Localization and dynamics of Rydberg atoms in silicon

Gabriel Aeppli, Paul Scherrer Institute

Impurities in silicon enable experiments that are generally performed for cold atoms. Because of the high dielectric constant and small conduction band mass, the wavefunctions are mesoscopic rather than atomic-scale, and so certain quantum effects are much easier to see than for ordinary atoms. Also, it is possible to exploit electronic devices for in-situ quantum state detection. Recent experiments, including the demonstration of giant multiphoton absorption due to Rydberg states, which illustrate these points are described.

A Local Quantum Phase Transition in YFe2Al10

Meigan Aronson, University of British Columbia

It is now well accepted that the suppression of ordered states, such as magnetism, can give rise to a novel state with highly anomalous metallic characteristics. It remains a challenge to understand the role of the quantum critical fluctuations associated with the T=0 phase transition in inducing this new state, and if there is feedback between the fluctuations and the essential properties of the quasiparticles in the non-Fermi liquid electronic state. A lack of detailed experimental results on suitable QC systems has slowed progress towards this understanding. The quasi-two dimensional metal YFe2Al10 is a very promising system, comprised of layers of nearly square nets of Fe atoms. Despite the strong divergence of the susceptibility,[)<[)<(T)~T-1.4 there is no evidence for magnetic order above 0.02 K. Inelastic neutron scattering measurements find that the scattering has no indication of incipient magnetic order, rather there is no measureable wave vector dependence, beyond that of the form factor. However, the scattering displays a strong energy divergence, and the Kramers-Kronig analysis indicates that it is these quantum critical excitations that are responsible for the divergence in the magnetic susceptibility. The scattering is also temperature independent, evidence that the imaginary part of the dynamical susceptibility [)<”(E,T) displays E/T scaling, where the absence of any characteristic energy scale beyond temperature is the hallmark of quantum critical systems. The quantum critical fluctuations are very strong in YFe2Al10, signaling that it is very close to a T=0 phase transition. The neutron scattering measurements reveal that the critical fluctuations are completely local, with each moment fluctuating incoherently, each with the same spectrum of excitations. These findings rule out the possibility that YFe2Al10 is near magnetic ordering, and instead it seems likely that the phase transition corresponds to a purely electronic phase transition, possibly an orbital selective Mott transition, where localized magnetic moments first emerge.

Study of the phase diagram of cuprates in an ultrafast fashion

Fabio Boschini, Quantum Matter Institute & University of British Columbia

The phase diagram  of copper-oxides  hosts  intertwined  phases as disparate  as high-temperature superconductivity, charge order and the pseudogap, making the precise description of each separate phase challenging. In the last decade the development of time-resolved techniques has offered a novel perspective for investigations of dynamical properties of quantum phases. In this regard, we recently demonstrated that time-resolved photoemission spectroscopy can disentangle the dynamics of phase fluctuations and charge excitation, establishing the dominant role of phase coherence in the emergence of high-temperature superconductivity in Bi-based cuprates [1]. In light of this, we employed this same dynamical approach to reveal the competition between charge-order and superconductivity in Y-based cuprates in the time-domain [2] and demonstrate the intimate relation between the well-known ≈70 meV electron-boson band renormalization (kink) and the superconducting gap [3]. Finally, time-resolved photoemission approach has allowed us to reveal unambiguously the relation of the pseudogap and short-range antiferromagnetic correlations in optimally-doped NCCO electron-doped cuprate [4]. In particular, this transient study provides clear evidence of the role of short-range correlations in defining the Fermi surface topology.

Unconventional superconductivity probed using uniaxial stress: an NMR application to Sr2RuO4

Stuart Brown, University of California, Los Angeles

The superconducting state of Sr2RuO4 has long been held up as a solid state analog to super uid 3He- A, with strong normal state correlations, and an odd-parity order parameter that also breaks time reversal symmetry. Known as the chiral p-wave state, a consequence of the proposed two-component order parameter is a split superconducting transition when subjected to in-plane magnetic fields or uniaxial stress. Recent studies instead revealed a steep rise and increase in transition temperature by a factor of 2.5 in stressed samples, motivating us to study the evolution of the normal and superconducting states using 17O NMR. Under stressed conditions, the normal state Knight shifts are consistent with tuning through a van Hove singularity, along with an associated enhancement of magnetic fluctuations. Deep in the superconducting state, a reduction of Knight shifts K is observed, indicating a drop in the spin polarization. Implications for the order parameter, in the context of a much broader set of experimental findings, are discussed.

Weyl-Kondo semimetal behavior in Ce3Bi4Pd3

Silke Bühler-Paschen, Vienna University of Technology

Strongly correlated electron systems are a treasure trove for exotic quantum phases and fluctuations, frequently featuring new functionalities of interest for technological exploitation. The underlying physics in heavy fermion systems is a delicate balance of the RKKY and Kondo interaction, which makes these materials highly tuneable by external parameters such as pressure or magnetic field. Recently, we have introduced spin-orbit coupling (SOC) as a new tuning parameter [1]. We have realized it by substituting heavy Pt by much lighter Pd in the noncentrosymmetric Kondo insulator Ce3Bi4Pt3, which leads to the formation of a strongly correlated semimetal state in Ce3Bi4Pd3, with thermodynamic signatures of a Weyl-Kondo semimetal [1,2]: The low-temperature electronic specific heat coefficient Cel/T of Ce3Bi4Pd3 is linear in T 2 - a sign of a linear electronic dispersion - with a slope that corresponds to an ultralow quasiparticle velocity. Recently, we have discovered that this phenomenon is accompanied by a giant spontaneous Hall effect and an associated even-in-field Hall resistivity [3]. As the material is entirely nonmagnetic, this is direct evidence of its Berry curvature in the absence of time-reversal symmetry breaking, and thus of its topological nature, in the form of tilted Kondo-driven Weyl nodes [3].

Recent progresses on heavy fermion quantum criticality: unconventional superconductivity and entwined degrees of freedom

Ang Cai, Rice University

Heavy fermions systems provide a valuable setting to study quantum criticality. Here, quantum critical point (QCP) is believed to go beyond the Landau paradigm, featuring the physics of Kondo destruction [1]. In this context, we report on recent developments regarding two prominent and overarching issues. First, we analyze the implications of the Kondo destruction quantum criticality for superconductivity, an issue that has been highlighted in particular by the observed high-Tc superconductivity in CeRhIn5 [2].

We have studied an Anderson lattice model using the recently developed Cluster Extended-DMFT approach. We have demonstrated that the Kondo destruction QCP is robust going from single-site EDMFT to the Cluster EDMFT [3], and shown that this type of QCP promotes high Tc unconventional superconductivity, with a maximum Tc on the order of several percent of the bare Kondo temperature [3]. Our results provide a natural understanding for the phase diagram of the Ce-115 systems, and highlight how unconventional superconductivity develops from a strange-metal normal state.

Second, we address how Kondo materials provide a setting to study the interplay between spin and other local degrees of freedom. Experiments performed in Vienna on cubic heavy fermion system Ce3Pd20Si6 uncovered evidence for two consecutive Fermi surface collapsing QCPs under a single parameter tuning [4]. We consider an SU(4) Bose-Fermi Kondo model with both spin and orbital degrees of freedom coupled to bosonic baths, which represents an effective model in the EDMFT approach of a multipolar Kondo lattice system associated with Gamma8 multiplets and Kugel–Khomskii type interactions. We find that a generic parameter tuning trajectory contains two QCPs, each being associated with the Kondo destruction of spin or orbital degree of freedom [4]. Our results reveal how quantum criticality simplifies the interplay between the entwined degrees of freedom and support electron localization as a unified framework for strongly correlated materials.

Ferromagnetic quantum Criticality in Overdoped cuprates

Sudip Chakravarty, University of California, Los Angeles

The extreme variability of observables across the phase diagram of the cuprate high temperature superconductors has remained a profound mystery, with no convincing explanation of the superconducting dome. While much attention has been paid to the underdoped regime of the hole- doped cuprates because of its proximity to a complex Mott insulating phase, little attention has been paid to the overdoped regime. Experiments are beginning to reveal that the phenomenology of the overdoped regime is just as puzzling. For example, the electrons appear to form a Landau Fermi liquid, but this interpretation is problematic; any trace of Mott phenomena, as signified by incommensurate antiferromagnetic fluctuations is absent and the uniform spin susceptibility shows a ferromagnetic upturn. We argued that [Competing ferromagnetism in high-temperature copper oxide superconductors, Angela Kopp, Amit Ghosal, and Sudip Chakravarty, PNAS April 10, 2007 104 (15) 6123-6127] many of these puzzles can be resolved if we assume that competing ferromagnetic fluctuations are simultaneously present with superconductivity, and the termination of the superconducting dome in the overdoped regime marks a quantum critical point beyond which there should be a genuine ferromagnetic phase at zero temperature.

Tunable  Mott  insulator  and  superconductivity  in  trilayer  graphene  on  boron  nitride moir superlattice

Guorui Chen, Lawrence Berkeley National Laboratory

Mott insulator is a central concept in strongly correlated physics and manifests when the repulsive Coulomb interaction between electrons dominates over their kinetic energy. A tunable Mott insulator, where the competition between the Coulomb interaction and the kinetic energy can be varied in situ, can provide an invaluable model system for the study of Mott physics. In this talk, I will discuss a general route to engineer strongly correlated physics in two-dimensional moiré superlattices, and show the experimental realization of a tunable Mott insulator in the ABC stacked trilayer graphene (TLG)/hBN moiré superlattice. The moiré superlattice in TLG/hBN heterostructures leads to narrow electronic minibands and allows for the observation of gate-tunable Mott insulator states at 1/4 and 1/2 fillings. In addition, signatures of superconductivity are observed at low temperature near the 1/4 filling Mott insulating state in the TLG/hBN heterostructures.

A theory about frustrated metals

Gang Chen, University of Hong Kong

Motivated by the experiments on the correlated materials with both local moments and conduction electrons, I propose a new model/theory to understand the interplay between these degrees of freedom. We find that the local moment scatters the conduction strongly and change the self energy and the transport properties of the metals. We also show how to manipulate the local moments, and thus indirectly modify the conduction electrons in some testable fashion. We discuss the general application of our theory.

Valley-dependent electron correlations and 'valley noise' in monolayer semiconductors

Scott Crooker, Los Alamos National Laboratory

Much of the current interest in atomically-thin transition metal dichalcogenide (TMD) semiconductors such as MoS2 and WSe2 is driven by the physics of coupled spin & valley degrees of freedom and the potential for new spin- and valley-based devices. This talk will discuss two recent optical studies that probe the valley-related physics and correlations of electrons and holes in monolayer TMD semiconductors.

In the first study, we demonstrate a new approach for exploring intrinsic valley dynamics by measuring noise correlations [1]. Under conditions of strict thermal equilibrium, we use optical Faraday rotation to passively “listen” to the thermodynamic fluctuations of the valley polarization in a Fermi sea of resident electrons or holes, due to their spontaneous scattering between the K and K’ valleys of the Brillouin zone. The noise correlation function reveals very long exponentially-decaying intrinsic valley relaxation, providing a viable route toward quantitative measurements of the truly intrinsic valley dynamics, free from any external perturbation, pumping, or excitation. In the second study, we measure the optical response of an electron Fermi sea in monolayer WSe2. Low energy features in the optical spectrum suggest the emergence of new quasiparticles due to coupling to short-wavelength inter-valley plasmons [2], which become discretized in large applied magnetic fields to 65 T.

Topological Spin Excitations in Honeycomb Ferromagnet CrI3

Pengcheng Dai, Rice University

In two-dimensional honeycomb ferromagnets, bosonic magnon quasiparticles (spin waves) may either behave as massless Dirac fermions or form topologically protected edge states. The key ingredient defining their nature is the next-nearest-neighbor Dzyaloshinskii-Moriya interaction that breaks the inversion symmetry of the lattice and discriminates chirality of the associated spin-wave excitations. Using inelastic neutron scattering, we find that spin waves of the insulating honeycomb ferromagnet CrI3(TC=61 K) have two distinctive bands of ferromagnetic excitations separated by a ~4 meV gap at the Dirac points. These results can only be understood by considering a Heisenberg Hamiltonian with Dzyaloshinskii-Moriya interaction, thus providing experimental evidence that spin waves in CrI3 can have robust topological properties potentially useful for dissipationless spintronic applications.

What Non-Equilibrium Photon Spectroscopies Can Tell Us About Materials

Tom Devereaux, Stanford University

Photon-based spectroscopies have had a significant impact on both fundamental science and applications by providing an efficient approach to investigate the microscopic physics of materials. Together with the development of synchrotron X-ray techniques, theoretical understanding of the spectroscopies themselves and the underlying physics that they reveal has progressed through advances in numerical methods and scientific computing. In this talk, I will provide an overview of ultrafast techniques for out-of-equilibrium spectroscopies, from characterizing equilibrium properties to generating transient or metastable states, mainly from a theoretical point of view.

Developments in Correlated Materials in Low Dimensions

Gregory Fiete, Northeastern University

In this talk, I will highlight recent theoretical and experimental developments in correlated materials in low dimensions, focusing on two-dimensional systems that exhibit interesting properties driven by electronic correlations. These include various forms of superconductivity, magnetism, charge density waves, topological properties, and the interplay of these. The aim of this brief talk is to set the stage for the following talks and related scientific discussion.

Supermetal

Liang Fu, Massachusetts Institute of Technology

I will describe properties of two-dimensional metals with divergent density of states due to Fermi surface singularity. Aside from the well-known van Hove singularity, I will introduce a wide array of Fermi surface singularities associated with high-order saddle points and power-law (instead of logrithmic) divergent density of states. These supermetals can be quantum critical, or form ordered states such as superconductivity and density wave at low temperature. Candidate examples of supermetal include magic-angle twisted bilayer graphene and bilayer ruthenates.

Triangular rare-earth based quantum magnets

Philipp Gegenwart, University of Augsburg

Rare-earth based triangular antiferromagnets provide a novel playground for frustrated magnetism and quantum spin liquid behavior of spin-orbit coupled moments. YbMgGaO4 features spin-orbit moments from a Kramers doublet, which remain fluctuating down to very low temperatures. We discuss thermodynamic, magnetic and inelastic neutron scattering experiments with particular emphasis on the effect of Mg2+ and Ga3+ site randomness, which reveal localized spinon- and valence-bond-type excitations [1]. We also present most recent data on the structurally disorder-free triangular quantum spin liquid candidate NaYbO2 [2] and discuss the potential of rare-earth frustrated magnets for adiabatic demagnetization cooling.

Kondo breakdown and non-Fermi liquid in frustrated Kondo lattice models

Tarun Grover, University of California, San Diego

One of the central themes in heavy fermion systems is the breakdown of Kondo screening as a function of external parameters [1,2]. One possible route to Kondo breakdown is via geometric frustration of local moments. For example, geometric frustration may cause local moments to enter a quantum spin liquid state thereby decoupling them from the conduction electrons at low energies [3]. Numerical study of models that may exhibit such physics is challenging, both due to the fermion sign problem and the sign problem that arises from geometric frustration. In this talk I will discuss recent progress in tackling such problems and show that a large class of frustrated Kondo lattice models can be formulated without any sign problem [4]. As an application, I will discuss a model where Dirac electrons on the honeycomb lattice are Kondo coupled to quantum spins on the kagome lattice [5]. I will show that in a certain parameter regime, one obtains a non-Fermi liquid phase where Kondo screening is not operative and spins form a topologically ordered quantum spin liquid state. I will discuss the properties of this non- Fermi liquid phase via conventional observables such as the spectral function, and also via mutual information between the electrons and the spins.

Thermalization and Possible Signatures of Quantum Chaos in Complex Crystalline Materials

Aharon Kapitulnik, Stanford University

Analyses of thermal diffusivity data on complex insulators and on strongly correlated electron systems hosted in similar complex crystal structures suggest that quantum chaos is a good description for thermalization processes in these systems, particularly in the high temperature regime where the many phonon bands and their interactions dominate the thermal transport. Here we observe that for such systems  diffusive  thermal  transport  is  controlled  by  a  universal  Planckian  time  scale  $\tau\sim\hbar/k_BT$, and a unique (butterfly) velocity $v_B$. Specifically, $v_B \approx v_{ph} $ for complex insulators, and $v_{ph} \lesssim v_B \ll v_{F}$ in the presence of strongly correlated itinerant electrons ($v_{ph}$ and $v_F$ are the phonons and electrons velocities respectively). For the complex correlated electron systems we further show that charge diffusivity, while also reaching the Planckian relaxation bound, is largely dominated by the Fermi velocity of the electrons, hence suggesting that it is only the thermal (energy) diffusivity that describes chaos diffusivity.

Topology and Correlation in Kitaev Materials

Yong-Baek Kim, University of Toronto

We discuss recent progress in theory and experiment on emergent correlated topological phases in Kitaev materials. Here the competition between different anisotropic spin-exchange interactions may lead to a number of exotic phases of matter. We investigate possible emergence of quantum spin liquid, topological magnons, and topological superconductivity in two and three dimensional systems. We make connections to existing and future experiments.

Weyl-Kondo Semimetal - Magnetism and Topology in Heavy-Fermion Systems

Hsin-Hua Lai, Rice University

Strongly correlated and topologically nontrivial insulators go back by decades to quantum Hall systems. By contrast, topological conductors with strong correlations are still being identified. Heavy fermion metals exemplify systems of strong correlations and are typically located at the edge of magnetism. Given their inherently strong spin-orbit couplings, heavy fermion metals present a natural setting to study how strong correlations in general and magnetic degrees of freedom in particular can produce correlated topological states. Recently, we have advanced a Weyl-Kondo semimetal phase [1] within a well-defined three-dimensional lattice model that breaks the inversion symmetry. The quasiparticles near the Weyl nodes develop out of the magnetic correlations in the form of Kondo effect, as do the surface states that feature Fermi arcs. Importantly, the Weyl nodes are pinned near the Fermi energy by the Kondo effect along with space group symmetry. This has allowed us to propose a key thermodynamic signature of the Weyl-Kondo semimetal phase, viz. the specific heat C going as T cubed with a prefactor enhanced by as much as 9 orders of magnitude compared to the expected value for weakly correlated systems. This thermodynamic signature has been identified in the heavy fermion semimetal Ce3Bi4Pd3 [2]. Furthermore, recent experimental finding of a spontaneous Hall effect in Ce3Bi4Pd3 provides direct evidence for the topological nature of this phase [3]. Our results open up a new avenue for the creation of correlated topological conductors by the fluctuations of magnetic degrees of freedom.

Novel thermal gradient induced charge transport in magnetic insulators with multiple-Q spin texture

Shizeng Lin, Los Alamos National Laboratory

Recent experiments have revealed the existence of multiple-Q spin texture, such as magnetic helix, bubble and skyrmion crystals, in magnetic insulators. We study the novel charge transport in these systems in the presence of a thermal gradient. The thermal gradient drives the multiple-Q spin texture into motion. The motion of the spin texture can pump charge both in the longitudinal and transverse directions. We find that the charge pump is associated with the non-trivial topology of the electronic state twisted by the magnetic spin texture. By introducing ancillary dimensions associated with the Goldstone mode of the spin texture, the system can map into higher dimensional quantum Hall systems. The direction of the charge current depends on the topological index of the quantum Hall systems.

Emergence of Flat-Band Magnetism and Half-Metallicity in Twisted Bilayer Graphene

Alejandro Lopez-Bezanilla, Los Alamos National Laboratory

Twisted bilayer graphene is a planar structure composed of two van der Waals bonded graphene sheets where one sheet is twisted with respect to the other. First-principles calculations are presented to demonstrate that dynamic band-structure engineering in twisted bilayer graphene is possible by controlling the chemical composition with extrinsic doping, the interlayer coupling strength with pressure, and the magnetic ordering with external electric field. The interplay between electronic and geometric degrees of freedom in compressed bilayers leads to an unbalanced distribution of charge density resulting in the spontaneous apparition of localized magnetic moments without disrupting the structural integrity of the bilayer. Weak exchange correlation between magnetic moments is estimated in large unit cells and evidence of half-metallicity provided.

Non-Fermi liquid at the FFLO quantum critical point

Ipsita Mandal, Cornell University

When a 2D superconductor is subjected to a strong in-plane magnetic field, Zeeman polarization of the Fermi surface can give rise to inhomogeneous FFLO order with a spatially modulated gap. Further increase of the magnetic field eventually drives the system into a normal metal state. We performed a renormalization group analysis of this quantum phase transition, starting from an appropriate low- energy theory. We computed one-loop flow equations within the controlled dimensional regularization scheme with fixed dimension of Fermi surface, expanding in \epsilon = 5 / 2 − d . We found a new stable non-Fermi liquid fixed point and will discuss its critical properties. One of the most interesting aspects of the FFLO non-Fermi liquid scenario is that the quantum critical point is potentially naked, with the scaling regime observable down to arbitrary low temperatures. In order to study this possibility, we also performed a general analysis of competing instabilities, which suggested that only charge density wave order is enhanced in the vicinity of the quantum critical point.

Stripes and Nematicity in a Hole-Doped Three-Orbital Spin-Fermion Model for Superconducting Cuprates

Adriana Moreo, University of Tennessee & Oak Ridge National Laboratory

Numerical studies of a spin-fermion model that captures the charge-transfer properties of Cu-based high critical temperature superconductors[1] reveal the spin and charge order in the parent compound and under hole doping. In clusters of dimension 16X4, that break the rotational symmetry, half-filled stripes are observed upon hole doping, i.e., n stripes of length 4 develop when 2n holes are introduced in the system. The antiferromagnetic order observed in the parent compound develops a pi-shift across each stripe and the magnetic structure factor peaks at momentum k=(π-δ,π) with δ=2πNh/2L where L=16 and Nh is the number of doped holes. The electronic charge is also modulated and the charge structure factor peaks at k=(2δ,0). In addition, orbital nematicity with <npx>-<npy>≠ 0 develops as electrons are removed from the system. These results indicate that the spin and charge distribution experimentally observed in hole-doped cuprates is captured by unbiased Monte Carlo studies of a doped charge-transfer insulator.

Possible Kitaev spin liquid physics and topological transitions in RuCl3

Stephen Nagler, Oak Ridge National Laboratory

The S=1/2 honeycomb lattice magnetic insulator alpha-RuCl3 has attracted attention as a possible manifestation of physics related to the Kitaev model. At low temperatures the material orders antiferromagnetically in the zigzag phase. Despite this ordering, neutron scattering and spectroscopic measurements have provided evidence for a fractionalized magnetic excitation spectrum. The magnetic order can be suppressed by a magnetic field of 7.5 Tesla applied appropriately in the honeycomb plane, and this results in a disordered quantum phase that is likely some sort of quantum spin liquid. This talk emphasizes the excitation spectrum as a function of field as measured with inelastic neutron scattering, along with diffraction and thermodynamic measurements. The data shows features that may be evidence for a high field topological phase transition.

Interacting Majorana fermions in strained nodal superconductors

Emilian Nica, Arizona State University

Landau levels have been predicted to emerge in systems with Dirac and Weyl nodal points under applied non-uniform strain. Here, we consider 2D, dxy spin-singlet (2D-S) and 3D p + - ip equal-spin triplet (3D-T) superconductors (SCs). We show that the topology of the zeroth Landau levels (LLs) depends on the direction of the applied strain. In 2D, strain along one axis of the Brillouin Zone (BZ) induces bulk zeroth LLs which are localized at the real-space boundary between two topologically distinct phases within the strained sample. Strain along the diagonal of the BZ leads to two zeroth LLs which are protected by a mirror symmetry, while the bulk of the sample is in a topologically-trivial phase. Similar conclusions hold for the 3D-T SCs. We demonstrate the spinful Majorana nature of the bulk gapless modes in all cases. Weak residual interactions of the repulsive Hubbard type lead to a ferromagnetic instability for 2D-S cases, where the pairing condensate and ferromagnetism coexist in the bulk. Furthermore, we show that 3D-T SCs under uniaxial strain exhibit emergent real-space Majorana fermions. While typical density-density interactions are suppressed, these 3D-T systems are likely to develop more exotic instabilities [1].

Using  ultrashort  optical  pulses  to  unravel  the  interplay  between  electronic  and magnetic phenomena in strongly correlated systems

Rohit Prasankumar, Los Alamos National Laboratory

A hallmark of correlated electron materials is the sensitivity of their properties to the interplay and coupling between different order parameters. For example, heavy fermion materials display a wide spectrum of low temperature states depending on the interactions between coherent heavy electrons and localized f-electron spins. Similarly, multiferroic oxides have attracted much attention in recent years due to their coexisting, sometimes coupled magnetic and electric orders, which could lead to controlling magnetism with an electric field and ferroelectricity (FE) with a magnetic field. In the past several years, we have demonstrated that ultrafast optical spectroscopy (UOS), at frequencies ranging from soft X-rays to terahertz, is a unique tool for exploring ME coupling in both canonical multiferroics (e.g.,HoMnO3) and multiferroic oxide heterostructures. We have also shown that UOS can provide insight into the electronic structure of heavy fermion materials that cannot be obtained through other experimental techniques. Here, I will describe our work in these areas, along with new concepts for ultrafast studies of these material systems.