Browsing by Subject "Disorder"

Now showing 1 - 6 of 6
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    Consequences of nematoelasticity in structurally disordered quantum materials
    (2024-05) Meese, William
    Electronic nematicity – a state of broken rotational symmetry but preserved translational symmetry – appears to be a general feature of quantum materials, and it often develops in the vicinity of unconventional superconductivity. Strains are conjugate fields for the electronic nematic order parameter – a relationship known as nematoelasticity. While external, homogeneous strains are useful tools in studying electronic nematicity, crystals simultaneously contain inhomogeneous strains because of structural disorder generated by crystalline defects. In this thesis, I demonstrate that these unavoidable, internal, random strains lead to a host of new electronic and relaxational behavior in electronic nematics. Electronic nematicity manifests as either an isolated instability or a vestigial one. The latter is a partially melted phase of some underlying primary order, meaning it is borne out of the fluctuations of the primary order parameter. In the former case, the impact of random strains has been studied before using the random-field Ising model (RFIM). In the case of vestigial nematicity, the RFIM description is incomplete, as random strain plays the simultaneous role of both a random field for the nematic order parameter and a random mass for the primary order parameter. I generalized the RFIM to a new model that is then applied to systems such as the iron pnictide superconductors. This disorder-free limit is built upon the Ashkin-Teller model, whose composite “Baxter” order parameter plays the role of the vestigial nematic comprised of two primary magnetic degrees of freedom. These magnetic degrees of freedom are mapped onto the interpenetrating Néel vectors in the pnictides. The Baxter variable is then subject to a random field, making this random “Baxter field” act as both a random field and a random mass. In analogy with the RFIM, the model is dubbed the random-Baxter-field model (RBFM), and massively-parallel Monte Carlo simulations were used to characterize its impact on nematicity. It was found that the random strains break the vestigial nematic order into domains while creating new correlations in the primary order parameters, enhancing their fluctuations. In a following experimental collaboration, electronic nematic domain formation was then used to explain optical dichroism measurements on the compound FePSe3 which showed evidence of vestigial 3-state Potts nematic domains. In the limit of nearly perfect crystals, however, neither the RFIM nor the RBFM account for the nature of long-ranged strains from crystalline defects. Recent heat capacity measurements on the nematic insulator, Tm1–xYxVO4, show that, even in the purest samples, random strains are ubiquitous and correlated. To this end, I reexamined nematoelastic interactions in structurally disordered elastic media. I developed a realistic description of elastically generated random strains from a veritable zoo of different crystalline defects, and used numerical simulations of dislocations to qualitatively account for the experimental results. It was found that even simple ensembles of defects – in this case, dislocations – can generate strain distributions that can fit the data just as well as conventional, uncorrelated random strains can without needing to overfit. These elastically generated random strains contain not only long-ranged and anisotropic correlations, but also higher-order statistics that are frequently omitted in random field models. This way of viewing random strains constitutes a new type of random field disorder and will lead to exciting new phenomena in future work.
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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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    Excitonic eigenstates of disordered semiconductor quantum wires: adaptive wavelet computation of eigenvalues for the electron-hole Schrödinger equation
    (University of Minnesota. Institute for Mathematics and Its Applications, 2011-09) Mollet, Christian; Kunoth, Angela; Meier, Torsten
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    Protocol Dependence of Spatially Inhomogeneous Magnetic Systems
    (2024-05) Freedberg, Jennifer
    The work presented in this dissertation studies the effects of measurement protocol on magnetic systems. There are two broad classes of metallic systems which will be described -- ferromagnets and spin glasses. The research presented demonstrates a clear protocol dependence for both for nonequilibrium dynamical magnetic materials such as spin glass and metallic ferromagnetic systems. Thus, for experimental reproducibility, it is necessary for one to specify the protocol used to prepare and take measurements. In a single crystal of CuMn 7.92 at.%, the out-of-equilibrium dynamics of aging, rejuvenation, and memory are explored. By using a double-waiting time protocol and quenching to the measuring temperatures, the underlying dynamics of the memory effect are able to be observed with no finite cooling rate effects. After quantifying the memory loss seen in glassy systems, previously proposed explanations were experimentally tested and a quantitative model developed. We find that coincident growth of spin glass correlations reduces the amount of memory retained, and that there is an additional length scale present whose ratio with size of the original correlated regions controls the severity of the memory loss. The effects of a finite cooling rate in spin glasses are then investigated as the temperature is swept continuously. We quantitatively find competing effects of aging and rejuvenation. This implies that the growth rates of glassy order between protocols utilizing quenches cannot naively be compared to protocols which use a finite rate of cooling. Additionally, we determine that the slower growth observed in the finite cooling rate protocols is due to rejuvenation, rather than cumulative aging. In four metallic ferromagnets, four paths to a net-zero magnetzation state are explored. This was done by demagnetizing samples using four different methods and then conducting the same measurements afterwards. The results indicated that the path to zero magnetization changes the behavior of the system, and thus the preparation of the initial state affected subsequent measurements.
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    Simulation data for: "Two parameter scaling in the crossover from symmetry class BDI to AI"
    (2022-08-01) Kasturirangan, Saumitran; Kamenev, Alex; Burnell, Fiona J; kastu007@umn.edu; Kasturirangan, Saumitran
    The transport statistics at finite energies near a quantum critical point in the presence of disorder were not well understood analytically. This was approached by performing extensive simulations of transport using the package KWANT for python for disordered 1D quantum chains and metallic arm-chair graphene nanoribbons. This dataset contains the resulting data for several system sizes, strengths, and energies. This was used to establish two-parameter scaling and characterize the transport statistics.