Suchergebnisse

arXiv:0906.0185v1 ; Superconducting persistent-current qubits are quantum-coherent artificial atoms with multiple, tunable energy levels. In the presence of large-amplitude harmonic excitation, the qubit state can be driven through one or more of the constituent energy-level avoided crossings. The resulting Landau-Zener- Stückelberg (LZS) transitions mediate a rich array of quantum-coherent phenomena. We review here three experimental works based on LZS transitions: Mach-Zehnder-type interferometry between repeated LZS transitions, microwave-induced cooling, and amplitude spectroscopy. These experiments exhibit a remarkable agreement with theory, and are extensible to other solid-state and atomic qubit modalities. We anticipate they will find application to qubit state-preparation and control methods for quantum information science and technology. © 2009 Springer Science+Business Media, LLC. ; This work was supported by AFOSR and LPS (F49620-01-1-0457) under the DURINT program, and by the U.S. Government. The work at Lincoln Laboratory was sponsored by the US DoD under Air Force Contract No. FA8721-05-C-0002. ; Peer Reviewed

The Creative Commons Attribution 3.0 Unported License to their work. ; We demonstrate a large enhancement of the spin accumulation in monolayer graphene following electron-beam induced deposition of an amorphous carbon layer at the ferromagnet-graphene interface. The enhancement is 104-fold when graphene is deposited onto poly(methyl metacrylate) (PMMA) and exposed with sufficient electron-beam dose to cross-link the PMMA, and 103-fold when graphene is deposited directly onto SiO2 and exposed with identical dose. We attribute the difference to a more efficient carbon deposition in the former case due to an increase in the presence of compounds containing carbon, which are released by the PMMA. The amorphous carbon interface can sustain very large current densities without degrading, which leads to very large spin accumulations exceeding 500 μeV at room temperature. ; We acknowledge the financial support from the Spanish Ministerio de Economía y Competitividad MINECO (MAT2010-18065) and the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC Grant Agreement No. 308023 SPINBOUND. M.V.C. acknowledges support from MINECO (RYC-2011-08319) and J.F.S. from AGAUR (Beatriu de Pinós program). ; Peer Reviewed

Graphene offers long spin propagation and, at the same time, a versatile platform to engineer its physical properties. Proximity-induced phenomena, taking advantage of materials with large spin-orbit coupling or that are magnetic, can be used to imprint graphene with large spin-orbit coupling and magnetic correlations. However, full understanding of the proximitized graphene and the consequences on the spin transport dynamics requires the development of unconventional experimental approaches. The investigation of the spin relaxation anisotropy, defined as the ratio of lifetimes for spins pointing out of and in the graphene plane, is an important step in this direction. This review discusses various methods for extracting the spin relaxation anisotropy in graphene-based devices. Within the experimental framework, current understanding on spin transport dynamics in single-layer and bilayer graphene is presented. Due to increasing interest, experimental results in graphene in proximity with high spin-orbit layered materials are also reviewed. ; This research was supported by the European Union's Horizon 2020 research and innovation programme Graphene Flagship Core 1 and 2 under Grant Agreement Nos. 696656 and 785219, respectively, by the European Research Council under grant agreement 306652 SPINBOUND, by the Spanish Ministry of Economy and Competitiveness, MINECO (Grant Nos. MAT2016-75952-R and SEV2017-0706 Severo Ochoa), and by the CERCA Programme and the Secretariat for Universities and Research, Knowledge Department of the Generalitat de Catalunya 2017 SGR 827. ; Peer reviewed

Recent experiments reporting an unexpectedly large spin Hall effect (SHE) in graphene decorated with adatoms have raised a fierce controversy. We apply numerically exact Kubo and Landauer-Büttiker formulas to realistic models of gold-decorated disordered graphene (including adatom clustering) to obtain the spin Hall conductivity and spin Hall angle, as well as the nonlocal resistance as a quantity accessible to experiments. Large spin Hall angles of ∼0.1 are obtained at zero temperature, but their dependence on adatom clustering differs from the predictions of semiclassical transport theories. Furthermore, we find multiple background contributions to the nonlocal resistance, some of which are unrelated to the SHE or any other spin-dependent origin, as well as a strong suppression of the SHE at room temperature. This motivates us to design a multiterminal graphene geometry which suppresses these background contributions and could, therefore, quantify the upper limit for spin-current generation in two-dimensional materials. ; This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 696656. S. R. acknowledges Funding from the Spanish Ministry of Economy and Competitiveness and the European Regional Development Fund (Project No. FIS2015-67767-P (MINECO/FEDER)), the Secretaria de Universidades e Investigación del Departamento de Economía y Conocimiento de la Generalidad de Cataluña, and the Severo Ochoa Program (MINECO, Grant No. SEV2013-0295). We acknowledge computational resources from PRACE and the Barcelona Supercomputing Center (Mare Nostrum), under Project No. 2015133194. J. M. M.-T. was supported as a Fulbright Colombia Scholar and by Colciencias (Departamento Administrativo de Ciencia, Tecnologia e Innovacion) of Colombia. B. K. N. was supported by United States National Science Foundation (NSF) Grant No. ECCS 1509094 and is grateful for the hospitality of Centre de Physique Théorique de Grenoble-Alpes where part of this work was done. S. O. V. acknowledges support from the European Research Council under Grant Agreement No. 308023 SPINBOUND. The supercomputing time was provided by Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by NSF Grant No. ACI-1053575. ; Peer Reviewed

Exposing graphene to a hydrogen post-etching process yields dendritic graphene shapes. Here, we demonstrate that similar dendritic structures can be achieved at long growth times without adding hydrogen externally. These shapes are not a result of a surface diffusion controlled growth but of the competing backward reaction (etching), which dominates the growth dynamics at long times due to an in situ rise in the hydrogen partial pressure. We have performed a systematic study on the growth of graphene as a function of time to identify the onset and gradual evolution of graphene shapes caused by etching and then demonstrated that the etching can be stopped by reducing the flow of hydrogen from the feed. In addition, we have found that the etching rate due to the in situ rise in hydrogen is strongly dependent on the confinement (geometrical confinement) of copper foil. Highly etched graphene with dendritic shapes was observed in unconfined copper foil regions while no etching was found in graphene grown in a confined reaction region. This highlights the effect of the dynamic reactant distribution in activating the in situ etching process during growth, which needs to be counteracted or controlled for large scale growth. ; This research was partially supported by the Spanish Ministry of Economy and Competitiveness, MINECO (under Contracts No. MAT2013-46785-P, No. MAT2016-75952-R, No. MAT2015-68307- P and Severo Ochoa No. SEV-2013-0295), by the European Research Council under Grant Agreement No. 306652 SPINBOUND, by the European Union's Horizon 2020 research and innovation programme under grant agreement No. 696656, and by the CERCA Programme and the Secretariat for Universities and Research, Knowledge Department of the Generalitat de Catalunya 2014 SGR 56. ZMG acknowledges support from MINECO FPI fellowship under Contract No. BES-2014-069925. ; Peer reviewed

The spin polarization induced by the spin Hall effect (SHE) in thin films typically points out of the plane. This is rooted on the specific symmetries of traditionally studied systems, not in a fundamental constraint. Recently, experiments on few-layer MoTe2 and WTe2 showed that the reduced symmetry of these strong spin-orbit coupling materials enables a new form of canted spin Hall effect, characterized by concurrent in-plane and out-of-plane spin polarizations. Here, through quantum transport calculations on realistic device geometries, including disorder, we predict a very large gate-tunable SHE figure of merit λsθxy≈1-50 nm in MoTe2 and WTe2 monolayers that significantly exceeds values of conventional SHE materials. This stems from a concurrent long spin diffusion length (λs) and charge-to-spin interconversion efficiency as large as θxy≈80%, originating from momentum-invariant (persistent) spin textures together with large spin Berry curvature along the Fermi contour, respectively. Generalization to other materials and specific guidelines for unambiguous experimental confirmation are proposed, paving the way toward exploiting such phenomena in spintronic devices. These findings vividly emphasize how crystal symmetry and electronic topology can govern the intrinsic SHE and spin relaxation, and how they may be exploited to broaden the range and efficiency of spintronic materials and functionalities. ; X.W. acknowledges the Agence National pour la Recherche Flagera GRANSPORT funding. ICN2 authors were supported by the European Union Horizon 2020 research and innovation programme under Grant Agreements No. 881603 (Graphene Flagship) and No. 824140 (TOCHA, H2020-FETPROACT-01-2018). ICN2 is funded by the CERCA Programme/Generalitat de Catalunya, and is supported by the Severo Ochoa program from Spanish Ministerio de Ciencia e Innovación (Grants No. SEV-2017-0706 and No. PID2019-111773RB-I00/AEI/10.13039/501100011033). V.M.P. acknowledges the support of the National Research Foundation Singapore under its Medium-Sized Centre Programme.

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY). ; The interference between repeated Landau-Zener transitions in a qubit swept through an avoided level crossing results in Stückelberg oscillations in qubit magnetization, a hallmark of the coherent strongly driven regime in two-level systems. The two-dimensional Fourier transforms of the resulting oscillatory patterns are found to exhibit a family of one-dimensional curves in Fourier space, in agreement with recent observations in a superconducting qubit. We interpret these images in terms of time evolution of the quantum phase of the qubit state and show that they can be used to probe dephasing mechanisms. ; We acknowledge partial support from W. M. Keck Foundation Center for Extreme Quantum Information Theory, and from the United States Government. The work of M. S. R. was supported by DOE CSGF, Grant No. DE-FG02-97ER25308, and the NSF. The work at Lincoln Laboratory was sponsored by the U.S. DOD under Air Force Contract No. FA8721-05-C-0002. ; Peer Reviewed

We performed x-ray magnetic circular dichroism (XMCD) measurements on heterostructures comprising topological insulators (TIs) of the (Bi,Sb)2(Se,Te)3 family and the magnetic insulator EuS. XMCD measurements allow us to investigate element-selective magnetic proximity effects at the very TI/EuS interface. A systematic analysis reveals that there is neither significant induced magnetism within the TI nor an enhancement of the Eu magnetic moment at such interface. The induced magnetic moments in Bi, Sb, Te, and Se sites are lower than the estimated detection limit of the XMCD measurements of ∼10−3μB/at. ; This research was supported by the European Union's Horizon 2020 FET-PROACTIVE project TOCHA under Grant Agreement 824140. The authors acknowledge funding by the European Research Council under Grant Agreement No. 306652 SPINBOUND, by the CERCA Programme/Generalitat de Catalunya 2017 SGR 827, by MINECO (under Contracts No. MAT2016-75952-R, No. MAT2016-78293-C6-2-R, No. PID2019-111773RBI00/AEI/10.13039/501100011033, No. PID2019- 107338RB-C65, and Severo Ochoa No. SEV-2017-0706) and the European Regional Development Fund (ERDF) under the program Interreg V-A España-Francia-Andorra (Contract No. EFA 194/16 TNSI). A. I. F. acknowledges funding from MINECO-Juan de la Cierva fellowship ref. IJCI-2015-25514 and from the European Union's Horizon 2020 People Programme (Marie Skłodowska Curie Actions) H2020-MSCA-IF-2017 under REA Grant Agreement No. 796925. F. B. acknowledges funding from MINECO Ramón y Cajal Program under Contract No. RYC-2015-18523. ; Peer reviewed

arXiv:0805.1552v1 ; The energy-level structure of a quantum system, which has a fundamental role in its behaviour, can be observed as discrete lines and features in absorption and emission spectra. Conventionally, spectra are measured using frequency spectroscopy, whereby the frequency of a harmonic electromagnetic driving field is tuned into resonance with a particular separation between energy levels. Although this technique has been successfully employed in a variety of physical systems, including natural and artificial atoms and molecules, its application is not universally straightforward and becomes extremely challenging for frequencies in the range of tens to hundreds of gigahertz. Here we introduce a complementary approach, amplitude spectroscopy, whereby a harmonic driving field sweeps an artificial atom through the avoided crossings between energy levels at a fixed frequency. Spectroscopic information is obtained from the amplitude dependence of the system's response, thereby overcoming many of the limitations of a broadband-frequency-based approach. The resulting 'spectroscopy diamonds', the regions in parameter space where transitions between specific pairs of levels can occur, exhibit interference patterns and population inversion that serve to distinguish the atom's spectrum. Amplitude spectroscopy provides a means of manipulating and characterizing systems over an extremely broad bandwidth, using only a single driving frequency that may be orders of magnitude smaller than the energy scales being probed. ; This work was supported by the Air Force Office of Scientific Research and the Laboratory for Physical Sciences (F49620-01-1-0457) under the Defense University Research Initiative in Nanotechnology programme, and by the US government. Thework at Lincoln Laboratory was sponsored by the US Department of Defence under Air Force Contract No. FA8721-05-C-0002. ; Peer Reviewed

We determine the spin-lifetime anisotropy of spin-polarized carriers in graphene. In contrast to prior approaches, our method does not require large out-of-plane magnetic fields and thus it is reliable for both low-and high-carrier densities. We first determine the in-plane spin lifetime by conventional spin precession measurements with magnetic fields perpendicular to the graphene plane. Then, to evaluate the out-of-plane spin lifetime, we implement spin precession measurements under oblique magnetic fields that generate an out-of-plane spin population. We find that the spin-lifetime anisotropy of graphene on silicon oxide is independent of carrier density and temperature down to 150 K, and much weaker than previously reported. Indeed, within the experimental uncertainty, the spin relaxation is isotropic. Altogether with the gate dependence of the spin lifetime, this indicates that the spin relaxation is driven by magnetic impurities or random spin-orbit or gauge fields. ; This research was partially supported by the European Research Council under Grant Agreement No. 308023 SPINBOUND, by the Spanish Ministry of Economy and Competitiveness, MINECO (under Contract No. MAT2013-46785-P and Severo Ochoa No. SEV-2013-0295), and by the Secretariat for Universities and Research, Knowledge Department of the Generalitat de Catalunya. M.V.C., J.F.S. and J.C. acknowledge support from the Ramón y Cajal, Juan de la Cierva and Beatriu de Pinós programs, respectively. F.B. acknowledges funding from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/2007-2013/ under REA Grant Agreement No. 624897. J.E.S. and J.V.d.V. acknowledge funding from the Methusalem Funding of the Flemish Government and the Research Foundation-Flanders (FWO). ; Peer Reviewed

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY). ; Transitions in an artificial atom, driven nonadiabatically through an energy-level avoided crossing, can be controlled by carefully engineering the driving protocol. We have driven a superconducting persistent-current qubit with a large-amplitude radio-frequency field. By applying a biharmonic wave form generated by a digital source, we demonstrate a mapping between the amplitude and phase of the harmonics produced at the source and those received by the device. This allows us to image the actual wave form at the device. This information is used to engineer a desired time dependence, as confirmed by the detailed comparison with a simulation. © 2009 The American Physical Society. ; This work was sponsored by the U.S. Government and the W. M. Keck Foundation Center for Extreme Quantum Information Theory. M.S.R. was supported by DOE CSGF, Grant No. DE-FG02-97ER25308, and the NSF. The work at Lincoln Laboratory was sponsored by the AFOSR under Air Force Contract No. FA8721-05-C-0002. ; Peer Reviewed

Scalable fabrication of magnetic 2D materials and heterostructures constitutes a crucial step for scaling down current spintronic devices and the development of novel spintronic applications. Here, we report on van der Waals (vdW) epitaxy of the layered magnetic metal Fe3GeTe2 (FGT) - a 2D crystal with highly tunable properties and a high prospect for room temperature ferromagnetism (FM) - directly on graphene by employing molecular beam epitaxy. Morphological and structural characterization confirmed the realization of large-area, continuous FGT/graphene heterostructure films with stable interfaces and good crystalline quality. Furthermore, magneto-transport and x-ray magnetic circular dichroism investigations confirmed a robust out-of-plane FM in the layers, comparable to state-of-the-art exfoliated flakes from bulk crystals. These results are highly relevant for further research on wafer-scale growth of vdW heterostructures combining FGT with other layered crystals such as transition metal dichalcogenides for the realization of multifunctional, atomically thin devices. ; They also acknowledge the provision of beamtime under the project HC-4068 at the European Synchrotron Radiation Facility (ESRF), located in Grenoble (France). ICN2 researchers acknowledge support from the European Union Horizon 2020 research and innovation programme under Grant Agreement No. 881603 (Graphene Flagship).

Nanosize pores can turn semimetallic graphene into a semiconductor and, from being impermeable, into the most efficient molecular-sieve membrane. However, scaling the pores down to the nanometer, while fulfilling the tight structural constraints imposed by applications, represents an enormous challenge for present top-down strategies. Here we report a bottom-up method to synthesize nanoporous graphene comprising an ordered array of pores separated by ribbons, which can be tuned down to the 1-nanometer range. The size, density, morphology, and chemical composition of the pores are defined with atomic precision by the design of the molecular precursors. Our electronic characterization further reveals a highly anisotropic electronic structure, where orthogonal one-dimensional electronic bands with an energy gap of ∼1 electron volt coexist with confined pore states, making the nanoporous graphene a highly versatile semiconductor for simultaneous sieving and electrical sensing of molecular species. ; This research was funded by the Centres de Recerca de Catalunya Programme–Generalitat de Catalunya, the Xunta de Galicia (Centro singular de investigacion de Galicia accreditation 2016–2019, ED431G/09), and the European Regional Development Fund. This research was supported by the Spanish Ministry of Economy and Competitiveness (under contract no. MAT2016-78293-C6-2-R, MAT2016-78293-C6-3-R, MAT2016-78293-C6-4-R, and MAT2016-75952-R and Severo Ochoa no. SEV-2013-0295); the Secretariat for Universities and Research, Knowledge Department of the Generalitat de Catalunya 2014 SGR 715, 2014 SGR 56, and 2017 SGR 827; the Basque Department of Education (contract no. PI-2016-1-0027); and the European Union's Horizon 2020 research and innovation program under grant agreement 696656. C.M. was supported by the Agency for Management of University and Research grants (AGAUR) of the Catalan government through the FP7 framework program of the European Commission under Marie Curie COFUND action 600385. ; Peer reviewed