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Fully Generalized Spin Models with Strain Effects of Kitaev Spin Liquid Candidate Materials
Authors:
Pureum Noh,
Hyunggeun Lee,
Myung Joon Han,
Eun-Gook Moon
Abstract:
The $KJΓΓ'$ spin model-originally derived for an ideal $P\bar{3}1m$ symmetric geometry-has long served as a central framework for understanding candidate Kitaev materials. In realistic crystals, however, this ideal geometry is seldom realized, either at low temperatures or under external perturbations, limiting the model's quantitative applicability. Here we introduce a fully generalized spin mode…
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The $KJΓΓ'$ spin model-originally derived for an ideal $P\bar{3}1m$ symmetric geometry-has long served as a central framework for understanding candidate Kitaev materials. In realistic crystals, however, this ideal geometry is seldom realized, either at low temperatures or under external perturbations, limiting the model's quantitative applicability. Here we introduce a fully generalized spin model, denoted $ε$-$KJΓΓ'$, that explicitly incorporates arbitrary lattice deformations $ε$. All spin-exchange interactions and their strain-dependent coefficients are obtained from density-functional theory (DFT) calculations and a microscopic derivation of coupling constants for materials based on $d^5$ transition-metal ions. For $α$-RuCl$_3$ under a strain of $3\%$, new emergent exchange channels acquire magnitudes comparable to their unstrained counterparts. Building on these parameters, we investigate strain-driven quantum phase transitions between competing magnetic states-including the zigzag order and the Kitaev quantum spin liquid (KQSL)-and identify a strain-induced topological transition within the KQSL states that offers a practical diagnostic of Kitaev physics. Furthermore, our symmetry analysis of the $ε$-$KJΓΓ'$ model is applicable to both $d^{5}$ ions, such as $α$-RuCl$_3$, and $d^{7}$ systems, including cobalt-based compounds.
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Submitted 29 May, 2025;
originally announced May 2025.
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Manipulating Topological Quantum Phase Transitions of Kitaev's Quantum Spin Liquids with Electric Fields
Authors:
Pureum Noh,
Kyusung Hwang,
Eun-Gook Moon
Abstract:
Highly entangled excitations such as Majorana fermions of Kitaev quantum spin liquids have been proposed to be utilized for future quantum science and technology, and a deeper understanding of such excitations has been strongly desired. Here we demonstrate that Majorana fermion's mass and associated topological quantum phase transitions in the Kitaev quantum spin liquids may be manipulated by usin…
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Highly entangled excitations such as Majorana fermions of Kitaev quantum spin liquids have been proposed to be utilized for future quantum science and technology, and a deeper understanding of such excitations has been strongly desired. Here we demonstrate that Majorana fermion's mass and associated topological quantum phase transitions in the Kitaev quantum spin liquids may be manipulated by using electric fields in sharp contrast to the common belief that an insulator is inert under weak electric fields due to charge energy gaps. Using general symmetry analysis with perturbation and exact diagonalization, we uncover the universal phase diagrams with electric and magnetic fields. We also provide distinctive experimental signatures to identify Kitaev quantum spin liquids with electric fields, especially in connection with the candidate materials such as $α$-RuCl3.
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Submitted 1 August, 2023;
originally announced August 2023.
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Majorana-fermion origin of the planar thermal Hall effect in the Kitaev magnet $α$-RuCl$_3$
Authors:
K. Imamura,
S. Suetsugu,
Y. Mizukami,
Y. Yoshida,
K. Hashimoto,
K. Ohtsuka,
Y. Kasahara,
N. Kurita,
H. Tanaka,
P. Noh,
J. Nasu,
E. -G. Moon,
Y. Matsuda,
T. Shibauchi
Abstract:
The field-induced quantum disordered state of layered honeycomb magnet $α$-RuCl$_3$ is a prime candidate for Kitaev spin liquids hosting Majorana fermions and non-Abelian anyons. Recent observations of anomalous planar thermal Hall effect demonstrate a topological edge mode, but whether it originates from Majorana fermions or bosonic magnons remains controversial. Here we distinguish these origins…
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The field-induced quantum disordered state of layered honeycomb magnet $α$-RuCl$_3$ is a prime candidate for Kitaev spin liquids hosting Majorana fermions and non-Abelian anyons. Recent observations of anomalous planar thermal Hall effect demonstrate a topological edge mode, but whether it originates from Majorana fermions or bosonic magnons remains controversial. Here we distinguish these origins from low-temperature measurements of high-resolution specific heat and thermal Hall conductivity with rotating in-plane fields. In the honeycomb bond direction, a distinct closure of the low-energy bulk gap is observed concomitantly with the sign reversal of the Hall effect. General discussions of topological bands show that this is the hallmark of an angle-rotation-induced topological transition of fermions, providing conclusive evidence for the Majorana-fermion origin of the thermal Hall effect in $α$-RuCl$_3$.
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Submitted 17 May, 2023;
originally announced May 2023.
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Construction of Entangled Many-body States via the Higgs Mechanism
Authors:
Pureum Noh,
Eun-Gook Moon
Abstract:
We provide a guiding principle to generate entanglement of quantum many-body states by applying key ideas of the Higgs mechanism to systems without gauge structures. Unitary operators associated with the Higgs mechanism are constructed, named as mean-operators, and employed to prepare entangled many-body states out of a trivial state. We uncover a symmetry-protectedtopological state with two Ising…
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We provide a guiding principle to generate entanglement of quantum many-body states by applying key ideas of the Higgs mechanism to systems without gauge structures. Unitary operators associated with the Higgs mechanism are constructed, named as mean-operators, and employed to prepare entangled many-body states out of a trivial state. We uncover a symmetry-protectedtopological state with two Ising symmetries on a square lattice and find entangled states with different symmetries and lattices. Plausible applications to quantum simulators such as Rydberg atoms and trapped ions, are also discussed, interpreting the mean-operators as the Ising coupling gates.
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Submitted 25 January, 2022; v1 submitted 17 January, 2022;
originally announced January 2022.
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Advancing Hybrid Quantum-Classical Algorithms via Mean-Operators
Authors:
Donggyu Kim,
Pureum Noh,
Hyun-Yong Lee,
Eun-Gook Moon
Abstract:
Entanglement in quantum many-body systems is the key concept for future technology and science, opening up a possibility to explore uncharted realms in an enormously large Hilbert space. The hybrid quantum-classical algorithms have been suggested to control quantum entanglement of many-body systems, and yet their applicability is intrinsically limited by the numbers of qubits and quantum operation…
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Entanglement in quantum many-body systems is the key concept for future technology and science, opening up a possibility to explore uncharted realms in an enormously large Hilbert space. The hybrid quantum-classical algorithms have been suggested to control quantum entanglement of many-body systems, and yet their applicability is intrinsically limited by the numbers of qubits and quantum operations. Here we propose a theory which overcomes the limitations by combining advantages of the hybrid algorithms and the standard mean-field-theory in condensed matter physics, named as mean-operator-theory. We demonstrate that the number of quantum operations to prepare an entangled target many-body state such as symmetry-protected-topological states is significantly reduced by introducing a mean-operator. We also show that a class of mean-operators is expressed as time-evolution operators and our theory is directly applicable to quantum simulations with $^{87}$Rb neutral atoms or trapped $^{40}$Ca$^+$ ions.
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Submitted 15 July, 2021;
originally announced July 2021.