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Pavol Szabó

Publications and source records attributed to Pavol Szabó.

10 recordsLinked to original sources
arXiv2026

Multigap superconductivity in Ising superconductors: The case of (LaSe)1.14(NbSe2)m misfit layer compounds

Strong spin-orbit coupling and broken inversion symmetry in transition metal dichalcogenides give rise to Ising superconductivity, a spin-protected pairing state first identified in monolayer NbSe$_2$ through in-plane critical fields far exceeding the Pauli limit. More recently, Ising superconductivity has been proposed as a potential route to unconventional and even topological superconductivity in bulk misfit compounds. Here, we investigate the superconducting order parameter of layered misfit compounds composed of alternating transition metal dichalcogenide and rocksalt layers, which host extremely doped, electronically decoupled NbSe$_2$ sheets within a three-dimensional crystal. Using directional scanning tunneling spectroscopy on the misfit superconductors (LaSe)$_{1.14}$(NbSe$_2$) and (LaSe)$_{1.14}$(NbSe$_2$)$_2$, we uncover a strongly anisotropic multigap superconducting state: a fragile gap on the $Γ$-centered Fermi-surface pocket coexists with a robust, intrinsic gap on the K and K$'$ pockets. These features are in quantitative agreement with momentum-resolved gaps $Δ(\mathbf{k})$ obtained from anisotropic Migdal-Eliashberg calculations. The marked fragility of the $Γ$-centered gap, combined with the strong sensitivity of the critical temperature to non-magnetic disorder, points to pairing beyond conventional $s$-wave symmetry, potentially involving a topological order parameter. These results establish NbSe$_2$-based misfit compounds as a tunable bulk platform for multigap, unconventional superconductivity, with Ising protection offering a promising route toward topological pairing.

cond-mat.supr-con↗
arXiv2026

Strong-coupling multigap superconductivity in the Heusler compound ScAu$_2$Al

ScAu$_2$Al is the Heusler superconductor with the highest reported transition temperature, Tc, close to 5 K. Here, we combine high-resolution ac calorimetry and electrical-transport measurements with first-principles-based Eliashberg calculations. The large specific-heat jump, $ΔC/{γ_nT_c} =$ 2.2, well above the weak-coupling BCS value of 1.43, together with an electron-phonon coupling constant $λ = $1.1, establishes ScAu2Al as a strong-coupling superconductor. The electronic specific heat is described better by a two-gap α model, with $2Δ_S/k_BT_c = $ 4.1 and $2Δ_L/k_BT_c = $ 4.8, than by a single-gap model. The thermodynamic upper critical field follows a conventional WHH-like temperature dependence with $B_{c2}(0) = 0.18$ T, whereas the resistively determined critical field reaches values about three times larger and exhibits a pronounced positive curvature. We attribute the enhanced resistive field scale to superconductivity in disordered grain-boundary and interfacial regions. The experimental results are supported by calculations of the heat capacity and upper critical field within the Eliashberg formalism using Fermi-surface and electron-phonon parameters obtained from first-principles calculations

cond-mat.supr-con↗
arXiv2026

Possible Proximity to Ferromagnetism in the V$_2$Ga$_5$ Superconductor

Superconductivity and ferromagnetism are generally competing ground states in $d$-electron systems, making their interplay of fundamental interest. We report a comprehensive study of high-quality single- and polycrystalline V$_2$Ga$_5$, a bulk type-II superconductor ($T_c = 3.54 \ K$) with a quasi-one-dimensional crystal structure, supplemented with density functional theory (DFT) calculations, suggesting possible proximity to ferromagnetic order. Below $T \approx 10 \ K$, magnetic susceptibility shows ZFC/FC splitting, along with saturation and hysteresis in $M(H)$. Moreover, electrical transport measurements reveal a magnetic-field-dependent resistivity upturn, while specific heat is enhanced in magnetic fields. DFT calculations show that the Fermi level in V$_2$Ga$_5$ is located at a peak in the density of states, with a small magnetic moment per unit cell comparable to the experimental value. Together, these results indicate the possibility that ferromagnetic correlations develop below $T \approx 10 \ K$, well above $T_c$, with long-range ferromagnetic order suppressed by the superconducting transition.

cond-mat.supr-con↗
arXiv2025

Direct evidence for two-gap superconductivity in hydrogen-intercalated titanium diselenide

Transition-metal dichalcogenides (TMDs) offer an extremely rich material platform in the exploration of unconventional superconductivity. The unconventional aspects include exotic coupling mechanisms such as the Ising pairing, a complex interplay with other electronic orders such as charge-density waves (CDWs), symmetry-breaking and topological effects, and non-trivial gap structures such as multi-gap and possible nodal phases. Among TMDs, titanium diselenide (1$T$-TiSe$_2$) is one of the most studied and debated cases. Hints to an anomalous structure of its superconducting order parameter have emerged over the years, possibly linked to its spatial texturing in real and reciprocal space due to the presence of a 2$\times$2$\times$2 CDW phase, or to a pressure-driven multi-band Fermi surface. However, a direct evidence for a non-trivial structure of the superconducting gap in this material is still lacking. In this work, we bring the first evidence for a two-gap structure in the recently-discovered H-intercalated TiSe$_2$ superconductor (with T$_c \simeq 3.6$ K) by an extensive experimental study that combines magnetotransport measurements, point-contact spectroscopy and scanning tunnel spectroscopy. We show that the temperature dependence of the upper critical field (for $\vec B \parallel c$) as well as the shape of the point-contact and tunneling spectra strongly suggest the existence of two distinct superconducting gaps, and can indeed all be fitted in a self-consistent way with the same gap amplitudes $Δ_1 = 0.26 \pm 0.12$ meV and $Δ_2 =0.62 \pm 0.18$ meV.

cond-mat.supr-con↗
arXiv2023

Suspended dry pick-up and flip-over assembly for van der Waals heterostructures with ultra-clean surfaces

Van der Waals heterostructures are an excellent platform for studying intriguing interface phenomena, such as moiré and proximity effects. Surface science techniques like scanning tunneling microscopy (STM) have proven a powerful tool to study such heterostructures but have so far been hampered because of their high sensitivity to surface contamination. Here, we report a dry polymer-based assembly technique to fabricate van der Waals heterostructures with atomically clean surfaces. The key features of our suspended dry pick-up and flip-over technique are 1) the heterostructure surface never comes into contact with polymers, 2) it is entirely solvent-free, 3) it is entirely performed in a glovebox, and 4) it only requires temperatures below 130$^{\circ}$. By performing ambient atomic force microscopy and atomically-resolved scanning tunneling microscopy on example heterostructures, we demonstrate that we can fabricate air-sensitive heterostructures with ultra-clean interfaces and surfaces. Due to the lack of polymer melting, the technique is further compatible with heterostructure assembly under ultra-high vacuum conditions, which promises ultimate heterostructure quality.

cond-mat.mes-hall↗
arXiv2023

One-dimensional topological superconductivity in a van der Waals heterostructure

One-dimensional (1D) topological superconductivity is a state of matter that is not found in nature. However, it can be realised, for example, by inducing superconductivity into the quantum spin Hall edge state of a two-dimensional topological insulator. Because topological superconductors are proposed to host Majorana zero modes, they have been suggested as a platform for topological quantum computing. Yet, conclusive proof of 1D topological superconductivity has remained elusive. Here, we employ low-temperature scanning tunnelling microscopy to show 1D topological superconductivity in a van der Waals heterostructure by directly probing its superconducting properties, instead of relying on the observation of Majorana zero modes at its boundary. We realise this by placing the two-dimensional topological insulator monolayer WTe$_2$ on the superconductor NbSe$_2$. We find that the superconducting topological edge state is robust against magnetic fields, a hallmark of its triplet pairing. Its topological protection is underpinned by a lateral self-proximity effect, which is resilient against disorder in the monolayer edge. By creating this exotic state in a van der Waals heterostructure, we provide an adaptable platform for the future realization of Majorana bound states. Finally, our results more generally demonstrate the power of Abrikosov vortices as effective experimental probes for superconductivity in nanostructures.

cond-mat.mes-hall↗
arXiv2023

Protection of Ising spin-orbit coupling in bulk misfit superconductors

Low-dimensional materials have remarkable properties that are distinct from their bulk counterparts. A paradigmatic example is Ising superconductivity that occurs in monolayer materials such as NbSe2 which show a strong violation of the Pauli limit. In monolayers, this occurs due to a combination of broken inversion symmetry and spin-orbit coupling that locks the spins of the electrons out-of-plane. Bulk NbSe2 is centrosymmetric and is therefore not an Ising superconductor. We show that bulk misfit compound superconductors, (LaSe)1.14(NbSe2) and (LaSe)1.14(NbSe2)2, comprised of monolayers and bilayers of NbSe2, exhibit unexpected Ising protection with a Pauli-limit violation comparable to monolayer NbSe2, despite formally having inversion symmetry. We study these misfit compounds using complementary experimental methods in combination with first-principles calculations. We propose theoretical mechanisms of how the Ising protection can survive in bulk materials. We show how some of these mechanisms operate in these bulk compounds due to a concerted effect of charge-transfer, defects, reduction of interlayer hopping, and stacking. This highlights how Ising superconductivity can, unexpectedly, arise in bulk materials, and possibly enable the design of bulk superconductors that are resilient to magnetic fields.

cond-mat.supr-con↗
arXiv2020

Yu-Shiba-Rusinov bands in ferromagnetic superconducting diamond

The combination of different exotic properties in materials paves the way for the emergence of their new potential applications. An example is the recently found coexistence of the mutually antagonistic ferromagnetism and superconductivity in hydrogenated boron-doped diamond, which promises to be an attractive system with which to explore unconventional physics. Here, we show the emergence of Yu-Shiba-Rusinov (YSR) bands with a spatial extent of tens of nanometers in ferromagnetic superconducting diamond using scanning tunneling spectroscopy. We demonstrate theoretically how a two-dimensional (2D) spin lattice at the surface of a three-dimensional (3D) superconductor gives rise to the YSR bands, and how their density-of-states profile correlates with the spin lattice structure. The established strategy to realize new forms of the coexistence of ferromagnetism and superconductivity opens a way to engineer the unusual electronic states and also to design better performing superconducting devices.

cond-mat.supr-con↗
arXiv2017

Novel graphene/Sn and graphene/SnOx hybrid nanostructures: Induced superconductivity and band gaps revealed by scanning probe measurements

The development of functional composite nanomaterials based on graphene and metal nanoparticles (NPs) is currently the subject of intense research interest. In this study we report the preparation of novel type of graphene/Sn and graphene/SnOx (1 < x < 2) hybrid nanostructures and their investigation by scanning probe methods. First, we prepare Sn NPs by evaporating 7 - 8 nm tin on highly oriented pyrolytic graphite substrates. Graphene/Sn nanostructures are obtained by transferring graphene on top of the tin NPs immediately after evaporation. We show by scanning tunnelling microscopy (STM) and spectroscopy (STS) that tin NPs reduce significantly the environmental p-type doping of graphene. Furthermore, we demonstrate by low-temperature STM and STS measurements that superconductivity is induced in graphene, either directly supported by Sn NPs or suspended between them. Additionally, we prepare SnOx NPs by annealing the evaporated tin at 500 ${^o}$C. STS measurements performed on hybrid graphene/SnOx nanostructures reveal the electronic band gap of SnOx NPs. The results can open new avenues for the fabrication of novel hybrid superconducting nanomaterials with designed structures and morphologies.

cond-mat.mes-hall↗