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Open Access Publications from the University of California

Scholarly Works (10 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2019)

This dissertation presents the results of several research projects on condensed matter systems that fall within the broad scope of quantum materials. These are compounds which possess emergent phenomena that would not be expected from conventional solid state theory. These include the topological semimetals Cd3As2, TaAs, and ZrTe5, as well as the frustrated antiferromagnet Fe1/3NbS2. In Cd3As2, a Dirac semimetal, focused ion beam microstructured devices were found to exhibit a new type of coherent electron orbit. This “Weyl orbit” involves the Fermi arc surface states. Although these states are a necessary consequence of the topological nature of Cd3As2, they had not previously been observed in electronic transport. In the Weyl semimetal TaAs, the same focused ion beam microstructuring techniques were found to induce superconductivity on the device surface due to the differential sputtering of tantalum and arsenic. Instead, mechanical polishing techniques were used to thin devices in order to observe signatures of surface-driven transport, likely stemming from Fermi arc states as well.

In ZrTe5, magnetization and magnetic torque measurements found a paramagnetic to diamagnetic crossover at the quantum limit magnetic field. This is the result of charge carriers entering a chiral, zeroth Landau level pinned at zero energy. This is a direct consequence of a topological band crossing, and therefore points to ZrTe5 as a Dirac semimetal. A possible transition out of this topological phase was also observed as a function of temperature.

Finally, at sufficiently low temperatures and high current densities, Fe1/3NbS2 was found to be a switchable antiferromagnet. A spin transfer torque produced by an applied current was found to rotate the antiferromagnetic order. The rotation of these moments is reflected in the anisotropic magnetoresistance, changing the resistance of the device. In this manner, microstructured devices of Fe1/3NbS2 form an antiferromagnetic memory bit with electronic write-in and read-out. The low current densities involved and tunability of the device response point to Fe1/3NbS2 as a promising platform for antiferromagnetic spintronics.

    Cover page: An Exploration of Quantum Materials: From Topological Semimetals to Antiferromagnetic Memory
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2020)
    Advances in controlling the correlated behaviour of transition metal dichalcogenides have opened a new frontier of many-body physics in two dimensions. A field where these materials have yet to make a deep impact is antiferromagnetic spintronics-a relatively new research direction promising technologies with fast switching times, insensitivity to magnetic perturbations and reduced cross-talk1-3. Here, we present measurements on the intercalated transition metal dichalcogenide Fe1/3NbS2 that exhibits antiferromagnetic ordering below 42 K (refs. 4,5). We find that remarkably low current densities of the order of 104 A cm-2 can reorient the magnetic order, which can be detected through changes in the sample resistance, demonstrating its use as an electronically accessible antiferromagnetic switch. Fe1/3NbS2 is part of a larger family of magnetically intercalated transition metal dichalcogenides, some of which may exhibit switching at room temperature, forming a platform from which to build tuneable antiferromagnetic spintronic devices6,7.
      Cover page: Electrical switching in a magnetically intercalated transition metal dichalcogenide
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2018)
      We combine transport, magnetization, and torque magnetometry measurements to investigate the electronic structure of ZrTe5 and its evolution with temperature. At fields beyond the quantum limit, we observe a magnetization reversal from paramagnetic to diamagnetic response, which is characteristic of a Dirac semi-metal. We also observe a strong non-linearity in the magnetization that suggests the presence of additional low-lying carriers from other low-energy bands. Finally, we observe a striking sensitivity of the magnetic reversal to temperature that is not readily explained by simple band-structure models, but may be connected to a temperature dependent Lifshitz transition proposed to exist in this material.
        Cover page: Thermodynamic signature of Dirac electrons across a possible topological transition in ZrTe5
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2020)
        While electrons moving perpendicular to a magnetic field are confined to cyclotron orbits, they can move freely parallel to the field. This simple fact leads to complex current flow in clean, low carrier density semi-metals, such as long-ranged current jets forming along the magnetic field when currents pass through point-like constrictions. Occurring accidentally at imperfect current injection contacts, the phenomenon of "current jetting" plagues the research of longitudinal magneto-resistance, which is particularly important in topological conductors. Here we demonstrate the controlled generation of tightly focused electron beams in a new class of micro-devices machined from crystals of the Dirac semi-metal Cd3As2. The current beams can be guided by tilting a magnetic field and their range tuned by the field strength. Finite element simulations quantitatively capture the voltage induced at faraway contacts when the beams are steered towards them, supporting the picture of controlled electron jets. These experiments demonstrate direct control over the highly non-local signal propagation unique to 3D semi-metals in the current jetting regime, and may lead to applications akin to electron optics in free space.
          Cover page: Magnetic electron collimation in three-dimensional semi-metals
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2020)
          Nematic order is the breaking of rotational symmetry in the presence of translational invariance. While originally defined in the context of liquid crystals, the concept of nematic order has arisen in crystalline matter with discrete rotational symmetry, most prominently in the tetragonal Fe-based superconductors where the parent state is four-fold symmetric. In this case the nematic director takes on only two directions, and the order parameter in such 'Ising-nematic' systems is a simple scalar. Here, using a spatially resolved optical polarimetry technique, we show that a qualitatively distinct nematic state arises in the triangular lattice antiferromagnet Fe1/3NbS2. The crucial difference is that the nematic order on the triangular lattice is a [Formula: see text] or three-state Potts-nematic order parameter. As a consequence, the anisotropy axes of response functions such as the resistivity tensor can be continuously reoriented by external perturbations. This discovery lays the groundwork for devices that exploit analogies with nematic liquid crystals.
            Cover page: Three-state nematicity in the triangular lattice antiferromagnet Fe1/3NbS2
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2017)
            By introducing a superconducting gap in Weyl or Dirac semimetals, the superconducting state inherits the nontrivial topology of their electronic structure. As a result, Weyl superconductors are expected to host exotic phenomena, such as nonzero-momentum pairing due to their chiral node structure, or zero-energy Majorana modes at the surface. These are of fundamental interest to improve our understanding of correlated topological systems, and, moreover, practical applications in phase-coherent devices and quantum applications have been proposed. Proximity-induced superconductivity promises to allow these experiments on nonsuperconducting Weyl semimetals. We show a new route to reliably fabricate superconducting microstructures from the nonsuperconducting Weyl semimetal NbAs under ion irradiation. The significant difference in the surface binding energy of Nb and As leads to a natural enrichment of Nb at the surface during ion milling, forming a superconducting surface layer (Tc ~ 3.5 K). Being formed from the target crystal itself, the ideal contact between the superconductor and the bulk may enable an effective gapping of the Weyl nodes in the bulk because of the proximity effect. Simple ion irradiation may thus serve as a powerful tool for the fabrication of topological quantum devices from monoarsenides, even on an industrial scale.
              Cover page: Inducing superconductivity in Weyl semimetal microstructures by selective ion sputtering
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2020)
              We study Shubnikov-de Haas oscillations in single crystals of TaAs and find a previously undetected two-dimensional quantum oscillation that does not belong to the bulk Fermi surface. We cannot find an impurity phase consistent with our observations, and extensive diffraction measurements have not shown the presence of known impurity phases. We conjecture that the frequency originates from surface states that are sensitive to surface disorder. One candidate is the interference of coherent quasiparticles traversing two distinct Fermi arcs on the [001] crystallographic surface. The frequency and effective mass quantitatively agree with predictions of density functional theory and previous angle-resolved photoemission spectroscopy measurements of the Fermi arcs.
                Cover page: Signatures of possible surface states in TaAs
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2016)
                Electrons in materials with linear dispersion behave as massless Weyl- or Dirac-quasiparticles, and continue to intrigue due to their close resemblance to elusive ultra-relativistic particles as well as their potential for future electronics. Yet the experimental signatures of Weyl-fermions are often subtle and indirect, in particular if they coexist with conventional, massive quasiparticles. Here we show a pronounced anomaly in the magnetic torque of the Weyl semimetal NbAs upon entering the quantum limit state in high magnetic fields. The torque changes sign in the quantum limit, signalling a reversal of the magnetic anisotropy that can be directly attributed to the topological nature of the Weyl electrons. Our results establish that anomalous quantum limit torque measurements provide a direct experimental method to identify and distinguish Weyl and Dirac systems.
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2019)
                  The discovery of quantum oscillations in hole- A nd electron-doped cuprate families has underscored the importance of the Fermi surface in cuprate superconductivity. While the observed quantum oscillations in both families have revealed the presence of reconstructed Fermi surfaces, there remains an important distinction between the two. In hole-doped cuprates the oscillations are thought to arise from the effects of a charge density wave, while in the electron-doped cuprates it is thought that these oscillations occur from an antiferromagnetically reconstructed Fermi surface, despite the fact that the oscillations are observed in overdoped compounds, far from the putative antiferromagnetic critical point. In this work we study thin films of Pr2CuO4±δ, whose apparent doping can be finely tuned by annealing, allowing studies of quantum oscillations in samples straddling the critical point. We show that even though there is a mass enhancement of the quasiparticles, there are only small changes to the Fermi surface itself, suggesting that charge order is a more likely origin, with electronic correlations that are strongly dependent on the structural parameters.
                    Cover page: Interplay of structure and charge order revealed by quantum oscillations in thin films of Pr2CuO4±δ
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2021)
                    The theory behind the electrical switching of antiferromagnets is premised on the existence of a well-defined broken symmetry state that can be rotated to encode information. A spin glass is, in many ways, the antithesis of this state, characterized by an ergodic landscape of nearly degenerate magnetic configurations, choosing to freeze into a distribution of these in a manner that is seemingly bereft of information. Here, we show that the coexistence of spin glass and antiferromagnetic order allows a novel mechanism to facilitate the switching of the antiferromagnet Fe1/3 + δNbS2, rooted in the electrically stimulated collective winding of the spin glass. The local texture of the spin glass opens an anisotropic channel of interaction that can be used to rotate the equilibrium orientation of the antiferromagnetic state. Manipulating antiferromagnetic spin textures using a spin glass' collective dynamics opens the field of antiferromagnetic spintronics to new material platforms with complex magnetic textures.
                      Cover page: Antiferromagnetic switching driven by the collective dynamics of a coexisting spin glass
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