R. Escudero

Tunnelling and point contact spectroscopy of the density of states in quasicrystalline alloys

Point contact and tunnelling experiments performed at low temperatures were used to study the electronic behaviour of the icosahedral quasicrystalline alloys AlPdRe, AlCuFe, and AlPdMn. With samples of high quality we observed at low temperatures a zero-bias anomaly that we related to the decrease of the electronic density of states (DOS) due to the electron-electron interaction. This interaction tends to diminish the DOS at the Fermi level and can be seen as the energy pseudogap of the alloy. Our experiments indicate that the DOS is strongly modified near and consists of a spiky feature in a broad pseudogap, with the width of the feature of the order of 100 meV or even larger for the AlPdRe, whereas it is as small as 20-22 meV for Al-Cu-Fe and 17-20 meV for Al-Pd-Mn. The broad pseudogap has widths larger than 400 meV for AlPdRe, whereas for AlCuFe it is about 80-90 meV and for AlPdMn it is of the order of 110-122 meV. The studies were performed on three samples of the compositions , , and . The junctions were of the types alloy-Au(In, Al) and alloy-insulator-Au(In, Al), and were studied at different temperatures between that of liquid nitrogen and 2 K, and even to 400 mK for the AlCuFe alloy.

Magnetization studies in quasi two-dimensional palladium nanoparticles encapsulated in a graphite host

Abstract:We show that the numerical method based on the off-equilibrium fluctuation-dissipation relation does work and is very useful and powerful in the study of disordered systems which show a very slow dynamics. We have verified that it gives the right information in the known cases (diluted ferromagnets and random field Ising model far from the critical point) and we used it to obtain more convincing results on the frozen phase of four-dimensional spin glasses. Moreover we used it to study the Griffiths phase of the diluted and the random field Ising models.In this work, magnetization studies on quasi bi-dimensional nanoparticles of Pd encapsulated in a graphite host as a function of magnetic field and temperature are presented. We found that an important magnetic signal can be attributed to the Pd nanoparticles. The obtained magnetic behaviour is interpreted in view of theoretical studies which predict ferromagnetism in slabs of Pd with some monolayers of thickness.

Single crystal synthesis of [(C6H5)4P]2[C70][I] by electrocrystallization and experimental determination of the g-value anisotropy of C70•- and C60•- at 4.2 K

We describe the synthesis by electrocrystallization and the crystal packing of [(C6H5)4P]2[C70][I]. Magnetic susceptibility measurements show Curie-Weiss behaviour. ESR measurements on single crystals of [(C6H5)4P]2[C70][I] and [(C6H5)4P]2[C60][I]x (0<x⪡1) allowed us to determine the g tensor principal values at 4.2 K for both radicals C60•- (gx = gy = 1.9970, gz = 1.9998, gav = 1.9979) and C70•- (gx = gy = 1.9996, gz (long axis) = 2.0150, gav = 2.0047).

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)

Structure and magnetic properties of the weak ferromagnet Sr2-xLaxIrO4

The 5d electron-based Sr2?xLaxIrO4 system (0?x?0.2) has been synthesized by a solid-state route. The x = 0 composition is a nonmetallic weak ferromagnet with a Curie temperature at about 240?K. The crystal structure behaviour and magnetic properties exhibited by this Sr2?xLaxIrO4 system can be explained on the basis of the extended character of the 5d electrons of the Ir cation and its valence states. Rietveld analysis of x-ray powder diffraction on Sr2IrO4 agrees well with previous structural neutron experiments. Furthermore, density functional theory (DFT) calculations predict that I41/acd represents a more stable crystal structure than K2NiF4 (I4/mmm). Electrical resistivity and magnetic properties of Sr2?xLaxIrO4 are strongly dependent on the Ir3+ content. The Sr2?xLaxIrO4 magnetic behaviour in the range of 2?240?K can be ascribed to a weak ferromagnet, produced by an array of canted antiferromagnetically ordered Ir4+ magnetic moments.

Superconducting energy gap of single-crystal Bi2Sr2Ca1Cu2O8+δ by electron tunneling

Abstract We report measurements of the superconducting energy gap in a Bi 2 Sr 2 Ca 1 Cu 2 O 8+δ single crystal using point contact electron tunneling. We show data related to the energy gap spectra in these new ceramics. The energy gap was determined to be around 27–32 meV. The results give values for the ratio 2Δ ( T = 4.6 K )/ k B T c (80 K ) of between 7.8 and 9.3. The data also reveal a reproducible structure at energies of 67 and 120 meV.

Magnetic anomaly in superconducting FeSe

Synthesis, electrical and magnetic characterization of a superconducting FeSe0.85 compound is reported. An anomaly in the magnetization against temperature around 90 K is observed. The magnetic characterization of a commercial compound with nominal FeSe stoichiometry is also presented. The overall magnetic behaviors as well as the magnetic anomaly in both compounds are discussed in terms of magnetic impurities and secondary phases.

Fitting of transport measurements in polycrystalline La2/3Ca1/3MnO3

Using a previous qualitative explanation to describe the transport properties of polycrystalline La2/3Ca1/3MnO3 thick films, we achieved a good fit of the temperature dependence of the resistance R(T). Depending on the sample, we have observed different metal–insulator (MI) transitions while the magnetic behavior is always similar. Small regions of depleted Tc adjacent to the grain boundary could have an important resistance contribution without affecting the magnetic properties in an appreciable manner. In this work, we achieve a quantitative explanation for the different transport behaviors that we have observed experimentally.

Electron tunneling in superconducting Ba0.6K0.4BiO3

Abstract We report tunneling measurements using superconducting bulk samples of Ba 0.6 K 0.4 BiO 3 and evaporated counterelectrodes of Sn. With our procedure, we find reproducible tunneling characteristics that show the gap feature and high frequency structure in the d I /d V versus V curves. This structure may be related to the phonon spectrum or to the excitations that give rise to superconductivity in these ceramic materials. The energy gap evolution with temperature follows BCS-like behavior, with ratio 2Δ/ K B T c at T =0 K in the range of intermediate coupling.

Influence of ferromagnetic thickness on structural and magnetic properties of exchange-biased manganite superlattices

The structural and magnetic properties of superlattice structures of alternating ferromagnetic La2∕3Ca1∕3MnO3 (F-LCMO) and antiferromagnetic La1∕3Ca2∕3MnO3 (AF-LCMO) layers were systematically studied as functions of F-LCMO layer thickness, tF. Samples were grown via a high-oxygen pressure sputtering process. Magnetic hysteresis measurements after field cooling revealed an exchange bias, Hex, at low temperatures in such superlattices. We found a correlation of the structural and magnetic properties with tF. In particular, we observed diminished resistance, increased metal-insulator transition temperature, TMI, as well as increased Curie temperature with increasing tF. Additionally, we found that the temperature dependence of Hex*tF for superlattices with the same antiferromagnetic layer thickness, tAF, is a unique function and independent of tF. We also find that the low-temperature saturation magnetization, MS, follows a power-law dependence with temperature, according to M0(1−BTα) with an exponent of α=...