Physics of Polymorphic Transitions in CeRuSn

We report a detailed study of the polymorphic transitions in ternary stannide CeRuSn on high quality single crystals through a combination of X-ray diffraction experiments conducted at 300, 275 and 120 K, and measurements of the thermal expansion, magnetization, and resistivity, along main crystallographic axes. In addition, the transition was followed as a function of pressure up to 0.8 GPa. The present X-ray diffraction data show that the room temperature polymorph consists of the lattice doubled along the c axis with respect to the CeCoAl-type structure consistent with previous reports. Upon cooling, the compound undergoes two successive transitions, first to a quintuple (290 K) and than to a triple CeCoAl superstructure at 225 K. The transitions are accompanied by a tremendous volume change due to a strong shrinking of the lattice along the c axis, which is clearly observed in thermal expansion. We advance arguments that the volume collapse originates from an increasing number of crystallographically inequivalent Ce sites and the change of ratio between the short and long Ce-Ru bonds. The observed properties of the polymorphic transition in CeRuSn are reminiscent of the transition in elementary Cerium, suggesting that similar physics, i.e., a Kondo influenced transition and strong lattice vibrations might be the driving forces

Tuning Heavy Fermion Systems into Quantum Criticality by Magnetic Field

We discuss a series of thermodynamic, magnetic and electrical transport experiments on the two heavy fermion compounds CeNi2Ge2 and YbRh2Si2 in which magnetic fields, B, are used to tune the systems from a Non-Fermi liquid (NFL) into a field-induced FL state. Upon approaching the quantum-critical points from the FL side by reducing B we analyze the heavy quasiparticle (QP) mass and QP-QP scattering cross sections. For CeNi2Ge2 the observed behavior agrees well with the predictions of the spin-density wave (SDW) scenario for three-dimensional (3D) critical spin-fluctuations. By contrast, the observed singularity in YbRh2Si2 cannot be explained by the itinerant SDW theory for neither 3D nor 2D critical spinfluctuations. Furthermore, we investigate the magnetization M(B) at high magnetic fields. For CeNi2Ge2 a metamagnetic transition is observed at 43 T, whereas for YbRh2Si2 a kink-like anomaly occurs at 10 T in M vs B (applied along the easy basal plane) above which the heavy fermion state is completely suppressed.Comment: 15 pages, 8 figures, submitted to Journal of Low Temperature Physics, special Series on "High Magnetic Field Facilities

Scaling approach to itinerant quantum critical points

Based on phase space arguments, we develop a simple approach to metallic quantum critical points, designed to study the problem without integrating the fermions out of the partition function. The method is applied to the spin-fermion model of a T=0 ferromagnetic transition. Stability criteria for the conduction and the spin fluids are derived by scaling at the tree level. We conclude that anomalous exponents may be generated for the fermion self-energy and the spin-spin correlation functions below $d=3$, in spite of the spin fluid being above its upper critical dimension.Comment: 3 pages, 2 figures; discussion of the phase space restriction modified and, for illustrative purposes, restricted to the tree-level analysis of the ferromagnetic transitio

Dissymmetrical tunnelling in heavy fermion metals

A tunnelling conductivity between a heavy fermion metal and a simple metallic point is considered. We show that at low temperatures this conductivity can be noticeably dissymmetrical with respect to the change of voltage bias. The dissymmetry can be observed in experiments on the heavy fermion metals whose electronic system has undergone the fermion condensation quantum phase transition.Comment: 7 pages, Revte

Universal Behavior of Heavy-Fermion Metals Near a Quantum Critical Point

The behavior of the electronic system of heavy fermion metals is considered. We show that there exist at least two main types of the behavior when the system is nearby a quantum critical point which can be identified as the fermion condensation quantum phase transition (FCQPT). We show that the first type is represented by the behavior of a highly correlated Fermi-liquid, while the second type is depicted by the behavior of a strongly correlated Fermi-liquid. If the system approaches FCQPT from the disordered phase, it can be viewed as a highly correlated Fermi-liquid which at low temperatures exhibits the behavior of Landau Fermi liquid (LFL). At higher temperatures $T$, it demonstrates the non-Fermi liquid (NFL) behavior which can be converted into the LFL behavior by the application of magnetic fields $B$. If the system has undergone FCQPT, it can be considered as a strongly correlated Fermi-liquid which demonstrates the NFL behavior even at low temperatures. It can be turned into LFL by applying magnetic fields $B$. We show that the effective mass $M^*$ diverges at the very point that the N\'eel temperature goes to zero. The $B-T$ phase diagrams of both liquids are studied. We demonstrate that these $B-T$ phase diagrams have a strong impact on the main properties of heavy-fermion metals such as the magnetoresistance, resistivity, specific heat, magnetization, volume thermal expansion, etc.Comment: Revtex, 11 pages, revised and accepted by JETP Let

Non Fermi Liquid behavior in the under-screened Kondo model

Using the Schwinger boson spin representation, we reveal a new aspect to the physics of a partially screened magnetic moment in a metal, as described by the spin $S$ Kondo model. We show that the residual ferromagnetic interaction between a partially screened spin and the electron sea destabilizes the Landau Fermi liquid, forming a singular Fermi liquid with a $1/ (T \ln ^{4} (T_{K}/T))$ divergence in the low temperature specific heat coefficient $C_{V}/T$. A magnetic field $B$ tunes this system back into Landau Fermi liquid with a Fermi temperature proportional to $B \ln^2 (T_K/B)$. We discuss a possible link with field-tuned quantum criticality in heavy electron materials.Comment: References corrected. Minor changes to tex

Long range order and two-fluid behavior in heavy electron materials

The heavy electron Kondo liquid is an emergent state of condensed matter that displays universal behavior independent of material details. Properties of the heavy electron liquid are best probed by NMR Knight shift measurements, which provide a direct measure of the behavior of the heavy electron liquid that emerges below the Kondo lattice coherence temperature as the lattice of local moments hybridizes with the background conduction electrons. Because the transfer of spectral weight between the localized and itinerant electronic degrees of freedom is gradual, the Kondo liquid typically coexists with the local moment component until the material orders at low temperatures. The two-fluid formula captures this behavior in a broad range of materials in the paramagnetic state. In order to investigate two-fluid behavior and the onset and physical origin of different long range ordered ground states in heavy electron materials, we have extended Knight shift measurements to URu$_2$Si$_2$, CeIrIn$_5$ and CeRhIn$_5$. In CeRhIn$_5$ we find that the antiferromagnetic order is preceded by a relocalization of the Kondo liquid, providing independent evidence for a local moment origin of antiferromagnetism. In URu$_2$Si$_2$ the hidden order is shown to emerge directly from the Kondo liquid and so is not associated with local moment physics. Our results imply that the nature of the ground state is strongly coupled with the hybridization in the Kondo lattice in agreement with phase diagram proposed by Yang and Pines.Comment: 9 pages, 13 figure

Break up of heavy fermions at an antiferromagnetic instability

We present results of high-resolution, low-temperature measurements of the Hall coefficient, thermopower, and specific heat on stoichiometric YbRh2Si2. They support earlier conclusions of an electronic (Kondo-breakdown) quantum critical point concurring with a field induced antiferromagnetic one. We also discuss the detachment of the two instabilities under chemical pressure. Volume compression/expansion (via substituting Rh by Co/Ir) results in a stabilization/weakening of magnetic order. Moderate Ir substitution leads to a non-Fermi-liquid phase, in which the magnetic moments are neither ordered nor screened by the Kondo effect. The so-derived zero-temperature global phase diagram promises future studies to explore the nature of the Kondo breakdown quantum critical point without any interfering magnetism.Comment: minor changes, accepted for publication in JPS

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