BaT2As2 single crystals (T = Fe,Co, Ni) and superconductivity upon Co-doping

The crystal structure and physical properties of BaFe₂As₂, BaCo₂As₂, and BaNi₂As₂ single crystals are surveyed. BaFe₂As₂ gives a magnetic and structural transition at T_N = 132(1) K, BaCo₂As₂ is a paramagnetic metal, while BaNi₂As₂ has a structural phase transition at T₀ = 131 K, followed by superconductivity below T_c = 0.69 K. The bulk superconductivity in Co-doped BaFe₂As₂ below T_c = 22 K is demonstrated by resistivity, magnetic susceptibility, and specific heat data. In contrast to the cuprates, the Fe-based system appears to tolerate considerable disorder in the transition metal layers. First principles calculations for BaFe_1.84Co_0.16As₂ indicate the inter-band scattering due to Co is weak.     

Superconductivity,magnetism and crystal chemistry of Ba1−xKxFe2As2

BaFe₂As₂ is the parent compound of the ‘122’ iron arsenide superconductors and crystallizes with the tetragonal ThCr₂Si₂-type structure, space group I4/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?xK_xFe₂As₂ 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?xK_xFe₂As₂ (x ? 0.3) by ⁵⁷Fe Mössbauer spectroscopy in contrast μSR results obtained with single crystals. The magnetic hyperfine interactions investigated by ⁵⁷Fe Mössbauer spectroscopy are discussed and compared to the ZrCuSiAs-type materials.     

Suppression of superconductivity in Zn-doped SmFeAsO0.8F0.2 system

A series of SmFe_1?xZn_xAsO_0.8F_0.2 samples with x = 0, 0.05, 0.1, 0.2 and 0.4 have been successfully synthesized using a solid state method. The lattice parameters are found to increase with increasing Zn doping content. The superconductivity has been definitely suppressed by Zn doping at Fe site with the transition temperature T_c being reduced from 52.5 K to 23.3 K for the sample of x = 0.05, and to 18.2 K for the sample of x = 0.1. For the samples with x > 0.1, the superconducting transition vanishes, and, at the meantime, the spin-density-wave anomaly recovers at 140 K. The metal to semiconductor transition is also observed in the SmFe_1?xZn_xAsO_0.8F_0.2 system. The behavior of SmFe_1?xZn_xAsO_0.8F_0.2 is very different from that of R_EFeAsO (R_E = rare earth metal), which reveals a very strong electron correlation in SmFe_1?xZn_xAsO_0.8F_0.2.     

Search for an effect of superconductivity on the phonons in Ba(Fe1−xCox)2As2

Inelastic neutron scattering was used to search for an influence of superconductivity on the phonons in optimally doped and in slightly overdoped Ba(Fe_1?xCo_x)₂As₂, x = 0.06 and x = 0.10. The study focused on phonons with energies close to the superconducting gap energy 2Δ because it is well known that such phonons will respond most strongly to the opening of the gap. We were able to obtain high quality data but nevertheless, we could not detect any influence of superconductivity on the phonons, neither on the linewidths nor on the frequencies. Our results imply that any coupling of low energy phonons to the electrons has to be very small, much smaller than observed in conventional superconductors with a high T_c. Our results are in line with the low coupling strength predicted by density functional theory for the investigated phonon branches.     

Superconductivity of the platinum doped 122 iron arsenide SrFe2−xPtxAs2

Pt doped 122 iron arsenide SrFe_1?xPt_xAs₂ (0 ? x ? 0.4) was successfully synthesized. The tetragonal unit-cell volume and the lattice constant a increase with increasing the Pt content, while c decreases, suggesting that the Fe ions are indeed replaced by Pt ions. By the Pt doping, the magnetic order of the parent phase is suppressed, and superconductivity emerges at approximately x = 0.15. T_c reaches the maximum of 16 K at x = 0.2. The compounds series can be a suitable subject to investigate role of the doped 5d state in the superconducting 3d Fe–As layer.     

Electrochemical supercapacitor studies of hierarchical structured Co2+-substituted SnO2 nanoparticles by a hydrothermal method

Hierarchical structured Co-doped SnO₂ nanoparticles are prepared by a low temperature hydrothermal process. The structural and surface morphologies of the SnO₂ and Sn_1?xCo_xO₂ nanoparticles are studied by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The Sn_1?xCo_xO₂ nanoparticles form with a tetragonal rutile structure during the hydrothermal process without further calcination. The pseudocapacitance behavior of the Sn_1?xCo_xO₂ nanoparticles is characterized by cyclic voltammetry (CV) in 1.0 M H₂SO₄ electrolyte. The specific capacitance (SC) is found to increase with an increase in cobalt content. A maximum SC of 840 F g^?1 is obtained for a Sn_0.96Co_0.04O₂ composite at a 10 mV s^?1 scan rate.     

Anomalous Zn- and Ni-substitution effects on superconductivity in the superconducting weak ferromagnets RuSr2RCu2O8 (R = Gd,Eu)

The effect of magnetic Ni and non-magnetic Zn impurities on superconducting transition temperature T_c in RuSr₂R(Cu_1?x(Ni, Zn)_x)₂O₈ with R = Gd or Eu (Ni- and Zn-substituted Ru1212Gd(Eu)) was extensively studied. It is found that the suppression rate dT_c/dx of RuSr₂R(Cu_1?x(Ni, Zn)_x)₂O₈ is comparable to that of underdoped YBa₂(Cu_1?x(Ni, Zn)_x)₃O_7?δ. The suppression of superconductivity in Ni-substituted Ru1212Eu samples is more significant than that in Zn-substituted ones, indicative of Ni being a more effective pair-breaker than Zn. In strong contrast, the magnetic Ni impurity atoms have a weaker effect on superconductivity than non-magnetic Zn atoms in Ru1212Gd, similar to what was observed in the high-T_c cuprates. These intriguing findings strongly suggest that the impurity-induced local disturbance of the 3d-spin correlation at Cu sites around Ni/Zn is distinctly different between Ru1212Gd and Ru1212Eu.     

Superconducting property of Zr–Co–Al–Nb metallic glasses

The superconducting property of Zr₅₅Co_(30?x)Al₁₅Nb_x (x = 0–20 at.%) metallic glasses fabricated by rapid solidification was investigated. The Zr₅₅Co_(30–x)Al₁₅Nb_x (x = 5–20 at.%) metallic glasses with a mixture structure of amorphous and nanocrystal phases exhibited superconductivity of T_c,on = 1.8–2.6 K. The maximum T_c,on = 2.6 K was obtained for the Zr₅₅Co₁₀Al₁₅Nb₂₀ metallic glass. This was attributable to the superconducting property of nanocrystalline particles contained in the Zr₅₅Co₁₀Al₁₅Nb₂₀ alloy. The increase of Nb element in the Zr₅₅Co_(30–x)Al₁₅Nb_x alloy led to the increase of T_c,on and the decrease of glass transition temperature. The glass transition temperature was between 704 and 749 K for the Zr₅₅Co_(30–x)Al₁₅Nb_x (x = 0–20 at.%) alloys. The temperature interval of supercooled liquid state was between 51 and 68 K for the Zr₅₅Co_(30–x)Al₁₅Nb_x (x = 0–20 at.%) alloys.     

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

A review of our investigations on single crystals of LnFeAsO_1?xF_x (Ln = La, Pr, Nd, Sm, Gd) and Ba_1?xRb_xFe₂As₂ is presented. A high-pressure technique has been applied for the growth of LnFeAsO_1?xF_x crystals, while Ba_1?xRb_xFe₂As₂ 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?xF_x crystals. Resistivity measurements on LnFeAsO_1?xF_x crystals show a significant broadening of the transition in high magnetic fields, whereas the resistive transition in Ba_1?xRb_xFe₂As₂ simply shifts to lower temperature. The critical current density for both compounds is relatively high and exceeds 2 × 10⁹ A/m² at 15 K in 7 T. The anisotropy of magnetic penetration depth, measured on LnFeAsO_1?xF_x crystals by torque magnetometry is temperature dependent and apparently larger than the anisotropy of the upper critical field. Ba_1?xRb_xFe₂As₂ crystals are electronically significantly less anisotropic. Point-Contact Andreev-Reflection spectroscopy indicates the existence of two energy gaps in LnFeAsO_1?xF_x. Scanning Tunneling Spectroscopy reveals in addition to a superconducting gap, also some feature at high energy (～20 meV).     

Optical and Raman spectroscopy studies on Fe-based superconductors

A brief review of optical and Raman studies on the Fe-based superconductors is given, with special emphasis on the competing phenomenon in this system. Optical investigations on ReFeAsO (Re = rare-earth element) and AFe₂As₂ (A = alkaline-earth metal) families provide clear evidence for the gap formation in the broken symmetry states, including the partial gaps in the spin-density wave states of parent compounds, and the pairing gaps in the superconducting states for doped compounds. Especially, the superconducting gap has an s-wave pairing lineshape in hole-doped BaFe₂As₂. Optical phonons at zone center detected by Raman and infrared techniques are classified for several Fe-based compounds. Related issues, such as the electron–phonon coupling and the effect of spin-density wave and superconducting transitions on phonons, are also discussed. Meanwhile, open questions including the T-dependent mid-infrared peak at 0.6–0.7 eV, electronic correlation, and the similarities/differences between high-T_c cuprates and Fe-based superconductors are also briefly discussed. Important results from other experimental probes are compared with optical data to better understand the spin-density wave properties, the superconductivity, and the multi-band character in Fe-based compounds.     

A semimetal model of the normal state magnetic susceptibility and transport properties of Ba(Fe1−xCox)2As2

A simple two-band 3D model of a semimetal is constructed to determine which normal state features of the Ba(Fe_1?xCo_x)₂As₂ superconductors can be qualitatively understood within this framework. The model is able to account in a semiquantitative fashion for the measured magnetic susceptibility, Hall, and Seebeck data, and the low temperature Sommerfeld coefficient for 0 < x < 0.3 with only three parameters for all x. The purpose of the model is not to fit the data but to provide a simple starting point for thinking about the physics of these interesting materials. Although many of the static magnetic properties, such as the increase of the magnetic susceptibility with temperature, are reproduced by the model, none of the spin-fluctuation dynamics are addressed. A general conclusion from the model is that the magnetic susceptibility of most semimetals should increase with temperatures.     

New superconducting RbFe2As2: A first-principles investigation

RbFe₂As₂ has recently been reported to be a bulk superconductor with T_c = 2.6 K in the undoped state, in contrast to undoped BaFe₂As₂ with a magnetic ground state. We present here the results of the first-principles calculations of the structural, elastic and electronic properties for this newest superconductor and discuss its behaviour in relation to other related systems.     

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?xK_xFe₂As₂ 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 2Δ_L/kT_c > 6–8. In the electron doped Ba(Fe_1?xCo_x)₂As₂ 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 2Δ/kT_c ～ 5 and 6, respectively.     

Sulfur substitution and pressure effect on superconductivity of α-FeSe

In this paper, sulfur substitution and pressure effect on superconductivity of α-FeSe has been investigated in Fe(Se_1 ? xS_x)_0.88 (x = 0.1, 0.2). For x = 0.1, the critical temperature T_c is slightly larger than that of non-substituted sample, in consistent with the pressure effect on the superconductivity of α-FeSe. However, with further increasing S content to x = 0.2, T_c decreases. Temperature dependent of specific heat showed that the structural transition seems to be suppressed. T_c for x = 0.2 can be further decreased by applying pressure of 5 kbar, in contrary to the pressure effect on α-FeSe. We suggest that, in addition to the suppression of structural transition, other factors like the increase of carrier concentration should be considered for understanding the pressure effect on the superconductivity of α-FeSe.     

A comparative study of different concentrations of pure Zn powder effects on synthesis,structure, magnetic and microwave-absorbing properties in mechanically-alloyed Ni–Zn ferrite

In this study, a powder mixture of Zn, Fe₂O₃ and NiO was used to produce different compositions of Ni_1−xZn_xFe₂O₄ (x=0.36, 0.5 and 0.64) nanopowders. High-energy ball milling with a subsequent heat treatment method was carried out. The XRD results indicated that for the content of Zn, x=0.64 a single phase of Ni–Zn ferrite was produced after 30 h milling while for the contents of Zn, x=0.36 and 0.5, the desired ferrite was formed after sintering the 30 h-milled powders at 500 °C. The average crystallite size decreased with increase in the Zn content. A DC electrical resistivity of the Ni–Zn ferrite, however, decreased with increase in the Zn content, its value was much higher than those samples prepared by the conventional ceramic route by using ZnO instead of Zn. This is attributed to smaller grains size which were obtained by using Zn. The FT-IR results suggested two absorption bands for octahedral and tetrahedral sites in the range of 350–700 cm⁻¹. The VSM results revealed that by increasing the Zn content from 0.36 to 0.5, a saturation magnetization reached its maximum value; afterwards, a decrease was observed for Zn with x=0.64. Finally, magnetic permeability and dielectric permittivity were studied by using vector network analyzer to explore microwave-absorbing properties in X-band frequency. The minimum reflection loss value obtained for Ni_0.5Zn_0.5Fe₂O₄ samples, about −34 dB at 9.7 GHz, making them the best candidates for high frequency applications.     

Effects of excessive Zn2+ ions on intrinsic magnetic and structural properties of Ni0.2Zn0.6Cu0.2Fe2O4 powder prepared by chemical coprecipitation method

Zn(OH)₂ is a kind of amphoteric compound. Therefore, for chemical coprecipitation method, the precipitation of Zn²⁺ ions may be incomplete if using NaOH as precipitator. In this study, single-phase powder specimens with a nominal composition Ni_0.2Zn_0.6Cu_0.2Fe₂O₄ were prepared with chemical coprecipitation method, and the effects of excessive Zn²⁺ content (x, x = 3%, 5%, 7%, 9%) in working solution on intrinsic magnetic and structural properties were studied by vibrating sample magnetometer and X-ray diffractometer, respectively. It was found that the magnetization when H_m = 398 kA/m (5000 Oe) reached a maximum when x = 5%, and then decreased with the increase of x, which was attributed to the effect of different amount of Zn²⁺ in A sites on the A–B and B–B exchange interaction. Moreover, it was found that the lattice parameter was affected by the Zn²⁺ and Fe³⁺ ions due to their different ion radius to a certain extent.     

Oxygen isotope effect in disordered underdoped and overdoped La2−xSrxCu1−yZnyO4 superconductors

The effect of oxygen isotopic substitution on the superconducting transition temperature has been studied for heavily underdoped and overdoped La_2?xSr_xCu_1?yZn_yO₄ compounds with different Zn contents in the CuO₂ plane. The effect of Zn on the isotope coefficient, α, was significantly more pronounced in the case of the underdoped (x = 0.09) compounds compared to the overdoped (x = 0.22) ones. The variation of α with disorder content can be described quite well within a model based solely on Cooper pair-breaking in the case of the underdoped compounds. This model fails to describe the behavior of α(y) for the overdoped samples, even though Zn still suppresses T_c very effectively at this hole (Sr) content, indicating that the Zn induced pair-breaking is still very much at play. We discuss the implications of these findings in details by considering the Zn induced magnetism, stripe correlations, and possible changes in the superconducting order parameter as hole content in the CuO₂ plane, p (≡x), is varied.     

Superconductivity in bulk T′-(La,Sm)2CuO4 prepared via a molten alkaline hydroxide route

We have synthesized La_2?xSm_xCuO₄ (0 ? x ? 2.0) with the Nd₂CuO₄ structure via a molten alkaline hydroxide route at temperatures as low as 400–480 °C. After reduction heat treatment in vacuum at 600–750 °C for removal of excess oxygen atoms at the interstitial apical site, superconductivity with T_c = 20–24 K was observed in the samples with x = 0.05–1.0. The superconducting volume fraction is nearly 100% for x = 0.3–0.7. Our results demonstrate that La_2?xSm_xCuO₄ with no nominal carrier doping is a bulk superconductor.     

The effect of copper concentration on structural,optical and dielectric properties of CuxZn1 − xS thin films

Cu_xZn_1 ? xS (x = 0, 0.25, 0.50, 0.75, 1) thin films were deposited on glass substrates using Successive Ionic Layer Adsorption and Reaction (SILAR) method at room temperature and ambient pressure. The copper concentration (x) effect on the structural, morphological and optical properties of Cu_xZn_1 ? xS thin films was investigated. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies showed that all the films exhibit polycrystalline nature and are covered well with glass substrates. The crystalline and surface properties of the films improved with increasing copper concentration. The energy bandgap values were changed from 2.07 to 3.67 eV depending on the copper concentration. The refractive index (n), optical static and high frequency dielectric constants (ε_o, ε_∞) values were calculated by using the energy bandgap values as a function of the copper concentration.     

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

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 Fe_1+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 ～0.2 eV. With doping, the structural and magnetic transitions are suppressed in favor of superconductivity, with superconducting transition temperatures up 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 Ce 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 Fe_1+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.