Relativistic Artificial Molecules Realized by Two Coupled Graphene Quantum Dots

Coupled quantum dots (QDs), usually referred to as artificial molecules, are important not only in exploring fundamental physics of coupled quantum objects, but also in realizing advanced QD devices. However, previous studies have been limited to artificial molecules with nonrelativistic fermions. Here, we show that relativistic artificial molecules can be realized when two circular graphene QDs are coupled to each other. Using scanning tunneling microscopy (STM) and spectroscopy (STS), we observe the formation of bonding and antibonding states of the relativistic artificial molecule and directly visualize these states of the two coupled graphene QDs. The formation of the relativistic molecular states strongly alters distributions of massless Dirac fermions confined in the graphene QDs. Because of the relativistic nature of the molecular states, our experiment demonstrates that the degeneracy of different angular-momentum states in the relativistic artificial molecule can be further lifted by external magnetic fields. Then, both the bonding and antibonding states are split into two peaks

Ballistic Graphene Cooper Pair Splitter

We report an experimental study of a Cooper pair splitter based on ballistic graphene multiterminal junctions. In a two transverse junction geometry, namely the superconductor-graphene-superconductor and the normal metal-graphene-normal metal, we observe clear signatures of Cooper pair splitting in the local as well as nonlocal electronic transport measurements. Our experimental data can be very well described by our beam splitter model. These results open up possibilities to design new entangled state detection experiments using ballistic Cooper pair splitters

Electronic Properties of Graphene in a Strong Magnetic Field

We review the basic aspects of electrons in graphene (two-dimensional graphite) exposed to a strong perpendicular magnetic field. One of its most salient features is the relativistic quantum Hall effect the observation of which has been the experimental breakthrough in identifying pseudo-relativistic massless charge carriers as the low-energy excitations in graphene. The effect may be understood in terms of Landau quantization for massless Dirac fermions, which is also the theoretical basis for the understanding of more involved phenomena due to electronic interactions. We present the role of electron-electron interactions both in the weak-coupling limit, where the electron-hole excitations are determined by collective modes, and in the strong-coupling regime of partially filled relativistic Landau levels. In the latter limit, exotic ferromagnetic phases and incompressible quantum liquids are expected to be at the origin of recently observed (fractional) quantum Hall states. Furthermore, we discuss briefly the electron-phonon coupling in a strong magnetic field. Although the present review has a dominating theoretical character, a close connection with available experimental observation is intended.Comment: 56 pages, 27 figures; published version with minor corrections and updated reference

Roadmap on quantum nanotechnologies

Quantum phenomena are typically observable at length and time scales smaller than those of our everyday experience, often involving individual particles or excitations. The past few decades have seen a revolution in the ability to structure matter at the nanoscale, and experiments at the single particle level have become commonplace. This has opened wide new avenues for exploring and harnessing quantum mechanical effects in condensed matter. These quantum phenomena, in turn, have the potential to revolutionize the way we communicate, compute and probe the nanoscale world. Here, we review developments in key areas of quantum research in light of the nanotechnologies that enable them, with a view to what the future holds. Materials and devices with nanoscale features are used for quantum metrology and sensing, as building blocks for quantum computing, and as sources and detectors for quantum communication. They enable explorations of quantum behaviour and unconventional states in nano- and opto-mechanical systems, low-dimensional systems, molecular devices, nano-plasmonics, quantum electrodynamics, scanning tunnelling microscopy, and more. This rapidly expanding intersection of nanotechnology and quantum science/technology is mutually beneficial to both fields, laying claim to some of the most exciting scientific leaps of the last decade, with more on the horizon

A first-principles study of bilayer 1T'-WTe2/CrI3: A candidate topological spin filter

The ability to manipulate electronic spin channels in 2D materials is crucial for realizing next-generation spintronics. Spin filters are spintronic components that polarize spins using external electromagnetic fields or intrinsic material properties like magnetism. Recently, topological protection from backscattering has emerged as an enticing feature through which the robustness of 2D spin filters might be enhanced. In this work, we propose and then characterize one of the first 2D topological spin filters: bilayer CrI3/1T'-WTe2. To do so, we use a combination of Density Functional Theory and maximally localized Wannier functions to demonstrate that the bilayer (BL) satisfies the principal criteria for being a topological spin filter; namely that it is gapless, exhibits charge transfer from WTe2 to CrI3 that renders the BL metallic despite the CrI3 retaining its monolayer ferromagnetism, and does not retain the topological character of monolayer 1T'-WTe2. In particular, we observe that the atomic magnetic moments on Cr from DFT are approximately 3.2 mB/Cr in the BL compared to 2.9 mB/Cr with small negative ferromagnetic (FM) moments induced on the W atoms in freestanding monolayer CrI3. Subtracting the charge and spin densities of the constituent monolayers from those of the BL further reveals spin-polarized charge transfer from WTe2 to CrI3. We find that the BL is topologically trivial by showing that its Chern number is zero. Altogether, this evidence indicates that BL 1T'-WTe2/CrI3 is gapless, magnetic, and topologically trivial, meaning that a terraced WTe2/CrI3 BL heterostructure in which only a portion of a WTe2 monolayer is topped with CrI3 is a promising candidate for a 2D topological spin filter. Our results further suggest that 1D chiral edge states may be realized by stacking strongly hybridized FM monolayers, like CrI3, atop 2D nonmagnetic Weyl semimetals like 1T'-WTe2

Relativistic Quantum Chaos

Generations of Ph.D. students at Arizona State University and later at Lanzhou University were involved in research on relativistic quantum chaos. They are: Dr. Ryan Yang, Dr. Xuan Ni, Dr. Guanglei Wang, Dr. Lei Ying, Dr. Rui Bao, Mr. Pei Yu, Mr. Ziyuan Li and Mr. Chengzhen Wang. We thank them. We would also like to thank Dr. Arje Nachman at the Air Force Office of Scientific Research and Dr. Michael Shlesinger from the Office of Naval Research for their great support over the years - without which the works on relativistic quantum chaos would not have been possible. YCL is currently supported by the Pentagon Vannevar Bush Faculty Fellowship program sponsored by the Basic Research Office of the Assistant Secretary of Defense for Research and Engineering and funded by the Office of Naval Research through Grant No. N00014-16-1-2828. LH was supported by NNSF of China under Grants No. 11422541 and No. 11775101.Peer reviewedPostprin

Neue Wege zur Erzeugung und Detektion nichtlokaler Cooperpaare in Festkörpernanostrukturen

New strategies to generate and to detect spin-entangled pairs of electrons in mesoscopic solid-state heterostructures are proposed. A bilayer-graphene device is designed in which Cooper pairs from an s-wave superconductor are injected into an electrostatically-defined topological channel. Due to a particular band structure, two electrons of a Cooper pair propagate in opposite directions and are hence spatially separated if atomic-scale defects are sparse. Neither energy filtering nor Coulomb repulsion are required. The device can be interpreted as a normal/superconducting/normal junction, in which emission of nonlocal singlets is equivalent to local Andreev reflection, in contrast to the widespread identification of Cooper pair splitting with crossed Andreev reflection. To detect entanglement, the Josephson current through two parallel single-level quantum dots is investigated at all occupations. Combining exact diagonalization and perturbation theory, signatures of nonlocal Cooper pair transport are identified in the critical current, a macroscopic quantity. The model reproduces recent experimental observations and predicts a nonlocal triplet ground state on the quantum dots due to a tunable superconductor-mediated exchange coupling, which, too, is visible in the critical current exactly if nonlocal Cooper pairs are transported. Entanglement detection is pursued, too, by converting spin-entangled Cooper pairs into polarization-entangled photons, which are subsequently probed by a Bell-type measurement. A closed emission cycle is constructed, such that no correlations between the photons and the electrons remain, which are detrimental for entanglement transfer. Inevitable side channels and imperfections are identified and accounted for by a particular measurement protocol. Furthermore, it is discussed how spin-charge separation can be used to extract signatures of nonlocal spin singlets from the average current through an electronic beam splitter. This complements a known mechanism based on exchange statistics by which only the more difficult-to-access noise is sensitive for entanglement. The device can be realized by two crossed nanowires or by quantum-Hall edge states in Corbino geometry. The thesis also contains a review on entanglement and on recent theoretical and experimental progress in solid-state systems towards entanglement detection and generation, focusing on Cooper pair splitting.Neue Strategien um spinverschränkte Elektronenpaare in mesoskopischen Festkörperheterostrukturen zu erzeugen und nachzuweisen werden vorgeschlagen. Eine Bilagengraphenstruktur wird entworfen, in der Cooperpaare von einem s-Wellensupraleiter in einen elektrisch definierten topologischen Kanal injiziert werden. Durch die spezielle Bandstruktur bewegen sich zwei Cooperpaarelektronen in entgegengesetzte Richtungen und werden räumlich getrennt, wenn die atomare Störstellendichte gering ist. Weder Energiefilter noch Coulombabstoßung werden benötigt. Die Struktur kann als normal/supraleitend/normal-Übergang interpretiert werden, in dem nichtlokale Singulettemission äquivalent zu lokaler Andreevreflexion ist, im Gegensatz zur üblichen Identifikation mit gekreuzter Andreevreflexion. Zur Verschränkungsdetektion wird der Josephsonstrom durch zwei parallele Einzustandquantenpunkte bei allen Besetzungen untersucht. Durch exakte Diagonalisierung und Störungstheorie werden Signaturen nichtlokalen Cooperpaartransports im kritischen Strom, einer makroskopischen Größe, identifiziert. Das Modell reproduziert aktuelle experimentelle Beobachtungen und sagt einen nichtlokalen Quantenpunkttriplettgrundzustand durch eine supraleitervermittelte Austauschkopplung vorher, der im kritischen Strom sichtbar ist, genau wenn nichtlokale Cooperpaare transportiert werden. Verschränkungsdetektion wird auch verfolgt, indem spinverschränkte Cooperpaare in polarisationsverschränkte Photonen konvertiert werden, die mit einer Bell-artigen Messung untersucht werden. Ein geschlossener Emissionszyklus wird konstruiert, sodass keine Korrelationen zwischen Photonen und Elektronen verbleiben, die die Verschränkungsübertragung behindern. Unabdingbare Nebeneffekte werden identifiziert und durch ein spezielles Messprotokoll behoben. Desweiteren wird diskutiert, wie Spin-Ladungstrennung verwendet werden kann, um Signaturen nichtlokaler Spinsinguletts aus dem mittleren Strom durch einen elektronischen Strahlteiler zu extrahieren. Dies ergänzt einen bekannten Mechanismus, bei dem, mithilfe der Austauschstatistik, nur das schwerer zugängliche Rauschen sensitiv für Verschränkung ist. Die Struktur ist durch gekreuzte Nanodrähte oder Quantenhallrandkanäle in Corbinogeometrie realisierbar. Die Dissertation enthält auch eine Zusammenfassung über Verschränkung und aktuelle theoretische und experimentelle Fortschritte in Festkörpersystemen bezüglich Verschränkungsdetektion und -erzeugung, insb. Cooperpaarspaltung