G. A. Levin

Spin suceptibility and specific heat bi-layer high-Tc cuprates: on the origin of scaling and universality

Abstract The anomalies in the spin susceptibility X s ( T ) and specific heat C V ( T ) in the high- T c cuprates are shown to have origin in a non-degenerate electron component. This arises when the chemical potential μ is tuned near the top of a 2D band submerged just below the Fermi surface. In bi-layer cuprates the model predicts a scaling of the form: X s (T, x) = Xs (W(x)/T) and C V(T, x)/T = γ(W(x)/T) , where doping-dependent scaling parameter W is determined by the location of μ relative to the top of the non-degenerate band. We compare the results of this model with experiments and find the predicted type of scaling in the susceptibility of under, optimally, and overdoped (TlSr 2 (Lu 1− x Ca x )Cu 2 O y samples. We also argue that the scaling curves X s ( y ) and γ ( y ) may be universal for a certain class of bi-layer cuprates.

Origin and consequences of the ‘gap’ in the cuprate normal state

Abstract We present an alternative to the spin gap as an explanation for the origin of the gap feature observed in the normal state of the high-Tc cuprates. We consider a model that has a non-degenerate as well as a degenerate component of charge carriers. The properties are governed by the location of the chemical potential relative to the top of the non-degenerate band. In the underdoped regime this translates into an energy gap for the non-degenerate carriers; this decreases with doping, vanishing at optimal doping. In the overdoped regime there emerges a small Fermi energy that governs the temperature dependence of properties. As illustrations we consider several spin and charge properties, namely spin susceptibility, entropy, in-plane resistivity, Hall effect and Hall angle.

Dynamics of flux creep in underdoped single crystals of Y{sub 1-x}Pr{sub x}Ba{sub 2}Cu{sub 3}O{sub 7-{delta}}

Transport as well as magnetic relaxation properties of the mixed state were studied on strongly underdoped Y_1-xPr_xBa_2Cu_3O_7-d crystals. We observed two correlated phenomena - a coupling transition and a transition to quantum creep. The distribution of transport current below the coupling transition is highly nonuniform, which facilitates quantum creep. We speculate that in the mixed state below the coupling transition, where dissipation is nonohmic, the current distribution may be unstable with respect to self-channeling resulting in the formation of very thin current-carrying layers.

Relationship between Conductivity and Phase Coherence Length in Cuprates

The large (102–105) and strongly temperature dependent resistive anisotropy η=(σab/σc)1/2 of cuprates perhaps holds the key to understanding their normal state in-plane σab and out-of-plane σc conductivities. It can be shown that η is determined by the ratio of the phase coherence lenghts li in the respective directions: . In layered crystals in which the out-of-plane transport is incoherent, lc is fixed, equal to the interlayer spacing. As a result, the T-dependence of η is determined by that of lab, and vice versa, the in-plane phase coherence lenght can be obtained directly by measuring the resistive anisotropy. We present data for hole-doped YBa2Cu3Oy (6.3<y<6.9) and Y1-xPrxBa2O7-δ(0<x≤0.55) and show that σcb of crystals with different doping levels can be well described by a two parameter universal function of the in-plane phase coherence length. In the electron-doped Nd2-xCexCuO4-y, the dependence σab(η) indicates a crossover from incoherent transport in the c-direction.

An improved analysis of resistivity from flux transformer geometry

Abstract We present two algorithms for the calculation of the resistivity tensor from current/voltage measurements for the two different contact configurations: “flux transformer” and “ I ‖ c ”. The application of this method is illustrated on data for an Y 0.78 Pr 0.22 Ba 2 Cu 3 O 7− δ single crystal.

QUANTUM CREEP IN Y1-XPRXBA2CU3O7-DELTA CRYSTALS : MAGNETIC RELAXATION ANDTRANSPORT

We report transport and magnetic relaxation measurements in the mixed state of strongly underdoped Y_{1-x}Pr_{x}Ba_{2}Cu_{3}O_{7} crystals. A transition from thermally activated flux creep to temperature independent quantum flux creep is observed in both transport and magnetic relaxation at temperatures T * 5 K. Flux transformer measurements indicate that the crossover to quantum creep is preceded by a coupling transition. Based on these observations we argue that below the coupling transition the current is confined within a very narrow layer beneath the current contacts.

REDUCTION OF FLUX CREEP BY HEAT PULSES

We investigated the possibility of reducing the giant flux creep in high-temperature superconductors by temporarily increasing the temperature T above the operating temperature T0, after the critical state is created at T0. This T increase leads to a supercritical state which decays rapidly so that, when T is decreased back to T0, the superconductor is in the subcritical state. We have found that both the maximum persistent current and the relaxation rate are hysteretic and differ substantially from the corresponding quantities measured after flux annealing.

In-plane and out-of-plane resistivities of PrBa2Cu3O7−δ and Y0.47Pr0.53Ba2Cu3O7−δ single crystals☆

Abstract We report out-of-plane and in-plane normal-state resistivities of twinned, fully-oxygenated single crystals of insulating PrBa2Cu3O7−δ and superconducting Y0.47Pr0.53Ba2Cu3O7−δ (Tc ≈ 18 K) measured by the flux transformer method. In the latter specimen, the onset of superconductivity was suppressed by a magnetic field of 9 T. The anisotropy ϱ c /ϱ ab = a +_bT −2 3 for both samples, increasing monotonically at low temperatures with no sign of saturation. The absence of saturation and the T −2 3 dependence indicate two-dimensional transport. These data also represent a direct observation of hopping conduction in a homogeneous superconductor.

Magnetotransport mechanisms in strongly underdoped YBa{sub 2}Cu{sub 3}O{sub x} single crystals

We report magnetoresistivity measurements on strongly underdoped YBa{sub 2}Cu{sub 3}O{sub x} (x=6.25 and 6.36) single crystals in applied magnetic fields H{parallel} c axis. We identify two different contributions to both in-plane {Delta}{rho}{sub ab}/{rho}{sub ab} and out-of-plane {Delta}{rho}{sub c}/{rho}{sub c} magnetoresistivities. The first contribution has the same sign as the temperature coefficient of the resistivity {partial_derivative} ln {rho}{sub i}/{partial_derivative}T (i={l_brace}c,ab{r_brace}). 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.

Is There a Unified Description of Conductivity of Layered Cuprates

AbstractWe present a novel approach to the analysis of the normal state in-plane