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G. Khaliullin

Publications and source records attributed to G. Khaliullin.

At least 37 records · Page 2Linked to original sources
arXiv2012

Giant Magnon Gap in Bilayer Iridate Sr3Ir2O7: Enhanced Pseudo-dipolar Interactions Near the Mott Transition

Using resonant inelastic x-ray scattering, we observe in the bilayer iridate Sr3Ir2O7, a spin-orbit 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 single-layer iridate Sr2IrO4. Analyzing the origin of these observations, we find that the giant magnon gap results from bond-directional pseudo-dipolar interactions that are strongly enhanced near the metal-insulator transition boundary. This suggests that novel magnetism, such as that inspired by the Kitaev model built on the pseudo-dipolar interactions, may emerge in small charge-gap iridates.

cond-mat.str-el↗
arXiv2012

Dimensionality Driven Spin-Flop Transition in Layered Iridates

Using resonant x-ray diffraction, we observe an easy c-axis collinear antiferromagnetic structure for the bilayer Sr$_3$Ir$_2$O$_7$, a significant contrast to the single layer Sr$_2$IrO$_4$ with in-plane canted moments. Based on a microscopic model Hamiltonian, we show that the observed spin-flop transition as a function of number of IrO$_2$ layers is due to strong competition among intra- and inter-layer bond-directional pseudo-dipolar interactions of the spin-orbit entangled $J_{eff}$=1/2 moments. With this we unravel the origin of anisotropic exchange interactions in a Mott insulator in the strong spin-orbit coupling regime, which holds the key to the various types of unconventional magnetism proposed in 5$d$ transition metal oxides.

cond-mat.str-el↗
arXiv2011

Effects of charge inhomogeneities on elementary excitations in La_{2-x}Sr_xCuO_4

Purely local experimental probes of many copper oxide superconductors show that their electronic states are inhomogeneous in real space. For example, scanning tunneling spectroscopic (STS) imaging shows strong variations in real space, and according to nuclear quadrupole resonance (NQR) studies the charge distribution in the bulk varies on the nanoscale. However, the analysis of the experimental results utilizing spatially-averaged probes often ignores this fact. We have performed a detailed investigation of the doping-dependence of the energy and line width and position of the zone-boundary Cu-O bond-stretching vibration in La_{2-x}Sr_xCuO_4 by inelastic neutron scattering. Both our new results as well as previously reported angle-dependent momentum widths of the electronic spectral function detected by angle-resolved photoemission can be reproduced by including the same distribution of local environments extracted from the NQR analysis.

cond-mat.supr-con↗
arXiv2011

Isospin Dynamics in Sr2IrO4 : Forging Links to Cuprate Superconductivity

We used resonant inelastic x-ray scattering to reveal the nature of magnetic interactions in Sr2IrO4, a 5d transition-metal oxide with a spin-orbit entangled ground state and Jeff=1/2 magnetic momemts, referred to as 'isospins'. The magnon dispersion in Sr2IrO4 is well described by an antiferromagnetic Heisenberg model with isospin one-half moments on a square lattice, which renders the low-energy effective physics of Sr2IrO4 much akin to that in superconducting cuprates. This is further supported by the observation of exciton modes in Sr2IrO4 whose dispersion is strongly renormalized by magnons, which can be understood by analogy to the hole propagation in the background of antiferromagnetically ordered spins in the cuprates.

cond-mat.str-el↗
arXiv2011

Intense paramagnon excitations in a large family of high-temperature superconductors

In the search for the mechanism of high-temperature superconductivity, intense research has been focused on the evolution of the spin excitation spectrum upon doping from the antiferromagnetic insulating to the superconducting states of the cuprates. Because of technical limitations, the experimental investigation of doped cuprates has been largely focused on low-energy excitations in a small range of momentum space. Here we use resonant inelastic x-ray scattering to show that a large family of superconductors, encompassing underdoped YBa$_2$Cu$_4$O$_8$ and overdoped YBa$_2$Cu$_3$O$_{7}$, exhibits damped spin excitations (paramagnons) with dispersions and spectral weights closely similar to those of magnons in undoped cuprates. %The results are in excellent agreement with the spin excitations obtained by exact diagonalization of the $\bf t-J$ Hamiltonian on finite-sized clusters. The comprehensive experimental description of this surprisingly simple spectrum permits quantitative tests of magnetic Cooper pairing models. A numerical solution of the Eliashberg equations for the magnetic spectrum of YBa$_2$Cu$_3$O$_{7}$ reproduces its superconducting transition temperature within a factor of two, a level of agreement comparable to Eliashberg theories of conventional superconductors.

cond-mat.supr-con↗
arXiv2009

Theory of Raman and Resonant Inelastic X-ray Scattering from Collective Orbital Excitations in YTiO$_3$

We present two different theories for Raman scattering and Resonant Inelastic X-ray Scattering (RIXS) in the low temperature ferromagnetic phase of YTiO$_3$ and compare this to the available experimental data. For description of the orbital ground-state and orbital excitations, we consider two models corresponding to two theoretical limits: one where the $t_{2g}$ orbitals are degenerate, and the other where strong lattice distortions split them. In the former model the orbitals interact through superexchange. The resulting superexchange Hamiltonian yields an orbitally ordered ground state with collective orbital excitations on top of it - the orbitons. In the orbital-lattice model, on the other hand, distortions lead to local dd-transitions between crystal field levels. Correspondingly, the orbital response functions that determine Raman and RIXS lineshapes and intensities are of cooperative or single-ion character. We find that the superexchange model yields theoretical Raman and RIXS spectra that fit very well to the experimental data.

cond-mat.str-el↗
arXiv2009

Momentum dependence of orbital excitations in Mott-insulating titanates

High-resolution resonant inelastic x-ray scattering has been used to determine the momentum dependence of orbital excitations in Mott-insulating LaTiO$_3$ and YTiO$_3$ over a wide range of the Brillouin zone. The data are compared to calculations in the framework of lattice-driven and superexchange-driven orbital ordering models. A superexchange model in which the experimentally observed modes are attributed to two-orbiton excitations yields the best description of the data.

cond-mat.str-el↗
arXiv2009

Low-energy Mott-Hubbard excitations in LaMnO_3 probed by optical ellipsometry

We present a comprehensive ellipsometric study of an untwinned, nearly stoichiometric LaMnO_3 crystal in the spectral range 1.2-6.0 eV at temperatures 20 K < T < 300 K. The complex dielectric response along the b and c axes of the Pbnm orthorhombic unit cell, ε^b(ν) and ε^c(ν), is highly anisotropic over the spectral range covered in the experiment. The difference between ε^b(ν) and ε^c(ν) increases with decreasing temperature, and the gradual evolution observed in the paramagnetic state is strongly enhanced by the onset of A-type antiferromagnetic long-range order at T_N = 139.6 K. In addition to the temperature changes in the lowest-energy gap excitation at 2 eV, there are opposite changes observed at higher energy at 4 - 5 eV, appearing on a broad-band background due to the strongly dipole-allowed O 2p -- Mn 3d transition around the charge-transfer energy 4.7 eV. Based on the observation of a pronounced spectral-weight transfer between low- and high-energy features upon magnetic ordering, they are assigned to high-spin and low-spin intersite d^4d^4 - d^3d^5 transitions by Mn electrons. The anisotropy of the lowest-energy optical band and the spectral weight shifts induced by antiferromagnetic spin correlations are quantitatively described by an effective spin-orbital superexchange model. An analysis of the multiplet structure of the intersite transitions by Mn e_g electrons allowed us to estimate the effective intra-atomic Coulomb interaction, the Hund exchange coupling, and the Jahn-Teller splitting energy between e_g orbitals in LaMnO_3. This study identifies the lowest-energy optical transition at 2 eV as an intersite d-d transition, whose energy is substantially reduced compared to that obtained from the bare intra-atomic Coulomb interaction.

cond-mat.str-el↗
arXiv2009

Turning a nickelate Fermi surface into a cuprate-like one through heterostructuring

Using the local density approximation and its combination with dynamical mean-field theory, we show that electronic correlations induce a single-sheet, cuprate-like Fermi surface for hole-doped 1/1 LaNiO3 /LaAlO3 heterostructures, even though both eg orbitals contribute to it. The Ni 3d 3z^2-1 orbital plays the role of the axial Cu 4s-like orbital in the cuprates. These two results indicate that "orbital engineering" by means of heterostructuring should be possible. As we also find strong antiferromagnetic correlations, the low-energy electronic and spin excitations in nickelate heterostructures resemble those of high-temperature cuprate superconductors.

cond-mat.str-el↗
arXiv2008

Mott Insulators in the Strong Spin-Orbit Coupling Limit: From Heisenberg to a Quantum Compass and Kitaev Models

We study the magnetic interactions in Mott-Hubbard systems with partially filled $t_{2g}$-levels and with strong spin-orbit coupling. The latter entangles the spin and orbital spaces, and leads to a rich variety of the low energy Hamiltonians that extrapolate from the Heisenberg to a quantum compass model depending on the lattice geometry. This gives way to "engineer" in such Mott insulators an exactly solvable spin model by Kitaev relevant for quantum computation. We, finally, explain "weak" ferromagnetism, with an anomalously large ferromagnetic moment, in Sr$_2$IrO$_4$.

cond-mat.str-el↗
arXiv2008

Spin, Orbital and Charge Order at the Interface between Correlated Oxides

The collective behavior of correlated electrons in the VO$_2-$interface layer of LaVO$_3$/SrTiO$_3$ heterostructure is studied within a quarter-filled $t_{2g}$-orbital Hubbard model on a square lattice. We argue that the ground state is ferromagnetic driven by the double exchange mechanism, and is orbitally and charge ordered due to a confined geometry and electron correlations. The orbital and charge density waves open gaps on the entire Fermi surfaces of all orbitals. The theory explains the observed insulating behavior of the $p$-type interface between LaVO$_3$ and SrTiO$_3$.

cond-mat.str-el↗
arXiv2008

Origin of spatial variation of pairing gap in Bi-based high-Tc cuprates

Recently, scanning tunneling microscopy on Bi-2212 cuprate superconductor has revealed a spatial variation of the energy gap that is directly correlated with a modulation of the apical oxygen position. We identify two mechanisms by which out-of-plane oxygens can modulate the pairing interaction within the CuO_2 layer: a covalency between the x^2-y^2 band and apical p-orbital, and a screening of correlation U by apical oxygen polarization. Both effects strongly depend on the apical oxygen position and their cooperative action explains the experiment.

cond-mat.supr-con↗
arXiv2008

Unusual electron correlations in NaxCoO2 due to the spin-state quasidegeneracy of cobalt ions

Recent studies exposed many remarkable properties of layered cobaltates NaxCoO2. Surprisingly, many-body effects have been found to increase at sodium-rich compositions of NaxCoO2 where one expects a simple, nearly free motion of the dilute S=1/2 holes doped into a band insulator NaCoO2. Here we discuss the origin of enigmatic correlations that turn a doped NaCoO2 into a strongly correlated electronic system. A minimal model including orbital degeneracy is proposed and its predictions are discussed. The model is based on a key property of cobalt oxides - the spin-state quasidegeneracy of CoO6 octahedral complex - which has been known, e.g., in the context of an unusual physics of LaCoO3 compound. Another important ingredient of the model is the 90-degree Co-O-Co bonding in NaxCoO2 which allows nearest-neighbor $t_{2g}-e_g$ hopping. This hopping introduces a dynamical mixture of electronic configurations $t_{2g}^6, S=0$ and $t_{2g}^5e_g^1, S=1$ of neighboring cobalt ions. We show that scattering of charge carriers on spin-state fluctuations suppresses their coherent motion and leads to the spin-polaron physics at $x\sim 1$. At larger doping when coherent fermionic bands are formed, the model predicts singlet superconductivity of extended s-wave symmetry. The presence of low-lying spin states of Co$^{3+}$ is essential for the pairing mechanism. Implications of the model for magnetic orderings are also discussed.

cond-mat.str-el↗
arXiv2008

The origin of strong correlations and superconductivity in Na$_x$CoO$_2$

We propose a minimal model resolving a puzzle of enigmatic correlations observed in sodium-rich Na$_x$CoO$_2$ where one expects a simple, free motion of the dilute $S=1/2$ holes doped into a band insulator NaCoO$_2$. The model also predicts singlet superconductivity at experimentally observed compositions. The model is based on a key property of cobalt oxides -- the spin-state quasidegeneracy of CoO$_6$ octahedral complex -- leading to an unusual physics of, {\it e.g.}, LaCoO$_3$. We show that correlated hopping between $t_{2g}$ and $e_g$ states leads to the spin-polaron physics at $x\sim 1$, and to an extended s-wave pairing at larger doping when coherent fermionic bands are formed.

cond-mat.str-el↗
arXiv2007

Doping induced magnetic clusters and Co spin state transition in Na_xCoO_2 with 0.78<x<.97

By muon spin rotation we investigated the magnetic properties of a series of highly Na doped NaxCoO2 single crystals with 0.78(1) 0.04 until they yield a bulk magnetic state near x=0.78. Evidence for a strong (likely geometrical) frustration of the magnetic order is obtained from the anomalous doping dependence of the spin fluctuation rate (above the ordering temperature) which is strongly enhanced at x=0.78 as compared to x=0.97.

cond-mat.supr-con↗
arXiv2007

Spin polaron theory for the photoemission spectra of layered cobaltates

Recently, strong reduction of the quasiparticle peaks and pronounced incoherent structures have been observed in the photoemission spectra of layered cobaltates. Surprisingly, these many-body effects are found to increase near the band insulator regime. We explain these unexpected observations in terms of a novel spin-polaron model for CoO_2 planes which is based on a fact of the spin-state quasidegeneracy of Co^{3+} ions in oxides. Scattering of the photoholes on spin-state fluctuations suppresses their coherent motion. The observed ``peak-dip-hump'' type lineshapes are well reproduced by the theory.

cond-mat.str-el↗
arXiv2007

Cu NQR Wipeout Effect versus Charge Pseudogap in Zn/Ni Doped NdBa$_{2}$Cu$_{3}$O$_{6+y}$

We report $^{63,65}$Cu NQR measurements on slightly underdoped NdBa$_{2}$Cu$_{3}$O$_{6+y}$ single crystals heavily doped by Ni and Zn impurities. Owing to the impurity doping superconductivity is fully suppressed in both cases. The Ni strongly enhances magnetic correlations and induces a wipeout of the NQR signal comparable to that found in stripe ordered lanthanum cuprates. In contrast, the magnetism is suppressed in the Zn doped sample where no wipeout effect is observed and the nuclear spin relaxation rate is reduced. Our findings are in a striking correspondence with the different impact of Ni and Zn impurities on the charge pseudogap evidenced by recent optical data, uncovering thereby a close relationship between the magnetic correlations and pseudogap phenomena.

cond-mat.supr-con↗