Malek Zareyan

Institute for Advanced Studies in Basic Sciences

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Ali G. Moghaddam

Institute for Advanced Studies in Basic Sciences

Wolfgang Belzig

University of Konstanz

Mir Vahid Hosseini

University of Zanjan

Babak Zare Rameshti

Iran University of Science and Technology

Leyla Majidi

Institute for Research in Fundamental Sciences

Jacob Linder

Norwegian University of Science and Technology

Yuli V. Nazarov

Delft University of Technology

Zahra Shomali

Tarbiat Modares University

A. N. Omelyanchouk

B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine

Asle Sudbø

Norwegian University of Science and Technology

Cooperative Institutions

Institute for Advanced Studies in Basic Sciences

Institute for Research in Fundamental Sciences

University of Zanjan

Norwegian University of Science and Technology

Centre for Advanced Study

Delft University of Technology

B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine

National Academy of Sciences of Ukraine

Norwegian Academy of Science and Letters

V. N. Karazin Kharkiv National University

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Oscillations of Andreev States in Clean Ferromagnetic Films

Physical Review Letters (2001)

Malek Zareyan Wolfgang Belzig Yuli V. Nazarov

We investigate the influence of the exchange field on the Andreev bound states in a ferromagnetic ( F) film backed on one side by a superconductor ( S). Our model accounts for diffusive reflection at the outer surface and possible backscattering at the FS interface. Phase shifting of the Andreev level by the exchange field results in an oscillatory behavior of the density of states of F as a function of the layer thickness. We show that our results agree quantitatively with recent experiments.

Andreev reflection

10.1103/physrevlett.86.308

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Citations (89)

Graphene-Based Electronic Spin Lenses

Physical Review Letters (2010)

Ali G. Moghaddam Malek Zareyan

We theoretically demonstrate the capability of a ferromagnetic-normal (FN) interface in graphene to focus an electron-wave with a certain spin direction. The essential feature is the negative refraction Klein tunneling, which is spin-resolved when the exchange energy of F graphene exceeds its Fermi energy. Exploiting this property, we propose a graphene NFN electronic spin lens through which an unpolarized electronic beam can be collimated with a finite spin-polarization. Our study reveals that magnetic graphene has the potential to be the electronic counterpart of the recently discovered photonic chiral meta-materials that exhibit a negative refractive index for only one direction of the circular polarization of the photon-wave.

Fermi energy

Photon polarization

10.1103/physrevlett.105.146803

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Citations (72)

Anisotropic RKKY interaction in spin-polarized graphene

Physical Review B (2013)

Fariborz Parhizgar Reza Asgari Saeed H. Abedinpour Malek Zareyan

We study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in the presence of spin polarized two-dimensional Dirac fermions. We show that an externally applied spin polarization along the $z$ axis mediates an anisotropic interaction which corresponds to an $XXZ$ model interaction between two magnetic moments. For undoped graphene, while the $x$ part of interaction keeps its constant ferromagnetic sign, its $z$ part oscillates with the distance $R$ of magnetic impurities. A finite doping causes both parts of the interaction to oscillate with $R$. We explore a beating pattern of oscillations of the RKKY interaction along armchair and zigzag lattice directions, which occurs for some certain values of the chemical potential. The two characteristic periods of the beating are determined by inverse of the difference and the sum of the chemical potential and the spin polarization.

RKKY interaction

Zigzag

Lattice (music)

10.1103/physrevb.87.125402

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On the self-consistent theory of Josephson effect in ballistic superconducting microconstrictions

Low Temperature Physics (1999)

Malek Zareyan Yu. A. Kolesnichenko A. N. Omelyanchouk

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 Tc the problem is treated self-consistently, with allowance for the distribution of the order parameter Δ(r) inside the contact. The closed integral equation for Δ in strongly inhomogeneous microcontact geometry (L≲ξ0, where ξ0 is the coherence length at T=0) replaces the differential Ginzburg–Landau equation. The critical current Ic(L) is expressed in terms of the solution of this integral equation. The limiting cases of L≪ξ0 and L≫ξ0 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 ultrashort channels with L≲aD (aD∼υF/ωD, where ωD is the Debye frequency) the corrections for the value of the critical current Ic (L=0) are sensitive to the strong-coupling effects.

Ginzburg–Landau theory

10.1063/1.593831

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Bound states of Dirac fermions in monolayer gapped graphene in the presence of local perturbations

Chinese Physics B (2016)

Mohsen Yarmohammadi Malek Zareyan

In graphene, conductance electrons behave as massless relativistic particles and obey an analogue of the Dirac equation in two dimensions with a chiral nature. For this reason, the bounding of electrons in graphene in the form of geometries of quantum dots is impossible. In gapless graphene, due to its unique electronic band structure, there is a minimal conductivity at Dirac points, that is, in the limit of zero doping. This creates a problem for using such a highly motivated new material in electronic devices. One of the ways to overcome this problem is the creation of a band gap in the graphene band structure, which is made by inversion symmetry breaking (symmetry of sublattices). We investigate the confined states of the massless Dirac fermions in an impured graphene by the short-range perturbations for local chemical potential and local gap. The calculated energy spectrum exhibits quite different features with and without the perturbations. A characteristic equation for bound states (BSs) has been obtained. It is surprisingly found that the relation between the radial functions of sublattices wave functions, i.e., , , and , , can be established by SO(2) group.

10.1088/1674-1056/25/6/068105

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Citations (23)

Spin-flip noise in a multiterminal spin valve

Physical Review B (2004)

Wolfgang Belzig Malek Zareyan

We study shot noise and cross-correlations in a four terminal spin-valve geometry using a Boltzmann-Langevin approach. The Fano factor (shot noise to current ratio) depends on the magnetic configuration of the leads and the spin-flip processes in the normal metal. In a four-terminal geometry, spin-flip processes are particularly prominent in the cross-correlations between terminals with opposite magnetization.

Fano factor

Spin valve

Flip angle

10.1103/physrevb.69.140407

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Citations (40)

Enhanced superconducting proximity effect in clean ferromagnetic domain structures: A quasiclassical Green’s function approach

Physical Review B (2006)

Maniya Maleki Malek Zareyan

We investigate the superconducting proximity effect in a clean magnetic structure consisting of two ferromagnetic layered domains with antiparallel magnetizations in contact with a superconductor. Within the quasiclassical Green's function approach we find that the penetration of the superconducting correlations into the magnetic domains can be enhanced as compared to the corresponding single domain structure. This enhancement depends on an effective exchange field which is determined by the thicknesses and the exchange fields of the two domains. The pair amplitude function oscillates spatially inside each domain with a period inversely proportional to the local exchange field. While the oscillations have a decreasing amplitude with distance inside the domain which is attached to the superconductor, they are enhancing in the other domain and can reach the corresponding normal metal value for a zero effective exchange field. We also find that the corresponding oscillations in the Fermi level proximity density of states as a function of the second domain's thickness has an growing amplitude over a range which depends on the effective exchange field. Our findings can be explained as the result of cancellation of the exchange fields induced phases gained by an electron inside the two domains with antiparallel magnetizations.

10.1103/physrevb.74.144512

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Proximity effect in ferromagnet/superconductor hybrids: From diffusive to ballistic motion

Physical Review B (2009)

Jacob Linder Malek Zareyan Asle Sudbø

We present an analytical study of the proximity effect in ferromagnet/superconductor (F/S) heterostructures, allowing for an arbitrary magnetic exchange energy as well as arbitrary impurity and spin-flip scattering rates within a quasiclassical approach. While previous studies mainly have focused on the clean or dirty limits, our results grant access to the regime of intermediate impurity concentrations, thus allowing us to probe the crossover from the clean to the dirty limit. We find that in the crossover regime, all possible symmetry correlations of the proximity-induced anomalous Green's function are induced in the ferromagnet. We also point out that the local density of states oscillates spatially not only for an F/S bilayer but also for a normal/superconductor (N/S) bilayer in the diffusive limit, a fact which appears to have gone unnoticed in the literature. Within the weak proximity-effect regime, we present compact analytical expressions valid for arbitrary exchange fields and impurity-scattering rates for (i) the local density of states in an F/S bilayer, (ii) the Josephson current in an S/F/S junction, and (iii) the critical temperature in an F/S/F multilayer. For all cases, we study in particular the crossover regime between diffusive and ballistic motion. Our results may be useful for analyzing experimental data in cases when the dirty limit is not fully reached, thus invalidating the use of the Usadel equation.

10.1103/physrevb.79.064514

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Citations (26)

Anisotropic minimal conductivity of graphene bilayers

Physical Review B (2009)

Ali G. Moghaddam Malek Zareyan

The 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 $\ensuremath{\theta}$ of the connected electrodes and finds a dependence on its length $L$ on the characteristic scale $\ensuremath{\ell}=\ensuremath{\pi}/\ensuremath{\Delta}k\ensuremath{\simeq}50\text{ }\text{nm}$ determined by the inverse of $k$-space distance of two Dirac points. The minimum conductivity increases from a universal isotropic value ${\ensuremath{\sigma}}_{\ensuremath{\perp}}^{\text{min}}=(8/\ensuremath{\pi}){e}^{2}/h$ for a short strip $L⪡\ensuremath{\ell}$ to a higher anisotropic value for longer strips, which in the limit of $L⪢\ensuremath{\ell}$ varies from $(7/3){\ensuremath{\sigma}}_{\ensuremath{\perp}}^{\text{min}}$ at $\ensuremath{\theta}=0$ to $3{\ensuremath{\sigma}}_{\ensuremath{\perp}}^{\text{min}}$ over an angle range $\ensuremath{\Delta}\ensuremath{\theta}\ensuremath{\sim}\ensuremath{\ell}/L$.

Brillouin zone

10.1103/physrevb.79.073401

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Citations (17)

Crossed Andreev reflection in superconducting graphene spin-valves: Spin-switch effect

arXiv (Cornell University) (2009)

Jacob Linder Malek Zareyan Asle Sudbø

We consider the non-local quantum transport properties of a graphene superconducting spin-valve. It is shown that one may create a spin-switch effect between perfect elastic co-tunneling (CT) and perfect crossed Andreev-reflection (CAR) for all bias voltages in the low-energy regime by reversing the magnetization direction in one of the ferromagnetic layers. This opportunity arises due the possibility of tuning the local Fermi-level in graphene to values equivalent to a weak, magnetic exchange splitting, thus reducing the Fermi surface for minority spins to a single point and rendering graphene to be half-metallic. Such an effect is not attainable in a conventional metallic spin-valve setup, where the contributions from CT and CAR tend to cancel each other and noise-measurements are necessary to distinguish these processes.

Andreev reflection

Spin valve

Fermi energy

10.48550/arxiv.0906.3738

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