arXiv subjects

D. Meier

Publications and source records attributed to D. Meier.

At least 37 records · Page 2Linked to original sources

arXiv2020

Effect of pseudo-cubic (111)-oriented orthorhombic substrate facets on perovskite oxide thin film synthesis

Strain engineering with different substrate facets is promising for tuning functional properties of thin film perovskite oxides. By choice of facet, different surface symmetries and chemical bond directions for epitaxial interfaces can be tailored. Here, preparation of well-defined pseudo-cubic (111)-oriented orthorhombic substrates of DyScO3 , GdScO3 , and NdGaO3 is reported. The choice of orthorhombic facet, (011)o or (101)o , both corresponding to pseudo-cubic (111)pc , gives vicinal surfaces with single or double (111pc layer terrace step heights, respectively, impacting subsequent thin film growth. Orthorhombic LaFeO3 epitaxy on the (101)o facet reveals a distinction between alternating (111)pc layers, both during and after growth. The observed differences are explained based on the oxygen octahedral tilt pattern relative to the two orthorhombic (111)pc surfaces. This robust structural detail in the orthorhombic perovskite oxides enables utilisation of different (111)pc facets for property engineering, through polyhedral connectivity control and cation coordination at epitaxial interfaces.

cond-mat.mtrl-sci↗

arXiv2020

Controlling local resistance via electric-field induced dislocations

Dislocations are one-dimensional (1D) topological line defects where the lattice deviates from the perfect crystal structure. The presence of dislocations transcends condensed matter research and gives rise to a diverse range of emergent phenomena [1-6], ranging from geological effects [7] to light emission from diodes [8]. Despite their ubiquity, to date, the controlled formation of dislocations is usually achieved via strain fields, applied either during growth [9,10] or retrospectively via deformation, e.g., (nano [11-14])-indentation [15]. Here we show how partial dislocations can be induced using local electric fields, altering the structure and electronic response of the material where the field is applied. By combining high-resolution imaging techniques and density functional theory calculations, we directly image these dislocations in the ferroelectric hexagonal manganite Er(Ti,Mn)O3 and study their impact on the local electric transport behaviour. The use of an electric field to induce partial dislocations is a conceptually new approach to the burgeoning field of emergent defect-driven phenomena and enables local property control without the need of external macroscopic strain fields. This control is an important step towards integrating and functionalising dislocations in practical devices for future oxide electronics.

cond-mat.mtrl-sci↗

arXiv2020

Intrinsic and extrinsic conduction contributions at nominally neutral domain walls in hexagonal manganites

Conductive and electrostatic atomic force microscopy (cAFM and EFM) are used to investigate the electric conduction at nominally neutral domain walls in hexagonal manganites. The EFM measurements reveal a propensity of mobile charge carriers to accumulate at the nominally neutral domain walls in ErMnO3, which is corroborated by cAFM scans showing locally enhanced d.c. conductance. Our findings are explained based on established segregation enthalpy profiles for oxygen vacancies and interstitials, providing a microscopic model for previous, seemingly disconnected observations ranging from insulating to conducting domain wall behavior. In addition, we observe variations in conductance between different nominally neutral walls that we attribute to deviations from the ideal charge-neutral structure within the bulk, leading to a superposition of extrinsic and intrinsic contributions. Our study clarifies the complex transport properties at nominally neutral domain walls in hexagonal manganites and establishes new possibilities for tuning their electronic response based on oxidation conditions, opening the door for domain-wall based sensor technology.

physics.app-ph↗

arXiv2020

Electrostatic potential mapping at ferroelectric domain walls by low-temperature photoemission electron microscopy

Low-temperature X-ray photoemission electron microscopy (X-PEEM) is used to measure the electric potential at domain walls in improper ferroelectric Er0.99Ca0.01MnO3. By combining X-PEEM with scanning probe microscopy and theory, we develop a model that relates the detected X-PEEM contrast to the emergence of uncompensated bound charges, explaining the image formation based on intrinsic electronic domain-wall properties. In contrast to previously applied low-temperature electrostatic force microscopy (EFM), X-PEEM readily distinguishes between positive and negative bound charges at domain walls. Our study introduces an X-PEEM based approach for low-temperature electrostatic potential mapping, facilitating nanoscale spatial resolution and data acquisition times in the order of 0.1-1 sec.

cond-mat.mtrl-sci↗

arXiv2019

Penetrating particle ANalyzer (PAN)

PAN is a scientific instrument suitable for deep space and interplanetary missions. It can precisely measure and monitor the flux, composition, and direction of highly penetrating particles ($> \sim$100 MeV/nucleon) in deep space, over at least one full solar cycle (~11 years). The science program of PAN is multi- and cross-disciplinary, covering cosmic ray physics, solar physics, space weather and space travel. PAN will fill an observation gap of galactic cosmic rays in the GeV region, and provide precise information of the spectrum, composition and emission time of energetic particle originated from the Sun. The precise measurement and monitoring of the energetic particles is also a unique contribution to space weather studies. PAN will map the flux and composition of penetrating particles, which cannot be shielded effectively, precisely and continuously, providing valuable input for the assessment of the related health risk, and for the development of an adequate mitigation strategy. PAN has the potential to become a standard on-board instrument for deep space human travel. PAN is based on the proven detection principle of a magnetic spectrometer, but with novel layout and detection concept. It will adopt advanced particle detection technologies and industrial processes optimized for deep space application. The device will require limited mass (~20 kg) and power (~20 W) budget. Dipole magnet sectors built from high field permanent magnet Halbach arrays, instrumented in a modular fashion with high resolution silicon strip detectors, allow to reach an energy resolution better than 10\% for nuclei from H to Fe at 1 GeV/n.

physics.space-ph↗

arXiv2018

Spincaloritronic measurements: a round robin comparison of the longitudinal spin Seebeck effect

The rising field of spin caloritronics focuses on the interactions between spin and heat currents in a magnetic material; the observation of the spin Seebeck effect opened the route to this branch of research. This paper reports the results of a round robin test performed by five partners on a single device highlighting the reproducibility problems related to the measurements of the spin Seebeck coefficient, the quantity that describes the strength of the spin Seebeck effect. This work stimulated the search for more reproducible measurement methods through the analysis of the systematic effects.

cond-mat.mes-hall↗

arXiv2018

Longitudinal spin Seebeck coefficient: heat flux vs. temperature difference method

The determination of the longitudinal spin Seebeck effect (LSSE) coefficient is currently plagued by a large uncertainty due to the poor reproducibility of the experimental conditions used in its measurement. In this work we present a detailed analysis of two different methods used for the determination of the LSSE coefficient. We have performed LSSE experiments in different laboratories, by using different setups and employing both the temperature difference method and the heat flux method. We found that the lack of reproducibility can be mainly attributed to the thermal contact resistance between the sample and the thermal baths which generate the temperature gradient. Due to the variation of the thermal resistance, we found that the scaling of the LSSE voltage to the heat flux through the sample rather than to the temperature difference across the sample greatly reduces the uncertainty. The characteristics of a single YIG/Pt LSSE device obtained with two different setups was $(1.143\pm0.007)\cdot 10^{-7}$ Vm/W and $(1.101\pm0.015)\cdot 10^{-7}$ Vm/W with the heat flux method and $(2.313\pm0.017)\cdot 10^{-7}$ V/K and $(4.956\pm0.005)\cdot 10^{-7}$ V/K with the temperature difference method. This shows that systematic errors can be considerably reduced with the heat flux method.

cond-mat.mes-hall↗

arXiv2018

Electrical half-wave rectification at ferroelectric domain walls

Ferroelectric domain walls represent multifunctional 2D-elements with great potential for novel device paradigms at the nanoscale. Improper ferroelectrics display particularly promising types of domain walls, which, due to their unique robustness, are the ideal template for imposing specific electronic behavior. Chemical doping, for instance, induces p- or n-type characteristics and electric fields reversibly switch between resistive and conductive domain-wall states. Here, we demonstrate diode-like conversion of alternating-current (AC) into direct-current (DC) output based on neutral 180$^{\circ}$ domain walls in improper ferroelectric ErMnO$_3$. By combining scanning probe and dielectric spectroscopy, we show that the rectification occurs for frequencies at which the domain walls are fixed to their equilibrium position. The practical frequency regime and magnitude of the output is controlled by the bulk conductivity. Using density functional theory we attribute the transport behavior at the neutral walls to an accumulation of oxygen defects. Our study reveals domain walls acting as 2D half-wave rectifiers, extending domain-wall-based nanoelectronic applications into the realm of AC technology.

cond-mat.mtrl-sci↗

arXiv2017

Electronic bulk and domain wall properties in B-site doped hexagonal ErMnO$_3$

Acceptor and donor doping is a standard for tailoring semiconductors. More recently, doping was adapted to optimize the behavior at ferroelectric domain walls. In contrast to more than a century of research on semiconductors, the impact of chemical substitutions on the local electronic response at domain walls is largely unexplored. Here, the hexagonal manganite ErMnO$_3$ is donor doped with Ti$^{4+}$. Density functional theory calculations show that Ti$^{4+}$ goes to the B-site, replacing Mn$^{3+}$. Scanning probe microscopy measurements confirm the robustness of the ferroelectric domain template. The electronic transport at both macro- and nanoscopic length scales is characterized. The measurements demonstrate the intrinsic nature of emergent domain wall currents and point towards Poole-Frenkel conductance as the dominant transport mechanism. Aside from the new insight into the electronic properties of hexagonal manganites, B-site doping adds an additional degree of freedom for tuning the domain wall functionality.

cond-mat.mtrl-sci↗

arXiv2017

Topological domain walls in helimagnets

A magnetic helix arises in chiral magnets with a wavelength set by the spin-orbit coupling. We show that the helimagnetic order is a nanoscale analog to liquid crystals, exhibiting topological structures and domain walls that are distinctly different from classical magnets. Using magnetic force microscopy and micromagnetic simulations, we demonstrate that - similar to cholesteric liquid crystals - three fundamental types of domain walls are realized in the helimagnet FeGe. We reveal the micromagnetic wall structure and show that they can carry a finite skyrmion charge, permitting coupling to spin currents and contributions to a topological Hall effect. Our study establishes a new class of magnetic nano-objects with non-trivial topology, opening the door to innovative device concepts based on helimagnetic domain walls.

cond-mat.str-el↗

arXiv2017

Global formation of topological defects in the multiferroic hexagonal manganites

The spontaneous transformations associated with symmetry-breaking phase transitions generate domain structures and defects that may be topological in nature. The formation of these defects can be described according to the Kibble-Zurek mechanism, which provides a generic relation that applies from cosmological to interatomic lengthscales. Its verification is challenging, however, in particular at the cosmological scale where experiments are impractical. While it has been demonstrated for selected condensed-matter systems, major questions remain regarding e.g. its degree of universality. Here we develop a global Kibble-Zurek picture from the condensed-matter level. We show theoretically that a transition between two fluctuation regimes (Ginzburg and mean-field) can lead to an intermediate region with reversed scaling, and we verify experimentally this behavior for the structural transition in the series of multiferroic hexagonal manganites. Trends across the series allow us to identify additional intrinsic features of the defect formation beyond the original Kibble-Zurek paradigm.

cond-mat.mtrl-sci↗

arXiv2017

Conductivity contrast and tunneling charge transport in the vortex-like ferroelectric domain patterns of multiferroic hexagonal YMnO3

We deduce the intrinsic conductivity properties of the ferroelectric domain walls around the topologically protected domain vortex cores in multiferroic YMnO3. This is achieved by performing a careful equivalent-circuit analysis of dielectric spectra measured in single-crystalline samples with different vortex densities. The conductivity contrast between the bulk domains and the less conducting domain boundaries is revealed to reach up to a factor 500 at room temperature, depending on sample preparation. Tunneling of localized defect charge carriers is the dominant charge-transport process in the domain walls that are depleted of mo-bile charge carriers. This work demonstrates that via equivalent-circuit analysis, dielectric spectroscopy can provide valuable information on the intrinsic charge-transport properties of ferroelectric domain walls, which is of high relevance for the design of new domain-wall-based microelectronic devices.

cond-mat.str-el↗

arXiv2017

$^{13}$CO/C$^{18}$O Gradients Across the Disks of Nearby Spiral Galaxies

We use the IRAM Large Program EMPIRE and new high-resolution ALMA data to measure 13CO(1-0)/C18O(1-0) intensity ratios across nine nearby spiral galaxies. These isotopologues of CO are typically optically thin across most of the area in galaxy disks, and this ratio allows us to gauge their relative abundance due to chemistry or stellar nucleosynthesis effects. Resolved 13CO/C18O gradients across normal galaxies have been rare due to the faintness of these lines. We find a mean 13CO/C18O ratio of 6.0$\pm$0.9 for the central regions of our galaxies. This agrees well with results in the Milky Way, but differs from results for starburst galaxies (3.4$\pm$0.9) and ultraluminous infrared galaxies (1.1$\pm$0.4). In our sample, the 13CO/C18O ratio consistently increases with increasing galactocentric radius and decreases with increasing star formation rate surface density. These trends qualitatively agree with expectations for carbon and oxygen isotopic abundance variations due to stellar nucleosynthesis, with a possible effect of fractionation.

astro-ph.GA↗

arXiv2016

Local dynamics of topological magnetic defects in the itinerant helimagnet FeGe

Chiral magnetic interactions induce complex spin textures including helical and conical spin waves, as well as particle-like objects such as magnetic skyrmions and merons. These spin textures are the basis for innovative device paradigms and give rise to exotic topological phenomena, thus being of interest for both applied and fundamental sciences. Present key questions address the dynamics of the spin system and emergent topological defects. Here we analyze the micromagnetic dynamics in the helimagnetic phase of FeGe. By combining magnetic force microscopy, single-spin magnetometry, and Landau-Lifschitz-Gilbert simulations we show that the nanoscale dynamics are governed by the depinning and subsequent motion of magnetic edge dislocations. The motion of these topologically stable objects triggers perturbations that can propagate over mesoscopic length scales. The observation of stochastic instabilities in the micromagnetic structure provides new insight to the spatio-temporal dynamics of itinerant helimagnets and topological defects, and discloses novel challenges regarding their technological usage.

cond-mat.mtrl-sci↗

arXiv2015

Dependence of transverse magneto-thermoelectric effects on inhomogeneous magnetic fields

Transverse magneto-thermoelectric effects are studied in permalloy thin films grown on MgO substrates. We find that small parasitic magnetic fields below 1 Oe can produce artifacts of the order of 1 % of the amplitude of the anisotropic magneto-thermopower which is also detected in the experiments. The measured artifacts reveal a new source of uncertainties for the detection of the transverse spin Seebeck effect. Taking these results into account we conclude that the contribution of the transverse spin Seebeck effect to the detected voltages is below the noise level of 20 nV.

cond-mat.mes-hall↗

arXiv2014

Imaging and characterization of conducting ferroelectric domain walls by photoemission electron microscopy

High-resolution X-ray photoemission electron microscopy (X-PEEM) is a well-established method for imaging ferroelectric domain structures. Here, we expand the scope of application of X-PEEM and demonstrate its capability for imaging and investigating domain walls in ferroelectrics with high-spatial resolution. Using ErMnO3 as test system, we show that ferroelectric domain walls can be visualized based on photo-induced charging effects and local variations in their electronic conductance can be mapped by analyzing the energy distribution of photoelectrons. Our results open the door for non-destructive, contract-free, and element-specific studies of the electronic and chemical structure at domain walls in ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2014

VLASSICK: The VLA Sky Survey in the Central Kiloparsec

At a distance of 8 kpc, the center of our Galaxy is the nearest galactic nucleus, and has been the subject of numerous key projects undertaken by great observatories such as Chandra, Spitzer, and Herschel. However, there are still no surveys of molecular gas properties in the Galactic center with less than 30" (1 pc) resolution. There is also no sensitive polarization survey of this region, despite numerous nonthermal magnetic features apparently unique to the central 300 parsecs. In this paper, we outline the potential the VLASS has to fill this gap. We assess multiple considerations in observing the Galactic center, and recommend a C-band survey with 10 micro-Jy continuum RMS and sensitive to molecular gas with densities greater than 10^4 cm^{-3}, covering 17 square degrees in both DnC and CnB configurations ( resolution ~5"), totaling 750 hours of observing time. Ultimately, we wish to note that the upgraded VLA is not just optimized for fast continuum surveys, but has a powerful correlator capable of simultaneously observing continuum emission and dozens of molecular and recombination lines. This is an enormous strength that should be fully exploited and highlighted by the VLASS, and which is ideally suited for surveying the center of our Galaxy.

astro-ph.GA↗

arXiv2013

Transverse spin Seebeck vs. Anomalous and Planar Nernst Effects in Permalloy Thin Films

Transverse magneto-thermoelectric effects are studied in permalloy thin films grown on MgO and GaAs substrates and compared to those grown on suspended SiN membranes. The transverse voltage along platinum strips patterned on top of the permalloy films is measured vs. the external magnetic field as a function of angle and temperature gradient. After the identification of the contribution of the planar and anomalous Nernst effects, we find an upper limit for the transverse spin Seebeck effect, which is several orders of magnitude smaller than previously reported.

cond-mat.mes-hall↗