Fe2As2 Single Crystals Research Articles

Crystal growth and pressure effect on the transport properties in 122-type (La,Na,K)Fe2As2 superconductors

Abstract High-quality single crystals are essential for probing the intrinsic properties of unconventional superconductors. In this work, we report the successful growth of (La,Na,K)Fe2As2 single crystals, a novel 122-type iron-based superconductor, using the self-flux method. The crystal structure and chemical composition were characterized through x-ray diffraction and energy-dispersive x-ray spectroscopy. The (La,Na,K)Fe2As2 single crystals exhibit bulk superconductivity, with a critical transition temperature Tc of approximately 22 K. Magnetization measurements reveal a second peak effect and a critical current density Jc ~ 5×105 A/cm2 at 2 K in a self-field. The upper critical field anisotropy was systematically studied within the framework of the anisotropic Ginzburg–Landau theory, yielding an anisotropy value of approximately 2.6. Furthermore, we employ the diamond anvil cell technique to investigate the pressure effect on (La,Na,K)Fe2As2. Our results demonstrate that superconductivity is gradually suppressed with increasing pressure, while the normal state resistivity at low temperatures evolves from non-Fermi liquid to Fermi liquid behavior. This observation suggests that (La,Na,K)Fe2As2 may be situated on the right of the quantum critical point or, at the very least, within the over-doped region. These findings provide a critical sample platform and experimental insights for advancing the understanding of the physical properties of the newly discovered (La,Na,K)Fe2As2 superconductors.

Growth and characterization of superconducting Ca1–x Na x Fe2As2 single crystals by NaAs-flux method

High-quality superconducting Ca1−x Na x Fe2As2 single crystals have been successfully grown by the NaAs-flux method, with sodium doping level x = 0.4–0.64. The typical sizes of these crystals are more than 10 mm in ab-plane and ∼ 0.1 mm along c-axis in thickness. X-ray diffraction, resistance and magnetization measurements are carried out to characterize the quality of these crystals. While no signature of magnetic phase transitions is detected in the normal state, bulk superconductivity is found for these samples, with a sharp transition at T c ranging from 19.8 K (x = 0.4) to 34.8 K (x = 0.64). The doping dependences of the c-axis parameter and T c are consistent with previous reports, suggesting a possible connection between the lattice parameters and superconductivity.

Dynamic crossover across the peak effect in BaK x Fe2As2 superconductor for H-axis and H-plane

The vortex matter properties of a BaK x Fe2As2 single crystal ( K) were studied, by employing both isofield and isothermal ac-susceptibility measurements, , in a wide range of frequencies and amplitudes of the applied ac magnetic field. The irreversibility line (), formally defined by the onset of the third harmonic, is recorded for both -axis and -plane. It can be reproduced from the empirical equation, , with and Tesla, Tesla. The isofield measurements of the first harmonic revealed a narrow diamagnetic peak, related to a local peak of the critical current below the irreversibility line for both -axis and -plane. The local peak for Tesla is transformed to a sudden drop before it completely disappears. Detailed ac-susceptibility measurements were conducted for frequencies ranging within f = 0.1–10 kHz. From these data, the pinning potential, U, is deduced both as function of temperature and dc magnetic field. These results revealed that the ac response of vortex matter exhibits three distinct dynamic behaviors. By employing a model proposed by Mikitik and Brandt (2001 Phys. Rev. B 64 184514), that is based on a Lindemann type criterion and the collective pinning theory, we reproduced the experimentally recorded vortex matter phase diagram by taking into account both thermal fluctuations and random point disorder. To this effect, we adopted a pinning mechanism, , = 1.1–1.5 and a Ginzburg number .

Generalized phenomenological model for the magnetic field penetration and magnetization hysteresis loops of a type-II superconductor

A generalized phenomenological model for the critical state of type-II superconductors with magnetic field parallel to the superconducting plate is proposed. This model considers the global magnetization including both the equilibrium magnetization from surface screening current and the non-equilibrium magnetization from bulk pinning in a self-consistent way. Our model can be used to simulate the magnetization-hysteresis-loops (MHLs) and flux penetrating process of different type-II superconductors, from low- to high-kappa values. Here we take an optimally doped Ba0.6K0.4Fe2As2 single crystal as a testing example. The model can fit the data quite well and several important parameters can be extracted from the fitting. Thus, the model can be extended to a general case for studying the magnetization and flux penetration in other type-II superconductors.

Growth and Characterizations of Iron-based Superconductor (Ba1-x Rb x ) Fe2As2 Single Crystals

Novel nematic states as well as remarkable superconducting properties were reported in iron-based superconductors. We synthesized high-quality single crystals of (Ba1-x Rb x )Fe2As2 close to the optimal composition of x ≈ 0.4, and have characterized normal state and superconducting properties for pristine and proton irradiated samples. The highest Tc was found to be ∼36.6 K for pristine samples. A maximum critical current density of 12 MA/cm2 was achieved in a proton irradiated sample, which is comparable to that in irradiated (Ba0.6K0.4) Fe2As2. Twin boundaries were observed optically in both non-doped and under-doped samples. We also attempted local magnetic field measurements induced by transport current in a sample with twin boundaries for future exploration of nematic states in different regions of doping.

Vortex creep activation energies and depinning currents in CaKFe4As4 and Ba0.6K0.4Fe2As2 revealed by AC susceptibility measurements

Systematic ac susceptibility measurements have been carried out to study the vortex dynamics in CaKFe4As4 and Ba0.6K0.4Fe2As2 single crystals under various temperatures, dc magnetic fields, ac field frequencies and amplitudes. The field-temperature phase diagrams were shown, and the characteristics of irreversibility line were also derived. The specific expressions of activation energy on the parameters of temperature (T), current density (J), and dc magnetic field (H) are obtained according to these data. The results indicate that both superconductors have similar functional expressions of activation energy and flux pinning behaviors. Though both CaKFe4As4 and Ba0.6K0.4Fe2As2 superconductors exhibit very strong flux pinning ability, the vortex pinning potential in CaKFe4As4 is slightly smaller than that in Ba0.6K0.4Fe2As2, which may result from its distorted FeAs4-tetrahedron in CaKFe4As4. The depinning critical current densities at the limit of low temperature and low field were also extrapolated, yielding the corresponding values of Jc0(0) ∼ 1.0 × 108 and 2.2 × 108 A cm−2 for CaKFe4As4 and Ba0.6K0.4Fe2As2 superconductors, respectively, which suggest potential applications.

Hybrid Nb/Al/Ba0.5K0.5Fe2As2 sandwich Josephson junctions

We fabricated hybrid Josephson junctions between a Ba0.5K0.5Fe2As2 (BKFA) single crystal and the conventional superconductor Nb with normal metal Al as the barrier. An in situ process was used to create clean BKFA/Al/Nb interfaces. Current transport is along the c-axis of BKFA. The current–voltage characteristics are slightly hysteretic at low temperatures. Except for some features indicative of resonant modes the shape of the current–voltage characteristics can be well described by the resistively and capacitvely shunted junction (RCSJ) model, with a product of the Josephson critical current and the normal state resistance of the junction exceeding 180 μV at low temperatures. The temperature dependence of the Josephson current is linear for temperatures down to 2 K. Under 40 GHz microwave irradiation integer Shapiro steps are observed. The power dependence of the Shapiro step-height is in good agreement with RCSJ simulations. Possible relations of our data with the symmetry of the superconducting order parameter of BKFA are discussed.

Synthesis of Iron-Based Superconductor Ba0.6K0.4Fe2As2 by Mechanical Alloying

A polycrystalline sample of superconductor Ba0.6K0.4Fe2As2 was synthesized by mechanical alloying, which was confirmed by the X-ray diffraction analysis. The sample heat treatment time is reduced more than twice in comparison with the heat treatment time in the solid-phase synthesis method. The superconducting transition temperature of the synthesized sample is 37 K, which is close to the critical temperature for the Ba0.6K0.4Fe2As2 single crystal.

Non-resonant Microwave Absorption and Boundary Current Response of Ba0.34K0.64Fe2As2 Superconducting Single Crystals

Boundary current response of Ba0.34K0.64Fe2As2 (BaK122) single crystals was probed by non-resonant microwave absorption (NRMA) as a function of 100-KHz modulation field at an intermediate temperature, namely, 25 K, which is 10 K below the actual Tc. The measurements were done for both field parallel and perpendicular to the iron arsenide plane configuration. A weak anisotropic effect is noted. However, it is observed that the boundary current response depends on the modulation field value and it increases as we increase the modulation field value from 1 to 9 G. Further, this boundary current response strongly depends on the microwave power. We have shown that these modulation field and microwave power dependence of the boundary current response of NRMA signals in BaK122 iron arsenide crystal can be qualitatively understood through the Dulcic model.

Comparative study of vortex dynamics in CaKFe4As4 and Ba0.6K0.4Fe2As2 single crystals

We investigate the vortex dynamics in two typical hole doped iron based superconductors CaKFe4As4 (CaK1144) and Ba0.6K0.4Fe2As2 (BaK122) with similar superconducting transition temperatures. It is found that the magnetization hysteresis loop exhibits a clear second peak effect in BaK122 in wide temperature region while it is absent in CaK1144. However, a second peak effect of critical current density versus temperature is observed in CaK1144, which is however absent in BaK122. The different behaviors of second peak effect in BaK122 and CaK1144 may suggest distinct origins of vortex pinning in different systems. Magnetization and its relaxation have also been measured by using dynamical and conventional relaxation methods for both systems. Analysis and comparison of the two distinct systems show that the vortex pinning is stronger and the critical current density is higher in BaK122 system. It is found that the Maley’s method can be used and thus the activation energy can be determined in BaK122 by using the time dependent magnetization in wide temperature region, but this is not applicable in CaK1144 system. Finally we present the different regimes with distinct vortex dynamics in the field-temperature diagram for the two systems.

Current-voltage characteristics and vortex dynamics of superconducting Ba0.54K0.46Fe2As2 single crystal

We have investigated the current (I) - voltage (V) characteristics and vortex dynamics of Ba0.54K0.46Fe2As2 single crystal (BaK122) in H‖ab-plane and H‖c-axis orientations. The single crystal has been prepared by conventional growth method using self flux and characterized by onset Tc = 38.5 K, ΔTc = 1.0 K (offset-onset criterion) and residual resistivity ratio (RRR) ≈ 8. Usually for high-Tc superconductors, I - V curve shows exponential behavior due to flux creep at low magnetic field and it changes to near-linear nature with increase of magnetic field due to flux flow. We have shown that I –V characteristics differ significantly in H‖ab orientations compared to H‖c orientation which might be due to the presence of second magnetization peak (SMP). We have also estimated the activation energy using the Arrhenius plot in the thermally activated flux flow (TAFF) regime from the temperature dependent resistivity curves in both field orientation and have shown the variation of activation energy with magnetic field.

Effects of heavy-ion irradiation on the microwave surface impedance of (Ba1−xKx)Fe2As2 single crystals

The electrodynamic response of Ba1−xKxFe2As2 single crystals at the microwave frequencies has been investigated by means of a coplanar resonator technique, at different values of non-magnetic disorder introduced into the samples by heavy-ion irradiation. The surface impedance Zs = Rs + iXs conforms to the classical skin effect above the critical temperature. Below Tc, Rs monotonically decreases while Xs shows a peak, which evolves as a function of the irradiation fluence. The disorder-dependent Zs(T) curves are analyzed within a two-fluid model, suitably modified to account for a finite quasiparticle fraction at T = 0. The analysis gives, for the unirradiated crystal, quasiparticle relaxation times τ that are in good agreement with previous literature. Smaller τ values are deduced for the disordered crystals, both in the normal and in the superconducting states. The limits of application of the model are discussed.

Structure and Anisotropy of the Superconducting Order Parameter in Ba0.65K0.35Fe2As2 Probed by Andreev Spectroscopy

The superconducting order parameters in optimally doped Ba0.65K0.35Fe2As2 single crystals have been directly measured using multiple Andreev reflection effect spectroscopy of superconductor–normal metal–superconductor break-junctions. We determine two superconducting gaps, which are nodeless in the k x k y -plane of the momentum space, and resolve a substantial in-plane anisotropy of the large gap. The temperature dependences of the gaps indicate a strong coupling within the bands where ΔL develops, a weak coupling in the condensate with the small gap ΔS, and a moderate interband interaction between the two condensates. The own critical temperatures of both condensates have been estimated (under the hypotherical assumption of zero interband interaction).

In-field Conductivity Fluctuations in Ba0.72K0.28Fe2As2 Single Crystals

Fluctuations in the conductivity of Ba0.72K0.28Fe2As2 single crystal are studied systematically by resistance measurements as a function of temperature and magnetic field. A clear Maki−Thompson and Aslamakov−Larkin (MT–AL) two- to three-dimensional (2D–3D) crossover is found on the excess conductivity (Δσ) curves as the temperature approaches the superconducting critical temperature, T c. 3D fluctuations in superconductivity are realized near T c that are well fitted to experimental data by the 3D Aslamazov–Larkin theory. The Maki–Thompson model shows a 2D conductivity fluctuation above the 2D-3D temperature transition, T 0, which depends on magnetic field. Results show that the 2D-3D dimensional crossover moves to lower temperature with increasing magnetic field. The values of the transition temperature and the crossover in the reduced temperature, ln(e 0), as functions of magnetic field were used to determine the coherence length and the lifetime, τ φ , of the fluctuational pairs at the temperature of 35 K. Analysis of the Ba0.72K0.28Fe2As2 single crystal gives a value of 3.76 × 10− 12 s for the τ φ in the absence of magnetic field and it decreases to 2.4 × 10− 12 s in magnetic field of 13 T.

Nematic superconducting state in iron pnictide superconductors

Nematic order often breaks the tetragonal symmetry of iron-based superconductors. It arises from regular structural transition or electronic instability in the normal phase. Here, we report the observation of a nematic superconducting state, by measuring the angular dependence of the in-plane and out-of-plane magnetoresistivity of Ba0.5K0.5Fe2As2 single crystals. We find large twofold oscillations in the vicinity of the superconducting transition, when the direction of applied magnetic field is rotated within the basal plane. To avoid the influences from sample geometry or current flow direction, the sample was designed as Corbino-shape for in-plane and mesa-shape for out-of-plane measurements. Theoretical analysis shows that the nematic superconductivity arises from the weak mixture of the quasi-degenerate s-wave and d-wave components of the superconducting condensate, most probably induced by a weak anisotropy of stresses inherent to single crystals.

Effects of disorder induced by heavy-ion irradiation on (Ba1\u2212xKx)Fe2As2 single crystals, within the three-band Eliashberg s\xb1 wave model

One of the open issues concerning iron-based superconductors is whether the s± wave model is able to account for the overall effects of impurity scattering, including the low rate of decrease of the critical temperature with the impurity concentration. Here we investigate Ba1−xKxFe2As2 crystals where disorder is introduced by Au-ion irradiation. Critical temperature, Tc, and London penetration depth, λL, were measured by a microwave resonator technique, for different values of the irradiation fluence. We compared experimental data with calculations made on the basis of the three-band Eliashberg equations, suitably accounting for the impurity scattering. We show that this approach is able to explain in a consistent way the effects of disorder both on Tc and on λL(T), within the s± wave model. In particular, a change of curvature in the low-temperature λL(T) curves for the most irradiated crystals is fairly well reproduced.

Field-driven transition in Ba1−xKxFe2As2 with splayed columnar defects

Columnar defects in type-II superconductors serve as artificial pinning centers, which lead to enhancement of critical current density. However, little is known about the details of vortex motion in iron-based superconductors with columnar defects. In this study, we performed magnetization measurements in Ba0.6K0.4Fe2As2 single crystals with splayed columnar defects for several directions of magnetic field in order to investigate an anomalous peak effect in the system. We report results of detailed magnetization measurements, and discuss possible origins of the field-driven transition.

Boundary Current Response in Ba 0 . 3 4 K 0 . 6 4 Fe2As2 Single Crystal Probed by Non-resonant Microwave Absorption

Non-resonant microwave absorption (NRMA) in superconducting materials has become a new experimental technique to probe and understand superconducting materials. For example, cuprate superconductors are well studied with this technique. At the same time, the technique is also evolving. This NRMA technique has been used to study magnetic shielding effects/boundary current in Ba0.34 K 0.64Fe2As2 (BaK122) single crystals of iron pnictide superconducting sample measured at 9.4 GHz below T c (4.2–32 K). It has been observed that a small modulation field used in NRMA experiment yields the boundary current response. We have established that the boundary current response depends on both the modulation amplitude and the temperature. At high modulation field amplitudes and temperatures close to T c, the boundary current response gets suppressed and the flux-modulated response dominates. At low temperatures far away from T c, only the boundary current response dominates.

Phase diagram and transport properties of Sb-doped Ca0.88La0.12Fe2As2 single crystals

The effects of isovalent Sb substitution on the superconducting properties of the Ca0.88La0.12Fe2(As1-y Sb y )2 system have been studied through electrical resistivity measurements. It is seen that the antiferromagnetic or structural transition is suppressed with Sb content, and a high-T c superconducting phase, accompanied by a low-T c phase, emerges at 0.02 ≤ y ≤ 0.06. In this intermediate-doping regime, normal-state transport shows non-Fermi-liquid-like behaviors with nearly T-linear resistivity above the high-T c phase. With further Sb doping, this high-T c phase abruptly vanishes for y > 0.06 and the conventional Fermi liquid is restored, while the low-T c phase remains robust against Sb impurities. The coincidence of the high-T c phase and non-Fermi liquid transport behaviors in the intermediate Sb-doping regime suggests that AFM fluctuations play an important role in the observed non-Fermi liquid behaviors, which may be intimately related to the unusual nonbulk high-T c phase in this system.

Penetration Depth and Quasiparticle Conductivity of Co- and K-Doped BaFe2As2Crystals, Investigated by a Microwave Coplanar Resonator Technique

In this work, an experimental approach toward reaching new insights into the mechanism of superconductivity in the iron-based compounds is presented. The temperature dependence of the penetration depth and of the microwave conductivity of high-quality Ba(Fe1-xCox)2As2 and (Ba1-xKx)Fe2As2 single crystals have been obtained by a 7.97-GHz superconducting coplanar resonator technique, in a perturbative approach. The proposed technique is suitable to obtain the absolute value of the penetration depth and shows high sensitivity to small samples. For the Co-doped BaFe2As2, the comparison of crystals with different shape factors allows distinguishing the in-plane and out-of-plane components of the penetration depth. Peaks in the normalized quasiparticle conductivity of both Co-doped and K-doped crystals are shown and discussed.