J. J. Heremans

Robustness of topologically protected surface states in layering of Bi2Te3 thin films.

Bulk Bi2Te3 is known to be a topological insulator. We investigate surface states of Bi2Te3(111) thin films of one to six quintuple layers using density-functional theory including spin-orbit coupling. We construct a method to identify topologically protected surface states of thin film topological insulators. Applying this method to Bi2Te3 thin films, we find that the topological nature of the surface states remains robust with the film thickness and that the films of three or more quintuple layers have topologically nontrivial surface states, which agrees with experiments.

Low-frequency noise in transport through quantum pOint contacts

We report the noise characteristics of quantum point contacts between 100 Hz and 100 kHz at 4.2 K. The noise consists of a 1/f component on top of a white background. The 1/f noise increases as the contact width decreases and shows peaks between the quantized resistance plateaus. The white noise background increases with current but is much lower than the full shot noise level, suggesting that shot noise is not generated in an ideal quantum point contact, where the electrons do not suffer backscattering as they enter and traverse the contact.

Mobility anisotropy of two‐dimensional hole systems in (311)A GaAs/AlxGa1−xAs heterojunctions

We have measured the low‐temperature mobility of high‐quality two‐dimensional hole systems confined at the (311)A GaAs/AlxGa1−xAs interface. Variables were the thickness of the spacer layer separating the carriers from the Si dopants, and the carrier sheet density. A large anisotropy in mobility is found between the [233] and [011] directions. While the high mobility [233] direction yields results analogous to the two‐dimensional electron case, we conclude that transport along [011] is almost entirely determined by anisotropic interface roughness scattering.

Use of superlattices to realize inverted GaAs/AlGaAs heterojunctions with low‐temperature mobility of 2×106 cm2/V s

Reproducible realization of high quality inverted interfaces (GaAs on AlGaAs) grown by molecular beam epitaxy is reported. Effective use of thin‐layer GaAs/AlAs superlattices in place of an AlGaAs barrier was made to reduce the number of impurities and the roughness at these interfaces. The low‐temperature (≂4 K) mobility for electrons at these interfaces is as high as 2×106 cm2/V s for an electron density of ≂5×1011 cm−2—a factor of four improvement over the highest mobility reported for inverted interfaces.

Observation of magnetic focusing in two‐dimensional hole systems

We report the first observation of transverse magnetic hole focusing in high quality two‐dimensional hole systems confined in square and triangular quantum wells grown on (311)A GaAs substrates. The results demonstrate ballistic hole transport over distances up to 11 μm and allow us to probe the constant energy contours in k‐space for these two types of confinement.

Comparative current–voltage characteristics of nicked and repaired λ-DNA

We report on current–voltage characteristics obtained at room temperature on λ-DNA molecules, modified at both ends with disulfide groups and spanning Au electrodes. A comparison is drawn between the characteristics of the λ-DNA, which after modification features a gap between 3′ and 5′ sites (nicked DNA), and λ-DNA where this gap has been ligated (repaired DNA). The repaired DNA double helices show a close-to-linear current–voltage characteristic, and a dc conductivity estimated at ∼1×10−3 S cm−1. In contrast, the nicked DNA shows pronouncedly nonlinear and rectifying behavior, with a conductivity gap of ∼3 eV. The low-field conductivity of the nicked DNA is approximately a factor 20 lower than the repaired DNA’s conductivity.

Spin-polarized reflection in a two-dimensional electron system

We present a method to create spin-polarized beams of ballistic electrons in a two-dimensional electron system in the presence of spin–orbit interaction. Scattering of a spin-unpolarized injected beam from a lithographic barrier leads to the creation of two fully spin-polarized side beams, in addition to an unpolarized specularly reflected beam. Experimental magnetotransport data on InSb∕InAlSb heterostructures demonstrate the spin-polarized reflection in a mesoscopic geometry.

High‐quality two‐dimensional electron system confined in an AlAs quantum well

We report the fabrication and characterization of a high‐quality two‐dimensional electron system in the X‐point valley of an AlAs quantum well. The modulation doped structure has a density of ns=2.5×1011 cm−2 and low‐temperature mobility μ=3×104 cm2/V s. Cyclotron resonance data reveal an effective mass mc=0.46m0, indicating that the X‐point conduction valleys with heavy in‐plane mass are occupied. In the magnetotransport data, we observe quantum Hall states at consecutive integral Landau‐level fillings (ν), implying that the degeneracy of these valleys is lifted. Our data at high magnetic fields show well‐developed fractional quantum Hall states at ν=1/3 and 2/3 with a gap of 1/3Δ=1.3K for the ν=1/3 state at B≊30 T.

Theory of high-temperature superconductivity and effective gravity

We argue that an approach involving effective gravity could play a crucial role in elucidating the properties of the high temperature superconducting materials. In particular we propose that the high critical temperature might be naturally explained in a framework constructed as a direct condensed matter analog of the Randall-Sundrum approach to a geometrization of the hierarchy problem in high-energy physics.

Characterization of films of La1−xSrxMnO3−δ grown by means of metal organic chemical vapor deposition

An effort to develop La manganites for possible sensor applications has led to the successful growth of La1−xBxMnO3−δ (B=Sr, Ca) thin films on single crystal MgO, Al2O3, and LaAlO3 substrates. Here we concentrate on Sr-doped material on LaAlO3 with Curie temperature Tc=262±2 K. Tc was determined from magnetization measurements using Arrott plots, and agrees well with the observed resistivity peak in zero applied magnetic field (H). This Tc corresponds closely to a Sr content x=0.17 reported in bulk materials. At H=60 kOe the maximum magnetoresistance Δρ/ρ≈0.95. No discernible hysteresis, as might be expected for structural phase transitions, was observed. When plotted as a function of reduced temperature T/Tc, the magnetization extrapolated to zero H is best fit by a Brillouin function with an effective spin value 〈S〉 much larger than the 〈S〉=1.9 indicated by the composition. This may be indicative of magnetic polaron formation, especially near the magnetic and transport transition temperatures. At temper...