arXiv subjects

Bernhard Holzapfel

Publications and source records attributed to Bernhard Holzapfel.

At least 19 recordsLinked to original sources

arXiv2024

Structural analysis and transport properties of [010]-tilt grain boundaries in Fe(Se,Te)

Understanding the nature of grain boundaries is a prerequisite for fabricating high-performance superconducting bulks and wires. For iron-based superconductors [e.g. Ba(Fe,Co)$_2$As$_2$, Fe(Se,Te), and NdFeAs(O,F)], the dependence of the critical current density $J_\mathrm{c}$ on misorientation angle ($θ_\mathrm{GB}$) has been explored on [001]-tilt grain boundaries, but no data for other types of orientations have been reported. Here, we report on the structural and transport properties of Fe(Se,Te) grown on CeO$_2$-buffered symmetric [010]-tilt roof-type SrTiO$_3$ bicrystal substrates by pulsed laser deposition. X-ray diffraction and transmission electron microscopy revealed that $θ_\mathrm{GB}$ of Fe(Se,Te) was smaller whereas $θ_\mathrm{GB}$ of CeO$_2$ was larger than that of the substrate. The difference in $θ_\mathrm{GB}$ between the CeO$_2$ buffer layer and the substrate is getting larger with increasing $θ_\mathrm{GB}$. For $θ_\mathrm{GB}\geq24^\circ$ of the substrates, $θ_\mathrm{GB}$ of Fe(Se,Te) was zero, whereas $θ_\mathrm{GB}$ of CeO$_2$ was continuously increasing. The inclined growth of CeO$_2$ can be explained by the geometrical coherency model. The $c$-axis growth of Fe(Se,Te) for $θ_\mathrm{GB}\geq24^\circ$ of the substrates is due to the domain matching epitaxy on (221) planes of CeO$_2$. Electrical transport measurements confirmed no reduction of inter-grain $J_\mathrm{c}$ for $θ_\mathrm{GB}\leq9^\circ$, indicative of strong coupling between the grains.

cond-mat.supr-con↗

arXiv2019

Novel method to study strain effect of thin films using a piezoelectric-based device and a flexible metallic substrate

For applying tensile or compressive uniaxial strain to functional thin films, we propose a novel approach in combining a piezoelectric-based device and a technical metallic substrate used widely in the 2nd generation coated conductors (i.e. superconducting tapes). A strain-induced shift of the superconducting transition temperature of 0.1 K for Co-doped BaFe2As2 was observed along [100] direction, corresponding to a uniaxial pressure derivative dTc/dp100=-4 K/GPa. For Mn3CuN, a uniaxial strain derivative along [100] direction of the Curie temperature dTC/de100=13 K/% was observed. The current approach is applicable to various functional thin films in a wide range of temperatures.

cond-mat.supr-con↗

arXiv2017

Observation of Zero Resistance in As-Electrodeposited FeSe

Superconducting FeSe films were electrochemically deposited on rolling-assisted biaxially textured substrate (RABiTS) tape. We observed zero resistivity in the as-electrodeposited FeSe film prepared on the RABiTS when the appropriate voltage was applied while it was dipped into the solution. When the RABiTS tape was dipped in the solution without applying voltage, a thin Se film was deposited on the substrate. The compositional ratio of the FeSe film got closer to the stoichiometric ratio with decreasing the dipping time before applying voltage.

cond-mat.supr-con↗

arXiv2017

High-field transport properties of a P-doped BaFe2As2 film on technical substrate

High temperature (high-Tc) superconductors like cuprates have superior critical current properties in magnetic fields over other superconductors. However, superconducting wires for high-field-magnet applications are still dominated by low-Tc Nb3Sn due probably to cost and processing issues. The recent discovery of a second class of high-Tc materials, Fe-based superconductors, may provide another option for high-field-magnet wires. In particular, AEFe2As2 (AE: Alkali earth elements, AE-122) is one of the best candidates for high-field-magnet applications because of its high upper critical field, Hc2, moderate Hc2 anisotropy, and intermediate Tc. Here we report on in-field transport properties of P-doped BaFe2As2 (Ba-122) thin films grown on technical substrates (i.e., biaxially textured oxides templates on metal tapes) by pulsed laser deposition. The P-doped Ba-122 coated conductor sample exceeds a transport Jc of 10^5 A/cm^2 at 15 T for both major crystallographic directions of the applied magnetic field, which is favourable for practical applications. Our P-doped Ba-122 coated conductors show a superior in-field Jc over MgB2 and NbTi, and a comparable level to Nb3Sn above 20 T. By analysing the E-J curves for determining Jc, a non-Ohmic linear differential signature is observed at low field due to flux flow along the grain boundaries. However, grain boundaries work as flux pinning centres as demonstrated by the pinning force analysis.

cond-mat.supr-con↗

arXiv2016

Hall-plot of the phase diagram for Ba(Fe1-xCox)2As2

The Hall effect is a powerful tool for investigating carrier type and density. For single-band materials, the Hall coefficient is traditionally expressed simply by $R_H^{-1} = -en$, where $e$ is the charge of the carrier, and $n$ is the concentration. However, it is well known that in the critical region near a quantum phase transition, as it was demonstrated for cuprates and heavy fermions, the Hall coefficient exhibits strong temperature and doping dependencies, which can not be described by such a simple expression, and the interpretation of the Hall coefficient for Fe-based superconductors is also problematic. Here, we investigate thin films of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ with compressive and tensile in-plane strain in a wide range of Co doping. Such in-plane strain changes the band structure of the compounds, resulting in various shifts of the whole phase diagram as a function of Co doping. We show that the resultant phase diagrams for different strain states can be mapped onto a single phase diagram with the Hall number. This universal plot is attributed to the critical fluctuations in multiband systems near the antiferromagnetic transition, which may suggest a direct link between magnetic and superconducting properties in the BaFe$_2$As$_2$ system.

cond-mat.supr-con↗

arXiv2015

Electrochemical deposition of FeSe on RABiTS tapes

FeSe film is successfully fabricated onto Rolling Assisted Biaxially Textured Substrate (RABiTS) tapes by an electrochemical deposition technique. The deposited FeSe films tend to become high crystallinity with a decrease in the applied voltage to -1.0 V, and the compositional ratio of Fe to Se approaches 1:1. The sample deposited at -1.0 V shows a superconducting transition approximately 8.0 K in the magnetic susceptibility.

cond-mat.supr-con↗

arXiv2014

Highly textured oxypnictide superconducting thin films on metal substrates

Highly textured NdFeAs(O,F) thin films have been grown on ion beam assisted deposition (IBAD)-MgO/Y2O3/Hastelloy substrates by molecular beam epitaxy. The oxypnictide coated conductors showed a superconducting transition temperature (Tc) of 43 K with a self-field critical current density (Jc) of 7.0 x 104 A/cm2 at 5 K, more than 20 times higher than powder-in-tube processed SmFeAs(O,F) wires. Albeit higher Tc as well as better crystalline quality than Co-doped BaFe2As2 coated conductors, in-field Jc of NdFeAs(O,F) was lower than that of Co-doped BaFe2As2. These results suggest that grain boundaries in oxypnictides reduce Jc significantly compared to that in Co-doped BaFe2As2 and, hence biaxial texture is necessary for high Jc.

cond-mat.supr-con↗

arXiv2013

Oxypnictide SmFeAs(O,F) superconductor: a candidate for high-field magnet applications

The recently discovered oxypnictide superconductor SmFeAs(O,F) is the most attractive material among the Fe-based superconductors due to its highest transition temperature of 56 K and potential for high-field performance. In order to exploit this new material for superconducting applications, the knowledge and understanding of its electro-magnetic properties are needed. Recent success in fabricating epitaxial SmFeAs(O,F) thin films opens a great opportunity to explore their transport properties. Here we report on a high critical current density of over 10^5 A/cm^2 at 45 T and 4.2 K for both main field orientations, feature favourable for high-field magnet applications. Additionally, by investigating the pinning properties, we observed a dimensional crossover between the superconducting coherence length and the FeAs interlayer distance at 30-40 K, indicative of a possible intrinsic Josephson junction in SmFeAs(O,F) at low temperatures that can be employed in electronics applications such as a terahertz radiation source and a superconducting Qubit.

cond-mat.supr-con↗

arXiv2013

Intrinsic pinning and the critical current scaling of clean epitaxial Fe(Se,Te) thin films

We report on the transport properties of clean, epitaxial Fe(Se,Te) thin films prepared on Fe-buffered MgO (001) single crystalline substrates by pulsed laser deposition. Near Tc a steep slope of the upper critical field for H||ab was observed (74.1 T/K), leading to a very short out-of-plane coherence length, ξc, of 0.2 nm, yielding 2ξc(0) approximately 0.4 nm. This value is shorter than the interlayer distance (0.605 nm) between Fe-Se(Te) planes, indicative of modulation of the superconducting order parameter along the c-axis. An inverse correlation between the power law exponent N of the electric field-current density (E-J) curve and the critical current density, Jc, has been observed at 4 K, when the orientation of H was close to the ab-plane. These results prove the presence of intrinsic pinning in Fe(Se,Te). A successful scaling of the angular dependent Jc and the corresponding exponent N can be realized by the anisotropic Ginzburg Landau approach with appropriate Γvalues 2~3.5. The temperature dependence of Γbehaves almost identically to that of the penetration depth anisotropy.

cond-mat.supr-con↗

arXiv2013

BaFe2As2/Fe bilayers with [001]-tilt grain boundary on MgO and SrTiO3 bicrystal substrates

Co-doped BaFe2As2 (Ba-122) can be realized on both MgO and SrTiO3 bicrystal substrates with [001]-tilt grain boundary by employing Fe buffer layers. However, an additional spinel (i.e. MgAl2O4) buffer between Fe and SrTiO3 is necessary since an epitaxial, smooth surface of Fe layer can not be grown on bare SrTiO3. Both types of bicrystal films show good crystalline quality.

cond-mat.supr-con↗

arXiv2013

Versatile fluoride substrates for Fe-based superconducting thin films

We demonstrate the growth of Co-doped BaFe2As2 (Ba-122) thin films on AEF2 (001) (AE: Ca, Sr, Ba) single crystal substrates using pulsed laser deposition. All films are grown epitaxially despite of a large misfit of -10.6% for BaF2 substrate. For all films a reaction layer is formed at the interface confirmed by X-ray diffraction and by transmission electron microscopy. The superconducting transition temperature of the film on CaF2 is around 27 K, whereas the corresponding values of the other films are around 21 K. The Ba-122 on CaF2 shows identical crystalline quality and superconducting properties as films on Fe-buffered MgO.

cond-mat.supr-con↗

arXiv2012

Bicrystalline Grain Boundary and Hybrid SNS Junctions Based on Ba-122 Thin Films

To investigate the properties of iron-based superconductors, we examined different kinds of Josephson junctions using Co-doped Ba-122 thin films. Based on bicrystalline SrTiO3 substrates we prepared grain boundary (GB) junctions, which showed clear Josephson effects. We present electrical measurements at varying temperatures and magnetic fields. Furthermore, we prepared hybrid junctions using thin film technique. Confined by a Ba-122 base electrode (S) and a PbIn counter electrode (S') two different geometries of hybrid junctions are formed: a planar SNS' junction with a gold barrier (N) in between and an edge-type junction with an interface-engineered barrier. All three kinds of junctions showed asymmetric resistively shunted junction-like (RSJ) behavior with ICRN products of 20.2 /mu V (GB junctions), 18.4 /mu V (planar SNS' junctions) and 12.3 /mu V (edge-type junctions), respectively. An excess current could be observed at all junctions.

cond-mat.supr-con↗

arXiv2012

The influence of the buffer layer architecture on transport properties for BaFe1.8Co0.2As2 films on technical substrates

A low and almost temperature independent resistance in the normal state and an anomalous peak effect within the normal-superconducting transition have been observed in BaFe$_{1.8}$Co$_{0.2}$As$_2$/Fe bilayers, prepared on IBAD-MgO/Y$_2$O$_3$ buffered technical substrates. A resistor network array sufficiently reproduces this effect, assuming an increase of the electrical conductance between tape and film with decreasing buffer layer thickness. Based on this model we evaluated the influence of this effect on the critical current density and successfully reconstructed the superconducting transition of the bilayer.

cond-mat.supr-con↗

arXiv2011

Generic Fe buffer layers for Fe-based superconductors: Epitaxial FeSe1-xTex thin films

Biaxially textured FeSe1-xTex films have been realized on Fe-buffered MgO substrates by pulsed laser deposition. Similar to the Fe/BaFe2As2 bilayers, the crystalline quality of FeSe1-xTex films exhibit a sharp out-of-plane and in-plane texture less than 0.9°. The Fe/FeSe1-xTex bilayers showed high superconducting transition temperatures of over 17 K. The angular-dependent critical current densities exhibit peaks positioned at H // ab similar to other pnictide thin films. The volume pinning force of FeSe1-xTex in this direction is very strong compared with that of Co-doped BaFe2As2, due to a good matching between the interlayer distance in the c direction and the out-of-plane coherence length.

cond-mat.supr-con↗

arXiv2011

Thickness dependence of structural and transport properties of Co-doped BaFe2As2 on Fe buffered MgO substrates

We have investigated the influence of the superconducting layer thickness, d, on the structural and transport properties of Co-doped BaFe2As2 films deposited on Fe-buffered MgO substrates by pulsed laser deposition. The superconducting transition temperature and the texture quality of Co-doped BaFe2As2 films improve with increasing d due to a gradual relief of the tensile strain. For d>90 nm an additional 110 textured component of Co-doped BaFe2As2 was observed, which leads to an upward shift in the angular dependent critical current density at H // c. These results indicate that the grain boundaries created by the 110 textured component may contribute to the c-axis pinning.

cond-mat.supr-con↗

arXiv2011

As-vacancies, local moments, and Pauli limiting in LaO_0.9F_0.1FeAs_(1-delta) superconductors

We report magnetization measurements of As-deficient LaO_0.9F_0.1FeAs_1-delta (delta about 0.06) samples with improved superconducting properties as compared with As-stoichiometric optimally doped La-1111 samples. In this As-deficient system with almost homogeneously distributed As-vacancies (AV), as suggested by the (75)As-nuclear quadrupole resonance (NQR) measurements,we observe a strong enhancement of the spin-susceptibility by a factor of 3-7. This observation is attributed to the presence of an electronically localized state around each AV, carrying a magnetic moment of about 3.2 mu_Bohr per AV or 0.8 mu_Bohr/Fe atom. From theoretical considerations we find that the formation of a local moment on neighboring iron sites of an AV sets in when the local Coulomb interaction exceeds a critical value of about 1.0 eV in the dilute limit. Its estimated value amounts to ~ 2.5 eV and implies an upper bound of ~ 2 eV for the Coulomb repulsion at Fe sites beyond the first neighbor-shell of an AV. Electronic correlations are thus moderate/weak in doped La-1111. The strongly enhanced spin susceptibility is responsible for the Pauli limiting behavior of the superconductivity that we observe in As-deficient LaO_0.9F_0.1FeAs_1-delta. In contrast, no Pauli limiting behavior is found for the optimally doped, As-stoichiometric LaO_0.9F_0.1FeAs superconductor in accord with its low spin susceptibility.

cond-mat.supr-con↗

arXiv2011

Modulated Martensite: Why it forms and why it deforms easily

Diffusionless phase transitions are at the core of the multifunctionality of (magnetic) shape memory alloys, ferroelectrics and multiferroics. Giant strain effects under external fields are obtained in low symmetric modulated martensitic phases. We outline the origin of modulated phases, their connection with tetragonal martensite and consequences for their functional properties by analysing the martensitic microstructure of epitaxial Ni-Mn-Ga films from the atomic to macroscale. Geometrical constraints at an austenite-martensite phase boundary act down to the atomic scale. Hence a martensitic microstructure of nanotwinned tetragonal martensite can form. Coarsening of twin variants can reduce twin boundary energy, a process we could follow from the atomic to the millimetre scale. Coarsening is a fractal process, proceeding in discrete steps by doubling twin periodicity. The collective defect energy results in a substantial hysteresis, which allows retaining modulated martensite as a metastable phase at room temperature. In this metastable state elastic energy is released by the formation of a 'twins within twins' microstructure which can be observed from the nanometre to millimetre scale. This hierarchical twinning results in mesoscopic twin boundaries which are diffuse, in contrast to the common atomically sharp twin boundaries of tetragonal martensite. We suggest that observed extraordinarily high mobility of such mesoscopic twin boundaries originates from their diffuse nature which renders pinning by atomistic point defects ineffective.

cond-mat.mtrl-sci↗

arXiv2010

Jc Scaling and Anisotropies in Co-doped Ba-122 thin films

We have successfully grown epitaxial, superconducting films of Ba(Fe1-xCox)2As2 (Ba-122)with x ~ 0.1. The films grow without observable correlated defects parallel to the c-axis, as confirmed by TEM. This is also reflected in the absence of a caxis peak in Jc(θ). In contrast to cuprate high-Tc superconductors such as YBa2Cu3O7-d or even Bi2Sr2Ca2Cu3O10-d, the pnictides show a rather low anisotropic behavior in their Jc(θ) behavior as well as in their upper critical fields, Hc2. As a multiband superconductor, Ba-122 exhibits a temperature dependent electronic mass anisotropy.

cond-mat.supr-con↗