arXiv subjects

H. Raffy

Publications and source records attributed to H. Raffy.

At least 19 recordsLinked to original sources

arXiv2024

Persistence of vortexlike phase fluctuations in underdoped to heavily overdoped Bi-2201 cuprates

The mechanism that controls the superconducting (SC) transition temperature $T_{\mathrm{c}}^{0}$ as a function of doping is one of the central questions in cuprate high-temperature superconductors. While it is generally accepted that $T_{\mathrm{c}}^{0}$ in underdoped cuprates is not determined by the scale of pairing but by the onset of global phase coherence, the role of phase fluctuations in the overdoped region has been controversial. Here, our transport measurements in perpendicular magnetic fields ($H$) on underdoped Bi-2201 reveal immeasurably small Hall response for $T>T_{\mathrm{c}}(H)$ as a signature of SC phase with vortexlike phase fluctuations. We find that the extent of such a regime in $T$ and $H$ is suppressed near optimal doping but becomes strongly enhanced in heavily overdoped Bi-2201. Our results thus show that vortexlike phase fluctuations play an important role in the field-tuned SC transition in the heavily overdoped region, in contrast to conventional mean-field Bardeen-Cooper-Schrieffer description. The unexpected nonmonotonic dependence of phase fluctuations on doping provides a new perspective on the SC transition in cuprates.

cond-mat.supr-con↗

arXiv2023

High magnetic field evolution of the in-plane angular magnetoresistance of electron-doped Sr1-xLaxCuO2 in the normal state

We studied the in-plane angular magnetoresistance (AMR), in the normal state, of underdoped superconducting Sr1-xLaxCuO2 , which has the simplest crystal structure among cuprates. The measurements of two underdoped thin films with different dopings were performed in intense magnetic field H (up to 22 T). The longitudinal magnetoresistance at temperature T is negative and scales with H/T. For both samples, the AMR is anisotropic and shows an unexpected dependence on H intensity. While at the low magnetic field, one observes essentially twofold AMR oscillations for the more doped sample, fourfold ones start to grow under the high magnetic field, resulting in the coexistence of the two. For the less doped film at the low magnetic field, both twofold and fourfold AMR components exist. With the increase of the magnetic field, the fourfold component survives a pi/4 phase shift, during which its amplitude vanishes, at a magnetic field Hc such as: 16 T < Hc < 17 T. As a result, at the high magnetic field above Hc, the angular dependence of the in-plane magnetoresistance turns out to be the same for both samples. We tentatively ascribe the above features to the presence of anti-ferromagnetism in the CuO2 planes of underdoped Sr1-xLaxCuO2.

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arXiv2023

Transport properties of very overdoped nonsuperconducting Bi2Sr2CuO6+d thin films

The transport properties, resistance, Hall effect, and low T magnetoresistance for very oxygen overdoped nonsuperconducting Bi2Sr2CuO6+d (Bi2201) thin films are reported. From 20 to 300K, the temperature dependence of the resistance is well described by a law of the form a+bT4/3, theoretically predicted to occur in the presence of ferromagnetic fluctuations. In addition, this prediction is reinforced by the analysis of the transverse and the longitudinal low T magnetoresistance. Interestingly, the presence of a weak disorder causing low T electronic localization allows us to evidence very short diffusion lengths, as observed in other systems with ferromagnetic fluctuations.

cond-mat.supr-con↗

arXiv2021

Transport signatures of the pseudogap critical point in the cuprate superconductor Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+δ}$

Five transport coefficients of the cuprate superconductor Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+δ}$ were measured in the normal state down to low temperature, reached by applying a magnetic field (up to 66T) large enough to suppress superconductivity. The electrical resistivity, Hall coefficient, thermal conductivity, Seebeck coefficient and thermal Hall conductivity were measured in two overdoped single crystals, with La concentration $x = 0.2$ ($T_{\rm c}=18$K) and $x = 0.0$ ($T_{\rm c}=10$K). The samples have dopings $p$ very close to the critical doping $p^{\star}$ where the pseudogap phase ends. The resistivity displays a linear dependence on temperature whose slope is consistent with Planckian dissipation. The Hall number $n_{\rm H}$ decreases with reduced $p$, consistent with a drop in carrier density from $n = 1+p$ above $p^{\star}$ to $n=p$ below $p^{\star}$. This drop in $n_{\rm H}$ is concomitant with a sharp drop in the density of states inferred from prior NMR Knight shift measurements. The thermal conductivity satisfies the Wiedemann-Franz law, showing that the pseudogap phase at $T = 0$ is a metal whose fermionic excitations carry heat and charge as do conventional electrons. The Seebeck coefficient diverges logarithmically at low temperature, a signature of quantum criticality. The thermal Hall conductivity becomes negative at low temperature, showing that phonons are chiral in the pseudogap phase. Given the observation of these same properties in other, very different cuprates, our study provides strong evidence for the universality of these five signatures of the pseudogap phase and its critical point.

cond-mat.supr-con↗

arXiv2018

Universal $T$-linear resistivity and Planckian limit in overdoped cuprates

The perfectly linear temperature dependence of the electrical resistivity observed as $T \rightarrow$ 0 in a variety of metals close to a quantum critical point is a major puzzle of condensed matter physics . Here we show that $T$-linear resistivity as $T \rightarrow$ 0 is a generic property of cuprates, associated with a universal scattering rate. We measured the low-temperature resistivity of the bi-layer cuprate Bi2212 and found that it exhibits a $T$-linear dependence with the same slope as in the single-layer cuprates Bi2201, Nd-LSCO and LSCO, despite their very different Fermi surfaces and structural, superconducting and magnetic properties. We then show that the $T$-linear coefficient (per CuO$_2$ plane), $A_1$, is given by the universal relation $A_1 T_F = h / 2e^2$, where $e$ is the electron charge, $h$ is the Planck constant and $T_F$ is the Fermi temperature. This relation, obtained by assuming that the scattering rate 1 / $τ$ of charge carriers reaches the Planckian limit whereby $\hbar / τ= k_B T$, works not only for hole-doped cuprates but also for electron-doped cuprates despite the different nature of their quantum critical point and strength of their electron correlations.

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arXiv2016

Destroying coherence in high temperature superconductors with current flow

The loss of single-particle coherence going from the superconducting state to the normal state in underdoped cuprates is a dramatic effect that has yet to be understood. Here, we address this issue by performing angle resolved photoemission spectroscopy (ARPES) measurements in the presence of a transport current. We find that the loss of coherence is associated with the development of an onset in the resistance, in that well before the midpoint of the transition is reached, the sharp peaks in the ARPES spectra are completely suppressed. Since the resistance onset is a signature of phase fluctuations, this implies that the loss of single-particle coherence is connected with the loss of long-range phase coherence.

cond-mat.supr-con↗

arXiv2015

Quasiparticles dynamics in high-temperature superconductors far from equilibrium: an indication of pairing amplitude without phase coherence

We perform time resolved photoelectron spectroscopy measurements of optimally doped $\tn{Bi}_2\tn{Sr}_2\tn{CaCu}_2\tn{O}_{8+δ}$ (Bi-2212) and $\tn{Bi}_2\tn{Sr}_{2-x}\tn{La}_{x}\tn{Cu}\tn{O}_{6+δ}$ (Bi-2201). The electrons dynamics show that inelastic scattering by nodal quasiparticles decreases when the temperature is lowered below the critical value of the superconducting phase transition. This drop of electronic dissipation is astonishingly robust and survives to photoexcitation densities much larger than the value sustained by long-range superconductivity. The unconventional behaviour of quasiparticle scattering is ascribed to superconducting correlations extending on a length scale comparable to the inelastic path. Our measurements indicate that strongly driven superconductors enter in a regime without phase coherence but finite pairing amplitude. The latter vanishes near to the critical temperature and has no evident link with the pseudogap observed by Angle Resolved Photoelectron Spectroscopy (ARPES).

cond-mat.supr-con↗

arXiv2013

Electronic phase diagram of high temperature copper oxide superconductors

In order to understand the origin of high-temperature superconductivity in copper oxides, we must understand the normal state from which it emerges. Here, we examine the evolution of the normal state electronic excitations with temperature and carrier concentration in Bi2Sr2CaCu2O8 using angle-resolved photoemission. In contrast to conventional superconductors, where there is a single temperature scale Tc separating the normal from the superconducting state, the high- temperature superconductors exhibit two additional temperature scales. One is the pseudogap scale T*, below which electronic excitations exhibit an energy gap. The second is the coherence scale Tcoh, below which sharp spectral features appear due to increased lifetime of the excitations. We find that T* and Tcoh are strongly doping dependent and cross each other near optimal doping. Thus the highest superconducting Tc emerges from an unusual normal state that is characterized by coherent excitations with an energy gap.

cond-mat.supr-con↗

arXiv2010

Penetration depth of electron-doped-infinite-layer Sr$_{0.88}$La$_{0.12}$CuO$_{2+x}$ thin films

The in-plane penetration depth of Sr$_{0.88}$La$_{0.12}$CuO$_{2+x}$ thin films at various doping obtained from oxygen reduction has been measured, using AC susceptibility measurements. For the higher doping samples, the superfluid density deviates strongly from the s-wave behavior, suggesting, in analogy with other electron-doped cuprates, a contribution from a nodal hole pocket, or a small gap on the Fermi surface such as an anisotropic s-wave order parameter. The low value of the superfluid densities, likely due to a strong doping-induced disorder, places the superconducting transition of our samples in the phase-fluctuation regime.

cond-mat.supr-con↗

arXiv2010

Anisotropy of the in-plane angular magnetoresistance of electron-doed Sr1-xLaxCuO2 thin films

Signatures of antiferromagnetism (AF) in the underdoped Ln2-xCexCuO4 (Ln = Nd, Pr,...) family are observed even for doping levels for which superconductivity exists. We have looked for a similar property in a different electron-doped cuprate family, Sr1-xLaxCuO2, which consists of CuO2 planes separated by Sr/La atoms, and is exempt of the possible influence of magnetic rare earth ions. We report in-plane magnetoresistance measurements in the normal state of underdoped, superconducting, c-axis oriented, epitaxial Sr1-xLaxCuO2 thin films. This probe is sensitive to spin arrangement and we find that the in-plane magnetoresistance, which is negative and does not saturate for T, exhibits an angular dependence when measured upon rotating a magnetic field within the CuO2 planes. The analysis reveals a superposition of fourfold and twofold angular oscillations. Both of these increase in amplitude with increasing field and decreasing and appear below a temperature, which gets higher with decreasing doping levels. Our results demonstrate that these magnetoresistance oscillations, also observed for the Ln2-xCexCuO4 (Ln = Nd, Pr,...) family and attributed to an AF signature, are, without ambiguity, a property of CuO2 planes. Besides, these oscillations vary with doping in an unusual way compared to previous results: fourfold oscillations are essentially present in the more underdoped samples while only twofold oscillations are visible in the less underdoped ones. This intriguing observation appears to be a consequence of spin dilution with increasing doping level.

cond-mat.supr-con↗

arXiv2009

Observation of a d-wave nodal liquid in highly underdoped Bi_2Sr_2CaCu_2O_{8+δ}

We use angle resolved photoemission spectroscopy to probe the electronic excitations of the non-superconducting state that exists between the antiferromagnetic Mott insulator at zero doping and the superconducting state at larger dopings in Bi_2Sr_2CaCu_2O_{8+δ}. We find that this state is a nodal liquid whose excitation gap becomes zero only at points in momentum space. Despite exhibiting a resistivity characteristic of an insulator and the absence of coherent quasiparticle peaks, this material has the same gap structure as the d-wave superconductor. We observe a smooth evolution of the spectrum across the insulator-to-superconductor transition, which suggests that high temperature superconductivity emerges when quantum phase coherence is established in a non-superconducting nodal liquid.

cond-mat.supr-con↗

arXiv2008

Magnetoresistance and Hall effect in e-doped superconducting SrLaCuO thin films

We have epitaxially grown c-axis oriented SrxLa1-xCuO2 thin films by rf sputtering on KTaO3 substrates with x = 0.12. The as-grown deposits are insulating and a series of superconducting films with various Tc(R=0) up to 26 K have been obtained by in-situ oxygen reduction. Transport measurements in the ab plane of these samples have been undertaken. We report original results on the temperature dependence of the Hall effect and on the anisotropic magnetoresistance (T > Tc). We discuss the magnitude of upper critical fields and anisotropy, the Hall effect, which presents changes of sign indicative of the existence of two types of carriers, the normal state magnetoresistance, negative in parallel magnetic field, a possible signature of spin scattering. These properties are compared to those of hole-doped cuprates, such as BiSr(La)CuO with comparable Tc.

cond-mat.supr-con↗

arXiv2008

Influence of oxygen reduction on the structural and electronic properties of electron-doped Sr1-xLaxCuO2 thin films

Single phase, c-axis oriented, e-doped, Sr1-xLaxCuO2 thin films were epitaxially grown on KTaO3 and DyScO3 substrates by reactive rf sputtering. As-grown films being insulating due to oxygen excess, oxygen reduction is necessary to observe superconductivity. Two different procedures were employed to reach superconductivity. On one hand an in-situ reduction process was conducted on a series of films deposited on both types of substrates. On the other hand, an ex-situ reduction procedure was performed sequentially on a single film deposited on DyScO3. The study of the influence of oxygen reduction on the structural and electronic properties of the thin films is presented and discussed.

cond-mat.supr-con↗

arXiv2008

Characterizing the insulator adjacent to the superconductor in Bi$_2$Sr$_{2-x}$La$_{x}$CuO$_{6+δ}$ ($x=0.3$)

The time-averaged and low--frequency noise transport properties were investigated in the vicinity of the superconductor--insulator transition for a Bi$_2$Sr$_{2-x}$La$_{x}$CuO$_{6+δ}$ ($x=0.3$) thin film. The results are consistent with a superconductor (metal) embedded in a strong insulator, the latter showing two-dimensional variable range hopping properties. The weak insulator behavior -- if any -- is attributed to the metallic inclusions only.

cond-mat.supr-con↗

arXiv2007

Protected nodes and the collapse of the Fermi arcs in high Tc cuprates

Angle resolved photoemission on underdoped Bi2Sr2CaCu2O8 reveals that the magnitude and d-wave anisotropy of the superconducting state energy gap are independent of temperature all the way up to Tc. This lack of T variation of the entire k-dependent gap is in marked contrast to mean field theory. At Tc the point nodes of the d-wave gap abruptly expand into finite length ``Fermi arcs''. This change occurs within the width of the resistive transition, and thus the Fermi arcs are not simply thermally broadened nodes but rather a unique signature of the pseudogap phase.

cond-mat.supr-con↗

arXiv2007

Resistance noise in Bi_2Sr_2CaCu_2O$_{8+δ}$

The resistance noise in a Bi_2Sr_2CaCu_2O$_{8+δ}$ thin film is found to increase strongly in the underdoped regime. While the increase of the raw resistance noise with decreasing temperature appears to roughly track the previously reported pseudogap temperature for this material, standard noise analysis rather suggests that the additional noise contribution is driven by the proximity of the superconductor-insulator transition.

cond-mat.supr-con↗

arXiv2007

Anomalous dispersion in the autocorrelation of ARPES data of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

We find that peaks in the autocorrelation of angle resolved photoemission spectroscopy data of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ in the superconducting state show dispersive behavior for binding energies smaller than the maximum superconducting energy gap. For higher energies, though, a striking anomalous dispersion is observed that is a consequence of the interaction of the electrons with collective excitations. In contrast, in the pseudogap phase, we only observe dispersionless behavior for the autocorrelation peaks. The implications of our findings in regards to Fourier transformed scanning tunneling spectroscopy data are discussed.

cond-mat.supr-con↗

arXiv2007

Influence of Disorder on the Hall Effect in Bi$_2$Sr$_2$CuO$_{6+δ}$

The in-plane resistivity and Hall coefficient have been measured for the single-layer compound Bi$_2$Sr$_2$CuO$_{6+δ}$ for the whole range of doping states. The deviation of the Hall coefficient, $R_H$, from a high-temperature linear behavior and the temperature dependence of the Hall angle are both only weakly dependent upon doping, contrasting with Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+δ}$ and Bi$_2$Sr$_2$CaCuO$_{8+δ}$. This is in contradiction with former proposals that the transverse transport detects the formation of incoherent Cooper pairs in the pseudogap state. Conversely, the analysis of the data using a phenomenological angular dependent scattering rate clearly allows to distinguish between underdoped and overdoped states, and we propose that the maximum in $R_H(T)$ simply arises due to the combination of a large isotropic scattering rate and an anisotropic temperature dependent one.

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