Quantum Griffiths effects and smeared phase transitions in metals: theory and experiment

In this paper, we review theoretical and experimental research on rare region effects at quantum phase transitions in disordered itinerant electron systems. After summarizing a few basic concepts about phase transitions in the presence of quenched randomness, we introduce the idea of rare regions and discuss their importance. We then analyze in detail the different phenomena that can arise at magnetic quantum phase transitions in disordered metals, including quantum Griffiths singularities, smeared phase transitions, and cluster-glass formation. For each scenario, we discuss the resulting phase diagram and summarize the behavior of various observables. We then review several recent experiments that provide examples of these rare region phenomena. We conclude by discussing limitations of current approaches and open questions.Comment: 31 pages, 7 eps figures included, v2: discussion of the dissipative Ising chain fixed, references added, v3: final version as publishe

Interconfigurational fluctuations in Ce cubic compounds

La susceptibilité magnétique de trente composés cubiques de cérium est analysée, en supposant une dépendance en température des fluctuations de valence. Le schéma des niveaux d'énergie obtenu par l'ajustement de la susceptibilité mesurée est utilisé pour calculer d'autres propriétés physiques. On a trouvé une corrélation entre le rayon ionique du cérium et l'énergie d'excitation interconfigurationnelle.The magnetic susceptibility of thirty Ce cubic compounds is analyzed, assuming a temperature dependence of the valence fluctuations. The energy level scheme, obtained by fitting the measured susceptibility is used to calculate other physical properties. A correlation between the Ce ionic radius and the interconfigurational excitation energy has been found

Low Temperature Specific Heat Of Thgd Spin Glass

The specific heat measurements of ThGd alloys, with Gd concentration ranging from 0.15% to 10.8% and temperatures between 0.4 and 8 K, are reported. The characteristic spin glass specific heat enhancement and its maximum was found linearly proportional to Gd concentration. At low temperatures a T2 dependence of the specific heat was observed and the entropy associated with the magnetic ordering was the 70% of the expected value for the spin S = 7 2. © 1979.299671673Ruderman, Kittel, (1954) Phys. Rev., 96, p. 99Coles, (1973) Amorphous Magnetism, p. 169. , Plenum Press, New YorkAnderson, (1973) Amorphous Magnetism, p. 1. , Plenum Press, New YorkMydosh, (1978) J. Mag. Magn. Mater., 7, p. 237Allain, Bonnerot, Caroli, Jesser, Raphael, Chaleur spécifique et susceptibilité magnétique des solutions solides diluées d'yttrium-gadolinium (1967) Journal de Physique, 28, p. 98Phillips, Matthias, Heat Capacity of Ferromagnetic Superconductors (1961) Physical Review, 122, p. 105Isaacs, (1973) Phys. Rev., 8 B, p. 3301Child, Koehler, Millhouse, (1968) J. Appl. Phys., 39, p. 1329Hennepof, Leenstra, (1972) Physica, 62, p. 155J. Sereni, Unpublished PhD. Thesis, Universidad de Cuyo (1976)Satoh, Kumagai, (1973) J. Phys. Soc. Japan, 34, p. 391Klein, (1968) Phys. Rev., 173, p. 552. , and refs. thereinThouless, Anderson, Palmer, Solution of 'Solvable model of a spin glass' (1977) Philosophical Magazine, 35, p. 593Sherrington, Kirkpatrik, (1975) Phys. Rev. Lett., 35, p. 1792Souletie, Tournier, (1969) J. Low Temp. Phys., 1, p. 95Luengo, Maple, (1973) Solid State Commun., 12, p. 75

Evidence of spin-density-wave to spin-glass transformation in ynd alloys

Low-temperature ac and de magnetic susceptibility and specific-heat measurements have been carried out on YNd di1ute alloys, where the Nd concentrations are 1.9, 2.5, 4.5, 6.8, and 9.0 at. %. The most concentrated alloys (4.5, 6.8, and 9.0 at. %) present Iong-range antiferromagnetic spin-density-wave (SDW) order be1ow the criticai temperature, but some residual frustration is 1ikely to produce a reentrant behavior at Iow temperatures, where the magnetic data show strong irreversibility effects and the specific heat indicates the occurrence of an anisotropy induced gap near zero frequency in the density of states of the magnetic excitations. For the less concentrated alloys (1.9 and 2.5 at. %), the interacting magnetic state shows typical features of spin-glass behavior. The evolution to this state is ascribed as being due to impurity disorder and frustration which induces a breaking up of the SDW coherence into small domains

SEARCHING FOR A QUANTUM CRITICAL POINT IN FERROMAGNETIC Ce2.15Pd1.95In0.9

Low temperature magnetic and thermal ( C m ) properties of the ferromagnetic (FM) alloys Ce 2.15 (Pd 1− x Rh x ) 1.95 In 0.9 were investigated in order to explore the possibility for tuning towards a quantum critical point (QCP) by doping Pd with Rh. As expected, the magnetic transition decreases from T = 4.1K at x = 0 with increasing Rh concentration. However, the phase boundary splits into two transitions, the upper being antiferromagnetic (AF) whereas the lower FM. The AF phase boundary extrapolates to T N = 0 for x cr ≈ 0.65 whereas the first order FM transition vanishes at x ≈ 0.3. The quantum critical character of the T N → 0 point is inferred from the divergent T dependence of the tail of C m /T observed in the x = 0.5 and 0.55 alloys, and the tendency to saturation of the maximum of C m ( T N )/ T currently observed in exemplary Ce compounds when T N → 0. Beyond the critical concentration the unit cell volume deviates from the Vegard´s law in coincidence with a strong increase of the Kondo temperature.Fil: Sereni, Julian Gustavo Renzo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Bajas Temperaturas; ArgentinaFil: Giovannini, M.. Università  di Genova; ItaliaFil: Gomez Berisso, Mariano. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Bajas Temperaturas; ArgentinaFil: Saccone, A.. Università  di Genova; Itali

Evidence of spin-density-wave to spin-glass transformation in ynd alloys

Low-temperature ac and de magnetic susceptibility and specific-heat measurements have been carried out on YNd di1ute alloys, where the Nd concentrations are 1.9, 2.5, 4.5, 6.8, and 9.0 at. %. The most concentrated alloys (4.5, 6.8, and 9.0 at. %) present Iong-range antiferromagnetic spin-density-wave (SDW) order be1ow the criticai temperature, but some residual frustration is 1ikely to produce a reentrant behavior at Iow temperatures, where the magnetic data show strong irreversibility effects and the specific heat indicates the occurrence of an anisotropy induced gap near zero frequency in the density of states of the magnetic excitations. For the less concentrated alloys (1.9 and 2.5 at. %), the interacting magnetic state shows typical features of spin-glass behavior. The evolution to this state is ascribed as being due to impurity disorder and frustration which induces a breaking up of the SDW coherence into small domains

Multiprobe high-pressure experiments in CePd_0.6Rh_0.4 and CePd₃

Results of recent multiprobe high-pressure experiments on ferromagnetic CePd0.6Rh0.4CePd0.6Rh0.4 and intermediate-valent CePd3CePd3 are presented. Simultaneous resistivity (ρρ), thermopower (S ), and ac heat capacity measurements show that the long-range ferromagnetic state of CePd0.6Rh0.4CePd0.6Rh0.4 vanishes in the proximity of a sharp valence-crossover pressure View the MathML sourcepV≈7.5GPa, i.e. before reaching a quantum critical point. However, a magnetic signal that is progressively suppressed is still detected at higher pressures. For CePd3CePd3, the results of simultaneous ρ(T,p)ρ(T,p) and S(T,p)S(T,p) measurements up to View the MathML source22GPa and down to the mK temperature range show a surprisingly weak pressure dependence