Impurity Band Conduction in a High Temperature Ferromagnetic Semiconductor

The band structure of a prototypical dilute ferromagnetic semiconductor, Ga$_{1-x}$Mn$_{x}$As, is studied across the phase diagram via optical spectroscopy. We prove that the Fermi energy ($E_{F}$) resides in a Mn induced impurity band (IB). This conclusion is based upon careful analysis of the frequency and temperature dependence of the optical conductivity ($\sigma_{1}(\omega,T)$). From our analysis of $\sigma_{1}(\omega,T)$ we infer a large effective mass ($m^*$) of the carriers, supporting the view that conduction occurs in an IB. Our results also provide useful insights into the transport properties of Mn-doped GaAs.Comment: 4 pages, 4 figure

Correlated metallic state of vanadium dioxide

The metal-insulator transition and unconventional metallic transport in vanadium dioxide (VO$_2$) are investigated with a combination of spectroscopic ellipsometry and reflectance measurements. The data indicates that electronic correlations, not electron-phonon interactions, govern charge dynamics in the metallic state of VO$_2$. This study focuses on the frequency and temperature dependence of the conductivity in the regime of extremely short mean free path violating the Ioffe-Regel-Mott limit of metallic transport. The standard quasiparticle picture of charge conduction is found to be untenable in metallic VO$_2$.Comment: 5 pages, 3 figure

Experimental demonstration of frequency-agile terahertz metamaterials

Metamaterials exhibit numerous novel effects1–5 and operate over a large portion of the electromagnetic spectrum6–10. Metamaterial devices based on these effects include gradientindex lenses11,12, modulators for terahertz radiation13–15 and compact waveguides16. The resonant nature of metamaterials results in frequency dispersion and narrow bandwidth operation where the centre frequency is fixed by the geometry and dimensions of the elements comprising the metamaterial composite. The creation of frequency-agile metamaterials would extend the spectral range over which devices function and, further, enable the manufacture of new devices such as dynamically tunable notch filters. Here, we demonstrate such frequency-agile metamaterials operating in the far-infrared by incorporating semiconductors in critical regions of metallic split-ring resonators. For this first-generation device, externa

Spin-Induced Optical Conductivity in the Spin-Liquid Candidate Herbertsmithite

We report a direct measurement of the low-frequency optical conductivity of large-area single-crystal herbertsmithite, a promising spin-liquid candidate material, by means of terahertz time-domain spectroscopy. In the spectral range below 1.4 THz, we observe a contribution to the real part of the in-plane conductivity σ[subscript ab](ω) from the spin degree of freedom. This spin-induced conductivity exhibits a power-law dependence on frequency σ[subscript ab](ω) ~ ω[superscript β] with β ≈ 1.4. Our observation is consistent with the theoretically predicted low-frequency conductivity arising from an emergent gauge field of a gapless U(1) Dirac spin liquid.United States. Dept. of Energy. Office of Basic Energy Sciences (Division of Materials Sciences and Engineering Grant DE-FG02-08ER46521)United States. Dept. of Energy. Office of Basic Energy Sciences (Division of Materials Sciences and Engineering Grant DE-SC0006423)United States. Dept. of Energy. Office of Basic Energy Sciences (Division of Materials Sciences and Engineering Grant DE-FG02-07ER46134