Quantum Transport and Electric-Field Effects in Layered Topological Semimetals and Magnetic Materials
This dissertation describes transport experiments on quantum devices in layered Dirac nodal line topological semimetals and antiferromagnetic materials down to a few layers. We used gate-induced effects to alter the transport properties of these materials. First, we introduced current annealing in topological semimetals to achieve high-quality devices. We demonstrate current annealing to substantially improve the electronic transport properties of 2D topological semimetal flakes. Contact resistance and resistivity were improved by factors up to 2,000,000 and 20,000, respectively, in devices based on exfoliated flakes of two topological semimetals, ZrSiSe and BaMnSb2. Using this method, carrier mobility in ZrSiSe improved by a factor of 3800, resulting in the observation of record-high mobility for exfoliated ZrSiSe. Quantum oscillations in annealed ZrSiSe appeared at magnetic fields as low as 5 T, and magnetoresistance increased by 10,000. We argue that a thermal process underlies this improvement. Finally, Raman spectroscopy and analysis of quantum oscillations in ZrSiSe indicate that the phonon modes and Fermi surface area are unchanged by current annealing. Through current annealing, we had access to suitable device quality, hence could investigate topological semimetals extensively. We studied Shubnikov de Haas oscillations focused on the unexpected frequency of 140 T that is unrelated to any known surface (450 T) or bulk (220 T) Fermi surface. We show that this frequency has a 2D nature by providing angle dependence and thickness dependence of quantum oscillations. Angle dependence of 140 T follows 1/cos(θ), in contrast to bulk frequency, 220 T, which shows no change with angle. In addition, thickness dependence shows that the relative amplitude of 140 T to 220 T exponentially increases by decreasing the thickness. In addition, we provide a comparison of phase analysis of 140 T with 220 T that shows the different origins of these frequencies. We speculate this 140 T frequency can be of Weyl orbit oscillations and extracted a Fermi arc with a length of 0.08 1/ ̊A within this picture. Third, we studied the gate-induced electrostatic field effect on ZrSiSe using ZrSiSe/hBN/- graphene heterostructure. We observed that the electrostatic field increases the resistivity away from charge neutrality point Vg = 0.8 V at magnetic fields below 2 T. In contrast, the electrostatic field reduces resistivity at fields above 2 T. In addition, we observed that gate voltage reduces the curvature of magnetoresistance. In addition, the amplitude of quantum oscillations reduces with an increase in gate voltage. The bulk frequency of quantum oscillations, 220 T, does not change with gate voltage, but the Weyl orbit frequency, 140 T, shows a quadratic red shift with the gate voltage. In addition to topological semimetals in few-layer limits that excited many physicists in the last decade, the interplay of magnetism and superconductivity as well as electrostatic field control of magnetic phase in 2D limit enchanted many physicists, including us. The interplay between magnetic materials and superconductors provides an exciting playground due to opposing requirements for electron spin alignment between superconductivity and magnetic order. In the fourth chapter, we studied NiPS3, an antiferromagnetic layered material with in-plane antiferromagnetic order, and FeSe, a layered superconductor, by fabricating superconduc- tor/antiferromagnet/superconductor (FeSe/NiPS3/FeSe) Josephson Junction. In addition, we fabricated a controlled Josephson Junction using graphite and a similar thickness of NiPS3, graphite/NiPS3/graphite, to have a fair comparison with the superconductor Josephson Junction. We showed that our devices were in a direct tunneling regime at temperatures lower than 20 K and 10 K for graphite-JJ and FeSe-JJ, respectively. It has been shown that NiPS3 undergoes a spin-flop transition in magnetic fields parallel to the ab plane and larger than 6 T. Although we failed to observe this spin-flop transition for fields parallel to the ab plane, we observed signatures of possible transition in magnetic fields parallel to the c axis for the similar magnetic field in our controlled graphite device. However, this observation is ambiguous because such a spin-flop transition was not observed in bulk measurements. Further, we observed steps in the temperature dependence of resistivity for FeSe-JJ; these steps do not exist in a graphite-JJ control device; hence we attribute them to FeSe-JJ. The steps in the temperature dependence of resistivity happen at 50 K and 10 K; this coincides with the transition temperature of FeSe to a superconductor. In addition, we observed anisotropic and unconventional magnetoresistance in temperatures between 50 K and 10 K. This anisotropic magnetoresistance dies at temperatures lower than 10 K when FeSe becomes superconducting. Finally, in the fifth chapter, we attempted to induce a magnetic phase transition in MnPSe3, FePS3, and VPS3 by adjusting the carrier density in these materials using an electrostatic field. Due to the spin filtering effect, such magnetic phase transition should accompany higher conductivity or even semimetallic behavior, but our devices failed to show such behavior. We fabricated devices using various contact metals, geometry, and gating techniques but achieving the long-sought after magnetic phase transition remained elusive in these materials. However, we successfully doped these materials using electrostatic field-induced effects.
2D semimetal, Fermi arc, Gate effect, Nanotechnology, topological semimetal, Weyl orbit, ZrSi, Condensed Matter Physics, Quantum Physics
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