arXiv subjects

Y. Gallais

Publications and source records attributed to Y. Gallais.

64 records · Page 4Linked to original sources

arXiv2007

The Fermi edge singularity of spin polarized electrons

We study the absorption spectrum of a two-dimensional electron gas (2DEG) in a magnetic field. We find that that at low temperatures, when the 2DEG is spin polarized, the absorption spectra, which correspond to the creation of spin up or spin down electron, differ in magnitude, linewidth and filling factor dependence. We show that these differences can be explained as resulting from creation of a Mahan exciton in one case, and of a power law Fermi edge singularity in the other.

cond-mat.mes-hall↗

arXiv2006

Absorption in the fractional quantum Hall regime: trion dichroism and spin polarization

We present measurements of optical interband absorption in the fractional quantum Hall regime in a GaAs quantum well in the range 0 < nu < 1. We investigate the mechanism of singlet trion absorption, and show that its circular dichroism can be used as a probe of the spin polarization of the ground state of the two-dimensional electron system (2DES). We find that at nu = 1/3 the 2DES is fully spin-polarized. Increasing the filling factor results in a gradual depolarization, with a sharp minimum in the dichroism near nu = 2/3. We find that in the range 0.5 < nu < 0.85 the 2DES remains partially polarized for the broad range of magnetic fields from 2.75 to 11 Tesla. This is consistent with the presence of a mixture of polarized and depolarized regions.

cond-mat.mes-hall↗

arXiv2006

Transition from Free to Interacting Composite Fermions away from $ν$=1/3

Spin excitations from a partially populated composite fermion level are studied above and below $ν=1/3$. In the range $2/7<ν<2/5$ the experiments uncover significant departures from the non-interacting composite fermion picture that demonstrate the increasing impact of interactions as quasiparticle Landau levels are filled. The observed onset of a transition from free to interacting composite fermions could be linked to condensation into the higher order states suggested by transport experiments and numerical evaluations performed in the same filling factor range.

cond-mat.mes-hall↗

arXiv2006

Spin excitations in the Fractional Quantum Hall regime at $ν\lesssim1/3$

We report inelastic light scattering experiments in the fractional quantum Hall regime at filling factors $ν\lesssim1/3$. A spin mode is observed below the Zeeman energy. The filling factor dependence of the mode energy is consistent with its assignment to spin flip excitations of composite fermions with four attached flux quanta ($ϕ$=4). Our findings reveal a composite fermion Landau level structure in the $ϕ$=4 sequence.

cond-mat.mes-hall↗

arXiv2006

Coupling between quasiparticles and a bosonic mode in the normal state of HgBa$_2$CuO$_{4+δ}$

We report a doping dependent study of the quasiparticles dynamics in HgBa$_2$CuO$_{4+δ}$ via Electronic Raman Scattering. A well-defined energy scale is found in the normal state dynamics of the quasiparticles over a broad doping range. It is interpreted as evidence for coupling between the quasiparticles and a collective bosonic mode whose energy scale depend only weakly with doping. We contrast this behavior with that of the superconducting gap whose amplitude near the node continuously decreases towards the underdoped regime. We discuss the implications of our findings on the nature of the collective mode and argue that electron-phonon coupling is the most natural explanation.

cond-mat.supr-con↗

arXiv2006

Two Energy Scales and two Quasiparticle Dynamics in the Superconducting State of Underdoped Cuprates

The superconducting state of underdoped cuprates is often described in terms of a single energy-scale, associated with the maximum of the (d-wave) gap. Here, we report on electronic Raman scattering results, which show that the gap function in the underdoped regime is characterized by two energy scales, depending on doping in opposite manners. Their ratios to the maximum critical temperature are found to be universal in cuprates. Our experimental results also reveal two different quasiparticle dynamics in the underdoped superconducting state, associated with two regions of momentum space: nodal regions near the zeros of the superconducting gap and antinodal regions. While antinodal quasiparticles quickly loose coherence as doping is reduced, coherent nodal quasiparticles persist down to low doping levels. A theoretical analysis using a new sum-rule allows us to relate the low-frequency-dependence of the Raman response to the temperature-dependence of the superfluid density, both controlled by nodal excitations.

cond-mat.supr-con↗

arXiv2005

Effects of Magnetic and non-Magnetic Impurities on the Superconducting State of YBa$_{2}$Cu$_{3}$O$_{7-δ}$

We report Electronic Raman Scattering measurements on optimally doped Zn and Ni substituted YBCO in A$_{1g}$ and B$_{1g}$ channels. We show that the superconducting gap energy is independant of magnetic Ni and non-magnetic Zn substitutions. On the contrary the collective A$_{1g}$ mode follows the critical temperature $T_c$ with two distinct slopes for Ni and Zn, and tracks the magnetic resonance seen in inelastic neutron scattering. We explain the unconventionnal energy dependence of the superconducting gap and discuss the behaviour of the A$_{1g}$ mode within magnetic and non-magnetic impurities.

cond-mat.supr-con↗

arXiv2005

Interplay between the pseudogap and superconductivity in HgBa$_2$CuO$_4$ single crystals

We report a doping dependant Electronic Raman Scattering (ERS) study of HgBa$_2$CuO$_{4+δ}$ (Hg-1201) single crystals. We investigate the dynamics of the antinodal and nodal quasiparticles. We show that the dynamical response of the antinodal quasiparticles is strongly reduced towards the underdoped regime in both the normal and superconducting states. When probing the nodal quasiparticles, we are able to distinguish between the energy scale of the pseudogap and that of the superconducting gap. A simple model relating the suppression of the dynamical response of the antinodal quasiparticles to fluctuations related to a competing phase is proposed.

cond-mat.supr-con↗

arXiv2003

Resonant Raman scattering in mercurate single crystals

We report resonant electronic Raman scattering in optimally doped single layer HgBa$_2$CuO$_{4+δ}$ (Hg-1201) and trilayer HgBa$_2$Ca$_2$Cu$_3$O$_{8+δ}$ (Hg-1223) single crystals. Analysis of the $B_{1g}$ and B$_{2g}$ channels in the superconducting state of Hg-1201 advocates for a gap having d-wave symmetry. In addition a resonant study $B_{1g}$ pair-breaking peak and the $A_{1g}$ peak suggests that the $A_{1g}$ peak is not directly related to the d-wave superconducting gap amplitude. Comparison with trilayer Hg-1223 demonstrates the universal behavior of this two energy scales in optimally doped cuprates.

cond-mat.supr-con↗

arXiv2002

Evidence for two distinct energy scales in the Raman spectra of YBa2(CuNi)O6.95

We report low energy electronic Raman scattering from Ni-substituted YBa2Cu3O6.95 single crystals with Tc ranging from 92.5 K to 78 K. The fully symmetrical A1g channel and the B1g channel which is sensitive to the dx2-y2 gap maximum have been explored. The energy of the B1g pair-breaking peak remains constant under Ni doping while the energy of the A1g peak scales with Tc (EA1g/Tc=5). Our data show that the A1g peak tracks the magnetic resonance peak observed in inelastic neutron scattering yielding a key explanation to the long-standing problem of the origin the A1g peak.

cond-mat.supr-con↗