Improvement of the accuracy of noise measurements by the two-amplifier correlation method.

We present a novel method for device noise measurement, based on a two-channel cross-correlation technique and a direct "in situ" measurement of the transimpedance of the device under test (DUT), which allows improved accuracy with respect to what is available in the literature, in particular when the DUT is a nonlinear device. Detailed analytical expressions for the total residual noise are derived, and an experimental investigation of the increased accuracy provided by the method is performed.

Unraveling quantum Hall breakdown in bilayer graphene with scanning gate microscopy.

Investigating the structure of quantized plateaus in the Hall conductance of graphene is a powerful way of probing its crystalline and electronic structure and will also help to establish whether graphene can be used as a robust standard of resistance for quantum metrology. We use low-temperature scanning gate microscopy to image the interplateau breakdown of the quantum Hall effect in an exfoliated bilayer graphene flake. Scanning gate images captured during breakdown exhibit intricate patterns where the conductance is strongly affected by the presence of the scanning probe tip. The maximum density and intensity of the tip-induced conductance perturbations occur at half-integer filling factors, midway between consecutive quantum Hall plateau, while the intensity of individual sites shows a strong dependence on tip-voltage. Our results are well-described by a model based on quantum percolation which relates the points of high responsivity to tip-induced scattering in a network of saddle points separating localized states.

Huge conductance peak caused by symmetry in double quantum dots.

We predict a huge interference effect contributing to the conductance through large ultraclean quantum dots of chaotic shape. When a double-dot structure is made such that the dots are the mirror image of each other, constructive interference can make a tunnel barrier located on the symmetry axis effectively transparent. We show (via theoretical analysis and numerical simulation) that this effect can be orders of magnitude larger than the well-known universal conductance fluctuations and weak localization (both less than a conductance quantum). A small magnetic field destroys the effect, massively reducing the double-dot conductance; thus a magnetic field detector is obtained, with a similar sensitivity to a SQUID, but requiring no superconductors.