The three central phenomena of cuprate (copper oxide) superconductors are linked by a common doping level p*-at which the enigmatic pseudogap phase ends and the resistivity exhibits an anomalous ...linear dependence on temperature, and around which the superconducting phase forms a dome-shaped area in the phase diagram
. However, the fundamental nature of p* remains unclear, in particular regarding whether it marks a true quantum phase transition. Here we measure 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
that includes p*. As a function of doping, we find that C
/T is strongly peaked at p* (where C
is the electronic contribution to C) and exhibits a log(1/T) dependence as temperature T tends to zero. These are the classic thermodynamic signatures of a quantum critical point
, as observed in heavy-fermion
and iron-based
superconductors at the point where their antiferromagnetic phase comes to an end. We conclude that the pseudogap phase of cuprates ends at a quantum critical point, the associated fluctuations of which are probably involved in d-wave pairing and the anomalous scattering of charge carriers.
The recently discovered superconductorUTe2, with a superconducting transition temperatureTcbetween 1.5 and 2 K, is attracting much attention due to strong suspicion of spin-triplet and topological ...superconductivity. Its properties under magnetic field are also remarkable, with field-reinforced (H∥b) and field-induced Hin the(b,c)plane superconducting phases. Here, we report the first complete thermodynamic determination of the phase diagram for fields applied along the three crystallographic directions. For field along the easyaaxis, we uncover a strong negative curvature of the upper critical field very close toTc, revealing a strong suppression of the pairing strength at low magnetic fields. By contrast, measurements performed up to 36 T along the hard magnetizationbaxis confirm a bulk field-reinforced superconducting phase. Most of all, they also reveal the existence of a phase transition line within the superconducting phase. Drastic differences occur between the low-field and high-field phases pointing to different pairing mechanisms. Detailed analysis suggests a possible transition between a low-field spin-triplet to high-field spin-singlet state, a unique case among superconductors.
The recent discovery of a charge order in underdoped YBa2Cu3Oy raised the question of the interplay between superconductivity and this competing phase. Understanding the normal state of ...high-temperature superconductors is now an essential step towards the description of the pairing mechanism in those materials and determining the upper critical field is therefore of fundamental importance. We present here a calorimetric determination of the field-temperature phase diagram in underdoped YBa2Cu3Oy single crystals. We show that the specific heat saturates in high magnetic fields. This saturation is consistent with a normal state without any significant superconducting contribution and a total Sommerfeld coefficient γN∼6.5±1.5 mJ mol(-1) K(-2) putting strong constraints on the theoretical models for the Fermi surface reconstruction.
We report the growth of high quality FeSe single crystals using chemical vapor transport based on an AlCl3 KCl eutectic and their physical properties were fully characterized by magnetic, transport ...and specific heat measurements. Their critical superconducting temperature at Tc ∼ 8.7 K (width < 0.7 K) and structural tetragonal-orthorhombic transition at Ts ∼ 85-90 K confirm the values already reported in the literature for similar crystals. In addition, the non-variation of Ts under high magnetic field (up to 14 T) determined by magnetic and specific heat measurements indicates that spin fluctuations are not directly involved in the structural transition. Finally, high temperature electrical resistivity measurements up to 600 K evidence a maximum around 350 K. The origin of this crossover from metallic behavior at low temperature to a semiconducting-like regime at high temperature is discussed and could be associated with a change of carrier density above 350 K. On the other hand, the tentative growth of Te-substituted Fe(Se1−xTex) crystals using the same growth method resulted in crystals with x(Te) < 1% and a slightly lower Tc compared to pure FeSe ones, which may be related to transition metal-chalcogen non-stoichiometry. Finally, Cr doping at the Fe site was also attempted. In contrast to previous reports in the literature, our results suggest that Cr does not substitute for Fe in the FeSe crystal but agglomerates in Cr-rich (Cr,Fe)Se2 inclusions, and Tc of these (Fe1−xCrx)Se crystals is not increased, but slightly decreased in comparison to stoichiometric FeSe crystals.
Homoepitaxial diamond layers doped with boron in the 10(20)-10(21) cm(-3) range are shown to be type II superconductors with sharp transitions (approximately 0.2 K) at temperatures increasing from 0 ...to 2.1 K with boron contents. The critical concentration for the onset of superconductivity in those 001-oriented single-crystalline films is about 5-7 10(20) cm(-3). The H-T phase diagram has been obtained from transport and ac-susceptibility measurements down to 300 mK.
Although the local resistivity of semiconducting silicon in its standard crystalline form can be changed by many orders of magnitude by doping with elements, superconductivity has so far never been ...achieved. Hybrid devices combining silicon's semiconducting properties and superconductivity have therefore remained largely underdeveloped. Here we report that superconductivity can be induced when boron is locally introduced into silicon at concentrations above its equilibrium solubility. For sufficiently high boron doping (typically 100 p.p.m.) silicon becomes metallic. We find that at a higher boron concentration of several per cent, achieved by gas immersion laser doping, silicon becomes superconducting. Electrical resistivity and magnetic susceptibility measurements show that boron-doped silicon (Si:B) made in this way is a superconductor below a transition temperature Tc 0.35 K, with a critical field of about 0.4 T. Ab initio calculations, corroborated by Raman measurements, strongly suggest that doping is substitutional. The calculated electron-phonon coupling strength is found to be consistent with a conventional phonon-mediated coupling mechanism. Our findings will facilitate the fabrication of new silicon-based superconducting nanostructures and mesoscopic devices with high-quality interfaces.
We have realized laser-doped all-silicon superconducting (S)/normal metal (N) bilayers of tunable thickness and dopant concentration. We observed a strong reduction of the bilayers' critical ...temperature when increasing the normal metal thickness, a signature of the highly transparent S/N interface associated to the epitaxial sharp laser doping profile. We extracted the interface resistance by fitting with the linearized Usadel equations, demonstrating a reduction of 1 order of magnitude from previous superconductor/doped Si interfaces. In this well-controlled crystalline system we exploited the low-resistance S/N interfaces to elaborate all-silicon lateral SNS junctions with long-range proximity effect. Their dc transport properties, such as the critical and retrapping currents, could be well understood in the diffusive regime. Furthermore, this work led to the estimation of important parameters in ultradoped superconducting Si, such as the Fermi velocity, the coherence length, or the electron-phonon coupling constant, fundamental to conceive all-silicon superconducting electronics.
We report on an extensive experimental and numerical study of the low-energy electronic properties of superconducting FeSe single crystals, using point-contact Andreev reflection Spectroscopy (PCAR), ...specific heat, and London penetration depth measurements. Taking explicitly into account Fermi surface anisotropy and recently suggested orbital-selective quasiparticle spectral weights Zi (i=dxy,dx2−y2,dxz,dyz,dz2), our calculations quantitatively account for all our measurements as well as data from the literature, assuming that Zyz>Zxz>Zxy. This study confirms the picture of a highly different quantum coherence of the Fe orbitals at the Fermi energy. In particular, the normal state properties (Sommerfeld coefficient and zero-temperature London penetration depth) strongly depend on the Zi values, which seem to be significantly sensitive to disorder.
We report on a detailed analysis of the transport properties and superconducting critical temperatures of boron-doped diamond films grown along the $\{100\}$ direction. The system presents a ...metal-insulator transition (MIT) for a boron concentration ($n_B$) on the order of $n_c \sim 4.5 \times 10^{20}$ cm$^{-3}$ in excellent agreement with numerical calculations. The temperature dependence of the conductivity and Hall effect can be well described by variable range hopping for $n_B n_c$) present a superconducting transition at low temperature. The zero temperature conductivity $\sigma_0$ deduced from fits to the data above the critical temperature ($T_c$) using a classical quantum interference formula scales as : $\sigma_0 \propto (n_B/n_c-1)^\nu$ with $\nu \sim 1$. Large $T_c$ values ($\geq 0.4$ K) have been obtained for boron concentration down to $n_B/n_c \sim 1.1$ and $T_c$ surprisingly mimics a $(n_B/n_c-1)^{1/2}$ law. Those high $T_c$ values can be explained by a slow decrease of the electron-phonon coupling parameter $\lambda$ and a corresponding drop of the Coulomb pseudo-potential $\mu^*$ as $n_B \rightarrow n_c$.
PDF
