Point-contact Spectroscopy Measurements Research Articles

Superconductivity of Co-Doped CaKFe4As4 Investigated via Point-Contact Spectroscopy and London Penetration Depth Measurements.

The iron-based superconductors (IBSs) of the recently discovered 1144 class, unlike many other IBSs, display superconductivity in their stoichiometric form and are intrinsically hole doped. The effects of chemical substitutions with electron donors are thus particularly interesting to investigate. Here, we study the effect of Co substitution in the Fe site of CaKFe4As4 single crystals on the critical temperature, on the energy gaps, and on the superfluid density by using transport, point-contact Andreev-reflection spectroscopy (PCARS), and London penetration depth measurements. The pristine compound (Tc≃36 K) shows two isotropic gaps whose amplitudes (Δ1 = 1.4-3.9 meV and Δ2 = 5.2-8.5 meV) are perfectly compatible with those reported in the literature. Upon Co doping (up to ≈7% Co), Tc decreases down to ≃20 K, the spin-vortex-crystal order appears, and the low-temperature superfluid density is gradually suppressed. PCARS and London penetration depth measurements perfectly agree in demonstrating that the nodeless multigap structure is robust upon Co doping, while the gap amplitudes decrease as a function of Tc in a linear way with almost constant values of the gap ratios 2Δi/kBTc.

Multiple transition temperature enhancement in superconducting TiNbMoTaW high entropy alloy films through tailored N incorporation

We report about the influence of nitrogen incorporation on the superconducting transition temperature TC of TiNbMoTaW high entropy alloy films deposited using high power impulse magnetron sputtering. By measuring the temperature dependence of resistivity of (TiNbMoTaW)Nx nitrides, we observe a significant increase of TC, from 0.62 K for x = 0 up to 5.02 K for x = 0.74. With further increase of x, TC is decreasing and reaches 1.08 K for x = 0.97. The eightfold TC enhancement seems to be associated with the incorporation of light N atoms into the face-centered cubic lattice and with the x dependent enhancement of the electron-phonon interaction, which may be related to the high configuration entropy in high entropy alloys. Measurements in magnetic field show that the upper critical fields Bc2 of (TiNbMoTaW)Nx with x > 0.15 provide Bc2/TC > 2 T/K ratios, which are above the weak-coupling pair breaking limit. Additional heat capacity and point contact spectroscopy measurements show that the superconductivity in the ∼1 μm thick films is bulk in nature, consistent with conventional weak-coupling phonon mediated superconductivity. The proposed strategy of nitrogen incorporation into high entropy alloys may pave a pathway towards tailoring their superconducting properties, especially their TC.

Spectroscopic evidence for the superconductivity of elemental metal Y under pressure

Very high applied pressure induces superconductivity with the transition temperature (Tc) exceeding 19 K in elemental yttrium, but relatively little is known about the nature of that superconductivity. From point-contact spectroscopy (PCS) measurements in a diamond anvil cell (DAC), a strong enhancement in the differential conductance is revealed near the zero-biased voltage owing to Andreev reflection, a hallmark of the superconducting (SC) phase. Analysis of the PCS spectra based on the extended Blonder-Tinkham-Klapwijk (BTK) model indicates two SC gaps at 48.6 GPa, where the large gap ΔL is 3.63 meV and the small gap ΔS is 0.46 meV. When scaled against a reduced temperature, both small and large SC gaps collapse on a single curve that follows the prediction from BCS theory. The SC gap-to-Tc ratio is 8.2 for the larger gap, and the initial slope of the upper critical field is −1.9 T/K, indicating that Y belongs to a family of strongly coupled BCS superconductors. The successful application of PCS to Y in DAC environments demonstrates its utility for future research on other pressure-induced high-Tc superconductors.

Emergent superconducting fluctuations in compressed kagome superconductor CsV3Sb5

The recent discovery of superconductivity (SC) and charge density wave (CDW) in kagome metals AV3Sb5 (A = K, Rb, Cs) provides an ideal playground for the study of emergent electronic orders. Application of moderate pressure leads to a two-dome-shaped SC phase regime in CsV3Sb5 accompanied by the destabilizing of CDW phase. Nonetheless, the nature of this pressure-tuned SC state and its interplay with the CDW are yet to be explored. Here, we perform soft point-contact spectroscopy (SPCS) measurements in CsV3Sb5 to investigate the evolution of superconducting order parameter with pressure. Surprisingly, we find that the superconducting gap is significantly enhanced between the two SC domes, at which the zero-resistance temperature is suppressed and the transition is remarkably broadened. Moreover, the temperature-dependence of the SC gap in this pressure range severely deviates from the conventional Bardeen-Cooper-Schrieffer (BCS) behavior, evidencing for strong Cooper pair phase fluctuations. These findings reveal the complex intertwining of the CDW with SC in the compressed CsV3Sb5, suggesting striking parallel to the cuprate superconductor La2−xBaxCuO4. Our results point to the essential role of charge degree of freedom in the development of intertwining electronic orders, and thus provide new constraints for theories.

Strong-coupling superconductivity in the kagome metal CsV3Sb5 revealed by soft point-contact spectroscopy

The nature of the superconducting state in kagome metals $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ is a key issue in need of experimental clarification. Here, we report on a study of the superconducting order parameter in the kagome superconductor ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ through simultaneous ``soft'' point-contact spectroscopy and resistivity measurements under both ambient and a hydrostatic pressure. Signatures of two-gap superconductivity are resolved in the soft point-contact spectra, accompanied by an asymmetric excess conductance above ${T}_{c}$. Quantitative analysis based on the two-dimensional Blonder-Tinkham-Klapwijk model reveals an ($s+s$)-wave superconducting gap with $2{\mathrm{\ensuremath{\Delta}}}_{0}/{k}_{B}{T}_{c}\ensuremath{\simeq}7.2$, placing ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ in the strong-coupling regime. The strong-coupling two-gap feature indicates a high electronic density of states (DOS) and possible existence of flat-band-driven multiple van Hove singularities (VHSs) at the Fermi level. The presence of asymmetric excess spectral conductance above ${T}_{c}$ hints at a modest electronic correlation in ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$. Under a hydrostatic pressure of 2.1 kbar, the nodeless multigap nature of the superconducting state remains, whereas both the larger gap and the excess spectral conductance are greatly suppressed, accompanied by an enhanced ${T}_{c}$. An estimate of the spectral-weighted gap ratio reveals a weakened coupling strength, indicative of a reduced total superconducting DOS upon pressure. Our results point to key roles of both flat-band-associated VHSs and electronic correlation in the onset of kagome superconductivity and shed some light on the interplay between charge-density-wave order and superconductivity in vanadium-based kagome superconductors.

Nodal multigap superconductivity in the anisotropic iron-based compound RbCa2Fe4As4F2

The 12442 compounds are a recently discovered family of iron-based superconductors, that share several features with the cuprates due to their strongly anisotropic structure, but are so far poorly understood. Here, we report on the gap structure and anisotropy of RbCa2(Fe1−xNix)4As4F2 single crystals, investigated by a combination of directional point-contact Andreev-reflection spectroscopy and coplanar waveguide resonator measurements. Two gaps were identified, with clear signatures of d-wave-like nodal structures which persist upon Ni doping, well described by a two-band d − d state with symmetry-imposed nodes. A large London penetration depth anisotropy was revealed, weakly dependent on temperature and fully compatible with the d − d model.

Point-contact spectroscopy of heavy fermion superconductors Ce2PdIn8 and Ce3PdIn11 in comparison with CeCoIn5

We report point-contact spectroscopy measurements on heavy fermion cousins CeCoIn5, Ce2PdIn8 and Ce3PdIn11 to systematically study the hybridization between f and conduction electrons. Below a temperature T*, the spectrum of each compound exhibits an evolving Fano-like conductance shape, superimposed on a sloping background, that suggests the development of hybridization between local f and itinerant conduction electrons in the coherent heavy fermion state below T*. We present a quantitative analysis of the conductance curves with a two-channel model to compare the tunneling process between normal metallic silver particles in our soft point-contact and heavy-fermion single crystals CeCoIn5, Ce2PdIn8 and Ce3PdIn11.

Point-contact spectroscopy on antiferromagnetic Kondo semiconductors CeT2Al10 (T = Ru and Os)**Project supported by the National Key Research and Development Program of China (Grant Nos. 2017YFA0303101 and 2016FYA0300402), the National Natural Science Foundation of China (Grant Nos. 11674279, 11774404, and 11374257), the Zhejiang Provincial Natural Science Foundation of China (Grant No. LR18A04001), and the Japan Society for the Promotion of

We have carried out point-contact spectroscopy (PCS) measurements on one family of antiferromagnetic Kondo semiconductor CeT2Al10 (T = Ru and Os) with a Nèel temperature TN ∼ 27.5 and 28.5 K, respectively. Their PCS conductance curves both exhibit a characteristic coherent double-peak-structure at temperatures below TN, signaling an AFM gap around the Fermi surface. The temperature dependent AFM gap Δ1 follows a Bardeen–Cooper–Schrieffer (BCS)-like mean-field behavior with a moderate gap anisotropy for PCS along different crystal axes. Another asymmetric gap-like feature is observed for both compounds at temperatures far below TN, which is consistent with opening of a new hybridization gap Δh inside the long-range ordered AFM state. Our results suggest a common itinerant nature of the anomalous AFM ordering, constraining theoretical models to explain the AFM origin in CeRu2Al10 and CeOs2Al10.

Study of the surface properties of NCCO electron-doped cuprate

Whenever one is interested in making high temperature superconductor-based devices, the goodness of the sample surface in terms of structural and electrical properties is a strong issue. In fact, it is well known that the surface of high Tc superconducting samples is not bulk-representative, due to air contamination and to the possible presence of oxygen vacancies. In addition, the quality of the surface layer results to be crucial in surface sensitive measurements as in X-ray photoelectron and Angle-resolved photoemission spectroscopy. Recently, some studies have been dedicated to the realization of devices based on electron-doped cuprates, bilayers and nanowires, showing the actual possibility to realize good quality junctions by using these cuprates. In this work, we report on the fabrication of thin films of the electron-doped Nd2−xCexCuO4±δ compound and analyze the surface natural barrier of as-grown films by means of point contact spectroscopy measurements. Suitable treatments of samples in an ozone rich atmosphere have been developed in order to improve the surface quality of the films. Auger electron spectroscopy has been used to monitor the effectiveness of these treatments.

Quasi-2D superconductivity in FeTe0.55Se0.45 ultrathin film

Iron-chalcogenide FeTe0.55Se0.45 was found to be a promising topological superconducting candidate recently, which may host Majorana bound state in the vortex core and thus attracts intensive research interests in this material. In this report, mechanically exfoliated FeTe0.55Se0.45 superconducting thin films close to the two-dimensional (2D) limit, i.e. sample thickness is on the order of coherence length, were studied systematically by means of electrical transport and point-contact Andreev-reflection spectroscopy (PCARS) measurements. The quasi-2D nature of FeTe0.55Se0.45 thin films is evidenced by the observation of Berezinskii–Kosterlitz–Thouless (BKT) transition and anisotropic upper critical fields in the vicinity of superconducting transition. Compared to bulk samples, we found that the superconducting transition temperature is only slightly suppressed even for films down to 5 nm. The superconducting gap symmetry remains unchanged and the gap size is weakly affected by tailoring thickness. Our findings suggest that the superconductivity of FeTe0.55Se0.45 thin films is rather robust against reduced dimensions. It provides a novel platform for device applications for quantum computations in combination with possible realization of Majorana modes in this material.

Low-temperature hydrogen absorption in metallic nanocontacts studied by point-contact spectroscopy measurements

We report on hydrogen (H) and deuterium (D) atoms absorption below T = 20 K in metallic palladium (Pd) via quantum tunnelling (QT). When a small bias voltage is applied between Pd nanocontacts that are immersed in liquid H2 (D2), the differential conductance spectra measured by point-contact spectroscopy change enormously. The results indicate H (D) absorption in Pd nanocontacts at the temperature where H (D) absorption due to thermal hopping process is not expected, and can be explained by QT. The QT occurs when the energy level of the potential well trapping the H (D) atom coincides with those not trapping the H (D) atom, and is assisted by phonons induced by ballistic electrons.

Emergence of higher order rotational symmetry in the hidden order phase of URuSi

Electrical resistivity measurements were performed as functions of temperature, magnetic field, and angle between the magnetic field and the c-axis of a URuSi single crystal. The resistivity exhibits a two-fold oscillation as a function of at high temperatures, which undergoes a 180-phase shift (sign change) with decreasing temperature at around 35 K. The hidden order transition is manifested as a minimum in the magnetoresistance and amplitude of the two-fold oscillation. Interestingly, the resistivity also showed four-fold, six-fold, and eight-fold symmetries at the hidden order transition. These higher order symmetries were also detected at low temperatures, which could be a sign of the formation of another pseudogap phase above the superconducting transition, consistent with recent evidence for a pseudogap from point-contact spectroscopy measurements and NMR. Measurements of the magnetisation of single crystalline URuSi with the magnetic field applied parallel and perpendicular to the crystallographic c-axis revealed regions with linear temperature dependencies between the hidden order transition temperature and about 25 K. This T-linear behaviour of the magnetisation may be associated with the formation of a precursor phase or ‘pseudogap’ in the density of states in the vicinity of 30–35 K.

Design and construction of a point-contact spectroscopy rig with lateral scanning capability.

The design and realization of a cryogenic rig for point-contact spectroscopy measurements in the needle-anvil configuration is presented. Thanks to the use of two piezoelectric nano-positioners, the tip can move along the vertical (z) and horizontal (x) direction and thus the rig is suitable to probe different regions of a sample in situ. Moreover, it can also form double point-contacts on different facets of a single crystal for achieving, e.g., an interferometer configuration for phase-sensitive measurements. For the later purpose, the sample holder can also host a Helmholtz coil for applying a small transverse magnetic field to the junction. A semi-rigid coaxial cable can be easily added for studying the behavior of Josephson junctions under microwave irradiation. The rig can be detached from the probe and thus used with different cryostats. The performance of this new probe has been tested in a Quantum Design PPMS system by conducting point-contact Andreev reflection measurements on Nb thin films over large areas as a function of temperature and magnetic field.

Fermi-Surface Topological Phase Transition and Horizontal Order-Parameter Nodes in CaFe2As2 Under Pressure.

Iron-based compounds (IBS) display a surprising variety of superconducting properties that seems to arise from the strong sensitivity of these systems to tiny details of the lattice structure. In this respect, systems that become superconducting under pressure, like CaFe2As2, are of particular interest. Here we report on the first directional point-contact Andreev-reflection spectroscopy (PCARS) measurements on CaFe2As2 crystals under quasi-hydrostatic pressure, and on the interpretation of the results using a 3D model for Andreev reflection combined with ab-initio calculations of the Fermi surface (within the density functional theory) and of the order parameter symmetry (within a random-phase-approximation approach in a ten-orbital model). The almost perfect agreement between PCARS results at different pressures and theoretical predictions highlights the intimate connection between the changes in the lattice structure, a topological transition in the holelike Fermi surface sheet, and the emergence on the same sheet of an order parameter with a horizontal node line.

Observation of superconductivity induced by a point contact on 3D Dirac semimetal Cd3As2 crystals

Three-dimensional (3D) Dirac semimetals, which possess 3D linear dispersion in the electronic structure as a bulk analogue of graphene, have lately generated widespread interest in both materials science and condensed matter physics. Recently, crystalline Cd3As2 has been proposed and proved to be a 3D Dirac semimetal that can survive in the atmosphere. Here, by using point contact spectroscopy measurements, we observe exotic superconductivity around the point contact region on the surface of Cd3As2 crystals. The zero-bias conductance peak (ZBCP) and double conductance peaks (DCPs) symmetric around zero bias suggest p-wave-like unconventional superconductivity. Considering the topological properties of 3D Dirac semimetals, our findings may indicate that Cd3As2 crystals under certain conditions could be topological superconductors, which are predicted to support Majorana zero modes or gapless Majorana edge/surface modes in the boundary depending on the dimensionality of the material.

Two-step fabrication technique of gold tips for use in point-contact spectroscopy.

For a successful point-contact spectroscopy (PCS) measurement, metallic tips of proper shape and smoothness are essential to ensure the ballistic nature of a point-contact junction. Until recently, the fabrication of Au tips suitable for use in point-contact spectroscopy has remained more of an art involving a trial and error method rather than an automated scientific process. To address these issues, we have developed a technique with which one can prepare high quality Au tips reproducibly and systematically. It involves an electronic control of the driving voltages used for an electrochemical etching of a gold wire in a HCl-glycerol mixture or a HCl solution. We find that a stopping current, below which the circuit is set to shut off, is a single very important parameter to produce an Au tip of desired shape. We present detailed descriptions for a two-step etching process for Au tips and also test results from PCS measurements using them.

Point-Contact Spectroscopy of Heavy Fermion Compounds CeCu6 and CeAl3 in Magnetic Field

We have carried out point-contact spectroscopy measurements on CeCu6 and CeAl3 to study these heavy fermion states. Temperature and magnetic field dependences of the point-contact spectra were observed in detail. At the lowest temperature of 0.4K, differential resistance spectra of CeCu6 and CeAl3 as function of bias voltage exhibit each distinct minimum structure and additional small peak structure. Temperature evolutions of these spectra started at temperatures related to the Kondo effect, although the minimum and the peak structure emerged in reverse between both spectra of two compounds. In magnetic fields, both structures monotonically change with no distinct change without a change of curvature of magnetic field dependence of magnitude of differential resistance at zero bias voltage at critical magnetic field of meta-magnetization. This behavior in magnetic field is different from that of CeAl2, which shows antiferromagnetic ordering. This result may suggest that PCS is able to discriminate between heavy fermion state and antiferromagnetic state.

Isotropic multi-gap superconductivity in BaFe1.9Pt0.1As2 from thermal transport and spectroscopic measurements

Thermal conductivity, point contact spectroscopy, angle-resolved photoemission and Raman spectroscopy measurements were performed on BaFe1.9Pt0.1As2 single crystals obtained from the same synthesis batch in order to investigate the superconducting energy gap structure using multiple techniques. Low temperature thermal conductivity was measured in the superconducting state as a function of temperature and magnetic field, revealing an absence of quasiparticle excitations in the limit up to 15 T applied magnetic fields. Point-contact Andreev reflection spectroscopy measurements were performed as a function of temperature using the needle-anvil technique, yielding features in the conductance spectra at both 2.5 meV and 7.0 meV scales consistent with a multi-gap scenario. Angle-resolved photoemission spectroscopy probed the electronic band structure above and below the superconducting transition temperature of Tc = 23 K, revealing an isotropic gap of magnitude meV on both electron and hole pockets. Finally, Raman spectroscopy was used to probe quasiparticle excitations in multiple channels, showing a threshold energy scale of 3 meV below Tc. Overall, we find strong evidence for an isotropic gap structure with no nodes or deep minima in this system, with a 3 meV magnitude gap consistently observed and a second, larger gap suggested by point-contact spectroscopy measurements. We discuss the implications that the combination of these results reveal about the superconducting order parameter in the BaFe2−xPtxAs2 doping system and how this relates to similar substituted iron pnictides.

Experimental evidence for s-wave pairing symmetry in superconducting Cu(x)Bi2Se3 single crystals using a scanning tunneling microscope.

Topological superconductors represent a newly predicted phase of matter that is topologically distinct from conventional superconducting condensates of Cooper pairs. As a manifestation of their topological character, topological superconductors support solid-state realizations of Majorana fermions at their boundaries. The recently discovered superconductor Cu(x)Bi(2)Se(3) has been theoretically proposed as an odd-parity superconductor in the time-reversal-invariant topological superconductor class, and point-contact spectroscopy measurements have reported the observation of zero-bias conductance peaks corresponding to Majorana states in this material. Here we report scanning tunneling microscopy measurements of the superconducting energy gap in Cu(x)Bi(2)Se(3) as a function of spatial position and applied magnetic field. The tunneling spectrum shows that the density of states at the Fermi level is fully gapped without any in-gap states. The spectrum is well described by the Bardeen-Cooper-Schrieffer theory with a momentum independent order parameter, which suggests that Cu(x)Bi(2)Se(3) is a classical s-wave superconductor contrary to previous expectations and measurements.

Non-monotonic behaviour of the superconducting order parameter in Nb/PdNi bilayers observed through point contact spectroscopy

Point contact spectroscopy measurements have been performed on Nb/PdNi bilayers in which the thickness of the Nb layer, dNb, was kept constant at 40 nm while the thickness of PdNi, dPdNi, was changed from 2 to 9 nm. Features related to the superconducting gap induced in the ferromagnet have been observed in the dV/dI versus V curves. These structures show a non-monotonic behaviour as a function of dPdNi as a consequence of the damped oscillatory behaviour of the superconducting order parameter in the ferromagnetic layer.