Evangelos Golias

Evangelos Golias
Lund University | LU · MAX IV Laboratory

PhD

About

79
Publications
13,512
Reads
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1,679
Citations
Additional affiliations
June 2017 - present
Freie Universität Berlin
Position
  • PostDoc Position
April 2014 - June 2017
Helmholtz-Zentrum Berlin für Materialien und Energie
Position
  • PostDoc Position
September 2010 - December 2014
National Centre of Scientific Research "Demokritos"
Position
  • Research Associate
Education
September 2010 - December 2013
National Technical University of Athens
Field of study
  • Nanotechnology
September 2009 - October 2011
September 1999 - March 2004

Publications

Publications (79)
Preprint
Full-text available
Antiferromagnets are promising candidates for ultrafast spintronic applications, leveraging current-induced spin-orbit torques. However, experimentally distinguishing between different switching mechanisms of the staggered magnetization (N\'eel vector) driven by current pulses remains a challenge. In an exemplary study of the collinear antiferromag...
Article
Full-text available
Nanoscale detection and control of the magnetic order underpins a spectrum of condensed-matter research and device functionalities involving magnetism. The key principle involved is the breaking of time-reversal symmetry, which in ferromagnets is generated by an internal magnetization. However, the presence of a net magnetization limits device scal...
Article
Antiferromagnetic materials hold potential for use in spintronic devices with fast operation frequencies and field robustness. Despite the rapid progress in proof-of-principle functionality in recent years, there has been a notable lack of understanding of antiferromagnetic domain formation and manipulation, which translates to either incomplete or...
Preprint
Full-text available
The unique features of altermagnetism allow for new ways of measuring and controlling the magnetic order parameter. Here we utilise these properties to provide an unprecedented level of information and control over the magnetic order from the micron to nanoscale.
Article
The growth, morphology, and structure of films on Cu(001) and Ag(001) are studied by means of low‐energy electron diffraction (LEED), medium‐energy electron diffraction, Auger electron spectroscopy, and scanning tunnelling microscopy. Different concentrations x from about 0.5 to 1 and thicknesses from 0.2 to 12.9 ML of are examined. For several val...
Article
We investigate the magneto-optical response of Co to an ultrashort laser excitation by x-ray resonant magnetic reflectivity (XRMR) employing circular polarization. The time-resolved reflectivities detected for opposite sample magnetization are separated into magnetic and nonmagnetic contributions, which contain information about the structural, ele...
Article
Full-text available
Local geometric control of basic synthesis parameters, such as elemental composition, is important for bottom-up synthesis and top-down device definition on-chip but remains a significant challenge. Here, we propose to use lithographically defined metal stacks for regulating the surface concentrations of freely diffusing synthesis elements on compo...
Article
Full-text available
Two-dimensional (2D) Dirac materials are electronically and structurally very sensitive to proximity effects. We demonstrate, however, the opposite effect: that the deposition of a monolayer 2D material could exercise a substantial influence on the substrate electronic structure. Here we investigate TiC(111) and show that a graphene overlayer produ...
Preprint
Full-text available
Phosphorene, a 2D allotrope of phosphorus, is technologically very appealing because of its semiconducting properties and narrow band gap. Further reduction of the phosphorene dimensionality may spawn exotic properties of its electronic structure, including lateral quantum confinement and topological edge states. Phosphorene atomic chains self-asse...
Article
Full-text available
Current pulse driven Néel vector rotation in metallic antiferromagnets is one of the most promising concepts in antiferromagnetic spintronics. We show microscopically that the Néel vector of epitaxial thin films of the prototypical compound Mn2Au can be reoriented reversibly in the complete area of cross shaped device structures using single curren...
Article
Metal-organic frameworks (MOFs) are crystalline and porous, molecular solids consisting of metal nodes and organic ligands. An interesting example is the MOF-2 system, where Cu2+ ions form antiferromagnetically coupled dimers to yield so-called paddlewheels. In the case of surface-anchored MOF-2 (SURMOF-2) systems the Cu2+ ions are connected via ca...
Preprint
Full-text available
Antiferromagnetic (AF) materials hold potential for use in spintronic devices with fast operation frequencies and robustness against magnetic field perturbations. However, the precise tuning of material properties such as magnetic anisotropy and domain structure is crucial for efficient device functionality, yet poorly understood in fully compensat...
Article
Full-text available
MAXPEEM, a dedicated photoemission electron microscopy beamline at MAX IV Laboratory, houses a state-of-the-art aberration-corrected spectroscopic photoemission and low-energy electron microscope (AC-SPELEEM). This powerful instrument offers a wide range of complementary techniques providing structural, chemical and magnetic sensitivities with a si...
Preprint
Full-text available
Current pulse driven Neel vector rotation in metallic antiferromagnets is one of the most promising concepts in antiferromagnetic spintronics. We show microscopically that the Neel vector of epitaxial thin films of the prototypical compound Mn2Au can be reoriented reversibly in the complete area of cross shaped device structures using single curren...
Article
Full-text available
We present a microscopic magnetic domain imaging study of single-shot all-optical magnetic toggle switching of a ferrimagnetic Gd 26 Fe 74 film with out-of-plane easy axis of magnetization by X-ray magnetic circular dichroism photoelectron emission microscopy. Individual linearly polarized laser pulses of 800 nm wavelength and 100 fs duration above...
Article
V-doped (Bi,Sb)2Te3 has a ten times higher magnetic coercivity than its Cr-doped counterpart and, therefore, is believed to be advantageous for the quantum anomalous Hall effect (QAHE). The QAHE requires the opening of a magnetic bandgap at the Dirac point. We do not find this gap by angle-resolved photoelectron spectroscopy down to 1 K. By x-ray m...
Preprint
We present a microscopic magnetic domain imaging study of single-shot all-optical magnetic toggle switching of a ferrimagnetic Gd$_{26}$Fe$_{74}$ film with out-of-plane easy axis of magnetization by x-ray magnetic circular dichroism photoelectron emission microscopy. Individual linearly polarized laser pulses of 800 nm wavelength and 100 fs duratio...
Article
Full-text available
Structure inversion asymmetry is an inherent feature of quantum confined heterostructures with non‐equivalent interfaces. It leads to a spin splitting of the electron states and strongly affects the electronic band structure. The effect is particularly large in topological insulators because the topological surface states are extremely sensitive to...
Article
Full-text available
Proximity to heavy sp -elements is considered promising for reaching a band gap in graphene that could host quantum spin Hall states. The recent report of an induced spin-orbit gap of 0.2 eV in Pb-intercalated graphene detectable by spin-resolved photoemission has spurred renewed interest in such systems (Klimovskikh et al 2017 ACS Nano 11 , 368)....
Article
Full-text available
We present evidence for an ultrafast optically induced ferromagnetic alignment of antiferromagnetic Mn in Co/Mn multilayers. We observe the transient ferromagnetic signal at the arrival of the pump pulse at the Mn L3 resonance using x-ray magnetic circular dichroism in reflectivity. The timescale of the effect is comparable to the duration of the e...
Article
Full-text available
We study the ultrafast demagnetization of Ni/NiMn and Co/NiMn ferromagnetic/antiferromagnetic bilayer systems after excitation by a laser pulse. We probe the ferromagnetic order of Ni and Co using magnetic circular dichroism in time-resolved pump-probe resonant x-ray reflectivity. Tuning the sample temperature across the antiferromagnetic ordering...
Preprint
Full-text available
V-doped (Bi,Sb)$_2$Te$_3$ has a ten times higher magnetic coercivity than its Cr-doped counterpart and therefore is believed to be a superior system for the quantum anomalous Hall effect (QAHE). The QAHE requires the opening of a magnetic band gap at the Dirac point. We do not find this gap by angle-resolved photoelectron spectroscopy down to 1 K....
Preprint
Full-text available
We study the ultrafast demagnetization of Ni/NiMn and Co/NiMn ferromagnetic/antiferromagnetic bilayer systems after excitation by a laser pulse. We probe the ferromagnetic order of Ni and Co using magnetic circular dichroism in time-resolved pump--probe resonant X-ray reflectivity. Tuning the sample temperature across the antiferromagnetic ordering...
Preprint
Full-text available
We present evidence for an ultrafast optically induced antiferromagnetic-to-ferromagnetic phase transition of Mn in Co/Mn multilayers. We observe the transient ferromagnetic signal at the arrival of the pump pulse at the Mn L$_3$ resonance using x-ray magnetic circular dichroism in reflectivity. The timescale of the effect is comparable to the dura...
Article
Full-text available
Vanadium doped Bi2−x Te2.4Se0.6 single crystals, with x = 0.015 and 0.03, are grown by the Bridgman method. Bandstructure characterization by angle‐resolved photoemission spectroscopy (ARPES) measurements shows gapless topological surface states for both Vanadium concentrations. The Van‐der‐Pauw resistivity, the Hall charge carrier density and the...
Article
Full-text available
X-ray absorption spectroscopy investigations of the spin-state switching of spin-crossover (SCO) complexes adsorbed on a highly-oriented pyrolytic graphite (HOPG) surface have shown so far that HOPG is a promising candidate to realize applications such as spintronic devices because of the stability of SCO complexes on HOPG and the possibility of hi...
Article
Full-text available
Epitaxial SnTe (111) is grown by molecular-beam epitaxy on Bi2Te3 substrates. Structural evaluation indicates that SnTe deviates from cubic due to in-plane compressive strain, which induces significant changes in the electronic band structure. More specifically, a pair of gapless crossings between the two uppermost valence bands occurs in k space a...
Article
Full-text available
We propose a novel mechanism of flat band formation based on the relative biasing of only one sublattice against other sublattices in a honeycomb lattice bilayer. The mechanism allows modification of the band dispersion from parabolic to "Mexican hat"-like through the formation of a flattened band. The mechanism is well applicable for bilayer graph...
Article
Full-text available
Most recently, theoretical calculations predicted the stability of a novel two-dimensional phosphorus honeycomb lattice named blue phosphorus. Here, we report on the growth of blue phosphorus on Au(111) and unravel its structural details using diffraction, microscopy and theoretical calculations. Most importantly, by utilizing angle-resolved photoe...
Preprint
Most recently, theoretical calculations predicted the stability of a novel two-dimensional phosphorus honeycomb lattice named blue phosphorus. Here, we report on the growth of blue phosphorus on Au(111) and unravel its structural details using diffraction, microscopy and theoretical calculations. Most importantly, by utilizing angle-resolved photoe...
Article
Full-text available
Phase change alloys are used for non-volatile random access memories exploiting the conductivity contrast between its amorphous and its meta-stable, crystalline phase. However, the surprisingly strong contrast has never been directly related to the electronic band structure of the material. Here, we employ in-situ photoelectron spectroscopy to map...
Article
Full-text available
We have synthesized single crystals of Dirac semimetal candidates AZnBi2 with A=Ba and Sr. In contrast to A=Sr, the Ba material displays a novel local Zn vacancy ordering, which makes the observation of quantum oscillations in out-of-plane magnetic fields possible. As a new Dirac semimetal candidate, BaZnBi2 exhibits small cyclotron electron mass,...
Preprint
We have synthesized single crystals of Dirac semimetal candidates AZnBi2 with A=Ba and Sr. In contrast to A=Sr, the Ba material displays a novel local Zn vacancy ordering, which makes the observation of quantum oscillations in out-of-plane magnetic fields possible. As a new Dirac semimetal candidate, BaZnBi2 exhibits small cyclotron electron mass,...
Article
Full-text available
Topological insulators constitute a new phase of matter protected by symmetries. Time-reversal symmetry protects strong topological insulators of the Z2 class, which possess an odd number of metallic surface states with dispersion of a Dirac cone. Topological crystalline insulators are merely protected by individual crystal symmetries and exist for...
Preprint
Phase change alloys are used for non-volatile random access memories exploiting the conductivity contrast between amorphous and metastable, crystalline phase. However, this contrast has never been directly related to the electronic band structure. Here, we employ photoelectron spectroscopy to map the relevant bands for metastable, epitaxial GeSbTe...
Article
Full-text available
To enhance the spin–orbit interaction in graphene by a proximity effect without compromising the quasi-free-standing dispersion of the Dirac cones means balancing the opposing demands for strong and weak graphene–substrate interaction. So far, only the intercalation of Au under graphene/Ni(1 1 1) has proven successful, which was unexpected since gr...
Article
Electronic structure has been studied in lightly electron doped correlated spin-orbit insulator Sr$_2$IrO$_4$ by angle-resolved photoelectron spectroscopy. We have observed coexistence of the lower Hubbard band and the in-gap band, the momentum dependence of the latter traces that of the band calculations without on-site Coulomb repulsion. The in-g...
Preprint
Electronic structure has been studied in lightly electron doped correlated spin-orbit insulator Sr$_2$IrO$_4$ by angle-resolved photoelectron spectroscopy. We have observed coexistence of the lower Hubbard band and the in-gap band, the momentum dependence of the latter traces that of the band calculations without on-site Coulomb repulsion. The in-g...
Article
Full-text available
Topologically nontrivial states reveal themselves in strongly spin-orbit coupled systems by Dirac cones. However, their appearance is not a sufficient criterion for a topological phase. In topological insulators, where these states protect surface metallicity, they are straightforwardly assigned based on bulk-boundary correspondence. On metals, whe...
Preprint
Using time-, spin- and angle-resolved photoemission, we investigate the ultrafast spin dynamics of hot electrons on the surface of the topological insulator Bi$_2$Te$_3$ following optical excitation by fs-infrared pulses. We observe two surface-resonance states above the Fermi level coexisting with a transient population of Dirac fermions that rela...
Preprint
Using time- and angle-resolved photoemission, we investigate the ultrafast response of excited electrons in the ternary topological insulator (Bi$_{1 x}$Sb$_{x}$)$_2$Te$_3$ to fs-infrared pulses. We demonstrate that at the critical concentration $x$=0.55, where the system becomes bulk insulating, a surface voltage can be driven at room temperature...
Article
Full-text available
Using time- and angle-resolved photoemission, we investigate the ultrafast response of excited electrons in the ternary topological insulator (Bi1-xSbx)2Te3 to fs-infrared pulses. We demonstrate that at the critical concentration x = 0.55, where the system becomes bulk insulating, a surface voltage can be driven at room temperature through the topo...
Article
Full-text available
Using time-, spin-, and angle-resolved photoemission, we investigate the ultrafast spin dynamics of hot electrons on the surface of the topological insulator Bi2Te3 following optical excitation by femtosecond-infrared pulses. We observe two surface-resonance states above the Fermi level coexisting with a transient population of Dirac fermions that...
Article
In this Rapid Communication we investigate the coupling between excited electrons and phonons in the highly anisotropic electronic structure of the prototypical topological insulator Bi$_2$Te$_3$. Using time- and angle-resolved photoemission spectroscopy we are able to identify the emergence and ultrafast temporal evolution of the longitudinal-opti...
Preprint
In this Rapid Communication we investigate the coupling between excited electrons and phonons in the highly anisotropic electronic structure of the prototypical topological insulator Bi$_2$Te$_3$. Using time- and angle-resolved photoemission spectroscopy we are able to identify the emergence and ultrafast temporal evolution of the longitudinal-opti...
Preprint
Magnetic doping is expected to open a band gap at the Dirac point of topological insulators by breaking time-reversal symmetry and to enable novel topological phases. Epitaxial (Bi$_{1-x}$Mn$_{x}$)$_{2}$Se$_{3}$ is a prototypical magnetic topological insulator with a pronounced surface band gap of $\sim100$ meV. We show that this gap is neither due...
Article
We reinvestigate the putative giant spin splitting at the surface of SrTiO$_3$ reported by Santander-Syro $et~al.$ [Nature Mat. 13, 1085 (2014)]. Our spin- and angle-resolved photoemission experiments on (001) oriented surfaces supporting a two-dimensional electron liquid with high carrier density show no detectable spin polarization in the photocu...
Preprint
We reinvestigate the putative giant spin splitting at the surface of SrTiO$_3$ reported by Santander-Syro $et~al.$ [Nature Mat. 13, 1085 (2014)]. Our spin- and angle-resolved photoemission experiments on (001) oriented surfaces supporting a two-dimensional electron liquid with high carrier density show no detectable spin polarization in the photocu...
Article
Most recently, black phosphorus has come into focus as a promising material for future applications in nanoelectronic devices due to its unique electronic and transport properties. Here, we use angle-resolved photoemission spectroscopy in conjunction with ab initio calculations within the framework of density-functional theory to disentangle surfac...
Article
Full-text available
Magnetic doping is expected to open a band gap at the Dirac point of topological insulators by breaking time-reversal symmetry and to enable novel topological phases. Epitaxial (Bi1-xMnx)2Se3 is a prototypical magnetic topological insulator with a pronounced surface band gap of 1/4100 meV. We show that this gap is neither due to ferromagnetic order...
Data
Supplementary Figures 1-21, Supplementary Tables 1-2, Supplementary Notes 1-9 and Supplementary References
Article
Full-text available
Three-dimensional topological insulators (TIs) are characterized by spin-polarized Dirac-cone surface states that are protected from backscattering by time-reversal symmetry. Control of the spin polarization of topological surface states (TSSs) using femtosecond light pulses opens novel perspectives for the generation and manipulation of dissipatio...
Preprint
Three-dimensional topological insulators (TIs) are characterized by spin-polarized Dirac-cone surface states that are protected from backscattering by time-reversal symmetry. Control of the spin polarization of topological surface states (TSSs) using femtosecond light pulses opens novel perspectives for the generation and manipulation of dissipatio...
Article
Full-text available
We analyze the strong hexagonal warping of the Dirac cone of Bi2Te3 by angle-resolved photoemission. Along ΓM, the dispersion deviates from a linear behavior meaning that the Dirac cone is warped outwards and not inwards. We show that this introduces an anisotropy in the lifetime broadening of the topological surface state which is larger along ΓK....
Article
Bi2Se3 topological insulators (TI) are grown on AlN (0001)/Si(111) substrates by molecular beam epitaxy. In a one-step growth at optimum temperature of 300(0)C, Bi2Se3 bonds strongly with AlN without forming interfacial reaction layers. This produces high epitaxial quality Bi2Se3 single crystals with a perfect registry with the substrate and abrupt...
Article
Full-text available
Ultrathin (sub-monolayer to 12 monolayers) AlN nanosheets are grown epitaxially by plasma assisted molecular beam epitaxy on Ag(111) single crystals. Electron diffraction and scanning tunneling microscopy provide evidence that AlN on Ag adopts a graphite-like hexagonal structure with a larger lattice constant compared to bulk-like wurtzite AlN. Thi...
Article
The electronic band structure of monolayer (4 × 4) silicene on Ag(111) is imaged by angle resolved photoelectron spectroscopy. A dominant hybrid surface metallic band is observed to be located near the bulk Ag sp-band which is also faintly visible. The two-dimensional character of the hybrid band has been distinguished against the bulk character of...
Article
Germanium deposited on single-crystal Ag(111) substrates at about 1/3-monolayer coverage forms a stable (√3×√3)R30∘ superstructure as seen by reflection high-energy electron diffraction. In situ angle resolved photoelectron spectroscopy reveals a rich band structure consisting of conelike features that cross the Fermi surface at the center (Γ̅ poin...
Article
Injection of metal atoms from a high-k oxide cap to a passivating GeO2 layer could ultimately affect the quality of Ge-based gate stacks. Here we investigate with first-principles calculations the incorporation of different metal atoms and their effect on the electronic properties of GeO2. We find that tetravalent Hf and Zr substitutional atoms hav...
Article
While compelling evidence for silicene on Ag (111) has been recently published [1], the existence of germanene remains elusive. We have performed MBE growth of (sub) monolayer Ge on single crystal Ag (111) substrates, supported by DFT calculations, with the aim to obtain germanene. RHEED data indicate a ( √3 x√3 )R30^0 superstructure, while in-situ...
Article
Changes in the electron trapping at the interface between Ge substrates and LaGeOx films grown by atomic O assisted molecular beam deposition are inferred upon post deposition annealing treatment on the as-deposited films from electrically detected magnetic resonance (EDMR) spectroscopy and from the electrical response of Pt/LaGeOx/Ge metal oxide s...
Conference Paper
Due to the high carrier mobility, Ge and III-V semiconductors are attractive as active channels for post-Si metal oxide semiconductor (MOS) devices. However, integration of gate dielectrics on high-mobility substrates is frequently jeopardized by the electrical activity of traps nearby the interface. Active traps determination at the interface betw...
Article
Volatilization of a Ge substrate may generate a large number of impurities inside the gate dielectric of a Ge-based device. Here we use density-functional theory calculations to probe the stability of Ge atoms and GeO molecules inside Al2O3 and Y2O3 high-k oxides. We identify the most stable impurity configurations and we show that both types of ex...
Article
GeO molecules are often emitted by Ge substrates under high-temperature annealing and, in the case of gate stacks, they diffuse through high-k oxides. Here we use first-principles quantum-mechanical calculations to probe the stability of these impurities in La2O3 and HfO2 and their effect on the electronic properties of the host systems. We find th...
Article
The molecular and atomic oxidation of molecular beam deposited Se passivating layers on Ge substrates was in situ investigated by X-ray photoelectron spectroscopy. It turns out that while Se is efficient in suppressing Ge oxidation upon molecular oxygen exposure, an extra thin Al layer is needed to protect the Ge surface from highly reactive atomic...
Article
In this work we demonstrate the fabrication and characterization of high performance junction diodes using annealing temperatures within the temperature range of 300–350°C. The low temperature dopant activation was assisted by a 50nm platinum layer which transforms into platinum germanide during annealing. The fabricated diodes exhibited high forwa...