Josephson Tunneling at the Atomic Scale

EPFL, 2015
Thèse EPFL

Résumé

The aim of this thesis is to characterize the properties of a Josephson junction in a Scanning Tunneling Microscope (STM) at millikelvin temperatures and to implement Josephson STM (JSTM) as a versatile probe at the atomic scale. To this end we investigate the I(V) tunneling characteristics of the Josephson junction in our STM at a base temperature of 15 mK by means of current-biased and voltage-biased experiments. We find that in the tunneling regime, the Josephson junction is operated in the dynamical Coulomb blockade (DCB) regime in which the sequential tunneling of Cooper pairs dominates the tunneling current. Employing P(E)-theory allows us to model experimental I(V) characteristics from voltage-biased experiments and determine experimental values of the Josephson critical current in agreement to theory. Moreover, we observe a breakdown of P(E)-theory for experiments at large values of the tunneling conductance on the order of the quantum of conductance, which could indicate that the coherent tunneling of Cooper pairs strongly contributes to the tunneling current in this limit. We also observe that the Josephson junction in an STM at temperatures well below 100 mK is highly sensitive to its electromagnetic environment that results from its tiny junction capacitance of a few femtofarads. The combination of the experimental results with numerical simulations reveals that the immediate environment of the Josephson junction in an STM is frequency-dependent and, additionally, that a typical STM geometry shares the electromagnetic properties of a monopole antenna with the STM tip acting as the antenna. Comparing the I(V) curves of voltage-biased and current-biased experiments, we observe that the time evolution of the phase is strongly effected by dissipation due to quasiparticle excitations. From investigations on the retrapping current we show first that the temporal evolution of the junction phase in our STM satisfies a classical equation of motion. Second, we can determine two different channels for energy dissipation of the junction phase. For tunneling resistance values RN< 150 kOhm the junction dissipates via Andreev reflections whereas for larger values of RN

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Chargement

EPFL, 2015
Thèse EPFL

Résumé

Official source

À propos de ce résultat

Concepts associés (14)

Concepts associés

Chargement

Publications associées (12)

Publications associées

Chargement

In this thesis we discuss the realization of a scanning tunneling microscope (STM) operating at temperatures below 20 mK. This is accomplished by attaching the STM to a dilution refrigerator. We can apply high magnetic fields up to 14 T perpendicular and 0.5 T parallel to the sample surface. A full ultra high vacuum (UHV) preparation chamber is attached to the cryostat allowing in situ preparation of the samples to investigate. This setup allows us to investigate low temperature phase transitions in solids and offers the possibility for ultimate energy resolution tunneling spectroscopy measurements on the atomic scale. The concept of realizing the mK-STM is given. A main point of concern was the design of the STM unit. The choice of materials should be well adapted for the use at very low temperatures and in high magnetic fields. A special concern are thermal expansion coefficients and thermal conductivity. In addition, we analyzed the variation of the tip sample distance for a given external excitation for our STM design and two variations of the Besoke-STM by using a one dimensional mass spring model. This gives us the possibility to extract an optimization criterion for our design to limit the response of the tip sample distance to vibrations that are transmitted to the STM. The external damping mechanism, which ensures a low mutual vibration level of the STM as well as the grounding concept for the electronics is explained. To characterize the STM at operating temperatures of 1.5 K, 800 mK and 15 mK we measured topography images of Au(111) and Cu(111) surfaces. This allows us to extract the stability between tip and sample. A statistical analysis of the noise in the tunneling current without activated feedback loop yields a stability of 2.7±0.1 pm in a frequency range f ≈ 0.03 – 25 Hz at 800 mK. To verify if tip and sample are cooled efficiently, we measured the temperature at their positions with additional thermometers. We achieved 17 mK at the tip and 20 mK at the sample position, respectively. The energy resolution of the STM setup was verified by taking tunneling spectra between a superconductor and a normal conducting metal. These spectra could be fitted using the BCS theory of superconductivity. From those ts we can extract the upper limit of the effective temperature of the electrons which is Teff = 87 ± 3 mK for our best measurement. This corresponds to an energy resolution of 3.5 kbTeff = 26 ± 1 µeV. Further, the tunneling conductance between two superconductors was investigated. A sub-gap structure, which can be related to Andreev reflections in asymmetric tunneling junction was observed. In addition, at zero bias voltage a peak due to the Josephson effect can be measured. Side peaks with intensities that scale with this zero bias conductance peak appear most probably due to the interaction of Cooper pairs with the electromagnetical environment in which the junction is placed. Investigating the dependence of the conductance between a superconducting tip and a normal conducting sample on externally applied magnetic fields leads to the appearance of shoulders in the coherence peaks. These shoulders shift linearly with the external magnetic field. Such an effect has been measured for planar tunnel junctions and can be explained with the lifting of the spin degeneracy of the quasi-particle density of states in a superconductor. It was observed for the first time by Meservey, Tedrow and Fulde. The fact, that we are now able to prepare STM tips showing the Meservey-Tedrow-Fulde effect enables us to measure the absolute spin polarization at EF with atomic resolution.

EPFL2011

Chargement

Scanning Tunneling Spectroscopy with Superconducting Junctions

Jacob Senkpiel

This thesis contains two major topics, the restriction of tunneling to only a few channels in the scanning tunneling microscope (STM) and the interaction of local magnetic impurities with superconductivity. At a temperature of 15mK, the quantum back-action of the electromagnetic environment in an STM junction becomes prominent. It influences the tunneling process, and by that inevitably also the spectroscopy of physical phenomena. We demonstrate that the macroscopic tip shape strongly defines this back-action. It can be reduced by increasing the tip wire diameter. This increases the capacitance of the junction, and thereby significantly enhances the spectroscopic energy resolution. Modeling this effect with P(E)-theory, we extrapolate that the electromagnetic environment of the junction influences measurements in the STM up to a temperature of 1K. This result helps establish a direct correspondence between the P(E)-model and the energy resolution of the STM. We further study the tunneling process by constructing a single-channel junction of an Al adatom on an Al(100) crystal and the single apex atom of an Al tip. We provide proof that the transport in this junction is strongly limited to a single channel by analyzing Andreev reflection spectra over a wide conductance range up to the quantum of conductance. With this junction we show how the Josephson effect deviates from the many channel and low transmission model by Ambegaokar and Baratoff. We also present a model, based on the full Andreev bound state relation for the few channel limit, which accounts for transmission dependencies and multiple Cooper pair tunneling. Modeling the Josephson effect in our junction this model reproduces the experimental data in great detail. Regarding the determination of the Josephson coupling energy in STM-experiments, we expect at least 0.6% and up to 2.6% deviation from the linear model at a conductance of 0.1G0 and up to 17% at 0.5G0. In the normal conducting state of this junction the environmental back-action manifests as a transmission reduction around zero bias, known as the dynamical Coulomb blockade (DCB). Here we test the predicted vanishing of the DCB for transmissions towards unity. Our data support this expectation. These results suggest that the transport process becomes less sensitive to the environmental back-action with increasing channel transmission. Concerning pair breaking potentials in a multi-band superconductor, we study Fe-doped NbSe2 with a V-tip. We demonstrate that Yu-Shiba-Rusinov (YSR) resonances emerge not only in the energy-gap but also outside of it, at the position of coherence peaks, where they are significantly broadened. We demonstrate a correspondence of the YSR-state lifetime to the imaginary part of the superconducting order parameter. To do so we compare the experimental peak-width to peak-energy-position dependence with a T-matrix scattering model, taking into account the two-band superconductivity of NbSe2, with inter-band coupling and magnetic background scattering. Our results show that YSR-resonances can be used to probe the imaginary part of the superconducting order parameter. We suspect that many asymmetries observed in spectra of the superconducting gap are related to this effect. We collate some early results of the local Josephson critical current in NbSe2. We find local variations around the embedded Fe impurities suggesting that the order parameter is reduced by about 20%.

EPFL2018

Chargement

Tunneling processes through magnetic impurities on superconducting surfaces: Yu-Shiba-Rusinov states and the Josephson effect

Haonan Huang

Magnetic impurities generate a wealth of phenomena on surfaces. On metals, conducting electrons screen the magnetic moment giving rise to the Kondo effect. On superconductors, the Yu-Shiba-Rusinov (YSR) states emerge inside the superconducting gap due to the exchange coupling. In this thesis, we use a scanning tunneling microscope (STM) with a base temperature of 10mK to study magnetic impurities on surfaces. This thesis contains two parts, the interplay between magnetic impurities and superconductivity as well as the tunneling processes between YSR states.Concerning the interplay between magnetic impurities and superconductivity, the single impurity Anderson model (SIAM) offers a consistent picture, in which the impurity substrate coupling is a central parameter of experimental relevance. For YSR phenomena, the mean field (MF) approximation is usually sufficient which gives analytical results for a quantitative interpretation of the experimental data. We tune the coupling by the atomic forces in the junction and quantitatively identify the key role of the coupling in the YSR energy. We further investigate multiple Andreev reflections (MARs) involving a single YSR state, confirming the absence of a spin forbidden family of MARs thereby proving the spin non-degeneracy of the YSR states experimentally.The presence of YSR impurities not only gives rise to peaks in the tunneling spectra, but also influences the superconducting ground state. At a small coupling, the impurity spin is unscreened, which results in a pi phase shift in the Josephson transport. At a large coupling, the impurity spin is screened and the phase is 0. The change of the YSR ground state qualifies as a quantum phase transition (QPT). We observe a significant step in the Josephson current when tuning the coupling across the QPT, which signals the 0-pi transition and the change of the ground state.We then go beyond the MF approximation and include correlation effects in the SIAM to fit the Kondo effect in the magnetic field. We present the scaling between the YSR energy and the Kondo temperature connected by the SIAM. Despite the close relation between the two phenomena, the question remains that to which extent the correlation effects persist in the superconducting state. On this, we show that the correlation effects manifest as an offset current in the YSR measurements.Conventionally, the STM tip only serves as a probe to unravel the sample properties. However, in principle the tip, sample and junction form one quantum system. With the capability to controllably introduce YSR states of desired properties on the tip apex (the YSR-STM), we construct a minimal tunnel junction between two discrete levels, a YSR state on the sample and a YSR state on the tip apex. The tunneling between two YSR states, Shiba-Shiba tunneling, features sharp current peaks, the area of which reveals the relaxation dynamics. We further show that the spin plays an important role in Shiba-Shiba tunneling and we find a behavior consistent with paramagnetic impurities. To conclude, this thesis presents multiple experimental aspects of a single YSR state quantitatively explained by the SIAM, both on the MF level and in the fully correlated situation. The possibility to functionalize the STM tip with a YSR state opens more possibilities, and we show the tunneling between two YSR states as a first application, laying the basis for possible extension of the YSR-STM to more scenarios.

EPFL2021

Chargement

Tunneling processes through magnetic impurities on superconducting surfaces: Yu-Shiba-Rusinov states and the Josephson effect

Haonan Huang

EPFL2021

EPFL2011

Scanning Tunneling Spectroscopy with Superconducting Junctions

Jacob Senkpiel

EPFL2018