86 research outputs found
Pressure dependence of the upper critical field of MgB2 and of YNi2B2C
We present measurements of H under pressure in MgB and in
YNiBC. The changes in the shape of H are interpreted within
current models and show the evolution of the main Fermi surface velocities
and electron-phonon coupling parameters with pressure. In
MgB the electron-phonon coupling strength of the nearly two dimensional
band, responsible for the high critical temperature, is more affected
by pressure than the band coupling, and the hole doping of the
band decreases. In YNiBC, the peculiar positive curvature of
H is weakened by pressure.Comment: 5 pages, 5 figure
Raman spectra of MgB2 at high pressure and topological electronic transition
Raman spectra of the MgB2 ceramic samples were measured as a function of
pressure up to 32 GPa at room temperature. The spectrum at normal conditions
contains a very broad peak at ~590 cm-1 related to the E2g phonon mode. The
frequency of this mode exhibits a strong linear dependence in the pressure
region from 5 to 18 GPa, whereas beyond this region the slope of the
pressure-induced frequency shift is reduced by about a factor of two. The
pressure dependence of the phonon mode up to ~ 5GPa exhibits a change in the
slope as well as a "hysteresis" effect in the frequency vs. pressure behavior.
These singularities in the E2g mode behavior under pressure support the
suggestion that MgB2 may undergo a pressure-induced topological electronic
transition.Comment: 2 figure
Role of the E2g phonon in the superconductivity of MgB2: a Raman scattering study
Temperature dependent Raman scattering studies in polycrystalline
MgB2(10<T<300 K)reveal that the E2g phonon does not experience any self energy
renormalization effect across the superconducting critical temperature Tc ~ 40
K. In contrast, most of the current theoretical models rely on the role of the
E2g phonon in the electron-phonon coupling mechanism of superconductivity in
MgB2. According to these models, a hardening of 12% is expected below Tc at the
Gamma point of the Brillouim zone. In the presence of our results, those models
must be reviewed. The analysis of the temperature dependence of the E2g phonon
frequency yields to a isobaric Gruneisen parameter of -1.2< gama(E2g)< 0.2,
smaller than the value of 3.9 obtained from isothermal Raman experiments under
pressure. It is suggested that this apparent disagreement can be explained in
terms of pressure induced changes of the topology of the Fermi surface. Finally
we notice that the phonon linewidth presents the expected two-phonon anharmonic
decay as a function of T and no anomalous temperature dependence of the
linewidth is observed near Tc.Comment: Published in Solid State Comm. 125, 499 (2003
High pressure photoinduced polymerization of the orthorhombic polymeric phase of C 60
Abstract The stability of the linear orthorhombic polymer of C 60 as a function of pressure has been studied by Raman scattering and X-ray measurements. The in situ Raman study shows an irreversible transition to a new phase occurring at pressures as low as $0.3 GPa. The specimens treated at pressure up to 3 GPa without laser irradiation do not show any structural changes after pressure release. The Raman spectrum of the new phase differs from those of the known 2D polymerized phases of C 60 . These data indicate that the simultaneous application of pressure and laser irradiation results in pressure photoinduced polymerization of the pristine polymeric chains of C 60 thus transforming it to a new polymeric phase of C 60
Disorder induced collapse of the electron phonon coupling in MgB observed by Raman Spectroscopy
The Raman spectrum of the superconductor MgB has been measured as a
function of the Tc of the film. A striking correlation is observed between the
onset and the frequency of the mode. Analysis of the data with
the McMillan formula provides clear experimental evidence for the collapse of
the electron phonon coupling at the temperature predicted for the convergence
of two superconducting gaps into one observable gap. This gives indirect
evidence of the convergence of the two gaps and direct evidence of a transition
to an isotropic state at 19 K. The value of the electron phonon coupling
constant is found to be 1.22 for films with T 39K and 0.80 for films with
T19K.Comment: 5 pages, 4 figure
Pressure Effects and Large Polarons in Layered MgB_2 Superconductor
We consider the dependence of the MgB_2 superconducting critical temperature
on the pressure. Our model exploits the influence of the large polarons on the
band structure of the layered MgB_2 superconductor. Namely, the hole
Pekar-Froehlich polarons form quasi two-dimensional potential wells in the
boron plane which shift the positions of the sigma- and pi-bands. This energy
shift depends on the pressure and the Cooper pairing of the correlated
sigma-electrons happens inside polaron wells. The results obtained are as
follows: dT_c/dp = -\alpha (5.2 \pm 0.9) K/GPa or dT_c/dp = -\alpha (6.9\pm
1.1) K/GPa for a different choice of the Grueneisen parameter. Being compared
with known experimental data they give us a resonable interval for the value of
the Froehlich electron-phonon coupling constant: \alpha = 0.15 - 0.45.Comment: 6 pages, 1 fig, LaTeX, subm. to Phys. Rev.
An Efficient, “Burn in” Free Organic Solar Cell Employing a Nonfullerene Electron Acceptor
A comparison of the efficiency, stability, and photophysics of organic solar cells employing poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3′″-di(2-octyldodecyl)-2,2′;5′,2″;5″,2′″-quaterthiophen-5,5′″-diyl)] (PffBT4T-2OD) as a donor polymer blended with either the nonfullerene acceptor EH-IDTBR or the fullerene derivative, [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as electron acceptors is reported. Inverted PffBT4T-2OD:EH-IDTBR blend solar cell fabricated without any processing additive achieves power conversion efficiencies (PCEs) of 9.5 ± 0.2%. The devices exhibit a high open circuit voltage of 1.08 ± 0.01 V, attributed to the high lowest unoccupied molecular orbital (LUMO) level of EH-IDTBR. Photoluminescence quenching and transient absorption data are employed to elucidate the ultrafast kinetics and efficiencies of charge separation in both blends, with PffBT4T-2OD exciton diffusion kinetics within polymer domains, and geminate recombination losses following exciton separation being identified as key factors determining the efficiency of photocurrent generation. Remarkably, while encapsulated PffBT4T-2OD:PC71BM solar cells show significant efficiency loss under simulated solar irradiation (“burn in” degradation) due to the trap-assisted recombination through increased photoinduced trap states, PffBT4T-2OD:EH-IDTBR solar cell shows negligible burn in efficiency loss. Furthermore, PffBT4T-2OD:EH-IDTBR solar cells are found to be substantially more stable under 85 °C thermal stress than PffBT4T-2OD:PC71BM devices
Photoluminescence Study of the Two‐Dimensional Tetragonal and Rhombohedral Polymers of C 60
INTERMOLECULAR INTERACTION AND ENERGETICAL SPECTRUM IN CRYSTALS OF AROMATIC HYDROCARBONS AND FULLERENS AT HIGH PRESSURE
The aim is to perform the complex experimental investigation of the intermolecular interaction, energetical spectrum and dynamics of the Fraenkel excitons in the crystals of the aromatic hydrocarbons and fullerens at high pressure. The experimantal investigation of depending matrix elements of the resonant and imcompressible volume interaction upon the distances between molecules in the crystals of the aromatic hydrocarbons has been performed firstly. The strong dependence of the Davidov's disjunction and shift of the exciton bands upon the distances between molecules has been discovered. The anomal baric dependence in the intensity of the exciton bands connected with configuration mixing of the exciton states has been discovered. The investigation of the quasi-resonance at high pressure has been performed, the critical parameters for spliting-out of the impurity level and position of the upper boundary in the exciton spectrum of the naphthaline crystal have been determined. The energetical spectrum, molecular dynamics and phase transitions at high pressure in the fullerite crystals have been investigated. Within the frames of the Rashb resonance theory the calculations of the position of the impurity bands and exciton localization amplitude on the impurity have been performed, and the comparison with results of the experiments at high pressure has been performed. The calculations of the baric dependence in the intensity of the exciton bands of naphthaline crystal taking configuration mixing within the frames of the Crage - MacLur model into consideration have been performed.Available from VNTIC / VNTIC - Scientific & Technical Information Centre of RussiaSIGLERURussian Federatio
Photopolymerization in the Fullerene Layers of the Molecular Donor–Acceptor Complex {Pt(nPr2dtc)2} · (C60)2
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