arXiv subjects

Elke Scheer

Publications and source records attributed to Elke Scheer.

33 records · Page 2Linked to original sources

arXiv2023

Microscopic theory of supercurrent suppression by gate-controlled surface depairing

Recently gate-mediated supercurrent suppression in superconducting nano-bridges has been reported in many experiments. This could be either a direct or an indirect gate effect. The microscopic understanding of this observation is not clear till now. Using the quasiclassical Green's function method, we show that a small concentration of magnetic impurities at the surface of the bridges can significantly help to suppress superconductivity and hence the supercurrent inside the systems while applying a gate field. This is because the gate field can enhance the depairing through the exchange interaction between the magnetic impurities at the surface and the superconductor. We also obtain a \emph{symmetric} suppression of the supercurrent with respect to the gate field, a signature of a direct gate effect. Future experiments can verify our predictions by modifying the surface with magnetic impurities.

cond-mat.mes-hall↗

arXiv2023

Interplay of Andreev reflection and Coulomb blockade in hybrid superconducting single electron transistors

We study the interplay between Coulomb blockade and superconductivity in a tunable superconductor-superconductor-normal metal single-electron transistor. The device is realized by connecting the superconducting island via an oxide barrier to the normal metal lead and with a break junction to the superconducting lead. The latter enables Cooper pair transport and (multiple) Andreev reflection. We show that those processes are relevant also far above the superconducting gap and that signatures of Coulomb blockade may reoccur at high bias while they are absent for small bias in the strong-coupling regime. Our experimental findings agree with simulations using a master equation approach in combination with the full counting statistics of multiple Andreev reflection.

cond-mat.supr-con↗

arXiv2023

Simulating bistable current-induced switching of metallic atomic contacts by electron-vibration scattering

We present a microscopic model, describing current-driven switching in metallic atomic-size contacts. Applying a high current through an atomic-size contact, creates a strong electronic nonequilibrium that excites vibrational modes by virtue of the electron-vibration coupling. Using density functional theory (DFT) in combination with the Landauer-Büttiker theory for phase-coherent transport, expressed in terms of nonequilibrium Green's functions (NEGFs), we study the current-induced forces arising from this nonequilibrium and determine those vibrational modes which couple most strongly to the electronic system. For single-atom lead (Pb) contacts we show specific candidates for bistable switches, consisting of two similar atomic configurations with differing electric conductance. We identify vibrational modes that induce a transition between these configurations. Our results reveal a possible origin of bistable switching in atomic-size contacts through excitation of vibrations by inelastic electron scattering and underline the power of the combined DFT-NEGF approach and statistical mechanics analysis of a Langevin equation to overcome the time-scale gap between atomic motion and rare switching events, allowing for an efficient exploration of the contacts' configurational phase space.

cond-mat.mes-hall↗

arXiv2022

Quantitative signal extraction in the dynamic range of nanomechanical systems by free and constrained fitting

We present a free and a constrained fitting procedure for quantitative signal extraction of nanomechanical systems in the dynamic range and for physical model testing. We demonstrate that applying the free-fitting procedure to the measured frequency response of silicon nitride (SiN) nanomembranes at varying pressure enables us to disentangle the intrinsic membrane vibration properties from the system response, thereby giving quantitative access to the eigenfrequency, quality factor, coupling strength between resonator and drive system, and to system noise. The validity of physical models for quantities such as excitation, fluctuations, and damping mechanisms can be verified by imposing additional mathematical links between different physical parameters as constraints in the constrained fitting procedure. We verify the performance of the constrained fitting procedure for the same samples tested in various experimental setups.

cond-mat.mes-hall↗

arXiv2021

Unconventional Meissner screening induced by chiral molecules in a conventional superconductor

The coupling of a superconductor to a different material often results in a system with unconventional superconducting properties. A conventional superconductor is a perfect diamagnet expelling magnetic fields out of its volume, a phenomenon known as Meissner effect. Here, we show that the simple adsorption of a monolayer of chiral molecules, which are non-magnetic in solution, onto the surface of a conventional superconductor can markedly change its diamagnetic Meissner response. By measuring the internal magnetic field profile in superconducting Nb thin films under an applied transverse field by low-energy muon spin rotation spectroscopy, we demonstrate that the local field profile inside Nb is considerably modified upon molecular adsorption in a way that also depends on the applied field direction. The modification is not limited to the chiral molecules/Nb interface, but it is long ranged and occurs over a length scale comparable to the superconducting coherence length. Zero-field muon spin spectroscopy measurements in combination with our theoretical analysis show that odd-frequency spin-triplet states induced by the chiral molecules are responsible for the modification of the Meissner response observed inside Nb. These results indicate that a chiral molecules/superconductor system supports odd-frequency spin-triplet pairs due to the molecules acting as a spin active layer and therefore they imply that such system can be used as a simpler alternative to superconductor/ferromagnet or superconductor/topological insulator hybrids for the generation and manipulation of unconventional spin-triplet superconducting states.

cond-mat.supr-con↗

arXiv2021

Mechanically Modulated Sideband and Squeezing Effects of Membrane Resonators

We investigate the sideband spectra of a driven nonlinear mode with its eigenfrequency being modulated at a low frequency (< 1 kHz). This additional parametric modulation leads to prominent antiresonance lineshapes in the sideband spectra, which can be controlled through the vibration state of the driven mode. We also establish a direct connection between the antiresonance frequency and the squeezing of thermal fluctuation in the system. Our work not only provides a simple and robust method for squeezing characterization but also opens a new possibility toward sideband applications.

physics.class-ph↗

arXiv2021

Persistent response in ultra-strongly driven mechanical membrane resonators

We study experimentally and theoretically the phenomenon of persistent response in ultra-strongly driven membrane resonators. This term denotes the development of a vibrating state with nearly constant amplitude over an extreme wide frequency range. We reveal the underlying mechanism of the persistent response state by directly imaging the vibrational state using advanced optical interferometry. We argue that the persistent state is related to the nonlinear interaction between higher order flexural modes and higher-order overtones of the driven mode. Finally, we propose a stability diagram for the different vibrational states that the membrane can adopt.

physics.app-ph↗

arXiv2019

Spatial modulation of nonlinear flexural vibrations of membrane resonators

We study the vibrational motion of membrane resonators upon strong drive in the strongly nonlinear regime. By imaging the vibrational state of rectangular siliconnitride membrane resonators and by analyzing the frequency response using optical interferometry, we show that upon increasing the driving strength, the membrane adopts a peculiar deflection pattern formed by concentric rings superimposed onto the drum head shape of the fundamental mode. Such a circular symmetry cannot be described as a superposition of a small number of excited linear eigenmodes. Furthermore, the different parts of the membrane oscillate at different multiples of the drive frequency, an observation that we denominate as 'localization of overtones'. We introduce a phenomenological model that is based on the coupling of a very small number of effective nonlinear oscillators, representing the different parts of the membrane, and that describes the experimental observations.

cond-mat.mes-hall↗

arXiv2018

Creation of equal-spin triplet superconductivity at the Al/EuS interface

In conventional superconductors, electrons of opposite spins are bound into Cooper pairs. However, when the superconductor is in contact with a non-uniformly ordered ferromagnet, an exotic type of superconductivity can appear at the interface, with electrons bound into three possible spin-triplet states. Triplet pairs with equal spin play a vital role in low-dissipation spintronics. Despite the observation of supercurrents through ferromagnets, spectroscopic evidence for the existence of equal-spin triplet pairs is still missing. Here we show a theoretical model that reveals a characteristic gap structure in the quasiparticle density of states which provides a unique signature for the presence of equal-spin triplet pairs. By scanning tunnelling spectroscopy we measure the local density of states to reveal the spin configuration of triplet pairs. We demonstrate that the Al/EuS interface causes strong and tunable spin-mixing by virtue of its spin-dependent transmission.

cond-mat.supr-con↗

arXiv2018

Vibrational modes of ultrathin carbon nanomembrane mechanical resonators

We report measurements of vibrational mode shapes of mechanical resonators made from ultrathin carbon nanomembranes (CNMs) with a thickness of approximately 1 nm. CNMs are prepared from electron irradiation induced cross-linking of aromatic self-assembled monolayers (SAMs) and the variation of membrane thickness and/or density can be achieved by varying the precursor molecule. Single- and triple-layer freestanding CNMs were made by transferring them onto Si substrates with square/rectangular orifices. The vibration of the membrane was actuated by applying a sinusoidal voltage to a piezoelectric disk on which the sample was glued. The vibrational mode shapes were visualized with an imaging Mirau interferometer using a stroboscopic light source. Several mode shapes of a square membrane can be readily identified and their dynamic behavior can be well described by linear response theory of a membrane with negligible bending rigidity. Applying Fourier transformations to the time-dependent surface profiles, the dispersion relation of the transverse membrane waves can be obtained and its linear behavior confirms the membrane model. Comparing the dispersion relation to an analytical model, the static stress of the membranes was determined and found to be caused by the fabrication process.

cond-mat.mtrl-sci↗

arXiv2016

Signature of magnetic-dependent gapless odd frequency states at superconductor/ferromagnet interfaces

The theory of superconductivity developed by Bardeen, Cooper and Schrieffer (BCS) explains the stabilization of electron pairs into a spin-singlet, even frequency, state by the formation of an energy gap within which the density of states is zero. At a superconductor interface with an inhomogeneous ferromagnet, a gapless odd frequency superconducting state is predicted, in which the Cooper pairs are in a spin-triplet state. Although indirect evidence for such a state has been obtained, the gap structure and pairing symmetry have not so far been determined. Here we report scanning tunnelling spectroscopy of Nb superconducting films proximity coupled to epitaxial Ho. These measurements reveal pronounced changes to the Nb subgap superconducting density of states on driving the Ho through a metamagnetic transition from a helical antiferromagnetic to a homogeneous ferromagnetic state for which a BCS-like gap is recovered. The results prove odd frequency spin-triplet superconductivity at superconductor/inhomogeneous magnet interfaces.

cond-mat.supr-con↗

arXiv2015

Force-noise spectroscopy by tunnelling current deflection sensing

An electro-mechanical setup for the measurement of AC-forces in a low-temperature tunnelling microscope has been developed, which enables extremely high force resolution. The crosstalk of vibrations onto the tunnelling current is used to measure the deflection of a force-sensing cantilever beam. We demonstrate its capability to measure the noise of the force at a tunnelling contact using polycrystalline Iridium. Depending on temperature, spring constant and current, a resolution in the range of $\rm {fN}/\sqrt{\rm Hz}$ is possible. We observe peak levels of the force-noise at the energy of the expected phonon maximal density of states, which suggests that inelastic transport processes contribute to force fluctuations.

cond-mat.mes-hall↗

arXiv2013

Influence of vibrations on electron transport through nanoscale contacts

In this article we present a novel semi-analytical approach to calculate first-order electron-vibration coupling constants within the framework of density functional theory. It combines analytical expressions for the first-order derivative of the Kohn-Sham operator with respect to nuclear displacements with coupled-perturbed Kohn-Sham theory to determine the derivative of the electronic density matrix. This allows us to efficiently compute accurate electron-vibration coupling constants. We apply our approach to describe inelastic electron tunneling spectra of metallic and molecular junctions. A gold junction bridged by an atomic chain is used to validate the developed method, reproducing established experimental and theoretical results. For octane-dithiol and octane-diamine single-molecule junctions we discuss the influence of the anchoring group and mechanical stretching on the inelastic electron tunneling spectra.

cond-mat.mes-hall↗

arXiv2010

Mechanical control of vibrational states in single-molecule junctions

We report on inelastic electron tunneling spectroscopy measurements carried out on single molecules incorporated into a mechanically controllable break-junction of Au and Pt electrodes at low temperature. Here we establish a correlation between the molecular conformation and conduction properties of a single-molecule junction. We demonstrate that the conductance through single molecules crucially depends on the contact material and configuration by virtue of their mechanical and electrical properties. Our findings prove that the charge transport via single molecules can be manipulated by varying both the molecular conformation (e.g., trans or gauche) and the contact material.

cond-mat.mes-hall↗

arXiv2009

Transport Channels in a Double Junction- coherent coupling changes the picture

Transport through a point contact is accurately modelled by assigning to the junction an ensemble of independent transport channels with possibly different transmissions. We here argue that for a series of two contacts, coherently coupled across an island, the transport channels are different from the ensembles that would describe each contact taken as stand-alone device. We further show that instead of two sets of channels with manifold cross-links over the island the double junction can be described by pairs of channels from both sides coherently coupled together, where each pair, however, has no coherent connection to the others. This finding will substantially simplify modelling transport by a Green's functions technique. Additional channels through only one junction may complete the picture. Finally we discuss how partial coherence across the island with an appropriate ansatz can be modelled in the same scheme.

cond-mat.mes-hall↗