N. P. Armitage

Progress and perspectives on electron-doped cuprates

Although the vast majority of high-

Weyl and Dirac semimetals in three-dimensional solids

T_c

Terahertz Response and Colossal Kerr Rotation from the Surface States of the Topological Insulator Bi2Se3

cuprate superconductors are hole-doped, a small family of electron-doped compounds exists. Under investigated until recently, there has been tremendous recent progress in their characterization. A consistent view is being reached on a number of formerly contentious issues, such as their order parameter symmetry, phase diagram, and normal state electronic structure. Many other aspects have been revealed exhibiting both their similarities and differences with the hole-doped compounds. This review summarizes the current experimental status of these materials, with a goal to providing a snapshot of our current understanding of electron-doped cuprates. When possible we put our results in the context of the hole-doped compounds. We attempt to synthesize this information into a consistent view on a number of topics important to both this material class as well as the overall cuprate phenomenology including the phase diagram, the superconducting order parameter symmetry, phase separation, pseudogap effects, the role of competing orders, the spin-density wave mean-field description of the normal state, and electron-phonon coupling.

Temporal correlations of superconductivity above the transition temperature in La2-xSrxCuO4 probed by terahertz spectroscopy

Weyl and Dirac semimetals are three-dimensional phases of matter with gapless electronic excitations that are protected by topology and symmetry. As three-dimensional analogs of graphene, they have generated much recent interest. Deep connections exist with particle physics models of relativistic chiral fermions, and, despite their gaplessness, to solid-state topological and Chern insulators. Their characteristic electronic properties lead to protected surface states and novel responses to applied electric and magnetic fields. The theoretical foundations of these phases, their proposed realizations in solid-state systems, and recent experiments on candidate materials as well as their relation to other states of matter are reviewed.

Quasi-Langmuir–Blodgett thin film deposition of carbon nanotubes

We report the THz response of thin films of the topological insulator Bi2Se3. At low frequencies, transport is essentially thickness independent showing the dominant contribution of the surface electrons. Despite their extended exposure to ambient conditions, these surfaces exhibit robust properties including narrow, almost thickness-independent Drude peaks, and an unprecedentedly large polarization rotation of linearly polarized light reflected in an applied magnetic field. This Kerr rotation can be as large as 65° and can be explained by a cyclotron resonance effect of the surface states.

A sudden collapse in the transport lifetime across the topological phase transition in (Bi1-xInx)2Se3

Above the superconducting temperature for a given material, correlations between pairs of electrons are already present. Many experiments indicate that such correlations may exist up to 100 K above the transition. However, a temporal probe of the superconducting fluctuations suggests that correlations only exist within a narrow temperature range.

Electron-phonon interaction and charge carrier mass enhancement in SrTiO3

The handling and manipulation of carbon nanotubes continues to be a challenge to those interested in the application potential of these promising materials. To this end, we have developed a method to deposit pure nonoriented nanotube films over large flat areas on substrates of arbitrary composition. The method bears some resemblance to the Langmuir–Blodgett deposition method used to lay down thin organic layers. We show that this redeposition technique causes no major changes in the films’ microstructure and that they retain the electronic properties of as-deposited films laid down on an alumina membrane.

Polarization modulation time-domain terahertz polarimetry

The quantum phase transition from a topological to a conventional insulator in In-doped Bi2Se3 occurs when the topological phase is destroyed by the hybridization of states on opposite surfaces. This is characterized by a sudden change in the transport lifetime, measured by means of optical spectroscopy.

Charge carrier interaction with a purely electronic collective mode: plasmarons and the infrared response of elemental bismuth.

We report a comprehensive THz, infrared and optical study of Nb-doped SrTiO3 as well as dc conductivity and Hall effect measurements. Our THz spectra at 7 K show the presence of an unusually narrow (<2 meV) Drude peak. For all carrier concentrations the Drude spectral weight shows a factor of three mass enhancement relative to the effective mass in the local density approximation, whereas the spectral weight contained in the incoherent midinfrared response indicates that the mass enhancement is at least a factor two. We find no evidence of a particularly large electron-phonon coupling that would result in small polaron formation.

Quantized Faraday and Kerr rotation and axion electrodynamics of a 3D topological insulator

We present high precision measurements of polarization rotations in the frequency range from 0.1 to 2.5 THz using a polarization modulation technique. A motorized stage rotates a polarizer at ~ 80 Hz, and the resulting modulation of the polarization is measured by a lock-in technique. We achieve an accuracy of 0.050° (900 μrad) and a precision of 0.02° (350 μrad) for small rotation angles. A detailed mathematical description of the technique is presented, showing its ability to fully characterize elliptical polarizations from 0.1 to 2.5 THz.