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The current-voltage characteristics of Superconducting Quantum Interference Devices (SQUIDs) are known to modulate as a function of applied magnetic field with a period of one flux quantum $Phi_0=h$/2e. Here we report on the fabrication and properties of SQUIDs modulating with a period of $1/2timesPhi_0$. The characteristics of these bicrystal SQUIDs are consistent with either a strong sin(2$phi$) component of the current-phase relation of the Josephson current, or with an interaction between the Cooper-pairs, causing an admixture of quartets to the condensate.
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We use a model of hard-core bosons to describe a SQUID built with two crystals of $d_{x^2-y^2}$-superconductors with orientations (100) and (110). Across the two faceted (100)/(110) interfaces, the structure of the superconducting order parameter leads to an alternating sign of the local Josephson coupling, and the possibility of quartet formation. Using a mapping of the boson model to an $XXZ$ model, we calculate numerically the energy of the system as a function of the applied magnetic flux, finding signals of $hc/4e$ oscillations in a certain region of parameters. This region has a large overlap to that at which binding of bosons exists.
We analyze the crossover from an hc/e-periodicity of the persistent current in flux threaded clean metallic rings towards an hc/2e-flux periodicity of the supercurrent upon entering the superconducting state. On the basis of a model calculation for a one-dimensional ring we identify the underlying mechanism, which balances the hc/e versus the hc/2e periodic components of the current density. When the ring circumference exceeds the coherence length of the superconductor, the flux dependence is strictly hc/2e periodic. Further, we develop a multi-channel model which reduces the Bogoliubov - de Gennes equations to a one-dimensional differential equation for the radial component of the wave function. The discretization of this differential equation introduces transverse channels, whose number scales with the thickness of the ring. The periodicity crossover is analyzed close the critical temperature.
Shubnikov-de Haas and de Haas-van Alphen effects have been measured in the underdoped high temperature superconductor YBa$_2$Cu$_3$O$_{6.51}$. Data are in agreement with the standard Lifshitz-Kosevitch theory, which confirms the presence of a coherent Fermi surface in the ground state of underdoped cuprates. A low frequency $F = 530 pm 10$ T is reported in both measurements, pointing to small Fermi pocket, which corresponds to 2% of the first Brillouin zone area only. This low value is in sharp contrast with that of overdoped Tl$_2$Ba$_2$CuO$_{6+delta}$, where a high frequency $F = 18$ kT has been recently reported and corresponds to a large hole cylinder in agreement with band structure calculations. These results point to a radical change in the topology of the Fermi surface on opposing sides of the cuprate phase diagram.
We report extensive measurements of quantum oscillations in the normal state of the Fe-based superconductor LaFePO, (Tc ~ 6 K) using low temperature torque magnetometry and transport in high static magnetic fields (45 T). We find that the Fermi surface is in broad agreement with the band-structure calculations with the quasiparticle mass enhanced by a factor ~2. The quasi-two dimensional Fermi surface consist of nearly-nested electron and hole pockets, suggesting proximity to a spin/charge density wave instability.
We report the observation of Shubnikov-de Haas oscillations in the underdoped cuprate superconductor YBa$_2$Cu$_4$O$_8$ (Y124). For field aligned along the c-axis, the frequency of the oscillations is $660pm 30$ T, which corresponds to $sim 2.4$ % of the total area of the first Brillouin zone. The effective mass of the quasiparticles on this orbit is measured to be $2.7pm0.3$ times the free electron mass. Both the frequency and mass are comparable to those recently observed for ortho-II YBa$_2$Cu$_3$O$_{6.5}$ (Y123-II). We show that although small Fermi surface pockets may be expected from band structure calculations in Y123-II, no such pockets are predicted for Y124. Our results therefore imply that these small pockets are a generic feature of the copper oxide plane in underdoped cuprates.
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