Multiscale approach to spin transport in magnetic multilayers

This article discusses two dual approaches to spin transport in magnetic multilayers: a direct, purely quantum, approach based on a Tight-Biding model (TB) and a semiclassical approach (Continuous Random Matrix Theory, CRMT). The combination of both approaches provides a systematic way to perform multi-scales simulations of systems that contain relevant physics at scales larger (spin accumulation, spin diffusion...) and smaller (specular reflexions, tunneling...) than the elastic mean free paths of the layers. We show explicitly that CRMT and TB give consistent results in their common domain of applicability

Mesoscopic Fluctuations of the Loschmidt Echo

We investigate the time-dependent variance of the fidelity with which an initial narrow wavepacket is reconstructed after its dynamics is time-reversed with a perturbed Hamiltonian. In the semiclassical regime of perturbation, we show that the variance first rises algebraically up to a critical time $t_c$, after which it decays. To leading order in the effective Planck's constant $\hbar_{\rm eff}$, this decay is given by the sum of a classical term $\simeq \exp[-2 \lambda t]$, a quantum term $\simeq 2 \hbar_{\rm eff} \exp[-\Gamma t]$ and a mixed term $\simeq 2 \exp[-(\Gamma+\lambda)t]$. Compared to the behavior of the average fidelity, this allows for the extraction of the classical Lyapunov exponent $\lambda$ in a larger parameter range. Our results are confirmed by numerical simulations.Comment: Final, extended version; to appear in Physical Review

Graphene-based heterojunction between two topological insulators

Quantum Hall (QH) and quantum spin Hall (QSH) phases have very different edge states and, when going from one phase to the other, the direction of one edge state must be reversed. We study this phenomena in graphene in presence of a strong perpendicular magnetic field on top of a spin-orbit (SO) induced QSH phase. We show that, below the SO gap, the QSH phase is virtually unaffected by the presence of the magnetic field. Above the SO gap, the QH phase is restored. An electrostatic gate placed on top of the system allows to create a QSH-QH junction which is characterized by the existence of a spin-polarized chiral state, propagating along the topological interface. We find that such a setup naturally provides an extremely sensitive spin-polarized current switch.Comment: 10 pages, 5 figure

Interplay between non equilibrium and equilibrium spin torque using synthetic ferrimagnets

We discuss the current induced magnetization dynamics of spin valves F0|N|SyF where the free layer is a synthetic ferrimagnet SyF made of two ferromagnetic layers F1 and F2 coupled by RKKY exchange coupling. In the interesting situation where the magnetic moment of the outer layer F2 dominates the magnetization of the ferrimagnet, we find that the sign of the effective spin torque exerted on the free middle layer F1 is controlled by the strength of the RKKY coupling: for weak coupling one recovers the usual situation where spin torque tends to, say, anti-align the magnetization of F1 with respect to the pinned layer F0. However for large coupling the situation is reversed and the spin torque tends to align F1 with respect to F0. Careful numerical simulations in the intermediate coupling regime reveal that the competition between these two incompatible limits leads generically to spin torque oscillator (STO) behavior. The STO is found in the absence of magnetic field, with very significant amplitude of oscillations and frequencies up to 50 GHz or higher

Semiclassical gaps in the density of states of chaotic Andreev billiards

The connection of a superconductor to a chaotic ballistic quantum dot leads to interesting phenomena, most notably the appearance of a hard gap in its excitation spectrum. Here we treat such an Andreev billiard semiclassically where the density of states is expressed in terms of the classical trajectories of electrons (and holes) that leave and return to the superconductor. We show how classical orbit correlations lead to the formation of the hard gap, as predicted by random matrix theory in the limit of negligible Ehrenfest time \tE, and how the influence of a finite \tE causes the gap to shrink. Furthermore, for intermediate \tE we predict a second gap below E=\pi\hbar /2\tE which would presumably be the clearest signature yet of \tE-effects.Comment: Refereed version. 4 pages, 3 figure

Semiclassical approach to the ac-conductance of chaotic cavities

We address frequency-dependent quantum transport through mesoscopic conductors in the semiclassical limit. By generalizing the trajectory-based semiclassical theory of dc quantum transport to the ac case, we derive the average screened conductance as well as ac weak-localization corrections for chaotic conductors. Thereby we confirm respective random matrix results and generalize them by accounting for Ehrenfest time effects. We consider the case of a cavity connected through many leads to a macroscopic circuit which contains ac-sources. In addition to the reservoir the cavity itself is capacitively coupled to a gate. By incorporating tunnel barriers between cavity and leads we obtain results for arbitrary tunnel rates. Finally, based on our findings we investigate the effect of dephasing on the charge relaxation resistance of a mesoscopic capacitor in the linear low-frequency regime

Displacement Echoes: Classical Decay and Quantum Freeze

Motivated by neutron scattering experiments, we investigate the decay of the fidelity with which a wave packet is reconstructed by a perfect time-reversal operation performed after a phase space displacement. In the semiclassical limit, we show that the decay rate is generically given by the Lyapunov exponent of the classical dynamics. For small displacements, we additionally show that, following a short-time Lyapunov decay, the decay freezes well above the ergodic value because of quantum effects. Our analytical results are corroborated by numerical simulations

DEVELOPMENT OF NEW ITU APPROACH TO INTERNATIONAL FREQUENCY SPECTRUM MANAGEMENT AND STANDARDIZATION PROCESSES

Mednarodna Unija za telekomunikacije (ITU) je specializirana organizacija Združenih Narodov za telekomunikacije s sedežem v Ženevi, ki med drugim skrbi za učinkovito in usklajeno upravljanje z radiofrekvenčnim spektrom (10kHz-3000GHz) in to na način, da so zagotovljene pravice in enake možnosti njegovega izkoriščanja vsem uporabnikom, državam članicam (193). Vse od njene ustanovitve leta 1865 je tehnološki napredek motiviral oblikovanje ITU mednarodnih standardov za izgradnjo in funkcioniranje telekomunikacijskih sistemov in omrežij in kasneje tudi za izkoriščanje radiofrekvenčnega spektra. Tehnološki razvoj je uravnaval dinamiko aktivnosti in pogojeval organizacijske spremembe in statutarne reforme. Strokovna priporočila pa so postala univerzalna referenca za uspešen razvoj in standardizacijo telekomunikacijskih sistemov. Pa vendar je že nekaj časa opaziti, da je učinkovitost ITU v vse večjem razkoraku z evolucijo in preobrazbo telekomunikacijskega okolja, pri čemer ni skrivnost, da so številni poskusi zadnjih let, kako uskladiti strukturo in delovanje ITU z dinamičnim razvojem okolja, bili neuspešni. Inertnost, pa tudi oportunizem različnih subjektov, je bila in je še vedno močna ovira, da bi se ITU uspešno spopadala z izzivi časa. Radiofrekvenčni spekter in orbitalne pozicije, ki pripadajo različnim radiofrekvenčnim alokacijam, so edinstvena naravna dobrina, katere usklajena in transparentna mednarodna regulativa je permanentna skrb vsake suverene države. Učinkoviti procesi standardizacije v globalnem okolju pa so ključ za uspešno pot k hitremu razvoju telekomunikacijske infrastrukture v času, ko je tehnološka konvergenčnost izbrisala meje med tehnologijo, telekomunikacijskimi sistemi in telekomunikacijskimi aplikacijami. Disertacija ponuja podrobno analizo aktivnosti ITU na dveh osrednjih segmentih, t.j. radiokomunikacije in standardizacija. Na osnovi dolgoletnega opazovanja in proučevanja dela in učinkovitosti ITU, zaključki nedvoumno kažejo na potrebo po strukturnih reformah, pot do katerih pa je zaradi politične dimenzije, ki je vedno prisotna v medvladnih organizacijah, izjemno zapletena. V sodobnem telekomunikacijskem okolju, kjer tehnološki razvoj prehiteva počasne vladne odločitve, so potrebne jasne strateške orientacije in trdna odločenost držav članic, da se pristopi k vsebinskim spremembam. S tem namenom je predstavljen tudi predlog novega konstitutivnega in organizacijskega modela ITU, katerega implementacija bi omogočila boljše pogoje za doseganje zastavljenih ciljev. Sedanje težave v funkcioniranju te specializirane organizacije OZN so v največji meri posledica njenega zaostalega organizacijskega koncepta, ki sodi v zgodovinsko okolje. Države članice imajo še vedno izključno vlogo pri pomembnih odločitvah, žal pa njihove vlade ne premorejo več strokovnega potenciala za doseganje znanstvenega napredka. V ta prostor so vstopili drugi kreativni deležniki (iz industrije in nevladnih organizacij) ki so postali gonilna sila vsega razvoja. To so tisti ki želijo ustvarjati v drugačnem okolju, kjer bodo suvereni v odločanju na področjih njihove izključne kompetentnosti. Pogosto zapletena razmerja med državami članicami z različnimi interesi, velikimi in malimi, razvitimi in manj razvitimi, sposobnimi in odsotnimi, egoističnimi in kooperativnimi – onemogočajo univerzalne rešitve, ko gre za učinkovito in transparentno uporabo radiofrekvenčnega spektra. Na področju standardizacije ICT sistemov in aplikacij so razmere še bolj zapletene, saj so ti procesi v zadnjih desetletjih spremenili vsebino zaradi potreb po hitrem regionalnem razvoju in predvsem zaradi hitrega tehnološkega napredka, kateremu tradicionalni in klasično počasni ITU pristop k standardizaciji ni mogel več slediti. Dinamika razvoja pa kliče po odprtju in inovacijah tistih, ki so teh inovacij sposobni in ki imajo za te aktivnosti tudi primerne finančne in intelektualne vire. Državne strukture so v standardizacijskih postopkih in razvoju postale irelevantne, čeprav se še vedno z veliko silo oklepajo starih prerogativov. V zadnjih dveh desetletjih se je v znanstvene študije, analize in v izpeljavo tehničnih programov nenapovedano in samovoljno vrinila še »politizacija dogovarjanja« na različnih nivojih mednarodnega tehničnega sodelovanja. Ta realnost je dodatno poslabšala razmere v sicer arhaičnih strukturah ITU. Cilj disertacije je nov pristop k mednarodni regulativi uporabe spektra in njegovi mednarodni zaščiti na način, da bo koriščenje frekvenčnega spektra kar najbolj učinkovito in usklajeno. Radiofrekvenčni spekter je univerzalna dobrina, katere uporabnost ni v lasti nikogar in kjer akademske študije in dosežki odpirajo nove dimenzije za njegovo praktično uporabo. V okolju globalnega dogovarjanja in univerzalnega izkoriščanja spektra so pomembni odgovori na spodnja vprašanja, ki nas lahko pripeljejo k novi paradigmi: Kako zagotoviti transparentnost in učinkovitost pri planiranju, razdeljevanju in dodeljevanju frekvenčnega spektra? Kako pritegniti h kreativnemu sodelovanju nove Partnerje (industrija in privatni sektor) in jim zagotoviti pogoje za inovativne rešitve v standardizaciji ICT. Kako ustvariti homogeno in konvergenčno okolje, ki bi zagotavljalo nenehno evolucijo ICT k racionalnim in globalnim usmeritvam pri določanju komunikacijske politike?The convergence of fast developing technologies (ICT), the globalisation of telecommunications in general and availability of new services, have profoundly changed the telecommunication environment and related national policies. The ITU constituencies are Member States and the Organisation strategic decisions are taken exclusively by the Governments. However, in the changing environment it became impossible to ignore the non-governmental partners’ views while discussing the global policies and action plans that should be leading to important decisions. The appearance of strategic alliances, privatization of telecommunication infrastructures and market liberalization are the phenomena that brought new elements in telecommunication development and had substantially modified the role of Governments. These phenomena influenced the Union operational model to great extent where voices of partners are claiming more balanced share in the decision making processes and rational distribution of responsibilities, i.e. between the administrations of Members States, national regulators, telecommunication operators and industry. The necessary transparency of regulatory policy together with simplified Frequency Spectrum Management and universally recognized standardisation activities may be endangered, if no constitutional reforms intervene in the next two or three plenipotentiary cycles. The situation is calling for open review of governmental responsibilities in order to make realistic projection of reforms that should allow ITU to preserve its technical sovereignty and global credibility through its important mission. The ITU structure and its mode of operation in the field of radiocommunications and ICT standardisation today appear to be archaic and not synchronized neither with the environment nor with Members and other user’s needs. National concern for harmonious development of Information and Communication Infrastructure (ICT) remain high on each Government agenda due to its substantial impact on national economies. There is strategic interest to build up and preserve well-coordinated international regulatory regime on Frequency Spectrum/Orbit Management and global/universal and transparent ICT standardisation processes. And the present thesis is a modest contribution to make it happen

The density of states of chaotic Andreev billiards

Quantum cavities or dots have markedly different properties depending on whether their classical counterparts are chaotic or not. Connecting a superconductor to such a cavity leads to notable proximity effects, particularly the appearance, predicted by random matrix theory, of a hard gap in the excitation spectrum of quantum chaotic systems. Andreev billiards are interesting examples of such structures built with superconductors connected to a ballistic normal metal billiard since each time an electron hits the superconducting part it is retroreflected as a hole (and vice-versa). Using a semiclassical framework for systems with chaotic dynamics, we show how this reflection, along with the interference due to subtle correlations between the classical paths of electrons and holes inside the system, are ultimately responsible for the gap formation. The treatment can be extended to include the effects of a symmetry breaking magnetic field in the normal part of the billiard or an Andreev billiard connected to two phase shifted superconductors. Therefore we are able to see how these effects can remold and eventually suppress the gap. Furthermore the semiclassical framework is able to cover the effect of a finite Ehrenfest time which also causes the gap to shrink. However for intermediate values this leads to the appearance of a second hard gap - a clear signature of the Ehrenfest time.Comment: Refereed version. 23 pages, 19 figure