Showing papers in "Physica C-superconductivity and Its Applications in 2009"

Pairing symmetry and pairing state in ferropnictides: theoretical overview

Abstract: We review the main ingredients for an unconventional pairing state in the ferropnictides, with particular emphasis on interband pairing due to magnetic fluctuations. Summarizing the key experimental prerequisites for such pairing, the electronic structure and nature of magnetic excitations, we discuss the properties of the s± state that emerges as a likely candidate pairing state for these materials and survey experimental evidence in favor of and against this novel state of matter.

The development of the superconducting PbO-type β-FeSe and related compounds

Abstract: An overview of the recent development of the superconducting FeSe1x and related compounds is presented. Methods to synthesize high purity polycrystalline samples, single crystals and thin films with preferred orientation are described. In addition to the synthesis of FeSe and FeSeTe, the effects of various partial chemical substitutions on Fe and Se/Te site are described. It was found that the effects of chemical doping to the Se-site or Fe-site are rather different. Ionic size of the dopant is found to play a critical role on the occurrence of superconductivity. We also review the physical properties, including transport, magnetic, and thermal properties. There exist interesting transport anomalies in the resistivity at low temperature, and it was found that a structural distortion at low temperature is critical to the occurrence of superconductivity in these materials. However, the exact origin of these observed anomalies is not clear, and the exact pairing symmetry in FeSe-based superconductors is also still in question. 2009 Published by Elsevier B.V.

Neutron studies of the iron-based family of high TC magnetic superconductors

Abstract: We review neutron scattering investigations of the crystal structures, magnetic structures, and spin dynamics of the iron-based RFe(As, P)(O, F) (R = La, Ce. Pr, Nd), (Ba,Sr,Ca)Fe2As2, and Fe1+x(Te-Se) systems. On cooling from room temperature all the undoped materials exhibit universal behavior, where a tetragonal-to-orthorhombic/monoclinic structural transition Occurs, below which the systems become antiferromagnets. For the first two classes of materials the magnetic structure within the a-b plane consists of chains of parallel Fe spins that are coupled antiferromagnetically in the orthogonal direction, with an ordered moment typically less than one Bohr magneton. Hence these are itinerant electron magnets, with a spin structure that is consistent with Fermi-surface nesting and a very energetic spin wave bandwidth similar to 0.2 eV. With doping, the structural and magnetic transitions are suppressed in favor of superconductivity, with Superconducting transition temperatures up to approximate to 55 K. Magnetic correlations are observed in the Superconducting regime, With a Magnetic resonance that follows the Superconducting order parameter just like the cuprates. The rare earth moments order antiferromagnetically at low T like 'conventional' Magnetic Superconductors, while the Cc crystal field linewidths are affected when superconductivity sets in. The application of pressure in CaFe2As2 transforms the system from a magnetically ordered orthorhombic material to a 'collapsed' non-magnetic tetragonal system. Tetragonal Fe1+xTe transforms to a low T monoclinic structure at small x that changes to orthorhombic at larger x, which is accompanied by a crossover from commensurate to incommensurate magnetic order. Se doping Suppresses the magnetic order, while incommensurate magnetic correlations are observed in the superconducting regime. (C) 2009 Elsevier B.V. All rights reserved.

A new Y-based HTSC with Tc above 100 K

Abstract: In search of finding the dominant mechanism in high temperature superconductivity phenomena, the Y3Ba5Cu8O18 compound was synthesized through the standard solid-state reaction technique. Characteristic XRD experiment was performed on the samples and was analyzed by the MAUD software refinement program. The analysis results indicate a 358 phase structure with the initial nominal stoichiometry. The electrical resistivity and its behavior under different magnetic field were measured. The electrical resistivity indicates the transition temperature T c onset = 102 K with transition width ΔTc = 2.4 K. This is the first observation of such a high transition temperature in the Y-based compound. Application of magnetic field leads the resistivity curve to spread out below transition region, and Tc (ρ = 0) shifts to lower temperatures. Also, a small broadening is observed by the application of high magnetic field in the T c onset region.

Electronic structure of Fe-based superconductors

Abstract: The electronic structure of the Fe-based superconductors is discussed, mainly from the point of view of first principles calculations in relation to experimental data. Comparisons and contrasts with cuprates are made. The problem of reconciling experiments indicating an s symmetry gap with experiments indicating line nodes is discussed and a possible resolution is given.

Structural, magnetic and superconducting phase transitions in CaFe2As2 under ambient and applied pressure

Abstract: At ambient pressure CaFe 2 As 2 has been found to undergo a first order phase transition from a high temperature, tetragonal phase to a low-temperature orthorhombic/antiferromagnetic phase upon cooling through T ∼ 170 K. With the application of pressure this phase transition is rapidly suppressed and by ∼0.35 GPa it is replaced by a first order phase transition to a low-temperature collapsed tetragonal, non-magnetic phase. Further application of pressure leads to an increase of the tetragonal to collapsed tetragonal phase transition temperature, with it crossing room temperature by ∼1.7 GPa. Given the exceptionally large and anisotropic change in unit cell dimensions associated with the collapsed tetragonal phase, the state of the pressure medium (liquid or solid) at the transition temperature has profound effects on the low-temperature state of the sample. For He-gas cells the pressure is as close to hydrostatic as possible and the transitions are sharp and the sample appears to be single phase at low temperatures. For liquid media cells at temperatures below media freezing, the CaFe 2 As 2 transforms when it is encased by a frozen media and enters into a low-temperature multi-crystallographic-phase state, leading to what appears to be a strain stabilized superconducting state at low temperatures.

The synthesis and characterization of LiFeAs and NaFeAs

Abstract: The newest homologous series of superconducting Fe-pnictides, LiFeAs (Li111) and NaFeAs (Na111) have been synthesized and investigated. Both crystallize with the layered tetragonal anti-PbFCl-type structure in P4 / nmm space group. Polycrystalline samples and single crystals of Li111 and Na111 display superconducting transitions at ∼18 K and 12–25 K, respectively. No magnetic order has been found in either compound, although a weak magnetic background is clearly in evidence. The origin of the carriers and the stoichiometric compositions of Li111 and Na111 were explored.

Single crystals of LnFeAsO1-xFx (Ln= La, Pr, Nd, Sm, Gd) and Ba1-xRbxFe2As2: growth, structure and superconducting properties

Abstract: 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 2 As 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 2 As 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 2 As 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 2 As 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).

High pressure studies on Fe-pnictide superconductors

Abstract: A review of high pressure studies on Fe-pnictide superconductors is given. The pressure effects on the magnetic and superconducting transitions are discussed for different classes of doped and undoped FeAs-compounds: ROFeAs (R = rare-earth), AeFe2As2 (Ae = Ca, Sr, Ba), and AFeAs (A = Li, Na). Pressure tends to decrease the magnetic transition temperature in the undoped or only slightly doped compounds. The superconducting Tc increases with low pressure for underdoped FeAs-pnictides, remains approximately constant for optimal doping, and decreases linearly in the overdoped range. The undoped LaOFeAs and AeFe2As2 become superconducting under pressure although non-hydrostatic pressure condition seems to play a role in CaFe2As2. The superconductivity in the (undoped) AFeAs is explained as a chemical pressure effect due to the volume contraction caused by the small ionic size of the A-elements. The binary FeSe shows the largest pressure coefficient of Tc in the Se-deficient superconducting phase.

Frustrated magnetic interactions, giant magneto–elastic coupling, and magnetic phonons in iron–pnictides

Abstract: We present a detailed first-principles study of Fe-pnictides with particular emphasis on competing magnetic interactions, structural phase transition, giant magneto–elastic coupling and its effect on phonons. The exchange interactions J i , j ( R ) are calculated up to ≈12 A from two different approaches based on direct spin-flip and infinitesimal spin-rotation. We find that J i , j ( R ) has an oscillatory character with an envelop decaying as 1/ R 3 along the stripe-direction while it is very short range along the diagonal direction and antiferromagnetic. A brief discussion of the neutron scattering determination of these exchange constants from a single crystal sample with orthorhombic-twinning is given. The lattice parameter dependence of the exchange constants, dJ i , j / da are calculated for a simple spin-Peierls like model to explain the fine details of the tetragonal–orthorhombic phase transition. We then discuss giant magneto–elastic effects in these systems. We show that when the Fe-spin is turned off the optimized c -values are shorter than experimental values by 1.4 A for CaFe 2 As 2 , by 0.4 A for BaFe 2 As 2 , and by 0.13 A for LaOFeAs. We explain this strange behavior by unraveling surprisingly strong interactions between arsenic ions, the strength of which is controlled by the Fe-spin state through Fe–As hybridization. Reducing the Fe-magnetic moment, weakens the Fe–As bonding, and in turn, increases As–As interactions, causing a giant reduction in the c -axis. These findings also explain why the Fe-moment is so tightly coupled to the As-z position. Finally, we show that Fe-spin is also required to obtain the right phonon energies, in particular As c-polarized and Fe–Fe in-plane modes that have been recently observed by inelastic X-ray and neutron scattering but cannot be explained based on non-magnetic phonon calculations. Since treating iron as magnetic ion always gives much better results than non-magnetic ones and since there is no large c -axis reduction during the normal to superconducting phase transition, the iron magnetic moment should be present in Fe-pnictides at all times. We discuss the implications of our results on the mechanism of superconductivity in these fascinating Fe-pnictide systems.

Japanese efforts on coated conductor processing and its power applications: New 5 year project for materials and power applications of coated conductors (M-PACC)

Abstract: Five years of Japanese national project (FY2003–FY2007) was ended last spring with remarkable success. The national project was originally aimed for development of coated conductors which have high superconductivity performance and long length enough to fabricate high temperature superconducting (HTS) electric power devices. Preliminary research and development of HTS electric power devices were carried out as well. A series of R&D results will be summarized and reviewed in this paper. The new 5 years Japanese national project has started last June (FY2008–FY2012) to develop HTS electric power applications including SMES, cables and transformers and to develop/produce coated conductors satisfying the requirements from the respective power devices. Collaborative R&D efforts by national laboratories, universities and private companies have been started with the supports of NEDO and METI. The accomplishment of the previous project will be summarized and the plans and goals of the new project will be presented in this paper.

Superconductivity of powder-in-tube Sr0.6K0.4Fe2As2 wires

Abstract: Nb-sheathed Sr0.6K0.4Fe2As2 superconducting wires have been fabricated using the powder-in-tube (PIT) method for the first time and the superconducting properties of the wires have been investigated. The transition temperature (Tc) of the Sr0.6K0.4Fe2As2 wires is confirmed to be as high as 35.3 K. Most importantly, Sr0.6K0.4Fe2As2 wires exhibit a very weak Jc-field dependence behavior even the temperature is very close to Tc. The upper critical field Hc2(0) value can exceed 140 T, surpassing those of MgB2 and all the low temperature superconductors. Such high Hc2 and superior Jc-field performance make the 122 phase SrKFeAs wire conductors a powerful competitor potentially useful in very high field applications.

Towards beyond 1 GHz NMR: Mechanism of the long-term drift of screening current-induced magnetic field in a Bi-2223 coil

Abstract: The screening current-induced magnetic field in the (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O x (Bi-2223) insert coil proposed for a beyond 1 GHz nuclear magnetic resonance (NMR) spectrometer may generate a long-term field drift, resulting in a loss of field-frequency lock operation and an inability to make high resolution NMR measurements. The measured screening current-induced magnetic field of a Bi-2223 double-pancake coil exhibits a hysteresis effect at 4.2 K that is reproduced by a numerical simulation based on a finite thickness rectangular superconductor bar model. The screening current-induced field at the coil center is of opposite polarity to that generated by the coil current, and thus the apparent field intensity shows a positive drift with time. On the contrary, the field at a coil end is of the same polarity as the coil field, and the apparent field intensity decreases with time. If we wait for ∼1000 h after coil excitation, the field drift rate approaches the field decay rate of the persistent current of 10 −8 h −1 , suitable for a long-term NMR measurement in a beyond 1 GHz NMR spectrometer.

Effect of current sweep reversal on the magnetic field stability for a Bi-2223 superconducting solenoid

Abstract: The effect of current sweep reversal on the temporal drift in magnetic field intensity for a Bi-2223 solenoid was investigated by experiment and using numerical simulation. Current sweep reversal, by as small as 1% of the peak current, was found to stabilize the drift in magnetic field intensity for a Bi-2223 tape solenoid. The field drift was due to flux creep in the Bi-2223 tape and the current sweep reversal formed a barrier for flux entrance at the upper and lower surface of the conductor, preventing flux creep. With a current reversal of several% of the peak current, the barrier formation extended over half of the solenoid and the magnetic field intensity became constant with time. The current sweep reversal technique should prove useful to stabilize an ultra-high field low/high-temperature superconducting nuclear magnetic resonance magnet operated at frequencies (field intensities) beyond 1 GHz (23.5 T).

Fabrication and characterization of iron pnictide wires and bulk materials through the powder-in-tube method

Abstract: The recent discovery of superconductivity in the iron-based superconductors with very high upper critical fields presents a new possibility for practical applications, but fabricating fine-wire is a challenge because of mechanically hard and brittle powders and the toxicity and volatility of arsenic. In this paper, we report the synthesis and the physical characterization of iron pnictide wires and bulks prepared by the powder-in-tube method (PIT). A new class of high- T c iron pnictide composite wires, such as LaFeAsO 1− x F x , SmFeAsO 1− x F x and Sr 1− x K x FeAs, has been fabricated by the in situ PIT technique using Fe, Ta and Nb tubes. Microscopy and X-ray analysis show that the superconducting core is continuous, and retains phase composition after wire drawing and heat treatment. Furthermore, the wires exhibit a very weak J c -field dependence behavior even at high temperatures. The upper critical field H c2 (0) value can exceed 100 T, surpassing those of MgB 2 and all the low temperature superconductors and indicating a strong potential for applications requiring very high field. These results demonstrate the feasibility of producing superconducting pnictide composite wire. We also applied the one-step PIT method to synthesize the iron-based bulks, due to its convenience and safety. In fact, by using this technique, we have successfully discovered superconductivity at 35 K and 15 K in Eu 0.7 Na 0.3 Fe 2 As 2 and SmCoFeAsO compounds, respectively. These clearly suggest that the one-step PIT technique is unique and versatile and hence can be tailored easily for other rare earth derivatives of novel iron-based superconductors.

Increase of critical current density with doping carbon nano-tubes in YBa2Cu3O7−δ

Abstract: The effects of carbon nano-tubes (CNTs) on the crystal structure and superconducting properties of YBa 2 Cu 3 O 7− δ (Y-123) compound were studied. Samples were synthesized using standard solid-state reaction technique by adding CNT up to 1 wt% and X-ray diffraction data confirm the single phase orthorhombic structure for all the samples. Current–voltage measurements in magnetic fields up to 9 T were used to study the pinning energy U J and critical current density J c as a function of magnetic field at fixed temperature. We find that while T c does not change much with the CNT doping (91–92 K), both U J and J c increase systematically up to 0.7 wt% CNT doping in a broad magnetic field ranges between 0.1 and 9 T and J c in the 0.7 wt% CNT doped sample is at least 10 times larger than that of the pure Y-123. The scanning electron microscope image shows that CNTs are forming an electrical-network between grains. These observations suggest that the CNT addition to the Y-123-compounds improve the electrical connection between superconducting grains to result in the J c increase.

FeTe as a candidate material for new iron-based superconductor

Abstract: Tetragonal FeSe is a superconductor with a transition temperature Tc of 8 K and shows a huge enhancement of Tc with applying pressure. Tetragonal FeTe has a structure very analogous to superconducting FeSe, but does not show superconducting transition. We investigated the pressure effect of resistivity on FeTe. The resistivity at room temperature decreased with increasing pressure. An anomaly in resistivity around 80 K shifted towards a lower temperature with increasing pressure.

New Nb multi-layer fabrication process for large-scale SFQ circuits

Abstract: We investigated the most suitable device structure for large-scale SFQ circuits and propose a new Nb 10-layer device structure that is composed of active layers including junctions at the top, PTL layers in the middle and DC power layers at the bottom. This device structure enables us to reduce the influence of the magnetic field due to large bias currents and to form a Nb/AlOx/Nb junction layer in the last part of the fabrication sequence. To achieve this structure, we developed a higher quality planarization that could remove the residual slight roughness after standard caldera planarization. We fabricated a diagnostic chip that is composed of test elements such as junctions, contacts, resistors and many kinds of process test patterns. We obtained sufficient characteristics for the diagnostic chips. Moreover, to evaluate the fabrication process, we designed and fabricated several shift registers. We confirmed the correct operation of an up to 2560-bit shift register having 10,281 junctions.

Development of REBCO superconducting power transformers in Japan

Abstract: In Japan we started a national project to develop a 66/6.9 kV–20 MVA transformer with REBCO superconducting tapes in 2006. This paper gives an overview of progress of the development of superconducting transformers in Japan and also describes the fundamental technologies studied before now to realize a 66/6.9 kV–20 MVA transformer as follows. To reduce the ac loss in REBCO superconducting thin tapes, authors proposed a new method different from the conventional technique of reducing the ac loss in superconducting multifilamentary wires. It consists of scribing process into a multifilamentary structure by laser or chemical etching, and a special winding process. Making a multilayered solenoidal coil with laser-scribed REBCO tapes, we verified the ac loss reduction in proportion to a filament width even in coil configuration. In addition, to realize a current capacity more than the rated secondary current of 2.4 kA, we first investigated the workability of REBCO tapes in the actual winding process with forming a transposed parallel conductor, where REBCO tapes were bent edgewise at transposing points. Making a test coil of a 24-strand parallel conductor, we verified no degradation of the critical current and nearly uniform current distribution among the tapes. The result suggests the applicability of the method of enhancing the current capacity by forming a parallel conductor with REBCO tapes. Further, to realize the dielectric strength regulated for the Japanese standards, i.e. lightning impulse withstand level of 350 kV and excess ac voltage of 140 kV, we made test coils and carried out dielectric breakdown tests. As a result, we got hold of the required insulation distance at the important points from the viewpoint of insulation design.

Generic phase diagram of “electron-doped” T′ cuprates

Abstract: We investigated the generic phase diagram of the electron doped superconductor, Nd 2− x Ce x CuO 4 , using films prepared by metal organic decomposition. After careful oxygen reduction treatment to remove interstitial O ap atoms, we found that the T c increases monotonically from 24 K to 29 K with decreasing x from 0.15 to 0.00, demonstrating a quite different phase diagram from the previous bulk one. The implication of our results is discussed on the basis of tremendous influence of O ap “impurities” on superconductivity and also magnetism in T′ cuprates. Then we conclude that our result represents the generic phase diagram for oxygen-stoichiometric Nd 2− x Ce x CuO 4 .

ARPES studies of the electronic structure of LaOFe (P,As)

Abstract: We report a comparison study of LaOFeP and LaOFeAs, two parent compounds of recently discovered iron-pnictide superconductors, using angle-resolved photoemission spectroscopy. Both systems exhibit some common features that are very different from well-studied cuprates. In addition, important differences have also been observed between these two ferrooxypnictides. For LaOFeP, quantitative agreement can be found between our photoemission data and the LDA band structure calculations, suggesting that a weak coupling approach based on an itinerant ground state may be more appropriate for understanding this new superconducting compound. In contrast, the agreement between LDA calculations and experiments in LaOFeAs is relatively poor, as highlighted by the unexpected Fermi surface topology around (π, π). Further investigations are required for a comprehensive understanding of the electronic structure of LaOFeAs and related compounds.

Ni2X2 (X = pnictide, chalcogenide, or B) based superconductors

Abstract: We review the properties of Ni-based superconductors which contain Ni2X2 (X = As, P, Bi, Si, Ge, B) planes, a common structural element found also in the recently discovered FeAs superconductors. Strong evidence for the fully gapped nature of the superconducting state has come from field dependent thermal conductivity results on BaNi2As2. Coupled with the lack of magnetism, the majority of evidence suggests that the Ni-based compounds are conventional electron–phonon mediated superconductors. However, the increase in Tc in LaNiAsO with doping is anomalous, and mimics the behavior in LaFeAsO. Furthermore, comparisons of the properties of Ni- and Fe-based systems show many similarities, particularly with regards to structure–property relationships. This suggests a deeper connection between the physics of the FeAs superconductors and the related Ni-based systems which deserves further investigation.

Scale-up of 2G wire manufacturing at American Superconductor Corporation

Abstract: American Superconductor Corporation (AMSC) has developed the base technology and a manufacturing line for initial volume production of low-cost second generation high temperature superconductor (2G HTS) wire for commercial and military applications. The manufacturing line is based on reel-to-reel processing of wide HTS strips using rolling assisted bi-axially textured substrate (RABiTS™) for the template and Metal Organic Deposition (MOD) for the HTS layer. AMSC’s wide strip process is a low cost manufacturing technology since multiple wires are produced in a single manufacturing pass by slitting the wide strip to narrower width in the last stage of the manufacturing process. Industry standard 4.4 mm wide wires are produced by laminating metallic foils, such as copper, stainless steel or any other material, to the HTS insert wire, and are chosen to tailor the electrical, thermal and mechanical properties of the wire for specific applications. The laminated, 4.4 mm wide wires are known as “344 superconductors.” In this paper, we summarize the status of AMSC’s manufacturing capability, the performance of the wire presently being produced, as well as the cost and technical advantages of AMSC’s manufacturing approach. In addition, future direction for research and development to improve electrical performance is presented.

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

Abstract: 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 2 As 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 ) 2 As 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.

Enhanced superconductivity in Sr2CuO4−v

Abstract: A critical review of previous investigations of the superconductivity with enhanced T c ∼ 95 K found in Sr 2 CuO 4− v shows that new physics occurs in a highly overdoped region of the cuprate phase diagram. Moreover, evidence is adduced from the literature that 30% of the oxygen sites in the CuO 2 layers are vacant; a conclusion which is at odds with the universally made assumption that superconductivity originates in stoichiometric CuO 2 layers. While further research is needed in order to identify the pairing mechanism(s) responsible for the enhanced T c , we suggest possible candidates.

Renormalization group analysis of competing orders and the pairing symmetry in Fe-based superconductors

Abstract: We analyze antiferromagnetism and superconductivity in novel Fe-based superconductors within the weak-coupling, itinerant model of electron and hole pockets near (0, 0) and ( π , π ) in the folded Brillouin zone. We discuss the interaction Hamiltonian, the nesting, the RG flow of the couplings at energies above and below the Fermi energy, and the interplay between SDW magnetism, superconductivity and charge orbital order. We argue that SDW antiferromagnetism wins at zero doping but looses to superconductivity upon doping. We show that the most likely symmetry of the superconducting gap is A 1 g in the folded zone. This gap has no nodes on the Fermi surface but changes sign between hole and electron pockets. We also argue that at weak coupling, this pairing predominantly comes not from spin fluctuation exchange but from a direct pair hopping between hole and electron pockets.

Anisotropic London penetration depth and superfluid density in single crystals of iron-based pnictide superconductors

Abstract: In- and out-of-plane magnetic penetration depths were measured in three iron-based pnictide superconducting systems. The “122” system was represented by electron-doped Ba(Fe 1− x Co x ) 2 As 2 with the doping through the whole phase diagram with x ≈ 0.038, 0.047, 0.058, 0.074 and 0.10 ( T c ranged from 13 to 24 K) and by hole-doped (Ba 1− x K x )Fe 2 As 2 with doping close to optimal, with measured x ≈ 0.45 ( T c ≈ 28 K) and an underdoped sample with x ≈ 0.15 ( T c ≈ 19 K). The “1111” system was represented by single crystals of NdFeAs(O 1− x F x ) with nominal x = 0.1 ( T c ≈ 43 K). All studied samples of both 122 systems show a robust power-law behavior, λ ( T ) ∝ T n , with the sample-dependent exponent n = 2–2.5, which is indicative of unconventional pairing. This scenario could be possible either through scattering in a S ± state or due to nodes in the superconducting gap. In the Nd-1111 system, the interpretation of the results is complicated by magnetism of the rare-earth ions. For all three systems, the anisotropy ratio, γ λ ≡ λ c / λ ab , was found to decrease with increasing temperature, whereas the anisotropy of the coherence lengths, γ ξ ≡ ξ ab / ξ c = H c 2 ⊥ c / H c 2 ‖ c , has been found to increase (both opposite to the trend in two-band MgB 2 ). The overall anisotropy of the pnictide superconductors is small, in fact much smaller than that of the cuprates (except YBa 2 Cu 3 O 7− x (YBCO)). The 1111 system is about two times more anisotropic than the 122 system. Our data and analysis suggest that the iron-based pnictides are complex superconductors in which a multiband three-dimensional electronic structure and strong magnetic fluctuations play important roles.

Superconductivity, magnetism and crystal chemistry of Ba1−xKxFe2As2

Abstract: BaFe 2 As 2 is the parent compound of the ‘122’ iron arsenide superconductors and crystallizes with the tetragonal ThCr 2 Si 2 -type structure, space group I 4/ mmm . A spin-density-wave transition at 140 K is accompanied by a symmetry reduction to space group Fmmm and simultaneously by antiferromagnetic ordering. Hole-doping induces superconductivity in Ba 1− x K x Fe 2 As 2 with a maximum T c of 38 K at x ≈ 0.4. The upper critical fields approach 75 T with rather small anisotropy of H c2 . At low potassium concentrations ( x ⩽ 0.2), superconductivity apparently co-exists with the orthorhombically distorted and magnetically ordered phase. At doping levels x ⩾ 0.3, the structural distortion and antiferromagnetic ordering is completely suppressed and the T c is maximized. No magnetically ordered domains could be detected in optimally doped Ba 1− x K x Fe 2 As 2 ( x ⩾ 0.3) by 57 Fe Mossbauer spectroscopy in contrast μ SR results obtained with single crystals. The magnetic hyperfine interactions investigated by 57 Fe Mossbauer spectroscopy are discussed and compared to the ZrCuSiAs-type materials.

Structural relaxation, metal-to-insulator transition and pseudo-gap in oxygen deficient НоBa2Cu3O7−δ single crystals

Abstract: The effect of annealing at room temperatures on the conductivity in the basis plane of oxygen deficient HoВа 2 Сu 3 О 7− δ single crystals, tempered from a temperature of 650 °C is investigated. We determine that during the annealing process an extension of the area of the linear dependence of the resistivity ρ ab ( T ) occurs. A shift downwards of the temperature interval corresponding to the metal-to-insulator transition and a considerable transformation of the form of the resistivity transfers to the superconducting state was observed. The absolute value of the temperature that the pseudo-gap regime commences is reduced by about 20 K. The results are attributed to labile oxygen ordering without changing the concentration of oxygen in the sample. The estimation of the cluster sizes is characterized by the differences in oxygen content. We perform a comparative analysis between the experimental data with the predictions of the high temperature superconductivity cluster theory.

Flux pinning by discontinuous columnar defects in 74 MeV Ag-irradiated YBa2Cu3O7 coated conductors

Abstract: Linear damage tracks are created in production-quality YBCO coated conductors by irradiation with 61–74 MeV Ag ions. The ion tracks are observed by transmission electron microscopy to be elongated but discontinuous. The in-field transport critical current ( I c ) is enhanced significantly for fields applied parallel to the irradiation direction with a broad peak appearing in the magnetic field-angle dependence of the critical current, coinciding with the irradiation direction. The zero-field I c is typically reduced somewhat, however annealing at 200 °C partially restores this and enhances the in-field I c even for field parallel to the irradiation direction. Lower energies tend to produce a weaker peak, but also retain the zero-field I c to a greater extent, consistent with a trend of greater discontinuity in the ion tracks.