Physical Nature of the Pseudogap Phase and Anomalous Transfer of Spectral Weight in Underdoped Cuprates

Abstract

It is shown that many anomalies observed in underdoped cuprates, including anomalous spectral weight transfer and a large pseudogap, appear to have a common nature due to both the cluster structure of the underdoped phase and the specific mechanism of superconducting pairing. The combined action of these factors leads to the fact that at a temperature T lying in a certain temperature range T_c < T < T*, the crystal contains small isolated clusters that can exist both in superconducting and normal states, randomly switching between them. In this case, below T_c with a very high probability, the cluster is in a superconducting state, and above T*, it is in a normal state, and the interval T_c < T < T* is the region of existence of the so-called pseudogap phase. The temperatures T_c and T* for YBa₂Cu₃O_6+δ were calculated depending on the doping level δ. The calculation results are in good agreement with the experiment without the use of fitting parameters. At a given T in the same temperature range, the time sequence of randomly arising superfluid density pulses from each cluster can be represented as a random process. The effective width Δω_eff of the spectrum of such a random process will be determined by a correlation time, i.e., the characteristic time between successive on/off superconductivity in two different clusters. This time, according to the estimate, is ~ 10⁻¹⁵ s, which corresponds to Δω_eff ~ 1 eV and explains the effect of spectral weight transfer to the high-frequency region. This approach also makes it possible to explain other anomalies observed in the vicinity of T_c: the reversibility of magnetization curves in a certain temperature range below T_c, the anomalous Nernst effect, and anomalous diamagnetism above T_c.