K. Rogacki

Al substitution in MgB2 crystals: Influence on superconducting and structural properties

Single crystals of Mg1-xAlxB2 have been grown at a pressure of 30 kbar using the cubic anvil technique. Precipitation free crystals with x 0.1. This is in a form of nonsuperconducting MgAlB4 domains in the structure of a superconducting Mg1-xAlxB2 matrix. Resistivity and magnetic investigations show the slight increase of the upper critical field, H-c2, for H parallel to c for the samples with small x, a significant reduction of the H-c2 anisotropy at lower temperatures, and a decrease of the residual resistance ratio value for Al substituted samples as compared to those of unsubstituted crystals. Superconducting gaps variation as a function of Al content, investigated with point contact spectroscopy for the series of the crystals with T-c in the range from 20 to 37 K, does not indicate on the merging of the gaps with decreasing T-c down to 20 K. It may be related to an appearance of the precipitation phase in the Mg1-xAlxB2 structure.

Thermal conductivity and electrical resistivity of the high-Tc superconductor YBa2Cu3O9−Δ

Abstract Thermal conductivity and electrical resistivity of single-phase YBa 2 Cu 2 O 9−Δ compound were measured in the superconducting ( T c =89.5 K) and normal states, in the temperature range 5–320 K. The electronic component of the total thermal conductivity was estimated to be 20%. The electrical resistivity changed linearly in the normal state up to highest measured temperature.

Simple calorimeter for temperature range 50–350 K

Abstract The construction of a fully automated adiabatic calorimeter for the temperature range 50–350 K is described. Appropriate operation of the calorimeter was tested by measuring the specific heat of a pure copper sample with a mass of 0.36 g. The measurements showed agreement within ±1.5% of the standard literature values. Some results obtained for niobium hydride are also reported.

Shape of spin density wave versus temperature in AFe 2As 2 (A=Ca, Ba, Eu): A Mössbauer study

Parent compounds AFe2As2 (A=Ca, Ba, Eu) of the 122 family of the iron-based superconductors have been studied by 57Fe Moessbauer spectroscopy in the temperature range 4.2 K - ~300 K. Spin density waves (SDW) have been found with some confidence. They are either incommensurate with the lattice period or the ratio of the respective periods is far away from ratio of small integers. SDW shape is very unconventional i.e. differs from the sinusoidal shape. Magnetic order starts with lowered temperature as narrow sheets of the significant electron spin density separated by areas with very small spin density. Magnetic sheets are likely to be ordered in the alternate anti-ferromagnetic fashion as the material as whole behaves similarly to collinear anti-ferromagnet. Further lowering of temperature simply expands sheet thickness leading to the near triangular SDW. Finally, sheets fill the whole available space and almost rectangular shape of SDW is reached. Substantial maximum amplitude of SDW appears at the temperature just below the magnetic onset temperature, and this maximum amplitude increases slightly with lowering temperature. The square root from the mean squared hyperfine field behaves versus temperature according to the universality class (1, 2), i.e., with the electronic spin space having dimensionality equal unity and the real space having dimensionality equal two. The more or less pronounced tail above transition temperature due to the development of incoherent SDW is seen.

Electronic structure parameters of Sm1+xBa2−xCu3Oy solid solution of orthorhombic and tetragonal structure

Abstract Two series of Sm 1+ x Ba 2− x Cu 3 O y samples were prepared, both exhibiting a change from orthorhombic to tetragonal structure. The first is a series of solid solution of increasing Sm/Ba ratio, the second is a series of exact “123” cation stoichiometry with lowering oxygen content. Several quantities, such as electrical resistivity in magnetic field up to 8T (ϱ), Hall carrier concentration ( n H ) and thermopower ( S ), were measured. Despite the different chemical nature of the deviation from the initial SmBa 2 Cu 3 O 6.85 composition in the two series, the transport properties of the normal state change in an analogous manner. No significant differences in the Hall mobility at room temperature were revealed between 90 K superconducting and non-metallic samples; however, a wide maximum was observed for both series for samples with intermediate T c values. The critical temperature ( T c ) is depressed with increasing deviation from the initial composition in both series; however, for the solid solution there is no clear 60 K plateau as for the stoichiometric series. The electronic density of states ( g ( E F )) was assessed from in-field T c measurements on the basis of the GLAG theory and, independently, from analysis of S and n H values at room temperature. Both methods revealed no significant differences in the behavior of the g ( E F ) between the solid solution and stoichiometric series. However, the g ( E F ) calculated by the first method decreases fast with deviation from the initial composition, whereas the results of the second method remain nearly constant over a wide range of composition factors. The value of the g ( E F ) obtained for the initial SmBa 2 Cu 3 O 6.85 sample from the jump of the specific heat at T c based on BCS theory also agrees well with the values calculated from transport measurements.

Interplay between magnetism and superconductivity in EuFe2−xCoxAs2studied by57Fe and151Eu Mössbauer spectroscopy

The compound EuFe(2-x)Co(x)As2 was investigated by means of the 57Fe and 151Eu Moessbauer spectroscopy versus temperature (4.2 - 300 K) for x=0 (parent), x=0.34 - 0.39 (superconductor) and x=0.58 (overdoped). It was found that spin density wave (SDW) is suppressed by Co-substitution, however it survives in the region of superconductivity, but iron spectra exhibit some non-magnetic component in the superconducting region. Europium orders anti-ferromagnetically regardless of the Co concentration with the spin re-orientation from the a-axis in the parent compound toward c-axis with the increasing replacement of iron by cobalt. The re-orientation takes place close to the a-c plane. Some trivalent europium appears in EuFe(2-x)Co(x)As2 versus substitution due to the chemical pressure induced by Co-atoms and it experiences some transferred hyperfine field from Eu2+. Iron experiences some transferred field due to the europium ordering for substituted samples in the SDW and non-magnetic state both, while the transferred field is undetectable in the parent compound. Superconductivity coexists with the 4f-europium magnetic order within the same volume. It seems that superconductivity has some filamentary character in EuFe(2-x)Co(x)As2 and it is confined to the non-magnetic component seen by the iron Moessbauer spectroscopy.

Effect of oxygen atom disordering on Tc OF YBa2Cu3O7 − x

Abstract The effects of oxygen deficiency and temperature-induced oxygen atom disordering on the critical temperature (Tc) of YBa2Cu3O7 − x are presented. Two possible influences are proposed and discussed to explain an extraordinary behaviour of Tc, observed in the oxygen-deficient region of the phase. The first influence is the effect of oxygen disordering on the b-axis Cu-O chains, which results in a state characteristic of a set of mutually perpendicular and inequivalently occupied Cu-O chains, which is possibly more favourable for superconductivity. The second is the influence of the Cu-O planes, which exist between the yttrium and the barium-ion layers, the contribution of which on Tc may exceed that of the Cu-O chains and, starting at a certain oxygen index, it may become dominating. Estimates on the upper value of Tc for 1:2:3 type compounds (165 K) and on the limit of superconductivity in the YBa2Cu3O7 − x phase have been made. They give 6.23, if determined by the oxygen index, or 3 × 10−3 if determined by the orthorombic distortion parameter p.

Magnetocaloric and thermomagnetic properties of Ni2.18Mn0.82Ga Heusler alloy in high magnetic fields up to 140 kOe

Measurements of the adiabatic temperature change (ΔT) and the specific heat transfer (ΔQ) of Ni2.18Mn0.82Ga Heusler alloy were taken in order to quantify the direct giant magnetocaloric effect of the alloy when it is in the vicinity of magneto-structural phase transition (PT) from paramagnetic austenite to ferromagnetic martensite, and their results are presented. A new vacuum calorimeter was used to simultaneously measure ΔT and ΔQ of magnetocaloric materials with a Bitter coil magnet in fields of up to H = 140 kOe. Other thermomagnetic properties of this alloy were investigated using standard differential scanning calorimetry and PPMS equipment. The maximal values of magnetocaloric effect in H = 140 kOe were found to be ΔT = 8.4 K at initial temperature 340 K and ΔQ = 4900 J/kg at 343 K. Using this direct method, we show that the alloy indeed demonstrates the largest value of ΔQ as compared with previously published results for direct measurements of magnetocaloric materials, even though at 140 kOe the ...

Thermal conductivity of YBa2Cu3O7−Δ influenced by oxygen content

Abstract The thermal conductivity of two samples of YBa 2 Cu 3 O 7−Δ with a significantly different oxygen content and therefore with different superconducting transition temperatures were measured. The results suggest that the electron-phonon coupling may be weak.

Enhancement of the Hall-Lorenz number in optimally doped YBa2Cu3O7−d

Electronic heat transport in the normal state of a high-quality single crystal of the optimally doped superconductor YBa2Cu3O6.95 was studied by measurements of longitudinal and transverse transport coefficients. For the temperature range from 100 to 300 K, the Hall-Lorenz number (Lxy) depends weakly on temperature and is about two times larger than the Sommerfeld value of the Lorenz number L0=π2/3. Our results can be interpreted using a Fermi liquid model when effects of the pseudogap that opens at the Fermi level are included. However, we find that the bipolaron model can also explain both the enhanced value and the weak temperature dependence of the Hall-Lorenz number.