arXiv subjects

Werner M. J. van Weerdenburg

Publications and source records attributed to Werner M. J. van Weerdenburg.

10 recordsLinked to original sources

arXiv2026

Collective excitation-mediated transport in nanoscale Josephson junctions that exhibit quantum confinement

Quantum confinement can strongly modify transport through Josephson junctions. Here, we study local tunneling transport through nanoscale Josephson junction stacks in the Coulomb blockade regime, where the metallic layers exhibit strong vertical quantum confinement. We find that quasiparticle transport is strongly enhanced by a collective excitation mode intrinsic to the junction and localized in the isolated metallic overlayer. We quantify both the collective-mode energy and the Coulomb gap and show that both exhibit strong layer-dependent modulation, consistent with the modulation of the underlying quantum well states. We further investigate how the collective excitation responds to various perturbations, including mechanical motion and an applied magnetic field. Our results suggest that this collective mode is sensitive to quasiparticles near the Fermi level and may therefore provide an indirect probe of the superconducting state.

cond-mat.supr-con↗

arXiv2026

Probing crystal-field modulations with magnetic adatoms on the incipient charge-density-wave superconductor $2H$-NbS$_2$

The interplay between multiple quantum phases in layered materials may lead to incipient quantum behavior, where the material's ground state is close to a phase transition and sensitive to local disorder. The transition metal dichalcogenide material $2H$-NbS$_2$ exhibits incipient charge-density-wave behavior along with a well-developed superconducting state, creating a scenario where the local lattice instabilities play a crucial role. Here we present how an individual magnetic atom on $2H$-NbS$_2$ can be applied as a local sensor to reveal hidden crystal-field modulations. By manipulating the adatom across the surface with the tip of a scanning tunneling microscope, we measure variations in the Yu-Shiba-Rusinov (YSR) excitation spectra and map the local environment around an intrinsic point defect. We find that while the superconducting state is spatially uniform, the YSR excitation energy strongly depends on the position of the atom. We determine that the main contribution to this effect originates from variations in the local crystal-field environment. These results establish a new approach to investigate crystal-field modulations at the atomic scale and reveal how defects and lattice instabilities shape the atomic landscape of an incipient charge-density-wave material.

cond-mat.mes-hall↗

arXiv2025

Odd-parity ground state in dilute Yu-Shiba-Rusinov dimers and chains

Magnetic adatoms on superconductors induce Yu-Shiba-Rusinov (YSR) states, which are key to the design of low-dimensional correlated systems and topological superconductivity. Competing magnetic interactions and superconducting pairing lead to a rich phase diagram. Using a scanning tunneling microscope (STM), we position Fe atoms on 2H-NbSe$_2$ to build a dimer with an odd-parity ground state, i.e., a partially screened YSR channel with the hybridized states spanning the Fermi level. This ground state makes the dimer a promising precursor for a topological YSR chain. By adding one atom at a time, we track the formation of YSR bands. The lowest-energy band crosses the Fermi level and we find strong site-dependent spectral variations especially at the chain's terminations. We attribute these features to quantum spin effects and ferromagnetic coupling influenced by the local chemical environment, rather than topological superconductivity or Majorana modes.

cond-mat.mes-hall↗

arXiv2024

Quantifying the quantum nature of high spin YSR excitations in transverse magnetic field

Excitations of individual and coupled spins on superconductors provide a platform to study quantum spin impurity models as well as a pathway toward realizing topological quantum computing. Here, we characterize, using ultra-low temperature scanning tunneling microscopy/spectroscopy, the Yu-Shiba-Rusinov (YSR) states of individual manganese phthalocyanine molecules with high spin character on the surface of an ultra-thin lead film in variable transverse magnetic field. We observe two types of YSR excitations, depending on the adsorption geometry of the molecule. Using a zero-bandwidth model, we detail the role of the magnetic anisotropy, spin-spin exchange, and Kondo exchange. We illustrate that one molecular type can be treated as an individual spin akin to an isolated spin on the metal center, whereas the other molecular type invokes a coupled spin system represented by a spin on the center and the ligand. Using the field-dependent evolution of the YSR excitations and comparisons to modeling, we describe the quantum phase of each of the molecules. These results provide an insight into the quantum nature of YSR excitations in magnetic field, and a platform to study spin impurity models on superconductors in magnetic field.

cond-mat.supr-con↗

arXiv2023

Stochastic syncing in sinusoidally driven atomic orbital memory

Stochastically fluctuating multi-well systems as physical implementations of energy-based machine learning models promise a route towards neuromorphic hardware. Understanding the response of multi-well systems to dynamic input signals is crucial in this regard. Here, we investigate the stochastic response of binary orbital memory states derived from individual Fe and Co atoms on a black phosphorus surface to sinusoidal input voltages. Using scanning tunneling microscopy, we quantify the state residence times for DC and AC voltage drive with various input frequencies. We find that Fe and Co atoms both exhibit features of synchronization to the AC input, but only Fe atoms demonstrate a significant frequency-dependent change in the time-averaged state occupations. By modeling the underlying stochastic process, we show that the frequency response of the system is directly related to the DC voltage dependence of the state asymmetry. This relation provides a tunable way to induce population changes in stochastic systems and lays the foundation for understanding the response of multi-well systems to dynamical input signals.

cond-mat.mes-hall↗

arXiv2022

Threefold enhancement of superconductivity and the role of field-induced odd-frequency pairing in epitaxial aluminum films near the 2D limit

BCS theory has been widely successful at describing elemental bulk superconductors. Yet, as the length scales of such superconductors approach the atomic limit, dimensionality as well as the environment of the superconductor can lead to drastically different and unpredictable superconducting behavior. Here, we report a threefold enhancement of the superconducting critical temperature and gap size in ultrathin epitaxial Al films on Si(111), when approaching the 2D limit, based on high-resolution scanning tunneling microscopy/spectroscopy (STM/STS) measurements. In magnetic field, the Al films show type II behavior and the Meservey-Tedrow-Fulde (MTF) effect for in-plane magnetic fields. Using spatially resolved spectroscopy, we characterize the vortex structure in the MTF regime and find strong deviations from the typical Abrikosov vortex. We corroborate these findings with calculations that unveil the role of odd-frequency pairing and a paramagnetic Meissner effect. These results illustrate two striking influences of reduced dimensionality on a BCS superconductor and present a new platform to study BCS superconductivity in large magnetic fields.

cond-mat.supr-con↗

arXiv2022

Gating orbital memory with an atomic donor

Orbital memory is defined by two stable valencies that can be electrically switched and read-out. To explore the influence of an electric field on orbital memory, we studied the distance-dependent influence of an atomic Cu donor on the state favorability of an individual Co atom on black phosphorus. Using low temperature scanning tunneling microscopy/spectroscopy, we characterized the electronic properties of individual Cu donors, corroborating this behavior with ab initio calculations based on density functional theory. We studied the influence of an individual donor on the charging energy and stochastic behavior of an individual Co atom. We found a strong impact on the state favorability in the stochastic limit. These findings provide quantitative information about the influence of local electric fields on atomic orbital memory.

cond-mat.mes-hall↗

arXiv2021

Quantifying the interplay between fine structure and geometry of an individual molecule on a surface

The pathway toward the tailored synthesis of materials starts with precise characterization of the conformational properties and dynamics of individual molecules. Electron spin resonance based scanning tunneling microscopy can potentially address molecular structure with unprecedented resolution. Here, we determine the fine structure and geometry of an individual TiH molecule, utilizing a combination of a newly developed mK ESR-STM in a vector magnetic field and ab initio approaches. We demonstrate a strikingly large anisotropy of the g-tensor unusual for a spin doublet ground state, resulting from a non-trivial orbital angular momentum stemming from the molecular ground state. We quantify the relationship between the resultant fine structure, hindered rotational modes, and orbital excitations. Our model system provides new avenues to determine the structure and dynamics of individual molecules.

cond-mat.mes-hall↗

arXiv2020

A scanning tunneling microscope capable of electron spin resonance and pump-probe spectroscopy at mK temperature and in vector magnetic field

In the last decade, detecting spin dynamics at the atomic scale has been enabled by combining techniques like electron spin resonance (ESR) or pump-probe spectroscopy with scanning tunneling microscopy (STM). Here, we demonstrate an ultra-high vacuum (UHV) STM operational at milliKelvin (mK) and in a vector magnetic field capable of both ESR and pump-probe spectroscopy. By implementing GHz compatible cabling, we achieve appreciable RF amplitudes at the junction while maintaining mK base temperature. We demonstrate the successful operation of our setup by utilizing two experimental ESR modes (frequency sweep and magnetic field sweep) on an individual TiH molecule on MgO/Ag(100) and extract the effective g-factor. We trace the ESR transitions down to MHz into an unprecedented low frequency band enabled by the mK base temperature. We also implement an all-electrical pump-probe scheme based on waveform sequencing suited for studying dynamics down to the nanoseconds range. We benchmark our system by detecting the spin relaxation time T1 of individual Fe atoms on MgO/Ag(100) and note a field strength and orientation dependent relaxation time.

cond-mat.mes-hall↗

arXiv2018

An orbitally derived single-atom magnetic memory

A single magnetic atom on a surface epitomizes the scaling limit for magnetic information storage. Indeed, recent work has shown that individual atomic spins can exhibit magnetic remanence and be read out with spin-based methods, demonstrating the fundamental requirements for magnetic memory. However, atomic spin memory has been only realized on thin insulating surfaces to date, removing potential tunability via electronic gating or distance-dependent exchange-driven magnetic coupling. Here, we show a novel mechanism for single-atom magnetic information storage based on bistability in the orbital population, or so-called valency, of an individual Co atom on semiconducting black phosphorus (BP). Distance-dependent screening from the BP surface stabilizes the two distinct valencies and enables us to electronically manipulate the relative orbital population, total magnetic moment and spatial charge density of an individual magnetic atom without a spin-dependent readout mechanism. Furthermore, we show that the strongly anisotropic wavefunction can be used to locally tailor the switching dynamics between the two valencies. This orbital memory derives stability from the energetic barrier to atomic relaxation and demonstrates the potential for high-temperature single-atom information storage.

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