T. Akachi

Crystalline structure and the superconducting properties of NbB2+x

The effect of boron excess in the structure and superconducting properties of NbB2 is reported. Rietveld refinements of the x-ray diffraction patterns indicate that boron excess induces significant changes in the Nb–B bond length, increasing the c-axis. In contrast, the B–B bond length remains essentially constant. Magnetization behaviour was studied in the temperature range from 2 to 15 K. We found that for (B/Nb)exp 2.20(2) of boron excess samples display superconductivity with a maximum TC of about 9.8 K at (B/Nb)exp = 2.34(1) .H igh pressure measurements in samples with two different boron contents reveal that TC decreases at different ratios, dTC/dP. Superconducting parameters were determined, indicating that NbB2+x is a type II superconductor. We correlated the change of TC with the evolution of th es tructural parameters and found that it coincides with theoretical predictions of band structure. (Some figures in this article are in colour only in the electronic version)

Study of the crystal structure, superconducting and magnetic properties of Ru1−xFexSr2GdCu2O8

Samples from the Ru1−xFexSr2GdCu2O8 system with x = 0, 0.025, 0.05, 0.075, 0.1 and 0.2, were prepared and their structural, superconducting and magnetic properties were studied. Rietveld refinement of the x-ray diffraction patterns shows that the Fe substitution occurs at both Ru and Cu sites. An increase of Fe concentration produces no significant changes in the Ru–O(3)–Ru bond angle, which is a measure of the rotation of the RuO6 octahedra around the c-axis, or in the Cu–O(1)–Ru bond angle , which is a measure of the canting of the RuO6 octahedra. On the other hand, the Cu–O(2)–Cu bond angle, which is a measure of the buckling of the CuO2 layer, has a slight tendency to decrease with increase of the Fe content. We found that both ferromagnetic and superconducting transition temperatures are reduced with increase of the Fe concentration. Analysis related to the decay of the superconducting and ferromagnetic states is presented.

Site occupancy and Tc degradation in iron substituted YBa2(Cu1−xFex)4O8+δ

Abstract A series of YBa2(Cu1−xFex)4O8+δ samples with x=0.00625, 0.0125, 0.01875, 0.025 and 0.05, were synthesized and characterized by resistance vs. temperature measurements, X-ray diffraction and by Mossbauer spectroscopy. In order to make the site assignment for the Fe atoms, a Rietveld refinement of the X-ray spectra was performed and a point charge calculation of the quadrupole splitting for all the possible oxygen environments around the two Cu sites was carried out, taking into account the two possible ionic states of the iron atoms that occupy those sites. We found that, for low level iron concentrations, the Fe atoms occupy only the Cu(1) sites of the structure in a fivefold pyramidal coordination, with the apical oxygen atom placed along the a-axis between the double chains of the unit cell. The presence of these extra oxygen atoms in the crystal structure could be related to the rapid Tc reduction as iron concentration increases.

Local magnetic fields in the Cu sites of YBa2Cus−xFexOy detected by Mössbauer spectroscopy

Abstract The temperature dependence of the Mossbauer parameters of the spectra of an YBa 2 Cu s−x Fe x O y superconducting sample, with x =0.125, together with a strong temperature dependent asymmetry in one of the three observed quadrupole doublets, suggests the existence of a small magnetic field in the positions of the Fe atoms, which are assumed to be only the Cu(1) sites of the 1-2-3 structure.

Effect of Fe substitution in the structure and superconducting properties of the (Y0.8Pr0.2)Ba2Cu4−xFexO8 system

Abstract The effects of iron substitutions on the superconducting and structural properties of (Y 0.8 Pr 0.2 )Ba 2 Cu 4− x Fe x O 8 were studied. As iron concentration increases, the rate at which T c diminishes with x is the same as for the YBa 2 Cu 4− x Fe x O 8 system, but faster than in the YBa 2 Cu 3− x Fe x O 7− δ system. The Mossbauer spectra of the different samples, together with Rietveld refinements of their corresponding X-ray spectra, indicate that the iron atoms occupy preferably the Cu(1) sites of the double (CuO) 2 chains in fivefold coordination; that is, due to their tri-valency, the iron atoms attract extra oxygen atoms, named O(5), that place themselves along the a -axis in between two adjacent planes of the double (CuO) 2 chains. As x is increased, the Cu(1)–O(4) bond length decreases, whereas the Cu(2)–O(4) one increases. Also, a reduction of the occupation factors of the oxygen atoms in the CuO 2 planes is observed. Some of these changes may be due to the presence of O(5) atoms, which in turn affect the charge-transfer mechanism between the (CuO) 2 double chains and the CuO 2 planes where superconductivity takes place, and could be responsible of the rate at which T c decreases.

Suppression of Tc in the (Y0.9Ca0.1)Ba2Cu4−xFexO8 system

In this paper, the effects produced by iron substitutions in the (Y0.9Ca0.1)Ba2Cu4−xFexO8 system on the superconducting and structural properties are studied. The Rietveld-fit of the crystal structure and Mossbauer spectroscopy results of (Y0.9Ca0.1)Ba2Cu4−xFexO8 samples indicate that the iron atoms occupy the Cu(1) sites of the (Cu–O)2 double chains in fivefold coordination at low iron concentrations. Besides at high iron concentrations the iron atoms occupy the Cu(1) sites of single Cu–O chains and Cu(2) sites in the CuO2 planes of the (Y0.9Ca0.1)Ba2Cu4O8 phase with structural defects. Simultaneously, as iron concentration increases, a faster decrease of Tc is observed in this material compared with the YBa2Cu3−xFexO7−δ system. According to the charge-transfer model proposed for YBa2Cu4O8 under pressure, the decrease in the Cu(1)–O(4) bond length in parallel to the increase in the Cu(2)–O(4) bond length may affect the charge-transfer mechanism leading to the suppression of Tc.

Anomalous behaviour of Moessbauer parameters of a YBa2Cu2.875Fe0.125Oδ superconductor around 110 K

Abstract Transmission Mo¨ssbauer spectra at various temperatures were obtained from YBa2Cu2.875Fe0.125Oδ powder samples. The spectra displayed three quadrupole doublets, one of which behaves differently from the other two. The experimental data allows to infer that the Fe atoms occupy only Cu(1) sites with definite valence states and in three different environments. The model put forward here is consistent with previous experiments.

About the ionic state of iron in the Cu sites of the Nd2−xCexCu2−yFeyO4−δ superconductor

Abstract The substitution of a small fraction of Cu atoms by Fe atoms in the copper oxide superconductors has proven to be useful in sensing, with Mossbauer spectroscopy, the local surroundings of the Cu sites of their structures. In this work we report the results of such substitution in the Nd 1.85 Ce 0.15 Cu 2 O 4−δ system. After annealing in argon at high pressure and temperature, a strong quadrupole doublet is observed indicating that the iron atoms have been incorporated into the superconductor structure.

Iron substitution in the Cu sites of the YBa2Cu4−xFexOδ structure studied by Mössbauer spectroscopy

Abstract The site distribution of 57 Fe in a series of YBa 2 Cu 4−x Fe x O δ superconducting samples ( x = 0.025, 0.05 and 0.075) was determined bu Mossbauer spectroscopy. Resistance vs. temperature studies were also performed in order to determined the influence that increasing concentration of Fe has on T c .

Evidence of High Energy Excitations in High TC Superconductors

Tunneling experiments were performed in Y-Ba-Cu-O samples that show a granular composition that allow the measurements of differential resistance using a conventional modulation technique. Typical tunneling spectra are produced from the samples. These curves show clear structure that could be associated to excitations coupled to the tunneling electrons. The energies of these excitations are of the order of hundreds of millivolts, far to high to be due to phonons. It is not clear yet if these excitations play a role in the superconducting properties of the material. However, some theoretical ideas are presented in order to decide about the possibility for a new mechanism that couples electrons and causes superconductivity.