arXiv subjects

Mohsen Hafez-Torbati

Publications and source records attributed to Mohsen Hafez-Torbati.

17 recordsLinked to original sources

arXiv2026

Altermagnetic Anomalous Hall Effect and Spin--Edge-Locked Chiral Modes in a Modified Kane--Mele--Hubbard Model

We establish a correlation-driven route to the altermagnetic anomalous Hall effect (AHE) and its associated \emph{spin--edge-locked} edge states in a modified Kane--Mele--Hubbard model. Using dynamical mean-field theory (DMFT), we show that, at half-filling, increasing the Hubbard interaction drives the system from a metallic paramagnetic phase hosting antichiral edge states into an insulating in-plane Néel-type antiferromagnetic phase, in which a residual antiunitary symmetry forbids the AHE. Hole doping induces a spin-flop transition to an out-of-plane Néel-type antiferromagnetic phase, thereby breaking this symmetry and generating a finite anomalous Hall conductivity that persists into the strongly correlated regime. Distinct from a conventional spin-polarized Hall response in ferromagnets, the altermagnetic AHE receives equal and additive contributions from the two symmetry-related spin sectors and is accompanied by spin--edge-locked chiral states. Our results demonstrate that carrier doping and spin-rotationally invariant Hubbard interactions are sufficient to realize the altermagnetic AHE, without invoking an explicitly Ising-like interaction, and provide a realistic microscopic route toward its realization in correlated transition metal dichalcogenides monolayers.

cond-mat.str-el↗

arXiv2026

High-Temperature Quantum Anomalous Hall Effect in Buckled Honeycomb Antiferromagnets

We propose Néel antiferromagnetic (AF) Mott insulators with a buckled honeycomb structure as potential candidates to host a high-temperature AF Chern insulator (AFCI). Using a generalized Kondo lattice model we show that the staggered potential induced by a perpendicular electric field due to the buckling can drive the AF Mott insulator to an AFCI phase. We address the temperature evolution of the Hall conductance and the chiral edge states. The quantization temperature $T_q$, below which the Hall conductance is quantized, depends essentially on the strength of the spin-orbit coupling and the hopping parameter, independent of the specific details of the model. The deviation of the Hall conductance from the quantized value $e^2/h$ above $T_q$ is found to be accompanied by a spectral broadening of the chiral edge states, reflecting a finite life-time, i.e., a decay. Using parameters typical for heavy transition-metal elements we predict that the AFCI can survive up to room temperature. We suggest Sr$_3$CaOs$_2$O$_9$ as a potential compound to realize a high-$T$ AFCI phase.

cond-mat.str-el↗

arXiv2025

From explicit to spontaneous charge order and the fate of antiferromagnetic quantum Hall state

The antiferromagnetic quantum Hall insulator (AFQHI), where one of the spin components is in the quantum Hall state and the other in the trivial state, is an established phase emerging as a result of the Hubbard repulsion in spinful quantum Hall systems. The stabilization of the AFQHI requires a charge order preventing the effect of the spin-flip transformation on the electronic state to be compensated by a space-group operation, and is often induced via an ionic potential. While one would naively expect the nearest-neighbor (NN) density-density interaction favoring spontaneous charge order to result in qualitatively similar phenomena, an analysis of the Haldane-Hubbard model extended by the NN interaction finds no AFQHI. Here, by considering an extended version of the Harper-Hofstadter-Hubbard model we go beyond the honeycomb structure and suggest that the realization of the AFQHI generically requires an explicit charge order and cannot be emerged through a spontaneous charge order. We unveil how the AFQHI disappears upon approaching from the explicit to the spontaneous charge ordering limit. Our findings shine more light on the stabilization conditions of the AFQHI which can guide the future optical lattice experiments searching for this intriguing magnetic topological insulator phase.

cond-mat.str-el↗

arXiv2024

Antiferromagnetic Chern insulator with large charge gap in heavy transition-metal compounds

Despite the discovery of multiple intrinsic magnetic topological insulators in recent years the observation of Chern insulators is still restricted to very low temperatures due to the negligible charge gaps. Here, we uncover the potential of heavy transition-metal compounds for realizing a collinear antiferromagnetic Chern insulator (AFCI) with a charge gap as large as 300 meV. Our analysis relies on the Kane-Mele-Kondo model with a ferromagnetic Hund coupling $J_{\rm H}$ between the spins of itinerant electrons and the localized spins of size $S$. We show that a spin-orbit coupling $λ_{\rm SO} \gtrsim 0.03t$, where $t$ is the nearest-neighbor hopping element, is already large enough to stabilize an AFCI provided the alternating sublattice potential $δ$ is in the range $δ\approx SJ_{\rm H}$. We establish a remarkable increase in the charge gap upon increasing $λ_{\rm SO}$ in the AFCI phase. Using our results we explain the collinear AFCI recently found in monolayers of CrO and MoO with charge gaps of 1 and $50$ meV, respectively. In addition, we propose bilayers of heavy transition-metal oxides of perovskite structure as candidates to realize a room-temperature AFCI if grown along the $[111]$ direction and subjected to a perpendicular electric field.

cond-mat.str-el↗

arXiv2024

Antiferromagnetic topological insulators in heavy-fermion systems

The cooperation of electronic correlation and spin-orbit coupling can stabilize magnetic topological insulators which host novel quantum phenomena such as the quantum anomalous Hall state also known as Chern insulator (CI). Here, we investigate the existence of magnetic topological insulators with antiferromagnetic (AF) order in heavy-fermion materials. Our analysis relies on the half-filled Kane-Mele-Kondo (KMK) model with the AF Kondo interaction $J_{\rm K}$ coupling the spin of itinerant electrons with a $S=1/2$ localized spin at each lattice site. We consider the Néel AF ordering with the local magnetization not only perpendicular ($z$-AF ordering) but also parallel ($xy$-AF ordering) to the honeycomb plane. We show that in the absence of an energy offset between the two sublattices of the honeycomb structure the system is always topologically trivial. There is a transition from the trivial $xy$-AF insulator ($xy$-AFI) to the trivial Kondo insulator (KI) upon increasing $J_{\rm K}$. We unveil that an alternating sublattice potential can lead to the stabilization of the $z$-AFCI and the $z$-AF quantum spin Hall insulator ($z$-AFQSHI). We address the charge excitations in the bulk as well as at the edges of the KMK model. We provide a systematic comparison between the size of the charge gap in the AFCI in heavy-fermion materials and the size of the charge gap in the AFCI in transition-metal compounds. Our findings can guide the future experimental studies searching for AF topological insulators in novel class of systems which can survive up to higher temperatures.

cond-mat.str-el↗

arXiv2022

Simplified approach to the magnetic blue shift of Mott gaps

The antiferromagnetic ordering in Mott insulators upon lowering the temperature is accompanied by a transfer of the single-particle spectral weight to lower energies and a shift of the Mott gap to higher energies (magnetic blue shift, MBS). The MBS is governed by the double exchange and the exchange mechanisms. Both mechanisms enhance the MBS upon increasing the number of orbitals. By performing a polynomial fit to numerical dynamical mean-field theory data we provide an expansion for the MBS in terms of hopping and exchange coupling of a prototype Hubbard-Kondo-Heisenberg model and discuss how the results can be generalized for application to realistic Mott or charge-transfer insulator materials. This allows estimating the MBS of the charge gap in real materials in an extremely simple way avoiding extensive theoretical calculations. The approach is exemplarily applied to $α$-MnTe, NiO, and BiFeO$_3$ and an MBS of about $130$ meV, $360$ meV, and $157$ meV is found, respectively. The values are compared with the previous theoretical calculations and the available experimental data. Our ready-to-use formula for the MBS simplifies the future studies searching for materials with a strong coupling between the antiferromagnetic ordering and the charge excitations, which is paramount to realize a coupled spin-charge coherent dynamics at a femtosecond time scale.

cond-mat.str-el↗

arXiv2022

Antiferromagnetic Chern insulator in centrosymmetric systems

An antiferromagnetic Chern insulator (AFCI) can exist if the effect of the time-reversal transformation on the electronic state cannot be compensated by a space group operation. The AFCI state with collinear magnetic order is already realized in noncentrosymmetric honeycomb structures through the Kane-Mele-Hubbard model. In this paper, we demonstrate the existence of the collinear AFCI in a square lattice model which preserves the inversion symmetry. Our study relies on the time-reversal-invariant Harper-Hofstadter-Hubbard model extended by a next-nearest-neighbor hopping term including spin-orbit coupling and a checkerboard potential. We show that an easy $z$-axis AFCI appears between the band insulator at weak and the easy $xy$-plane AF Mott insulator at strong Hubbard repulsion provided the checkerboard potential is large enough. The close similarity between our results and the results obtained for the noncentrosymmetric Kane-Mele-Hubbard model suggests the AFCI as a generic consequence of spin-orbit coupling and strong electronic correlation which exists beyond a specific model or lattice structure. An AFCI with the electronic and the magnetic properties originating from the same strongly interacting electrons is promising candidate for a strong magnetic blue shift of the charge gap below the Néel temperature and for realizing the quantum anomalous Hall effect at higher temperatures so that applications for data processing become possible.

cond-mat.str-el↗

arXiv2021

Double-Exchange Enhanced Magnetic Blue-Shift of Mott Gaps

A substantial energy gap of charge excitations induced by strong correlations is the characteristic feature of Mott insulators. We study how the Mott gap is affected by long-range antiferromagnetic order. Our key finding is that the Mott gap is increased by the magnetic ordering: a magnetic blue-shift (MBS) occurs. Thus, the effect is proportional to the exchange coupling in the leading order in the Hubbard model. In systems with additional localized spins the double-exchange mechanism induces an additional contribution to the MBS which is proportional to the hopping in the leading order. The coupling between spin and charge degrees of freedom bears the potential to enable spin-to-charge conversion in Mott systems on extreme time scales determined by hopping and exchange only, since a spin-orbit mediated transfer of angular momentum is not involved in the process. In view of spintronic and magnonic applications, it is highly promising to observe that several entire classes of compounds show exchange and double-exchange effects. Exemplarily, we show that the magnetic contribution to the band-gap blue-shift observed in the optical conductivity of $α$-MnTe is correctly interpreted as the MBS of a Mott gap.

cond-mat.str-el↗

arXiv2021

Lattice symmetry and emergence of antiferromagnetic quantum Hall states

Strong local interaction in systems with non-trivial topological bands can stabilize quantum states such as magnetic topological insulators. We investigate the influence of the lattice symmetry on the possible emergence of antiferromagnetic quantum Hall states. We consider the spinful Harper-Hofstadter model extended by a next-nearest-neighbor (NNN) hopping which opens a gap at half-filling and allows for the realization of a quantum Hall insulator. The quantum Hall insulator has the Chern number $\mathcal{C}=2$ as both spin components are in the same quantum Hall state. We add to the system a staggered potential $Δ$ along the $\hat{x}$-direction favoring a normal insulator and the Hubbard interaction $U$ favoring a Mott insulator. The Mott insulator is a Néel antiferromagnet for small and a stripe antiferromagnet for large NNN hopping. We investigate the $U$-$Δ$ phase diagram of the model for both small and large NNN hoppings. We show that while for large NNN hopping there exists a $\mathcal{C}=1$ stripe antiferromagnetic quantum Hall insulator in the phase diagram, there is no equivalent $\mathcal{C}=1$ Néel antiferromagnetic quantum Hall insulator at the small NNN hopping. We discuss that a $\mathcal{C}=1$ antiferromagnetic quantum Hall insulator can emerge only if the effect of the spin-flip transformation cannot be compensated by a space group operation. Our findings can be used as a guideline in future investigations searching for antiferromagnetic quantum Hall states.

cond-mat.str-el↗

arXiv2020

Spin-imbalance-induced transverse magnetization in the Hofstadter-Hubbard model

The fermionic, time-reversal invariant Hofstadter-Hubbard model with a population difference between the two spin states is investigated. In the strongly interacting regime, where the system can be described by an effective spin model, we find an exotic spin structure by means of classical Monte-Carlo calculations. Remarkably, this spin structure exhibits a transverse net magnetization perpendicular to the magnetization induced by the population imbalance. It is thus inherently different from canted antiferromagnetism. We further investigate effects of quantum fluctuations within the dynamical mean-field approximation and obtain a rich phase diagram including ferromagnetic, anti-ferromagnetic, ferrimagnetic, and transverse magnetization phases.

cond-mat.quant-gas↗

arXiv2020

Interaction-driven topological phase transitions in fermionic SU($3$) systems

We consider SU($3$) fermions on the triangular lattice in the presence of a gauge potential which stabilizes a quantum Hall insulator (QHI) at the density of one particle per lattice site. We investigate the effect of the Hubbard interaction, favoring magnetic long-range order, and a three-sublattice potential (TSP), favoring a normal insulator (NI), on the system. For weak TSP we find that the Hubbard interaction drives the QHI into a three-sublattice magnetic Mott insulator (MMI). For intermediate values of TSP we identify two transition points upon increasing the Hubbard interaction. The first transition is from the NI to the QHI and the second transition is from the QHI to the MMI. For large values of the TSP a charge-ordered magnetic insulator (COMI) emerges between the NI and the QHI, leading to an interaction-driven COMI-to-QHI transition.

cond-mat.str-el↗

arXiv2019

Competing Charge and Magnetic Order in Fermionic Multi-Component Systems

We consider the fermionic SU($3$) Hubbard model on the triangular lattice in the presence of a three-sublattice staggered potential which provides the possibility to investigate the competition of charge and magnetic order in three-component systems. We show that depending on the strength of the staggered potential $Δ$, the Hubbard interaction $U$ destabilizes the band insulator (BI) at small $U$ into the Mott insulator (MI) at large $U$ in three different ways with different intermediate phases. This leads to a rich phase diagram in the $U$-$Δ$ plane. Our results indicate that multi-component systems show not only exotic states in the Mott regime as has been considered previously, but also interesting competition between charge and magnetic orders.

cond-mat.str-el↗

arXiv2019

Artificial $SU(3)$ Spin-Orbit Coupling and Exotic Mott Insulators

Motivated by recent progress in the realization of artificial gauge fields and $SU(N)$ Mott insulators using alkaline-earth-like atoms in optical lattices, we develop an unbiased $SU(N)$ real-space dynamical mean-field theory (DMFT) approach to study the effect of spin-orbit coupling and onsite Hubbard interaction $U$ on $SU(3)$ fermionic systems. We investigate the behavior of the local magnetization, double occupancies, and the triple occupancy versus the Hubbard interaction across the metal to Mott insulator transition. We map out the magnetic phase diagram in the large-$U$ limit and show that the spin-orbit coupling can stabilize long-range orders such as ferromagnet, spiral, and stripes with different orientations in $SU(3)$ Mott insulators.

cond-mat.str-el↗

arXiv2018

Density-wave steady-state phase of dissipative ultracold fermions with nearest-neighbor interactions

In this work we investigate the effect of local dissipation on the presence of density-wave ordering in spinful fermions with both local and nearest-neighbor interactions as described by the extended Hubbard model. We find density-wave order to be robust against decoherence effects up to a critical point where the system becomes homogeneous with no spatial ordering. Our results will be relevant for future cold-atom experiments using fermions with non-local interactions arising from the dressing by highly-excited Rydberg states, which have finite lifetimes due to spontaneous emission processes.

cond-mat.quant-gas↗

arXiv2017

Singlet exciton condensation and bond-order-wave phase in the extended Hubbard model

The competition of interactions implies the compensation of standard mechanisms which leads to the emergence of exotic phases between conventional phases. The extended Hubbard model (EHM) is a fundamental example for the competition of the local Hubbard interaction and the nearest-neighbor density-density interaction, which at half-filling and in one dimension leads to a bond order wave (BOW) between a charge density wave (CDW) and a quasi-long-range order Mott insulator (MI). We study the full momentum-resolved excitation spectrum of the one dimensional EHM in the CDW phase and clarify the relation between different elementary energy gaps. We show that the CDW-to-BOW transition is driven by the softening of a singlet exciton at momentum $π$. The BOW is realized as the condensate of this singlet exciton.

cond-mat.str-el↗

arXiv2017

Massive spinons in $S=1/2$ spin chains: spinon-pair operator representation

Spinons are among the generic excitations in one-dimensional spin systems, they can be massless or massive. The quantitative description of massive spinons poses a considerable challenge in spite of various variational approaches. We show that a representation in terms of hopping and Bogoliubov spinon processes, which we call "spinon-pair" operators, and their combination is possible. We refer to such a representation as second quantized form. Neglecting terms which change the number of spinons yields the variational results. Treating the bilinear and quartic terms by continuous unitary transformations leads to considerably improved results. Thus, we provide the proof-of-principle that systems displaying massive spinons as elementary excitations can be treated in second quantization based on spinon-pair representation.

cond-mat.str-el↗

arXiv2016

Orientational bond and Néel order in the two-dimensional ionic Hubbard model

Unconventional phases often occur where two competing mechanisms compensate. An excellent example is the ionic Hubbard model where the alternating local potential $δ$, favoring a band insulator (BI), competes with the local repulsion $U$, favoring a Mott insulator (MI). By continuous unitary transformations we derive effective models in which we study the softening of various excitons. The softening signals the instability towards new phases that we describe on the mean-field level. On increasing $U$ from the BI in two dimensions, we find a bond-ordered phase breaking orientational symmetry due to a d-wave component. Then, antiferromagnetic order appears coexisting with the d-wave bond order. Finally, the d-wave order vanishes and a Néel-type MI persists.

cond-mat.str-el↗