Intrinsic coupling of orbital excitations to spin fluctuations in Mott insulators

We show how the general and basic asymmetry between two fundamental degrees of freedom present in strongly correlated oxides, spin and orbital, has very profound repercussions on the elementary spin and orbital excitations. Whereas the magnons remain largely unaffected, orbitons become inherently coupled with spin fluctuations in spin-orbital models with antiferromagnetic and ferro-orbital ordered ground states. The composite orbiton-magnon modes that emerge fractionalize again in one dimension, giving rise to spin-orbital separation in the peculiar regime where spinons are faster than orbitons.     

Ultrafast optical nonlinearity in the quasi-one-dimensional mott insulator Sr2CuO3

We report strong instantaneous photoinduced absorption in the quasi-one-dimensional Mott insulator Sr2CuO3 in the IR spectral region. The observed photoinduced absorption is to an even-parity two-photon state that occurs immediately above the absorption edge. Theoretical calculation based on a two-band extended Hubbard model explains the experimental features and indicates that the strong two-photon absorption is due to a very large dipole coupling between nearly degenerate one- and two-photon states. Room temperature picosecond recovery of the optical transparency suggests the strong potential of Sr2CuO3 for all-optical switching.     

Nonlinear magnetic susceptibility and microwave spin dynamics of R2CuO4 quasi-2D antiferromagnets (R=Eu, Pr, Gd)

An experimental study of nonlinear ac magnetic susceptibility and microwave spin dynamics of R₂CuO₄ quasi-2D Heisenberg antiferromagnets (R=Eu, Pr, Gd) has been carried out. The data obtained can be accounted for if one assumes the existence of a random-field (RF) state in the Eu₂CuO₄ and Pr₂CuO₄ tetragonal crystals within the 77⩽T⩽350 K range covered. If this is so, 3D antiferromagnetic order persists only within limited regions, while in CuO₂ layers there are 2D Heisenberg antiferromagnetic spin fluctuations with large correlation lengths. In the Gd₂CuO₄ crystal there exists, besides uniform 3D antiferromagnetic long-range order with weak ferromagnetism, an admixture of an RF-type state. Fiz. Tverd. Tela (St. Petersburg) 41, 1437–1444 (August 1999)     

Electronic structure of (LaSr)2CuO4 and (NdCe)2CuO4

The electronic structure of (LaSr)₂CuO₄ and (NdCe)₂CuO₄ are compared. It is concluded that in both materials after doping by holes and electrons respectively the Fermi energy is at the top of the valence band. This explains many symmetrical properties and in particular the photoemission experiments.Dedicated to Prof. P. Kienle on the occasion of his 60th birthday     

Ultrafast optical spin injection into image-potential states of Cu(001)

We report the observation of a net spin polarization in the n=1 image-potential state at the Cu(001) surface. The spin polarization is achieved by spin-selective multiphoton excitation of electrons from the spin-orbit split Cu d bands to the image-potential state using circularly polarized ultrafast light pulses. We show that by tuning the exciting photon energy, we can adjust the resonant coupling of the image-potential state to d bands of different double-group symmetry. This allows us to tune the spin polarization injected into the image-potential state.     

Magnetic properties and antiferromagnetic Cu ordering in Pr2CuO4

Bulk-magnetisation measurements and neutron-scattering experiments were performed both on a polycrystalline sample and on a single-crystal of Pr₂CuO₄ in the temperature range from 1.5 to 300 K. Pr₂CuO₄ crystallizes with theT (Nd₂CuO₄)-type structure. We observed antiferromagnetic ordering of the Cu moments belowT _N =(190±2) K in a single crystal and belowT _N =(250±10)K in powder material. The magnetic unit cell dimensions area _m =2a _o ,c _m =c _o ;the Cu moments are oriented in the basal plane with a magnetic saturation moment of _Cu=(0.45±0.12) _B . Pr³⁺ does not order magnetically above 1.5K due to its crystal-field induced singlet ground state as verified by inelastic neutron scattering.     

Raman scattering in the Mott insulators LaTiO3 and YTiO3: evidence for orbital excitations

Raman scattering is used to observe pronounced electronic excitations around 230 meV--well above the two-phonon range--in the Mott insulators LaTiO3 and YTiO3. Based on the temperature, polarization, and photon energy dependence, the modes are identified as orbital excitations. The observed profiles bear a striking resemblance to magnetic Raman modes in the insulating parent compounds of the superconducting cuprates, indicating an unanticipated universality of the electronic excitations in transition metal oxides.     

Ultrafast momentum-dependent response of electrons in antiferromagnetic EuFe2As2 driven by optical excitation

Employing the momentum sensitivity of time- and angle-resolved photoemission spectroscopy we demonstrate the analysis of ultrafast single- and many-particle dynamics in antiferromagnetic EuFe(2)As(2). Their separation is based on a temperature-dependent difference of photoexcited hole and electron relaxation times probing the single-particle band and the spin density wave gap, respectively. Reformation of the magnetic order occurs at 800 fs, which is 4 times slower compared to electron-phonon equilibration due to a smaller spin-dependent relaxation phase space.     

Study of carrier doping across the parent Mott insulator La2CuO4

Ce substituted La₂CuO₄ single crystals are investigated to try doping electrons into the parent Mott insulator. Transport properties of slightly Ce substituted La₂CuO₄ show that carriers are still holes activated from an impurity level of which activation energy is the same as the parent La₂CuO₄.     

The role of the nuclear spin in optical pumping withD 2-light

Optical pumping withD ₂-light provides an excellent means for studying collisional relaxation in the excited² P _3/2-state of alkali atoms. Collisional relaxation of orientations in that state very sensitively affects the spin orientation in the ground state. All these orientations may be easily created by absorption of σ⁺- or σ{?{-light. At a certain strength of the relaxation realized by a certain buffer gas pressure, the spin orientation in the ground state even vanishes, providedD ₂-light is used for excitation. The condition for this situation is derived from the set of rate equations which governs the evolution of all the orientations involved. These conditions very markedly depend on the nuclear spin valueI. The validity of this dependence has been checked by magnetic decoupling of the nuclear spin and observing the associated shift of the pressure for vanishing spin orientation.     

Jahn-Teller effect and the orbital ground state of Cu2+ ions in Eu2CuO4 and La2CuO4 crystals

Local ion displacements and their anharmonicity in Eu₂CuO₄ and La₂CuO₄ crystals are studied by high-precision x-ray diffractometry. A symmetry analysis and the temperature behavior of the probability density function of Cu²⁺ ions reveal a degenerate ground orbital state in the Eu₂CuO₄ crystal (tetragonal doublet d _xz, d _yz). The pattern and anharmonicity of the local displacements are found to be determined not only by lattice vibrations but by 2D antiferromagnetic spin fluctuations in the CuO₂ sheet as well. By contrast, in the La₂CuO₄ crystal the orbital ground state of the Cu²⁺ ion is a singlet (d _z 2). The pattern of Eu³⁺ local displacements and their effect on the properties of the Cu subsystem in the Eu₂CuO₄ crystal are also studied. Fiz. Tverd. Tela (St. Petersburg) 39, 1600–1608 (September 1997)     

High energy spin dynamics of La2CuO4 and La1.9Sr0.1CuO4

The magnetic excitations in La₂CuO₄ and La_1.9Sr_0.1CuO₄ were studied by inelastic neutron scattering up to energies of 76 meV. For the pure sample, the results forE15 meV can be well described by conventional spin wave theory with a spin wave velocity c=0.89±0.07 eVÅ. For lower energies, the observed intensities were somewhat higher than expected from spin wave theory and did not follow the Bose-Einstein factor. For the doped sample, the linewidth in constant-E scans at smallE shows a very short correlation length of =7.5 Å only, which is considerably below the value expected from the concentration dependence reported by other authors. An increase of the linewidth for largeE indicates a reduced spin-wave stiffness when compared to the undoped material. However, the use of a spin-wave picture may not be appropriate as the standard expression for damped spin waves in a paramagnetic material is in serious conflict with experiment.     

Pseudogap in doped Mott insulators is the near-neighbor analogue of the Mott gap

We show that the strong-coupling physics inherent to the insulating Mott state in 2D leads to a jump in the chemical potential upon doping and the emergence of a pseudogap in the single-particle spectrum below a characteristic temperature. The pseudogap arises because any singly occupied site not immediately neighboring a hole experiences a maximum energy barrier for transport equal to t(2)/U, t the nearest-neighbor hopping integral and U the on-site repulsion. The resultant pseudogap cannot vanish before each lattice site, on average, has at least one hole as a near neighbor. The ubiquity of this effect in all doped Mott insulators suggests that the pseudogap in the cuprates has a simple origin.