6 research outputs found
Ubiquitous diffraction resonances in positronium formation from fullerenes
Due to the dominant electron capture by positrons from the molecular wall and
the spatial dephasing across the wall-width, a powerful diffraction effect
universally underlies the positronium (Ps) formation from fullerenes. This
results into trains of resonances in the Ps formation cross section as a
function of the positron beam energy, producing uniform structures in recoil
momenta in analogy with classical single-slit diffraction fringes in the
configuration space. The prediction opens a hitherto unknown avenue of Ps
spectroscopy with nanomaterials.Comment: 6 pages, 3 figures, submitte
Strain-tuned quantum criticality in electronic Potts-nematic systems
Motivated by recent observations of threefold rotational symmetry breaking in
twisted moir\'e systems, cold-atom optical lattices, quantum Hall systems, and
triangular antiferromagnets, we phenomenologically investigate the
strain-temperature phase diagram of the electronic 3-state Potts-nematic order.
While in the absence of strain the quantum Potts-nematic transition is
first-order, quantum critical points (QCP) emerge when uniaxial strain is
applied, whose nature depends on whether the strain is compressive or tensile.
In one case, the nematic amplitude jumps between two non-zero values while the
nematic director remains pinned, leading to a symmetry-preserving meta-nematic
transition that terminates at a quantum critical end-point. For the other type
of strain, the nematic director unlocks from the strain direction and
spontaneously breaks an in-plane twofold rotational symmetry, which in twisted
moir\'e superlattices triggers an electric polarization. Such a piezoelectric
transition changes from first to second-order upon increasing strain, resulting
in a quantum tricritical point. Using a Hertz-Millis approach, we show that
these QCPs share interesting similarities with the widely studied Ising-nematic
QCP. The existence of three minima in the nematic action also leaves
fingerprints in the strain-nematic hysteresis curves, which display multiple
loops. At non-zero temperatures, because the upper critical dimension of the
3-state Potts model is smaller than three, the Potts-nematic transition is
expected to remain first-order in 3D, but to change to second-order in 2D. As a
result, the 2D strain-temperature phase diagram displays two first-order
transition wings bounded by lines of critical end-points or tricritical points,
reminiscent of the phase diagram of metallic ferromagnets. We discuss how our
results can be used to unambiguously identify spontaneous Potts-nematic order.Comment: 17 pages, 7 figure
Elastocaloric evidence for a multicomponent superconductor stabilized within the nematic state in Ba(FeCo)As
The iron-based high- superconductors exhibit rich phase diagrams with
intertwined phases, including magnetism, nematicity and superconductivity. The
superconducting in many of these materials is maximized in the regime of
strong nematic fluctuations, making the role of nematicity in influencing the
superconductivity a topic of intense research. Here, we use the AC
elastocaloric effect (ECE) to map out the phase diagram of
Ba(FeCo)As near optimal doping. The ECE signature at
on the overdoped side, where superconductivity condenses without any nematic
order, is quantitatively consistent with other thermodynamic probes that
indicate a single-component superconducting state. In contrast, on the slightly
underdoped side, where superconductivity condenses within the nematic phase,
ECE reveals a second thermodynamic transition proximate to and below . We
rule out magnetism and re-entrant tetragonality as the origin of this
transition, and find that our observations strongly suggest a phase transition
into a multicomponent superconducting state. This implies the existence of a
sub-dominant pairing instability that competes strongly with the dominant
instability. Our results thus motivate a re-examination of the pairing
state and its interplay with nematicity in this extensively studied iron-based
superconductor, while also demonstrating the power of ECE in uncovering
strain-tuned phase diagrams of quantum materials