Point Contact Andreev Reflection Spectra Research Articles

Anisotropic s-Wave Gap in the Vicinity of a Quantum Critical Point in Superconducting BaFe2(As1–x P x )2 Single Crystals: A Study of Point-Contact Spectroscopy

We report on soft c-axis point-contact Andreev reflection (PCAR) spectroscopy combining with resistivity measurements on BaFe2(As0.7P0.3)2, to elucidate the superconducting gap structure in the vicinity of the quantum critical point. A double peak at the gap edge plus a dip feature at zero-bias has been observed on the PCAR spectra, indicative of the presence of a nodeless gap in BaFe2(As0.7P0.3)2. Detailed analysis within a sophisticated theoretical model reveals an anisotropic gap with deep gap minima. The PCARs also feature additional structures related to the electron–bosonic coupling mode. Using the extracted superconducting energy gap value, a characteristic bosonic energy Ω b and its temperature dependence are obtained, comparable with the spin-resonance energy observed in neutron scattering experiment. These results indicate a magnetism-driven quantum critical point in the BaFe2(As1−x P x )2 system.

Point-contact Andreev reflection measurements on ZrRuAs single crystals

ZrRuAs single crystals with hexagonal crystal structure, which is a kind of non-centrosymmetric superconductor with relatively high transition temperature, is predicted to be a topological superconductor candidate. To explore the exotic properties of ZrRuAs, the superconducting gap of ZrRuAs needs to be measured. In this work, point-contact Andreev reflection (PCAR) spectroscopy is used to investigate the superconductivity of ZrRuAs single crystals. The double conductance peaks in the PCAR spectra are observed along different crystal directions. The superconducting gap can be quantitatively analyzed by using the modified Blonder–Tinkham–Klapwijk model based on the s-wave superconductivity. The temperature dependence of the extracted superconducting gap follows the Bardeen–Cooper–Schrieffer theory. Our results suggest the isotropic s-wave superconductivity in ZrRuAs.

Sub-kelvin Andreev reflection spectroscopy of superconducting gaps in FeSe

Point contact Andreev reflection studies have been conducted on FeSe single crystals by lowering the temperatures down to 0.5 K. The point contact Andreev reflection spectra were analyzed in the framework of the two-band model. As a result, the presence of two anisotropic superconducting gaps in FeSe were certainly established and their BCS-like temperature dependencies were obtained. The weights of each gap have been determined and the anisotropy parameter has been calculated. It is shown, that sub-kelvin temperatures are necessary to ascertain details of the superconducting gap structure, especially for multiband materials when Andreev reflection spectroscopy is applied.

Point contact Andreev reflection studies of a non-centro symmetric superconductor Re6Zr

Re6Zr, a non-centrosymmetric superconductor is an interesting system as recent experimental evidence suggests that the superconducting state breaks time reversal symmetry. This implies a mixing of spin singlet-triplet states leading to a complex order parameter in this system. Here, we report point contact Andreev Reflection (PCAR) measurements on a single crystal of Re6Zr (superconducting transition temperature (Tc) = 6.78 K). We observe multiple gap features in the PCAR spectra which depends on the type of tip and contact. Spectral features appear at voltages 1.0 ± 0.1 mV, 0.75 ± 0.05 mV and 0.45 ± 0.1 mV suggesting that there are at least more than one band contributing to superconductivity. However, strong surface inter-band scattering is possibly responsible for the uncertainty in observing them together distinctly in a single contact in the PCAR measurements. Interestingly, the bulk gap (Δ = 1.95kBTc = 1.1 meV) is occasionally observed in PCAR spectra, mostly with ferromagnetic tips. The gap features associated with the other two smaller gaps disappear at the bulk Tc. In addition, no anisotropy in the upper critical field was observed. Our results suggest an unconventional superconducting order in this compound: Multiband singlet states dominated by inter-band pairing which break the time reversal symmetry or singlet mixed with triplet states.

Distinction between critical current effects and intrinsic anomalies in the point-contact Andreev reflection spectra of unconventional superconductors**Project supported by the National Key Basic Research Program of China (Grant Nos. 2015CB921000, 2016YFA0300301, and 2017YFA0302902), the National Natural Science Foundation of China (Grant Nos. 11674374 and 1474338), the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (Grant No. QYZDB-SSW-SLH008), the

In this work, we discuss the origin of several anomalies present in the point-contact Andreev reflection spectra of (Li1−xFex)OHFeSe, LiTi2O4, and La2−xCexCuO4. While these features are similar to those stemming from intrinsic superconducting properties, such as Andreev reflection, electron-boson coupling, multigap superconductivity, d-wave and p-wave pairing symmetry, they cannot be accounted for by the modified Blonder–Tinkham–Klapwijk (BTK) model, but require to consider critical current effects arising from the junction geometry. Our results point to the importance of tracking the evolution of the dips and peaks in the differential conductance as a function of the bias voltage, in order to correctly deduce the properties of the superconducting state.

Critical analysis of soft point contact Andreev reflection spectra between superconducting films and pressed In

We present a critical analysis of an alternative technique of point contact Andreev reflection (PCAR) spectroscopy used to extract energy resolved information of superconductors which is based on making ‘soft-contacts’ between superconductors and indium. This technique is not sensitive to mechanical vibrations and hence can be used in a cryogen free platform increasing its accessibility to users having no access to cryogenic liquids. Through our experiments on large number of superconducting films we show that the PCAR spectra below the Tc of In show sub-harmonic gap structures consistent with the theory of multiple Andreev reflection (MAR) and a zero bias conductance (ZBC) anomaly associated with the Josephson supercurrent. Furthermore, we demonstrate that large contact resistance with low transparency ballistic contacts in the PCAR regime are required to obtain reliable spectroscopic data. One limitation of the technique arises for low contact resistance junctions where the superconducting proximity effect (SPE) reduces the value of the superconducting energy gap.

Spin polarization measurements in ferromagnetic SrRuO3 using point-contact Andreev reflection technique

Using the point-contact Andreev reflection (PCAR) technique, we have performed spin polarization measurements in a polycrystalline SrRuO3 to study the difference of the spin polarization between the ballistic and diffusive transport samples. PCAR spectra were measured with Nb tips for T = 4.2 K with changing the contact area mechanically. The results were well fitted by the modified Blonder-Tinkham-Klapwijk (mBTK) model. We estimate the spin polarization P ≈ 0.59, which is slightly larger than that of a single crystal film with P ≈ 0.51 due to the change of transport property.

The Order-Parameter Symmetry and Fermi Surface Topology of 122 Fe-Based Superconductors: A Point-Contact Andreev-Reflection Study

We report on the results of directional point-contact Andreev-reflection (PCAR) measurements in Ba(Fe1−x Co x )2As2 single crystals and epitaxial c-axis oriented films with x=0.08 as well as in Ca(Fe1−x Co x )2As2 single crystals with x=0.06. The PCAR spectra are analyzed within the two-band 3D version of the Blonder–Tinkham–Klapwijk model for Andreev reflection we recently developed, and that makes use of an analytical expression for the Fermi surface that mimics the one calculated within the Density-Functional Theory (DFT). The spectra in Ca(Fe0.94Co0.06)2As2 unambiguously demonstrate the presence of nodes or zeros in the small gap. In Ba(Fe0.92Co0.08)2As2, the ab-plane spectra in single crystals can be fitted by assuming two nodeless gaps, but this model fails to fit the c-axis ones in epitaxial films. All these results are discussed in comparison with recent theoretical predictions about the occurrence of accidental 3D nodes and the presence of “hot spots” in the gaps of 122 compounds.

Andreev bound state and multiple energy gaps in the noncentrosymmetric superconductor BiPd

We report directional point contact Andreev reflection (PCAR) measurements on high-quality single crystals of the noncentrosymmetric superconductor BiPd. The PCAR spectra measured on different crystallographic faces of the single crystal clearly show the presence of multiple superconducting energy gaps. For point contacts with low resistance, in addition to the superconducting gap feature, a pronounced zero-bias conductance peak is observed. These observations provide strong evidence for the presence of an unconventional order parameter in this material.

Effect of Transport-Induced Charge Inhomogeneity on Point-Contact Andreev Reflection Spectra at Ferromagnet–Superconductor Interfaces

We investigate the transport properties of a ferromagnet-superconductor interface within the framework of a modified three-dimensional Blonder-Tinkham-Klapwijk formalism. In particular, we propose that charge inhomogeneity forms via two unique transport mechanisms, namely, evanescent Andreev reflection and evanescent quasiparticle transmission. Furthermore, we take into account the influence of charge inhomogeneity on the interfacial barrier potential and calculate the conductance as a function of bias voltage. Point-contact Andreev reflection (PCAR) spectra often show dip structures, large zero-bias conductance enhancement, and additional zero-bias conductance peak. Our results indicate that transport-induced charge inhomogeneity could be a source of all these anomalous characteristics of the PCAR spectra.

Multigap Superconductivity and Strong Electron-Boson Coupling in Fe-Based Superconductors: A Point-Contact Andreev-Reflection Study ofBa(Fe1−xCox)2As2Single Crystals

Directional point-contact Andreev-reflection measurements in Ba(Fe(1-x)Co(x))2As2 single crystals (T(c) = 24.5 K) indicate the presence of two superconducting gaps with no line nodes on the Fermi surface. The point-contact Andreev-reflection spectra also feature additional structures related to the electron-boson interaction, from which the characteristic boson energy Ω(b)(T) is obtained, very similar to the spin-resonance energy observed in neutron scattering experiments. Both the gaps and the additional structures can be reproduced within a three-band s ± Eliashberg model by using an electron-boson spectral function peaked at Ω(0) = 12 meV ≃ Ω(b)(0).

Possible microscopic origin of large broadening parameter in point Andreev reflection spectroscopy

We report on the low frequency noise in ballistic point-contacts between a silver tip and a niobium foil. The ballistic nature is confirmed by point-contact Andreev reflection (PCAR) spectroscopy. As has been found by others, a broadening parameter Γ needs to be introduced to fit the PCAR spectra. For contacts with higher resistance, a larger Γ is required to fit the PCAR spectra and we find that random two level fluctuations dominate the noise spectra. This finding suggests that two level fluctuations contribute to the broadening of the gap features.

Point contact Andreev reflection spectroscopy of superconducting energy gaps in 122-type family of iron pnictides

A brief overview of the superconducting energy gap studies on 122-type family of iron pnictides is given. It seems that the situation in the hole doped Ba 1− x K x Fe 2As 2 is well resolved. Most of the measurements including the presented here point contact Andreev reflection spectra agree on existence of multiple nodeless gaps in the excitation spectrum of this multiband system. The gaps have basically two sizes – the small one with a strength up to the BCS weak coupling limit and the large one with a very strong coupling with 2 Δ L / kT c > 6–8. In the electron doped Ba(Fe 1− x Co x ) 2As 2 the most of the experiments including our point contact measurements reveal in quite broadened spectra only a single gap with a strong coupling strength. The high precision ARPES measurements on this system identified two gaps but very close to each other, both showing a strong coupling with 2 Δ/ kT c ∼ 5 and 6, respectively.

Conductance features in point contact Andreev reflection spectra

Point contact Andreev reflection (PCAR) spectroscopy is a common technique fordetermining the spin polarization of a ferromagnetic sample. The polarization is extractedby measuring the bias dependence of the conductance of a metallic/superconducting pointcontact. Under ideal conditions, the conductance is dominated by Andreev reflection andthe Blonder–Tinkham–Klapwijk (BTK) model can be used to extract a value for thepolarization. However, PCAR spectra often exhibit unwanted features in theconductance that cannot be appropriately modelled with the BTK theory. Inthis paper we isolate some of these unwanted features and show that any furtherextraction of the spin polarization from these non-ideal spectra proves unreliable.Understanding the origin of these features provides an objective criterion forrejection of PCAR spectra unsuitable for fitting with the modified BTK model.

Extracting the diffusivity ratio from point contact Andreev reflection spectroscopy and upper critical field measurements in MgB2

The diffusivity ratio η, which measures the relative intraband scattering in the π and σ bands in MgB2 has been determined by fitting the Hc2 (T) at T ~ Tcand by Point Contact Andreev Reflection. We find a satisfactory agreement between the values for η obtained by both methods for c-axis orientated MgB2 thin films. Point contact Andreev Reflection was then applied to bulk MgB2 containing Mg vacancies. Spectra obtained in zero field indicate a distribution of the two gaps Δσ,π but no merging of the values with increased magnesium deficiency. Spectra fitted as a function of field are consistent with an increase in π intraband scattering with increasing magnesium deficiency. Measurement of the point contact Andreev reflection spectra as a function of temperature revealed features not immediately expected from current theoretical models.

Possibilities and limitations of point-contact spectroscopy for measurements of spin polarization

Major issues that limit the applicability of point-contact Andreev reflection (PCAR) spectroscopy for measurements of transport spin polarization $(P)$ are investigated. We show that the analysis of the PCAR spectra on the basis of the generalized Blonder-Tinkham-Klapwijk model may lead to degenerate solutions; i.e., the straightforward determination of $P$ from the multiparameter fitting is ambiguous. We have developed a refined method of analysis that reduces the ambiguity. This method is applied to several test cases (copper, NiMnSb, ${\mathrm{Co}}_{2}\mathrm{Mn}\mathrm{Si}$, ${\mathrm{Sr}}_{2}\mathrm{Fe}\mathrm{Mo}{\mathrm{O}}_{6}$), and it is found to work in materials with low and high $P$.

Analysis of point-contact Andreev reflection spectra in spin polarization measurements

We present a systematic analysis of point-contact Andreev reflection (PCAR) spectra for ferromagnetic materials, using both modeling and experimental data. We emphasize the importance of consistent data analysis to avoid possible misinterpretation of the data. We consider the relationship between ballistic and diffusive transport, the effect of different transport regimes on spin polarization measurements, and the importance of unambiguous identification of the type of transport regime. We find that in a realistic parameter range, the analysis of PCAR spectra of purely diffusive character by a ballistic model yield approximately the same (within $\ensuremath{\sim}3%$) values of the spin polarization and the barrier strength $Z$ larger by $\ensuremath{\sim}0.5--0.6$. We also consider the dependence of polarization values on $Z$, and have shown by simple modeling that letting the superconducting gap vary as an adjustable parameter can result in a spurious dependence of the spin-polarization ${P}_{c}$ on $Z$. At the same time we analyzed the effects of finite $Z$ on the apparent value of ${P}_{c}$ measured by the PCAR technique, using a large number of examples from both our own measurements and from the literature. We conclude that there is a system-dependent variation in ${P}_{c}\phantom{\rule{0.2em}{0ex}}(Z)$, presumably due to spin-flip scattering at the interface. However, the exact type of this dependence is hard to determine with any statistical certainty.

Role of critical current on the point-contact Andreev reflection spectra between a normal metal and a superconductor

The point contact spectrum between a normal metal and a superconductor often shows unexpected sharp dips in the conductance at voltage values larger than the superconducting energy gap. These dips are not predicted in the Blonder-Tinkham-Klapwizk (BTK) theory, commonly used to analyse these contacts. We present here a systematic study of these dips in a variety of contacts between different combinations of a superconductor and a normal metal. From the correlation between the characteristics of these dips with the contact area, we can surmise that such dips are caused by the contact not being in the ballistic limit. An analysis of the possible errors introduced while analysing such a spectrum with the standard BTK model is also presented.