We performed nuclear magnetic resonance (NMR) measurements to investigate the evolution of spin-density-wave (SDW) and superconducting (SC) states upon electron doping in CaFe_{1-x}Co_{x}AsF, which exhibits an intermediate phase diagram between those of LaFeAsO_{1-x}F_x and Ba(Fe_{1-x}Co_x)_2As_2. We found that homogeneous coexistence of the incommensurate SDW and SC states occurs only in a narrow doping region around the crossover regime, which supports S_{+-}-wave symmetry. However, only the structural phase transition survives upon further doping, which agrees with predictions from orbital fluctuation theory. The transitional features upon electron doping imply that both spin and orbital fluctuations are involved in the superconducting mechanism.
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…
Low-temperature specific heat (SH) is measured on the 1111-type CaFe_{0.88}Co_{0.12}AsF single crystals under different magnetic fields. A clear SH jump with the height ΔC/T|_Tc = 10.4 mJ/mol K^2 was observed at the superconducting transition temperature T_c. The electronic SH coefficient Δγ(B) increases linearly with the field below 5 T and a kink is observed around 5 T, indicating a multi-gap feature in the present system. Such a sign is also reflected in the Tc-B data. A detailed analysis shows that this behavior can be interpreted in terms of a two-gap scenario with the ratio Δ_L=Δ_S = 2:8…
Gap symmetry and structure are crucial issues in understanding the superconducting mechanism of unconventional superconductors. Here we report an in-depth investigation on the out-of-plane lower critical field $H_{c1}^{c}$ of fluorine-based 1111 system superconductor CaFe$_{0.88}$Co$_{0.12}$AsF with $T_c$ = 21 K. A pronounced two-gap feature is revealed by the kink in the temperature dependent $H_{c1}^c(T)$ curve. The magnitudes of the two gaps are determined to be $Δ_1$ = 0.86 meV and $Δ_2$ = 4.48 meV, which account for 74% and 26% of the total superfluid density respectively. Our results sug…
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…
Millimeter sized single crystals of CaFe$_{1-x}$Co$_x$AsF were grown using a self-flux method. It is found that high-quality single crystals can be grown from three approaches with different initial raw materials. The chemical compositions and crystal structure were characterized carefully. The c-axis lattice constant is suppressed by the Co substitution. Superconductivity with the critical transition $T_c$ as high as 21 K was confirmed by both the resistivity and magnetic susceptibility measurements in the sample with $x$ = 0.12. Moreover, it is found that $T_c$ can be enhanced for about 1 K …
The anisotropy of the Fe-based superconductors is much smaller than that of the cuprates and the theoretical calculations. A credible understanding for this experimental fact is still lacking up to now. Here we experimentally study the magnetic-field-angle dependence of electronic resistivity in the superconducting phase of iron-based superconductor CaFe$_{0.882}$Co$_{0.118}$AsF, and find the strongest anisotropy effect of the upper critical field among the iron-based superconductors based on the framework of Ginzburg-Landau theory. The evidences of energy band structure and charge density dis…
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…
The relationship between antiferromagnetic spin fluctuations and superconductivity has become a central topic of research in studies of superconductivity in the iron pnictides. We present unambiguous evidence of the absence of magnetic fluctuations in the non-superconducting collapsed tetragonal phase of CaFe2As2 via inelastic neutron scattering time-of-flight data, which is consistent with the view that spin fluctuations are a necessary ingredient for unconventional superconductivity in the iron pnictides. We demonstrate that the collapsed tetragonal phase of CaFe2As2 is non-magnetic, and dis…
Co-doping of lanthanum and phosphorus in CaFe2As2 induces superconductivity at 45 K. This superconducting transition temperature is higher than the 38 K transition in Ba1-xKxFe2As2, which is the maximum found thus far among the 122 phases. Superconductivity with a substantial shielding volume fraction was observed at 0.12 $\leq$ x $\leq$ 0.18 and y = 0.06 in Ca1-xLaxFe2(As1-yPy)2. The superconducting phase of the present system seems to be not adjacent to an antiferromagnetic phase.
We report $^{75}$As NMR measurements in CaFe$_2$As$_2$, made under applied pressures up to 0.83 GPa produced by a standard clamp pressure cell. Our data reveal phase segregation of paramagnetic (PM) and antiferromagnetic (AFM) phases over a range of pressures, with the AFM phase more than 90 % dominant at low temperatures. In situ RF susceptibility measurements indicate the presence of superconductivity. $^{75}$As spin-lattice relaxation experiments indicate that the $^{75}$As nuclei sample the superconductivity while in the magnetically-ordered environment.
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 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…
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…
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…
We examined doping effect of 3d transition metal elements (TM: Cr, Mn, Co, Ni, and Cu) at the Fe site of a quaternary fluoroarsenide CaFeAsF, an analogue of 1111-type parent compound LaFeAsO. The anomaly at ~120 K observed in resistivity (rho) vs. temperature (T) plot for the parent compound is suppressed by the doping of each TM element. Furthermore, Co- and Ni-doping (CaFe1-xTMxAsF, TM = Co, Ni) induces superconductivity with a transition temperature maximized at the nominal x = 0.10 for Co (22 K) and at x = 0.05 for Ni (12 K). These optimal doping levels may be understood by considering tha…
In this paper we report the synthesis of iron-based superconductors CaFe2-xRhxAs2 using one-step solid state reaction method, which crystallizes in the ThCr2Si2-type structure with a space group I4/mmm. The systematic evolution of the lattice constants demonstrates that the Fe ions are successfully replaced by the Rh. By increasing the doping content of Rh, the spin-density-wave (SDW) transition in the parent compound is suppressed and superconductivity emerges. The maximum superconducting transition temperature is found at 18.5 K with the doping level of x = 0.15. The temperature dependence o…
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 predict that CaFeAs$_2$, a newly discovered iron-based high temperature (T$_c$) superconductor, is a staggered intercalation compound that integrates topological quantum spin hall (QSH) and superconductivity (SC). CaFeAs$_2$ has a structure with staggered CaAs and FeAs layers. While the FeAs layers are known to be responsible for high T$_c$ superconductivity, we show that with spin orbital coupling each CaAs layer is a $Z_{2}$ topologically nontrivial two-dimensional QSH insulator and the bulk is a 3-dimensional weak topological insulator. In the superconducting state, the edge states in th…
75As-zero-field nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements are performed on CaFe2As2 under pressure. At P = 4.7 and 10.8 kbar, the temperature dependences of nuclear-spin-lattice relaxation rate (1/T1) measured in the tetragonal phase show no coherence peak just below Tc(P) and decrease with decreasing temperature. The superconductivity is gapless at P = 4.7 kbar but evolves to that with multiple gaps at P = 10.8 kbar. We find that the superconductivity appears near a quantum critical point under pressures in the range 4.7 kbar < P < 10.8 kbar. Both el…
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…
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…
We present a detailed study of the combined effects of Sr substitution and hydrostatic pressure on the ground-state properties of CaFe$_2$As$_2$. Measurements of the electrical resistance and magnetic susceptibility, both at ambient and finite pressure $P$ $\leq$ 2$\,$GPa, were performed on Ca$_{1-x}$Sr$_x$Fe$_2$As$_2$ single crystals grown out of Sn flux. We find that upon Sr substitution the ranges of stability of both the structural-magnetic transition and the transition into the non-magnetic collapsed tetragonal phase are shifted to higher pressure levels with the latter moving at a higher…
Magnetization, susceptibility, specific heat, resistivity, neutron and x-ray diffraction have been used to characterize the properties of single crystalline CaFe2-xNixAs2 as a function of Ni doping for x varying from 0 to 0.1. The combined first-order structural and magnetic phase transitions occur together in the undoped system at 172 K, with a small decrease in the area of the a-b plane along with an abrupt increase in the length of the c-axis in the orthorhombic phase. With increasing x the ordered moment and transition temperature decrease, but the transition remains sharp at modest doping…
CaFe$_2$As$_2$ has been found to be exceptionally sensitive to the application of hydrostatic pressure and superconductivity has been found to exist in a narrow pressure region that appears to be at the interface between two different phase transitions. The pressure - temperature ($P - T$) phase diagram of CaFe$_2$As$_2$ reveals that this stoichiometric, highly ordered, compound can be easily tuned to reveal all the salient features associated with FeAs-based superconductivity without introducing any disorder. Whereas at ambient pressure CaFe$_2$As$_2$ does not superconduct for $T > 1.8$ K and…
Fe-based superconductors have drawn much attention during the last decade due to the finding of superconductivity in materials containing the magnetic element, Fe, and the coexistence of superconductivity & magnetism. Extensive study of the electronic structure of these systems suggested dominant role of d states in their electronic properties, whereas the cuprate superconductors show major role of the ligand derived states. In this article, we review some of our results on the electronic structure of these fascinating systems employing high resolution photoemission spectroscopy. The combined …
Resistivity, magnetization and microscopic $^{75}$As nuclear magnetic resonance (NMR) measurements in the antiferromagnetically ordered state of the iron-based superconductor parent material CaFe$_2$As$_2$ exhibit anomalous features that are consistent with the collective freezing of domain walls. Below $T^*\approx 10$ K, the resistivity exhibits a peak and downturn, the bulk magnetization exhibits a sharp increase, and $^{75}$As NMR measurements reveal the presence of slow fluctuations of the hyperfine field. These features in both the charge and spin response are strongly field dependent, ar…
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 …
Imaging local magnetic field distribution in a single crystal of superconductor CaFe1.94Co0.06As2 shows the presence of anomalous remnant magnetization within Meissner like regions of the sample. Close to the superconducting transition temperature we find a coexistence of superconductivity and magnetic correlations. While the magnetic correlations are enhanced with the lowering of temperature, interestingly the area over which the magnetic phase exists shrinks at the expense of a growing superconducting phase in the sample. The coexistence of two phases deep inside the superconducting phase in…
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…
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…
The interplay between structural phase and electronic correlations has been an intriguing topic of research. An prominent example is the pressure-induced uncollapsed to collapsed tetragonal phase transition observed in CaFe$_2$As$_2$, which is accompanied with the emergence of superconductivity in the collapsed phase. Recently, a very similar structural phase transition was discovered in LaFe$_2$As$_2$, but in contrast to CaFe$_2$As$_2$, superconductivity was only observed in the uncollapsed phase, not the collapsed phase. Previous studies have attributed this puzzling observation to the diffe…
High-pressure superconductivity in a rare-earth doped Ca0.86Pr0.14Fe2As2 single crystalline sample has been studied up to 12 GPa and temperatures down to 11 K using designer diamond anvil cell under a quasi-hydrostatic pressure medium. The electrical resistance measurements were complemented by high pressure and low temperature x-ray diffraction studies at a synchrotron source. The electrical resistance measurements show an intriguing observation of superconductivity under pressure, with Tc as high as ~51 K at 1.9 GPa, presenting the highest Tc reported in the intermetallic class of 1-2-2 iron…
Hydrogen is the simplest bipolar element and its valence state can be controlled from +1 to -1. We synthesized the 1111-type CaFeAsH and LnFeAsO1-xHx (Ln = lanthanide) with the ZrCuSiAs type structure by a high-pressure synthesis method. The position and valence state of the substituted H were determined by neutron diffraction and density functional theory calculations. The close similarity in the structural and electrical properties of CaFeAsH and CaFeAsF indicated the formation of the hydride ion (H-), which is isovalent with the fluoride ion (F-), in the 1111-type iron arsenides. When some …
The temperature-dependence of the in-plane optical properties of (CaFe$_{1-x}$Pt$_{x}$As)$_{10}$Pt$_{3}$As$_{8}$ have been investigated for the undoped ($x=$0) parent compound, and the optimally-doped ($x=$0.1) superconducting material ($T_{c}\simeq$ 12 K) over a wide frequency range. The optical conductivity has been described using two free-carrier (Drude) components, in combination with oscillators to describe interband transitions. At room temperature, the parent compound may be described by a strong, broad Drude term, as well as a narrow, weaker Drude component. Below the structural and m…
The recent discovery and subsequent developments of FeAs-based superconductors have presented novel challenges and opportunities in the quest for superconducting mechanisms in correlated-electron systems. Central issues of ongoing studies include interplay between superconductivity and magnetism as well as the nature of the pairing symmetry reflected in the superconducting energy gap. In the cuprate and RE(O,F)FeAs (RE = rare earth) systems, the superconducting phase appears without being accompanied by static magnetic order, except for narrow phase-separated regions at the border of phase bou…
Coupling among conduction electrons (e.g. Zhang-Rice singlet) are often manifested in the core level spectra of exotic materials such as cuprate superconductors, manganites, etc. These states are believed to play key roles in the ground state properties and appear as low binding energy features. To explore such possibilities in the Fe-based systems, we study the core level spectra of a superconductor, CaFe$_{1.9}$Co$_{0.1}$As$_2$ (CaCo122) in the CaFe$_2$As$_2$ (Ca122) family employing high-resolution hard $x$-ray photoemission spectroscopy. While As core levels show almost no change with dopi…
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…
Iron-based superconductors Ba0.7Rb0.3Fe2As2 and CaFe1.92Co0.08As2 of the '122' family have been investigated by means of the 14.41-keV Moessbauer transition in 57Fe versus temperature ranging from the room temperature till 4.2 K. A comparison is made with the previously investigated parent compounds BaFe2As2 and CaFe2As2. It has been found that Moessbauer spectra of these superconductors are composed of the magnetically split component due to development of spin density wave (SDW) and non-magnetic component surviving even at lowest temperatures. The latter component is responsible for supercon…
Recent investigations of the superconducting iron-arsenide families have highlighted the role of pressure, be it chemical or mechanical, in fostering superconductivity. Here we report that CaFe2As2 undergoes a pressure-induced transition to a non-magnetic, volume "collapsed" tetragonal phase, which becomes superconducting at lower temperature. Spin-polarized total-energy calculations on the collapsed structure reveal that the magnetic Fe moment itself collapses, consistent with the absence of magnetic order in neutron diffraction.
The antiferromagnet CaFe$_2$As$_2$ does not become superconducting when subject to ideal hydrostatic pressure conditions, where crystallographic and magnetic states also are well defined. By measuring electrical resistivity and magnetic susceptibility under quasi-hydrostatic pressure, however, we find that a substantial volume fraction of the sample is superconducting in a narrow pressure range where collapsed tetragonal and orthorhombic structures coexist. At higher pressures, the collapsed tetragonal structure is stabilized, with the boundary between this structure and the phase of coexistin…
Single crystals of Ca(Fe1-xRux)2As2 (0<x<0.065) and Ca1-yLay(Fe0.973Ru0.027)2As2 (0<y<0.2) have been synthesized and studied with respect to their structural, electronic and magnetic properties. The partial substitution of Fe by Ru induces a decrease of the c-axis constant leading for x<0.023 to a suppression of the coupled magnetic and structural (tetragonal to orthorhombic) transitions. At x_cr=0.023 a first order transition to a collapsed tetragonal (CT) phase is found, which behaves like a Fermi liquid and which is stabilized by further increase of x. The absence of superconductivity near …
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…
The observation of non-bulk superconductivity with an unexpectedly high onset transition temperature Tc up to ~ 49 K in non-superconducting single crystalline CaFe2As2 upon rare-earth doping has raised interesting questions concerning its origin. Several possibilities including interfacial mechanism have been proposed to account for the unusual observation. In an attempt to differentiate such propositions, we have carried out a systematic compositional and magnetic study on single crystals of nominal (Ca1-xPrx)Fe2As2 with 0 <= x <= 0.13 throughout the solubility range of Pr, which covers both …
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 …
We use inelastic neutron scattering to show that the spin waves in the iron chalcogenide Fe$_{1.05}$Te display novel dispersion clearly different from those in the related iron pnictide systems. By fitting the spin waves to a Heisenberg Hamiltonian, we extract magnetic exchange couplings that are dramatically different from both predictions by density functional calculations and measurements on the iron pnictide CaFe$_2$As$_2$. While the nearest-neighbor exchange couplings in CaFe$_2$As$_2$ and Fe$_{1.05}$Te are quite different, their next-nearest-neighbor exchange couplings are similar. These…
Superconductivity in Iron-Arsenic based pnictides emerges in close proximity to an antiferromagnetic (AFM)ordered parent state and the AFM phase overlaps with superconducting (SC) phase in some pnictides for certain range of doping. CaFe2-xNixAs2 belongs to this category, where both the phases overlap. Here we use scanning tunneling microscopy and spectroscopy to investigate the local electronic properties of uderdoped CaFe1.96Ni0.04As2 single crystals in the vicinity of the boundary of the two phases. Both resistivity and magnetic measurements show that a tiny portion (~ 1.2 %) of this compou…
To explore the origin of the unusual non-bulk superconductivity with a Tc up to 49 K reported in the rare-earth-doped CaFe2As2 , the chemical composition, magnetization, specific heat, resistivity, and annealing effect are systematically investigated on nominal (Ca1-xRx)Fe2As2 single crystals with different x's and R = La, Ce, Pr, and Nd. All display a doping-independent Tc once superconductivity is induced, a doping-dependent low field superconducting volume fraction f, and a large magnetic anisotropy η in the superconducting state, suggesting a rather inhomogeneous superconducting state in a…
Antiferromagnetism is relevant to high temperature (high-Tc) superconductivity because copper oxide and iron arsenide high-Tc superconductors arise from electron- or hole-doping of their antiferromagnetic (AF) ordered parent compounds. There are two broad classes of explanation for the phenomenon of antiferromagnetism: in the local moment picture, appropriate for the insulating copper oxides, AF interactions are well described by a Heisenberg Hamiltonian; while in the itinerant model, suitable for metallic chromium, AF order arises from quasiparticle excitations of a nested Fermi surface. Ther…
Using various transport and magnetotransport probes we study the coexistence of spin-density wave and superconductor states in (TMTSF)2ClO4 at various degrees of ClO4 anions ordering. In the two-phase complex state when both superconductivity and spin-density wave are observed in transport, we find prehistory effects, enhancement of the superconducting critical field, and strong spatial anisotropy of the superconducting state. These features are inconsistent with the conventional model of structural inhomogeneities produced by anion ordering transition. We reveal instead that superconductor an…
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…
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…
From first principles calculations we unravel surprisingly strong interactions between arsenic ions in iron-pnictides, the strength of which is controlled by the Fe-spin state. Reducing the Fe-magnetic moment, weakens the Fe-As bonding, and in turn, increases As-As interactions, causing giant reduction in the c-axis. For CaFe$_2$As$_2$ system, this reduction is as large as 1.4 Å. Since the large c-reduction has been recently observed only under high-pressure\cite{cTphase}, our results suggest that the iron magnetic moment should be present in Fe-pnictides at all times at ambient pressure. Fina…
We study theoretically the shift of the superconducting transition temperature (Tc) under uniaxial compression in beta-type organic superconductors, beta-(BEDT-TTF)2I3 and beta-(BDA-TTP)2X[X=SbF6,AsF6], in order to clarify the electron correlation, the spin frustration and the effect of dimerization. The transfer integrals are calculated by the extended Huckel method assuming the uniaxial strain and the superconducting state mediated by the spin fluctuation is solved using Eliashberg's equation with the fluctuation-exchange approximation. The calculation is carried out on both the dimerized (o…
We present a calorimetric study on single crystals of Ca(Fe1-xCox)2As2 (x = 0, 0.032, 0.051, 0.056, 0.063, and 0.146). The combined first order spin-density wave/structural transition occurs in the parent CaFe2As2 compound at 168 K and gradually shifts to lower temperature for low doping levels (x = 0.032 and x = 0.051). It is completely suppressed upon higher doping x = 0.056. Simultaneously, superconductivity appears at lower temperature with a transition temperature around Tc = 14.1 K for Ca(Fe0.937Co0.063)2As2. The phase diagram of Ca(Fe0.937Co0.063)2As2 has been derived and the upper crit…
The presence of macroscopic phase separation into superconducting and magnetic phases in LaFeAsO$_{1-x}$F$_x$ (LFAOF) and CaFe$_{1-x}$Co$_x$AsF (CFCAF) is demonstrated by muon spin rotation (muSR) measurement across their phase boundaries ($x=0.06$ for LFAOF and $x=0.075$--0.15 for CFCAF). In LFAOF, both magnetism and superconductivity develop simultaneously below a common critical temperature, $T_m =~ T_c =~ 18$ K, where the magnetism is characterized by strong randomness. A similar, but more distinct segregation of these two phases is observed in CFCAF, where the magnetic phase retains $T_m$…
We report a systematic study of structural and transport properties in single crystals of Ba(Fe_(1-x)Ru_x)_2As_2 for x ranging from 0 to 0.5. The isovalent substitution of Fe by Ru leads to an increase of the a parameter and a decrease of the c parameter, resulting in a strong increase of the AsFeAs angle and a decrease of the As height above the Fe planes. Upon Ru substitution, the magnetic order is progressively suppressed and superconductivity emerges for x > 0.15, with an optimal Tc ~ 20K at x = 0.35 and coexistence of magnetism and superconductivity between these two Ru contents. Moreover…
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 use high resolution angle resolved photoemission spectroscopy and density functional theory with experimentally obtained crystal structure parameters to study the electronic properties of CaKFe4As4. In contrast to related CaFe2As2 compounds, CaKFe4As4 has high Tc of 35K at stochiometric composition. This presents unique opportunity to study properties of high temperature superconductivity of iron arsenic superconductors in absence of doping or substitution. The Fermi surface consists of three hole pockets at $Γ$ and two electron pockets at the $M$ point. We find that the values of the super…
