Dr. Andrew Mounce

Center For Integrated Nanotechnologies

Sandia National Laboratory

Host: Gannon

Title:  Nanoscale Magnetometry Using Nitrogen Vacancies in Diamond

Abstract: The properties that make nitrogen vacancies in diamond (NVs) good qubits, for example long coherence times at room temperature, also make them excellent sensors for magnetic fields. In this presentation, I will discuss the properties of nitrogen vacancies and how to utilize them for magnetometry of devices and materials with DC magnetic fields and fluctuating fields up to GHz frequencies. To be useful as sensors, NVs must be activated close to the surface of diamond but near the surface nitrogen has a low probability of incorporating with a vacancy to create a magnetically sensitive NV defect. With this in mind, I will present our efforts toward improving NV activation yield to optimize sensitivity.  Finally, I will discuss our recent results as examples of the utility of using nitrogen vacancies for wide-field magnetic imaging for applied physics problems and NV detected nuclear magnetic resonance toward fundamental physics problems.

Dr. Andrew Mounce

Center For Integrated Nanotechnologies

Sandia National Laboratory

Host: Gannon

Title:  Nanoscale Magnetometry Using Nitrogen Vacancies in Diamond

Abstract: The properties that make nitrogen vacancies in diamond (NVs) good qubits, for example long coherence times at room temperature, also make them excellent sensors for magnetic fields. In this presentation, I will discuss the properties of nitrogen vacancies and how to utilize them for magnetometry of devices and materials with DC magnetic fields and fluctuating fields up to GHz frequencies. To be useful as sensors, NVs must be activated close to the surface of diamond but near the surface nitrogen has a low probability of incorporating with a vacancy to create a magnetically sensitive NV defect. With this in mind, I will present our efforts toward improving NV activation yield to optimize sensitivity.  Finally, I will discuss our recent results as examples of the utility of using nitrogen vacancies for wide-field magnetic imaging for applied physics problems and NV detected nuclear magnetic resonance toward fundamental physics problems.

Prof. Morten Eskildsen

Department of Physics

University of Notre Dame

Host: Gannon

Title: Fission induced vortex lattice disordering in UPt₃

Abstract:  Subjecting a type-II superconductor to a magnetic field will cause the formations of quantized vortices.  Due to their repulsive interaction the vortices will, in an ideal situation, arrange themselves into a perfectly order vortex lattice (VL).  In reality, however, thermal effects and/or pinning to material defects are present, and the balance between these competing factors determine both the structural and dynamic properties of vortex matter.  This leads to a rich phase diagram comprised of both ordered and disordered solid phases and vortex liquids.  While transitions between the different phases are driven by changes of intensive quantities such as the magnetic field or temperature, their locations in the phase diagram are sensitive to the amount of defects in the host superconductor.  This provides an experimental handle with which one can tune the vortex matter phase diagram, e.g by introducing impurities during the material synthesis or by bombardment with heavy ions to create columnar defects post growth.

Here we report on small-angle neutron scattering (SANS) studies of vortices in the topological superconductor UPt₃, and specifically how the VL in this material undergoes a gradual disordering as it is subjected to a beam of cold neutrons.  The disordering occurs on a time scale of tens of minutes, and is attributed to local heating events caused by neutron induced fission of ²³⁵U, which temporarily heat regions of the sample above the critical temperature.  Vortices in the affected regions remain in a disordered configuration after re-cooling, which is most likely a quenched vortex glass.  Moreover, the rate of disordering is proportional to the magnetic field, suggesting a direct relation to collective VL properties such as the elastic moduli.  While the VL 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 damped field oscillation, indicating that no permanent radiation damage of the UPt₃ crystal occur within experimental time scales.  The results demonstrate a novel avenue for vortex matter studies, allowing an introduction of localized and reversible quenched disorder.

Prof. Morten Eskildsen

Department of Physics

University of Notre Dame

Host: Gannon

Title: Fission induced vortex lattice disordering in UPt₃

Abstract:  Subjecting a type-II superconductor to a magnetic field will cause the formations of quantized vortices.  Due to their repulsive interaction the vortices will, in an ideal situation, arrange themselves into a perfectly order vortex lattice (VL).  In reality, however, thermal effects and/or pinning to material defects are present, and the balance between these competing factors determine both the structural and dynamic properties of vortex matter.  This leads to a rich phase diagram comprised of both ordered and disordered solid phases and vortex liquids.  While transitions between the different phases are driven by changes of intensive quantities such as the magnetic field or temperature, their locations in the phase diagram are sensitive to the amount of defects in the host superconductor.  This provides an experimental handle with which one can tune the vortex matter phase diagram, e.g by introducing impurities during the material synthesis or by bombardment with heavy ions to create columnar defects post growth.

Here we report on small-angle neutron scattering (SANS) studies of vortices in the topological superconductor UPt₃, and specifically how the VL in this material undergoes a gradual disordering as it is subjected to a beam of cold neutrons.  The disordering occurs on a time scale of tens of minutes, and is attributed to local heating events caused by neutron induced fission of ²³⁵U, which temporarily heat regions of the sample above the critical temperature.  Vortices in the affected regions remain in a disordered configuration after re-cooling, which is most likely a quenched vortex glass.  Moreover, the rate of disordering is proportional to the magnetic field, suggesting a direct relation to collective VL properties such as the elastic moduli.  While the VL 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 damped field oscillation, indicating that no permanent radiation damage of the UPt₃ crystal occur within experimental time scales.  The results demonstrate a novel avenue for vortex matter studies, allowing an introduction of localized and reversible quenched disorder.

Prof. Bruno Uchoa

Department of Physics and Astronomy

University of Oklahoma

Host: Kaul

Title: Unbounded hydrodynamics in nodal-line semimetals

Abstract: The ratio between the shear viscosity and the entropy η/s is considered a universal measure of the strength of interactions in quantum systems. This quantity was conjectured to have a universal lower bound (1/4π)h/kB, which indicates a very strongly correlated quantum fluid. After a general overview on quantum hydrodynamics, which describes the long wavelength deviations of local thermal equilibrium, I will address the quantum kinetic theory for a nodal-line semimetal in the hydrodynamic regime. I will show that the ratio between the shear viscosity and the entropy is unbounded, scaling towards zero with decreasing temperature in the perturbative limit. Due to the large unscreened Fermi surface represented by the nodal-line, the phase space for collisions is greatly enhanced compared to either conventional relativistic systems and metals, resulting in a short hydrodynamic scattering time that is nearly temperature independent (up to logarithmic scaling corrections) and set by the radius of the nodal line. I suggest that the lower bound criteria should be modified to account for unscreened relativistic systems with a Fermi surface.

Prof. Bruno Uchoa

Department of Physics and Astronomy

University of Oklahoma

Host: Kaul

Title: Unbounded hydrodynamics in nodal-line semimetals

Abstract: The ratio between the shear viscosity and the entropy η/s is considered a universal measure of the strength of interactions in quantum systems. This quantity was conjectured to have a universal lower bound (1/4π)h/kB, which indicates a very strongly correlated quantum fluid. After a general overview on quantum hydrodynamics, which describes the long wavelength deviations of local thermal equilibrium, I will address the quantum kinetic theory for a nodal-line semimetal in the hydrodynamic regime. I will show that the ratio between the shear viscosity and the entropy is unbounded, scaling towards zero with decreasing temperature in the perturbative limit. Due to the large unscreened Fermi surface represented by the nodal-line, the phase space for collisions is greatly enhanced compared to either conventional relativistic systems and metals, resulting in a short hydrodynamic scattering time that is nearly temperature independent (up to logarithmic scaling corrections) and set by the radius of the nodal line. I suggest that the lower bound criteria should be modified to account for unscreened relativistic systems with a Fermi surface.

Professor Herb Fertig

Department of Physics

Indiana University

Host: Kaul

Title:

Exciton Drift from Quantum Geometry

Abstract:

In some situations, excitons – bound particle-hole pairs above an insulating ground state – carry an electric dipole moment, allowing them to be manipulated via coupling to an electric field. Excitons in two-dimensional systems turn out to be fully determined by the quantum geometry of its eigenstates, through a quantity which we call the dipole curvature.  The dipole curvature arises naturally in the semiclassical equations of motion of an exciton in an electric field, yielding a drift velocity akin to that expected for excitons in crossed electric and magnetic fields, even in the absence of a real magnetic field.  In real magnetic fields it yields corrections to the drift velocity expected based on Lorentz invariance.  The effect arises naturally in systems where the environments of the electron and hole are sufficiently different, and is particularly relevant for interlayer excitons in heterostructures -- bilayers of different materials.  We discuss estimates of these effects for simple heterostructure models, including graphene and transition-metal dichalcogenide layers, both with and without magnetic fields.  The last of these turn out to be particularly promising platforms for observing these effects.   We discuss how this quantum geometric drift velocity might be observed in experiment, and possible further consequences that follow from the semiclassical dynamics.

Professor Herb Fertig

Department of Physics

Indiana University

Host: Kaul

Title:

Exciton Drift from Quantum Geometry

Abstract:

In some situations, excitons – bound particle-hole pairs above an insulating ground state – carry an electric dipole moment, allowing them to be manipulated via coupling to an electric field. Excitons in two-dimensional systems turn out to be fully determined by the quantum geometry of its eigenstates, through a quantity which we call the dipole curvature.  The dipole curvature arises naturally in the semiclassical equations of motion of an exciton in an electric field, yielding a drift velocity akin to that expected for excitons in crossed electric and magnetic fields, even in the absence of a real magnetic field.  In real magnetic fields it yields corrections to the drift velocity expected based on Lorentz invariance.  The effect arises naturally in systems where the environments of the electron and hole are sufficiently different, and is particularly relevant for interlayer excitons in heterostructures -- bilayers of different materials.  We discuss estimates of these effects for simple heterostructure models, including graphene and transition-metal dichalcogenide layers, both with and without magnetic fields.  The last of these turn out to be particularly promising platforms for observing these effects.   We discuss how this quantum geometric drift velocity might be observed in experiment, and possible further consequences that follow from the semiclassical dynamics.

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).