Noise and electrical properties of YBCO nanostructures

This thesis work deals with the investigation of noise properties in cuprate High critical Temperature Superconductor (HTS) YBCO nanoscale devices. Here the aim is to get a better understanding of nanoscale fluctuations in the normal state of HTS from which superconductivity evolves. The observation of fluctuations in the electronic properties might offer valuable clues toward the microscopic mechanism leading to superconductivity in HTS, which still represent one of the main unsolved problems in solid-state physics. In this respect, the YBCO nanodevices are implemented as tools to obtain new experimental signatures, which can deliver new insights about the complex properties of HTS materials. Since cuprate HTS undergo various nano-scale ordering transitions upon cooling and variation of hole doping, being able to study transport properties on the nanoscale is of utmost importance. In this respect, resistance noise properties of YBCO nanowires are studied as a function of temperature and hole doping. Indications of nematic fluctuations, that is local time dependent fluctuations of the in-plane conductivity anisotropy, have been observed in a wide temperature range above the superconducting transition. The observed fluctuations might be related to so-called charge stripe fluctuations, which represent a possible microscopic mechanism for superconductivity in these materials.However, the interest in HTS nanostructures is not purely academic. The technological application of YBCO weak links in SQUID, is a major focus of research in the field. In this thesis, we present a novel fabrication process of HTS weak links: the nanoscale Grooved Dayem Bridge (GDB). Here, the layout of the bridge and the weak link inside the bridge are realized during one single lithography process on a YBCO film grown on a single crystal substrate. This results in high-quality weak links with IcRn products as high as 550 \ub5V and differential resistances much larger than those observed in bare Dayem bridges at T=77 K. Moreover, the GDB greatly simplifies the fabrication procedure compared to grain boundary based JJs. We have used YBCO GDBs as novel nanoscale building blocks in HTS SQUID magnetometers coupled to an in plane pickup loop, which have been characterized via transport and noise measurements at T= 77 K. These devices exhibit large voltage modulations (ΔV =27-50 \ub5V), low values of white magnetic flux noise, 6 \ub5Φ0/\sqrt{Hz}, and corresponding magnetic field noise, 63 fT/\sqrt{Hz}, at T=77 K. Therefore, GDB based SQUIDs combine the nanofabrication advantages and the device reproducibility, which are typical of Dayem bridges, with the performances, i.e. low magnetic flux and field noise, of state-of-the-art SQUIDs based on grain boundary JJs. The achieved magnetic field noise paves the way for the realization of a single layer YBCO magnetometer with magnetic field noise below 20 fT/\sqrt(Hz)

Probing the phase diagram of cuprates with YBa2_2Cu3_3O7−δ_{7-\delta} thin films and nanowires

We have grown and characterized 30 nm thick YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) films, deposited by pulsed laser deposition on both MgO (110) and SrTiO$_3$ (001) substrates, which induce opposite strain to the superconducting layer. By carefully tuning the in-situ post-annealing oxygen pressure, we achieved, in a reproducible way, films at different oxygen doping, spanning from the slightly overdoped down to the strongly underdoped region of the phase diagram. The transport properties of the films, investigated through resistance versus temperature measurements, are in perfect qualitative agreement with single crystals. Starting from these films, we have also successfully fabricated nanowires with widths down to 65 nm, at different oxygen doping. The nanostructures exhibit characteristic temperatures (as the critical temperature $T_{\mathrm{c}}$ and the pseudogap temperature $T^*$) similar to those of the as-grown films and carry critical current densities $J_{\mathrm{c}}$ close to the critical depairing value, limited by vortex entry. This implies that the superconducting and the normal state properties of underdoped YBCO are preserved in our films, and they can be studied as a function of the dimensionality of the system, down to the nanoscale.Comment: 11 pages, 9 figures, submitted to Phys. Rev. Material

Electromigration tuning of the voltage modulation depth in YBa2Cu3O7-delta nanowire-based SQUIDs

Oxygen electromigration applied to a YBa2Cu3O7-delta nanowire can be used to tune its electrical properties. Here, we apply electromigration to YBCO nanowire-based superconducting quantum interference devices (SQUIDs) and study its effect on the voltage modulation depth of the devices. Using a dc electromigration current we replenish the oxygen of the weak links, improving the critical current symmetry of the SQUIDs. AC current electromigration is used to reduce the doping level of the weak links, thus reducing their critical current and increasing differential resistance. Both type of electromigration processes are found to improve the SQUIDs performance, although the best results are obtained with ac biased electromigration, which improved the voltage modulations of the SQUIDs by a factor as high as 8. This procedure can be instrumental to fine tune ex-situ the properties of superconducting electronics where a large number of weak links are required

Simulation of a Novel Bridge MEMS-PZT Energy Harvester for Tire Pressure System

Self-powering is becoming an important issue for autonomous sensor systems. By having an on-the-go power source the life span increases in comparison to a limited battery source. In this paper, simulation of an innovative design for a piezoelectric energy harvester for Tire Pressure Measurement System (TPMS) is presented. The MEMS-based thin-film PZT harvester structure is in the form of a bridge with a big central seismic mass and multiple electrodes. This design takes the advantage of the S-profile bending and a short beam length to concentrate the piezoelectric effect in a small segment along the beam and maximize the power output for a given displacement. From simulation in Comsol Multiphysics, the 9mm x 5mm bridge, seismic mass of 8.7mg and resonance frequency of 615Hz, generates 1 mu W by mechanical pulses excitation equivalent to driving at 60 km/h (roughly 180G)

Fabrication and electrical transport characterization of high quality underdoped YBa2Cu3O7-δ nanowires

We present the fabrication and electrical transport characterization of underdoped YBa2Cu3O7-δnanowires. The nanowires have been realized without any protective capping layer and theyshow transport properties similar to those of the parent thin film, demonstrating that they havenot been damaged by the nanopatterning. The current-voltage characteristics of the underdopednanowires show large hysteretic voltage switching at the critical current, in contrast to theflux-flow like characteristics of optimally doped nanostructures, indicating the formation of aself-stabilizing hot spot. These results open up new possibilities for using the underdopednanowires as single photon detectors and for exploring the underdoped side of the YBa2Cu3O7-δphase diagram at the nanoscale

Mapping the Phase Diagram of a YBa2Cu3 O7-δ Nanowire Through Electromigration

We use electromigration (EM) to tune the oxygen content of YBa2Cu3O7-δ (YBCO) nanowires. During EM, the dopant oxygen atoms in the nanowire are moved under the combined effect of electrostatic force and Joule heating. The EM current can be tuned to either deplete or replenish nanowires with oxygen, allowing fine tuning of its hole-doping level. Electrical transport measurements and Kelvin probe microscopy corroborate good homogeneity of the doping level along the electromigrated nanowires. Thus, EM provides an effective method to study transport properties of YBCO in a wide doping range at the nanoscale in one and the same device

High transparency Bi2Se3 topological insulator nanoribbon Josephson junctions with low resistive noise properties

Bi$_2$Se$_3$ nanoribbons, grown by catalyst-free Physical Vapour Deposition, have been used to fabricate high quality Josephson junctions with Al superconducting electrodes. The conductance spectra (dI/dV) of the junctions show clear dip-peak structures characteristic of multiple Andreev reflections. The temperature dependence of the dip-peak features reveals a highly transparent Al/Bi$_2$Se$_3$ topological insulator nanoribbon interface and Josephson junction barrier. This is supported by the high values of the Bi$_2$Se$_3$ induced gap and of I$_c$R$_n$ (I$_c$ critical current, R$_n$ normal resistance of the junction) product both of the order of 160 $\mu$eV, a value close to the Al gap. The devices present an extremely low relative resistance noise below 1$\times$10$^{-12}$ $\mu$m$^2$/Hz comparable to the best Al tunnel junctions, which indicates a high stability in the transmission coefficients of transport channels. The ideal Al/Bi$_2$Se$_3$ interface properties, perfect transparency for Cooper pair transport in conjunction with low resistive noise make these junctions a suitable platform for further studies of the induced topological superconductivity and Majorana bound states physics.Comment: The following article has been accepted by Applied Physics Letter

SQUID magnetometer based on Grooved Dayem nanobridges and a flux transformer

We report noise measurements performed on a SQUID magnetometer implementing Grooved Dayem nanobridge of YBCO as weak-links. The SQUID shows magnetic flux noise as low as 10 $\mu \Phi_0$/Hz$^{0.5}$. The magnetometer is realized by coupling the SQUID to a flux transformer with a two-level coupling scheme using a flip-chip approach. This improves the effective area of the SQUID and result in a magnetic field noise of 50 fT/Hz$^{0.5}$ at T=77 K.Comment: 4 pages, 4 figure

Current-phase relation of a short multi-mode Bi2Se3 topological insulator nanoribbon Josephson junction with ballistic transport modes

We used the asymmetric superconducting quantum interference device (SQUID) technique to extract the current phase relation (CPR) of a Josephson junction with a 3D-topological insulator (3D-TI) Bi2Se3 nanobelt as the barrier. The obtained CPR shows deviations from the standard sinusoidal CPR with a pronounced forward skewness. At temperatures below 200 mK, the junction skewness values are above the zero temperature limit for short diffusive junctions. Fitting of the extracted CPR shows that most of the supercurrent is carried by ballistic topological surface states (TSSs), with a small contribution of diffusive channels primarily due to the bulk. These findings are instrumental in engineering devices that can fully exploit the properties of the topologically protected surface states of 3D TIs

Improved noise performance of ultrathin YBCO Dayem bridge nanoSQUIDs

We have fabricated YBa2Cu3O7-delta (YBCO) nano superconducting quantum interference devices (nanoSQUIDs), realized in Dayem bridge configuration, on films with thickness down to 10 nm. The devices, which have not been protected by a Au capping layer during the nanopatterning, show modulations of the critical current as a function of the externally applied magnetic field from 300 mK up to the critical temperature of the nanobridges. The absence of the Au shunting layer and the enhancement of the sheet resistance in ultrathin films lead to very large voltage modulations and transfer functions, which make these nanoSQUIDs highly sensitive devices. Indeed, by using bare YBCO nanostructures, we have revealed an upper limit for the intrinsic white flux noise level S-Phi,w(1/2