Session F37: Iridates and Kitaev Materials

Live

With the emergence of antiferromagnetic (AF) spintronics in recent years, Iridates have attracted great interests because of their exotic ground state with magneto transport properties, driven by the interaction between strong spin-orbit coupling (SOC) and electron correlations. As a prominent example, Sr₂IrO₄ is a quasi-two-dimensional Jeff =1/2 canted AF Mott insulator. Similar to S=1/2 moments, J_eff=1/2 moments have no single-ion anisotropy. They are however also very different in that the J_eff=1/2 moments can form significant inter-site quadrupoles that are highly sensitive to lattice distortion via the so-called ‘pseudo-JT’ effect. The strength and symmetry of the pseudospin-lattice coupling can be externally controlled by the application of in situ strain. In this work, we investigate the tuning of the metamagnetic transition of the AF structure in Sr₂IrO₄ by applying anisotropic strain of the B_2g symmetry. By measuring the magnetoresistance (MR) and angular dependence of MR at different magnitude of in situ anisotropic strain, we observed clear shifts of the critical field of the metamagnetic transition. X-ray resonance magnetic scattering (XRMS) is performed to reveal the underlying modulation of the AF structure.   

Live

Kitaev magnets have gained a huge amount of interest in recent years due to the possibility of such materials hosting a highly-entangled quantum spin liquid (QSL) ground state. Such materials are characterized by a honeycomb lattice structure and can be analytically solved by mapping the localized spins into itinerant and localized Majorana excitations. Interestingly, recent experiments and numerics have shown that, within a finite-temperature range, the QSL near the "pure" Kitaev limit yields an exotic, metal-like phase with finite Sommerfeld coefficient. In this work, we describe the apparent Sommerfeld coefficient and exotic T-dependence in the specific heat of Ag3LiIr2O6 via a Landau-Fermi liquid theory of itinerant Majorana-like quasiparticles. By considering the effects of interaction on the statistical energy of these Majorana quasiparticles, the resulting specific heat is found to be dominated by a quadratic-temperature dependence with a strong non-analytic contribution, in agreement with present experiments on the silver-lithium iridate at zero and finite external magnetic field.   

Live

Experimental identification of fractionalized quasiparticles in quantum spin liquids is a long-sought goal in correlated topological phases. Since conventional spectroscopic probes (e.g., neutron scattering) cannot directly couple to each individual fractionalized excitation, one can only measure a broad continuum in energy due to a multitude of fractionalized quasiparticles decaying from a single spin-flip. In this talk, we discuss how two-dimensional nonlinear spectroscopy can sharply reveal distinctive signatures of fractionalized excitations in quantum spin liquids by considering the example of the exactly solvable Kitaev honeycomb model. We demonstrate the existence of a number of salient features of the Majorana fermions and fluxes in two-dimensional nonlinear spectrum, which provide crucial information about such excitations.   

Live

Frustrated magnets with highly degenerate ground states are at the heart of hunting exotic states of matter. Recent studies in spin-orbit coupled honeycomb magnets have generated immense interest in bond-dependent interactions, appreciating a symmetric off-diagonal Γ interaction which exhibits a macroscopic degeneracy in the classical limit. Here, we study a generic spin model and discover a novel chiral-spin ordering with spontaneous broken time-reversal symmetry near the dominant Γ region. Furthermore, gapless excitations revealed by a vanishing energy gap and a finite central charge on cylinders are presented. A putative chiral spin liquid (CSL) via melting of classical large-unit-cell magnetic orders is proposed. Guided by ab-initio calculations, we suggest a practical route towards a sizeable Γ region in α-RuCl₃, paving the way for a future search of CSL in real materials.   

Live

Multicritical points are extremely difficult to pinpoint and study numerically owing to the large correlation lengths and sensitive dependence of multicritical exponents on parameter values. We study the experimentally accessible problem of the frustrated triangular lattice Ising antiferromagnet with ferromagnetic second and third nearest neighbours using highly efficient Monte-Carlo worm algorithms. We show that the width in temperature of the two-step melting transition of the three-sublattice ordered phase stabilised by ferromagnetic second-nearest neighbours can be tuned by the strength of the third-nearest neighbour ferromagnetic interactions. Increasing third-nearest neighbour ferromagnetic interactions causes the two-step melting phase to pinch-off into a multicritical point. Further increase in the third-nearest neighbour interactions makes the melting weakly first-ordered. We are able to numerically pinpoint the location of the multicritical point and calculate multicritical exponents to show that this multicritical point belongs to the universality class of a Z6 parafermionic CFT. We extend our analysis to the frustrated Kagome Ising antiferromagnet wherein a similar two-step melting of the three-sublattice ordered phase can be tuned into a multicritical pinch-off.   

Live

Emergent phenomenon cooperatively driven by electronic correlation and spin-orbit coupling is at the forefront of condensed matter physics. In 3d complex oxide systems, the orbital degeneracy can usually be lifted via the Jahn-Teller Effect. While in 5d systems even the orbital singlet can be highly sensitive to lattice distortion due to the presence of strong SOC. Here we show that, by applying anisotropic strain up to only 0.05%, we can in-situ modulate the metamagnetic transition field by almost 300% of the spin-orbit-coupled Mott insulator Sr₂IrO_4, which is an important representative of the 5d transition metal oxides. Our simultaneous measurements of resonant x-ray scattering and transport reveal that this drastic response originates from the complete strain-tuning of the transition between the spin-flop and spin-flip limits, and is always accompanied by large elasto- and magneto-conductance. This enables electrically controllable and electronically detectable metamagnetic switching. The resulted strain-magnetic field phase diagram reveals that the strain introduces C₄-symmetry-breaking magnetic anisotropy to the square lattice via pseudospin-lattice coupling, directly demonstrating the pseudo-Jahn-Teller effect of the spin-orbit-coupled correlated system. The extracted coupling strength is much weaker than the superexchange interactions, yet crucial for the spontaneous symmetry-breaking, affording the remarkably efficient strain-control. This work shows a promising avenue to effectively manipulate antiferromagnetic materials in the field of iridates, and more broadly functional 5d transition metal oxides by playing emergent phenomena with in situ strain.   

Live

Using the first-principles density functional approach, here, we investigate electronic and magnetic
properties of newly discovered double perovskite Lu2NiIrO6 under the cooperative effect of
Coulomb interaction (U) for Ni-3d and Ir-5d elements and spin orbit coupling (SOC). This system
is ferrimagnetic (FIM) and half-metallic (HM) ground state in generalized gradient approximation
(GGA) and GGA+U, whereas SOC breaks HM and changes it into Mott-insulator with the band gap
of 0.19 eV in close agreement with experimental band gap in the (010)-direction. When Ni is
replaced with Fe, it’s magnetic ground state changes into anti-ferromagnetic (AFM) state with zero
spin magnetization in the unit cell and 1.0 eV band gap in the (100)-direction. Similarly, if Ir-5d
element is replaced with Ru-4d element, its FIM ground state remains unchanged with band gap of
0.47 eV in the (100)-direction.   

Live

Na₂MO₃ based Mott insulators have been studied recently as potential candidates to realize Kitaev spin model. 4d⁵ or 5d⁵ systems on a Honeycomb lattice exhibit spin-orbit assisted Mott insulating ground states. At the strong SOC regime, the ground state for these ions in an octahedral crystal field is dominated by J_eff =1/2 states resulting in bond-dependent anisotropic magnetic exchange in the plane, essential to realize the Kitaev spin model. However, candidates to realize the Kitaev spin model are not limited to 4d/5d systems. Lanthanide elements with significant SOC and inherent anisotropy can also be considered in this context. In this talk, I will present our work on a lanthanide based, Kitaev material candidate with a potential J_eff =1/2 ground state. Synthesis and structure of the material will be discussed. Physical property measurements coupled with inelastic neutron scattering results will be utilized to understand the single-ion characteristics of the lanthanide and to unveil the rich low-temperature physics of the material.

Daum, M. J.; Ramanathan, A.; Kolesnikov, A. I.; Calder, S. A.; Mourigal, M.; La Pierre, H. S., Collective excitations in the tetravalent lanthanide honeycomb antiferromagnet, Na₂PrO₃. arXiv preprint arXiv:2010.06741 2020.   

Not Participating

Quantum Fisher information (QFI) is a concept from quantum metrology
that has gained extensive recent application to condensed matter physics, pri-
marily because it is easily accessible in neutron scattering experiments and can
be used to quantify the amount of multipartite entanglement in a system at
finite temperatures. We present results for the QFI in the Kitaev honeycomb
model (KHM), where the derivatives of the QFI with respect to the driving
parameter detect the presence of the gapped-gapless phase transition by. We
compute the scaling of the divergence of the second derivatives of the QFI on the approach to the critical
point, and show that the diverging QFI can be understood in terms of diverg-
ing length scales in the two point correlation functions for the site and bond
magnetization operators. In the former case, this correlation length is the two
point correlator of the Majorana degrees of freedom.   

Live

Iridium containing oxides have been continually attracting interest for studying emergent phases of matter. My talk focuses on the magnetic interactions in the iridate compounds Sr₃(Ni,Co)IrO₆, which have the highest coercive magnetic fields of up to 55 T of any materials to our knowledge. These compounds belong to a class of Ir⁴⁺ oxides in which the spin-orbit coupling, exchange interaction, crystal electric field energy scale and Hund’s rule U are within an order of magnitude. This results in the highly entangled spin-orbital J_eff = ½ state of Ir. The origin of this high coercive field is thought to be the 5d Ir⁴⁺ ion and its strong and unusual spin-orbit (S-O) coupling. However, the precise origin of the high coercive field, magnetic interactions, magnetic ordering and role of domain dynamics in these compounds are not yet understood. I present the result of our magnetization and, x-ray spectroscopy and dichroism studies to shed light on these questions.   

Live

The nature of the ground state in the frustrated spin-1/2 Cu-based kagome compounds such as herbertsmithite ZnCu₃(OH)₆Cl₂ has been the subject of intense discussion for many years. The knowledge on the nature of the vibrational modes in this system and the magnetoelastic coupling would be of major help to resolve the magnetic ground states of these materials. We investigate the lattice dynamics in herbertsmithite ZnCu₃(OH)₆Cl₂ and a newly synthesized Cu-based Kagome compound Y₃Cu₉(OH)₁₉Cl₈ [1] by a combination of infrared spectroscopy measurements and ab initio density functional theory calculations. The results provide an unambiguous assignment of infrared-active lattice vibrations involving in-plane and out-of-plane atom displacements in the kagome layers and indicate the presence of a strong magnetoelastic coupling to the spin system [2]. We further discuss similarities and differences between ZnCu₃(OH)₆Cl₂ and Y₃Cu₉(OH)₁₉Cl₈.
[1] P. Puphal, M. Bolte, D. Sheptyakov, et al., J. Mater. Chem. C, 5, 2629 (2017).
[2] Y. Li, A. Pustogow, M. Bories, et al., Phys. Rev. B 101, 161115 (R) (2020).   

On Demand

The insulator spin chain compound Ba₃Ir₄O₁₀ is a new candidate for a frustrated quantum spin liquid, which was reported to have a stabilized quantum spin liquid state down to 0.2 K. The spin liquid is extraordinarily sensitive to small perturbations. The replacement of approximately 2% of barium with strontium or growth of the samples in the magnetic field readily lift the frustration parameter and this doped material has long-range order from an antiferromagnetic transition at 130 K. We will present results of Raman scattering experiments on Sr-doped, undoped and magnetic field-grown samples of Ba₃Ir₄O₁₀, which demonstrate damping of the phonons by magnetic fluctuations. The results will be interpreted in terms of magnon-phonon coupling due to strong spin-orbit interaction.   

On Demand

A recent observation of anomalous Hall Effect (AHE) in antiferromagnets has suggested that the ferromagnetic ordering is not a necessary condition. Accordingly, a theoretical study showed that higher-rank multipoles formed by a cluster of spins (cluster multipoles) can generate the anomalous Hall Effect without magnetization. Despite such an intriguing proposal, it has seldom been investigated to control the unconventional AHE by including the cluster multipoles. Here, we demonstrate that a strain can manipulate the hidden Berry curvature effect by inducing the higher-order cluster multipoles in spin-orbit-coupled antiferromagnets. Observing the large AHE on fully strained antiferromagnetic Nd₂Ir₂O₇ thin films, we prove that strain-induced cluster T₁-octupoles are the only source of observed AHE. Our results provide a previously unidentified pathway for generating the unconventional AHE via strain-induced magnetic structures and establish a platform for exploring undiscovered topological phenomena via strain in correlated materials.