Spin transport in topological insulator-based nanostructures

Kristina Vaklinova
EPFL, 2017
Thèse EPFL

Résumé

With the advent of spintronics, significant research effort is made to realize efficient injection, transport, manipulation, and detection of spin. The material platforms and device architectures suitable for this purpose 3D topological insulators (TIs) are a class of Dirac materials, which possess 2D spin-polarized helical surface states due to their strong SO coupling. This property offers research possibilities in several directions, specifically (i) to gain a deeper understanding of the electronic properties of massless Dirac fermions hosted by the surface states; (ii) to exploit the spin-momentum locking of the surface states towards efficient spin-charge conversion; and (iii) to investigate the interplay of the surface states with magnetic materials, superconductors, and 2D materials that could lead to the discovery of new surface and interface phenomena with exciting technological prospects. The main challenge in the field of 3D TIs is posed by the difficulty to selectively access the surface states in electrical transport experiments. This issue arises from the intrinsic bulk doping, which leads to a contribution of the bulk states to the conductivity. Furthermore, while they are prospective as efficient spin current generators, the efficiency of the electrical detection of spin currents is limited due to pronounced spin relaxation and dephasing. This motivates the realization of 3D TIs with improved properties and to engineer 3D TI-2D material heterostructures, whose interfaces would enable efficient spin-charge manipulation. The present thesis aims at the characterization of various Dirac materials by magnetotransport experiments at low temperatures, as well as the nanofabrication and investigation of spintronics devices based on 3D TIs. In the first experimental part, the electronic properties of four different 3D TIs are investigated - Sb2Te3 and Bi2Te2Se, which are grown in a vapor-solid process, ZrTe5, grown as single crystals in a Czochralski process, and finally the naturally occurring mineral Aleksite. In the second part, Bi2Te2Se is chosen for the fabrication of lateral spin valve devices, in which electrical detection of charge current-induced spin polarization due to the spin-momentum locking of the 2D TI surface states is demonstrated. Spin measurements are performed for devices with different coupling strength between the FM detector and the TI channel. Subsequently, the spin signal is investigated for the first time in the presence of a hBN tunnel barrier. An inversion of the spin signal is observed, which depends on the resistance of the hBN/FM/TI interface. The third experimental part addresses the spin generation properties of Bi2Te2Se in a van der Waals TI/graphene heterostructure, in which the TI acts as a spin injector, while the injected spin propagates within the graphene channel underneath and is detected non-locally by a ferromagnetic electrode on top of the graphene. This is a first-time demonstration of spin injection and detection in a heterostructure, which combines the best properties of two Dirac materials, determined by the presence of SO coupling in the TI and the lack thereof in graphene. These results are examples of emergent phenomena on 2D surfaces and interfaces, which could be further investigated in vertical and lateral heterostructures utilizing the spin Hall, Rashba, or Edelstein effect for efficient spin-charge conversion towards all-spin-based information technology.

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https://infoscience.epfl.ch/record/227322?ln=fr

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Kristina Vaklinova
EPFL, 2017
Thèse EPFL

Résumé

Official source

https://infoscience.epfl.ch/record/227322?ln=fr

À propos de ce résultat

Concepts associés (15)

Concepts associés

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Publications associées (9)

Publications associées

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Spin and Charge transport in Dirac Materials

Alexander Ronald Hoyer

The digitalization of our world is proceeding with every new technical product placed on the market. By making them smarter, e.g. linking them together and equipping them with sensors, a vast mass of data is collected. Processing and storing this information is particularly challenging as it should be done in real time and, ideally with a handheld device. This requires logic devices to be very fast, low on power consumption and rather small. The latter requirement is leading towards a transition from the classical physics domain to a domain where quantum effects like tunneling and interference start to dominate the behavior. Thus for miniaturization to continue, it is necessary to adapt new techniques that work in the quantum regime. One promising approach to achieve all of this is to exploit the spin degree of freedom an electron possesses, opening up the field of spin electronics (spintronics). As first principle operation of such devices has been demonstrated with silicon, new materials may possess attributes that better suite the requirements of spintronics devices. Graphene, a two dimensional carbon allotrope, is such a candidate as it shows low spin orbit coupling, which theoretical allows high spin life times to be achieved. Furthermore the 2D nature of graphene allows to control and tailor the electronic and spin properties by external means such as proximity and chemical functionalization. On this basis low temperature magnetotransport measurements are performed to explore the effect of functionalization of graphene with 4-nitrophenyl diazonium tetrafluoroborate within this thesis. Basic measurements in Hall bar geometry show an introduction of disorder due to the functionalization. Additional weak localization, a quantum effect that gives information on the phase coherence length of the electrons, is observed after functionalization and used to explore the electronic properties of the tailored graphene. To address the spin properties of graphene its spin bath needs to be controlled and manipulated. This is done within this thesis by all electrical means and requires the fabrication of spin valve structures on graphene. Two strategies to inject spin polarized currents into graphene are pursued within this framework. First spin valve measurements on functionalized graphene are promising regarding the manipulation of spin properties. The generation of the spin polarized currents is still a big issue. Traditionally, ferromagnets are used as leads to fulfill this task. Latest studies show that topological insulators (TI), a new class of materials, could be better suited to fulfill this task as they give rise to fully spin polarized currents. Conceptually these materials possess a band gap that is crossed by spin polarized bands. Crystals of nanometer size of Tin(II)telluride, a potential TI, are grown in a PVD process and electrically contacted to look for fingerprints of these unusual states. Charge and spin transport experiments show the presence of two types of charge carriers and a high intrinsic p-doping. Finally, a new 3^He system is set up within this thesis and a concept for low noise measurements is implemented that integrates a home build signal generation and amplification stage.

EPFL2016

Chargement

Two-dimensional (2D) materials, in particular graphene and transition metal dichalcogenides (TMDC), have attracted great scientific interest over the last decade, revealing exceptional mechanical, electrical and optical properties. Owing to their layered nature, subnanometer-thick single-layer forms of these crystals can be chemically grown or mechanically isolated, representing an ultimately scaled-down platform for further miniaturization of electronic devices. Indeed, the large family of 2D materials contains hundreds of members, including metals, semiconductors, insulators, and others, covering the whole spectrum of potential applications. Availability of all necessary building blocks for the construction of all-2D solid-state devices makes this platform ideal for fabrication of ultrathin, transparent and flexible devices. Extensively investigated graphene has demonstrated excellent electrical properties. However, the absence of a bandgap is an obstacle for its interaction with light and therefore limits applications in optics. In this aspect, atomically thin TMDC semiconductors appear to be an alternative, more promising platform, as they possess a direct band gap in the visible range in the monolayer form. Despite being only three atoms thick, these semiconductors demonstrate exceptionally large light absorption, efficient light emission, and strong light-matter interaction, attracting justified interest from the optics community. We will focus on their potential applications for optoelectronics in Chapter 4. Even more, broken inversion symmetry and strong spin-orbit coupling in single-layer TMDC crystals reveal a new quantum number, the so-called valley index. Indeed, this unique degree of freedom of charge carriers is known for some materials since the 1970s. However, in the case of 2D semiconductors, spin-valley locking mechanism and valley contrasting optical selection rules open new ways for addressing, manipulation and sensing of this pseudospin, opening the whole new field of valleytronics. Due to the large splitting of the valence band, spin and valley degree of freedom become locked in these atomically thin materials. We employ this unique feature in Chapter 5 for indirect injection of spins into graphene by pumping valley polarized carriers in an adjacent TMDC monolayer. Being gapless, graphene does not allow direct optical injection of spins. Another striking feature of 2D materials is their unique ability to be stacked in vertical heterostructures with strong electrical coupling between layers. The weak van der Waals force, which keeps components together, provides freedom in the assembly process, so that a lattice mismatch or a twist angle becomes unimportant in the first approximation. This provides a novel platform for harvesting complementary properties of the constituent materials, allowing the engineering of new artificial meta-materials as we demonstrate in Chapter 6. Furthermore, synergistic effects in van der Waals heterostructures enable completely new properties and phenomena, which do not exist in the single materials, with twist angle being an important knob for tuning these effects. We observe and exploit such novel phenomena arising in heterostructures of 2D semiconductors in the last chapter of this thesis. On the way to the realization of practical optoelectronic and valleytronic applications, this thesis studies fundamental aspects of rich spin-valley physics of atomically thin semiconductors

EPFL2019

Chargement

Charge transport in proximitized graphene within van der Waals heterostructures

Katharina Polyudov

Since their discovery, graphene and other 2D materials have become a subject of intense research in condensed matter physics. Especially the vast possibilities of combining those materials into heterostructures are promising for the discovery of novel physical phenomena. The heterostructures accessible through state-of-the-art techniques have suitably clean interfaces between the layers. Graphene has gained a lot of attention due to its remarkable mechanical stability and extremely high electron mobility. However, the zero bandgap of graphene is a major bottleneck for implementing this 2D material into most electronic applications. The ability to tune the properties of graphene by proximity effects has shifted the focus of graphene research towards the combination of graphene with other van der Waals materials. The main objective of the present thesis was to explore for two different types of graphene-based van der Waals heterostructure whether fingerprints of electronic proximity effects can be traced in their low temperature magnetotransport properties. The first part of the thesis deals with heterostructures of graphene and Bi2Te2Se (BTS). The strong spin-orbit coupling of BTS makes it a three-dimensional topological insulator with topological surface states which are protected by time-reversal symmetry. At the same time, BTS is promising to exert a pronounced spin-orbit proximity effect on graphene, and thereby to open a band gap and/or introduce a spin texture in the latter. The graphene/BTS heterostructures were fabricated by the direct growth of BTS on graphene to ensure a clean interface between both materials and correspondingly a good electronic coupling between them. Analysis of the weak localization effect observed in the magnetoconductivity revealed the presence of enhanced SOC in the proximitized graphene. The second part of the thesis focuses on heterostructures wherein graphene is combined with a-RuCl3 which has recently gained a lot of attention as a potential quantum spin liquid system. Previous studies on graphene/a-RuCl3 heterostructures found an unusual temperature evolution of the quantum oscillation amplitude, whose origin remained unclear. The magnetotransport data collected in this thesis point toward two possible origins for this behavior, namely spin fluctuations associated with the magnetic transition into an antiferromagnetic phase at the Néel temperature of approximately 7 K, and the hybridization-induced formation of heavy (flat) bands, both of which are likely to depend on the a-RuCl3 layer thickness. In addition, heterostructures comprising an a-RuCl3 monolayer were found to display a unusual gate dependence of the quantum oscillations, further corroborating the importance of the a-RuCl3 thickness.

EPFL2020

Chargement

Charge transport in proximitized graphene within van der Waals heterostructures

Katharina Polyudov

EPFL2020

EPFL2019

Spin and Charge transport in Dirac Materials

Alexander Ronald Hoyer

EPFL2016