Point Contact Andreev Reflection Spectroscopy Research Articles

Point-Contact Andreev-Reflection Spectroscopy in the Fe-based Superconductor LaFeAsO1−x F x

Point-contact Andreev-reflection (PCAR) spectroscopy measurements have been performed in the Fe-based superconductor LaFeAsO1−x F x in order to investigate the energy gaps. The Andreev spectra show clear and reproducible features that could be related to two nodeless gaps. Their values, as determined by fitting the conductance curves measured in junctions with local T c =27.3–28.6 K within the two-band BTK model, are Δ 1=2.75–3.8 meV for the small gap and Δ 2=7.9–10.2 meV for the larger one, respectively. The absence of zero-bias conductance peaks and considerations regarding the nonperfectly directional current injection in PCAR experiments and the polycrystalline nature of the samples rule out the possibility of nodal gaps in the superconductor. Moreover, the conductance curves show pseudogap-like features coexisting, at low temperature, with superconductivity and disappearing close to the Néel temperature of the parent compound, T N≈140 K.

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.

Single crystals of LnFeAsO 1−xF x (Ln = La, Pr, Nd, Sm, Gd) and Ba 1−xRb xFe 2As 2: Growth, structure and superconducting properties

A review of our investigations on single crystals of LnFeAsO 1− x F x (Ln = La, Pr, Nd, Sm, Gd) and Ba 1− x Rb x Fe 2As 2 is presented. A high-pressure technique has been applied for the growth of LnFeAsO 1− x F x crystals, while Ba 1− x Rb x Fe 2As 2 crystals were grown using a quartz ampoule method. Single crystals were used for electrical transport, structure, magnetic torque and spectroscopic studies. Investigations of the crystal structure confirmed high structural perfection and show incomplete occupation of the (O, F) position in superconducting LnFeAsO 1− x F x crystals. Resistivity measurements on LnFeAsO 1− x F x crystals show a significant broadening of the transition in high magnetic fields, whereas the resistive transition in Ba 1− x Rb x Fe 2As 2 simply shifts to lower temperature. The critical current density for both compounds is relatively high and exceeds 2 × 10 9 A/m 2 at 15 K in 7 T. The anisotropy of magnetic penetration depth, measured on LnFeAsO 1− x F x crystals by torque magnetometry is temperature dependent and apparently larger than the anisotropy of the upper critical field. Ba 1− x Rb x Fe 2As 2 crystals are electronically significantly less anisotropic. Point-Contact Andreev-Reflection spectroscopy indicates the existence of two energy gaps in LnFeAsO 1− x F x . Scanning Tunneling Spectroscopy reveals in addition to a superconducting gap, also some feature at high energy (∼20 meV).

Point-contact Andreev-reflection spectroscopy in ReFeAsO 1−xF x (Re = La, Sm): Possible evidence for two nodeless gaps

A deep understanding of the character of superconductivity in the recently discovered Fe-based oxypnictides ReFeAsO 1− x F x (Re = rare-earth) necessarily requires the determination of the number of the gaps and their symmetry in k space, which are fundamental ingredients of any model for the pairing mechanism in these new superconductors. In the present paper, we show that point-contact Andreev-reflection spectroscopy experiments performed on LaFeAsO 1− x F x (La-1111) polycrystals with T c ∼ 27 K and SmFeAsO 0.8F 0.2 (Sm-1111) polycrystals with T c ∼ 53 K gave differential conductance curves exhibiting two peaks at low bias and two additional structures (peaks or shoulders) at higher bias voltages, an experimental situation quite similar to that observed by the same technique in pure and doped MgB 2. The single-band Blonder–Tinkham–Klapwijk model is totally unable to properly fit the conductance curves, while the two-gap one accounts remarkably well for the shape of the whole experimental dI/ dV vs. V curves. These results give direct evidence of two nodeless gaps in the superconducting state of ReFeAsO 1− x F x (Re = La, Sm): a small gap, Δ 1, smaller than the BCS value (2 Δ 1/ k B T c ∼ 2.2–3.2) and a much larger gap Δ 2 which gives a ratio 2 Δ 2/ k B T c ∼ 6.5–9.0. In Sm-1111 both gaps close at the same temperature, very similar to the bulk T c , and follow a BCS-like behaviour, while in La-1111 the situation is more complex, the temperature dependence of the gaps showing remarkable deviations from the BCS behaviour at T close to T c . The normal-state conductance reproducibly shows an unusual, but different, shape in La-1111 and Sm-1111 with a depression or a hump at zero bias, respectively. These structures survive in the normal state up to T ∗ ∼ 140 K, close to the temperatures at which structural and magnetic transitions occur in the parent, undoped compound.

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.

Investigation of superconducting gap structure in TbFeAsO0.9F0.1 using point contact Andreev reflection

Bulk samples of TbFeAsO0.9F0.1 (Tcon = 50 K) were measured by point contact Andreev reflection spectroscopy. The spectra show unambiguous evidence for multiple gap-like features plus the presence of high-bias shoulders. By measuring the spectra as a function of temperature with both gold and superconducting niobium tips, we establish that the gap-like features are associated with the superconducting order parameter in this material. We discuss whether the well-defined zero-bias conductance peak that we observe infrequently is associated with a nodal superconducting order parameter.

Evidence for two-gap superconductivity inBa0.55K0.45Fe2As2from directional point-contact Andreev-reflection spectroscopy

Directional point-contact Andreev-reflection spectroscopy measurements on the ${\text{Ba}}_{0.55}{\text{K}}_{0.45}{\text{Fe}}_{2}{\text{As}}_{2}$ single crystals are presented. The spectra show significant differences when measured in the $ab$ plane in comparison with those measured in the $c$ direction of the crystal. In the latter case only a reduced point-contact conductance around zero bias has been revealed persisting well above ${T}_{c}$ and probably related to the structural and magnetic transitions in the system. Within the $ab$ plane two superconducting energy gaps are detected below ${T}_{c}$. Here a reduced conductance above ${T}_{c}$ could also be found. The fits of the $ab$-plane data to the superconducting $s$-wave two-gap model indicate that the smaller gap has a size below the BCS value while the large gap reveals much higher coupling strength.

Effect of Li–Al co-doping on the energy gaps ofMgB2

We studied the effects of co-doping with Li and Al on the energy gaps ofMgB2 by performing point-contact Andreev-reflection spectroscopy (PCAR) in polycrystallineMg1−x(AlαLi1−α)xB2 sampleswith x≤0.4. Even though the lattice parameters and the critical temperatureof the compound simply scale with the effective Al contentαx, irrespective of the Li concentration, the energy gaps do not. Inparticular, for a given effective Al content, the comparison withMg1−y(Al)yB2 withy = αx shows that theσ bandgap is practicallythe same while the π bandgap is higher. A clear gap merging is observed in the most doped sample(x = 0.4)when Tc<20 K. The results are discussed within the two-band Eliashberg theory and compared to theoutcomes of first-principles calculations of the effects of Li and Al co-doping on theelectronic structure of magnesium diboride.

Magnetic phase transformations and superconductivity in Dy0.8Y0.2Rh4B4

The results of experimental studies of the magnetic and the superconducting properties of the compound Dy0.8Y0.2Rh4B4 with tetragonal body-centered crystal structure of the perovskite type (LuRu4B4) are presented. It is shown that the compound undergoes a paramagnet-ferrimagnet phase transition at the Curie temperature TC≈30.5K and its magnetic compensation temperature Tcomp≈17K. According to resistance measurements, the compound becomes a ferrimagnetic superconductor at Tconset≈5.9K which undergoes a ferrimagnet-antiferromagnet phase transition at TN≈2.7K while still remaining a superconductor. The specific heat exhibits a sharp maximum at this temperature. Point-contact Andreev reflection spectroscopy is used to measure the temperature and field dependences of the order parameter Δ(T,H) and the temperature dependence of the upper critical field Hc2(T). The dependences obtained differ radically from those generally accepted for conventional superconductors. The results obtained are discussed in connection with the possibility of triplet pairing in Dy0.8Y0.2Rh4B4.

Study of spin fluctuations in Ni3±xAl1∓x using point contact Andreev reflection spectroscopy

We report point contact Andreev reflection (PCAR) spectroscopy studies on Ni3±xAl1∓x with composition range spanning the ferromagnetic to paramagnetic phase boundary. PCAR studies performed using Nb tip as counterelectrode reveal that the superconducting quasiparticle lifetime (τ) and superconducting energy gap (Δ) decrease with increasing spin fluctuation in the normal metal electrode. Our study reveals that PCAR could be a useful probe to study spin fluctuations in systems that are on the verge of magnetic instability.

Point-contact Andreev-reflection spectroscopy in segregation-freeMg1−xAlxB2 singlecrystals up to x = 0.32

We present new results of point-contact Andreev-reflection (PCAR) spectroscopy in single-phaseMg1−xAlxB2 singlecrystals with x up to 0.32. Fitting the conductance curves of our point contacts with the two-bandBlonder–Tinkham–Klapwijk model allowed us to extract the gap amplitudesΔσ and Δπ. The gap values agree rather well with other PCAR results for Al-doped crystals and polycrystals upto x = 0.2 reported in the literature, and extend them to higher Al contents. In thelow-doping regime, however, we observed an increase in the small gapΔπ onincreasing x (or decreasing the local critical temperature of the junctions,TcA) which is not as clearly found for other samples. On further decreasingTcA below 30 K, both the gaps decrease and, up to the highest doping levelx = 0.32 anddown to TcA = 12 K, no gap merging is observed. A detailed analysis of the data within the two-band Eliashbergtheory shows that this gap trend can be explained as being mainly due to the band filling andto an increase in the interband scattering which is necessary to account for the increase inΔπ at low Alcontents (x<0.1). We suggest interpreting the following decrease ofΔπ for TcA<30 K as being governed by the onset of the inhomogeneity and disorder in the Al distributionthat partly mask the intrinsic effects of doping and are not taken into account in standardtheoretical approaches.

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.

Point contact Andreev reflection studies on the low [formula omitted] ferromagnet [formula omitted] using a superconducting Nb tip

We report measurement of transport spin polarization ( P t ) using point contact Andreev reflection (PCAR) spectroscopy on NdNi 5, a ferromagnet with Curie temperature of T Curie ∼ 7.7 K , using superconducting Nb tip ( T C ∼ 9.2 K ) . Detail analysis of the spectra shows that the normalized value of P t ( = P t ( T ) / P t ( T = 0 ) ) for contacts with different Z follows closely the temperature variation of spontaneous magnetization, though the value of P t decreases with increasing Z. We also observe a large decrease in superconducting quasiparticle lifetime arising from critical spin fluctuations close to the Curie temperature of the ferromagnet.

Point-contact Andreev reflection spectroscopic study of the superconducting gap structure in LuNi 2B 2C

We employ the point-contact Andreev reflection spectroscopy technique in LuNi 2B 2C ( T c ∼ 16.0 K ) single crystals along three different crystallographic orientations as a function of magnetic field and temperature. Our data taken on the [0 0 1] and [1 1 0] faces show gap values at ∼ 2 K of ∼ 2.4 meV , and ∼ 2.6 meV , respectively, as analyzed by the one-gap Blonder–Tinkham–Klapwijk model. Both the energy gap and the Dynes broadening parameter exhibit similar dependences on magnetic field along the two directions. Data taken as a function of temperature follow the BCS-like temperature dependence yielding 2 Δ 0 / k B T c ∼ 3.4 and ∼ 3.6 , respectively. The measured gap values in [1 0 0] direction vary from 1.6 to 2.5 meV at low temperatures and disappear at the same bulk T c .

Spin polarization of Ni2MnIn and Ni80Fe20 determined by point-contact Andreev spectroscopy

We present point-contact Andreev reflection (PCAR) spectroscopy on Permalloy (Ni80Fe20) and on the half-metallic Heusler alloy Ni2MnIn. The thin Permalloy films are deposited on Si, the Ni2MnIn films on Si as well as on in situ cleaved (110) surfaces of InAs. A highly conductive layer under the ferromagnetic film almost eliminates a series resistance and thus facilitates the determination of the spin polarization from the differential conductance curves. We obtain a spin polarization of Permalloy of 35%. The spin polarization of Ni2MnIn depends on the substrate, presumably due to the growth of different crystal structures. It is shown that the surface sensitive PCAR spectroscopy cannot determine the spin polarization of the bulk material of half-metals where the degree of spin polarization strongly depends on the crystal structure.

Point-contact Andreev spectroscopy on thin [formula omitted] Heusler films

Heusler films with L 2 1 and B2 structure are deposited simultaneously on amorphous carbon films, Si(1 0 0) surfaces, and in situ cleaved InAs(1 1 0) surfaces by coevaporation of Ni and the alloy MnIn. Morphology, structure, and stoichiometry are investigated with transmission-electron microscopy, electron diffraction, and X-ray spectroscopy. The almost perfect lattice match supports highly oriented growth of Ni 2 MnIn on InAs, which is proven by electron diffraction under grazing incidence. The electrical resistivity of thin films on Si show metallic behavior. At temperatures of liquid helium point-contact Andreev reflection spectroscopy is performed on films grown on Si(1 0 0) and in situ cleaved InAs(1 1 0) surfaces yielding spin polarizations comparable to the ones of Fe, Ni, Co, and permalloy ( Ni 80 Fe 20 ) .

Temperature dependence of transport spin polarization inNdNi5from point-contact Andreev reflection

We report a study in which Point contact Andreev reflection (PCAR) spectroscopy using superconducting Nb tip has been carried out on NdNi5, a ferromagnet with a Curie temperature of TC~7.7K. The measurements were carried out over a temperature range of 2-9K which spans across the ferromagnetic transition temperature. From an analysis of the spectra, we show that (i) the temperature dependence of the extracted value of transport spin polarization closely follows the temperature dependence of the spontaneous magnetization; (ii) the superconducting quasiparticle lifetime shows a large decrease close to the Curie temperature of the ferromagnet. We attribute the latter to the presence of strong ferromagnetic spin fluctuations in the ferromagnet close to the ferromagnetic transition temperature.

Electrical resistivity and Andreev reflection spectroscopy of the superconducting oxide spinelLiTi2O4

We measured resistivity and Andreev reflection spectroscopy on $\mathrm{Li}{\mathrm{Ti}}_{2}{\mathrm{O}}_{4}$ polycrystalline samples $({T}_{c}\ensuremath{\approx}12.3\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ synthesized by a new method. Resistivity shows the metallic temperature dependence and is well fitted by the Bloch-Gr\"uneisen theory above $90\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This is consistent with the picture of electron-phonon scattering. Below $90\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and above ${T}_{c}$ it shows ${T}^{2}$ temperature dependence suggesting Fermi liquid behavior above ${T}_{c}$. From the ${H}_{c2}\ensuremath{-}T$ phase diagram the upper critical field ${H}_{c2}(0)$ is estimated to be about $11.6\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ based on the Ginzburg-Landau theory. Point-contact Andreev reflection spectroscopy gives $2{\ensuremath{\Delta}}_{0}∕{k}_{B}{T}_{C}\ensuremath{\approx}4$ and reveals that $\mathrm{Li}{\mathrm{Ti}}_{2}{\mathrm{O}}_{4}$ is a typical intermediate-coupling $s$-wave BCS superconductor. We also observed the resistive hump in a normal state and the field-induced upturn near the transition region of the $\ensuremath{\rho}(T)$ curve on the aged samples. In comparison with the fresh one, it suggests that the above exotic $\ensuremath{\rho}(T)$ behavior is not an intrinsic character of the pure stoichiometric $\mathrm{Li}{\mathrm{Ti}}_{2}{\mathrm{O}}_{4}$ compound.

Vortex overlapping in a BCS type-II superconductor revealed by Andreev reflection spectroscopy

The point-contact Andreev reflection spectroscopy for a simple BCS type-II superconductor was investigated in pure niobium (Nb). The small broadening factor of $\ensuremath{\Gamma}∕\ensuremath{\Delta}(0)&lt;0.09$ was achieved by careful preparation of the junctions. Then we attempted to use the recently proposed two-channel model [Phys. Rev. B 72, 012502 (2005)] to explain the spectra measured in various magnetic fields for such low-$\ensuremath{\Gamma}$ case. It was found that this rigid vortex core model is appropriate below a crossover field ${H}^{*}$, above which it is not adequate due to the onset of substantial vortex overlapping effect.

Comparison of free surface polarization of NiMnSb and Co2MnSi

We present a systematic study of the polarization of the transport current from a variety of NiMnSb and Co2MnSi thin films and bulk material using point contact Andreev reflection spectroscopy. The simple analysis suggests that the free surface polarization of NiMnSb is within error 10% lower than that of Co2MnSi. In either material the measured polarization is rather insensitive to key physical and material properties. We use a two channel model to rule out the influence that stray magnetic field from the ferromagnet might have on the measurements presented.