Ferromagnet-superconductor Junctions Research Articles

Thermoelectric detection of Andreev states in unconventional superconductors

The authors show that a thermoelectric current in ferromagnet-superconductor junctions flows due to Andreev processes and is sensitive to surface states arising in unconventional superconductors, changing sign in their presence.

Electrical and thermal transport in antiferromagnet-superconductor junctions

We demonstrate that antiferromagnet-superconductor (AF-S) junctions show qualitatively different transport properties than normal metal-superconductor (N--S) and ferromagnet-superconductor (F-S) junctions. We attribute these transport features to presence of two new scattering processes in AF--S junctions, i.e., specular reflection of holes and retroreflection of electrons. Using the Blonder-Tinkham-Klapwijk formalism, we find that the electrical and thermal conductance depend nontrivially on antiferromagnetic exchange strength. Furthermore, we show that the interplay between the N\'eel vector direction and the interfacial Rashba spin-orbit coupling leads to a large anisotropic magnetoresistance. The unusual transport properties make AF--S interfaces unique among the traditional condensed-matter-system-based superconducting junctions.

Charge transport in silicene-based ferromagnetic p-wave superconductor junctions

Using Blonder-Tinkham-Klapwijk (BTK) formalism we studied the transport properties in silicene-based normal/ferromagnetic p-wave superconductor (N/FpS) junctions. Our study shows that the amplitude of the normalized conductance strongly depends on the pairing symmetry in superconductor such as px, py and px+ipy-wave symmetries, the applied electric field (EZ) and the exchange field (h) in FpS. In N/FpxS-silicene junctions the zero-bias conductance peak is obtained due to the Andreev bound states. When the electric field and magnetic field are parallel, called P configuration, there is one conductance dip (CD) for the voltage eV=lEz−λSO with λSO the spin obit coupling term and 2l the distance between two sublattices in the buckled silicene. In the anti-parallel (AP) configuration there is one CD at eV=lEz−λSO for 0<lEz−λSO<h and for lEz−λSO≥h there are two CDs at eV=h and eV=lEz−λSO, respectively. A sudden transition of conductance from zero to maximum (henceforth referred to as“switching of conductance”) at eV=lEz−λSO occurs in both N/FpyS-silicene junction and N/Fpx+ipyS-silicene junction. In N/FpyS-silicene junction the Andreev resonant peak appears at eV=h for lEz−λSO−h=0.1Δ00 in the AP case, where Δ00 is the zero temperature gap of the FpS. In N/Fpx+ipyS-silicene junctions the maximum value of the conductance appears at eV=Δ00−h in P junction while in AP junction at eV=Δ00+h.

Spin-selective coupling to Majorana zero modes in mixed singlet and triplet superconducting nanowires

We theoretically investigate the transport properties of a quasi one dimensional ferromagnet-superconductor junction where the superconductor consists of mixed singlet and triplet pairings. We show that the relative orientation of the stoner field ($\bf{\tilde{h}}$) in the ferromagnetic lead and the $\bf{d}$ vector of the superconductor acts like a on-off switch for the zero bias conductance of the device. In the regime, where triplet pairing amplitude dominates over the singlet counterpart (topological phase), a pair of Majorana zero modes appear at each end of the superconducting part of the nanowire. When $\bf{\tilde{h}}$ is parallel or anti-parallel to the $\bf{d}$ vector, transport gets completely blocked due to blockage in pairing while, when $\bf{\tilde{h}}$ and $\bf{d}$ are perpendicular to each other, the zero energy two terminal differential conductance spectra exhibits sharp transition from $4e^2/h$ to $2e^2/h$ as the magnetization strength in the lead becomes larger than the chemical potential indicating the spin selective coupling of pair of Majorana zero modes to the lead.

Fabrication of Optimized Superconducting Phase Inverters Based on Superconductor–Ferromagnet–Superconductor $$\pi $$ π -Junctions

We have implemented a trilayer technological approach to fabricate $$\mathrm {Nb}{-}\mathrm {Cu}_{0.47}\mathrm {Ni}_{0.53}{-} \mathrm {Nb}$$ superconducting phase inverters ( $$\pi $$ -junctions) with enhanced critical current. Within this technique, all three layers of the superconductor–ferromagnet–superconductor junction deposited in a single vacuum cycle that have allowed us to obtain $$\pi $$ -junctions with critical current density up to 20 kA/cm $$^2$$ . The value achieved is a factor of 10 higher than for the step-by-step method used in earlier works. Our additional experiments have shown that this difference is related to a bilayered CuNi/Cu barrier used in the case of the step-by-step technique and interlayer diffusion at the CuNi/Cu interface. We show that the interlayer diffusion can be utilized for fine tuning of the $$0{-}\pi $$ transition temperature of already fabricated junctions. The results obtained open new opportunities for the CuNi-based phase inverters in digital and quantum Josephson electronics.

Graphene based superconducting junctions as spin sources for spintronics

We investigate spin-polarized transport in graphene-based ferromagnet-superconductor junctions within the Blonder-Tinkham-Klapwijk formalism by using spin-polarized Dirac-Bogoliubov-de-Gennes equations. We consider superconductor in spin-singlet s-wave pairing state and ferromagnet is modeled by an exchange field with energy of Ex. We have found that graphene-based junctions can be used to produce highly spin-polarized current in different situations. For example, if we design a junction with high Ex and EF compared to order parameter of superconductor, then one can have a large spin-polarized current which is tunable in magnitude and sign by bias voltage and Ex. Therefore graphene-based superconducting junction can be used in spintronic devices in alternative to conventional junctions or half-metallic ferromagnets. Also, we have found that the calculated spin polarization can be used as a tool to distinguish specular Andreev reflection (SAR ) from the conventional Andreev reflection (CAR ) such that in the case of CAR, spin polarization in sub-gap region is completely negative which means that spin-down current is greater than spin-up current. When the SAR is dominated, the spin polarization is positive at all bias-voltages, which itself shows that spin-up current is greater than spin-down current.

Andreev reflection and subgap conductance in monolayer MoS2 ferromagnet/s and d-wave superconductor junction

The accurate and proper form of electron-hole excitations and corresponding Dirac-like spinors of superconducting monolayer molybdenum disulfide are exactly obtained. Andreev reflection and resulting subgap conductance in a MoS2-based ferromagnet-superconductor (F/S) junction are particularly calculated in terms of dynamical characteristics of structure. Due to the spin-splitting energy gap in the valence band and also nondegenerate K and K′ valleys, the ferromagnetic exchange energy sh can cause a distinct behavior of Andreev reflection process between spin-up and spin-down charge carriers in the different valleys. In order to occur the retro Andreev reflection, the chemical potential is necessarily fixed in a determined range. Given here one-particle superconducting bispinors enable us to explicitly involve the anisotropic superconducting gap ΔS under electron-hole conversion, i.e., taking place in d-wave asymmetry. Dependence of the Andreev process on the electron incidence angle at the interface is explicitly explored in the presence of such superconducting pair potential.

Magnetoanisotropic Andreev reflection in ferromagnet-superconductor junctions.

Andreev reflection spectroscopy of ferromagnet-superconductor (FS) junctions [corrected] is an important probe of spin polarization. We theoretically investigate spin-polarized transport in FS junctions in the presence of Rashba and Dresselhaus interfacial spin-orbit fields and show that Andreev reflection can be controlled by changing the magnetization orientation. We predict a giant in- and out-of-plane magnetoanisotropy of the junction conductance. If the ferromagnet is highly spin polarized-in the half-metal limit-the magnetoanisotropic Andreev reflection depends universally on the spin-orbit fields only. Our results show that Andreev reflection spectroscopy can be used for sensitive probing of interfacial spin-orbit fields in a FS junction.

Charge and Spin Wiedemann–Franz Law in Superconductor–Ferromagnet–Superconductor Junction

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.

Current-voltage characteristics of low-resistance tunnel structures ferromagnet-insulator-superconductor Co2CrAl–I–Pb

A physical model of tunneling processes leading to a nonequilibrium superconducting state in ferromagnet-superconductor junctions is proposed and confirmed experimentally. The model is based on the analysis of experimental current-voltage characteristics and the determination of junction voltage dependences of the energy gap, the effective chemical potential and the effective temperature of quasiparticles in the modified energy distribution of quasiparticles as well as on taking into account the change in the density of electron states due to destruction of the Cooper pairs, caused by the proximity effect.

Enhanced Andreev reflection in gapped graphene

We theoretically demonstrate unusual features of superconducting proximity effect in gapped graphene that presents a pseudospin symmetry-broken ferromagnet with a net pseudomagnetization. We find that the presence of a band gap makes the Andreev conductance of graphene superconductor/pseudoferromagnet (S/PF) junction to behave similar to that of a graphene ferromagnet-superconductor junction. The energy gap ${\ensuremath{\Delta}}_{N}$ can enhance the pseudospin inverted Andreev conductance of S/PF junction to reach a limiting maximum value for ${\ensuremath{\Delta}}_{N}\ensuremath{\gg}\ensuremath{\mu}$, which depending on the bias voltage can be larger than the value for the corresponding junction with no energy gap. We further demonstrate a damped-oscillatory behavior for the local density of states of the PF region of S/PF junction and a long-range crossed Andreev reflection process in PF/S/PF structure with antiparallel alignment of pseudomagnetizations of PFs, which confirm that, in this respect, the gapped normal graphene behaves like a ferromagnetic graphene.

Kondo effect and spin-active scattering in ferromagnet-superconductor junctions

We study the interplay of superconducting and ferromagnetic correlations on charge transport in different geometries with a focus on both a quantum point contact as well as a quantum dot in the even and the odd state with and without spin-active scattering at the interface. In order to obtain a complete picture of the charge transport we calculate the full counting statistics in all cases and compare the results with experimental data. We show that spin-active scattering is an essential ingredient in the description of quantum point contacts. This holds also for quantum dots in an even charge state whereas it is strongly suppressed in a typical Kondo situation. We explain this feature by the strong asymmetry of the hybridisations with the quantum dot and show how Kondo peak splitting in a magnetic field can be used for spin filtering. For the quantum dot in the even state spin-active scattering allows for an explanation of the experimentally observed mini-gap feature.

Josephson effect between p-wave superconductors with different orbital pairing symmetries

The Josephson effects in the ferromagnetic superconductor junctions of the equal spin pairing symmetry are investigated by a Furusaki–Tsukada-like formula. It is shown that the internal phase difference between the two different anisotropic superconductors plays an important role on the Josephson current. The current is predicted to be an oscillation function of the macroscopic phase difference with the period 2 π, π, or π / 2 depending on the different orbital pairing symmetries of the superconductors of the junction.

Electric flow through an ideal ferromagnet–superconductor junction

It is investigated the possibility of controlling the electric flow through a ferromagnet–superconductor junction by spin polarization, within a simple, ideal model of a perfect ferromagnetic–superconductor junction. The ferromagnetic and superconducting properties as well as the Andreev reflection are briefly reviewed and the electrical resistance of the junction is computed both in the diffusive and ballistic regime for the ferromagnetic sample. It is shown that the resistance of the junction increases with increasing magnetization, including both positive or negative jumps on passing from the ballistic to the diffusive regime.

Electron transport in a ferromagnet-superconductor junction on graphene

In a usual ferromagnet connected with a superconductor, the exchange potential suppresses the superconducting pairing correlation. We show that this common knowledge does not hold in a ferromagnet-superconductor junction on a graphene. When the chemical potential of a graphene is close to the conical point of energy dispersion, the exchange potential rather assists the charge transport through a junction interface. The loose-bottomed electric structure causes this unusual effect.

Coherence effects in S/I/N/I/FS tunnel junctions

The dc Josephson effect in superconductor / insulator / normal metal / insulator /ferromagnetic superconductor junctions has been studied. We calculate the dc Josephson current based on the Bogoliubov de Gennes equation. The Josephson current is derived as a function of exchange field in ferromagnetic superconductor, normal metal thickness and insulating barrier strength. It is found that there exists an oscillation relation between the critical Josephson current and the normal metal thickness. The oscillation amplitude decreases as the thickness of the normal metal increases or the exchange field augments.

Theory of the tunneling spectroscopy of ferromagnetic superconductors

We study tunneling conductance in normal metal / insulator / ferromagnetic superconductor junctions. The tunneling spectra show a clear difference between spin-singlet s-wave pairing, spin-triplet opposite spin pairing and spin-triplet equal spin pairing: These pairings exhibit, respectively, gap struture, double peak structure and zero bias peak in the spectra. The obtained result may serve as a tool for determining the pairing symmetry of ferromagnetic superconductors.

Probing Magnetic Anisotropy and Spin Polarization in Spintronic Materials

Magnetic anisotropy and spin polarization are fundamental parameters in ferromagnetic materials that have use in spintronic device applications. As the need for screening properties of new magnetic materials rises, it is important to have measurement probes for quantities such as anisotropy and spin polarization. We have developed two unconventional yet powerful techniques to study these parameters. A resonant RF transverse susceptibility method is used to map the characteristic anisotropy and switching fields over a wide range in temperature and magnetic fields. For studies of spin polarization, the phenomenon of Andreev reflection across ferromagnet-superconductor junctions is used to extract values of the transport spin polarization. The effectiveness of these approaches is demonstrated in candidate spintronic materials such as half-metallic CrO/sub 2/ thin films and arrays of monodisperse, single-domain Fe nanoparticles.

Subgap transport in ferromagnet-superconductor junctions due to magnon-assisted Andreev reflection

We propose a process of magnon-assisted Andreev reflection at a ferromagnetic metal-superconductor $(\mathrm{FS})$ interface, which consists of the simultaneous injection of a Cooper pair from the superconductor and the emission of a magnon inside the ferromagnet. At low temperature this process represents an additional channel for subgap transport across an $\mathrm{FS}$ interface, which lifts restrictions on the current resulting from the necessity to match spin-polarized current in the ferromagnet with spinless current in the superconductor. For a junction between a superconductor and a ferromagnet with an arbitrary degree of polarization, the inelastic magnon-assisted Andreev reflection process would manifest itself as a nonlinear addition to the $I(V)$ characteristics, which is asymmetric with respect to the sign of the bias voltage and is related to the density of states of magnons in the ferromagnet. Expressions for the subgap $I(V)$ characteristics are given for arbitrary interfacial quality while the limiting cases of uniformly transparent and disordered interfaces are discussed in detail.

Magnon-assisted Andreev transport across ferromagnet–superconductor junctions

We study subgap transport due to Andreev reflection in a ferromagnet–superconductor junction at low temperature. The mismatch of spin polarized current in the ferromagnet and spinless current in the superconductor results in an additional contact resistance which can be reduced by magnon emission in the ferromagnet. We find that magnon-assisted Andreev reflection produces a non-linear contribution to the asymmetric part of the conductance, G( V)− G(− V), which is related to the density of states of magnons in the ferromagnet.