Using 23Na NMR measurements on sodium cobaltates at intermediate dopings (0.44<=x<=0.62), we establish the qualitative change of behavior of the local magnetic susceptibility at x*=0.63-0.65, from a low x Pauli-like regime to the high x Curie-Weiss regime. For 0.5<=x<=0.62, the presence of a maximum T* in the temperature dependence of the susceptibility shows the existence of an x-dependent energy scale. T_1 relaxation measurements establish the predominantly antiferromagnetic character of spin correlations for x<x*. This contradicts the commonly assumed uncorrelated Pauli behavior in this x range and is at odds with the observed ferromagnetic correlations for x>x*. It is suggested that at a given x the ferromagnetic correlations might dominate the antiferromagnetic ones above T*. From 59Co NMR data, it is shown that moving towards higher x away from x=0.5 results in the progressive appearance of nonmagnetic Co3+ sites, breaking the homogeneity of Co states encountered for x<=0.5. The main features of the NMR-detected 59Co quadrupolar effects, together with indications from the powder x-ray diffraction data, lead us to sketch a possible structural origin for the Co3+ sites. In light of this ensemble of new experimental observations, a new phase diagram is proposed, taking into account the systematic presence of correlations and their x-dependence.
We present the first angle-resolved photoemission (ARPES) measurement of Fermi Surface in the "misfit" cobaltate [Bi2Ba2O4].[CoO2]~2. This compound contains the same triangular Co planes as Na cobaltates, but in a different 3D environment. Our data establish the similarity of their electronic structure. We propose that the peculiar lineshape of all cobaltates is of the "peak-dip-hump" type, due to strong many-body effects. We detect a progressive transfer of spectral weight from the quasiparticle feature near Ef to a broad hump in misfit phases where Ba is replaced by Sr or Ca. This indicates …
The structure of the low-energy electronic states in layered cobaltates is considered starting from the Mott insulating limit. We argue that the coherent part of the wave-functions and the Fermi-surface topology at low doping are strongly influenced by spin-orbit coupling of the correlated electrons on the $t_{2g}$ level. An effective t-J model based on mixed spin-orbital states is radically different from that for the cuprates, and supports unconventional, pseudospin-triplet pairing.
A natural explanation for the carrier concentration-dependent electronic behavior in the layered cobaltates emerges within correlated-electron Hamiltonians with finite on-site and significant nearest neighbor hole-hole Coulomb repulsions. The nearest neighbor repulsion decreases hole double-occupancy below hole density 1/3, but increases the same at higher hole densities. Our conclusion is valid for both single-band and three-band extended Hubbard Hamiltonians, and sheds light on concentration-dependent $e_g^\prime$ hole occupancy within the latter.
The apparently counterintuitive carrier concentration-dependent electronic properties of layered cobaltates have attracted wide interest. Here we point out that very similar carrier-concentration dependence has previously been noted in strongly correlated quasi-one dimensional (quasi-1D) organic charge-transfer solids. The normal states of both families can be understood, over the entire range of carrier concentration of interest, within the extended Hubbard Hamiltonian with significant intersite Coulomb interaction. As with the charge-transfer solids, superconductivity in the cobaltates appea…
The single-layer perovskite cobaltates have attracted enormous attention due to the recent observation of hour-glass shaped magnetic excitation spectra which resemble the ones of the famous high-temperature superconducting cuprates. Here, we present an overview of our most recent studies of the spin and charge correlations in floating-zone grown cobaltate single crystals. We find that frustration and a novel kind of electronic and magnetic nano phase separation are intimately connected to the appearance of the hour-glass shaped spin excitation spectra. We also point out the difference between …
Several experimental and theoretical studies in cobaltates suggest the proximity of the system to charge ordering (CO). We show, qualitatively, in the frame of a $t-V$ model coupled to phonons that optical phonon modes at the $K$ and $M$ points of the Brillouin zone, which involves only $O$-ions displacement around a $Co$-ion, are good candidates to display anomalies due to the CO proximity. If by increasing of $H_2O$ content the system is pushed closer to CO, the mentioned phonon modes should show softening and broadening.
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…
A 59Co NMR study of CoO2, the x=0 end member of AxCoO2 (A = Na, Li...) cobaltates, reveals a metallic ground state, though with clear signs of strong electron correlations: low-energy spin fluctuations develop at wave vectors q different from 0 and a crossover to a Fermi-liquid regime occurs below a characteristic temperature T*~7 K. Despite some uncertainty over the exact cobalt oxidation state n this material, the results show that electronic correlations are revealed as x is reduced below 0.3. The data are consistent with NaxCoO2 being close to the Mott transition in the x -> 0 limit.
We present an explanation for the puzzling spectral and transport properties of layered cobaltates close to the band-insulator limit, which relies on the key effect of charge ordering. Blocking a significant fraction of the lattice sites deeply modifies the electronic structure in a way that is shown to be quantitatively consistent with photoemission experiments. It also makes the system highly sensitive to interactions (especially to intersite ones), hence accounting for the strong correlations effects observed in this regime, such as the high effective mass and quasiparticle scattering rate.…
We posit that there exist deep and fundamental relationships between the above seemingly very different materials. The carrier concentration-dependences of the electronic behavior in the conducting organic charge-transfer solids and layered cobaltates are very similar. These dependences can be explained within a single theoretical model, - the extended Hubbard Hamiltonian with significant nearest neighbor Coulomb repulsion. Interestingly, superconductivity in the cobaltates seems to be restricted to bandfilling exactly or close to one-quarter, as in the organics. We show that dynamic Jahn-Tell…
Recently the unusual metallic state with a substantially non-uniform distribution of a charge and mag\-ne\-tic density in CoO$_2$ planes was found experimentally in the Na$_x$CoO$_2$ compound with $x>0.6$. We have investigated an origin of such electron disproportionation in the lamellar sodium cobaltates by calculating the ion states as a function of a strength of the electron correlations in the $d$(Co)-shells within the GGA+U approximation for the system with a realistic crystal structure. It was found that the nonuniformity of spin and charge densities are induced by an ordering of the sod…
Previous ARPES experiments in NaxCoO2 reported both a strongly renormalized bandwidth near the Fermi level and moderately renormalized Fermi velocities, leaving it unclear whether the correlations are weak or strong and how they could be quantified. We explain why this situation occurs and solve the problem by extracting clearly the coherent and incoherent parts of the band crossing the Fermi level. We show that one can use their relative weight to estimate self-consistently the quasiparticle weight Z, which turns out to be very small Z=0.15 +/- 0.05. We suggest this method could be a reliable…
We present a numerical treatment of a spin-orbital polaron model for Na_xCoO_2 at small hole concentration (0.7 < x < 1). We demonstrate how the polarons account for the peculiar magnetic properties of this layered compound: They explain the large susceptibility; their internal degrees of freedom lead both to a negative Curie-Weiss temperature and yet to a ferromagnetic intra-layer interaction, thereby resolving a puzzling contradiction between these observations. We make specific predictions on the momentum and energy location of excitations resulting from the internal degrees of freedom of t…
Macroscopic magnetization, muon spin rotation (muSR), and NMR measurements were carried out to study magnetism in the Na1CoO2 cobaltate. Using SQUID measurements, Na1CoO2 is shown to have a bulk magnetic susceptibility much lower and flatter than that of NaxCoO2 with x=0.7-0.9. In fact, muSR yields a signal of mostly nonmagnetic origin, which is attributed to the x=1 phase. The intrinsic cobalt spin susceptibility corresponding to this x=1 phase is measured using Na NMR. It is indeed found to be almost zero, in agreement with a low-spin 3+ charge state of all cobalt atoms. This single state of…
Measurements of polarization and temperature dependent soft x-ray absorption have been performed on Na_xCoO_2 single crystals with x=0.4 and x=0.6. They show a deviation of the local trigonal symmetry of the CoO_6 octahedra, which is temperature independent in a temperature range between 25 K and 372 K. This deviation was found to be different for Co^{3+} and Co^{4+} sites. With the help of a cluster calculation we are able to interpret the Co L_{23}-edge absorption spectrum and find a doping dependent energy splitting between the t_{2g} and the e_g levels (10Dq) in Na_xCoO_2.
We present a comparative study of CoO2 layers in the Bi-misfit and NaxCoO2 cobaltates. Co NMR measures the intrinsic susceptibility of the Co layers and is not affected by spurious contributions. At low dopings where room-temperature thermopower (TEP) is large, Curie-Weiss susceptibilities are observed in both materials. But NMR and muSR experiments find neither charge nor spin order down to low temperatures in Bi-misfit, in contrast to the case of NaxCoO2. This demonstrates that metallicity, charge and magnetic orders are specific of the Na layers in NaxCoO2 whereas strong correlations are ge…
We have grown single crystals of Na$_x$Ca$_y$CoO$_2$ and determined their superstructures as a function of composition using neutron and x-ray diffraction. Inclusion of Ca$^{2+}$ stabilises a single superstructure across a wide range of temperatures and concentrations. The superstructure in the Na$^+$ layers is based on arrays of divacancy clusters with Ca$^{2+}$ ions occupying the central site, and it has an ideal concentration Na$_{4/7}$Ca$_{1/7}$CoO$_2$. Previous measurements of the thermoelectric properties on this system are discussed in light of this superstructure. Na$_{4/7}$Ca$_{1/7}$C…
We propose that unconventional superconductivity in hydrated sodium cobaltate $Na_xCoO_2$ results from an interplay of electronic correlations and electron-phonon interactions. On the basis of the $t-V$ model plus phonons we found evidences for a) unconventional superconductivity, b) realistic values of $T_c$ and c) the dome shape existing near $x \sim 0.35$. This picture is obtained for $V$ close to the critical Coulomb repulsion $V_c$ which separates the uniform Fermi liquid from $\sqrt{3} \times \sqrt{3}$ CDW ordered phase.
We study with ARPES the electronic structure of CoO2 slabs, stacked with rock-salt (RS) layers exhibiting a different (misfit) periodicity. Fermi Surfaces (FS) in phases with different doping and/or periodicities reveal the influence of the RS potential on the electronic structure. We show that these RS potentials are well ordered, even in incommensurate phases, where STM images reveal broad stripes with width as large as 80Å. The anomalous evolution of the FS area at low dopings is consistent with the localization of a fraction of the electrons. We propose that this is a new form of electroni…
Layered misfit cobaltate [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$], which emerged as an important thermoelectric material~[A. C. Masset et al. Phys. Rev. B, 62, 166 (2000)], has been explored extensively in the last decade for the exact mechanism behind its high Seebeck coefficient. Its complex crystal and electronic structures have inhibited consensus among such investigations. This situation has arisen mainly due to difficulties in accurate identification of the chemical state, spin state, and site symmetries in its two subsystems (rocksalt [Ca$_2$CoO$_3$] and triangular [CoO$_2$]). By employing res…
We observe quasi-static incommensurate magnetic peaks in neutron scattering experiments on layered cobalt oxides La2-xSrxCoO4 with high Co oxidation states that have been reported to be paramagnetic. This enables us to measure the magnetic excitations in this highly hole-doped incommensurate regime and compare our results with those found in the low-doped incommensurate regime that exhibit hourglass magnetic spectra. The hourglass shape of magnetic excitations completely disappears given a high Sr doping. Moreover, broad low-energy excitations are found, which are not centered at the incommens…
We report on a member of the spin-disordered honeycomb lattice antiferromagnet in a quasi-one-dimensional cobaltate Ba_3Co_2O_6(CO_3)_0.7. Resistivity exhibits as semimetallic along the face-sharing CoO6 chains. Magnetic susceptibility shows strongly anisotropic Ising-spin character with the easy axis along the chain due to significant spin-orbit coupling and a trigonal crystal field. Nevertheless, ^135Ba NMR detects no indication of the long-range magnetic order down to 0.48 K. Marginally itinerant electrons possess large entropy and low-lying excitations with a Wilson ratio R_W = 116, which …
The quest for Kitaev quantum spin liquids has led to great interest in honeycomb quantum magnets with strong spin-orbit coupling. It has been recently proposed that even Mott insulators with $3d$ transition metal ions, having nominally weak spin-orbit coupling, can realize such exotic physics. Motivated by this, we study the rhombohedral honeycomb cobaltates CoTiO$_3$, BaCo$_2$(PO$_4$)$_2$, and BaCo$_2$(AsO$_4$)$_2$, using $\textit{ab initio}$ density functional theory, which takes into account realistic crystal field distortions and chemical information, in conjunction with exact diagonalizat…
Effects of Fe-substitution on the crystal structure and magnetic correlations of the geometrically frustrated antiferromagnets YBaCo4-xFexO7+δ (x = 0, 0.2, 0.4, 0.5, 0.6, and 0.8) have been studied by neutron diffraction, Mössbauer spectroscopy, and ac susceptibility. The compounds YBaCo4-xFexO7+δ have a special layered-type crystal structure with an alternating Kagomé (6c site) and triangular (2a site) layers along the c axis. Fe3+ ions are found to be substituted at both the crystallographic 2a and 6c sites of Co ions. Mössbauer results show a high spin state of Fe3+ ions in a tetrahedral co…
Spin-state transitions are the hallmark of rare-earth cobaltates. In order to understand them, it is essential to identify all relevant parameters which shift the energy balance between spin states, and determine their trends. We find that Δ, the eg-t2g crystal-field splitting, increases by ~250 meV when increasing pressure to 8 GPa and by about 150 meV when cooling from 1000K to 5K. It changes, however, by less than 100 meV when La is substituted with another rare earth. Also the Hund's rule coupling J_avg is about the same in systems with very different spin-state transition temperature, lik…
NaxCoO2 has emerged as a material of exceptional scientific interest due to the potential for thermoelectric applications, and because the strong interplay between the magnetic and superconducting properties has led to close comparisons with the physics of the high-Tc cuprates. The density, x, of the sodium in the intercalation layers can be altered electrochemically, directly changing the number of conduction electrons on triangular Co layers. Recent electron diffraction measurements reveal a kaleidoscope of Na+ ion patterns as a function of concentration. Here we use single-crystal neutron d…
Puzzling properties of sodium cobaltates Na${}_x$CoO${}_2$ become an issue of numerous recent theoretical and experimental investigations because of very high thermopower and intricate phase diagram of this material. Experiments indicated Na${}_{2/3}$CoO${}_2$ to have a kagome-like cobalt sublattice, which implies additional complexity of this strongly correlated system. In the present work we have employed the ab-initio GGA+U modeling method to investigate spin states of Na${}_{2/3}$CoO${}_2$, using an exact crystal cell established in the experiment. Stability of sodium imprint for concentra…
Dominant Kitaev Interaction and Field-induced Quantum Disordered Phase in the Cobaltate Na$_2$Co$_2$TeO$_6$
Institute for Solid State Physics, University of Tokyo（东京大学固体物理研究所）；School of Physics, Southeast University（东南大学物理学院）；Anhui Key Laboratory of Low-Energy Quantum Materials and Devices, High Magnetic Field Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences（中国科学院合肥物质科学研究院强磁场科学中心）；School of Physical Science and Engineering, Beijing Jiaotong University（北京交通大学物理科学与工程学院）；Center for Quantum Materials, Seoul National University（首尔大学量子材料中心）；School of Physics, Georgia Institute of Technology（佐治亚理工学院物理学院）；Institute of Theoretical Physics, Chinese Academy of Sciences（中国科学院理论物理研究所）
We report a state-of-the-art photoemission (ARPES) study of high quality single crystals of the NaxCoO2 series focusing on the fine details of the low-energy states. The Fermi velocity is found to be small (< 0.5 eV.A) and only weakly anisotropic over the Fermi surface at all dopings setting the size of the pair wavefunction to be on the order of 10-20 nanometers. In the low doping regime the exchange inter-layer splitting vanishes and two dimensional collective instabilities such as 120-type fluctuations become kinematically allowed. Our results suggest that the unusually small Fermi velocity…
We report the structural and magnetic properties of a new class of cobaltates with the chemical formula (BaSr)4-xLa2xCo4O15 (x = 0, 0.5 and 1). These compounds crystallize in a hexagonal structure in which cobalt ions are distributed among two distinct crystallographic sites with different oxygen coordination. Three Co-O tetrahedra and one octahedron are linked by shared oxygen atoms to form Co4O15 clusters, which are packed together into a honeycomb-like network. Partial substitution of Sr and/or Ba atoms by La allows one to adjust the degree of Co valence mixing, but all compositions remain …
We study the interplay between superconductivity and antiferromagnetic spin fluctuations in electron-doped cobaltate Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O based on the kinetic energy driven superconductivity mechanism. We show that the superconducting state is governed by both charge carrier pairing and quasiparticle coherent, and displays a common dome-shaped phase diagram in agreement with experimental results. By calculating the dynamical spin structure factor, we theoretically find that the magnetic excitation shows a commensurate resonance peak, which locates at antiferromagnetic wave vector $…
Neutron scattering experiments were performed on single crystals of layered cobalt-oxides La2-xCaxCoO4 (LCCO) to characterize the charge and spin orders in a wide hole-doping range of 0.3<x<0.8. For a commensurate value of x=0.5 in (H,0,L) plane, two types of superlattice reflections concomitantly appear at low temperature; one corresponds to a checkerboard charge ordered pattern of Co2+/Co3+ ions and the other is magnetic in origin. Further, the latter magnetic-superlattice peaks show two types of symmetry in the reflections, suggesting antiferromagnetic-stacking (AF-S) and ferromagnetic-stac…
Within the charge-spin separation fermion-spin theory, we have shown that the mechanism of superconductivity in the electron doped cobaltate Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O is ascribed to its kinetic energy. The dressed fermions interact occurring directly through the kinetic energy by exchanging magnetic excitations. This interaction leads to a net attractive force between dressed fermions, then the electron Cooper pairs originating from the dressed fermion pairing state are due to the charge-spin recombination, and their condensation reveals the superconducting ground state. The superconduc…
Recent experiments on the Co-based $d^7$ honeycomb materials BaCo$_2$(AsO$_4$)$_2$ and BaCo$_2$(PO$_4$)$_2$ have drawn renewed interest to XXZ models with competing interactions beyond the nearest neighbor exchange. While the case of dominant antiferromagnetic exchange has been extensively studied, the actual materials exhibit a ferromagnetic nearest neighbor exchange. Here we show that such a sign change of the dominant nearest neighbor coupling has substantial consequences on the phase diagram. In particular, the nature of the quantum disordered phase of interest changes from a plaquette val…
In order to investigate the relationship between superconductivity and magnetism in bilayer-hydrate cobaltate Na_x(H_3O)_zCoO_2 \cdot yH_2O, Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements were performed on three different samples, which demonstrate various ground states at low temperatures. The appearance of small internal fields is observed in the NQR spectra below approximately 6 K on one of the samples that possesses the largest c-axis length and the highest NQR frequency. The other two samples exhibit superconducting transition in zero magnetic fiel…
This paper reviews the current progress on searching the Kitaev spin liquid state in 3d electron systems. Honeycomb cobaltates were recently proposed as promising candidates to realize the Kitaev spin liquid state, due to the more localized wave functions of 3d ions compared with that of 4d and 5d ions, and also the easy tunability of the exchange Hamiltonian in favor of Kitaev interaction. Several key parameters that have large impacts on the exchange constants, such as the charge-transfer gap and the trigonal crystal field, are identified and discussed. Specifically, tuning crystal field eff…
We consider possible routes to superconductivity in hydrated cobaltates Na_xCoO_2.yH_2O on the basis of the t-J-V model plus phonons on the triangular lattice. We studied the stability conditions for the homogeneous Fermi liquid (HFL) phase against different broken symmetry phases. Besides the sqrt(3)xsqrt(3)-CDW phase, triggered by the nearest-neighbour Coulomb interaction V, we have found that the HFL is unstable, at very low doping, against a bond-ordered phase due to J. We also discuss the occurrence of phase separation at low doping and V. The interplay between the electron-phonon interac…
$^{59}$Co NMR spectra in oriented powders of Na$_{0.35}$CoO$_{2}$ and in its hydrated superconducting phase (HSC) Na$_{0.35}$CoO$_{2}$,1.3H$_{2}$O reveal a single electronic Co state with identical $T$ independent NMR shift tensor. These phases differ markedly from Na$_{0.7}$CoO$_{2}$, in which we resolve 3 types of Co sites. The large T variation of their spin susceptibilities $χ^{s}$ and the anisotropy of the orbital susceptibility $χ^{orb}$ allow us to conclude that charge disproportionation occurs, in a non magnetic Co$^{3+}$ and two magnetic sites with about 0.3 and 0.7 holes in the $t_{2…
In this study, we synthesized single crystals of Na$_{x}$CoO$_{2}$ with $x\sim0.8$ using the optical floating zone technique. A thorough electrochemical treatment of the samples permitted us to control the de-intercalation of Na to obtain single crystal samples of stable Na ordered phases with $x=0.5-0.8$. Comparisons of the bulk magnetic properties with those observed in the Na ordered powder samples confirmed the high quality of these single crystal phases. The ab plane resistivity was measured for the Na ordered samples and it was quite reproducible for different sample batches. The data we…
Using elastic and inelastic neutron scattering, we investigated the evolution of the local atomic structure and lattice dynamics of La(1-x)Sr(x)CoO3 (x=0-0.5) as it crosses over with x from an insulator to a ferromagnetic metal (FMM). Our pair density function analysis indicates that in the paramagnetic insulating (PMI) phase for all x, spin activation of Co3+ ions induces local static Jahn-Teller (JT) distortions. The size of the JT lattice increases almost linearly with x. However, in the FMM phase, static JT distortions are absent for x <= 30 %. This coincides with narrowing of hw=22 and 24…
Engineered $Na_{0.75}CoO_2$ is considered a prime candidate to achieve high-efficiency thermoelectric systems to regenerate electricity from waste heat. In this work, three elements with outmost electronic configurations, (1) an open d shell (Ni), (2) a closed d shell (Zn), and (3) a half filled f shell (Eu) with maximum unpaired electrons, were selected to outline the dopants' effects on electronic and crystallographic structures of $Na_{0.75}CoO_2$. Systematic $ab$ $initio$ density functional calculations with $DMOL^3$ package showed that the Ni and Zn were more stable when substituting Co w…
To shed new light on the mechanism of superconductivity in sodium cobalt oxide bilayer-hydrate (BLH), we perform a density functional calculation with full structure optimization for BLH and its related nonsuperconducting phase, monolayer hydrate (MLH). We find that these hydrates have similar band structures, but a notable difference can be seen in the $a_{1g}$ band around the Fermi level. While its dispersion in the $z$ direction is negligibly small for BLH, it is of the order of 0.1 eV for MLH. This result implies that the three dimensional feature of the $a_{1g}$ band may be the origin for…
Co nuclear-quadrupole-resonance (NQR) measurements were performed on various bilayered hydrate cobaltate Na_x(H_3O)_zCoO_2\cdot yH_2O with different values of the superconducting and magnetic-ordering temperatures, T_c and T_M, respectively. From measurements of the temperature and sample dependence of the NQR frequency, it was revealed that the NQR frequency is changed by the change of the electric field gradient (EFG) along the c axis ν_{zz} rather than the asymmetry of EFG within the ab-plane. In addition, it is considered that the change of ν_{zz} is gaverned mainly by the trigonal distort…
In the quest for materials hosting Kitaev spin liquids, much of the efforts have been focused on the fourth- and fifth-row transition metal compounds, which are spin-orbit coupling assisted Mott insulators. Here, we study the structural and magnetic properties of 3$d$ transition metal oxides, Na$_2$Co$_2$TeO$_6$ and Na$_3$Co$_2$SbO$_6$. The partial occupancy of sodium in former compound is addressed using a cluster expansion, and a honeycomb lattice of sodiums is found to be energetically favored. Starting from the \textit{ab initio} band structures, a many-body second order perturbation theor…
We have synthesized and characterized different stable phases of sodium cobaltates Na$_{x}$CoO$_{2}$ with sodium content $0.65<x<0.80$. We demonstrate that $^{23}$Na NMR allows to determine the difference in the susceptibility of the phases and reveals the presence of Na order in each phase. $^{59}$Co NMR experiments give clear evidence that Co charge disproportionation is a dominant feature of Na cobaltates. Only a small fraction ($\approx$ 25%) of cobalts are in a non-magnetic Co$^{3+}$ charge state whereas electrons delocalize on the other cobalts. The magnetic and charge properties of the …
We present a combined experimental and theoretical x-ray absorption spectroscopy (XAS) study of the new class of cobaltates YBaCo4O7 and YBaCo3MO7 (M= Al, Fe). The focus is on the local electronic and magnetic properties of the transition metal ions in these geometrically frustrated kagome compounds. For the mixed valence cobaltate YBaCo4O7, both the Co2+ and Co3+ are found to be in the high spin state. The stability of these high spin states in tetrahedral coordination is compared with those in the more studied case of octahedral coordination. For the new compound YBaCo3FeO7, we find exclusiv…
We systematically analyze the influence of the superconducting gap symmetry and the electronic structure on the dynamical spin susceptibility in superconducting Na_xCoO_2$\cdot y$H_2O within a three different models: the single a_{1g}-band model with nearest-neighbor hoppings, the realistic three-band t_{2g}-model with, and without e'_g pockets present at the Fermi surface. We show that the magnetic response in the normal state is dominated by the incommensurate antiferromagnetic spin density wave fluctuations at large momenta in agreement with experimental temperature dependence of the spin-l…
The surprise discovery of superconductivity below 5K in sodium cobalt oxides when hydrated with water has caught the attention of experimentalists and theorists alike. Most explanations for its occurence have focused heavily on the properties of some small elliptically shaped pockets predicted to be the electronically dominant Fermi surface sheet, but direct attempts to look for them have instead cast serious doubts over their existence. Here we present evidence that these pockets do indeed exist, based on bulk measurements of the electron momentum distribution in unhydrated and hydrated sodiu…
We study the exchange interactions and resulting magnetic phases in the honeycomb cobaltates. For a broad range of trigonal crystal fields acting on Co2+ ions, the low-energy pseudospin-1/2 Hamiltonian is dominated by bond-dependent Ising couplings that constitute the Kitaev model. The non-Kitaev terms nearly vanish at small values of trigonal field Δ, resulting in spin liquid ground state. Considering Na3Co2SbO6 as an example, we find that this compound is proximate to a Kitaev spin liquid phase, and can be driven into it by slightly reducing Δby \sim 20 meV, e.g., via strain or pressure cont…
An hour-glass shaped magnetic excitation spectrum appears to be an universal characteristic of the high-temperature superconducting cuprates. Fluctuating charge stripes or alternative band structure approaches are able to explain the origin of these spectra. Recently, an hour- glass spectrum has been observed in an insulating cobaltate, thus, favouring the charge stripe scenario. Here we show that neither charge stripes nor band structure effects are responsible for the hour-glass dispersion in a cobaltate within the checkerboard charge ordered regime of La2-xSrxCoO4. The search for charge str…
[Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$], a two dimensional misfit metallic compound, is famous for its rich phases accessed by temperature, $i.e.$ high temperature spin-state transition, metal-insulator transition (MIT) at intermediate temperature ($\sim$ 100 K) and low temperature spin density wave (SDW). It enters into SDW phase below T$_{MIT}$ which becomes long range at 27 K. Information on the independent role of misfit layers (rocksalt/Ca$_2$CoO$_3$ \& triangular/CoO$_2$) in these phases is scarce. By combining a set of complementary macroscopic (DC magnetization and resistivity) and microscop…
Single crystals of sodium cobaltates Na$_{x}$CoO$_{2}$ with $x \approx 0.8$ were grown by the floating zone technique. Using electrochemical Na de-intercalation method we reduced the sodium content in the as-grown crystals down to pure phase with 22 K Néel temperature and $x \approx 0.77$. The $^{59}$Co NMR study in the paramagnetic state of the $T_{N}=22$ K phase permitted us to evidence that at least 6 Co sites are differentiated. They could be separated by their magnetic behaviour into three types: a single site with cobalt close to non-magnetic Co$^{3+}$, two sites with the most magnetic c…
We have performed Co-nuclear quadrupole resonance (NQR) studies on Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O compounds with different Na ($x$) and hydrate ($y$) contents. Two samples with different Na contents but nearly the same $T_c$ values ($x$ = 0.348, $T_c$ = 4.7 K ; $x$ = 0.339, $T_c$ = 4.6 K) were investigated. The spin-lattice relaxation rate $1/T_1$ in the superconducting (SC) and normal states is almost the same for the two samples except just above $T_c$. NQR measurements were also performed on different-hydrate-content samples with different $T_c$ values, which were prepared from the same N…
We present X-ray spectroscopic evidence for the evolution of valence-specific spin states and tetragonal distortions in single-layer cobaltates. Measurements of Co $L_3$-edge resonant inelastic X-ray scattering reveal the $t_{2g}$ electronic structure of Co for hole-doped La$_{2-x}$Sr$_x$CoO$_4$ ($x$ = 0.5, 0.7 and 0.8). As the Sr-doping $x$ increases, the tetragonal splitting of the $t_{2g}$ states of high-spin Co$^{2+}$ decreases, whereas that of low-spin Co$^{3+}$ increases and the fraction of high-spin Co$^{3+}$ increases. The results enable us to clarify the origin of the change of magnet…
