Searcharxiv⌕ Search

arXiv subjects

M. Bode

Publications and source records attributed to M. Bode.

At least 19 recordsLinked to original sources
arXiv2020

Observation of tunable single-atom Yu-Shiba-Rusinov states

The coupling of a spin to an underlying substrate is the basis for a plethora of phenomena. In the case of a metallic substrate, Kondo screening of the adatom magnetic moment can occur. As the substrate turns superconducting, an intriguing situation emerges where the pair breaking due to the adatom spins leads to Yu-Shiba-Rusinov bound states, but also intertwines with Kondo phenomena. Through scanning tunneling spectroscopy, we analyze the interdependence of Kondo screening and superconductivity. Our data obtained on single Fe adatoms on Nb(110) show that the coupling and the resulting YSR states are strongly adsorption site-dependent and reveal a quantum phase transition at a Kondo temperature comparable to the superconducting gap. The experimental signatures are rationalized by combined density functional theory and continuous-time quantum Monte-Carlo calculations to rigorously treat magnetic and hybridization effects on equal footing.

cond-mat.supr-con↗
arXiv2019

Structural and magnetic properties of 3d transition metal oxide chains on the (001) surfaces of Ir and Pt

We present a survey of the structural and magnetic properties of submonolayer transition metal dioxides on the (001) surfaces of the heavy face-centered cubic (fcc) noble metals Ir and Pt performed by spin-averaged scanning tunneling microscopy (STM) and spin-polarized (SP-)STM. Our STM results confirm that deposition of Co, Fe, Mn, and Cr on the (2 {\times} 1) oxygen-reconstructed Ir(001) surface leads to the formation of quasi one-dimensional chains with a (3 {\times} 1) unit cell. As recently predicted by density functional theory [Ferstl et al., Phys. Rev. Lett. 117, 046101 (2016)], our SP-STM images of FeO2 and MnO2 on Ir(001) show a two-fold periodicity along the chains which is characteristic for an antiferromagnetic coupling along the chains. In addition, these two materials also exhibit spontaneous, permanent, and long-range magnetic coupling across the chains. Whereas we find a ferromagnetic inter-chain coupling for FeO2/Ir(001), the magnetic coupling of MnO2 on Ir(001) appears to be a non-collinear 120{\deg} spin spiral, resulting in a (9 {\times} 2) magnetic unit cell. On Pt(001) patches of (3 {\times} 1)-reconstructed oxide chains could only be prepared by transition metal (Co, Fe, and Mn) deposition onto the cold substrate and subsequent annealing in an oxygen atmosphere. Again SP-STM on MnO2/Pt(001) reveals a very large, (15 {\times} 2) magnetic unit cell which can tentatively be explained by a commensurate 72{\deg} spin spiral. Large scale SP-STM images reveal a long wavelength spin rotation along the MnO2 chain.

cond-mat.mtrl-sci↗
arXiv2018

Systematics of electronic and magnetic properties in the transition metal doped Sb$_2$Te$_3$ quantum anomalous Hall platform

The quantum anomalous Hall effect (QAHE) has recently been reported to emerge in magnetically-doped topological insulators. Although its general phenomenology is well established, the microscopic origin is far from being properly understood and controlled. Here we report on a detailed and systematic investigation of transition-metal (TM)-doped Sb$_2$Te$_3$. By combining density functional theory (DFT) calculations with complementary experimental techniques, i.e., scanning tunneling microscopy (STM), resonant photoemission (resPES), and x-ray magnetic circular dichroism (XMCD), we provide a complete spectroscopic characterization of both electronic and magnetic properties. Our results reveal that the TM dopants not only affect the magnetic state of the host material, but also significantly alter the electronic structure by generating impurity-derived energy bands. Our findings demonstrate the existence of a delicate interplay between electronic and magnetic properties in TM-doped TIs. In particular, we find that the fate of the topological surface states critically depends on the specific character of the TM impurity: while V- and Fe-doped Sb$_2$Te$_3$ display resonant impurity states in the vicinity of the Dirac point, Cr and Mn impurities leave the energy gap unaffected. The single-ion magnetic anisotropy energy and easy axis, which control the magnetic gap opening and its stability, are also found to be strongly TM impurity-dependent and can vary from in-plane to out-of-plane depending on the impurity and its distance from the surface. Overall, our results provide general guidelines for the realization of a robust QAHE in TM-doped Sb$_2$Te$_3$ in the ferromagnetic state.

cond-mat.mes-hall↗
arXiv2017

Universal response of the type-II Weyl semimetals phase diagram

The discovery of Weyl semimetals represents a significant advance in topological band theory. They paradigmatically enlarged the classification of topological materials to gapless systems while simultaneously providing experimental evidence for the long-sought Weyl fermions. Beyond fundamental relevance, their high mobility, strong magnetoresistance, and the possible existence of even more exotic effects, such as the chiral anomaly, make Weyl semimetals a promising platform to develop radically new technology. Fully exploiting their potential requires going beyond the mere identification of materials and calls for a detailed characterization of their functional response, which is severely complicated by the coexistence of surface- and bulk-derived topologically protected quasiparticles, i.e., Fermi arcs and Weyl points, respectively. Here, we focus on the type-II Weyl semimetal class where we find a stoichiometry-dependent phase transition from a trivial to a non-trivial regime. By exploring the two extreme cases of the phase diagram, we demonstrate the existence of a universal response of both surface and bulk states to perturbations. We show that quasi-particle interference patterns originate from scattering events among surface arcs. Analysis reveals that topologically non-trivial contributions are strongly suppressed by spin texture. We also show that scattering at localized impurities generate defect-induced quasiparticles sitting close to the Weyl point energy. These give rise to strong peaks in the local density of states, which lift the Weyl node significantly altering the pristine low-energy Weyl spectrum. Visualizing the microscopic response to scattering has important consequences for understanding the unusual transport properties of this class of materials. Overall, our observations provide a unifying picture of the Weyl phase diagram.

cond-mat.mes-hall↗
arXiv2016

Mesoscopic electron focusing in topological insulators

The particle wave duality sets a fundamental correspondence between optics and quantum mechanics. Within this framework, the propagation of quasiparticles can give rise to superposition phenomena which, like for electromagnetic waves, can be described by the Huygens principle. However, the utilization of this principle by means of propagation and manipulation of quantum information is limited by the required coherence in time and space. Here we show that in topological insulators, which in their pristine form are characterized by opposite propagation directions for the two quasiparticles spin channels, mesoscopic focusing of coherent charge density oscillations can be obtained at large nested segments of constant energy contours by magnetic surface doping. Our findings provide evidence of strongly anisotropic Dirac fermion-mediated interactions. Even more remarkably, the validity of our findings goes beyond topological insulators but applies for systems with spin orbit lifted degeneracy in general. It demonstrates how spin information can be transmitted over long distances, allowing the design of experiments and devices based on coherent quantum effects in this fascinating class of materials.

cond-mat.mes-hall↗
arXiv2015

Experimental realization of a topological p-n junction by intrinsic defect-grading

A junction between an n- and p-type semiconductor results in the creation of a depletion region whose properties are at the basis of nowadays electronics. If realized using topological insulators as constituent materials, p-n junctions are expected to manifest several unconventional effects with great potential for applications. Experimentally, all these fascinating properties remained unexplored so far, mainly because prototypical topological PNJs, which can be easily realized and investigated, were not readily available. Here, we report on the creation of topological PNJs which can be as narrow as few tenths of nm showing a built-in potential of 110meV. These junctions are intrinsically obtained by a thermodynamic control of the defects distribution across the crystal. Our results make Bi2Te3 a robust and reliable platform to explore the physics of topological p-n junction.

cond-mat.mes-hall↗
arXiv2015

Scattering properties of the three-dimensional topological insulator Sb2Te3: Coexistence of topologically trivial and non-trivial surface states with opposite spin-momentum helicity

The binary chalcogenides Bi2Te3 and Bi2Se3 are the most widely studied topological insulators. Although the quantum anomalous Hall effect has recently been observed in magnetically doped Sb2Te3 this compound has been studied to a much lesser extend. Here, by using energy resolved quasiparticle interference mapping, we investigate the scattering events of pristine Sb2Te3 surfaces. We find that, in addition to the Dirac fermions, another strongly spin polarized surface resonance emerges at higher energies in the unoccupied electronic states. Although the two surface states are of different origin, i.e. topologically protected and trivial, respectively, both show strongly directional scattering properties and absence of backscattering. Comparison with ab-initio calculations demonstrates that this is a direct consequence of their spin momentum locked spin texture which is found to exhibit an opposite rotational sense for the trivial state and the Dirac state.

cond-mat.mes-hall↗
arXiv2012

A possible binary system of a stellar remnant in the high magnification gravitational microlensing event OGLE-2007-BLG-514

We report the extremely high magnification (A > 1000) binary microlensing event OGLE-2007-BLG-514. We obtained good coverage around the double peak structure in the light curve via follow-up observations from different observatories. The binary lens model that includes the effects of parallax (known orbital motion of the Earth) and orbital motion of the lens yields a binary lens mass ratio of q = 0.321 +/- 0.007 and a projected separation of s = 0.072 +/- 0.001$ in units of the Einstein radius. The parallax parameters allow us to determine the lens distance D_L = 3.11 +/- 0.39 kpc and total mass M_L=1.40 +/- 0.18 M_sun; this leads to the primary and secondary components having masses of M_1 = 1.06 +/- 0.13 M_sun and M_2 = 0.34 +/- 0.04 M_sun, respectively. The parallax model indicates that the binary lens system is likely constructed by the main sequence stars. On the other hand, we used a Bayesian analysis to estimate probability distributions by the model that includes the effects of xallarap (possible orbital motion of the source around a companion) and parallax (q = 0.270 +/- 0.005, s = 0.083 +/- 0.001). The primary component of the binary lens is relatively massive with M_1 = 0.9_{-0.3}^{+4.6} M_sun and it is at a distance of D_L = 2.6_{-0.9}^{+3.8} kpc. Given the secure mass ratio measurement, the companion mass is therefore M_2 = 0.2_{-0.1}^{+1.2} M_sun. The xallarap model implies that the primary lens is likely a stellar remnant, such as a white dwarf, a neutron star or a black hole.

astro-ph.EP↗
arXiv2011

OGLE-2005-BLG-018: Characterization of Full Physical and Orbital Parameters of a Gravitational Binary Lens

We present the analysis result of a gravitational binary-lensing event OGLE-2005-BLG-018. The light curve of the event is characterized by 2 adjacent strong features and a single weak feature separated from the strong features. The light curve exhibits noticeable deviations from the best-fit model based on standard binary parameters. To explain the deviation, we test models including various higher-order effects of the motions of the observer, source, and lens. From this, we find that it is necessary to account for the orbital motion of the lens in describing the light curve. From modeling of the light curve considering the parallax effect and Keplerian orbital motion, we are able to measure not only the physical parameters but also a complete orbital solution of the lens system. It is found that the event was produced by a binary lens located in the Galactic bulge with a distance $6.7\pm 0.3$ kpc from the Earth. The individual lens components with masses $0.9\pm 0.3\ M_\odot$ and $0.5\pm 0.1\ M_\odot$ are separated with a semi-major axis of $a=2.5 \pm 1.0$ AU and orbiting each other with a period $P=3.1 \pm 1.3$ yr. The event demonstrates that it is possible to extract detailed information about binary lens systems from well-resolved lensing light curves.

astro-ph.SR↗
arXiv2010

OGLE-2005-BLG-153: Microlensing Discovery and Characterization of A Very Low Mass Binary

The mass function and statistics of binaries provide important diagnostics of the star formation process. Despite this importance, the mass function at low masses remains poorly known due to observational difficulties caused by the faintness of the objects. Here we report the microlensing discovery and characterization of a binary lens composed of very low-mass stars just above the hydrogen-burning limit. From the combined measurements of the Einstein radius and microlens parallax, we measure the masses of the binary components of $0.10\pm 0.01\ M_\odot$ and $0.09\pm 0.01\ M_\odot$. This discovery demonstrates that microlensing will provide a method to measure the mass function of all Galactic populations of very low mass binaries that is independent of the biases caused by the luminosity of the population.

astro-ph.SR↗
arXiv2009

A Cold Neptune-Mass Planet OGLE-2007-BLG-368Lb: Cold Neptunes Are Common

We present the discovery of a Neptune-mass planet OGLE-2007-BLG-368Lb with a planet-star mass ratio of q=[9.5 +/- 2.1] x 10^{-5} via gravitational microlensing. The planetary deviation was detected in real-time thanks to the high cadence of the MOA survey, real-time light curve monitoring and intensive follow-up observations. A Bayesian analysis returns the stellar mass and distance at M_l = 0.64_{-0.26}^{+0.21} M_\sun and D_l = 5.9_{-1.4}^{+0.9} kpc, respectively, so the mass and separation of the planet are M_p = 20_{-8}^{+7} M_\oplus and a = 3.3_{-0.8}^{+1.4} AU, respectively. This discovery adds another cold Neptune-mass planet to the planetary sample discovered by microlensing, which now comprise four cold Neptune/Super-Earths, five gas giant planets, and another sub-Saturn mass planet whose nature is unclear. The discovery of these ten cold exoplanets by the microlensing method implies that the mass ratio function of cold exoplanets scales as dN_{\rm pl}/d\log q \propto q^{-0.7 +/- 0.2} with a 95% confidence level upper limit of n < -0.35 (where dN_{\rm pl}/d\log q \propto q^n). As microlensing is most sensitive to planets beyond the snow-line, this implies that Neptune-mass planets are at least three times more common than Jupiters in this region at the 95% confidence level.

astro-ph.EP↗
arXiv2009

Interpretation of Strong Short-Term Central Perturbations in the Light Curves of Moderate-Magnification Microlensing Events

To improve the planet detection efficiency, current planetary microlensing experiments are focused on high-magnification events searching for planetary signals near the peak of lensing light curves. However, it is known that central perturbations can also be produced by binary companions and thus it is important to distinguish planetary signals from those induced by binary companions. In this paper, we analyze the light curves of microlensing events OGLE-2007-BLG-137/MOA-2007-BLG-091, OGLE-2007-BLG-355/MOA-2007-BLG-278, and MOA-2007-BLG-199/OGLE-2007-BLG-419, for all of which exhibit short-term perturbations near the peaks of the light curves. From detailed modeling of the light curves, we find that the perturbations of the events are caused by binary companions rather than planets. From close examination of the light curves combined with the underlying physical geometry of the lens system obtained from modeling, we find that the short time-scale caustic-crossing feature occurring at a low or a moderate base magnification with an additional secondary perturbation is a typical feature of binary-lens events and thus can be used for the discrimination between the binary and planetary interpretations.

astro-ph.EP↗
arXiv2009

X-ray and UV observations of nova V598 Puppis between 147 and 255 days after outburst

Aims: The launch of Swift has allowed many more novae to be observed regularly over the X-ray band. Such X-ray observations of novae can reveal ejecta shocks and the nuclear burning white dwarf, allowing estimates to be made of the ejecta velocity. Methods: We analyse XMM-Newton and Swift X-ray and UV observations of the nova V598 Pup, which was initially discovered in the XMM-Newton slew survey. These data were obtained between 147 and 255 days after the nova outburst, and are compared with the earlier, brighter slew detection. Results: The X-ray spectrum consists of a super-soft source, with the soft emission becoming hotter and much fainter between days ~147 and ~172 after the outburst, and a more slowly declining optically thin component, formed by shocks with kT ~ 200-800 eV (corresponding to velocities of 400-800 km s^-1). The main super-soft phase had a duration of less than 130 days. The Reflection Grating Spectrometer data show evidence of emission lines consistent with optically thin emission of kT ~100 eV and place a limit on the density of the surrounding medium of log(n_e/cm^-3) < 10.4 at the 90 % level. The UV emission is variable over short timescales and fades by at least one magnitude (at lambda ~ 2246-2600 angstrom) between days 169 and 255.

astro-ph.HE↗
arXiv2009

Mass measurement of a single unseen star and planetary detection efficiency for OGLE 2007-BLG-050

We analyze OGLE-2007-BLG-050, a high magnification microlensing event (A ~ 432) whose peak occurred on 2 May, 2007, with pronounced finite-source and parallax effects. We compute planet detection efficiencies for this event in order to determine its sensitivity to the presence of planets around the lens star. Both finite-source and parallax effects permit a measurement of the angular Einstein radius \theta_E = 0.48 +/- 0.01 mas and the parallax \pi_E = 0.12 +/- 0.03, leading to an estimate of the lens mass M = 0.50 +/- 0.14 M_Sun and its distance to the observer D_L = 5.5 +/- 0.4 kpc. This is only the second determination of a reasonably precise (<30%) mass estimate for an isolated unseen object, using any method. This allows us to calculate the planetary detection efficiency in physical units (r_\perp, m_p), where r_\perp is the projected planet-star separation and m_p is the planet mass. When computing planet detection efficiency, we did not find any planetary signature and our detection efficiency results reveal significant sensitivity to Neptune-mass planets, and to a lesser extent Earth-mass planets in some configurations. Indeed, Jupiter and Neptune-mass planets are excluded with a high confidence for a large projected separation range between the planet and the lens star, respectively [0.6 - 10] and [1.4 - 4] AU, and Earth-mass planets are excluded with a 10% confidence in the lensing zone, i.e. [1.8 - 3.1] AU.

astro-ph.EP↗
arXiv2009

A systematic fitting scheme for caustic-crossing microlensing events

We outline a method for fitting binary-lens caustic-crossing microlensing events based on the alternative model parameterisation proposed and detailed in Cassan (2008). As an illustration of our methodology, we present an analysis of OGLE-2007-BLG-472, a double-peaked Galactic microlensing event with a source crossing the whole caustic structure in less than three days. In order to identify all possible models we conduct an extensive search of the parameter space, followed by a refinement of the parameters with a Markov Chain-Monte Carlo algorithm. We find a number of low-chi2 regions in the parameter space, which lead to several distinct competitive best models. We examine the parameters for each of them, and estimate their physical properties. We find that our fitting strategy locates several minima that are difficult to find with other modelling strategies and is therefore a more appropriate method to fit this type of events.

astro-ph.GA↗
arXiv2008

RoboNet-II: Follow-up observations of microlensing events with a robotic network of telescopes

RoboNet-II uses a global network of robotic telescopes to perform follow-up observations of microlensing events in the Galactic Bulge. The current network consists of three 2m telescopes located in Hawaii and Australia (owned by Las Cumbres Observatory) and the Canary Islands (owned by Liverpool John Moores University). In future years the network will be expanded by deploying clusters of 1m telescopes in other suitable locations. A principal scientific aim of the RoboNet-II project is the detection of cool extra-solar planets by the method of gravitational microlensing. These detections will provide crucial constraints to models of planetary formation and orbital migration. RoboNet-II acts in coordination with the PLANET microlensing follow-up network and uses an optimization algorithm ("web-PLOP") to select the targets and a distributed scheduling paradigm (eSTAR) to execute the observations. Continuous automated assessment of the observations and anomaly detection is provided by the ARTEMiS system.

astro-ph↗
arXiv2008

OGLE-2005-BLG-071Lb, the Most Massive M-Dwarf Planetary Companion?

We combine all available information to constrain the nature of OGLE-2005-BLG-071Lb, the second planet discovered by microlensing and the first in a high-magnification event. These include photometric and astrometric measurements from Hubble Space Telescope, as well as constraints from higher order effects extracted from the ground-based light curve, such as microlens parallax, planetary orbital motion and finite-source effects. Our primary analysis leads to the conclusion that the host of Jovian planet OGLE-2005-BLG-071Lb is an M dwarf in the foreground disk with mass M= 0.46 +/- 0.04 Msun, distance D_l = 3.3 +/- 0.4 kpc, and thick-disk kinematics v_LSR ~ 103 km/s. From the best-fit model, the planet has mass M_p = 3.8 +/- 0.4 M_Jup, lies at a projected separation r_perp = 3.6 +/- 0.2 AU from its host and so has an equilibrium temperature of T ~ 55 K, i.e., similar to Neptune. A degenerate model less favored by \Delta\chi^2 = 2.1 (or 2.2, depending on the sign of the impact parameter) gives similar planetary mass M_p = 3.4 +/- 0.4 M_Jup with a smaller projected separation, r_\perp = 2.1 +/- 0.1 AU, and higher equilibrium temperature T ~ 71 K. These results from the primary analysis suggest that OGLE-2005-BLG-071Lb is likely to be the most massive planet yet discovered that is hosted by an M dwarf. However, the formation of such high-mass planetary companions in the outer regions of M-dwarf planetary systems is predicted to be unlikely within the core-accretion scenario. There are a number of caveats to this primary analysis, which assumes (based on real but limited evidence) that the unlensed light coincident with the source is actually due to the lens, that is, the planetary host. However, these caveats could mostly be resolved by a single astrometric measurement a few years after the event.

astro-ph↗