Pressure-dependent electrical resistivity and ac-calorimetry measurements on single crystals of the heavy-fermion antiferromagnet U(2)Zn(17) at pressures to 5.5 GPa reveal that the low temperature magnetic order changes above ∼3 GPa. The Neél temperature (T(N) = 9.7 K at ambient pressure) decreases slowly for pressures below 3 GPa, but above this pressure a new magnetic state develops at T(M)≈8.7 K. This magnetic phase becomes more stable with pressure increase (dT(M)/dP = 1 K GPa(-1), T(M) = 10.5 K at 5.3 GPa). The heavy-electron state in U(2)Zn(17) is robust against pressure--the electronic specific heat coefficient γ≈0.4 J mol(-1) K(-2) is nearly pressure independent for both magnetic phases. Magnetic ac-susceptibility measurements show that the pressure-induced state is not ferromagnetic.

The Kondo lattice system Ce Zn0.66 Sb2 is studied by electrical resistivity and ac magnetic susceptibility measurements at several pressures. At P=0 kbar, ferromagnetic and antiferromagnetic transitions appear at 3.6 and 0.8 K, respectively. The electrical resistivity at TN dramatically changes from the Fisher-Langer type (ferromagneticlike) to the Suezaki-Mori (SM) type near 17 kbar, i.e., from a positive divergence to a negative divergence in the temperature derivative of the resistivity. The pressure-induced SM-type anomaly, which shows thermal hysteresis, is easily suppressed by a small magnetic field (1.9 kOe for 19.8 kbar), indicating a weakly first-order nature of the transition. By subtracting a low-pressure data set, we directly compare the resistivity anomaly with the SM theory without any assumption on backgrounds, where the negative divergence in dρ dT is ascribed to enhanced critical fluctuations in the presence of superzone gaps. © 2005 The American Physical Society.

Measurements of electrical resistivity at applied pressures up to 55 kbar have been carried out on single crystals of the ferromagnet CeNiSb 3 revealing a complex magnetic phase diagram. The ferromagnetism at ambient pressure at T c=6 K first increases with pressure up to 25 kbar, then decreases slightly for P > 25 kbar. In the pressure range 35 kbar≤P≤55 kbar, a second anomaly in the resistivity is found at T M2, below another higher temperature magnetic phase that is conjectured to be different than the ferromagnetism present at lower pressure. Changes in the physical properties such as the residual resistivity, and low temperature maximum in ρ mag(T) suggest a modification of the electronic structure where the two magnetic phases coexist with one another above 35 kbar. The critical pressure for the suppression of the lower temperature phase is estimated to be P cmag2≃55 kbar, while the increase of both the T 2 coefficient of the electrical resistivity A and residual resistivity ρ 0 is suggestive of a quantum critical point associated with the higher temperature magnetic phase at P cmag1∼60 kbar. ©2005 The American Physical Society.

The f-electron compound CeAuSb 2, which crystallizes in the ZrCuSi 2-type tetragonal structure, orders antiferromagnetically between 5 and 6.8 K, where the antiferromagnetic transition temperature T N depends on the occupancy of the Au site. Here we report the electrical resistivity and heat capacity of a high-quality crystal CeAuSb 2 with T N of 6.8 K, the highest for this compound. The magnetic transition temperature is initially suppressed with pressure, but is intercepted by a new magnetic state above 2.1 GPa. The new phase shows a dome shape with pressure and coexists with another phase at pressures higher than 4.7 GPa. The electrical resistivity shows a T2 Fermi-liquid behavior in the complex magnetic state, and the residual resistivity and the T2 resistivity coefficient increases with pressure, suggesting the possibility of a magnetic quantum critical point at a higher pressure. © 2012 American Physical Society.

β-UB2C exhibits itinerant ferromagnetism below T c 75 K; whereas, UNiSi2 exhibits ferromagnetism of localized uranium moments below Tc 95 K and Kondo lattice behaviour at higher temperatures. We have found that ferromagnetism in both compounds is quenched at high pressure. In β-UB2C the Curie temperature continuously approaches zero at 2.95 GPa, where resistivity and specific heat reveal the behaviour similar to that observed in Ce-based antiferromagnets near quantum critical points. In UNiSi2 the Curie temperature decreases gradually to 25 K at 5.34 GPa. Above this pressure hysteretic phenomena appear in the temperature dependences of resistivity between 5 and 17 K, signalling a change in the magnetic order. No signature of magnetism was found above 5.5 GPa down to 1.15 K. Kondo lattice behaviour of resistivity with the enhanced residual resistivity is observed in UNiSi2 above 5.5 GPa. © Published under licence by IOP Publishing Ltd.

Electrical resistivity and ac-calorimetric measurements reveal a complex magnetic phase diagram for single crystals of the ferromagnetic Kondo-lattice compound CeAgSb2 at high pressures up to 80 kbar. The ferromagnetic order at TC = 9.6 K at ambient pressure is completely suppressed at a critical pressure PC = 35 kbar. Another magnetic transition, possibly antiferromagnetic, found above 27 kbar, attains a maximum value TN 6 K at 44 kbar and then appears to be completely suppressed by 50 kbar. Thermodynamic and transport measurements in the ferromagnetic state indicate an energy gap Δ 30 K in the spin-wave excitation spectrum at ambient pressure which decreases to Δ 10 K at P = 30 kbar. No superconductivity is observed at ambient pressure above T 0.1 K, under applied pressure in the ferromagnetic state (P = 28.5 kbar), nor in the antiferromagnetic state (P = 33 − 46 kbar) above 0.3 K. © 2003 The American Physical Society.

The temperature dependence of the specific heat and of the resistivity under pressure has been measured for single crystals of the semiconductor Ce3 Au3 Sb4. The transport data follow an exponential activation and variable range hopping at low T, consistent with weak disorder and localization, while C / T has a - ln T dependence with large entropy. Thus the properties of Ce3 Au3 Sb4 are very different from those of ordinary Kondo insulators. © 2007.

In four classes of materials - the layered copper oxides, organics, iron pnictides and heavy-fermion compounds - an unconventional superconducting state emerges as a magnetic transition is tuned towards absolute zero temperature, that is, towards a magnetic quantum critical point (QCP). In most materials, the QCP is accessed by chemical substitution or applied pressure. CeCoIn 5 is one of the few materials that are 'born' as a quantum critical superconductor and, therefore, offers the opportunity to explore the consequences of chemical disorder. Cadmium-doped crystals of CeCoIn 5 are a particularly interesting case where Cd substitution induces long-range magnetic order, as in Zn-doped copper oxides. Applied pressure globally suppresses the Cd-induced magnetic order and restores bulk superconductivity. Here we show, however, that local magnetic correlations, whose spatial extent decreases with applied pressure, persist at the extrapolated QCP. The residual droplets of impurity-induced magnetic moments prevent the reappearance of conventional signatures of quantum criticality, but induce a heterogeneous electronic state. These discoveries show that spin droplets can be a source of electronic heterogeneity and emphasize the need for caution when interpreting the effects of tuning a correlated system by chemical substitution. © 2014 Macmillan Publishers Limited.

The magnetic, thermal, and transport properties of single crystals of the narrow gap semiconductor Ce3 Au3 Sb4 have been studied. Transport data are consistent with an exponential activation and variable range hopping at low temperatures, which is characteristic of weak disorder and localization. The specific heat and the magnetic susceptibility data suggest the existence of a large density of states of localized states in the gap. The physics is then different from standard Kondo insulators. From its unique physical properties, we propose a three band model (valence, conduction, and f bands) with weak disorder that explains most of the experimental findings. © 2007 The American Physical Society.