Curie-like paramagnetism due to incomplete Zhang-Rice singlet formation in La2−xSrxCuO

Abstract

In an effort to elucidate the origin of the Curie-like paramagnetism that is generic for heavilyoverdoped cuprates, we have performed high transverse-field muon spin rotation (TF-µSR) measurements of La2−xSrxCuO4 single crystals over the Sr content range 0.145 ≤ x ≤ 0.33. We show that the x-dependence of the previously observed field-induced broadening of the internal magnetic field distribution above the superconducting transition temperature Tc reflects the presence of two distinct contributions. One of these becomes less pronounced with increasing x and is attributed to diminishing antiferromagnetic correlations. The other grows with increasing x, but decreases above x ∼ 0.30, and is associated with the Curie-like term in the bulk magnetic susceptibility χ. In contrast to the Curie-like term, however, this second contribution to the TF-µSR line width extends back into the underdoped regime. Our findings imply a coexistence of antiferromagnetically correlated and paramagnetic moments, with the latter becoming dominant beyond x ∼ 0.185. This suggests that the doped holes do not neutralize all Cu spins via the formation of Zhang-Rice singlets. Moreover, the paramagnetic component of the TF-µSR line width is explained by holes progressively entering the Cu 3d x 2 −y 2 orbital with doping.

Search for ferromagnetic order in overdoped copper-oxide superconductors

“…The size of this contribution can be estimated from the measured magnetic susceptibility. In overdoped LSCO ( x  = 0.33), the paramagnetic susceptibility1215 (including Pauli, Curie and other terms) is χ  = 4 × 10 −6  emu/cm 3 at T = 5 K. The corresponding magnetization in the applied 1.15 T magnetic field is ~0.05 emu/cm 3 , which is ~40 times smaller than the magnetization we measured in the x  = 0.35 LSCO film (~2 emu/cm3). Therefore we conclude that the paramagnetic effect is not very important for our observations; the magnetic contrast for the most part originates from static magnetic moments.…”

“…Hence, we chose the Sr doping level x  = 0.27, 0.29 and 0.35 to cover both the superconducting and metallic states in the vicinity of SMT. Note that the Curie-like magnetic susceptibility peaks at x  = 0.30 in bulk LSCO12, which corresponds to x  = 0.35 in our thin films due to the epitaxial strain; therefore the ferromagnetic order should be at the maximum at x  = 0.35, if it exists. After the deposition we anneal the films in situ in atmosphere of pure ozone, in order to minimize the density of oxygen vacancies.…”

“…Measurements of the magnetic susceptibility χ in overdoped LSCO and Tl-2201 [refs 8, 9, 10, 11, 12] revealed the appearance of an additional term proportional to 1/ T . This Curie-like paramagnetic susceptibility originates from localized magnetic moments that are fluctuating at finite temperature and can be polarized by a magnetic field.…”

“…The dramatic evolution of Fermi surface with overdoping in LSCO may provide a chance for magnetic moments to be interacting and correlated in a preferred direction, in some composition window. Several experimental observations provide some support to this scenario8910111213141516. The temperature dependence of magnetic susceptibility ( χ ) in overdoped LSCO crystals indicated the absence of local magnetic moments below the critical doping x c  ≈ 0.18, and dramatic increase in their density with further overdoping89101112.…”

Universal inhomogeneous magnetic-field response in the normal state of cuprate high-Tcsuperconductors

“…[112,[122][123][124], resembling a Curie-like term in the magnetic susceptibility [115]. In fact, there appears to be a Curie-like contribution even at lower dopings, which becomes more pronounced with increased hole-doping [115].…”