Characteristic signatures of quantum criticality driven by geometrical frustration

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Science Advances  24 Apr 2015:
Vol. 1, no. 3, e1500001
DOI: 10.1126/sciadv.1500001
  • Fig. 1 Critical and noncritical control parameters for a two-dimensional geometrically frustrated lattice of magnetic moments.

    Black circles indicate Ce atoms in the quasi-kagome plane of CeRhSn. Uniaxial pressure parallel to the a axis (pa) deforms the equilateral triangular units and will reduce the geometrical frustration, whereas pressure perpendicular to the plane (pc) leaves frustration unchanged.

  • Fig. 2 Evidence of a frustration-induced QCP in CeRhSn.

    (A) Thermal expansion coefficient divided by temperature α/T versus temperature measured along the a and c directions, as indicated by red and blue symbols. The inset shows the Grüneisen parameter Γ = BVmβ/C, where B = 105 GPa is the bulk modulus of isostructural UCoAl (39), Vm = 1.36 × 10−4 m3/mol is the molar volume, β = 2αa + αc is the volume thermal expansion, and C denotes specific heat. The black dotted line indicates a power-law divergence. (B) Specific heat divided by temperature C/T as a function of temperature. Solid and open circles indicate data measured at zero field and at 2 T applied parallel to the a axis, respectively. Up to 2 T, there is no appreciable nuclear Schottky contribution visible even at lowest measured temperatures.

  • Fig. 3 Divergence of the magnetic Grüneisen parameter ΓH.

    ΓH/H as a function of temperature at various magnetic fields applied parallel to the c axis.

  • Fig. 4 Spin flop crossover in the spin liquid state of CeRhSn.

    (A) Magnetic Grüneisen ratio ΓH as a function of the magnetic field applied parallel to the a axis. The inset shows a temperature versus magnetic field phase diagram, where the gray dotted line has been obtained from inflection points of ΓH(H), and the blue and red lines indicate anomalies in the field dependence of the electronic specific heat. (B) Electronic specific heat divided by temperature Cel/T versus magnetic field applied parallel to the a axis at constant temperatures. The nuclear contribution has been subtracted from the raw data. Cel/T data at 0.15, 0.2, 0.3, and 0.6 K are shifted vertically for clarity by 0.15, 0.3, 0.45, and 0.6 J/mol K2, respectively. The blue and red dotted lines indicate the positions of specific heat anomalies.

  • Fig. 5 Possible scenario for the metamagnetic crossover in CeRhSn.

    T-H-Q phase diagram with a spin flop transition between two magnetically ordered states (8). The parameter Q indicates the strength of quantum fluctuations induced by geometrical frustration. A line of bicritical points (BCP, in red) separates two distinct magnetically ordered states. The quantum bicritical point (QBCP) is the point where TBCP approaches zero temperature. The purple solid and dotted lines indicate Hm and the metamagnetic crossover, respectively. The isostructural heavy fermion antiferromagnet YbAgGe is positioned near the QBCP, and its T-H phase diagram near Hm is illustrated by solid green lines (8). In this material, field-induced quantum critical behavior arises from the nearby QBCP. Paramagnetic CeRhSn displays a zero-field QCP induced by Q (solid red circle) and field-driven metamagnetic crossover (dotted purple line) of spin flop nature.

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