Critical Field Measurements Research Articles

Two-fold symmetric superconductivity in the Kagome superconductor RbV3Sb5

The recently discovered kagome superconductors offer a promising platform for investigating intertwined orders and novel states, including topology, superconductivity, charge density waves, and more. The interplay among these orders can spontaneously break rotational symmetry, giving rise to exotic phenomena such as nematicity or even nematic superconductivity. Here we present our findings on the two-fold symmetric superconductivity in thin-flake RbV3Sb5 in response to direction-dependent in-plane magnetic fields, in contrast to the inherent six-fold structural symmetry of the crystal lattice. The two-fold symmetry was evidenced through a combination of magnetoresistance transport experiments, critical magnetic field measurements, and observations of anisotropic superconducting gaps. Additionally, by altering the experimental configuration, we also detected the presence of six-fold symmetric components superimposed on the two-fold symmetry at the boundary between normal and superconducting states. Our results underscore the correlation-driven symmetry-breaking phenomena and emphasize the potential of this correlated kagome family as a promising platform for investigating intertwined orders, including unconventional superconductivity.

Upper Critical Field and Tunneling Spectroscopy of Underdoped Na(Fe,Co)As Single Crystals.

A comprehensive study of superconducting properties of underdoped NaFe0.979Co0.021As single crystals by a combination of upper critical field measurements and incoherent multiple Andreev reflection effect (IMARE) spectroscopy is presented. The Hc2(T) temperature dependences are measured at magnetic fields up to 16 T with in-plane and out-of-plane field directions and considered within single-band and two-band models in order to estimate the Hc2(0) value. In IMARE spectroscopy probes, the magnitude, characteristic ratio, and temperature dependence of the superconducting order parameters (ΔL,S(T)) are determined locally and directly. A possible k-space anisotropy of the large superconducting gap is demonstrated. We show that usage of a quadruple of λij0 coupling constants obtained in the IMARE experiment can significantly reduce the number of free parameters required to fit the Hc2(T) dependence within a two-band approach (from six to two).

Superconducting Fourfold Fe(Te,Se) Film on Sixfold Magnetic MnTe via Hybrid Symmetry Epitaxy

Epitaxial Fe(Te,Se) thin films have been grown on various substrates but never been grown on magnetic layers. Here we report the epitaxial growth of fourfold Fe(Te,Se) film on a sixfold antiferromagnetic insulator, MnTe. The Fe(Te,Se)/MnTe heterostructure shows a clear superconducting transition at around 11 K, and the critical magnetic field measurement suggests the origin of the superconductivity to be bulk-like. Structural characterizations suggest that the uniaxial lattice match between Fe(Te,Se) and MnTe allows a hybrid symmetry epitaxy mode, which was recently discovered between Fe(Te,Se) and Bi2Te3. Furthermore, the Te/Fe flux ratio during deposition of the Fe(Te,Se) layer is found to be critical for its superconductivity. Now that superconducting Fe(Te,Se) can be grown on two related hexagonal platforms, Bi2Te3 and MnTe, this result opens a new possibility of combining topological superconductivity of Fe(Te,Se) with the rich physics in the intrinsic magnetic topological materials (MnTe)n(Bi2Te3)m family.

Anomalous Critical Field Dependence of Cr-Nb-Cr Trilayers on Niobium Thickness

Cr-Nb-Cr trilayer thin films were deposited using DC/RF sputtering with Cr layer thickness maintained at 7.5 nm and Nb thickness varying from 30 nm to 229 nm, in order to study the interplay of antiferromagnetism and superconductivity. Various viewpoints have been expressed in the literature with respect to analyzing the anomalous properties that have been identified in superconductor/magnetic thin films. The study of Cr-Nb-Cr thin films were taken up to identify the existence of similar anomalous superconducting properties on account of antiferromagnetic-superconductor interfaces. X-ray reflectivity measurements showed high quality of thin film structure with minimum surface and interface roughness. Transport measurements down to 2K were carried out in the presence of magnetic fields up to 2 T applied perpendicular to the trilayers to deduce Tc. The suppression of Tc due to Cr layer is larger than proximity effect of similar metal films. It could be explained by proximity effect using antiferromagnetic Cr layer. Upper critical field measurements show a distinct nonmonotonic dependence of upper critical magnetic field and the slope on Nb layer thickness. The analysis due to WHH theory to deduce upper critical fields for the trilayer thin films studied did not match with experimental values. Although studies performed on Cr-Nb-Cr trilayers did not show any anomaly in Tc, it clearly showed a depression of Tc much largerthan proximity effect, non monotonic behavior in Bc2(0) and dBc2/dTc behavior with Nb layer thickness.

Impact of Threading Dislocations in GaN Power Switching Devices

Thick epitaxial (3-15 microns) GaN power switching devices are known to contain a high density of crystal defects, and especially threading dislocations in the epitaxial layer. The impact of these defects on device performance and reliability is a topic of ongoing research. This paper presents state-of-the-art methods for analyzing threading dislocation density, growth, and quantification of their impact on leakage current, blocking voltage, and peak E field. The effects of substrate type, epitaxial thickness, and processing techniques on dislocation density have been investigated to provide a roadmap for design and performance optimization of switching two terminal devices. Experimental and simulated data are used in conjunction to model dislocation impact on electrical performance characteristics and to identify techniques to minimize defect density.The experimental results have been achieved for 3 and 10 micron thick epitaxy as well as initial results for 15 micron epitaxy. The substrate was bulk GaN, and the epitaxy was grown by HPVPE by Kyma Corporation. The DMOS test structure investigated showed state of the art current voltage characteristics for a 10 micron n- epitaxy. Significant deviations were observed between the measured doping profile and the simulated profile, using a critical field of 3 MV/cm. For the DMOS device with epi doping of 8 × 1015 \U0001d450\U0001d45a−3 and epi thickness of 8 \U0001d707\U0001d45a. The device breaks down at approximately 2020 \U0001d449. At 2000 \U0001d449, the peak field is roughly 3.11 \U0001d440\U0001d449/\U0001d450\U0001d45a. For a device with epi doping of 8 × 1015 \U0001d450\U0001d45a−3 and epi thickness of 15 \U0001d707\U0001d45a. The device breaks down at approximately 3020 \U0001d449. At 3000 \U0001d449, the peak field is roughly 3.14 \U0001d440\U0001d449/\U0001d450\U0001d45a. The observed trend is significant since it shows that by increasing the epi thickness, also results in an increase in the peak field. For thin GaN epi of 3 microns, a peak breakdown field of 2.77 MV/cm was measured. These three date points will allow one to determine a relationship between peak critical field and epi thickness, which is a major requirement for designing vertical GaN high voltage switches.We report images from transmission x-ray topographs and TEM investigations as well as the correlation with critical breakdown field measurements. The transmission x-ray topograph shows that the GaN is characterized by a nearly uniform distribution of strain centers which are likely bundles of threading screw and edge dislocations. Strain free centers are then regions of relatively low dislocation densities. The threading dislocations are part of dislocation loops which are present in the GaN substrate and migrate through the interface. The cross-sectional TEM image shows partial dislocation loops through the thickness of the GaN sample.Figure: X-Ray Topography of GaN/GaN showing a nearly uniform distribution of strain centers. The Grazing incidence topograph is similar to the transmission image, the grazing image also shows the nearly uniform distribution of strain centers. The X ray image shows that individual threading dislocations are not resolved showing a relatively low dislocation density in those regions. Figure 1

Synthesis and physical properties of the theoretically predicted spin-triplet superconductor Li0.9Mo6O17

We report the single crystal growth and characterization of the quasi-one-dimensional superconductor Li0.9Mo6O17 via temperature-gradient flux method. The grown single crystals show a clear ab plane identified by the x-ray diffraction (XRD) pattern. Temperature dependent resistivities reveal a metallic to semiconducting crossover at TM= 24 K followed by a superconducting transition at Tc= 2.2 K for ρa and ρc. In addition, the upper critical fields demonstrate a large anisotropy with Hc2b>Hc2a>Hc2c both at ρa and ρc. Particularly, an upper critical field Hc2b of about 16.2 T at zero temperature limit was deduced from the field dependence of resistivity measurements, which is notably larger than the estimated Pauli paramagnetic limit 3.1 T and supports the existence of the spin-triplet superconducting pairing and unconventional superconductivity in Li0.9Mo6O17. The XRD, resistivities and upper critical field measurements all imply a high quality of the as-grown Li0.9Mo6O17 samples. Furthermore, the interlayer and in-plane magnetoresistivity (MR) up to 60 T reveal the possible phase transition driven by the density-wave gap suppression and Zeeman split effect in the high field state.

A Case Study of Soil Moisture and Infiltration after an Urban Fire

There is an increased risk of future fire disturbances due to climate change and anthropogenic activity. These disturbances can impact soil moisture content and infiltration, which are important antecedent conditions for predicting rainfall–runoff processes in semi-arid regions. Yet these conditions are not well documented. This case study provides critical field measurements and information, which are needed to improve our understanding of mechanisms such as precipitation and temperature that lead to the variability of soil properties and processes in urban and burned landscapes. In June 2018, a fire burned a portion of the riparian zone in Alvarado Creek, an urban tributary of the San Diego River in California, United States. This fire provided an opportunity to observe soil moisture content and infiltration for one year after the fire. Three transects (one burned and two unburned) were monitored periodically to evaluate the complex spatial and temporal dynamics of soil moisture and infiltration patterns. Average dry season soil moisture content was less than five percent volume water content (%VWC) for all transects, and the burned transect exhibited the lowest %VWC during the wet season. Infiltration rates displayed a high degree of spatial and temporal variability. However, the location with the highest burn severity had the lowest average infiltration rate. The observed differences between the burned and unburned transects indicate that the fire altered hydrologic processes of the landscape and reduced the ability of the soil to retain water during the wet season. This research provides the first high-resolution soil moisture and infiltration field analysis of an urban fire-disturbed stream in southern California and a method to characterize post-fire hydrologic conditions for rainfall–runoff processes.

Direct measurement of resistivity in destructive pulsed magnetic fields

A simple method for measuring electrical resistivity under destructive pulsed magnetic fields is presented. This method uses pick-up voltage as the power source to allow the measurement of the absolute value of resistivity in ultra-high magnetic fields above 100 T. The experimental setup and its operation are described in detail, and its performance is demonstrated using critical field measurements of thin-film FeSe0.5Te0.5 samples. Possible scientific applications of this setup in high magnetic fields as well as in any other environment with a high field sweep rate are also discussed.

Tunneling-Spectroscopy Evidence for Two-Gap Superconductivity in a Binary Mo-Re Alloy

Expectation of fundamentally new phenomena owing to multiband superconductivity stimulates a search for novel materials with, at least, two distinct gaps in the superconducting state. Recent temperature-dependent electronic heat capacity and lower critical field measurements for molybdenum-rhenium alloys have revealed signatures of two-gap superconductivity. In this letter, we report a direct evidence of two superconducting energy gaps in a Mo1-xRex alloy films probed by single-electron tunneling spectroscopy. Earlier, it was found that the superconducting transition temperature Tc in the alloys with very close atomic concentrations of Mo and Re is about an order of magnitude higher than Tc’s of the constituent elements. Our observation of two distinct gaps in this system testifies in favor of the scenario that relates this enhancement to the interband interaction rather than to specific modifications of the molybdenium phonon spectrum after alloying with rhenium, i.e., a heavy mass defect.

Broken time-reversal symmetry in superconducting Pr1−xLaxPt4Ge12

The superconducting state of the filled skutterudite alloy series Pr$_{1-x}$La$_{x}$Pt$_{4}$Ge$_{12}$ has been systematically studied by specific heat, zero-field muon spin relaxation ($\mu$SR), and superconducting critical field measurements. An additional inhomogeneous local magnetic field, indicative of broken time-reversal symmetry (TRS), is observed in the superconducting states of the alloys. For $x \lesssim 0.5$ the broken-TRS phase sets in below a temperature $T_m$ distinctly lower than the superconducting transition temperature $T_c$. For $x \gtrsim 0.5$ $T_m \approx T_c$. The local field strength decreases as $x \to 1$, where LaPt$_{4}$Ge$_{12}$ is characterized by conventional pairing. The lower critical field $H_{c1}(T)$ of PrPt$_{4}$Ge$_{12}$ shows the onset of a second quadratic temperature region below $T_q \sim T_m$. Upper critical field $H_{c2}(T)$ measurements suggest multiband superconductivity, and point gap nodes are consistent with the specific heat data. In Pr$_{1-x}$La$_{x}$Pt$_{4}$Ge$_{12}$ only a single specific heat discontinuity is observed at $T_c$, in contrast to the second jump seen in PrOs$_{4}$Sb$_{12}$ below $T_c$. These results suggest that superconductivity in PrPt$_{4}$Ge$_{12}$ is characterized by a complex order parameter.

Determination of the Bardeen–Cooper–Schrieffer material parameters of the HIE-ISOLDE superconducting resonator

Superconducting material parameters of the Nb film coatings on the quarter-wave resonator for the HIE-ISOLDE project were studied by fitting experimental results with the Bardeen–Cooper–Schrieffer Mattis–Bardeen (BCS-MB) theory. We pointed out a strong correlation among fitted estimators of material parameters in the BCS-MB theory, and used a procedure to reduce the uncertainty by merging two χ2 distributions of the surface resistance and effective penetration depth. In this procedure, unlike previous studies, BCS coherence length (ξ0) and London penetration depth (λL) were not fixed at their literature values in the calculation because in our film, whose thickness is from 2 to 13 μm, they may take altered values as a consequence of microstructual defects. Since surface resistance and penetration depth have similar dependencies on coherence length and mean free path, the effects of the correlation between the estimators of these two parameters could not be mitigated by just combining surface resistance and penetration depth data. We used upper critical field measurement by SQUID magnetometry to provide a complementary constraint to these RF measurements, and this allowed all the material parameters to be obtained by fitting the experimental data. In the best performing cavity, the determined parameters are ξ0 = 29 nm, λL = 26 nm, mean free path l = 99 nm, and coupling strength Δ0/kBTc = 1.7. The coherence length is slightly shorter than for clean bulk Nb (39 nm) in the literature although the Nb film is thick and rather bulk-like. A poorly performing cavity showed weaker coupling constant Δ0/kBTc = 1.5 which may indicate film contamination during the coating process.

Experimental evidence of a very thin superconducting layer in epitaxial indium nitride

Indium nitride is one of the very few semiconductors which is known to have a superconducting phase at temperatures of Tc > 1 K. Superconductivity occurs in a window of carrier densities of approximately 1018–1020 cm−3. This is a very low density when compared to other superconductors (i.e. metals, alloys, high Tc oxides) and thus raises interesting fundamental questions as well as technological possibilities. In this paper we address one key question about the dimensionality of the superconducting state of InN by using angle dependent critical field measurements. Our samples were grown by two different growth techniques (chemical vapour deposition and plasma-assisted molecular beam epitaxy) on c-oriented sapphire, with and without a GaN buffer layer. In both cases we find that for film thicknesses much larger than the coherence length d ≫ ξ, the angle dependence of the critical field (down to T < 280 mK) with respect to the c-axis continues to be clearly two-dimensional, demonstrating a characteristic cusp when the angle crosses 90° with respect to the c-axis. This indicates that the superconducting electrons are most likely confined to a layer much thinner than the thickness of the InN film. Further we find the magnitude of the gap to be 2Δ(0)/kBTc = 3.6, very close to the BCS prediction.

Speciated online PM&amp;lt;sub&amp;gt;1&amp;lt;/sub&amp;gt; from South Asian combustion sources – Part 1: Fuel-based emission factors and size distributions

Abstract. Combustion of biomass, garbage, and fossil fuels in South Asia has led to poor air quality in the region and has uncertain climate forcing impacts. Online measurements of submicron aerosol (PM1) emissions were conducted as part of the Nepal Ambient Monitoring and Source Testing Experiment (NAMaSTE) to investigate and report emission factors (EFs) and vacuum aerodynamic diameter (dva) size distributions from prevalent but poorly characterized combustion sources. The online aerosol instrumentation included a “mini” aerosol mass spectrometer (mAMS) and a dual-spot eight-channel aethalometer (AE33). The mAMS measured non-refractory PM1 mass, composition, and size. The AE33-measured black carbon (BC) mass and estimated light absorption at 370 nm due to organic aerosol or brown carbon. Complementary gas-phase measurements of carbon dioxide (CO2), carbon monoxide (CO), and methane (CH4) were collected using a Picarro Inc. cavity ring-down spectrometer (CRDS) to calculate fuel-based EFs using the carbon mass balance approach. The investigated emission sources include open garbage burning, diesel-powered irrigation pumps, idling motorcycles, traditional cookstoves fueled with dung and wood, agricultural residue fires, and coal-fired brick-making kilns, all of which were tested in the field. Open-garbage-burning emissions, which included mixed refuse and segregated plastics, were found to have some of the largest PM1 EFs (3.77–19.8 g kg−1) and the highest variability of the investigated emission sources. Non-refractory organic aerosol (OA) size distributions measured by the mAMS from garbage-burning emissions were observed to have lognormal mode dva values ranging from 145 to 380 nm. Particle-phase hydrogen chloride (HCl) was observed from open garbage burning and was attributed to the burning of chlorinated plastics. Emissions from two diesel-powered irrigation pumps with different operational ages were tested during NAMaSTE. Organic aerosol and BC were the primary components of the emissions and the OA size distributions were centered at ∼80 nm dva. The older pump was observed to have significantly larger EFOA than the newer pump (5.18 g kg−1 compared to 0.45 g kg−1) and similar EFBC. Emissions from two distinct types of coal-fired brick-making kilns were investigated. The less advanced, intermittently fired clamp kiln was observed to have relatively large EFs of inorganic aerosol, including sulfate (0.48 g kg−1) and ammonium (0.17 g kg−1), compared to the other investigated emission sources. The clamp kiln was also observed to have the largest absorption Ångström exponent (AAE = 4) and organic carbon (OC) to BC ratio (OC : BC = 52). The continuously fired zigzag kiln was observed to have the largest fraction of sulfate emissions with an EFSO4 of 0.96 g kg−1. Non-refractory aerosol size distributions for the brick kilns were centered at ∼400 nm dva. The biomass burning samples were all observed to have significant fractions of OA and non-refractory chloride; based on the size distribution results, the chloride was mostly externally mixed from the OA. The dung-fueled traditional cookstoves were observed to emit ammonium, suggesting that the chloride emissions were partially neutralized. In addition to reporting EFs and size distributions, aerosol optical properties and mass ratios of OC to BC were investigated to make comparisons with other NAMaSTE results (i.e., online photoacoustic extinctiometer (PAX) and off-line filter based) and the existing literature. This work provides critical field measurements of aerosol emissions from important yet under-characterized combustion sources common to South Asia and the developing world.

Onset of a Two-Dimensional Superconducting Phase in a Topological-Insulator-Normal-Metal Bi_{1-x}Sb_{x}/Pt Junction Fabricated by Ion-Beam Techniques.

Inducing superconductivity in a topological insulator can lead to novel quantum effects. However, experimental approaches to turn a topological insulator into a superconductor are limited. Here, we report on superconductivity in topological insulator Bi_{0.91}Sb_{0.09} induced via focused ion-beam deposition of a Pt thin film. The superconducting phase exhibits a Berezinski-Kosterlitz-Thouless transition, demonstrative of its two-dimensional character. From the in-plane upper critical field measurements, we estimate the superconducting thickness to be ∼17 nm for a 5.5-μm-thick sample. Our results provide evidence that the interface superconductivity could originate from the surface states of Bi_{0.91}Sb_{0.09}.

Modelling Ephemeral Gully Erosion from Unpaved Urban Roads: Equifinality and Implications for Scenario Analysis.

Modelling gully erosion in urban areas is challenging due to difficulties with equifinality and parameter identification, which complicates quantification of management impacts on runoff and sediment production. We calibrated a model (AnnAGNPS) of an ephemeral gully network that formed on unpaved roads following a storm event in an urban watershed (0.2 km2) in Tijuana, Mexico. Latin hypercube sampling was used to create 500 parameter ensembles. Modelled sediment load was most sensitive to the Soil Conservation Service (SCS) curve number, tillage depth (Td), and critical shear stress (τc). Twenty-one parameter ensembles gave acceptable error (behavioural models), though changes in parameters governing runoff generation (SCS curve number, Manning’s n) were compensated by changes in parameters describing soil properties (TD, τc, resulting in uncertainty in the optimal parameter values. The most suitable parameter combinations or “behavioural models” were used to evaluate uncertainty under management scenarios. Paving the roads increased runoff by 146–227%, increased peak discharge by 178–575%, and decreased sediment load by 90–94% depending on the ensemble. The method can be used in other watersheds to simulate runoff and gully erosion, to quantify the uncertainty of model-estimated impacts of management activities on runoff and erosion, and to suggest critical field measurements to reduce uncertainties in complex urban environments.

Zeeman-limited superconductivity in crystalline Al films

We report the evolution of the Zeeman-mediated superconducting phase diagram (PD) in ultrathin crystalline Al films. Parallel critical field measurements, down to 50 mK, were made across the superconducting tricritical point of films ranging in thickness from 7 to 30 ML. The resulting phase boundaries were compared with the quasiclassical theory of a Zeeman-mediated transition between a homogeneous BCS condensate and a spin-polarized Fermi liquid. Films thicker than $\ensuremath{\sim}20\phantom{\rule{0.16em}{0ex}}\mathrm{ML}$ showed good agreement with theory, but thinner films exhibited an anomalous PD that cannot be reconciled within a homogeneous BCS framework.

90 K superconductivity of clean Pb1212 epitaxial films

A single-phase (Pb1212) epitaxial film with has been grown using a two-step technique, which allows lattice relaxation near the film/substrate interface and reduction of Pb vacancies. Using the upper critical field measurement, the coherence lengths are derived to be 20 and 4.3 Å, along the ab-plane and c-axis, respectively. The value of the irreversibility field is found to be close to that of YBa2Cu3O7. High magnetic field measurement has revealed normal state resistivity below and metallic behavior in the underdoped region.

Kramers non-magnetic superconductivity in LnNiAsO superconductors

We investigated a series of nickel-based oxyarsenides LnNiAsO (Ln = La, Ce, Pr, Nd, Sm) compounds. CeNiAsO undergoes two successive anti-ferromagnetic transitions at TN1 = 9.3 K and TN2 = 7.3 K; SmNiAsO becomes an anti-ferromagnet below TN ≃ 3.5 K; NdNiAsO keeps paramagnetic down to 2 K but orders anti-ferromagnetically below TN ≃ 1.3 K. Superconductivity was observed only in Kramers non-magnetic LaNiAsO and PrNiAsO with Tc = 2.7 K and 0.93 K, respectively. The superconductivity of PrNiAsO is further studied by upper critical field and specific heat measurements, which reveal that PrNiAsO is a weakly coupled Kramers non-magnetic superconductor. Our work confirms that the nickel-based oxyarsenide superconductors are substantially different in mechanism to iron-based ones, and are likely to be described by the conventional superconductivity theory.

Angular variation of the magnetoresistance of the superconducting ferromagnet UCoGe

We report a magnetoresistance study of the superconducting ferromagnet UCoGe. The data, taken on single-crystalline samples, show a pronounced structure at $B^* = 8.5$~T for a field applied along the ordered moment $m_0$. Angle dependent measurements reveal this field-induced phenomenon has an uniaxial anisotropy. Magnetoresistance measurements under pressure show a rapid increase of $B^*$ to 12.8~T at 1.0~GPa. We discuss $B^*$ in terms of a field induced polarization change. Upper critical field measurements corroborate the unusual S-shaped $B_{c2}(T)$-curve for a field along the $b$-axis of the orthorhombic unit cell.

Epitaxial growth of ultra-thin NbN films on AlxGa1−xN buffer-layers

The suitability of AlxGa1−xN epilayers to deposit onto ultra-thin NbN films has been demonstrated for the first time. High quality single-crystal films with 5 nm thickness confirmed by high resolution transmission electron microscopy (HRTEM) have been deposited in a reproducible manner by means of reactive DC magnetron sputtering at elevated temperatures and exhibit critical temperatures (Tc) as high as 13.2 K and residual resistivity ratio (RRR) ∼1 on hexagonal GaN epilayers. On increasing the Al content x in the AlxGa1−xN epilayer above 20%, a gradual deterioration of Tc to 10 K was observed. Deposition of NbN on bare silicon substrates served as a reference and comparison. Excellent spatial homogeneity of the fabricated films was confirmed by R(T) measurements of patterned micro-bridges across the entire film area. The superconducting properties of these films were further characterized by critical magnetic field and critical current measurements. It is expected that the employment of GaN material as a buffer-layer for the deposition of ultra-thin NbN films will prospectively benefit terahertz electronics, particularly hot electron bolometer (HEB) mixers.