arXiv subjects

Stefan Ilić

Publications and source records attributed to Stefan Ilić.

14 recordsLinked to original sources

arXiv2026

Interplay between Superconductivity and Altermagnetism in Disordered Materials and Heterostructures

We study the interplay between superconductivity and altermagnetism in disordered systems using recently derived quantum kinetic transport equations. Starting from this framework, we derive the Ginzburg-Landau free energy and identify, in addition to the conventional pair-breaking term, a coupling between the spin and the spatial variation of the superconducting order parameter. Two distinct effects emerge from this coupling. The first is a nonlinear magnetoelectric effect, in which a supercurrent (i.e., a phase gradient) induces a spin texture; this contribution is quadratic in the phase gradient. The second effect arises when the magnitude, rather than the phase, of the superconducting order parameter varies in space, likewise leading to a finite magnetization. We show that these two contributions compete in the case of an Abrikosov vortex, where both the amplitude and phase of the order parameter vary spatially. The effect associated with amplitude variations also gives rise to a proximity-induced magnetization (PIM) in hybrid structures composed of a superconductor (S) and an altermagnet (AM). Using quasiclassical theory, we analyze the PIM in diffusive S/AM bilayers and S/AM/S Josephson junctions, and determine the induced magnetization profiles. In Josephson junctions, where both the PIM and the nonlinear magnetoelectric effect coexist, we further predict the occurrence of $0$-$π$ transitions.

cond-mat.supr-con↗

arXiv2026

Exploring the conventional and anomalous Josephson effects at arbitrary disorder strength in systems with spin-dependent fields

We present a theory of the Josephson current in superconductor-normal metal-superconductor (SNS) junctions in the presence of generic spin-dependent fields, such as spin-orbit coupling (SOC), Zeeman fields, and altermagnetism. We consider systems with arbitrary disorder strength, going beyond the usual diffusive and ballistic approximations. Using the linearized quasiclassical Eilenberger equation, we derive a compact expression for the Josephson current, which is then applied to various situations of experimental interest. First, we investigate the evolution of the Josephson critical current in an applied magnetic field in the presence of Rashba and Dresselhaus SOC, and discuss how this dependence can be used to probe SOC in the junction. We then study the anomalous Josephson ($φ_0$) effect in systems with Rashba SOC and show that it remains robust over a wide range of disorder strength, and can even be enhanced by moderate disorder in sufficiently long junctions. Finally, we investigate the Josephson current in disordered junctions with altermagnets, and show how the $0$-$π$ transition in such systems is suppressed by disorder. Our results may be useful for describing experimental setups with high-mobility samples, which nevertheless always contain some amount of disorder, and where neither purely ballistic nor diffusive approximations are adequate.

cond-mat.supr-con↗

arXiv2026

Detection of spin- and valley-polarized states in van der Waals materials via thermoelectric and non-reciprocal transport

We predict thermoelectric and current rectification effects in hybrid junctions formed by Ising superconductors and materials hosting valley-polarized states. Both effects originate from the interplay of intrinsic Ising spin-orbit coupling, spin-splitting from an exchange or Zeeman field, and valley polarization. The resulting transport signatures provide experimentally accessible probes of valley-polarized states in van der Waals heterostructures, such as junctions of few-layer transition metal dichalcogenides and twisted bilayer or rhombohedral graphene.

cond-mat.mes-hall↗

arXiv2025

Josephson anomalous vortices

We show that vortices with circulating current, related with odd-frequency triplet pairing, appear in Josephson junctions where the barrier is a weak ferromagnet with strong spin-orbit coupling. By symmetry analysis we show that there is an additional term - a rotary invariant - in the superconducting free energy which allows for magnetoelectric effects even when the previously considered Lifshitz invariant vanishes. Using a microscopic model based on a modified Usadel equation incorporating those effects, we show that the size, shape, and position of these vortices can be controlled by manipulating Rashba spin-orbit coupling in the weak link, via gates, and we suggest that these vortices could be detected via scanning magnetometry techniques. We also show that the transverse triplet components of the pairing amplitudes can form a texture.

cond-mat.supr-con↗

arXiv2025

Gate-tunable polarity inversions and three-fold rotation symmetry of the superconducting diode effect

The superconducting diode effect is an asymmetry in the critical current with respect to the supercurrent polarity. One impetus driving recent interest in the effect is its dependence on intrinsic or microscopic symmetry breaking mechanisms. Here, we study the superconducting diode effect in gated planar Josephson junctions fabricated on a superconductor--semiconductor heterostructure under an in-plane magnetic field. We observe two gate-driven inversions of the diode polarity in the vicinity of zero field, as well as a third-harmonic component in the dependence of the diode efficiency on the in-plane field angle. We analyze the Lifshitz invariant for an arbitrary spin--orbit coupling and show that multiple polarity inversions are possible in the presence of both linear and cubic Dresselhaus terms, where the Rashba parameter varies monotonically with gate voltage. Numerical calculations of the diode efficiency further reveal the presence of higher harmonics in its field-angle dependence in the presence of spin--orbit coupling.

cond-mat.mes-hall↗

arXiv2024

Weak localization at arbitrary disorder strength in systems with generic spin-dependent fields

We present a theory of weak localization (WL) in the presence of generic spin-dependent fields, including any type of spin-orbit coupling, Zeeman fields, and non-homogeneous magnetic textures. We go beyond the usual diffusive approximation, considering systems with short-range disorder of arbitrary strength, and obtain a compact expression for the weak localization (WL) correction to the conductivity in terms of the singlet-triplet polarization operator in momentum space. The latter can be directly related to the solution of the quasiclassical Eilenberger equation for superconducting systems. This formulation presents an intuitive framework to explore how the interplay of various spin-dependent fields drives weak (anti) localization. We apply our results to study in-plane magnetoconductivity in systems with spin-orbit coupling, and in newly discovered altermagnets. Our results enable straightforward calculation of the WL conductivity at arbitrary disorder strength, which can be particularly useful for interpreting experiments on high-mobility samples.

cond-mat.mes-hall↗

arXiv2023

Ubiquitous Superconducting Diode Effect in Superconductor Thin Films

The macroscopic coherence in superconductors supports dissipationless supercurrents which could play a central role in emerging quantum technologies. Accomplishing unequal supercurrents in the forward and backward directions would enable unprecedented functionalities. This nonreciprocity of critical supercurrents is called superconducting (SC) diode effect. We demonstrate strong SC diode effect in conventional SC thin films, such as niobium and vanadium, employing external magnetic fields as small as 1 Oe. Interfacing the SC layer with a ferromagnetic semiconductor EuS, we further accomplish non-volatile SC diode effect reaching a giant efficiency of 65%. By careful control experiments and theoretical modeling, we demonstrate that the critical supercurrent nonreciprocity in SC thin films could be easily accomplished with asymmetrical vortex edge/surface barriers and the universal Meissner screening current governing the critical currents. Our engineering of the SC diode effect in simple systems opens door for novel technologies. Meanwhile, we reveal the ubiquity of Meissner screening effect induced SC diode effect in superconducting films, which should be eliminated with great care in the search of exotic superconducting states harboring finite-momentum Cooper pairing.

cond-mat.supr-con↗

arXiv2022

Reducing the frequency of the Higgs mode in a helical superconductor coupled to an LC-circuit

We show that the amplitude, or Higgs mode of a superconductor with strong spin-orbit coupling and an exchange field, couples linearly to the electromagnetic field. Furthermore, by coupling such a superconductor to an LC resonator, we demonstrate that the Higgs resonance becomes a regular mode at frequencies smaller than the quasiparticle energy threshold $2Δ$. We finally propose and discuss a possible experiment based on microwave spectroscopy for an unequivocal detection of the Higgs mode. Our approach may allow visualizing Higgs modes also in more complicated multiband superconductors with a coupling between the charge and other electronic degrees of freedom.

cond-mat.supr-con↗

arXiv2022

Emergence of large spin-charge interconversion at an oxidized Cu/W interface

Spin-orbitronic devices can integrate memory and logic by exploiting spin-charge interconversion (SCI), which is optimized by design and materials selection. In these devices, such as the magnetoelectric spin-orbit (MESO) logic, interfaces are crucial elements as they can prohibit or promote spin flow in a device as well as possess spin-orbit coupling resulting in interfacial SCI. Here, we study the origin of SCI in a Py/Cu/W lateral spin valve and quantify its efficiency. An exhaustive characterization of the interface between Cu and W electrodes uncovers the presence of an oxidized layer (WO$_x$). We determine that the SCI occurs at the Cu/WO$_x$ interface with a temperature-independent interfacial spin-loss conductance of $G_{||} \approx$ 20 $\times$ 10$^{13} Ω^{-1}m^{-2}$ and an interfacial spin-charge conductivity $σ_{SC}=-$1610 $Ω^{-1}cm^{-1}$ at 10 K ($-$830 $Ω^{-1}cm^{-1}$ at 300 K). This corresponds to an efficiency given by the inverse Edelstein length $λ_{IEE}=-$0.76 nm at 10 K ($-$0.4 nm at 300 K), which is remarkably larger than in metal/metal and metal/oxide interfaces and bulk heavy metals. The large SCI efficiency at such an oxidized interface is a promising candidate for the magnetic readout in MESO logic devices.

cond-mat.mes-hall↗

arXiv2022

Theory of the supercurrent diode effect in Rashba superconductors with arbitrary disorder

We calculate the non-reciprocal critical current and quantify the supercurrent diode effect in Rashba superconductors with arbitrary disorder, using the quasiclassical Eilenberger equation. The non-reciprocity is caused by the helical superconducting state, which appears when both inversion and time-reversal symmetries are broken. In the absence of disorder, we find a very strong diode effect, with the non-reciprocity exceeding 40% at optimal temperatures, magnetic fields and spin-orbit coupling. We establish that the effect persists even in the presence of strong disorder. We show that the sign of the diode effect changes as magnetic field and disorder are increased, reflecting the changes in the nature of the helical state.

cond-mat.supr-con↗

arXiv2022

Anomalous Andreev interferometer: Study of an anomalous Josephson junction coupled to a normal wire

Josephson junctions (JJs), where both time-reversal and inversion symmetry are broken, exhibit a phase shift $ϕ_0$ in their current-phase relation. This allows for an anomalous supercurrent to flow in the junction even in the absence of a phase bias between the superconductors. We show that a finite phase shift also manifests in the so-called Andreev interferometers - a device that consists of a mesoscopic conductor coupled to the $ϕ_0$-junction. Due to the proximity effect, the resistance of this conductor is phase-sensitive - it oscillates by varying the phase of the JJ. As a result, the quasiparticle current $I_{\mathrm{qp}}$ flowing through the conductor has an anomalous component, which exists only at finite $ϕ_0$. Thus, the Andreev interferometry could be used to probe the $ϕ_0$ effect. We consider two realizations of the $ϕ_0$-junction and calculate $I_{\mathrm{qp}}$ in the interferometer: a superconducting structure with spin-orbit coupling and a system of spin-split superconductors with spin-polarized tunneling barriers.

cond-mat.supr-con↗

arXiv2021

Current rectification in junctions with spin-split superconductors

Spin-split superconductors exhibit an electron-hole asymmetric spin-resolved density of states, but the symmetry is restored upon averaging over spin. On the other hand, asymmetry appears again in tunneling junctions of spin-split superconductors with a spin-polarized barrier. As demonstrated recently in both theory and experiment, this fact leads to a particularly strong thermoelectric effect in superconductor-ferromagnet structures. In this work we show another important effect stemming from the electron-hole asymmetry - current rectification. We calculate the charge current in spin-polarized tunnel junctions of normal metal and a spin-split superconductor with AC and DC voltage bias. In the DC case, the I-V curve is not fully antisymmetric and has a voltage-symmetric component due to spin polarization. This translates to the existence of a rectified current in the AC case, which is proportional to the spin polarization of the junction and strongly depends on the frequency of the applied bias. We discuss possible applications of the rectification effect, including a diode for superconducting electronics and radiation detectors. The analysis of the rectified charge current is supplemented by the discussion of heat current and relevant noise correlators, where electron-hole asymmetry also plays an important role, and which are useful for applications in detectors.

cond-mat.supr-con↗

arXiv2021

Coexistence of superconductivity and spin-splitting fields in superconductor/ferromagnetic insulator bilayers of arbitrary thickness

Ferromagnetic insulators can induce a strong exchange field in an adjacent superconductor via the magnetic proximity effect. This manifests as spin-splitting of the BCS density of states of the superconductor, an important ingredient for numerous superconducting spintronics applications and the realization of Majorana fermions. A crucial parameter that determines the magnitude of the induced spin-splitting in FI/S bilayers is the thickness of the S layer $d$: In very thin samples, the superconductivity is suppressed by the strong magnetism. By contrast, in very thick samples, the spin splitting is absent at distances away from the interface. In this work, we calculate the density of states and critical field of FI/S bilayers of arbitrary thickness. From here, we determine the range of parameters of interest for applications, where the exchange field and superconductivity coexist. We compare our theory with the tunneling spectroscopy measurements in several EuS/Al/AlO$_x$/Al samples. If the Al film in contact with the EuS is thinner than a certain critical value, we do not observe superconductivity, whereas, in thicker samples, we find evidence of a first-order phase transition induced by an external field. The complete transition is preceded by a regime in which normal and superconducting regions coexist. We attribute this mixed phase to inhomogeneities of the Al film thickness and the presence of superparamagnetic grains at the EuS/Al interface with different switching fields. Our results demonstrate on the one hand, the important role of the S layer thickness, which is particularly relevant for the fabrication of high-quality samples suitable for applications. On the other hand, the agreement between theory and experiment demonstrates the accuracy of our theory, which, originally developed for homogeneous situations, is generalized to highly inhomogeneous systems.

cond-mat.supr-con↗

arXiv2020

A Unified Description of Spin Transport, Weak Antilocalization and Triplet Superconductivity in Systems with Spin-Orbit Coupling

The Eilenberger equation is a standard tool in the description of superconductors with an arbitrary degree of disorder. It can be generalized to systems with linear-in-momentum spin-orbit coupling (SOC), by exploiting the analogy of SOC with a non-abelian background field. Such field mixes singlet and triplet components and yields the rich physics of magnetoelectric phenomena. In this work we show that the application of this equation extends further, beyond superconductivity. In the normal state, the linearized Eilenberger equation describes the coupled spin-charge dynamics. Moreover, its resolvent corresponds to the so called Cooperons, and can be used to calculate the weak localization corrections. Specifically, we show how to solve this equation for any source term and provide a closed-form solution for the case of Rashba SOC. We use this solution to address several problems of interest for spintronics and superconductivity. Firstly, we study spin injection from ferromagnetic electrodes in the normal state, and describe the spatial evolution of spin density in the sample, and the complete crossover from the diffusive to the ballistic limit. Secondly, we address the so-called superconducting Edelstein effect, and generalize the previously known results to arbitrary disorder. Thirdly, we study weak localization correction beyond the diffusive limit, which can be a valuable tool in experimental characterization of materials with very strong SOC. We also address the so-called pure gauge case where the persistent spin helices form. Our work establishes the linearized Eilenberger equation as a powerful and a very versatile method for the study of materials with spin-orbit coupling, which often provides a simpler and more intuitive picture compared to alternative methods.

cond-mat.mes-hall↗