NMR study of the magnetic and metal-insulator transitions in Na0.5CoO2: a nesting scenario

Co and Na NMR are used to probe the local susceptibility and charge state of the two Co sites of the Na-ordered orthorhombic Na(0.5)CoO(2). Above T(N) = 86 K, both sites display a similar T dependence of the spin shift, suggesting that there is no charge segregation into Co(3+) and Co(4+) sites. Below T(N), the magnetic long range commensurate order found is only slightly affected by the metal-insulator transition at T(MIT) = 51 K. Furthermore, the electric field gradient at the Co site does not change at these transitions, indicating the absence of charge ordering. All these observations can be explained by successive spin-density wave induced by two nestings of the Fermi surface specific to the x = 0.5 Na ordering.     

Low to high spin-state transition induced by charge ordering in antiferromagnetic YBaCo2O5

The oxygen-deficient double perovskite YBaCo2O5, containing corner-linked CoO5 square pyramids as principal building units, undergoes a paramagnetic to antiferromagnetic spin ordering at 330 K. This is accompanied by a tetragonal to orthorhombic distortion. Below 220 K orbital ordering and long-range Co(2+)/Co(3+) charge ordering occur as well as a change in the Co2+ spin state from low to high spin. This transition is shown to be very sensitive to the oxygen content of the sample. To our knowledge this is the first observation of a spin-state transition induced by long-range orbital and charge ordering.     

Tailor-made electronic and magnetic properties in one-dimensional pure and Y-substituted Ca3Co2O6

Full-potential density-functional calculations show that the electronic structure of one-dimensional ferrimagnetic Ca3Co2O6 varies from metal to half metal to insulator as its magnetic ordering changes from the ferrimagnetic through the ferromagnetic to the paramagnetic state. The present Letter is the first to establish the occurrence of half metallicity in one-dimensional oxides. Moreover, the electronic and magnetic properties of this material can be tuned by substitution of Y for Ca, as shown by our detailed study on Ca(3-x)YxCo2O6 (x = 0, 0.3, 0.75, and 1). The Co ions are in two different valence states [Co4+ (low-spin) and Co2+ (high-spin)], and hence the occurrence of charge ordering in addition to spin ordering is established. For specific Y concentrations we predict a rarely seen combination of ferromagnetic and insulating behavior.     

Charge ordering and magnetopolarons in Na0.82CoO2

Using spectral ellipsometry, we measured the dielectric function of a Na(0.82(2))CoO2 crystal that exhibits bulk antiferromagnetism with T(N)=19.8 K. We identify two prominent transitions as a function of temperature. The first one at 280 K involves marked changes of the electronic and lattice responses that are indicative of charge ordering in the CoO2 layers. The second transition occurs around T(N)=19.8 K and reveals sizable spin-charge coupling. The data are discussed in terms of charge ordering and formation of magnetopolarons due to a charge-induced spin-state transition of adjacent Co3+ ions.     

Charge ordering in charge-compensated Na0.41CoO2 by oxonium ions

Charge ordering behavior is observed in the crystal prepared through the immersion of the Na_0.41CoO₂ crystal in distilled water. Discovery of the charge ordering in the crystal with Na content less than 0.5 indicates that the immersion in water brings about the reduction of the Na_0.41CoO₂. The formal valence of Co changes from +3.59 estimated from the Na content to +3.50, the same as that in Na_0.5CoO₂. The charge compensation is confirmed to arise from the intercalation of the oxonium ions as occurred in the superconducting sodium cobalt oxide bilayer-hydrate [K. Takada, et al. J. Mater. Chem. 14 (2004) 1448]. The charge ordering is the same as that observed in Na_0.5CoO₂. It suggests that the Co valence of +3.50 is necessary for the charge ordering.     

Disproportionation, metal-insulator transition, and critical interaction strength in Na(1/2)CoO2

Charge disproportionation (CD) and spin differentiation in Na(1/2)CoO2 are studied using the correlated band local-density approximation + Hubbard U (LDA+U) approach. The simultaneous CD and gap opening seen previously is followed in detail through a first-order charge disproportionation transition 2Co(3.5+)-->Co3++Co4+. Disproportionation in the Co a(g) orbital results in half of the ions (Co3+) becoming electronically and magnetically dead, transforming the quarter-filled a(g) system into a half-filled subsystem that subsequently undergoes the observed charge ordering or metal-insulator transition. Comparing with data in the x approximately 0.3 regime suggests the system has moved into the multiband regime where the effective Coulomb repulsion U-->U(eff)=U/sqrt[3] strongly lessens correlation effects.     

Characterization of the charge ordering state by maximum entropy method

In this work we introduce the use of a combined Rietveld refinement and Maximum Entropy Method (MEM), as a helpful new tool to gain insight into the charge ordering pattern present in CO-transition metal compounds. In particular we show the study made on two CO oxyborates A₂OBO₃ (A = Mn and Fe): (a) the Mn-oxyborate, that exhibits a strong charge lattice coupling, and whose charge ordering pattern had been previously satisfactory established from BVS analysis of single crystal X-ray diffraction data and that we have used as a test to confirm the validity of the new method; (b) the weakly charge-lattice coupled Fe-oxyborate, a compound not suitable for the BVS approach and for which nevertheless this method gives evidence for a new charge ordering pattern.     

X-ray photoelectron studies of changes in the electronic structure upon phase transitions in Co–Ni and Co–Fe alloys

The changes in the electronic structure of Co–Ni and Co–Fe systems upon phase transitions are studied. X-ray photoelectron study of the valence-band spectra and the parameters of the multiplet splitting of Co, Ni and Fe 3s spectra is carried out at different temperatures. It is established that the ordering–separation phase transition in Co–Ni alloys takes place in the temperature range of 600–700°C. As opposed to Co–Ni alloys, in the Fe–Co alloy, ordering–separation–ordering phase transitions are observed. High-temperature ordering of the Fe₅₀Co₅₀ alloy is observed above 1200°C. The transition from ordering to separation is shown to lead to changes in the d electron spectra of the valence band and in the parameters of the multiplet splitting of the 3s spectra.     

Charge and orbital ordering in Na0.5CoO2

The ground state of Na_0.5CoO₂ has been calculated using the full potential local orbital method and local density approximation plus Hubbard U (); the results demonstrate that charge and orbital ordering evidently exist in the present system in association with the antiferromagnetic state. Notable structural features observed between 300 and 30 K have been carefully examined using in situ TEM investigations, a superstructure with a wave vector of Q1=a^∗/2, becoming commonly visible below , can be interpreted as the charge/orbital ordering on the Co1 and Co2 sites. Moreover, we have also observed another notable superstructure with Q2=a^∗/4 below the phase transition of , which suggests a more complicated orbital ordered state existing at lower temperatures.     

Charged vortex structure in high-temperature superconductors

By using a model Hamiltonian with competing antiferromagnetic (AFM) spin density wave (SDW) and d-wave superconductivity orders, the effect of next-nearest-neighbour (nnn) hopping on spin and charge structures in high-temperature superconductors is investigated at finite temperatures. For an optimally doped sample, we find that the AFM order magnitude in the vortex core is firstly enhanced and then suppressed, accompanied with a ``positively → negatively → positively" charged vortex structure transition with increasing nnn hopping strength, which implies that the AFM order is unnecessarily bounded to an electron-rich vortex core. In addition, a charge ordering pattern with four negatively charged peaks localized in a small region is also found around the vortex core centre without net charge. Recent scanning-tunneling-microscopy experimental observations of the checkerboard structure are hopefully understood.     

Electronic texture of the thermoelectric oxide Na0.75CoO2

From 59Co and 23Na NMR, we demonstrate the impact of the Na+ vacancy ordering on the cobalt electronic states in Na0.75CoO2: at long time scales, there is neither a disproportionation into 75% Co3+ and 25% Co4+ states, nor a mixed-valence metal with a uniform Co3.25+ state. Instead, the system adopts an intermediate configuration in which 30% of the lattice sites form an ordered pattern of localized Co3+ states. Above 180 K, an anomalous mobility of specific Na+ sites is found to coexist with this electronic texture, suggesting that the formation of the latter may contribute to stabilizing the Na+ ordering. Control of the ion doping in these materials thus appears to be crucial for fine-tuning of their thermoelectric properties.     

Raman scattering study of anomalous spin, charge, and lattice dynamics in the charge-ordered phase of Bi1-xCaxMnO3 (x > 0.5)

We report an inelastic light scattering study of the effects of charge ordering on the spin, charge, and lattice dynamics of Bi1-xCaxMnO3 (x>0.5). We find that charge ordering results in anomalous phonon behavior, such as the appearance of "activated" modes. More significantly, however, the transition to the charge-ordered phase results in the appearance of a quasielastic scattering response with the symmetry of the spin-chirality operator ( T(1g)); this scattering response is thus indicative of magnetic or chiral spin fluctuations in the antiferromagnetic charge-ordered phase.     

59Co NMR evidence for charge ordering below T_(CO) approximately 51 K in Na0.5CoO2

The CoO2 layers in NaxCoO2 may be viewed as a spin S=1/2 triangular-lattice doped with charge carriers. The underlying physics of the cobaltates is very similar to that of the high T_(c) cuprates. We will present unequivocal 59Co NMR evidence that below T_(CO) approximately 51 K, the insulating ground state of the itinerant antiferromagnet Na0.5CoO2 (T_(N) approximately 86 K) is induced by charge ordering.     

Charge ordering and coexistence of charge fluctuations in quasi-two-dimensional organic conductors theta-(BEDT-TTF)2X

We present a scenario for the peculiar coexistence of charge fluctuations observed in quasi-2D 1/4-filled organic conductors theta-(BEDT-TTF)2X in the quantum critical regime where the charge ordering is suppressed down to zero temperature. The scenario is explored in the extended Hubbard model including electron-phonon couplings on an anisotropic triangular lattice. We find that the coexisting fluctuations emerge from two different instabilities, the "Wigner crystallization on a lattice" driven by the off-site Coulomb repulsion and the charge-density-wave formation due to the nesting of the Fermi surface, not from phase competition or real-space inhomogeneity. This mechanism explains the contrastive temperature dependence of two fluctuations in experiments.     

Direct evidence of the charge ordered phase transition of indium nanowires on Si111

A controversial issue of the driving force for the phase transition of the one-dimensional (1D) metallic In wires on Si(111) is studied by low-temperature scanning tunneling microscopy and spectroscopy. The energy gap opening and the longitudinal charge ordering through charge transfer at the Fermi level are unambiguously observed. The vacancy defects induce a local charge ordering decoupled from a lattice distortion above T(c), and pin the phase of charge order below T(c). All these results below and above T(c) including the detailed features such as local fluctuations strongly support the 1D charge-density-wave mechanism for the phase transition.     

Charge and spin structure in of YBa(2)Cu(3)O(6.35)

Neutron scattering has been used to measure the charge and spin structure in the highly underdoped superconductor YBa(2)Cu(3)O(6.35). Incommensurate static charge ordering is found that remains at high temperatures. The magnetic pattern is complex with a resonance and incommensurate structure observed at low temperatures. The results clarify the role of striped phases in YBa(2)Cu(3)O(6+x) superconductors.     

The magnetic behavior of Co atoms on the surface and in the interior of the noble metals Au,Ag and Cu

The magnetism of Co atoms in the interior and on the surface of the noble metals has been studied by means of the anomalous Hall effect. On the surface of all three noble metals Co possesses a magnetic moment even at lowest studied coverages of 0.02 atomic layers. Ferromagnetic ordering appears at Co thicknesses of about one atomic layer. Co as a bulk impurity is non-magnetic in Cu whereas it is magnetic in Ag.     

Experimental evidence of coupling interaction between charge ordering and spin ordering

In order to experimentally probe into the complicated interaction between charge ordering and spin ordering in manganites,two sets of samples Nd0.5Sr0.5Mn1-xGaxO3(NSMGO) and Nd 0.5Sr0.5Mn1-y CryO3(NSMCO)(0.0 x,y 0.075),have been studied by means of electrical transport,magnetization,electron spin resonance and transmission electron microscopy analysis.By comparing the influence of Cr-doping and Ga-doping in the Nd0.5Sr0.5MnO3(NSMO) system,large difference between the evolution of charge ordering temperature T co in the Cr-doping and the Ga-doping cases is found.In the NSMCO system,the CE-type antiferromagnetic(AFM)/charge ordering(CO) phase disappears with only 2.5 percent Cr doping;but in the NSMGO system,the CE-type AFM/CO phase always exists.This phenomenon indicates that the charge ordering formation is dominated by the spin ordering.As a result,it is experimentally proved that there is strong coupling interaction between charge ordering and spin ordering in NSMO system.     

Correlation-driven charge order at the interface between a Mott and a band insulator

To study digital Mott insulator LaTiO3 and band insulator SrTiO3 interfaces, we apply correlated band theory within the local density approximation including a Hubbard U to (n, m) multilayers, 1相似文献   

Wigner crystallization in (DI-DCNQI)2Ag detected by synchrotron radiation X-Ray diffraction

The low-temperature electronic structure of the quarter-filled, quasi-one-dimensional (Q1D) system (DI-DCNQI)2Ag is revealed using synchrotron radiation x-ray diffraction. In spite of the interchain frustration in the twofold superstructure along the 1D chain, the body-centered tetragonal "charge ordering" structure, which consists of 4k_{F} charge ordering columns and 4k_{F} bond order wave columns, is realized. This is the first example of the Q1D system having plural kinds of columns as its ground state. This charge ordered structure is regarded as a Wigner crystal caused by intercolumn Coulomb repulsion.