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Revealing the Origin of Time-reversal Symmetry Breaking in Fe-chalcogenide Superconductor FeTe1-xSex
Authors:
Camron Farhang,
Nader Zaki,
Jingyuan Wang,
Genda Gu,
Peter D. Johnson,
Jing Xia
Abstract:
Recently evidence has emerged in the topological superconductor Fe-chalcogenide FeTe1-xSex for time-reversal symmetry breaking (TRSB), the nature of which has strong implications on the Majorana zero modes (MZM) discovered in this system. It remains unclear however whether the TRSB resides in the topological surface state (TSS) or in the bulk, and whether it is due to an unconventional TRSB superc…
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Recently evidence has emerged in the topological superconductor Fe-chalcogenide FeTe1-xSex for time-reversal symmetry breaking (TRSB), the nature of which has strong implications on the Majorana zero modes (MZM) discovered in this system. It remains unclear however whether the TRSB resides in the topological surface state (TSS) or in the bulk, and whether it is due to an unconventional TRSB superconducting order parameter or an intertwined order. Here by performing in superconducting FeTe1-xSex crystals both surface-magneto-optic-Kerr effect (SMOKE) measurements using a Sagnac interferometer and bulk magnetic susceptibility measurements, we pinpoint the TRSB to the TSS, where we also detect a Dirac gap. Further, we observe surface TRSB in non-superconducting FeTe1-xSex of nominally identical composition, indicating that TRSB arises from an intertwined surface ferromagnetic (FM) order. The observed surface FM bears striking similarities to the two-dimensional (2D) FM found in 2D van der Waals crystals, and is highly sensitive to the exact chemical composition, thereby providing a means for optimizing the conditions for Majorana particles that are useful for robust quantum computing.
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Submitted 10 January, 2023; v1 submitted 14 August, 2022;
originally announced August 2022.
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On the Role of Defects in the Electronic Structure of MnBi$_{2-x}$Sb$_x$Te$_4$
Authors:
Daniel Nevola,
Kevin F. Garrity,
Nader Zaki,
Jiaqiang Yan,
Hu Miao,
Sugata Chowdhury,
Peter D. Johnson
Abstract:
Elemental substitution is a proven method of Fermi level tuning in topological insulators, which is needed for device applications. Through static and time resolved photoemission, we show that in MnBi$_2$Te$_4$, elemental substitution of Bi with Sb indeed tunes the Fermi level towards the bulk band gap, making the material charge neutral at 35\% Sb concentration. For the first time, we are able to…
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Elemental substitution is a proven method of Fermi level tuning in topological insulators, which is needed for device applications. Through static and time resolved photoemission, we show that in MnBi$_2$Te$_4$, elemental substitution of Bi with Sb indeed tunes the Fermi level towards the bulk band gap, making the material charge neutral at 35\% Sb concentration. For the first time, we are able to directly probe the excited state band structure at this doping level, and their dynamics, which show that the decay channels at the Fermi level are severely restricted. However, elemental substitution widens the surface state gap, which we attribute to the increase in antisite defects resulting from Sb substitution. This hypothesis is supported by DFT calculations that include defects, which show a sensitivity of the topological surface state to their inclusion. Our results emphasize the need for defect control if MnBi$_{2-x}$Sb$_x$Te$_4$ is to be used for device applications.
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Submitted 28 June, 2022;
originally announced June 2022.
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Ultrafast Melting of Superconductivity in an Iron-Based Superconductor
Authors:
Dan Nevola,
Nader Zaki,
John M. Tranquada,
Weiguo Yin,
Genda Gu,
Qiang Li,
Peter D. Johnson
Abstract:
Intense debate has recently arisen regarding the photoinduced changes to the iron-chalcogenide superconductors, including the enhancement of superconductivity and a metastable state. Here, by employing high energy resolution, we directly observe the melting of superconductivity on ultrafast timescales. We demonstrate a distinctly nonequilibrium response on short timescales, where the gap fills in…
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Intense debate has recently arisen regarding the photoinduced changes to the iron-chalcogenide superconductors, including the enhancement of superconductivity and a metastable state. Here, by employing high energy resolution, we directly observe the melting of superconductivity on ultrafast timescales. We demonstrate a distinctly nonequilibrium response on short timescales, where the gap fills in prior to the destruction of the superconducting peak, followed by a metastable response. We propose that the former is due to pair phase decoherence and speculate that the latter is due to the increase in double stripe correlations that are known to compete with superconductivity. Our results add to exciting new developments on the iron-based superconductors, indicating that the photoinduced metastable state possibly competes with superconductivity.
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Submitted 17 September, 2022; v1 submitted 1 March, 2022;
originally announced March 2022.
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Magnetic, superconducting, and topological surface states on Fe$_{1+y}$Te$_{1-x}$Se$_{x}$
Authors:
Yangmu Li,
Nader Zaki,
Vasile O. Garlea,
Andrei T. Savici,
David Fobes,
Zhijun Xu,
Fernando Camino,
Cedomir Petrovic,
Genda Gu,
Peter D. Johnson,
John M. Tranquada,
Igor A. Zaliznyak
Abstract:
The idea of employing non-Abelian statistics for error-free quantum computing ignited interest in recent reports of topological surface superconductivity and Majorana zero modes (MZMs) in FeTe$_{0.55}$Se$_{0.45}$. An associated puzzle is that the topological features and superconducting properties are not observed uniformly across the sample surface. Understanding and practical control of these el…
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The idea of employing non-Abelian statistics for error-free quantum computing ignited interest in recent reports of topological surface superconductivity and Majorana zero modes (MZMs) in FeTe$_{0.55}$Se$_{0.45}$. An associated puzzle is that the topological features and superconducting properties are not observed uniformly across the sample surface. Understanding and practical control of these electronic inhomogeneities present a prominent challenge for potential applications. Here, we combine neutron scattering, scanning angle-resolved photoemission spectroscopy (ARPES), and microprobe composition and resistivity measurements to characterize the electronic state of Fe$_{1+y}$Te$_{1-x}$Se$_{x}$. We establish a phase diagram in which the superconductivity is observed only at sufficiently low Fe concentration, in association with distinct antiferromagnetic correlations, while the coexisting topological surface state occurs only at sufficiently high Te concentration. We find that FeTe$_{0.55}$Se$_{0.45}$ is located very close to both phase boundaries, which explains the inhomogeneity of superconducting and topological states. Our results demonstrate the compositional control required for use of topological MZMs in practical applications.
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Submitted 14 December, 2020;
originally announced December 2020.
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Optical and photoemission investigation of structural and magnetic transitions in the iron-based superconductor Sr$_\mathbf{0.67}$Na$_\mathbf{0.33}$Fe$_\mathbf{2}$As$_\mathbf{2}$
Authors:
R. Yang,
J. W. Huang,
N. Zaki,
I. Pletikosic,
Y. M. Dai,
H. Xiao,
T. Valla,
P. D. Johnson,
X. J. Zhou,
X. G. Qiu,
C. C. Homes
Abstract:
We report the temperature-dependent optical conductivity and ARPES studies of the iron-based superconductor (SC) Sr$_{0.67}$Na$_{0.33}$Fe$_2$As$_2$ in the high-temperature tetragonal paramagnetic phase; below the structural and magnetic transitions at $T_{\rm N}\simeq$125 K in the orthorhombic spin-density-wave (SDW)-like phase, and $T_r\simeq$42 K in the reentrant tetragonal double-Q magnetic pha…
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We report the temperature-dependent optical conductivity and ARPES studies of the iron-based superconductor (SC) Sr$_{0.67}$Na$_{0.33}$Fe$_2$As$_2$ in the high-temperature tetragonal paramagnetic phase; below the structural and magnetic transitions at $T_{\rm N}\simeq$125 K in the orthorhombic spin-density-wave (SDW)-like phase, and $T_r\simeq$42 K in the reentrant tetragonal double-Q magnetic phase where both charge and SDW order exist; and below the SC transition at $T_c\simeq$10 K. The free-carrier component in the optical conductivity is described by two Drude contributions; one strong and broad, the other weak and narrow. The broad Drude component decreases dramatically below $T_{\rm N}$ and $T_r$, with much of its strength being transferred to a bound excitation in the mid-infrared, while the narrow Drude component shows no anomalies at either of the transitions, actually increasing in strength at low temperature while narrowing dramatically. The behavior of an infrared-active mode suggests zone-folding below $T_r$. Below $T_c$ the dramatic decrease in the low-frequency optical conductivity signals the formation of a SC energy gap. ARPES reveals hole-like bands at the center of the Brillouin zone (BZ), with both electron- and hole-like bands at the corners. Below $T_{\rm N}$, the hole pockets at the center of the BZ decrease in size, consistent with the behavior of the broad Drude component; while below $T_r$ the electron-like bands shift and split, giving rise to a low-energy excitation in the optical conductivity at ~20 meV. The magnetic states, with resulting SDW and charge-SDW order, respectively, lead to a significant reconstruction of the Fermi surface that has profound implications for the transport originating from the electron and hole pockets, but appears to have relatively little impact on the SC in this material.
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Submitted 30 December, 2019; v1 submitted 7 October, 2019;
originally announced October 2019.
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Time Reversal Symmetry Breaking in the Fe-Chalcogenide Superconductors
Authors:
N. Zaki,
G. Gu,
A. M. Tsvelik,
C. Wu,
P. D. Johnson
Abstract:
Topological superconductivity has been sought for in a variety of heterostructure systems, the interest being that a material displaying such a phenomenon could prove to be the ideal platform to support Majorana fermions, which in turn could be the basis for advanced qubit technologies. Recently the high Tc family of superconductors, $FeSe_{x}Te_{1-x}$, have been shown to exhibit the property of t…
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Topological superconductivity has been sought for in a variety of heterostructure systems, the interest being that a material displaying such a phenomenon could prove to be the ideal platform to support Majorana fermions, which in turn could be the basis for advanced qubit technologies. Recently the high Tc family of superconductors, $FeSe_{x}Te_{1-x}$, have been shown to exhibit the property of topological superconductivity and further, evidence has been found for the presence of Majorana fermions. We have studied the interplay of topology, magnetism and superconductivity in the $FeSe_{x}Te_{1-x}$ family using high-resolution laser-based photoemission. At the bulk superconducting transition, a gap opens at the chemical potential as expected. However, a second gap is observed to open at the Dirac point in the topological surface state. The associated mass acquisition in the topological state points to time-reversal symmetry breaking, probably associated with the formation of ferromagnetism in the surface layer. The presence of intrinsic ferromagnetism combined with strong spin-orbit coupling provides an ideal platform for a range of exotic topological phenomena.
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Submitted 6 November, 2020; v1 submitted 26 July, 2019;
originally announced July 2019.
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On the interplay of paramagnetism and topology in the Fe-based High Tc Superconductors
Authors:
J. D. Rameau,
N. Zaki,
G. D. Gu,
P. D. Johnson,
M. Weinert
Abstract:
The high Tc superconductor FeTe0.55Se0.45 has recently been shown to support a surface state with topological character. Here we use low-energy laser-based ARPES with variable light polarization, including both linear and circular polarization, to re-examine the same material and the related FeTe0.7Se0.3, with larger Te concentration. In both cases we observe the presence of a surface state displa…
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The high Tc superconductor FeTe0.55Se0.45 has recently been shown to support a surface state with topological character. Here we use low-energy laser-based ARPES with variable light polarization, including both linear and circular polarization, to re-examine the same material and the related FeTe0.7Se0.3, with larger Te concentration. In both cases we observe the presence of a surface state displaying linear dispersion in a cone-like configuration. The use of circular polarization confirms the presence of helical spin structure. These experimental studies are compared with theoretical studies that account for the local magnetic effects related to the paramagnetism observed in this system in the normal state. In contrast to previous studies we find that including the magnetic contributions is necessary to bring the chemical potential of the calculated electronic band structure naturally into alignment with the experimental observations.
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Submitted 25 March, 2019;
originally announced March 2019.
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Temperature-Driven Topological Transition in 1T'-MoTe2
Authors:
Ayelet Notis Berger,
Erick Andrade,
Alex Kerelsky,
Drew Edelberg,
Jian Li,
Zhijun Wang,
Lunyong Zhang,
Jaewook Kim,
Nader Zaki,
Jose Avila,
Chaoyu Chen,
Maria C Asensio,
Sang-Wook Cheong,
Bogdan A. Bernevig,
Abhay N. Pasupathy
Abstract:
The topology of Weyl semimetals requires the existence of unique surface states. Surface states have been visualized in spectroscopy measurements, but their connection to the topological character of the material remains largely unexplored. 1T'-MoTe2, presents a unique opportunity to study this connection. This material undergoes a phase transition at 240K that changes the structure from orthorhom…
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The topology of Weyl semimetals requires the existence of unique surface states. Surface states have been visualized in spectroscopy measurements, but their connection to the topological character of the material remains largely unexplored. 1T'-MoTe2, presents a unique opportunity to study this connection. This material undergoes a phase transition at 240K that changes the structure from orthorhombic (putative Weyl semimetal) to monoclinic (trivial metal), while largely maintaining its bulk electronic structure. Here we show from temperature-dependent quasiparticle interference measurements that this structural transition also acts as a topological switch for surface states in 1T'-MoTe2. At low temperature, we observe strong quasiparticle scattering, consistent with theoretical predictions and photoemission measurements for the surface states in this material. In contrast, measurements performed at room temperature show the complete absence of the scattering wavevectors associated with the trivial surface states. These distinct quasiparticle scattering behaviors show that 1T'-MoTe2 is ideal for separating topological and trivial electronic phenomena via temperature dependent measurements.
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Submitted 18 December, 2017;
originally announced December 2017.
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The cuprate phase diagram and the influence of nanoscale inhomogeneities
Authors:
Nader Zaki,
Hongbo Yang,
Jon Rameau,
Helmut Claus,
David G. Hinks,
Peter D. Johnson
Abstract:
The phase diagram associated with the high Tc superconductors is complicated by an array of different ground states. The parent material represents an antiferromagnetic insulator but with doping superconductivity becomes possible with transition temperatures previously thought unattainable. The underdoped region of the phase diagram is dominated by the so-called pseudogap phenomena whereby in the…
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The phase diagram associated with the high Tc superconductors is complicated by an array of different ground states. The parent material represents an antiferromagnetic insulator but with doping superconductivity becomes possible with transition temperatures previously thought unattainable. The underdoped region of the phase diagram is dominated by the so-called pseudogap phenomena whereby in the normal state the system mimics superconductivity in its spectra response but does not show the complete loss of resistivity associated with the superconducting state. An understanding of this regime presents one of the great challenges for the field. In the present study we revisit the structure of the phase diagram as determined in photoemission studies. By careful analysis of the role of nanoscale inhomogeneities in the overdoped region we are able to more carefully separate out the gaps due to the pseudogap phenomena from the gaps due to the superconducting transition. Within a mean field description we are thus able to link the magnitude of the gap directly to the Heisenberg exchange interaction term, $J\sum{s_i \cdot s_j}$, contained in the $t-J$ model. This approach provides a clear indication that the pseudogap is that associated with spin singlet formation.
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Submitted 17 May, 2018; v1 submitted 4 August, 2017;
originally announced August 2017.
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Tuning the Electronic Structure of Monolayer Graphene/MoS2 van der Waals Heterostructures via Interlayer Twist
Authors:
Wencan Jin,
Po-Chun Yeh,
Nader Zaki,
Daniel Chenet,
Ghidewon Arefe,
Yufeng Hao,
Alessandro Sala,
Tevfik Onur Mentes,
Jerry I. Dadap,
Andrea Locatelli,
James Hone,
Richard M. Osgood Jr
Abstract:
We directly measure the electronic structure of twisted graphene/MoS2 van der Waals heterostructures, in which both graphene and MoS2 are monolayers. We use cathode lens microscopy and microprobe angle-resolved photoemission spectroscopy measurements to image the surface, determine twist angle, and map the electronic structure of these artificial heterostructures. For monolayer graphene on monolay…
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We directly measure the electronic structure of twisted graphene/MoS2 van der Waals heterostructures, in which both graphene and MoS2 are monolayers. We use cathode lens microscopy and microprobe angle-resolved photoemission spectroscopy measurements to image the surface, determine twist angle, and map the electronic structure of these artificial heterostructures. For monolayer graphene on monolayer MoS2, the resulting band structure reveals the absence of hybridization between the graphene and MoS2 electronic states. Further, the graphene-derived electronic structure in the heterostructures remains intact, irrespective of the twist angle between the two materials. In contrast, however, the electronic structure associated with the MoS2 layer is found to be twist-angle dependent; in particular, the relative difference in the energy of the valence band maximum at Γ and K of the MoS2 layer varies from approximately 0 to 0.2 eV. Our results suggest that monolayer MoS2 within the heterostructure becomes predominantly an indirect bandgap system for all twist angles except in the proximity of 30 degrees. This result enables potential bandgap engineering in van der Waals heterostructures comprised of monolayer structures.
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Submitted 16 November, 2015; v1 submitted 2 October, 2015;
originally announced October 2015.
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Quasiparticle Interference, quasiparticle interactions and the origin of the charge density-wave in 2H-NbSe$_{2}$
Authors:
C. J. Arguello,
E. P. Rosenthal,
E. F. Andrade,
W. Jin,
P. C. Yeh,
N. Zaki,
S. Jia,
R. J. Cava,
R. M. Fernandes,
A. J. Millis,
T. Valla,
R. M. Osgood Jr.,
A. N. Pasupathy
Abstract:
We show that a small number of intentionally introduced defects can be used as a spectroscopic tool to amplify quasiparticle interference in 2H-NbSe$_{2}$, that we measure by scanning tunneling spectroscopic imaging. We show from the momentum and energy dependence of the quasiparticle interference that Fermi surface nesting is inconsequential to charge density wave formation in 2H-NbSe$_{2}$. We d…
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We show that a small number of intentionally introduced defects can be used as a spectroscopic tool to amplify quasiparticle interference in 2H-NbSe$_{2}$, that we measure by scanning tunneling spectroscopic imaging. We show from the momentum and energy dependence of the quasiparticle interference that Fermi surface nesting is inconsequential to charge density wave formation in 2H-NbSe$_{2}$. We demonstrate that by combining quasiparticle interference data with additional knowledge of the quasiparticle band structure from angle resolved photoemission measurements, one can extract the wavevector and energy dependence of the important electronic scattering processes thereby obtaining direct information both about the fermiology and the interactions. In 2H-NbSe$_{2}$, we use this combination to show that the important near-Fermi-surface electronic physics is dominated by the coupling of the quasiparticles to soft mode phonons at a wave vector different from the CDW ordering wave vector.
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Submitted 22 August, 2014; v1 submitted 19 August, 2014;
originally announced August 2014.
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The Nature of Surface States on Vicinal Cu (775): An STM and Photoemission Study
Authors:
Nader Zaki,
Kevin Knox,
Richard M. Osgood,
Peter D. Johnson,
Jun Fujii,
Ivana Vobornik,
Giancarlo Panaccione
Abstract:
We report ARPES and a set of in situ STM measurements on a narrow-terrace-width vicinal Cu(111) crystal surface, Cu(775), whose vicinal cut lies close to the transition between terrace and step modulation. These measurements show sharp zone-folding (or Umklapp) features with a periodicity in k||, indicating that the predominant reference plane is that of Cu(775), i.e. that the surface is predomina…
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We report ARPES and a set of in situ STM measurements on a narrow-terrace-width vicinal Cu(111) crystal surface, Cu(775), whose vicinal cut lies close to the transition between terrace and step modulation. These measurements show sharp zone-folding (or Umklapp) features with a periodicity in k||, indicating that the predominant reference plane is that of Cu(775), i.e. that the surface is predominately step-modulated. Our measurements also show variation in Umklapp intensity with photon energy, which is consistent with prior ARPES experiments on other vicinal Cu(111) surfaces and in agreement with our designation of the state as being step modulated. The measurements also show a weak terrace-modulated state, which, based on several characteristics, we attribute to the presence of terrace widths larger than the ideal terrace width. By measuring the intensity ratio of the two distinct surface-state modulations from PE and the terrace-width distribution from STM, we derive a value for the terrace width, at which the surface-state switches between the two modulations.
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Submitted 22 May, 2014;
originally announced May 2014.
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Atom-Wide Co Wires on Cu(775) at Room Temperature
Authors:
Nader Zaki,
Denis Potapenko,
Peter D. Johnson,
Richard M. Osgood
Abstract:
We report on a new surface phase of the Co-vicinal-Cu(111) system which exhibits self-assembled uniform Co quantum wires that are stable at 300K. STM-imaging measurements show that wires will self-assemble within a narrow range of Co coverage and, within this range, the wires increase in length as coverage is increased. The STM images show that the wires form along the leading edge of the step ris…
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We report on a new surface phase of the Co-vicinal-Cu(111) system which exhibits self-assembled uniform Co quantum wires that are stable at 300K. STM-imaging measurements show that wires will self-assemble within a narrow range of Co coverage and, within this range, the wires increase in length as coverage is increased. The STM images show that the wires form along the leading edge of the step rise, differentiating it from previously theoretically predicted atomic-wire phases. The formation of relatively long laterally un-encapsulated one- and two-atom wires also differentiates it from past experimentally observed step-island formation. Furthermore, our experiments also show directly that the Co wires coexist with another Co phase that had been previously predicted for growth on Cu(111). Our observations allow us to comment on the formation kinetics of the atomic-wire phase and on the fit of our data to a recently developed lattice-gas model.
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Submitted 22 May, 2014;
originally announced May 2014.
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The failure of DFT-based computations for a stepped-substrate-supported correlated Co wire
Authors:
Nader Zaki,
Hyowon Park,
Richard M. Osgood,
Andrew J. Millis,
Chris A. Marianetti
Abstract:
Density functional theory (DFT) has been immensely successful in its ability to predict physical properties, and, in particular, structures of condensed matter systems. Here, however, we show that DFT qualitatively fails to predict the dimerized structural phase for a monatomic Co wire that is self-assembled on a vicinal, i.e. stepped, Cu(111) substrate. To elucidate the nature of this failure, we…
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Density functional theory (DFT) has been immensely successful in its ability to predict physical properties, and, in particular, structures of condensed matter systems. Here, however, we show that DFT qualitatively fails to predict the dimerized structural phase for a monatomic Co wire that is self-assembled on a vicinal, i.e. stepped, Cu(111) substrate. To elucidate the nature of this failure, we compute the energetics of a Co chain on a Cu surface, step, notch, and embedded in bulk. The results demonstrate that increasing Co coordination extinguishes the dimerization, indicating that the failure of DFT for Co on the Cu step arises from excessive hybridization, which both weakens the ferromagnetic correlations that drive the dimerization and increases the bonding that opposes dimerization. Additionally, we show that including local interactions via DFT+U or DFT+DMFT does not restore the dimerization for the step-substrate supported wire, though the Co wire does dimerize in DFT+DMFT for the isolated vacuum case. This system can serve as a benchmark for future electronic structure methods.
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Submitted 5 December, 2013;
originally announced December 2013.
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Spin-exchange-induced dimerization of an atomic 1-D system
Authors:
Nader Zaki,
Chris A. Marianetti,
Danda P. Acharya,
Percy Zahl,
Peter Sutter,
Junichi Okamoto,
Peter D. Johnson,
Andrew J. Millis,
Richard M. Osgood
Abstract:
Using low-temperature scanning tunneling microscopy, we demonstrate an unambiguous 1-D system that surprisingly undergoes a CDW instability on a metallic substrate. Our ability to directly and quantitatively measure the structural distortion of this system provides an accurate reference for comparison with first principles theory. In comparison to previously proposed physical mechanisms, we attrib…
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Using low-temperature scanning tunneling microscopy, we demonstrate an unambiguous 1-D system that surprisingly undergoes a CDW instability on a metallic substrate. Our ability to directly and quantitatively measure the structural distortion of this system provides an accurate reference for comparison with first principles theory. In comparison to previously proposed physical mechanisms, we attribute this particular 1-D CDW instability to a ferromagnetic state. We show that though the linear arrayed dimers are not electronically isolated, they are magnetically independent, and hence can potentially serve as a binary spin-memory system.
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Submitted 2 August, 2012;
originally announced August 2012.
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Trapping Surface Electrons on Graphene Layers and Islands
Authors:
D. Niesner,
Th. Fauster,
J. I. Dadap,
N. Zaki,
K. R. Knox,
P. -C. Yeh,
R. Bhandari,
R. M. Osgood,
M. Petrović,
M. Kralj
Abstract:
We report the use of time- and angle-resolved two-photon photoemission to map the bound, unoccupied electronic structure of the weakly coupled graphene/Ir(111) system. The energy, dispersion, and lifetime of the lowest three image-potential states are measured. In addition, the weak interaction between Ir and graphene permits observation of resonant transitions from an unquenched Shockley-type sur…
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We report the use of time- and angle-resolved two-photon photoemission to map the bound, unoccupied electronic structure of the weakly coupled graphene/Ir(111) system. The energy, dispersion, and lifetime of the lowest three image-potential states are measured. In addition, the weak interaction between Ir and graphene permits observation of resonant transitions from an unquenched Shockley-type surface state of the Ir substrate to graphene/Ir image-potential states. The image-potential-state lifetimes are comparable to those of mid-gap clean metal surfaces. Evidence of localization of the excited electrons on single-atom-layer graphene islands is provided by coverage-dependent measurements.
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Submitted 29 December, 2011; v1 submitted 9 September, 2011;
originally announced September 2011.