One of the intriguing characteristics of honeycomb lattices is the appearance of a pseudomagnetic field as a result of mechanical deformation. In the case of graphene, the Landau quantization resulting from this pseudomagnetic field has been measured using scanning tunneling microscopy. Here we show that a signature of the pseudomagnetic field is a local sublattice symmetry breaking observable as a redistribution of the local density of states. This can be interpreted as a polarization of graphene's pseudospin due to a strain induced pseudomagnetic field, in analogy to the alignment of a real spin in a magnetic field. We reveal this sublattice symmetry breaking by tunably straining graphene using the tip of a scanning tunneling microscope. The tip locally lifts the graphene membrane from a SiO support, as visible by an increased slope of the I(z) curves. The amount of lifting is consistent with molecular dynamics calculations, which reveal a deformed graphene area under the tip in the shape of a Gaussian. The pseudomagnetic field induced by the deformation becomes visible as a sublattice symmetry breaking which scales with the lifting height of the strained deformation and therefore with the pseudomagnetic field strength. Its magnitude is quantitatively reproduced by analytic and tight-binding models, revealing fields of 1000 T. These results might be the starting point for an effective THz valley filter, as a basic element of valleytronics.

We calculate the local density of states (LDOS) for an infinite graphene sheet with a single centro-symmetric out-of-plane deformation, in order to investigate measurable strain signatures on graphene. We focus on the regime of small deformations and show that the strain-induced pseudomagnetic field induces an imbalance of the LDOS between the two triangular graphene sublattices in the region of the deformation. Real space imaging reveals a characteristic six-fold symmetry pattern where the sublattice symmetry is broken within each fold, consistent with experimental and tight-binding observations. The open geometry we study allows us to make use of the usual continuum model of graphene and to obtain results independent of boundary conditions. We provide an analytic perturbative expression for the contrast between the LDOS of each sub-lattice, showing a scaling law as a function of the amplitude and width of the deformation. We confirm our results by a numerically exact iterative scattering matrix method.

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Ultrashort, coherent x-ray pulses of a free-electron laser are used to holographically image the magnetization dynamics within a magnetic domain pattern after creation of a localized excitation via an optical standing wave. We observe a spatially confined reduction of the magnetization within a couple of hundred femtoseconds followed by its slower recovery. Additionally, the experimental results show evidence of a spatial evolution of magnetization, which we attribute to ultrafast transport of nonequilibrium spin-polarized electrons for early times and to a fluence-dependent remagnetization rate for later times. DOI: 10.1103/PhysRevLett.112.217203 PACS numbers: 75.78.Jp, 42.40.Kw, 75.25.−j, 78.70.Ck Progress in the field of light-induced, ultrafast manipulation of magnetic order has recently led to all-optical, ultrafast magnetic switching [1][2][3] and to an increased control of its dynamics by designing tailored nanostructured samples [4][5][6][7] as well as by exploiting nanoscale magnetic inhomogeneities [8,9]. The influence of interfaces between different materials and magnetic domain boundaries has cast doubt on our theoretical understanding of the underlying fundamental mechanism responsible for femtosecond magnetization dynamics. The model explaining the ultrafast loss of magnetic order after optical excitation by (e.g., electron-phonon or impurity-mediated) spin-flip scattering events [10] has in part been challenged by an approach based on nonlocal superdiffusive spin transport [11]. In spite of their very different microscopic origins, both have been successful in explaining a wide range of experimental data, suggesting that both mechanisms play an important role and that their respective magnitudes depend on the specific experimental conditions [7]. More specifically, in the case of superdiffusive spin transport, energy-and spin-dependent electron lifetimes and velocities induce spin-polarized currents, leading to significant ultrafast spatial rearrangement of magnetic order.To gain control of magnetization dynamics and all-optical switching in the lateral dimension, one relies on nanometer localization of the optical excitation, as well as detailed knowledge on how (spin-polarized) electron currents lead to a spatial transfer of magnetization. Technologically this plays an important role not only for all-optical approaches, but also for heat-assisted magnetic recording, which has the potential to increase the magnetic recording density by lowering the coercitivity of high-anisotropy materials [12]. Necessary to this end, it is required to deliver the optical energy to a sub-100-nm spot size, i.e., far beyond the diffraction limit of optical light. The most successful approaches include localization of the evanescent light from near-field optical probes [13], using metallic plates to excite surface plasmons [14,15] or a combination thereof [16].Here, we implement time-resolved Fourier transform holography (FTH) [17] and exploit x-ray magnetic circular dichroism (XMCD) to directly image the magnet...

We develop a theoretical description of interaction-induced phenomena in an electronic MachZehnder interferometer formed by integer quantum Hall edge states (with ν = 1 and 2 channels) out of equilibrium. Using the non-equilibrium functional bosonization framework, we derive an effective action which contains all the physics of the problem. We apply the theory to the model of a shortrange interaction and to a more realistic case of long-range Coulomb interaction. The theory takes into account interaction-induced effects of dispersion of plasmons, charging, and decoherence. In the case of long-range interaction we find a good agreement between our theoretical results for the visibility of Aharonov-Bohm oscillations and experimental data.