31,497 research outputs found
A numerical renormalization group approach to dissipative quantum impurity systems
No full textDie fortschreitende Miniaturisierung elektronischer Bauteile stößt an ihre Grenzen. Die einzelnen Strukturen, welche für die Herstellung integrierter Schaltungen in verschiedenen Halbleitern benötigt werden, schrumpfen immer mehr und könnten in den nächsten Jahren die Größenordnung einzelner Atomen erreichen. Spätestens dann wird die Physik der Nanostrukturen auch im Alltag durch neuartige Technologie an Bedeutung gewinnen.
Die vorliegende Doktorarbeit beschäftigt sich mit der Physik von Quantenstörstellenmodellen. Diese Art der Modelle verbindet ein identischer Aufbau: die Störstelle besteht aus einem relativ simplen Teilsystem aus nur wenigen Atomen (z.B. einem einzelnen Molekül), welches meist exakt beschrieben werden kann. Die Komplexität der Beschreibung entsteht erst aus der Anbindung dieser Störstelle an eine sehr große Anzahl quantenmechanischer Teilchen. Diese Teilchen können, im Fall einer Anbindung an elektronische Leiter, Elektronen sein. Genauso gut kann es sich aber auch um quantisierte Schwingungen (sogenannte Phononen) oder andere Teilchen, die einer bosonischen Statistik genügen, handeln. Diese große Anzahl wechselwirkender Teilchen macht eine exakte Lösung der Heisenberg-Gleichungen auch numerisch unmöglich. Ebenso ist es in vielen Fällen nicht möglich die auftretenden physikalischen Effekte, wie zum Beispiel den Kondo Effekt, bei dem eine große Anzahl von Elektronen aus dem fermionischen Bad eine "Wolke'' bildet, welche das freie magnetische Moment der Störstelle abschirmt, störungstheoretisch zu beschreiben und zu erklären.
Die Arbeit ist wie folgt strukturiert: Im ersten Teil wird der dominante physikalische Effekt, der Kondo Effekt, eingeführt und verständlich gemacht. Des Weiteren werden in diesem Teil die experimentellen Fortschritte, die eine kohärente Kontrolle einzelner Spins ermöglichen, anhand einer Auswahl aktueller experimenteller Arbeiten erläutert. Zu guter letzt werden zwei weitere experimentelle Arbeiten vorgestellt, die einen kurzen Einblick in das spannende Gebiet der molekularen Elektronik bieten.
Der zweite Teil der Arbeit führt die Methode ein mithilfe derer ein Großteil der Ergebnisse erlangt wurden: die numerische Renormierungsgruppe. Die numerische Renormierungsgruppe ist eine Methode, die im Gegensatz zu vielen anderen Methoden der Renormierungsgruppe, frei von störungstheoretischer Behandlung aller Systemparameter ist. Sie hat sich deshalb, seit ihrer erstmaligen Anwendung in den 1970er Jahren, zu einem Standardwerkzeug der computergestützten Physik entwickelt. Mit ihrer Hilfe lassen sich Korrelationseffekte in Quantenstörstellenmodellen bei Temperaturen nahe des absoluten Nullpunktes sehr gut beschreiben.
Nach einer allgemeinen Einführung in die Grundkonzepte sowohl für fermionische als auch für bosonische Bäder, werden relevante kürzlich eingeführte Erweiterungen der numerischen Renormierungsgruppe erläutert. Hierzu zählen die Dichte-Matrix Numerische Renormierungsgruppe, die geschickte Wahl einer vollständigen Eigenbasis des Fock-Raumes, die zeitabhängige Numerische Renormierungsgruppe, sowie die Berücksichtigung endlicher Temperaturen und einiger Tricks zur Verbesserung der Genauigkeit der berechneten Ergebnisse.
Der dritte Teil der Aabeit beinhaltet die Anwendung der numerischen Renormierungsgruppen-Methoden auf verschiedene Quantenstörstellenmodelle. Dies ist der Hauptteil und beleuchtet die wissenschaftliche Arbeit, welche während der vergangenen Jahre geleistet wurde. Die untersuchten Modelle werden jeweils erläutert und separat motiviert, bevor die Ergebnisse der eingehenden Analysen dargelegt werden.
Dieser Hauptteil lässt sich in folgende Kapitel, die einen Überblick über die untersuchten Themenbereiche erlauben, unterteilen:
- Ferromagnetisches Kondo Modell,
- Spin-Dynamik im Anisotropen Kondo und Spin-Boson Modell,
- Zwei gekoppelte Spins in einem Bosonischen Bad,
- Dekohärenz in einem Aharanov-Bohm Interferometer.The miniaturization of electronics is reaching its limits. Structures necessary to build integrated circuits from semiconductors are shrinking and could reach the size of only a few atoms within the next few years. It will be at the latest at this point in time that the physics of nanostructures gains importance in our every day life. This thesis deals with the physics of quantum impurity models. All models of this class exhibit an identical structure: the simple and small impurity only has few degrees of freedom. It can be built out of a small number of atoms or a single molecule, for example. In the simplest case it can be described by a single spin degree of freedom, in many quantum impurity models, it can be treated exactly. The complexity of the description arises from its coupling to a large number of fermionic or bosonic degrees of freedom (large meaning that we have to deal with particle numbers of the order of 10^{23}). An exact treatment thus remains impossible. At the same time, physical effects which arise in quantum impurity systems often cannot be described within a perturbative theory, since multiple energy scales may play an important role. One example for such an effect is the Kondo effect, where the free magnetic moment of the impurity is screened by a "cloud" of fermionic particles of the quantum bath.
The Kondo effect is only one example for the rich physics stemming from correlation effects in many body systems. Quantum impurity models, and the oftentimes related Kondo effect, have regained the attention of experimental and theoretical physicists since the advent of quantum dots, which are sometimes also referred to as as artificial atoms. Quantum dots offer a unprecedented control and tunability of many system parameters. Hence, they constitute a nice "playground" for fundamental research, while being promising candidates for building blocks of future technological devices as well.
Recently Loss' and DiVincenzo's p roposal of a quantum computing scheme based on spins in quantum dots, increased the efforts of experimentalists to coherently manipulate and read out the spins of quantum dots one by one. In this context two topics are of paramount importance for future quantum information processing: since decoherence times have to be large enough to allow for good error correction schemes, understanding the loss of phase coherence in quantum impurity systems is a prerequisite for quantum computation in these systems. Nonequilibrium phenomena in quantum impurity systems also have to be understood, before one may gain control of manipulating quantum bits.
As a first step towards more complicated nonequilibrium situations, the reaction of a system to a quantum quench, i.e. a sudden change of external fields or other parameters of the system can be investigated. We give an introduction to a powerful numerical method used in this field of research, the numerical renormalization group method, and apply this method and its recent enhancements to various quantum impurity systems.
The main part of this thesis may be structured in the following way:
- Ferromagnetic Kondo Model,
- Spin-Dynamics in the Anisotropic Kondo and the Spin-Boson Model,
- Two Ising-coupled Spins in a Bosonic Bath,
- Decoherence in an Aharanov-Bohm Interferometer
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Achievement of sensing single spin with the aid of Kondo resonance in quantum dot connected to ferromagnetic electrodes
No full textWe theoretically propose the detection of spin states of magnetic atom or molecule in proximity to a quantum dot by the Kondo effect with ferromagnetic electrodes, which can be switched in parallel or antiparallel alignments. The relative orientation of spin to the magnetization of electrode can be evidentially tracked from the spin splitting in Kondo peak of differential conductance. The experimental realization is discussed. (c) 2008 American Institute of Physics.http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=PARTNER_APP&SrcAuth=LinksAMR&KeyUT=WOS:000256485700028&DestLinkType=FullRecord&DestApp=ALL_WOS&UsrCustomerID=8e1609b174ce4e31116a60747a720701Physics, AppliedSCI(E)EI0ARTICLE18null9
Wpływ efektu domieszkowania na stan izolatorów Kondo CeNiSn i CeRhSb
Get PDFW rozprawie przedstawiono badania struktury elektronowej i własności termodynamicznych izolatorów Kondo (IK). Do opis stanu podstawowego zastosowano model Andersona dla periodycznej sieci Kondo. Zbadano wpływu domieszkowania na stabilność przerwy energetycznej w znanych już wcześniej izolatorach Kondo CeRhSb i CeNiSn metalami przejściowymi z grupy metali 3d, 4d oraz podstawianie Ce atomami La lub Sb atomami Sn lub In. Wykazano, że domieszkowanie prowadzi do zmiany liczby elektronów walencyjnych, zmienia też efekt hybrydyzacji pomiędzy elektronami f i pasmem przewodnictwa. Wpływ energii hybrydyzacji f-s i liczby elektronów walencyjnych na stan podstawowy izolatora przeanalizowano w oparciu o pomiary struktury elektronowej wyznaczonej przy pomocy metody XPS i porównano z obliczeniami ab initio.
W rozprawie wykazano, że stan izolatora Kondo powstaje w domieszkowanych układach CeRhSb i CeNiSn w zakresie stężeń domieszki, gdzie efekt hybrydyzacji jest silny, w miarę wzrostu domieszkowania hybrydyzacja f-s maleje, równocześnie zanika przerwa półprzewodnikowa na poziomie Fermiego. Potwierdzono, że zjawisko zaniku przerwy w miarę wzrostu domieszkowania ma charakter uniwersalny dla izolatorów Kondo.
Słowa kluczowe: ciężkie fermiony, silnie skorelowane elektrony, izolator Kondo, ekranowanie typu Kondo, antyferromagnetyzm, periodyczny model Andersona
Kondo temperature of magnetic impurities at surfaces
No full textBased on the experimental observation that only the close vicinity of a magnetic impurity at metal surfaces determines its Kondo behavior, we introduce a simple model which explains the Kondo temperatures observed for cobalt adatoms at the (111) and (100) surfaces of Cu, Ag, and Au. Excellent agreement between the model and scanning tunneling spectroscopy experiments is demonstrated. The Kondo temperature is shown to depend on the occupation of the d level determined by the hybridization between the adatom and the substrate with a minimum around single occupancy.</p
Kondo correlation and spin-flip scattering in spin-dependent transport through a quantum dot coupled to ferromagnetic leads
No full textWe investigate the linear and nonlinear dc transport through an
interacting quantum dot connected to two ferromagnetic electrodes
around Kondo regime with spin-flip scattering in the dot. Using a
slave-boson mean-field approach for the Anderson Hamiltonian
having finite on-site Coulomb repulsion, we find that a spin-flip
scattering always depresses the Kondo correlation at arbitrary
polarization strength in both parallel and antiparallel alignment
of the lead magnetization and that it effectively reinforces the
tunneling-related conductance in the antiparallel configuration.
For systems deep in the Kondo regime, the zero-bias single Kondo
peak in the differential conductance is split into two peaks by
the intradot spin-flip scattering; while for systems somewhat
further from the Kondo center, the spin-flip process in the dot
may turn the zero-bias anomaly into a three-peak structure
Kondo effect of single Co adatoms on Cu surfaces
No full textThe Kondo resonance of Co adatoms on the Cu(100) and Cu(111)
surfaces has been studied by scanning tunneling spectroscopy.
We demonstrate the scaling of the Kondo temperature T-K with
the host electron density at the magnetic impurity. The
quantitative analysis of the tunneling spectra reveals that the
Kondo resonance is dominated by the Cu bulk electrons. While at
the Cu(100) surface both tunneling into the hybridized
localized state and into the substrate conduction band
contribute to the Kondo resonance, the latter channel is found
to be dominant for Cu(111)
Fermi surface study of CeRu2Si2 above the Kondo temperature
No full textThe 2-dimensional angular correlation of the positron annihilation radiation (2D-ACAR) of the heavy fermion system CeRu2Si2 was measured above the Kondo temperature TK. The 3-dimensional electron-positron momentum density rho (p) was reconstructed from five projections via the Cormack [1] and a modified Fourier-transform-based method [2]. After the 3D Lock-Crisp-West transformation [3], the resulting k-space density rho (k) was tested against band structure calculations treating the f-electrons as itinerant (f-band) and localised (f-core). Our preliminary results show unoccupied regions centred at the Z point of the Brillouin zone (BZ), predicted by both models and consistent with the de Haas van Alphen experiments. Furthermore, we observe large and connected electron-like columnar structures along the X-P direction. The f-band calculation predicts that these structures are much more prominent than in the f-core calculation and more similar to those reported by our measurements. Therefore, we suggest that in CeRu2Si2 the itinerant character of the f-electrons persists above TK
Exploring Emptiness: An Investigation of MA and MU in My Sonic Composition Practice
No full textThe commentary investigates Japanese aesthetics of space, silence and emptiness - ma and mu - that informed my compositional practice during the research period 2012 - 2015. The portfolio comprises text compositions and sound installations in which forms of micro events and sustained events are employed. Throughout, the emphasis is on my personal engagement with, and manifestation of emptiness that concerns a particular model of listening and perception.
Chapter 1 discusses six primary research areas: ma and mu, material, text, form, listening and perception. Firstly, I introduce ma and mu by examining noh culture and Zeami's teaching of senu hima (where there is no-action) in the context of my personal approaches to music. The following subjects are then used to contextualise my PhD practice by means of examples from various composers and visual artists. Here, these particular and enigmatic concepts are explored through Japanese art as well as Western contemporary works by Alvin Lucier, Eliane Radigue and those of the Wandelweiser collective.
Part 2 provides contextual commentaries on selected compositions from the portfolio that mostly articulate my aesthetics in relation to the topics covered in Chapter 1. koso koso addresses my methodologies to investigate the essence of senu hima, followed by treow that discusses my approach to materials and the importance of space. I move on to grade two and grade two extended in order to examine text scores, and then, look into Espèces d'espaces 03 and 04 as examples of musical forms that I employ.
Finally, listening and perception are investigated through the compositions gnome and con.de.structuring. Throughout, I describe how my works explore emptiness as a result of my particular emphasis on listening over composing
Node-line Dirac semimetal manipulated by Kondo mechanism in nonsymmorphic CePtSi
Get PDFDirac node lines (DNLs) are characterized by Dirac-type linear crossings
between valence and conduction bands along one-dimensional node lines in the
Brillouin zone (BZ). Spin-orbit coupling (SOC) usually shifts the degeneracy at
the crossings thus destroys DNLs, and so far the reported DNLs in a few
materials are non-interacting type, making the search for robust interacting
DNLs in real materials appealing. Here, via first-principle calculations, we
reveal that Kondo interaction together with nonsymmorphic lattice symmetries
can drive a robust interacting DNLs in a Kondo semimetal CePt_2Si_2, and the
feature of DNLs can be significantly manipulated by Kondo behavior in different
temperature regions. Based on the density function theory combining dynamical
mean-field theory (DFT+DMFT), we predict a transition to Kondo-coherent state
at coherent temperature T_coh= 80 K upon cooling, verified by temperature
dependence of Ce-4f self-energy, Kondo resonance peak, magnetic susceptibility
and momentum-resolved spectral. Below T_coh, well-resolved narrow heavy-fermion
bands emerge near the Fermi level, constructing clearly visualized interacting
DNLs locating at the BZ boundary, in which the Dirac fermions have strongly
enhanced effective mass and reduced velocity. In contrast, above a crossover
temperature T_KS =600 K, the destruction of local Kondo screening drives
non-interacting DNLs which are comprised by light conduction electrons at the
same location. These DNLs are protected by lattice nonsymmorphic symmetries
thus robust under intrinsic strong SOC. Our proposal of DNLs which can be
significantly manipulated according to Kondo behavior provides an unique
realization of interacting Dirac semimetals in real strongly correlated
materials, and serves as a convenient platform to investigate the effect of
electronic correlations on topological materials.Comment: 9 pages, 9 figure
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