Amit Keren: Magnetic "Isotope Effect" in Cuprates

Amit Keren (Technion, Israel) told us about an accumulation of 10 years of
research by his group, of a family of YBCO-like high Tc cuprates called
CLBLCO, = (Ca_x La_{1-x})(Ba_{1.75-x}La_{0.25+x})Cu_3O_y.

While most systematic studies
of cuprates involve changing just one doping parameter, such as the oxygen
concentration y, CLBLCO presents a unique opportunity to continuously vary TWO
parameters, x (family index) and y (oxygen concentration), without
significantly disturbing the structure or varying the disorder in the CuO_2
planes. In fact, the primary effect of changing x on the CuO plane is to
slightly vary the copper-oxygen buckling angle which is known to change the
magnetic superexchange constant.

Amit showed his group's mu-SR data for the superconducting transition
temperature Tc(x,y), the spin freezing temperature Tg(x,y) at intermediate
doping, and Neel temperature T_N(x,y) at low doping. Moreover, the 2D AFM
exchange J(x) was extracted from the Neel temperature (by fitting the
T-dependent staggered magnetization to estimate the interlayer exchange).

At first, the data seems scattered on the (T,y) phase diagram. Amit chose to
collapse the data by rescaling all transition temperatures by T_c^{max}(x), and
also rescaling the y axis by an "effective doping" Delta p = K(x) (y-y_max),
which collapsed all the Tc(Delta p) "domes" onto one universal curve.
Collapsing the Tc domes is hardly surprising. However the same axes rescaling
completely collapses the -magnetic- freezing transitions onto one curve as
well!

The conclusion is that T_c^{max}(x) \propto J(x).

Apparently, the data collapse indicates that a single energy scale determines
both antiferromagnetic and SC ordering temperatures!

This blogger feels that this finding, although simple, is far from obvious. It
puts a serious constraint on theoretical mechanisms of cuprate
superconductivity: One would naively expect more than just J, to determine T_c (say e.g. some
additional kinetic or interaction energy might be important). These scales have
no apparent reason to stay proportional to each other as the two material parameters x, and y,
are independently varied.


Amit also showed uniform susceptibility data, which was used to define
the "pseudogap temperature" T*(x,y). While this energy scale did not precisely
collapse by rescaling the axes, it seemed to follow for some reason the 3D Neel
temperature T_N, which depends on magnetic interactions both in and out of the CuO planes.