182 research outputs found
Equilibrium structure and shape of Ag and Pt nanoparticles grown on silica surfaces: from experimental investigations to the determination of a metal-silica potential
A combination of experimental and numerical calculations on metallic silver
and platinum nanoparticles deposited on silica substrates is presented, with a
focus on the metal-substrate interactions. Experimentally, the nanoparticles
are elaborated under ultra-high vacuum and characterized by Grazing-Incidence
Small-Angle X-ray Scattering (GISAXS) and High Resolution Transmission
Electronic Microscopy (HRTEM) to determine their structure and morphology, and
in particular their aspect ratio (height/diameter) which quantifies the
metal-substrate interaction. Numerically, the interactions between the metal
and the silica species are modeled with the Lennard-Jones (12, 6) potential,
with two parameters for each metal and silica species. The geometric parameters
were found in the literature, while the energetic parameters were determined
from our experimental measurements of the aspect ratio. The parameters are:
{\sigma}_{Ag-O} = 0.278 nm, {\sigma}_{Ag-Si} = 0.329 nm, {\epsilon}_{Ag-O} = 75
meV, and {\epsilon}_{Ag-Si} = 13 meV for Ag-silica and {\sigma}_{Pt-O} = 0.273
nm, {\sigma}_{Pt-Si} = 0.324 nm, {\epsilon}_{Pt-O} = 110 meV, and
{\epsilon}_{Pt-Si} = 18 meV for Pt-silica. The proposed Ag-silica potential
reproduces quantitatively the unexpected experimental observation of the
variation of the aspect ratio for Ag nanoparticles larger than 5 nm, which has
been interpreted as a consequence of the silica roughness. The nanoparticle
orientation, structure and disorder are also considered. This metal-silica
potential for Ag and Pt should be helpful for further studies on pure metals as
well as their alloys.Comment: 35p, 12 figure
Fermi liquid behavior of the in-plane resistivity in the pseudogap state of YBa_2Cu_4O_8
Our knowledge of the ground state of underdoped hole-doped cuprates has
evolved considerably over the last few years. There is now compelling evidence
that inside the pseudogap phase, charge order breaks translational symmetry
leading to a reconstructed Fermi surface made of small pockets. Quantum
oscillations, [Doiron-Leyraud N, et al. (2007) Nature 447:564-568], optical
conductivity [Mirzaei SI, et al. (2013) Proc Natl Acad Sci USA 110:5774-5778]
and the validity of Wiedemann-Franz law [Grissonnache G, et al. (2016) Phys.
Rev. B 93:064513] point to a Fermi liquid regime at low temperature in the
underdoped regime. However, the observation of a quadratic temperature
dependence in the electrical resistivity at low temperatures, the hallmark of a
Fermi liquid regime, is still missing. Here, we report magnetoresistance
measurements in the magnetic-field-induced normal state of underdoped
YBa_2Cu_4O_8 which are consistent with a T^2 resistivity extending down to 1.5
K. The magnitude of the T^2 coefficient, however, is much smaller than expected
for a single pocket of the mass and size observed in quantum oscillations,
implying that the reconstructed Fermi surface must consist of at least one
additional pocket.Comment: Main + SI : published versio
Correlation between Fermi surface transformations and superconductivity in the electron-doped high- superconductor NdCeCuO
Two critical points have been revealed in the normal-state phase diagram of
the electron-doped cuprate superconductor NdCeCuO by exploring
the Fermi surface properties of high quality single crystals by high-field
magnetotransport. First, the quantitative analysis of the Shubnikov-de Haas
effect shows that the weak superlattice potential responsible for the Fermi
surface reconstruction in the overdoped regime extrapolates to zero at the
doping level corresponding to the onset of superconductivity.
Second, the high-field Hall coefficient exhibits a sharp drop right below
optimal doping where the superconducting transition
temperature is maximum. This drop is most likely caused by the onset of
long-range antiferromagnetic ordering. Thus, the superconducting dome appears
to be pinned by two critical points to the normal state phase diagram.Comment: 9 pages; 7 figures; 1 tabl
Universal magnetic structure of the half-magnetization phase in Cr-based spinels
Using an elastic neutron scattering technique under a pulsed magnetic field
up to 30 T, we determined the magnetic structure in the half-magnetization
plateau phase in the spinel CdCrO. The magnetic structure has a cubic
32 symmetry, which is the same as that observed in HgCrO. This
suggests that there is a universal field induced spin-lattice coupling
mechanism at work in the Cr-based spinels.Comment: 4 pages, 4 figure
Shubnikov-de Haas oscillations in YBa_2Cu_4O_8
We report the observation of Shubnikov-de Haas oscillations in the underdoped
cuprate superconductor YBaCuO (Y124). For field aligned along the
c-axis, the frequency of the oscillations is T, which corresponds
to % of the total area of the first Brillouin zone. The effective
mass of the quasiparticles on this orbit is measured to be times
the free electron mass. Both the frequency and mass are comparable to those
recently observed for ortho-II YBaCuO (Y123-II). We show that
although small Fermi surface pockets may be expected from band structure
calculations in Y123-II, no such pockets are predicted for Y124. Our results
therefore imply that these small pockets are a generic feature of the copper
oxide plane in underdoped cuprates.Comment: v2: Version of paper accepted for publication in Physical Review
Letters. Only minor changes to the text and reference
Angle-dependence of quantum oscillations in YBa2Cu3O6.59 shows free spin behaviour of quasiparticles
Measurements of quantum oscillations in the cuprate superconductors afford a
new opportunity to assess the extent to which the electronic properties of
these materials yield to a description rooted in Fermi liquid theory. However,
such an analysis is hampered by the small number of oscillatory periods
observed. Here we employ a genetic algorithm to globally model the field,
angular, and temperature dependence of the quantum oscillations observed in the
resistivity of YBa2Cu3O6.59. This approach successfully fits an entire data set
to a Fermi surface comprised of two small, quasi-2-dimensional cylinders. A key
feature of the data is the first identification of the effect of Zeeman
splitting, which separates spin-up and spin-down contributions, indicating that
the quasiparticles in the cuprates behave as nearly free spins, constraining
the source of the Fermi surface reconstruction to something other than a
conventional spin density wave with moments parallel to the CuO2 planes.Comment: 8 pages, 4 figure
Lifshitz critical point in the cuprate superconductor YBa2Cu3Oy from high-field Hall effect measurements
The Hall coefficient R_H of the cuprate superconductor YBa2Cu3Oy was measured
in magnetic fields up to 60 T for a hole concentration p from 0.078 to 0.152,
in the underdoped regime. In fields large enough to suppress superconductivity,
R_H(T) is seen to go from positive at high temperature to negative at low
temperature, for p > 0.08. This change of sign is attributed to the emergence
of an electron pocket in the Fermi surface at low temperature. At p < 0.08, the
normal-state R_H(T) remains positive at all temperatures, increasing
monotonically as T \to 0. We attribute the change of behaviour across p = 0.08
to a Lifshitz transition, namely a change in Fermi-surface topology occurring
at a critical concentration p_L = 0.08, where the electron pocket vanishes. The
loss of the high-mobility electron pocket across p_L coincides with a ten-fold
drop in the conductivity at low temperature, revealed in measurements of the
electrical resistivity at high fields, showing that the so-called
metal-insulator crossover of cuprates is in fact driven by a Lifshitz
transition. It also coincides with a jump in the in-plane anisotropy of ,
showing that without its electron pocket the Fermi surface must have strong
two-fold in-plane anisotropy. These findings are consistent with a
Fermi-surface reconstruction caused by a unidirectional spin-density wave or
stripe order.Comment: 16 pages, 13 figures, see associated Viewpoint: M. Vojta, Physics 4,
12 (2011
Evidence for a small hole pocket in the Fermi surface of underdoped YBa2Cu3Oy
The Fermi surface of a metal is the fundamental basis from which its
properties can be understood. In underdoped cuprate superconductors, the Fermi
surface undergoes a reconstruction that produces a small electron pocket, but
whether there is another, as yet undetected portion to the Fermi surface is
unknown. Establishing the complete topology of the Fermi surface is key to
identifying the mechanism responsible for its reconstruction. Here we report
the discovery of a second Fermi pocket in underdoped YBa2Cu3Oy, detected as a
small quantum oscillation frequency in the thermoelectric response and in the
c-axis resistance. The field-angle dependence of the frequency demonstrates
that it is a distinct Fermi surface and the normal-state thermopower requires
it to be a hole pocket. A Fermi surface consisting of one electron pocket and
two hole pockets with the measured areas and masses is consistent with a
Fermi-surface reconstruction caused by the charge-density-wave order observed
in YBa2Cu3Oy, provided other parts of the reconstructed Fermi surface are
removed by a separate mechanism, possibly the pseudogap.Comment: 23 pages, 5 figure
Hall, Seebeck, and Nernst Coefficients of Underdoped HgBa2CuO4+d: Fermi-Surface Reconstruction in an Archetypal Cuprate Superconductor
Charge density-wave order has been observed in cuprate superconductors whose
crystal structure breaks the square symmetry of the CuO2 planes, such as
orthorhombic YBa2Cu3Oy (YBCO), but not so far in cuprates that preserve that
symmetry, such as tetragonal HgBa2CuO4+d (Hg1201). We have measured the Hall
(R_H), Seebeck (S), and Nernst coefficients of underdoped Hg1201 in magnetic
fields large enough to suppress superconductivity. The high-field R_H(T) and
S(T) are found to drop with decreasing temperature and become negative, as also
observed in YBCO at comparable doping. In YBCO, the negative R_H and S are
signatures of a small electron pocket caused by Fermi-surface reconstruction,
attributed to charge density-wave modulations observed in the same range of
doping and temperature. We deduce that a similar Fermi-surface reconstruction
takes place in Hg1201, evidence that density-wave order exists in this
material. A striking similarity is also found in the normal-state Nernst
coefficient, further supporting this interpretation. Given the model nature of
Hg1201, Fermi-surface reconstruction appears to be common to all hole-doped
cuprates, suggesting that density-wave order is a fundamental property of these
materials
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