Single Crystal Of Ba0 Research Articles

Anisotropic properties of Ba1−xSrxFe12O19 single crystals

Single crystals of Ba1−xSrxFe12O19 with x = 0, 0.25, 0.5, 0.75, and 1 were grown from a Na2CO3 flux.

Approaching the ultimate superconducting properties of (Ba,K)Fe2As2 by naturally formed low-angle grain boundary networks

The most effective way to enhance the dissipation-free supercurrent in the presence of a magnetic field for type II superconductors is to introduce defects that act as artificial pinning centers (APCs) for vortices. For instance, the in-field critical current density of doped BaFe2As2 (Ba122), one of the most technologically important Fe-based superconductors, has been improved over the last decade by APCs created by ion irradiation. The technique of ion irradiation has been commonly implemented to determine the ultimate superconducting properties. However, this method is rather complicated and expensive. Here, we report a surprisingly high critical current density and strong pinning efficiency close to the crystallographic c-axis for a K-doped Ba122 epitaxial thin film without APCs, achieving performance comparable to ion-irradiated K-doped Ba122 single crystals. Microstructural analysis reveals that the film is composed of columnar grains with widths of approximately 30–60 nm. The grains are rotated around the b- (or a-) axis by 1.5° and around the c-axis by −1°, resulting in the formation of low-angle grain boundary networks. This study demonstrates that the upper limit of in-field properties reached in ion-irradiated K-doped Ba122 is achievable by grain boundary engineering, which is a simple and industrially scalable manner.

Fabrication of Corner-Like Josephson Junctions Based on Pnictide Single crystals

In order to study fundamental properties of pnictide superconductors such as the pairing symmetry, we have fabricated corner-like Josephson devices. Their configuration contains two SNS* (superconductor/barrier/superconductor*) thin-film junctions in different crystal directions. We have used multi-band Co-doped Ba-122 single crystals as the base electrode (S), evaporated lead films as a counter electrode (S*) made of a conventional s-wave superconductor and normal-metal (N) layers as the barrier. The junctions exhibit symmetric I-V characteristics with IcRN values about 0.975 μV. Ic is the critical current, and RN is the normal-state resistance. The observed Fraunhofer pattern-like dependence of Ic on the magnetic field indicates homogeneity of the junctions studied.

Lu-atom-ordered oxonitridoaluminosilicate Ba0.9Ce0.1LuAl0.2Si3.8N6.9O0.1.

A single crystal of Ba0.9Ce0.1LuAl0.2Si3.8N6.9O0.1 (barium cerium lutetium aluminosilicate nitride oxide) was obtained by heating a mixed powder of Ba3N2, Si3N4, Al, Lu2O3, and CeO2 at 2173 K for 1 h under N2 gas at 0.85 MPa. X-ray single-crystal structure analysis revealed that the title oxynitride is hexa-gonal (lattice constants: a = 6.0378 (5) Å, c = 9.8133 (9) Å; space group: P63 mc) and isostructural with BaYbSi4N7. (Ba,Ce) and Lu atoms occupy twelvefold and sixfold coordination sites, respectively.

Low-field vortex melting in a single crystal of Ba0.6K0.4Fe2As2

Theoretically, the vortex melting phenomenon occurs at both low and high magnetic fields at a fixed temperature. While the high field melting has been extensively investigated in high Tc cuprates, the low field melting phenomena in the presence of disorder hasn't been well explored. Using bulk magnetization measurement and high-sensitivity differential magneto-optical imaging technique, we detect a low-field vortex melting phenomenon in a single crystal of Ba0.6K0.4Fe2As2. The low field melting is accompanied by a significant change in local magnetization ~ 3 G, which decreases with increasing applied field. The observed vortex melting phenomena is traced on a field temperature phase diagram and which lies very close to theoretically predicted Lindemann criteria based low field melting line. Our analysis shows a Lindemann number cL = 0.14 associated with the low field melting. Imaging of low-field vortex melting features shows the process nucleates via formation of extended finger-like projections which spreads across the sample with increasing field or temperature, before entering into an interaction-dominated vortex solid phase regime. Magnetization scaling analysis shows that the dimensionality of melting vortex state is close to one. Angular dependence of bulk magnetization hysteresis loop in our sample shows the presence of extended defects. From our studies, we propose the sample contains a peculiar geometry of extended defects arranged in a plane in the sample, with these planes extending through the sample thickness. In the weak intervortex interaction limit, we argue that reduced vortex dimensionality due to pinning by these peculiar extended defect planes strongly enhances thermal fluctuations. It is these extended defects planes, which we propose are promoting low dimensional vortex melting in the pnictide system.

Doping dependence of the pinning efficiency in K-doped Ba122 single crystals prior to and after fast neutron irradiation

A sharp peak was observed in the doping dependence of the critical current density, Jc, in potassium doped Ba122 single crystals. This behavior is in contrast to the doping dependence of the transition temperature, Tc, which varies much more smoothly around its maximum. We performed fast neutron irradiation on the crystals in order to find out whether the Jc peak results from intrinsic properties or the particular defect landscape. Fast neutrons are known to introduce defects up to a size of a few nanometers, which have proven to be more efficient for flux pinning than the crystallographic defects in the pristine crystals. We find that the peak in Jc shifts to higher doping levels after the irradiation, broadens, and roughly follows the shape of the Tc curve. Moreover, a power law between Jc and Tc is observed in the irradiated crystals, which can be explained by relations between fundamental parameters and Tc observed previously in iron-based superconductors. This power law does not hold for the pristine crystals which indicates that the doping dependence of Jc results from an enhanced pinning efficiency in the under-doped area of the phase diagram.

Anomalous Metal Interface Effect of Iron-based Superconductors

We report the results of the Andreev spectroscopy of the (Co-doped Ba-122 single crystals)/metal/Pb junctions with a metal layer of Al, Pb, Ag, and Sn. We observed Andreev-like spectra for each metal layer and the junctions were identified to be (superconductor)/(normal metal) type. The spectra showed different behaviors for each metal layer. Especially in the case of Sn, the junction showed complex structure where different S/N type features were folded. We consider this behavior is due to a chemical reaction, in the interface between an iron-based superconductor and a layering metal.

On the structure of the superconducting order parameter in high-temperature Fe-based superconductors

We discuss the synthesis, characterization, and comprehensive study of Ba-122 single crystals with various substitutions and various superconducting transition temperatures. We use five complementary techniques to obtain a self-consistent set of data on the superconducting properties of Ba-122. A major conclusion of our work is the coexistence of two superconducting condensates differing in the electron–boson coupling strength. The two gaps that develop in distinct Fermi surface sheets are nodeless in the kxky plane and exhibit s-wave symmetry; the two-band model suffices for the description of the main parameters of the superconducting state. A moderate interband coupling and a considerable Coulomb repulsion in the description of the two-gap superconducting state of barium pnictides favor the s++ model.

Planar Hybrid Josephson Junctions Based on Ba-122 Single Crystals

We prepared SIS' (insulating barrier I, different superconducting electrodes S, and S') hybrid Josephson junctions on the polished surface of Ba-122 single crystals as a base electrode and used Pb as a thin-film counter electrode. We obtained $I_{c}R_{N}$ -products up to 1.15 mV and got remarkable $I_{c}R_{N}\mbox{-}T$ dependence. The influence of microwave irradiation to the junctions will be discussed.

Single-Crystal Growth of Ba1−xKxFe2As2 by KAs Self-Flux Method

Single crystals of Ba1−xKxFe2As2 with 0.15 ≤ x ≤ 1 have been successfully synthesized by a KAs self-flux method. The potassium (K) concentration x of the grown crystals was systematically changed by changing the mixing ratio of Ba to Fe in the starting materials. The crystals have flat surfaces corresponding to the crystallographic (001) planes, whose planar dimensions increase with decreasing thickness when x increases. The superconducting transition temperature systematically changes with x, ranging from 38 K at x = 0.4 to 3.4 K at x = 1.0, in good agreement with the results for polycrystalline samples.

Effect of growth mechanisms on the deformation of a unit cell and polarization reversal in barium–strontium titanate heterostructures on magnesium oxide

The effect of a growth mechanism on the unit cell strain and the related change in the properties of single-crystal Ba0.8Sr0.2TiO3 films grown on MgO substrates according to the Frank–van der Merwe and Volmer–Weber growth mechanisms is studied. The unit cell strain is shown to depend substantially on the film thickness and the growth mechanism. It is found that the same film–substrate pair can be used to vary stresses in the film from two-dimensional tensile to compressive stresses due to a change in the growth mechanism and the film thickness.

Preparation of hybrid Josephson junctions on Co-doped Ba-122 single crystals

In this paper we present a method for processing a hybrid Josephson junction on Co-doped (Ba-122) single crystals with a thin film Pb-counter electrode and a barrier layer of . This includes the leveling and polishing of the crystals and structuring them with thin film techniques such as photo lithography, sputtering and ion beam etching. The junctions show hysteretical resistively and capacitively shunted junction-like I–V characteristics with an -product of about 800 .

Superconducting properties of hole doped Ba0.6Li0.4Fe2As2 single crystal

We report the emergence of superconductivity in Li doped Ba-122 single crystals grown by the Bridgman method. The superconducting transition temperature Tc,onset is around 19 K. The specific heat capacity C/T shows a weak anomaly near Tc. The value of ΔC/γnTc is smaller than the value predicted in BCS theory indicating a multigap nature of the sample. The magnetic measurements show that the lower critical field Hc1(T) exhibits a linear temperature dependence, with a pronounced change of the Hc1(T) curvature around 0.4Tc and Hc1(0) ≈ 430 Oe in the Ba0.6Li0.4Fe2As2 single crystal. Furthermore, temperature dependence of the penetration depth λ(T) follows a power law (~Tn) below 0.4Tc which predicts possible S±-wave pairing in a Ba0.6Li0.4Fe2As2 superconductor. Over a wide range of temperatures, the Jc(H) exhibits a relation with α = 0.5 ~ 0.6 for H || c and H || ab which indicates random defects in the sample. We found that the temperature dependence of the critical current density Jc(T) can be fitted well with the -type pinning model, whose origin is attributed to spatial variations of charge carrier mean free path . We suggest that the large mismatch in the ionic radius of Ba and Li can affect the irreversible magnetic properties of the Ba0.6Li0.4Fe2As2 single crystal without any structural transition.

Microscopic coexistence of antiferromagnetism and superconductivity in hole-doped Iron-pnictides Ba0.77K0.23Fe2As2 revealed by NMR

We have performed nuclear magnetic resonance (NMR) measurements on an underdoped single-crystal Ba0.77K0.23Fe2As2 with Tc = 16.5 K. Below TN = 46 K, an internal magnetic field splits the NMR peaks of H ∥ c and shifts those of H ∥ a to higher frequencies. The nuclear spin-lattice relaxation rate 1/T1 measured at the central peak with H ∥ a shows a distinct decrease below Tc(μ0H = 12 T) = 16 K. Our results clearly show that antiferromagnetic order and superconductivity coexist at a microscopical length scale.

Pressure-induced superconductivity in Ba0.5Sr0.5Fe2As2

High-pressure electrical resistance measurements have been performed on single crystal Ba0.5Sr0.5Fe2As2 platelets to pressures of 16 GPa and temperatures down to 10 K using designer diamond anvils under quasi-hydrostatic conditions with an insulating steatite pressure medium. The resistance measurements show evidence of pressure-induced superconductivity with an onset transition temperature at ∼31 K and zero resistance at ∼22 K for a pressure of 3.3 GPa. The transition temperature decreases gradually with increasing pressure before completely disappearing for pressures above 12 GPa. The present results provide experimental evidence that a solid solution of two 122-type materials, i.e., Ba1−xSrxFe2As2 (0 < x < 1), can also exhibit superconductivity under high pressure.

Microscopic coexistence of antiferromagnetic order and superconductivity in Ba0.77K0.23Fe2As2

We report ^{75}As nuclear magnetic resonance studies on an underdoped single-crystal Ba0.77K0.23Fe2As2 with Tc = 16.5 K. Below T_N = 46 K, the NMR peaks for H//c split and those for H //a shift to higher frequencies, which indicates that an internal magnetic field along the c axis develops below T_N. The spin-lattice relaxation rate 1/T_1 measured at the shifted peak with H//a, which experiences the internal field, shows a distinct decrease below Tc({\mu}_0H = 12 T) = 16 K, following a T^3 relation at low temperatures. Our results show unambiguously that antiferromagnetic order and superconductivity coexist microscopically. The unusual superconducting state with the coexisting magnetism is highlighted.

Giant magnetic flux jumps in single crystals of Ba0.6K0.4Fe2As2

Giant magnetic flux jumps are observed in magnetic hysteresis loops of Ba0.6K0.4Fe2As2 single crystals. The size of the flux jumps, which appear only at low temperatures (T&amp;lt;4 K), is so large that it can transform the whole superconducting state into the normal state. The recovery rate to the superconducting state is rather slow, although the superconducting state is almost fully recovered. We show that theoretical predictions based on the adiabatic approach with nonlocal electrodynamics give a good explanation of the flux jumps observed in the single crystals of Ba0.6K0.4Fe2As2.

One-step method to grow Ba0.6K0.4Fe2As2 single crystals without fluxing agent

Single crystals of Ba0.6K0.4Fe2As2 have been successfully grown without using any fluxing agent through a simple one-stepmethod. X-ray diffraction patterns demonstrate that they have high crystalline quality andc-axis orientation. The onset transition temperature is up to 38 K,while the zero resistivity temperature reaches 36.7 K. Both theR–T andM–T data show very sharp superconducting transitions and the transition width is about 0.4 K.We also found that the samples possess very large current carrying ability and high uppercritical field, indicating potential applications requiring very high field. The above simpleand safe one-step technique of single crystal growth can be effective in other systems ofFe-based superconductors.

Softening of Bond Stretching Phonon Mode in Ba1−x K x BiO3 Superconductor

Single crystals of Ba1−xKxBiO3 compound for different values of x (0≤x≤0.6) from insulator to superconducting region have been grown by electrochemical method. The crystals have been characterized by powder X-ray diffraction and Laue X-ray to determine the crystal structure, phases and potassium concentration. The phonon dispersion of the crystals in (100) direction has been investigated by high-resolution inelastic X-ray scattering. The phonon dispersion for low energy region is almost similar for all crystals measured in this study, while the higher energy modes shift to higher energy by increasing the potassium concentration. Anomalous softening of highest energy phonon has been observed across the insulator–superconductor transition around q∼0.25. Observation of the softening in the superconducting samples indicates a correlation between the softening and superconductivity in this system.

Disorder effects and current percolation in FeAs-based superconductors

We report the influence of atomic disorder introduced by sequentialneutron irradiation on the basic superconducting properties, flux pinningand grain connectivity. Two different polycrystalline Sm-1111 samples (SmFeAsO1 − xFx) and two Ba-122single crystals (BaFe1.8Co0.2As2) were investigated. The monotonic decrease of the transition temperature with neutronfluence degrades the upper critical field, at least in the investigated temperature region.Pinning, on the other hand, is largely improved, with a different optimal defectconcentration (fluence) in the two materials. The analysis of the current flow in thepolycrystalline samples reveals weak link behavior in the majority of grain connections andthe existence of stronger grain connections. The density of the latter seems to be close tothe percolation threshold (i.e. the minimum fraction for a continuous current path). Bothtypes of connections are sensitive to disorder and degrade upon neutron irradiation.