arXiv subjects

Jozef Kačmarčík

Publications and source records attributed to Jozef Kačmarčík.

7 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

AC calorimetric study of magneto-quantum oscillations in anisotropic multiband V$_2$Ga$_5$ superconductor

Unlike de Haas--van Alphen measurements, heat-capacity magneto-quantum oscillations directly probe the oscillatory bulk quasiparticle density of states. Here, we report the observation of MQOs in V$_2$Ga$_5$ single crystals studied via highly sensitive ac calorimetry. The strongest MQO signal is observed for a magnetic field applied along the vanadium chains, in excellent agreement with de Haas--van Alphen magnetization data. A single dominant frequency of 126.6 T resolved by fast Fourier transform confirms the true bulk origin of the elliptical $γ$ Fermi-surface pocket located near the Z point of the Brillouin zone. The angular dependence of the FFT frequency closely tracks the anisotropy of the $γ$ pocket, as supported by first-principles calculations. Analysis of the temperature- and field-dependent MQO amplitudes allows the precise determination of the effective cyclotron mass, Dingle temperature, quantum relaxation time, carrier mobility, and electron mean free path. Furthermore, we demonstrate that the net Berry flux is invariant with respect to the magnetic-field orientation, as a consequence of a conserved hybridization phase twist within the $γ$ pocket. These findings establish ac calorimetry as a powerful macroscopic probe of topological orbital hybridization in complex intermetallics.

cond-mat.supr-con↗

arXiv2026

Band-Selective Tunneling and Anisotropic Multiband Superconductivity in V$_2$Ga$_5$

Multiband superconductors with structural anisotropy offer a fertile ground for exploring unconventional quantum states, yet disentangling their directional pairing characteristics remains a formidable challenge. Here, we present a comprehensive thermodynamic and spectroscopic study of the tetragonal intermetallic superconductor $\text{V}_2\text{Ga}_5$ ($T_{\rm c} \approx 3.5$~K), combining first-principles electronic structure calculations with highly sensitive AC calorimetry and directional low-temperature scanning tunneling spectroscopy. By constructing a self-consistent, anisotropic multiband singlet $s$-wave pairing model within the fully symmetric $A_{1g}$ representation, we successfully reconcile the experimental specific heat and upper critical field anomalies. Crucially, we reveal that the apparent reversal of bulk gap hierarchies in directional tunneling experiments is a direct consequence of band-selective tunneling. This effect is governed by an elegant interplay between localized Fermi velocity 'hot spots' and specific Fermi surface topologies, rather than raw thermodynamic gap magnitudes alone. Our findings provide a clear microscopic picture of direction-dependent, band-selective tunneling in a highly uniaxial anisotropic superconductor, demonstrating how orientation-dependent transport constraints shape the observable signatures of multiband quantum condensates.

cond-mat.supr-con↗

arXiv2025

Ising superconductivity in noncentrosymmetric bulk NbSe2

Ising superconductivity allows in-plane upper critical magnetic fields to vastly surpass Pauli limit by locking the antiparallel electron spins of Cooper pairs in the out-of-plane direction. It was first explicitly demonstrated in fully two-dimensional monolayers of transition metal dichalcogenides with large spin-orbit coupling and broken inversion symmetry. Since then, several studies have shown that it can be present in layered bulk materials, too. In our previous study, we have clarified the underlying microscopic mechanism of Ising superconductivity in bulk, based on a reduced electronic coupling between superconducting layers due to intercalation by insulating layers and restricted inversion symmetry. But earlier studies suggest that in some transition metal dichalcogenide polytypes Pauli paramagnetic limit is violated even without intercalation. Here, using heat capacity measurements we unambiguously demonstrate, that the pristine noncentrosymmetric bulk 4Ha-NbSe2 polytype significantly violates the Pauli limit. The band structure parameters obtained from ab initio calculations using the experimentally determined crystal structure are used in the theoretical model which provides the microscopic mechanism of the Ising protection based solely on broken inversion symmetry.

cond-mat.supr-con↗

arXiv2020

Wide critical fluctuations of the field-induced phase transition in graphite

In the immediate vicinity of the critical temperature (T$_c$) of a phase transition, there are fluctuations of the order parameter, which reside beyond the mean-field approximation. Such critical fluctuations usually occur in a very narrow temperature window in contrast to Gaussian fluctuations. Here, we report on a study of specific heat in graphite subject to high magnetic field when all carriers are confined in the lowest Landau levels. The observation of a BCS-like specific heat jump in both temperature and field sweeps establishes that the phase transition discovered decades ago in graphite is of the second-order. The jump is preceded by a steady field-induced enhancement of the electronic specific heat. A modest (20 percent) reduction in the amplitude of the magnetic field (from 33 T to 27 T) leads to a threefold decrease of T$_c$ and a drastic widening of the specific heat anomaly, which acquires a tail spreading to two times T$_c$. We argue that the steady departure from the mean-field BCS behavior is the consequence of an exceptionally large Ginzburg number in this dilute metal, which grows steadily as the field lowers. Our fit of the critical fluctuations indicates that they belong to the $3DXY$ universality class, similar to the case of $^4$He superfluid transition.

cond-mat.str-el↗

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↗