Frustrated interactions on the Centred and Breathing Pyrochlore Lattices The pyrochlore lattice offers a route to realizing unconventional states of matter from magnetic monopoles to a quantum spin liquid described by emergent quantum electrodynamics. I will discuss the properties of spin models on the experimentally-realizable centred pyrochlore lattice, where there is an additional site at the centre of every tetrahedron. First, in the classical Heisenberg model, the low energy properties are governed by the interplay of ferrimagnetic correlations on the one hand and a disordered spin liquid on the other, giving rise to a variety of ordered, partially ordered and spin liquid ground states. The correlations in the spin liquid can be understood in terms of an emergent gas of electrostatic charges. Adding dipolar interactions, we find that this state well reproduces the experimentally-observed magnetic properties of the metal-organic framework material [Mn(ta)2] at finite temperature. Second, I will discuss possibilities to arrive at an effective low energy description for the spin=1/2 quantum XXZ model. Third, for the breathing pyrochlore lattice, I will show how the synergy between humans and machine learning algorithms enabled us to understand the full structure of the q=W phase, which is the ordering phase of the classical rank-2 U(1) spin liquid.
In this seminar I will discuss two topics I studied during my PhD thesis. In the first part, I present an experiment dealing with domain wall expansion under a magnetic field in an ultra thin magnetic film. I observe a creep regime in the growth of the domain which I characterize and compare with theoretical predictions. I also briefly discuss how the findings of such experiment could impact other disordered systems out of equilibrium. In the second, I study the non linear flow curve of a yield stress fluids embedded in a porous medium and I introduce an exactly solvable model that allows to understand how the non-linearities in the flow emerge.
Atomistic simulations based on the first-principles of quantum mechanics can nowadays reach unprecedented length and time scales. Especially when dealing with weakly bonded systems, new methodologies that allow the treatment of electrons and nuclei as quantum particles become indispensable. I will give an overview of simulation methods that are applicable for large system sizes and that can capture the quantum nature of nuclei in anharmonic potential energy landscapes . I will discuss how they can be connected to first-principles electronic structure and machine-learning approaches . Applications where the quantum nature of the nuclei become indispensable to explain experimentally observable quantities like tunneling rate constants including dissipation .  M. Rossi, J. Chem. Phys. 154, 170902 (2021)  A. M. Lewis, A. Grisafi, M. Ceriotti, and M. Rossi, J. Chem. Theory Comput. 17, 7203-7214 (2021)  Y. Litman, et al., J. Chem. Phys. 156, 194106 (2022)
Ab initio molecular dynamics (AIMD) simulations with imaginary-time path integrals offer accuracy and flexibility required to address the complex structure and dynamics of various hydrogen-bonded systems including nuclear quantum effects. However, the need for ab initio energies and forces evaluated on the fly using advanced quantum electronic structure methods to drive such simulations limits the achievable system sizes and simulation times. As the new simulation state of the art, the computational requirements of AIMD simulations can be mitigated by replacing explicit quantum electronic structure with machine learning potentials (MLPs) that allow us to run efficient simulations of extensive systems while maintaining the accuracy of the underlying ab initio potential and thus addressing them at realistic conditions. In this talk, I will discuss our advancements in simulation methodology and its application in simulations of three hydrogen-bonding systems. The methodological part will revolve around finding efficient approaches to generate training sets for MLP generation using active learning strategies. The application part will initially focus on two closely related solutions in hydrogen-bonding solvents. First, motivated by the organic chemistry of solvated electrons, we will explore the behavior of the benzene radical anion in liquid ammonia, aiming to describe its electronic structure, stability, and geometry including its dynamic Jahn-Teller distortions. Removing the negative charge, we will then discuss path integral simulations of neutral benzene in liquid ammonia and liquid water, where we recognize that the so-called π-hydrogen bonding between the aromatic solute and the polar solvent represents a prominent solvation motive. The simulated trajectories will be used to answer questions about this exotic type of hydrogen bond's structure, dynamics, cooperativity, and vibrational spectroscopy. Understanding the nuances of π-hydrogen bonding is important for appropriate modeling of the solvation of aromatic species, particularly aromatic residues at biological interfaces. Finally, we will touch upon the realm of surface physics in order to discuss a recently studied hydrogen-bonded assembly of organic molecules that was proposed to exhibit fundamentally quantum-behaving proton-transfer reactivity at ultralow temperatures. We take a step towards explaining this behavior using path-integral instanton theory to find consistent conclusions.
My overarching research objective is to develop a comprehensive understanding of various emergent and dynamic phenomena at subcellular scale considering their minimal chemo- mechanistic representation and the fact that these systems are fundamentally out of thermodynamic equilibrium. In this spirit, I have been investigating subnuclear systems under active perturbation, cytoplasmic biomolecular condensates like stress granules, active cytoskeletal structures like stress fibers. In this seminar, I plan to focus on my computational and theoretical work on active subnuclear medium [1, 2], and if time permits, I may briefly discuss my other ongoing projects. Spatial organization of chromatin inside a eukaryotic cell nucleus and the spatiotemporal coordination of the chromatin with various subnuclear condensates (SNCs, e.g., transcription factories, paraspeckles) play crucial roles in genome regulation. However, a plethora of enzymes act inside the nucleus that actively perturb the medium. To investigate the effect of such active perturbation on the subnuclear medium, I developed a polymer physics (Brownian dynamics) framework and simulate it using two self-developed highly parallelized computation codes. The active perturbation has been implemented in a non-localized fashion that transiently link and pass two chromatic segments (mimicking the enzyme Topoisomerase-II) and thereby perturb the medium. Using this framework (copolymer setting), we first studied the phase separation of the euchromatic and the heterochromatic segments, where the activity was considered in the euchromatic segments only . Interestingly, we find characteristic phase separation morphologies, namely wall-like organization of euchromatin and negative nematic ordering of the euchromatin segments. A mean-field formalism based on a simplified equilibrium model-system can explain the physical mechanism underlying this phase separation, but could not explain the characteristic morphologies. The existence of these characteristic morphologies highlight the critical role of active perturbation in chromatin organization. Following a similar active polymer framework (homopolymer setting), where the chromatin is considered inside the nucleus together with a model- SNC, we studied the dynamics of the subnulear medium . Using numerical simulations and a versatile effective model, we show that the SNC-dynamics in such a complex medium can be described as a combination of three dynamical modes, which we have linked with the three different physical aspects of the imbedding medium. We find that, under the above-mentioned active perturbation, a slow mode associated with remodeling of chromatic meshes enhances SNC- dynamics. This may provide an insight into the role of global active perturbation on regulating the target-searching process of a SNC in such a complex medium.