16 research outputs found
Magnetoresistance of semi-metals: the case of antimony
Large unsaturated magnetoresistance has been recently reported in numerous
semi-metals. Many of them have a topologically non-trivial band dispersion,
such as Weyl nodes or lines. Here, we show that elemental antimony displays the
largest high-field magnetoresistance among all known semi-metals. We present a
detailed study of the angle-dependent magnetoresistance and use a
semi-classical framework invoking an anisotropic mobility tensor to fit the
data. A slight deviation from perfect compensation and a modest variation with
magnetic field of the components of the mobility tensor are required to attain
perfect fits at arbitrary strength and orientation of magnetic field in the
entire temperature window of study. Our results demonstrate that large orbital
magnetoresistance is an unavoidable consequence of low carrier concentration
and the sub-quadratic magnetoresistance seen in many semi-metals can be
attributed to field-dependent mobility, expected whenever the disorder
length-scale exceeds the Fermi wavelength.Comment: Supplementary material on reques
Sawtooth lattice multiferroic BeCrO: Non-collinear magnetic structure and multiple magnetic transitions
Noncollinear magnetic structures and multiple magnetic phase transitions in a
sawtooth lattice antiferromagnet consisting of Cr are experimentally
identified in this work, thereby proposing the scenario of magnetism-driven
ferroelectricity in a sawtooth lattice. The title compound, BeCrO,
displays three magnetic phase transitions at low temperatures, at
7.5 K, at 25 K and at 26 K,
revealed through magnetic susceptibility, specific heat and neutron diffraction
in this work. These magnetic phase transitions are found to be influenced by
externally applied magnetic fields. Isothermal magnetization curves at low
temperatures below the magnetic transitions indicate the antiferromagnetic
nature of \bco\ with two spin-flop-like transitions occurring at
29 kOe and 47 kOe. Our high-resolution X-ray
and neutron diffraction studies, performed on single crystal and powder samples
unambiguously determined the crystal structure as orthorhombic . By
performing the magnetic superspace group analysis of the neutron diffraction
data at low temperatures, the magnetic structure in the temperature range
is determined to be the polar magnetic space group,
with a cycloidal magnetic propagation vector
= (0, 0, 0.090(1)). The magnetic structure in the newly
identified phase below , is determined as
with the magnetic propagation vector = (0, 0, 0.908(1)). The
cycloidal spin structure determined in our work is usually associated with
electric polarization, thereby making \bco\ a promising multiferroic belonging
to the sparsely populated family of sawtooth lattice antiferromagnets.Comment: 24 pages, 5 figures, accepte
Effect of surface polishing and oxidization induced strain on electronic order at the Verwey transition in Fe3O4
International audienceFollowing the controversy between two previous publications (Lorenzo et al 2008 Phys. Rev. Lett. 101 226401 and Garcia et al 2009 Phys. Rev. Lett. 102 176405), we report on the influence of mechanical polishing, and subsequent sample storage, on the electronic order at the Verwey transition of highly pure magnetite, Fe3O4, by resonant x-ray scattering. Contrary to expectations, mechanically polishing the surface induces an inhomogeneous micron deep dead layer, probably of powdered Fe3O4. In addition, we have found that polishing the sample immediately before the experiment influences and favors the appearance of long range order electronic correlations, whereas samples polished well in advance have their electronic order quenched. Conversely, lattice distortions associated with the Verwey transition appear less affected by the surface state. We conclude that mechanical polishing induces stresses at the surface that may propagate into the core of the single crystal sample. These strains relax with time, which affects the different order parameters, as measured by x-ray resonant diffraction
Optical conductivity of cuprates in a new light
Understanding the physical properties of unconventional superconductors as
well as of other correlated materials presents a formidable challenge. Their
unusual evolution with doping, frequency, and temperature, has frequently led
to non-Fermi-liquid (non-FL) interpretations. Optical conductivity is a major
challenge in this context. Here, the optical spectra of two archetypal
cuprates, underdoped HgBaCuO and optimally-doped
BiSrCaCuO, are interpreted based on the standard Fermi
liquid (FL) paradigm. At both dopings, perfect frequency-temperature FL scaling
is found to be modified by the presence of a second, gapped electronic
subsystem. This non-FL component emerges as a well-defined mid-infrared
spectral feature after the FL contribution -- determined independently by
transport -- is subtracted. Temperature, frequency and doping evolution of the
MIR feature identifies a gapped rather than dissipative response. In contrast,
the dissipative response is found to be relevant for pnictides and ruthenates.
Such an unbiased FL/non-FL separation is extended across the cuprate phase
diagram, providing a natural explanation why the superfluid density is
attenuated on the overdoped side. Thus, we obtain a unified interpretation of
optical responses and transport measurements in all analyzed physical regimes
and all analyzed compounds.Comment: 43 pages, 13 figure
Inverse correlation between quasiparticle mass and Tc in a cuprate high-Tc superconductor
Close to a zero-temperature transition between ordered and disordered electronic phases, quantum fluctuations can lead to a strong enhancement of electron mass and to the emergence of competing phases such as superconductivity. A correlation between the existence of such a quantum phase transition and superconductivity is quite well established in some heavy fermion and iron-based superconductors, and there have been suggestions that high-temperature superconductivity in copper-oxide materials (cuprates) may also be driven by the same mechanism. Close to optimal doping, where the superconducting transition temperature Tc is maximal in cuprates, two different phases are known to compete with superconductivity: a poorly understood pseudogap phase and a charge-ordered phase. Recent experiments have shown a strong increase in quasiparticle mass m* in the cuprate YBa2Cu3O7-δ as optimal doping is approached, suggesting that quantum fluctuations of the charge-ordered phase may be responsible for the high-Tc superconductivity. We have tested the robustness of this correlation between m* and Tc by performing quantum oscillation studies on the stoichiometric compound YBa2Cu4O8 under hydrostatic pressure. In contrast to the results for YBa2Cu3O7-δ, we find that in YBa2Cu4O8, the mass decreases as Tc increases under pressure. This inverse correlation between m* and Tc suggests that quantum fluctuations of the charge order enhance m* but do not enhance Tc
Universal quantum oscillations in the underdoped cuprate superconductors
The metallic state of the underdoped high-Tc cuprates has remained an enigma:
How may seemingly disconnected Fermi surface segments, observed in zero
magnetic field as a result of the opening of a partial gap (the pseudogap),
possess conventional quasiparticle properties? How do the small Fermi-surface
pockets evidenced by the observation of quantum oscillations (QO) emerge as
superconductivity is suppressed in high magnetic fields? Such QO, discovered in
underdoped YBa2Cu3O6.5 (Y123) and YBa2Cu4O8 (Y124), signify the existence of a
conventional Fermi surface (FS). However, due to the complexity of the crystal
structures of Y123 and Y124 (CuO2 double-layers, CuO chains, low structural
symmetry), it has remained unclear if the QO are specific to this particular
family of cuprates. Numerous theoretical proposals have been put forward to
explain the route toward QO, including materials-specific scenarios involving
CuO chains and scenarios involving the quintessential CuO2 planes. Here we
report the observation of QO in underdoped HgBa2CuO4+{\delta} (Hg1201), a model
cuprate superconductor with individual CuO2 layers, high tetragonal symmetry,
and no CuO chains. This observation proves that QO are a universal property of
the underdoped CuO2 planes, and it opens the door to quantitative future
studies of the metallic state and of the Fermi-surface reconstruction
phenomenon in this structurally simplest cuprate.Comment: 17 pages, 5 figure
Onset of exciton-exciton annihilation in single-layer black phosphorus
International audienc