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.     

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.     

Spin-singlet clusters in the ladder compound NaV2O5

The space group of alpha(')-NaV2O5 turns below T(c) = 34 K from Pmmn with all V sites equivalent, into Fmm2 with three independent vanadium sites per layer. This is incompatible with models of charge ordering into V4+ and V5+. Our structure determination indicates that the phase transition consists of a charge ordering with three distinct valence states, formally V4+, V4.5+, and V5+. The singlet formation is not associated with dimerization on the spin ladder, but with the formation of spin clusters. Finally, we ascribe the quadrupling of the c axis to the large polarizability of the V2O5 skeleton.     

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.     

Charge and orbital order in Fe3O4

Charge and orbital ordering in the low-temperature monoclinic structure of magnetite (Fe3O4) is investigated using the local spin density approximation with Coulomb interaction correction method. While the difference between t(2g) minority occupancies of Fe(2+)(B) and Fe(3+)(B) cations is large and gives direct evidence for charge ordering, the screening is so effective that the total 3d charge disproportion is rather small. The charge order has a pronounced [001] modulation, which is incompatible with the Anderson criterion. The orbital order agrees with the Kugel-Khomskii theory.     

Unconventional anisotropic superexchange in alpha'-NaV2O5

The strong line broadening observed in electron spin resonance on NaV2O5 is found to originate from an unusual type of the symmetric anisotropic exchange interaction with simultaneous spin-orbit coupling on both sites. The microscopically derived anisotropic exchange constant is almost 2 orders of magnitude larger than the one obtained from conventional estimations. Based on this result we systematically evaluate the anisotropy of the ESR linewidth in terms of the symmetric anisotropic exchange only, and we find microscopic evidence for precursor effects of the charge ordering already below 150 K.     

Ferromagnetic Lieb-Mattis theorem

We prove ferromagnetic ordering of energy levels for XXX Heisenberg chains with spins of arbitrary magnitude, thus extending our previous result for the spin-1/2 chain. Ferromagnetic ordering means that the minimum energies in the invariant subspaces of fixed total spin are monotone decreasing as a function of the total spin. This result provides a ferromagnetic analogue of the well-known theorem by Lieb and Mattis about ordering of energy levels in antiferromagnetic and ferrimagnetic systems on bipartite graphs.     

Spin, charge, and orbital orderings in iron-based superconductors

In this article, we briefly review spin, charge, and orbital orderings in iron-based superconductors, as well as the multi-orbital models. The interplay of spin, charge, and orbital orderings is a key to understand the high temperature superconductivity. As an illustration, we use the two-orbital model to show the spin and charge orderings in iron-based superconductors based on the mean-field approximation in real space. The typical spin and charge orderings are shown by choosing appropriate parameters, which are in good agreement with experiments. We also show the effect of Fe vacancies, which can introduce the nematic phase and interesting magnetic ground states. The orbital ordering is also discussed in iron-based superconductors. It is found that disorder may play a role to produce the superconductivity.     

Zero temperature phase diagram of the ferromagnetic Kondo lattice

We study numerically the one-dimensional ferromagnetic Kondo lattice, a model widely used to describe nickel and manganese perovskites. Due to the competition between double and super-exchange, we find a region where the formation of magnetic islands induces a charge-ordered state. This ordering is present even in the absence of any inter-site Coulomb repulsion and presents an insulating gap associated to the charge structure. We study the metal–insulator transition induced by a magnetic field which removes simultaneously both charge and spin orderings. This new mechanism should be taken into account in theories of charge ordering involving spin degrees of freedom.     

La0.3Ca0.7Mn1－xVxO3体系的有序相和再入型自旋玻璃行为研究

通过对La_0.3Ca_0.7Mn_1－xV_xO₃(x=0.05,0.10,0.134,0.20)体系的M-T曲线、ρ-T曲线、ESR谱的测量,研究了Mn位掺V对La_0.3Ca_0.7MnO₃体系电荷序和自旋序的影响.结果表明,当0.05≤x≤0.134时, 体系存在电荷有序(CO)相,其自旋序随温度降低发生顺磁(PM)-电荷有序(CO)-反铁磁(AFM)变化.当x＝0.20时,CO相逐步融化,在40 K发生自旋玻璃转变,表现出再入型的自旋玻璃行为,低温下的基态存在着多种复杂的磁相互作用之间的竞争机理. 关键词： 电荷有序 自旋序 相分离 再入型自旋玻璃行为     

Conductivity and specific heat anomalies at the low temperature transition in the stoichiometric YFe2O4

Anomalies were found in electrical conductivity and specific heat at the two-step transition in the stoichiometric YFe₂O₄. It is concluded that the transition is a new type of Verwey transition: i.e., ordering of ionic charge, Fe²⁺ and Fe³⁺, accompanied by lattice distortion and antiferromagnetic spin ordering.     

Evidence for charge glasslike behavior in lightly doped La2-xSrxCuO4 at low temperatures

A c-axis magnetotransport and resistance noise study in La_(1.97)Sr_(0.03)CuO_(4) reveals clear signatures of glassiness, such as hysteresis, memory, and slow, correlated dynamics, but only at temperatures (T) well below the spin glass transition temperature T_(sg). The results strongly suggest the emergence of charge glassiness, or dynamic charge ordering, as a result of Coulomb interactions.     

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.     

Spin frustration,charge ordering,and enhanced antiferromagnetism in TMTTF2SbF6

We theoretically investigate the effects of charge order and spin frustration on the spin ordering in TMTTF salts. Using first-principles band calculations, we find that a diagonal inter-chain transfer integral t_q1, which causes spin frustration between the inter-chain dimers in the dimer-Mott insulating state, strongly depends on the choice of anion. Within the numerical Lanczos exact diagonalization method, we show that the ferroelectric charge order changes the role of t_q1 from the spin frustration to the enhancement of the two-dimensionality in spin sector. The results indicate that t_q1 assists the cooperative behavior between charge order and antiferromagnetic state observed in TMTTF₂SbF₆.     

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 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.     

Phase diagram for stripes in YBa2Cu3O6+x superconductors

Neutron scattering has been used to measure the charge and spin structure in the YBa₂Cu₃O_6+x superconductors. Incommensurate static charge ordering is found at low doping levels while only charge fluctuations are found at higher doping. The spin structure is complex with both a commensurate resonance and incommensurate structure observed at low temperatures. The scattering results are used to construct a phase diagram for stripes in the YBa₂Cu₃O_6+x system.     

Infrared conductivity from spin and charge density waves: (TMTSF) 2X

Infrared conductivity from an incommensurate spin density wave occurs due to even-order charge density wave harmonics which interact with the host lattice. Phonon states within the density-wave-induced energy gap for single-particle excitations lead to conductivity much different from that of an incommensurate charge density wave including counter-ion ordering. The conductivity expected for relaxed and quenched states of (TMTSF)₂ClO₄ is discussed.     

Spin current through a tunnel junction

We derive an expression for the spin current through a tunnel barrier in terms of many-body Green’s functions. The spin current has two possible contributions. One is associated with angular momentum transfer due to spin-polarized charge current crossing the junction. If there are magnetic moments on both sides of the tunnel junction, due to spin accumulation or ferromagnetic ordering, then there is a second contribution related to the exchange coupling between the moments.