arXiv subjects

B. Michon

Publications and source records attributed to B. Michon.

12 recordsLinked to original sources

arXiv2022

Reconciling scaling of the optical conductivity of cuprate superconductors with Planckian resistivity and specific heat

Materials tuned to a quantum critical point display universal scaling properties as a function of temperature $T$ and frequency $\omega$. A long-standing puzzle regarding cuprate superconductors has been the observed power-law dependence of optical conductivity with an exponent smaller than one, in contrast to $T$-linear dependence of the resistivity and $\omega$-linear dependence of the optical scattering rate. Here, we present and analyze resistivity and optical conductivity of La$_{2-x}$Sr$_x$CuO$_4$ with $x=0.24$. We demonstrate $\hbar\omega/k_{\mathrm{B}} T$ scaling of the optical data over a wide range of frequency and temperature, $T$-linear resistivity, and optical effective mass proportional to $\sim \ln T$ corroborating previous specific heat experiments. We show that a $T,\omega$-linear scaling Ansatz for the inelastic scattering rate leads to a unified theoretical description of the experimental data, including the power-law of the optical conductivity. This theoretical framework provides new opportunities for describing the unique properties of quantum critical matter.

cond-mat.str-el↗

arXiv2021

The spectral weight of hole doped cuprates across the pseudogap critical point

One of the most widely discussed features of the cuprate high Tc superconductors is the presence of a pseudogap in the normal state. Recent transport and specific heat measurements have revealed an abrupt transition at the pseudogap critical point, denoted p*, characterized by a drop in carrier density and a strong mass enhancement. In order to give more details about this transition at p*, we performed low-temperature infrared spectroscopy in the normal state of cuprate superconductors La2-xSrxCuO4 (LSCO) and La1.8-xEu0.2SrxCuO4 (Eu-LSCO) for doping contents across the pseudogap critical point p* (from p = 0.12 to 0.24). Through the complex optical conductivity we can extract the spectral weight, K*, of the narrow Drude peak due the coherent motion of the quasi-particles, and the spectral weight enclosed inside the mid-infrared (MIR) band, KMIR, caused by coupling of the quasi-particles to collective excitations of the many-body system. K* is smaller than a third of the value predicted by band calculations, and KMIR forms a dome as a function of doping. We observe a smooth doping dependence of K* through p*, and demonstrate that this is consistent with the observed doping dependence of the carrier density and the mass enhancement. We argue that the superconducting dome is the result of the confluence of two opposite trends, namely the increase of the density of the quasi-particles and the decrease of their coupling to the collective excitations as a function of doping.

cond-mat.supr-con↗

arXiv2020

Unconventional free charge in the correlated semimetal Nd2Ir2O7

Nd2Ir2O7 is a correlated semimetal with the pyrochlore structure, in which competing spin-orbit coupling and electron-electron interactions are believed to induce a time-reversal symmetry broken Weyl semimetal phase characterized by pairs of topologically protected Dirac points at the Fermi energy. However, the emergent properties in these materials are far from clear, and exotic new states of matter have been conjectured. Here we demonstrate optically that at low temperatures the free carrier spectral weight is proportional to T^2 where T is the temperature, as expected for massless Dirac electrons. However, we do {\em not} observe the corresponding T^3 term in the specific heat. That the system is not in a Fermi liquid state is further corroborated by the "Planckian" T-linear temperature dependence of the momentum relaxation rate and the progressive opening of a correlation-induced gap at low temperatures. These observations can not be reconciled within the framework of band theory of electron-like quasiparticles and point toward the effective decoupling of the charge transport from the single particle sector.

cond-mat.str-el↗

arXiv2018

Unusual interplay between superconductivity and field-induced charge order in YBa2Cu3Oy

We present a detailed study of the temperature (T) and magnetic field (H) dependence of the electronic density of states (DOS) at the Fermi level, as deduced from specific heat and Knight shift measurements in underdoped YBa2Cu3Oy. We find that the DOS becomes field-independent above a characteristic field H_{DOS} and that the H_{DOS}(T) line displays an unusual inflection near the onset of the long range 3D charge-density wave order. The unusual S-shape of H_{DOS}(T) is suggestive of two mutually-exclusive orders that eventually establish a form of cooperation in order to coexist at low T. On theoretical grounds, such a collaboration could result from the stabilisation of a pair-density wave state, which calls for further investigations in this region of the phase diagram

cond-mat.supr-con↗

arXiv2018

Wiedemann-Franz law and abrupt change in conductivity across the pseudogap critical point of a cuprate superconductor

The thermal conductivity $\kappa$ of the cuprate superconductor La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$ was measured down to 50 mK in seven crystals with doping from $p=0.12$ to $p=0.24$, both in the superconducting state and in the magnetic field-induced normal state. We obtain the electronic residual linear term $\kappa_0/T$ as $T \to 0$ across the pseudogap critical point $p^{\star}= 0.23$. In the normal state, we observe an abrupt drop in $\kappa_0/T$ upon crossing below $p^{\star}$, consistent with a drop in carrier density $n$ from $1 + p$ to $p$, the signature of the pseudogap phase inferred from the Hall coefficient. A similar drop in $\kappa_0/T$ is observed at $H=0$, showing that the pseudogap critical point and its signatures are unaffected by the magnetic field. In the normal state, the Wiedemann-Franz law, $\kappa_0/T=L_0/\rho(0)$, is obeyed at all dopings, including at the critical point where the electrical resistivity $\rho(T)$ is $T$-linear down to $T \to 0$. We conclude that the non-superconducting ground state of the pseudogap phase at $T=0$ is a metal whose fermionic excitations carry heat and charge as conventional electrons do.

cond-mat.supr-con↗

arXiv2018

Thermodynamic signatures of quantum criticality in cuprates

The three central phenomena of cuprate superconductors are linked by a common doping $p^{\star}$, where the enigmatic pseudogap phase ends, around which the superconducting phase forms a dome, and at which the resistivity exhibits an anomalous linear dependence on temperature as $T \to 0$. However, the fundamental nature of $p^{\star}$ remains unclear, in particular whether it marks a true quantum phase transition. We have measured the specific heat $C$ of the cuprates Eu-LSCO and Nd-LSCO at low temperature in magnetic fields large enough to suppress superconductivity, over a wide doping range across $p^{\star}$. As a function of doping, we find that the electronic term $C_{\rm el}$ is strongly peaked at $p^{\star}$, where it exhibits a $-T$log$T$ dependence as $T \to 0$. These are the classic signatures of a quantum critical point, as observed in heavy-fermion and iron-based superconductors where their antiferromagnetic phase ends. We conclude that the pseudogap phase of cuprates ends at a quantum critical point, whose associated fluctuations are most likely involved in the $d$-wave pairing and the anomalous scattering.

cond-mat.supr-con↗

arXiv2017

Magnetic and thermodynamic properties of Cu$_x$TiSe$_2$ single crystals

We present a detailed study of the phase diagram of copper intercalated TiSe$_2$ single crystals, combining local Hall-probe magnetometry, tunnel diode oscillator technique (TDO), specific-heat, and angle-resolved photoemission spectroscopy measurements. A series of the Cu$_x$TiSe$_2$ samples from three different sources with various copper content $x$ and superconducting critical temperatures $T_c$ have been investigated. We first show that the vortex penetration mechanism is dominated by geometrical barriers enabling a precise determination of the lower critical field, $H_{c1}$. We then show that the temperature dependence of the superfluid density deduced from magnetic measurements (both $H_{c1}$ and TDO techniques) clearly suggests the existence of a small energy gap in the system, with a coupling strength $2\Delta_s \sim [2.4-2.8]k_BT_c$, regardless of the copper content, in puzzling contradiction with specific heat measurements which can be well described by one single large gap $2\Delta_l \sim [3.7-3.9]k_BT_c$. Finally, our measurements reveal a non-trivial doping dependence of the condensation energy, which remains to be understood.

cond-mat.supr-con↗

arXiv2017

Anisotropy of the Seebeck Coefficient in the Cuprate Superconductor YBa$_{2}$Cu$_{3}$O$_{y}$: Fermi-Surface Reconstruction by Bidirectional Charge Order

The Seebeck coefficient $S$ of the cuprate YBa$_{2}$Cu$_{3}$O$_{y}$ was measured in magnetic fields large enough to suppress superconductivity, at hole dopings $p = 0.11$ and $p = 0.12$, for heat currents along the $a$ and $b$ directions of the orthorhombic crystal structure. For both directions, $S/T$ decreases and becomes negative at low temperature, a signature that the Fermi surface undergoes a reconstruction due to broken translational symmetry. Above a clear threshold field, a strong new feature appears in $S_{\rm b}$, for conduction along the $b$ axis only. We attribute this feature to the onset of 3D-coherent unidirectional charge-density-wave modulations seen by x-ray diffraction, also along the $b$ axis only. Because these modulations have a sharp onset temperature well below the temperature where $S/T$ starts to drop towards negative values, we infer that they are not the cause of Fermi-surface reconstruction. Instead, the reconstruction must be caused by the quasi-2D bidirectional modulations that develop at significantly higher temperature.

cond-mat.supr-con↗

arXiv2017

Puzzling evidence for surface superconductivity in the layered dichalcogenide Cu$_{10\%}$TiSe$_2$

We report on specific heat and magnetotransport measurements performed on superconducting Cu$_{10\%}$TiSe$_2$ single crystals. We show that superconductivity persists in transport measurements up to magnetic fields $H_R$ well above the upper critical field $H_{c2}$ deduced from the calorimetric measurements. Surprisingly this "surface" superconductivity is present for all magnetic field orientations, either parallel or perpendicular to the layers. For $H\|ab$, the temperature dependence of the $H_R/H_{c2}$ ratio can be well reproduced by solving the Ginzburg-Landau equations in presence of a surface layer with reduced superconducting properties. Unexpectedly this temperature dependence does not depend on the field orientation.

cond-mat.supr-con↗

arXiv2016

Fermi-surface transformation across the pseudogap critical point of the cuprate superconductor La$_{1.6-x}$Nd$_{0.4}$Sr$_{x}$CuO$_4$

The electrical resistivity $\rho$ and Hall coefficient R$_H$ of the tetragonal single-layer cuprate Nd-LSCO were measured in magnetic fields up to $H = 37.5$ T, large enough to access the normal state at $T \to 0$, for closely spaced dopings $p$ across the pseudogap critical point at $p^\star = 0.235$. Below $p^\star$, both coefficients exhibit an upturn at low temperature, which gets more pronounced with decreasing $p$. Taken together, these upturns show that the normal-state carrier density $n$ at $T = 0$ drops upon entering the pseudogap phase. Quantitatively, it goes from $n = 1 + p$ at $p = 0.24$ to $n = p$ at $p = 0.20$. By contrast, the mobility does not change appreciably, as revealed by the magneto-resistance. The transition has a width in doping and some internal structure, whereby R$_H$ responds more slowly than $\rho$ to the opening of the pseudogap. We attribute this difference to a Fermi surface that supports both hole-like and electron-like carriers in the interval $0.2 < p < p^\star$, with compensating contributions to R$_H$. Our data are in excellent agreement with recent high-field data on YBCO and LSCO. The quantitative consistency across three different cuprates shows that a drop in carrier density from $1 + p$ to $p$ is a universal signature of the pseudogap transition at $T=0$. We discuss the implication of these findings for the nature of the pseudogap phase.

cond-mat.supr-con↗

arXiv2015

Critical Doping for the Onset of Fermi-Surface Reconstruction by Charge-Density-Wave Order in the Cuprate Superconductor La$ _{2-x} $Sr$_{x} $CuO$ _{4}$

The Seebeck coefficient $S$ of the cuprate superconductor La$ _{2-x} $Sr$_{x} $CuO$ _{4}$ (LSCO) was measured in magnetic fields large enough to access the normal state at low temperatures, for a range of Sr concentrations from $x = 0.07$ to $x = 0.15$. For $x = 0.11$, 0.12, 0.125 and 0.13, $S/T$ decreases upon cooling to become negative at low temperatures. The same behavior is observed in the Hall coefficient $R_{H}(T)$. In analogy with other hole-doped cuprates at similar hole concentrations $p$, the negative $S$ and $R_{H}$ show that the Fermi surface of LSCO undergoes a reconstruction caused by the onset of charge-density-wave modulations. Such modulations have indeed been detected in LSCO by X-ray diffraction in precisely the same doping range. Our data show that in LSCO this Fermi-surface reconstruction is confined to $0.085 < p < 0.15$. We argue that in the field-induced normal state of LSCO, charge-density-wave order ends at a critical doping $p_{\rm CDW} = 0.15 \pm 0.005$, well below the pseudogap critical doping $p^\star \simeq 0.19$.

cond-mat.supr-con↗

arXiv2015

Wiedemann-Franz law in the underdoped cuprate superconductor YBa2Cu3Oy

The recent detection of charge-density modulations in YBa2Cu3Oy and other cuprate superconductors raises new questions about the normal state of underdoped cuprates. In one class of theories, the modulations are intertwined with pairing in a dual state, expected to persist up to high magnetic fields as a vortex liquid. In support of such a state, specific heat and magnetisation data on YBa2Cu3Oy have been interpreted in terms of a vortex liquid persisting above the vortex-melting field Hvs at T = 0. Here we report high-field measurements of the electrical and thermal Hall conductivities in YBa2Cu3O6.54 that allow us to probe the Wiedemann-Franz law, a sensitive test of the presence of superconductivity in a metal. In the T = 0 limit, we find that the law is satisfied for fields immediately above Hvs. This rules out the existence of a vortex liquid and it places strict constraints on the nature of the normal state in underdoped cuprates.

cond-mat.supr-con↗