arXiv subjects

Onur Erten

Publications and source records attributed to Onur Erten.

At least 37 records · Page 2Linked to original sources

arXiv2024

Tuning the electronic and magnetic properties of NiBr$_2$ via pressure

Transition metal dihalides (MX$_2$, M= transition metal, X= halide) have attracted much attention recently due to their intriguing low-dimensional magnetic properties. Particular focus has been placed in this family in the context of multiferroicity -- a common occurrence in MX$_2$ compounds that adopt non-collinear magnetic structures. One example of helimagnetic multiferroic material in the dihalide family is represented by NiBr$_2$. Here, we study the evolution of the electronic structure and magnetic properties of this material under pressure using first-principles calculations combined with Monte Carlo simulations. Our results indicate there is significant magnetic frustration in NiBr$_2$ due to the competing interactions arising from its underlying triangular lattice. This magnetic frustration increases with pressure and is at the origin of the helimagnetic order. Further, pressure causes a sizable increase in the interlayer interactions. Our Monte Carlo simulations show that a large (3-fold) increase in the helimagnetic transition temperature can be achieved at pressures of around 15 GPa. This indicates that hydrostatic pressure can indeed be used as a tuning knob to increase the magnetic transition temperature of NiBr$_2$.

cond-mat.mtrl-sci↗

arXiv2024

Theory of moiré magnetism and multi-domain spin textures in twisted Mott insulator - semimetal heterobilayers

Motivated by the recent experimental developments in van der Waals heterostructures, we investigate the emergent magnetism in Mott insulator - semimetal moiré superlattices by deriving effective spin models and exploring their phase diagram by Monte Carlo simulations. Our analysis indicates that the stacking-dependent interlayer Kondo interaction can give rise to different types of magnetic order, forming domains within the moiré unit cell. In particular, we find that the AB (AA) stacking regions tend to order (anti)ferromagnetically for an extended range of parameters. The remaining parts of the moiré unit cell form ferromagnetic chains that are coupled antiferromagnetically. We show that the decay length of the Kondo interaction can control the extent of these phases. Our results highlight the importance of stacking-dependent interlayer exchange and the rich magnetic spin textures that can be obtained in van der Waals heterostructures.

cond-mat.str-el↗

arXiv2024

Nematic heavy fermions and coexisting magnetic order in CeSiI

Motivated by the recent discovery of magnetism and heavy quasiparticles in van der Waals material CeSiI, we develop an effective model that incorporates the conduction electrons residing at the silicene layer interacting with the local moments of the Ce ions. Ce sites are arranged on two layers of triangular lattices, above and below the silicene layer, and they are located at the center of the honeycomb lattice. This arrangement results in an effective extended Kondo interaction along with a predominant ferromagnetic RKKY interaction. Via mean-field theory of Abrikosov fermions, our analysis indicates that the ground state of the monolayer can exhibit a non-magnetic nematic heavy fermion phase that breaks $C_6$ rotational symmetry for small Heisenberg exchange and a magnetically ordered phase for large Heisenberg exchange. For intermediate values, a coexistence of magnetic order and a uniform heavy Fermi liquid is stabilized where they reside on separate Ce layers. We show that this phase can further be enhanced by an external electric field. Our results provide a natural mechanism for the coexistence of magnetic order and heavy fermions in CeSiI and highlight the possibility of unconventional non-magnetic heavy fermions with broken rotational symmetry.

cond-mat.str-el↗

arXiv2024

Thermodynamics of vison crystals in an anisotropic quantum spin liquid

Using unbiased Monte Carlo simulations and variational analysis, we present the ground state and finite temperature phase diagrams of an exactly solvable spin-orbital model with Kitaev-type interactions on a square lattice. We show that an array of new gapped and gapless vison crystals -- characterized by the periodic arrangement of $\mathbb{Z}_2$ flux excitations -- can be stabilized as a function of external magnetic field and exchange anisotropy. In particular, we discover a variety of `quarter phases' wherein new sixteen-site periodic patterns emerge, with only a quarter of the fluxes adopting 0-flux configurations. In contrast, the rest remain in $π$-flux configurations. Vison crystals break translational symmetry and undergo finite temperature phase transitions. We investigate the finite temperature properties of these phases and report the corresponding critical and crossover temperatures. Our results reveal an array of novel phases in exactly solvable extensions of the Kitaev model, wherein local and topological orders can coexist.

cond-mat.str-el↗

arXiv2024

Vison crystals, chiral and crystalline phases in the Yao-Lee model

We study the phase diagram of the Yao-Lee model with Kitaev-type spin-orbital interactions in the presence of Dzyaloshinskii-Moriya interactions and external magnetic fields. Unlike the Kitaev model, the Yao-Lee model can still be solved exactly under these perturbations due to the enlarged local Hilbert space. Through a variational analysis, we obtain a rich ground state phase diagram that consists of a variety of vison crystals with periodic arrangements of background Z2 flux (i.e. visons). With an out-of-plane magnetic field, these phases have gapped bulk and chiral edge states, characterized by a Chern number ν and an associated chiral central charge c_{-} = ν/2 of edge states. We also find helical Majorana edge states that are protected by magnetic mirror symmetry. Our results spotlight the tunability and the accompanying rich physics in exactly-solvable spin-orbital generalizations of the Kitaev model.

cond-mat.str-el↗

arXiv2023

Topological and magnetic phase transitions in the bilayer Kitaev-Ising model

We investigate the phase diagram of a bilayer Kitaev honeycomb model with Ising interlayer interactions, deriving effective models via perturbation theory and performing Majorana mean-field theory calculations. We show that a diverse array of magnetic and topological phase transitions occur, depending on the direction of the interlayer Ising interaction and the relative sign of Kitaev interactions. When two layers have the same sign of the Kitaev interaction, a first-order transition from a Kitaev spin liquid to a magnetically ordered state takes place. The magnetic order points along the Ising axis and it is (anti)ferromagnetic for (anti)ferromagnetic Kitaev interactions. However, when two layers have opposite sign of the Kitaev interaction, we observe a notable weakening of magnetic ordering tendencies and the Kitaev spin liquid survives up to a remarkably larger interlayer exchange. Our mean-field analysis suggests the emergence of an intermediate gapped $\mathbb{Z}_2$ spin liquid state, which eventually becomes unstable upon vison condensation. The confined phase is described by a highly frustrated $120^\circ$ compass model. We furthermore use perturbation theory to study the model with the Ising axis pointing along $\hat{z}$-axis or lying in the $xy$-plane. In both cases, our analysis reveals the formation of 1D Ising chains, which remain decoupled in perturbation theory, resulting in a subextensive ground-state degeneracy. Our results highlight the interplay between topological order and magnetic ordering tendencies in bilayer quantum spin liquids.

cond-mat.str-el↗

arXiv2023

Theory of moiré magnetism in twisted bilayer $α$-RuCl$_3$

Twisted heterostructures of van der Waals materials have received much attention for their many remarkable properties. Here, we present a comprehensive theory of the long-range ordered magnetic phases of twisted bilayer $α$-RuCl$_3$ via a combination of first-principles calculations and atomistic simulations. While a monolayer exhibits zigzag antiferromagnetic order with three possible ordering wave vectors, a rich phase diagram is obtained for moiré superlattices as a function of interlayer exchange and twist angle. For large twist angles, each layer spontaneously picks a single zigzag ordering wave vector, whereas, for small twist angles, the ground state involves a combination of all three wave vectors in a complex hexagonal domain structure. This multi-domain order minimizes the interlayer energy while enduring the energy cost due to the domain wall formation. Our results indicate that magnetic frustration due to stacking-dependent interlayer exchange in moiré superlattices can be used to tune the magnetic ground state and enhance quantum fluctuations in $α$-RuCl$_3$.

cond-mat.mtrl-sci↗

arXiv2023

Revival of antibiskyrmionic magnetic phases in bilayer NiI$_2$

Magnetic skyrmions are topologically protected spin textures with potential applications in memory and logic devices. Skyrmions have been commonly observed in systems with Dzyaloshinskii-Moriya interaction due to broken inversion symmetry. Yet, recent studies suggest that skyrmions can also be stabilized in systems with inversion symmetry such as Ni-based dihalides due to magnetic frustration. In this article, we employ atomistic simulations to investigate chiral magnetic phases in bilayers of NiI$_2$ and NiBr$_2$. We show that the antiferromagnetic interlayer coupling introduces an additional magnetic frustration and gives rise to a variety of novel spin textures with different topological charges. Specifically for NiI$_2$, we observe that the skyrmions with the in-plane component of spins wrapping around twice (biskyrmions) have an enhanced stability compared to the monolayer case. We also study the polarization induced by the non-colinear magnetic order in NiI$_2$ bilayers and show that the polarization of the topologically nontrivial phases is negligible compared to the spiral phases. Thus, we conclude that polarization measurements can be an indirect route for detecting skyrmions in upcoming experiments.

cond-mat.mes-hall↗

arXiv2023

Magnetic fragmentation and fractionalized Goldstone modes in a bilayer quantum spin liquid

We study the phase diagram of a bilayer quantum spin liquid model with Kitaev-type interactions on a square lattice. We show that the low energy limit is described by a $π$-flux Hubbard model with an enhanced SO(4) symmetry. The antiferromagnetic Mott transition of the Hubbard model signals a magnetic fragmentation transition for the spin and orbital degrees of freedom of the bilayer. The fragmented "Néel order" features a non-local string order parameter for an in-plane Néel component, in addition to an anisotropic local order parameter. The associated quantum order is characterized by an emergent $\mathbb{Z}_{2} \times \mathbb{Z}_{2}$ gauge field when the Néel vector is along the $\hat{z}$ direction, and a $\mathbb{Z}_2$ gauge field otherwise. We underpin these results with a perturbative calculation, which is consistent with the field theory analysis. We conclude with a discussion on the low energy collective excitations of these phases and show that the Goldstone boson of the $\mathbb{Z}_{2} \times \mathbb{Z}_{2}$ phase is fractionalized and non-local.

cond-mat.str-el↗

arXiv2022

Kitaev spin-orbital bilayers and their moiré superlattices

We determine the phase diagram of a bilayer, Yao-Lee spin-orbital model with inter-layer interactions (J), for several stackings and moiré superlattices. For AA stacking, a gapped Z2 quantum spin liquid phase emerges at a finite Jc. We show that this phase survives in the well-controlled large-J limit, where an isotropic honeycomb toric code emerges. For moiré superlattices, a finite-q inter-layer hybridization is stabilized. This connects inequivalent Dirac points, effectively `untwisting' the system. Our study thus provides insight into the spin-liquid phases of bilayer spin-orbital Kitaev materials.

cond-mat.str-el↗

arXiv2022

Skyrmion formation in Ni-based Janus dihalide monolayers: Interplay between magnetic frustration and Dzyaloshinskii-Moriya interaction

We present a comprehensive theory of the magnetic phases in monolayer nickel-based Janus dihalides (NiIBr, NiICl, NiBrCl) through a combination of first-principles calculations and atomistic simulations. Phonon band structure calculations and finite temperature molecular dynamics simulations show that Ni-dihalide Janus monolayers are stable. The parameters of the interacting spin Hamiltonian extracted from ab initio calculations show that nickel-dihalide Janus monolayers exhibit varying degrees of magnetic frustration together with a large Dzyaloshinskii-Moriya interaction due to their inherent inversion symmetry breaking. The atomistic simulations reveal a delicate interplay between these two competing magnetic interactions in giving rise to skyrmion formation.

cond-mat.str-el↗

arXiv2021

Shadow surface states in topological Kondo insulators

The surface states of 3D topological insulators in general have negligible quantum oscillations when the chemical potential is tuned to the Dirac points. In contrast, we find that topological Kondo insulators can support surface states with an arbitrarily large Fermi surface when the chemical potential is pinned to the Dirac point. We illustrate that these Fermi surfaces give rise to finite-frequency quantum oscillations, which can become comparable to the extremal area of the unhybridized bulk bands. We show that this occurs when the crystal symmetry is lowered from cubic to tetragonal in a minimal two-orbital model. We label such surface modes as `shadow surface states'. Moreover, we show that the sufficient next-nearest neighbor out-of-plane hybridization leading to shadow surface states can be self-consistently stabilized for tetragonal topological Kondo insulators. Consequently, shadow surface states provide an important example of high-frequency quantum oscillations beyond the context of cubic topological Kondo insulators.

cond-mat.str-el↗

arXiv2021

Topological Quantum Many-Body Scars in Quantum Dimer Models on the Kagome Lattice

We present a class of quantum dimer models on the kagome lattice with full translational invariance that feature a quantum many-body scar state of analytically known entanglement properties within their spectra. Using exact diagonalization on lattices of up to 60 sites, we show that non-scar states conform to the eigenstate thermalization hypothesis. Specifically, we show that energies are distributed according to the Gaussian ensemble expected of their respective symmetry sector, illustrate the existence of the scar from bipartite entanglement properties, and demonstrate revival phenomena in studies of fidelity dynamics.

cond-mat.str-el↗

arXiv2021

Superconductivity in a quintuple-layer square-planar nickelate

Since the discovery of high-temperature superconductivity in the copper oxide materials, there have been sustained efforts to both understand the origins of this phase and discover new cuprate-like superconducting materials. One prime materials platform has been the rare-earth nickelates and indeed superconductivity was recently discovered in the doped compound Nd$_{0.8}$Sr$_{0.2}$NiO$_2$. Undoped NdNiO$_2$ belongs to a series of layered square-planar nickelates with chemical formula Nd$_{n+1}$Ni$_n$O$_{2n+2}$ and is known as the 'infinite-layer' ($n = \infty$) nickelate. Here, we report the synthesis of the quintuple-layer ($n = 5$) member of this series, Nd$_6$Ni$_5$O$_{12}$, in which optimal cuprate-like electron filling ($d^{8.8}$) is achieved without chemical doping. We observe a superconducting transition beginning at $\sim$13 K. Electronic structure calculations, in tandem with magnetoresistive and spectroscopic measurements, suggest that Nd$_6$Ni$_5$O$_{12}$ interpolates between cuprate-like and infinite-layer nickelate-like behavior. In engineering a distinct superconducting nickelate, we identify the square-planar nickelates as a new family of superconductors which can be tuned via both doping and dimensionality.

cond-mat.supr-con↗

arXiv2021

Matrix-pairing states in the alkaline Fe-selenide superconductors: exotic Josephson junctions

True to their unconventional nature, multi-band alkaline Fe-selenides and, more recently, the heavy-fermion CeCu$_{2}$Si$_{2}$ have shown signatures of fully-gapped but sign-changing superconductivity (SC). A two-orbital pairing state, called $sτ_{3}$, with non-trivial matrix structure, was proposed as a candidate able to reconcile the seemingly contradictory properties of these SC's. Motivated by the non-trivial orbital structure of the proposed $sτ_{3}$ state, which has orbital-selective pairing structure, we study prototypical Josephson junctions where at least one of the leads is in a SC state of this kind. An analysis of these junctions in the limit of two degenerate orbitals (bands) and with a simple form of junction hybridization reveals several remarkable properties. One is the emergence of gapless, purely electron- and hole-like bound states for $sτ_{3}-N-sτ_{3}$ junctions with arbitrary global phase difference between the leads, and likewise for $sτ_{3}-N-I$ junctions. The other is the absence of static Josephson currents when both leads are SC's. In both of these signatures, $sτ_{3}$ junctions are dramatically different from conventional Josephson junctions. We also find that the gapless bound states are protected by an orbital-exchange symmetry, although the protection is not topological. Junctions which break this symmetry, such as $sτ_{3}-N-s$, have gapped Andreev bound states. In general, the Josephson effect also re-emerges once the degeneracy of the two orbitals is lifted. We support these conclusions via analytical and numerical results for the bound states, together with microscopic calculations of the Josephson current. Our results indicate that junctions involving $sτ_{3}$ pairing in alkaline Fe-selenides's will generically have bound states with a small gap together with a greatly suppressed Josephson current.

cond-mat.supr-con↗

arXiv2021

Moiré skyrmions and chiral magnetic phases in twisted CrX$_{3}$ (X $=$ I, Br, Cl) bilayers

We present a comprehensive theory of the magnetic phases in twisted bilayer Cr-trihalides through a combination of first-principles calculations and atomistic simulations. We show that the stacking-dependent interlayer exchange leads to an effective moiré field that is mostly ferromagnetic with antiferromagnetic patches. A wide range of noncollinear magnetic phases can be stabilized as a function of the twist angle and Dzyaloshinskii-Moriya interaction as a result of the competing interlayer antiferromagnetic coupling and the energy cost for forming domain walls. In particular, we demonstrate that for small twist angles various skyrmion crystal phases can be stabilized in both CrI$_3$ and CrBr$_3$. Our results provide an interpretation for the recent observation of noncollinear magnetic phases in twisted bilayer CrI$_3$ and demonstrate the possibility of engineering further nontrivial magnetic ground states in twisted bilayer Cr-trihalides.

cond-mat.str-el↗

arXiv2021

Skyrmions in twisted van der Waals magnets

Magnetic skyrmions in 2D chiral magnets are in general stabilized by a combination of Dzyaloshinskii-Moriya interaction and external magnetic field. Here, we show that skyrmions can also be stabilized in twisted moiré superlattices in the absence of an external magnetic field. Our setup consists of a 2D ferromagnetic layer twisted on top of an antiferromagnetic substrate. The coupling between the ferromagnetic layer and the substrate generates an effective alternating exchange field. We find a large region of skyrmion crystal phase when the length scales of the moiré periodicity and skyrmions are compatible. Unlike chiral magnets under magnetic field, skyrmions in moiré superlattices show enhanced stability for the easy-axis (Ising) anisotropy which can be essential to realize skyrmions since most van der Waals magnets possess easy-axis anisotropy.

cond-mat.str-el↗

arXiv2020

Theory of $(s+id)$ pairing in mixed-valent correlated metals

Motivated by the recent discovery of superconductivity in square-planar nickelates as well as by longstanding puzzling experiments in heavy-fermion superconductors, we study Cooper pairing between correlated $d$-electrons coupled to a band of weakly-correlated electrons. We perform self-consistent large N calculations on an effective $t-J$ model for the $d$-electrons with additional hybridization. Unlike previous studies of mixed-valent systems, we focus on parameter regimes where both hybridized bands are relevant to determining the pairing symmetry. For sufficiently strong hybridization, we find a robust $s+id$ pairing which breaks time-reversal and point-group symmetries in the mixed-valent regime. Our results illustrate how intrinsically multi-band systems such as heavy-fermions can support a number of highly non-trivial pairing states. They also provide a putative microscopic realization of previous phenomenological proposals of $s+id$ pairing and suggest a potential resolution to puzzling experiments in heavy-fermion superconductors such as U$_{1-x}$Th$_x$Be$_{13}$ which exhibit two superconducting phase transitions and a full gap at lower temperatures.

cond-mat.supr-con↗