arXiv subjects

Z. Ropka

Publications and source records attributed to Z. Ropka.

At least 19 recordsLinked to original sources

arXiv2009

Specific heat and orbital moment of CoO from first-principles atomistic calculations

We have for the first time calculated low-energy electronic structure both in paramagnetic and magnetic state as well as zero-temperature properties and thermodynamics. We consistently described magnetic properties of CoO in agreement with its insulating ground state. The orbital moment of 1.42 mu_B gives 35 contribution to the total moment of 4.04 mu_B at T =0 K. We have calculated from this low-energy electronic structure the temperature dependence of the specific heat being in nice agreement with experimental data. In our approach CoO is an insulator independently on distortions and the magnetic order.

cond-mat.str-el↗

arXiv2006

Magnetic properties and the electronic structure of LiCoO_2

We have described properties of LiCoO_2 within the Quantum Atomistic Solid State (QUASST) theory taking into account very strong electron correlations, predominantly of the intra-atomic origin, spin-orbit coupling and the detailed local crystallographic surroundings and its symmetry. Properties of LiCoO_2 are consistently explained together with NaCoO_2 and LaCoO_3 - in all of these compounds the Co^3+ ions occur in the low-spin state. This low-spin state is the effect of the relatively strong crystal-field interactions (B_4^z = +320 K <=> 10Dq = 3.3 eV) and is a manifestation of the very large orbital moment of the Co^3+ ion.

cond-mat.str-el↗

arXiv2006

NiO - from first principles

We have calculated from first-principles the octupolar interactions of the Ni2+ ion in NiO, which gives the theoretical basis for the ionic description of properties of NiO with fully localized strongly-correlated eight d electrons. A failure of the up-now first principles ionic calculations for NiO was largely due to too small values taken for the octupolar moment of the transition-metal atom, largely generated by too small value for . Our many-electron crystal-field based approach enables successful calculations of the electronic structure and magnetic properties both in the paramagnetic and in magnetically-ordered state as well as zero-temperature properties and thermodynamics.

cond-mat.str-el↗

arXiv2006

Comment on Phys. Rev. Lett.'s paper "All-Electron Self-Consistent GW Approximation: Application to Si, MnO, and NiO": band vs localized description of NiO

In contrary to authors of Phys. Rev. Lett. 93, 126406 (2004) claiming "the band picture to be a reasonable starting point for the description of the electronic structure of NiO, much better than the ligand-field picture", we argue that the many-electron CEF approach is physically adequate starting point for discussion of the electronic structure and magnetism of NiO.

cond-mat.str-el↗

arXiv2006

The Jahn-Teller theorem for 3d ions and their compounds

In contrary to customarily consideration of the Jahn-Teller theorem for 3d-ion compounds in the orbital space only we point out that it has to be considered in the spin-orbital space. It is despite the weakness of the intraatomic spin-orbit coupling. A direct motivation for this paper is an erroneous claim of a recent Phys. Rev. Lett. 96 (2006) 027201 paper that the high-spin t2g4eg2 state of the 3d6 configuration is not Jahn-Teller active.

cond-mat.str-el↗

arXiv2006

Comment on a Phys. Rev. Lett. paper: the origin of the excited state in LaCoO3

In contrary to a claim of the recent Phys. Rev. Lett. 96 (2006) 027201 paper we maintain that the first excited state in LaCoO3 is the high-spin (HS) state (a lowest quasi-triplet from the octahedral subterm 5T2g of the 5D term, Phys. Rev. B 67 (2003) 172401) in agreement with the Tanabe-Sugano diagram.

cond-mat.str-el↗

arXiv2006

LaCoO3 - from first principles

We have performed calculations of the electronic structure of LaCoO3 from first principles, assuming the atomistic construction of matter and the electrostatic origin of the crystal-field splitting. In our atomic-like approach QUASST the d electrons of the Co3+ ion in LaCoO3 form the highly-correlated atomic-like system 3d6 with the singlet ground state 1A1 (an octahedral subterm from the 1I term) and the excited octahedral subterm 5T2g of the 5D term. In the spin-orbital space, being physically adequate, this high-spin state is Jahn-Teller active. We take the ESR experiment of Noguchi et al., Phys. Rev. B 66, 094404 (2002), as confirmation of the existence of the discrete electronic structure for 3$d$ electron states in LaCoO3 in the meV scale postulated in QUASST.

cond-mat.str-el↗

arXiv2006

Crystal field, spin-orbit coupling and magnetism in a ferromagnet YTiO3

Magnetic properties of stechiometric YTiO3 has been calculated within the single-ion-based paradigm taking into account the low-symmetry crystal field and the intra-atomic spin-orbit coupling of the Ti3+ ion. Despite of the very simplified approach the calculations reproduce perfectly the value of the magnetic moment and its direction as well as temperature dependence of the magnetic susceptibility chi(T). It turns out that the spin-orbit coupling is fundamentally important for 3d magnetism and magnetic properties are determined by lattice distortions.

cond-mat.str-el↗

arXiv2005

Comment on a Phys. Rev. Lett. paper: "All-Electron Self-Consistent GW Approximation: Application to Si, MnO, and NiO". Magnetic moment of NiO

We claim that any approach neglecting the spin-orbit coupling and the orbital magnetism is not physically adequate for 3d oxides, including NiO, and that in reaching "excellent agreement" in a Phys. Rev. Lett. 93, 126406 (2004) paper too small experimental value of 1.9 muB has been taken for the Ni magnetic moment despite publication of a new experimental value of 2.2 muB, at 300 K yielding 2.6 muB at T = 0 K, already in a year of 1998.

cond-mat.str-el↗

arXiv2005

Comment on a Phys. Rev. Lett. paper: 94 (2005) 146402: Orbital symmetry and Electron Correlation in NaxCoO2

We argue that the electronic structure considered in a Phys. Rev. Lett. paper 94 (2005) 146402 of the Co3+ ion in the CoO6 octahedron of NaxCoO2 is completely wrong. The presented Fig. 1 is redrawn here as Fig. 1. For physically adequate electronic structure it is necessary to take into account strong intra-atomic electron correlations and spin-orbit coupling. For Co3+ ions there are 15 low lying many-electron states within 0.1 eV as can be obtained in the many-electron CEF approach.

cond-mat.str-el↗

arXiv2005

On the crystal field in the modern solid-state theory

We point out the high physical correctness of the use and the concept of the crystal-field approach, even if is used to metallic magnetic materials of transition-metal 3d/4f/5f compounds. We discuss the place of the crystal-field theory in modern solid-state physics and we point out the necessity to consider the crystal-field approach with the spin-orbit coupling and strong electron correlations, as a contrast to the single-electron version of the crystal field customarily used for 3d electrons. We have extended the strongly-correlated crystal-field theory to a Quantum Atomistic Solid-State Theory (QUASST) to account for the translational symmetry and inter-site spin-dependent interactions indispensable for formation of magnetically-ordered state. We have correlated macroscopic magnetic and electronic properties with the atomic-scale electronic structure for ErNi5, UPd2Al3, FeBr2, LaCoO3 and LaMnO3. In QUASST we have made unification of 3d and rare-earth compounds in description of the low-energy electronic structures and magnetism of open 3d-/4f-/5f-shell electrons. QUASST offers consistent description of zero-temperature properties and thermodynamic properties of 4f-/5f-/3d-atom containing compounds. Our studies indicate that it is the highest time to unquench the orbital magnetism in 3d oxides.

cond-mat.str-el↗

arXiv2005

The atomic-start description of NiO

We have calculated magnetic properties and the electronic structure of NiO both in the paramagnetic and in magnetically-ordered state as well as zero-temperature properties and thermodynamics within the strongly-correlated crystal-field approach. It is in agreement with a Mott's suggestion that NiO is an insulator due to strong electron correlations. We have quantified crystal-field, spin-orbit and magnetic interactions of the Ni2+ ion in NiO. We have obtained that E_dd >> E_CF(=2.0 eV) >> E_{s-o}(=0.29 eV) > E_mag(=0.07 eV). The orbital moment of 0.54 mu_B amounts at 0 K, in the magnetically-ordered state, to about 20% of the total moment (2.53 mu_B). Our studies indicate that it is the highest time to "unquench" orbital magnetic moment in 3d solid-state physics and the necessity to take always into account strong intra-atomic correlations among d electrons and the intra-atomic spin-orbit coupling. Pacs: 75.25.+z, 75.10.Dg Keywords: Crystalline Electric Field, 3d oxides, magnetism, spin-orbit coupling NiO

cond-mat.str-el↗

arXiv2004

Are there crystal field levels in UPd2Al3? We answer, THERE ARE

We claim that crystal field (CEF) levels exist in UPd2Al3 in contrary to a recent claim of Hiess et al. (cond-mat/0411041) and Bernhoeft et al. (cond-mat/0411042), that there is no experimental evidence for discrete crystal field levels in this superconducting heavy-fermion antiferromagnet. We claim that excitations revealed by Krimmel et al. (J. Phys.: Condens. Matter 8 (1996) 1677 in inelastic-neutron-scattering (INS) studies are i) crystal-field excitations described by us within ii) the 5f3 (U3+) configuration. Moreover, our 5f3 (U3+) scheme, presented in Physica B 276-278 (2000) 803 and in Czech. J. Phys. 54 (2004) D295, provides a clear physical explanation for the 1.7 meV excitation (magnetic exciton) as associated to the removal of the Kramers-doublet ground state degeneracy in the antiferromagnetic state. The crystal-field theory completed by strong intra-atomic correlations and intersite spin-dependent interactions to the Quantum Atomistic Solid State Theory (QUASST), offers the meV energy scale needed for description of magnetic and electronic properties of compounds containing open-shell 3d, 4f, 5f atoms. The derived set of CEF parameters for the U3+ state reproduces both the INS excitations, temperature dependence of the heat capacity, large uranium magnetic moment as well as its direction. PACS: 71.70.E, 75.10.D Keywords: Crystalline Electric Field, Heavy fermion, magnetism, UPd2Al3

cond-mat.str-el↗

arXiv2004

The low-energy electronic structure and the orbital magnetism in NiO

The orbital and spin moment of the Ni2+ ion in NiO has been calculated within the quasi-atomic approach. The orbital moment of 0.46 mu_B amounts at 0 K, in the magnetically-ordered state, to about 20% of the total moment (2.45 mu_B). For this outcome, being in nice agreement with the recent experimental finding of the orbital moment, taking into account the intra-atomic spin-orbit coupling is indispensable. PACS: 75.25.+z, 75.10.Dg Keywords: Crystalline Electric Field, 3d oxides, magnetism, NiO

cond-mat.str-el↗

arXiv2004

Quadrupolar interactions in heavy fermion metal YbRh2Si2

We describe the experimentally revealed by Sichelschmidt et al, Phys. Rev. Lett. 91 (2003) 156401, g tensor, g_perpendicular=3.561 and g_parallel=0.17, at 5 K by means of crystal field interactions of the 4f13 configuration of the Yb3+ ion of YbRh2Si2 in a slightly orthorhombically distorted tetragonal crystal field. We have shown that the temperature dependence of the quadrupolar interactions Q(T) of the Yb nucleous will help to distinguish between Gamma_7 and Gamma_6 ground state. For the Gamma_7 ground state Q(T) is expected to exhibits an anomalous dependence. Keywords: heavy fermion, crystal field, quadrupolar moment,YbRh2Si2

cond-mat.str-el↗

arXiv2004

Violation of the Curie law in Na2V3O7 as the crystal-field and spin-orbit coupling effect

We have shown that the observed drastic violation of the Curie-Weiss law in Na2V3O7, reported in Phys. Rev. Lett. 90 (2003) 167202, is caused by conventional crystal-field interactions and the intra-atomic spin-orbit coupling of the V4+ ion. The fine discrete electronic structure of the 3d1 configuration, with a substantial orbital moment, is the reason for anomalous low-temperature properties of Na2V3O7. According to the Quantum Atomistic Solid-State Theory (QUASST) Na2V3O7 is expected to exhibit pronounced heavy-fermion phenomena at low temperatures. This study confirm our earlier claim that the orbital moment has to be unquenched in description of 3d-atom compounds. PACS: 71.70.E, 75.10.D Keywords: crystal-field interactions, spin-orbit coupling, orbital moment, Na2V3O7

cond-mat.str-el↗

arXiv2004

Comment on Phys. Stat. Sol. (b) 236 (2003) 281 paper by A. M. Oles "Orbital ordering and orbital fluctuations in transition metal oxides"

We argue that the 3A2 state considered by Oles in Phys. Stat. Sol. (b) 236 (2003) 281 for the d2 system occurring in the V3+ ion in V2O3 and LaVO3 as well as in Ti2+ ion in TiO and in many other oxides is wrong. The proper ground state is 3T1g - its 9-fold degeneracy is further split in a crystal by intra-atomic spin-orbit interactions and lattice distortions.

cond-mat.str-el↗

arXiv2004

Strongly correlated crystal-field approach to Mott insulator LaCoO3

Our success in description of recent electron-spin-resonance results on Mott insulator LaCoO3, Phys. Rev. B 67 (2003) 172401, lies in taking into account strong electron correlations among d electrons and the relativistic spin-orbit coupling. In the developed by us Quantum Atomistic Solid State Theory (QUASST) we assume that the atomic-like integrity of the 3d^6 system is preserved in the Co^3+ ion in LaCoO3 and that intra-atomic correlations are much stronger than crystal field interactions. We conclude that in LaCoO3 there is no intermediate spin state as came out from band-structure calculations. The excited states originate from the high-spin 5T2g term, being 12 meV above the ground 1A1 state. We are convinced that many-electron CEF approach with strong correlations and the atomic-scale orbital magnetism is physically adequate approach to 3d oxides. Keywords: Mott insulator, crystal field, spin-orbit coupling, LaCoO3

cond-mat.str-el↗