30 research outputs found
Experimental Evidence for Static Charge Density Waves in Iron Oxypnictides
In this Letter we report high-resolution synchrotron x-ray powder diffraction and transmission electron microscope analysis of Mn-substituted LaFeAsO samples, demonstrating that a static incommensurate modulated structure develops across the low-temperature orthorhombic phase, whose modulation wave vector depends on the Mn content. The incommensurate structural distortion is likely originating from a charge-density-wave instability, a periodic modulation of the density of conduction electrons associated with a modulation of the atomic positions. Our results add a new component in the physics of Fe-based superconductors, indicating that the density wave ordering is charge driven
Berry curvature unravelled by the Nernst effect in MnGe
The discovery of topological quantum materials represents a striking
innovation in modern condensed matter physics with remarkable fundamental and
technological implications. Their classification has been recently extended to
topological Weyl semimetals, i.e., solid state systems which exhibit the
elusive Weyl fermions as low-energy excitations. Here we show that the Nernst
effect can be exploited as a sensitive probe for determining key parameters of
the Weyl physics, applying it to the non-collinear antiferromagnet MnGe.
This compound exhibits anomalous thermoelectric transport due to enhanced Berry
curvature from Weyl points located extremely close to the Fermi level. We
establish from our data a direct measure of the Berry curvature at the Fermi
level and, using a minimal model of a Weyl semimetal, extract for the first
time the Weyl point energy and their distance in momentum-space
Strain, Young's modulus, and structural transition of EuTiO3 thin films probed by micro-mechanical methods
EuTiO3 (ETO) is a well-known complex oxide mainly investigated for its
magnetic properties and its incipient ferro-electricity. In this work, we
demonstrate the realization of suspended micro-mechanical structures, such as
cantilevers and micro-bridges, from 100 nm-thick single-crystal epitaxial ETO
films deposited on top of SrTiO3(100) substrates. By combining profile analysis
and resonance frequency measurements of these devices, we obtain the Young's
modulus, strain, and strain gradients of the ETO thin films. Moreover, we
investigate the ETO anti-ferro-distorsive transition by temperature-dependent
characterizations, which show a non-monotonic and hysteretic mechanical
response. Comparison between experimental and literature data allows us to
weight the contribution from thermal expansion and softening to the tuning
slope, while a full understanding of the origin of such a wide hysteresis is
still missing. We also discuss the influence of oxygen vacancies on the
reported mechanical properties by comparing stoichiometric and oxygen-deficient
samples.Comment: 8 pages, 5 figures; 7 Supplementary Material section
Calorimetric evidence for two phase transitions in BaKFeAs with fermion pairing and quadrupling states
Theoretically, materials that break multiple symmetries allow, under certain
conditions, the formation of four-fermion condensates above the superconducting
critical temperature. Such states can be stabilized by phase fluctuations.
Recently a fermionic quadrupling condensate that breaks the time-reversal
symmetry was reported in BaKFeAs [V. Grinenko
et al., Nat. Phys. 17, 1254 (2021)]. Evidence for the new state of matter comes
from muon-spin rotation, transport, thermoelectric, and ultrasound experiments.
Observing a specific heat anomaly is a very important signature of a transition
to a new state of matter. However, a fluctuation-induced specific heat
singularity is usually very challenging to resolve from a background of other
contributions. Here, we report on detecting two anomalies in the specific heat
of BaKFeAs at zero magnetic field. The
anomaly at the higher temperature is accompanied by the appearance of a
spontaneous Nernst effect, indicating broken time-reversal () symmetry.
The second anomaly at the lower temperature coincides with the transition to a
zero resistance state, indicating superconductivity breaking the gauge
symmetry. Our data provide calorimetric evidence for the phase formation
above the superconducting phase transition.Comment: 12 pages, 3 figures and Supplementary informatio
Recommended from our members
Elastoresistivity of Heavily Hole-Doped 122 Iron Pnictide Superconductors
Nematicity in heavily hole-doped iron pnictide superconductors remains controversial. Sizeable nematic fluctuations and even nematic orders far from magnetic instability were declared in RbFe2As2 and its sister compounds. Here, we report a systematic elastoresistance study of a series of isovalent- and electron-doped KFe2As2 crystals. We found divergent elastoresistance on cooling for all the crystals along their [110] direction. The amplitude of elastoresistivity diverges if K is substituted with larger ions or if the system is driven toward a Lifshitz transition. However, we conclude that none of them necessarily indicates an independent nematic critical point. Instead, the increased nematicity can be associated with another electronic criticality. In particular, we propose a mechanism for how elastoresistivity is enhanced at a Lifshitz transition
Recommended from our members
Highly efficient modulation doping: A path toward superior organic thermoelectric devices
We investigate the charge and thermoelectric transport in modulation-doped large-area rubrene thin-film crystals with different crystal phases. We show that modulation doping allows achieving superior doping efficiencies even for high doping densities, when conventional bulk doping runs into the reserve regime. Modulation-doped orthorhombic rubrene achieves much improved thermoelectric power factors, exceeding 20 μW m−1 K−2 at 80°C. Theoretical studies give insight into the energy landscape of the heterostructures and its influence on qualitative trends of the Seebeck coefficient. Our results show that modulation doping together with high-mobility crystalline organic semiconductor films is a previosly unexplored strategy for achieving high-performance organic thermoelectrics
Bereziskii-Kosterlitz-Thouless transition in the Weyl system \ce{PtBi2}
Symmetry breaking in topological matter became, in the last decade, a key
concept in condensed matter physics to unveil novel electronic states. In this
work, we reveal that broken inversion symmetry and strong spin-orbit coupling
in trigonal \ce{PtBi2} lead to a Weyl semimetal band structure, with unusually
robust two-dimensional superconductivity in thin fims. Transport measurements
show that high-quality \ce{PtBi2} crystals are three-dimensional
superconductors (600~mK) with an isotropic critical field
(50~mT). Remarkably, we evidence in a rather thick flake
(60~nm), exfoliated from a macroscopic crystal, the two-dimensional nature of
the superconducting state, with a critical temperature ~mK and highly-anisotropic critical fields. Our results reveal a
Berezinskii-Kosterlitz-Thouless transition with ~mK and
with a broadening of Tc due to inhomogenities in the sample. Due to the very
long superconducting coherence length in \ce{PtBi2}, the
vortex-antivortex pairing mechanism can be studied in unusually-thick samples
(at least five times thicker than for any other two-dimensional
superconductor), making \ce{PtBi2} an ideal platform to study low dimensional
superconductivity in a topological semimetal