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Malek Zareyan

Publications and source records attributed to Malek Zareyan.

27 records · Page 2Linked to original sources
arXiv2008

Long range Josephson coupling through ferromagnetic graphene

We study the Josephson effect in graphene-based ballistic superconductor-ferromagnet-superconductor (SFS) junctions. We find an oscillatory Josephson coupling $I_c R_N$ of F graphene whose amplitude is nonvanishing for a half-metallic graphene, increases for the exchange fields $h$ above the Fermi energy $E_{F}$ and shows only a slow damping at strong exchange fields $h\gg E_{F}$. We interpret this long range Josephson coupling as the result of the exchange mediated Andreev-Klein process at FS interfaces which enhances the induced antiparallel-spin superconducting correlations in F graphene by increasing $h$ above $E_{F}$. We further demonstrate the existence of regular temperature induced transitions between 0 and $π$ couplings in the plane of $T$ and $h$ where the phase boundaries have distinct shapes at the two regimes of $h$ below and above $E_{F}$.

cond-mat.mes-hall↗
arXiv2008

Andreev-Klein reflection in graphene ferromagnet-superconductor junctions

We show that Andreev reflection in a junction between ferromagnetic (F) and superconducting (S) graphene regions is fundamentally different from the common FS junctions. For a weakly doped F graphene with an exchange field $h$ larger than its Fermi energy $E_{\rm F}$, Andreev reflection of massless Dirac fermions is associated with a Klein tunneling through an exchange field p-n barrier between two spin-split conduction and valence subbands. We find that this Andreev-Klein process results in an enhancement of the subgap conductance of a graphene FS junction by $h$ up to the point at which the conductance at low voltages $eV\ll Δ$ is greater than its value for the corresponding non-ferromagnetic junction. We also demonstrate that the Andreev reflection can be of retro or specular types in both convergent and divergent ways with the reflection direction aligned, respectively, closer to and farther from the normal to the junction as compared to the incidence direction.

cond-mat.mes-hall↗
arXiv2008

Anisotropic minimal conductivity of graphene bilayers

Fermi line of bilayer graphene at zero energy is transformed into four separated points positioned trigonally at the corner of the hexagonal first Brillouin zone. We show that as a result of this trigonal splitting the minimal conductivity of an undoped bilayer graphene strip becomes anisotropic with respect to the orientation $θ$ of the connected electrodes and finds a dependence on its length $L$ on the characteristic scale $\ell=π/Δk\simeq 50 nm$ determined by the inverse of k-space distance of two Dirac points. The minimum conductivity increases from a universal isotropic value $σ^{min}_{\bot}=(8/π)e^2/h$ for a short strip $L\ll \ell$ to a higher anisotropic value for longer strips, which in the limit of $L\gg \ell$ varies from $(7/3)σ^{min}_{\bot}$ at $θ=0$ to $3σ^{min}_{\bot}$ over an angle range $Δθ\sim \ell/L$.

cond-mat.mes-hall↗
arXiv2006

Josephson effect in mesoscopic graphene strips with finite width

We study Josephson effect in a ballistic graphene strip of length $L$ smaller than the superconducting coherence length and arbitrary width $W$. We find that the dependence of the critical supercurrent $I_{c}$ on $W$ is drastically different for different types of the edges. For \textit{smooth} and \textit{armchair} edges at low concentration of the carriers $I_{c}$ decreases monotonically with decreasing $W/L$ and tends to a constant minimum for a narrow strip $W/L\lesssim1$. The minimum supercurrent is zero for smooth edges but has a finite value $eΔ_{0}/\hbar$ for the armchair edges. At higher concentration of the carriers, in addition to this overall monotonic variation, the critical current undergoes a series of peaks with varying $W$. On the other hand in a strip with \textit{zigzag} edges the supercurrent is half-integer quantized to $(n+1/2)4eΔ_{0}/\hbar$, showing a step-wise variation with $W$.

cond-mat.mes-hall↗
arXiv2003

Unconventional superconducting states induced in a ferromagnet by a d-wave superconductor

We develop a quasi-classical theory for the superconducting proximity effect in a ballistic ferromagnetic layer in contact with a d-wave superconductor. In agreement with recent experiments we find that the density of states oscillate around the normal state value with varying the thickness of the ferromagnetic layer. We show that the phase, the amplitude, and the period of these oscillations depend on the orientation of the superconductor. This effect reveals spatial oscillations and anisotropy of the induced superconducting correlations in the ferromagnet.

cond-mat.supr-con↗
arXiv1999

Ballistic Four-Terminal Josephson Junction: Bistable States and Magnetic Flux Transfer

The macroscopic quantum interference effects in ballistic Josephson microstructures are investigated. The studied system consists of four bulk superconductors (terminals) which are weakly coupled through the mesoscopic rectangular normal metal (two dimensional electron gas). We show that nonlocal electrodynamics of ballistic systems leads to specific current-phase relations for the mesoscopic multiterminal Josephson junction. The nonlocal coupling of supercurrents produces the "dragging" effect for phases of the superconducting order parameter in different terminals. The composite Josephson junction, based on this effect, exhibits the two -level system behaviour controlled by the external magnetic flux. The effect of magnetic flux transfer in a system of nonlocally coupled superconducting rings is studied.

cond-mat.supr-con↗
arXiv1999

On the Selfconsistent Theory of Josephson Effect in Ballistic Superconducting Microconstrictions

The microscopic theory of current carrying states in the ballistic superconducting microchannel is presented. The effects of the contact length L on the Josephson current are investigated. For the temperatures T close to the critical temperature T_c the problem is treated selfconsistently, with taking into account the distribution of the order parameter $Δ(r)$ inside the contact. The closed integral equation for $Δ$ in strongly inhomogeneous microcontact geometry ($L\lesssim ξ_{0}, ξ_{0}$ is the coherence length at T=0) replaces the differential Ginzburg-Landau equation. The critical current $I_{c}(L)$ is expressed in terms of solution of this integral equation. The limiting cases of $L\ll ξ_{0}$ and $L\gg ξ_{0}$ are considered. With increasing length L the critical current decreases, although the ballistic Sharvin resistance of the contact remains the same as at L=0. For ultra short channels with $L\lesssim a_{D}$ ($a_{D}\sim v_{F}/ω_{D}, ω_{D}$ is the Debye frequency) the corrections to the value of critical current I_c(L=0) are sensitive to the strong coupling effects.

cond-mat.supr-con↗
arXiv1998

Coherent current states in mesoscopic four-terminal Josephson junction

A theory is offered for the ballistic 4-terminal Josephson junction. The studied system consists of a mesoscopic two-dimensional normal rectangular layer which is attached in each side to the bulk superconducting banks (terminals). The relation between the currents through the different terminals, which is valid for arbitrary temperatures and junction sizes, is obtained. The nonlocal coupling of the supercurrents leads to a new effect, specific for the mesoscopic weak link between two superconducting rings; an applied magnetic flux through one of the rings produces a magnetic flux in the other ring even in the absence of an external flux through the other one. The phase dependent distributions of the local density of Andreev states, of the supercurrents and of the induced order parameter are obtained. The "interference pattern" for the anomalous average inside the two dimensional region can be regulated by the applied magnetic fluxes or the transport currents. For some values of the phase differences between the terminals, the current vortex state and the two dimensional phase slip center are appeared.

cond-mat.mes-hall↗