85 research outputs found
Small-q Phonon Mediated Unconventional Superconductivity in the Iron Pnictides
We report self-consistent calculations of the gap symmetry for the iron-based
high-temperature superconductors using realistic small-q phonon mediated
pairing potentials and four-band energy dispersions. When both electron and
hole Fermi surface pockets are present, we obtain the nodeless state
that was first encountered in a spin-fluctuations mechanism picture. Nodal gap
structures such as and and even a p-wave
triplet state, are accessible upon doping within our phononic mechanism. Our
results resolve the conflict between phase sensitive experiments reporting a
gap changing sign attributed previously only to a non-phononic mechanism and
isotope effect measurements proving the involvement of phonons in the pairing.Comment: Final version. Corrected typos. Reference adde
Nematicity from mixed S_{+-} + d_{x^2-y^2} states in iron-based superconductors
We demonstrate that in iron-based superconductors, the extended S_{+-} SC
state coexists with the d_{x^2-y^2} state under generic conditions. The mixed
S_{+-} + d_{x^2-y^2} SC is a natural nematic state in which the tetragonal
symmetry C_4 is broken to C_2 explaining puzzling findings of nematic SC in
FeSe films [Science 332, 1410 (2011)]. Moreover, we report the possibility of a
first order transition at low-T from the nematic S_{+-} + d_{x^2-y^2} state to
the pure d_{x^2-y^2} state induced by the Zeeman magnetic field proposing an
original experimental strategy for identifying our mixed nematic state in FeSe
films. Extrapolating our findings, we argue that nematicity in non
superconducting states of underdoped and undoped pnictides may reflect mixed
S_{+-} + d_{x^2-y^2} Density Wave states.Comment: Improvements and corrections in the texte, references adde
Evolution of multi-gap superconductivity in the atomically thin limit: Strain-enhanced three-gap superconductivity in monolayer MgB
Starting from first principles, we show the formation and evolution of
superconducting gaps in MgB at its ultrathin limit. Atomically thin MgB
is distinctly different from bulk MgB in that surface states become
comparable in electronic density to the bulk-like - and -bands.
Combining the ab initio electron-phonon coupling with the anisotropic
Eliashberg equations, we show that monolayer MgB develops three distinct
superconducting gaps, on completely separate parts of the Fermi surface due to
the emergent surface contribution. These gaps hybridize nontrivially with every
extra monolayer added to the film, owing to the opening of additional coupling
channels. Furthermore, we reveal that the three-gap superconductivity in
monolayer MgB is robust over the entire temperature range that stretches up
to a considerably high critical temperature of 20 K. The latter can be boosted
to 50 K under biaxial tensile strain of 4\%, which is an enhancement
stronger than in any other graphene-related superconductor known to date.Comment: To appear in Phys. Re
Magnetic-field-induced chiral hidden order in URu2Si2
Two of the most striking and yet unresolved manifestations of the hidden
order (HO) in URu2Si2, are associated on one hand with the double-step
metamagnetic transitions and on the other with the giant anomalous Nernst
signal. Both are observed when a magnetic field is applied along the c-axis.
Here we provide for the first time a unified understanding of these puzzling
phenomena and the related field-temperature (B-T) phase diagram. We demonstrate
that the HO phase at finite fields can be explained with a chiral dxy+idx2-y2
spin density wave, assuming that the zero field HO contains only the
time-reversal symmetry preserving idx2-y2 component. We argue that the presence
of the field-induced chiral HO can be reflected in a distinctive non-linear
B-dependence of the Kerr angle, when a Kerr experiment is conducted for finite
fields. This fingerprint can be conclusive for the possible emergence of
chirality in the HO.Comment: 8 pages and 9 figures main text + 6 pages supplementary material.
Philosophical Magazine: Special Issue: Focused Issue on Hidden Order in
URu2Si2 (May 2014
Magnetic field induced pattern of coexisting condensates in the superconducting state of CeCoIn
In usual superconductors the carriers form Cooper pairs that have zero total
momentum meaning that the superfluid density is homogeneous. We know for
decades that it is a priori possible to observe at high fields the
Fulde-Ferrel-Larkin-Ovchinikov (FFLO) state in which the superfluid density
exhibits a field dependent modulation. The search for such inhomogeneous
condensates in materials is the subject of tremendous excitement especially
after the discovery of a promising high-field superconducting (HFSC) phase in
CeCoIn. However, subsequent neutron and NMR experiments contradicted the
FFLO picture in CeCoIn establishing a puzzling coupling of the distinct
HFSC state with a magnetic modulation. Here we show that, a novel type of
exotic state is observed at high fields in CeCoIn in which a pattern of
coexisting condensates manifests. The specific pattern includes the d-wave
singlet SC state, the staggered -triplet SC state and Spin Density Waves
(SDW). Because of particle-hole asymmetry these three condensates may either
appear separately or all three together providing a new perspective on
antiferromagnetic superconductors that may include high- cuprates and
pniktides. The field induced transition to the pattern state in CeCoIn is
only a paradigm of a generic mechanism that prevents the elimination of the
dominating condensate by the field via the formation of a pattern of
condensates. This opens new avenues for the search of exotic states not only in
electronic systems, but also in other systems where inhomogeneous condensates
are actively discussed like trapped atomic fermi gases and dense quark matter
Unconventional superconducting phases in a correlated two-dimensional Fermi gas of nonstandard quasiparticles: a simple model
We discuss a detailed phase diagram and other microscopic characteristics on
the applied magnetic field - temperature (H_a-T) plane for a simple model of
correlated fluid represented by a two-dimensional (2D) gas of heavy
quasiparticles with masses dependent on the spin direction and the effective
field generated by the electron correlations. The consecutive transitions
between the Bardeen-Cooper-Schrieffer (BCS) and the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phases are either continuous or
discontinuous, depending on the values of H_a and T. In the latter case, weak
metamagnetic transitions occur at the BCS-FFLO boundary. We single out two
different FFLO phases, as well as a reentrant behaviour of one of them at high
fields. The results are compared with those for ordinary Landau quasiparticles
in order to demonstrate the robustness of the FFLO states against the BCS state
for the case with spin-dependent masses (SDM). We believe that the mechanism of
FFLO stabilization by SDM is generic: other high-field low-temperature (HFLT)
superconducting phases benefit from SDM as well.Comment: 10 pages, 4 figure
Fulde-Ferrell-Larkin-Ovchinnikov phase in the presence of pair hopping interaction
The recent experimental support for the presence of the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase in the CeCoIn5 directed the
attention towards the mechanisms responsible for this type of
superconductivity. We investigate the FFLO state in a model where
on--site/inter--site pairing coexists with repulsive pair hopping interaction.
The latter interaction is interesting in that it leads to pairing with nonzero
momentum of the Cooper pairs even in the absence of the external magnetic field
(the so-called eta-pairing). It turns out that depending on the strength of the
pair hopping interaction the magnetic field can induce one of two types of the
FFLO phase with different spatial modulations of the order parameter. It is
argued that the properties of the FFLO phase may give information about the
magnitude of the pair hopping interaction. We also show that eta-pairing and
d-wave superconductivity may coexist in the FFLO state. It holds true also for
superconductors which in the absence of magnetic field are of pure d-wave type.Comment: 16 pages, 8 figure
Antiferromagnetic Phases in the Fulde-Ferrell-Larkin-Ovchinnikov State of CeCoIn_5
The antiferromagnetic (AFM) order in the Fulde-Ferrell-Larkin-Ovchinnikov
(FFLO) superconducting state is analyzed on the basis of a Ginzburg-Landau
theory. To examine the possible AFM-FFLO state in CeCoIn_5, we focus on the
incommensurate AFM order characterized by the wave vector Q = Q_{0} \pm q_inc
with Q_0 =(\pi,\pi,\pi) and q_inc \parallel [110] or [1-10] in the tetragonal
crystal structure. We formulate the two component Ginzburg-Landau theory and
investigate the two degenerate incommensurate AFM order. We show that the
pinning of AFM moment due to the FFLO nodal planes leads to multiple phases in
magnetic fields along [100] or [010]. The phase diagrams for various coupling
constants between the two order parameters are shown for the comparison with
CeCoIn_5. Experimental results of the NMR and neutron scattering measurements
are discussed.Comment: 6pages, Proceedings of ICHE2010, To appear in J. Phys. Soc. Jpn.
Supp
Ginzburg-Landau Analysis for the Antiferromagnetic Order in the Fulde-Ferrell-Larkin-Ovchinnikov Superconductor
Incommensurate antiferromangetic (AFM) order in the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductor is investigated on the
basis of the Ginzburg-Landau theory. We formulate the two component
Ginzburg-Landau model to discuss two degenerate incommensurate AFM states in
the tetragonal crystal structure. Owing to the broken translation symmetry in
the FFLO state, a multiple phase diagram of single-q phase and double-q phase
is obtained under the magnetic field along [100] or [010] direction. Magnetic
properties in each phase are investigated and compared with the neutron
scattering and NMR measurements for a heavy fermion superconductor CeCoIn_5. An
ultrasonic measurement is proposed for a future experimental study to identify
the AFM-FFLO state. The field orientation dependence of the AFM order in
CeCoIn_5 is discussed.Comment: 8 page
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