Thursdays 4:00 p.m. 104 Physics or online via Zoom.
Contact colloquium organizer Dr. Hyunsoo Kim at email@example.com for the Zoom link.
(Link to main colloquium page)
A day without yesterday: Measuring the primordial Universe
Of all the questions humanity ever pondered, perhaps the most profound is, "Where did all of this come from?". Modern cosmology tries to answer this question by examining and measuring the Universe itself. The dawn of the 21st century has brought with it a plethora of observational data that spans the entire history of the Universe, increasing exponentially our understanding of the cosmos and in particular its initial phases. The Universe started with a Big Bang, some finite time ago, giving birth to space and time and everything we have ever observed today. One of the biggest challenges within this framework was to explain the relation between the observable Large Scale Structures, namely galaxies, galaxy clusters and superclusters, with the interactions that took place in the initial phases of the Universe. The zoology of inflationary models is large and until now observations have not managed to constrain it. The best way to understand the origin of inflation, and thus the Universe, is by measuring the statistical distribution of the primordial fluctuations and their deviation from randomness. In this colloquium, I am going to show how much information, on the primordial Universe, is located within the statistics of halos by implementing a new, simulation-based, methodology.
Cosmological probe combination for current and future surveys
Recent progress in observational cosmology and the establishment of ΛCDM have relied on the combination of different cosmological probes. These probes are not independent, since they all measure the same physical fields. The resulting cross-correlations allow for a robust test of the cosmological model through the consistency of different physical tracers and for the identification of systematics. Integrated analyses taking into account both the auto- as well as the cross-correlations between probes therefore present a promising analysis method for both current as well as future data.
In this talk, I will present an integrated analysis of CMB temperature anisotropies, CMB lensing, galaxy clustering and weak lensing. I will then outline possible ways in which to extend joint analyses to optimally benefit from upcoming data, and present results on photometric galaxy clustering in the Hyper Suprime Cam DR1 data as well as a new method to combine thermal Sunyaev-Zel’dovich selected cluster number counts and weak gravitational lensing.
Imaging Molecular Dynamics: A theorist’s perspective
Making molecular movies has been one of the main goals of ultrafast science. Tremendous research progress has been made due to advancements in experiments, but theories have also played a crucial role in interpreting the experimental observables. In this colloquium, I will discuss the theoretical development needed to push for the next-generation probe scheme using intense few-cycle infrared (IR) laser pulses. Recent breakthroughs in the theory of strong field sequential double ionization (SDI) of molecules will be discussed. The capability of the SDI probe scheme will be demonstrated by tracking the elusive coherence in the charge migration process in molecules. Other probe schemes using intense IR fields, such as high-harmonics spectroscopy, will also be discussed.
Correlated nonequilibrium dynamics of many-body systems: from polarons to dipolar gases
Cold atom quantum simulators have opened the pathway to observe and study novel phases of matter and nonequilibrium phenomena providing also strong links to quantum technologies and quantum information processing. In this sense, many-body dynamics of correlated quantum gases constitutes one of the most appealing problems in modern quantum physics. In this colloquium, I will discuss two characteristic examples of many-body dynamics. Namely, quasiparticle formation in short-range interacting atomic mixtures and exotic phases of matter arising in long-range anisotropic dipolar gases. Both of these platforms are at the forefront of contemporary cold atom experiments and can find broader applications ranging from condensed matter systems to chemistry and biophysics. Specifically, the dynamical formation of the polaron, an impurity dressed by the elementary excitations of its host, will be described when spinor impurities are immersed in a Bose gas. Monitoring experimentally relevant observables, such as the structure factor, we will unveil the dynamical birth, excitation and death of Bose polarons, their induced interactions and the Ohmic character of the host. As a next step, we will switch gears to explore gases of high spin quantum numbers where the involvement of quantum fluctuations materializes in exotic phases of matter including supersolid and droplet states. Utilizing a fastly rotating external magnetic field, we will map out the underlying phase diagram and explicate that tuning the field orientation provides a new way to control the emergent phases and transitions irrespectively of the dimension. Monitoring the dynamics across the underlying phase boundaries we will expose the spontaneous generation of supersolids and droplet lattices.
Title: Mass-imbalanced ultracold gases - A window to universal few-body physics
Ultracold atomic gaseous matter offers novel possibilities to investigating several aspects of fundamental physics. In this vibrant field experiment and theory provide an in depth physical insight and at the heart of this synergy lies few-body physics. The understanding of few-body processes allows to identify those physical mechanisms that universally apply to many different systems owing to a minimal set of parameters.
For example, Vitaly V. Efimov, in 1970s predicted a class of stable quantum states of particle triplets when the pairwise two-body constituents alone are unstable. Although the initial proposal was in the context of nuclear physics, the first experimental confirmation occurred in 2006 by observing the Efimov states in an ultracold gas of Cs atoms. Since then many theoretical and/or experimental proposals showcased the universal aspects of the Efimov states in the context of nuclear, atomic, or condensed matter physics. Therefore, in this colloquium I will present my theoretical efforts to extend the concept of universality of the Efimov physics. More specifically, I will focus on the few-body collisional aspects of mass-imbalanced systems and the corresponding emergent Efimovian idiosyncrasies. The necessary conditions, as well as, the physical implications of the extended universality in atomic mixtures of unequal masses will also be discussed.
Rydberg atoms in ultracold environments: quantum simulations of nano-technology and decoherence
Rydberg Atoms in highly excited electronic states with principal quantum numbers 30-100 are versatile elements of the toolkit of ultracold atomic physics due to their extreme properties. They are usually created within an ambient ultracold gas and sometimes even in Bose-Einstein condensates. While the effect of that environment can often be negligible, there are conditions where it will be crucial. We discuss how this can be used to tune, guide and record the quantum dynamics of Rydberg systems.
Within a BEC at nano-K temperatures, the environment is so cold that Rydberg atoms survive the extreme scenario that thousands of condensed ground-state atoms are inside the Rydberg electron orbital volume. These condensate atoms interact with the electron, which can be exploited to infer Rydberg dynamics similar to the tracking of high energy particles by a bubble or cloud chamber. We show how the condensate can thus record molcular dynamics involving Rydberg states and offer a direct glimpse at the entangled states responsible for decoherence.
In uncondensed, more dilute gases at the much higher temperatures of micro-K, the ambient medium can controllably interact with embedded Rydberg atoms by also coupling the gas atoms to Rydberg states. We show how the resultant interactions allow non-destructive probes of the embedded Rydberg states through disrupted electro-magnetically-induced transparency, and thus introduce controllable decoherence for quantum simulations of nano-technologies e.g. for energy transport in photosynthetic light harvesting.
Dynamic Critical Phenomena and Self-Organized Criticality at the Superfluid Transition in 4He
Just last week, the First Law of Thermodynamics (which applies only to static systems) has been systematically modified to account for out-of-equilibrium dynamics1. The superfluid transition in liquid 4He is possibly the best experimental realization of an ideal second-order critical point. When driven away from equilibrium by a heat flux, this static critical point crosses over into a fascinating nonlinear region, where the liquid’s divergent thermophysical properties are modified by the nonlinear dynamics near the superfluid transition2. I will show our experimental results for this cross-over, and in another experimental configuration, the transition into a dynamical state of Self-Organized Criticality (SOC) near the superfluid transition3. These experiments have resulted from metrological advances in open-system calorimetry, thermometry, and transport measurements, which may make a new class of future astrophysical measurements feasible4. Recently, the ubiquitous applicability of SOC, as advanced originally by its inventor5, has been applied to describe the evolutionary dynamics of the brain6. The future of non-equilibrium thermodynamics may realize a big impact across the natural sciences.
1. Paul A. Cassak, et al., Phys. Rev. Lett. 130, 085201 (2023).
2. P. K. Day, et al., Phys. Rev. Lett. 81, 2474 (1998).
3. W. A. Moeur, et al., Phys. Rev. Lett. 78, 2421 (1997).
4. C. J. Green, et al., J. Low Temp. Phys. 138, 871 (2005).
5. Per Bok, How Nature Works (Oxford University Press, 1996); P. Bok, C. Tang, and K. Wiesenfeld, Phys. Rev. Lett. 59, 381 (1987).
6. D. Plenz, et al., Front. Phys. 9, (2021) https://doi.org/10.3389/fphy.2021.639389; J. Hesse and T. Gross, Front. Syst. Neurosci. 8, (2014) https://doi.org/10.3389/fnsys.2014.00166
The rich landscape of intertwined electronic phases in quantum materials
Rafael M. Fernandes
University of Minnesota
Quantum materials encompass a wide family of systems that display many fascinating phenomena,
from high-temperature superconductivity to topological order. They stand out not only as
promising candidates for new technological applications, but also as windows into the fundamental
microscopic properties of interacting electrons, whose collective behavior can be very different
from the behavior of an individual electron. Macroscopically, these electronic correlations are
manifested by the emergence of complex phase diagrams displaying a plethora of electronic states
that are not independent, but intertwined. In this talk, I will present a theoretical framework that
captures the intricate interplay between electronic states of matter that may seem unrelated at first
sight. Based on the concept of vestigial orders, it generalizes to the quantum realm concepts
common to the description of liquid crystals. More specifically, in this approach, thermal or
quantum fluctuations cause an electronically ordered state to partially melt in multiple stages,
leading to the emergence of two or more intertwined phases with comparable energy scales. This
framework not only sheds new light on the known phase diagrams of various quantum materials,
such as iron-based superconductors, but it also provides new insights into the experimental
realization of exotic states, such as a charge-4e superconducting phase in twisted bilayer graphene.
Skyrmion Textures in Magnetic Materials
C. D. Batista
Department of Physics and Astronomy, University of Tennessee, Knoxville, TN, USA
Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA
Shull-Wollan Center, Oak Ridge National Laboratory, Oak Ridge, TN, USA
Inspired by the work Herman Hemholtz, William Thompson proposed in 1867 that atoms could be vortices in aether. While later experiments put this proposal out of business, thinking of topological solitons as emerging building blocks or artificial atoms is very appealing. Indeed, more recent developments, that started around the 1960's, have demonstrated that nature has plenty of room for finding updated versions of aether. The aether of quantum magnets is the vector field of magnetic moments, whose topological solitons can be regarded as emergent mesoscale atoms. Like real atoms, these solitons form periodic arrays or crystals whose organizing principles are dictated by symmetry, anisotropy and competing microscopic interactions. These magnetic textures generate an effective magnetic field, coupled to the orbital degrees of freedom of conduction electrons, that can reach astronomical values. We will see how these topological magnetic structures emerge in real materials and how the quantum mechanical nature of spins can lead to richer skyrmion textures than the one that have been observed so far.
Nonlinear interrogation of quantum materials: why higher order response tells you more
The nonlinear response of quantum materials contains a wealth of information that is often hidden in the linear regime. Examples include second-harmonic generation (SHG) as a sensitive probe of electronic symmetry, and higher-order conductivities that provide insights into the quantum geometry of Bloch states and their Berry curvature distribution. Two-dimensional coherent THz and Raman spectroscopy are two other powerful nonlinear probes of low-energy excitations in quantum materials, which have recently become available in several labs. We review experimental progress and introduce an intuitive theoretical description of these methods in terms of Liouville quantum pathways. We then theoretically show how they can directly probe quasiparticle properties in the Kitaev honeycomb spin liquid and provide direct evidence for the emergence of localized Majorana excitations trapped by vison pairs.
Peter P. Orth is a theoretical condensed matter physicist who works on understanding and predicting properties of quantum materials. He explores quantum materials both in and away from equilibrium using analytical field theory and quantum computing algorithms. Peter P. Orth is currently Professor in the Department of Physics at Saarland University, Germany. He is also Affiliate Associate Professor in the Department of Physics and Astronomy at Iowa State University and Contributing Scientist at Ames Laboratory. He received a Diploma in Physics at the Universität Heidelberg in 2007, and a Ph.D. at Yale University in 2011 under the supervision of Prof. Karyn Le Hur. He held postdoctoral positions at the Karlsruhe Institute of Technology (KIT) and at the University of Minnesota. He serves as deputy director of the DOE EFRC "Center for the Advancement of Topological Semimetals", where he investigates new ways to realize desired band topologies by controlling magnetic order, and is a co-leader in the quantum algorithm thrust of the DOE National Quantum Initiative Center on "Superconducting Quantum Materials and Systems" (SQMS), where he develops and implements quantum computing algorithms for many-body systems.
Femtosecond spin-to-charge current conversion in FeCo/graphene nanobilayers excited by femtosecond optical laser pulse
In early 2000s, it was experimentally demonstrated the possibility of creating pure spin-polarized currents by using spin–orbit coupling (SOC). In non-magnetic materials with the large SOC this phenomenon of conversation of the charge current to spin current is known as the Spin Hall Effect. Graphene is a natural 2DEG system with a unique band structure and optical properties, and it has been demonstrated as a very attractive material in a huge number of applications. However, its extremely low, on the order of few μeV, intrinsic spin-orbit coupling limits its applicability in spintronic applications, where the manipulation of spin is required. Several approaches to enhance the strength of SOC in graphene have been proposed. They include addition of small amounts of covalently bound hydrogen atoms, fluorine functionalization, or decorating graphene with heavy ion atoms. Additionally, SOC in graphene can be increased by placing graphene in contact with a 3D ferromagnetic material via hybridization between the p states of graphene and 3d states of a ferromagnet. This approach has a special interest from the perspective of using ferromagnet/graphene bilayers as transient THz spintronic emitters. This talk presents generation of THz bandwidth (single picosecond in duration) electromagnetic transients from a FeCo/graphene heterostructure triggered by 100-fs-wide optical laser pulses. It will be shown that the transient THz emission originates form the Inverse Rashba-Edelstein Effect, which implies the active role of FeCo in SOC enhancement in graphene. The results of circularly polarized light experiments probing the peculiarities of the graphene band structure spin texture and its impact on the ferromagnetic exchange-coupling it will be also discussed.
Fuller Prize Finalists 2023:
Stripe order, impurities, and symmetry breaking in a 3-dimensional model of a diluted frustrated magnet, James Elverson (Thomas Vojta)
Synthesis of Transition Metal Dichalcogenides under varying lab conditions, Sadracd Games (Yew San Hor)
Towards understanding Dust Attenuation of Emission Lines with Illustris TNG Galaxies, Andrew Madsen (Shun Saito)
Magnetic properties of diluted hexaferrites, Logan Sowadski, Sean Anderson, Cameron Lerch (Julia Medvedeva, Thomas Vojta)
Influence of Momentum Spectrometer Resolution on Fully Differential Data of Atomic Collisions, McGowan Toombs (Daniel Fischer)
Physical Properties of Rare Earth Digermanides, Kelci Graville (Halyna Hodovanets)