We measured the transport properties and susceptibility of single crystals Ca$_{1-x}$La$_x$Fe$_2$As$_2$(x=0, 0.05, 0.1, 0.15, 0.19 and 0.25). Large in-plane resistivity anisotropy similar to that in Co-doped 122 iron-pnictides is observed although no transition metals were introduced in the FeAs-plane. The in-plane resistivity anisotropy gradually increases with La doping below T$_{SDW}$, being different from the hole-doped 122 superconductors. The susceptibilities of the samples show that La doping leads to suppression of SDW and induces a Curie-Weiss-like behavior at low temperature, which is much stronger than the other 122 iron-based superconductors.
We report anisotropic dc magnetic susceptibility $χ(T)$, electrical resistivity $ρ(T)$, and heat capacity $C(T)$ measurements on the single crystals of CaFe$_{2-x}$Co$_x$As$_2$ for $x$ = 0 and 0.06. Large sized single crystals were grown by the high temperature solution method with Sn as the solvent. For the pure compound with $x$ = 0, a high temperature transition at 170 K is observed which is attributed to a combined spin density wave (SDW) ordering and a structural phase transition. On the other hand, for the Co-doped samples for $x$ = 0.06, the SDW transition is suppressed while supercondu…
We report a breakdown of the scaling of the antiferromagnetic transition temperature T_N on chemical doping for Pt-doped CaFe2As2. The substitution of Pt is ineffective in the reduction of antiferromagnetic ordering as well as in inducing superconductivity in Ca(Fe1-xPtx)2As2. The Pt-doped CaFe2As2 that does not exhibit superconductivity will, however, provide us an ideal opportunity to elucidate the role of chemical doping in the occurrence of superconductivity in iron-based materials.
We have computed complete electronic structure of new Pt-doped superconductor CaFe1-xPtxAs2. Our findings for the parent compound of this superconductor agree well with experimental predictions.
Large single crystals of La-doped CaFe2As2 were successfully synthesized by the FeAs self-flux method. The x-ray diffraction patterns suggest high crystalline quality and c-axis orientation. By substitution of trivalent La3+ ions for divalent Ca2+, the resistivity anomaly in the parent compound CaFe2As2 is completely suppressed and a superconducting transition reaches the value of 42.7 K, which is higher than that of about 30 K reported in Saha S. R. et al., arXiv:1105.4798v1. The upper critical field has been determined with the magnetic field along ab-plane and c-axis, yielding an anisotropy…
In rare-earth doped single crystalline CaFe2As2, the mysterious small volume fraction which superconducts up to 49 K, much higher than the bulk Tc ~ 30s K, has prompted a long search for a hidden variable that could enhance the Tc by more than 30% in iron-based superconductors of the same structure. Here we report a chemical, structural, and magnetic study of CaFe2As2 systematically doped with La, Ce, Pr, and Nd. Coincident with the high Tc phase, we find extreme magnetic anisotropy, accompanied by an unexpected doping-independent Tc and equally unexpected superparamagnetic clusters associated…
Aliovalent rare earth substitution into the alkaline earth site of CaFe2As2 single-crystals is used to fine-tune structural, magnetic and electronic properties of this iron-based superconducting system. Neutron and single crystal x-ray scattering experiments indicate that an isostructural collapse of the tetragonal unit cell can be controllably induced at ambient pressures by choice of substituent ion size. This instability is driven by the interlayer As-As anion separation, resulting in an unprecedented thermal expansion coefficient of $180\times 10^{-6}$ K$^{-1}$. Electrical transport and ma…
Ca(Fe$_{1-x}$Rh$_x$)$_2$As$_2$ undergoes successive phase transitions with increasing Rh doping in the $T$ $=$ 0 limit. The antiferromagnetic-metal phase with orthorhombic structure at 0.00 $\le$ $x$ $\le$ 0.020 is driven to a superconducting phase with uncollapsed-tetragonal (ucT) structure at 0.020 $\le$ $x$ $\le$ 0.024; a non-superconducting collapsed-tetragonal (cT) phase takes over at $x$ $\geq$ 0.024. The breakdown of Fermi-liquid transport is observed in the ucT phase above $T_{\rm c}$. In the adjacent cT phase, Fermi-liquid transport is restored along with a disappearance of supercondu…
The effects of pressure generated in a liquid medium, clamp, pressure cell on the in-plane and c-axis resistance, temperature-dependent Hall coefficient and low temperature, magnetoresistance in CaFe2As2 are presented. The T - P phase diagram, including the observation of a complete superconducting transition in resistivity, delineated in earlier studies is found to be highly reproducible. The Hall resistivity and low temperature magnetoresistance are sensitive to different states/phases observed in CaFe2As2. Auxiliary measurements under uniaxial, c-axis, pressure are in general agreement with…
The presence of macroscopic phase separation between the superconducting and magnetic phases in \cfcaf is demonstrated by muon spin rotation (muSR) measurements conducted across their phase boundaries (x=0.05-0.15). The magnetic phase tends to retain the high transition temperature (T_m > T_c), while Co-doping induces strong randomness. The volumetric fraction of superconducting phase is nearly proportional to the Co content $x$ with constant superfluid density. These observations suggest the formation of superconducting "islands" (or domains) associated with Co ions in the Fe$_2$As$_2$ layers…
We use multi-scale techniques to determine the extent of local inhomogeneity and superconductivity in Ca$_{0.86}$Pr$_{0.14}$Fe$_{2}$As$_{2}$ single crystal. The inhomogeneity is manifested as a spatial variation of praseodymium concentration, local density of states, and superconducting order parameter. We show that the high-$T_{c}$ superconductivity emerges from clover-like defects associated with Pr dopants. The highest $T_{c}$ is observed in both the tetragonal and collapsed tetragonal phases, and its filamentary nature is a consequence of non-uniform Pr distribution that develops localized…
The transport properties, phase diagram, and dopant distribution are investigated in systematically Nd doped CaFe_2As_2 single crystals. Coexistence of two superconducting (SC) phases with different critical transition temperature (T_c) is observed. The low-T_c phase emerges as x >= 0.031, and the T_c value increases to its maximum value of about 20 K at x = 0.083, the maximum doping level in our study. As x >= 0.060, the high-T_c phase with a T_c value of about 40 K is observed. The structure transition (STr) from tetragonal to orthorhombic phase vanishes suddenly around x = 0.060, where a ne…
We report the detection of unusual superconductivity up to 49 K in single crystalline CaFe2As2 via electron-doping by partial replacement of Ca by rare-earth. The superconducting transition observed suggests the possible existence of two phases: one starting at ~ 49 K, which has a low critical field ~ 4 Oe, and the other at ~ 21 K, with a much higher critical field > 5 T. Our observations are in strong contrast to previous reports of doping or pressurizing layered compounds AeFe2As2 (or Ae122), where Ae = Ca, Sr or Ba. In Ae122, hole-doping has been previously observed to generate superconduct…
We report the synthesis and basic physical properties of single crystals of CaFe2As2, an isostructural compound to BaFe2As2 which has been recently doped to produce superconductivity. CaFe2As2 crystalizes in the ThCr2Si2 structure with lattice parameters a = 3.907(4) A and c = 11.69(2) A. Magnetic susceptibility, resistivity, and heat capacity all show a first order phase transition at T_0 171 K. The magnetic susceptibility is nearly isotropic from 2 K to 350 K. The heat capacity data gives a Sommerfeld coefficient of 8.2 +- 0.3 mJ/molK2, and does not reveal any evidence for the presence of hi…
The effects of simultaneous Sb doping on the superconductivity of 112-type Ca1-xRExFeAs2 (RE = La, Ce, Pr, and Nd) were studied through measurements of the magnetization and electrical resistivity. In Sb-free materials, the superconducting transition temperature Tc of the La-doped sample was 35 K, while those of the Pr- and Nd-doped samples were ~10 K; no superconductivity was observed in the Ce-doped sample. Sb doping increased the Tc of all RE-doped samples: Tc increased to 47, 43, 43, and 43 K for RE = La, Ce, Pr, and Nd, respectively. We also found that the enhanced superconductivity resul…
In this paper we report the superconductivity above 40 K in the electron doping single crystal Ca1-xRexFe2As2 (Re = La, Ce, Pr). The x-ray diffraction patterns indicate high crystalline quality and c-axis orientation. the resistivity anomaly in the parent compound CaFe2As2 is completely suppressed by partial replacement of Ca by rare earth and a superconducting transition reaches as high as 43 K, which is higher than the value in electron doping FeAs-122 compounds by substituting Fe ions with transition metal, even surpasses the highest values observed in hole doping systems with a transition …
TThe parent compounds of iron-arsenide superconductors, $A$Fe$_{2}$As$_{2}$ ($A$=Ca, Sr, Ba), undergo a tetragonal to orthorhombic structural transition at a temperature $T_{\mathrm{TO}}$ in the range 135 to 205K depending on the alkaline earth element. Below $T_{\mathrm{TO}}$ the free standing crystals split into equally populated structural domains, which mask intrinsic, in-plane, anisotropic properties of the materials. Here we demonstrate a way of mechanically detwinning CaFe$_{2}$As$_{2}$ and BaFe$_{2}$As$_{2}$. The detwinning is nearly complete, as demonstrated by polarized light imaging…
La-substituted CaFeAsH,(Ca1-xLax)FeAsH, were synthesized by the solid state reaction at 1173 K under high pressure of 2.5 GPa. The r-T anomaly corresponding to tetragonal to orthorhombic structural transition was suppressed by La-substitution and then the superconductivity was observed at an electron doping content of Ne > 0.05. The maximum onset Tc of 47.4 K was reached for Ne = 0.18, which is significant increase compared to 23 K achieved by direct electron doping to the FeAs-layer via Co-substitution to the Fe-site. This value of Tc value fits well to the phenomenological relation between T…
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…
The electronic and magnetic properties of the new hydride superconductor CaFeAsH, which superconducts up to 47 K when electron-doped with La, and the isovalent alloy system CaFeAsH$_{1-x}$F$_x$ are investigated using density functional based methods. The $\vec Q = (π,π,0)$ peak of the nesting function $ξ(\vec q)$ is found to be extremely strong and sharp, and additional structure in $ξ(\vec q)$ associated with the near-circular Fermi surfaces (FSs) that may impact low energy excitations is quantified. The unusual band introduced by H, which shows strong dispersion perpendicular to the FeAs lay…
Recently discovered (CaFe$_{1-x}$Pt$_x$As)$_{10}$Pt$_3$As$_8$ and (CaFeAs)$_{10}$Pt$_{4-y}$As$_8$ superconductors are very similar materials having the same elemental composition and structurally similar superconducting FeAs slabs. Yet the maximal critical temperature achieved by changing Pt concentration is approximately twice higher in the latter. Using angle-resolved photoemission spectroscopy(ARPES) we compare the electronic structure of their optimally doped compounds and find drastic differences. Our results highlight the sensitivity of critical temperature to the details of fermiology a…
We report detailed measurements of composition as well as temperature dependence of the phonon density-of-states in a new series of FeAs compounds with composition CaFe1\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12). The composition as well as temperature dependence of phonon spectra for CaFe\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12) compounds have been measured using time of flight IN4C and IN6 spectrometers at ILL, France. The comparison of phonon spectra at 300 K in these compounds shows that acoustic phonon modes up to 12 meV harden in the doped compounds in comparison to the parent CaFeAsF. While inter…
A Neutron Powder Diffraction (NPD) experiment has been performed to investigate the structural phase transition and magnetic order in CaFe1-xCoxAsF superconductor compounds (x = 0, 0.06, 0.12). The parent compound CaFeAsF undergoes a tetragonal to orthorhombic phase transition at 134(3) K, while the magnetic order in form of a spin-density wave (SDW) sets in at 114(3) K. The antiferromagnetic structure of the parent compound has been determined with a unique propagation vector k = (1,0,1) and the Fe saturation moment of 0.49(5)uB aligned along the long a-axis. With increasing Co doping, the lo…
We study antimony doping effects in the iron-based superconductor CaFe(Sb$_{x}$As$_{1-x}$)$_{2}$ by using the first-principle calculation. The calculations reveal that the substitution of the doped antimony atom into As of the chainlike As layers is more stable than that in FeAs layers. This prediction can be checked by experiments. Our results suggest that doping homologous elements into the chainlike As layers existing only in novel 112 system is responsible for rising up the critical temperature. We discuss antimony doping effects on the electronic structure. It is found that the calculated…
We investigated the homogeneous coexistence of spin-density-wave (SDW) and superconducting (SC) states via 75As-nuclear magnetic resonance (NMR) in CaFe(1-x)Co(x)AsF and found that the electronic and magnetic properties of this compound are intermediate between those of LaFeAsO(1-x)F(x) and Ba(Fe(1-x)Co(x))2As2. For 6% Co-doped samples, the paramagnetic spectral weight completely disappears in the crossover regime between the SDW and SC phases followed by the anomalous behavior of relaxation rate (1/T1), implying that the two phases are not segregated. The 59Co-NMR spectra show that spin momen…
We report a combination of Fe K$β$ x-ray emission spectroscopy and $ab$-intio calculations to investigate the correlation between structural and magnetic degrees of freedom in CaFe$_{2}$(As$_{1-x}$P$_{x} $)$_{2}$. The puzzling temperature behavior of the local moment found in rare earth-doped CaFe$_{2}$As$_{2}$ [\textit{H. Gretarsson, et al., Phys. Rev. Lett. {\bf 110}, 047003 (2013)}] is also observed in CaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$. We explain this phenomenon based on first-principles calculations with scaled magnetic interaction. One scaling parameter is sufficient to describe quanti…
CaFeAs2 is a parent compound of recently discovered 112-type iron-based superconductors. It is predicted to be a staggered intercalation compound that naturally integrates both quantum spin Hall insulating and superconducting layers and an ideal system for the realization of Majorana modes. We performed a systematical angle-resolved photoemission spectroscopy and first-principle calculation study of the slightly electron-doped CaFeAs2. We found that the zigzag As chain of 112-type iron-based superconductors play a considerable role in the low-energy electronic structure, resulting in the chara…
Both cuprates and iron-based superconductors demonstrate nematicity, defined as the spontaneous breaking of rotational symmetry in electron systems. The nematic state can play a role in the high-transition-temperature superconductivity of these compounds. However, the microscopic mechanism responsible for the transport anisotropy in iron-based compounds remains debatable. Here, we investigate the electronic anisotropy of CaFeAsF by measuring its interlayer resistivity under magnetic fields with varying field directions. Counterintuitively, the interlayer resistivity was larger in the longitudi…
A well-known feature of CaFe$_{2}$As$_{2}$-based superconductors is the pressure-induced collapsed tetragonal phase that is commonly ascribed to the formation of an interlayer As-As bond. Using detailed X-ray scattering and spectroscopy, we find that Rh-doped Ca$_{0.8}$Sr$_{0.2}$Fe$_{2}$As$_{2}$ does not undergo a first-order phase transition and that local Fe moments persist despite the formation of interlayer As-As bonds. Our density functional theory calculations reveal that the Fe-As bond geometry is critical for stabilizing magnetism and that the pressure-induced drop in the $c$ lattice p…
The effects of anneal-tuning of single-crystalline samples of CaFe$_2$As$_2$ synthesized via a molten Sn flux method are investigated using x-ray diffraction, chemical composition, electrical transport and magnetic susceptibility measurements in order to understand the role of growth conditions on the resultant phase diagram. Previous studies of CaFe$_2$As$_2$ crystals synthesized using a self-flux (FeAs) method revealed an ability to tune the structural and magnetic properties of this system by control of post-synthesis annealing conditions, resulting in an ambient pressure phase diagram that…
Utilizing the high stability of calcium and rare earth hydrides, CaFeAsF1-xHx (x = 0.0-1.0) and SmFeAsO1-xHx (x = 0.0-0.47) have been first synthesized using high pressure to form hydrogen-substituted 1111 type iron-arsenide superconductors. Neutron diffraction and density functional calculations have demonstrated that the hydrogens are incorporated as H- ions occupying F- sites in the blocking layer of CaFeAsF. The resulting CaFeAsF1-xHx is non-superconducting, whereas SmFeAsO1-xHx is a superconductor, with an optimal Tc = 55 K at x 0.2. It was found that up to 40% of the O2- ions can be repl…
High-quality and sizable single crystals are crucial for studying the intrinsic properties of unconventional superconductors, which are lacking in the 1111 phase of the Fe-based superconductors. Here we report the successful growth of CaFeAsF single crystals with the sizes of 1-2 mm using the self-flux method. Owning to the availability of the high-quality single crystals, the structure and transport properties were investigated with a high reliability. The structure was refined by using the single-crystal x-ray diffraction data, which confirms the reports earlier on the basis of powder data. …
Superconductivity research is like running a marathon. Three decades after the discovery of high-Tc cuprates, there have been mass data generated from transport measurements, which bring fruitful information. In this review, we give a brief summary of the intriguing phenomena reported in electron-doped cuprates from the aspect of electrical transport as well as the complementary thermal transport. We attempt to sort out common features of the electron-doped family, e.g. the strange metal, negative magnetoresistance, multiple sign reversals of Hall in mixed state, abnormal Nernst signal, comple…
We present experimental evidence for a dramatic suppression of the phononic thermal conductivity of rare earth and Sr doped La_2CuO_4. Remarkably, this suppression correlates with the occurrence of superconductivity. Conventional models for the phonon heat transport fail to explain these results. In contrast, a straightforward explanation is possible in terms of static and dynamic stripe correlations of holes and spins.
Systematic analysis of the planar resistivity, Hall effect and cotangent of the Hall angle for the electron-doped cuprates reveals underlying Fermi-liquid behavior even deep in the antiferromagnetic part of the phase diagram. The transport scattering rate exhibits a quadratic temperature dependence, and is nearly independent of doping, compound and carrier type (electrons vs. holes), and hence universal. Our analysis moreover indicates that the material-specific resistivity upturn at low temperatures and low doping has the same origin in both electron- and hole-doped cuprates.
Recent nuclear magnetic resonance (NMR) measurements revealed the coexistence of stripe-type antiferromagnetic (AFM) and ferromagnetic (FM) spin correlations in both the hole- and electron-doped BaFe$_2$As$_2$ families of iron-pnictide superconductors by a Korringa ratio analysis. Motivated by the NMR work, we investigate the possible existence of FM fluctuations in another iron pnictide superconducting family, Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$. We re-analyzed our previously reported data in terms of the Korringa ratio and found clear evidence for the coexistence of stripe-type AFM and FM spin co…
Platelet-like single crystals of the Ca(Fe1-xCox)2As2 series having lateral dimensions up to 15 mm and thickness up to 0.5 mm were obtained from the high temperature solution growth technique using Sn flux. Upon Co doping, the c-axis of the tetragonal unit cell decreases, while the a-axis shows a less significant variation. Pristine CaFe2As2 shows a combined spin-density-wave and structural transition near T = 166 K which gradually shifts to lower temperatures and splits with increasing Co-doping. Both transitions terminate abruptly at a critical Co-concentration of xc = 0.075. For x \geq 0.05…
Resistivity and magnetization measurements are used for studying the transverse sliding of AF domain boundaries in lightly doped La_{2-x}Sr_{x}CuO_{4}. We discuss that it is the freezing of the transverse boundary motion that is responsible for the appearance of ``spin-glass'' features at low temperatures.
The charge transport of electron doped Mott insulators on a triangular lattice is investigated within the t-J model based on the partial charge-spin separation fermion-spin theory. The conductivity spectrum shows a low-energy peak and the unusual midinfrared band, while the resistivity is characterized by a crossover from the high temperature metallic-like to the low temperature insulating-like behavior, in qualitative agreement with experiments. Our results also show that the mechanism that cause this unusual charge transport is closely related with a competition between the kinetic energy an…
Within the t-J model, the heat transport of electron-doped cobaltates is studied based on the fermion-spin theory. It is shown that the temperature dependent thermal conductivity is characterized by the low temperature peak located at a finite temperature. The thermal conductivity increases monotonously with increasing temperature at low temperatures T $<$ 0.1$J$, and then decreases with increasing temperature for higher temperatures T $>$ 0.1$J$, in qualitative agreement with experimental result observed from Na$_{x}$CoO$_{2}$ .
Behavior of ρ_{ab}(T) and R_{H}(T) is presented for LSCO and YBCO single crystals in the lightly hole-doped antiferromagnetic region, with an emphasis on the R_{H}(T) data. In both systems, R_{H} is virtually constant at moderate temperatures and tends to increase at low temperatures. Since essentially the same behavior of ρ_{ab}(T) and R_{H}(T) is observed in both LSCO and YBCO, we discuss that the in-plane charge transport properties are universal among the cuprates in the lightly-doped regime and that the R_{H}(T) data we obtained represent the genuine behavior of the Hall effect in this re…
The 2D lattice gas model with competing short and long range interactions is appliedused for calculation of the incoherent charge transport in the classical strongly-correlated charge segregated polaronic state. We show, by means of Monte-Carlo simulations, that at high temperature the transport is dominated by hopping of the dissociated correlated polarons, where with thetheir mobility is inversely proportional to the temperature. At the temperatures below the clustering transition temperature the bipolaron transport becomes dominant. The energy barrier for the bipolaron hopping is determined…
In-plane and out-of-plane thermal conductivities (κ_{ab} and κ_{c}) are measured on single crystals of pure, 1%-hole-doped, and 1%-Zn-doped La_{2}CuO_{4}. The roles of magnons and the spin stripes in the heat transport in these samples are discussed. Comparison with the heat transport in CuGeO_{3}, which shows similar κ(T) behavior as that of La_{2}CuO_{4}, gives us a lesson of how the heat transport can probe the difference in the spin ground state.
Both the in-plane and the out-of-plane resistivities (ρ_{ab} and ρ_{c}) are measured in high-quality La_{2-x}Sr_{x}CuO_{4} (LSCO) single crystals in the lightly- to moderately-doped region, x = 0.01 to 0.10, and the resistivity anisotropy is determined. In all the samples studied, the anisotropy ratio ρ_{c}/ρ_{ab} quickly increases with decreasing temperature, although in non-superconducting samples the strong localization effect causes ρ_{c}/ρ_{ab} to decrease at low temperatures. Most notably, it is found that ρ_{c}/ρ_{ab} at moderate temperatures (100 - 300 K) is almost completely independe…
In order to separate electronic and phonon mechanisms of thermal transport in high temperature superconducting Bi2212, we measured thermal conductivity of single crystals of Y-doped Bi2212, an insulating analogue of pure Bi2212, in the temperature range between 40 mK and 2 K. Temperature dependence of the thermal conductivity of insulating samples changes from T$^{1.7}$ between 1 and 2 K to $T^{2.6}$ below 200 mK, indicating saturation of the phonon mean free path with decreasing temperature. Overall, thermal conductivity of the insulating samples (which is due entirely to phonons) is strongly…
Detailed behavior of the magnetoresistance (MR) is studied in lightly doped antiferromagnetic La_{1.99}Sr_{0.01}CuO_{4}, where, thanks to the weak ferromagnetic moment due to spin canting, the antiferromagnetic (AF) domain structure can be manipulated by the magnetic field. The MR behavior demonstrates that CuO_2 planes indeed contain anti-phase AF domain boundaries in which charges are confined, forming anti-phase stripes. The data suggest that a high magnetic field turns the anti-phase stripes into in-phase stripes, and the latter appear to give better conduction than the former, which chall…
We report a systematic study of normal state Nernst effect in the electron-doped cuprates Pr$_{2-x}$Ce$_x$CuO$_{4-δ}$ over a wide range of doping (0.05$\leq x \leq$0.21) and temperature. At low temperatures, we observed a notable vortex Nernst signal above T$_c$ in the underdoped films, but no such normal state vortex Nernst signal is found in the overdoped region. The superconducting fluctuations in the underdoped region are most likely incoherent phase fluctuations as found in hole-doped cuprates. At high temperatures, a large normal state Nernst signal is found at dopings from slightly unde…
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 pressure for underdoped FeAs-pnictides, remains approximately constant for optimal doping, and decreases linearly in the overdoped range. The undoped LaOFeA…
We report that intrinsic transport and magnetic properties, and their anisotropy from high quality single crystal $CaFe_2As_2$. The resistivity anisotropy ($ρ_c/ρ_{ab}$) is $\sim 50 $, and less than 150 of $BaFe_2As_2$, which arises from the strong coupling along c-axis due to an apparent contraction of about 0.13 nm compared to $BaFe_2As_2$. Temperature independent anisotropy indicates that the transport in ab plane and along c-axis direction shares the same scattering mechanism. In sharp contrast to the case of parent compounds $ROFeAs$ (R=rare earth) and $MFe_2As_2$ (M=Ba and Sr), spin-dens…
We studied the transport properties of meso-junctions of semiconducting (Sm) highly doped Si with different superconductors (Sc) through point contact Andreev reflection (PCAR) spectroscopy. Spectra of low transparency point contacts between Si and In showed an enhancement in the superconducting energy gap of In. This was due to the effect of an additional gap arising from the Schottky barrier at the Sm-Sc interface. For higher transparency Si-Nb and Si-Pb point contacts, no gap enhancement was observed though there were weak sub gap features. These were due to proximity induced interface supe…
Measurements of longitudinal and transverse transport coefficients were performed in the normal state of a high-quality single crystal of optimally-doped superconductor YBa2Cu3O6.95. We found the Hall-Lorenz number (Lxy) depends only weakly on temperature, thus no appreciable violation of the Wiedemann-Franz law has been observed. However, Lxy is about two times larger than the Sommerfeld's value of the Lorenz number. Our results can be interpreted consistently if we assume that the electronic system of YBa2Cu3O6.95 behaves like a Fermi-liquid with a pseudo-gap that opens at the Fermi level.
Single crystalline CaFe2As2 and (Ca1-xNax)Fe2As2 polycrystals (0 < x < 0.66) are synthesized and characterized using structural, magnetic, electronic transport, and heat capacity measurements. These measurements show that the structural/magnetic phase transition in CaFe2As2 at 165 K is monotonically suppressed by the Na doping and that superconductivity can be realized over a wide doping region. For 0.3 < x < 0.36, the magnetic susceptibilities indicate the possible coexistence of the spin density wave (SDW) and superconductivity. Superconducting phases corresponding to the Na doping level in …
In order to clarify the origin of anomalous superconductivity in (Ca,RE)Fe2As2 system, Pr doped and Pr,Co co-doped CaFe2As2 single crystals were grown by the FeAs flux method. These samples showed two-step superconducting transition with Tc1 = 25~42 K, and Tc2 < 16 K, suggesting that (Ca,RE)Fe2As2 system has two superconducting components. Post-annealing performed for these crystals in evacuated quartz ampoules at various temperatures revealed that post-annealing at ~400°C increased the c-axis length for all samples. This indicates that as-grown crystals have a certain level of strain, which i…
Fe-clad BaxK1-xFe2As2 superconducting tapes were fabricated by the ex situ powder-in-tube method combined with a short high-temperature annealing technique. The effect of annealing time and different dopants on the transport properties of the BaxK1-xFe2As2 tapes were systematically studied. By co-doping with Ag and Pb, the transport critical current density Jc of BaxK1-xFe2As2 tapes was significantly improved in whole field region and the highest transport Jc was up to 1.4x10^4 A/cm^2 (Ic=100 A) at 4.2K in self field. It is proposed that the superior Jc in the co-doped samples are due to the c…
We report resistivity, Hall coefficient, current-voltage characteristics, and magneto-optical imaging measurements of epitaxial Co-doped BaFe_{2}As_{2} thin films deposited on MgO(001) substrate. The Hall resistivity of the films has a substantial contribution arising from anomalous Hall effect of ferromagnetic components. The critical current density (J_{c}) of the films is ~2 MA/cm^{2} at low temperatures. Differential magneto-optical images of the remanent state give similar J_{c} values and also exhibit presence of extended defects in the film.
Thermal transport measurements have been performed on single-crystalline Co-doped BaFe2As2 down to 0.1 K and under magnetic fields up to 7 T. Significant peak anomalies are observed in both thermal conductivity and thermal Hall conductivity below Tc as an indication of the enhancement of the quasiparticle mean-free path. Moreover, we find a sizable residual T-linear term in thermal conductivity, possibly due to a finite quasiparticle density of states in the superconducting gap induced by impurity pair breaking. Our findings support a pairing symmetry compatible with the theoretically predicte…
We have measured the thermoelectric power, S, and resistivity, rho, of pure and Al-doped MgB2. S is positive and increases linearly with temperature above the superconducting transition temperature, Tc. Deviations from the linear dependence appear at higher temperature, T>T0 approx 160 K. Tc and T0 both decrease with Al doping whereas the slope of S(T) in the linear range increases with the Al content. The data are discussed in terms of doping induced changes of the Fermi surface and the density of states at the Fermi level.
