Ce 4f Electrons Research Articles

Magnetism and superconductivity in strongly correlated CeRhIn5

Specific heat studies of CeRhIn5 as functions of pressure and magnetic field have been used to explore the relationship between magnetism and unconventional superconductivity, both of which involve the 4f electron of Ce. Results of these studies cannot be understood as a simple competition for Fermi-surface states and require a new conceptual framework.

CeFePO: A Heavy Fermion Metal with Ferromagnetic Correlations

The ground state properties of CeFePO, a homologue of the new high temperature superconductors RFePnO1-xFx, were studied by means of susceptibility, specific heat, resistivity, and NMR measurements on polycrystals. All the results demonstrate that this compound is a magnetically nonordered heavy fermion metal with a Kondo temperature TK approximately 10 K, a Sommerfeld coefficient gamma=700 mJ/mol K2, and a mass enhancement factor of the order of 50. Analysis of the susceptibility data and of the spin relaxation time indicates that the strong electronic correlation effects originate from the Ce-4f electrons rather than from Fe-3d electrons. An enhanced Sommerfeld-Wilson ratio R=5.5 as well as a Korringa product S0/T1TK2 approximately 0.065 well below 1 indicate the presence of ferromagnetic correlations. Therefore, CeFePO appears to be on the nonmagnetic side of a ferromagnetic instability.

First-Principles Study on the Effects of Zr Dopant on the CO Adsorption on Ceria

With use of the first-principles density functional theory with the inclusion of the on-site Coulomb interaction of the Ce4f electrons (DFT+U) method, the adsorption of CO on the (110) surface of a stoichiometric Ce0.75Zr0.25O2 system is studied systematically. It is found that, (1) in addition to the carbonate-like CO3 structures, there exists a new adsorption species (a bent CO2− structure) on the Ce0.75Zr0.25O2(110) surface, and (2) the Zr-dopant can enhance the adsorption of CO and improve the CO oxidation process on the ceria surface by promoting the desorption of CO2.

Superconductivity at 41 K and Its Competition with Spin-Density-Wave Instability in LayeredCeO1−xFxFeAs

A series of layered CeO1-xFxFeAs compounds with x=0 to 0.20 are synthesized by the solid state reaction method. Similar to the LaOFeAs, the pure CeOFeAs shows a strong resistivity anomaly near 145 K, which was ascribed to the spin-density-wave instability. F doping suppresses this instability and leads to the superconducting ground state. Most surprisingly, the superconducting transition temperature could reach as high as 41 K. Such a high T_{c} strongly challenges the classic BCS theory based on the electron-phonon interaction. The closeness of the superconducting phase to the spin-density-wave instability suggests that the magnetic fluctuation plays a key role in the superconducting pairing mechanism. The study also reveals that the Ce 4f electrons form local moments and are ordered antiferromagnetically below 4 K, which could coexist with superconductivity.

Direct Observation of Dispersive Kondo Resonance Peaks in a Heavy-Fermion System

Ce 4d-4f resonant angle-resolved photoemission spectroscopy was carried out to study the electronic structure of strongly correlated Ce 4f electrons in a quasi-two-dimensional nonmagnetic heavy-fermion system CeCoGe1.2Si0.8. For the first time, dispersive coherent peaks of an f state crossing the Fermi level, the so-called Kondo resonance, are directly observed together with the hybridized conduction band. Moreover, the experimental band dispersion is quantitatively in good agreement with a simple hybridization-band picture based on the periodic Anderson model. The obtained physical quantities, i.e., coherent temperature, Kondo temperature, and mass enhancement, are comparable to the results of thermodynamic measurements. These results manifest an itinerant nature of Ce 4f electrons in heavy-fermion systems and clarify their microscopic hybridization mechanism.

Electronic structure of CenMmIn2m+3n, where n = 1, 2; m = 0, 1;M = Co, Rh or Ir: experiment and calculations

We present a detailed study of the electronic structure of theCenMmIn2m+3n (M = Co, Rh,Ir; n = 1, 2and m = 0, 1) series of Ce intermetallic compounds and of the referenceLaIn3 compound. The ground state of these heavy-fermion (HF) materials can be tunedbetween antiferromagnetic (AF) and superconducting (SC), with pressure ordoping as the tuning parameter. Performing the x-ray photoelectron spectroscopy(XPS) measurements on the Ce 3d core levels as well as on the valence bandstates we analyse the dependence of both the Ce 4f band character and physicalproperties on the kind of transition metal atom M and on the number ofCeIn3 layersintervened by the MIn2 layers in the investigated family of compounds. We draw a parallel between theXPS valence band spectra and ab initio band structure calculations based onthe full-potential linearized augmented plane-wave (FP-LAPW) method. Weanalyse changes in valence band states within the whole family of materials. Wecompare the experimental magnetic moments on Ce atoms with the theoretical onescalculated within different approximations for exchange–correlation potential.Finally, we have shown that the Ce 4f electrons participate in bonding formation forall investigated compounds. Our study indicates that the observed changes inthe 4f band on-site hybridization energy result from the reconstruction of thecharge density distribution driven by transition metal atoms inserted into theCeIn3 structure.

Luminescence of Intraporous Cerium(III) Complexes in Alkyl Sulfonic Mesoporous Silica

Mesoporous silica materials derivatized with propyl-sulfonic groups (SBA-15) with a monovalent cation-exchange capacity of ∼1 meq/g were exchanged with Ce(III) ions to ∼0.5 meq/g to form Ce(III)SBA material displaying high-intensity purple photoluminescence. XPS and NIR-DRS analysis suggests the Ce(III) ion is bound to two proximal propyl-sulfonate groups and capped with a hydroxyl in a complex [≡Si−(CH2)3SO3]2Ce(III)OH·xH2O (x > 4), where the coordinated water can be almost completely removed by anoxic dehydration at 350 C to x < 0.2. The white hydrated (x ≥ 4) CeSBA and the brown-colored dehydrated (x < 0.2) CeSBA exhibit fluorescence with comparable integrated intensities at 368 and 394 nm, respectively. The Ce(III) 4f electron is located by XPS above the top of the Si 3p O 2p valence band, thus identifying the Ce(III) 4f orbital as the HOMO. Photoluminescence emission and excitation spectra of the 5d → 4f transition display Stokes shifts and structure sensitivity consistent with a direct transition pr...

Tuning LDA+U for electron localization and structure at oxygen vacancies in ceria

We examine the real space structure and the electronic structure (particularly Ce4f electron localization) of oxygen vacancies in CeO(2) (ceria) as a function of U in density functional theory studies with the rotationally invariant forms of the LDA+U and GGA+U functionals. The four nearest neighbor Ce ions always relax outwards, with those not carrying localized Ce4f charge moving furthest. Several quantification schemes show that the charge starts to become localized at U approximately 3 eV and that the degree of localization reaches a maximum at approximately 6 eV for LDA+U or at approximately 5.5 eV for GGA+U. For higher U it decreases rapidly as charge is transferred onto second neighbor O ions and beyond. The localization is never into atomic corelike states; at maximum localization about 80-90% of the Ce4f charge is located on the two nearest neighboring Ce ions. However, if we look at the total atomic charge we find that the two ions only make a net gain of (0.2-0.4)e each, so localization is actually very incomplete, with localization of Ce4f electrons coming at the expense of moving other electrons off the Ce ions. We have also revisited some properties of defect-free ceria and find that with LDA+U the crystal structure is actually best described with U=3-4 eV, while the experimental band structure is obtained with U=7-8 eV. (For GGA+U the lattice parameters worsen for U>0 eV, but the band structure is similar to LDA+U.) The best overall choice is U approximately 6 eV with LDA+U and approximately 5.5 eV for GGA+U, since the localization is most important, but a consistent choice for both CeO(2) and Ce(2)O(3), with and without vacancies, is hard to find.

Normal state properties at a field-tuned quantum-critical point in the heavy-fermion superconductor [formula omitted

CeRhIn 5 is a prototypical heavy-fermion antiferromagnet where the localized 4f electron of Ce hybridizes weakly with ligand electrons. Applying pressure suppresses the antiferromagnetism and induces unconventional superconductivity when the same Ce 4f electrons become delocalized and participate in the formation of Cooper pairs. The emergent superconducting phase unavoidably hides the existence of quantum-critical point, preventing direct access to it. Measurements of the heat capacity and electrical resistivity under magnetic field and pressure reveal unusual normal state properties at a projected quantum-critical point ( P 2 ) : divergence in C / T and sublinear temperature dependence in the resistivity.

Band structure and Fermi surface of heavy Fermion compounds Ce 2TIn 8[formula omitted] studied by angle-resolved photoemission spectroscopy

We report a systematic study on the electronic structure of Ce-based heavy Fermion materials Ce 2TIn 8 ( T = Rh , Co, Ir) by high-resolution angle-resolved (ARPES) and 4d–4f resonant (RPES) photoemission spectroscopies. We have experimentally determined the band structure and Fermi surface of these three compounds, and found that the Fermi surfaces of three compounds are very similar to each other, having quasi two-dimensional cylindrical Fermi surfaces centered at the M(A) point in the Brillouin zone. RPES results have confirmed the localized picture and, in addition, revealed that Ce 4f electrons in Ce 2RhIn 8 have a relatively stronger localized nature than those in Ce 2CoIn 8. This difference in the localized character explains well the difference in the magnetic property of these compounds.

Electronic interaction effects in rare‐earth alloys from 2D‐ACAR experiments

AbstractMeasurement of the two dimensional angular correlation of the electron‐positron annihilation radiation (2D‐ACAR) complemented with ab‐initio calculations can provide decisive information about the character of the f‐electrons in rare earth compounds and are the prerequisite to the study of electron correlations. We provide examples of systems where good approximations of the archetype f‐electron localized and f‐electron itinerant behaviours, apply. i) In the case of the antiferromagnetic heavy fermion and superconductor CeIn3 the multisheet Fermi Surface (FS), reconstructed from our measurements in the paramagnetic phase, agrees closely with the predictions of band structure calculations regarding the Ce 4f electrons as fully localized. ii) On the other hand, our studies of the antiferromagnet actinide based UGa3 in the paramagnetic phase, compared with calculations which include the effects due to the non uniform positron density and the electron‐positron correlations, produce a substantial evidence that an unconstrained 5f‐electron itinerant description applies. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Photoemission study on heavy fermion superconductors

We have performed angle-resolved photoemission experiments for heavy fermion superconductors CeIrIn5 and UPd2Al3. It was found that the Ce 4f electrons in CeIrIn5 are nearly localized, but the small itinerant component also exists in the ARPES spectra. For UPd2Al3, on the other hand, the strongly dispersive bands of U 5f origin were observed, suggesting that the U 5f electronic states are dominated by the itinerant character. We have found that these two compounds have very different natures of f-electrons although both of them show the coexistence or competition of the superconductivity and the magnetic ordering.

Kondo Effect and Antiferromagnetism in CeCu2Ge2: An Electronic Structure Study

Electronic structure results are presented for the heavy fermion compound CeCu2Ge2. The main focus of the present paper is interplay of Kondo effect and antiferromagnetic instability. We examine properties such as the dispersion of the conduction states and the Fermi surface topology. Electronic band structure calculations are compared treating the Ce 4f electron as localized (atomic-like) and as itinerant (band-like). The results suggest that the incipient Kondo effect as reflected in the transfer of f spectral weight to the low-energy regime is important for the formation of the observed long-range magnetic order.

Electronic and magnetic states of Ce 4f electrons in CeRh 3B 2

CeRh 3B 2 is a ferromagnet with exceptionally high Curie temperature ( T C = 115 K) among the Ce compounds with no transition metal magnetic element. Magnetic circular dichroism of soft X-ray photoabsorption (XMCD) has revealed that the orbital angular momentum of Ce 4f electron is quenched to some extent. In addition, preferential occupation of the Ce 4f orbital pointing to the c-axis has been confirmed by the soft X-ray photoabsorption measurements with different geometries. On the other hand, resonant photoemission around the Ce 3d → 4f photoabsorption edge has been adopted in order to disentangle the Ce 4f and Rh 4d components in the valence band. The Rh 4d spectrum is qualitatively consistent with the band structure calculation. Although the Ce 4f spectrum has a peak near the Fermi level, the spectral shape is qualitatively different from that of the typical Kondo materials and is rather similar to that of itinerant Ce 4f systems.

Three-dimensional Fermi-surface of [formula omitted] studied by angle-resolved photoemission

Angle-resolved photoemission spectroscopy on a quasi-two-dimensional strongly correlated Ce 4f electron compound CeTe 2 has been performed to elucidate the importance of the three-dimensionality at the electronic structure in understanding its anomalous physical properties such as a complicated charge density wave formation. We have found that the Fermi surface as well as the band structure show the characteristic three-dimensionalities as follows: (1) a systematic reduction of the spectral weight at the hole-like Fermi surface along the Γ M line away from the ZR line suggests the imperfect nesting along the [ 1 1 0 ] axis; (2) the anomalous dispersive feature which is suggestive of the “band-folding” along the Γ Z line. The observed three-dimensionality indicates the existence of the three-dimensional nesting vector on CeTe 2 .

Pressure-induced antiferromagnetic superconductivity in [formula omitted]: A [formula omitted]-NQR study under pressure

We report on antiferromagnetic (AF) properties of pressure-induced superconductivity in CeNiGe 3 via the Ge 73 nuclear-quadrupole-resonance (NQR) measurements under pressure ( P). The NQR-spectrum measurements have revealed that the incommensurate antiferromagnetic ordering is robust against increasing P with the increase of ordered moment and ordering temperature. Nevertheless the measurements of nuclear spin-lattice relaxation rate ( 1 / T 1 ) have pointed to the onset of superconductivity as a consequence of Ce-4f electrons delocalized by applying P. The emergence of superconductivity under the development of AF order suggests that a novel type of superconducting mechanism works in this compound.

Angle-resolved photoemission spectroscopy study of [formula omitted

The Ce 4f electronic structure of a CDW system CeTe 2 has been investigated using high-resolution angle-resolved photoemission spectroscopy (ARPES). By employing the fine-structure (FS) resonance, the Ce 4f ARPES spectra have been measured. The FS resonance ARPES shows a typical two-peak structure with the peaks at ∼ - 4 and ∼ - 1 eV , in which the latter peak shifts with increasing emission angle by ∼ 300 meV . The Ce 4f states have the negligibly small intensity near E F , reflecting the minor contribution from Ce 4f electrons to the metallic ground state of CeTe 2 . This implies that Te(1) 5p and Ce 5d electrons are involved in the CDW formation in CeTe 2 .

Electronic structure of [formula omitted] studied by Ce [formula omitted] resonant photoemission

We have performed Ce 4 d – 4 f resonant photoemission (RPE) spectroscopy on the isostructural heavy fermion system CeNiGe 2 - x Si x ( 0 ⩽ x ⩽ 1 ) . With increasing x, the unit volume decreases by about 5% and the ground state originating in Ce 4f character varies from an antiferromagnetic regime (AFM, 0 ⩽ x ⩽ 0.8 ) to a quantum critical point (QCP, x = 1 ). RPE data reveals that the cf-hybridization intensity becomes stronger as the ground state of Ce 4f electron changes from AFM to QCP, which is exclusively caused by the increase of chemical pressure due to the reduction of lattice constant in CeNiGe 2 - x Si x system.

Fermi surfaces of the half-Heusler compounds [formula omitted

We report on the Fermi surface in the correlated half-Heusler compounds Ce 1 - x La x BiPt . In CeBiPt, as well as in Ce 0.95 La 0.05 BiPt , we find a temperature-dependent Fermi-surface topology. In addition, we observe a field-induced change of the electronic band structure as discovered by electrical-transport measurements in pulsed magnetic fields. For magnetic fields above ∼ 25 T , in a simple one band picture, the charge-carrier concentration determined from Hall-effect measurements increases nearly 30%, whereas the Shubnikov-de Haas (SdH) signal disappears at the same field. In the non-4f compound LaBiPt the Fermi surface remains unaffected, suggesting that these features are intimately related to the Ce 4f electrons. Electronic band–structure calculations point to a 4f-polarization-induced change of the Fermi-surface topology.

Bulk 3-D Fermi surfaces of [formula omitted] proved by soft X-ray ARPES

Bulk sensitive angle-resolved photoemission spectroscopy (ARPES) study was performed to probe the three-dimensional bulk Fermi surfaces of CeRu 2 Ge 2 in the paramagnetic phase, which is not accessible by the de Haas–van Alphen (dHvA) studies. Since Ce 4f electrons of CeRu 2 Ge 2 are thought to be localized, we compared the observed Fermi surfaces with the band-structure calculation of 4f electron localized model LaRu 2 Ge 2 . The observed electronic states are slightly different from both the band calculation for LaRu 2 Ge 2 and dHvA measurement for CeRu 2 Ge 2 in the ferromagnetic phase. Magnetic interaction and Ce 4f hybridization should be considered to fully understand the differences in electronic structures.