounds, PHCC and DTN. Bond disorder was introduced in by fractional 
chemical substitution of superexchange mediating Cl ions for Br [1]. In PHCC, 
we find by triple axis and time-of-flight spectroscopy that bond disorder 
increases the average spin gap and reduces the magnon bandwidth. The most 
notable effect of bond disorder are increased magnon lifetimes which can be 
correlated with the single-magnon density of states function [2]. At the same 
time low temperature ESR data suggests formation of gapless S=1 excitations 
on the fraction of bonds where both Cl ions are replaced [3]. However, 
unbiasly extracted critical exponents in the vicinity of field induced 
ordering transition show little or no modifications of scaling properties. 
Although disordered DTN has been proposed as the prime candidate for 
observing magnetic Bose Glass phase [4], our recent neutron diffraction and 
bulk thermodynamic data suggests critical behavior is affected considerably 
less than theory and previous experiments suggest [5]. We are left with a 
critical question whether the Bose Glass phase and changes in scaling 
properties are observable near quantum critical points in real disordered 
materials altogether?<br /> <br /> [1] T. Yankova et al., Crystals for 
neutron scattering studies of quantum magnetism. Philosophical Magazine, 92, 
2629 - 2647 (2012)<br /> [2] D. H&uuml;vonen et al., Excitations in a quantum 
spin liquid with random bonds. Physical Review B, 86, 214408 (2012)<br /> [3] 
V.N. Glazkov et al., Formation of gapless triplets in the bond-doped spin-gap 
antiferromagnet (C4H12N2)(Cu2Cl6). Journal of Physics: Condensed Matter, 
486002 (2014)<br /> [4] R. Yu et al., Bose glass and Mott glass of 
quasiparticles in a doped quantum magnet, Nature 489, 379 (2012).<br /> [5] 
E. Wulf et al., Criticality in a disordered quantum antiferromagnet studied 
by neutron diffraction. Physical Review B, 88, 174418 (2013); E. Wulf et al., 
to be submitted (2014)</p>


Minden érdeklõdõt szívesen látunk!