arXiv subjects

Tohid Farajollahpour

Publications and source records attributed to Tohid Farajollahpour.

10 recordsLinked to original sources

arXiv2026

Local phase-space Berry curvature and Hall transport in textured twisted bilayer graphene

Slow twist-angle and heterostrain textures in twisted bilayer graphene provide a natural route to phase-space Berry geometry. We show that a purely geometric tetrad/shift sector does not generate mixed Berry curvature once the spin connection is treated consistently. By contrast, a projected textured mini-Dirac cone in twisted bilayer graphene acquires genuine mixed phase-space curvature. We analyze a controlled local Hall-bar limit with a one-dimensional texture and mirror $M_x$. In that limit the pseudogauge gradient renormalizes only the longitudinal conductivity along the textured direction, while a dc nonlinear Hall response appears only when an additional valley-odd tilt generates a Berry-curvature dipole. Both geometric responses peak at the same local filling, $μ_{\rm loc}=\sqrt{2}\,m$, providing a gate-tunable fingerprint of their common origin. All local-cone parameters and their texture susceptibilities are extracted from a heterostrained Bistritzer-MacDonald model at $θ=1.3^\circ$. The tilt invoked in the transport estimates corresponds to heterostrain of only $0.03\%-0.2\%$, below values routinely imaged in devices. The resulting theory provides a local analytic framework for textured moiré Dirac materials and cleanly separates geometric, texture-induced, and transport-level ingredients relevant to realistic TBG.

cond-mat.mes-hall↗

arXiv2026

Parity-driven RKKY decoupling and anomalous $1/R$ Dzyaloshinskii-Moriya interaction in $p$-wave magnets

Unconventional $p$-wave magnets, characterized by an odd-parity momentum-dependent spin splitting, offer a fundamentally distinct paradigm for non-collinear spintronics. Here, we theoretically investigate the Ruderman-Kittel-Kasuya-Yosida indirect exchange in a two-dimensional $p$-wave magnet subjected to Rashba spin-orbit coupling. Using an analytical real-space Green's function formalism, we uncover a parity-driven spatial decoupling in the magnetic response. Because of the odd-parity exchange field, the out-of-plane Ising interaction is structurally insulated from the macroscopic $p$-wave modulation, oscillating isotropically at the shifted Fermi wavevector. Conversely, the in-plane Heisenberg components exhibit pronounced, directionally tunable spatial beating. Beyond collinear exchange, the hybridized bands generate a highly tunable, three-component Dzyaloshinskii-Moriya interaction alongside symmetric off-diagonal anisotropies. We reveal that the out-of-plane chiral twisting is driven by the massive, nonrelativistic $p$-wave momentum shift, while the in-plane chiral components are strictly relativistic. Furthermore, the competition between the $p$-wave nodal geometry and the Rashba gap drives an anomalous, dimension-reducing crossover, in which the in-plane chiral components follow a 1D-like $1/R$ spatial decay along the nodal lines over an extended intermediate-distance window before ultimately recovering the conventional 2D $1/R^2$ asymptote. These findings establish $p$-wave magnets as promising platforms for engineering robust, directionally tunable non-collinear spin textures.

cond-mat.mes-hall↗

arXiv2025

Light-induced charge and spin Hall currents in materials with $C_4K$ symmetry

Berry curvature, a momentum space property, can manifest itself in current responses. The well-known anomalous Hall effect in time-reversal-breaking systems arises from a Berry curvature monopole. In time-reversal-invariant materials, a second-order Hall conductivity emerges from a Berry curvature dipole. Recently, it has been shown that a Berry curvature quadrupole induces a third-order ac Hall response in systems that break time reversal ($K$) and a fourfold rotational ($C_{4}$) symmetry, while remaining invariant under the combination of the two ($C_{4}K$). In this letter, we demonstrate that incident light can induce a $\rm dc$ Hall current in such systems, driven by the Berry curvature quadrupole. We consider a combination of a static $\rm dc$ electric field and an ac light-induced electric field. We calculate the current perpendicular to both the static electric field and the fourfold axis. Remarkably, the induced current is generically spin-polarized. A net charge current appears for light that is linearly or elliptically polarized, but not for circular polarization. In contrast, the spin current remains unchanged when the polarization of light is varied. This allows for rich possibilities such as generating a spin current by shining circularly polarized light on an altermagnetic material. We demonstrate this physics using a two-dimensional toy model for altermagnets.

cond-mat.mes-hall↗

arXiv2020

Synthetic non-Abelian gauge fields and gravitomagnetic effects in tilted Dirac cone systems

In planar tilted Dirac cone systems, the tilt parameter can be made space-dependent by either a perpendicular displacement field, or by chemical substitution in certain systems. We show that the symmetric partial derivative of the tilt parameter generates non-Abelian synthetic gauge fields in these systems. The small velocity limit of these gauge forces corresponds to Rashba and Dresselhaus spin-orbit couplings. At the classical level, the same symmetric spatial derivatives of tilt contribute to conservative, Lorentz-type and friction-like forces. The velocity dependent forces are odd with respect to tilt and therefore have opposite signs in the two valleys when the system is inversion symmetric. Furthermore, toggling the chemical potential between the valence and conduction bands reverses the sign of the all these classical forces, which indicates these forces couple to the electric charge of the carriers. As such, these forces are natural extensions of the electric and magnetic forces in the particular geometry of the tilted Dirac cone systems.

cond-mat.mes-hall↗

arXiv2019

Sound of Fermi arcs

Using Green's function of semi-infinite Weyl semimetals we find that the collective charge excitations of Fermi arcs in undoped Weyl semi-metals are linearly dispersing gapless plasmon modes. The gaplessness comes from proper consideration of the deep penetration of surface states at the end of Fermi arcs into the interior of Weyl semimetal. The linear dispersion -- rather than square root dispersion of {\it pure 2D} electron systems with extended Fermi surface -- arises from the strong anisotropy introduced by the Fermi arc itself, due to which the continuum of surface particle-hole excitations in this system will have strong resemblence to the one-dimensional electron systems. This places the Fermi arc electron liquid in between the 1D and 2D electron liquids. Contamination with the particle-hole excitations of the bulk gives rise to the damping of the Fermi arc sounds.

cond-mat.str-el↗

arXiv2019

Optical conductivity of triple point fermions

As a low-energy effective theory on non-symmorphic lattices, we consider a generic triple point fermion Hamiltonian which is parameterized by an angular parameter $λ$. We find strong $λ$ dependence in both Drude and interband optical absorption of these systems. The deviation of the $T^2$ coefficient of the Drude weight from Dirac/Weyl fermions can be used as a quick way to optically distinguish the triple point degeneracies from the Dirac/Weyl degeneracies. At the particular $λ=π/6$ point, we find that the "helicity" reversal optical transition matrix element is identically zero. But deviating from this point, the helicity reversal emerges as an absorption channel.

cond-mat.str-el↗

arXiv2019

The $8Pmmn$ borophene sheet: A solid-state platform for space-time engineering

We construct the most generic Hamiltonian of the $8Pmmn$ structure of borophene sheet in presence of spin-orbit, as well as background electric and magnetic fields. In addition to spin and valley Hall effects, this structure offers a framework to conveniently manipulate the resulting "tilt" of the Dirac equation by applying appropriate electric fields. Therefore, the tilt can be made space-, as well as time-dependent. The border separating the low-field region with under-tilted Dirac fermions from the high-field region with over-tilted Dirac fermions will correspond to a black-hole horizon. In this way, space-time dependent electric fields can be used to design the metric of the resulting space-time felt by electrons and holes satisfying the tilted Dirac equation. Our platform offers a way to generate analogues of gravitational waves by electric fields (instead of mass sources) which can be detected in solid state spectroscopies as waves of enhanced superconducting correlations.

cond-mat.mes-hall↗

arXiv2018

Greens function of semi-infinite Weyl semimetals

We classify all possible boundary conditions (BCs) for a Weyl material into two classes: (i) BC that mixes the spin projection but does not change the chirality attribute, and (ii) BC that mixes the chiralities. All BCs are parameterized with angular variables that can be regarded as mixing angles between spins or chiralities. Using the Greens function method, we show that these two BCs faithfully reproduce the Fermi arcs. The parameters are ultimately fixed by the orientation of Fermi arcs. We build on our classification and show that in the presence of a background magnetic field, only the second type BC gives rise to non-trivial Landau orbitals.

cond-mat.str-el↗

arXiv2017

The role of the strain induced population imbalance in Valley polarization of graphene: Berry curvature perspective

Real magnetic and lattice deformation gauge fields have been investigated in honeycomb lattice of graphene. The coexistence of these two gauges will induce a gap difference between two valley points ($K$ and $K'$) of system. This gap difference allows us to study the possible topological valley Hall current and valley polarization in the graphene sheet. In the absence of magnetic field, the strain alone could not generate a valley polarization when the Fermi energy coincides exactly with the Dirac points. Since in this case there is not any imbalance between the population of the valley points. In other words each of these gauges alone could not induce any topological valley-polarized current in the system at zero Fermi energy. Meanwhile at non-zero Fermi energies population imbalance can be generated as a result of the external strain even at zero magnetic field. In the context of Berry curvature within the linear response regime the valley polarization (both magnetic free polarization, $Π_0$, and field dependent response function, $χ_α$) in different values of gauge fields of lattice deformation has been obtained.

cond-mat.mes-hall↗

arXiv2016

Semiconductor of spinons: from Ising band insulator to orthogonal band insulator

Within the ionic Hubbard model, electron correlations transmute the single-particle gap of a band insulator into a Mott gap in the strong correlation limit. However understanding the nature of possible phases in between these two extreme insulating phases remains an outstanding challenge. We find two strongly correlated insulating phases in between the above extremes: (i) The insulating phase just before the Mott phase can be viewed as gapping a non-Fermi liquid state of spinons through staggered ionic potential. The quasi-particles of underlying spinons are orthogonal to physical electrons and hence they do not couple to photoemission probes, giving rise to "ARPES-dark" state due to which the ARPES gap will be larger than optical and thermal gap. (ii) The correlated insulating phase just after the normal band insulator corresponds to the ordered phase of slave Ising spins (Ising insulator) where charge configuration is controlled by an underlying Ising variable which indirectly couples to external magnetic field and hence gives rise to additional temperature and field dependence in semiconducting properties. In the absence of tunability for the Hubbard $U$, such a temperature and field dependence can be conveniently employed to achieve further control on the transport properties of Ising-based semiconductors. The rare earth monochalcogenide semiconductors where the magneto-resistance is anomalously large can be a candidate system for the ordered phase of Ising variable where pairs of charge bosons are condensed in the background. Combining present results with our previous dynamical mean field theory study, we argue that the present picture holds if the ionic potential is strong enough to survive the downward renormalization of the ionic potential caused by Hubbard $U$.

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