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Physics Special Student's Colloquium | The Racah Institute of Physics

Physics Special Student's Colloquium

Date: 
Mon, 05/04/202112:00-13:30
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Two Speakers:  1. Hen Alpern, Racah Institute of Physics, HUJI
Title: " Unconventional superconductivity and magnetic-related states induced in a conventional superconductor by nonmagnetic chiral molecules"
Abstract:
 Hybrid ferromagnetic/superconducting systems are well known for hosting intriguing phenomena such as emergent triplet superconductivity at their interfaces and the appearance of in-gap, spin-polarized Yu-Shiba-Rusinov (YSR) surface-states bound to magnetic impurities. In this work we demonstrate that similar phenomena can be induced in a conventional superconductor upon chemisorbing non-magnetic chiral molecules. By applying scanning tunneling spectroscopy, we show that the singlet-pairing s-wave order parameter of Nb, NbN and NbSe2 is significantly altered upon the adsorption of chiral polyalanine alpha-helix molecules. The tunneling spectra exhibit zero-bias conductance peaks embedded inside gaps or gap-like features, suggesting the emergence of a triplet-pairing component, corroborated by fits to theoretical spectra. Conductance spectra measured on devices comprising NbSe2 flakes over which these chiral molecules were adsorbed, exhibit, in some cases, in-gap states nearly symmetrically positioned around zero bias. These states shift apart with magnetic field, akin to YSR states, as corroborated by theoretical simulations. Other samples show evidence for a collective phenomenon of hybridized YSR-like states giving rise to unconventional superconductivity, manifested in the conductance spectra by the appearance of a zero bias conductance peak that diminishes, but does not split, with magnetic field, consistent with p-wave triplet superconductivity. The transition between these two scenarios appears to be governed by the density of adsorbed molecules. Finally, low-energy muon spin rotation/relaxation (LE-μSR) data demonstrate a strong modification of the screening supercurrent distribution deep inside a Nb film upon adsorption of chiral molecules, providing evidence for unconventional chiral-induced superconductivity. The adsorption-modified local magnetic field profile inside the Nb film monitored by LE-mSR, a measure of the screening modification, is well fitted to a model calculation where the molecular layer is considered as an insulating spin-active interface that is proximity-coupled to the Nb film, inducing there triplet superconductivity. Moreover, the zero-field LE-mSR measurements suggest the appearance of a state of broken time-reversal symmetry in this hybrid system.

2.   Daniel Cohen, ,  Racah Institute of Physics, HUJI
Title: "Nano scale NMR – the quest for enhanced sensitivity"
Abstract:
 Nuclear Magnetic Resonance (NMR) is a well-established field that greatly contributed to fundamental science as well as the development of practical applications - from study of molecular structures to the development of the MRI machine.

Classic NMR devices require both the sample size and the external magnetic field to be large in order to generate a measurable signal. Effectively, this means that the signal is averaged over many molecules, which results in a limited precision and resolution of the technique.

Nano scale NMR is a recent extension of the field that explores the use of quantum sensors to overcome the signal-to-noise problem of minute samples, which holds the promise for superior resolution.

In recent years, Nitrogen-Vacancy centers in a diamond have been used to measure the NMR spectra of various samples with great success.

However, the use of a quantum sensor for enhanced sensitivity to magnetic fields comes at the price of enhanced sensitivity to magnetic noise, which limits the senor’s coherence time and consequently the precision. The main source of noise in experiments using liquid samples stems from the diffusion of the nuclear spins that causes the magnetic field generated by the sample to fluctuate.

In this talk, I will discuss the effects of this noise on state-of-the-art experiments in the field and present results that show that, in fact, the diffusion noise leads to long lasting correlations that might have a dramatic effect on resolution measurements. I will then show preliminary experimental results from our collaborators that support our theoretical model.