Phase Diagram Of High-temperature Superconductors Research Articles

Anomalous Criticality in the Electrical Resistivity of La 2– x Sr x CuO 4

The presence or absence of a quantum critical point and its location in the phase diagram of high-temperature superconductors have been subjects of intense scrutiny. Clear evidence for quantum criticality, particularly in the transport properties, has proved elusive because the important low-temperature region is masked by the onset of superconductivity. We present measurements of the low-temperature in-plane resistivity of several highly doped La2-xSrxCuO4 single crystals in which the superconductivity had been stripped away by using high magnetic fields. In contrast to other quantum critical systems, the resistivity varies linearly with temperature over a wide doping range with a gradient that scales monotonically with the superconducting transition temperature. It is maximal at a critical doping level (pc) approximately 0.19 at which superconductivity is most robust. Moreover, its value at pc corresponds to the onset of quasi-particle incoherence along specific momentum directions, implying that the interaction that first promotes high-temperature superconductivity may ultimately destroy the very quasi-particle states involved in the superconducting pairing.

Comparative infrared study of optimally doped and underdopedLa2−xSrxCuO4 single crystals

The temperature dependence of the optical spectra of twoLa2−xSrxCuO4 single crystals was investigated for both in-plane and out-of-plane directions. For the underdoped(x = 0.08) single crystal, the in-plane optical conductivity that was analysed by ageneralized Drude formalism shows a suppression of the scattering rate1/τ(ω) and an increasedeffective mass m* as the temperature decreases at low frequencies. Since this behaviour can be explained by thepseudogap effect, it is concluded that the pseudogap is not present at the optimal doping(x = 0.15). This result isconfirmed by the c-axis optical conductivity, which decreases only for the underdoped single crystal. Theabsence of the pseudogap at the optimal composition is in accord with the quantum criticalpoint model and it can explain the phase diagram of high-temperature superconductors.

Neutron scattering investigations of the Abrikosov state of high-temperature superconductors

The magnetic phase diagram of high-temperature superconductors can contain many exotic vortex phases not observed in conventional superconducting materials. For example, the familiar vortex lattice may melt at high temperatures into a vortex liquid. The influence of defects, which pin the vortices, is of particular interest from both a theoretical and an experimental point of view. We have used small angle neutron scattering (SANS) to probe the order of the vortex system on a microscopic scale and have succeeded, for the first time, to measure a well-ordered vortex lattice (VL) structure at all doping regimes of La 2− x Sr x CuO 4. In the optimally to overdoped regime a field-induced transition from hexagonal to square coordination is reported. The possible connections of our SANS results to photoemission data, as well as the implications for various competing theoretical models will be discussed.

Explanation of the phase diagram of high-temperature superconductors in terms of the model ofnegative-U centres superconductivity

This paper demonstrates qualitatively how a unified explanation ofthe wide variety of different regions in a typical phase diagram ofhigh-temperature superconductors (HTSC) can be given in terms of the model ofnegative-U centres superconductivity. The existence of four regions (pseudogap,superconducting, non-Fermi liquid and Fermi liquid) in the phase diagram andall the transitions between these regions can be explained by assuming thatnegative-U centres exist in HTSC and qualitatively taking into account their thermodynamic anddirect quantum-mechanical interaction with ordinary electrons from the valence band. It isfound that the principal parameter determining the properties of HTSC in thenegative-U centres model is the relative concentration of electrons belonging to thenegative-U centres.The negative-U centres naturally determine both the superconducting properties of HTSC systems and theentire phase diagram in the normal state.

Anomalous Fermi liquid behavior of overdoped high-Tcsuperconductors

According to a generic temperature versus carrier-doping $(T\text{\ensuremath{-}}p)$ phase diagram of high-temperature superconductors (HTSC), it has been proposed that as doping increases to the overdoped region, they approach gradually a conventional (canonical) Fermi liquid. However, Hall effect measurements in several systems reported by different authors show a still strong $T$ dependence in overdoped samples. We report here electrical transport measurements of ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ thin films presenting a temperature dependence of the Hall constant, ${R}_{H}$, which does not present a gradual transition towards the $T$-independent behavior of a canonical Fermi liquid. Instead, the $T$ dependence passes by a minimum near optimal doping and then increases again in the overdoped region. We discuss the theoretical predictions from two representative Fermi liquid models and show that they cannot give a satisfactory explanation to our data. We conclude that this region of the phase diagram in YBCO, as in most HTSC, is not a canonical Fermi liquid, therefore we call it an anomalous Fermi liquid.

Comment on “Study of the multicritical point in the vortex phase diagram(H∥c)of a weakly pinned crystal ofYBa2Cu3O7−δ”

The continuity between the thermally driven first-order vortex lattice melting observed at high temperatures and the disorder-driven transition appearing in the low-temperature region of the vortex phase diagram of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) is still controversial. Pal et al. have reported a rich vortex phase diagram for relatively clean YBCO crystals, in which, in qualitative agreement with a recent theoretical proposal, the end point of the vortex lattice melting is located well above the disorder-driven transition line. However, the results presented by Pal et al. suggest the existence of domains of different quenched disorder degree in the investigated crystal, and in these conditions the global magnetic measurements cannot lead to definite conclusions about the vortex phase diagram of high-temperature superconductors.

Critical behavior in the low-energy spin fluctuations in high-T(c) superconductors

We study the spin fluctuations of pure and Zn-substituted La2 − xSrxCuO4 using the muon spin relaxation technique. Superconductivity is found to coexist with low-frequency spin fluctuations over a large region of the superconducting phase diagram. The characteristic temperature of spin fluctuations detected by μSR decreases with increasing x and vanishes above a critical doping xc ≈ 0.19. This value of xc coincides with the doping at which the normal state pseudogap extrapolates to zero. These results are discussed in terms of a quantum transition that separates the superconducting phase diagram of high-temperature superconductors into two distinct ground states.

High-field generic phase diagram of high-temperature superconductors

The temperature-dependent resistivity of (Y 1− z Pr z )Ba 2Cu 3O x epitaxial thin films has been measured in zero-field and at very-high-pulsed magnetic fields (F. Herlach et al., Physica B 216 (1996) 161). It is shown that the zero-field normal-state resistivity above T c for various levels of hole doping p for the (Y 1− z Pr z )Ba 2Cu 3O x cuprate superconductors can be scaled onto one single universal curve (B. Wuyts et al., Phys. Rev. B 53 (1996) 9418). An energy scale Δ, the resistivity ρ Δ and the residual resistivity ρ 0 are suitable scaling parameters. The universal curve exhibits a first region (I) of linear ρ( T) at high temperatures T> T*, a super-linear ρ( T) region at intermediate temperatures T MI< T< T* (II) and a low-temperature insulating-like ρ( T) region (III) at T< T MI. This low-temperature regime is masked by the onset of superconductivity at T= T c. The existence of a universal metallic ρ( T) curve is interpreted as a strong indication of one single mechanism dominating the scattering of the charge carriers in these materials. In order to gain access to the low-temperature T< T c part of the normal-state transport properties (regions II and III), experiments in high magnetic fields are performed. These experiments allowed suppressing the superconducting state thus retaining the normal-conducting high-field normal-state properties. Our experiments revealed that the ground state of YBa 2Cu 3O x (for x<6.8) is of an insulating nature with the resistivity increasing as temperature is lowered (G.S. Boebinger et al., Phys. Rev. Lett. 77 (1996) 5417; Y. Ando et al., Phys. Rev. Lett. 77 (1996) 2065; Phys. Rev. B 56 (1997) R8530). All experiments shown in this work are understood as consequences of a low-dimensional quantum transport mechanism in combination with scattering of a magnetic origin (V.V. Moshchalkov, Sol. State Commun. 86 (1993) 715; Cond-Mat/9802281; V.V. Moshchalkov et al., Europhys. Lett. 46 (1999) 75).

Is there an ideal phase diagram for high-temperature superconductors?

Abstract Powerful new/old experimental evidence supports the filamentary model, for the metal-insulator transition in randomly doped, uncompensated impurity-band semiconductors. The resulting phase diagram, which contains an intermediate ‘X’ phase, closely resembles the usual phase diagram for high-temperature superconductors, leading to the identification of the X phase (which is often called a ‘non-Fernii liquid’) as an anisotropic 3d filamentary metal. An important by-product of the new/old evidence is that it leads to a quantitative, parameter-free estimate of the strength of the dopant electron-ion interaction energies, which in the impurity band case are 25 times larger than those of bonding valence electrons, corresponding to a maximum T c ∼ 150 K. A phase diagram is predicted for an ‘ideal’ high-temperature superconductor which is characteristically different from that currently sketched in experimental papers; observation of such a phase diagram would constitute strong evidence in favour of the filamentary model.

Is there an ideal phase diagram for high-temperature superconductors?

Is there an ideal phase diagram for high-temperature superconductors?

In-plane and out-of-plane magnetoresistance in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} with various carrier densities

The in-plane and out-of-plane magnetoresistance of Bi2Sr2CaCu2O8+xsingle crystals was investigated in magnetic fields of 12 T, oriented parallel to the c axes. Crystals with different carrier concentrations, corresponding to the underdoped, optimally doped, and overdoped region of the phase diagram of high-temperature superconductors, were prepared by variation of the oxygen content. Above the critical temperature, the magnetoresistance is positive for in-plane and negative for out-of-plane currents in all samples. With recent theories for superconducting order-parameter fluctuations, the in-plane magnetoresistance can be described well for all samples. Only the out-of-plane magnetoresistance of the underdoped crystal shows distinct deviations from the theoretical predictions and appears to be influenced by the ‘pseudo-gap’.

Topological order in the phase diagram for high-temperature superconductors with point defects

Applying a Lindemann like criterion obtained previously by Kierfeld et al. [J. Kierfeld, T. Nattermann, T. Hwa, Phys. Rev. B 55 (1997) 626], we estimate the magnetic field and temperature for a high- T c superconductor, at which a topologically ordered vortex glass phase becomes unstable with respect to a disorder-induced formation of dislocations. The employed criterion is shown to be equivalent to a conventional phenomenological Lindemann criterion including the values for the numerical factors, i.e., for the Lindemann number. The positional correlation length of the topologically ordered vortex glass is calculated.

A model for high-temperature superconductors using the extended Hubbard hamiltonian

We derive a method to study the phase diagram for high-temperature superconductors (HTCS's). Our starting point is the Hubbard hamiltonian with a weak attractive interaction to obtain the formation of bound pairs. We consider this attractive potential at different positions for different compounds. We then construct a wave function of the BCS type by a variational method using the Fourier transform of this extended Hubbard potential and then derive an energy gap equation which can be applied to the s wave and d wave channel. This simple approach allows us to obtain the critical temperature as a function of the doping concentration which gives very good agreement with the experimental phase diagrams of YBaCuO and La(Sr,Ba)CuO compounds.

Influence of processing on the low-field magnetic phase diagram of high-temperature superconductors

Low AC field magnetic phase diagrams in the (hAC,T) plane have been deduced from AC susceptibility measurements for YBa2Cu3O7- delta sintered polycrystals, magnetically melt-textured polycrystals and single crystals. It is shown that dissipative regimes appear in all cases above a critical line located well below the irreversibility line, and the strongest shifts of the critical line against the irreversibility line are observed in magnetically melt-textured polycrystals. The relative positions of the irreversibility lines show that the increase of jc in magnetically melt-textured polycrystals is relevant to the creation of pinning sites with high activation energies.