336 research outputs found
Ferromagnetic quantum phase transition in SrCaRuO thin films
The ferromagnetic quantum phase transition in the perovskite ruthenate
SrCaRuO is studied by low-temperature magnetization and
electrical resistivity measurements on thin films. The films were grown
epitaxially on SrTiO substrates using metalorganic aerosol deposition and
characterized by X-ray diffraction and room temperature scanning tunneling
microscopy. High residual resistivity ratios of 29 and 16 for and ,
respectively, prove the high quality of the investigated samples. We observe a
continuous suppression of the ferromagnetic Curie temperature from K
at towards at . The analysis of the electrical
resistivity between 2 and 10 K reveals and behavior at and , respectively. For undoped CaRuO, the measurement has
been extended down to 60 mK, revealing a crossover to behavior around 2
K, which suggests a Fermi-liquid ground state in this system.Comment: 3 pages, 4 Figures, Manuscript for Proceedings of the International
Conference on Quantum Criticality and Novel Phases (QCNP09, Dresden
Berry Phase in Cuprate Superconductors
Geometrical Berry phase is recognized as having profound implications for the
properties of electronic systems. Over the last decade, Berry phase has been
essential to our understanding of new materials, including graphene and
topological insulators. The Berry phase can be accessed via its contribution to
the phase mismatch in quantum oscillation experiments, where electrons
accumulate a phase as they traverse closed cyclotron orbits in momentum space.
The high-temperature cuprate superconductors are a class of materials where the
Berry phase is thus far unknown despite the large body of existing quantum
oscillations data. In this report we present a systematic Berry phase analysis
of Shubnikov - de Haas measurements on the hole-doped cuprates
YBaCuO, YBaCuO, HgBaCuO, and the
electron-doped cuprate NdCeCuO. For the hole-doped materials, a
trivial Berry phase of 0 mod is systematically observed whereas the
electron-doped NdCeCuO exhibits a significant non-zero Berry
phase. These observations set constraints on the nature of the high-field
normal state of the cuprates and points towards contrasting behaviour between
hole-doped and electron-doped materials. We discuss this difference in light of
recent developments related to charge density-wave and broken time-reversal
symmetry states.Comment: new version with added supplementary informatio
Bulk evidence for single-gap s-wave superconductivity in the intercalated graphite superconductor CYb
We report measurements of the in-plane electrical resistivity and the
thermal conductivity of the intercalated graphite superconductor
CYb to temperatures as low as /100. When a field is applied along the
c-axis, the residual electronic linear term evolves in an
exponential manner for . This activated behaviour
establishes the order parameter as unambiguously s-wave, and rules out the
possibility of multi-gap or unconventional superconductivity in this system.Comment: 4 pages, 4 figs, submitted to Phys. Rev. Let
Quantum critical scaling at the edge of Fermi liquid stability in a cuprate superconductor
In the high temperature cuprate superconductors, the pervasiveness of
anomalous electronic transport properties suggests that violation of
conventional Fermi liquid behavior is closely tied to superconductivity. In
other classes of unconventional superconductors, atypical transport is well
correlated with proximity to a quantum critical point, but the relative
importance of quantum criticality in the cuprates remains uncertain. Here we
identify quantum critical scaling in the electron-doped cuprate material
La2-xCexCuO4 with a line of quantum critical points that surrounds the
superconducting phase as a function of magnetic field and charge doping. This
zero-temperature phase boundary, which delineates a metallic Fermi liquid
regime from an extended non-Fermi liquid ground state, closely follows the
upper critical field of the overdoped superconducting phase and gives rise to
an expanse of distinct non Fermi liquid behavior at finite temperatures.
Together with signatures of two distinct flavors of quantum fluctuations, this
suggests that quantum criticality plays a significant role in shaping the
anomalous properties of the cuprate phase diagram.Comment: 16 pages, 3 figures + supplementary materia
Quantum oscillations in from an incommensurate -density wave order
We consider quantum oscillation experiments in
from the perspective of an incommensurate
Fermi surface reconstruction using an exact transfer matrix method and the
Pichard-Landauer formula for the conductivity. The specific density wave order
considered is a period-8 -density wave in which the current density is
unidirectionally modulated. The current modulation is also naturally
accompanied by a period-4 site charge modulation in the same direction, which
is consistent with recent magnetic resonance measurements. In principle Landau
theory also allows for a period-4 bond charge modulation, which is not
discussed, but should be simple to incorporate in the future. This scenario
leads to a natural, but not a unique, explanation of why only oscillations from
a single electron pocket is observed, and a hole pocket of roughly twice the
frequency as dictated by two-fold commensurate order, and the corresponding
Luttinger sum rule, is not observed. However, it is possible that even higher
magnetic fields will reveal a hole pocket of half the frequency of the electron
pocket or smaller. This may be at the borderline of achievable high field
measurements because at least a few complete oscillations have to be clearly
resolved.Comment: 8 pages, 7 figure
Onset of a boson mode at superconducting critical point of underdoped YBa2Cu3Oy
The thermal conductivity of underdoped \Y was measured in the limit as a function of hole concentration across the superconducting
critical point at = 5.0%. ``Time doping'' was used to resolve the
evolution of bosonic and fermionic contributions with high accuracy. For , we observe an additional contribution to
which we attribute to the boson excitations of a phase with long-range spin or
charge order. Fermionic transport, manifest as a linear term in , is
seen to persist unaltered through , showing that the state just below
is a thermal metal. In this state, the electrical resistivity varies
as log and the Wiedemann-Franz law is violated
Quantum critical point for stripe order: An organizing principle of cuprate superconductivity
A spin density-wave quantum critical point (QCP) is the central organizing
principle of organic, iron-pnictide, heavy-fermion and electron-doped cuprate
superconductors. It accounts for the superconducting Tc dome, the
non-Fermi-liquid resistivity, and the Fermi-surface reconstruction. Outside the
magnetically ordered phase above the QCP, scattering and pairing decrease in
parallel as the system moves away from the QCP. Here we argue that a similar
scenario, based on a stripe-order QCP, is a central organizing principle of
hole-doped cuprate superconductors. Key properties of Eu-LSCO, Nd-LSCO and YBCO
are naturally unified, including stripe order itself, its QCP, Fermi-surface
reconstruction, the linear-T resistivity, and the nematic character of the
pseudogap phase.Comment: Written for a special issue of Physica C on "Stripes and electronic
liquid crystal
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