Colloquium Calendar

Cloudy and Hazy Worlds in the Era of JWST

Abstract:

Aerosols are everywhere. I will discuss two avenues where our understanding of such photochemical hazes and condensation clouds has advanced for exoplanetary atmospheres. Both kinds of aerosols fundamentally shape the atmospheric chemistry of a variety of exoplanets, with subsequent impacts on observations from Hubble to JWST to upcoming missions. First, I will present results on the properties of photochemical haze particles produced from laboratory studies and the ways we may begin untangling these properties with JWST’s instrumentation for the most promising planetary targets. Second, I will focus on updates to our understanding of exoplanet clouds. Clouds made of silicate materials are thought to be the dominant cloud species that affects our interpretations of hot Jupiters, but the underlying laboratory data typically used for such interpretation does not fully capture the complexity of these materials. I will discuss my recent efforts to properly account for this complexity by considering mineral polymorphs and non-spherical cloud particle models. Properly accounting for the full chemical and physical complexity of both condensate and photochemical aerosol particles in exoplanet atmospheres will let us use them as atmospheric tracers of planetary conditions.

Hidden Engines: Uncovering the Workings of the Nearest Galaxy Centers

Abstract:

Centers of galaxies are some of the most extreme objects in our universe: hosting starbursts and active supermassive black holes that can launch jets and winds far outside the compact galaxy nucleus. The effects of the unique interactions between stars, gas, and black holes that occur here don’t just stay confined to these small regions: they have an outsized influence on the overall evolution of galaxies as a whole. At just 8.1 kpc away, the center of the Milky Way is unparalleled in its proximity, making it the best laboratory for detailed studies of the processes that govern and define galaxy nuclei. However, the Galactic center also presents a big challenge for these studies: it is a relatively quiet environment. Few stars are forming in this region, and the black hole is not active. Clearly, it hasn’t always been this way: from the Fermi Bubbles to hundred-year old echoes of X-ray bursts there are many relics of an active past in the center of our own Milky Way. We also know our Galaxy center likely won’t stay quiet for long: it contains a sizable reservoir of molecular gas that is the fuel for future star formation and black hole accretion. In this talk I will present the results of research following the gas and its properties from kiloparsec to sub-parsec scales to understand why the Galactic center is so quiet right now and what the future holds. Finally, I will discuss ongoing work to increase the sample size of galaxy nuclei with parsec-scale gas measurements, and what this means for putting the Galactic center in context with its more active neighbors. 

Title

Abstract:

Controlling Exciton Dynamics at the Nanoscale

Abstract:

Two of our recent works on controlling exciton dynamics at the nanoscale will be presented. In the first part of the talk, I will discuss our recent efforts in using organic molecule/2D heterostructures to build nanoscale periodic potentials akin to the moiré patterns found in 2D heterostructures but have a much shorter periodicity and a tunable lattice symmetry. These heterostructures can host a higher density of long-lived excitons, which would favor the formation of exotic phases such as high temperature Bose-Einstein condensates. In the second part of the talk, I will present our recent measurements on a class of organic semiconductors known as non-fullerene acceptors (NFAs), which have been used recently in high performance organic photovoltaics. I will discuss how electron delocalization can be constrained at the nanoscale by the local morphology, which can in turn promote entropy-driven exciton dissociation. This mechanism allows excitons to acquire energy from the environment, which facilitates the enthalpy-uphill photon-to-free carrier conversion process.

Atomically resolved studies of unconventional quantum phases in 2D materials and heterostructures

Abstract:

The search for novel quantum phases in 2D materials is rapidly expanding: It is driven by the interest in robust quantum anomalous Hall insulators, topological superconductivity, correlated electronic states, and fractional statistics and by the prospect of quantum simulation in solid state. Unconventional, inherently quantum behavior has been observed in layered and twisted graphene heterostructures, multilayered homo- and heterobilayer transition metal dichalcogenides (TMDs), in surface and quasi-2D layers in 3D materials, and nanopatterned devices. To progress further, the field relies on tunable systems to study phase transitions and on atomic resolution to correlate the phases with local physical and electronic properties.  In this colloquium, I will showcase recent developments in the field of tunable 2D platforms, highlighting twisted moiré systems, topological 2D materials, and atomic manipulation. Scanning tunneling microscopy (STM) has proved crucial for untangling competing quantum phases and deeply understanding the foundational elements driving their physics. Through high-resolution magnetic-field scanning tunneling spectroscopy, we have demonstrated the importance of the fine details of quantum geometry in these novel 2D platforms. Specifically, I will report on our discovery of the emergent anomalously large orbital magnetic susceptibility in twisted double bilayer graphene, along with the orbital magnetic moment. I will also discuss the potential in the field of quantum materials of combining STM, molecular beam epitaxy (MBE), and stacked 2D devices. As an example, I will present STM data on a back-gateable MBE-grown thin film of the quantum spin Hall insulator WTe2.

A new twist on graphite: Band structure engineering and topological electron crystals in graphene moiré systems

Abstract:

Strongly correlated and topological phases in condensed matter systems are at the cutting edge of fundamental physics studies, as well as being promising candidates for the next generation of technological capabilities like quantum computing. In recent years, a remarkable amount of progress has been made in creating and controlling such phases by introducing a small twist angle or lattice mismatch between two dimensional (2D) materials. These systems, called moiré systems, have facilitated the surprising discovery of strongly correlated phases where one might not expect them (e.g. superconductivity in “magic-angle” twisted bilayer graphene) or long-sought new physics (e.g. the fractional quantum anomalous Hall effect (FQAHE) in twisted MoTe2). However, much of the work in this rapidly developing field have focused on the case where the constituent 2D materials of the moiré system are monolayers, or at most bilayers. I will show that this restriction to one or two atomic layers is unnecessarily limiting. Surprising new phenomenology can be realized in graphitic moiré systems, where at least one component is three-layers or more. Most notably, we find that a new type of electron crystallization can occur that spontaneously breaks the moiré translational symmetry and has dissipationless edge modes, analogous to a topological version of a Wigner crystal. Our results suggest that these topological electron crystals 1) are at least somewhat common across multilayer graphene moiré systems, 2) can have uniquely tunable properties, and 3) closely compete with the newly discovered FQAHE. Understanding this competition, as well as the novel phenomenology of the topological electron crystal phase, will be of fundamental interest in future studies of strongly correlated topological systems.

New Phenomena Around Supermassive Black Holes: Dynamics and Chemistry

Abstract:

In 1982 Blandford and Payne predicted that magnetic fields are fundamental for accretion onto supermassive black holes (SMBHs). Magnetic field lines anchored in the disk accelerate a wind via the centrifugal force, allowing for the angular momentum to be transferred out of the system and gas to accrete onto the central compact object generating an active galactic nucleus (AGN). The wind can form a few Schwarzschild radii from the SMBH up to the nuclear torus. Almost a half century later, the detailed mechanisms of SMBH growth are still a passionate area of research. Astronomers currently debate whether winds are fuelled by jets, mechanical winds, or radiation, with magnetic processes being the least accepted explanation. Here, I present detailed ALMA observations of the most compact and opaque galactic nuclei in the universe, appropriately named compact obscure nuclei (CONs). CONs represent a significant phase of galactic nuclear growth, with opaque and compact centers (r <100 pc), that conceal growing SMBHs. The analysis of these observations reveal a wind that exceeds theoretical maximum momentum for an AGN feedback powered galactic wind. The extreme momentum implies the existence of a magneto hydrodynamic (MHD) wind. The wind is highly collimated and rotates out to 100pc above the galactic nucleus. Furthermore, abundances of complex organic molecules rival SGR b2 and Galactic hot cores in the nucleus. These results imply that growth of SMBHs is very similar to the growth of hot cores or protostars, and feedback from an AGN is not necessary to drive a galactic wind.

Radiation Resilience: Unveiling the Secrets of Halide Perovskites Solar Cells for Space

Abstract:

Halide perovskites have been demonstrated to be excellent semiconductors for optoelectronic devices. The photoelectric conversion efficiency of perovskite solar cells is now over 25%, close to that of commercial applications. Yet, there are many of the challenges experienced by halide perovskite materials, such as degradation caused by exposure to oxygen or water. These issues might not apply in space environments. However, there, the solar cell must survive radiation exposure. I will present our ab initio molecular dynamics (AIMD) simulations for different halide perovskites to investigate their response to low-energy radiation. The simulations help estimate non-ionizing radiation damage in materials, the primary degrader of optoelectronic properties under radiation environments. These efforts would allow for a better understanding of the radiation hardness of materials

The Brave Nu World

Abstract:

The discovery of nonzero neutrino masses shook the particle physics world at the turn of the 21st century and remains the only concrete evidence that there is something missing from the standard model of particle physics. I provide an overview of the current status of neutrino physics, including some of the open questions and the many avenues are pursuing to answer them.

Learning to shine: Understanding and controlling optomechanical coupling and nonlinear phononics effects in complex materials

Abstract: 

Materials under laser illumination heat up. More specifically, they heat up *eventually*: a few tens of picoseconds after being illuminated, some materials are nowhere near a thermal state, and, instead, display coherent oscillations in the gigahertz to terahertz frequency range.  In recent years, the control of these light-induced states has been a focal point of a field sometimes referred to as "non-linear phononics", with  the outstanding challenge of the field to discover specific illumination protocols (e.g.. a specific sequence of light pulses) that stabilize non-equilibrium states of matter otherwise unstable in thermodynamics equilibrium. 

In this talk, I will present our recent progress in modeling, understanding, and controlling the atomic-scale, non-thermal response of materials to visible light.  Using first-principles methods as an approximation to the time-dependent Schrödinger equation, I will show how light can result in non-thermal excitations of the atomic lattice in simple materials [1], and how simple classical models of this response based on generalized Langevin equation can capture the light-induced dynamics [2]. I will then show how non-thermal effects become prominent in more complex materials, such as broken-symmetry materials such as Charge Density Wave and Peierls materials [3,4], and derive a theory of the coupling of light with structural order parameter in broken symmetry materials through impulsive Raman scattering, an effect confirmed experimentally. 

I will conclude showing how this effect can be deterministically controlled [4] to engineer states of matter with large non-linear optical susceptibility using reinforcement learning [5,6].

Relevant references:

[1] Phys. Rev. Lett. 119, 136602 (2017) ; [2] Nature Photonics volume 13, pages 425–430 (2019) ; [3] arXiv:2105.08874  ; [4] Phys. Rev. Materials 8, 015202 (2024) ; [5] Nature Communications volume 13, Article number: 3251 (2022)] [6] in preparation

Title

Abstract:

Title

Abstract:

Title

Abstract: