Spin-gap Behavior Research Articles

Superconductive phonon anomalies in high- Tc cuprates

We consider the effects of spin-phonon coupling within the slave-boson mean-field treatment of the extended t- J model. With no additional assumptions the theory gives a semi-quantitative account of the frequency and linewidth anomalies of certain phonon modes in YBa 2Cu 3O 7 at the superconducting transition. It contains also a good correspondence with observed spin-gap behaviour and isotope effect measurements.

C13NMR studies of the normal and superconducting states of the organic superconductor κ-(ET)2Cu[N(CN)2]Br

The authors report $^{13}\mathrm{C}$ NMR spin-lattice relaxation rates 1/${\mathit{T}}_{1}$ and Knight shifts ${\mathit{K}}_{\mathit{S}}$ in the quasi-two-dimensional organic superconductor \ensuremath{\kappa}-(ET${)}_{2}$Cu[N(CN${)}_{2}$]Br (${\mathit{T}}_{\mathit{c}}$=11.6 K), for an aligned single crystal. The normal-state behavior is reminiscent of the high-${\mathit{T}}_{\mathit{c}}$ cuprates, in which antiferromagnetic fluctuations and spin-gap behavior dominate. In the superconducting state, the data rule out the BCS electron-phonon mechanism as the source of the superconductivity, but support an unconventional pairing state with possible nodes in the gap function.

Doping Level Dependence of Spin-Gap Behavior inY(Ba1-xLax)2Cu3OyStudied by Cu-NQR

In order to investigate the hole doping dependence of the spin-gap behavior in Y123 system, the temperature dependence of the nuclear spin-lattice relaxation time T 1 of Cu(2) site was measured in ...

Carrier Concentration Dependence of the Pseudo Spin Gap Behavior inLaBa2Cu3Oy

The carrier-concentration dependence of the pseudo spin gap behaviors in the optimum and the overdoped regions of the LaBa 2 Cu 3 O y system has been studied by NMR. In the temperature dependences of 1/ T 1 T , it was observed that the spin gap behavior, which is known to exist in the underdoped region, is suppressed in the overdoped region. This seems to confirm the expectation that the spin gap behavior is specific to the underdoped region. However, LaBa 2 Cu 3 O 6.96 , which has T c as high as 95 K, still exhibits a clear spin-gap-like behavior. This fact suggests that the optimum hole concentration to maximize T c does not necessarily coincide with that where the spin gap behavior disappears in this system.

Nearly antiferromagnetic Fermi-liquid description of magnetic scaling and spin-gap behavior.

We present a Fermi-liquid description of magnetic scaling and spin-gap behavior in strongly correlated electron systems. We show that a gap in the spin-excitation spectrum is a natural consequence of the existence of a second energy scale of magnetic origin, in systems of itinerant, but nearly localized electron spins, such as are found in the cuprate superconductors and heavy-electron systems. A second energy scale leads to a frequency-dependent restoring force ${\mathit{f}}^{\mathit{a}}$(q,\ensuremath{\omega}) that becomes stronger as the frequency increases. A spin gap at Q=(\ensuremath{\pi}/a,\ensuremath{\pi}/a) is a consequence of these frequency-dependent vertex corrections; it takes on a constant value when the antiferromagnetic correlation length becomes a constant. We show that the spin-spin response function reduces to that recently introduced by Barzykin to describe the damping by particle-hole excitations of the spin-wave spectrum found in the N=\ensuremath{\infty} quantum nonlinear \ensuremath{\sigma} model. We discuss fully doped and underdoped cuprate superconductors, the appearance of a pseudogap of magnetic origin in the quasiparticle density of states in the underdoped system, and the constraints nuclear magnetic resonance and neutron-scattering experiments impose on the magnitude and temperature dependence of the Fermi-liquid parameters.

Is spin-gap behavior near T c in cuprates an evidence for d xy pairing? A role of superconducting fluctuations

The spin susceptibility at the antiferromagnetic wave vector of the normal state of high- T c cuprates is calculated on the basis of the mode-mode coupling theory of the spin fluctuation in 2D taking the superconducting fluctuation into account. The spin-gap behavior is obtained for s or d xy pairing but not for extended- s or d x 2−y 2 pairing when the Fermi surface is technically “nested”. This result together with the consideration on the effect of impurity scatterings may suggest d xy pairing and lead to a view point to understand the superconducting mechanism from charge-transfer fluctuations.

Spin-gap behavior in high-Tc oxides observed by nuclear spin-lattice relaxation

From the temperature dependence of the nuclear spin-lattice relaxation rate, 1/T1T, in the planar Cu(2) sites, it is found that a highly enhanced and strongly temperature-dependent relaxation process due to antiferromagnetic Cu spin fluctuations exists in all high-Tc superconducting oxides. The data also exhibit the opening of a gap in the low-lying magnetic excitations with an energy comparable to the superconducting gap. In contrast to this T1 behavior, the temperature variations of the indirectly coupled nuclear spin-spin relaxation rate, 1/T2g, and the anisotropic Knight shifts Kα have quite an independent signature; the former shows continuous increase toward Tc and the latter in the low low-doping regime decreases from a much higher temperature than that at which the spin-gap is seen in 1/T1T. These features indicate that the generalized spin susceptibility, χ(q,ω), has very characteristic temperature variations for different q and ω regions.

Phase Diagram ofS=1/2 Antiferromagnetic Heisenberg Model on a Dimerized Square Lattice

The ground state energy, the energy gap and the Fourier transform of the staggered spin correlation function S ( Q ) at zero temperature are investigated for the S =1/2 antiferromagnetic Heisenberg model on a dimerized square lattice with the help of the spin-wave approximations, the second-order perturbation expansion and the quantum Monte Carlo simulations. In contrast to one-dimensional case, the spin gap appears above a finite critical value of the dimerization parameter. δ c ≃0.303, where the second-order transition takes place. The antiferromagnetic long range order exists below δ c . The critical exponent of the order parameter in the antiferromagnetically ordered state is estimated by quantum Monte Carlo calculation to be ≃(δ c -δ ) β with β=0.27±0.04, while the correlation length in the spin gap region is scaled as ≃(δ-δ c ) -ν with ν=1.36±0.05. All of them are different from the estimates in one-dimension, ν=1 and the spin-wave approximation, β=ν =1. Possible relevance of our result for the spin gap behavior in high- T c superconductors is also discussed.

NMR studies of high temperature superconductors

Results of NMR experiments mostly in YBa 2Cu 4O 7−δ are reviewed. The normal state data revealed two important aspects of the magnetic properties, namely, the temperature dependent antiferromagnetic Cu spin correlations and the spin gap behavior, the latter being observed in the reduced oxygen material. These features appear to be the general properties of many high T c cuprates. The Cu nuclear relaxation behavior in the superconducting state show some anomalous feature which remains to be explained.

Pairing and spin gap in the normal state of short coherence length superconductors.

We study the normal state of the 2D attractive Hubbard model using quantum Monte Carlo simulations. We show that singlet pairing correlations develop above ${\mathit{T}}_{\mathit{c}}$, and the normal state of a short coherence length superconductor deviates from a canonical Fermi liquid. In the intermediate U regime, the spin susceptibility ${\mathrm{\ensuremath{\chi}}}_{\mathit{s}}$ is strongly temperature dependent, and the low-frequency spectral weight, as measured by the NMR relaxation rate 1/${\mathit{T}}_{1}$T, is shown to track ${\mathrm{\ensuremath{\chi}}}_{\mathit{s}}$. This provides a simple, qualitative explanation for the spin-gap behavior observed in several high-${\mathit{T}}_{\mathit{c}}$ systems.

63Cu and17O NMR studies on the magnetic and superconducting properties of YBa2Cu3O7−y

Results of63Cu and17O NMR experiments in YBa2Cu3O7 and YBa2Cu3O6.63 are reviewed. The normal state data revealed two important aspects of the magnetic properties of these materials, namely, the temperature dependent antiferromagnetic Cu spin correlations and the spin gap behavior, the latter being observed in the reduced oxygen material. These features appear to be the general properties of many high-T c cuprates. Anomalous temperature dependence of the anisotropy of the Cu relaxation rate was found in the superconducting state of YBa2Cu3O7, which can be explained by a d-wave pairing model.

Critical and Gaussian fluctuation effects in the specific heat and conductivity of high-Tc, superconductors

Abstract We report measurements of specific heat and electrical resistivity for three high-Tc, oxides, YBa2Cu3O7-δ for 0⩽ δ ⩽ 0·18, (Ca0·8Y0·2)Sr2(Tl0·5Pb0·5)Cu2O7 and Tl1·8Ba2Ca2·2Cu3O10 for which superconducting fluctuations increase rapidly as the interplane coupling is reduced. The coherence lengths ξab and ξe, and Ginzburg temperatures τG are deduced from the data. For YBa2Cu3O7-δ we observe a factor of three decrease in ξe, and an order of magnitude increase in τG between δ = 0 and 0.1, resulting in a progression from relatively weak three-dimensional fluctuations for δ = 0 to strong two-dimensional (2D) critical fluctuations over a wide temperature range for δ⩾,0·1. Large values of τG and strong 2D critical behaviour are also observed in the other two compounds. We discuss the relevance of these results to the spin-gap behaviour observed above Tc , in other properties of oxygendepleted YBa2Cu3O7-δ. Our values of ξ are lower than those deduced from Hc2 measurements, raising the possibility that addi...

Nuclear relaxation behavior of the superconducting cuprates: Bi2Sr2CaCu2O8.

Nuclear-magnetic-resonance data are presented and analyzed for the high-${\mathit{T}}_{\mathit{c}}$ compound ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ for two oxygen doping levels. Both sample conditions lead to spin-gap behavior for the NMR shift, with a precursive downturn in the data at Tg${\mathit{T}}_{\mathit{c}}$. In addition, the relaxation times ${\mathit{T}}_{1}$ obey the relation (${\mathit{T}}_{1}$T${)}^{\mathrm{\ensuremath{-}}1}$\ensuremath{\propto}${\mathit{K}}^{\mathit{s}}$(T) at low temperatures (T\ensuremath{\lesssim}100 K), where ${\mathit{K}}^{\mathit{s}}$(T) is the spin paramagnetic shift. This relation, which is also obeyed by other superconductors, is argued to be related to the spin-gap effects and thus incompatible with a Fermi-liquid approach to the understanding of these systems.