arXiv subjects

Roberto Kraus

Publications and source records attributed to Roberto Kraus.

8 recordsLinked to original sources

arXiv2017

The effect of different in-chain impurities on the magnetic properties of the spin chain compound SrCuO$_2$ probed by NMR

The S=1/2 Heisenberg spin chain compound SrCuO2 doped with different amounts of nickel (Ni), palladium (Pd), zinc (Zn) and cobalt (Co) has been studied by means of Cu nuclear magnetic resonance (NMR). Replacing only a few of the S=1/2 Cu ions with Ni, Pd, Zn or Co has a major impact on the magnetic properties of the spin chain system. In the case of Ni, Pd and Zn an unusual line broadening in the low temperature NMR spectra reveals the existence of an impurity-induced local alternating magnetization (LAM), while exponentially decaying spin-lattice relaxation rates $T_1^{-1}$ towards low temperatures indicate the opening of spin gaps. A distribution of gap magnitudes is proven by a stretched spin-lattice relaxation and a variation of $T_1^{-1}$ within the broad resonance lines. These observations depend strongly on the impurity concentration and therefore can be understood using the model of finite segments of the spin 1/2 antiferromagnetic Heisenberg chain, i.e. pure chain segmentation due to S = 0 impurities. This is surprising for Ni as it was previously assumed to be a magnetic impurity with S = 1 which is screened by the neighboring copper spins. In order to confirm the S = 0 state of the Ni, we performed x-ray absorption spectroscopy (XAS) and compared the measurements to simulated XAS spectra based on multiplet ligand-field theory. Furthermore, Zn doping leads to much smaller effects on both the NMR spectra and the spin-lattice relaxation rates, indicating that Zn avoids occupying Cu sites. For magnetic Co impurities, $T_1^{-1}$ does not obey the gap like decrease, and the low-temperature spectra get very broad. This could be related to the increase of the Neel temperature which was observed by recent muSR and susceptibility measurements, and is most likely an effect of the impurity spin $S\neq0$.

cond-mat.str-el↗

arXiv2016

Orbital breathing effects in the computation of x-ray d-ion spectra in solids by ab initio wave-function-based methods

In existing theoretical approaches to core-level excitations of transition-metal ions in solids relaxation and polarization effects due to the inner core hole are often ignored or described phenomenologically. Here we set up an ab initio computational scheme that explicitly accounts for such physics in the calculation of x-ray absorption and resonant inelastic x-ray scattering spectra. Good agreement is found with experimental transition-metal $L$-edge data for the strongly correlated $d^9$ cuprate Li$_2$CuO$_2$, for which we determine the absolute scattering intensities. The newly developed methodology opens the way for the investigation of even more complex $d^n$ electronic structures of group VI B to VIII B correlated oxide compounds.

cond-mat.str-el↗

arXiv2016

Probing inter- and intrachain Zhang-Rice excitons in Li$_2$CuO$_2$ and determining their binding energy

Cuprate materials, like those hosting high temperature superconductivity, represent a famous class of materials where the correlations between the strongly entangled charges and spins produce complex phase diagrams. Several years ago the Zhang-Rice singlet was proposed as a natural quasiparticle in hole-doped cuprates. The occurance and binding energy of this quasiparticle, consisting of a pair of bound holes with antiparallel spins on the same CuO$_4$ plaquette, depends on the local electronic interactions, which are fundamental quantities for understanding the physics of the cuprates. Here, we employ state-of-the-art Resonant Inelastic X-ray Scattering (RIXS) to probe the correlated physics of the CuO$_4$ plaquettes in the quasi-one dimensional chain cuprate Li$_2$CuO$_2$. By tuning the incoming photon energy to the O $K$-edge, we populate bound states related to the Zhang-Rice quasiparticles in the RIXS process. Both intra- and interchain Zhang-Rice singlets are observed and their occurrence is shown to depend on the nearest-neighbor spin-spin correlations, which are readily probed in this experiment. We also extract the binding energy of the Zhang-Rice singlet and identify the Zhang-Rice triplet excitation in the RIXS spectra.

cond-mat.str-el↗

arXiv2015

Electron-lattice interactions strongly renormalize the charge transfer energy in the spin-chain cuprate Li$_2$CuO$_2$

Strongly correlated insulators are broadly divided into two classes: Mott-Hubbard insulators, where the insulating gap is driven by the Coulomb repulsion $U$ on the transition-metal cation, and charge-transfer insulators, where the gap is driven by the charge transfer energy $Δ$ between the cation and the ligand anions. The relative magnitudes of $U$ and $Δ$ determine which class a material belongs to, and subsequently the nature of its low-energy excitations. These energy scales are typically understood through the local chemistry of the active ions. Here we show that the situation is more complex in the low-dimensional charge transfer insulator Li$_\mathrm{2}$CuO$_\mathrm{2}$, where $Δ$ has a large non-electronic component. Combining resonant inelastic x-ray scattering with detailed modeling, we determine how the elementary lattice, charge, spin, and orbital excitations are entangled in this material. This results in a large lattice-driven renormalization of $Δ$, which significantly reshapes the fundamental electronic properties of Li$_\mathrm{2}$CuO$_\mathrm{2}$.

cond-mat.str-el↗

arXiv2015

Orbital control of effective dimensionality: from spin-orbital fractionalization to confinement in the anisotropic ladder system CaCu2O3

Fractionalization of an electronic quasiparticle into spin, charge and orbital parts is a fundamental and characteristic property of interacting electrons in one dimension. However, real materials are never strictly one-dimensional and the fractionalization phenomena are hard to observe. Here we studied the spin and orbital excitations of the anisotropic ladder material CaCu2O3, whose electronic structure is not one-dimensional. Combining high-resolution resonant inelastic x-ray scattering experiments with theoretical model calculations we show that: (i) spin-orbital fractionalization occurs in CaCu2O3 along the leg direction x through the xz orbital channel as in a 1D system; and (ii) no fractionalization is observed for the xy orbital, which extends in both leg and rung direction, contrary to a 1D system. We conclude that the directional character of the orbital hopping can select different degrees of dimensionality. Using additional model calculations, we show that spin-orbital separation is generally far more robust than the spin-charge separation. This is not only due to the already mentioned selection realized by the orbital hopping, but also due to the fact that spinons are faster than the orbitons.

cond-mat.str-el↗

arXiv2013

Femtosecond dynamics of magnetic excitations from resonant inelastic x-ray scattering in CaCu2O3

Taking spinon excitations in the quantum antiferromagnet CaCu2O3 as an example, we demonstrate that femtosecond dynamics of magnetic excitations can be probed by direct resonant inelastic x-ray scattering (RIXS). To this end, we isolate the contributions of single and double spin-flip excitations in experimental RIXS spectra, identify the physical mechanisms that cause them and determine their respective timescales. By comparing theory and experiment, we find that double spin flips need a finite amount of time to be generated, rendering them sensitive to the core-hole lifetime, whereas single spin flips are to a very good approximation independent of it. This shows that RIXS can grant access to time-domain dynamics of excitations and illustrates how RIXS experiments can distinguish between excitations in correlated electron systems based on their different time dependence.

cond-mat.str-el↗

arXiv2012

Determining the Short-Range Spin Correlations in Cuprate Chain Materials with Resonant Inelastic X-ray Scattering

We report a high-resolution resonant inelastic soft x-ray scattering study of the quantum magnetic spin-chain materials Li2CuO2 and CuGeO3. By tuning the incoming photon energy to the oxygen K-edge, a strong excitation around 3.5 eV energy loss is clearly resolved for both materials. Comparing the experimental data to many-body calculations, we identify this excitation as a Zhang-Rice singlet exciton on neighboring CuO4-plaquettes. We demonstrate that the strong temperature dependence of the inelastic scattering related to this high-energy exciton enables to probe short-range spin correlations on the 1 meV scale with outstanding sensitivity.

cond-mat.str-el↗

arXiv2011

Unoccupied electronic structure of TiOCl studied by x-ray absorption near-edge spectroscopy

We study the unoccupied electronic structure of the spin-1/2 quantum magnet TiOCl by x-ray absorption near-edge spectroscopy (XANES) at the Ti L and O K edges. Data is acquired both in total electron and fluorescence yield mode (TEY and FY, respectively). While only the latter allows to access the unconventional low-temperature spin-Peierls (SP) phase of TiOCl the signal is found to suffer from significant self-absorption in this case. Nevertheless, we conclude from FY data that effects of the SP distortion on the electronic structure are absent within experimental accuracy. The similarity of room-temperature FY and TEY data, the latter not being obscured by self-absorption, allows us to use TEY spectra for comparison with simulations. These are performed by cluster calculations in D$_{4h}$ and D$_{2h}$ symmetries using two different codes. We extract values of the crystal-field splitting (CFS) and parametrize our results in often found notations by Slater, Racah and Butler. In all cases, good agreement with published values from other studies is found.

cond-mat.str-el↗