Residual Resistivity Research Articles

Optimizing the spin Hall effect in Pt-based binary alloys

We present multicode calculations for the spin Hall effect in binary Pt-based alloys, where we explore the viability of alloying the archetype spin Hall material Pt with a large set of metals [Al, Ag, Au, Cu, Hf (hcp), Hf (fcc), Ir, Pd] in order to optimize the charge to spin current conversion for practical applications. To capture intrinsic and extrinsic mechanisms in material-specific calculations, we employ different first-principles codes based on density functional theory in the framework of Green's-function-based multiple scattering approaches. Capturing the transport properties within the relativistic and fully quantum mechanical Kubo-Bastin formalism as well as the semiclassical Boltzmann approach allows for a better understanding of the microscopic physics as well as a larger set of reliable data for the key transport parameters. If available, we compare our results to experimental data, where we generally find good agreement. As we access the full concentration range, we are able to identify the optimal doping regime, which will depend on the binary alloy but generally falls within a region of 60–90 at.% of Pt. When including the unavoidable experimental residual resistivities, the maximum spin Hall angle that we identified is 13% in Al0.2Pt0.8 and Hf0.1Pt0.9, which is comparable to the best spin Hall angles experimentally found in good metal systems. The longitudinal resistivities in this regime go beyond 70µΩ cm, which is compatible with metallic-based magnetic random access memory devices. Published by the American Physical Society 2024

Single Crystal Growth and Nano-Structure Study in a Topological Dirac Metal, CoTe2-δ

A single crystal of a topological material, CoTe2-δ, has been grown via the chemical vapor transport method for a structural and electronic transport study. Single-crystal X-ray diffraction, powder X-ray diffraction, and high-resolution scanning electron microscope measurements confirm the high quality of the as-grown single crystals. In a high-resolution scanning electron microscopy study, a clear layered feature of the trigonal CoTe2-δ crystal was observed. Fractal features and mosaic-type nanostructures were observed on the as-grown surface and cleaved surface, respectively. The trigonal CoTe2-δ demonstrates a metallic ground state in transport measurements, with a typical carrier’s concentration in a 1021 cm−3 magnitude and a residual resistivity ratio of 1.6. Below 10 K, trigonal CoTe2-δ contains quite complicated magnetoresistance behavior as a result of the competing effect between Dirac states and possible spin fluctuations.

Separation in Fe–Ni and Fe–Ni–P alloys under electron irradiation and isochronous annealing

The annealing of radiation defects in the Fe–34.7at.%Ni and Fe–34.6at.%Ni–0.1at.%P alloys irradiated with 5 MeV electrons is studied by the method of residual electrical resistivity. It is shown that, during annealing after irradiation at 80 K, vacancies become mobile in the region of 220 K with a migration energy of about 0.6 eV. In this case, vacancy-impurity complexes (clusters) are formed, and in the H36 alloy vacancy clusters are formed. Under irradiation at room temperatures, vacancy defects accumulate in the form of vacancy clusters. The dissociation of these clusters at 350–550 K results in the appearance of freely migrating vacancies and enhanced self-diffusion. This leads to radiation-accelerated ordering processes in the Fe–Ni and Fe–Ni–P alloys. At temperatures around 800 K, homogenization of the solid solution occurs. The details and stages of the dissociation of vacancy-impurity complexes are discussed.

Self-protected high-temperature superconducting demonstrator magnet for particle detectors

A high temperature superconducting (HTS) demonstration coil has been developed in the frame of the Experimental Physics department Research and Development program at CERN The magnet extends the recent experimental demonstration of aluminium-stabilised HTS conductors and supports the development of future large scale detector magnets. The HTS magnet has five turns and an open bore diameter of 230 mm. Up to 30 K, the coil was measured to be fully superconducting across four central turns at 4.4 kA, the maximum available current of existing power supply. The central magnetic field is 0.113 T, the peak field on the conductor is 1.2 T and the coil has a stored magnetic energy of 0.1 kJ. A 3D-printed aluminium alloy (Al10SiMg) cylinder acts both as a stabiliser and a mechanical support for the superconductor. The resistivity of Al10SiMg was measured at cryogenic temperatures, and has a residual resistivity ratio of approximately 2.5. The ability to solder ReBCO tapes (a stack of four REBCO tapes, 4 mm wide, Fujikura) to Al10SiMg stabiliser, electroplated with copper and tin, forming a coil, is demonstrated using tin-lead solder at 188 ∘C. The HTS magnet was proven to be stable when superconductivity was broken locally using a thin-film heater. Despite voids in the solder joint between HTS and stabiliser, no degradation of the magnet’s performance was observed after 12 thermal cycles and locally quenching the magnet. A numerical model of the transient behaviour of solenoid with partially shorted turns is developed and validated against measurements. Our work experimentally and numerically validates that using an aluminium alloy as a stabiliser for HTS tapes can result in a stable, lightweight and transparent magnet.

Anisotropic positive linear and sub-linear magnetoresistivity in the cubic type-II Dirac metal Pd3In7

We report a transport study on Pd3In7 which displays multiple Dirac type-II nodes in its electronic dispersion. Pd3In7 is characterized by low residual resistivities and high mobilities, which are consistent with Dirac-like quasiparticles. For an applied magnetic field (μ0H) having a non-zero component along the electrical current, we find a large, positive, and linear in μ0H longitudinal magnetoresistivity (LMR). The sign of the LMR and its linear dependence deviate from the behavior reported for the chiral-anomaly-driven LMR in Weyl semimetals. Interestingly, such anomalous LMR is consistent with predictions for the role of the anomaly in type-II Weyl semimetals. In contrast, the transverse or conventional magnetoresistivity (CMR for electric fields E⊥μ0H) is large and positive, increasing by 103−104 % as a function of μ0H while following an anomalous, angle-dependent power law {rho }_{{{{rm{xx}}}}}propto {({mu }_{0}H)}^{n} with n(θ) ≤ 1. The order of magnitude of the CMR, and its anomalous power-law, is explained in terms of uncompensated electron and hole-like Fermi surfaces characterized by anisotropic carrier scattering likely due to the lack of Lorentz invariance.

Huge Peltier conductivity in valence fluctuating material Yb3Si5

We report the low temperature thermoelectric properties in the valence fluctuating system Yb3Si5 using high-quality single crystals. The coexistence of the tiny residual resistivity of 0.06 μΩ cm and the considerable Seebeck coefficient of 9 μV K−1 at 7 K leads to a huge Peltier conductivity of 40 A cm−1 K−1 and a power factor of 40 mW m−1 K−2. These pronounced characteristics are possibly attributed to the uncompensated semimetallic state in addition to the valence fluctuation of Yb ions. The large size of Yb3Si5 single crystal, which is typically ϕ 0.5–1 mm in diameter and 10 mm in length, enables the realistic application to the cryogenic thermoelectric device.

Verification of Wiedemann–Franz Law in Silver with Moderate Residual Resistivity Ratio

Electrical and thermal transport were studied in a vacuum-annealed polycrystalline silver wire with residual resistivity ratio 200 – 400, in the temperature range 0.1-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-$$\\end{document}1.2 K and in magnetic fields up to 5 T. Both at zero field and at 5 T the wire exhibits the Wiedemann–Franz law with the fundamental Lorenz number, contrary to an earlier report (Gloos et al., in Cryogenics 30:14–18, 1990). Our result demonstrates that silver is an excellent material for thermal links in ultra-low-temperature experiments operating at high magnetic fields.

Enhanced TC in SrRuO3/DyScO3(110) thin films with high residual resistivity ratio

Epitaxial untwinned SrRuO3 thin films were grown on (110)-oriented DyScO3 substrates by molecular-beam epitaxy. We report an exceptional sample with a residual resistivity ratio (RRR), ρ [300 K]/ρ [4 K] of 205 and a ferromagnetic Curie temperature, TC, of 168.3 K. We compare the properties of this sample to other SrRuO3 films grown on DyScO3(110) with RRRs ranging from 8.8 to 205, and also compare it to the best reported bulk single crystal of SrRuO3. We determine that SrRuO3 thin films grown on DyScO3(110) have an enhanced TC as long as the RRR of the thin film is above a minimum electrical quality threshold. This RRR threshold is about 20 for SrRuO3. Films with lower RRR exhibit TCs that are significantly depressed from the intrinsic strain-enhanced value.

Metal–insulator–superconductor transition in nickelate-based heterostructures driven by topotactic reduction

The discovery of superconductivity in doped infinite-layer nickelates has attracted great interest recently. Here, a metal–insulator–superconductor transition is demonstrated by engineering the process of topotactic reduction. By employing topotactic reduction, a superconducting Nd0.8Sr0.2NiO2 layer is obtained from high-quality Nd0.8Sr0.2NiO3, which is characterized by layer-by-layer growth and low room-temperature resistivity, rather than the low-quality Nd0.8Sr0.2NiO3 with mixed phases. Moreover, an insulating intermediate state is uncovered within the transition from metallic Nd0.8Sr0.2NiO3 to superconducting Nd0.8Sr0.2NiO2, corresponding to a non-monotonic modulation of resistivity driven by topotactic reduction. In the incompletely reduced Nd0.8Sr0.2NiO2+δ, residual oxygen atoms in the Nd/Sr plane disrupt the long-range order of the infinite-layer structure, resulting in suppressed superconductivity with a low transition temperature and non-zero residual resistivity. On the other hand, the superconductivity is optimized in the fully reduced sample, where a sign change in the Hall coefficient is always observed at low temperatures. Our study highlights the diverse electronic states achievable by controlling topotactic reduction, providing valuable insight into the understanding and manipulating of superconductivity in infinite-layer nickelates.

Topological Hall effect and the transition of the anomalous Hall effect mechanism in hexagonal alloys Mn3.1− xFexGe0.9 (x = 1.6, 1.8, 2.0)

Hexagonal Mn3Ge, with both kagome lattice and triangular antiferromagnetism, has gained significant attention due to its large anomalous Hall effect (AHE), resulting from the non-vanishing Berry phase. In this study, we present the magnetic and anomalous transport properties of a series hexagonal D019 type Fe-doped Mn3Ge alloys with the composition of Mn3.1−xFexGe0.9 (x = 1.6, 1.8, 2.0). The ferromagnetic interactions gradually increase with increasing Fe content. The longitudinal resistivity of all alloys exhibits a typical metallic behavior, increasing with temperature from 5 to 390 K. The residual resistivity decreases from 120.4 to 67.8 μΩ·cm as x increases from 1.6 to 2.0. A temperature-driven Lifshitz transition and a spin reorientation have been observed in the x = 1.6 alloy. Topological Hall effect accompanied by the spin reorientation is demonstrated. The maximum value of the topological Hall resistivity ρxyT is approximately 0.16 μΩ·cm. The relationship of ρxyA∝ ρxx in x = 1.6 alloy suggests that the extrinsic skew scattering predominantly contributes to the AHE mechanism. In the case of x = 1.8 and 2.0, both intrinsic and extrinsic factors contribute to the AHE. The anomalous Hall conductivity of our polycrystalline samples at room temperature is comparable to that of single crystal Mn3Ge, which is advantageous for practical applications. This study reveals the effectiveness of chemical engineering in tailoring nontrivial spin textures and the AHE.

Low temperature spin relaxation length exceeding 3 μm in highly conductive copper channels

Despite extensive studies of spin transport in metallic structures, it remains a challenge to achieve spin relaxation length well above 1 μm in metals even at low temperatures. We explore nonlocal spin transport in Cu channels with a cross section of 0.5 × 0.5 μm2, which exhibit superior values of electrical conductivity and residual resistivity ratio (RRR). Based on structures fabricated in a single batch, we found an average spin relaxation length of λCu=3.2±0.7μm and an average spin relaxation time of τs = 120 ± 50 ps at 30 K. Substantial variations of λCu, RRR, and resistivity ρCu are found among the structures and the three quantities correlate well to one another. The most conductive Cu channel in the batch yields λCu=5.3±0.8μm and τs=250±80ps. These superior values exceed expectations for metals and can be attributed to reduced spin relaxation from grain boundaries and surfaces.

Weak dimensionality dependence of charge density wave transition in 2H-NbSe2

The effect of dimensionality on charge density wave (CDW) transition temperature (TCDW) in 2H-NbSe2 is still under debate. Raman measurements uncovered highly enhanced TCDW for few-layer samples, while scanning tunneling microscopy results suggest comparable value of bulk crystals. Here, we obtained high-quality crystals of 2H-NbSe2 with residual resistivity ratio up to 120 and processed thin flakes by mechanical exfoliation. Electrical resistance measurements were carried out on crystals with different thickness to monitor the dimensionality dependence of TCDW, superconducting Tc, and upper critical field Hc2. It is revealed that when the bulk crystal evolves into few layers, the TCDW only increases slightly, though the variations of Tc and upper critical field Hc2 are consistent with previous results. The observed weak dependence of long-range CDW order on dimensionality agrees well with the recent theoretical calculations on anharmonic spectra. These results reconcile experiment and theory, and thus shed light on the mechanism of CDW for thin flakes of 2H-NbSe2.

Epitaxial α-Ta (110) film on a-plane sapphire substrate for superconducting qubits on wafer scale

Realization of practical superconducting quantum computing requires many qubits of long coherence time. Compared to the commonly used Ta deposited on c-plane sapphire, which occasionally form α-Ta (111) grains and β-tantalum grains, high quality Ta (110) film can grow epitaxial on a-plane sapphire because of the atomic relationships at the interface. Well-ordered α -Ta (110) film on wafer-scale a-plane sapphire has been prepared. The film exhibits high residual resistance ratio. Transmon qubits fabricated using these film shows relaxation times exceeding 150 μs. The results suggest Ta film on a-plane sapphire is a promising choice for long coherence time qubit on wafer scale.

Growth, magnetic, transport and electronic properties of Co2TiSi Heusler alloy thin films

We have grown thin films of Co2TiSi (CTS) Heusler alloy on n-type Si (100) at different substrate temperatures by electron beam evaporator under a high vacuum. The structural, magnetic, and electric characterizations were carried out and found a strong correlation between the properties and the growth temperatures of the samples. The crystallinity and magnetic moment both were significantly improved with increasing growth temperatures from room temperature to 400 °C. The saturation magnetization of 1.85 µB at 10 K is close to the theoretical Slater–Pauling value. A large value of spin wave stiffness constant ( D = 7.2 meV- nm2) of CTS film manifests the robustness of magnetization. The temperature-dependent longitudinal resistivity demonstrated different scattering phenomenon like weak localization, electron-magnon, electron-election, and electron-phonon interactions. A decrease in residual resistivity (109–49 µΩ-cm) and an increase in residual resistivity ratio (1.2–1.39) indicate an enhancement of crystallinity and structural order of CTS films. By first principle study using DFT calculation we have analyzed the effect of atomic site disorder on electronic properties of both bulk and thin film (220) surface form of CTS Heusler alloy. It has been noticed that the ordered bulk structure shows perfect half-metallic nature but for thin film the spin polarization and moment both are deviated from identical value. The spin polarization and magnetic moment both are reduced a lot by introducing atomic site disorder in L21 ordered structure.

Adsorption-Controlled Growth and Magnetism in Epitaxial SrRuO3 Films.

Controlling defect densities in SrRuO3 films is the cornerstone for probing the intricate relationship among its structural, electrical, and magnetic properties. We combine film growth, electrical transport, and magnetometry to demonstrate the adsorption-controlled growth of phase-pure, epitaxial, and stoichiometric SrRuO3 films on SrTiO3 (001) substrates using solid source metal-organic molecular beam epitaxy. Across the growth window, we show that the anomalous Hall curves arise from two distinct magnetic domains. Domains with similar anomalous Hall polarities generate the stepped feature observed within the growth window, and those with opposite polarities produce the hump-like feature present exclusively in the highly Ru-poor film. We achieve a residual resistivity ratio (RRR = ρ300K/ρ2K) of 87 in a 50 nm-thick, coherently strained, and stoichiometric SrRuO3 film, the highest reported value to date on SrTiO3 (001) substrates. We hypothesize further improvements in the RRR through strain engineering to control the tetragonal-to-orthorhombic phase transformation and the domain structure of SrRuO3 films.

Investigation of the deposition of α-tantalum (110) films on a-plane sapphire substrate by molecular beam epitaxy for superconducting circuit

Polycrystalline α-tantalum (110) films deposited on the c-plane sapphire substrate by sputtering are used in superconducting qubits nowadays. However, these films always occasionally form other structures, such as α-tantalum (111) grains and β-tantalum grains. To improve the film quality, we investigate the growth of α-tantalum (110) films on the a-plane sapphire substrate under varying conditions by molecular beam epitaxy technology. The optimized α-tantalum (110) film is a single crystal, with a smooth surface and atomically flat metal–substrate interface. The film with thickness of 30 nm shows a Tc of 4.12 K and a high residual resistance ratio of 9.53. The quarter wavelength coplanar waveguide resonators fabricated with the 150 nm optimized α-tantalum (110) film exhibit intrinsic quality factor of over one million under single photon excitation at millikelvin temperature.

High-quality superconducting α-Ta film sputtered on the heated silicon substrate

Intrigued by the discovery of the long lifetime in the α-Ta/Al2O3-based Transmon qubit, researchers recently found α-Ta film is a promising platform for fabricating multi-qubits with long coherence time. To meet the requirements for integrating superconducting quantum circuits, the ideal method is to grow α-Ta film on a silicon substrate compatible with industrial manufacturing. Here we report the α-Ta film sputter-grown on Si (100) with a low-loss superconducting TiNx buffer layer. The α-Ta film with a large growth temperature window has a good crystalline character. The superconducting critical transition temperature (Tc) and residual resistivity ratio (RRR) in the α-Ta film grown at 500 °C are higher than that in the α-Ta film grown at room temperature (RT). These results provide crucial experimental clues toward understanding the connection between the superconductivity and the materials' properties in the α-Ta film and open a new route for producing a high-quality α-Ta film on silicon substrate for future industrial superconducting quantum computers.

Effect of Strain on the Resistivity and Thermal Conductivity of High Purity Niobium

High purity niobium (Nb) is a technologically important material for large-scale accelerator and nano-scale quantum computing applications in microwave frequency range. The high thermal conductivity and low resistivity of Nb are critical to the high performance at temperature range of 0.01 - 4.0 K. The presence of interstitials such as O, N, H, and C act as scattering centers and alter the mean free path, reducing resistivity and thermal conductivity, and contributing significantly to Nb's thermal performance for temperatures of 2.0 K and above. The residual resistivity ratio (RRR), defined as the ratio of the normal state resistivity at 300 K to that at 4.2 K (Nb, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$T_{c}$</tex-math></inline-formula> = 9.2 K), is an accepted direct estimate of the impurity content of fully recrystallized Nb. Complete re-crystallization of Nb is challenging unless very high temperatures are employed, which is often impractical, hence, in practice, dislocation and dislocation structures impact the thermal performance of Nb due to strong phonon scattering contributions. This paper reports on the degradation of thermal conductivity and RRR of high purity Nb large grain, single crystal with fixed impurity, varying strain, and dislocation content levels. Experimental thermal conductivity data fits the Boltzmann transport equation incorporating dislocation density.

Fabrication of a 64-Pixel TES Microcalorimeter Array With Iron Absorbers Uniquely Designed for 14.4-keV Solar Axion Search

If a hypothetical elementary particle called an axion exists, to solve the strong CP problem, a 57Fe nucleus in the solar core could emit a 14.4-keV monochromatic axion through the M1 transition. If such axions are once more transformed into photons by a 57Fe absorber, a transition edge sensor (TES) X-ray microcalorimeter should be able to detect them efficiently. We have designed and fabricated a dedicated 64-pixel TES array with iron absorbers for the solar axion search. In order to decrease the effect of iron magnetization on spectroscopic performance, the iron absorber is placed next to the TES while maintaining a certain distance. A gold thermal transfer strap connects them. We have accomplished the electroplating of gold straps with high thermal conductivity. The residual resistivity ratio (RRR) was over 23, more than eight times higher than a previous evaporated strap. In addition, we successfully electroplated pure-iron films of more than a few micrometers in thickness for absorbers and a fabricated 64-pixel TES calorimeter structure.

RRP Nb3Sn Subelement Shear Dependence on Hexagonal Subelement Stack Orientation and the Strand's Position Within a Rutherford Cable

Superconducting strands built into Rutherford cables undergo high deformation at the cable edges. This deformation negatively impacts subelement integrity and subsequently the Residual Resistance Ratio (RRR) of the stabilization Cu in Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands after heat treatment due to compromised diffusion barriers [1]. Rutherford cables for the high luminosity upgrade of the LHC at CERN (HL-LHC) use Restack Rod Process (RRP) Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires with a 108/127 subelement stack. Numerous transverse cross-sectional samples from the cables made under the HL-LHC Accelerator Upgrade Project (“HL-LHC AUP”) [2] have been mounted, polished, and analyzed with light microscopy. The number of sheared subelements within a strand was seen to depend not only on a strand's position in the overall cable (i.e. center vs. edge), but also on its specific position within the cable edge. Moreover, the rotational angle of the subelement stack within a strand, relative to the cable's centerline plane, appears to play a role. There is a need to understand these dependencies, as they might impact the applicability of any criterion for a maximum number of sheared subelements (typically < 15% of subelements) applied to series production quality assurance, often imposed to ensure acceptable RRR degradation. By gathering data across many HL-LHC AUP cables and utilizing a more statistical approach, we aim to establish more accurate success criteria for future R&D cables using RRP wire when the available cable runs (and thus samples) are limited.