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  • Post
    A categorical order theory of pulse scheduling in gate-based quantum computing
    (2023) Andersson, Axel; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Bylander, Jonas; Dobsicek, Miroslav
    We define a preorder on a vector space of complex valued integrable functions on the non-negative real numbers. This preorder is then used to develop a scheduling theory for microwave pulse schedules with an application for quantum computer experiments on superconducting circuits. The scheduling theory is further developed in a categorical framework using a subcategory of Ord, the category of preordered sets and order-preserving mappings between them. The developed theory is then applied to create a Python library which translates IBM OpenPulse schedules to Quantify schedules. Further, this library was then used to conduct single qubit characterisation experiments. Performed experiments include: resonator spectroscopy, two-tone spectroscopy, Rabi oscillation, relaxation time (T1) and qubit state discrimination experiments.
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    Studying local orders in YBCO by nanoscale confinement
    (2023) D'Alessio, Andrea; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Lombardi, Floriana; Arpaia, Riccardo
    In 1986 scientists were amazed by the discovery of materials that conducted electricity without any resistance at temperatures much higher than previously thought, an accomplishment awarded the Nobel Prize in Physics already the following year. This led to great excitement and the hope one would be able to design materials that would be ‘superconducting’, as the phenomenon is called, even at room temperature, the holy grail of superconductivity research. The possibility to dispense the use of expensive and cumbersome cooling would indeed revolutionize many technologies like electrified transports, and also lay the foundation for the next generation of green innovations. However, despite intense research, many open questions remain and the mechanism behind high temperature superconductivity still represent a puzzle, far from being fully understood. Recently, the most common idea is that the comprehension of the superconducting mechanism cannot prescind from the understanding of the normal state of these materials. This is also unconventional, and populated by a constellation of local orders, characterized by nanoscale lengths: here, charge, spin, lattice and orbitals have a role, but their entwining and mutual relations are still not fully understood. The core of cuprate high-Tc superconductors (HTS) is represented by the CuO2 planes, where superconductivity sets in and where all the symmetry breaking orders reside. In order to succeed in understanding these fascinating materials, an innovative strategy is to confine the CuO2 planes at the nanoscale in HTS, and to use strain and confinement as a knob to tune the orders both in the superconducting and in the normal state. This can be done only if HTS are shrunk in thin film form, preserving the bulk quality. In this way, confining the orders on the same scale of their characteristic lengths, one may expect the locality of charge, spin and current orders to be enhanced, possibly simplifying the physics at play. The confinement can be obtained in two ways, either by nanopatterning c-axis oriented HTS thin film, where the CuO2 planes take the shape of the nanostructures, or depositing a-axis oriented HTS nm-thin films, where the CuO2 planes form nanoribbons, constrained on either side by vacuum and the underlying substrate. In this thesis work we have followed both of these strategies, using c-axis and a-axis oriented YBa2Cu3O7−δ thin films.
  • Post
    Towards probing zero energy bound states with circuit QED in topological insulator junctions
    (2023) Alcalde Herraiz, Núria; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Bauch, Thilo; Pullukattuthara Surendran, Ananthu; Bauch, Thilo; Lombardi, Floriana
    Majorana fermions are aimed to be observed in a spin-non-degenerate system where particle-hole symmetry is imposed. Such a landscape was proposed by Fu and Kane to be formed at the interface between a topological insulator and a superconductor. Topological insulators are insulating materials in the bulk, but that present metallic states with spin non-degeneracy at the surface. These metallic states in the vicinity of an ordinary superconductor become superconducting by proximity effect imposing the particle-hole symmetry, hence an scenario were Majorana fermions observation is possible. Taking first steps towards Majorana engineering, the aim of the thesis is to study the energy band structure of a STIS junction. The STIS junction studied was made from bismuth selenide nanowires and aluminium electrode contacts and then embedded in a RF-SQUID loop coupled to a resonator. This configuration allowed to characterize the band structure of the STIS junction probing its susceptibility through microwave reflectometry measurements. The experimental data was then compared to an STIS short ballistic junction model obtaining a relaxation parameter γD = 2 GHz and a microwave transitions rate of γND = 3 GHz for probing frequency 4.20 GHz and γND = 10 GHz at 8.16 GHz.
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    Carbon Nanotube Networks as Thermally Conducting Layers
    (2022) Juteräng, David; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Liu, Johan; Fu, Yifeng
    Flexible and thermally conductive materials with microfabricated structures are important in a number of different research fields. Different approaches for integration of such materials into functioning devices have been implemented in a plethora of ways. Carbon nanotube networks have been the subject of many studies due to their remarkable physical properties, including high thermal conductivity, high electron mobility, high Young’s modulus and their flexibility, but challenges still remain. One hurdle to overcome is the lack of efficient bonds between nanotubes in meshes. In this project, the viability of a nickel/carbon nanotube network have been investigated in the context of a potential thermal spreading hybrid material. Carbon nanotubes of with different lengths were grown on silicon substrates, dispersed in acetone and mixed into solutions containing Nickel-oxide particles. The blends were deposited onto new Silicon substrates where they formed networks. The Nickel particles stuck to strands and bundles of nanotubes, forming bridges between them. Thermal treatment of the networks were performed at different time scales in order to study the effects of annealing on the networks. The characteristics of the Ni/CNT networks were finally investigated using scanning electron microscopy and Raman spectroscopy in order to study potential changes within them. An increase of the D-peak/G-peak intensity ratio corresponding to longer thermal treatment of the substrates were concluded to be a plausible indicator of increased bonding between the Ni-particles and CNTs. In addition, a simulation was made of a CNT-CNT electron tunneling junction. This was done in order to provide the theoretical backround for the challenges regarding CNT meshes. The lack of chemical bonds between tubes were calculated to increase the resistance of a square CNT thin film by approximately 150%.
  • Post
    La2-xSrxCuO4 thin films and nanostructures to study local ordering phenomena in a striped superconductor
    (2022) Biagi, Marco; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Kalaboukhov, Alexei; Arpaia, Riccardo
    Since their discovery in 1986, cuprates high critical temperature superconductors (HTS) represent one of the most fascinating class of materials in condensed matter physics. Understanding the underlying mechanism behind high-Tc superconductivity is a real challenge, which could results in the possibility to design in the future a room-temperature superconductor, a technological holy grail allowing an energy-efficiency revolution, and the large-scale realization of applications such as magnetically levitated trains and quantum computers. The strong electron-electron correlations in HTS lead to the formation of exotic charge and spin orders such as charge density waves (CDW) and spin density waves (SDW), that are respectively charge and spin density periodic spatial modulations. In La-based cuprates, as La2-xSrxCuO4 (LSCO), CDW and SDW are characterized, in a well-defined portion of the phase diagram, by a well-defined relation of periodicity, forming the so-called stripe order. The understanding of these local orders is crucial, since they have been recently reported to be responsible for the superconducting and normal state of HTS. Their nature can be effectively investigated in thin films, where the strain induced by the substrate, and the confinement of the HTS at the nanoscale, have been proven to be two powerful knobs to manipulate these orders and understand their mutual interaction. The fabrication of HTS nanostructures is a very challenging task, and up to now relevant results were obtained mainly for YBa2Cu3O7-δ. We optimized the growth of 20 nm thick optimally doped LSCO thin films on LaSrAlO4 (001) substrates by Pulsed Laser Deposition. The films are smooth, as confirmed by atomic force microscopy and reflection high-energy electron diffraction, and highly crystalline, as confirmed by X-ray diffraction. Our best films show a Tc ∼ 39 K, comparable to the bulk value. Finally, we realize LSCO nanowires down to 50 nm width. We measure their Jc values and study the Jc(T). To prove the high degree of homogeneity of our nanowires, we compare the value of J0 c , obtained by fitting the Jc(T) with the Bardeen expression, to the Ginzburg-Landau theoretical limit for the depairing current Jv, due to vortex motion. Our results pave the way for the study of LSCO ground state at the nanoscale.