arXiv subjects

Patrick Härtl

Publications and source records attributed to Patrick Härtl.

4 recordsLinked to original sources

arXiv2026

Moiré-resonant surface state in ultrathin RuO$_2$

RuO$_2$ has emerged as a prototypical candidate for altermagnetism. In the face of daunting evidence for magnetic order in the bulk, the focus naturally shifted to surfaces and ultrathin films, where Coulomb interactions are dimensionally quenched and electron correlations strongly enhanced. Here, we examine atomically ordered, ultrathin RuO$_2$(110) grown on Ru(0001) using a combination of scanning tunneling microscopy (STM), density functional theory, and density matrix renormalization group methods. We observe a nonmagnetic charge order that is imprinted by the incommensurate moiré stacking with the substrate and enhanced by the electronic Fermi surface scattering within the flat-band surface state. We further identify a nonmagnetic, metastable $c(2 \times 2)$ surface reconstruction that arises from surface phonon softening and can be toggled reversibly via STM tip manipulation. Spin-polarized STM measurements, however, reveal no evidence of magnetic order on the RuO$_2$(110) surface. Our findings of a nonmagnetic charge-modulation position ultrathin RuO$_2$(110) as an intriguing platform for exploring moiré-assisted electronic orders.

cond-mat.mtrl-sci↗

arXiv2025

Sizable superconducting gap and anisotropic chiral topological superconductivity in the Weyl semimetal PtBi$_2$

Topological superconductors offer a fertile ground for realizing Majorana zero modes -- topologically protected, zero-energy quasiparticles that are resilient to local perturbations and hold great promise for fault-tolerant quantum computing. Recent studies have presented encouraging evidence for intrinsic topological superconductivity in the Weyl semimetal trigonal PtBi$_2$, hinting at a robust surface phase potentially stable beyond the McMillan limit. However, due to substantial spatial variations in the observed superconducting (SC) gap $Δ$ the nature of the underlying order parameter $Δ$($k$) remained under debate. Here we report the realization of sizable surface SC gaps ($Δ> 10\,\mathrm{meV}$) in PtBi$_2$, exhibiting remarkable spatial uniformity from hundreds of nanometers down to the atomic level, as revealed by scanning tunneling microscopy and spectroscopy. Building on this spatial homogeneity -- indicative of long-range phase coherence -- we uncover previously unobserved low-energy Andreev bound states (ABSs) that ubiquitously emerge within the SC gap across the surface. Theoretical simulations that closely reproduce the experimental spectra, reveal an anisotropic chiral pairing symmetry of $Δ$($k$), and further suggest that the observed ABSs are of topological origin. The combination of a large, nontrivial pairing gap and accessible surface states establishes PtBi$_2$ as a compelling platform for investigating topological superconductivity and its associated Majorana modes.

cond-mat.supr-con↗

arXiv2023

Probing spin-dependent charge transport at single-nanometer length scales

The coherent transport of charge and spin is one key requirement of future devices for quantum computing and communication. Scattering at defects or impurities may seriously reduce the coherence of quantum-mechanical states, thereby affecting device functionality. While numerous methods exist to experimentally assess charge transport, the real-space detection of a material's spin transport properties with nanometer resolution remains a challenge. Here we report on a novel approach which utilizes a combination of spin-polarized scanning tunneling microscopy (SP-STM) and the recently introduced molecular nanoprobe (MONA) technique. It relies on the local injection of spin-polarized charge carriers from a magnetic STM tip and their detection by a single surface-deposited phthalocyanine molecule via reversible electron-induced tautomerization events. Based on the particular electronic structure of the Rashba alloy BiAg$_2$ which is governed by a spin-momentum-locked surface state, we proof that the current direction inverses as the tip magnetization is reversed by an external field. In a proof-of-principle experiment we apply SP-MONA to investigate how a single Gd cluster influences the spin-dependent charge transport of the Rashba surface alloy.

cond-mat.mes-hall↗

arXiv2022

Magnetic domain structure of epitaxial Gd films grown on W(110)

We present a detailed real-space spin-polarized scanning tunneling microscopy (SP-STM) study of the magnetic domain structure of Gd(0001) films epitaxially grown on W(110). To find optimal preparation conditions, the influence of the substrate temperature during deposition and of the post-growth annealing temperature was investigated. Our results show that the lowest density of surface defects, such as step edges as well as screw and edge dislocations, is obtained for room temperature deposition and subsequent annealing at 900\,K. SP-STM data reveal small-sized magnetic domains at lower annealing temperatures, evidently caused by pinning at grain boundaries and other crystalline defects. The coverage-dependent magnetic domain structure of optimally prepared Gd films was systematically investigated. For low coverage up to about 80 atomic layers (AL) we observe $μ$m-sized domains separated by domain walls which are oriented approximately along the $[1{\bar 1}0]$ direction of the underlying W substrate. Above a critical film thicknesses $Θ_{\text{crit}} \approx (100 \pm 20)$\,AL, we identify stripe domains, indicative for a spin reorientation transition from in-plane to out-of-plane. In agreement with existing models, the periodicity of the stripe domains increases the further the coverage exceeds $Θ_{\text{crit}}$. While the orientation of the stripe domains is homogeneous over large distances just above $Θ_{\text{crit}}$, we find a characteristic zig-zag pattern at $Θ\gtrsim 200$\,AL and irregular stripe domains beyond 500\,AL. Intermediate minima and maxima of the magnetic signal indicate the nucleation of branching domains. The results are discussed in terms of various contributions to the total magnetic energy, such as the magneto-crystalline, the magneto-static, and the magneto-elastic energy density.

cond-mat.mtrl-sci↗