152 research outputs found
Optical absorption in the strong coupling limit of Eliashberg theory
We calculate the optical conductivity of superconductors in the
strong-coupling limit. In this anomalous limit the typical energy scale is set
by the coupling energy, and other energy scales such as the energy of the
bosons mediating the attraction are negligibly small. We find a universal
frequency dependence of the optical absorption which is dominated by bound
states and differs significantly from the weak coupling results. A comparison
with absorption spectra of superconductors with enhanced electron-phonon
coupling shows that typical features of the strong-coupling limit are already
present at intermediate coupling.Comment: 10 pages, revtex, 4 uuencoded figure
Impact of Few-Layered Graphene Plates on Structure and Properties of an Epoxy Resin
Impact of few-layered graphene (FLG) plates on the properties of the ED-20 epoxy resin is experimentally studied by using various techniques. The average dimensions of FLG plates are estimated as about 50 nm in thickness and 5 ΞΌm in both width and length. The FLG-mass-loading, Cf, in the nanocomposites is ranged from 0.01% to 5%. On the whole, the FLG plates promote an improvement in the thermal and chemical resistivities of the nanocomposites due to the chemical binding of the epoxy macromolecular chains to the free carbon bonds located on lateral verges of the plates. On the other hand, the FLG plates worsen the tensile strength of the nanocomposites at Cf > 0.01%, whereas the dynamic elastic moduli undergo small variations.Π‘ ΠΈΡΠΏΠΎΠ»ΡΠ·ΠΎΠ²Π°Π½ΠΈΠ΅ΠΌ ΡΠ°Π·Π»ΠΈΡΠ½ΡΡ
ΠΌΠ΅ΡΠΎΠ΄ΠΈΠΊ ΡΠΊΡΠΏΠ΅ΡΠΈΠΌΠ΅Π½ΡΠ°Π»ΡΠ½ΠΎ ΠΈΡΡΠ»Π΅Π΄ΠΎΠ²Π°Π½ΠΎ Π²Π»ΠΈΡΠ½ΠΈΠ΅ ΠΌΠ½ΠΎΠ³ΠΎΡΠ»ΠΎΠΉΠ½ΡΡ
Π³ΡΠ°ΡΠ΅Π½ΠΎΠ²ΡΡ
(ΠΠ‘Π) ΠΏΠ»Π°ΡΡΠΈΠ½ΠΎΠΊ Π½Π° ΡΠ²ΠΎΠΉΡΡΠ²Π° ΡΠΏΠΎΠΊΡΠΈΠ΄Π½ΠΎΠΉ ΡΠΌΠΎΠ»Ρ ΠΠ-20. Π‘ΡΠ΅Π΄Π½ΠΈΠ΅ ΡΠ°Π·ΠΌΠ΅ΡΡ ΠΠ‘Π-ΠΏΠ»Π°ΡΡΠΈΠ½ΠΎΠΊ ΡΠΎΡΡΠ°Π²Π»ΡΠ»ΠΈ ΠΏΡΠΈΠΌΠ΅ΡΠ½ΠΎ 5 ΠΌΠΊΠΌ Π² Π΄Π»ΠΈΠ½Ρ ΠΈ ΡΠΈΡΠΈΠ½Ρ ΠΈ 50 Π½ΠΌ ΠΏΠΎ ΡΠΎΠ»ΡΠΈΠ½Π΅. ΠΠ°ΡΡΠΎΠ²Π°Ρ Π½Π°Π³ΡΡΠ·ΠΊΠ° Cf ΠΠ‘Π-ΠΏΠ»Π°ΡΡΠΈΠ½ΠΎΠΊ Π² Π½Π°Π½ΠΎΠΊΠΎΠΌΠΏΠΎΠ·ΠΈΡΠ°Ρ
ΠΈΠ·ΠΌΠ΅Π½ΡΠ»Π°ΡΡ ΠΎΡ 0,01% Π΄ΠΎ 5%. Π ΡΠ΅Π»ΠΎΠΌ, ΠΠ‘Π-ΠΏΠ»Π°ΡΡΠΈΠ½ΠΊΠΈ ΡΠΏΠΎΡΠΎΠ±ΡΡΠ²ΡΡΡ ΡΠ»ΡΡΡΠ΅Π½ΠΈΡ ΠΊΠ°ΠΊ ΡΠ΅ΡΠΌΠΈΡΠ΅ΡΠΊΠΎΠΉ, ΡΠ°ΠΊ ΠΈ Ρ
ΠΈΠΌΠΈΡΠ΅ΡΠΊΠΎΠΉ ΡΡΠΎΠΉΠΊΠΎΡΡΠΈ Π½Π°Π½ΠΎΠΊΠΎΠΌΠΏΠΎΠ·ΠΈΡΠΎΠ² Π²ΡΠ»Π΅Π΄ΡΡΠ²ΠΈΠ΅ Ρ
ΠΈΠΌΠΈΡΠ΅ΡΠΊΠΎΠ³ΠΎ ΡΠ²ΡΠ·ΡΠ²Π°Π½ΠΈΡ ΠΌΠ°ΠΊΡΠΎΠΌΠΎΠ»Π΅ΠΊΡΠ»ΡΡΠ½ΡΡ
ΡΠ΅ΠΏΠΎΡΠ΅ΠΊ ΡΠΌΠΎΠ»Ρ ΡΠΎ ΡΠ²ΠΎΠ±ΠΎΠ΄Π½ΡΠΌΠΈ ΡΠ³Π»Π΅ΡΠΎΠ΄Π½ΡΠΌΠΈ ΡΠ²ΡΠ·ΡΠΌΠΈ, Π»ΠΎΠΊΠ°Π»ΠΈΠ·ΠΎΠ²Π°Π½Π½ΡΠΌΠΈ Π½Π° Π±ΠΎΠΊΠΎΠ²ΡΡ
Π³ΡΠ°Π½ΡΡ
ΠΏΠ»Π°ΡΡΠΈΠ½ΠΎΠΊ. Π‘ Π΄ΡΡΠ³ΠΎΠΉ ΡΡΠΎΡΠΎΠ½Ρ, ΠΠ‘Π-ΠΏΠ»Π°ΡΡΠΈΠ½ΠΊΠΈ ΡΡ
ΡΠ΄ΡΠ°ΡΡ ΠΏΡΠ΅Π΄Π΅Π» ΡΠ°Π·ΡΡΡΠ΅Π½ΠΈΡ Π½Π°Π½ΠΎΠΊΠΎΠΌΠΏΠΎΠ·ΠΈΡΠΎΠ² ΠΏΡΠΈ Π½Π°ΠΏΠΎΠ»Π½Π΅Π½ΠΈΡΡ
Cf > 0,01, Π² ΡΠΎ Π²ΡΠ΅ΠΌΡ ΠΊΠ°ΠΊ Π΄ΠΈΠ½Π°ΠΌΠΈΡΠ΅ΡΠΊΠΈΠ΅ ΡΠΏΡΡΠ³ΠΈΠ΅ ΠΌΠΎΠ΄ΡΠ»ΠΈ ΠΏΡΠ΅ΡΠ΅ΡΠΏΠ΅Π²Π°ΡΡ Π½Π΅Π·Π½Π°ΡΠΈΡΠ΅Π»ΡΠ½ΡΠ΅ ΠΈΠ·ΠΌΠ΅Π½Π΅Π½ΠΈΡ.Π Π²ΠΈΠΊΠΎΡΠΈΡΡΠ°Π½Π½ΡΠΌ ΡΡΠ·Π½ΠΈΡ
ΠΌΠ΅ΡΠΎΠ΄ΠΈΠΊ Π΅ΠΊΡΠΏΠ΅ΡΠΈΠΌΠ΅Π½ΡΠ°Π»ΡΠ½ΠΎ Π΄ΠΎΡΠ»ΡΠ΄ΠΆΠ΅Π½ΠΎ Π²ΠΏΠ»ΠΈΠ² Π±Π°Π³Π°ΡΠΎΡΠ°ΡΠΎΠ²ΠΈΡ
Π³ΡΠ°ΡΠ΅Π½ΠΎΠ²ΠΈΡ
(ΠΠ¨Π) ΠΏΠ»Π°ΡΡΠ²ΠΎΠΊ Π½Π° Π²Π»Π°ΡΡΠΈΠ²ΠΎΡΡΡ Π΅ΠΏΠΎΠΊΡΠΈΠ΄Π½ΠΎΡ ΡΠΌΠΎΠ»ΠΈ ΠΠ-20. Π‘Π΅ΡΠ΅Π΄Π½Ρ ΡΠΎΠ·ΠΌΡΡΠΈ ΠΠ¨Π-ΠΏΠ»Π°ΡΡΠ²ΠΎΠΊ ΡΠΊΠ»Π°Π΄Π°Π»ΠΈ ΠΏΡΠΈΠ±Π»ΠΈΠ·Π½ΠΎ 5 ΠΌΠΊΠΌ Ρ Π΄ΠΎΠ²ΠΆΠΈΠ½Ρ ΡΠ° ΡΠΈΡΠΈΠ½Ρ Ρ 50 Π½ΠΌ Ρ ΡΠΎΠ²ΡΠΈΠ½Ρ. ΠΠ°ΡΠΎΠ²Π΅ Π½Π°Π²Π°Π½ΡΠ°ΠΆΠ΅Π½Π½Ρ Cf ΠΠ¨Π-ΠΏΠ»Π°ΡΡΠ²ΠΎΠΊ Ρ Π½Π°Π½ΠΎΠΊΠΎΠΌΠΏΠΎΠ·ΠΈΡΠ°Ρ
Π·ΠΌΡΠ½ΡΠ²Π°Π»ΠΎΡΡ Π²ΡΠ΄ 0,01% Π΄ΠΎ 5%. Π£ ΡΡΠ»ΠΎΠΌΡ, ΠΠ¨Π-ΠΏΠ»Π°ΡΡΠ²ΠΊΠΈ ΡΠΏΡΠΈΡΠΈΠ½ΡΡΡΡ ΠΏΠΎΠ»ΡΠΏΡΠ΅Π½Π½Ρ ΡΠΊ ΡΠ΅ΡΠΌΡΡΠ½ΠΎΡ, ΡΠ°ΠΊ Ρ Ρ
Π΅ΠΌΡΡΠ½ΠΎΡ ΡΡΡΠΉΠΊΠΎΡΡΠΈ Π½Π°Π½ΠΎΠΊΠΎΠΌΠΏΠΎΠ·ΠΈΡΡΠ² Π²Π½Π°ΡΠ»ΡΠ΄ΠΎΠΊ Ρ
Π΅ΠΌΡΡΠ½ΠΎΠ³ΠΎ Π·Π²βΡΠ·ΡΠ²Π°Π½Π½Ρ ΠΌΠ°ΠΊΡΠΎΠΌΠΎΠ»Π΅ΠΊΡΠ»ΡΡΠ½ΠΈΡ
Π»Π°Π½ΡΡΠΆΠΊΡΠ² ΡΠΌΠΎΠ»ΠΈ Π· Π²ΡΠ»ΡΠ½ΠΈΠΌΠΈ ΠΠ°ΡΠ±ΠΎΠ½ΠΎΠ²ΠΈΠΌΠΈ Π·Π²βΡΠ·ΠΊΠ°ΠΌΠΈ, ΡΠΎ Π»ΠΎΠΊΠ°Π»ΡΠ·ΠΎΠ²Π°Π½Ρ Π½Π° Π±ΡΡΠ½ΠΈΡ
Π³ΡΠ°Π½ΡΡ
ΠΏΠ»Π°ΡΡΠ²ΠΎΠΊ. Π ΡΠ½ΡΠΎΠ³ΠΎ Π±ΠΎΠΊΡ, ΠΠ¨Π-ΠΏΠ»Π°ΡΡΠ²ΠΊΠΈ ΠΏΡΠΈΠ·Π²ΠΎΠ΄ΡΡΡ Π΄ΠΎ ΠΏΠΎΠ³ΡΡΡΠ΅Π½Π½Ρ ΠΌΠ΅ΠΆΡ ΠΌΡΡΠ½ΠΎΡΡΠΈ Π½Π°Π½ΠΎΠΊΠΎΠΌΠΏΠΎΠ·ΠΈΡΡΠ² Π·Π° Π½Π°ΠΏΠΎΠ²Π½Π΅Π½Ρ Cf > 0,01%, Ρ
ΠΎΡΠ° Π΄ΠΈΠ½Π°ΠΌΡΡΠ½Ρ ΠΏΡΡΠΆΠ½Ρ ΠΌΠΎΠ΄ΡΠ»Ρ Π·Π°Π·Π½Π°ΡΡΡ Π½Π΅Π·Π½Π°ΡΠ½ΠΈΡ
Π·ΠΌΡΠ½
Surface effects in multiband superconductors. Application to MgB
Metals with many bands at the Fermi level can have different band dependent
gaps in the superconducting state. The absence of translational symmetry at an
interface can induce interband scattering and modify the superconducting
properties. We dicuss the relevance of these effects to recent experiments in
MgB
Point-contact spectroscopy of the nickel borocarbide superconductor YNi2B2C in the normal and superconducting state
Point-contact (PC) spectroscopy measurements of YNi2B2C single crystals in
the normal and superconducting (SC) state (T_c=15.4K) for the main
crystallographic directions are reported. The PC study reveals the
electron-phonon interaction (EPI) spectral function with dominant phonon
maximum around 12 meV and further weak structures (hump or kink) at higher
energy at about 50 meV. No "soft" modes below 12 meV are resolved in the normal
state. The PC EPI spectra are qualitatively similar for the different
directions. Contrary, directional study of the SC gap results in
\Delta_[100]=1.5 meV for the a direction and \Delta_[001]=2.3 meV along the c
axis; however the critical temperature T_c in PC in all cases is near to that
in the bulk sample. The value 2\Delta_[001]/kT_c=3.6 is close to the BCS value
of 3.52, and the temperature dependence \Delta_[001](T) is BCS-like, while the
for small gap \Delta_[100](T) is below BCS behavior at T>T_c/2 similarly as in
the two-gap superconductor MgB2. It is supposed that the directional variation
\Delta can be attributed to a multiband nature of the SC state in YNi2B2C.Comment: 9 pages, 10 figures, to be published in a special issue of J. Low
Temp. Phys. in honour of Prof. H. von Loehneyse
Nonlocal Effects and Shrinkage of the Vortex Core Radius in YNi2B2C Probed by muSR
The magnetic field distribution in the vortex state of YNi2B2C has been
probed by muon spin rotation (muSR). The analysis based on the London model
with nonlocal corrections shows that the vortex lattice has changed from
hexagonal to square with increasing magnetic field H. At low fields the vortex
core radius, rho_v(H), decreases with increasing H much steeper than what is
expected from the sqrt(H) behavior of the Sommerfeld constant gamma(H),
strongly suggesting that the anomaly in gamma(H) primarily arises from the
quasiparticle excitations outside the vortex cores.Comment: 4 pages, 4 figures, submitted to Phys. Rev.
Full Relativistic Electronic Structure and Fermi Surface Sheets of the First Honeycomb-Lattice Pnictide Superconductor SrPtAs
We report full-potential density functional theory (DFT)-based {\it ab
initio} band structure calculations to investigate electronic structure
properties of the first pnictide superconductor with a honeycomb-lattice
structure: SrPtAs. As a result, electronic bands, density of states, Fermi
velocities and the topology of the Fermi surface for SrPtAs are obtained. These
quantities are discussed in comparison to the first available experimental
data. Predictions for future measurements are provided
Enhancement of the upper critical field by nonmagnetic impurities in dirty two-gap superconductors
Quasiclassic Uzadel equations for two-band superconductors in the dirty limit
with the account of both intraband and interband scattering by nonmagnetic
impurities are derived for any anisotropic Fermi surface. From these equations
the Ginzburg-Landau equations, and the critical temperature are obtained.
An equation for the upper critical field, which determines both the temperature
dependence of and the orientational dependence of
as a function of the angle between and the c-axis is
obtained. It is shown that the shape of the curve essentially
depends on the ratio of the intraband electron diffusivities and ,
and can be very different from the standard one-gap dirty limit theory. In
particular, the value can considerably exceed ,
which can have important consequences for applications of . A scaling
relation is proposed which enables one to obtain the angular dependence of
from the equation for at . It is shown
that, depending on the relation between and , the ratio of the upper
critical field for and can both increase and decrease as the temperature decreases. Implications
of the obtained results for are discussed
Tunneling spectroscopy in the magnetic superconductor TmNi2B2C
We present new measurements about the tunneling conductance in the
borocarbide superconductor TmNiBC. The results show a very good
agreement with weak coupling BCS theory, without any lifetime broadening
parameter, over the whole sample surface. We detect no particular change of the
tunneling spectroscopy below 1.5K, when both the antiferromagnetic (AF) phase
and the superconducting order coexist.Comment: Submitted to Phys. Rev. B, Rapid Communication
Specific Heat Study of the Magnetic Superconductor HoNi2B2C
The complex magnetic transitions and superconductivity of HoNi2B2C were
studied via the dependence of the heat capacity on temperature and in-plane
field angle. We provide an extended, comprehensive magnetic phase diagram for B
// [100] and B // [110] based on the thermodynamic measurements. Three magnetic
transitions and the superconducting transition were clearly observed. The 5.2 K
transition (T_{N}) shows a hysteresis with temperature, indicating the first
order nature of the transition at B=0 T. The 6 K transition (T_{M}), namely the
onset of the long-range ordering, displays a dramatic in-plane anisotropy:
T_{M} increases with increasing magnetic field for B // [100] while it
decreases with increasing field for B // [110]. The anomalous anisotropy in
T_{M} indicates that the transition is related to the a-axis spiral structure.
The 5.5 K transition (T^{*}) shows similar behavior to the 5.2 K transition,
i.e., a small in-plane anisotropy and scaling with Ising model. This last
transition is ascribed to the change from a^{*} dominant phase to c^{*}
dominant phase.Comment: 9 pages, 11 figure
Phonon-mediated anisotropic superconductivity in the Y and Lu nickel borocarbides
We present scanning tunneling spectroscopy and microscopy measurements at low
temperatures in the borocarbide materials RNi2B2C (R=Y, Lu). The characteristic
strong coupling structure due to the pairing interaction is unambiguously
resolved in the superconducting density of states. It is located at the
superconducting gap plus the energy corresponding to a phonon mode identified
in previous neutron scattering experiments. These measurements also show that
this mode is coupled to the electrons through a highly anisotropic
electron-phonon interaction originated by a nesting feature of the Fermi
surface. Our experiments, from which we can extract a large electron-phonon
coupling parameter lambda (between 0.5 and 0.8), demonstrate that this
anisotropic electron-phonon coupling has an essential contribution to the
pairing interaction. The tunneling spectra show an anisotropic s-wave
superconducting gap function.Comment: 5 pages, 3 figure
- β¦