Title: Spintronics in Quantum Materials

Abstract: Recent advancements in spintronic techniques originally developed for spin-based devices now enable us to study fundamental spin physics of various quantum materials with unprecedented spin-current control and measurement, opening a new area of theoretical and experimental investigation of quantum systems. In the first half of the talk, we will discuss the development history of spintronics with a concrete example of the evolution of the domain-wall technology in spintronics. In the second part, we will introduce an emerging research area of spin transport in quantum materials which is fueled by the aforementioned advanced spintronic techniques. As examples, we will discuss our researches on quantum phase slips in spin transport through quantum spin chains [1], which shows how spintronic techniques can be used for probing elusive quantum materials, and long-range spin transport mediated by a vortex liquid in superconductors [2], which shows that quantum materials can provide novel platforms for efficient spin-transport devices.

[1] S. K. Kim and Y. Tserkovnyak, “Topological Effects on Quantum Phase Slips in Superfluid Spin Transport,” Phys. Rev. Lett. 116, 127201 (2016)

[2] S. K. Kim, R. Myers, and Y. Tserkovnyak, "Nonlocal Spin Transport Mediated by a Vortex Liquid in Superconductors," Phys. Rev. Lett. 121, 187203 (2018)

Title: 

Why Isn’t God Ambidextrous?: Chirality in Nature and the Role it Plays in Physics, Chemistry, and Biology

Title: Approaching the Attosecond keV X-ray Frontier

Abstract: It was demonstrated experimentally in 2001 that the cutoff of high harmonic spectrum could be extended by increasing the center wavelength of driving lasers. In recent years, mJ level, few-cycle, carrier-envelope phase stabilized lasers at 1.6 to 2.1 mm have been developed for generating attosecond X-rays pulses in the water window (282-533 eV). When a 3 mJ, 12 fs laser at 1.7 mm laser was used to implement polarization gating, isolated soft X-rays reaching the carbon K-edge (282 eV) were generated in our laboratory. Isolated X-ray pulses with 53-as duration were characterized by attosecond streaking measurements. Such ultrabroadband light sources are now being used in time-resolved X-ray absorption near edge structure measurements for studying charge dynamics in atoms, molecules and solids. The rapid progress in the development of mid-infrared (MIR) and long wavelength infrared (LWIR), few-cycle, carrier-envelope phase stable lasers with mJ pulse energy at a kilohertz repetition rate opens the door for the generation of the isolated keV attosecond X-ray pulses.

Coherent Control and Attosecond Dynamics with Pulsed XUV and IR Radiation

Abstract: The enormous advances in the generation of advanced light sources have enabled the exploration of the ultrafast dynamics in atoms and molecules, thereby promising a rich field of possibilities in the control of matter.  One aim of quantum coherent control is to steer electronic motion in atoms and molecules in specific directions or locations.  Recently, experimental manipulation of the photoelectron angular distribution (PAD) was achieved using the Free-Electron Laser (FEL) at FERMI in Trieste (Italy) by controlling the relative time-delay between the fundamental and second harmonic of a linearly polarized femtosecond extreme ultraviolet (XUV) pulse to an unprecedented precision of three attoseconds. [1]

We present a variety of schemes by which control of the PAD asymmetry can be achieved, such as interfering one-photon and two-photon ionization pathways in a region of an intermediate resonance, overlapping the XUV pulse with an infrared (IR) field, or using circularly polarized light.  Employing circularly polarized light opens up a number of particularly interesting possibilities in the study of multi-photon ionization processes.  For example, a circular dichroism is revealed in an ionization scheme for which an XUV pulse ionizes helium and then sequentially pumps the remaining electron to an oriented excited state of He⁺, while an overlapping optical field, which can either be co-rotating or counter-rotating with respect to the XUV field, further ionizes the residual He⁺ ion via multiphoton absorption [2].

Few-cycle elliptically polarized pulses can be employed in so-called “attoclock experiments” to investigate the tunneling time in strong-field ionization to test the claim that tunneling ionization in atomic hydrogen is instantaneous [3].  Recently, we used a one-dimensional model with linearly polarized light in a short-range Yukawa potential to show that the often-used picture of a probability flow traversing the entire barrier from the inner to the outer classically allowed regions is fundamentally flawed [4].  An update on the latest development, an experimental attoclock experiment on atomic hydrogen [5], will also be presented.

 [1] K. C. Prince et al., Nature Photonics 81 (2016) 043408.

 [2] M. Ilchen et al.,  Phys. Rev. Lett. 118 (2017) 013002.

 [3] L. Torlina et al., Nature Physics 11 (2015) 502.

 [4] N. Douguet and K. Bartschat, Phys. Rev. A 97 (2018) 013402.

 [5] Satya Sainadh et al., arXiv 1707.05445 (2018).

Title: Ghost Hunting: The Story of Neutrinos and the Cosmos

Abstract: Neutrinos are one of the most abundant particles in the Universe and are constantly streaming through our bodies unnoticed. Because of their extreme shyness to interact with ordinary matter, it takes extraordinary ingenuity and humongous detectors to catch these ghostlike particles in earthly experiments. This was a rewarding endeavor and led to the discovery of neutrino oscillations -- the first-ever laboratory evidence for physics beyond the Standard Model. However, the neutrino game is far from being over; rather, this is just the beginning of a remarkable journey into the enchanting land of neutrinos, with many current and upcoming experiments poised to make fundamental discoveries. We will discuss how these teeny-tiny neutrinos could help us unravel some well-kept secrets of the cosmos. In particular, we will highlight some recent exciting news coming from Antarctica that could revolutionize our understanding of the Universe.

Title: Attosecond physics: faster than a New York minute 

Abstract: The genesis of light pulses with attosecond (10⁻¹⁸ seconds) durations signifies a new frontier in time-resolved physics. The scientific importance is obvious: the time-scale necessary for probing the motion of an electron(s) in the ground state is attoseconds (atomic unit of time = 24 as). The availability of attosecond pulses would allow, for the first time, the study of the time-dependent dynamics of correlated electron systems by freezing the electronic motion, in essence exploring the structure with ultra-fast snapshots, then following the subsequent evolution using pump-probe techniques.

This talk will examine the fundamental principles of attosecond formation by Fourier synthesis of a high harmonic comb and phase measurements using two-color techniques. Quantum control of the spectral phase, critical to attosecond formation, has its origin in the fundamental response of an atom to an intense electromagnetic field. We will interpret the laser-atom interaction using a semi-classical model. Finally, the comparison of recent measurements with the predictions of strong-field scaling will be used to show that high energy photons with inherently shorter bursts can be created using long wavelength fundamental fields.

Title: New Directions in Scattering and Photodissociation

Abstract: We will describe current work in two directions in our laboratory. In one, we present the first observation of photofragment alignment produced by circularly polarized photolysis light, obtained in photodissociation of a planar polyatomic molecule. This alignment arises via a new mechanism involving coherent excitation of two mutually perpendicular in-plane transition dipole moment components. The alignment is described by a new anisotropy parameter, γ₂’, that was isolated by a unique laser polarization geometry. The determination of the parameter γ₂’ was realized in ozone photolysis at 

Curious cases of resonances in gas-phase fullerenes

Abstract:

Empty fullerenes and atom-encaging endofullerenes are quintessential symmetric molecules exhibiting stability in the room temperature. This property endows them with the quality of being tested for spectroscopies, otherwise inaccessible with regular atoms/molecules. Probing the response of these nanosystems to electromagnetic radiations or lepton impacts are classic directions. Conventional spectroscopies of recording photoelectron counts as the photon frequency varies reveal varieties of resonances for gas-phase fullerenes. Some of these resonances originate from correlated electron motions leading to plasmons. More contemporary spectroscopies can measure the time-of-flight of the photoelectron from its production site in the molecule to the detector and utilizes a Wigner-clock based on the knowledge of the photoelectron phase. This talk will show that the information from the knowledge of the Wigner-time is consistent with the underlying correlative dynamics at the plasmon. Another exotic group of resonances to be addressed includes photoexcitation at one site of the molecule and its subsequent decay at a different site, as well as a coherent admixture of this pathway with local Auger processes. Finally, by choosing antiparticles like positrons to collide with fullerenes the emergence of novel diffraction resonances in the formation of matter-antimatter bound systems, the positroniums, will be discussed. All these theoretical studies have promise of experimental measurements.

Title: A tale of two states: the complex relationship between superconductivity and magnetism in quantum materials

Abstract: Much of our current technology was enabled by our understanding of semiconductors, whose electrons behave collectively in a similar way as how an individual electron does. In contrast, a hallmark of quantum materials is the emergence of unusual collective electronic behaviors that give rise to fascinating phenomena with unique potential for novel applications. A posterchild is the phenomenon of high-temperature superconductivity, by which materials carry electric currents without dissipation at relatively high temperatures. An important clue to elucidate this highly debated state of matter comes from the observation that it tends to appear in close proximity to the very different phenomenon of magnetism. These two states seem to live a love-hate relationship, displaying a mixture of competition and cooperation. In this talk, I will discuss new and exciting progress on this problem enabled by recent Quantum Monte Carlo simulations of an effective low-energy model.