Upgrade of a low-temperature scanning tunneling microscope for electron-spin resonance

Electron spin resonance with a scanning tunneling microscope (ESR-STM) combines the high energy resolution of spin resonance spectroscopy with the atomic scale control and spatial resolution of STM. Here we describe the upgrade of a helium-3 STM with a 2D vector-field magnet (B-z = 8.0 T, B-x = 0.8 T) to an ESR-STM. The system is capable of delivering radio frequency (RF) power to the tunnel junction at frequencies up to 30 GHz. We demonstrate magnetic field-sweep ESR for the model system TiH/MgO/Ag(100) and find a magnetic moment of (1.004 +/- 0.001)mu(B). Our upgrade enables to toggle between a DC mode, where the STM is operated with the regular control electronics, and an ultrafast-pulsed mode that uses an arbitrary waveform generator for pump-probe spectroscopy or reading of spin-states. Both modes allow for simultaneous radiofrequency excitation, which we add via a resistive pick-off tee to the bias voltage path. The RF cabling from room temperature to the 350 mK stage has an average attenuation of 18 dB between 5 and 25 GHz. The cable segment between the 350 mK stage and the STM tip presently attenuates an additional 34(-3)(+5) dB from 10 to 26 GHz and 38(-2)(+3) dB between 20 and 30 GHz. We discuss our transmission losses and indicate ways to reduce this attenuation. We finally demonstrate how to synchronize the arrival times of RF and DC pulses coming from different paths to the STM junction, a prerequisite for future pulsed ESR experiments. Published under license by AIP Publishing.

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

Magnetism of Single Adatoms and Small Adsorbed Clusters Investigated by Means of Low-Temperature STM

Quentin Dubout

This thesis works reports on the magnetic properties of single transition metal atoms and hydrogenated compounds adsorbed on crystal surfaces. It is an experimental study, based on scanning tunneling microscopy (STM), initiated by the project of demonstrating the existence of magnetic remanence for single adsorbed atoms (adatoms). All the measurements have been performed with a scanning tunneling microscope operating at 0.4 K and equipped with 8.5 T superconducting coils. The investigated systems are cobalt atoms, pure and hydrogenated (cobalt hydrides), adsorbed on Pt(111) and graphene on Pt(111) surfaces. In the first case, we have immediately been confronted to the presence of cobalt hydrides on the surface. Three types of hydrides (CoH, CoH2, and CoH3) have been identified, quite different from the clean adatoms, and readily distinguished by their tunnel spectrum (STS). Twoofthesehydridesdisplaylow-energyvibrationalmodes(1−30meV),identifiedbytheir absence of response to a magnetic field, the effect of an isotopic substitution, and by comparison with known systems. The three hydrides can be deprotonated applying a sufficient voltage between tip and sample. CoH2 on Pt(111) presents a Kondo resonance, due to a half-integer spin (S = 3/2) and a peculiar magnetic anisotropy. This system corresponds to the first demonstration of the appearance of the Kondo effect upon hydrogen adsorption. The results of several attempts to observe remanence for single cobalt adatoms on Pt(111) with spin-polarized scanning tunneling microscopy (SP-STM) are also presented and discussed. The second system differs from the first one in the presence of a graphene layer on the Pt surface. The first experimental determination of the magnetic moment and magnetic anisotropy of adatoms on graphene is reported in this thesis. A magnetic moment of (2.2 ± 0.4) μB was measured by spin-excitation spectroscopy (SES), as well as a hard magnetization axis with an anisotropy energy of 8.1 ± 0.4 meV. This value, comparable to the record value of single cobalt atoms on Pt(111), has been mainly attributed to the strong hybridization between cobalt and graphene. Ab initio calculations confirmed our observations. Graphene thus represents an extremely promising substrate for future applications in nanoscale magnetism and spintronics. This system also exhibits three different cobalt hydrides in addition to the clean atoms. Their respective chemical composition, CoH, CoH2, and CoH3, was determined by direct comparison between high resolution STM images and simulated images obtained from ab initio calculated atomic structures. A controlled deprotonation method is presented. CoH3 is the only hydride to display spin excitations. Its magnetic moment is comparable to that of clean cobalt, it presents a hard magnetization axis as well, however, its magnetic anisotropy energy is sensibly lower, 1.7 ± 0.05 meV. The other two hydrides display vibrational excitations only. The effect of hydrogen on the magnetic properties of metal adatoms is therefore very strong in this system as well, and must be taken into account in future studies, hydrogen being a very common contaminant in ultra-high vacuum set-ups. Finally, a detailed study of the structure graphene mono- and bilayers on Ru(0001) was realized. Two different stackings, AB and AA, have been identified for the bilayers, the last one not observed to date for this system. The moiré structure of monolayer graphene and of this last type of bilayer has been determined: (11.57×11.57)R4.3◦ and (10.54×10.54)R4.7◦, respectively. Moreover, C-C bond distortions of more than 10%, observed in the STM images, have been studied in detail. A quantitative comparison with ab initio calculations allowed to identify their origin as the tilt of the graphene π-orbitals. These distortions are therefore purely artificial. This is an important result for future scanning tunneling microscopy studies of graphene.

EPFL2013

Construction d'un microscope à effet tunnel à basse température et études d'impuretés magnétiques en surfaces

Laurent Claude

This thesis reports on the design and construction of a novel experimental setup for the study of surface magnetic systems. This setup consists of a scanning tunneling microscope (STM) designed for ultra high vacuum (UHV) operating at T ≲ 1 K and strong magnetic fields (0 - 8.5 T). Such extreme experimental conditions have influenced the choice of the construction materials and components as well as the general design principles. This work comprises the design, realization and first tests of the STM, cryostat, damping system, sample preparation UHV chambers and transfer system. In parallel to this new setup realization, we have investigated the magnetic properties of dilute transition metal impurities deposited on non-magnetic metal surfaces. Measurements of those systems were performed with x-ray magnetic circular dichroism (XCMD). We have started by adressing the host electronic density effect on magnetic properties of the impurities. In order to make contact with the Friedel [1] and Anderson [2] models, 3d impurities (Fe and Ni) were deposited on alkali metal substrates (K, Na et Li) whose electronic structure is close to the Fermi electron gas. The gradual increase of the host electronic density with decreasing atomic weight going from K to Li allow us to study the effects of s - d electron hybridization on the magnitude magnetic moment of the impurity . In particular, we show that the magnetic moment of Fe is strongly reduced in going from K to Li, while the Ni moment disappears on Na and Li. The XMCD data also show that the orbital magnetic moment is atomic-like on K, but compared to the spin moment, significantly reduces as the substrate electron density increases. Finally, we report on a variable temperature STM study of the nucleation and growth processes of Pd/Pt(111). The experimental results are analyzed on the basis of mean field theory applied to surface diffusion processes and solved by numerical integration. This study allows us to highlight the different atomistic processes that contribute to the nucleation and growth of a prototype epitaxial system.

EPFL2005