research
Quantum magnetism
Intercalant-mediated Kitaev exchange in Ag3LiIr2O6
The recently synthesized Ag3LiIr2O6 has been proposed as a Kitaev magnet in proximity to the quantum spin liquid phase. We explore its microscopic Hamiltonian and magnetic ground state using many-body quantum chemistry methods and exact diagonalization techniques. Our calculations establish a dominant bond dependent ferromagnetic Kitaev exchange between Ir sites and find that the inclusion of Ag 4d orbitals in the configuration interaction calculations strikingly enhances the Kitaev exchange. Furthermore, using exact diagonalization of the nearest-neighbor fully anisotropic J−K−Γ Hamiltonian, we obtain the magnetic phase diagram as a function of further neighbor couplings. We find that the antiferromagnetic off-diagonal coupling stabilizes long range order, but the structure factor calculations suggest that the material is very close to the quantum spin liquid phase and the ordered state can easily collapse into a liquid by small perturbations such as structural distortion or bond disorder.
Strong effect of hydrogen order on magnetic Kitaev interactions in H3LiIr2O6
Very recently a quantum liquid was reported to form in H3LiIr2O6, an iridate proposed to be a close realization of the Kitaev honeycomb model. To test this assertion we perform detailed quantum chemistry calculations to determine the magnetic interactions between Ir moments. We find that weakly bond dependent ferromagnetic Kitaev exchange dominates over other couplings, but still is substantially lower than in Na2IrO3. This reduction is caused by the peculiar position of the interlayer species: removing hydrogen cations next to a Ir2O2 plaquette increases the Kitaev exchange by more than a factor of 3 on the corresponding Ir ─ Ir link. Consequently, any lack of hydrogen order will have a drastic effect on the magnetic interactions and strongly promote spin disordering.
Large off-diagonal exchange couplings and spin liquid states in C3-symmetric iridates
Iridate oxides on a honeycomb lattice are considered promising candidates for realization of quantum spin liquid states. We investigate the magnetic couplings in a structural model for a honeycomb iridate K2IrO3, with C3 point-group symmetry at the Ir sites, which is an end member of the recently synthesized iridate family KxIryO2. Using ab initio quantum chemical methods, we elucidate the subtle relationship between the real space symmetry and magnetic anisotropy and show that the higher point-group symmetry leads to high frustration with strong magnetic anisotropy driven by the unusually large off-diagonal exchange couplings (Γ's) as opposed to other spin-liquid candidates considered so far. Consequently, large quantum fluctuations imply lack of magnetic ordering consistent with the experiments. Exact diagonalization calculations for the fully anisotropic K − J − Γ Hamiltonian reveal the importance of the off-diagonal anisotropic exchange couplings in stabilizing a spin liquid state and highlight an alternative route to stabilize spin liquid states for ferromagnetic K.
Pressure-induced dimerization and valence bond crystal formation in the Kitaev-Heisenberg magnet α−RuCl3
Magnetization and high-resolution x-ray diffraction measurements of the Kitaev-Heisenberg material α−RuCl3 reveal a pressure-induced crystallographic and magnetic phase transition at a hydrostatic pressure of p ∼0.2 GPa. This structural transition into a triclinic phase is characterized by a very strong dimerization of the Ru-Ru bonds, accompanied by a collapse of the magnetic susceptibility. Ab initio quantum-chemistry calculations disclose a pressure-induced enhancement of the direct 4d−4d bonding on particular Ru-Ru links, causing a sharp increase of the antiferromagnetic exchange interactions. These combined experimental and computational data show that the Kitaev spin-liquid phase in α−RuCl3 strongly competes with the crystallization of spin singlets into a valence bond solid.
Evolution of electronic and magnetic properties of Sr2IrO4 under strain
Motivated by properties-controlling potential of the strain, we investigate strain dependence of structure, electronic, and magnetic properties of Sr2IrO4 using complementary theoretical tools: ab-initio calculations, analytical approaches (rigid octahedra picture, Slater-Koster integrals), and extended 𝑡−J model. We find that strain affects both Ir-Ir distance and Ir-O-Ir angle, and the rigid octahedra picture is not relevant. Second, we find fundamentally different behavior for compressive and tensile strain. One remarkable feature is the formation of two subsets of bond- and orbital-dependent carriers, a compass-like model, under compression. This originates from the strain-induced renormalization of the Ir-O-Ir superexchange and O on-site energy. We also show that under compressive (tensile) strain, Fermi surface becomes highly dispersive (relatively flat). Already at a tensile strain of 1.5%, we observe spectral weight redistribution, with the low-energy band acquiring almost purely singlet character. These results can be directly compared with future experiments.
Influence of multiplet structure on Sr2IrO4 photoemission spectra
Most of the low-energy effective descriptions of spin-orbit driven Mott insulators consider spin-orbit coupling (SOC) as a second-order perturbation to electron-electron interactions. However, when SOC is comparable to anisotropic Hund's coupling, such as in Ir, the validity of this formally weak SOC approach is not a priori known. Depending on the relative strength of SOC and anisotropic Hund's coupling, different descriptions of the multiplet structure should be employed in the weak and strong SOC limits, viz. LS and jj coupling schemes, respectively. We investigate the implications of both the coupling schemes on the low-energy effective t−J model and calculate the angle-resolved photoemission (ARPES) spectra using self-consistent Born approximation. In particular, we obtain the ARPES spectra of quasi-two-dimensional square-lattice iridate Sr2IrO4 in both weak and strong SOC limits. The differences in the limiting cases are understood in terms of the composition and relative energy splittings of the multiplet structure. Our results indicate that the LS coupling scheme yields better agreement with the experiment, thus providing an indirect evidence for the validity of LS coupling scheme for iridates. We also discuss the implications for other metal ions with strong SOC.
Spin–orbit coupling, orbitally entangled antiferromagnetic order, and collective spin–orbital excitations in Sr2VO4
With electron filling n= 1 in the Sr2VO4 compound, the octahedrally coordinated orbitals are strongly active when the tetragonal distortion induced crystal field is tuned by external agent such as pressure. Considering the full range of crystal field induced tetragonal splitting in a realistic three-orbital model, collective spin–orbital excitations are investigated using the generalized self-consistent plus fluctuation approach. At ambient pressure, an entangled orbital + antiferromagnetic order is found to be stabilized beyond a critical value (∼ 30 meV) of spin–orbit coupling which is in the realistic range for 3d ions. The behavior of the calculated energy scales of collective excitations with crystal field is consistent with that of the transition temperatures with pressure as obtained from susceptibility and resistivity anomalies in high-pressure studies.
Journal of Physics: Condensed Matter 35 (4), 045801 (2022)
Coupled spin-charge order in frustrated itinerant triangular magnets.
We uncover four spin-charge ordered ground states in the strong coupling limit of the Kondo lattice model on triangular geometry. The results are obtained using Monte Carlo simulations, with a classical treatment of localized moments. Two of the states at one-third electronic filling (n = 1/3) consist of decorated ferromagnetic chains coupled antiferromagnetically with the neighboring chains. The third magnetic ground state is noncollinear, consisting of antiferromagnetic chains separated by a pair of canted ferromagnetic chains. An even more unusual magnetic ground state, a variant of the 120∘ Yafet-Kittel phase, is discovered at n = 2/3. These magnetic orders are stabilized by opening a gap in the electronic spectrum: a “band effect.” All the phases support modulations in the electronic charge density due to the presence of magnetically inequivalent sites. In particular, the charge ordering pattern found at n= 2/3 is observed in various triangular lattice systems, such as 2H−AgNiO2, 3R−AgNiO2, and NaxCoO2.
Doped frustrated molecular magnets
MMs with triangular and tetrahedral motifs are particularly appealing due to frustration, leading to degeneracy in the ground state. Here, using Kondo description of a doped molecular magnet, we show that introducing electrons in magnetic clusters can lead to multiple spin states due to a subtle interplay of geometrical frustration effects, electron itinerancy and Kondo coupling. The various low- and high-spin states are stable over a wide range of parameters, leading to a rich phase diagram. We are now studying the role of asymmetry and Dzyaloshinskii-Moriya interactions in stabilizing multiple spin states in frustrated magnetic clusters.
Endohedral metallofullerenes Ln2@C2n (2n >= 74):
A combined experimental and theoretical effort into a systematic investigation of the series of endohedral metallofullerenes (EMFs) Ln2@Cn2 with Ln2 = Gd2, Tb2, Dy2, Ho2, Er2, TbGd and TbY reveals not only that Tb2@C80 is an excellent single molecule magnet (SMM), but also highlights the role of structural symmetry in forming a was carried out experimentally and theoretically (using DFT, quantum chemistry and effective Hamiltonian). Our work highlights the importance of composition and anisotropy in obtaining good SMMs.
Modelling magnetodynamics
With single-molecule magnets research on the rise as a result of recent advantages in the field, like remarkable high blocking temperatures up to 60 Kelvin [Nature, 548, 439, 2017], gigantic coercivity up to 80 Tesla [Nat Commun., 10, 571, 2019], magnetization stability in the thin films, further applications are seriously in the scope. The possible venue here is to develop a theory of magnetic moment manipulation and control at the microscopic level. Theory of optimal control in quantum dynamics in complex systems is well-developed. For example, the uses of density matrix techniques have been well summarized already in the early ‘60s by Fano, Haar, and many others. Thus, in many respects, the task is to reframe that research into the language of the problem at hand, and into familiar terms for the community. Recently, it was already proven the Redfield reduced density matrix techniques are applicable for slow-relaxing single-molecule magnets [Nat Commun., 8, 14620, 2017]. In our recent contribution [PCCP,20, 11656, 2018], we have outlined the use of Lindblad dynamics in combination with a few axioms in the rationalization of the relaxation behavior of single-molecule magnets. In this report we put this approach in the context of the magentodynamics theory, showing the close connection to the Landau-Lifshitz-Gilbert model and presenting further elaboration for the proposed method.
Discovery, crystal growth, and characterization of garnet Eu2PbSb2Zn3O12
Single crystal specimens of previously unknown garnet Eu2PbSb2Zn3O12 were grown in a reactive PbO:PbF2 flux medium. The crystals were characterized by a combination of X‐ray crystallography, magnetization measurements, and the optical techniques of Raman, photoluminescence, and UV/Vis spectroscopy. The material exhibits Van Vleck paramagnetism associated with the J = 0 state of Eu3+, which was possible to accurately fit to a theoretical model. Band structure calculations were performed and compared to the experimental band gap of 1.98 eV. The crystals demonstrate photoluminescence associated with the 4f 6 configuration of the Eu3+ ions sitting at the distorted 8‐coordinate garnet A site. The title compound represents a unique quinary contribution to a relatively unexplored area of rare earth bearing garnet crystal chemistry.
Fermionic representation for the ferromagnetic Kondo lattice model: Diagrammatic study of spin-charge coupling effects on magnon excitations.
A purely fermionic representation is introduced for the ferromagnetic Kondo lattice model that allows conventional diagrammatic tools to be employed to study correlation effects. Quantum 1/S corrections to magnon excitations are investigated by using a systematic inverse-degeneracy expansion scheme that incorporates correlation effects in the form of self-energy and vertex corrections, while explicitly preserving the continuous spin-rotation symmetry. Magnon self-energy is studied in the full range of interaction strength and is shown to result in strong magnon damping and anomalous softening for zone-boundary modes, which accounts for several zone-boundary anomalies observed in recent spin-wave measurements of ferromagnetic manganites.
Magnetic excitations in iron pnictides
Spin wave dispersion and damping are investigated in the metallic SDW state of different itinerant electron models including a small interlayer hopping. Magnetic excitations in iron pnictides are shown to be well understood in terms of physical mechanisms characteristic of metallic magnets, such as carrier-induced ferromagnetic spin couplings, intra-band particle-hole excitations, and the spin-charge coupling mechanism, which is also important in ferromagnetic manganites