Nanovezetékeken alapuló áramkörök gyártása és kvantum effektusainak elektron transzport vizsgálata = Fabrication and Electron Transport Study of Nanowire based Quantum Devices

Az anyagtudomány fejlődésének köszönhetően számos nanoméretű objektum létrehozására nyílik mód napjainkban, ilyenek például a félvezető nanopálcák (NW). Kis méretük miatt ezek a NW-k nagyon ígéretesek kvantum elektronikai célokra. Kvantum elektronika a modern szilárdtestfizika dinamikusan fejlődő területe, melynek fő célja elektromos áramkörök kvantum mechanikai szabadsági fokainak kontrollálása és kiolvasása, mint amilyen egy mesterséges atomba (QD) zárt elektron spinje. Jelen projektben nanoméretű áramkörök készítésére és kvantum effektusainak alacsony hőmérsékleti vizsgálatára alkalmas infrastruktúrát építettünk ki. Vizsgáltuk InAs NW-kból és szupravezető (S) ill. ferromágneses (F) elektródák kombinációjából készített áramköröket. InAs NW-ban kialakított dupla kvantum dotot szupravezetőhöz csatolva megmutattuk, hogy Cooper- párokat lehet térben szeparálni, ami megnyitja az utat mobil elektronokból álló Einstein Podolsky Rosen párok keltésére és összefonódottságuk tanulmányozására. Másrészről, ha F vezetéket kapcsolunk InAs NW-ban kialakított QD-hoz, akkor a ferromágnesség lokális kicserélődési téret kelt a mesterséges atomon. Megmutattuk, hogy ez a tér erősen függ a QD töltés állapotától. Sőt a tér még előjelet is válthat ugyanazon töltés állapotban, lehetővé téve az alap állapot spinjének megfordítását kapu feszültség segítségével. Megmutattuk, hogy egy ilyen F-QD rendszer hatékony spin áram erősítőként tud funkcionálni. | Recent development in material science allowed the synthesis of various nanoscale objects, like semiconductor nanowires (NW). Due to their small size, these NWs are very attractive for quantum electronic purpose. Quantum electronics is a dynamically progressing field of modern solid state physics, where the central goal is to manipulate and read out quantum mechanical degrees of freedom of circuits, like the spin of an electron trapped into a quantum dot (QD). In the present project the fabrication and transport characterization infrastructure have been established, which allows the production of nanocircuits and exploring their quantum effects in cryogenic measurements. We have investigated various InAs NW based electric circuits, where superconducting (S) and ferromagnetic (F) leads were attached to the wire. Coupling S lead to NW based double QD system, we have demonstrated, that Cooper pairs can be separated, which opens the way for Einstein Podolsky Rosen pair generation and entanglement analysis of mobile electrons. On the other hand attaching F lead to a QD, ferromagnetism penetrates into the dot inducing a local exchange field. We have shown that this exchange field strongly depends on the charge state of the QD. Furthermore it can even change sign for the same state, which allows the spin reversal of the ground state of the QD by gate voltage. We have shown that such an F- quantum dot system act as an efficient spin current amplifier

Spin polarizáció nanoszerkezetekben = Spin polarization in nanostructures

A projekt keretében spin-polarizációs jelenségeket tanulmányoztunk Andrejev- és MOKE-spektroszkópiával. Saját építésű berendezéssel különböző ferromágneses anyagokban maghatároztuk a vezetési elektronok spin-polarizációjának értékét. Új eljárást dolgoztunk ki a spin-diffúziós hossz hőmérsékletfüggésének és telítési értékének meghatározására, működését modellkísérletekben demonstráltuk. A spintronikai alkalmazások szempontjából ígéretes anyagok széles körét tanulmányoztuk MOKE spektroszkópiával. Vizsgáltuk a szupravezető-fém határátmenet mezoszkópikus transzportjelenségeit, és megmutattuk, hogy a diffúzív tartomány fáziskoherens térfogatának növekedésével proximity-szupravezetés jön létre. Részletesen tanulmányoztuk az ionos vezetéstől származó ún. memrisztor jelenséget Ag-S felületi rétegen létrehozott kontaktusokban. A 3-5 nm mérettartományban kialakított kontaktusokban extrém nagy áramsűrűségnél rezisztív kapcsolási jelenséget figyeltünk meg. Elsőként állítottunk elő fémes tulajdonságú ionos nano-kapcsolókat, melyekről megmutattuk, hogy mindkét állapotában közel ballisztikus vezetés valósul meg, és nagysebességű kapcsolásra alkalmas ( <10 ns). A projekt keretében összesen 15 publikáció készült. Nívós nemzetközi folyóiratokban jelent meg 12 publikáció, impakt faktor összegük: 64.485. A fémes Ag-S memrisztor kapcsolókra vonatkozó legújabb eredmények a Material Research Society 2011-es konferenciáján kerültek bemutatásra. | In the framework of the project we studied spin polarization phenomena by Andrejev and MOKE spectroscopies. The degree of spin polarization has been determined in different ferromagnetic materials by applying a home made instrument. A new method has been worked out to determine the spin diffusion length, and the applicability was demonstrated by model experiments. A broad variety of materials promising for spintronic applications were studied by MOKE spectroscopy. The mesoscopic transport phenomena of superconductor-metal interface were investigated and the presence of proximity superconduction have been shown to occur with the increase of the volume of the phase coherent diffusive regions. The memristor effect originating form ionic conduction was investigated in details on contacts prepared at the surface layer of Ag-S thin films. Applying contacts in the 3-5 nm range and extremely large current densities allowed to observe the resistive switching phenomenon. We were the first to prepare ionic nano-switches of metallic character and we have shown that close to ballistic conduction is realized in both states of the switch and it is appropriate for high frequency (<10 ns) switching. In the framework of the project 15 publications were prepared. 12 of them appeared in high level international scientific journals, the comulated impact factor is: 64.485

From Andreev to Majorana bound states in hybrid superconductor-semiconductor nanowires

Electronic excitations above the ground state must overcome an energy gap in superconductors with spatially-homogeneous s-wave pairing. In contrast, inhomogeneous superconductors such as those with magnetic impurities or weak links, or heterojunctions containing normal metals or quantum dots, can host subgap electronic excitations that are generically known as Andreev bound states (ABSs). With the advent of topological superconductivity, a new kind of ABS with exotic qualities, known as Majorana bound state (MBS), has been discovered. We review the main properties of ABSs and MBSs, and the state-of-the-art techniques for their detection. We focus on hybrid superconductor-semiconductor nanowires, possibly coupled to quantum dots, as one of the most flexible and promising experimental platforms. We discuss how the combined effect of spin-orbit coupling and Zeeman field in these wires triggers the transition from ABSs into MBSs. We show theoretical progress beyond minimal models in understanding experiments, including the possibility of different types of robust zero modes that may emerge without a band-topological transition. We examine the role of spatial non-locality, a special property of MBS wavefunctions that, together with non-Abelian braiding, is the key to realizing topological quantum computation.Comment: Review. 23 pages, 8 figures, 1 table. Shareable published version by Springer Nature at https://rdcu.be/b7DWT (free to read but not to download

Broadband microwave spectroscopy of semiconductor nanowire-based Cooper-pair transistors

The Cooper-pair transistor (CPT), a small superconducting island enclosed between two Josephson weak links, is the atomic building block of various superconducting quantum circuits. Utilizing gate-tunable semiconductor channels as weak links, the energy scale associated with the Josephson tunneling can be changed with respect to the charging energy of the island, tuning the extent of its charge fluctuations. Here, we directly demonstrate this control by mapping the energy level structure of a CPT made of an indium arsenide nanowire (NW) with a superconducting aluminum shell. We extract the device parameters based on the exhaustive modeling of the quantum dynamics of the phase-biased nanowire CPT and directly measure the even-odd parity occupation ratio as a function of the device temperature, relevant for superconducting and prospective topological qubits.Comment: Published version. Supplementary Information is available as ancillary file, raw data and calculations can be downloaded from http://doi.org/10.4121/uuid:5d54f11b-6774-4ae4-96cf-25e6a91927e

Observation of the 4π\pi-periodic Josephson effect in indium arsenide nanowires

Quantum computation by non-Abelian Majorana zero modes (MZMs) offers an approach to achieve fault tolerance by encoding quantum information in the non-local charge parity states of semiconductor nanowire networks in the topological superconductor regime. Thus far, experimental studies of MZMs chiefly relied on single electron tunneling measurements which leads to decoherence of the quantum information stored in the MZM. As a next step towards topological quantum computation, charge parity conserving experiments based on the Josephson effect are required, which can also help exclude suggested non-topological origins of the zero bias conductance anomaly. Here we report the direct measurement of the Josephson radiation frequency in InAs nanowires with epitaxial aluminium shells. For the first time, we observe the $4\pi$-periodic Josephson effect above a magnetic field of $\approx 200\,$mT, consistent with the estimated and measured topological phase transition of similar devices.Comment: Published version. Supplementary Information is available as ancillary file, raw data and calculations can be downloaded from http://dx.doi.org/10.4121/uuid:1f936840-5bc2-40ca-8c32-1797c12cacb

Observation of 2e-periodic Supercurrents in Nanowire Single-Cooper-Pair Transistors

Parity control of superconducting islands hosting Majorana zero modes (MZMs) is required to operate topological qubits made from proximitized semiconductor nanowires. We, therefore, study parity effects in hybrid InAs-Al single-Cooper-pair transistors (SCPTs) as a first step. In particular, we investigate the gate-charge supercurrent modulation and observe a consistent 2$e$-periodic pattern indicating a general lack of low-energy subgap states in these nanowires at zero magnetic field. In a parallel magnetic field, an even-odd pattern develops with a gate-charge spacing that oscillates as a function of field demonstrating that the modulation pattern is sensitive to the presence of a single subgap state. In addition, we find that the parity lifetime of the SCPT decreases exponentially with magnetic field as the subgap state approaches zero energy. Our work highlights the important role that intentional quasiparticle traps and superconducting gap engineering would play in topological qubits that require quenching of the island charge dispersion.Comment: 8 pages, 4 figures, supplemental material included as ancillary fil

Parity transitions in the superconducting ground state of hybrid InSb-Al Coulomb islands

The number of electrons in small metallic or semiconducting islands is quantized. When tunnelling is enabled via opaque barriers this number can change by an integer. In superconductors the addition is in units of two electron charges (2e), reflecting that the Cooper pair condensate must have an even parity. This ground state (GS) is foundational for all superconducting qubit devices. Here, we study a hybrid superconducting-semiconducting island and find three typical GS evolutions in a parallel magnetic field: a robust 2e-periodic even-parity GS, a transition to a 2e-periodic odd-parity GS,and a transition from a 2e- to a 1e-periodic GS. The 2e-periodic odd-parity GS persistent in gate-voltage occurs when a spin-resolved subgap state crosses zero energy. For our 1e-periodic GSs we explicitly show the origin being a single zero-energy state gapped from the continuum, i.e. compatible with an Andreev bound states stabilized at zero energy or the presence of Majorana zero modes