Rhenium diselenide (ReSe2) is a layered indirect gap semiconductor for which micromechanical cleavage can produce monolayers consisting of a plane of rhenium atoms with selenium atoms above and below. ReSe2 is unusual among the transition-metal dichalcogenides in having a low symmetry; it is triclinic, with four formula units per unit cell, and has the bulk space group P1̅. Experimental studies of Raman scattering in monolayer, few-layer, and bulk ReSe2 show a rich spectrum consisting of up to 16 of the 18 expected lines with good signal strength, pronounced in-plane anisotropy of the intensities, and no evidence of degradation of the sample during typical measurements. No changes in the frequencies of the Raman bands with layer thickness down to one monolayer are observed, but significant changes in relative intensity of the bands allow the determination of crystal orientation and of monolayer regions. Supporting theory includes calculations of the electronic band structure and Brillouin zone center phonon modes of bulk and monolayer ReSe2 as well as the Raman tensors determining the scattering intensity of each mode. It is found that, as for other transition-metal dichalcogenides, Raman scattering provides a powerful diagnostic tool for studying layer thickness and also layer orientation in few-layer ReSe2.

We report on the magnetoresistance of a two-dimensional electron gas subjected to an abrupt magnetic field gradient arising from a ferromagnetic stripe fabricated at its surface. A sharp resistance resonance effect is observed at B(p) due to the formation of two types of magnetic edge states that drift in opposite directions perpendicular to the magnetic field gradient for BB(p). A semiclassical drift-diffusion model gives a good description of the effects of the magnetic confinement on both the diagonal and off-diagonal components of the resistivity tensor.

A scanning Hall probe microscope is used to study flux pinning in a thin superconducting Pb film covering a square array of single-domain Co dots with in-plane magnetization. We show that single flux quanta of opposite sign thread the superconducting film below T(c) at the opposite poles of these dipoles. Depending on the polarity of the applied field, flux lines are attracted to a specific pole of the dipoles, due to the direct interaction with the vortexlike structures induced by the local stray field.

Scanning Hall probe microscopy has been used to search for spontaneous fields at the well-defined edges of large mesoscopic disks, etched into the ab surface of very high quality single crystal Sr 2 RuO 4. Such fields are predicted to exist at locations of broken translational symmetry as a consequence of the proposed two-component spin-triplet chiral order parameterd = 0 (k x ± ik y)ẑ. We find no evidence for such fields and impose an upper limit of ±2.5 mG on their magnitude. We do, however, observe an abrupt apparent loss of strong bulk pinning and a change in the screening behavior above H ∼ 25 Oe. At high fields (H > 25 Oe) pronounced magnetic screening by the disks is very well described by a model containing only strong edge currents, and bulk critical currents do not appear to play a significant role. Our results are discussed in terms of relevant theoretical predictions.

The Hall conductivity of a high-mobility two-dimensional electron gas (2DEG) has been investigated in a distribution of quantized magnetic flux tubes (vortices) formed at a type-II superconducting "gate" layer. A pronounced suppression of the Hall effect was observed for long Fermi wavelengths (as compared to the submicron vortex size) indicating a situation where electrons are diffracted by the flux quanta. In contrast, for shorter Fermi wavelengths the Hall conductivity has been found to be insensitive to the extreme inhomogeneity of the magnetic field and determined by the average field.PACS numbers: 72.10.Fk, 73.40.Qv, 73.50.Jt There has been much interest in the last few years in a hybrid system in which an extremely inhomogeneous magnetic field created by a type-II superconductor is projected down onto a two-dimensional electron gas (2DEG) below [1][2][3][4][5][6][7]. This system can be fabricated by gating a GaAs/GaAlAs heterostructure 2DEG with a type-II superconducting film. An applied magnetic field is segregated within and near the superconductor into a distribution of magnetic flux tubes (vortices) with characteristic diameter d = 2X (^^0.1 jum and is the magnetic-field penetration depth). A point of fundamental interest here is the fact that the size of the magnetic flux tubes can be much smaller than transport relaxation lengths, in which case a vortex may be considered as a magnetic string, i.e., a flux tube of negligibly small cross-sectional area. Nonlocal weak localization [2,3] and electrodynamic coupling between a superconductor and the 2DEG [4] were recently reported.In this Letter we report ballistic electron transport through a random distribution of vortices. When one considers such a ballistic transport it is not clear a priori how to obtain transport coefficients from the rather complicated electron motion. Fortunately, the picture is strikingly simplified when account is taken of the fact that electrons are only influenced by the vortices along a small part of their paths and, hence, vortices can be considered as additional scatterers introduced into the 2DEG [5,6,8]. We wish to emphasize two fundamental features of these scatterers: their asymmetric and essentially quantum character. The asymmetry results from the vector action of the magnetic field while the quantum character is clearly highlighted by the equivalence between the quasiclassical angle of deflection of the electrons due to the Lorentz force, p = Xp/d (see below), and the characteristic angle of diffraction of a wave with wavelength X? at an obstacle of size d. In addition, scattering at the vector potential outside the magnetic-field region should be taken into account [6,7,9]. When all these features are considered a question which naturally arises is whether the system exhibits a Hall effect and, if so, what is its magnitude.The scattering efficiency at the flux tubes is a function of Xp/d [6,9] and we have tried to vary this parameter over a maximum range in our experiment. To this end, a set of GaAlojAsoj/GaAs het...