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AG Scheer
Mesoscopic Systems

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Low Temperature Group

Here we present some of the low-temperature experiments performed in our group. We investigate fundamental questions of mesoscopic systems, as described on the Research site. 

Thermo-voltage of nano-thermocouples

Dr. Bastian Kopp, Thomas Möller

Study of thermoelectric effects in nanostructures gives important additional information about charge transport, also regarding possible life-time limiting phenomena and applications for the conversion of light energy via heat into electrical energy. The scope of this project is to gain deep insight into the charge transport mechanism, by studying thermo-voltage effects of metallic atomic-sized contacts, at low temperatures. Using a mechanically-controlled break junction (MCBJ) mechanism, we are able to use a laser as a heat source for the creation of a temperature difference (∆T), while measuring the thermo-voltage obtained through the junction. A technique to create and detect the temperature gradient was established.

 a) Spatially resolved measurements of the thermo-voltage across the junction. The data is presented over an optical image. b) A temperature difference (∆T) map, in which ∆T is measured in every lasing position across the sample. The orange structure represents the gold structure.

 Funding: SFB767

Low temperature scanning tunneling microscopy study of nanostructure including measurements of shot noise

Hong Zheng

The subject of the project is the investigation of the electronic transport in nanostructures with the help of low-temperature scanning probe techniques. The aim is to reveal the nature of the transport which is required for the development of functional atomic and molecular devices in future nanoscale devices. One of the most powerful and theoretically best studied ways for electronic correlations measurement is the electronic shot noise. It’s the second cumulant of the full counting statistics of the current.

Previously we have developed a technique to simultaneously determine the geometrical structure and the shot noise of atomic-scale structures with sub-atomic special resolution. The system consists of three main parts: home-made low-temperature STM with very high thermal, mechanical and electronic stability, electronics and low noise pre-amplifier. And we will further develop the technique and apply it to reveal the electronic correlation and the charge transport mechanism in atomic-size structures.

At present stage we work on TiN film samples demonstrating superconductor-insulator transition and try to find out the mechanism of this transition which has been under debate for decades.

Electronic transport and magnetisation effects in atomic-scale transition metal junctions

Figure 1:Measurements of palladium a) Magneto-conductance in a field perpendicular to the current at a single atomic contact. b) Anisotropic magneto-conductance of two different contacts with a cos2 fit.
Florian Strigl, Martin Keller, Elke Scheer & Torsten Pietsch

The ongoing downsizing of electronic circuits leads to the question of where the ultimate limit would be. Naturally, the smallest possible current transporting structures are molecules or single atoms. The same accounts for magnetic storage units. While band-magnets hit the so called superparamagnetic limit, the tendency for magnetic order in reduced dimensions can be enhanced for other materials. For the transition metals platinum, palladium and iridium there are predictions of magnetism in atomic size contacts1,2,3.
The aim of this project is to identify this magnetism in the magnetic field dependence of the conductance of atomic contacts produced via the mechanical controllable break junction method. For platinum it was possible to find evidence for magnetism by Strigl et al.4, but different contributions from geometrical structure, spin-orbit-coupling, or spin-dependent scattering could not be discriminated. Nevertheless one can conclude that for about 30% of the atomic contacts in Pt the magnetic moments are aligned non-collinearly. The further aims are to reproduce these measurements also in other transition metals, to get further insight in the interplay of different contributions.
The first set of measurements of palladium contacts at different conductance values has been completed, showing similarities to the previous Pt-measurements. Figure 1a and b show typical magneto- and anisotropic magneto-conductance curves for palladium. Further measurements on iridium atomic contacts are planned. We hope to get more information about the unquenching of the d-orbitals and a model of the magnetism in reduced dimensions of these transition metals.

(1) A. Delin: Physical Review B 2003, 8, 144434
(2) K. Smeloava: Physical Review B 2008, 77, 033408
(3) A. Delin: Physical Review Letter 2004, 92, 057201
(4) F. Strigl: Nature Communications 2015, 6, 6172
(5) K. Bolotin: Physical Review Letter 2006, 97, 127202

Scanning tunneling spectroscopy to probe odd-triplet contributions to the long-ranged proximity effect

S. Diesch1, C. Sürgers2, D. Beckmann2, P. Machon1, W. Belzig1, E. Scheer1

[1] Universität Konstanz, Konstanz, Germany
[2] Karlsruhe Institute of Technology, Karlsruhe, Germany

In conventional superconductors, electrons are bound in singlet Cooper pairs, i.e. with opposite spin. More recently, experiments on superconductor-ferromagnet-systems have shown Cooper pairs tunneling through ferromagnetic layers, indicating Cooper pairs of equal spin, thus corresponding to a long-range triplet proximity effect [1]. Most experimental evidence for triplet superconductivity comes from observations of the thickness dependence of the Josephson current through a ferromagnetic barrier, and there now is an increasing amount of direct spectroscopic evidence (e.g. our results measured on niobium/holmium multilayers [2]) to test the existing theoretical models [3].

This project aims to analyze the DOS of thin films of the ferromagnetic insulator europium sulfide on superconducting aluminum or vice versa, using a scanning tunneling microscope in spectroscopy mode at 280 mK and in varying magnetic fields. We observe significant broadening of the superconducting energy gap and a variety of sub-gap structures induced by the presence of the ferromagnet. We interpret our findings based on the diffusive theory [4,5] and a more advanced circuit theory model [4].

References

[1] F. S. Bergeret, Phys. Rev. Lett. 86, 4096 (2001)

[2] A. Di Bernardo, Nat. Comm. 6:8053 (2015)

[3] F. Hübler, Phys. Rev. Lett. 109, 87004 (2012)

[4] P. Machon, Phys. Rev. Lett. 110, 047002 (2013)

[5] J. Linder, Phys. Rev. B 81, 214504 (2010)

Size Controlled Luminescent Semiconductor-Metal Hybrid Nanostructures: Interplay between Size, Optical and Transport Properties Explored on the Single Particle Level

Figure 1: Scanning tunneling spectroscopy (STS) of individual silicon nanoparticle of ~ 5 nm
Dr. Tuhin Basu

Silicon-metal hybrid nanoscale objects with controlled dimension and desired morphology are exceptionally promising candidates for exploring fundamental physics of small-systems. They provide useful functionalities by coupling size-dependent properties of the individual nano-components. In this project, an attempt will be implemented to synthesize hybrid nanoscale objects consisting of silicon quantum dots and noble metal (gold and silver) nanoparticles by facile one pot wet-chemical strategy. Due to exciton-plasmon coupling in these silicon-metal hetero-nanostructures, a modification in band structure is expected that leads to a complex and competitive decay dynamics of excitons. As a consequence the photophysical and electrical properties of this hybrid nanoscale objects will certainly change. Importantly this band structure can be altered by changing size, shape of the individual component and their overall orientation. Unfortunately characterization of the ensemble of these hybrid nanoscale objects will only give an overall average effect in which the determination of exact reason behind the property modification is not feasible. Therefore in this project we will try to find out the exact decay dynamics of the excitons and their role in property modification by combining optical (steady state and time resolved) and electrical property investigation in an ensemble (by making a colloidal suspension) as well as in single particle level. Thus we can substantiate the precise correlation between size-shape-orientation of these hybrid nanoscale objects and their modified intrinsic properties. Finally utilization of these silicon-metal hybrid nanoscale objects will be attempted as an efficient energy harvesting thermoelectric material.   

 

Large oscillations of the magnetoresistance in nanopatterned thin superconducting films

A SEM image of a double network aluminum sample.
A magnetoresistance measurements on a double network of aluminum.
F. Strigl, formerly C. Espy

Little and Parks showed with their experiments that the transition temperature of a superconducting cylinder oscillates in an external magnetic field with a period of Φ = h/2e [1]. Theoretical studies have found that there should be a h/e periodicity in ring-shaped or networked nano-ring unconventional superconductors that can arise via various mechanisms when the ring diameter becomes smaller than the zero-temperature coherence length, ξ0. Three proposed mechanisms that might give rise to h/e periodicity include the Aharonov-Bohm effect [2], a mechanism based on the dependence of the Cooper pair's internal energy on its motion [3], or finally via the formation of quasi-particles [4]. There is even speculation that this periodicity should be seen in rings made out of aluminum, a conventional superconductor, with diameters smaller than the coherence length [5].

Carillo et al. investigated this proposed periodicity in a single YBCO loop [6], Gammel et al. investigated simple networks of YBCO [7], and Sochnikov et al. began investigating this proposed periodicity in double network patterns of LSCO [8]. None of the investigations found the sought after periodicity and the amplitude of the observed oscillations was much larger than one would expect from the Little-Parks effect. Sochnikov et al. propose that this enhanced amplitude is due to interaction between thermally excited moving vortices and the oscillating persistent currents in the loops. Based on their experiments on a niobium ladder, Berdiyorov et al. propose that the enhanced amplitude is rather due to interaction between the applied current and the Meissner currents in the superconductor [9].

The aim of this project is to perform the first experiments looking for h/e-periodicity in various geometries of elemental superconductors. This data will help answer the question whether this behavior is or is not unique to high-Tc superconductors, thereby adding another important piece to the puzzle of the mechanism of high-Tc superconductivity. This project is performed in collaboration with the Institute for Superconductivity at Bar-Ilan University in Israel. 

[1] W. A. Little and R. D. Parks. Phys. Rev. Lett. 9, 9 (1962)

[2] T.-C. Wei and P. M. Goldbart Phys. Rev. B 7, 224512 (2008)

[3] V. Vakaryuk Phys. Rev. Lett. 101, 167002 (2008)

[4] F. Loder et al. New J. Phys. 11, 075005 (2009)

[5] F. Loder et al. Phys. Rev. B. 78, 174526 (2008)

[6] I. Sochnikov et al. Nature Nanotechnology 5, 516–519 (2010)

Interplay between Superconductivity and Charging Effects

Calculated IV-curves
Calculated IV-curves for two different parameter sets
T. Lorenz, E. Scheer, formerly U. Schröter and P. Kar Choudhury

The goal of this project is to investigate the behavior of a breakjunction in a single-electron-transitor-like circuit, that means put in series with a tunnel junction such that an island of small capacitance is formed between the junctions. In point-contacts with channels of rather high transmission, like breakjunctions, higher-order Andreev reflections become of importance. The principal question is whether the island and the tunnel junction will cause a suppression of multiple Andreev reflection (MAR) in the breakjunction due to Coulomb blockade, or whether transport even involving MAR can be established without requiring high island charging energies. For modelling, a Green's functions algorithm, known to describe single arbitrary transmission junctions, is merged with a rate equation method to account for the island charging. In calculated current-voltage curves, there is no unique step pattern. A two-fold threshold is found for the main onset, which can be at eV=2Δ or eV=2Ec.

Orthodox theory is not valid any more with a contact of higher transmission. Allowing transport to keep coherence all across the island through both junctions or not will also make a difference. A further challenge is to include transport of Cooper pairs into the modeling formalism, which can so far only handle quasi-particles in the superconducting state.

Öffnet externen Link im aktuellen FensterPhys. Rev. B (74), 245301 (2006)
Öffnet externen Link im aktuellen FensterPhys. Rev. B (76), 205104 (2007)
Öffnet externen Link im aktuellen FensterJ. Phys. A: Math. Theor. (41), 265202 (2008)
Öffnet externen Link im aktuellen FensterJ. Phys. A: Math. Theor. (41), 375203 (2008)

 

 

 

Atomic Contacts at High Currents

D. Weber

Funding by DFG

We investigate electromigration effects in atomic-size contacts of aluminum (Al), lead (Pb), niobium (Nb) and gold (Au), fabricated with the mechanically controllable break junction technique at temperatures T < 1 K. We observe discrete current-driven conductance changes ΔG due to rearrangements at the atomic scale.1 In particular situations, a reversible switching between two conductance values is observed (>500 repetitions) which we attribute to the formation of preferred atomic configurations (see figure). An increasing current is driven through an atomic contact until a jump in the conductance value signals an atomic rearrangement. The same procedure is repeated in the opposite current direction, and so on until the system is trapped in a bistable switching state.

At low temperatures, Al, Pb and Nb become superconducting, and so called multiple Andreev reflections can be observed. This effect gives the possibility to observe individual conductance channels in the contact2 which is shown in Fig 2.

[1]    Schirm, C.; Matt, M.; Pauly, F.; Cuevas, J. C.; Nielaba, P.; Scheer, E. Nat. Nanotechnol. 2013, 8 (9), 645–648.

[2]     Scheer, E.; Joyez, P.; Esteve, D.; Urbina, C.; Devoret, M. H. Phys. Rev. Lett. 1997, 78 (18), 3535.

Fig 1: Conductance (top) and applied current (bottom) of an single-atom aluminum contact over time.
Fig 2: Electronic channel contribution of a bistable configuration of an aluminum contact extracted of MAR spectra.