arXiv subjects

Y. Gallais

Publications and source records attributed to Y. Gallais.

At least 19 recordsLinked to original sources

arXiv2022

Spin singlet and quasiparticles excitations in cuprate superconductors

We followed step by step the transition from an antiferromagnetic (AF) Mott insulator to a superconducting (SC) metal in the Bi$_2$Sr$_2$CaCu$_{2}$O$_{8+δ}$ (Bi-2212) cuprate using the electronic Raman scattering spectroscopy. This was achieved by tracking the doping dependence of the spin singlet excitation originate from the AF Mott insulator, the normal state quasiparticles excitation related to the mobile charge carriers and the Bogoliubov quasiparticles related to the SC gap. We show that the signature of the pseudogap phase which develops during this transition, can be interpreted as the blocking of charge carriers by the enhancement of the antiferromagnetic correlations as the temperature drops. We find that the energy scale of the pseudogap, $Δ_{\textrm{pg}}(p)$, closely follows the one of the spin singlet excitation, $Δ_{\textrm{sse}}(p)$, with doping $p$. The quasiparticles lifetime considerably increases with doping when the pseudogap collapses. We reveal that the maximum amplitude of the SC gap, $Δ_{\textrm{sc}}^{\textrm{max}}$ and the SC transition temperature \Tc are linked in an extended range of doping such as $Δ_{\textrm{sc}}^{\textrm{max}}(p) \propto Δ_{\textrm{sse}}(p)\, T_c(p)$. This relation suggests that the AF correlations play a key role in the mechanism of superconductivity.

cond-mat.supr-con↗

arXiv2022

Nematic fluctuations mediated superconductivity revealed by anisotropic strain in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

Anisotropic strain is an external field capable of selectively addressing the role of nematic fluctuations in promoting superconductivity. We demonstrate this using polarization-resolved elasto-Raman scattering to probe the evolution of nematic fluctuations under strain in the normal and superconducting states of the paradigmatic iron-based superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. In the non-superconducting parent compound BaFe$_2$As$_2$ we observe a strain-induced suppression of the nematic susceptibility which follows the expected behavior of an Ising order parameter under a symmetry breaking field. For the superconducting compound, the suppression of the nematic susceptibility correlates with the decrease of the superconducting critical temperature $T_c$. Our results indicate a significant contribution of nematic fluctuations to electron pairing and validate theoretical scenarios of enhanced $T_c$ near a nematic quantum critical point.

cond-mat.supr-con↗

arXiv2021

Elasto-Raman scattering: arsenic optical phonon as a probe of nematicity in BaFe$_2$As$_2$

We report a Raman scattering study of nematic degrees of freedom in the iron-based superconductor parent compound BaFe$_2$As$_2$ under tunable uniaxial strain. We demonstrate that the polarization resolved arsenic (As) phonon intensity can be used to monitor the nematic order parameter as a function of both temperature and strain. At low temperature in the nematic ordered phase we use it to track the continuous and reversible orientation of nematic domains under variable strain. At higher temperature, the evolution of the As phonon intensity under strain reflects an enhanced nematic susceptibility close to the nematic transition $T_S$. Its temperature dependence under strong strain follows qualitatively the expected behavior of an Ising order parameter under a symmetry breaking field. Our elasto-Raman study illustrates the interest of combining selective anisotropic strain with a symmetry resolved probe like Raman scattering. Elasto-Raman scattering can be applied to a wide variety of quantum materials where uniaxial strain tunes electronic orders.

cond-mat.supr-con↗

arXiv2021

Orbital dichotomy of Fermi liquid properties in Sr$_2$RuO$_4$ revealed by Raman spectroscopy

We report a polarization-resolved Raman spectroscopy study of the orbital dependence of the quasiparticles properties in the prototypical multi-band Fermi liquid Sr\textsubscript{2}RuO\textsubscript{4}. We show that the quasiparticle scattering rate displays $ω^{2}$ dependence as expected for a Fermi liquid. Besides, we observe a clear polarization-dependence in the energy and temperature dependence of the quasiparticle scattering rate and mass, with the $d_{xz/yz}$ orbital derived quasiparticles showing significantly more robust Fermi liquid properties than the $d_{xy}$ orbital derived ones. The observed orbital dichotomy of the quasiparticles is consistent with the picture of Sr\textsubscript{2}RuO\textsubscript{4} as a Hund's metal. Our study establishes Raman scattering as a powerful probe of Fermi liquid properties in correlated metals.

cond-mat.str-el↗

arXiv2021

Exploration of the Hg-based cuprate superconductors by Raman spectroscopy under hydrostatic pressure

The superconducting phase of the $\mathrm{HgBa}_2\mathrm{CuO}_{4+δ}$ (Hg-1201) and $\mathrm{HgBa}_2\mathrm{Ca}_2\mathrm{Cu}_3\mathrm{O}_{8+δ}$ (Hg-1223) cuprates has been investigated by Raman spectroscopy under hydrostatic pressure. Our analysis reveals that the increase of $T_c$ with pressure is slower in Hg-1223 cuprate compared to the Hg-1201 due to a charge carrier concentration imbalance (accentuated by pressure) between the $\mathrm{CuO}_2$ layers of Hg-1223. We find that the energy variation under pressure of the apical oxygen mode from which the charge carriers are transferred to the $\mathrm{CuO}_2$ layers, is the same for both the Hg-1223 and Hg-1223 cuprates and it is controlled by the inter-layer compressibility. At last, we show that the binding energy of the Cooper pairs related to the maximum amplitude of the $d-$ wave superconducting gap at the anti-nodes, does not follow $T_c$ with pressure. It decreases while $T_c$ increases. In the particular case of Hg-1201, the binding energy collapses from 10 to 2 $K_B T_c$ as the pressure increases up to 10 GPa. These direct spectroscopic observations joined to the fact that the binding energy of the Cooper pairs at the anti-nodes does not follow $T_c$ either with doping, raises the question of its link with the pseudogap energy scale which follows the same trend with doping.

cond-mat.supr-con↗

arXiv2021

Possible observation of the signature of the bad metal phase and its crossover to a Fermi liquid in K(BEDT-TTF)2Cu(NCS)2 bulk and nanoparticles by Raman scattering

K(BEDT-TTF)2Cu(NCS)2 has been investigated by Raman scattering in both bulk and nanoparticle compounds. Phonon modes from 20 to 1600 cm-1 have been assigned. Focusing on the unexplored low frequency phonons, a plateau in frequencies is observed in the bulk phonons between 50 and 100 K and assigned to the signature of the bad metal phase. Nanoparticles of K(BEDT-TTF)2Cu(NCS)2 exhibit anomalies at 50 K associated to the crossover from a bad metal to a Fermi liquid whose origins are discussed.

cond-mat.str-el↗

arXiv2021

Amplitude mode of charge density wave in TTF[Ni(dmit)2]2 observed by electronic Raman scattering

We measured the optical signature of the charge density waves (CDWs) in the multiband conductor TTF[Ni(dmit)2]2 by electronic Raman scattering. At low energies, a hump develops below 60 K. This hump is associated to the amplitude mode of the CDW with an energy around 9 meV. Raman symmetry-resolved measurements show that the CDW amplitude mode is anisotropic and that the CDW can be associated to the band nesting of Ni(dmit)2 chains.

cond-mat.str-el↗

arXiv2020

Lattice-Shifted Nematic Quantum Critical Point in FeSe$_{1-x}$S$_x$

We report the evolution of nematic fluctuations in FeSe$_{1-x}$S$_x$ single crystals as a function of Sulfur content $x$ across the nematic quantum critical point (QCP) $x_c\sim$ 0.17 via Raman scattering. The Raman spectra in the $B_{1g}$ nematic channel consist of two components, but only the low energy one displays clear fingerprints of critical behavior and is attributed to itinerant carriers. Curie-Weiss analysis of the associated nematic susceptibility indicates a substantial effect of nemato-elastic coupling which shifts the location of the nematic QCP. We argue that this lattice-induced shift likely explains the absence of any enhancement of the superconducting transition temperature at the QCP. The presence of two components in the nematic fluctuations spectrum is attributed to the dual aspect of electronic degrees of freedom in Hund's metals, with both itinerant carriers and local moments contributing to the nematic susceptibility.

cond-mat.supr-con↗

arXiv2020

Universal relationship between the energy scales of the pseudogap phase, the superconducting state and the charge density wave order in copper oxide superconductors

We report the hole doping dependencies of the pseudogap phase energy scale, $2Δ_{\rm PG}$, the anti-nodal (nodal) superconducting energy scales $2Δ^{AN}_{\rm SC}$ ($2Δ^{N}_{\rm SC}$) and the charge density wave energy scale, $2Δ_{\rm CDW}$. They have been extracted from the electronic Raman responses of distinct copper oxide families. For all the cuprates studied, we reveal universal doping dependencies which suggest that $2Δ_{\rm PG}$, $2Δ^{AN}_{\rm SC}$ and $2Δ_{\rm CDW}$ are governed by common microscopic interactions and that these interactions become relevant well above the superconducting transition at $T_c$. In sharp contrast, $2Δ^N_{\rm SC}$ tracks the doping dependence of $T_c$, appearing to be controlled by a different kind of interactions than the energy scales above.

cond-mat.supr-con↗

arXiv2020

Nematic Fluctuations in the Cuprate Superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

Establishing the presence and the nature of a quantum critical point in their phase diagram is a central enigma of the high-temperature superconducting cuprates. It could explain their pseudogap and strange metal phases, and ultimately their high superconducting temperatures. Yet, while solid evidences exist in several unconventional superconductors of ubiquitous critical fluctuations associated to a quantum critical point, in the cuprates they remain undetected until now. Here using symmetry-resolved electronic Raman scattering in the cuprate Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$, we report the observation of enhanced electronic nematic fluctuations near the endpoint of the pseudogap phase. While our data hint at the possible presence of an incipient nematic quantum critical point, the doping dependence of the nematic fluctuations deviates significantly from a canonical quantum critical scenario. The observed nematic instability rather appears to be tied to the presence of a van Hove singularity in the band structure.

cond-mat.supr-con↗

arXiv2020

Superconducting gap and nematic resonance at the quantum critical point observed by Raman scattering in $\mathrm{BaFe_{2}}(\mathrm{As}_{1-x}\mathrm{P}_{x}\mathrm{)_{2}}$

We report comprehensive temperature and doping-dependences of the Raman scattering spectra for $\mathrm{BaFe_{2}}(\mathrm{As}_{1-x}\mathrm{P}_{x}\mathrm{)_{2}}$ ($x =$ 0, 0.07, 0.24, 0.32, and 0.38), focusing on the nematic fluctuation and the superconducting responses. With increasing $x$, the bare nematic transition temperature estimated from the Raman spectra reaches $T =$ 0 K at the optimal doping, which indicates a quantum critical point (QCP) at this composition. In the superconducting compositions, in addition to the pair breaking peaks observed in the $A_{\mathrm{1g}}$ and $B_{\mathrm{1g}}$ spectra, another strong $B_{\mathrm{1g}}$ peak appears below the superconducting transition temperature which is ascribed to the nematic resonance peak. The observation of this peak indicates significant nematic correlations in the superconducting state near the QCP in this compound.

cond-mat.supr-con↗

arXiv2019

Magnetic transitions in CaMn7O12 : a Raman observation of spin-phonon couplings

The quadruple Calcium manganite (CaMn7O12) is a multiferroic material that exhibits a giant magnetically-induced ferroelectric polarization which makes it very interesting for magnetoelectric applications. Here, we report the Raman spectroscopy study on this compound of both the phonon modes and the low energy excitations from 4 K to room temperature. A detailed study of the Raman active phonon excitations shows that three phonon modes evidence a spin-phonon coupling at TN2 = 50 K. In particular, we show that the mode at 432 cm-1 associated to Mn(B)O6 (B position of the perovskite) rotations around the [111] cubic diagonal is impacted by the magnetic transition at 50 K and its coupling to the new modulation of the Mn spin in the (a,b) plane. At low energies, two large low energy excitations are observed at 25 and 47 cm-1. The first one disappears at 50 K and the second one at 90 K. We have associated these excitations to electro-magneto-active modes.

cond-mat.str-el↗

arXiv2018

The role of the rare earth in the lattice and magnetic coupling in multiferroic h-HoMnO3

We used Raman scattering to study the lattice and magnetic excitations in the hexagonal HoMnO3 single crystals. The E2 phonon mode at 237 cm-1 is affected by the magnetic order. This mode is related to the displacement of Mn and O ions in a-b plane and modulates the Mn-O-Mn bond angles in a-b plane and the in-plane Mn-Mn superexchange interaction. The mode at 269 cm-1 associated to the displacement of the apical Ho3+ ions along the c direction presents an abrupt change of slope at TN showing that the role of the rare earth ions can not be neglected in the magnetic transition. We have identified magnon and crystal field excitations. The temperature dependence of the magnetic excitations has been compared to the Mn and Ho moment and indicates that the exchange interaction pattern between Mn and Ho atoms drives the uniaxial anisotropy gap above the Mn-spin-rotation transition.

cond-mat.str-el↗

arXiv2018

Intimate link between Charge Density Wave, Pseudogap and Superconducting Energy Scales in Cuprates

The cuprate high temperature superconductors develop spontaneous charge density wave (CDW) order below a temperature $T_{CDW}$ and over a wide range of hole doping (p). An outstanding challenge in the field is to understand whether this modulated phase is related to the more exhaustively studied pseudogap and superconducting phases. To address this issue it is important to extract the energy scale $Δ_{CDW}$ associated with the charge modulations, and to compare it with the pseudogap (PG) $Δ_{PG}$ and the superconducting gap $Δ_{SC}$. However, while $T_{CDW}$ is well-characterized from earlier works little has been known about $Δ_{CDW}$ until now. Here, we report the extraction of $Δ_{CDW}$ for several cuprates using electronic Raman spectroscopy. Crucially, we find that, upon approaching the parent Mott state by lowering $p$, $Δ_{CDW}$ increases in a manner similar to the doping dependence of $Δ_{PG}$ and $Δ_{SC}$. This shows that CDW is an unconventional order, and that the above three phases are controlled by the same electronic correlations. In addition, we find that $Δ_{CDW} \approx Δ_{SC}$ over a substantial doping range, which is suggestive of an approximate emergent symmetry connecting the charge modulated phase with superconductivity.

cond-mat.supr-con↗

arXiv2018

Anisotropic Kondo pseudo-gap in URu2Si2

A polarized electronic Raman scattering study reveals the emergence of symmetry dependence in the electronic Raman response of single crystalline URu$_{2}$Si$_{2}$ below the Kondo crossover scale $T_K\sim100K$. In particular, the development of a coherent Kondo pseudo-gap predominantly in the E$_g$ channel highlights strong anisotropy in the Kondo physics in URu$_{2}$Si$_{2}$ that has previously been neglected in theoretical models of this system. A calculation of the Raman vertices demonstrates that the strongest Raman vertex does indeed develop within the E$_g$ channel for interband transitions and reaches a maximum along the diagonals of the Brillouin zone, implying a d-wave-like geometry for the Kondo pseudo-gap. Below the hidden order phase transition at $T_{HO}= 17.5K$, the magnitude of the pseudo-gap is found to be enhanced. Moreover, the anisotropy of the pseudo-gap is similar in form to that proposed for the chiral d-wave (E$_g$) superconducting state that appears below $T_c=1.5K$.

cond-mat.str-el↗

arXiv2018

Unambiguous connection between the Fermi surface topology and the pseudogap in Bi$_{2}$Sr$_2$CaCu$_2$O$_{8+d}$

We study the behavior of the pseudogap in overdoped Bi$_{2}$Sr$_2$CaCu$_2$O$_{8+d}$ by electronic Raman scattering (ERS) and angle-resolved photoemission spectroscopy (ARPES) on the same single crystals. Using both techniques we find that, unlike the superconducting gap, the pseudogap related to the anti-bonding band vanishes above the critical doping p$_c$ = 0.22. Concomitantly, we show from ARPES measurements that the Fermi surface of the anti-bonding band is hole-like below pc and becomes electron-like above p$_c$. This reveals that the appearance of the pseudogap depends on the Fermi surface topology in Bi$_{2}$Sr$_2$CaCu$_2$O$_{8+d}$ , and more generally, puts strong constraint on theories of the pseudogap phase.

cond-mat.supr-con↗

arXiv2017

Strain and Magnetic Field Induced Spin-Structure Transitions in Multiferroic BiFeO3

The magnetic-field-dependent spin ordering of strained BiFeO3 films is determined using nuclear resonant scattering and Raman spectroscopy. The critical field required to destroy the cycloidal modulation of the Fe spins is found to be significantly lower than in the bulk, with appealing implications for field-controlled spintronic and magnonic devices.

cond-mat.mtrl-sci↗

arXiv2017

Vertical temperature-boundary of the pseudogap under the superconducting dome of the Bi2Sr2CaCu2O8+d phase-diagram

Combining electronic Raman scattering experiments with cellular dynamical mean field theory, we present evidence of the pseudogap in the superconducting state of various hole-doped cuprates. In Bi2Sr2CaCu2O8+d we track the superconducting pseudogap hallmark, a peak-dip feature, as a function of temperature T and doping p, well beyond the optimal one. We show that, at all temperatures under the superconducting dome, the pseudogap disappears at the doping pc, between 0.222 and 0.226, where also the normal-state pseudogap collapses at a Lifshitz transition. This demonstrates that the superconducting pseudogap boundary forms a vertical line in the T-p phase diagram.

cond-mat.supr-con↗