We report the first NMR investigation of spin dynamics in the overdoped nonsuperconducting regime of Ba(Fe1-xCox)2As2 up to x=0.26. We demonstrate that the absence of interband transitions with large momentum transfer Q{AF} approximately (pi/a,0) between the hole and electron Fermi surfaces results in complete suppression of antiferromagnetic spin fluctuations for x greater than or approximately 0.15. Our experimental results provide direct evidence for a correlation between T{c} and the strength of Q{AF} antiferromagnetic spin fluctuations.

The superconductor Sr4V2O6Fe2As2 with transition temperature at 37.2 K has been fabricated. It has a layered structure with the space group of p4/nmm, and with the lattice constants a = 3.9296Å and c = 15.6732Å. The observed large diamagnetization signal and zero-resistance demonstrated the bulk superconductivity. The broadening of resistive transition was measured under different magnetic fields leading to the discovery of a rather high upper critical field. The results also suggest a large vortex liquid region which reflects high anisotropy of the system. The Hall effect measurements revealed dominantly electron-like charge carriers in this material. The superconductivity in the present system may be induced by oxygen deficiency or the multiple valence states of vanadium.Since the discovery of superconductivity 1 at 26 K in oxy-arsenide LaF eAsO 1−x F x , tremendous attention has been paid to searching new superconductors in this family. Among the superconductors with several different structures, 2,3,4,5,6 the highest T c has been raised to 55-56 K 7,8,9,10,11 in doped oxy-iron-arsenides (F-doped LnFeAsO, the so-called 1111 phase, Ln=rare earth elements) or the fluoride derivative iron-arsenides (Lndoped AEFeAsF, AE=alkaline earth elements).12 The superconductivity can also be induced by applying a high pressure to the undoped parent samples.13,14 Although it remains unclear what governs the mechanism of superconductivity in the FeAs-based system, it turns out to be clear that the parent phase is accompanied by an antiferromagnetic (AF) order and the superconductivity can be induced by suppressing this magnetic order. A typical example was illustrated in the (Ba, Sr)F e 2 As 2 (so-called 122) system, the AF order is suppressed and superconductivity was induced by either doping K to the Ba or Sr sites, 2,15,16 or doping Co to the Fe sites. 17,18 On the other hand, superconductivity was also found in the parent phase of FeP-based system, such as LaFePO (T c = 2.75K)19 , or in LiFeAs. 3,4 Very recently superconductivity at about 17 K was found in another FeP based parent compound Sr 4 Sc 2 O 6 Fe 2 P 2 (so-called 42622).20 Due to the absence of the AF order in the superconductors mentioned above, one naturally questions whether the AF order is a prerequisite for the superconductivity in the iron-pnictide system. As far as we know, no superconductivity was detected in the parent phase of some FeAs-based compounds, including the 1111, 122 and the recently discovered 42622 and 32522 phases. 21,22,23,24,25 Although some trace of superconductivity was reported in the doped FeAs-based 42622 or 32522 compounds, the high-T c superconductivity was not supported by a clear large diamagnetization signal. 22,23 In this Letter, we report the discovery of superconductivity at about 37.2 K in the new compound Sr 4 V 2 O 6 Fe 2 As 2 . This work presents the unambiguous evidence for high temperature superconductivity in the FeAs-based 42622 system. The polycrystalline samples were synthesized by using a two-step solid state re...

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...

The topological materials have attracted much attention for their unique electronic structure and peculiar physical properties. ZrTe5 has host a long-standing puzzle on its anomalous transport properties manifested by its unusual resistivity peak and the reversal of the charge carrier type. It is also predicted that single-layer ZrTe5 is a two-dimensional topological insulator and there is possibly a topological phase transition in bulk ZrTe5. Here we report high-resolution laser-based angle-resolved photoemission measurements on the electronic structure and its detailed temperature evolution of ZrTe5. Our results provide direct electronic evidence on the temperature-induced Lifshitz transition, which gives a natural understanding on underlying origin of the resistivity anomaly in ZrTe5. In addition, we observe one-dimensional-like electronic features from the edges of the cracked ZrTe5 samples. Our observations indicate that ZrTe5 is a weak topological insulator and it exhibits a tendency to become a strong topological insulator when the layer distance is reduced.

Broadband, efficient and fast conversion of light to electricity is crucial for sensing and clean energy. Here we reveal the largest observed bulk photo-voltaic effect (BPVE), an intrinsic mechanism predicted to be ultrafast and exceed the Shockley-Quiesser limit. This discovery results from combining recent developments in the connection of BPVE to topology, Weyl semimetals and focused-ion beam fabrication. Our room temperature observation of the first BPVE in the mid-IR, is enabled by microscopic devices of the Weyl semimetal TaAs. Detailed symmetry analysis enables unambiguous separation of this response from competing photothermal effects. The size and wavelength range of the shift current offers new opportunities in optical detectors, clean energy, and topology, while directly demonstrating the utility of Weyl semimetals for applications.Converting light to electricity is crucial for clean energy, imaging, communications, chemical 1 arXiv:1712.04951v2 [cond-mat.mes-hall]

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.

The mechanism of high-temperature superconductivity in the iron-based superconductors remains an outstanding issue in condensed matter physics. The electronic structure plays an essential role in dictating superconductivity. Recent revelation of distinct electronic structure and high-temperature superconductivity in the single-layer FeSe/SrTiO3 films provides key information on the role of Fermi surface topology and interface in inducing or enhancing superconductivity. Here we report high-resolution angle-resolved photoemission measurements on the electronic structure and superconducting gap of an FeSe-based superconductor, (Li0.84Fe0.16)OHFe0.98Se, with a Tc at 41 K. We find that this single-phase bulk superconductor shows remarkably similar electronic behaviours to that of the superconducting single-layer FeSe/SrTiO3 films in terms of Fermi surface topology, band structure and the gap symmetry. These observations provide new insights in understanding high-temperature superconductivity in the single-layer FeSe/SrTiO3 films and the mechanism of superconductivity in the bulk iron-based superconductors.

Unconventional superconductivity usually originates from several strongly coupled degrees of freedom, such as magnetic, charge and elastic. A highly anisotropic electronic phase, not driven by lattice degrees of freedom, has been proposed in some of these superconductors, from cuprates to iron-based compounds. In the iron pnictide BaFe 2 As 2 , this nematic phase arises in the paramagnetic phase and is present for wide doping and temperature ranges. Here we probe the in-plane electronic anisotropy of electron-and hole-doped BaFe 2 As 2 compounds. Unlike other materials, the resistivity anisotropy behaves very differently for electron-and hole-type dopants and even changes sign on the hole-doped side. This behaviour is explained by Fermi surface reconstruction in the magnetic phase and spin-fluctuation scattering in the paramagnetic phase. This unique transport anisotropy unveils the primary role played by magnetic scattering, demonstrating the close connection between magnetism, nematicity and unconventional superconductivity.