## Seminars

### Condensed Matter Physics Seminars

The Physics Condensed Matter Seminars for the Spring Semester 2023 will be held IAMM 147 on Wednesdays 10:20-11:10. The seminars begin for graduate students with an introduction to the course on January 25, with the first full seminar on February 1.** We will meet for discussions and interactions preceding the seminar at 10am, outside IAMM 147, where coffee and cookies will be available. **

The Seminars Series is part of the course PHYS599 and are organized by Alan Tennant who can be reached on dtennant@utk.edu.

If you would like further discussions with the seminar speakers then during their visits then please contact their host for scheduling.

**January 25 Introduction to Course (students only)** A. Tennant (venue IAMM 147)

**February 1 Quantum phase transitions, entanglement and density functional theory**, Lianao Wu, University of the Basque Country (host Mike Guidry) (venue IAMM 310)

Abstract: Density functional theory (DFT) is shown to provide a novel conceptual and computational framework for entanglement in interacting many-body quantum systems. DFT can, in particular, shed light on the intriguing relationship between quantum phase transitions and entanglement. We use DFT concepts to express entanglement measures in terms of the first or second derivative of the ground state energy. We illustrate the versatility of the DFT approach via a variety of analytically solvable models. As a further application we discuss entanglement and quantum phase transitions in the case of mean field approximations for realistic models of many-body systems.

**February 8 Partons as unique ground states of quantum Hall parent Hamiltonians**, Gerardo Ortiz, Indiana University Bloomington (host Cristian Batista) (venue IAMM 147)

I will describe our attempts to construct quantum Hall parent Hamiltonians with multiple-Landau-level orbitals and their relation to parton wave functions whose excitations display either Abelian or non-Abelian braiding statistics. The emergent Entangled Pauli Principle (EPP), which defines the “DNA” of the quantum Hall fluid, is behind the exact determination of the topological characteristics of the fluid, including charge and braiding statistics of excitations, and effective edge theory descriptions. The DNA associated with fractional quantum Hall states admits a tensor network structure of finite bond dimension that emerges via root level entanglement and encodes all universal properties of the fluid.

**February 15 Tunneling Andreev Reflection – new quantitative microscopy of superconductors with atomic resolution**, Petro Maksimovich, Center for Nanophase Materials Science, Oak Ridge National Laboratory (host Wonhee Ko) (venue IAMM 147).

Andreev reflection is an established method to probe the existence of superconductivity, and, crucially, the symmetry of the order parameter. However, the popular point contact method for the measurement of Andreev reflection (PCAR) necessitates formation of good mechanical contact, which severely limits spatial resolution and introduces complicated interpretation, particularly in the important case of unconventional and multi-band superconductors.

In this talk, we will present our recently developed tunneling Andreev reflection (TAR) technique – a new experimental approach to quantify Andreev reflection through a tunnel junction [1]. The technique utilizes the fundamental connection between the physics of the scattering process and the strength of exponential non-linearity of the tunneling current, and therefore adopts the renormalized tunneling current decay rate as the experimental observable. Due to this choice of the observable, TAR enables robust theory-experiment comparison, detailed quantification of the presence and symmetry of the superconducting order parameter, and even quantification of the broadening mechanisms that compete with superconductivity. Remarkably, TAR actually reveals that at least two distinct mechanisms of Andreev reflection are possible in the tunnel junction, dramatically enhancing our ability to probe unconventional superconductivity. We will demonstrate the present capabilities of TAR using classical superconductors, and then extend the discussion toward contested order parameter symmetries in established, but still debated, FeSe superconductor. TAR unambiguously confirms the sign-changing order parameter in this material, with atomic-scale resolution, and further reveals complete suppression of superconductivity along the nematic twin boundaries above 1.2 K. Locally suppressed superconductivity, in turn, explains the peculiar vortex templating effect exerted by twin boundaries – essentially causing recrystallization of the vortex glass phase – and it will likely dramatically modify vortex dynamics compared to uniform medium.

Research sponsored by Division of Materials Science and Engineering, Basic Energy Sciences, Office of Science, US Department of Energy. SPM experiments were carried out as part of a user project at the Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, a US Department of Energy Office of Science User Facility.

1. W. Ko, J. Lado, P. Maksymovych, Nano Lett. 22 (2022) 4042. 2. W. Ko, E. Dumitrescu, P. Maksymovych, Phys. Rev. Res. **3** (2021) 033248 3. P. Maksymovych, J. Yan, B. C. Sales, J. Wang, Phys. Rev. Res. 4 (2022) 033058

**February 22 Elastoresistivity measurement of kagome metal CsV3Sb5**, Zhaoyu Liu, University of Washington (host Jian Liu) (venue IAMM 147).

The recently discovered kagome metal CsV3Sb5 has attracted enormous attention due to its nontrivial topological electronic structure and intertwined symmetry-broken states [1,2]. One central question is the nature of the broken symmetry associated with the charge density wave (CDW) onsets at T ~ 90K [3,4]. In this talk,I will present the measurement of elastoresistivity and elastocaloric effect of CsV3Sb5. Using three different techniques, the differential, the modified Montgomery and the transverse method, we precisely decomposed the elastoresistivity coefficient into different symmetrychannels. We found that the isotropic elastoresistivity coefficient (i.e., m11+m12) increases substantiallybelow the charge density wave transition temperatureand becomes several times larger than the nearly temperature-independent anisotropic coefficient (i.e.,m11-m12). Our results suggest that the charge density wave phase in CsV3Sb5 either does not break rotational symmetry or its broken rotational symmetry is decoupled from the anisotropic strain.

[1] B. R. Ortiz et al., New Kagome Prototype Materials: Discovery of KV3Sb5, RbV3Sb5, and CsV3Sb5, Physical Review Materials 3, 94407 (2019). [2] S. Y. Yang et al., Giant, Unconventional Anomalous Hall Effect in the Metallic Frustrated Magnet Candidate, KV3Sb5, Science Advances 6, 1 (2020). [3] L. Nie et al., Charge-Density-Wave-Driven Electronic Nematicity in a Kagome Superconductor, Nature 604, 7904 (2022). [4] Y. Xu, Z. Ni, Y. Liu, B. R. Ortiz, S. D. Wilson, B. Yan, L. Balents, and L. Wu, Three-state nematicity and magneto-optical Kerr effect in the charge density waves in kagome superconductors, Nat. Phys. 18, 1470–1475 (2022).

**March 1 Complementary theoretical approaches to modeling pyrochlore spin liquid candidates,** Shu Zhang, Max Planck Institute for the Physics of Complex Systems, Dresden (host Yang Zhang) (venue IAMM 147).

Quantum spin liquids (QSLs) are enigmatic phases of matter defined by topological orders and the absence of symmetry breaking. In searching for material candidates of QSLs, a central challenge is to connect detailed theoretical modeling with experimental results. On the one hand, a large amount of data are available from material characterization. On the other hand, they often lack distinct features due to the absence of conventional phase transition and conventional quasiparticles with sharp dispersions. In this talk, I would like to discuss the progress in using complementary theoretical approaches to model the Hamiltonian and the spin dynamics of pyrochlore spin liquid candidates, taking the example of NaCaNi2F7 and Ce2Zr2O7. The case of spin-1 NaCaNi2F7 is well described by a weakly perturbed antiferromagnetic Heisenberg Hamiltonian. By comparing the linear spin wave theory, molecular dynamics simulation, analytical stochastic model, and inelastic neutron scattering data, our study reveals a picture of spin-wave driven exploration of the ground state through a large degenerate manifold. Our more recent work on Ce2Zr2O7 combines exact diagonalization and molecular dynamics simulation, which suggests a model Hamiltonian hosting an exotic pi-flux U(1) QSL.

**March 8 Dynamics and out of equilibrium phenomena in a topological magnet**, Santiago Grigera, Instituto de Fisica de Liquidos y Sistemas Biologicos, UNLP-CONICET La Plata, Argentina (VIRTUAL ONLY) (host Alan Tennant) (Virtual Only)

The spin ice compound Dy2Ti2O7 stands out as the first magnet in three dimensions exhibiting a topological regime, with its tell-tale emergent fractionalized magnetic monopole excitations. Its fate at low temperatures, where theoretically predicted order has not been observed in experiments, has remained an intriguing open question. This talk will present experiments that probe this low temperature phase, such as the measurement of magnetic noise, neutron scattering under different cooling protocols and the creation of magnetic avalanches, and discuss modeling and simulations that shed light on the intriguing dynamics and out of equilibrium physics in this system.

**March 15 Spring break – No seminar.**

**March 22 Quantum correlations and entanglement witnesses in condensed matter**, Pontus Laurell, Dept. Physics, University of Tennessee (host Alan Tennant) (venue IAMM 147).

Entanglement and other nonclassical correlations are ubiquitous in quantum many-body systems. They play a prominent role in theories of important condensed matter phenomena such as novel phases of matter and topological order, and also represent resources to be harnessed in quantum information applications. Yet there has been a distinct lack of viable methods to detect these correlations in the solid state, impeding our ability to identify suitable materials and to unravel their secrets. In this talk I will describe complementary approaches we have taken towards finding useful measures of these properties, which can both be modeled theoretically and measured experimentally in a model-independent fashion, by making use of information “hidden” in dynamical correlations or spectroscopic data. By employing entanglement witnesses, quantities akin to order parameters for certain classes of entangled states, we have been able to witness entanglement in quantum spin chains [1,2] and the Hubbard chain [3]. Fourier transforms into real space and time [3,4] offer new perspectives, providing access both to the spatial structure of quantum correlations and to nonlinear local short-time dynamics in materials. Besides their experimental potential, these approaches can provide new theoretical insights and hints for modeling of quantum materials, and reflect the broad value of quantum information ideas to condensed matter.

[1] A. Scheie, P. Laurell, A. M. Samarkoon, B. Lake, S. E. Nagler, G. E. Granroth, S. Okamoto, G. Alvarez & D. A. Tennant: Phys. Rev. B 103, 224434 (2021). [2] P. Laurell, A. Scheie, C. J. Mukherjee, M. M. Koza, M. Enderle, Z. Tylczynski, S. Okamoto, R. Coldea, D. A. Tennant & G. Alvarez: Phys. Rev. Lett. 127. 037201 (2021). [3] P. Laurell, A. Scheie, D. A. Tennant, S. Okamoto, G. Alvarez & E. Dagotto: Phys. Rev. B 106, 085110 (2022). [4] A. Scheie, P. Laurell, B. Lake, S. E. Nagler, M. B. Stone, J.-S. Caux & D. A. Tennant: Nat. Commun. 13, 5796 (2022).

**March 29 Experiments on quantum spin liquids (tentative)**, Stephen E. Nagler, Dept. of Physics, University of Tennessee, and Oak Ridge National Laboratory (host Alan Tennant) (venue IAMM 147)

**April 5 Hidden Emergent Locality in Quasiparticle Spectrum**, Kun Chen, Flatiron Institute – Simons Foundation, New York (host Yang Zhang) (venue IAMM 147)

Abstract: In this research, we utilize cutting-edge techniques from quantum field theory and numerical techniques to study the structure of quasiparticle spectrum of valence electrons. Our analysis of the renormalized action of the electron liquid uncovers hidden emergent localities in the electron self-energy and electron-electron interaction, opening new possibilities for developing more accurate parameterizations with various potential applications. In particular, by linearizing the self-energy parameterization, we simplify the Dyson equation to a Kohn-Sham-type eigen-problem, which enables a high-throughput, beyond-DFT method for predicting band structures, z-factors and effective massess in real materials with increased accuracy and efficiency.

**April 12 Searching for superconducting materials with Majorana corner modes**, Yi-Ting Hsu, University of Notre Dame (host Ruixing Zhang) (venue IAMM 147)

**Abstract:** Majorana corner modes can exist in topological superconductors (TSCs) protected by certain crystalline symmetries. In this talk, I will show that Majorana zero modes can occur at the corners of two-dimensional inversion-symmetric superconductors *without* utilizing proximity effect. Using mathematically derived topological invariants, I will present how to obtain comprehensive recipes and conduct systematic searches over material databases to identify promising TSC materials with corner Majoranas. By combining DFT band structure calculations and pairing symmetry analyses, I will discuss concrete material candidates, focusing on monolayer transition metal dichalcogenides.

**April 19 Order from and by disorder: routes to spin and orbital ordering on the highly frustrated FCC lattice**, Kemp Plumb, Brown University (host Steve Johnston) (venue IAMM 147)

**Abstract:** When spin orbit coupling is on equal footing with electronic correlations, the spatially anisotropic character of d-orbitals in transition metal compounds can lead to many and diverse quantum phases of matter. The most celebrated example is the Kitaev model for j=1/2 Kramers doublets on the honeycomb lattice. However, coupling of spin and orbital fluctuations gives rise to many more possibilities, including multi-polar order, spin liquids, and/or spin orbitals liquids. In this talk, I will discuss the spin and orbital dynamics of two model face centered cubic antiferromagnets: GaTa_{4}Se_{8} that realizes an orbitally active j=3/2 model; and K_{2}IrCl_{6} that realizes a j=½ antiferromagnet. GaTa_{4}Se_{8} is a cluster Mott insulating lacunar spinel, where electronic correlations, spin orbit coupling, and orbital degeneracy act in concert to produce several interesting magnetic and orbital phases. I will discuss neutron scattering measurements on this material that reveal a dynamical spin-orbital state preceding an order-disorder type spin-orbital ordering transition. Spin-orbit coupling quenches out the classical Jahn-Teller mechanism in GaTa_{4}Se_{8} and results in a novel spin-orbital valence bond ground state. K_{2}IrCl_{6}is a highly frustrated antiferromagnet with dominant Heisenberg and Kitaev interactions. I will present inelastic neutron scattering measurements on this compound that reveal a magnetic excitation spectrum highly renormalized by quantum fluctuations. Our data indicate that a dominant magnetic order is selected in K_{2}IrCl_{6} through a quantum order-by-disorder mechanism, but that two symmetry distinct magnetic orders coexist in pristine samples. These results showcase the power of neutron scattering to reveal spin and orbital dynamics in quantum materials.** **

**April 26 New insights into molecular origins of learning and memory: evidence for long term potentiation in biological membranes **(interdisciplinary seminar with biophysics) Dima Bolmatov, Dept. of Physics, University of Tennessee (host Alan Tennant) (venue IAMM 147)

**Abstract**: Biological supramolecular assemblies, such as biological membranes, have been used to demonstrate signal processing via short-term synaptic plasticity (STP) in the form of paired pulse facilitation (PPF) and depression (PPD), emulating the brain’s efficiency and flexible cognitive capabilities [1]. However, STP memory states in lipid bilayers are volatile and cannot be stored or accessed over relevant periods of time, a key requirement for learning and memory. Using droplet interface bilayers (DIBs) composed of lipids, water and hexadecane, and an electrical stimulation “training” protocol featuring repetitive sinusoidal current-voltage cycling, I will show that DIBs display memcapacitive properties that undergo long-term synaptic plasticity in the form of long-term potentiation (LTP) associated with capacitive energy storage in the phospholipid bilayer [2]. The time scales for the physical changes associated with LTP range between minutes and hours. LTP is the result of molecular and membrane structural changes to the zwitterionic lipid headgroups and the dielectric properties of the lipid bilayer that result from the buildup of an increasingly asymmetric ion charge distribution at the bilayer interfaces. The results provide compelling evidence that in the absence of peptides or proteins, lipid bilayers are capable of LTP [2], emulating hippocampal LTP formation observed in mammals and birds [3]. Moreover, the results support the interpretation that the lipid bilayer provides a model for understanding the molecular basis of biological memory. This newly developed model offers a novel therapeutic target for brain diseases that do not respond to drugs targeting proteins and as a platform for artificial neural network developments and memory computing using crossbar architectures of two-terminal passive circuit memory elements.

[1]. Najem, J. S. *et al*. Dynamical nonlinear memory capacitance in biomimetic membranes. Nat. Commun. 2019, 10 (1), 1-11. [2]. Scott, H. L.; Bolmatov, D.; Podar, P. T.; Liu, Z.; Kinnun, J. J.; Doughty, B.; Lydic, R.; Sacci, R. L.; Collier, C. P.; Katsaras, J. Evidence for long-term potentiation in phospholipid membranes. Proc. Nat. Acad. Sci. 2022, 119 (50), e2212195119. [3]. Bliss, T. V.; Lømo, T. Long‐lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit following stimulation of the perforant path. J. Physiol. 1973, 232 (2), 331-356.

**May 3 Discovery and Innovation at the Quantum Science Center**, Travis Humble, Director, Quantum Science Center, Oak Ridge National Laboratory (host Adrian Del Maestro/Alan Tennant) (venue IAMM 147)

Abstract: The Quantum Science Center is a focal point within our national quantum information science (QIS) ecosystem for scientific discoveries and innovations. As a partnership of 16 institutions, our goal is to overcome key roadblocks in the resilience, controllability, and scalability of quantum technologies by integrating research from many different disciplines to advance discoveries in the fundamental science of QIS toward practical applications that impact economic and national security. Central to our efforts is the creation, detection, and control of quantum information through research in quantum materials, quantum algorithms, and quantum sensors. In this talk, I will review our current goals and progress for next generation QIS capabilities including the development of quantum materials and devices for storing and processing quantum information, quantum sensors and testbeds for detecting and characterizing highly entangled quantum states, and quantum simulators that use topological protection for scientific applications of quantum computing.

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