Probing the surface- and interface-sensitive momentum-resolved electronic structure of advanced quantum materials and interfaces

Authors: Arian Arab (Temple University)
Co-authored by industrial partner: No

Type: Thesis

State: Published (Approved)
Published: May 2019

Abstract: In this dissertation, we used a combination of synchrotron-based x-ray spectroscopic techniques such as angle-resolved photoelectron spectroscopy (ARPES), soft x-ray ARPES, hard x-ray photoelectron spectroscopy (HAXPES), and soft x-ray absorption spectroscopy (XAS) to investigate momentum-resolved and angle-integrated electronic structure of advanced three- and two-dimensional materials and interfaces. The results from the experiments were compared to several types of state-of-the-art first-principles theoretical calculations. In the first part of this dissertation we investigated the effects of spin excitons on the surface states of samarium hexaboride (SmB6), which has gained a lot of interest since it was proposed to be a candidate topological Kondo insulator. Here, we utilized high-resolution (overall resolution of approximately 3 meV) angle-resolved and angle-integrated valence-band photoemission measurements at cryogenic temperatures (1.2 K and 20 K) to show evidence for a V-shaped density of states of surface origin within the bulk gap of SmB6. Our temperature-dependent measurements of the valence-band spectra revealed a sharp feature appearing within the bulk gap of SmB6 at low temperatures. We attribute this feature to a resonance caused by the spin-exciton scattering in SmB6, which destroys the protection of surface states due to time-reversal invariance and spin-momentum locking. The near-Fermi-energy spin-exciton-driven scattering is thermally activated and only appears below the temperature of about 25 K. This temperature is considerably lower than the temperature at which the bulk hybridization gap is first observed. Therefore, it is plausible that the formation of the Fermi-liquid, which is responsible for the surface conduction should only occur at very low temperatures and may be responsible for the plateau in the resistivity at 5 K. In the second part of this dissertation we investigated the electronic structure of a strongly-correlated oxide NdNiO3 grown along the unconventional pseudocubic [111] direction and buried under 4 unit cells (u.c.) of an insulating oxide LaAlO3. Over the last several decades transition-metal oxides (TMO) have been demonstrated to host a wide variety of strongly-correlated-electron phenomena, such as metal-insulator transitions and high-temperature superconductivity, induced by chemical doping and/or various external stimuli. Until now, majority of the work has been focused on systems grown along the pseudocubic [001] direction. Recently, however, several theoretical proposals have been put forward to utilize TMO heterostructures consisting of a few u.c. grown epitaxially along the [111] direction. Here, we realized the first momentum-resolved soft x-ray ARPES measurement of the valence-band electronic structure of artificial graphene-like Mott crystal NdNiO3 [111] buried under four u.c. of an insulating oxide LaAlO3. Our measurements of the buried Ni 3d states near the Fermi level exhibit excellent agreement with the first-principles calculations and establish the realization of an antiferro-orbital order in this artificial lattice. Such ‘engineered’ electronic structure is unique to this quazi-2D crystal and cannot be realized either in the bulk or thin-film nickelates grown along the conventional [001] direction. Our findings open the door for engineering novel polarized Mott-electronic ground states in rare-earth nickelates, as well as other strongly-correlated transition-metal oxides. From the technical perspective, we demonstrate that soft x-ray ARPES can be used to measure the momentum-resolved electronic structure of ultrathin (2 u.c.) layers, buried within complex oxide heterostructures.

Journal Keywords: ARPES; Hard x-ray; Heterostructure; Kondo Insulators; Nickelates; Photoemission

Subject Areas: Physics, Materials

Instruments: I09-Surface and Interface Structural Analysis