Ultrasonic investigation of a heavy fermion compound YbAgGe

Ubiquity of amplitude-modulated magnetic ordering in the H−T phase diagram of the frustrated non-Fermi-liquid YbAgGe

“…A coupling between the CE levels and the phonons may be one possibility, which has been seen in CeAuAl 3 for instance [29]. A strong coupling between CE levels and lattice vibrations would affect the elastic constants of YbAgGe, and has indeed been observed in previous reports [6]. Here, the softening of the elastic constants at H crit was attributed to the broadening of the 4 f level to gain Zeeman energy, which in turn yields a suppressed heavy-fermion behavior.…”

“…Several topical themes of modern solid-state researchfrustrated magnetism, non-Fermi-liquid behavior, and fieldinduced quantum criticality-converge in YbAgGe, a system that has been in the focus of experimental studies for more than a decade [1][2][3][4][5][6][7][8][9][10][11][12].…”

“…However, the c − d phase boundary has been intensively studied because of numerous anomalous properties. The Hall resistivity is enhanced [2], the sign of the Grüneisen parameter changes [7], the magnetization peaks [19], and the elastic constants soften [6]. Detailed magnetocaloric measurements [7] classify the sharp transition at T BCP ≈ 0.3 K and H BCP ≈ 4.5 T as a bicritical point (BCP), which may be tuned to become quantum critical.…”

Preserved entropy and fragile magnetism

“…In both cases the H = 0 T ordering occurs at relatively accessible temperatures and the magnetic field scales are neither unreasonably small, nor large, allowing for measurement in standard, lab-based systems. The YbAgGe H − T phase diagram has evolved over the past decade as the result of a slew of thermodynamic, transport and microscopic measurements [9,[12][13][14][15][16][17][18][19][20][21]. For the magnetic field applied in the easy plane of this hexagonal material there is a cascade of at least three ordered states, labeled a, b, c in figure 1(a), followed by phase d, which has a linear, low temperature resistivity over the whole field range.…”