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Quantum phase transitions in the quasi-1D Ising-like antiferromagnet BaCo2V2O8

Abstract : This manuscript is devoted to the study of quantum phase transitions in the BaCo2V2O8 oxide, a quasi-one dimensional antiferromagnet consisting of spin chains of cobalt magnetic ions carrying an effective spin S = 1/2 showing a strong Ising-like anisotropy. To achieve this, we have studied BaCo2V2O8 under the effect of different physical parameters.Our first study concerned the effect of a transverse magnetic field, i.e. applied perpendicularly to the Ising axis. It has been shown that when BaCo2V2O8 is subjected to such a field, an effective staggered magnetic field is induced perpendicularly to both the Ising-axis and the uniform applied field. Using neutron scattering experiments (both elastic and inelastic) compared to numerical calculations, we have proved that this staggered field competes with the Ising-like anisotropy. This leads to a very original quantum phase transition. Our system can actually be mapped onto a quantum field theory called “double sine-Gordon model”, describing the competition between two dual topological excitations. We have thus shown that BaCo2V2O8 under a transverse magnetic field is the first experimental realization of such a theory.The second study was devoted to the effect of a longitudinal magnetic field, i.e. a field applied parallel to the Ising-axis. The spin-dynamics have been investigated by means of inelastic neutron scattering experiments and it has been shown that above a critical field of 4 T, it corresponds to the one expected for a Tomonaga Luttinger liquid phase (TLL). Moreover, numerical calculations have shown that, because of the Ising-like anisotropy in this system, the majority of the spectral weight in the energy spectrum is carried by longitudinal spin fluctuations. This result is the signature of a quantum behavior without classical analogous with low energy longitudinal fluctuations driving the physics of the system. Finally, this is the first time that the dispersion spectrum for an Ising-like spin 1/2 chain could be probed in this TLL phase.The last two studies are preliminary work. The phase diagram of BaCo2V2O8 has been probed by calorimetric measurements under pressure and under a longitudinal magnetic field. Pressures up to 10 GPa have been obtained using a diamond anvil cell. We have then performed specific heat measurements allowing us to investigate the effect of pressure on the Hamiltonian of BaCo2V2O8 through its (H, P, T) phase diagram. Finally, we have also started to study the effect of the substitution of magnetic ions Co2+ by non-magnetic impurities Mg2+. The neutron diffraction experiments under a longitudinal magnetic field have shown that the critical temperature and critical field decrease proportionally to the concentration of impurities. The spin-dynamics at zero-field has also been investigated and reveals the appearance of non-dispersive magnetic modes, which possibly comes from the finite size effect of the spin chains segmented by the non-magnetic impurities.In conclusion, our experimental studies associated to numerical calculations allowed us to unveil a very rich physics in this model compound for the study of quantum magnetism and quantum phase transitions.
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https://tel.archives-ouvertes.fr/tel-02063688
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Submitted on : Monday, March 11, 2019 - 1:43:09 PM
Last modification on : Saturday, October 17, 2020 - 3:00:19 AM
Long-term archiving on: : Wednesday, June 12, 2019 - 2:47:47 PM

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  • HAL Id : tel-02063688, version 1

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Quentin Faure. Quantum phase transitions in the quasi-1D Ising-like antiferromagnet BaCo2V2O8. Strongly Correlated Electrons [cond-mat.str-el]. Université Grenoble Alpes, 2018. English. ⟨NNT : 2018GREAY063⟩. ⟨tel-02063688⟩

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