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Universal Structural Influence on the 2D Electron Gas at SrTiO$_3$ Surfaces
Authors:
Eduardo B. Guedes,
Stefan Muff,
Walber H. Brito,
Marco Caputo,
Nicholas C. Plumb,
J. Hugo Dil,
Milan Radović
Abstract:
The two-dimensional electron gas found at the surface of SrTiO$_3$ and related interfaces has attracted significant attention as a promising basis for oxide electronics. In order to utilize its full potential, the response of this 2DEG to structural changes and surface modification must be understood in detail. Here, we present a study of the detailed electronic structure evolution of the 2DEG as…
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The two-dimensional electron gas found at the surface of SrTiO$_3$ and related interfaces has attracted significant attention as a promising basis for oxide electronics. In order to utilize its full potential, the response of this 2DEG to structural changes and surface modification must be understood in detail. Here, we present a study of the detailed electronic structure evolution of the 2DEG as a function of sample temperature and surface step density. By comparing our experimental results with \textit{ab initio} calculations, we found that a SrO-rich surface layer is a prerequisite for electronic confinement. We also show that local structure relaxations cause a metal-insulator transition of the system around 135~K. Our study presents a new and simple way of tuning the 2DEG via surface vicinality and identifies how the operation of prospective devices will respond to changes in temperature.
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Submitted 12 February, 2021;
originally announced February 2021.
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Single spin-polarised Fermi surface in SrTiO$_3$ thin films
Authors:
Eduardo B. Guedes,
Stefan Muff,
Mauro Fanciulli,
Andrew P. Weber,
Marco Caputo,
Zhiming Wang,
Nicholas C. Plumb,
Milan Radović,
J. Hugo Dil
Abstract:
The 2D electron gas (2DEG) formed at the surface of SrTiO$_3$(001) has attracted great interest because of its fascinating physical properties and potential as a novel electronic platform, but up to now has eluded a comprehensible way to tune its properties. Using angle-resolved photoemission spectroscopy with and without spin detection we here show that the band filling can be controlled by growi…
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The 2D electron gas (2DEG) formed at the surface of SrTiO$_3$(001) has attracted great interest because of its fascinating physical properties and potential as a novel electronic platform, but up to now has eluded a comprehensible way to tune its properties. Using angle-resolved photoemission spectroscopy with and without spin detection we here show that the band filling can be controlled by growing thin SrTiO$_3$ films on Nb doped SrTiO$_3$(001) substrates. This results in a single spin-polarised 2D Fermi surface, which bears potential as platform for Majorana physics. Based on our results it can furthermore be concluded that the 2DEG does not extend more than 2 unit cells into the film and that its properties depend on the amount of SrO$_x$ at the surface and possibly the dielectric response of the system.
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Submitted 25 May, 2020; v1 submitted 20 August, 2019;
originally announced August 2019.
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Spin-resolved electronic response to the phase transition in MoTe$_2$
Authors:
Andrew P. Weber,
Philipp Rüßmann,
Nan Xu,
Stefan Muff,
Mauro Fanciulli,
Arnaud Magrez,
Philippe Bugnon,
Helmuth Berger,
Nicholas C. Plumb,
Ming Shi,
Stefan Blügel,
Phivos Mavropoulos,
J. Hugo Dil
Abstract:
The semimetal MoTe$_2$ is studied by spin- and angle- resolved photoemission spectroscopy to probe the detailed electronic structure underlying its broad range of response behavior. A novel spin-texture is uncovered in the bulk Fermi surface of the non-centrosymmetric structural phase that is consistent with first-principles calculations. The spin-texture is three-dimensional, both in terms of mom…
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The semimetal MoTe$_2$ is studied by spin- and angle- resolved photoemission spectroscopy to probe the detailed electronic structure underlying its broad range of response behavior. A novel spin-texture is uncovered in the bulk Fermi surface of the non-centrosymmetric structural phase that is consistent with first-principles calculations. The spin-texture is three-dimensional, both in terms of momentum dependence and spin-orientation, and is not completely suppressed above the centrosymmetry-breaking transition temperature. Two types of surface Fermi arc are found to persist well above the transition temperature. The appearance of a large Fermi arc depends strongly on thermal history, and the electron quasiparticle lifetimes are greatly enhanced in the initial cooling. The results indicate that polar instability with strong electron-lattice interactions exists near the surface when the bulk is largely in a centrosymmetric phase.
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Submitted 1 July, 2018;
originally announced July 2018.
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The topological surface state of $α$-Sn on InSb(001) as studied by photoemission
Authors:
M. R. Scholz,
V. A. Rogalev,
L. Dudy,
F. Reis,
F. Adler,
J. Aulbach,
L. J. Collins-McIntyre,
L. B. Duffy,
H. F. Yang,
Y. L. Chen,
T. Hesjedal,
Z. K. Liu,
M. Hoesch,
S. Muff,
J. H. Dil,
J. Schäfer,
R. Claessen
Abstract:
We report on the electronic structure of the elemental topological semimetal $α$-Sn on InSb(001). High-resolution angle-resolved photoemission data allow to observe the topological surface state (TSS) that is degenerate with the bulk band structure and show that the former is unaffected by different surface reconstructions. An unintentional $p$-type doping of the as-grown films was compensated by…
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We report on the electronic structure of the elemental topological semimetal $α$-Sn on InSb(001). High-resolution angle-resolved photoemission data allow to observe the topological surface state (TSS) that is degenerate with the bulk band structure and show that the former is unaffected by different surface reconstructions. An unintentional $p$-type doping of the as-grown films was compensated by deposition of potassium or tellurium after the growth, thereby shifting the Dirac point of the surface state below the Fermi level. We show that, while having the potential to break time-reversal symmetry, iron impurities with a coverage of up to 0.25 monolayers do not have any further impact on the surface state beyond that of K or Te. Furthermore, we have measured the spin-momentum locking of electrons from the TSS by means of spin-resolved photoemission. Our results show that the spin vector lies fully in-plane, but it also has a finite radial component. Finally, we analyze the decay of photoholes introduced in the photoemission process, and by this gain insight into the many-body interactions in the system. Surprisingly, we extract quasiparticle lifetimes comparable to other topological materials where the TSS is located within a bulk band gap. We argue that the main decay of photoholes is caused by intraband scattering, while scattering into bulk states is suppressed due to different orbital symmetries of bulk and surface states.
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Submitted 29 November, 2017;
originally announced November 2017.
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Observation of Wannier-Stark localization at the surface of BaTiO$_3$ films by photoemission
Authors:
Stefan Muff,
Nicolas Pilet,
Mauro Fanciulli,
Andrew P. Weber,
Christian Wessler,
Zoran Ristic,
Zhiming Wang,
Nicholas C. Plumb,
Milan Radovic,
J. Hugo Dil
Abstract:
Observation of Bloch oscillations and Wannier-Stark localization of charge carriers is typically impossible in single-crystals, because an electric field higher than the breakdown voltage is required. In BaTiO$_3$ however, high intrinsic electric fields are present due to its ferroelectric properties. With angle-resolved photoemission we directly probe the Wannier-Stark localized surface states of…
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Observation of Bloch oscillations and Wannier-Stark localization of charge carriers is typically impossible in single-crystals, because an electric field higher than the breakdown voltage is required. In BaTiO$_3$ however, high intrinsic electric fields are present due to its ferroelectric properties. With angle-resolved photoemission we directly probe the Wannier-Stark localized surface states of the BaTiO$_3$ film-vacuum interface and show that this effect extends to thin SrTiO$_3$ overlayers. The electrons are found to be localized along the in-plane polarization direction of the BaTiO$_3$ film.
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Submitted 6 October, 2017;
originally announced October 2017.
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Spin-resolved electronic structure of ferroelectric α-GeTe and multiferroic Ge1-xMnxTe
Authors:
J. Krempasky,
M. Fanciulli,
J. Minár,
W. Khan,
M. Muntwiler,
F. Bertran,
S. Muff,
A. P. Weber,
V. N. Strocov,
V. V. Volobuiev,
G. Springholz,
J. H. Dil
Abstract:
Germanium telluride features special spin-electric effects originating from spin-orbit coupling and symmetry breaking by the ferroelectric lattice polarization, which opens up many prospectives for electrically tunable and switchable spin electronic devices. By Mn doping of the α-GeTe host lattice, the system becomes a multiferroic semiconductor possessing magnetoelectric properties in which the e…
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Germanium telluride features special spin-electric effects originating from spin-orbit coupling and symmetry breaking by the ferroelectric lattice polarization, which opens up many prospectives for electrically tunable and switchable spin electronic devices. By Mn doping of the α-GeTe host lattice, the system becomes a multiferroic semiconductor possessing magnetoelectric properties in which the electric polarization, magnetization and spin texture are coupled to each other. Employing spin- and angle-resolved photoemission spectroscopy in bulk- and surface-sensitive energy ranges and by varying dipole transition matrix elements, we disentangle the bulk, surface and surface-resonance states of the electronic structure and determine the spin textures for selected parameters. From our results, we derive a comprehensive model of the α-GeTe surface electronic structure which fits experimental data and first principle theoretical predictions and we discuss the unconventional evolution of the Rashba-type spin splitting upon manipulation by external B- and E-fields.
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Submitted 14 August, 2017;
originally announced August 2017.
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Operando imaging of all-electric spin texture manipulation in ferroelectric and multiferroic Rashba semiconductors
Authors:
J. Krempaský,
S. Muff,
J. Minár,
N. Pilet,
M. Fanciulli,
A. P. Weber,
V. V. Volobuiev,
M. Gmitra,
C. A. F. Vaz,
V. Scagnoli,
G. Springholz,
J. H. Dil
Abstract:
The control of the electron spin by external means is a key issue for spintronic devices. Using spin- and angle-resolved photoemission spectroscopy (SARPES) with three-dimensional spin detection, we demonstrate operando electrostatic spin manipulation in ferroelectric GeTe and multiferroic Ge1-xMnxTe. We not only demonstrate for the first time electrostatic spin manipulation in Rashba semiconducto…
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The control of the electron spin by external means is a key issue for spintronic devices. Using spin- and angle-resolved photoemission spectroscopy (SARPES) with three-dimensional spin detection, we demonstrate operando electrostatic spin manipulation in ferroelectric GeTe and multiferroic Ge1-xMnxTe. We not only demonstrate for the first time electrostatic spin manipulation in Rashba semiconductors due to ferroelectric polarization reversal, but are also able to follow the switching pathway in detail, and show a gain of the Rashba-splitting strength under external fields. In multiferroic Ge1-xMnxTe operando SARPES reveals switching of the perpendicular spin component due to electric field induced magnetization reversal. This provides firm evidence of effective multiferroic coupling which opens up magnetoelectric functionality with a multitude of spin-switching paths in which the magnetic and electric order parameters are coupled through ferroelastic relaxation paths. This work thus provides a new type of magnetoelectric switching entangled with Rashba-Zeeman splitting in a multiferroic system.
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Submitted 26 July, 2017;
originally announced July 2017.
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Universal response of the type-II Weyl semimetals phase diagram
Authors:
P. Rüßmann,
A. P. Weber,
F. Glott,
N. Xu,
M. Fanciulli,
S. Muff,
A. Magrez,
P. Bugnon,
H. Berger,
M. Bode,
J. H. Dil,
S. Blügel,
P. Mavropoulos,
P. Sessi
Abstract:
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…
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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.
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Submitted 1 June, 2017;
originally announced June 2017.
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Observation of a two-dimensional electron gas at CaTiO$_3$ film surfaces
Authors:
Stefan Muff,
Mauro Fanciulli,
Andrew P. Weber,
Nicolas Pilet,
Zoran Ristic,
Zhiming Wang,
Nicholas C. Plumb,
Milan Radovic,
J. Hugo Dil
Abstract:
The two-dimensional electron gas at the surface of titanates gathered attention due to its potential to replace conventional silicon based semiconductors in the future. In this study, we investigated films of the parent perovskite CaTiO$_3$, grown by pulsed laser deposition, by means of angular-resolved photoelectron spectroscopy. The films show a c(4x2) surface reconstruction after the growth tha…
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The two-dimensional electron gas at the surface of titanates gathered attention due to its potential to replace conventional silicon based semiconductors in the future. In this study, we investigated films of the parent perovskite CaTiO$_3$, grown by pulsed laser deposition, by means of angular-resolved photoelectron spectroscopy. The films show a c(4x2) surface reconstruction after the growth that is reduced to a p(2x2) reconstruction under UV-light. At the CaTiO$_3$ film surface, a two-dimensional electron gas (2DEG) is found with an occupied band width of 400 meV. With our findings CaTiO$_3$ is added to the group of oxides with a 2DEG at their surface. Our study widens the phase space to investigate strontium and barium doped CaTiO$_3$ and the interplay of ferroelectric properties with the 2DEG at oxide surfaces. This could open up new paths to tailor two-dimensional transport properties of these systems towards possible applications.
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Submitted 26 May, 2017;
originally announced May 2017.
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Selective probing of hidden spin-polarized states in inversion-symmetric bulk MoS2
Authors:
E. Razzoli,
T. Jaouen,
M. -L. Mottas,
B. Hildebrand,
G. Monney,
A. Pisoni,
S. Muff,
M. Fanciulli,
N. C. Plumb,
V. A. Rogalev,
V. N. Strocov,
J. Mesot,
M. Shi,
J. H. Dil,
H. Beck,
P. Aebi
Abstract:
Spin- and angle-resolved photoemission spectroscopy is used to reveal that a large spin polarization is observable in the bulk centrosymmetric transition metal dichalcogenide MoS2. It is found that the measured spin polarization can be reversed by changing the handedness of incident circularly-polarized light. Calculations based on a three-step model of photoemission show that the valley and layer…
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Spin- and angle-resolved photoemission spectroscopy is used to reveal that a large spin polarization is observable in the bulk centrosymmetric transition metal dichalcogenide MoS2. It is found that the measured spin polarization can be reversed by changing the handedness of incident circularly-polarized light. Calculations based on a three-step model of photoemission show that the valley and layer-locked spin-polarized electronic states can be selectively addressed by circularly-polarized light, therefore providing a novel route to probe these hidden spin-polarized states in inversion-symmetric systems as predicted by Zhang et al. [Nature Physics 10, 387 (2014)].
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Submitted 24 January, 2017;
originally announced January 2017.
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Entanglement and manipulation of the magnetic and spin-orbit order in multiferroic Rashba semiconductors
Authors:
J. Krempasky,
S. Muff,
F. Bisti,
M. Fanciulli,
H. Volfová,
A. Weber,
N. Pilet,
P. Warnicke,
H. Ebert,
J. Braun,
F. Bertran,
V. V. Volobuev,
J. Minár,
G. Springholz,
J. H. Dil,
V. N. Strocov
Abstract:
The interplay between electronic eigenstates, spin, and orbital degrees of freedom, combined with fundamental breaking of symmetries is currently one of the most exciting fields of research. Multiferroics such as (GeMn)Te fulfill these requirements providing unusual physical properties due to the coexistence and coupling between ferromagnetic and ferroelectric order in one and the same system. Her…
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The interplay between electronic eigenstates, spin, and orbital degrees of freedom, combined with fundamental breaking of symmetries is currently one of the most exciting fields of research. Multiferroics such as (GeMn)Te fulfill these requirements providing unusual physical properties due to the coexistence and coupling between ferromagnetic and ferroelectric order in one and the same system. Here we show that multiferroic (GeMn)Te inherits from its parent ferroelectric α-GeTe compound a giant Rashba splitting of three-dimensional bulk states which competes with the Zeeman spin splitting induced by the magnetic exchange interactions. The collinear alignment of ferroelectric and ferromagnetic polarization leads to an opening of a tunable Zeeman gap of up to 100 meV around the Dirac point of the Rashba bands, coupled with a change in spin texture by entanglement of magnetic and spin-orbit order. Through applications of magnetic fields, we demonstrate manipulation of spin- texture by spin resolved photoemission experiments, which is also expected for electric fields based on the multiferroic coupling. The control of spin helicity of the bands and its locking to ferromagnetic and ferroelectric order opens fascinating new avenues for highly multifunctional multiferroic Rashba devices suited for reprogrammable logic and/or nonvolatile memory applications.
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Submitted 5 June, 2016; v1 submitted 1 June, 2016;
originally announced June 2016.
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Spin polarization and attosecond time delay in photoemission from spin degenerate states of solids
Authors:
Mauro Fanciulli,
Henrieta Volfová,
Stefan Muff,
Jürgen Braun,
Hubert Ebert,
Jan Minár,
Ulrich Heinzmann,
J. Hugo Dil
Abstract:
After photon absorption, electrons from a dispersive band of a solid require a finite time in the photoemission process before being photoemitted as free particles, in line with recent attosecondresolved photoemission experiments. According to the Eisenbud-Wigner-Smith model, the time delay is due to a phase shift of different transitions that occur in the process. Such a phase shift is also at th…
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After photon absorption, electrons from a dispersive band of a solid require a finite time in the photoemission process before being photoemitted as free particles, in line with recent attosecondresolved photoemission experiments. According to the Eisenbud-Wigner-Smith model, the time delay is due to a phase shift of different transitions that occur in the process. Such a phase shift is also at the origin of the angular dependent spin polarization of the photoelectron beam, observable in spin degenerate systems without angular momentum transfer by the incident photon. We propose a semi-quantitative model which permits to relate spin and time scales in photoemission from condensed matter targets and to better understand spin- and angle-resolved photoemission spectroscopy (SARPES) experiments on spin degenerate systems. We also present the first experimental determination by SARPES of this time delay in a dispersive band, which is found to be greater than 26 as for electrons emitted from the sp-bulk band of the model system Cu(111).
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Submitted 29 September, 2016; v1 submitted 27 January, 2016;
originally announced January 2016.
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Sputtering induced re-emergence of the topological surface state in Bi$_2$Se$_3$
Authors:
Raquel Queiroz,
Gabriel Landolt,
Stefan Muff,
Bartosz Slomski,
Thorsten Schmitt,
Vladimir N. Strocov,
Jianli Mi,
Bo Brummerstedt Iversen,
Philip Hofmann,
Jürg Osterwalder,
Andreas P. Schnyder,
J. Hugo Dil
Abstract:
We study the fate of the surface states of Bi$_2$Se$_3$ under disorder with strength larger than the bulk gap, caused by neon sputtering and nonmagnetic adsorbates. We find that neon sputtering introduces strong but dilute defects, which can be modeled by a unitary impurity distribution, whereas adsorbates, such as water vapor or carbon monoxide, are best described by Gaussian disorder. Remarkably…
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We study the fate of the surface states of Bi$_2$Se$_3$ under disorder with strength larger than the bulk gap, caused by neon sputtering and nonmagnetic adsorbates. We find that neon sputtering introduces strong but dilute defects, which can be modeled by a unitary impurity distribution, whereas adsorbates, such as water vapor or carbon monoxide, are best described by Gaussian disorder. Remarkably, these two disorder types have a dramatically different effect on the surface states. Our soft x-ray ARPES measurements combined with numerical simulations show that unitary surface disorder pushes the Dirac state to inward quintuplet layers, burying it below an insulating surface layer. As a consequence, the surface spectral function becomes weaker, but retains its quasiparticle peak. This is in contrast to Gaussian disorder, which smears out the quasiparticle peak completely. At the surface of Bi$_2$Se$_3$, the effects of Gaussian disorder can be reduced by removing surface adsorbates using neon sputtering, which, however, introduces unitary scatterers. Since unitary disorder has a weaker effect than Gaussian disorder, the ARPES signal of the Dirac surface state becomes sharper upon sputtering.
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Submitted 18 December, 2015;
originally announced December 2015.
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Observation of Fermi arc spin texture in TaAs
Authors:
B. Q. Lv,
S. Muff,
T. Qian,
Z. D. Song,
S. M. Nie,
N. Xu,
P. Richard,
C. E. Matt,
N. C. Plumb,
L. X. Zhao,
G. F. Chen,
Z. Fang,
X. Dai,
J. H. Dil,
J. Mesot,
M. Shi,
H. M. Weng,
H. Ding
Abstract:
We have investigated the spin texture of surface Fermi arcs in the recently discovered Weyl semimetal TaAs using spin- and angle-resolved photoemission spectroscopy. The experimental results demonstrate that the Fermi arcs are spin-polarized. The measured spin texture fulfills the requirement of mirror and time reversal symmetries and is well reproduced by our first-principles calculations, which…
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We have investigated the spin texture of surface Fermi arcs in the recently discovered Weyl semimetal TaAs using spin- and angle-resolved photoemission spectroscopy. The experimental results demonstrate that the Fermi arcs are spin-polarized. The measured spin texture fulfills the requirement of mirror and time reversal symmetries and is well reproduced by our first-principles calculations, which gives strong evidence for the topologically nontrivial Weyl semimetal state in TaAs. The consistency between the experimental and calculated results further confirms the distribution of chirality of the Weyl nodes determined by first-principles calculations.
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Submitted 25 October, 2015;
originally announced October 2015.
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Observation of correlated spin-orbit order in a strongly anisotropic quantum wire system
Authors:
C. Brand,
H. Pfnür,
G. Landolt,
S. Muff,
J. H. Dil,
Tanmoy Das,
C. Tegenkamp
Abstract:
Quantum wires with spin-orbit coupling provide a unique opportunity to simultaneously control the coupling strength and the screened Coulomb interactions where new exotic phases of matter can be explored. Here we report on the observation of an exotic spin-orbit density wave in Pb-atomic wires on Si(557) surfaces by mapping out the evolution of the modulated spin-texture at various conditions with…
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Quantum wires with spin-orbit coupling provide a unique opportunity to simultaneously control the coupling strength and the screened Coulomb interactions where new exotic phases of matter can be explored. Here we report on the observation of an exotic spin-orbit density wave in Pb-atomic wires on Si(557) surfaces by mapping out the evolution of the modulated spin-texture at various conditions with spin- and angle-resolved photoelectron spectroscopy. The results are independently quantified by surface transport measurements. The spin polarization, coherence length, spin dephasing rate, and the associated quasiparticle gap decrease simultaneously as the screened Coulomb interaction decreases with increasing excess coverage, providing a new mechanism for generating and manipulating a spin-orbit entanglement effect via electronic interaction. Despite clear evidence of spontaneous spin-rotation symmetry breaking and modulation of spin-momentum structure as a function of excess coverage, the average spin-polarization over the Brillouin zone vanishes, indicating that time-reversal symmetry is intact as theoretically predicted.
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Submitted 22 July, 2015;
originally announced July 2015.
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Surface versus bulk contributions to the giant Rashba splitting in the ferroelectric α-GeTe(111) semiconductor
Authors:
J. Krempaský,
H. Volfová,
S. Muff,
N. Pilet,
G. Landolt,
M. Radović,
M. Shi,
D. Kriegner,
V. Holý,
J. Braun,
H. Ebert,
F. Bisti,
V. A. Rogalev,
V. N. Strocov,
G. Springholz,
J. Minár,
J. H. Dil
Abstract:
In systems with broken inversion symmetry spin-orbit coupling (SOC) yields a Rashba-type spin splitting of electronic states, manifested in a k-dependent splitting of the bands. While most research had previously focused on 2D electron systems, recently a three-dimensional (3D) form of such Rashba-effect was found in a series of bismuth tellurohalides. Whereas these materials exhibit a very large…
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In systems with broken inversion symmetry spin-orbit coupling (SOC) yields a Rashba-type spin splitting of electronic states, manifested in a k-dependent splitting of the bands. While most research had previously focused on 2D electron systems, recently a three-dimensional (3D) form of such Rashba-effect was found in a series of bismuth tellurohalides. Whereas these materials exhibit a very large spin-splitting, they lack an important property concerning functionalization, namely the possibility to switch or tune the spin texture. This limitation can be overcome in a new class of functional materials displaying Rashba-splitting coupled to ferroelectricity: the ferroelectric Rashba semiconductors (FERS). Using spin- and angle-resolved photoemission spectroscopy (SARPES) we show that GeTe(111) forms a prime member of this class, displaying a complex spin-texture for the Rashba-split surface and bulk bands arising from the intrinsic inversion symmetry breaking caused by the ferroelectric polarization of the bulk (FE). Apart from pure surface and bulk states we find surface-bulk resonant states (SBR) whose wavefunctions entangle the spinors from the bulk and surface contributions. At the Fermi level their hybridization results in unconventional spin topologies with cochiral helicities and concomitant gap opening. The GeTe(111) surface and SBR states make the semiconductor surface conducting. At the same time our SARPES data confirm that GeTe is a narrow-gap semiconductor, suggesting that GeTe(111) electronic states are endowed with spin properties that are theoretically challenging to anticipate. As the helicity of the spins in Rashba bands is connected to the direction of the FE polarization, this work paves the way to all-electric non-volatile control of spin-transport properties in semiconductors.
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Submitted 17 March, 2015;
originally announced March 2015.
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Response of the topological surface state to surface disorder in TlBiSe$_2$
Authors:
Florian Pielmeier,
Gabriel Landolt,
Bartosz Slomski,
Stefan Muff,
Julian Berwanger,
Andreas Eich,
Alexander A. Khajetoorians,
Jens Wiebe,
Ziya S. Aliev,
Mahammad B. Babanly,
Roland Wiesendanger,
Jürg Osterwalder,
Evgeniy V. Chulkov,
Franz J. Giessibl,
J. Hugo Dil
Abstract:
Through a combination of experimental techniques we show that the topmost layer of the topo- logical insulator TlBiSe$_2$ as prepared by cleavage is formed by irregularly shaped Tl islands at cryogenic temperatures and by mobile Tl atoms at room temperature. No trivial surface states are observed in photoemission at low temperatures, which suggests that these islands can not be re- garded as a cle…
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Through a combination of experimental techniques we show that the topmost layer of the topo- logical insulator TlBiSe$_2$ as prepared by cleavage is formed by irregularly shaped Tl islands at cryogenic temperatures and by mobile Tl atoms at room temperature. No trivial surface states are observed in photoemission at low temperatures, which suggests that these islands can not be re- garded as a clear surface termination. The topological surface state is, however, clearly resolved in photoemission experiments. This is interpreted as a direct evidence of its topological self-protection and shows the robust nature of the Dirac cone like surface state. Our results can also help explain the apparent mass acquisition in S-doped TlBiSe$_2$.
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Submitted 4 February, 2015;
originally announced February 2015.
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Direct observation of the spin texture in strongly correlated SmB6 as evidence of the topological Kondo insulator
Authors:
N. Xu,
P. K. Biswas,
J. H. Dil,
R. S. Dhaka,
G. Landolt,
S. Muff,
C. E. Matt,
X. Shi,
N. C. Plumb,
M. Radovic,
E. Pomjakushina,
K. Conder,
A. Amato,
S. V. Borisenko,
R. Yu,
H. -M. Weng,
Z. Fang,
X. Dai,
J. Mesot,
H. Ding,
M. Shi
Abstract:
The concept of a topological Kondo insulator (TKI) has been brought forward as a new class of topological insulators in which non-trivial surface states reside in the bulk Kondo band gap at low temperature due to the strong spin-orbit coupling [1-3]. In contrast to other three-dimensional (3D) topological insulators (e.g. Bi2Se3), a TKI is truly insulating in the bulk [4]. Furthermore, strong elec…
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The concept of a topological Kondo insulator (TKI) has been brought forward as a new class of topological insulators in which non-trivial surface states reside in the bulk Kondo band gap at low temperature due to the strong spin-orbit coupling [1-3]. In contrast to other three-dimensional (3D) topological insulators (e.g. Bi2Se3), a TKI is truly insulating in the bulk [4]. Furthermore, strong electron correlations are present in the system, which may interact with the novel topological phase. Applying spin- and angle-resolved photoemission spectroscopy (SARPES) to the Kondo insulator SmB6, a promising TKI candidate, we reveal that the surface states of SmB6 are spin polarized, and the spin is locked to the crystal momentum. Counter-propagating states (i.e. at k and -k) have opposite spin polarizations protected by time-reversal symmetry. Together with the odd number of Fermi surfaces of surface states between the 4 time-reversal invariant momenta in the surface Brillouin zone [5], these findings prove, for the first time, that SmB6 can host non-trivial topological surface states in a full insulating gap in the bulk stemming from the Kondo effect. Hence our experimental results establish that SmB6 is the first realization of a 3D TKI. It can also serve as an ideal platform for the systematic study of the interplay between novel topological quantum states with emergent effects and competing order induced by strongly correlated electrons.
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Submitted 30 July, 2014;
originally announced July 2014.
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Interband spin-orbit coupling between anti-parallel spin states in Pb quantum well states
Authors:
Bartosz Slomski,
Gabriel Landolt,
Stefan Muff,
Fabian Meier,
Jürg Osterwalder,
J. Hugo Dil
Abstract:
Using spin and angle-resolved photoemission spectroscopy we investigate a momentum region in Pb quantum well states on Si(111) where hybridization between Rashba-split bands alters the band structure significantly. Starting from the Rashba regime where the dispersion of the quasi-free two-dimensional electron gas is well described by two spin-polarized parabolas, we find a breakdown of the Rashba…
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Using spin and angle-resolved photoemission spectroscopy we investigate a momentum region in Pb quantum well states on Si(111) where hybridization between Rashba-split bands alters the band structure significantly. Starting from the Rashba regime where the dispersion of the quasi-free two-dimensional electron gas is well described by two spin-polarized parabolas, we find a breakdown of the Rashba behavior which manifests itself (i) in a spin splitting that is no longer proportional to the in-plane momentum and (ii) in a reversal of the sign of the momentum splitting. Our experimental findings are well explained by including interband spin-orbit coupling that mixes Rashba-split states with anti-parallel rather than parallel spins. Similar results for Pb/Cu(111) reveal that the proposed hybridization scenario is independent on the supporting substrate.
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Submitted 2 June, 2013;
originally announced June 2013.
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Separating the bulk and surface n- to p-type transition in the topological insulator GeBi(4-x)SbxTe7
Authors:
Stefan Muff,
Fabian von Rohr,
Gabriel Landolt,
Bartosz Slomski,
Andreas Schilling,
Robert J. Cava,
Jürg Osterwalder,
J. Hugo Dil
Abstract:
We identify the multi-layered compound GeBi4Te7 to be a topological insulator with a freestanding Dirac point, slightly above the valence band maximum, using angle-resolved photoemission spectroscopy (ARPES) measurements. The spin polarization satisffies the time reversal symmetry of the surface states, visible in spin-resolved ARPES. For increasing Sb content in GeBi(4-x)SbxTe7 we observe a trans…
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We identify the multi-layered compound GeBi4Te7 to be a topological insulator with a freestanding Dirac point, slightly above the valence band maximum, using angle-resolved photoemission spectroscopy (ARPES) measurements. The spin polarization satisffies the time reversal symmetry of the surface states, visible in spin-resolved ARPES. For increasing Sb content in GeBi(4-x)SbxTe7 we observe a transition from n- to p-type in bulk sensitive Seebeck coefficient measurements at a doping of x = 0.6. In surface sensitive ARPES measurements a rigid band shift is observed with Sb doping, accompanied by a movement of the Dirac point towards the Fermi level. Between x = 0.8 and x = 1 the Fermi level crosses the band gap, changing the surface transport regime. This difference of the n- to p-type transition between the surface region and the bulk is caused by band bending effects which are also responsible for a non-coexistence of insulating phases in the bulk and in the near surface region.
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Submitted 15 November, 2013; v1 submitted 29 April, 2013;
originally announced April 2013.
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Surface and bulk Fermiology and band dispersion in non-centrosymmetric BiTeI
Authors:
Gabriel Landolt,
Sergey V. Eremeev,
Yury M. Koroteev,
Bartosz Slomski,
Stefan Muff,
Masaki Kobayashi,
Vladimir N. Strocov,
Thorsten Schmitt,
Ziya S. Aliev,
Mahammad B. Babanly,
Imamaddin R. Amiraslanov,
Evgueni V. Chulkov,
Jurg Osterwalder,
J. Hugo Dil
Abstract:
BiTeI has a layered and non-centrosymmetric structure where strong spin-orbit interaction leads to a giant spin splitting in the bulk bands. Here we present high-resolution angle-resolved photoemission (ARPES) data in the UV and soft x-ray regime that clearly disentangle the surface from the bulk electronic structure. Spin-resolved UV-ARPES measurements on opposite, non-equivalent surfaces show id…
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BiTeI has a layered and non-centrosymmetric structure where strong spin-orbit interaction leads to a giant spin splitting in the bulk bands. Here we present high-resolution angle-resolved photoemission (ARPES) data in the UV and soft x-ray regime that clearly disentangle the surface from the bulk electronic structure. Spin-resolved UV-ARPES measurements on opposite, non-equivalent surfaces show identical spin structures, thus clarifying the surface state character. Soft x-ray ARPES data clearly reveal the spindle-torus shape of the bulk Fermi surface, induced by the spin-orbit interaction.
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Submitted 10 April, 2012;
originally announced April 2012.
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Local modularity measure for network clusterizations
Authors:
Stefanie Muff,
Francesco Rao,
Amedeo Caflisch
Abstract:
Many complex networks have an underlying modular structure, i.e., structural subunits (communities or clusters) characterized by highly interconnected nodes. The modularity $Q$ has been introduced as a measure to assess the quality of clusterizations. $Q$ has a global view, while in many real-world networks clusters are linked mainly \emph{locally} among each other (\emph{local cluster-connectiv…
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Many complex networks have an underlying modular structure, i.e., structural subunits (communities or clusters) characterized by highly interconnected nodes. The modularity $Q$ has been introduced as a measure to assess the quality of clusterizations. $Q$ has a global view, while in many real-world networks clusters are linked mainly \emph{locally} among each other (\emph{local cluster-connectivity}). Here, we introduce a new measure, localized modularity $LQ$, which reflects local cluster structure. Optimization of $Q$ and $LQ$ on the clusterization of two biological networks shows that the localized modularity identifies more cohesive clusters, yielding a complementary view of higher granularity.
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Submitted 13 October, 2005; v1 submitted 10 March, 2005;
originally announced March 2005.