O. Trovarelli

The break-up of heavy electrons at a quantum critical point

The point at absolute zero where matter becomes unstable to new forms of order is called a quantum critical point (QCP). The quantum fluctuations between order and disorder that develop at this point induce profound transformations in the finite temperature electronic properties of the material. Magnetic fields are ideal for tuning a material as close as possible to a QCP, where the most intense effects of criticality can be studied. A previous study on the heavy-electron material YbRh2Si2 found that near a field-induced QCP electrons move ever more slowly and scatter off one another with ever increasing probability, as indicated by a divergence to infinity of the electron effective mass and scattering cross-section. But these studies could not shed light on whether these properties were an artefact of the applied field, or a more general feature of field-free QCPs. Here we report that, when germanium-doped YbRh2Si2 is tuned away from a chemically induced QCP by magnetic fields, there is a universal behaviour in the temperature dependence of the specific heat and resistivity: the characteristic kinetic energy of electrons is directly proportional to the strength of the applied field. We infer that all ballistic motion of electrons vanishes at a QCP, forming a new class of conductor in which individual electrons decay into collective current-carrying motions of the electron fluid.

Hall-effect evolution across a heavy-fermion quantum critical point

A quantum critical point (QCP) develops in a material at absolute zero when a new form of order smoothly emerges in its ground state. QCPs are of great current interest because of their singular ability to influence the finite temperature properties of materials. Recently, heavy-fermion metals have played a key role in the study of antiferromagnetic QCPs. To accommodate the heavy electrons, the Fermi surface of the heavy-fermion paramagnet is larger than that of an antiferromagnet. An important unsolved question is whether the Fermi surface transformation at the QCP develops gradually, as expected if the magnetism is of spin-density-wave (SDW) type, or suddenly, as expected if the heavy electrons are abruptly localized by magnetism. Here we report measurements of the low-temperature Hall coefficient (RH)—a measure of the Fermi surface volume—in the heavy-fermion metal YbRh2Si2 upon field-tuning it from an antiferromagnetic to a paramagnetic state. RH undergoes an increasingly rapid change near the QCP as the temperature is lowered, extrapolating to a sudden jump in the zero temperature limit. We interpret these results in terms of a collapse of the large Fermi surface and of the heavy-fermion state itself precisely at the QCP.

Magnetic-field induced quantum critical point in YbRh2Si2

We report low-temperature calorimetric, magnetic, and resistivity measurements on the antiferromagnetic (AF) heavy-fermion metal YbRh(2)Si(2) ( T(N)=70 mK) as a function of magnetic field B. While for fields exceeding the critical value B(c0) at which T(N)-->0 the low-temperature resistivity shows an AT2 dependence, a 1/(B-B(c0)) divergence of A(B) upon reducing B to B(c0) suggests singular scattering at the whole Fermi surface and a divergence of the heavy quasiparticle mass. The observations are interpreted in terms of a new type of quantum critical point separating a weakly AF ordered from a weakly polarized heavy Landau-Fermi liquid state.

Temperature Dependence of the Kondo Resonance and Its Satellites in CeCu2Si2

We present high-resolution photoemission spectroscopy studies on the Kondo resonance of the strongly correlated Ce system CeCu2Si2. By exploiting the thermal broadening of the Fermi edge we analyze position, spectral weight, and temperature dependence of the low-energy 4f spectral features, whose major weight lies above the Fermi level E(F). We also present theoretical predictions based on the single-impurity Anderson model using an extended noncrossing approximation, including all spin-orbit and crystal field splittings of the 4f states. The excellent agreement between theory and experiment provides strong evidence that the spectral properties of CeCu2Si2 can be described by single-impurity Kondo physics down to T approximately 5 K.

Complex magnetic order in EuRh2Si2

Abstract We present resistivity, magnetoresistance, magnetisation and heat capacity results on EuRh2Si2. Magnetic susceptibility and specific heat data confirm the reported magnetic transition at 25 K and reveal an additional magnetic transition at 23.5 K. Isothermal magnetization reveal a metamagnetic transition at a rather low field, Bm≈0.1 T. The resistivity data show a large decrease of ρ at the lowest transition at 23.5 K, and the metamagnetic transition lead to a large negative magnetoresistance at 2 K. The temperature dependence of the specific heat below the transition is unusual, being proportional to T over a significant temperature range.

An unconventional metallic state in YbRh2(Si1-xGex)2: a high pressure study

We present a detailed pressure study of the electrical resistivity ρ(T) and the specific heat C(T) of the non-Fermi-liquid (NFL) compound YbRh2Si2 and of ρ(T) for a single crystal in which 5 at.% of Si is replaced by isoelectronic Ge. The magnetic phase diagram is deduced up to p ~ = 2.5 GPa. A comparison of the effects of the volume change introduced by doping and/or by hydrostatic pressure will be given. We show that the NFL behaviour observed in ρ(T) as well as the magnetic phase diagram are not influenced by the disorder introduced by alloying.

Coexistence of magnetic order and heavy fermion behavior in Ce7X3

Abstract Low temperature specific heat and magnetic susceptibility measurements on Ce 7 X 3 (X = Rh, Ni) compounds are reported. Ce 7 Rh 3 shows an onset of spontaneous magnetization at 7.2 K. About 40% of the expected R in 2 entropy gain is found in the paramagnetic phase, related to a γ HT = 1.1 J/K 2 mol specific heat coefficient. In Ce 7 Ni 3 an antiferromagnetic transition is observed at 1.7 K, superimposed to a large C P / T electronic contribution, described by a γ HT = 0.9 J/K 2 mol coefficient for T > 10 K. These results are interpreted in terms of the three different crystal sublattices of Ce. The entropy gain related to each transition allows to identify the behaviour of each sublattice. In the case of Ce 7 Ni 3 one sublattice behaves as a heavy fermion system, with a value of C P / T = 3.3 J/K 2 Ce at. Such a behaviour is confirmed under an applied magnetic field.

Transport and magnetic properties of new ternary Ce2T3X9-compounds (T = Rh, Ir, X = Al, Ga)

Abstract Measurements of thermoelectric power, electrical resistivity, and magnetic susceptibility on new Ce 2 T 3 X 9 -compounds (T=Rh, Ir; X=Al, Ga) are presented. These compounds crystallize in the orthorhombic Y 2 Co 3 Ga 9 -structure. The transport data show for each compound two characteristic anomalies, around 100 K and around 20 K, respectively. In contrast to many Ce-systems, where such anomalies can be ascribed to crystalline-field and Kondo effects, respectively, for these systems the temperature dependence of the susceptibility and the specific heat suggest an intermediate valent Ce-state which shows additional low energy interactions.

Role of Ce sublattices in the thermal and magnetic properties of Ce7X3 (X=Ni, Ru, Rh, Ir, Pd, and Pt) compounds

The role of the Ce sublattices in the thermal and magnetic properties of the Ce7X3 (X=Ni, Ru, Rh, Ir, Pd, and Pt) family of compounds is studied by means of ac and dc magnetic susceptibility, magnetization and mainly specific-heat experiments in applied magnetic field. The experimental data show that in these compounds there is coexistence of magnetic order, heavyfermion and intermediate-valence behavior, which is interpreted in terms of the contribution of the three different sublattices present in the crystalline structure of Th7Fe3-type (denoted by 1CeI, 3CeII, and 3CeIII). From the available volume of the CeIII atoms in their crystallographic environment it is found that sublattice CeIII has an intermediate-valence behavior, whereas from entropic considerations sublattices CeI and CeII are identified as responsible for the magnetic order or heavy-fermion behavior, depending on the Ce-ligand electronic structure. This systematics evidences a clear correlation between the thermal and magnetic properties of these compounds and the position of the respective Ce-ligands in the periodic table, through the particular sensitivity of Ce to the environmental conditions.

Coexistence of superconductivity and antiferromagnetism in the heavy-fermion superconductor CeCu2(Si1-xGex)2 probed by means of Cu nuclear quadrupole resonance - a test case for the SO(5) theory

We report on the basis of Cu-NQR measurements that superconductivity (SC) and antiferromagnetism (AF) coexist on a microscopic level in CeCu_{2}(Si_{1-x}Ge_{x})_{2}, once a tiny amount of 1%Ge (x = 0.01) is substituted for Si. This coexistence arises because Ge substitution expands the unit-cell volume in nearly homogeneous CeCu2Si2 where the SC coexists with slowly fluctuating magnetic waves. We propose that the underlying exotic phases of SC and AF in either nearly homogeneous or slightly Ge substituted CeCu2Si2 are accountable based on the SO(5) theory that unifies the SC and AF. We suggest that the mechanism of the SC and AF is common in CeCu2Si2.