49 research outputs found
Charge Nematicity and Electronic Raman Scattering in Iron-based Superconductors
We review the recent developments in electronic Raman scattering measurements
of charge nematic fluctuations in iron-based superconductors. A simple
theoretical framework of a -wave Pomeranchuk transition is proposed in order
to capture the salient features of the spectra. We discuss the available Raman
data in the normal state of 122 iron-based systems, particularly Co doped
BaFeAs, and we show that the low energy quasi-elastic peak, the
extracted nematic susceptibility and the scattering rates are consistent with
an electronic driven structural phase transition. In the superconducting state
with a full gap the quasi-elastic peak transforms into a finite frequency
nematic resonance, evidences for which are particularly strong in the electron
doped systems. A crucial feature of the analysis is the fact that the
electronic Raman signal is unaffected by the acoustic phonons. This makes Raman
spectroscopy a unique probe of electronic nematicity.Comment: Invited review to a special issue in Iron-based superconductor
Raman Scattering as a Selective Probe of Chiral Electronic Excitations in Bilayer Graphene
We report a symmetry resolved electronic Raman scattering (ERS) study of a
bilayer graphene device under gate voltage. We show that the ERS continuum is
dominated by interband chiral excitations of symmetry and displays a
characteristic Pauli-blocking behavior similar to the monolayer case.
Crucially, we show that non-chiral excitations make a vanishing contribution to
the Raman cross-section due to destructive interference effects in the Raman
amplitude matrix elements. This is in a marked contrast to optical absorption
measurements and opens interesting venues for the use of Raman scattering as a
selective probe of chiral degrees of freedom in topological matter and other 2D
crystals
Rapid collapse of spin waves in non-uniform phases of the second Landau level
The spin degree of freedom in quantum phases of the second Landau level is
probed by resonant light scattering. The long wavelength spin wave, which
monitors the degree of spin polarization, is at the Zeeman energy in the fully
spin-polarized state at =3. At lower filling factors the intensity of the
Zeeman mode collapses indicating loss of polarization. A novel continuum of
low-lying excitations emerges that dominates near =8/3 and =5/2.
Resonant Rayleigh scattering reveals that quantum fluids for break up
into robust domain structures. While the state at =5/2 is considered to be
fully polarized, these results reveal unprecedented roles for spin degrees of
freedom.Comment: 4 pages, 5 figure
Higgs-mode radiance and charge-density-wave order in 2H-NbSe
Despite being usually considered two competing phenomena, charge-density-wave
and superconductivity coexist in few systems, the most emblematic one being the
transition metal dichalcogenide 2H-NbSe. This unusual condition is
responsible for specific Raman signatures across the two phase transitions in
this compound. While the appearance of a soft phonon mode is a well-established
fingerprint of the charge-density-wave order, the nature of the sharp sub-gap
mode emerging below the superconducting temperature is still under debate. In
this work we use the external pressure as a knob to unveil the delicate
interplay between the two orders, and consequently the nature of the
superconducting mode. Thanks to an advanced extreme-conditions Raman technique
we are able to follow the pressure evolution and the simultaneous collapse of
the two intertwined charge density wave and superconducting modes. The
comparison with microscopic calculations in a model system supports the
Higgs-type nature of the superconducting mode and suggests that
charge-density-wave and superconductivity in 2H-NbSe involve mutual
electronic degrees of freedom. These findings fill knowledge gap on the
electronic mechanisms at play in transition metal dichalcogenides, a crucial
step to fully exploit their properties in few-layers systems optimized for
devices applications
Soft Spin Wave Near nu=1: Evidence for a Magnetic Instability in Skyrmion Systems
The ground state of the two dimensional electron gas near =1 is
investigated by inelastic light scattering measurements carried down to very
low temperatures. Away from =1, the ferromagnetic spin wave collapses and
a new low-energy spin wave emerges below the Zeeman gap. The emergent spin wave
shows soft behavior as its energy increases with temperature and reaches the
Zeeman energy for temperatures above 2 K. The observed softening indicates an
instability of the two dimensional electron gas towards a magnetic order that
breaks spin rotational symmetry. We discuss our findings in light of the
possible existence of a Skyrme crystal.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
Collapse of critical nematic fluctuations in FeSe under pressure
We report the evolution of the electronic nematic susceptibility in FeSe via
Raman scattering as a function of hydrostatic pressure up to 5.8 GPa where the
superconducting transition temperature reaches its maximum. The
critical nematic fluctuations observed at low pressure vanish above 1.6 GPa,
indicating they play a marginal role in the four-fold enhancement of at
higher pressures. The collapse of nematic fluctuations appears to be linked to
a suppression of low energy electronic excitations which manifests itself by
optical phonon anomalies at around 2 GPa, in agreement with lattice dynamical
and electronic structure calculations using local density approximation
combined with dynamical mean field theory. Our results reveal two different
regimes of nematicity in the phase diagram of FeSe under pressure: a d-wave
Pomeranchuk instability of the Fermi surface at low pressure and a magnetic
driven orthorhombic distortion at higher pressure.Comment: 7 pages, 4 figures. Supplementary Material available upon reques