Effective mass (solid-state physics)

About: Effective mass (solid-state physics) is a research topic. Over the lifetime, 12539 publications have been published within this topic receiving 295485 citations.

Condensation, excitation, pairing, and superfluid density in high-Tc superconductors: the magnetic resonance mode as a roton analogue and a possible spin-mediated pairing

Abstract: To find a primary factor determining T c and a pairing mechanism in high-T c cuprates, we combine the muon spin relaxation results on n s /m* (superconducting carrier density/effective mass), accumulated over the last 17 years, with the results from neutron and Raman scattering, scanning tunnelling microscopy, specific heat, Nernst effect, and angle-resolved photoemission spectroscopy measurements. We identify the neutron magnetic resonance mode as an analogue of the roton minimum in the superfluid 4 He, and argue that n s /m* and the resonance mode energy ∞ω res play a primary role in determining T c in the underdoped region. We propose a picture wherein roton-like excitations in the cuprates appear as a coupled mode, which has resonance modes for spin and charge responses at different momentum transfers but the same energy transfer, as detected respectively by means of the neutron S = I mode and the Raman S = 0 A 1 g mode. We shall call this the 'hybrid spin/charge roton'. After discussing the role of dimensionality in condensation, we propose a generic phase diagram for the cuprates with spatial phase separation in the overdoped region as a special case of the Bose-Einstein to Bardeen-Cooper-Schrieffer crossover conjecture where the superconducting coupling is lost rapidly in the overdoped region. Using a microscopic model of charge motion resonating with antiferromagnetic spin fluctuations, we propose the possibility that the hybrid spin/charge roton and higher-energy spin fluctuations mediate the superconducting pairing. In this model, the resonance modes can be viewed as a meson analogue and the 'dome' shape of the phase diagram can be understood as a natural consequence of departure from the competing Mott insulator ground state via carrier doping.

Influence of metal–metal bonds on electron spectra of MoO2 and WO2

Abstract: Core and valence level photoemission and electron energy loss (EELs) spectra of MoO2 and WO2 have been measured. Metal–metal bonding in these distorted rutile dioxides splits the metal 4d or 5d conduction band into two components, with a significantly bigger splitting for WO2 than MoO2. The O 2p bandwidth is also found to be bigger for WO2 then MoO2. Plasmon loss peaks below 2 eV show that the effective mass ratios for electrons not involved in σ metal–metal bonding are much greater than unity. The photoemission and EELS data both suggest that metal–metal π bonding is important in these compounds. Comparison between photoemission spectra of WO2 and oxygen-deficient Na0.65WO3 –y suggests that structure evident in the bandgap of the latter compound may be associated with metal–metal bonding allowed by oxygen deficiency.

Repulsive polarons and itinerant ferromagnetism in strongly polarized Fermi gases

Abstract: We analyze the properties of a single impurity immersed in a Fermi sea. At positive energy and scattering lengths, we show that the system possesses a well-defined but metastable excitation, the repulsive polaron, and we calculate its energy, quasiparticle residue and effective mass. From a thermodynamic argument we obtain the number of particles in the dressing cloud, illustrating the repulsive character of the polaron. Identifying the important 2- and 3-body decay channels, we furthermore calculate the lifetime of the repulsive polaron. The stability conditions for the formation of fully spin polarized (ferromagnetic) domains are then examined for a binary mixture of atoms with a general mass ratio. Our results indicate that mass imbalance lowers the critical interaction strength for phase-separation, but that very short quasiparticle decay times will complicate the experimental observation of itinerant ferromagnetism. Finally, we present the spectral function of the impurity for various coupling strengths and momenta.

Electronic States of Boron and Phosphorus in Diamond

Abstract: The electronic states of boron and phosphorus in diamond have been studied by infrared absorption and photo-thermal ionisation spectroscopies. High quality boron doped synthetic diamond (p-type conductive) and phosphorus-doped CVD diamond film (n-type conductive) were used for this study. In the case of boron-doped diamond, the four main excited states of the bound hole follow a Rydberg series, suggesting that boron has a hydrogen-like behaviour, with a weak splitting of the excited states. The consistent values of the optical ionisation energy (E0 = 382 meV), of an “average” effective mass (m* = 0.74m0) and of the Bohr radius of the ground state (a* = 4.1 A) deduced from the Rydberg series support this suggestion. The comparison with the effective mass approximation, applied for acceptor states in diamond, suggests that the top of the valence band of diamond is different from that of silicon and germanium. In the case of phosphorus-doped diamond, two excited states of the bound electron have been observed for the first time, at 523 and 562 meV from the ground level. The good agreement with the effective mass approximation suggests that phosphorus is a shallow donor, and allows us to propose a first value of the optical ionisation energy of phosphorus in diamond of about 600 meV, consistent with Hall effect measurements.

Coulomb correlation effects in zinc monochalcogenides

Abstract: Electronic structure and band characteristics for zinc monochalcogenides with zinc-blende- and wurtzite-type structures are studied by first-principles density-functional-theory calculations with different approximations. It is shown that the local-density approximation underestimates the band gap and energy splitting between the states at the top of the valence band, misplaces the energy levels of the Zn-3d states, and overestimates the crystal-field-splitting energy. The spin-orbit-coupling energy is found to be overestimated for both variants of ZnO, underestimated for ZnS with wurtzite-type structure, and more or less correct for ZnSe and ZnTe with zinc-blende-type structure. The order of the states at the top of the valence band is found to be anomalous for both variants of ZnO, but is normal for the other zinc monochalcogenides considered. It is shown that the Zn-3d electrons and their interference with the O-2p electrons are responsible for the anomalous order. The effective masses of the electrons...