2,996 research outputs found
Magnetic properties of two-phase superconductors
We have recently proposed a theoretical model for superconductors endowed
with two distinct superconducting phases, described by two scalar order
parameters which condensate at different critical temperatures. On analyzing
the magnetic behavior of such systems, we have found some observable
differences with respect to the case of ordinary Ginzburg-Landau
superconductors. In particular, at low temperature the London penetration
length is strongly reduced and the Ginzburg-Landau parameter k becomes a
function of temperature. By contrast, in the temperature region between the two
phase transitions k is constant and the system is a type-I or a type-II
superconductor depending on the ratio between the critical temperatures.Comment: revtex, 5 pages, 1 eps figur
Noether symmetries for two-dimensional charged particle motion
We find the Noether point symmetries for non-relativistic two-dimensional
charged particle motion. These symmetries are composed of a quasi-invariance
transformation, a time-dependent rotation and a time-dependent spatial
translation. The associated electromagnetic field satisfy a system of
first-order linear partial differential equations. This system is solved
exactly, yielding three classes of electromagnetic fields compatible with
Noether point symmetries. The corresponding Noether invariants are derived and
interpreted
Infrared properties of MgAlBC) single crystals in the normal and superconducting state
The reflectivity of -oriented MgAl(BC) single crystals has been measured by means of infrared
microspectroscopy for cm. An increase with doping of
the scattering rates in the and bands is observed, being more
pronounced in the C doped crystals. The -band plasma frequency also
changes with doping due to the electron doping, while the -band one is
almost unchanged. Moreover, a interband excitation, predicted
by theory, is observed at eV in the undoped sample,
and shifts to lower energies with doping. By performing theoretical calculation
of the doping dependence , the experimental observations can be
explained with the increase with electron doping of the Fermi energy of the
holes in the -band. On the other hand, the band density of
states seems not to change substantially. This points towards a reduction
driven mainly by disorder, at least for the doping level studied here. The
superconducting state has been also probed by infrared synchrotron radiation
for cm in one pure and one C-doped sample. In the
undoped sample ( = 38.5 K) a signature of the -gap only is observed.
At = 0.08 ( = 31.9 K), the presence of the contribution of the
-gap indicates dirty-limit superconductivity in both bands.Comment: 12 pages, 9 figure
Spin dynamics and disorder effects in the S=1/2 kagome Heisenberg spin liquid phase of kapellasite
We report Cl NMR, ESR, SR and specific heat measurements on the
frustrated kagom\'e magnet kapellasite,
CuZn(OH)Cl, where a gapless spin liquid phase is
stabilized by a set of competing exchange interactions. Our measurements
confirm the ferromagnetic character of the nearest-neighbour exchange
interaction and give an energy scale for the competing interactions K. The study of the temperature-dependent ESR lineshift reveals a
moderate symmetric exchange anisotropy term , with %. These
findings validate a posteriori the use of the Heisenberg
model to describe the magnetic properties of kapellasite [Bernu et al., Phys.
Rev. B 87, 155107 (2013)]. We further confirm that the main deviation from this
model is the severe random depletion of the magnetic kagom\'e lattice by 27%,
due to Cu/Zn site mixing, and specifically address the effect of this disorder
by Cl NMR, performed on an oriented polycrystalline sample.
Surprisingly, while being very sensitive to local structural deformations, our
NMR measurements demonstrate that the system remains homogeneous with a unique
spin susceptibility at high temperature, despite a variety of magnetic
environments. Unconventional spin dynamics is further revealed by NMR and
SR in the low-, correlated, spin liquid regime, where a broad
distribution of spin-lattice relaxation times is observed. We ascribe this to
the presence of local low-energy modes.Comment: 15 pages, 11 figures. To appear in Phys. Rev.
Second discontinuity in the specific heat of two-phase superconductors
The recently proposed theoretical model of superconductors endowed with two
distinct superconducting phases and critical temperatures is further analyzed
by introducing two distinct order parameters, described by two scalar fields
which condensate at different temperatures. We find some deviations in basic
thermodynamical quantities with respect to the Ginzburg-Landau one-phase
superconductors. In particular, by contrast to the usual case where only one
jump in specific heat takes place at the normal-superconductor transition
temperature, we actually predict an additional discontinuity for C_V when
passing from a superconducting phase to the other one.Comment: revtex, 4 pages, 1 figur
Thermal conductivity of MgB in the superconducting state
We present thermal conductivity measurements on very pure and dense bulk
samples, as indicated by residual resistivity values as low as 0.5 mW cm and
thermal conductivity values higher than 200 W/mK. In the normal state we found
that the Wiedemann Franz law, in its generalized form, works well suggesting
that phonons do not contribute to the heat transport. The thermal conductivity
in the superconducting state has been analysed by using a two-gap model. Thank
to the large gap anisotropy we were able to evaluate quantitatively intraband
scattering relaxation times of and bands, which depend on the
disorder in different way; namely, as the disorder increases, it reduces more
effectively the relaxation times of than of bands, as
suggested by a recent calculation [1].Comment: 12 pages, 5 figure
Flux flow resistivity in the two-gap superconductivity
We investigate the flux flow state in a two-gap superconductor in which two
s-wave gaps with different amplitudes exist on two separate Fermi surfaces. The
flux flow resistivity is obtained on the basis of the Bardeen-Stephen relation
and the result agrees well with the anomalous field dependence of the flow
resistivity recently observed in the two-gap superconductor MgB2. Some typical
properties of the vortex in this system are also discussed.Comment: 5 pages, 1 figure. Some typos are corrected. Some comments are added.
To be published in J. Phys. Soc. Jp
Study of temperature dependent atomic correlations in MgB
We have studied the evolution with temperature of the local as well as the
average crystal structure of MgB using the real-space atomic pair
distribution function (PDF) measured by high resolution neutron powder
diffraction. We have investigated the correlations of the B-B and B-Mg nearest
neighbor pair motion by comparing, in the wide temperature range from T=10 K up
to T=600 K, the mean-square displacements (MSD) of single atoms with the
mean-square relative displacements (MSRD) obtained from the PDF peak
linewidths. The results show that the single atom B and Mg vibrations are
mostly decoupled from each other, with a small predominance of positive (in
phase) correlation factor for both the B-B and B-Mg pairs. The small positive
correlation is almost temperature independent, in contrast with our theoretical
calculations; this can be a direct consequence of the strong decay processes of
the anharmonic phonons
Large N study of extreme type II superconductors in a magnetic field
The large N analysis of an extreme type II superconductor is revisited. It is
found that the phase transition is of second-order in dimensions 4 < d < 6. For
the physical dimension d=3 no sign of phase transition is found, contrary to
early claims.Comment: Revtex, 7 pages, no figure
- …