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Mapping g-factors and complex intervalley coupling in Si/SiGe by conveyor-mode shuttling
Authors:
Mats Volmer,
Tom Struck,
Arnau Sala,
Jhih-Sian Tu,
Stefan Trellenkamp,
Davide Degli Esposti,
Giordano Scappucci,
Hendrik Bluhm,
Łukasz Cywiński,
Lars R. Schreiber
Abstract:
As silicon spin qubit chips are increasing in qubit number and area, methods for the screening of qubit related material parameters become vital. Here we demonstrate the two-dimensional mapping of small variations of the electron g-factor of quantum dots formed in planar Si/SiGe quantum wells with precision better than $10^{-3}$ and with nanometer lateral resolution. We scan the electron g-factor…
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As silicon spin qubit chips are increasing in qubit number and area, methods for the screening of qubit related material parameters become vital. Here we demonstrate the two-dimensional mapping of small variations of the electron g-factor of quantum dots formed in planar Si/SiGe quantum wells with precision better than $10^{-3}$ and with nanometer lateral resolution. We scan the electron g-factor across a 40 nm $\times$ 400 nm area and observe two g-factors per QD site which obey a striking symmetry and bimodal distribution across the area. These two g-factors relate to valley states of the electron in the quantum dot in agreement with a recent theoretical model. Using conveyor-belt shuttling of entangled electron spin pairs, complementary to the mapping of the local valley-splitting, we map the g-factor. We compare g-factor and valley splitting maps measured on the same device, and extract the complex intervalley coupling parameter along the shuttle trajectories applying a theoretical model of g-factor dependence on intervalley coupling. These maps will allow unprecedented insights into the spin-valley dynamics during qubit manipulation, readout and shuttling and serve as a benchmark for the engineering of Si/SiGe heterostructures for large-scale quantum chips.
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Submitted 24 July, 2026; v1 submitted 2 March, 2026;
originally announced March 2026.
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Numerical simulation of coherent spin-shuttling in a QuBus with charged defects
Authors:
Nils Ciroth,
Arnau Sala,
Ran Xue,
Lasse Ermoneit,
Thomas Koprucki,
Markus Kantner,
Lars R. Schreiber
Abstract:
Recent advances in coherent conveyor-mode spin qubit shuttling are paving the way for large-scale quantum computing platforms with qubit connectivity achieved by spin qubit shuttles. We developed a simulation tool to investigate numerically the impact of device imperfections on the spin-coherence of conveyor-mode shuttling in Si/SiGe. We simulate the quantum evolution of a mobile electron spin-qub…
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Recent advances in coherent conveyor-mode spin qubit shuttling are paving the way for large-scale quantum computing platforms with qubit connectivity achieved by spin qubit shuttles. We developed a simulation tool to investigate numerically the impact of device imperfections on the spin-coherence of conveyor-mode shuttling in Si/SiGe. We simulate the quantum evolution of a mobile electron spin-qubit under the influence of sparse and singly charged point defects placed in the Si/SiGe heterostructure in close proximity to the shuttle lane. We consider different locations of a single charge defect with respect to the center of the shuttle lane, multiple orbital states of the electron in the shuttle with $g$-factor differences between the orbital levels, and orbital relaxation induced by electron-phonon interaction. With this simulation framework, we identify the critical defect density of charged point defects in the heterostructure for conveyor-mode spin qubit shuttle devices and quantify the impact of a single defect on the coherence of a qubit.
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Submitted 3 December, 2025;
originally announced December 2025.
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The origin of magnetism in a supposedly nonmagnetic osmium oxide
Authors:
S. Agrestini,
F. Borgatti,
P. Florio,
J. Frassineti,
D. Fiore Mosca,
Q. Faure,
B. Detlefs,
C. J. Sahle,
S. Francoual,
J. Choi,
M. Garcia-Fernandez,
K. -J. Zhou,
V. F. Mitrovic,
P. M. Woodward,
G. Ghiringhelli,
C. Franchini,
F. Boscherini,
S. Sanna,
and M. Moretti Sala
Abstract:
A supposedly nonmagnetic 5d$^1$ double perosvkite oxide is investigated by a combination of spectroscopic and theoretical methods, namely resonant inelastic X-ray scattering, X-ray absorption spectroscopy, magnetic circular dichroism, and multiplet ligand field calculations. We found that the large spin-orbit coupling admixes the 5d $t_{2g}$ and $e_g$ orbitals, covalency raises the 5d population w…
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A supposedly nonmagnetic 5d$^1$ double perosvkite oxide is investigated by a combination of spectroscopic and theoretical methods, namely resonant inelastic X-ray scattering, X-ray absorption spectroscopy, magnetic circular dichroism, and multiplet ligand field calculations. We found that the large spin-orbit coupling admixes the 5d $t_{2g}$ and $e_g$ orbitals, covalency raises the 5d population well above the nominal value, and the local symmetry is lower than $O_h$. The obtained electronic interactions account for the finite magnetic moment of Os in this compound and, in general, of 5d$^1$ ions. Our results provide direct evidence of elusive Jahn-Teller distortions, hinting at a strong electron-lattice coupling.
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Submitted 22 January, 2024;
originally announced January 2024.
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Mapping of valley-splitting by conveyor-mode spin-coherent electron shuttling
Authors:
Mats Volmer,
Tom Struck,
Arnau Sala,
Bingjie Chen,
Max Oberländer,
Tobias Offermann,
Ran Xue,
Lino Visser,
Jhih-Sian Tu,
Stefan Trellenkamp,
Łukasz Cywiński,
Hendrik Bluhm,
Lars R. Schreiber
Abstract:
In Si/SiGe heterostructures, the low-lying excited valley state seriously limit operability and scalability of electron spin qubits. For characterizing and understanding the local variations in valley splitting, fast probing methods with high spatial and energy resolution are lacking. Leveraging the spatial control granted by conveyor-mode spin-coherent electron shuttling, we introduce a method fo…
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In Si/SiGe heterostructures, the low-lying excited valley state seriously limit operability and scalability of electron spin qubits. For characterizing and understanding the local variations in valley splitting, fast probing methods with high spatial and energy resolution are lacking. Leveraging the spatial control granted by conveyor-mode spin-coherent electron shuttling, we introduce a method for two-dimensional mapping of the local valley splitting by detecting magnetic field dependent anticrossings of ground and excited valley states using entangled electron spin-pairs as a probe. The method has sub-μeV energy accuracy and a nanometer lateral resolution. The histogram of valley splittings spanning a large area of 210 nm by 18 nm matches well with statistics obtained by the established but time-consuming magnetospectroscopy method. For the specific heterostructure, we find a nearly Gaussian distribution of valley splittings and a correlation length similar to the quantum dot size. Our mapping method may become a valuable tool for engineering Si/SiGe heterostructures for scalable quantum computing.
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Submitted 29 December, 2023;
originally announced December 2023.
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Superconductor-semiconductor hybrid capacitance with a nonlinear charge-voltage profile
Authors:
Joachim Lauwens,
Lars Kerkhofs,
Arnau Sala,
Bart Sorée
Abstract:
Electronic devices that work in the quantum regime often employ hybrid nanostructures to bring about a nonlinear behaviour. The nonlinearity that these can provide has proven to be useful, in particular, for applications in quantum computation. Here we present a hybrid device that acts as a capacitor with a nonlinear charge-voltage relation. The device consists of a nanowire placed between the pla…
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Electronic devices that work in the quantum regime often employ hybrid nanostructures to bring about a nonlinear behaviour. The nonlinearity that these can provide has proven to be useful, in particular, for applications in quantum computation. Here we present a hybrid device that acts as a capacitor with a nonlinear charge-voltage relation. The device consists of a nanowire placed between the plates of a coplanar capacitor, with a co-parallel alignment. At low temperatures, due to the finite density of states on the nanowire, the charge distribution in the capacitor is uneven and energy-dependent, resulting in a charge-dependent effective capacitance. We study this system analytically and numerically, and show that the nonlinearity of the capacitance is significant enough to be utilized in circuit quantum electrodynamics. The resulting nonlinearity can be switched on, modulated and switched off by an external potential, thus making this capacitive device highly versatile for uses in quantum computation.
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Submitted 15 June, 2023;
originally announced June 2023.
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Probing the graphene/substrate interaction by electron tunneling decay
Authors:
Virginia Carnevali,
Alessandro Sala,
Pietro Biasin,
Mirco Panighel,
Giovanni Comelli,
Maria Peressi,
Cristina Africh
Abstract:
The electronic properties of graphene can be modified by the local interaction with a selected metal substrate. To probe this effect, Scanning Tunneling Microscopy is widely employed, particularly by means of local measurement via lock-in amplifier of the differential conductance and of the field emission resonance. In this article we propose an alternative, reliable method of probing the graphene…
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The electronic properties of graphene can be modified by the local interaction with a selected metal substrate. To probe this effect, Scanning Tunneling Microscopy is widely employed, particularly by means of local measurement via lock-in amplifier of the differential conductance and of the field emission resonance. In this article we propose an alternative, reliable method of probing the graphene/substrate interaction that is readily available to any STM apparatus. By testing the tunneling current as function of the tip/sample distance on nanostructured graphene on Ni(100), we demonstrate that I(z) spectroscopy can be quantitatively compared with Density Functional Theory calculations and can be used to assess the nature of the interaction between graphene and substrate. This method can expand the capabilities of standard STM systems to study graphene/substrate complexes, complementing standard topographic probing with spectroscopic information.
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Submitted 28 April, 2023; v1 submitted 27 April, 2023;
originally announced April 2023.
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Line Shapes of Electric Dipole Spin Resonance in Pauli Spin Blockade
Authors:
Arnau Sala,
Jeroen Danon
Abstract:
Electric dipole spin resonance (EDSR) is a commonly used tool for manipulation and spectroscopy of quantum-dot-based spin qubits. When an EDSR experiment is embedded in a transport setup and Pauli spin blockade is used as means for spin-state read-out, then measured resonant responses in the leakage current indeed carry information about the level structure of the system under study. However, the…
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Electric dipole spin resonance (EDSR) is a commonly used tool for manipulation and spectroscopy of quantum-dot-based spin qubits. When an EDSR experiment is embedded in a transport setup and Pauli spin blockade is used as means for spin-state read-out, then measured resonant responses in the leakage current indeed carry information about the level structure of the system under study. However, the actual line shape of these current resonances differs substantially from experiment to experiment, varying from being symmetric to asymmetric and from being a peak to a dip, a thorough understanding of which is still lacking. Here, we investigate theoretically the detailed line shape of EDSR-induced resonances in the leakage current in the regime of spin blockade, and we connect different line shapes to the different underlying physical mechanisms that can enable the EDSR. We carry out both numerical and analytical investigations, producing simple analytic expressions that give insight in the physics at play. Our results thus provide a means to extract more information about the detailed system parameters of quantum dots hosting spin qubits from an EDSR experiment than just their level structure based on the location of the resonances.
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Submitted 20 August, 2021; v1 submitted 25 May, 2021;
originally announced May 2021.
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"Inside Out" Growth Method for High-Quality Nitrogen-Doped Graphene
Authors:
Sara Fiori,
Daniele Perilli,
Mirco Panighel,
Cinzia Cepek,
Aldo Ugolotti,
Alessandro Sala,
Hongsheng Liu,
Giovanni Comelli,
Cristiana Di Valentin,
Cristina Africh
Abstract:
High-quality nitrogen-doped graphene on nickel is prepared by exploiting both the catalytic properties of nickel and the solubility of nitrogen atoms into its bulk. Following the standard chemical vapor deposition procedure, a previously nitrogen-doped nickel substrate is exposed to carbon-containing precursors so that nitrogen atoms, segregating to the surface, remain trapped in the growing graph…
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High-quality nitrogen-doped graphene on nickel is prepared by exploiting both the catalytic properties of nickel and the solubility of nitrogen atoms into its bulk. Following the standard chemical vapor deposition procedure, a previously nitrogen-doped nickel substrate is exposed to carbon-containing precursors so that nitrogen atoms, segregating to the surface, remain trapped in the growing graphene network. Morphological and chemical characterization by scanning tunneling microscopy and X-ray photoelectron spectroscopy demonstrates that the process yields a flat, wide, continuous nitrogen-doped graphene layer. Experimental results are combined with a thorough density functional theory investigation of possible structural models, to obtain a clear description at the atomic scale of the various configurations of the nitrogen atoms observed in the graphene mesh. This growth method is potentially scalable and suitable for the production of high-performance nano-devices with well-defined nitrogen centers, to be exploited as metal-free carbon-based catalysts in several applicative fields such as electrochemistry, energy storage, gas storage/sensing or wastewater treatment.
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Submitted 3 October, 2020; v1 submitted 21 September, 2020;
originally announced September 2020.
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Molecular anchoring stabilizes low valence Ni(I)TPP on copper against thermally induced chemical changes
Authors:
Henning Maximilian Sturmeit,
Iulia Cojocariu,
Matteo Jugovac,
Albano Cossaro,
Alberto Verdini,
Luca Floreano,
Alessandro Sala,
Giovanni Comelli,
Stefania Moro,
Matus Stredansky,
Manuel Corva,
Erik Vesselli,
Peter Puschnig,
Claus Michael Schneider,
Vitaliy Feyer,
Giovanni Zamborlini,
Mirko Cinchetti
Abstract:
Many applications of molecular layers deposited on metal surfaces, ranging from single-atom catalysis to on-surface magnetochemistry and biosensing, rely on the use of thermal cycles to regenerate the pristine properties of the system. Thus, understanding the microscopic origin behind the thermal stability of organic/metal interfaces is fundamental for engineering reliable organic-based devices. H…
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Many applications of molecular layers deposited on metal surfaces, ranging from single-atom catalysis to on-surface magnetochemistry and biosensing, rely on the use of thermal cycles to regenerate the pristine properties of the system. Thus, understanding the microscopic origin behind the thermal stability of organic/metal interfaces is fundamental for engineering reliable organic-based devices. Here, we study nickel porphyrin molecules on a copper surface as an archetypal system containing a metal center whose oxidation state can be controlled through the interaction with the metal substrate. We demonstrate that the strong molecule-surface interaction, followed by charge transfer at the interface, plays a fundamental role in the thermal stability of the layer by rigidly anchoring the porphyrin to the substrate. Upon thermal treatment, the molecules undergo an irreversible transition at 420 K, which is associated with an increase of the charge transfer from the substrate, mostly localized on the phenyl substituents, and a downward tilting of the latters without any chemical modification
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Submitted 18 June, 2020;
originally announced June 2020.
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Highly tunable exchange-only singlet-only qubit in a GaAs triple quantum dot
Authors:
Arnau Sala,
Jørgen Holme Qvist,
Jeroen Danon
Abstract:
We propose an implementation of a singlet-only spin qubit in a GaAs-based triple quantum dot with a (1,4,1) charge occupation. In the central multi-electron dot, the interplay between Coulomb interaction and an out-of-plane magnetic field creates an energy spectrum with a tunable singlet-triplet splitting, which can be exploited to create a six-particle singlet-only qubit with a qubit splitting th…
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We propose an implementation of a singlet-only spin qubit in a GaAs-based triple quantum dot with a (1,4,1) charge occupation. In the central multi-electron dot, the interplay between Coulomb interaction and an out-of-plane magnetic field creates an energy spectrum with a tunable singlet-triplet splitting, which can be exploited to create a six-particle singlet-only qubit with a qubit splitting that can straightforwardly be tuned over tens of $μ$eV by adjusting the external magnetic field. We confirm the full exchange-based electric control of the qubit and demonstrate its superior coherence properties due to its singlet-only nature.
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Submitted 19 November, 2019;
originally announced November 2019.
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Imaging at the mesoscale (LEEM, PEEM)
Authors:
Alessandro Sala
Abstract:
*To be published in Springer Handbook of Surface Science (Springer Verlag) [Preprint]*
The capability to display images containing chemical, magnetic and structural information and to perform spectroscopy and diffraction from a μm-sized area makes cathode lens electron microscopy one of the most used and reliable techniques to analyze surfaces at the mesoscale. Thanks to its versatility, LEEM/PE…
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*To be published in Springer Handbook of Surface Science (Springer Verlag) [Preprint]*
The capability to display images containing chemical, magnetic and structural information and to perform spectroscopy and diffraction from a μm-sized area makes cathode lens electron microscopy one of the most used and reliable techniques to analyze surfaces at the mesoscale. Thanks to its versatility, LEEM/PEEM systems are currently employed to study model systems in the fields of nanotechnology, nanomagnetism, material science, catalysis, energy storage, thin films and 2D materials. In the following chapter, we will present a brief but complete review of this class of instruments. After an historical digression in the introducing section, we will show first the basic operating principles of a simple setup and then the elements that can be added to improve the performances. Later, two sections will be dedicated to LEEM and PEEM respectively. In both cases, a theoretical discussion on the contrast mechanisms will prelude to a showcase of the operating modes of the instrument, with clear examples that will show the best performances available nowadays. Finally, a brief discussion about the future developments of cathode lens electron microscopy will close the chapter.
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Submitted 4 December, 2018;
originally announced December 2018.
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Universal scaling behavior of the upper critical field in strained FeSe0.7Te0.3 thin films
Authors:
Feifei Yuan,
Vadim Grinenko,
Kazumasa Iida,
Stefan Richter,
Aurimas Pukenas,
Werner Skrotzki,
Masahito Sakoda,
Michio Naito,
Alberto Sala,
Marina Putti,
Aichi Yamashita,
Yoshihiko Takano,
Zhixiang Shi,
Kornelius Nielsch,
Ruben Huehne
Abstract:
Revealing the universal behaviors of iron-based superconductors (FBS) is important to elucidate the microscopic theory of superconductivity. In this work, we investigate the effect of in-plane strain on the slope of the upper critical field Hc2 at the superconducting transition temperature Tc (i.e. -dHc2/dT) for FeSe0.7Te0.3 thin films. The in-plane strain tunes Tc in a broad range, while the comp…
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Revealing the universal behaviors of iron-based superconductors (FBS) is important to elucidate the microscopic theory of superconductivity. In this work, we investigate the effect of in-plane strain on the slope of the upper critical field Hc2 at the superconducting transition temperature Tc (i.e. -dHc2/dT) for FeSe0.7Te0.3 thin films. The in-plane strain tunes Tc in a broad range, while the composition and disorder are almost unchanged. We show that -dHc2/dT scales linearly with Tc, indicating that FeSe0.7Te0.3 follows the same universal behavior as observed for pnictide FBS. The observed behavior is consistent with a multiband superconductivity paired by interband interaction such as sign change s+- superconductivity.
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Submitted 13 September, 2018;
originally announced September 2018.
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Leakage and dephasing in $^{28}$Si-based exchange-only spin qubits
Authors:
Arnau Sala,
Jeroen Danon
Abstract:
Exchange-only spin qubits hosted in $^{28}$Si-based triple quantum dots do not suffer from decoherence caused by randomly fluctuating nuclear-spin ensembles and can be relatively robust against electrical noise when operated at a sweet spot. Remaining sources of decoherence are qubit relaxation, leakage out of the qubit subspace, and dephasing due to residual effects of charge noise, the latter tw…
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Exchange-only spin qubits hosted in $^{28}$Si-based triple quantum dots do not suffer from decoherence caused by randomly fluctuating nuclear-spin ensembles and can be relatively robust against electrical noise when operated at a sweet spot. Remaining sources of decoherence are qubit relaxation, leakage out of the qubit subspace, and dephasing due to residual effects of charge noise, the latter two of which are the focus of this work. We investigate spin-orbit-mediated leakage rates to the three-spin ground state accompanied by virtual (i) tunneling, (ii) orbital excitation, and (iii) valley excitation of an electron. We find different power-law dependencies on the applied magnetic field $B$ for the three mechanisms as well as for the two leakage rates, ranging from $\propto B^5$ to $\propto B^{11}$, and identify the sweet spot as a point of minimal leakage. We also revisit the role of electrical noise at the sweet spot, and show that it causes a decay of coherent qubit oscillations that follows a power law $\propto 1/t$ (as opposed to the more common exponential decay) and introduces a $π/2$ phase shift.
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Submitted 7 November, 2018; v1 submitted 29 August, 2018;
originally announced August 2018.
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Bloch-point-mediated topological transformations of magnetic domain walls in cylindrical nanowires
Authors:
Olivier Fruchart,
A Wartelle,
B Trapp,
M Staňo,
Christophe Thirion,
S Bochmann,
J Bachmann,
M Foerster,
L Aballe,
O. Menteş,
A Locatelli,
A Sala,
L Cagnon,
J. -C Toussaint
Abstract:
Cylindrical nanowires made of soft magnetic materials, in contrast to thin strips, may host domain walls of two distinct topologies. Unexpectedly, we evidence experimentally the dynamic transformation of topology upon wall motion above a field threshold. Micromagnetic simulations highlight the underlying precessional dynamics for one way of the transformation, involving the nucleation of a Bloch-p…
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Cylindrical nanowires made of soft magnetic materials, in contrast to thin strips, may host domain walls of two distinct topologies. Unexpectedly, we evidence experimentally the dynamic transformation of topology upon wall motion above a field threshold. Micromagnetic simulations highlight the underlying precessional dynamics for one way of the transformation, involving the nucleation of a Bloch-point singularity, however, fail to reproduce the reverse process. This rare discrepancy between micromagnetic simulations and experiments raises fascinating questions in material and computer science.
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Submitted 9 July, 2018; v1 submitted 28 June, 2018;
originally announced June 2018.
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Creation of entangled atomic states by an analogue of the Dynamical Casimir Effect
Authors:
K. Lange,
J. Peise,
B. Lücke,
T. Gruber,
A. Sala,
A. Polls,
W. Ertmer,
B. Juliá-Díaz,
L. Santos,
C. Klempt
Abstract:
If the boundary conditions of the quantum vacuum are changed in time, quantum field theory predicts that real, observable particles can be created in the initially empty modes. Here, we realize this effect by changing the boundary conditions of a spinor Bose-Einstein condensate, which yields a population of initially unoccupied spatial and spin excitations. We prove that the excitations are create…
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If the boundary conditions of the quantum vacuum are changed in time, quantum field theory predicts that real, observable particles can be created in the initially empty modes. Here, we realize this effect by changing the boundary conditions of a spinor Bose-Einstein condensate, which yields a population of initially unoccupied spatial and spin excitations. We prove that the excitations are created as entangled excitation pairs by certifying continuous-variable entanglement within the many-particle output state.
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Submitted 29 August, 2018; v1 submitted 7 May, 2018;
originally announced May 2018.
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Fe intercalation under graphene and hexagonal boron nitride in-plane heterostructure on Pt(111)
Authors:
Igor Píš,
Silvia Nappini,
Federica Bondino,
Tevfik Onur Menteş,
Alessandro Sala,
Andrea Locatelli,
Elena Magnano
Abstract:
Metal nanostructures confined between sp2 hybridized 2D materials and solid supports are attracting attention for their potential application in new nanotechnologies. Model studies under well-defined conditions are valuable for understanding the fundamental aspects of the phenomena under 2D covers. In this work we investigate the intercalation of iron atoms through a single layer of mixed graphene…
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Metal nanostructures confined between sp2 hybridized 2D materials and solid supports are attracting attention for their potential application in new nanotechnologies. Model studies under well-defined conditions are valuable for understanding the fundamental aspects of the phenomena under 2D covers. In this work we investigate the intercalation of iron atoms through a single layer of mixed graphene and hexagonal boron nitride on Pt(111) using a combination of spectroscopic and microscopic techniques. Thermally activated diffusion of iron proceeds preferentially under graphene and only partially under hexagonal boron nitride areas. When oxygen is coadsorbed with iron, the intercalation rate is higher, and formation of B2O3 and oxygenated B-C species is observed. Our results suggest the possibility of confining ferromagnetic layers under heterostructures of graphene and hexagonal boron nitride with potential technological implications in the fields of spintronics, magnetic data storage or chemistry under 2D covers.
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Submitted 11 April, 2018;
originally announced April 2018.
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Electronic properties of WS$_2$ on epitaxial graphene on SiC(0001)
Authors:
Stiven Forti,
Antonio Rossi,
Holger Büch,
Tommaso Cavallucci,
Francesco Bisio,
Alessandro Sala,
Tevfik Onur Menteş,
Andrea Locatelli,
Michele Magnozzi,
Maurizio Canepa,
Kathrin Müller,
Stefan Link,
Ulrich Starke,
Valentina Tozzini,
Camilla Coletti
Abstract:
This work reports an electronic and micro-structural study of an appealing system for optoelectronics: tungsten disulphide WS$_2$ on epitaxial graphene (EG) on SiC(0001). The WS$_2$ is grown via chemical vapor deposition (CVD) onto the EG. Low-energy electron diffraction (LEED) measurements assign the zero-degree orientation as the preferential azimuthal alignment for WS$_2$/EG. The valence-band (…
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This work reports an electronic and micro-structural study of an appealing system for optoelectronics: tungsten disulphide WS$_2$ on epitaxial graphene (EG) on SiC(0001). The WS$_2$ is grown via chemical vapor deposition (CVD) onto the EG. Low-energy electron diffraction (LEED) measurements assign the zero-degree orientation as the preferential azimuthal alignment for WS$_2$/EG. The valence-band (VB) structure emerging from this alignment is investigated by means of photoelectron spectroscopy measurements, with both high space and energy resolution. We find that the spin-orbit splitting of monolayer WS$_2$ on graphene is of 462 meV, larger than what is reported to date for other substrates. We determine the value of the work function for the WS$_2$/EG to be 4.5$\pm$0.1 eV. A large shift of the WS$_2$ VB maximum is observed as well , due to the lowering of the WS$_2$ work function caused by the donor-like interfacial states of EG. Density functional theory (DFT) calculations carried out on a coincidence supercell confirm the experimental band structure to an excellent degree. X-ray photoemission electron microscopy (XPEEM) measurements performed on single WS$_2$ crystals confirm the van der Waals nature of the interface coupling between the two layers. In virtue of its band alignment and large spin-orbit splitting, this system gains strong appeal for optical spin-injection experiments and opto-spintronic applications in general.
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Submitted 15 September, 2017;
originally announced September 2017.
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The influence of the in-plane lattice constant on the superconducting transition temperature of FeSe0.7Te0.3 thin films
Authors:
Feifei Yuan,
Kazumasa Iida,
Vadim Grinenko,
Paul Chekhonin,
Aurimas Pukenas,
Werner Skrotzki,
Masahito Sakoda,
Michio Naito,
Alberto Sala,
Marina Putti,
Aichi Yamashita,
Yoshihiko Takano,
Zhixiang Shi,
Kornelius Nielsch,
Ruben Huehne
Abstract:
Epitaxial Fe(Se,Te) thin films were prepared by pulsed laser deposition on (La0.18Sr0.82)(Al0.59Ta0.41)O3 (LSAT), CaF2-buffered LSAT and bare CaF2 substrates, which exhibit an almost identical in-plane lattice parameter. The composition of all Fe(Se,Te) films were determined to be FeSe0.7Te0.3 by energy dispersive X-ray spectroscopy, irrespective of the substrate. Albeit the lattice parameters of…
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Epitaxial Fe(Se,Te) thin films were prepared by pulsed laser deposition on (La0.18Sr0.82)(Al0.59Ta0.41)O3 (LSAT), CaF2-buffered LSAT and bare CaF2 substrates, which exhibit an almost identical in-plane lattice parameter. The composition of all Fe(Se,Te) films were determined to be FeSe0.7Te0.3 by energy dispersive X-ray spectroscopy, irrespective of the substrate. Albeit the lattice parameters of all templates have comparable values, the in-plane lattice parameter of the FeSe0.7Te0.3 films varies significantly. We found that the superconducting transition temperature (Tc) of FeSe0.7Te0.3 thin films is strongly correlated with their a-axis lattice parameter. The highest Tc of over 19 K was observed for the film on bare CaF2 substrate, which is related to unexpectedly large in-plane compressive strain originating mostly from the thermal expansion mismatch between the FeSe0.7Te0.3 film and the substrate.
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Submitted 9 June, 2017;
originally announced June 2017.
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Magnetic skyrmions in confined geometries : effect of the magnetic field and the disorder
Authors:
Roméo Juge,
Soong-Geun Je,
Dayane de Souza Chaves,
Stefania Pizzini,
Liliana D. Buda-Prejbeanu,
Lucia Aballe,
Michael Foerster,
Andrea Locatelli,
Tevfik Onur Menteş,
Alessandro Sala,
Francesco Maccherozzi,
Sarnjeet S. Dhesi,
Stéphane Auffret,
Gilles Gaudin,
Jan Vogel,
Olivier Boulle
Abstract:
We report on the effect of the lateral confinement and a perpendicular magnetic field on isolated room-temperature magnetic skyrmions in sputtered Pt/Co/MgO nanotracks and nanodots. We show that the skyrmions size can be easily tuned by playing on the lateral dimensions of the nanostructures and by using external magnetic field amplitudes of a few mT, which allow to reach sub-100 nm diameters. Our…
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We report on the effect of the lateral confinement and a perpendicular magnetic field on isolated room-temperature magnetic skyrmions in sputtered Pt/Co/MgO nanotracks and nanodots. We show that the skyrmions size can be easily tuned by playing on the lateral dimensions of the nanostructures and by using external magnetic field amplitudes of a few mT, which allow to reach sub-100 nm diameters. Our XMCD-PEEM observations also highlight the important role of the pinning on the skyrmions size and stability under an out-of-plane magnetic field. Micromagnetic simulations reveal that the effect of local pinning can be well accounted for by considering the thin film grain structure with local anisotropy variations and reproduce well the dependence of the skyrmion diameter on the magnetic field and the geometry.
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Submitted 13 June, 2017; v1 submitted 6 June, 2017;
originally announced June 2017.
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Flux-closure domains in high aspect ratio electroless-deposited CoNiB nanotubes
Authors:
Michal Staňo,
Sandra Schaefer,
Alexis Wartelle,
Maxime Rioult,
Rachid Belkhou,
Alessandro Sala,
Tevfik Onur Menteş,
Andrea Locatelli,
Laurent Cagnon,
Beatrix Trapp,
Sebastian Bochmann,
Sylvain Martin,
Eric Gautier,
Jean-Christophe Toussaint,
Wolfgang Ensinger,
Olivier Fruchart
Abstract:
We report the imaging of magnetic domains in ferromagnetic CoNiB nanotubes with very long aspect ratio, fabricated by electroless plating. While axial magnetization is expected for long tubes made of soft magnetic materials, we evidence series of azimuthal domains. We tentatively explain these by the interplay of anisotropic strain and/or grain size, with magneto-elasticity and/or anisotropic inte…
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We report the imaging of magnetic domains in ferromagnetic CoNiB nanotubes with very long aspect ratio, fabricated by electroless plating. While axial magnetization is expected for long tubes made of soft magnetic materials, we evidence series of azimuthal domains. We tentatively explain these by the interplay of anisotropic strain and/or grain size, with magneto-elasticity and/or anisotropic interfacial magnetic anisotropy. This material could be interesting for dense data storage, as well as curvature-induced magnetic phenomena such as the non-reciprocity of spin-wave propagation.
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Submitted 14 May, 2018; v1 submitted 21 April, 2017;
originally announced April 2017.
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The exchange-only singlet-only spin qubit
Authors:
Arnau Sala,
Jeroen Danon
Abstract:
We propose a feasible and scalable quantum-dot-based implementation of a singlet-only spin qubit which is to leading order intrinsically insensitive to random effective magnetic fields set up by fluctuating nuclear spins in the host semiconductor. Our proposal thus removes an important obstacle for further improvement of spin qubits hosted in high-quality III-V semiconductors such as GaAs. We show…
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We propose a feasible and scalable quantum-dot-based implementation of a singlet-only spin qubit which is to leading order intrinsically insensitive to random effective magnetic fields set up by fluctuating nuclear spins in the host semiconductor. Our proposal thus removes an important obstacle for further improvement of spin qubits hosted in high-quality III-V semiconductors such as GaAs. We show how the resulting qubit could be initialized, manipulated, and read out by electrical means only, in a way very similar to a triple-dot exchange-only spin qubit. Due to the intrinsic elimination of the effective nuclear fields from the qubit Hamiltonian, we find an improvement of the dephasing time $T_2^*$ of several orders of magnitude as compared to similar existing spin qubits.
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Submitted 21 June, 2017; v1 submitted 6 February, 2017;
originally announced February 2017.
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Shortcut to adiabaticity in spinor condensates
Authors:
Arnau Sala,
David López Núñez,
Joan Martorell,
Luigi De Sarlo,
Tilman Zibold,
Fabrice Gerbier,
Artur Polls,
Bruno Juliá-Díaz
Abstract:
We devise a method to shortcut the adiabatic evolution of a spin-1 Bose gas with an external magnetic field as the control parameter. An initial many-body state with almost all bosons populating the Zeeman sublevel $m=0$, is evolved to a final state very close to a macroscopic spin-singlet condensate, a fragmented state with three macroscopically occupied Zeeman states. The shortcut protocol, obta…
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We devise a method to shortcut the adiabatic evolution of a spin-1 Bose gas with an external magnetic field as the control parameter. An initial many-body state with almost all bosons populating the Zeeman sublevel $m=0$, is evolved to a final state very close to a macroscopic spin-singlet condensate, a fragmented state with three macroscopically occupied Zeeman states. The shortcut protocol, obtained by an approximate mapping to a harmonic oscillator Hamiltonian, is compared to linear and exponential variations of the control parameter. We find a dramatic speedup of the dynamics when using the shortcut protocol.
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Submitted 14 July, 2016;
originally announced July 2016.
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Room temperature chiral magnetic skyrmion in ultrathin magnetic nanostructures
Authors:
Olivier Boulle,
Jan Vogel,
Hongxin Yang,
Stefania Pizzini,
Dayane de Souza Chaves,
Andrea Locatelli,
Tevfik Onur Menteş Alessandro Sala,
Liliana D. Buda-Prejbeanu,
Olivier Klein,
Mohamed Belmeguenai,
Yves Roussigné,
Andrey Stashkevich,
Salim Mourad Chérif,
Lucia Aballe,
Michael Foerster,
Mairbek Chshiev,
Stéphane Auffret,
Ioan Mihai Miron,
Gilles Gaudin
Abstract:
Magnetic skyrmions are chiral spin structures with a whirling configuration. Their topological properties, nanometer size and the fact that they can be moved by small current densities have opened a new paradigm for the manipulation of magnetisation at the nanoscale. To date, chiral skyrmion structures have been experimentally demonstrated only in bulk materials and in epitaxial ultrathin films an…
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Magnetic skyrmions are chiral spin structures with a whirling configuration. Their topological properties, nanometer size and the fact that they can be moved by small current densities have opened a new paradigm for the manipulation of magnetisation at the nanoscale. To date, chiral skyrmion structures have been experimentally demonstrated only in bulk materials and in epitaxial ultrathin films and under external magnetic field or at low temperature. Here, we report on the observation of stable skyrmions in sputtered ultrathin Pt/Co/MgO nanostructures, at room temperature and zero applied magnetic field. We use high lateral resolution X-ray magnetic circular dichroism microscopy to image their chiral Néel internal structure which we explain as due to the large strength of the Dzyaloshinskii-Moriya interaction as revealed by spin wave spectroscopy measurements. Our results are substantiated by micromagnetic simulations and numerical models, which allow the identification of the physical mechanisms governing the size and stability of the skyrmions.
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Submitted 10 January, 2016;
originally announced January 2016.
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Tuning the Electronic Structure of Monolayer Graphene/MoS2 van der Waals Heterostructures via Interlayer Twist
Authors:
Wencan Jin,
Po-Chun Yeh,
Nader Zaki,
Daniel Chenet,
Ghidewon Arefe,
Yufeng Hao,
Alessandro Sala,
Tevfik Onur Mentes,
Jerry I. Dadap,
Andrea Locatelli,
James Hone,
Richard M. Osgood Jr
Abstract:
We directly measure the electronic structure of twisted graphene/MoS2 van der Waals heterostructures, in which both graphene and MoS2 are monolayers. We use cathode lens microscopy and microprobe angle-resolved photoemission spectroscopy measurements to image the surface, determine twist angle, and map the electronic structure of these artificial heterostructures. For monolayer graphene on monolay…
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We directly measure the electronic structure of twisted graphene/MoS2 van der Waals heterostructures, in which both graphene and MoS2 are monolayers. We use cathode lens microscopy and microprobe angle-resolved photoemission spectroscopy measurements to image the surface, determine twist angle, and map the electronic structure of these artificial heterostructures. For monolayer graphene on monolayer MoS2, the resulting band structure reveals the absence of hybridization between the graphene and MoS2 electronic states. Further, the graphene-derived electronic structure in the heterostructures remains intact, irrespective of the twist angle between the two materials. In contrast, however, the electronic structure associated with the MoS2 layer is found to be twist-angle dependent; in particular, the relative difference in the energy of the valence band maximum at Γ and K of the MoS2 layer varies from approximately 0 to 0.2 eV. Our results suggest that monolayer MoS2 within the heterostructure becomes predominantly an indirect bandgap system for all twist angles except in the proximity of 30 degrees. This result enables potential bandgap engineering in van der Waals heterostructures comprised of monolayer structures.
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Submitted 16 November, 2015; v1 submitted 2 October, 2015;
originally announced October 2015.
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Proposal for a transmon-based quantum router
Authors:
Arnau Sala,
M. Blaauboer
Abstract:
We propose an implementation of a quantum router for microwave photons in a superconducting qubit architecture consisting of a transmon qubit, SQUIDs and a nonlinear capacitor. We model and analyze the dynamics of operation of the quantum switch using quantum Langevin equations in a scattering approach and compute the photon reflection and transmission probabilities. For parameters corresponding t…
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We propose an implementation of a quantum router for microwave photons in a superconducting qubit architecture consisting of a transmon qubit, SQUIDs and a nonlinear capacitor. We model and analyze the dynamics of operation of the quantum switch using quantum Langevin equations in a scattering approach and compute the photon reflection and transmission probabilities. For parameters corresponding to up-to-date experimental devices we predict successful operation of the router with probabilities above 94%.
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Submitted 20 May, 2016; v1 submitted 18 September, 2015;
originally announced September 2015.
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Dependences on RE of Superconducting Properties of Transition Metal co-doped (Ca,RE)FeAs2 with RE = La-Gd
Authors:
Hiroyuki Yakita,
Hiraku Ogino,
Alberto Sala,
Tomoyuki Okada,
Akiyasu Yamamoto,
Kohji Kishio,
Akira Iyo,
Hiroshi Eisaki,
Jun-ichi Shimoyama
Abstract:
Dependence of superconducting properties of (Ca,RE)(Fe,TM)As2 [(Ca,RE)112, TM: Co, Ni)] on RE elements (RE = La-Gd) was systematically investigated. Improvement of superconducting properties by Co or Ni co-doping was observed for all (Ca,RE)112, which is similar to Co-co-doped (Ca,La)112 or (Ca,Pr)112. Tc of Co-co-doped samples decreased from 38 K for RE = La to 29 K for RE = Gd with decreasing io…
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Dependence of superconducting properties of (Ca,RE)(Fe,TM)As2 [(Ca,RE)112, TM: Co, Ni)] on RE elements (RE = La-Gd) was systematically investigated. Improvement of superconducting properties by Co or Ni co-doping was observed for all (Ca,RE)112, which is similar to Co-co-doped (Ca,La)112 or (Ca,Pr)112. Tc of Co-co-doped samples decreased from 38 K for RE = La to 29 K for RE = Gd with decreasing ionic radii of RE3+. However, Co-co-doped (Ca,Eu)112 showed exceptionally low Tc = 21 K probably due to the co-existence of Eu3+ and Eu2+ suggested by longer interlayer distance dFe-Fe of (Ca,Eu)112 than other (Ca,RE)112.
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Submitted 27 May, 2015;
originally announced May 2015.
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Co-co-doping effect for (Ca,RE)FeAs2 sintered bulk
Authors:
Hiroyuki Yakita,
Hiraku Ogino,
Alberto Sala,
Tomoyuki Okada,
Kohji Kishio,
Akira Iyo,
Hiroshi Eisaki,
Jun-ichi Shimoyama
Abstract:
Superconducting properties of Co-co-doped (Ca,RE)FeAs2 ((Ca,RE)112: RE = La, Pr) were investigated. Co-co-doping increased Tc of (Ca,Pr)112 while Mn-co-doping suppressed superconductivity of (Ca,RE)112. Co-co-doped (Ca,La)112 showed large diamagnetic screening and sharper superconducting transition than Co-free (Ca,La)112. Tczero observed in resistivity measurements increased from 14 K to 30 K by…
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Superconducting properties of Co-co-doped (Ca,RE)FeAs2 ((Ca,RE)112: RE = La, Pr) were investigated. Co-co-doping increased Tc of (Ca,Pr)112 while Mn-co-doping suppressed superconductivity of (Ca,RE)112. Co-co-doped (Ca,La)112 showed large diamagnetic screening and sharper superconducting transition than Co-free (Ca,La)112. Tczero observed in resistivity measurements increased from 14 K to 30 K by Co-co-doping, while Tconset was not increased. The critical current density (Jc) of Co-co-doped (Ca,La)112 were approximately 2.1 x 104 Acm-2 and 3.2 x 103 Acm-2 at 2 K and 25 K, respectively, near zero field. These relatively high Jcs and large diamagnetic screening observed in susceptibility measurement as for polycrystalline bulks suggest bulk superconductivity of Co-co-doped (Ca,RE)112 compounds.
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Submitted 27 May, 2015;
originally announced May 2015.
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Influence of substrate type on transport properties of superconducting FeSe0.5Te0.5 thin films
Authors:
Feifei Yuan,
Kazumasa Iida,
Marco Langer,
Jens Hänisch,
Ataru Ichinose,
Ichiro Tsukada,
Alberto Sala,
Marina Putti,
Ruben Hühne,
Ludwig Schultz,
Zhixiang Shi
Abstract:
FeSe0.5Te0.5 thin films were grown by pulsed laser deposition on CaF2, LaAlO3 and MgO substrates and structurally and electro-magnetically characterized in order to study the influence of the substrate on their transport properties. The in-plane lattice mismatch between FeSe0.5Te0.5 bulk and the substrates shows no influence on the lattice parameters of the films, whereas the type of substrates af…
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FeSe0.5Te0.5 thin films were grown by pulsed laser deposition on CaF2, LaAlO3 and MgO substrates and structurally and electro-magnetically characterized in order to study the influence of the substrate on their transport properties. The in-plane lattice mismatch between FeSe0.5Te0.5 bulk and the substrates shows no influence on the lattice parameters of the films, whereas the type of substrates affects the crystalline quality of the films and, therefore, the superconducting properties. The film on MgO showed an extra peak in the angular dependence of critical current density Jc(θ) at θ = 180° (H || c), which arises from c-axis defects as confirmed by transmission electron microscopy. In contrast, no Jc(θ) peaks for H || c were observed in films on CaF2 and LaAlO3. Jc(θ) can be scaled successfully for both films without c-axis correlated defects by the anisotropic Ginzburg-Landau (AGL) approach with appropriate anisotropy ratio γJ. The scaling parameter γJ is decreasing with decreasing temperature, which is different from what we observed in FeSe0.5Te0.5 films on Fe-buffered MgO substrates.
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Submitted 15 April, 2015;
originally announced April 2015.
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Synthesis and physical properties of Ca1-xRExFeAs2 with RE = La ~ Gd
Authors:
Alberto Sala,
Hiroyuki Yakita,
Hiraku Ogino,
Tomoyuki Okada,
Akiyasu Yamamoto,
Kohji Kishio,
Shigeyuki Ishida,
Akira Iyo,
Hiroshi Eisaki,
Masaya Fujioka,
Yoshihiko Takano,
Marina Putti,
Jun-ichi Shimoyama
Abstract:
Synthesis of a series of layered iron arsenides Ca1-xRExFeAs2 (112) was attempted by heating at 1000 C under a high-pressure of 2 GPa. The 112 phase successfully forms with RE = La, Ce, Nd, Sm, Eu and Gd, while Tb, Dy and Ho substituted and RE free samples does not contain the 112 phase. The Ce, Nd, Sm, Eu and Gd doped Ca1-xRExFeAs2 are new compounds. All of them exhibit superconducting transition…
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Synthesis of a series of layered iron arsenides Ca1-xRExFeAs2 (112) was attempted by heating at 1000 C under a high-pressure of 2 GPa. The 112 phase successfully forms with RE = La, Ce, Nd, Sm, Eu and Gd, while Tb, Dy and Ho substituted and RE free samples does not contain the 112 phase. The Ce, Nd, Sm, Eu and Gd doped Ca1-xRExFeAs2 are new compounds. All of them exhibit superconducting transition except for the Ce doped sample. The behaviour of the critical temperature, with the RE ionic radii have been investigated.
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Submitted 10 April, 2014;
originally announced April 2014.
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Highly effective and isotropic pinning in epitaxial Fe(Se,Te) thin films grown on CaF2 substrates
Authors:
V. Braccini,
S. Kawale,
E. Reich,
E. Bellingeri,
L. Pellegrino,
A. Sala,
M. Putti,
K. Higashikawa,
T. Kiss,
B. Holzapfel,
C. Ferdeghini
Abstract:
We report on the isotropic pinning obtained in epitaxial Fe(Se,Te) thin films grown on CaF2 (001) substrate. High critical current density values larger than 1 MA/cm2 in self field in liquid helium are reached together with a very weak dependence on the magnetic field and a complete isotropy. Analysis through Transmission Electron Microscopy evidences the presence of defects looking like lattice d…
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We report on the isotropic pinning obtained in epitaxial Fe(Se,Te) thin films grown on CaF2 (001) substrate. High critical current density values larger than 1 MA/cm2 in self field in liquid helium are reached together with a very weak dependence on the magnetic field and a complete isotropy. Analysis through Transmission Electron Microscopy evidences the presence of defects looking like lattice disorder at a very small scale, between 5 and 20 nm, which are thought to be responsible for such isotropic behavior in contrast to what observed on SrTiO3, where defects parallel to the c-axis enhance pinning in that direction
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Submitted 4 September, 2013;
originally announced September 2013.
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A new approach for improving global critical current density in Fe(Se0.5Te0.5) polycrystalline materials
Authors:
A. Palenzona,
A. Sala,
C. Bernini,
V. Braccini,
M. R. Cimberle,
C. Ferdeghini,
G. Lamura,
A. Martinelli,
I. Pallecchi,
G. Romano,
M. Tropeano,
R. Fittipaldi,
A. Vecchione,
A. Polyanskii,
F. Kametani,
M. Putti
Abstract:
A novel method to prepare bulk Fe(Se0.5Te0.5) samples is presented, based on a melting process and a subsequent annealing treatment. With respect to the standard sintering technique, it produces much more homogeneous and denser samples, characterized by large and well interconnected grains. The resulting samples exhibit optimal critical temperature values, sharp resistive and magnetic transitions,…
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A novel method to prepare bulk Fe(Se0.5Te0.5) samples is presented, based on a melting process and a subsequent annealing treatment. With respect to the standard sintering technique, it produces much more homogeneous and denser samples, characterized by large and well interconnected grains. The resulting samples exhibit optimal critical temperature values, sharp resistive and magnetic transitions, large magnetic hysteresis loops and high upper critical fields are observed. Interestingly, the global critical current density is much enhanced as compared to the values reported in literature for bulk samples of the same 11 family, reaching about 103 A/cm2 at zero field at 4.2 K as assessed by magnetic, transport and magneto-optical techniques. Even more importantly, its field dependence turns out to be very weak, such that at μ_{0}H = 7 T it is suppressed only by a factor \sim2.
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Submitted 21 May, 2012; v1 submitted 16 May, 2012;
originally announced May 2012.
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Isoelectronic Ru substitution at Fe-site in Sm(Fe1-xRux)As(O0.85F0.15) compound and its effects on structural, superconducting and normal state properties
Authors:
M. Tropeano,
M. R. Cimberle,
C. Ferdeghini,
G. Lamura,
A. Martinelli,
A. Palenzona,
I. Pallecchi,
A. Sala,
M. Putti,
F. Bernardini,
M. Monni,
S. Massidda,
I. Sheikin
Abstract:
In this work we present a systematic experimental and theoretical study of the structural, transport and superconducting properties of Sm(Fe1-xRux)As(O0.85F0.15) polycrystalline samples as a function of Ru content (x) ranging from 0 to 1. The choice of Ru as isoelectronic substitution at Fe site of F-doped compounds allows to better clarify the role of structural disorder in modifying the normal a…
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In this work we present a systematic experimental and theoretical study of the structural, transport and superconducting properties of Sm(Fe1-xRux)As(O0.85F0.15) polycrystalline samples as a function of Ru content (x) ranging from 0 to 1. The choice of Ru as isoelectronic substitution at Fe site of F-doped compounds allows to better clarify the role of structural disorder in modifying the normal and superconducting properties of these newly discovered multiband superconductors. Two different regions are identified: the Fe-rich phase (x<0.5) where superconducting and normal state properties are strongly affected by disorder induced by Ru substitution; the Ru-rich phase (x>0.5) where the system is metallic and strongly compensated and the presence of Ru frustrates the magnetic moment on Fe ions. Here the lack of magnetic features and related spin fluctuations may be the cause for the suppression of superconductivity.
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Submitted 12 April, 2010;
originally announced April 2010.