XBEND · eXtreme BENDing strain induced novel interfaces in single crystal cantilevers of strongly correlated metals

The ground state of correlated materials is controlled by tuning parameters such as pressure or strain, which are usually applied homogeneously to crystals. Here we aim for strong yet controlled strain gradient fields in microstructured correlated crystals that selectively transform parts of the structure into different electronic ground states. XBENDs ambition is to open a new field of research into gradient quantum matter, defined as a highly non-linear regime in which gradients lead to qualitatively new behavior as compared to simple spatial distributions of bulk phases.The concrete goal is a rational design of new electronic properties at interfaces between different correlated ground states within a single crystal – akin to the interface states between different materials in heterostructures. Specifically, we aim to frustrate interactions and/or competing orders in unconventional superconductors at this interface, with the goal to enhance the transition temperature Tc. We will create interfaces between (1) ferro-polar and paraelectric phases in crystals of doped SrTiO3; (2) oppositely detwinned domains in underdoped Ba(Fe,Co)2As2 crystals; (3) hidden-order and magnetic phases in URu2Si2. Contrasting their nature and physical responses to those expected from trivial phase mixtures will signal that XBEND has entered a non-linear gradient-driven regime. Technically, we will fabricate free-standing single crystal cantilevers from as-grown crystals using Focused Ion Beam (FIB) machining. The cantilevers will be functionalized to allow cryogenic measurements of magnetoresistance while applying bending strain by pushing them with a piezoelectric motor. XBEND aims for extreme strain gradients in the 10%/micron range, yet our proof-of-concept already demonstrated 2%/micron. The induced microscopic strain/domain pattern will be probed by X-Ray microdiffraction at DESY/PETRA III, and a complete finite-element-model will capture the elasto-resistive response.