175 research outputs found
Competing charge density waves and temperature-dependent nesting in 2H-TaSe2
Multiple charge density wave (CDW) phases in 2H-TaSe2 are investigated by
high-resolution synchrotron x-ray diffraction. In a narrow temperature range
immediately above the commensurate CDW transition, we observe a multi-q
superstructure with coexisting commensurate and incommensurate order
parameters, clearly distinct from the fully incommensurate state at higher
temperatures. This multi-q ordered phase, characterized by a temperature
hysteresis, is found both during warming and cooling, in contrast to previous
reports. In the normal state, the incommensurate superstructure reflection
gives way to a broad diffuse peak that persists nearly up to room temperature.
Its position provides a direct and accurate estimate of the Fermi surface
nesting vector, which evolves non-monotonically and approaches the commensurate
position as the temperature is increased. This behavior agrees with our recent
observations of the temperature-dependent Fermi surface in the same compound
[Phys. Rev. B 79, 125112 (2009)]
Crossover from weak to strong pairing in unconventional superconductors
Superconductors are classified by their pairing mechanism and the coupling
strength, measured as the ratio of the energy gap to the critical temperature,
Tc. We present an extensive comparison of the gap ratios among many single- and
multiband superconductors from simple metals to high-Tc cuprates and iron
pnictides. Contrary to the recently suggested universality of this ratio in
Fe-based superconductors, we find that the coupling in pnictides ranges from
weak, near the BCS limit, to strong, as in cuprates, bridging the gap between
these two extremes. Moreover, for Fe- and Cu-based materials, our analysis
reveals a universal correlation between the gap ratio and Tc, which is not
found in conventional superconductors and therefore supports a common
unconventional pairing mechanism in both families. An important consequence of
this result for ferropnictides is that the separation in energy between the
excitonic spin-resonance mode and the particle-hole continuum, which determines
the resonance damping, no longer appears independent of Tc.Comment: 15 pages, 3 figures, 5 tables with an exhaustive overview of the
published gap and spin-resonance measurements in Fe-based superconductors.
New in V3: updated references. To be published in Phys. Rev.
Dispersion and damping of zone-boundary magnons in the noncentrosymmetric superconductor CePt3Si
Inelastic neutron scattering (INS) is employed to study damped spin-wave
excitations in the noncentrosymmetric heavy-fermion superconductor CePt3Si
along the antiferromagnetic Brillouin-zone boundary in the low-temperature
magnetically ordered state. Measurements along the (1/2 1/2 L) and (H H 1/2-H)
reciprocal-space directions reveal deviations in the spin-wave dispersion from
the previously reported model. Broad asymmetric shape of the peaks in energy
signifies strong spin-wave damping by interactions with the particle-hole
continuum. Their energy width exhibits no evident anomalies as a function of
momentum along the (1/2 1/2 L) direction, which could be attributed to
Fermi-surface nesting effects, implying the absence of pronounced commensurate
nesting vectors at the magnetic zone boundary. In agreement with a previous
study, we find no signatures of the superconducting transition in the magnetic
excitation spectrum, such as a magnetic resonant mode or a superconducting spin
gap, either at the magnetic ordering wavevector (0 0 1/2) or at the zone
boundary. However, the low superconducting transition temperature in this
material still leaves the possibility of such features being weak and therefore
hidden below the incoherent background at energies ~0.1 meV, precluding their
detection by INS
Quantitative assessment of pinning forces and the superconducting gap in NbN thin films from complementary magnetic force microscopy and transport measurements
Epitaxial niobium-nitride thin films with a critical temperature of Tc=16K
and a thickness of 100nm were fabricated on MgO(100) substrates by pulsed laser
deposition. Low-temperature magnetic force microscopy (MFM) images of the
supercurrent vortices were measured after field cooling in a magnetic field of
3mT at various temperatures. Temperature dependence of the penetration depth
has been evaluated by a two-dimensional fitting of the vortex profiles in the
monopole-monopole model. Its subsequent fit to a single s-wave gap function
results in the superconducting gap amplitude Delta(0) = 2.9 meV = 2.1*kB*Tc, in
perfect agreement with previous reports. The pinning force has been
independently estimated from local depinning of individual vortices by lateral
forces exerted by the MFM tip and from transport measurements. A good
quantitative agreement between the two techniques shows that for low fields, B
<< Hc2, MFM is a powerful and reliable technique to probe the local variations
of the pinning landscape. We also demonstrate that the monopole model can be
successfully applied even for thin films with a thickness comparable to the
penetration depth.Comment: 6 pages, 6 figures, 2 table
Surface properties of SmB6 from x-ray photoelectron spectroscopy
We have investigated the properties of cleaved SmB single crystals by
x-ray photoelectron spectroscopy. At low temperatures and freshly cleaved
samples a surface core level shift is observed which vanishes when the
temperature is increased. A Sm valence between 2.5 - 2.6 is derived from the
relative intensities of the Sm and Sm multiplets. The B/Sm
intensity ratio obtained from the core levels is always larger than the
stoichiometric value. Possible reasons for this deviation are discussed. The B
signal shows an unexpected complexity: an anomalous low energy component
appears with increasing temperature and is assigned to the formation of a
suboxide at the surface. While several interesting intrinsic and extrinsic
properties of the SmB surface are elucidated in this manuscript no clear
indication of a trivial mechanism for the prominent surface conductivity is
found
Magnetic-field and doping dependence of low-energy spin fluctuations in the antiferroquadrupolar compound Ce(1-x)La(x)B(6)
CeB(6) is a model compound exhibiting antiferroquadrupolar (AFQ) order, its
magnetic properties being typically interpreted within localized models. More
recently, the observation of strong and sharp magnetic exciton modes forming in
its antiferromagnetic (AFM) state at both ferromagnetic and AFQ wave vectors
suggested a significant contribution of itinerant electrons to the spin
dynamics. Here we investigate the evolution of the AFQ excitation upon the
application of an external magnetic field and the substitution of Ce with
non-magnetic La, both parameters known to suppress the AFM phase. We find that
the exciton energy decreases proportionally to T_N upon doping. In field, its
intensity is suppressed, while its energy remains constant. Its disappearance
above the critical field of the AFM phase is preceded by the formation of two
modes, whose energies grow linearly with magnetic field upon entering the AFQ
phase. These findings suggest a crossover from itinerant to localized spin
dynamics between the two phases, the coupling to heavy-fermion quasiparticles
being crucial for a comprehensive description of the magnon spectrum.Comment: Extended version with a longer introduction and an additional figure.
6 pages and 5 figure
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