Browsing by Subject "Superconductivity"

Now showing 1 - 9 of 9
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    Continuos Doping of La2CuO4+x Thin Films
    (2015-09) Kinney, Joseph
    Finding more efficient ways of exploring the doping phase diagrams of high temperature superconductors as well as probing the fundamental properties of these materials are essential ingredients for driving the discovery of new materials. We use a doping technique involving gating with ionic liquids to systematically and continuously tune the Tc of superconducting La2CuO4+x thin films. We probe both the transport properties and the penetration depth of these samples and find that Homes scaling, lambda^-2 ~ sigma*Tc, is obeyed, consistent with these materials being in the dirty limit. This result is independent of the precise mechanism for the gating process as all of the parameters of the scaling relationship are determined by direct measurements on the films.
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    Disorder effects and non-equilibrium dynamics on the electronic orders of strongly correlated materials
    (2019-08) Cui, Tianbai
    Strongly correlated materials offer promising prospects for numerous applications, from superconductivity to quantum information processing. The exotic electronic properties arise from the collective behavior due to strong electron-electron correlation. This leads to the complex phase diagram of strongly correlated materials consists of multiple distinct yet intertwined electronic orders, for examples spin density-wave, charge density-wave, nematic order, and superconductivity. Most theoretical studies of this delicate balance between different electronic orders in strongly correlated systems assume disorder is absent and equilibrium is reached, which sometimes makes comparison with experiments challenging. In this thesis, I will surpass these assumptions to show how disorder dramatically changes the way electronic orders develop, and also demonstrate that non-equilibrium perturbations enable us to understand different dynamics in various timescales and to search for new physical behaviors which are absent in equilibrium. In particular, I will discuss the rare region effect in inhomogeneous systems and show how it changes the critical behaviors of nematic and magnetic quantum phase transitions. I will propose a self-consistent perturbative approach to study the dynamics of the superconducting gap at the picosecond time scales after driven out of equilibrium. Using this approach, I will show that the dynamics of the multi-band superconductor is distinct from the single-band conventional superconductors. I will also elaborate on the damping and relaxation effects on the gap dynamics within the electronic system at picosecond time scales.
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    Disorder in quasi-1D topological phases and gapless superconductivity in 2D
    (2023-02) Kasturirangan, Saumitran
    We consider how disorder affects the electronic transport and localization in the vicinity of a topological phase transition in quasi-1D. This is studied for one of the most elementary examples of a topological insulator, the SSH chain. At the topological phase transition, the addition of disorder that respects the chiral symmetry of the system keeps it at a critical point. The electronic wavefunctions at zero energy are not exponentially localized, as one would expect in 1D. The conventional Fokker-Planck approach governing the evolution of transport statistics and the associated single parameter scaling, breaks down in describing the crossover of statistics from class BDI to class AI. We show that a second parameter, the product of energy and relaxation time, is required to capture this crossover. This is demonstrated using data collapse of numerically obtained transport. The regimes of transport behavior are characterized and appear to be universal. These results are used to study zigzag graphene nanoribbons, which is a topological semi-metal. The edge states have a power-law dispersion depending on the width and it is shown that the system is at a topological multicritical point. Upon adding hopping disorder, the transport, density of states, and localization length all obey the same behavior as the SSH chain at criticality, when re-scaled. The edge states are found to be energetically stable and remain close to the boundary. However, they are localized at any non-zero energy. We consider the implications of an out-of-plane field on superconductivity in monolayer NbSe$_2$. We find that the strong Ising spin-orbit coupling arising from the broken inversion symmetry, results in mixing the singlet and triplet components of the superconducting gap. On increasing the magnetic field strength, is it possible to find a gapless superconductor with Bogoliubov Fermi surfaces if the triplet pairing is sufficiently large.
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    Electrostatic modification of novel materials
    (2013-10) Lee, Yeonbae
    Electric double layer transistor configurations have been employed to electrostatically modify the physical properties of two novel materials; single crystals of insulating strontium titanate (SrTiO3) and thin films of amorphous indium oxide (a-InO). First the results of doping SrTiO3 over broad ranges of temperature and carrier concentration employing an ionic liquid as the gate dielectric are reported. The surprising results are, with increasing carrier concentration, an apparent carrier-density dependent conductor-insulator transition, a regime of the anomalous Hall effect, suggesting magnetic ordering, and finally the appearance of superconductivity. The possible appearance of magnetic order near the boundary between the insulating and superconducting regimes is reminiscent of effects associated with quantum critical behavior in some complex compounds. Secondly, the evolution with carrier concentration of the electrical properties of a-InO thin films has been studied. Carrier variations of up to 7 x 1014 carriers-cm-2 were achieved again using an ionic liquid as a gate dielectric. The superconductor-insulator transition was traversed, and both the magnitude and the position of large magnetoresistance peak found in the insulating regime were modified. The systematic variation of the magnetoresistance peak with charge concentration was found to be qualitatively consistent with a simulation based on a model involving granularity.
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    Superconductivity and Magnetism in Strontium Titanate Thin Films and Heterostructures
    (2020-06) Ayino, Yilikal
    In this thesis we present our investigation of magnetism and superconductivity in SrTiO3 thin films and heterostructures in the polar/non-polar interface of NdTiO3/SrTiO3, using milli-Kelvin transport measurements. In Chapter 2 we describe our experimental method, including our low temperature transport measurement setup. In Chapter 3 we present our investigation of the magnetic properties of NdTiO3/SrTiO3 using milli-Kelvin temperature magnetotransport measurements. We observe large negative magnetoresistance (in excess of -90%) and robust hysteresis. We interpret our data in the context of spin-dependent transport and argue for the presence of ferromagnetic ordering in this anti-ferromagnetic/ paramagnetic interface. In addition, using time dependent magnetoresistance measurement we observe butterfly-shaped transit hysteretic features near zero-magnetic field. Such butterfly-shaped hysteretic features are previously interpreted as evidence for interfacial magnetism. We argue that these transient hysteretic signals are consistent with magnetocaloric effects arising from magnetic materials extrinsic to the sample. In Chapter 4 we discuss our investigation of effects of paramagnetic pair breaking and spin-orbital coupling on multi-band superconductivity. Using low temperature critical field measurements, we observe robust evidence for multi-band superconductivity from the unconventional dependence of critical field on temperature. We observe a pronounced positive curvature of the critical field curve as function temperature. Furthermore, we observe that the out-of-plane critical field exceeds the Pauli limit, while typically the out-of-plane critical field saturates about an order of magnitude smaller than the Pauli limit. We interpret this as due to the enhancement of the critical field due to multi-band superconductivity, as has been theoretically predicted. Furthermore, we propose an original model for critical field for multi-band superconductors, which includes orbital pair breaking, paramagnetic pair breaking and spin-orbital coupling. We find excellent agreement of this model with our data and that this model has wide reaching applications to any multi-band superconductors in the dirty limit, in which all these effects are present. Chapter 5 focuses on the comparison of effects of magnetic impurity scattering and non-magnetic impurity scattering in the context of multi-band superconductors. We did extensive measurements of magnetically doped (Nd-doped STO) and non-magnetically doped (La-doped STO) samples. We measured samples with carrier densities spanning about an order magnitude, for both sets of samples. We find that for magnetically doped samples the superconducting dome (critical temperature as a function of carrier density) is shifted to lower densities. As a result, at lower densities we observe that the critical temperatures of magnetically doped samples are higher than those of non-magnetically doped samples, while at higher densities it is the opposite. Analysis of out-of-plane critical field measurements reveals the multi-band nature of superconductivity in both sets of samples. In–plane critical field data reveal the importance of spin-orbital coupling in Nd doped samples. We argue that the reason why, at lower densities, Nd doped samples have higher critical temperature despite being magnetically doped is the suppression of magnetic scattering by spin-orbital coupling. Furthermore, we studied several very high mobility samples at extremely low densities (<1018 cm-3) and we did not observe superconductivity down to less than 30mK, contrary to observations reported in literature on oxygen vacancy doped samples.
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    Superconductivity at a quantum critical point: A theoretical approach to the understanding of unconventional superconductors in strongly correlated systems.
    (2021-08) Wu, Yiming
    The complexity of strongly correlated electronic system is manifested by the interplay of multiple electronic orders, among which superconductivity is one of the most interesting phases. In experimentally observed phase diagrams for materials such as heavy fermion compounds, cuprates and iron based superconductors, superconductivity is close to other electronic orders such as ferromagnetism, anti-ferromagnetism and nematicity etc. This fact brings about interests of studying the role of a possible underlying quantum critical point(QCP) in determining the unusual properties of these materials. Here we consider an itinerant fermion system which is close to a QCP. Because of the closeness, the collective boson mode due to the order parameter fluctuations will couple to low energy fermions and mediate the fermion-fermion interaction. This effective interaction simultaneously gives rise to two competing fate for the fermions: On one hand it can lead to SC if the there is any pairing instability in at least one pairing channel. On the other hand, the same interaction also diminish fermion coherence and results in non Fermi liquid behavior. These two tendencies compete with each other, in a sense that SC gaps out low energy fermions and reduces the self energy , while non Fermi liquid tends to destroy fermion coherence and is detrimental to SC. In order to capture this story we adopt the approach that Eliashberg first used when he studied the electron-phonon coupling system, i.e. we approximate the fermion self energy by neglecting the vertex corrections, which is controllable when the vertex is parametrically smaller. We further assume the interaction depends only on frequency via a dynamical exponent $\gamma$, namely $V(\Omega_m)\propto 1/|\Omega_m|^\gamma$. Based on this model, we unveil many special properties of SC state on both imaginary and real frequency axis, including the ‘gap closing’ behavior observed in cuprates. As a unique feature of pairing at a QCP, we find there exists an infinite set of solutions to the gap equation, corresponding to different local minima in free energy. This set becomes a continuous one at a special case $\gamma=2$, which corresponds to phonon-mediated pairing interaction with a vanishingly small phonon frequency. We also studied the odd-frequency pairing state from this model, and find there is no zero bias peak in the quasiparticle density of states which was considered as an evidence of odd- frequency pairing. At last, in addition to mean field analysis, the superconducting phase fluctuation is also discussed.
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    Transport in Superconducting/Ferromagnetic Heterostructures
    (2019-07) Moen, Evan
    In this thesis I present my research on spin and charge transport in ferromagnet, superconductor (F=S) heterostructures using a self-consistent, clean limit theory. The goal is to characterize realistic samples. The primary focus is on the F1=N=F2=S superconducting spin valve. I also consider the S1=F1=N=F2=S2 ferromagnetic Josephson structures. We solve the Bogoliubov deGennes equations (BdG) using a self-consistent, numerical approach and determine the thermodynamic quantities such as the pair potential. For the charge transport, we use the Blonder-Tinhkam-Kapwijk (BTK) method to determine the conductance G. We study the conductance features and their dependence on the physical parameters such as the layer thicknesses and interfacial quality of the sample. The main results are the dependence of G on the misalignment angle of the magnetizations in F2 relative to F1, which constitutes a 'valve eect'. The valve eect in F=S structures is due to the proximity eect, which is angularly dependent. The critical bias (CB), equal to the gap energy, is non-monotonic with due to this proximity eect. The conductance features are split for incoming spin-up and spin-down electrons, which leads to a subgap (below CB) peak in the total conductance. This subgap peak is dependent on the intermediate F2 layer thickness and ferromagnetic exchange eld h in which the peak position oscillates between zero bias and the CB with a periodicity of =h. These subgap peaks are resistant to high interfacial barriers and lead to a monotonic angular dependence on in the peak maxima. In the S1=F1=N=F2=S2 quasiparticle conductance, there are multiple subgap peaks with similar oscillations in the peak positions. In addition, the conductance peak position oscillates with by a quarter phase between the parallel and antiparallel conguration. We also study the spin transport in the F1=N=F2=S system for realistic parameters. The spin transport quantities are not conserved due to the spin transfer torque (STT) within the ferromagnetic layers, and are spatially dependent. There exists a critical bias feature in which no spin current penetrates the S layer for biases below the CB, and the STT becomes quasilinear for biases above the critical bias.
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    Transport properties of superconducting nanostructures.
    (2012-07) Snyder, Stephen David
    This work is concerned with the transport properties of superconducting nanorings at extremely low temperatures in magnetic field. The goal of this work was to experimentally observe a prediction on the crossover from h/2e to h/e period oscillations of transition temperature when the size of the ring becomes small relative to the superconducting coherence length. Impurities in the aluminum nanostructures studied here hinder the direct observation of this crossover. However, the proper direction to take in future experiments on this subject has been evaluated and firmly established. Along the way, an interesting effect has been observed in the form of a high resistance state in superconducting nanorings. This is remarkable because it has a resistance higher than the normal state resistance even though it is superconducting. Therefore, it seems to be phase coherent even though it is resistive. There have been other similar observations in the literature in wires and disks but not rings. It can be explained in terms of nonequilibrium relaxation of quasiparticles near normal-superconductor interfaces that occur naturally in such constricted structures. The relevant physics of this is discussed.
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    Transport Studies on Gated Three-Terminal Josephson Junctions
    (2022-08) Graziano, Gino
    This thesis covers experimentally the properties of gate-tunable three-terminal Josephsonjunctions based on a superconductor-proximitized two-dimensional semiconductor. Specifically, InAs quantum wells with an epitaxially grown aluminum capping layer. Multiterminal Josephson junctions have recently attracted experimental work following several theoretical predictions of emergent topological physics. Namely, that topologically protected states may manifest themselves in the generalized multiterminal Andreev bound state spectrum of such devices, as a function of applied voltages and relative phase between the superconducting leads. The theoretical efforts focus on the case of multiple superconducting leads in contact with a small central scattering region characterized by a scattering matrix. Detection of the topological Andreev bound states via quantized transconductance is predicted to be possible at low bias when there are few conductance modes between each terminal, and for a subset of possible scattering matrices. In this work, three-terminal Josephson devices are fabricated, and subsequently measured at millikelvin temperatures. The electronic transport characteristics of a top-gated Y-shaped three-terminal Josephson junction are measured, as well as their gate dependencies. These behaviors are explained by combining qualitative, analytical, and simulation based approaches. Further, three-terminal Josephson junctions with closely-spaced independent gates are studied. The effect of asymmetric gating by these independent gates is elaborated and supported by simulations. These devices also show access to a few-conductance-mode regime via quantum point contact-like gating, moving closer to the strict requirements proposed by topological multiterminal Josephson junction theory. Lastly, a gate-tunable superconducting Josephson diode effect is measured and described in a three-terminal Josephson junction. It is demonstrated that this effect is fundamental to multiterminal Josephson devices, and in principle independent of the material used.