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Register a new userMagnetoconductivity due to Quantum Interference in Strongly Underdoped YBa_2Cu_3O_x
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E. Cimpoiasu
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We report magnetoconductivity measurements on YBa_2Cu_3O_x (x=6.25 and 6.36) single crystals. Our main result is that both the in-plane Delta_sigma_ab and out-of-plane Delta_sigma_c magnetoconductivities exhibit the field dependence characteristic of two-dimensional quantum interference in applied magnetic fields H parallel to the c-axis. Namely, Delta_sigma_c,ab are proportional to ln(H/H_0), with Delta_sigma_c/sigma_c substantially greater than Delta_sigma_ab/sigma_ab. We interpret this result as evidence of interlayer incoherence in these crystals, so that the phase-coherent trajectories are mostly confined to one bilayer.
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We report magnetoresistivity measurements on strongly underdoped YBa_2Cu_3O_x (x=6.25, 6.36) single crystals in applied magnetic fields H || c-axis. We identify two different contributions to both in-plane and out-of-plane magnetoresistivities. The first contribution has the same sign as the temperature coefficient of the resistivity partial ln(rho_i)/partial T (i={c,ab}). This contribution reflects the incoherent nature of the out-of-plane transport. The second contribution is positive, quadratic in field, with an onset temperature that correlates to the antiferromagnetic ordering.
The phenomenological Greens function developed in the works of Yang, Rice and Zhang has been very successful in understanding many of the anomalous superconducting properties of the deeply underdoped cuprates. It is based on considerations of the resonating valence bond spin liquid approximation and is designed to describe the underdoped regime of the cuprates. Here we emphasize the region of doping, $x$, just below the quantum critical point at which the pseudogap develops. In addition to Luttinger hole pockets centered around the nodal direction, there are electron pockets near the antinodes which are connected to the hole pockets by gapped bridging contours. We determine the contours of nearest approach as would be measured in angular resolved photoemission experiments and emphasize signatures of the Fermi surface reconstruction from the large Fermi contour of Fermi liquid theory (which contains $1+x$ hole states) to the Luttinger pocket (which contains $x$ hole states). We find that the quasiparticle effective mass renormalization increases strongly towards the edge of the Luttinger pockets beyond which it diverges.
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.
The underlying physics of the magnetic-field-induced resistive state in high temperature cuprate superconductors remains a mystery. One interpretation is that the application of magnetic field destroys the d-wave superconducting gap to uncover a Fermi surface that behaves like a conventional (i.e.Fermi Liquid) metal (1). Another view is that an applied magnetic field destroys long range superconducting phase coherence, but the superconducting gap amplitude survives (2, 3). By measuring the specific heat of ultra-clean YBa2Cu3O6.56 (YBCO 6.56), we obtain a measure of the quasi-particle density of states from the superconducting state well into the magnetic-field-induced resistive state. We have found that at very high magnetic fields the specific heat exhibits both the conventional temperature dependence and quantum oscillations expected for a Fermi Liquid. On the other hand, the magnetic field dependence of the quasi-particle density of states follows a sqrt{H} behavior that persists right through the zero-resistance transition, evidencing the fully developed d-wave superconducting gap over the entire magnetic field range measured. The coexistence of these two phenomena pose a rigorous thermodynamic constraint on theories of high-magnetic-field resistive state in the cuprates.
We report quantum oscillations in underdoped YBa2Cu3O6.56 over a significantly large range in magnetic field extending from 24 to 101 T, enabling three well-spaced low frequencies at 440 T, 532 T, and 620 T to be clearly resolved. We show that a small nodal bilayer coupling that splits a nodal pocket into bonding and antibonding orbits yields a sequence of frequencies, F0 - {Delta}F, F0, and F0 + {Delta}F and accompanying beat pattern similar to that observed experimentally, on invoking magnetic breakdown tunneling at the nodes. The relative amplitudes of the multiple frequencies observed experimentally in quantum oscillation measurements are shown to be reproduced using a value of nodal bilayer gap quantitatively consistent with that measured in photoemission experiments in the underdoped regime.
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