Terahertz response of microfluidic-jetted fabricated 3D flexible metamaterials

Conventional materials exhibit some restrictions on their electromagnetic properties. Especially in terahertz region, for example, materials that exhibit magnetic response are far less common in nature than materials that exhibit electric response. However, materials can be designed, namely artificial man-made metamaterials that exhibit electromagnetic properties that are not found in natural materials by adjusting, for example, the dielectric, magnetic or structural parameters of the constituent elements. This dissertation demonstrates the use of new fabrication techniques to construct metamaterials in THz range via a material deposition system. The metamaterials are fabricated by stacking alternative layers with conventional designs such as single ring- split ring resonators (SRR) and microstrips to form a 3D metamaterial structure. Conductive nano-particle Ag, Cu and semiconductor polymer fluids are used as structural mediums. The metamaterials are fabricated on polyimide substrate. Their flexible nature will be advantageous in future device innovations. In order to obtain electromagnetic resonance in the terahertz range, the dimensions of the single ring-SRR and microstrips are first approximated by analytical methods and then confirmed by numerical simulation. The fabricated metamaterials are then characterized in transmission mode using Time-domain THz Spectroscopy (THz-TDS) in the 0.1 to 2 THz range

Rotational Symmetry Breaking in a Trigonal Superconductor Nb-Doped Bi₂Se₃

The search for unconventional superconductivity has been focused on materials with strong spin-orbit coupling and unique crystal lattices. Doped bismuth selenide (Bi2Se3) is a strong candidate, given the topological insulator nature of the parent compound and its triangular lattice. The coupling between the physical properties in the superconducting state and its underlying crystal symmetry is a crucial test for unconventional superconductivity. In this paper, we report direct evidence that the superconducting magnetic response couples strongly to the underlying trigonal crystal symmetry in the recently discovered superconductor with trigonal crystal structure, niobium (Nb)-doped Bi2Se3. As a result, the in-plane magnetic torque signal vanishes every 60°. More importantly, the superconducting hysteresis loop amplitude is enhanced along one preferred direction, spontaneously breaking the rotational symmetry. This observation indicates the presence of nematic order in the superconducting ground state of Nb-doped Bi2Se3

Multiple Fermi Surfaces in Superconducting Nb-Doped Bi₂Se₃

Topological insulator Bi2Se3 has shown a number of interesting physical properties. Doping Bi2Se3 with copper or strontium has been demonstrated to make the material superconducting and potentially even a topological superconductor. The recent discovery of superconducting niobium-doped Bi2Se3 reveals an exciting new physical phenomenon, the coexistence of superconductivity and magnetic ordering, as well as signatures of an odd-parity p-wave superconducting order. To understand this new phenomenon, a detailed knowledge of the electronic structure is needed. We present an observation of quantum oscillations in the magnetization (the de Haas-van Alphen effect) of Nb-doped Bi2Se3. In the fully superconducting crystal, two distinct orbits are observed, in sharp contrast to Bi2Se3, Cu-doped Bi2Se3, and Sr-doped Bi2Se3. The multiple frequencies observed in our quantum oscillations, combined with our electrical transport studies, indicate the multi-orbit nature of the electronic state of Nb-doped Bi2Se3

Terahertz study of trichloroanisole by time-domain spectroscopy,”

a b s t r a c t The THz transmission spectra of three trichloroanisole (TCA) compounds (2,3,4-TCA, 2,4,6-TCA and 2,5,6-TCA) are measured using terahertz time-domain spectroscopy (THz-TDS). The spectrum of 2,4,6-TCA in the solid phase displays several discrete absorption peaks over frequency range from 0.1 to 1.5 THz, with significant peaks observed at 0.6, 0.95 and 1.2 THz. A weak absorption peak is observed near 0.2-0.3 THz. Computational chemistry using the B3LYP density functional method is used to study structure and internal rotations in 2,4,6-TCA, where results strongly suggest that frequencies of the CH 3 O-carbon(benzene) internal rotor of the methoxy group correspond to the observed spectra

Self-Intercalation Tunable Interlayer Exchange Coupling in a Synthetic Van Der Waals Antiferromagnet

One of the most promising avenues in 2D materials research is the synthesis of antiferromagnets employing 2D van der Waals (vdW) magnets. However, it has proven challenging, due in part to the complicated fabrication process and undesired adsorbates as well as the significantly deteriorated ferromagnetism at atomic layers. Here, the engineering of the antiferromagnetic (AFM) interlayer exchange coupling between atomically thin yet ferromagnetic CrTe2 layers in an ultra-high vacuum-free 2D magnetic crystal, Cr5Te8 is reported. By self-introducing interstitial Cr atoms in the vdW gaps, the emergent AFM ordering and the resultant giant magnetoresistance effect are induced. A large negative magnetoresistance (10%) with a plateau-like feature is revealed, which is consistent with the AFM interlayer coupling between the adjacent CrTe2 main layers in a temperature window of 30 K below the Néel temperature. Notably, the AFM state has a relatively weak interlayer exchange coupling, allowing a switching between the interlayer AFM and ferromagnetic states at moderate magnetic fields. This work represents a new route to engineering low-power devices that underpin the emerging spintronic technologies, and an ideal laboratory to study 2D magnetism

Quantum Oscillations in the Topological Superconductor Candidate Cu₀.₂₅Bi₂Se₃

Quantum oscillations are generally studied to resolve the electronic structure of topological insulators. In Cu0.25Bi2Se3, the prime candidate of topological superconductors, quantum oscillations are still not observed in magnetotransport measurement. However, using torque magnetometry, quantum oscillations (the de Haas-van Alphen effect) were observed in Cu0.25Bi2Se3. The doping of Cu in Bi2Se3 increases the carrier density and the effective mass without increasing the scattering rate or decreasing the mean free path. In addition, the Fermi velocity remains the same in Cu0.25Bi2Se3 as that in Bi2Se3. Our results imply that the insertion of Cu does not change the band structure and that conduction electrons in Cu doped Bi2Se3 sit in the linear Dirac-like band

Terahertz response of microfluidic-jetted three-dimensional flexible metamaterials

We demonstrate the fabrication and characterization of three-dimensional (3D) metamaterials in the terahertz (THz) range using the microfluidic-jetted technique. This technique has proven a convenient technique to fabricate metamaterial structures at the micrometer scale. The metamaterials are fabricated using dodecanethiol functionalized gold nanoparticles on flexible polyimide substrates. The metamaterials consist of alternate layers of single split-ring resonator and microstrip arrays that are stacked to form a 3D metamaterial medium. The fabricated metamaterials, with lattice sizes of 180 μm, are characterized using THz time-domain spectroscopy within 0.1 to 2 THz in the transmission mode. Numerical simulation is performed to calculate the effective metamaterials parameter

Quantum Oscillations in CuₓBi₂Se₃ in High Magnetic Fields

CuxBi2Se3 has drawn much attention as the leading candidate to be the first topological superconductor and the realization of coveted Majorana particles in a condensed matter system. However, there has been increasing controversy about the nature of its superconducting phase. This study sheds light on ambiguity in the normal-state electronic state by providing a complete look at the quantum oscillations in magnetization in CuxBi2Se3 at high magnetic fields up to 31 T. Our study focuses on the angular dependence of the quantum oscillation pattern in a low carrier concentration. As the magnetic field tilts from the crystalline c axis to the ab plane, the change of the oscillation period follows the prediction of the ellipsoidal Fermi surface. As the doping level changes, the 3D Fermi surface becomes quasicylindrical at high carrier density. Such a transition is potentially a Lifshitz transition of the electronic state in CuxBi2Se3

Phase Transitions of Dirac Electrons in Bismuth

The Dirac Hamiltonian, which successfully describes relativistic fermions, applies equally well to electrons in solids with linear energy dispersion, for example, in bismuth and graphene. A characteristic of these materials is that a magnetic field less than 10 tesla suffices to force the Dirac electrons into the lowest Landau level, with resultant strong enhancement of the Coulomb interaction energy. Moreover, the Dirac electrons usually come with multiple flavors or valley degeneracy. These ingredients favor transitions to a collective state with novel quantum properties in large field. By using torque magnetometry, we have investigated the magnetization of bismuth to fields of 31 tesla. We report the observation of sharp field-induced phase transitions into a state with striking magnetic anisotropy, consistent with the breaking of the threefold valley degeneracy