" Pseudogap state from quantum criticality"
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Upon application of an external tuning parameter, a magnetic state can be driven
to a normal metal state at zero temperature. This phenomenon is known as quantum
criticality and leads to fascinating responses in thermodynamics and transport
of the compound. In the standard picture, a single quantum critical point occurs
at zero temperature, which results in a nontrivial critical behaviour in its
vicinity. Here we show that in two dimensions the scenario is considerably more
complex due to the enormous amount of quantum fluctuations. Instead of the
single point separating the antiferromagnet from the normal metal, we have
discovered a broad region between these two phases where the magnetic order is
destroyed but certain areas of the Fermi surface are closed by a large gap. This
gap reflects the formation of a novel quantum state characterised by a
superposition of d-wave superconductivity and a quadrupole-density wave, id est
a state in which an electron quadrupole density spatially oscillates with a
period drastically different from the one of the original spin-density wave. At
moderate temperatures both orders co-exist at short distances but thermal
fluctuations destroy the long-range order. Below a critical temperature the
fluctuations are less essential and superconductivity becomes stable. This new
phenomenon may shed some light on the origin of the mysterious pseudogap state
and of the high-temperature transition into the superconducting state in
cuprates. Our results demonstrate that quantum phase transitions between
antiferromagnets and normal metals in layered materials may be the proper
playground for search of new high temperature superconductors.