Search results for "State"

Competition of electron-phonon mediated superconductivity and Stoner magnetism on a flat band

2018

The effective attractive interaction between electrons, mediated by electron-phonon coupling, is a well-established mechanism of conventional superconductivity. In metals exhibiting a Fermi surface, the critical temperature of superconductivity is exponentially smaller than the characteristic phonon energy. Therefore, such superconductors are found only at temperatures below a few kelvin. Systems with flat energy bands have been suggested to cure the problem and provide a route to room-temperature superconductivity, but previous studies are limited to only BCS models with an effective attractive interaction. Here we generalize Eliashberg's theory of strong-coupling superconductivity to syst…

Field dependence of the vortex-core sizes in dirty two-band superconductors

2019

We study the structure of Abrikosov vortices in two-band superconductors for different external magnetic fields and different parameters of the bands. The vortex core size determined by the coherence lengths are found to have qualitatively different behaviour from that determined by the quasiparticle density of states spatial variation. These different vortex core length scales coincide near the upper critical field, while the discrepancy between them becomes quite significant at lower fields. Within the diffusive approximation we demonstrate several generic regimes in the field dependence of the vortex core sizes determined by the disparity of diffusion constants in the two bands.

Comparison of the Richardson and BCS models of superconductivity based on calculations of ground state energies

2017

Suprajohtavuuden Richardsonin ja BCS mallien vertailu perustilojen laskettujen energioiden perusteella. Suprajohtavuus on edelleen aktiivinen tutkimuksen alue, koska sen tarjoamista mahdollisuuksista huolimatta sitä ei vielä ymmärretä kokonaisvaltaisesti. Tässä pro gradu -tutkielmassa käyn läpi perusteet kahdelle suprajohtavuuden mallille, Richardsonin ja BCS malleille. BCS teoria on ensimmäinen ja käytetyin mikroskooppinen suprajohtavuuden teoria. Richardsonin malli on harvemmin käytetty malli, josta saadaan redusoidun BCS Hamiltonin operaattorin tarkat ominaistilat. Lasken molempien mallien perustilojen energiat sekä redusoidulle että täydelle BCS Hamiltonin operaattorille. Sama lasketaan…

Electron-phonon interaction in flat-band superconductivity

2017

Parhaiten tunnettu suprajohtavuuden syntymekanismi perustuu fononien välittämään vetovoimaan elektronien välillä. Tässä tutkielmassa tutkin fononien välittämää suprajohtavuutta systeemeissä, jossa elektronivyöt ovat tasomaisia. Tasovyöllä elektronien dispersio on erittäin heikko, jolloin tilatiheys on tavanomaista suurempi. Tämän takia suprajohtavuus tasovyöllä on tavanomaista voimakkaampi silloin kun elektronien välinen vetovoima on heikko. Eliashbergin teoria on elektroni–fononi-suprajohtavuuden teoria, joka ottaa luonnollisella tavalla fononien äärellisen nopeuden huomioon elektronien välisessä vuorovaikutuksessa. Pohjustuksena tasovyösuprajohtavuuteen perehdyn Eliashbergin teoriaan ensi…

Regionalizm zagrożeniem czy wyzwaniem dla demokratycznego państwa prawa? Kilka uwag w sprawie Katalonii

2018

Aspiracje Katalończyków do przekształcenia wspólnoty autonomicznej wpływają na koncepcję strukturalną państwa autonomicznego, nierozerwalną jedność narodu hiszpańskiego wyrażoną w art. 2, jak również przypisaną ludowi hiszpańskiemu suwerenność na mocy art. 1.2. Unia Europejska nie ingeruje w rozwiązania systemowe państw członkowskich. Problem stopnia decentralizacji, autonomii czy podziału zadań na linii władze centralne – poziom regionalny pozostawia kompetencji władz krajowych. Konstruktywnej dyskusji – w obliczu Brexitu, referendum w Szkocji, powracającej perspektywy Catalanexitu czy też innych silnych ruchów regionalnych – wydaje się jednak obawiać, a jest to problem, którego nie można …

Using altered states of consciousness in improvisational music therapy : the potential of resonance frequency breathing

2018

Improvisational music therapy is a type of creative arts therapy in which clients are encouraged to express themselves through the symbolic and non-verbal medium of music, by creating free music improvisations together with the therapist. The present research aims at investigating whether the efficacy of this method can be further enhanced by purposefully accessing the ego-quieting properties of altered states of consciousness. This work proceeds in three main steps. First, the notions of consciousness and its alteration are examined and reformulated, resulting in a more comprehensive theoretical model. Then, existing methods for inducing altered states of consciousness are evaluated in ter…

Spontaneous Raman Scattering Spectrum of Gaseous IrF6 in the Ground Electronic State

1996

Singular tori as attractors of four-wave-interaction systems

2009

We study the spatiotemporal dynamics of the Hamiltonian four-wave interaction in its counterpropagating configuration. The numerical simulations reveal that, under rather general conditions, the four-wave system exhibits a relaxation process toward a stationary state. Considering the Hamiltonian system associated to the stationary state, we provide a global geometrical view of all the stationary solutions of the system. The analysis reveals that the stationary state converges exponentially toward a pinched torus of the Hamiltonian system in the limit of an infinite nonlinear medium. The singular torus thus plays the role of an attractor for the spatiotemporal wave system. The topological pr…

Impulsive excitation of high vibrational states in I2–Xe complex on the electronic ground state

2010

Abstract High vibrational states, up to ν  = 22, are excited and investigated on the ground electronic state of a 1:1 I 2 –Xe complex isolated in solid Kr using femtosecond CARS technique and spontaneous resonant Raman measurements. The results show that this system is a promising candidate for investigations of coherent control of bimolecular reactions by using vibrational wavepackets on the ground electronic state.

Saddle index properties, singular topology, and its relation to thermodynamic singularities for aϕ4mean-field model

2004

We investigate the potential energy surface of a ${\ensuremath{\phi}}^{4}$ model with infinite range interactions. All stationary points can be uniquely characterized by three real numbers ${\ensuremath{\alpha}}_{+},{\ensuremath{\alpha}}_{0},{\ensuremath{\alpha}}_{\ensuremath{-}}$ with ${\ensuremath{\alpha}}_{+}+{\ensuremath{\alpha}}_{0}+{\ensuremath{\alpha}}_{\ensuremath{-}}=1$, provided that the interaction strength $\ensuremath{\mu}$ is smaller than a critical value. The saddle index ${n}_{s}$ is equal to ${\ensuremath{\alpha}}_{0}$ and its distribution function has a maximum at ${n}_{s}^{\mathrm{max}}=1∕3$. The density $p(e)$ of stationary points with energy per particle $e$, as well as…