arXiv subjects

Jochen Mannhart

Publications and source records attributed to Jochen Mannhart.

36 records · Page 2Linked to original sources

arXiv2019

High-temperature-grown buffer layer boosts electron mobility in epitaxial La-doped BaSnO$_3$/SrZrO$_3$ heterostructures

By inserting a SrZrO$_3$ buffer layer between the film and the substrate, we demonstrate a significant reduction of the threading dislocation density with an associated improvement of the electron mobility in La:BaSnO$_3$ films. A room temperature mobility of 140 cm$^2$ V$^{-1}\text{s}^{-1}$ is achieved for 25-nm-thick films without any post-growth treatment. The density of threading dislocations is only $4.9\times 10^{9}$ cm$^{-2}$ for buffered films prepared on (110) TbScO$_3$ substrates by pulsed laser deposition.

cond-mat.mtrl-sci↗

arXiv2019

Magnetic Properties of Epitaxially-grown $SrRuO_3$ Nanodots

We present the fabrication and exploration of arrays of nanodots of $SrRuO_3$ with dot sizes between 500 nm and 15 nm. Down to the smallest dot size explored, the samples were found to be magnetic with a maximum of the Curie temperature $T_C$ achieved by dots of 30 nm diameter. This peak in $T_C$ is associated with a dot-size-induced relief of the epitaxial strain, as evidenced by scanning transmission electron microscopy.

cond-mat.mes-hall↗

arXiv2018

Coherently strained epitaxial YBa$_2$Cu$_3$O$_{7-δ}$ films grown on NdGaO$_3$ (110)

YBa$_2$Cu$_3$O$_{7-δ}$ is a good candidate to systematically study high-temperature superconductivity by nanoengineering using advanced epitaxy. An essential prerequisite for these studies are coherently strained YBa$_2$Cu$_3$O$_{7-δ}$ thin films, which we present here using NdGaO$_3$ (110) as a substrate. The films are coherent up to at least 100 nm thickness and have a critical temperature of 89$\pm$1 K. The $a$ and $b$ lattice parameters of the YBa$_2$Cu$_3$O$_{7-δ}$ are matched to the in-plane lattice parameters of NdGaO$_3$ (110), resulting in a large reduction of the orthorhombicity of the YBa$_2$Cu$_3$O$_{7-δ}$. These results imply that a large amount of structural disorder in the chain layers of YBa$_2$Cu$_3$O$_{7-δ}$ is not detrimental to superconductivity.

cond-mat.supr-con↗

arXiv2018

Modulation of superconducting transition temperature in LaAlO$_{3}$/SrTiO$_{3}$ by SrTiO$_{3}$ structural domains

The tetragonal domain structure in SrTiO$_{3}$ (STO) is known to modulate the normal-state carrier density in LaAlO$_{3}$/SrTiO$_{3}$ (LAO/STO) heterostructures, among other electronic properties, but the effect of STO domains on the superconductivity in LAO/STO has not been fully explored. Using a scanning SQUID susceptometer microscope to map the superconducting response as a function of temperature in LAO/STO, we find that the superconducting transition temperature is spatially inhomogeneous and modulated in a pattern that is characteristic of structural domains in the STO.

cond-mat.supr-con↗

arXiv2018

Single-gap superconductivity and dome of superfluid density in Nb-doped SrTiO3

SrTiO$_3$ exhibits a superconducting dome upon doping with Nb, with a maximum critical temperature \mbox{$T_\mathrm{c} \approx 0.4$~K}. Using microwave stripline resonators at frequencies from 2 to 23~GHz and temperatures down to 0.02~K, we probe the low-energy optical response of superconducting SrTiO$_3$ with charge carrier concentration from 0.3 to $2.2\times 10^{20}$~cm$^{-3}$, covering the majority of the superconducting dome. We find single-gap electrodynamics even though several electronic bands are superconducting. This is explained by a single energy gap $2Δ$ due to gap homogenization over the Fermi surface consistent with the low level of defect scattering in Nb-doped SrTiO$_3$. Furthermore, we determine $T_\mathrm{c}$, $2Δ$, and the superfluid density as a function of charge carrier concentration, and all three quantities exhibit the characteristic dome shape.

cond-mat.supr-con↗

arXiv2018

Independence of surface morphology and reconstruction during the thermal preparation of perovskite oxide surfaces

Using a CO$_2$ laser to directly heat the crystals from the back side, SrTiO$_3$ substrates may be thermally prepared in situ for epitaxy without the need for ex-situ etching and annealing. We find that the formation of large terraces with straight steps at 900-1100 °C is independent from the formation of the ideal surface reconstruction for epitaxy, which requires temperatures in excess of 1200 °C to complete. The process may be universal, at least for perovskite oxide surfaces, as it also works, at different temperatures, for LaAlO$_3$ and NdGaO$_3$, two other widely used oxide substrate materials.

cond-mat.mtrl-sci↗

arXiv2017

Design and realization of a sputter deposition system for the \textit{in situ-} and \textit{in operando-}use in polarized neutron reflectometry experiments

We report on the realization of a sputter deposition system for the in situ- and in operando-use in polarized neutron reflectometry experiments. Starting with the scientific requirements, which define the general design considerations, the external limitations and boundaries imposed by the available space at a neutron beamline and by the neutron and vacuum compatibility of the used materials, are assessed. The relevant aspects are then accounted for in the realization of our highly mobile deposition system, which was designed with a focus on a quick and simple installation and removability at the beamline. Apart from the general design, the in-vacuum components, the auxiliary equipment and the remote control via a computer, as well as relevant safety aspects are presented in detail.

physics.ins-det↗

arXiv2017

In-gap features in superconducting LaAlO$_3$-SrTiO$_3$ interfaces observed by tunneling spectroscopy

We identified quasiparticle states at well-defined energies inside the superconducting gap of the electron system at the LaAlO$_3$-SrTiO$_3$ interface using tunneling spectroscopy. The states are found only in a number of samples and depend upon the thermal-cycling history of the samples. The states consist of a peak at zero energy and other peaks at finite energies, symmetrically placed around zero energy. These peaks disappear, together with the superconducting gap, with increasing temperature and magnetic field. We discuss the likelihood of various physical mechanisms that are known to cause in-gap states in superconductors and conclude that none of these mechanisms can easily explain the results. The conceivable scenarios are the formation of Majorana bound states, Andreev bound states, or the presence of an odd-frequency spin triplet component in the superconducting order parameter.

cond-mat.supr-con↗

arXiv2017

In-situ Polarized Neutron Reflectometry: Epitaxial Thin Film Growth of Fe on Cu(001) by DC Magnetron Sputtering

The step-wise growth of epitaxial Fe on Cu(001)/Si(001), investigated by in-situ polarized neutron reflectometry is presented. A sputter deposition system was integrated into the neutron reflectometer AMOR at the Swiss neutron spallation source SINQ, which enables the analysis of the microstructure and magnetic moments during all deposition steps of the Fe layer. We report on the progressive evolution of the accessible parameters describing the microstructure and the magnetic properties of the Fe film, which reproduce known features and extend our knowledge on the behavior of ultrathin iron films.

cond-mat.mtrl-sci↗

arXiv2016

The mechanism of spin-orbit coupling in a 2D oxide interface

The presence of spin-orbit coupling drives the anomalous magnetotransport at oxide interfaces and forms the basis for numerous intriguing properties of these 2D electron systems, such as topologically protected phases or anti-localization. For many of those systems, the identification of the underlying coupling mechanism is obfuscated by multi-band effects. We therefore analyze the transport of LaAlO$_3$/SrTiO$_3$ interfaces under high pressures, a technique to single out the multi-band contributions. We argue that the observed magnetoresistance is due to quantum interference and not related to Coulomb interaction. Therefore, this system is an excellent candidate to generate a metal-insulator transition of the long-sought symplectic 2D universality class. It is shown that the spin-orbit coupling can be linked unambiguously to the band structure with a cubic (Dresselhaus-like) rather than a linear (Rashba-like) spin-orbit band splitting.

cond-mat.str-el↗

arXiv2016

Quantum-Matter Heterostructures

Combining the power and possibilities of heterostructure engineering with the collective and emergent properties of quantum materials, quantum-matter heterostructures open a new arena of solid-state physics. Here we provide a review of interfaces and heterostructures made of quantum matter. Unique electronic states can be engineered in these structures, giving rise to unforeseeable opportunities for scientific discovery and potential applications. We discuss the present status of this nascent field of quantum-matter heterostructures, its limitations, perspectives, and challenges.

cond-mat.mtrl-sci↗

arXiv2016

Artificial Atoms Based on Correlated Materials

Low-dimensional electron systems fabricated from quantum matter have in recent years become available and are being explored with great intensity. This article gives an overview of the fundamental properties of such systems and summarizes the state of the field. We furthermore present and consider the concept of artificial atoms fabricated from quantum materials, anticipating remarkable scientific advances and possibly important applications of this new field of research. The surprising properties of these artificial atoms and of molecules or even of solids assembled from them are presented and discussed.

cond-mat.str-el↗

arXiv2015

Electron-phonon Coupling and the Superconducting Phase Diagram of the LaAlO3-SrTiO3 Interface

The superconductor at the LaAlO3-SrTiO3 interface provides a model system for the study of two-dimensional superconductivity in the dilute carrier density limit. Here we experimentally address the pairing mechanism in this superconductor. We extract the electron-phonon spectral function from tunneling spectra and conclude, without ruling out contributions of further pairing channels, that electron-phonon mediated pairing is strong enough to account for the superconducting critical temperatures. Furthermore, we discuss the electron-phonon coupling in relation to the superconducting phase diagram. The electron-phonon spectral function is independent of the carrier density, except for a small part of the phase diagram in the underdoped region. The tunneling measurements reveal that the increase of the chemical potential with increasing carrier density levels off and is zero in the overdoped region of the phase diagram. This indicates that the additionally induced carriers do not populate the band that hosts the superconducting state and that the superconducting order parameter therefore is weakened by the presence of charge carriers in another band.

cond-mat.supr-con↗

arXiv2013

Locally enhanced conductivity due to the tetragonal domain structure in LaAlO$_{3}$/SrTiO$_{3}$ heterointerfaces

The ability to control materials properties through interface engineering is demonstrated by the appearance of conductivity at the interface of certain insulators, most famously the {001} interface of the band insulators LaAlO$_{3}$ and TiO$_{2}$-terminated SrTiO$_{3}$ (STO). Transport and other measurements in this system show a plethora of diverse physical phenomena. To better understand the interface conductivity, we used scanning superconducting quantum interference device microscopy to image the magnetic field locally generated by current in an interface. At low temperature, we found that the current flowed in conductive narrow paths oriented along the crystallographic axes, embedded in a less conductive background. The configuration of these paths changed on thermal cycling above the STO cubic-to-tetragonal structural transition temperature, implying that the local conductivity is strongly modified by the STO tetragonal domain structure. The interplay between substrate domains and the interface provides an additional mechanism for understanding and controlling the behaviour of heterostructures.

cond-mat.mtrl-sci↗

arXiv2013

Tuning the Ultrafast Spin Dynamics in Carrier-Density-Controlled Ferromagnets

Ultrafast strengthening or quenching of the ferromagnetic order of semiconducting Eu1-xGdxO was achieved by resonant photoexcitation. The modification of the magnetic order is established within 3 ps as revealed by optical second harmonic generation. A theoretical analysis shows that the response is determined by the interplay of chemically and optically generated carriers in a nonequilibrium scenario beyond the three-temperature model. General criteria for the design of spintronics materials with tunable ultrafast spin dynamics are given.

cond-mat.str-el↗

arXiv2012

Giant third-order magneto-optical rotation in ferromagnetic EuO

A magnetization-induced rotation in the third-order nonlinear optical response is observed in out-of-plane-magnetized epitaxial EuO films. We discuss the relation of this nonlinear magneto-optical rotation to the linear Faraday rotation. It is allowed in all materials but, in contrast to the linear Faraday rotation, not affected by the reduction of the thickness of the material. Thus, the third-order magneto-optical rotation is particularly suitable for probing the magnetization of functional magnetic materials such as ultra-thin films and multilayers.

cond-mat.str-el↗

arXiv2009

Calculation of the Capacitances of Conductors -- Perspectives for the Optimization of Electronic Devices

The equation describing the capacitance of capacitors is determined. It is shown that by optimizing the material of the conducting electrodes, the capacitance of capacitors reaching the quantum regime can be substantially enhanced or reduced. Dielectric capacitors with negative total capacitances are suggested and their properties analyzed. Resulting perspectives to enhance the performance of electronic devices are discussed.

cond-mat.mtrl-sci↗

arXiv2008

Flux Periodicities in Loops of Nodal Superconductors

Supercurrents in superconducting flux threaded loops are expected to oscillate with the magnetic flux with a period of hc/2e. This is indeed true for s-wave superconductors larger than the coherence length xi_0. Here we show that for superconductors with gap nodes, there is no such strict condition for the supercurrent to be hc/2e rather than hc/e periodic. For nodal superconductors, the flux induced Doppler shift of the near nodal states leads to a flux dependent occupation probability of quasi-particles circulating clockwise and counter clockwise around the loop, which leads to an hc/e periodic component of the supercurrent, even at zero temperature. We analyze this phenomenon on a cylinder in an approximative analytic approach and also numerically within the framework of the BCS theory. Specifically for d-wave pairing, we show that the hc/e periodic current component decreases with the inverse radius of the loop and investigate its temperature dependence.

cond-mat.supr-con↗