Small Fermi Energy Research Articles

Asymmetric fermi surfaces for magnetic schrödinger operators

We consider Schrödinger operators with periodic magnetic field having zero flux through a fundamental cell of the period lattice. We show that, for a generic small magnetic field and a generic small Fermi energy, the corresponding Fermi surface is convex and not invariant under inversion in any point.

Asymmetric Fermi surfaces for magnetic Schrödinger operators

We consider Schrodinger operators with periodic magnetic field having zero flux through a fundamental cell of the period lattice. We show that, for a generic small magnetic field and a generic small Fermi energy, the corresponding Fermi surface is convex and not invariant under inversion in any point.

Origin and consequences of the ‘gap’ in the cuprate normal state

Abstract We present an alternative to the spin gap as an explanation for the origin of the gap feature observed in the normal state of the high-Tc cuprates. We consider a model that has a non-degenerate as well as a degenerate component of charge carriers. The properties are governed by the location of the chemical potential relative to the top of the non-degenerate band. In the underdoped regime this translates into an energy gap for the non-degenerate carriers; this decreases with doping, vanishing at optimal doping. In the overdoped regime there emerges a small Fermi energy that governs the temperature dependence of properties. As illustrations we consider several spin and charge properties, namely spin susceptibility, entropy, in-plane resistivity, Hall effect and Hall angle.

Temperature dependence of the bend resistance of composite fermions in narrow cross junctions

The bend resistance in narrow cross junctions exhibits dips near filling factors v = 1 2 p . We show that the temperature dependence of the dip near v = 1 2 is consistent with the interpretation that the dips arise due to the ballistic transmission of composite fermions. At temperatures higher than 0.3 K the temperature dependence is strong reflecting small Fermi energy of the composite fermions. The bend resistance saturates at lower temperatures as expected for the ballistic transmission of fermionic particles.

Transport properties of composite fermions in narrow cross junctions

The bend resistance in narrow cross junctions has been investigated in the fractional quantum Hall regime. Dips in the resistance due to the ballistic transmission of composite fermions have been observed near v = 1/2 and 1/4. The temperature dependence of the dips is strong reflecting the small Fermi energy of the composite fermions.

Ballistic transport of composite fermions in narrow cross junctions

The transport properties of the half-filled Landau level in crossed-wire junctions are presented. A dip in the bend resistance due to ballistic transmission of composite fermions has been observed near upsilon =1/2 and 1/4. The temperature dependence of the dips is found to be strong due to the small Fermi energy of the composite fermions. The dips are found to shift to lower magnetic fields, indicating that the effective magnetic field for the composite fermions in the narrow channel is not uniform.

Magnetoresistance and Hall effect of sodium-hydride-Graphite ternary intercalation compounds

The electronic structure of stage 5 sodium hydride-graphite intercalation compounds is investigated by means of magnetoresistance and Shubnikov-de Haas oscillations. The small concentration of graphitic π electron carriers and the absence of intercalate carriers, which are related to the small Fermi energy EF ≈ 0.6eV, are considered to be brought about by the strong ionicity of the NaH intercalates, in contrast with the case of isostructural potassium hydride-graphite intercalation compounds which have metallic hydrogen layers in the KH intercalates with large concentration of graphitic π carriers.

Nonadiabatic superconductivity: Electron-phonon interaction beyond Migdal's theorem.

We generalize Eliashberg's equations to include nonadiabatic effects like vertex corrections, cross phonon scattering, and others arising from the breakdown of Migdal's theorem. This generalization is imposed by the fact that all high ${T}_{c}$ superconductors (oxides, fullerene compounds, etc.) have a very small Fermi energy ( ${E}_{F}$). Nonadiabatic effects show a complex structure as a function of the exchanged frequency and momenta. In particular, a predominance of small momentum scattering leads to positive contributions with respect to ${T}_{c}$. This situation is actually realized in a correlated Fermi liquid with small ${E}_{F}$ that naturally leads to an enhancement of ${T}_{c}$ and to various other consequences for the phenomenology of both the superconductive and normal phases.

Study of superconducting fluctuations in superconductors with strong fluctuations

The effect of superconducting fluctuations is studied in an approximation beyond the Hartree-Fock approximation for the one-particle Green function and the random-phase approximation for the propagator of the superconducting fluctuations (HF-RPA) in the clean limit. The pair susceptibility is obtained in a form consistent with the self-energy correction in the sense that the phase invariance of the system is not violated. This is done by the systematic use of the Ward-Takahashi relations derived from the phase invariance. Superconducting fluctuations greatly reduce the transition temperature from the mean-field value in a superconductor with a small Fermi energy and a large BCS coupling constant.

Conservation of momentum, and its consequences, in interband resonant tunneling.

We have observed a drastic change with temperature (300--4 K) in the line shape of the current-voltage characteristics of GaSb-AlSb-InAs-AlSb-GaSb heterostructures, which below \ensuremath{\simeq}50 K develop a kink at low biases that is most pronounced at 4 K. This behavior is a direct consequence of the conservation of parallel momentum of electrons and holes participating in interband resonant tunneling, a conservation law that is apparent only at low temperatures because of the small hole Fermi energy. Confirmation of this interpretation is provided by experiments under a magnetic field perpendicular to the tunnel current that show a large peak in the conductance at 6 T. This is the field at which the hole distribution has gained enough parallel momentum to equal the Fermi momentum of electrons.

On the Temperature Dependence of the Hall Resistance in Low Carrier Density Metals

A model calculation of the Hall factor for a metallic system with a small Fermi energy has revealed that it can lead to a significant temperature dependence of Hall coefficient even when a single parabolic band contributes to electrical conduction. The present result gives a natural explanation for the behavior of temperature dependence of the transport properties observed in GaAs/Al x Ga 1- x As superlattices.

Positron-annihilation lifetime in the high-T c Oxides

The superconducting transition affects the positron-annihilation lifetime in the high-Tc oxides. This effect is due to the unusual properties of the oxides in the normal state and, in particular, to the small Fermi energy. The value of the shift also depends on the interlayer distance.

Gutzwiller approximation for the Emery model.

Normal-state properties of the two-dimensional model proposed by Emery for copper-oxide superconductors are studied within the Gutzwiller approximation. For larger intrasite repulsion it is found that hybridization produces quasihole bands with a reduced kinetic energy. For hole densities between 1 and 2, the reduction factor q is proportional to (epsilon/sub 0//sup -1/+(u-epsilon/sub 0/)/sup -1/)/sup 2/ where epsilon/sub 0/ = deltaepsilon/t, u = U/t, deltaepsilon is the difference in energy between the copper and the oxygen levels, and t is the hybridization energy. These results differ qualitatively from those for the heavy-fermion case where q is exponentially small, resulting in small Fermi energy (large effective mass). In the present case holes are only moderately heavy so that the Fermi energy is large.

Nearly Gapless Singlet-Pairing in Pure System of Heavy Fermion Superconductor

Heavy fermion superconductors, recently found in compounds with I-electrons (such as CeCu2Si2 and UBed, have attracted much attention.i)-6) One of the recent· issues is concerning the type of Cooper pairing; singlet (even parity) or triplet (odd parity) type? Although a nature of heavy fermion itself has not been completely clarified from a theoretical point of view, it is likely from experimental evidences that heavy fermions behave as a fermi liquid with very small Fermi energy of order TK. There was an attempt to explain, along this line, heavy fermion superconductors within the singlet BCS scheme.1l Since the motion of heavy fermion is slower than those of light-electrons and ions, the Coulomb interaction between heavy fermions is screened instantaneously by rapid motions of the latter; so that the interaction potential in k-space between heavy fermions is given by Vk=4Ke 2 /k €(k, 0»0, where €(k, 0) is the static dielectric constant. The reason why the triplet case is considered to be a likely possibility seems to be that Vk is positive for the whole regions of k-space. Indeed, several theorists have recently claimed that the triplet pairing is the most likely possibility.S),9) However, the present authors recently pointed out that s-wave pairing is still possible owing to the spatial correlations among particles, at least in principle, even if Vk is always positive. IO) The experimental results concerning the above issues are also very controversiaL For CeCu2Si., Assmus et aL 5) claimed that the Landau parameter Foa is positive which is in favour of singlet pairing_ On the other hand, Ott et aL6) recently showed that the low temperature specific heat of UBel3 exhibits T 3-dependence, which is in favour of the p-wave superconductivity of ABM-type. Moreover, Kitaoka et aL ll) recently found an unusual nuclear relaxation for CeCu2Si2 which cannot be understood by neither s-wave scheme of simple BCS-type nor p-wave scheme of ABMtype. The purpose of this paper is to point out that a singlet-pairing with nearly gapless excitations is possible for a model in which the interaction between fermions on the same site is strongly repulsive while that on the neighbour sites is appropriately attractive, and that consequences of this model are consistent with existing experiments of heavy fermion superconductors. In particular, a puzzling behavior of nuclear relaxation of CeCu2Si. found by Kitaoka et aL ll) is qualitatively explained by this model. On the basis of a quasi-particle theory of Kondo lattice,10).12) we simulate a heavy fermion system as a half-filled band of fermions with a Hamiltonian

Tunneling into Low-Carrier-Density Superconductors

A theory of tunneling between a normal metal and a low-carrier-density superconductor at zero temperature is developed. Because of the small Fermi energy of low-carrier-density superconductors and the energy dependence of the tunneling matrix elements for these junctions, the conductance-versus-voltage curves are quite different from those for tunneling between a metal and a high-density (metallic) superconductor. The tunneling displays both the voltage-dependent conductance associated with tunneling between a metal and a degenerate semiconductor and the peaks in conductance arising from the large quasiparticle density of states at voltages slightly larger than the superconducting energy gap.

Electron Tunneling between a Metal and a Semiconductor: Characteristics of Al-Al2O3-SnTe and −GeTe Junctions

The process of electron tunneling from a metal to a semiconductor through an insulating layer is considered. Theoretical current-voltage expressions have been obtained, in particular, for the case when the semiconductor is degenerate p type and the conduction band of the insulator provides the dominant tunneling barrier. The existence of an energy gap and a relatively small Fermi energy in the semiconductor makes the current highly asymmetrical with respect to the polarity of the applied voltage. Experimentally, junctions of Al-Al2O3-SnTe and −GeTe have been fabricated and their I-V characteristics measured at 4.2°K. Good agreement is observed between the theoretical and experimental current behavior, from which energy parameters of the semiconductors and the barrier relations of the junctions have been obtained.

Oscillatory Tunneling Current through Thin-Film Insulating Barriers in a Magnetic Field

The tunneling current between two conductors through an intermediate thin-film insulating barrier in the presence of a magnetic field parallel to the direction of tunneling has been calculated. It is shown that the Fowler—Nordheim tunnel current decreases as the magnetic field is increased and, in certain cases, contains a component which oscillates with a period proportional to 1/H. The oscillations should be observable when the electrons in the material from which the electrons tunnel have a small Fermi energy, a small effective mass, and a long relaxation time. Bismuth, lead telluride, and graphite are examples of materials in which the criteria for observing oscillations are realized.