Research

Effects of Disorder on the Electronic and Magnetic Properties of Quantum Materials

Disorder can have remarkably disparate consequences in superconductors.  In crystals and thick films, defects can have a favorable effect, dramatically boosting the material’s supercurrent carrying capacity.  On the contrary, in films and circuits, defects can have unfavorable effects: disorder can destroy superconductivity in ultrathin films (driving a phase transition to an insulating state) and induce microwave losses in superconducting quantum circuits.  The effects inherently depend on the dynamics of electron pairs (integral to the superconducting state) and vortices (flux lines that enter upon exposure to magnetic fields) through a landscape of potential wells defined by disorder. The overarching goal of my research is to study the effects of disorder on the electronic and magnetic properties of quantum materials and devices.  Specifically, my research group will study vortex-defect interactions in superconductors, skyrmion-defect interactions in magnetic materials, and study the effects of a material’s microstructure on decoherence in superconducting circuits.

Areas of Research

Vortex Dynamics In Superconductors

Superconductors appear in an astonishing variety of applications.  Integral to many energy efficiency initiatives, they appear in power cables, wind turbine generators, and low-dissipation computing components.  Superconducting qubits are one of the leading schemes for quantum computers, which may achieve revolutionary computing speeds and unprecedented levels of information security. In fact, superconducting technology already plays a role in other areas of national security, specifically in gamma ray detectors of clandestine nuclear material and surge protectors imperative for resilient electric grid technology.   In the field of medicine, MRI machines employ superconducting magnets, and superconducting quantum interference devices monitor neural activity. Furthermore, superconductors are an enabling technology for studying fundamental physics: they are used in accelerator magnets (thus energizing breakthroughs in particle physics), bolometers for x-ray astronomy, and qubits that can simulate quantum mechanics.

These successes highlight the current and potential impact of superconductors.  However, all technologically useful ones contain vortices, whose motion is pervasively detrimental to performance.  My specialty is vortex physics. My immediate research goal is to learn how to design superconducting materials and devices to mitigate the deleterious effects of vortices.  To achieve this, I will study vortex dynamics in a wide range of materials, devices, and regimes to move towards predictive design, transforming a field that heavily relies on trial-and-error.

Effects of Disorder on Skyrmion Dynamics in Magnetic Materials

Conventional electronics are based on transport of charge carriers.  Yet control and measurement of spin requires less energy, which is one reason why spintronics are predicted to become the mainstay of next generation electronics.   Magnetic skyrmions are topological excitations defined by a winding configuration of magnetic moments in magnetic materials. They are promising candidates for the information carrier exploited in spintronic devices because smaller currents are required to manipulate them than other magnetic textures.  Striking similarities between skyrmion and vortex dynamics make it possible to apply concepts and use experimental setups from the more mature field of superconducting vortex physics to understand the behavior of skyrmions. For example, skyrmions also form a triangular lattice in the absence of disorder, are mobilized by a critical current, and undergo disorder mediated collective interactions and glassiness. Yet there are striking differences between the two systems — e.g., unlike vortices, skyrmions experience a significant Magnus force and appear at room temperature.   The field is in its infancy; skyrmions were first observed at room temperature as recently as 2015-2016 and the first evidence of current induced motion occurred shortly thereafter.  We will study correlations between skyrmion size and velocity, as well as the robustness of uniform skyrmion motion to disorder in magnetic multilayers (e.g., Ir/Co/Pt) in patterned Hall bars.  Additionally, we will look for evidence of quantum and thermal creep.  Last, we will apply techniques that proved successful in controlling disorder in superconductors to similarly improve properties in devices exploiting skyrmions.

Energy Loss Mechanisms in Superconducting Quantum Circuits

The immense excitement sparked by recent increases in the number of superconducting quantum bits (qubits) implemented in prototype quantum computers has been accompanied by a remarkable surge in commercialization efforts. However, quantum supremacy is not simply inevitable; daunting challenges must still be surmounted, namely error correction and decoherence. To truly surpass the capabilities of classical hardware, the number of implemented qubits and the amount of time information is retained in qubits – the coherence time – must increase. The latter requires reducing coupling to decoherence mechanisms, dissipative irreversible interactions with the environment.  Limited knowledge about the microscopic origin of decoherence mechanisms and effects of disorder continues to hinder scalability and reproducibility.

The most significant contributors to decoherence are material defects that form unwanted two-level systems (TLFs). These parasitic TLFs can be modeled as double-well potentials containing a barrier through which atoms or ions quantum tunnel. If charged, TLFs can act as dipoles thus interact with oscillating electric fields in the device. The microscopic origin of TLFs – specifically, the type of disorder creating the energy minima – has yet to be identified. That is, we cannot predict whether a particular defect will act as a TLF nor whether it will alter circuit properties. Another important mechanism is dissipation from excitations from quasiparticles (unpaired electrons). Though theory predicts that quasiparticle populations should be negligible at temperatures far below the superconducting transition temperature, experiments have revealed high populations of quasiparticles at the millikelvin temperatures at which these circuits operate. The source is typically unknown; suspicions point to surprisingly disparate possibilities — cosmic rays, blackbody photons emitted from warmer regions of the cryostat, and heat release from TLF dynamics.

Grants

  1. CAREER: Skyrmion-Vortex Interactions in Ferromagnet-Superconductor Heterostructures(2021-2026)
    National Science Foundation, Award # 2046925 (PI)
  2. Superconducting quantum circuits based on epitaxial nitrides (2020-2022)
    NREL Laboratory Directed Research and Development (LDRD) (Co-PI)
  3. MRI: Acquisition of an Automated, Variable Temperature and Magnetic Field Multi-property Measurement System (2019-2022)
    National Science Foundation, Award # 1917860 (PI)
  4. Quantum and Thermal Creep of Skyrmions and Superconducting Vortices (2019-2022)
    National Science Foundation, Award # 1905909 (PI)
  5. QLCI-CG: The Open Quantum Frontier Institute Conceptualization Grant (2020-2021)
    National Science Foundation, Award # 1936835 (Co-PI)