The electrical resistivity (Rxx) and Hall resistivity (Rxy) of LaFeAsO1-xFx have been measured over a wide fluorine doping range 0 =< x =< 0.14 using 60 T pulsed magnets. While the superconducting phase diagram (Tc, x) displays the classic dome-shaped structure, we find that the resistive upper critical field (Hc2) increases monotonically with decreasing fluorine concentration, with the largest Hc2 >= 75 T for x = 0.05. This is reminiscent of the composition dependence in high-Tc cuprates and might correlate with opening of a pseudo-gap in the underdoped region. Further, the temperature dependence of Hc2(T) for superconducting samples can be understood in terms of multi-band superconductivity. Rxy data for non-superconducting samples show non-linear field dependence, which is also consistent with a multi-carrier scenario.
The crystal structures of superconducting LaO1-xFxBiS2 (x ~ 0.23, 0.46) were determined by single-crystal X-ray diffraction analysis. Their space group was P4/nmm. Distortion of the Bi-S plane changed when the F content was increased from 0.23 to 0.46, and a nearly flat Bi-S plane was formed at x ~ 0.46. Computational calculations supported this effect of F doping on distortion of Bi-S plane. LaO1-xFxBiS2 with higher F contents were computationally predicted to be thermodynamically more unstable under ambient pressure. We discussed the bonding, conductivities, and synthetic routes of LaO1-xFxB…
We report the resistivity of a series of fluorine-doped SmFeAsO1-xFx polycrystalline superconductors in magnetic fields up to 60T. For underdoped samples (x < 0.15), the low temperature resistive state is characterized by pronounced magneto-resistance and a resistive upturn at low temperatures. The "insulating behavior" is characterized by a log-T divergence observed over a decade in temperature. In contrast, the normal state for samples with doping x > 0.15 display metallic behavior with little magnetoresistance, where intense magnetic fields broaden the superconducting transition rather than…
We have investigated the fluorine-doping dependence of lattice constants, transports and specific heat for polycrystalline LaFePO1-xFx. F doping slightly and monotonically decreases the in-plane lattice parameter. In the normal state, electrical resistivity at low temperature is proportional to the square of temperature and the electronic specific heat coefficient has large value, indicating the existence of moderate electron-electron correlation in this system. Hall coefficient has large magnitude, and shows large temperature dependence, indicating the low carrier density and multiple carrier…
We succeeded in growing single crystalline SmFeAsO1-xFx with exceptionally high fluorine concentration by using a CsCl flux method. Comparing to conventional flux methods, this method can introduce about double the amount of fluorine into the oxygen site. The obtained single crystal shows the highest superconducting transition temperature (Tc = 57.5 K) in single crystalline iron pnictides. In addition, the residual resistivity ratio (RRR) is almost three times as large as that of previously reported single crystals. This suggests that our single crystals are suitable for investigation of the i…
We report the results from resistivity and magnetic measurements on polycrystalline Ce oxypnictide (CeFeAsO1-xFx) samples where x spans from 0.13 to 0.25. We find that the orbital limiting field is as high as 150 T and it systematically decreases with increasing doping. The Maki parameter is greater than one across the phase diagram and the large Maki parameter suggests that orbital and Pauli limiting effects contribute to the upper critical field. The broadening of the superconducting transition in the resistivity data was interpreted using the thermally activated flux flow (TAFF) model where…
Raman spectra of polycrystalline NdFeAsO1-xFx (x=0.0, 0.1, 0.2) compound have been systematically investigated as functions of temperature and fluorine concentration. Scanning electron microscopic and Raman microscopic characterization demonstrates that the polycrystalline samples mainly contain two phases, i.e. superconductor NdFeAsO1-xFx compound and a MnP-type FeAs phase, with dissimilar characteristic Raman bands. It was found that fluorine doping leads to structure disorder in the insulator Nd-O layers and high temperature coefficient of Fe-As vibrational mode.
A series of polycrystalline SmO1-xFxFeAs bulks (x=0.15, 0.2, 0.3 and 0.4) were prepared by the conventional solid state reaction. Resistivity, susceptibility, magnetic hysteresis, critical current density and microstructure of these samples have been investigated. It is found that critical transition temperature Tc increases steadily with increasing fluorine content, with the highest onset Tc=53 K at x=0.4. On the other hand, the superconductivity seems correlated with lattice constants; that is, Tc rises with the shrinkage of a-axis while resistivity increases with the enlargement of c-axis. …
The fluorine-doped rare-earth iron oxyarsenides, REFeAsO1-xFx (RE =rare earth) have recently emerged as a new family of high-temperature superconductors with transition temperatures (Tc) as high as 55 K (refs 1-4). Early work has provided compelling evidence that the undoped parent materials exhibit spin-density-wave (SDW) antiferromagnetic order and undergo a structural phase transition from tetragonal to orthorhombic crystal symmetry upon cooling.5 Both the magnetic and structural instabilities are suppressed upon doping with fluoride ions before the appearance of superconductivity.6,7 Here …
The layered compound LaOBiSSe was successfully synthesized by a two-step solid-state reaction method, which is isostructural with the quaternary compound LaOBiS2 indicated by the powder X-ray diffraction characterization, and the crystal lattice parameters are slightly expanded by the substitution of Se atoms. Electrical transport measurement shows that LaOBiSSe exhibits semiconducting behavior similar with the parent compound LaOBiS2, while by F-doping the samples can be much more easily turned to be metallic and superconducting at low temperature with a small amount of F. Under the optimal c…
Here we report the superconductivity in the LaFeAsO1-xFx system prepared by high pressure synthesis. The highest onset superconducting transition temperature (Tc) in this La-based system is 41.0 K with the nominal composition of LaFeAsO1-xFx (x = 0.6), which is higher than that reported previously by ambient pressure synthesis. The increase of Tc can be attributed to the further shrinkage of crystal lattice that causes the stronger chemical pressure on the Fe-As plane, which is induced by the increased F-doping level under high pressure synthesis.
SmFeAsO1-xFx tapes were prepared using three kinds of starting materials. It shows that the starting materials have an obvious effect on the impurity phases in final superconducting tapes. Compared with the other samples, the samples fabricated by SmAs, FeO, Fe2As, and SmF3 have the smallest arsenide impurity phase and voids. As a result, these samples possess much denser structure and better grain connectivity. Moreover, among the three kinds of samples fabricated in this work, this kind of sample has the highest zero-resistivity temperature ~40 K and largest critical current density ~4600 A/…
The lattice properties at low temperatures of two samples of NdFeAsO1-xFx (x=0.05 and 0.25) have been examined in order to investigate possible structural phase transition that may occur in the optimally doped superconducting sample with respect to the non-superconducting low-F concentration compound. In order to detect small modifications in the ion displacements with temperature micro-Raman and high resolution synchrotron powder diffraction measurements were carried out. No increase of the width of the (220) or (322) tetragonal diffraction peaks and microstrains could be found in the superco…
Measurements of the electrical resistivities, Hall coefficients, thermoelectric powers, electronic specific heat coefficients have been carried out for samples of LnFe1-yMyAsO1-xFx (Ln=La, Nd; M=Co, Mn; x=0.11) obtained by M atom dopings to the superconducting LnFeAsO1-xFx (Ln1111) system. The NMR longitudinal relaxation rates 1/T1 have also been measured for samples of LaFe1-yCoyAsO1-xFx with various x values. Co atoms doped to the superconducting LnFeAsO1-xFx are nonmagnetic, and the Tc-suppression by the Co atoms has been found to be too weak to understand by the pair breaking effect expect…
Since the discovery of copper oxide superconductor in 1986 [1], extensive efforts have been devoted to the search of new high-Tc superconducting materials, especially high-Tc systems other than cuprates. The recently discovered quaternary superconductor La[O1-xFx]FeAs with the superconducting critical transition Tc of 26 K [2], which has a much simple layered structure compared with cuprates, has attracted quick enthusiasm and is going to become a new high-Tc system [3-6]. Here we report the discovery of bulk superconductivity in the praseodymium-arsenide oxides Pr[O1-xFx]FeAs with an onset dr…
The recently discovered quaternary arsenide oxide superconductor La[O1-xFx]FeAs with the superconducting critical transition temperature (Tc) of 26 K [1], has been quickly expanded to another high-Tc superconducting system beyond copper oxides by the replacement of La with other rare earth elements, such as Sm, Ce, and Pr etc. [2-4], and the Pr[O1-xFx]FeAs has become to be the first non-cuprate superconductor that holding a Tc above 50 K. All these arsenide (including phosphide) superconductors formed in a same tetragonal layered structure with the space group P4/nmm which has an alternant sta…
We report inelastic neutron scattering measurements of crystal field transitions in PrFeAsO, PrFeAsO0.87F0.13, and NdFeAsO0.85F0.15. Doping with fluorine produces additional crystal field excitations, providing evidence that there are two distinct charge environments around the rare earth ions, with probabilities that are consistent with a random distribution of dopants on the oxygen sites. The 4f electrons of the Pr3+ and Nd3+ ions have non-magnetic and magnetic ground states, respectively, indicating that the enhancement of Tc compared to LaFeAsO1-xFx is not due to rare earth magnetism.
High temperature superconductors with a Tc above 40 K have been found to be strongly correlated electron systems and to have a layered structure. Guided by these rules, Kamihara et al. discovered a Tc up to 26 K in the layered La(O1-xFx)FeAs. By replacing La with tri-valence rare-earth elements RE of smaller ionic radii, Tc has subsequently been raised to 41-52 K. Many theoretical models have been proposed emphasizing the important magnetic origin of superconductivity in this compound system and a possible further Tc-enhancement in RE(O1-xFx)FeAs by compression. This later prediction appears t…
We report measurements of resistivity, magnetoresistivity, Hall effect, Seebeck coefficient, infrared reflectivity of undoped SmFeAsO and lightly doped SmFeAs(O0.93F0.07) oxypnictides. All the properties measured on SmFeAsO are characterized by clear signatures of the magnetic instability. A self-consistent picture emerges in which below the magnetic transition carrier condensation occurs due to the opening of spin density wave (SDW) gap. This is accompanied by the mobility increase of not gapped carriers due to the suppression of electron-electron scattering. SmFeAs(O0.93F0.07) exhibits an in…
We report on the synthesis and microstructural studies of fluorine based iron doped superconductors. We have successfully synthesized the fluorine based new superconducting material LaO1-xFxFeAs by choosing comparatively lower temperature and longer synthesis duration. The superconducting transition temperature that we have achieved is 27.5 K which is observed at doping level of x=0.2. The structural microstructural characterizations have been done by employing XRD, SEM and TEM techniques. The SEM and TEM micrographs reveal the layered structure.
A comparative study of thermal properties of SmFeAsO, SmFeAs(O0.93F0.07) and SmFeAs(O0.85F0.15) samples is presented. Specific heat and thermal conductivity show clear evidences of the spin density wave (SDW) ordering below TSDW 135 K in undoped SmFeAsO. At low level of F-doping, SmFeAs(O0.93F0.07), SDW ordering is suppressed and superconducting features are not yet optimally developed in both specific heat and thermal conductivity. At optimal level of F-doping SmFeAs(O0.85F0.15) anomalies related to the superconducting transition are well noticeable. By a compared analysis of doped and undope…
The magnetic and transport behaviors of cerium substituted iron oxy-arsenide superconductor with x = 0.1 to 0.4 fluoride (F) doping have been investigated in this report. Temperature dependent susceptibility and resistivity measurements showed the 0.1 F-doped sample (CeFeAsO0.9F0.1) has a superconducting transition temperature (Tc) of around 30 K. With increasing doping beyond x = 0.2 Tc saturates to around 40 K. Temperature dependent susceptibility measured in different magnetic fields for the under-doped sample showed Meissner effect in low field and the diamagnetism is still visible up to 1…
Electrical resistivities, Hall coefficients and thermoelectric powers have been measured for polycrystalline samples of LaFe1-yCoyAsO1-xFx (x=0.11) with various values of y. The results show that there exists clear distinction of these transport behaviors between the superconducting and nonsuperconducting metallic regions of y divided by the boundary value yc~0.05. We have found that the behaviors in both regions are very similar to those of high-Tc Cu oxides in the corresponding phases. If they reflect, as in the case of Cu oxides, effects of strong magnetic fluctuations, the energy scale of …
The F doped LaFeAsO, a recently discovered superconductor with the high Tc of 26 K, has been studied by the resistivity, magnetic susceptibility, and heat capacity measurements in the F doping range from 0 to 0.14 (x in LaFeAsO1-xFx). In the low temperature region, a T3lnT term in the heat capacity and a T2 term in the magnetic susceptibility, which are derived from the spin fluctuation, are observed. The nearly ferromagnetic nature evidenced by a large Wilson ratio (6.5 for x = 0, and 11.2 for x = 0.025) suggests that the superconductivity in the LaFeAsO system is mediated by ferromagnetic sp…
In layered single band systems, the interlayer conductivity depends on the orientation of the in-plane magnetic field and takes maximum values when the magnetic field is perpendicular to flat regions of the Fermi surface. Extending this known results to multi-band systems, we propose an experiment to extract information about their Fermi surface topology. We discuss application of the formula to a FeAs-based superconductor, LaFeAsO$_{1-x}$F$_x$. We show that the magnetically ordered state in the parent compound is clearly distinguished from the paramagnetic state by the oscillation period in t…
Here, we present a de Haas-van Alphen (dHvA) effect1 study on the newly discovered LaFeAsO1-xFx compounds2,3 in order to unveil the topography of the Fermi surface associated with their antiferromagnetic and superconducting phases, which is essential for understanding their magnetism, pairing symmetry and superconducting mechanism. Calculations 4 and surface-sensitive measurements 5,6,7 provided early guidance, but lead to contradictory results, generating a need for a direct experimental probe of their bulk Fermi surface. In antiferromagnetic LaFeAsO1-xFx 8,9 we observe a complex pattern in t…
Colossal magnetoresistance (CMR) is a rare phenomenon in which the electronic resistivity of a material can be decreased by orders of magnitude upon application of a magnetic field. Such an effect could be the basis of the next generation of magnetic memory devices. Here we report CMR in the antiferromagnetic oxypnictide NdMnAsO1-xFx as a result of competition between an antiferromagnetic insulating phase with strong electron correlations and a paramagnetic semiconductor upon application of a magnetic field. The discovery of CMR in antiferromagnetic Mn2+ oxypnictide materials could open up an …
A review of our investigations on single crystals of LnFeAsO1-xFx (Ln=La, Pr, Nd, Sm, Gd) and Ba1-xRbxFe2As2 is presented. A high pressure technique has been applied for the growth of LnFeAsO1-xFx crystals, while Ba1-xRbxFe2As2 crystals were grown using 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 less than full occupation of the (O, F) position in superconducting LnFeAsO1-xFx crystals. Resistivity measurements on LnFeAsO1-xFx…
We report on the synthesis and structural/ microstructural studies of iron based fluorine doped LaOFeAs superconductors. We have successfully synthesized fluorine doped superconducting LaO1-xFxFeAs materials by choosing lower temperature (1150^0C) and longer synthesis duration (60 hours) as compared to the standard values of these employed in the pioneering first contribution [Kamihara et al 2008 J Am Chem Soc 130 3296]. Decrease of lattice parameters as determined by x-ray diffraction confirm the substitution of fluorine. The superconducting transition temperature is 27.5 K which is observed …
WO3-xFx (x < 0.45) perovskite-like oxyfluorides were prepared by a chemically reducing fluorination route using the polymer polytetrafluoroethylene (Teflon). The symmetry of the crystal structures of WO3-xFx changes from monoclinic to tetragonal to cubic as the fluorine content increases. Fluorine doping changes insulating WO3 to a metallic conductor, and superconductivity (Tc = 0.4 K) was discovered in the samples with fluorine contents of 0.41 < x < 0.45. This easy fluorination method may be applicable to other systems and presents an opportunity for finding new oxyfluoride superconductors.
In order to unravel a role of doping in the iron-based superconductors, we investigated the in-plane resistivity for BaFe$_2$As$_2$ doped at either of the three different lattice sites, Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, BaFe$_2$(As$_{1-x}$P$_x$)$_2$, and Ba$_{1-x}$K$_x$Fe$_2$As$_2$, focusing on the doping effect in the low-temperature antiferromagnetic/orthorhombic (AFO) phase. A major role of doping in the high-temperature paramagnetic/tetragonal (PT) phase is known to change the Fermi surface by supplying charge carriers or by exerting chemical pressure. In the AFO phase, we found a clear corr…
We have prepared the superconducting system LaFe1-yCoyAsO1-xFx (x=0.11) and carried out measurements of their electrical resistivities r and superconducting diamagnetisms. 75As- and 135La-NMR studies have also been carried out. The Knight shift observed for 75As has been found to be suppressed by the superconductivity, while for 135La, the shift is almost insensitive to the superconductivity. This result presents rather strong experimental evidence for the singlet pairing. The Co-doping effect on the superconducting transition temperature Tc is not so significant as expected for superconductor…
Millimeter-sized single crystals of LaFeAsO, LaFeAsO1-xFx, and LaFe1-xCoxAsO were grown in NaAs flux at ambient pressure. The detailed growth procedure and crystal characterizations are reported. The as-grown crystals have typical dimensions of 3 * 4 * 0.05-0.3 mm3 with the crystallographic c-axis perpendicular to the plane of the plate-like single crystals. Some crystals manifest linear dimensions as large as 4-5 mm. X-ray and neutron single crystal scattering confirmed that LaFeAsO crystals exhibit a structural phase transition at Ts ~ 154 K and a magnetic phase transition at TSDW ~ 140 K. T…
Bulk polycrystalline samples, SmFeAsO and the iso-structural superconducting SmFeAsO0.80F0.20 are explored through resistivity with temperature under magnetic field ρ(T, H), AC and DC magnetization (M-T), and Specific heat (Cp) measurements. The Resistivity measurement shows superconductivity for x = 0.20 sample with Tc(onset) ~ 51.7K. The upper critical field, [Hc2(0)] is estimated ~3770kOe by Ginzburg-Landau (GL) theory. Broadening of superconducting transition in magneto transport is studied through thermally activated flux flow in applied field up to 130 kOe. The flux flow activation energ…
Iron arsenide superconductors based on the material LaFeAsO1-xFx are characterized by a two-dimensional Fermi surface (FS) consisting of hole and electron pockets yielding structural and antiferromagnetic transitions at x = 0. Electron doping by substituting O2- with F- suppresses these transitions and gives rise to superconductivity with a maximum Tc = 26 K at x = 0.1. However, the over-doped region cannot be accessed due to the poor solubility of F- above x = 0.2. Here we overcome this problem by doping LaFeAsO with hydrogen. We report the phase diagram of LaFeAsO1-xHx (x < 0.53) and, in add…
Theoretical calculations under the scheme of WIEN2k computer package had been performed on the pristine compound LaOFeAs, as well as on LaO1-xFxFeAs. One factor crucial to the manifestation of superconductivity seems to be fluorine doping; hence we search for its effect. Paying close attention to Fe d-orbitals upon doping of F by O, at first, Fe d-orbitals have dz2 symmetry, changing gradually to something akin to dx2-y2 which seems more prone to conductivity than the former one. This issue agree with the accepted results happened in the copper-based high-temperature superconductors.
We report ab-plane Hall Effect and magnetoresistivity measurements on La2-xCexCuO4 thin films as a function of doping for magnetic fields up to 14T and temperatures down to 1.8K. A dramatic change in the low temperature (1.8 K) normal state Hall coefficient is found near a doping Ce=0.14. This, along with a nonlinear Hall resistance as a function of magnetic field, suggests that the Fermi surface reconstructs at a critical doping of Ce= 0.14. A competing antiferromagnetic phase is the likely cause of this Fermi surface reconstruction. Low temperature linear-in-T resistivity is found at Ce=0.14…
Temperature-dependent in-plane, $ρ_a(T)$, and inter-plane, $ρ_c(T)$, resistivities were measured for the iron-arsenide superconductor (Ba$_{1-x}$K$_x$)Fe $_2$As$_2$ over a broad doping range from parent compound to optimal doping $T_c\approx 38 K$, $0\leq x \leq 0.35$. The coupled magnetic/structural transition at $T_{SM}$ is clearly observed for samples with $T_c <$26 K ($x <0.25$), however its effect on resistivity is much weaker than in the electron-doped Ba(Fe$_{1-x}$Co$_x$)Fe $_2$As$_2$, and the transition leads only to a decrease of resistivity. In addition to the feature at $T_{SM}$, th…
Large high-quality single crystals of hole-doped iron-based superconductor (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ were grown over a broad composition range $0.22 \leq x \leq 1$ by inverted temperature gradient method. We found that high soaking temperature, fast cooling rate, and adjusted temperature window of the growth are necessary to obtain single crystals of heavily K doped crystals (0.65$\leq x \leq$ 0.92) with narrow compositional distributions as revealed by sharp superconducting transitions in magnetization measurements and close to 100% superconducting volume fraction. The crystals were exten…
Complete electronic phase diagram and enhanced superconductivity in fluorine-doped PrFeAsO1-xFx
Institute of High Pressure Physics (IHPP), Polish Academy of Sciences（波兰科学院高压物理研究所）；Faculty of Physics, Warsaw University of Technology（华沙理工大学物理学院）；Institute of Physics, Polish Academy of Sciences（波兰科学院物理研究所）
Low-temperature synthesis is a promising and potentially effective method for improving superconducting properties. We report on the fabrication of polycrystalline samples of SmFeAsO1-xFx with nominal x content varying in a wide range of x = 0-0.35 synthesized at 900 deg C. This synthesis temperature is around 300 deg C lower than the conventional synthesis temperature. The variation in the lattice parameters and transition temperature (Tc) of various F-doped samples indicates that reduction of the unit cell volume (V) seems to be the main reason for the rise of Tc up to 57.8 K. Magnetoresista…
Temperature-dependent resistivity is studied in single crystals of iron-arsenide superconductor Na$_{1-δ}$Fe$_{1-x}$Co$_x$As for electrical current directions along, $ρ_a (T)$, and transverse, $ρ_c (T)$, to the Fe-As layers. Doping with Co increases stability of this compound to reaction with the environment and suppresses numerous features in both $ρ_a(T)$ and $ρ_c(T)$ compared to the stoichiometric NaFeAs. Evolution of $ρ_a (T)$ with $x$ follows a universal trend observed in other pnictide superconductors, exhibiting a $T$-linear temperature dependence close to the optimal doping and develop…
The effect of electron doping by trivalent charge state rare-earth ion (RE = La, Ce, Pr, and Nd) substitutions on the superconductivity in BaFe2As2 was examined using epitaxial films. Each of the RE substitutions suppressed the resistivity anomaly associated with the magnetic/structural phase transitions, leading to the resistivity drops and superconductivity transitions. Bulk superconductivity was observed at the maximum onset critical temperature (Tconset) of 22.4 K for La-doping and 13.4 K for Ce-doping, while only broad resistivity drops were observed at 6.2 K for Pr-doping and 5.8 K for N…
Temperature-dependent inter-plane resistivity, $ρ_c(T)$, was measured systematically as a function of transition metal substitution in the iron-arsenide superconductors Ba(Fe$_{1-x}$T$_x$)$_2$As$_2$, $T$= Ni, Pd, Rh. The data are compared with the behavior found in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, revealing resistive signatures of pseudogap. In all compounds we find resistivity crossover at a characteristic pseudogap temperature $T^*$ from non-metallic to metallic temperature dependence on cooling. Suppression of $T^*$ proceeds very similar in cases of Ni and Pd doping and much faster than in s…
The variation of TC in MgB2 has been systematically investigated for monovalent cation substitution in Mg1-xLixB2 and in Mg1-xCuxB2 and simultaneous cation and Carbon substitution in Mg0.80Li0.20B2-xCx and Mg0.95Cu0.05B2-xCx using dc resistivity and ac susceptibility techniques. TC seems to be uniquely determined by the electron count in the sample, being constant at the MgB2 value for electron counts lower than MgB2 but rapidly decreasing for larger electron counts. The Debye temperature extracted from fitting normal state resistivity to the Bloch-Gruneisen formula shows no systematics with T…
Temperature-dependent inter-plane resistivity, $ρ_c(T)$, was measured in hole-doped iron-arsenide superconductor (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ over a doping range from parent compound to optimal doping $T_c\approx 38~K$, $0\leq x \leq 0.34$. Measurements were undertaken on high-quality single crystals grown from FeAs flux. The coupled magnetic/structural transition at $T_{SM}$ leads to clear accelerated decrease of $ρ_c(T)$ on cooling in samples with $T_c <$26~K ($x <0.25$). This decrease in hole-doped material is in notable contrast to an increase in $ρ_c(T)$ in the electron-doped Ba(Fe$_{1-x…
We investigate the effect of Ni doping on the Fe-site in single crystals of the magnetic superconductor RbEuFe$_4$As$_4$ for doping concentrations of up to 4%. A clear suppression in the superconducting transition temperature is observed in specific heat, resistivity and magnetization measurements. Upon Ni-doping, the resistivity curves shift up in a parallel fashion indicating a strong increase of the residual resistivity due to scattering by charged dopand atoms while the shape of the curve and thus the electronic structure appears largely unchanged. The observed step $ΔC/T_c$ at the superco…
Resistivity, Hall effect, magnetoresistance and DC magnetization were measured in Mn and Zn doped Ba$_{0.5}$K$_{0.5}$Fe$_{2}$As$_{2}$ samples. It is found that the Mn-doping can depress the superconducting transition temperature drastically with a rate of $ΔT_c$/Mn-1% = -4.2 K, while that by Zn-doping is negligible. Detailed analysis reveals that the Mn-doping enhances the residual resistivity ($ρ_0$) significantly, and induces strong local magnetic moments ($\sim$ 2.58 $μ_B$) which play as pair breakers. While the impurity scattering measured by $ρ_0$ in the Zn-doped samples is much weaker, a…
The effects of planar hole concentration, p, and in-plane disorder, Zn (y), on the DC resistivity, r(T), of sintered samples of Y_{1-x}Ca_xBa_2(Cu_{1-y}Zn_y)_3O_{7-d} were investigated over a wide doping range by changing both the oxygen deficiency (d) and Ca content (x). From the r(T,p) data we extracted characteristic crossover temperatures on the underdoped and overdoped sides, T* and T_m respectively, above which r(T) is linear. We compare our results with a number of other polycrystalline, thin film and single crystal cuprate superconductors and find similar behavior in the p-dependence o…
The change in the lattice parameters or the lattice disorder is claimed as a cause of the slight reduction in the transition temperature by carbon doping in MgB2. In this work, an extensive investigation on the effects of carbohydrate doping has been carried out. It is found that not only the a-axis but also the c-axis lattice parameter increases with the sintering temperature. A linear relation between the unit cell volume and the critical temperature is observed. Compared with the well known correlation between the lattice strain and the critical temperature, the X-ray peak broadening itself…
The effect of nanoscale-SiC doping of MgB2 was investigated using transport and magnetic measurements. It was found that there is a clear correlation between the critical temperature Tc, the resistivity r, the residual resistivity ratio, RRR = R(300K)/R(40K), the irreversibility field H* and the alloying state in the samples. SiC-doping introduced many nano-scale precipitates, provoking an increase of r(40K) from 1 mW-cm (RRR = 15) for the clean limit sample to 300 mW-cm (RRR = 1.75) for the SiC-doped sample, leading to significant enhancement of Hc2 and H* with only minor effect on Tc. EELS a…
We measured the in-plane resistivity anisotropy in the underdoped Ca$_{1-x}$Na$_x$Fe$_2$As$_2$ single crystals. The anisotropy (indicated by $ρ_{\rm b} - ρ_{\rm a}$) appears below a temperature well above magnetic transition temperature $T_{\rm N}$, being positive ($ρ_{\rm b} - ρ_{\rm a} > 0$) as $x\leq$ 0.14. With increasing the doping level to $x$ = 0.19, an intersection between $ρ_{\rm b}$ and $ρ_{\rm a}$ is observed upon cooling, with $ρ_{\rm b} - ρ_{\rm a} < 0$ at low-temperature deep inside a magnetically ordered state, while $ρ_{\rm b} - ρ_{\rm a}> 0$ at high temperature. Subsequently, …
The doping and temperature dependence of the Hall coefficient, RH, and ab-plane resistivity in the normal state down to 350mK is reported for oriented films of the electron-doped high-Tc superconductor Pr2-xCexCuO4+d. The doping dependence of b (r=r0+AT^b) and R_sub_H (at 350 mK) suggest a quantum phase transition at a critical doping near x=0.165.
We studied the temperature and magnetic field dependence of vortex dissipation and critical current in the mixed-state of unconventional superconducting alloys Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($0.044 \leq x \leq 0.100$) through current-voltage measurements. Our results reveal that all the electric field $E$ vs current density $j$ curves in the Ohmic regime merge to one point ($j_0,E_0$) and that there is a simple relationship between the critical current density $j_c$ and flux-flow resistivity $ρ_{\rm ff}$: $ρ_{\rm ff}/ρ_{\rm n} = (1- j_{c}/j_{0})^{-1}$, where $ρ_{\rm n}=E_0/j_0$ is the normal-…
The influence of irradiation by electrons with energies of $0.5-2.5$\,MeV at temperatures of about $10$\,K on the basal-plane resistivity of the YBa$_2$Cu$_3$O$_{7-δ}$ single crystals is investigated in the range from $T_c$ to $300$\,K. The resistivity temperature dependence is determined by defects arising due to the irradiation. These defects directly affect the superconducting transition, decreasing $T_c$ and increasing the transition width without significant distortions of its shape. The resulting defects also lead to an increase in the Debye temperature due to a reduction of the anisotro…
We report the successful growth and the impurity scattering effect of single crystals of Na(Fe$_{0.97-x}$Co$_{0.03}$T$_x$)As (T=Cu, Mn). The temperature dependence of DC magnetization at high magnetic fields is measured for different concentrations of Cu and Mn. Detailed analysis based on the Curie-Weiss law indicates that the Cu doping weakens the average magnetic moments, while doping Mn enhances the local magnetic moments greatly, suggesting that the former may be non- or very weak magnetic impurities, and the latter give rise to magnetic impurities. However, it is found that both doping Cu…
Polycrystalline Sr1-xNdxFeAsF samples were prepared at various Nd-doping levels using both a stoichiometric mixture of the starting materials and in slight excess amounts of FeAs. Susceptibility and resistivity of the samples were studied down to 4 K revealing a probable coexistence of superconductivity and a magnetic ordering. Temperature dependence of resistivity for all the Nd-doped samples shows the presence of a transition below 15 K most likely originating from the magnetic ordering of Nd moments, while the spin-density-wave anomaly at 175 K survives up to 0.35 Nd-doping. Superconductivi…
Resistivity, Hall effect and magnetoresistance have been investigated systematically on single crystals of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ ranging from undoped to optimally doped regions. A systematic evolution of the quasiparticle scattering has been observed. It is found that the resistivity in the normal state of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ is insensitive to the potassium doping concentration, which is very different from the electron doped counterpart Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$, where the resistivity at 300 K reduces to half value of the undoped one when the system is optimally doped…
We have successfully synthesized the fluoride-arsenide compounds Ca$_{1-x}$RE$_x$FeAsF (RE=Nd, Pr; x=0, 0.6). The x-ray powder diffraction confirmed that the main phases of our samples are Ca$_{1-x}$RE$_x$FeAsF with the ZrCuSiAs structure. By measuring resistivity, superconductivity was observed at 57.4 K in Nd-doped and 52.8 K in Pr-doped samples with x=0.6. Bulk superconductivity was also proved by the DC magnetization measurements in both samples. Hall effect measurements revealed hole-like charge carriers in the parent compound CaFeAsF with a clear resistivity anomaly below 118 K, while th…
