@phdthesis{Pakkayil2017,
  author    = {Pakkayil, Shijin Babu},
  title     = {Towards ferromagnet/superconductor junctions on graphene},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-153863},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2017},
  abstract  = {This thesis reports a successful fabrication and characterisation of ferromagnetic/superconductor junction (F/S) on graphene. The thesis preposes a fabrication method to produce F/S junctions on graphene which make use of ALD grown Al2O3 as the tunnel barrier for the ferromagnetic contacts. Measurements done on F/G/S/G/F suggests that by injecting spin polarised current into the superconductor, a spin imbalance is created in the quasiparticle density of states of the superconductor which then diffuses through the graphene channel. The observed characteristic curves are similar to the ones which are already reported on metallic ferromagnet/superconductor junctions where the spin imbalance is created using Zeeman splitting. Further measurements also show that the curves loose their characteristic shapes when the temperature is increased above the critical temperature (Tc) or when the external magnetic field is higher then the critical field (Hc) of the superconducting contact. But to prove conclusively and doubtlessly the existence of spin imbalance in ferromagnet/superconductor junctions on graphene, more devices have to be made and characterised preferably in a dilution refrigerator.},
  subject      = {Graphen},
  language  = {en}
}
@unpublished{Reiss2012,
  author    = {Reiss, Harald},
  title     = {Time scales and existence of time holes in non-transparent media},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-73554},
  year      = {2012},
  abstract  = {The analysis presented in this paper applies to experimental situations where observers or objects to be studied, all at stationary positions, are located in environments the optical thickness of which is strongly different. Non-transparent media comprise thin metallic films, packed or fluidised beds, superconductors, the Earth's crust, and even dark clouds and other cosmological objects. The analysis applies mapping functions that correlate physical events, e, in non-transparent media, with their images, f(e), tentatively located on standard physical time scale. The analysis demonstrates, however, that physical time, in its rigorous sense, does not exist under non-transparency conditions. A proof of this conclusion is attempted in three steps: i) the theorem "there is no time without space and events" is accepted, (ii) images f[e(s,t)] do not constitute a dense, uncountably infinite set, and (iii) sets of images that are not uncountably infinite do not create physical time but only time-like sequences. As a consequence, mapping f[e(s,t)] in non-transparent space does not create physical analogues to the mathematical structure of the ordered, dense half-set R+ of real numbers, and reverse mapping, f-1f[e(s,t)], the mathematical inverse problem, would not allow unique identification and reconstruction of original events from their images. In these cases, causality as well as invariance of physical processes under time reversal, might be violated. An interesting problem is whether temporal cloaking (a time hole) in a transparent medium, as very recently reported in the literature, can be explained by the present analysis. Existence of time holes could perhaps be possible, not in transparent but in non-transparent media, as follows from the sequence of images, f[e(s,t)], that is not uncountably infinite, in contrast to R+. Impacts are expected for understanding physical diffusion-like, radiative transfer processes and stability models to protect superconductors against quenchs. There might be impacts also in relativity, quantum mechanics, nuclear decay, or in systems close to their phase transitions. The analysis is not restricted to objects of laboratory dimensions.},
  subject      = {Zeitrichtung},
  language  = {en}
}
@unpublished{Reiss2012,
  author    = {Reiss, Harald},
  title     = {Physical time and existence of time holes in non-transparent media},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-67268},
  year      = {2012},
  abstract  = {The analysis presented in this paper applies to experimental situations where observers or objects to be studied (both stationary, with respect to each other) are located in environments the optical thickness of which is strongly different. By their large optical thickness, non-transparent media are clearly distinguished from their transparent counterparts. Non-transparent media comprise thin metallic films, packed or fluidised beds, the Earth's crust, and even dark clouds and other cosmological objects. As a representative example, a non-transparent slab is subjected to transient disturbances, and a rigorous analysis is presented whether physical time reasonably could be constructed under such condition. The analysis incorporates mapping functions that correlate physical events, e, in non-transparent media, with their images, f(e), tentatively located on a standard physical time scale. The analysis demonstrates, however, that physical time, in its rigorous sense, does not exist under non-transparency conditions. A proof of this conclusion is attempted in three steps: i) the theorem "there is no time without space and events" is accepted, (ii) images f[e(s,t)] do not constitute a dense, uncountably infinite set, and (iii) sets of images that are not uncountably infinite do not create physical time but only time-like sequences. As a consequence, mapping f[e(s,t)] in non-transparent space does not create physical analogues to the mathematical structure of the ordered, dense half-set R+ of real numbers, and reverse mapping, f-1f[e(s,t)] would not allow unique identification and reconstruction of original events from their images. In these cases, causality and determinism, as well as invariance of physical processes under time reversal, might be violated. Existence of time holes could be possible, as follows from the sequence of images, f[e(s,t)], that is not uncountably infinite, in contrast to R+. Practical impacts are expected for understanding physical diffusion-like, radiative transfer processes, stability models to protect superconductors against quenchs or for description of their transient local pair density and critical currents. Impacts would be expected also in mathematical formulations (differential equations) of classical physics, in relativity and perhaps in quantum mechanics, all as far as transient processes in non-transparent space would be concerned. An interesting problem is whether temporal cloaking (a time hole) in a transparent medium, as very recently reported in the literature, can be explained by the present analysis. The analysis is not restricted to objects of laboratory dimensions: Because of obviously existing radiation transfer analogues, it is tempting to discuss consequences also for much larger structures in particular if an origin of time is postulated.},
  subject      = {Strahlungstransport},
  language  = {en}
}
@phdthesis{Reinthaler2015,
  author    = {Reinthaler, Rolf Walter},
  title     = {Charge and Spin Transport in Topological Insulator Heterojunctions},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-135611},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2015},
  abstract  = {Over the last decade, the field of topological insulators has become one of the most vivid areas in solid state physics. This novel class of materials is characterized by an insulating bulk gap, which, in two-dimensional, time-reversal symmetric systems, is closed by helical edge states. The latter make topological insulators promising candidates for applications in high fidelity spintronics and topological quantum computing. This thesis contributes to bringing these fascinating concepts to life by analyzing transport through heterostructures formed by two-dimensional topological insulators in contact with metals or superconductors. To this end, analytical and numerical calculations are employed. Especially, a generalized wave matching approach is used to describe the edge and bulk states in finite size tunneling junctions on the same footing. The numerical study of non-superconducting systems focuses on two-terminal metal/topological insulator/metal junctions. Unexpectedly, the conductance signals originating from the bulk and the edge contributions are not additive. While for a long junction, the transport is determined purely by edge states, for a short junction, the conductance signal is built from both bulk and edge states in a ratio, which depends on the width of the sample. Further, short junctions show a non-monotonic conductance as a function of the sample length, which distinguishes the topologically non-trivial regime from the trivial one. Surprisingly, the non-monotonic conductance of the topological insulator can be traced to the formation of an effectively propagating solution, which is robust against scalar disorder. The analysis of the competition of edge and bulk contributions in nanostructures is extended to transport through topological insulator/superconductor/topological insulator tunneling junctions. If the dimensions of the superconductor are small enough, its evanescent bulk modes can couple edge states at opposite sample borders, generating significant and tunable crossed Andreev reflection. In experiments, the latter process is normally disguised by simultaneous electron transmission. However, the helical edge states enforce a spatial separation of both competing processes for each Kramers' partner, allowing to propose an all-electrical measurement of crossed Andreev reflection. Further, an analytical study of the hybrid system of helical edge states and conventional superconductors in finite magnetic fields leads to the novel superconducting quantum spin Hall effect. It is characterized by edge states. Both the helicity and the protection against scalar disorder of these edge states are unaffected by an in-plane magnetic field. At the same time its superconducting gap and its magnetotransport signals can be tuned in weak magnetic fields, because the combination of helical edge states and superconductivity results in a giant g-factor. This is manifested in a non-monotonic excess current and peak splitting of the dI/dV characteristics as a function of the magnetic field. In consequence, the superconducting quantum spin Hall effect is an effective generator and detector for spin currents. The research presented here deepens the understanding of the competition of bulk and edge transport in heterostructures based on topological insulators. Moreover it proposes feasible experiments to all-electrically measure crossed Andreev reflection and to test the spin polarization of helical edge states.},
  subject      = {Topologischer Isolator},
  language  = {en}
}
@phdthesis{Jacobs2003,
  author    = {Jacobs, Arne},
  title     = {Andreev-Streuung, Josephson-Bloch-Oszillationen und Zener-Tunneln in Heterokontakten aus Normal- und Supraleitern},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-9237},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2003},
  abstract  = {Die vorliegende Arbeit beleuchtet verschiedene Aspekte des Ladungstransports in Heterokontakten aus Normal- (N) und Supraleitern (S) im Rahmen des Bogoliubov-de Gennes-Formalismus. Dabei ist der bestimmende Prozeß die Andreev-Streuung: die Streuung von Elektronen in L{\"o}cher, bzw. umgekehrt, an r{\"a}umlichen Variationen des supraleitenden Paarpotentials unter Erzeugung, bzw. Vernichtung, eines Cooperpaares und damit der Induktion eines Suprastroms. Befindet sich ein Supraleiter zwischen zwei normalleitenden Bereichen, so wandelt sich der an der einen NS-Phasengrenze durch Andreev-Streuung induzierte Suprastrom an der anderen NS-Phasengrenze wieder in einen durch Quasiteilchen getragenen Strom um. Diese Umwandlung erfolgt durch den Einfall eines Quasiteilchens, dessen Charakter dem des auf der gegen{\"u}berliegenden Seite des Supraleiters einfallenden Quasiteilchens entgegengerichtet ist, wie anhand von Wellenpaket-Rechnungen explizit gezeigt wird. Ersetzt man den Supraleiter durch einen mesoskopischen SNS-Kontakt, ist die Vielteilchen-Konfiguration in der mittleren N-Schicht phasenkoh{\"a}rent und daher verschieden von den unkorrelierten Quasiteilchen-Anregungen, die die verschobene Fermi-Kugel in den normalleitenden Zuleitungen bilden. Die Josephson-Str{\"o}me, die durch die Quasiteilchen in der mittleren N-Schicht getragen werden, werden unter zwei verschiedenen Modellannahmen berechnet: Im einen Fall werden nur Streuzust{\"a}nde als Startzust{\"a}nde betrachtet, im anderen, bei gleichzeitiger Ber{\"u}cksichtigung eines normalstreuenden Potentials, nur gebundene Zust{\"a}nde. Der SNS-Kontakt wird durch eine supraleitend/halbleitende Heterostruktur modelliert, deren Parameter-Werte sich an den Experimenten der Gruppe von Herbert Kroemer in Santa Barbara orientieren. Wenn die supraleitenden Bereiche ohne normalleitende Zuleitungen direkt mit einem Reservoir von Cooperpaaren verbunden sind, fallen nur Quasiteilchen in Streuzust{\"a}nden aus den supraleitenden B{\"a}nken auf die NS-Phasengrenzen des Kontaktes ein. Mit den Normalleiter-Wellenfunktionen, die sich bei Anlegen einer Spannung V aus diesen Startzust{\"a}nden entwickeln, wird die Josephson-Wechselstromdichte in der Mitte der N-Schicht bei der Temperatur T = 2,2 K berechnet. Die Stromdichte weist spannungsabh{\"a}ngige Oszillationen in der Zeit auf, deren Periode das Inverse der Josephson-Frequenz ist. Alle Stromdichten zeigen bei kleinen Spannungen einen steilen Anstieg ihres Betrages, der durch Quasiteilchen zustandekommt, die durch das elektrische Feld aus dem Kondensat kommend in den Paarpotentialtopf hineingezogen werden und dort bei kleinen Spannungen eine große Zahl von Andreev-Streuungen erfahren, wobei sie bei jedem Elektron-Loch-Zyklus die Ladung 2e durch die N-Schicht transportieren. Im zweiten betrachteten Fall wird unter Ber{\"u}cksichtigung von Normalstreuung der Gesamtzustand des Systems zu jedem Zeitpunkt durch eine Superposition von gebundenen Zust{\"a}nden ausgedr{\"u}ckt. Die Energie dieser gebundenen Zust{\"a}nde ist abh{\"a}ngig von der Phasendifferenz Phi zwischen den supraleitenden Schichten. F{\"u}r Werte der Phasendifferenz von ganzzahligen Vielfachen von Pi sind Zust{\"a}nde entgegengerichteter Impulse paarweise entartet. Das normalstreuende Potential mischt diese Zust{\"a}nde, hebt ihre Entartung auf und f{\"u}hrt zu Energiel{\"u}cken: Es bilden sich Energieb{\"a}nder im Phi-Raum, die formal den Bloch-B{\"a}ndern von Kristallen im Wellenzahlraum entsprechen. Wird eine {\"a}ußere Spannung angelegt, so {\"a}ndert sich die Phasendifferenz gem{\"a}ß der Josephson-Gleichung mit der Zeit und die Quasiteilchen oszillieren in ihren jeweiligen Phi-Bloch-B{\"a}ndern: Diese Josephson-Bloch-Oszillationen ergeben den "normalen" Josephson-Wechselstrom, der zwischen positiven und negativen Werten schwingt und im zeitlichen Mittel Null ist. Zus{\"a}tzlich k{\"o}nnen die Quasiteilchen durch Zener-Tunneln --- wie der analoge Prozeß in der Halbleiterphysik genannt wird --- in h{\"o}here B{\"a}nder {\"u}bergehen. W{\"a}hrend sich die Richtung der Josephson-Stromdichte zu den Zeiten minimaler Energiel{\"u}cke umkehrt, hat die Zener-Tunnel-Stromdichte nach einem Tunnel-Prozeß das gleiche Vorzeichen, das die Josephson-Stromdichte vor dem Tunnel-Prozeß hatte. Wenn die angelegte Spannung hinreichend groß ist und gen{\"u}gend Quasiteilchen in das h{\"o}here Band tunneln, {\"u}berkompensiert die Zener-Tunnel-Stromdichte in der Halbperiode nach dem Tunnel-Prozeß die Josephson-Stromdichte, und die Gesamtstromdichte schwingt wieder in dieselbe Richtung wie vor dem Zener-Tunneln. Somit hat sich gewissermaßen die Periode halbiert: Die Gesamtstromdichte schwingt mit der doppelten Josephson-Frequenz. Allen untersuchten Aspekten des Ladungstransports durch Heterokontakte aus Normal- und Supraleitern ist eines gemein: Der f{\"u}r ihr Verst{\"a}ndnis fundamentale Prozeß ist die Andreev-Streuung.},
  subject      = {Supraleiter},
  language  = {de}
}