73 research outputs found
Electric Control of Spin Transitions at the Atomic Scale
Electric control of spins has been a longstanding goal in the field of solid
state physics due to the potential for increased efficiency in information
processing. This efficiency can be optimized by transferring spintronics to the
atomic scale. We present electric control of spin resonance transitions in
single molecules by employing electron spin resonance scanning tunneling
microscopy (ESR-STM). We find strong bias voltage dependent shifts in the ESR
signal of about ten times its linewidth, which is due to the electric field
induced displacement of the spin system in the tunnel junction. This opens up
new avenues for ultrafast control of coupled spin systems, even towards atomic
scale quantum computing, and expands on understanding and optimizing spin
electric coupling in bulk materials.Comment: 11 pages, 13 figures, including supplementary informatio
Robustness of Yu-Shiba-Rusinov resonances in presence of a complex superconducting order parameter
Robust quantum systems rely on having a protective environment with minimized
relaxation channels. Superconducting gaps play an important role in the design
of such environments. The interaction of localized single spins with a
conventional superconductor generally leads to intrinsically extremely narrow
Yu-Shiba-Rusinov (YSR) resonances protected inside the superconducting gap.
However, this may not apply to superconductors with nontrivial, energy
dependent order parameters. Exploiting the Fe-doped two-band superconductor
NbSe, we show that due to the nontrivial relation between its complex
valued and energy dependent order parameters, YSR states are no longer
restricted to be inside the gap. They can appear outside the gap (i. e. inside
the coherence peaks), where they can also acquire a substantial intrinsic
lifetime broadening. T-matrix scattering calculations show excellent agreement
with the experimental data and relate the intrinsic YSR state broadening to the
imaginary part of the host's order parameters. Our results suggest that
non-thermal relaxation mechanisms contribute to the finite lifetime of the YSR
states, even within the superconducting gap, making them less protected against
residual interactions than previously assumed. YSR states may serve as valuable
probes for nontrivial order parameters promoting a judicious selection of
protective superconductors.Comment: 11 pages, 8 figures, including supporting informatio
Single Channel Josephson Effect in a High Transmission Atomic Contact
The Josephson effect in scanning tunneling microscopy (STM) is an excellent
tool to probe the properties of the superconducting order parameter on a local
scale through the Ambegaokar-Baratoff (AB) relation. Using single atomic
contacts created by means of atom manipulation, we demonstrate that in the
extreme case of a single transport channel through the atomic junction
modifications of the current-phase relation lead to significant deviations from
the linear AB formula relating the critical current to the involved gap
parameters. Using the full current-phase relation for arbitrary channel
transmission, we model the Josephson effect in the dynamical Coulomb blockade
regime because the charging energy of the junction capacitance cannot be
neglected. We find excellent agreement with the experimental data. Projecting
the current-phase relation onto the charge transfer operator shows that at high
transmission multiple Cooper pair tunneling may occur. These deviations become
non-negligible in Josephson-STM, for example, when scanning across single
adatoms.Comment: 9 pages, 6 figures, including supplementary informatio
Microwave-assisted tunneling and interference effects in superconducting junctions under fast driving signals
The following article appeared in Physical Review B 101.13 (2020): 134507. DOI:10.1103/PhysRevB.101.134507. Open access publication funded by the Max Planck SocietyAs scanning tunneling microscopy is pushed towards fast local dynamics, a quantitative understanding of tunnel junctions under the influence of a fast ac driving signal is required, especially at the ultralow temperatures relevant to spin dynamics and correlated electron states. We subject a superconductor-insulator-superconductor junction to a microwave signal from an antenna mounted in situ and examine the dc response of the contact to this driving signal. Quasiparticle tunneling and the Josephson effect can be interpreted in the framework of Tien-Gordon theory. The situation is more complex when it comes to higher-order effects such as multiple Andreev reflections. Microwave-assisted tunneling unravels these complex processes, providing deeper insights into tunneling than are available in a pure dc measurementJ.C.C. acknowledges funding from the Spanish Ministry of Economy and Competitiveness (MINECO) (Contract No. FIS2017-84057-P). J.A. acknowledges support from the IQST and the German Science Foundation (DFG) under Grant No. AN336/11-1. This work was funded in part by the ERC Consolidator Grant AbsoluteSpin (Grant No. 681164
Extracting the Transport Channel Transmissions in Scanning Tunneling Microscopy using the Superconducting Excess Current
Transport through quantum coherent conductors, like atomic junctions, is
described by the distribution of conduction channels. Information about the
number of channels and their transmission can be extracted from various
sources, such as multiple Andreev reflections, dynamical Coulomb blockade, or
shot noise. We complement this set of methods by introducing the
superconducting excess current as a new tool to continuously extract the
transport channel transmissions of an atomic scale junction in a scanning
tunneling microscope. In conjunction with ab initio simulations, we employ this
technique in atomic aluminum junctions to determine the influence of the
structure adjacent to the contact atoms on the transport properties.Comment: 8 pages, 9 figures, including supporting informatio
Extracting transport channel transmissions in scanning tunneling microscopy using superconducting excess current
Transport through quantum coherent conductors, such as atomic junctions, is described by conduction
channels. Information about the number of channels and their transmissions can be extracted from various
sources, such as multiple Andreev reflections, dynamical Coulomb blockade, or shot noise. We complement
this set of methods by introducing the superconducting excess current as a new tool to continuously extract the
transport channel transmissions of an atomic scale junction in a scanning tunneling microscope. In conjunction
with ab initio simulations, we employ this technique in atomic aluminum junctions to determine the influence of
the structure adjacent to the contact atoms on the transport propertie
Combining Electron Spin Resonance Spectroscopy with Scanning Tunneling Microscopy at High Magnetic Fields
Magnetic media remain a key in information storage and processing. The
continuous increase of storage densities and the desire for quantum memories
and computers pushes the limits of magnetic characterisation techniques.
Ultimately, a tool which is capable of coherently manipulating and detecting
individual quantum spins is needed. The scanning tunnelling microscope (STM) is
the only technique which unites the prerequisites of high spatial and energy
resolution, low temperature and high magnetic fields to achieve this goal.
Limitations in the available frequency range for electron spin resonance STM
(ESR-STM) mean that many instruments operate in the thermal noise regime. We
resolve challenges in signal delivery to extend the operational frequency range
of ESR-STM by more than a factor of two and up to 100GHz, making the Zeeman
energy the dominant energy scale at achievable cryogenic temperatures of a few
hundred millikelvin. We present a general method for augmenting existing
instruments into ESR-STMs to investigate spin dynamics in the high-field limit.
We demonstrate the performance of the instrument by analysing inelastic
tunnelling in a junction driven by a microwave signal and provide proof of
principle measurements for ESR-STM.Comment: 8 pages, 7 figure
Tunneling dynamics between superconducting bound states at the atomic limit
Despite plenty of room at the bottom, there is a limit to the miniaturization
of every process. For charge transport this is realized by the coupling of
single discrete energy levels at the atomic scale. Here, we demonstrate
sequential tunneling between parity protected Yu-Shiba-Rusinov (YSR) states
bound to magnetic impurities located on the superconducting tip and sample of a
scanning tunneling microscope at 10 mK. We reduce the relaxation of the excited
YSR state to the bare minimum and find an enhanced lifetime for single
quasiparticle levels. Our work offers a way to characterize and to manipulate
coupled superconducting bound states, such as Andreev levels, YSR states, or
Majorana bound states.Comment: 14 pages, 10 figures incl. supplementary informatio
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