arXiv subjects

Balázs Újfalussy

Publications and source records attributed to Balázs Újfalussy.

10 recordsLinked to original sources

arXiv2026

Influence of Magnetic Order on Proximity-Induced Superconductivity in Mn Layers on Nb(110) from First Principles

We investigate the influence of magnetic order on the proximity-induced superconducting state in the Mn layers of a Mn-Nb(110) heterostructure by using a first-principles method. For this study, we use the recently developed Bogoliubov-de Gennes (BdG) solver for superconducting heterostructures [Csire et al., Phys. Rev. B 97, 024514 (2018)] within the first-principles calculations based on multiple scattering theory and the screened Korringa-Kohn-Rostoker (SKKR) Green's function method. In our calculations, we first study the normal-state density of states (DOS) in the single- and double-Mn-layer heterostructures, and calculate the induced magnetic moments in the Nb layers. Next, we compute the momentum-resolved spectral functions in the superconducting state for the heterostructure with a single Mn layer, and find bands crossing the Fermi level within the superconducting (SC) gap. We also study the SC state DOS in the single- and double-Mn-layer heterostructures and compare some of our results with experimental findings, revealing secondary gaps, plateau-like regions, and central V-shaped in-gap states within the bulk SC Nb gap that are magnetic-order-dependent. Finally, we compute the singlet and internally antisymmetric triplet (IAT) order parameters for each layer for both heterostructures, and find an order of magnitude difference in the induced singlet part of the SC order parameter in the Mn layer/s between the FM and AFM cases in favor of the AFM pairing with the maximum still being only 4.44% of the bulk Nb singlet order parameter value. We also find a negligible induced triplet part, yet comparable to the induced singlet values, indicating some singlet-triplet mixing in the Mn layer/s.

cond-mat.supr-con↗

arXiv2025

Coexistence of topologically trivial and non-trivial Yu-Shiba-Rusinov bands in magnetic atomic chains on a superconductor

Majorana zero modes (MZMs) have been proposed as a promising basis for Majorana qubits offering great potential for topological quantum computation. Such modes may form at the ends of a magnetic atomic chain on a superconductor. Typically only a single MZM may be present at one end of the chain, but symmetry may protect multiple MZMs at the same end. Here, we study the topological properties of Yu-Shiba-Rusinov (YSR) bands of excitations in Mn chains constructed on a Nb(110) and on a Ta(110) substrate using first-principles calculations and scanning tunneling microscopy and spectroscopy experiments. We demonstrate that even and odd YSR states with respect to mirroring on the symmetry plane containing the chain have different dispersions, and both of them may give rise to MZMs separately. Although the spin-orbit coupling leads to a hybridization between the bands, multiple MZMs may still exist due to the mirror symmetry. These findings highlight the influence of symmetries on interpreting the spectroscopic signatures of candidates for MZMs.

cond-mat.supr-con↗

arXiv2023

Topological superconductivity from first-principles I: Shiba band structure and topological edge states of artificial spin chains

Magnetic chains on superconductors hosting Majorna Zero Modes (MZMs) attracted high interest due to their possible applications in fault-tolerant quantum computing. However, this is hindered by the lack of a detailed, quantitative understanding of these systems. As a significant step forward, we present a first-principles computational approach based on a microscopic relativistic theory of inhomogeneous superconductors applied to an iron chain on the top of Au-covered Nb(110) to study the Shiba band structure and the topological nature of the edge states. Contrary to contemporary considerations, our method enables the introduction of quantities indicating band inversion without fitting parameters in realistic experimental settings, holding thus the power to determine the topological nature of zero energy edge states in an accurate ab-initio based description of the experimental systems. We confirm that ferromagnetic Fe chains on Au/Nb(110) surface do not support any separated MZM; however, a broad range of spin-spirals can be identified with robust zero energy edge states displaying signatures of MZMs. For these spirals, we explore the structure of the superconducting order parameter shedding light on the internally antisymmetric triplet pairing hosted by MZMs. We also reveal a two-fold effect of spin-orbit coupling: although it tends to enlarge the topological phase regarding spin spiraling angles, however, it also extends the localization of MZMs. Due to the presented predictive power, our work fills a big gap between the experimental efforts and theoretical models while paving the way for engineering platforms for topological quantum computation.

cond-mat.supr-con↗

arXiv2023

Topological superconductivity from first-principles II: Effects from manipulation of spin spirals $-$ Topological fragmentation, braiding, and Quasi-Majorana Bound States

Recent advances in electron spin resonance techniques have allowed the manipulation of the spin of individual atoms, making magnetic atomic chains on superconducting hosts one of the most promising platform where topological superconductivity can be engineered. Motivated by this progress, we provide a detailed, quantitative description of the effects of manipulating spins in realistic nanowires by applying a first-principles-based computational approach to a recent experiment: an iron chain deposited on top of Au/Nb heterostructure. As a continuation of the first part of the paper experimentally relevant computational experiments are performed in spin spiral chains that shed light on several concerns about practical applications and add new aspects to the interpretation of recent experiments. We explore the stability of topological zero energy states, the formation and distinction of topologically trivial and non-trivial zero energy edge states, the effect of local changes in the exchange fields, the emergence of topological fragmentation, and the shift of Majorana Zero Modes along the superconducting nanowires opening avenues toward the implementation of a braiding operation.

cond-mat.supr-con↗

arXiv2021

Magnetically-textured superconductivity in elemental Rhenium

Recent $μ$SR measurements revealed remarkable signatures of spontaneous magnetism coexisting with superconductivity in elemental rhenium. Here we provide a quantitative theory that uncovers the nature of the superconducting instability by incorporating every details of the electronic structure together with spin-orbit coupling and multi-orbital physics. We show that conventional $s$-wave superconductivity combined with strong spin-orbit coupling is inducing even-parity odd-orbital spin triplet Cooper pairs, and in presence of a screw axis Cooper pairs' migration between the induced equal-spin triplet component leads to an exotic magnetic state.

cond-mat.supr-con↗

arXiv2020

Real-space multiple scattering theory for superconductors with impurities

We implement the Bogoliubov-de Gennes (BdG) equation in real-space using the screened Korringa-Kohn-Rostoker (KKR) method. This allows us to solve, self-consistently, the superconducting state for 3d crystals including substitutional impurities with a full normal-state DFT band structure. We apply the theoretical framework to bulk Nb with impurities. Without impurities, Nb has an anisotropic gap structure with two distinct peaks around the Fermi level. In the presence of non-magnetic impurities those peaks are broadened due to the scattering between the two bulk superconducting gaps, however the peaks remain separated. As a second example of self-consistent real-space solutions of the BdG equations we examine superconducting clusters embedded within a non-superconducting bulk metallic host. This allows us to estimate the coherence length of the superconductor and we show that, within our framework, the coherence length of the superconductor is related to the inverse of the gap size, just as in bulk BCS theory.

cond-mat.supr-con↗

arXiv2020

Quantitative Theory of Triplet Pairing in the Unconventional Superconductor LaNiGa$_2$

The exceptionally low-symmetry crystal structures of the time-reversal symmetry breaking superconductors LaNiC$_2$ and LaNiGa$_2$ lead to an internally-antisymmetric non-unitary triplet (INT) state as the only possibility compatible with experiments. We argue that this state has a distinct signature: a double-peak structure in the Density of States (DOS) which resolves in the spin channel in a particular way. We construct a detailed model of LaNiGa$_2$ capturing its electronic band structure and magnetic properties ab initio. The pairing mechanism is described via a single adjustable parameter. The latter is fixed by the critical temperature $T_c$ allowing parameter-free predictions. We compute the electronic specific heat and find excellent agreement with experiment. The size of the ordered moment in the superconducting state is compatible with zero-field muon spin relaxation experiments and the predicted spin-resolved DOS suggests the spin-splitting is within the reach of present experimental technology.

cond-mat.supr-con↗

arXiv2019

Gap Anisotropy in Multiband Superconductors Based on Multiple Scattering Theory

We implement the Bogoliubov-de Gennes (BdG) equation in a screened Korringa-Kohn-Rostoker (KKR) method for solving, self-consistently, the superconducting state for 3d crystals. This method combines the full complexity of the underlying electronic structure and Fermi surface geometry with a simple phenomenological parametrisation for the superconductivity. We apply this theoretical framework to the known s-wave superconductors Nb, Pb, and MgB$_2$. In these materials multiple distinct peaks at the gap in the density of states were observed, showing significant gap anisotropy which is in good agreement with experiment. Qualitatively, the results can be explained in terms of the k-dependent Fermi velocities on the Fermi surface sheets exploiting concepts from BCS theory.

cond-mat.supr-con↗

arXiv2016

First-principles approach to thin superconducting slabs and heterostructures

We present a fully first-principles method for superconducting thin films. The layer dependent phonon spectrum is calculated to determine the layer dependence of the electron-phonon coupling for such systems, which is coupled to the Kohn-Sham-Bogoliubov-de Gennes equations, and it is solved in a parameter free way. The theory is then applied to different surface facets of niobium slabs and to niobium-gold heterostructures. We investigate the dependence of the transition temperature on the thickness of the slabs and the inverse proximity effect observed in thin superconducting heterostructures.

cond-mat.supr-con↗

arXiv2015

Multiple scattering theory for superconducting heterostructures

We generalize the screened Korringa-Kohn-Rostoker (SKKR) method for solving the corresponding Kohn-Sham-Bogoliubov-de Gennes (KSBdG) equations for surfaces and interfaces. As an application of the newly developed theory we study the quasiparticle spectrum of Au overlayers on a Nb(100) host. We find that, within the superconducting gap region, the quasiparticle spectrum consists of Andreev bound states (ABS) with a dispersion which is closely connected to the underlying electronic structure of the overlayer. We also find that the spectrum has a strongly k-dependent induced gap. The properties of the gap is discussed in relation to the thickness of the overlayer, and it is shown that certain states do not participate in the Andreev scattering process.

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