High Residual Resistivity Ratio Research Articles

Microcalorimeter Absorber Optimization for ATHENA and LEM

High quantum efficiency (QE) X-ray absorbers are needed for future X-ray astrophysics telescopes. The Advanced Telescope for High ENergy Astrophysics (ATHENA) mission requirements for the X-ray Integral Field Unit (X-IFU) instrument dictate, at their most stringent, that the absorber achieve vertical QE > 90.6% at 7 keV and low total heat capacity, 0.731 pJ/K. The absorber we have designed is 313 µm square composed of 1.05 μm Au and 5.51 μm electroplated Bi films (Barret et al. in Exp Astron 55:373–426, 2023). Overhanging the TES, the absorber is mechanically supported by 6 small legs whose 5 μm diameter is tuned to the target thermal conductance for the device. Further requirements for the absorber for X-IFU include a > 40% reflectance at wavelengths from 1 to 20 μm to reduce shot noise from infrared radiation from higher temperature stages in the cryostat. We meet this requirement by capping our absorbers with an evaporated Ti/Au thin film. Additionally, narrow gaps between absorbers are required for high fill fraction, as well as low levels of fine particulate remaining on the substrate and zero shorts between absorbers that may cause thermal crosstalk. The Light Element Mapper (LEM) is an X-ray probe concept optimized to explore the soft X-ray emission from 0.2 to 2.0 keV. These pixels for LEM require high residual resistance ratio (RRR) thin 0.5 µm Au absorbers to thermalize uniformly and narrow < 2 μm gaps between pixels for high areal fill fraction. This paper reports upon technology developments required to successfully yield arrays of pixels for both mission concepts and presents first testing results of devices with these new absorber recipes.

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.

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.

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.

Liquid helium-cooled high-purity copper coil for generation of long pulsed magnetic fields

To generate long-duration pulsed magnetic fields with low energy consumption, we present a practical setup that implements an electromagnet made of high-purity copper (99.9999%). The resistance of the high-purity copper coil decreases from 171 mΩ (300 K) to 19.3 mΩ (77.3 K) and to below ∼0.15 mΩ (4.2 K), indicating a high residual resistance ratio of 1140 and a substantial reduction in Joule loss at low temperature. Using a 157.5 F electric-double-layer-capacitor bank with a charged voltage of 100 V, a pulsed magnetic field of 19.8 T with a total field duration of more than 1 s is generated. The field strength of the liquid helium-cooled high-purity copper coil is approximately double that of a liquid nitrogen-cooled one. The low resistance of the coil and the resultant low Joule heating effect explain the improvements in accessible field strength. The low electric energy used for field generation warrants further investigation on low-impedance pulsed magnets consisting of high-purity metals.

Development of High Strength and High RRR Aluminum-Stabilizer for Superconducting Cable in CEPC Detector Magnet

For the development of a large-aperture superconducting detector magnet in circular electron-positron collider (CEPC), super-conducting cables consisting of Nb-Ti/Cu wires and aluminum stabilizer are essential in both handling large electromagnetic forces during normal operation and redistributing the operation current of tens of kiloampere in case of a quench. High-purity aluminum stabilizer offers extremely high residual resistivity ratio (RRR) for quench protections, however, is too ductile to withstand huge Lorentz forces. Doping technology have been studied in detail in this paper to improve the yield strength of 4N8-aluminium (99.998%) while minimizing the reduction of RRR. All aluminum-alloy samples are characterized by a GM cryocooler for the measurement of RRR and by the standard tensile test facility for the measurement of 0.2% yield strength. It is found that by doping in Ni-0.025% Be-0.025%, annealing at 430 °C, cold-working hardening (∼21%), and curing at 130 °C for 15 h, the resultant aluminum alloy obtains an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p0.2</sub> of 75 MPa at room temperature and a RRR of 417, meeting the basic design requirements of the superconducting detector magnet for CEPC.

Manufacturing studies and rf test results of the 1.3 GHz fundamental power coupler prototypes

The superconducting radiofrequency electron linear accelerator of the Shanghai HIgh repetition rate XFEL aNd Extreme light facility (SHINE) will use 600 TESLA-style cavities with 1.3 GHz fundamental power couplers for continuous-wave operation. The first batch of 26 1.3 GHz coupler prototypes has been fabricated from three domestic manufacturers for qualification. Several key manufacturing processes have been developed and qualified, including high residual resistivity ratio copper plating, vacuum brazing of ceramic windows, electron beam welding, and titanium nitride coating. In addition, uniform quality control and inspection processes have been established to ensure the power couplers meet the requirements for the intended use in the SHINE project. All the 1.3 GHz coupler prototypes have been power conditioned with 14-kW traveling wave and 7-kW standing wave rf in continuous-wave mode. Even higher power levels have been demonstrated with 20-kW traveling wave and 10-kW standing wave rf, which indicates their robustness.11 MoreReceived 27 July 2022Accepted 7 November 2022DOI:https://doi.org/10.1103/PhysRevAccelBeams.25.113501Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasSuperconducting RFAccelerators & Beams

Bayesian optimization with experimental failure for high-throughput materials growth

A crucial problem in achieving innovative high-throughput materials growth with machine learning, such as Bayesian optimization (BO), and automation techniques has been a lack of an appropriate way to handle missing data due to experimental failures. Here, we propose a BO algorithm that complements the missing data in optimizing materials growth parameters. The proposed method provides a flexible optimization algorithm that searches a wide multi-dimensional parameter space. We demonstrate the effectiveness of the method with simulated data as well as in its implementation for actual materials growth, namely machine-learning-assisted molecular beam epitaxy (ML-MBE) of SrRuO3, which is widely used as a metallic electrode in oxide electronics. Through the exploitation and exploration in a wide three-dimensional parameter space, while complementing the missing data, we attained tensile-strained SrRuO3 film with a high residual resistivity ratio of 80.1, the highest among tensile-strained SrRuO3 films ever reported, in only 35 MBE growth runs.

First Observation of the de Haas–van Alphen Effect and Fermi Surfaces in the Unconventional Superconductor UTe2

We report the first observation of the de Haas–van Alphen (dHvA) effect in the novel spin-triplet superconductor UTe2 using high quality single crystals with a high residual resistivity ratio (RRR) over 200. The dHvA frequencies, named α and β, are detected for the field directions between c- and a-axes. The frequency of branch β increases rapidly with the field angle tilted from c- to a-axis, while branch α splits, owing to the maximal and minimal cross-sectional areas from the same Fermi surface. Both dHvA branches, α and β reveal two kinds of cylindrical Fermi surfaces with a strong corrugation at least for branch α. The angular dependence of the dHvA frequencies is in very good agreement with that calculated by the generalized gradient approximation (GGA) method taking into account the on-site Coulomb repulsion of U = 2 eV. It indicates that the main Fermi surfaces are experimentally detected. The observed cyclotron effective masses are large in the range from 32 to 57 m0. They are approximately 10–20 times lager than the corresponding band masses, consistent with the mass enhancement obtained from the Sommerfeld coefficient, γ and the calculated density of states at the Fermi level. The local density approximation (LDA) calculations of ThTe2 assuming U4+ with the 5f2 localized model are in less agreement with our experimental results, in spite of the prediction for two cylindrical Fermi surfaces, suggesting a mixed valence states of U4+ and U3+ in UTe2.

Novel synthesis approach for “stubborn” metals and metal oxides

Advances in physical vapor deposition techniques have led to a myriad of quantum materials and technological breakthroughs, affecting all areas of nanoscience and nanotechnology which rely on the innovation in synthesis. Despite this, one area that remains challenging is the synthesis of atomically precise complex metal oxide thin films and heterostructures containing "stubborn" elements that are not only nontrivial to evaporate/sublimate but also hard to oxidize. Here, we report a simple yet atomically controlled synthesis approach that bridges this gap. Using platinum and ruthenium as examples, we show that both the low vapor pressure and the difficulty in oxidizing a "stubborn" element can be addressed by using a solid metal-organic compound with significantly higher vapor pressure and with the added benefits of being in a preoxidized state along with excellent thermal and air stability. We demonstrate the synthesis of high-quality single crystalline, epitaxial Pt, and RuO2 films, resulting in a record high residual resistivity ratio (=27) in Pt films and low residual resistivity, ∼6 μΩ·cm, in RuO2 films. We further demonstrate, using SrRuO3 as an example, the viability of this approach for more complex materials with the same ease and control that has been largely responsible for the success of the molecular beam epitaxy of III-V semiconductors. Our approach is a major step forward in the synthesis science of "stubborn" materials, which have been of significant interest to the materials science and the condensed matter physics community.

Proximity effect in [Nb(1.5 nm)/Fe(x)]10/Nb(50 nm) superconductor/ferromagnet heterostructures.

We have investigated the structural, magnetic and superconduction properties of [Nb(1.5 nm)/Fe(x)]10 superlattices deposited on a thick Nb(50 nm) layer. Our investigation showed that the Nb(50 nm) layer grows epitaxially at 800 °C on the Al2O3(1−102) substrate. Samples grown at this condition possess a high residual resistivity ratio of 15–20. By using neutron reflectometry we show that Fe/Nb superlattices with x < 4 nm form a depth-modulated FeNb alloy with concentration of iron varying between 60% and 90%. This alloy has weak ferromagnetic properties. The proximity of this weak ferromagnetic layer to a thick superconductor leads to an intermediate phase that is characterized by a suppressed but still finite resistance of structure in a temperature interval of about 1 K below the superconducting transition of thick Nb. By increasing the thickness of the Fe layer to x = 4 nm the intermediate phase disappears. We attribute the intermediate state to proximity induced non-homogeneous superconductivity in the structure.

Strain relaxation induced transverse resistivity anomalies in SrRuO3 thin films

Here, we report a magnetotransport study of high-quality $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_{3}$ thin films with high residual resistivity ratios grown by reactive oxide molecular-beam epitaxy. The transverse resistivity exhibits clear anomalies which are typically believed to be signatures of the topological Hall effect and the presence of magnetic skyrmions. By systematically investigating these anomalies as a function of temperature, field, film thickness, and excitation currents we verify that these anomalies originate from a two-channel anomalous Hall effect arising from magnetic inhomogeneities in the samples and not from an intrinsic topological Hall effect. Using a combination of magnetic circular dichroism, scanning transmission electron microscopy, x-ray diffraction, and magnetotransport, we discover that strain relaxation effects in the films are the origin of these inhomogeneities. Our results shed light on the recently reported Hall effect anomalies in $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_{3}$ thin films and heterostructures and provide an approach to determine whether such anomalies arise from an intrinsic topological Hall effect or from extrinsic mechanisms.

Evidence for topological semimetallicity in a chain-compound TaSe3

Among one-dimensional transition-metal trichalcogenides, TaSe3 is unconventional in many respects. One is its strong topological semimetallicity as predicted by first-principles calculations. We report the experimental investigations of the electronic properties of one-dimensional-like TaSe3 single crystals. While the b-axis electrical resistivity shows good metallicity with a high residual resistivity ratio greater than 100, an extremely large magnetoresistance is observed reaching ≈7 × 103% at 1.9 K for 14 T. Interestingly, the magnetoresistance follows the Kohler’s rule with nearly quadratic magnetic field dependence, consistent with the electron–hole compensation scenario as confirmed by our Hall conductivity data. Both the longitudinal and Hall conductivities show Shubnikov-de Haas oscillations with two frequencies: Fα ≈ 97 T and Fβ ≈ 186 T. Quantitative analysis indicates that Fα results from the two-dimensional-like electron band with the non-trivial Berry phase [1.1π], and Fβ from the hole band with the trivial Berry phase [0(3D) − 0.16π(2D)]. Our experimental findings are consistent with the predictions based on first-principles calculations.

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.

Machine-learning-assisted thin-film growth: Bayesian optimization in molecular beam epitaxy of SrRuO3 thin films

Materials informatics exploiting machine learning techniques, e.g., Bayesian optimization (BO), have the potential to reduce the number of thin-film growth runs for optimization of thin-film growth conditions through incremental updates of machine learning models in accordance with newly measured data. Here, we demonstrated BO-based molecular beam epitaxy (MBE) of SrRuO3, one of the most intensively studied materials in the research field of oxide electronics, mainly owing to its unique nature as a ferromagnetic metal. To simplify the intricate search space of entangled growth conditions, we ran the BO for a single condition while keeping the other conditions fixed. As a result, high-crystalline-quality SrRuO3 film exhibiting a high residual resistivity ratio of over 50 as well as strong perpendicular magnetic anisotropy was developed in only 24 MBE growth runs in which the Ru flux rate, growth temperature, and O3-nozzle-to-substrate distance were optimized. Our BO-based search method provides an efficient experimental design that is not as dependent on the experience and skills of individual researchers, and it reduces experimental time and cost, which will accelerate materials research.

Study on High Residual Resistance Ratio (RRR) of Wire in Channel Superconductor Wire

This paper studies the process and technology to obtain high Residual Resistance Ratio (RRR) of Wire in Channel (WIC) NbTi/Cu superconducting wires through three methods. The first method is online annealing on medium frequency furnace using NbTi/Cu wire and C10100 copper channel wire in hard state before soldering. The second method uses C10100 copper channel wire to anneal in the vacuum furnace, followed by soldering using NbTi/Cu wire and C10100 copper channel wire in annealing state. And the third method uses C11000 copper channel wire of higher oxygen content and NbTi/Cu wire to solder directly. The results show that the three methods can improve the RRR value. Specifically, with the increase of power, the first method can raise the RRR of WIC. With the increase of anneal temperature, the second method can raise the RRR of C10100 copper channel wire and WIC wire. Finally, the third method can promote the RRR of C11000 copper channel wire and WIC wire to relatively higher levels, which is excellent for mass production.

Improvement in the Crystallographic Phase Content and Superconducting Properties of Mechanically Alloyed MgB2

In this paper, the results of the superconductivity evaluation of MgB2 intermetallic-based superconducting materials prepared through mechanical alloying are reported. The formation of the MgB2 intermetallic phase took place through an intermediate phase reaction at the solid-state reaction temperature, which tends to favor single-phase materials. The annealing of mechanically milled Mg and B powders resulted in the highest mass fraction of MgB2 observed in this study, reaching more than 90% after annealing at 900 °C for 1 h. At that point, there were still second phases present consisting of Mg, MgO, and Fe2B. Improved superconducting properties were obtained in samples sintered at temperatures of 850 °C and 900 °C that had a low resistivity, low critical temperature gap values, and a high residual resistivity ratio (RRR).

Synthesis science of SrRuO3 and CaRuO3 epitaxial films with high residual resistivity ratios

Epitaxial SrRuO3 and CaRuO3 films were grown under an excess flux of elemental ruthenium in an adsorption-controlled regime by molecular-beam epitaxy (MBE), where the excess volatile RuOx (x = 2 or 3) desorbs from the growth front leaving behind a single-phase film. By growing in this regime, we were able to achieve SrRuO3 and CaRuO3 films with residual resistivity ratios (ρ300 K/ρ4 K) of 76 and 75, respectively. A combined phase stability diagram based on the thermodynamics of MBE (TOMBE) growth, termed a TOMBE diagram, is employed to provide improved guidance for the growth of complex materials by MBE.

RRR characteristics of niobium along the welding directions for SRF cavities

Prototype cavities for the superconducting LINAC, named RAON, have been made and tested by the rare isotope science project (RISP) in South Korea. The cavities are quarter-wave resonators (QWRs), half-wave resonators (HWRs) and two types of single-spoke resonators (SSR1, SSR2). All cavities have been made of niobium (Nb) of high residual resistance ratio (RRR) grade. The RRR value decreased (degraded) during electron-beam welding due to the incorporation of impurities from the surroundings. Therefore, the RRR value must be maintained to ensure the cavity’s performance. Conventional e-beam welding, a process to join two niobium parts thermally, has been performed so that the cavity can be fabricated in a structurally-favorable way without considering a preferential welding direction. Thus, we analyzed the RRR characteristics in terms of the welding direction, if any favorable direction existed, to improve the RRR value. Also, we analyzed the RRR results as a function of the vacuum level, the distance from welding center, and the type of welded side. In this study, the RRR along the welding direction, including as a function of the welded side and the vacuum level, will be discussed.

Physical properties optimization of polycrystalline LiFeAs

We present a study of parameter optimization for synthesizing truly stoichiometric polycrystalline LiFeAs. Stoichiometric LiFeAs has been prepared in a very broad range of synthesis temperature (200–900°C) under otherwise exactly the same conditions, and has been characterized by structural, magnetic, transport, nuclear quadrupole resonance (NQR), and specific heat measurements. Our study showed that the LiFeAs phase is formed at 200°C with a large amount of impurity phases. The amount of these impurity phases reduces with increasing synthesis temperature and the clean LiFeAs phase is obtained at a synthesis temperature of 600°C. Magnetic susceptibility and resistivity measurements confirmed that the superconducting properties such as the critical temperature Tc, and the upper critical field Hc2 do not depend on the synthesis temperature (≤700°C), remaining at almost the same value of ∼19K and ∼40T, respectively. However, the width ΔTc of the transition and the NQR line width decrease with increasing the synthesis temperature and reached to minimum value for the synthesis temperature of 600°C. Our careful analysis suggests that the best sample obtained at 600°C is optimal concerning the low resistivity, high residual resistivity ratio (RRR), low ΔTc, high Tc and Hc2, and a small NQR line width with values which are comparable to that reported for LiFeAs single crystals. Specific heat measurements confirmed the bulk superconducting nature of the samples. The Hc2 value estimated from the specific heat is consistent with that of the resistivity measurements. Concisely, 600°C synthesis temperature yields optimal high quality polycrystalline LiFeAs bulk samples. Further improvement of the quality of the sample prepared at 600°C could be obtained by a controlled slow cooling process. Microstructural analysis reveals that the abundance of micro-cracks becomes strongly reduced by the slow cooling process, resulting in an increase in clean and well-connected grain boundaries.