arXiv subjects

Theo A. Costi

Publications and source records attributed to Theo A. Costi.

5 recordsLinked to original sources

arXiv2024

Singlet-Triplet Kondo Effect in Blatter Radical Molecular Junctions: Zero-bias Anomalies and Magnetoresistance

The Blatter radical has been suggested as a building block in future molecular spintronic devices due to its radical character and expected long-spin lifetime. However, whether and how the radical character manifests itself in the charge transport and magnetotransport properties seems to depend on the environment or has not yet been studied. Here, we investigate single-molecule junctions of the Blatter radical molecule in a mechanically controlled break junction device at low temperature. Differential conductance spectroscopy on individual junctions shows two types of zero-bias anomalies attributed to Kondo resonances revealing the ability to retain the open-shell nature of the radical molecule in a two-terminal device. Additionally, a high negative magnetoresistance is also observed in junctions without showing a zero-bias peak. We posit that the high negative magnetoresistance is due to the effect of a singlet-triplet Kondo effect under magnetic field originating from a double-quantum-dot system consisting of a Blatter radical molecule with a strong correlation to a second side-coupled molecule. Our findings not only provide the possibility of using the Blatter radical in a two-terminal system under cryogenic conditions but also reveal the magnetotransport properties emerging from different configurations of the molecule inside a junction.

cond-mat.mes-hall↗

arXiv2024

Probing the spin polarization of an Anderson impurity

We report spin-polarized scanning tunneling microscopy measurements of an Anderson impurity system in MoS$_{2}$ mirror twin boundaries, where both the quantum confined impurity state and the Kondo resonance resulting from the interaction with the substrate are accessible. Using a spin-polarized tip, we observe magnetic field induced changes in the peak heights of the Anderson impurity states as well as in the magnetic field-split Kondo resonance. Quantitative comparison with numerical renormalization group calculations provides evidence of the notable spin polarization of the spin-resolved impurity spectral function under the influence of a magnetic field. Moreover, we extract the field and temperature dependence of the impurity magnetization from the differential conductance measurements and demonstrate that this exhibits the universality and asymptotic freedom of the $S=1/2$ Kondo effect. This work shows that mirror twin boundaries can be used as a testing ground for theoretical predictions on quantum impurity models.

cond-mat.mes-hall↗

arXiv2023

Modulated Kondo screening along magnetic mirror twin boundaries in monolayer MoS2 on graphene

A many-body resonance emerges at the Fermi energy when an electron bath screens the magnetic moment of a half-filled impurity level. This Kondo effect, originally introduced to explain the abnormal resistivity behavior in bulk magnetic alloys, has been realized in many quantum systems over the past decades, such as quantum dots, quantum point contacts, nanowires, single-molecule transistors, heavy-fermion lattices, down to adsorbed single atoms. Here we describe a unique Kondo system which allows us to experimentally resolve the spectral function consisting of impurity levels and Kondo resonance in a large Kondo temperature range, as well as their spatial modulation. Our experimental Kondo system, based on a discrete half-filled quantum confined state within a MoS2 grain boundary, in conjunction with numerical renormalization group calculations, enables us to test the predictive power of the Anderson model which is the basis of the microscopic understanding of Kondo physics.

cond-mat.mes-hall↗

arXiv2022

Magnetic Field Universality of the Kondo Effect Revealed by Thermocurrent Spectroscopy

Probing the universal low temperature magnetic field scaling of Kondo-correlated quantum dots via electrical conductance has proved to be experimentally challenging. Here, we show how to probe this in nonlinear thermocurrent spectroscopy applied to a molecular quantum dot in the Kondo regime. Our results demonstrate that the bias-dependent thermocurrent is a sensitive probe of universal Kondo physics, directly measures the splitting of the Kondo resonance in a magnetic field, and opens up possibilities for investigating nanosystems far from thermal and electrical equilibrium.

cond-mat.mes-hall↗

arXiv2009

Observation of the underscreened Kondo effect in a molecular transistor

We present the first quantitative experimental evidence for the underscreened Kondo effect, an uncomplete compensation of a quantized magnetic moment by conduction electrons, as originally proposed by Nozieres and Blandin. The device consists of an even charge spin S = 1 molecular quantum dot, obtained by electromigration of C60 molecules into gold nanogaps and operated in a dilution fridge. The persistence of logarithmic singularities in the low temperature conductance is demonstrated by a comparison to the fully screened configuration obtained in odd charge spin S = 1/2 Coulomb diamonds. We also discover an extreme sensitivity of the underscreened Kondo resonance to magnetic field, that we confirm on the basis of numerical renormalization group calculations.

cond-mat.mes-hall↗