In recent years, Weyl semi-metals have attracted a lot of interest in topological condensed matter, for instance for their significant potential application in quantum electronics, as the coupling between Weyl semi-metals and superconductivity, either intrinsically in the material or at the interface of a heterostructure, gives rise to a new type of topological superconductivity, which could be used to perform quantum computation operations free from decoherence. In this thesis, we investigate the low temperature magneto-transport properties of trigonal-PtBi2, a layered material, both in the presence of quantum confinement (exfoliated nano-structures) and in its absence (macro-structures). We report band structure calculations showing that trigonal-PtBi2 is a type-I Weyl semi-metal with multiple bands at the Fermi level. Shubnikov-de-Haas oscillations in macrostructures confirm the contribution to transport of carriers from multiple pockets, and magneto-transport measurements show an unusual angular dependence of the magnetoresistance with the field, which might be a manifestation of the large anisotropy of the topological band. We also report the existence of a large planar Hall effect in nano-structures, which is one of the predicted manifestations of Weyl physics. At very low temperature, below 1K, trigonal-PtBi2 becomes superconducting. We investigate the superconducting state in both macro- and nano-structures, and find that quantum confinement in nano-structures makes the superconductivity become two-dimensional. This result is confirmed by the characterization of a Berezinskii–Kosterlitz–Thouless (BKT) transition in nano-structures. This transition is very robust, as it occurs in nano-structures five times thicker than what had previously been reported for any BKT transitions in the literature. We also report on the impact of inhomogeneities on the superconducting transition.

A superconducting nanowire single-photon detector (SNSPD) is an emerging, and today commercially available technology, for photon-counting and quantum cryptography. Yet, the photon detection event is not fully understood and current modeling efforts require substantial computational resources which motivates studies of simpler models.  This thesis introduces a model for vortex dynamics in thin-layered superconductors, such as SNSPDs, using a simplified approach, which leads to a 2D Coulomb gas model where the vortices are modeled as electrostatic charges. The model is carefully constructed from the method of images to describe a wire with open boundary conditions and an applied supercurrent. Subsequently, equilibrium and non-equilibrium properties are sampled with the Metropolis-Hastings algorithm and further analyzed and discussed. The suggested model is shown to be effective and successfully reproduces expected SNSPD behavior; most importantly critical behavior and voltage pulses which are directly measured during detection events. In conclusion, a 2D Coulomb gas model can be a preferred alternative for modeling vortex dynamics in SNSPDs at a small computational cost, motivating further development and studies. / Supraledande nanotråd-enfotondetektorer (SNSPD:er) är en framväxande och idag kommersiellt tillgänglig teknologi som används för räknande av fotoner samt inom kvantkryptografi. Ändå är fotondetektionshändelsen inte helt förstådd och de nuvarande modelleringar kräver substantiell datorkraft vilket motiverar studier av enklare modeller. Det här examensarbetet introducerar en model för vortexdynamik i tunnskiktade supraledare såsom SNSPD:er genom ett förenklat tillvägagångssätt som leder till en 2D Coulombgas-modell där ett vortex modelleras som en elektrisk laddning. Modellen är noggrant konstruerad med öppna randvillkor och den så kallade frysta spegelbildsmetoden samt en pålagd superström. Då samlas mätvärden in på jämvikts- samt icke-jämviktsegenskaper hos systemet som vidare analyseras, jämförs och diskuteras. Den föreslagna modellen visas vara effektiv och reproducerar framgångsrikt förväntat beteende hos SNSPD:er; framför allt kritiskt beteende och spänningstoppar som direkt uppmäts i en fysisk detektionshändelse. Sammanfattningsvis, kan en 2D Coulombgas-modell vara ett föredraget alternativ för att modellera vortexdynamik hos SNSPD:er för en liten beräkningskostnad, vilket motiverar fler studier av detta.

Motivated by the recent experimental progress in the search for Majorana fermions, we identify signatures of topological superconductivity and propose realistic experiments to observe these signatures. In the first part of this thesis, we study charge transport through a topological superconductor with a pair of Majorana end states, coupled to leads via quantum dots with resonant levels. The nonlocality of the Majorana bound states opens the possibility of Cooper pair splitting with nonlocal shot noise. In the space of quantum dot energy levels, we find a characteristic four-peaked cloverlike pattern for the strength of noise due to Cooper pair splitting, distinct from the single ellipsoidal peak found in the absence of Majorana end states. Semiconductor-superconductor hybrid systems are promising candidates for the realiza- tion Majorana fermions and topological order in solid state devices. In the second part, we show that the topological order is mirrored in the excitation spectra and can be observed in nonlinear Coulomb blockade transport through a ring-shaped nanowire. Especially, the ex- citation spectrum is almost independent of magnetic flux in the topologically trivial phase but acquires a characteristic h/e magnetic flux periodicity in the nontrivial phase. The transition between the trivial and nontrivial phase is reflected in the closing and reopening of an excitation gap. In the third part, we investigate characteristic features in the spin response of doped three-dimensional topological insulators with odd-parity unequal-spin superconducting pairing, which are predicted to have gapless Majorana surface modes. These Majorana modes contribute to the spin response, giving rise to a characteristic temperature behavior of the Knight shift and the spin-lattice relaxation time in magnetic resonance experiments.

<p>Conventional computing is based on the concept of bits which are classical entities that are either 0 or 1 and can be represented by stable magnets. The field of quantum computing relies on qubits which are a complex linear combination of 0 and 1. Recently, the concept of probabilistic computing with probabilistic (<em>p-</em>)bits was introduced where <em>p-</em>bits are robust classical entities that fluctuate between 0 and 1. <em>P-</em>bits can be naturally represented by low-barrier nanomagnets. Probabilistic computers (<em>p-</em>computers) based on <em>p-</em>bits are domain-based hardware accelerators for Monte Carlo algorithms that can efficiently address probabilistic tasks like sampling, optimization and machine learning. </p> <p>In this dissertation, starting from the intrinsic physics of nanomagnets, we show that a compact hardware implementation of a <em>p-</em>bit based on stochastic magnetic tunnel junctions (s-MTJs) can operate at high-speeds in the order of nanoseconds, a prediction that has recently received experimental support.</p> <p>We then move to the system level and illustrate by simulation and by experiment how multiple interconnected <em>p-</em>bits can be utilized to train a Boltzmann machine built with hardware <em>p-</em>bits. We observe that even non-ideal s-MTJs can be utilized for probabilistic computing when combined with hardware-aware learning.</p> <p>Finally, we show how to build a <em>p-</em>computer to accelerate a wide variety of problems ranging from optimization and sampling to quantum computing and machine learning. The common theme for all these applications is the underlying Monte Carlo and Markov chain Monte Carlo algorithms and their parallelism enabled by a unique <em>p-</em>computer architecture.</p>

La supraconductivité est intensément étudiée en physique de la matière condensée pour ses éventuelles applications. En effet, ce phénomène est caractérisé macroscopiquement par des propriétés remarquables, mais pour le moment, son exploitation est limitée par la nécessité de refroidir ces matériaux à des températures cryogéniques. Après la découverte de H₃S, dont la température de transition est de 200 K, un renouveau d’intérêt est apparu pour les matériaux supraconducteurs conventionnels. Pour ce composé, le couplage entre électrons et phonons est à l’origine de l’appariement électronique, une condition nécessaire à la supraconductivité. L’énergie associée à ce couplage se trouve généralement dans la gamme des infrarouges lointains, voire des THz, faisant de la spectroscopie infrarouge un outil idéal pour étudier ce mécanisme. Cette thèse présente les études de deux matériaux supraconducteurs dans des conditions expérimentales extrêmes de pression ou de température, permise grâce à la forte brillance du rayonnement synchrotron. Pour la phase supraconductrice H₃S à des pressions supérieures à 150 GPa, l’environnement en cellule à enclumes de diamant exclut la plupart des techniques pour déterminer la nature du mécanisme mais les études optiques restent adaptées. Les résultats spectroscopiques présentés ici démontrent un fort couplage entre électrons et phonons, qui explique l’origine d’une si haute température de transition. Des mesures complémentaires visant à caractériser NaCl sous pression sont également décrites car ce matériau est couramment utilisé comme transmetteur de pression dans les cellules `a haute pression, notamment pour le supraconducteur H₃S. La deuxième étude rapporte des résultats spectroscopiques dans le THz sur des couches nanométriques de Nb, dont les températures de transition supraconductrice sont de 4,5 K et 6,8 K. Ces mesures confirment que la nature conventionnelle du mécanisme subsiste au sein de ces films minces quasi-bidimensionnels. Pour ce travail, un ensemble instrumental permettant la mesure spectroscopique de matériaux jusqu’à des températures de 200 mK a été entièrement développé autour d’un cryostat à démagnétisation adiabatique. / Superconductivity is highly studied in condensed matter physics for its potential applications. Indeed, this phenomenon is macroscopically characterized by remarkable properties, but generally occurs in materials at cryogenic temperature thus limitating their exploitation. Recently, renewed interest has appeared for conventional superconducting materials with the discovery of H₃S, whose transition temperature is at 200 K. For this compound, the coupling between electrons and phonons is at the origin of the electronic pairing, a necessary condition for superconductivity. The associated energy for this coupling belongs to the far infrared range, even THz, making infrared spectroscopy an ideal tool to study the mechanism. This thesis presents the studies of two superconducting materials in extreme experimental conditions of pressure and temperature, allowed by the high brilliance of synchrotron radiation. For the superconducting phase H₃S under pressures superior to 150 GPa, the environment of diamond anvil cell excludes most of the techniques to determine the nature of the mechanism but the optical studies remain adapted. The spectroscopic results presented here demonstrate a strong coupling between electrons and phonons, which explains the origin of such a high transition temperature. Complementary measurements aiming at characterizing NaCl under pressure is also described because this material is often used as a pressure transmitting medium in high pressure cells, in particular for the superconducting H₃S. The second study reports spectroscopic results in the THz on nanometric layers of Nb, whose superconducting transition temperatures are 4,5 K and 6,8 K. These measurements confirm that the conventional nature of the mechanism subsists within these quasi-bidimensional thin films. For this work, an instrumental ensemble allowing the spectroscopic measurements of materials down to 200 mK has been entirely developed based on an adiabatic demagnetization cryostat.

This work presents a comprehensive study on single crystalline LaPt2Si2, in which superconductivity and a charge density wave (CDW) coexist. The usage of density functional theory (DFT) modeling and Inelastic Neutron Scattering has been the primary form of investigation, in order to determine all the characteristic features of the sample taken under consideration. From the results one can observe that the Fermi surface nesting is the primary contributor for the CDW wavevector ~qCDW = (1/3, 0, 0). In addition, the phonon density of states present two typical energy levels, with soft modes in the Pt3-Pt4 layer coherent with the presence of a CDW. The superconducting temperature has been estimated at Tc = 1.6 K. The experimental data from the inelastic instrument High Resolution Chopper Spectrometer (HRC) at the J-PARC neutron source are in good agreement with the theoretical simulations, showing the same energy levels for the polarization phonon modes (from 4 to 18 meV and from 32 to 42 meV). / Denna rapport presenterar en omfattande studie av enkristalls LaPt2Si2 i vilken supraledning och en laddningsdensitetsvåg (CDW) samexisterar. Användandet av DFT-modellering och neutronspridning har varit de huvudsakliga undersökningsmetoderna, för att bestämma alla karakteristiska drag hos det undersökta provet. Från resultaten kan observeras att den inneslutna Fermiytan är den huvudsakliga bidragaren till CDW-vågvektorn~qCDW = (1/3, 0, 0). Vidare visar den närvarande fonontillståndsdensiteten två typiska energinivåer, med mjuka lägen i Pt3-Pt4-skiktet, som stämmer överens med närvaron av en CDW. Den supraledande temperaturen har uppskattats till Tc = 1.6 K. Experimentella data från det inelastiska instrumentet HRC vid J-PARCs neutronkälla stämmer väl överens med teoretiska simuleringar, som visar samma energinivåer för polarisationsfononlägena (från 4 till 18 meV och från 32 till 42 meV).

Theoretical aspects of multicomponent superconductivity and systemswith competing interactions are studied using Monte Carlo techniques.Motivated by recent experimental and theoretical results of complex struc-ture formation of vortices in multicomponent systems, possible vortex struc-ture formations due to vortex interactions that are not purely attractive orrepulsive are considered. Vortex structures such as clusters, superclusters,hierarchical structure formation, stripes, gossamer patters, glassy phases, aswell as checkerboard lattices and loops are demonstrated to be possible.The order of the superconducting phase transition is considered for multi-component lattice London superconductors. The phase transition is demon-strated to be either rst-order or continuous depending on the strength of asymmetry-breaking Josephson intercomponent interaction. It is argued thatthe rst-order phase transition is caused by a vortex phase separation due toa uctuation-induced attractive interaction between vortex lines. / <p>QC 20151117</p>

<p dir="ltr"><b>Tellurium (Te) stands out as an elemental narrow-bandgap semiconductor characterized by its distinctive chiral crystal structure. The interplay between fundamental symmetries and the topological properties of electrons has garnered significant attention in the scientific community. With its unique chiral crystal structure featuring three Tellurium atoms spiraling within a single unit cell, Tellurium offers a singular material system. This system provides an exceptional opportunity to explore the novel quantum and topological transport properties of electrons. Hydrothermally grown two-dimensional (2D) Te with a thickness of several nanometers gives us an opportunity to precisely control the carrier density and the carrier type in Te using gate voltage. In this dissertation, the spin-orbit coupling (SOC) of Te is quantitatively analyzed using the weak anti-localization effect. The strong SOC also gives rise to the Weyl point at the band edge of the conduction band. The topological nontrivial band structure of Te is characterized by a π phase shift in the Shubnikov-de Haas (SdH) oscillations. Due to the high mobility, the quantum Hall effect is measured with low spin and valley Landau levels controlled by an electric and magnetic field. Bilayer charge transferable quantum Hall states of Weyl fermions is observed in a wide Te quantum well. The topological phase transition from a semiconductor to Weyl semimetal under high pressure is studied up to 2.47 GPa. The chirality of 2D Te is separated by the hot sulfuric acid etching technique. The spin configuration and topological charge of the Weyl node exhibit a reversal in different chiralities, leading to an inverse in nonlinear responses, encompassing both electrical (nonreciprocal transport in the longitudinal direction and nonlinear planar Hall effect in the transvers direction) and optical phenomena (circular photogalvanic effect and circular photovoltaic effect). Our results unveil the topological nature of the Tellurium (Te) band structures, offering a promising avenue for controlling charge and spin transport within the chiral degree of freedom.</b></p>

Doped IrTe2 is considered a platform for topological superconductivity and therefore receives currently a lot of interest. In addition, the superconductivity in these materials exists in close vicinity to electronic order and the formation of molecular orbital crystals, which we explore here by means of high-pressure single crystal x-ray diffraction in combination with density functional theory. Our crystallographic refinements provide detailed information about the structural evolution as a function of applied pressure up to 42 GPa. Using this structural information for density functional theory calculations, we show that the local multicenter bonding in IrTe2 is driven by changes in the Ir-Te-Ir bond angle. When the electronic order sets in, this bond angle decreases drastically, leading to a stabilization of a multicenter molecular orbital bond. This unusual local mechanism of bond formation in an itinerant material provides a natural explanation for the different electronic orders in IrTe2. It further illustrates the strong coupling of the electrons with the lattice and is most likely relevant for the superconductivity in this material.

<p dir="ltr">Since the discovery of graphene in 2004, numerous other materials with intriguing electronic, optical, and magnetic properties have been found to be layered and exfoliatable down to atomic thickness. Owing to their weak interlayer coupling, mediated only by van der Waals forces, this new class of 2-dimensional materials, also known as van der Waals (vdW) materials, allows layer-by-layer stacking, overcoming some of the limitations of growth techniques. In particular, the growing inventory of vdW materials has expanded to include magnetic materials, further broadening the possibilities of novel devices based on stacked heterostructures. These magnetic heterostructures can find applications in spintronics and memory devices and may be combined with other vdW materials with optical properties for applications in optoelectronics. In this thesis, we assembled heterostructures via mechanical transfer or growth to modify the magnetism in these vdW materials. We used various optical and electrical techniques to probe the modified magnetism or its effects on the novel heterostructure. Thus, we observed the emergence of the magnetic proximity effect on the topological insulator BiSbTeSe₂ after dry transferring a thin flake of Cr₂Ge₂Te₆ on top, taking steps towards the observation of novel topological phases, such as the quantum Hall insulator. Additionally, we demonstrated an increased Curie temperature and magnetic anisotropy, effectively enhancing the magnetism, in thin flakes of Cr₂Ge₂Te₆ and Cr₂Si₂Te₆ after sputtering NiO or MgO. Finally, noting that the effect of modified magnetism in Cr2Ge2Te6 after sputtering NiO or MgO is induced due to wrinkle formation and strain, we further reproduce similar wrinkle formation on other 2D materials such as hBN, graphite, and 2D antiferromagnets (XPS₃, (X= Mn, Fe, Ni), CrSBr, RuCl₃). We used polarized Raman spectroscopy to characterize the induced biaxial strain in hBN and showed that such wrinkle formation can lead to moderately (up to 1.4% strain) spatially inhomogeneous and anisotropic strain profiles. These efforts demonstrate the versatility of tailoring the properties of these vdW materials.</p>