High-quality polycrystalline SmFeAsO0.75 was synthesized with a superconducting transition width less than 1 K, and the electronic behavior was systematically studied by transport and specific heat measurements. An obvious superconducting jump was witnessed, together with a very small normalized superconducting jump, which is much smaller than expected by the BCS theory. A strong temperature dependent Hall coefficient was found and attributed to the partial gapping of the Fermi surface up to the temperature of 160 K which was predicted and supported by the emergence of the pseudogap. The charge-carrier density as well as the effective mass were also obtained and discussed in detail.
In their comment Calandra \textit{et al} \cite{Calandra}, assert two points: (1) the estimate of charge transfer from Li to graphene layers in LiC$_6$ in our letter \cite{Pan2011c} is incorrect because of the three dimensional (3D) character of the electronic structure in bulk LiC$_6$; (2) our main claim that the superconductivity in graphite intercalation compounds (GICs) is graphene-sheet-driven is therefore invalid.
Raman spectroscopy of a cobalt-doped iron-pnictide superconductor reveals the complex electronic structure of the superconducting state in this material. [Viewpoint on Phys. Rev. B 80, 180510 (2009), arXiv:0910.0898]
We have performed photoemission studies of the electronic structure in LiC$_6$ and KC$_8$, a non-superconducting and a superconducting graphite intercalation compound, respectively. We have found that the charge transfer from the intercalant layers to graphene layers is larger in KC$_8$ than in LiC$_6$, opposite of what might be expected from their chemical composition. We have also measured the strength of the electron-phonon interaction on the graphene-derived Fermi surface to carbon derived phonons in both materials and found that it follows a universal trend where the coupling strength and…
We have measured the superconducting critical temperature (Tc) and the diamagnetic susceptibility of La2-xSrxCuO4 single crystals in various magnetic fields. We observed a field-induced evolution from an apparent Tc phase to an intrinsic Tc1 = 15 K or Tc2 = 30 K phase characterized by "magic" hole concentration which is commensurate with that of a two dimensional electronic lattice. The onset Tc of the intrinsic superconducting phases remains robust up to H = 5 Tesla. We suggest that the intrinsic superconducting phases at "magic" doping concentrations are the pristine electronic phases of hig…
The voltage-carrying state of superconducting NbTiN nanowires, used for single-photon detectors, is analyzed. Upon lowering the current, the wire returns to the superconducting state in a steplike pattern, which differs from sample to sample. Elimination of geometrical inhomogeneities, such as sharp corners, does not remove these steplike features. They appear to be intrinsic to the material. Since the material is strongly disordered, electronic inhomogeneities are considered as a possible cause. A thermal model, taking into account random variations of the electronic properties along the wire…
We study coherent quantum transport through a superconducting film connected to normal-metal electrodes. Simple expressions for the differential conductance and the local density of states are obtained in the clean limit and for transparent interfaces. Quasiparticle interference causes periodic vanishing of the Andreev reflection at the energies of geometrical resonances, subgap transport, and gapless superconductivity near the interfaces. Application of the results to spectroscopic measurements of the superconducting gap and the Fermi velocity is analyzed.
A new concept for nonvolatile superconducting memories is proposed. The devices combine ferromagnetic dots for the storage of the data and Josephson junctions for their readout. Good scalability is expected for large scale integration. First memory cells have been implemented using 3 $μ$m-Nb-technology and permalloy dots. Nonvolatile data storage at 300 K was demonstrated.
We study the field-angle resolved electronic Raman scattering in 2-dimensional d-wave superconducting vortex states theoretically by quasi-classical approximation, the so-called Doppler-shift method. An analytic expression is obtained for the field angle dependence of the Raman scattering amplitude at zero temperature. After numerical integration, we obtain the scattering intensity for various field angles by changing the Raman shift energy. Field-angle resolved electronic Raman scattering turns out to be an effective method for probing unconventional superconducting gap structures. It shows a…
This paper gives a detailed numerical study of the superconducting electronic heat transport in the unconventional multiband superconductor Strontium Ruthenate Sr$_2$RuO$_4$. The study demostrates that a model with different nodal structures on different sheets of the Fermi surface is able to describe quantitatively experimental heat transport data. The contribution of the density of states DOS is given for each sheet of the Fermi surface and the total contribution is also calculated. Finally, a discussion of the universal character of the electronic heat transport in unconventional supercondu…
Thermo-electric transport at the nano-scale is a rapidly developing topic, in particular in superconductor-based hybrid devices. In this review paper, we first discuss the fundamental principles of electronic cooling in mesoscopic superconducting hybrid structures, the related limitations and applications. We review recent work performed in Grenoble on the effects of Andreev reflection, photonic heat transport, phonon cooling, as well as on an innovative fabrication technique for powerful coolers.
We report electronic structure calculations for Ta$_2$PdS$_5$, which is a layered superconductor containing heavy elements and displaying an upper critical field, $H_{c2}(0)$, $\sim$3 times higher than the estimated Pauli limit. We show that this is a multiband superconductor that is most likely in the strong coupling regime. This provides an alternative explanation to strong spin orbit scattering for the high upper critical field.
Recently we have proposed a s+g-wave model for the superconductivity in borocarbides, YNi2B2C and LuNi2B2C. In the present paper we first summarize thermodynamic properties of s+g-wave model. Then we shall analyse the recent Raman spectra data of RNi2B2C (with R=Lu and Y) by Yang et al. The present model appears to describe salient features of the Raman spectra.
DFT simulation based ab-initio approach has been executed for investigating the comparative study of the physical properties of superconducting disilicide materials YT$_2$Si$_2$ (T= Co, Ni, Ru, Rh, Pd, Ir). This is the first comparative theoretical investigation of these materials, which is done through Cambridge Serial Total Energy Package module.
Our recent year's studies of the prototypal FeSe and molecule-intercalated (Li,Fe)OHFeSe superconductor systems are briefly reviewed here, with emphasis on experimental observations of the link between the superconductivity and normal-state electronic property in the single crystals and films. These samples were successfully synthesized by our recently developed soft-chemical hydrothermal methods, which are also briefly described. Particularly in the Mn-doped high-Tc (Li,Fe)OHFeSe film, a strong enhancement of the superconducting critical current density was achieved, which is promising for pr…
We have performed temperature (T) - dependent laser-photoemission spectroscopy of antiferromagnetic (AF) superconductor ErNi2B2C to study the electronic-structure evolution reflecting the interplay between antiferromagnetism and superconductivity. The spectra at the superconducting (SC) phase show a very broad spectral shape. T-dependent SC gap shows a sudden deviation from the BCS prediction just below TN. This observation can be well explained by the theoretical model and thus represents characteristic bulk electronic structure of the AF SC phase for the first time.
We report the electrical transport in vertical Josephson tunnel junctions (area 400 $μm$$^2$) using GdBa$_2$Cu$_3$O$_7$$_{-δ}$ electrodes and SrTiO$_3$ as an insulating barrier (with thicknesses between 1 nm and 4 nm). The results show Josephson coupling for junctions with SrTiO$_3$ barriers of 1 nm and 2 nm. The latter displays a Josephson of 8.9 mV at 12 K. This value is larger than the usually observed in planar arrays of junctions. Our results are promising for the development of superconducting electronic devices in the terahertz regime.
By means of DFT-based first-principles calculations, we examine two polymorphs of the newly synthesized 1111-like MgFeSeO as possible new superconducting systems. We have found that the polymorph with blocks [MgO], where Mg atoms are placed in the centers of O4 tetrahedra, is dynamically unstable - unlike the ZrCuSiAs-type polymorph with oxygen atoms placed in the centers of Mg4 tetrahedra. The characterization of this material covers the structural, elastic properties, electronic band structure, density of electronic states, and Fermi surface. Our calculations suggest that a high critical tem…
We study electronic properties of a superconducting topological insulator whose parent material is a topological insulator. We calculate the temperature dependence of the specific heat and spin susceptibility for four promising superconducting pairings proposed by L. Fu and E. Berg (Phys. Rev. Lett. 105, 097001). Since the line shapes of temperature dependence of specific heat are almost identical among three of the four pairings, it is difficult to identify them simply from the specific heat. On the other hand, we obtain wide varieties of the temperature dependence of spin susceptibility for …
We present details of carrier properties in high quality ${\rm Ba(Fe_{1-x}Co_{x})_2As_2}$ single crystals obtained from electronic Raman scattering. The experiments indicate a strong band and momentum anisotropy of the electron dynamics above and below the superconducting transition highlighting the importance of complex band-dependent interactions. The presence of low energy spectral weight deep in the superconducting state suggests a gap with accidental nodes which may be lifted by doping and/or impurity scattering. When combined with other measurements, our observation of band and momentum …
Recent scanning tunneling microscopy on BSCCO 2212 has revealed a substantial spatial supermodulation of the energy gap in the superconducting state. We propose that this gap modulation is due to the superlattice modulations of the atoms in the structure, and hence the parameters in a microscopic model of the CuO2 plane. The gap modulation is estimated using renormalized mean field theory for a t-t'-J model on a superlattice. The results compare well with experiment.
Electronic structure of newly synthesized single crystals of calcium iron arsenide doped with sodium with Tc ranging from 33 to 14 K has been determined by angle-resolved photoemission spectroscopy (ARPES). The measured band dispersion is in general agreement with theoretical calculations, nonetheless implies absence of Fermi surface nesting at antiferromagnetic vector. A clearly developing below Tc strongly band-dependant superconducting gap has been revealed for samples with various doping levels. BCS ratio for optimal doping, $2Δ/k_{\rm B}T_{\rm c}=5.5$, is substantially smaller than the nu…
First principle FLAPW-GGA calculations have been performed with the purpose to understand the peculiarities of band structure and Fermi surface topology for recently discovered superconductor: Sr4V2Fe2As2O - in comparison with isostructural phase Sr4Sc2Fe2As2O6. Our main finding is that the replacement of Sc on vanadium leads to drastic transformation of electronic, magnetic and conductive properties of these materials: as against non-magnetic Sr4Sc2Fe2As2O6 which is formed from non-magnetic conducting [Fe2As2] and insulating [Sr4Sc2O6] blocks, Sr4V2Fe2As2O6 consists from non-magnetic conducti…
High-$T_c$ Berezinskii-Kosterlitz-Thouless transition in 2D superconducting systems with coupled deep and quasi-flat electronic bands with van Hove singularities
The electronic structure of the giant vortex states in a mesoscopic superconducting disk is studied in a dirty limit using the Usadel approach. The local density of states profiles are shown to be strongly affected by the effect of quasiparticle (QP) tunneling between the states localized in the vortex core and the ones bound to the sample edge. Decreasing temperature leads to a crossover between the edge-dominated and core-dominated regimes in the magnetic field dependence of the tunneling conductance. This crossover is discussed in the context of the efficiency of quasiparticle cooling by th…
Precursor MgB2 thin films were prepared on sapphire substrates by magnetron sputtering. Influence of ex-situ annealing process on superconducting MgB2 thin films roughness is discussed. Optimized annealing process of MgB precursor thin films in vacuum results in smooth superconducting MgB2 thin films with roughness below 10 nm, critical temperature Tcon = 31 K and transition width DTc less than 1 K. Nano-bridges based on the superconducting MgB2 thin films using optical and Focused Ion Beam lithography were prepared. Critical current density jc (4.2 K) measured on 50 nm wide strip was 7.3x106 …
We report the electronic structure and related properties of the superconductor Ta2PdSe5 as determined from density functional calculations. The Fermi surface has two disconnected sheets, both derived from bands of primarily chalcogenide p states. These are a corrugated hole cylinder and and a heavier complex shaped electron sheet. The sheets contain 0.048 holes and a compensating number of electrons per formula unit, making the material a semimetallic superconductor. The results support the presence of two band superconductivity, although a discrepancy in the specific heat is noted. This disc…
We study the surface state of a doped topological crystalline insulator in the superconducting state. Motivated by Sn$_{1-x}$In$_x$Te, we consider fully gapped pair potentials and calculate the surface spectral function. It is found that mirror-protected zero-energy Andreev bound states appear at the (001) surface and that these states can move along the mirror symmetric line on the surface Brillouin zone. We also show that the surface Andreev bound state changes systematically with doping due to the presence of the Dirac surface state in the normal state.
We present in this paper the conductance maps at 100 mK in the disordered polycrystalline film of titanium nitride (TiN). At 5 nm, the film is close to quasi-two dimensional limit and exhibits features pertaining to the superconductor to insulator transition. We measured conductance maps at zero field and at 4 T, which represent the superconducting and the normal phase, respectively. The conductance map at 4 T is uniform, in which the conductance behavior, with logarithmic variation, resembles to the disorder enhanced electron-electron interaction in the two dimensional metallic phase. At low …
Although correlated electronic-structure calculations explain very well the normal state of Sr$_2$RuO$_4$, its superconducting symmetry is still unknown. Here we construct the spin and charge fluctuation pairing interactions based on its correlated normal state. Correlations significantly reduce ferromagnetic in favor of antiferromagnetic fluctuations and increase inter-orbital pairing. From the normal-state Eliashberg equations, we find spin-singlet $d$-wave pairing close to magnetic instabilities. Away from these instabilities, where charge fluctuations increase, we find two time-reversal sy…
We explored by electronic Raman scattering the superconducting state of Bi-2212 single crystal by performing a fine tuned doping study. We found three distinct energy scales in A1g, B1g and B2g symmetries which show three distinct doping dependencies. Above p=0.22 the three energies merge, below p=0.12, the A1g scale is no more detectable while the B1g and B2g scales become constant in energy. In between, the A1g and B1g scales increase monotonically with under-doping while the B2g one exhibits a maximum at p=0.16. The three superconducting energy scales appear to be an universal feature of ho…
NMR spectra and nuclear spin-lattice relaxation rate $T_1^{-1}$ of $^{11}$B have been measured in superconducting polycrystalline MgB$_{2}$ with $T_C^{ons}\cong 39.5K$. It is shown that $(T_{1}T)^{-1}$ and the Knight shift $K_{s}$ are independent of temperature and nearly isotropic above $T_C$. Both of these quantities are decreased gradually in going to superconducting state. According NMR data the density of states (DOS) near the Fermi level is flat at the scale $\approx$ 500K. Some conclusions on the orbital content of the DOS at the Fermi level was done and compared with the results of the…
The electronic Raman scattering has been investigated in optimally oxygen doped YBa$_{2}$Cu$_{3}$O$_{7-δ}$ single crystals as well as in crystals with non-magnetic, Zn, and magnetic, Ni, impurities. We found that the intensity of the A$_{1g}$ peak is impurity independent and their energy to $T_{c}$ ratio is almost constant ($2Δ/k_{B}T_{c}\sim5$). Moreover, the signal at the B$_{1g}$ channel is completely smeared out when non-magnetic Zn impurities are present. These results are qualitatively interpreted in terms of the Zeyher and Greco's theory that relates the electronic Raman scattering in t…
Due to the large spin-orbital coupling in the layered 5d-transition metal chalcogenides compound, the occurrence of superconductivity in Ir2-xPdxTe2 offers a good chance to search for possible topological superconducting states in this system. We did comprehensive studies on the superconducting properties and electronic structures of single crystalline Ir0.95Pd0.05Te2 samples. The superconducting gap size, critical fields and coherence length along different directions were experimentally determined. Macroscopic bulk measurements and microscopic low temperature scanning tunneling spectroscopy …
Pure electronic Raman spectra with no phonon structures superimposed to the electronic continuum, are reported, in optimally doped HgBa_{2}CaCu_{2}O_{6+δ} single crystals (T_{c }=126 K). As a consequence, the spectra in the A_{1g }, B_{1g } and B_{2g } symmetries, including the crucial low energy frequency dependence of the electronic scattering, are directly and reliably measured. The B_{2g } and, most strikingly, the B_{1g } spectra exhibit a strong intrinsic linear term, which suggests that the nodes are shifted from the [110] and [1\bar{1}0] directions, a result inconsistent with a pure d_…
We discuss very low temperature experiments on superconducting micro-coolers made of a double Normal metal - Insulator - Superconductor junction. We investigate with a high resolution the differential conductance of the micro-cooler as well as of additional probe junctions. There is an explicit crossover between the single quasi-particle current and the phase-coherent Andreev current. We establish a thermal model by considering the thermal contribution due to the Andreev current. The related increase of the electron temperature is discussed, including the influence of several parameters like t…
Motivated by the recent discovery of superconductivity on the heterointerface $LaAlO_{3}/SrTiO_{3}$, we theoretically investigate its local electronic structures near an impurity considering the influence of Rashba-type spin-orbit interaction (RSOI) originated in the lack of inversion symmetry. We find that local density of states near an impurity exhibits the in-gap resonance peaks due to the quasiparticle scattering on the Fermi surface with the reversal sign of the pairing gap caused by the mixed singlet and RSOI-induced triplet superconducting state. We also analyze the evolutions of densi…
The electronic structures of the ground state for several different superconducting materials, such as cuprates, conventional 3-dimensional superconductors, doped semiconductors and low-dimensional systems, are quite different and sometimes in contrast to what is supposed to make a superconductor. Properties like the Fermi-surface (FS) topology, density-of-states (DOS), stripes, electron-phonon coupling ($λ_{ep}$) and spin fluctuations ($λ_{sf}$) are analyzed in order to find clues to what might be important for the mechanism of superconductivity. A high DOS at $E_F$ is important for standard …
We investigated the annealing effects on the physical properties of SrFe$_2$(As$_{1-x}$P$_x$)$_2$ ($x=0.35$). The superconducting transition temperature ($T_c$) increased from 26 K to 33 K by annealing. The X-ray diffraction measurement suggested that the annealed crystals have the shorter/longer $a/c$-axes and the larger pnictogen height $h_\mathrm{Pn}$. This must be linked to the $T_c$-enhancement by annealing. Moreover, it was found that the post-annealing decreased the electronic specific heat coefficient at $T$=0 K, $γ_r$, and changed the magnetic field ($H$) dependence from sub-linear $γ…
Polarisation-resolved Raman scattering measurements were performed on MgB_2 single crystals to determine the magnitude, symmetry and temperature dependence of the superconducting gap. A single sharp peak due to Cooper pair breaking appears in the electronic continuum below Tc, reaching an average maximum Raman shift of 105 +/- 1 cm^{-1} (2 Delta(0) / k_{B}Tc = 3.96 +/- 0.09) and showing up to 5 cm^{-1} anisotropy between polarised and depolarised spectra. The temperature dependence of 2 Delta follows that predicted from BCS theory, while the anisotropy decreases with decreasing temperature. It…
We review electronic transport in superconducting junctions with Sr$_2$RuO$_4$. Transport measurements provide evidence for chiral domain walls and, therefore, chiral superconductivity in superconducting Sr$_2$RuO$_4$, but so far, the symmetry of the underlying superconducting state remains inconclusive. Further studies involving density of states measurements and spin-polarised transport in local/non--local Sr$_2$RuO$_4$ junctions with magnetic materials could lead to fundamental discoveries and a better understanding of the superconducting state.
A simple theoretical model is proposed to describe the recent experimental results on formation of induced superconducting state and anomalous tunneling characteristics in selectively doped multilayered nanostructures based on La$_2$CuO$_4$ perovskite. In particular, it is shown that the structure composed from the nominally non-superconducting (undoped and overdoped) layers turns to be superconducting with superconductivity confined to narrow regions near the interfaces, in agreement with the experimental observations.
We report electronic structure calculations for the layered centrosymmetric superconductor LaNiGa$_2$, which has been identified as having a possible triplet state based on evidence for time reversal symmetry breaking. The Fermi surface has several large sheets and is only moderately anisotropic, so that the material is best described as a three dimensional metal. These include sections that are open in the in-plane direction as well as a section that approaches the zone center. The density of states is high and primarily derived from Ga $p$ states, which hybridize with Ni $d$ states. Comparin…
The discoveries of superconductivity in the heavily-boron doped semiconductors diamond (C:B) in 2004 and silicon (Si:B) in 2006 have renewed the interest in the physics of the superconducting state of doped semiconductors. Recently, we discovered superconductivity in the closely related ''mixed'' system heavily boron-doped silcon carbide (SiC:B). Interestingly, the latter compound is a type-I superconductor whereas the two aforementioned materials are type-II. In this paper we present an extensive analysis of our recent specific-heat study, as well as the band structure and expected Fermi surf…
Using high-resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy, the atomic and low energy electronic structure of the Sr-doped superconducting topological insulators (SrxBi2Se3) was studied. Scanning tunneling microscopy shows that most of the Sr atoms are not in the van der Waals gap. After Sr doping, the Fermi level was found to move further upwards when compared with the parent compound Bi2Se3, which is consistent with the low carrier density in this system. The topological surface state was clearly observed, and the position of the Dirac poin…
The electronic structure and superconducting gap structure are prerequisites to establish microscopic theories in understanding the superconductivity mechanism of iron-based superconductors. However, even for the most extensively studied optimally-doped (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$, there remain outstanding controversies on its electronic structure and superconducting gap structure. Here we resolve these issues by carrying out high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements on the optimally-doped (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$ superconductor using both He…
Electronic Raman scattering measurements have been performed on $HgBa_{2}Ca_{2}Cu_{3}O_{8+δ} $ single crystals in the superconducting state. Pure electronic Raman spectra with no phonon structures hindering the analysis of the electronic continuum have been obtained. As a consequence, the spectra in the pure $B_{1g} $ and $B_{2g} $ symmetries are directly and reliably analyzed and the pure $A_{1g} $ contribution can be easily identified. Below the critical temperature $T_{c}, $ two electronic structures at $2Δ\sim 6.4 k_{B}T_{c} $ and $2Δ\sim 9.4 $ $k_{B}T_{c} $ are clearly seen. Both are obse…
Electronic structures of orthorhombic ternary nickel silicides: superconducting Lu2Ni3Si5 and its non-superconducting counterpart, Y2Ni3Si5, have been calculated employing the fully-relativistic and full-potential local-orbital method within the density functional theory. Our investigations were focused particularly on the band structures and Fermi surfaces (FSs), being very similar for both ternaries. It appears that their FSs exist in four bands and contain electronlike and holelike three-dimensional sheets and small pockets, which suggests a presence of two- or even multi-band superconducti…
We have investigated the influence of Fe-excess on the electrical transport and magnetism of Fe1+yTe0.5Se0.5 (y=0.04 and 0.09) single crystals. Both compositions exhibit resistively determined superconducting transitions (Tc) with an onset temperature of about 15 K. From the width of the superconducting transition and the magnitude of the lower critical field Hc1, it is inferred that excess of Fe suppresses superconductivity. The linear and non-linear responses of the ac-susceptibility show that the superconducting state for these compositions is inhomogeneous. A possible origin of this phase …
We construct minimal electronic models for a newly discovered superconductor LaO$_{1-x}$F$_x$BiS$_2$ ($T_c=$ 10.6K) possessing BiS$_2$ layers based on first principles band calculation. First, we obtain a model consisting of two Bi $6p$ and two S $3p$ orbitals, which give nearly electron-hole symmetric bands. Further focusing on the bands that intersect the Fermi level, we obtain a model with two $p$ orbitals. The two bands (per BiS$_2$ layer) have quasi-one-dimensional character with a double minimum dispersion, which gives good nesting of the Fermi surface. At around $x\sim 0.5$ the topology…
Cooper's formalism for fermionic pairing has been revisited considering upto 3rd neighbour hopping terms, firstly with a Fermi liquid like background on a square lattice keeping in mind the overdopped cuprates. Then the whole scheme is repeated with a Marginal Fermi liquid- like background, taking into account the self energy correction of the mobile electrons to include a more realitic density of states in the calculation. Detailed comparison of our theoretical results with those from experiments strongly supports the Marginal Fermi liquid- like character of the normal phase with exciton medi…
The type II Dirac semimetal PdTe$_2$ is unique in the family of topological parent materials because it displays a superconducting ground state below 1.7 K. Despite wide speculations on the possibility of an unconventional topological superconducting phase, tunneling and heat capacity measurements revealed that the superconducting phase of PdTe$_2$ follows predictions of the microscopic theory of Bardeen, Cooper and Shriefer (BCS) for conventional superconductors. The superconducting phase in PdTe$_2$ is further interesting because it also displays properties that are characteristics of type-I…
The electronic Raman scattering in overdoped (Y,Ca)Ba2Cu3Oy was investigated with changing hole concentration in the superconducting state. It was found that the superconducting responses such as the pair-breaking peaks in the A1g and B1g spectra and the anisotropy of the pair-breaking peak in XX and YY polarizations radically change at around the carrier doping p=0.19. Since both a- and c-axis resistivities strongly suggest the closing of pseudogap at p~0.18, the observed change at p=0.19 in superconducting Raman response is attributed to the electronic crossover due to the collapse of the ps…
The performance of hybrid superconducting electronic coolers is usually limited by the accumulation of hot quasi-particles in the superconducting leads. This issue is all the more stringent in large-scale and high-power devices, as required by applications. Introducing a metallic drain connected to the superconducting electrodes via a fine-tuned tunnel barrier, we efficiently remove quasi-particles and obtain electronic cooling from 300 mK down to 130 mK with a 400 pW cooling power. A simple thermal model accounts for the experimental observations.
The microscopic nature of an insulating state in the vicinity of a superconducting state, in the presence of disorder, is a hotly debated question. While the simplest scenario proposes that Coulomb interactions destroy the Cooper pairs at the transition, leading to localization of single electrons, an alternate possibility supported by experimental observations suggests that Cooper pairs instead directly localize. The question of the homogeneity, granularity, or possibly glassiness of the material on the verge of this transition is intimately related to this fundamental issue. Here, by combini…
Thermal conductivity of superconducting MgB2 was studied in both the superconducting and the normal state region. The latter is almost equally determined by the electronic - and the lattice contribution to the total thermal conductivity. In the superconducting state, however, the lattice contribution is larger. The electronic thermal conductivity below Tc was derived from the experimental data considering the Bardeen-Rickayzen-Tewordt theory together with the model of Geilikman. The analysis shows that electron scattering on static imperfections dominates.
Superconducting and normal state transport properties in iron pnictides are sensitive to disorder and impurity scattering. By investigation of Ba(Fe1-xCox)2As2 thin films with varying Co concentration, we demonstrate that in the dirty limit the superconducting dome in the electronic phase diagram of Ba(Fe1-xCox)2As2 shifts towards lower doping concentrations, which differs significantly from observations in single crystals. We show that especially in the underdoped regime superconducting transition temperatures higher than 27 K are possible.
We report a detailed study of the electronic and structural properties of the 39K superconductor \mgbtwo and of several related systems of the same family, namely \mgalbtwo, \bebtwo, \casitwo and \cabesi. Our calculations, which include zone-center phonon frequencies and transport properties, are performed within the local density approximation to the density functional theory, using the full-potential linearized augmented plane wave (FLAPW) and the norm-conserving pseudopotential methods. Our results indicate essentially three-dimensional properties for these compounds; however, strongly two-…
