arXiv subjects

Jozef Haniš

Publications and source records attributed to Jozef Haniš.

6 recordsLinked to original sources

arXiv2026

Machine learning protocol to identify pairing symmetries via quasiparticle interference imaging in Ising superconductors

Identifying the pairing symmetry in unconventional superconductors is essential for reliably characterizing their superconducting states and for enabling their integration into realistic quantum devices. Here, we introduce a machine-learning-guided strategy to determine pairing symmetry from quasiparticle interference (QPI) data, which integrates first-principles calculations, tight-binding modeling, and symmetry-based classification of the superconducting pairing function. We demonstrate the approach on monolayer NbSe2 as an experimentally accessible probe of superconductivity in real materials, within a single scalar-impurity Bogoliubov-de Gennes framework. Our analysis shows that the QPI-to-parameter inverse problem can be solved with high accuracy for most superconducting pairing channels in this setting, indicating that QPI carries rich, learnable information about the superconducting gap structure. Taken together, these results demonstrate that machine-learning-assisted QPI analysis provides a promising pathway for precise learning of superconducting pairing functions in quantum materials.

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↗

arXiv2024

Distinguishing nodal and nonunitary superconductivity in quasiparticle interference of an Ising superconductor with Rashba spin-orbit coupling: an example of NbSe$_2$

The NbSe$_2$ monolayer with Rashba spin-orbit coupling represents a paradigmatic example of an Ising superconductor on a substrate. Using a single-band model and symmetry analysis, we present general superconducting pairing functions beyond the nearest-neighbor approximation, uncovering new types of gap functions, including the nodal singlet gap function and the triplet non-unitary pairing function that breaks time-reversal symmetry. The non-unitarity builts in the asymmetrical band dispersion in the superconducting quasiparticle energy spectra. Performing exact T-matrix calculations of quasiparticle interference due to a single scalar impurity scattering, we found that the interference patterns possess characteristic features distinguishing the type of pairing and possible nematic and chiral symmetry violations.

cond-mat.supr-con↗

arXiv2024

Probing type-II Ising pairing using the spin-mixing parameter

The immunity of Ising superconductors to external magnetic fields originates from a spin locking of the paired electrons to an intrinsic Zeeman-like field. The spin-momentum locking in non-centrosymmetric crystalline materials leads to type-I Ising pairing in which the direction of the intrinsic field can be deduced from the spin expectation values. Conversely, in centrosymmetric crystals the electron spins locked to the orbitals can form Ising type-II pairs consisting of spin-orbit split doublets. Due to time-reversal symmetry, the doublets are spin degenerate, making it difficult to read the spin polarization of bands and the direction of spin-orbit fields. Here we present an efficient approach to determine the direction of the intrinsic field using the spin-mixing parameter $b^2$. Using first principles calculations based on the density functional theory, we study monolayer transition metal dichalcogenide superconductors PdTe$_2$, NbTe$_2$, and TiSe$_2$ with the 1T structure. We calculate $b^2$ for individual Fermi pockets and provide a general picture of possible Ising type-II pairing within the full Brillouin zone. In order to complement our first principles results, we use group theory to provide a detailed picture of spin-orbit coupling and spin mixing in the relevant bands forming Fermi pockets. We demonstrate that contrary to the anticipated effects of spin-orbit locking, not every spin-orbit split spin doublet actively participates in Ising pairing. Finally, by connecting the spin-mixing parameter $b^2$ with the intrinsic out-of-plane Zeeman field we estimate the upper in-plane critical magnetic field.

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↗