Search results for "SUPERCONDUCTIVITY"
showing 10 items of 983 documents
On/off switching of bit readout in bias-enhanced tunnel magneto-Seebeck effect.
2014
Thermoelectric effects in magnetic tunnel junctions are currently an attractive research topic. Here, we demonstrate that the tunnel magneto-Seebeck effect (TMS) in CoFeB/MgO/CoFeB tunnel junctions can be switched on to a logic 1 state and off to 0 by simply changing the magnetic state of the CoFeB electrodes. We enable this new functionality of magnetic tunnel junctions by combining a thermal gradient and an electric field. This new technique unveils the bias-enhanced tunnel magneto-Seebeck effect, which can serve as the basis for logic devices or memories in a green information technology with a pure thermal write and read process. Furthermore, the thermally generated voltages that are re…
Doping effects on the magnetic frustration in the honeycomb iridates
2015
We investigate the doping effects of magnetic and nonmagnetic impurities injected to the honeycomb iridate sample of Na2IrO3 . Both the doping result in changing the ordering temperature as well as the Curie-Weiss temperature of the parent sample as a consequence of enhancement of the lattice frustration, screening of the Ir atoms and spin-orbit effects that reflects in the susceptibility and specific heat measurements. Our findings are corroborated by a detailed comparative study of various magnetic and nonmagnetic impurity atoms that have notable effects on different electronic properties of the doped compounds.
Thermal properties in low dimensional structures below 1 K
2009
In this thesis thermal properties of low dimensional structures were experimentally studied at low temperatures with the help of tunnel junction thermometry and the Joule heating technique. The main objects of study were electron-phonon coupling in disordered thin metal films and phonon transport in suspended silicon nitride membranes. Our aim has been to clarify the effect of the phonon dimensionality, i.e. the effect of boundaries to the phonon modes and the transition from 3D to 2D phonons. The dimensionality cross over had not been observed before this work even though it is fabricationally a standard procedure to create the low dimensional environments for nanoscale applications and de…
Polaronic relaxation in perovskites
1995
We report a low-temperature loss anomaly in several oxidic perovskites such as ${\mathrm{KTaO}}_{3}$, ${\mathrm{KTaO}}_{3}$:Nb, ${\mathrm{SrTiO}}_{3}$, ${\mathrm{SrTiO}}_{3}$:Ca, ${\mathrm{PbTiO}}_{3}$:La, Cu, and ${\mathrm{BaTiO}}_{3}$:La. We show that this anomaly arises from a low-frequency dielectric relaxation. The activation energy and the relaxation time of this process are nearly the same for all the investigated perovskites disregarding their composition, texture, and ferroelectric properties. We thus ascribe the loss anomaly to the localization of polarons on residual defects. Although the dielectric losses in ${\mathrm{SrTiO}}_{3}$ and ${\mathrm{SrTiO}}_{3}$:Ca are qualitatively …
Properties of the Phonon Gas in Ultrathin Membranes at Low Temperature
1998
We analyze heat conduction by phonons in ultrathin membranes by constructing a new theoreticalframework which implies a crossover from a bulk three-dimensional phonon distribution into a quasi-two-dimensional distribution when the temperature is lowered. We calculate the corresponding changesin the relevant thermodynamic quantities. At the end we make a comparison to experimental data.[S0031-9007(98)07273-1]
First-Principles Simulation of Substitutional Defects in Perovskites
2000
The results of supercell calculations of electronic structure and related properties of substitutional impurities in perovskite oxides KNbO3 and KTaO3 are discussed. For Fe impurities in KNbO3, the results obtained in the local density approximation (LDA) and in the LDA+U approach (that allows an ad hoc treatment of nonlocality in exchange-correlation) are compared, and different impurity charge configurations are discussed. The study of off-centre Li defects in incipient ferroelectric KTaO3 have been done by the appropriately parametrized Intermediate Neglect of Differential Overlap (INDO) method. The interaction energies of two off-centre impurities in different relative configurations ar…
Ab Initio Thermodynamics of Oxygen Vacancies and Zinc Interstitials in ZnO.
2015
ZnO is an important wide band gap semiconductor with potential application in various optoelectronic devices. In the current contribution, we explore the thermodynamics of oxygen vacancies and zinc interstitials in ZnO from first-principles phonon calculations. Formation enthalpies are evaluated using hybrid DFT calculations, and phonons are addressed using the PBE and the PBE+U functionals. The phonon contribution to the entropy is most dominant for oxygen vacancies, and their Gibbs formation energy increases when including phonons. Finally, inclusion of phonons decreases the Gibbs formation energy difference of the two defects and is therefore important when predicting their equilibrium c…
Pseudogap in high-temperature superconductors from realistic Fr\"ohlich and Coulomb interactions
2012
It has been recently shown that the competition between unscreened Coulomb and Fr\"{o}hlich electron-phonon interactions can be described in terms of a short-range spin exchange $J_p$ and an effective on-site interaction $\tilde{U}$ in the framework of the polaronic $t$-$J_p$-$\tilde{U}$ model. This model, that provides an explanation for high temperature superconductivity in terms of Bose-Einstein condensation (BEC) of small and light bipolarons, is now studied as a charged Bose-Fermi mixture. Within this approximation, we show that a gap between bipolaron and unpaired polaron bands results in a strong suppression of low-temperature spin susceptibility, specific heat and tunneling conducta…
High temperature superconductivity from realistic long-range Coulomb and Fr\"ohlich interactions
2012
In the last years ample experimental evidence has shown that charge carriers in high-temperature superconductors are strongly correlated but also coupled with lattice vibrations (phonons), signaling that the true origin of high-Tc superconductivity can only be found in a proper combination of Coulomb and electron-phonon interactions. On this basis, we propose and study a model for high-Tc superconductivity, which accounts for realistic Coulomb repulsion, strong electron-phonon (Fr\"ohlich) interaction and residual on-site (Hubbard \tilde{U}) correlations without any ad-hoc assumptions on their relative strength and interaction range. In the framework of this model, which exhibits a phase tr…
Melting the Superconducting State in the Electron Doped Cuprate Pr$_{1.85}% $Ce$_{0.15}$CuO$_{4-\delta}$ with Intense near-infrared and Terahertz Pul…
2016
We studied the superconducting (SC) state depletion process in an electron doped cuprate Pr$_{1.85}$Ce$_{0.15}$CuO$_{4-\delta}$ by pumping with near-infrared (NIR) and narrow-band THz pulses. When pumping with THz pulses tuned just above the SC gap, we find the absorbed energy density required to deplete superconductivity, $A_{dep}$, matches the thermodynamic condensation energy. Contrary, by NIR pumping $A_{dep}$ is an order of magnitude higher, despite the fact that the SC gap is much smaller than the energy of relevant bosonic excitations. The result implies that only a small subset of bosons contribute to pairing.