Publications (133)607.39 Total impact
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ABSTRACT: We study the effects of electron doping in Mott insulators containing d^4 ions such as Ru4+, Os4+, Rh5+, and Ir5+ with J=0 singlet ground state. Depending on the strength of the spinorbit coupling, the undoped systems are either nonmagnetic or host an unusual, excitonic magnetism arising from a condensation of the excited J=1 triplet states of t_2g^4. We find that the interaction between Jexcitons and doped carriers strongly supports ferromagnetism, converting both the nonmagnetic and antiferromagnetic phases of the parent insulator into a ferromagnetic metal, and further to a nonmagnetic metal. Close to the ferromagnetic phase, the lowenergy spin response is dominated by intense paramagnon excitations that may act as mediators of a triplet pairing.  [Show abstract] [Hide abstract]
ABSTRACT: Magnetism in transitionmetal compounds (TMCs) has traditionally been associated with spin degrees of freedom, because the orbital magnetic moments are typically largely quenched. On the other hand, magnetic order in 4f and 5delectron systems arises from spin and orbital moments that are rigidly tied together by the large intraatomic spinorbit coupling (SOC). Using inelastic neutron scattering on the archetypal 4delectron Mott insulator Ca$_2$RuO$_4$, we report a novel form of excitonic magnetism in the intermediatestrength regime of the SOC. The magnetic order is characterized by ``soft'' magnetic moments with large amplitude fluctuations manifested by an intense, lowenergy excitonic mode analogous to the Higgs mode in particle physics. This mode heralds a proximate quantum critical point separating the soft magnetic order driven by the superexchange interaction from a quantumparamagnetic state driven by the SOC. We further show that this quantum critical point can be tuned by lattice distortions, and hence may be accessible in epitaxial thinfilm structures. The unconventional spinorbitallattice dynamics in Ca$_2$RuO$_4$ identifies the SOC as a novel source of quantum criticality in TMCs.  [Show abstract] [Hide abstract]
ABSTRACT: Raman scattering experiments on stoichiometric, Mottinsulating LaTiO$_3$ over a wide range of excitation energies reveal a broad electronic continuum which is featureless in the paramagnetic state, but develops a gap of $\sim 800$ cm$^{1}$ upon cooling below the N\'eel temperature $T_N = 146$ K. In the antiferromagnetic state, the spectral weight below the gap is transferred to welldefined spectral features due to spin and orbital excitations. Lowenergy phonons exhibit pronounced Fano anomalies indicative of strong interaction with the electron system for $T > T_N$, but become sharp and symmetric for $T < T_N$. The electronic continuum and the marked renormalization of the phonon lifetime by the onset of magnetic order are highly unusual for Mott insulators and indicate liquidlike correlations between spins and orbitals. 
Article: Direct Evidence for Dominant Bonddirectional Interactions in a Honeycomb Lattice Iridate Na2IrO3
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ABSTRACT: Heisenberg interactions are ubiquitous in magnetic materials and have been prevailing in modeling and designing quantum magnets. Bonddirectional interactions offer a novel alternative to Heisenberg exchange and provide the building blocks of the Kitaev model, which has a quantum spin liquid (QSL) as its exact ground state. Honeycomb iridates, A2IrO3 (A=Na,Li), offer potential realizations of the Kitaev model, and their reported magnetic behaviors may be interpreted within the Kitaev framework. However, the extent of their relevance to the Kitaev model remains unclear, as evidence for bonddirectional interactions remains indirect or conjectural. Here, we present direct evidence for dominant bonddirectional interactions in antiferromagnetic Na2IrO3 and show that they lead to strong magnetic frustration. Diffuse magnetic xray scattering reveals broken spinrotational symmetry even above Neel temperature, with the three spin components exhibiting nanoscale correlations along distinct crystallographic directions. This spinspace and realspace entanglement directly manifests the bonddirectional interactions, provides the missing link to Kitaev physics in honeycomb iridates, and establishes a new design strategy toward frustrated magnetism.  [Show abstract] [Hide abstract]
ABSTRACT: We have explored the hidden symmetries of a generic fourparameter nearestneighbor spin model, allowed in honeycomb lattice compounds under trigonal compression. Our method utilizes a systematic algorithm to identify all dual transformations of the model that map the Hamiltonian on itself, changing the parameters and providing exact links between different points in its parameter space. We have found the complete set of points of hidden SU(2) symmetry at which seemingly highly anisotropic model can be mapped back on the Heisenberg model and inherits therefore its properties such as the presence of gapless Goldstone modes. The procedure used to search for the hidden symmetries is quite general and may be extended to other bondanisotropic spin models and other lattices, such as the triangular, kagome, hyperhoneycomb, or harmonichoneycomb lattices. We apply our findings to the honeycomb lattice iridates Na2IrO3 and Li2IrO3, and illustrate how they help to identify plausible values of the model parameters that are compatible with the available experimental data.  [Show abstract] [Hide abstract]
ABSTRACT: We show, how in principle, a coherent coupling between two superconductors of opposite parity can be realised in a threelayer oxide heterostructure. Due to strong intraionic spinorbit coupling in the middle layer, singlet Cooper pairs are converted into triplet ones and vice versa. The result is a large enhancement of the triplet order parameter, as well as a Josephson current between the even and odd parity superconductors that persist well beyond the native triplet critical temperature.  [Show abstract] [Hide abstract]
ABSTRACT: We report a neutron scattering study of the magnetic order and dynamics of the bilayer perovskite Sr$_3$Fe$_2$O$_7$, which exhibits a temperaturedriven metalinsulator transition at 340 K. We show that the Fe$^{4+}$ moments adopt incommensurate spiral order below $T_\text{N}=115$ K and provide a comprehensive description of the corresponding spin wave excitations. The observed magnetic order and excitation spectra can be well understood in terms of an effective spin Hamiltonian with interactions ranging up to third nearestneighbor pairs. The results indicate that the helical magnetism in Sr$_3$Fe$_2$O$_7$ results from competition between ferromagnetic doubleexchange and antiferromagnetic superexchange interactions whose strengths become comparable near the metalinsulator transition. They thus confirm a decadesold theoretical prediction and provide a firm experimental basis for models of magnetic correlations in strongly correlated metals.  [Show abstract] [Hide abstract]
ABSTRACT: In condensed matter systems, out of a large number of interacting degrees of freedom emerge weakly coupled quasiparticles (QPs), in terms of which most physical properties are described. The lack of identification of such QPs is a major barrier for understanding myriad exotic properties of correlated electrons, such as unconventional superconductivity and nonFermi liquid behaviours. Here we report the observation of a composite particle in a quasitwodimensional spin1/2 antiferromagnet Sr2IrO4an exciton dressed with magnonsthat propagates with the canonical characteristics of a QP: a finite QP residue and a lifetime longer than the hopping time scale. The dynamics of this chargeneutral excitation mirrors the fundamental process of the analogous onehole propagation in the background of spins1/2, and reveals the same intrinsic dynamics that is obscured for a single, chargedhole doped into twodimensional cuprates.  [Show abstract] [Hide abstract]
ABSTRACT: We study the magnetic order and excitations in strong spinorbit coupled, Van Vlecktype, $d^4$ Mott insulators on a square lattice. Extending the previous work, we include the tetragonal crystal field splitting and explore its effects on magnetic phase diagram and magnon spectra. Two different ordered phases, with inplane and outofplane orientation of the staggered moments, are found for the higher and lower values of the crystal field splitting, respectively. The magnetic excitation spectra for paramagnetic and magnetically ordered phases are calculated and discussed in the context of a candidate spinorbit $d^4$ Mott insulator Ca$_2$RuO$_4$.  [Show abstract] [Hide abstract]
ABSTRACT: We report a highfield electron spin resonance study in the subTHz frequency domain of a single crystal of ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ that has been recently proposed as a prototypical spinorbital Mott insulator. In the antiferromagnetically ordered state with noncollinear spin structure that occurs in this material at ${T}_{\mathrm{N}}$ $$\approx${}$ 240 K we observe both the ``low'' frequency mode due to the precession of weak ferromagnetic moments arising from a spin canting, and the ``high'' frequency modes due to the precession of the antiferromagnetic (AFM) sublattices. Surprisingly, the energy gap for the AFM excitations appears to be very small, amounting to only 0.83 meV. This suggests a rather isotropic Heisenberg dynamics of interacting Ir${}^{4+}$ effective spins despite the spinorbital entanglement in the ground state.  [Show abstract] [Hide abstract]
ABSTRACT: We study the quantum manybody instabilities of the $t J_{\mathrm{K}}  J_{\mathrm{H}}$ KitaevHeisenberg Hamiltonian on the honeycomb lattice as a minimal model for a doped spinorbit Mott insulator. This spin$1/2$ model is believed to describe the magnetic properties of the layered transitionmetal oxide Na$_2$IrO$_3$. We determine the groundstate of the system with finite chargecarrier density from the functional renormalization group (fRG) for correlated fermionic systems. To this end, we derive fRG flowequations adapted to the lack of full spinrotational invariance in the fermionic interactions, here represented by the highly frustrated and anisotropic Kitaev exchange term. Additionally employing a set of Ward identities for the KitaevHeisenberg model, the numerical solution of the flow equations suggests a rich phase diagram emerging upon doping charge carriers into the groundstate manifold ($\mathbb{Z}_2$ quantum spin liquids and magnetically ordered phases). We corroborate superconducting triplet $p$wave instabilities driven by ferromagnetic exchange and various singlet pairing phases. For filling $\delta > 1/4$, the $p$wave pairing gives rise to a topological state with protected Majorana edgemodes. For antiferromagnetic Kitaev and ferromagnetic Heisenberg exchange we obtain bondorder instabilities at van Hove filling supported by nesting and densityofstates enhancement, yielding dimerization patterns of the electronic degrees of freedom on the honeycomb lattice. Further, our flow equations are applicable to a wider class of model Hamiltonians.  [Show abstract] [Hide abstract]
ABSTRACT: We report a highfield electron spin resonance study in the subTHz frequency domain of a single crystal of Sr$_2$IrO$_4$ that has been recently proposed as a prototypical spinorbital Mott insulator. In the antiferromagnetically (AFM) ordered state with noncollinear spin structure that occurs in this material at $T_{\rm N} \approx 240$ K we observe both the "low" frequency mode due to the precession of weak ferromagnetic moments arising from a spin canting, and the "high" frequency modes due to the precession of the AFM sublattices. Surprisingly, the energy gap for the outofplane AFM excitations appears to be very small, amounting to 0.83 meV only. This suggests a rather isotropic Heisenberg dynamics of interacting Ir$^{4+}$ effective spins despite the spinorbital entanglement in the ground state.  [Show abstract] [Hide abstract]
ABSTRACT: In Mott insulators with the t_{2g}^{4} electronic configuration such as of Re^{3+}, Ru^{4+}, Os^{4+}, and Ir^{5+} ions, spinorbit coupling dictates a Van Vlecktype nonmagnetic ground state with an angular momentum J=0, and the magnetic response is governed by gapped singlettriplet excitations. We derive the exchange interactions between these excitons and study their collective behavior on different lattices. In perovskites, a conventional Bose condensation of excitons into a magnetic state is found, while an unexpected onedimensional behavior supporting spinliquid states emerges in honeycomb lattices, due to the bond directional nature of exciton interactions in the case of 90° dpd bonding geometry.  [Show abstract] [Hide abstract]
ABSTRACT: We have investigated the excitation spectra of $j_{eff}$=$\frac{1}{2}$ Mott insulator Na$_2$IrO$_3$. Taking into account a relativistic multiplet structure of Ir ions, we have calculated the optical conductivity $\sigma(\omega)$ and resonant inelastic xray scattering (RIXS) spectra, which manifest different features from those of a canonical $j_{eff}$=$\frac{1}{2}$ system Sr$_2$IrO$_4$.Distinctly from the twopeak structure in Sr$_2$IrO$_4$, $\sigma(\omega)$ in Na$_2$IrO$_3$ has a broad single peak dominated by interband transitions from $j_{eff}$=$\frac{3}{2}$ to $\frac{1}{2}$. RIXS spectra exhibit the spinorbit (SO) exciton that has a twopeak structure arising from the crystalfield effect, and the magnon peak at energies much lower than in Sr$_2$IrO$_4$. In addition, a small peak near the optical absorption edge is found in RIXS spectra, originating from the coupling between the electronhole ($e$$h$) excitation and the SO exciton. Our findings corroborate the validity of the relativistic electronic structure and importance of both itinerant and local features in Na$_2$IrO$_3$.  [Show abstract] [Hide abstract]
ABSTRACT: We explore the phase diagram of spinorbit Mott insulators on a honeycomb lattice, within the KitaevHeisenberg model extended to its full parameter space. Zigzagtype magnetic order is found to occupy a large part of the phase diagram of the model, and its physical origin is explained as due to interorbital t_{2g}e_{g} hopping. The magnetic susceptibility, spin wave spectra, and zigzag order parameter are calculated and compared to the experimental data, obtaining thereby the spin coupling constants in Na_{2}IrO_{3} and Li_{2}IrO_{3}.  [Show abstract] [Hide abstract]
ABSTRACT: Based on the microscopic model including spinorbit coupling, onsite Coulomb and Hund's interactions, as well as crystal field effects, we investigate the magnetic and optical properties of Sr_{2}IrO_{4}. Taking into account all intermediate state multiplets generated by virtual hoppings of electrons, we calculate the isotropic, pseudodipolar, and DzyaloshinskyMoriya coupling constants, which describe the experiment quite well. The optical conductivity σ(ω) evaluated by the exact diagonalization method shows two peaks at ∼0.5 and ∼1.0 eV in agreement with experiment. The twopeak structure of σ(ω) arises from the unusual Fanotype overlap between the electronhole continuum of the J_{eff}=1/2 band and the intrasite spinorbit exciton observed recently in Sr_{2}IrO_{4}.  [Show abstract] [Hide abstract]
ABSTRACT: Using resonant inelastic xray scattering, we observe in the bilayer iridate Sr_{3}Ir_{2}O_{7}, a spinorbit coupling driven magnetic insulator with a small charge gap, a magnon gap of ≈92 meV for both acoustic and optical branches. This exceptionally large magnon gap exceeds the total magnon bandwidth of ≈70 meV and implies a marked departure from the Heisenberg model, in stark contrast to the case of the singlelayer iridate Sr_{2}IrO_{4}. Analyzing the origin of these observations, we find that the giant magnon gap results from bonddirectional pseudodipolar interactions that are strongly enhanced near the metalinsulator transition boundary. This suggests that novel magnetism, such as that inspired by the Kitaev model built on the pseudodipolar interactions, may emerge in small chargegap iridates.  [Show abstract] [Hide abstract]
ABSTRACT: We explore the phase diagram of spinorbit Mott insulators on a honeycomb lattice, within the KitaevHeisenberg model extended to its full parameter space. Zigzagtype magnetic order is found to occupy a large part of the phase diagram of the model, and its physical origin is explained as due to interorbital t2geg hopping. Magnetic susceptibility and spin wave spectra are calculated and compared to the experimental data, obtaining thereby the spin coupling constants in Na2IrO3 and Li2IrO3.  [Show abstract] [Hide abstract]
ABSTRACT: We propose a minimal model describing magnetic behavior of Febased superconductors. The key ingredient of the model is a dynamical mixing of quasidegenerate spin states of Fe2+ ion by intersite electron hoppings, resulting in an effective local spin Seff. The moments Seff tend to form singlet pairs, and may condense into a spin nematic phase due to the emergent biquadratic exchange couplings. The longrange ordered part m of Seff varies widely, 0<=m<=Seff, but magnon spectra are universal and scale with Seff, resolving the puzzle of large but fluctuating Femoments. Unusual temperature dependences of a local moment and spin susceptibility are also explained.  [Show abstract] [Hide abstract]
ABSTRACT: Using resonant xray diffraction, we observe an easy caxis collinear antiferromagnetic structure for the bilayer Sr3Ir2O7, a significant contrast to the single layer Sr2IrO4 with inplane canted moments. Based on a microscopic model Hamiltonian, we show that the observed spinflop transition as a function of number of IrO2 layers is due to strong competition among intra and interlayer bonddirectional pseudodipolar interactions of the spinorbit entangled J(eff)=1/2 moments. With this we unravel the origin of anisotropic exchange interactions in a Mott insulator in the strong spinorbit coupling regime, which holds the key to the various types of unconventional magnetism proposed in 5d transition metal oxides.
Publication Stats
4k  Citations  
607.39  Total Impact Points  
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Institutions

19962015

Max Planck Institute for Solid State Research
Stuttgart, BadenWürttemberg, Germany


19942007

Russian Academy of Sciences
Moskva, Moscow, Russia


2005

Universiteit Twente
 Department of Computational Materials Science (CMS)
Enschede, Overijssel, Netherlands


1999

University of Groningen
 Materials Science Group
Groningen, Groningen, Netherlands


1997

Max Planck Institute of Physics
München, Bavaria, Germany


1995

Max Planck Institute for Dynamics of Complex Technical Systems
Magdeburg, SaxonyAnhalt, Germany
