Anomalous Transfer of Spectral Weight and Pseudogap in Underdoped Cuprate HTSC’s

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 Tc < 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 Tc with a very high probability the cluster is in a superconducting state, and above T* it is in a normal state, and the interval Tc < T < T* is the region of the so-called pseudogap phase. 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 Δωeff 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 Δωeff ~ 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 Tc: the reversibility of magnetization curves in a certain temperature range below Tc, the anomalous Nernst effect and anomalous diamagnetism above Tc.