Mercator Fellow 2016 and 2017

Prof. Dr. Juan Carlos Cuevas

Prof. Dr. Juan Carlos Cuevas
Departamento de Física Teórica de la Materia Condensada 05
Facultad de Ciencias
Universidad Autónoma de Madrid (Spain)

Period: July - December 2016 and July - December 2017

The SFB 767 hosted Prof. Dr. Juan Carlos Cuevas, Associate Professor at the Universidad Autónoma de Madrid (E), as Mercator Fellow for two periods of 6 months in Konstanz, in 2016 and 2017.

Prof. Cuevas is one of the leading theoreticians in several fields connected to the SFB 767.
In 2011 he was awarded an “InnoLecture” guest professorship by the Carl-Zeiss-Foundation, which he already spent at the University of Konstanz. He is also the author of the first textbook on molecular electronics (Molecular Electronics: An Introduction to Theory and Experiment) together with Prof. Dr. Elke Scheer of the SFB 767.
In his scientific career, Prof. Cuevas has made fundamental contributions to the theory of molecular electronics, superconducting nanocontacts and to nanoplasmonics.

In molecular electronics he was one of the first to push forward the theoretical approach combining first-principles methods with non-equilibrium Green’s function techniques. Corroborated by model-based intuitive investigations of transport he was able to shed light on numerous experimental observations in the field – often in direct cooperation with experimental groups. In this field he has long-standing connections the PIs E. Scheer (C02) and F. Pauly (C13 and A08). In addition the projects C04 and C14 are also working experimentally in this field. Prof. Cuevas’ expertise on mesoscopic superconducting transport was instrumental for the understanding of the so-called multiple Andreev reflections in superconducting contacts out of equilibrium and investigations of spectral properties of the proximity effect. He has a long-standing collaboration with W. Belzig (C03) in this area. Furthermore, superconducting substrates and leads are used experimentally in C02 and C04.

Important publications related to the fellowship in Konstanz

Local Density of States in Clean 2D SNS Heterostructures

Abstract: Motivated by recent advances in the fabrication of Josephson junctions in which the weak link is made of a low-dimensional nonsuperconducting material, we present here a systematic theoretical study of the local density of states (LDOS) in a clean two-dimensional normal metal (N) coupled to two s-wave superconductors (S). To be precise, we employ the quasiclassical theory of superconductivity in the clean limit, based on Eilenberger's equations, to investigate the phase-dependent LDOS as a function of factors such as the length or the width of the junction, a finite reflectivity, and a weak magnetic field. We show how the spectrum of Andreev bound states that appear inside the gap shape the phase-dependent LDOS in short and long junctions. We discuss the circumstances when a gap appears in the LDOS and when the continuum displays a significant phase dependence. The presence of a magnetic flux leads to a complex interference behavior, which is also reflected in the supercurrent-phase relation. Our results agree qualitatively with recent experiments on graphene SNS junctions. Finally, we show how the LDOS is connected to the supercurrent that can flow in these superconducting heterostructures and present an analytical relation between these two basic quantities.

D. Nikolic*, J.C. Cuevas, W. Belzig*
Phys. Rev. Research 1, 033031 (2019)
DOI: 10.1103/PhysRevResearch.1.033031

[* = SFB767 members]

Peltier Cooling in Molecular Junctions - Nature Nanotechnology 13 (2018)

This article has been highlighted on the cover of the February 2018 issue of Nature Nanotechnology.

Abstract: The study of thermoelectricity in molecular junctions is of fundamental interest for the development of various technologies including cooling (refrigeration) and heat-to-electricity conversion. Recent experimental progress in probing the thermopower (Seebeck effect) of molecular junctions has enabled studies of the relationship between thermoelectricity and molecular structure. However, observations of Peltier cooling in molecular junctions—a critical step for establishing molecular-based refrigeration—have remained inaccessible. Here, we report direct experimental observations of Peltier cooling in molecular junctions. By integrating conducting-probe atomic force microscopy with custom-fabricated picowatt-resolution calorimetric microdevices, we created an experimental platform that enables the unified characterization of electrical, thermoelectric and energy dissipation characteristics of molecular junctions. Using this platform, we studied gold junctions with prototypical molecules (Au–biphenyl-4,4′-dithiol–Au, Au–terphenyl-4,4′′-dithiol–Au and Au–4,4′-bipyridine–Au) and revealed the relationship between heating or cooling and charge transmission characteristics. Our experimental conclusions are supported by self-energy-corrected density functional theory calculations. We expect these advances to stimulate studies of both thermal and thermoelectric transport in molecular junctions where the possibility of extraordinarily efficient energy conversion has been theoretically predicted.  

L. Cui, R. Miao, K. Wang, D. Thompson, L. A. Zotti, J. C. Cuevas, E. Meyhofer and P. Reddy
Nature Nanotech. 13, 122 (2018)
DOI: 10.1038/s41565-017-0020-z

Microwave Spectroscopy Reveals the Quantum Geometric Tensor ... arXiv:1810.11227 (2018)

Abstract: Topology is providing new insight into condensed matter physics problems. Concepts like Chern numbers and their relation to physical phenomena have become very familiar, but actually, key quantities like the quantum geometric tensor, which provides a much deeper information about quantum states, remain experimentally difficult to access. Recently it has been shown that multiterminal superconducting junctions constitute an ideal playground to mimic topological systems in a controlled manner. Here, we theoretically study the spectrum of Andreev bound states in topological Josephson matter and demonstrate that the full information of the quantum geometric tensor of the ground state manifold can be extracted with the help of microwave spectroscopy. In particular, we develop the concept of artificially polarized microwaves, which can be used to obtain both the quantum metric tensor and the Berry curvature. The quantized integrated absorption provides a direct evidence of topological quantum properties of the Andreev states.

R.L. Klees, G. Rastelli*, J.C. Cuevas, W. Belzig*
arXiv:1810.11227 (2018)

[* = SFB767 members]

Enhancing Near-Field Radiative Heat Transfer with Si-based Metasurfaces - Phys. Rev. Lett. 118 (2017)

Abstract: We demonstrate in this work that the use of metasurfaces provides a viable strategy to largely tune and enhance near-field radiative heat transfer between extended structures. In particular, using a rigorous coupled wave analysis, we predict that Si-based metasurfaces featuring two-dimensional periodic arrays of holes can exhibit a room-temperature near-field radiative heat conductance much larger than any unstructured material to date. We show that this enhancement, which takes place in a broad range of separations, relies on the possibility to largely tune the properties of the surface plasmon polaritons that dominate the radiative heat transfer in the near-field regime.

V. Fernández-Hurtado, F. J. García-Vidal, Shanhui Fan, and J. C. Cuevas
Phys. Rev. Lett. 118, 203901 (2017)
DOI: 10.1103/PhysRevLett.118.203901

Quantized Thermal Transport in Single Atom Junctions - Science 355 (2017)

Abstract: Thermal transport in individual atomic junctions and chains is of great fundamental interest because of the distinctive quantum effects expected to arise in them. By using novel, custom-fabricated, picowatt-resolution calorimetric scanning probes, we measured the thermal conductance of gold and platinum metallic wires down to single-atom junctions. Our work reveals that the thermal conductance of gold single-atom junctions is quantized at room temperature and shows that the Wiedemann-Franz law relating thermal and electrical conductance is satisfied even in single-atom contacts. Furthermore, we quantitatively explain our experimental results within the Landauer framework for quantum thermal transport. The experimental techniques reported here will enable thermal transport studies in atomic and molecular chains, which will be key to investigating numerous fundamental issues that thus far have remained experimentally inaccessible.

L. Cui, W. Jeong, S. Hur, M. Matt*, J. C. Klöckner*, F. Pauly*, P. Nielaba*, J. C. Cuevas, E. Meyhofer, and P. Reddy
Science 355, 1192 (2017)
DOI: 10.1126/science.aam6622

[* = SFB767 members]

Thermal Conductance of Metallic Atomic-Size Contacts ... Phys. Rev. B 96 (2017)

Abstract: Motivated by recent experiments [Science 355, 1192 (2017); Nat. Nanotechnol. 12, 430 (2017)], we present here an extensive theoretical analysis of the thermal conductance of atomic-size contacts made of three different metals, namely gold (Au), platinum (Pt), and aluminum (Al). The main goal of this work is to elucidate the role of phonons in the thermal transport through these atomic contacts as well as to study the validity of the Wiedemann-Franz law, which relates the electrical and the thermal conductance. For this purpose, we have employed two different custom-developed theoretical approaches. The first one is a transport method based on density functional theory (DFT) that allows one to accurately compute the contributions of both electrons and phonons to the thermal transport in few-atom-thick contacts. The second technique is based on a combination of classical molecular dynamics (MD) simulations and a tight-binding model that enables the efficient calculation of the electronic contribution to the thermal conductance of atomic contacts of larger size. Our DFT-based calculations show that the thermal conductance of few-atom contacts of Au and Pt is dominated by electrons, with phonons giving a contribution typically below 10% of the total thermal conductance, depending on the contact geometry. For these two metals we find that the small deviations from the Wiedemann-Franz law, reported experimentally, largely stem from phonons. In the case of Al contacts we predict that the phononic contribution can be considerably larger with up to 40% of the total thermal conductance. We show that these differences in the phononic contribution across metals originate mainly from their distinct Debye energies. On the other hand, our MD-based calculations demonstrate that the electronic contribution to the thermal conductance follows very closely the Wiedemann-Franz law, irrespective of the material and the contact size. Finally, the ensemble of our results consistently shows that the reported observation of quantized thermal transport at room temperature is restricted to few-atom contacts of Au, a monovalent metal in which the transport is dominated by the s valence orbitals. In the case of multivalent metals like Pt and Al this quantization is statistically absent due to the fact that additional orbitals contribute to the transport with conduction channels that have intermediate transmissions between 0 and 1, even in the case of single-atom contacts.

J.C. Klöckner*, M. Matt*, P. Nielaba*, F. Pauly*, J.C. Cuevas
Phys. Rev. B 96, 205405 (2017)
DOI: 10.1103/PhysRevB.96.205405

[* = SFB767 members]

Length Dependence of the Thermal Conductance of Alkane-Based Single-Molecule Junctions, Phys. Rev. B 94 (2016)

Abstract: Motivated by recent experiments, we present here a systematic ab initio study of the length dependence of the thermal conductance of single-molecule junctions. We make use of a combination of density functional theory with nonequilibrium Green's function techniques to investigate the length dependence of the phonon transport in single-alkane chains, contacted with gold electrodes via both thiol and amine anchoring groups. Additionally, we study the effect of the substitution of the hydrogen atoms in the alkane chains by heavier fluorine atoms to form polytetrafluoroethylenes. Our results demonstrate that (i) the room-temperature thermal conductance is fairly length independent for chains with more than 5 methylene units and (ii) the efficiency of the thermal transport is strongly influenced by the strength of the phononic metal-molecule coupling. Our study sheds light on the phonon transport in molecular junctions, and it provides clear guidelines for the design of molecular junctions for thermal management.

J.C. Klöckner*, M. Bürkle, J.C. Cuevas, F. Pauly*
Phys. Rev. B 94, 205425 (2016)
DOI: 10.1103/PhysRevB.94.205425

[* = SFB767 members]

  1. J. Senkpiel, S. Dambach, M. Etzkorn, R. Drost, C. Padurariu, B. Kubala, W. Belzig, A. Levy Yeyati, J.C. Cuevas, J. Ankerhold, C.R. Ast, K. Kern
    Single Channel Josephson Effect in a High Transmission Atomic Contact
  2. W. Schosser, L.A. Zotti, J.C. Cuevas, F. Pauly
    Doping Hepta-Alanine with Tryptophan: A Theoretical Study of its Effect on the Electrical Conductance of Peptide-Based Single-Molecule Junctions
    J. Chem. Phys. 150, 174705 (2019)
  3. E. Moncada-Villa, A.I. Fernández-Domínguez, J. C. Cuevas
    Magnetic-Field Controlled Anomalous Refraction in Doped Semiconductors
    J. Opt. Soc. Am. B 36, 935 (2019)
  4. V. Fernández-Hurtado, A.I. Fernández-Domínguez J. Feist, F.J. Garcia-Vidal, J.C. Cuevas
    Super-Planckian Far-Field Radiative Heat Transfer
    Phys. Rev. B 97, 045408 (2018)
  5. R.M. Abraham Ekeroth, P. Ben-Abdallah, J.C. Cuevas, A. García-Martín
    Anisotropic Thermal Magnetoresistance for an Active Control of Radiative Heat Transfer
    ACS Photonics 5, 705 (2018)
  6. J.C. Klöckner, J.C. Cuevas, F. Pauly
    Transmission Eigenchannels for Coherent Phonon Transport
    Phys. Rev. B 97, 155432 (2018)
  7. J.A. Fereiro, X. Yu, I. Pecht, M. Sheves, J.C. Cuevas, D. Cahen
    Tunneling Explains Efficient Electron Transport via Protein Junctions
    Proc. Natl. Acad. Sci. USA 115, E4577 (2018)
  8. V. Fernández-Hurtado, A.I. Fernández-Domínguez J. Feist, F.J. García-Vidal, J.C. Cuevas
    Exploring the Limits of Super-Planckian Far-Field Radiative Heat Transfer Using 2D Materials
    ACS Photonics 5, 3082 (2018)
  9. L. Cui, W. Jeong, V. Fernández-Hurtado, J. Feist, F.J. Garcia-Vidal, J.C. Cuevas, E. Meyhofer, P. Reddy
    Study of Radiative Heat Transfer in Angström- and nanometre-sized gaps
    Nature Comm. 8, 14479 (2017)
  10. J.C. Klöckner, R. Siebler, J.C. Cuevas, F. Pauly
    Thermal Conductance and Thermoelectric Figure of Merit of C60-based Single-Molecule Junctions: Electrons, Phonons, and Photons
    Phys. Rev. B 95, 245404 (2017)
  11. R.M. Abraham Ekeroth, A. García-Martín, J.C. Cuevas
    Thermal Discrete Dipole Approximation for the Description of Thermal Emission and Radiative Heat Transfer of Magneto-Optical Systems
    Phys. Rev. B 95, 235428 (2017)
  12. J.C. Klöckner, J.C. Cuevas, F. Pauly
    Tuning the Thermal Conductance of Molecular Junctions with Interference Effects
    Phys. Rev. B 96, 245419 (2017)