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Fermi Surface (fermi + surface)

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Physics and Astronomy100%

Selected Abstracts

Feshbach shape resonance for high Tc superconductivity in superlattices of nanotubes

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 11 2006
Antonio Bianconi
Abstract The case of a Feshbach shape resonance in the pairing mechanism for high Tc superconductivity in a crystalline lattice of doped metallic nanotubes is described. The superlattice of doped metallic nanotubes provides a superconductor with a strongly asymmetric gap. The disparity and different spatial locations of the wave functions of electrons in different subbands at the Fermi level should suppress the single electron impurity interband scattering giving multiband superconductivity in the clean limit. The Feshbach resonances will arise from the component single-particle wave functions out of which the electron pair wave function is constructed: pairs of wave functions which are time inverse of each other. The Feshbach shape resonance increases the critical temperature by tuning the chemical potential at the Lifshitz electronic topological transition (ETT) where the Fermi surface of one of the bands changes from the one dimensional (1D) to the two dimensional (2D) topology (1D/2D ETT). (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

High-field de Haas,van Alphen effect in GdIn3

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2003
Z. Kletowski
Abstract Results of the de Haas,van Alphen effect investigations are reported for the antiferromagnetically ordered compound GdIn3. For the first time data on the dHvA frequencies and their angular dependencies are shown for magnetic field higher than the metamagnetic transition field. The list of detected frequencies and related cyclotron masses is given. Among many Fermi surface branches observed, only the j, b and d correspond to theoretically predicted and experimentally observed branches for the nonmagnetic LaSn3. The metamagnetic transition taking place at 30 T in the GdIn3 influences some branches of Fermi surface, mostly those in the (110) plane and close to the [111] direction. All effective masses found are lower than m0 and close to those found for the LaSn3. [source]

Nonadiabatic electron,phonon effects in low carrier density superconductors

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2005
E. Cappelluti
Abstract Different families of unconventional superconductors present a low charge carrier density as a common trait, suggesting that the low charge density can be at the basis of a unifying picture for different superconductors. In the past years we have suggested that the electron,phonon interaction can be responsible for a high- Tc superconducting pairing in a nonadiabatic regime, where nonadiabatic effects are triggered on by the small electronic Fermi energy associated with the low charge density character. A coherent picture of such a framework requires however reconciling the low charge density and the small Fermi energy with a finite metallic character (sizable density of states and large Fermi surfaces). In this paper we investigate the peculiar conditions which are needed to be encountered in order to fulfill these requirements. We discuss the specific case of fullerenes, cuprates and MgB2 alloys by analyzing their specific structural and electronic properties The comparison between these materials and simple instructive models permits to underline the different routes to reconcile these characteristics in different compounds. In cuprates and fullerenes the interplay between small Fermi energies and large Fermi surface is strictly connected with strong electronic correlation effects. A comprehensive understanding of these issues can be useful to the future search for new nonadiabatic high- Tc materials. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Spin dependent distribution and Fermi surface of the perovskite manganite compound La0.7Sr0.3MnO3 via 2D-ACAR measurements,

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2004
A. S. Hamid
Abstract Using 2D angular correlation of positron annihilation radiation (ACAR) experiment, we have performed a systematic study of the spin dependent and Fermi surface of the colossal magneto-resistance CMR La0.7Sr0.3MnO3. The measurements have been carried out using a re-versal magnetic field direction (parallel and anti-parallel to the direction of motion of the polarized posi-trons). The measured spectra have been investigated in the momentum space as well as in the wave vector space. They revealed information about the hybridization effect of Mn(3d eg1) and O(2p) like states. Further, the results showed that the majority spin electrons predominated at the Fermi level. From another perspective, the Fermi surface of La0.7Sr0.3MnO3 revealed a cuboids hole surface centered on R point and a spheroid electron surface centered on , point. A comparison with the earlier results showed qualitative agreement. However, the current results could reveal the dimension of the electron surface centered on , point that was predicted in the earlier 2D-ACAR measurements. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

The electronic Fermi surface e3 of aluminium under moderate compression

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 3 2003
Helmut BrossArticle first published online: 11 NOV 200
Abstract Both in LDA and in GGA the electronic structure of aluminium is evaluated by use of the MAPW selfconsistent scheme and with different exchange correlation functionals. Near and above the equilibrium lattice constant the electronic sheet of the Fermi surface, e3, is found to be quite similar to the model originally proposed by Ashcroft. However, even moderate compressions induce a drastic variation. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Spin triplet superconductivity in Sr2RuO4

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2003
Karol I. Wysoki
Abstract Sr2RuO4 is at present the best candidate for being a superconducting analogue of the triplet superfluidity in 3He. This material is a good (albeit correlated) Fermi liquid in the normal state and an exotic superconductor below Tc. The mechanism of superconductivity and symmetry of the order parameter are the main puzzling issues of on-going research. Here we present the results of our search for a viable description of the superconducting state realised in this material. Our calculations are based on a three-dimensional effective three-band model with a realistic band structure. We have found a state with non-zero order parameter on each of the three sheets of the Fermi surface. The corresponding gap in the quasi-particle spectrum has line or point nodes on the , and , sheets and is complex with no nodes on the , sheet. This state describes remarkably well a number of existing experiments including power low temperature dependence of the specific heat, penetration depth, thermal conductivity etc. The stability of the state with respect to disorder and different interaction parameters are also analyzed briefly. [source]

Electronic transport through large quantum dots in the Kondo regime

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2003
P. Stefa
Abstract Conductance through a large two-level quantum dot is investigated theoretically in the strong coupling regime. In large quantum dots the separation between discrete levels becomes smaller than the level width due to strong hybridization with electrodes. In such circumstances, apart from strong electronic correlations in the quantum dot, the indirect interaction between both the spatial levels comes into play. It takes place in lateral quantum dots, where the spatial level index is not conserved during the hybridization process with electrodes. This interaction shifts the Kondo resonance peak in the density of states out of the Fermi surface and alters its intensity. This feature can be observed in the differential conductance dependence vs. bias voltage. The virtual inter-level mixing is suppressed for temperatures above the Kondo temperature of the system. The results of theoretical predictions are compared with the results of experimental conductance measurements performed on large quantum dots and some non-typical conductance features are clarified. [source]

Electronic structure of BaFe2As2 as obtained from DFT/ASW first-principles calculations

ANNALEN DER PHYSIK, Issue 8 2010
U. Schwingenschlögl
We use ab-initio calculations based on the augmented spherical wave method within density functional theory to study the magnetic ordering and Fermi surface of BaFe2As2, the parent compound of the hole-doped iron pnictide superconductors (K,Ba)Fe2As2, for the tetragonal I4/mmm as well as the orthorhombic Fmmm structure. In comparison to full potential linear augmented plane wave calculations, we obtain significantly smaller magnetic energies. This finding is remarkable, since the augmented spherical wave method, in general, is known for a most reliable description of magnetism. [source]

Nonadiabatic electron,phonon effects in low carrier density superconductors

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2005
E. Cappelluti
Abstract Different families of unconventional superconductors present a low charge carrier density as a common trait, suggesting that the low charge density can be at the basis of a unifying picture for different superconductors. In the past years we have suggested that the electron,phonon interaction can be responsible for a high- Tc superconducting pairing in a nonadiabatic regime, where nonadiabatic effects are triggered on by the small electronic Fermi energy associated with the low charge density character. A coherent picture of such a framework requires however reconciling the low charge density and the small Fermi energy with a finite metallic character (sizable density of states and large Fermi surfaces). In this paper we investigate the peculiar conditions which are needed to be encountered in order to fulfill these requirements. We discuss the specific case of fullerenes, cuprates and MgB2 alloys by analyzing their specific structural and electronic properties The comparison between these materials and simple instructive models permits to underline the different routes to reconcile these characteristics in different compounds. In cuprates and fullerenes the interplay between small Fermi energies and large Fermi surface is strictly connected with strong electronic correlation effects. A comprehensive understanding of these issues can be useful to the future search for new nonadiabatic high- Tc materials. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Dynamical adjustment of propagators in Renormalization Group flows

ANNALEN DER PHYSIK, Issue 3 2007
M. Salmhofer
Abstract A class of continuous renormalization group flows with a dynamical adjustment of the propagator is introduced and studied theoretically for fermionic and bosonic quantum field theories. The adjustment allows to include self,energy effects nontrivially in the denominator of the propagator and to adapt the scale decomposition to a moving singularity, and hence to define flows of Fermi surfaces in a natural way. These flows require no counterterms, but the counterterms used in earlier treatments can be constructed using them. The influence of propagator adjustment on the strong,coupling behaviour of flows is examined for a simple example, and some conclusions about the strong coupling behaviour of renormalization group flows are drawn. [source]