arXiv subjects

Hae-Young Kee

Publications and source records attributed to Hae-Young Kee.

At least 37 records · Page 2Linked to original sources

arXiv2024

Static and fluctuating zigzag order, and possible signatures of Kitaev physics, in torque measurements of $α$-RuCl${_3}$

We have measured magnetic torque on a $T_N=7$ K single crystal of $α$-RuCl$_3$, as a function of the field angle in the $ab$-plane, focusing on temperatures between 2 and 20 K and fields from 0 to 9 T. We find a number of features, many of which can be classified by their angular periodicity. The sample shows an oscillation with a period of 180$^\circ$ (i.e.\ two-fold periodicity) and within the magnetically ordered zigzag phase there is a 60$^\circ$ period (i.e.\ six-fold) sawtooth pattern, which can be explained by reorientation of the zigzag domains as the crystal rotates in the applied field. We argue that the six-fold sawtooth and the two-fold sinusoidal signals arise from distinct regions of the crystal. Suppressing the zigzag order with an applied field above $\sim8$ T at low temperature, a six-fold {\sl sinusoidal} signal remains, suggesting that there is fluctuating zigzag order in the putative field-induced quantum spin liquid state. Finally, in testing theoretical results which predict a torque response with divergent slope across C$_2$-preserving $b$-axes (B1-axis), we find no features like that predicted for Ising topological order. Instead we find features at low temperatures and fields just above the zigzag phase across the non-C$_2$-preserving $b$-axes (B2-axes). Interpretation of this feature is complicated by the development of other similar signatures nearby at slightly lower fields, and by clear enhancement with thermal cycling. Additionally, we contrast the torque response of $T_N \sim$ 7 K and 14 K samples.

cond-mat.str-el↗

arXiv2024

Giant anisotropic magnetoresistance in few-layer α-RuCl3 tunnel junctions

The spin-orbit assisted Mott insulator $α$-RuCl3 is proximate to the coveted quantum spin liquid (QSL) predicted by the Kitaev model. In the search for the pure Kitaev QSL, reducing the dimensionality of this frustrated magnet by exfoliation has been proposed as a way to enhance magnetic fluctuations and Kitaev interactions. Here, we perform angle-dependent tunneling magnetoresistance (TMR) measurements on ultrathin $α$-RuCl3 crystals with various layer numbers to probe their magnetic, electronic and crystal structure. We observe a giant change in resistance - as large as ~2500% - when the magnetic field rotates either within or out of the $α$-RuCl3 plane, a manifestation of the strongly anisotropic spin interactions in this material. In combination with scanning transmission electron microscopy, this tunneling anisotropic magnetoresistance (TAMR) reveals that few-layer $α$-RuCl3 crystals remain in the high-temperature monoclinic phase at low temperature. It also shows the presence of a zigzag antiferromagnetic order below the critical temperature TN ~ 14 K, which is twice the one typically observed in bulk samples with rhombohedral stacking. Our work offers valuable insights into the relation between the stacking order and magnetic properties of this material, which helps lay the groundwork for creating and electrically probing exotic magnetic phases like QSLs via van der Waals engineering.

cond-mat.str-el↗

arXiv2024

Finite-momentum and field-induced pairings in orbital-singlet spin-triplet superconductors

Finite-momentum pairing in a Pauli-limited spin-singlet superconductor arises from the pair-breaking effects of an external Zeeman field, a mechanism which is not applicable in odd-parity spin-triplet superconductors. However, in multiorbital systems, the relevant bands originating from different orbitals are usually separated in momentum space, implying that orbital-singlet pairing is a natural candidate for a finite-momentum pairing state. We show that finite-momentum pairing arises in even-parity orbital-singlet spin-triplet superconductors via the combination of orbitally-nontrivial kinetic terms and Hund's coupling. The finite-momentum pairing is then suppressed with an increasing spin-orbit coupling, stabilizing a uniform pseudospin-singlet pairing. We also examine the effects of the magnetic field and find field-induced superconductivity at large fields. We apply these findings to the multiorbital superconductor with spin-orbit coupling, Sr$_{2}$RuO$_{4}$ and show that a finite-momentum pseudospin-singlet state appears between the uniform pairing and normal states. Future directions of inquiry relating to our findings are also discussed.

cond-mat.supr-con↗

arXiv2024

Intertwined van-Hove Singularities as a Mechanism for Loop Current Order in Kagome Metals

Recent experiments on Kagome metals AV$_3$Sb$_5$ (A=Cs,Rb,K) indicated spontaneous time-reversal symmetry breaking in the charge density wave state in the absence of static magnetization. The loop current order (LCO) is proposed as its cause, but a microscopic model explaining the emergence of LCO through electronic correlations has not been firmly established. We show that the coupling between van-Hove singularities (vHS) with distinct mirror symmetries is a key ingredient to generate LCO ground state. By constructing an effective model, we find that when multiple vHS with opposite mirror eigenvalues are close in energy, the nearest-neighbor electron repulsion favors a ground state with coexisting LCO and charge bond order. It is then demonstrated that this mechanism applies to the Kagome metals AV$_3$Sb$_5$. Our findings provide an intriguing mechanism of LCO and pave the way for a deeper understanding of complex quantum phenomena in Kagome systems.

cond-mat.str-el↗

arXiv2024

Transforming from Kitaev to Disguised Ising Chain: Application to CoNb$_2$O$_6$

For many years, CoNb$_2$O$_6$ has served as an exemplar of the one-dimensional Ising model. However, recent experimental and theoretical analyses challenge its applicability to this material. Prior to that, a tailored spin model for 3d$^7$ systems such as Co$^{2+}$, known as the $JKΓ$ model, has emerged, featuring Heisenberg $(J)$, Kitaev $(K)$, and Gamma $(Γ)$ interactions. While these interactions are permitted by the symmetry of the system, their role in CoNb$_2$O$_6$ remains enigmatic. We present a microscopic theory based on spin-orbit entangled J$_{\textrm{eff}}$ = 1/2 states, aimed at elucidating the roles of Kitaev and Gamma interactions in shaping Ising anisotropy. Leveraging strong coupling theory, we identify a dominant ferromagnetic Kitaev interaction. Furthermore, by comparing dynamical structure factors obtained via exact diagonalization with those from inelastic neutron scattering experiments, we find an antiferromagnetic $Γ$ interaction, which dictates the Ising axis and explains the mechanism behind moment pinning. Our theory suggests that CoNb$_2$O$_6$ represents a rare one-dimensional Kitaev chain with significant ferromagnetic Kitaev and antiferromagnetic Gamma interactions.

cond-mat.str-el↗

arXiv2024

Contrasting twisted bilayer graphene and transition metal dichalcogenides for fractional Chern insulators: an emergent gauge picture

The recent experimental discovery of the zero-field fractional Chern insulator (FCI) in twisted $\mathrm{MoTe_2}$ moiré superlattices has sparked immense interest in this exotic topological quantum state. The FCI has also been observed in previous experiments in magic angle twisted bilayer graphene (TBG) under a finite magnetic field of about 5 Tesla. Generally, the stabilization of FCI requires fine-tuning the topological band to satisfy certain conditions. It would still be helpful to have an intuitive picture to understand the different behaviors in twisted $\mathrm{MoTe_2}$ and TBG. Here, we compare them through the lens of emergent gauge fields. In TBG, the system can be mapped to two Dirac fermions coupled to emergent gauge fields with opposite signs. In contrast, the twisted $\mathrm{MoTe_2}$ reduces to a hole with parabolic dispersion coupled to an emergent gauge field. This contrasting gauge structure provides a new perspective on the observed difference: the zero-field FCI is stable in $\mathrm{MoTe_2}$ but absent in TBG. Based on this understanding, we will explore potential strategies for stabilizing FCI in both moiré superlattices.

cond-mat.mes-hall↗

arXiv2024

Beyond Kitaev physics in strong spin-orbit coupled magnets

We review the recent advances and current challenges in the field of strong spin-orbit coupled Kitaev materials, with a particular emphasis on the physics beyond the exactly-solvable Kitaev spin liquid point. To that end, we give a comprehensive overview of the most relevant exchange interactions in $d^5$ and $d^7$ iridates and similar compounds, an exposition of their microscopic origin, and a systematic attempt to map out the most interesting correlated regimes of the multi-dimensional parameter space, guided by powerful symmetry and duality transformations as well as by insights from wide-ranging analytical and numerical studies. We also survey recent exciting results on quasi-1D models and discuss their relevance to higher-dimensional models. Finally, we highlight some of the key questions in the field as well as future directions.

cond-mat.str-el↗

arXiv2024

Twice Hidden String Order and Competing Phases in the Spin-1/2 Kitaev-Gamma Ladder

Finding the Kitaev spin liquid in candidate materials involves understanding the entire phase diagram, including other allowed interactions. One of these interactions, called the Gamma ($Γ$) interaction, causes magnetic frustration and its interplay with the Kitaev ($K$) interaction is crucial to comprehend Kitaev materials. Due to the complexity of the combined $KΓ$ model, quasi-one dimensional models have been investigated. While several disordered phases are found in the 2-leg ladder, the nature of the phases are yet to be determined. Here we focus on the disordered phase near the antiferromagnetic $Γ$ limit (denoted by A$Γ$ phase) next to the ferromagnetic Kitaev phase. We report a distinct non-local string order parameter characterizing the A$Γ$ phase, different from the string order parameter in the Kitaev phase. This string order parameter becomes evident only after two unitary transformation, referred to as a twice hidden string order parameter. The related entanglement spectrum, edge states, magnetic field responses, and the symmetry protecting the phase are presented, and its relevance to the two-dimensional Kitaev materials is discussed. Two newly identified disordered phases in the phase diagram of $KΓ$ ladder is also reported.

cond-mat.str-el↗

arXiv2023

Ultrafast spin dynamics in the proximate quantum spin liquid α-RuCl3

α-RuCl3 is a Kitaev material suggested to be a proximate quantum spin liquid in a certain temperature and magnetic field range. Nonequilibrium measurements of transient dynamics have been proposed to detect fractionalized particles that emerge in the spin liquid and to possibly drive the system into novel photoinduced magnetic states that cannot be accessed by conventional equilibrium probes. Here we study ultrafast spin dynamics of photoinduced excitations in α-RuCl3 using pump-probe transient grating spectroscopy. In the real part of the complex transient reflectance change ΔR/R, we observe the long-range antiferromagnetic correlation near the Néel temperature. Most intriguingly, above the Néel temperature in the Kitaev paramagnetic phase, we reveal a photoexcitation component sensitive to the in-plane magnetic field in the imaginary part of ΔR/R. This component exhibits two distinct lifetimes of about tens of picoseconds. This photoexcitation component may be connected to novel photoexcited states in the Kitaev quantum spin liquid, and its lifetimes likely reflect the dynamics of unconventional spin excitations in the Kitaev model.

cond-mat.str-el↗

arXiv2023

Fate of multipolar physics in $5d^2$ double perovskites

In a cubic environment, the ground state of spin-orbit coupled $5d^2$ ions is a non-Kramers $E_g$ doublet, which hosts quadrupole and octupole moments. A series of $5d^2$ osmium double perovskites Ba$_2M$OsO$_6$ (M = Mg, Ca, Zn, Cd) have recently been proposed to exhibit multipolar orders. We investigate the structural properties of these materials using $\textit{ab}$-$\textit{initio}$ calculations and find that the cubic structure is unstable for the Cd compound while the Mg, Ca, and Zn materials retain $Fm\bar{3}m$ symmetry. We show that Ba$_2$CdOsO$_6$ favours a rhombohedral $R\bar{3}$ structure characterized by $a^-a^-a^-$ octahedral tiltings as indicated by unstable $\mathcal{T}_{1g}$ phonon modes. Trigonal distortions split the excited $T_{2g}$ triplet into an $E'_g$ doublet and an $A_g$ singlet, which may cross energy levels with the $E_g$ doublet and suppress the multipolar physics. We find a window where $E_g$ remains the lowest energy state under trigonal distortion, enabling the emergence of multipole phases in non-cubic crystal environments.

cond-mat.str-el↗

arXiv2023

Spontaneous dimerization, spin-nematic order, and deconfined quantum critical point in a spin-1 Kitaev chain with tunable single-ion anisotropy

The Kitaev-type spin chains have been demonstrated to be fertile playgrounds in which exotic phases and unconventional phase transitions are ready to appear. In this work, we use the density-matrix renormalization group method to study the quantum phase diagram of a spin-1 Kitaev chain with a tunable negative single-ion anisotropy (SIA). When the strength of the SIA is small, the ground state is revealed to be a spin-nematic phase which escapes conventional magnetic order but is characterized by a finite spin-nematic correlation because of the breaking spin-rotational symmetry. As the SIA increases, the spin-nematic phase is taken over by either a dimerized phase or an antiferromagnetic phase through an Ising-type phase transition, depending on the direction of the easy axis. For large enough SIA, the dimerized phase and the antiferromagnetic phase undergo a ``Landau-forbidden" continuous phase transition, suggesting new platform of deconfined quantum critical point in spin-1 Kitaev chain.

cond-mat.str-el↗

arXiv2023

Creating long-range entangled Majorana pairs: from spin-1/2 twisted Kitaev to generalized XY chains

Entangling a pair of far-distant qubits in many-body systems has been a challenging task in quantum computing. A robust entanglement was predicted in the rainbow states and generating nonlocal Bell pairs protected by a mirror symmetry was recently proposed. We investigate a way to create entangled Majorana fermions in the spin-1/2 Kitaev chain with open boundary conditions. The spin-1/2 Kitaev chain, a one-dimensional version of the honeycomb lattice with bond-dependent Ising interactions, has a macroscopic degeneracy related to the zero modes containing non-local spin strings. We show that applying a pair pulse sequence on the central unit cell of the chain promotes long distance spin correlations and maximal bipartite entanglement entropy. We extend this method to the generalized bond-dependent spin-1/2 chain by introducing another set of Majorana fermions and make a comparison to the entangled Bell pairs. The time to reach maximal bipartite entanglement entropy is shorter in the Kitaev chain as the zero modes do not participate in the entangled pairs. An application of our results to a recently proposed twisted Kitaev chain, CoNb2O6, is presented and future directions are also discussed.

cond-mat.str-el↗

arXiv2023

Field-induced Kitaev multipolar liquid in spin-orbit coupled $d^{2}$ honeycomb Mott insulators

The Kitaev model, characterized by bond-dependent Ising spin interactions among spin-orbit entangled dipole moments in Mott insulators, offered a new approach to quantum spin liquids. Motivated by another type of bond-dependent interaction among quadrupole moments in $5d^2$ Mott insulators, we provide a microscopic route to uncover the Kitaev multipolar liquid, featuring fractionalized excitations out of non-Kramers doublets carrying multipole moments. The key ingredient is the magnetic field that allows for bond-anisotropic quadrupole-octupole interactions via mixing with the excited triplet states. The conditions to realize signatures of this phase in real materials are also discussed.

cond-mat.str-el↗

arXiv2023

Origin of $π$-shifted three-dimensional charge density waves in kagome metal AV$_3$Sb$_5$

Understanding the nature of charge density wave (CDW) and superconductivity in kagome metal AV$_3$Sb$_5$ (A=Cs,Rb,K) is a recent subject of intensive study. Due to the presence of van Hove singularities, electron-electron interaction has been suggested to play an important role in the formation of such broken symmetry states. Recent experiments show that the CDW order is three-dimensional and it is staggered across different kagome layers. However, the experimental interpretation for the precise structure of CDW varies in terms of whether it is the star of David (SD), inverse star of David (ISD) or the alternation of the two among neighboring layers. In this work, we show that the origin of these distinct CDW orders can be understood in a unified picture by considering intra- and inter-layer electron-electron interactions as well as the coupling between electrons and lattice distortions. Utilizing an effective 9-band model with V $d$ orbitals and out-of-plane Sb $p$ orbitals, it is demonstrated that the repulsive electron-electron interaction favors charge bond order which induces either SD or ISD upon including lattice distortions. As the inter-layer interaction is introduced, $π$-shifted CDW develops with the staggered ordering along the $c$-axis. We also find that the phase with alternating SD and ISD can be stabilized as the ground state under strong inter-layer interaction.

cond-mat.str-el↗

arXiv2023

Islands of Chiral Solitons in Integer Spin Kitaev Chains

An intriguing chiral soliton phase has recently been identified in the $S$=1/2 Kitaev spin chain. Here we show that for $S$=1,2,3,4,5 an analogous phase can be identified, but contrary to the $S$=1/2 case the chiral soliton phases appear as islands within the sea of the polarized phase. In fact, a small field applied in a general direction will adiabatically connect the integer spin Kitaev chain to the polarized phase. Only at sizable intermediate fields along symmetry directions does the soliton phase appear centered around the special point $h^\star_x$=$h^\star_y$=$S$ where two exact product ground-states can be identified. The large $S$ limit can be understood from a semi-classical analysis, and variational calculations provide a detailed picture of the $S$=1 soliton phase. Under open boundary conditions, the chain has a single soliton in the ground-state which can be excited, leading to a proliferation of in-gap states. In contrast, even length periodic chains exhibit a gap above a twice degenerate ground-state. The presence of solitons leaves a distinct imprint on the low temperature specific heat.

cond-mat.str-el↗

arXiv2023

Reconciling $π$ phase shift in Josephson junction experiments with even-parity superconductivity in Sr$_2$RuO$_4$

The superconducting state of Sr$_2$RuO$_4$ was once thought to be a leading candidate for $p$-wave superconductivity. A constant Knight shift below the transition temperature provided evidence for spin-triplet pairing, and a $π$ phase shift observed in Josephson junction tunneling experiments suggested odd-parity pairing, both of which are described by $p$-wave states. However, with recent experiments observing a significant decrease in the Knight shift below the transition temperature, signifying a spin-singlet state, the odd-parity results are left to be reconciled. In this work, we show that an even-parity pseudospin-singlet state originating from interorbital pairing via spin-orbit coupling can explain what has been assumed to be evidence for an odd-parity state. In the presence of small mirror symmetry breaking, interorbital pairing is uniquely capable of displaying odd-parity characteristics required to explain these experimental results. Further, we discuss how these experiments may be used to differentiate the proposed pairing states of Sr$_2$RuO$_4$.

cond-mat.supr-con↗

arXiv2023

Thermal Hall conductivity of $α$-RuCl$_3$

Thermal Hall conductivity originating from topological magnons is observed in the Kitaev candidate $α$-RuCl$_3$ in broad intervals of temperature and in-plane magnetic field, raising questions on the role of the Majorana mode in heat conduction.

cond-mat.str-el↗

arXiv2022

Non-Kitaev vs. Kitaev Honeycomb Cobaltates

Recently, honeycomb cobaltates with 3$d^7$ were proposed to display Kitaev physics despite weak spin-orbit coupling. However, other theoretical and experimental works found leading XXZ Heisenberg and negligible Kitaev interactions in BaCo$_2$(AsO$_4$)$_2$ (BCAO), which calls for a further study to clarify the origin of the discrepancies. Here we derive the analytical expressions of the spin model using strong-coupling perturbation theory. With tight binding parameters obtained by {\it ab-initio} calculations for idealized honeycomb BCAO, we find that the largest intraorbital $t_{2g}-t_{2g}$ exchange path, which was assumed to be small in the earlier theory proposal, leads to a ferromagnetic (FM) Heisenberg interaction. This becomes the dominant interaction, as other $t_{2g}-e_g$ and $e_g-e_g$ contributions almost cancel each other. Exactly the same assumed-to-be-small channel also generates an antiferromagnetic Kitaev interaction, which then cancels a FM Kitaev interaction from $t_{2g}-e_g$ paths, resulting in a small Kitaev interaction. Under the trigonal distortion, the preeminent isotropic Heisenberg becomes an anisotropic XXZ model as expected, which is the case for BCAO. However, in Na$_3$Co$_2$SbO$_6$ the intraorbital $t_{2g}-t_{2g}$ hopping is smaller and comparable to the $t_{2g}-e_g$ hopping, leading to a delicate competition between enhanced Kitaev and reduced Heisenberg interactions.

cond-mat.str-el↗