Neutron scattering study of novel magnetic order in Na0.5CoO2

We report polarized and unpolarized neutron scattering measurements of the magnetic order in single crystals of Na0.5CoO2. Our data indicate that below TN=88 K the spins form a novel antiferromagnetic pattern within the CoO2 planes, consisting of alternating rows of ordered and nonordered Co ions. The domains of magnetic order appear to be closely coupled to the domains of Na ion order, consistent with such a twofold symmetric spin arrangement. Magnetoresistance and anisotropic susceptibility measurements further support this model for the electronic ground state.     

Infrared spectroscopy of the charge ordering transition of Na0.5CoO2

We report infrared reflectivity study of charge ordering in a Na0.5CoO2 single crystal. In comparison with x=0.7 and 0.85 compounds, we found that the effective carrier density increases systematically with decreasing Na contents. The charge ordering transition only affects the optical spectra below 1000 cm(-1). A hump near 800 cm(-1) develops below 100 K, which is accompanied by the appearance of new lattice modes as well as the strong antiresonance feature of phonon spectra. These observations signify a polaronic characteristic of charge carriers. Below T(co), an optical gap develops at the magnitude of 2Delta approximately 3.5k(B)T(co) (T相似文献   

Charge and spin ordering in insulating Na0.5CoO2: effects of correlation and symmetry

Ab initio band theory including correlations due to intra-atomic repulsion is applied to study charge disproportionation and charge and spin ordering in insulating Na0.5CoO2. Various ordering patterns (zigzag and two striped) for four-Co supercells are analyzed before focusing on the observed "out-of-phase stripe" pattern of antiferromagnetic Co4+ spins along charge-ordered stripes. This pattern relieves frustration and shows distinct analogies with the cuprate layers: a bipartite lattice of antialigned spins, with axes at 90degrees angles. Substantial distinctions with cuprates are also discussed, including the tiny gap of a new variant of "charge-transfer" type within the Co 3d system.     

Fermi Surface Topology of Na0.5CoO2 from the Hybrid Density Functional

The Fermi surface topology of Na0.5CoO2 is studied using the hybrid density functional theory. We first study a single （CoO2）^0.5- layer model with the percentage of the nonlocal Hartree-Fock exchange changing from 0% to 20%. The results show that only when the mixed nonlocal Hartree-Fock exchange is between 1% and 5%, the Fermi surface topology is similar to the experimental one. With 3% HF exchange in the hybrid density functional, considering the effects of Na ions in the Na0.5CoO2 system, we find that the Fermi surface is split to double holes and small gaps open near the intersections between the Brillouin zone and the Fermi surface. Our results show that both the amounts of the nonlocal Hartree-Fock exchange in the hybrid density functional and the Na ions have much influence on the Fermi surface topology.     

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.     

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.     

Mössbauer study of thermal magnetic transitions in Cu0.5Zn0.5Fe2O4

The dependence of the Mössbauer spectra of the ferrite Cu_0.5Zn_0.5Fe₂O₄ on the temperature is investigated. The profiles of the hyperfine magnetic fields distributionP(H) are obtained from the spectra by a decomposition method. The results are interpreted by assuming a variation of the magnetic correlation radius in the region of the thermal magnetic transition.     

Anomalous quasiparticle renormalization in Na0.73CoO2: role of interorbital interactions and magnetic correlations

We report an angular resolved photoemission study of NaxCoO2 with x approximately 0.73 where it is found that the renormalization of the quasiparticle (QP) dispersion changes dramatically upon a rotation from GammaM to GammaK. The comparison of the experimental data to the calculated band structure reveals that the quasiparticle renormalization is most pronounced along the GammaK direction, while it is significantly weaker along the GammaM direction. We discuss the observed anisotropy in terms of multiorbital effects and point out the relevance of magnetic correlations for the band structure of NaxCoO2 with x approximately 0.75.     

Structure and magnetic properties of colossal magnetoresistance compound Tb0.5Sr0.5CoO3

The structure and the magnetic properties of the doped rare earth cobaltite systems are of recent interest owing to the CMR phenomenon that occur in them. In this paper, we investigate the structure and magnetic properties of Tb_0.5Sr_0.5CoO₃ solid solution, for the first time, using neutron powder diffraction technique. The sample Tb_0.5Sr_0.5CoO₃ is found to crystallize in orthorhombic (Pbnm) symmetry. The unit cell volume and Co—O bond length reduce with temperature. The calculatede _g bandwidth obtained from structural parameters turns out to be 0.989 eV. Low temperature neutron diffraction profiles exhibit a magnetic contribution to the fundamental Bragg peaks indicating a ferromagnetic ordering belowT _c. The results are compared with Co—O—Co bond angles and Co—O bond length of La_0.5Sr_0.5CoO₃, highlighting the ionic size effects on substitution of Tb ion for La in the compound.     

Shubnikov-de Haas oscillations and the magnetic-field-induced suppression of the charge ordered state in Na(0.5)CoO2

We have performed electrical transport measurements at low temperatures and high magnetic fields in Na(0.5)CoO2 single crystals. Shubnikov-de Haas oscillations corresponding to only 1% of the area of the orthorhombic Brillouin zone were clearly observed, indicating that most of the original Fermi surface vanishes at the charge-ordering (CO) transition. In-plane magnetic fields were found to suppress strongly the CO state. For fields rotated within the conducting planes, we observe angular magnetoresistance oscillations whose periodicity changes from twofold to sixfold at the transition.     

Quasiparticle dispersion and heat capacity of Na0.3CoO2: a dynamical mean-field theory study

We use the dynamical mean-field theory to calculate the Fermi surface and heat capacity for Na0.3CoO2. We resolve the conflicting outcomes of previous calculations by demonstrating that the nature of the calculated Fermi surface depends sensitively upon the bare Hamiltonian, and, in particular, the crystal-field splitting. By calculating both the Fermi surface and the heat capacity, we show that the only conclusion consistent with angle-resolved photoemission and heat capacity measurements is that the e'g pockets are not present at the Fermi surface.     

Low-temperature electronic and magnetic transitions in the antiferromagnetic semiconductor Cr0.5Mn0.5S

Experimental-theoretical studies were carried out of the electrical and magnetic properties of the antiferromagnetic semiconductor Cr_0.5Mn_0.5S in the temperature range 4.2–300 K. A magnetic antiferromagnetic-ferrimagnetic phase transition was observed along with a semimetalsemiconductor electronic transition. Monte Carlo calculations indicate that the changes in the type of magnetic ordering and conductivity are due to the cooperative Jahn-Teller effect caused by the Cr₂₊ ions. Fiz. Tverd. Tela (St. Petersburg) 41, 1660–1664 (September 1999)     

23Na NMR study of the local order in the Na1/2Bi1/2TiO3 structure in a weak magnetic field

The orientation dependences of the second-order quadrupole shifts of the central component in the ²³Na NMR spectrum were studied in the temperature range 293–760 K. The profile of the spectral distribution is calculated using various models of the Na_1/2Bi_1/2TiO₃ structure. The calculations agree with the experimental data for the monoclinic structure of a polar cluster with two Na displacement components: a displacement along the [111] _p direction and a small displacement statistically or dynamically disordered over six equally probable [100] _p -type directions. Tetragonal-phase nuclei and monoclinic clusters with a very small displacement component along the [111] _p direction are found to coexist and have close energies over the temperature range 580–610 K. The results obtained provide new information concerning the character of the diffuse phase transition at 610 K.     

Fermi surface evolution and Luttinger theorem in Na(x)CoO2: a systematic photoemission study

We report a systematic angle-resolved photoemission study on Na(x)CoO2 for a wide range of Na concentrations (0.3 < or = x < or = 0.72). In all the metallic samples at different x, we observed (i) only a single holelike Fermi surface centered around gamma and (ii) its area changes with x according to the Luttinger theorem. We also observed a surface state that exhibits a larger Fermi surface area. The e'(g) band and the associated small Fermi surface pockets near the K points predicted by band calculations are found to sink below the Fermi energy in a manner almost independent of the doping and temperature.     

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.     

Statistics of resonances and delay times: a criterion for metal-insulator transitions

We study the distributions of the normalized resonance widths P(Gamma;) and delay times P(tau;) for 3D disordered tight-binding systems at the metal-insulator transition (MIT) by attaching leads to the boundary sites. Both distributions are scale invariant, independent of the microscopic details of the random potential and the number of channels. Theoretical considerations suggest the existence of a scaling theory for P(Gamma;) in finite samples, and numerical calculations confirm this hypothesis. Based on this, we give a new criterion for the determination and analysis of the MIT.     

31P NMR study of magnetic phase transitions of MnP single crystal under 2 GPa pressure

Superconductivity on the border of the long-range magnetic order has been discovered in MnP under high pressures. In order to investigate the nature of the magnetic properties adjacent to the superconducting state, we performed zero-field 31P NMR for MnP single crystal under ambient and hydrostatic pressure of 2 GPa, respectively. Radio frequency power level was used to determine whether NMR signal originates from a helical state or not. When 2 GPa pressure was applied, the signal from helical state exists even above 160 K, while that from the ferromagnetic phase was not observed. Our NMR results indicate that the magnetic phase which is adjacent to the superconducting state is in a helical magnetic structure.     

Magnetic ordering and spin waves in Na0.82CoO2

NaxCoO2, the parent compound of the recently synthesized superconductor Na(x)CoO(2):yH(2)O, exhibits bulk antiferromagnetic order below approximately 20 K for 0.75相似文献