Low Residual Resistivity Ratio Research Articles

HfTe2: Enhancing Magnetoresistance Properties by Improvement of the Crystal Growth Method.

We present a systematic study on the magnetotransport properties of HfTe2 single crystals grown by different synthetic protocols. Both chemical vapor transport (CVT) as well as the self-flux method were applied. Depending on the synthetic procedure the crystal quality is reflected by the residual resistivity ratio (RRR). The best CVT grown crystal shows a RRR of 262, while the crystal with the highest quality obtained with the Te self-flux method exhibits a value of 404. The superiority of the self-flux method can be traced back to its ability to reduce the amount of Zr as main contaminant more effectively compared to chemical vapor transport. The large RRR value is reflected in the magnetoresistance (MR) effect which reaches more than 9400%, outperforming the data published for HfTe2. The benefit of the self-flux approach was tested for WTe2 and a RRR of 2525 was reached significantly surpassing the data reported in literature. Crystals of both high and low RRR were compared with respect to the magnetotransport properties, i.e., transverse magnetoresistance and the Hall effect. The major factor determining the maximum value of the MR is the carrier mobility which is severely affected by the preparation conditions, while the carrier balance remains virtually unaffected.

Thermoelectric properties of Heusler-type Ru2VAl1−xGax alloys

Electrical and thermal transport properties of the Heusler-type alloys, Ru2VAl1−xGax (x = 0.0–1.0) were studied by means of the electrical resistivity, Seebeck coefficient, and thermal conductivity measurements. All studied compounds show weak metallic characteristics with a low residual resistivity ratio. In addition, the Ru2VAl1−xGax alloys with x ≤ 0.75 show an n-type thermoelectric conduction from 10 to 300 K, while Ru2VGa displays p-type conduction. The estimated Fermi energy of these materials is higher than 0.5 eV, endorsing their metallic character. From the thermal conductivity study, we noticed that low-temperature thermal conductivity decreases with increasing Ga content for x ≤ 0.5 and then increases with further Ga substitution. This observation is essentially due to the change in the phonon scattering processes as a result of the substitution of heavier Ga atoms into the Al sites of Ru2VAl. It is important that an enhanced thermoelectric figure of merit ZT was found in Ru2VAl0.25Ga0.75, about seven times higher than that in Ru2VAl.

Degradation of the Cu Residual Resistivity Ratio in Cr-Plated Composite Nb<sub>3</sub>Sn Wires

Some of the bronze-route Nb3Sn wires produced for the ITER toroidal field magnets have values of the Cu residual resistivity ratio (RRR) lower than the specification (RRR > 100) when reacted with the longest ITER heat treatment cycle (cycle A: high-temperature plateau 200 h at 650 ◦C). As the low RRR value was suspected to be due to the Cr plating, CERN has carried out extensive investigations on two different Cr-plated Nb3Sn wires in order to assess how the RRR is influenced by the Cr plating. Each type of wire was reacted with three heat treatments differing for the duration of the high-temperature plateau (from 100 to 200 h). The presence of Cr in the Cu stabilizer was investigated using a transmission electron microscope together with energy-dispersive X-ray spectrometry. In the most critical wire reacted for 200 h, CrS nanoparticles and Cr in solid solution (0.02 at.%) with Cu were found up to 13 μm from the Cr–Cu interface. At larger distance, we could not quantify the Cr concentrations because of the detector sensitivity limit (about 0.016 at.% Cr in Cu). We provide a model that can explain the measured RRR degradation purely in terms of the Cr contamination that we observed.

Reply to “Comment on `Details of Sample Dependence and Transport Properties of URu2Si2”'

The comment from Butch et al. gives the view of physicists who have carefully studied the effects of hydrostatic pressure (P) conditions on a specific crystal. In our article, we do not discuss this problem but we concentrate on the single crystal properties. The main reason is that it is a delicate point as pressure experiments are usually conducted with a constant volume in a clamp cell. For example, the combined effects of the solidification of the pressure medium and even feedback at the phase transition [for example here, hidden order (HO)] when the material may expand or contract in different directions can cause a departure from ideal hydrostatic conditions. As an observed experimental rule, the best approach to hydrostaticity is given when the crystal size is small in compared with the volume of the pressure cell, a condition which is never realized in pressure inelastic neutron scattering experiments. A key point in a detailed study of the pressure conditions to succeed is to realize comparative measurements in the same pressure cell as used in neutron scattering, as well as in situ pressure measurements. Such condition was realized in neutron scattering experiments, where a strain gauge was glued on the crystal face to trace where macroscopic phase transition occurs, and even to estimate pressure inhomogeneity. The work presented in our article was realized to characterize the sample quality at most and to be able to select samples for quantum oscillation experiments (Shubnikov–de Haas (SdH) and de Haas–van Alphen effect) to precisely determine the feedback between HO and antiferromagnetic (AF) properties on the Fermi Surface. The residual resistivity ratio (RRR) for URu2Si2 is a simple and an effective criterion for selecting the best material to enhance the quantum oscillation signal, even if RRR is estimated from the value at 2K and validity of Matthiesen’s rule can be discussed. In our experience, for example, no SdH signal was detected for samples with low RRR of below 50. It should be noted that complementary information is given by specific heat measurement. Because of the large extinction observed in the neutron diffraction pattern of crystals, the determination of an absolute value of the extrinsic tiny sublattice magnetization via the intensity of ‘‘residual’’ AF reflections is a difficult and delicate task. Electron microscopy up to now has not been sensitive enough in the characterization of strongly correlated uranium intermetallic compounds, as shown by contradictory results on UPt3 for example. 7,8) As suggested in our paper, electronic correlation may generate a new type of extended defects. In the article by Butch et al., the neutron diffraction experiments were realized under isobar conditions where the pressure in the pressure cell was estimated from the pressure applied through a capillary. At least for us, it is not obvious whether the isobar conditions are realized on the crystal as no direct measurement of the pressure is achieved on the crystal’s location in the pressure cell at low temperatures. Furthermore, the resistivity measurement was realized independently in a classical piston cylinder cell in a commercial cryostat with a conventional medium of a 1 : 1 volume mixture of n-pentane/isoamyl alcohol. The interesting point is that the disappearance of HO and of superconductivity occurs at Px, as already obtained from ac susceptibility measurements. Our experimental observation of P measurements on different crystals indicates that the collapse of the SC transition never coincides with Px obtained through resistivity measurements and varies from crystal to crystal with no simple relation with the RRR value. This is quite different from the data obtained by Butch et al. which shows a coincidence with Px. Figure 1 shows the resistivity data of (a) the crystal examined by the Butch et al. and (b) a crystal with RRR 1⁄4 270. As can be seen, a nice anomaly at HO is detected in (a), but quite different behavior is detected between the single crystals (a) and (b) at low temperatures T < 10K. It must be noted that in our article the RRR domain near 30 is the range in which the power law dependence seems to reach a minimum, suggesting a correlation between the electronic mean free path and the 1 10 10

Robust properties of the superconducting ferromagnet UCoGe

The angular dependent magnetoresistance of the superconducting ferromagnet UCoGe is studied in detail at 40 mK to fields of 18 T in a single crystal sample with a low residual resistivity ratio [R(300 K)/R(1 K)=RRR=5]. A ferro-to-ferrimagnetic transition near 9 T that depends only on the field component along the uniaxial magnetic c axis is observed in both transport and susceptibility. Reverse hysteresis in the magnetoresistance in the critical field region indicates a coupling of the superconductivity to the magnetization for fields aligned closely to the ab plane. These results corroborate and advance previous findings on higher residual resistivity ratio samples.

Self-Field Effects in Magneto-Thermal Instabilities for Nb-Sn Strands

Recent advancements in the critical current density of conductors, coupled with a large effective filament size, have drawn attention to the problem of magneto-thermal instabilities. At low magnetic fields, the quench current of such high strands is significantly lower than their critical current because of the above-mentioned instabilities. An adiabatic model to calculate the minimum current at which a strand can quench due to magneto-thermal instabilities is developed. The model is based on an dasiaintegralpsila approach already used elsewhere . The main difference with respect to the previous model is the addition of the self-field effect that allows to describe premature quenches of non-magnetized strands and to better calculate the quench current of strongly magnetized strands. The model is in good agreement with experimental results at 4.2 K obtained at Fermilab using virgin Modified Jelly Roll (MJR) strands with a low residual resistivity ratio (RRR) of the stabilizing copper. The prediction of the model at 1.9 K and the results of the tests carried out at CERN, at 4.2 K and 1.9 K, on a 0.8 mm Rod Re-Stack Process (RRP) strand with a low RRR value are discussed. At 1.9 K the test revealed an unexpected strand performance at low fields that might be a sign of a new stability regime.

STM study of surfaces of kish graphite doped by iron

Iron doping effect on kish graphite (KG) flakes at 2800°C was studied by observing structural defects, pores and hillocks, at different depth of the specimens under STM and by measuring residual resistivity ratio (RRR) values. A low RRR value of the KG flakes indicates a high density of defects in the specimen. The defects were formed due to the diffusion of iron clusters, which were assumed to occur across and along basal planes of the graphite; the former created pores and the latter hillocks. In KG flakes heat-treated at 2800°C, the increase in the number of the hillocks can be explained by the diffusion to the outer parts of iron impurities trapped in the KG flakes along the basal planes of the graphite. Iron doping with a small change in RRR values led to the formation of pores at near depth of the flake, with mainly diffusion of iron to the inside of the flake. Iron doping with a large change in RRR values led to the diffusion of iron to the inside of the flake following mainly by the reversed diffusion of iron, forming hillocks.

Structure, magnetic and electrical properties of ScFe xAl 12− x alloys and their relation to the UFe xAl 12− x system

ScFe x Al 12− x compounds exist for 4⩽ x⩽6. However, a single-phase alloy is only obtained for x=4. The x=7 sample contains considerable amounts of unidentified phases. The alloys with x⩾5 are ferromagnetic, whereas that with x=4 is antiferromagnetic. Its magnetization versus field curve at 4.2 K has, however, ferromagnetic character. The electrical resistivity of the investigated materials shows metallic character with a rather low residual-resistivity ratio. The obtained results are discussed in relation to the UFe x Al 12− x system.

Single-Crystal Studies of the Chevrel-Phase Superconductor LaxMo6Se8: II: Physical and Superconducting Properties

Single crystals of LaxMo6Se8have been grown and some of their magnetic, transport, and superconducting properties studied. The electrical resistivity is characterized by its high value at room temperature, its low residual resistivity ratio, and a pronounced negative curvature at high temperatures. Comparison with the isostructural compounds Mo3Se4(Mo6Se8) and LaMo6S8shows that this behavior is due to structural as well as to extrinsic features (e.g., brittleness due to weak intercluster bondings). The position of the Fermi level near a peak of the density of states plays an important role in the normal-state physical properties, fixing the functional forms of both resistivity and magnetic susceptibility. The superconducting state is mainly characterized by a strong lanthanum concentration dependence of the critical temperatureTc, by quite definite granular effects, and by a very high critical field (Hc2(0)∼55 T). The intragrain critical current density, as estimated by magnetic mesurements, is relatively high (4×104A/cm2at zero field and 1.7 K), three times larger than the one obtained for the void compound Mo3Se4. This fact is due to a higher density of pinning centers in the ternary compound because of microstructural features such as microcracks or crystal defects caused by the extreme brittleness of the crystals.