Hole-rich Domains Research Articles

Influence of chemical pressure effects on nonlinear thermal conductivity of intrinsically granular superconductors

Using a two-dimensional model of capacitively coupled Josephson junction arrays (created by a network of twin boundary dislocations with strain fields acting as an insulating barrier between hole-rich domains in underdoped crystals), we study the influence of chemical pressure (∇μ) on nonlinear (i.e., ∇T dependent) thermal conductivity (NLTC) of an intrinsically granular superconductor. Quite a substantial enhancement of NLTC is predicted when intrinsic chemoelectric field Eμ∝∇μ closely matches the externally produced thermoelectric field ET∝∇T. The estimates of the model parameters suggest a realistic possibility to experimentally monitor this effect in nonstoichiometric high-TC superconductors.

Hole-pair hopping in arrangements of hole-rich/hole-poor domains in a quantum antiferromagnet

We study the motion of holes in a doped quantum antiferromagnet in the presence of arrangements of hole-rich and hole-poor domains such as the stripe-phase in high-$T_C$ cuprates. When these structures form, it becomes energetically favorable for single holes, pairs of holes or small bound-hole clusters to hop from one hole-rich domain to another due to quantum fluctuations. However, we find that at temperature of approximately 100 K, the probability for bound hole-pair exchange between neighboring hole-rich regions in the stripe phase, is one or two orders of magnitude larger than single-hole or multi-hole droplet exchange. As a result holes in a given hole-rich domain penetrate further into the antiferromagnetically aligned domains when they do it in pairs. At temperature of about 100 K and below bound pairs of holes hop from one hole-rich domain to another with high probability. Therefore our main finding is that the presence of the antiferromagnetic hole-poor domains act as a filter which selects, from the hole-rich domains (where holes form a self-bound liquid), hole pairs which can be exchanged throughout the system. This fluid of bound hole pairs can undergo a superfluid phase ordering at the above mentioned temperature scale.

Chemomagnetism, magnetoconcentration effect, and “fishtail” anomaly in chemically induced granular superconductors

Within a 2D model of Josephson junction arrays (created by 2D network of twin boundary dislocations with strain fields acting as insulating barrier between hole-rich domains in underdoped crystals), a few novel effects expected to occur in intrinsically granular material are predicted including: (i) Josephson chemomagnetism (chemically induced magnetic moment in zero applied magnetic field) and its influence on a low-field magnetization (chemically induced paramagnetic Meissner effect), and (ii) magnetoconcentration effect (creation of oxygen vacancies in applied magnetic field) and its influence on a high-field magnetization (chemically induced analog of "fishtail" anomaly). The conditions under which these effects can be experimentally measured in non-stoichiometric high-T_c superconductors are discussed.

Magnetic and structural instabilities in the stripe-phase region of La1.875Ba0.125 - ySryCuO4 (0 y 0.1)

Zero-field positive muon spin relaxation (ZF-µ+SR) experiments were performed to investigate the magnetic properties of the low-temperature structural modifications of the La1.875Ba0.125 - ySryCuO4 ( y = 0.0, 0.025, 0.050, 0.075, 0.100) series, in which the total hole concentration is close to 1 / 8. Together with high-resolution time-of-flight neutron powder diffraction measurements, the results imply that the interplay among lattice distortions, doping and superconductivity is intimately related to the magnetic correlations of the Cu spins, which are interpreted in terms of a stripe-phase model. Materials with y 0.075 exhibit an incomplete structural transition at Td 2, from a low-temperature orthorhombic (LTO, Bmab) to a low-temperature tetragonal (LTT, P42 / ncm) phase. Diffraction patterns collected while approaching Td 2 reveal a rapid reduction of the orthorhombicity in the LTO phase with a residual fraction always surviving to low temperatures. Different behaviour is shown by the composition with y = 0.1. Rietveld analysis shows coexistence of the LTO phase with a less distorted low-temperature orthorhombic phase (LTO-2, Pccn). In the LTT phase, a fraction of muons senses regions with purely static spin correlations due to an incommensurate spin-density wave. The muon spin depolarization suggests that in the spatially separated remnant LTO, hole-rich domains, dynamical correlations of charge density give rise to superconductivity. As the spin fluctuations become static at y 0.075, the London penetration depth, calculated from the transverse-field µ+SR depolarization rate, , reveals a decrease in the superconducting carrier density. The magnetic freezing temperature, Tf is suppressed and the magnetic phase fraction shrinks as y increases, whereas the superconducting correlations persist in a larger sample volume. The disappearance of long-range magnetic order at y = 0.1 and the growth of a quasi-static component correlate well with the presence of the LTO-2 microstructure which behaves as a buffer phase out of which the LTT domains become dominant in the Ba-rich compositions.

Interlayer coupling and the thermopower of cuprate superconductors.

A review of the extant literature on the thermopower (TEP) of the cuprate superconductors reveals a transition from polaronic to homogeneous itinerant electron behavior with increasing hole concentration in the ${\mathrm{CuO}}_{2}$ sheets. Superconductivity appears at the compositions of crossover from one regime to the other, and the normal state of the superconductors retains a heterogeneous electronic structure of mobile hole-rich and hole-poor domains, which introduces both a statistical and a transport component to the Seebeck coefficient. A primary manifestation of the transport component is an enhancement term \ensuremath{\delta}\ensuremath{\alpha}(T) having a maximum at a ${\mathit{T}}_{\mathrm{max}}$=100--140 K, which is characteristic of a strong coupling of the charge carriers to optical rather than acoustic phonons. Comparison of the TEP for ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4}$ vs ${\mathrm{La}}_{0.85}$${\mathrm{Sr}}_{1.15}$${\mathrm{GaCuO}}_{5}$ and ${\mathrm{La}}_{1.8}$${\mathrm{Sr}}_{0.2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{6+\mathrm{\ensuremath{\delta}}}$ vs ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ indicates that an increase in the c-axis coupling between neighboring ${\mathrm{CuO}}_{2}$ sheets increases the size of the polaron or hole-rich domains in the underdoped compositions, thereby lowering the room-temperature value of the Seebeck coefficient \ensuremath{\alpha}(300 K) and increasing the magnitude of \ensuremath{\delta}\ensuremath{\alpha}(T). These observations implicate a strong elastic component, enhanced by electron-lattice interactions, in the c-axis coupling as well as in the formation of larger nonadiabatic polarons and their interactions within the ${\mathrm{CuO}}_{2}$ planes. They also signal that caution must be exercised in any application of a ``universal plot'' of \ensuremath{\alpha}(300 K) versus hole concentration p per Cu atom in a ${\mathrm{CuO}}_{2}$ plane to obtain a value of p from TEP data. \textcopyright{} 1996 The American Physical Society.