arXiv subjects

George Jackeli

Publications and source records attributed to George Jackeli.

17 recordsLinked to original sources

arXiv2026

Magnetic field-driven phase switching in the antiferromagnetic Mott insulator Ca$_3$(Ru$_{0.99}$Ti$_{0.01}$)$_2$O$_7$

A bandwidth-controlled antiferromagnetic Mott-insulating phase in Ca$_3$(Ru$_{1-x}$Ti$_x$)$_2$O$_7$ is realized through isovalent substitution at the Ru site. For a dilute substitution with only 1% Ti, the Mott insulator ground state remains nearly degenerate with the ground state of pristine Ca$_3$Ru$_2$O$_7$, where the Ru moments are ferromagnetically aligned within the metallic RuO$_2$ bilayers stacked in an antiferromagnetic fashion. The exceptionally shallow free energy landscape of this doped compound arises from intertwined electron-electron and electron-lattice interactions. This makes its magnetic and transport properties highly sensitive to external perturbations. We systematically investigated magnetic-field-induced phase switching in Ca$_3$(Ru$_{0.99}$Ti$_{0.01}$)$_2$O$_7$ to explore its magnetic $H$-$T$ phase diagram. With the field applied along the easy $b$-axis, parallel to the antiferromagnetic moments, the magnetization exhibits a first-order spin-flop transition at $\approx $ 6 T, indicating reorientation of the Ru moments perpendicular to the field. The transition is accompanied by a decrease in the electrical resistance, but the spin-flop phase remains insulating. Above 10.5 T, all Ru moments align with the $b$-axis, resulting in a forced ferromagnetic metallic phase. In contrast, neither spin-flop nor forced-ferromagnetic phases are observed up to 14 T, when the field is applied along the $a$-axis. While the electronic kinetic energy and the electron-lattice coupling contribute to the free-energy balance of this system, the resulting $H$-$T$ phase diagram is remarkably simple and closely resembles that of a canonical anisotropic antiferromagnet, albeit with substantially renormalized critical fields.

cond-mat.str-el↗

arXiv2023

Quantum critical Bose gas in the two-dimensional limit in the honeycomb antiferromagnet YbCl$_3$ under magnetic fields

BEC is a quantum phenomenon, where a macroscopic number of bosons occupy the lowest energy state and acquire coherence at low temperatures. It is realized not only in $^4$He and dilute atomic gases, but also in quantum magnets, where hardcore bosons, introduced by the Matsubara-Matsuda transformation of spins, condense. In 3D antiferromagnets, an XY-type long-range ordering (LRO) occurs near a magnetic-field-induced transition to a fully polarized state (FP) and has been successfully described as a BEC in the last few decades. An attractive extension of the BEC in 3D magnets is to make their 2D analogue. For a strictly 2D system, BEC cannot take place due to the presence of a finite density of states at zero energy, and a Berezinskii-Kosterlitz-Thouless (BKT) transition may instead emerge. In a realistic quasi-2D magnet consisting of stacked 2D magnets, a small but finite interlayer coupling stabilizes marginal LRO and BEC, but such that 2D physics, including BKT fluctuations, is still expected to dominate. A few systems were reported to show such 2D-limit BEC, but at very high magnetic fields that are difficult to access. The honeycomb $S$ = 1/2 Heisenberg antiferromagnet YbCl$_3$ with an intra-layer coupling $J\sim$ 5 K exhibits a transition to a FP state at a low in-plane magnetic field of $H_{\rm s}$ = 5.93 T. Here, we demonstrate that the LRO right below $H_{\rm s}$ is a BEC in the 2D-limit stabilized by an extremely small interlayer coupling $J_{\perp}$ of 10$^{-5}J$. At the quantum critical point Hs, we capture 2D-limit quantum fluctuations as the formation of a highly mobile, interacting 2D Bose gas in the dilute limit. A much-reduced effective boson-boson repulsion Ueff as compared with that of a prototypical 3D system indicates the presence of a logarithmic renormalization of interaction unique to 2D.

cond-mat.mtrl-sci↗

arXiv2022

Anisotropic exchange and non-collinear antiferromagnets on a noncentrosymmetric fcc structure as in the half-Heuslers

One of the signatures of the face-centered cubic (fcc) antiferromagnet as a typical example of a geometrically frustrated system is the large ground state degeneracy of the classical nearest neighbor and next-nearest neighbor Heisenberg (isotropic) model on this lattice. In particular, collinear states are degenerate with non-collinear and non-coplanar ones: this degeneracy is accidental and is expected to be lifted by anisotropic exchange interactions. In this work, we derive the most general nearest and next-nearest neighbor exchange model allowed by the space-group symmetry of the noncentrosymmetric half-Heusler compounds, which includes three anisotropic terms: the so-called Kitaev, Gamma and Dzyaloshinskii-Moriya interactions -- most notably, the latter is allowed by the breaking of inversion symmetry in these materials and has not been previously been studied in the context of the fcc lattice. We compute the resulting phase diagram and show how the different terms lift the ground state degeneracy of the isotropic model, and lay emphasis on finding regimes where multi-q (non-collinear/non-coplanar) states are selected by anisotropy. We then discuss the role of quantum fluctuations and the coupling to a magnetic field in the ground state selection, and show that these effects can stabilize non-coplanar (triple-q) states. These results suggest that some half-Heusler antiferromagnets might host rare non-collinear/non-coplanar orders, which may in turn explain the unusual transport properties detected in these semimetals.

cond-mat.str-el↗

arXiv2021

$\mathrm{SU}(4)$-Symmetric Quantum Spin-Orbital Liquids on Various Lattices

An emergent $\mathrm{SU}(4)$ symmetry discovered in the microscopic model for $d^1$ honeycomb materials [M.~G.~Yamada, M.~Oshikawa, and G.~Jackeli, Phys. Rev. Lett. \textbf{121}, 097201 (2018).] has enabled us to tailor exotic $\mathrm{SU}(4)$ models in real materials. In the honeycomb structure, the emergent $\mathrm{SU}(4)$ Heisenberg model would potentially have a quantum spin-orbital liquid ground state due to the \textit{multicomponent frustration}, and we can expect similar spin-orbital liquids also in three-dimensinal versions of the honeycomb lattice. In such quantum spin-orbital liquids, both the spin and orbital degrees of freedom become fractionalized and entangled together due to the strong frustrated interactions between them. Similarly to spinons in pure quantum spin liquids, quantum spin-orbital liquids can host not only spinon excitations, but also fermionic \textit{orbitalon} excitations at low temperature.

cond-mat.str-el↗

arXiv2020

Magnetic and Electronic Properties of Spin-Orbit Coupled Dirac Electrons on a $(001)$ Thin Film of Double Perovskite Sr$_2$FeMoO$_6$

We present an interacting model for the electronic and magnetic behavior of a strained $(001)$ atomic layer of Sr$_2$FeMoO$_6,$ which shows room-temperature ferrimagnetism and magnetoresistance with potential spintronics application in the bulk. We find that the strong spin-orbit coupling in the molybdenum 4$d$ shell gives rise to a robust ferrimagnetic state with an emergent spin-polarized electronic structure consisting of flat bands and four massive or massless Dirac dispersions. Based on the spin-wave theory, we demonstrate that the magnetic order remains intact for a wide range of doping, leading to the possibility of exploring flat band physics, such as Wigner crystallization in electron-doped Sr$_{2-x}$La$_{x}$FeMoO$_6.$

cond-mat.str-el↗

arXiv2019

Giant Magnetoelastic-Coupling Driven Spin-Lattice Liquid State in Molybdate Pyrochlores

We propose the idea of a spin-lattice liquid, in which spin and lattice degrees of freedom are strongly coupled and remain disordered and fluctuating down to low temperatures. We show that such a state arises naturally from a microscopic analysis of a class of molybdate pyrochlore compounds, and is driven by a giant magnetoelastic effect. Finally, we argue that this could explain some of the experimental features of Y$_2$Mo$_2$O$_7$.

cond-mat.str-el↗

arXiv2019

Kitaev quantum spin liquid - concept and materialization

A decade ago, Alexei Kitaev proposed an exactly solvable $S$ = 1/2 model on a two-dimensional honeycomb lattice, where the spins fractionalize into Majorana fermions and form a topological quantum spin liquid (QSL) in the ground state. It was soon recognized that a family of complex iridium oxides, as well as ruthenium chloride, with honeycomb structure are magnetic insulators and accommodate essential ingredients of the Kitaev model, due to the interplay of electron correlation and spin-orbit coupling. This initiated a race to materialize the Kitaev QSL and to capture the signature of Majorana fermions. In this review, we provide a wide perspective of this rapidly growing field, including theory, materials and experiment. We first summarize the theoretical background of the Kitaev QSL ground state and its materialization using spin-orbital-entangled $J_{\rm eff}$ = 1/2 moments. This is followed by an overview of candidate materials and their magnetic properties, including Na$_2$IrO$_3$, $α$, $β$, $γ$-Li$_2$IrO$_3$,$α$-RuCl$_3$ and H$_3$LiIr$_2$O$_6$. Finally, we review the latest exciting progress in the search for the Kitaev QSL. In particular, H$_3$LiIr$_2$O$_6$ and $α$-RuCl$_3$ in applied magnetic field show signatures of the QSL state, and $α$-RuCl$_3$ has unusual magnetic excitations and thermal transport properties that are consistent with spin fractionalization.

cond-mat.str-el↗

arXiv2018

Emergent $\mathrm{SU}(4)$ Symmetry in $α$-ZrCl$_3$ and Crystalline Spin-Orbital Liquids

While the enhancement of the spin-space symmetry from the usual $\mathrm{SU}(2)$ to $\mathrm{SU}(N)$ is promising for finding nontrivial quantum spin liquids, its realization in magnetic materials remains challenging. Here we propose a new mechanism by which the $\mathrm{SU}(4)$ symmetry emerges in the strong spin-orbit coupling limit. In $d^1$ transition metal compounds with edge-sharing anion octahedra, the spin-orbit coupling gives rise to strongly bond-dependent and apparently $\mathrm{SU}(4)$-breaking hopping between the $J_\textrm{eff}=3/2$ quartets. However, in the honeycomb structure, a gauge transformation maps the system to an $\mathrm{SU}(4)$-symmetric Hubbard model. In the strong repulsion limit at quarter filling, as realized in $α$-ZrCl$_3,$ the low-energy effective model is the $\mathrm{SU}(4)$ Heisenberg model on the honeycomb lattice, which cannot have a trivial gapped ground state and is expected to host a gapless spin-orbital liquid. By generalizing this model to other three-dimensional lattices, we also propose crystalline spin-orbital liquids protected by this emergent $\mathrm{SU}(4)$ symmetry and space group symmetries.

cond-mat.str-el↗

arXiv2018

Pressure-tuned magnetic interactions in honeycomb Kitaev materials

A range of honeycomb-lattice compounds has been proposed and investigated in the search for a topological Kitaev spin liquid. However, sizable Heisenberg interactions and additional symmetry-allowed exchange anisotropies in the magnetic Hamiltonian of these potential Kitaev materials push them away from the pure Kitaev spin-liquid state. Particularly the Kitaev-to-Heisenberg coupling ratio is essential in this respect. With the help of advanced quantum-chemistry methods, we explore how the magnetic coupling ratios depend on pressure in several honeycomb compounds (Na$_2$IrO$_3$, $β$-Li$_2$IrO$_3$, and $α$-RuCl$_3$). We find that the Heisenberg and Kitaev terms are affected differently by uniform pressure or strain: the Kitaev component increases more rapidly than the Heisenberg counterpart. This provides a scenario where applying pressure or strain can stabilize a spin liquid in such materials.

cond-mat.str-el↗

arXiv2017

Quantum gap and spin-wave excitations in the Kitaev model on a triangular lattice

We study the effects of quantum fluctuations on the dynamical generation of a gap and on the evolution of the spin-wave spectra of a frustrated magnet on a triangular lattice with bond-dependent Ising couplings, analog of the Kitaev honeycomb model. The quantum fluctuations lift the subextensive degeneracy of the classical ground-state manifold by a quantum order-by-disorder mechanism. Nearest-neighbor chains remain decoupled and the surviving discrete degeneracy of the ground state is protected by a hidden model symmetry. We show how the four-spin interaction, emergent from the fluctuations, generates a spin gap shifting the nodal lines of the linear spin-wave spectrum to finite energies.

cond-mat.str-el↗

arXiv2017

Spin-Orbit Dimers and Non-Collinear Phases in $d^1$ Cubic Double Perovskites

We formulate and study a spin-orbital model for a family of cubic double perovskites with $d^1$ ions occupying a frustrated fcc sublattice. A variational approach and a complimentary analytical analysis reveal a rich variety of phases emerging from the interplay of Hund's and spin-orbit couplings (SOC). The phase digram includes non-collinear ordered states, with or without net moment, and, remarkably, a large window of a non-magnetic disordered spin-orbit dimer phase. The present theory uncovers the physical origin of the unusual amorphous valence bond state experimentally suggested for Ba$_2B$MoO$_6$ ($B$=Y,Lu), and predicts possible ordered patterns in Ba$_2B$OsO$_6$ ($B$=Na,Li) compounds.

cond-mat.str-el↗

arXiv2016

Model analysis of magnetic susceptibility of Sr$_2$IrO$_4$ - 2D $J_{\rm eff}$ = 1/2 Heisenberg system with competing interlayer couplings

We report the analysis of magnetic susceptibility $χ$($T$) of Sr$_2$IrO$_4$ single crystal in the paramagnetic phase. We formulate the theoretical susceptibility based on isotropic Heisenberg antiferromagnetism incorporating the Dzyaloshinsky-Moriya interaction exactly, and include the interlayer couplings in a mean-field approximation. $χ$($T$) above $T_{\rm N}$ was found to be well described by the model, indicating the predominant Heisenberg exchange consistent with the microscopic theory. The analysis points to a competition of nearest and next-nearest neighbor interlayer couplings, which results in the up-up-down-down configuration of the in-plane canting moments identified by the diffraction experiments.

cond-mat.str-el↗

arXiv2016

Phase diagram and spin correlations of the Kitaev-Heisenberg model: Importance of quantum effects

We explore the phase diagram of the Kitaev-Heisenberg model with nearest neighbor interactions on the honeycomb lattice using the exact diagonalization of finite systems combined with the cluster mean field approximation, and supplemented by the insights from the linear spin-wave and second--order perturbation theories. This study confirms that by varying the balance between the Heisenberg and Kitaev term, frustrated exchange interactions stabilize in this model four phases with magnetic long range order: Néel phase, ferromagnetic phase, and two other phases with coexisting antiferromagnetic and ferromagnetic bonds, zigzag and stripy phases. They are separated by two disordered quantum spin-liquid phases, and the one with ferromagnetic Kitaev interactions has a substantially broader range of stability as the neighboring competing ordered phases, ferromagnetic and stripy, have very weak quantum fluctuations. Focusing on the quantum spin-liquid phases, we study spatial spin correlations and dynamic spin structure factor of the model by the exact diagonalization technique, and discuss the evolution of gapped low-energy spin response across the quantum phase transitions between the disordered spin liquid and magnetic phases with long range order.

cond-mat.str-el↗

arXiv2015

Quantum order by disorder in the Kitaev model on a triangular lattice

We identify and discuss the ground state of a quantum magnet on a triangular lattice with bond-dependent Ising-type spin couplings, that is, a triangular analog of the Kitaev honeycomb model. The classical ground-state manifold of the model is spanned by decoupled Ising-type chains, and its accidental degeneracy is due to the frustrated nature of the anisotropic spin couplings. We show how this subextensive degeneracy is lifted by a quantum order-by-disorder mechanism and study the quantum selection of the ground state by treating short-wavelength fluctuations within the linked cluster expansion and by using the complementary spin-wave theory. We find that quantum fluctuations couple next-nearest-neighbor chains through an emergent four-spin interaction, while nearest-neighbor chains remain decoupled. The remaining discrete degeneracy of the ground state is shown to be protected by a hidden symmetry of the model.

cond-mat.str-el↗

arXiv2012

On the Origin of Zigzag Magnetic Order in Iridium Oxide Na2IrO3

We explore the phase diagram of spin-orbit Mott insulators on a honeycomb lattice, within the Kitaev-Heisenberg model extended to its full parameter space. Zigzag-type magnetic order is found to occupy a large part of the phase diagram of the model, and its physical origin is explained as due to interorbital t2g-eg hopping. Magnetic susceptibility and spin wave spectra are calculated and compared to the experimental data, obtaining thereby the spin coupling constants in Na2IrO3 and Li2IrO3.

cond-mat.str-el↗

arXiv2010

Kitaev-Heisenberg Model on Honeycomb Lattice: Possible Exotic Phases in Iridium Oxides $A_2$IrO$_3$

We derive and study a spin one-half Hamiltonian on a honeycomb lattice describing the exchange interactions between Ir$^{4+}$ ions in a family of layered iridates $A_2$IrO$_3$ ($A$=Li,Na). Depending on the microscopic parameters, the Hamiltonian interpolates between the Heisenberg and exactly solvable Kitaev models. Exact diagonalization and a complementary spin-wave analysis reveal the presence of an extended spin-liquid phase near the Kitaev limit and a conventional Néel state close to the Heisenberg limit. The two phases are separated by an unusual stripy antiferromagnetic state, which is the exact ground state of the model at the midpoint between two limits.

cond-mat.str-el↗

arXiv2009

Magnetically Hidden Order of Kramers Doublets in $d^1$ Systems: Sr$_2$VO$_4$

We formulate and study an effective Hamiltonian for low-energy Kramers doublets of $d^1$-ions on a square lattice. We find that the system exhibits a magnetically hidden order in which the expectation values of the local spin and orbital moments both vanish. The order parameter responsible for a time-reversal symmetry breaking has a composite nature and is a spin-orbital analog of a magnetic octupole. We argue that such a hidden order is realized in the layered perovskite Sr$_2$VO$_4$.

cond-mat.str-el↗