53 research outputs found
Concepts relating magnetic interactions, intertwined electronic orders and strongly correlated superconductivity
Unconventional superconductivity (SC) is said to occur when Cooper pair
formation is dominated by repulsive electron-electron interactions, so that the
symmetry of the pair wavefunction is other than isotropic s-wave. The strong,
on-site, repulsive electron-electron interactions that are the proximate cause
of such superconductivity are more typically drivers of commensurate magnetism.
Indeed, it is the suppression of commensurate antiferromagnetism (AF) that
usually allows this type of unconventional superconductivity to emerge.
Importantly, however, intervening between these AF and SC phases, intertwined
electronic ordered phases of an unexpected nature are frequently discovered.
For this reason, it has been extremely difficult to distinguish the microscopic
essence of the correlated superconductivity from the often spectacular
phenomenology of the intertwined phases. Here we introduce a model conceptual
framework within which to understand the relationship between antiferromagnetic
electron-electron interactions, intertwined ordered phases and correlated
superconductivity. We demonstrate its effectiveness in simultaneously
explaining the consequences of antiferromagnetic interactions for the
copper-based, iron-based and heavy-fermion superconductors, as well as for
their quite distinct intertwined phases.Comment: Main text + 11 figure
Discovery of a Cooper-Pair Density Wave State in a Transition-Metal Dichalcogenide
To search for evidence that Cooper-pair density wave (PDW) states can occur
in transition metal dichalcogenides (TMD) we use atomic-resolution scanned
Josephson-tunneling microscopy (SJTM). Implementing an innovative SJTM
technique, we detect and visualize a PDW state in the canonical TMD NbSe.
Although its wavevectors are indistinguishable from those of the preexisting
charge density wave (CDW) state, simultaneous atomic-scale imaging of the CDW
and PDW demonstrates that their spatial arrangements are incongruent. By
contrast, the PDW and the superconductive state are unmistakably coupled, as
evidenced by their mutual decay into a superconducting vortex core. Despite the
atomic-scale dissimilarity of simultaneous CDW and PDW images, large-scale
visualization of their relative phase yields a characteristic
value . This reveals an inter-state discommensuration
between the CDW and PDW by one crystal unit cell, as the atomic-scale
disjunction mechanism. Finally, because many TMDs sustain both CDW and
superconducting states, the detection and imaging of a PDW in NbSe presages
abundant new PDW physics
A Supercooled Spin Liquid State in the Frustrated Pyrochlore Dy2Ti2O7
A "supercooled" liquid develops when a fluid does not crystallize upon
cooling below its ordering temperature. Instead, the microscopic relaxation
times diverge so rapidly that, upon further cooling, equilibration eventually
becomes impossible and glass formation occurs. Classic supercooled liquids
exhibit specific identifiers including microscopic relaxation times diverging
on a Vogel-Tammann-Fulcher (VTF) trajectory, a Havriliak-Negami (HN) form for
the dielectric function, and a general Kohlrausch-Williams-Watts (KWW) form for
time-domain relaxation. Recently, the pyrochlore Dy2Ti2O7 has become of
interest because its frustrated magnetic interactions may, in theory, lead to
highly exotic magnetic fluids. However, its true magnetic state at low
temperatures has proven very difficult to identify unambiguously. Here we
introduce high-precision, boundary-free magnetization transport techniques
based upon toroidal geometries and gain a fundamentally new understanding of
the time- and frequency-dependent magnetization dynamics of Dy2Ti2O7. We
demonstrate a virtually universal HN form for the magnetic susceptibility, a
general KWW form for the real-time magnetic relaxation, and a divergence of the
microscopic magnetic relaxation rates with precisely the VTF trajectory. Low
temperature Dy2Ti2O7 therefore exhibits the characteristics of a supercooled
magnetic liquid; the consequent implication is that this translationally
invariant lattice of strongly correlated spins is evolving towards an
unprecedented magnetic glass state, perhaps due to many-body localization of
spin.Comment: Version 2 updates: added legend for data in Figures 4A and 4B;
corrected equation reference in caption for Figure 4
Commensurate period Charge Density Modulations throughout the Pseudogap Regime
Theories based upon strong real space (r-space) electron electron
interactions have long predicted that unidirectional charge density modulations
(CDM) with four unit cell (4) periodicity should occur in the hole doped
cuprate Mott insulator (MI). Experimentally, however, increasing the hole
density p is reported to cause the conventionally defined wavevector of
the CDM to evolve continuously as if driven primarily by momentum space
(k-space) effects. Here we introduce phase resolved electronic structure
visualization for determination of the cuprate CDM wavevector. Remarkably, this
new technique reveals a virtually doping independent locking of the local CDM
wavevector at throughout the underdoped phase diagram of the
canonical cuprate . These observations have significant
fundamental consequences because they are orthogonal to a k-space (Fermi
surface) based picture of the cuprate CDM but are consistent with strong
coupling r-space based theories. Our findings imply that it is the latter that
provide the intrinsic organizational principle for the cuprate CDM state
Imaging Orbital-selective Quasiparticles in the Hund's Metal State of FeSe
Strong electronic correlations, emerging from the parent Mott insulator
phase, are key to copper-based high temperature superconductivity (HTS). By
contrast, the parent phase of iron-based HTS is never a correlated insulator.
But this distinction may be deceptive because Fe has five active d-orbitals
while Cu has only one. In theory, such orbital multiplicity can generate a
Hund's Metal state, in which alignment of the Fe spins suppresses inter-orbital
fluctuations producing orbitally selective strong correlations. The spectral
weights of quasiparticles associated with different Fe orbitals m should
then be radically different. Here we use quasiparticle scattering interference
resolved by orbital content to explore these predictions in FeSe. Signatures of
strong, orbitally selective differences of quasiparticle appear on all
detectable bands over a wide energy range. Further, the quasiparticle
interference amplitudes reveal that , consistent with
earlier orbital-selective Cooper pairing studies. Thus, orbital-selective
strong correlations dominate the parent state of iron-based HTS in FeSe.Comment: for movie M1, see
http://www.physik.uni-leipzig.de/~kreisel/osqp/M1.mp4, for movie M2, see
http://www.physik.uni-leipzig.de/~kreisel/osqp/M2.mp4, for movie M3, see
http://www.physik.uni-leipzig.de/~kreisel/osqp/M3.mp4, for movie M4, see
http://www.physik.uni-leipzig.de/~kreisel/osqp/M4.mp4, for movie M5, see
http://www.physik.uni-leipzig.de/~kreisel/osqp/M5.mp
Multi-Atom Quasiparticle Scattering Interference for Superconductor Energy-Gap Symmetry Determination
Complete theoretical understanding of the most complex superconductors
requires a detailed knowledge of the symmetry of the superconducting energy-gap
, for all momenta on the Fermi surface
of every band . While there are a variety of techniques for determining
, no general method existed to measure the signed
values of . Recently, however, a new technique based
on phase-resolved visualization of superconducting quasiparticle interference
(QPI) patterns centered on a single non-magnetic impurity atom, was introduced.
In principle, energy-resolved and phase-resolved Fourier analysis of these
images identifies wavevectors connecting all k-space regions where
has the same or opposite sign. But use of a single
isolated impurity atom, from whose precise location the spatial phase of the
scattering interference pattern must be measured is technically difficult. Here
we introduce a generalization of this approach for use with multiple impurity
atoms, and demonstrate its validity by comparing the
it generates to the determined from single-atom
scattering in FeSe where energy-gap symmetry is established. Finally,
to exemplify utility, we use the multi-atom technique on LiFeAs and find
scattering interference between the hole-like and electron-like pockets as
predicted for of opposite sign
Imaging atomic-scale effects of high-energy ion irradiation on superconductivity and vortex pinning in Fe(Se,Te)
Maximizing the sustainable supercurrent density, Jc, is crucial to high
current applications of superconductivity and, to achieve this, preventing
dissipative motion of quantized vortices is key. Irradiation of superconductors
with high-energy heavy ions can be used to create nanoscale defects that act as
deep pinning potentials for vortices. This approach holds unique promise for
high current applications of iron-based superconductors because Jc
amplification persists to much higher radiation doses than in cuprate
superconductors without significantly altering the superconducting critical
temperature. However, for these compounds virtually nothing is known about the
atomic scale interplay of the crystal damage from the high-energy ions, the
superconducting order parameter, and the vortex pinning processes. Here, we
visualize the atomic-scale effects of irradiating FeSexTe1-x with 249 MeV Au
ions and find two distinct effects: compact nanometer-sized regions of crystal
disruption or 'columnar defects', plus a higher density of single atomic-site
'point' defects probably from secondary scattering. We show directly that the
superconducting order is virtually annihilated within the former while
suppressed by the latter. Simultaneous atomically-resolved images of the
columnar crystal defects, the superconductivity, and the vortex configurations,
then reveal how a mixed pinning landscape is created, with the strongest
pinning occurring at metallic-core columnar defects and secondary pinning at
clusters of pointlike defects, followed by collective pinning at higher fields.Comment: Main text (14 pages, 5 figures) and supplementary information (6
pages, 7 figures
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