Filamentary Superconductivity Research Articles

Effects of pressure and magnetic field on superconductivity in ZrTe3: local pair-induced superconductivity

In this work, the origin of the highly anisotropic superconducting transition in ZrTe3, where the resistance along the a axis, Ra, is reduced at 4 K but those along the b axis, Rb, and axis, Rc′, are reduced at 2 K, was explored with the application of a magnetic field and pressure by the electrical resistance measurements. We found that the behavior of the upper critical field and its anisotropy as well as the pressure dependence determined by the Ra measurements are quite similar to those of Rb. Moreover, the excess conductivity for Rb indicates anomalous behavior. These results support an unconventional origin for the anisotropic transition rather than conventional superconducting fluctuation. The reduction in Ra is due to filamentary superconductivity (SC) induced by locally bound electron pairs (local pairs), which correspond to bi-polarons, and the transition of Rb corresponds to the emergence of bulk SC originating from the Cooper pairs triggered by the transfer of the local pairs.

Synthesis and transport properties of 112-type iron pnictide superconductors Ca1-xCexFe1-yCoyAs2

We studied the electronic transport properties in the 112-type Ca1-xCexFe1-yCoyAs2 (0.18 ≤ x ≤ 0.27, y = 0, 0.025) single crystals. Upon Ce doping on the Ca site, it is found that the antiferromagnetic(AFM)/structural phase transition is enhanced and a weak superconductivity, possibly of filamentary superconductivity in origin, emerges. However, only a small amount of Co co-doping in Ca0.8Ce0.2Fe0.975Co0.025As2 suppresses the AFM/structural transition completely and stabilizes a bulk superconductivity with Tc = 34.5 K. Intriguingly, Hall measurements reveal a sign change of RH only in Ca0.73Ce0.27FeAs2 around ∼80 K, which is presumably due to the band reconstruction driven by the AFM/structural transition. For other dopings, Hall coefficient (RH) is negative and shows nonmonotonic T-dependence with a broad minimum feature. Moreover, the magnetoresistance (MR) is found to be progressively suppressed with increasing Ce concentrations. While the size of MR tends to saturate at low T for Ca0.8Ce0.2Fe0.975Co0.025As2, it turns downward with decreasing temperatures below ∼80 K for all Co-free samples. Finally, the Kohler's rule is strongly violated only below a characteristic temperature TK, suggesting either a change of carrier number or exotic anisotropic scattering mechanism below TK in this system.

Structure and property correlations in FeS

For iron-sulfide (FeS), we investigate the correlation between the structural details, including its dimensionality and composition, with its magnetic and superconducting properties. We compare, theoretically and experimentally, the two-dimensional (2D) layered tetragonal (“t-FeS”) phase with the 3D hexagonal ("h-FeS") phase. X-ray diffraction reveals iron-deficient chemical compositions of t-Fe0.93(1)S and h-Fe0.84(1)S that show no low-temperature structural transitions. First-principles calculations reveal a high sensitivity of the 2D structure to the electronic and magnetic properties, predicting marginal antiferromagnetic instability for our compound (sulfur height of zS= 0.252) with an ordering energy of about 11meV/Fe, while the 3D phase is magnetically stable. Experimentally, h-Fe0.84S orders magnetically well above room temperature, while t-Fe0.93S shows coexistence of antiferromagnetism at TN= 116 and filamentary superconductivity below Tc= 4K. Low temperature neutron diffraction data reveals antiferromagnetic commensurate ordering with wave vector km=(0.25,0.25,0) and 0.46(2)µB/Fe. Additionally, neutron scattering measurements were used to find the particle size and iron vacancy arrangement of t-FeS and h-FeS. The structure of iron sulfide has a delicate relationship with the superconducting transition; while our sample with a = 3.6772(7)Å is a filamentary superconductor coexisting with an antiferromagnetic phase, previously reported samples with a > 3.68Å are bulk superconductors with no magnetism, and those with a≈3.674Å show magnetic properties.

Absence of nematic order in the pressure-induced intermediate phase of the iron-based superconductorBa0.85K0.15Fe2As2

The hole doped Fe-based superconductors Ba$_{1-x}$A$_x$Fe$_2$As$_2$ (where A=Na or K) show a particular rich phase diagram. It was observed that an intermediate re-entrant tetragonal phase forms within the orthorhombic antiferromagnetically-ordered stripe-type spin density wave state above the superconducting transition [S. Avci et al., Nature Comm. 5, 3845 (2014), A. E. B\"ohmer et al., arXiv:1412.7038v2]. A similar intermediate phase was reported to appear if pressure is applied to underdoped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ [E. Hassinger et al., Phys. Rev. B 86, 140502(R) (2012)]. Here we report data of the electric resistivity, Hall effect, specific heat, and the thermoelectrical Nernst and Seebeck coefficients measured on a Ba$_{0.85}$K$_{0.15}$Fe$_2$As$_2$ single crystal under pressure up to 5.5 GPa. The data reveals a coexistence of the intermediate phase with filamentary superconductivity. The Nernst coefficient shows a large signature of nematic order that coincides with the stripe-type spin density wave state up to optimal pressure. In the pressure-induced intermediate phase the nematic order is removed, thus confirming that its nature is a re-entrant tetragonal phase.

Magnetoresistivity and filamentary superconductivity in nickel-doped BaFe2As2**Project supported by the National Basic Research Program of China (Grant Nos. 2012CB821400, 2012CB921302, and 2015CB921303) and the National Natural Science Foundation of China (Grant Nos. 11274237, 91121004, 51228201, 11004238, and 11374011).

We present magnetotransport studies on a series of BaFe2−xNixAs2 (0.03 ≤ x ≤ 0.10) single crystals. In the underdoped (x = 0.03) non-superconducting sample, the temperature-dependent resistivity exhibits a peak at 22 K, which is associated with the onset of filamentary superconductivity (FLSC). FLSC is suppressed by an external magnetic field in a manner similar to the suppression of bulk superconductivity in an optimally-doped (x = 0.10) compound, suggesting the same possible origin as the bulk superconductivity. Our magnetoresistivity measurements reveal that FLSC persists up to the optimal doping and disappears in the overdoped regime where the long-range antiferromagnetic order is completely suppressed, pointing to a close relation between FLSC and the magnetic order.

Heat transport study of field-tuned quantum criticality inCeIrIn5

The in-plane electrical resistivity, rho, and thermal conductivity, kappa, of the heavy-fermion superconductor CeIrIn5 were measured down to 40mK in magnetic fields up to 11 T applied along the c axis. For all fields above Hc2=4T of filamentary superconductivity, we find that the ratio of heat and charge conductivities in the T to 0 limit obeys the Wiedemann-Franz law, kappa/T=L0/rho, where L0 = 2.45*10^-8 WOhmK^-2 is the Sommerfeld value of the Lorenz number. The temperature-dependent parts of both the electrical and thermal resistivity,w = T/L0 kappa, follow the functional dependence expected for the Fermi liquid theory of metals with rho - rho_0 = AT^2, w - w_0 = BT^2, with rho0 = w0 and B ~ 2A. The coefficient B does not show a significant field dependence even upon approaching Hc2 = 0.4 T of the bulk superconducting state. The weak response to the magnetic field is in stark contrast with the behavior found in the closely related CeCoIn5, in which the field-tuned quantum critical point coincides with Hc2. The value of the electron-electron mass enhancement, as judged by the A and B coefficients, is about one order of magnitude reduced in CeIrIn5 as compared to CeCoIn5 (in spite of the fact that the zero field gamma_0 in CeIrIn5 is twice as large as gamma_0 in CeCoIn5), which suggests that the material is significantly farther away from the magnetic quantum critical point at bulk Hc2 and at all of the studied fields. A suppressed Kadowaki-Woods ratio in CeIrIn5 compared to CeCoIn5 suggests a notably more localized nature of f electrons in the compound.

A Different Approach to High-Tc Superconductivity: Indication of Filamentary-Chaotic Conductance and Possible Routes to Room Temperature Superconductivity

The empirical relation of between the transition temperature of optimum doped superconductors Tco and the mean cationic charge , a physical paradox, can be recast to strongly support fractal theories of high-Tc superconductors, thereby applying the finding that the optimum hole concentration of σo = 0.229 can be linked with the universal fractal constant δ1 = 8.72109… of the renormalized quadratic Henon map. The transition temperature obviously increases steeply with a domain structure of ever narrower size, characterized by Fibonacci numbers. However, also conventional BCS superconductors can be scaled with δ1, exemplified through the energy gap relation kBTc ≈ 5Δ0/δ1, suggesting a revision of the entire theory of superconductivity. A low mean cationic charge allows the development of a frustrated nano-sized fractal structure of possibly ferroelastic nature delivering nano-channels for very fast charge transport, in common for both high-Tc superconductor and organic-inorganic halide perovskite solar materials. With this backing superconductivity above room temperature can be conceived for synthetic sandwich structures of less than 2+. For instance, composites of tenorite and cuprite respectively tenorite and CuI (CuBr, CuCl) onto AuCu alloys are proposed. This specification is suggested by previously described filamentary superconductivity of “bulk” CuO1﹣x samples. In addition, cesium substitution in the Tl-1223 compound is an option.

Reversible tuning of the collapsed tetragonal phase transition inCaFe2As2by separate control of chemical pressure and electron doping

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 x_cr is consistent with truly hydrostatic pressure experiments on undoped CaFe2As2. Starting in the CT regime at x=0.027 we investigate the additional effect of electron doping by partial replacement of Ca by La. Most remarkably, with increasing y the CT phase transition is destabilized and the system is tuned back into a tetragonal ground state at y>0.08. This effect is ascribed to a weakening of interlayer As-As bonds by electron doping. Upon further electron doping filamentary superconductivity with Tc of 41 K at y=0.2 is observed.

Signatures of filamentary superconductivity in antiferromagnetic BaFe2As2 single crystals

In this paper, we present ac susceptibility and magnetotransport measurements on aged single crystals of the ferropnictide parent compound, BaFe2As2 with a paramagnetic-to-antiferromagnetic transition temperature of 134 K. The ac susceptibility shows the clear onset of a partial diamagnetic response with an onset temperature, commensurate with a subtle downturn in resistivity at approximately 20 K. Below 20 K the magnetotransport shows in-plane anisotropy, magnetic-field history dependence and a hysteretic signature. Above 20 K the crystals show the widely reported high-field linear magnetoresistance. An enhanced noise signature in ac susceptibility is observed above 20 K, which varies in character with amplitude and frequency of the ac signal. The hysteresis in magnetoresistance and the observed sensitivity of the superconducting phase to the amplitude of the ac signal are indicative characteristics of granular or weakly linked filamentary superconductivity. These features taken together with the observed noise signature above suggests a link between the formation of the superconducting filamentary phase and the freezing of antiphase domain walls, known to exist in these materials.

On the nature of filamentary superconductivity in metal-doped hydrocarbon organic materials

AbstractHigh temperature superconductivity in K-doped 1,2:8,9-dibenzenopentacene (C

Superconductivity up to 10 K in the Sr(Pd1−xPtx)2Ge2 layer system

Metastable filamentary superconductivity with Tc = 10.2 K was observed in SrPt2Ge2. This is the highest Tc reported outside the iron-based 122-layer compounds. The stable phase for this compound is a non-superconducting monoclinic LaPt2Ge2-type structure with lattice parameters a = 0.4401(4) nm, b = 0.4435(4) nm, c = 0.9764(10) nm, β = 90.5°–90°. Weak diamagnetic shielding signal indicates that superconductivity is not from the major monoclinic phase, but from the minor BaFe2As2-type body-center-tetragonal (122-bct) phase with a=0.44 nm, c=0.98 nm,β=90°. Investigation on the pseudoternary system Sr(Pd1−xPtx)2Ge2 suggests that the stable bct to monoclinic structure phase transition occurs near x = 0.15. For x&amp;lt;0.15 with bct phase, bulk Tc increases from 3.2 K for SrPd2Ge2 to 3.6 K for SrPd1.8Pt0.2Ge2. For x&amp;gt;0.15 with metastable filamentary bct phase, Tc increases from 7.5 K for x = 0.5 to 9.2 K for x = 0.7 and reaches a Tc maximum of 10.2 K for SrPt2Ge2. High Tc is due to a strong quasi-2D Pt-Ge-Pt 5dxz,yz-4p-5dxz,yz hybridization in the Pt-Ge layer with squeezed-along-c-axis PtGe4 tetrahedral crystal field and Pt-5d8 spin-orbital interaction in the metastable filamentary bct phase and is another example of multi-band (dxz/dyz/dxy) non-conventional s-wave superconductor.

A crossover in the phase diagram of NaFe1−xCoxAs determined by electronic transport measurements

We report electronic transport measurements on single crystals of the NaFe1−xCoxAs system. We found that the cotangent of the Hall angle, cot θH, follows T4 for the parent compound with filamentary superconductivity and T2 for the heavily overdoped non-superconducting sample. However, it exhibits approximately T3-dependence in all the samples with bulk superconductivity, suggesting that this behavior is associated with bulk superconductivity in ferropnictides. A deviation develops below a characteristic temperature T* well above the structural and superconducting transitions, accompanied by a departure from power-law temperature dependence in resistivity. The doping dependence of T* resembles the crossover line of the pseudogap phase in cuprates.

Filamentary superconductivity across the phase diagram of Ba(Fe,Co)2As2

We show magnetotransport results on Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ ($0.0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.13$) single crystals. We identify the low-temperature resistance step at 23 K in the parent compound with the onset of filamentary superconductivity (FLSC), which is suppressed by an applied magnetic field in a similar manner to the suppression of bulk superconductivity (SC) in doped samples. FLSC is found to persist across the phase diagram until the long-range antiferromagnetic order is completely suppressed. A significant suppression of FLSC occurs for $0.02&lt;x&lt;0.04$, the doping concentration where bulk SC emerges. Based on these results and the recent report of an electronic anisotropy maximum for 0.02 $\ensuremath{\leqslant}x\ensuremath{\leqslant}$ 0.04 [Chu et al., Science 329, 824 (2010)], we speculate that, besides spin fluctuations, orbital fluctuations may also play an important role in the emergence of SC in iron-based superconductors.

Mixed bulk-filament nature in superconductivity of the charge-density-wave conductor ZrTe3

We have shown that the superconducting transitions in ZrTe${}_{3}$ are successive from filamentary to bulk, based on experimental results obtained for a superconducting specific-heat anomaly and the anisotropic superconducting transition curve of resistivity. A small jumplike superconducting specific-heat anomaly with a large and widely extended tail was observed to closely follow the anisotropic zero-resistance transition. We concluded that the jumplike anomaly signifies the onset of the long-range order of Cooper pairs with the opening of a superconducting gap at ${T}_{C}$, while the large and widely extended tail indicates behavior induced by Cooper pairs with a very short coherence length. As a limit for a very short coherence length, we propose local pairs with Bose characteristics. As a result, we can understand that the filamentary superconductivity is caused by the local pairs, and the large and widely extended tail is in a crossover region between superconductivity induced by local pairs and that induced by Cooper pairs. Based on a discussion about the origin of the change in the pair coupling from the starting of local pairing to Cooper pairing in ZrTe${}_{3}$, we conclude that ``mixed bulk-filament superconductivity'' results from unique electronic structural changes in the quasi-1D+3D (D, dimensional) Fermi surfaces after the CDW transition.

Evidence for filamentary superconductivity nucleated at antiphase domain walls in antiferromagnetic CaFe2As2

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 ${T}^{*}\ensuremath{\approx}10$ 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, are fully suppressed by ${H}^{*}\ensuremath{\approx}15$ T, and suggest the presence of filamentary superconductivity nucleated at the antiphase domain walls in this material.

Superconductivity in single-crystal [formula omitted

We measure the superconducting transition of Y In3 by resistivity, susceptibility, and specific heat. Despite using high-quality single-crystal samples, the transitions detected by the three techniques are shifted from each other in temperature, suggesting a region of filamentary superconductivity. We discuss the possible implications for filamentary superconductivity in unconventional superconductors.

Moisture-induced superconductivity inFeTe0.8S0.2

Moisture-induced superconductivity was observed in FeTe0.8S0.2. With exposing the sample to the air, the zero resistivity temperature and the superconducting volume fraction were enhanced up to 7.2 K and 48.5 %, respectively, while the as-grown sample showed only filamentary superconductivity. We concluded that the causes of the evolution of superconductivity were water-related ions and/or molecules, because only the sample kept in water at room temperature for several days showed superconductivity. The speed of evolution of superconductivity was strongly enhanced by immersing the sample into the hot water.

Domain walls at the spin-density-wave endpoint of the organic superconductor(TMTSF)2PF6under pressure

We report a comprehensive investigation of the organic superconductor ${(\text{TMTSF})}_{2}{\text{PF}}_{6}$ in the vicinity of the endpoint of the spin-density-wave metal phase transition where phase coexistence occurs. At low temperature, the transition of metallic domains toward superconductivity is used to reveal the various textures. In particular, we demonstrate experimentally the existence of one-dimensional and two-dimensional (2D) metallic domains with a crossover from a filamentary superconductivity mostly along the ${c}^{\ensuremath{\star}}$ axis to a 2D superconductivity in the ${b}^{\ensuremath{'}}c$-plane perpendicular to the most conducting direction. The formation of these domain walls may be related to the proposal of a soliton phase in the vicinity of the critical pressure of the ${(\text{TMTSF})}_{2}{\text{PF}}_{6}$ phase diagram.

On the origin of the double superconducting transition in overdoped YBa 2Cu 3O x

The superconducting transition in a single overdoped, detwinned YBa 2Cu 3O x (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 K transition likely arises from filamentary superconductivity in a minority optimally doped phase (<1%) of YBCO located at or near the crystal surface.

Transport and magnetic anomalies below70Kin a hydrogen-cycled Pd foil with a thermally grown oxide

Electron transport and magnetic properties have been studied in a deformed $12.5\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{m}$-thick Pd foil with a thermally-grown oxide and a low residual concentration of hydrogen. This foil was deformed by cycling across the Pd hydride miscibility gap and the residual hydrogen was trapped at dislocation cores. Anomalies of both resistance and magnetic susceptibility have been observed below $70\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, indicating the appearance of excess conductivity and a diamagnetic response that we interpret in terms of filamentary superconductivity. These anomalies are attributed to a condensed hydrogen-rich phase at dislocation cores near the $\mathrm{Pd}\text{\ensuremath{-}}\mathrm{Pd}$ oxide interface.