1,721,309 research outputs found
Dynamic separation of electron excitation and lattice heating during the photoinduced melting of the periodic lattice distortion in 2H-TaSe2
Full text linkThe photoinduced structural dynamics in 2H-TaSe2 in the charge-density wave (CDW) state is investigated using MeV ultrafast electron diffraction. By simultaneously tracking both the melting of the periodic lattice distortion (PLD) associated with the CDW and the lattice heating, following an impulsive photoexcitation, the separate contributions of electronic excitation and lattice thermalization to the melting process are disentangled in the time domain. Two distinct time-constants, reflecting the corresponding individual dynamics of the subsystems, are observed. Our experimental results demonstrate that the PLD in 2H-TaSe2 is first suppressed promptly by the electronic excitation and scattering, and then subsequently by lattice thermalization through electron-phonon interaction, on a much longer time scale. This latter leads to the final, full melting of the PLD. (C) 2013 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License
Short-range spin- and pair-correlations: A variational wave-function
No full textA many-body wave-function is postulated, which is sufficiently general to describe superconducting pair-correlations, and/or spin-correlations, which can occur either as long-range order or as finite-range correlations. The proposed wave-function appears to summarize some of the more relevant aspects of the rich phase-diagram of the high-T-c cuprates. Some of the states represented by this wave-function are reviewed: for superconductivity in the background of robust anti-ferromagnetism, the Cooper-pairs are shown to be a superposition of spinquantum numbers S = 0 and 1. If the anti-ferromagnetism is weak, a continuous super-symmetric rotation is identified connecting s-wave superconductivity to anti-ferromagnetism. (C) 2003 Elsevier B.V. All rights reserved
Optically induced coherent voltage oscillations in K0.3MoO3
No full textOptical induced transient changes of the electrical conductivity are investigated in the non-linear transport regime of the blue bronze K0.3MoO3 below its Peierls transition using the impulsive infrared excitation of a free-electron laser at energies above the Peierls gap. The transients of the sample voltage have typical relaxation times of 0.5-2 ms superimposed by coherent voltage oscillations, which exhibit lifetimes of up 100 ms. The frequency of those voltage oscillation is found to be time dependent. This, in comparison with voltage oscillations purely driven by static electrical fields, enables the study of quasiparticle dynamics in the charge density condensate of the blue bronze
Magnetic heat transport in low-dimensional quantum magnets
Full text linkIn laag-dimensionale kwantummagneten wordt warmte voornamelijk langs één as verplaatst door middel van transport van magnetische energie. Verder zijn deze materialen elektrisch isolerend en is de warmtegeleiding langs de goede as vergelijkbaar met die van een metaal. Ze bieden nieuwe mogelijkheden voor het slim koelen van micro-elektronica. Wanneer warmte langs één as wordt afgevoerd, kan de dichtheid van componenten langs de andere assen groter zijn dan wanneer een drie-dimensionale warmtegeleider zou zijn gebruikt.
Dit onderzoek concentreert zich op twee materialen; de spinketen SrCuO2, waarin de magnetische structuur een keten vormt, en de spinladder Ca9La5Cu24O41, waarin deze een ladder vormt. Een nieuwe techniek is ontwikkeld om de warmtegeleiding optisch te meten, met als voordelen ten opzichte van een conventionele techniek dat het een zeer snelle techniek is, dat de minimale afstand waarover kan worden gemeten ordegroottes kleiner is en dat er tijdsopgelost kan worden gemeten. Naast metingen aan pure materialen, zijn er ook metingen gedaan in een realistische ‘device’ configuratie.
Het is gebleken dat de warmtegeleiding in de spinladdermaterialen sterk tijdsafhankelijk is ten gevolge van een zwakke interactie tussen de magnetische excitaties en de trillingen van het kristalrooster. Deze eigenschap kan gebruikt worden om op een direkte manier de interactie tussen kristaltrillingen en magnetische excitaties te meten.
In low-dimensional quantum magnets heat is mainly transported along one axis by the transport of magnetic energy. Furthermore, these materials are electrically insulating and the heat transport along the good axis is comparable to that of a metal. They offer new possibilities for smart cooling of microelectronics. When heat is transported along one axis, the density of components along other axes can be larger than when a three-dimensional heat conductor would be used.
This research concentrates on two materials; the spin chain SrCuO2, in which the magnetic structure forms a chain, and the spin ladder Ca9La5Cu24O41, in which it forms a ladder. A new technique to measure heat transport optically is developed, which has the advantages over a conventional technique that it is a very fast technique, that the minimum distance over which one can measure is orders of magnitude smaller and that one can measure in a time-resolved fashion. Apart from measurements on pure materials, measurements on a realistic ‘device’ configuration have been performed.
It turns out the heat transport in spin ladder materials is strongly time-dependent because of a weak interaction between the magnetic excitations and the lattice vibrations. This property can be used to measure the interaction between crystal vibrations and magnetic excitations in a direct way.
Induced excitations in some metal oxides
Full text linkAn optical spectrometer may be a tool unfamiliar to an outsider of the optics community. It can be regarded as a natural evolution of the way we perceive the surrounding environment. Light coming from the sun is reflected or transmitted by objects. The human eye, by measuring this reflected or transmitted light, gains some information about the objects themselves, such as their color, brightness, and shape. Using the same principles, an optical spectrometer is designed to gain even more information. It has a source of light used to send light on some objects, and a hi-tech detector to measure the transmitted or reflected light.
After the material has been measured, a relation needs to be established between its optical response and its main physical properties. In this thesis we consider the case of solid state materials. These are materials where the atoms are in close proximity to each other, and arranged in repetitive patterns on macroscopic distances, to make a compact structures (a good example is NaCl, kitchen salt). To establish the above mentioned relation the following procedure is followed. First the optical response (transmission, reflectivity, absorption, etc.) at each frequency in the incident light is measured. The result in quantified in terms of the so called dielectric constant of the bulk ε(∞). In a second step, this function is related to the main physical propertied of the measured solid state material, using the knowledge of this particular structure, determined before by different type of measurements, such as for instance X-ray diffraction. Today, optical spectroscopy has become one of the most important standard tools for investigating novel condensed matter materials.
The aim of the work presented in this thesis is to investigate the low energy excitation spectra and the (induced) phase transitions in transition metal oxides.
La2@C72: Metal-Mediated Stabilization of a Carbon Cage
Full text linkIn this study, we report production, isolation, and characterization for the relatively small endohedral metallofullerene, La2@C72. As described, La2@C72 is readily isolated from conventional electric-arc-generated carbon/metal soot. This new species was purified by HPLC chromatography and characterized by laser desorption mass spectrometry and UV-vis spectroscopy. The mass spectrum also demonstrates the presence of the monometal species, La@C72, but the absence of empty-cage C72. Since empty-cage C72 has not been successfully isolated to date, the results of the present study are in agreement with the argument for metal-mediated stabilization of the C72 carbon cage by lanthanum ions. The chromatographic retention data suggest that the electronic structure of La2@C72 is consistent with a (La3+)2@C726- species and the prediction of a relatively small dipole moment.
Spin and phonon dynamics in layered cu-based organic-inorganic hybrids
Full text linkSnelle vooruitgang in technologie motiveert verder onderzoek om materialen te ontwerpen met multifunctionele eigenschappen. De organische-anorganische hybriden zijn veelbelovende stoffen, veelzijdig en goedkoop. In dit proefschrift bestudeer ik een familie van hybriden die ferromagnetische en ferro-elektrische eigenschappen combineren. Het belangrijkste kenmerk van deze materialen is de gelaagdheid van de structuur: Cu-bevattende anorganisch moleculen vormen lagen, gescheiden door organische moleculen.
Mijn studies richten zich op twee zeer belangrijke eigenschappen voor toepassingen: het energie transport, fundamenteel voor warmteafvoer en de magnetisatie controle, met toepassingen in magnetische geheugens.
Zowel de energietransport experimenten als de magnetisatie manipulatie werden uitgevoerd met optische methoden.
Deze optische niet-destructieve methoden maken het mogelijk processen te onderzoek tot op een femtoseconde tijdschaal (een biljardste van een seconde).
We vinden dat in de nabijheid van fase-overgangen,vibratietoestanden de eigenschappen van het energietransport in het materiaal sterk kunnen beïnvloeden.
In het bijzonder biedt onze analyse de mogelijkheid tot bestudering van de fundamentele koppeling tussen vibratietoestanden.
Betreffende manipulatie van de magnetisatie met licht, beschrijven we een nieuwe methode om de magnetisatie te verbeteren.
De methode is toepasbaar op gelaagde structuren en potentieel functioneel tot op kamertemperatuur.
Rapid advances in technology motivate further research to design materials with multifunctional properties. The organic-inorganic hybrids are promising compounds, versatile and cheap. In this thesis I study a family of hybrids which combine ferromagnetic and ferroelectric properties. The main characteristic of these materials is the layered character of the structure: the Cu-based inorganic constituents form layers, separated by organic molecules.
My studies focus on two very important properties for applications: the energy transport, fundamental for heat dissipation, and the magnetization control, with applications to magnetic memories.
Both energy transport and magnetization manipulation experiments have been performed by optical methods. These non-destructive methods allow the investigation of processes down to the femtosecond timescales (a quadrillionth of a second).
It is found that vibrational modes, in proximity to phase transitions, can strongly influence the energy transport properties of the material. In particular our analysis allows the study of fundamental coupling between vibrations.
Concerning light-manipulation of the magnetization we describe a novel method to enhance the magnetization. The method is applicable to layered structures and potentially functional up to the room temperature regime.
Unconventional energetics of the pseudogap state and superconducting state in high-T-c cuprates
No full textSubpicosecond shifting of the photonic band gap in a three-dimensional photonic crystal
Full text linkWe demonstrate spectral shifting of the photonic band gap in a three-dimensional photonic crystal within a time of less than 350 fs. Single 120 fs high-power optical pulses are capable to induce the transition from the semiconductor to the metallic phase of VO2 in the pores of our artificial silica opal. The phase transition produces a substantial decrease of the real part of the effective refractive index of the photonic crystal and shifts the spectral position of the photonic band gap. (C) 2005 American Institute of Physics
- …
