Welcome to the Ashoori Group

Published May 16th in Science Express: our new paper on the behavior of Dirac electrons in the presence of a moire superlattice potential

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

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.

This MIT News article describes it in detail.

"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 LaAlO₃/SrTiO₃ 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

"Very Large Capacitance Enhancement in a Two-Dimensional Electron System", Science 323, 825-828 (13 May 2011)

Increases in the gate capacitance of field-effect transistor structures allow the production of lower-power devices that are compatible with higher clock rates, driving the race for developing high-κ dielectrics. However, many-body effects in an electronic system can also enhance capacitance. Onto the electron system that forms at the LaAlO₃/SrTiO₃ interface, we fabricated top-gate electrodes that can fully deplete the interface of all mobile electrons. Near depletion, we found a greater than 40% enhancement of the gate capacitance. Using an electric-field penetration measurement method, we show that this capacitance originates from a negative compressibility of the interface electron system. Capacitance enhancement exists at room temperature and arises at low electron densities, in which disorder is strong and the in-plane conductance is much smaller than the quantum conductance.

See also:  MITnews, May 13th 2011

"Anomalous structure in the single particle spectrum of the fractional quantum Hall effect", Nature 464, 566-570 (25 March 2010).

The single particle spectrum of a system reveals the energies of its quasiparticles; charged, long lived excitations. We measure the single particle spectrum of a 2D electron gas in the fractional quantum Hall regime. Despite the intensive study this system has received in the past, the structure of this spectrum is completely unexpected; the features we previously observed in the quantum Hall effect break up into a series of sharp features that disperse linearly in energy with density.

For an overview of the technique, you should look at our recent work, or Oliver's PhD thesis.

A new wafer with a wider quantum well than we've used previously has begun providing fantastic new data. Single atom fluctuations in the width of the quantum well broaden TDCS features in energy. With the wider quantum well, these width variations are less important. The new spectra reveal quantum Hall physics in astounding detail, as shown in the data taken at 1 Tesla above (compare with that in our recent Nature paper. At higher fields, we are beginning to understand how the fractional quantum Hall effect reveals itself in the single particle spectrum, while at lower fields we're pinning down the properties of a previously unobserved coupled electron-plasmon excitation.

Our recent TDCS work (below) has been well received. Charles Day, with Physics Today, produced an excellent writeup in "Search and Discovery". This made the cover of their Japanese affiliate Parity (as shown to the right). Anne Trafton at MIT's press office also produced an extraordinarily understandable summary, including photos of the new dilution refrigerator we're moving the experiment into. A shorter version of this appeared in MIT's Tech Talk.

"High-resolution spectroscopy of two-dimensional electron systems", Nature 448, 176-179 (12 July 2007).

By probing the single-particle spectrum of a two-dimensional electron gas, we reveal the intricate and beautiful energy structure far from the Fermi surface. The measured spectra show a host of different physical phenomena, making them a fantastic resource for understanding these complex systems.

For an overview, you should look at this press release. For more information, have a look at our capacitance spectroscopy page, Oliver's simulations page, or his home page

"Imaging Transport Resonances in the Quantum Hall Effect", Physical Review Letters 95 136804 (2005)

People have long understood that localization plays a fundamental role in the quantum Hall effect. However, the microscopic nature of the localized states is inaccessible to bulk transport measurments: what these localized states actually are is still a mystery. In our work, we use a charge-sensitive scanning probe to measure the transport properties of localized states in the quantum Hall effect on a microscopic scale.

To learn more, have a look at our SPM Imaging page, or Gary Steele's thesis.

Gary's thesis is quite large; if you have a slow connection, you might look at this version with compressed images.