Dr. Igor Zaliznyak

Division of Condensed Matter Physics and Materials Science

Brookhaven National Laboratorty

Host: Gannon

Abstract:

While most solids expand when heated, some materials show the opposite behavior: negative thermal expansion (NTE). NTE is common in polymers and biomolecules, where it stems from the entropic elasticity of an ideal, freely-jointed chain. The origin of NTE in solids had been widely believed to be different, with phonon anharmonicity and specific lattice vibrations that preserve geometry of the coordination polyhedra – rigid unit motions (RUMs) – as leading contenders for explaining NTE. Our neutron scattering study of a simple cubic NTE material, ScF₃, overturns this consensus [1]. We observe that the correlation in the positions of the neighboring fluorine atoms rapidly fades on warming, indicating an uncorrelated thermal motion, which is only constrained by the rigid Sc-F bonds. These experimental findings lead us to a quantitative, quasi-harmonic theory of NTE in terms of entropic elasticity of a Coulomb floppy network crystal, which is applicable to a broad range of open framework solids featuring floppy network architecture [2]. The theory is in remarkable agreement with experimental results in ScF₃, accurately describing NTE, phonon frequencies, entropic compressibility, and structural phase transition governed by entropic stabilization of criticality. We thus find that NTE in a family of insulating ceramics stems from the same simple and intuitive physics of entropic elasticity of an under-constrained floppy network that has long been appreciated in soft matter and polymer science, but broadly missed by the “hard” condensed matter community. Our results reveal the formidable universality of the NTE phenomenon across soft and hard matter [1,2].

[1] D. Wendt, et al., Sci. Adv. 5: eaay2748. (2019).

[2] A. V. Tkachenko, I. A. Zaliznyak. arXiv:1908.11643 (2019).

Dr. Igor Zaliznyak

Division of Condensed Matter Physics and Materials Science

Brookhaven National Laboratorty

Host: Gannon

Abstract:

While most solids expand when heated, some materials show the opposite behavior: negative thermal expansion (NTE). NTE is common in polymers and biomolecules, where it stems from the entropic elasticity of an ideal, freely-jointed chain. The origin of NTE in solids had been widely believed to be different, with phonon anharmonicity and specific lattice vibrations that preserve geometry of the coordination polyhedra – rigid unit motions (RUMs) – as leading contenders for explaining NTE. Our neutron scattering study of a simple cubic NTE material, ScF₃, overturns this consensus [1]. We observe that the correlation in the positions of the neighboring fluorine atoms rapidly fades on warming, indicating an uncorrelated thermal motion, which is only constrained by the rigid Sc-F bonds. These experimental findings lead us to a quantitative, quasi-harmonic theory of NTE in terms of entropic elasticity of a Coulomb floppy network crystal, which is applicable to a broad range of open framework solids featuring floppy network architecture [2]. The theory is in remarkable agreement with experimental results in ScF₃, accurately describing NTE, phonon frequencies, entropic compressibility, and structural phase transition governed by entropic stabilization of criticality. We thus find that NTE in a family of insulating ceramics stems from the same simple and intuitive physics of entropic elasticity of an under-constrained floppy network that has long been appreciated in soft matter and polymer science, but broadly missed by the “hard” condensed matter community. Our results reveal the formidable universality of the NTE phenomenon across soft and hard matter [1,2].

[1] D. Wendt, et al., Sci. Adv. 5: eaay2748. (2019).

[2] A. V. Tkachenko, I. A. Zaliznyak. arXiv:1908.11643 (2019).

Professor Pengcheng Dai

Department of Physics and Astronomy

Rice University

Host: Gannon

Title:  Magnetic order and spin excitations in two-dimensional honeycomb lattice van der Waals ferromagnetic CrI3

Abstract:  We use neutron scattering to study magnetic order and spin excitations in honeycomb lattice van der Waals ferromagnet CrI3. We show that ferromagnetic phase transition is first order in nature, and controlled by spin-orbit coupling (SOC) induced magnetic anisotropy, instead of magnetic exchange coupling as in a conventional ferromagnet. We discuss the role of the next-nearest-neighbor Dzyaloshinskii-Moriya interaction and Kitaev’s interaction plays on the spin-wave spectra and the possible presence of topological spin excitations in CrI3.

Professor Pengcheng Dai

Department of Physics and Astronomy

Rice University

Host: Gannon

Title:  Magnetic order and spin excitations in two-dimensional honeycomb lattice van der Waals ferromagnetic CrI3

Abstract:  We use neutron scattering to study magnetic order and spin excitations in honeycomb lattice van der Waals ferromagnet CrI3. We show that ferromagnetic phase transition is first order in nature, and controlled by spin-orbit coupling (SOC) induced magnetic anisotropy, instead of magnetic exchange coupling as in a conventional ferromagnet. We discuss the role of the next-nearest-neighbor Dzyaloshinskii-Moriya interaction and Kitaev’s interaction plays on the spin-wave spectra and the possible presence of topological spin excitations in CrI3.

Professor Erik Henriksen

Department of Physics

Washington University in St. Louis

Host: Kaul

Title:  a-RuCl3 as a 2d crystalline acceptor: modulation doping, pn junctions, and the pursuit of Veselago's lens

Abstract: a-RuCl3 is a layered antiferromagnetic Mott insulator widely thought to host a quantum spin liquid state related to the Kitaev QSL. It can be exfoliated down to monolayer thicknesses and incorporated into van der Waals heterostructures along with graphene and myriad other atomically thin materials. Proximity of a-RuCl3 to graphene leads to a significant charge transfer between the two that, surprisingly, persists even when a thin insulating layer is inserted between them, a phenomenon analogous to modulation doping in epitaxially-grown semiconductors. In addition to constituting a new method of charge control in van der Waals stacks, we are using this to generate near-atomically-sharp pn junctions in graphene that may enable a realization of the electron-optic version of Veselago's lens. 

Professor Erik Henriksen

Department of Physics

Washington University in St. Louis

Host: Kaul

Title:  a-RuCl3 as a 2d crystalline acceptor: modulation doping, pn junctions, and the pursuit of Veselago's lens

Abstract: a-RuCl3 is a layered antiferromagnetic Mott insulator widely thought to host a quantum spin liquid state related to the Kitaev QSL. It can be exfoliated down to monolayer thicknesses and incorporated into van der Waals heterostructures along with graphene and myriad other atomically thin materials. Proximity of a-RuCl3 to graphene leads to a significant charge transfer between the two that, surprisingly, persists even when a thin insulating layer is inserted between them, a phenomenon analogous to modulation doping in epitaxially-grown semiconductors. In addition to constituting a new method of charge control in van der Waals stacks, we are using this to generate near-atomically-sharp pn junctions in graphene that may enable a realization of the electron-optic version of Veselago's lens. 

Professor Alannah Hallas

Department of Physics and Astronomy

Stewart Blusson Quantum Matter Institute

University of British Columbia

Host:  Gannon

Title: Intertwined Dipolar and Quadrupolar Correlations in the Pyrochlore Tb2Ge2O7

Abstract:  Rare earth pyrochlores are model systems for the study of highly frustrated magnetism, in large part, because the majority of these materials have a clean separation of energy scales between their single-ion properties and their collective interactions. Terbium pyrochlores represent the exception, where a low energy crystal field excitation profoundly affects the spin interactions and imbues these materials with especially complex phase behavior. We find that the magnetic states of the terbium pyrochlores evolve over three distinct temperature regimes, strongly suggesting a universality to their low temperature phase behavior. In this talk, I will present a comprehensive heat capacity and neutron scattering study of Tb2Ge2O7 where, for the first time, we have been able to elucidate the nature of each of these three regimes. We find that the complex phase behavior of the terbium pyrochlores originates from not only the usual geometric frustration but also frustration due to the competition between magnetic dipoles and electric quadrupoles.

Professor Alannah Hallas

Department of Physics and Astronomy

Stewart Blusson Quantum Matter Institute

University of British Columbia

Host:  Gannon

Title: Intertwined Dipolar and Quadrupolar Correlations in the Pyrochlore Tb2Ge2O7

Abstract:  Rare earth pyrochlores are model systems for the study of highly frustrated magnetism, in large part, because the majority of these materials have a clean separation of energy scales between their single-ion properties and their collective interactions. Terbium pyrochlores represent the exception, where a low energy crystal field excitation profoundly affects the spin interactions and imbues these materials with especially complex phase behavior. We find that the magnetic states of the terbium pyrochlores evolve over three distinct temperature regimes, strongly suggesting a universality to their low temperature phase behavior. In this talk, I will present a comprehensive heat capacity and neutron scattering study of Tb2Ge2O7 where, for the first time, we have been able to elucidate the nature of each of these three regimes. We find that the complex phase behavior of the terbium pyrochlores originates from not only the usual geometric frustration but also frustration due to the competition between magnetic dipoles and electric quadrupoles.

Professor Hitesh Changlani

Department of Physics 

Florida State University

and The National High Magnetic Field Laboratory

Host:  Kaul

Abstract:

Geometrically frustrated magnets harbor novel phases of strongly interacting quantum matter, including those with fractionalized excitations, such as quantum spin liquids.  While there is tremendous progress on understanding their ground state properties, I will primarily focus on their dynamics by highlighting two directions that my group is pursuing. First, I will present our work (done in collaboration with experimentalists) on a newly synthesized pyrochlore, NaCaNi2F7, which we find to be an almost ideal realization of a spin-1 three-dimensional highly frustrated antiferromagnet with no magnetic order and a continuum of excitations, as seen in inelastic neutron scattering [1]. We determine its effective Hamiltonian and show the presence of characteristic "pinch points”, along with good quantitative agreement at intermediate energy scales, from three different theoretical techniques [2]. In the second part of my talk, I switch my focus to the dynamical non-equilibrium effect of ``quantum scarring” which was first reported in a one dimensional Rydberg atom setup [3], with no known real material analog. I demonstrate that this effect is not restricted to 1D, and can be realized in higher dimensional systems [4], which I explain with the help of an exactly solvable point (that we recently discovered) in the XXZ-Heisenberg model on the frustrated kagome lattice [5]. Within the framework of this proposal, I suggest what would be needed to realize scarring in real materials.

[1] K. W. Plumb, H.J. Changlani, A. Scheie, S. Zhang, J. Krizan, J. A. Rodriguez-Rivera, Y.Qiu, B.Winn, R.J. Cava, C.L. Broholm, Nature Physics, 15, 54-59 (2019)

[2] S. Zhang, H.J. Changlani, K. Plumb, O. Tchernyshyov, R. Moessner, Phys. Rev. Lett. 122, 167203 (2019)

[3] H. Bernien et al., Nature 551, 579–584 (2017); C. Turner et al., Nature Physics 14, 745-749 (2018)

[4] K. Lee, R. Melendrez, A. Pal, H.J. Changlani, Phys. Rev. B 101, 241111(R) (2020)

[5] H.J. Changlani, D. Kochkov, K. Kumar, B. Clark, E. Fradkin, Phys. Rev. Lett. 120, 117202 (2018)

Professor Hitesh Changlani

Department of Physics 

Florida State University

and The National High Magnetic Field Laboratory

Host:  Kaul

Abstract:

Geometrically frustrated magnets harbor novel phases of strongly interacting quantum matter, including those with fractionalized excitations, such as quantum spin liquids.  While there is tremendous progress on understanding their ground state properties, I will primarily focus on their dynamics by highlighting two directions that my group is pursuing. First, I will present our work (done in collaboration with experimentalists) on a newly synthesized pyrochlore, NaCaNi2F7, which we find to be an almost ideal realization of a spin-1 three-dimensional highly frustrated antiferromagnet with no magnetic order and a continuum of excitations, as seen in inelastic neutron scattering [1]. We determine its effective Hamiltonian and show the presence of characteristic "pinch points”, along with good quantitative agreement at intermediate energy scales, from three different theoretical techniques [2]. In the second part of my talk, I switch my focus to the dynamical non-equilibrium effect of ``quantum scarring” which was first reported in a one dimensional Rydberg atom setup [3], with no known real material analog. I demonstrate that this effect is not restricted to 1D, and can be realized in higher dimensional systems [4], which I explain with the help of an exactly solvable point (that we recently discovered) in the XXZ-Heisenberg model on the frustrated kagome lattice [5]. Within the framework of this proposal, I suggest what would be needed to realize scarring in real materials.

[1] K. W. Plumb, H.J. Changlani, A. Scheie, S. Zhang, J. Krizan, J. A. Rodriguez-Rivera, Y.Qiu, B.Winn, R.J. Cava, C.L. Broholm, Nature Physics, 15, 54-59 (2019)

[2] S. Zhang, H.J. Changlani, K. Plumb, O. Tchernyshyov, R. Moessner, Phys. Rev. Lett. 122, 167203 (2019)

[3] H. Bernien et al., Nature 551, 579–584 (2017); C. Turner et al., Nature Physics 14, 745-749 (2018)

[4] K. Lee, R. Melendrez, A. Pal, H.J. Changlani, Phys. Rev. B 101, 241111(R) (2020)

[5] H.J. Changlani, D. Kochkov, K. Kumar, B. Clark, E. Fradkin, Phys. Rev. Lett. 120, 117202 (2018)