Browsing by Author "Hakonen, Pertti, Prof., Aalto University, Department of Applied Physics, Finland"
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Item Adiabatic melting experiment: ultra-low temperatures in helium mixtures(Aalto University, 2020) Riekki, Tapio S.; Tuoriniemi, Juha, Dr., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; µKI group; Perustieteiden korkeakoulu; School of Science; Hakonen, Pertti, Prof., Aalto University, Department of Applied Physics, FinlandMixture of the two stable helium isotopes, 3He and 4He, is a versatile system to study at low temperatures. It is a mixture of two fundamentally different quantum mechanical particles: fermions and bosons. Bosonic 4He component of the dilute mixture is known to become superfluid at about 2 K, while superfluidity of the dilute fermionic 3He component has not yet been observed. The transition is anticipated to occur at temperatures below 0.0001 K (i.e. 100 uK). To reach such ultra-low temperatures, new cooling methods need to be developed, one of which is the main subject of this thesis. Current, well-established, cooling methods rely on external cooling, where a metallic coolant is used to decrease temperature in a liquid helium sample. Their performance is limited by rapidly increasing thermal boundary resistance. Our novel adiabatic melting method relies on internal cooling process, where both the coolant and the sample are same helium. First, we create a phase-separation in the mixture by increasing its pressure to about 25 times the atmospheric pressure. This solidifies the 4He component, and we ideally end up with a system of pure solid 4He and pure liquid 3He. The phase-separated system is then precooled by conventional methods, after which the solid is melted. This allows 4He to mix with 3He again in heat absorbing process, resulting in a saturated mixture with about 8% molar 3He concentration. In theory, the mixing can reduce temperature by more than a factor 1000, but external heat leaks and imperfect phase-separation reduced this to the factor 5-7 in this work. We study the performance of the melting method under various conditions, such as different melting rates, various total amount of 3He, and alternate configurations of the setup. We also developed a computational model of the system, which was needed to evaluate the lowest achieved temperatures, as the mechanical oscillators used for thermometry had already become insensitive. For it, we studied the thermal coupling parameters of our system, including thermal boundary resistances and 3He thermal conductivity. The lowest resolved temperature was (90 +- 20) uK, still above the superfluid transition of the 3He component of the mixture. We also present suggestions for future improvements for the setup.Item Dynamics and vortex structures in topological superfluid ³He at ultra-low temperatures and under confinement(Aalto University, 2019) Mäkinen, Jere; Eltsov, Vladimir, Dr., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; ROTA - Topological superfluids; Perustieteiden korkeakoulu; School of Science; Hakonen, Pertti, Prof., Aalto University, Department of Applied Physics, FinlandSymmetry and its breaking are central to the modern understanding of the physical world. In conjunction with a handful of judiciously chosen experiments and topological reasoning, they have guided us to formulating fundamental laws in the context of relativistic quantum field theory as well as the theory of possible states of matter, their phase transitions and universal behavior. Topological defects, such as quantized vortices, affect the behavior at macroscopic scales. The range of possible defects is governed by the broken symmetries and topology of the system. The symmetries contained by the normal phase of ³He are exceptionally rich, and support a variety of symmetry-breaking phase transitions to macroscopically coherent, superfluid, states. Due to the existence of these symmetry-breaking phase transitions and the zoo of topological and nontopological defects allowed by the different superfluid phases, ³He can be used as a model system for a wide range of research topics ranging from turbulence and topological quantum computing to cosmology and grand unification scenarios. This thesis focuses on experimental studies on properties of linear topological defects born due to breaking of the U(1) symmetry at the superfluid transition - quantized vortices. At ultra-low temperatures in the superfluid B phase we quantify the interpretation of vortex imaging techniques based on Andreev scattering of thermal quasiparticles and study the response of an equilibrium vortex array to perturbations, namely to a rapid spin-down of the rotating drive. Superfluid B phase of ³He provides a unique platform to study non-equilibrium hydrodynamics as the ratio of the inertial and viscous forces may be tuned by orders of magnitude by changing the temperature. Recent advances in experimental techniques have demonstrated that under engineered confinement the topology of the superfluid may be altered, resulting in novel superfluid phases. These phases provide experimental access to new types of topological and nontopological defects, which are also studied in this thesis. We demonstrate that a new type of vortex - the half-quantum vortex (HQV), is stabilized in the superfluid polar phase. Taking advantage of the phase diagram of ³He under confinement, we demonstrate that HQVs may be transferred to superfluid phases with polar distortion - with striking consequences. In the polar-distorted A phase the HQVs have been predicted to harbor isolated Majorana fermions that in 2D systems obey non-Abelian statistics, while in the polar-distorted B phase the HQVs survive as "walls bounded by strings" - composite defects predicted in cosmological context decades ago.Item Electronic transport and mechanical resonance modes in graphene(Aalto University, 2017) Oksanen, Mika; Teknillisen fysiikan laitos; Department of Applied Physics; Nano group; Perustieteiden korkeakoulu; School of Science; Hakonen, Pertti, Prof., Aalto University, Department of Applied Physics, FinlandGraphene is a two-dimensional sheet of carbon atoms arranged in a hexagonal honeycomb lattice. After the first electronic devices made from single layer graphene were demonstrated in 2004, a tremendous interest has arisen around it. It is driven by the unique properties of graphene: it has a linear band structure so that the charge carriers behave like massless relativistic particles and very few materials can rival its mechanical, electronic, and optical properties, and most remarkably they all exist in a single substance. Consequently, graphene has been envisioned to either replace many currently used materials, or to enable conceptually new applications. Various aspects of graphene physics and devices has been studied in this Thesis. Transport experiments were performed on high quality suspended graphene samples, where the intrinsic performance limits can be probed. At millikelvin temperatures, the ballistic conductance in monolayer graphene manifests in electron wave interference reminiscent of Fabry-Pérot resonances in optical cavities. These interference patterns were analyzed to reveal therenormalization of Fermi velocity in graphene at low charge densities. At high bias voltage regime, shot noise thermometry was employed to study the electron-phonon scattering mechanisms. In suspended monolayer graphene, hot electron relaxation was found to be dominated by two-phonon scattering from thermally excited ripples, whereas in bilayer graphene the intrinsic optical phonons played the dominant role. Graphene is the ultimate material for nonlinear, tunable two-dimensional nanoelectromechanical systems. In this Thesis, methods to fabricate and measure graphene mechanical resonators were developed. In addition, diamond-like carbon resonators were studied, which may be a promising alternative for applications utilizing multilayer graphene membranes.Item Helical waves on quantized vortices(Aalto University, 2017) Hietala, Niklas; Hänninen, Risto, Dr., Aalto University, Department of Applied Physics, Finland; Eltsov, Vladimir, Dr., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science; Hakonen, Pertti, Prof., Aalto University, Department of Applied Physics, FinlandDue to quantum mechanical constraints, the vortices in superfluid helium-4 are line-like objects. This makes the study of hydrodynamics simpler. Rotational flow is possible only in the presence of these quantized vortices. In the zero temperature limit, superfluids resemble inviscid ideal fluids better than perhaps any other system. This makes them a convenient model system to study. Kelvin waves are helical perturbations of a vortex. Since any small perturbation of a straight vortex can be expressed as a sum of helical modes, they are the most basic excitations of a vortex. In superfluid turbulence Kelvin waves play a crucial role in the energy dissipation in the lowest temperatures. In classical fluids, helical vortices appear in wakes behind turbines and propellers. We study how Kelvine waves are generated due to an axial flow of the normal component. We show that the critical flow velocity for the amplification of a Kelvin wave depends both on the wavelength and the amplitude of the helix. We also study the interactions of nearby helical vortices in the absence of mutual friction. This work is also relevant to thin-cored helical vortices in classical fluids. We also consider the possible methods of identification of Kelvin waves for complicated vortex configurations. For classical fluids, helicity has proven to be a useful quantity. If the vorticity is restricted to vortex tubes, it is tied to the knottedness and twisting of the vortex tubes. One could think that for superfluids, where the vorticity is concentrated on line-like objects, helicity would also be a useful quantity. However, since a line cannot be twisted, it is not as straightforward to give a similar interpretation to the helicity. Helicity can be defined to the superfluids, but it turns out to be always zero. The main method used in this work is the vortex filament model. This model has turned out to be a valuable tool, since experiments provide only a limited amount of information about the vortex configurations. Besides the full model based on Biot-Savart law, we use the well-known local induction approximation. It is a useful method, but it has its limitations. The local induction approximation may be extended to include approximative non-local interactions. This model we used in our study on a recurrence phenomenon involving Kelvin waves. We found that the approximative model gives a good match with full Biot-Savart simulations for small amplitude Kelvin waves.Item Higgs bosons, half-quantum vortices, and Q-balls : an expedition in the ³He universe(Aalto University, 2017) Autti, Samuli; Eltsov, Vladimir, Docent, Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Low Temperature Laboratory, ROTA; Perustieteiden korkeakoulu; School of Science; Hakonen, Pertti, Prof., Aalto University, Department of Applied Physics, FinlandFor several decades, superfluid helium has provided an almost unparalleled frontier of quantum physics research, especially in topics related to macroscopic quantum phenomena. While nowadays systems studied in quantum physics are becoming increasingly artificially engineered - the quest for quantum computers and quantum technology in general being one of the driving forces of this development - traditionally both superfluid 4He and 3He have provided a natural but versatile window to their quantum nature. Superfluid 3He, the subject of this thesis, is a quantitatively understood macroscopic quantum system of considerable yet manageable mathematical complexity. In this thesis I show how such versatility translates into a variety of emergent phenomena, touching many seemingly distant fields in physics such as cosmology. Approaching superfluid helium-3 experimentally requires sophisticated cooling and probing methodology. We have used a rotating ultra-low-temperature refrigerator, where the superfluid sample is cooled down by a nuclear demagnetisation stage and probed using nuclear magnetic resonance spectroscopy (NMR). One particularly useful NMR instrument can be constructed by trapping a Bose-Einstein condensate (BEC) of magnon quasiparticles within the superfluid. We have used such condensates in probing a variety of delicate phenomena such as other spin wave modes, including Higgs modes. We have also observed propagation of self-trapped Q-ball solitons making use of magnon BECs. These achievements required careful experimental and numerical studies of the magnon BEC. The spectrum of the magnon BEC is controlled by the profile of the external magnetic field and by the spatial variation of the superfluid order parameter. The main part of the relaxation is due to temperature dependent spin transport in the normal component of the liquid, which allows translating the magnon BEC read-out into in-situ thermometer. As perhaps the most pubilicized topic of this thesis, I explain how we discovered half-quantum vortices in superfluid helium-3, a manifestation of the topological versatility of this system. Nobel laureate Anthony Leggett recently listed finding these vortices the most important remaining open problem in the ultra-low-temperature superfluid physics. In general, the very name of quantum physics refers to the observation that fundamental concepts such as energy and momentum are quantised in the microscopic world, that is, changes in the quantities describing a physical system can only occur in finite steps, integer multiples one quantum. Therefore finding vortices carrying only half-a-quantum of circulation is not only intriguing, but manifests deep understanding of the underlying physics and quantum physics in general.Item Interference, noise, and correlation phenomena in quantum devices(Aalto University, 2022) Manninen, Juuso; Teknillisen fysiikan laitos; Department of Applied Physics; NANO group; Perustieteiden korkeakoulu; School of Science; Hakonen, Pertti, Prof., Aalto University, Department of Applied Physics, FinlandIn this work, I explore several aspects of optomechanical systems coupled to their environments, nonclassical correlations created in such systems, and enhancing the very optomechanical coupling to the ultrastrong coupling limit. I also study micromechanical resonators revealing the de Haas—van Alphen effect in graphene. Two-level system defects are often found interacting with superconducting microwave setups warranting the study the of their effects on the dynamics of an optomechanical system. I derive a quantum Langevin equation formulation for a general system-environment interaction in terms of the system degrees of freedom, and apply this framework to a cavity surrounded by two-level system defects. Nonlinear dissipation and parametric effects, observed experimentally in other works, are found. I consider environmental noise effects, in a proposal for an optomechanical Bell inequality test, where nonclassical correlations between cavity fields are generated by a massive mechanical resonator. The violation of the CHSH Bell inequality is predicted with noise and system parameters compatible with modern technological capabilities. The quantum nature of mechanical motion can further be explored in the ultrastrong optomechanical coupling regime, where the intrinsic nonlinearity of the coupling is observable. To this end, I present a scheme for a microwave circuit setup that is capable of enhancing the optomechanical radiation pressure and cross-Kerr single-photon couplings by several orders of magnitude. Additionally, I study a graphene resonator setup operated as a magnetometer, where the quantum Hall effect and the de Haas—van Alphen effect are observed for the massless Dirac fermions of graphene via the mechanical motion. The general measurement scheme can also be generalized for other conducting two-dimensional materials.Item Microwave Noise Measurements on Nanoelectronic Devices(Aalto University, 2020) Elo, Teemu; Teknillisen fysiikan laitos; Department of Applied Physics; NANO group; Perustieteiden korkeakoulu; School of Science; Hakonen, Pertti, Prof., Aalto University, Department of Applied Physics, FinlandMicrowave noise and its correlations are important characterization methods in quantum nanophysics. Correlation of current fluctuations can be used to study Hanbury Brown – Twiss (HBT) exchange effect caused by indistinguishability of charge carriers in multiterminal conductors, although this effect has received little experimental attention prior to the two studies included in this thesis. In addition, this thesis presents a correlation measurement system for shot noise and a lumped-element Josephson parametric amplifier (JPA). Shot noise and its correlation were measured using a two-channel system which was designed for 600–900 MHz frequency band and utilized USB-interfaced software-defined radio receivers, digitizing 2 megasamples per second with 8 bit resolution. The sensitivity of the system was close to the theoretical value for the given sample rate. The system was complemented with a cryogenic low-noise amplifier, achieving 21 dB gain with 570–920 MHz bandwidth and 7 K noise temperature. The HBT exchange effect was studied in lithographically patterned disordered graphene samples in cross and box geometries. Both samples showed distinct negative HBT exchange correction whose value changed significantly at low charge carrier density. The experimental results for graphene cross matched with an analytical circuit theory model which combined noise sources of four diffusive arms and a semiballistic central dot. The box was modeled numerically as a two-dimensional diffusive conductor including additional contributions to noise from contact resistances. The experimental results for the box fell between the calculated ones for elastic and hot electron transport. This is attributed to the presence of a crossover regime or features intrinsic to diffusive graphene. JPAs add significantly less noise to the signal than semiconductor amplifiers, and hence noise measurements would benefit from a wideband JPA as a preamplifier. In addition, a simple JPA circuit can be easily modified for the needs of experiments studying the quantum vacuum. The presented JPA was realized as a parallel LC resonator consisting of a superconducting quantum interference device (SQUID) and an interdigital capacitor. The JPA was fabricated with a single electron beam lithography step followed by double-angle evaporation of aluminum. The JPA achieved 20 dB gain with 95 MHz bandwidth around 5.3 GHz, and the noise temperature was close to the one-photon quantum limit.Item Microwave-coupled superconducting devices for sensing and quantum information processing(Aalto University, 2015) Vesterinen, Visa; Seppä, Heikki, Research Prof., VTT Technical Research Centre of Finland Ltd, Finland; Hassel, Juha, Dr., VTT Technical Research Centre of Finland Ltd, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science; van Dijken, Sebastiaan, Prof., Aalto University, Department of Applied Physics, Finland; Hakonen, Pertti, Prof., Aalto University, Department of Applied Physics, FinlandSuperconducting circuits and devices have unique properties that make them interesting from both theoretical and practical perspective. In a superconductor cooled below its critical temperature, electrons bound in Cooper pairs have the ability to carry current without dissipation. A structure where the Cooper pairs are coherently tunneling across a weak link is called a Josephson junction (JJ). The dissipationless and non-linear character of the JJ has found applications, e.g., in microwave amplifiers and quantum circuits. These two subjects are closely related since superconducting quantum bits (qubits) are artificial atoms with a transition spectrum in the microwave range. Mediated by microwave photons, qubit readout in circuit quantum electrodynamics (cQED) architecture requires signal boosting with a low-noise preamplifier. In this thesis, a new type of ultrasensitive JJ microwave amplifier was characterized and its noise performance was found to be close to a bound set by quantum mechanics. The amplifier uses the intrinsic negative differential resistance of a current-biased JJ. This work also addressed a challenge related to the scalability of the cQED architecture when the qubits are weakly anharmonic. In a frequencycrowded multi-qubit system, driving individual qubits may cause leakage into non-computational levels of the others. Leakage-avoiding single-qubit Wah-Wah control was implemented. At maximum gate speed corresponding to the frequency crowding, microwave control of two transmon qubits on a 2D cQED quantum processor was decoherence limited. The results disclose the usefulness of Wah-Wah in a future quantum computing platform. Quasiparticles are excitations from the paired superconducting ground state of conduction electrons. As the third topic, the generation-recombination dynamics of quasiparticles was employed in sensing. In electrodynamical terms, superconducting thin films have kinetic inductance from the inertia of the Cooper pairs and resistive dissipation from the quasiparticles. If the film is a part of an electrical resonator, quasiparticle density steers its microwave eigenfrequency and quality factor. In this work, submillimetre-wave radiation and external magnetic field were first converted into quasiparticlegenerating temperature variations and screening currents in a superconductor, respectively. In the two devices called kinetic inductance bolometer and magnetometer, the corresponding changes in resonator parameters were read out to extract the encoded signal. Sensor characterization indicated potential for high sensitivity and low noise. Future applications of the bolometer and the magnetometer include security screening and biomagnetism, respectively. Here, multiplexability in frequency domain facilitates the scale-up to large sensor arrays.Item Nanoelectromechanical resonators and nanoconfinement in quantum fluids(Aalto University, 2023) Kamppinen, Timo; Eltsov, Vladimir, Dr., Aalto University, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; ROTA - Topological Quantum Fluids; Perustieteiden korkeakoulu; School of Science; Hakonen, Pertti, Prof., Aalto University, Department of Applied Physics, FinlandModern nanofabrication techniques have led to tremendous advances in many fields of physics. In particular, new superfluid phases of helium-3 have recently been discovered in engineered geometries that sculpture the order parameter of the topological superfluid on the nanometer scale. Detailed studies are required to reveal the physical properties of the new superfluid phases, and to identify the mechanisms that stabilize them. Recently, nanomechanical sensors for superfluid helium-4 have also emerged. They are promising tools for studying, for instance, the fast dynamics of quantized vortices. Incorporating nanoelectromechanical resonators to a superfluid environment pose several challenges involving acoustic dissipation, interaction with nearby surfaces, nonlinearity, and understanding of the device-fluid interactions in mesoscopic objects. In this thesis, low-frequency nanoelectromechanical resonators for studying the superfluids helium-4 and helium-3 are developed. To minimize the effect of walls, freestanding aluminum devices are fabricated on an opening in the substrate. The devices are characterized in vacuum, in helium-4 gas, and in superfluid helium-4. Good understanding of the device-intrinsic properties such as tunneling two-level systems is achieved, and device-fluid interactions are found to be in good agreement with the theories derived for macroscopic objects. Additionally, we use nuclear magnetic resonance spectroscopy to study the polar phase of superfluid helium-3, stabilized between oriented strands of nanometer-scale diameter and spacing. We show that the stability of the polar phase against scattering from non-magnetic impurities is protected by an extension of the Anderson theorem, albeit the system is an unconventional p-wave state with anisotropic gap. We verify experimentally that the superfluid gap in the polar phase has a cubic temperature dependence, which is a direct consequence of the Dirac nodal line in the spectrum of the Bogoliubov quasiparticles. The confining strands pin vortices strongly, allowing us to measure the density of vortices created in the transition to the superfluid state. We show that an applied bias field suppresses the number of vortices created by the Kibble-Zurek mechanism, providing a shortcut to adiabaticity in this system. The uncovered intrinsic properties of the nanoelectromechanical sensors aid in the design of new devices, and the good understanding of the device-fluid interactions demonstrated in the thesis pave way for future experiments, for instance on vortex dynamics. In helium-3, studies on vortex dynamics enable probing the properties of the fermionic vortex-core-bound states, including the elusive Majorana zero mode. In future, nanoengineered sensors and nanoengineered confinement of helium-3 could be combined, allowing for example experiments on dynamics of the synthetic electromagnetic and gravitational fields.Item Oscillations on helium surfaces(Aalto University, 2015) Manninen, Matti S.; Tuoriniemi, Juha, Doc., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Low Temperature Laboratory, µKI group; Perustieteiden korkeakoulu; School of Science; Hakonen, Pertti, Prof., Aalto University, Department of Applied Physics, FinlandHelium is light, chemically inert monatomic noble gas with either four (4He) or three (3He) nucleons. Both isotopes can be liquefied by lowering the temperature down to few kelvins. Further cooling does not solidify the liquid. Instead, quantum mechanics begins to play a significant role. Particles are divided into two classes, bosons and fermions, whose properties differ in fundamental ways. Helium represents both of these classes as the number of elementary particles is even in bosonic 4He and odd in fermionic 3He. When normal liquid is cooled down, it becomes more viscous, and the movement of an immersed object becomes increasingly damped. However, in bosonic 4He the damping begins to diminish at around 2 K. This transition from normal liquid into superfluid phase origins from quantum mechanics. As fermionic 3He obeys different statistics, also the superfluid transition temperature differs by several orders of magnitude: 3He becomes superfluid at temperatures around 1 mK. In the low temperature limit the damping is caused by rarefied gas of thermally excited quasiparticles. In this thesis standing waves on the free surface of superfluid 4He and 3He were studied experimentally. The measured damping of the waves reflects the boson or fermion character of the medium. Measurements were in reasonable agreement with theoretical prediction of damping due to thermal quasiparticles. Also the resonance frequencies were consistent with the previously measured value of the surface tension. The same experimental setup was also used to study saturated solution of 3He-4He mixture. The molar volume was measured at pressures up to the melting pressure. The melting pressure depends on only one variable, and therefore it can be used for thermometry. When temperature is 294 mK two liquid and two solid phases coexist. This quadruple point can serve as a fixed temperature point.Item Thermodynamics and coherence in quantum systems(Aalto University, 2023) Kirsanov, Nikita; Teknillisen fysiikan laitos; Department of Applied Physics; NANO group (Quantum Circuits and Correlations); Perustieteiden korkeakoulu; School of Science; Hakonen, Pertti, Prof., Aalto University, Department of Applied Physics, FinlandQuantum thermodynamics, an emerging interdisciplinary field, bridges the gap between quantum mechanics and classical thermodynamics. This thesis delves into quantum thermodynamic phenomena and their real-world applications across multiple physical domains, placing a particular emphasis on quantum coherence. The work includes theoretical and experimental studies of thermoelectric and heat currents in hybrid superconducting structures, originating from coherent Cooper pair splitting and elastic cotunneling effects. Furthermore, the work examines entropy dynamics in correlated qubit systems, utilizing the mathematical formalism and results of quantum information theory. Finally, a global-distance quantum key distribution scheme is proposed. The solution leverages coherent optical states and is based on the quantum foundations of the Second Law of Thermodynamics. The work demonstrates the practical impact of the addressed thermodynamic phenomena, laying a foundation for future applications.Item Topological superconductivity in magnetic adatom lattices(Aalto University, 2016) Röntynen, Joel; Ojanen, Teemu, Dr., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science; Hakonen, Pertti, Prof., Aalto University, Department of Applied Physics, FinlandTopological matter has emerged as one of the most prominent research fronts in condensed matter physics over the past three decades. The discovery of the role of topology in materials has shaped our fundamental understanding of how the constituents of matter organize themselves to produce various phases. Topology in these systems manifests as boundary states and exotic quasiparticles, whose intriguing properties are anticipated to facilitate various technological applications. In this thesis I have contributed to the search for topological superconductivity, which is expected to support localized, particle-like excitations called Majorana bound states. Majorana bound states break the dichotomy of bosons and fermions by obeying non-Abelian exchange statistics. Hence a Majorana bound state would be a manifestation of a fundamentally new type of physics. Furthermore, Majorana braiding is envisioned to be utilized in topologically protected quantum computing, which could revolutionize the future of computing. The experimental discovery of Majorana bound states is an outstanding goal in condensed matter physics at the moment. The systems investigated in this thesis consists of magnetic adsorbed atoms (adatoms) deposited on top of a conventional superconductor. In publications I and II we investigated the appearance of Majorana bound states in adatom chains. The main result in publication I is that coupled chains are more likely to exhibit Majorana bound states than uncoupled chains. In publication II we showed that a supercurrent can be used to control the topological phase, which could be helpful for the manipulation of Majorana bound states. In publications III and IV we showed that two-dimensional adatom structures support a generalization of px+ipy superconductivity, making it an interesting addition to the list of materials with unconventional superconductivity. The complex, mosaic-like structure of the topological phase diagram is remarkably rich due to long-range electron hopping. The number of propagating Majorana modes at the boundary is given by a topological invariant called a Chern number. We predicted that for typical experimentally available materials this number can be much larger than unity. The abundance of various topological phases with a large number of protected edge states makes the studied system potentially one of the richest topological materials discovered so far. Since two-dimensional structures in such systems are next in line to be studied experimentally, magnetic adatom structures provide a promising platform for realizing exotic phases of matter of fundamental interest.Item Transport Experiments on Suspended Graphene Devices(Aalto University, 2018) Laitinen, Antti; Teknillisen fysiikan laitos; Department of Applied Physics; Nano group; Perustieteiden korkeakoulu; School of Science; Hakonen, Pertti, Prof., Aalto University, Department of Applied Physics, FinlandIn this thesis, sophisticated conductance and noise measurements were employed for studying electron transport through suspended graphene devices in order to understand the fundamental properties of graphene. The experiments were conducted at low temperatures down to T = 10 mK, and at high magnetic fields up to B = 9 T on suspended graphene devices. In these devices, graphene is connected only to the metallic contacts leaving the graphene flake intact of outside disturbances, and close to ideal theoretical behavior. The work was divided into two segments: quantum transport studies in the zero magnetic field using rectangular bi- and monolayer graphene devices, and magnetotransport measurements at high magnetic fields on Corbino ring devices. In the case of the rectangular graphene devices, a model for contact doping in monolayer graphene by the metal leads was developed first. This facilitated understanding of the transport through the whole device and served as a basis for understanding the origin of the observed Fabry-Pérot resonances. The resonances were used to demonstrate the phase-coherent transport and long mean free path in the devices. Two sets of noise measurements were performed on these devices. First, low frequency 1/f noise measurements on suspended bilayer graphene (BLG) devices revealed extremely low flicker noise levels that was contributed to the substrate-free form of the devices and the effective screening of fluctuations in BLG. The low intrinsic noise level was exploited in a gas sensing application, where adsorbed gases were detected through the extra noise caused by molecules that had landed on the device. In the later set of noise measurements at higher frequencies, f = 600 - 900 MHz, noise thermometry was employed for characterization of the electron-phonon coupling in bi- and monolayer graphene. Finally, suspended graphene Corbino devices were developed for studying integer and fractional quantum Hall effect (IQHE and FQHE). The observed FQHE was explained with the established theory of composite fermions. Based on the measurements, it was concluded that the composite fermions in graphene are Dirac particles with cyclotron mass around one electron rest mass. At very high fields and low charge carrier densities, evidence of Wigner crystallization was obtained. Additionally, the breakdown of quantum Hall effect was studied at the filling factor ν = 0 in the middle of the lowest Landau level. Zener tunneling between Landau sublevels was found to facilitate the breakdown at fields below 7 T, while a more standard behavior due to bootstrapped electron heating was observed at higher fields.
