“Spin Dynamics and Unconventional Coulomb Phase in Nd2Zr2O7“

General ILL webinar 

Organised by College 5B

Thursday 8 July 2021 at 03:00 pm

ZOOM link

By Sylvain Petit

LLB, Université Paris-Saclay

1)  LLB, Université Paris-Saclay, CNRS, CEA, CE-Saclay, F-91191 Gif-sur-Yvette, France
2)  Institut Néel, CNRS and Université Grenoble Alpes, 38000 Grenoble, France
3)  Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
4)  Institut Laue Langevin, F-38042 Grenoble, France
5)  Université Grenoble Alpes, CEA, IRIG, MEM, MDN, 38000 Grenoble, France

Geometrical frustration is well known to be one of the key ingredients leading to unconventional states of matter, especially in magnetism. Among them, spin ice and more generally Coulomb phases have attracted significant interest. These can be considered as an original state of matter formed by disordered degenerate configurations where local degrees of freedom remain strongly constrained at the local scale by an organizing principle. In the case of spin ice, these degrees of freedom are Ising spins, sitting on the sites of a pyrochlore lattice formed of corner sharing tetrahedra and aligned along the <111> axes, which connect the corners of the tetrahedra to their center. The organizing principle, the “ice rule,” states that each tetrahedron should have two spins pointing in and two out, in close analogy with the hydrogen position in water ice. Importantly, the idea that this local constraint can be considered as the conservation law of an “emergent” magnetic flux (div B = 0) was quickly imposed. Quantum fluctuations can cause this flux to change with time, giving rise to an emergent electric field, and eventually to an emergent quantum electromagnetism [1]. Despite much work, however, experimental evidence for this enigmatic physics remains elusive. Indeed, the conditions for the realization of this so-called quantum spin ice state are drastic: transverse terms have to be sizable in the Hamiltonian to enable fluctuations out of the local Ising axes, but should remain small enough to prevent the stabilization of classical phases, called Higgs phases, characterized by ordered components perpendicular to these axes.

In the recent years, we have studied the Nd based pyrochlore Nd2Zr2O7 pyrochlore, which offers the opportunity to approach this issue. Despite a positive Curie temperature, indicating the existence of ferromagnetic interactions, a partial “all-in–all-out” (AIAO) antiferromagnetic ordering is observed below TN ≈ 300 mK. Furthermore, the spin wave excitation spectrum is quite original, and especially a flat “spin ice” mode [2].

Our recent studies suggest however that just above TN (and below 1 K), this compound hosts a correlated state, which could be a remarkable novel example of the Coulomb phase. This phase is described by a “two-in–two- out” rule as in spin ice, but built on a degree of freedom different from the conventional <111> Ising one [3].

Furthermore, to check the robustness of this peculiar state, we have studied the role of defects on the ground state and the excitations, using two types of substitutions: titanium instead of zirconium, to directly affect the magnetic interactions, and lanthanum instead of neodymium to reduce the number of magnetic atoms. We show that defects have a low impact on the crystal electric field scheme (inelastic neutron scattering 2T @LLB and Panther@ILL). We have established the (H,T) phase diagram of these doped compounds by measuring the magnetization and the magnetic structure (D23 @ILL, G41 @LLB), showing that the AIAO phase is reinforced by doping. Surprisingly, the excitations are still well defined in spite of disorder (IN5@ILL) and the flat mode is preserved, with the same characteristic gap energy [4].

[1] M. J. P. Gingras and P. A. McClarty, Rep. Prog. Phys. 77, 056501 (2014).
[2] E. Lhotel and al, PRL 115, 197202 (2015), S. Petit and al, Nature Physics 12, 746 (2016)
[3] M. Léger et al to appear in PRL (2021)
[4] M. Léger et al to appear in PRB (2021)