arXiv subjects

Katerina Vaxevani

Publications and source records attributed to Katerina Vaxevani.

6 recordsLinked to original sources

arXiv2026

Parity-dependent coupling of molecular spin chains to a superconductor

Topological order can fractionalize the quantum numbers of the underlying particles. A paradigmatic example is the spin-1/2 states at the edges of an antiferromagnetic integer-spin chain, protected by a topological Haldane gap in the bulk and mutually coupled in short chains. Owing to their topological protection, they are natural building blocks for hybrid spin-superconductor quantum systems. Whether fractionalization survives the coupling to a superconducting condensate, however, remains an open question. Here we grow molecular Haldane chains of antiferromagnetically coupled spin-1 triangulene units on a proximitized Au(111)/Nb(110) surface and resolve a parity-dependent coupling of their spin-1/2 edge states to the superconducting condensate by scanning tunnelling spectroscopy. Odd-length chains host Yu-Shiba-Rusinov bound states inside the superconducting gap, originating from the net S=1 ground state, whereas even-length chains form an S=0 ground state decoupled from the superconductor. A two-site superconductor model reveals that this alternation arises from the sign and strength of the inter-edge interaction, a mechanism independently validated by extra-gap spin excitations in tunnelling spectra. Collective many-body spin excitation modes are also detected decoupled from the superconductor by the much larger Haldane gap. The length-tunable coupling of the edge spins to the superconductor opens a route toward molecular spin qubits based on $π$-conjugated carbon architectures.

cond-mat.mes-hall↗

arXiv2025

Local control of parity and charge in nanoscale superconducting lead islands

Small superconducting islands can exhibit charge quantization, where Coulomb interactions compete with Cooper pairing. Using scanning tunneling spectroscopy, we probe this interplay by measuring the charging energy ($E_C$) and the pairing energy ($Δ$) of individual nano-islands. Below a critical island size, where $E_C > Δ$, we observe a crossover between even and odd parity ground states. By applying controlled voltage pulses, we continuously tune the island's electrostatic potential and map the full charge-parity landscape. These results demonstrate tunable superconducting ground states, offering a potential platform for qubit design and control.

cond-mat.mes-hall↗

arXiv2025

Epitaxial growth of gold films on the elemental superconductors V(100), Nb(100) and Nb(110)

Quantum technologies require a new generation of superconducting electronic devices and circuitry. However, the superconducting materials used to construct them are restricted to a class of bulk superconductors. Gold films grown in contact with superconducting materials can exhibit superconducting correlations through the proximity effect, with various possible implementations in quantum technology. Here, we study the growth of flat Au films on various surfaces of the elemental superconductors vanadium and niobium through a combination of low-temperature scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). In particular, we investigate the growth morphology and composition as a function of temperature and coverage. We find that gold films can grow flat and oxygen-free when annealed to sufficiently high temperatures; however, they are also susceptible to partial intermixing with the substrate elements. Low-temperature scanning tunneling spectroscopy (STS) measurements elucidate the emergence of a proximitized superconducting gap at the gold surface. Additionally, we demonstrate the survival of the magnetic state of FeTPP-Cl molecules on the surface of these gold films, proving that they behave as a good support for probing molecular magnetism. We found that the exchange interaction between the molecular spin and the superconducting condensate can be inferred by the measurement of sub-gap Yu-Shiva-Rusinov states. Our work shows that proximitised Au films constitute a promising platform to explore on-superconducting-surface synthesis and the interaction between superconductivity and magnetism in a large spin-orbit coupling environment.

cond-mat.supr-con↗

arXiv2024

Tuning the Spin Interaction in Non-planar Organic Diradicals Through Mechanical Manipulation

Open-shell polycyclic aromatic hydrocarbons (PAHs) represent promising building blocks for carbon-based functional magnetic materials. Their magnetic properties stem from the presence of unpaired electrons localized in radical states of $π$ character. Consequently, these materials are inclined to exhibit spin delocalization, form extended collective states, and respond to the flexibility of the molecular backbones. However, they are also highly reactive, requiring structural strategies to protect the radical states from reacting with the environment. Here, we demonstrate that the open-shell ground state of the diradical 2-OS survives on a Au(111) substrate as a global singlet formed by two unpaired electrons with anti-parallel spins coupled through a conformational dependent interaction. The 2-OS molecule is a protected derivative of the Chichibabin's diradical, featuring a non-planar geometry that destabilizes the closed-shell quinoidal structure. Using scanning tunneling microscopy (STM), we localized the two interacting spins at the molecular edges, and detected an excited triplet state a few millielectronvolts above the singlet ground state. Mean-field Hubbard simulations reveal that the exchange coupling between the two spins strongly depends on the torsional angles between the different molecular moieties, suggesting the possibility of influencing the molecule's magnetic state through structural changes. This was demonstrated here using the STM tip to manipulate the molecular conformation, while simultaneously detecting changes in the spin excitation spectrum. Our work suggests the potential of these PAHs for a new class of all-carbon spin-crossover materials.

cond-mat.mes-hall↗

arXiv2022

Pair excitations of a quantum spin on a proximitized superconductor

A magnetic impurity interacting with a superconductor develops a rich excitation spectrum formed by superposition of quasiparticles and spin states, which appear as Yu-Shiba-Rusinov and spin-flip excitations in tunneling spectra. Here, we show that tunneling electrons can also excite a superconducting pair-breaking transition in the presence of magnetic impurities, which is hidden for electrons on bare superconductors. Combining scanning tunneling spectroscopy with theoretical modeling, we map the excitation spectrum of a Fe-porphyrin molecule on the Au/V(100) proximitized surface into a manifold of many-body excitations and follow their behavior across a parity-changing transition. Pair excitations emerge in the tunneling spectra as peaks outside the gap in the strong interaction regime, scaling with the pair correlation. Our results unravel the quantum nature of magnetic impurities on superconductors and prove that pair excitations are parity detectors for magnetic impurities.

cond-mat.mes-hall↗

arXiv2022

Extending the spin excitation lifetime of a magnetic molecule on a proximitized superconductor

Magnetic molecules deposited on surfaces are a promising platform to individually address and manipulate spins. Long spin excitation lifetimes are necessary to utilize them in quantum information processing and data storage. Normally, coupling of the molecular spin with the conduction electrons of metallic surfaces causes fast relaxation of spin excitations into the ground state. However, the presence of superconducting paring effects in the substrate can protect the excited spin from decaying. In this work, we show that a proximity-induced superconducting gold film can sustain spin excitations of a FeTPP-Cl molecule for more than 80ns. This long value was determined by studying inelastic spin excitations of the S=5/2 multiplet of FeTPP-Cl on Au films over V(100) using scanning tunneling spectroscopy. The spin lifetime decreases with increasing film thickness, in apparent connection with the gradual gap-closing of a pair of de Gennes-Saint James resonances found inside the superconducting gap. Our results elucidate the use of proximitized gold electrodes for addressing quantum spins on surfaces, envisioning new routes for tuning the value of their spin lifetime.

cond-mat.mes-hall↗