Session D70: Novel 5d Materials

Discovery of Honeycomb and Kagomé Quantum Magnets

In this talk, I will present recent results on the discovery of kagome and honeycomb quantum magnets. Particular focus will be placed on the development of cobalt and rare earth based honeycomb materials, and the similarities and differences between integer and half-integer spin ions. The discovery of bidirectional twisting in MgCo₆Ge₆ will also be discussed and put in context. If time permits, I will also report on emerging materials discoveries that have the potential to reveal Kitaev spin liquid ground states.   

Nuclearmagnetic resonance studies of double perovskite Mott insulating Ba2LiOsO6

Double perovskites with 5d transition metal ions offer a flexible environment to study the interplay of comparable electron correlations and spin-orbit coupling. Mott insulating osmium compounds present an opportunity to study the resulting delicate emergent phases from such competing interactions. Here, we present detailed nuclear magnetic resonance (NMR) measurements on Ba₂LiOsO₆ single crystals. The nature of its magnetic transition at low temperature and its metamagnetic transition at 6 T will be discussed. The results are compared to the isostructural compound Ba₂NaOsO₆.   

Untangling the structural, magnetic dipole, and charge multipolar orders in Ba2MgReO6 and Cs2TaCl6

We present density-functional calculations of hidden multipolar orders in 5d transition-metal double perovskites, using Ba2MgReO6 and Cs2TaCl6 as examples. Recent studies have suggested the existence of unconventional types of ordering, not detectable using typical probes, in these 5d double perovskites, stemming from the interplay of spin-orbit coupling, crystal-field splitting, and electron-electron repulsion. These "hidden orders" are formed from ordered anisotropies in the electronic charge and magnetization density distributions that can be described within multipole expansions beyond the usual dipolar distributions and include ordered charge quadrupoles and magnetic octupoles, as well as higher-order multipoles.

We also present an experimental study using resonant elastic x-ray scattering to directly probe the existence of charge quadrupoles in Cs2TaCl6. In addition to shedding light on the detailed behavior of Ba2MgReO6 and Cs2TaCl6, our analysis provides a novel insight into the crystal structure-multipolar order relationship of 5d double perovskites.

Materials Theory, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland

Institute of Physics, Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland}   

High-pressure synthesis, crystal structure, and physical properties of perovskite KOsO3

The syntheses of LiOsO₃ and NaOsO₃ have brought some fresh input to the long-standing dialogs of ferroelectric metal and the metal-insulator transition. The LiOsO₃ with the LiNbO₃-type structure has been believed to be the best candidate for the ferroelectric metal proposed by Anderson and Blount [1]. Whether the metal-insulator transition in the orthorhombic perovskite NaOsO₃ [2] can be attributed to either a Mott transition or a Slater transition is still controversial [3,4].  KOsO₃ in this family has been synthesized under a high-pressure and high-temperature condition for the first time. As the geometric tolerance factor is close to 1, the perovskite KOsO₃ crystallizes in a cubic structure (Pm-3m). The cubic phase remains stable for a large range of the Na doping in K_1-xNa_xOsO₃. The perovskite KOsO₃ and the Na-doped KOsO₃ samples were characterized by the measurements of magnetic susceptibilities, thermoelectric power, and specific heat. In the talk, we will present the measurement results and the magnetic structure by neutron diffraction.

    

Octupolar order and Ising quantum criticality tuned by strain and dimensionality: Application to $d$-orbital Mott insulators

Recent experiments have discovered multipolar orders in a variety of $d$-orbital Mott insulators. Motivated by uncovering the exchange interactions which underlie octupolar order proposed in the Osmate ($5d^2$) double perovskites, we study a two-site model using exact diagonalization on a five-orbital Hamiltonian, incorporating spin-orbit coupling (SOC) and interactions, and including both intra-orbital and inter-orbital hopping. Using an exact Schrieffer-Wolff transformation, we then extract an effective pseudospin Hamiltonian for the non-Kramers doublets, uncovering dominant ferrooctupolar coupling driven by the interplay of two distinct intra-orbital hopping terms. Our work highlights the importance of higher order contributions to the exchange couplings coming from the small gap between the non-Kramers doublet and excited triplets in these systems. Using classical Monte Carlo simulations on the face-centered cubic lattice, we obtain a ferrooctupolar transition temperature ($T_c$) which is in good agreement with experiments on the osmate double perovskites. We also explore the impact of uniaxial strain and dimensional tuning via ultrathin films, which are shown to induce a transverse field on the Ising octupolar order. This suppresses $T_c$ and potentially allows one to access octupolar Ising quantum critical points. We discuss possible implications of our results for a broader class of materials which may host such non-Kramers doublet ions.   

Magnetization dynamics fingerprints of an excitonic condensate (t2g)4 magnet

The competition between spin-orbit coupling λ and electron-electron interaction U leads to a plethora of novel states of matter, extensively studied in materials such as ruthenates and iridates. Recent theoretical studies predicted that excitonic magnets -- the antiferromagnetic state due to the condensation of excitons (bounded electron-hole pairs) -- can be found in the ground-state of spin-orbit-coupled (t_2g)⁴  Hubbard models [1,2]. We present a detailed study of the magnetic excitations in that excitonic magnet, employing one-dimensional chains (via density matrix renormalization group) and small two-dimensional clusters (via Lanczos).  First we show that the low-energy spectrum is dominated by a dispersive (acoustic) magnonic mode, with extra features arising from the λ=0 state in the phase diagram. Second, and more importantly, we found a novel magnetic excitation forming a high-energy optical mode with the highest intensity at wavevector q→0 .  These unique fingerprints of the excitonic magnet are important in the analysis of neutron and RIXS experiments.

[1] N. Kaushal et al., Phys. Rev. B 101, 245147 (2020), Phys. Rev. B 96, 155111(2017),  Phys. Rev. B 99, 155115 (2019).

[2] G. Khalliulin, Phys. Rev. Lett. 111, 197201 (2013).

[3] N. Kaushal et al., arXiv:2110.11828 (2021) .   

Small polaron formation in 5d spin-orbit coupled omsates

Small polarons (SP) have been thoroughly investigated in 3d transition metal oxides [1]. However, very little is known about these quasi-particles in 5d systems, since the more delocalised nature of the 5d orbitals reduces the strength of the Electronic Correlation (EC), making SP formation in these compounds rather unexpected. The spin-orbit coupled Dirac-Mott insulator Ba₂NaOsO₆ (BNOO) represents a candidate material for enabling polaron formation in a relativistic background, due to the relatively large EC (U ~ 3 eV) and Jahn-Teller activity [2]. We verify this hypothesis by combining first principles calculations with nuclear magnetic resonance (NMR) and muons measurements. We find that excess electrons injected into BNOO through chemical doping are trapped in Os sites and distort the local phonon field, typical hallmark of SP formation. SP are subjected to thermally activated hopping, revealed by anomalous peaks in the spin-lattice and spin-spin relaxation rates, attributed to fluctuation of the electric field gradients driven by a charge-related time dependent perturbation.

References

[1] C. Franchini et al., Nat. Rev. Mater., 1-27 (2021) 10.1038/s41578-021-00289-w.

[2] D. Fiore Mosca et al., Phys. Rev. B 103, 104401 (2021) 10.1103/PhysRevB.103.104401.   

Ferro-octupolar order in d2 double perovskites of Osmium from first principles

Conflicting interpretations of experimental data preclude the understanding of the quantum magnetic state of spin-orbit coupled d² double perovskites. Whether the ground state is a Janh-Teller-distorted order of quadrupoles or the hitherto elusive octupolar order remains debated. We resolve this uncertainty through direct calculations of all-rank inter-site exchange interactions and inelastic neutron scattering cross-section for the d² double perovskite series Ba₂MOsO₆  (M= Ca, Mg, Zn). Using advanced many-body first principles methods we show that the ground state is formed by ferro-ordered octupoles and is dominated by superexchange interactions within the ground-state E_g doublet. Computed ordering temperature of the single second-order phase transition is consistent with experimentally observed material-dependent trends. We further investigate the electronic, structural and magnetic properties of such compounds by purely Density Functional Theory (DFT) calculations with a new approach that consists in the constrain of the onsite density matrix, as obtained via DFT +  Dynamical Mean Field Theory calculations. We prove that this method is able to reproduce the ferro-octupolar order and we compare this result to conventional DFT dipolar solutions.   

Flat-band ferroelectricity first discovered in HfO2

Ferroelectricity has been known to originate from the lowest-energy Gamma-phonon of a dispersed band.  Therefore, ferroelectricity is believed since it’s discovery in 1920 that at least finite-sized domains (10~100nm) that has the Gamma characteristics are required to stabilize and switch the ferroelectric dipoles. Here, we break the 100-years belief by introducing flat-band physics into the history of ferroelectricity. We discovered that, for the first time, flat bands in polar phonon exist surprisingly in ferroelectric HfO₂ and showed that they induce irreducibly localized dipoles of a few angstroms width which is the record-high small. More strikingly, these extremely localized dipoles are individually switchable by local electric fields and thus now one can circumvent the formation of the conventionally large domains for the ferroelectric switching. We can directly switch the ultimately-small unit-cell-scale dipoles and finally pave a way to achieve densest memory devices in the commercial ferroelectric compatible with Si-technology [1].

[1] “Scale-free ferroelectricity induced by flat phonon bands in HfO₂” Lee et al., Science 369, 1343 (2020).