496 research outputs found
Doping driven Small-to-Large Fermi surface transition and d-wave superconductivity in a two-dimenional Kondo lattice
We study the two-dimensional Kondo lattice model with an additional
Heisenberg exchange between localized spins. In a first step we use mean-field
theory with two order parameters. The first order parameter is a complex
pairing amplitude between conduction electrons and localized spins which
describes condensation of Kondo (or Zhang-Rice) singlets. A nonvanishing value
implies that the localized spins contribute to the Fermi surface volume. The
second order parameter describes singlet-pairing between the localized spins
and competes with the Kondo-pairing order parameter. Reduction of the carrier
density in the conduction band reduces the energy gain due to the formation of
the large Fermi surface and induces a phase transition to a state with strong
singlet correlations between the localized spins and a Fermi surface which
comprises only the conduction electrons. The model thus shows a doping-driven
change of its Fermi surface volume. At intermediate doping and low temperature
there is a phase where both order parameters coexist, which has a gapped large
Fermi surface and d-wave superconductivity. The theory thus qualitatively
reproduces the phase diagram of cuprate superconductors. In the second part of
the paper we show how the two phases with different Fermi surface volume emerge
in a strong coupling theory applicable in limit of large Kondo exchange. The
large-Fermi-surface phase corresponds to a `vacuum' of localized Kondo singlets
with uniform phase and the quasiparticles are spin-1/2 charge fluctuations
around this fully paired state. In the small-Fermi-surface phase the
quasiparticles correspond to propagating Kondo-singlets or triplets whereby the
phase of a given Kondo-singlet corresponds to its momentum. In this picture a
phase transition occurs for low filling of the conduction band as well.Comment: Revtex file, 17 pages, 14 eps-figure
Theory of the Lightly Doped Mott Insulator
A theory for the Hubbard model appropriate in the limit of large U/t, small
doping away from half-filling and short-ranged antiferromagnetic spin
correlations is presented. Despite the absence of any broken symmetry the Fermi
surface takes the form of elliptical hole pockets centered near (pi/2,pi/2)
with a volume proportional to the hole concentration. Short range
antiferromagnetic correlations render the nearest neighbor hopping almost
ineffective so that only second or third nearest neighbor hopping contributes
appreciably to the dispersion relation.Comment: 9 pages, 3 figure
Excitons in Mott insulators
Motivated by recent Raman and resonant inelastic X-ray scattering experiments
performed for Mott insulators, which suggest formation of excitons in these
systems, we present a theory of exciton formation in the upper Hubbard band.
The analysis based on the spin polaron approach is performed in the framework
of an effective t-J model for the subspace of states with one doubly occupied
site. Our results confirm the existence of excitons and bear qualitative
resemblance to experimental data despite some simplifications in our approach.
They prove that the basic underlying mechanismof exciton formation is the same
as that which gives rise to binding of holes in weakly doped antiferromagnets.Comment: 4 pages, 1 figur
Magnetic probe for material characterization at optical frequencies
Rapid development of novel, functional metamaterials made of purely dielectric, plasmonic, or composite structures which exhibit tunable optical frequency magnetic responses creates a need for new measurement techniques. We propose a method of actively measuring magnetic responses, i.e. magnetic dispersion, of such metamaterials within a wide range of optical frequencies with a single probe by exciting individual elementary cells within a larger matrix. The probe is made of a tapered optical fiber with a radially corrugated metal coating. It concentrates azimuthally polarized light in the near-field below the apex into a subwavelength size focus of the longitudinal magnetic field component. An incident azimuthally polarized beam propagates in the core until it reaches the metal stripes of constant angular width running parallel to the axis. For a broad frequency range light-to-plasmon coupling is assured as the lattice constant changes with the radius due to constant angular width. Bound plasmonic modes in slits between the metal stripes propagate toward the apex where circular currents in stripes and displacement currents in slits generate a strong longitudinal magnetic field. The energy density of the longitudinal magnetic component in the vicinity of the axis is much stronger than that of all the other components combined, what allows for pure magnetic excitation of magnetic resonances rather than by the electric field. The scattered signal is then measured in the far-field and analyzed
Evidence for Charging Effects in CdTe/CdMgTe Quantum Point Contacts
Here we report on fabrication and low temperature magnetotransport
measurements of quantum point contacts patterned from a novel two-dimensional
electron system - CdTe/CdMgTe modulation doped heterostructure. From the
temperature and bias dependence we ascribe the reported data to evidence for a
weakly bound state which is naturally formed inside a CdTe quantum
constrictions due to charging effects. We argue that the spontaneous
introduction of an open dot is responsible for the replacement of flat
conductance plateaus by quasi-periodic resonances with amplitude less than
2e^{2}/h, as found in our system. Additionally, below 1 K a pattern of weaker
conductance peaks, superimposed upon wider resonances, is also observed.Comment: 4 pages, 4 figure
Bi-metal coated aperture SNOM probes
Aperture probes of scanning near-field optical microscopes (SNOM) offer resolution which is limited by a sum of the aperture diameter at the tip of a tapered waveguide probe and twice the skin depth in metal used for coating. An increase of resolution requires a decrease of the aperture diameter. However, due to low energy throughput of such probes aperture diameters usually are larger than 50 nm. A groove structure at fiber core-metal coating interface for photon-to-plasmon conversion enhances the energy throughput 5-fold for Al coated probes and 30-fold for Au coated probes due to lower losses in the metal. However, gold coated probes have lower resolution, first due to light coupling from the core to plasmons at the outside of the metal coating, and second due to the skin depth being larger than for Al. Here we report on the impact of a metal bilayer of constant thickness for coating aperture SNOM probes. The purpose of the bilayer of two metals of which the outer one is aluminum and the inner is a noble metal is to assure low losses, hence larger transmission. Using body-of-revolution finite-difference time-domain simulations we analyze properties of probes without corrugations to measure the impact of using a metal bilayer and choose an optimum bi-metal configuration. Additionally we investigate how this type of metalization works in the case of grooved probes
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