DAVID SANCHEZ - Academia.edu (original) (raw)

Papers by DAVID SANCHEZ

Physical Review B, 2006

We predict the occurrence of Fano lineshapes in a semiconductor quantum wire with local spinorbit... more We predict the occurrence of Fano lineshapes in a semiconductor quantum wire with local spinorbit (Rashba) coupling. We show that the Rashba interaction acts in a strictly one dimensional channel as an attractive impurity, leading to the formation of purely bound states. In a quasione dimensional system these bound states couple to the conduction ones through the Rashba intersubband mixing, giving rise to pronounced dips in the linear conductance plateaus. We give exact numerical results and propose an approximate model capturing the main ingredients of the effect.

Physical Review Letters, 2004

We investigate departures of the Onsager relations in the nonlinear regime of electronic transpor... more We investigate departures of the Onsager relations in the nonlinear regime of electronic transport through mesoscopic systems. We show that the nonlinear current-voltage characteristic is not an even function of the magnetic field due only to the magnetic-field dependence of the screening potential within the conductor. We illustrate this result for two types of conductors: A quantum Hall bar with an antidot and a chaotic cavity connected to quantum point contacts. For the chaotic cavity we obtain through random matrix theory an asymmetry in the fluctuations of the nonlinear conductance that vanishes rapidly with the size of the contacts.

Physical Review Letters, 2012

Fluctuation relations are derived in systems where the spin degree of freedom and magnetic intera... more Fluctuation relations are derived in systems where the spin degree of freedom and magnetic interactions play a crucial role. The form of the non-equilibrium fluctuation theorems relies in the assumption of a local balance condition. We demonstrate that in some cases the presence of magnetic interactions violates this condition. Nevertheless, fluctuation relations can be obtained from the micro-reversibility principle sustained only at equilibrium as a symmetry of the cumulant generating function for spin currents. We illustrate the spintronic fluctuation relations for a quantum dot coupled to partially polarized helical edges states . 73.50.Fq, 73.63.Kv Introduction. Non-equilibrium fluctuation theorems (FTs) [1-3], widely used for macroscopic systems, are based on the thermodynamics governing the physical processes when they evolve forward and backward in time. The boundary conditions for the forward and the time-reversed processes determine the balance condition for the entropy exchange and therefore the form of the fluctuation theorem . The applicability of the non-equilibrium FTs to quantum systems has become an exciting problem and, in particular, to the case of the charge transfer phenomena in mesoscopic systems in the context of the full counting statistics . Interestingly, relations akin to the fluctuation-dissipation theorem have been formulated beyond the linear response regime . These fluctuation relations relate nonequilibrium fluctuation and dissipation coefficients for phase-coherent conductors. However, the role of a genuine quantum property such as the spin degree of freedom in the fluctuation relations has not been yet investigated in detail. Our motivation is not only fundamental since the electronic spin offers enormous advantages to create devices with unusual and extraordinary new functionalities . The purpose of this work is thus to generalize the fluctuation relations for spintronic systems.

Physical Review Letters, 2013

We investigate nonlinear transport properties of quantum conductors in response to both electrica... more We investigate nonlinear transport properties of quantum conductors in response to both electrical and thermal driving forces. Within the scattering approach, we determine the nonequilibrium screening potential of a generic mesoscopic system and find that its response is dictated by particle and entropic injectivities which describe the charge and entropy transfer during transport. We illustrate our model analyzing the voltage and thermal rectification of a resonant tunneling barrier. Importantly, we discuss interaction induced contributions to the thermopower in the presence of large temperature differences.

Physical Review Letters, 2010

When a biased conductor is put in proximity with an unbiased conductor a drag current can be indu... more When a biased conductor is put in proximity with an unbiased conductor a drag current can be induced in the absence of detailed balance. This is known as the Coulomb drag effect. However, even in this situation far away from equilibrium where detailed balance is explicitly broken, theory predicts that fluctuation relations are satisfied. This surprising effect has, to date, not been confirmed experimentally. Here we propose a system consisting of a capacitively coupled double quantum dot where the nonlinear fluctuation relations are verified in the absence of detailed balance. PACS numbers: 73.23.-b 72.70.+m 73.63.Kv

Physical Review B, 2005

We investigate the two-terminal nonlinear conductance of a Coulomb-blockaded quantum dot attached... more We investigate the two-terminal nonlinear conductance of a Coulomb-blockaded quantum dot attached to chiral edge states. Reversal of the applied magnetic field inverts the system chirality and leads to a different polarization charge. As a result, the current-voltage characteristic is not an even function of the magnetic field. We show that the corresponding magnetic-field asymmetry arises from single-charge effects and vanishes in the limit of high temperature.

Physical Review B, 2009

We determine the shot noise asymmetry of a quantum dot under reversal of an external magnetic fie... more We determine the shot noise asymmetry of a quantum dot under reversal of an external magnetic field. The dot is coupled to edge states which invert their chirality when the field is reversed, leading to a magnetoasymmetric electrochemical potential in the nanostructure. Surprisingly, we find an exact relation between the magnetoasymmetries corresponding to the nonlinear conductance and the shot noise to leading order in the applied bias, implying a higher-order fluctuation-dissipation relationship. Our calculations also show a magnetoasymmetry of the full probability distribution of the transferred charge.

Mathematical Biosciences, 1984

Journal of Statistical Physics, 2010

We analyze a macroscopic model with a maximal density constraint which describes short range repu... more We analyze a macroscopic model with a maximal density constraint which describes short range repulsion in biological systems. This system aims at modeling finite-size particles which cannot overlap and repel each other when they are too close. The parts of the fluid where the maximal density is reached behave like incompressible fluids while lower density regions are compressible. This paper investigates the transition between the compressible and incompressible regions. To capture this transition, we study a one-dimensional Riemann problem and introduce a perturbation problem which regularizes the compressible-incompressible transition. Specific difficulties related to the non-conservativity of the problem are discussed.

Lecture Notes in Computer Science, 2007

One of the hardest tasks in any healthcare application is the management of knowledge. Organisati... more One of the hardest tasks in any healthcare application is the management of knowledge. Organisational information as well as medical concepts should be represented in an appropriate way in order to improve interoperability among existing systems, to allow the implementation of knowledge-based intelligent systems, or to provide high level support to healthcare professionals. This paper proposes the inclusion of an especially designed ontology into an agent-based medical platform called HeCaSe2. The ontology has been constructed as an external resource, allowing agents to coordinate complex activities defined in any clinical guideline.

Studies in Computational Intelligence, 2010

Agent technology has emerged as a promising technology to develop complex and distributed systems... more Agent technology has emerged as a promising technology to develop complex and distributed systems. They have been extensively applied in a wide range of healthcare areas in the last 10 years. This chapter introduces the basic aspects and properties of agents and multi-agent systems and argues their adequacy to solve different kinds of problems in healthcare. It also describes several healthcare areas in which this technology has been successfully applied, introducing, in each case, some of the most relevant works. The explanation is illustrated with a more detailed description of two agent-based healthcare applications in which the authors have been involved. One of them automates the enactment of clinical guidelines by coordinating several healthcare entities of a medical organization. The other is a Web-based knowledge-driven platform that supports the execution of personalized Home Care services. The benefits that agent technology brings to those approaches are commented and several lines of future research are enounced.

Lecture Notes in Computer Science, 2005

Accessing up-to-date information in a fast and easy way implies the necessity of information mana... more Accessing up-to-date information in a fast and easy way implies the necessity of information management tools to explore and analyse the huge number of available electronic resources. The Web offers a large amount of valuable information, but its human-oriented representation and its size makes extremely difficult any kind of computer-based processing. In this paper, a combination of distributed AI and information extraction techniques is proposed to tackle this problem. In particular, we have designed a multiagent system that composes ontologies from taxonomies of terms. Moreover, the obtained ontology is used to represent, in a structured way, the currently available web resources. The paper analyses the application of this approach in some examples in the medical domain.

IFIP International Federation for Information Processing, 2006

Many public key infrastructure (PKI) approaches have been proposed in the recent years to secure ... more Many public key infrastructure (PKI) approaches have been proposed in the recent years to secure mobile ad hoc networks (MANETs). We present a new hybrid key management infrastructure, which combines the concepts of PKIs for MANET with trusted-third-party based infrastructures. In our hybrid approach, the underlying PKI is merely used to set-up initial trust of nodes in a MANET, and, thus, generate a random trust graph connecting all the nodes of the MANET. Then, MANET nodes cooperate to securely distribute trust information and symmetric keys to other nodes through the shortest trust path. The hybrid key management infrastructure enables the same security services as a normal PKI yet key establishment and node-to-node authentication, as demonstrated by our performance analysis, is substantially improved in terms of computational and communication efficiency. We also discuss the security level of the hybrid approach.

Lecture Notes in Computer Science, 2006

... they are close to each other but disconnected from MSNs, they can cooperatively check forrepl... more ... they are close to each other but disconnected from MSNs, they can cooperatively check forreplicated identities by ... J. Newsome, E. Shi, D. Song and A. Perrig: The sybil attack in sensor net-works: analysis & ... Y. Zhang and W. Lee: Intrusion Detection in Wireless Ad-Hoc Networks ...

International Workshop on Security, 2005

Abstract Key management is paramount for mobile sensor network (MSN) security. Due to the charact... more Abstract Key management is paramount for mobile sensor network (MSN) security. Due to the characteristics of MSNs, key management must enable direct (without intermediaries) key establishment between two arbitrary nodes. We apply combinatorial design theory to ...

Knowledge-Based Systems, 2011

The information content (IC) of a concept provides an estimation of its degree of generality/conc... more The information content (IC) of a concept provides an estimation of its degree of generality/concreteness, a dimension which enables a better understanding of concept's semantics. As a result, IC has been successfully applied to the automatic assessment of the semantic similarity between concepts. In the past, IC has been estimated as the probability of appearance of concepts in corpora. However, the applicability and scalability of this method are hampered due to corpora dependency and data sparseness. More recently, some authors proposed IC-based measures using taxonomical features extracted from an ontology for a particular concept, obtaining promising results. In this paper, we analyse these ontology-based approaches for IC computation and propose several improvements aimed to better capture the semantic evidence modelled in the ontology for the particular concept. Our approach has been evaluated and compared with related works (both corpora and ontology-based ones) when applied to the task of semantic similarity estimation. Results obtained for a widely used benchmark show that our method enables similarity estimations which are better correlated with human judgements than related works.

IEEE Intelligent Systems, 2000

EURASIP Journal on Wireless Communications and Networking, 2011

Secure time synchronization is a paramount service for wireless sensor networks (WSNs) constitute... more Secure time synchronization is a paramount service for wireless sensor networks (WSNs) constituted by multiple interconnected body area networks (BANs). We propose a novel approach to securely and efficiently synchronize nodes at BAN level and/or WSN level. Each BAN develops its own notion of time. To this effect, the nodes of a BAN synchronize with their BAN controller node. Moreover, controller nodes of different BANs cooperate to agree on a WSN global and/or to transfer UTC time. To reduce the number of exchanged synchronization messages, we use an environmental-aware time prediction algorithm. The performance analysis in this paper shows that our approach exhibits very advanced security, accuracy, precision, and low-energy trade-off. For comparable precision, our proposal outstands related clock synchronization protocols in energy efficiency and risk of attacks. These results are based on computations.