title: Bose glass and Mott glass of quasiparticles in a doped quantum magnet.
abstract: The low-temperature states of bosonic fluids exhibit fundamental quantum effects at the macroscopic scale: the best-known examples are Bose-Einstein condensation and superfluidity, which have been tested experimentally in a variety of different systems. When bosons interact, disorder can destroy condensation, leading to a 'Bose glass'. This phase has been very elusive in experiments owing to the absence of any broken symmetry and to the simultaneous absence of a finite energy gap in the spectrum. Here we report the observation of a Bose glass of field-induced magnetic quasiparticles in a doped quantum magnet (bromine-doped dichloro-tetrakis-thiourea-nickel, DTN). The physics of DTN in a magnetic field is equivalent to that of a lattice gas of bosons in the grand canonical ensemble; bromine doping introduces disorder into the hopping and interaction strength of the bosons, leading to their localization into a Bose glass down to zero field, where it becomes an incompressible Mott glass. The transition from the Bose glass (corresponding to a gapless spin liquid) to the Bose-Einstein condensate (corresponding to a magnetically ordered phase) is marked by a universal exponent that governs the scaling of the critical temperature with the applied field, in excellent agreement with theoretical predictions. Our study represents a quantitative experimental account of the universal features of disordered bosons in the grand canonical ensemble.
authors.affiliation: Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA.
authors.forename: Rong
authors.initials: R
authors.lastname: Yu
authors.forename: Liang
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authors.lastname: Yin
authors.forename: Neil S
authors.initials: NS
authors.lastname: Sullivan
authors.forename: J S
authors.initials: JS
authors.lastname: Xia
authors.forename: Chao
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authors.lastname: Huan
authors.forename: Armando
authors.initials: A
authors.lastname: Paduan-Filho
authors.forename: Nei F
authors.initials: NF
authors.lastname: Oliveira
authors.forename: Stephan
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authors.lastname: Haas
authors.forename: Alexander
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authors.lastname: Steppke
authors.forename: Corneliu F
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authors.lastname: Miclea
authors.forename: Franziska
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authors.lastname: Weickert
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authors.lastname: Movshovich
authors.forename: Eun-Deok
authors.initials: ED
authors.lastname: Mun
authors.forename: Brian L
authors.initials: BL
authors.lastname: Scott
authors.forename: Vivien S
authors.initials: VS
authors.lastname: Zapf
authors.forename: Tommaso
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authors.lastname: Roscilde