BaFe2As2 Single Crystals Research Articles

The Superconductivity in Bi-Doped BaFe2As2 Single Crystals.

We have successfully synthesized a series of Bi-doped BaFe2As2 high-quality single crystals for the first time. X-ray diffraction (XRD) patterns show an expansion of lattice parameter c with Bi doping, indicating a negative pressure effect. By investigating the resistivity, a Fermi liquid (FL) to non-Fermi liquid (NFL) crossover is observed from normal state to antiferromagnetic order state, accompanied by three superconducting transitions labeled as SC I, SC II and SC III, which are supposed to be induced by three superconducting realms with various Bi concentrations. Thus, we propose that the NFL behavior is closely related to the presence of superconductivity. The magnetic susceptibility measurements further speculate that the SC I and SC III phases should exhibit filamentary superconductivity while the SC II exhibits a possible bulk superconductivity of TC~7 K.

Enhanced flux pinning by refined structural/magnetic domains in nickel-doped BaFe2As2 single crystals

The critical current density at high magnetic fields of high-temperature superconducting single crystals and films is mainly determined by the flux pinning force arising from interactions between fluxons and crystalline defects or secondary phases. However, iron-based superconductors, which are acknowledged as promising candidates for high-field applications, suffer from a lack of strong flux pinning centers. It leads to a critical current density (Jc) inferior to that of iron-based superconducting films. In this work, we applied a simple annealing method to tailor flux pinning structures in slightly underdoped BaFe2-xNixAs2 single crystals without significantly compromising superconductivity. We propose that the probable strengthened orthorhombic distortion by post-annealing could generate twin domain boundaries or short-range antiferromagnetic clusters with sizes comparable to the in-plane coherence length. The inclusion of the structural/magnetic structures enhances electron scattering but also provides potent flux pinning centers. Ultimately, the Jc is enhanced by 2.5 times in the samples with optimized pinning landscapes. Our studies highlight the importance of further exploring underdoped iron-based superconductors, in which magnetic/orthorhombic phases that compete and coexist with superconductivity may also act as effective pinning centers.

Competitive and cooperative electronic states in Ba(Fe1\u2212xTx)2As2 with T\u2009=\u2009Co, Ni, Cr

The electronic inhomogeneities in Co, Ni, and Cr doped BaFe2As2 single crystals are compared within three bulk property regions: a pure superconducting (SC) dome region, a coexisting SC and antiferromagnetic (AFM) region, and a non-SC region. Machine learning is utilized to categorize the inhomogeneous electronic states: in-gap, L-shape, and S-shape states. Although the relative percentages of the states vary in the three samples, the total volume fraction of the three electronic states is quite similar. This is coincident with the number of electrons (Ni0.04 and Co0.08) and holes (Cr0.04) doped into the compounds. The in-gap state is confirmed as a magnetic impurity state from the Co or Ni dopants, the L-shape state is identified as a spin density wave which competes with the SC phase, and the S-shape state is found to be another form of magnetic order which constructively cooperates with the SC phase, rather than competing with it.

Comprehensive Eliashberg analysis of microwave conductivity and penetration depth of K-, Co-, and P-substituted BaFe2As2

We report on the combined experimental and theoretical analysis of the microwave-frequency electromagnetic response of BaFe2As2 single crystals with different substitutions: K in the Ba site (hole doping), Co in the Fe site (electron doping), and P in the As site (isovalent substitution). Measurements using a coplanar resonator technique lead to the experimental determination of the penetration depth and microwave conductivity as a function of temperature. The whole set of data is analyzed within a self-consistent three-band $s_{\pm}$-wave Eliashberg approach, able to account for all the main observed features in the different properties. Besides the validation of the model itself, the comparison between experiment and theory allows discussing the possible role of the Fe-As planes in defining the superconducting properties of these compounds, the relevance of coherence effects, and the presence of nodes in the superconducting order parameter.

In-plane and out-of-plane properties of a BaFe2As2 single crystal

Anisotropy of transport and magnetic properties of parent compounds of iron based superconductors is a key ingredient of superconductivity. In this work, we investigate in-plane and out-of-plane properties, namely thermal, electric, thermoelectric transport and magnetic susceptibility in a high quality BaFe2As2 single crystal of the 122 parent compound, using a combined experimental and theoretical approach. Combining the ab initio calculation of the band structure and the measured in-plane and out-of-plane resistivity, we evaluate the scattering rates which turn out to be strongly anisotropic and determined by spin excitations in the antiferromagnetic state.The observed anisotropy of thermal conductivity is discussed in terms of anisotropy of sound velocities which we estimate to be . Remarkably, we find that thermal conductivity is characterized by a sizeable electronic contribution at low temperature, which is ascribed to the high purity of our crystal.

Characterizing the Josephson Effect on Ba-122 Single-Crystal Junctions

This paper presents c-direction hybrid Josephson junctions with base electrodes of Co:BaFe2As2 single crystals. To realize Josephson junctions, it applies standard photolithography technologies and a newly developed surface polishing method that was used. As conventional isotropic s-wave superconductor counter electrodes used double layers of Pb/In. The artificial barrier for the tunneling junctions consists of a thin sputtered Ti layer or a thermally evaporated Al layer; both layers were used as an insulator barrier. For characterizing the Josephson effects with TiOx barrier, we could realize IcRN products of about 13.7 μV and unconventional Ic-T dependencies. The formation of Shapiro steps under the influence of microwave irradiation and magnetic field to the junctions will be discussed as well.

Anisotropy of the Seebeck and Nernst coefficients in parent compounds of the iron-based superconductors

In-plane longitudinal and transverse thermoelectric phenomena in two parent compounds of iron-based superconductors are studied. Namely, the Seebeck (S) and Nernst (n) coefficients were measured in the temperature range 10 - 300 K for BaFe2As2 and CaFe2As2 single crystals that were detwinned in-situ. The thermoelectric response shows sizeable anisotropy in the spin density wave state (SDW) for both compounds, while some dissimilarities in the vicinity of the SDW transition can be attributed to the different nature of the phase change in BaFe2As2 and CaFe2As2. Temperature dependences of S and n can be described within a two-band model that contains a contribution from highly mobile, probably Dirac, electrons. The Dirac band seems to be rather isotropic, whereas most of the anisotropy in the transport phenomena could be attributed to "regular" hole-like charge carriers. We also observe that the off-diagonal element of the Peltier tensor axy is not the same for the a and b orthorhombic axes, which indicates that the widely used Mott formula is not applicable to the SDW state of iron-based superconductors.

Structural, electro-magnetic, and optical properties of Ba(Fe,Ni)2As2 single-crystal thin film

We investigated the superconducting transition temperature (Tc), critical current density (Jc) and optical properties of optimally doped Ba(Fe0.95Ni0.05)2As2 (Ni-Ba122) single-crystalline epitaxial thin films grown by pulsed laser deposition for the first time. The Tc at zero resistivity was about 20.5 K and the Jc at self-field and 4.2 K was 2.8 MA cm−2 calculated by the Bean model. The superconducting properties such as Tc and Jc of thin films are comparable to those of bulk single-crystal samples. The superfluid plasma frequency (λp,S) of Ni-Ba122 thin film is ∼7033 cm−1 obtained by optical spectroscopic technique. Based on this plasma frequency, we obtained the London penetration depth (λL), ∼226 nm at 8 K, which is comparable to those of optimally Co- and K-doped BaFe2As2 single crystals.

On the determination of hardness and elastic modulus in BaFe2As2 lamellar-like material

Abstract

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.

Role of magnetism in superconductivity ofBaFe2As2: Study of5dAu-doped crystals

We investigate properties of BaFe2As2 (122) single crystals upon gold doping, which is the transition metal with the highest atomic weight. The Au substitution into the FeAs-planes of 122 crystal structure (Au-122) is only possible up to a small amount of ~3%. We find that 5d is more effective in reducing magnetism in 122 than its counter 3d Cu, and this relates to superconductivity. We provide evidence of short-range magnetic fluctuations and local-lattice inhomogeneities that may prevent strong percolative superconductivity in Ba(Fe1-xAux)2As2.

Enhancement of the critical current density by increasing the collective pinning energy in heavy ion irradiated Co-doped BaFe2As2 single crystals

We investigate the effect of heavy ion irradiation (1.4 GeV Pb) on the vortex matter in Ba(Fe0.92Co0.08)2As2 single crystals by superconducting quantum interference device (SQUID) magnetometry. The defects created by the irradiation are discontinuous amorphous tracks, resulting in an effective track density smaller than 25% of the nominal doses. We observe large increases in the critical current density (Jc), ranging from a factor of ∼3 at low magnetic fields to a factor of ∼10 at fields close to 1 T after irradiation with a nominal fluence of BΦ = 3.5 T. From the normalized flux creep rates (S) and the Maley analysis, we determine that the Jc increase can be mainly attributed to a large increment in the pinning energy, from <50 K to ≈500 K, while the glassy exponent μ changes from ∼1.5 to <1. Although the enhancement of Jc is substantial in the entire temperature range and S is strongly suppressed, the artificial pinning landscape induced by the irradiation does not modify significantly the crossover to fast creep in the field-temperature vortex phase diagram.

Combined external pressure and Cu-substitution studies on BaFe2As2 single crystals

We report a combined study of external pressure and Cu-substitution on BaFe2As2 single crystals grown by the in-flux technique. At ambient pressure, the Cu-substitution is known to suppress the spin density wave (SDW) phase in pure BaFe2As2(TSDW ≈ 140 K) and to induce a superconducting (SC) dome with a maximum transition temperature . This is much lower than the Tc ∼ 15–28 K achieved in the case of Ru, Ni and Co substitutions. Such a lower Tc is attributed to a Cu2+ magnetic pair-breaking effect. The latter is strongly suppressed by applied pressure, as shown herein, Tc can be significantly enhanced by applying high pressures. In this work, we investigated the pressure effects on Cu2+ magnetic pair-breaking in the BaFe2−xCuxAs2 series. Around the optimal concentration (xopd = 0.11), all samples showed a substantial increase of Tc as a function of pressure. Yet for those samples with a slightly higher doping level (over-doped regime), Tc presented a dome-like shape with maximum Tc ≃ 8 K. Remarkably interesting, the under-doped samples, e.g. x = 0.02 display a maximum pressure induced Tc ≃ 30 K which is comparable to the maximum Tc's found for the pure compound under external pressures. Furthermore, the magnetoresistance effect as a function of pressure in the normal state of the x = 0.02 sample also presented an evolution consistent with the screening of the Cu2+ local moments. These findings demonstrate that the Cu2+ magnetic pair-breaking effect is completely suppressed by applying pressure in the low concentration regime of Cu2+ substituted BaFe2As2.

Raman Spectroscopy of Ba(Fe1−x Mn x )2As2

Raman scattering measurements on iron–pnictide Mn-doped BaFe2As2 single crystals are reported. Single crystals were grown out of a FeAs self-flux using conventional high-temperature solution growth and characterized by X-ray diffraction, atomic force microscopy, and Raman. Raman spectra were obtained at room temperature and 77 K on ab- and a(b)c-planes. Two of four phonon modes allowed by symmetry were found and identified. It was observed that the scattering intensity of A1g mode and the frequencies of the A1g and B1g phonons are dependent upon doping of Mn. The dependence of scattering intensity and frequency of A1g mode on Mn doping might indicate that the Mn ion also occupies the As site.

Site specific spin dynamics in BaFe2As2: tuning the ground state by orbital differentiation.

The role of orbital differentiation on the emergence of superconductivity in the Fe-based superconductors remains an open question to the scientific community. In this investigation, we employ a suitable microscopic spin probe technique, namely Electron Spin Resonance (ESR), to investigate this issue on selected chemically substituted BaFe2As2 single crystals. As the spin-density wave (SDW) phase is suppressed, we observe a clear increase of the Fe 3d bands anisotropy along with their localization at the FeAs plane. Such an increase of the planar orbital content is interestingly independent of the chemical substitution responsible for suppressing the SDW phase. As a consequence, the magnetic fluctuations in combination with this particular symmetry of the Fe 3d bands are propitious ingredients for the emergence of superconductivity in this class of materials.

Magnetic phase diagram and critical current of BaFe2As2 single crystals with hole- and electron-doping

We report on comparative study of magnetic phase diagram and critical current of the hole- and electron-doped BaFe2As2 single crystals with close values of superconducting critical temperature, Tc, (slightly underdoped Ba0.64K0.36Fe2As2 with Tc=25K and optimally doped BaFe1.9Ni0.1As2 with Tc=20K) obtained from measurements of the temperature dependence of ac-susceptibility and isothermal irreversible magnetization loops, M(H), in magnetic fields parallel to the c-axis of the crystal. From ac-susceptibility measurements we get estimation of a slope of the upper critical field, Hc2, in dependence on temperature, dHc2/dT ≈ −4.2T/K for BaFe1.9Ni0.1As2 single crystal and Hc2/dT ≈ −1.75T/K for Ba0.64K0.36Fe2As2 sample that in accordance with Werthamer, Helfand, and Hohenberg (WHH) model gives Hc2(0) = 0.69Tc((dHc2)dT) ≈ 56T for BaFe1.9Ni0.1As2 sample and lower value of Hc2(0) ≈ 31T for Ba0.64K0.36Fe2As2 crystal. However, obtained from M(H) measurements temperature dependence of the irreversibility field, Hirr(T), for BaFe1.9Ni0.1As2 crystal located below the one for Ba0.64K0.36Fe2As2 crystal. Furthermore, at T=4.2K and higher temperatures our results for critical current density, Jc, calculated from M(H) curves clearly show slower reduction of Jc with increasing field in even underdoped Ba0.64Ka0.36Fe2As2 sample compared to optimally doped BaFe1.9Ni0.1As2 crystal demonstrating higher capacity of K-doped 122 compounds for production of superconducting cables and wires with high critical current in strong magnetic fields.

High field nuclear magnetic resonance in transition metal substituted BaFe2As2

We report high field 75As nuclear magnetic resonance (NMR) measurements on Co and Cu substituted BaFe2As2 single crystals displaying same structural/magnetic transition T0≃128 K. From our anisotropy studies in the paramagnetic state, we strikingly found virtually identical quadrupolar splitting and consequently the quadrupole frequency νQ≃2.57(1) MHz for both compounds, despite the claim that each Cu delivers 2 extra 3d electrons in BaFe2As2 compared to Co substitution. These results allow us to conclude that a subtle change in the crystallographic structure, particularly in the Fe–As tetrahedra, must be the most probable tuning parameter to determine T0 in this class of superconductors rather than electronic doping. Furthermore, our NMR data around T0 suggest coexistence of tetragonal/paramagnetic and orthorhombic/antiferromagnetic phases between the structural and the spin density wave magnetic phase transitions, similarly to what was reported for K-doped BaFe2As2 [Urbano et al., Phys. Rev. Lett. 105, 107001 (2010)].

Transport critical current measurements on a Cu-substituted BaFe2As2 superconductor

The critical current density Jc is a crucial parameter to establish the actual technological potential of a superconducting (SC) material. Furthermore, being proportional to the SC gap parameter, it can reveal important information about the microscopic nature of the SC state in a given material. The FeAs-based class of SC materials has been a focus of intense scientific investigation lately, but direct investigation of Jc by transport measurements is rather scarce in literature. For these materials, it is very interesting to map Jc as a function of their distinct SC tuning parameters such as applied pressure and chemical substitution. In this work, detailed investigation of the field, temperature, and pressure dependences of transport critical current density Jc for Cu-substituted BaFe2As2 single crystals is reported. In this particular material, Cu-substitution has a strong magnetic pair breaking effect. However, with increasing pressure, this sample shows an almost twofold increase of Tc, from 3.2 K to 6.9 K, which is followed by an increase in Jc. These observations are discussed considering the presence of magnetic pinning centers in the Fe-As plane, which, in principle, could suggest effective routes to increase Jc in the this class of materials.

Pressure effects on magnetic pair-breaking in Mn- and Eu-substituted BaFe2As2

We report a combined study of hydrostatic pressure (P ≤ 25 kbar) and chemical substitution on the magnetic pair-breaking effect in Eu- and Mn-substituted BaFe2As2 single crystals. At ambient pressure, both substitutions suppress the superconducting (SC) transition temperature (Tc) of BaFe2–xCoxAs2 samples slightly under the optimally doped region, indicating the presence of a pair-breaking effect. At low pressures, an increase of Tc is observed for all studied compounds followed by an expected decrease at higher pressures. However, in the Eu dilute system, Tc further increases at higher pressure along with a narrowing of the SC transition, suggesting that a pair-breaking mechanism reminiscent of the Eu Kondo single impurity regime is being suppressed by pressure. Furthermore, Electron Spin Resonance (ESR) measurements indicate the presence of Mn2+ and Eu2+ local moments and the microscopic parameters extracted from the ESR analysis reveal that the Abrikosov–Gor'kov expression for magnetic pair-breaking in a conventional sign-preserving superconducting state cannot describe the observed reduction of Tc.

Positron probing of electron momentum re-distribution at the superconducting transition in Ba(Fe1−xCox)2As2 single crystals

Superconducting Ba(Fe0.943Co0.057)2As2 single crystals with Tc (R=0)=19.5K, and non-superconducting BaFe2As2 single crystals have been studied by coincidence Doppler broadening of positron annihilation radiation line-shape (coincidence DBPARL) down to ~14K. This appears to be the first reporting of positron probing of any Fe-based superconductor. The superconducting sample shows, on cooling below ~40K and towards Tc, a sharp decrease of S, the line-shape parameter, which gives the fraction of suitably defined low momentum electrons as probed by the positrons. No such decrease of S for the non-superconducting sample indicates the effect to be induced by superconductivity. The ratio curve analysis of the coincidence DBPARL spectra suggests that in the superconducting state the positrons are annihilating less with the 3d electrons of Fe and Co and more with the 5s and 4d electrons of Ba. In addition, a novel double measurement of coincidence DBPARL for the a–b plane and planes normal to the a–b plane of the single crystalline sample shows a nearly isotropic distribution of the lower electron momentum in this Fe-based superconductor.