319 research outputs found
Isotropic three-dimensional gap in the iron-arsenide superconductor LiFeAs from directional heat transport measurements
The thermal conductivity k of the iron-arsenide superconductor LiFeAs (Tc ~
18K) was measured in single crystals at temperatures down to T~50mK and in
magnetic fields up to H=17T, very close to the upper critical field Hc2~18T.
For both directions of the heat current, parallel and perpendicular to the
tetragonal c-axis, a negligible residual linear term k/T is found as T ->0,
revealing that there are no zero-energy quasiparticles in the superconducting
state. The increase in k with magnetic field is the same for both current
directions and it follows closely the dependence expected for an isotropic
superconducting gap. There is no evidence of multi-band character, whereby the
gap would be different on different Fermi-surface sheets. These findings show
that the superconducting gap in LiFeAs is isotropic in 3D, without nodes or
deep minima anywhere on the Fermi surface. Comparison with other iron-pnictide
superconductors suggests that a nodeless isotropic gap is a common feature at
optimal doping (maximal Tc).Comment: 4 pages, 3 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
Universal heat conduction in the iron-arsenide superconductor KFe2As2 : Evidence of a d-wave state
The thermal conductivity of the iron-arsenide superconductor KFe2As2 was
measured down to 50 mK for a heat current parallel and perpendicular to the
tetragonal c-axis. A residual linear term (RLT) at T=0 is observed for both
current directions, confirming the presence of nodes in the superconducting
gap. Our value of the RLT in the plane is equal to that reported by Dong et al.
[Phys. Rev. Lett. 104, 087005 (2010)] for a sample whose residual resistivity
was ten times larger. This independence of the RLT on impurity scattering is
the signature of universal heat transport, a property of superconducting states
with symmetry-imposed line nodes. This argues against an s-wave state with
accidental nodes. It favors instead a d-wave state, an assignment consistent
with five additional properties: the magnitude of the critical scattering rate
for suppressing Tc to zero; the magnitude of the RLT, and its dependence on
current direction and on magnetic field; the temperature dependence of the
thermal conductivity.Comment: To appear in Physical Review Letter
Dynamics of electrons in the quantum Hall bubble phases
In Landau levels N > 1, the ground state of the two-dimensional electron gas
(2DEG) in a perpendicular magnetic field evolves from a Wigner crystal for
small filling of the partially filled Landau level, into a succession of bubble
states with increasing number of guiding centers per bubble as the filling
increases, to a modulated stripe state near half filling. In this work, we show
that these first-order phase transitions between the bubble states lead to
measurable discontinuities in several physical quantities such as the density
of states and the magnetization of the 2DEG. We discuss in detail the behavior
of the collective excitations of the bubble states and show that their spectra
have higher-energy modes besides the pinned phonon mode. The frequencies of
these modes, at small wavevector k, have a discontinuous evolution as a
function of filling factor that should be measurable in, for example, microwave
absorption experiments.Comment: 13 pages, 7 figures. Corrected typos in eqs. (38),(39),(40
The fermi arc and fermi pocket in cuprates in a short-range diagonal stripe phase
In this paper we studied the fermi arc and the fermi pocket in cuprates in a
short-range diagonal stripe phase with wave vectors , which
reproduce with a high accuracy the positions and sizes of the fermi arc and
fermi pocket and the superstructure in cuprates observed by Meng et
al\cite{Meng}. The low-energy spectral function indicates that the fermi pocket
results from the main band and the shadow band at the fermi energy. Above the
fermi energy the shadow band gradually departs away from the main band, leaving
a fermi arc. Thus we conclude that the fermi arc and fermi pocket can be fully
attributed to the stripe phase but has nothing to do with pairing.
Incorporating a d-wave pairing potential in the stripe phase the spectral
weight in the antinodal region is removed, leaving a clean fermi pocket in the
nodal region.Comment: 5 pages, 6 figure
From d-wave to s-wave pairing in the iron-pnictide superconductor (Ba,K)Fe2As2
The nature of the pairing state in iron-based superconductors is the subject
of much debate. Here we argue that in one material, the stoichiometric iron
pnictide KFe2As2, there is overwhelming evidence for a d-wave pairing state,
characterized by symmetry-imposed vertical line nodes in the superconducting
gap. This evidence is reviewed, with a focus on thermal conductivity and the
strong impact of impurity scattering on the critical temperature Tc. We then
compare KFe2As2 to Ba0.6K0.4Fe2As2, obtained by Ba substitution, where the
pairing symmetry is s-wave and the Tc is ten times higher. The transition from
d-wave to s-wave within the same crystal structure provides a rare opportunity
to investigate the connection between band structure and pairing mechanism. We
also compare KFe2As2 to the nodal iron-based superconductor LaFePO, for which
the pairing symmetry is probably not d-wave, but more likely s-wave with
accidental line nodes
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
Topological crossovers near a quantum critical point
We study the temperature evolution of the single-particle spectrum
and quasiparticle momentum distribution of homogeneous
strongly correlated Fermi systems beyond a point where the necessary condition
for stability of the Landau state is violated, and the Fermi surface becomes
multi-connected by virtue of a topological crossover. Attention is focused on
the different non-Fermi-liquid temperature regimes experienced by a phase
exhibiting a single additional hole pocket compared with the conventional
Landau state. A critical experiment is proposed to elucidate the origin of NFL
behavior in dense films of liquid He.Comment: 7 pages, 6 figure
Fermi-surface reconstruction by stripe order in cuprate superconductors
Quantum oscillations have revealed the presence of a small pocket in the
Fermi surface of the cuprate superconductor YBCO, whose nature and origin are
the subject of much debate. Interpretations include electron and hole pockets;
scenarios include Fermi-surface reconstruction by antiferromagnetism,
d-density-wave order, and stripe order. Here we report quantum oscillations in
the Seebeck and Nernst coefficients of YBCO and show, from the magnitude and
sign of the Seebeck coefficient, that they come from an electron pocket. Using
measurements of the Seebeck coefficient as a function of hole doping p, we show
that the evolution of the Fermi surface in YBCO is the same as in Eu-LSCO, a
cuprate where stripe order (a modulation of spin and charge densities) is well
established. The electron pocket is most prominent where stripe order is
strongest, at p = 1/8. This shows that Fermi-surface reconstruction is a
generic mechanism of underdoped cuprates, intimately related to stripe order.Comment: 15 pages, 5 figures, Supplementary information now integrated into
articl
Broken rotational symmetry in the pseudogap phase of a high-Tc superconductor
The nature of the pseudogap phase is a central problem in the quest to
understand high-Tc cuprate superconductors. A fundamental question is what
symmetries are broken when that phase sets in below a temperature T*. There is
evidence from both polarized neutron diffraction and polar Kerr effect
measurements that time- reversal symmetry is broken, but at temperatures that
differ significantly. Broken rotational symmetry was detected by both
resistivity and inelastic neutron scattering at low doping and by scanning
tunnelling spectroscopy at low temperature, but with no clear connection to T*.
Here we report the observation of a large in-plane anisotropy of the Nernst
effect in YBa2Cu3Oy that sets in precisely at T*, throughout the doping phase
diagram. We show that the CuO chains of the orthorhombic lattice are not
responsible for this anisotropy, which is therefore an intrinsic property of
the CuO2 planes. We conclude that the pseudogap phase is an electronic state
which strongly breaks four-fold rotational symmetry. This narrows the range of
possible states considerably, pointing to stripe or nematic orders.Comment: Published version. Journal reference and DOI adde
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