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]]>Measurement of the ground-state spin polarization of quantum systems offers great potential for the discovery and characterization of correlated electronic states. However, spin polarization measurements have mainly involved optical and NMR techniques that perturb the delicate ground states and, for quantum Hall systems, have provided conflicting results. Here we present spin-resolved pulsed tunnelling (SRPT) that precisely determines the phase diagram of the ground-state spin polarization as a function of magnetic field and Landau level (LL) filling factor (*ν*) with negligible perturbation to the system. Our phase diagram shows a variety of polarized, unpolarized and topological spin states in the lowest (*N* = 0) LL, which can largely be described by a weakly interacting composite fermion (CF) model. However, the phase diagram shows unexpected behaviour in the *N* = 1 LL. We observe fully polarized *ν* = 5/2 and 8/3 states but a partially depolarized *ν* = 7/3 state. This behaviour deviates from the conventional theoretical picture of weakly interacting fractional quasiparticles, but instead suggests unusual electronic correlations and the possibility of new non-Abelian phases. The results establish SRPT as a powerful technique for investigating correlated electron phenomena.

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The single-particle spectral function measures the density of electronic states in a material a function of both momentum and energy, providing central insights into strongly correlated electron phenomena. Here we demonstrate a high-resolution method for measuring the full momentum-and energy-resolved electronic spectral function of a two-dimensional (2D) electronic system embedded in a semiconductor. The technique remains operational in the presence of large externally applied magnetic fields and functions even for electronic systems with zero electrical conductivity or with zero electron density. Using the technique on a prototypical 2D system, a GaAs quantum well, we uncover signatures of many-body effects involving electron-phonon interactions, plasmons, polarons, and a phonon analog of the vacuum Rabi splitting in atomic systems.

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Published in Nature Physics, advance online publication DOI:10.1038/nphys3979

]]>http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12800.html

(also available on the arXiv here)

Low-dimensional electronic systems have traditionally been obtained by electrostatically confining electrons, either in heterostructures or in intrinsically nanoscale materials such as single molecules, nanowires, and graphene. Recently, a new paradigm has emerged with the advent of symmetry-protected surface states on the boundary of topological insulators, enabling the creation of electronic systems with novel properties. For example, time reversal symmetry (TRS) endows the massless charge carriers on the surface of a three-dimensional topological insulator with helicity, locking the orientation of their spin relative to their momentum. Weakly breaking this symmetry generates a gap on the surface, resulting in charge carriers with finite effective mass and exotic spin textures. Analogous manipulations of the one-dimensional boundary states of a two-dimensional topological insulator are also possible, but have yet to be observed in the leading candidate materials. Here, we demonstrate experimentally that charge neutral monolayer graphene displays a new type of quantum spin Hall (QSH) effect, previously thought to exist only in TRS topological insulators, when it is subjected to a very large magnetic field angled with respect to the graphene plane. Unlike in the TRS case, the QSH presented here is protected by a spin-rotation symmetry that emerges as electron spins in a half-filled Landau level are polarized by the large in-plane magnetic field. The properties of the resulting helical edge states can be modulated by balancing the applied field against an intrinsic antiferromagnetic instability, which tends to spontaneously break the spin-rotation symmetry. In the resulting canted antiferromagnetic (CAF) state, we observe transport signatures of gapped edge states, which constitute a new kind of one-dimensional electronic system with tunable band gap and associated spin-texture.

]]>**Published June 21st in Science**: our new paper on the behavior of graphene Dirac electrons in the presence of a moiré superlattice potential.

http://www.sciencemag.org/content/early/2013/05/15/science.1237240

See this nice perspective in the same issue from Michael Fuhrer.

Also, see MIT News for a nice description of the work.

Van der Waals heterostructures comprise a new class of artificial materials formed by stacking atomically-thin planar crystals. Here, we demonstrate band structure engineering of a van der Waals heterostructure composed of a monolayer graphene flake coupled to a rotationally-aligned hexagonal boron nitride substrate. The spatially-varying interlayer atomic registry results both in a local breaking of the carbon sublattice symmetry and a long-range moire' superlattice potential in the graphene. This interplay between short- and long-wavelength effects results in a band structure described by isolated superlattice minibands and an unexpectedly large band gap at charge neutrality, both of which can be tuned by varying the interlayer alignment. Magnetocapacitance measurements reveal previously unobserved fractional quantum Hall states reflecting the massive Dirac dispersion that results from broken sublattice symmetry. At ultra-high fields, integer conductance plateaus are observed at non-integer filling factors due to the emergence of the Hofstadter butterfly in a symmetry-broken Landau level.

]]>"Observations of plasmarons in a two-dimensional system: Tunneling measurements using time-domain capacitance spectroscopy", *Phys. Rev. B* **85**, 081306(R) (2012)

Calculations of the single-particle density of states (SPDOS) of electron liquids have long predicted that there exist two distinct charged excitations: the usual quasiparticle consisting of an electron or hole, and a plasmaron consisting of a hole resonantly bound to real plasmons in the Fermi sea. Using tunneling spectroscopy to measure the SPDOS of a 2D electronic system, we demonstrate the detection of a plasmaron in a 2D system in which electrons have mass. With the application of a magnetic field we discover unpredicted magnetoplasmarons which resemble Landau levels with a negative index.

Two bright features (they look like green diagonal lines) appear in the figure on the right. The upper feature corresponds to the band-edge of a 2D electron system. The lower feature exists below the band-edge, and this feature is the long elusive "plasmaron".

]]>"Coexistence of magnetic order and two-dimensional superconductivity at LaAlO_{3}/SrTiO_{3} interfaces", *Nature Physics* 7, 762–766 (04 September 2011).

Combining high-resolution magnetic torque magnetometry and transport measurements, we report here magnetization measurements providing direct evidence of magnetic ordering of the two-dimensional electron liquid at the LAO/STO interface. The magnetic ordering exists from well below the superconducting transition to up to 200 K, and is characterized by an in-plane magnetic moment. Surprisingly, despite the presence of this magnetic ordering, the interface superconducts below 120 mK. This is unusual because conventional superconductivity rarely exists in magnetically ordered metals. Our results suggest that there is either phase separation or coexistence between magnetic and superconducting states. The coexistence scenario would point to an unconventional superconducting phase as the ground state.

See also:

Nature Physics *News and Views* by A. J. Millis