86 research outputs found
Precision microwave dielectric and magnetic susceptibility measurements of correlated electronic materials using superconducting cavities
We analyze microwave cavity perturbation methods, and show that the technique
is an excellent, precision method to study the dynamic magnetic and dielectric
response in the frequency range. Using superconducting cavities, we
obtain exceptionally high precision and sensitivity for measurements of
relative changes. A dynamic electromagnetic susceptibility
is introduced, which
is obtained from the measured parameters: the shift of cavity resonant
frequency and quality factor . We focus on the case of a
spherical sample placed at the center of a cylindrical cavity resonant in the
mode. Depending on the sample characteristics, the magnetic
permeability , the dielectric permittivity and
the complex conductivity can be extracted from
. A full spherical wave analysis of the cavity perturbation
is given. This analysis has led to the observation of new phenomena in novel
low dimensional materials.Comment: 16 pages, 5 figure
Microwave properties of superconducting MgB2
Measurements of 10 GHz microwavesurface resistance, Rs, of dense MgB2wire and pellet are reported. Significant improvements are observed in the wire with reduction of porosity. The data lie substantially above the theoretical estimates for a pure Bardeen–Cooper–Schrieffer s-wave superconductor. However, the Rs (20 K) of the wire is an order of magnitude lower than that of polycrystalYBa2Cu3O6.95 and matches with single-crystal YBa2Cu3O6.95. The results show promise for the use of MgB2 in microwave applications
Fate of Quasiparticle at Mott Transition and Interplay with Lifshitz Transition Studied by Correlator Projection Method
Filling-control metal-insulator transition on the two-dimensional Hubbard
model is investigated by using the correlator projection method, which takes
into account momentum dependence of the free energy beyond the dynamical
mean-field theory. The phase diagram of metals and Mott insulators is analyzed.
Lifshitz transitions occur simultaneously with metal-insulator transitions at
large Coulomb repulsion. On the other hand, they are separated each other for
lower Coulomb repulsion, where the phase sandwiched by the Lifshitz and
metal-insulator transitions appears to show violation of the Luttinger sum
rule. Through the metal-insulator transition, quasiparticles retain nonzero
renormalization factor and finite quasi-particle weight in the both sides of
the transition. This supports that the metal-insulator transition is caused not
by the vanishing renormalization factor but by the relative shift of the Fermi
level into the Mott gap away from the quasiparticle band, in sharp contrast
with the original dynamical mean-field theory. Charge compressibility diverges
at the critical end point of the first-order Lifshitz transition at finite
temperatures. The origin of the divergence is ascribed to singular momentum
dependence of the quasiparticle dispersion.Comment: 24 pages including 10 figure
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