arXiv subjects

L. H. Tjeng

Publications and source records attributed to L. H. Tjeng.

107 records · Page 6Linked to original sources

arXiv2006

Valence, spin, and orbital state of the Co ions in the one-dimensional Ca3Co2O6: an x-ray absorption and magnetic circular dichroism study

We have investigated the valence, spin, and orbital state of the Co ions in the one-dimensional cobaltate Ca3Co2O6 using x-ray absorption and x-ray magnetic circular dichroism at the Co-L2,3 edges. The Co ions at both the octahedral Co_oct and trigonal Co_trig sites are found to be in a 3+ state. From the analysis of the dichroism we established a low-spin state for the Co_oct and a high-spin state with an anomalously large orbital moment of 1.7 muB at the Co3+ trig ions. This large orbital moment along the c-axis chain and the unusually large magnetocrystalline anisotropy can be traced back to the double occupancy of the d2 orbital in trigonal crystal field.

cond-mat.str-el↗

arXiv2006

The spin state transition in LaCoO$_{3}$; revising a revision

Using soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co-$L_{2,3}$ edge we reveal that the spin state transition in LaCoO$_{3}$ can be well described by a low-spin ground state and a triply-degenerate high-spin first excited state. From the temperature dependence of the spectral lineshapes we find that LaCoO$_{3}$ at finite temperatures is an inhomogeneous mixed-spin-state system. Crucial is that the magnetic circular dichroism signal in the paramagnetic state carries a large orbital momentum. This directly shows that the currently accepted low-/intermediate-spin picture is at variance. Parameters derived from these spectroscopies fully explain existing magnetic susceptibility, electron spin resonance and inelastic neutron data.

cond-mat.str-el↗

arXiv2006

Orbitally driven spin-singlet dimerization in $S$=1 La$_{4}$Ru$_{2}$O$_{10}$

Using x-ray absorption spectroscopy at the Ru-$L_{2,3}$ edge we reveal that the Ru$^{4+}$ ions remain in the $S$=1 spin state across the rare 4d-orbital ordering transition and spin-gap formation. We find using local spin density approximation + Hubbard U (LSDA+U) band structure calculations that the crystal fields in the low temperature phase are not strong enough to stabilize the $S$=0 state. Instead, we identify a distinct orbital ordering with a significant anisotropy of the antiferromagnetic exchange couplings. We conclude that La$_{4}$Ru$_{2}$O$_{10}$ appears to be a novel material in which the orbital physics drives the formation of spin-singlet dimers in a quasi 2-dimensional $S$=1 system.

cond-mat.str-el↗

arXiv2005

Electronic structure of $RE$AuMg and $RE$AgMg ($RE$ = Eu, Gd, Yb)

We have investigated the electronic structure of the equiatomic EuAuMg, GdAuMg, YbAuMg and GdAgMg intermetallics using x-ray photoelectron spectroscopy. The spectra revealed that the Yb and Eu are divalent while the Gd is trivalent. The spectral weight in the vicinity of the Fermi level is dominated by the mix of Mg $s$, Au/Ag $sp$ and $RE$ $spd$ bands, and not by the $RE$ $4f$. We also found that the Au and Ag $d$ bands are extraordinarily narrow, as if the noble metal atoms were impurities submerged in a low density $sp$ metal host. The experimental results were compared with band structure calculations, and we found good agreement provided that the spin-orbit interaction in the Au an Ag $d$ bands is included and correlation effects in an open $4f$ shell are accounted for using the local density approximation + Hubbard $U$ scheme. Nevertheless, limitations of such a mean-field scheme to explain excitation spectra are also evident.

cond-mat.other↗

arXiv2005

Soft x-ray magnetic circular dichroism study on Gd-doped EuO thin films

We report on the growth and characterization of ferromagnetic Gd-doped EuO thin films. We prepared samples with Gd concentrations up to 11% by means of molecular beam epitaxy under distillation conditions, which allows a very precise control of the doping concentration and oxygen stoichiometry. Using soft x-ray magnetic circular dichroism at the Eu and Gd M4,5 edges, we found that the Curie temperature ranged from 69 K for pure stoichiometric EuO to about 170 K for the film with the optimal Gd doping of around 4%. We also show that the Gd magnetic moment couples ferromagnetically to that of Eu.

cond-mat.str-el↗

arXiv2005

Nature of magnetism in Ca$_3$Co$_2$O$_6$

We find using LSDA+U band structure calculations that the novel one-dimensional cobaltate Ca$_3$Co$_2$O$_6$ is not a ferromagnetic half-metal but a Mott insulator. Both the octahedral and the trigonal Co ions are formally trivalent, with the octahedral being in the low-spin and the trigonal in the high-spin state. The inclusion of the spin-orbit coupling leads to the occupation of the minority-spin $d_{2}$ orbital for the unusually coordinated trigonal Co, producing a giant orbital moment (1.57 $μ_{B}$). It also results in an anomalously large magnetocrystalline anisotropy (of order 70 meV), elucidating why the magnetism is highly Ising-like. The role of the oxygen holes, carrying an induced magnetic moment of 0.13 $μ_{B}$ per oxygen, for the exchange interactions is discussed.

cond-mat.str-el↗

arXiv2005

Orbital-assisted metal-insulator transition in VO$_{2}$

We found direct experimental evidence for an orbital switching in the V 3d states across the metal-insulator transition in VO$_{2}$. We have used soft-x-ray absorption spectroscopy at the V $L_{2,3}$ edges as a sensitive local probe, and have determined quantitatively the orbital polarizations. These results strongly suggest that, in going from the metallic to the insulating state, the orbital occupation changes in a manner that charge fluctuations and effective band widths are reduced, that the system becomes more 1-dimensional and more susceptible to a Peierls-like transition, and that the required massive orbital switching can only be made if the system is close to a Mott insulating regime.

cond-mat.str-el↗

arXiv2005

Controlling orbital moment and spin orientation in CoO layers by strain

We have observed that CoO films grown on different substrates show dramatic differences in their magnetic properties. Using polarization dependent x-ray absorption spectroscopy at the Co L$_{2,3}$ edges, we revealed that the magnitude and orientation of the magnetic moments strongly depend on the strain in the films induced by the substrate. We presented a quantitative model to explain how strain together with the spin-orbit interaction determine the 3d orbital occupation, the magnetic anisotropy, as well as the spin and orbital contributions to the magnetic moments. Control over the sign and direction of the strain may therefore open new opportunities for applications in the field of exchange bias in multilayered magnetic films.

cond-mat.str-el↗

arXiv2005

Spectroscopy of stripe order in La1.8Sr0.2NiO4 using resonant soft x-ray diffraction

Strong resonant enhancements of the charge-order and spin-order superstructure-diffraction intensities in La1.8Sr0.2NiO4 are observed when x-ray energies in the vicinity of the Ni L2,3 absorption edges are used. The pronounced photon-energy and polarization dependences of these diffraction intensities allow for a critical determination of the local symmetry of the ordered spin and charge carriers. We found that not only the antiferromagnetic order but also the charge-order superstructure resides within the NiO2 layers; the holes are mainly located on in-plane oxygens surrounding a Ni2+ site with the spins coupled antiparallel in close analogy to Zhang-Rice singlets in the cuprates.

cond-mat.str-el↗

arXiv2004

Determination of the orbital moment and crystal field splitting in LaTiO$_{3}$

Utilizing a sum-rule in a spin-resolved photoelectron spectroscopic experiment with circularly polarized light, we show that the orbital moment in LaTiO$_3$ is strongly reduced both below and above the Néel temperature. Using Ti $L_{2,3}$ x-ray absorption spectroscopy as a local probe, we found that the crystal field splitting in the $t_{2g}$ subshell is about 0.12-0.30 eV. This large splitting does not facilitate the formation of an orbital liquid.

cond-mat.str-el↗

arXiv2003

Magnetic versus crystal field linear dichroism in NiO thin films

We have detected strong dichroism in the Ni $L_{2,3}$ x-ray absorption spectra of monolayer NiO films. The dichroic signal appears to be very similar to the magnetic linear dichroism observed for thicker antiferromagnetic NiO films. A detailed experimental and theoretical analysis reveals, however, that the dichroism is caused by crystal field effects in the monolayer films, which is a non trivial effect because the high spin Ni $3d^{8}$ ground state is not split by low symmetry crystal fields. We present a practical experimental method for identifying the independent magnetic and crystal field contributions to the linear dichroic signal in spectra of NiO films with arbitrary thicknesses and lattice strains. Our findings are also directly relevant for high spin $3d^{5}$ and $3d^{3}$ systems such as LaFeO$_{3}$, Fe$_{2}$O$_{3}$, VO, LaCrO$_{3}$, Cr$_{2}$O$_{3}$, and Mn$^{4+}$ manganate thin films.

cond-mat.str-el↗

arXiv2003

A different look at the spin state of Co$^{3+}$ ions in CoO$_{5}$ pyramidal coordination

Using soft-x-ray absorption spectroscopy at the Co-$L_{2,3}$ and O-$K$ edges, we demonstrate that the Co$^{3+}$ ions with the CoO$_{5}$ pyramidal coordination in the layered Sr$_2$CoO$_3$Cl compound are unambiguously in the high spin state. Our result questions the reliability of the spin state assignments made so far for the recently synthesized layered cobalt perovskites, and calls for a re-examination of the modeling for the complex and fascinating properties of these new materials.

cond-mat.str-el↗

arXiv2003

Growth and properties of strained VOx thin films with controlled stoichiometry

We have succeeded in growing epitaxial films of rocksalt VOx on MgO(001) substrates. The oxygen content as a function of oxygen flux was determined using 18O2-RBS and the vanadium valence using XAS. The upper and lower stoichiometry limits found are similar to the ones known for bulk material (0.8<x<1.3). From the RHEED oscillation period a large number of vacancies for both vanadium and oxygen were deduced, i.e. ~16% for stoichiometric VO. These numbers are, surprisingly, very similar to those for bulk material and consequently quite strain-insensitive. XAS measurements reveal that the vacancies give rise to strong low symmetry ligand fields to be present. The electrical conductivity of the films is much lower than the conductivity of bulk samples which we attribute to a decrease in the direct overlap between t2g orbitals in the coherently strained layers. The temperature dependence of the conductivity is consistent with a variable range hopping mechanism.

cond-mat.str-el↗

arXiv2002

Direct observation of electron doping in La0.7Ce0.3MnO3 using x-ray absorption spectroscopy

We report on a X-ray absorption spectroscopic (XAS) study on a thin film of La0.7Ce0.3MnO3, a manganite which was previously only speculated to be an electron doped system. The measurements clearly show that the cerium is in the Ce(IV) valence state and that the manganese is present in a mixture of Mn2+ and Mn3+ valence states. These data unambiguously demonstrate that La0.7Ce0.3MnO3 is an electron doped colossal magnetoresistive manganite, a finding that may open up new opportunities both for device applications as well as for further basic research towards a better modelling of the colossal magnetoresistance phenomenon in these materials.

cond-mat.str-el↗

arXiv2002

Exchange Splitting and Charge Carrier Spin Polarization in EuO

High quality thin films of the ferromagnetic semiconductor EuO have been prepared and were studied using a new form of spin-resolved spectroscopy. We observed large changes in the electronic structure across the Curie and metal-insulator transition temperature. We found that these are caused by the exchange splitting of the conduction band in the ferromagnetic state, which is as large as 0.6 eV. We also present strong evidence that the bottom of the conduction band consists mainly of majority spins. This implies that doped charge carriers in EuO are practically fully spin polarized.

cond-mat.str-el↗

arXiv2001

Experimental evidence of the spin selection rule in KLL Auger transition

With on- and off- resonant excitation photons, spin-resolved Auger electron spectra of epitaxial CrO$_{2}$ thin films show an experimental evidence of the spin-selective KLL Auger decay. The on-resonance O KLL Auger electrons are found to be highly spin-polarized, while the off-resonance ones with almost zero spin polarization. These results lead to conclude that the two-hole final state in KLL Auger decay is a spin-singlet. Applications to spin-resolved absorption spectroscopy are discussed.

cond-mat.str-el↗

arXiv2001

Work function changes in the double layered manganite La1.2Sr1.8Mn2O7

We have investigated the behaviour of the work function of La1.2Sr1.8Mn2O7 as a function of temperature by means of photoemission. We found a decrease of 55 +/- 10 meV in going from 60 K to just above the Curie temperature (125 K) of the sample. Above T_C the work function appears to be roughly constant. Our results are exactly opposite to the work function changes calculated from the double-exchange model by Furukawa, but are consistent with other measurements. The disagreement with double-exchange can be explained using a general thermodynamic relation valid for second order transitions and including the extra processes involved in the manganites besides double-exchange interaction.

cond-mat.str-el↗