We report a systematic investigation, together with a theoretical analysis, of the resistivity and Hall effect in single crystals of Ba(Fe1−xCox)2As2, over a wide doping range. We find a surprisingly great disparity between the relaxation rates of the holes and the electrons, in excess of an order of magnitude in the low-doping, lowtemperature regime. The ratio of the electron to hole mobilities diminishes with temperature and doping (away from the magnetically ordered state) and becomes more conventional. We also find a straightforward explanation of the large asymmetry (compared to cuprates) of the superconducting dome: in the underdoped regime the decisive factor is the competition between AF and superconductivity (SC), while in the overdoped regime the main role is played by degradation of the nesting that weakens the pairing interaction. Our results indicate that spin-fluctuations due to interband electron-hole scattering play a crucial role not only in the superconducting pairing, but also in the normal transport.PACS numbers: 74.20. Rp, 74.25.Ha, 74.70.Dd The discovery in the last year of new iron-based superconductors [1] provided a tempting analogy with high-T c cuprates. Indeed, a simple comparison between phase diagrams reveals, particularly clearly for the BaFe 2 As 2 family[2, 3, 4], a couple interesting similarities with the cuprates: first and foremost, the parent compound is an antiferromagnet (AFM), and spin fluctuations appear important for carrier pairing. Second, the superconductivity (SC) appears with either hole or electron doping, at a finite doping level, and forms a dome-shaped region in the phase diagram, as in cuprates.A closer look, however, reveals equally striking differences: Indeed, unlike the cuprates, the parent compoundis in pnictides are metals that support quantum oscillations [5,6], and the Coulomb correlations appear to be weak [7]. Second, unlike cuprates, superconductivity can be induced without doping, by external or chemical pressure [8]. Finally, the superconducting dome is very asymmetric [9,10]. And, probably most importantly, electronic structure in cuprates is formed essentially by one band, while in the pnictiodes multiband effects are of primary importance.The doping dependence of the evolution of the multiband electronic structure and its relationship to AFM, spin fluctuations, and SC is the key to the physics of the high-T c ferropnictides. Systematic Hall coefficient and resistivity measurements are clearly well-suited to provide useful insight into these issues. In this Letter we select BaFe 2 As 2 for a systematic study of the Hall effect and resistivity. Through quantitative analysis of the experimental data, combined with theoretical calculations, we establish a unified view of the doping induced evolution of SC and AFM, as well as the ramifications for the pairing mechanism. The crystals were grown by self-flux method using FeAs as the flux; the details are described elswhere [11,12]. The main advantage of the 122 system [2, 3, 4] is that it allows fab...

Magnetization and its relaxation have been measured in Ba(Fe1−xCox)2As2 single crystals at various doping levels ranging from very underdoped to very overdoped regimes. Sizable magnetization relaxation rate has been observed in all samples, indicating a moderate vortex motion and relatively small characteristic pinning energy. Detailed analysis leads to the following conclusions: (1) A prominent second-peak (SP) effect was observed in the samples around the optimal doping level (x ≈ 0.08), but it becomes invisible or very weak in the very underdoped and overdoped samples;(2) The magnetization relaxation rate is inversely related to the transient superconducting current density revealing the non-monotonic field and temperature dependence through the SP region; (3) A very sharp magnetization peak was observed near zero field which corresponds to a much reduced relaxation rate; (4) A weak temperature dependence of relaxation rate or a plateau was found in the intermediate temperature region. Together with the treatment of the Generalized-Inversion-Scheme, we suggest that the vortex dynamics is describable by the collective pinning model. Finally, vortex phase diagrams were drawn for all the samples showing a systematic evolution of vortex dynamics.

We report the specific heat (SH) measurements on single crystals of hole doped FeAs-based superconductor Ba 0.6 K 0.4 Fe 2 As 2 . It is found that the electronic SH coefficient γ e (T ) is not temperature dependent and increases almost linearly with the magnetic field in low temperature region. These point to a fully gapped superconducting state. Surprisingly the sharp SH anomaly ∆C/T | Tc reaches a value of 98 mJ/molK 2 suggesting a very high normal state quasiparticle density of states (γ n ≈ 63mJ/molK 2 ). A detailed analysis reveals that the γ e (T ) cannot be fitted with a single gap of s-wave symmetry due to the presence of a hump in the middle temperature region. However, our data indicate that the dominant part of the superconducting condensate is induced by an s-wave gap with the magnitude of about 6 meV. PACS numbers: 74.20.Rp, 74.25.Bt, 65.40.Ba, 74.70.Dd The discovery of high temperature superconductivity in the FeAs-based system has stimulated enormous interests in the field of condensed matter physics and material sciences [1]. The superconductivity has not only been discovered in the electron doped samples, but also in the hole-doped ones [2,3]. The central issues concerning the superconductivity mechanism are about the symmetry and the magnitude of the superconducting gap. The experimental results obtained so far are, however, highly controversial. The low temperature specific heat (SH) measurements in the F-doped LaFeAsO samples revealed a nonlinear magnetic field dependence of the SH coefficient γ e , which was attributed to the presence of a nodal gap [4]. This was later supported by many other measurements based on µSR [5,6,7], NMR[8, 9, 10], magnetic penetration [11] and point contact Andreev spectrum (PCAS) [12]. On the other hand, the PCAS on the Fdoped SmFeAsO indicated a feature of s-wave gap [13], some measurements [14,15,16,17] also gave support to this conclusion. It is important to note that most of the conclusions drawn for a nodal gap were obtained on the electron doped LnFeAsO samples (abbreviated as FeAs-1111, Ln stands for the rare earth elements) which are characterized by a low charge carrier density and thus low superfluid density [18]. For the FeAs-1111 phase, it is very difficult to grow crystals with large sizes, therefore most of the measurements on the pairing symmetry so far were made on polycrystalline samples. This is much improved in the (Ba, S r) 1−x K x Fe 2 As 2 (denoted as FeAs-122) system since sizable crystals can be achieved [19,20]. Preliminary data by angle resolved photoemission spectroscopy (ARPES) on these crystals show two groups of superconducting gaps (∆ 1 ≈ 12 meV, ∆ 2 ≈ 6 meV) all with s-wave symmetry [21,22,23]. It is known that the surface of this type of single crystals decay or reconstruct very quickly, this may give obstacles to get repeatable data when using the surface sensitive tools. Thus solid conclusions about the gap symmetry and magnitude from bulk measurements are highly desired.Specific heat (SH) is one of the powerful tools to meas...

Stripe order related in-plane fourfold symmetric superconductivity in La1.45Nd0.4Sr0.15CuO4 single crystal

No abstract

Abstract:A robust zero-energy bound state (ZBS) in a superconductor, such as a Majorana or Andreev bound state, is often a consequence of non-trivial topological or symmetry related properties, and can provide indispensable information about the superconducting state. Here we use scanning tunneling microscopy/spectroscopy to demonstrate, on the atomic scale, that an isotropic ZBS emerges at the randomly distributed interstitial excess Fe sites in the superconducting Fe(Te,Se). This ZBS is localized with a short decay length of ~ 10 Å, and surprisingly robust against a magnetic field up to 8 Tesla, as well as perturbations by neighboring impurities. We find no natural explanation for the observation of such a robust zero-energy bound state, indicating a novel mechanism of impurities or an exotic pairing symmetry of the iron-based superconductivity.Main Text: Superconductivity arises from the macroscopic quantum condensation of electron pairs. The symmetry of the wave-function of these pairs is one of the most essential aspects of the microscopic pairing mechanism. Since the impurity-induced local density of states (DOS) is sensitive to the pairing symmetry, it can be used to test the symmetry of the order parameter and to probe the microscopic pairing mechanism. Being a local probe with atomic resolution, scanning tunneling microscopy/spectroscopy (STM/S) (1) has played a key role in this respect, especially in the study of high-TC cuprate superconductors (2,3).Since its discovery, new compounds of iron-based superconductor (IBSC) continue to be found. However, the pairing symmetry remains a central unresolved issue. So far,

For a type-II superconductor, when the applied magnetic field is higher than the lower critical value H c1 , the magnetic flux will penetrate into the superconductor and form quantized vortices, which usually are arranged in an Abrikosov lattice. For the newly discovered iron pnictide superconductors, previous measurements have shown that, in electron-doped BaFe 2 As 2 , the vortices form a highly disordered structure 1-3 . In addition, the density of states (DOS) within the vortex cores 1 do not exhibit the Andreev bound states in conventional superconductors 4 -8 . In this Letter, we report the observation of a triangular vortex lattice and the Andreev bound states in hole-doped BaFe 2 As 2 by using a low temperature scanning tunneling microscope (STM). Detailed study of the vortex cores reveals that the spectrum of the Andreev bound states inside the vortex core exhibits a distinct spatial evolution: at the center of the vortex core, it appears as a single peak at 0.5 mV below the Fermi-energy; away from the core center, it gradually evolves into two sub-peaks and they eventually fade out. The drastic differences between the vortex cores of the electron-doped and hole-doped counterparts are illusive to the pairing mechanism of the iron pnictide superconductors.Magnetic flux quantization is one of the important quantum phenomena in the mixed

Abstract. -We present point-contact spectroscopy data for junctions between a normal metal and the newly discovered F-doped superconductor LaO0.9F 0.1−δ FeAs (F-LaOFeAs). A zero-bias conductance peak was observed and its shape and magnitude suggests the presence of Andreev bound states at the surface of F-LaOFeAs, which provides a possible evidence of an unconventional pairing symmetry with a nodal gap function. The maximum gap value ∆0 ≈ 3.9 ± 0.7meV was determined from the measured spectra, in good agreement with the recent experiments on specific heat and lower critical field.