Borocarbide Superconductor YNi2B2C Research Articles

Field angle dependence of the zero-energy density of states in unconventional superconductors: analysis of the borocarbide superconductor YNi2B2C

We investigate the field-angle-dependent zero-energy density of states for YNi2B2C with using realistic Fermi surfaces obtained by band calculations. Both the 17th and 18th bands are taken into account. For calculating the oscillating density of states, we adopt the Kramer-Pesch approximation, which is found to improve accuracy in the oscillation amplitude. We show that superconducting gap structure determined by analyzing STM experiments is consistent with thermal transport and heat capacity measurements.

Evidence of multiband superconductivity in the quaternary borocarbide superconductor YNi2B2C using directional point-contact spectroscopy

In this paper, we review our recent investigations on the gap anisotropy of the quaternary borocarbide superconductor YNi2B2C using directional point-contact spectroscopy. Through a detailed study of the temperature and magnetic field dependence of the superconducting energy gaps we show that the gap anisotropy in this material originates from electrons on different Fermi sheets having very different Fermi velocities. The gap anisotropy in this material is therefore well explained through a multiband scenario where electrons in different k-directions have very different electron–phonon coupling strength.

Point-Contact Spectroscopy of the Borocarbide Superconductor YNi2B2C in the Normal and Superconducting State

Point-contact (PC) spectroscopy measurements of YNi2B2C single crystals in the normal and superconducting (SC) state (T_c=15.4K) for the main crystallographic directions are reported. The PC study reveals the electron-phonon interaction (EPI) spectral function with dominant phonon maximum around 12 meV and further weak structures (hump or kink) at higher energy at about 50 meV. No "soft" modes below 12 meV are resolved in the normal state. The PC EPI spectra are qualitatively similar for the different directions. Contrary, directional study of the SC gap results in \Delta_[100]=1.5 meV for the a direction and \Delta_[001]=2.3 meV along the c axis; however the critical temperature T_c in PC in all cases is near to that in the bulk sample. The value 2\Delta_[001]/kT_c=3.6 is close to the BCS value of 3.52, and the temperature dependence \Delta_[001](T) is BCS-like, while the for small gap \Delta_[100](T) is below BCS behavior at T>T_c/2 similarly as in the two-gap superconductor MgB2. It is supposed that the directional variation \Delta can be attributed to a multiband nature of the SC state in YNi2B2C.

Magnetic-field dependence of superconducting energy gaps inYNi2B2C: Evidence of multiband superconductivity

We present results of in field directional point contact spectroscopy (DPCS) study in the quaternary borocarbide superconductor YNi2B2C, which is characterized by a highly anisotropic superconducting gap function. For I||a, the superconducting energy gap (D), decreases linearly with magnetic field and vanishes around 3.25T which is well below the upper critical field (Hc2~6T) measured at the same temperature (2.2K). For I||c, on the other hand, D decreases weakly with magnetic field but the broadening parameter (G) increases rapidly with magnetic field with the absence of any resolvable feature above 3.5T. From an analysis of the field variation of energy gaps and the zero bias density of states we show that the unconventional gap function observed in this material could originate from multiband superconductivity.

Observation of dendritic flux instabilities in YNi2B2C thin films

Magneto-optical imaging and magnetization measurements performed on thin films of the borocarbide superconductor YNi2B2C reveal the occurrence of magnetic flux instabilities upon reducing the applied magnetic field towards the remanent state. In contrast to other low-Tc materials such as Nb and MgB2, where similar instabilities occur in both increasing and decreasing magnetic fields, dendritic flux patterns are observed in YNi2B2C for decreasing fields only. Also in the magnetization measurements, a distinct asymmetry is evident between increasing and decreasing fields. The effect does not depend on the sweep rate of the field, but is strongly dependent on the maximum field applied before reduction. The observation of spontaneous flux instabilities in this additional family of low-temperature superconductors suggests that the responsible mechanism is universal to this class of materials.

Anisotropy of the superconducting gap of the borocarbide superconductor YNi2B2C with ultrasonic attenuation.

The ultrasonic attenuation alpha of the highly anisotropic s-wave superconductor YNi2B2C has been measured for all the symmetrically independent elastic modes to explore the location of the zero superconducting gap region on the Fermi surface. The attenuation of the longitudinal mode shows a pronounced anisotropy in the superconducting state: While alpha shows a thermally activated behavior along [110] and [001] directions, it shows T-linear dependence along [100]. These results together with those for the transverse modes demonstrate the presence of point nodes or zero-gap regions along [100] and [010] directions. This is a clear demonstration of ultrasonic attenuation as a powerful probe for the structure of the anisotropic superconducting gap.

Evidence for nodal quasiparticles in the nonmagnetic superconductor YNi2B2C via field-angle-dependent heat capacity.

Field-angle dependent heat capacity of the nonmagnetic borocarbide superconductor YNi2B2C reveals a clear fourfold oscillation, the first observation of its kind. The observed angular variations were analyzed as a function of magnetic field angle, field-intensity, and temperature to provide its origin. The quantitative agreement between experiment and theory strongly suggests that we are directly observing nodal quasiparticles generated along <100> by the Doppler effect. The results demonstrate that field-angle heat capacity can be a powerful tool in probing the momentum-space gap structure in unconventional superconductors such as high T(c) cuprates, heavy-fermion superconductors, etc.

Evidence of point nodes in superconducting gap of borocarbide superconductor YNi2B2C

In order to determine the superconducting gap structure of the borocarbide superconductor YNi2B2C, we have measured the angular variation of the c-axis thermal conductivity κzz in magnetic field rotated within the ab-planes. A clear fourfold symmetry with narrow cusps was observed in the angular variation of κzz. These results provide a strong evidence that the gap function has point nodes which are located along the a- and b-axes of the crystal.

Four-fold anisotropy of the basal plane magnetization of YNi 2B 2C: nonlocal effects

We present studies of the vortex state magnetization M of a single crystal of the nonmagnetic borocarbide superconductor YNi 2B 2C. With magnetic field H applied in the square basal plane of the tetragonal unit cell, M has a four-fold anisotropy. This π/2 periodicity occurs deep in the superconductive mixed state, with an unusual field dependence, for a range of temperatures T < T c. The observations are well explained by a generalized London model incorporating non-local electrodynamics, using parameters from complementary experiments.

Nonlocal effects in the superconductor YNi 211B 2C

Flux lines repel each other, thus a hexagonal lattice which maximizes the distance between them is generally the lowest energy configuration. In the rare-earth nickel borocarbide superconductor YNi 2B 2C, square lattices of flux lines have been observed when the field is parallel to the c-axis. Using the c-axis penetration depth as a gauge, we find that the cross-section of an individual flux line has a four-fold symmetry, which makes square lattices energetically favored over the more prosaic hexagonal ordering. The dependence of the penetration depth within the basal plane is discussed.

Shubnikov-de Haas oscillation and magnetoresistance in YNi 2B 2C

The electrical resistivity of single crystalline boro-carbide superconductor YNi 2B 2C has been measured down to 80 mK and under magnetic fields up to 17 T. The electrical resistivity shows quadratic temperature dependence with a moderate T 2 coefficient of 1.6 × 10 −9 Ω- cm K −2 . With applied fields above H C2 , a positive magnetoresistance, which is proportional to fields, has been observed. The resistance as a function of fields exhitits an oscillatory behavior with minuscule amplitude whose periodicity of 517 T for the first time in this compound. This periodicity is consistent with the α-branch observed by the de-Haas-van Alphen effect [1,2].

Fermi surface studies of the borocarbide superconductor YNi2B2C

The Fermi surface of the nonmagnetic borocarbide superconductor YNi2B2C was studied by the de Haas-van Alphen (dHvA) effect. Five branches of dH-vA frequencies are observed in the normal state for B ∥ c and the angular dependence was investigated with magnetic fields in the (110) plane. Together with the effective cyclotron masses, mc, the result is discussed in the context of electronic band-structure calculations.

Vortex response and critical fields observed via RF penetration depth measurements on the superconductor YNi 2B 2C

Measurements of the RF penetration depth λ( T, H, θ) are used to study the superconducting order parameter, vortex dynamics in the mixed state and delineate critical fields in the borocarbide superconductor YNi 2B 2C. The lower critical field has an anomalous T dependence, H c1( T) = 1.12[1 − ( T/ T c)]kOe, which is, however, consistent with independent superfluid density measurements at microwave frequencies. The vortex response is dominated by viscous flux flow, indicative of extremely weak pinning, and is parameterized by a field scale H c2, eff. The angular dependence of the vortex contribution λ(θ) is in good agreement with the Coffey-Clem model. Structure is seen in the depairing transition in the vicinity of the upper critical field, with the existence of well-defined critical fields H c2 α , H c2 β and H c2 γ , with the vortex field scale H c2, eff closest to H c2 β . Overall the measurements indicate that YNi 2B 2C has a rich and unusual field dependence of its transport parameters.

High pressure study of borocarbide superconductor YNi 2B 2C at room temperature

High pressure behaviour of YNi 2B 2C at room temperature is investigated by electrical resistivity, thermopower and X-ray diffraction incorporating imaging plate. These studies do not indicate any structural transition up to 16.4 GPa. Bulk modulus of the material is measured to be 200 GPa and estimated to be 270 GPa from the theoretical calculations based on TB-LMTO method.

Shuvnikov-de haas oscillation in YNi2B2C

The electrical resistivity and the magnetoresistance have been studied in single crystalline boro-carbide superconductor YNi2B2C at temperatures down to 80 mK and in magnetic fields up to 19 T. The electrical resistivity at low temperatures exhibits the quadratic behavior in temperature with a modestT2 coefficient ofA=1.6×10−9Ω cm/K2. In magnetic fields aboveHc2, a positive magnetoresistance with a linear field dependence, Δ ρ/ρ=aH, wherea=0.0289 μ Ω cm/T, was found. In this region, with fields applied parallel to the c-axis, the Shuvnikov-de Haas oscillation, whose periodicity is 517 T was observed for the first time in this compound. This result is consistent with the α-branch observed by the de-Haas van Alphen effect [1].

13C NMR in YNi2B2C

Pulsed NMR study of13C has been carried out in the quaternary borocarbide superconductor YNi2B2C. The13C nuclear spin-lattice relaxation rate, 1/T 1, in an unoriented powder sample has been measured between 20 and 300K in the normal states. TheT 1 T=constant behavior is found between 20 and 100K which suggests the existence of density of states at Fermi level at carbon site. Furthermore, an anormalous decrease in 1/T 1 T with increasing temperature similar to11B NMR is found above 100K.

De Hass - van Alphen oscillations in the normal and superconducting states of the boro-carbide superconductor YNi 2B 2C

The de Hass - van Alphen (dHvA) measurements in the normal and mixed states of the boro-carbide superconductor YNi 2B 2C are reported. Five dHvA branches are observed in the normal state with magnetic fields in the (010) plane. They can be accounted for by the electronic band-structure calculation by Lee et al. A clear dHvA oscillation is also observed in the mixed state. The quasi-particle scattering rate in the mixed state is discussed in connection with the superconducting gap.