Session RR03: V: Frustrated Magnetism

Magnetic fragmentation in pyrochlores

Pyrochlore iridates of formula R₂Ir₂O₇ with two interpenetrating pyrochlore lattices (corner-sharing tetrahedra) filled with a rare-earth R³⁺ and a magnetic metal Ir⁴⁺ are interesting materials in the field of frustrated magnetism. The iridium sublattice indeed orders at rather high temperature (~100 K) in an antiferromagnetic configuration called all-in all-out and imposes in turn an all-in all-out staggered effective magnetic field on the rare-earth pyrochlore lattice. This leads to the stabilization of new phases. For example, an exotic fragmented state of monopole crystal [1], where order and disorder coexist, is evidenced in Ho₂Ir₂O₇ [2] and Dy₂Ir₂O₇ [3]. In this new state of matter, the magnetic moment fragments into an antiferromagnetic ordered part and a persistently fluctuating Coulomb phase. This state has been studied through experimental and theoretical approach and I will detail the results obtained on both Ho and Dy compounds. I will also address the changes of the ground state observed when Ru⁴⁺, another magnetic metal ion, is substituted to the iridium in the Ho based pyrochlore. We propose a novel kind of fragmentation in this Ru based system.

[1] M. E. Brooks-Bartlett, S. T. Banks, L. D. C. Jaubert, A. Harman-Clarke, P. C. W. Holdsworth, Magnetic-moment fragmentation and monopole crystallization. Phys. Rev. X 4, 011007 (2014).

[2] E. Lefrançois, V. Cathelin, E. Lhotel, J. Robert, P. Lejay, C.V. Colin, B. Canals, F. Damay, J. Ollivier, B. Fa°k, L.C. Chapon, R. Ballou, V. Simonet, Magnetic charge injection in spin ice: a new way to fragmentation", Nature Communications 8, 209 (2017).

[3] V. Cathelin, E. Lefrançois, J. Robert, P. C. Guruciaga, C. Paulsen, D. Prabhakaran, P. Lejay, F. Damay, J. Ollivier, B. Fa°k, L. C. Chapon, R. Ballou, V. Simonet, P. C. W. Holdsworth, and E. Lhotel, Magnetic fragmentation in Dy pyrochlore iridate, Phys. Rev. Research Rapid Com. 2, 032073(R) (2020).

    

How clean can polymeric transferred monolayer MoS2 be?

Contamination-free transfer of monolayer MoS₂ (1L-MoS₂) is extremely beneficial for the fabrication of 1L-MoS₂ based flexible devices and integration into Si technology. Among the existing polymeric film transfer techniques of 1L-MoS₂, the surface-energy-assisted transfer technique is well-known. The technique uses polystyrene (PS) polymer as a carrier polymer, which is advantageous for providing a better mechanical support layer. Also, the transferred film is devoid of any optically visible polymer leftovers, unlike the film transferred by wet chemical etching methods. However, the existence of the microscopic PS residues on the transferred 1L-MoS₂ was not reported to date. In this work, the surface of 1L-MoS₂ was analyzed to establish the topographic modifications induced by the transfer process. The atomic force microscopy images reveal the presence of polymer residues which induces a mean roughness of ~ 5 nm. Additionally, high-resolution transmission electron microscopy analysis also reveals the presence of PS residues on transferred film. The A_1g phonon mode of the transferred film was blue-shifted (2.4 cm^-1), and FWHM (1 cm^-1) was increased. The Raman correlation plot establishes that the surface-energy-assisted transfer technique induces n-type doping to the 1L-MoS₂. The increase in film carrier concentration is because of the electrons transferred from PS polymer to 1L-MoS₂. Our findings are crucial for fabricating any high-performance flexible 2D devices.

    

Scalable Single-crystalline Metal Array for Tunable Emission of Van der Waals Materials

Van der Waals (vdW) materials, including h-BN and transition metal dichalcogenide (TMD), are ideal platform to explore quantum optics, Morie physics and many body effects.[1-3] Diverse applications and fundamental issues of the vdW are significant to the emission behavior and performances. Tunable emission of the vdW materials remains a long-lasting challenging issue.[4] Here, we achieved the self-assembly of the single-crystalline gold nanostructures to realize a scalable array for tunable and enhanced emission in the vdW. Synthesis, characterizations, and manipulation of the sub-100 nm single-crystalline nanostructures are presented. The scalable array is integrated with the representative vdW materials to demonstrate tunable emission and study the plasmonic effects. This study indicates that the deterministic nano-metal array is potential for quantum optics and information system. 

 

1.         Aharonovich, I., D. Englund, and M. Toth, Solid-state single-photon emitters. Nature Photonics, 2016. 10(10): p. 631-641.

2.         Behura, S.K., et al., Moiré physics in twisted van der Waals heterostructures of 2D materials. Emergent Materials, 2021. 4(4): p. 813-826.

3.         Sang, Y., et al., Tuning of Two-Dimensional Plasmon–Exciton Coupling in Full Parameter Space: A Polaritonic Non-Hermitian System. Nano Letters, 2021. 21(6): p. 2596-2602.

4.         Mendelson, N., et al., Engineering and Tuning of Quantum Emitters in Few-Layer Hexagonal Boron Nitride. ACS Nano, 2019. 13(3): p. 3132-3140.

  

    

Light-Matter Interplay in the Exciton-Photon Hybrid Configurations in the Flatland

Manipulating the interplay between matter and photonic structure unravel abundant possibilities in the fundamental understandings and practical applications, namely the surface-enhanced Raman spectroscopy and nanolaser. In this presentation, I will first show our new understanding of the Fano-type asymmetry deviated from the Rabi-type asymmetry in the exciton-plasmon hybrid system, which is experimentally confirmed with two-dimensional (2D) layered WSe₂ coupled to plasmonic lattice. To overcome the large Ohmic loss of plasmonic material, in the second project we apply the chemical vapor deposition (CVD) bottom-up method to fabricate the metaphotonic structure based on the bulk transition metal dichalcogenides (TMDCs). More specifically, we realize the magnetic-type surface lattice resonance (M-SLR) in the one-dimensional (1D) MoS₂ metaphotonic structure with extremely low material loss. Bright Mie modes and self-coupled anapole-exciton polaritons with unambiguous anti-crossing behavior are also realized in 2D MoS₂ metaphotonic structures. However, the aforementioned TMDCs structure does not demosntrate photoluminescence properties. By combining the multilayer (ML) TMDCs to the designed TMDCs metaphotonic structures, we are able to manipulate the polarization and direction of the photoluminescence from the ML TMDCs. Moreover, the valley degree of freedom of monolayer TMDCs will possibly result in abundant new physical phenomena when they are coupled to the chiral TMDCs structures.

 

    

Long-range spin transport in a system of indirect excitons in van der Waals MoSe2 /WSe2 heterostructure

Excitons in 2D van der Waals transition-metal dichalcogenide (TMD) heterostructures are characterized by high binding energies and determine optical properties of TMD heterostructures. Spatially indirect excitons (IXs), also known as interlayer excitons, are composed of electrons and holes in separated layers. TMD heterostructures are characterized by strong moiré superlattice potentials for IXs [1,2]. The long IX lifetimes make possible IX transport over long distances. In this contribution, we present the observation of a long-range spin transport in a system of IXs in MoSe₂ /WSe₂ heterostructure. The spin transport decay distance reaches and exceeds 100 microns. We measured the range of parameters, including the exciton density and temperature, of the long-range spin transport.   

Frustrated magnetism in triangular layered MGa2S4 (M=Ni,Fe) compounds

The perfect triangular lattice is the simplest 2D magnetic lattice to study magnetic frustration which can lead to exotic quantum phenomena. In this work we focus on MGa₂S₄ compounds with a magnetic ion M = Ni, Fe. Both systems crystallize in a perfect layered triangular Fe/Ni-lattice separated by non-magnetic Ga and S atoms. Recent neutron and magnetic susceptibility experiments¹ suggest strong exchange couplings for both systems.

Using ab-initio methods (total-energy mapping analysis and “projED”²) we extract an effective magnetic Hamiltonian including magnetic interactions up to the third next nearest neighbors and single ion anisotropy. We discuss the comparison between neutron scattering intensity of the model for the pristine crystal and experimental results where mixing of the M and Ga cations was suggested.

[1] K. Guratinder et al., Phys. Rev. B 104, 064412 (2021)

[2] K. Riedl et al., Phys. Status Solidi B 256, 1800684 (2019)   

Four-spin ring exchange in triangular lattice compounds

In the quest for exotic magnetic states, triangular lattice compounds offer a rich playground.  An important question in this context is, which exchange terms are necessary to construct an appropriate minimal magnetic model. For materials where the hopping amplitudes t are of comparable magnitude to the Coulomb repulsion U, higher order terms such as the four-spin ring exchange become significant. In this talk, we discuss ab-initio four-spin ring exchange for S=1/2 triangular lattice compounds, which fall in this regime.

A significant ring exchange may result from effectively reduced U, which is realized in κ-phase organic Mott insulators. Here, ring exchange is argued to suppress conventional magnetic order in a QSL/VBS candidate material. In contrast, in a recently synthesized organic compound we confirm that ring exchange induces a 'spin vortex crystal' order [1].

Another option to enter this regime are large hopping amplitudes due to heavy ligand elements. This situation is necessarily accompanied with strong spin-orbit coupling, resulting in anisotropic ring exchange that forms a potential new avenue for exotic magnetic states.

[1] Riedl et al., Phys. Rev. Lett. 127, 147204 (2021).   

Magnon dispersion and Edge states in pyrochlore Heisenberg magnets with anisotropy

We analyse the structure of edge states for the [111], [100] and [110] terminations of the pyrochlore lattice for a general Hamiltonian containing Heisenberg exchange, Dzyaloshinskii-Moriya interactions and spin-ice anisotropy. For different parameters of a chosen range, for both ferromagnetic and antiferromagnetic exchange we study the magnon dispersion for different phases namely ferromagnetic splay phase, all-in-all-out phase and a coplanar phase. In all of these phases depending on the direction of termination the magnon spectrum can be reciprocal or non reciprocal. We locate the region of non-reciprocity in the surface Brillouin zone. We also lay out the details of the edge localised magnons in these phases. The distinct topological structures in the bulk bands, like point degeneracies and nodal lines in all of the phases considered are discussed in detail. Using an analysis of the magnonic Berry curvature (like the evaluation of the topological charges and chern numbers) in the bulk geometry we give an account of correlations between the bulk and the boundary spectra.