arXiv subjects

C. Darie

Publications and source records attributed to C. Darie.

6 recordsLinked to original sources

arXiv2025

From hierarchical triangular spin liquid to multi-$q$ spin texture in spinel GeFe$_2$O$_4$

Combining macroscopic measurements, neutron scattering and modeling, we identify in the GeFe$_2$O$_4$ spinel a correlated paramagnetic state resulting from the predominance of third-neighbor antiferromagnetic interactions. These interactions materialize 4 isolated families of triangular planes with 120$^{\circ}$ spins emerging from the underlying pyrochlore lattice. At lower temperatures, a phase transition occurs from this hierarchical spin liquid to a non-coplanar spin texture that is characterized by 6 propagating vectors. This unusual multi-$q$ order is triggered by the presence of weaker interactions up to the sixth neighbors. The system is remarkably successful in coupling the different triangular planes while maintaining their two-dimensional 120$^{\circ}$ order. Our study highlights the hierarchy of interactions involved in GeFe$_2$O$_4$, which is singular among spinel compounds since first-neighbor interactions are only a small fraction of the dominant third neighbor ones.

cond-mat.str-el↗

arXiv2017

Evidence for a spinon Fermi surface in the triangular S=1 quantum spin liquid Ba$_3$NiSb$_2$O$_9$

Inelastic neutron scattering is used to study the low-energy magnetic excitations in the spin-1 triangular lattice of the 6H-B phase of Ba$_3$NiSb$_2$O$_9$. We study two powder samples: Ba$_3$NiSb$_2$O$_9$ synthesized under high pressure and Ba$_{2.5}$Sr$_{0.5}$NiSb$_2$O$_9$ in which chemical pressure stabilizes the 6H-B structure. The measured excitation spectra show broad gapless and nondispersive continua at characteristic wave vectors. Our data rules out most theoretical scenarios that have previously been proposed for this phase, and we find that it is well described by an exotic quantum spin liquid with three flavors of unpaired fermionic spinons, forming a large spinon Fermi surface.

cond-mat.str-el↗

arXiv2013

Inhomogeneous magnetism in the doped kagome lattice of LaCuO2.66

The hole-doped kagome lattice of Cu2+ ions in LaCuO2.66 was investigated by nuclear quadrupole resonance (NQR), electron spin resonance (ESR), electrical resistivity, bulk magnetization and specific heat measurements. For temperatures above ~180 K, the spin and charge properties show an activated behavior suggestive of a narrow-gap semiconductor. At lower temperatures, the results indicate an insulating ground state which may or may not be charge ordered. While the frustrated spins in remaining patches of the original kagome lattice might not be directly detected here, the observation of coexisting non-magnetic sites, free spins and frozen moments reveals an intrinsically inhomogeneous magnetism. Numerical simulations of a 1/3-diluted kagome lattice rationalize this magnetic state in terms of a heterogeneous distribution of cluster sizes and morphologies near the site-percolation threshold.

cond-mat.str-el↗

arXiv2006

Spin excitations in the antiferromagnet NaNiO2

In NaNiO2, Ni3+ ions form a quasi two dimensional triangular lattice of S = 1=2 spins. The magnetic order observed below 20K has been described as an A type antiferromagnet with ferro- magnetic layers weakly coupled antiferromagnetically. We studied the magnetic excitations with the electron spin resonance for frequencies 1-20 cm-1, in magnetic fields up to 14 T. The bulk of the results are interpreted in terms of a phenomenological model involving bi-axial anisotropy for the spins: a strong easy-plane term, and a weaker anisotropy within the plane. The direction of the easy plane is constrained by the collective Jahn-Teller distortion occurring in this material at 480 K.

cond-mat.str-el↗

arXiv2004

Decoupling of orbital and spin degrees of freedom in Li1-xNaxNiO2

In the Li1-xNaxNiO2 solid solutions three different single phase regions exist: for x > 0.9, forx = 0.7 and for x < 0.3. Although the intermediate compound does not show the cooperative Jahn-Teller transition of NaNiO2, its magnetic properties remain very similar with, in particular, the low temperature 3D magnetic ordering. Therefore, the strong coupling between orbital and spindegrees of freedom, characteristic of other oxides like perovskites, and usually invoked to explainthe absence of both long-range orbital and magnetic ordering in LiNiO2, seems not to take placein these layered compounds with 90-degree bonds. We also discuss the relevance of the O crystal fieldsplitting induced by the trigonal distortion, in generating AFM Ni-Ni in-plane interactions.

cond-mat.str-el↗

arXiv2002

Macroscopic Anisotropy and Symmetry Breaking in the Pyrochlore Antiferromagnet Gd$_{2}$Ti$_{2}$O$_{7}$}

In the Heisenberg antiferromagnet $Gd_2Ti_2O_7$, the exchange interactions are geometrically frustrated by the pyrochlore lattice structure. This ESR study reveals a strong temperature dependent anisotropy with respect to a [111] body diagonal below a temperature $T_A=80$ K, despite the spin only nature of the $Gd^{3+}$ ion. Anisotropy and symmetry breaking can nevertheless appear through the superexchange interaction. The presence of short range planar correlation restricted to specific Kagomé planes is sufficient to explain the two ESR modes studied in this work.

cond-mat.str-el↗