arXiv subjects

Zahra Faraei

Publications and source records attributed to Zahra Faraei.

12 recordsLinked to original sources

arXiv2026

Twist-Tunable Paramagnetic Superconductivity in $d$-wave Altermagnet/Superconductor Heterostructures

The interplay between twist-angle engineering and unconventional magnetism provides a powerful new route to control quantum phenomena. We theoretically investigate a heterostructure comprising a $d$-wave superconductor proximitized by a two-dimensional $d$-wave altermagnet. We reveal that the momentum-space mismatch between the superconducting gap nodes and the altermagnetic spin-splitting nodes generates a robust, twist-tunable odd-frequency spin-triplet pairing. Consequently, the macroscopic electromagnetic response of the system can be tuned from a conventional diamagnetic Meissner state to an anomalous paramagnetic Meissner effect driven entirely by the interfacial twist angle. For a $d_{x^2-y^2}$ altermagnet, the paramagnetic response is maximized at perfect alignment ($\phi=0$) and completely suppressed at a maximal twist of $\phi=\pi/4$, while a $d_{xy}$ altermagnet exhibits the exact complementary behavior. Our results establish twisted altermagnetic heterostructures as a versatile platform for engineering odd-frequency pairing and macroscopic superconducting phases.

cond-mat.supr-con↗

arXiv2026

DC conductivity of tilted Dirac Fermions across the Lifshitz Transition: short- versus long-range impurities

We theoretically investigate the DC conductivity of two-dimensional tilted Dirac systems subject to short- and long-range impurity scattering. Using the Kubo formalism, we systematically study transport across the subcritical (Type I), critical, and overcritical (Type II) tilt regimes. In the subcritical phase, short-range impurities yield a frequency-independent conductivity that decreases monotonically with tilt. Conversely, long-range Coulomb scattering results in a strongly energy-dependent conductivity governed by a tilt-independent scattering rate. At the Lifshitz transition ($t = 1$), the transport signatures of these impurities diverge fundamentally: the van Hove singularity in the density of states induces a localized conductivity dip for short-range disorder, but a pronounced macroscopic peak for Coulomb impurities. In the overcritical regime, an ultraviolet momentum cutoff is required to regularize the open Fermi surface, leading to distinct behaviors for each impurity type. Notably, the conductivity perpendicular to the tilt direction ($\sigma_{xx}$) exhibits a cutoff-dependent, non-monotonic peak near $t = \sqrt{2}$ for short-range defects, while it decays monotonically with increasing tilt for long-range scattering. For both potentials, the conductivity along the tilt axis ($\sigma_{yy}$) increases without bound, revealing extreme transport anisotropy. For long-range impurities, the energy dependence of the conductivity becomes nearly quadratic and linear for Type I and II, respectively. Furthermore, vertex corrections vanish identically at the Lifshitz transition for both impurity types. Finally, we provide a unified geometric framework for these phenomena, establishing the tilt parameter as a powerful knob for engineering macroscopic transport in Dirac materials.

cond-mat.mes-hall↗

arXiv2024

Interplay of Tilt and Axion Fields in Topological Superconductors: Anisotropy in the Meissner Effect

Topological superconductors host gapless surface states that fundamentally alter their electromagnetic response through the axion field term $\theta \vec{E}\cdot\vec{B}$, arising from the topological magnetoelectric effect. In this work, we investigate the electromagnetic properties of a three-dimensional topological Weyl superconductor by leveraging its theoretical mapping to a four-dimensional topological insulator with s-wave superconducting boundaries. By incorporating the tilt of Weyl cones into this model, we demonstrate that the tilt vector $\vec\zeta$ anisotropically modifies the axion field profile near the surface, leading to a tilt-enhanced Meissner effect and anomalous magnetic penetration depths. We show that the magnetic field component perpendicular to the tilt direction exhibits a non-exponential, hypergeometric decay dictated by the interplay between the axion term and $\vec\zeta$, while the parallel component remains largely tilt-insensitive--a hallmark of axion-mediated anisotropy absent in trivial superconductors. Remarkably, all tilt-dependent electromagnetic responses follow a universal scaling law, revealing a fundamental symmetry in the system's behavior. Furthermore, we predict a tilt-dependent planar Hall current at the surface, directly tied to the topological surface states.

cond-mat.supr-con↗

arXiv2021

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 $\Delta_s+i\gamma^5\Delta_5$ where $\Delta_s$ is the usual order parameter and $\Delta_5$ is the pseudo-scalar order parameter. After factoring out the $U(1)$ phase $e^{i\phi}$, this form of superconductivity admits yet additional complex structure in the plane of $(\Delta_s,\Delta_5)$. The polar angle $\chi$ in this plane dubbed chiral angle will be locked to the $U(1)$ phase $\phi$. We propose a synthetic setup based on stacking of topological insulators (TIs) and superconductors (SCs). Alternatively flux biasing the superconductors with a fluxes $\pm\Phi$ leads to $\Delta_5=\Delta_0 \sin(\chi)$, where $\Delta_0$ is the superconducting order parameter of the SC layers, and the chiral angle $\chi=\Phi/\Phi_0$ is directly given by the flux $\Phi$ in units of the flux quantum $\Phi_0=h/(2e)$. This can be used as a building block to construct a two-dimensional Josephson array. In this setup $\chi$ will be a background field defining a pseudoscalar $\Delta_5$ that can be tuned to desired configuration. While in a uniform background field $\Delta_5$ the dynamics of $\phi$ is given by standard XY model and its associated vortices, a {\em staggered} background $\pm\Delta_5$ (or equivalently $\chi$ and $\chi+\pi$ in alternating lattice sites) creates a new set of minima for the $\phi$ field that will support half-vortex excitations. An isolated single synthetic "half-vortex" in the $\chi$ field in an otherwise uniform background will bind a $\phi$-half-vortex. This is similar to the way a p-wave superconducting vortex core binds a Majorana fermion.

cond-mat.supr-con↗

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 $(1-\zeta^2)^{-\alpha}$ with $\alpha\approx 9$ where $0\le \zeta < 1$ is the tilt parameter. We demonstrate that such a strong enhancement for $\zeta\lesssim 1$ 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) $\tilde\rho$ from the Korringa relation, we show that it is related to the DOS $\rho$ of the tilted cone dispersion by the "redshift factor" $\tilde\rho=\rho/\sqrt{1-\zeta^2}$. We interpret this relation as NMR manifestation of an emergent underlying spacetime structure in TCWSMs.

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 $\vec\zeta=(\zeta_x,\zeta_y)$. 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 $\phi$ 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 $\zeta=1$ 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 $\zeta\approx 1$, and (ii) a non-trivial dependence on the angle $\theta$ of the the tilt vector $\vec\zeta$.

cond-mat.supr-con↗

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

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↗

arXiv2019

Induced superconductivity in Fermi arcs

When the interface of a superconductor (SC) with Weyl semimetal (WSM) supports Fermi arcs, the chirality blockade eliminates the induction of superconductivity into the bulk. This leaves the Fermi arc states as the only low-energy degrees of freedom in the proximity problem. Therefore the SC|WSM system will be a platform to probe transport properties that only involve the Fermi arcs. With a boundary condition that flips the spin at the boundary, we find a $Z_2$ protected Bogoliubov Fermi contour (BFC) around which the Bogoliubov quasi-particles disperse linearly. The resulting BFC and excitations around it leave a distinct $T^2$ temperature dependence in their contribution to specific heat. Furthermore, the topologically protected BFC being a Majorana Fermi surface gives rise to a zero-bias peak the strength of which characteristically depends on the length of Fermi arc and tunneling strength. For the other BC that flips the chirality at the interface, instead of BFCs we have Bogoliubov- Weyl nodes whose location depends on the tunneling strength.

cond-mat.supr-con↗

arXiv2018

NMR diagnosis of pseudo-scalar superconductivity in 3D Dirac materials

Recently observed 4{\pi} periodic Andreev bound states in three dimensional Dirac materials are attributed to convnetional superconducting pairing. Our alternative explanation in terms of a novel form of parity breaking pseudo-scalar superconducting order can be sharply diagnosed by nuclear magnetic resonance (NMR) relaxation rate. The left-right symmetry breaking of the pseudo-scalar superconductivity can be directly probed as an anti-peak structure below TC in sharp contrast to the conventional Hebel-Slichter peak.

cond-mat.supr-con↗

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

Superconducting proximity in three dimensional Dirac materials: odd-frequency, pseudoscalar, pseudovector and tensor-valued superconducting orders

We find that a conventional s-wave superconductor in proximity to three dimensional Dirac material (3DDM), to all orders of perturbation in tunneling, induces a combination of s and p-wave pairing only. We show that the Lorentz invariance of the superconducting pairing prevents the formation of Cooper pairs with higher orbital angular momenta in the 3DDM. This no-go theorem acquires stronger form when the probability of tunneling from the conventional superconductor to positive and negative energy states of 3DDM are equal. In this case all the p-wave contribution except for the lowest order, identically vanish and hence we obtain an exact result for the induced p-wave superconductivity in 3DDM. Fierz decomposing the superconducting matrix we find that temporal component of the vector superconducting order and spatial components of the pseudo-vector order are odd-frequency pairing. We find that the latter is odd with respect to exchange of position and chirality of the electrons in the Cooper pair and is spin-triplet which is necessary for NMR detection of such an exotic pseudo- vector pairing. Moreover, we show that the tensorial order breaks into a polar vector and an axial vector and both of them are conventional pairing except for being spin-triplet. According to our study, for gapless 3DDM the tensorial superconducting order will be the only order which is odd with respect to the chemical potential μ. Therefore we predict that a transverse p-n junction binds Majorana fermions. This effect can be used to control the neutral Majorana fermions with electric fields.

cond-mat.supr-con↗