We present the pressure-temperature phase diagram La$_5$Co$_2$Ge$_3$ up to $\sim$ 5\,GPa, which was constructed from magnetization, resistivity and specific heat measurements. At ambient pressure, La$_5$Co$_2$Ge$_3$ is an itinerant ferromagnet with a Curie temperature $T_\textrm C\sim$ 4\,K. Upon increasing pressure up to $\sim$ 1.7\,GPa, $T_\textrm C$ is suppressed down to $\sim$ 3\,K. Upon further increasing pressure, our results suggest that La$_5$Co$_2$Ge$_3$ enters a different low-temperature ground state. The corresponding transition temperature, $T^*$, has a nonmonotonic pressure dependence up to $\sim$ 5\,GPa. Our results demonstrate that the ferromagnetic quantum critical point in La$_5$Co$_2$Ge$_3$ is avoided by the appearance of a different, likely magnetically ordered state that has an antiferromagnetic component.
LaCrGe$_3$ has attracted attention as a paradigm example of the avoidance of ferromagnetic (FM) quantum criticality in an itinerant magnet. By combining thermodynamic, transport, x-ray and neutron scattering as well as $μ$SR measurements, we refined the temperature-pressure phase diagram of LaCrGe$_3$. We provide thermodynamic evidence (i) for the first-order character of the FM transition when it is suppressed to low temperatures and (ii) for the formation of new phases at high pressures. From our microscopic data, we infer that short-range FM ordered clusters exist in these high-pressure pha…
We have investigated single crystals of two substitution series Ce(Ru$_{1-x}$Fe$_{x}$)PO and CeFe(As$_{1-y}$P$_{y}$)O in the vicinity to the quantum critical material CeFePO by means of magnetic-susceptibility and specific-heat measurements. We observe an antiferromagnetic ground state in the vicinity of the quantum critical point, with pronounced metamagnetic transitions for $H\parallel c$, which is the magnetically hard direction. Our results verify that a ferromagnetic quantum critical point is avoided in substituted CeFePO, because we clearly demonstrate that the ferromagnetic ground state…
Cubic CeZn shows a structural phase transition under pressure, and it modifies the ground state from an antiferromagnetic (AFM) state to a ferromagnetic (FM) state. To investigate how the FM state terminates at a quantum phase transition, we measured the electrical resistivity under pressure for a single crystal CeZn. The transition temperature into the FM state decreases monotonously with increasing pressure, accompanied by the pronounced Kondo effect, but a drastic change in the field response occurs before the ordered phase terminates. This result suggests that the FM quantum critical point…
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 expecte…
Cerium 4f electronic spin dynamics in single crystals of the heavy-fermion system CeFePO is studied by means of ac-susceptibility, specific heat and muon-spin relaxation ($μ$SR). Short-range static magnetism occurs below the freezing temperature Tg ~ 0.7 K, which prevents the system from accessing the putative ferromagnetic quantum critical point. In the $μ$SR, the sample-averaged muon asymmetry function is dominated by strongly inhomogeneous spin fluctuations below 10 K and exhibits a characteristic time-field scaling relation expected from glassy spin dynamics, strongly evidencing cooperativ…
CeRuPO is a rare example of a ferromagnetic (FM) Kondo-lattice system. External pressure suppresses the ordering temperature to zero at about $p_c\approx3$ GPa. Our ac-susceptibility and electrical-resistivity investigations evidence that the type of magnetic ordering changes from FM to antiferromagnetic (AFM) at about $p^*\approx0.87$ GPa. Studies in applied magnetic fields suggest that ferromagnetic and antiferromagnetic correlations compete for the ground state at $p>p^*$, but finally the AFM correlations win. The change in the magnetic ground-state properties is closely related to the pres…
We review the evidence supporting valence-fluctuation mediated superconductivity in CeCu2Si2 and CeCu2Ge2, where Tc reaches 2.4 K at high pressure. In these systems the valence and magnetic critical points, at p_V and p_c respectively, are well separated. Characteristic signatures associated with both phenomena are distinct. In contrast, the valence and spin fluctuation regions appear much closer in most Ce based compounds. Concerning d-transition metals, superconductivity in pure iron emerges in the pressure window 15-30 GPa with the onset of Tc up to almost 3 K. All relevant observations poi…
We demonstrate that the cluster-glass state emerges as ferromagnetic quantum criticality is avoided in itinerant ferromagnet Sr1-x(La0.5K0.5)xRuO3. In this compound, the ferromagnetic order is suppressed by increasing x, and then disappears at the critical concentration: x=0.5. In this x range, the present study reveals that no prominent feature is ascribed to the quantum critical fluctuations in specific heat. Instead, ac magnetic susceptibility exhibits a broad peak due to spontaneous spin freezing, and the peak temperature depends significantly on the frequency of the applied ac magnetic fi…
We report resistivity measurements under pressure for Kondo-lattice ferromagnet CeRh$_6$Ge$_4$, and present that a quantum ferromagnetic (FM) phase transition is easily achieved. In most clean metallic ferromagnets, a quantum critical point (QCP) at zero field is avoided by changing the FM transition to a discontinuous transition or to an antiferromagnetic transition. In CeRh$_6$Ge$_4$, to the contrary, the Curie temperature of 2.5 K decreases continuously as increasing pressure without any clear signature that the transition changes to first order. The obvious non Fermi liquid behavior is obs…
We report the temperature-pressure-magnetic field phase diagram of the ferromagnetic Kondo-lattice CeTiGe$_3$ determined by means of electrical resistivity measurements. Measurements up to $\sim$ 5.8 GPa reveal a rich phase diagram with multiple phase transitions. At ambient pressure, CeTiGe$_3$ orders ferromagnetically at $T_\text{C}$ = 14 K. Application of pressure suppresses $T_\text{C}$, but a pressure induced ferromagnetic quantum criticality is avoided by the appearance of two new successive transitions for $p$ $>$ 4.1 GPa that are probably antiferromagnetic in nature. These two transiti…
We present a detailed analysis of the non-analytic structure of the free energy for the itinerant ferromagnet near the quantum critical point in two and three dimensions. We analyze a model of electrons with an isotropic dispersion interacting through a contact repulsion. A fermionic version of the quantum order-by-disorder mechanism allows us to calculate the free energy as a functional of the dispersion in the presence of homogeneous and spiralling magnetic order. We re-sum the leading divergent contributions, to derive an algebraic expression for the non-analytic contribution to free energy…
LaCrGe$_3$ is an itinerant ferromagnet with a Curie temperature of $T_{\rm c}$ = 85 K and exhibits an avoided ferromagnetic quantum critical point under pressure through a modulated antiferromagnetic phase as well as tri-critical wing structure in its temperature-pressure-magnetic field ($T$-$p$-$H$) phase diagram. In order to understand the static and dynamical magnetic properties of LaCrGe$_3$, we carried out $^{139}$La nuclear magnetic resonance (NMR) measurements. Based on the analysis of NMR data, using the self-consistent-renomalization (SCR) theory, the spin fluctuations in the paramagn…
We present the electrical resistivity data under application of pressures up to $\sim$ 26 GPa and down to 50 mK temperatures on YbFe$_2$Zn$_{20}$. We find a pressure induced magnetic phase transition with an onset at $p_c$=18.2$\pm$0.8 GPa. At ambient pressure, YbFe$_2$Zn$_{20}$ manifests a heavy fermion, nonmagnetic ground state and the Fermi liquid behavior at low temperatures. As pressure is increased, the power law exponent in resistivity, $n$, deviates significantly from Fermi liquid behavior and tends to saturate with $n$ = 1 near $p_c$. A pronounced resistivity maximum, $T_\text{max}$, …
The Kondo effect in ferromagnetic quantum critical CeRh$_6$Ge$_4$
Max Planck Institute for Chemical Physics of Solids（马克斯·普朗克固体化学物理研究所）；Deutsches Elektronen-Synchrotron DESY（德国电子同步辐射中心）；Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室）；National Yang Ming Chiao Tung University（国立阳明交通大学）；National Synchrotron Radiation Research Center（国家同步辐射研究中心）；Technische Universität Braunschweig（布伦瑞克工业大学）
We study a generalization of the two-dimensional transverse-field Ising model, combining both ferromagnetic and antiferromagnetic two-body interactions, that hosts exact global and local Z2 gauge symmetries. Using exact diagonalization and stochastic series expansion quantum Monte Carlo methods, we confirm the existence of the topological phase in line with previous theoretical predictions. Our simulation results show that the transition between the confined topological phase and the deconfined paramagnetic phase is of first-order, in contrast to the conventional Z2 lattice gauge model in whic…
We combine structural and magnetic measurements to compare the different magnetic phase diagrams between the pressure and substitution studies in CeTiGe3. We report on the structural, magnetic, and electrical transport properties of single crystals of CeTi1-xVxGe3 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.9, and 1), and of polycrystalline samples (x = 0.5, 0.6, 0.7, 0.8), as well as structural properties of CeTiGe3 under pressure up to 9 GPa. The ferromagnetic ordering in CeTiGe3 is suppressed with V doping in CeTi1-xVxGe3, and suggests a possible ferromagnetic quantum critical point near x = 0.45. We per…
Quantum critical points (QCPs) emerge when a 2nd order phase transition is suppressed to zero temperature. In metals the quantum fluctuations at such a QCP can give rise to new phases including unconventional superconductivity. Whereas antiferromagnetic QCPs have been studied in considerable detail ferromagnetic (FM) QCPs are much harder to access. In almost all metals FM QCPs are avoided through either a change to 1st order transitions or through an intervening spin-density-wave (SDW) phase. Here, we study the prototype of the second case, NbFe$_2$. We demonstrate that the phase diagram can b…
