Papers by Daniel Huertas-hernando
We consider two mechanisms of spin relaxation in disordered graphene. i) Spin relaxation due to c... more We consider two mechanisms of spin relaxation in disordered graphene. i) Spin relaxation due to curvature spin orbit coupling caused by ripples. ii) Spin relaxation due to the interaction of the electronic spin with localized magnetic moments at the edges. We obtain analytical expressions for the spin relaxation times, tau_SO and tau_J due to both mechanisms and estimate their values for realistic parameters of graphene samples. We obtain that spin relaxation originating from these mechanisms is very weak and spin coherence is expected in disordered graphene up to samples of length L ~ 1 micron.
We study the production of spatially separated entangled electrons in ferromagnetic leads from Co... more We study the production of spatially separated entangled electrons in ferromagnetic leads from Cooper pairs in a superconducting lead. We give a complete description of the elementary charge transfer processes, i) transfer of Cooper pairs out of the superconductor by Andreev reflection and ii) distribution of the entangled quasiparticles among the ferromagnetic leads, in terms of their statistics. The probabilities that entangled electrons flow into spatially separated leads are completely determined by experimentally measurable conductances and polarizations. Finally, we investigate how currents, noise and cross correlations are affected by transport of entangled electrons.
Magnetoelectronics is mainly digital, i.e. governed by up and down magnetizations. In contrast, a... more Magnetoelectronics is mainly digital, i.e. governed by up and down magnetizations. In contrast, analogue magnetoelectronics makes use of phenomena occuring for non-collinear magnetization configurations. Here we review theories which have recently been applied to the transport in non-collinear magnetic nanostructures in two and multiterminal structures, viz. random matrix and circuit theory. Both are not valid for highly transparent systems in a resistive environment like perpendicular metallic spin valves. The solution to this problem is a renormalization of the conventional and spin-mixing conductance parameters.
Semiclassical theories of electron and spin transport in metallic magnetic structures are reviewe... more Semiclassical theories of electron and spin transport in metallic magnetic structures are reviewed with emphasis on the role of disorder and electronic band structures in the current perpendicular to the interface plane (CPP) transport configuration.
arXiv: Superconductivity, 2002
The circuit theory of mesoscopic transport provides a unified framework to describe spin-dependen... more The circuit theory of mesoscopic transport provides a unified framework to describe spin-dependent or superconductivity-related phenomena. We extend this theory to hybrid systems of normal metals, ferromagnets and superconductors. Our main result is an expression for the current through an arbitrary contact between two general isotropic "nodes", which is suitable to describe the presence of superconducting and ferromagnetic elements in the system, as well as magnetically active interfaces/contacts. In certain cases (weak ferromagnet and magnetic tunnel junction) we derive transparent and simple results for the matrix current.
The European Regulators' Group for Electricity and Gas, ERGEG, has published Guidelines of Good P... more The European Regulators' Group for Electricity and Gas, ERGEG, has published Guidelines of Good Practice on Electricity Balancing Markets Integration (GGP-EBMI). The ERGEG document of September 09 [4] is the main source for defining the precondition for future cross border balancing. The national regulators will of course have a central role in the development of and the governance of the future market rules. 2.1.1 Security of supply The TSOs are responsible to ensure system security within their control area. Cross-border balancing shall not jeopardise system security. 2.1.2 Market based Mechanisms The purpose of the balancing markets is to secure balance between supply and demand of the system in short term in an economically-efficient manner. Hence, balancing markets shall operate in a market-oriented way. For reasons of overall efficiency, the selection of bids shall be based on the merit order of the balancing offers as well as network constraints. Any deviation from the merit order shall only be accepted when it is necessary to maintain system security. 2.1.3 Effective Competition A well-functioning balancing market shall be robust to any exercise of market power. 2.1.4 Impact of Cross-Border Trade Cross-border balancing shall not lead to withdrawal of interconnection capacity from the market players and neither shall it limit opportunities for cross-border trade. Comment: In special case where HVDC capacity is reserved for cross-border balancing, procurement of reserve capacity shall be subject of evaluation of the TSOs, in accordance with criteria defined by ENTSO-E and approved by regulators in a transparent way.
Bulletin of the American Physical Society, 2010
Using scattering theory we investigate the transport properties of three-terminal devices where o... more Using scattering theory we investigate the transport properties of three-terminal devices where one terminal is superconducting and two are normal metals. The contributions from electron transfer (ET) and crossed Andreev reflection (CAR) to the non-local signal between the two normal terminals are identified. We compute ET and CAR numerically for asymmetric devices. For symmetric devices, analytical expressions for ET and
Wind Energy, 2014
ABSTRACT The expected increase of wind power production in the North and Baltic Seas will substan... more ABSTRACT The expected increase of wind power production in the North and Baltic Seas will substantially increase the variability of the generation portfolio in Northern Europe. Access to available resources of flexible power production will be necessary to support the power system against this variability. Since the Nordic hydro-based power system can provide such resources, a stronger interconnection between continental Europe and the Nordic region seems to be beneficial. This paper assesses the challenges related to wind power production variability, especially offshore, in the North and Baltic Seas. Assessment on the transmission grid needed for both harvesting the available wind production located far away from load centres and to enable the optimal use of hydropower flexibility is studied in a long-term cost-benefit analysis. Special focus is devoted to the role of an offshore grid structure and the impact of onshore grid constraints. The analysis includes two interrelated simulation steps. The first step focuses on the strategic use of hydro energy in the day-ahead market, where detailed modelling of water courses and hydro production in the Nordic region is considered. Then, in a second step, flow-based simulations are conducted on a detailed grid model for the whole European system. The results show that long-term strategies for the expansion of offshore and onshore grids must be defined in a coordinated way to ensure optimal developments. Copyright © 2014 John Wiley & Sons, Ltd.
Physical Review Letters, 2008
We study the supercurrent in a superconductor/ferromagnet/superconductor graphene junction. In co... more We study the supercurrent in a superconductor/ferromagnet/superconductor graphene junction. In contrast to its metallic counterpart, the oscillating critical current in our setup decays only weakly upon increasing exchange field and junction width. We find an unusually large residual value of the supercurrent at the oscillatory cusps due to a strong deviation from a sinusoidal current-phase relationship. Our findings suggest a very efficient device for dissipationless supercurrent switching.
2012 IEEE Power and Energy Society General Meeting, 2012
The large remote offshore wind clusters that are planned in the North Sea will most likely be con... more The large remote offshore wind clusters that are planned in the North Sea will most likely be connected with a meshed HVDC grid. Power will mostly flow from the offshore wind clusters to shore, creating asymmetrical requirements for the HVDC links that will consist of several parallel HVDC systems. To realise an asymmetrical link, some of those systems could be designed unidirectional, resulting in possible changes and simplifications (especially to the protection system). Assessment of a future scenario has shown that 42% of the HVDC systems can only be operated unidirectional. The remaining systems could in theory be used both directions, but power flow optimisation has shown, that this will in many cases not happen. A first cost calculation has shown that almost 6% of the investment cost can be saved when asymmetric design is implemented. This indicates the need to consider asymmetric design and to develop unidirectional HVDC systems.
We study the proximity effect in hybrid structures consisting of superconductor and ferromagnetic... more We study the proximity effect in hybrid structures consisting of superconductor and ferromagnetic insulator separated by a normal diffusive metal (S/N/FI structures). These stuctures were proposed to realize the absolute spin-valve effect. We pay special attention to the gaps in the density of states of the normal part. We show that the effect of the ferromagnet is twofold: It not only shifts the density of states but also provides suppression of the gap. The mechanism of this suppression is remarkably similar to that due to magnetic impurities. Our results are obtained from the solution of one-dimensional Usadel equation supplemented with boundary conditions for matrix current at both interfaces.
Wiley Interdisciplinary Reviews: Energy and Environment, 2016
Journal of Magnetism and Magnetic Materials, May 1, 2001
We solve the di!usion equation in the normal metal of ferromagnet}normal metal}ferromagnet (F}N}F... more We solve the di!usion equation in the normal metal of ferromagnet}normal metal}ferromagnet (F}N}F) systems with non-collinear magnetizations. It is shown that the total conductance of the system in the presence of a magnetic "eld can be asymmetric with respect to time reversal. This asymmetry is associated with precession processes described by the imaginary part of the recently introduced mixing conductance.
Physical Review Letters, 2009
We investigate how spins relax in intrinsic graphene. The spin-orbit coupling arises from the ban... more We investigate how spins relax in intrinsic graphene. The spin-orbit coupling arises from the band structure and is enhanced by ripples. The orbital motion is influenced by scattering centers and ripple-induced gauge fields. Spin relaxation due to Elliot-Yafet and Dyakonov-Perel mechanisms and gauge fields in combination with spin-orbit coupling are discussed. In intrinsic graphene, the Dyakonov-Perel mechanism and spin flip due to gauge fields dominate and the spin-flip relaxation time is inversely proportional to the elastic scattering time. The spin-relaxation anisotropy depends on an intricate competition between these mechanisms. Experimental consequences are discussed.
In closed dots the evolution of a Coulomb-blockade conductance peak height as a function of an ex... more In closed dots the evolution of a Coulomb-blockade conductance peak height as a function of an external parameter (such as a magnetic field or the shape of the dot) can be studied by, e.g., calculating the correlation between the peak-height for different values of the parameter. Parametric correlations of the peak height in a chaotic or weakly disordered quantum dot
We study the finite temperature Coulomb blockade peak motion in a quantum dot in the presence of ... more We study the finite temperature Coulomb blockade peak motion in a quantum dot in the presence of a parallel magnetic field. Parallel field dependence of Coulomb blockade peak position has been suggested as an experimental technique to determine the ground-state spin of a quantum dot [1]. We find that in the presence of an exchange interaction, the peak motion is
We solve the di!usion equation in the normal metal of ferromagnet}normal metal}ferromagnet (F}N}F... more We solve the di!usion equation in the normal metal of ferromagnet}normal metal}ferromagnet (F}N}F) systems with non-collinear magnetizations. It is shown that the total conductance of the system in the presence of a magnetic "eld can be asymmetric with respect to time reversal. This asymmetry is associated with precession processes described by the imaginary part of the recently introduced mixing conductance.
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Papers by Daniel Huertas-hernando