These changes, explicitly determined using dft calculations, explain variations of the depth of phonon dispersion pd anomalies with strong correlation to t c 5 6, directly influence superconductivity and establish a direct connection to variations in temperature. Electronic band structure equations and fermi surface. If we assume that the potential of the metal lattice is the same everywhere, the fermi liquid of conduction electrons is isotropic. Because of its derivation from the homogeneous electron gas, the. The highresolution photoemission spectra show that superconducting gaps with values of 5. In some sense, it is exactly opposite to the case 6. Deviations in f in mgb2 can beremoved by a small mutual shift of sigma and pi bands.
This type of surface analysis has been carried out since, as is well known, for the fabrication of superconducting devices based on superconducting nanostructures where the quality of the starting surface is often of crucial importance to the optimal operation of the device. This and subsequent studies reveal a profound rearrangement of the fermi surface in underdoped cuprates. Except for effects of randomness of atomic positions. A system of conduction electrons can be thought of as a fermi liquid. The distribution function of the quasi particles is the fermi function. We provide a comparison of accurate full potential band calculations of the fermi surfaces areas and masses of mgb2 and zrb2 with. A quantum statistical theory of the superconductivity in mgb2 is developed regarding it as a member of the graphite intercalation compound. Wills physics laboratory, university of bristol, tyndall avenue, bristol bs8 1tl, united kingdom. Karpinski3 1bell laboratories, alcatellucent, murray hill, new jersey 07974, usa 2department of physics and materials research laboratory, university of illinois at urbanachampaign, urbana, illinois 61801, usa.
Mgb2, or magnesium diboride, is known for its unique superconductivity. Fermi surface topology and the upper critical field in twoband superconductors. The fermi energy is the highest energy state of a particle in a system near absolute zero temperature. The mechanism of this superconducting material has been explored and the electronic structure and properties of mgb2 have been studiedthe huge research population caused by mgb2 is inseparable from the. Recent remarkable progress of both energy and momentum resolutions in angleresolved photoemission spectroscopy arpes enables us to experimentally determine the fermi surface with considerable accuracy. The electronphonon coupling is found to be very anisotropic 4 being the strongest. Physics fermisurface reconstruction and the origin of. This filled region is known as an electron pocket and the fermi surface here is known as an electron orbit. Strongly enhancend anisotropic upper critical field in. Comparison of effective masses lead to orbit averaged. Elk code is an allelectron fullpotential linearized augmentedplanewave fplapw code for determining the properties of crystalline solids. Box 270116, d01171 dresden, germany received 22 february 2002. The discrepancies in areas in mgb2 can be removed by a shift of sigmabands downward with respect to pibands by 0. In terms of its composition, mgb2 differs strikingly from most lowtemperature superconductors, which feature mainly transition metals.
The sigmaband is modeled as a distorted cylinder and the piband as a halftorus, with parameters determined from bandstructure calculations. All the bands that are fully filled below the fermi energy are irrelevant to lowenergy physics. After twenty years of effort, definitive quantum oscillations that could be used to map the fermi surface were finally observed in a hightemperature cuprate superconductor in 2007. Calculation of electron transport properties of mgb2 unit cell. The theorists graph of mgb2s extraordinary fermi surface a way of visualizing the highestenergy states its electrons can occupy clearly shows the two populations of electrons and the different energies needed to break their superconducting pairs a graph that incidentally gives rise to the nicknames red and blue electrons. A firstprinciples approach to the eliashberg formalism has shown that the stronger gap. The dependence of rs at 15 ksolid symbol and 20 k open symbol on a.
The purpose of the present work is to study in detail the electronic band structure of mgb2 in comparison with that of. The only band we care about is the highest band, where the fermi surface rest in. The electronic band structures of superconducting mgb2 and. Firstprinciples study of the 0001mgb2 surface finished in mg and b. The cause of the reconstruction, and its implication for the origin of hightemperature superconductivity, is. The evolution of the fermi surface is also obtained from the greens function method for different values of. Matter 19 2007 355003 s souma and t takahashi phonons con. By knowing the fermi surface of different materials one could predict their properties and. Detailed electronic structure studies on superconducting mgb2. Based on a detailed modeling of the fermi surface topology of mgb2 we calculated the anisotropy of the upper critical field bc2 within the two gap model. Akimitsu, superconductivity at 39 k in magnesium diboride. Based on a detailed modeling of the fermi surface topology of mgb 2 we calculated the anisotropy of the upper critical field b c2 within the two gap model. Ef and the higher tc in conventional superconductors is related to large nef values as well as.
Pressure dependence of superconducting properties of mgb2. In the plane perpendicular to the caxis, bs form a honeycomb lattice with the nearest neighbour. This channel almost completely dominates the tunneling images and spectra for caxisoriented samples and not the twodimensional. Mgb2 is an impressive elph superconductor in spite of using only 3% of its phonons effectively. The fermi surface and sheetdependent superconducting gap.
Recent experimental data of anisotropic magnetoresistivity measured in mgb2 films have shown an intriguing behaviour. Fermi surface and a small or negligible gap on the. Fermi surface is an abstract geometrical representation of the all available electronic states in various metals which are responsible for the electrical, thermal and magnetic properties. In this elk simulation, the surfaces from different bands are separated in the 3d plot. Electron density response to phonon dynamics in mgb2. Using this value in the mcmillan expression for t c with. Effect of the foursheet fermi surface on magnetoresistivity of mgb2 by i. Fermi surface topology and vortex state in mgb2 request pdf. Recent experimental data of anisotropic magnetoresistivity measured in mgb 2 films have shown an intriguing behaviour. Its superconducting mechanism is primarily described by bcs theory. Near the k h and m l points, we observed a strong dispersive peak that approaches e f, as emphasized by the blue dots. Superconductivity, electronic band structure, density functional theory, fermi surface, phonon dispersions, fermi level, fermi energy, kgrid, effective masses, mgb2. Fermi energy only exists with respect to particles called fermions. The quasiparticles are described by the fermi dirac statistics.
The origin of this effect is due to the faster decay of the electronic states associated with the boron p x and p y orbitals compared to those associated with the boron p. The origin of the anomalous superconducting properties of mgb2. In order to explain such phenomenology, in this work, we extend our previous analyses on multiband transverse magnetoresistivity in magnesium diboride, by calculating its. Firstprinciples study of the 0001mgb2 surface finished. The fermi surface geometry of twobands, combining the effect of the normallike electronic state on the small gap band at high fields, produces characteristic behavior in the anisotropy of c and. Free surfaces recast superconductivity in fewmonolayer mgb2 core. A most unusual superconductor and how it works berkeley lab. The shape of the fermi surface is derived from the periodicity and symmetry of the crystalline lattice and from the occupation of electronic energy bands. Magnesium diboride is the inorganic compound with the formula mgb 2. Strongly enhancend anisotropic upper critical field in carbon substituted mgb2.
Magnesium diboride is the inorganic compound with the formula mgb2. We provide a comparison of accurate full potential band calculations of. Fermi surfaces i want to talk about fermi surfaces every metal has one, and they are all unique were going to lead into talking about an exceptional case between a metal and an insulator, where there is no fermi surface. In particular, the tubular structures with character are very strongly coupled to the e 2g phonon mode, corresponding to a bb bondstretching in the boron planes 9,10. Superconducting phase diagram of single crystal mgb2. Quantum statistical theory of superconductivity in mgb2. Based on the strong anisotropy of electronphonon coupling between.
In terms of its composition, mgb 2 differs strikingly from most lowtemperature superconductors, which feature mainly transition metals. The compound has attracted attention because it becomes superconducting at 39 k. Fermi surfaces university of oxford department of physics. Pdf fermi surface topology and the upper critical field. The existence of a fermi surface is a direct consequence of the pauli exclusion. The fermi pockets are just the fermi surface in this highest band. Superconducting properties of mgb2 from first principles.
Microwave and terahertz surface resistance of mgb 2 thin films 619 fig. Carrington a1, meeson pj, cooper jr, balicas l, hussey ne, yelland ea, lee s, yamamoto a, tajima s, kazakov sm, karpinski j. The fermi velocity is the velocity that corresponds to the fermi energy of a particle. Microwave and terahertz surface resistance of mgb2 thin. Pdf fermisurfer is a newly developed fermisurface viewer designed to. Mgb2 is characterized by a complex fermi surface consisting of disconnected sections. Pdf strongly enhancend anisotropic upper critical field. The theorists graph of mgb2s extraordinary fermi surface a way of visualizing the highestenergy states its electrons can occupy clearly shows the two populations of electrons and the different energies needed to break their superconducting pairs a graph that incidentally gives rise to the nicknames red and blue. The sc gaps have been assigned to distinctive leggetts collective mode in a multiband mgb 2 superconductor g. Recent remarkable progress of both energy and momentum resolutions in angleresolved photoemission spectroscopy arpes enables us to experimentally determine the fermi surface with considerable. Request pdf fermi surface topology and vortex state in mgb2 based on a detailed modeling of the fermi surface topology of mgb2 we calculated the anisotropy of the upper critical field bc2. We compare calculated fermi surface sheet fss areas f and masses of mgb2 and zrb2 with dhva data. Giant anharmonicity and a bcs theory of unexpected 40 k. In condensed matter physics, the fermi surface is the surface in reciprocal space which separates occupied from unoccupied electron states at zero temperature.
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