Session D41: Emergent magnetism in correlated electron systems I

Z2xZ2 magnetic domains and magnetic moment disproportionation in the spin-orbit Mott insulator Sr2IrO4

Sr2IrO4 is representative of the new class of materials dubbed “spin-orbit Mott insulators” which hosts a variety of unconventional magnetism that arise from spin-orbit entangled moments, or pseudospins. A prominent example is the possible realization of a Kitaev spin liquid when pseudospins are arranged in a honeycomb lattice. In this talk, I will discuss on a new aspect of pseudospins; they show propensity to spontaneously disproportionate and lead to rich magnetic structures where symmetrically equivalent sites are decorated with magnetic moments of different magnitudes. We start by showing that the observed Z2xZ2 domain structure cannot be explained by the known magnetic structure and necessarily requires mixing of two order parameters that belong to two different irreducible representations of the high symmetry phase, which in response to the magnetic moment disproportionation distorts to a low symmetry phase. We argue that this phenomenon may be found more generally in other systems with spin-orbit entangled magnetic moments and hence should be considered when solving for the magnetic structure.
  

Exploring magnetic frustration in a square-lattice metallic system: spin fluctuations in 15% Fe-doped CaCo2As2

Magnetic frustration can arise in a square lattice of magnetic moments from competition between nearest-neighbor (J₁) and next-nearest-neighbor (J₂) exchange, with η = J₁/2J₂ governing the magnetic ground state. Remarkably, our inelastic neutron scattering (INS) measurements have shown that CaCo_1.86As₂ is a unique example of a metallic square-lattice system with nearly perfect frustration (|η| ≈ 1) despite possessing A-type antiferromagnetic (AF) order below a Néel temperature of T_N = 52 K. Our recent neutron diffraction data on Ca(Co_1-xFe_x)_2-yAs₂ show that the A-type AF order is suppressed past x = 0.12, and here we give results from INS experiments made at T = 6 K on single-crystal samples of x = 0.15. Instead of the walls of magnetic fluctuations found for x = 0, we rather find ferromagnetic-like fluctuations emerging from zone centers that, depending on the energy, is quickly suppressed with increasing momentum transfer. We discuss our results in terms of the changing degree of frustration with increasing x.   

Nonreciprocal directional dichroism effect in Ni3TeO6 in the toroidal geometry

We bring together high magnetic field techniques, optical spectroscopy, and first principles electronic structure calculations to reveal high-energy, broadband nonreciprocal directional dichroism in Ni₃TeO₆. We focus on the toroidal geometry where polarization is perpendicular to the chiral spin arrangement, and light is propagating orthogonal to both. We employ circularly polarized as well as unpolarized light to fully explore the symmetry properties and eigenstates of the system as well as potential for photonics applications. Due to the spectral range of our work, we demonstrate nonreciprocal effects in the Ni²⁺ d-to-d on-site excitations as well as the phonon side bands that appear on the leading edge of these structures. In addition to being a peculiar and fundamental light-matter interaction in low-symmetric crystals, nonreciprocal directional dichroism can support one-way transparency, optical rectifiers, and high fidelity holograms – just to name a few.   

Magnetic structure in square cupola compound Ba(TiO)Cu4(PO4)4: 31P NMR Study

The magnetic structure of the antiferromagnetic square cupola compound Ba(TiO)Cu₄(PO₄)₄ single crystal is studied with ³¹P nuclear magnetic resonance techniques. The magnetic hyperfine shift K shows a clear splitting at the Neel temperature T_N = 9.5 K, where the resonance splits into 2 lines when the external field is oriented along c-axis and into 4 lines when the field is along a-axis. In paramagnetic region K(T) follows temperature dependence of the magnetic susceptibility χ(T). From K vs χ plot we determined hyperfine field values H^a_hf = 765 mT/μ_B and H^c_hf = 740 mT/μ_B for magnetic field oriented along a- and c-axis, respectively. From the rotation of the single crystal in external magnetic field we determined 8 different orientations of K-tensor in paramagnetic region. In antiferromagnetic state at T = 6 K we found that the local field at phosphorus is mainly due to dipolar field of coppers. Here the rotation of single crystal shows 8 different orientations of the local field B_int = 38 ± 2 mT. Experimental orientation of B_int corresponds to the calculation of dipolar fields at phosphorus assuming magnetic quadrupolar configuration of magnetic moments Γ₃(1).   

The Origin of Ising Magnetism in Ca3Co2O6 Unveiled by Orbital Imaging

One-dimensional CoO₆ chains in Ca₃Co₂O₆ arranged in a triangular lattice give rise to an Ising-like magnetism with an intriguing quantum tunneling staircase structure in its magnetization. To resolve the underlying local electronic configuration of the Co ions we applied s-core-level non-resonant inelastic x-ray scattering (s-NIXS), a new technique that is capable of imaging the shape of the 3d orbitals in real space. The orbital shapes that we found established unequivocally that both Co sites (octahedral and trigonal prismatic) are in a 3+ valence state (i.e. 3d⁶); the trigonal Co site has a high-spin configuration while the octahedral Co site is low spin. Interestingly, we directly ‘see’ that it is the complex d₂ orbital that is stabilized by the prismatic trigonal coordination, which naturally explains the Ising magnetism in the system. Utilizing this ability to image electron orbitals, and thus directly relating the orbital occupation with the local crystal structure—without the need for theoretical modeling—is essential for modeling magnetic properties. This is especially true in situations where one would like to make use of the delicate balance of competing interactions to stabilize a particular orbital state for a desired or optimized physical property.   

Finite, disordered chains of local and correlated moments in transition metal oxides

The structures of Bi₂CrAl₃O₉ and Li₂Mn₂(MoO₄)₃ are similar in that they both contain quasi-linear chains of octahedrally-coordinated atomic positions that are partially occupied by 3d transition metals and partially occupied by a non-magnetic species. In the absence of long range occupancy ordering, this configuration gives rise to finite chains of moments with statistically varying lengths. We report the results of in situ diffraction experiments probing the synthetic pathways that yield high quality single crystals of these compounds. Ex situ UV/Vis diffuse reflectance spectroscopy measurements establish the magnitude of the charge gap and the local configuration of the magnetic species. Scaling of the magnetization at T < 10 K provides a probe to differentiate between independently fluctuating local moments and collective excitations along the chains.   

Complex magnetism and magnetic compensation in metal tellurate (Mn,Ni)3TeO6

The two of metal tellurate, Ni₃TeO₆ and Mn₃TeO₆, have rhombohedral lattice (Ni = R3, Mn = R-3), and they shows antiferromagnetic ordering with spin-induced ferroelectricity. However, we grown (Mn,Ni)₃TeO₆ single crystal, and found monoclinc structure, such as Co₃TeO₆. From magnetic susceptibility, and specific heat measurements, we discover ferrimagnetic transition and magnetic compensation state. These results significantly different from both end-member Ni₃TeO₆ and Mn₃TeO₆ and require further investigations of its magnetic structure.   

High-energy nonreciprocal directional dichroism in a chiral magnet

Nonreciprocal directional dichroism is an unusual light-matter interaction that gives rise to diode-like behavior in low symmetry materials. The chiral varieties are particularly scarce due to the requirements for strong spin-orbit coupling, broken time reversal symmetry, and a chiral axis. We bring together magneto-optical spectroscopy and first principles calculations to reveal high energy, broad band nonreciprocal directional dichroism in Ni₃TeO₆with special focus on behavior in the metamagnetic phase above 52 T. In addition to demonstrating this effect in the magnetochiral configuration, we explore the transverse magnetochiral orientation in which applied field and light propagation are orthogonal to the chiral axis and by so doing, uncover an additional configuration with a nonreciprocal response in the visible part of the spectrum. In a significant conceptual advance, we use first principles methods to analyze how the Ni²⁺ d-to-d on-site excitations develop magnetoelectric character and present a microscopic model that unlocks the door to theory-driven discovery of chiral magnets with nonreciprocal properties.   

Probing Spin-Excitaions above the Ground State of 1T-TaS2

Spin-spin interactions can lead to exotic ground states with emergent excitations in frustrated quantum magnets. Such a system is the transition metal dichalcogenide 1T-TaS₂ .While the material is electrically insulating and exhibits no magnetic ordering down to milikelvin temperatures, the specific heat has a linear fermionic-type contribution that suggests a large band-width. We analyse the low temperature specific heat data and torque magnetometry measurements to isolate the contribution of the spin excitations to the magnetization from the large background response. The magnetic torque is strongly anisotropic and has a singular response at low magnetic fields which we attribute to a continuous disassociation of singlet pairs. This picture is also supported by the perfect collapse of the specific heat data at various temperatures and magnetic fields onto a universal curve predicted for singlets with a random distribution of antiferromagnetic coupling strengths. We interpret our data in the framework of a quantum spin liquid ground state having a large bandwidth with minority spins forming an array of singlet pairs with a continuum of coupling constants. Finally, despite the presumed large spinon Fermi surface we could not observe quantum oscillations of the magnetization.   

Competition of itinerant magnetic states in SrCo2As2 from RPA spin susceptibility

The square-lattice layered 122-type cobalt pnictides ACo₂Pn₂ (A=Ca, Sr, Eu; Pn=As, P) are metals with fascinating magnetic properties reminiscent of local moment frustration. Among those, SrCo₂As₂ and CaCo₂As₂ stand out due to the competing nature of their in-plane magnetic fluctuations. While CaCo₂As₂ exhibits A-type antiferromagnetic order with ferromagnetically ordered Co planes, inelastic neutron scattering has revealed signs of extreme frustration towards stripe-like antiferromagnetism. SrCo₂As₂ does not order down to T=0.05 K, but shows an intricate competition between ferro- and stripe-like magnetic fluctuations instead. While some of these observations can be rationalized with J₁-J₂-type local moment models, several experimental facts cannot be reconciled within such a local moment picture such an approximate description. Here, we perform a fully itinerant analysis of the magnetic properties of these materials based on a tight-binding model obtained from DFT calculations for the bandstructure. We report our findings on the static transverse spin susceptibility for the resulting multiorbital Hubbard model within the random-phase approximation RPA approximation.   

Quantum-to-classical correspondence in two-dimensional Heisenberg models

The quantum-to-classical correspondence (QCC) in spin models is a puzzling phenomenon
where the static susceptibility of a quantum system agrees with its classical-system counterpart,
at a different corresponding temperature, within the systematic error at a sub-percent level.
We employ the bold diagrammatic Monte Carlo method to explore the universality of QCC by
considering three different two-dimensional spin-1/2 Heisenberg models. In particular,
we reveal the existence of QCC in two-parametric models.   

Theoretical Prediction of Bulk Rashba Effect in Pyroelectric Antiferromagnet BiCoO3

Rashba effect, a type of spin-momentum locking phenomena, commonly occurs at interfaces or surfaces, while it is nowadays well established that Rashba-like band splitting may also occur in non-centrosymmetric (chiral, polar or ferroelectric) bulk materials. Here we are taking a step further by considering a noncentrosymmetric antiferromagnetic oxide as a playground for bulk Rashba effect. In ferroelectric and antiferromagnetic systems, i.e., multiferroic systems, spin-flipping symmetry operations may exist (such as a mirror symmetry operation) which behave as time-reversal symmetry, thus enforcing two-fold degeneracy at specific high-symmetric points and allowing bulk Rashba effect to emerge. As a prototypical example, ab-initio DFT calculations of antiferromagnetic BiCoO₃ in the polar structure reveal that a large Rashba-like spin splitting occurs at the conduction band bottom, having a large weight from Bi-p orbital states [1]. In this presentation, we also show that the spin texture of such a multiferroic can be modulated by applying a magnetic field.
[1] Kunihiko Yamauchi, Paolo Barone, Silvia Picozzi, arXiv:1910.06758 (Oct2019).   

Fermion Hubbard model on non-bipartite lattices: flux problem, gauge fields and emergent chirality

On some one dimensional (1D) and 2D non-bipartite lattices, we study both free and Hubbard interacting lattice fermions when there are some magnetic fluxes threaded or appropriate gauge fields coupled. On one hand, we focus on finding out the optimal flux which minimizes the energy of fermions at specific fillings. For spin-1/2 fermions at half-filling on a ring lattice with an odd number of sites, the optimal flux is determined to be ±π/2. We prove this conclusion for Hubbard interacting fermions by means of a generalized reflection positivity technique. It can further lead to some applications towards 2D non-bipartite lattices such as triangle and Kagome. At half-filling, the optimal flux pattern on the triangular lattice can be ascertained to be [π/2, π/2]. We find that chirality emerges in these optimal flux states. On the other hand, we verify these conclusions and further study other fillings away from half with the numerical exact diagonalization (ED) method and find that, when it deviates from half-filling, Hubbard interactions can even alter the optimal flux patterns on these lattices.