Possible origin of 60-K plateau in the YBa2Cu3O(6+y) phase diagram

We study a model of YBa2Cu3O(6+y) to investigate the influence of oxygen ordering and doping imbalance on the critical temperature Tc(y) and to elucidate a possible origin of well-known feature of YBCO phase diagram: the 60-K plateau. Focusing on "phase only" description of the high-temperature superconducting system in terms of collective variables we utilize a three-dimensional semi microscopic XY model with two-component vectors that involve phase variables and adjustable parameters representing microscopic phase stiffnesses. The model captures characteristic energy scales present in YBCO and allows for strong anisotropy within basal planes to simulate oxygen ordering. Applying spherical closure relation we have solved the phase XY model with the help of transfer matrix method and calculated Tc for chosen system parameters. Furthermore, we investigate the influence of oxygen ordering and doping imbalance on the shape of YBCO phase diagram. We find it unlikely that oxygen ordering alone can be responsible for the existence of 60-K plateau. Relying on experimental data unveiling that oxygen doping of YBCO may introduce significant charge imbalance between CuO2 planes and other sites, we show that simultaneously the former are underdoped, while the latter -- strongly overdoped almost in the whole region of oxygen doping in which YBCO is superconducting. As a result, while oxygen content is increased, this provides two counter acting factors, which possibly lead to rise of 60K plateau. Additionally, our result can provide an important contribution to understanding of experimental data supporting existence of multicomponent superconductivity in YBCO.Comment: 9 pages, 8 figures, submitted to PRB, see http://prb.aps.or

Quantum effects in a superconducting glass model

We study disordered Josephson junctions arrays with long-range interaction and charging effects. The model consists of two orthogonal sets of positionally disordered $N$ parallel filaments (or wires) Josephson coupled at each crossing and in the presence of a homogeneous and transverse magnetic field. The large charging energy (resulting from small self-capacitance of the ultrathin wires) introduces important quantum fluctuations of the superconducting phase within each filament. Positional disorder and magnetic field frustration induce spin-glass like ground state, characterized by not having long-range order of the phases. The stability of this phase is destroyed for sufficiently large charging energy. We have evaluated the temperature vs charging energy phase diagram by extending the methods developed in the theory of infinite-range spin glasses, in the limit of large magnetic field. The phase diagram in the different temperature regimes is evaluated by using variety of methods, to wit: semiclassical WKB and variational methods, Rayleigh-Schr\"{o}dinger perturbation theory and pseudospin effective Hamiltonians. Possible experimental consequences of these results are briefly discussed.Comment: 17 pages REVTEX. Two Postscript figures can be obtained from the authors. To appear in PR

Local dissipation effects in two-dimensional quantum Josephson junction arrays with magnetic field

We study the quantum phase transitions in two-dimensional arrays of Josephson-couples junctions with short range Josephson couplings (given by the Josephson energy) and the charging energy. We map the problem onto the solvable quantum generalization of the spherical model that improves over the mean-field theory method. The arrays are placed on the top of a two-dimensional electron gas separated by an insulator. We include effects of the local dissipation in the presence of an external magnetic flux f in square lattice for several rational fluxes f=0,1/2,1/3,1/4 and 1/6. We also have examined the T=0 superconducting-insulator phase boundary as function of a dissipation alpha for two different geometry of the lattice: square and triangular. We have found critical value of the dissipation parameter independent on geometry of the lattice and presence magnetic field.Comment: accepted to PR

SO(5) superconductor in a Zeeman magnetic field: Phase diagram and thermodynamic properties

In this paper we present calculations of the SO(5) quantum rotor theory of high-T$_{c}$ superconductivity in Zeeman magnetic field. We use the spherical approach for five-component quantum rotors in three-dimensional lattice to obtain formulas for critical lines, free energy, entropy and specific heat and present temperature dependences of these quantities for different values of magnetic field. Our results are in qualitative agreement with relevant experiments on high-T$_{c}$ cuprates.Comment: 4 pages, 2 figures, to appear in Phys. Rev. B, see http://prb.aps.or

A generalized spherical version of the Blume-Emery-Griffits model with ferromagnetic and antiferromagnetic interactions

We have investigated analitycally the phase diagram of a generalized spherical version of the Blume-Emery-Griffiths model that includes ferromagnetic or antiferromagnetic spin interactions as well as quadrupole interactions in zero and nonzero magnetic field. We show that in three dimensions and zero magnetic field a regular paramagnetic-ferromagnetic (PM-FM) or a paramagnetic-antiferromagnetic (PM-AFM) phase transition occurs whenever the magnetic spin interactions dominate over the quadrupole interactions. However, when spin and quadrupole interactions are important, there appears a reentrant FM-PM or AFM-PM phase transition at low temperatures, in addition to the regular PM-FM or PM-AFM phase transitions. On the other hand, in a nonzero homogeneous external magnetic field $H$, we find no evidence of a transition to the state with spontaneous magnetization for FM interactions in three dimensions. Nonethelesss, for AFM interactions we do get a scenario similar to that described above for zero external magnetic field, except that the critical temperatures are now functions of $H$. We also find two critical field values, $H_{c1}$, at which the reentrance phenomenon dissapears and $H_{c2}$ ($H_{c1}\approx 0.5H_{c2}$), above which the PM-AFM transition temperature vanishes.Comment: 21 pages, 6 figs. Title changed, abstract and introduction as well as section IV were rewritten relaxing the emphasis on spin S=1 and Figs. 5 an 6 were improved in presentation. However, all the results remain valid. Accepted for publication in Phys. Rev.

Three-dimensional Josephson-junction arrays in the quantum regime

We study the quantum phase transition properties of a three-dimensional periodic array of Josephson junctions with charging energy that includes both the self and mutual junction capacitances. We use the phase fluctuation algebra between number and phase operators, given by the Euclidean group E_2, and we effectively map the problem onto a solvable quantum generalization of the spherical model. We obtain a phase diagram as a function of temperature, Josephson coupling and charging energy. We also analyze the corresponding fluctuation conductivity and its universal scaling form in the vicinity of the zero-temperature quantum critical point.Comment: 9 pages, LATEX, three PostScript figures. Submitted to Phys. Rev. Let

History Dependent Phenomena in the Transverse Ising Ferroglass: the Free Energy Landscape

In this paper we investigate the relationship between glassy and ferromagnetic phases in disordered Ising ferromagnets in the presence of transverse magnetic fields, $\Gamma$. Iterative mean field simulations probe the free energy landscape and suggest the existence of a glass transition as a function of $\Gamma$ which is distinct from the Curie temperature. New experimental field-cooled and zero-field-cooled data on LiHo$_x$Y$_{1-x}$F$_4$ provide support for our theoretical picture.Comment: 4 pages RevTex; 5 figure

Critical charge instability on verge of the Mott transition and the origin of quantum protection in high-TcT_c cuprates

The concept of topological excitations and the related ground state degeneracy are employed to establish an effective theory of the superconducting state evolving from the Mott insulator for high-Tc cuprates. Casting the Coulomb interaction in terms of composite-fermions via the gauge flux attachment facility, we show that instanton events in the Matsubara "imaginary time," labeled by topological winding numbers, are essential configurations of the phase field dual to the charge. In analogy to the usual phase transition that is characterized by a sudden change of the symmetry, the topological phase transitions are governed by a discontinuous change of the topological numbers signaled by the divergence of the zero-temperature topological susceptibility. This defines a quantum criticality ruled by topologically conserved numbers rather than the Landau principle of the symmetry breaking. We show that in the limit of strong correlations topological charge is linked to the average electronic filling number and the topological susceptibility to the electronic compressibility of the system. We exploit the impact of these nontrivial U(1) instanton phase field configurations for the cuprate phase diagram which displays the "hidden" quantum critical point covered by the superconducting lobe in addition to a sharp crossover between a compressible normal "strange metal" state and a region characterized by a vanishing compressibility, which marks the Mott insulator. Finally, we argue that the existence of robust quantum numbers explains the stability against small perturbation of the system and attributes to the topological "quantum protectorate" as observed in strongly correlated systems.Comment: 23 pages, 12 figure

Quantum critical point and scaling in a layered array of ultrasmall Josephson junctions

We have studied a quantum Hamiltonian that models an array of ultrasmall Josephson junctions with short range Josephson couplings, $E_J$, and charging energies, $E_C$, due to the small capacitance of the junctions. We derive a new effective quantum spherical model for the array Hamiltonian. As an application we start by approximating the capacitance matrix by its self-capacitive limit and in the presence of an external uniform background of charges, $q_x$. In this limit we obtain the zero-temperature superconductor-insulator phase diagram, $E_J^{\rm crit}(E_C,q_x)$, that improves upon previous theoretical results that used a mean field theory approximation. Next we obtain a closed-form expression for the conductivity of a square array, and derive a universal scaling relation valid about the zero--temperature quantum critical point. In the latter regime the energy scale is determined by temperature and we establish universal scaling forms for the frequency dependence of the conductivity.Comment: 18 pages, four Postscript figures, REVTEX style, Physical Review B 1999. We have added one important reference to this version of the pape

A Dynamical Study of the Quantum p=2 Spherical Model

We present a dynamical study of the disordered quantum p=2 spherical model at long times. Its phase behavior as a function of spin-bath coupling, strength of quantum fluctuations and temperature is characterized, and we identify different paramagnetic and coarsened regions. A quantum critical point at zero temperature in the limit of vanishing dissipation is also found. Furthermore we show analytically that the fluctuation-dissipation theorem is obeyed in the stationary regime.Comment: 13 pages, 4 figures; published versio