Application of Coupled Oscillator Model to Terahertz and Optical Control of Plasmon Induced Opacity in Coupled Metamaterials

Terahertz (THz) frequencies of the electromagnetic spectrum have been underutilized when compared to neighboring microwave and infrared frequencies, largely due to the difficulties controlling and detecting these fields. In a step toward gaining control over THz frequencies, my advisor Dr. Yun-Shik Lee’s group experimentally demonstrated optical-pulse THz-control over a plasmonic metamaterial. To complement the experiments, I theoretically model the system using a coupled linear oscillator model to describe plasmonic oscillations of the metamaterial. The parameters of this oscillator model are fitted to the THz-Time domain spectroscopy results from the experimental counterpart without the presence of an optical pulse. Enhanced control of the THz response was experimentally demonstrated by the group by optically exciting charge carriers in a GaAs substrate, which was found to interfere with the plasmon-induced opacity in a predictable manner. Furthermore, these experiments show that plasmon-induced opacity can be reintroduced in the photoconductive sample by increasing incident THz field strength. These observations are modelled theoretically by introducing time dependence to the damping rates of the coupled linear oscillator model. The relation between the oscillator damping rates and THz field strength is attributed to phonon-assisted intervalley tunneling in large THz fields, which involves a decrease in substrate conductivity as electrons tunnel to side valleys with larger effective mass. A simple description of intervalley scattering is done using a two state population model where only electrons in the lower-energy -valley state contribute to substrate conductivity. The final results of my model replicate the THz response of the composite metamaterial sample with and without optical excitation, as well for differing THz field strengths. We observe the oscillation of THz transmission through the sample over sub-picosecond shifts in the optical pulse delay time, which is a consequence of intervalley scattering induced by plasmonic oscillations in the metamaterial. Finally, the size and temporal locations of these modulations in transmission are shown to be intimately related with transport properties of the substrate, such as intervalley scattering rates and mean-free times

Observation of multiple van Hove singularities and correlated electronic states in a new topological ferromagnetic kagome metal NdTi3Bi4

Kagome materials have attracted enormous research interest recently owing to its diverse topological phases and manifestation of electronic correlation due to its inherent geometric frustration. Here, we report the electronic structure of a new distorted kagome metal NdTi3Bi4 using a combination of angle resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations. We discover the presence of two at bands which are found to originate from the kagome structure formed by Ti atoms with major contribution from Ti dxy and Ti dx2-y2 orbitals. We also observed multiple van Hove singularities (VHSs) in its electronic structure, with one VHS lying near the Fermi level EF. In addition, the presence of a surface Dirac cone at the G point and a linear Dirac-like state at the K point with its Dirac node lying very close to the EF indicates its topological nature. Our findings reveal NdTi3Bi4 as a potential material to understand the interplay of topology, magnetism, and electron correlation.Comment: 7 pages, 4 figure

Observation of gapless nodal-line states in NdSbTe

Lanthanide (Ln) based systems in the ZrSiS-type nodal-line semimetals have been subjects of research investigations as grounds for studying the interplay of topology with possible magnetic ordering and electronic correlations that may originate from the presence of Ln 4f electrons. In this study, we carried out a thorough study of a LnSbTe system - NdSbTe - by using angle-resolved photoemission spectroscopy along with first-principles calculations and thermodynamic measurements. We experimentally detect the presence of multiple gapless nodal-line states, which is well supported by first-principles calculations. A dispersive and an almost non-dispersive nodal-line exist along the bulk X-R direction. Another nodal-line is present well below the Fermi level across the G- M direction, which is formed by bands with high Fermi velocity that seem to be sensitive to light polarization. Our study provides an insight into the electronic structure of a new LnSbTe material system that will aid towards understanding the connection of Ln elements with topological electronic structure in these systems.Comment: 34 pages, 12 figures; Supplemental Material include

Observation of multiple flat bands and topological Dirac states in a new titanium based slightly distorted kagome metal YbTi3Bi4

Kagome lattices have emerged as an ideal platform for exploring various exotic quantum phenomena such as correlated topological phases, frustrated lattice geometry, unconventional charge density wave orders, Chern quantum phases, superconductivity, etc. In particular, the vanadium based nonmagnetic kagome metals AV3Sb5 (A= K, Rb, and Cs) have seen a flurry of research interest due to the discovery of multiple competing orders. Here, we report the discovery of a new Ti based kagome metal YbTi3Bi4 and employ angle-resolved photoemission spectroscopy (ARPES), magnetotransport in combination with density functional theory calculations to investigate its electronic structure. We reveal spectroscopic evidence of multiple flat bands arising from the kagome lattice of Ti with predominant Ti 3d character. Through our calculations of the Z2 indices, we have identified that the system exhibits topological nontriviality with surface Dirac cones at the Gamma point and a quasi two-dimensional Dirac state at the K point which is further confirmed by our ARPES measured band dispersion. These results establish YbTi3Bi4 as a novel platform for exploring the intersection of nontrivial topology, and electron correlation effects in this newly discovered Ti based kagome lattice.Comment: 8 pages, 5 figure

Observation of flat and weakly dispersing bands in a van der Waals semiconductor Nb3Br8 with breathing kagome lattice

Niobium halides, Nb3X8 (X = Cl,Br,I), which are predicted two-dimensional magnets, have recently gotten attention due to their breathing kagome geometry. Here, we have studied the electronic structure of Nb3Br8 by using angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. ARPES results depict the presence of multiple flat and weakly dispersing bands. These bands are well explained by the theoretical calculations, which show they have Nb d character indicating their origination from the Nb atoms forming the breathing kagome plane. This van der Waals material can be easily thinned down via mechanical exfoliation to the ultrathin limit and such ultrathin samples are stable as depicted from the time-dependent Raman spectroscopy measurements at room temperature. These results demonstrate that Nb3Br8 is an excellent material not only for studying breathing kagome induced flat band physics and its connection with magnetism, but also for heterostructure fabrication for application purposes.Comment: 24 pages, 12 figures, Supplemental Material include

Observation of momentum-dependent charge density wave gap in a layered antiferromagnet GdTe3{\textrm{Gd}}{\textrm{Te}}_{3} Gd Te 3

Abstract Charge density wave (CDW) ordering has been an important topic of study for a long time owing to its connection with other exotic phases such as superconductivity and magnetism. The $$R{\textrm{Te}}_{3}$$ R Te 3 (R = rare-earth elements) family of materials provides a fertile ground to study the dynamics of CDW in van der Waals layered materials, and the presence of magnetism in these materials allows to explore the interplay among CDW and long range magnetic ordering. Here, we have carried out a high-resolution angle-resolved photoemission spectroscopy (ARPES) study of a CDW material $${\textrm{Gd}}{\textrm{Te}}_{3}$$ Gd Te 3 , which is antiferromagnetic below $$\sim \mathrm {12~K}$$ ∼ 12 K , along with thermodynamic, electrical transport, magnetic, and Raman measurements. Our ARPES data show a two-fold symmetric Fermi surface with both gapped and ungapped regions indicative of the partial nesting. The gap is momentum dependent, maximum along $${\overline{\Gamma }}-\mathrm{\overline{Z}}$$ Γ ¯ - Z ¯ and gradually decreases going towards $${\overline{\Gamma }}-\mathrm{\overline{X}}$$ Γ ¯ - X ¯ . Our study provides a platform to study the dynamics of CDW and its interaction with other physical orders in two- and three-dimensions