arXiv subjects

Philipp Keßler

Publications and source records attributed to Philipp Keßler.

5 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↗

arXiv2024

Epitaxial RuO$_2$ and IrO$_2$ films by pulsed laser deposition on TiO$_2$(110)

We present a systematic growth study of epitaxial RuO$_2$(110) and IrO$_2$(110) on TiO$_2$(110) substrates by pulsed laser deposition. We describe the main challenges encountered in the growth process, such as a deteriorating material flux due to laser induced target metallization or the delicate balance of under- vs over-oxidation of the 'stubborn' Ru and Ir metals. We identify growth temperatures and oxygen partial pressures of 700 K, $1\times 10^{-3}$ mbar for RuO$_2$ and 770 K, $5\times 10^{-4}$ mbar for IrO$_2$ to optimally balance between metal oxidation and particle mobility during nucleation. In contrast to IrO$_2$, RuO$_2$ exhibits layer-by-layer growth up to 5 unit cells if grown at high deposition rates. At low deposition rates, the large lattice mismatch between film and substrate fosters initial 3D island growth and cluster formation. In analogy to reports for RuO$_2$ based on physical vapor deposition, we find these islands to eventually merge and growth to continue in a step flow mode, resulting in highly crystalline, flat, stoichiometric films of RuO$_2$(110) (up to 30 nm thickness) and IrO$_2$(110) (up to 13 nm thickness) with well defined line defects.

cond-mat.mtrl-sci↗

arXiv2024

Absence of magnetic order in RuO$_2$: insights from $μ$SR spectroscopy and neutron diffraction

Altermagnets are a novel class of magnetic materials besides ferro- and antiferromagnets, where the interplay of lattice and spin symmetries produces a magnetic order that is staggered both in coordinate as well as momentum space. The metallic rutile oxide RuO$_2$, long believed to be a textbook Pauli paramagnet, recently emerged as a workhorse altermagnet when resonant X-ray and neutron scattering studies reported nonzero magnetic moments and long-range collinear order. While experiments on thin films seem consistent with altermagnetic behavior, the origin and size of magnetic moments in RuO$_2$ still remain controversial. Here we show that RuO$_2$ is nonmagnetic, regardless if as bulk or thin film. Employing muon spin spectroscopy as a highly sensitive probe of local magnetic moments complemented by density functional theory, we find at most $1.4 \times 10^{-4} $ $μ_B$/Ru in bulk RuO$_2$ and at most $7.5 \times 10^{-4}$ $μ_B$/Ru in epitaxial films. In their essence, these moments reflect the detection limit of our spectrometers and are orders of magnitude smaller than previously reported neutron results, i.e., the moments previously assumed to rationalize altermagnetic behavior. Our own neutron diffraction measurements on RuO$_2$ single crystals identify multiple scattering as a likely source for this discrepancy.

cond-mat.mtrl-sci↗

arXiv2023

Saturation of the anomalous Hall effect at high magnetic fields in altermagnetic RuO2

Observations of the anomalous Hall effect in RuO$_2$ and MnTe have demonstrated unconventional time-reversal symmetry breaking in the electronic structure of a recently identified new class of compensated collinear magnets, dubbed altermagnets. While in MnTe the unconventional anomalous Hall signal accompanied by a vanishing magnetization is observable at remanence, the anomalous Hall effect in RuO$_2$ is excluded by symmetry for the Néel vector pointing along the zero-field [001] easy-axis. Guided by a symmetry analysis and ab initio calculations, a field-induced reorientation of the Néel vector from the easy-axis towards the [110] hard-axis was used to demonstrate the anomalous Hall signal in this altermagnet. We confirm the existence of an anomalous Hall effect in our RuO$_2$ thin-film samples whose set of magnetic and magneto-transport characteristics is consistent with the earlier report. By performing our measurements at extreme magnetic fields up to 68 T, we reach saturation of the anomalous Hall signal at a field $H_{\rm c} \simeq$ 55 T that was inaccessible in earlier studies, but is consistent with the expected Néel-vector reorientation field.

cond-mat.mes-hall↗

arXiv2023

Stabilizing an atomically thin quantum spin Hall insulator at ambient conditions: Graphene-intercalation of indenene

Atomic monolayers on semiconductor surfaces represent a new class of functional quantum materials at the ultimate two-dimensional limit, ranging from superconductors [1, 2] to Mott insulators [3, 4] and ferroelectrics [5] to quantum spin Hall insulators (QSHI) [6, 7]. A case in point is the recently discovered QSHI indenene [7, 8], a triangular monolayer of indium epitaxially grown on SiC(0001), exhibiting a $\sim$120meV gap and substrate-matched monodomain growth on the technologically relevant $μ$m scale [9]. Its suitability for room-temperature spintronics is countered, however, by the instability of pristine indenene in air, which destroys the system along with its topological character, nullifying hopes of ex-situ processing and device fabrication. Here we show how indenene intercalation into epitaxial graphene offers effective protection from the oxidizing environment, while it leaves the topological character fully intact. This opens an unprecedented realm of ex-situ experimental opportunities, bringing this monolayer QSHI within realistic reach of actual device fabrication and edge channel transport.

cond-mat.mtrl-sci↗