In a dielectric-barrier discharge between diametrically opposite sides of a narrow tube, discharge filaments stabilize at regular intervals along the tube's length. Three types of periodic patterns are observed, as is a disordered state in which filaments fire at apparently random positions and times. Time-resolved current measurements indicate that for each spatial pattern, a particular number of discharge stages occur during the voltage half-cycle. A preliminary model of the pattern-formation dynamics is described, motivating further work on time-resolved imaging and investigations of surface charge distributions.

We report scanning Hall probe imaging of ErNi 2 B 2 C in the superconducting, antiferromagnetic, and weakly ferromagnetic regimes in magnetic fields up to 20 Oe, well below H c1 , with two results. First, imaging isolated vortices shows that they spontaneously rearrange on cooling through the antiferromagnetic transition temperature T N = 6 K to pin on twin boundaries, forming a striped pattern. Second, a weak, random magnetic signal appears in the ferromagnetic phase below T WFM = 2.3 K, and no spontaneous vortex lattice is present down to 1.9 K. We conclude that ferromagnetism coexists with superconductivity either by forming small ferromagnetic domains or with oscillatory variation of the magnetization on sub-penetration-depth length scales.

We report scanning Hall probe microscopy of highly underdoped superconducting YBa 2 Cu 3 O 6+x with T c ranging from 5 to 15 K, which showed distinct flux bundles with less than one superconducting flux quantum ͑⌽ 0 ͒ through the sample surface. The sub-⌽ 0 features occurred more frequently for lower T c , were more mobile than conventional vortices, and occurred more readily when the sample was cooled with an in-plane field component. We show that these features are consistent with kinked stacks of pancake vortices.

We have studied quantum interference between electrons and holes in a split-ring gold interferometer with graphene arms, one of which contained a pn junction. The carrier type, the pn junction, and the phase of the oscillations in a magnetic field were controlled by a top gate placed over one of the arms. We observe clear Aharonov-Bohm oscillations at the Dirac point and away from it, regardless of the carrier type in each arm. We also find clear oscillations when one arm of the interferometer contains a single pn junction, allowing us to study the interplay of the Aharonov-Bohm effect and Klein tunneling.

Effects of disorder on the electronic transport properties of graphene are strongly affected by the Dirac nature of the charge carriers in graphene. This is particularly pronounced near the Dirac point, where relativistic charge carriers cannot efficiently screen the impurity potential. We have studied time-dependent conductance fluctuations and magnetoresistance in graphene in the close vicinity of the Dirac point. We show that the fluctuations are due to the quantum interference effects due to scattering on impurities, and find an unusually large reduction of the relative noise power in magnetic field, possibly indicating that an additional symmetry plays an important role in this regime.In disordered electronic systems, quantum corrections to the conductance arise due to quantum interference between paths of electrons scattered on random impurities. In the absence of a magnetic field, the electron paths that traverse the loops in a clockwise fashion interfere constructively with the counterclockwise paths through the same loops, resulting in a small change in the conductance. Specifically, the backscattered paths (the paths that return to their origin) lead to a correction to the average conductance of the system, known as weak localization (WL) [1][2][3]. Magnetic field adds a different phase factor to the paths that are identical, but traversed in the opposite sense, removing the WL corrections, but one still observes the universal conductance fluctuations (UCF) as a function of magnetic field or chemical potential, which arise from adding the interference contributions from all possible paths [4,5]. The quantum interference contribution to the conductance also fluctuates if the impurity configuration changes over time, leading to time-dependent conductance fluctuations that are expected to cause 1/f noise [6][7][8].In graphene, the quantum interference phenomena are affected by the pseudospin and valley degrees of freedom [9,10]. Conservation of pseudospin precludes backscattering, suppressing WL and causing weak antilocalization (WAL) [11], while intervalley scattering restores the WL [12][13][14]. Additional effects, such as defects and corrugations, can completely suppress the quantum corrections [15]. Depending on the carrier density and the nature of the disorder, all three regimes (WL, WAL and the suppression of quantum corrections) are observed experimentally [16,17]. UCF in graphene depend on the carrier density and the nature of the impurity scattering, but can also depend on the details and the geometry of the sample [18][19][20]. In particular, strong intervalley scattering is found to suppress UCF [18,19], in contrast to its effect on WL. However, the majority of the theoretical and experimental work on quantum corrections has focused on the regime away from the Dirac point. In the close vicinity to the Dirac point (at low doping and low temperatures), the relativistic Dirac quasiparticles are unable to screen the long-range Coulomb interactions in the usual way, altering electron-electron inte...