Superconducting coherence length of hole-doped cuprates obtained from electron–boson spectral density function

Electron–boson spectral density functions (EBSDFs) can be obtained from measured spectra using various spectroscopic techniques, including optical spectroscopy. EBSDFs, known as glue functions, are suggested to have a magnetic origin. Here, we investigated EBSDFs obtained from the measured optical spectra of hole-doped cuprates with wide doping levels, from underdoped to overdoped cuprates. The average frequency of an EBSDF provides the timescale for the spin fluctuations to form Cooper pairs. This timescale is directly associated with retarded interactions between electrons. Using this timescale and Fermi velocity, a reasonable superconducting coherence length, which reflects the size of the Cooper pair, can be extracted. The obtained coherence lengths were consistent with those measured via other experimental techniques. Therefore, the formation of Cooper pairs in cuprates can be explained by spin fluctuations, the timescales of which appear in EBSDFs. Consequently, EBSDFs provide crucial information on the timescale of the microscopic mechanism of Cooper pair formation.

Superconducting Coherence Length of Hole-doped Cuprates Obtained From Electron-boson Spectral Density Function

Electron{boson spectral density functions (EBSDFs) can be obtained from measured spectra using various spectroscopic techniques, including optical spectroscopy. EBSDFs, known as glue functions, have a magnetic origin. Here, we investigated EBSDFs obtained from the measured optical spectra of hole-doped cuprates with wide doping levels, from underdoped to overdoped cuprates. The average frequency of an EBSDF provides the timescale for the spin fluctuations to form Cooper pairs. This timescale is directly associated with retarded interactions betweenelectrons. Using this timescale and Fermi velocity, a reasonable superconducting coherence length, which reflects the size of the Cooper pair, can be extracted. The obtained coherence lengths were consistent with those measured via other experimental techniques. Therefore, the formation of Cooper pairs in cuprates can be explained by spin fluctuations, the timescales of which appear in EBSDFs. Consequently, EBSDFs provide crucial information on the timescale of the microscopic mechanism of Cooper pair formation.

Bereziskii-Kosterlitz-Thouless transition in the Weyl system PtBi2

Symmetry breaking in topological matter became, in the last decade, a key concept in condensed matter physics to unveil novel electronic states. In this work, we reveal that broken inversion symmetry and strong spin-orbit coupling in trigonal PtBi2 lead to a Weyl semimetal band structure, with unusually robust two-dimensional superconductivity in thin fims. Transport measurements show that high-quality PtBi2 crystals are three-dimensional superconductors (Tc≈600 mK) with an isotropic critical field (Bc≈50 mT). Remarkably, we evidence in a rather thick flake (60 nm), exfoliated from a macroscopic crystal, the two-dimensional nature of the superconducting state, with a critical temperature Tc≈370 mK and highly-anisotropic critical fields. Our results reveal a Berezinskii-Kosterlitz-Thouless transition with TBKT≈310 mK and with a broadening of Tc due to inhomogenities in the sample. Due to the very long superconducting coherence length ξ in PtBi2, the vortex-antivortex pairing mechanism can be studied in unusually-thick samples (at least five times thicker than for any other two-dimensional superconductor), making PtBi2 an ideal platform to study low dimensional superconductivity in a topological semimetal.

A Josephson junction based on a highly disordered superconductor/low-resistivity normal metal bilayer

We calculate the current–phase relation (CPR) of a SN-S-SN Josephson junction based on a SN bilayer of variable thickness composed of a highly disordered superconductor (S) and a low-resistivity normal metal (N) with proximity-induced superconductivity. In such a junction, the N layer provides both a large concentration of phase in the weak link and good heat dissipation. We find that when the thickness of the S and the N layer and the length of the S constriction are about the superconducting coherence length the CPR is single-valued and can be close to a sinusoidal shape. The product I c R n can reach Δ(0)/2|e| (I c is the critical current of the junction, R n is its normal-state resistance, Δ(0) is the superconductor gap of a single S layer at zero temperature). Our calculations show, that the proper choice of the thickness of the N layer leads both to nonhysteretic current–voltage characteristics even at low temperatures and a relatively large product I c R n.

A holographic model of d-wave superconductor vortices with Lifshitz scaling

We study analytically the [Formula: see text]-wave holographic superconductors with Lifshitz scaling in the presence of external magnetic field. The vortex lattice solutions of the model have also been obtained with different Lifshitz scaling. Our results imply that holographic [Formula: see text]-wave superconductor is indeed a type II one even for different Lifshitz scaling. This is the same as the conventional [Formula: see text]-wave superconductors in the Ginzburg–Landau (GL) theory. Our results also indicates that the dynamical exponent [Formula: see text] cannot affect the droplet solutions, and the vortex lattice solutions receive its effects only in the radial part. This naively implies that it does not have direct influence on the shape of vortex lattice even after the higher-order corrections are taken into consideration (away from the phase transition point [Formula: see text]). However, it has effects on the upper critical magnetic field [Formula: see text] through the fact that a larger [Formula: see text] results in a smaller [Formula: see text] and therefore influences the size (characterized by [Formula: see text]) of the vortex lattices. Furthermore, close comparisons between our results and those of the GL theory reveal the fact that the upper critical magnetic field [Formula: see text] is inversely proportional to the square of the superconducting coherence length [Formula: see text], regardless of the anisotropy between space and time.

Is There a Metamaterial Route to High Temperature Superconductivity?

Superconducting properties of a material such as electron-electron interactions and the critical temperature of superconducting transition can be expressed via the effective dielectric response functionεeff(q,ω) of the material. Such a description is valid on the spatial scales below the superconducting coherence length (the size of the Cooper pair), which equals ∼100 nm in a typical BCS superconductor. Searching for natural materials exhibiting larger electron-electron interactions constitutes a traditional approach to high temperature superconductivity research. Here we point out that recently developed field of electromagnetic metamaterials deals with somewhat related task of dielectric response engineering on sub-100 nm scale. We argue that the metamaterial approach to dielectric response engineering may considerably increase the critical temperature of a composite superconductor-dielectric metamaterial.

Single-Mode Superconductivity in LaAlO3/SrTiO3 Nanostructures

ABSTRACTThe properties of superconductors at the extreme limits of dimensionality are of fundamental interest. The interface of LaAlO3 and SrTiO3 hosts a quasi-two-dimensional superconductor below Tc≈200 mK. Here we report superconductivity in nanowire-shaped structures created at the LaAlO3/SrTiO3 interface using conductive atomic force microscope lithography. Nanowire cross-sections are small compared to the superconducting coherence length in LaAlO3/SrTiO3 (w <<ξSC∼100 nm), placing them in the quasi-1D regime. The ability to “write” fully superconducting nanostructures on an insulating LaAlO3/SrTiO3 “canvas” opens possibilities for the development of new families of superconducting nanoelectronics. Four-terminal transport measurements suggest that in some devices both the normal and superconducting states are confined to a single quantum channel.

Fabrication and Electric Transport in MgB2 Doped with Nanosized Carbon-Based Core-Shell Structures

We present the fabrication and electric properties of MgB2 ceramic samples doped with nanosized spheres, 4-8 nm, of graphite with a metallic core. The samples were prepared using the spark plasma sintering technique. The size of the additive is comparable to the superconducting coherence length. The short processing time limits the diffusion of the carbon while keeping the core intact. Therefore, in addition to the doping with carbon, the metallic core, which has the size smaller than the superconducting coherence length, create pinning centers which might improve the dissipationless electric transport. The results are analyzed in the framework of different pinning models.