Bicrystal Substrates Research Articles

A washer type high temperature superconducting dc-SQUID magnetometer with bicrystal junctions on SrTiO 3 substrate

The c-oriented YBCO thin film was epitaxially grown on 10 × 10 mm 2 SrTiO 3 bicrystal substrate using pulsed laser deposition. The film has few outgrowths on the surface with the roughness of several nanometers in the scope of 20 μm. A washer type dc-SQUID magnetometer with bicrystal junctions was patterned using standard photolithography and wet-etching methods. The fabricated magnetometer with the effective area of 0.09 mm 2 has a white noise of 333 fT/Hz 1/2 without superconducting shield, and can be effectively applied in eddy current non-destructive evaluation. Flaws over 10 mm below the surface of the conducting aluminum plates can be clearly identified and located by mapping the magnetic field distribution in an unshielded magnetic environment.

Magnetoresistance in bicrystal Fe 3O 4 thin films

An epitaxial magnetite (Fe 3O 4) film has been deposited by the pulsed laser deposition (PLD) technique on a bicrystal substrate. Magnetoresistance measurements on patterned stripes (width=20 μm) across the bicrystal grain boundary have not revealed a distinguishable grain boundary contribution. Post-annealing of the film has a strong influence on the relaxation degree of the film and on the density of antiphase boundaries. Magnetotransport measurements of the micrometer-patterned annealed film show significantly enlarged high-field magnetoresistance values at low temperatures.

THz Josephson properties of grain boundary YBaCuO junctions on symmetric, tilted bicrystal sapphire substrates

Superconducting Josephson junctions with high characteristic voltages (IcRn larger than 4 mV at 4.2 K) are fabricated by depositing YBa2Cu3O7−x on tilted sapphire bicrystal substrates, where the tilting axis is along the grain boundary. The good junction quality and low microwave losses in sapphire gave high frequency response well into the THz region. High quality YBa2Cu3O7−x epitaxial films were deposited on tilted sapphire substrates with CeO2 buffer layers by pulsed laser deposition. YBaCuO films have smaller tilt angles, from 1.0° up to 10.3°, compared to inclination angles of the substrates from 1.5° to 13.6°. X-ray diffraction shows only a single orientation of the films in the a-b plane, as well as an absence of a-axis particles and outgrowths. Critical temperatures as high as Tc=88.5−89.0K and ΔTc⩽1.5K were obtained in all films. The grain boundary in a common high-Tc superconducting junction is usually much less straight than in the in-plane rotated bicrystal substrate, depressing Josephson currents. The waviness of the artificial grain boundary in a tilted bicrystal film is three times less than in an in-plane (untilted) bicrystal. Tilted Josephson junctions of widths from 1.5 to 6 μm were tested at temperatures from 0.26 to 77 K. IcRn products as high as 4.5 mV were observed at T=4.2K. Shapiro steps were observed at voltages over 3 mV under 300 GHz irradiation. Josephson radiation from the tilted junction was measured at frequencies up to 1.7 THz by a cryogenic bolometer. Suppressing the critical current with a magnetic field can separate Josephson radiation and thermal radiation. A parabolic dependence of the response on bias voltage for thermal radiation corresponds to an increase of junction temperature from 260 mK at zero bias to 3 K at 1 mV bias.

Midgap state-based π-junctions for digital applications

Midgap state-based YBa2Cu3O7−x π-junctions have been fabricated by a focused ion-beam system using 45° symmetric [001] tilt SrTiO3 bicrystal substrates. Measurements, performed by inserting the junctions in a superconducting loop (as a double-phase sensitive test), show both an unconventional nonmonotonic temperature dependence of the Josephson current, with a local minimum at a crossover temperature T* and, around T*, a half flux quantum shift in the critical current versus magnetic field modulations, clear signs of a 0–π crossover with temperature. Such results demonstrate that conventional 45° symmetric grain boundary junctions may have potential for applications, from digital circuits to quantum computing.

Magnetic field dependence of conductance and conduction noise of an artificial grain boundary junction in a La 0.67Ba 0.33MnO 3 epitaxial thin film

A single artificial grain boundary in La 0.67Ba 0.33MnO 3 (LBMO) thin film has been prepared by depositing the film on a bicrystal substrate using laser ablation technique. We investigated the magnetic field dependence of magnetoresistance and conductance–voltage characteristics of the grain boundary at 77 K. A decrease of nonlinearity of current–voltage characteristics was observed upon application of magnetic field. The results are explained by assuming the presence of two different types of parallel conducting channels (metallic and highly resistive) across the grain boundary. The analysis of the results reveals that the application of magnetic field suppresses magnetic disorders at the grain boundary region and increases metallic conduction channels across the grain boundary. The temperature dependence of the conduction noise of the bicrystal grain boundary was measured at 0 and 1.5 kG magnetic field and compared with a microbridge on the LBMO film having no grain boundary. The presence of the grain boundary was found to enhance noise by one order of magnitude. The noise of a bicrystal grain boundary showed a decrease in the presence of 1.5 kG magnetic field for T<210 K. This decrease of noise confirms that the application of a magnetic field induces more metallic channels across the grain boundary.

New type of second-order high- Tc SQUID planar gradiometer

We have proposed a novel type of high- T c superconducting quantum interference device (SQUID) planar gradiometer that could measure the mixed partial derivative of magnetic fields ∂ 2 B z/ ∂x ∂y . The device consists of two parts, one is the signal pickup antenna formed by four square pickup loops, and another one is a SQUID directly coupled to the common line which separates the antenna loops. The four pickup loops were arranged to a two-dimensional array. The loops in the diagonal direction are connected in series. The total current in the common line in this configuration is directly proportional to the mixed partial derivative of the magnetic field. We have made a device of such design on a STO bicrystal substrate and tested the device in gradient field.

Preparation and properties of NdBa 2Cu 3O 7− y grain boundary Josephson junctions on bicrystal MgO substrates by tri-phase epitaxy

Grain boundary Josephson junctions (GBJJs) with high-quality NdBa 2Cu 3O 7− y (NBCO) thin films were fabricated on SrTiO 3/BaTiO 3 buffered MgO (100) bicrystal substrates by using a novel process of tri-phase epitaxy (TPE). This film growth method is based on an application of pulsed laser deposition (PLD) under compositions and conditions predetermined from the relevant thermodynamic phase diagram. The film grain boundary was sharp and along with the bicrystal substrate boundary. A sub-micron size microbridge was patterned on the NBCO/SrTiO 3/BaTiO 3 films across the tilted grain boundary by electron beam lithography and focused ion beam etching. The junctions exhibited resistively shunted junction (RSJ) like current–voltage ( I– V) characteristics at temperatures up to 90 K. The critical current ( I c) of 3.3 mA at 4.2 K was completely depressed by 20 GHz microwave radiation. Microwave induced steps were clearly observed over 2 mV at 10 K with voltages corresponding to the Josephson frequency–voltage relationship.

High- Tc dc-SQUID magnetometer and its application in eddy current non-destructive evaluation

We have fabricated and tested a high- T c dc-SQUID magnetometer patterned on a YBCO thin film deposited on a SrTiO 3 bicrystal substrate. The measurement of noise spectrum at 77 K showed that the magnetometer has a white noise of 333 fT/Hz 1/2 without superconducting shield, corresponding to a flux sensitivity of 14.5 μΦ 0/Hz 1/2. We constructed a test system for eddy current non-destructive evaluation of conducting aluminum plates by using the magnetometer. Flaws over 10 mm below the surface can be clearly identified. By mapping the magnetic field distribution, we can locate the flaw position. The signals caused by flaw were expanded due to the use of double-D type driving coil with a wider central slit. The experimental results confirmed that this test system could operate in a magnetic unshielded environment.

비자기 차폐환경에서 미세자기신호 측정을 위한 고온 초전도 SQUID 자력계의 설계 및 제작

We have designed and fabricated the YBCO single layer directly-coupled SQUID magnetometers for the purpose of magnetocardiography in a magnetically disturbed environment. The SQUID magnetometers were designed three different types of pickup coil such as solid type, PL type I and PL type II for further stable fluxed-locked-loop operation without magnetic shielding. Magnetometer was fabricated with a single layer YBCO thin film deposited on STO(100) bicrystal substrate with misorientation angle of <TEX>$30^{\circ}$</TEX>. We have achieved a magnetic field noise BN of 30 fT/<TEX>$Hz^{1/2}$</TEX> at 100 Hz, and less than 70 fT/<TEX>$Hz^{1/2}$</TEX> at 1 Hz. The PL type II SQUIDs have exhibited the most stable fluxed-locked-loop operation in a magnetically unshielded environment.

Self-organized current transport through low-angle grain boundaries inYBa2Cu3O7−δthin films studied magnetometrically

The critical current density flowing across low angle grain boundaries in YBa$_2$Cu$_3$O$_{7-\delta}$ thin films has been studied magnetometrically. Films (200 nm thickness) were deposited on SrTiO$_3$ bicrystal substrates containing a single [001] tilt boundary, with angles of 2, 3, 5, and 7 degrees, and the films were patterned into rings. Their magnetic moments were measured in applied magnetic fields up to 30 kOe at temperatures of 5 - 95 K; current densities of rings with or without grain boundaries were obtained from a modified critical state model. For rings containing 5 and 7 degree boundaries, the magnetic response depends strongly on the field history, which arises in large part from self-field effects acting on the grain boundary.

The microstructure of La 0.67 Sr 0.33 MnO 3 grain boundaries on SrTiO 3 bicrystal substrates

The aim of this study is to characterize the grain boundary microstructure of artificial La0.67Sr0.33MnO3 (LSMO) grain boundaries obtained by epitaxial growth on SrTiO3 (STO) bicrystal substrates. The films were deposited on STO bicrystal substrates using pulsed laser deposition. The substrates were (001) oriented and the boundaries were symmetrical [001]-tilt boundaries with nominal tilt angle 24° and 37° respectively. The microstructures were characterized using a transmission electron microscope. The thickness of the LSMO films was 32 and 37 nm respectively. The films were relatively smooth and the interface between the substrate and the films was sharp without any evidence of chemical reaction or interdiffusion.

The microstructure of La0.67Sr0.33MnO3 grain boundaries on SrTiO3 bicrystal substrates

We report measurements of the electrical and magnetic properties of bilayers of YBa2Cu3O7 (YBCO) and La2/3Ca1/3MnO3 (LCMO) deposited on SrTiO3 substrates by Pulsed Laser Deposition. We find distinct differences in the properties depending on the order of the deposition, with the lower layer always being degraded. In particular, the YBCO has a Tc suppressed to ∼ 60 K when it is covered by LCMO. We argue that the effect is not due to an electronic interaction, but results from a reduction in the oxygen content of the YBCO by the LCMO.

High-Tc dc-SQUID gradiometers in flip-chip configuration

We describe a new design of a gradiometric flip-chip antenna, which is inductively coupled to a dc-SQUID gradiometer. Both components are patterned out of thin films of the high-Tc superconductor YBa2Cu3O7−x (YBCO). For the flip-chip antenna, a 40 mm × 10 mm SrTiO3 single crystalline substrate is used, while the gradiometer sensors are prepared on 10 mm × 10 mm SrTiO3 bicrystal substrates. Special attention is paid to the inductive coupling between the flip-chip antenna and the read-out gradiometer antenna. We investigate different designs of coupling loops in order to optimize the coupling inductance between both components of the sensor. With optimized coupling the sensor achieves a field-gradient resolution of 12 fT cm−1 Hz−1/2 in the white noise region and of 310 fT cm−1 Hz−1/2 at 1 Hz in the unshielded laboratory environment.

Spin-polarized injection into YBaCu3O7 grain boundary junctions

YBa 2 Cu 3 O 7 / SrTiO 3 / La 0.67 Ca 0.33 MnO 3 trilayer thin films grown on bicrystal substrates were patterned into a structure that allows the injection of spin-polarized current into the region of a grain boundary junction (GBJ). The transfer length LT was determined to ensure that injection was over the entire device width, in contrast to previously reported experiments. Currents up to some 100 times the junction critical current I0 and area density 108 A m−2, have been injected into the GBJ region by using an offset current method. The I0(B) characteristics of the GBJ under injection suggest that the suppression of I0 can be explained by a combination of heating and self-field effects, without any need to invoke the spin-polarized nature of the injected current. We find no evidence of direct injection into the junction.

Spread of critical currents in thin-film YBa/sub 2/Cu/sub 3/O/sub 7-x/ bicrystal junctions and faceting of grain boundary

The statistical distributions of critical currents in series arrays of YBa/sub 2/Cu/sub 3/O/sub 7-x/ grain-boundary junctions have been studied by low-temperature laser scanning microscopy. A set of arrays has been fabricated on [110] NdGaO/sub 3/ bicrystal substrates with misorientation angles from 2 /spl times/ 10/spl deg/ up to 2 /spl times/ 26/spl deg/ and patterned to the widths from 1.7 up to 5 micrometers. The critical current values of the individual junctions in the array have been obtained by focusing a laser beam on each junction and measuring the bias current at which the maximum laser-induced voltage response has appeared on the array. The measured critical current distributions have been demonstrated to be close to a log-normal Gauss function. A spread of this distribution has been found to increase with a bicrystal angle. YBa/sub 2/Cu/sub 3/O/sub 7-x/ grain-boundary topography has been studied by atomic force microscopy. From results of these measurements we suppose that the maximum values of critical current density might be assigned to the symmetrical facets of grain boundary.

A four channel HTS SQUID NDE system

We have constructed an NDE system using an array of four HTS SQUID sensors. The sensor array was fabricated on a SrTiO/sub 3/ bi-crystal substrate. The SQUID pickup loops were held perpendicular to the device under test. The lateral dimension of the SQUID was 20 micrometers and the noise was typically less than 30 pT/Hz/sup 1/2/ at 10 Hz. We used modulation FLL circuits for SQUID readout. The effect by modulation and feedback coupling between adjacent channels was investigated by measuring noise spectrum and output change with and without operating adjacent channels. We applied the system to map the magnetic field generated by current distribution and magnetic inclusions in a working circuit as well as a current carrying metal pattern.

Faceting and critical current densities of [001] high-angle tilt boundaries in YBCO films

The [001] high-angle tilt boundaries in YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) thin films deposited on SrTiO/sub 3/ (STO) bicrystal substrates have been studied by Scanning Squid Microscopy (SSM) and transmission electron microscopy (TEM). Generally critical current density across a boundary decreases with increasing misorientation. However, misorientation angle is found to be insufficient to predict the J/sub c/ behavior of a YBCO grain boundary. The two inclination angles of the adjacent grains are found to be important parameters. In two separate cases of same misorientation but different inclinations, the boundaries exhibit drastically different superconducting behaviors and this has been observed by SSM imaging of Josephson vortices. The YBCO film boundary is not an exact copy of the design boundary of the substrate bicrystal template. YBCO boundaries meander due to island formation during film growth. These microscopically wavy boundaries contain straight segments of atomistic facets. Three energetic criteria that affect faceting have been identified. The particular criterion that a facet plane cannot be too deviated from the design boundary plane (/spl phi/<28/spl deg/) explains the strong effect of the design inclination angles on J/sub c/ despite extensive faceting. The original scattered values of J/sub c/ each calculated from individual fluxes of the same design boundary are likely to be a bona fide effect corresponding to individual facet having its own J/sub c/ and /spl lambda//sub j/ values.

YBa/sub 2/Cu/sub 3/O/sub X/ submicron Josephson junctions on bicrystal substrates

A technological process based on e-beam lithography and ion beam etching of YBa/sub 2/Cu/sub 3/O/sub X/ (YBCO) film through carbon mask is applied to fabricate a series of YBCO dc SQUIDs on SrTiO/sub 3/ 30/spl deg/-bicrystal substrates. The width of Josephson junctions used in SQUIDs ranges from 0.6 /spl mu/m to 1 /spl mu/m. The character of the temperature dependence of the junctions critical current enables one to confirm the realization of d-wave pairing mechanism as well as the presence of localized states in the grain boundary region. The diffraction pattern of submicron junctions is discussed in relation to their further application in SQUIDs for task of scanning SQUID microscopy in high magnetic fields.

Feasibility studies of ultra-small Josephson junctions for qubits

Most proposed realizations of a high temperature superconductor (HTS) qubit require the use of very small Josephson junctions. The properties of bicrystal junctions are especially interesting since they make it possible to implement several types of flux qubits in a relatively simple way. We have developed a technique that allows us to produce high quality sub-micrometer junctions in a reproducible way using bicrystal technology. We have successfully fabricated and characterized a large number of YBCO junctions and SQUIDs with bridge width as small as 0.2 micrometer on 0/spl deg/-3/spl deg/, 0/spl deg/-40/spl deg/ and 0/spl deg/-45/spl deg/ bicrystal STO substrates. The properties of these junctions have been extensively examined at temperatures down to 20 mK. The effects of external magnetic fields on these structures have been investigated. Figures of merit for the proposed qubits were also extracted from these measurements.

Fabrication of grain boundary Josephson junction on top layer of YBCO multilayer using chemical mechanical planarization

To improve the sensitivity of a high-temperature superconducting quantum interference device (SQUID), it is necessary to fabricate a SQUID based on a multilayer. The trilayer (YBCO/STO/YBCO) was fabricated on a STO(1 0 0) bicrystal substrate. The characteristics of Josephson junctions fabricated with a misorientation angle of 24° in the top and base of the trilayer were compared to clarify whether superconductivity of the junction in the base layer was degraded. We studied a dependence of top layer junction characteristics on misorientation angles (24°, 30° and 36.8°). It is shown that a Josephson junction with a 30° bicrystal substrate has a higher I c R n product and a higher normal resistance R n. The SQUID was fabricated on the top layer with a 30° bicrystal substrate.