Showing 1–47 of 47 results for author: Levy, G

Isolating Exciton Dissociation Pathways in ReSe$_{\text{2}}$

Authors: Bradley G. Guislain, Rysa Greenwood, Matteo Michiardi, Giorgio Levy, Sergey Zhdanovich, Jerry Icban Dadap, Sydney K. Y. Dufresne, Arthur K. Mills, Dario Armanno, Shawn Lapointe, Francesco Goto, Nicolas Gauthier, Fabio Boschini, Andrea Damascelli, Ziliang Ye, David J. Jones

Abstract: Strongly bound excitons dominate the optical response in many van der Waals semiconductors, yet distinguishing between the different microscopic processes governing exciton dissociation remains challenging. Using time- and angle-resolved photoemission spectroscopy (TR-ARPES), we independently track exciton and band-edge carrier populations in bulk ReSe$_{\text{2}}$ under resonant excitation. By st… ▽ More Strongly bound excitons dominate the optical response in many van der Waals semiconductors, yet distinguishing between the different microscopic processes governing exciton dissociation remains challenging. Using time- and angle-resolved photoemission spectroscopy (TR-ARPES), we independently track exciton and band-edge carrier populations in bulk ReSe$_{\text{2}}$ under resonant excitation. By studying the fluence dependence and polarization-controlled exciton density dependence of the exciton dissociation process, we distinguish between competing processes and identify exciton photoionization as the microscopic dissociation mechanism. These results establish a population-resolved strategy for resolving exciton-to-carrier conversion pathways in strongly excitonic materials. △ Less

Submitted 13 April, 2026; originally announced April 2026.

Direct Fabrication of a Superconducting Two-Dimensional Electron Gas on KTaO3(111) via Mg-Induced Surface Reduction

Authors: Chun Sum Brian Pang, Bruce A. Davidson, Fengmiao Li, Mohamed Oudah, Peter C. Moen, Steef Smit, Cissy T. Suen, Simon Godin, Sergey A. Gorovikov, Marta Zonno, Pinder Dosanjh, Sergey Zhdanovich, Giorgio Levy, Matteo Michiardi, Alannah M. Hallas, George A. Sawatzky, Robert J. Green, Andrea Damascelli, Ke Zou

Abstract: Two-dimensional electron gases (2DEGs) at the surfaces of KTaO3 have become an exciting platform for exploring strong spin-orbit coupling, Rashba physics, and low-carrier-density superconductivity. Yet, a large fraction of reported KTaO3-based 2DEGs has been realized through chemically complex overlayers that both generate carriers and can obscure the native electronic structure, making spectrosco… ▽ More Two-dimensional electron gases (2DEGs) at the surfaces of KTaO3 have become an exciting platform for exploring strong spin-orbit coupling, Rashba physics, and low-carrier-density superconductivity. Yet, a large fraction of reported KTaO3-based 2DEGs has been realized through chemically complex overlayers that both generate carriers and can obscure the native electronic structure, making spectroscopic access to the underlying 2DEG challenging. Here, we demonstrate a simple and direct method to generate a superconducting 2DEG on KTaO3(111) using Mg-induced surface reduction in molecular-beam epitaxy (MBE). Mg has an extremely low sticking coefficient at elevated temperatures, enabling the formation of an ultrathin (less than 1-2 monolayers) MgO layer that is transparent to soft x-ray photoemission spectroscopy (XPS) and angle-resolved photoemission spectroscopy (ARPES). This allows direct measurement of the surface chemistry and low-energy electronic structure of the pristine reduced surface without the need for a several-nanometer-thick capping layer. XPS shows clear reduction of Ta5+ to lower oxidation states, while ARPES reveals a parabolic Ta 5d conduction band with an approximately 150 meV bandwidth and additional subband features arising from quantum confinement. Transport measurements confirm a superconducting transition below 0.7 K. Together, these results demonstrate a chemically straightforward and controllable pathway for fabricating spectroscopically accessible superconducting 2DEGs on KTaO3(111), and provide a powerful new platform for investigating the mechanisms underlying orientation-dependent superconductivity in KTaO3-based oxide interfaces. △ Less

Submitted 20 April, 2026; v1 submitted 21 December, 2025; originally announced December 2025.

Comments: 6 pages, 4 figures

Magneto-Optical Study of Chiral Magnetic Modes in NiI$_{2}$: Direct Evidence for Kitaev Interactions

Authors: Kartik Panda, Chaebin Kim, Daniel Bazyliansky, Javier Taboada-Gutiérrez, Florian Le Mardelé, Jan Dzian, Guy Levy, Jae Ha Kim, Youjin Lee, Bumchan Park, Martin Mourigal, Jae Hoon Kim, Alexey B. Kuzmenko, Milan Orlita, Je-Geun Park, Nimrod Bachar

Abstract: Bond-dependent magnetic interactions, particularly those described by the Kitaev model, have emerged as a key pathway toward realizing unconventional magnetic states such as quantum spin liquids and topologically nontrivial excitations, including skyrmions. These interactions frustrate conventional magnetic order and give rise to rich collective behavior that continues to challenge both theory and… ▽ More Bond-dependent magnetic interactions, particularly those described by the Kitaev model, have emerged as a key pathway toward realizing unconventional magnetic states such as quantum spin liquids and topologically nontrivial excitations, including skyrmions. These interactions frustrate conventional magnetic order and give rise to rich collective behavior that continues to challenge both theory and experiment. While Kitaev physics has been extensively explored in the context of honeycomb magnets, direct evidence for its role in real materials remains scarce. Magnetic van der Waals (vdW) materials have emerged as a versatile platform for exploring low-dimensional electrical, magnetic, and correlated electronic phenomena, and provide a fertile ground for potential applications ranging from spintronics to multiferroic devices and quantum information technologies. Here, we demonstrate, through magneto-transmission, Faraday angle rotation, and magnetic circular dichroism measurements, that the magnetic excitation spectrum of NiI$_2$, a van der Waals multiferroic material, is more accurately captured by a Kitaev-based spin model than by the previously invoked helical spin framework. △ Less

Submitted 8 November, 2025; originally announced November 2025.

Quenching of excitons at grain boundaries in C60 thin films

Authors: Rysa Greenwood, Bradley G. Guislain, MengXing Na, Alexandra B. Tully, Sergey Zhdanovich, Jerry Icban Dadap, Sydney K. Y. Dufresne, Vanessa King, Jiabin Yu, Giorgio Levy, Arthur K. Mills, Matteo Michiardi, Andrea Damascelli, Sarah A. Burke, David J. Jones

Abstract: Exciton lifetimes play a critical role in the performance of organic optoelectronic devices. In this work, we investigate how the presence of multiple rotational domains, and therefore grain boundaries, impacts exciton dynamics in thin films of C60/Au(111) using time and angle-resolved photoemission spectroscopy (TR-ARPES). We find that films with multiple rotational domains exhibit shorter excito… ▽ More Exciton lifetimes play a critical role in the performance of organic optoelectronic devices. In this work, we investigate how the presence of multiple rotational domains, and therefore grain boundaries, impacts exciton dynamics in thin films of C60/Au(111) using time and angle-resolved photoemission spectroscopy (TR-ARPES). We find that films with multiple rotational domains exhibit shorter exciton lifetimes and evidence of exciton-exciton annihilation, even when one domain predominates. Scanning tunneling microscopy (STM) measurements reveal electronic structure changes resulting from a locally reduced dielectric constant at grain boundaries, providing a mechanism for lifetime reduction through exciton funneling and other additional decay channels. These findings highlight the critical role of film quality in determining intrinsic exciton lifetimes, and show that minuscule amounts of disorder that are nearly undetectable by ensemble measurements can significantly impact dynamics. These results imply that precise structural control is essential for optimize the performance of organic optoelectronic devices. △ Less

Submitted 30 June, 2025; originally announced July 2025.

Comments: 19 pages, 3 figures

Enhanced coherence and layer-selective charge order in a trilayer cuprate superconductor

Authors: S. Smit, M. Bluschke, P. Moen, N. Heinsdorf, E. Zavatti, G. Bellomia, S. Giuli, S. K. Y. Dufresne, C. T. Suen, V. Zimmermann, C. Au-Yeung, S. Zhdanovich, J. I. Dadap, M. Zonno, S. Gorovikov, H. Lee, C-T. Kuo, J-S. Lee, D. Song, S. Ishida, H. Eisaki, B. Keimer, M. Michiardi, I. S. Elfimov, G. Levy , et al. (3 additional authors not shown)

Abstract: Trilayer cuprates hold the record for the highest superconducting critical temperatures ($T_{\text{c}}$), yet the underlying mechanism remains elusive. Using time- and angle-resolved photoemission spectroscopy (tr-ARPES), we uncover a striking interplay between charge order, superconducting gap magnitude, and quasiparticle coherence in Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$ (Bi2223). This constitutes… ▽ More Trilayer cuprates hold the record for the highest superconducting critical temperatures ($T_{\text{c}}$), yet the underlying mechanism remains elusive. Using time- and angle-resolved photoemission spectroscopy (tr-ARPES), we uncover a striking interplay between charge order, superconducting gap magnitude, and quasiparticle coherence in Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$ (Bi2223). This constitutes ARPES-based evidence of charge order on the inner CuO$_2$ plane, as confirmed via resonant x-ray scattering (RXS); in addition, the same inner plane hosts a superconducting gap significantly larger than that of the overdoped outer planes, firmly establishing it as underdoped. Unexpectedly, despite its underdoped nature, the inner plane also exhibits an exceptional degree of quasiparticle coherence; suppressing charge-order fluctuations further enhances this, making it comparable to that of the overdoped outer planes at elevated electronic temperatures. These findings, supported by complementary three-layer single-band Hubbard calculations, reveal a unique interlayer mechanism in which both pairing strength and phase coherence are optimized when interfacing planes with distinct hole concentrations, providing new microscopic insight into the record $T_{\text{c}}$ of Bi2223. △ Less

Submitted 2 June, 2025; originally announced June 2025.

Selective electron-phonon coupling strength from nonequilibrium optical spectroscopy: The case of MgB$_2$

Authors: S. Mor, F. Boschini, E. Razzoli, M. Zonno, M. Michiardi, G. Levy, N. D. Zhigadlo, P. C. Canfield, G. Cerullo, A. Damascelli, C. Giannetti, S. Dal Conte

Abstract: The coupling between quasiparticles and bosonic excitations rules the energy transfer pathways in condensed matter systems. The possibility of inferring the strength of specific coupling channels from their characteristic time scales measured in nonequilibrium experiments is still an open question. Here, we investigate MgB$_2$, in which conventional superconductivity at temperatures as high as 39… ▽ More The coupling between quasiparticles and bosonic excitations rules the energy transfer pathways in condensed matter systems. The possibility of inferring the strength of specific coupling channels from their characteristic time scales measured in nonequilibrium experiments is still an open question. Here, we investigate MgB$_2$, in which conventional superconductivity at temperatures as high as 39 K is mediated by the strong coupling between the conduction electrons and the E$_{2g}$ phonon mode. By means of broadband time-resolved optical spectroscopy, we show that this selective electron-phonon coupling dictates the nonequilibrium optical response of MgB$_2$, at early times ($<$100 fs) after photoexcitation. Furthermore, based on an effective temperature model analysis, we estimate its contribution to the total electron-boson coupling function extracted from complementary equilibrium spectroscopy approaches, namely optical reflectivity and ARPES. The coupling strength with the E$_{2g}$ phonon modes is thus estimated to be $λ\simeq$ 0.56, which is approximately half of the total coupling constant, in agreement with ab-initio calculations from the literature. As a benchmark, broadband time-resolved optical spectroscopy is performed also on the isostructural and non-superconducting compound AlB$_2$, showing that the nonequilibrium optical response relaxes on a slower time scale due to the lack of strongly-coupled phonon modes. Our findings demonstrate the possibility to resolve and quantify selective electron-phonon coupling from nonequilibrium optical spectroscopy. △ Less

Submitted 4 February, 2026; v1 submitted 4 March, 2025; originally announced March 2025.

Universal electronic structure of multi-layered nickelates via oxygen-centered planar orbitals

Authors: Christine C. Au-Yeung, X. Chen, S. Smit, M. Bluschke, V. Zimmermann, M. Michiardi, P. C. Moen, J. Kraan, C. S. B. Pang, C. T. Suen, S. Zhdanovich, M. Zonno, S. Gorovikov, Y. Liu, G. Levy, I. S. Elfimov, M. Berciu, G. A. Sawatzky, J. F. Mitchell, A. Damascelli

Abstract: Superconductivity has been demonstrated in the family of multi-layered nickelates La$_3$Ni$_2$O$_7$ and La$_4$Ni$_3$O$_{10}$. Key questions remain open regarding the low-energy electronic states that support superconductivity in these compounds. Here we take advantage of the natural polymorphism between bilayer (2222) and alternating monolayer-trilayer (1313) stacking sequences that arises in bulk… ▽ More Superconductivity has been demonstrated in the family of multi-layered nickelates La$_3$Ni$_2$O$_7$ and La$_4$Ni$_3$O$_{10}$. Key questions remain open regarding the low-energy electronic states that support superconductivity in these compounds. Here we take advantage of the natural polymorphism between bilayer (2222) and alternating monolayer-trilayer (1313) stacking sequences that arises in bulk La$_3$Ni$_2$O$_7$ crystals, and by employing angle-resolved photoemission spectroscopy (ARPES) we identify a universal low-energy electronic structure in this family of materials. We observe the fingerprint of a doping-dependent spin-density wave (SDW) instability -- strong and coherent enough to reconstruct the Fermi surface, both by gapping out regions of the low-energy electronic structure as well as translating the $β$ pocket by a vector $Q_{tβ}$ consistent with the results of previous neutron and x-ray scattering experiments. Using an effective tight-binding model, we simulate the spectral weight distribution observed in our ARPES dichroism experiments and establish that the low-energy electronic phenomenology is dominated by oxygen-centered planar orbitals, which evolve from the $d_{3x^2-r^2}$ and $d_{3y^2-r^2}$ symmetry characteristic of 3-spin polarons (3SP) to the familiar $d_{x^2-y^2}$ Zhang-Rice singlets (ZRS) that support high-temperature superconductivity in cuprates. By inclusion of magnetic moments on plaquettes of oxygen orbitals in our model, we show that ZRS-like states mediate the SDW. Combined with the observation that oxygen annealing is required to induce superconductivity in both thin films and bulk La$_3$Ni$_2$O$_7$, this demonstrates that the ZRS population dictates whether the ground state favors density-wave order or superconductivity -- with hole doping suppressing the former and stabilizing the latter, as in the cuprates. △ Less

Submitted 29 September, 2025; v1 submitted 27 February, 2025; originally announced February 2025.

Comments: 15 pages, 4 figures

Journal ref: Nat. Phys. (2026)

Kink in cuprates: the role of the low-energy density of states

Authors: E. Razzoli, F. Boschini, M. Zonno, M. X. Na, M. Michiardi, M. Schneider, E. H. da Silva Neto, S. Gorovikov, R. D. Zhong, J. Schneeloch, G. D. Gu, S. Zhdanovich, A. K. Mills, G. Levy, D. J. Jones, C. Giannetti, A. Damascelli

Abstract: The 40-70 meV band-structure renormalization (so-called kink) in high-temperature cuprate superconductors - which has been mainly interpreted in terms of electron-boson coupling - is observed to be strongly suppressed both above the superconducting transition temperature and under optical excitation. We employ equilibrium and time- and angle-resolved photoemission spectroscopy, in combination with… ▽ More The 40-70 meV band-structure renormalization (so-called kink) in high-temperature cuprate superconductors - which has been mainly interpreted in terms of electron-boson coupling - is observed to be strongly suppressed both above the superconducting transition temperature and under optical excitation. We employ equilibrium and time- and angle-resolved photoemission spectroscopy, in combination with Migdal-Eliashberg simulations, to investigate the suppression of the near-nodal kink in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. We show that the $\sim$30$\%$ decrease of the kink strength across the superconducting-to-normal-state phase transition can be entirely accounted for by the filling of the superconducting gap, without additional consideration of temperature-dependent electron-boson coupling. Our findings demonstrate that consideration of changes in the density of states is essential to quantitatively account for the band structure renormalization effects in cuprates. △ Less

Submitted 8 December, 2023; originally announced December 2023.

Critical Role of Disorder for Superconductivity in the Series of Epitaxial Ti(O,N) Films

Authors: Fengmiao Li, Oliver Dicks, Myung-Geun Han, Solveig Aamlid, Giorgio Levy, Ronny Sutarto, Chong Liu, Hsiang-Hsi Kung, Oleksandr Foyevstov, Simon Godin, Bruce A. Davidson, Andrea Damascelli, Yimei Zhu, Christoph Heil, Ilya Elfimov, George A. Sawatzky, Ke Zou

Abstract: Realizing experimental control of superconductivity is of paramount importance to advancing both basic research and technological applications. Disorder, generally existing in most superconductors, intricately interacts with Cooper pairs and also impacts the performance of quantum devices. In this paper, we report the study of a series of Ti(O,N) crystalline films prepared via molecular beam epita… ▽ More Realizing experimental control of superconductivity is of paramount importance to advancing both basic research and technological applications. Disorder, generally existing in most superconductors, intricately interacts with Cooper pairs and also impacts the performance of quantum devices. In this paper, we report the study of a series of Ti(O,N) crystalline films prepared via molecular beam epitaxy (MBE). We discover that substituting nitrogen (N) for oxygen (O) in TiO, namely TiO(N), considerably increases the normal-state conductivity and the superconducting transition temperature Tc. The Tc of TiO(N) falling between those of TiO (about 0.5 K) and TiN (about 6 K) is contrary to their comparable Tc predicted by the Migdal Eliasberg theory. It is found that their resistivity vs temperature obeys the Mooij rule, known as the characteristic of metallic glasses. Density functional theory (DFT) calculations demonstrate that strong disorder severely scatters the Bloch electron waves at nonzero momenta, which consequently weakens electron-phonon coupling in TiO(N). △ Less

Submitted 24 November, 2024; v1 submitted 4 October, 2023; originally announced October 2023.

Mixed-valence state in the dilute-impurity regime of La-substituted SmB$_6$

Authors: Marta Zonno, Matteo Michiardi, Fabio Boschini, Giorgio Levy, Klara Volkaert, Davide Curcio, Marco Bianchi, Priscila F. S. Rosa, Zachary Fisk, Philip Hofmann, Ilya S. Elfimov, Robert J. Green, George A. Sawatzky, Andrea Damascelli

Abstract: Homogeneous mixed-valence (MV) behaviour is one of the most intriguing phenomena of $f$-electron systems. Despite extensive efforts, a fundamental aspect which remains unsettled is the determination of the limiting cases for which MV emerges. Here we address this question for SmB$_6$, a prototypical MV system characterized by two nearly-degenerate Sm$^{2+}$ and Sm$^{3+}$ configurations. By combini… ▽ More Homogeneous mixed-valence (MV) behaviour is one of the most intriguing phenomena of $f$-electron systems. Despite extensive efforts, a fundamental aspect which remains unsettled is the determination of the limiting cases for which MV emerges. Here we address this question for SmB$_6$, a prototypical MV system characterized by two nearly-degenerate Sm$^{2+}$ and Sm$^{3+}$ configurations. By combining angle resolved photoemission spectroscopy (ARPES) and x-ray absorption spectroscopy (XAS), we track the evolution of the mean Sm valence, $v_{Sm}$, in the Sm$_x$La$_{1-x}$B$_6$ series. Upon substitution of Sm ions with trivalent La, we observe a linear decrease of valence fluctuations to an almost complete suppression at $x$$\,$=$\,$0.2, with $v_{Sm}$$\,$$\sim$$\,$2; surprisingly, by further reducing $x$, a re-entrant increase of $v_{Sm}$ develops, approaching the value of $v_{imp}$$\,$$\sim$$\,$2.35 in the dilute-impurity limit. Such observation departs from a monotonic evolution of $v_{Sm}$ across the whole series, as well as from the expectation of its convergence to an integer value for $x$$\,$$\rightarrow$$\,$0. Our ARPES and XAS results, complemented by a phenomenological model, demonstrate an unconventional evolution of the MV character in the Sm$_x$La$_{1-x}$B$_6$ series, paving the way to further theoretical and experimental considerations on the concept of MV itself, and its influence on the macroscopic properties of rare-earth compounds in the dilute-to-intermediate impurity regime. △ Less

Submitted 3 September, 2024; v1 submitted 11 September, 2023; originally announced September 2023.

Comments: 4 figures

Journal ref: Nature Communications 15, 7621 (2024)

A versatile laser-based apparatus for time-resolved ARPES with micro-scale spatial resolution

Authors: Sydney K. Y. Dufresne, Sergey Zhdanovich, Matteo Michiardi, Bradley G. Guislain, Marta Zonno, Sean Kung, Giorgio Levy, Arthur K. Mills, Fabio Boschini, David J. Jones, Andrea Damascelli

Abstract: We present the development of a versatile apparatus for a 6.2 eV laser-based time and angle-resolved photoemission spectroscopy with micrometer spatial resolution (time-resolved $μ$-ARPES). With a combination of tunable spatial resolution down to $\sim$11 $μ$m, high energy resolution ($\sim$11 meV), near-transform-limited temporal resolution ($\sim$280 fs), and tunable 1.55 eV pump fluence up to… ▽ More We present the development of a versatile apparatus for a 6.2 eV laser-based time and angle-resolved photoemission spectroscopy with micrometer spatial resolution (time-resolved $μ$-ARPES). With a combination of tunable spatial resolution down to $\sim$11 $μ$m, high energy resolution ($\sim$11 meV), near-transform-limited temporal resolution ($\sim$280 fs), and tunable 1.55 eV pump fluence up to $\sim$3 mJ/cm$^2$, this time-resolved $μ$-ARPES system enables the measurement of ultrafast electron dynamics in exfoliated and inhomogeneous materials. We demonstrate the performance of our system by correlating the spectral broadening of the topological surface state of Bi$_2$Se$_3$ with the spatial dimension of the probe pulse, as well as resolving the spatial inhomogeneity contribution to the observed spectral broadening. Finally, after in-situ exfoliation, we performed time-resolved $μ$-ARPES on a $\sim$30 $μ$m few-layer-thick flake of transition metal dichalcogenide WTe$_2$, thus demonstrating the ability to access ultrafast electron dynamics with momentum resolution on micro-exfoliated and twisted materials. △ Less

Submitted 8 September, 2023; originally announced September 2023.

Triggering a global density wave instability in graphene via local symmetry-breaking

Authors: Amy C. Qu, Pascal Nigge, Stefan Link, Giorgio Levy, Matteo Michiardi, Parsa L. Spandar, Tiffany Matthé, Michael Schneider, Sergey Zhdanovich, Ulrich Starke, Christopher Gutiérrez, Andrea Damascelli

Abstract: Two-dimensional quantum materials offer a robust platform for investigating the emergence of symmetry-broken ordered phases owing to the high tuneability of their electronic properties. For instance, the ability to create new electronic band structures in graphene through moiré superlattices from stacked and twisted structures has led to the discovery of several correlated and topological phases.… ▽ More Two-dimensional quantum materials offer a robust platform for investigating the emergence of symmetry-broken ordered phases owing to the high tuneability of their electronic properties. For instance, the ability to create new electronic band structures in graphene through moiré superlattices from stacked and twisted structures has led to the discovery of several correlated and topological phases. Here we report an alternative method to induce an incipient symmetry-broken phase in graphene at the millimetre scale. We show that an extremely dilute concentration ($<\!0.3\% $) of surface adatoms can self-assemble and trigger the collapse of the graphene atomic lattice into a distinct Kekulé bond density wave phase, whereby the carbon C-C bond symmetry is broken globally. Using complementary momentum-resolved techniques such as angle-resolved photoemission spectroscopy (ARPES) and low-energy electron diffraction (LEED), we directly probe the presence of this density wave phase and confirm the opening of an energy gap at the Dirac point. We further show that this Kekulé density wave phase occurs for various Fermi surface sizes and shapes, suggesting that this lattice instability is driven by strong electron-lattice interactions. Our results demonstrate that dilute concentrations of self-assembled adsorbed atoms offer an attractive alternative route towards designing novel quantum phases in two-dimensional materials. △ Less

Submitted 23 April, 2022; originally announced April 2022.

Comments: 15 pages, 5 figures

Journal ref: Science Advances, 8, eabm5180 (2022)

Unveiling the underlying interactions in Ta2NiSe5 from photo-induced lifetime change

Authors: Denis Golez, Sydney K. Y. Dufresne, Min-Jae Kim, Fabio Boschini, Hao Chu, Yuta Murakami, Giorgio Levy, Arthur K. Mills, Sergey Zhdanovich, Masahiko Isobe, Hidenori Takagi, Stefan Kaiser, Philipp Werner, David J. Jones, Antoine Georges, Andrea Damascelli, Andrew J. Millis

Abstract: We present a generic procedure for quantifying the interplay of electronic and lattice degrees of freedom in photo-doped insulators through a comparative analysis of theoretical many-body simulations and time- and angle-resolved photoemission spectroscopy (TR-ARPES) of the transient response of the candidate excitonic insulator Ta2NiSe5. Our analysis demonstrates that the electron-electron interac… ▽ More We present a generic procedure for quantifying the interplay of electronic and lattice degrees of freedom in photo-doped insulators through a comparative analysis of theoretical many-body simulations and time- and angle-resolved photoemission spectroscopy (TR-ARPES) of the transient response of the candidate excitonic insulator Ta2NiSe5. Our analysis demonstrates that the electron-electron interactions dominate the electron-phonon ones. In particular, a detailed analysis of the TRARPES spectrum enables a clear separation of the dominant broadening (electronic lifetime) effects from the much smaller bandgap renormalization. Theoretical calculations show that the observed strong spectral broadening arises from the electronic scattering of the photo-excited particle-hole pairs and cannot be accounted for in a model in which electron-phonon interactions are dominant. We demonstrate that the magnitude of the weaker subdominant bandgap renormalization sensitively depends on the distance from the semiconductor/semimetal transition in the high-temperature state, which could explain apparent contradictions between various TR-ARPES experiments. The analysis presented here indicates that electron-electron interactions play a vital role (although not necessarily the sole one) in stabilizing the insulating state. △ Less

Submitted 12 December, 2021; originally announced December 2021.

The Three-Dimensional Electronic Structure of LiFeAs: Strong-coupling Superconductivity and Topology in the Iron Pnictides

Authors: Ryan P. Day, MengXing Na, Manuel Zingl, Berend Zwartsenberg, Matteo Michiardi, Giorgio Levy, Michael Schneider, Doug Wong, Pinder Dosanjh, Tor M. Pedersen, Sergey Gorovikov, Shun Chi, Ruixing Liang, Walter N. Hardy, Douglas A. Bonn, Sergey Zhdanovich, Ilya S. Elfimov, Andrea Damascelli

Abstract: Amongst the iron-based superconductors, LiFeAs is unrivalled in the simplicity of its crystal structure and phase diagram. However, our understanding of this canonical compound suffers from conflict between mutually incompatible descriptions of the material's electronic structure, as derived from contradictory interpretations of the photoemission record. Here, we explore the challenge of interpret… ▽ More Amongst the iron-based superconductors, LiFeAs is unrivalled in the simplicity of its crystal structure and phase diagram. However, our understanding of this canonical compound suffers from conflict between mutually incompatible descriptions of the material's electronic structure, as derived from contradictory interpretations of the photoemission record. Here, we explore the challenge of interpretation in such experiments. By combining comprehensive photon energy- and polarization- dependent angle-resolved photoemission spectroscopy (ARPES) measurements with numerical simulations, we establish the providence of several contradictions in the present understanding of this and related materials. We identify a confluence of surface-related issues which have precluded unambiguous identification of both the number and dimensionality of the Fermi surface sheets. Ultimately, we arrive at a scenario which supports indications of topologically non-trivial states, while also being incompatible with superconductivity as a spin-fluctuation driven Fermi surface instability. △ Less

Submitted 27 September, 2021; originally announced September 2021.

Comments: 8 pages, 4 figures

Report number: Phys. Rev. B 105, 155142

Experimental determination of superexchange energy from two-hole spectra

Authors: Giorgio Levy, Maayan Yaari, Tom Z. Regier, Amit Keren

Abstract: We follow the evolution of Copper and Oxygen two-hole excitations, in optimally doped (Ca$_{x}$La$_{1-x}$)(Ba$_{1.75-x}$La$_{0.25+x} $)Cu$_{3}$O$_{y}$ for $x=0.1$ and $x=0.4$. The spectra have contributions from band states as well as a localized multiplet structure. From their identification, we determine the intrashell Coulomb interaction $U$ for Oxygen and Copper sites. These results allow us t… ▽ More We follow the evolution of Copper and Oxygen two-hole excitations, in optimally doped (Ca$_{x}$La$_{1-x}$)(Ba$_{1.75-x}$La$_{0.25+x} $)Cu$_{3}$O$_{y}$ for $x=0.1$ and $x=0.4$. The spectra have contributions from band states as well as a localized multiplet structure. From their identification, we determine the intrashell Coulomb interaction $U$ for Oxygen and Copper sites. These results allow us to estimate the atomic superexchange coupling $J$ suggesting a positive correlation between the maximal superconducting critical temperature $T_\text{C}^{max}$ and $J$. △ Less

Submitted 19 July, 2021; originally announced July 2021.

Comments: 6 pages, 3 figures, one supplementary material

Optical manipulation of Rashba-split 2-Dimensional Electron Gas

Authors: M. Michiardi, F. Boschini, H. -H. Kung, M. X. Na, S. K. Y. Dufresne, A. Currie, G. Levy, S. Zhdanovich, A. K. Mills, D. J. Jones, J. L. Mi, B. B. Iversen, Ph. Hofmann, A. Damascelli

Abstract: In spintronic devices, the two main approaches to actively control the electrons' spin degree of freedom involve either static magnetic or electric fields. An alternative avenue relies on the application of optical fields to generate spin currents, which promises to bolster spin-device performance allowing for significantly faster and more efficient spin logic. To date, research has mainly focused… ▽ More In spintronic devices, the two main approaches to actively control the electrons' spin degree of freedom involve either static magnetic or electric fields. An alternative avenue relies on the application of optical fields to generate spin currents, which promises to bolster spin-device performance allowing for significantly faster and more efficient spin logic. To date, research has mainly focused on the optical injection of spin currents through the photogalvanic effect, and little is known about the direct optical control of the intrinsic spin splitting. Here, to explore the all-optical manipulation of a material's spin properties, we consider the Rashba effect at a semiconductor interface. The Rashba effect has long been a staple in the field of spintronics owing to its superior tunability, which allows the observation of fully spin-dependent phenomena, such as the spin-Hall effect, spin-charge conversion, and spin-torque in semiconductor devices. In this work, by means of time and angle-resolved photoemission spectroscopy (TR-ARPES), we demonstrate that an ultrafast optical excitation can be used to manipulate the Rashba-induced spin splitting of a two-dimensional electron gas (2DEG) engineered at the surface of the topological insulator Bi$_{2}$Se$_{3}$. We establish that light-induced photovoltage and charge carrier redistribution -- which in concert modulate the spin-orbit coupling strength on a sub-picosecond timescale -- can offer an unprecedented platform for achieving all optically-driven THz spin logic devices. △ Less

Submitted 2 June, 2022; v1 submitted 19 May, 2021; originally announced May 2021.

Journal ref: Nature Communications 13, 3096 (2022)

Physical properties and electronic structure of single-crystal KCo$_2$As$_2$

Authors: D. J. Campbell, B. Wilfong, M. P. Zic, G. Levy, M. X. Na, T. M. Pedersen, S. Gorovikov, P. Y. Zavalij, S. Zhdanovich, A. Damascelli, E. E. Rodriguez, J. Paglione

Abstract: We present a method for producing high quality KCo2As2 crystals, stable in air and suitable for a variety of measurements. X-ray diffraction, magnetic susceptibility, electrical transport and heat capacity measurements confirm the high quality and an absence of long range magnetic order down to at least 2 K. Residual resistivity values approaching 0.25 $μΩ$~cm are representative of the high qualit… ▽ More We present a method for producing high quality KCo2As2 crystals, stable in air and suitable for a variety of measurements. X-ray diffraction, magnetic susceptibility, electrical transport and heat capacity measurements confirm the high quality and an absence of long range magnetic order down to at least 2 K. Residual resistivity values approaching 0.25 $μΩ$~cm are representative of the high quality and low impurity content, and a Sommerfeld coefficient $γ$ = 7.3 mJ/mol K$^2$ signifies weaker correlations than the Fe-based counterparts. Together with Hall effect measurements, angle-resolved photoemission experiments reveal a Fermi surface consisting of electron pockets at the center and corner of the Brillouin zone, in line with theoretical predictions and in contrast to the mixed carrier types of other pnictides with the ThCr2Si2 structure. A large, linear magnetoresistance of 200\% at 14~T, together with an observed linear and hyperbolic, rather than parabolic, band dispersions are unusual characteristics of this metallic compound and may indicate more complex underlying behavior. △ Less

Submitted 10 February, 2021; v1 submitted 7 October, 2020; originally announced October 2020.

Comments: 9 pages, 4 figures

Ubiquitous suppression of the nodal coherent spectral weight in Bi-based cuprates

Authors: M. Zonno, F. Boschini, E. Razzoli, M. Michiardi, M. X. Na, S. Dufresne, T. M. Pedersen, S. Gorovikov, S. Gonzalez, G. Di Santo, L. Petaccia, M. Schneider, D. Wong, P. Dosanjh, Y. Yoshida, H. Eisaki, R. D. Zhong, J. Schneeloch, G. D. Gu, A. K. Mills, S. Zhdanovich, G. Levy, D. J. Jones, A. Damascelli

Abstract: High-temperature superconducting cuprates exhibit an intriguing phenomenology for the low-energy elementary excitations. In particular, an unconventional temperature dependence of the coherent spectral weight (CSW) has been observed in the superconducting phase by angle-resolved photoemission spectroscopy (ARPES), both at the antinode where the d-wave paring gap is maximum, as well as along the ga… ▽ More High-temperature superconducting cuprates exhibit an intriguing phenomenology for the low-energy elementary excitations. In particular, an unconventional temperature dependence of the coherent spectral weight (CSW) has been observed in the superconducting phase by angle-resolved photoemission spectroscopy (ARPES), both at the antinode where the d-wave paring gap is maximum, as well as along the gapless nodal direction. Here, we combine equilibrium and time-resolved ARPES to track the temperature dependent meltdown of the nodal CSW in Bi-based cuprates with unprecedented sensitivity. We find the nodal suppression of CSW upon increasing temperature to be ubiquitous across single- and double-layer Bi cuprates, and uncorrelated to superconducting and pseudogap onset temperatures. We quantitatively model both the lineshape of the nodal spectral features and the anomalous suppression of CSW within the Fermi-Liquid framework, establishing the key role played by the normal state electrodynamics in the description of nodal quasiparticles in superconducting cuprates. △ Less

Submitted 10 September, 2020; originally announced September 2020.

Comments: 6 pages, 3 figures; Supplementary Materials available upon request

Journal ref: Phys. Rev. B 103, 155109 (2021)

Establishing non-thermal regimes in pump-probe electron-relaxation dynamics

Authors: MengXing Na, Fabio Boschini, Arthur K. Mills, Matteo Michiardi, Ryan P. Day, Berend Zwartsenberg, Giorgio Levy, Sergey Zhdanovich, Alexander F. Kemper, David J. Jones, Andrea Damascelli

Abstract: Time- and angle-resolved photoemission spectroscopy (TR-ARPES) accesses the electronic structure of solids under optical excitation, and is a powerful technique for studying the coupling between electrons and collective modes. One approach to infer electron-boson coupling is through the relaxation dynamics of optically-excited electrons, and the characteristic timescales of energy redistribution.… ▽ More Time- and angle-resolved photoemission spectroscopy (TR-ARPES) accesses the electronic structure of solids under optical excitation, and is a powerful technique for studying the coupling between electrons and collective modes. One approach to infer electron-boson coupling is through the relaxation dynamics of optically-excited electrons, and the characteristic timescales of energy redistribution. A common description of electron relaxation dynamics is through the effective electronic temperature. Such a description requires that thermodynamic quantities are well-defined, an assumption that is generally violated at early delays. Additionally, precise estimation of the non-thermal window -- within which effective temperature models may not be applied -- is challenging. We perform TR-ARPES on graphite and show that Boltzmann rate equations can be used to calculate the time-dependent electronic occupation function, and reproduce experimental features given by non-thermal electron occupation. Using this model, we define a quantitative measure of non-thermal electron occupation and use it to define distinct phases of electron relaxation in the fluence-delay phase space. More generally, this approach can be used to inform the non-thermal-to-thermal crossover in pump-probe experiments. △ Less

Submitted 10 September, 2020; originally announced September 2020.

Comments: 18 pages, 10 figures

Journal ref: Physical Review B 102, 184307, 2020

Direct determination of mode-projected electron-phonon coupling in the time-domain

Authors: MengXing Na, Arthur K. Mills, Fabio Boschini, Matteo Michiardi, Benjamin Nosarzewski, Ryan P. Day, Elia Razzoli, Alexander Sheyerman, Michael Schneider, Giorgio Levy, Sergey Zhdanovich, Thomas P. Devereaux, Alexander F. Kemper, David J. Jones, Andrea Damascelli

Abstract: Ultrafast spectroscopies have become an important tool for elucidating the microscopic description and dynamical properties of quantum materials. In particular, by tracking the dynamics of non-thermal electrons, a material's dominant scattering processes -- and thus the many-body interactions between electrons and collective excitations -- can be revealed. Here we present a new method for extracti… ▽ More Ultrafast spectroscopies have become an important tool for elucidating the microscopic description and dynamical properties of quantum materials. In particular, by tracking the dynamics of non-thermal electrons, a material's dominant scattering processes -- and thus the many-body interactions between electrons and collective excitations -- can be revealed. Here we present a new method for extracting the electron-phonon coupling strength in the time domain, by means of time and angle-resolved photoemission spectroscopy (TR-ARPES). This method is demonstrated in graphite, where we investigate the dynamics of photo-injected electrons at the K point, detecting quantized energy-loss processes that correspond to the emission of strongly-coupled optical phonons. We show that the observed characteristic timescale for spectral-weight-transfer mediated by phonon-scattering processes allows for the direct quantitative extraction of electron-phonon matrix elements, for specific modes, and with unprecedented sensitivity. △ Less

Submitted 1 August, 2019; v1 submitted 14 February, 2019; originally announced February 2019.

Comments: 19 pages, 4 figures

Journal ref: Science 366, 1231 (2019)

Room temperature strain-induced Landau levels in graphene on a wafer-scale platform

Authors: P. Nigge, A. C. Qu, É. Lantagne-Hurtubise, E. Mårsell, S. Link, G. Tom, M. Zonno, M. Michiardi, M. Schneider, S. Zhdanovich, G. Levy, U. Starke, C. Gutiérrez, D. Bonn, S. A. Burke, M. Franz, A. Damascelli

Abstract: Graphene is a powerful playground for studying a plethora of quantum phenomena. One of the remarkable properties of graphene arises when it is strained in particular geometries and the electrons behave as if they were under the influence of a magnetic field. Previously, these strain-induced pseudomagnetic fields have been explored on the nano- and micrometer-scale using scanning probe and transpor… ▽ More Graphene is a powerful playground for studying a plethora of quantum phenomena. One of the remarkable properties of graphene arises when it is strained in particular geometries and the electrons behave as if they were under the influence of a magnetic field. Previously, these strain-induced pseudomagnetic fields have been explored on the nano- and micrometer-scale using scanning probe and transport measurements. Heteroepitaxial strain, in contrast, is a wafer-scale engineering method. Here, we show that pseudomagnetic fields can be generated in graphene through wafer-scale epitaxial growth. Shallow triangular nanoprisms in the SiC substrate generate strain-induced uniform fields of 41 T. This enables the observation of strain-induced Landau levels at room temperature, as detected by angle-resolved photoemission spectroscopy, and confirmed by model calculations and scanning tunneling microscopy measurements. Our work demonstrates the feasibility of exploiting strain-induced quantum phases in two-dimensional Dirac materials on a wafer-scale platform, opening the field to new applications. △ Less

Submitted 22 November, 2019; v1 submitted 1 February, 2019; originally announced February 2019.

Comments: Revised version as published in Science Advances

Journal ref: Science Advances 5, eaaw5593 (2019)

Emergence of pseudogap from short-range spin-correlations in electron doped cuprates

Authors: F. Boschini, M. Zonno, E. Razzoli, R. P. Day, M. Michiardi, B. Zwartsenberg, P. Nigge, M. Schneider, E. H. da Silva Neto, A. Erb, S. Zhdanovich, A. K. Mills, G. Levy, C. Giannetti, D. J. Jones, A. Damascelli

Abstract: Electron interactions are pivotal for defining the electronic structure of quantum materials. In particular, the strong electron Coulomb repulsion is considered the keystone for describing the emergence of exotic and/or ordered phases of quantum matter as disparate as high-temperature superconductivity and charge- or magnetic-order. However, a comprehensive understanding of fundamental electronic… ▽ More Electron interactions are pivotal for defining the electronic structure of quantum materials. In particular, the strong electron Coulomb repulsion is considered the keystone for describing the emergence of exotic and/or ordered phases of quantum matter as disparate as high-temperature superconductivity and charge- or magnetic-order. However, a comprehensive understanding of fundamental electronic properties of quantum materials is often complicated by the appearance of an enigmatic partial suppression of low-energy electronic states, known as the pseudogap. Here we take advantage of ultrafast angle-resolved photoemission spectroscopy to unveil the temperature evolution of the low-energy density of states in the electron-doped cuprate Nd$_{\text{2-x}}$Ce$_{\text{x}}$CuO$_{\text{4}}$, an emblematic system where the pseudogap intertwines with magnetic degrees of freedom. By photoexciting the electronic system across the pseudogap onset temperature T*, we report the direct relation between the momentum-resolved pseudogap spectral features and the spin-correlation length with an unprecedented sensitivity. This transient approach, corroborated by mean field model calculations, allows us to establish the pseudogap in electron-doped cuprates as a precursor to the incipient antiferromagnetic order even when long-range antiferromagnetic correlations are not established, as in the case of optimal doping. △ Less

Submitted 17 August, 2019; v1 submitted 18 December, 2018; originally announced December 2018.

Comments: 17 pages, 3 figures

Journal ref: npj Quantum Mater. 5, 6 (2020)

Role of matrix elements in the time-resolved photoemission signal

Authors: F. Boschini, D. Bugini, M. Zonno, M. Michiardi, R. P. Day, E. Razzoli, B. Zwartsenberg, E. H. da Silva Neto, S. dal Conte, S. K. Kushwaha, R. J. Cava, S. Zhdanovich, A. K. Mills, G. Levy, E. Carpene, C. Dallera, C. Giannetti, D. J. Jones, G. Cerullo, A. Damascelli

Abstract: Time- and angle-resolved photoemission spectroscopy accesses the ultrafast evolution of quasiparticles and many-body interactions in solid-state systems. However, the momentum- and energy-resolved transient photoemission intensity may not be unambiguously related to the intrinsic relaxation dynamics of photoexcited electrons. In fact, interpretation of the time-dependent photoemission signal can b… ▽ More Time- and angle-resolved photoemission spectroscopy accesses the ultrafast evolution of quasiparticles and many-body interactions in solid-state systems. However, the momentum- and energy-resolved transient photoemission intensity may not be unambiguously related to the intrinsic relaxation dynamics of photoexcited electrons. In fact, interpretation of the time-dependent photoemission signal can be affected by the transient evolution of both the one-electron removal spectral function as well as the photoemission dipole matrix elements. Here we investigate the topological insulator Bi$_{1.1}$Sb$_{0.9}$Te$_2$S to demonstrate, by means of a careful probe-polarization study, the transient contribution of matrix elements to the time-resolved photoemission signal. △ Less

Submitted 15 October, 2018; originally announced October 2018.

Comments: 7 pages, 3 figures

Journal ref: New J. Phys. 22 023031 (2020)

Influence of Spin Orbit Coupling in the Iron-Based Superconductors

Authors: R. P. Day, G. Levy, M. Michiardi, B. Zwartsenberg, M. Zonno, F. Ji, E. Razzoli, F. Boschini, S. Chi, R. Liang, P. K. Das, I. Vobornik, J. Fujii, D. A. Bonn, W. N. Hardy, I. S. Elfimov, A. Damascelli

Abstract: We report on the influence of spin-orbit coupling (SOC) in the Fe-based superconductors (FeSCs) via application of circularly-polarized spin and angle-resolved photoemission spectroscopy. We combine this technique in representative members of both the Fe-pnictides and Fe-chalcogenides with ab initio density functional theory and tight-binding calculations to establish an ubiquitous modification of… ▽ More We report on the influence of spin-orbit coupling (SOC) in the Fe-based superconductors (FeSCs) via application of circularly-polarized spin and angle-resolved photoemission spectroscopy. We combine this technique in representative members of both the Fe-pnictides and Fe-chalcogenides with ab initio density functional theory and tight-binding calculations to establish an ubiquitous modification of the electronic structure in these materials imbued by SOC. The influence of SOC is found to be concentrated on the hole pockets where the superconducting gap is generally found to be largest. This result contests descriptions of superconductivity in these materials in terms of pure spin-singlet eigenstates, raising questions regarding the possible pairing mechanisms and role of SOC therein. △ Less

Submitted 20 July, 2018; v1 submitted 17 November, 2017; originally announced November 2017.

Comments: For supplementary information, see http://qmlab.ubc.ca/ARPES/PUBLICATIONS/articles.html

Journal ref: Phys. Rev. Lett. 121, 076401 (2018)

Collapse of superconductivity in cuprates via ultrafast quenching of phase coherence

Authors: F. Boschini, E. H. da Silva Neto, E. Razzoli, M. Zonno, S. Peli, R. P. Day, M. Michiardi, M. Schneider, B. Zwartsenberg, P. Nigge, R. D. Zhong, J. Schneeloch, G. D. Gu, S. Zhdanovich, A. K. Mills, G. Levy, D. J. Jones, C. Giannetti, A. Damascelli

Abstract: The possibility of driving phase transitions in low-density condensates through the loss of phase coherence alone has far-reaching implications for the study of quantum phases of matter. This has inspired the development of tools to control and explore the collective properties of condensate phases via phase fluctuations. Electrically-gated oxide interfaces, ultracold Fermi atoms, and cuprate supe… ▽ More The possibility of driving phase transitions in low-density condensates through the loss of phase coherence alone has far-reaching implications for the study of quantum phases of matter. This has inspired the development of tools to control and explore the collective properties of condensate phases via phase fluctuations. Electrically-gated oxide interfaces, ultracold Fermi atoms, and cuprate superconductors, which are characterized by an intrinsically small phase-stiffness, are paradigmatic examples where these tools are having a dramatic impact. Here we use light pulses shorter than the internal thermalization time to drive and probe the phase fragility of the Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ cuprate superconductor, completely melting the superconducting condensate without affecting the pairing strength. The resulting ultrafast dynamics of phase fluctuations and charge excitations are captured and disentangled by time-resolved photoemission spectroscopy. This work demonstrates the dominant role of phase coherence in the superconductor-to-normal state phase transition and offers a benchmark for non-equilibrium spectroscopic investigations of the cuprate phase diagram. △ Less

Submitted 2 April, 2018; v1 submitted 7 July, 2017; originally announced July 2017.

Comments: 24 pages, 9 figures, Main Text and Supplementary Information

Journal ref: Nature Materials (2018)

Correct Brillouin zone and electronic structure of BiPd

Authors: Alexander Yaresko, Andreas P. Schnyder, Hadj M. Benia, Chi-Ming Yim, Giorgio Levy, Andrea Damascelli, Christian R. Ast, Darren C. Peets, Peter Wahl

Abstract: A promising route to the realization of Majorana fermions is in non-centrosymmetric superconductors, in which spin-orbit-coupling lifts the spin degeneracy of both bulk and surface bands. A detailed assessment of the electronic structure is critical to evaluate their suitability for this through establishing the topological properties of the electronic structure. This requires correct identificati… ▽ More A promising route to the realization of Majorana fermions is in non-centrosymmetric superconductors, in which spin-orbit-coupling lifts the spin degeneracy of both bulk and surface bands. A detailed assessment of the electronic structure is critical to evaluate their suitability for this through establishing the topological properties of the electronic structure. This requires correct identification of the time-reversal-invariant momenta. One such material is BiPd, a recently rediscovered non-centrosymmetric superconductor which can be grown in large, high-quality single crystals and has been studied by several groups using angular resolved photoemission to establish its surface electronic structure. Many of the published electronic structure studies on this material are based on a reciprocal unit cell which is not the actual Brillouin zone of the material. We show here the consequences of this for the electronic structures and show how the inferred topological nature of the material is affected. △ Less

Submitted 23 January, 2018; v1 submitted 10 April, 2017; originally announced April 2017.

Comments: Originally a comment on 1505.03466/1610.03431 and 1609.08947. 4 pages plus an appendix with literature crystal structure refinements

Journal ref: Phys. Rev. B 97, 075108 (2018)

Observation of Dirac surface states in the noncentrosymmetric superconductor BiPd

Authors: H. M. Benia, E. Rampi, C. Trainer, C. M. Yim, A. Maldonado, D. C. Peets, A. Stoehr, U. Starke, K. Kern, A. Yaresko, G. Levy, A. Damascelli, C. R. Ast, A. P. Schnyder, P. Wahl

Abstract: Materials with strong spin-orbit coupling (SOC) have in recent years become a subject of intense research due to their potential applications in spintronics and quantum information technology. In particular, in systems which break inversion symmetry, SOC facilitates the Rashba-Dresselhaus effect, leading to a lifting of spin degeneracy in the bulk and intricate spin textures of the Bloch wave func… ▽ More Materials with strong spin-orbit coupling (SOC) have in recent years become a subject of intense research due to their potential applications in spintronics and quantum information technology. In particular, in systems which break inversion symmetry, SOC facilitates the Rashba-Dresselhaus effect, leading to a lifting of spin degeneracy in the bulk and intricate spin textures of the Bloch wave functions. Here, by combining angular resolved photoemission (ARPES) and low temperature scanning tunneling microscopy (STM) measurements with relativistic first-principles band structure calculations, we examine the role of SOC in single crystals of noncentrosymmetric BiPd. We report the detection of several Dirac surface states, one of which exhibits an extremely large spin splitting. Unlike the surface states in inversion-symmetric systems, the Dirac surface states of BiPd have completely different properties at opposite faces of the crystal and are not trivially linked by symmetry. The spin-splitting of the surface states exhibits a strong anisotropy by itself, which can be linked to the low in-plane symmetry of the surface termination. △ Less

Submitted 3 August, 2016; originally announced August 2016.

Comments: 6 page letter, + supplementary material

Journal ref: Physical Review B 94, 121407 (2016)

Evidence for superconductivity in Li-decorated monolayer graphene

Authors: Bart Ludbrook, Giorgio Levy, Pascal Nigge, Marta Zonno, Michael Schneider, David Dvorak, Christian Veenstra, Sergey Zhdanovich, Douglas Wong, Pinder Dosanjh, Carola Straßer, Alexander Stohr, Stiven Forti, Christian Ast, Ulrich Starke, Andrea Damascelli

Abstract: Monolayer graphene exhibits many spectacular electronic properties, with superconductivity being arguably the most notable exception. It was theoretically proposed that superconductivity might be induced by enhancing the electron-phonon coupling through the decoration of graphene with an alkali adatom superlattice [Profeta et al. Nat. Phys. 8, 131-134 (2012)]. While experiments have indeed demonst… ▽ More Monolayer graphene exhibits many spectacular electronic properties, with superconductivity being arguably the most notable exception. It was theoretically proposed that superconductivity might be induced by enhancing the electron-phonon coupling through the decoration of graphene with an alkali adatom superlattice [Profeta et al. Nat. Phys. 8, 131-134 (2012)]. While experiments have indeed demonstrated an adatom-induced enhancement of the electron-phonon coupling, superconductivity has never been observed. Using angle-resolved photoemission spectroscopy (ARPES) we show that lithium deposited on graphene at low temperature strongly modifies the phonon density of states, leading to an enhancement of the electron-phonon coupling of up to $λ\!\simeq\!0.58$. On part of the graphene-derived $π^*$-band Fermi surface, we then observe the opening of a $Δ\!\simeq\!0.9$ meV temperature-dependent pairing gap. This result suggests for the first time, to our knowledge, that Li-decorated monolayer graphene is indeed superconducting with $T_c\!\simeq\!5.9 K$. △ Less

Submitted 29 August, 2015; v1 submitted 24 August, 2015; originally announced August 2015.

Comments: Accepted. A high-resolution version with supplementary material can be found at http://qmlab.ubc.ca/ARPES/PUBLICATIONS/Articles/Graphene_Li.pdf

Journal ref: PNAS 112, 11795 (2015)

Effect of Pt substitution on the electronic structure of AuTe2

Authors: D. Ootsuki, K. Takubo, K. Kudo, H. Ishii, M. Nohara, N. L. Saini, R. Sutarto, F. He, T. Z. Regier, M. Zonno, M. Schneider, G. Levy, G. A. Sawatzky, A. Damascelli, T. Mizokawa

Abstract: We report a photoemission and x-ray absorption study on Au1-xPtxTe2 (x = 0 and 0.35) triangular lattice in which superconductivity is induced by Pt substitution for Au. Au 4f and Te 3d core-level spectra of AuTe2 suggests a valence state of Au2+(Te2)2-, which is consistent with its distorted crystal structure with Te-Te dimers and compressed AuTe6 otahedra. On the other hand, valence-band photoemi… ▽ More We report a photoemission and x-ray absorption study on Au1-xPtxTe2 (x = 0 and 0.35) triangular lattice in which superconductivity is induced by Pt substitution for Au. Au 4f and Te 3d core-level spectra of AuTe2 suggests a valence state of Au2+(Te2)2-, which is consistent with its distorted crystal structure with Te-Te dimers and compressed AuTe6 otahedra. On the other hand, valence-band photoemission spectra and pre-edge peaks of Te 3d absorption edge indicate that Au 5d bands are almost fully occupied and that Te 5p holes govern the transport properties and the lattice distortion. The two apparently conflicting pictures can be reconciled by strong Au 5d/Au 6s-Te 5p hybridization. Absence of a core-level energy shift with Pt substitution is inconsistent with the simple rigid band picture for hole doping. The Au 4f core-level spectrum gets slightly narrow with Pt substitution, indicating that the small Au 5d charge modulation in distorted AuTe2 is partially suppressed. △ Less

Submitted 11 October, 2014; originally announced October 2014.

Comments: 13 pages, 4 figures, accepted by Physical Review B

Journal ref: Phys. Rev. B, 90,144515 (2014)

Observation of Distinct Bulk and Surface Chemical Environments in a Topological Insulator under Magnetic doping

Authors: Ivana Vobornik, Giancarlo Panaccione, Jun Fujii, Zhi-Huai Zhu, Francesco Offi, Benjamin R. Salles, Francesco Borgatti, Piero Torelli, Jean Pascal Rueff, Denis Ceolin, Alberto Artioli, Manju Unnikrishnan, Giorgio Levy, Massimiliano Marangolo, Mamhoud Eddrief, Damjan Krizmancic, Huiwen Ji, Andrea Damascelli, Gerrit van der Laan, Russell G. Egdell, Robert J. Cava

Abstract: The influence of magnetic dopants on the electronic and chemical environments in topological insulators (TIs) is a key factor when considering possible spintronic applications based on topological surface state properties. Here we provide spectroscopic evidence for the presence of distinct chemical and electronic behavior for surface and bulk magnetic doping of Bi2Te3. The inclusion of Mn in the b… ▽ More The influence of magnetic dopants on the electronic and chemical environments in topological insulators (TIs) is a key factor when considering possible spintronic applications based on topological surface state properties. Here we provide spectroscopic evidence for the presence of distinct chemical and electronic behavior for surface and bulk magnetic doping of Bi2Te3. The inclusion of Mn in the bulk of Bi2Te3 induces a genuine dilute ferromagnetic state, with reduction of the bulk band gap as the Mn content is increased. Deposition of Fe on the Bi2Te3 surface, on the other hand, favors the formation of iron telluride already at coverages as low as 0.07 monolayer, as a consequence of the reactivity of the Te-rich surface. Our results identify the factors that need to be controlled in the realization of magnetic nanosystems and interfaces based on TIs. △ Less

Submitted 13 August, 2014; originally announced August 2014.

Journal ref: J. Phys. Chem. C 118, 12333, 2014

Bond-Order and the Role of Ligand States in Stripe-Modulated IrTe2

Authors: K. Takubo, R. Comin, D. Ootsuki, T. Mizokawa, H. Wadati, Y. Takahashi, G. Shibata, A. Fujimori, R. Sutarto, F. He, S. Pyon, K. Kudo, M. Nohara, G. Levy, I. Elfimov, G. A. Sawatzky, A. Damascelli

Abstract: The coupled electronic-structural modulations of the ligand states in IrTe$_2$ have been studied by x-ray absorption spectroscopy (XAS) and resonant elastic x-ray scattering (REXS). Distinctive pre-edge structures are observed at the Te-$M_{4,5}$ (3$d$ $\rightarrow$ 5$p$) absorption edge, indicating the presence of a Te 5$p$-Ir 5$d$ covalent state near the Fermi level. An enhancement of the REXS s… ▽ More The coupled electronic-structural modulations of the ligand states in IrTe$_2$ have been studied by x-ray absorption spectroscopy (XAS) and resonant elastic x-ray scattering (REXS). Distinctive pre-edge structures are observed at the Te-$M_{4,5}$ (3$d$ $\rightarrow$ 5$p$) absorption edge, indicating the presence of a Te 5$p$-Ir 5$d$ covalent state near the Fermi level. An enhancement of the REXS signal near the Te 3$d$ $\rightarrow$ 5$p$ resonance at the $Q\!=\!(1/5,0,-1/5)$ superlattice reflection is observed below the structural transition temperature $T_s\sim$ 280 K. The analysis of the energy-dependent REXS lineshape reveals the key role played by the spatial modulation of the covalent Te 5$p$-Ir 5$d$ bond-density in driving the stripe-like order in IrTe$_2$, and uncovers its coupling with the charge and/or orbital order at the Ir sites. The similarity between these findings and the charge-ordering phenomenology observed in the high-T$_c$ superconducting cuprates suggests that the iridates may harbor similar exotic phases. △ Less

Submitted 2 June, 2014; v1 submitted 29 May, 2014; originally announced May 2014.

Comments: 13 pages, 10 figures

Journal ref: Phys. Rev. B 90, 081104(R) (2014)

Polarity-driven surface metallicity in SmB$_6$

Authors: Z. -H. Zhu, A. Nicolaou, G. Levy, N. P. Butch, P. Syers, X. F. Wang, J. Paglione, G. A. Sawatzky, I. S. Elfimov, A. Damascelli

Abstract: By a combined angle-resolved photoemission spectroscopy and density functional theory study, we discover that the surface metallicity is polarity-driven in SmB$_6$. Two surface states, not accounted for by the bulk band structure, are reproduced by slab calculations for coexisting B$_6$ and Sm surface terminations. Our analysis reveals that a metallic surface state stems from an unusual property,… ▽ More By a combined angle-resolved photoemission spectroscopy and density functional theory study, we discover that the surface metallicity is polarity-driven in SmB$_6$. Two surface states, not accounted for by the bulk band structure, are reproduced by slab calculations for coexisting B$_6$ and Sm surface terminations. Our analysis reveals that a metallic surface state stems from an unusual property, generic to the (001) termination of all hexaborides: the presence of boron $2p$ dangling bonds, on a polar surface. The discovery of polarity-driven surface metallicity sheds new light on the 40-year old conundrum of the low-temperature residual conductivity of SmB$_6$, and raises a fundamental question in the field of topological Kondo insulators regarding the interplay between polarity and nontrivial topological properties. △ Less

Submitted 19 November, 2013; v1 submitted 11 September, 2013; originally announced September 2013.

Comments: 5 pages, 4 figures. A high-resolution version with supplementary material can be found at: http://www.phas.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/SmB6

Journal ref: Phys. Rev. Lett. 111, 216402 (2013)

Sign inversion in the superconducting order parameter of LiFeAs inferred from Bogoliubov quasiparticle interference

Authors: Shun Chi, S. Johnston, G. Levy, S. Grothe, R. Szedlak, B. Ludbrook, Ruixing Liang, P. Dosanjh, S. A. Burke, A. Damascelli, D. A. Bonn, W. N. Hardy, Y. Pennec

Abstract: Quasiparticle interference (QPI) by means of scanning tunneling microscopy/spectroscopy (STM/STS), angle resolved photoemission spectroscopy (ARPES), and multi-orbital tight bind- ing calculations are used to investigate the band structure and superconducting order parameter of LiFeAs. Using this combination we identify intra- and interband scattering vectors between the hole (h) and electron (e)… ▽ More Quasiparticle interference (QPI) by means of scanning tunneling microscopy/spectroscopy (STM/STS), angle resolved photoemission spectroscopy (ARPES), and multi-orbital tight bind- ing calculations are used to investigate the band structure and superconducting order parameter of LiFeAs. Using this combination we identify intra- and interband scattering vectors between the hole (h) and electron (e) bands in the QPI maps. Discrepancies in the band dispersions inferred from previous ARPES and STM/STS are reconciled by recognizing a difference in the $k_z$ sensitivity for the two probes. The observation of both h-h and e-h scattering is exploited using phase-sensitive scattering selection rules for Bogoliubov quasiparticles. From this we infer an s$_\pm$ gap structure, where a sign change occurs in the superconducting order parameter between the e and h bands. △ Less

Submitted 14 March, 2014; v1 submitted 20 August, 2013; originally announced August 2013.

Comments: Replaced previous version after the paper was accepted for publication in PRB

Journal ref: Phys. Rev. B 89, 104522 (2014)

Deconstruction of Resolution Effects in Angle-Resolved Photoemission

Authors: G. Levy, W. Nettke, B. M. Ludbrook, C. N. Veenstra, A. Damascelli

Abstract: We study how the energy and momentum resolution of angle-resolved photoemission spectroscopy (ARPES) affects the linewidth, Fermi crossing, velocity, and curvature of the measured band structure. Based on the fact that the resolution smooths out the spectra, acting as a low-pass filter, we develop an iterative simulation scheme that compensates for resolution effects and allows the fundamental phy… ▽ More We study how the energy and momentum resolution of angle-resolved photoemission spectroscopy (ARPES) affects the linewidth, Fermi crossing, velocity, and curvature of the measured band structure. Based on the fact that the resolution smooths out the spectra, acting as a low-pass filter, we develop an iterative simulation scheme that compensates for resolution effects and allows the fundamental physical parameters to be accurately extracted. By simulating a parabolic band structure of Fermi-liquid quasiparticles, we show that this method works for an energy resolution up to 100 meV and a momentum resolution equal to twice the energy resolution scaled by the Fermi velocity. Our analysis acquires particular relevance in the hard and soft x-ray regimes, where a degraded resolution limits the accuracy of the extracted physical parameters, making it possible to study how the electronic excitations are modified when the ARPES probing depth increases beyond the surface. △ Less

Submitted 8 August, 2014; v1 submitted 11 June, 2013; originally announced June 2013.

Comments: A high-resolution version can be found at: http://www.phas.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/ARPES_resolution.pdf

Journal ref: Phys. Rev. B 90, 045150 (2014)

Layer-by-layer entangled spin-orbital texture of the topological surface state in Bi2Se3

Authors: Z. -H. Zhu, C. N. Veenstra, G. Levy, A. Ubaldini, P. Syers, N. P. Butch, J. Paglione, M. W. Haverkort, I. S. Elfimov, A. Damascelli

Abstract: We study Bi2Se3 by polarization-dependent angle-resolved photoemission spectroscopy (ARPES) and density-functional theory slab calculations. We find that the surface state Dirac fermions are characterized by a layer-dependent entangled spin-orbital texture, which becomes apparent through quantum interference effects. This explains the discrepancy between the spin polarization from spin-resovled AR… ▽ More We study Bi2Se3 by polarization-dependent angle-resolved photoemission spectroscopy (ARPES) and density-functional theory slab calculations. We find that the surface state Dirac fermions are characterized by a layer-dependent entangled spin-orbital texture, which becomes apparent through quantum interference effects. This explains the discrepancy between the spin polarization from spin-resovled ARPES - ranging from 20 to 85% - and the 100% value assumed in phenomenological models. It also suggests a way to probe the intrinsic spin texture of topological insulators, and to continuously manipulate the spin polarization of photoelectrons and photocurrents all the way from 0 to +/-100% by an appropriate choice of photon energy, linear polarization, and angle of incidence. △ Less

Submitted 21 May, 2013; v1 submitted 19 December, 2012; originally announced December 2012.

Comments: 5 pages, 4 figures. A high-resolution version with supplementary material can be found at: http://www.phas.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/BiSe_P.pdf

Journal ref: Phys. Rev. Lett. 110, 216401 (2013)

Observation of charge accumulation and onsite Coulomb repulsion at transition metal impurities in the iron pnictides

Authors: R. Kraus, V. Bisogni, L. Harnagea, S. Aswartham, S. Wurmehl, G. Levy, I. S. Elfimov, B. Büchner, G. A. Sawatzky, J. Geck

Abstract: We report a combined valence band photoemission and Auger spectroscopy study of single crystalline Ca(Fe,Co)2As2 and Ba(Fe,TM)2As2 with TM=Ni or Cu. The valence band photoemission data show directly that the TM-states move to higher binding energies with increasing atomic number, contributing less and less to the states close to the Fermi level. Furthermore, the 3d8 final state of the LVV Auger de… ▽ More We report a combined valence band photoemission and Auger spectroscopy study of single crystalline Ca(Fe,Co)2As2 and Ba(Fe,TM)2As2 with TM=Ni or Cu. The valence band photoemission data show directly that the TM-states move to higher binding energies with increasing atomic number, contributing less and less to the states close to the Fermi level. Furthermore, the 3d8 final state of the LVV Auger decay, which is observed for Ni and Cu, unambiguously reveals the accumulation of charge at these impurities. We also show that the onsite Coulomb interaction on the impurity strongly increases when moving from Co over Ni to Cu. Our results quantify the impurity potentials and imply that the superconducting state is robust against impurity scattering. △ Less

Submitted 28 October, 2012; originally announced October 2012.

Determining the Surface-To-Bulk Progression in the Normal-State Electronic Structure of Sr2RuO4 by Angle-Resolved Photoemission and Density Functional Theory

Authors: C. N. Veenstra, Z. -H. Zhu, B. Ludbrook, M. Capsoni, G. Levy, A. Nicolaou, J. A. Rosen, R. Comin, S. Kittaka, Y. Maeno, I. S. Elfimov, A. Damascelli

Abstract: In search of the potential realization of novel normal-state phases on the surface of Sr2RuO4 - those stemming from either topological bulk properties or the interplay between spin-orbit coupling (SO) and the broken symmetry of the surface - we revisit the electronic structure of the top-most layers by ARPES with improved data quality as well as ab-initio LDA slab calculations. We find that the cu… ▽ More In search of the potential realization of novel normal-state phases on the surface of Sr2RuO4 - those stemming from either topological bulk properties or the interplay between spin-orbit coupling (SO) and the broken symmetry of the surface - we revisit the electronic structure of the top-most layers by ARPES with improved data quality as well as ab-initio LDA slab calculations. We find that the current model of a single surface layer (\surd2x\surd2)R45° reconstruction does not explain all detected features. The observed depth-dependent signal degradation, together with the close quantitative agreement with LDA+SO slab calculations based on the LEED-determined surface crystal structure, reveal that (at a minimum) the sub-surface layer also undergoes a similar although weaker reconstruction. This points to a surface-to-bulk progression of the electronic states driven by structural instabilities, with no evidence for Dirac and Rashba-type states or surface magnetism. △ Less

Submitted 12 February, 2013; v1 submitted 18 May, 2012; originally announced May 2012.

Comments: 4 pages, 4 figures, 1 table. Further information and PDF available at: http://www.phas.ubc.ca/~quantmat/ARPES/PUBLICATIONS/articles.html

Journal ref: Phys. Rev. Lett. 110, 097004 (2013)

Na2IrO3 as a spin-orbit-assisted antiferromagnetic insulator with a 340 meV gap

Authors: R. Comin, G. Levy, B. Ludbrook, Z. -H. Zhu, C. N. Veenstra, J. A. Rosen, Yogesh Singh, P. Gegenwart, D. Stricker, J. N. Hancock, D. van der Marel, I. S. Elfimov, A. Damascelli

Abstract: We study Na2IrO3 by ARPES, optics, and band structure calculations in the local-density approximation (LDA). The weak dispersion of the Ir 5d-t2g manifold highlights the importance of structural distortions and spin-orbit coupling (SO) in driving the system closer to a Mott transition. We detect an insulating gap Δ_gap = 340 meV which, at variance with a Slater-type description, is already open at… ▽ More We study Na2IrO3 by ARPES, optics, and band structure calculations in the local-density approximation (LDA). The weak dispersion of the Ir 5d-t2g manifold highlights the importance of structural distortions and spin-orbit coupling (SO) in driving the system closer to a Mott transition. We detect an insulating gap Δ_gap = 340 meV which, at variance with a Slater-type description, is already open at 300 K and does not show significant temperature dependence even across T_N ~ 15 K. An LDA analysis with the inclusion of SO and Coulomb repulsion U reveals that, while the prodromes of an underlying insulating state are already found in LDA+SO, the correct gap magnitude can only be reproduced by LDA+SO+U, with U = 3 eV. This establishes Na2IrO3 as a novel type of Mott-like correlated insulator in which Coulomb and relativistic effects have to be treated on an equal footing. △ Less

Submitted 12 November, 2012; v1 submitted 19 April, 2012; originally announced April 2012.

Comments: Accepted in Physical Review Letters. Auxiliary and related material can be found at: http://www.phas.ubc.ca/~quantmat/ARPES/PUBLICATIONS/articles.html

Journal ref: Phys. Rev. Lett. 109, 266406 (2012)

Probing the role of Co substitution in the electronic structure of iron-pnictides

Authors: G. Levy, R. Sutarto, D. Chevrier, T. Regier, R. Blyth, J. Geck, S. Wurmehl, L. Harnagea, H. Wadati, T. Mizokawa, I. S. Elfimov, A. Damascelli, G. A. Sawatzky

Abstract: The role of Co substitution in the low-energy electronic structure of Ca(Fe$_{0.944}$Co$_{0.056}$)$_2$As$_2$ is investigated by resonant photoemission spectroscopy and density functional theory. The Co 3d-state center-of-mass is observed at 250 meV higher binding energy than Fe's, indicating that Co posses one extra valence electron, and that Fe and Co are in the same 2+ oxidation state. Yet, sign… ▽ More The role of Co substitution in the low-energy electronic structure of Ca(Fe$_{0.944}$Co$_{0.056}$)$_2$As$_2$ is investigated by resonant photoemission spectroscopy and density functional theory. The Co 3d-state center-of-mass is observed at 250 meV higher binding energy than Fe's, indicating that Co posses one extra valence electron, and that Fe and Co are in the same 2+ oxidation state. Yet, significant Co character is detected for the Bloch wavefunctions at the chemical potential, revealing that the Co 3d electrons are part of the Fermi sea determining the Fermi surface. This establishes the complex role of Co substitution in CaFe2As2, and the inadequacy of a rigid-band shift description. △ Less

Submitted 26 March, 2012; originally announced March 2012.

Comments: 4 pages, 4 figures

Journal ref: Phys. Rev. Lett. 109, 077001 (2012)

Surface-enhanced charge-density-wave instability in underdoped Bi2201

Authors: J. A. Rosen, R. Comin, G. Levy, D. Fournier, Z. -H. Zhu, B. Ludbrook, C. N. Veenstra, A. Nicolaou, D. Wong, P. Dosanjh, Y. Yoshida, H. Eisaki, G. R. Blake, F. White, T. T. M. Palstra, R. Sutarto, F. He, A. Frano, Y. Lu, B. Keimer, G. A. Sawatzky, L. Petaccia, A. Damascelli

Abstract: Neutron and x-ray scattering experiments have provided mounting evidence for spin and charge ordering phenomena in underdoped cuprates. These range from early work on stripe correlations in Nd-LSCO to the latest discovery of charge-density-waves in YBCO. Both phenomena are characterized by a pronounced dependence on doping, temperature, and an externally applied magnetic field. Here we show that t… ▽ More Neutron and x-ray scattering experiments have provided mounting evidence for spin and charge ordering phenomena in underdoped cuprates. These range from early work on stripe correlations in Nd-LSCO to the latest discovery of charge-density-waves in YBCO. Both phenomena are characterized by a pronounced dependence on doping, temperature, and an externally applied magnetic field. Here we show that these electron-lattice instabilities exhibit also a previously unrecognized bulk-surface dichotomy. Surface-sensitive electronic and structural probes uncover a temperature-dependent evolution of the CuO2 plane band dispersion and apparent Fermi pockets in underdoped Bi2201, which is directly associated with an hitherto-undetected strong temperature dependence of the incommensurate superstructure periodicity below 130K. In stark contrast, the structural modulation revealed by bulk-sensitive probes is temperature independent. These findings point to a surface-enhanced incipient charge-density-wave instability, driven by Fermi surface nesting. This discovery is of critical importance in the interpretation of single-particle spectroscopy data and establishes the surface of cuprates and other complex oxides as a rich playground for the study of electronically soft phases. △ Less

Submitted 9 May, 2013; v1 submitted 11 November, 2011; originally announced November 2011.

Comments: A high-resolution version with supplementary material can be found at http://www.physics.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/Bi2201_PG_Q2.pdf

Journal ref: Nature Communications 4, 1977 (2013)

Rashba spin-splitting control at the surface of the topological insulator Bi2Se3

Authors: Z. -H. Zhu, G. Levy, B. Ludbrook, C. N. Veenstra, J. A. Rosen, R. Comin, D. Wong, P. Dosanjh, A. Ubaldini, P. Syers, N. P. Butch, J. Paglione, I. S. Elfimov, A. Damascelli

Abstract: The electronic structure of Bi2Se3 is studied by angle-resolved photoemission and density functional theory. We show that the instability of the surface electronic properties, observed even in ultra-high-vacuum conditions, can be overcome via in-situ potassium deposition. In addition to accurately setting the carrier concentration, new Rashba-like spin-polarized states are induced, with a tunable,… ▽ More The electronic structure of Bi2Se3 is studied by angle-resolved photoemission and density functional theory. We show that the instability of the surface electronic properties, observed even in ultra-high-vacuum conditions, can be overcome via in-situ potassium deposition. In addition to accurately setting the carrier concentration, new Rashba-like spin-polarized states are induced, with a tunable, reversible, and highly stable spin splitting. Ab-initio slab calculations reveal that these Rashba state are derived from the 5QL quantum-well states. While the K-induced potential gradient enhances the spin splitting, this might be already present for pristine surfaces due to the symmetry breaking of the vacuum-solid interface. △ Less

Submitted 2 June, 2011; originally announced June 2011.

Comments: A high-resolution version can be found at http://www.physics.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/BiSe_K.pdf

Journal ref: Phys. Rev. Lett. 107, 186405 (2011)

Reply to "Comment on arXiv:1012.1484v1 Structural origin of apparent Fermi surface pockets in angle-resolved photoemission of Bi_2Sr_{2-x}La_xCuO_{6+δ} by King et al."

Authors: P. D. C. King, J. A. Rosen, W. Meevasana, A. Tamai, E. Rozbicki, R. Comin, G. Levy, D. Fournier, Y. Yoshida, H. Eisaki, K. M. Shen, N. J. C. Ingle, A. Damascelli, F. Baumberger

Abstract: Reply to comment by Zhou et al. (arXiv:1012.3602) on arXiv:1012.1484 / Phys. Rev. Lett. 106, 127005 (2011). Reply to comment by Zhou et al. (arXiv:1012.3602) on arXiv:1012.1484 / Phys. Rev. Lett. 106, 127005 (2011). △ Less

Submitted 10 May, 2011; originally announced May 2011.

Comments: 2 pages

Structural origin of apparent Fermi surface pockets in angle-resolved photoemission of Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+δ}$

Authors: P. D. C. King, J. A. Rosen, W. Meevasana, A. Tamai, E. Rozbicki, R. Comin, G. Levy, D. Fournier, Y. Yoshida, H. Eisaki, K. M. Shen, N. J. C. Ingle, A. Damascelli, F. Baumberger

Abstract: We observe apparent hole pockets in the Fermi surfaces of single-layer Bi-based cuprate superconductors from angle-resolved photoemission (ARPES). From detailed low-energy electron diffraction measurements and an analysis of the ARPES polarization-dependence, we show that these pockets are not intrinsic, but arise from multiple overlapping superstructure replicas of the main and shadow bands. We f… ▽ More We observe apparent hole pockets in the Fermi surfaces of single-layer Bi-based cuprate superconductors from angle-resolved photoemission (ARPES). From detailed low-energy electron diffraction measurements and an analysis of the ARPES polarization-dependence, we show that these pockets are not intrinsic, but arise from multiple overlapping superstructure replicas of the main and shadow bands. We further demonstrate that the hole pockets reported recently from ARPES [Meng et al, Nature 462, 335 (2009)] have a similar structural origin, and are inconsistent with an intrinsic hole pocket associated with the electronic structure of a doped CuO$_2$ plane. The nature of the Fermi surface topology in the enigmatic pseudogap phase therefore remains an open question. △ Less

Submitted 7 December, 2010; originally announced December 2010.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. Lett. 106, 127005 (2011)

Loss of nodal quasiparticle integrity in underdoped YBa2Cu3O6+x

Authors: D. Fournier, G. Levy, Y. Pennec, J. L. McChesney, A. Bostwick, E. Rotenberg, R. Liang, W. N. Hardy, D. A. Bonn, I. S. Elfimov, A. Damascelli

Abstract: Arguably the most intriguing aspect of the physics of cuprates is the close proximity between the record high-Tc superconductivity (HTSC) and the antiferromagnetic charge-transfer insulating state driven by Mott-like electron correlations. These are responsible for the intimate connection between high and low-energy scale physics, and their key role in the mechanism of HTSC was conjectured very ea… ▽ More Arguably the most intriguing aspect of the physics of cuprates is the close proximity between the record high-Tc superconductivity (HTSC) and the antiferromagnetic charge-transfer insulating state driven by Mott-like electron correlations. These are responsible for the intimate connection between high and low-energy scale physics, and their key role in the mechanism of HTSC was conjectured very early on. More recently, the detection of quantum oscillations in high-magnetic field experiments on YBa2Cu3O6+x (YBCO) has suggested the existence of a Fermi surface of well-defined quasiparticles in underdoped cuprates, lending support to the alternative proposal that HTSC might emerge from a Fermi liquid across the whole cuprate phase diagram. Discriminating between these orthogonal scenarios hinges on the quantitative determination of the elusive quasiparticle weight Z, over a wide range of hole-doping p. By means of angle-resolved photoemission spectroscopy (ARPES) on in situ doped YBCO, and following the evolution of bilayer band-splitting, we show that the overdoped metal electronic structure (0.25<p<0.37) is in remarkable agreement with density functional theory and the Z=2p/(p+1) mean-field prediction. Below p~0.10-0.15, we observe the vanishing of the nodal quasiparticle weight Z_N; this marks a clear departure from Fermi liquid behaviour and -- consistent with dynamical mean-field theory -- is even a more rapid crossover to the Mott physics than expected for the doped resonating valence bond (RVB) spin liquid. △ Less

Submitted 22 July, 2010; originally announced July 2010.

Comments: Nature Physics, in press. A high-resolution version can be found at http://www.physics.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/YBCO_Z.pdf

Journal ref: Nature Physics 6, 905 (2010)

First Scanning Tunnelling Spectroscopy on Bi2Sr2Ca2Cu3O10 (Bi2223) single crystals

Authors: Martin Kugler, Giorgio Levy, Enrico Giannini, Alex Piriou, Alfred A. Manuel, Christian Hess, Oystein Fischer

Abstract: We report the first low temperature scanning tunnelling microscopy and spectroscopy study of high quality Bi2223 crystals. We present atomic resolution and show spectroscopic data acquired on two different samples. In one case, for Tc= 109K and a transition width of only 1K, we obtained an extremely homogeneous sample with a gap value of 60 meV over at least 50 nm. In the other case, the respect… ▽ More We report the first low temperature scanning tunnelling microscopy and spectroscopy study of high quality Bi2223 crystals. We present atomic resolution and show spectroscopic data acquired on two different samples. In one case, for Tc= 109K and a transition width of only 1K, we obtained an extremely homogeneous sample with a gap value of 60 meV over at least 50 nm. In the other case, the respective parameters were Tc= 111K with a transition width of 1.7K and yielded a slightly less homogeneous sample with a gap of 45 meV. We evidence strong similarities with Bi2Sr2CaCu2O8 (Bi2212) and discuss the doping level of our samples. △ Less

Submitted 10 March, 2005; originally announced March 2005.

Comments: 8 pages, 6 figures. Submitted to the proceedings of SNS04 published as a Special Issue of the Journal of Physics and Chemistry of Solids

Four-fold structure of vortex core states in Bi2Sr2CaCu2O8 (Bi2212)

Authors: Giorgio Levy, Martin Kugler, Alfred A. Manuel, Oystein Fischer, Ming Li

Abstract: We present a detailed study of vortex core spectroscopy in slightly overdoped Bi2Sr2CaCu2O8 using a low temperature scanning tunneling microscope. Inside the vortex core we observe a four-fold symmetric modulation of the local density of states with an energy-independent period of (4.3\pm 0.3)a0. Furthermore we demonstrate that this square modulation is related to the vortex core states which ar… ▽ More We present a detailed study of vortex core spectroscopy in slightly overdoped Bi2Sr2CaCu2O8 using a low temperature scanning tunneling microscope. Inside the vortex core we observe a four-fold symmetric modulation of the local density of states with an energy-independent period of (4.3\pm 0.3)a0. Furthermore we demonstrate that this square modulation is related to the vortex core states which are located at ~6 meV. Since the core-state energy is proportional to the superconducting gap magnitude, our results strongly suggest the existence of a direct relation between the superconducting state and the local electronic modulations in the vortex core. △ Less

Submitted 9 March, 2005; originally announced March 2005.

Comments: 5 pages, 4 figures. Submitted to Physical Review Letters

Scanning Tunneling Spectroscopy on Single Crystal MgB2

Authors: M. R. Eskildsen, M. Kugler, G. Levy, S. Tanaka, J. Jun, S. M. Kazakov, J. Karpinski, O. Fischer

Abstract: We report on the results of scanning tunneling spectroscopy measurements on single crystals of Mg2. Tunneling was performed both parallel and perpendicular to the crystalline c-axis. In the first case, a single superconducting gap (Delta_pi = 2.2 meV) associated with the pi-band is observed. Tunneling parallel to the ab-plane reveals an additional, larger gap (Delta_sigma ~ 7 meV) originating in… ▽ More We report on the results of scanning tunneling spectroscopy measurements on single crystals of Mg2. Tunneling was performed both parallel and perpendicular to the crystalline c-axis. In the first case, a single superconducting gap (Delta_pi = 2.2 meV) associated with the pi-band is observed. Tunneling parallel to the ab-plane reveals an additional, larger gap (Delta_sigma ~ 7 meV) originating in the highly two-dimensional sigma-band. Vortex imaging in the pi-band was performed with the field and tunnel current parallel to the c-axis. The vortices have a large core size compared to estimates based on Hc2, and show an absence of localized states in the core. Furthermore, superconductivity between the vortices is rapidly suppressed by an applied field. A comparison to specific heat measurements is performed. △ Less

Submitted 15 September, 2002; originally announced September 2002.

Comments: 12 pages, 7 figs. Submitted to Physica C

Journal ref: Physica C, vol. 385, pp. 169-176 (2003)