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Register a new userDisorder, Metal-Insulator crossover and Phase diagram in high-Tc cuprates
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Florence Rullier-Albenque
Publication date
2008
fields
Physics
and research's language is
English
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We have studied the influence of disorder induced by electron irradiation on the normal state resistivities $rho(T)$ of optimally and underdoped YBa2CuOx single crystals, using pulsed magnetic fields up to 60T to completely restore the normal state. We evidence that point defect disorder induces low T upturns of rho(T) which saturate in some cases at low T in large applied fields as would be expected for a Kondo-like magnetic response. Moreover the magnitude of the upturns is related to the residual resistivity, that is to the concentration of defects and/or their nanoscale morphology. These upturns are found quantitatively identical to those reported in lower Tc cuprates, which establishes the importance of disorder in these supposedly pure compounds. We therefore propose a realistic phase diagram of the cuprates, including disorder, in which the superconducting state might reach the antiferromagnetic phase in the clean limit.
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We present the influences of electronic and magnetic correlations and doping evolution on the groundstate properties of recently discovered superconductor Ba$_{2}$CuO$_{4-delta}$ by utilizing the Kotliar-Ruckenstein slave boson method. Starting with an effective two-orbital Hubbard model (Scalapino {it et al.} Phys. Rev. {bf B 99}, 224515 (2019)), we demonstrate that with increasing doping concentration, the paramagnetic (PM) system evolves from two-band character to single-band ones around the electron filling n=2.5, with the band nature of the $d_{3z^{2}-r^{2}}$ and $d_{x^{2}-y^{2}}$ orbitals to the $d_{x^{2}-y^{2}}$ orbital, slightly affected when the electronic correlation U varies from 2 to 4 eV. Considering the magnetic correlations, the system displays one antiferromagnetically metallic (AFM) phase in $2<n<2.16$ and a PM phase in $n>2.16$ at U=2 eV, or two AFM phases in $2<n<2.57$ and $2.76<n<3$, and a PM phase in $2.57<n<2.76$ respectively, at U=4 eV. Our results show that near realistic superconducting state around n=2.6 the intermediate correlated Ba$_{2}$CuO$_{3,2}$ should be single band character, and the s-wave superconducting pairing strength becomes significant when U$>$2 eV, and crosses over to d-wave when U$>$2.2 eV.
We have studied the doping dependence of the in-plane and out-of-plane superfluid density, rho^s(0), of two monolayer high-Tc superconductors, HgBa_2CuO_{4+delta} and La_{2-x}Sr_xCuO_4, using the low frequency ac-susceptibility and the muon spin relaxation techniques. For both superconductors, rho^s(0) increases rapidly with doping in the under- and optimally doped regime and becomes nearly doping independent above a critical doping, p_c = 0.20.
We show that the resistivity in each phase of the High-Tc cuprates is a special case of a general expression derived from the Kubo formula. We obtain, in particular, the T-linear behavior in the strange metal (SM) and upper pseudogap (PG) phases, the pure $T^2$, Fermi liquid (FL) behavior observed in the strongly overdoped regime as well as the $T^{1+delta}$ behavior that interpolates both in the crossover. We calculate the coefficients: a) of $T$ in the linear regime and show that it is proportional to the PG temperature $T^*(x)$; b) of the $T^2$-term in the FL regime, without adjusting any parameter; and c) of the $T^{1.6}$ term in the crossover regime, all in excellent agreement with the experimental data. From our model, we are able to infer that the resistivity in cuprates is caused by the scattering of holes by excitons, which naturally form as holes are doped into the electron background.
Cuprate high-T_c superconductors on the Mott-insulating side of optimal doping (with respect to the highest T_cs) exhibit enigmatic behavior in the non-superconducting state. Near optimal doping the transport and spectroscopic properties are unlike those of a Landau-Fermi liquid. For carrier concentrations below optimal doping a pseudogap removes quasi-particle spectral weight from parts of the Fermi surface, and causes a break-up of the Fermi surface into disconnected nodal and anti-nodal sectors. Here we show that the near-nodal excitations of underdoped cuprates obey Fermi liquid behavior. Our optical measurements reveal that the dynamical relaxation rate 1/tau(omega,T) collapses on a universal function proportional to (hbar omega)^2+(1.5 pi k_B T)^2. Hints at possible Fermi liquid behavior came from the recent discovery of quantum oscillations at low temperature and high magnetic field in underdoped YBa2Cu3O6+d and YBa2Cu4O8, from the observed T^2-dependence of the DC ({omega}=0) resistivity for both overdoped and underdoped cuprates, and from the two-fluid analysis of nuclear magnetic resonance data. However, the direct spectroscopic determination of the energy dependence of the life-time of the excitations -provided by our measurements- has been elusive up to now. This observation defies the standard lore of non-Fermi liquid physics in high T_c cuprates on the underdoped side of the phase diagram.
The mechanism of high temperature superconductivity is not resolved for so long because the normal state of cuprates is not yet understood. Here we show that the normal state pseudo-gap exhibits an unexpected non-monotonic temperature dependence, which rules out the possibility to describe it by a single mechanism such as superconducting phase fluctuations. Moreover, this behaviour, being remarkably similar to the behaviour of the charge ordering gap in the transition-metal dichalcogenides, completes the correspondence between these two classes of compounds: the cuprates in the PG state and the dichalcogenides in the incommensurate charge ordering state reveal virtually identical spectra of one-particle excitations as function of energy, momentum and temperature. These results suggest that the normal state pseudo-gap, which was considered to be very peculiar to cuprates, seems to be a general complex phenomenon for 2D metals. This may not only help to clarify the normal state electronic structure of 2D metals but also provide new insight into electronic properties of 2D solids where the metal-insulator and metal-superconductor transitions are considered on similar basis as instabilities of particle-hole and particle-particle interaction, respectively.
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