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Mecanismos de alargamento de níveis de Landau em grafeno = um estudo por espalhamento Raman = Landau level broadening mechanisms in graphene: a Raman scattering study / Landau level broadening mechanisms in graphene : a Raman scattering studyArdito, Fábio Machado 1984- 28 August 2018 (has links)
Orientadores: Eduardo Granado Monteiro da Silva, Fernando Iikawa / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-28T03:52:30Z (GMT). No. of bitstreams: 1
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Previous issue date: 2015 / Resumo: O célebre efeito Hall quântico semi-inteiro no grafeno é uma consequência direta dos níveis de Landau característicos previstos pela equação de Dirac. Apesar de se saber que os níveis de Landau devam ser tão estreitos quanto possível para que os efeitos de mecânica quântica relativística sejam realçados, pouca informação sobre os mecanismos mais relevantes que levam ao alargamento destes níveis em grafeno está disponível atualmente. Nesta tese, experimentos de micro-Raman a baixas temperaturas e campos magnéticos de até 15~T foram feitos em tipos diferentes de amostras de grafeno: monocamadas e bicamadas depositadas sobre um substrato de Si/SiO$_2$; e multicamadas produzidas sobre SiC. As larguras das transições interbanda entre níveis de Landau são obtidas pela análise do amortecimento das ressonâncias magneto-fônon que ocorrem quando transições opticamente ativas entre níveis de Landau cruzam a energia do fônon E$_{2g}$. A partir de uma análise quantitativa das larguras dependentes do índice de cada transição, encontramos que amostras produzidas por meios distintos mostram mecanismos de alargamento de níveis de Landau amplamente diferentes. O alargamento na amostra com multicamadas é causado por uma distribuição nas velocidades de Fermi ($\delta v_F \sim 8.7\times10^4$~m/s) possivelmente devida a flutuações residuais nas interações entre camadas, enquanto o alargamento em monocamadas é devido, principalmente, à presença de pseudo-campos magnéticos ($\delta B_{strain}=1.7$~T para nossa amostra) criados por tensões na rede através de interações com o substrato. A busca por métodos de fabricação de grafeno com qualidade eletrônica superior precisa levar em conta estes dois tipos de mecanismos de alargamento / Abstract: The famous half-integer quantum hall effect in graphene is a direct consequence of the characteristic Landau levels predicted by the Dirac equation. Albeit minimal Landau level widths are mandatory to reinforce the relativistic quantum effects under low magnetic fields, little information on the most relevant mechanisms leading to Landau level broadening in graphene samples is presently available. In this thesis, micro-Raman experiments at low temperatures and fields up to 15 T were performed on distinct graphene samples, namely a natural single-layer and a bilayer graphene flake deposited over a Si/SiO$_2$ substrate; and a multilayer epitaxial graphene sample produced on SiC. The interband Landau level transition widths are obtained by an analysis of the damping of the magnetophonon resonances that occur when optically active Landau level transitions cross the energy of the $E_{2g}$ phonon. We find, by means of a quantitative analysis of the index-dependent Landau level transition widths, that graphene samples produced by such distinct routes show largely different dominant Landau level broadening mechanisms. In fact, the broadening in multilayer epitaxial graphene is caused by a distribution of Fermi velocities ($\delta v_F \sim 8.7\times10^4$~m/s), possibly due to fluctuations of residual interlayer graphene interactions, while the Landau level broadening in single-layer graphene is mainly due to the presence of random strain-induced pseudo-magnetic fields ($\delta B_{strain}=1.7$ T for our sample) caused by interactions with the substrate. Search for production methods of graphene with optimal electronic quality must take into account both possible Landau level broadening mechanisms / Doutorado / Física / Doutor em Ciências / CAPES
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Magneto-optical confirmation of Landau level splitting in a GaN/AlGaN 2DEG grown on bulk GaNSchmult, Stefan, Solovyev, Victor V., Wirth, Steffen, Großer, Andreas, Mikolajick, Thomas, Kukushkin, Igor V. 05 October 2022 (has links)
Landau level splitting in a two-dimensional electron gas (2DEG) confined in an ultrapure GaN/AlGaN heterostructure grown by molecular beam epitaxy on bulk GaN is verified spectroscopically. The Landau level fan reconstructed from magneto-photoluminescence (PL) data yields an effective mass of 0.24m₀ for the 2D electrons. Narrow excitonic PL line widths < 100 μeV, an atomically flat surface of the layer stack, as well as the absence of the 2DEG in the dark environment, are important ancillary experimental findings while focusing on magneto-PL investigations of the heterostructure. Simultaneously recorded Shubnikov-de Haas and magneto-PL intensity oscillations under steady UV illumination exhibit an identical frequency and allow for two independent ways of determining the 2D density.
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Ferromagnetic phase transitions in neutron starsDiener, Jacobus Petrus Willem 12 1900 (has links)
Thesis (PhD)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: We consider the ferromagnetic phase in pure neutron matter as well as charge neutral, betaequilibrated
nuclear matter. We employ Quantum Hadrodynamics, a relativistic field theory
description of nuclear matter with meson degrees of freedom, and include couplings between the
baryon (proton and neutron) magnetic dipole moment as well as between their charge and the
magnetic field in the Lagrangian density describing such a system. We vary the strength of the
baryon magnetic dipole moment till a non-zero value of the magnetic field, for which the total
energy density of the magnetised system is at a minimum, is found. The system is then assumed
to be in the ferromagnetic state.
The ferromagnetic equation of state is employed to study matter in the neutron star interior.
We find that as the density increases the ferromagnetic field does not increase continuously, but
exhibit sudden rapid increases. These sudden increases in the magnetic field correspond to shifts
between different configurations of the charged particle’s Landau levels and can have significant
observational consequences for neutron stars. We also found that although the ferromagnetic
phase softens the neutron star equation of state it does not significantly alter the star’s massradius
relationship.
The properties of magnetised symmetric nuclear matter were also studied. We confirm that
magnetised matter tends to be more proton-rich but become more weakly bound for stronger
magnetic fields. We show that the behaviour of the compressibility of nuclear matter is influenced
by the Landau quantisation and tends to have an oscillatory character as it increases with
the magnetic field. The symmetry energy also exhibits similar behaviour. / AFRIKAANSE OPSOMMING: In hierdie studie het ons die ferromagnetiese fase in suiwer neutronmaterie, sowel as in ladingsneutrale,
beta-ge¨ekwilibreerde neutronstermaterie, ondersoek. Vir die doeleindes het ons die
Kwantum Hadrodinamika-model van kernmaterie gebruik. Dit is ’n relatiwistiese, veldteoretiese
model wat mesone inspan om die interaksies tussen die protone en neutrone te bemiddel. Om
die impak van die magneetveld te bestudeer, sluit ons ’n koppeling tussen die barioonlading en
die magneetveld, asook barioondipoolmoment en die magneetveld, in by die Lagrange digtheid
wat ons sisteem beskryf. Om die ferromagnetiese fase te ondersoek, varieer ons die sterkte van
die barioondipoolmoment om ’n nie-nul waarde van die magneetveld wat energie digtheid sal
minimeer te vind.
Die ferromagnetiese toestandsvergelyking word toegepas op materie aan die binnekant van die
neutronster en die impak hiervan op die waarneembare eienskappe van die ster word ondersoek.
Ons vind dat die ferromagnetiese magneetveld nie kontinu toeneem soos die digtheid verhoog
nie. Die skielike toenames in die magneetveld is die gevolg van die sisteem wat die konfigurasie
van die gelaaide deeltjies se Landau-vlakke skielik verander en dit kan beduidende waarneembare
gevolge vir die ster inhou. Ons vind ook dat die ferromagnetiese fase die toestandsvergelyking
versag, maar dat die versagting die massa-radius verhouding van die ster nie grootliks beïnvloed
nie.
Die eienskappe van gemagnetiseerde kernmaterie word ook ondersoek. Ons bevestig dat gemagnetiseerde
materie meer proton-ryk, maar minder sterk gebind word. Ons wys dat die saampersbaarheid
van kernmaterie deur die teenwoordigheid van Landau-vlakke beïnvloed word en
ossilerend saam met die magneetveld toeneem. Die simmetrie-energie manifesteer ook soortgelyke
gedrag.
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Superconductivity problems with multiple Ginzburg-Landau order parametersGeyer, Jani 12 1900 (has links)
Thesis (MSc)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: Two problems in the field of materials-based condensed matter physics, specifically in the field
of superconductivity, are studied theoretically. In both problems, where each is of current exper-
imental interest, an extension of Ginzburg-Landau theory is used to describe a physical system,
with focus on the energy associated to the interface(s) occurring in the respective systems.
The first physical system under consideration is that of a two-band superconductor. Using
Ginzburg-Landau theory for two-band superconductors, the interface energy ¾s between normal
and superconducting states coexisting at the thermodynamic critical magnetic field is determined.
From the theoretical and numerical analysis of the interface energy, it is found that close to the
transition temperature, where the Ginzburg-Landau theory is applicable, the two-band problem
maps onto an effective single band problem. This finding puts into question the possibility of
intermediate, so called type-1.5 superconductivity, in the regime where the Ginzburg-Landau
theory applies.
The second physical system is that of a system with competing superconductivity and anti-
ferromagnetism. From Ginzburg-Landau theory for such competing systems in a thermodynamic
critical magnetic field, it is shown that two possible interfaces can occur: an interface between
a pure anti-ferromagnetic state and a pure superconducting state; and an interface between a
state with coexisting superconductivity and anti-ferromagnetism and a pure anti-ferromagnetic
state. The energy associated to both these interfaces is analysed theoretically and numerically
from which the boundary between type-I and type-II superconductivity is obtained for certain
specific cases. / AFRIKAANSE OPSOMMING: Twee probleme in die veld van materiaal-gebaseerde gekondenseerde materie fisika, spesifiek in
die veld van supergeleiding, word teoreties bestudeer. In beide probleme, albei tans van eksper-
imentele belang, word ’n fisiese sisteem beskryf deur ’n uitbreiding van enkel-band Ginzburg-
Landau teorie, met fokus op die energie geassosieer met die koppelvlak(ke) wat in die onderskeie
sisteme aangetref word.
Die eerste fisiese sisteem wat beskou word is die van ’n twee-band supergeleier. Deur van
Ginzburg-Landau teorie vir twee-band supergeleiers gebruik te maak, word die koppelvlak energie
¾s tussen die gelyktydig bestaande normaal- en supergeleidende toestand in die termodinamiese
kritieke magneetveld bepaal. Deur beide teoretiese en numeriese analieses word bepaal dat na
aan die oorgangstemperatuur, waar Ginzburg-Landau teorie geldig is, die twee-band probleem op
’n effektiewe een-band probleem afbeeld. Hierdie bevinding bevraagteken dus die moontlikheid
van onkonvensionele, of sogenaamde tipe-1.5 supergeleiding, vir gevalle waar Ginzburg-Landau
teorie geldig is.
Die tweede fisiese siteem wat beskou word is ’n sisteem met kompeterende supergeleiding en
anti-ferromagnetisme. Met behulp van Ginzburg-Landau teorie vir sulke sisteme in ’n termod-
inamiese kritiese magneetveld word gewys dat daar twee moontlike koppelvlakke kan ontstaan:
’n koppelvlak tussen ’n uitsluitlik anti-ferromagnetiese toestand en ’n uitsluitlik supergeleidende
toestand; sowel as ’n koppelvlak tussen ’n uitsluitlik anti-ferromagnetiese toestand en ’n toes-
tand van beide supergeleiding en anti-ferromagnetisme. Die energie geassosieer met beide hierdie
koppelvlakke word teoreties en numeries geanaliseer wat lei tot ’n beskrywing van die grenslyn
tussen tipe-I en tipe-II supergeleiding in sekere spesifieke gevalle.
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A duality construction for interacting quantum Hall systemsKriel, Johannes Nicolaas 03 1900 (has links)
Thesis (PhD)--University of Stellenbosch, 2011. / ENGLISH ABSTRACT: The fractional quantum Hall effect represents a true many-body phenomenon in which the
collective behaviour of interacting electrons plays a central role. In contrast to its integral
counterpart, the appearance of a mobility gap in the fractional quantum Hall regime is due
entirely to the Coulomb interaction and is not the result of a perturbed single particle gap.
The bulk of our theoretical understanding of the underlying many-body problem is based on
Laughlin’s ansatz wave function and the composite fermion picture proposed by Jain. In the
latter the fractional quantum Hall effect of interacting electrons is formulated as the integral
quantum Hall effect of weakly interacting quasiparticles called composite fermions. The composite
fermion picture provides a qualitative description of the interacting system’s low-energy
spectrum and leads to a generalisation of Laughlin’s wave functions for the electron ground
state. These predictions have been verified through extensive numerical tests.
In this work we present an alternative formulation of the composite fermion picture within
a more rigorous mathematical framework. Our goal is to establish the relation between the
strongly interacting electron problem and its dual description in terms of weakly interacting
quasiparticles on the level of the microscopic Hamiltonian itself. This allows us to derive an
analytic expression for the interaction induced excitation gap which agrees very well with existing
numerical results. We also formulate a mapping between the states of the free particle and
interacting descriptions in which the characteristic Jastrow-Slater structure of the composite
fermion ansatz appears naturally. Our formalism also serves to clarify several aspects of the
standard heuristic construction, particularly with regard to the emergence of the effective magnetic
field and the role of higher Landau levels. We also resolve a long standing issue regarding
the overlap of unprojected composite fermion trial wave functions with the lowest Landau level
of the free particle Hamiltonian. / AFRIKAANSE OPSOMMING: Die fraksionele kwantum Hall-effek is ’n veeldeeltjie verskynsel waarin die kollektiewe gedrag
van wisselwerkende elektrone ’n sentrale rol speel. In teenstelling met die heeltallige kwantum
Hall-effek is die ontstaan van ’n energie gaping in die fraksionele geval nie ’n enkeldeeltjie
effek nie, maar kan uitsluitlik aan die Coulomb wisselwerking toegeskryf word. Die teoretiese
raamwerk waarbinne hierdie veeldeeltjie probleem verstaan word is grootliks gebaseer op
Laughlin se proefgolffunksie en die komposiete-fermion beeld van Jain. In laasgenoemde word
die fraksionele kwantum Hall-effek van wisselwerkende elektrone geformuleer as die heeltallige
kwantum Hall-effek van swak-wisselwerkende kwasi-deeljies wat as komposiete-fermione bekend
staan. Hierdie beeld lewer ’n kwalitatiewe beskrywing van die wisselwerkende sisteem se
lae-energie spektrum en lei tot ’n veralgemening van Laughlin se golffunksies vir die elektron
grondtoestand. Hierdie voorspellings is deur verskeie numeriese studies geverifieer.
In hierdie tesis ontwikkel ons ’n alternatiewe formulering van die komposiete-fermion beeld
binne ’n strenger wiskundige raamwerk. Ons doel is om die verband tussen die sterk-wisselwerkende
elektron sisteem en sy duale beskrywing in terme van swak-wisselwerkende kwasi-deeltjies op
die vlak van die mikroskopiese Hamilton-operator self te realiseer. Hierdie konstruksie lei tot ’n
analitiese uitdrukking vir die opwekkingsenergie wat baie goed met bestaande numeriese resultate
ooreenstem. Ons identifiseer ook ’n afbeelding tussen die vrye-deeltjie en wisselwerkende
toestande waarbinne die Jastrow-Slater struktuur van die komposiete-fermion proefgolffunksies
op ’n natuurlike wyse na vore kom. Verder werp ons formalisme nuwe lig op kwessies binne die
standaard heuristiese konstruksie, veral met betrekking tot die oorsprong van die effektiewe
magneetveld en die rol van ho¨er effektiewe Landau vlakke. Ons lewer ook uitspraak oor die
vraagstuk van die oorvleueling van ongeprojekteerde komposiete-fermion golffunksies met die
laagste Landau vlak van die vrye-deeltjie Landau probleem.
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Topology and mass generation mechanisms in abelian gauge field theoriesBertrand, Bruno 09 September 2008 (has links)
Among a number of fundamental issues, the origin of inertial mass remains one of the major open problems in particle physics. Furthermore, topological effects related to non perturbative field configurations are poorly understood in those gauge theories of direct relevance to our physical universe. Motivated by such issues, this Thesis provides a deeper understanding for the appearance of topological effects in abelian gauge field theories, also in relation to the existence of a mass gap for the gauge interactions. These effects are not accounted for when proceeding through gauge fixings as is customary in the literature.
The original Topological-Physical factorisation put forth in this work enables to properly identify in topologically massive gauge theories (TMGT) a topological sector which appears under formal limits within the Lagrangian formulation. Our factorisation then allows for a straightforward quantisation of TMGT, accounting for all the topological features inherent to such dynamics. Moreover dual actions are constructed while preserving the gauge symmetry also in the presence of dielectric couplings. All the celebrated mass generation mechanisms preserving the gauge symmetry are then recovered but now find their rightful place through a network of dualities, modulo the presence of topological terms generating topological effects. In particular a dual formulation of the famous Nielsen-Olesen vortices is constructed from TMGT. Within a novel physically equivalent picture, these topological defects are interpreted as dielectric monopoles.
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Énergétique des vortex et comportement magnétique de la supraconductivité SO(5)Juneau, Michaël January 2002 (has links)
Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.
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Ginzbutrg-Landau theory with hidden order parameter applied to interface superconductivity / TEORIA DE GINZBURG-LANDAU COM PARÃMETRO DE ORDEM ESCONDIDO APLICADA AO ESTUDO DA SUPERCONDUTIVIDADE DE INTERFACEVICTOR NOCRATO MOURA 21 February 2017 (has links)
Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / In recent years, several experiments have been reported in which interface superconductivity was observed in heterostructures of different materials, inclunding non-superconductors. The origin of this superconductivity has not yet been elucidated and there is no well-established theory to explain this phenomenon. In 2015 a model based on the Ginzburg-Landau theory was proposed that would explain the interface superconductivity phenomenon assuming a system with two order parameters. It has been proposed that the order parameter characterizing the bulk material with a defective or doped layer permits the formation of a second parameter which competes with the former and prevails over it in the vicinity of the interface. The superconductivity at the interface is then explained by the growth of this second order parameter only in this region, remaining still ``hidden" inside the bulk. The model was applied to a one-dimensional system with an interface, which presented a surprising result: the ``hidden" superconductivity appers in quantized critical temperatures, this allowing the existence of several eigenstates of the system, with different critical temperatures. In this dissertation, we use this model and investigate the unfolding of hidden superconductivity and its quantized temperatures. We observe that the interfaces resemble one-dimensional quantum wells, with the critical temperature playing the role of the energy in the quantum case. Following this idea we use numerical methods to solve the Ginzburg-Landau equations for a system with an arbitrary number of parallel interfaces. Our results show that in this case, the critical temperatures are quantized and degenerate when the interfaces are very separated, but it has its degeneracy broken when we approach the interfaces, as it happens in a lattice of square wells. We then proposed a tight-binding model to estimate critical temperatures on parallel interfaces and verified the validity of this approximation through the numerical solution of the complete problem. We also analyze the vortex states for a square two-dimensional defect, verifying the possibility of creating or destroying vortices in the region of `` hidden" superconductivity through an external magnetic field. / Nos Ãltimos anos foram reportados diversos experimentos em que a supercondutividade de interface foi observada em heteroestruturas de diferentes materiais, inclusive em nÃo-supercondutores extit{a priori}. A origem dessa supercondutividade ainda nÃo foi elucidada e nÃo existe uma teoria bem estabelecida para explicar esse fenÃmeno. Em 2015 foi proposto um modelo com base na teoria de Ginzburg-Landau que explicaria o fenÃmeno de supercondutividade de interface assumindo um sistema com dois parÃmetros de ordem. Foi proposto que o parÃmetro de ordem que caracteriza o material extit{bulk} com uma camada defeituosa, ou dopada, permite a formaÃÃo de um segundo parÃmetro que compete com o primeiro e prevalece sobre ele nas proximidades da interface. A supercondutividade na interface à entÃo explicada pelo crescimento deste segundo parÃmetro de ordem apenas nesta regiÃo, permancecendo ainda ``escondido" dentro do extit{bulk}. O modelo foi aplicado para um sistema unidimensional com uma interface, apresentando um resultado surpreendente: a supercondutividade escondida aparece em temperaturas crÃticas quantizadas, podendo entÃo existir vÃrios autoestados do sistema, com diferentes temperaturas crÃticas. Nessa dissertaÃÃo utilizamos esse modelo e investigamos os desdobramentos da supercondutividade escondida e suas temperaturas quantizadas. Percebemos que as interfaces assemelham-se com poÃos quÃnticos unidimensionais, com a temperatura crÃtica fazendo o anÃlogo ao da energia no caso quÃntico. Seguindo essa ideia utilizamos mÃtodos numÃricos para resolver as equaÃÃes de Ginzburg-Landau para um sistema com um nÃmero arbitrÃrio de interface paralelas. Nossos resultados mostram que neste caso, as temperaturas crÃticas, alÃm de quantizadas, sÃo degeneradas quando as interfaces estÃo muito separadas, mas tem essa degenerescÃncia quebrada quando aproximamos as interfaces, como ocorre em uma rede de poÃos quadrados. Propusemos entÃo um modelo tipo extit{tight-binding} para estimar temperaturas crÃticas em interfaces paralelas e verificamos a validade dessa aproximaÃÃo atravÃs da soluÃÃo numÃrica do problema completo. Analisamos tambÃm os estados de vÃrtices para um defeito bidimensional quadrado, verificando a possibilidade de se criar ou destruir vÃrtices na regiÃo de supercondutividade escondida atravÃs de um campo magnÃtico externo.
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Supersymmetric Landau ModelsBeylin, Andrey V 05 August 2011 (has links)
This work is focused on the different supersymmetric extensions of the Landau model. We aim to fully solve each model and describe its energy levels, wavefunctions, Hilbert space and define a norm on it, as well as find symmetry generators and transformations with respect to them. Several possible generalizations were considered before. It was found for Landau model on the so called Superflag manifold as well as planar Superflag and Superplane Landau models that standard norm on the Hilbert space is not positive definite. Later for planar cases it was found that it is possible to fix this by introducing a new norm which will be invariant and positive definite. Surprisingly this procedure brings up "hidden" symmetries for the known super Landau models. In the dissertation we apply the same procedure for Landau model on superpshere and Superflag manifolds. It turns out that superpsherical Landau model is equivalent to the Superflag model with one of the parameters fixed. Because the model on superpshere can be recovered from the Superflag we will do calculations of corrected norm only for the Superflag. After this we develop a different generalization of the Superplane Landau model. Starting with Lagrangian in a superfield form we introduce two arbitrary functions of superfields K(Φ) and V(Φ) into the Lagrangian. We follow with the component form of Lagrangian. The quantization of the model is possible, and we will show that there is a reparametrization which turn equation of motion of the first scheme into the second set. Standard metric is again non-positive definite and we apply already known procedure to correct it. It will not be possible to solve Schrodinger equations in general with undefined K and V, so we consider one specific case which give us Landau model on a sphere with N = 2 supersymmetry, which put it apart from the superspherical Landau model, which have a superpshere for a target space but do not possess supersymmetry.
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Bounds on One--Dimensional Exchange Energies with Application to Lowest Landau Band Quantum Mechanicsrseiring@ap.univie.ac.at 12 February 2001 (has links)
No description available.
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