Role of hexagonal boron nitride in protecting ferromagnetic nanostructures from oxidation

Ferromagnetic contacts are widely used to inject spin polarized currents into non-magnetic materials such as semiconductors or 2-dimensional materials like graphene. In these systems, oxidation of the ferromagnetic materials poses an intrinsic limitation on device performance. Here we investigate the role of ex-situ transferred chemical vapour deposited hexagonal boron nitride (hBN) as an oxidation barrier for nanostructured cobalt and permalloy electrodes. The chemical state of the ferromagnets was investigated using X-ray photoemission electron microscopy owing to its high sensitivity and lateral resolution. We have compared the oxide thickness formed on ferromagnetic nanostructures covered by hBN to uncovered reference structures. Our results show that hBN reduces the oxidation rate of ferromagnetic nanostructures suggesting that it could be used as an ultra-thin protection layer in future spintronic devices.Comment: 7 pages, 6 figure

Large-scale BN tunnel barriers for graphene spintronics

We have fabricated graphene spin-valve devices utilizing scalable materials made from chemical vapor deposition (CVD). Both the spin-transporting graphene and the tunnel barrier material are CVD-grown. The tunnel barrier is realized by h-BN, used either as a monolayer or bilayer and placed over the graphene. Spin transport experiments were performed using ferromagnetic contacts deposited onto the barrier. We find that spin injection is still greatly suppressed in devices with a monolayer tunneling barrier due to resistance mismatch. This is, however, not the case for devices with bilayer barriers. For those devices, a spin relaxation time of 260 ps intrinsic to the CVD graphene material is deduced. This time scale is comparable to those reported for exfoliated graphene, suggesting that this CVD approach is promising for spintronic applications which require scalable materials.Comment: 13 pages, 3 figure

Elektron transzport molekuláris nanostruktúrákban = Electron transport in molecular nanostructures

Az OTKA K76010 kutatói pályázat keretében nanokontaktusok viselkedését vizsgáltuk, melyekben az áram egyetlen atomon vagy egyetlen molekulán keresztül folyik. A kutatás keretében kifejlesztettünk egy molekulák kontrollált adagolására alkalmas mérőrendszert, mely egyedi molekulák kontaktálását teszi lehetővé folyékony hélium hőmérsékleten. Szupravezető subgap módszerrel karakterizáltunk különböző anyagból készült atomi méretű kontaktusokat és elméleti csoportokkal együttműködve részt vettünk egy vezetőképesség hisztogramok szimulálására alkalmas módszer fejlesztésében, melynek megbízhatóságát a subgap módszer segítségével egyedi vezetési csatornák szintjén ellenőriztük. Kifejlesztettünk egy új statisztikai módszert, melynek segítségével atomi és molekuláris kontaktusokban kialakuló konfigurációk közötti korrelációk vizsgálhatók. Ezen módszer segítségével számos atomi és molekuláris nanovezeték viselkedését sikerült a szokásos vezetőképesség hisztogram technikánál részletesebben feltérképezni. | Within the OTKA K76010 project nano-scale junctions were investigated, in which the current flows along single atoms or molecules. A novel measurement setup was developed which is capable of a controlled dosing of molecules to a liquid helium temperature break junction device so that single molecules can be contacted in a well controlled manner. With the help of superconducting subgap structure measurements various metal atomic sized contacts were characterized. In collaboration with theory groups a novel method was developed to simulate conductance histograms. With the help of subgap structure measurements the reliability of this method was tested on the level of individual channel transmission eigenvalues. Beside, we have introduced a novel statistical analysis method based on the two-dimensional cross-correlation histogram analysis of conductance traces. This method provides new information about the relation of different junction configurations that occur during the formation and evolution of metal and single-molecule junctions. The correlation method was successfully applied to study the behavior a several atomic and single-molecule structures beyond conductance histograms

Breaking the quantum PIN code of atomic synapses

Atomic synapses represent a special class of memristors whose operation relies on the formation of metallic nanofilaments bridging two electrodes across an insulator. Due to the magnifying effect of this narrowest cross-section on the device conductance, a nanometer scale displacement of a few atoms grants access to various resistive states at ultimately low energy costs, satisfying the fundamental requirements of neuromorphic computing hardware. Yet, device engineering lacks the complete quantum characterization of such filamentary conductance. Here we analyze multiple Andreev reflection processes emerging at the filament terminals when superconducting electrodes are utilized. Thereby the quantum PIN code, i.e. the transmission probabilities of each individual conduction channel contributing to the conductance of the nanojunctions is revealed. Our measurements on Nb$_2$O$_5$ resistive switching junctions provide a profound experimental evidence that the onset of the high conductance ON state is manifested via the formation of truly atomic-sized metallic filaments

Bevezetés a hegeli filozófiába = Introduction to Hegel`s Philosophy

Az eredmények 2 csoportba sorolhatók: 1. a projekt lezárt részei, 2. a közvetlenül befejezés előtt álló részek, ill. további munkálatokat és támogatást igénylő részek. 1. A projekt lezárt részei: 1.1. Az aktuális Hegel. Tanulmánykötet. Szerk. Rózsa Erzsébet és M. Quante. Előszó: H. Lenk és Fehér M. István. Fordította: Makk Norbert, Drimál István, Bóta Péter, Novák Zoltán. Kossuth Egyetemi Kiadó Debrecen 2003. 1.2. Erzsébet Rózsa: Versöhnung und System. Zu Grundmotiven von Hegels praktischer Philosophie. Fink Verlag 2005. Monographie. (Ennek a könyvnek a bemutatójára 2006 májusában kerül sor a berlini Collegium Hungaricumban, a Magyar Évad keretében.) 1.3. Erzsébet Rózsa: Individualität und Soziabilität. Studien zu Hegels Auffassung des modernen Individuums. Megjelenés előtt: Mentis Verlag, Paderborn. 2.A projekt közvetlenül befejezés előtt álló részei: 2.1. Hegel: 1826-os művészetfilozófiai előadások. Ford. Zoltai Dénes. Kontrollfordítás szükséges, ill. a nyomdai költségek biztosítása. 2.2. Hegel: Válogatás a magyarul még nem publikált ifjúkori írásokból. Szerk. Rózsa Erzsébet és Ch. Jamme. Ford. Andrejka Zoltán, Bóta Péter, Drimál István, Makk Norbert. Nyersfordítás, ami még sok további munkát igényel. Ezen kívül a kiadási költségek finanszírozását is biztosítani kell. 2.3. Rózsa Erzsébet: A modern individuum drámája. Hegel-tanulmányok. Ford. Makk Norbert. A kötet megjelentetési költségeit elő kell teremteni. | The achievements of the project can be divided into two groups: 1. closed parts of the project, 2. parts almost closed, or parts that require additional elaboration and support. 1. Closed parts: 1.1. Az aktuális Hegel. Essays. Eds. Rózsa Erzsébet and M. Quante. Preface: H. Lenk and Fehér M. István. Transl.: Makk Norbert, Drimál István, Bóta Péter, Novák Zoltán. Kossuth Egyetemi Kiadó, Debrecen 2003. 1.2. Erzsébet Rózsa: Versöhnung und System. Zu Grundmotiven von Hegels praktischer Philosophie. Fink Verlag 2005. Monography. (The book will be presented in May 2006, in the Collegium Hungaricum of Berlin, as part of the Hungarian Season.) 1.3. Erzsébet Rózsa: Individualität und Soziabilität. Studien zu Hegels Auffassung des modernen Individuums. Mentis Verlag, Paderborn. To be published soon. 2. Parts almost closed: 2.1. Hegel: 1826-os művészetfilozófiai előadások. Transl. Zoltai Dénes. Supervisory translation and publishing costs are needed. 2.2. Hegel: Válogatás a magyarul még nem publikált ifjúkori írásokból. Eds. Rózsa Erzsébet and Ch. Jamme. Transl. Andrejka Zoltán, Bóta Péter, Drimál István, Makk Norbert. Raw translation that requires further elaboration. Besides, the costs of the publication should be financed. 2.3. Rózsa Erzsébet: A modern individuum drámája. Hegel-tanulmányok. Transl. Makk Norbert. The book's publishing costs should be raised

In-situ strain tuning in hBN-encapsulated graphene electronic devices

Using a simple setup to bend a flexible substrate, we demonstrate deterministic and reproducible in-situ strain tuning of graphene electronic devices. Central to this method is the full hBN encapsulation of graphene, which preserves the exceptional quality of pristine graphene for transport experiments. In addition, the on-substrate approach allows one to exploit strain effects in the full range of possible sample geometries and at the same time guarantees that changes in the gate capacitance remain negligible during the deformation process. We use Raman spectroscopy to spatially map the strain magnitude in devices with two different geometries and demonstrate the possibility to engineer a strain gradient, which is relevant for accessing the valley degree of freedom with pseudo-magnetic fields. Comparing the transport characteristics of a suspended device with those of an on-substrate device, we demonstrate that our new approach does not suffer from the ambiguities encountered in suspended devices

Snake Trajectories in Ultraclean Graphene p-n Junctions

Snake states are trajectories of charge carriers curving back and forth along an interface. There are two types of snake states, formed by either inverting the magnetic field direction or the charge carrier type at an interface. Whereas the former has been demonstrated in GaAs-AlGaAs heterostructures, the latter has become conceivable only with the advance of ballistic graphene where a gapless p-n interface governed by Klein tunneling can be formed. Such snake states were hidden in previous experiments due to limited sample quality. Here we report on magneto-conductance oscillations due to snake states in a ballistic suspended graphene p-n-junction which occur already at a very small magnetic field of 20mT. The visibility of 30% is enabled by Klein collimation. Our finding is firmly supported by quantum transport simulations. We demonstrate the high tunability of the device and operate it in different magnetic field regimesComment: Accepted for publication in Nature Communication

Strong nonlocal tuning of the current-phase relation of a quantum dot based Andreev molecule

Multiple systems hosting Andreev molecular states have been proposed and studied, consisting of closely spaced Josephson junctions modeled as ballistic channels. We show that replacing the ballistic channels in the weak link of the Josephson junctions with quantum dots (QD), leads to a very exciting, rich phase diagram. It shows a strong nonlocal Josephson effect: as one junction is tuned the current-phase relation of the other junction is modified. This architecture hosts $0 - \pi$ transitions and shows a tunable anomalous phase-shift $\phi_0$ , nonlocally controlled in both cases, without relying on spin-orbit interaction or Zeeman fields. In addition significant superconducting diode effect can also be observed. The presented non-local current-phase relation can be used as a signature of the formation of an Andreev molecular state, as well as to introduce new ways to tune quantum architectures