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Register a new userThe superconducting gaps in FeSe studied by soft point-contact Andreev reflection spectroscopy
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FeSe single crystals have been studied by soft point-contact Andreev-reflection spectroscopy. Superconducting gap features in the differential resistance dV/dI(V) of point contacts such as a characteristic Andreev-reflection double-minimum structure have been measured versus temperature and magnetic field. Analyzing dV/dI within the extended two-gap Blonder-Tinkham-Klapwijk model allows to extract both the temperature and magnetic field dependence of the superconducting gaps. The temperature dependence of both gaps is close to the standard BCS behavior. Remarkably, the magnitude of the double-minimum structure gradually vanishes in magnetic field, while the minima position only slightly shifts with field indicating a weak decrease of the superconducting gaps. Analyzing the dV/dI(V) spectra for 25 point contacts results in the averaged gap values <Delta_L> = 1.8+/-0.4meV and <Delta_S>=1.0+/-0.2 meV and reduced values 2<Delta_L>/kTc=4.2+/-0.9 and 2<Delta_S>/kTc=2.3+/-0.5 for the large (L) and small (S) gap, respectively. Additionally, the small gap contribution was found to be within tens of percent decreasing with both temperature and magnetic field. No signatures in the dV/dI spectra were observed testifying a gapless superconductivity or presence of even smaller gaps.
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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.
The newly discovered oxypnictide family of superconductors show very high critical temperatures of up to 55K. Whilst there is growing evidence that suggests a nodal order parameter, point contact Andreev reflection spectroscopy can provide crucial information such as the gap value and possibly the number of energy gaps involved. For the oxygen deficient NdFeAsO0.85 with a Tc of 45.5K, we show that there is clearly a gap value at 4.2K that is of the order of 7meV, consistent with previous studies on oxypnictides with lower Tc. Additionally, taking the spectra as a function of gold tip contact pressure reveals important changes in the spectra which may be indicative of more complex physics underlying this structure.
The superconducting energy gap and quasiparticle density of states (DOS) is measured by means of Andreev-reflection spectroscopy in normal metal - superconductor point contacts. It is found that both in paramagnetic and in antiferromagnetically ordered states (Tm- and Dy-compounds) the DOS has the BCS-like form while in Er- the dip in the temperature dependence of the gap is observed at the Neel temperature. In Ho-compound, the non-BCS-like form of DOS is observed in the temperature region where the incommensurate spin density waves exist along the axes a and c . Inelastic point-contact spectroscopy performed on these compounds shows that the low-frequency phonons are important for the superconducting state which are strongly mixed with the magnetic excitation branches in magnetic superconductors.
Using intrinsic multiple Andreev reflections effect (IMARE) spectroscopy, we studied SnS contacts in the layered oxypnictide superconductors Sm$_{1-x}$Th$_x$OFeAs with various thorium doping and critical temperatures $T_C = 21-54$ K. We observe a scaling between both superconducting gaps and $T_C$. The determined BCS-ratio for the large gap $2Delta_L/k_BT_C = 5.0-5.7$ and its eigen BCS-ratio (in a hypothetical case of zero interband coupling) $2Delta_L/k_BT_C^L = 4.1-4.6$ both exceeding the weak-coupling limit 3.52, and for the small gap $2Delta_S/k_BT_C = 1.2-1.6$ remain nearly constant within all the $T_C$ range studied. The temperature dependences $Delta_{L,S}(T)$ agree well with a two-band BCS-like Moskalenko and Suhl model. We prove intraband coupling to be stronger than interband coupling, whereas and Coulomb repulsion constants $mu^{ast}$ are finite in Sm-based oxypnictides.
Our previous point-contact Andreev reflection studies of the heavy-fermion superconductor CeCoIn$_5$ using Au tips have shown two clear features: reduced Andreev signal and asymmetric background conductance [1]. To explore their physical origins, we have extended our measurements to point-contact junctions between single crystalline heavy-fermion metals and superconducting Nb tips. Differential conductance spectra are taken on junctions with three heavy-fermion metals, CeCoIn$_5$, CeRhIn$_5$, and YbAl$_3$, each with different electron mass. In contrast with Au/CeCoIn$_5$ junctions, Andreev signal is not reduced and no dependence on effective mass is observed. A possible explanation based on a two-fluid picture for heavy fermions is proposed. [1] W. K. Park et al., Phys. Rev. B 72 052509 (2005); W. K. Park et al., Proc. SPIE-Int. Soc. Opt. Eng. 5932 59321Q (2005); W. K. Park et al., Physica C (in press) (cond-mat/0606535).
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