arXiv subjects

P. Nagel

Publications and source records attributed to P. Nagel.

15 recordsLinked to original sources

arXiv2026

From perovskite to infinite-layer nickelates: hole concentration from x-ray absorption

The difficulty of determining cation concentrations and oxygen stoichiometry in infinite-layer nickelate thin films has so far prevented clear experimental identification of the nickel electron configuration in the superconducting phase. We used soft x-ray absorption spectroscopy to study the successive changes in PrNiO$_x$ thin films at various intermediate stages of topotactic reduction with $x=2-3$. By comparing the Ni-$L$ edge spectra to single and double cluster ligand-field calculations, we find that none of our samples exhibit a pure $d^9$ configuration. Our quantitative analysis using the charge sum rule shows that even when films are maximally reduced, the averaged number of nickel $3d$ holes is 1.35. Superconducting samples have even higher values, calling into question the previously assumed limit of hole doping. Concomitant changes in the oxygen $K$-edge absorption spectra upon reduction indicate the presence of oxygen $2p$ holes, even in the most reduced films. Overall, our results suggest a complex interplay of hole doping mechanisms resulting from self-doping effects and oxygen non-stoichiometry.

cond-mat.supr-con↗

arXiv2017

SU(3) sphaleron: Numerical solution

We complete the construction of the sphaleron $\widehat{S}$ in $SU(3)$ Yang-Mills-Higgs theory with a single Higgs triplet by solving the reduced field equations numerically. The energy of the $SU(3)$ sphaleron $\widehat{S}$ is found to be of the same order as the energy of a previously known solution, the embedded $SU(2)\times U(1)$ sphaleron $S$. In addition, we discuss $\widehat{S}$ in an extended $SU(3)$ Yang-Mills-Higgs theory with three Higgs triplets, where all eight gauge bosons get an equal mass in the vacuum. This extended $SU(3)$ Yang-Mills-Higgs theory may be considered as a toy model of quantum chromodynamics without quark fields and we conjecture that the $\widehat{S}$ gauge fields play a significant role in the nonperturbative dynamics of quantum chromodynamics (which does not have fundamental scalar fields but gets a mass scale from quantum effects).

hep-ph↗

arXiv2016

Of Substitution and Doping: Spatial and Electronic Structure in Fe Pnictides

A highly intriguing aspect in iron-pnictide superconductors is the composition-dependent electronic structure, in particular the question if and how charge carriers are introduced to the system upon substitution of Ba by alkali metals or of Fe by other transition metals, TM. We report on a systematic study of spatial structure and electronic states by x-ray diffraction and x-ray absorption on a large number of compositions in the (Ba,K)(Fe,TM)2As2 family. The coherent combination of detailed structural information with an in-depth analysis of the electronic structure allows us to sensitively disentangle (charge-carrier) "doping" effects from "substitutional" effects. Results include a doping character that is site-decoupled, as well as TM 3d energy-level schemes that exhibit non-standard level sequences and even t2-e level crossings. Our study indicates that doping per se seems to play a lesser role than expected for pnictide superconductivity and magnetism.

cond-mat.supr-con↗

arXiv2014

X-ray absorption spectroscopy study of the electronic and magnetic proximity effects in YBa$_2$Cu$_3$O$_{7}$/La$_{2/3}$Ca$_{1/3}$MnO$_3$ and La$_{2-x}$Sr$_x$CuO$_4$/La$_{2/3}$Ca$_{1/3}$MnO$_3$ multilayers

With x-ray absorption spectroscopy we investigated the orbital reconstruction and the induced ferromagnetic moment of the interfacial Cu atoms in YBa$_2$Cu$_3$O$_{7}$/La$_{2/3}$Ca$_{1/3}$MnO$_3$ (YBCO/LCMO) and La$_{2-x}$Sr$_{x}$CuO$_4$/La$_{2/3}$Ca$_{1/3}$MnO$_3$ (LSCO/LCMO) multilayers. We demonstrate that these electronic and magnetic proximity effects are coupled and are common to these cuprate/manganite multilayers. Moreover, we show that they are closely linked to a specific interface termination with a direct Cu-O-Mn bond. We furthermore show that the intrinsic hole doping of the cuprate layers and the local strain due to the lattice mismatch between the cuprate and manganite layers are not of primary importance. These findings underline the central role of the covalent bonding at the cuprate/manganite interface in defining the spin-electronic properties.

cond-mat.str-el↗

arXiv2012

Magnetic proximity effect in $\mathbf{YBa_2Cu_3O_7/La_{2/3}Ca_{1/3}MnO_3}$ and $\mathbf{YBa_2Cu_3O_7/LaMnO_{3+δ}}$ superlattices

Using neutron reflectometry and resonant x-ray techniques we studied the magnetic proximity effect (MPE) in superlattices composed of superconducting YBa$_2$Cu$_3$O$_7$ (YBCO) and ferromagnetic-metallic (FM-M) La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) or ferromagnetic-insulating (FM-I) LaMnO$_{3+δ}$ (LMO). We find that the MPE strongly depends on the electronic state of the manganite layers, being pronounced for the FM-M LCMO and almost absent for FM-I LMO. We also detail the change of the magnetic depth profile due to the MPE and provide evidence for its intrinsic nature.

cond-mat.str-el↗

arXiv2012

Electronic structure of single-crystalline Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$ probed by x-ray absorption spectroscopy: Evidence for isovalent substitution of Fe$^{2+}$ by Co$^{2+}$

The substitutional dependence of valence and spin-state configurations of Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($x =$ 0, 0.05, 0.11, 0.17, and 0.38) is investigated with near-edge x-ray absorption fine structure at the $L_{2,3}$ edges of Fe, Co, and As. The present data provide direct spectroscopic evidence for an effectively isovalent substitution of Fe$^{2+}$ by Co$^{2+}$, which is in contrast to the widely assumed Co-induced electron-doping effect. Moreover, the data reveal that not only does the Fe valency remain completely unaffected across the entire doping range, but so do the Co and As valencies as well. The data underline a prominent role of the hybridization between (Fe,Co) 3$d_{xy}$, $d_{xz}$, $d_{yz}$ orbitals and As $4s/4p$ states for the band structure in $A$(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and suggest that the covalency of the (Fe,Co)-As bond is a key parameter for the interplay between magnetism and superconductivity.

cond-mat.supr-con↗

arXiv2011

Spectroscopic study of impurities and associated defects in nanodiamonds from Efremovka (CV3) and Orgueil (CI) meteorites

The results of spectroscopic and structural studies of phase composition and of defects in nanodiamonds from Efremovka (CV3) and Orgueil (CI) chondrites indicate that nitrogen atomic environment in meteoritic nanodiamonds (MND) is similar to that observed in synthetic counterparts produced by detonation and by the Chemical Vapour Deposition (CVD)-process. Most of the nitrogen in MND appears to be confined to lattice imperfections, such as crystallite/twin boundaries and other extended defects, while the concentration of nitrogen in the MND lattice is low. It is suggested that the N-rich sub-population of MND grains may have been formed with high growth rates in environments rich in accessible N (i.e., N in atomic form or as weakly bonded compounds). For the first time the silicon-vacancy complex (the "silicon" defect) is observed in MND by photoluminescence spectroscopy.

cond-mat.mes-hall↗

arXiv2010

Orbital-Resolved Partial Charge Transfer from the Methoxy Groups of Substituted Pyrenes in Complexes with Tetracyanoquinodimethane - a NEXAFS Study

It is demonstrated that the near-edge X-ray absorption fine structure (NEXAFS) provides a powerful local probe of functional groups in novel charge transfer (CT) compounds. Microcrystals of tetra- and hexamethoxypyrene as donors with the strong acceptor tetracyanoquinodimethane (TMPx/HMPx - TCNQy) were grown from solution via vapour diffusion in different stoichiometries x:y = 1:1, 1:2 and 2:1. Owing to the element specificity of NEXAFS, the oxygen and nitrogen K-edge spectra are direct spectroscopic fingerprints of the donating and accepting moieties. The orbital selectivity of the NEXAFS resonances allows to precisely elucidate the participation of specific orbitals in the charge-transfer process. In the present case charge is transferred from methoxy-orbitals 2e (PI*) and 6a1 (SIGMA*) to the cyano-orbitals b3g and au (PI*) and - to a weaker extent - to b1g and b2u (SIGMA*). The occupation of 2e reflects the anionic character of the methoxy groups. Surprisingly, the charge transfer increases with increasing HMP content of the complex. As additional indirect signature, all spectral features of the donor and acceptor are shifted to higher and lower photon energies, respectively. Providing quantitative access to the relative occupation of specific orbitals, the approach constitutes the most direct probe of the charge-transfer mechanism in organic salts found so far. Although demonstrated for the specific example of pyrene-derived donors with the classical acceptor TCNQ, the method is very versatile and can serve as routine probe for novel CT-complexes on the basis of functionalized polycyclic aromatic hydrocarbons.

cond-mat.mtrl-sci↗

arXiv2010

YBa$_2$Cu$_3$O$_7$/La$_{0.7}$Ca$_{0.3}$MnO$_3$ bilayers: Interface coupling and electric transport properties

Heteroepitaxially grown bilayers of ferromagnetic La$_{0.7}$Ca$_{0.3}$MnO$_3$ (LCMO) on top of superconducting YBa$_2$Cu$_3$O$_7$ (YBCO) thin films were investigated by focusing on electric transport properties as well as on magnetism and orbital occupation at the interface. Transport measurements on YBCO single layers and on YBCO/LCMO bilayers, with different YBCO thickness $d_Y$ and constant LCMO thickness $d_L=50$\,nm, show a significant reduction of the superconducting transition temperature $T_c$ only for $d_Y<10$\,nm,with only a slightly stronger $T_c$ suppression in the bilayers, as compared to the single layers. X-ray magnetic circular dichroism (XMCD) measurements confirm recently published data of an induced magnetic moment on the interfacial Cu by the ferromagnetically ordered Mn ions, with antiparallel alignment between Cu and Mn moments. However, we observe a significantely larger Cu moment than previously reported, indicating stronger coupling between Cu and Mn at the interface. This in turn could result in an interface with lower transparency, and hence smaller spin diffusion length, that would explain our electric transport data, i.e.smaller $T_c$ suppression. Moreover, linear dichroism measurements did not show any evidence for orbital reconstruction at the interface, indicating that a large change in orbital occupancies through hybridization is not necessary to induce a measurable ferromagnetic moment on the Cu atoms.

cond-mat.supr-con↗

arXiv2009

Soft x-ray magnetic circular dichroism of LaCoO$_{3}$, La$_{0.7}$Ce$_{0.3}$CoO$_{3}$, and La$_{0.7}$Sr$_{0.3}$CoO$_{3}$ films: Evidence for Co valency-dependent magnetism and magnetic anisotropy

Epitaxial thin films of undoped LaCoO$_{3}$\@, of electron-doped La$_{0.7}$Ce$_{0.3}$CoO$_{3}$\@, and of hole-doped La$_{0.7}$Sr$_{0.3}$CoO$_{3}$\@ exhibit ferromagnetic order with a transition temperature $T_{\rm{C}}$ $\approx$ 85 K, 22 K, and 240 K, respectively. The spin-state structure for these compounds was studied by soft x-ray magnetic circular dichroism at the Co $L_{2,3}$ and O $K$ edges. It turns out that a higher spin state of the Co$^{3+}$ ions is responsible for the magnetism in LaCoO$_{3}$\@ while for La$_{0.7}$Ce$_{0.3}$CoO$_{3}$ the Co$^{3+}$ ions are in a low-spin state and the ferromagnetism is predominantly determined by Co$^{2+}$\@. For La$_{0.7}$Sr$_{0.3}$CoO$_{3}$\@, on the other hand, the magnetism originates from higher spin states of Co$^{3+}$ and Co$^{4+}$\@. Moreover, a strong magnetic anisotropy is observed for all systems.

cond-mat.str-el↗

arXiv2008

Suppression of spin-state transition in epitaxially strained LaCoO_{3}

Epitaxial thin films of LaCoO_{3} (E-LCO) exhibit ferromagnetic order with a transition temperature T_c = 85 K, while polycrystalline thin LaCoO_{3} films (P-LCO) remain paramagnetic. The temperature-dependent spin-state structure for both E-LCO and P-LCO was studied by x-ray absorption spectroscopy at the Co L_{2,3} and O K edges. Considerable spectral redistributions over temperature are observed for P-LCO. The spectra for E-LCO, on the other hand, do not show any significant changes for temperatures between 30 K and 450 K at both edges, indicating that the spin state remains constant and that the epitaxial strain inhibits any population of the low-spin (S = 0) state with decreasing temperature. This observation identifies an important prerequisite for ferromagnetism in E-LCO thin films.

cond-mat.str-el↗

arXiv2003

Two Pseudogaps in the Cuprates: Meingast et al. Reply

In classical superconductors Cooper-pair formation and phase coherence occur simultaneously as the temperature is lowered below Tc. In high-temperature superconductors (HTSC), on the other hand, the small superfluid density and low associated phase stiffness of the superconducting condensate are expected to lead to a separation of the Cooper-pair formation and the phase-coherence temperatures, especially in underdoped materials [3]. The only real phase transition in this scenario is the 3d-XY phase-ordering transition at Tc [3,4]. In our Letter [2] we showed that Tc in underdoped and optimally doped YBCO is just such a phase-ordering temperature, and then the question naturally arises - where do the Cooper pairs form? The observed 3d-XY scaling of our thermal expansion data over a wide temperature range [2] suggests that pairing occurs at temperatures considerably above Tc, and it thus appeared quite natural for us to associate the opening of the pseudogap at T*.

cond-mat.supr-con↗

arXiv2002

Magnetic Field Dependent Coherent Polarization Echoes in Glasses

The unexpected finding of a strong magnetic field dependence of the dielectric properties of insulating glasses at very low temperatures has been a puzzling problem since its discovery. Several attempts have been made to explain this striking phenomenon. In order to obtain information on the origin of the magnetic field effects we have studied coherent properties of atomic tunneling systems in glasses by polarization echo experiments in magnetic fields. Our results clearly favor a model based on the assumption that nuclear quadrupoles play a crucial role for the observed magnetic field dependence.

cond-mat.dis-nn↗

arXiv2000

Phase fluctuations and the pseudogap in YBa2Cu3Ox

The thermodynamics of the superconducting transition is studied as a function of doping using high-resolution expansivity data of YBa2Cu3Ox single crystals and Monte-Carlo simulations of the anisotropic 3D-XY model. We directly show that Tc of underdoped YBa2Cu3Ox is strongly suppressed from its mean-field value (Tc-MF) by phase fluctuations of the superconducting order parameter. For overdoped YBa2Cu3Ox fluctuation effects are greatly reduced and Tc ~ Tc-MF . We find that Tc-MF exhibits a similar doping dependence as the pseudogap energy, naturally suggesting that the pseudogap arises from phase-incoherent Cooper pairing.

cond-mat.supr-con↗

arXiv1998

Intermolecular bond stability of C60 dimers and 2D pressure-polymerized C60

The thermal stability of C60 dimers and 2D pressure-polymerized C60 is studied using high-resolution capacitance dilatometry. The transformation of both the dimer and the polymer phases back to 'normal' C60 is excellently described by a simple thermally activated process, with activation energies of 1.75 1 0.1 eV (dimer) and 1.9 1 0.2 eV (polymer). These results are compared to previous data of 1D-polymerized C60 and photo-polymerized C60. The thermal expansivity of the 2D-polymer phase is as much as a factor of ten smaller than that of pure C60 and approaches values for diamond.

cond-mat↗