Prof. Sara Haravifard

Department of Physics

Duke University

Host: Kaul

Title: Investigating real-world quantum spin liquid candidates

Abstract: The quantum spin liquid (QSL) state is an exotic state of matter featuring a high degree of entanglement and lack of long-range magnetic order in the zero-temperature limit.  Recently, Yb-based triangular lattice antiferromagnets have garnered significant interest as possible QSL candidates, however, the presence of chemical disorder in real-world compounds has made directly measuring the Hamiltonian parameters challenging. To further elucidate role of chemical disorder and to explore phase diagram of these materials, we present neutron scattering and high-resolution magnetometry measurements of YbMgGaO4 and YbZnGaO4, covering a broad range of applied magnetic field at low temperature. We use key observations of the magnetic phase crossover to motivate an exploration of the field- and parameter-dependent phase diagram, providing an expanded view of the available magnetic states in applied field. More broadly, our approach demonstrates a means of pursuing QSL candidates where Hamiltonian parameters might otherwise be obscured by disorder. 

Prof. Adrian Del Maestro

Department of Physics and Astronomy

University of Tennessee

Host: Kaul

Title: Nanoscale confinement towards a one-dimensional superfluid

Abstract: In one spatial dimension, enhanced thermal and quantum fluctuations should preclude the existence of any long range ordered superfluid phase of matter.  Instead, the quantum liquid should be described at low energies by an emergent hydrodynamic framework known as Tomonaga-Luttinger liquid theory.  In this talk I will present details on some orthogonal but complimentary experimental and theoretical searches for this behavior in helium-4 including: (1) pressure driven superflow through nanopores, and (2) the excitation spectrum of a confined superfluid inside nano-engineered porous silica-based structures. For flow experiments, we have devised a framework that is able to quantitatively describe dissipation at the nanoscale leading to predictions for the critical velocity borne out by recent superflow measurements in nanopores.  In confined porous media, with radii reduced via pre-plating with rare gases, I will discuss ab initio simulations of phase and density correlations inside the pore that are in agreement with recent neutron scattering measurements.   Taken together, these results indicate significant progress towards the experimental observation of a truly one-dimensional quantum liquid.

This work was supported by the NSF through grants DMR-1809027 and DMR-1808440.

Dr. Paul McClarty

Max Planck Institute for the Physics of Complex Systems

Dresden, Germany

Host: Kaul

Title: Topological Magnons

Abstract: Coherent magnetic excitations can have nontrivial topological features. In this talk, I'll give examples of "gapped" and "gapless" band topology of magnons including Kitaev-Heisenberg honeycomb models as analogues of Chern insulators and Dirac nodal lines in CoTiO3. Then I'll talk about features of band topology that are special to magnon systems: (i) the robustness of topological edge states in the presence of interactions; (ii) the visualization of spin-momentum locking in neutron scattering; (iii) the non-Hermitian topology of spontaneous magnon decay.

Dr. Lucia Steinke

National High Magnetic Field Laboratory High B/T Facility

University of Florida

Host: Gannon

Title: Towards a universal measurement platform for calorimetric and thermal transport measurements at the combined extremes of high magnetic fields and sub-millikelvin temperatures

Abstract: The NHMFL High B/T facility at the University of Florida in Gainesville pursues the mission to enable user experiments at the combined extremes of high magnetic fields B and ultra-low temperatures (ULT) below one millikelvin. While almost all measurement techniques like electrical transport or magnetometry require adaptations to be successfully performed in this environment, calorimetry and thermal transport are particularly challenging due to a lack of suitable thermometry. Existing thermometers typically either lack ULT sensitivity, are incompatible with high magnetic fields, or have too high masses and long thermalization times to allow for measurements of small samples. Specific heat and thermal transport experiments at ULT and high B - especially on small crystals - would be essential to studies of quantum criticality, superconductors with very low transition temperatures, or to provide evidence for a quantum spin liquid state. To meet increased user demand for such experiments, our group is developing fast and compact ULT thermometers to form the basis of a universal measurement platform for calorimetry and thermal transport of a variety of samples. The measurement principle relies on the well-established temperature dependent viscosity of liquid ³He that is probed by tracking the resonance of quartz tuning forks immersed in the liquid. I will report on recent progress in the miniaturization of such tuning fork thermometers (TFT), and present results from field and temperature dependent performance tests. 

The National High Magnetic Field Laboratory is supported by the National Science Foundation through NSF DMR-1644779 and the State of Florida.

Prof. Divine Kumah

Department of Physics

North Carolina State University

Host: Gannon

Title: Tuning Transport and Magnetism at Polar Oxide Interfaces

Abstract: Atomic-scale interactions at the interfaces between polar and non-polar transition metal oxides have led to the realization of exciting phenomena including two-dimensional electron gases, superconductivity and interfacial magnetism. However, these interactions may lead to the suppression of electronic and magnetic ordering at interfaces with strong structure-property relationships. By imaging the atomic structure of the interface between polar LaSrMnO3 (LSMO) and non-polar SrTiO3, we identify interfacial structural distortions which are correlated with thickness-dependent metal-insulator and ferromagnetic-paramagnetic transitions in the rare-earth manganites. We show that these structural distortions can be tuned by inserting polarity-matched spacer layers at the LSMO interfaces leading to a stabilization of ferromagnetism in LSMO layers as thin as two unit cells. The stabilized magnetism is found to be independent of strain. We employ a combination of synchrotron X-ray diffraction, temperature-dependent magnetization measurements and X-ray magnetic circular dichroism to elucidate the interplay between structural and spin degrees of freedom in the rare-earth manganites.[1,2] Additionally, we find that by tuning growth and post-growth processing conditions, a two-dimensional electron gas (2DEG) forms at the interface between antiferromagnetic LaCrO3 and SrTiO3 providing a route to design all-oxide heterostructures which couple magnetic ordering with the mobile carriers within the 2DEG.[3] These results demonstrate the intimate role of picometer-scale structural distortions on the physical properties of transition metal oxides and have important implications for designing novel quantum materials.

References



[1] Koohfar et. al., npj Quantum Materials 4 (1), 25 (2019)

[2] Koohfar et. al., Physical Review B 101 (6), 064420 (2020)

[3] Al-Tawhid et. al., AIP Advances 10 (4), 045132 (2020)

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. 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.

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).