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Register a new userMagnetic field dependence of the density of states in the multiband superconductor $beta$-Bi$_2$Pd
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We present very low temperature scanning tunneling microscopy (STM) experiments on single crystalline samples of the superconductor $beta$-Bi$_2$Pd. We find a single fully isotropic superconducting gap. However, the magnetic field dependence of the intervortex density of states is higher than the one expected in a single gap superconductor, and the hexagonal vortex lattice is locked to the square atomic lattice. Such increase in the intervortex density of states and vortex lattice locking have been found in superconductors with multiple superconducting gaps and anisotropic Fermi surfaces. We compare the upper critical field $H_{c2}(T)$ obtained in our sample with previous measurements and explain available data within multiband supercondutivity. We propose that $beta$-Bi$_2$Pd is a single gap multiband superconductor. We anticipate that single gap multiband superconductivity can occur in other compounds with complex Fermi surfaces.
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Electronic states in the gap of a superconductor inherit intriguing many-body properties from the superconductor. Here, we create these in-gap states by manipulating Cr atomic chains on the $beta$-Bi$_2$Pd superconductor. We find that the topological properties of the in-gap states can greatly vary depending on the crafted spin chain. These systems make an ideal platform for non-trivial topological phases because of the large atom-superconductor interactions and the existence of a large Rashba coupling at the Bi-terminated surface. We study two spin chains, one with atoms two-lattice-parameter apart and one with square-root-of-two lattice parameters. Of these, only the second one is in a topologically non-trivial phase, in correspondence with the spin interactions for this geometry.
We report the observation of half-integer magnetic flux quantization in mesoscopic rings of superconducting $beta$-Bi$_2$Pd thin films. The half-quantum fluxoid manifests itself as a $pi$ phase shift in the quantum oscillation of the critical temperature. This result verifies unconventional superconductivity of $beta$-Bi$_2$Pd, in accord with the expectation of a topological superconductor. We also discuss the strong indication that $beta$-Bi$_2$Pd is a spin-triplet superconductor.
We studied $ab$-plane transport properties in single crystals of the superconductor $beta$-FeSe up to 16 T. In the normal state, below 90 K, the crystals present a strongly anisotropic positive magnetoresistance that becomes negligible above that temperature. In the superconducting state (T$_c$=8.87(5) K) the upper critical field anisotropy $H$$_{c2}$$parallel$$ab$ / $H$$_{c2}$$parallel$$c$ changes with temperature and the angular dependence of the dissipation for fixed temperatures and fields reflects a strongly anisotropic behavior. Our results make evident that multiband effects are needed to describe the measured transport properties. We model the magnetoresistance and upper critical field behavior with a two-band model showing that the diffusivities ratio parameter remains unchanged going from the normal to the superconducting state.
2$H$-TaSe$_2$ has been one of unique transition metal dichalcogenides exhibiting several phase transitions due to a delicate balance among competing electronic ground states. An unusual metallic state at high-$T$ is sequentially followed by an incommensurate charge density wave (ICDW) state at $approx$ 122 K and a commensurate charge density wave (CCDW) state at $approx$ 90 K, and superconductivity at $T_{rm{C}}sim$0.14 K. Upon systematic intercalation of Pd ions into TaSe$_2$, we find that CCDW order is destabilized more rapidly than ICDW to indicate a hidden quantum phase transition point at $x$$sim$0.09-0.10. Moreover, $T_{rm{C}}$ shows a dramatic enhancement up to 3.3 K at $x$ = 0.08, $sim$24 times of $T_{rm{C}}$ in 2$H$-TaSe$_2$, in proportional to the density of states $N(E_F)$. Investigations of upper critical fields $H_{c2}$ in single crystals reveal evidences of multiband superconductivity as temperature-dependent anisotropy factor $gamma_H$ = $H_{c2}^{ab}$/$H_{c2}^{c}$, quasi-linear increase of $H_{c2}^{c}(T)$, and an upward, positive-curvature in $H_{c2}^{ab}(T)$ near $T_{rm{C}}$. Furthermore, analysis of temperature-dependent electronic specific heat corroborates the presence of multiple superconducting gaps. Based on above findings and electronic phase diagram vs $x$, we propose that the increase of $N(E_F)$ and effective electron-phonon coupling in the vicinity of CDW quantum phase transition should be a key to the large enhancement of $T_{rm{C}}$ in Pd$_x$TaSe$_2$.
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.
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