In this paper we report a detailed $μ^{+}$SR and {}$^{19}$F-NMR study of the La$_{0.7}$Y$_{0.3}$FeAsO$_{1-x}$F$_{x}$ class of materials. Here, the diamagnetic La$_{1-y}$Y$_{y}$ substitution increases chemical pressure and, accordingly, sizeably enhances the optimal superconducting transition temperature. We investigate the magnetic-superconducting phase transition by keeping the Y content constant ($y = 0.3$) and by varying the F content in the range $0.025 \leq x \leq 0.15$. Our results show how magnetism and superconductivity coexist for $x = 0.065$. Such coexistence is due to segregation of the two phases in macroscopic regions, resembling what was observed in LaFeAsO$_{1-x}$F$_{x}$ materials under applied hydrostatic pressure. This scenario is qualitatively different from the nanoscopic coexistence of the two order parameters observed when La is fully substituted by magnetic rare-earth ions like Sm or Ce.
The Sm$_{0.95}$La$_{0.05}$FeAsO$_{0.85}$F$_{0.15}$ compound is a quasi-2D layered superconductor with a superconducting transition temperature T$_c$ = 52 K. Due to the Fe spin-orbital related anisotropic exchange coupling (antiferromagnetic or ferromagnetic fluctuation), the tetragonal microcrystalline powder can be aligned at room temperature using the field-rotation method where the tetragonal $\it{ab}$-plane is parallel to the aligned magnetic field B$_{a}$ and $\it{c}$-axis along the rotation axis. Anisotropic superconducting properties with anisotropic diamagnetic ratio $χ_{c}/χ_{ab}\sim$…
The structure and electronic density of states in layered LnFeAsO$_{1-x}$F$_x$ (Ln=La,Sm; $x$=0.0, 0.125, 0.25) are investigated using density functional theory. For the $x$=0.0 system we predict a complex potential energy surface, formed by close-lying single-well and double-well potentials, which gives rise to the tetragonal-to-orthorhombic structural transition, appearance of the magnetic order, and an anomaly in the specific heat capacity observed experimentally at temperatures below $\sim$140--160 K. We propose a mechanism for these transitions and suggest that these phenomena are generic…
Superconducting transition under pressure ($P$) has been investigated for optimum-doped $R$FeAsO$_{\rm 1-x}$F$_{\rm x}$ ($R$=La, Ce$-$Sm) by dc magnetization measurements. For $R$=La, $T_{\rm c}$ is found to be pressure independent up to $P$$\sim$3.0 GPa and then shows a monotonic decrease with increasing pressure. The plateau width decreases for the system with smaller lattice constants, and shrinks to almost zero for $R$=Sm. From the $T_{\rm c}$($P$) data, we construct the $T_{\rm c}$ evolution map on the height of the As atom from the Fe-plane $h_{\rm As}$ versus lattice constant $a$ or $c$…
$^{19}$F NMR measurements in optimally electron-doped La$_{1-y}$Y$_{y}$Fe$_{1-x}$Mn$_{x}$AsO$_{0.89}$F$_{0.11}$ superconductors are presented. In these materials the effect of Mn doping on the superconducting phase is studied for two series of compounds ($y= 0$ and $y= 0.2$) where the chemical pressure is varied by substituting La with Y. In the $y=0.2$ series superconductivity is suppressed for Mn contents an order of magnitude larger than for the $y=0$ series. For both series a peak in the $^{19}$F NMR nuclear spin-lattice relaxation rate $1/T_1$ emerges upon Mn doping and gets significantly…
The recent realisations of hydrogen doped $Ln$FeAsO ($Ln$=Nd and Sm) superconducting epitaxial thin films call for further investigation of their structural and electrical transport properties. Here, we report on the microstructure of a NdFeAs(O,H) epitaxial thin film and its temperature, field, and orientation dependencies of the resistivity and the critical current density $J_{\rm c}$. The superconducting transition temperature $T_{\rm c}$ is comparable to NdFeAs(O,F). Transmission electron microscopy investigation supported that hydrogen is homogenously substituted for oxygen. A high self-f…
Here we report pressure effect on superconducting transition temperature (Tc) of ReFeAsO0.85 (Re= Sm and Nd) system without fluorine doping. In-situ measurements under high pressure showed that Tc of the two compounds decrease monotonously over the pressure range investigated. The pressure coefficients dTc/dP in SmFeAsO0.85 and Nd FeAsO0.85 were different, revealing the important influence of the deformation in layers on Tc. Theoretical calculations suggested that the electron density of states decrease with increasing pressure, following the same trend of experimental data.
Systematic P-NMR studies on LaFe(As_{1-x}P_x)(O_{1-y}F_y) with y=0.05 and 0.1 have revealed that the antiferromagnetic spin fluctuations (AFMSFs) at low energies are markedly enhanced around x=0.6 and 0.4, respectively, and as a result, Tc exhibits respective peaks at 24 K and 27 K against the P-substitution for As. This result demonstrates that the AFMSFs are responsible for the increase in Tc for LaFe(As_{1-x}P_x)(O_{1-y}F_y) as a primary mediator of the Cooper pairing. From a systematic comparison of AFMSFs with a series of (La_{1-z}Y_z)FeAsO_δ compounds in which Tc reaches 50 K for z=0.95,…
Using a variational Monte Carlo method, we study competitions of strong electron-electron and electron-phonon interactions in the ground state of Holstein-Hubbard model on a square lattice. At half filling, an extended intermediate metallic or weakly superconducting (SC) phase emerges, sandwiched by antiferromagnetic and charge order (CO) insulating phases. By the carrier doping into the CO insulator, the SC order dramatically increases for strong electron-phonon couplings, but largely hampered by wide phase separation (PS) regions. Superconductivity is optimized at the border to the PS.
We present tunneling data from optimally-doped, superconducting BaFe1.86Co0.14As2 and its parent compound, BaFe2As2. In the superconductor, clear coherence-like peaks are seen across the whole field of view, and their analysis reveals nanoscale variations in the superconducting gap value, Delta. The average magnitude of 2Delta is ~7.4 kBTC, which exceeds the BCS weak coupling value for either s- or d-wave superconductivity. The characteristic length scales of the deviations from the average gap value, and of an anti-correlation discovered between the gap magnitude and the zero bias conductance…
57Fe-Mössbauer studies of superconducting Rb0.8Fe1.6Se2.0 with TC = 32.4 K were performed on single-crystalline and polycrystalline samples in the temperature range 4.2 K to 295 K. They reveal the presence of 88% magnetic and 12% non-magnetic Fe2+ species with the same polarization dependence of their hyperfine spectra. The magnetic species are attributed to the 16i sites of the \surd5 x \surd5 x 1 superstructure, and non-magnetic species to a nano-sized phase postulated from recent studies of superconducting KxFe2-ySe2 systems. The 57Fe-spectrum of a single-crystalline sample in an external f…
We show that biaxial strain induces alternating tetragonal superconducting and orthorhombic nematic domains in Co substituted CaFe2As2. We use Atomic Force, Magnetic Force and Scanning Tunneling Microscopy (AFM, MFM and STM) to identify the domains and characterize their properties, finding in particular that tetragonal superconducting domains are very elongated, more than several tens of micron long and about 30 nm wide, have the same Tc than unstrained samples and hold vortices in a magnetic field. Thus, biaxial strain produces a phase separated state, where each phase is equivalent to what …
We synthesized large superconducting single crystal Ca0.77La0.18Fe0.90As2 ('112' type) of 2 millimeter size. Scanning electron microscopy (SEM) measurements revealed that bright and dark stripes alternately spread on the surface of crystals, indicating possible existence of intrinsic phase separation. Temperature (T) -dependent resistivity, Hall effect and magneto-resistance (MR) were measured with magnetic field (H) applied to different directions of crystal. The upper critical field (Hc2) anisotropy was estimated with a moderate value around 2.8. Positive Hall coefficient (RH) and anisotropi…
We have investigated the influence of Fe-excess on the electrical transport and magnetism of Fe1+yTe0.5Se0.5 (y=0.04 and 0.09) single crystals. Both compositions exhibit resistively determined superconducting transitions (Tc) with an onset temperature of about 15 K. From the width of the superconducting transition and the magnitude of the lower critical field Hc1, it is inferred that excess of Fe suppresses superconductivity. The linear and non-linear responses of the ac-susceptibility show that the superconducting state for these compositions is inhomogeneous. A possible origin of this phase …
Structural investigations on the K0.8Fe1.6+xSe2 superconducting materials have revealed remarkable micro-stripes arising evidently from the phase separation. Two coexisted structural phases can be characterized by modulations of q1 = 1/5[a*+3b*], the antiferromagnetic phase K0.8Fe1.6Se2, and q2 = 1/2[a*+b*], the superconducting phase K0.75Fe2Se2, respectively. These stripe patterns likely result from the anisotropic assembly of superconducting particles along the [110] and [1-10] direction. In addition to the notable stripe structures, a nano-scale phase separation also appears in present supe…
K$_x$Fe$_{2-y}$Se$_2$ exhibits an iron-vacancy ordering at $T_{\rm s} {\sim}270°$C and separates into two phases: a minor superconducting (iron-vacancy-disordered) phase and a major non-superconducting (iron-vacancy-ordered) phase. The microstructural and superconducting properties of this intermixture can be tuned by an appropriate control of the quenching process through $T_{\rm s}$. A faster quenching rate leads to a finer microstructure and a suppression of formation of the non-superconducting phase by up to 50%. Nevertheless, such a faster cooling rate does induce a monotonic reduction in…
The effect of hydrostatic pressure up to 2 GPa on the superconductiong transitions in La{2-x}Sr{x}CuO{4+delta} is investigated. The ambient and high pressure properties of two series of samples with x=0 and x=0.015 and 0<delta<0.1 are characterized and compared by ac-susceptibility measurements. At ambient pressure both sets of samples fit into the same phase diagram as a function of the total hole concentration, n_h. For n_h<0.085 there is a single superconducting transition (T_{c} approx 30 K) with an unusually large pressure coefficient, dT_c{30}/dp approx 10K/GPa. At higher hole density (n…
We have measured mesoscopic superconducting Au$_{0.7}$In$_{0.3}$ rings prepared by e-beam lithography and sequential deposition of Au and In at room temperature followed by a standard lift-off procedure. In samples showing no Little-Parks resistance oscillations, highly unusual double resistance anomalies, two resistance peaks found near the onset of superconductivity, were observed. Although resistance anomaly featuring a single resistance peak has been seen in various mesoscopic superconducting samples, double resistance anomalies have never been observed previously. The dynamical resistance…
We have used scanning micro x-ray diffraction to characterize different phases in superconducting K$_{x}$Fe$_{2-y}$Se$_2$ as a function of temperature, unveiling the thermal evolution across the superconducting transition temperature (T$_c\sim$32 K), phase separation temperature (T$_{ps}\sim$520 K) and iron-vacancy order temperature (T$_{vo}\sim$580 K). In addition to the iron-vacancy ordered tetragonal magnetic phase and orthorhombic metallic minority filamentary phase, we have found a clear evidence of the interface phase with tetragonal symmetry. The metallic phase is surrounded by this int…
Temperature dependent single-crystal x-ray diffraction (XRD) in transmission mode probing the bulk of the newly discovered K0.8Fe1.6Se2 superconductor (Tc = 31.8 K) using synchrotron radiation is reported. A clear evidence of intrinsic phase separation at 520 K between two competing phases, (i) a first majority magnetic phase with a ThCr2Si2-type tetragonal lattice modulated by the iron vacancy ordering and (ii) a minority non-magnetic phase having an in-plane compressed lattice volume and a weak superstructure, is reported. The XRD peaks due to the Fe vacancy ordering in the majority phase di…
We demonstrate the successful fabrication on CaF$_2$ substrates of FeSe$_{1-x}$Te$_{x}$ films with $0 \le x \le 1$, including the region of $0.1 \le x \le 0.4$, which is well known to be the "phase-separation region", via pulsed laser deposition which is a thermodynamically non-equilibrium method. In the resulting films, we observe a giant enhancement of the superconducting transition temperature, $T_\mathrm{c}$, in the region of $0.1 \le x \le 0.4$: the maximum value reaches 23 K, which is approximately 1.5 times as large as the values reported for bulk samples of FeSe$_{1-x}$Te$_{x}$. We pre…
An unusual thermal-magnetic hysteresis was observed between a minor magnetic transition around 120 K and the main one at 80 K in superconducting RuSr2(R,Ce)2Cu2O10-delta (Ru1222R) samples, where R = Gd or Eu, down to a submicron length-scale. The observation suggests a possible phase-separation and is consistent with the very small but universal demagnetizing factor observed, which is difficult to reconcile with the canted spin-structure previously proposed. In such a scenario, the unusual superconducting properties of the Ru-based cuprates can also be understood naturally.
To understand the interplay of d-wave superconductivity and antiferromagnetism in the cuprates, we consider a two-dimensional extended Hubbard model with nearest neighbor attractive interaction. Free energy of the homogeneous (coexisting superconducting and antiferromagnetic) state calculated a s a function of the band filling shows a region of of phase separation. The phase separation caused by the intersite attractive force leads to novel insights into salient features of the pseudogap phase diagram. In particular, the upper crossover curve can be identified with the phase separation boundar…
We consider the case of a density-driven metal-superconductor transition in the proximity of an electronic phase separation. In particular we investigate the interplay between superconducting fluctuations and density fluctuations, which become quantum critical when the electronic phase separation vanishes at zero temperature into a quantum critical point. In this situation the critical dynamical density fluctuations strongly affect the dynamics of the Cooper pair fluctuations, which acquire a more singular character with a z=3 dynamical critical index. This gives rise to a scenario that possib…
Heterostructures made of itinerant ferromagnets and superconductors are studied. In contrast to most previous models, ferromagnetism is not enforced as an external Zeeman field but induced in a correlated single-band model (CSBM) that displays itinerant ferromagnetism as a mean-field ground state. This allows us to investigate the influence of an adjacent superconducting layer on the properties of the ferromagnet in a self-consistent Bogoliubov-de Gennes approach. The CSBM displays a variety features not present in the Zeeman exchange model that influence the behavior of order parameters close…
We propose the use of normal and Andreev resonances in normal-superconducting structures to generate divergent beams of nonlocally entangled electrons. Resonant levels are tuned to selectively transmit electrons with specific values of the perpendicular energy, thus fixing the magnitude of the exit angle. When the normal metal is a ballistic two-dimensional electron gas, the proposed scheme guarantees arbitrarily large spatial separation of the entangled electron beams emitted from a finite interface. We perform a quantitative study of the linear and nonlinear transport properties of some suit…
The kinetic energy of superconducting electrons in an ultrathin, doubly connected superconducting cylinder, determined by the applied flux, increases as the cylinder diameter decreases, leading to a destructive regime around half-flux quanta and a superconductor to normal metal quantum phase transition (QPT). Regular step-like features in resistance vs. temperature curves taken at fixed flux values were observed near the QPT in ultrathin Al cylinders. It is proposed that these features are most likely resulted from a phase separation near the QPT in which normal regions nucleate in a homogeneo…
Structural phase separation in the form of partially disordered stripes, with characteristic length scales in the nanometer range, is observed for superconducting BaPb$_{1-x}$Bi$_x$O$_3$. The evolution of the superconducting coherence length with composition relative to the size of these stripes suggests an important role of the nanostructure in determining the shape of the superconducting dome. It is proposed that the maximum $T_c$ is determined by a kind of "optimal inhomogeneity", characterized by a crossover from an inhomogeneous macroscopic superconductor to a granular superconductor for …
Intermediate states near normal-superconducting transition at ~100mK, important for a sensitive thermometer in x-ray calorimetry, have been studied by an imaging technique based on Low Temperature Scanning Synchrotron Microscopy (LTSSM). In conjunction with iridium superconducting transition-edge sensors operating in electrothermal feedback mode, the LTSSM visualizations reveal that a normal-superconducting phase separation takes place in the actual intermediate states.
Due to the competition of two anion orders, (TMTSF)2ReO4, presents a phase coexistence between semiconducting and metallic (superconducting) regions (filaments or droplets) in a wide range of pressure. In this regime, the superconducting upper critical field for H parallel to both c* and b' axes present a linear part at low fields followed by a divergence above a cross-over field. This cross-over corresponds to the 3D-2D decoupling transition expected in filamentary or granular superconductors. The sharpness of the transition also demonstrates that all filaments are of similar sizes and self o…
We show that the main features of the cuprates superconductors phase diagram can be derived considering the disorder as a key property of these materials. Our basic point is that the high pseudogap line is an onset of phase separation which generates compounds made up of regions with distinct doping levels. We calculate how this continuous temperature dependent phase separation process occurs in high critical temperature superconductors (HTSC) using the Cahn-Hilliard approach, originally applied to study alloys. Since the level of phase separation varies for different cuprates, it is possible …
Fe1+xTe0.5Se0.5 is the archetypical iron-based superconductor. Here we show that the superconducting state is controlled by the stacking of its anti-PbO layers, such that homogeneous ordering hinders superconductivity and the highest volume fractions are observed in phase separated structures as evidenced by either a distribution of lattice parameters or microstrain.
hrough neutron diffraction experiments, including spin-polarized measurements, we find a collinear incommensurate spin-density wave with propagation vector $ \mathbf k = $ ($0.4481(4) \, \,0 \, \, \frac1 2$) at base temperature in the superconducting parent compound Fe$_{1+x}$Te. This critical concentration of interstitial iron corresponds to $x \approx 12%$ and leads crystallographic phase separation at base temperature. The spin-density wave is short-range ordered with a correlation length of 22(3) Å, and as the ordering temperature is approached its propagation vector decreases linearly in …
We made point-contact measurements with Au tips on La_{2-x}Sr_{x}CuO_{4} samples with 0.08 < x < 0.20 to investigate the relationship between superconducting gap and pseudogap. We obtained junctions whose conductance curves presented typical Andreev reflection features at all temperatures from 4.2 K up to T_c^A close to the bulk T_c. Their fit with the BTK-Tanaka-Kashiwaya model gives good results if a (s+d)-wave gap symmetry is used. The doping dependence of the low temperature dominant isotropic gap component Delta_{s} follows very well the T_{c} vs. x curve. These results support the separa…
We report an in-plane optical spectroscopy study on the iron-selenide superconductor K$_{0.75}$Fe$_{1.75}$Se$_2$. The measurement revealed the development of a sharp reflectance edge below T$_c$ at frequency much smaller than the superconducting energy gap on a relatively incoherent electronic background, a phenomenon which was not seen in any other Fe-based superconductors so far investigated. Furthermore, the feature could be noticeably suppressed and shifted to lower frequency by a moderate magnetic field. Our analysis indicates that this edge structure arises from the development of a Jose…
We report the low-temperature electronic and magnetic properties of the alkali metal-organic solvent intercalated iron selenide superconductor Lix(C5H5N)yFe2-zSe using muon-spin-spectroscopy measurements. The zero-field \muSR results indicate that nearly half of the sample is magnetically ordered and there is a spatial phase separation between the superconducting and the magnetic fraction. The transverse-field \muSR results show that the temperature dependence of the penetration depth λ(T) in the mixed state of Lix(C5H5N)yFe2-zSe can be explained using a two-gap s+s-wave model with gap values …
We study the quantum transition from an antiferromagnet to a superconductor in a model for electron- and hole-doped cuprates by means of a variational cluster perturbation theory approach. In both cases, our results suggest a tendency towards phase separation between a mixed antiferromagnetic-superconducting phase at low doping and a pure superconducting phase at larger doping. However, in the electron-doped case the energy scale for phase separation is an order of magnitude smaller than for hole doping. We argue that this can explain the different pseudogap and superconducting transition scal…
We have studied the current-voltage characteristics of superconducting weak links in which the coupling medium is the 2-D electron gas in InAs-based semiconductor quantum wells, with relatively large (typically 0.5 micron) separations between niobium electrodes. The devices exhibit Josephson-like current-voltage characteristics; however, the falloff of the differential resistance with decreasing temperature is thermally activated, and is orders of magnitude slower than for more conventional weak links. Most unexpectedly, the thermal activation energies are found to be proportional to the width…
While the parent compounds of the cuprate high temperature superconductors (high-Tc's) are Mott insulators, the iron-pnictide high-Tc's are in the vicinity of a metallic spin density wave (SDW) state, which highlights the difference between these two families. However, insulating parent compounds were identified for the newly discovered KxFe2-ySe2. This raises an intriguing question as to whether the iron-based high-Tc's could be viewed as doped Mott insulators like the cuprates. Here we report angle-resolved photoemission spectroscopy (ARPES) evidence of two insulating and one semiconducting …
We propose a phenomenological model for high-$T_{c}$ superconductors (HTSC) assuming: (1) a microscopic phase separation between superconducting and normal-metal areas in the overdoped region; and (2) existence of a homogeneous superconducting phase only below the pseudo-gap $T^{*}$ line, which shows a sharp reduction towards $T^{*}\sim 0$ at a mildly overdoped critical concentration $x_{c}$. This model explains anomalous doping and temperature dependences of $n_{s}/m^{*}$ (superconducting carrier density / effective mass) observed in several overdoped HTSC systems. We point out an analogy to …
Recent discovery of superconducting (SC) ternary iron selenides has block antiferromagentic (AFM) long range order. Many experiments show possible mesoscopic phase separation of the superconductivity and antiferromagnetism, while the neutron experiment reveals a sizable suppression of magnetic moment due to the superconductivity indicating a possible phase coexistence. Here we propose that the observed suppression of the magnetic moment may be explained due to the proximity effect within a phase separation scenario. We use a two-orbital model to study the proximity effect on a layer of block A…
A model of superconductivity is proposed taking into account repulsive particle interaction, mesoscopic phase separation and softening of crystalline lattice. These features are typical of many high-temperature superconductors. The main results obtained for the model are: (i) phase separation is possible only if repulsive forces play a significant role; (ii) the critical temperature as a function of the superconducting phase fraction can have non-monotonic behaviour; (iii) superconductivity is possible in heterophase systems even when it would be forbidden in pure samples. These results are in…
We report investigation on Fe1+yTexSe1-x single crystals by using scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). Both nonsuperconducting samples with excess iron and superconducting samples demonstrate nanoscale phase separation and chemical inhomogeneity of Te/Se content, which we attribute to a miscibility gap. The line scan EELS technique indicates ~20% or less fluctuation of Te concentration from the nominal compositions.
The properties of the superconducting ferromagnets RuSr2(Gd/Eu)Cu2O8 and RuSr2(Ln/Ce)2Cu2O10 are reviewed. The problem of coexistence and competition between magnetic and superconducting states is discussed. A model of phase separation into ferromagnetic and anti-ferromagnetic structures is proposed.
Current and field profiles, and magnetization and ac losses are calculated for arrays of infinitely long superconducting tapes in the critical state in a perpendicularly applied magnetic field. The tapes are arranged both horizontally and in a matrix configuration, which is the geometry found in many actual high-T_c superconducting tapes. The finite thickness of the tapes and the effects of demagnetizing fields are considered. Systematic results for the magnetization and ac losses of the tapes are obtained as function of the geometry and separation of the constituent tapes. Results allow to un…
In this paper we determine the phase diagrams (for $T=0$ as well as $T>0$) of the Penson-Kolb-Hubbard model for two dimensional square lattice within Hartree-Fock mean-field theory focusing on investigation of superconducting phases and possibility of the occurrence of the phase separation. We obtain that the phase separation, which is a state of coexistence of two different superconducting phases (with $s$-wave and $η$-wave symmetries), occurs in define range of the electron concentration. In addition, increasing temperature can change the symmetry of the superconducting order parameter (from…
The Gaussian fluctuations in slave-boson mean-field theory of $t-J$ model is analyzed in this paper in the low-doping regime where the superconducting to normal (pseudo-gap phase) transition is driven by vanishing of bose-condensation amplitude. By eliminating the boson and constraint fields exactly in the linear response regime we show that the Gaussian theory describes a Fermi liquid superconductor where the superconducting to normal transition is actually a spin-charge separation transition characterized by a change of Landau parameter from $F_1>-1$ in the superconducting phase to $F_1=-1$ …
We have grown a single crystal of La_2-x_Sr_x_CuO_4_ in which the Sr concentration, x, continuously changes from 0.24 to 0.29 in the overdoped regime and obtained many pieces of single crystals with different x values by slicing the single crystal. From detailed measurements of the magnetic susceptibility, chi, of each piece, it has been found that the absolute value of chi at the measured lowest temperature 2 K, |chi_2K_|, on field cooling rapidly decreases with increasing x as well as the superconducting (SC) transition temperature. As the value of |chi_2K_| is regarded as corresponding to t…
The nature of the phase separation in KxFe2-ySe2 single crystals is not well understood. Here, temperature dependent SEM images provide compelling evidence that phase separation corresponds to a nucleation and growth process rather than a spinodal decomposition. The superconducting phase is the remnant of high temperature phase after iron vacancy order-disorder transition. Three-dimension (3D) spatial distribution of superconducting phase in the iron vacancy ordered matrix was revealed by using focused ion beam scanning electron microscopy (FIB-SEM). Our results clearly show that superconducti…
Two correlated superconducting phases are identified in layered superconductor BaTi2(Sb1-xBix)2O (x=0.16), with the superconducting transition temperatures of TC = 6 K (the high TC phase) and 3.4 K (the low TC Phase), respectively. The 6 K superconducting phase appears firstly in the as-prepared sample and can decay into the low TC phase by exposing to ambient atmosphere for certain duration. Especially the high TC phase can reappear from the decayed sample with the low TC phase by vacuum annealing. It is also found that the CDW/SDW order occurs only with the 6 K superconducting phase. These n…
We propose a new phase of matter, an electronic phase separation transition that starts near the upper pseudogap and segregates the holes into high and low density domains. The Cahn-Hilliard approach is used to follow quantitatively this second order transition. The resulting grain boundary potential confines the charge in domains and favors the development of intragrain superconducting amplitudes. The zero resistivity transition arises only when the intergrain Josephson coupling $E_J$ is of the order of the thermal energy and phase locking among the superconducting grains takes place. We show…
High Tc superconductivity in FeAs-based multilayers (pnictides), evading temperature decoherence effects in a quantum condensate, is assigned to a Feshbach resonance (called also shape resonance) in the exchange-like interband pairing. The resonance is switched on by tuning the chemical potential at an electronic topological transition (ETT) near a band edge, where the Fermi surface topology of one of the subbands changes from 1D to 2D topology. We show that the tuning is realized by changing i) the misfit strain between the superconducting planes and the spacers ii) the charge density and iii…
We report the results of a zero-field muon spin relaxation (ZF-muSR) study of superconducting Ba1-xKxFe2As2 (0.5 < x < 0.9) in search of weak spontaneous internal magnetic fields associated with proposed time-reversal-symmetry breaking mixed pairing states. The measurements were performed on polycrystalline samples, which do not exhibit the mesoscopic phase separation previously observed in single crystals of Ba1-xKxFe2As2. No evidence of spontaneous internal magnetic fields is found in any of the samples at temperatures down to T ~ 0.02 K.
Measurements of thermal conductivity ($κ$) versus temperature and doping for several cuprate superconductors are discussed. Suppressed values of the normal-state $κ$ and the slope change in $κ$ at $T_c$, observed near 1/8 doping in both YBa$_2$Cu$_3$O$_{6+x}$ and Hg cuprates, are attributed to local lattice distortions and the suppression of the superconducting condensate, respectively. Both phenomena are proposed to arise from small domains of localized planar holes, presumably a manifestation of phase separation. It is suggested that the phase behavior of $κ$ reflects stripe dynamics and the…
Superconducting quantum phase transitions tuned by disorder (d), paramagnetic impurity (MI) and perpendicular magnetic field (B) have been studied in homogeneously disordered ultrathin a-Pb films. The MI-tuned transition is characterized by progressive suppression of the critical temperature to zero and a continuous transition to a weakly insulating normal state with increasing MI density. In all important aspects, the d-tuned transition closely resembles the MI-tuned transition and both appear to be fermionic in nature. The B-tuned transition is qualitatively different and probably bosonic. I…
High quality Rb0.8Fe1.6+xSe2 single crystals are grown by a one step melting growth method. Superconductivity has been observed in all samples with x more than 0.05 and the maximum critical temperature Tc~32 K has been obtained in samples with x from 0.1 to 0.2 from electronic resistivity measurement. A superconducting dome in the electronic phase diagram of Tc as a function of Fe content has been observed for the first time in this AxFeySe2 (A=K,Rb) superconducting system. Two coexisting phases with structural modulation vectors of q1=1/5 [3a*+b*] and q2=1/2[a*+b*] have been observed in all s…
We show that on superconducting spherical nanoshells, the co-existence of the Meissner state with a variety of vortex patterns drives the phase transition to higher magnetic fields. The spherical geometry leads to a Magnus-Lorentz force pushing the nucleating vortices and antivortices towards the poles, overcoming local pinning centers, preventing vortex-antivortex recombination and leading to the appearance of a Meissner belt around the sphere's equator. In sufficiently small and thin spherical shells paramagnetic vortex states can be stable, enabling spatial separation of freely moving shell…
Neutron scattering from high-quality YBa2Cu3O6.33 (YBCO6.33) single crystals with a Tc of 8.4 K shows no evidence of a coexistence of superconductivity with long-range antiferromagnetic order at this very low, near-critical doping of p~0.055. However, we find short-range three dimensional spin correlations that develop at temperatures much higher than Tc. Their intensity increases smoothly on cooling and shows no anomaly that might signify a Neel transition. The system remains subcritical with spins correlated over only one and a half unit cells normal to the planes. At low energies the short-…
The superconducting transition in a single overdoped, detwinned YBa2Cu3Ox (YBCO) crystal is studied using four different probes. Whereas the AC and DC magnetic susceptibilities find a dominant transition at 88 K with a smaller effect near 92 K, the specific heat and electrical resistivity reveal only a single transition at 88 K and 92 K, respectively. Under hydrostatic pressures to 0.60 GPa these two transitions shift in opposite directions, their separation increasing. The present experiments clearly show that the bulk transition lies at 88 K and originates from fully oxygenated YBCO; the 92 …
