Abstracts

Simon Billinge

Columbia University

When Hard Materials Act Soft: Local Symmetry Breaking in Quantum Material, How to Find It, and Why You Should Care About It

Modern materials under study for next generation technologies, such as for energy conversion and storage, environmental remediation and health, are highly complex, often heterogeneous and nano structured. A full understanding of the structure requires us to go beyond crystallography and to study the local structure, which is a major experimental challenge. There are recently emerging powerful experimental developments, for example, using the atomic pair distribution function technique (PDF), among others. In this talk I will focus on bulk materials, in particular quantum materials, that have distorted local structures, a potentially large class of materials where, nonetheless this property has been largely overlooked. In particular, I will focus on materials where atomic or bonding orbitals are electronically active, driving the local atomic distortions. I will describe a new language we are developing for classifying these materials, and new modeling tools that are under development to reveal the local structures.

Collin Broholm

Johns Hopkins University

Rare Earth Magnetism in Non-centrosymmetric Semi-metals

I shall discuss and contrast two distinct forms of rare earth magnetism in non-centrosymmetric semi-metals that we have studied using neutron scattering techniques. In CeRu4Sn6 there is no phase transition down to T=0.2 K and the magnetic excitation spectrum is gapless and critical through much of the Brillouin zone [1]. This observation of quantum criticality in a stoichiometric intermetallic without tuning suggests the possibility of a quantum critical phase. NdAlSi does have a magnetic phase transition to incommensurate magnetic order at T=7.2 K followed by a transition to commensurate ferrimagnetism at T=3.3 K [2]. I shall discuss the relation of these magnetic structures to topologically protected Weyl points in the electronic band structure of NdAlSi.

This work was supported as part of the Institute for Quantum Matter, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0019331.

[1] “Pristine quantum criticality in a Kondo semimetal,” W. T. Fuhrman, A. Sidorenko, J. Hänel, H. Winkler, A. Prokofiev, J. A. Rodriguez-Rivera, Y. Qiu, P. Blaha, Q. Si, C. L. Broholm, and S. Paschen, Sci. Adv. 7, eabf0134, (2021).

[2] “Incommensurate magnetism mediated by Weyl fermions in NdAlSi” Jonathan Gaudet, Hung-Yu Yang, Santu Baidya, Baozhu Lu, Guangyong Xu, Yang Zhao, Jose A. Rodriguez, Christina M. Hoffmann, David E. Graf, Darius H. Torchinsky, Predrag Nikolić, David Vanderbilt, Fazel Tafti, and Collin L. Broholm, arXiv:2012.12970, (2020).

Morten Eskildsen

University of Notre Dame

Broken Time-reversal Symmetry in the Topological Superconductor UPt3

Subjecting a type-II superconductor to a magnetic field will cause the formations of quantized vortices. The vortices introduce singularities in the order parameter and may be used as probes of the superconducting state in the host material. Moreover, the structural and dynamical properties of vortex matter is of both fundamental interest as well as practical importance. Here I will discuss our small-angle neutron scattering studies of the vortex lattice in UPt3 from both of the above-mentioned perspectives.

Identification of broken time-reversal symmetry (BTRS), a key component of chiral symmetry, of the superconducting order parameter has presented a challenge in bulk superconductors. The two leading candidates for bulk chiral superconductors are UPt3 and Sr2RuO4, although evidence for comes largely from surface-sensitive measurements and have recently been called into question for the latter. In our SANS studies of UPt3 we discovered a previously unknown non-monotonic VL rotation in the so-called B-phase with increasing field [1]. Furthermore, the VL rotation depends on the field history, demonstrating that the vortices possess an internal degree of freedom and providing direct evidence for bulk BTRS in this material.

The UPt3 VL undergoes a gradual disordering on a time scale of tens of minutes as it is subjected to a beam of cold neutrons [2]. The disordering is due to local heating events caused by neutron induced fission of 235U, which leaves an increasing fraction of the sample in a quenched vortex glass state. The disordering rate is proportional to the vortex density, suggesting a direct relation to collective VL properties such as the elastic moduli. While the system does not spontaneously re-order once the local heating has been dissipated, it is possible to re-anneal the VL by the application of a small-amplitude field oscillation. This shows that no permanent radiation damage of the UPt3 crystal occur within experimental time scales. Our results demonstrate a novel avenue for vortex matter studies, allowing an introduction of localized and reversible quenched disorder.

References

1. K. E. Avers et al., Nat. Phys. 16, 531-535 (2020).

2. K. E. Avers et al., arXiv:2103.09843.

Bruce Gaulin

McMaster University

Quantum Spin Liquid States and the Dipole-Octuple Pyrochlore Ce2Zr2O7

Abstract

Alexander Grutter

National Institute of Standards and Technology (NIST) Center for Neutron Research

Unraveling Proximity and Topology at Interfaces with Next Generation Neutron Reflectometry

Whether it originates in crystallinity, magnetism, or electronic structure, interfacial symmetry breaking represents one of the most powerful tools for the realization of new quantum materials with advanced functionality. Mismatches in band topology and time reversal symmetry across interfaces have been harnessed to open gaps in the surface states of topological insulators or to induce topological transitions. Heterostructures interfacing superconductors with a quantum anomalous hall insulator (QAHI) have been reported to exhibit signatures of Majorana fermions, while two-dimensional systems with strong spin-orbit interactions have long been suspected of harboring skyrmions at interfaces with perpendicular magnetic materials. In all of these cases, our understanding of the underlying physics has hinged critically on the ability to precisely isolate the properties of the interface from the bulk of the system. By decomposing the magnetic and electronic properties on a layer-by-layer and element-resolved basis, new quantum material systems may be robustly understood and designed. In this talk, I will discuss our recent progress in applying polarized neutron reflectometry in concert with X-ray scattering, spectroscopy and electron microscopy to uniquely identify modeling solutions in the complex and challenging parameter space of topologically nontrivial heterostructures. The discussion will particularly focus on the new capabilities enabled by CANDOR, the new polychromatic neutron reflectometer at the NIST Center for Neutron Research. A highly intensity-limited technique, neutron reflectometry has been critically hampered by the long measurement times necessary to probe the trace magnetic signals in quantum materials. By implementing a polychromatic beam with multiplexed energy analyzing detectors, CANDOR allows for multiple orders of magnitude intensity gains, allowing even more sensitive measurements to be performed in hours instead of days.

Sajna Hameed

University of Minnesota

Nature of the Phase Transitions in La-substituted and Ca-Doped YTiO3

The Mott insulating rare-earth titanates (RTiO3, R being a rare-earth ion) exhibit myriad interesting spin-orbital phases that arise from a strong coupling among spin, lattice, electronic and orbital degrees of freedom. Here, we report on a comprehensive study of the nature of the different phase-transitions exhibited by isovalently substituted and hole-doped ferromagnetic Mott insulator YTiO3. This includes a ferromagnet-antiferromagnet phase transition/cross-over in the isovalently doped system Y1-xLaxTiO3 at x ~ 0.3, a ferromagnet-paramagnet phase transition in the hole-doped system Y1-yCayTiO3 at y ~ 0.2, and an insulator-metal transition at y ~ 0.35. Through combined neutron diffraction, x-ray absorption spectroscopy and muon spin rotation measurements, we find that the thermal magnetic phase transitions and the insulator-metal transition have an inherent first-order nature [1-3]. Fresh insight into the spin-exchange interaction is obtained via additional inelastic neutron scattering measurements and theoretical calculations [3]. We will also report on the effects of uniaxial elastic strain on the ferromagnetic transition temperatures in Y1-xLaxTiO3 and Y1-yCayTiO3 [4].

This work was supported by the Department of Energy through the University of Minnesota Center for Quantum Materials under Grant No. DE-SC0016371.

[1] S. Hameed et al., arXiv:2103.08565 (2021)

[2] S. Hameed et al., arXiv:2103.08566 (2021)

[3] S. Hameed et al., manuscript in preparation (2021)

[4] A. Najev et al., arXiv:2105.06695 (2021)

Mingda Li

Massachusetts Institute of Technology

Theory and machine learning enhanced neutron scattering for emergent topological materials

Topological materials contain robust electronic states against perturbation and have promising energy and information applications. However, the fingerprint of nontrivial topology onto neutron spectra is not clear. In this presentation, I will introduce how quantum field theories and machine learning can aid neutron scattering to probe topological materials with one example each. For topological Weyl semimetals, field-theoretical calculation predicted the presence of Kohn anomaly, that the electronic topology leaves hallmark phonon softening at specific points in Brillouin zone, which enables the inelastic scattering probe that agrees with predictions. For topological insulator heterostructures, machine learning is shown to capture the small but correlated signals for polarized neutron reflectometry to amplify the small magnetic proximity effect. I will conclude by showing the increasingly important roles theory and machine learning can play to study topological materials through neutron scattering.

Peter Littlewood

University of Chicago

Long Range Dynamical Correlations Near Structural Phase Transitions in Ferroelectrics and Ferroelastics

All electronically driven phase transitions are coupled to local ionic displacements, sometimes as their principal manifestation (e.g. charge density waves, ferroelectrics) and sometimes as a side effect (e.g. magnetism, metal to insulator transitions).

The coupling to the lattice induces elastic strain fields, which have intrinsic long-range interactions that cannot be screened. When strain fields are produced as a secondary order parameter in phase transitions - as for example in ferroelectrics - this produces unexpected consequences for the dynamics of order parameter fluctuations, including the generation of a gap in what would otherwise have been expected to be Goldstone modes.

This talk will pick examples in two classes of materials. One is the transition metal oxide perovskites where coupling of the fundamental order parameter to octahedral rotations gives rise to large entropic effects that can shift the transition temperature by hundreds of degrees K , essentially by exploiting the physics of jammed solids. A second example is the ferroelectric GeTe, where the transition to the cubic phase is accompanied by the development of long-range anisotropic fluctuations.

Modern neutron and X-ray scattering methods are well placed to reveal these correlations, and to provide insight to tune materials properties by manipulating the compositional makeup independent of electronic physics.

In collaboration with G Guzman-Verri, University of Costa Rica, and S Kimber, Université de Bourgogne

Despina Louca

University of Virginia

Exotic Magnetoresistive Materials

Topological superconductors (TSC) can host exotic quasiparticles such as Majorana fermions, poised as the fundamental qubits of quantum computers. TSC’s are predicted to form a superconducting gap in the bulk, and gapless surface/edges states which can lead to the emergence of Majorana zero energy modes. A candidate TSC is the layered dichalcogenide MoTe2, a type-II Weyl (semi)metal in the non-centrosymmetric orthorhombic (Td) phase. It becomes superconducting upon cooling below 0.25 K, while under pressure, superconductivity extends well beyond the structural boundary between the orthorhombic and monoclinic (1T) phases. Here, we show that under pressure, coupled with the electronic band transition across the Td to 1T phase boundary, a new phase we call Td* appears as the volume fraction of the Td phase decreases in the co-existence region. Td* is centrosymmetric with a four-layer unit cell and AABB layer stacking (and its twin, ABBA). In the region of space where Td* appears, Weyl nodes are destroyed. Td* disappears upon entering the monoclinic phase as a function of temperature or on approaching the suppression of the orthorhombic phase under pressure above 1 GPa. Our calculations in the orthorhombic phase under pressure show significant band tilting around the Weyl nodes that most likely changes the spin-orbital texture of the electron and hole pockets near the Fermi surface under pressure that may be linked to the observed suppression of magnetoresistance with pressure.

Martin Mourigal

Georgia Institute of Technology

Magnon Pairing, Interactions, and Decay in the Spin-orbital Magnet FeI2

One of the scientific frontiers in quantum magnetism is the discovery and understanding of quantum entangled and topologically ordered states in real bulk materials. At the focal point of the experimental investigation of these quantum spin networks is the identification of fractionalized excitations in transport and spectroscopic measurements. Inelastic neutron scattering has proved a powerful technique to reveal such signatures in a variety of systems ranging from quasi-1D magnets to kagome compounds and more. Recent and on-going developments with neutron scattering instrumentation have allowed the characterization of magnetic excitations in entire volumes of momentum-energy space with high resolution. This has prompted revisiting long overlooked materials in search for exotic spin dynamics despite seemingly classical magnetically ordered ground-states. In this talk, I will discuss such experiments on a long-known material, FeI2, and show how high-fidelity modeling brings new insights on its spin dynamics [1]. I will describe the mechanism that endows low energy quadrupolar fluctuations in FeI2 with large spectral weight and how these can be completely understood using a SU(3) representation of spin degrees of freedom. I will discuss the consequence of the having several quasiparticles as the low-energy degrees of freedom in this system [2].

This work was supported by DOE/BES under award DE-SC-0018660.

[1] X. Bai, S.-S. Zhang, Z. L. Dun, H. Zhang, Q. Huang, H. D. Zhou, M. B. Stone, A. I. Kolesnikov, F. Ye, C. D. Batista, M. Mourigal, “Hybridized quadrupolar excitations in the frustrated and spin-anisotropic magnet FeI2 ”, Nature Physics 17, 467-472 (2021), https://doi.org/10.1038/s41567-020-01110-1.

[2] A. Legros, S.-S. Zhang, X. Bai, H. Zhang, Z. L. Dun, W. A. Phelan, C. D. Batista, M. Mourigal, and N. P. Armitage, “Observation of 4- and 6-magnon bound-states in the spin-anisotropic frustrated antiferromagnet FeI2”, Submitted (November 2020), https://arxiv.org/abs/2012.04205.

Sebastian Mühlbauer

Technische Universität München

Vortex Matter Beyond the Standard SANS Approach

Superconducting vortex lattices can be regarded as macroscopic lattices, formed by topological entities. Analogous to condensed matter, a large variety of phases is also observed, resembling their particle like character and reflecting the underlying physical properties. Moreover, vortex matter represents ideal model systems for questions of general importance like self-organization, pattern formation and also domain nucleation and growth.

Neutron scattering provides an ideal tool for the investigation of vortex matter in bulk samples. Going beyond the standard SANS approach [1], we present an overview how to address the static and dynamic properties of superconducting vortex matter by means of neutron grating interferometry (nGI) [2,3,4], time-resolved small angle neutron scattering (TISANE) [5], ultra small-angle neutron scattering (USANS) [2,3] and the neutron resonance spin echo spectroscopy technique (MIEZE). We combine our SANS measurements with molecular dynamics simulations of superconducting vortex matter [6] and an in-situ transport setup [7] to study orthogonal flow phenomena. With our combined approach, we are able to cover a wide range of length scales from nm to mm and time-scales from seconds to ps.

[1] S. Mühlbauer et al., Reviews of Modern Physics 91, 015004, (2019)

[2] A. Backs et al., Phys. Rev. B 100, 064503, (2019)

[3] T. Reimann et al., Phys Rev. B 96, 144506. (2017)

[4] T. Reimann et al., Nature communications 6, article number 8813, (2015)

[5] S. Mühlbauer et al., Phys. Rev. B 83, 184502 (2011)

[6] A. Backs et al, in prep. (2021)

[7] X. Brems et al., arXiv:2104.07967 (2021)

Raymond Osborn

Argonne National Laboratory

More is Different: Real Space Maps of Structural Correlations in Quantum Materials

Recent advances in both neutron and x-ray instrumentation, such as the SNS diffractometer, Corelli, allow the collection of large volumes of diffuse scattering in crystalline materials with high efficiency. This enables new methods of interrogating the data that provide information on structural correlations within quantum materials, both long and short-range, that does not depend on complex simulations. In particular, it is now possible to transform S(Q) into three-dimensional pair distribution functions, providing model-independent "images" of nanoscale disorder in real space. By eliminating Bragg peaks before the transformation, these 3D-∆PDF measurements reveal structural correlations directly, displaying only the probabilities of interatomic vectors that deviate from the average structure. I will discuss two examples drawn from recent x-ray experiments. In the first, 3D-∆PDF maps reveal the collapse of 3D order at the metal-insulator transition of doped VO2 and the emergence of extended 2D correlations resulting from a novel form of geometric frustration. In the second, structural distortions due to a charge-density-wave in Sr3Rh4Sn13 are shown to persist above Tc, consistent with an order-disorder transition. I will also show an example of 3D-∆PDF analysis on Corelli.

Work supported by the U.S. DOE, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division.

Damjan Pelc

University of Zagreb and University of Minnesota

Structural Inhomogeneity in Oxide Superconductors

It has long been known that some of the most prominent complex oxides display significant structural and electronic inhomogeneity. The nature and role of this inhomogeneity, however, has been intensively debated. In this talk, I will present new insights into intrinsic structural inhomogeneity from state-of-the-art diffuse neutron and x-ray scattering experiments, in two representative material systems: the cuprate high-temperature superconductors [1] and strontium titanate [2]. Our ongoing, systematic studies of cuprate local structure reveal intriguing short-range correlated disorder; in strontium titanate, we have been able to manipulate extended defects using plastic deformation, with striking effects on the electronic subsystem. These results show that structural inhomogeneity is an essential ingredient in the physics of these oxides, and that it can be used to tune their electronic properties, with implications for a wide range of important quantum materials.

The work at the University of Minnesota was supported by the Department of Energy through the University of Minnesota Center for Quantum Materials under DE-SC0016371.

[1] D. Pelc et al., arxiv:2103.05482 (2021)

[2] S. Hameed et al., arxiv:2005.00514 (2020)

Natalia Perkins

University of Minnesota

Magnon-spinon Dichotomy in the Kitaev Hyperhoneycomb β-Li2IrO3

The three-dimensional hyperhoneycomb iridate beta-Li2IrO3 is perhaps the most intriguing example of Kitaev materials with complex magnetic ground state and the intricate interplay between the Kitaev exchange and external magnetic fields. In addition, it shows signatures of Majorana fermions with long coherent times at intermediate and higher energy scales (observed in RIXS), similar to the observation of spinons in quasi-1D spin chains. We present a theoretical study of the response of beta-Li2IrO3 under external magnetic fields in different directions. The results are based on the minimal nearest-neighbor J-K-Gamma model and reveal a rich intertwining of field-induced phases and magnetic phase transitions.

Kate Ross

Colorado State University

Microscopics of Quantum Annealing in the Disordered Dipolar Ising Ferromagnet LiHoxY1-xF4

The technique of “quantum annealing” (QA) involves using quantum fluctuations to find the global minimum of a rugged energy landscape. For some problems it has been shown to produce faster optimization than thermal annealing (TA), and it has been adopted as one technique used for quantum computing (adiabatic quantum computing). Conceptually, QA is often framed in the context of the disordered transverse field Ising model, where a magnetic field applied perpendicular to the Ising axis tunes the quantum fluctuations and enables a “better” (lower energy) spin configuration to be obtained via quantum tunneling. A celebrated material example of this model, LiHo0.45Y0.55F4, was shown decades ago to exhibit faster dynamics after a QA protocol, compared to a TA protocol. However, little is known about the actual process of optimization involved and ultimately what the optimal spin configurations are like.

We have set out to understand the microscopics of QA in LiHo0.45Y0.55F4 using diffuse magnetic neutron scattering. We performed the same protocols as initially used to demonstrate QA in this material, and find that the QA protocol results in what appears to be the equilibrium state, whereas TA results in a state that continues to evolve over time. This is as expected if QA indeed provides an optimization “speed up” compared to TA. However, we also clearly observe evidence that the transverse field does more than just introduce quantum fluctuations; namely, it produces random longitudinal fields, which had been previously studied theoretically and experimentally. Thus, while the material does respond to QA differently than TA, it is not a simple annealing problem; the energy landscape being optimized is changing as the optimization proceeds. Understanding this version of quantum annealing could be of interest in the context of adiabatic quantum computing, possibly for designing new algorithms, or for accounting for unwanted experimental effects.

Alan Tennant

Oak Ridge National Laboratory

New Ways to Look at Quantum Materials with Neutrons: From Entanglement Witnesses to Machine Learning

Information theory and machine learning provide two new approaches to understanding scattering data. A place where such methods is needed is in understanding quantum materials that host unusual phases and excitations. We have recently applied these to quantum magnets and frustrated systems where quantum entanglement and topological effects are important. By applying quantum information theory, we show how the quantum Fisher information as well as one- and two-tangle can be used to detect the degree of entanglement in materials, as well as their use as a model agnostic approach to scattering experiments. Machine learning provides an alternative approach where large scale modeling and complex data can be integrated together. This approach is shown to be advantageous by providing a more automated approach to difficult analysis challenges as well as being able to dovetail with advances in modeling of quantum systems.

John Tranquada

Brookhaven National Laboratory

Neutron Scattering Reveals a Spatially-modulated Spin Gap in Cuprate Superconductors

In the stripe-ordered phase of La2-xBaxCuO4, the antiphase alignment of neighboring spin stripes decouples them from the low-energy spin excitations within the charge stripes [1], similar to the decoupling of 1D spin excitations in stripe-order La1.67Sr0.33NiO4 [2]. In the cuprate case, the charge stripes behave as 2-leg spin-1/2 ladders in which the doped holes are confined as pairs [3] for energies less than the singlet-triplet excitation energy. Above that energy scale, individual holes become deconfined; their motion strongly damps the spin correlations, resulting in a spin-spin correlation length of about one lattice spacing [4]. In order to achieve spatially-uniform superconductivity, it is necessary to avoid spin-stripe order and to gap the incommensurate, antiphase spin fluctuations. Evidence of this comes from the empirical observation that the incommensurate spin gap provides an upper limit on the coherent superconducting gap for all studied cuprates [5].

1. J. M. Tranquada, Adv. Phys. (accepted); arXiv:2102.02257

2. A. M. Merritt et al., Phys. Rev. B 100, 195122 (2019)

3. Yangmu Li et al., Sci. Adv. 5, eaav7686 (2019)

4. Guangyong Xu et al., Phys. Rev. B 76, 014508 (2007)

5. Yangmu Li et al., Phys. Rev. B 98, 224508 (2018)

Zhentao Wang

University of Minnesota

Strain-tunable Metamagnetic Critical End-point in Mott Insulating Rare-earth Titanates

Rare-earth titanates are Mott insulators whose magnetic ground state -- antiferromagnetic (AFM) or ferromagnetic (FM) -- can be tuned by the radius of the rare-earth element. Here, we combine phenomenology and first-principles calculations to shed light on the generic magnetic phase diagram of a chemically-substituted titanate on the rare-earth site that interpolates between an AFM and a FM state. Octahedral rotations present in these perovskites cause the AFM order to acquire a small FM component -- and vice-versa -- removing any multi-critical point from the phase diagram. However, for a wide parameter range, a first-order metamagnetic transition line terminating at a critical end-point survives inside the magnetically ordered phase. Similarly to the liquid-gas transition, a Widom line emerges from the end-point, characterized by enhanced fluctuations. In contrast to metallic ferromagnets, this metamagnetic transition involves two symmetry-equivalent and insulating canted spin states. Moreover, instead of a magnetic field, we show that uniaxial strain can be used to tune this transition to zero-temperature, inducing a quantum critical end-point.

This work was supported by the Department of Energy through the University of Minnesota Center for Quantum Materials under Grant No. DE-SC0016371.

Stephen Wilson

University of California, Santa Barbara

Neutron Scattering Insights into Unconventional Metals Neighboring Mott Instabilities in Thin Film Titanates

Thin film heterostructures involving rare earth titanates have given rise to a host of unconventional electronic states, ranging from strange metal phases to tunable, topologically nontrivial metals. Here I will discuss neutron scattering experiments exploring two classes of perovskite titanate thin films (1) thin film heterostructures interfacing Mott states (SmTiO3 and GdTiO3) with a band insulator (SrTiO3) and (2) a carrier-tuned Weyl semi-metal candidate (Eu1-xSmxTiO3). The goal will be to highlight insights gained via neutron scattering measurements of these systems and how neutrons can inform models of their unusual phase behaviors.