The crystal and magnetic structure of LaTiO3 has been studied by x-ray and neutron diffraction techniques using nearly stoichiometric samples. We find a strong structural anomaly near the antiferromagnetic ordering, TN =146 K. In addition, the octahedra in LaTiO3 exhibit an intrinsic distortion which implies a splitting of the t2g-levels. Our results indicate that LaTiO3 should be considered as a Jahn-Teller system where the structural distortion and the resulting level splitting are enhanced by the magnetic ordering.LaTiO 3 has been studied already in the seventies and was thought to be a text book example of a Mottinsulator with antiferromagnetic order [1]. Ti is in its three-valent state with a single electron in the t 2g -orbitals of the 3d-shell. The titanate is hence an electron analog to the cuprates with a single hole in the 3d-shell. However, the t 2g -orbitals in the LaTiO 3 are less Jahn-Teller active and, therefore, the orbital moment may not be fully quenched in the titanate. The physics of the orbital degree of freedom has recently reattracted attention to this material [2,3].The ordered moment in LaTiO 3 amounts to 0.46 µ B which is much smaller than the value of 1µ B expected for a single electron with quenched orbital moment [4]. Quantum fluctuations can explain only about 15% reduction in the 3D-case. A straight-forward explanation could be given in terms of spin-orbit coupling, as an unquenched orbital moment would align antiparallel to the spin-moment in the titanate. However, in a recent neutron scattering experiment the magnon spin gap was observed at 3.3 meV, and it was argued that the strong interaction of an orbital moment with the crystal lattice implies a much larger value for the spin gap [2]. An orbital contribution to the ordered moment in LaTiO 3 was hence excluded. On the basis of standard theories, however, even the G-type antiferromagnetic ordering in LaTiO 3 may not be explained without a spinorbit coupling. Instead one expects ferromagnetism [5,6] related with the orbital degeneracy. Under the assumption of a specific structural distortion, Moshizuki and Imada recently presented a successful model for the antiferromagnetic order in LaTiO 3 [7]. However, there is no experimental evidence for such a distortion. The puzzling magnetic properties of LaTiO 3 led Khaliullin and Maekawa to suggest a novel theoretical description for RETiO 3 based on the idea of an orbital liquid. They were able to explain many of the magnetic characteristics of LaTiO 3 [3], but the presumed orbital fluctuations have not been observed [8]. Therefore, magnetism in LaTiO 3 still remains an open issue.We have reanalyzed the crystal and magnetic structure of LaTiO 3 by x-ray and by neutron diffraction samples with almost perfect stoichiometry. First, we find a clear structural anomaly at the Néel-ordering and, second, the shape of the octahedra in this compound is not ideal but distorted. From these observations we conclude that LaTiO 3 has to be considered as a soft Jahn-Teller system thereby explaining m...

We present an investigation of the influence of structural distortions in charge-carrier doped La1−xMxCoO3 by substituting La 3+ with alkaline earth metals of strongly different ionic sizes, that is M = Ca 2+ , Sr 2+ , and Ba 2+ , respectively. We find that both, the magnetic properties and the resistivity change non-monotonously as a function of the ionic size of M. Doping La1−xMxCoO3 with M = Sr 2+ yields higher transition temperatures to the ferromagnetically ordered states and lower resistivities than doping with either Ca 2+ or Ba 2+ having a smaller or larger ionic size than Sr 2+ , respectively. From this observation we conclude that the different transition temperatures and resistivities of La1−xMxCoO3 for different M (of the same concentration x) do not only depend on the varying chemical pressures. The local disorder due to the different ionic sizes of La 3+ and M 2+ play an important role, too.

Analysis of the structural, transport, and superconducting properties of Nd-doped La2 "Sr Cu04 reveals a critical tilt angle of the Cu06 octahedra for the disappearance of superconductivity in the low temperature tetragonal phase. Our results indicate a strong inAuence of the tilt of the Cu06 octahedra on the electronic properties, suggesting the importance of spin-orbit coupling for the destruction of superconductivity and for the stabilization of a magnetic state.

It is argued that in the mixed state of a type II superconductor, because of the difference of the chemical potential in a superconducting versus normal state, the vortex cores may become charged.The extra electron density is estimated. The extra charge contributes to the dynamics of the vortices; in particular, it can explain in certain cases the change of the sign of the Hall coefficient below T, . frequently observed in the high temperature superconductors.The transition of a system to a superconducting state leads to a change of the chemical potential of the electrons [1 -3]. In a material which has two electronic subsystems, one of which becomes superconducting, the other remaining normal, this change of the chemical potential causes a charge redistribution between these subsystems below T,[2]. This is qualitatively easy to understand: Below T, there is an energy gain for the condensed charge carriers, and it is therefore energetically favorable to transfer some charge carriers from the normal to the condensate region.The effect is of general character and should occur in any superconductor.However, the charge transfer is determined by the magnitude of (6/eF), where 5 is the energy gap and eF the Fermi energy. Therefore the charge redistribution is, in particular, important in the high temperature superconductors (HTSCs), because of their relatively large value of 6/eF. It was shown in Ref.[2] that it can explain several anomalies observed in the HTSCs at and below T, .We want to point out in this Letter that the change of the chemical potential below T, may also lead to charging of a vortex core in the mixed state of a type II superconductor. Assuming that the vortex core is a region of normal metal surrounded by superconducting material the corresponding difference in the chemical potential leads to a redistribution of the electrons. The extra charge of the cores gives rise to an additional force on vortices; in particular, for an appropriate sign of the charge this force can lead to a sign change of the Hall coefficient, which is frequently observed experimentally in the HTSCs [4 -7].The theoretical treatment carried out in [1,2] gives the following expression for the change of the chemical potential p, of the electrons below T, for a model with a constant densit of states:~' (T) p(T) = po-(1) 4p, o for a less than half-filled band. For p, o ) D/2, where D is the bandwidth, the term -5 /4p, o in (1) is substituted by + 5 /4(Dp, o). For the general case of an arbitrary density of states N(e) the corresponding formula has the form [2] 1 BN p(T) = po c~'(T), N(eF) ae (2) There is yet another contribution to the chemical potentialthe kinetic energy of the superfluid motion around the vortex core [10,11] ns f11v r 6p, = -' n 2 (4) Here n, and n are the superAuid and total electron densities. Note that the terms (1) and (4) perfectly match at r = s: The condition that the kinetic energy of the superAuid motion equals the condensation energy, or that the corresponding velocity is equal to the depairing velocity,...

We present a study of the structure, the electric resistivity, the magnetic susceptibility, and the thermal expansion of La1−xEuxCoO3. LaCoO3 shows a temperature-induced spin-state transition around 100 K and a metal-insulator transition around 500 K. Partial substitution of La 3+ by the smaller Eu 3+ causes chemical pressure and leads to a drastic increase of the spin gap from about 190 K in LaCoO3 to about 2000 K in EuCoO3, so that the spin-state transition is shifted to much higher temperatures. A combined analysis of thermal expansion and susceptibility gives evidence that the spin-state transition has to be attributed to a population of an intermediate-spin state without orbital degeneracy for x < 0.5 and with orbital degeneracy for larger x. In contrast to the spin-state transition, the metal-insulator transition is shifted only moderately to higher temperatures with increasing Eu content, showing that the metal-insulator transition occurs independently from the spin-state distribution of the Co 3+ ions. Around the metal-insulator transition the magnetic susceptibility shows a similar increase for all x and approaches a doping-independent value around 1000 K indicating that well above the metal-insulator transition the same spin state is approached for all x.