Teaching at Chalmers
- Floriana Lombardi: MCC015 – Superconducting devices, fundamentals and applications (7.5 HP master level course)
- Thilo Bauch: FKA 173 – Quantum Optics and Quantum Information (7.5 HP master level course)
Eric Andersson Wahlberg
Reshaping the phase diagram of YBa2Cu3O7-δ through strain in ultrathin films and nanowires (2021)
This thesis aims at adding important pieces to the puzzle of understanding the physics of the High critical Temperature Superconductor (HTS) cuprates, where despite over 30 years of intense research many open questions remain. The HTS cuprates are characterized by an incredibly complex phase diagram with multiple intertwined local orders, such as charge density waves (CDW). The superconducting state originates from an enigmatic state called “strange metal”. One of the defining properties of the strange metal is a surprisingly simple linear temperature dependence of the resistivity, a possible consequence of the strong electron-electron correlations in these materials. This behavior cannot be accounted for by the conventional theories of electric transport and calls for new theoretical and experimental approaches that can give more hints about its true nature. This thesis describes new experiments that are able to highlight the physics of the strange metal by studying the physical mechanisms that lead to its breakdown, using nanoscale YBa2Cu3O7-δ (YBCO) thin films and devices. So far, a detailed temperature-doping phase diagram describing the complex properties of the HTS cuprates was available only for single crystals. The first part of the thesis shows that we can reproduce all the main features of the HTS phase diagram for YBCO thin films and nanowires. By reconstructing the surface of the substrates, with high temperature annealing, we are able to grow highly strained, untwinned films. These films are instrumental for studying anisotropic transport properties of both the strange metal and the superconducting state. In the second part of the thesis we have studied the evolution of the T-linear resistivity in the strained films as a function of the thickness and of the doping. In ultrathin and underdoped YBCO films the strange metal phase is restored when the CDW order, detected by resonant inelastic X-ray scattering, is suppressed. This observation points towards an intimate connection between the onset of CDW and the breakdown of the T-linear resistivity in underdoped cuprates, a link that was missing until now. Finally, the thesis describes how the phase diagram of YBCO is reshaped for thin films and devices at the nanoscale, and in particular how the superconducting transition is enhanced by the suppression of CDW order in the underdoped regime. We also show that the dynamics of the phase-slip phenomenon in ultrathin nanowires becomes very different in the direction where the CDW order is suppressed. These results highlight the competing nature of superconductivity and charge order. Overall, the research presented in the thesis work, demonstrates how strain control and nanoscale dimensions allow to manipulate the ground state of HTS which is an important step to disclose the mechanism for high critical temperature superconductivity.
Noise and electrical properties of YBCO nanostructures (2020)
This thesis work deals with the investigation of noise properties in cuprate High critical Temperature Superconductor (HTS) YBa2Cu3O7−δ (YBCO) nanoscale devices. Here the aim is to get a better understanding of nanoscale fluctuations in the normal state of HTSs from which superconductivity evolves. The observation of fluctuations in the electronic properties might offer valuable clues toward the microscopic mechanism leading to superconductivity in HTSs, 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 superconducting quantum interference devices (SQUIDs), is a major focus of research in the field of superconducting sensors such as ultra-sensitive magnetometers. 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 μV 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 (∆V= 27−50 μV), low values of white magnetic flux noise, 6 μΦ0/√Hz, and corresponding magnetic field noise, 63 fT/√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/√Hz.
YBa2Cu3O7−γ nanogaps: A path toward hybrid junction (2017)
This thesis paves the way for the realization of hybrid Josephson junctions based on YBa2Cu3O7−γ (YBCO) nanogaps. This allows one to explore a much richer Josephson physics where the superconducting properties of YBCO, like the d-wave order parameter and large superconducting gap, are proximized into the 2-dimensional Dirac materials such as graphene, or topological insulators. The core part of the thesis discusses a complicated nanopatterning technology employed to realize encapsulated YBCO nanogaps where the electrodes preserve superconducting properties close to the bulk. To assess the quality of the nanogaps, we promoted three sets of YBCO nanowires fabricated by applying different lithographic processes. Using a thermally activated vortex-entry model applied to resistance vs. temperature measurements close to the critical temperature Tc allows us to determine the maximum damage the nanowires undergo during the patterning. We found that the effective width of the nanogap is of the order of 100 nm at Tc while retaining the geometrical value of about 35 nm at lower temperatures. The effect of an ex-situ ozone annealing is investigated both on YBCO films and nanowires. Studying the X-ray diffraction pattern on bare films revealed that the ozonation is a practical method to oxygenate YBCO films. To probe the effectiveness of this method on nanostructures, we compared the critical current density and the broadening of the resistive transition close to Tc measured before and after the ozone treatment. We conclude that the ozone treatment is instrumental in recovering very high-quality superconducting properties inside the nanostructures, which were degraded by the oxygen out-diffusion occurring during the nanopatterning. The feasibility of the encapsulated YBCO nanogaps for hybrid Josephson devices is demonstrated by bridging them with thin Au films. We successfully fabricated and studied planar Superconductor-Normal-Superconductor (SNS) Josephson junctions made of YBCO high-TC superconductor (S) and Au normal metal (N). Our junctions showed Fraunhofer-like modulation patterns and Shapiro steps when they are irradiated by microwave radiation. By using a double barrier SINI′S model, we have studied the transport properties of the critical current in temperature. The results of the fitting suggest that the Cooper pair transport is mainly across the 5 nm Ti sticking layer, underneath of the Au film bridge, due to the ozone treatment. We promoted the growth of fully untwinned high-quality a-axis oriented YBCO films on LaSrGaO4 substrates using PrBa2Cu3O7−γ as a buffer layer. Such films allow for a strong induced superconductivity in proximity based multilayer devices due to the large coherence length along the a-axis, compared to the c-axis film based devices. YBCO nanowires at different angles with respect to the [0,1,0] direction of the substrate are realized to investigate the in-plane anisotropy of the critical current density. The in-plane anisotropy could be explained by considering the anisotropy in the coherence length ξ and London penetration depth λL along the b- and c- axis film.
Decoherence and noise in High critical Temperature Superconducting quantum nanodevices (2016)
The thesis deals with the investigation of dissipation mechanisms and noise in superconducting quantum nanodevices made of the cuprate High critical Temperature Superconductor (HTS) YBa2Cu3O7-δ (YBCO). The main aim is to get a better understanding of the microscopic physical mechanism leading to superconductivity in HTSs, which still represents one of the main unsolved problems in solid-state physics. In this respect, YBCO nanodevices are used as tools to design new experiments, which can deliver deeper insights about the complex properties of HTS materials. In the first part of the thesis transport and noise properties of nanowire based YBCO Superconducting QUantum Interference Devices (SQUIDs) are presented. NanoSQUID devices, with transport properties close to the pristine bulk material, are potential tools to investigate fundamental physics of the YBCO by looking e.g. at the fluxoid quantization. Moreover, because of the measured ultra low magnetic flux white noise (below 1 μΦ0/√Hz), these devices are very attractive for applications as magnetic flux sensors. Flux noise properties are not degraded when employing an inductively coupled, mainly via kinetic inductance, large pick-up loop. This allows to improve magnetic field sensitivity,with possible applications in magnetic field imaging, such as magnetoencephalography. The second part of the thesis discusses the employment of an all-YBCO transmon quantum circuit based on bi-epitaxial grain boundary Josephson junctions to study decoherence mechanisms intrinsic of the material. First, microwave losses from all the materials involved in the fabrication process, including the dielectric substrates and the YBCO itself, are investigated at very low temperatures and microwave powers. The reported results demonstrate the feasibility of a YBCO transmon. Then, the quantum coherence of the device, extracted from spectroscopy measurements, is studied in the presence of high magnetic field and a comparison to DC-transport properties is made. A significant improvement of the coherence time is observed with the application of an external magnetic field, compatible with the occurrence of a fully gapped superconducting state. The main dissipation source has been identified in the presence of a resistive shunt in the junction barrier.
YBa2Cu3O7−δ nanowires to study nanoscale ordering in High-Tc Superconductors (2016)
We present the realization of YBa2Cu3O7−δ (YBCO) nanowires for both basic physics studies, which could help to elucidate the microscopic mechanisms of High critical Temperature Superconductors (HTS), and for novel applications. The first part of the thesis describes an improved nanopatterning procedure, based on e-beam lithography in combination with an amorphous carbon mask and a very gentle Ar+ ion etching. By using this procedure, nanowires preserving pristine superconducting properties, characterized by critical current densities very close to the theoretical depairing limit, have been achieved. These structures represent model systems for the study of HTS: their superconducting properties are very close to the as grown films. We have grown YBCO thin films covering a broad oxygen doping range going from the slightly overdoped regime down to the strongly underdoped region of the phase diagram. We have been able to trace the entire underdoped part of the phase diagram, and to show that the most peculiar features encountered in single crystals are well reproduced in our thin films. The core of the thesis describes an experiment done to study the effect of the nanoscale ordering on the superconducting properties of our nanowires. The presence of a local charge density wave (CDW) order has been recently demonstrated to be ubiquitous among all the cuprate families; if associated to a local modulations of Cooper pair density, CDWs might affect the absolute value of the critical current density of nanowires patterned at different in plane angles. We have used YBCO nanowires, fabricated on untwinned films, as a function of the oxygen doping and with dimensions of the same order as the CDW correlation length. By measuring the current voltage characteristic (IVC) of nanowires with the same width, patterned at different angles with respect to a fixed in plane direction of the substrate, we have revealed a cosinusoidal modulation of the critical current density for the narrowest width w (of the order of 65 nm). This dependence, that smears out for wider nanowire dimensions, can represent one of the first evidence of the existence of a pair density wave in YBCO. In the second part of the thesis we have focused on applications. YBCO nanoSQUIDs, employing very short nanowires in the so-called Dayem bridge configuration, have been fabricated and characterized. These devices, working in the whole temperature range from 300 mK up to the critical temperature Tc (close to 85 K), have revealed an ultra low white noise, below 1 μΦ0/√Hz above 10 kHz, corresponding to a predicted spin sensitivity of 50 μB per √Hz.The homogeneity of the nanowires has also given a boost to the realization of devices, aimed at the detection of single photons. We have shown that in ultrathin (7-8 unit cell thick) YBCO nanowires a hot spot is formed within the wire: as a consequence, the nanowires are driven from the superconducting directly to the normal state, analogously to NbN nanowires commonly used as single photon detectors.
Approaching the depairing critical current in superconducting YBa2Cu3O7−x nanowires (2013)
The microscopic origin of High critical Temperature Superconductivity is still an open issue in condensed matter physics. In these complex oxides electrons self-organize in ways qualitatively different from those of conventional metals and insulators. The study of these materials at the nanoscale can help to shed light on various ordering and phase transitions giving new hints into the origin of high critical temperature superconductivity. In this thesis a systematic study of the transport properties of nanowires made of High critical Temperature Superconductor (HTS) YBa2Cu3O7−δ (YBCO) is presented. A soft nanopatterning technology for the fabrication of these nanowires has been implemented to preserve a homogeneous character of YBCO nanostructures. Two interesting observations are reported regarding the critical current density Jc carried by the nanowires: 1) Its value increases by reducing the nanowire width w. 2) For the smallest wires with w≃40 nm the value of Jc approaches for the first time the theoretical depairing limit indicating nanostructure with properties very close to as grown films. The behavior of the critical current density as a function of width has been explained in terms of current crowding at the inner corners at the connection between the nanowires and the wide electrodes. YBCO nanowire based nanoSQUIDs have been also fabricated. The nano-SQUIDs can work in the whole temperature range from 300 mK to 80 K. Critical current modulation as a function of an externally applied magnetic field at different temperatures has been studied and the screening parameter βL has been extracted. The experimental and simulated values of βL are in good agreement indicating a Josephson-like behavior in the whole temperature range.
Nanoscale YBa2Cu3O7−δ devices – Searching for a subdominant order parameter using a single electron transistor (2012)
The microscopic origin of High critical Temperature Superconductivity (HTS) is still an open issue in condensed matter physics. It is believed that by exploring the quasiparticle energy spectrum one can learn about the mechanism promoting the superconducting state. In this thesis we have developed a nanoscale spectroscopic tool, an all superconducting YBa2Cu3O7−δ (YBCO) Single Electron Transistor, allowing us to obtain information from the quasiparticle spectrum of an entire nanometer scale island. In this experiment we find a fully gapped superconductivity which strongly depends on the externally applied magnetic field. This finding shows that the order parameter is not purely dx2−y2 with nodes, instead it has an additional subdominant imaginary component which lifts the zero energy quasiparticles. The realization of the transistor has required the engineering of nanaoscale YBCO Josephson grain boundary (GB) junctions with stringent demands on the transport properties. Part of the work in this thesis has been devoted to the development and characterization of two methods to fabricate nanoscale GB junctions. A conventional method based on e-beam lithography and ion milling and a new soft nanopatterning technique. The new method is based on the phase competition between superconducting YBCO and insulating green phase at the grain boundary. This has allowed the creation of junctions with minimal damage in the fabrication process. Together, the two methods create grain boundaries that span a large range of critical current densities and normal resistivities, which can be employed in various applications.
Grain Boundary Josephson π-Junctions – Correlation between microstructure and transport properties for novel devices (2010)
The d-wave symmetry of the order parameter in high temperature superconductors has led to a new Josephson phenomenology that can be used to design novel superconducting devices both in the classical quantum regime. Because of the “π” shift of the phase of the order parameter between two orthogonal directions, Josephson “π” junctions characterized by an additional π phase shift in the current phase relation can be realized. At the same time a pinned half integer spontaneous flux (semifluxon) can be observed in frustrated systems consisting of a specific arrangement of nominally “0” and “π” Josephson junctions. In this thesis we have engineered biepitaxial grain boundary Josephson junctions to explore these new possibilities. The biepitaxial technique has been chosen because it allows a large flexibility in the sample design and in the junctions’ performances. To obtain junctions with reproducible parameters the microstructure of the grain boundaries has been systematically investigated by Transmission Electron Microscopy and correlated to the electrical transport properties. We have fabricated corner shaped “0-π” junctions where one facet is a regular “0” junction and the other facet is a “π” junction. Such arrangement is an example of frustrated system and it is interesting to study the static and dynamic properties of the spontaneously nucleated semifluxon at the discontinuity point. A major concern in working with grain boundary junctions is the unavoidable presence of faceting on the submicron scale along the interface profile. An important question was to understand whether the microfacets could obscure the 0-π behavior of the corner junctions. We have therefore, investigated 0-π junctions with different grain boundary angles; they have systematically showed the expected complementary behavior for the Josephson current as a function of the external magnetic field compared to conventional “0” junctions. Moreover, spontaneous magnetic flux has been detected at the phase discontinuity point by Scanning SQUID (Superconducting QUantum Interference Device) Microscope investigations. The influence of the semifluxon on the dynamics of the junction manifests itself by unconventional resonance steps in the current voltage characteristics. We have detected resonance steps for both 0 and 0-π junctions and correlated their frequency to the presence of a semifluxon in the system. This work has shown that grain boundary junctions with specific properties can be fabricated in a reproducible way by the biepitaxial technique and that the faceting on a microscale does not obscure novel d-wave effects that are important for applications.
d-wave Josephson Junctions – Dynamics and Applications in the Quantum Regime (2005)
The properties of d-wave YBCO Josephson junctions and dc-SQUIDS with high misorientation angles have been studied experimentally. The predominant d-wave symmetry of the pairing wave function in high-T, superconductors has, to a large extent, applications, also makes it possible to fabricate devices with novel properties that may be suitable for applications in the quantum regime; one particularly interesting property is a strong second harmonic component in the current-phase relation of junctions with the electrodes oriented in a node-lobe arrangement: so-called 0-45° junctions. The work has been focused on the properties of grain-boundary 0°-45° dc-SQUIDS: Their dc-properties as well as the effects of a strong second harmonic component on the dynamics have been studied experimentally and modeled numerically. The experimental results can be explained by taking into account the meandering of the grain boundary which in combination with a strong orientational dependence of the transport properties due to the d-wave symmetry results in interfaces with properties that change dramatically over a length scale of 50-100 nm. The properties of grain-boundary YBCO 0°-45° junctions have been studied in the quantum regime. Macroscopic quantum tunneling has been demonstrated in these junctions and microwave spectroscopy is here used to verify the presence of discrete energy levels. The quality factor of the junction has been experimentally determined to 40±10, a much higher value than previously believed to be possible in high-T, structures. Finally, the possibility of using high-Tc structures to build qubits is discussed. There is now a renewed interest in this possibility, especially in light of our recent results which demonstrate that the inherent dissipation mechanisms are much less detrimental to coherent phenomena than previously feared.
YBCO nanowires for ultrasensitive magnetic flux detectors and optical applications (2014)
In this licentiate thesis, we present the realization of YBa2Cu3O7−δ (YBCO) nanowires, with cross sections down to 40×50 nm2, and their employment in nanoSQUIDs and optical sensors. We have overcome the limiting issues in establishing reliable nanofabrication routines for YBCO, due to the chemical instability (related to oxygen out-diffusion), and the extreme sensitivity to defects and disorder due to very short coherence length of this material. Through an improved nanopatterning procedure, based on e-beam lithography in combination with an amorphous carbon mask and a very gentle Ar+ ion etching, by keeping a capping layer of Au on the top of the nanostructures, we have achieved nanowires preserving “pristine” superconducting properties, very close to the as-grown films. The benchmark of their quality is given by the very high critical current densities they carry (up to 10^8 A/cm2), which are very close to the theoretical depairing limit. Therefore, these structures represent model system to study High Critical Temperature Superconductors, since the properties they show are strictly related to the superconducting material, scaled to the nanoscale, and they are not ruled by damages and defects associated to the nanopatterning procedure. Different experiments have been done, employing these nanowires in more complex devices. We have fabricated and characterized YBCO nano superconducting quantum interference devices, employing very short nanowires in the so-called Dayem bridge configuration. These devices, working in the whole temperature range from 300 mK up to their critical temperature (≈82 K), have revealed an ultra white low-noise below 1 μΦ0/√Hz above 10 kHz, corresponding to a predicted spin sensitivity of only 50 μB per √Hz. These properties make our devices very attractive for many applications, as for the investigation of the magnetic moment in small ensembles of spins in a wide range of temperatures and magnetic fields. We have realized YBCO nanorings, measuring the magnetoresistance close to Tc. Large oscillations have been measured, which can be associated to the vortex dynamics triggering the nanowires to the resistive state. The Fast Fourier Transform spectra have shown, for the nanorings with narrower line width, a single sharp peak: this peak, associated to h/2e periodicity, as predicted for optimally doped YBCO, represents a clear evidence of a uniform vorticity of the order parameter inside the rings, confirming the high degree of homogeneity of our structures. The homogeneity of our nanowires, together with the fast relaxation times of the YBCO, gave the boost for the realization of devices, aimed at the detection of single photons. This goal required the use of very long wires (up to 10 μm), covering large areas (≈300 μm^2). Crucial requirement is the homogeneity of the superconducting properties within different regions of each device. This requirement has to be achieved without using a Au capping layer. We have therefore engineered YBCO nanowires capped with a ferromagnetic La0.7Sr0.3MnO3 layer, showing encouraging homogeneity properties. We have done preliminary photoresponse measurements on these devices, detecting signals whose main origin appears to be mostly bolometric.
Unconventional Properties of YBCO Thin Films and GB Josephson Devices for Novel Applications (2009)
This thesis deal with YBa2Cu3O7-δ, (YBCO), thin films and Josephson junction, (JJ), devices for quantum applications. We have investigated if low dissipation and an unconventional Josephson Current Phase Relation, (CPR) with multiple harmonics can be combined in JJ:s suitable for qubit applications. The focus has also been on the anisotropy of the intrinsic properties in YBCO, such as the magnetic penetration depth, λ and the d-wave order parameter, influence the dynamics of grain boundary, (GB), JJ:s. We have studied if standard nano lithographic techniques could successfully be applied to HTS materials and if it was possible to observe single electronic effects in HTS YBCO GB structures. The properties of bicrystal d-wave YBCO Superconducting QUantum Interference Devices, (dc-SQUIDS), characterized by a mutual 45°  rotation and an additional c-axis tilt of GB, have been investigated in the temperature range 50 mK – 30 K. The experiments showed that an unconventional CPR and low dissipation can be combined within the same structure. We have found that the complicated dynamics of these SQUIDS can be understood if the effects of a sin2φ current component in the effective CPR and the inclusion of π-facets in the GB are taken into account. The full in-plane angular dependence of the inductance in (103) YBCO films was determined by operating geometrically identical SQUIDS, with different orientation with respect to the  direction of the (103) film, in direct current injection mode. The SQUIDS were realized by the bi-epitaxial technique. We found that the inductance can differ a factor 20 in the extreme cases where transport either is along the ab-planes or in the c-axis direction of the film. From these measurements we have determined the London penetration depths in ab-direction, λab, and C-direction, λc, and their temperature dependence. Finally we have demonstrated Coulomb blockade in YBCO Single Electronic Transistor devices and that it indeed is possible to obtain a sub 100 nm line width for superconducting YBCO nanowires with our amorphous carbon technique.
Dynamics of high-angle grain boundary YBCO Josephson junctions (2004)
This thesis describes experimental investigations of properties of high-angle YBa2Cu3O7−δ (YBCO) bicrystal Josephson junctions and SQUIDS fabricated on SrTiO3 substrates. The main focus of the investigation has been on the effects of the predominant d-wave symmetry of the superconducting wave function in YBCO on transport properties and dynamics. At a high-angle grain-boundary interface between two high-temperature superconductors Andreev states can form, the current carried by these states has π-periodic component which flows in a direction opposite of the usual Josephson current. Asymmetric high-angle grain boundaries also exhibit a critical current which is four orders of magnitude lower than symmetric lower-angle junctions, this can be attributed to the node-lobe arrangement of the the superconducting order parameter. High-angle grain-boundary dc-SQUIDS have been studied which exhibit unusual dynamics such as a relative “shift” of the positions of the positive and negative modulation and a highly non-sinusoidal dependence on the external field. This behavior vanished when moving to very narrow junctions. These result are explained using a semi-classical model which assumes the presence of a 2nd harmonic in the current-phase relation. Numerical simulations confirm that this model is in qualitative agreement with experimental results. The properties of sub-micron sized junctions have also been studied. These exhibit some unusual behavior. These junctions have been used to study the tunnelling spectra since the high normal resistance means that it is possible to study energies close to the gap. Finally, some general properties of high-angle Josephson Junctions are discussed. It is argued that some seemingly inconsistent experimental results can be explained using a multi-channel model which accounts for the wiggling and faceting of the interface.
Synthesis and electrical characterization of topological insulator Bi2Se3 nanobelts (2019)
In this work a growth process has been developed for growing nanobelts of Bi2Se3. The advantage of nanobelts lies in the high surface to volume ratio which reduces the effect of the bulk due to Se vacancies in transport measurement. The experiments performed include: the development of a PVD process for the growth of the nanobelts, followed by SEM and EDX analysis to study the nanobelts and their stoichiometry and electrical characterization of the grown nanobelts. Electronic transport properties are studied by fabricating devices with Hall bar geometry. Measurement in these devices include: longitudinal resistance as a function of temperature, longitudinal and transverse resistance as a function of magnetic field in cryogenic temperatures of 5K. From these measurements, Shubnikov-de-Haas oscillations are studied and various parameters are extracted such as: frequency of oscillations, 2D carrier densities and mobilities. Transverse resistance data provides information on the total 2D and 3D carrier densities and Hall mobilities.
Chirag Murendranath Patil
Single Photon Characterization of (103)-oriented YBa2Cu3O7−x Coplanar Waveguide Resonator (2015)
The strong electron-electron correlation makes the cuprate high critical-temperature superconductors (HTS) to behave as unconventional metals, where conventional electron band theory fails to describe the various electronic and magnetic properties of those materials. The most prominent property of HTSs is the superconducting state, whose order parameter symmetry is known to be dominated by a d-wave. Up to date no microscopic theory exists, which could explain the occurrence of superconductivity in these materials. However, the low energy quasiparticle excitation spectrum in these materials is believed to hold key information about the microscopic mechanism leading to the phenomenon of superconductivity in HTS. Superconducting quantum devices are powerful tools to study the quasiparticle excitation spectrum. In fact, a tiny fully developed superconducting gap (20 μeV) has been observed by studying the electronic transport in an all-HTS single electron transistor. This experiment unveiled a subdominant complex (s-wave) part in the order parameter besides the d-wave component. A complementary experiment thereof would be the study of relaxation times in a HTS transmon qubit, which should scale with the quasiparticle density of states in the HTS material. The transmon is a type of artificial two-level system (TLS), realized by a capacitively shunted Josephson junction, which is embedded in a superconducting resonator. By probing the microwave spectrum of this two-level system using single photons inside the resonator one can obtain information about quasiparticle induced relaxation processes in the TLS at the qubit transition frequency. Here the resonator acts like a filter towards the environment protecting the transmon from any noise source outside the resonator. Since this device is operated at higher frequencies (around 5 Ghz, which corresponds to an energy of 20 μeV), it is characterized at corresponding low temperatures (T ≈ 120 mK). The key ingredient of the HTS transmon is the Josephson junction. High quality junctions using HTS can be realized using the bi-epitaxial grain boundary junction. Here, the junction is localized at the interface of (001) oriented YBCO and a (103) oriented YBCO electrode. In order to study quasiparticle induced relaxation processes in a YBCO transmon, all other loss mechanisms such as dielectric losses of the substrate and conductor losses of the YBCO need to be known and characterized in advance. The microwave properties of (001) YBCO films are studied and fairly well understood, whereas the microwave properties of (103) YBCO films are yet to be revealed; especially in the low temperature and low power limit (single microwave photon limit). So it is appropriate to study the high frequency dissipation properties of YBCO in the framework of the transmon. By patterning a resonator device using (103) YBCO, its microwave dissipation properties can be investigated within the scope of the transmon design i.e, measuring in the millikelvin temperature and probing in the single photon limit. In this thesis, the resonator is implemented adopting a Coplanar waveguide (CPW) design to understand the different loss mechanisms in the preceding context. The temperature dependence of the resonance frequency and the quality factor (of CPW) is then related to the available models describing various dissipation mechanisms. This work is further directed towards exploring the possibilities of using this CPW resonator as a sensitive magnetic field sensor. This is achieved by studying the variation of the (kinetic) inductance of the superconductor by applying an external magnetic field and also performing some noise measurements to determine its sensitivity. These tasks are the main focus of this thesis work. The first chapter gives a brief introduction to the phenomenological aspects of superconductivity, modeling and design of the coplanar waveguide resonator and properties of a superconducting microwave resonator. Chapter 2 gives a catalogue of the various clean room process steps involved in the fabrication of the resonator followed by the high frequency measurement setup to probe the resonator device. The measured results are analyzed and discussed with conclusion in the final chapters.
Zenas Van Veldhoven
Engineered Tunnel Barriers for a YBa2Cu3O7−δ Single Electron Transistor (2014)
Despite the great scientific effort, the microscopic origin of superconductivity in the high-temperature superconductor YBa2Cu3O7−δ is still not understood. In recent work, Gustafsson D. et al. discovered that the superconducting order parameter in a YBa2Cu3O7−δ mesoscopic island did not have a purely d-wave symmetry, as it is commonly assumed. By using a Single-Electron Transistor (SET) made of a YBa2Cu3O7−δ source, mesoscopic island, drain and gate, they discovered that the YBa2Cu3O7−δ island had a fully gapped superconductivity. This indicates the existence of a subdominant imaginary order parameter in YBa2Cu3O7−δ that gaps the nodes of the d-wave order parameter. In order to investigate the doping dependence of this subdominant order parameter, one should use a new design for the SET, with normal metal source, drain and gate. To have a working SET, the tunnel resistance between the electrodes and the islands needs to be higher than the quantum resistance. The goal of this work is to engineer the YBa2Cu3O7−δ-Au tunnel junction with a sufficiently high tunnel resistance. For that, tunnel barriers between YBa2Cu3O7−δ and Au were created by an Ar ion milling procedure of the surface using different etching times and different voltages. The current-voltage characteristics of the Au-YBa2Cu3O7−δ junctions were investigated and the contact resistivity between YBa2Cu3O7−δ and Au was measured. The surface etching seems to create low transparency interfaces. In addition, a normal YBa2Cu3O7−δ layer is formed at the surface, which can be turned superconducting by ozone treatment. Suitable tunnel barriers for a YBa2Cu3O7−δ SET can be created using mild surface etching and ozonation.
Doping dependent transport in YBCO nanostructures: insights into the microscopic mechanism for high critical temperature superconductivity (2013)
The microscopic mechanism responsible for superconductivity in high critical temperature superconductors (HTSs), almost three decades after their discovery, still remains unknown. It is widely believed that studies in the underdoped (UD) regime of these materials could shed light on this unresolved question. In this thesis project, a controllable and reproducible Pulsed Laser Deposition (PLD) growth of underdoped YBa2Cu3O7−δ (YBCO) films was done by changing only the post-annealing pressure. X ray diffractometry (XRD) 2θ-ω scans of the films have shown a continuous YBCO unit cell expansion as the pressure decreased, indicating a reduction in the doping level of our films. Rather sharp resistance vs. temperature transitions obtained in our films indicate a high level of homogeneity. First steps towards optimization of the surface properties of the films have been also undertaken. A soft-patterning technique developed previously in our group, preserving homogeneity of submicron structures, was employed for nanorings’ patterning on the optimally doped films. Little Parks (LP) experiments were conducted on rings with different sizes. Cooper pairs have been identified as the predominant charge carriers in all the rings, as expected at the optimal doping level.
Transport Properties of Bi2Te3 and Proximity Effect with Aluminum superconductors (2013)
Topological insulators are materials with an insulating bulk and a gap-less metallic surface. On the surface the energy dispersion is linear and described by an odd number of Dirac cones. The interest for these materials was renewed recently when room temperature topological insulators among the bismuth compounds was discovered. Intensive research in the last years is focusing on observing the signatures of the topological surface. However, it is difficult to isolate from the bulk and the effects observed can have alternative interpretations. So far the surface states have not been totally distinguished from the bulk. Therefore topological insulators need further characterization and this thesis is a part of that research. The two main focuses were to characterize the transport properties of molecular beam epitaxy grown Bi₂Te₃ thin films and Bi₂Se₃ single crystal with Hall effect and proximity effect. The Hall measurements of Bi₂Te₃ were used as feedback to the growers in collaboration to achieve better quality thin films. The Bi₂Te₃ showed negative charge carriers and the volume carrier concentration was improved from 1×10²¹ cm³ to 4.4×10¹⁸ cm⁸. The mobility was improved from 150 cm2/Vs to 5500 cm2/Vs. For Bi₂Se₃ samples the typical values were 1.3 x 10¹⁹ cm³ and 5100 cm2/Vs, which was comparable with the best Bi₂Te₃ films. The properties of the topological insulators Bi₂Te₃ and Bi₂Se₃ were also investigated using proximity induced superconductivity in Josephson junctions and superconducting quantum interference devices with aluminum contacts at temperatures down to 20 mK. The Josephson coupling was confirmed by the response in microwave radiation and magnetic field. The height of the observed steps corresponded well to integer Shapiro steps. The response of the devices in magnetic field showed expected Fraunhofer patterns, where the effective areas for both the Josephson junctions and the superconducting quantum interference devices was in good agreement with the design. In addition the temperature dependence of the junctions was examined and evaluated in the clean and dirty regimes. The critical current scaled with the temperature, according to simulations of the resistively shunted junction model. To further characterize the Bi₂Te₃ thin film topography and spectroscopy was measured, describing the roughness of the film and indicating a Dirac cone around 200 mV.
Microwave and noise properties of YBCO films and Josephson junctions for quantum circuit applications (2010)
The high-critical temperature superconductivity (HTS) of cuprates has been the subject of intensive research in the last few decades. At the time of this writing, a complete microscopic theory that can fully explain the superconductive properties of HTS is still missing. It has been established that the order parameter of HTS is mainly of d-wave shape. Therefore it was long assumed that nodal quasi-particles would make it impossible to observe quantum behaviour in HTS. However, recent experiments have shown that HTS Josephson junctions can exhibit macroscopic quantum effects. Indeed, the fabrication of HTS Josephson junctions has been very important to understand the unconventional symmetry of the order parameter. Moreover, Josephson junctions are a fundamental building block for superconducting circuits and offer very interesting quantum applications. In our research group, we are interested in the intrinsic dissipation in a YBCO Josephson junction using the transmon qubit design. The environmental noise in the transmon design is strongly suppressed. The junction is embedded in a co-planar waveguide resonator structure. The dissipation in the junction is then obtained by measuring the relaxation time of the coupled system (junction and resonator). The realization of YBCO Josephson junctions with multilayer technology has proven to be a very difficult task. Problems arise due to the very short coherence length of YBCO and the surface instability of the material. Instead, artificial grain boundary junctions fabricated with the biepitaxial technique have shown great flexibility. The grain boundary is obtained by inducing different growth orientations with a patterned seed layer. In this way, junctions can be realized at arbitrary positions on the same chip. To fulfill the main requirements of the transmon design, the width of the junction must be below 100 nm. When the Josephson junction is patterned to these small sizes by ion-beam milling, the YBCO at the edges of the junction suffers extensive damage and the superconductivity is severely degraded due to oxygen out diffusion. This damage can be prevented by using a new soft nano-patterning approach (“green phase” technique). It combines conventional nanofabrication techniques with principles of self-assembly. Before realizing a full-YBCO transmon qubit, several tasks need to be fulfilled: • The grain boundary of the junction must be characterized in detail. The transport of Cooper pairs and quasi-particles through the grain boundary is not very well known, and we need to gain more insight in those mechanisms. • The other sources of dissipation in the transmon design must be analyzed, in particular the dissipation in the large electrodes of the junction. Determining this is essential to extract the purely intrinsic dissipation of the junction from the relaxation time. These two tasks are the focus of my thesis work. The first chapter gives a brief introduction on superconductivity, the Josephson effect and the working principle of the transmon qubit. Chapter 2 handles the basic properties of YBCO and the fabrication of YBCO thin films with different crystal orientations. The fabrication of nanosized junctions with the green phase technique is also explained. In chapter 3, the uniformity of the grain boundary and the noise properties of biepitaxial Josephson junctions are measured for different grain boundary angles. The high-frequency properties of YBCO will be analyzed in chapter 4. This is done by fabricating YBCO resonators and measuring their quality factor at various temperatures. These measurements give information about the temperature dependence of the London penetration depth and the surface resistance for film grown with different orientations.
Nanosize HTS Josephson junctions by a new self-assembly technique (2009)
The discovery of High Temperature Superconductors (HTS) has risen an immediate interest among the scientific community. Materials like YBa2Cu3O7−x (YBCO), with critical temperature above the boiling point of liquid nitrogen, were considered particularly attractive for a number of electronic applications. However, soon after the discovery of HTS scientists realized the difficulties they had to overcome to fabricate HTS Josephson junctions, the building block for superconductor based electronics, in a reproducible way. This is due to the peculiar properties of HTS compounds. The coherence length ξ, which can be seen as the distance over which Cooper pairs retain phase coherence, is much smaller in HTS than in LTS and typically of the order of 1 nm. Consequently, the superconducting order parameter is extremely sensitive to chemical and structural changes even on the nm scale. A careful control of the interface between the barrier and the electrodes is therefore crucial to fabricate junctions with uniform properties. Moreover, the transport in HTS is highly anisotropic: as an example, for YBCO the conductivity along the c axis is two orders of magnitude smaller than that in the (a,b) planes. Finally, the difficulties in growing epitaxial multilayers structures hindered the fabrication of Superconductor-Insulator-Superconductor (SIS) sandwich-like junctions. Nevertheless, a number of techniques has been developed to fabricate HTS junctions: several of them employ Artificial Grain Boundaries (AGBs) between two superconducting electrodes with different crystallographic orientations. The advances in fabrication techniques has led to a renewed interest toward applications of HTS, in particular to quantum computation. Most proposed realizations of a HTS quantum bit (qubit) require the use of nanometer size Josephson junctions. The performances of other superconducting devices like Superconducting Quantum Interference Devices (SQUIDs) would also have a benefit from a decrease of the device dimensions down to the nanoscale, for example in terms of a reduced value of flux noise. However, a common problem in fabricating GB junctions on the nanoscale is the lateral damage in the superconducting films induced by the ion milling stages usually performed to define the dimensions of the junctions. The structural and chemical changes induced by ion beams shrink the effective width of the superconducting structures: this eventually sets a limit – around 200 nm – to the narrowest Josephson junctions which can be defined without loss of the superconducting properties. In order to realize HTS junctions with reproducible properties on the nanoscale, it is therefore crucial to minimize the excessive damage due to patterning by ion milling. This thesis has focused on studying the feasibility of a new approach to shrink the width of HTS Josephson junctions down to ≈ 100 nm. The idea is to take advantage of the presence of secondary insulating phases within YBCO films grown on specific substrates. The nucleation of one of these phases, Y2BaCuO5 (green phase), can be enhanced by using proper substrates, low deposition temperatures and by the presence of GBs. The goal is to determine particular growth conditions for YBCO where nanometer size GB junctions are embedded within an insulating matrix made by green phase grains. In the following chapters we will: • introduce superconductivity, with a focus on Josephson effect (Chapter 1), • give a brief overview of YBCO properties and of techniques employed to fabricate HTS junctions (Chapter 2), • describe the YBCO film growth with reference to our samples (Chapter 3), • explain in detail a new approach to fabricate nanosized junctions (Chapter4), • analyze the morphology of our samples, focusing on the presence of precipitates within the films and at the AGBs (Chapter 5), • present and discuss the transport measurements on our junctions (Chapter 6).
Mudassar Mumtaz Vrik
High Frequency Properties of the Oxide materials at millikelvin temperatures for HTS devices in the quantum limit (2009)
In this thesis we used coplanar wave guide resonator (CPW) technique for many different reasons. • Coplanar waveguides have a simple layer structure without any deposition of insulator layer. • It has better planar connection with quantum circuits. • Resonance frequency of coplanar waveguide transmission lines is less sensitive to the kinetic inductance of the films as compared to the geometrical inductance of the structure. •For a given substrate with certain thickness, different cross sections of transmission line with constant characteristic impedance ZL=50Ω can be designed to match the resonator impedance with the impedance of all coaxial cables used in measurement setup and other microwave equipment. A CPW resonator is constituted by a conductor separated from a pair of ground planes, all on the same plane, atop a dielectric medium and coupling capacitors to couple the resonators with the outside world. In this thesis quarter wavelength resonators, with interdigitated coupling capacitors at one end and shorted at the other, are made. Such types of resonators are characterized by measuring the reflection coefficient S11. Once we have the S11 parameter the total quality factor Qtot of the resonator can easily be calculated whose reciprocal is the sum of the reciprocals of the internal and external quality factors of the resonator. By extracting the external quality factor we can find the internal quality factor which is mainly due to the dielectric losses. Information about the magnetic penetration depth λL of the films has been obtained from temperature dependent measurements of the resonance frequencies of the resonators.
Feasibility Study of an All High Tc Single Electron Transistor (2008)
At the time of this writing the physics responsible for the superconductivity of copper oxide compound materials is still not understood and object of very exciting research. Different theories have been proposed to explain the reason of a so high critical temperature in such superconductors, but none of them has been capable to fully predict the broad range of phenomena related to this phase transition. The scientific community seems to agree on one thing: the definition of the symmetry of the order parameter in high-Tc superconductors is the first fundamental prerequisite for a full comprehension of the physics of these compounds. The realization of junctions with Josephson properties has been responsible for great improvements in understanding high-Tc superconductors. We will focus, among the different technologies capable of producing junctions with high-Tc compounds, especially on the so called grain boundary junctions, realized by carefully engineering the grain boundary between two diversely oriented crystals of superconducting compound. Because of the high anisotropy of the order parameter, it is possible to realize a weak-coupling between the superconducting order parameter in the two electrodes, necessary condition for the onset of the Josephson effect. The realization of such junctions has allowed experiments which determined, once for all, the presence of the d-wave symmetry as dominant component of the order parameter in most copper-oxide based superconductors. In particular, experiments performed on grain boundary junctions realized with YBa2Cu3O7−δ – i.e. the superconductor employed in this thesis work – have given evidence of the presence of a s+d-wave admixture of the superconducting order parameter. Anyway, according to theory any complex linear combination of d-wave and s-wave is possible and, therefore, the presence of an imaginary s-wave component of the order parameter should not to be excluded. Moreover, the existence of such component would be very interesting for the realization of an artificial two level system, based on the quantum dynamics of single Josephson junctions. To better understand the physics of cuprate compound superconductors, different experiments have been proposed and realized, all of them sharing the capability of mapping directly the superconducting order parameter in the different directions of the momentum space. Within such framework, we propose the realization of a Single Electron Transistor realized with high-Tc gran boundary junctions. This work tries to understand if two technologies used to realize grain boundary junctions- YBCO/YSZ bicrystal technology and YBCO/STO/MgO biepitaxial technology – are suitable for such single-electronics application. A Single Electron Transistor (SET) is constituted by a series of two small Josephson junctions capacitively coupled to a gate electrode. The physical principle which enables its operation is the Coulomb blockade of tunneling, i.e. the impossibility of tunneling single electrons through the junctions because of the effect of the Coulomb repulsion. For the Coulomb blockade of tunneling to be effective, every tunnel junction has to comply two fundamental constraints. • The junction’s normal resistance RN – i.e. the resistance of the junction measured at very high bias voltages – has to be higher than 25 kΩ, so that quantum fluctuations are ineffective in promoting tunneling events. • For thermal fluctuations to be unsuccessful in fostering single electron tunneling, the capacitance of each junction has to be smaller than e^2/2k_B*T. To give an idea of the order of magnitude, such capacitance should be smaller than 1 fF for temperatures close to 1 K. Understanding whether the realization of an SET is feasible or not in a given technological platform means trying to understand if it is possible to realize junctions which satisfy the above mentioned restrictions. To characterize the normal resistance of the grain boundary Josephson junctions a simple measurement of the current-voltage characteristic of the junction can be used. On the other hand, measuring junction capacitances of the order of 1 fF is not straightforward. In this work, the junction capacitance has been extracted from the measurement of the current-voltage characteristics of Superconducting Quantum Interference Devices (SQUID), acquired at different values of the external magnetic field. A SQUID is constituted by two Josephson junctions connected by two superconducting leads in a ring-like geometry. By applying an external magnetic field it is possible to strongly modify the current-voltage characteristic of the device and generate AC circulating currents in the loop which, under certain circumstances, can interfere non linearly with the LC resonator formed by the superconducting ring inductance and the junction capacitances. At evenly spaced values of the magnetic flux linked to the loop, such interaction can cause a current step to appear in the current voltage characteristic of the SQUID. The voltage at which such step occurs is related to the product LC. By estimating the SQUID inductance by separate means, it is possible to have an accurate measurement of the capacitance of the junctions of the SQUID. Such capacitances can further be used, together with the information of the normal resistance, to understand the biggest cross-section of a junction which permits to realize a Single Electron Transistor. In the following chapters we will: 1. give a brief overview of the physics of superconductors, of the SETS and of the SQUIDS, with a detailed description on possible interactions between the SQUID and LC resonators (Chapter 1), 2. describe briefly the fabrication procedure and the measurement setup used to realize and characterize SQUIDS and SET prototypes (Chapter 2). 3. summarize the most significant results of the SQUID characterization together with the measurements of the SET prototypes, concluding with a suggestion for a junction dimension suitable for high-Tc single electronics applications (Chapter 3).
D-wave effects in YBa2Cu3O7−δ Josephson devices (2004)
It is well established that the symmetry of the order parameter (OP) in high temperature superconductors (HTS) is predominantly d-wave. The difference in phase between the lobes of the d-wave OP makes possible to fabricate π-junctions with an intrinsic phase shift of π of the superconducting phase. π-junctions have been suggested for the fabrication of improved Rapid Single Flux Quantum (RSFQ) logic cells and other novel devices like sensors and circuits where they can be an important complement to the standard junctions in the design. We have fabricated YBa2Cu3O7−δ (YBCO) grain boundary junctions (GBJ) using a biepitaxial technique with an intermediate seed layer for the creation of the grain boundaries. The use of such technique allows us to have more creative designs compared to the case of a bicrystal grain boundary junction. We have measured the critical current as a function of the GB angle. The observed dependence proves the influence of the d-wave symmetry in the transport properties of the junctions. However, the detection of a π-junction requires it to be integrated in a loop with at least one ordinary junction and one π-junction. We have fabricated de superconducting quantum interference devices (SQUIDS) where one of the junctions is an ordinary 0-junction and the other is a π-junction and measured the transport properties. π-SQUIDS are considered to be the building block for new innovative electronics.