Doping and Magnetic Field Dependence of In-Plane Tunneling intoYBa2Cu3O7−

Y. Dagan

¹

,

G. Deutscher

²

Abstract

We present tunneling measurements into (1,1,0) YBa(2)Cu(3)O(7-x) films at various doping levels around the optimum. We find that, above a certain doping level near optimum doping, a spontaneous zero bias conductance peak splitting, delta, appears. It increases with doping. It also increases with magnetic field applied along the c axis, for both underdoped and overdoped films. The low field susceptibility chi = d delta /dH/(H-->0) is maximum, possibly diverging when the spontaneous value of delta goes to zero. These results suggest a transition from a pure d(x(2)-y(2)) to a d+id(xy) or d+is order parameter.

Local and macroscopic tunneling spectroscopy ofY1−xCaxBa<

“…However, some other (nominally) optimally doped YBCO films prepared by laser ablation 11 did show splitting with small peak-to-peak separations, up to δ s =1.2 meV. It seems thus that a transition to a state of broken time reversal symmetry takes place (at least at 4.2 K) at a critical doping around optimal, in agreement with previous results 24,25 .…”

“…Furthermore, the values of the splits obtained by the two distinct tunneling methods are in good agreement with each other. These data confirm those of Dagan and Deutscher obtained on oxygen-overdoped (110) YBCO 24 , as well as the preliminary results obtained by Deutscher et al 25 and Kohen et al 22 on Ca-doped films. Evidence for a d+is (or d+id`) OP is found not only in [110] tunneling, but also in tunneling spectra that we have measured in off-nodal directions.…”

“…On the other hand, a subdominant s-wave OP that was also proposed 3 does not remove (only shifts) the nodes, and therefore has only little effect on the ZBCP. Spontaneous splitting of the ZBCP (in zero magnetic field) was observed by a few groups 5,11,[21][22][23][24] .…”

“…Deutscher and collaborators have shown that such splitting takes place only beyond some critical doping level, close to optimal 21,24,25 , and the dependence of the split on the magnetic field (both magnitude and direction) is better explained by a d+id` OP 9, 24, 25 . Sharoni et al also noted that ZBCP splitting is not observed for underdoped YBCO films 11 .…”

Spontaneously broken time-reversal symmetry in high-temperature superconductors

“…Ever since, there has been an ongoing debate whether there exists a low-temperature phase that breaks time-reversal (T ) symmetry, in addition to the reflection symmetry of the crystal broken by the d-wave order parameter itself. Several experiments on tunnelling and charge transport support a phase transition into a state with broken T -symmetry at low temperatures [3][4][5][6][7][8]. On the other hand efforts to detect the concomitant spontaneous magnetic field have failed, or at best have put severe restrictions on how strong the subdominant order can be [9][10][11][12][13].…”