Program

This talk presents results from a method of contactless pulsed tunneling spectroscopy for two-dimensional electron systems (2DES) that: (1) injects electrons at well defined energies while eliminating the effects of heating in the tunneling spectra; (2) functions even when the 2DES is insulating; and (3) allows precision extraction of the tunneling density of states without distortions due to gating effects. The method also allows detection of relaxation times for electrons that are put into high energy states and observation of the effect of the system ground state on this relaxation. I will give an overview of the method and key results in energy relaxation, lifetime broadening of levels, observation of features from many-body states, and momentum and spin selectivity.

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Photonic systems form a special platform for topological physics, that is relatively easily designable and accessible. Photonic crystals can be fabricated with designed spatial symmetry and band dispersions. Eigen wavefunctions, band structures and correlations often can be measured directly through spectroscopic methods. Going beyond topological photonics in the non-interacting limit, dissipation and nonlinearity introduced through matter medium can lead to complex phases and rich topological phenomena. We will discuss examples of experimental, dissipative nonlinear topological photonic systems.

First, we discuss how optical probes can reveal various aspects of magnetism in 2D materials. In particular, we report the first observation of quantum anomalous Hall states in twisted bilayer WSe₂, made possible by optically measuring the average magnetization ⟨S⟩. Next, we explain how one can access higher-order observables such as ⟨S·S⟩ and ⟨S·(S×S)⟩ by measuring higher-order correlations of scattered photons. Such an approach could enable unambiguous detection of chiral spin liquids. Finally, inspired by progress in cold atom systems, we introduce a method to optically engineer magnetic Hamiltonians, such as a generalized Heisenberg model.

Experimental progress in quantum computing has yielded exciting new tools for probing and manipulating superconducting systems. I will present two studies which, leveraging these tools, explore the non-equilibrium physics of Josephson quantum circuits. The first study uncovers the kinetics of photons, making connections to Luttinger liquids, quantum optics, and classical fluids. The second study discovers scaling behavior in the non-equilibrium response of arrays near the superconductor-insulator quantum phase transition.

Recent discoveries of various light-induced long-lived synthetic topological polar structures in PbTiO3/SrTiO3 superlattices highlight the power of structural and functional couplings in creating new ordered phases. Since the superlattices are on the verge of polarization reorganization, these materials may be exceptionally susceptible to dynamical lattice distortion under resonant phonon excitations. A synergy between materials design (superlattices) and light control (mid-infrared pumping) would create exciting opportunities to discover new quantum phases. Here I will present our recent work on various PbTiO3/SrTiO3 superlattices with mid-infrared excitations.

Transport measurements are considered a gold standard for identifying novel states of matter by providing direct and quantitative information on low energy responses. While electrical transport is widely used to investigate charge-carrying (quasi)particles, it remains an experimental challenge to measure transport of emergent charge-neutral modes, which lie at the heart of numerous quantum phenomena. In this talk, I will discuss our recent efforts to investigate space-and-time-resolved transport of neutral collective modes in two-dimensional flatband systems through ultrafast imaging. This non-equilibrium approach overcomes intrinsic limits of steady-state measurements and allows us to simultaneously capture and separate multiple neutral modes from their distinct propagation behaviors, such as the phase and amplitude modes of U(1) symmetry-broken phases.

Chiral phononics is an emerging field that utilizes the angular momentum and effective magnetic fields produced by circularly polarized lattice vibrations to manipulate the properties of quantum materials. In this talk, I will present a methodology to control phonon angular momentum through phonon-polariton formation in terahertz and mid-infrared cavities. By tuning the frequencies of the polariton branches, we are able to generate phononic Lissajous trajectories on demand that produce staggered magnetic fields and enable dynamical and selective crystallographic symmetry breaking. We further use chiral cavities to induce and lift phonon degeneracy, allowing us to resonantly enhance coupling to magnetic degrees of freedom. Our results create an avenue towards cavity chiral phononics.

Light-matter interaction in quantum materials is one of the critical aspects that elucidates their intriguing properties. In particular, the terahertz (THz) frequency range is of great interest as it allows us to access rich low-energy excitations in such materials. Recent advancements in generating an intense THz pulse enabled the investigations of nonlinear light-matter interaction, which can provide information unreachable by the linear light-matter coupling. More recently, THz 2D coherent spectroscopy (THz 2DCS) has emerged as a new technique to disentangle different nonlinear optical processes of magnons, phonons, and plasmons. Yet, understanding the THz 2DCS spectra is still in its infancy.
In this talk, I will present the recent results of THz 2DCS in the case of a conventional superconductor NbN to elucidate the light-matter interaction of the amplitude collective excitation of the superconducting order parameter, namely the Higgs mode [1]. Using broad-band THz pulses as light sources, we observed a third-order nonlinear optical response whose power spectrum peaked at twice the superconducting gap energy 2∆. With narrow-band THz pulses, a THz nonlinear signal was identified at the driving frequency Ω and displayed a resonant enhancement at temperature when Ω = 2∆. General theoretical considerations show that such resonance can only stem from a disorder-activated paramagnetic light-matter interaction. Numerical simulations demonstrate that even for a small amount of disorder, the Ω = 2∆ resonance is dominated by the superconducting amplitude mode over the investigated disorder levels. We further investigated the THz 2DCS response in the case of MgB2, a multigap counterpart of NbN [2]. I will discuss the essential difference in THz nonlinear responses between MgB2 and NbN. These studies demonstrated the ability of THz 2DCS to explore the physics of the amplitude Higgs mode in superconductors, which would open a new door to studying collective excitations in other types of unconventional superconductors.
[1] K. Katsumi, J. Fiore, M. Udina, R. Romero III, D. Barbalas, J. Jesudasan, P. Raychaudhuri, G. Seibold, L. Benfatto, and N. P. Armitage, Phys. Rev. Lett. 132, 256903 (2024). [2] K. Katsumi, J. Liang, R. Romero III, K. Chen, X. Xi, and N. P. Armitage, arXiv:2411.10852 (2024).

Judicious engineering of the quantum vacuum surrounding a material inside a cavity can lead to detectable and nonintuitive modifications of its electronic and vibrational states, producing a vacuum-dressed material. Breaking specific material symmetry with a cavity provides a unique approach to inducing a new ground state in quantum materials. Particularly intriguing is the coupling of a material with a circularly polarized cavity field, which can break time-reversal symmetry (TRS) and lead to topological bands. This has spurred significant interest in the development of chiral optical cavities that feature broken TRS, especially in the terahertz (THz) frequency range, where various large oscillator-strength resonances exist. In this talk, I will present a novel design [1] and experimental demonstration of the THz chiral photonic-crystal cavity (PCC) that achieves broken TRS using a magnetoplasma in a lightly doped semiconductor. By leveraging the nonreciprocal properties of a THz magnetoplasma and the low effective mass of electrons in InSb, we confined a cavity mode with a single circular polarization at 0.67 THz in the presence of a small (> 0:3 T) magnetic field, achieving a Q-factor of 60 in a proof-of-principle device. Temperature, magnetic field, and polarization-dependent experiments and simulations validate the proof-of-concept chiral cavity with broken TRS, making it well-suited for studies of chiral light–matter interactions in the THz range. In addition, I will discuss preliminary results on chiral phonon polaritons. Chiral phonons carry a finite magnetic moment that results in the phonon Zeeman effect, which has been observed in several materials with phonon magnetic moment values ranging from hundredths to several Bohr magnetons. Such large phonon magnetic moments result in large magnetic fields of > 1 T (experimental), and > 100 T (predicted) when chiral phonons are driven by light. Here, we explore the formation of chiral phonon polaritons in a chiral cavity in the ultrastrong coupling regime in films of PbTe [1, 2].

Two-dimensional (2D) materials have extraordinary potential to control light and light-matter interactions on an atomic scale. Recently, twisted 2D materials have drawn considerable attention due to their capability of inducing moiré superlattices and the discovery electronic correlated phases. Various nanoscale optoelectronic probing scheme, utilizing infrared and terahertz radiation, are presented, revealing the materials topology, photoconversion and interaction effects. Furthermore, by probing modulated 2D materials on the nanoscale, record-small nanoscale polaritonic cavities are revealed, as well as the formation of nanoscale hypercrystals, exhibiting negative refraction and topological interface states.

Optical driving of certain materials has been shown to induce transient optical properties reminiscent of superconductivity at temperatures above the equilibrium transition temperature. While additional measurements of their electrical, magnetic, and nonlinear optical properties have significantly added to our understanding of these fascinating phenomena, important aspects of their microscopic properties and formation mechanisms require a more sophisticated probe to resolve. In this talk, I will introduce multidimensional terahertz spectroscopy (MDTS) as a new probe of superconducting order that is poised to address these challenges. I will present our recent experiments on the cuprate superconductor La2-xSrxCuO4 that demonstrate the power of MDTS to elucidate superconducting disorder and their higher-order correlations, and present an outlook for applying these capabilities to driven superconducting states.

Measuring the femtosecond dynamics of complex order parameters using a real-space probe has long been a long-coveted goal. To tackle this challenge, over the past decades different variations of pump-probe techniques that integrate pulsed-laser light with an STM have been developed. In this talk I introduce a new multimodal STM-based pump-probe instrument that allows for the measurement of both phonon and carrier dynamics through a combination of time-resolved optical pump-probe reflectance, time-resolved tunneling, and time-resolved point-contact measurements. We apply these techniques to investigate the amplitude and phase excitations in an unconventional charge density wave (CDW) insulator. Specifically, our aim is to detect collective phase oscillations—phasons—in an incommensurate CDW, which gain mass via Coulomb interactions. Although massive phasons were theorized many years ago, only recently has THz emission from the unconventional CDW insulator (TaSe₄)₂I been observed and attributed to this mode. I will present time-domain tunnelling and point-contact measurements revealing 0.22 THz oscillations with temperature dependence consistent with a longitudinal phason acquiring mass via the Higgs mechanism. Interestingly, a second mode at 0.11 THz, with similar intensity, also emerges. Comparison with simultaneous time resolved reflectance data suggests that this is a parametrically amplified, massless transverse phason that competes with and suppresses the amplitudon seen in OPPR at the same frequency. Finally, I’ll discuss how these techniques open new opportunities for studying dynamic phenomena in complex materials.

In this talk I will demonstrate two new aspects of angle-resolved photoemission spectroscopy (ARPES) in addressing key problems in modern condensed matter physics. In the first aspect, I will show how Floquet-Bloch manipulation using time-resolved ARPES reveals a hidden Dirac gap in the topological antiferromagnet MnBi2Te4. Here, the time-periodic field of a mid-infrared pump pulse is used to modify the structure of the surface states of MnBi2Te4. Surprisingly, opposite helicities of circularly polarized light result in substantially different Dirac mass gaps in the antiferromagnetic phase despite the equilibrium Dirac cone being massless, which is explained by a Dirac fermion with a random mass. In the second aspect, I will discuss the development of a new photoemission technique – Double-ARPES – that measures the momentum and energy distribution of entangled pairs of electrons photoemitted from a material upon absorption of a single photon. I will show how D-ARPES can be used to determine the Cooper pair structure in unconventional superconductors, and will present preliminary data demonstrating Cooper pair emission in the high-Tc superconductor Bi-2212 and the pair density wave candidate material EuRbFe4As4.

The remarkable properties of transition metal dicalcogenide multilayers have reinvigorated the study of many important questions in quantum materials, including the long-sought "excitonic insulator" phase of matter. Motivated in part by possible realizations in these compounds I will described work done over the last few years on steady state and transiently driven excitonic condensates in and out of optical cavities, presenting predictions both for new superradiant phases and for novel observables that may reveal the presence of driven condensates. This work is supported in part by the Pro-QM Energy Research Frontier Center of the US Department of Energy (DE-SC0019443) and is reported in Phys. Rev. Lett. 132 266001 (2024), Phys. Rev. Lett. 133 217002 (2024) and arXiv:2410.22116

In the seemingly empty vacuum, a hidden world thrives: a vibrant landscape of electromagnetic fields that perpetually flicker in and out of existence. Harnessing this restless "nothingness" promises a novel route for controlling solid-state systems. Embedding quantum materials within deep sub-wavelength terahertz cavities drastically amplifies their interaction with these vacuum fields, giving rise to hybrid light–matter states with uniquely tailored properties. In this talk, I will first introduce a new experimental platform specifically designed to overcome limitations that have hindered progress in exploring extreme light-matter coupling and spectroscopy of van der Waals heterostructures. I will then show how cavity photons can reshape interactions and engineer collective behaviour in quantum materials under both equilibrium and non-equilibrium conditions.

Optically excited quantum materials exhibit nonequilibrium states with remarkable emergent properties, but these phenomena are usually transient, decaying on picosecond timescales and limiting practical applications. Advancing the design and control of nonequilibrium phases requires the development of targeted strategies to achieve long-lived, metastable phases. Here, we report the discovery of symmetry-protected electronic metastability in the model cuprate ladder Sr14Cu24O41. Using femtosecond resonant x-ray scattering and spectroscopy, we show that this metastability is driven by a transfer of holes from chain-like charge reservoirs into the ladders. This ultrafast charge redistribution arises from the optical dressing and activation of a hopping pathway that is forbidden by symmetry at equilibrium. Relaxation back to the ground state is hence suppressed after the pump coherence dissipates. Our findings highlight how dressing materials with electromagnetic fields can dynamically activate terms in the electronic Hamiltonian, and provide a rational design strategy for nonequilibrium phases of matter.

A few years ago, an intriguing new spin based logic-in-memory architecture, MESO, was described which used magnetoelectric multiferroics (ME) and spin-orbit (SO) metallic oxides as key building blocks. Over the past year, there have been some new developments in SOT based manipulation of magnets. Particularly, the role of epitaxy and atomically perfect interfaces with spin and/or orbital current enhanced oxides has been shown to significantly impact the spin-to-charge conversion (or vice versa). We are studying spin transport in La-BFO using a combination of NV imaging and spin Hall measurements. Over the past year, we have discovered the powerful role of magnon confinement as a pathway to enhance spin transport (and thus the spin-to-charge conversion efficiency) by 100X. This talk will give you a summary of our progress so far.

In high-Tc cuprate superconductors (SCs), the elucidation of the interplay between the charge order and the superconductivity is considered to provide pivotal insights on the understanding of high-Tc superconductivity. In this talk, we will report on the light-induced nonequilibrium dynamics in the charge order phases of La2-x-yNdySrxCuO4 including the static stripe phase and dynamical charge density wave phase with changing the Nd and Sr contents by using optical pump-terahertz probe spectroscopy. We found that the plasma edge appears in the c-axis THz reflectivity spectrum after the photoexcitation far above Tc, with its frequency coinciding with the Josephson plasma frequency. Based on the experimental results, we discuss the origin of the observed plasma edge and the interplay between the charge order and the superconductivity in La-based cuprate SCs.

In this talk, I will discuss our recent progress on optical control of integer and fractional quantum anomalous Hall effects in twisted MoTe2 bilayer.

Strong interactions between particles can lead to emergent collective excitations. These phenomena have been extensively established in electronic systems, but are also expected to occur for gases of neutral particles like spin waves, also known as magnons, in magnets. In a hydrodynamic regime where magnons are strongly interacting, they can form a slow collective density mode – in analogy to sound waves in water – with characteristic low-frequency signatures. While such a mode has been predicted in theory, its signatures have yet to be observed experimentally. In this work, we isolate exfoliated sheets of CrCl3 where magnon interactions are strong, and develop a technique to measure its collective magnon dynamics via the quantum coherence of nearby Nitrogen-Vacancy (NV) centers in diamond. By measuring the magnetic fluctuations emitted by thin multilayer CrCl3 in the presence of a variable-frequency drive field, we observe spectroscopic evidence for this two-dimensional magnon sound mode.

Excitation of ordered phases produces quasiparticles and collective modes, as exemplified by magnons that emerge from magnetic order, with applications in information transmission3 and quantum interconnects. Extending this paradigm to ferroelectric materials suggests the existence of ferrons, i.e. fundamental quanta of the collective excitation of ferroelectric order, as theoretically explored recently. While coherent magnons are observed in a broad range of experiments, coherent ferrons have eluded experimental detection. This discrepancy is particularly intriguing given that electric dipole interactions (FE) are inherently stronger than their magnetic counterparts (FM), F_E⁄F_M = (α^2∙ε_r )^(-1)≫1, where  (= 1/137) is the fine structure constant and r the high frequency relative dielectric constant. Here, we report the generation and transport of coherent ferrons in the van der Waals (vdW) ferroelectric material NbOI2. By launching collective oscillations of the ferroelectric dipoles using a short laser pulse, we identify coherent ferrons from intense and narrow-band terahertz (THz) emission and observe their propagations along the polar direction at extremely hypersonic velocities exceeding 105 m/s. The THz emission is a second-order nonlinear process that requires ferroelectric order, as is confirmed in the structurally related ferroelectric WO2Br2 and non-ferroelectric TaOBr2. The discovery of coherent ferrons paves the way for numerous applications, including narrow-band THz emission, ferronic information processing, and quantum interconnects.