arXiv subjects

Danilo Nikolić

Publications and source records attributed to Danilo Nikolić.

13 recordsLinked to original sources

arXiv2026

Anomalous Superfluid Response in Altermagnetic Superconductors

We report on the emergence of the anomalous (paramagnetic) superfluid response in altermagnetic superconductors at arbitrary impurity concentrations. Due to anisotropic gapless superconductivity, altermagnetic superconductors with an out-of-plane Zeeman field display an anisotropic paramagnetic Meissner effect. The effect is strongest for parallel altermagnetic and Zeeman exchange field vectors and in the clean sample. The presence of nonmagnetic impurities leads to isotropisation and, consequently, weakens the effect; however, the paramagnetic response sustains intermediate amounts of impurities in the system. As demonstrated in recent experiments, microwave superfluid stiffness measurements can serve as a sensitive probe of gapless superconductivity.

cond-mat.supr-con↗

arXiv2026

Spin-polarized supercurrents and Josephson diode effect in altermagnets

We present a systematic theoretical study of the Josephson effect in junctions consisting of a d-wave altermagnet (AM) placed between two BCS superconductors (SC). In general, the SC/AM interfaces are spin-active and modeled by spin-dependent $δ$ potentials, allowing for an arbitrary direction of the local exchange field vector. The model is formulated within the fully quantum (Gor'kov) and quasiclassical (Eilenberger) Green's function technique, applied to two distinct cases of (i) a weakly spin-polarized AM (exchange field much smaller compared to the Fermi energy) and (ii) a strongly spin-polarized AM (exchange field comparable to the Fermi energy). We apply our model to the SC/AM/SC geometry, accounting for the Josephson current-phase relation (CPR). In the weakly spin-polarized regime, the CPR displays the normal Josephson effect. Irrespective of the orientation of the altermagnet, the junction undergoes the $0-π$ transition. Depending on the orientation, the system displays the features similar to those of a ferromagnetic or an antiferromagnetic junction. To investigate the spin-polarized currents and nonreciprocal transport as the central results of the present work, we put the main focus on the strongly spin-polarized regime. Within this regime, we distinguish two cases. A coplanar exchange field profile across the junctions displays the normal Josephson effect; however, with a pure and stable long-range second harmonic in the CPR. In contrast, a noncoplanar exchange field profile gives rise to the so-called quantum geometric phases across the junction, leading to the absence of the phase-inversion center in the Josephson CPR. As a result, a Josephson diode effect emerges with a significant charge diode efficiency larger than 30% and a perfect spin diode efficiency of 100%.

cond-mat.supr-con↗

arXiv2025

Necessary conditions for spin-resolved Josephson diode effect across strongly spin-polarized magnetic materials

We present a set of necessary conditions for the appearance of charge and spin Josephson diode effects across strongly spin-polarized inhomogeneous magnetic materials (FM) placed between two spin-singlet superconductors. Noncoplanarity of the FM's spin texture gives rise to quantum geometric phases, $Δφ'$, that enter the Josephson current-phase relation (CPR) similarly to the superconducting phase difference, resulting in charge and spin Josephson diode effects. Our study shows that such effects appear if the CPR possesses no phase-inversion center, achieved under the following conditions. First, noncoplanarity of the spin texture is necessary to break the spatial inversion symmetry. Second, both spin bands have to contribute to the transport, i.e., the effect is absent in half-metallic junctions. Third, different band-specific densities of states are required, and this condition is ensured by the strong spin polarization of the FM. Finally, higher harmonics in the CPR are necessary, i.e., the effect is absent in the tunneling limit. However, even in this case, the CPR must not have a phase-inversion center, which is ensured by the restriction of the quantum geometric phase to values $Δφ'\neq kπ/2, k\in\mathbb{Z}$. We formulate a minimal phenomenological model that incorporates all these points, qualitatively illustrating our theory.

cond-mat.supr-con↗

arXiv2025

Purely even harmonic Josephson current due to crossed pair transmission across strongly spin-polarized materials

We revisit the problem of the second harmonic generation in the current-phase relation across ferromagnetic bilayers placed between BCS superconductors. In particular, we consider a strongly spin-polarized metallic ferromagnet coupled to two superconducting leads via thin spin-active (left) and non-spin-active (right) insulating layers. The system is examined in the framework of the quasiclassical Green$^\prime$s function formalism, both in the ballistic (Eilenberger) and the diffusive (Usadel) limit. Strong spin polarization allows for neglecting short-range mixed-spin correlations, and the Josephson supercurrent across the ferromagnet is fully mediated by long-range equal-spin triplet correlations. Using a diagrammatic technique for ballistic propagators introduced in Refs. [1-3], we describe the relevant Andreev processes responsible for the effective conversion of two spin-singlet Cooper pairs in the superconductor into two $\uparrow\uparrow$ and $\downarrow\downarrow$ pairs in the ferromagnet. Contrary to the naive picture of direct conversion, we show that the lowest order process involves four Cooper pairs in the superconductor, among which three are incoming, and one is outgoing, giving rise to net charge transport of 4e across the non-spin-active interface. The self-consistent numerical treatment of the diffusive junction, typically more relevant in experiments, confirms this picture quantitatively.

cond-mat.supr-con↗

arXiv2025

Theory of quantum-geometric charge and spin Josephson diode effects in strongly spin-polarized hybrid structures with noncoplanar spin textures

We present a systematic study of the spin-resolved Josephson diode effect (JDE) in strongly spin-polarized ferromagnets (sFM) coupled to singlet superconductors (SC) via ferromagnetic insulating interfaces (FI). All metallic parts are described in the framework of the quasiclassical Usadel Green's function theory applicable to diffusive systems. The interfaces are characterized by an S-matrix obtained for a model potential with exchange vectors pointing in an arbitrary direction with respect to the magnetization in the sFM. Our theory predicts a large charge Josephson diode effect with an efficiency exceeding $33\%$ and a perfect spin diode effect with $100\%$ efficiency. To achieve these the following conditions are necessary: (i) a noncoplanar profile of the three magnetization vectors in the system and (ii) different densities of states of spin-$\uparrow$ and spin-$\downarrow$ bands in the sFM achieved by a strong spin polarization. The former gives rise to the quantum-geometric phase, $Δφ$, that enters the theory in a very similar manner as the superconducting phase difference across the junction, $Δχ$. We perform a harmonic analysis of the Josephson current in both variables and find symmetries between Fourier coefficients allowing an interpretation in terms of transfer processes of multiple equal-spin Cooper pairs across the two ferromagnetic spin bands. We point out the importance of crossed pair transmission processes. Finally, we study a spin-switching effect of an equal-spin supercurrent by reversing the magnetic flux in a SQUID device incorporating the mentioned junction and propose a way for measuring it.

cond-mat.supr-con↗

arXiv2025

Quantum-geometric spin and charge Josephson diode effects

We present a general mechanism for large charge and spin Josephson diode effects in strongly spin-polarized superconductor-ferromagnet hybrid structures with a noncoplanar spin texture, formulated in terms of quantum-geometric phases. We present necessary conditions for this effect to occur, and show numerical results for disordered materials, relevant for applications. We calculate Josephson diode efficiencies for both charge- and spin-diodes and show that a spin-diode efficiency of 100% can be reached. Finally, we present a SQUID device that can switch between nearly pure spin-up and spin-down equal-spin supercurrents across the ferromagnet by reversing the flux. These findings establish functionalities that are absent for coplanar spin textures.

cond-mat.supr-con↗

arXiv2025

Spin-resolved Josephson diode effect through strongly spin-polarized conical magnets

We present a theoretical study of the spin-resolved Josephson diode effect in junctions comprising strongly spin-polarized conical magnets (FM) coupled to singlet superconductors (SC). The system is treated by making use of the Gor$^\prime$kov and quasiclassical Green$^\prime$s function methods. Modeling the SC/FM interfaces as spin-dependent $δ$-potentials, we apply our model to an SC/FM/SC junction and account for the Josephson current-phase relation (CPR). The nontrivial coupling between the spin bands in the conical magnet gives rise to a strong Josephson diode effect with an efficiency greater than 40%. The effect essentially depends on the quantum spin-geometric phase that enters the Josephson CPR in a very similar manner to the superconducting phase difference. The former is generated non-locally by the intrinsically noncoplanar spin arrangement of the conical magnet, which breaks the time-reversal and inversion symmetries. Strong spin polarization and a helical pitch of the conical magnet comparable to the superconducting coherence length are essential for the effect. We perform a harmonic analysis of the Josephson CPR and interpret the effect in terms of coherent transfer of multiple equal-spin triplet Cooper pairs across the conical magnet.

cond-mat.supr-con↗

arXiv2024

Signature of long-ranged spin triplets across a two-dimensional superconductor/helimagnet van der Waals interface

The combination of a superconductor with a magnetically inhomogeneous material has been established as an efficient mechanism for the generation of long-ranged spin-polarized (spin-triplet) Cooper pairs. Evidence for this mechanism, however, has been established based on studies done on three-dimensional systems, where the strong bonds existing at the interface between the superconductor and the magnetic material should in principle enhance proximity effects and strengthen any electronic correlations. Here, we fabricate devices based on van der Waals stacks of flakes of the two-dimensional superconductor $NbS_2$ combined with flakes of $Cr_{1/3}NbS_2$, which has a built-in magnetic inhomogeneity due to its helimagnetic spin texture at low temperatures. We find that the critical temperature of these vdW bilayers is strongly dependent on the magnetic state of $Cr_{1/3}NbS_2$, whose degree of magnetic inhomogeneity can be controlled via an applied magnetic field. Our results demonstrate evidence for the generation of long-ranged spin-triplet pairs across the $Cr_{1/3}NbS_2$/$NbS_2$ vdW interface.

cond-mat.supr-con↗

arXiv2023

Microscopic theory of supercurrent suppression by gate-controlled surface depairing

Recently gate-mediated supercurrent suppression in superconducting nano-bridges has been reported in many experiments. This could be either a direct or an indirect gate effect. The microscopic understanding of this observation is not clear till now. Using the quasiclassical Green's function method, we show that a small concentration of magnetic impurities at the surface of the bridges can significantly help to suppress superconductivity and hence the supercurrent inside the systems while applying a gate field. This is because the gate field can enhance the depairing through the exchange interaction between the magnetic impurities at the surface and the superconductor. We also obtain a \emph{symmetric} suppression of the supercurrent with respect to the gate field, a signature of a direct gate effect. Future experiments can verify our predictions by modifying the surface with magnetic impurities.

cond-mat.mes-hall↗

arXiv2023

DC Josephson effect between two Yu-Shiba-Rusinov bound states

Motivated by recent experiments [Nat. Phys. $\textbf{16}$, 1227 (2020)], we present here a theoretical study of the DC Josephson effect in a system comprising two magnetic impurities coupled to their respective superconducting electrodes and which exhibit Yu-Shiba-Rusinov (YSR) states. We make use of a mean-field Anderson model with broken spin symmetry to compute the supercurrent in this system for an arbitrary range of parameters (coupling between the impurities, orientation of the impurity spins, etc.). We predict a variety of physical phenomena such as (i) the occurrence of multiple $0-π$ transitions in the regime of weak coupling that can be induced by changing the energy of the YSR states or the temperature; (ii) the critical current strongly depends on the relative orientation of the impurity spins and it is maximized when the spins are either parallel or antiparallel, depending on the ground state of the impurities; and (iii) upon increasing the coupling between impurities, triplet superconductivity is generated in the system and it is manifested in a highly nonsinusoidal current-phase relation. In principle, these predictions can be tested experimentally with the existing realization of this system and the main lessons of this work are of great relevance for the field of superconducting spintronics.

cond-mat.supr-con↗

arXiv2023

Optimized proximity thermometer for ultrasensitive detection: Role of an ohmic electromagnetic environment

We propose a mesoscopic thermometer for ultrasensitive detection based on the proximity effect in superconductor-normal metal (SN) heterostructures. The device is based on the zero-bias anomaly due to the inelastic Cooper pair tunneling in an SNIS junction (I stands for an insulator) coupled to an ohmic electromagnetic (EM) environment. The theoretical model is done in the framework of the quasiclassical Usadel Green's formalism and the dynamical Coulomb blockade. The usage of an ohmic EM environment makes the thermometer highly sensitive down to very low temperatures, $T \lesssim 5~$mK. Moreover, defined in this way, the thermometer is stable against small but nonvanishing voltage amplitudes typically used for measuring the zero-bias differential conductance in experiments. Finally, we propose a simplified view, based on an analytic treatment, which is in very good agreement with numerical results and can serve as a tool for the development, calibration, and optimization of such devices in future experiments in quantum calorimetry.

cond-mat.mes-hall↗

arXiv2022

Interference phenomena in Josephson junctions with ferromagnetic bilayers: Spin-triplet correlations and resonances

We study the Josephson effect in planar $SF_1F_2S$ junctions that consist of conventional $s$-wave superconductors ($S$) connected by two metallic monodomain ferromagnets ($F_1$ and $F_2$) with arbitrary transparency of interfaces. We solve the scattering problem in the clean limit based on the Bogoliubov-de Gennes equation for both spin-singlet and odd in frequency spin-triplet pairing correlations. We calculate numerically the Josephson current-phase relation $I(ϕ)$. While the first harmonic of $I(ϕ)$ is completely generated by spin-singlet and short-range spin-triplet superconducting correlations, for noncollinear magnetizations of ferromagnetic layers the second harmonic has an additional long-range spin-triplet component. Therefore, for strong ferromagnetic influence, the long-range spin-triplet contribution to the second harmonic dominates. We find an exception due to the geometric resonance for equal ferromagnetic layers when the first harmonic is strongly enhanced. Both first and second harmonic amplitudes oscillate with ferromagnetic layer thicknesses due to $0-π$ transitions. We study the influence of interface transparencies and find additional resonances for finite transparency of interface between ferromagnetic layers.

cond-mat.supr-con↗

arXiv2020

Electron cooling by phonons in superconducting proximity structures

We investigate the electron-phonon cooling power in disordered electronic systems with a special focus on mesoscopic superconducting proximity structures. Employing the quasiclassical Keldysh Green's function method, we obtain a general expression for the cooling power perturbative in the electron-phonon coupling, but valid for arbitrary electronic systems out of equilibrium. We apply our theory to several disordered electronic systems valid for an arbitrary relation between the thermal phonon wavelength and the electronic mean free path due to impurity scattering. Besides recovering the known results for bulk normal metals and BCS superconductors, we consider two experimentally relevant geometries of superconductor-normal metal proximity contacts. Both structures feature a significantly suppressed cooling power at low temperatures related to the existence of a minigap in the quasiparticle spectrum. This improved isolation low cooling feature in combination with the high tunability makes such structures highly promising candidates for quantum calorimetry

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