arXiv subjects

Mehdi Kargarian

Publications and source records attributed to Mehdi Kargarian.

At least 19 recordsLinked to original sources

arXiv2026

Topological hybridisation of plasmons with ferrimagnetic magnons

We study the formation of hybrid plasmon-magnon modes in a heterostructure comprising a monolayer semiconductor with strong Rashba spin-orbit coupling -- specifically, Janus transition-metal dichalcogenides (TMDs) -- and an insulating ferrimagnet, such as yttrium iron garnet-based compounds. Using a combined microscopic-macroscopic framework for plasmon-magnon coupling, we show that plasmons and magnons strongly hybridize over both GHz and THz frequency ranges, enabling experimental access well above cryogenic temperatures. Moreover, the developed approach provides an efficient and natural classification of the topology of the hybrid modes, rooted in the phase winding of the plasmon-magnon coupling induced by spin-momentum locking and the associated chiral winding of the electronic spin along the Fermi contours. Finally, we identify experimentally accessible manifestations of the hybridization, such as topological interface modes and an anomalous thermal Hall response.

cond-mat.mes-hall↗

arXiv2025

Spin liquid phase in the Hubbard model: Luttinger-Ward analysis of the slave-rotor formalism

We propose an approach for studying the spin liquid phase of the Hubbard model on the triangular lattice by combining the Baym-Kadanoff formalism with the slave rotor parton construction. This method enables the computation of a series of two-body Feynman diagrams for the Luttinger-Ward (LW) functional using a one-loop truncation. This approach enables us to study the U(1) quantum spin liquid phase characterized by a spinon Fermi surface and to derive the Green's functions for spinons, chargons, and electrons. Our findings extend beyond the standard mean-field approximation by accounting for the effects of gauge field fluctuations. The spatial components of the U(1) gauge field are equivalently represented by interactions that incorporate corrections from the spinon-chargon two-particle random phase approximation. This framework effectively captures the long-range correlations inherent to the U(1) quantum spin liquid and combines non-perturbative quantum field theory with the projective construction, providing new insights into the study of quantum spin liquids and other strongly correlated electron systems. We demonstrate that our approach correctly computes the low-temperature linear temperature dependence of the specific heat in the U(1) spin liquid, in agreement with the behavior expected for a Fermi surface. Moreover, this approach reproduces the resonant peaks in the Mott gap, as observed in cobalt atoms on single-layer 1T-TaSe2.

cond-mat.str-el↗

arXiv2025

Superconductivity and a van Hove singularity confined to the surface of a topological semimetal

The interplay between electronic topology and superconductivity is the subject of great current interest in condensed matter physics. For example, superconductivity induced on the surface of topological insulators is predicted to be triplet in nature, while the interplay between electronic correlations and topology may lead to unconventional superconductivity as in twisted bilayer graphene. Here, we unveil an unconventional two-dimensional superconducting state in the recently discovered Dirac nodal line semimetal ZrAs2 which is exclusively confined to the top and bottom surfaces within the crystal's ab plane. As a remarkable consequence of this emergent state, we observe a Berezinskii-Kosterlitz-Thouless (BKT) transition, the hallmark of two-dimensional superconductivity. Notably, this is the first observation of a BKT transition on the surface of a three-dimensional system. Furthermore, employing angle-resolved photoemission spectroscopy and first-principles calculations, we find that these same surfaces also host a two-dimensional van Hove singularity near the Fermi energy. The proximity of van Hove singularity to the Fermi level leads to enhanced electronic correlations contributing to the stabilization of superconductivity at the surface of ZrAs2, a unique phenomenon among topological semimetals. The surface-confined nature of the van Hove singularity, and associated superconductivity, realized for the first time, opens new avenues to explore the interplay between low-dimensional quantum topology, correlations, and superconductivity in a bulk material without resorting to the superconducting proximity effect.

cond-mat.supr-con↗

arXiv2025

Probing the collective excitations of excitonic insulators in an optical cavity

The light--matter interaction in optical cavities offers a promising ground to create hybrid states and manipulate material properties. In this work, we examine the effect of light-matter coupling in the excitonic insulator phase using a quasi one-dimensional lattice model with two opposite parity orbitals at each site. We show that the model allows for a coupling between the collective phase mode and cavity photons. Our findings reveal that the collective mode of the excitonic state significantly impacts the dispersion of the cavity mode, giving rise to an avoiding band crossing in the photon dispersion. This phenomenon is absent in trivial and topological insulator phases and also in phonon-mediated excitonic insulators, underscoring the unique characteristics of collective excitations in excitonic insulators. Our results demonstrate the significant impact of light-matter interaction on photon propagation in the presence of excitonic collective excitations.

cond-mat.mes-hall↗

arXiv2024

Pomeranchuk instability of a topological crystal

Nematic quantum fluids appear in strongly interacting systems and break the rotational symmetry of the crystallographic lattice. In metals, this is connected to a well-known instability of the Fermi liquid-the Pomeranchuk instability. Using scanning tunneling microscopy, we identified this instability in a highly unusual setting: on the surface of an elemental topological metal, arsenic. By directly visualizing the Fermi surface of the surface state via scanning tunneling spectroscopy and photoemission spectroscopy, we find that the Fermi surface gets deformed and becomes elliptical at the energies where the nematic state is present. Known instances of nematic instability typically need van-Hove singularities or multi-orbital physics as drivers. In contrast, the surface states of arsenic are essentially indistinguishable from well-confined isotropic Rashba bands near the Fermi level, rendering our finding the first realization of Pomeranchuk instability of the topological surface state.

cond-mat.str-el↗

arXiv2024

Spinon Kondo lattice in quantum spin liquids

Motivated by recent experimental observations of Kondo resonances in cobalt atoms on single layer 1T-TaSe$_{2}$, we theoretically investigate the effect of coupling a U(1) quantum spin liquid with a spinon Fermi surface to a lattice of Anderson impurities. Within the slave-rotor formalism, we find that above a critical coupling strength between the spin liquid and impurity lattice, the spinons hybridize to form heavy quasiparticles near the Fermi level, realizing a {\it spinon Kondo lattice phase} analogous to heavy fermion materials. Using the Bethe-Salpeter equation and accounting for emergent gauge fluctuations, we compute the spectral density and density of states, revealing the formation of spinon-chargon bound states in the spinon Kondo lattice phase. We characterize the thermodynamic and spectroscopic signatures of this phase, demonstrating specific heat and neutron scattering responses distinct from a pure quantum spin liquid. Our findings establish the spinon Kondo lattice as a framework to study the rich physics of spin liquids.

cond-mat.str-el↗

arXiv2024

Stoner ferromagnetism, correlated metal and thermoelectricity in partially flat-band materials

Recent discovery of correlated electronic phases in twisted heterostructures raised a surge of interests in studying models and materials with flat bands where the electronic excitations are nearly dispersionless in momentum space. As such, the kinetic energy is quenched and the correlations are enhanced, giving rise to a plethora of unusual magnetic, superconducting and transport behaviors. Finding materials whose energy bands are completely flat is rather challenging, yet those whose dispersion is flat only in a portion of the momentum space might be more accessible in material search. In this work, we propose a partially flat-band system on a square lattice. Using the Hubbard model, it is demonstrated that the suppression of the electronic kinetic energy in the flat portion of the band dispersion drives the system to Stoner ferromagnetism even at very weak interactions, i.e., much smaller than the bandwidth, with significantly enhanced Curie temperature. While the low-energy magnon modes are well defined collective excitations, flat magnon bands can be observed at high energies. We show that the strong interaction leads to reduction of the flat portion of the magnon band. However, tuning the chemical potential at a strong interaction regime may lead to spin density wave at finite wave vectors. Then, focusing on the non-magnetic correlated phase and using dynamical mean-field theory, we demonstrate the appearance of a flat-band induced sharp peak in the density of states in addition to the correlation-induced Mott bands. Furthermore, the large seebeck coefficient and the figure of merit of the proposed partially flat-band model, compared to symmetric regular band models, put them in the category of efficient thermoelectric materials.

cond-mat.str-el↗

arXiv2023

Phase diagram of the Kitaev-Hubbard model: $\mathbb{Z}_2$ slave-spin and QMC approaches

Recent experiments show that the ground state of some layered materials with localized moments is in close proximity to the Kitaev spin liquid, calling for a proper model to describe the measurements. The Kitaev-Hubbard (KHu) model is the minimal model that captures the essential ingredients of these systems; it yields the Kitaev-Heisenberg spin model at the strong coupling limit and contains the charge fluctuations present in these materials as well. Despite its relevance, the phase diagram of the KHu model has not been rigorously revealed yet. In this work, we study the full phase diagram of the KHu model using the $\mathbb{Z}_2$ slave-spin mean-field theory as well as the auxiliary field quantum Monte Carlo on rather large systems and at low temperatures. The Mott transition is signaled by a vanishing quasiparticle weight evaluated using the slave-spin construction. Moreover, we demonstrate that there are multiple magnetic phase transitions within the Mott phase including magnetically ordered phases and most notably a quantum spin liquid phase for $1.0 \lesssim t^{\prime}/t \lesssim 1.11 $ at $U/t=5$.

cond-mat.str-el↗

arXiv2023

Optical drive of amplitude and phase modes in excitonic insulators

Motivated by recent interests in exploring excitonic condensate as the ground state of some narrow-bandgap semiconductors such as transition metal dichalcogenides and layered chalcogenide material Ta$_2$NiSe$_5$, in this work we theoretically study the dynamics of condensate in response to periodically driven laser fields with different polarizations and intensities. In particular, we consider laser light beams with bicircular and circular polarizations breaking the time-revesal symmetry, and linear polarization. We show that the amplitude of the condensate oscillates in time during irradiating by light with a magnitude depending on the light intensity. The dynamics survives even after the light is switched off. The phase mode however changes linearly with time for a condensate originating from purely electronic correlations. We further show that in the presence of electron-phonon coupling the linear-in-time behavior is replaced by a harmonically oscillating behavior, a manifestation of gapped phase modes due to relative band charge symmetry breaking. Furthermore, we show that the primarily electronic and primarily lattice cases corresponding to strong and weak electron-phonon coupling, respectively, reveal distinct dynamics of the condensate, an observation which can modify the optical response of an excitonic insulator by stimulating amplitude and phase modes in the former case.

cond-mat.str-el↗

arXiv2023

Exposing nontrivial flat bands and superconducting pairing in infinite-layer nickelates

Flat bands coupled with magnetism and topological orders near or at the Fermi level are well known to drive exotic correlation physics and unconventional superconductivity. Here, based on first-principles modeling combined with an in-depth symmetry analysis, we reveal the presence of topological flat bands involving low-energy Ni-$3d_{z^2}$ states in the recently discovered superconductor LaNiO$_{2}$. Our analysis demonstrates that LaNiO$_2$ is an Axion insulator with $\mathbb{Z}_{4} = 2$ and that it supports topological crystalline insulating states protected by the glide mirror symmetries. The topological flat bands in LaNiO$_{2}$ are also shown to host odd-parity superconductivity. Our study indicates that the nickelates would provide an interesting materials platform for exploring the interplay of flat bands, topological states, and superconductivity.

cond-mat.supr-con↗

arXiv2023

Unconventional superconducting pairing in a B20 Kramers Weyl semimetal

Topological superconductors present an ideal platform for exploring nontrivial superconductivity and realizing Majorana boundary modes in materials. However, finding a single-phase topological material with nontrivial superconducting states is a challenge. Here, we predict nontrivial superconductivity in the pristine chiral metal RhGe with a transition temperature of 5.8 K. Chiral symmetries in RhGe enforce multifold Weyl fermions at high-symmetry momentum points and spin-polarized Fermi arc states that span the whole surface Brillouin zone. These bulk and surface chiral states support multiple type-II van Hove singularities that enhance superconductivity in RhGe. Our detailed analysis of superconducting pairing symmetries involving Chiral Fermi pockets in RhGe, indicates the presence of nontrivial superconducting pairing. Our study establishes RhGe as a promising candidate material for hosting mixed-parity pairing and topological superconductivity.

cond-mat.supr-con↗

arXiv2022

Phase transition and fractionalization in superconducting Kondo lattice model

Topology, symmetry, electron correlations, and the interplay between them have formed the cornerstone of our understanding of quantum materials in recent years and are used to identify new emerging phases. While the first two give a fair understanding of noninteracting and, in many cases, weakly interacting wave function of electron systems, the inclusion of strong correlations could change the picture substantially. The Kondo lattice model is a paradigmatic example of the interplay of electron correlations and conduction electrons of a metallic system, describing heavy fermion materials and also fractionalized Fermi liquid pertaining to an underlying gauge symmetry and topological orders. In this work, we study a superconducting Kondo lattice model, a network of 1D Kitaev superconductors Kondo coupled to a lattice of magnetic moments. Using slave-particle representation of spins and exact numerical calculations, we obtain the phase diagram of the model in terms of Kondo coupling $J_K$ and identify a topological order phase for $J_{K} J_{K}^c$, where $J_{K}^c$ is the critical point. Setting the energy scales of electron hopping and pairing to unity, the mean-field theory calculations achives $J_{K}^c=2$ and in exact numerics we found $J_{K}^c\simeq 1.76$, both of which show that the topological order is a robust phase. We argue that in terms of slave particles, the compensated phase corresponds to an invertible phase, and a Mott insulating transition leads to a topological order phase. Furthermore, we show that in the regime $J_{K}<J_{K}^c$ in addition to the low-energy topological states, a branch of subgap states appears inside the superconducting gap.

cond-mat.str-el↗

arXiv2022

Equatorial magnetoplasma waves

Due to its rotation, Earth traps a few equatorial ocean and atmospheric waves, including Kelvin, Yanai, Rossby, and Poincare modes. It has been recently demonstrated that the mathematical origin of equatorial waves is intricately related to the nontrivial topology of hydrodynamic equations describing oceans or the atmosphere. In the present work, we consider plasma oscillations supported by a two-dimensional electron gas confined at the surface of a sphere or a cylinder. We argue that in the presence of a uniform magnetic field, these systems host a set of equatorial magnetoplasma waves that are counterparts to the equatorial waves trapped by Earth. For a spherical geometry, the equatorial modes are well developed only if their penetration length is smaller than the radius of the sphere. For a cylindrical geometry, the spectrum of equatorial modes is weakly dependent on the cylinder radius and overcomes finite-size effects. We argue that this exceptional robustness can be explained by destructive interference effects. We discuss possible experimental setups, including grains and rods composed of topological insulators (e.g., Bi2Se3) or metal-coated dielectrics (e.g., Au2S).

cond-mat.mes-hall↗

arXiv2022

Designing $\mathbb{Z}_2$ and $\mathbb{Z}_2 \times \mathbb{Z}_2$ topological orders in networks of Majorana bound states

Topological orders have been intrinsically identified in a class of systems such as fractional quantum Hall states and spin liquids. Accessing such states often requires extreme conditions such as low temperatures, high magnetic fields, pure samples, etc. Another approach would be to engineer the topological orders in systems with more accessible ingredients. In this work, we present networks of Majorana bound states, which are currently accessible in semiconductor nanowires proximitized to conventional superconductors, and show that the effective low-energy theory is topologically ordered. We first demonstrate the main principles in a lattice made of Kitaev superconducting chains comprising both spin species. The lattice is coupled to free magnetic moments through the Kondo interaction. We then show that at the weak coupling limit, effective ring spin interactions are induced between magnetic moments with a topological order enjoying a local $\mathbb{Z}_2 \times \mathbb{Z}_2$ gauge symmetry. We then show that the same topological order and also the $\mathbb{Z}_2$ one can be engineered in architecture patterns of semiconductor nanowires hosting Majorana bound states. The basic blocks of patterns are the time-reversal Majorana Cooper boxes coupled to each other by metallic leads, and the Majorana states are allowed to tunnel to quantum dots sitting on the vertices of the lattices. In the limit of strong onsite Coulomb interactions, where the charge fluctuations are suppressed, the magnetic moments of dots on the square and honeycomb lattices are described by topologically ordered spin models with underlying $\mathbb{Z}_2$ and $\mathbb{Z}_2 \times \mathbb{Z}_2$ gauge symmetries, respectively. Finally, we show that the latter topological order can also be realized in a network of purely Majorana zero modes in the absence of coupling to quantum dots.

cond-mat.supr-con↗

arXiv2021

Topological spin-plasma waves

The surface of a topological insulator hosts Dirac electronic states with the spin-momentum locking, which constrains spin orientation perpendicular to electron momentum. As a result, collective plasma excitations in the interacting Dirac liquid manifest themselves as coupled charge- and spin-waves. Here we demonstrate that the presence of the spin component enables effective coupling between plasma waves and spin waves at interfaces between the surface of a topological insulator and insulating magnet. Moreover, the helical nature of spin-momentum locking textures provides the phase winding in the coupling between the spin and plasma waves that makes the spectrum of hybridized spin-plasma modes to be topologically nontrivial. We also show that such topological modes lead to a large thermal Hall response.

cond-mat.mes-hall↗

arXiv2021

Hybrid topological magnon-phonon modes in honeycomb and kagome lattices

Magnons and phonons are two fundamental neutral excitations of magnetically ordered materials which can significantly dominate the low-energy thermal properties. In this work we study the interplay of magnons and phonons in honeycomb and Kagome lattices. When the mirror reflection with respect to the magnetic ordering direction is broken, the symmetry-allowed in-plane Dzyaloshinskii-Moriya (DM) interaction will couple the magnons to the phonons and the magnon-polaron states are formed. Besides, both lattice structures also allow for an out-of-plane DM interaction rendering the uncoupled magnons to be topological. Our aim is to study the interplay of such topological magnons with phonons. We show that the hybridization between magnons and phonons can significantly redistribute the Berry curvature among the bands. Especially, we found that the topological magnon band becomes trivial while the hybridized states at lower energy acquire Berry curvature strongly peaked near the avoided crossings. As such the thermal Hall conductivity of topological magnons shows significant changes due to coupling to the phonons.

cond-mat.str-el↗

arXiv2020

Excitonic insulator phase and dynamics of condensate in a topological one-dimensional model

We employ mean-field approximation to investigate the interplay between the nontrivial band topology and the formation of excitonic insulator (EI) in a one-dimensional chain of atomic $s-p$ orbitals in the presence of repulsive inter-orbital Coulomb interaction. We find that our model, in a non-interacting regime, admits topological and trivial insulator phases, whereas, in strong Coulomb interaction limit, the chiral symmetry is broken and the system undergoes a topological-excitonic insulator phase transition. The latter phase transition stems from an orbital pseudomagnetization and band inversion around $k=0$. Our findings show that contrary to the topological insulator phase, electron-hole bound states do not form exciton condensate in the trivial band insulator phase due to lack of band inversion. Interestingly, the EI phase in low $s-p$ hybridization limit hosts a Bardeen-Cooper-Schrieffer (BCS)/Bose-Einstein condensation (BEC) crossover. Irradiated by a pump pulse, our findings reveal that the oscillations of exciton states strongly depend on the frequency of the laser pulse. We further explore the signatures of dynamics of the exciton condensate in optical measurements.

cond-mat.str-el↗

arXiv2020

Effects of dynamical noises on Majorana bound states

The nonlocal nature of unpaired Majorana bound states (MBSs) in topological superconductors can be exploited to create topologically protected qubits and perform gate operations fault-tolerantly via braidings. However, the time-dependent noises induced by coupling to an environment which is inevitable in any realistic system could spoil the topological protection. In this work, we study the effects of various dynamical noises such as Lorentzian, thermal, and quantum point contact on the MBSs in the recently proposed one-dimensional topological superconductors. We begin by investigating the Kitaev p-wave superconductors and examine the effects of long-range hopping and pairing on the transition rate of MBSs. We found that, especially, the long-range pairings significantly reduce the transition rate of bound states. Then, we consider the recently discovered topological superconducting nanowires and magnetic chains. Our findings are consequential for the recent attempts to manipulate MBSs. In particular, for the latter two experimentally realized systems we argue how low magnetic/Zeeman fields and strong spin-orbit coupling make the MBSs more robust to noises.

cond-mat.supr-con↗