Superconducting films of Ca-Sr-Bi-Cu oxides have been prepared by coevaporation of CaF2, SrF2, Bi, and Cu, followed by post-oxidation in wet O2. The films were characterized by four-probe resistivity measurements, Rutherford backscattering, transmission electron microscopy, x-ray diffraction, and Hall measurements. Zero resistance was achieved at ∼80 K, although evidence of traces of superconductivity at higher temperatures was seen in resistivity and Hall data. The critical current at 4.2 K was 1.0×106 A cm−2. The films were epitaxial on 〈100〉 and 〈110〉 SrTiO3 substrates. The electrical and structural properties of the films were insensitive to film composition over a wide range of stoichiometries.
In two-dimensional hole systems confined to wide GaAs quantum wells, where the heavy- and light-hole states are close in energy, we observe a very unusual crossing of the lowest two Landau levels as the sample is tilted in a magnetic field. At a magic tilt angle $\ensuremath{\theta}\ensuremath{\simeq}{34}^{\ensuremath{\circ}}$, which surprisingly is independent of the well width or hole density, in a large filling factor range near $\ensuremath{\nu}=1$, the lowest two levels are nearly degenerate as evidenced by the presence of two-component quantum Hall states. Remarkably, a quantum Hall state is seen at $\ensuremath{\nu}=1$, consistent with a correlated ${\mathrm{\ensuremath{\Psi}}}_{111}$ state.
Composite fermions (CFs), exotic quasiparticles formed by pairing an electron and an even number of magnetic flux quanta, emerge at high magnetic fields in an interacting electron system, and can explain phenomena such as the fractional quantum Hall state (FQHS) and other many-body phases. CFs possess an effective mass $({m}_{\text{CF}})$ whose magnitude is inversely related to the most fundamental property of a FQHS, namely its energy gap. We present here experimental measurements of ${m}_{\text{CF}}$ in ultrahigh quality two-dimensional electron systems confined to GaAs quantum wells of varying thickness. An advantage of measuring ${m}_{\text{CF}}$ over gap measurements is that mass values are insensitive to disorder and are therefore ideal for comparison with theoretical calculations, especially for high-order FQHS. Our data reveal that ${m}_{\text{CF}}$ increases with increasing well width, reflecting a decrease in the energy gap as the electron layer becomes thicker and the in-plane Coulomb energy softens. Comparing our measured masses with available theoretical results, we find significant quantitative discrepancies, highlighting that more rigorous and accurate calculations are needed to explain the experimental data.
We report on the cascade of quantum phase transitions exhibited by tunnel-coupled edge states across a quantum Hall line junction. We identify a series of quantum critical points between successive strong and weak tunneling regimes in the zero-bias conductance. Scaling analysis shows that the conductance near the critical magnetic fields $B_{c}$ is a function of a single scaling argument $|B-B_{c}|T^{-\kappa}$, where the exponent $\kappa = 0.42$. This puzzling resemblance to a quantum Hall-insulator transition points to importance of interedge correlation between the coupled edge states.
The Wigner crystal, an ordered array of electrons, is one of the very first proposed many-body phases stabilized by the electron-electron interaction. We examine this quantum phase with simultaneous capacitance and conductance measurements, and observe a large capacitive response while the conductance vanishes. We study one sample with four devices whose length scale is comparable with the crystal's correlation length, and deduce the crystal's elastic modulus, permittivity, pinning strength, etc. Such a systematic quantitative investigation of all properties on a single sample has a great promise to advance the study of Wigner crystals.
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