arXiv subjects

S. A. Jafari

Publications and source records attributed to S. A. Jafari.

At least 19 recordsLinked to original sources

arXiv2025

Systematic cRPA study of two-dimensional MA$_2$Z$_4$ materials: From unconventional screening to correlation-driven instabilities

Understanding the interplay between screening, electronic correlations, and collective excitations is essential for the design of two-dimensional quantum materials. Here, we present a comprehensive first-principles study of more than 60 MA$_2$Z$_4$ monolayers, encompassing semiconducting, metallic, cold-metallic, magnetic, and topological phases. Using the constrained random phase approximation (cRPA), we compute material-specific effective Coulomb interaction parameters $U$, $U'$, and $J$, including their spatial dependence across distinct correlated subspaces defined by local coordination and crystal symmetry. In semiconducting compounds, long-range nonlocal interactions persist, revealing unconventional screening and suggesting strong excitonic effects beyond simple dielectric models. In cold-metallic systems, sizable long-range Coulomb interactions remain despite the presence of free carriers, highlighting their atypical metallic screening. Among 33-valence-electron compounds, we find $U_{\mathrm{eff}} > W$ in the $β_2$ phase, indicating proximity to charge-density-wave or Mott instabilities. Several V- and Nb-based systems exhibit intermediate-to-strong correlation strength, with $U/W > 1 $ in multiple cases. Using cRPA-derived Stoner parameters, we identify magnetic instabilities in various V-, Nb-, Cr-, and Mn-based compounds. Finally, selected cold-metallic systems display plasmon dispersions that deviate from the conventional $\sqrt{q}$ behavior, revealing nearly non-dispersive low-energy modes. These results position MA$_2$Z$_4$ monolayers as a versatile platform for investigating correlation-driven instabilities and emergent collective behavior in two dimensions.

cond-mat.str-el↗

arXiv2024

Moving frame theory of zero-bias photocurrent on the surface of topological insulators

Motivated by observations of zero-biased photocurrent on the surface of topological insulators, we show that the in-plane effective magnetic field $\tilde B$ implements a moving frame transformation on the topological insulators' helical surface states. As a result, photo-excited electrons on the surface undergo a Galilean boost proportional to the effective in-plane magnetic field $\tilde B$. The boost velocity is transversely proportional to $\tilde B$. This explains why the experimentally observed photocurrent depends linearly on $\tilde B$. Our theory while consistent with the observation that at leading order the effect does not depend on the polarization of the incident radiation, at next leading order in $\tilde B$ predicts a polarization dependence in both parallel and transverse directions to the polarization. We also predict two induced Fermi surface effects that can serve as further confirmation of our moving frame theory. Based on the estimated value $ζ\approx 0.34$ of the tilt parameter for a magnetic fields of $\tilde B\sim 3$T, our geometric picture qualifies the surface Dirac cone of magnetic topological insulators as an accessible platform for the synthesis and experimental investigation of strong synthetic gravitational phenomena.

cond-mat.mes-hall↗

arXiv2024

Tilted Dirac Cones in Two-Dimensional Materials: Impact on Electron Transmission and Pseudospin Dynamics

This study is devoted to the profound implications of tilted Dirac cones on the quantum transport properties of two-dimensional (2D) Dirac materials. These materials, characterized by their linear conic energy dispersions in the vicinity of Dirac points, exhibit unique electronic behaviors, including the emulation of massless Dirac fermions and the manifestation of relativistic phenomena such as Klein tunneling. Expanding beyond the well-studied case of graphene, the manuscript focuses on materials with tilted Dirac cones, where the anisotropic and tilted nature of the cones introduces additional complexity and richness to their electronic properties. The investigation begins by considering a heterojunction of 2D Dirac materials, where electrons undergo quantum tunneling between regions with upright and tilted Dirac cones. The role of tilt in characterizing the transmission of electrons across these interfaces is thoroughly examined, shedding light on the influence of the tilt parameter on the transmission probability and the fate of the pseudospin of the Dirac electrons, particularly upon a sudden change in the tilting. We also investigate the probability of reflection and transmission from an intermediate slab with arbitrary subcritical tilt, focusing on the behavior of electron transmission across regions with varying Dirac cone tilts. The study demonstrates that for certain thicknesses of the middle slab, the transmission probability is equal to unity, and both reflection and transmission exhibit periodic behavior with respect to the slab thickness.

cond-mat.mes-hall↗

arXiv2023

Synthetic complex Weyl superconductors, chiral Josephson effect and synthetic half-vortices

We show that the most generic form of spin-singlet superconducting order parameter for chiral fermions is of the $Δ_s+iγ^5Δ_5$ where $Δ_s$ is the usual order parameter and $Δ_5$ is the pseudo-scalar order parameter. After factoring out the $U(1)$ phase $e^{iϕ}$, this form of superconductivity admits yet additional complex structure in the plane of $(Δ_s,Δ_5)$. The polar angle $χ$ in this plane dubbed chiral angle will be locked to the $U(1)$ phase $ϕ$. We propose a synthetic setup based on stacking of topological insulators (TIs) and superconductors (SCs). Alternatively flux biasing the superconductors with a fluxes $\pmΦ$ leads to $Δ_5=Δ_0 \sin(χ)$, where $Δ_0$ is the superconducting order parameter of the SC layers, and the chiral angle $χ=Φ/Φ_0$ is directly given by the flux $Φ$ in units of the flux quantum $Φ_0=h/(2e)$. This can be used as a building block to construct a two-dimensional Josephson array. In this setup $χ$ will be a background field defining a pseudoscalar $Δ_5$ that can be tuned to desired configuration. While in a uniform background field $Δ_5$ the dynamics of $ϕ$ is given by standard XY model and its associated vortices, a {\em staggered} background $\pmΔ_5$ (or equivalently $χ$ and $χ+π$ in alternating lattice sites) creates a new set of minima for the $ϕ$ field that will support half-vortex excitations. An isolated single synthetic "half-vortex" in the $χ$ field in an otherwise uniform background will bind a $ϕ$-half-vortex. This is similar to the way a p-wave superconducting vortex core binds a Majorana fermion.

cond-mat.supr-con↗

arXiv2023

Kinetic theory of {\it tilted} Dirac cone materials

We formulate the Boltzmann kinetic equations for interacting tilted Dirac fermions in two space dimensions characterized by a tilt parameter $0\leζ<1$. Solving the linearized Boltzmann equation, we find that the broadening of the Drude pole is enhanced by $κ(ζ)\times(1-ζ^2)^{-1/2}$, where the $κ$ is interaction-induced enhancement factor. The intensity of the Drude pole is also anisotropically enhanced by $(1-ζ^2)^{-1}$. The ubiquitous "redshift" factors $(1-ζ^2)^{1/2}$ can be regarded as a manifestation of an underlying spacetime structure in such solids. The additional broadening $κ$ indicates that interaction effects are more pronounced for electrons in a $ζ$-deformed Minkowski spacetime of tilted Dirac fermions.

cond-mat.str-el↗

arXiv2023

Tilt induced vortical response and mixed anomaly in inhomogeneous Weyl matter

We propose a non-dissipative transport effect and vortical response in Weyl semimetals in the presence of spatial inhomogeneities, namely a spatially varying tilt of the Weyl cones. We show that when the spectrum is anisotropic and tilted due to spatial lattice variations, one is confronted with generalized quantum anomalies due to the effective fields stemming from the tilt structure. In particular, we demonstrate that the position-dependent tilt parameter induces local vorticity, thus generating a chiral vortical effect even in the absence of rotation or magnetic fields. As a consequence, it couples to the electric field and thus contributes to the anomalous Hall effect.

cond-mat.mes-hall↗

arXiv2022

Holographic Hydrodynamics of {\it Tilted} Dirac Materials

We present a gravity dual to a quantum material with tilted Dirac cone in 2+1 dimensional spacetime. In this many-body system the electronics degrees of freedom are strongly-coupled, constitute a Dirac fluid and admit an effective hydrodynamic description. The holographic techniques are applied to compute the thermodynamic variables and hydrodynamic transports of a fluid on the boundary of an asymptotically anti de Sitter spacetime with a boosted black hole in the bulk. We find that these materials exhibit deviations from the normal Dirac fluid which rely on the tilt of the Dirac cone. In particular, the shear viscosity to entropy density ratio is reduced and the KSS bound is violated in this system. This prediction can be experimentally verified in two-dimensional quantum materials ({\it e.g.} organic $α$-({BEDT}-{TTF})$_2$I$_3$ and $8Pmmn$ borophene) with tilted Dirac cone.

hep-th↗

arXiv2022

Tunning the tilt of a Dirac cone by atomic manipulations: application to 8Pmmn borophene

We decipher the microscopic mechanism of the formation of tilt in the two-dimensional Dirac cone of $8Pmmn$ borphene. In our ab-initio calculations, we identify relevant low-energy degrees of freedom on the $8Pmmn$ lattice and find that these atomic orbitals reside on an effective honeycomb lattice (inner sites), while the high-energy degrees of freedom reside on the rest of the $8Pmmn$ lattice (ridge sites). Integrating out the high-energy atomic orbitals, gives rise to remarkably large effective further neighbor hoppings on the coarse grained "honeycomb graph" of inner sites that determine the location and tilt of the Dirac cone. Molecular orbitals -- that can be modified by atomic manipulations -- are responsible for the creation of further neighbor edges on the honeycomb graph that controls the tilt of the resulting Dirac cone. This leads to an effective tight-binding model on a parent honeycomb graph that facilitates numerical modeling of various effects such as disorder/interactions/symmetry-breaking for tilted Dirac cone fermions. Since the tilt is a proxy to spacetime metric, our result offers a robust perspective on the fabrication of desired emergent spacetime structure and synthesis of geometric forces at ambient conditions that are likely to enhance our control on the movement of electrons in electric/optical devices.

cond-mat.mtrl-sci↗

arXiv2022

Electron Currents from Gradual Heating in Tilted Dirac Cone Materials

Materials hosting tilted Dirac/Weyl fermions provide an emergent spacetime structure for the solid state physics. They admit a geometric description in terms of an effective spacetime metric. Using this metric that is rooted in the long-distance behavior of the underlying lattice, we formulate the hydrodynamic theory for tilted Dirac/Weyl materials in $2+1$ spacetime dimensions. We find that the mingling of space and time through the off-diagonal components of the metric gives rise to: (i) heat and electric currents in response to the $temporal$ gradient of temperature, $\partial_t T$ and (ii) a non-zero symmetric Hall-like conductance $σ^{ij}\propto ζ^iζ^j$ where $ζ^j$ parameterize the tilt in $j$'th space direction. The finding (i) above that can be demonstrated in the laboratory in state of the art cooling/heating rate settings, implies that the non-trivial emergent spacetime geometry in these materials empowers them with a fascinating capability to harvest the naturally available sources of $\partial_t T$ of hot deserts to produce electric energy. We further find a tilt-induced contribution to the conductivity which is an offspring of Drude pole and can be experimentally disentangled from the Drude pole itself.

cond-mat.mes-hall↗

arXiv2022

Undamped inter-valley paramagnons in doped graphene

We predict the existence of an undamped collective spin excitation in doped graphene in the paramagnetic regime, referred to as paramagnons. Since the electrons and the holes involved in this collective mode reside in different valleys of the band structure, the momentum of these inter-valley paramagnons is given by the separation of the valleys in momentum space. The energy of the inter-valley paramagnons lies in the void region below the continuum of inter-band single-particle electron-hole excitations that appears when graphene is doped. The paramagnons are undamped due to the lack of electron-hole excitations in this void region. Their energy strongly depends on doping concentration, which can help to identify them in future experiments.

cond-mat.mes-hall↗

arXiv2021

Circuit realization of tilted Dirac cone: A platform for fabrication of curved spacetime geometry on a chip

We present a LC circuit model that supports tilted {\it Dirac cone} in its spectrum. The tilt of the Dirac cone is specified by the parameters of the model consisting of mutual inductance between the neighboring sites and a capacitance C0 at every lattice site. These parameters can be completely measured by impedance spectroscopy. Given that a tilted Dirac cone can be described by a background spacetime metric, the impedance spectroscopy can perfectly provide (local) information about the metric of the spacetime. Non-uniform spatial dependence of the mutual inductance or capacitance induces non-trivial geometrical structure on the emergent spacetime.

cond-mat.str-el↗

arXiv2021

Strength of effective Coulomb interaction in two-dimensional transition-metal Halides MX$_2$ and MX$_3$ (M=Ti, V, Cr, Mn, Fe, Co, Ni; X=Cl, Br, I)

We calculate the strength of the effective onsite Coulomb interaction (Hubbard $U$) in two-dimensional (2D) transition-metal (TM) dihalides MX$_2$ and trihalides MX$_3$ (M=Ti, V, Cr, Mn, Fe, Co, Ni; X=Cl, Br, I) from first principles using the constrained random-phase approximation. The correlated subspaces are formed from $t_{2g}$ or $e_g$ bands at the Fermi energy. Elimination of the efficient screening taking place in these narrow bands gives rise to sizable interaction parameters U between the localized $t_{2g}$ ($e_g$) electrons. Due to this large Coulomb interaction, we find $U/W >1$ (with the band width $W$) in most TM halides, making them strongly correlated materials. Among the metallic TM halides in paramagnetic state, the correlation strength $U/W$ reaches a maximum in NiX$_2$ and CrX$_3$ with values much larger than the corresponding values in elementary TMs and other TM compounds. Based on the Stoner model and the calculated $U$ and $J$ values, we discuss the tendency of the electron spins to order ferromagnetically.

cond-mat.str-el↗

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↗

arXiv2020

Fast nuclear spin relaxation rates in tilted cone Weyl semimetals: Redshift factors from Korringa relation

Spin lattice relaxation rate is investigated for 3D tilted cone Weyl semimetals (TCWSMs). The nuclear spin relaxation rate is presented as a function of temperature and tilt parameter. We find that the relaxation rate behaves as $(1-ζ^2)^{-α}$ with $α\approx 9$ where $0\le ζ< 1$ is the tilt parameter. We demonstrate that such a strong enhancement for $ζ\lesssim 1$ that gives rise to very fast relaxation rates, is contributed by the combined effect of a new hyperfine interactions arising from the tilt itself, and the anisotropy of the ellipsoidal Fermi surface. Extracting an effective density of states (DOS) $\tildeρ$ from the Korringa relation, we show that it is related to the DOS $ρ$ of the tilted cone dispersion by the "redshift factor" $\tildeρ=ρ/\sqrt{1-ζ^2}$. We interpret this relation as NMR manifestation of an emergent underlying spacetime structure in TCWSMs.

cond-mat.mes-hall↗

arXiv2020

Generalization of Lieb-Wu wave function inspired by one-dimensional ionic Hubbard model

With the ionic Hubbard model (IHM) in mind, we construct a non-trivial generalization of the Bethe ansatz (BA) wave function which naturally generalizes the Lieb-Wu wave function with an ionic parameter $Δ$, and reduces to Lieb-Wu solution in the limit $Δ\to 0$. The resulting two-particle scattering matrix satisfies the Yang-Baxter equation. To the extent that the unit cells with more than two electrons (Choy-Haldane issue) are avoided on average, our wave function represents an effective soluiton for the one-dimensional IHM. The Choy-Haldane issue restricts the validity of our solution to low-filling and large $U\gtrsim 4$. This regime is attainable in cold atom realizations of the IHM. For this regime, we numerically solve the generalized Bethe equations and compute the ground state energy in the thermodynamic limit.

cond-mat.str-el↗

arXiv2020

Undamped transverse electric mode in undoped two-dimensional tilted Dirac cone materials

Transverse electric (TE) modes can not propagate through the conducting solids. This is because the continuum of particle-hole excitations of conductors contaminates with the TE mode and dampes it out. But in solids hosting tilted Dirac cone (TDC) that admit a description in terms of a modified Minkowski spacetime, the new spacetime structure remedies this issue and therefore a tilted Dirac cone material (TDM) supports the propagation of an undamped TE mode which is sustained by density fluctuations. The resulting TE mode propagates at fermionic velocities which strongly confines the mode to the surface of the two-dimensional (2D) TDM.

cond-mat.mes-hall↗

arXiv2019

Electrically charged Andreev modes in two-dimensional tilted Dirac cone systems

In a graphene-based Josephson junction, the Andreev reflection can become specular which gives rise to propagating Andreev modes. These propagating Andreev modes are essentially charge neutral and therefore they transfer energy but not electric charge. One main result of this work is that when the Dirac theory of graphene is deformed into a tilted Dirac cone, the breaking of charge conjugation symmetry of the Dirac equation renders the resulting Andreev modes electrically charged. We calculate an otherwise zero charge conductance arising solely from the tilt parameters $\vecζ=(ζ_x,ζ_y)$. The distinguishing feature of such a form of charge transport from the charge transport caused by normal electrons is their dependence on the phase difference $ϕ$ of the two superconductors which can be experimentally extracted by employing a flux bias. Another result concerns the enhancement of Josephson current in a regime where instead of propagating Andreev modes, localized Andreev levels are formed. In this regime, we find enhancement by orders of magnitude of the Josephson current when the tilt parameter is brought closer and closer to $ζ=1$ limit. We elucidate that, the enhancement is due to a combination of two effects: (i) enhancement of number of transmission channels by flattening of the band upon tilting to $ζ\approx 1$, and (ii) a non-trivial dependence on the angle $θ$ of the the tilt vector $\vecζ$.

cond-mat.supr-con↗

arXiv2019

Electrodynamics of tilted Dirac/Weyl materials: A unique platform for unusual surface plasmon polaritons

The electrodynamics of Weyl semimetals (WSMs) is an extension of Maxwell's theory where in addition to field strength tensor $F_{μν}$, an axion field enters the theory which is parameterized by a four-vector $b^μ=(b_0,\bf b)$. In the tilted Weyl matter (TWM) an additional set of parameters ${\bfζ}=(ζ_x,ζ_y,ζ_z)$ enter the theory that can be encoded into the metric of the spacetime felt by electrons in TWM. This allows an extension of Maxwell's electrodynamics that describes electric and magnetic fields in TWMs and tilted Dirac material (TDM) when $b^μ=0$. The tilt parameter $\bfζ$ appearing as off-diagonal metric entries mixing time and space components mingles $\bf E$ and $\bf B$ fields whereby modifies the inhomogeneous Maxwell's equations. Surface plasmon polariton (SPP) in these systems describes the propagation of electromagnetic waves at the {\it interface of two different spacetime geometries}. In the case of TDM, we find a characteristic dependence of SPP spectrum on the tilt parameter $ζ$ which can be used map $ζ$ from SPP measurements. In the case of TWM, depending on whether the interface with vacuum supports a Fermi arc or not, and whether the propagation direction is along the Fermi arc or transverse to it, we find many unusual spectral features for SPP modes. Our detailed study of the dependence of SPP spectra on the arrangements of three vectors $(\bf b, q,ζ)$, the first two of which are at our control, can be utilized to map the tilt characteristics and Fermi arc characteristics from SPP measurements.

cond-mat.str-el↗