Low dimensional materials constitute an exciting and unusually tunable platform for investigation of correlated states. Here I will present our results on transport measurements of high quality few-layer graphene and black phosphorus devices. In Bernal-stack trilayer grapehne, we observe tunable integer and fractional quantum Hall states, and quantum parity effect at the charge neutrality point. In tetralayer graphene, we have observed a large intrinsic gap at half filling, up to 80 meV, that arises from electronic interactions in rhombohedral stacking, and multiple Lifshitz transitions in Bernal stacking. Lastly, I will discuss our recent observation of robust long distance spin transport through the antiferromagnetic state in graphene.

The advent of nanofabrication has opened new venues for controlling vortex matter, which is responsible for the electro-magnetic response of all applied superconductors. In particular, nano-hole structures with a variety of intriguing patterns have emerged as a versatile platform for controlling and optimizing vortex pinning in superconductors for enhanced critical current. Magnetic field pinning of vortices with meso and nanoscale magnetic structures has also shown great potential for in-situ manipulation of vortex behavior. Here, I will briefly review the vortex response to a variety of nanostructured hole-arrays in superconductors and in particular, demonstrate that a random pattern, an often-overlooked vortex pinning system, can lead to a significant critical current enhancement over a wide magnetic field range. I will also demonstrate the use of ferromagnetic strips on a superconductor to mimic a vortex triode device and lastly, introduce a novel nano-magnetic patterned structure based on artificial spin-ice rules to realize a globally reconfigurable and locally writable magnetic structure that can subsequently be used to control single flux quanta in a superconducting film. The novel ferromagnetic/superconducting hetero-structure enables switchable and reversible rectification effect of the critical current and furthermore, enables the experimental study of geometric frustration in a flux quanta system.

This work was supported by the Department of Energy, Office of Basic Energy Sciences which also funds Argonne’s Center for Nanoscale Materials (CNM) where the nano-and magnetic patterning and morphological analysis were performed.

Host: DeLong

All fields have golden ages.  One which is now underway is to create a "Quark-Gluon Plasma" by the collisions of heavy ions at very high energies. This is interesting not just for the intrinsic beauty of the subject, but because of the sociology of the field: a billion dollar machine constructed because it could be, and where experiments with hundreds of physicists delivered a vast quantity of beautiful results.

I begin by reviewing modern field theories: first Abelian (photons, which comprise light), then non-Abelian (the modern theory of neutrons and protons).  In both cases, theories which can be written down in one line yield amazing complexity.



The field is underpinned by the ability to do numerical simulations of the fundamental theory in thermal equilibrium, which I summarize.  I also discuss theoretical understanding, including computations in exactly soluble models (AdS/CFT).



I then turn to experiment, and describe the basic evidence that at the heavy ion colliders at Brookhaven and at CERN, there is a qualitatively new phase created, a Quark-Gluon Plasma.

In 1937, Ettore Majorana introduced the concept of what are now fittingly called Majorana fermions -- fermionic particles that are their own antiparticles. Nowadays an active search for condensed-matter analogues of these elusive objects is well underway, motivated by both the prospect of revealing new facets of quantum mechanics and longer-term quantum computing applications. This talk will survey recent advances in this pursuit. In particular, I will describe strategies for "engineering" Majorana platforms from simple building blocks, preliminary experimental successes, and future milestones that reveal foundational aspects of Majorana physics directly relevant for quantum computation. 

Recent developments in the study of quantum gravity have revealed a surprising and beautiful connection between quantum entanglement and the geometry of spacetime. This discovery offers a new perspective on old puzzles concerning black holes, and may lead to a profoundly new way of thinking about the emergence of spacetime from fundamental quantum-mechanical building blocks. I will describe these developments, explaining along the way the necessary background in quantum gravity and quantum information theory.

Host: Pranjal Nayak

In science, new advances and insights often emerge from the confluence of different ideas coming from what appeared to be disconnected research areas. The theme of my colloquium will review an ongoing collision between the three topics listed in my title which has been generating interesting new insights about the nature of quantum gravity, as well as variety of other fields, eg, condensed matter physics and quantum field theory.

In addition to carrying a quantized electrical charge, electrons also possess a quantized angular momentum, or spin.  In ordinary charge-based electronics, the spins are usually randomly oriented and they do not play a role in device function.  However, in the last ten years there have been significant advances in understanding how to control electron spins in useful ways.  Spin currents can now be used, for example, to apply torques within magnetic memory devices that are more than 1000 times stronger than charge-current-generated magnetic fields.  I will discuss some of this progress, with an emphasis on recent discoveries that magnetic devices can be manipulated with record-breaking efficiencies using strong spin-orbit coupling in heavy metals and topological insulators.

Spectroscopic investigations are essential to understand intriguing phenomena in many areas of physics.  In particular, recent advanced spectroscopic tools such as resonant inelastic x-ray scattering and photoemission spectroscopy play an important role in condensed matter physics.  While research on thin-films or superlattices is expected to reveal hidden novel physical properties, previous inelastic spectroscopic investigations have been unable to detect these properties due to the small cross-section.

In this colloquium, I will discuss advanced inelastic light scattering (Raman spectroscopic) studies of two-magnon dynamics in Sr₂IrO₄ thin-films as functions of strain and temperature.  Recent experimental studies of Sr₂IrO₄ thin-films have contradicted the widely-accepted Glazer description of octahedral tilting and rotation in perovskite oxides.  However, our experimental observations and density functional theory calculations show that the multi-orbital nature of the J_eff = ½ state is crucial to understanding the magnetic and electronic properties of 5d transition-metal oxides.  This study demonstrates how advanced spectroscopic tools improve our fundamental understanding of strongly correlated, spin-orbit coupled electrons, enabling us to explore the novel phase diagrams of these systems.

Dualities are a powerful concept in quantum field theory, helping us to identify the correct low energy degrees of freedom. In this talk several old and many new dualities in 2+1 dimensions will be shown to all follow from one conjectural base pair, with potential applications to the physics of the quantum Hall effect.  

Host: Sumit Das