**SPEAKER ABSTRACTS (in alphabetical order by presenter)**

**LA-UR-19-23286**

**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.

**Cooperative**** valence dynamics of the Anderson Lattice observed by resonant inelastic x-ray scattering**

Maerin Rahn, Los Alamos National Laboratory

In rare earth intermetallics with weakly bound f-electrons and a Kondo energy scale much larger than magnetic exchange or crystal field splittings, the screening of local moments may result in a non- magnetic Fermi liquid ground state. At low temperatures, the quantum fluctuations between magnetic and non-magnetic valence configurations can acquire a cooperative (lattice) character, which has a profound impact on a number of bulk physical properties. On a phenomenological basis, a sound understanding of this phenomenon has been achieved. However, a microscopic theory of this prototype coherently entangled quantum many body state remains an outstanding challenge. The cooperative character of such Anderson Lattices is accessible by momentum-resolved spectroscopies, such as angle- resolved photoemission and inelastic neutron scattering. These methods probe single-particle excitations in the charge and magnetic channels, respectively. By contrast, novel soft x-ray resonant inelastic x-ray scattering (RIXS) experiments provide a more subtle point of view, coupling to both charge and spin degrees of freedom in a non-trivial way. If calculations of the underlying Kramers-Heisenberg term on a basis of strongly correlated electronic bands could be achieved, this could unlock unprecedented microscopic insight into one of the longest-standing issues in quantum matter.

**Nematicity**** and quantum**** ****criticality in CeRhIn5 probed by dilatometry**

Priscila Rosa, Los Alamos National Laboratory

CeRhIn5, a heavy-fermion antiferromagnet, exhibits remarkable behavior when subjected to either high magnetic fields or high applied pressures. From the superconducting dome around a putative unconventional quantum critical point at 2:3 GPa to the recently discovered XY nematic phase at H* > 30 T, this material serves as a platform for the study of strongly correlated phenomena. In this talk, high-resolution thermal expansion and magnetostriction measurements will be used to probe CeRhIn5 at extremes. Optical fibers containing fiber Bragg grating (FBG) sensors are used to measure the length change in millimeter-sized single crystals subjected to DC fields to 45 T. An FBG optical apparatus for measuring thermal expansion under applied hydrostatic pressures to 2.5 GPa will also be presented. Our results support a scenario in which anisotropic hybridization sets the stage for electronic nematicity and quantum criticality in CeRhIn5.

**A Novel Strongly Spin-Orbit Coupled Quantum Dimer Magnet: Yb2Si2O7**

Kate Ross, Colorado State University

The quantum dimer magnet (QDM) is the canonical example of 'quantum magnetism'. This state consists of entangled nearest-neighbor spin dimers and often exhibits a field-induced 'triplon' Bose-Einstein condensate (BEC) phase. I will discuss a new QDM in the strongly spin-orbit coupled, distorted honeycomb-lattice material Yb2Si2O7 [1]. Single crystal neutron scattering, specific heat, and ultrasound velocity measurements reveal a gapped singlet zero field ground state with sharp, dispersive excitations. We find a field-induced magnetically ordered phase reminiscent of a BEC phase, with exceptionally low critical fields of Hc1 ~0.4 T and Hc2 ~1.4 T. Using inelastic neutron scattering we observe a Goldstone mode that persists throughout the entire field-induced magnetically ordered phase, suggestive of the spontaneous breaking of U(1) symmetry expected for a triplon BEC. However, in contrast to other well- known cases of this phase, the high-field (H > 1.2T) part of the phase diagram in Yb2Si2O7 is interrupted by an unusual regime signaled by a change in the field dependence of the ultrasound velocity and net magnetization, as well as the disappearance of a sharp anomaly in the specific heat. These measurements raise the question of how anisotropy in strongly spin-orbit coupled materials modifies the field induced phases of QDMs.

**Strange Metal Transport Properties of ****Electron-Doped**** ****La2-xCexCuO4**

Tarapada Sarkar, University of Maryland-College Park

We report measurements of resistivity, Hall Effect, magnetoresistance and thermopower in the electron- doped cuprate La2-xCexCuO4 for 0.19≥ x ≥0.08 as a function of temperature. The surprising new and unconventional results are:

- The normal state magnetoresistance exhibits an anomalous linear-in-H behavior [1] at the same dopingandtemperaturewherea linear-in-T resistivity was previously observed for H>Hc2 [2],i.e.above the Fermi surface reconstruction at x =0.14 up to the end of the superconducting dome (x~ 0.175). For doping above the dome conventional Fermi liquid behavior is found.
- The normal state Seebeck coefficient, S/T, exhibits an unconventional low temperature –lnT dependence at the same doping where linear-in-T and linear-in-H resistivity is found [3]. Conventional S/T = constant behavior is found above the superconducting dome.
- The normal state resistivity above Tc, from 80 K to 400 K, follows an anomalous ~A(x)T2behavior at zero field for all doping(x), with no indication of a MIR limit [4].

We conclude that conventional Fermi liquid theory cannot explain any of these results. Moreover, the magnitude of the anomalous magnetoresistance and thermopower scales with Tc, suggesting that the origin of the superconductivity is correlated with the anomalous normal state properties. If time allows, we will discuss the surprising origin of the QCP at the end of the SC dome [5].

**Is Superfluid 3He-A a Precursor to Magnetically Ordered Solid 3He?**

*James** Sauls, Northwestern University*

*Liquid 3 He is a strongly correlated Fermi liquid with heavy quasiparticles that become superconducting at low temperatures. There are two broken symmetry phases, both of which are spin-triplet, p-wave BCS condensates. The bulk of the pressure-temperature phase diagram is occupied by the time-reversal invariant B phase, a condensate of entangled spin-triplet, p-wave Cooper pairs with a pair amplitude jBi = Y1;-1(p)j ""i+Y1;+1(p)j ##i+Y1;0(p)j "# + #"i . This is the ground state predicted by Balian and Werthamer in 1963 based on weak-coupling BCS theory for p-wave pairing valid for any pressure. By contrast, the high pressure A phase is a condensate of antiferromagnetically ordered, chiral p-wave Cooper pairs, jAi = Y1;+1(p) (j ""i+j ##i) . Thus, the A phase breaks time-reversal and mirror reflection symmetries, as well as gauge, spin and orbital rotational symmetries.*

Despite our detailed understanding of the physical properties of the phases of superfluid 3 He, a quantitative theory of the pairing mechanism, phase diagram and thermodynamics of the high-pressure superfluid phases has been elusive. Above the tri-critical pressure of pPCP = 21 bar, the A phase is stabilized in a window of temperatures, TAB < T < Tc , separated from the B phase by a pressure and temperature dependent first-order transition at TAB(p) . The stability of the A phase requires a microscopic pairing theory based on strong-correlation physics that goes beyond weak-coupling BCS theory. The “feedback” model proposed by Anderson and Brinkman in which spin-triplet pairing correlations modify the spin-fluctuation-mediated pairing interaction based on paramagnon exchange was a key insight pointing towards a mechanism to stabilize the equal-spin-pairing A phase over the B phase. However, paramagnon exchange theory fails to provide quantitative predictions for the stability of the A phase, specifically the pressure-temperature phase diagram.

I present a strong-coupling theory of superfluid 3 He based on a generalized fluctuation-mediated theory of paring, combined with next-to-leading order corrections to weak-coupling pairing theory based on quasiparticle-quasparticle interactions that accurately describes the thermodynamic potentials for the A and B phases at all pressures and temperatures below Tc(p).

The interaction potentials that describe the quasiparticle scattering amplitudes exhibit a broad ferromagnetic spin-fluctuation peak near q = 0, reminiscent of paramagnon theory, but also resonances corresponding to antiferromagnetic spin-fluctuations and density fluctuations at wavevector Q = 2kf = 0: 82. This wavevector corresponds to a reciprocal lattice vector of bcc solid 3 He at melting pressure. The results provide a quantitative strong-coupling theory for the stability of the A phase, and imply that liquid 3 He at high pressures is an almost localized Fermi liquid near a Mott transition, and suggests that the equal-spin pairing A phase is the precursor of the UUDD phase of solid 3He.

**Magnetic Topological Semimetal in Square-Net materials**

*Leslie** Schoop, Princeton University*

*Materials containing the structural motif of a square-net, have been heavily investigated in respect to their topological semimetal nature. While many of these are nonmagnetic, magnetism can be introduced to these compounds due to their range of chemical tunability. In this talk I will introduce recently developed guidelines that characterize square net materials based on chemical concepts and are suitable to separate trivial ones form topological ones, just by considering atomic distances and electron counts. I will also discuss our recent progress in synthesizing and characterizing magnetically ordered members of this family. *

**Mapping the Fermi Surface in HgBa2CuO4+d with Angular Magnetoresistance**

*Katherine Schreiber, Los Alamos National Laboratory*

The Fermi surfaces of underdoped cuprates are reconstructed by charge density waves. As a result, the Fermi surfaces may be quite complex, featuring small pockets and interlayer warping. The shape of the reconstructed Fermi surface, including warping of the surface along the c-axis, provides important information towards discerning the symmetry of the charge density waves. With this knowledge, the relationship between the charge density wave and superconductivity may become better understood. In this work, we present a study of the Fermi surface in underdoped HgBa2CuO4+d (Hg1201) samples, obtained through angular magnetoresistance measurements. Transport along the c-axis was measured in pulsed magnetic fields of up to 65 T, applied along many polar and azimuthal angles in order to map out the Fermi surface, over a broad range of temperatures. We discuss the implication of our measurements for the shape of the Fermi surface, including the interlayer warping.

**Novel electronic nematicity in (Ba,Rb)Fe2As2 iron-based superconductors**

*Takasada** Shibauchi, University of Tokyo*

*Electronic nematicity, a correlated state that spontaneously breaks rotational symmetry, is observed in several layered quantum materials. In contrast to their liquid-crystal counterparts, the nematic director cannot usually point in an arbitrary direction (XY nematics), but is locked by the crystal to discrete directions (Ising nematics), resulting in strongly anisotropic fluctuations above the transition. Here, we report on the observation of isotropic XY-nematic fluctuations, via elastoresistance measurements, in hole-doped Ba*_{1−x}Rb* _{x}*Fe

*Rajiv** Singh, University of California, Davis*

*Spin-S Kitaev models on the Honeycomb lattice share many features of the soluble spin-half Kitaev model. They have conserved Z_2 fluxes on each elementary hexagonal plaquette and spin-spin correlations are zero beyond nearest-neighbor. We present a study of the thermodynamic properties of these quantum spin-liquids using high-temperature series expansions and thermal pure-quantum methods. We find evidence for plateaus in entropy, where the value of the entropy equals half the total entropy, which implies a highly degenerate low-energy manifold. Anisotropy in the Kitaev couplings destroys these plateaus and instead produces robust plateaus at much smaller entropy values. We discuss possible reasons for the origin of these plateaus. *

**Unconventional superconductivity? The strange case of CeCu2Si2**

*Frank** Steglich, Max Planck Institute for Chemical Physics of Solids*

**Exotic spin-orbital entangled phases in 4d and 5d transition metal oxides**

*Hidenori** Takagi, Max Planck Institute for Solid State Research*

*The exploration of novel phases of interacting electrons (correlated electrons) has long been a major stream of condensed-matter research. Many-body interactions among electrons give rise to a huge variety of phases, grouped into electron-solid, -liquid-crystal, -liquid and -gas states. The wealth of possibilities arises from a complicated interplay of lattice geometry, quantum effects and the multiple degrees of freedom of the electron (charge, spin and orbital). In the past, the two dominant areas of exploration have been the 3d transition-metal (TM) oxides and the 4f intermetallic compounds but recently 5d TM oxides and related compounds have emerged as the next arena of correlated-electron physics. Significant new physics is expected due to the presence of a large spin-orbit coupling in heavy 5d elements, tying together the otherwise independent spin and orbital degrees of freedom. This can be of order 0.5eV and is often larger than the crystal-field splitting of the orbital states, resulting in a spin-orbital-entangled state of correlated electrons. The nature of the spin-orbital entanglement depends significantly on the d-electron number and the chemical bonding, and it is anticipated that, in combination with electron correlations, a rich variety of novel electronic phases are waiting to be discovered. To name just a few, the proposed phases include Kitaev quantum spin liquids, correlated topological semimetals, excitonic magnets and multipolar-ordered states. *

In this talk, I will present our recent exploration of such exotic faces of spin-orbital entangled matter in 5d (and 4d) transition metal oxides. Topics will include the following.

- Spin-orbital quantum liquid on honeycomb lattice in 5d
^{5}H_{3}LiIr_{2}O_{6}[1]. - J
_{eff}= 0 Mott insulator [2] in 4d^{4 }Ru^{4+}oxides and proximity to excitonic magnetism. - Multipolar ordering in 5d
^{1}Ta^{4+}chlorides [3].

** **

**Revisiting the dimensional crossover observed in the BEC material BaCuSi2O6 with new experimental results on Sr-doped samples**

*Franziska Weickert, Florida State University*

Thermodynamic investigations of pure BaCuSi2O6 close to *H*c1 uncovered BEC behavior in the critical exponent of the phase boundary. Furthermore, they revealed a dimensional crossover from 3D for *T *> 1K towards 2D for lower temperatures [4] that is unique for BaCuSi2O6 compared to other BEC materials. This unexpected discovery is most surprising, because low-energy coupling terms are usually amplified close to quantum critical points, which increases rather than decreases the dimension when lowering the temperature [1]. First interpretations argued that frustrated interlayer exchange causes the dimensional reduction by effectively decoupling the dimer layers along the *c*-axis [5]. In contrast, a theoretical study based on DFT calculations excluded magnetic frustration to be present in this material [6] and opened up again the discussion on the effect of structural lattice distortion on the dimensionality of the BEC.

In the presentation, we discuss experimental results on newly synthesized single crystals BaCuSi2O6 doped with 10% Sr [7,8], which do not exhibit a structural phase transition at 90K and allow to investigate BEC in the absence of lattice distortions. We establish the *H-T *phase diagram based on magnetization, specific heat and magnetocaloric effect (MCE) measurements in pulsed magnetic fields revealing a dome-shaped BEC phase with slightly lower critical fields compared to pure BaCuSi2O6 as displayed in Fig. 1. We furthermore discuss magnetic torque measurements carried out in high DC fields up to 31T and down to 0.3K that address the dimensional crossover in the critical exponent of the phase boundary close to *H*c1.

** **

**Role of Orbital Physics in Iron Chalcogenides**

*Ming** Yi, Rice University*

*Electron correlation effects give rise to a variety of emergent phenomena in quantum materials—high temperature superconductivity, electronic nematicity, Mott insulating phase, magnetism. The family of Fe(Se,Te) superconductors plays a remarkable host to all of these phenomena in different parameter regimes. In this talk, I will present angle-resolved photoemission results on two aspects of electron correlation effects in this material family—i) orbital-selective Mott insulating behaviors towards the FeTe end of the phase diagram, and ii) electronic nematicity in completely detwinned FeSe. Both examples showcase the phenomenal way that correlation effects rewrite the low energy electronic states of a material system, and reveal the exceptional role the orbital degree of freedom plays in composing the fundamental physics in iron chalcogenide superconductors. *

*Rong** Yu, Renmin University of China*

*Electronic order in general, and electronic nematic order in particular, has been the topic of considerable interest in the area of iron-based superconductors. New type of questions continues to emerge. In this talk, I will focus on two recent developments along this direction, with a particular attention paid to the role of electron correlations. First, I will show that the multiorbital aspect of electron correlations, in the form of orbital selectivity, interplays with the nematicity in a striking way [1]. Illustrated in the context of the bulk FeSe, I will show that a finite nematic order helps to stabilize an orbital selective Mott phase. Moreover, when the various types of bond and site nematic orders are combined, there exists a surprisingly large orbital selectivity between the xz and yz orbitals even though the associated band splitting is relatively small. These results explain the seemingly unusual observation of strong orbital selectivity in the nematic phase of FeSe [2]. In the second example, I will show, based on a Ginzburg-Landau theory with symmetry analysis, that the spin correlations in the system allow for a variety of nematic orders, in particular an unusual $B_{2g}$ nematicity [3]. Using qualitative considerations as well as microscopic calculations, I will discuss the types of magnetic fluctuations that stabilize this $B_{2g}$ nematicity and how our proposed mechanism provides a natural understanding of the recent experimental observations in the heavily hole doped iron pnictides (Rb,Cs)Fe$_2$As$_2$ [4,5]. *

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