Dr. Vladimir Strokov

Vladimir Strokov

Spectroscopy of Novel Materials group. Beamline scientist at the ADRESS beamline of SLS responsible for scientific research and instrumental development in the field of soft-X-ray Angle-Resolved Photoelectron Spectroscopy (ARPES).

Paul Scherrer Institute
Forschungsstrasse 111
5232 Villigen PSI
Switzerland

Biography

Vladimir N. Strocov has graduated from the Physics Faculty of St. Petersburg State University (Russia) in 1986, and defended his PhD degree with the Research Institute of Physics of St. Petersburg State University in 1990. The scientific carrier was followed by multiple periods of research work with Chalmers University of Technology (Sweden), Universität des Saarlandes (Germany), Université de la Méditerranée (France) and Universität Augsburg (Germany). Development of the experimental and theoretical methodology of Very-Low-Energy Electron Diffraction (VLEED) as a novel spectroscopic method to probe the dispersions and lifetimes of the photoemission final states, in many cases strongly deviating from the free-electron-like model and containing non-trivial excited-state self-energy effects. This information is of vital importance for reliable determination of three-dimensional electronic dispersions from the ARPES experiment. The habilitation thesis "Very-Low-Energy Electron Diffraction Spectroscopy as a Method for the Band Structure Investigation" was defended with the Institute of Analytic Instrumentation, St. Petersburg, in 2020. Since 2004 a beamline scientist at the group of Spectroscopy on Novel Materials, Laboratory for Condensed Matter, Photon Science Division of Paul Scherrer Institute, with the instrumental and research activity focused on soft-X-ray ARPES at the Swiss Light Source. This activity has resulted in pushing the soft-X-ray ARPES technique to previously unthinkable applications to electronic structure of buried nanostructure and impurity systems actual for electronic and spintronic devices. As of mid-2020, more than 210 publications in refereed journals and monographs, including 33 publications in Physical Review Letters, 24 in the Nature-group journals and 3 invited reviews.

Institutional Responsibilities

Leading scientific projects on soft-X-ray Angle-Resolved Photoelectron Spectroscopy (ARPES) with its main applications to electron correlations, 2D and 3D materials, heterostructures and impurity systems. Leading design, construction, commissioning, software and operation of the high-resolution soft-X-ray beamline Advanced RESonance Spectroscopies (ADRESS) beamline at SLS including the ARPES endstation. Optics design of high-resolution beamlines and X-ray spectrometers. Leading development of the energy- and angle-multichannel spin analyzer iMott. Support of the soft-X-ray ARPES users.

Scientific Research

Electronic structure of bulk materials, nanostructures and impurity systems, including the dynamic many-body properties and electron-boson coupling. Experimental studies using electron and X-ray spectroscopies supported by computational analysis. X-ray and electron spectroscopy instrumentation (synchrotron-radiation beamlines, high-resolution X-ray and electron spectrometers). The experimental studies focus on ARPES in the soft-X-ray photon energy range. This technique combines the electron-momentum resolution of ARPES with enhanced photoelectron escape depth, concomitant sharp definition of the three-dimensional momentum, and resonant photoexcitation delivering elemental and chemical state specificity. The advanced instrumentation at the ADRESS beamline has allowed stretching this technique to the most photon-hungry cases of buried nanostructures and impurities, which are at the heart of modern electronic and spintronic devices. An ongoing project on the high-efficiency multichannel spin detector iMott will allow access to spin texture of buried nanostructures and impurities. The materials of particular interest include (1) strongly correlated transition-metal oxides (cuprates, titanates, etc). New non-trivial quantum states emerging in their heterostructures offer new functionalities for future quantum devices; (2) Semiconductor interfaces with other semiconductors, topological materials and superconductors. The latter promise realization of Majorana fermions for quantum computing; (3) Magnetically doped semiconductors and topological materials, where resonant soft-X-ray ARPES resolves embedding of the impurity states into the host electronic structure. 

Selected Publications

Selected publications for the last 5 years. For an extensive overview we kindly refer you to our publication repository DORA.

Concept of a multichannel spin-resolving electron analyzer based on Mott scattering. V. N. Strocov, V. N. Petrov & J. H. Dil, J. Synchr. Rad. 22 (2015) 708

The concept of a multichannel electron spin detector based on optical imaging principles and Mott scattering (iMott) is presented. A multichannel electron image produced by a standard angle-resolving (photo) electron analyzer or microscope is re-imaged by an electrostatic lens at an accelerating voltage of 40 kV onto the Au target. Quasi-elastic electrons bearing spin asymmetry of the Mott scattering are imaged by magnetic lenses onto position-sensitive electron CCDs whose differential signals yield the multichannel spin asymmetry image. Fundamental advantages of this concept include acceptance of inherently divergent electron sources from the electron analyzer or microscope focal plane as well as small aberrations achieved by virtue of high accelerating voltages, as demonstrated by extensive ray-tracing analysis. The efficiency gain compared with the single-channel Mott detector can be a factor of more than 104 which opens new prospects of spin-resolved spectroscopies in application not only to standard bulk and surface systems (Rashba effect, topological insulators, etc.) but also to buried heterostructures. The simultaneous spin detection combined with fast CCD readout enables efficient use of the iMott detectors at X-ray free-electron laser facilities.

Fermi Surface of Three-Dimensional La1−xSrxMnO3 Explored by Soft-X-Ray ARPES: Rhombohedral Lattice Distortion and its Effect on Magnetoresistance. L. L. Lev, J. Krempaský, U. Staub, V. A. Rogalev, T. Schmitt, M. Shi, P. Blaha, A. S. Mishchenko, A. A. Veligzhanin, Y. V. Zubavichus, M. B. Tsetlin, H. Volfová, J. Braun, J. Minár & V. N. Strocov, Phys. Rev. Lett. 114 (2015) 237601

Electronic structure of the three-dimensional colossal magnetoresistive perovskite La1−xSrxMnO3 has been established using soft-x-ray angle-resolved photoemission spectroscopy with its intrinsically sharp definition of three-dimensional electron momentum. The experimental results show much weaker polaronic coupling compared to the bilayer manganites and are consistent with the theoretical band structure including the empirical Hubbard parameter U. The experimental Fermi surface unveils the canonical topology of alternating three-dimensional electron spheres and hole cubes, with their shadow contours manifesting the rhombohedral lattice distortion. This picture has been confirmed by one-step photoemission calculations including displacement of the apical oxygen atoms. The rhombohedral distortion is neutral to the Jahn-Teller effect and thus polaronic coupling, but affects the double-exchange electron hopping and thus the colossal magnetoresistance effect.

Fermi states and anisotropy of Brillouin zone scattering in the decagonal Al–Ni–Co quasicrystal. V. A. Rogalev, O. Gröning, R. Widmer, J. H. Dil, F. Bisti, L. L. Lev, T. Schmitt & V. N. Strocov. Nature Comm. 6 (2015) 8607

Quasicrystals (QCs) are intermetallic alloys that have excellent long-range order but lack translational symmetry in at least one dimension. The valence band electronic structure near the Fermi energy EF in such materials is of special interest since it has a direct relation to their unusual physical properties. However, the Fermi surface (FS) topology as well as the mechanism of QC structure stabilization are still under debate. Here we report the first observation of the three-dimensional FS and valence band dispersions near EF in decagonal Al70Ni20Co10 (d-AlNiCo) QCs using soft X-ray angle-resolved photoemission spectroscopy. We show that the FS, formed by dispersive Al sp-states, has a multicomponent character due to a large contribution from high-order bands. Moreover, we discover that the magnitude of the gap at the FS related to the interaction with Brillouin zone boundary (Hume–Rothery gap) critically differs for the periodic and quasiperiodic directions.

Polaronic metal state at the LaAlO3/SrTiO3 interface. C. Cancellieri, A. S. Mishchenko, U. Aschauer, A. Filippetti, C. Faber, O. S. Barišić, V. A. Rogalev, T. Schmitt, N. Nagaosa & V. N. Strocov, Nature Comm. 7 (2016) 10386

Interplay of spin, charge, orbital and lattice degrees of freedom in oxide heterostructures results in a plethora of fascinating properties, which can be exploited in new generations of electronic devices with enhanced functionalities. The paradigm example is the interface between the two band insulators LaAlO3 and SrTiO3 that hosts a two-dimensional electron system. Apart from the mobile charge carriers, this system exhibits a range of intriguing properties such as field effect, superconductivity and ferromagnetism, whose fundamental origins are still debated. Here we use soft-X-ray angle-resolved photoelectron spectroscopy to penetrate through the LaAlO3 overlayer and access charge carriers at the buried interface. The experimental spectral function directly identifies the interface charge carriers as large polarons, emerging from coupling of charge and lattice degrees of freedom, and involving two phonons of different energy and thermal activity. This phenomenon fundamentally limits the carrier mobility and explains its puzzling drop at high temperatures.

Entanglement and manipulation of the magnetic and spin–orbit order in multiferroic Rashba semiconductors. J. Krempaský, S. Muff, F. Bisti, M. Fanciulli, H. Volfová, A. Weber, N. Pilet, P. Warnicke, H. Ebert, J Braun, F. Bertran, V. V. Volobuiev, J. Minár, G. Springholz, J. H. Dil & V. N. Strocov. Nature Comm. 7 (2016) 13071

Entanglement of the spin–orbit and magnetic order in multiferroic materials bears a strong potential for engineering novel electronic and spintronic devices. Here, we explore the electron and spin structure of ferroelectric a-GeTe thin films doped with ferromagnetic Mn impurities to achieve its multiferroic functionality. We use bulk-sensitive soft-X-ray angle-resolved photoemission spectroscopy (SX-ARPES) to follow hybridization of the GeTe valence band with the Mn dopants. We observe a gradual opening of the Zeeman gap in the bulk Rashba bands around the Dirac point with increase of the Mn concentration, indicative of the ferromagnetic order, at persistent Rashba splitting. Furthermore, subtle details regarding the spin–orbit and magnetic order entanglement are deduced from spin-resolved ARPES measurements. We identify antiparallel orientation of the ferroelectric and ferromagnetic polarization, and altering of the Rashba-type spin helicity by magnetic switching. Our experimental results are supported by first-principles calculations of the electron and
spin structure.

Weakly-Correlated Nature of Ferromagnetism in Nonsymmorphic CrO2 Revealed by Bulk-Sensitive Soft-X-Ray ARPES. F. Bisti, V. A. Rogalev, M. Karolak, S. Paul, A. Gupta, T. Schmitt, G. Güntherodt, V. Eyert, G. Sangiovanni, G. Profeta & V. N. Strocov. Phys. Rev. X 7 (2017) 041067

Chromium dioxide CrO2 belongs to a class of materials called ferromagnetic half-metals, whose peculiar aspect is that they act as a metal in one spin orientation and as a semiconductor or insulator in the opposite one. Despite numerous experimental and theoretical studies motivated by technologically important applications of this material in spintronics, its fundamental properties such as momentum-resolved electron dispersions and the Fermi surface have so far remained experimentally inaccessible because of metastability of its surface, which instantly reduces to amorphous Cr2O3. In this work, we demonstrate that direct access to the native electronic structure of CrO2 can be achieved with soft-x-ray angle-resolved photoemission spectroscopy whose large probing depth penetrates through the Cr2O3 layer. For the first time, the electronic dispersions and Fermi surface of CrO2 are measured, which are fundamental prerequisites to solve the long debate on the nature of electronic correlations in this material. Since density functional theory augmented by a relatively weak local Coulomb repulsion gives an exhaustive description of our spectroscopic data, we rule out strong-coupling theories of CrO2. Crucial for the correct interpretation of our experimental data in terms of the valence-band dispersions is the understanding of a nontrivial spectral response of CrO2 caused by interference effects in the photoemission process originating from the non-symmorphic space group of the rutile crystal structure of CrO2.

Orbital ordering of the mobile and localized electrons at oxygen-deficient LaAlO3/SrTiO3 interfaces. A. Chikina, F. Lechermann, M.-A. Husanu, M. Caputo, C. Cancellieri, X. Wang, T. Schmitt, M. Radovic & V. N. Strocov. ACS Nano 12 (2018) 7927

Interfacing different transition-metal oxides opens a route to functionalizing their rich interplay of electron, spin, orbital, and lattice degrees of freedom for electronic and spintronic devices. Electronic and magnetic properties of SrTiO3-based interfaces hosting a mobile two-dimensional electron system (2DES) are strongly influenced by oxygen vacancies, which form an electronic dichotomy, where strongly correlated localized electrons in the in-gap states (IGSs) coexist with noncorrelated delocalized 2DES. Here, we use resonant soft-X-ray photoelectron spectroscopy to prove the eg character of the IGSs, as opposed to the t2g character of the 2DES in the paradigmatic LaAlO3/SrTiO3 interface. We furthermore separate the dxy and dxz/dxz orbital contributions based on deeper consideration of the resonant photoexcitation process in terms of orbital and momentum selectivity. Supported by a self-consistent combination of density functional theory and dynamical mean field theory calculations, this experiment identifies local orbital reconstruction that goes beyond the conventional eg-vs-t2g band ordering. A hallmark of oxygen-deficient LaAlO3/SrTiO3 is a significant hybridization of the eg and t2g orbitals. Our findings provide routes for tuning the electronic and magnetic properties of oxide interfaces through “defect engineering” with oxygen vacancies.

k-space imaging of anisotropic two-dimensional electron gas in GaN-based high-electron-mobility transistor heterostructures. L. L. Lev, I. O. Maiboroda, M.-A. Husanu, E. S. Grichuk, N. K. Chumakov, I. S. Ezubchenko, X. Wang, T. Schmitt, M. L. Zanaveskin, V. G. Valeyev & V. N. Strocov. Nature Comm. 9 (2018) 2653

Nanostructures based on buried interfaces and heterostructures are at the heart of modern semiconductor electronics as well as future devices utilizing spintronics, multiferroics, topological effects, and other novel operational principles. Knowledge of electronic structure of these systems resolved in electron momentum k delivers unprecedented insights into their physics. Here we explore 2D electron gas formed in GaN/AlGaN high-electron-mobility transistor heterostructures with an ultrathin barrier layer, key elements in current high-frequency and high-power electronics. Its electronic structure is accessed with angle-resolved photoelectron spectroscopy whose probing depth is pushed to a few nanometers using soft-X-ray synchrotron radiation. The experiment yields direct k-space images of the electronic structure fundamentals of this system—the Fermi surface, band dispersions and occupancy, and the Fourier composition of wavefunctions encoded in the k-dependent photoemission intensity. We discover significant planar anisotropy of the electron Fermi surface and effective mass connected with relaxation of the interfacial atomic positions, which translates into nonlinear (high-field) transport properties of the GaN/AlGaN heterostructures as an anisotropy of the saturation drift velocity of the 2D electrons.

Electronic phase separation at LaAlO3/SrTiO3 interfaces tunable by oxygen deficiency. V. N. Strocov, A. Chikina, M. Caputo, M.-A. Husanu, F. Bisti, D. Bracher, T. Schmitt, F. Miletto Granozio, C. A. F. Vaz & F. Lechermann. Phys. Rev. Materials 3 (2019) 106001

Electronic phase separation is crucial for the fascinating macroscopic properties of the LaAlO3/SrTiO3 (LAO/STO) paradigm oxide interface, including the coexistence of superconductivity and ferromagnetism. We investigate this phenomenon using angle-resolved photoelectron spectroscopy (ARPES) in the soft-x-ray energy range, where the enhanced probing depth combined with resonant photoexcitation allow us access to fundamental electronic structure characteristics – momentum-resolved spectral function, dispersions and ordering of energy bands, Fermi surface – of buried interfaces. Our experiment uses x-ray irradiation of the LAO/STO interface to tune its oxygen deficiency, building up a dichotomic system where mobile weakly correlated Ti t2g electrons coexist with localized strongly correlated Ti eg ones. The ARPES spectra dynamics under x-ray irradiation shows a gradual intensity increase under constant Luttinger count of the Fermi surface. This fact identifies electronic phase separation (EPS) where the mobile electrons accumulate in conducting puddles with fixed electronic structure embedded in an insulating host phase, and allows us to estimate the lateral fraction of these puddles. We discuss the physics of EPS invoking a theoretical picture of oxygen-vacancy clustering, promoted by the magnetism of the localized Ti eg electrons, and repelling of the mobile t2g electrons from these clusters. Our results on the irradiation-tuned EPS elucidate the intrinsic one taking place at the stoichiometric LAO/STO interfaces.

Electron-lattice interaction boost on the verge of metal-insulator transition in oxides. M.-A. Husanu, L. Vistoli, C. Verdi, A. Sander, V. Garcia, J. Rault, F. Bisti, L. L. Lev, T. Schmitt, F. Giustino,  A. S. Mishchenko, M. Bibes & V. N. Strocov. Communications Physics 3 (2020) 62

Many transition metal oxides (TMOs) are Mott insulators due to strong Coulomb repulsion between electrons, and exhibit metal-insulator transitions (MITs) whose mechanisms are not always fully understood. Unlike most TMOs, minute doping in CaMnO3 induces a metallic state without any structural transformations. This material is thus an ideal platform to explore band formation through the MIT. Here, we use angle-resolved photoemission spectroscopy to visualize how electrons delocalize and couple to phonons in CaMnO3. We show the development of a Fermi surface where mobile electrons coexist with heavier carriers, strongly coupled polarons. The latter originate from a boost of the electron-phonon interaction (EPI). This finding brings to light the role that the EPI can play in MITs even caused by purely electronic mechanisms. Our discovery of the EPI-induced dichotomy of the charge carriers explains the transport response of Ce-doped CaMnO3 and suggests strategies to engineer quantum matter from TMOs.

Books

Spectroscopy of Complex Oxide Interfaces: Photoemission and Related Spectroscopies. Eds. C. Cancellieri and V. N. Strocov, Springer Verlag (2018)