Journal of Magnetism and Magnetic Materials (03048853)588
We investigate energy spectra of Bi honeycomb quantum dots (BiQDs) considering buckled hexagonal and triangular structures with zigzag and armchair edges. We apply the four-orbital tight-binding approach. By using of the probability density of states, we distinguish some edge states in the regime of the band gap of Bi monolayer. We see that the number of edge states of zigzag (armchair) triangular BiQDs is more than the number of edge states of zigzag (armchair) hexagonal BiQDs. Also, we investigated energy spectra of hexagonal and triangular BiQDs with zigzag and armchair edges in the presence of the perpendicular magnetic fields. In the low and strong magnetic fields, there are relativistic Landau levels in the electronic spectra of BiQDs (in the hexagonal and triangular boundary conditions). Moreover, we show that edge atoms have more important roles in the formation of the line connectors between i and |i+1| Landau levels of hexagonal BiQDs than ones that are in the triangular BiQDs. In the middle numbers of the magnetic fields, more states collapse into Landau levels in which bulk atoms have a significant role to constitute them in both hexagonal and triangular BiQDs. Furthermore as the same as Bi nanoribbon spectrum, there are some helical edge modes in the band gap regions of Bi monolayer in which edge atoms are responsible to form them. These helical edge modes have topological properties. © 2023 Elsevier B.V.
Physica B: Condensed Matter (09214526)659
Using the multi-orbital (sp3) tight-binding (TB) model, we have obtained the band structure of the Antimony monolayer with a hexagonal array of holes. These structures are named Antimony antidot lattice (AAL). In our tight-binding model, the interaction of the nearest and next-nearest neighbors of atoms has been considered. Our numerical results present that band structures of the system depend on the size of embedded holes in the Antimony monolayer as points of the antidot lattice. Also, we have shown that AALs with a large radius of holes are almost conductors with tiny energy gaps. Besides, we have represented that spin–orbit coupling magnificently affects the band gap for Antimony antidot lattice in the different ranges of hole radius. Moreover, we have investigated the effect of distance between holes of AALs on the band structure and energy gap. Also, the variations of the band gap of Antimony antidot by changing the inserted strain on it have been investigated. Furthermore taking into account the effect of strain, we have compared the band structure and energy gap of antidot lattice (Highlightslayer with hole) and ideal sheet (monolayer without hole). Finally, the effect of substrate on the electronical properties of Antimony monolayer with holes (Antidot) and without holes (ideal sheet) is analyzed. © 2023
Nanotechnology (09574484)34(46)
We have studied a 5-terminal system consisting of three single level quantum dots (QDs) that are in contact with their respective reservoirs. In addition to the intra-dot Coulomb interaction, the electron in the dot affected by an inter-dot Coulomb repulsion from its adjacent QD. We describe this system by an Anderson type model Hamiltonian and apply the Greens function method to study the transport properties of the system. Since we are interested in temperatures higher than the Kondo temperature, we use the equations of motion technique to calculate Green’s functions. Numerical analysis shows that there is a correlation between the transport characteristics of the lower and upper dot and we can change the conductivity of the lower dot only by varying the parameters of the upper dot and vice versa. We demonstrated that the middle dot play the role of the switch on/off of this correlation. Also, we investigated the effect of thermoelectric properties. We found that the inter-dot Coulomb interaction can improve the thermoelectric performance of the system. © 2023 IOP Publishing Ltd.
Physica E: Low-Dimensional Systems and Nanostructures (13869477)139
In this paper, the scattering from a spin impurity for the edge state of the Kane–Mele model has been calculated via the Lippmann–Schwinger approach. To solve this problem analytically, we first approximately calculate the edge states of the Kane–Mele model and use it to obtain Green's function. We then calculate the scattering matrix of an electron from a spin impurity with Heisenberg interaction. Finally, we generalize this method to a double impurity model and use it to calculate the entanglement between two spins of two impurities. © 2022 Elsevier B.V.
Nanotechnology (09574484)32(37)
Armchair phosphorene nanoribbons (APNRs) are known to be semiconductors with an indirect bandgap. Here, we propose to introduce new states in the gap of APNRs by creating a periodic structure of vacancies (antidots). Based on the tight-binding model, we show that a periodic array of vacancies or nanopores leads to the formation of an impurity band inside the gap region. We first present an analytical expression for the dispersion relation of an impurity band induced by hybridization of bound states associated with each single vacancy defect. Then, we increase the size of vacancy defects to include a bunch of atoms and theoretically investigate the effect of nanopores size and their spacing on electronic band structure, carrier transmission function, and thermoelectric properties. Our analysis of the power generation rate and thermoelectric efficiency of these structures reveals that an ANPR can be used as a superb thermoelectric power generation module. © 2021 IOP Publishing Ltd.
Physics Letters, Section A: General, Atomic and Solid State Physics (03759601)384(17)
We have performed a tight-binding calculation of the band structure of Bi/Si sheets on which hexagonal holes are embedded. We named this kind of structure as Bismuth/Silicene antidot lattice (BAL/SAL). We have considered the semi-empirical sp3 tight-binding model on which the nearest and next-nearest neighbors of atoms have been taken into account. Our numerical results show that the size of holes in the two-dimensional sheets as the antidot lattice points are effective on the band structures and also transport properties of the system. Also, we have observed that BALs are almost conductors with very small band gaps for the large radius of circles, but about SALs for the large radius of holes, we obtain a considerable band gap, and conductivity is reduced. Besides, we have seen that, for the different radius of holes in the monolayer of Silicene, the presence of spin-orbit coupling in our band gaps obtained by tight-binding is not very important. On the contrary to Silicene antidot lattice, SOC interaction has a considerable effect on the band gap for Bismuth antidot lattice for the various radiuses of holes in their monolayer. Also, we have investigated the effects of the distance between holes on BAL/SAL band gaps and their band structures. Finally, we have obtained the band gap of Bismuth/Silicene antidot under strain, and we have compared results of sheet and antidot in the presence of strain. © 2020 Elsevier B.V.
Nanotechnology (09574484)31(37)
The thermoelectric power generation efficiency of a bismuth monolayer nanoribbon has been studied theoretically. We calculate the conductance of such a structure using the multi-orbital tight-binding model and also recursive Green's function method, in the presence of a substrate and on-site potential. For the case of the SiC substrate-supported bismuth nanoribbon and by proper selection of on-site potential, a boxcar shape conductance in terms of energy has been obtained. Using the Landauer-Büttiker formalism in the non-linear response regime, we calculate heat and charge currents at low temperatures. By calculation of the electrical output power and power conversion thermoelectric efficiency, we have illustrated that such a structure can operate at high thermoelectric efficiency and also a considerable power generation rate. © 2020 IOP Publishing Ltd.
Materials Research Express (20531591)6(6)
We have theoretically studied ballistic graphene-based Ferromagnetic/Superconductor (FG/SG) junction operated as thermoelectric generators. Ferromagnetism and superconductivity in graphene are assumed to be induced by superconducting and ferromagnetic electrodes placed on the top of the graphene sheet. We calculate the normal and Andreev reflection probabilities using the Blonder-Tinkham-Klapwijk (BTK) formalism to solve a four-dimensional version of the Dirac-Bogoliubov-de Gennes equation. By a special choice of system parameters it is possible to achieve a condition in which thermal conductance declines significantly while electrical conductance increases. We also calculate output power and thermoelectric power-conversion efficiency at low temperature based on such a structure and show that the power generation efficiency is close to the Carnot limit. © 2019 IOP Publishing Ltd.
Journal of Magnetism and Magnetic Materials (03048853)488
The simultaneous realization of full valley and spin polarized current is one of the major challenges in the spintronics and valleytronics graphene devices. In this paper, we propose a superlattice of ferromagnetic gapped graphene with spin-orbit coupling (SOC) for the generation of simultaneous remarkable full valley and spin polarized current without need for strain and magnetic vector potential. It is found that a gate-controllable spin and valley polarized current can be achieved by the proposed superlattice device. More importantly, the spin and valley polarizations are oscillatory functions of the gate voltage and the directions of polarized currents can be electrically controlled by changes in the gate voltage, which is crucial for development of multiple-valued logic graphene-based spintronics and valleytronics devices. Furthermore, in case of one barrier structure, we show that at the Fermi energy approaching the spin-orbit energy gap, there is a gate-tunable strong spin-valley filtering effect which is attractive for spin-valley sensing applications. © 2019 Elsevier B.V.
Materials Research Express (20531591)6(9)
Low temperature thermoelectric cooling properties of superlattice structured nanowires have been investigated theoretically. A boxcar shape transmission function can be obtained by special choice of the barriers number, their widths and spacing, material composition, and bias voltage. Charge and heat currents have been calculated using the Landauer formalism in the nonlinear response regime. Also we have calculated the rate of cooling, electric power consumption and cooling coefficient of performance (COP) of a thermoelectric cooler using such a transmission function. Our results show that the COP of such a structure remains superb also when operating at a remarkable cooling rate. © 2019 IOP Publishing Ltd.
Journal of Semiconductors (16744926)39(8)
We have developed a π-orbital tight-binding Hamiltonian model taking into account the nearest neighbors to study the effect of antidot lattices (two dimensional honeycomb lattice of atoms including holes) on the band structure of silicene and silicon carbide (SiC) sheets. We obtained that the band structure of the silicene antidote superlattice strongly depends on the size of embedded holes, and the band gap of the silicene antidot lattice increases by increasing of holes diameter. The band gap of SiC antidot lattice, except for the lattice of the small unit cell, is independent of the holes diameter and also depends on the distance between holes. We obtained that, the band gap of the SiC antidote lattice is the same as the band gap of the corresponding sheet without hole. Also, the electronic properties of the SiC antidot superlattice occupied either by carbon or by silicon atoms are investigated, numerically. Furthermore, we study the effect of occupation of graphene antidot by Si atoms and vice versa. Also, we have calculated the band structure of graphene and silicene antidot lattice filled by Si + C atoms. Finally, we compute the band structure of the SiC antidote lattice including the holes which are filled by C or by Si atoms. Really, in this paper we have generalized the method of paper[38] about graphene antidot with empty holes to the cases of filled holes by different atoms and also to the case of silicene and silicon carbide antidot lattices. © 2018 Chinese Institute of Electronics.
Physica C: Superconductivity and its Applications (09214534)549pp. 113-115
We theoretically study the tunneling conductance of a normal/d-wave superconductor silicene junction using Blonder–Tinkham–Klapwijk (BTK) formalism. We discuss how the conductance spectra are affected by changing the chemical potential (μN) in the normal silicene region. It is obtained that the amplitude of the spin/valley-dependent Andreev reflection (AR) and charge conductance (G) of the junction can be strongly modulated by the orientation angle of superconductive gap (β) and perpendicular electric field (Ez). We demonstrate that the charge conductance exhibits an oscillatory behavior as a function of β by a period of π/2. Remarkably, variation of μN strongly modifies the amplitude of the oscillations and periodically there are transport gaps in the G−β oscillations for a range of μN. These findings suggest that one may experimentally tune the transport properties of the junction through changing β, Ez and μN. © 2018 Elsevier B.V.
Nouri n., N.,
Bieniek m., ,
Brzezińska m., ,
Modarresi m., ,
Zia borujeni s., ,
Rashedi, G.R.,
Wójs a., ,
Potasz p., Physics Letters, Section A: General, Atomic and Solid State Physics (03759601)382(40)pp. 2952-2958
We investigate topological phases in two-dimensional Bi/Sb honeycomb crystals considering planar and buckled structures, both freestanding and deposited on a substrate. We use the multi-orbital tight-binding model and compare results with density functional theory calculations. We distinguish topological phases by calculating topological invariants, analyzing edge states properties of systems in a ribbon geometry and studying their entanglement spectra. We show that coupling to the substrate induces transition to the Z2 topological insulator phase. It is observed that topological crystalline insulator (TCI) phase, found in planar crystals, exhibits an additional pair of edge states in both energy spectrum and entanglement spectrum. Transport calculations for TCI phase suggest robust quantized conductance even in the presence of crystal symmetry-breaking disorder. © 2018 Elsevier B.V.
Nouri n., N.,
Potasz p., ,
Zia borujeni s., ,
Wójs a., ,
Rashedi, G.R. Acta Physica Polonica A (05874246)132(2)pp. 313-315
Optical properties of transition metal dichalcogenides monolayer of MoS2 are analyzed using multi-orbital tight-binding models with only Mo d-orbitals (three-band model) and with an inclusion of S p-orbitals (six-band model). We look at band structures, momentum matrix elements between valence and conduction band, and joint optical density of states. Good agreement between the two models is shown in a vicinity of K point of the Brillouin zone. On line connecting K and Γ points, a local conduction band minimum at Q point is recovered only by six-band model in agreement with density functional theory and experimental results. We show that optical transitions at this point are active for both light polarizations. A peak in joint optical density of states is also seen at this point suggesting its potentially important role in a proper description of excitonic effects.
Physica E: Low-Dimensional Systems and Nanostructures (13869477)85pp. 324-333
We have developed a tight-binding Hamiltonian model to study the band structure of group-III nitride nanostructures, AlN, GaN and InN in the form of monolayer sheets, nanotubes (NT) and nanoribbons (NR). We have considered the semi-empirical sp3d5s* tight-binding model taking into account the nearest neighbors. Based on our numerical results we obtained that d-orbitals are important for correct calculation of group-III nitride band structures. Also we have observed that the AlN, GaN and InN graphene-like sheets are semiconductors with indirect band gaps of 5.28 eV, 2.62 eV and 1.66 eV, respectively. We have seen that, our tight binding band gaps are larger than the corresponding band gaps obtained by DFT calculation. Also, we have investigated the effects of chirality of nanotubes and nanoribbons on the nitrides band structures and their gaps. Then, we have studied the influence of nanotube diameter and width of nanoribbon on the band structure. We observed that nanotubes and nanoribbons as the corresponding unzipped nanotubes have the same band structure. Moreover, we understand that the electronic properties are almost insensitive to diameter for nanotubes but width of nanoribbons is effective on the band structure. © 2016 Elsevier B.V.
Journal of Applied Physics (10897550)122(4)
We study the tunneling conductance of a silicene-based ferromagnet/insulator/superconductor (FIS) junction by the use of the spin-dependent Dirac-Bogoliubov de-Gennes equation. We demonstrate that the conductance spectra are strongly affected by exchange energy h, Fermi energy EF, and external perpendicular electric field Ez. In the thin barrier limit of insulator silicene IS, the zero-bias charge conductance of the FIS silicene junction oscillates as a function of barrier strength χG. It is shown that the period of oscillations changes from π/2 to π corresponding to undoped and doped silicene. Remarkably, in contrast to that of the graphene FIS junction where the conductance only vanishes at the exchange energy h=EF, here due to the buckled structure of silicene, there is a transport gap region for the range of h values and the magnitude of such a gap region can be controlled by Ez. Moreover, it is found that by appropriate choice of h and Ez, it is possible to achieve a fully spin and valley-polarized charge conductance through the FIS silicene junction. This property suggests experimentally measuring the Fermi energy of silicene. © 2017 Author(s).
Superlattices and Microstructures (10963677)102pp. 202-211
We theoretically study the tunneling conductance of a normal/d-wave superconductor silicene junction using Blonder-Tinkham-Klapwijk (BTK) formalism. We discuss in detail how the conductances spectra are affected by inducing d-wave superconducting pairing symmetry in the buckled silicene. It is obtained that the amplitude of the spin/valley-dependent Andreev reflection and subgap conductance of the junction can be strongly modulated by the orientation angle of superconductive gap (β) and perpendicular electric field (EZ), suggesting that one may experimentally tune the transport properties of the junction through changing β and EZ. We demonstrate that the subgap conductance exhibits an oscillatory behavior as a function of the orientation angle of superconductive gap (β) with a period of π/2 and by increasing the insulating gap of silicene, the charge conductance oscillations suppress. Remarkably, due to the buckled structure of silicene at the maximum orientation angle of the d-wave superconducting β=π/4, we found a very distinct behavior from the graphene-based NS junction where the charge conductance is insensitive to the bias energy. In addition, the Andreev reflection and subgap conductance can be switched on and off by applying electric field. © 2016 Elsevier Ltd
Rashidian z., ,
Lorestaniweiss z., ,
Hajati, Y.,
Rezaeipour s., ,
Rashedi, G.R. Journal of Magnetism and Magnetic Materials (03048853)442pp. 15-24
In this paper, based on the transfer-matrix method, we investigate the transport properties of Dirac fermions through a superlattice of ferromagnetic/normal/ferromagnetic (FNF) buckled silicene junction where an electrostatic gate potential U is attached to the normal region. It is found that owing to buckled structure of silicene, the transmission probabilities and consequently valley-resolved conductance of the junction can be turned on or off by adjusting electric field strength, the number of barriers, and electrostatic gate voltage. Remarkably, the fully valley polarized current can be achieved by increasing the number of barriers in the proposed device. The effect of the number of barriers on the total charge conductance Gc of such a junction versus barrier length has also been investigated and it is shown that by increasing the number of barriers the amplitudes of Gc oscillations decrease. It is also found that Fano factor strongly modulated by applying electric field, number of barriers, and gate voltage. In particular, in presence of an electrostatic gate potential, Fano factor reaches the full Poissonian value F=1, which signifies that transport is forbidden (T→0) and pure tunneling occurs in this junction. © 2017 Elsevier B.V.
Physica B: Condensed Matter (09214526)468pp. 61-65
In this paper the impact of repetition of graphene-based ferromagnet-normal(FN) regions on the transport properties of system is studied, theoretically. Here, the purpose is to generalize the Fert-Grünberg FNF multijunction to a graphene-based one. Thus, parallel and antiparallel magnetization alignments of ferromagnetic regions are considered. The concept of quantum well is used to calculate the transmission probability using the Dirac-Bogoliubov-de Gennes equations. Charge, spin and thermal conductances are investigated for multilayer junctions in terms of magnetization alignments, strength of ferromagnets and thickness of ferromagnet regions. Finally charge, spin and thermal Giant magnetoresistances (GMR) are calculated for the above-mentioned structures. It is obtained that the spin conductance of antiparallel setups is zero, thus it can be used as a spin valve. © 2015 Elsevier B.V.
European Physical Journal D (14346060)69(7)
Abstract We propose a method to generate stationary entanglement between two macroscopic vibrating elements (micro-mechanical resonators (MRs)), via a transmission line resonator (TLR) field mode, where the MRs are coupled to the TLR capacitively. In this paper two situations are studied; (i) a driving on TLR field with an external microwave pulse, (ii) driving on TLR field and simultaneous driving on two MRs. Here, the entanglement is quantified by the logarithmic negativity. As our proposed system is a continuous variable system, the logarithmic negativity is defined in terms of covariance matrix. We have shown that the second case leads to much stronger entanglement, even at a few milli Kelvin temperatures. © 2015 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.
Chinese Physics B (16741056)24(4)
We investigate the quantum transport property in gapped graphene-based ferromagnetic/normal/ferromagnetic (FG/NG/FG) junctions by using the Dirac-Bogoliubov-de Gennes equation. The graphene is fabricated on SiC and BN substrates separately, so carriers in FG/NG/FG structures are considered as massive relativistic particles. Transmission probability, charge, and spin conductances are studied as a function of exchange energy of ferromagnets (h), size of graphene gap, and thickness of normal graphene region (L) respectively. Using the experimental values of Fermi energy in the normal graphene part (EFN ∼ 400 meV) and energy gap in graphene (260 meV for SiC and 50 meV for BN substrate), it is shown that this structure can be used for both spin-up and spin-down polarized current. The latter case has different behavior of gapped FG/NG/FG from that of gapless FG/NG/FG structures. Also perfect charge giant magnetoresistance is observed in a range of EFN - mvF2 < h < EFN + mvF2. © 2015 Chinese Physical Society and IOP Publishing Ltd.
Rashidian z., ,
Mojarabian f.m., ,
Bayati p., ,
Rashedi, G.R.,
Ueda a., ,
Yokoyama t., T. Journal of Physics Condensed Matter (09538984)26(25)
We theoretically investigate the transport properties of bilayer graphene junctions, where the ferromagnetic strips are attached to the middle region of the graphene sheet. In these junctions, we can control the band gap and the band structure of the bilayer graphene by using the bias voltage between the layers and the exchange field induced on the layers. The conductance and Fano factor (F ) are calculated by the Landauer-Buttiker formula. It is found that when the voltage between the layers or the exchange field are tuned, the pseudodiffusive (F = 1/3) transport turns into tunneling (F = 1) or ballistic transport (F = 0). By tuning the potential difference between the layers, one can control the spin polarization of the current. © 2014 IOP Publishing Ltd Printed in the UK.
Journal of Magnetism and Magnetic Materials (03048853)362pp. 36-41
Using the extended Blonder-Tinkham-Klapwijk formalism, the spin-dependent transport properties in a graphene-based normal metal/ferromagnetic barrier/d-wave superconductor (NFBd-wave) junction have been studied theoretically. Here, we have mainly studied the influences of spin-dependent barrier and rotation angle of d-wave superconducting order parameter (α) on the charge and spin conductance. It is found that the rotation angle has a strong effect on the amplitude and phase of the charge conductance oscillations. As a remarkable result, we obtained that because of the spin-dependent barrier (FB), the rotation angle cannot suppress the zero-bias charge conductance and for the maximum rotation angle α=π/4, the charge conductance shows oscillatory behavior which is different from similar non-spin-dependent barrier junctions. We have also shown that the spin filtering application of this junction is drastically changed by the rotation angle α. As α increases, the spin filtering application enhances, being strongest for α=π/4. At last, we propose an experimental setup to detect our predicted effects. © 2014 Elsevier B.V.
Journal of Alloys and Compounds (09258388)593pp. 235-241
The topological band structure of the CuxAu1- xInTe2 alloys with (x = 0, 0.25, 0.5, 0.75, 1) have been studied by the first principles study in the framework of the density functional theory (DFT) using the generalized gradient approximation (GGA), modified Becke-Johnson (mBJ) and Engel-Vosko generalized gradient approximation (GGA-EV). Our studies show that the topological phase of CuxAu 1-xInTe2 is sensitive to x and type of exchange correlation potentials. The inverted band order occurs with x < 0.25 for mBJ and x < 0.5 for GGA-EV and x > 1 for GGA in band structure. Moreover, the band order of the alloys has been investigated under hydrostatic pressure. The calculated results strength is fitted to several second order polynomial equations for each of approximation. © 2013 Elsevier B.V. All rights reserved.
Journal of the Physical Society of Japan (13474073)82(6)
In this paper, the influence of superconducting proximity effect on the thermal, charge and spin transport in a nanoscale superconductor-ferromagnet- superconductor (SFS) junction is investigated. These calculations are done in the clean limit and in the frame-work of quasiclassical Eilenberger equations. Then by using the obtained density of states (DOS), we obtain the thermal and electrical and spin conductances for SFS junction. Also, we investigate validity of the Wiedemann-Franz (WF) law in this SFS structure. © 2013 The Physical Society of Japan.
Soltani renani, M.,
Ezatabadipour h., H.,
Jalali j., J.,
Darabi, P.,
Azizi e., E.,
Rashedi, G.R. European Physical Journal D (14346060)67(12)
In this paper,we introduce a system containing of two qubits interacting with a cavity which interacted with a reservoir. Using the Fano technique we will show this system is equivalent with two qubits which are interacting with a common heat bath. We also add a laser field and the behavior of this system is investigated when the qubits are dissipative. In this way we show that the presence of laser field can generate a high entanglement in this system, in other word the pump of energy using the laser field can compensate for the lose of dissipation of qubits. We also show that our system is almost insensible to temperature. © 2013 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.
Journal of Applied Physics (10897550)111(12)
We study the magnetoresistance (MR) and spin transport in a graphene-based ferromagnetic/ferromagnetic barrier/s-wave superconductor (FF BS) junction for two regimes including retro-Andreev reflection (AR) (E F =100Δ 0) and retroreflection crossing over to specular Andreev reflection (E F=Δ 0). We observed that the variation of exchange energy in the ferromagnetic region h 1 has a stronger effect on the amplitude of the MR oscillations rather than length, exchange energy, and effective gate potential in the F B region in both regimes. In the E F =100Δ 0 regime, the higher values of exchange energy h 1 approach E F, which decrease the amplitude of the MR oscillations with respect to F B length. By increasing the exchange energy up to h 1>E F, a phase shift occurs for the peaks of the MR curves and the amplitude of the MR oscillations reaches its maximum value at the exchange energy h c =2E F+U 0. Furthermore, in the E F =100Δ 0 regime, at the exchange energy h 1=eV, the amplitude of the MR oscillations approaches its maximum value. At this value of energy of carriers eV +h 1 - E F (Dirac point), the sign of the energy will be reversed and a transition from the retroreflection to the specular Andreev reflection occurs. Thus, in this regime (E F =100Δ 0), by observing the maximum of the MR at specified values of eV and h 1, we can measure the Fermi energy of the graphene. While in the E F =100Δ 0 regime, the MR versus bias energy can be tuned from the positive to negative values by varying h 1, but in the E F =100Δ 0 regime, the MR shows only the positive values. In addition, we have investigated the spin conductance in both regimes. © 2012 American Institute of Physics.
JOURNAL OF APPLIED PHYSICS (00218979)112(3)
Journal of Applied Physics (10897550)112(11)
In the present paper, a study on graphene-based ferromagnetic/d-wave superconductor (Fd) junction using the Blonder-Tinkham-Klapwijk formalism and Dirac-Bogoliubov De-Gennes equation has been done. The effect of rotation of order parameter on the transport properties of this structure is investigated. This rotation is related to the orientation of high temperature superconductor crystal such as YBaCuO coexisting with a graphene monolayer. As the main result of this paper, we obtain the zero bias conductance peak as a fingerprint of unconventional superconductivity for both of spin conductance and charge conductance. Also, we obtain that the thermal conductance of this Fd junction is a linear function of temperature at the low temperatures. This later case is similar to the Wiedemann-Franz law for metal at small temperatures. © 2012 American Institute of Physics.
Journal of Applied Physics (10897550)112(1)
By depositing a superconductor on gapped graphene (graphene grown on SiC substrate), the motion of quasiparticles in this superconductor is explained by the massive Dirac equation. In this paper, we study the spin dependent transport properties of graphene-based ferromagnetic/insulator/superconductor (FIS) junction and graphene-based ferromagnetic/ferromagnetic barrier/superconductor (FF BS) junction in which only the superconducting region is deposited on the gapped graphene and the other graphene regions are gapless. We found that in the graphene-based FIS junction and in the thin barrier approximation, by opening the energy gap in the superconducting region, the charge conductance is an oscillatory function of barrier strength (χ G), despite the large Fermi energy mismatch between ferromagnetic and superconductor regions. As an important result, we analytically obtained that for the normal incident of charge carriers, this junction is not totally transparent. This means that the second characteristic of Klein tunneling is not satisfied due to the massive Dirac fermions carrying the current in the superconductor region. For the graphene-based FF BS junction, opening the energy gap causes a phase shift as large as π 2 to appear between the peaks of charge conductance for parallel and antiparallel configuration versus χ G. Interestingly, we obtained that by increasing the energy gap in this junction, the magnetoresistance increases and by approaching the energy gap to the Fermi energy of the superconductor, it reaches its maximum value (more than -150). This characteristic shows that this junction can be a suitable candidate for application in the graphene-based spintronics devices. © 2012 American Institute of Physics.
Physica C: Superconductivity and its Applications (09214534)477pp. 1-5
In this paper, we investigate the charge and spin currents in the Josephson junction between two superconductors with nonunitary order parameters. Two nonunitary spin-triplet superconductors sandwich a thin film of normal metal and constitute a Josephson junction. We study the transport properties of this superconductor-normal metal-superconductor (SNS) Josephson junction using the quasiclassical Eilenberger equation in the clean limit. An external phase difference between order parameters and a misorientation between two gap vectors of two superconductors exist. Influence of the misorientation between left and right superconducting gap vectors and thickness of normal metal and Josephson phase difference between two order parameters on the charge and spin transport in the junction, are studied, theoretically. © 2012 Elsevier B.V. All rights reserved.
Physica E: Low-Dimensional Systems and Nanostructures (13869477)44(3)pp. 647-653
In this paper the parallel and antiparallel graphene based Ferromagnet-Normal-Ferromagnet (FNF) structures are investigated theoretically. Effect of parallel and antiparallel alignments strength of ferromagnets and thickness of normal region and temperature on the charge, spin and thermal conductances are studied. A survey on Giant magnetoresistance (GMR) has been done and we have shown that, conductances of parallel and antiparallel structures are different. In this paper, we study and calculate all kinds of the GMR including the charge-GMR, thermal-GMR and also spin-GMR for a FNF systems. Although the charge-GMR is important and useful in fabrication magnetic information storage has been investigated in many works but few papers exist about thermal-GMR and spin-GMR. Also with consideration spin current we observed that, in definite strength of ferromagnetic film and in the presence of charge current, spin current is zero. This latter case can be used as a spin-valve. © 2011 Elsevier B.V.
Physica E: Low-Dimensional Systems and Nanostructures (13869477)43(4)pp. 966-970
We study the tunneling magneto-transport properties of the Ferromagnetic Insulator-Normal Insulator-Ferromagnetic Insulator (FNF) and Ferromagnetic Insulator-Barrier Insulator-Ferromagnetic Insulator (FBF) junctions on the surface of topological insulator in which in-plane magnetization directions of both ferromagnetic sides can be parallel and antiparallel. We derive analytical expressions for electronic conductances of the two mentioned junctions with both parallel and antiparallel directions of magnetization and using them calculate the magnetoresistance of the two junctions. We use thin barrier approximation for investigating the FBF junction. We find that although magnetoresistance of the FNF and FBF junctions are tunable by changing the strength of magnetization texture, they show different behaviors with variation of magnetization. In contrast to the magnetoresistance of FNF, magnetoresistance of FBF junctions shows very smooth enhance by increasing the strength of magnetization. We suggest an experimental set up to detect our predicted effects. © 2010 Elsevier B.V. All rights reserved.
Chinese Physics B (16741056)20(12)
In this paper the influence of superconducting correlations on the thermal and charge conductances in a normal metal - superconductor (NS) junction in the clean limit is studied theoretically. First we solve the quasiclassical Eilenberger equations, and using the obtained density of states we can acquire the thermal and electrical conductances for the NS junction. Then we compare the conductance in a normal region of an NS junction with that in a single layer of normal metal (N). Moreover, we study the Wiedemann - Franz (WF) law for these two cases (N and NS). From our calculations we conclude that the behaviour of the NS junction does not conform to the WF law for all temperatures. The effect of the thickness of normal metal on the thermal conductivity is also theoretically investigated in the paper. © 2011 Chinese Physical Society and IOP Publishing Ltd.
Journal of Physics Condensed Matter (09538984)23(27)
Charge and spin Josephson currents in a ballistic superconductor- ferromagnet-superconductor junction with spin-triplet pairing symmetry are studied using the quasiclassical Eilenberger equation. The gap vector of superconductors has an arbitrary relative angle with respect to magnetization of the ferromagnetic layer. We clarify the effects of the thickness of the ferromagnetic layer and the magnitude of the magnetization on the Josephson charge and spin currents. We find that the 0-π transition can occur when the misorientation angle between the exchange field of the ferromagnetic layer and the d-vector is smaller than π/4. We also show how spin current flows due to misorientation between the exchange field and the d-vector. © 2011 IOP Publishing Ltd.
Low Temperature Physics (1063777X)36(3)pp. 205-209
The charge and spin transport properties of a clean TS-N-TS Josephson junction (triplet superconductor-normal metal-triplet superconductor) are studied using the quasiclassical Eilenberger equation for the Green's function. The effects of the thickness of normal layer between superconductors on the spin and charge currents are investigated. The effect of misorientation between the triplet superconductors which creates the spin current is the main subject of this paper. It is shown that for some values of the phase difference between the superconductors a spin current exists in the absence of charge current and vice versa. © 2010 American Institute of Physics.
Iranian Journal of Physics Research (16826957)10(2)pp. 141-144
Charge and spin transport properties of a clean $SNS$ Josephson junction (triplet superconductor-normal metal-triplet superconductor) are studied using the quasiclassical Eilenberger equation of Green's function. Our system consists of two p-wave superconducting crystals separated by a Copper nano layer. Effects of thickness of normal layer between superconductors on the spin and charge currents are investigated. Also misorientation between triplet superconductors which creates the spin current is another subject of this paper.
Fizika Nizkikh Temperatur (01326414)36(3)pp. 262-267
Charge and spin transport properties of a clean TS-N-TS Josephson junction (triplet superconductor-normal metal-triplet superconductor) are studied using the quasiclassical Eilenberger equation for Green's function. Effects of thickness of normal layer between superconductors on the spin and charge currents are investigated. The effect of a misorientation between triplet superconductors which creates the spin current is the main subject of this paper. It is shown that for some values of phase difference between superconductors the spin current exists in the absence of charge current and vice versa. © 2010 G. Rashedi, Y. Rahnavard, and Yu.A. Kolesnichenko.
Iranian Journal of Physics Research (16826957)10(2)pp. 135-139
In this paper, we investigate the coherent quantum transport in grapheme-based ferromagnet-superconductor-ferromagent junctions within the framework of BCS theory using DBdG quasiparticles equation.The coherency with the finite size of superconductor region has two characteristic features subgap electron transport and oscillations of differential conductance. we show that periodic vanishing of the Andreev reflection at the energies called geometrical resonances above the superconducting gap is a striking consequence of quasiparticles interference. We suggest to make devices that produce polarized spin-current with possible applications in spintronics.
Chinese Physics B (16741056)19(10)
In this paper, a dc Josephson junction between borocarbide superconductors has been studied theoretically. The s+g-wave pairing symmetry which is observed in rare earth complex of borocarbides has a huge anisotropy and is an interesting form of unconventional superconductivity. We calculate the Josephson current in a superconductor- insulator-superconductor (SIS) Josephson junction with s+g-wave superconducting pairing symmetry. In our planar junction c-axis is parallel to an interface with finite transparency but ab-planes of two tetragonal superconductors are misorientated by angle α. We obtain that the Josephson current is strongly dependent on mis-orientation between the left and the right ab-planes. An insulator sandwiched between two superconductors which acts as a potential barrier is demonstrated by a transparency coefficient. The effects of the potential barrier and the mis-orientation on the current are studied analytically and numerically. Occurrence of 0-π transition in this s+g-wave junction is investigated in this paper. A comparison between d-wave Josephson junction and s+g-wave one is also made in the present paper. © 2010 Chinese Physical Society and IOP Publishing Ltd.
Iranian Journal of Physics Research (16826957)9(4)pp. 337-340
In this paper, a dc Josephson junction between two singlet superconductors (d-wave and s-wave) with arbitrary reflection coefficient has been investigated theoretically. For the case of high Tc superconductors, the c-axes are parallel to an interface with finite transparency and their ab-planes have a mis-orientation. The physics of potential barrier will be demonstrated by a transparency coefficient via which the tunneling will occur. We have solved the nonlocal Eilenberger equations and obtained the corresponding and suitable Green functions analytically. Then, using the obtained Green functions, the current-phase diagrams have been calculated. The effect of the potential barrier and misorientation on the currents is studied analytically and numerically. It is observed that, the current phase relations are totally different from the case of ideal transparent Josephson junctions between d-wave superconductors and two swave superconductors. This apparatus can be used to demonstrate d-wave order parameter in high Tc superconductors.
Physical Review B - Condensed Matter and Materials Physics (10980121)82(1)
We study the thermodynamic properties of a dirty ferromagnetic S |F| S Josephson junction with s -wave superconducting leads in the low-temperature regime. We employ a full numerical solution with a set of realistic parameters and boundary conditions, considering both a uniform and nonuniform exchange field in the form of a Bloch domain wall. Our main result is that it is possible to strongly modify the electronic specific heat of the system by changing the phase difference between the two superconducting leads from zero up to nearly π. This is explained in terms of the proximity-altered density of state in the ferromagnetic region, and we discuss possible methods for experimental detection of the predicted effect. © 2010 The American Physical Society.
Journal of Applied Physics (10897550)108(8)
We study theoretically behavior of thermal massless Dirac fermions inside graphene-based Ferromagnetic Insulator d -wave/ s -wave superconductor (F |I| d and F I S) junctions in the ballistic regime. We employ Dirac-Bogoliubov-de Genne formalism and use Dirac-BdG wave functions within the ferromagnetic, normal, and superconducting regions and appropriate boundary conditions, to derive Andreev and normal reflection coefficients. By employing the obtained Andreev and normal reflection coefficients, thin barrier approximation and also appropriate values of parameters, we investigate characteristics of heat current through the F |I| d and F I S junctions. We find that for s -wave superconductors, thermal conductance () oscillates sinusoidally versus strength of barrier in the thin barrier approximation. The finding persists for all values of α, orientation of d -wave superconductor crystal in the k -space, for values of α very close to π /4, magnitude of the oscillations suppress completely. By increasing temperature, the thermal conductance increases exponentially for small values of α and for larger values of superconductor crystal orientation angle the thermal conductance modifies and interestingly for α=π /4 exhibits linear behavior, i.e., T which is similar to Wiedemann-Franz law for metals in low temperatures. While increasing the temperature move minimum value of the thermal conductance with respect to strength of magnetization texture h/ Δ0 towards smaller values of h/ Δ0, increasing the superconductor crystal orientation angle, α from 0 to π /4 enhances whole values of the thermal conductance and consequently the minimum value move towards up. At last we suggest an experimental setup for verifying these findings. © 2010 American Institute of Physics.
Journal of Physics Condensed Matter (09538984)22(41)
We study transport properties and local density of states in a clean superconductor-normal metal-superconductor Josephson junction with triplet f-wave superconductors, based on the Eilenberger equation. Effects of the thickness of normal metal and a misorientation between the gap vector of the superconductors on the spin and charge currents are investigated. A spin current, which arises from the misalignment of the d-vectors, in the absence of charge current for some values of phase difference between the superconductors is found. We also find unconventional behavior of the spin current associated with the 0-π transition. The misalignment of the d-vectors also gives rise to a zero energy peak in the density of states in the normal metal. © 2010 IOP Publishing Ltd.
Physica C: Superconductivity and its Applications (09214534)470(17-18)pp. 703-708
In this paper, conductance of spin and electron in graphene-based ferromagnet - superconductor (FS) and parallel and antiparallel ferromagnet-superconductor-ferromagnet (FSF) junctions are studied. Using the Dirac-Bogoliubov-de Gennes equations, Andreev and normal reflections are obtained and then using these coefficients, conductance of spin and electrons are calculated at the FS interface(s) analytically. As a result, both the energy dependence of spin and charge differential conductances are investigated and a comparison between electron and spin transport is done in this paper. Effect of exchange energy of ferromagnet h on conductances is studied too. © 2010 Elsevier B.V. All rights reserved.
Alidoust, M.,
Linder, J.,
Rashedi, G.R.,
Yokoyama t., T.,
Sudbø, A. Physical Review B - Condensed Matter and Materials Physics (10980121)81(1)
We study numerically the properties of spin transport and charge transport in a current-biased nanoscale diffusive superconductor/ferromagnet/ superconductor junction when the magnetization texture is nonuniform. Specifically, we incorporate the presence of a Bloch/Neel domain walls and conical ferromagnetism, including the role of spin-active interfaces. The superconducting leads are assumed to be of the conventional s -wave type. In particular, we investigate how the 0-π transition is influenced by the inhomogeneous magnetization texture and focus on the particular case where the charge current vanishes while the spin current is nonzero. In the case of a Bloch/Neel domain wall, the spin current can be seen only for one component of the spin polarization, whereas in the case of conical ferromagnetism the spin current has the three components. This is in contrast to a scenario with a homogeneous exchange field, where the spin current vanishes completely. We explain all of these results in terms of the interplay between the triplet anomalous Green's function induced in the ferromagnetic region and the local direction of the magnetization vector in the ferromagnet. Interestingly, we find that the spin current exhibits discontinuous jumps at the 0-π transition points of the critical charge current. This is seen both in the presence of a domain wall and for conical ferromagnetism. We explain this result in terms of the different symmetry obeyed by the current-phase relation when comparing the charge and spin currents. Specifically, we find that whereas the charge current obeys the well-known relation Ic (φ) =- Ic (-φ), the spin current satisfies Is (φ) = Is (-φ), where φ is the superconducting phase difference. © 2010 The American Physical Society.
Journal of Applied Physics (10897550)107(12)
We present an investigation of heat transport in gapless graphene-based ferromagnetic/singlet superconductor/ferromagnetic junctions. We find that unlike the uniform increase in the thermal conductance versus temperature, the thermal conductance exhibits intensive oscillatory behavior versus width of sandwiched s-wave superconducting region between the two ferromagnetic layers. This oscillatory form rises from interference of the massless Dirac fermions in graphene. Also we find that thermal conductance versus exchange field h displays a minimal value at h/Ef ≃1 within the low temperature regime where this finding demonstrates that propagating modes of the Dirac fermions in this value reach to their minimum numbers and verify the previous results for electronic conductance. We find that for thin widths of superconducting region, the thermal conductance versus temperature shows linear increment, i.e.,F ∝ T. At last we propose an experimental setup to detect our predicted effects. © 2010 American Institute of Physics.
Journal of Physics Condensed Matter (09538984)21(7)
In this paper, the non-local mixing of coherent current states in d-wave superconducting banks is investigated. The superconducting banks are connected via a ballistic point contact. The banks have mis-orientation and phase difference. Furthermore, they are subjected to a tangential transport current along the ab plane of d-wave crystals and parallel to the interface between the superconductors. The effects of mis-orientation and external transport current on the current-phase relations and current distributions are the subjects of this paper. It is observed that, at values of phase difference close to 0, π and 2π, the current distribution may have a vortex-like form in the vicinity of the point contact. The current distribution of the above-mentioned junction between d-wave superconductors is totally different from the junction between s-wave superconductors. The interesting result which this study shows is that spontaneous and Josephson currents are observed for the case of = 0. © 2009 IOP Publishing Ltd.
Journal of Physics: Conference Series (17426588)97(1)
In this paper, a dc Josephson junction between two singlet superconductors (d-wave and s-wave) with arbitrary reflection coefficient has been investigated theoretically following the famous paper [Y. Tanaka and S. Kashiwaya 1996 Phys. Rev. B 53, R11957]. For the case of High Tc superconductors, the c-axes are parallel to an interface with finite transparency and their ab-planes have a mis-orientation. The effect of transparency and mis-orientation on the currents is studied both analytically and numerically. It is observed that, the current phase relations are totally different from the case of ideal transparent Josephson junctions between d-wave superconductors and two s-wave superconductors. This apparatus can be used to demonstrate d-wave order parameter in High Tc superconductors © 2008 IOP Publishing Ltd.
Physica C: Superconductivity and its Applications (09214534)451(1)pp. 31-37
The spin current in the Josephson junction as a weak-link (interface) between misoriented triplet superconductors is investigated theoretically for the models of the order parameter in UPt3. Green functions of the system are obtained from the quasiclassical Eilenberger equations. The analytical results for the charge and spin currents are illustrated by numerical calculations for the certain misorientation angles of gap vector of superconductors. As the main result of this paper, it is found that, at some values of the phase difference, at which the charge current is exactly zero, the spin current has its maximum value. Furthermore, it is shown that the origin of spin current is the misorientation between gap vectors of triplet superconductors. © 2006 Elsevier B.V. All rights reserved.
Fizika Nizkikh Temperatur (01326414)31(6)pp. 634-639
A stationary Josephson effect in a weak link between misorientated nonunitary triplet superconductors is investigated theoretically. The non-self-consistent quasiclassical Eilenberger equation for this system is solved analytically. As an application of this analytical calculation, the current-phase diagrams are plotted for the junction between two nonunitary bipolar f-wave superconducting banks. A spontaneous current parallel to the interface between superconductors is observed. Also, the effect of misorientation between crystals on the Josephson and spontaneous currents is studied. Such experimental investigations of the current-phase diagrams can be used to test the pairing symmetry in the above-mentioned superconductors.
Superconductor Science and Technology (09532048)18(4)pp. 482-488
Recently, the '(p + h)-wave' form of pairing symmetry has been proposed for the superconductivity in the PrOs4Sb12 compound (Parker et al 2004 Preprint cond-mat/0407254). In the present paper, a stationary Josephson junction as a weak-link between PrOs4Sb 12 triplet superconductors is theoretically investigated. The quasiclassical Eilenberger equations are analytically solved. The spin and charge current-phase diagrams are plotted, and the effect of misorientation between crystals on the spin current and spontaneous and Josephson currents is studied. It is found that such experimental investigations of the current-phase diagrams can be used to test the pairing symmetry in the above-mentioned superconductors. It is shown that this apparatus can be applied as a polarizer for the spin current. © 2005 IOP Publishing Ltd.
Superlattices and Microstructures (10963677)35(1-2)pp. 155-155
Physical Review B - Condensed Matter and Materials Physics (10980121)69(2)
The influence of electron reflection on dc Josephson effect in a ballistic point contact with transport current in the banks is considered theoretically. The effect of finite transparency on the vortexlike currents near the contact and at the phase difference φ=π, which has been predicted recently [Yu. A. Kolesnichenko, A. N. Omelyanchouk, and S. N. Shevchenko, Phys. Rev. B. 67 172504 (2003)], is investigated. We show that at low temperatures even a small reflection on the contact destroys the mentioned vortexlike current states, which can be restored by increasing the temperature. © 2004 The American Physical Society.
