Spatially Resolved Magnetization in the Bose-Einstein Condensed State of BaCuSi2O6: Evidence for Imperfect Frustration

TL;DR

This study uses NMR to investigate the magnetization profile in BaCuSi2O6, revealing that imperfect frustration leads to partial condensation in less populated layers of the Bose-Einstein condensed phase.

Contribution

It provides the first detailed NMR analysis showing that perfect frustration is slightly lifted, causing partial condensation in the layered structure of BaCuSi2O6.

Findings

Different local magnetizations in layers near Hc1

Partial condensation occurs due to lifted frustration

NMR data aligns with a model of interacting bosons

Abstract

In order to understand the nature of the two-dimensional Bose-Einstein condensed (BEC) phase in BaCuSi2O6, we performed detailed 63Cu and 29Si NMR above the critical magnetic field, Hc1= 23.4 T. The two different alternating layers present in the system have very different local magnetizations close to Hc1; one is very weak, and its size and field dependence are highly sensitive to the nature of inter-layer coupling. Its precise value could only be determined by "on-site" 63Cu NMR, and the data are fully reproduced by a model of interacting hard-core bosons in which the perfect frustration associated to tetragonal symmetry is slightly lifted, leading to the conclusion that the population of the less populated layers is not fully incoherent but must be partially condensed.