Page 48 - ILLS Annual Report 2018

MAGNETISM
Quentin Faure. French
University Grenoble Alpes, CEA – INAC, Institut Néel, and ILL
‘I am finishing my PhD on quantum magnetism, more specifically on antiferromagnetic anisotropic spin chains. This work encompasses neutron scattering, numerical calculations and
thermodynamic measurements under pressure. Theoretical input was provided by T. Giamarchi and S. Takayoshi.’
Topological quantum phase transition in the Ising-like antiferromagnetic spin chain BaCo2V2O8
Single-crystal diffractometer D23
Three-axis spectrometers ThALES and IN12
The universal concept of phase transitions, usually characterised by a symmetry breaking, has played a crucial role in
the description of many phenomena in condensed matter physics. However, since the seminal ideas of Berezinskii, Kosterlitz and Thouless (Nobel Prize in Physics, 2016), topological excitations are at the heart of
our understanding of a novel class of phase transitions. Here, we provide evidence of
a quantum phase transition separating two unconventional magnetic phases characterised by different kinds of topological excitations.
Figure 1
a) Zero-field magnetic structure (blue arrows) of a single Co2+ screw chain of BaCo2V2O8 (blue and red spheres are Co and O respectively).
b) Applying a magnetic field along b creates an additional staggered field along a (red arrows), which produces a new rotated magnetic arrangement shown by the blue arrows (the O atoms are no longer represented in this figure, for sake of clarity).
AUTHORS
Q. Faure (UGA, CEA-INAC, Institut Néel and ILL) S. Petit (LLB, Saclay, France)
V. Simonet (Institut Néel, Grenoble, France)
S. Raymond (CEA-INAC, Grenoble, France)
M. Boehm (ILL)
B. Grenier (UGA, CEA-INAC, Grenoble, France)
ARTICLE FROM
Nat. Phys. (2018)—doi: 10.1038/s41567-018-0126-8
REFERENCES
[1] S. Kimura et al., J. Phys. Soc. Jpn. 82 (2013) 033706 [2] Q. Faure et al., Nat. Phys. 14 (2018) 867
[3] B. Grenier et al., Phys. Rev. Lett. 114 (2015) 017201
Topology has brought new insight to our understanding
of condensed matter physics. For instance, topological phase transitions exhibit no symmetry breaking and
are characterised by topological defects / excitations (discontinuities that cannot be removed by continuous deformation unless annihilated by their anti-defects). In most cases, those transitions are controlled by a single type of topological object, for instance solitons (kinks and 360 ° rotation of a pendulum, for instance) or vortices. However, there are richer and less studied classes of topological phase transitions in which two different sets of topological excitations compete with each other.
This is what we found in the magnetic oxide BaCo2V2O8. In this material, which consists of chains of Co2+ carrying effective anisotropic spin-1/2, the spins are strongly coupled antiparallel to each other in the ‘Ising’ c-direction (see figure 1a). Because of crystallographic peculiarities, applying a transverse uniform magnetic field H along
b produces an additional transverse and staggered magnetic field along a [1]. The latter competes with the Ising anisotropy and forces the spins to bend along a (see Figure 1b), as evidenced by neutron diffraction on D23, driving a phase transition beyond Hc = 10 T [2].
ANNUAL REPORT 2018
a) H = 0 b) H > 10T
H∥b
c ab