title: Dimensional reduction at a quantum critical point.
abstract: Competition between electronic ground states near a quantum critical point (QCP)--the location of a zero-temperature phase transition driven solely by quantum-mechanical fluctuations--is expected to lead to unconventional behaviour in low-dimensional systems. New electronic phases of matter have been predicted to occur in the vicinity of a QCP by two-dimensional theories, and explanations based on these ideas have been proposed for significant unsolved problems in condensed-matter physics, such as non-Fermi-liquid behaviour and high-temperature superconductivity. But the real materials to which these ideas have been applied are usually rendered three-dimensional by a finite electronic coupling between their component layers; a two-dimensional QCP has not been experimentally observed in any bulk three-dimensional system, and mechanisms for dimensional reduction have remained the subject of theoretical conjecture. Here we show evidence that the Bose-Einstein condensate of spin triplets in the three-dimensional Mott insulator BaCuSi2O6 (refs 12-16) provides an experimentally verifiable example of dimensional reduction at a QCP. The interplay of correlations on a geometrically frustrated lattice causes the individual two-dimensional layers of spin-(1/2) Cu2+ pairs (spin dimers) to become decoupled at the QCP, giving rise to a two-dimensional QCP characterized by linear power law scaling distinctly different from that of its three-dimensional counterpart. Thus the very notion of dimensionality can be said to acquire an 'emergent' nature: although the individual particles move on a three-dimensional lattice, their collective behaviour occurs in lower-dimensional space.
authors.affiliation: Geballe Laboratory for Advanced Materials and Department of Applied Physics, Stanford University, Stanford, California 94305, USA. suchitra@stanfordalumni.org
authors.forename: S E
authors.initials: SE
authors.lastname: Sebastian
authors.forename: N
authors.initials: N
authors.lastname: Harrison
authors.forename: C D
authors.initials: CD
authors.lastname: Batista
authors.forename: L
authors.initials: L
authors.lastname: Balicas
authors.forename: M
authors.initials: M
authors.lastname: Jaime
authors.forename: P A
authors.initials: PA
authors.lastname: Sharma
authors.forename: N
authors.initials: N
authors.lastname: Kawashima
authors.forename: I R
authors.initials: IR
authors.lastname: Fisher