Anomalous behaviour of the in-plane electrical conductivity of the layered superconductor κ\kappa-(BEDT-TTF)2_2Cu(NCS)2_2

The quasiparticle scattering rates in high-quality crystals of the quasi-two-dimensional superconductor $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$ ~are studied using the Shubnikov-de Haas effect and MHz penetration-depth experiments. There is strong evidence that the broadening of the Landau-levels is primarily caused by spatial inhomogeneities, indicating a quasiparticle lifetime for the Landau states $\gg 3$ ps. In contrast to the predictions of Fermi-liquid theory, the scattering time derived from the intralayer conductivity is found to be much shorter ($0.14-0.56$ ps)

How close can one approach the Dirac point in graphene experimentally?

The above question is frequently asked by theorists who are interested in graphene as a model system, especially in context of relativistic quantum physics. We offer an experimental answer by describing electron transport in suspended devices with carrier mobilities of several 10^6 cm^2V^-1s^-1 and with the onset of Landau quantization occurring in fields below 5 mT. The observed charge inhomogeneity is as low as \approx10^8 cm^-2, allowing a neutral state with a few charge carriers per entire micron-scale device. Above liquid helium temperatures, the electronic properties of such devices are intrinsic, being governed by thermal excitations only. This yields that the Dirac point can be approached within 1 meV, a limit currently set by the remaining charge inhomogeneity. No sign of an insulating state is observed down to 1 K, which establishes the upper limit on a possible bandgap

Magnetotransport in a pseudomorphic GaAs/GaInAs/GaAlAs heterostructure with a Si delta-doping layer

Magnetotransport properties of a pseudomorphic GaAs/Ga0.8In0.2As/Ga0.75Al0.25As heterostructure are investigated in pulsed magnetic fields up to 50 T and at temperatures of T=1.4 K and 4.2 K. The structure studied consists of a Si delta-layer parallel to a Ga0.8In0.2As quantum well (QW). The dark electron density of the structure is n_e=1.67x 10^16 m^-2. By illumination the density can be increased up to a factor of 4; this way the second subband in the Ga0.8In0.2As QW can become populated as well as the Si delta-layer. The presence of electrons in the delta-layer results in drastic changes in the transport data, especially at magnetic fields beyond 30 T. The phenomena observed are interpreted as: 1) magnetic freeze-out of carriers in the delta-layer when a low density of electrons is present in the delta-layer, and 2) quantization of the electron motion in the two dimensional electron gases in both the Ga0.8In0.2As QW and the Si delta-layer in the case of high densities. These conclusions are corroborated by the numerical results of our theoretical model. We obtain a satisfactory agreement between model and experiment.Comment: 23 pages, RevTex, 11 Postscript figures (accepted for Phys. Rev. B

Performance characteristics of In0.6Ga0.4As/In0.52Al0.48As modulation‐doped field‐effect transistor monolithically integrated with In0.53Ga0.47As p‐i‐n photodiodes

Pseudomorphic In0.6Ga0.4As/In0.52Al0.48As 1 μm gate modulation‐doped field‐effect transistors have been monolithically integrated with In0.53Ga0.47As photodiodes for front‐end photoreceivers using one‐step molecular‐beam epitaxy and lithography techniques. A 1‐μm thick undoped In0.52Al0.48As layer is used to isolate the two devices. The transistors are characterized by gm(ext) =500 mS/mm and fT =9 GHz. The temporal response of the photodiodes is characterized by a linewidth of 60 ps. The eye pattern of the photoreceiver circuit for 1.7 Gbit/s pseudorandom optical signal is open and it is expected that the circuit can perform at bandwidths up to 2.5 GHz. Measured bandwidths of ∼6.5 GHz are obtained by using regrowth.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70086/2/JAPIAU-68-4-1918-1.pd

Exchange enhancement of the Landau-level separation for two-dimensional electrons in GaAs/Ga1-xAlxAs heterojunctions

Exchange enhancement of the Landau-level separation of up to 30% has been observed in three high mobility GaAs/Ga1-xAlxAs single heterojunctions. Analysis of the amplitude of Shubnikov-de Haas oscillations as a function of temperature and magnetic field has allowed measurement of this enhancement at filling factors as high as nu = 100