Physics and Astronomy Condensed Matter Seminar

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

Prof. Kate Ross

Department of Physics

Colorado State University

Host: Kaul

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

Abstract:  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.
Date:
-
Location:
Zoom

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

Prof. Kate Ross

Department of Physics

Colorado State University

Host: Kaul

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

Abstract:  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.
Date:
-
Location:
Zoom

Investigating real-world quantum spin liquid candidates

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. 

 

Date:
-
Location:
Zoom

Investigating real-world quantum spin liquid candidates

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. 

 

Date:
-
Location:
Zoom

Nanoscale confinement towards a one-dimensional superfluid

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.

Date:
-
Location:
Zoom

Nanoscale confinement towards a one-dimensional superfluid

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.

Date:
-
Location:
Zoom

Topological Magnons

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.

Date:
-
Location:
Zoom

Topological Magnons

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.

Date:
-
Location:
Zoom

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

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.

 

Date:
-
Location:
Zoom

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

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.

 

Date:
-
Location:
Zoom