Layered materials exhibit a plethora of fascinating properties. The challenge is to make the mate... more Layered materials exhibit a plethora of fascinating properties. The challenge is to make the materials into epitaxial films, preferably integrated with mature technological platforms to facilitate their potential applications. Progress in this direction can establish the film thickness as a valuable parameter to control various phenomena, superconductivity in particular. Here, a synthetic route to epitaxial films of SrAlSi, a layered superconducting electride, on silicon is designed. A set of films ranging in thickness is synthesized employing a silicene‐based template. Their structure and superconductivity are explored by a combination of techniques. Two regimes of TC dependence on the film thickness are identified, the coherence length being the crossover parameter. The results can be extended to syntheses of other honeycomb‐lattice ternary compounds on Si or Ge exhibiting superconducting, magnetic, and other properties.
ABSTRACT The temperature dependence of the electrical resistance and thermopower of La2− x SrxCuO... more ABSTRACT The temperature dependence of the electrical resistance and thermopower of La2− x SrxCuO4+δ single crystals with x ≅ 0.003 and δ<0.05 has been studied in the temperature range from 100 to 400 K. All crystals exhibiting two-dimensional hopping conductivity via neighboring acceptor sites in the CuO2 plane show a significant difference in the charge transfer below and above the Néel temperature T N. This difference indicates that the loss of a two-sublattice antiferromagnetic order strongly affects the charge transport in the CuO2 plane. The obtained data lead to a conclusion that the crystal above T N occurs in a resonance valence bond state of the Bose type.
Muon spin rotation experiments in zero magnetic field and magnetization measurements have been ca... more Muon spin rotation experiments in zero magnetic field and magnetization measurements have been carried out in a single crystal of spin S =1/ 2 double chain cuprate LiCu 2 O 2 over a temperature range of 2-300 K. In the antiferromagnetic state we find a bound state of an electron around the muon-the magnetic polaron-with the electron wave function confined within R = 0.55Ϯ 0.05 nm. Electron localization in this form persists up to 31 K, well above T N = 24.7 K.
Journal of Experimental and Theoretical Physics, May 1, 1995
The temperature dependence of the ac conductivity Ï=Ïâ+AÏ{sup s} of the sample YBaâCuâO{sub 6.2} ... more The temperature dependence of the ac conductivity Ï=Ïâ+AÏ{sup s} of the sample YBaâCuâO{sub 6.2} was investigated at low frequencies. An anomalous temperature dependence of the term AÏ{sup s} was found: specifically, s(T) is observed to have a minimum. It was shown that the observed singularity can be explained on the basis of a model that assumes tunneling of large-radius polarons, for which the overlap of regions of lattice distortion is large. Estimates are obtained for the radius and binding energy of a polaron. 11 refs., 5 figs.
Regular submonolayer structures of Eu atoms coupled with Ge(110) exhibit magnetic states. Submono... more Regular submonolayer structures of Eu atoms coupled with Ge(110) exhibit magnetic states. Submonolayer structures on different substrates can host 2D magnetic phases, suggesting the phenomenon to be rather general.
Ferromagnetic insulators are widely employed to induce magnetic phenomena in adjacent layers via ... more Ferromagnetic insulators are widely employed to induce magnetic phenomena in adjacent layers via proximity effect. This approach could make non-magnetic materials (ranging from silicon to graphene) available for spintronic applications. Eu chalcogenides, EuO in particular, are highly efficient spin generators but suffer from low Curie temperatures. Here, experiments aimed at increase in EuO by its integration with the ferromagnetic metal Gd are reported. The epitaxial bilayers Gd/EuO are synthesized on different substrates and characterized by a combination of diffraction and microscopy techniques. Their magnetic structureestablished with magnetization and transport measurements as well as elementselective X-ray magnetic circular dichroism studycomprises coupled magnetic orders of EuO and Gd. EuO is robust against proximity effectsits is still low, increased at most by a few tens of K. Nevertheless, the results encourage further studies of proximity-enhanced ferromagnetism to extend the range of applications of ultrathin layers of EuO in spintronics.
Electron doping of the Kondo insulator YbB 12 has been achieved by substitution of tetravalent Zr... more Electron doping of the Kondo insulator YbB 12 has been achieved by substitution of tetravalent Zr in the newly synthesized Yb 0.8 Zr 0.2 B 12 solid solution. Neutron-scattering measurements of the spin dynamics are reported, together with electrical resistivity and ac magnetic-susceptibility data. Zr doping transforms the spin-gap spectral response into a pseudogap shape and makes its temperature evolution much smoother than in the Yb 1−x Lu x B 12 family studied previously. The results obtained are discussed in connection with the local bound-state picture for the Kondo insulator.
Addition of magnetism to spectacular properties of graphene may lead to novel topological states ... more Addition of magnetism to spectacular properties of graphene may lead to novel topological states and design of spin logic devices enjoying low power consumption. A significant progress is made in defect-induced magnetism in graphene-selective elimination of ௭ orbitals (by vacancies or adatoms) at triangular sublattices tailors graphene magnetism. Proximity to a magnetic insulator is a less invasive way, which is being actively explored now. Integration of graphene with the ferromagnetic semiconductor EuO has much to offer, especially in terms of proximity-induced spin-orbit interactions. Here, we synthesize films of EuO on graphene using reactive molecular beam epitaxy. Their quality is attested by electron and X-ray diffraction, cross-sectional electron microscopy, Raman and magnetization measurements. Studies of electron transport reveal a magnetic transition at ܶ * ≈ 220 K, well above the Curie temperature 69 K of EuO. Up to ܶ * , the dependence ܴ ௫௬ ሺܤሻ is strongly non-linear suggesting the
Imprinting magnetism into graphene may lead to unconventional electron states and enable the desi... more Imprinting magnetism into graphene may lead to unconventional electron states and enable the design of spin logic devices with low power consumption. The ongoing active development of 2D magnets suggests their coupling with graphene to induce spin‐dependent properties via proximity effects. In particular, the recent discovery of submonolayer 2D magnets on surfaces of industrial semiconductors provides an opportunity to magnetize graphene coupled with silicon. Here, synthesis and characterization of large‐area graphene/Eu/Si(001) heterostructures combining graphene with a submonolayer magnetic superstructure of Eu on silicon are reported. Eu intercalation at the interface of the graphene/Si(001) system results in a Eu superstructure different from those formed on pristine Si in terms of symmetry. The resulting system graphene/Eu/Si(001) exhibits 2D magnetism with the transition temperature controlled by low magnetic fields. Negative magnetoresistance and the anomalous Hall effect in the graphene layer provide evidence for spin polarization of the carriers. Most importantly, the graphene/Eu/Si system seeds a class of graphene heterostructures based on submonolayer magnets aiming at applications in graphene spintronics.
An analysis of the data published on some high-temperature superconducting systems has revealed a... more An analysis of the data published on some high-temperature superconducting systems has revealed a universal dependence of the Neel and critical superconducting transition temperatures on the 2D conductivity of CuO 2 planes in these compounds
High carrier mobility of both electrons and holes is found in nanofilms of layered SrAl2Si2 integ... more High carrier mobility of both electrons and holes is found in nanofilms of layered SrAl2Si2 integrated with silicon. The salient feature of its atomic structure is anionic bilayers [Al2Si2]2−, isostructural and isoelectronic to bilayer silicene.
Recent discoveries of intrinsic two-dimensional (2D) magnets open up vast opportunities to addres... more Recent discoveries of intrinsic two-dimensional (2D) magnets open up vast opportunities to address fundamental problems in condensed matter physics, giving rise to applications from ultra-compact spintronics to quantum computing. The ever-growing material landscape of 2D magnets lacks, however, carbon-based systems, prominent in other areas of 2D research. Magnetization measurements of the Eu/graphene compound—a monolayer of the EuC 6 stoichiometry—reveal the emergence of 2D ferromagnetism but detailed studies of competing magnetic states are still missing. Here, we employ element-selective X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) to establish the magnetic structure of monolayer EuC 6 . The system exhibits the anomalous Hall effect, negative magnetoresistance, and magnetization consistent with a ferromagnetic state but the saturation magnetic moment (about 2.5 µ B /Eu) is way too low for the half-filled f -shells of Eu 2+ ions. Combined XAS/XMCD studies at the Eu L 3 absorption edge probe the EuC 6 magnetism in high fields and reveal the nature of the missing magnetic moments. The results are set against XMCD studies in Eu/silicene and Eu/germanene to establish monolayer EuC 6 as a prominent member of the family of Eu-based 2D magnets combining the celebrated graphene properties with a strong magnetism of europium.
Integration of crystalline oxides with silicon provides a versatile platform to extend and advanc... more Integration of crystalline oxides with silicon provides a versatile platform to extend and advance silicon technology. The interface between oxide and Si controls the structure and functional properties of the resulting material. In particular, the formation of a submonolayer metal phase on silicon is the standard approach to stabilize the epitaxial growth of oxides. However, fundamental questions—a) whether the interface transforms in the process of the synthesis; and b) if it is possible to control the interface and its electronic structure by varying the submonolayer template—remain unanswered. The present study employs MBE synthesis of EuO and SrO on Si(001) to demonstrate that the structure of the oxide/Si interface does not depend on the type of the template, its symmetry, and stoichiometry. Chemical transformations of the templates converging into the same 2D product are detected in situ by electron diffraction. Then, the common interfacial structure of 1D periodicity is visualized by high‐resolution electron microscopy. The study provides insights into the process of oxide integration with silicon but also sets the limits in designing oxide/Si interfaces.
Intrinsic two-dimensional (2D) magnetism has been demonstrated in various materials scaled down t... more Intrinsic two-dimensional (2D) magnetism has been demonstrated in various materials scaled down to a single monolayer. However, the question is whether 2D magnetism extends beyond the monolayer limit, to chemical species formed by sparse but regular 2D arrays of magnetic atoms. Here we show that sub-monolayer superstructures of Eu atoms self-assembled on the silicon surface exhibit strong magnetic signals. Robust easy-plane magnetism is discovered in both one- and two-dimensionally ordered structures with Eu coverage of half monolayer and above. The emergence of 2D magnetism manifests itself by a strong dependence of the effective transition temperature on weak magnetic fields. The results constitute a versatile platform for miniaturization of 2D magnetic systems and seed an expandable class of atomically thin magnets for applications in information technologies.
Abstract 2D magnets hold high promise for unconventional quantum phases, exploration of fundament... more Abstract 2D magnets hold high promise for unconventional quantum phases, exploration of fundamental physics of magnetism, ultra-compact devices based on van der Waals heterostructures. Graphene is a constant inspiration for design of 2D materials; stoichiometric graphene-based compounds with magnetic atoms give rise to 2D ferromagnets. Unfortunately, addition of magnetic species induces high chemical reactivity. Here, we encapsulate a monolayer Eu lattice between graphene monolayers to form a chemically inert van der Waals 2D ferromagnet. The material is produced by Eu intercalation into few-layer graphene, as confirmed by a combination of diffraction and microscopy techniques. The emergence of ferromagnetism is witnessed by magnetization and electron transport measurements. The effective Curie temperature is controlled by weak magnetic fields manifesting the 2D nature of the magnetic state. The results expand the family of 2D magnets and establish a versatile platform for synthesis of its new members by metal intercalation.
Layered materials exhibit a plethora of fascinating properties. The challenge is to make the mate... more Layered materials exhibit a plethora of fascinating properties. The challenge is to make the materials into epitaxial films, preferably integrated with mature technological platforms to facilitate their potential applications. Progress in this direction can establish the film thickness as a valuable parameter to control various phenomena, superconductivity in particular. Here, a synthetic route to epitaxial films of SrAlSi, a layered superconducting electride, on silicon is designed. A set of films ranging in thickness is synthesized employing a silicene‐based template. Their structure and superconductivity are explored by a combination of techniques. Two regimes of TC dependence on the film thickness are identified, the coherence length being the crossover parameter. The results can be extended to syntheses of other honeycomb‐lattice ternary compounds on Si or Ge exhibiting superconducting, magnetic, and other properties.
ABSTRACT The temperature dependence of the electrical resistance and thermopower of La2− x SrxCuO... more ABSTRACT The temperature dependence of the electrical resistance and thermopower of La2− x SrxCuO4+δ single crystals with x ≅ 0.003 and δ<0.05 has been studied in the temperature range from 100 to 400 K. All crystals exhibiting two-dimensional hopping conductivity via neighboring acceptor sites in the CuO2 plane show a significant difference in the charge transfer below and above the Néel temperature T N. This difference indicates that the loss of a two-sublattice antiferromagnetic order strongly affects the charge transport in the CuO2 plane. The obtained data lead to a conclusion that the crystal above T N occurs in a resonance valence bond state of the Bose type.
Muon spin rotation experiments in zero magnetic field and magnetization measurements have been ca... more Muon spin rotation experiments in zero magnetic field and magnetization measurements have been carried out in a single crystal of spin S =1/ 2 double chain cuprate LiCu 2 O 2 over a temperature range of 2-300 K. In the antiferromagnetic state we find a bound state of an electron around the muon-the magnetic polaron-with the electron wave function confined within R = 0.55Ϯ 0.05 nm. Electron localization in this form persists up to 31 K, well above T N = 24.7 K.
Journal of Experimental and Theoretical Physics, May 1, 1995
The temperature dependence of the ac conductivity Ï=Ïâ+AÏ{sup s} of the sample YBaâCuâO{sub 6.2} ... more The temperature dependence of the ac conductivity Ï=Ïâ+AÏ{sup s} of the sample YBaâCuâO{sub 6.2} was investigated at low frequencies. An anomalous temperature dependence of the term AÏ{sup s} was found: specifically, s(T) is observed to have a minimum. It was shown that the observed singularity can be explained on the basis of a model that assumes tunneling of large-radius polarons, for which the overlap of regions of lattice distortion is large. Estimates are obtained for the radius and binding energy of a polaron. 11 refs., 5 figs.
Regular submonolayer structures of Eu atoms coupled with Ge(110) exhibit magnetic states. Submono... more Regular submonolayer structures of Eu atoms coupled with Ge(110) exhibit magnetic states. Submonolayer structures on different substrates can host 2D magnetic phases, suggesting the phenomenon to be rather general.
Ferromagnetic insulators are widely employed to induce magnetic phenomena in adjacent layers via ... more Ferromagnetic insulators are widely employed to induce magnetic phenomena in adjacent layers via proximity effect. This approach could make non-magnetic materials (ranging from silicon to graphene) available for spintronic applications. Eu chalcogenides, EuO in particular, are highly efficient spin generators but suffer from low Curie temperatures. Here, experiments aimed at increase in EuO by its integration with the ferromagnetic metal Gd are reported. The epitaxial bilayers Gd/EuO are synthesized on different substrates and characterized by a combination of diffraction and microscopy techniques. Their magnetic structureestablished with magnetization and transport measurements as well as elementselective X-ray magnetic circular dichroism studycomprises coupled magnetic orders of EuO and Gd. EuO is robust against proximity effectsits is still low, increased at most by a few tens of K. Nevertheless, the results encourage further studies of proximity-enhanced ferromagnetism to extend the range of applications of ultrathin layers of EuO in spintronics.
Electron doping of the Kondo insulator YbB 12 has been achieved by substitution of tetravalent Zr... more Electron doping of the Kondo insulator YbB 12 has been achieved by substitution of tetravalent Zr in the newly synthesized Yb 0.8 Zr 0.2 B 12 solid solution. Neutron-scattering measurements of the spin dynamics are reported, together with electrical resistivity and ac magnetic-susceptibility data. Zr doping transforms the spin-gap spectral response into a pseudogap shape and makes its temperature evolution much smoother than in the Yb 1−x Lu x B 12 family studied previously. The results obtained are discussed in connection with the local bound-state picture for the Kondo insulator.
Addition of magnetism to spectacular properties of graphene may lead to novel topological states ... more Addition of magnetism to spectacular properties of graphene may lead to novel topological states and design of spin logic devices enjoying low power consumption. A significant progress is made in defect-induced magnetism in graphene-selective elimination of ௭ orbitals (by vacancies or adatoms) at triangular sublattices tailors graphene magnetism. Proximity to a magnetic insulator is a less invasive way, which is being actively explored now. Integration of graphene with the ferromagnetic semiconductor EuO has much to offer, especially in terms of proximity-induced spin-orbit interactions. Here, we synthesize films of EuO on graphene using reactive molecular beam epitaxy. Their quality is attested by electron and X-ray diffraction, cross-sectional electron microscopy, Raman and magnetization measurements. Studies of electron transport reveal a magnetic transition at ܶ * ≈ 220 K, well above the Curie temperature 69 K of EuO. Up to ܶ * , the dependence ܴ ௫௬ ሺܤሻ is strongly non-linear suggesting the
Imprinting magnetism into graphene may lead to unconventional electron states and enable the desi... more Imprinting magnetism into graphene may lead to unconventional electron states and enable the design of spin logic devices with low power consumption. The ongoing active development of 2D magnets suggests their coupling with graphene to induce spin‐dependent properties via proximity effects. In particular, the recent discovery of submonolayer 2D magnets on surfaces of industrial semiconductors provides an opportunity to magnetize graphene coupled with silicon. Here, synthesis and characterization of large‐area graphene/Eu/Si(001) heterostructures combining graphene with a submonolayer magnetic superstructure of Eu on silicon are reported. Eu intercalation at the interface of the graphene/Si(001) system results in a Eu superstructure different from those formed on pristine Si in terms of symmetry. The resulting system graphene/Eu/Si(001) exhibits 2D magnetism with the transition temperature controlled by low magnetic fields. Negative magnetoresistance and the anomalous Hall effect in the graphene layer provide evidence for spin polarization of the carriers. Most importantly, the graphene/Eu/Si system seeds a class of graphene heterostructures based on submonolayer magnets aiming at applications in graphene spintronics.
An analysis of the data published on some high-temperature superconducting systems has revealed a... more An analysis of the data published on some high-temperature superconducting systems has revealed a universal dependence of the Neel and critical superconducting transition temperatures on the 2D conductivity of CuO 2 planes in these compounds
High carrier mobility of both electrons and holes is found in nanofilms of layered SrAl2Si2 integ... more High carrier mobility of both electrons and holes is found in nanofilms of layered SrAl2Si2 integrated with silicon. The salient feature of its atomic structure is anionic bilayers [Al2Si2]2−, isostructural and isoelectronic to bilayer silicene.
Recent discoveries of intrinsic two-dimensional (2D) magnets open up vast opportunities to addres... more Recent discoveries of intrinsic two-dimensional (2D) magnets open up vast opportunities to address fundamental problems in condensed matter physics, giving rise to applications from ultra-compact spintronics to quantum computing. The ever-growing material landscape of 2D magnets lacks, however, carbon-based systems, prominent in other areas of 2D research. Magnetization measurements of the Eu/graphene compound—a monolayer of the EuC 6 stoichiometry—reveal the emergence of 2D ferromagnetism but detailed studies of competing magnetic states are still missing. Here, we employ element-selective X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) to establish the magnetic structure of monolayer EuC 6 . The system exhibits the anomalous Hall effect, negative magnetoresistance, and magnetization consistent with a ferromagnetic state but the saturation magnetic moment (about 2.5 µ B /Eu) is way too low for the half-filled f -shells of Eu 2+ ions. Combined XAS/XMCD studies at the Eu L 3 absorption edge probe the EuC 6 magnetism in high fields and reveal the nature of the missing magnetic moments. The results are set against XMCD studies in Eu/silicene and Eu/germanene to establish monolayer EuC 6 as a prominent member of the family of Eu-based 2D magnets combining the celebrated graphene properties with a strong magnetism of europium.
Integration of crystalline oxides with silicon provides a versatile platform to extend and advanc... more Integration of crystalline oxides with silicon provides a versatile platform to extend and advance silicon technology. The interface between oxide and Si controls the structure and functional properties of the resulting material. In particular, the formation of a submonolayer metal phase on silicon is the standard approach to stabilize the epitaxial growth of oxides. However, fundamental questions—a) whether the interface transforms in the process of the synthesis; and b) if it is possible to control the interface and its electronic structure by varying the submonolayer template—remain unanswered. The present study employs MBE synthesis of EuO and SrO on Si(001) to demonstrate that the structure of the oxide/Si interface does not depend on the type of the template, its symmetry, and stoichiometry. Chemical transformations of the templates converging into the same 2D product are detected in situ by electron diffraction. Then, the common interfacial structure of 1D periodicity is visualized by high‐resolution electron microscopy. The study provides insights into the process of oxide integration with silicon but also sets the limits in designing oxide/Si interfaces.
Intrinsic two-dimensional (2D) magnetism has been demonstrated in various materials scaled down t... more Intrinsic two-dimensional (2D) magnetism has been demonstrated in various materials scaled down to a single monolayer. However, the question is whether 2D magnetism extends beyond the monolayer limit, to chemical species formed by sparse but regular 2D arrays of magnetic atoms. Here we show that sub-monolayer superstructures of Eu atoms self-assembled on the silicon surface exhibit strong magnetic signals. Robust easy-plane magnetism is discovered in both one- and two-dimensionally ordered structures with Eu coverage of half monolayer and above. The emergence of 2D magnetism manifests itself by a strong dependence of the effective transition temperature on weak magnetic fields. The results constitute a versatile platform for miniaturization of 2D magnetic systems and seed an expandable class of atomically thin magnets for applications in information technologies.
Abstract 2D magnets hold high promise for unconventional quantum phases, exploration of fundament... more Abstract 2D magnets hold high promise for unconventional quantum phases, exploration of fundamental physics of magnetism, ultra-compact devices based on van der Waals heterostructures. Graphene is a constant inspiration for design of 2D materials; stoichiometric graphene-based compounds with magnetic atoms give rise to 2D ferromagnets. Unfortunately, addition of magnetic species induces high chemical reactivity. Here, we encapsulate a monolayer Eu lattice between graphene monolayers to form a chemically inert van der Waals 2D ferromagnet. The material is produced by Eu intercalation into few-layer graphene, as confirmed by a combination of diffraction and microscopy techniques. The emergence of ferromagnetism is witnessed by magnetization and electron transport measurements. The effective Curie temperature is controlled by weak magnetic fields manifesting the 2D nature of the magnetic state. The results expand the family of 2D magnets and establish a versatile platform for synthesis of its new members by metal intercalation.
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Papers by Oleg Parfenov