Fractional quantum Hall effect measures at zero g factor

Fractional quantum Hall effect energy gaps have been measured as a function of Zeeman energy. The gap at ν = 1/3 decreases as the g factor is reduced by hydrostatic pressure. This behavior is similar to that at ν = 1 and shows that the excitations are spinlike. At small Zeeman energy, the excitation is consistent with the reversal of 3 spins and may be interpreted as a small composite Skyrmion. At 20 kbar, where g has changed sign, the 1/3 gap appears to increase again

Non equilibrium anisotropic excitons in atomically thin ReS2

We present a systematic investigation of the electronic properties of bulk and few layer ReS2 van der Waals crystals using low temperature optical spectroscopy. Weak photoluminescence emission is observed from two non-degenerate band edge excitonic transitions separated by similar to 20 meV. The comparable emission intensity of both excitonic transitions is incompatible with a fully thermalized (Boltzmann) distribution of excitons, indicating the hot nature of the emission. While DFT calculations predict bilayer ReS2 to have a direct fundamental band gap, our optical data suggests that the fundamental gap is indirect in all cases

Phase-Transition-Induced Carrier Mass Enhancement in 2D Ruddlesden-Popper Perovskites

There is a variety of possible ways to tune the optical properties of 2D perovskites, though the mutual dependence between different tuning parameters hinders our fundamental understanding of their properties. In this work we attempt to address this issue for (C$_n$H$_{2n+1}$NH$_3$)$_2$PbI$_4$ (with n=4,6,8,10,12) using optical spectroscopy in high magnetic fields up to 67T. Our experimental results, supported by DFT calculations, clearly demonstrate that the exciton reduced mass increases by around 30% in the low temperature phase. This is reflected by a 2-3 fold decrease of the diamagnetic coefficient. Our studies show that the effective mass, which is an essential parameter for optoelectronic device operation, can be tuned by the variation of organic spacers and/or moderate cooling achievable using Peltier coolers. Moreover, we show that the complex absorption features visible in absorption/transmission spectra track each other in magnetic field providing strong evidence for the phonon related nature of the observed side bands.S.D.S acknowledges the Royal Society and Tata Group (UF150033). The work was supported by a Royal Society International Exchanges Cost Share award (IEC\R2\170108). The authors thank EPSRC for funding through grant EP/M05143/

Symmetry Breakdown in Franckeite: Spontaneous Strain, Rippling, and Interlayer Moire

Franckeite is a naturally occurring layered mineral with a structure composed of alternating stacks of SnS2-like and PbS-like layers. Although this superlattice is composed of a sequence of isotropic two-dimensional layers, it exhibits a spontaneous rippling that makes the material structurally anisotropic. We demonstrate that this rippling comes hand in hand with an inhomogeneous in-plane strain profile and anisotropic electrical, vibrational, and optical properties. We argue that this symmetry breakdown results from a spatial modulation of the van der Waals interaction between layers due to the SnS2-like and PbS-like lattices incommensurability

Spin and valley quantum Hall ferromagnetism in graphene

In a graphene Landau level (LL), strong Coulomb interactions and the fourfold spin/valley degeneracy lead to an approximate SU(4) isospin symmetry. At partial filling, exchange interactions can spontaneously break this symmetry, manifesting as additional integer quantum Hall plateaus outside the normal sequence. Here we report the observation of a large number of these quantum Hall isospin ferromagnetic (QHIFM) states, which we classify according to their real spin structure using temperature-dependent tilted field magnetotransport. The large measured activation gaps confirm the Coulomb origin of the broken symmetry states, but the order is strongly dependent on LL index. In the high energy LLs, the Zeeman effect is the dominant aligning field, leading to real spin ferromagnets with Skyrmionic excitations at half filling, whereas in the `relativistic' zero energy LL, lattice scale anisotropies drive the system to a spin unpolarized state, likely a charge- or spin-density wave.Comment: Supplementary information available at http://pico.phys.columbia.ed

Quantum Hall and insulating states of a 2-D electron-hole system

We review recent results from magneto-transport studies of an InAs/GaSb based electron-hole system. In high magnetic field, the system shows two types of overall behaviour depending on whether the effective occupancy nu(c) - nu(h) = nu(eff) is zero or finite. When nu(e) - nu(h) is finite the resistivity has a metallic temperature dependence and the Hall resistance is quantized at a finite value. When nu(e) - nu(h) = 0 however, the resistivity shows an insulating temperature dependence. Since the condition nu(e) - nu(h) = nu(eff) for a given value of nu(eff) can be achieved with different values of the individual occupancies, the system can oscillate between insulating and metallic behaviour with magnetic field.In the insulating states where nu(e) - nu(h) - 0, the system exhibits novel behaviour, where the Hall resistance becomes symmetric under field reversal, and fluctuates reproducibly with magnetic field. Geometry dependence measurements show that in this state the sample interior becomes extremely insulating, and conduction is dominated by the mesa edge. The transport is one dimensional, and the chirality usually associated with the Hall effect is lost.Current driven breakdown is studied for quantum Hall states where nu(e) - nu(h) is finite. There is a strong dependence of the breakdown behaviour on channel width and the relative electron and hole densities. This suggests a strong involvement of the sample interior in stark contrast to the insulating states. (C) 2003 Published by Elsevier B.V