Condensed Matter Seminar

Complex geometrical phases in quantum materials probed by transport

Professor Hiro Nakamura

Department of Physics

University of Arkansas

Host: Seo

Title: Complex geometrical phases in quantum materials probed by transport

Abstract: Recent advances in condensed matter physics brought the phase part of wavefunction in the forefront, as exemplified by the Berry phase in graphene. However, experimentally probing complex phases in a momentum space is not easy. In this talk, we present how advanced transport techniques such as quantum interference and planer Hall effect could shed light on higher-order and/or anisotropic quantum phases in solids. We show recent application of these techniques to three-dimensional (3D) topological antiperovskites, which points to a unique spin/pseudospin texture in this material [1,2]. Preliminary results from a 2D material with distinct spin texture (few-layer WSe2) also showcase an impact of different anisotropy of such phase patterns.

1. H. Nakamura et al., Nature Comm. 11, 1161 (2020).

2. D. Huang, H. Nakamura, and H. Takagi, arXiv:2101.05512 (2021)

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Complex geometrical phases in quantum materials probed by transport

Professor Hiro Nakamura

Department of Physics

University of Arkansas

Host: Seo

Title: Complex geometrical phases in quantum materials probed by transport

Abstract: Recent advances in condensed matter physics brought the phase part of wavefunction in the forefront, as exemplified by the Berry phase in graphene. However, experimentally probing complex phases in a momentum space is not easy. In this talk, we present how advanced transport techniques such as quantum interference and planer Hall effect could shed light on higher-order and/or anisotropic quantum phases in solids. We show recent application of these techniques to three-dimensional (3D) topological antiperovskites, which points to a unique spin/pseudospin texture in this material [1,2]. Preliminary results from a 2D material with distinct spin texture (few-layer WSe2) also showcase an impact of different anisotropy of such phase patterns.

1. H. Nakamura et al., Nature Comm. 11, 1161 (2020).

2. D. Huang, H. Nakamura, and H. Takagi, arXiv:2101.05512 (2021)

Date:
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Location:
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Critical charge fluctuations and superconductivity in magnetic environments

Prof. Yashar Komijani

Department of Physics

University of Cincinnati

Host: Gannon/Kaul

 

Title: Critical charge fluctuations and superconductivity in magnetic environments

Abstract:

Quantum electronic matter has long been understood in terms of two limiting behaviors of electrons: one of delocalized metallic states, and the other of localized magnetic states. Heavy fermions are miniature high-Tc superconductors whose small energy scales provide the possibility of tuning the ground state between these two limits and enable accessing the strange metallic behavior which develops at the brink of localization. I will discuss the attempts [1,2] to map out the phase diagram of heavy-fermions using dynamical large-N method and the Schwinger boson representation of the spins. I will then highlight the recent observation of quantum critical point and strange metal behavior in the stoichiometric ferromagnetic heavy-fermion CeRh6Ge4 [3]. This is surprising as the abrupt change in the patterns of entanglement and the Fermi surface that usually accompanies singular charge fluctuations are absent in a ferromagnet. I will argue that the innocuous easy-plane magnetic anisotropy that is present in this system, produces triplet resonating valence bond (tRVB) states, which lead to a highly entangled ordered phase, similar to a magnetically-frustrated anti-ferromagnet. Doping such a tRVB host provides a route towards realizing triplet superconductivity in a magnetic environment [4].

References: [1] Y. Komijani, P. Coleman, PRL 120, 157206 (2018); ibid 122, 217001 (2019).

[2] J. Wang, Y.-Y. Chang, C.-Y. Mou, S. Kirchner, C.-H. Chung, PRB 102, 115133 (2019).

[3] B. Shen, Y. Zhang, Y. Komijani, M. Nicklas, R. Borth, A. Wang, Y. Chen, Z. Nie, R. Li, X. Lu, H. Lee, M. Smidman, F. Steglich, P. Coleman, H. Yuan, Nature 579, 51 (2020).

[4] P. Coleman, Y. Komijani, E. König, PRL 125, 077001 (2020).

 

 

 

 

Date:
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Location:
Zoom

Critical charge fluctuations and superconductivity in magnetic environments

Prof. Yashar Komijani

Department of Physics

University of Cincinnati

Host: Gannon/Kaul

 

Title: Critical charge fluctuations and superconductivity in magnetic environments

Abstract:

Quantum electronic matter has long been understood in terms of two limiting behaviors of electrons: one of delocalized metallic states, and the other of localized magnetic states. Heavy fermions are miniature high-Tc superconductors whose small energy scales provide the possibility of tuning the ground state between these two limits and enable accessing the strange metallic behavior which develops at the brink of localization. I will discuss the attempts [1,2] to map out the phase diagram of heavy-fermions using dynamical large-N method and the Schwinger boson representation of the spins. I will then highlight the recent observation of quantum critical point and strange metal behavior in the stoichiometric ferromagnetic heavy-fermion CeRh6Ge4 [3]. This is surprising as the abrupt change in the patterns of entanglement and the Fermi surface that usually accompanies singular charge fluctuations are absent in a ferromagnet. I will argue that the innocuous easy-plane magnetic anisotropy that is present in this system, produces triplet resonating valence bond (tRVB) states, which lead to a highly entangled ordered phase, similar to a magnetically-frustrated anti-ferromagnet. Doping such a tRVB host provides a route towards realizing triplet superconductivity in a magnetic environment [4].

References: [1] Y. Komijani, P. Coleman, PRL 120, 157206 (2018); ibid 122, 217001 (2019).

[2] J. Wang, Y.-Y. Chang, C.-Y. Mou, S. Kirchner, C.-H. Chung, PRB 102, 115133 (2019).

[3] B. Shen, Y. Zhang, Y. Komijani, M. Nicklas, R. Borth, A. Wang, Y. Chen, Z. Nie, R. Li, X. Lu, H. Lee, M. Smidman, F. Steglich, P. Coleman, H. Yuan, Nature 579, 51 (2020).

[4] P. Coleman, Y. Komijani, E. König, PRL 125, 077001 (2020).

 

 

 

 

Date:
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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:
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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:
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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