Nonlinear conductivity in CaRuO3 thin films measured by short current pulses

Metals near quantum critical points have been predicted to display universal out-of equilibrium behavior in the steady current-carrying state. We have studied the non-linear conductivity of high-quality CaRuO$_3$ thin films with residual resistivity ratio up to 57 using micro-second short, high-field current pulses at low temperatures. Even for the shortest pulses of $5\mu$s, Joule heating persists, making it impossible to observe a possible universal non-linearity. Much shorter pulses are needed for the investigation of universal non-linear conductivity.Comment: 9 pages, 7 figure

Temperatur- und winkelabhängiger Magnetotransport in SrRuO3/SrIrO3 Heterostrukturen

Mittels einer in Eigenbau realisierten Metallorganischen Aerosol Depositions Anlage wurden Heterostrukturen der Perowskite SrRuO3 und SrIrO3 auf (001)c orientierten SrTiO3 Substrate gewachsen. Durch Röntgenbeugungsexperimente und Transmissionsmikroskopie-Aufnahmen wurde das phasenreine Wachstum der Dünnfilme überprüft, wogegen das Wachstum mit glatten Oberflächen durch Rasterkraftmikroskopie-Aufnahmen nachgewiesen wurde. Durch SQUID-Magnetometrie konnte eine bevorzugte Richtung der Magnetisierung senkrecht zur Probenoberfläche beobachtet werden, die auf eine leichte magnetische Achse mit out-of-plane Orientierung hindeutet. Gegenüber der ferromagnetischen Ordnungstemperatur TC=150K von epitaktisch verspannten SrRuO3 Dünnfilmen reduziert sich die Ordnungstemperatur der Heterostruktur [(SrRuO3)5/(SrIrO3)2]k mit abnehmender Anzahl an Wiederholungen der Bilagenstruktur (SrRuO3)5/(SrIrO3)2. Temperatur- und winkelabhängige Hall-Effekt Messungen zeigen neben dem normalen und anomalen Hall-Effekt Anzeichen für einen zusätzlichen Beitrag, der als topologischer Hall-Effekt identifiziert wurde. Mit Hilfe dessen konnte die Bildung von Néel-Skyrmionen an der Grenzfläche zwischen SrRuO3 und SrIrO3 unterhalb von 80K beobachtet werden. Diese zeigen eine große Stabilität gegenüber einem aus der Probenoberfläche verdrehten Magnetfeld, aber keine Richtungsabhängigkeit der Magnetfeldrotation in Bezug zur Stromrichtung. Die Skyrmionenstabilität hängt dabei von der Temperatur- und Winkelabhängigkeit der beiden unabhängigen Beiträge zum anomalen Hall-Effekt ab, die ein deutlich unterscheidbares Schaltverhalten zeigen. Diese zwei unterschiedlichen Leitungskanäle tragen ebenfalls zum anisotropen Magnetwiderstand (AMR) bei, dessen qualitativer Verlauf der Temperatur- und Winkelabhängigkeit mittels einer aufgesetzten Modellfunktion beschrieben werden konnte. Die starke magnetische Anisotropie lies sich durch Drehungen der Magnetisierung aus der Probennormalen in die Filmebene senkrecht und parallel zum Strom bestimmen. Die magnetische leichte Achse ist dabei aus der Normalen heraus gedreht und richtet sich erst bei stärkeren Magnetfeldern entlang dieser aus.By using a self-constructed metal-organic aerosol deposition chamber we have grown thin film heterostructures of the perovskites SrRuO3 and SrIrO3 on (001)c oriented SrTiO3 substrates. X-ray diffraction and transmission electron microscopy were used to check the phase purity of the heterostructures, whereas the growth of smooth surfaces was examined by atomic force microscopy. The orientation of an easier magnetization direction perpendicular to the film surface was found with SQUID magnetometry, indicating an easy-axis direction perpendicular to the surface. Compared to the ferromagnetic ordering temperature $TC=150K of epitaxially strained SrRuO3 thin films the ordering temperature of the heterostructure [(SrRuO3)5/(SrIrO3)2]k reduces with decreasing number of repetitions of the bilayer structure (SrRuO3)5/(SrIrO3)2. Temperature- and angle-dependent Hall effect measurement showing in addition to the ordinary and extraordinary Hall effect an additional contribution which was identified as the topological Hall effect. The formation of Néel-type skyrmions at the interface between SrRuO3 and SrIrO3 below 80K could be deduced from this extra contribution. These show large stability against a magnetic field twisted out of the sample surface, but do not show any directional dependence of the magnetic field rotation in relation to the current direction. The skyrmion stability depends on the temperature- and field-dependence of the two independent contributions of the extraordinary Hall effect which show a clearly distinguishable switching behavior. These two independent channels contribute also to the anisotropic magnetoresistance (AMR). The temperature and angle-dependence of the AMR could be described in good aggreement with the model function setted up. Rotating the magnetic field from the out-of-plane direction parallel and perpendicular to the current demonstrate the strong magnetic anisotropy. The magnetic easy axis is rotated from the sample normal and turns towards the out-of-plane direction by applying stronger magnetic fields

The impact of primordial black holes on the stellar mass function of ultra-faint dwarf galaxies

If primordial black holes constitute the dark matter, stars forming in dark-matter dominated environments with low velocity dispersions, such as ultra-faint dwarf galaxies, may capture a black hole at birth. The capture probability is non-negligible for primordial black holes of masses around $10^{20}$g, and increases with stellar mass. Moreover, infected stars are turned into virtually invisible black holes on cosmologically short timescales. Hence, the number of observed massive main-sequence stars in ultra-faint dwarfs should be suppressed if the dark matter was made of asteroid-mass primordial black holes. This would impact the measured mass distribution of stars, making it top-light (i.e. depleted in the high-mass range). Using simulated data that mimic the present-day observational power of telescopes, we show that already existing measurements of the mass function of stars in local ultra-faint dwarfs could be used to constrain the fraction of dark matter composed of primordial black holes in the -- currently unconstrained -- mass range of $10^{19}-10^{21}$g.Comment: Updated to match the MNRAS version. The mean captured PBH number (figure 1) was corrected. Most figures as well as some parts of the text are slightly modifie

Angular dependence of Hall effect and magnetoresistance in SrRuO3−SrIrO3 heterostructures

Perovskite SrRuO$_3$ is a prototypical itinerant ferromagnet which allows interface engineering of its electronic and magnetic properties. We report synthesis and investigation of atomically flat artificial multilayers of SrRuO$_3$ with the spin-orbit semimetal SrIrO$_3$ in combination with band-structure calculations with a Hubbard $U$ term and topological analysis. They reveal an electronic reconstruction and emergence of flat Ru-4d$_{xz}$ bands near the interface, ferromagnetic interlayer coupling and negative Berry-curvature contribution to the anomalous Hall effect. We analyze the Hall effect and magnetoresistance measurements as a function of the field angle from out of plane towards in-plane orientation (either parallel or perpendicular to the current direction) by a two-channel model. The magnetic easy direction is tilted by about $20^\circ$ from the sample normal for low magnetic fields, rotating towards the out-of-plane direction by increasing fields. Fully strained epitaxial growth enables a strong anisotropy of magnetoresistance. An additional Hall effect contribution, not accounted for by the two-channel model is compatible with stable skyrmions only up to a critical angle of roughly $45^\circ$ from the sample normal. Within about $20^\circ$ from the thin film plane an additional peak-like contribution to the Hall effect suggests the formation of a non-trivial spin structure.Comment: to be published in Phys. Rev.

Strong terahertz third-harmonic generation by kinetic heavy quasiparticles in CaRuO3

We report on time-resolved nonlinear terahertz spectroscopy of a strongly correlated ruthenate, CaRuO3, as a function of temperature, frequency, and terahertz field strength. Third-harmonic radiation for frequencies up to 2.1 THz is observed evidently at low temperatures below 80 K, where the low-frequency linear dynamical response deviates from the Drude model and a coherent heavy quasiparticle band emerges by strong correlations associated with the Hund’s coupling. Phenomenologically, by taking an experimentally observed frequency-dependent scattering rate, the deviation of the field driven kinetics from the Drude behavior is reconciled in a time-dependent Boltzmann description, which allows an attribution of the observed third-harmonic generation to the terahertz field driven nonlinear kinetics of the heavy quasiparticles