The first point-contact spectroscopy measurements on MgCNi_3 single crystals of very good quality are presented. The measurements have been performed in the temperature range from 1.5 K up to 10 K and at magnetic fields up to 6 T. The superconducting energy gap Δ of the system is found to be scattered between 1 and 1.2 meV which gives the 2Δ/k T_c ratio between BCS weak-coupling value of ~3.5 and a strong coupling of about 4.2. The excess current evaluated in magnetic field indicates the presence of a single energy gap.
Theory of self-induced resonances in asymmetric two-junction interferometer device is presented. An extension of previous theoretical approaches contains inclusions from several asymmetries: the Josephson current ε, capacitances χand dissipation ρpresented in an equivalent circuit. Our theory can be useful to determine asymmetry parameters always present in lightly damped asymmetric SQUIDs made from low- and high-T_C materials.
The influence of the microwave power on the internal Josephson Junction System (JJS) in granular high T_{c}, superconductors was investigated using EPR methodology. Josephson Microwave Absorption signal was used to monitor local temperature of the Josephson Junction System while the bulk sample temperature was measured by a thermocouple. The effect of the JJS overheating is discussed as the consequence of an interaction between JJS, microwave field and the bulk sample.
The anisotropic microwave absorption in the presence of alternative magnetic field has been studied for the first time in superconducting super-lattices based on PbTe-PbS, which are layered anisotropic systems similar to high T_{c} superconductors. A new method of study has been used. The microwave response was detected under broad-band conditions and compared with the results of synchronous detection. All the features which have been observed in high T_{c} materials are clearly seen here.
A generalized superconducting interferometer, comprising a parallel arrangement of (lumped) inductances and series (lumped) Josephson junctions is considered. Such a system can be seen as the building block of a simplified model of a high-T_{c} superconductor with its haphazard distribution of Josephson weak links on grain boundaries and lattice defects. It is shown that the system properties can be self-consistently derived from a properly defined potential energy function, taking account of the energies of the system, its current source and external magnetic field. In particular, by solving a stationary problem for this function relative to conditions of constant current bias and constant magnetic flux applied to the system, the critical current of the interferometer can be determined in function of the applied flux. Stationary phase relations and their impact on other system variables are discussed in detail. The theory is applied to the simplest possible system exhibiting all discussed properties, i.e. an interferometer composed of two junctions in series and one junction in parallel.
We report on the first experimental observations of an anomalous increase in the resistance of a semiconductor-super conductor contact in the vicinity of the critical temperature T_{c}. The effect is found in lead-doped InSb thin films having inclusions of lead of 1-3 μm in diameter: The observed effect can be a model that explains similar resistance increase observed sometimes in the high T_{c} superconductors.
Point contact spectroscopy experiments were performed on both single crystals Bi_{2}Sr_{2}CaCu_{2}O_{8+y} and BiSrCaCuO/Ag sintered material using gold tips. The spectra exhibit tunneling structure, which are interpreted in terms of the energy gap structure 2Δ = 58 meV at 4.2 K. We observed the strong Andreev reflection of the Au-BiSrCaCuO/Ag boundary.
We propose to realize MoRe/SiO_x(W)/MoRe hybrid junctions by using self-organization effects for the creation of quantum dots (tungsten clusters) in the semiconductor barriers consisting of a mixture of silicon and silicon oxide. Current-voltage characteristics of the MoRe/SiO_x(W)/MoRe samples have been measured in a wide voltage range from -900 to 900 mV at temperatures from 4.2 to 77 K. At low temperatures and for a comparatively small W content in the hybrid barrier, the heterostructures exhibited current-voltage curves of an unusual shape. Single or several current peaks caused by electron tunneling through the allowed states in the barrier have been observed in the transport characteristics. With increasing temperature, superconducting fluctuations in the MoRe electrodes become unimportant, and the current-voltage curve of a heterostructure follows the Ohm law. At last, we present theoretical description of the charge transport in such inhomogeneous systems with account of many-electron processes.
We have considered a model of n-layer high-temperature cuprates of homologous series like HgBa_2Ca_{n-1}Cu_nO_{2+2n+δ} to determine the dependence of the critical temperature T_c(n) on the number n of Cu-O planes in an elementary cell. Focusing on the description of the high-temperature superconducting system in terms of the collective phase variables, we have studied a semi-microscopic anisotropic three-dimensional vector XY model of stacked copper-oxide layers with adjustable parameters representing microscopic in-plane and out-of-plane phase stiffnesses. The model captures the layered composition and block structure along c-axis of superconducting homologous series. Implementing the spherical closure relation we have solved the phase XY model exactly with the help of transfer matrix method for vector variables. The calculated dependence of the critical temperature T_c(n) on the block size n is monotonic with n.
We study the influence of electron correlations on nonequilibrium transport through the quantum dots coupled between one metallic and one superconducting electrode. Such type of nanodevices are characterized by the induced on-dot pairing spread from the superconducting lead (proximity effect) and effectively responsible for mixing the particle with hole excitations. On the other hand, strong Coulomb repulsion between the opposite spin electrons tends to suppress a double occupancy of the quantum dots competing with the on-dot superconducting order. The Coulomb interactions can also cause a screening of the quantum dot spin by itinerant electrons of the metallic lead giving rise to formation of the Kondo resonance. We analyze interplay of such phenomena for the setup of double quantum dots where the quantum interference (such as Fano) effects influence the subgap electron transport through the Andreev scattering.
It is shown that a time-reversal invariant topological superconductivity can be realized in a quasi-one-dimensional structure, which is fabricated by filling the superconducting materials into the periodic channel of dielectric matrices like zeolite and asbestos under high pressure. The topological superconducting phase sets up in the presence of large spin-orbit interactions when s-wave intra-wire and d-wave inter-wire pairings take place. Kramers pairs of Majorana bound states emerge at the edges of each wire. The time-reversal topological superconductor belongs to DIII class of symmetry with a Z₂ invariant.
Point-contact spectroscopy studies of the superconducting energy gap and the electron-phonon coupling mechanism are performed on the boron rich YB_6 and ZrB_{12} single crystals. The obtained values of the superconducting energy gaps suggest the strong coupling with 2 Δ/k_{B}T_{c}≈ 4.2 for YB_6 and 2 Δ/k_{B}T_{c}≈ 4.15 for ZrB_{12}. We have observed the dominant soft phonon modes mediating superconductivity in the both samples at energy ≈ 8 meV for YB_6 and ≈ 11 meV in ZrB_{12}, respectively.
It is shown that the inclusion of junctions characterized by non-sinusoidal current - phase relationship in the systems composed of multiple Josephson junctions - results in the appearance of additional system phase states. Numerical simulations and stability considerations confirm that these phase states can be realized in practice. Moreover, spontaneous formation of the grain boundary junctions in high-T_c superconductors with non-trivial current-phase relations due to the d-wave symmetry of the order parameter is probable. Switching between the phase states of multiple grain boundary junction systems can lead to additional 1/f noise in high-T_c superconductors.
Point contact spectroscopy results are presented on the electron underdoped Ba(Fe_{0.96}Co_{0.04})_2As_2 single crystals. Two superconducting energy gaps with coupling values 2 Δ_1 ∿ kT_{c} ≈ 2.55 and 2 Δ_2 ∿ kT_{c} ≈ 11 at T_{c} = 15.5 K have been observed in the point contact spectra. The temperature dependence of the normal state background of the point contact spectra observed between T_{c} and T_{N} indicates antiferromagnetic origin of the V-shaped minimum at zero bias.
We describe the quantum interference effects in the nanodevice consisting of the double quantum dot coupled to the metallic and superconducting electrodes. In such heterostructures the superconducting properties are spread to the quantum dot due to the proximity effect. We investigate the density of states and anomalous Andreev conductance of the interfacial quantum dot exploring the conditions necessary for appearance of the Fano-type lineshapes. We also consider the electron correlations and discuss an interplay between the Coulomb blockade and the Fano-type interference.
We study the homogeneously disordered MoC thin films with thicknesses of 10 and 5 nm and the superconducting transition temperatures near 6 and 4 K, significantly decreased as compared to the bulk T_{c}=8.32 K due to a disorder. The scanning tunnelling spectroscopy reveals in the thicker sample a BCS superconducting energy gap Δ with a broadening parameter Γ equal to about 10 per cent of Δ. Remarkably, Γ increases with temperature. The thinner, more disordered sample shows a gapped superconducting density of states but without any coherence peaks at the gap edge, which could not be approximated by the BCS DOS. Moreover, the reduced DOS around the Fermi level persists above the resistive transition temperature reminding the pseudo-gap known from high-T_{c} cuprates.
We consider a fractional Josephson junction mediated by a quantum dot in which the Zeeman field arising from the magnetic fields driving left and right wires into topological phase can be tuned. Both fields, forming an angle Θp, can be rotated in the common plane perpendicular to the spin-orbit field in the wires. For Θp=0 the dot can be regarded as effectively non-interacting due to the large Zeeman splitting, whereas for Θp ≤sssimπ electron interactions are switched on the dot, affecting Majorana states. The tunnel electrode, weakly coupled to the dot from the top, allows to probe their density of states via conductance measurement. We show that electron interactions renormalize Majorana peak and introduce characteristic asymmetry in the gate voltage dependence of the transverse zero-bias conductance through the dot.
We study spectral properties of a quantum dot attached to two superconductors with nonzero phase difference. The system is described as a single-impurity Anderson model coupled to BCS superconducting leads. We utilize diagrammatic perturbation expansion in the Coulomb interaction to capture relevant physical phenomena, particularly the effect of the Coulomb interaction on the Andreev bound states present in the electronic spectrum. Results of the Hartree-Fock and the random phase approximations at zero temperature are presented.
Proofs are given that by resorting to the discretization of the superconducting phase variable leads to the conversion of the eigenvalue equation of a mesoscopic Josephson junction under a dc voltage into a generalized version of the Harper equation with anisotropy parameter. A full conversion proceeds, however, in terms of selected parameters. Classical limits and further generalizations are also shortly discussed.
We consider theoretically a junction between two topological superconducting wires, mediated by a quantum dot. The wires are modelled by the Kitaev chains tuned into topological phase, which possess unpaired Majorana states at their ends. We derive the low energy Hamiltonian of the model. The Majorana states closer to the dot convert into the Dirac fermion inside the dot, forming fractional Josephson junction. The dot is additionally weakly coupled to the normal tunneling probe allowing transport measurement through the dot. When the topological wires are short, the unpaired Majorana end-states can hybridize inside the wire forming an extended Dirac fermionic state. It yields the destruction of the extended state in the dot. We discuss the dependence of the spectral density of the dot and its conductance on superconducting phase. We show that the conservation of parity of the junction, crucial for successful measurement of the fractional effect, can be assured by the gate voltage manipulation of the dot level position and that in case of an unpaired Majorana state in the junction a half conductance quantum can be observed.
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