SrTiO3 Bicrystal Substrates Research Articles

Superconducting (Ba,K)Fe2As2 epitaxial films on single and bicrystal SrTiO3 substrates

The realization of single crystal and bicrystal films of superconducting materials is of great interest because they allow the investigation of the intragrain performance as well as the understanding of potential limitations in the grain boundary transparency. For many years, the realization of a high-quality (Ba,K)Fe2As2 film has been challenging. Here, the realization of (Ba,K)Fe2As2 epitaxial thin films on single crystal SrTiO3(001) and [001]-tilt-type SrTiO3 bicrystal substrates with high superconducting properties is demonstrated. The epitaxial growth of (Ba,K)Fe2As2 was enabled by implementing an undoped BaFe2As2 buffer layer between the SrTiO3 substrate and (Ba,K)Fe2As2 film. The film exhibits a high Tc of 38.0 K and an extremely high Jc of 14.3 MA/cm2 at 4.2 K. Artificial grain boundaries of (Ba,K)Fe2As2 were also successfully achieved on bicrystals with misorientation angles up to 36.8° by the same preparation methods. The artificial grain boundaries exhibited an identical Tc of 38.0 K and an excellent transfer of the grain orientation from the bicrystal substrates with high crystallinity comparable to that of the high-quality Ba(Fe,Co)2As2 films. This enables the investigation of the intrinsic (Ba,K)Fe2As2 grain boundary nature, which will clarify its potential for superconducting applications, like Josephson junctions, wires, and magnets.

Variation of the performance of YBa[formula omitted]Cu[formula omitted]O[formula omitted] radio frequency superconducting quantum interference device with bicrystal grain boundary angle

High transition temperature (high-Tc) superconducting quantum interference devices (SQUIDs) based on bicrystal grain boundary junctions have been utilized in a wide range of applications. To explore the effect of the bicrystal grain boundary angle on the property of the high-Tc SQUID, we fabricated and characterized YBa2Cu3O7−δ (YBCO) radio frequency (rf) SQUIDs using [001]-tilt grain boundary junctions on SrTiO3 (STO) bicrystal substrates with three different misorientation angles of θ=18∘, 24°, and 30°. Unlike the device of θ=18∘ showing only the triangular waveform of the voltage–flux (V−Φ) characteristics, for device of θ=24∘ and 30°, the shape of the V−Φ curve can be tuned from approximately triangular into nearly rectangular by adjusting the working point of the device. It suggests that, as θ increases from 18° to 30°, the operation of the device may change from the hysteretic mode into the nonhysteretic mode. This is backed by electrical transport measurements of YBCO bicrystal junctions on separate STO substrates, which help to reveal that lying behind the indicated changeover of the SQUID operation mode is the exponential decrease of the junction critical current density Jc as θ increases. The white noise of the device, which decreases with increasing flux-to-voltage transfer coefficient ∂V/∂Φ, shows a large drop as θ increases to 24°. For both 24° and 30° devices, the white field noise at 77 K is about 90 fT/Hz1/2 at respective optimum working points. For device of 30° with nearly rectangular V−Φ characteristics showing the largest ∂V/∂Φ among the fabricated samples, the white noise region extends below 10 Hz and a field noise of 145 fT/Hz1/2 at 1 Hz is shown, demonstrating the potential of obtaining high-performance bicrystal YBCO rf SQUIDs with this grain boundary angle.

Atomic-Scale Mechanism of Grain Boundary Effects on the Magnetic and Transport Properties of Fe3O4 Bicrystal Films.

In strongly correlated materials, change of the local lattice configuration is expected to tune or even generate new properties otherwise in the ideal bulk materials. For highly spin-polarized materials, the spin-dependent transport is sensitive to the local magnetic structure. Here, the artificial grain boundaries (GBs) with different tilt angles are produced in Fe3O4 films using SrTiO3 bicrystal substrates. The saturation magnetization of Fe3O4 bicrystal films is enhanced. The detailed atomic structural results combining with density functional theory calculations reveal that the elongated FeA-O bond length at GBs resulting in the reduction of charge transfer reduces the FeA magnetic moments, which enhances the total magnetic moments of Fe3O4. The in-plane rotation of the Fe3O4 lattice on bicrystal substrates alters the magnetization processes. Especially, the Fe3O4 bicrystal film with a tilt angle of 22.6° shows strong in-plane magnetic anisotropy due to the zigzag GBs. The altered magnetic anisotropy of Fe3O4 bicrystal films enhances the anisotropic magnetoresistance. The findings reveal the mechanism of GBs on the magnetic and transport properties and manifest that the strategy of utilizing GBs can tune the physical properties in highly spin-polarized materials.

Properties of Fe(Se, Te) Bicrystal Grain Boundary Junctions, SQUIDs, and Nanostrips

We have fabricated and characterized single junctions, dc Superconducting Quantum Interference Devices (SQUIDs), and nanostrips made by Fe(Te, Se) superconductive thin films. SrTiO 3 bicrystal substrates with different misorientation angles have been used for the fabrication of Josephson junctions and SQUIDs. Nanostrips have been realized growing thin films on CaF 2 substrates. The interaction between fluxons and antifluxons has been taken into account for the evaluation of the pinning energy values.

Fe(Se,Te) superconducting quantum interference devices

We have fabricated and analyzed bicrystal grain boundary superconducting quantum interference devices made by Fe(Se,Te) superconductors, grown on [001] tilt SrTiO3 bicrystal substrates. We have realized several devices using the so-called hole-type geometry, with different loop inductances and different junction widths. Modulations of the voltage under an external applied magnetic field have been observed for three samples. Voltage amplitudes of the order of few microvolt have been measured. The maximum magnetic flux-to-voltage transfer parameter value that we have observed has been . The experimental data are analyzed in the famework of existing theoretical models.

Introduction of an artificial domain wall into BiFeO3 thin film using SrTiO3 bicrystal substrate

For the future realization of nanoelectronic devices using domain walls (DWs) in ferroelectric thin films as a functional element, the artificial introduction and position control of DWs in BiFeO3 (BFO) thin films using SrTiO3 bicrystal substrates are demonstrated. The BFO thin film follows the bicrystal nature of SrTiO3, and consequently the boundary with an out-of-plane misorientation angle of 8° is formed immediately above that of the substrate. The boundary shows a charged nature induced by a head-to-head polarization configuration. The BFO bicrystal film exhibits a well-saturated polarization hysteresis loop equivalent to those of single-crystalline films. The piezoresponse scanning force microscopy of polarization switching suggests ferroelastic relaxation-mediated 180° switching through 71° switching. Although the boundary corresponds to a low-angle symmetry boundary with misfit dislocations, experimental results show that it behaves just like normal DWs by crystallographic twinning. Therefore, it is concluded that the artificially introduced boundary can be considered as a charged 101° DW.

Grain boundary junctions of FeSe0.5Te0.5 thin films on SrTiO3 bi-crystal substrates

Grain boundary junctions were fabricated in the epitaxial FeSe0.5Te0.5 thin films on [001] tilt SrTiO3 bi-crystal substrates with a CeO2 buffer layer. Critical current densities across the junctions with different mis-orientation angles of 4°, 7°, 15°, and 24° were measured at magnetic fields up to 30 T. It was found that the 4° and 7° junctions carry critical current densities comparable to that of the intra-grain film while those of the 15° and 24° junctions were suppressed drastically. A critical mis-orientation angle of around 9° was identified that separates the strong coupling region from the weak link region. We found that the critical current densities across the grain boundary with a 24° mis-orientation angle are modulated by the magnetic field, indicating a Josephson Effect. This junction is estimated to be in the intermediate-size regime with an effective transverse junction width L ∼ 2.6–2.8 μm and a Josephson penetration depth λJ ∼ 1.2 μm.

Grain boundary engineering with gold nanoparticles

We investigated high-TC grain boundary Josephson junctions with and without incorporated gold nanoparticles. Pulsed laser deposition was used for the deposition of YBa2Cu3O7−δ thin films on SrTiO3 bicrystal substrates with different grain boundary angles. During the deposition process, single-crystalline nanoparticles self-assembled from a thin gold layer which was sputtered on the substrate before the YBCO deposition. The interaction between nanoparticles and thin film growth significantly influences the quality of the YBCO films [1]. The critical current density and the critical temperature of the superconducting films can be increased in a defined manner. Furthermore, the nanoparticles influence the growth conditions in the region of the grain boundary and thus the properties of the later patterned Josephson junctions. The comparison between Josephson junctions with and without nanoparticles on the same substrate shows a reduction of the critical current IC and an increase of the normal state resistance RN for all investigated types of grain boundaries in the areas with gold nanoparticles. In some cases we even found an increase of the resulting ICRN product. We present the influence of light irradiation on the properties of the Josephson junctions.

Amplitude sensitive experiment of pairing symmetry in d0–d0 submicron Y–Ba–Cu–O bicrystal grain boundary junctions

We have fabricated and analyzed submicron YBa2Cu3O7−x grain boundary Josephson junctions grown on [100] tilt SrTiO3 bicrystal substrates. We present an experiment sensitive to the amplitude of the order parameter. To this aim, we have measured electrical properties of [100] tilt bicrystal YBa2Cu3O7−x grain boundary junctions with nominal widths of 700 nm and 300 nm. Junctions are fabricated so that positive lobes of the d-wave electrodes face one another (d0–d0 junction). We demonstrate that, in such devices, the temperature dependences of the critical current may be accounted for by very high-transparency junction barriers, in which the influence of nodes in the pair potential is an essential element. We based our analysis on a recent theoretical model that, starting from the Bogoliubov–de Gennes equations, takes into account the presence of Andreev bound states in layered superconductors, with Cu–O planes tilted with respect the substrate plane, as is the case of [100] tilt grain boundary junctions.

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.

Effects of self-assembled gold nanoparticles on YBa2Cu3O7−δ thin films and devices

In our work we prepared YBa2Cu3O7−δ (YBCO) thin films with self-assembled gold nanoparticles on SrTiO3 (STO) substrates. We carried out different experiments to determine the effects on the crystallographic properties of the YBCO matrix as well as of the gold nanoparticles. Furthermore, we investigated how the particles influence the superconducting parameters of the film, e.g. the critical temperature TC and the critical current density jC. To ascertain jC we employed magneto-optical Faraday microscopy. In addition, the YBCO film was deposited and structured on STO bi-crystal substrates, thus producing grain boundary Josephson junctions. We studied those junctions with respect to the normal state resistance RN, and the dependence of the critical current IC on the temperature T as well as on the magnetic flux Φ. Finally, we prepared direct current superconducting quantum interference device (dc-SQUID) gradiometers and embedded gold nanoparticles at well-defined areas such as only the antenna or the SQUID region. We measured the flux noise in a shielded environment using an ac-bias reversal technique and compared it with that of sensors without gold nanoparticles. Thus, we demonstrate a new preparation method and an innovative application of gold nanoparticles.

]Influence of misorientation angle on third harmonic voltages V3 induced by YBCO thin films on bi-crystal substrates

Abstract YBa2Cu3Oy thin films have been deposited on SrTiO3 bi-crystal substrates with various misorientation angles and a STO single-crystal substrate by a pulsed laser deposition method. The magnetic field dependence of third harmonic voltage V3 vs. coil current I0 was measured by the third harmonic voltage method when the coils were mounted on the crystal grain boundary. The V3 increases monotonically with increasing I0 in the single-crystal substrate. However, the V3 increases strangely with increasing I0 in the bi-crystal substrate with low-angle grain boundary in low-magnetic fields. On the other hand, the V3 increases monotonically in the bi-crystal substrates with high-angle grain boundary.

Inductive measurement of in-field critical current density of YBCO thin film on aSrTiO3 bicrystal substrate using the third harmonic voltage method

The spatial distribution and the magnetic field dependence of the critical current densityJc for aYBa2Cu3Oy (YBCO) thin filmdeposited on a SrTiO3 (STO) bicrystal substrate have been investigated using the third harmonic voltage method. Whena pick-up coil is mounted on the area without a crystal grain-boundary, the third harmonic voltageV3 increases monotonically with increasing coil currentI0 over a certain threshold value. On the other hand, when the pick-upcoil is mounted on the area with a crystal grain-boundary, theV3–I0 curve shows a strange behavior. However, theV3–I0 curves show a monotonic increase when a magnetic field over 0.07 T is applied to theYBCO thin film, because the shielding current hardly flows across the crystalgrain-boundary in the large magnetic field. The magnetic field dependence ofJc is also measured using the four-probe method to clarify the strange behavior of theV3–I0 curve in the third harmonic voltage method.

Engineering of YBa2Cu3O7–δ grain boundary Josephson junctions by Au nanocrystals

AbstractWe prepared and investigated grain boundary Josephson junctions based on SrTiO3 bicrystal substrates. During the deposition of YBa2Cu3O7–δ (YBCO) gold nanocrystals forming from an intermediate gold layer can modify the crystalline structure and thus the properties of the YBCO grain boundaries. The variation of the film thickness of the Au seed layer changes the growth conditions of the YBCO film and the Au nanocrystals. The values of the characteristic ICRN product do not change whereas the values of the critical current IC decrease.magnified image(© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improved charge conduction in cobalt-phthalocyanine thin films grown along 36.8° boundary of SrTiO3 bicrystals

We have investigated temperature dependence of resistivity (ρ) and current-voltage characteristics of cobalt-phthalocyanine (CoPc) films deposited (i) along the 36.8° boundary (ATB) and (ii) in the single crystal region (SCR) of SrTiO3 bicrystal substrates. The analyses of ρ-T data show that while ATB films exhibit critical behavior, SCR films follow insulating three-dimensional variable-range-hopping mechanism. The mobility (μ) at 300 K has been estimated to be ∼148 and 3.8×10−3 cm2 V−1 s−1, respectively for ATB and SCR films. A very high μ for ATB films indicates that the bicrystal boundary acts as a template for the ordering of CoPc molecules.

Characterization of dual high transition temperature superconducting quantum interference device first-order planar gradiometers on a chip

The dual first-order planar gradiometers of high transition temperature (high-Tc) superconducting quantum interference devices (SQUIDs) were designed and fabricated on a 10×10 mm2 SrTiO3 bicrystal substrate. Each gradiometer consists of four bare SQUIDs which are connected to two symmetric pickup loops. Any two of the SQUIDs can be connected in series to obtain better performance. In this study, by selecting the coupling direction of SQUID, a balance resolution of 0.08% was achieved. And, a second-order electronic planar gradiometer was composed of these two first-order gradiometers. Experimental data showed that the noise performance of the second-order electronic planar gradiometer composed of two planar first-order gradiometers can reach 15 μΦ0/Hz1/2 at 1 kHz in an unshielded environment, which is comparable to the noise level of a magnetometer in a shielded environment.

High-Tc SQUID gradiometer system for immunoassays

A high-Tc dc SQUID (superconducting quantum interference device) gradiometer wasdeveloped for magnetic immunoassays where magnetic nanoparticles are usedas markers to detect biological reactions. The gradiometer was fabricated on a5 × 10 mm2 SrTiO3 bicrystal substrate and has a gradiometer resolution of2.1 pT cm−1 Hz−1/2. A magnetic signal was detected from a sample of1 µl ofFe3O4 nanoparticlesin a 40 mg ml−1 solution kept in a microcavity fabricated on Si wafers withSi3N4 membranes using MEMS (micro-electro-mechanical-systems) technology. Itwas found that volumes as small as 0.3 nl in principle would be detectablewith our present device. This corresponds to a total number of particles of2.2 × 107. The estimated average dipole moment per particle is4.8 × 10−22 A m2. We are aiming at reading out immunoassays by detecting the Brownian relaxation ofmagnetic nanoparticles, and we also intend to integrate MEMS technology into our system.

Investigation of the effect of thermal cycling on the device performance ofYBa2Cu3O7−δ DC-SQUIDs

We investigated the effect of thermal cycling on the operational performance ofYBa2Cu3O7−δ (YBCO) direct current superconducting quantum interference devices (DC-SQUIDs) fabricated onto24° SrTiO3 (STO) bicrystal substrates. The devices under investigation consist of directly coupledDC-SQUID magnetometer configurations. Thin films having 200 nm thicknesseswere deposited by dc-magnetron sputtering and device patterns were made bya standard lithography process and chemical etching. The SQUIDs having4 µm-wide grain boundary Josephson junctions (GBJJs) were characterized by means of criticalcurrents, peak-to-peak output voltages and noise levels, depending on the thermal cycles.In order to achieve a protective layer for the junctions against the undesired effects ofthermal cycles and ambient atmosphere during the room temperature storage, the deviceswere coated with a 400 nm thick YBCO layer at room temperature. Since the second layerof amorphous YBCO is completely electrically insulating, it does not affect the operation ofthe junctions and pick-up coils of magnetometers. This two-layered configuration ensuresthe protection of the junctions from ambient atmosphere as well as from theeffect of water molecules interacting with the film structure during each thermalcycle.

Integrated high-TC radio frequency superconducting quantum interference device using SrTiO3 bicrystal substrate resonators

In contrast to the previous high-TC radio frequency (rf) superconducting quantum interference device (SQUID) magnetometer, in which the rf SQUID is placed face to face with the substrate resonator, a simple integrated washer-type rf SQUID magnetometer based on a single SrTiO3 bicrystal substrate resonator is designed and fabricated in this work. The magnetometer combines the rf SQUID, flux focusing, and resonator into a single chip. This integrated washer-type rf SQUID magnetometer offers a superior advantage in integrating the device to a scanning SQUID detection system in which the distance between the sample and SQUID can be minimized.

Multifunctional design of high-transition-temperature directly coupled superconducting-quantum-interference-device magnetometers on a chip

We have designed four multifunctional high-transition-temperature directly coupled superconducting-quantum-interference-device (SQUID) magnetometers on a 10×10mm2 SrTiO3 bicrystal substrate. Each magnetometer is composed of two serial bare SQUIDs. These magnetometers can be connected to four kinds of first-order electronic planar gradiometers or two kinds of second-order electronic planar gradiometers by using analog subtracting electronic circuits. The noise spectra of each magnetometer, two first-order electronic planar gradiometers, and a second-order planar gradiometer were measured. We measured the small magnetic signal generated from a double D coil to show that this design could supplement the deficiency of a magnetometer.