Bose glass and Mott glass of quasiparticles in a doped quantum magnet.

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.

Ferromagnetic Quantum Critical Point Avoided by the Appearance of Another Magnetic Phase in LaCrGe_{3} under Pressure.

The temperature-pressure phase diagram of the ferromagnet LaCrGe_{3} is determined for the first time from a combination of magnetization, muon-spin-rotation, and electrical resistivity measurements. The ferromagnetic phase is suppressed near 2.1 GPa, but quantum criticality is avoided by the appearance of a magnetic phase, likely modulated, AFM_{Q}. Our density functional theory total energy calculations suggest a near degeneracy of antiferromagnetic states with small magnetic wave vectors Q allowing for the potential of an ordering wave vector evolving from Q=0 to finite Q, as expected from the most recent theories on ferromagnetic quantum criticality. Our findings show that LaCrGe_{3} is a very simple example to study this scenario of avoided ferromagnetic quantum criticality and will inspire further study on this material and other itinerant ferromagnets.

Rhombohedrally distorted γ-brasses Cr(1-x)Fe(x)Ga.

A series of rhombohedrally distorted γ-brass structures containing a mixture of magnetically active 3d elements, Cr and Fe, Cr(1-x)Fe(x)Ga, is investigated crystallographically. These structures consist of chains of trans-face-sharing Ga-centered transition metal icosahedra. Neutron powder diffraction specifically on Cr(0.5)Fe(0.5)Ga, which could be prepared as a single phase material, gave lattice constants (11 K) a = 12.5172(2) Šand c = 7.8325(2) Šand a refined composition of Cr(0.502(6))Fe(0.498)Ga = Cr(6.523)Fe(6.477)Ga(13) and revealed partial ordering of Cr and Fe atoms among three crystallographic sites. Magnetic susceptibility and magnetization studies of Cr(0.5)Fe(0.5)Ga showed the onset of magnetic ordering at ca. 25 K. Theoretical calculations suggested both site-energy and bond-energy factors influencing the Cr/Fe distribution. Heteroatomic interactions significantly affect exchange interactions and create low local magnetic moments. Models created to mimic Cr(0.5)Fe(0.5)Ga showed ferromagnetic Fe-Fe and antiferromagnetic Cr-Fe interactions, with an overall ferrimagnetic ordering.