Spin-orbit proximity in van der Waals heterostructures

165 p.In this thesis, graphene/transition metal dichalcogenides van der Waals hetero-structures were used to develop a device that combines Hall probes with ferromagnetic electrodes. With it, the spin Hall effect in graphene induced by spin-orbit coupling proximity with MoS2 and WSe2 could unambiguously be demonstrated. The Hanle precession of the non-local resistance not only gives convincing experimental proof but also allows the quantification of the spin transport and the spin-to-charge conversion. The fact that both occur in different parts of the same material gives rise to a high efficiency for the voltage output up to room temperature. Additionally, the control by applying a gate voltage was shown in graphene proximitized with WSe2, enabling a record efficiency measured of around 40 nm. Additionally, in a graphene/WSe2 lateral spin valve, coherent, electrically controllable spin precession in the absence of an external magnetic field was achieved, even in the diffusive regime

Electronic and spintronic devices using two-dimensional materials

179 p. El contenido del capítulo 8 está sujeto a confidencialidadEver since in 2004 atomically-thin two-dimensional van der Waals materials became available to the scientific community, at the reach of manual microexfoliation techniques, their implementation in novel device structures and concepts promised disruptive new applications and motivated research in a vast range of fields.Confined to the thinnest possible thickness, electrons in these materials exhibit a plethora of electronic properties, from semiconducting MoS2, to superconductor NbSe2, dielectric BN, and, jack-of-all trades, graphene.In this thesis, we explore fundamental and applied aspects of chemical vapor deposition (CVD) graphene, MoS2, and WSe2 using electronic device structures that use them as transporting channel, namely field-effect transistors (FETs), Hall bars, and diodes.MoS2 is a n-type semiconducting 2D vdW that complements one of the weak aspects of graphene-based transistors, which is the small ratio between the maximum current output and of the minimum current output of the transistors. Using MoS2 we identify an electron doping constraint for performing stable magnetotransport measurements, and we investigate the origins of the strong current fluctuations of the FETs. We study the low-frequency noise (LFN) of the current output of devices made with different layer thicknesses, and use the strong light-matter interactions of MoS2 to employ photodoping techniques together with the electrostatic gating to dope the channel. By converging all these conditions, we are able to discern the mechanism behind the different types of LFN noise reported in literature for MoS2, while at the same time identifying a LFN crossover driven by photodoping.With p-type semiconducting WSe2 we optimize the electron and hole transport properties of ambipolar FETs by considering BN as a top and bottom interface substrate and encapsulation layer, respectively. By doing so, we areable to address to some extent the strong hysteretic effects that adversely affect the operation of WSe2 FETs on oxide substrates, and improve the overall device performance.The versatility of CVD graphene allows us to do both applied and fundamental studies, both related to spintronics and electronics.The unique properties of graphene make it a core material in the search of full-electrical approaches to generate, transport, and detect spin currents without the use of magnetic elements. Using a Hall-bar shaped sample, non-local signals in graphene have been demonstrated to be associated with spin transport. In our case, we use the large area availability of CVD graphene to study non-local effects in an unlikely scenario for the transport of spins. We study the non-local signals of millimeter sized Hall-bars of CVD graphene, and by doing a systematic study as a function of device scale, from macro-to-microscale we identify a mechanism that cannot be connected with spin diffusion that also leads to large signals. By evaluating the microscopic details of the samples, and the different effects observed, we propose a mechanism mediated by grain boundaries to drive such effects.In a more applied manner, we use CVD graphene for two other types of devices. First, we study the use of graphene as an electrode material for lateral and vertical field-effect transistors that operate using organic channels, and determine that the low density of states of graphene allows for unscreened electric fields to reach the organic layer and enable the transistor operation in the vertical geometry.The second applied study is the large-scale fabrication of diodes using CVD graphene. Benefiting from the ultra-thin cross section of graphene, and using a lateral geometry we demonstrate the reliable fabrication of lateral metal/insulator/graphene diodes. The time constants determined from the direct-current analysis place the operation of the fabricated devices in the THz range. Additionally, the material combination considered enabled large current densities based on field-emission processes.CICnanoGUNE : nanoscience cooperative research cente

Subcycle terahertz nanoscopy of ultrafast interlayer dynamics in van der Waals heterostructures

Tunneling is one of the most fundamental manifestations of quantum mechanics determining elementary physical processes, chemical reaction pathways and shaping life as we know it. Moreover, it is crucial for modern data storage and electronics, and is essential for highly efficient solar technology. In this work, we introduce a novel, non-invasive concept to resolve electron tunneling on the relevant length- and timescales that even works on insulating samples. The central idea is to monitor the evolution of the local polarizability of electron-hole pairs during the tunneling process with evanescent terahertz nearfields, which are detected with subcycle temporal resolution. In a proof of concept, we resolve the ultrafast interlayer dynamics in van der Waals heterobilayers utilizing our new technique of subcycle contact-free nanoscopy to access the full life cycle of photo-induced spatially separated interlayer electron-hole pairs. Our approach builds on the drastic change of the polarizability of the electron-hole pairs during interlayer tunneling as explained by ab initio density functional theory calculations. We confirm the temporal dynamics using a complementary terahertz emission experiment that is directly linked to the ultrafast charge separation. Moreover, we reveal pronounced variations of the local formation and annihilation of interlayer excitons on deeply subwavelength, nanometer lengthscales. Such contact-free nanoscopy of tunneling-induced dynamics should be universally applicable to conducting and non-conducting samples and reveal how ultrafast transport processes shape functionalities in a wide range of condensed matter systems

Electronic and spintronic devices using two-dimensional materials

179 p. El contenido del capítulo 8 está sujeto a confidencialidadEver since in 2004 atomically-thin two-dimensional van der Waals materials became available to the scientific community, at the reach of manual microexfoliation techniques, their implementation in novel device structures and concepts promised disruptive new applications and motivated research in a vast range of fields.Confined to the thinnest possible thickness, electrons in these materials exhibit a plethora of electronic properties, from semiconducting MoS2, to superconductor NbSe2, dielectric BN, and, jack-of-all trades, graphene.In this thesis, we explore fundamental and applied aspects of chemical vapor deposition (CVD) graphene, MoS2, and WSe2 using electronic device structures that use them as transporting channel, namely field-effect transistors (FETs), Hall bars, and diodes.MoS2 is a n-type semiconducting 2D vdW that complements one of the weak aspects of graphene-based transistors, which is the small ratio between the maximum current output and of the minimum current output of the transistors. Using MoS2 we identify an electron doping constraint for performing stable magnetotransport measurements, and we investigate the origins of the strong current fluctuations of the FETs. We study the low-frequency noise (LFN) of the current output of devices made with different layer thicknesses, and use the strong light-matter interactions of MoS2 to employ photodoping techniques together with the electrostatic gating to dope the channel. By converging all these conditions, we are able to discern the mechanism behind the different types of LFN noise reported in literature for MoS2, while at the same time identifying a LFN crossover driven by photodoping.With p-type semiconducting WSe2 we optimize the electron and hole transport properties of ambipolar FETs by considering BN as a top and bottom interface substrate and encapsulation layer, respectively. By doing so, we areable to address to some extent the strong hysteretic effects that adversely affect the operation of WSe2 FETs on oxide substrates, and improve the overall device performance.The versatility of CVD graphene allows us to do both applied and fundamental studies, both related to spintronics and electronics.The unique properties of graphene make it a core material in the search of full-electrical approaches to generate, transport, and detect spin currents without the use of magnetic elements. Using a Hall-bar shaped sample, non-local signals in graphene have been demonstrated to be associated with spin transport. In our case, we use the large area availability of CVD graphene to study non-local effects in an unlikely scenario for the transport of spins. We study the non-local signals of millimeter sized Hall-bars of CVD graphene, and by doing a systematic study as a function of device scale, from macro-to-microscale we identify a mechanism that cannot be connected with spin diffusion that also leads to large signals. By evaluating the microscopic details of the samples, and the different effects observed, we propose a mechanism mediated by grain boundaries to drive such effects.In a more applied manner, we use CVD graphene for two other types of devices. First, we study the use of graphene as an electrode material for lateral and vertical field-effect transistors that operate using organic channels, and determine that the low density of states of graphene allows for unscreened electric fields to reach the organic layer and enable the transistor operation in the vertical geometry.The second applied study is the large-scale fabrication of diodes using CVD graphene. Benefiting from the ultra-thin cross section of graphene, and using a lateral geometry we demonstrate the reliable fabrication of lateral metal/insulator/graphene diodes. The time constants determined from the direct-current analysis place the operation of the fabricated devices in the THz range. Additionally, the material combination considered enabled large current densities based on field-emission processes.CICnanoGUNE : nanoscience cooperative research cente

Terahertz Spintronics with Antiferromagnetic Insulators

The existence of the THz gap in the electromagnetic spectrum is not only preventing the advancement of several technologies but also hindering research and developmental activities due to a lack of research facilities operating in the gap region. There is a plethora of materials with dynamics lying in the THz gap region whose study could potentially lead to the development of new technologies for the generation, detection, and processing of THz signals. Antiferromagnets are gaining recent interest due to their high frequency dynamics lying in the THz region, and their potential uses as active elements in THz spintronics devices have been suggested. This dissertation focuses on the study of insulating antiferromagnets for their potential use in future THz spintronic devices. The first chapter is the introductory one. In the second chapter we focus on the development of a state-of-the-art continuous polarization tunable quasi-optical measurement system operating in the frequency range 220GHz-1.1THz, at temperatures 5K-300K and a maximum magnetic field of up to 9T. The operation of this custom designed system is discussed, and initial results from the test measurements on MnF2 single crystals verify its capabilities. In the third chapter we discuss results from a detailed spectroscopic study performed on two stoichiometric compounds of a novel two-dimensional antiferromagnetic insulator from the MnBi2Te4(Bi2Te3)n family with n=1 and 2. The motion of the antiferromagnetic modes with the direction of applied magnetic field reveal the anisotropic nature of the system, characterized by an easy magnetic symmetry axis and a corrugated hard plane that change slightly with the stoichiometry variation. Our results show how the transition temperature also varies between n=1 and n=2 compounds, indicating that the exchange interaction originating from the antiferromagnetic order changes with the interlayer configuration. In the last chapter we discuss our results on single electron transistor measurements where we realize a stable 1- and 2-input single molecule logic gates

第一原理計算と角度分解光電子分光によるCr₂O₃超薄膜およびCrTe₂薄膜の研究

広島大学(Hiroshima University)博士(理学)Doctor of Sciencedoctora

Quantum magnonics: when magnon spintronics meets quantum information science

Spintronics and quantum information science are two promising candidates for innovating information processing technologies. The combination of these two fields enables us to build solid-state platforms for studying quantum phenomena and for realizing multi-functional quantum tasks. For a long time, however, the intersection of these two fields was limited. This situation has changed significantly over the last few years because of the remarkable progress in coding and processing information using magnons. On the other hand, significant advances in understanding the entanglement of quasi-particles and in designing high-quality qubits and photonic cavities for quantum information processing provide physical platforms to integrate magnons with quantum systems. From these endeavours, the highly interdisciplinary field of quantum magnonics emerges, which combines spintronics, quantum optics and quantum information science.Here, we give an overview of the recent developments concerning the quantum states of magnons and their hybridization with mature quantum platforms. First, we review the basic concepts of magnons and quantum entanglement and discuss the generation and manipulation of quantum states of magnons, such as single-magnon states, squeezed states and quantum many-body states including Bose-Einstein condensation and the resulting spin superfluidity. We discuss how magnonic systems can be integrated and entangled with quantum platforms including cavity photons, superconducting qubits, nitrogen-vacancy centers, and phonons for coherent information transfer and collaborative information processing. The implications of these hybrid quantum systems for non-Hermitian physics and parity-time symmetry are highlighted, together with applications in quantum memories and high-precision measurements. Finally, we present an outlook on the opportunities in quantum magnonics.Comment: 93 pages, 35 figures, Physics Reports (in press

Gate-defined superconducting nanostructures in bilayer graphene weak links

State-of-the-art edge-connected graphene/hexagonal boron nitride van der Waals heterostructures provide low contact resistivity, high charge carrier mobilities as well as a large mean free path. In combination with their high device geometry flexibility they appear thus to be predestined for realizing high-quality tunable weak links in Josephson junctions, which can be readily implemented into superconducting circuits for quantum technological applications. However, designing gate-controlled nanostructures in monolayer graphene remains a serious challenge due to its lack of a band gap which hinders the confinement of charge carriers. The present thesis aims to address this shortcoming by establishing bilayer graphene as a suitable alternative. Unlike the single-layer relative, bilayer graphene offers the opportunity to open an electronic band gap by breaking the layer symmetry which is possible with the ease of exposing electric displacement fields across the two layers. In this regard, employing the combination of locally defined back and top gate architectures allows to design electrostatically induced nanostructures based on spatial band structure engineering. In this thesis, at first the realization of a gate-tunable charge carrier confinement is presented. The formation of the constriction is demonstrated by means of superconducting magneto-interferometry measurements. Building on the successfully induced electrostatic confinement and in combination with a more sophisticated double top gate structure, a fully operable quantum point contact is implemented within the bilayer graphene weak link. When the junction is measured in the normal state, quantized conductance is observed due to the formation of one-dimensional subbands. Though, unlike in other material systems we here explore the complexity of the degeneracy of spin, valley and unusual mini-valley quantum degrees of freedom. In final measurements, the quantum point contact is probed in the superconducting state. The measured critical current through the junction displays a discrete variation directly correlated to the quantized steps in the normal state conductance. These results pave the way towards the study of individual Andreev bound levels through this superconducting quantum point contact. In conclusion, the presented work demonstrates the implementation of electrostatically tunable superconducting nanostructures in bilayer graphene weak links which serves as a platform for the design of more complex electronic circuits