Date: January 20th, 2020
Time: 12:30-15:30

QNS will meet Science Counselors and Deputies of EU Countries + Switzerland and Norway.
During their visit we present of our center and provide the labs tour.

More detailed schedule will be updated soon!

Academic Affiliation: Leiden University, Netherlands

Research Area: Develop and use novel techniques for STM to investigate quantum matter.

Talk date: January 21, 2020

Charge trapping and super-Poissonian noise centers in a cuprate superconductor

In this talk I will present new insight into the mystery of highly anisotropic transport in a cuprate high-temperature superconductor and show that these materials can trap additional charges. Above the superconducting transition these materials are perfectly metallic along the crystal planes (ab-plane), but are insulating in the c-axis, with ratios between in-plane and perpendicular resistance exceeding 104. This anisotropy has been identified as one of the key mysteries in the cuprates and has been connected to the mechanism of high-temperature superconductivity. I will show how we employ our newly developed scanning noise spectroscopy technique, for which we combined a scanning tunneling microscope (STM) with a novel MHz frequency amplifier to bring noise-spectroscopy measurements to the atomic scale
[1]. We discover surprisingly large deviations from the expected Poissonian noise of uncorrelated electron transport in this strongly correlated material. A behavior that is familiar from highly polarizable insulators and represents strong evidence for trapping of charge in the charge reservoir layers of the cuprates. Our measurements suggest a picture of metallic layers separated by polarizable insulators within a three-dimensional superconducting state. We will show how these new observations connect to the mystery of the anisotropic transport in this high-temperature superconductor, shedding new light onto this issue [2].

References:
[1] K.M. Bastiaans et al. Rev. Sci. Instrum. 89, 093709 (2018).
[2] K.M. Bastiaans et al. Nature Physics 14, 1183 (2018).

Academic Affiliation: Leiden University, Netherlands

Research Area: Develop and use novel techniques for STM to investigate quantum matter.

Talk date: January 22, 2020

A strongly inhomogeneous superfluid in an iron-based superconductor

Although the possibility of spatial variations in the superfluid of unconventional, strongly correlated superconductors has been suggested, it is not known whether such inhomogeneities – if they exist – are driven by disorder, strong scattering, or other factors. In this talk I will show how we use Josephson scanning tunneling microscopy to reveal a strongly inhomogeneous superfluid in the iron-based superconductor FeTe_0.55Se_0.45. By simultaneously acquiring the topographic and electronic properties, we find that this inhomogeneity in the superfluid is not caused by structural disorder or strong inter-pocket scattering, and does not correlate with variations in the energy of the Cooper pair-breaking gap. Instead, we see a clear spatial correlation between superfluid density and the quasiparticle strength, defined as the height of the coherence peak, on a local scale. [1]

References:
[1] D. Cho*, K.M. Bastiaans* et al. Nature 571, 541 (2019)

Academic Affiliation: C2N, Department of Nanoelectronics, CNRS, France

Research Area: STM spectroscopy, Superconductivity, Topological Insulators

Research stay period: February 2020 – July 2020

Academic Affiliation: Research Center Jülich, Germany

Research Area: Understing fundamental properties on the nanoscale with the focus on magnetic excitations and correlated systems.

Research stay period: February 09 – 22, 2020

During the visit Prof. Ternes will give a talk in our Center

Sensing dark spins with a scanning tunneling microscope

Probing the spin of individual atoms and molecules on surfaces by means of inelastic electron tunneling spectroscopy (IETS) has since its introduction [1] developed to a very successful and widely used method in scanning tunneling microscopy [2]. Recently, IETS measurements have been also taken simultaneously on two magnetic impurities, one attached to the apex at the tip, the other one on the sample surface, revealing correlation-driven transport asymmetries, reminiscent of spinpolarized transport in a magnetic field [3]. In this experiment only the spin on the surface was spectroscopically active while the one on the tip was spectroscopically dark.

Here now I will show how the transport is significantly altered when the tunneling electron is interacting with both spins simultaneously; the one on sample and the one on the tip apex. Different to measurements on two S = 1spins which only showed the expected steps in the differential conductance at each individual excitation and the sum of both excitations [4] we observe complex IETS data when we use a functionalized tip apex with a high S = 3/2 Kondo system to probe a S = 2 Fe adatom on the CuN surface. To successfully simulate such spectrum, Kondo as well as potential scattering processes have to be taken into account whereby strong interference effects due to the fermionic nature of the tunneling electrons lead to novel selection rules [5].

The understanding of the transport rules is important because it enables one to use well understood surface supported spins to calibrate the spin at the ill defined tip apex. Successively, such spin can then be used as a well characterized mobile sensor with unprecedented spacial resolution [6].

References:
[1] A.J. Heinrich, J.A. Gupta, C.P. Lutz, and D.M. Eigler, Science 306, 466 (2004).
[2] M. Ternes, New J. Phys. 17, 063016 (2015).
[3] M. Muenks, P. Jacobson, M. Ternes, and K. Kern, Nature Comm. 8, 14119 (2017).
[4] M. Ormaza, et al., Nano Lett. 17, 1877 (2017).
[5] M. Ternes, C.P. Lutz, A.J. Heinrich, and W.-D. Schneider, arXiv:1908.08267 [cond-mat.meshall].
[6] B. Verlhac, et al., Science 366, 623 (2019).

The information about the talk time and location will be updated soon!

Academic Affiliation: Princeton University, USA

Research Area: At the forefront of quantum materials research is the goal to understand how new quantum phenomena can emerge from the topology of electronic wavefunctions or correlations arising from electron-electron interactions. The Yazdani Lab has contributed significantly to this research paradigm by harnessing the power of high-resolution scanning tunneling microscopy (STM) techniques to directly visualize electronic wavefunctions in topological and correlated quantum materials.

Research stay period: February 12 – 13, 2020

More detailed information about the visit will be updated soon!