Details: |
"Experimental evidence for Bose Einstein condensation of triplets in spin dimer
compounds"
Spin dimer compounds are based on strongly coupled pairs
of spins (dimers), which by virtue of the crystal structure only weakly
interact with each other. For spin ½ the ground state is a product of
singlets and there is a gap to excited triplet states. A field-tuned quantum
critical point (QCP) separates the low-field quantum paramagnetic state
from a state characterized by long-range magnetic order. In the absence
of terms in the spin Hamiltonian that explicitly break U(1) symmetry,
this phase transition can be interpreted as a Bose-Einstein condensation
(BEC) of delocalized triplets. Despite considerable theoretical interest
in this class of material, there are very few examples available for experimental
study. In this talk I will describe the high field behavior of two new
spin dimer compounds that we have recently been able to synthesize; A_{2}Cu(BO_{3})_{2} (where A = Sr & Ba) and BaCuSi_{2}O_{6} (also known as "Han purple").
The spin
gap in A_{2}Cu(BO_{3})_{2} is rather large
(7.4 and 7.9 meV for A = Sr and Ba respectively),
all but precluding direct measurements of the ordered state. On the other
hand, BaCuSi_{2}O_{6} has a smaller spin gap of just 3.1 meV,
corresponding
to a critical field H_{c1} of approximately 23 T. Consequently, we have been
able to probe the ordering transition, determining the critical exponent
governing the approach of the phase boundary towards the QCP at H_{c1}. Our
results show that the phase transition is indeed in the BEC universality
class. Moreover, we have found evidence for a dramatic cross-over to 2-dimensional
behavior close to the QCP; a consequence of the unique geometry of the
BaCuSi_{2}O_{6} body-centered tetragonal lattice. |